Life cycle assessment of hydrogen production, storage, and utilization toward sustainability

In the pursuit of sustainable energy solutions, hydrogen emerges as a promising candidate for decarbonization. The United States has the potential to sell wind energy at a record‐low price of 2.5 cents/kWh, making hydrogen production electricity up to four times cheaper than natural gas. Hydrogen's appeal stems from its highly exothermic reaction with oxygen, producing only water as a byproduct. With an energy content equivalent to 2.4 kg of methane or 2.8 kg of gasoline per kilogram, hydrogen boasts a superior energy‐to‐weight ratio compared to fossil fuels. However, its energy‐to‐volume ratio, exemplified by liquid hydrogen's 8.5 MJ.L−1 versus gasoline's 32.6 MJ.L−1, presents a challenge, requiring a larger volume for equivalent energy. In addition, this review employs life cycle assessment (LCA) to evaluate hydrogen's full life cycle, including production, storage, and utilization. Through an examination of LCA methodologies and principles, the review underscores its importance in measuring hydrogen's environmental sustainability and energy consumption. Key findings reveal diverse hydrogen production pathways, such as blue, green, and purple hydrogen, offering a nuanced understanding of their life cycle inventories. The impact assessment of hydrogen production is explored, supported by case studies illustrating environmental implications. Comparative LCA analysis across different pathways provides crucial insights for decision‐making, shaping environmental and sustainability considerations. Ultimately, the review emphasizes LCA's pivotal role in guiding the hydrogen economy toward a low‐carbon future, positioning hydrogen as a versatile energy carrier with significant potential.


| INTRODUCTION
In the face of escalating energy demands and the challenges posed by conventional fossil fuel resources, the world grapples with finding sustainable solutions.Hydrogen (H 2 ) emerges as a versatile energy carrier, offering emission-free solutions with its high specific energy, elevated combustion temperatures, and substantial storage capacity (Bellocchi et al., 2023;Pathak et al., 2023).The drive toward environmentally sustainable and energy-efficient hydrogen production gains momentum, spotlighting green hydrogen and blue hydrogen from water electrolysis and steam methane reforming (SMR), respectively.Blue hydrogen, integrating CO 2 capture with reforming, serves as a transitional solution in the shift from fossil fuels to a hydrogen economy, while green hydrogen, produced through electrolysis with renewable energies, takes the forefront in large-scale projects (Lei et al., 2021;Yuan et al., 2023).
Hydrogen's recognized potential lies in its high energy density and clean-burning attributes, suitable for diverse applications from transportation to power generation.Unlike fossil fuels, hydrogen's combustion produces no harmful emissions, aligning with climate change mitigation routes.Recent interest in hydrogen reflects its role in the low-carbon economy, exploring new technologies for this versatile fuel's potential.The high energy density of hydrogen, crucial in weight-sensitive transportation, coupled with its flexibility as a gas or liquid, positions it as a promising fuel source across various applications.Hydrogen fuel cells offer clean and efficient electricity generation, with the overall process emitting only water vapor.The unique attributes of hydrogen make it a flexible energy carrier, positioned to assume a pivotal role in the worldwide energy scenario in the coming years, specifically in the realms of power generation, transportation, and industrial uses (Abbas et al., 2023;Hassan et al., 2023a;Hassan et al., 2023b).
Much like electricity, H 2 acts as an energy carrier rather than a direct energy source.Its vital role resides in efficiently storing renewable energies and mitigating wastage during periods of non-utilization owing to its capacity for storage, usability, and transportability.However, the conventional color-coding system oversimplifies distinctions, assuming that green hydrogen always exhibits lower carbon emissions than blue or gray hydrogen, which is not universally accurate.Blue hydrogen, despite releasing no carbon during its process or at the point of use, is perceived as less safe than green hydrogen.An example is the emission of greenhouse gases like CH 4 , SO x , NO x , and CO 2 during the growth or thermochemical conversions of bioenergy feedstocks associated with green hydrogen.The carbon capture and storage technique in blue hydrogen significantly reduces toxic emissions.Yet, the production of photovoltaic (PV) panels, a key renewable energy technology, contributes to a substantial carbon footprint, generating hazardous waste throughout its lifecycle from material extraction to energy-intensive manufacturing processes.
Additionally, the 30-year lifespan of solar panels necessitates careful management as special waste at the end of their life (Osman et al., 2022).Despite the dominance expected for green hydrogen in the sustainable energy system of the future, blue hydrogen remains a vital player in expediting the energy transition (Yuan et al., 2023).In addition, the transition to sustainable energy systems requires a comprehensive assessment of hydrogen energy and the hydrogen economy.
Therefore, understanding the environmental impacts and sustainability of hydrogen as an energy carrier is crucial.A comprehensive assessment entails evaluating the entire life cycle of hydrogen, including its production, storage, distribution, and utilization stages.This approach, known as life cycle assessment (LCA), provides insights into the environmental footprint of hydrogen and allows for a thorough examination of its sustainability (Osman et al., 2022;Weidner et al., 2023).By conducting a detailed analysis of the LCA of hydrogen, we can gain a deeper understanding of the different production pathways, their associated life cycle inventories, and the environmental impacts they pose.This assessment extends beyond production and explores storage and distribution technologies, considering infrastructure requirements, energy inputs, and their environmental implications.Moreover, an evaluation of the environmental performance of hydrogen must be conducted, particularly in its diverse applications, such as power production and transportation.
The novelty and originality of this work lie in its comprehensive application of LCA methodology to evaluate the environmental sustainability and energy consumption of hydrogen systems throughout their entire life cycle.The study goes beyond examining hydrogen production alone and extends its analysis to include storage, distribution, and utilization stages.This holistic approach provides valuable insights into the environmental footprint of hydrogen and offers a thorough examination of its sustainability.Furthermore, the review explores different pathways of hydrogen production, including blue, green, purple and biohydrogen, and investigates their life cycle inventories.By conducting a comparative analysis of these pathways, the study aims to provide decision-making support and influence the future trajectory of the hydrogen economy based on insights from LCAs.The work also identifies key environmental and sustainability considerations associated with hydrogen production, storage, and utilization.By highlighting the challenges, limitations, and implications of hydrogen energy systems, the study contributes to a deeper understanding of the environmental impacts and potential of hydrogen as a versatile energy vector.

| HYDROGEN PRODUCTION
Hydrogen production is broadly classified into three main methods: blue, green, and purple.Blue H 2 utilizes coal gasification or natural gas coupled with carbon capture and storage (CCS) to alleviate carbon emissions.On the other hand, green H 2 is generated through the electrolysis process using renewable energy sources like wind or solar power, emphasizing a sustainable and environmentally friendly approach (Dash et al., 2023;Ishaq et al., 2022).Purple hydrogen, a relatively newer concept, combines blue and green methods by using renewable energy for the process but integrating CCS to address any remaining emissions.However, the cost of hydrogen production poses a significant obstacle, particularly for green hydrogen.Steam reforming-based production is about three times more costly than natural gas-based production per unit of energy.
Electrolysis, with electricity, costs around 5 cents/kWh and is nearly twice as expensive as natural gas-based methods.Recent reports, like the one from Renewable World Energy, suggest the potential for the United States to sell wind energy at a record-low price of 2.5 cents/kWh.This could make hydrogen production electricity less expensive than that from natural gas by fourfold.Despite cost challenges, hydrogen can be blended with natural gas for easier transportation through existing pipelines.Additionally, this blending can help reduce CO 2 emissions from current natural gas reforming plants (Ishaq et al., 2022).
It is worth noting that biohydrogen, a carbon-free and high-energy-dense fuel, holds promise for clean energy production, though its complexity poses challenges for optimization (Alagumalai et al., 2023).It is generated biologically through various methods like fermentation (Sarangi & Nanda, 2020;Wang et al., 2012), biophotolysis (Ghirardi et al., 2014;Javed et al., 2022), and microbial electrolysis cells (Cardeña et al., 2019;Gautam et al., 2023), utilizing biowastes, thus reducing costs and pollution, as shown in Figure 1.This sustainable approach also addresses waste management and land pollution issues.With its potential to be derived from non-depletable resources like waste, biohydrogen garners considerable interest as a future energy carrier.Microbial electrolysis stands as the most common method for biohydrogen production, utilizing microbial fuel cells to convert organic matter into electricity and then hydrogen gas.Photobiological, dark fermentation and photo fermentative methods also utilize biological organisms for hydrogen production, gaining popularity for their sustainability and cost-effectiveness.The key to cost-effective biohydrogen production lies in efficiently converting complex organic feedstock into fermentable glucose.Different biomass types require specific pretreatment methods; for instance, lignocellulosic materials necessitate thorough pretreatment due to their complex composition of cellulose, hemicellulose, and lignin (Alagumalai et al., 2023).

| Blue hydrogen
Blue hydrogen, derived from fossil fuels through the utilization of CCS, emerges as a cost-effective alternative, particularly in comparison to green hydrogen (Noussan et al., 2020).The development of this technology has garnered international interest, with many countries aiming to promote its implementation due to its economic feasibility in industry.It serves as a transition energy carrier, enabling the utilization of existing infrastructure and offering a sustainable solution for fossil fuel-dependent nations like Canada, Iran, Norway, Qatar, the Russian Federation, and the United States (Yu et al., 2021).Notably, the Quest Carbon Capture and Storage Project in Western Canada, operated by Royal Dutch/ Shell, stands as the world's pioneering commercial-scale carbon capture, utilization, and storage project dedicated to blue hydrogen production.In its initial year, the facility surpassed expectations by storing one million tons of carbon dioxide ahead of schedule, accumulating a total of five million tons from 2015 to 2020 (Rock et al., 2017;Yu et al., 2021).
Alternative technologies for blue hydrogen production, influenced by factors like feedstock availability and economic viability, are pivotal for ensuring its sustainability.A crucial factor among these considerations is the availability of feedstock within a region to meet market demands, serving as a pivotal element for sustainable H 2 production.Emphasizing the significance, it is crucial to acknowledge that blue hydrogen is generated through thermal processes, converting natural gas or coal into hydrogen and incorporating CCS.
In contrast, gray hydrogen utilizes thermal sources similar to blue hydrogen but excludes CCS.Notably, natural gas, widely employed for gray hydrogen production, stands out due to its substantial methane content, establishing it as a reliable source of hydrogen (Di Lullo et al., 2021).In regions abundant with natural gas, like the United States and Canada, its availability presents minimal or negligible challenges for hydrogen production.Notably, both countries held substantial proven reserves of natural gas, totaling 512 trillion cubic feet at the start of 2019, significantly influencing global usage patterns.This abundance of natural gas resources in these jurisdictions underscores their significant impact on the global natural gas landscape (Oni et al., 2022).
Conventional hydrogen production facilities are integrating carbon capture and utilization systems to reduce CO 2 emissions.These methods, often derived from fossil fuels, primarily involve SMR for methane oxidation and natural gas reforming (Hantoko et al., 2024).During processing, desulfurization eliminates hydrogen sulfide (H 2 S) from raw natural gas, with subsequent steps involving coal gasification and air separation techniques.This intricate process aims to efficiently extract hydrogen while managing byproducts like CO 2 and H 2 S (Dash et al., 2023;Sazali, 2020).There is a growing interest in technologies like natural gas decarbonization (NGD) and autothermal reforming (ATR) for the production of blue hydrogen.These methods are expected to potentially integrate into the future array of hydrogen production technologies.Although not entirely novel, these technologies have found applications in various industrial processes (Oni et al., 2022).ATR technique has found practical application in commercial settings, including air-blown units for ammonia (NH 3 ) generation from natural gas and oxygen-blown units for methanol.These processes yield hydrogen along with syngas, which possess a composition conducive to H 2 production.
In contrast, NGD has historical roots dating back to the 1920s, primarily employed for producing carbon black from hydrocarbons like natural gas through thermal decomposition into C and H 2 .The resulting gases, containing H 2 , are combusted to supply heat for the overall process (Keipi et al., 2018).The challenge with this thermal decomposition process primarily revolves around efficiently removing the produced carbon.Instances of reactor fouling, clogging, and catalyst deactivation pose significant hurdles in the catalytic approach to NGD (Geißler et al., 2015;Hasnan et al., 2020;Oni et al., 2022).
In terms of environmental impacts and sustainability considerations, blue hydrogen production offers a notable reduction in carbon emissions compared to traditional natural gas-based methods, although residual emissions remain from carbon capture processes and methane leakage during extraction and transportation.However, SMR, a common method for blue hydrogen production, requires substantial water usage, potentially straining local water resources in water-scarce regions.Sustainability hinges on effective carbon capture and storage technologies, necessitating diligent monitoring and maintenance to prevent leakage.Blue hydrogen serves as a transitionary solution, aiding in short-term carbon emission reduction while renewable energy infrastructure is developed.

| Green hydrogen
The trajectory of H 2 production is increasingly leaning toward green H 2 obtained through water electrolysis, which involves splitting water into hydrogen and oxygen encompasses various types, including alkaline electrolysis, recognized for its maturity and broader commercial adoption (Aboukalam da Cruz et al., 2023;Hu et al., 2022).This approach, characterized by a low energy cost of approximately 286 kJ/mol of hydrogen, is currently undergoing substantial research and development efforts to diminish existing costs and advance technological capabilities.Green hydrogen is anticipated to achieve an average cost of $2/kg H 2 by 2030, with potential forecasts of $1.5/kg H 2 contingent upon improved and more affordable electrolyzers and a levelized cost of electricity at $20/MWh (Boretti, 2021).Countries worldwide, including Australia, India, China, Japan, Bangladesh, and Germany, focus on green hydrogen in their energy transitions, supported by the World Bank Group.Despite these strengths, alkaline electrolysis faces challenges related to hydrogen purity, current densities, operating pressures, and corrosivity.Efforts persist to enhance its efficiency, reduce resistance, and address these limitations (Sharshir et al., 2024).
Different electrolyzer technologies possess distinct attributes and challenges (Zainal et al., 2024).Alkaline electrolyzers (AEL) are known for reliability but struggle with efficiency at low loads.Polymer electrolyte membrane electrolyzers (PEMEL) boast a compact design but rely on costly materials like platinum and iridium, which have recently seen price hikes.Anionic exchange membrane electrolyzers (AEMEL) attempt to combine AEL and PEMEL advantages but face challenges related to membrane resistance and lifespan.Solid oxide electrolyzer cells (SOECs), operating at high temperatures, consume less energy but grapple with technological and material resilience concerns (Squadrito et al., 2023).
Efforts to promote green hydrogen focus on AEL and PEMEL electrolysis technologies, aiming for capital expenditure (CAPEX) reduction through affordable materials and operating expenditure (OPEX) reduction through lower consumption and electricity costs (Proost, 2019).However, the growing emphasis on electrolysis in global energy policies raises concerns about potential shifts in equilibrium among countries due to large-scale technology implementations.Attention is also directed toward critical raw materials that could significantly impact production costs, particularly for technologies reliant on limited geographical resources (Kiemel et al., 2021;Squadrito et al., 2023).
Regarding environmental impacts and sustainability considerations, green hydrogen production boasts minimal direct carbon emissions as it harnesses renewable energy sources, yet the carbon footprint can fluctuate based on the LCA of renewable energy generation technologies.However, the production of renewable energy infrastructure necessitates land and raw materials, potentially impacting the environment.Sustainable development hinges on scalable and available renewable energy sources, requiring ongoing investment in infrastructure for a reliable energy supply.Additionally, enhancing the efficiency of electrolysis processes and utilizing renewable energy sources effectively play pivotal roles in reducing the carbon footprint of green hydrogen production.

| Purple hydrogen
Purple hydrogen, generated from nuclear electricity, serves as an alternative production method not heavily emphasized in European hydrogen strategies but holds practical potential in regions like China and Russia (Ajanovic et al., 2022;El-Emam et al., 2020;Ping et al., 2018;Zhiznin et al., 2020).Produced through electrolysis using atomic current, this type of hydrogen offers advantages not readily available in conventional energy sources.Nuclear power plants, with zero-carbon emissions, boast a potential operating capacity exceeding 90% and require minimal land compared to renewables (Mohideen et al., 2023;Pinsky et al., 2020;Younas et al., 2022).
Hydrogen production can be integrated with nuclear power plants through various methods, utilizing the heat and electricity they generate (Wang et al., 2019).Processes include nuclear energy-assisted SMR, low-temperature electrolysis, high-temperature steam electrolysis, and thermochemical cycles for water splitting (Mohideen et al., 2023;Nadaleti et al., 2022).These pathways show varying efficiencies and emissions reduction impacts, with thermochemical cycles, especially copper-chloride and sulfur-iodine methods, showing promise (Granovskii et al., 2008).Pilot projects, such as the $20 million initiative by the U.S. Department of Energy in Arizona, aim to pave the way for large-scale nuclear hydrogen production, targeting a cost of $1/kg through nuclear power (Mohideen et al., 2023;Yue et al., 2021).The different pathways depend on whether nuclear power is utilized as an electrical or thermal energy source (Muellner et al., 2021).
Nuclear reactors, typically base-load electricity suppliers, have the potential to incorporate hydrogen production facilities to store excess energy.This integration supports both high-and low-temperature hydrogen electrolysis technologies.However, despite uranium resources lasting around 90 years globally, constraints related to limited uranium-235 supply and technical factors may hinder the substantial expansion of nuclear-based hydrogen production (Muellner et al., 2021;Pani c et al., 2022).
In terms of environmental impact and sustainability considerations, purple hydrogen production stands out for its lower carbon emissions compared to blue hydrogen, as it incorporates renewable energy sources.However, there are still residual emissions from natural gas utilization and carbon capture processes.It is essential to carefully manage natural gas resources and consider the environmental impacts of renewable energy technologies.Moreover, the hybrid nature of purple hydrogen offers a promising pathway, leveraging existing natural gas infrastructure while gradually transitioning to renewables to reduce carbon emissions.This approach necessitates thorough planning, coordination, and investment to effectively integrate renewable energy sources with existing infrastructure and optimize production efficiency and sustainability.

| Explanation of life cycle assessment and its key principles
LCA proves to be a valuable tool for examining the environmental sustainability of a system and delving into its energy consumption patterns (Yin et al., 2023).The main goal of LCA is waste recovery's environmental performance evaluation, minimizing hazardous material emissions, and enhancing carbon neutrality (Ozturk et al., 2023;Xu et al., 2023).As a holistic technique, LCA provides insights into environmental, social factors, and economic aspects, allowing decision-makers to make more sustainable choices (Peña et al., 2021).Following the ISO 14040 and 14044 standards, LCA methodology comprises four key stages (Liu et al., 2023): (1) defining goals and scope, (2) life cycle inventory analysis, (3) impact evaluation, and (4) clarification (Jamil et al., 2023).
Figure 2 illustrates the LCA procedures based on ISO14040.Goal and scope definition is a key step in the LCA process to determine the purpose and scope of the study, specify the system boundaries, and identify potential indirect effects.This establishes the functional unit and the processes that will be assessed, as well as laying the foundation for the assessment.The system boundary indicates certain processes that can be retained or eliminated after the procedure and typically can be illustrated using flow diagrams of inputs and outputs.A quantitative assessment of the work involved in the method can be made through the operational division (Jamil et al., 2023).The functional unit serves as a reference unit to provide normalizable inventory data.It is generally regarded as an indicator of product volume, even though it is also affected by some parameters such as energy, site area, distance, and economic significance.
At the inventory analysis stage, comprehensive data records are assembled within the specified scope of the system.It may include input data such as resources, raw materials, machinery, and power, and output data such as main and secondary products, waste, and contaminants.Furthermore, indirect effects, such as those related to the environment and society or land use, are gathered and notated.The impact assessment is conducted under ISO 14042 and consists of three phases, including definition and selection, categorization, and characterization (Sitepu et al., 2020).Through this step, classification and quantitative analysis are carried out on environmental impacts.During this step, the consequences associated with the identified inputs and outputs in inventory analysis are evaluated and assigned to specific impact classifications, such as human health, resource depletion, and climate change.Based on the assessment's results, findings are interpreted, conclusions are drawn, and recommendations are made at the final stage.It provides insightful environmental performance of the system for sustainable development and improvements (Liu et al., 2023).

| Challenges and limitations of applying life cycle assessment to hydrogen energy systems
Ongoing research in hydrogen energy is enhancing LCAs for better environmental insights.According to an internationally developed LCA guide by the hydrogen industry (Lozanovski et al., 2011), purity, pressure, temperature, and production capacity are some of the main factors for each method of hydrogen production, which should be evaluated in the LCA studies on hydrogen energy (Wilkinson et al., 2023).Hydrogen energy systems face several challenges and limitations, including hydrogen production, distribution, and utilization.LCA results in hydrogen production are often based on secondary life cycle inventory data obtained from literature, simulation, or lab experiments, which are not sensitivity-analyzed (Arfan et al., 2023).This resulted in different conclusions regarding the most sustainable method of hydrogen production from previous studies.
Depending on the assumptions made, the data used, and the specific context of the assessment, specific LCA results can vary.Limited availability and accuracy of data at certain stages of the hydrogen life cycle, especially emerging technologies, like electrolysis or advanced reforming processes, can introduce uncertainties into the assessment.Defining system boundaries for hydrogen energy systems is challenging and subjective, whereas the complexity of the system, as well as the multiple pathways for production and end use, make it difficult to establish appropriate boundaries.Hydrogen can be produced through various methods, from natural gas (SMR) (Massarweh et al., 2023), coal (Jiang et al., 2022), biomass (Bin Abu Sofian et al., 2024), or renewable energy sources (Amin et al., 2022;Gapp & Pfeifer, 2023), resulting in various environmental impacts, which have a significant influence on the overall assessment.Energy sources, such as fossil fuels and renewable energy, resource availability, geographical location, and climate conditions used in hydrogen production affect the environmental performance and LCA results.There are also challenges associated with hydrogen storage methods (e.g., compressed gas, liquid, or solid-state storage) and transportation modes.As part of the evaluation of LCA, it is imperative to consider end-of-life policy considerations.However, there is considerable difficulty in assessing the end-of-life phase of hydrogen systems, including the disposal or recycling of equipment and infrastructure.Furthermore, shifts in regulations, incentives, or market conditions may not be easily incorporated into the assessment.
Despite these challenges, ongoing research and standardization efforts aim to improve LCA accuracy and reliability for hydrogen energy systems.Addressing these limitations is crucial for meaningful insights into hydrogen's environmental sustainability as an energy carrier.Additionally, continuous updates to LCA methodologies and data repositories can enhance the applicability and precision of assessments.

| LIFE CYCLE ASSESSMENT OF HYDROGEN PRODUCTION
Evaluating the life cycle impacts of H 2 production is a crucial element in assessing its environmental footprint and sustainability.This evaluation involves examining the environmental consequences at every stage of hydrogen production, from resource extraction to end-use application.Life cycle impact assessment (LCIA) methodologies are employed to quantify the environmental burdens associated with various production pathways.LCIA considers factors like resource utilization, greenhouse gas emissions, air and energy consumption, water pollution, and waste generation throughout the entire hydrogen production process.
Various methods for hydrogen production carry distinct environmental implications.Fossil fuel-based routes, like SMR, result in higher carbon emissions due to hydrocarbon combustion.Researchers worldwide have put forth diverse clean hydrogen production methods that harness water as a source.These methods include water splitting through photoelectrochemical cells, thermochemical water splitting, and water electrolysis, representing promising pathways for environmentally friendly H 2 generation (Liu et al., 2019;Sadeghi & Ghandehariun, 2023).

| Case studies
Recent LCA studies examining hydrogen production and usage often lack comprehensive discussions about various environmental impacts (Osman et al., 2022).While renewable energy sources offer zero-carbon operations in energy generation, their extensive infrastructure has significant lifecycle impacts, notably stemming from materials like structural steel and end-of-life treatment of hazardous materials.Additionally, comparisons among different hydrogen production pathways suggest a potential shift in burden from climate change considerations (gray and blue pathways) to concerns regarding mineral resource ecotoxicity and scarcity (Kwak et al., 2020).
In this context, Weidner et al. (2023) deployed a prospective life cycle analysis to evaluate the environmental sustainability of large-scale hydrogen production.Their study, which focused on producing 500 Mt/yr of hydrogen, compared various production options and assessed their life cycle impacts.The analysis considered factors such as human health burdens, the planetary boundaries framework, global economic impacts, and production costs reflective of environmental consequences.The findings revealed that the climate change impacts of anticipated H 2 production levels surpassed the allotted planetary boundary by 3.3-5.4times.Only green H 2 produced from wind energy remained within the specified boundary.However, the study noted that human health, along with other environmental impacts, were comparatively less severe.The research underscored the importance of addressing metal depletion and ecotoxicity impacts linked to green H 2 production.In terms of cost, it was observed that blue H 2 , derived from fossil fuels with carbon capture, incurred the highest environmental damages, resulting in an increase in cost from approximately $2 to $5 per kilogram of hydrogen.
Conversely, green hydrogen from solar PV exhibited the lowest environmental costs, with the cost decreasing from around $7 to $3 per kilogram when applying prospective life cycle analysis.These findings underscore the importance of considering the life cycle impacts of H 2 production methods and the necessity of transitioning to low-carbon alternatives.The study suggests implementing low-carbon standards and reevaluating high-energy lifestyles.By offering a complete assessment of hydrogen environmental sustainability, the research contributes valuable insights to the ongoing debate surrounding blue and green H 2 .
In another study, Delpierre et al. (2021) implemented an ex-ante LCA to assess the environmental impacts of large-scale hydrogen generation using two electrolysis technologies: Polymer electrolyte membrane (PEM) and alkaline electrolyte (AE).The study also incorporated exploratory scenarios to analyze the potential upscaling effects and development of hydrogen production systems in the Netherlands by 2050.The study reveals that both PEM and AE systems share similar environmental impacts, differing by less than 7% across categories, excluding "ozone depletion."The electrolyzer contributes only 10% to overall impacts, while over 90% is attributed to electricity origin.The research underscores the urgency of transitioning to sustainable hydrogen production, particularly through water electrolysis with renewable energy sources.It acknowledges challenges in large-scale implementation, providing a framework for assessing emerging technologies through a combination of quantitative LCA and qualitative scenario analysis.This approach aids decision-making in early technology development and contributes to understanding the potential environmental gains in a future hydrogen economy.
Hosseinzadeh et al. ( 2022) implemented a thorough evaluation of diverse hydrogen production methods utilizing bio-waste as a renewable energy source, considering both technological and economic aspects, as well as environmental impacts.The research investigated dark fermentation, photo-fermentation, solid-state fermentation, microbial electrolysis cells (MEC), gasification, pyrolysis, and plasma processes.In terms of technology, dark fermentation demonstrated superior performance, with hybrid dark fermentation incorporating photo-fermentation and MEC exhibiting the highest efficiency, producing approximately 1 L of hydrogen per gram of organic waste.Economically, gasification and fermentation emerged as the most cost-effective methods for hydrogen production, with costs around $2 and $2.3 per gram, respectively.Plasma ($2.4 per gram), pyrolysis ($2.6 per gram), MEC ($2.8 per gram), and photo-fermentation ($3.5 per gram) were comparatively more expensive.Regarding environmental impact, the fermentation process had the lowest greenhouse gas emissions at 15 kg of CO 2-eq /kg H 2 produced.On the other hand, gasification, along with MEC and plasma processes, followed with slightly higher emissions.The study emphasized that gasification stands out as the most mature process, boasting the highest technology readiness level for potential commercial applications.Karaca et al. (2020) carried out an LCA to compare the environmental implications of various H 2 production methods utilizing nuclear energy.The research examined five impact categories: abiotic depletion potential (ADP), global warming potential (GWP), human toxicity potential (HTP), acidification potential (AP), and ozone depletion potential (ODP).The assessed methods of generation encompassed conventional electrolysis (CE), high-temperature electrolysis (HTE), as well as 3-, 4-, and 5-step Cu-Cl cycles.The LCA results revealed GWP values for hydrogen production methods as follows: HTE of 0.4768 kg CO 2 -eq/kg H 2 , CE of 0.7071 kg CO 2 -eq/kg H 2 , 3-step Cu-Cl of 1.320 kg CO 2eq/kg H 2 , 4-step Cu-Cl of 1.201 kg CO 2 -eq/kg H 2 , and 5-step Cu-Cl of 1.346 kg CO 2 -eq/kg H 2 .The study identified HTE as the most environmentally friendly method among the evaluated options.However, it noted that the fuel cycle exhibited the highest environmental impact within the stages of hydrogen production.Regarding specific production methods, H 2 production via boiling water reactor-high-temperature electrolysis (BWR-HTE) demonstrated lower impacts than production via pressurized water reactor-high-temperature electrolysis (PWR-HTE) in all impact categories except ODP.BWR-HTE showed higher ODP due to the recirculation cycle for water purification.These findings highlight the varied environmental impacts of nuclear-based H 2 production methods, emphasizing the importance of considering specific impact categories in sustainability assessments when choosing a production method.
Zhang et al. ( 2022) carried out a comprehensive LCA of 3 solar energy-based H 2 generation methods, which included PEM water electrolysis together with photothermal power generation, PEM water electrolysis coupled with PV power production, and thermochemical water splitting with S-I cycle coupled with solar photothermal technology.Four environmental aspects were assessed: AP, GWP, nutrient enrichment potential, and ODP.The research concluded that the thermochemical water splitting method with the S-I cycle and solar photothermal technology had the lowest environmental impact.Specifically, its GWP and AP were 1.02 kg CO 2 -eq and 6.56E-3 kg SO 2 -eq, respectively, indicating significant advantages over other methods.The study highlighted key factors for reducing environmental impact in hydrogen production, such as utilizing photothermal technology with the S-I cycle, employing PV power generation for PEM water electrolysis, minimizing pollution during plant construction, and extending system lifetime.Overall, the research underscores the importance of considering life cycle impacts and showcases the potential of solar energy-based methods to mitigate environmental effects in hydrogen production.
In another study, Pawłowski et al. (2023) conducted a study centered on LCA of H 2 production.Their focus was specifically on a solar-to-hydrogen plant utilizing Anion Exchange Membrane (AEM) electrolyzers, and the study considered the unique climate conditions in Poland.The authors assessed the carbon footprint of the plant using AEM electrolysis, considering greenhouse gas emissions throughout its life cycle.The study revealed a GWP range for the plant between 2.73 and 4.34 kgCO 2 -eq, indicating relatively low emissivity of hydrogen produced from solar energy, particularly with the innovative AEM technology.Regarding economic profitability, the research underscored the dependence on external subsidies.As an evolving technology, the AEM electrolysis of green hydrogen from PV was not yet cost-competitive without such support.The study highlighted the importance of evaluating the hydrogen production environmental impact, especially for emerging technologies, and emphasized LCA as a comprehensive approach for assessing emissions in different hydrogen production pathways, facilitating effective technology comparisons.Palmer et al. (2021) performed an LCA and net energy analysis of large-scale H 2 production through electrolysis powered by solar PV.The objective was to assess the environmental impacts and net energy balance of this green H 2 production method, comparing it with the prevalent SMR process.The study, considering operational sensitivities, identified solar modules as the primary contributor to environmental impacts in the life cycle of solar-electrolysis due to the materials and processes involved in their manufacturing.Under baseline conditions, greenhouse gas emissions from solar-electrolysis were approximately one-quarter of those from SMR.However, sensitivity analysis revealed that under reasonably anticipated conditions, greenhouse gas emissions from solar-electrolysis could be comparable to SMR.In terms of net energy balance, the results indicated that the net energy of solar-electrolysis was lower than that of fossil fuels, but the uncertainties in the values warranted further attention.Overall, the study underscores the importance of integrating LCA and net energy analysis into project planning to ensure that hydrogen production aligns with green production goals.It highlights the necessity of considering factors beyond cost reduction, such as wind or solar resource variability and strength, global supply chain impacts, real-world operating performance, and the availability of critical minerals.
In their recent publication, Jolaoso et al. ( 2024) conducted an LCA of a SOEC hydrogen production system, where the LCA aimed to quantify environmental impacts and greenhouse gas emissions associated with various energy sources used in the system to advance sustainability in a green economy.The study unveiled significant findings, highlighting a 28% reduction in the carbon footprint in the SOEC system with optimized heat integration, emphasizing the importance of system design for enhanced sustainability.Solar PV energy emerged as an environmentally benign option for SOEC operations, with the analysis revealing that environmental damage from a bituminous power plant was, on average, 700% higher than that of solar PV.The research underscored the crucial role of balance of plant design in achieving sustainable large-scale hydrogen production, emphasizing careful consideration of the steam generation process and SOEC manufacturing due to their high environmental impact potential.Overall, the LCA of the integrated H 2 production SOEC system provides valuable insights into the environmental performance of different energy sources, emphasizing the necessity of conducting LCAs and net energy analyses for evaluating new hydrogen technologies and facilitating informed comparisons to promote sustainability in a green economy.Li et al. (2020a) presented a study focused on the LCA of hydrogen production, specifically comparing hydrogen produced from the natural gas steam reforming process (NGSH) and biomass staged-gasification (BSGH).The authors introduced a modified exergetic life cycle environmental impact model, combining LCA and exergy analysis for a quantitative assessment of resource and pollution-related environmental impacts.The study comprehensively evaluated the two hydrogen production processes using indicators of resource and environmental performance.The comparison revealed significant advantages of BSGH in renewability and environmental sustainability over NGSH.BSGH outperforms in terms of GWP and photochemical ozone formation (POF), indicating its environmental friendliness.However, it is noted that BSGH had slightly larger thermal pollution compared to NGSH, attributed to excessive hot flue gas emissions in the BSGH process.The sensitivity analysis emphasized the crucial role of tar conversion efficiency in the staged-gasification process, which exerts a more pronounced influence on the overall cumulative exergy consumption and environmental metrics than biomass combustion efficiency.Li et al. (2020b) carried out a comprehensive study involving LCA and techno-economic analysis of biomass-tohydrogen (BTH) production utilizing methane tri-reforming (MTR) from corn straw.The primary objectives were to assess energy consumption, environmental performance, and economic feasibility for BTH with and without MTR.The results indicated that BTH with MTR exhibited higher energy efficiency, reaching 17.08%, compared to BTH without MTR.The main contributor to energy loss was identified as the Claus unit.Optimal operating conditions for MTR were determined to be a reaction temperature of 800 C and an operating pressure of 0.1 MPa.Despite a 10.97% increase in total capital investment for BTH with MTR, the production cost decreased by 10.12%, resulting in a static payback period of approximately 4.72 years.Environmental impact assessment revealed that BTH without MTR had 3.09% lower energy consumption and 7.85% lower greenhouse gas emissions compared to BTH with MTR.LCA suggested that BTH with MTR could achieve negative carbon dioxide emissions within the context of the natural carbon cycle, signifying a noteworthy advancement in clean H 2 production with low carbon emissions along with high energy savings.In conclusion, the research demonstrated the viability and environmental friendliness of BTH production with MTR.The study's insights into energy efficiency, economic feasibility, and environmental impact contribute significantly to the development of sustainable H 2 production technologies.Vilbergsson et al. (2023) conducted an LCA of hydrogen production in Iceland, comparing it with locally produced hydrogen in Austria and Belgium.The study aimed to assess the potential of remote green hydrogen production in decarbonizing European industries.The results revealed that hydrogen production in Iceland, utilizing local renewable energy sources, particularly through polymer electrolyte membrane electrolysis, led to significantly lower greenhouse gas emissions compared to Austria and Belgium.Specifically, hydrogen production in Iceland resulted in more than 13 times lower greenhouse gas emissions compared to Austria and over 21 times lower emissions compared to Belgium.The energy mix used in the process was identified as the primary factor influencing the GWP of hydrogen production.While the transportation of hydrogen had a minor impact on the overall carbon footprint, liquefaction during transportation emerged as the most significant contributor.The research highlighted the importance of considering both spatial and temporal perspectives in assessing remote hydrogen production.It also underscored that intermittent operation of hydrogen production systems significantly reduced full load hours, thereby increasing the carbon footprint of green hydrogen.These findings suggest that remotely produced hydrogen, particularly from Iceland's renewable energy sources, can play a crucial role in decarbonizing carbon-intensive industries in Europe.The study supports the potential of hydrogen as a versatile energy carrier in achieving global decarbonization goals and emphasizes the necessity for further development of hydrogen infrastructure and trade networks to enable a global hydrogen economy.
In a case study focused on Romania, Chisalita et al. ( 2023) conducted a cradle-to-gate LCA to estimate the environmental impact of H 2 production using innovative chemical looping technologies.Three looping innovations chemical looping hydrogen generation (CLH), sorption-enhanced chemical looping reforming (SECLR), and sorption-enhanced reforming (SER), were compared.The study also evaluated these looping technologies contrary to conventional hydrogen generation methods, both with and without carbon dioxide capture, as well as green H 2 generation.The LCA analysis considered various environmental impact indicators, revealing that CLH exhibited a lower environmental burden in comparison to the standard blue hydrogen production methodology.CLH outperformed SER and SECLR in 11 out of 12 environmental indicators.Green H 2 production showed a minor environmental impact, except in categories such as human toxicity, terrestrial ecotoxicity, and mineral depletion.The study suggests that the implementation of chemical looping technologies, particularly CLH, has the potential to significantly reduce the environmental impact of H 2 production.This research underscores the promise of these technologies in achieving a more environmentally friendly hydrogen production process.Akhtar et al. (2023) performed an in-depth cradle-to-gate social life cycle assessment on the production of green H 2 through water electrolysis, utilizing renewable electricity sourced from wind farms and solar PV.The study encompassed seven countries, including China, Australia, Oman, Saudi Arabia, South Africa, Chile, and the United States.Its objective was to identify social hotspots and geopolitical implications within the emerging hydrogen economy, with a specific focus on Sustainable Development Goals (SDGs) and various social indicators.The social life cycle assessment findings revealed that the production of green H 2 in South Africa posed the uppermost risks to multiple social indicators, including issues encompassing concerns such as child labor, association and bargaining rights, fair salary, the gender wage gap, and unemployment.Nevertheless, in other nations, the peril to social indicators experienced a substantial reduction when essential equipment was manufactured domestically rather than imported.The study underscored that the complexity of the green H 2 supply chain, with components sourced globally, contributed to inferior performance in social indicators compared to conventional hydrogen production.It emphasizes the necessity for globally standardized regulations to avert colonialism within the prospective hydrogen economy.Moreover, the research emphasized the importance of developing domestic green H 2 supply chains and enhancing working conditions in certain sectors within individual countries.This aligns with the SDGs and fosters sustainable development.Zheng et al. (2024) examined and focused on the prospective technology paths for medium-and long-term H 2 production, as well as energy supply scenarios in the Province of Guangdong, China.The study underscores the significance of hydrogen energy supply for regional energy development and the imperative to explore optimal production paths for reducing CO 2 emissions.It evaluates five H 2 production routes, including industrial byproducts and wind, coal, hydro-, and PV power, assessing key technologies and their technical maturity.Notably, the research employs the LCA method to evaluate the environmental impact of H 2 production technologies.
Various studies have previously examined different H 2 production methods based on energy efficiency, environmental impacts, and cost indicators.For instance, Dincer and Acar (2015) recognized the generation of hydrogen through PV power as the most ecologically sustainable method, while Safari and Dincer (2020) emphasized that hydrogen production through natural gas reforming is the most energy-efficient and cost-effective method.However, the study emphasizes the challenges associated with the high cost and low technical maturity of green hydrogen production routes, specifically those involving wind power and PV.
Regarding biohydrogen production, numerous studies have assessed its environmental impacts through LCA, with a focus on fossil energy consumption, greenhouse gases emissions, and other factors.Since commercial biohydrogen production is limited, these LCAs primarily rely on data from lab-scale or pilot-scale experiments and process simulations.Lignocellulosic biomass serves as the primary material for biohydrogen production.Despite the challenges posed by the lack of commercial-scale data, LCA studies provide valuable insights into the environmental performance of biohydrogen production processes (Ganeshan et al., 2023).Figure 3 illustrates the system boundaries and factors considered for LCA of hydrogen production through biomass gasification.The LCA primarily focuses on the raw material stage, energy acquisition, and manufacturing stages involved in the production process.The LCA aims to assess the environmental impacts associated with the utilization of various biomass feedstocks.
In this context, Wulf and Kaltschmitt (2013) discussed the LCA of biohydrogen production from various biomass sources for transportation purposes.The study primarily focuses on analyzing greenhouse gas emissions, emissions with acidification potential, and fossil energy demand associated with different biohydrogen technologies.The researchers evaluated six hydrogen production pathways using different biomass feedstocks: gasification of softwood, gasification of wood from short rotation coppice, steam reforming of biomethane (two pathways), pyroreforming of glycerol (a byproduct of biodiesel production), and alkaline water electrolysis with electricity from biomass cogeneration plants fired with straw.The assessment considered the entire process chain, from biomass cultivation to biohydrogen production.The results indicated that hydrogen produced from woody biomass, particularly via gasification, resulted in the lowest emissions due to the low emissions associated with biomass provision.Short rotation coppice exhibited the highest biomass potential for hydrogen production, while biomass from forests also showed significant potential.The study contributes to the evaluation of biohydrogen as a potential fuel for the transportation sector, considering its impact on reducing greenhouse gas emissions.The authors conducted an analysis based on LCA standards, assessing environmental impact categories such as anthropogenic climate change, acidification, and cumulative energy demand.However, it is worth noting that the research focuses specifically on the German context and relies on assumptions and data available at the time of the study.
In addition, LCA demonstrates the potential of waste-to-hydrogen technologies, particularly gasification and fermentation, in significantly reducing carbon emissions associated with biohydrogen production.Lui et al. (2022) investigated the potential of waste-to-hydrogen technologies for producing biohydrogen and managing waste sustainably.The study applies the LCA approach to compare the carbon-saving potential of two main waste-to-hydrogen technologies, namely gasification and fermentation, with the conventional method of SMR for hydrogen production.The results reveal significant reductions in CO 2 -eq emissions per kg of H 2 for waste-to-hydrogen technologies, ranging from 50% to 69% compared to SMR.Gasification of municipal solid waste and waste wood exhibited lower GWP (4.99 and 4.11 kg CO 2 -eq/kg H 2 ) than dark fermentation treating wet waste (6.6 kg CO 2 -eq/kg H 2 ) and combined dark and photo fermentation (6.4 kg CO 2 -eq/kg H 2 ).The study also highlights the lower distance emissions of waste-to-hydrogen-based fuel cell electric buses (0.33-0.44 kg CO 2 -eq/km) compared to the SMR-based scenario (0.89 kg CO 2 -eq/km).
In a recent study, Arfan et al. (2023) explored the environmental and economic dimensions of biohydrogen production from biowaste and biomass in Sweden.Employing LCA with SimaPro software and the CML-IA, 2001 method, the analysis covers 10 environmental impact categories.Results reveal that hydrogen production from biowaste exhibits higher climate impact and freshwater eutrophication compared to biomass but fares better in ozone depletion and other categories.The study underscores the influence of feedstock-to-biogas/pyrolysis-oil yields ratio and energy source on environmental impacts, particularly climate change and human toxicity.Sensitivity analysis highlights these factors' significance.Economically, hydrogen production from biowaste costs between €0.45 and €2.76 per kilogram over 20 years, while biomass ranges from €0.54 to €3.31.Despite biomass's advantages in certain impacts, biowaste emerges as a favorable choice when considering overall impact categories and life cycle cost perspectives.This study contributes valuable insights applicable not only in Europe but globally.
Table 1 presents a comprehensive overview of different sources of hydrogen feedstocks (such as biomass, water, and fossil fuels), their purity levels, data sources, life cycle inventory (LCI) methods, and GWP assessments.The sources vary in terms of their purity, with some being less than 99.9% pure and others equal to or greater than 99.9%.Data sources include literature, laboratory data, process simulations, and plant data, among others.LCI methods range from Cradle-Gate to Gate-Gate assessments, with various models such as ReCiPe, CML, and IPCC used for GWP calculations.The table provides insights into the diverse methods and data utilized in assessing the environmental impact of hydrogen feedstock sources, crucial for understanding their sustainability and contribution to global warming.
In conclusion, the LCA of hydrogen production provides valuable insights into the diverse environmental implications associated with different methods.Notably, fossil fuel-based pathways, particularly SMR, significantly contribute to carbon emissions, emphasizing the urgency of transitioning to cleaner and more sustainable energy sources.The emergence of renewable energy-based pathways, such as solar or wind-powered electrolysis, presents an environmentally friendly alternative with lower greenhouse gas emissions.As the global energy sector strives to replace depleted fossil fuels, hydrogen assumes a pivotal role as a carbon-free and high-energy-density solution.Although a substantial portion of current hydrogen production relies on fossil fuels, advancements in clean hydrogen production methods, especially those utilizing water as a source, hold promise for a sustainable future.
Technologies like water electrolysis, thermochemical water splitting, and water splitting through photoelectrochemical cells provide environmentally conscious options for hydrogen production.However, it is crucial to recognize the delicate balance between achieving zero-carbon operations and addressing the environmental impacts associated with the extensive infrastructure required for renewable energy generation.Additionally, the life cycle analysis highlights potential burden-shifting among environmental impact categories, emphasizing the need for comprehensive evaluations to inform sustainable decision-making in the evolving landscape of hydrogen production.Furthermore, biohydrogen production from waste presents a promising and eco-friendly energy solution.By harnessing biowaste and biomass, it provides a renewable source of hydrogen while tackling waste management issues.Though production may have some environmental impacts, ongoing research and technological progress are improving efficiency and minimizing these drawbacks.Additionally, the economic feasibility of producing biohydrogen from waste is growing, making it an appealing choice for a more sustainable energy transition.
T A B L E 1 Diverse sources, purity levels, and assessments highlight the hydrogen feedstock source's sustainability.carrier lies in its intensely exothermic reaction with oxygen, producing only water.With an energy content equivalent to 2.4 kg of CH 4 or 2.8 kg of gasoline per kilogram, hydrogen boasts a superior energy-to-weight ratio compared to fossil fuels.However, its energy-to-volume ratio, exemplified by liquid hydrogen's 8.5 MJ.L À1 versus gasoline's 32.6 MJ.L À1 , presents a challenge, requiring a larger volume for equivalent energy.Ongoing research in hydrogen storage aims to enhance energy density, addressing this challenge and minimizing system volume limitations (Ball & Wietschel, 2009;Dematteis et al., 2021;Lai et al., 2015).The technologies of hydrogen storage encompass numerous techniques and procedures designed to safely store H 2 , rendering it applicable for diverse purposes.Three primary storage technologies comprise solid-state storage, liquid storage, and gaseous storage (Zhang et al., 2024).Gaseous storage encompasses both underground H 2 storage and compressed H 2 storage.The former entails storing the gas in subsurface geological formations, for instance, salt caverns or reservoirs.Conversely, the latter storage involves the containment of hydrogen as a gas at elevated pressure within cylinders or storage vessels (Bellosta von Colbe et al., 2019;Elberry et al., 2021).Liquid storage involves the procedure of cooling and liquefying H 2 at exceptionally low temperatures, which is then stored in cryogenic tanks that are specially designed to withstand low temperatures and securely contain the liquid (Aziz, 2021;Yatsenko et al., 2022).This approach provides a higher energy density in comparison to gaseous storage (Zhang et al., 2024).Solid-state storage methods utilize materials like metal hydrides (Drawer et al., 2024), ammonia (Kojima, 2024), liquid-organic hydrogen carriers (Tsogt et al., 2024), metal-organic frameworks (Bao et al., 2023), and porous carbon materials (Thanh et al., 2023).This method allows for storage at reduced temperatures and pressures, potentially decreasing infrastructure needs and improving energy density (Boateng & Chen, 2020).

Hydrogen
Efforts to develop efficient H 2 storage materials are crucial, where challenges herein lie in ensuring secure and efficient storage to promote widespread adoption.Designing materials with attributes such as structural integrity, porosity, surface area, capacity, stability, and safety is fundamental to advancing hydrogen storage technology's pivotal role in clean energy applications (Mondal et al., 2024).The choice of hydrogen storage form, whether it is gaseous, liquid, or a two-phase combination, depends on the application.Liquid storage, despite reducing some volumetric penalties, still faces challenges regarding energy density compared to conventional fuels.The lower volumetric energy density necessitates greater storage space per megajoule of energy, leading to stronger, larger, and more complex apparatus for hydrogen storage.From a design perspective, this results in a snowball effect of cost, maintenance, and weight penalties associated with hydrogen storage media, especially in transportation, where available space, size, and weight significantly influence vehicle design and performance.Addressing these challenges is crucial to optimizing hydrogen storage technology for diverse end-user applications (Tashie-Lewis & Nnabuife, 2021).

| Life cycle assessment of hydrogen storage
LCA of H 2 storage technologies is needed for determining their environmental sustainability and identifying areas for improvement.It allows for the evaluation of the environmental impacts associated with different storage methods, including liquid hydrogen storage, solid-state storage, and compressed gas storage.This assessment plays a pivotal role in understanding the overall efficiency and environmental impact of hydrogen energy systems.By applying LCIA methodologies, a systematic approach can be taken to assess the environmental implications of hydrogen storage technologies.Factors such as energy density, safety, and infrastructure requirements are considered when examining various storage methods.The examination of the life cycle impact of hydrogen storage is crucial in promoting environmentally responsible practices within the realm of emerging energy solutions.

| Case studies
The scientific literature extensively covers LCAs related to energy storage systems, particularly those involving hydrogen-based technologies.However, these studies present challenges in terms of comparability due to their inherent heterogeneity.Various factors contribute to this heterogeneity, including a multitude of assumptions, insufficient primary data on certain components, and the utilization of diverse LCA approaches (Bobba et al., 2020;Gandiglio et al., 2022).Despite these challenges, LCA stands out as a well-established methodology, providing a comprehensive framework for evaluating the potential environmental impacts of product systems.Consequently, a substantial number LCA studies focusing on H 2 energy systems exists in the literature.These studies contribute significantly to our understanding of the environmental implications associated with hydrogen-based technologies (Valente et al., 2017).In this context, Bionaz et al. ( 2022) conducted a comprehensive LCA of a renewable H 2 -based energy storage system, specifically examining the Froan islands in Norway.The study compared the environmental sustainability and techno-economic feasibility of the renewable hydrogen-based system (REMOTE) with diesel-based configurations and submarine connections to the mainland grid.Results indicated that the REMOTE system had significantly lower climate impacts, with an environmental impact of 148.2 kgCO 2-eq/MWh, compared to diesel-based configurations with a much higher impact of 1090.9 kgCO 2-eq/MWh.The scenario involving a sea cable had an impact of 113.7 kgCO 2-eq/ MWh, influenced by the low-carbon Norwegian electricity intensity.The study conducted a sensitivity analysis, considering factors such as the length of the sea cable along with electricity CO 2 emission intensity, revealing the significant influence of local conditions on the results.Additionally, the REMOTE system was identified as the most cost-effective solution for providing electricity to the insular community, emphasizing its potential for both environmental and economic benefits.Gandiglio et al. (2022) carried out an LCA to assess the environmental sustainability of renewables integrated with an H 2 -battery storage system for a remote off-grid community, comparing it with the current diesel generator-based scenario.The LCA considered various impact categories, including photochemical ozone formation, climate change, ozone depletion, acidification, fossil resource use, and marine and terrestrial eutrophication.Results revealed that the renewable energy system with hydrogen-battery storage exhibited significantly lower environmental impacts than the dieselbased solution, accounting for less than 10% of the impact in almost all categories.Specifically, in terms of climate change, the renewable scenario emitted 0.197 kg CO 2-eq/kWh, while the diesel-based scenario emitted 1.73 kg CO 2-eq/ kWh, translating to an 89% reduction in greenhouse gas emissions.Over the plant's 25-year lifetime, the renewable solution had the potential to save approximately 6570 tons of CO 2-eq.Despite slightly higher values in indicators such as mineral and metal resource use, water usage, and freshwater eutrophication for the renewable scenario, the overall environmental benefits were substantial.Agostini et al. (2018) carried out an LCA comparing a solid-state H 2 storage tank as an auxiliary power unit for a light-duty vehicle with type III and IV tanks.The results showed that factoring in electricity consumed for hydrogen gas compression, the solid-state system had similar greenhouse gases and primary energy demand but higher resource depletion.In terms of costs, the auxiliary power unit with a solid-state tank was markedly more expensive, about 1.5-2 times the cost of other tank systems (III and IV).The study suggested potential improvements by reducing structural materials in both the solid-state hydrogen tank and the balance of plant for enhanced environmental and economic performance.
In a recently published article, Rong et al. (2024) carried out a comprehensive techno-economic analysis of H 2 storage and transportation systems, focusing on the LCA of different modes.The objective of the study was to economically transport H 2 from the facility to the refueling station, considering the daily demand for hydrogen and the distance of transportation.Four main modes were analyzed: liquid hydrogen, pipeline hydrogen, compressed gas hydrogen, and liquid organic hydrogen carriers.The researchers evaluated the economic feasibility of point-to-point scenarios and 1to-N relay scenarios, with the latter showing potential for improved economic applicability and reduced unit costs through multiple storage and transportation stages.The study identified optimal storage and transportation modes based on factors like transportation distance and daily hydrogen demand, considering 14 feasible combinations.Overall, the research contributes to the design and application of economically viable hydrogen storage and transportation systems, addressing challenges associated with storage and transportation to promote the widespread use of hydrogen energy.
In the context of rural electrification, Galvez et al. (2012) focused on optimizing autonomous hybrid systems with H 2 storage, aiming for economic efficiency while considering environmental impact through LCA.The study employed a multi-objective optimization approach utilizing the Hybrid Optimization Model for Electric Renewable programs.In a case study within a Cuban rural community, the researchers utilized their proposed algorithm to obtain a compromise solution.This solution featured a 25.4 kW wind turbine with 8 kW of PV panels.Another solution, termed HOGA, incorporated a 76-kW wind turbine with 21 kW of PV panels.Both solutions prioritized hydrogen storage over battery storage for energy storage.The analysis of greenhouse gases in the life cycle of each system component was conducted, generating a set of solutions considering the net present cost and net avoided emissions in the system life cycle as objective functions.Overall, the research underscores the importance of integrating the environmental dimension into the design and evaluation of hybrid systems.By incorporating LCA and optimizing based on multiple objectives, the study aims to offer economically viable and environmentally sustainable solutions for rural electrification, emphasizing the benefits and feasibility of hydrogen storage.Mori et al. (2021) aimed to assess the design and life cycle of a micro-grid energy system for a mountain hut, specifically focusing on the integration of hydrogen storage for seasonal energy storage.The study considered eight different configurations of the stand-alone energy system and evaluated them based on economic, technical, and environmental criteria.The results showed that the micro-grid configurations incorporating a hydrogen-storage system achieved a significant decrease of more than 70% in environmental impacts compared to the initial state.This indicates that the use of hydrogen for seasonal energy storage in mountain huts is more favorable from an environmental perspective than battery storage.In terms of LCA, the study analyzed the environmental impacts of the microgrid configurations from the cradle to the gate.The LCA results supported the findings that the hydrogen-storage system-based configurations had substantial environmental benefits.However, it should be noted that the hydrogen storage system configurations required a larger investment compared to battery-storage systems.
As we explored these studies, it becomes apparent that incorporating hydrogen storage into diverse energy systems can markedly decrease environmental impacts and greenhouse gas emissions.The ongoing pursuit of efficient storage materials and systems, coupled with comprehensive LCAs, holds the key to advancing sustainable energy solutions and optimizing the role of hydrogen in our transition to a more environmentally conscious energy landscape.

| HYDROGEN UTILIZATION
The need for hydrogen spans diverse sectors that consume energy, including industry, commercial enterprises, transportation, and residential applications.Additionally, hydrogen plays a vital role as a crucial raw material in various industrial processes, such as the production of methanol and ammonia, food processing, metal treatment, glassmaking, and petroleum refining.As per the International Energy Agency (IEA), the worldwide demand for H 2 reached approximately 73.9 million metric tons annually in 2018.This demand was primarily fueled by the refining industry, contributing 38.2 million metric tons per year, with the ammonia industry following closely at 31.5 million metric tons per year (Levin & Chahine, 2010;Massarweh et al., 2023;Rambhujun et al., 2020).Major countries with substantial hydrogen requirements include the European Union, China, the United States, Japan, India, and South Korea.All of these nations have adopted H 2 strategies and made investments in the advancement of fuel cell technology.Additionally, developing nations like Morocco, Chile, and Namibia are actively positioning themselves as suppliers of low-carbon hydrogen.Meanwhile, traditional fossil fuel-exporting countries such as Oman, Australia, the United Arab Emirates, and Saudi Arabia are exploring clean hydrogen as a means to diversify their economies (Giuli, 2022).
The hydrogen economy concept embodies the safe, economical, and sustainable use of hydrogen as a zero-emission fuel for vehicles, industries, and power plants, see Figure 4. Currently, around 70 million tons of hydrogen are generated each year from natural gas, primarily through steam reforming, constituting 6% of global natural gas utilization (Okolie et al., 2021).This idea revolves around ensuring the secure and cost-effective application of hydrogen as an emission-free fuel for automobiles, industries, and power generation, underscoring the pivotal role of fuel cells as a key means of harnessing hydrogen as an energy source.Ongoing progress in this area aims to decrease expenses while augmenting the efficiency and endurance of catalytic procedures (Javaid, 2021).
Hydrogen presents promise as a clean and efficient energy carrier despite concerns like NO x emissions and storage limitations.It is used in combustion devices and fuel cell systems, producing only water as a byproduct.While hydrogen improves thermodynamic performance in spark ignition engines, challenges like engine knock and NO x emissions persist.Hydrogen-fueled internal combustion engines face limitations like reduced efficiency and prolonged ignition delay.Direct injection methods offer precise fuel delivery into diesel engine cylinders, improving combustion, power output, and fuel economy.This approach minimizes cold-start emissions and optimizes engine performance by controlling fuel amount per cylinder.Direct injection's advantages include better dispersion and mixing of hydrogen with air, enhancing combustion and reducing emissions.Although hydrogen has a higher flame speed than hydrocarbon fuels, it doesn't always translate to higher thermal efficiency.Despite emitting more NO x , hydrogen-fueled diesel engines can mitigate this with various strategies like exhaust gas recirculation and lean NO x traps (Hosseini et al., 2023).Overall, direct injection methods offer superior combustion control and efficiency, making them promising for hydrogen-fueled diesel engines.
The fast advancement of H 2 technology alongside escalating energy requirements has prompted numerous countries to formulate domestic hydrogen roadmaps.The potential of H 2 and fuel cells in meeting the surging societal energy demands across various sectors has driven its integration into national strategies in many nations.Notably, Japan's government has strategically prioritized H 2 energy as a national initiative, establishing a mature industrial chain that leads to technological advancements and commercialization.Japan's initiatives include a substantial production volume, encompassing more than 300,000 sets of Ene-farm cogeneration systems, over 10,000 parts of Toyota Mirai, and a network of more than 100 hydrogen refueling stations (Fan et al., 2021;Wang et al., 2020).
These strategic roadmaps, evident in various nations such as Korea, Japan, and Singapore, share common developmental objectives, including stability, low carbon emissions, high efficiency, low costs, and large-scale production.Furthermore, beyond addressing environmental sustainability, energy-scarce developed nations are exploring hydrogen as a potential solution to address long-term concerns about energy availability.Japan has developed a comprehensive plan for 2020-2030 that includes stationary fuel cells, H 2 production, fuel cell vehicles (FCVs), and commercial uses.Likewise, developing nations such as China are actively investing in renewable energy and H 2 infrastructure, especially in FCVs and H 2 refueling stations, to align with their sustainability ambitions and address the increasing energy need (Fan et al., 2021).
Several countries have recently introduced significant hydrogen initiatives and funding aimed at aiding the expansion, operation, and widespread adoption of H 2 technologies.For example, the Australian government has committed over 100 million AUD to support H 2 research and pilot projects.In China, a Ten Cities program for H 2 transport has been initiated, with Wuhan expected to become the first Chinese Hydrogen City by 2025.Moreover, countries such as the UK, France, Germany, and several others, including Belgium, Austria, Brazil, Italy, India, the Netherlands, Norway, South Korea, New Zealand, South Africa, Saudi Arabia, and the United States, have allocated substantial funds and projects to advance low-carbon hydrogen technologies across various industrial and transportation sectors (Massarweh et al., 2023).

| Life cycle assessment of hydrogen utilization
The LCA of hydrogen utilization plays a critical role in assessing its environmental performance and guiding the development of sustainable hydrogen-based technologies.By analyzing different aspects, such as hydrogen production methods (e.g., electrolysis, SMR), transportation, storage, and various utilization pathways (e.g., fuel cells, combustion), the LCA provides insights into the associated environmental impacts.This assessment helps in identifying areas where improvements can be made to enhance the overall environmental performance of hydrogen utilization.It enables the comparison of different technologies and strategies, such as renewable hydrogen production versus fossil fuel-based hydrogen production or different hydrogen storage and distribution systems.
Moreover, the LCA of hydrogen utilization allows for the consideration of indirect impacts, such as the environmental consequences of energy sources used in hydrogen production or the emissions associated with infrastructure development.It provides a comprehensive perspective on the environmental trade-offs and synergies of adopting hydrogen as an energy carrier.By conducting an LCA, policymakers, researchers, and industry stakeholders can make informed decisions about the deployment of hydrogen technologies, considering both the environmental benefits and potential drawbacks.This knowledge is crucial for shaping sustainable energy policies, optimizing hydrogen supply chains, and fostering the development of low-carbon hydrogen-based systems.

| Case studies
The case studies presented the LCA of hydrogen utilization, covering diverse applications such as hydrogen FCVs in China, renewable hydrogen in California, hydrogen fuel cell buses in Argentina, hydrogen fuel cells in the shipping sector, and various aspects of hydrogen technologies, including PEMFC stack sustainability and hydrogen transport options.These studies incorporate LCA methodologies, examining the environmental, economic, and social dimensions of hydrogen utilization.The exploration spans different regions and applications, providing a comprehensive understanding of the sustainability implications associated with the diverse uses of hydrogen.Lu et al. (2022) performed an LCA to assess the energy efficiency of hydrogen fuel cell vehicles (HFCVs) in China, integrating insights from domestic and foreign scholars and industry data.The study compares HFCVs with various vehicles, including conventional gasoline, diesel, natural gas, methanol, and battery electric vehicles.Results show HFCV well-to-wheel energy efficiency ranging from 6.8% to 29.2%, depending on the hydrogen source and transport mode.Efficiency is highest (21.9%-29.2%)when hydrogen is produced through water electrolysis with renewable power.Scenarios involving methanol and grid power electrolysis exhibit lower efficiency (6.8%-12.5%).Hydrogen transportation mode choice is linked to production and usage site distance; long-tube trailers suit short-medium distances, while liquefaction or pipeline is effective for distances exceeding 800 km.The study emphasizes the impact of hydrogen production, feedstock production, vehicle operation and on well-to-wheel energy efficiency, suggesting that increasing hydrogen transportation capacity has a less significant effect.Overall, the research provides insights into HFCV life cycle energy efficiency, highlighting the importance of hydrogen source and transport mode considerations and the potential for greener options with renewable power in hydrogen production.
In California, Sinha and Brophy (2021) performed an LCA comparing the environmental impacts of renewable H 2 production for FCVs to battery electric vehicles along with internal combustion engine automobiles.Focusing on potential projects in northern and southern California, the study analyzed both the fuel cycle (production and transport of renewable H 2 ) and the vehicle cycle (FCV operation).The LCA results showed a life cycle carbon footprint for renewable FCVs using renewable H 2 ranging from 105 to 149 g CO 2 -eq/vehicle km, comparable to battery electric vehicles charged from the California grid (0.132 kg CO 2-eq/automobile km) and half that of compact internal combustion cars.The study underscores the crucial role of renewable electricity in determining the carbon footprint of renewable H 2 production (1.5 to 6 kg CO 2-eq/kg hydrogen).The research underscores the significance of policy instruments such as the California Air Resources Board Low Carbon Fuel Standard, which incentivizes renewable hydrogen production.Overall, the study provides insights into the potential of renewable hydrogen for decarbonizing California's transportation sector.It emphasizes the importance of integrating renewable electricity as a key strategy for achieving decarbonization goals.
In Argentina, Iannuzzi et al. (2021) performed an LCA to compare hydrogen fuel cell buses with diesel engine buses in Rosario, analyzing their energy and environmental impacts.The study considered various hydrogen production processes, distinguishing between renewable ("green hydrogen") and non-renewable ("gray hydrogen") sources.LCA results demonstrated significant greenhouse gas emissions reductions with hydrogen fuel, potentially avoiding at least 70% of emissions based on Argentina's current grid emission factor.This aligns with European Union sustainability criteria for biofuels.The study emphasized that hydrogen production accounted for about 80% of the energy consumption in the hydrogen life cycle, underscoring the importance of using renewable sources for sustainable transport.Biomass raw materials, like rapid-cut densified poplar energy plantations and post-industrial wood residues, were identified as competitive options for decarbonizing transport.In terms of energy demand, biogas reforming technology showed 47% less energy-intensive than solid biomass gasification.Overall, the LCA results supported hydrogen fuel cell vehicles, particularly when fueled by renewable hydrogen, as a sustainable option for transportation decarbonization, contributing to reduced greenhouse gas emissions and environmental impact.Jang et al. (2022) performed an LCA to assess the environmental impacts of hydrogen fuel cells in the shipping sector.The study employed a parametric trend LCA methodology, evaluating around 2000 ships involved in domestic and international services.Various hydrogen production methods (SMR, coal gasification, methanol cracking, and wind energy electrolysis) and three fuel cell systems (PEMFC, MCFC, and SOFC) were analyzed.The research identified SMR and coal gasification as having significant environmental potential over their lifetime, potentially surpassing conventional diesel or liquefied natural gas (LNG) products.Notably, the study emphasized the significance of conducting the entire life cycle, highlighting that the upstream phase of LNG production could generate around 100-fold more emissions than the downstream phase.The results were shortened into lifetime ecological indicators, aiding stakeholders in decision-making and informing regulatory frameworks for a green hydrogen maritime economy.The study advocates for a holistic LCA approach, emphasizing its comprehensive assessment of fuel cell environmental impacts throughout their life cycle, beyond existing regulatory methods, such as the ship energy efficiency management plan, along with the energy efficiency design index.Mori et al. (2023) conducted a life cycle sustainability assessment focusing on eco-design for a 48 kW PEMFC stack.The study assessed environmental, economic, and social impacts, revealing a carbon footprint of 1160 kg CO 2 -equivalent, primarily attributed to platinum (63.5% of total value).The production cost for 50,000 PEMFC stacks per year was determined to be €2233/stack.Platinum production in South Africa has emerged as a sustainability hotspot.Eco-design actions led to a 31% short-term and 46% long-term reduction in climate change impact, while a 54% reduction was achieved with 95% recycled platinum.The study emphasized the importance of circular and sustainable approaches in hydrogen technology development, advocating for eco-(re)design to meet environmental and social principles.By integrating various aspects, the research aimed to support the sustainable development of hydrogen-related products, contributing to carbon neutrality goals.Overall, it highlighted the potential for reducing the environmental impact of PEMFC stacks through eco-design actions and underscored the importance of sustainable practices in hydrogen technology deployment.
In a comparative environmental LCA study for public buses, Lubecki et al. (2023) assessed the sustainability of hydrogen fuel, electricity, and diesel for public bus transportation.The study aimed to identify sustainable development pathways by analyzing environmental impacts and GWP associated with different energy carriers.The LCA model considered energy carrier consumption during bus operation, electricity sources for electric buses, and hydrogen production.Various energy sources, including wind, solar, waste, and grid electricity, were examined.The study covered the life cycles of energy carriers and associated infrastructure.Results indicated that modernizing conventional bus fleets using hydrogen and electric pathways could significantly reduce environmental impact, especially in terms of GWP.
The utilization of low-and zero-emission energy sources, such as wind and solar, played a pivotal role.The research emphasized the importance of reducing energy carrier consumption to further enhance the environmental performance of bus transportation.Additionally, it highlighted the dual benefits of hydrogen and electricity production systems based on waste-to-energy plants, addressing sustainable waste management while meeting transport demands.
For H 2 transport and distribution choices, Wulf et al. (2018) conducted an LCA to evaluate the environmental impacts of different H 2 supply chains.Three supply chain architectures were analyzed: (a) compressed H 2 storage in salt caverns with pipelines, (b) liquid organic hydrogen carriers, and (c) pressurized hydrogen truck transport.Results indicated that pipeline solutions generally had the least environmental impact across the most assessed categories for all cases.Truck transport outperformed in a few impact categories only for short distances (100 km).For truck transport, liquid organic hydrogen carriers showed more environmental influences than pressurized gas in 7 out of 14 sorts, with differences decreasing for longer distances.Seasonal hydrogen storage had minimal environmental influence across all impact categories, regardless of transport distance or hydrogen demand.The study highlighted the strong scaling effects favoring pipeline networks' good performance and emphasized that the most cost-optimal solution might not be the most environmentally benign option.The research aimed to fill a gap by specifically addressing environmental issues associated with diverse hydrogen transport and distribution technologies, complementing previous studies that focused on economic aspects.
Regarding hydrogen-fueled diesel engines, El Hannach et al. ( 2019) examined the environmental and economic impacts of implementing hydrogen and diesel dual-fuel solutions in heavy-duty trucks.The study aims to address the high greenhouse gases emissions and air pollutant emissions associated with diesel combustion in the transportation sector.By retrofitting existing fuel injection systems and onboard fuel storage, diesel can be blended with hydrogen to reduce emissions.The LCA conducted in the study reveals that the implementation of hydrogen and diesel dual-fuel solutions leads to a significant reduction in emissions in proportion to the diesel displacement ratio.Moreover, the use of hydrogen fuel offers potential cost savings, especially when hydrogen pricing is below C$4/kg.The research suggests that utilizing low-cost waste hydrogen can immediately reduce emissions and operational costs for existing truck fleets, serving as an economical bridge solution for sustainable heavy-duty freight.
For hydrogen-fueled engines, Kolahchian Tabrizi et al. (2024) recently explored the retrofitting of diesel engines with H 2 as a potential solution for decarbonizing non-electrified railways.The authors employ one-dimensional engine modeling and advanced numerical simulations to evaluate the performance and environmental impact of hydrogenfueled engines.The research includes LCA analysis with a Well-to-Wheel system boundary to assess the carbon footprint of the trains.The findings indicate that the hydrogen-fueled engine exhibits reduced traction power compared to the baseline diesel engine, with a decrease from 600 to 400 kW.Although the energy consumption of the hydrogenfueled engine on a real train mission profile is approximately two times that of the diesel version, the LCA shows a significant reduction in equivalent CO 2 emissions.The engine fueled with PV-based green hydrogen demonstrates a 56% reduction in CO 2 emissions.Substituting diesel with low-carbon hydrogen can potentially decrease the train's carbon footprint from 4.27 to less than 2 kg CO 2 -eq/km.The study emphasizes that moderately modified engines can be a promising solution for decarbonizing non-electrified railway sections.The research highlights the importance of considering the life cycle impacts and carbon footprint when evaluating alternative fuel options for transportation systems.
Collectively, the case studies underscore the intricate interplay of technological choices, energy sources, and geographical considerations in determining the environmental viability of hydrogen utilization.From transportation modes to industrial applications, each study contributes valuable insights into optimizing the life cycle of hydrogen fuel cells.Key findings emphasize the importance of renewable energy sources, efficient production methods, and strategic transportation solutions.As the world seeks cleaner alternatives, these case studies provide a holistic understanding of the challenges and opportunities in harnessing hydrogen's potential across various sectors, paving the way for sustainable energy solutions.The production, storage, and utilization of hydrogen may cause negative environmental consequences.LCA studies investigated the different aspects associated with the environmental sustainability of large-scale hydrogen production and utilization.The advancement of microalgal biofuels has the potential to fulfill 30% of the global energy demand without causing any adverse effects on the environment (Kowthaman et al., 2022).Nonetheless, further research attempts are necessary to address the limitations associated with CO 2 capture using microalgae.For instance, the cultivation process is vulnerable to extreme environmental conditions such as temperature, pH, and salinity (Li et al., 2023).Norsker et al. (2011) carried out a cost analysis on the production of biomass using various bioenergy production processes involving flat panel photobioreactors, open ponds and horizontal tubular photobioreactors.Based on this analysis, the costs associated with biomass production, including dewatering, were determined to be €5.96,€4.15, and €4.95/ kg for these processes.Additionally, optimization efforts were undertaken to enhance the photosynthetic efficiency of these processes, improve mixing and irradiation conditions, and reduce costs related to medium and carbon dioxide.As a result, the price of biomass production was reduced to €0.68/kg.
Moreover, Somers et al. (2021) introduced an innovative approach employing mixotrophic growth, which combines the advantages of both photoautotrophic and heterotrophic growth.This novel process substantially improved the economic feasibility and overall efficiency of the operation.The study revealed that the suggested process attained a substrate generation rate of 0.7 g-biomass per g-substrate, with a cost of $921 per dry metric ton and a productivity of 0.8 kg/m 3 /day (Somers et al., 2021).In summary, the results indicated that the closed system incurs higher production costs compared to the contemporary system, while the latter has higher operational costs than the former (Ighalo et al., 2022).
Based on a techno-economic assessment, the cost of producing methane through the utilization of green hydrogen in combination with carbon dioxide is higher than that of natural gas (€0.66 vs €0.17/Nm 3 ); however, at the same time, it is lower than that of biomethane (€1/Nm 3 ) (Chirone et al., 2022).The proton exchange membrane electrolyzer used in the process mentioned above was the primary contributor to these costs, representing around 90% of the total equipment expenses.The primary cost for proton exchange membrane operation is electricity costs, which account for more than 99% of total operational expenses.Additionally, wastewater treatment, primarily generated during the condensation of steam in the methanation process, has a minimal cost of about 0.1% of operational expenses.The maintenance expenses constitute only approximately 0.12% of the overall operating costs, which are initially estimated to be 10% of equipment costs and could increase due to the relatively short lifespan of electrolyzers.The proposed system leads to a methane production cost of approximately €0.66/Nm 3 .
In comparison, the cost of natural gas exploration and production in Europe was €0.17/Nm 3 in 2020.Moreover, the cost of methane produced using conventional methanation systems, excluding the two interconnected fluidized beds, was recently suggested to be around €0.51/Nm 3 .The comparative assessment of the environmental effects of this system indicates that methane production through the proposed system results in reduced environmental impacts compared to other categories such as photochemical ozone formation, terrestrial acidification, marine and terrestrial eutrophication, resource use related to energy carriers, and the impact categories of respiratory inorganics (Chirone et al., 2022).Although the proposed system enhances methane production from maize silage in categories such as climate change, ecotoxicity in freshwater, land use, ionizing radiation, and non-cancer human health, it exhibits environmental drawbacks in the remaining environmental categories (Chirone et al., 2022).Electricity generation from wind represents the primary source of environmental impacts of the proton exchange membrane unit in the system, with a contribution ranging from 35% in the ionizing radiation category to approximately 100% in the water scarcity, resource use-mineral and metal, land use, and cancer human health categories (Chirone et al., 2022).
The environmental impacts associated with hydrogen utilization for port logistic activities demonstrated that a significant reduction in greenhouse gases could be achieved through the use of green H 2 when compared with conventional diesel-based logistics (Mio et al., 2023).Gürbüz et al. (2021) investigated the impact of euro diesel-hydrogen dual fuel combustion on the performance and environmental-economic indicators of a small turbojet engine used in unmanned air vehicles.Experimental studies were conducted on a JetCat P80-SE turbojet engine, varying the hydrogen flow rate, and substituting euro diesel fuel with hydrogen at different fractions.The results indicate significant improvements in specific energy consumption and emissions.At 40,000 rpm, a 6.5% reduction in specific energy consumption was achieved with 15% hydrogen energy fractions, while a 33% reduction in CO emissions was observed.At 100,000 rpm, the reduction in specific energy consumption was 1.2%, and CO emissions decreased by 54%.The range of CO 2 emissions decreased by 16%-18% across the engine speeds.Although there was an increase in hydrocarbon and NO x emissions with higher hydrogen energy fractions, the overall environmental-economic indicators showed improvement due to the reduction in CO 2 and CO emissions.This study demonstrates the potential benefits of diesel-hydrogen dual-fuel combustion in terms of performance and environmental impact.Additionally, the most sustainable and environmentally friendly green hydrogen technology was found to be alkaline electrolysis cells (Mio et al., 2023).
The associated environmental impacts of supercritical water gasification-based microalgae systems were compared with other systems in terms of GWP values to estimate greenhouse gas emissions.It was found that the GWP values of supercritical water gasification-based microalgae systems range from 0.69 to 0.87 kg CO 2 -eq/kg ammonia, providing a positive effect in carbon emission reduction when compared to conventional fossil energy conversion systems (Liu et al., 2021).
Green hydrogen carriers such as methanol and formic acid displayed higher global warming impact values compared to H 2 liquefaction.However, in the case of electrochemical formic acid production, there was a 39% reduction in global warming impact values when utilizing wind electricity (Kim et al., 2023).
The LCA evaluation of carbon footprint showed a significant potential for lowering greenhouse gas emissions through electron beam plasma methane pyrolysis technology, ranging from 1.9 to 6.4 kg CO 2 -eq./kgH 2 , depending on the energy source (Kerscher et al., 2021).This indicates a notable improvement in comparison to state-of-the-art hydrogen production technologies, which have a carbon footprint of 10.8 kg CO 2 -eq/kg H 2 .
The LCA for the ammonia synthesis from hydrogen through thermo-catalytic solar cracking of LNG was performed using GaBi software (Mohamed et al., 2021).The anticipated global warming potential with 30-year continuous operations is roughly 616 g CO 2-eq/kg ammonia, indicating a potential reduction of up to 69.2% in global warming potential compared to the global average.On the other hand, the analysis showed that the system generates approximately 3.32E-2 kg 1,4-DB-eq and 5.96E-4 kg PM2.5-eq/kg ammonia in terms of impact categories associated with human toxicity and the formation of fine particulate matter.
The supercritical water gasification system for oilfield wastewater showed superior exergy efficiency (53.43%) and reduced GWP (5.52 kg CO 2 -eq/kg H 2 ) in comparison to other alternative supercritical water gasification systems (Peng et al., 2024).This demonstrates the system's potential as an effective and environmentally friendly technology for hydrogen production from wastewater.
LCA of hydrogen FCVs and natural gas vehicles was conducted under various hydrogen production pathways (Cui et al., 2023).The study demonstrated that the use of wind power in water electrolysis for hydrogen production has higher environmental performance.The environmental indicators such as acidification potential, GWP, eutrophication potential, and photochemical ozone creation potential were found to be 91.44%,73.33%, 96.47%, and 93.39%, respectively, which are lower than the processes involving coal synthetic natural gas vehicles.
The life-cycle analysis of the chemical looping combustion method indicated that the primary source of greenhouse gas emissions originated from natural gas production and transport, contributing to more than 70% of the total emissions (He et al., 2021).The calculated life cycle greenhouse gas emissions were 129.98 kg/CO 2 -eq/MWh, where the CO 2 recompression process was the second-largest contributor, followed by gas leakage between the fuel reactor and air reactor as the third significant factor.The use of chemical looping combustion technology for hydrogen production demonstrated an overall system improvement, with an 18.19% decrease in energy consumption compared to traditional standalone production systems and a noteworthy 46.87% reduction in life-cycle emissions.
The social impacts of hydrogen projects are multifaceted, encompassing aspects such as community engagement, employment generation, and public health considerations.While these projects offer opportunities for economic growth and job creation, they also present challenges, such as land use changes and safety concerns (Almaraz et al., 2024).Prioritizing community involvement, ensuring equitable access to employment opportunities, and implementing robust safety measures are vital for realizing the potential benefits of hydrogen technologies while mitigating potential risks.Overall, a holistic approach that considers social implications alongside technological advancements is essential for fostering sustainable and responsible development in the hydrogen sector.

| Policy, practical implications, and future directions for improving the hydrogen economy based on life cycle assessment insights
An economy based on hydrogen can achieve a substantial reduction in greenhouse gas emissions and increase energy supply.The recent LCA studies were integrated with techno-economic assessments to explore various options for improving the hydrogen economy as follows: • Various approaches have reduced the cost of bioenergy production from microalgae.For instance, volatile fatty acids from food waste were employed as a carbon substrate to enhance the heterotrophic cultivation of microalgae.This strategy aimed to boost lipid production, consequently lowering the overall cost of biodiesel production to $2.3 per gallon (Fei et al., 2015).The high energy cost for the production of methane from green hydrogen highlights the significant reliance of economic viability on anticipated reductions in electricity costs or the efficient use of waste energy (Kerscher et al., 2021).• Enhancements to the efficiency of the proton exchange membrane electrolyzer will be needed for hydrogen production in the future to reduce the high electricity consumption, which will lead to a substantial positive impact on both the economic and environmental aspects (Chirone et al., 2022).• There is a possibility to reuse wastewater as a supply for the proton exchange membrane electrolyzer, following a purification process to reduce economical costs and mitigate the plant's impact on freshwater consumption.• Future cost predictions of hydrogen production revealed the profitability of plasma pyrolysis technology of levelized cost of hydrogen below 1.50 V/kg H 2 , thereby reinforcing its economic viability in real applications (Kerscher et al., 2021).In general, plasma pyrolysis technology demonstrates economic advantages for hydrogen production compared to water electrolysis due to its lower specific energy consumption.In particular, the use of renewable energy in electron beam plasma pyrolysis technology could further increase its economic viability.• It is necessary to improve cogeneration hydrogen production systems by increasing the high-temperature resistance of the oxygen carrier in the future (Fan et al., 2018;Zhu et al., 2018).This has the potential to raise the temperature at the turbine inlet, leading to improved energy efficiency and consequently enhancing both economic and environmental viability.• Life cycle emissions can be reduced during hydrogen production systems by minimizing energy consumption in recompression and addressing leakage concerns during the exploration and transportation of natural gas (He et al., 2021).• The LCA methodology utilized in this study offers valuable insights into the environmental impact of hydrogen throughout its complete life cycle.This comprehension is vital for evaluating the sustainability of hydrogen energy systems and pinpointing areas where environmental enhancements are feasible.Decision-makers can leverage this data to prioritize production, storage, and utilization pathways that align with environmental conservation goals.• The research examines various hydrogen production pathways, including blue, green, purple, and biohydrogen methods.Through a comparative analysis of these pathways, policymakers and stakeholders can assess their life cycle inventories and environmental ramifications.This assessment aids in the selection of the most sustainable and effective production techniques, considering factors like local resources, energy availability, and environmental concerns.
• The thorough evaluation of hydrogen systems through LCA offers decision support to a range of stakeholders, encompassing industry stakeholders, investors, and researchers.This support extends to aiding in the selection of suitable production and storage technologies, optimizing energy efficiency, mitigating environmental consequences, and pinpointing avenues for future research and development efforts.• The research underscores the challenges, limitations, and implications associated with hydrogen energy systems.
This has the potential to stimulate cooperation among academia, industry, and governments to tackle these challenges, surmount limitations, and foster innovation in hydrogen technologies.By harnessing insights from LCA, stakeholders can collaborate to devise greener and more effective methods for hydrogen production, storage, and utilization.

| Challenges
• Energy efficiency: One of the primary challenges in hydrogen energy systems is ensuring energy efficiency throughout the entire life cycle.The production, storage, and utilization of hydrogen require energy inputs, and optimizing the efficiency of each stage is crucial to achieving a sustainable and economically viable system.Improving the energy efficiency of hydrogen production methods, such as electrolysis or steam methane reforming, is essential to minimize energy losses and increase overall system efficiency.• Infrastructure limitations: Another issue is the lack of infrastructure for hydrogen energy, including refueling stations and storage facilities.Experts need to concentrate on constructing and implementing the necessary infrastructure to facilitate the widespread adoption of hydrogen energy (Olabi et al., 2021).• Cost: The cost of hydrogen production, storage, and utilization is another significant challenge.Currently, hydrogen production from renewable sources is relatively expensive compared to fossil fuel-based production methods.The high cost is primarily attributed to the capital-intensive infrastructure required for renewable energy systems and the limited economies of scale.Lowering the cost of hydrogen production and making it economically competitive with conventional energy sources is crucial for its widespread adoption (Abe et al., 2019).• Material compatibility: Hydrogen's high reactivity can lead to embrittlement and other forms of deterioration in certain materials used for production, storage, and usage.Research efforts should focus on developing hydrogen-compatible materials capable of withstanding the harsh conditions associated with hydrogen applications (Le et al., 2024).• Energy density: Hydrogen possesses a lower energy density compared to typical fossil fuels, resulting in larger storage requirements for the same amount of energy.Researchers should prioritize the development and enhancement of hydrogen storage devices to tackle this challenge effectively (Osman et al., 2024).• Safety considerations: Due to hydrogen's highly combustible nature, addressing safety concerns related to its production, storage, and transportation is crucial.It is imperative to develop safe and reliable methods and techniques for handling hydrogen (Agaton et al., 2022).

| Opportunities
• Environmental sustainability: Hydrogen energy systems provide an opportunity for enhanced environmental sustainability.LCA methodologies help evaluate the environmental footprint of hydrogen throughout its entire life cycle, guiding sustainable decision-making.By identifying and addressing environmental challenges associated with hydrogen production, storage, and utilization, the industry can strive for continuous improvement, minimizing environmental impacts and ensuring a sustainable energy future.• Energy transition and economic opportunities: The transition to a hydrogen-based economy presents significant economic opportunities.The establishment of hydrogen production, storage, distribution, and utilization infrastructure creates new industries and job opportunities.The growth of the hydrogen sector can stimulate economic development, attract investments, and foster innovation in technologies and supporting industries.Moreover, the export potential of hydrogen can contribute to energy independence and international trade, positioning countries as leaders in the global energy market.• Renewable energy and versatile applications: Renewable energy sources like wind and solar power not only offer the opportunity to produce hydrogen, reducing greenhouse gas emissions and integrating renewables into the energy mix, but hydrogen also serves as an energy storage solution, enabling the integration of intermittent renewables into the grid, while hydrogen fuel cell cars show promise in replacing gasoline vehicles, reducing emissions, and improving air quality, prompting researchers to focus on advancing this technology (Le et al., 2024).• Technological advancements: The development and deployment of hydrogen energy systems drive technological advancements.Research and innovation in hydrogen production, storage, and utilization technologies are accelerating, leading to improved efficiency, cost reduction, and scalability.Advancements in electrolysis, fuel cell technology, hydrogen storage materials, and infrastructure solutions contribute to the optimization and commercial viability of hydrogen energy systems.

| CONCLUSION AND RECOMMENDATIONS
This comprehensive review underscores the environmental considerations and sustainability potential of hydrogen as an adaptable energy carrier, driven by the imperative to combat climate change and curb carbon emissions.Through rigorous life cycle assessment (LCA), the study investigates diverse production pathways, storage technologies, and utilization applications, yielding crucial insights.Notably, various hydrogen production methods, spanning blue, green, purple and biohydrogen, are scrutinized, with green hydrogen emerging as particularly advantageous due to its diminished carbon footprint, courtesy of electrolysis with renewable energies.Nevertheless, the intricate nuances of the entire life cycle, encompassing the environmental ramifications of renewable energy technologies, warrant thorough examination.Essential to optimizing energy efficiency and curbing waste, hydrogen storage technologies are highlighted, emphasizing the significance of evaluating infrastructure needs, energy inputs, and associated environmental impacts.Moreover, hydrogen's role in enabling clean and efficient electricity generation is pivotal for industry-wide decarbonization efforts.Throughout, comparative LCA analyses underscore the necessity of a holistic approach, considering factors like material extraction, manufacturing processes, and waste management.Consequently, this comprehensive assessment equips decision-makers with invaluable insights, facilitating the transition toward a low-carbon economy by positioning hydrogen as a pivotal component of the global energy landscape.Looking ahead, the study identifies several avenues for future research, advocating further exploration of emerging production pathways like purple hydrogen and biohydrogen.Additionally, the development of innovative storage technologies holds promise for enhancing efficiency and safety.By optimizing hydrogen utilization across diverse sectors and fostering international collaborations, stakeholders can expedite the transition to a sustainable, energy-efficient hydrogen economy.Policymakers are urged to leverage these findings in shaping regulations, incentives, and investment strategies that propel the adoption of environmentally friendly hydrogen systems, thereby advancing global sustainability objectives.

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I G U R E 2 LCA framework based on ISO14040.

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I G U R E 4 Hydrogen's versatile applications across various sectors.

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| KEY ENVIRONMENTAL, SUSTAINABILITY CONSIDERATIONS, AND FUTURE PERSPECTIVES 7.1 | Identification of key environmental, sustainability considerations, and social impacts