Trends in Research and Development for CO2 Capture and Sequestration

Technological and medical advances over the past few decades epitomize human capabilities. However, the increased life expectancies and concomitant land-use changes have significantly contributed to the release of ∼830 gigatons of CO2 into the atmosphere over the last three decades, an amount comparable to the prior two and a half centuries of CO2 emissions. The United Nations has adopted a pledge to achieve “net zero”, i.e., yearly removing as much CO2 from the atmosphere as the amount emitted due to human activities, by the year 2050. Attaining this goal will require a concerted effort by scientists, policy makers, and industries all around the globe. The development of novel materials on industrial scales to selectively remove CO2 from mixtures of gases makes it possible to mitigate CO2 emissions using a multipronged approach. Broadly, the CO2 present in the atmosphere can be captured using materials and processes for biological, chemical, and geological technologies that can sequester CO2 while also reducing our dependence on fossil-fuel reserves. In this review, we used the curated literature available in the CAS Content Collection to present a systematic analysis of the various approaches taken by scientists and industrialists to restore carbon balance in the environment. Our analysis highlights the latest trends alongside the associated challenges.


■ INTRODUCTION
Carbon dioxide (CO 2 ) is a critical component for plant life and thus animal and human life. Combustion of carbon-containing fuels to CO 2 allows humans to live almost anywhere on Earth and provides power for industrial production. However, as shown in Figure 1, rapid growth of atmospheric CO 2 concentrations has undesired consequences, including global warming, where the past 40 years have seen temperatures rise at a rate (0.18°C/decade) that is more than twice that (0.08°C /decade) in the 100 years prior. 1 CO 2 emissions from naturally pre-existing and anthropogenic activities outweigh uptake and sequestration pathways, resulting in a cumulative buildup of atmospheric CO 2 levels. Thus, global warming, atmospheric CO 2 levels, and world population are interconnected. 2−4 Today, our global population is almost 8 billion and atmospheric CO 2 levels are ∼417 ppm (ppm). 5 Land use changes and fossil-fuels usage account for ∼40 billion metric tons/year CO 2 emissions globally as of 2021. 6,7 In comparison, the world population was ∼2.5 billion in 1950 and the global CO 2 emissions were estimated at ∼11.5 billion metric tons/ year. According to the predictions by an Intergovernmental Panel for Climate Change (IPCC), global warming is likely to reach 1.5°C between 2030 and 2052 (relative to preindustrial levels between 1850 and 1900). Due to the far-reaching ecological consequences the international community adopted significant CO 2 -emissions reduction targets ("race to zero" by 2050) in 2015 at the 21st Conference of Parties on Climate Change. 8 Given these societal challenges and because CO 2 is nontoxic, nonflammable, plentiful, and a renewable carbon source, it has been utilized as a sustainable feedstock as part of multipronged approaches to decarbonize the atmosphere via capture and biological, chemical, and geological sequestration or storage. Carbon or CO 2 Capture and Storage (CCS) technologies are focused on reducing the amounts of CO 2 released into the atmosphere by separating it from other gases, compressing, transporting and finally storing the captured CO 2 far away from the atmosphere, avoiding any leakage back into the ecosystem. 10,11 The high costs associated with these technologies have limited large-scale annual capture and storage capacity to only about 0.1% of global CO 2 emissions, 12 but this number is predicted to go up to 19% by 2050. 13 CO 2 in its purified or impure form (i.e., present in gas mixtures) can be directly utilized as is (enhanced oil recovery, food and beverage industries) or as a raw material for conversion into other substances via geological (carbonates/soda ash, rock and saline formations, minerals, coalbeds, aquifers, etc.), biological (carbohydrates, lipids, proteins, secondary metabolites, etc.), and chemical (carbon monoxide, alkanes, alcohols, alkenes, acids, etc.) processes via carbon or CO 2 Capture and Utilization (CCU) technologies. 14 Figure 2 shows the carbon cycle and human contributions to CO 2 emissions and potentially useful technologies for capture and sequestration of CO 2 .
In this paper, we present a systematic analysis of the latest trends in CO 2 capture and sequestration research utilizing data from the CAS Content Collection of journal and patent publications spanning the years 2001−2021. We focus on technologies capable of reducing global atmospheric CO 2 levels. The current global CO 2 emissions are at about 40 gigatons/year; therefore, a technology should have the potential to sequester gigaton quantities of CO 2 each year. 15 We first provide an overview of publication trends by time, country/region, and keywords. Then, the technologies, methods, materials, and chemical substances involved in CO 2 separation, capture, sequestration and use are discussed and their publication trends analyzed to provide an overview of current developments in CO 2 mitigation strategies.

■ RESEARCH TRENDS IN CO 2 CAPTURE AND SEQUESTRATION
The CAS Content Collection is the largest human-curated collection of published scientific knowledge, In this work, about 18500 CO 2 capture and sequestration-related documents containing relevant terms in title, abstract, or CASindexed areas were found with publication years between 2001 and 2021 (see the Supporting Information for method descriptions of search strategy and terminologies). We chose to use a sample of available documents rather than a comprehensive set of documents in order to ensure that trends in CCS research are accurately represented. Figure 3 shows the annual publication volume related to CO 2 capture and sequestration. Overall publication numbers increased rapidly in the early 2000s, slowed down after mid-2010s, and recently experienced rapid growth. The initial steady increase can be attributed to the urgency of reducing atmospheric CO 2 levels triggered by global efforts. However, the absence of strong support in carbon capture and storage projects evident in small investment and economic incentives given to the CCS process compared to other technologies may have been the cause of stabilizing publication numbers afterward. Alternatively, we see an interdependence between oil prices and climate policies; low oil prices are likely to make  Figure 2. Global processes contributing to atmospheric CO 2 emissions (red arrows) and sequestration (green arrows), and global efforts to mitigate net emissions through technologies classified into different disciplines of science. Global average CO 2 fluxes for the decade 2011−2020 are shown for processes that significantly impact the global carbon cycle via CO 2 release (red boxes) or capture (green boxes), as numbers of gigatons (Gt) CO 2 per year. 6 the expenses of CO 2 capture technologies difficult to tolerate, even with the use of captured CO 2 in enhanced oil recovery to offset capture costs. It can also be seen from Figure 3A that most of the documents are journal publications. Patent publications only account for ∼10% in this document pool, whereas on average, patents account for a third of the total documents in the CAS Content Collection within similar time frame, suggesting a relatively small commercial interest in this research topic. 16 However, the distinct count of concepts being introduced to this field showed a steep increase in the last 10 years, especially in journal publications ( Figure 3B), suggesting new ideas or methods being tested.
We also grouped the publications according to the country/ region of the first author affiliated organization (journal publications) or the country/region of the patent assignee (patent publications). Figure 4 shows that the same ten countries/regions are responsible for the largest numbers of both journal and patent publications. Publication trends over the past 20 years show that China has steady growth in both journal and patent publications, while India shows similar growth in journal publications; these increases may be driven by the increasing CO 2 emissions in both China and India ( Figure 5). In CCS literature, the United States shows a small peak in the years of 2013 and 2014 and constant publication numbers since 2015. It is clear that researchers from China are driving publication trends in CCS, as removing articles from China-based authors showed publication numbers remaining roughly constant from 2015 to 2019 before increasing in 2020 and 2021 ( Figure 6).

■ CARBON CAPTURE PROCESSES
In this section we focus on the methods and materials used for CO 2 capture and separation from other gases in industrial settings. Annual volumes of related publications from 2000 to 2021 are shown in Figure 7, where the pattern is similar to that for total publications related to CO 2 sequestration and utilization ( Figure 3A). Publication numbers were low prior to 2007, increased sharply afterward, peaked in the early 2010s, and stabilized afterward. The publication trends for CO 2 separation and capture (Figure 7) are more abrupt than the  overall publication trends shown in Figure 3A but likely originate from similar causes. CO 2 capture and separation technologies are closer to implementation than other CO 2 sequestration research and are more sensitive to economic incentives and oil prices, consistent with the observed publication trends. This section will begin with an introduction to carbon capture systems (industrial setting for flue gas handling), followed by reviews of capture methods (physical/ chemical processes and materials used).
Carbon Capture Systems. The removal of CO 2 from power plant flue gases, the single largest source of human CO 2 emissions, has been a major focus of carbon dioxide capture research. 17−19 The three most widely studied techniques are postcombustion capture, precombustion capture, and oxy-fuel combustion capture.
Postcombustion Capture. Most thermal power plants generate electricity by burning fuel (most often pulverized coal or natural gas) in air to release its thermal energy. The heat boils water into steam which propels a turbine to generate electricity; the combustion generates CO 2 and water vapor, which, along with nitrogen from the air, are major components of the flue gas. Postcombustion capture, the most popular method, removes CO 2 from this gas mixture. It can be straightforwardly retrofitted to existing power plants and is the only commercialized carbon capture technique. The major disadvantage of postcombustion CO 2 capture is that flue gas, diluted with large amounts of nitrogen carried over from air, has low pressure and low CO 2 concentration, making the separation difficult and energy intensive.
Precombustion Capture. Precombustion capture is carried out in power plants where fossil fuels are utilized differently. A limited amount of pure oxygen is supplied, with or without steam, to partially oxidize the fuel, producing synthesis gas (syngas) comprised primarily of carbon monoxide, hydrogen, and some carbon dioxide. This hot gas mixture contains thermal and chemical energy, which are converted to electricity through a steam turbine and a gas turbine, respectively. The whole process, termed the integrated gasification combined cycle (IGCC), is more energy efficient than combustion and has simpler emission control. 20 If CO 2 capture is desired, the cooled syngas, instead of going straight to the gas turbine, is subjected to a water-gas-shift reaction to convert carbon monoxide into CO 2 . The CO 2 /H 2 mixture then goes through a separation unit to remove CO 2 and produce high-purity hydrogen. The CO 2 /H 2 mixture has a simple composition, is at high pressure, and contains CO 2 in high concentration, making CO 2 separation much easier and less energy intensive than postcombustion capture. However, retrofitting power plants for precombustion carbon capture is much more difficult than for postcombustion, and the production of pure oxygen for partial oxidation is energy intensive.
Oxy-Fuel Combustion Capture. As the term implies, in oxyfuel combustion the fuel is combusted in pure oxygen instead of air. The flue gas generated thus comprises predominantly CO 2 and water vapor, which are easily separated by condensation of water. The biggest challenge facing oxy-fuel combustion is the high energy and cost required to produce pure oxygen, which is used in much larger quantities than in IGCC. The simplified schemes for the three CO 2 capture techniques are shown in Figure 8.
Chemical Looping Combustion Capture. In this emerging CO 2 capture technology, treatment of a fuel with a metal oxide partially reduces the latter while yielding a waste stream containing only CO 2 and H 2 O. 21 The reduced metal oxide is then oxidized with air to regenerate metal oxide, which is returned to the fuel stream to complete the cycle. Chemical looping may be viewed as a variant of oxy-fuel combustion in which a metal oxide to metal transition acts as an oxygen   carrier, obviating the expensive process for generating pure oxygen. However, the chemical looping combustion of solid and liquid fuels is much more complicated than the combustion of gaseous fuels. In addition, the fluidized bed reactors needed for combustion are complex, while the processes and apparatus needed to move solids between the combustion and reoxidation chambers are complicated and not easy to optimize, making the technology expensive to use. The above four CO 2 capture processes are compared in Table 1.
Direct Air Capture. An alternative to CO 2 capture from industrial plants is to capture it directly from the environment. 22 Direct air capture (DAC) can be carried out by absorption or adsorption and has the potential to achieve negative emissions if clean energy is used in the process without generating extra CO 2 . 23 DAC plants are small and can be placed where needed, such as near carbon storage, use, or emission sites. 24,25 DAC projects have recently been strongly supported by substantial funding, and 19 DAC plants have been established worldwide. 26,27 Unfortunately, because of the very low CO 2 concentration in the atmosphere (412 ppm), the theoretical energy required to capture one ton of CO 2 from air is several times that of capturing it from power plant emissions, 28 and the current cost of capture is higher than the price of CO 2 ; thus, DAC is not profitable today.
The numbers of publications related to the different processes for CO 2 capture are shown in Figure 9, where postcombustion capture has a significantly higher publication volume than all other processes. Publication volumes for the three primary techniques all increased starting in the late 2000s but peaked in the early or mid 2020s, displaying a generally decreasing trend afterward. Chemical looping and direct air capture, the newest emerging technologies, have low but increasing publication volumes, consistent with their lack of technical maturity.
Methods for Capturing CO 2 from Flue Gas. The most studied methods for separating CO 2 from gas mixtures include: (1) absorption into solvents, (2) adsorption into porous solid adsorbents, and (3) filtration using membranes.
Absorption. Absorption of CO 2 may be carried out chemically or physically. In chemical absorption, an alkali absorbent solution is brought into contact with the gas mixture   to neutralize CO 2 and form carbamate or bicarbonate salts. 29,30 The resulting solution is then transferred to a regenerator (reboiler) to release the CO 2 and recover the solvent. Monoethanolamine (MEA) is the most widely used absorbent and is the only one currently used in commercial applications. 31 Other widely studied amines include diethanolamine (DEA), 32 methyldiethanolamine (MDEA), 33 piperazine, 34 and 2-amino-2-methyl-1-propanol (AMP). 35 Aminebased absorption is effective even for low-pressure streams with low CO 2 concentrations, making it particularly suitable for postcombustion capture. Their drawbacks include limited thermal and oxidative stability, 36 the high thermal energy required for solvent regeneration, 37 solvent evaporation, and the corrosiveness of the absorbents. 38 The numbers of studies related to CO 2 capture using the most popular amines are shown in Figure 10. MEA has clearly been the most studied absorbent over time, while the publication volumes for most amines peaked in the mid-2010s and then decreased.
Electrochemical solvent regeneration, where CO 2 is stripped from the stream by electrochemically generated copper ions, has been reported. 39,40 The processes were found to be more energy efficient than thermal regeneration and can be carried out at normal temperatures, thus minimizing solvent loss and thermal degradation.
Ammonia is more stable, less expensive, and less corrosive than other amines. 41 However, ammonia boils at −33°C and has a high vapor pressure, leading to rapid loss of absorbent and deterioration of absorption capacity. Ammonia loss can be mitigated by absorption of CO 2 at low temperature, 42 but the absorption efficiency is compromised and the energy requirements for cooling decrease energy efficiency. Washing with water or acids also improves ammonia retention 43,44 but generates large amounts of wastewater or chemical waste.
Potassium carbonate acts as an absorber by reacting with CO 2 to form potassium bicarbonate. 45 It shares most of ammonia's advantages while being nonvolatile, enabling CO 2 absorption at much higher temperatures. 46 Its major disadvantage is a low absorption rate owing to poor CO 2 mass transfer. Amine and amino acid promoters that form intermediates with CO 2 to facilitate the generation of bicarbonate ions have been investigated. 47 Grimekis et al. demonstrated that adding piperazine and MEA improved absorption rates and CO 2 solubility at the same time, while MDEA and glycine significantly impacted CO 2 solubility. 47,48 Li et al. reported enhancements in both absorption and desorption by using glycine or lysine as promoters. 49 The total journal and patent publication volumes for MEA, potassium carbonate, and ammonia are shown in Figure 10B. Both K 2 CO 3 and ammonia have much lower journal publication numbers than MEA; interestingly, there are more patents on ammonia than on MEA.
Besides being studied as additives for other absorbents, amino acid salts have also attracted attention as CO 2 absorbents themselves. They share the benefits of amines and carbonates and have high absorption capacity along with low toxicity and vapor pressure. 50 Their major disadvantage is the easy formation of precipitates upon CO 2 absorption due to their ionic nature, which complicates heat and mass transfer. 51 For direct air capture (DAC), due to the low CO 2 concentration in the air, the leading absorbents used are strong base solutions such as KOH or NaOH. 52 The use of strong base absorbents means large amounts of energy are needed to separate CO 2 from the absorbent, which exacerbates the high cost of DAC. Mahmoudkhani et al. achieved significant reduction in absorbent regeneration energy and temperature by using sodium trititanate in place of calcium hydroxide for causticization. 53 More recently, Shu et al. reported an electrochemical process using an electrochemical cell having a pH gradient, allowing for reduction in energy consumption as well as simultaneous desorption and regeneration. 54 Computer-aided molecular design has been conducted to identify new structures or commercially available substances that have not been explored as CO 2 absorbents. Salazar et al. studied 50 amines that were prescreened using solubility and boiling point data, selecting three that showed much lower theoretical reboiler duty than MEA for postcombustion capture. 55 Papadopoulos et al. modeled the solubility and partial pressure of CO 2 and identified both new as well as commercially available alternative amines that outperformed MEA in overall absorption/desorption cycles. 56 Physical absorption relies on physical dissolution of CO 2 as the driving force. 57 The method is effective only at high pressure and lower temperature and thus is much more suitable for precombustion capture. However, physical absorption using noncorrosive solvents is much less demanding on equipment than chemical absorption and has been practiced commercially. In addition, regeneration of physical solvents is much less energy intensive since the dissolved CO 2 can be easily released through depressurization or moderate heating. Commonly studied good CO 2 solvents include methanol, 58 Selexol (polyethylene glycol dimethyl ether), 59 Nmethyl-2-pyrrolidone (NMP), 60 and propylene carbonate. 61 The advantages and limitations of these solvents are summarized in Borhani's review. 62 All four solvents have similar CO 2 solubilities at 25°C. In general, solvents with higher molecular weights such as Selexol have lower vapor pressures, leading to less solvent loss, but suffer from high viscosities which impair mass transfer. Methanol, despite its low cost, has a very high vapor pressure, necessitating refrigeration and water washing to minimize solvent loss. Propylene carbonate's vapor pressure is higher than Selexol's but is still low enough to not require water washing. Because it dissolves hydrogen poorly, propylene carbonate is selective for CO 2 in precombustion capture. Chen et al. reported optimized propylene carbonate solutions containing 2-methylimidazole and ethylene glycol, which demonstrated significantly improved CO 2 solubility and selectivity over hydrogen, methane, and nitrogen. 61 The publication numbers related to different physical absorbents are shown in Figure 11, where methanol dominates the other three absorbents in both journal and patent publication volumes. The much lower document numbers compared to those in Figure 10B indicates physical absorption's lower popularity in comparison to chemical absorption.
Ionic liquids (ILs) are salts with low melting points, enabling them to stay in a liquid state within wide temperature ranges during normal applications. Typical examples are imidazolium salts such as 1-butyl-3-methylimidazolium tetrafluoroborate ([BmIm][BF 4 ]). 63 They have recently drawn strong interest as alternative CO 2 solvents thanks to their very low vapor pressure, lower flammability, good thermal stability, and structural tunability. 64 However, physical properties such as high viscosity combined with the high cost of ILs has limited research into and commercial application of ILs. 65 Adsorption. In adsorption, porous solid adsorbents with large surface areas bind CO 2 . Adsorption methods are compatible with precombustion, postcombustion, and direct air capture. The solid materials used are more stable, less toxic, and easier to handle compared to liquid absorbents. The most studied adsorbents include carbon (activated carbon, biochar, charcoal, etc.), zeolites, and metal−organic frameworks (MOFs), whose advantages and limitations are summarized in Table S1.
Recent research on adsorbents is focused on improving CO 2 uptake and adsorption kinetics and enhancing dimensional stability and reusability, as well as overcoming moisture sensitivity (for zeolites and MOFs). 66−69 Optimizing adsorbents for CO 2 uptake and selectivity does not necessarily guarantee their applicability in real applications. The design of temperature-swing (favorable for postcombustion capture) or pressure-swing (favorable for precombustion capture) adsorption−desorption cycles, as well as different reactor configurations, all pose specific performance and stability requirements on adsorbents and can induce uncertainties in the practical success of an adsorbent that performed well in the lab. 70 More sophisticated modeling to efficiently screen the numerous possible structures (particularly for MOFs) and to predict their performance under complex processing conditions is needed to increase the industrial application potential of CO 2 adsorption.
Voskian et al. reported an electrochemical device for CO 2 adsorption, utilizing a polyanthraquinone−carbon nanotube composite electrode, where CO 2 is captured via reductive addition to the quinones and released by discharging. 71,72 The electro-swing adsorption−desorption process is more energy efficient than temperature-swing and pressure-swing cycles, and the compact electrochemical cells are easy to fabricate and scale up.
From the trends of publications adopting various adsorbents ( Figure 12A), it can be seen that, while publications on zeolites increased little after 2012, those related to carbon had steady growth since 2007. Figure 12B shows the total publication volumes for the adsorbents, where the extremely small patent number for MOFs is worth noting, albeit consistent with the fact that related research is more focused on lab studies of new MOF structures.
Membranes. CO 2 capture by membrane filtration is still an emerging technology, mainly due to low gas permeabilities and  consequent poor separation efficiencies. 73 Membrane-based processes offer lower material costs and operational simplicity and flexibility. Applications in precombustion and postcombustion have both been widely studied. 74,75 For precombustion capture (CO 2 /H 2 separation), H 2 passes through the membrane to the other side (permeate side), leaving CO 2 at the feed side, whereas in postcombustion capture (CO 2 /N 2 separation), separation is achieved by CO 2 preferentially passing through the membrane. Separation mechanisms are different depending on the membrane material and the gas stream, including (1) size sieving, where the membrane's pore size is large enough to allow only the smaller gas molecule to pass through, (2) surface diffusion, where the surfaces of the membrane and pores are occupied by one gas through preferential adsorption and become inaccessible to the other gas, which therefore tends to stay at the feed side while the more adsorbable gas moves to the other side, and (3) solution diffusion, where the more soluble gas preferentially dissolves into and then diffuses through the membrane. 76 While the first two mechanisms work for porous membranes, the third occurs during separation using dense membranes. The most studied membranes can be classified into dense inorganic membranes, porous inorganic membranes, and polymer membranes, as summarized in Table S2.
Most studied polymer membranes have been nonporous (dense) membranes. However, emerging materials such as conjugated microporous polymers, 77 polymers of intrinsic microporosity, 78 and thermally rearranged polymers, 79 where pores with controlled architectures are introduced to organic polymers to improve CO 2 permeability and CO 2 /N 2 selectivity, have recently intrigued researchers. Polymer membrane matrices with inorganic fillers, combining the permeability and thermal stability of inorganic materials with mechanical strength and processability of polymers, have also shown promise. 80 Husna et al. prepared surface-modified UiO-66-NH 2 by grafting an anhydride-terminated polyimide onto the MOF. The modified filler had improved compatibility with microporous polyimide matrices, and the blended membranes demonstrated improved resistance to thermal aging and plasticization, with CO 2 permeability and CO 2 /N 2 selectivity surpassing the Robeson 2008 upper bound. 81 The publication trends of CO 2 capture using polymer membranes and inorganic membranes, as well as their overall publication volumes, are individually shown in Figure 13. Here, inorganic membranes surpass polymer membranes in both journal and patent volumes, with remarkable growth in the most recent years. It should be noted that the actual relative prevalence of inorganic membrane studies is likely even higher�in our analysis method, documents containing polymer substances are deemed polymer-membrane-related, yet one cannot rule out the possibilities of inorganic membranes studied in these documents. The journal and patent publication volumes of some of the most studied polymer and inorganic membranes are shown in Figure 14.

Comparisons of Methods and Concept Map Analysis.
The three carbon capture methods (absorption, adsorption, membrane) are compared in Table 2.
The numbers of publications related to CO 2 capture using absorption, adsorption, and membranes from 2001 to 2021 are shown in Figure 15. Here, absorption-related studies grew substantially up to 2014 and then decreased, whereas adsorption and membranes kept growing, albeit at slower paces after 2010. Absorption has been studied in the patent literature more frequently than adsorption. This observation is consistent with absorption capture being relatively more mature and closer to industrial applications. Membrane separation, on the other hand, has much lower numbers for all publication types compared to the other two techniques, with patent publication volume several times lower than that of absorption, consistent with it being an emerging technology.
To get insights into the prevalence of various CO 2 capture methods (absorption, adsorption, membranes) studied in different processes (postcombustion and precombustion), the numbers of publications involving co-occurrences of the corresponding terms are shown in Figure 16, which suggest that absorption has been studied the most for postcombustion capture; for precombustion, on the other hand, the three methods have almost equal shares of publications. This is to be expected, given that precombustion produces streams with high pressure and high CO 2 concentration that are relatively easy for all separation methods, whereas the dilute CO 2 in postcombustion's CO 2 /N 2 stream favors absorption capture.  The occurrence frequency of chemical preparation of substances in publications can be one indicator of a certain field's technical maturity. The publication trends and total publication volumes for studies involving the synthesis of at least one substance are shown in Figure 17. The numbers of preparative studies involving adsorption capture had the fastest rate of increase ( Figure 17A) as well as the highest total publication volume ( Figure 17B, orange portion of bars) compared to those related to absorption and membranes. 19.6% of all adsorption-related publications concern synthesis, also the highest of all capture methods, with the ratio being only 7.8% for absorption studies. Membrane separation, commonly considered an emerging technology, features a lower percentage of preparative studies than adsorption; researchers have likely focused more on modification of existing materials and membrane fabrication and characterization.
To further shed light on the status of development in CO 2 capture and sequestration, we have also analyzed the prevalence and connections between different concepts    occurring in related publications. The results are shown in Figure 18, where the size of a node reflects the number of times the corresponding concept occurred in the literature, lines between every two nodes denote co-occurrences in the same publication, and distances between nodes indicate the frequencies at which the concepts co-occurred. One interesting observation of the graph is that the concept "simulation and modeling" is nearer to "absorption" than "adsorption", the latter instead being adjacent to common adsorbent characterizations such as "pore size" and "surface structure". The relative material and operation simplicity for absorption capture likely explains the prevalence of its modeling studies.
To help understand recent advancements in real applications of carbon capture, some examples of existing or planned operations of various capture methods are listed in Table 3. As the information shows, carbon capture from flue gas using amine solvents has strong commercial prospects, but direct air capture is also receiving attention, despite being considered the most expensive and energy intensive.

■ CO 2 SEQUESTRATION METHODS
Once CO 2 is captured, it can be sequestrated and stored by chemical or geological processes. CO 2 can also be sequestered biologically, where carbon capture and sequestration are accomplished in one step by living organisms. Recent research progresses and publication trends in these methods will be discussed.
Biological CO 2 Sequestration. Natural biological CO 2 fixation via plant photosynthesis accounts for the largest CO 2 influx (440 gigatons/year) from the earth's atmosphere, of  DOE-funded project outside Bakersfield, California, will capture CO 2 from a gas-fired power plant, using a solvent system developed by Fluor 86 Shell's Cansolv technology for postcombustion capture will be fitted to the gas-emitting stacks of the VPI Immingham power station in the UK, to capture up to 95% of the CO 2 in the flue gas; the system was also installed in a Canadian power plant to capture 1 million tonnes annually 87 ExxonMobil Low Carbon Solutions will develop 20 CCS projects with an initial investment of $3 billion 88 solid sorbents CCS firm Svante raised $75 million to develop nanoporous MOF sorbent to capture CO 2 from flue gas and from the air 88 which 2−3% remains locked in the land for decades. 8,15 In addition, about 50% of this amount is fixed by marine primary producers. 89 Biological CO 2 fixation reactions are highly selective and often require little resources, spurring interest in developing biomimetic and biobased technologies for CO 2 capture and sequestration. 90 The past decade has witnessed a rapid increase in the number of related journal publications (Figure 19), while recently, viable and cost-effective negativeemissions technologies have been developed, collectively referred to as Bioenergy with Carbon Capture and Storage (BECCS), for utilizing biomass (derived from biologically fixed CO 2 ) as the energy source to capture and permanently store CO 2 . 91−94 According to a report published in 2019, 95 five facilities were utilizing this technology to capture ∼1.5 million tons of CO 2 per year. However, a recent study predicts that BECCS has a global potential to sequester up to 5.2 gigatons of CO 2 . 95 The accelerated publication activity related to BECCS over the last 6 years reflects these recent developments ( Figure  20). Biological-System Level CO 2 Sequestration Studies. Primary producers are known to utilize six pathways for CO 2 fixation, which represent billions of years of evolution and optimization for survival and reproduction of the host organisms (Table 4). 96,97 Because of their importance, prior research efforts focused on these biological systems have been useful starting points to capture and sequester atmospheric CO 2 . 12,15,98−101 Among the six CO 2 fixation pathways, the reductive pentose phosphate pathway or the Calvin−Benson− Bassham (CBB) cycle is the prevalent mechanism used by all plants and algae, and most autotrophic bacteria. It is also the most economically relevant pathway. 102 The other five pathways are only present in a small number of bacteria or archaea but nevertheless provide clues regarding the unique environments in which the host organisms thrive. 103 Recent efforts include engineering natural CO 2 fixation pathways in non-native organisms 104−110 and engineering synthetic CO 2 fixation pathways into organisms. 111−115 An emerging new concept combines microbial or algal cell factories with electrochemistry to directly convert CO 2 . 116 Each approach represents a promising line of investigation potentially leading to the capture of gigaton quantities of CO 2 from the atmosphere using natural biological hosts.
The most important challenge in employing photosynthetic organisms for CO 2 capture lies in scaling up the process. Because this involves the use of photobioreactors, open ponds, or raceway ponds, scaling is hindered by large surface area requirements, light requirements, low productivity, and contamination possibilities. 117 Closed systems have been proposed to overcome some of these limitations. 118 Recently, chemoautotrophic organisms including Ralstonia eutropha have been exploited for industrial applications due to the CBB pathway that works with other pathways to sequester CO 2 into bioplastics. 119,120 Furthermore, growth of this organism at scale is simple and can be genetically engineered. 121−129 In our curated list of publications, a significant number of the biology-related publications (∼1600 out of ∼3900) had identifiable terms in the abstract that could be linked to photosynthetic organisms that use the CBB pathway for CO 2 fixation. 70% of these documents were published in the past decade, suggestive of the recent focus on photosynthetic organisms as the biological chassis of choice for CO 2remediation studies (Figure 21).
Bacteria and algae have rapidly become popular natural biosystems of choice for CO 2 -sequestration studies due to their extremely versatile metabolic capabilities, shorter lifecycles, natural abundance, simpler growth requirements, and bioremediation potential and recent advances in genetic manipulation capabilities (Figure 22). Analysis of a subset of our curated publication data set comprising 1343 journal articles and 103 patents published between 2017 and 2021 indicated that it is becoming increasingly attractive to utilize bacteria (mostly cyanobacteria) and algae as cellular factories to sequester CO 2 because they can deliver a sustainable and renewable platform to produce biofuels and high-value products, wastewater and flue-gas remediation, and biomitiga-   tion of unwanted nutrients. 130−138 In one study, the authors report the use of a novel metagenomic approach to analyze the microbial communities in a cold subsurface high-CO 2 aquifer 2 fixation. Metabolic analyses at the organism level provided insights into the biochemical cycles that support subsurface life under the extreme condition of CO 2 saturation, which predominantly involved the use of CBB and WL pathways in tandem. 136 Agricultural and forestry-related activities contribute significantly to global CO 2 emissions. Due to their longer life cycles and less amenability to genetic modification, CO 2 sequestration studies have not focused on the production of bioproducts using CO 2 fixation in plants. Instead, CO 2 sequestration studies utilizing plants have focused on using plant biomass, especially from energy crops, as sustainable and renewable feedstocks for fermentation or biochar production. 92,139−142 The overall publication trends for biological CO 2 sequestration research indicate a growing interest in using both natural and engineered biosystems and enzymes as modules to capture CO 2 in innovative ways and convert it into useful bioproducts, while reducing our dependence on fossil fuel. It is notable that the past few years have seen a number of these applications also integrate flue gas and bioremediation into CO 2 -sequestration strategies.
Chemical Methods for CO 2 Sequestration. Chemical methods for CO 2 sequestration are methods that convert carbon dioxide by chemical means into other materials such as mineral carbonates or concrete which sequester carbon for significant periods of time. CO 2 may also be converted into reduced forms that can be used either as fuels or in the manufacture of organic compounds or fuels which sequester carbon dioxide for a shorter span.
Concrete. 4.4 billion tons of concrete is manufactured worldwide, 143 which generates 7−8% of total human CO 2 emissions. 144 Concrete is made from water, cement, and rocks or sand (aggregate). Cement is prepared from limestone (calcium carbonate, CaCO 3 ), silica (SiO 2 ), iron ore, furnace slag, and clay or slate. 145 Heating the mixture at high temperature (1800°C) drives off carbon dioxide to generate calcium oxide and calcium silicates. 60% of CO 2 emissions in concrete manufacture comes from the decarbonation of limestone, and the remaining 40% comes from the energy needed to make the cement. 146 The calcium salts in powdered cement react with water at the time of use to form a paste containing calcium hydroxides and silicates, which adhere to the aggregate and bind it into a single mass. 145 Over time, the calcium hydroxides in concrete absorb carbon dioxide from the atmosphere, forming more stable calcium carbonates which strengthen the concrete in the weeks after installation and over the service life of the structure. Between 10 and 30% of CO 2 emitted during cement manufacture is reabsorbed during its service life. 146 Carbon dioxide can also be added during concrete pouring to incorporate more CO 2 and to increase concrete strength. When concrete structures are demolished, the concrete can be broken into aggregates which can be recycled into new concrete, reducing concrete's energy consumption. Concrete wastes also absorb carbon dioxide when left exposed to air, but only 1% of concrete wastes are left exposed long enough to absorb significant amounts of CO 2 . 146 Reduction of CO 2 emissions can be obtained by improving the efficiency of heating or using renewable energy sources for cement production, by capturing CO 2 liberated in cement manufacture, by carbonating concrete during installation, and by allowing concrete wastes to remain exposed to air during demolition. In addition, the recycling of concrete to form aggregate may reduce the amount of cement needed for new construction.
While concrete with no net CO 2 emissions is possible using these advances, it requires most of the concrete service lifetime to reach carbon neutrality and requires additional exposure of concrete wastes to air to absorb CO 2 . 146 Carbonation (addition of additional CO 2 to concrete while setting) is unlikely to be used unless it increases concrete strength. 147,148 The use of concrete containing recycled aggregates may require modified processes to install and may require more expensive reinforcing materials or equipment, 149 although it could reduce CO 2 emissions by up to 50% over new concrete manufacture. Reducing the carbon footprint of concrete to zero, however, is likely to require replacement of concrete with other less carbon-intensive materials.
Mineral Carbonation. Mineral carbonation is the sequestration of carbon dioxide by forming stable metal carbonate salts such as calcium and magnesium carbonates. 150 Sequestration can be performed either below ground (in situ) or above ground (ex situ) using excavated minerals or metal salts. Natural minerals containing calcium or magnesium oxides or silicates such as wollastonite, olivine, and serpentine will absorb carbon dioxide to form carbonates, as will ammonia or  other bases. Cheap wastes like steel slag, mining wastes such as asbestos and nickel tailings, red mud from alumina manufacturing, waste ash from sources such as incinerators, and alkaline paper mill waste can also absorb CO 2 but may require careful handling to prevent environmental contamination.
Mineral carbonation can be performed directly (by treatment of the dry or slurried minerals with carbon dioxide) or indirectly (by conversion of the minerals to metal oxides or hydroxides followed by carbonation). Direct carbonation in the solid phase is limited by mass transport and is generally slow unless high-surface-area absorbents are used. Carbonation of minerals in aqueous solution is fast, but the dissolution of minerals in water is slow. The solubility of minerals in water is improved with acids, with hydrochloric or acetic acid being the most common acids used, 150 but both acids are corrosive and difficult or impossible to recover, increasing the costs of their use further.
Mineral carbonation in some cases yields valuable materials. Precipitated calcium carbonate (PCC), for example, has been sold for $320/ton, while ultrapure calcium carbonate obtained from carbonation can yield revenue of >$9000/ton. 150,151 The use of mineral-carbonation-derived carbonates, however, would only sequester a small fraction of human CO 2 emissions.
Ex situ mineral carbonation is likely to be an economical way to sequester carbon dioxide if waste products (such as concrete wastes or ash) are used as sources for metal carbonates. Most indirect methods result in uneconomical carbonation. If temporary sequestration is desired, the processes can be made profitable by selling the carbonates (particularly pure and ultrapure CaCO 3 ), but the market for carbonates is much smaller than the scale that would be needed to capture a significant fraction of human carbon dioxide emissions. In situ methods are likely to be permanent methods for CO 2 sequestration, requiring minimal monitoring, and are economical, but sequestrated carbon dioxide is difficult or impossible to reintegrate into the carbon cycle.
Technologies for geological and carbonate-forming methods of CO 2 mitigation are likely more mature than those of other chemical methods, and their costs and benefits are better known. Of the keywords searched, the largest number of documents discussed carbonation and mineralization ( Figure  23). References to carbonation are high but stabilize after 2011, while publications involving concrete for CO 2 sequestration follow a different pattern. The number of articles on concrete is significantly smaller (though some concrete articles may be included in documents discussing carbonation). The lower level of interest in CaCO 3 than in other products of CO 2 reduction may be evidence that in situ mineralization has attracted more interest than ex situ mineralization.
The advantages and disadvantages of concrete carbonation, as well as examples of their applications, are listed in Table 5.
Geological Sequestration of Carbon Dioxide. Carbon geosequestration relates to the process of injecting captured carbon dioxide in deep porous geologic formations for longterm storage. Captured CO 2 is compressed to elevated pressures, converted into a supercritical fluid, and then transported mostly by pipelines to the injection site. 152 Any method used for geological storage of CO 2 should be able to store it for a minimum of 1000 years with a leakage rate of less than 0.1% per year.
Most estimates suggest that sufficient capacity exists to store many thousands of gigatons (Gt) of CO 2 with only a small risk of surface leakage in the following 10000 years. However, the level of uncertainty of these estimates depends on the formation (type and heterogeneity), the physical and chemical processes accompanying CO 2 storage, the method being used to determine the storage capacity, and the amount of available data. 153,154 Several assessments of regional storage capacity were conducted in Europe, China, Japan, Canada, and the United States, yet making direct comparisons of their results poses a problem due to their different underlying assumptions. A method to better assess the CO 2 storage capacity worldwide using globally available data sets was developed at MIT as part of a larger project to use Integrated Assessment Models (IAMs). 155 Their Economic Prediction and Policy Analysis (EPPA) model estimated between 8000 and 55000 gigatons of accessible geologic storage capacity for carbon dioxide using current storage technology and that storage capacity is not a limiting factor for carbon dioxide sequestration technology in most regions even if stringent emissions reductions are required.
The multiple requirements for site selection and successful long-term CO 2 storage include (1) large capacity for storage of the site, (2) high porosity and permeability in the reservoir, (3) sealing caprock, (4) no fault planes near the site of injection and low seismicity, (5) deeper than 800 m (about 2600 ft) so CO 2 remains supercritical, (6) wellbore construction must withstand long-term storage without compromising caprock sealing capacity, 156 (7) easily accessible and monitored site, and (8) subhydrostatic pore pressure. 157 Other considerations include distance from CO 2 sources, population density and local public acceptance, reliability of the storage operation, legal accessibility, and the deployment model used. 158 Therefore, site options for geological sequestration of CO 2 include saline aquifers (porous reservoirs that contain saltwater), 159 unmineable coal sites, shales and underground depleted oil and gas reservoirs, 160−162 declining oil and gas fields, 163,164 deep ocean waters, ocean floor or sediments, 165−167 and basalts or reactive rock formations. 168 CO 2 sequestration via solid gas hydrates (clathrates), including storage in deep oceanic basins, sediments under the sea floor, permafrost regions, methane hydrate reservoirs, and depleted oil and gas fields partially saturated with water has received increased attention in the past years due to its potential storage capacity in the hundreds of thousands of Gt. 167 Deep saline aquifers are one of the best candidates for CO 2 storage because they are widespread, have large storage capacity and ideal geologic properties, cannot be used for human consumption or agriculture, and are isolated from the environment. However, this process involves complicated reactions among CO 2 , brine solution, and rock formations, which could potentially affect the integrity and storage efficiency of the well over the long term. 153,170 The efficiency of trapping mechanisms and the movement of CO 2 through the rock are strongly influenced by the CO 2 −brine−rock wettability, the pressure and temperature, salinity, and dissolved ions. 171 These trapping processes take place at different rates and over many years, even thousands of years.
Several million tons of CO 2 were injected in saline formations at several successful sites without issue: the Sleipner and Snohvit projects in the North Sea, 172 the Quest project in Canada using the Basal Cambrian Sands, 173 and the Mt. Simon sandstone in Illinois. However, CO 2 injection was stopped at one site in In Salah, Algeria, due to caprock fracture. These projects indicate that CO 2 storage can be safely accomplished if site selection, injection and postinjection operations, and monitoring of the formation are rigorously evaluated and implemented.
Besides these storage sites, CO 2 has been used extensively in the past 40 years in enhanced oil recovery (EOR) operations. 174 Typically, oil recovery increases by 10−15% with EOR due to the solubility of CO 2 in oil. Up to two-thirds of the injected CO 2 returns with the extracted oil and is usually reinjected into the reservoir to minimize operating costs and trap more CO 2 in the oil reservoir. The major drawback of CO 2 storage using EOR is that today's processes use naturally occurring CO 2 (i.e., CO 2 that was previously underground) due to its lower costs compared to CO 2 from anthropogenic sources. Also, EOR projects are driven by the economics of oil production and not by CO 2 storage and do not take advantage of the full potential of the oil field to store additional CO 2 once no additional oil can be extracted.
Similar to EOR, injection of CO 2 in tight gas sands, shales, and coal seams is used to recover gas by displacement in a process called enhanced gas recovery (EGR). 175,176 CO 2 storage in coalbeds is quite different from storage in oil and gas fields or saline formations because the trapping mechanism is by adsorption as opposed to storage in rock pore space. Here, CO 2 is preferentially adsorbed onto the coal micropore surface, displacing the existing methane. 177 The use of former fossil-fuel reservoirs for geosequestration of CO 2 is attractive for many reasons. Rock reservoirs have sufficient porosity and permeability to promote massive CO 2 volume injections, while oil and gas fields have a geological barrier preventing upward migration and leakage of CO 2 into shallower formations (proven by having stored hydrocarbons for thousands to millions of years without appreciable leakage). Meanwhile, the existing infrastructure and industrial setup required for fluid injection can be utilized while the reservoirs have already been geologically characterized, tested, and monitored. Moreover, revenues from the produced gas/oil can be used to help offset the current high costs of CO 2 sequestration. However, sequestering CO 2 while extracting oil or gas is probably not the best way to mitigate the environmental impact of CO 2 emissions.
Selected terms "aquifer", "saline", "brine", "geological", "shale", "seam", "caprock", "underground storage", "deep sea storage", "seismic", and "clathrate" related to geological storage  Figure 24). Individual search terms showed similar trends except for "shale" and "clathrate", whih displayed an upward movement in the past 4 years, albeit in fewer numbers (data not shown). Research on CO 2 storage in the form of clathrates is still in development, with limited field data available, but remains promising due to its potential for high volumes of CO 2 to be sequestered. While the "geological" term was used most frequently in publications, as expected, the "aquifer", "saline", and "brine" terms returned more publications than the rest of the search terms, reflecting more interest in this specific storage site ( Figure 25). This trend is replicated in a network diagram showing the top 1000 co-occurring concepts within documents with the term "aquifer" appearing as the top geological term in comparison with the rest of the geological search terms that we used (data not shown). In the same network diagram, the simulation and modeling concept indicated a strong affiliation with geological processes, as expected, since numerical programs are used for assessment of the storage capacity of the formation as well as for estimation of geological CO 2 storage security for leakage risks. Main Issues and Potential Leakage Pathways. Once injected in a well, CO 2 plumes will rise via buoyant forces, due to lower density than its surroundings. CO 2 then spreads laterally upon encountering caprock until it finds a gap. Fault planes or fracture networks near the injection zone increase the possibility of gaps, which would be potentially dangerous to life in the surrounding area. CO 2 can potentially migrate into shallow groundwater aquifers and compromise water quality by releasing trace metals such as Sr, Zn, Co, and Ba and organic compounds and/or change the water's pH. 178,179 Deep coal seams that are not economically viable sources for coal mining are generally used for sequestration of CO 2 . Despite the many advantages of these sites, the injected CO 2 may chemically and physically alter the coal matrix and induce its swelling and mobilization of polycyclic aromatic hydrocarbons (PAHs) in the coal seam. 180 This mobilization of PAHs may cause environmental issues, as PAHs are harmful even at relatively low concentrations.
Induced seismicity is also a cause for concern, but it is not expected to be a significant problem at geological CO 2 storage sites if good engineering practices are followed. 181 The measures generally taken to alleviate such effects consist of fluid pressure management. For example, the injection of CO 2 is often conducted simultaneously with the coextraction of formation brine in saline aquifers or extraction of oil/gas at EOR/EGR sites, which can control the amplitude of the overpressure in the reservoir and along faults. 182 CO 2 pipelines pose a risk to local population and the environment, as the presence of water and other impurities within CO 2 may lead to operational problems related to corrosion, gas hydrate, and ice formation and thus accidental release of CO 2 . The exact levels of impurities will vary depending on the source and capture process. Therefore, apart from dehydration, gas treatment is required. The level of impurities that can be tolerated will depend on the storage method (or end use) and the transportation method. Other challenges to CO 2 transport through pipelines consist of pipeline design and maintaining the CO 2 in a supercritical phase. 152 Regulation of CO 2 Injection and Environmental Monitoring. Estimation and quantitative predictions of geological CO 2 storage security suggest geological storage is a secure, resilient, and feasible option for reducing global climate change even when applying worst-case values for each scenario. 183 CO 2 becomes safer and more secure the longer it stays in the ground due to a range of physical processes, with mineralization being the ultimate goal as trapping of CO 2 becomes permanent.
Best practices include monitoring of the injection process and deploying surface and subsurface sensing technologies to allow for risk assessment and mitigation of potential release of CO 2 from wellbores, faults, and other migration pathways, including CO 2 leakage from pressurized pipelines during transport. 153,184 Monitoring allows leak detection with enough warning to minimize the amount lost, and to quantify the leak size. Simulations are also used in predicting the pressure buildup in the formation, fluid flow, and geomechanical and geochemical processes at the injection site. Research focused on improving the fundamental understanding and modeling of various aspects of geological storage and monitoring of CO 2 has been carried out over the past decade. 185 The Safe Drinking Water Act (SDWA) requires the EPA to regulate underground injection activities to prevent contamination of underground sources of drinking water (USDW). EPA has issued regulations for six classes of underground  . Global publication trends for journal publications that contain "aquifer", "saline"/"brine", and "geology"/"geological" search terms.
injection wells. Class II wells are used to inject fluids related to oil and gas production, including injection of CO 2 for EOR. Class VI wells are used to inject CO 2 for geological storage. 186,187 To protect potable water, EPA requires that carbon storage project owners applying for permits define an Area of Review (AoR) in which all risks to underground sources of drinking water and the leakage potential of legacy wells located within the AoR be identified. The AoR is an estimate of the project footprint and is used to develop monitoring plans to ensure protection of USDWs. 188 Either the area of review is assigned a fixed radius (depending on the well type) or it is defined using computational modeling as the edge of the pressure front, whichever is larger. A suggested possibility to reduce the uncertainty of long-term storage of CO 2 and to decrease the impact of wells on the migrating CO 2 plume is to inject CO 2 below the maximum penetration of most wells.

■ CONCLUSIONS
The past two decades have seen dramatic growth in research and application of CO 2 capture methods and subsequent chemical, biological, and geological sequestration. Absorption using amine solutions is the most mature CO 2 capture method and the only one in large-scale applications, whereas persistent research interest in absorption and membrane filtration is evident despite challenges in industrial applications. Postcombustion has drawn by far the most research interest owing to its lower cost and relative ease to retrofit existing plants, but it only favors absorption capture methods. Precombustion methods, on the other hand, can accommodate any of the capture methods because of the easier separation of CO 2 from their gas streams and their flexibilities. Although overall publication volumes related to CO 2 capture largely stopped growing since the mid-2010s, the trends are not universal for all specific fields, and continuous publication growth can still be observed for some methods and materials.
Carbon Capture and Storage technologies are attractive to industries such as fossil-fuel extraction and cement, steel, and fertilizer production, as they can continue to function, and CCS receives greater attention because of the ability to allow business as usual. However, CCS is seen as controversial by some environmental groups, as this technology seems to perpetuate fossil-fuel exploration and risks delaying decarbonization efforts.
The use of biomass via BECCS to capture carbon is likely a rapidly deployable and effective method to sequester CO 2 at low cost without major alterations in land use. Enzymes, particularly RubisCO and carbonic anhydrase, provide an intermediate strategy for CO 2 capture and an alternative to physical and chemical capture methods. Of the chemical methods, mineral carbonation (likely ex situ) may provide the most expedient method to capture CO 2 emissions, while concrete carbonation may be useful if it improves concrete strength and reduces overall concrete use.
Injection of large quantities of CO 2 into underground reservoirs where it can be securely and permanently stored can be successfully achieved with economic incentives to accelerate field-scale applications of CO 2 sequestration. Significant advances in site characterization, monitoring, and leak assessment and management have occurred in the past 10 years. Legal and regulatory protocols have also been put into place in the US. Over time, the leakage risk decreases while the permanence of the storage increases, but the effectiveness of a site to securely store CO 2 at a geological time scale is very difficult to define. Moreover, uncertainties persist over the liabilities of parties after the site is closed. Nevertheless, scaling up and worldwide deployment and coordination of these technologies and strategies should be the focus for upcoming years. Since high-purity CO 2 streams are required for storage, future research will also have to address ways to reduce the cost of CO 2 capture and sequestration processes to be costcompetitive with other carbon-free options. ■ ASSOCIATED CONTENT
Advantages and disadvantages of common CO 2 adsorbents, comparison of different types of membranes for CO 2