Waste-to-Wealth: Wheat-Based Porous Electrodes for Electrochemical Energy Storage Devices

Energy production/storage as one of the world’s leading issues has been widely studied. This significant problem can be solved by using disposable/low-cost biomass materials. Electrochemical energy storage (EES) devices including supercapacitors, rechargeable batteries, and hybrid EES devices have been extensively developed in recent years. The EES devices have been recognized as a proper answer to the energy storage problem in the world. Numerous studies have been accomplished to develop biomass-based and biochar-based EES devices to decrease environmental pollution and production costs. The most important part of the EES devices are electrodes including the cathode and anode. According to recent studies, biochar-based electrodes have considerable electrochemical properties. Wheat is one of the most important parts of the human diet. The wheat wastes have amazing electrochemical properties to be used as a precursor of electrochemical energy storage (EES) electrodes including supercapacitors, batteries, and hybrid EES devices. The benign/low-cost wheat wastes especially wheat straw and wheat husk have been used to fabricate wheat-based biochar materials. The electrochemical properties of the wheat-based biochar electrodes (cathode and anode) in the EES devices have determined that these benign/low-cost EES electrodes reduce production costs and obtain acceptable electrochemical performance and environmentally friendly procedures.

The trouble with energy production and storage has amazingly increased in recent years because of the huge development of societies. The environmental damage in recent decades leads to increase attention to environmental protection studies. These hazardous damages are due to several problems like ozone layer depletion, global warming, climate change, and living organisms' extinction. According to recent studies on environmental pollution, one of the best methods to decrease environmental pollution is by producing energy and chemical substances from renewable resources and benign procedures. Produce and storage of energy are considered crucial subjects in the world. One of the best solutions for environmental protection is to produce and store energy by using disposable biomass materials. Using disposable/low-cost biomass materials, producing and storing renewable energy reduces production costs and reduces environmental pollution. In recent years, energy storage has swiftly developed as a hot topic in engineering and materials science. An appropriate energy storage device can improve energy storage efficiency and reduce storage costs. Therefore, energy storage by biomass materials has been widely examined in recent years to protect the environment and decrease production costs.
Electrochemical energy as a high-efficient and benign type of energy has been swiftly developed in recent decades. 1 Electrochemical energy is low-cost and benign energy with less toxicity and greenhouse gas emissions, improving environmental protection. State-of-the-art studies have provided high-efficient renewable energies. Therefore, renewable resources have gradually replaced nonrenewable hydrocarbon energy sources (like coal, oil, and gas). The electrochemical energy storage devices (EES) including supercapacitors, rechargeable batteries, and hybrid EES devices have been developed appropriately because these EES devices provide several advantages. The combination of biomass conversion technology and electrochemical energy has provided low-cost/high-performance EES devices with several significant advantages, such as benign/low-cost production, highly efficient storage devices, and low environmental pollution.
Batteries are one of the oldest human inventions in materials science. The first generation of batteries was fabricated in the Parthian empire about 200-160 B.C. They used an ancient terracotta pot constituent an iron rod inside a copper cylinder as two battery electrodes in vinegar as an acid-based electrolyte. 2 The batteries have a high power density that plays a significant role in electrical devices. Over the course of 250 years, capacitors have evolved. 3 Also, supercapacitors have a lower energy density and higher power density than batteries. The combination of supercapacitors and batteries is the hybrid EES device that simultaneously contains higher energy/power densities. Hybrid EES devices like ion-capacitors have been considered the next generation of EES devices because of the better energy/power densities. Sodium-ion (Na-ion) and lithium-ion (Li-ion) capacitors are considered the best hybrid EES devices with appropriate electrochemical properties.
The four main parts of the EES devices are electrodes, binder, electrolyte, (anode and cathode), and membrane (separator). Numerous studies have been done to enhance these four parts to acquire better EES devices. The EES electrodes are considered the most significant part of the EES devices. The role of a binder is to consist (bind) of electrodes like a nanocomposite structure, but the electrodes can take this role as binder-free devices. 4 Tremendous efforts have been made to improve the EES electrodes (cathodes and anodes) via inorganic materials, 5 organic materials, inorganicorganic hybrid materials, and renewable/low-cost biomass materials. [6][7][8] All main parts of the EES devices can be fabricated from renewable biomass materials. 9 The electrolyte, binder, and membrane of the EES devices can be prepared from organic compounds through the extraction/conversion of biomass materials (fauna and flora) such as lignin, chitosan, chitin, and cellulose. Also, the biochar materials have been prepared through the pyrolysis (carbonization) of disposable biomass materials (like agricultural/ livestock wastes or food wastes). The biochar should be modified to obtain better EES electrodes with higher electrochemical properties. Graphite's theoretical specific capacity (372 mAh g −1 ) is unacceptable and cannot meet the high-energy-density requirements for advanced electric devices/vehicles. 10 But the modified biochar-based electrodes can achieve higher electrochemical properties. Hence, numerous studies have been done to obtain the EES devices' electrodes, electrolyte, binder, and separator from renewable/lowcost biomass materials. Biomass materials are conveniently found throughout the world. The biomass materials have two significant parameters, including low-cost and benign features. Plants are z E-mail: h.parsimehr1991@gmail.com considered one of the most abundant biomass resources on Earth. The plants have been cultivated/grown throughout the Earth and constitute the main part of humans' diets. One of the most significant parts of plants is the crops. There are seven significant crops, including wheat, rice, maize, barley, rye, millet, and sorghum. These seven crops have been considered the most important portion of humans' diets. These seven crops are widely cultivated throughout the world. These valuable septet crops have been cultivated for several reasons, including producing human food, livestock feed, producing chemical substances, and renewable energy. Producing energy and materials from crops is considered the prominent application of crop cultivation. One of the most abundantly cultivated crops in the world is wheat. Wheat (Triticum) is the main human food source in different countries. According to the Food and Agriculture Organization of the United Nations (FAO), the global production of wheat in 2021 is equal to 778.6 million metric tons. Also, the top-ten wheat producers in the world are China, India, Russia, the United States, Canada, France, Ukraine, Pakistan, Germany, and Australia. The wheat production of these ten countries is illustrated in Fig. 1. The world's production of 778.6 million metric tonnes of wheat in 2021 determines the importance of wheat as well as the massive amount of wheat products and wheat wastes. China, India, Russia, and the USA are the most important wheat producers in the world. These four countries have produced more than 361.4 million metric tonnes of wheat in 2021 (46.4% of all wheat production in the world). Millions of tonnes of wheat waste are produced annually around the world. According to the FAO, China is the biggest wheat producer in the world. China produced more than 131.7 million metric tonnes of wheat in 2021. This huge amount equals 16.9% of all wheat production in the world. The second largest wheat producer is India, which produced 93.5 million metric tonnes of wheat in 2021 (12.0% of all wheat production in the world). The third largest wheat producer in the world in 2021 is Russia, producing 73.3 million metric tonnes which equals 9.4% of all wheat production in the world.
Wheat production has steadily increased around the world ( Fig. 2A) to meet human demand (Fig. 2B). Wheat production will be increased in the coming years to provide one of the essential humans' foodstuffs. Also, battery production, especially for electric vehicles, has swiftly increased in recent years. The development of faster batteries in the next decade ( Fig. 2C) will continue to meet rising demand. According to the US Department of Energy (Fig. 2C), battery demand will increase in the future. 11 Biomass materials like wheat wastes have the appropriate potential to produce chemical substances and green energy. 12 The massive amount of wheat wastes in the world as a benign/low-cost and renewable resource can be used to produce chemicals and energy. Several studies have been accomplished to produce biofuels and chemicals from wheat wastes. 13,14 Wheat is primarily composed of lignocellulosic substances. Also, several organic compounds and biopolymers like polysaccharides have been extracted from wheat and wheat wastes. There are two major methods to prepare chemical compounds from wheat. The first one is extraction via physicochemical methods. 15 Second method is the conversion of wheat waste to chemicals via the fermentation process. 16 Using wheat wastes as renewable resources reduces costs and protects the environment. Pyrolyzed wheat (wheat biochar) has numerous applications. As a result, the number of studies on the wheat biochar applications such as adsorption of hazardous materials 17 and preparation of the electrochemical energy systems 18 has increased  in recent years. Different parts and derivatives of the wheat, including straw, bran, flour, stalk, husk, and starch have been used to prepare wheat-based biochar (Tables I and III). In this review article, all studies that have used wheat-based biochar as EES electrodes have been examined, and the best electrochemical properties have been determined.

EES Electrodes Preparation from Wheat-Biochar
The biomass materials need to be pyrolyzed (increasing the carbon content) to be suitable for EES electrodes. 19,20 Therefore, numerous studies have been done to optimize the biomass carbonization process. The most significant parameters in the carbonization process, including heating rate, heating time, hating temperature, and physicochemical treatments have considerable influences on the electrochemical properties of biochar substances. 8 The first important parameter in biochar materials is the specific surface area (SSA) value. The electrochemical properties of biochar materials are directly related to their SSA contents. Hence, most of the studies have tried to increase the SSA value of biochar materials. 9 There are six carbonization methods for biomass materials, including thermal pyrolysis (TP), microwave pyrolysis (MP), hydrothermal pyrolysis (HP), molten salt pyrolysis (MSP), and catalytic pyrolysis (CP). Also, the last method is a hybrid of the other methods. 8 Also, the physicochemical treatment of biochar (especially with KOH) and post-carbonization of biochar are two useful methods that have been widely developed to improve the electrochemical properties and SSA content of the biochar materials. 9 The surface modification of the materials has a huge influence on the materials. 21 The surface modification of the biochar materials leads to a significant improvement in their electrochemical properties. The pyrolysis atmosphere (PA) and modification method (MM) via different studies have been reported in the tables to determine the optimized process for the biochar preparation. The modification with water (H 2 O) in the tables indicates the hydrothermal carbonization of the biochar. Finding the optimized carbonization method with the best electrochemical properties can lead to future studies on biochar-based EES electrodes for high-performance EES devices. Wheat waste carbonization is comparable to that of other biomass materials. The wheat biochar should be modified to obtain higher electrochemical properties. There is no discernible difference in the carbonization of wheat and other biomass materials. Hence, the electrochemical performance of each biochar materials is more related to the carbonization conditions and physicochemical modifications. But overall, the electrochemical properties of the modified biochar materials are better than graphite, as explained before. Therefore, attention to biochar materials like wheat waste biochar has increased.
Battery electrodes.-The electrochemical properties of the battery electrodes made from wheat-biochar have been reported in Table I. The modification method (MM), pyrolysis atmosphere (PA), and pyrolysis temperature (PT) have been reported in the tables to determine the best pyrolysis parameters. The durability of the EES devices as a significant parameter has been reported in the tables.  The capacitance's durability over cycles is regarded as an important parameter for evaluating the EES device's performance during charge-discharge cycles. Hence, the cycle stability (CS) and cycle stability percent (CSP) of the EES devices are reported in the tables.
To obtain durable EES electrodes, the last cycle's capacitance content should be closer to the first cycle's capacitance content. The cycle stability performance (CSP) is calculated by the following formula.
( ) = × CSP % Final discharge capacity Initial discharge capacity 100 Also, the initial discharge capacity (IDC) of batteries, as an important parameter for rechargeable batteries, has been examined in Table I. The best electrochemical properties of Table I have been  reported in Table II. According to Table II, the modification of biochar (especially with NaOH and KOH) has a considerable influence on the electrochemical properties of the EES electrodes. Also, different parts of wheat have acceptable electrochemical properties in comparison to graphite.
According to Table I, most studies have focused on the anode for Li-ion batteries because they are more common than the other batteries. In addition, the Li-S cathode and Na-ion anode are ranked second and third, respectively. Hence, wheat biochar-based electrodes for the Na-ion anode and Li-S cathode should be further developed in the next studies. The physicochemical modifications of biomass and biochar materials have a great influence on the SSA value and electrochemical properties of the biochar-based EES electrodes. Therefore, the physicochemical modification techniques should be more developed in the next studies to increase the battery's performance. Hence, the wheat waste biochar can be used as high-performance, low-cost, and benign EES electrodes. Most publications have focused on anodes for Li-ion batteries. Therefore, the anodes for other rechargeable batteries like Na-ion and the cathodes, especially for Li-S batteries, should be further developed in the next studies.
Supercapacitor electrodes.-The electrochemical properties of the supercapacitor electrodes made from wheat biochar have been reported in Table III. The specific capacitance (SC), gravimetric energy density (ED), and gravimetric power density (PD) have been reported in Table III. Reporting gravimetric ED and PD is more common than reporting volumetric values. Hence, the gravimetric values have been reported in Table III. Also, PA, PT, MM, CS, and CSP have been explained in the previous section. The wheat wastes can be used for other parts of the supercapacitor, such as the electrolyte from wheat starch. 43 Hence, wheat waste, a low-cost and renewable resource, can be used to fabricate supercapacitors. The best electrochemical properties of Table III have been determined in  Table IV. According to Table IV, chemical modifications of wheat biochar, especially with KOH, have considerably improved the electrochemical properties as supercapacitor electrodes. Hence, different chemical modifications should be studied to find better modification techniques.
The doping technique, as a novel and efficient method to improve the electrochemical properties of the 2D carbon substances, has been swiftly developed in recent years. The nitrogen-doped porous carbon materials used as EES electrodes have considerable electrochemical properties. 74 The functional role of nitrogen in nitrogen-doped carbon materials has been determined using X-ray photoelectron spectroscopy (XPS). The covalent bonds of nitrogen with carbon in the biochar structure can be determined by the XPS spectrum. Also, the amounts of different N species on the biochar surface can be calculated by XPS analysis. Nitrogen content in biochar is an essential parameter of the electrochemical properties of nitrogendoped carbon because the chemical and electrochemical properties have been provided by the N doping process. The chemical reactions at the surface of the nitrogen-doped carbon in the electrochemical testing process have been determined by the following two reactions: 63 The reactive nitrogen species in nitrogen-doped carbon materials lead to significantly increased electrochemical properties of biochar materials. Therefore, chemical modifications that increase the formation of N-doped carbon materials can improve the electrochemical properties of nitrogen-doped biochar materials. 63 Hybrid EES devices electrodes.-The hybrid EES devices, such as Li-ion and Na-ion capacitors, are a combination of rechargeable batteries and supercapacitors. 75 Higher energy and power densities in the hybrid EES devices lead to increase interest in these novel EES devices. The number of publications on the biochar-based electrodes for the hybrid EES devices is much lower that for batteries and supercapacitors. Only one study reported wheat-based biochar as the electrode of the hybrid EES device. 76 The electrochemical properties of the biochar-based hybrid electrode are reported in Table V.

Perspective of Wheat-Based EES Devices
The wheat wastes, especially wheat straw, can be used to prepare chemicals, renewable energy, and energy storage devices. The biochar from wheat wastes can be used to fabricate the EES electrodes. Also, another part of the EES devices, including biocompatible binder, 77 electrolyte, 78 and, electrocatalyst 79 can be prepared by the extraction/conversion of wheat wastes. Therefore, wheat waste, as a low-cost and benign resource with high economic efficiency, can be used to create wealth from waste. This waste-towealth strategy that conformed to a circular economy led to a reduction in environmental pollution. The main issue with the biochar-based electrodes is the electrochemical properties of these low-cost materials. Most of the studies have focused on the modification methods of biochar to find the best ones. Also, several studies have tried to improve the SSA value of biochar materials because of the great importance of the surface chemistry and specific surface area of the application carbon materials. 80 Table VI   modification to increase the SSA value. However, these modifications should be more developed to reach SSA values above 4000 m 2 g −1 .
Also, other advanced applications such as biocompatibility, flexibility, stimuli-responsiveness, stretchability, transparency, and wearability have a dramatically positive influence on the performance and applications of the EES devices. 81 Therefore, the next studies on biochar-based EES devices should combine biochar applications with these significant properties to fabricate highperformance and low-cost EES devices.

Conclusions
According to FAO reports, the global production of wheat in 2021 is equal to 778.6 million metric tons. These huge number indicates the massive amount of wheat wastes in the world. The pyrolysis of wheat wastes leads to production of biochar that can be used to fabricate the EES electrodes for batteries, supercapacitors, and hybrid EES devices. The carbonization parameters and modification methods have a considerable influence on the electrochemical performance of the biochar materials. The acceptable electrochemical properties of wheat biochar as EES electrodes determined this low-cost and benign resource can be used to fabricate electrodes (anode and cathode) for EES devices to reduce the production costs and environmental pollution.