The Effectiveness of Membrane Materials in Green Alternative Energy and Environmental Technologies

2.1.

Polymer electrolyte membrane fuel cell (PEMFC)

PEMFC converts the fuel chemical (such as hydrogen) energy into electricity [24]. Polymer PEMFC is a power conversion device for energy. Because of several attractive properties such as high effectiveness and close to nil emissions, it was considered to be one of the most promising energy generation technologies for transport, stationary and portable applications [25]. A negatively charged electrode (anode) and a positively charged cathode are separated by a polymer electrolyte in the form of a membrane in the PEMFC. This solution has the advantages of high current density, low operating temperature, quick start-up time, low weight, and dimensions. The anode receives hydrogen, whereas the cathode receives oxygen. The membrane is only pierced by protons from the anode to the cathode, whereas electrons travel from the anode to the cathode in the external circuit, bypassing the membrane. As previously stated, the input substrates are 2H₂ (hydrogen) and O₂ (oxygen), and the end product is 2H₂O (water). However, crucial issues such as high cost and low durability continue to stymie PEMFC’s widespread commercialization. Thus, the production of advanced materials such as high electroactive catalysts and alternative polymer exchange membranes will dramatically boost the cell efficiency of PEMFCs at a fraction of the cost of traditional materials (Pt/C and perfluorinated membranes), facilitating their commercialization [26].

Planar PEMFC is equivalent to a fuel cell class with minimized volume and weight, which reduces physical burden while increasing the density [27]. Planar PEMFC usually has open-air cathodes to permit a passive, self-respiring operation with no air/O₂ fan. The equivalent circuits (EC) approach is the most frequently used model of PEMFC impedance-based diagnostic tools [28]. Simplicity and versatility, to accommodate a large variety of operational conditions and low calculation costs, make up the chief advantage of this model. In the last few years, several onboard monitoring techniques based on EC models have been proposed.

2.2.

The fuel cell catalysts

The catalyst for hydrogen oxidation at the elevator and oxygen returns at the cathode is an essential component for the efficient operation of the fuel cell [29]. The catalyst in PEFC fuel cells consists of a layer of platinum nanoparticles distributed in a carbon matrix with a large surface area. The colloidal dispersion of platinum nanoparticles aided by carbon and dissolved ionic polymer particles of the membrane causes them to arrange themselves (i.e., the particles) into a compound of different phases during manufacture, enabling electrons, protons, and gases to pass through [30]. The electric current and the movement of reactants in the cell cause the return and oxidation reactions to move away from electrochemical equilibrium, resulting in irregular spatial distributions. Such shifts lead to a loss of voltage in the cell and reduce cell efficiency by between 30% and 40% [31]. Therefore, the goal is to find highly effective catalysts and create layered structures that mitigate these losses. The location and attachment of the porous substrate or nanostructure on the catalyst is critical since it determines the stability of the bonds between the catalyst and its substrate and the stability of the catalyst layer structure and carrier properties [32]. Theoretical estimates suggest that the yield or efficiency of catalysts in current fuel cells is less than 10%. It is noteworthy that Mark Debe and his co-workers have developed catalytic layers that take the form of a thin film with a nano structure and have higher efficiency and better durability than the current designs [33]. We expect to increase return rates by ten to a hundred times if we consider the number of these examples in the engineering of stimulation at the nanoscale level and in designing their layers. Although platinum is chosen as a catalyst in most types of fuel cells due to its stability and activity, a less costly alternative should be sought. As a result, palladium took its position. Reduced particle size is one method for increasing the active surface areas of catalysts. Many studies have employed nanotechnology to create porous nanocatalysts that eliminate the need for a bulk catalyst and a gas diffusion layer. Consequently, PEFC fuel cells need as little as ten to 100 times less platinum, which reduces the cost of fuel cell stacks by 25% while maintaining the same amount of output power. Incorporated Pt catalysts, also known as non-Pt catalysts, reduces the cost of Pt [34]. It remains crucial to maintain or improve catalyst activity and durability when Pt loading is reduced or eliminated. With the current modern technology, the most significant PEMFC-based electrocatalysts are still nanoparticles distributed on carbon supports. However, these catalysts suffer from performance degradation during practical operation because of the high voltage, acidic and oxidative environment in PEMFC [34]. Although other modes such as decomposition, dissolution and toxicity of Pt particles have contributed to failure, the corrosion of the carbon support materials has been identified as the imperative cause of the weakness of the catalyst. In the case of the cathode catalyst, oxidation of the carbon support may occur in the presence of oxygen, resulting in the separation of the Pt molecules and, as a result, a decrease in fuel cell efficiency. For the anode catalyst, the carbon supportive can also be oxidized in an acute deficiency of hydrogen [35]. Because of these degradation processes, the stability of the Pt catalyst was below the lowest durability target of 5,000 h for automotive applications, based on the PEMFC compound test monitored by the United States Department of Energy [36]. Because of the marketing power of fuel cells, the need for materials that are both intrinsically stable and easy to replace existing carbon stents becomes urgent not only to extend the serving life but also to enhance the reliability and reduce the total lifetime cost of the PEMFC. Significant efforts have been made to explore stable alternatives to replace the carbon materials (Vulcan XC-72R and Ketjen) currently used with fuel cell catalysts [37]. These are based on some basic requirements, including high surface area, preferred dispersion of catalytic metals, high oxidation resistance, high electrochemical stability under fuel cell operating conditions, as well as high electrical conductivity. The challenges are that the support materials hardly meet these requirements at the same time. While graphene has been studied only recently, carbon materials with one-dimensional nanostructures (CNTs and carbon nanofibers-CNFs) have long received attention as catalytic support because of their unique structure and properties [38]. It was concluded that structural defects play an important role in improving catalytic activity. However, the erosion of carbon materials always begins at detecting sites. The enhanced durability of Pt/CNTs was observed for CNTs with a good graphite structure and is attributed to the strong resistance of CNTs to corrosion and the specific interaction between Pt nanoparticles and CNTs (the positional electrons of CNTs and Pt d-electron particles) [39].

The highest methanol oxidation mass activity reached 323 A/g with a cycling rate of 50 mV/s for Pt/GCNFs/graphite electrode in 2 mol/l CH₃OH +1 mol/l H₂SO₄ solutions. The Pt/GCNFs/graphite electrodes had high oxidation activity (129.5 A/g) and good stability even at low Pt loading (8.79 μg/cm²). Catalytic activity loss after 500 cycles can be due either to methanol consumption, toxicity, and a structural change in platinum nanoparticles or disturbances in the potential of the aqueous solutions [40]. The highest current density is 55.4 mA/cm² higher than long-term cycling ends, and 91% of the value of maximum peak current (60.94 mA/m²) after the Pt/GCNTs/ graphite electrode has been stored in a twin distilling water during 24 h [41]. The distribution and morphology of nanostructures of noble metal on advanced materials help in achieving high-performance catalysts effectively. The promising catalyst support materials should fulfill the following characteristics (1) high specific surface area and efficient interaction with metal nanoparticles (NPs) to achieve high and stable dispersion (2) excellent stability under harsh electrochemical conditions to keep the structural integrity of the composites; (3) high electrical conductivity to promote electron transfer during the electrode reaction process [42]. As has been demonstrated repeatedly in experiments, supporting materials in the catalytic system can significantly enhance electrocatalytic activity and reduce noble metal consumption [43]. Given the high graphitization of the inherent carbon material surface, it is hard to achieve direct deposition of highly dispersed NPs with high efficiency of loading on such surfaces [44]. The surface functionality of carbon materials usually is indispensable to activate the inert surface (producing more anchorage sites for NPs). Two pathways exist to change the carbon surface: (1) the covalent fastening by chemicals in reactions to the 𝛼-combined carbon skeleton, and (2) non-covalent interactions or packaging of different organic functional fragments. Supported Pt and Pt alloy catalysts showed excellent performance in electrode reactions, because of the high and uniform dispersion of the catalytic metals and improved mass transfer. To improve electrical conductivity and stability, mesoporous carbon should be treated by high-temperature annealing and graphite catalytic [45]. However, these treatment strategies must be implemented at the expense of specific porosity and surface area, which makes it difficult to deposit metal nanoparticles. To address the issue of carbon oxidation, a major effort has been made in recent years to develop non-carbon materials to support catalysts, including nitrites, carbides, mesophoric silicates, conductive polymers, and metal oxides [46]. Although there has been some progress, there are still significant disadvantages compared with carbon materials such as low electronic conductivity, high acidic solubility, and low surface area. Some non-carbon materials have intrinsic inertness in high temperature and electrochemical corrosive environments, which improves the stability of the supports and the supported catalysts [47].

The durability of the catalyst is mainly determined by the corrosion resistance of the support material. Graphite reinforcement is useful for improving durability. The abuse of nitrogenous doping has been known to improve the stability of carbon materials. It was found that the high nitrogen content would reduce the stability of carbonaceous substances, because of the structural disturbance caused by N in the carbon lattice [48]. With an increase in the nitrogen content, the resistance to oxidation of the Amesbury carbon could decrease because excessive doping of nitrogen may accelerate the conversion of the oxidized carbon oxidation gas. The durability of the catalyst is also determined by the interaction of the support with Pt molecules. It is believed that increasing the degree of graphite support to carbon leads to enhanced sites on carbon, which act as binding centers for Pt nanoparticles. Thus, graphite to a higher degree strengthens the interaction between the PT molecules and the carbon support [49].

2.3.

Layers of gaseous diffusion

The gas diffusion layer serves as an electrical conductor between the carbon-supported catalyst and the current collector plates. Thin gas diffusion layers with the lowest resistance to electricity are preferred [50]. Between the catalyst layer and the two oxygen and hydrogen gas inlets in the fuel cell is a porous gas-carrying layer. This layer allows the reactants to leak into the catalyst layer while allowing electrons to move away from the anode and extract water from the cathode. Otherwise, the flow of reactants at both electrodes would be impeded. So that the gas-carrying layer is with caudate and holes ranging between 10 and 100 μm, while the layer with micro-holes has dimensions of hundreds of nanometers. Improving the performance of fuel cells to perfection is a multi-level issue, from the molecular dynamics of the transport of protons across the membrane and the chemical reactions taking place in the catalyst to the macroscopic properties of the complex media that control gas scattering, water permeability, and charge transport. At the level of the entire fuel cell or stack, these mechanisms interact with each other complexly that may lead to undesirable effects such as the irregular distribution of the reactants, the ineffective use of the catalyst besides the flooding of the cell with water, or the dehydration and depletion of the fuel.

It is known that 2D graphene can function as a proton exchange membrane in fuel cells. Among the many beneficial properties of graphene (sturdy material, electrical conductivity, self-repair ability, flexibility, etc.), it will take billions of years for hydrogen, the smallest molecule, to penetrate the graphene monolayer [51]. This makes it a considerable substance-containing material, especially for the small gas molecules known to escape quickly into the atmosphere. For example, hydrogen cars need to store their fuel inside a container that does not diffuse hydrogen, which is proving challenging.

It has been found that monomers of graphene and boron nitride are very permeable to thermal protons under ambient conditions [52]. This means that thin films can be manufactured, the most reliable ones as graphene are a full-proof gas separator. The hydrogen collected was in small volumes, but these are only the first steps—proof of concept. Imagine a car using hydrogen assembled from the air, plus fuel stored in tanks—it would be exciting.

Great efforts went into the mass production of graphene, for use in many applications, thanks to its extended properties, and if the industry falters in making it in large quantities, we may witness a whole new technological revolution [53].

Between the main macroporous layer and the catalyst layer, a microporous layer is often added. This microporous layer’s function is to help with the distribution of reacting gases, improve mechanical compatibility between layers, minimize contact resistance, and improve water management. Carbon is used for electrical conductivity, and PTFE is used for hydrophobicity in this microporous layer. The commercial Gas diffusion layers (GDLs’) in-plane and through-plane resistivity are between mΩ cm and 6–20 mΩ cm², respectively [54]. The GDL microstructure determines the plane and aircraft’s resistances. The flat resistance determines the cell strength, and the flat resistance affects the reaction uniformity. The GDL’s position in the anode is not the same as its role in the cathode. For instance, cathode water must be removed from the catalyst surface easily for flood prevention (i.e., liquid water formation). The liquid water prevents catalyst sites from entering the catalyst and inhibits oxygen. The anode must retain some water so that the membrane does not dry up and this applies in particular to the dry anode gas stream.

Hydrogen storage nanomaterials such as carbon tubes have been developed as they have a higher storage capacity than ordinary solids (table 1).

Titanium is now used in Liquid/gas diffusion layers (LGDLs) on the PEMEC anode side. It has a thickness of more than 300 m and a substantial electrical conductive route as well as fluidic resistance [55]. Furthermore, their random architectures make controlling the water/electron/thermal distribution difficult, and their intricate pore shape leads to exceptional interfacial contact resistance. For the first time, titanium in thin and well-tunable LGDLs with flat interfacial surfaces are produced and deployed in a PEMEC, demonstrating enhanced multifunctional performance over traditional LGDLs. At a current density of 2.0 A/cm², the operating voltages were as low as 1.69 V, with an efficiency of up to 88% and efficiency by more than 9% [56]. Mo et al. [56] conducted both ex-situ and in-situ characterizations to better understand the mechanisms. The well-tunable LGDL with flat surface characteristics might also be used to increase the performance and efficiency of PEMFCs. Furthermore, reducing the LGDL thickness from 350 m of typical LGDLs to 25 m would significantly reduce the weight and volume of PEMEC stacks, thus opening up new avenues for future development of low-cost PEMECs with improved performance. Its customizable properties, such as pore size, pore shape, pore distribution, and permeability, will be extremely useful for constructing PEMEC models and testing simulations of PEMECs with optimal and repeatable performance [57].

3.1.

Green hydrogen and fuel cells

Climate change is one of the most critical environmental issues that concern governments today, given the economic, social, and health risks this issue entails. Experts agree that vigorous international attempts must be made to limit global warming to prevent the negative impact of climate change on our planet’s environment. Global carbon emissions must be reduced to zero by the year 2050, prompting some governments to take immediate measures to adopt alternative energy means [67]. Green hydrogen is one method that has gained momentum in recent times for its reliance on producing hydrogen from sewage waste [68]. Hydrogen and fuel cells are commonly recognized as one of the most important energy options for the twenty-first century. These technologies will greatly contribute to reduced environmental impact, increased energy security (and variety), and the development of new energy sectors. Transport, distributed heat and power generating, and energy storage systems may all benefit from hydrogen and fuel cells. The shift from a carbon-based (fossil fuel) energy system to a hydrogen-based economy, on the other hand, entails considerable scientific, technological, and economical challenges to the deployment of hydrogen and fuel cells as future clean energy solutions.

Hydrogen can be generated in a variety of ways, including electrolysis, which involves breaking down water into oxygen and hydrogen using an electric current (figure 1). Green or “renewable” hydrogen is created when the energy used in the process comes from a renewable source such as wind [69].

Currently, three significant technological obstacles must be solved to shift from a carbon-based (fossil fuel) energy system to a hydrogen-based one. First, the cost of producing and delivering efficient and sustainable hydrogen must be greatly decreased. Second, new generations of hydrogen storage technologies must be created for both automotive and stationary uses. Finally, the cost of hydrogen-based fuel cells and other devices must be decreased.

Hydrogen is labeled gray, blue, and green, according to the amount of carbon emitted during the industrially prepared hydrogen. Gray hydrogen is hydrogen that is made from fossil sources like natural gas. Hydrogen derived from low-carbon sources is known as blue hydrogen. Green hydrogen is hydrogen that is made from renewable energy sources [70]. Unfortunately, gray hydrogen accounts for about 95% of the hydrogen produced in the world today. The natural gas formation mainly produced gray hydrogen in industrial applications. In a combustion chamber with the presence of a co-factor, natural gas that has been treated with water vapor is used at high temperatures, ranging from 800–1700 °C. This method gives off a lot of carbon dioxide. The fact remains that much of the world’s hydrogen production is linked to natural gas. Blue hydrogen (low carbon dioxide), is more expensive, as most of the carbon dioxide emissions are captured and stored, up to 90%, during gray hydrogen production [71]. The most common method for producing green hydrogen is by the electrolysis of water, in which water molecules are broken down into oxygen and hydrogen. Electrolysis is a proven method, but it is not mature enough for widespread use, and it is relatively expensive. The cost of producing green hydrogen is two to three times higher than that of gray hydrogen produced from natural gas, so it is not competitive [72]. Even though green hydrogen plays a minor role in the overall energy mix, accounting for just 0.1% of global hydrogen output in 2020, costs for generating green hydrogen could drop by as much as 64% by 2040 [73]. According to the degree to which hydrogen is free of carbon, there are three principal forms of hydrogen: (1) electrolysis created Green hydrogen with renewable energy sources, including solar and wind. (2) Blue hydrogen is made by evaporating methane, the principal component of natural gas rock, and removing the carbon during the process. (3) Gray hydrogen is made by reforming methane with steam without removing the fuel. According to the International Energy Agency, grey hydrogen, which is derived from fossil fuel sources, accounts for 95% of the total global supply of hydrogen, and electrolysis produces the rest they together account for around 4% of final energy consumption [74]. In 2019, the total global demand for hydrogen was about 120 million metric tons (60% for pure hydrogen and 40% for hydrogen-based fuels), with an estimated economic value of $135.5 billion [75]. According to an European plan, 6 GW of electrical hydrogen analyzer facilities will be launched in the first phase (2020–2024) to generate up to one million tons of green hydrogen, with the second phase (2025–2030) increasing production capacity by 40 GW to produce up to ten million tons of green hydrogen, and the European Union will spend between 180 and 470 billion euros to encourage the use of green hydrogen until 2050 [76]. By replacing gas and coal with clean electricity, current technology, such as used in the production of solar and wind energy, can reduce carbon dependence in the energy sector by up to 85%. Some other economic sectors, such as shipping and manufacturing, face more difficulty moving into electricity dependence. They usually require a fuel that has a high energy density or the heat it produces at high temperatures. However, green hydrogen (GH) is the future in these sectors. GH is one of the four technologies necessary to achieve the goal of the Paris Climate Agreement, which is to reduce over 10 billion tons of carbon dioxide emissions per year in the industrial sectors that pose the biggest challenge such as the mining, construction and building, and chemical industries [76].

Figure 2 depicts an assessment of future cost trends for green and blue hydrogen based on BNEF data [77]. The graphic shows the expenses in terms of hydrogen mass (on the left axis) as well as energy content, which takes into account hydrogen’s lower heating value (120 MJ/kg, or 33.3 kWh/kg).

3.2.

Microbial fuel cells (MFCs)

By 2050, the world’s population is expected to reach 9.5 billion people. Given that most of our current energy is generated from fossil fuels, providing adequate, sustainable electricity while mitigating climate change is a significant challenge [78]. One idea that has gained momentum in recent years is generating electricity using bacteria in devices called microbial fuel cells (MFCs). These fuel cells rely on the capacity of some naturally occurring microorganisms that can exchange electrons to generate electricity [79]. It can feed this process using materials called substrates to include the organic materials found in the wastewater. MFCs use a system of anodes and cathodes—electrodes that pass a current either in or out. MFC systems comprise an anode chamber and a cathode chamber separated by a membrane. Bacteria grow on the anode and convert the substrates into carbon dioxide, protons, and electrons [80].

The electrons are then moved to the cathode chamber through an external circuit, while the protons move through the membrane. The reaction between protons and electrons in the cathode chamber absorbs oxygen and produces vapor. Electrons can flow out as long as the substrates are shifted, which is what electricity means. There are some advantages in electricity generation from MFCs, systems can be developed in any place; they create a lower sludge than traditional wastewater treatment methods, including active sludge systems [81]. They can be small, but they can be modular in construction for larger systems; they have a high salt tolerance. It is possible to work at room temperature. The availability of a wide range of renewable substrates that can be used to generate electricity in MFCs has the potential to revolutionize future electricity production. These substrates include urine and organic matter in wastewater. Inorganic waste, like sulfides, and even gaseous pollutants, excrete into the soil (root secretions). A renewable substrate that MFCs can use to generate electricity plants root secretions in so-called plant MFCs. When plants grow, they produce carbohydrates such as glucose, some of which are excreted into the root system [82]. Microorganisms near the roots convert carbohydrates into protons, electrons, and carbon dioxide. In the MFC plant, protons are transported through a membrane and recombined with oxygen to complete the electron transfer circuit. By connecting the load to the circuit, the generated electricity can be harnessed. Multifunctional plant cells could revolutionize electricity production in isolated communities without access to the grid. In cities, streets can be lit with trees. Another type of microbial fuel cell is microbial desalination cells. These devices use bacteria to generate electricity, for example, from wastewater, while desalinating the water at the same time [83]. The water to be desalinated is placed in a chamber confined between the anode and cathode chambers of MFCs using negative (anion) and positive (positive) ions membranes. When the bacteria in the anode compartment consume the wastewater, protons are released. These protons cannot pass through the anion membrane, so the negative ions from the saltwater are transferred to the anode chamber. Protons are consumed in the cathode, and thus the positive ions of the salt are moved into the cathode chamber which causes the middle chamber to desalinate [84]. The ions released into the anode and cathode chambers help to improve the efficiency of electricity generation. Anaerobic digestion microorganisms are used to break down degradable materials, or waste without oxygen to recover energy from wastewater by producing biogas, which is made of methane, the principal component of natural gas [85]. Nevertheless, this process is usually ineffective. Research shows that the microbial groups used within these digesters share electrons with the so-called electron transfer between species—which opens the possibility of using positive energy to influence metabolism [86].