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| Fig. 1: Schematic of a microbial fuel cell. Microbes digest fuel to facilitate oxidation at the anode in order to produce a current. (Source: Wikimedia Commons) |
Fuel cells are commonly thought of as both a replacement for traditional batteries or storage of electrical energy and as an environmentally sustainable alternative to fossil fuels for electricity generation. Hydrogen fuel cells are the most popular candidate for fuel cell power generation. As a fuel source candidate, hydrogen is appealing for its high energy content relative to its weight, availability, and absence of carbon emission. [1] Methane-based fuel cells are an alternative to Hydrogen fuel cells that utilize hydrocarbons to generate electricity. [2] Methane fuel cells are widely viewed as a transitional fuel source, best used to help transition to a hydrogen fuel cell based economy. [3] These fuel cells are appealing due to the ready availability of hydrocarbons, both as anthropogenic pollutants from sources such as landfills, and as byproducts of fossil fuel power generation. [4,5] Re-use of these hydrocarbons can help offset carbon emissions from anthropogenic sources as well as the potential to generate electricity with relatively low carbon emission compared to the burning of fossil fuels. [2]
Natural gas in particular is seen as a promising source of fuel for methane fuel cells. Extensive infrastructure to extract, transport, and store this fuel already exists due to existing extensive use of natural gas as a fuel source. [2] Methane fuel cells provide a potential improved means of extracting power from natural gas using existing technology, both through reduced carbon emissions and potential increased electricity yield per unit fuel. [2] Most electricity produced using methane in the modern world involves burning the methane. [3] Combustion of methane gas produces heat, expanding air around the combusting methane. The most efficient power plants on the power grid in the United States are combined-cycle power plants, with efficiencies ranging from 45% to 57%. [6] Much of the inefficiency of plants such as these comes from the intermediary stages between existing chemical potential in methane and electricity, such as the conversion from heat to air expansion, which tends to lose energy to heat loss to the surrounding system. [3] Conversion of methane into electricity using fuel cells requires fewer intermediary steps, which could provide increased efficiency of conversion. [7]
Here, I will focus on two proposed methods for extracting electricity from methane using fuel cells, chemical catalysis and microbial conversion.
Methane fuel cells based on chemical catalysis utilize catalysts such as solid ceramic catalysts or excised nanoparticles. [7] Direct oxidation of methane at operating temperatures below 500°C is generally slow. [2] Catalysts are necessary to speed up the oxidation of carbon within the fuel cells. One of the more promising developments in recent methane fuel cell research has come in the form of integrating a bimetallic anode with a nanofiber cathode, along with an efficient reforming catalyst. [2] This method allows for a high-performance fuel cell to operate around 500°C, with peak power density of 3.7 W/m2. [2] The nanostructured cathode and anode allow for high throughput of oxygen reduction, which, combined with the ability of the catalyst to encourage steam reforming, produce a fuel cell promising lower cost and improved stability in comparison to other solid-state methane fuel cells. [2,7,8] However, the low power density and still-high operating temperature still render this fuel cell a poor candidate for widespread commercial adoption.
Microbial methane fuel cells utilize microbes to digest methane to generate electric current. These cells generally utilize methane-digesting microbes as catalysts to reduce methane as part of traditional fuel cell setups (see Fig. 1). [9] Use of microbes to directly digest methane into electricity has historically been limited due to the difficulty of culturing microbes used to catalyze these reactions. [10] Direct conversion of methane to electricity through reverse methanogenesis has shown recent promise with fuel cells able to reach a peak power density of 164 mW/m2. [11] A more effective means of biocatalysis of methane into electricity may come from a dual-stage process in which methane is first converted into an intermediary fuel, which is then digested by microbes to produce methane. [12] Digestion of this intermediary fuel rather than direct digestion of methane can allow for the use of more easily cultured bacteria, as well as easier transportation of the intermediary fuel, as methane is notoriously difficult to transport losslessly. [10] Two-stage systems can also take advantage of fuels that can be digested at higher densities than methane, leading to higher overall power densities within fuel cells. Study on such two-stage methane conversion fuel cells has demonstrated a peak power density of 426 mW/m2. [10] Even with this relatively high peak power density compared to single-stage conversion through either biocatalysis or chemical catalysis, these dual stage anodes are not panacea solutions for the issues of methane fuel cells. While they can use bacteria and fuels that are easier to scale than direct conversion, large-scale synthesis of both electrodes and cathodes needed for these cells, as well as long-term operation present lingering issues. [10]
Methane fuel cells, both chemically and biologically catalyzed present an opportunity for efficient conversion of methane into electricity. These fuel cells can potentially eliminate inefficiency through more direct conversion of methane into power, and produce fewer greenhouse gas emissions per unit of electricity produced than most traditional power plants. While these fuel cells are appealing due to their potential advantages over traditional means of power production, chemically and biologically catalyzed fuel cells are still ripe for further improvement, with order of magnitude improvements in performance potentially attainable. [11]
© Emily Thierstein. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] M. K. Singla, P. Nijhawan, and A. S. Oberoi, "Hydrogen Fuel and Fuel Cell Technology For Cleaner Future: A Review," Environ. Sci. Pollut. Res. 28, 15607 (2021).
[2] Y. Chen et al., "A Robust Fuel Cell Operated on Nearly Dry Methane at 500°C Enabled By Synergistic Thermal Catalysis and Electrocatalysis," Nat. Energy 3, 1042 (2018).
[4] M. F. M. Abushammala et al., "Regional Landfills Methane Emission Inventory in Malaysia," Waste Manage. Res. 29, 863 (2011).
[5] S. A. Saadabadi et al., "A Solid Oxide Fuel Cell Fuelled By Methane Recovered From Groundwater," J. Clean. Prod. 91, 125877 (2021).
[7] K. Hong et al., "Direct Methane Protonic Ceramic Fuel Cells With Self-Assembled Ni-Rh Bimetallic Catalyst," Nat. Commun. 14, 7485 (2023).
[3] R. F. B. de Souza et al., "Partial Methane Oxidation in Fuel Cell-Type Reactors for Co-Generation of Energy and Chemicals: A Short Review," Catalysts 12, 217 (2022).
[6] K. Storm, Industrial Construction Estimating Manual (Gulf Professional Publishing, 2020), pp 95-159.
[8] Z. Lin et al., "Catalyst Layer Supported Solid Oxide Fuel Cells Running on Methane," J. Power Sources 507, 230317 (2021).
[9] Z. J. Ren, "Microbial Fuel Cells: Running on Gas," Nat. Energy 2, 17093 (2017).
[10] S. Kondaveeti et al., "Methane as a Substrate for Energy Generation Using Microbial Fuel Cells," Indian J. Microbiol. 59, 121 (2019).
[11] M. J. McAnulty et al., "Electricity From Methane By Reversing Methanogenesis," Nat. Commun. 8, 15419 (2017).
[12] J. Myung, P. E. Saikaly, and B. E. Logan, "A Two-Staged System to Generate Electricity in Microbial Fuel Cells Using Methane," Chem. Eng. J. 352, 262 (2018).
