Microbial Fuel Cells and Microbial Electrolyzers

Abstract

Microbial fuel cells are electrochemical devices that use microbes as catalysts instead of inorganic catalysts to drive the anodic and/or cathodic reactions to produce electricity. 1 The field of microbial fuel cells (MFCs) was initially focused on wastewater treatment, but has evolved into a much more diverse field of research called Bioelectrochemical Systems (BES). Microbial electrolyzers or microbial electrolysis cells (MECs) are a different manifestation of BES that generate hydrogen using less than half the voltage or electrical energy needed for conventional water electrolysis. The reduced electrical input is enabled by the chemical energy that comes from organic or reduced inorganic substrates that serve as the feedstock for hydrogen production in MECs. Electrons are extracted from the substrates and converted into hydrogen at the cathode, operating at room temperature. The cathode catalysts can be metal-based or biological in nature. Figure 1 shows a generalized configuration of the BES. The versatility of the BES platform shown in the figure illustrates the potential of this approach to produce not only electricity and hydrogen but also biofuels, chemicals and bioproducts. Electrocatalysis-Biocatalysis SynergyFuel cells have set a high mark for energy efficiency among the various renewable energy production options that have evolved over the last few years. The high efficiency comes from the molecular nature of electrocatalysis. In comparison to thermocatalysis, where vibrational energy is used to increase the rate of reaction, electrocatalysis has a significant advantage, because voltage is the driving force for reactions in addition to temperature, which minimizes the energy losses to the environment, thereby improving the conversion efficiency. Biocatalysis is another low temperature catalytic process that works at the molecular level by overcoming the energy of activation via structural re-combination of reactants that self-assemble into catalytic sites. Combining electrocatalysis with biocatalysis in the bioelectrochemical approach employed in BES results in a significant synergy, improving efficiency significantly. This is because of the continuity of the electron flow from the substrate to the product, afforded first, via the biocatalytic reaction, and then by the electrocatalytic reaction, resulting in a continuous path within the electrical circuit. This mechanism is supported by the relatively recent discovery of electrical communication between biological systems and electrodes. Biological nanowires have been identified to enable efficient electron conductance between inorganic substrates and organic or biological entities. Pili-based nanowires have been identified in Geobacter sulfurreducens and Shewanella oneidensis, which are capable of electron transfer from microbes to an anode. 2,3Direct interfacing of electron producers and electron sinks has given rise to this high degree of efficiency evidenced in microbial fuel cells. Similarly, MECs also have high efficiency, shown by the generati...

Unravelling biocomplexity of electroactive biofilms for producing hydrogen from biomass

“…Strategies such as dark fermentation have made progress but experience low yields and carbon losses to side products and can struggle with more complex streams, while photofermentation poses operational and design challenges (Singh et al ., ). Bioelectrochemical systems offer a novel way to solve these problems by recruiting biocatalysis and electrocatalysis for efficient conversion of complex biomass resources (Borole, , , ).…”