Mechanisms of Charge Transfer during Bio-Cathodic Electro-Synthesis of CO2-Neutral Methane
Sreemoyee Ghosh Ray1 and Makarand M Ghangrekar2*
1PKSinha Center for Bioenergy, Indian Institute of Technology, India
2Department of Civil Engineering, Indian Institute of Technology, India
Submission:May 15, 2017; Published: June 27, 2017
*Correspondence Address: M Ghangrekar, Department of Civil Engineering, Indian Institute of Technology, Kharagpur-721302, India, Tel: +91-3222-283440; Fax: 91-3222-282254; Email: firstname.lastname@example.org
How to cite this article: Sreemoyee G R, Makarand M G. Mechanisms of Charge Transfer during Bio-Cathodic Electro-Synthesis of CO2-Neutral Methane. Adv Biotech & Micro. 2017; 3(5): 555619. DOI: 10.19080/AIBM.2017.03.555625.
Exploiting fossil-fuels to meet the increasing per capita energy consumption is leading to generation of green house gasses. Massive CO2 emission causes an alarming impact on global warming. Sustainable solutions are hence required to be explored to capture and re-utilize CO2 to valuable products. Application of Bio-electrochemical systems (BES) is a novel approach based on electrochemical redox processes, capable of converting the chemical energy stored in biodegradable organic matter by catalytic activity of microorganisms to electrical energy or using electrical energy it can synthesis organic compounds from CO2. This review briefly discusses the salient features of different electron transfer mechanisms and microbial pathways for cathodic generation of bio-methane in BES.
Bioelectrochemical approaches provide an attractive solution for microbial electro-synthesis of bio-methane, which is having a greater prospect to become an energy source and an energy carrier as well . Methane-producing bio-electrochemical system (BES) offers advantages of producing such CO2-neutral methane (Figure 1), where the process is independent of biomass. Energy from the (excess) renewable electricity can be stored in the form of produced methane. One of the key principles of BES systems is the use of microorganisms as bio-catalyst, which helps in executing diversified bio-chemical reduction and oxidation reactions [2,3]. A more recent application of electron transfer from electrode to microorganisms, where current is getting consumed, enables the possibilities of biological reduction of oxidized pollutants in bioremediation systems [4-6], biological reduction of nitrate to nitrogen gas  and microbial electrosynthesis for production of a wide array of valuable fuels and reduced bio-chemical compounds [8,9].
Methane producing BES consists of both anodic and cathodic compartments, which were comprised of respective anodic and cathodic electrode and separated by proton exchange membrane. Oxidation of organic substrates takes place in the anodic chamber and the fate of oxidation reaction depends on the type of substrates used. For example, upon acetate oxidation, protons and electrons get liberated and the process generates two moles of carbon di-oxide (Equation 1).
Electrons are transferred by redox active mediators or conductive bacterial pilli to anode and flow through the external circuit to cathode; whereas, protons are transferred through membrane to maintain the electro-neutrality of the system. Application of bio-cathode in microbial electrolysis cells (MEC), a variant of BES, enables the growth of microorganisms, which catalyses the reduction of CO2 generated in the anodic chamber, combined with electrons and protons, to generate methane. Methane generation by hydrogenotrophic methanogens takes places via two classical pathways of extracellular electron transfer (EET) .
In microbial electro-synthesis process, during mediated electron transfer, hydrogenotrophic methanogenesis is dependent on biotic/abiotic H2 production (Figure 2). The intermediate H2 production can be achieved electrochemically (Equation 2) or bio-electrochemically i.e., by the activity of hydrogenase enzyme present in electro-active H2 producing microorganisms. The produced hydrogen then can be consumed by H2-utilizing methanogens in presence of excess CO2 to produce methane under applied cathode potential of around - 0.5 V vs. SHE (Equation 3) [3,11].
The major disadvantage of this process was found to be the use of expensive cathode catalyst for achieving enhanced electro-catalytic activity for H2 evolution. Moreover, considering overpotential and internal resistance developed during electrohydrogenesis it requires 0.5-1V to carry out the process effectively. Bio-methane production can also be facilitated by acetate and formate producing bacteria (Figure 2), where the intermediate products are again re-utilized by methanogenic bacteria to produce methane as reaction end-product .
An investigation on direct metabolic pathways for EET driven methane production was first reported by Cheng et al.  where the ability of microorganisms to produce methane from CO2 reduction by using an electrode as direct electron donor was depicted while using mixed methanogenic inoculum and referred to as electro-methanogenesis process (Figure 2). Reaction of electron transport is catalyzed by membrane-bound compounds, which can use the energy difference between donor and acceptor (depending on the difference in redox potentials, AE) and facilitate the establishment of ion-gradient across the membrane. Trans-membrane ion transport is assisted by membrane-localized protein complexes (such as cytochrome c and terminal oxidases/ reductases) or bacterial conductive pilli (nano-wires), which can transport electrons to the final electron acceptors [14,15]. The standard potential for methane production via electro-methanogenesis (ECat =- 0.24 V vs. SHE at pH of 7) is lower than the H2 production via indirect EET (electro-hydrogenesis, ECat = - 0.41 V vs. SHE), which makes the former reaction to be energetically more efficient to occur . Hence, electro-methanogenesis can be carried out in a singlechamber anaerobic systems like UASB reactor to construct localized methane producing MECs, which can be regarded as a potentially applicable device to increase the overall methane yield .
The CO2 and electrons generated in the anodic chamber during biological oxidation of organic matter can be reutilized during the cathodic generation of methane. The possible electrochemical reduction of CO2 to CH4 in the cathodic chamber happens according to the following equation (Equation 4):
Methane production at more negative potential within the range of - 0.65 to - 0.9 V vs. SHE, both via direct EET and abiotically produced H2 gas via hydrogenotrophic methanogenesis, was compared and the relative contributions of these two mechanisms were reported to be highly dependent on the set cathodic potential . It was also found that the biotic cathode with mixed enriched culture of biocatalyst enhanced current densities compared to the abiotic cathode and generated small amount of abiotic H2. However, the study could not reveal the inter-species H2 transfer between electro-active H2 producing microorganisms and H2 utilizing methanogens.
A more recent interest has been revealed on mechanisms by which exchange of electrons happens through electrically conductive biological connections of filamentous appendages and conductive extra-cellular polymeric substances (EPS) present in the intercellular spaces of syntrophic microbial partners in the form of aggregates [8,18,19]. Such bacteria can therefore participate in direct interspecies electron transfer (DIET), which can be attributed by their ability to form extracellular electrical connections. The knowledge cultivation on DIET mechanism, which is yet to be explored completely, is important to understand the energy exchange and to expand the metabolic capabilities of anaerobic microbial community
The well-known mechanism of hydrogen interspecies electron transfer (HIT) depicts the ability of electron donating microbial species (hydrogen producing bacteria) to reduce protons to H2, which is consumed by electron accepting partner species (methanogens) for reduction of any electron acceptor, such as CO2 . DIET has been documented among Geobactor sp. and between co-culture of Geobactor and methanogenic species [21,22].
Research on cathodic reaction in BESs exploiting bio- cathodic electro-synthesis of methane, as alternative fuel, has been intensified over the last few decades. However, detailed understanding on charge transfer mechanisms are immensely needed to bring an insight of bio-electrochemical approaches for methane generation and recovery. Moreover, many scientific and technical challenges are yet to be addressed to make this technology more eco-friendly, economical and feasible for commercial applications, which can be regarded as an important step to mitigate the future requirement of clean energy.
Authors sincerely acknowledge the Ministry of New and Renewable Energy, Government of India (Ministry sanction letter no. 10/14/2010-P&C, dated 26/03/2012) for providing fellowship to the first author.