On the Edge of Research and Technological Application: A Critical Review of Electromethanogenesis

Abstract

The conversion of electrical current into methane (electromethanogenesis) by microbes represents one of the most promising applications of bioelectrochemical systems (BES). Electromethanogenesis provides a novel approach to waste treatment, carbon dioxide fixation and renewable energy storage into a chemically stable compound, such as methane. This has become an important area of research since it was first described, attracting different research groups worldwide. Basics of the process such as microorganisms involved and main reactions are now much better understood, and recent advances in BES configuration and electrode materials in lab-scale enhance the interest in this technology. However, there are still some gaps that need to be filled to move towards its application. Side reactions or scaling-up issues are clearly among the main challenges that need to be overcome to its further development. This review summarizes the recent advances made in the field of electromethanogenesis to address the main future challenges and opportunities of this novel process. In addition, the present fundamental knowledge is critically reviewed and some insights are provided to identify potential niche applications and help researchers to overcome current technological boundaries.

Keywords: BES technology; biocathode; bioelectrochemistry; methane; methanogenesis; microbial electrolysis cell; power-to-gas.

Figure 1

Scientific publications dealing with methane production catalyzed by minerals (catalytic methanation) or microbes (electromethanogenesis) published from 2000 to the beginning of 2017. Additionally, the number of citations of electromethanogesis-related papers is also shown. This data was extracted from Scopus database using the keywords “methane production bioelectrochemical systems”, “electromethanogenesis”, “methane bioelectrosynthesis”, bioelectrochemical methane production”, “electromethanosynthesis”, “methanogenesis bioelectrochemical system” for electromethanogenesis and “catalytic methanation” for catalytic methane formation. (Search date: 4 January 2017).

Figure 2

Historical overview of major achievements towards methane production via BESs. In 1987 Daniels et al. reported for the first time the capability of some methanogens to use elemental iron as an electron donor and reduce CO₂ into CH₄ [26]. 12 years later, in 1999 Park et al. used a BES with pure and mixed cultures of H₂-consuming bacteria to produce methane from CO₂ [27]. They used neutral red as the sole source of reducing power, thus replacing H₂ as the sole electron donor source. In 2008, works of Clauwaert et al. and Rozendal et al. on H₂ production at the cathode [28] and the placement of cathodic biofilm for H₂ and CH₄ production [29], served as a precursor for the birth of the term [8] just one year later by Cheng et al. During the same year 2009 the first patent based on electromethanogenesis was registered by Cheng et al. [30] and Villano et al. increased the methane production with the enrichment of hydrogenophilic methanogens in the cathodic community [13]. The possible electron transfer mechanisms between microbes and the cathode were reviewed by Rosenbaum et al. in 2011 [16], the same year in which Morita et al. demonstrated the enhancement of electron transfer between methanogens with conductive aggregates in anaerobic digester [31]. Proofs of concept for further application of electromethanogenesis were conducted by Tartakovsky et al. [32] and Xu et al. [33] in 2011 and 2014 respectively, demonstrating methanogenesis enhancement in anaerobic digesters and biogas purification. The importance of the presence of bacteria species in the microbial community to enhance methane production in BESs was highlighted by Van Eerten-jansen [19]. Afterwards, in 2014 Rotaru et al. provided further knowledge of indirect electron transfer routes between microbes for the reduction of CO₂ into CH₄ [34,35]. Batlle-Vilanova et al. elucidated the biotic and abiotic hydrogen yield in a biocathode [36], directly linked with the methane yield in electromethanogenic reactors. Electron transfer mechanisms in a bioelectrochemical biogas upgrading process were deciphered in 2015 by Batlle-Vilanova et al. as a first step for further scaling-up of such electromethanogenesis-based technology [14]. Recently, Shi et al. reviewed the extracellular electron transfer mechanisms between microorganisms and minerals, as a basic for designing future methane producing BESs [37].

Figure 3

Described electromethanogenesis routes within mixed culture biocathodes. Sizes of the circles do not correspond to any proportions. 1. Direct electromethanogenesis; 2. Abiotic H₂ production; 3. Biotic H₂ production; 4. Bioelectrochemical acetate production; 5. Bioelectrochemical formate production; 6. Hydrogenotrophic methanogenesis; 7. Mediated acetate production; 8. Acetoclastic methanogenesis; 9. Mediated formate production; 10. Indirect methane production from formate. AM: Acetoclastic methanogen; EM: Electromethanogen (includes species capable to perform direct and mediated electromethanogenesis); HM: Hydrogenotrophic methanogen; HPM: Hydrogen-producing microorganism; APM: Acetate-producing microorganism; FPM: Formate-producing microorganism; SM: Syntrophic microorganism; Mred: Reduced mediator; Mox: Oxidized mediator.

Figure 4

Proposed electron transfer mechanisms within the biocathode compartment. Even though H₂ is the only intermediate molecule (bio)electrochemically produced on the surface of the electrode shown in the figure, formate or other molecules must be also considered to play the same role as electron carrier. Sizes of the circles do not correspond to any proportions.