Bioelectrochemical methane production from CO2

Nowadays, most of our energy and fuels are produced from fossil resources. Fossil resources are, however, finite and their use results in emissions that affect the environment and human health. For reasons of energy security and environmental sustainability, there is therefore a need to produce energy and fuels from renewable resources. However, currently several challenges need to be overcome before renewable resources can be implemented on a large scale for the production of renewable energy and fuels. At the moment, all the renewable resources can be converted into electricity. However, renewable electricity is often produced intermittently. Therefore, excess renewable electricity, when supply does not meet demand, needs to be stored not to get lost. On the other hand, fuels can currently only be produced directly from biomass. There are, however, discussions about whether sufficient biomass can be produced in a sustainable way to cover the global demand.

A methane-producing Bioelectrochemical system (BES) is a novel technology to store excess renewable electricity in the form of methane, independent of biomass. Key principle of the methane-producing BES is the use of microorganisms as catalysts for the reduction of CO2 and electricity into methane. At the start of this thesis, the methane-producing BES was at its infancy, and for implementation of the technology a more thorough understanding of the technology was needed. Therefore, the aim of this thesis was to investigate the principles and perspectives of bioelectrochemical methane production from CO2. Focus was on the main bottlenecks limiting system’s performance.

In Chapter 2, the performance of a flat-plate methane-producing BES that was operated for 188 days was studied. The methane production rate and energy efficiency were investigated with time to elucidate the main bottlenecks limiting system’s performance. Using water as electron donor at the anode, methane production rate was 0.12 mL CH4/m2 cathode per day and overall energy efficiency was 3.1% at          -0.55 V vs. Normal Hydrogen Electrode (NHE) cathode potential during continuous operation. Analysis of the internal resistance showed that in the short term, cathode and anode losses were dominant, but with time also pH gradient and transport losses became important.

Since the cathode energy losses were dominant, in Chapter 3, the microbial community that catalyses the reduction of CO2 into methane was studied. The microbial community was dominated by three phylotypes of methanogenic archaea, being closely related to Methanobacterium palustre and Methanobacterium aarhusense, and six phylotypes of bacteria. Besides methanogenic archaea, the bacteria seemed to be associated with methane production, producing hydrogen as intermediate. Biomass density varied greatly with part of the electrode being covered by a thick biofilm, whereas only clusters of biomass were found on other parts of the electrode.

Based on the microbial community it seemed that methane was produced indirectly using hydrogen as electron donor. Therefore, the electron transfer mechanisms of bioelectrochemical methane production were investigated in Chapter 4. Understanding the electron transfer mechanisms could give insight in methods to steer the process towards higher rate. A mixed culture methane-producing biocathode was developed that produced 5.2 L methane/m2 cathode per day at -0.7 V vs. NHE cathode potential. To elucidate the formation of intermediates, methanogenic archaea in the biocathode were inhibited with 2-bromethanesulfonate. Methane was primarily produced indirectly using hydrogen and acetate as electron donor, whereas methane production via direct electron transfer hardly occurred.

Besides producing methane, a BES could also be used to produce higher value organics, such as medium chain fatty acids. Currently, medium chain fatty acids are produced by fermenting (low-grade) organic biomass using an external electron donor, such as hydrogen and/or ethanol. A BES could provide the electrons in-situ, either as the electrode directly or indirectly via hydrogen. In Chapter 5, medium chain fatty acids production in a BES at -0.9 V vs. NHE cathode potential was demonstrated, without addition of an external electron mediator. Caproate (six carbon atoms), butyrate (four carbon atoms), and smaller fractions of caprylate (eight carbon atoms) were the main products formed from acetate (two carbon atoms). In-situ produced hydrogen was likely electron donor for the reduction of acetate. Electron and carbon balances revealed that 45% of the electrons in electric current and acetate, and 31% of the carbon in acetate were recovered in the formed products.

In Chapter 6, the present performance of methane-producing BESs was evaluated. Analysis of the performances reported in literature did not reveal an increase with time. Based on the main bottlenecks that limit system’s performance as found in this thesis, methods to increase performance were discussed. Besides, we showed that our envisioned first application is to upgrade CO2 in biogas of anaerobic digestion to additional methane. Finally, the feasibility of production of higher-value organics, such as medium chain fatty acids, in BES was discussed.