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Microbially catalyzed electricity driven bioproduction from CO2 at the
cathode in bioelectrochemical systems

Final Report Summary - MICROBIOELECTROSYN (Microbially catalyzed electricity driven bioproduction from CO2 at the<br/>cathode in bioelectrochemical systems)

The extensive energy consumption by exploiting fossil fuels is leading to massive CO2 emissions into the atmosphere. This greenhouse gas severely influences the global warming and consequently the climate. Sustainable solutions are therefore being constantly explored to deal with CO2 emissions. Initial attempts were focused mostly on the capture and storage of CO2 (referred to as carbon capture and storage (CCS)). The recognition of the fact that CO2 is a valuable source of carbon which is available in abundance and it can be channelled for the production of useful chemicals is leading to a gradual transitioning of research efforts from CCS to carbon capture and utilization (CCU) approach. CCU aims at on-site conversion of captured CO2 into valuable products. The electricity-driven production using microorganisms (also referred to as microbial electrosynthesis (MES)) is an emerging approach in the context of CCU for the sustainable bioproduction from CO2. Several advantages such as less land, water and nutrient requirements along with the possibility of producing multicarbon compounds makes this bioproduction approach promising.
The energy needed to facilitate such conversions can be used from the renewable sources, which in fact is available in excess intermittently and is difficult to store. MES in this context also offers a novel way to channel the renewable energy and store it in the form of useful chemicals. It typically relies on the capability of microorganisms to fix CO2 at high rates by consuming reducing equivalents released from the electrode as their energy source in bioelectrochemical systems (BESs). Considerable fundamental and technological challenges are foreseen in converting CO2 to products via MES. For instance, from a microbiological perspective, there is limited information on the identity of suitable microbial populations for these processes. Very little is known on the role of microbial communities in direct or indirect catalysis at cathodes of the BESs. So far, there is no developed model or process of MES for furthering implementation in an engineered environment.
Considering the aforementioned aspects, the aim of the project was to demonstrate and develop a bioelectrochemical platform for the MES of acetate by anaerobic CO2 fixation in bioelectrochemical systems. To achieve this goal, the key objectives were categorized as: i) microbial enrichments for electricity-driven production of acetate from CO2, ii) identification of the enriched microbial communities at cathode involved in efficient bioproduction, and iii) developing a bioelectrochemical production process. The overview and the key research outcomes of this project are depicted in Figure 1. As a first step, an effective approach for enriching a robust and reproducible microbial community at autotrophic conditions was developed. The use of enriched microbial population was then successfully demonstrated in two different reactor types for reproducible and instant start-up of acetate bioproduction process from CO2. Finally, a continuous bioelectrochemical reactor platform was demonstrated for acetate production at high rates. The combined use of i) a galvanostatic approach for continuous supply of reducing equivalents and ii) an enriched microbial culture as an inoculum enabled rapid start-up of electrosynthetic process, up to 6 g L-1 acetate concentration and high average acetate production rates (close to 20 g m-2 d-1 = 1 g L-1d-1) in continuous biocathode reactors. These are the highest, reproducible acetate production rates achieved so far with unmodified carbon-based electrodes in MES processes, irrespective of the microbial inoculum source, the potentiostatic or galvanostatic control, and the reactor design. In addition to providing a promising strategy for enriching a robust microbial community for fixing CO2, the results achieved in this project expand the knowledge base on electricity-driven bioproduction processes. The successful demonstration of continuous MES process emphasize the possibility of fast conversion of CO2 and excess electrical energy into storable chemicals such as acetate in the framework of CCU concept.
Based on the acetate production rates achieved with the enriched mixed microbial community in this project, either a batch production process or a continuous production platform can be envisioned. The hydrogen-based bioproduction in this study suggests that a useful electricity-driven bioproduction system could be developed regardless of the presence or the absence of direct electron uptake mechanism by microorganisms. The use of a galvanostatic mode approach with an active start-up microbial inoculum with no or limited methanogenic activity will ensure a dedicated process without interference of side reactions.
Volumetric production rates of acetate via H2:CO2 conversions at high pressures (fermentation) by a genetically modified homoacetogenic bacteria Acetobacterium woodii have already reached to 28 g L-1 d-1, a much higher rate than what has been achieved via MES thus far. Furthermore, taking into account the high energy investment and other factors (capital and operational costs), the production of acetate via MES is not yet economically viable. These facts indicate the need for further development of MES to position it as an economically viable concept. Therefore, besides optimization of several operational parameters, further exploration of cheaper and efficient electrode materials, novel microorganisms, reactor designs and bioproduction of higher value products such as longer chain organic acids and secondary alcohols is needed to bring this concept potentially into practice.
This project has contributed to this research area by creating and strengthening knowledge based novel bioproduction processes, enriching the scientific base for developing such processes, developing the human potential and skills required for innovations in such bioproduction approaches, and strengthening the European research institute in the proposed frontier research area. This bioproduction approach not only helps to reduce greenhouse gases but also averts release of any additional fossil based emissions. On the long term, CO2 and renewable electricity are the only abundant sources of carbon and energy, respectively. These aspects potentially create an opportunity for MES approach to become a key technology in future bioproduction and contribute to the advancements of CCU concept. Electricity-driven bioproduction using microorganisms is therefore not only of economic and ecological importance but also of societal importance.
The achievements in CO2-based bioproduction will certainly contribute to the knowledge based bioeconomy initiatives worldwide. Importantly, no societal challenges or controversies are identified with this bioproduction approach so far thus avoiding any deterrence in further research and development efforts. Further research advancements are expected to improve the production of fine chemicals starting from CO2 and electricity as well as generate scientific and commercial outcome along with both environmental and economic benefits for the industries and society at large. Moreover, this approach is expected to be more sustainable than existing biorefineries. If CO2 mitigation becomes an urgent global concern, such approaches which are currently uneconomic have potential to become economically viable on a longer term.

Report based on:
- Patil, S.A. Arends, J.B.A. Vanwonterghem, I., van Meerbergen, J., Guo K., Tyson, G.W. and Rabaey, K. (2015): Selective enrichment establishes a stable performing community for microbial electrosynthesis of acetate from CO2. Environmental Science & Technology, DOI: 10.1021/es506149d
- Patil S. A., Arends J.B.A. Guo K. and Rabaey K. (2015): Continuous and long-term operation of mixed culture biocathodes leads to high rate microbial electrosynthesis of acetate from CO2. In preparation.

• Relevant contact details for further information:
Dr. Sunil A. Patil: sunil.patil@ugent.be; sunilmicro12@gmail.com
Prof. Korneel Rabaey: korneel.rabaey@ugent.be

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