Global warming is caused to a large extend by CO2 emitted in human activities based on fossil fuels burning. To fight against its effects, large emitting countries have agreed to i) decrease CO2 emissions and ii) develop carbon-neutral technologies to produce (bio)fuels and commodities. In this sense, significant efforts are being made to investigate both CO2 capture and storage and novel conversion technologies based on chemical, photochemical, electrochemical, biologic or inorganic processes.
Microbial electrochemical technologies (METs) are a promising approach to uptake and reduce the CO2 in-situ by using renewable electricity. Microorganisms grown on electrodes under autotrophic conditions use CO2 as electron acceptor while an electrode supplies electrons in the form of electricity. In the process, named as microbial electrosynthesis (MES), different compounds are obtained depending on the metabolic possibilities of the microorganisms present in the system. The potential of this approach is high: an enriched culture of selected electroactive microorganisms can steer the CO2 transformation into high added-value compounds. Nevertheless, several knowledge gaps still exist.
This PhD. thesis investigated reliable operational procedures for the monitoring of the performances of METs to produce suitable substrates for economically viable downstream applications. The cathodes of two different designs of bioelectrochemical systems (BESs), tubular and flat-plate, were inoculated with an enriched culture of a carboxydotrophic strain and operated until stable conversion of CO2 into acetate, ethanol and small amounts of butyrate.
Results obtained are published in international scientific journals (Green Chemistry, 21, Issue 3, 2019, 684-691) and are highly valuable to steer METs development:
- Tubular BES achieved a concomitant production of ethanol and acetate, which were found crucial for triggering the production of longer carbon chain carboxylates and alcohols in, for example, a coupled chain elongation bioreactor. Flat-plate BES showed constant acetate production and high resilience and robustness to unexpected operational episodes.
- Coulombic efficiencies and overall production rates were higher in the flat-plate design, which suggests the need to improve the manoeuvrability by setting threshold values of key parameters that switch between target metabolic pathways.
- Improving the reactor design, mass transport limitation, together with reaching a high maturity of the electroactive community turned out to be crucial to obtain more reduced compounds from CO2 and electricity.
- Continuous in-line monitoring of key parameters (pH, CO2 dissolved and partial pressure of hydrogen) revealed variations in the current signal and pH values that were correlated with CO2 depletion and the transition from acetogenesis to solventogenesis in the enriched culture. In addition, new inoculation and feeding strategies, based on previous electrode enrichment with an electroactive biofilm and avoiding periods with low availability of reducing power, showed promising results that should be addressed in future research on CO2 bio-electrorecycling.
- In-line monitoring of pH and electron consumption are meaningful operational variables to differentiate between the carboxylate and alcohol production, which opens the door to develop new approaches to control the bio-electrorecycling of CO2 into biofuels by METs.