Charting a narrow course for direct electron uptake-facilitated electromicrobial production

Abstract

Electromicrobial production (EMP) processes based on CO2-fixing microbes that directly accept electrons from a cathode have received significant attention in the past decade. However, fundamental questions about the performance limits and viability of this strategy remain unanswered. Here, we sought to determine what would be necessary for such a system to compete with alternative sustainable production technologies based on H2-mediated EMP and traditional bioprocessing with crop feedstocks. Using global warming potential as the metric for comparison, we show that each EMP process can outperform sugarcane-based sucrose production. Following a stoichiometric and energetic analysis, direct electron uptake-based EMP would need to achieve a current density >48 mA/cm2to reach parity with the H2- mediated system. Because this is currently only practical with a gas diffusion electrode (GDE) architecture, we developed a physical model of the proposed bio-GDE and used it to determine the conditions that a microbial catalyst would experience in a reactor. Our analysis demonstrates that unavoidable inefficiencies in the reactor (e.g., kinetic overpotentials and Ohmic losses) require additional energy input, increasing the breakeven current density to ∼91 mA/cm2. At this current density, the microbial catalyst would need to withstand a pH >10.4 and a total salinity >18.8%. Because currently-known electroautotrophs are not adapted to such extreme conditions, we discuss potential improvements to reactor design that may alleviate these challenges, and consider the implications these results have on the engineerability and feasibility of direct electron uptake-based EMP.