1D model simulations suggest biofilms can suppress cathodic H2 bubble formation

Abstract

Microbial electrosynthesis (MES) electrochemically generates H2 for the reduction of CO2 into acetate by acetogens. So far, the importance of a biofilm on the cathode of such systems remains under debate. Here, we investigated the role of cathodic biofilms through computational modelling. We developed MES-BIOFILM, a comprehensive one-dimensional model to simulate steady-state H2 and CO2 concentration gradients within cathodic biofilms, while accounting for the limited solubility of H2. The effects of key biofilm properties, including thickness, cell density and H2 consumption rate were analysed. Overall, our simulations indicate that cathodic biofilms play an important role in (1) minimizing H2 loss from the system through H2 consumption within the biofilm; and (2) suppressing H2 bubble formation by maintaining the H2 concentrations at the cathode surface below saturation. Our model predicts that the current density at which H2 bubbles begin to form can be increased by 5.5-fold relative to a bare electrode when biofilms exhibit a high cell density and high H2 consumption rate. This effect requires biofilms of at least 50 m thickness, whereas CO2 diffusion limitations arise above 200 m. These findings highlight the critical role of biofilms to increase MES performance, while requiring further experimental validation.