1D model simulations suggest biofilms can suppress cathodic H ₂ bubble formation

Microbial electrosynthesis (MES) electrochemically generates H ₂ for the reduction of CO ₂ 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 H ₂ and CO ₂ concentration gradients within cathodic biofilms, while accounting for the limited solubility of H ₂ . The effects of key biofilm properties, including thickness, cell density and H ₂ consumption rate were analysed. Overall, our simulations indicate that cathodic biofilms play an important role in (1) minimizing H ₂ loss from the system through H ₂ consumption within the biofilm; and (2) suppressing H ₂ bubble formation by maintaining the H ₂ concentrations at the cathode surface below saturation. Our model predicts that the current density at which H ₂ 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 H ₂ consumption rate. This effect requires biofilms of at least 50 µm thickness, whereas CO ₂ diffusion limitations arise above 200 µm. These findings highlight the critical role of biofilms to increase MES performance, while requiring further experimental validation.