Tuning hydrogen versus methane production on sustainable biochar-based cathodes by MEC voltage control
In the energy transition framework, fossil fuels will be replaced by renewable energy mainly produced from intermittent sources like solar and wind, thus efficient electricity storage will be crucial. Bioelectrochemical Power-to-Hydrogen offers a promising storage solution, however its development is hindered by high costs and lower productivity compared to conventional H2 production methods. New research is needed to develop, characterise and select novel sustainable and effective materials. In this context, carbon-derived cathodes are a valid alternative to metal-based catalysts.This study evaluated five cathodes, stainless steel and four carbon-based electrodes, for H2 production in microbial electrolysis cells (MECs) at two voltages (600 and 800 mV). The goal was to assess their performance and the impact of methanogenesis, an undesired competing reaction.The Olive Mill Waste-biochar (OMW) cathode achieved an H2 yield of 257 mL L-1 d-1, demonstrating the potential of non-commercial biochar. However, stainless steel (SSM) and Carbon Black (CB) performed better at 800 mV, producing 496 and 493 mL L-1 d-1, respectively. Cyclic voltammetry (CV) and next-generation sequencing (NGS) analyses linked lower H2 yields to microbial cathode colonization, particularly hydrogen-oxidizing bacteria. Additionally, 600 mV polarization resulted in lower H2 productivity and higher CH4 production for some cathodes, including OMW, Black Pearl (BP), and CB, highlighting the importance of optimizing voltage to enhance H2 production and purity. Overall, carbon-based cathodes resulted a viable alternative to metal-based ones for H2 production in MEC systems with the potential to reduce electrode manufacturing costs.