Tuning Hydrogen versus Methane Production on Sustainable Biochar-Based Cathodes in Microbial Electrolysis Cells by Voltage Control

Due to the intermittency of solar and wind energy generation, efficient energy storage solutions are essential to ensure a global transition to renewable energy sources. Bioelectrochemical Power-to-Hydrogen systems are a promising storage pathway, yet their development is limited by high costs and low productivity compared to conventional hydrogen production. Novel, sustainable, and cost-effective materials, such as carbon-based electrodes, can help to overcome these challenges. This study evaluates five cathodes for hydrogen and methane production in microbial electrolysis cells (MECs) operated at 600 and 800 mV: stainless steel mesh (SSM), two custom-made biochars derived from olive mill waste (OMW-1, OMW-2), and two commercial carbon-based materials (Carbon Black and Black Pearls). OMW-1 achieved a H₂ yield of 257 ± 62 mL L^-1 d^-1 at 800 mV, showing the potential of noncommercial biochar. CB and SSM performed better, reaching 493 ± 57 and 496 ± 9 mL L^-1 d^-1 H₂, respectively. Cyclic voltammetry and next-generation sequencing revealed that hydrogen-oxidizing bacteria colonization negatively impacted H₂ yields. At 600 mV, increased CH₄ production was observed for OMW-2, BP, and CB. Energetically, OMW-2 (3.0 ± 0.2 kWh L^-1 d^-1) performed comparably to CB and BP (both 3.3 kWh L^-1 d^-1), outperforming SSM at both voltages. These findings support the viability of carbon-based cathodes as sustainable alternatives to metal-based ones with the potential to reduce electrode costs while maintaining or improving energy productivity.