Biochar Cathodes for Bioelectrochemical Systems: Understanding the Effect of Material Heterogeneity on Performance for Abiotic Hydrogen Evolution Reaction

Granular carbon‐based cathodes in carbon dioxide‐reducing bioelectrochemical systems (CO 2 ‐reducing BES) feature high biocompatibility and stability. Wood‐based biochar is gaining popularity in (bio)electrochemical applications due to its sustainability and reduced environmental impact. Yet, previous studies primarily examined lab‐scale biochars. This study investigates how heterogeneity of industrial‐scale granular biochars (GBs) influences their electrocatalytic activity for hydrogen evolution reaction (HER) in the nexus of CO 2 ‐reducing BES. Significant variations are identified in overpotentials for HER at −1 mA cm −2 ( η ‐1 mA cm −2 ) among the GB‐based cathodes. Beechwood‐derived GB pyrolyzed at 740 °C shows the lowest η ‐1 mA cm −2 (223.6 ± 30.0 mV), outperforming birchwood‐derived GB at 700 °C (503.5 ± 4.9 mV) and granular graphite (608.3 ± 19.5 mV). Despite its superior performance, beechwood‐based GB shows high heterogeneity. Such heterogeneity underlies different physicochemical properties, likely due to uneven temperature distribution in industrial pyrolysis. The remarkable performance of beechwood‐based GB pyrolyzed at 740 °C is attributed to its higher electrical conductivity, higher degree of carbonization, favorable H/C ratios, higher disorder in carbonaceous structure, and suitable porosity. The results highlight the influence of the wood type, the importance of systematic GB characterization, and the necessity to optimize industrial‐scale biochar production to achieve homogeneous and high‐performance biochar.