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 (CO₂-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 CO₂-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.