Graphite granules as microbial electrodes: Determining the relevant surface area

Fixed-bed electrode reactors represent a promising low-cost configuration for microbial electrochemical technologies, particularly bioremediation. To assess performance and guide development, it is essential to quantify the relevant electrode area. This is challenging for fixed-bed electrodes composed of coarse carbonaceous materials like graphite granules, granular activated carbon, or biochar, because gas adsorption/desorption coupled to Brunauer-Emmett-Teller (BET) theory likely overestimates microbially accessible areas. We compared four methods for determining the surface area of graphite granules: the copper underpotential deposition and cyclic voltammetry analysis, both providing electrochemical active surface area; BET measurement for total area; and profilometry combined with volumetry for topographic area. By cultivating Geobacter sulfurreducens biofilms on single graphite granules and smooth graphite plates, it was demonstrated that profilometry, combined with volumetry, yields the most realistic results (1.5×10 −3 ±3×10 −4 m 2 g −1 or 1.11±0.33 cm² per granule) for calculating current densities, while BET measurements (0.8±0.2 m 2 g −1 ) caused significant overestimation. This was verified by helium ion microscopy of G. sulfurreducens biofilms on single graphite granules, demonstrating limited colonization of open pores (diameter 9.0±3.0 µm) likely due to mass-transfer limitations. The results indicate that topographic surface area from profilometry is most suitable for quantifying the relevant electrode surface area in fixed-bed reactors. • Surface area of graphite granules was determined by four distinct methods • Mass-specific surface area from profilometry (1.5 × 10 -3 m² g⁻¹) is most realistic • Surface area from BET analysis (0.8 m² g⁻¹) overestimates microbially relevant area • HIM images show that open pores (9±3 µm) remain uncolonized by G. sulfurreducens • Profilometry enables direct fixed-bed vs. monolithic electrode reactor comparison