Three-Dimensional Sulfite Oxidase Bioanodes Based on Graphene Functionalized Carbon Paper for Sulfite/O2 Biofuel Cells

We have developeda three-dimensional (3D) graphene electrode suitable for the immobilization of human sulfite oxidase (hSO), which catalyzes the electrochemical oxidation of sulfite via direct electron transfer (DET). The electrode is fabricated bydrop-casting graphene-polyethylenimine (G-P) composites on carbon papers (CPs) precoated with graphene oxide (GO). The negatively charged hSO can be adsorbed electrostatically on the positively charged matrix (G-P) on CP electrodes coated with GO (CPG), with a proper orientation for accelerated DET. Notably, further electrochemical reduction of G-P on CPG electrodes leads to a 9-fold increase of the saturation catalytic current density (j_m) for sulfite oxidation reaching 24.4 ± 0.3 μA cm^–2, the highest value among reported DET-based hSO bioelectrodes. The increased electron transfer rate plays a dominating role in the enhancement of direct enzymatic current because of the improved electric contact of hSO withthe electrode. The optimized hSO bioelectrode shows a significant catalytic rate (k_cat: 25.6± 0.3 s^–1) and efficiency (k_cat/K_m: 0.231 ± 0.003 s^–1 μM^–1) compared to the reported hSO bioelectrodes. The assembly of the hSO bioanode and a commercial platinum biocathode allows the construction of sulfite/O₂ enzymatic biofuel cells (EBFCs) with flowing fuels. The optimized EBFC displays an open-circuit voltage (OCV) of 0.64 ± 0.01 V and a maximum power density of 61 ± 6 μW cm^–2 (122 ± 12 mW m^–3) at 30 °C, which exceeds the best reported value by more than 6 times.