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

We have developed a three-dimensional (3D) graphene electrode suitable for the immobilization of human sulfite oxidase ( h SO), which catalyzes the electrochemical oxidation of sulfite via direct electron transfer (DET). The electrode is fabricated by drop-casting graphene-polyethylenimine (G-P) composites on carbon papers (CPs) precoated with graphene oxide (GO). The negatively charged h SO 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 h SO bioelectrodes. The increased electron transfer rate plays a dominating role in the enhancement of direct enzymatic current because of the improved electric contact of h SO with the electrode. The optimized h SO 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 h SO bioelectrodes. The assembly of the h SO bioanode and a commercial platinum biocathode allows the construction of sulfite/O 2 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.