Effects of pore size and surface properties of MgO-templated carbon on the performance of bilirubin oxidase–modified oxygen reduction reaction cathode

Highlights: • Bilirubin oxidase (BOD)-loaded MgO-templated carbon electrodes were prepared. • Effects of pore size and structure on electrode ORR performance were probed. • Oxygen reduction current density increased with increasing electrode pore size. • Pore size affected the amount and distribution of BOD. -- Abstract: Enzymatic biofuel cells (BFCs) directly convert the chemical energy produced by oxidizing fuel into electricity using enzymes as electrocatalysts. However, these cells are characterized by low output and poor long-term durability, which highlights the need for further performance improvement, especially on the cathode side. Herein, we clarify the effect of the pore size and surface morphology of MgO-templated carbon (MgOC) as an electrode material on the amount of electrochemically active bilirubin oxidase (BOD) available on the cathode. This enzyme is widely used as an electrocatalyst for the four-electron cathodic reduction of oxygen. The amount of electrochemically active enzyme can be increased by using an electrode of larger pore size and by increasing the incubation time for enzyme adsorption. However, excess enzyme prevents the mass transfer of O₂ to the enzyme adsorbed on the electrode surface. The MgOC surface was electrochemically modified using several substituted aromatic amines to enhance the interaction between the active sites on BOD and the electrode. Our results showed that the use of 6-amino-2-naphthoic acid as the promoter increased the interfacial electron transfer rate between BOD and the carbon surface and enhanced the stability.