Performance evaluation of enzymatic biofuel cells using a new cathodic catalyst containing hemin and poly acrylic acid promoting the oxygen reduction reaction

A new cathodic catalyst is introduced for enzymatic biofuel cells (EBCs) by enhancing the oxygen reduction reaction (ORR) consisting of the glucose oxidation reaction (GOR and partial ORR) and H₂O₂ reduction reaction (HRR). In this catalyst, glucose oxidase (GOx) and hemin are used for promoting both the partial ORR and HRR, respectively. The hemin is attached onto polyethyleneimine (PEI) and carbon nanotubes (CNTs) (CNT/[PEI/hemin]), while GOx is conjugated with poly acrylic acid (PAA) (PAA/GOx). The two structures are then connected to each other ({CNT/[PEI/hemin]}/{PAA/GOx}). According to electrochemical evaluations of this catalyst, the onset potential is positively shifted (0.4 V vs. Ag/AgCl) and the current density increases considerably (522 μA cm^-2) by the use of the PEI/hemin composite and PAA/GOx conjugate. The maximum power density of an EBC using this catalyst is 255 ± 2.8 μW cm^-2, which is a comparable level to previously reported excellent results. This is due to the facile relay between the partial ORR and HRR by the (i) proper linkage between GOx and hemin through a cascade type preparation, (ii) four step redox reaction of Fe ions within hemin and (iii) use of water swellable PAA facilitating glucose mass transfer. It is observed that the upper water swellable PAA/GOx layer promotes smooth glucose mass transfer of glucose fuel and produces a sufficient amount of H₂O₂ by the partial ORR. The produced H₂O₂ is further reduced by the HRR at positive potential. The chemical structure and the degree of enzyme immobilization of the best catalyst are examined by EDS, XPS and glucose colorimetric assay.