Development and investigation of carbon-based cathode catalysts for polymer electrolyte membrane fuel cells

Abstract

Alternatives for Pt-based catalysts for oxygen reduction reactions (ORRs) in polymer electrolyte membrane fuel cells (PEMFCs), especially carbon-based catalysts were synthesized and characterized. In the acidic media, carbon-based catalysts show relatively much lower ORR activity compared to Pt-based cata-lysts; therefore, improving their performance is one of the major objectives for commercializing PEMFCs. In this study, heteroatom (boron, nitrogen, phosphorus, and sulfur) doped carbon-based catalysts were synthe-sized to improve the ORR performance. P and/or S heteroatoms are additionally doped into NDC to improve its ORR activity in acidic media. The binary and ternary doping of P and/or S in the NDC produces many edge sites and increases the portion of pyridinic-N sites in the carbon materials, which is favourable for the ORRs. Moreover, the addition of P and/or S enhances the asymmetry of the atomic charge density in C atoms and strengthens the adsorption of oxygen molecules on the carbon atoms. The ORR activity of the carbon materials is significantly modified through the additional doping of P and/or S in the NDC. In particular, S-doping in NDC reveals 3.1 times higher mass activity (mA mgnon-metal-1) compared to that of NDC at 0.75 V (vs. RHE), and derives the four-electron pathway in ORRs with high stability. As a catalyst for ORRs in acidic media, graphene was modified by doping of N and additional do-pants, B and P. In the modification steps, DCDA, boric acid, and phosphoric acid were used as doping sources for N, B, and P, respectively, and it was confirmed that the heteroatoms were doped evenly on the graphene surface. Regardless of the dopant types, the graphene-based catalysts revealed high onset potentials, ~0.86 ± 0.01 V (vs. RHE). However, additional doping of B and P in NGr significantly improved catalytic activity in ORRs. The NGr showed 0.45 mA mg-1 of mass activity at 0.75 V, but BNGr and PNGr indicated 0.53 and 0.80 mA mg-1 of mass acti...