Electrocatalysts for Oxygen Reduction Reaction Synthesized from Co-Facial Cobalt Porphyrin and Graphene Oxide

Abstract

Polymer electrolyte fuel cells (PEFCs) are used as energy devices, which produce no CO2. Because the Oxygen reduction reaction (ORR) is slow, it limits performance of PEFCs. The ORR involves four-electron reduction (O2 + 4H+ + 4e- → 2H2O) or two-electron reduction (O2 + 2H+ + 2e- → H2O2). The ideal reaction is four-electron reduction. When hydrogen peroxide reacts metal ions in PEFCs, hydroxyl radicals are formed. The radical species induce decomposition of polymer electrolyte membrane in PEFCs. Thereby, the selectivity of the four-electron reduction is an important factor for electrocatalyst development. In addition, electrocatalysts with higher ORR activity are needed. At the present, platinum group metal alloys (PGMs) are used as ORR electrocatalysts in PEFCs. Since PGMs are expensive and their reserves are limited, electrocatalysts based on earth abundant elements should be prepared. Non-PGM electrocatalysts for the ORR have been intensively studied. For example, Fe- or Co-based electrocatalysts prepared in pyrolysis from carbon and Fe- or Co-based materials.[1,2]These non-PGM electrocatalysts are known to exhibit high catalytic activity for the ORR. Herein we report the synthesis of a non-PGM electrocatalyst with co-facial cobalt active sites and its catalytic activity and selectivity for the ORR. The catalyst was prepared in the pyrolysis process at 900oC for 45 s from two precursors: graphene oxide and a co-facial cobalt porphyrin ion pair. The co-facial cobalt porphyrin ion pair have high selectivity of the four-electron reduction.[3,4] To study the ORR catalytic activity and selectivity, linear sweep voltammetry of the electrocatalyst was recorded at three pHs (pH 1, pH 7, and pH 13) under oxygen. The ORR activity improved with increasing pH and the activity at pH 13 was higher than that of Pt/C. The electrocatalyst showed the highest four-electron selectivity (< 10% H2O2) at pH 13. We conclude that the ORR activity and selectivity of the electrocatalyst are improved by pyrolysis process.