High-Loading Cobalt Oxide Coupled with Nitrogen-Doped Graphene for Oxygen Reduction in Anion-Exchange-Membrane Alkaline Fuel Cells

Abstract

A new nanocomposite catalyst consisting of high-loading cobalt oxide (CoO) on nitrogen-doped reduced graphene oxide (rGO) for oxygen reduction reaction (ORR) was prepared in this work. Its high activity for the ORR in alkaline electrolyte was determined using the rotating disk electrode technique, and further confirmed in real alkaline membrane fuel cells. A combination of physicochemical characterization (e.g., X-ray absorption and X-ray photoelectron spectra) and density functional theory (DFT) calculation suggests that cobalt(II) cations in the composite catalyst may coordinate with the pyridinic nitrogen atoms doped into graphene planes, most likely the active species for the ORR. Especially, the DFT calculations indicate that a stable rGO(N)–Co(II)–O–Co(II)–rGO(N) structure can be formed in the nitrogen-doped graphene catalyst. Kinetic parameter analysis shows a high selectivity of four-electron reduction on the composite catalyst during the ORR with an average electron transfer number of 3.75. A synergistic effect between the rGO(N) and CoO may exist, yielding a much higher catalytic activity on the CoO/rGO(N) catalyst, compared to either rGO(N) or CoO controls. The novel synthesis procedure utilizing rGO(N) to further couple Co(II) yields a high loading of Co species (24.7 wt %). Thus, a relatively thinner cathode in fuel cell can accommodate more active Co species and facilitate O2 transfer. Due to the high intrinsic activity and efficient mass transport, the CoO–rGO(N) ORR catalyst achieved approaching performance to state-of-the-art Pt/C cathodes in anion-exchange-membrane alkaline fuel cells.