Bifunctional catalytic activity of solvothermally synthesized CeO₂ nanosphere/NiO nanoflake nanocomposites

Abstract

The design and development of bifunctional electrocatalysts that can catalyze both the oxygen evolution reaction and oxygen reduction reaction (OER/ORR) is a burning issue for energy conversion and storage technologies. Herein, we demonstrate the bifunctional activity of CeO₂ nanospheres embedded in NiO nanoflakes. The CeO₂/NiO nanocomposites are synthesized solvothermally by varying their molar ratios. Among the synthesized nanocomposites, CeO₂/NiO-2 exhibits an outstanding OER/ORR activity compared to those of individual counterparts and other compositions and outperforms the benchmark ORR catalyst Pt/C and the OER catalyst IrO₂ in terms of stability. The CeO₂/NiO-2 nanocomposite shows the low onset potential of 1.47 and 0.8 V (vs reversible hydrogen electrode) for OER and ORR, respectively. Moreover, a low potential difference (i.e., ΔE) of 0.86 V between the potential for 10 mA cm ^-2 OER current density and the ORR half-wave potential for CeO₂/NiO-2 makes it an efficient bifunctional oxygen electrocatalyst. The excellent bifunctional activity of the nanocomposite is ascribed to the electronic synergy between CeO₂ and NiO, which leads to the formation of an increased number of oxygen vacancy defects as well as more accessible active sites at an optimum molar ratio of CeO₂ and NiO. The unique morphology of CeO₂ nanospheres embedded in NiO nanoflakes facilitates the exposure of surface-active sites with the bifunctional nature of the catalyst. The practical applicability of the CeO₂/NiO-2 nanocomposite is demonstrated with a Zn–air battery, which delivers 1.41 V open-circuit voltage and 105.0 mW cm ^-2 peak power density. It has long-term cycling stability for 22 h with negligible voltaic efficiency loss.