Kirkendall effect induced the formation of hollow Co2P in Co-N-C for ORR, OER, HER and flexible Zn–Air battery

Enhancing the exposure of active sites and optimizing the electronic structure of the catalysts are indispensable for efficient catalyst design, yet remains challenging in the renewable energy field. Herein, a sequential pyrolyzing and phosphating approach for a well-designed GO/CoTPP gel precursor has been explored to prepare a Co-based carbon catalyst containing single-atom Co-Nx and hollow Co2P active species (Co2P/Co-N-C) for multifunctional electrocatalytic reactions. Kirkendall effect occurring during the template-free one-pot phosphating approach induced the formation of the hollow Co2P structure in the catalyst, simultaneously optimizing the electron structure and allowing the active sites to be exposed to a greater extent. DFT calculations demonstrated that the Co2P and Co-N4 structures enable Co2P/Co-N-C to have a suitable d-band center depending on the electron transfer from Co2P to Co-Nx, which enhances the adsorption of O* and the dissociation of OH* in oxygen electrode reactions, thereby facilitates catalytic reactions. As a result, the Co2P/Co-N-C exhibits excellent trifunctional activity with a half-wave potential of 0.89 V in alkaline for oxygen reduction reaction (ORR), overpotentials of 330 mV for oxygen evolution reaction (OER) and 190 mV for hydrogen evolution reaction (HER), respectively, which are among the best results reported to date for Co-containing carbon-based catalysts. Furthermore, the flexible Zinc-air battery with Co2P/Co-N-C electrodes also demonstrates a high open circuit voltage of 1.35 V, which is applied to build the self-powered device for water splitting continuously. Our work provides a facile strategy to prepare high-performance trifunctional catalysts by morphology/structure control of metal-based NPs on carbon-based materials.