CoxP@Co3O4 Nanocomposite on Cobalt Foam as Efficient Bifunctional Electrocatalysts for Hydrazine-Assisted Hydrogen Production

The kinetically sluggish oxygen evolution reaction (OER) at an anode has always been the bottleneck in the large-scale application of electrocatalytic water splitting to produce ecofriendly and sustainable hydrogen. Therefore, replacing the OER with hydrazine oxidation reaction (HzOR), which requires a lower theoretical potential, has been considered as a more energy-efficient strategy. Herein, a novel bifunctional CoxP@Co3O4 nanocomposite with grass-like and block-like structures was fabricated on Co foam (defined as P-Co3O4/Co) via a facile hydrothermal synthesis for Co3O4 and the sodium hypophosphite-phosphorization method for cobalt phosphides. Compared with the Co3O4 precursor on Co foam, the heterogeneous P-Co3O4/Co, composed of a mixture of CoP, Co2P, and Co3O4, possessed superb electrochemical catalytic activity for both the hydrogen evolution reaction and HzOR in 1.0 M KOH and 0.3 M hydrazine medium. Low overpotentials of 106 and 129 mV were required to deliver current densities of 10 and 200 mA cm(-2), respectively. Meanwhile, potentials of -100 and -83 mV are needed to drive current densities of 10 and 200 mA cm(-2), respectively, which exceed those of almost recently reported catalysts. The excellent performance can be attributed to the fact that the synergistic effect between the presence of multiphase of CoxP/Co3O4 and the three-dimensional porous Co foam substrate makes the as-synthesized catalyst possess a large specific surface area and fast charge/mass transport. Density functional theory calculations unravel that the phosphorization strategy can not only regulate the electronic structure of pristine Co3O4, enhancing the electronic conductivity, but also optimize the adsorption/desorption strength of H* and alter the free energy change of the dehydrogenation kinetics of NH2NH2*. Meanwhile, a low cell voltage of 1 V was achieved to deliver a current density of 948 mA cm(-2) when P-Co3O4/Co behaved as both the cathode and anode simultaneously, which was superior to most of the nonprecious metal-based catalysts.