Co_<i>x</i>P@Co₃O₄ Nanocomposite on Cobalt Foam as Efficient Bifunctional Electrocatalysts for Hydrazine-Assisted Hydrogen Production

Abstract

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.