Cobalt Nanocube Electrocatalysts Comprising Sub-Nanometer Oxide-Phosphide Hetero-Interface for Highly Efficient Water Electrolysis

Abstract

Hydrogen production from electrochemical water splitting is known as an eco-friendly energy conversion process, which has drawn attention for a few decades. Typically, the water electrolysis is divided into PEM and AEM electrolysis where acidic and alkaline electrolyte are used, respectively. In case of PEM water electrolysis, the acidic water splitting reveals critical drawback of poor counterpart OER efficiency. For this reason, AEM water electrolysis in alkaline condition has huge advantages in high efficiency of full cell owing to the lower overpotential for oxygen evolving anode, and accessibility of using transition metal-based materials for each anode and cathode. However, alkaline HER is 2-3 orders of magnitude slower than acidic HER due to the lack of proton nearby the surface.Thus, it is necessary to find highly efficient alkaline HER electrocatalysts for the future commercialization of AEM electrolyzer.Herein, by phosphidation of 10 nm Co3O4 nanocube (CO) loaded on nickel foam (NF) substrate, unique heterojunction interface of CoP and Co3O4 within 10 nanometers were realized, which converged both the excellent electrocatalytic performance of CoP and high stability of Co3O4. Unique sub-nanometer oxide-phosphide heterojunction was discovered via inverse fast fourier transform (IFFT), which confirmed the phase transition alongside the particle, while maintaining its nanocube morphology. X-ray photoelectron spectroscopy (XPS) was brought out to further confirm the co-exsistence of cobalt oxide and phosphide materials with respect to the chemical structure.Experimental investigations revealed the excellent water splitting activity of the hybrid-phosphidated Co3O4 (H-PCO) catalyst, only requiring an overpotential of 162, 171 and 395 mV to reach 100 mA cm-2, each for acidic HER, alkaline HER, and alkaline OER, respectively. H-PCO/PNF showed excellent stability over 20 hours under various working conditions, which was superior to that of conventionally fabricated CoP catalysts. Such outstanding alkaline water electrolysis performance and stability stems from the evolution of new active sites are expected at the interface because the interface has different chemical and electronic structures compared to bulk materials. In addition, the scalability of the electrocatalyst was demonstrated where a 34.56 cm-2 catalyst foam was uniformly fabricated As a result, all 15 points of as-prepared H-PCO electrode showed negligible distribution of the overpotentials at 100 mA cm-2, and the large unit-cell scale measurement showed remarkably improved HER performance compared to bare Ni foam. Applicability of the electrocatalyst was demonstrated by the construction of unbiased PV-EC water splitting system, where a high STH efficiency of 11.5% over 100 hours was achieved with the help of Si solar cells.In conclusion, we developed water splitting electrode containing hybrid interface of cobalt oxide and phosphide in sub-10 nm cube structure. The hybrid structure of H-PCO comprising sub-nanometer hetero-interface between cobalt oxide and phosphide evoluted synergetic effect that greatly improved either HER performance, pH indenpendency, or long-term stability, which outperformed single CO and CoP electrodes. We believe our rationally designed hetero-interfacial electrocatalyst can outperform previously established electrocatalysts via its high efficiency, versatility, and scalability.