Electrocatalytic Performance and Stability of Nanostructured Fe–Ni Pyrite-Type Diphosphide Catalyst Supported on Carbon Paper

A simple and effective method to prepare an active and stable nanostructured working electrode for electrochemical water splitting is described. Specifically, mixed Fe–Ni diphosphide was prepared by sputtering a 200-nm-thick layer of Permalloy onto carbon paper gas diffusion layer followed by gas transport phosphorization reaction. The mass density of the resultant diphosphide phase was established to be ≈1.1 mg/cm 2 . Energy-dispersive X-ray microanalysis shows that the actual elemental composition of the resultant ternary electrocatalyst is approximately Fe 0.2 Ni 0.8 P 2, while the powder X-ray diffraction analysis confirms that the electrocatalyst crystallizes in NiP 2 cubic pyrite-like structure. As a cathode for hydrogen evolution reaction (HER) in acidic and alkaline electrolytes, this earth-abundant electrode has exchange current densities of 6.84 × 10 –3 and 3.16 × 10 –3 mA/cm 2 and Tafel slopes of 55.3 and 72.2 mV/dec, respectively. As an anode for oxygen evolution reaction (OER) in alkaline electrolyte, the electrode shows an exchange current density of 2.88 × 10 –4 mA/cm 2 and Tafel slope of 49.3 mV/dec. The observed high activity of the electrode correlates well with its electronic structure, which was assessed by density functional theory (DFT) calculations. The stability of Fe 0.2 Ni 0.8 P 2 electrocatalyst in HER and OER was evaluated by means of accelerated degradation test and chronopotentiometry. The results of these experiments elucidate partial dissolution and entire chemical transformation of Fe 0.2 Ni 0.8 P 2 as the main mechanisms of the electrode degradation during HER and OER, respectively. Overall, our findings could facilitate the composition-based design of active, stable, and durable phosphide electrodes for electrochemical water splitting.