Structural evolution and durability of ultrafine NiFe phosphide nanoparticle/carbon composite films in water oxidation at high current densities

Abstract

NiFe phosphide (NiFe-P) is a highly active precatalyst for the oxygen evolution reaction (OER), but its compositional and structural changes during sustained electrolysis have not been thoroughly understood. Moreover, the size control of NiFe-P particles remains challenging yet desirable without multistep synthesis or surface capping agents. To realize this, flame aerosol synthesis (FAS) is a promising method due to its short particle residence time, tunable redox environment, and good scalability. Herein, the one-step FAS of NiFe-P is reported for the first time. With the controlled coformation of carbon, interlaced NiFe-P nanoplates and <5 nm NiFe-P nanoparticles are selectively synthesized on nickel foam in 10 min. At the anodic potential of OER, NiFe-P transforms into highly active (oxy)hydroxides in situ by surface oxidation and dephosphorylation. The overpotential of the optimal film at 500 mA cm-2 increases at only 0.28 mV h-1 over 100 h, making it among the most durable NiFe-based catalysts reported. Post-OER cyclic voltammetry, double-layer capacitance, and inductively coupled plasma mass spectrometry (ICP-MS) measurements indicate performance degradation to be mainly caused by the selective leaching of Fe from the (oxy)hydroxides. Counterintuitively, slight structural instability of the films induced by the electrochemical removal of carbon enhances durability by keeping a relatively stable surface Fe content through a self-refreshing mechanism.