Hetero-Layered Ni-Fe Hydroxide/Oxide Nanostructures Generated on Stainless-Steel Substrate for Efficient Alkaline Water Splitting.

Abstract

Highly active and inexpensive anode materials are required for large-scale hydrogen production using alkaline water electrolysis (AWE). Here, hetero-layered nanostructures of Ni-Fe hydroxide/oxide with a high activity for an oxygen evolution reaction (OER) were synthesized on a 316 stainless steel (SS) substrate through constant current density electrolysis. The thicknesses, morphologies, and compositions of the nanostructures, generated through dealloying and surface oxidation of the SS elements with severe oxygen micro-bubble evolution, were dependent on the electrolysis time. Nano-structural analyses showed that Ni-Fe hydroxide/oxide hetero-layered nanostructures were generated during the initial stage of electrolysis, growing nanofiber-like Ni-Fe hydroxide layers with increasing electrolysis time up to 5 h. The prolonged electrolysis resulted in densification of the nanofiber structures. The OER overpotential at 10 mA/cm2 was estimated to be 254 mV at 20 °C, demonstrating better performance than that of a standard OER catalyst, e.g., Ir oxide, and obtaining the value of Ni-Fe layered double hydroxide (LDH). Furthermore, the OER property surpassed the Ni-Fe LDH catalysts at high current density regions greater than 100 mA/cm2. Moreover, stable electrolysis was achieved for 20 h at conditions similar to that of the practical AWE of 400 mA/cm2 in 20 and 75 ℃ solution. Therefore, the simple surface modification method could synthesize highly active nanostructures for alkaline water splitting anodes.