NiFe inverse opal electrocatalysts for the efficient oxygen evolution reaction

Abstract

NiFe alloy is one of the high efficient bifunctional electrocatalysts for water splitting in an alkaline solution. Unlike the noble metal such as Ir and Ru oxide, NiFe is earth abundance, low-cost, high-stability in an alkaline solution. Thanks to these advantages, nanostructured NiFe electrocatalysts for highly enhanced oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) performance have been developed constantly. However, few studies about the correlation between surface area of nanostructures and OER enhancement have been carried out. In this research, we fabricated nano porous NiFe catalysts using inverse opal structures for further enhanced OER performance and to find out the correlation between increased surface area of nanostructures and enhanced OER activity. Overpotential for OER of NiFe inverse opal structures is reduced at ~77 mV compared to NiFe planar structures. And strangely OER enhancement of 10 layers is rather decreasing compared to its 5 layers. We think that it is a problem from bubble overpotential and the distance mismatch between diffusion length of ions in water and size of nanostructures. Also, we optimized thickness layers of NiFe inverse opal structures by comparing OER and HER performance. Optimized NiFe inverse opal electrolyzer showed the dramatic enhancement that it reduces ~200 mV overpotential for overall water splitting. Furthermore, for application to solar water splitting, we fabricated the unassisted water splitting device consisted of NiFe inverse opal electrolyzer and series-connected heterojunction with intrinsic thin-layer (HIT) Si cells which exhibits the 9.54 % solar-to-hydrogen (STH) efficiency. In addition, this device is maintained at ~9.54 % without any degradation for 24 hours. NiFe inverse opal electrolyzer and series connected HIT Si solar cells are suitable configuration for ideal water splitting system because it is composed of only earth-abundant and low-cost materials with high efficiency.