Valence Alignment of Mixed Ni–Fe Hydroxide Electrocatalysts through Preferential Templating on Graphene Edges for Enhanced Oxygen Evolution

Abstract

Engineering the metal−carbon heterointerface has become an increasingly important route toward achieving cost-effective and highperforming electrocatalysts. The specific properties of graphene edge sites, such as the high available density of states and extended unpaired π-bonding, make it a promising candidate to tune the electronic properties of metal catalysts. However, to date, understanding and leveraging graphene edge− metal catalysts for improved electrocatalytic performance remains largely elusive. Herein, edge-rich vertical graphene (er-VG) was synthesized and used as a catalyst support for Ni−Fe hydroxides for the oxygen evolution reaction (OER). The hybrid Ni−Fe/er-VG catalyst exhibits excellent OER performance with a mass current of 4051 A g −1 (at overpotential η = 300 mV) and turnover frequency (TOF) of 4.8 s −1 (η = 400 mV), outperforming Ni−Fe deposited on pristine VG and other metal foam supports. Angle-dependent X-ray absorption spectroscopy shows that the edge-rich VG support can preferentially template Fe−O units with a specific valence orbital alignment interacting with the unoccupied density of states on the graphene edges. This graphene edge−metal interaction was shown to facilitate the formation of undersaturated and strained Fe-sites with high valence states, while promoting the formation of redox-activated Ni species, thus improving OER performance. These findings demonstrate rational design of the graphene edge−metal interface in electrocatalysts which can be used for various energy conversion and chemical synthesis reactions.

Carbon-Based Composites as Electrocatalysts for Oxygen Evolution Reaction in Alkaline Media

“…To modify pristine vertical graphene, directly after the growth of the material, the plasma chamber was degassed and pumped down to ~10 −3 Pa. Argon was introduced again to the hot chamber and the VG was post-treated for 5 min. As a result, edge-rich vertical graphene was produced as support for the electrochemically active phase [ 80 ]. To form a nanoarray-structurization and N-doping before use as a support for NiFe-P particles, commercial carbon paper was cleaned in ethanol, acetone, and water.…”

“…Potentiostatic electrodeposition was sustained for 100 s with an applied potential of 1.1 V vs. Ag/AgCl using a graphite rod counter electrode. After electrodeposition, the samples were washed thoroughly in water and ethanol [ 80 ].…”

“…From the point of view of the carbon component, the highest reported activities in OER are, in general, obtained for the composites with graphene or reduced graphene oxide [ 80 , 100 ], and N or S,N-co-doped graphene materials [ 96 , 150 , 151 , 152 ]. Interestingly, similarly high reactivities can be obtained with composites built with amorphous carbons without [ 147 , 153 ], or with alien atom doping [ 139 , 154 ].…”

“…This specific interaction of the graphene edge–metal component resulted in the formation of beneficial undersaturated and strained Fe-sites with high valence states, and at the same time, it boosted the formation of redox-activated Ni species. These two effects resulted in an improved OER reactivity [ 80 ]. For the Ni@Pt core–shell nanoplates and Ni/rGO composites, the overpotential of Ni/rGO is lower than pure Ni and the enhanced performance of Ni/rGO was ascribed to the possible effect of the rGO coatings.…”

“…Various carbon structures are used, such as rGO or CNTs, usually doped with nitrogen to provide the desired electrical properties. Another common trait of the well-performing materials is the ability to stabilize high valence states of the TM cations in the active phase [ 80 , 108 , 109 ].…”