Valence Alignment of Mixed Ni–Fe Hydroxide Electrocatalysts through Preferential Templating on Graphene Edges for Enhanced Oxygen Evolution
Constantine Tsounis, Xunyu Lu, Nicholas M. Bedford, Bijil Subhash, Lars Thomsen, Qingran Zhang, Zhipeng Ma, Kostya Ostrikov, Avi Bendavid, Jason Scott, Rose Amal, Zhaojun Han
Abstract
Engineering the metal–carbon heterointerface has become an increasingly important route toward achieving cost-effective and high-performing 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.