Rational Design of Graphene-Supported Single Atom Catalysts for Hydrogen Evolution Reaction

TLDR: In this paper, an activity correlation with catalysts, electronic structure, in order to clarify the origin of reactivity for a series of transition metals supported on nitrogen-doped graphene as SACs for hydrogen evolution reaction (HER) by a combination of density functional theory calculations and electrochemical measurements.

Abstract: DOI: 10.1002/aenm.201803689 On the other hand, the limited fossil fuel reserves coupled with sustainable development vision call for the development of new green technologies for energy production.[1] Hydrogen, an abundant, renewable, and highly dense energy source, has been considered as a potential alternative sustainable energy source.[2] The ideal way to produce hydrogen of high purity and in large quantities is by the electrolytic reduction of water via hydrogen evolution reaction (HER). Naturally, HER has a high energy barrier (known as overpotential, ɳ, the minimum potential required to produce hydrogen above its thermodynamic value), which demands effective catalysts to overcome. Amongst all HER catalysts, platinum is the most efficient to date with a small overpotential in acidic solutions. However, the high cost and scarcity of platinum limit its application for industrial production of hydrogen.[3] Thus, the proper choice of an active, efficient, and durable electrocatalyst from earth’s abundant sources remains a major challenge in energy research. In recent years, tremendous effort has been devoted to the invention of new types of heterogeneous electrocatalysts, based on a variety of nonprecious transition metals, including Co, Ni, Mo, Fe, and their derivatives (i.e., nitrides, The proper choice of nonprecious transition metals as single atom catalysts (SACs) remains unclear for designing highly efficient electrocatalysts for hydrogen evolution reaction (HER). Herein, reported is an activity correlation with catalysts, electronic structure, in order to clarify the origin of reactivity for a series of transition metals supported on nitrogen-doped graphene as SACs for HER by a combination of density functional theory calculations and electrochemical measurements. Only few of the transition metals (e.g., Co, Cr, Fe, Rh, and V) as SACs show good catalytic activity toward HER as their Gibbs free energies are varied between the range of –0.20 to 0.30 eV but among which Co-SAC exhibits the highest electrochemical activity at 0.13 eV. Electronic structure studies show that the energy states of active valence dz orbitals and their resulting antibonding state determine the catalytic activity for HER. The fact that the antibonding state orbital is neither completely empty nor fully filled in the case of Co-SAC is the main reason for its ideal hydrogen adsorption energy. Moreover, the electrochemical measurement shows that Co-SAC exhibits a superior hydrogen evolution activity over Ni-SAC and W-SAC, confirming the theoretical calculation. This systematic study gives a fundamental understanding about the design of highly efficient SACs for HER.