Broader Context

Abstract

We report superior hydrogen evolution activity of metal-free boro-carbonitride (BCN) catalysts. The highly positive onset potential (À56 mV vs. RHE) and the current density of 10 mA cm À2 at an overpotential of 70 mV exhibited by a carbon-rich BCN with the composition BC 7 N 2 demonstrates the extraordinary electrocatalytic activity at par with Pt. Theoretical studies throw light on the cause of high activity of this composition. The high activity and good stability of BCNs surpass the characteristics of other metal-free catalysts reported in recent literature. Generation of hydrogen from water is potentially an important means of using solar energy for the benefit of mankind. The hydrogen evolution reaction (HER) is not only a vital part of electrochemical water splitting but also provides a way to understand the underlying mechanism of electron transfer processes in electrocatalysis. It is well known that platinum supported on carbon exhibits very good electrocatalytic HER activity. 1-6 Other transition metal catalysts have also been tried for HER, 7-17 but it would be ideal to have a metal-free catalyst for the purpose, partly because of the scarcity and high cost of Pt. 18 Most of the photocatalysts and electrocatalysts reported so far suffer from low quantum efficiency as well as stability. A composite of carbon nitride (C 3 N 4) and nitrogen-doped graphene (NG) has recently been reported to possess unique properties for successful electrocatalytic H 2 production. 19 Films of porous C 3 N 4 layers with NG have also been shown to display excellent electrochemical HER performance with a highly positive onset potential 20 and high exchange current density and stability comparable to platinum. The use of bimetallic core-shell electro-catalysts with carbonitrides for fuel cell applications has also been reported. 21-23 Other materials which have shown promising electrocatalytic HER activity are MoS 2 nanoparticles grown on graphene 24 and Mo 2 C-carbon nanocomposites. 25 We considered it most appropriate to investigate the electrochemical HER activity of borocarbonitrides, B x C y N z , which have been shown to have impressive surface and catalytic properties 26,27 and are also low cost materials. These materials are nanoplatelets containing graphene and BN domains, possibly along with BCN rings. 28-30 They contain B-C, B-N, C-N, CC bonds but no B-B and N-N bonds. They would have defect sites (e.g. sp 3-C, Stone-Wales defects) in the carbon network which can act as active sites (as nucleophiles for H + ions) for electron transfer reactions. B x C y N z is different from B, N-codoped graphene in some ways such as thermal stability and the presence of covalent BN domains in the carbon matrix. Although excess of