A Hybrid Material Based on Molybdenum Carbide and Nitrogen Doped Carbon for Electrochemical Hydrogen Evolution

Abstract

The increase in the global consumption of fossil fuel has adverse impact on the economy and environment. Alternate clean and sustainable energy sources are necessary to meet the future energy requirement. Hydrogen is a promising eco-friendly fuel alternative to the traditional fossil fuel. Hydrogen can be generated by photochemical, electrochemical and photoelectrochemical methods using suitable catalyst. Pt-based catalysts have been conventionally used for the electrochemical hydrogen evolution. However, the use of such catalyst is not encouraged due to high cost and scarcity of Pt. In recent years, several less expensive and earth-abundant electrocatalysts, such as transition metal chalcogenides, phosphides, nitrides and carbides have drawn much attention.1-2 Among them, transition metal carbide, particularly molybdenum carbide (Mo2C) received enormous attention. The electrocatalytic activity of Mo2C can be tuned by controlling the electronic structure, shapes, size and morphology. Achieving high catalytic activity at low overpotential is a main challenge with metal carbide-based electrocatalysts. We demonstrate a new chemical route for the shape-controlled synthesis of a hybrid material derived from N-doped carbon and Mo2C (Mo2C-N-C) for the electrochemical hydrogen evolution reaction (HER) in acidic and basic medium. The synthetic procedure involves the facile complexation of Mo-precursor with cationic polymer, poly(diallyldimethyammonim chloride) (PDDA) and the subsequent carburization at 800 0C under optimized condition. The hybrid material was characterized by analytical techniques including XRD, XPS, Raman, TEM, etc. Electron microscopic measurements evidence the growth of nanowire bundles with length of 2-3 µm. The electrocatalytic activity was evaluated by recording the polarization curves in 0.5 M H2SO4 and 1 M KOH. We could achieve the benchmark current density of 10 mA/cm2 in acidic and alkaline pH at an overpotential of 136 and 183 mV, respectively. The catalytic performance is comparable to the existing Mo2C-based catalysts.3 The catalytic activity was further evaluated by calculating the Tafel analysis. A Tafel slope of 56 and 61 mV/dec was observed in acidic and alkaline pH, respectively. The Tafel analysis suggests that the electroctalytic hydrogen evolution involves Volmer-Heyrovsky mechanism. The hybrid material is highly durable and it retains its initial activity even after 1000 cycles. The superior performance can be explained by considering the surface morpholgy of Mo2C as well as the synergistic effect between the carbide catalyst and N-doped carbon.