Enhanced hydrogen evolution reactivity on $${\mathrm{Mo}}_2{\mathrm{C}}$$ Mo
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Bull Mater Sci (2020)43:321 https://doi.org/10.1007/s12034-020-02230-3
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Enhanced hydrogen evolution reactivity on Mo2 C–Mo2 N composites DEBDIPTO ACHARYA1, , KASINATH OJHA2, , NISHA MAMMEN1,3, PREETI DAGAR4, SOURAV MONDAL1, ASHOK K GANGULI2,4 and SHOBHANA NARASIMHAN1,* 1
School of Advanced Materials and Theoretical Sciences Unit, Jawaharlal Nehru Centre for Advanced Scientific Research, Bangalore 560064, India 2 Department of Chemistry, Indian Institute of Technology Delhi, Hauz Khas, New Delhi 110016, India 3 Department of Physics, Nanoscience Center, University of Jyva¨skyla¨, Jyva¨skyla¨ 40014, Finland 4 Institute of Nano Science and Technology, Mohali, Punjab 160062, India *Author for correspondence ([email protected]) These authors contributed equally to this study. MS received 31 December 2019; accepted 20 March 2020 Abstract. We have studied the electrolysis of water, by performing a combined experimental and theoretical study of the hydrogen evolution reaction (HER) capability of Mo2 C–Mo2 N composites. Experimentally, we have synthesized nanowires with varying Mo2 C:Mo2 N ratios. We have found that the composites show good HER activity in an acidic medium, that is superior to that of either pristine Mo2 C or Mo2 N. These experimental results are supported by ab initio density functional theory calculations. Interestingly, we find that it is vital to incorporate van der Waals corrections to accurately reproduce the experimentally observed structural transition from an orthorhombic to tetragonal phase as x, the N concentration in Mo2 C1x Nx , is increased. By computing Gibbs free energy for H adsorption on Mo2 C1x Nx surfaces, our calculations confirm the experimental finding that mixed systems have superior HER activity to pristine systems, with N-rich systems being most active. Keywords. Water splitting; hydrogen evolution; molybdenum carbide; molybdenum nitride; density functional theory; electrolysis.
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Introduction
The growing concern regarding pollution and global warming caused by fossil fuel combustion has led to an acceleration in research towards finding alternative renewable and sustainable energy sources [1,2]. Hydrogen is a promising alternative to fossil fuels, as it has the highest energy density per unit mass, and water is the only byproduct formed when hydrogen is combusted in an engine or transformed into electricity in a fuel cell. Even though hydrogen is known to be the most abundant element in the universe, it is not easily found in its free molecular form on Earth, and needs to be extracted from hydrocarbons or through electrolysis of water. Electrochemical water splitting is divided into two halfcell reactions: hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). HER is the cathodic reaction (2Hþ þ 2e ! H2 ) and OER is the anodic reaction
(2H2 O ! O2 þ 4Hþ þ 4e ). Both HER and OER require catalysts to lower electrochemical overpotential. State-of-the-art HER electrocatalysts are mainly Pt-based mat
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