Partially amorphized MnMoO 4 for highly efficient energy storage and the hydrogen evolution reaction† Xiaodong Yan, a Lihong Tian, ab James Murowchick c and Xiaobo Chen * a Engineering the crystallinity of materials has been proved to be an efficient strategy to improve the material’s properties in many appli- cations. Herein, we demonstrate the successful transformation of electrochemically inert MnMoO 4 into highly active bifunctional electrode materials for supercapacitors and as catalysts for the hydrogen evolution reaction through hydrogenation (hydrogen reduction at elevated temperatures). The hydrogenated MnMoO 4 is partially amorphized with a one-fold increase in the electrochemically active surface area (ECSA). A 17-fold increase in specific capacitance is achieved, and the onset overpotential to drive the hydrogen evolution reaction markedly decreased to 105 mV from 194 mV. The highly enhanced electrochemical properties are likely due to the amorphous components and highly enhanced ECSA, which expose more active sites and enhance the charge-transfer kinetics on the surface. Introduction Solar and wind energies are among the most desired renewable energy sources to compensate the depletion of fossil fuels and even to completely replace them in the future. However, the intermittency of solar and wind cannot provide a stable and round-the-clock energy supply. Conversion of solar and wind energies into electricity may hold the hope of a feasible pathway to their utilization wherever and whenever energy is needed. Electrical energy storage/conversion devices are also needed for the transportation and utilization of electricity. Supercapacitors are regarded as one of such promising electrical energy storage/ conversion devices due to their long cycle lifetimes, rapid charge–discharge capabilities and high power densities. 1–5 Nevertheless, commercial supercapacitors, usually composed of activated carbons, are still restricted to only a few niche markets due to their limited energy densities. 1,6 Therefore, new electrode materials with high specic capacitances have been intensively explored. For instance, manganese oxide, 7–11 cobalt oxide, 12–15 and functionalized nanostructured carbons 16–19 have recently attracted much attention owing to their tuneable electro- chemical activities. Mixed metal oxides such as MMoO 4 (M ¼ Ni, Co and Mn) are also of high interest to be investigated as supercapacitor electrode materials. 20–23 Among them, MnMoO 4 seems to be less attractive due to its relatively low specic capacitance. 20–22 Structural modication may provide an opportunity to tune the capacitive properties of MnMoO 4 . Recently, hydrogenation or heat treatment under a hydrogen environment at elevated temperatures has been shown as a powerful approach in modifying the properties of different nanomaterials. For example, the optical, electronic, electrical and photocatalytic properties of titanium dioxide nano- materials have been largely altered by hydrogenation towards various applications, 24–26 and the capacitive properties of hydrogenated MnO 2 , MoO 3 and TiO 2 have been remarkably enhanced. 11,27,28 Therefore, hydrogenation treatment may be a good approach to improve the capacitive properties of MnMoO 4 . Another promising way to overcome the intermittency of solar and wind energies is to convert them into hydrogen through photocatalytic and electrolytic water splitting. 29–33 Although great progress has been achieved, 29,34,35 photocatalytic water splitting is still far from practical application due to its low solar-to-hydrogen conversion efficiency. Water electrolysis is regarded as a feasible means to produce hydrogen on a large scale. One of the key components for water electrolysis to produce hydrogen are the electrocatalysts used for the hydrogen evolution reaction (HER). The state-of-the-art HER catalyst is Pt, with the drawbacks of scarcity and high cost. Thus, earth- abundant, low-cost and highly active electrocatalysts are highly preferred. 32,33 So far, transition metal based catalysts, such as transition metal dichalcogenides, 36–38 lithium transition metal a Department of Chemistry, University of Missouri – Kansas City, Kansas City, Missouri, USA. E-mail: chenxiaobo@umkc.edu b Hubei Collaborative Innovation Center for Advanced Organochemical Materials, Ministry-of-Education Key Laboratory for the Synthesis and Applications of Organic Functional Molecules, Hubei University, Wuhan, Hubei 430062, China c Department of Geosciences, University of Missouri – Kansas City, Kansas City, Missouri 64110, USA † Electronic supplementary information (ESI) available: Detailed experimental procedures and supplementary gures. See DOI: 10.1039/c6ta00744a Cite this: J. Mater. Chem. A, 2016, 4, 3683 Received 26th January 2016 Accepted 15th February 2016 DOI: 10.1039/c6ta00744a www.rsc.org/MaterialsA This journal is © The Royal Society of Chemistry 2016 J. Mater. Chem. A, 2016, 4, 3683–3688 | 3683 Journal of Materials Chemistry A COMMUNICATION Published on 15 February 2016. Downloaded by Yale University Library on 01/03/2016 17:35:00. View Article Online View Journal | View Issue