Molybdenum boride and carbide catalyze hydrogen evolution in both acidic and basic solutions.

Sunlight-driven water splitting is one of the most attractive methods for solar energy conversion and storage. To achieve efficient water splitting, active catalysts for the hydrogen evolution reaction (HER) are required. Although platinum and a few other precious metals are highly active HER catalysts, their high cost and low abundance are severe hurdles for large-scale applications. The limitations of precious HER catalysts prompts the search for and development of non-precious catalysts. Molybdenum-based materials and molecules have drawn much attention recently. In particular, molybdenum sulfides have been shown to exhibit high HER activity. There is experimental and computational evidence that the active sites in these catalysts are located at the Mo edge, where there are unsaturated sulfur atoms. These sulfur atoms can adsorb hydrogen atoms and mediate hydrogen evolution. Herein, we report that molybdenum boride (MoB) and carbide (Mo2C) are active HER catalysts. To our knowledge, this is the first time MoB has been shown to be a HER catalyst. AlthoughMo2C has been shown to be a good support for Pt in HER, its own HER activity has not been studied in detail. Our results are interesting in several aspects: 1) The catalysts are non-precious and commercially available. They are also active under both acidic and basic conditions, which are rare properties for HER catalysts. 2) As the structures of molybdenum boride and carbide are different from that of MoS2, the activity of these catalysts is not easily explained by the presence of unsaturated edge sites. The results presented here might invoke new theoretical and experimental investigations of molybdenum-containing HER catalysts. 3) The work might inspire the development of molecular catalysts containing unusual boride and carbide ligands. MoB and Mo2C particles were purchased from commercial sources and their compositions were confirmed by inductively coupled plasma optical emission spectrometry (ICP-OES). XRD measurements showed that the MoB particles were mainly in the a-form (tetragonal), whereas theMo2C particles were in the b-form (hexagonal; Supporting Information, Figure S1). According to SEM images, the particle size is in the range of 1–3 mm (Figure S2). The MoB and Mo2C particles were deposited onto carbon-paste electrodes so that their HER activity could be measured by electrochemistry. An activation process was observed for MoB at pH ca. 0, and for Mo2C at pH ca. 0 and 14. During this activation process, the activity of a freshly-prepared electrode gradually increased over the first several polarization measurements (Figure S3). Normally the catalytic current became stable after five scans. No activation process was observed for MoB at pH ca. 14. Figure 1 shows the 10th (and thus stable) polarization curves of MoB and Mo2C-based electrodes at pH ca. 0 (1m H2SO4) and pH ca. 14 (1m KOH).