Amorphous molybdenum sulfide films are efficient hydrogen evolution catalysts in water. The films are prepared via simple electro-polymerization procedures and are characterized by XPS, electron microscopy and electronic absorption spectroscopy. Whereas the precatalysts could be MoS 3 or MoS 2 , the active form of the catalysts is identified as amorphous MoS 2 . Significant geometric current densities are achieved at low overpotentials (e.g., 15 mA cm À2 at h ¼ 200 mV) using these catalysts. The catalysis is compatible with a wide range of pHs (e.g., 0 to 13). The current efficiency for hydrogen production is quantitative. A 40 mV Tafel slope is observed, suggesting a rate-determining ion+atom step. The turnover frequency per active site is calculated. The amorphous molybdenum sulfide films are among the most active non-precious hydrogen evolution catalysts.

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Molybdenum sulfide materials have been shown as promising non-precious catalysts for hydrogen evolution. This paper describes the study of the promotional effects of certain transition metal ions on the activity of amorphous MoS 3 films. Ternary metal sulfide films, M-MoS 3 (M ¼ Mn, Fe, Co, Ni, Cu, Zn), have been prepared by cyclic voltammetry of aqueous solutions containing MCl 2 and (NH 4 ) 2 [MoS 4 ]. Whereas the Mn-, Cu-, and Zn-MoS 3 films show similar or only slightly higher catalytic activity as the MoS 3 film, the Fe-, Co-, and Ni-MoS 3 films are significantly more active. The promotional effects of Fe, Co, and Ni ions exist under both acidic and neutral conditions, but the effects are more pronounced under neutral conditions. Up to a 12-fold increase in exchange current density and a 10-fold increase in the current density at an overpotential of 150 mV are observed at pH ¼ 7. It is shown that Fe, Co, and Ni ions promote the growth of the MoS 3 films, resulting a high surface area and a higher catalyst loading. These changes are the main contributors to the enhanced activity at pH ¼ 0. However, at pH ¼ 7, Fe, Co, and Ni ions appear to also increase the intrinsic activity of the MoS 3 film.

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S2 Experimental Section Physical measurements Electrochemical measurements were recorded on an EG&G Princeton AppliedResearch Potentiostat/Galvanostat model 273. EQCM measurements were performed with home-made equipment using a home-made potentiostat (Wenking-type). The home-made potentiostat was connected to an EG&G 175 Universal Programmer or a TTI Function/Arbitrary/Pulse generator model TG2511. A three-electrode configuration was used.For polarization and electrolysis measurements, a platinum wire was used as the counter electrode and a home-made Ag/AgCl (KCl saturated) electrode was used as the reference electrode. The reference electrode was placed in a position very close to the working electrode, often with the aid of a Luggin tube. In case of electrochemical film deposition, a fresh titanium wire was used as the counter electrode. Potentials were referenced to a standard hydrogen electrode (SHE) or reversible hydrogen electrode (RHE), respectively, by adding a value of (0.197+0.059·pH) V. Gas chromatography measurements were conducted on a Perkin-Elmer Clarus 400 GC with a TCD detector and a 5 Å molecular sieves packed column or on a home-made gas analyzer equipped with a Valco microvolume dual filament TCD detector and a HayeSep DB 100/120 mesh, 30 ft. long, 2mm internal diameter, stainless steel packed column (part number G3591-80088 -Agilent Technologies). UV-Vis spectra were recorded using an Ocean Optics USB 2000+ spectrophotometer connected to a homebuilt dual lamp (D 2 and tungsten filament lamps) optic fiber light source. X-ray photoelectron spectroscopy (XPS) data were collected by an Axis Ultra (Kratos Analytical, Manchester, UK) under ultra-high vacuum condition (>10 -8 Torr), using a monochromatic Al K  X-ray source (1486.6 eV), in the Surface Analysis Laboratory of CIME at EPFL. The source power was maintained at 150W (10 mA, 15 kV) Gold (Au 4f 7/2 ) and copper (Cu 2p 3/2 ) lines at 84.0 and 932.6 eV, respectively, were used for calibration, and the adventitious carbon 1s peak at 284.8 eV as an internal standard to compensate for any charging effects. After a Shirley background subtraction, the spectra were fitted with Gaussian/Lorentzian product functions.