Operando synthesis of macroporous molybdenum diselenide films for electrocatalysis of the hydrogen-evolution reaction

The catalytically inactive components of a film have been converted, through an operando method of synthesis, to produce a catalyst for the reaction that the film is catalyzing. Specifically, thin films of molybdenum diselenide have been synthesized using a two-step wet-chemical method, in which excess sodium selenide was first added to a solution of ammonium heptamolydbate in aqueous sulfuric acid, resulting in the spontaneous formation of a black precipitate that contained molybdenum triselenide (MoSe₃), molybdenum trioxide (MoO₃), and elemental selenium. After purification and after the film had been drop cast onto a glassy carbon electrode, a reductive potential was applied to the precipitate-coated electrode. Hydrogen evolution occurred within the range of potentials applied to the electrode, but during the initial voltammetric cycle, an overpotential of 400 mV was required to drive the hydrogen-evolution reaction at a benchmark current density of 10 mA cm^-2. The overpotential required to evolve hydrogen at the benchmark rate progressively decreased with subsequent voltammetry cycles, until a steady state was reached at which only 250 mV of overpotential was required to pass 10 mA cm^-2 of current density. During the electrocatalysis, the catalytically inactive components in the as-prepared film were (reductively) converted to MoSe₂ through an operando method of synthesis of the hydrogen-evolution catalyst. The initial film prepared from the precipitate was smooth, but the converted film was completely covered with pores 200 nm in diameter. The porous MoSe₂ film was stable while being assessed by cyclic voltammetry for 48 h, and the overpotential required to sustain 10 mA cm^-2 of hydrogen evolution increased by <50 mV over this period of operation.