Solar Fuels: Visible-Light-Driven Generation of Dihydrogen at p-Type Silicon Electrocatalysed by Molybdenum Hydrides

Solar Fuels: Visible-Light-Driven Generation of Dihydrogen at p-Type Silicon Electrocatalysed by Molybdenum Hydrides:

Abstract

We show that a robust molybdenum hydride system can sustain photoelectrocatalysis of a hydrogen evolution reaction at boron-doped, hydrogen-terminated, p-type silicon. The photovoltage for the system is about 600–650 mV and the current densities, which can be sustained at the photocathode in non-catalytic and catalytic regimes, are similar to those at a photoinert vitreous carbon electrode. The kinetics of electrocatalysed hydrogen evolution at the photocathode are also very similar to those measured at vitreous carbon—evidently visible light does not significantly perturb the catalytic mechanism. Importantly, we show that the doped (1–10 Ω cm) p-type Si can function perfectly well in the dark as an ohmic conductor and this has allowed direct comparison of the cyclic voltammetric behaviour of the response of the system under dark and illuminated conditions at the same electrode. The p-type Si we have employed optimally harvests light energy in the 600–700 nm region and with 37 mW cm⁻² illumination in this range; the light to electrochemical energy conversion is estimated to be 2.8 %. The current yield of hydrogen under broad tungsten halide lamp illumination at 90 mW cm⁻² is (91±5) % with a corresponding chemical yield of (98±5) %.

In the light or dark with Moly at p-type Si: A molybdenum hydride/p-type Si system provides a stable platform for probing photoredox and photoelectrocatalytic behaviour. Whilst the kinetics of hydrogen evolution are slow, the Mo hydride catalyst offers options for considerable molecular tailoring. Cyclic voltammetry of an Mo electrocatalyst at the same p-type Si electrode in the dark (blue trace) and light (red trace) show a 640 mV photovoltage gain.