Hydrogen-evolution characteristics of Ni-Mo-coated, radial junction, n+p-silicon microwire array photocathodes

The photocathodic H-2-evolution performance of Ni-Mo-coated radial n(+)p junction Si microwire (Si MW) arrays has been evaluated on the basis of thermodynamic energy-conversion efficiency as well as solar cell figures of merit. The Ni-Mo-coated n(+)p-Si MW electrodes yielded open-circuit photovoltages (V-oc) of 0.46 V, short-circuit photocurrent densities (J(sc)) of 9.1 mA cm(-2), and thermodynamically based energy-conversion efficiencies (eta) of 1.9% under simulated 1 Sun illumination. Under nominally the same conditions, the efficiency of the Ni-Mo-coated system was comparable to that of Pt-coated n(+)p-Si MW array photocathodes (V-oc = 0.44 V, J(sc) = 13.2 mA cm(-2), eta = 2.7%). This demonstrates that, at 1 Sun light intensity on high surface area microwire arrays, earth-abundant electrocatalysts can provide performance comparable to noble-metal catalysts for photoelectrochemical hydrogen evolution. The formation of an emitter layer on the microwires yielded significant improvements in the open-circuit voltage of the microwire-array-based photocathodes relative to Si MW arrays that did not have a buried n(+)p junction. Analysis of the spectral response and light-intensity dependence of these devices allowed for optimization of the catalyst loading and photocurrent density. The microwire arrays were also removed from the substrate to create flexible, hydrogen-evolving membranes that have potential for use in a solar water-splitting device.