(Invited) Hydrogen Evolution from Water Using Modified Chalcogenide Photocathodes

Abstract

Copper chalcopyrite semiconductors of Cu(In,Ga)(S,Se)2 include a wide range of compounds that are of interest for photoelectrochemical (PEC) water splitting due to their band gaps in the range of 1.0 eV to 2.5 eV by varying the alloy ratio. The conduction band of the compounds is also well positioned above the reduction potential of water, enabling them to be used as photocathodes for H2 generation. Two of important factors required for photocathodes of PEC water splitting are photocurrent (photocurrent density) and onset potential (photovoltage), i.e., photocathodes having positive onset potentials coupled with high photocurrents are promised in order to induce water splitting by using a tandem system without applied bias energy. For chalcopyrite materials applied to the photocathode part, this can be achieved through band engineering by altering the composition of the alloy. In this study, wide-gap Cu(In,Ga)S2 (CIGS) films with various amounts of Ga were fabricated using spray pyrolysis followed by sulfurization. Photoelectrochemical water splitting properties of photocathodes based on these films were evaluated after modification with a CdS layer and Pt nanoparticles. Both photocurrent densities and onset potentials of the photocathodes were gradually improved by increase in the amount of Ga in the CIGS films up to a Ga/(In+Ga) ratio of 0.25 using 0.1 M Na2SO4 (pH 9) as an electrolyte under illumination of simulated sunlight (AM 1.5G). Further inclusion of Ga in the CIGS film was detrimental for both photocurrent density and onset potential. The maximum photocurrent density of 6.8 mA cm-2 (at 0 V vs. RHE) and the highest photocurrent onset potential of 0.84 V vs. RHE were obtained by using such photocathode. Achievements of a relatively wide interface band gap of the CIGS/CdS heterointerface and formation of relatively large grains in the CIGS with Ga/(In+Ga) ratio of 0.25 were likely to be responsible for such superior water reduction properties. The preliminary result of band engineering via replacement of Cu with Ag in the Cu(In,Ga)(S,Se)2 system in relation with its application as photocathode also will be presented.