Design And Synthesis Of Cu?In,Ga?Se₂ Photocathodes For Solar Water Splitting

Among various types of new energy resources,solar energy has been considered as one of the most promising choices.Recently,there has been signicant interestes in photoelectrochemical?PEC?water splitting using semiconductor photoelectrodes,since this represents a promising means of directly converting solar energy into chemical energy by producing hydrogen.Cu?In,Ga?Se₂?CIGS?has drawn great concern as a promising material for PEC hydrogen evolution reaction?HER?.CIGS has controllable bandgap values by tuning the composition,offering great flexibility in forming favorable energy band positions.However,the requirement of CIGS photocathodes is much more demanding than that of the solar cell counterparts,mainly due to the difficulty of charge transfer and surface reaction at the electrode/electrolyte interface,as well as the instability of CIGS electrode in aqueous solutions.Therefore,it is urgently demanded to enhance the PEC performance of CIGS photocathodes.Firstly,taking CIGS/CdS/TiO₂ as a prototype,we propose an original two-step platinization method to independently control the spatial dispersion and particle size of Pt nano-particles.When only photoelectrodeposition?PED?or sputter process is conducted,the CIGS electrode shows poor PEC performance.When the two-step platinization method is applied,the fill factor of CIGS electrode is improved from 10%to 34.6%and a photocurrent of 25 mA cm^-2 at 0 V_RHE under AM 1.5G.This two-step platinization method possesses universality and can be applied in many other photocathodes to improve their PEC performance.Secondly,we passivate the defect sites at the CdS/TiO₂ interface by an ultrathin Al₂O₃ layer,thus facilitating electron transport in the bulk of electrode.Upon Al₂O₃passivation,the electrode achieves an ABPE of 6.6%in neutral electrolyte?1 M phosphate buffer,pH 6.8?with a long-term stability up to 8 h.When applied in acidic electrolyte?1 M HClO₄,pH 0?,this photocathode achieves an ABPE value of 9.3%with nearly no deactivation in 2 h duration.This is the best PEC performance ever reported among chalcopyrite-based photocathodes.Finally,we construct a Cd-free CIGS photocathode to further increase the stability of electrode and photocurrent density in electrolyte.TiO₂ is chosen as the promising candidate to decrease parasitic absorption.Al₂O₃ passivation layer together with the post annealing process can further optimize the interface between CIGS and TiO₂,thus realizing a saturated photocurrent of 30 mA cm^-2 with a long-term stability up to 30 h in acidic electrolyte.