Failure Modes of Platinized pn⁺-GaInP Photocathodes for Solar-Driven H₂ Evolution

The long-term stability for the hydrogen-evolution reaction (HER) of homojunction pn⁺-Ga_0.52In_0.48P photocathodes (band gap = 1.8 eV) with an electrodeposited Pt catalyst (pn⁺-GaInP/Pt) has been systematically evaluated in both acidic and alkaline electrolytes. Electrode dissolution during chronoamperometry was correlated with changes over time in the current density-potential (J-E) behavior to reveal the underlying failure mechanism. Pristine pn⁺-GaInP/Pt photocathodes yielded an open-circuit photopotential (E_oc) as positive as >1.0 V vs the potential of the reversible hydrogen electrode (RHE) and a light-limited current density (J_ph) of >12 mA cm^-2 (1-sun illumination). However, E_oc and J_ph gradually degraded at either pH 0 or pH 14. The performance degradation was attributed to three different failure modes: (1) gradual thinning of the n⁺-emitter layer due to GaInP dissolution in acid; (2) active corrosion of the underlying GaAs substrate at positive potentials causing delamination of the upper GaInP epilayers; and (3) direct GaAs/electrolyte contact compromising the operational stability of the device. This work reveals the importance of both substrate stability and structural integrity of integrated photoelectrodes toward stable solar fuel generation.