Co-Catalyst Epitaxy on Semiconductor Support for Stable and Efficient Photoelectrochemical Water Splitting Under Concentrated Solar Light

Hydrogen (H₂) emerges as a clean energy solution produced through solar water splitting, providing a sustainable alternative to carbon-emitting fossil fuels. Over the years, many semiconductor photoelectrodes have effectively harnessed solar energy to produce green H₂ via photoelectrochemical (PEC) water splitting. These photoelectrodes, when exposed to sunlight, save voltage compared to electrocatalysts that operate in the dark. However, unlike electrochemical reactions, the maximum photocurrent density (J_ph) is limited by the amount of photogenerated charge carriers in the semiconductors, which restricts H₂ production rate. To overcome this, concentrated solar light can be used to increase the number of photogenerated charge carriers. This approach has been validated through the integration of photovoltaic-electrocatalyst (PV-EC) devices, which also help reduce the costs associated with semiconductor light absorbers, co-catalysts, and electricity. Despite the advancements in PV-EC systems for H₂ production, there hasn't been a detailed study on the performance and long-term stability of a photoelectrode that can simultaneously absorb light, excite electrons, separate charge carriers, and catalyze the hydrogen evolution reaction (HER) under concentrated solar light. Thus, fundamental studies are necessary to better understand the PEC HER processes and to develop more efficient and durable photoelectrodes capable of fully utilizing concentrated solar energy.<br/>Photoelectrodes are typically made by applying Pt co-catalysts onto semiconductor materials. While Pt co-catalysts are highly effective for the hydrogen evolution reaction (HER), their low adhesion has caused instability and limited the long-term functionality of photoelectrodes. Researchers have explored various solutions to enhance stability, such as using reduced graphene oxide binders, metal oxide overlayers, or hydrogel protection to encapsulate Pt NPs, preventing their detachment. These passivation layers prevent the agglomeration and detachment of Pt co-catalysts and suppressed the photocorrosion of photoelectrodes, highlighting the importance of considering both mechanical and chemical factors when designing photoelectrodes. However, these approaches can block active sites and hinder the mass transfer of reactants and products. There is still a significant need for innovative strategies to anchor Pt co-catalysts onto photoelectrodes for efficient and stable PEC water splitting, especially as the challenge may become more pronounced when using concentrated solar light to accelerate the H₂ production rate.<br/>In this study, we demonstrate a stable and efficient method for producing high-yield H₂ using Pt nanoparticles (NPs) decorated on GaN nanowires (NWs) grown on an n⁺-p Si photoelectrode. Under concentrated solar light at 640 mW/cm², the pristine Pt/GaN/Si system exhibited a high photocurrent density at 0 V vs. reversible hydrogen electrode (V_RHE) (J₀) of over 100 mA/cm². However, this performance degraded within 0.5 hours before stabilizing. The rate of performance degradation was significantly faster under concentrated solar light compared to conventional 100 mW/cm² illumination. Surface chemical and microstructure analysis revealed that concentrated solar light caused rapid surface modifications and the removal of some Pt NPs. However, Pt co-catalysts with an epitaxial relationship with GaN NWs remained strongly anchored on the surface, even after vigorous H₂ gas evolution. This finding led us to redeposit Pt NPs on the reacted surface of the photoelectrode, where more anchoring sites were available, resulting in enhanced HER activity and stability. This work identifies the stable bonding form of Pt NPs on single crystalline GaN NWs and elucidates strategies to develop efficient and durable photoelectrodes that operate under concentrated solar light.