Concentrated irradiation enables the production of solar fuels using smaller and inexpensive devices, resulting in decreased costs for solar fuel production. Additionally, by carefully selecting optimal operating conditions, it might be possible to enhance the performance of photoelectrochemical (PEC) devices [1]. A large proportion of the research on semiconducting materials for solar-driven water splitting has focused on their behaviour at room temperature and under 1 sun illumination. Hence, the utilization of high photon fluxes presents new possibilities for solar fuel production, as it has been demonstrated for integrated PV+EC devices [2]. Nevertheless, many obstacles hinder the design and testing of PEC cells intended for operation under concentrated irradiation. The potentially advantageous synergy between higher temperatures and irradiation makes the system complex and difficult to study.

In that regard, the effect of temperature alone on the properties of photoelectrodes has been reported recently for α-Fe₂O₃ [3], showing increased photocurrent densities at high potentials and increased recombination rates at low electrode potentials. In a similar study, BiVO₄ photoelectrodes have shown substantial increase in photocurrent densities across all electrode potentials [4]. The behaviour of photoelectrodes under high irradiations has been performed under relatively low irradiances (<30 kW m^‑2, c.f. 1 sun ≈ 1 kW m^-2) and usually for very stable photoelectrodes, e.g. Fe₂O₃ [5].

The combined effects of temperature and high irradiations on the properties of photoelectrode materials has been rarely investigated experimentally. With the aim to tackle this challenge, we designed and developed a PEC cell for operation under high irradiations (30 – 360 kW m^-2) complemented with a multiphysics model to predict the surface temperature, and the effects of ohmic drop due to bubble evolution, substrate resistance, and current density distributions. The High-Flux PEC cell (HF-PEC) comprises a stainless body steel acting as counter electrode, a high transmission quartz window and a modified Luggin capillary for the reference electrode. To avoid extreme temperatures at the surface of the photoelectrode, the electrolyte was recirculated at the back of the substrate to keep it at lower temperatures. Two photoelectrode materials were investigated: Sn-doped α-Fe₂O₃ and BiVO₄, both deposited on FTO or Ti substrates via spray pyrolysis.

In the case of Ti|α-Fe₂O₃ photoelectrodes, it was found that at high temperatures under 1 sun illumination, the charge transfer efficiencies decreased with temperature at low potentials (< 1.2 V vs. RHE), but increased at higher potentials; in contrast, impedance spectroscopy under high irradiations indicated that charge transfer efficiencies increased for all electrode potentials, even when the surface of the photoelectrode was at higher temperatures. This suggests, that although e-h recombination kinetics increased with temperature, the increased charge transport kinetics and higher exciton concentration could ultimately favour the charge transfer at the semiconductor-electrolyte interface. For FTO|BiVO₄ photoelectrodes, it was found that photocurrent densities do not scale proportionately with irradiance, and that degradation rates for this material are the result of complex interactions between increased transfer efficiencies, high temperatures and high photocurrent densities; it was found that the highest (irradiance-normalized) current density for BiVO₄ was achieved at ca. 120 kW m^-2.