Decoupling the Effects of Temperature and High Photon Fluxes in Photoelectrochemical Water Splitting

Abstract

The production of solar fuels by using concentrated irradiation allows for smaller and cheaper devices, and ultimately lower fuel production costs, with the added benefit of increased performances if operating conditions are chosen judiciously [1]. Photoelectrochemical (PEC) water splitting has been studied mostly at room temperature and under 1 sun illumination, hence there are still unrealized possibilities for solar fuel production by using high photon fluxes. However, the potentially advantageous synergy between higher temperatures and irradiation, also renders the system complex and difficult to study. The effect of temperature alone on photoelectrode performances has been reported for several semiconducting materials, e.g. Fe2O3 [2] and BiVO4 [3]; while the effect of high photon fluxes has been investigated separately up to 30 suns [4]. The combined effects of temperature and high irradiations on the performance of PEC devices has been rarely investigated experimentally, this is mostly due to the complexity of decoupling the effects due to increased surface temperature, photon fluxes and degradation rates.With the aim to address this challenge, we developed a PEC cell for operation at high irradiations, 30 – 360 kW m-2, along with a multiphysics model to predict the increased surface temperature and the inevitable effects related to ohmic drop, bubble evolution and current density distribution. The PEC cell was equipped with thermocouples to measure the temperature of the photoelectrode and electrolyte at different positions inside the device. Additionally, the effects of temperature alone were investigated under 1 sun by heating the electrolyte between 20 and 55 °C. Two photoelectrodes were investigated: BiVO4 and Sn-doped Fe2O3, deposited via spray pyrolysis on FTO or Ti foil.Our findings suggest that the losses due to bubbles and ohmic drop could account for up to 90% and 50% of the losses in photocurrent densities for FTO and Ti substrates, respectively. And although, increased temperatures improve the charge carrier transport, electrolyte conductivity and oxygen evolution kinetics and thermodynamics, these effects could be offset by the aforementioned losses. For hematite operating at high temperatures (55 °C) under 1 sun illumination, it was found that the surface charge transfer efficiency decreases at low electrode potentials; in contrast, impedance spectroscopy under high irradiation indicates that the charge transfer efficiencies actually increases, even when the surface of the photoelectrodes were at higher temperatures. This suggests that although e-h recombination kinetics increases with temperature, the increased charge transport kinetics and higher exciton concentration could ultimately favor the charge transfer at the photoelectrode | electrolyte interface, see Fig 1a. In the case of BiVO4, the rate of photocurrent decay does not follow a linear relationship with increased irradiation, and its performance can be predicted by a transient model accounting for dissolution and changing thicknesses. Our results at higher irradiations reveal that the highest normalized current density for BiVO4 was achieved at ca. 120 kW m-2 as shown in Fig. 1b.Hence, the electrolyte flow and the irradiation concentration should be carefully chosen in order to keep the photoelectrode surface at appropriate temperatures, while also preventing the buildup of bubbles and current density distribution. This work shows the promise of using the synergy of temperature and high photon fluxes for PEC water splitting, while also warning about engineering and material challenges that should be addressed for a successful application of this approach for solar fuel production.