Thermoelectric–Photoelectrochemical Water Splitting under Concentrated Solar Irradiation

Photoelectrochemical (PEC) systems utilize sunlight to directly convert water and carbon dioxide into useful chemicals like hydrogen, synthesis gas, or ethanol. Such devices may lower the costs of sustainable fuel production by integrating light harvesting and catalysis in one compact device.^[1] However, light absorption suffers losses from thermalization and the inability to use low-energy photons, which limits the overall solar-to-chemical conversion efficiency.^[2,3] Here, we demonstrate that PEC reactors can utilize this waste heat by integrating thermoelectric modules, which provide additional voltage under concentrated light irradiation.^[3] While most single semiconductors require external bias, we already accomplish unassisted water splitting under 2 sun irradiation by wiring a BiVO₄ photoanode to a thermoelectric element, whereas the photocurrent of a perovskite-BiVO₄ tandem system is enhanced 1.7-fold at 5 sun. This strategy is particularly suitable for photoanodes with more positive onset potentials like hematite, with thermoelectric-perovskite-Fe₂O₃ systems achieving a 29.7× overall photocurrent increase at 5 sun over conventional perovskite-Fe₂O₃ devices without light concentration. This thermal management approach provides a universal strategy to facilitate widespread solar fuel production, as light concentration increases output, reduces the reactor size and cost, and may enhance catalysis.^[3]<br/>We further reveal the recent success of thermoelectric composites in other material science applications. While thermoelectric pastes consisting of conductive powders in a polymer matrix display limited power factors,^[4] their hydrophobicity proves essential for effective moisture protection of PEC devices and solar cells. Accordingly, a graphite epoxy paste encapsulant extends the lifetime of metal halide perovskite photocathodes to 4 days under operation in water,^[5] and increases the stability of a BiOI photocathode for H₂ evolution from minutes to months.^[6] This material allows the manufacturing of flexible, lightweight devices, which float on water similar to lotus leaves.^[7] Those devices are compatible with large-scale, automated fabrication processes, which present the most potential towards future real-world applications.^[8]<br/><br/>[1] Andrei, V.; Roh, I.; Yang, P. <i>Sci. Adv.</i> 2023, 9, eade9044.<br/>[2] Andrei, V.; Bethke, K.; Rademann, K. <i>Energy Environ. Sci.</i> 2016, 9, 1528–1532.<br/>[3] Pornrungroj, C.; Andrei, V.; Reisner, E. <i>J. Am. Chem. Soc.</i> 2023, 145, 13709–13714.<br/>[4] Andrei, V.; Bethke, K.; Rademann, K. <i>Phys. Chem. Chem. Phys.</i> 2016, 18, 10700–10707.<br/>[5] Pornrungroj, C.; Andrei, V et al. <i>Adv. Funct. Mater.</i> 2021, 31, 2008182.<br/>[6] Andrei, V.; Jagt, R. A. et al. <i>Nat. Mater. </i>2022, 21, 864–868.<br/>[7] Andrei, V.; Ucoski, G. M. et al. <i>Nature</i> 2022, 608, 518–522.<br/>[8] Sokol, K. P.; Andrei, V. <i>Nat. Rev. Mater.</i> 2022, 7, 251–253.