Bias-Free Solar to Chemical Conversion using Perovskite Photocathodes

Considering the increasing consequences of climate change, utilizing fuels synthesized by mere use of solar flux, water, and ambient air seems to be advanced option for the simultaneous capture and storage of vast amounts of solar energy at ambient temperature and pressure. Solar hydrogen production is one of ultimate technologies needed to realize a carbon-neutral, sustainable society [1]. However, an energy-intensive water oxidation half-reaction together with the poor performance of conventional inorganic photocatalysts have been big hurdles for practical solar hydrogen production [2]. In first part of work, we present a photoelectrochemical cell with a record high photocurrent density of 19.8 mA cm⁻² for hydrogen production by utilizing a high-performance organic-inorganic halide perovskite as a panchromatic absorber and lignocellulosic biomass as an alternative source of electrons working at lower potentials. Additionally, the system was able to generate H₂without applying any additional bias while utilising raw waste itself [3].<br/><br/>To further draw the application of perovskite photoelectrodes, we applied the photocathode to generate bias-free hydrogen peroxide (H₂O₂). It is an eco-friendly oxidant and a promising energy source comparable to H₂. It stands an irrefutable chance due to its wide application for wastewater treatment, the paper industry, and pulp bleaching with numerous in-situ production of other chemicals including propylene. However, currently, more than 95 % of H₂O₂ production relies on the anthraquinone oxidation process [4], which requires a significant energy supply at several steps and numerous organic chemicals. In addition, precious Pd-based catalysts and high-pressure H₂ used during the hydrogenation reaction increase the cost of the process and introduce many safety issues. Therefore, developing single-step oxygen reduction to H₂O₂, with a simple and eco-friendly method can pave a promising route [5]. However, utilising inorganic semiconductors impedes solar-to-chemical conversion efficiency (SCC) lower than 1 % due to several factors including their low absorption coefficient, sluggish kinetics owing to the charge diffusion length, and low stability and selectivity of the H₂O₂ production’s electrocatalyst. In second part of work, we utilized an inorganic-organic-based lead halide perovskite photocathode for demonstrating bias-free solar H₂O₂ production without the need for additional bias or sacrificial agents. To further improve the stability and selectivity of this photocathode, we passivated the photocathode with oxidised buckypaper as the H₂O₂ electrocatalyst and the field’s metal as an intimate junction for the subsequent transfer of photogenerated electrons towards oxygen reduction reaction. With such an efficient strategy, an SCC of ~1.463 % is attained in such a system, attaining 100 % selectivity toward H₂O₂ for 12 h. Moreover, the integrated photocathode was able to produce a stable photocurrent for 45 h continuously under one-sun simulated conditions [6].<br/><br/><b>References:</b><br/>[1] Turner, J. A. Sustainable hydrogen production. Science 305, 972–974 (2004).<br/>[2] Jiao, Y., Zheng, Y., Jaroniec, M. & Qiao, S. Z. Design of electrocatalysts for oxygen-and hydrogen-involving energy conversion reactions. Chem. Soc. Rev. 44, 2060–2086 (2015).<br/>[3] Mehrotra, R., et al. Bias-free solar hydrogen production at 19.8 mA cm⁻² using perovskite photocathode and lignocellulosic biomass. Nat Commun 13, 5709 (2022).<br/>4] Perry, S. C. et al. Electrochemical synthesis of hydrogen peroxide from water and oxygen. Nat. Rev. Chem. 3, 442–458 (2019).<br/>[5] Hong, Y. et al. Unassisted photocatalytic H2O2 production under visible light by fluorinated polymer-TiO₂ heterojunction. Chem. Eng. J. 418, 129346 (2021).<br/>[6] Mehrotra, R. et. al. Unassisted selective solar hydrogen peroxide production by an oxidised buckypaper-integrated perovskite photocathode. Nat Commun 12, 6644 (2021).