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Driving chemistry at photoelectrodes and electrodes Li, Tengfei

Abstract

Photoelectrochemical and electrochemical cells drive redox reactions by electricity with and without light, respectively. Water splitting is a widely-studied electrolytic reaction, where oxygen and hydrogen are produced at the anode and cathode, respectively. This technology, however, is limited by the relatively low value of products. The anodic production of oxygen usually requires a significantly higher electrical potential than the theoretical potential and actually holds very little economic value. The product of cathode reaction, hydrogen, may also be a poor target product because the hydrogen made by water electrolysis costs twice as much as the hydrogen extracted from fossil fuels in most markets. Therefore, a better selection of reactant and target product is required to make a value-added photoelectrochemical or electrochemical reaction. I show herein that the challenges for anode reaction in a photoelectrochemical cell can be overcomed by two strategies: lowering the amount of electricity required to produce oxygen; or simply driving alternative chemistry that forms products with higher value than oxygen. A bismuth vanadate (BiVO₄) photoanode is used to develop these two strategies. The photoelectrochemical activity of BiVO₄ is improved (i.e. a lower potential is required to drive water oxidation) by exposure to ultraviolet light radiation. I then use this BiVO₄ photoanode to drive alternative anodic reaction, organic oxidation, and generate organic products that are more valuable than oxygen. A variety of organic transformations are demonstrated on BiVO₄ photoanode, including alcohol oxidation, C-H oxidation and lignin decomposition. I also study an alternative cathode reaction, CO₂ reduction, to replace hydrogen evolution reaction. I demonstrate that pairing cathodic CO₂ reduction with anodic organic oxidation in a single electrochemical cell can make valuable carbon products on both electrodes. I also question a presupposition in many previous CO₂ reduction studies that CO₂ is the best (or even the only) carbon species for cathodic reduction. The direct reductions of bicarbonate and carbonate into CO are demonstrated in an electrochemical flow cell without CO₂ gas feed, which bypasses the energy-intensive step to first thermally extract CO₂ gas in carbon capture and utilization process and suggests bicarbonate/carbonate might be a better cathode reactant than CO₂.

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