Efficient photoelectrochemical oxidation of benzyl alcohol in a microchannel flow cell using compact and mesoporous TiO₂ photoanodes

Photoelectrochemical (PEC) conversion presents a viable strategy for reducing the external bias required in conventional electrochemical methods for organic molecule valorization. However, the efficiency of PEC processes is largely dependent on photoelectrode characteristics, specifically light absorption and charge transport properties. These properties are crucial for efficient generation of charge carriers and photocurrent for driving reactions. Herein, we report efficient PEC conversion of benzyl alcohol to benzaldehyde using tailored TiO₂ photoanodes of both compact and mesoporous film morphologies in a continuous-flow PEC cell. Notably, our PEC flow cell was designed on a microscale to facilitate superior mass transfer. Parameters such as applied potential, electrolyte flow rate, channel width, and photoanode characteristics were systematically investigated to elucidate their impact on conversion efficiency and selectivity in the PEC oxidation process. The results indicated that at an applied potential of 3.5 V and an electrolyte flow rate of 0.05 ml min^-1, the microchannel with a width of 0.1 mm, which corresponds to a residence time of 1.6 min, achieved benzyl alcohol conversion exceeding 80%. Comparative analyses with traditional batch H-cells revealed over a fivefold increase in benzyl alcohol conversion in the microchannel flow cell with the mesoporous TiO₂ photoanode. Additionally, the flow cell exhibited approximately threefold higher benzaldehyde selectivity compared to batch reactors employing the compact TiO₂ photoanode. Overall, this work demonstrates the potential of continuous-flow microchannels with tailored photoanodes for achieving efficient and rapid PEC conversions, promising advancements in sustainable organic transformations.