Unassisted Switchable Dual‐Photoelectrode Devices Utilizing p‐n Carbon Quantum Dots as “Semiconductor Electrolytes”: Optimization Between H ₂ O ₂ and Solar Electricity Production

Switchable self-driven photoelectrochemical (PEC) devices are developed to boost H₂O₂ or electricity generation under visible-light illumination, in which p-n type carbon quantum dots (N-CQDs) is applied as conceptually-new "semiconductor electrolytes". The N-CQDs contains N-dopants, and both negatively- and positively-charged surface groups. This allows N-CQDs to act as the electrolyte and to interact with both a BiVO₄ photoanode and a Cu₂O photocathode. In a two-compartment cell with a separating membrane, N-CQDs can dynamically form p-n heterojunctions with the photoanode or the photocathode, facilitating charge separation. In this setup, the fine-tuned electronic structure of N-CQDs promotes the two-electron reactions with water or O₂ to produce H₂O₂, achieving a rate of 28 µm min^-1 and Faradic efficiency exceeding 80%. Switching into a one-compartment cell, N-CQDs promotes four-electron charge transfer and stabilizes the photoelectrodes, giving electricity output for over 120 h. This control over electron transfer, selectivity, and durability cannot be achieved using traditional electrolytes.