Integrated Photoelectrode and Electrolyte Engineering via Carbon Quantum Dots for Self‐Powered H ₂ O/O ₂ ‐Mediated Portable Photoelectrochemical Cells

ABSTRACT We developed N‐doped carbon quantum dots (N‐CQDs) as both photocathode modifiers and electrolytes for self‐powered portable photoelectrochemical (PEC) cells to generate electricity under visible‐light illumination. Using betaine‐type Meldonium precursor, and either ethylenediamine, N,N ‐dimethylformamide or NH ₃ ·H ₂ O as a nitrogen source deliver three different N‐CQDs featuring both surface‐negatively‐charged and positively‐charged groups. Due to structural‐directing template functionality of ethylenediamine, N‐CQDs(en) incorporates the highest pyridinic‐N content, which facilitates the charge conductivity and fine‐regulates the reaction selectivity within Csp ² ‐frameworks. As semiconductor‐coatings electrodeposited on Cu ₂ O, N‐CQDs(en) integrate with Cu ₂ O into heterojunctions (N‐CQDs(en)/Cu ₂ O) to improve charge separation and promote 4e ⁻ oxygen‐reduction into water. Importantly, encapsulating an aqueous solution containing N‐CQDs(en) in a gelatin/sodium L‐pyroglutamate‐derived conductor gives a quasi‐solid‐state electrolyte that facilitates the charge migration, improving the electrodes‐electrolytes interfacial incompatibility, while also possibly helping to in situ complement active sites on the modified photocathode. Coupled with a FeNiOOH/FeN‐decorated BiVO ₄ photoanode, enabling the efficient 4e ⁻ water oxidation, the complete system establishes a self‐sustaining H ₂ O–O ₂ –H ₂ O cycle. The resulting PEC cell shows impressive electricity output for over 120 h under irradiation, enough to power some small electronics. Unlike conventional photovoltaics, this cell is moisture‐tolerant, oxidation‐resistant and concurrently harnesses light and chemical energy, presenting a new paradigm for next‐generation light‐to‐electricity conversion.