Regeneration of Nicotinamide Adenine Dinucleotide Phosphate by a Chlorophyll a‑Coated TiO₂ Film Electrode

A TiO₂ electrode was coated with chlorophyll a to regenerate nicotinamide adenine dinucleotide phosphate (NADPH), which can enhance the photovoltages of the electrodes for photoelectrochemical capacitors. The photovoltage of an uncoated TiO₂ electrode was high during the first cycle but then steadily reduced owing to the oxidization of NADPH in the electrolyte during the photo-charge–discharge cycling. By contrast, a chlorophyll a-coated TiO₂ electrode maintained high photovoltages for 100 cycles. Residual NADPH concentrations after 100 cycles increased from 73% to 90% because of the coating, demonstrating that NADPH was regenerated by photoexcited chlorophyll a similar to a photosynthetic reaction in nature.