Photoelectrosynthesis of adipic acid coupled with energy storage in an open-loop flow battery

Photoelectrochemical (PEC) oxidation of cyclohexanone (CYC) to adipic acid offers a new sustainable synthesis approach, while efficient and selective transformation continues to pose a significant challenge. Moreover, traditional PEC systems typically require both sunlight and an external bias to ensure efficient and stable chemicals production, resulting in low energy utilization efficiency and 1-fold application scenarios. Herein, we construct a high-performance photoanode for PEC synthesis of adipic acid from CYC by modifying TiO₂ nanorods array with NiFeCu (oxy)hydroxides (NiFeCu(OH)₂/TiO₂), achieving 5.6 μmol cm^-2 h^-1 adipic acid production with 95.7% selectivity at 1.0 V vs. reversible hydrogen electrode (RHE), outperforming the selectivity of the reported works for PEC CYC oxidation to adipic acid at low applied potentials. We demonstrate that the Cu modification in NiFe(OH)₂/TiO₂ promotes the generation of adsorbed hydroxyl radicals (OH*) to form reactive Ni/Fe^{2+ δ} -OH* species, facilitating the activation of C _α -H bonds in CYC and the subsequent C-C cleavage to obtain adipic acid. It is notable that we achieved PEC CYC oxidation with redox flow batteries (denoted as PEC open-loop flow battery), in which value-added adipic acid is produced and electricity is discharged spontaneously (at a voltage of ∼1.3 V with a capacity of 1.1 Ah L^‒1). This work underscores the potential for sustainable light-driven adipic acid synthesis and solar energy storage.