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

Abstract 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 onefold 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% high selectivity at 1.0 V vs. RHE, outperforming the selectivity of the reported works for PEC CYC oxidation to adipic acid at low applied potentials. We demonstrated that the Cu modification in NiFe(OH)2/TiO2 promotes the generation of adsorbed hydroxyl radicals (OH*) to form reactive Ni/Fe2+δ−OH* species, facilitating the activation of Cα−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.