Anion-enhanced electrosynthesis of adipic acid: dynamic creation of NiOOH-SeO₃2−for cyclohexanone adsorption and C_β–H activation

The electrocatalytic oxidation reaction of cyclohexanone (CHN) to adipic acid (COR), a crucial precursor for nylon-66, is impeded by weak substrate adsorption and sluggish C_β–H activation over a conventional Ni(OH)₂ catalyst. Anion enhancement has been widely employed in such organic electrooxidations, yet the anion effect remains poorly understood. Here, we design NiSe_x as an anion-enhanced catalyst and uncover that in-situ generated SeO₃2− plays a triple role in COR, namely facilitating NiOOH formation, enhancing adsorption, and uniquely facilitating C_β–H activation. In-situ Raman and electrochemical analyses reveal the transformation of NiSe_x into a selenite-adsorbed NiOOH phase, which promotes the formation of active Ni3+ species. Decoupling mechanistic studies demonstrate that SeO₃2− not only strengthens CHN adsorption but also, uniquely among the anions tested, facilitates the rate-determining C_β–H cleavage step and further COR process. Consequently, NiSe_x achieves a current density of 87 mA cm−2 at 1.54 V versus reversible hydrogen electrode, outperforming Ni(OH)₂ by a factor of 3.6, as well as NiS_x and NiP_x, whose anion lacks this specific C_β–H activation enhancement. This work establishes a triple role of SeO₃2− and provides a decoupled framework for understanding anion effects for electrooxidation.