Microenvironment Manipulation via Enzyme-Inspired Ligand Engineering Boosting Fatty Alcohol Electrocatalytic Oxidation

Alcohol electrocatalytic oxidation represents a promising green method for producing carboxylic acid products. Additionally, it serves as a viable alternative to the oxygen evolution reaction (OER), contributing to reduced energy consumption in water electrolysis and improving the economic feasibility of hydrogen production. In the realm of electrocatalyst, nickel hydroxide (Ni(OH)₂) stands out for its low cost and excellent electrocatalytic activity, widely applied in the electro-oxidation of short-chain alcohols such as methanol and ethanol. However, the electrocatalytic oxidation of long-chain fatty alcohols, which offers higher added value, remains a significant challenge. The extremely low solubility of long-chain fatty alcohols in aqueous electrolyte results in a low concentration of accessible reactants on the catalyst surface, thereby restricting reaction rates due to mass transport limitation.<br/><br/>In natural enzyme, the active center is typically surrounded by the second coordination sphere where specific ligand groups can modulate the local microenvironment near the active center. This arrangement enhances the affinity between reactants and the active center, consequently accelerating reaction rates. Building upon this concept, our work investigates nickel-based metal organic frameworks (Ni-MOFs) as material platform designed to mimic enzyme microenvironments. Ni-MOFs were prepared through the assembly of nickel metal ions and highly tunable carboxylate organic ligands via solvothermal methods. Ex-situ X-ray photoelectron spectroscopy (XPS) analysis indicates that Ni-MOFs undergo reconstruction under anodic reaction conditions, transforming into Ni(OH)₂, which is considered as the actual catalytic active species. Notably, Raman spectra and attenuated total reflectance infrared spectroscopy (ATR-IR) reveal that some carboxylate ligands persist in the second coordination sphere after the reconstruction.<br/><br/>Through modulating the carboxylate ligand hydrophobicity, we create a hydrophobic microenvironment that facilitates the local enrichment of fatty alcohols at active sites. This strategy significantly boosts the activity and selectivity of fatty alcohol electrocatalytic oxidation while mitigating competitive OER. Experimental findings demonstrate that Ni-MOFs outperform pure Ni(OH)₂ for the electrocatalytic oxidation of the model substrate octanol, achieving a more than five-fold increase in octanoic acid yield and a Faradaic efficiency exceeding 85%. The enzyme-inspired ligand engineering strategy proposed in this work lays the groundwork for designing efficient electrocatalytic oxidation catalysts and holds promise for application in various electrocatalytic reactions involving water-immiscible reactants.