Ligand-engineered Ni-TCNQ framework with synergistic adsorption and charge transfer for efficient HMF oxidation-HER coupling

The electrocatalytic oxidation of 5-hydroxymethylfurfural (HMF) offers an eco-friendly route to produce 2,5-furandicarboxylic acid (FDCA). Coupling this anodic reaction with the hydrogen evolution reaction provides a sustainable alternative to oxygen evolution reaction in water electrolysis, enhancing energy efficiency while generating green hydrogen. However, conventional nickel-based catalysts suffer from weak substrate adsorption and limited active-site utilization. Here, we introduce a ligand-engineered charge-transfer complex, Ni(TCNQ) 2 (H 2 O) 2 , in which π-conjugated TCNQ ligands enable strong π–π stacking with the furan ring of HMF to enrich substrate adsorption, while the electron-accepting nature of TCNQ mediates metal-ligand charge transfer to regulate the Ni 2+ /Ni 3+ redox couple. This dual function significantly accelerates reaction kinetics and stabilizes intermediates, enabling Ni(TCNQ) 2 (H 2 O) 2 to deliver 100 mA·cm −2 at 1.38 V and nearly 100 % FDCA yield. Mechanistic insights from X-ray photoelectron spectroscopy, operando Raman, and density functional theory (DFT) confirm that ligand-driven adsorption and electron interactions accelerate the catalytic process. DFT free-energy profiles further demonstrate that Ni(TCNQ) 2 (H 2 O) 2 smooths the reaction energy landscape, lowering the thermodynamic barriers compared with Ni(OH)₂. This work underscores the pivotal role of ligand engineering in tuning catalytic interfaces for efficient biomass electrooxidation and green hydrogen co-production in integrated renewable energy systems.