Enhanced Electrochemical CO₂ Reduction to Formate over Phosphate-Modified In: Water Activation and Active Site Tuning

Electrochemical CO₂ reduction reaction (CO₂RR) offers a sustainable strategy for producing fuels and chemicals. However, it suffers from sluggish CO₂ activation and slow water dissociation. In this work, we construct a (P-O)^δ- modified In catalyst that exhibits high activity and selectivity in electrochemical CO₂ reduction to formate. A combination of in situ characterizations and kinetic analyses indicate that (P-O)^δ- has a strong interaction with K⁺(H₂O)_n, which effectively accelerates water dissociation to provide protons. In situ attenuated total reflectance surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS) measurements together with density functional theory (DFT) calculations disclose that (P-O)^δ- modification leads to a higher valence state of In active site, thus promoting CO₂ activation and HCOO* formation, while inhibiting competitive hydrogen evolution reaction (HER). As a result, the (P-O)^δ- modified oxide-derived In catalyst exhibits excellent formate selectivity across a broad potential window with a formate Faradaic efficiency as high as 92.1 % at a partial current density of ~200 mA cm^-2 and a cathodic potential of -1.2 V vs. RHE in an alkaline electrolyte.