Unveiling pH‐Dependent Adsorption Strength of *CO2− Intermediate over High‐Density Sn Single Atom Catalyst for Acidic CO2‐to‐HCOOH Electroreduction

The acidic electrochemical CO₂ reduction reaction (CO₂RR) for direct formic acid (HCOOH) production holds promise in meeting the carbon-neutral target, yet its performance is hindered by the competing hydrogen evolution reaction (HER). Understanding the adsorption strength of the key intermediates in acidic electrolyte is indispensable to favor CO₂RR over HER. In this work, high-density Sn single atom catalysts (SACs) were prepared and used as catalyst, to reveal the pH-dependent adsorption strength and coverage of *CO₂ ^- intermediatethat enables enhanced acidic CO₂RR towards direct HCOOH production. At pH=3, Sn SACs could deliver a high Faradaic efficiency (90.8 %) of HCOOH formation and a corresponding partial current density up to -178.5 mA cm^-2. The detailed in situ attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopic studies reveal that a favorable alkaline microenvironment for CO₂RR to HCOOH is formed near the surface of Sn SACs, even in the acidic electrolyte. More importantly, the pH-dependent adsorption strength of *CO₂ ^- intermediate is unravelled over the Sn SACs, which in turn affects the competition between HER and CO₂RR in acidic electrolyte.