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

Abstract The acidic electrochemical CO 2 reduction reaction (CO 2 RR) 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 CO 2 RR 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 *CO 2 − intermediatethat enables enhanced acidic CO 2 RR 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 CO 2 RR to HCOOH is formed near the surface of Sn SACs, even in the acidic electrolyte. More importantly, the pH‐dependent adsorption strength of *CO 2 − intermediate is unravelled over the Sn SACs, which in turn affects the competition between HER and CO 2 RR in acidic electrolyte.