Unveiling pH-Dependent Adsorption Strength of *CO 2 - Intermediate over High-Density Sn Single Atom Catalyst for Acidic CO 2 -to-HCOOH Electroreduction.

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.