Sponge-inspired catalyst design for durable acidic CO2 reduction at low K+ concentration.

Acidic electrochemical CO2 reduction (CO2RR) typically requires K+ ions to create a local H+-depleted microenvironment, suppressing competing hydrogen evolution reaction (HER). Excessive localized K+ causes salt precipitation, compromising electrolysis stability. Achieving stable operation with high Faradaic efficiency (FE) at low K+ concentrations remains a crucial challenge for conventional nanomaterials. Inspired by water-trapping function of sponges, we design a three-dimensional interconnected porous cubic SnO2 electrocatalyst (SnO2 sponge) that confines OH- within porous channels to consume proton influx from the bulk, enabling durable acidic CO2RR towards formic acid (HCOOH). Theoretical and experimental studies reveal the SnO2 sponge sustains substantially higher OH- concentration than dispersed SnO2 nanoparticles. At pH 1.82, the SnO2 sponge achieves 94.5% FEHCOOH at 800 mA cm-2. With only 0.075 M K+, it retains 95.2% FEHCOOH at 400 mA cm-2. Notably, it enables continuous HCOOH production at 400 mA cm-2 with 97.7% FEHCOOH for over 390 h without cleaning. This work provides a promising strategy for durable and efficient CO2RR in acidic media with low K+ concentrations.