<p>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% FE_HCOOH at 800 mA cm^–2. With only 0.075 M K⁺, it retains 95.2% FE_HCOOH at 400 mA cm^–2. Notably, it enables continuous HCOOH production at 400 mA cm^–2 with 97.7% FE_HCOOH for over 390 h without cleaning. This work provides a promising strategy for durable and efficient CO₂RR in acidic media with low K⁺ concentrations.</p>