Unraveling Water Balances behind Salt Formation in CO2 Reduction Electrolyzers

Salt precipitation in CO2 electrolyzers remains a critical barrier to long-term stability, yet the underlying water-salt transport and balance are poorly understood. Here, we established a quantitative mass balance framework to account for water entering and leaving the cathode chamber from all possible sources and across different phases, thereby elucidating the origin of bicarbonate salt formation. We revealed that although the inlet CO2 stream typically flows through a water bubbler, it is often not fully saturated with water vapor and thus results in significant liquid water evaporation inside the cathode chamber. Although water crossover from the anolyte to the cathode is, in principle, sufficient to dissolve the cotransported K+ as carbonate species (or even bicarbonate in some cases), evaporation reduces the effective local liquid water-to-K+ ratio, leading to salt precipitation. Conversely, excessive humidification suppresses salt formation but induces electrode wetting and flooding, highlighting a fundamental trade-off between dehydration-induced crystallization and overhumidification-driven mass transport limitations. Together, these findings establish water management as a governing principle for stabilizing CO2 electrolyzers and provide engineering guidelines for controlling salt formation in electrochemical reactor systems.