Electrochemical CO₂ reduction systems operating under industrially relevant conditions inherently experience significant self-heating due to ohmic losses and activation overpotentials, driving operation toward elevated temperatures. While beneficial for reducing cell voltages, this thermal increase directly impacts the local reaction environment within membrane electrode assemblies (MEAs), particularly through water transport phenomena that alter the microenvironment of Cu gas diffusion electrodes (GDEs) [1].

Here, we investigate the influence of temperature (ambient to 80 °C) on copper-based CO₂ electrolyzers across catholyte-fed and zero-gap configurations at industrially relevant current densities (100–300 mA cm^-2). Increasing temperature significantly lowers the cell voltage (3.94 → 3.37 V at 100 mA cm^-2) due to improved ionic conductivity and faster charge-transfer kinetics [2]. However, this energetic benefit is accompanied by a severe loss in selectivity, with ethylene (C₂H₄) Faradaic efficiency (FE) dropping from 19 to 3.5 %, while hydrogen evolution becomes dominant due to reduced CO₂ solubility and accelerated competing reactions [3].

At intermediate conditions (40–50 °C, 150 mA cm^-2), a favorable regime emerges, where C₂H₄ selectivity improves by ~15 percentage points, indicating a balance between kinetics and transport. At higher temperatures (>60 °C), catholyte-fed systems exhibit rapid instability (<20 min), driven by bubble accumulation, flooding, and three-phase boundary degradation [4].

Zero-gap electrolyzers mitigate these limitations by reducing liquid accumulation and stabilizing interfacial transport. At 300 mA cm^-2, a 7.2 h stability test shows a decline in C₂H₄ FE from 22.03 to 14.7% (FE_C2+ ~29 %), corresponding to a 33.2 % decrease while maintaining energy efficiency toward C₂₊ products above 10 %. This decay correlates with significant water transport (9.5 mL; 1.32 mL h^-1), indicating strong electro-osmotic drag affecting the cathode microenvironment.

Reducing the current density to 200 mA cm^-2 enables a 26 h test, where C₂H₄ FE decreases from ~20 to ~13% (≈ 35 %), while maintaining FE_C2+ > 30% and EE_C2+ > 11%. The cell voltage increases moderately (2.92 → 3.05 V; 3.46 mV h^-1), alongside a significantly lower water transport rate (0.37 mL h^-1), highlighting improved stability through controlled water management.

Overall, temperature emerges as a system-level design parameter, while water transport within MEAs plays a central role in governing stability and selectivity. Achieving efficient and durable CO₂ electrolysis requires integrated thermal–architectural co-design to control the catalyst microenvironment under industrial conditions [5].