Recovering Ethylene Selectivity in CO ₂ Electrolyzers Operated at Elevated Temperatures by Overcoming Thermal-Induced Water Restructuring

High Resolution Image Download MS PowerPoint Slide Large-scale membrane-electrode-assembly (MEA) CO 2 electrolyzer stacks inevitably operate under heated conditions, yet the mechanistic origin of temperature-dependent selectivity shift on Cu and strategies to sustain the C 2 H 4 selectivity remain poorly understood. Here, we show that, in MEA cells with 0.1 M KHCO 3 anolyte, increasing temperature strengthens the interfacial hydrogen-bond network, thereby suppressing water dissociation to form surface-bound *H. The resulting imbalance between *H and *CO 2 surface coverages limits *CO hydrogenation and C–C coupling, shifting the selectivity from C 2 H 4 toward CO and displacing the current density for peak C 2+ Faradaic efficiency (FE) to higher values. By rebalancing the relative hydrogenation environment associated with the surface coverages of *H and *CO 2 through control of CO 2 partial pressure, C 2 H 4 selectivity can be recovered under elevated temperatures, reaching a peak C 2 H 4 FE of 55–60% between 50 and 80 °C, comparable to or exceeding ambient performance. Alternatively, concentrated anolytes efficiently mitigate temperature sensitivity by weakening the hydrogen-bond network and thus improve the FE of C 2 H 4 . These insights establish a mechanistic basis for temperature-induced selectivity control of Cu-based catalysts in scalable, heated CO 2 electrolyzers.