Tuning Cu Nanocluster Size for Methane Production in a Bipolar Membrane CO₂ Electrolyzer

The acidic electrochemical CO2 reduction reaction (CO2RR) offer potential for enhanced carbon utilization efficiency in hydrocarbon production, but its selectivity is constrained by the competing hydrogen evolution reaction (HER) and the poor C2/C1 product control. Herein, we demonstrate that precise tailoring of copper nanoclusters enables exceptional HER suppression while promoting selective CO2-to-CH4 conversion under acidic conditions (pH 2). Cu36 nanoclusters achieve superior methane selectivity across a wide current density range (300–700 mA cm–2), achieving a CH4 partial current density (jCH4) of 288 mA cm–2. By analyzing the correlation between post-activation CO2RR performance and crystal structure, we reveal that the structural evolution of Cu nanoclusters significantly influences their product distribution under electrochemical conditions, and confirm that ligands stabilize the Cu36 cluster to avoid significant aggregation. Ligand-stabilized Cu exhibits enhanced stability in acidic media, maintaining operation for 70 h at 300 mA cm–2. Our approach addresses key acidic CO2RR challenges by simultaneously delivering high selectivity, activity, and stability─critical metrics for practical electrochemical carbon conversion.