Facet-Selective Bromide Adsorption at High-index Facets of Copper Sulfides for Improved Electrochemical Nitrogen Reduction Activity and Selectivity

Abstract

Electrochemical nitrogen reduction reaction (eNRR) is a promising sustainable alternative to the energy-intensive Haber–Bosch process for ammonia (NH₃) synthesis. However, simultaneously achieving high activity and selectivity remains challenging due to competing hydrogen evolution reaction (HER). In this study, a series of copper sulfide catalysts were investigated for eNRR under ambient conditions. Copper sulfides featuring highly crystalline low-index (1 1 1) facets exhibited significant NH₃ production; however, the presence of high-index facets led to a reduction in Faradaic efficiency (F.E.) due to their higher activity toward the competing HER. To mitigate this intrinsic trade-off between activity and selectivity, bromide ions were introduced to selectively adsorb onto the high-index planes, effectively suppressing HER. Optimizing the bromide ion coverage on CuSₓ catalysts resulted in a maximum NH3 yield of 1.2 μmol h–¹ cm–² (JNH3 = 0.198 mA/cm2) and a F.E. of 23.34% at −0.6 VRHE, marking improvements of 84.0% in activity and 316% in selectivity compared to untreated catalysts. It was verified that bromide ions selectively passivate HER-prone surfaces without blocking the active sites responsible for nitrogen adsorption. This study highlights that facet-selective surface engineering using halide ions can simultaneously enhance both activity and selectivity in eNRR. It provides a rational design principle for improving catalytic performance in complex reaction environments where multiple pathways compete.