Electrochemical CO₂ reduction reaction (CO₂RR) is a promising technology for mitigating global warming and storing renewable energy. Designing low-cost and efficient electrocatalysts with high selectivity is a priority to facilitate CO₂ conversion. Halide ion (F^–, Cl^–, Br^–, I^–) modified electrocatalysts is a potential strategy to promote CO₂ reduction and suppress the competitive hydrogen evolution reaction (HER). Therefore, a comprehensive review of the role and mechanism of halide ions in the CO₂RR process can help better guide the future design of efficient electrocatalysts. In this review, we first discuss the role of halide ions on the structure and morphology of electrocatalysts. Secondly, the relationship between the halide ions and the valence states of the active sites on the catalyst surface is further elaborated on. Thirdly, the mechanisms of halide in enhancing CO₂ conversion efficiency are also summarized, including the involvement of halide ions in electron transfer and their influence on the reaction pathway. Finally, we conclude with a summary and future outlook.

Being a typical state of the art heterogeneous catalyst, supported noble metal catalyst often demonstrates enhanced catalytic properties. However, a facile synthetic method for realizing large-scale and low-cost supported noble metal catalyst is strictly indispensable. To this end, by making use of the strong metal–support interaction (SMSI) and mechanochemical reaction, we introduce an efficient synthetic route to obtain ultrafine Pt and Ir nanoclusters immobilized on diverse substrates by wet chemical milling. We further demonstrate the scaling-up effect of our approach by large-scale ball-milling production of Pt nanoclusters immobilized on TiO₂ substrate. The synthesized Pt/Ir@Co₃O₄ catalysts exhibit superior oxygen evolution reaction (OER) performance with only 230 and 290 mV overpotential to achieve current density of 10 and 100 mA·cm⁻², beating the catalytic performance of Co₃O₄ supported Pt or Ir clusters and commercial Ir/C. It is envisioned that the present work strategically directs facile ways for fabricating supported noble metal heterogeneous catalysts.