Electron delocalization of robust high-nuclear bismuth-oxo clusters for promoted CO 2 electroreduction.

The integration of high activity, selectivity and stability in one electrocatalyst is highly desirable for electrochemical CO 2 reduction (ECR), yet it is still a knotty issue. The unique electronic properties of high-nuclear clusters may bring about extraordinary catalytic performance; however, construction of a high-nuclear structure for ECR remains a challenging task. In this work, a family of calix[8]arene-protected bismuth-oxo clusters (BiOCs), including Bi 4 (BiOC-1/2), Bi 8 Al (BiOC-3), Bi 20 (BiOC-4), Bi 24 (BiOC-5) and Bi 40 Mo 2 (BiOC-6), were prepared and used as robust and efficient ECR catalysts. The Bi 40 Mo 2 cluster in BiOC-6 is the largest metal-oxo cluster encapsulated by calix[8]arenes. As an electrocatalyst, BiOC-5 exhibited outstanding electrochemical stability and 97% Faraday efficiency for formate production at a low potential of -0.95 V vs. RHE, together with a high turnover frequency of up to 405.7 h -1 . Theoretical calculations reveal that large-scale electron delocalization of BiOCs is achieved, which promotes structural stability and effectively decreases the energy barrier of rate-determining *OCHO generation. This work provides a new perspective for the design of stable high-nuclear clusters for efficient electrocatalytic CO 2 conversion.