drawing attention as a promising technology to mitigate excess anthropogenic CO 2 emissions because it can convert carbon dioxide into high-value-added chemicals and fuels in an eco-friendly and economical way. [2] Among suitable metals, Cu (and its compounds) is the only one capable of producing high value-added C 2+ products, such as C 2 H 4 and C 2 H 5 OH, with high selectivity through CO dimerization. [3] However, the C 2+ selectivity of Cu-based electrocatalysts can be further improved by tailoring the active sites of Cu through doping, [4] tuning the catalyst particle shapes and sizes, [5] and increasing the number of low coordination sites, for example, the grain boundary density. [6] In particular, the use of mixed oxidation states (i.e., Cu + and Cu 0 ) has been suggested as an effective means to achieve high C 2+ product selectivity by lowering the energy barrier for CO dimerization. [7] However, Cu + is not stable and is easily converted to metallic Cu (Cu 0 ) at the negative applied potentials used for the CO 2 RR. As a result, many strategies have been developed to sustain the mixed oxidation state of Cu during the CO 2 RR. [8] For instance, doping Cu with B atoms can induce the formation of Cu + under CO 2 RR conditions, and increase its stability. [4a,9] Nevertheless, it is still challenging to achieve controllable Cu + coverage at the catalytically active Cu top-surface.Ceria (CeO 2 ) is a reducible metal oxide and an excellent support for metal catalysts in many catalytic processes, such as the water-gas shift reaction, [10] CO oxidation, [11] and CO 2 hydrogenation reaction. [12] As a catalyst support, CeO 2 can disperse metals at various scales uniformly, from the nanometre in scale to single atoms. [13] In addition, CeO 2 forms strong metal-support interactions (SMSIs) with the active metals, which allows the formation of a partially charged metal at the metal-CeO 2 interface. [14] These unique characteristics of CeO 2 can also promote the electrocatalytic CO 2 RR activity of metals such as Au and Cu supported on CeO 2 . In particular, Cu on ceria (Cu-CeO 2 ) shows wide tunability in CO 2 RR product selectivity, and this is dependent on the sizes and compositions of Cu-CeO 2 . For example, CeO 2 can stabilize a single-atom (SA) Cu in the Cu 2+ valence state and generate CH 4 with a faradaic efficiency (FE) of Ceria (CeO 2 ) is one of the most extensively used rare earth oxides. Recently, it has been used as a support material for metal catalysts for electrochemical energy conversion. However, to date, the nature of metal/CeO 2 interfaces and their impact on electrochemical processes remains unclear. Here, a Cu-CeO 2 nanorod electrochemical CO 2 reduction catalyst is presented. Using operando analysis and computational techniques, it is found that, on the application of a reductive electrochemical potential, Cu undergoes an abrupt change in solubility in the ceria matrix converting from less stable randomly dissolved single atomic Cu 2+ ions to (Cu 0 ,Cu 1+ ) nanoclusters. Unlike single ...