The progress on synthesis of colloidal nanocrystals (NCs) has enabled researchers to engineer crystalline nanoparticles over many aspects including composition, size, morphology, crystal structure, surface functionalities, and so on. The rendering unique chemical and physical properties of these precisely engineered colloidal NCs make them superior to their bulk counterparts in many applications, especially for electrochemical reduction reactions that are currently extensively investigated to resolve global energy and environmental issues. Herein we present the recent progress of colloidal NCs and their roles in electrochemical reduction reactions, such as oxygen reduction reaction (ORR), hydrogen evolution reaction (HER), and carbon dioxide reduction reaction (CO2RR). In this feature article, we first introduce the colloidal NCs on the synthesis of colloidal NCs with controlled size, shape, composition and structure. We then focus on the emerging concept in colloidal NCs synthesis, as well as the self-assembly of colloidal NCs to superlattice structures. Afterwards, we discuss the fundamentals and representative strategies in designing colloidal NC structures for superior catalytic performance in ORR, HER and CO2RR. In the end, we provide a perspective on the future opportunities of colloidal NCs in electrochemical reduction reaction applications.

Cubic cobalt oxide nanocrystals (NCs) with bridged-multi-octahedral structures were prepared by a cooperative mechanism between the particle-based oriented attachment and the atom-mediated crystal growth. The obtained bridged-multi-octahedral NCs show high crystallinity and Co-terminated {111} facets enclosing the octahedrons. Compared to conventional cobalt oxide prepared hydrothermally, this bridged-multi-octahedral NC structure exhibits enhanced electrocatalytic performances towards oxygen evolution and reduction reactions, which is attributed to their preferential exposure of the Co-terminated {111} facets with a low Co coordination number, high electrochemically active surface area, and the reduced charge transfer resistance from the catalytic active sites to the underlying electrode, thus suggesting the tuning of crystal growth for the electrocatalytic enhancement.

Cubic cobalt oxide nanocrystals (NCs) with bridged-multi-octahedral structures were prepared by a cooperative mechanism between the particle-based oriented attachment and the atom-mediated crystal growth. The obtained bridged-multi-octahedral NCs show high crystallinity and Co-terminated {111} facets enclosing the octahedrons. Compared to conventional cobalt oxide prepared hydrothermally, this bridged-multi-octahedral NC structure exhibits enhanced electrocatalytic performances towards oxygen evolution and reduction reactions, which is attributed to their preferential exposure of the Co-terminated {111} facets with a low Co coordination number, high electrochemically active surface area, and the reduced charge transfer resistance from the catalytic active sites to the underlying electrode, thus suggesting the tuning of crystal growth for the electrocatalytic enhancement.