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With the rapid development of economy, the past decades have experienced more and more severe energy depletion and environmental pollution issues, hence it is urgent to develop more environmental-friendly energy devices, such as fuel cells, metal-air batteries, water electrolysis and so on. However, such devices have long been suffering from the sluggish reaction kinetics and high energy barriers, plus the conventional electrocatalysts used in these devices mostly are noble-metal-based materials, such as Pt/C, IrO 2 , and RuO 2. These noble-metal-based electrocatalysts possess significant disadvantages such as high price, limited reserves, and undesirable stability, and these factors together hinder their large-scale industrial application and the inhomogeneity of the catalyst structure at the atomic level also impose great challenges to disclose the underlying catalytic mechanism. Noble metal nanoclusters, as a promising type of electrocatalyst with definitive composition and structure, have been attracting increasingly research efforts. This review aims to summarize the recent achievements of molecular metal nanoclusters employed in electrocatalytic processes, with particular elaboration on oxygen reduction reaction (ORR), hydrogen evolution reaction (HER), oxygen evolution reaction (OER), as well as to unravel the catalytic mechanism and establish the relationship between its structure and functionality. Specifically, the size effect, the metal core configuration, charge effect, size effect, ligand effect, and metal-ligand binding motifs of the metal clusters that would impact the electrocatalytic performance are comprehensively discussed. In the end, the outlook and perspective including challenges and opportunities are proposed. We anticipate this review would be beneficial for gaining a deeper understanding of engineering nanoclusters for electrocatalysis and to expand its application in electrocatalysis. • The advantages using molecular metal clusters for electrocatalysis are highlighted. • Recent advances in ORR, HER, and OER by metal clusters are summarized. • Grand challenges and great opportunities in this field are elaborated. • It can stimulate more research efforts to advance this rapidly booming regime. [ABSTRACT FROM AUTHOR]

• A facile strategy to fabricate NiCo 2 Se 4 /NiCoS 4 heterostructure is developed. • NiCo 2 Se 4 /NiCoS 4 exhibited superior OER performance to IrO 2. • NiCo 2 Se 4 /NiCoS 4 showed excellent performance in water splitting and Zn-air battery. • It paves a path for preparing low cost and efficient catalysts with multifunctionalities. Rational design and constructing multifunctional electrocatalysts featuring with low cost and high efficiency is critical for promoting the implement of sustainable energy devices such as water splitting and zinc air batteries. Herein, we report a facile means to prepare a selenide/sulfide hetero-structured NiCo 2 Se 4 /NiCoS 4 catalyst for oxygen evolution reaction (OER), hydrogen evolution reaction (HER), overall water splitting (OWS), and Zn-air batteries (ZABs). The NiCo 2 Se 4 /NiCoS 4 heterostructure showed excellent OER and HER performance, evidenced by the small overpotential of 248 mV and 180 mV @ 10 mA cm−2 for OER and HER, and superior long-term stability to the benchmark IrO 2 and Pt/C catalyst for OER and HER, respectively. It also exhibited a potential of 1.660 V @ 10 mA cm−2 in the practical OWS test, close to the Pt/C + IrO 2 catalyst. Furthermore, when employed as air-cathode catalyst for ZABs, the NiCo 2 Se 4 /NiCoS 4 modified battery exhibited the narrow charge-discharge voltage gap of 0.98 V @ 50 mA cm−2 and a maximal specific capacity of 693.17 mA h g−1, outperforming the IrO 2 counterpart. Such excellent performance can be attributed to the advantageous structural merit of the NiCo 2 Se 4 nanoflowers, and especially the heterostructure interfaces created in NiCo 2 Se 4 /NiCoS 4 , which significantly boosted the catalytic synergistic effects. This study can offer a new avenue to design and prepare abundant-element-based cost effective, efficient and durable electrocatalysts with multifunctionalities that hold great promises to be applied in electrochemical devices. Image, graphical abstract [ABSTRACT FROM AUTHOR]