Oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are the two most important reactions in rechargeable metal‐air battery, a promising technology to meet the energy requirements for various applications. The development of low‐cost, highly efficient and stable bifunctional ORR/OER catalysts is critical for a large‐scale application of this technology. In this review, the authors first introduce the fundamentals of bifunctional ORR/OER electrocatalysis in alkaline electrolyte. Various types of nanostructured materials as bifunctional ORR/OER catalysts including metal oxide, hydroxide and sulfide, functional carbon material, metal, and their composites are then reviewed. The crucial factors that can be used to tune the activity of the catalyst towards ORR/OER are summarized, including (1) phase, morphology, crystal facet, defect, mixed‐metal and strain engineering for metal oxide; (2) heteroatom doping, topological defects, and formation of metal‐N‐C structure for carbon material; (3) alloy effect for metal. These experiences lay the foundation for large scale application of metal‐air battery and can also effectively guide the rational design of catalysts for other electrocatalytic reactions.

to the efficiency loss of the overall electrochemical process. [2] Therefore, efficient OER electrocatalyst is a cornerstone for the sustainable energy storage and conversion technologies. [3] Noble-metal-based catalysts such as Ru and Ir oxides have been reported to be excellent OER catalysts; [4] however, their low natural abundance and higher cost render their widespread commercial utilization impractical.[5] Thus, developing efficient, durable, and cost-effective catalytic materials for OER is crucial, but so far still remains a great challenge.Recently, various metal-metalloid compound materials such as chalcogenides, nitrides, and phosphides have been reported to exhibit promising OER electrocatalytic activities. [6] This has been attributed to the charge transfer between different elements and the modified electronic structures, and consequently lowers the kinetic energy barriers of the electrochemical processes. [7] In this regard, to extend such applications on metal borides is reasonable, wherein metal borides share certain properties with other metal-metalloids, such as bonding schema in metal phosphides. [8] Recently, the application of monometallic borides such as cobalt boride, iron boride, and nickel boride as well as cobalt-borate-based graphene hybrid as oxygen-evolving catalyst has been reported to exhibit promising electrocatalytic activity for OER in alkaline media. [9] However, multimetal-metalloid boron-based material, herein referred to as an amorphous quaternary metal boride, for simplicity, as oxygen-evolving electrocatalyst, reported to date is very limited. Thus, it is still of great interest to develop metal-boride-based OER catalysts and further to explore their catalytic activities. The as-identified approach to enhance electrochemical OER properties in metalmetalloid material opens a new avenue in related applications for energy devices that involve OER such as water electrolysis and metal-air batteries.Despite the complicated structure of amorphous material, well-characterized amorphous nanomaterial as oxygen-evolving material has received attention due to the unique properties such as higher catalytic selectivity and activity. [10] A great number of under-coordinated metal atoms, and hence abundant defects, in amorphous nanomaterial may provide more reactive sites at the catalyst surface, and as a result, facilitating the binding of hydroxyls and thus enhancing OER performance. [11] Furthermore, these amorphous nanomaterials have attracted much attention in other electrochemical applications Cost-effective and efficient oxygen-evolving electrocatalysts are urgently required for energy storage and conversion technologies. In this work, an amorphous trimetallic boride nanocatalyst (Fe-Co-2.3Ni-B) prepared by a simple approach is reported as a highly efficient oxygen evolution reaction electrocatalyst. It exhibits an overpotential (η) of 274 mV to deliver a geometric current density (j geo ) of 10 mA cm −2 , a small Tafel slope of 38 mV dec −1 , and excellent long-term durabil...

Molecularly sized and heterogeneous Ni-Fe sites on graphene serve as efficient electrocatalysts.

We report that a structurally simple molecular 1,10-phenanthroline-Cu complex on a mesostructured graphene matrix can be active and selective toward CO2 reduction over H2 evolution in an aqueous solution. The active sites consisted of Cu(I) center in a 2 distorted trigonal bipyramidal geometry, which enabled the adsorption of CO2 with η 1 -COO-like configuration to commence the catalysis, with a turnover frequency of ~45 s -1 at -1 V vs reversible hydrogen electrode. Using in-situ infrared spectroelectrochemical investigation, we demonstrated that the Cu complex was reversibly heterogenized near graphene surface via potential control. An increase of electron density in the complex was observed as a result of the interaction from the electric field, which further tuned the electron distribution in the neighboring CO2. It was also found that the mesostructure of graphene matrix favored CO2 reduction on the Cu center over hydrogen evolution by limiting mass transport from the bulk solution to the electrode surface.Received: ((will be filled in by the editorial staff)) Revised: ((will be filled in by the editorial staff))

A highly efficient and durable electrocatalyst of Pd hydride nanocubes encapsulated within 2D amorphous Ni‐B nanosheets is reported. The PdH0.43 nanocubes are first synthesized via a simple N,N‐dimethylformamide thermal treatment. The as‐synthesized PdH0.43 nanocubes are then encapsulated in 2D amorphous NiB nanosheets by NaBH4 reduction in the presence of nickel species. During the NaBH4 treatment, the PdH0.43 can be further transformed into PdH0.706 due to the presence of endogenous H2. Electrochemical studies demonstrate that the degree of hydride of Pd nanocubes (PdHx) plays an important role in the enhancement of their oxygen reduction reaction (ORR) activity. With increasing x value, both the activity and stability increase significantly. At 0.90 V versus reversible hydrogen electrode, the ORR activity of PdH0.706 @Ni‐B reaches 1.05 A mgPd−1, which is nearly five times higher than that of the state‐of‐the‐art Pt catalysts. Accelerated durability tests show that even after 10 000 potential cycles, there is negligible shift in their half‐wave potential and no shape and structure change occurs, indicating the incorporation of amorphous 2D Ni‐B nanosheets can greatly improve their stability without compromising their activity. The present study illustrates the importance of high degree of hydride and presence of amorphous Ni‐B nanosheets on the enhancement of ORR activity for Pd‐based electrocatalyst.