Cobalt boride

Abstract: It is demonstrated that amorphous cobalt boride (Co2B) prepared by the chemical reduction of CoCl2 using NaBH4 is an exceptionally efficient electrocatalyst for the oxygen evolution reaction (OER) in alkaline electrolytes and is simultaneously active for catalyzing the hydrogen evolution reaction (HER). The catalyst achieves a current density of 10 mA cm−2 at 1.61 V on an inert support and at 1.59 V when impregnated with nitrogen-doped graphene. Stable performance is maintained at 10 mA cm−2 for at least 60 h. The optimized catalyst, Co2B annealed at 500 °C (Co2B-500) evolves oxygen more efficiently than RuO2 and IrO2, and its performance matches the best cobalt-based catalysts reported to date. Co2B is irreversibly oxidized at OER conditions to form a CoOOH surface layer. The active form of the catalyst is therefore represented as CoOOH/Co2B. EXAFS observations indicate that boron induces lattice strain in the crystal structure of the metal, which potentially diminishes the thermodynamic and kinetic barrier of the hydroxylation reaction, formation of the OOH* intermediate, a key limiting step in the OER.

Abstract: Engineered polycrystalline electrodes are critical to the cycling stability and safety of lithium-ion batteries, yet it is challenging to construct high-quality coatings at both the primary- and secondary-particle levels. Here we present a room-temperature synthesis route to achieve a full surface coverage of secondary particles and facile infusion into grain boundaries, and thus offer a complete ‘coating-plus-infusion’ strategy. Cobalt boride metallic glass was successfully applied to a Ni-rich layered cathode LiNi0.8Co0.1Mn0.1O2. It dramatically improved the rate capability and cycling stability, including under high-discharge-rate and elevated-temperature conditions and in pouch full-cells. The superior performance originates from a simultaneous suppression of the microstructural degradation of the intergranular cracking and of side reactions with the electrolyte. Atomistic simulations identified the critical role of strong selective interfacial bonding, which offers not only a large chemical driving force to ensure uniform reactive wetting and facile infusion, but also lowers the surface/interface oxygen activity, which contributes to the exceptional mechanical and electrochemical stabilities of the infused electrode. Coating is commonly used to improve electrode performance in batteries, but it is challenging to achieve and maintain complete coverage of electrode particles during cycling. Here the authors present a coating-and-infusion approach on Ni-rich cathodes that effectively retards stress corrosion cracking.