energy conversion and storage. [1,2] Developing efficient electrocatalysts that can effectively enhance the sluggish kinetic processes are particularly important to the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) at low overpotentials. [3,4] Platinum group metals and noble metal oxides (e.g., IrO 2 , RuO 2 ) are considered as pioneering HER and OER catalysts, respectively. However, the large-scale applications are limited by the scarcity and high cost of these materials. [5,6] Recently, a great deal of effort and progress has been made toward the development of earth-abundant, highly efficient, and durable HER and OER catalysts, such as transition metal chalcogenides, [7][8][9] phosphides, [10][11][12] nitrides, [13][14][15] and carbides [16][17][18] (for HER), and transition metal oxide, [19][20][21] hydroxide/oxyhydroxide, [22][23][24] phosphate, [25][26][27] and carbon materials [28][29][30] (for OER). Due to the thermodynamic convenience and practical application in proton-exchange membrane or alkaline electrolyzers, these HER and OER catalysts generally exhibit high activity in strongly acidic and basic conditions, separately; thus pairing the two type catalysts in an integrated electrolyzer with high efficiency and stability for overall water splitting is difficult due to the mismatch of electrolyte pH. [31,32] There is Developing efficient, durable, and earth-abundant electrocatalysts for both hydrogen and oxygen evolution reactions is important for realizing largescale water splitting. The authors report that FeB 2 nanoparticles, prepared by a facile chemical reduction of Fe 2+ using LiBH 4 in an organic solvent, are a superb bifunctional electrocatalyst for overall water splitting. The FeB 2 electrode delivers a current density of 10 mA cm −2 at overpotentials of 61 mV for hydrogen evolution reaction (HER) and 296 mV for oxygen evolution reaction (OER) in alkaline electrolyte with Tafel slopes of 87.5 and 52.4 mV dec −1 , respectively. The electrode can sustain the HER at an overpotential of 100 mV for 24 h and OER for 1000 cyclic voltammetry cycles with negligible degradation. Density function theory calculations demonstrate that the boron-rich surface possesses appropriate binding energy for chemisorption and desorption of hydrogen-containing intermediates, thus favoring the HER process. The excellent OER activity of FeB 2 is ascribed to the formation of a FeOOH/ FeB 2 heterojunction during water oxidation. An alkaline electrolyzer is constructed using two identical FeB 2 -NF electrodes as both anode and cathode, which can achieve a current density of 10 mA cm −2 at 1.57 V for overall water splitting with a faradaic efficiency of nearly 100%, rivalling the integrated state-of-the-art Pt/C and RuO 2 /C.

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Highly efficient and stable electrocatalysts, particularly those that are capable of multifunctionality in the same electrolyte, are in high demand for the hydrogen evolution reaction (HER), oxygen evolution reaction (OER), and oxygen reduction reaction (ORR). In this work, highly monodisperse CoP and Co P nanocrystals (NCs) are synthesized using a robust solution-phase method. The highly exposed (211) crystal plane and abundant surface phosphide atoms make the CoP NCs efficient catalysts toward ORR and HER, while metal-rich Co P NCs show higher OER performance owing to easier formation of plentiful Co P@COOH heterojunctions. Density functional theory calculation results indicate that the desorption of OH* from cobalt sites is the rate-limiting step for both CoP and Co P in ORR and that the high content of phosphide can lower the reaction barrier. A water electrolyzer constructed with a CoP NC cathode and a Co P NC anode can achieve a current density of 10 mA cm at 1.56 V, comparable even to the noble metal-based Pt/C and RuO /C pair. Furthermore, the CoP NCs are employed as an air cathode in a primary zinc-air battery, exhibiting a high power density of 62 mW cm and good stability.

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