Highly Porous Nickel-Phosphorous Electrode Synthesized By a Facile Electrodeposition Method for Efficient Alkaline Electrolysis

Water electrolysis is an electrolysis reaction of water, which can produce a large amount of hydrogen environmentally friendly. The purposes of water electrolysis are responding to current hydrogen demand and storing electric energy produced from renewable energy such as solar or wind power to solve their intermittency. Water electrolysis reaction is divided into two half reactions; hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). The theoretical equilibrium potential of the water electrolysis is 1.23 V, but the electrolysis reaction of the actual water requires much higher voltage due to the slow kinetics of oxygen and hydrogen reaction. Therefore, in order to increase hydrogen production efficiency, it is necessary to develop a catalyst having high activity and low cost for OER and HER. At present, transition metal-based materials such as Fe, Co, and Ni are mainly studied as high efficiency catalysts for HER and OER for alkaline water electrolysis. Those transition metals are being studied in the form of various compounds such as oxide/hydroxide, phosphide, sulfide and other transition metal-based materials. Among various type of compounds, Nickel phosphides/phosphorous have drawn attention for use in the alkaline water electrolysis system due to their chemical stability, conductivity, and bifunctional catalytic activity towards HER and OER. Different properties between Ni and P such as electronegativity and atomic radius induce the modulation of the Ni electronic structure. The modulated electronic structure of Ni can tailor the adsorption properties of adsorbates participating in HER and OER, and accelerate the kinetics of both reactions. However, since most of Ni phosphide/phosphorous catalysts require long time for synthesis process, including high temperature heat treatment, which acts as a hurdle to apply to industry field. In this study, we have developed the Ni-P electrodes for cathode and anode for alkaline electrolysis. An electrodeposition method has been applied, because it is a facile and reproducible technique that can be carried out at room temperature. The synthesis procedure of the N i -P electrodes only needs 10 seconds, and hydrogen bubbles vigorously generated during synthesis act as a dynamic template, resulting in a porous structure of Ni-P. The surface area of highly porous Ni-P (HP Ni-P) was 3.42 times larger than that of the flat Ni electrode. The HP Ni-P exhibits remarkable electro-catalytic activity and stability towards the HER and OER in alkaline solutions. For the HER, overpotentials at -10 and -100 mA/cm 2 were 97 and 145 mV, respectively, which are superior catalytic activity than that of Pt/C. For the OER, HP Ni-P shows higher catalytic activity than IrO 2 at the entire current region; overpotentials at 10 and 100 mA/cm 2 were 286 and 331 mV. The durability of HP Ni-P was also superior to the precious metal-based catalysts (Pt/C, IrO 2 ) in the long-term chronopotentiometry test. Our experimental and theoretical studies demonstrate that the formation of Ni-P bond induces the lack of electrons of Ni in Ni-P and increases hydrogen adsorption sites. In addition, electronic structure change of Ni in Ni-P promotes water dissociation and lowers hydrogen adsorption energy. These changes enable the Volmer-Tafel reaction and accelerate the HER kinetics. Furthermore, the oxidation of P during OER could help to accelerate the OER rate. Fig 1. SEM images of (a), (b) highly porous Ni-P, and (c), (d) highly porous Ni Fig 2. (a) HER and (b) OER activities of HP Ni-P, HP Ni and precious metal-based catalysts in 1 M KOH. Figure 1