Porous Nickel-Phosphorous Electrode Synthesized By Fast-Simple Electrodeposition Process for Efficient Overall Water Splitting
Authors: Song DongHoon , Cho EunAe 
Published: February 2020

Water electrolysis is an electrolysis reaction of water, and it is a technology that 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 iron (Fe), cobalt (Co), and nickel (Ni) are mainly studied as high efficiency and low cost catalysts for oxygen and hydrogen evolution reactions for alkaline water electrolysis. Those transition metals are being studied in the form of various compounds such as oxide, carbide, nitride and phosphide. Among various type of compounds, transition metal phosphides (TMP) have drawn much attention for use in the alkaline water electrolysis system. These TMP catalysts have relatively good catalytic activity and stability for both OER and HER, and have high electrical conductivity compared to oxide based catalyst. In addition, TMP catalysts can be synthesized using non-toxic precursors as compared to nitride and carbide based catalysts. However, since most of TMP catalysts requires several hours to days for synthesis process, including high temperature heat treatment processes, the efficiency problem has not been solved.
In this study, we have developed a method for producing nickel-phosphorus electrodes for high efficiency oxygen and hydrogen evolution reaction by fast-simple electrodeposition method. The synthesis procedure of the Ni-P electrodes ends in a few seconds at room temperature. High cathodic current density was applied to promote the generation of hydrogen bubbles and the deposition of Ni-P, thereby obtaining a highly porous structure. Hydrogen bubbles played an important role in forming the porous Ni–P layer by acting as self-dynamic templates. The surface area of highly porous Ni-P (HP Ni-P) was 50 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, overpotential at 10 mA/cm2 was 180 mV, which is slightly higher than that of Pt/C, but HP Ni-P required a lower overpotential than that of Pt/C at the high current region. For the OER, HP Ni-P shows higher catalytic activity than IrO2
at the entire current region; overpotential at 10mA/cm2 was 287 mV. The durability of HP Ni-P was also superior to the precious metal-based catalysts (Pt/C, IrO2) and Ni electrode in the long-term chronopotentiometry test. This result is ascribed to increase in surface area by hydrogen bubble generation and electronic structure change of Ni. Electronic structure change may lead to change in adsorption energy of intermediate adsorbates such as H*, OH*, O* and OOH*, which are participated in the steps of HER and OER, and can promote the kinetics of reactions in alkaline media.

Figure 1