A commercialization of green hydrogen is necessary to construct a hydrogen economy. In this context, a water electrolysis which is composed by the hydrogen evolution reaction and its anodic counter reaction is evaluated as an optimal generation method of green hydrogen. As compared with replaceable oxidative reaction such as oxygen evolution reaction (OER) and chlorine evolution reaction (COR), the development of efficient electrocatalyst for HER, which is an irreplaceable cathodic reaction of water electrolysis, became the most urgent assignment in both of acidic and alkaline water electrolysis.

Acidic HER is divided by two steps: adsorption and desorption of proton, while the difference of Gibbs free energy between the first (M + proton, M means reaction site of catalyst) and final (M + H₂) state is zero. Thus, each reaction rate of adsorption and desorption step decreases when the proton adsorbed free energy shows positive and negative value, respectively. Furthermore, an ideal catalyst shows the proton adsorbed free energy of zero. In particularly, alkaline HER needs to separate water molecule to proton and hydride to supply the proton. Therefore, it showed a different free energy at the intermediate though an equal catalyst due to the addition of hydride absorbed free energy, but it also need a zero-like free energy. In this context, platinum (Pt) based catalyst is known as the most zero-like proton adsorbed free energy in alkaline media, however its high cost owing to a high rarity of Pt becomes the research gap of HER catalyst.

As a substitution of Pt based catalyst, transition metal phosphide (TMP) has received a lot of attention due to its efficient electron transfer between metal and phosphorus. Furthermore, nickel phosphide (Ni-P) is evaluated as the most suitable catalyst for HER due to the high catalytic activity and practical usefulness. The nickel (Ni) has lower electronegativity than phosphorus (P), and the electron transfers from Ni to P. Therefore, a proton and hydride adsorbed at the negatively charged P and positively charged Ni, respectively. Because the proton desorption step becomes a rate determining step of Ni without P for HER catalyst, the higher interaction between adsorbed proton and hydride increase the kinetic of not only desorption step but also overall HER. Based on the theoretical advantage, a lot of researchers are studying to find the most suitable ratio between Ni and P. For instance, J. Zhang formed P vacancy on Ni₁₂P₅ catalyst for alkaline HER to balance the kinetic of adsorption and desorption of proton, while the Ni₁₂P₅ compound has negative proton adsorbed free energy without the P vacancy. [1] Although the formation of P vacancy increased the proton adsorbed free energy, there was limitation on variance of P ratio.

Unlike the crystalline Ni-P compound, an amorphous Ni-P compound which is fabricated by an electroplating process is facile to control the ratio between Ni and P. However, the electroplated Ni-P has a critical limitation in increasing P ratio up to about 25 at% because P-rich amorphous Ni-P has too low electric conductivity to be electroplated. Furthermore, ‘isolated Ni’ which has low bonding with P is formed due to nonuniformity of crystal structure between Ni and P, then the isolated Ni has lower redox potential than Ni-P. A galvanic corrosion between the grain of isolated Ni and Ni-P becomes a critical cause of high degradation rate of amorphous Ni-P catalyst. In this study, we suggest a surficial engineering process to control the ratio between Ni and P after the electroplating of Ni-P compound on carbon fiber paper (Ni-P/CFP) catalyst to improve the activity and stability for HER in alkaline media.

Using the difference of redox potential between the isolated Ni and Ni-P, the surficial engineering is designed as a dealloying cycle of metal alloy to selectively remove the isolated Ni. As a result, The Ni-P/CFPx10 (10 times dealloying-cycled Ni-P/CFP catalyst) catalysts show higher P content of 29.6 at% on the surface (24.9 at% before dealloying treatment). Furthermore, the surficial engineering arouses the recrystallization of Ni-P compound, thus the permeated amount of P increases according to the number of surficial engineering cycle. Because of the mentioned influences, the Ni-P/CFP catalyst shows higher catalytic activity and durability after the surficial engineering. In particular, the Ni-P/CFPx10 catalysts shows the 58 mV lower overpotential at -10 mA cm^-2 the pristine Ni-P/CFP catalyst (247 mV) in alkaline HER.

[1] Jingjing Duan, Sheng Chen, C8sar A. Ort&z-Ledln, Mietek Jaroniec, and Shi-Zhang Qiao, Phosphorus Vacancies that Boost Electrocatalytic Hydrogen Evolution by Two Orders of Magnitude, Angew. Chem. Int. Ed., 59, 2020, 8181.