Electrochemical Surface Treatment to Enhance the Activity and Stability of Ni-P/CFP Electrocatalysts in Hydrogen Evolution Reaction
Authors: Jo Seunghyun , Oh Hyunjun , Eom Kwang Sup 
Published: July 2022

To construct an efficient and eco-friendly hydrogen economy, it is necessary to improve a cost competitiveness of green hydrogen. In this respect, a water electrolysis has been considered as an ideal production method of green hydrogen, where hydrogen evolution reaction (HER) is an irreplaceable cathodic reaction, while the various oxidative reactions such as oxygen evolution reaction (OER) or chlorine evolution reaction (CER) can be coupled as an anodic reaction. Because the HER becomes an essential reaction of water electrolysis, to facilitate efficient HER is one of the most important goal in a water electrolysis, in particular, to adopt and design the cost-effective HER catalysts are needed to lower the high activation energy of HER.
As a theoretical and practical researches, a platinum (Pt) based compound is known as the most effective catalyst which has a zero-like proton adsorbed free energy in both of acidic and 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 among the various non-noble metal based catalyst. Among them, 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 the Ni to P. In other words, a proton and hydride adsorbed at the negatively charged P and positively charged Ni, respectively. Because homogeneous Ni metal has a lower kinetic of proton desorption than adsorption, the higher interaction between adsorbed proton and hydride increases the kinetic of not only desorption step but also overall HER. However, in conventional fabrication methods of Ni-P such as hydrothermal and electroplating, it is difficult to control the ratio of Ni and P, which might maximize the kinetics and moreover, stability.
In this context, 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 (CFP) for the efficient HER catalyst in both of acidic and alkaline media. Because ‘isolated Ni’ which has a low bonding with P has different redox potential with 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 and Ni-P/CFPx20 (10, 20 times dealloying-cycled Ni-P/CFP catalyst) catalysts show higher P content of 29.6 and 36.5 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. In particular, the Ni-P/CFPx20 and Ni-P/CFPx10 catalysts shows the 55 mV and 58 mV lower overpotential at -10 mA cm-2 the pristine Ni-P/CFP catalyst (179 mV for acidic HER and 247 mV for alkaline HER) in acidic and alkaline HER, respectively.

Figure 1