Improving the Stability of the Nickel Phosphide HER Catalyst Using Metal Additives in Acidic Media
Authors: Oh Hyunjun , Eom Kwang Sup 
Published: November 2022

Energy and the environment are key issues for the development of sustainable society. Renewable energies such as wind, solar, tidal, and biomass energy are clean and it can replace fossil fuels. However, since renewable energy is produced intermittently according to region, time, and season, it also requires the systems for conversion, storage, and transport of surplus energy. In response to this need, hydrogen has received much attention as an energy carrier due to its high gravimetric energy density and long-term storage possibility. The hydrogen production through water splitting is a promising strategy to convert excess electrical energy generated from renewable energy into high-density energy source in the form of hydrogen gas (H2). Moreover, the produced hydrogen can be used as electrical energy via the fuel cells with high energy conversion efficiency and zero pollution. However, in this technology, the problematic point is the high price of hydrogen production, and it is mostly due to low efficiency and kinetics of water splitting=To overcome this problem, it is essential to develop a high-performance catalyst for water splitting. Among the various candidates of the catalytic materials, the Pt-based catalyst has the best performance especially in the hydrogen evolution reaction (HER), but it is needed to be substituted into non-noble metal based ones due to a cost problem.
In particular, transition metal phosphides (TMPs) such as Ni-P are receiving much attention due to its remarkable catalytic performance approaching that of Pt-based one. [1]. Specifically, in Ni-P, the electrons are transferred from Ni to P due to the difference in electronegativity, so that the energy of the HER intermediate can be controlled to have the optimal activity. Moreover, the binding site of Ni-P acts as a new active site, increasing the performance more. Nevertheless, the major issue to be solved for commercialization of Ni-P is the low stability due to the dissolution of Ni, especially in acidic media where its corrosion potential is lower than HER. Specially, the Ni dissolution is most severe in the isolated Ni phases, which is relatively unbounded with P [2].
In this context, we suggest a stability enhancement method of NiP by adopting the metallic stabilizer. Specifically, We prepared NiP-Cu by substituting Cu into the isolated Ni sites (Fig. 1a). We believed that the higher reduction potential of Cu might increase the overall corrosion resistance of the NiP-based material. As a result (Fig1. c), the corrosion potential of NiP-Cu was measured to be -333 mV, which is 12 mV higher than the pristine Ni-P. Also, in the accelerated degradation test (ADT) including potential sweep, the NiP-Cu showed three times lower degradation rate (3 mV/h)than NiP (12 mV/h), as shown in Fig1c-d.

References

[1] P. Liu, and J. Rodriquez, J. Am. Chem. Soc.,2005, 127, 42, 14871-14878
[2] A. Kucernak, and V. Sundaram, J. Mater. Chem. A, 2014, 2, 17435-17445.
[3] KE. Ayers, KW. Harrison, ECS Trans., 2013, 50, 35

Figure 1