Electrosynthesis and Performance of Homogeneously Mesoporous Ni-Rich Ni-Pt Thin Films for Efficient Hydrogen Evolution Reaction in Acidic Media

Abstract

The development of green hydrogen technology requires abundant electrocatalyst materials to (partially) replace costly platinum while maintaining the catalytic efficiency and durability. Nickel is an excellent choice for the partial replacement of Pt for water splitting, and is widely used for electrocatalysis in alkaline media. Although Ni cannot provide an activity identical to Pt, specific nanostructures aimed at a high surface-to-volume ratio, such as nanoporous structures, can significantly improve catalytic efficiency. However, in acidic media, the use of non-noble metals for electrocatalysis requires extensive durability tests to confirm the stability of the alloy. Additionally, the impact of hydrogen itself is often overlooked, as hydrogen is known to cause hydrogen embrittlement in Ni and its alloys.This contribution deals with the synthesis of a homogeneously mesoporous Ni-Pt alloy with high catalytic efficiency (Fig. 1). By electrodeposition from aqueous media, Ni-rich Ni-Pt alloy thin films are synthesised in a wide compositional range (between 60 and 99 at% Ni), depending on the deposition potential. The addition of the amphiphilic block copolymer Pluronic P-123 to the electrolyte formulation enables a micelle-assisted deposition process, resulting in films with homogeneous porosity with an approximate pore size of 10 nm irrespective of composition. Moreover, all thin films are single-phase and nanocrystalline [1], and show excellent performance under hydrogen evolution reaction (HER) in acidic media [2].As determined by Rutherford backscattering (RBS), the as-deposited Ni-Pt thin films with film thickness between 180 nm and 260 nm exhibit Pt enrichment at the surface of 1-2 nm in depth, after which Ni and Pt exhibit constant composition over depth. After hydrogen evolution in 0.5 M H2SO4, a gradient in Ni/Pt ratio over the film thickness is established, indicating preferential leaching of Ni into the electrolyte, and resulting in higher Pt content at the surface. Due to the porosity of the thin films, this leaching affects the entire film thickness. A chronopotentiometric durability test (24 h) reveals that the overpotential stabilises over time, indicating that the leaching of Ni is suppressed once a certain Pt/Ni ratio is attained in the thin films. In addition, the HER electrocatalysis is accompanied by a significant hydrogen uptake, determined by elastic recoil detection analysis (ERDA). Depending on the electrodeposition parameters, the as-deposited thin films already exhibit significant hydrogen concentrations between 2 and 10 at%. After the HER durability test, local cracking is observed by SEM, related to an excess of hydrogen in the thin films. Despite the cracking, no significant detachment of the films from the substrate is observed. The hydrogen contained in the films is assumed to be interstitial and does not affect the metallic nature of the Ni-Pt films. An X-ray photoelectron spectroscopy (XPS) post-analysis shows that the fraction of metallic Ni with respect to Ni in higher oxidation states is higher after exposure to the HER than for the as-prepared surface. Acknowledgement: Financed by the European Union – NextGenerationEU.[1] K. Eiler, J. Fornell, C. Navarro-Senent, E. Pellicer, J. Sort, Nanoscale, 2020, 12, 7749[2] K. Eiler, S. Suriñach, J. Sort, E. Pellicer, Appl. Catal. B, 2020, 265, 118597Figure 1: SEM micrograph of a mesoporous Ni93Pt7 thin film. Figure 1