Critical Enhancement Effects of Ni Content on Electrocatalytic Hydrogen Evolution by Highly Homogeneous PtNi_<i>m</i>O_<i>x</i> Composite Films Prepared Using a Mixed Metal-Imidazole Casting Method

Development of efficient and durable electrocatalysts for the hydrogen evolution reaction (HER) is eagerly desired to construct a practical water splitting system. Rigidly adhering PtNimOx composite films (m is a Ni ratio versus Pt) with different Ni contents were synthesized on a fluorine-doped tin oxide conducting glass (FTO) substrate by a mixed metal-imidazole casting (MiMIC) method using 1-methylimidazole (MeIm). An X-ray diffraction (XRD) pattern of the PtNi0.2Ox (denoted as PtNi0.2Ox(w/o)) film prepared without MeIm showed intense peaks for the face-centered cubic (fcc) Pt crystalline phase, which are close to those for a neat Pt film. However, the PtNimOx films showed that the peaks for fcc Pt became broad and less prominent even at m = 0.025, showing that only 2.5% of the Ni content effectively suppressed the formation of the Pt crystalline domain. Pt and Ni elements were homogeneously dispersed without their domains at the nanostructure level in the PtNi0.2Ox film. In contrast, Pt and Ni elements were inhomogeneously distributed, forming their distinct domains in the PtNi0.2Ox(w/o) film. The X-ray absorption fine structure spectroscopy (XAFS) data suggested that the PtNi0.2Ox film composed of PtOx aligns with the Pt0 bulk and partially oxidized NiII centers. However, the X-ray photoelectron spectra (XPS) data indicated the unique surface states of exclusive PtII and NiIII in the PtNi0.2Ox film. The HER performance of the PtNimOx electrode was drastically improved by only a low content of m = 0.025 to exhibit excellent performance with a η10 of 43 mV and a Tafel slope of 42 mV dec–1 at m = 0.2. The mass activity (0.14 A cm–2 mgPt–1) of PtNi0.2Ox at η = 50 mV was 23.3, 2.3, and 100 times higher than that of neat Pt, PtNi0.2Ox(w/o), and commercial 10 wt % Pt/C films on a FTO electrode, which is advantageously comparable to that of the state-of-the-art Pt-based electrocatalysts.