Raney multi-metallic electrodes from regular crystalline and quasi-crystalline precursors: I. Cu-stabilized Ni/Mo cathodes for hydrogen evolution in acid

Multi-metallic alloys comprising Al (65-75 at.%), Ni, Cu (Co), and Mo, were prepared in regular crystalline (RC) and quasicrystalline (QC) forms, and subjected to alkali leaching to create Ni-rich Raney electrocatalysts. Afte r acid-conditioning, these were tested as cathodes in pressed-powder form f or hydrogen evolution and for corrosion resistance in 0.5 M aqueous H2SO4. RC precursors are multi-phase mixtures with a predominant hexagonal Ni2-xCu xAl3 structure. Increasing Cu level exacerbates the intrinsic resistance of Ni2Al3 to leaching in alkali. In contrast, QCs are mainly single-phase mat erials which are readily leached regardless of Cu content. Only Mo-containi ng samples have mass-specific activities for H-2 evolution which approach t hose of 'platinized', and carbon-supported, Pt control electrodes. A curren t density benchmark of 70 mA cm(-2) was attained at modest overpotentials, viz., - 115 to -220 mV/NHE. Since there is no evidence for Ni/Mo alloy afte r leaching, it appears that the synergy operates merely by intimate physica l contact of these components. Mo is not incorporated in either of the main RC or QC phases, but forms instead a separate ternary phase of provisional composition Ni16Mo10Al74. Before cathodic polarization, the active (leache d) form consists of fcc Ni/Cu (or Ni/Cu/Co) mesoporous alloy with a grain s ize of similar to 2 microns, interpenetrated by an ultra-fine dispersion of oxidized Mo. The presence of minor levels of Cu and Mo together impart res pectable stability, though not yet sufficient for technical application. Th e alloys have slightly positive rest potentials, and passivate (corrosion c urrents < 1 mA cm(-2)) up to a breakdown potential beyond + 200 mV/NHE. Pos t-alkali vacuum annealing at 400 degreesC markedly improves the resistance to corrosion. XPS shows surface enrichment of Cu and Mo, present mainly in oxidation states I and IV, respectively, after etching. The route from nove l QC precursors is of practical benefit in leading to more efficient leachi ng, and yields active forms with somewhat higher resistance to corrosion in acid. (C) 2001 Elsevier Science Ltd. All rights reserved.