Morphology and Utilization of Smooth Hydrogen-Evolving Raney Nickel Cathode Coatings and Porous Sintered-Nickel Cathodes.

The utilization of the inner surfaces of hydrogen-evolving, porous, sintered-nickel electrodes and Raney nickel-coated electrodes was investigated and compared by steady-state voltammetry, impedance spectroscopy, coulometric determination of catalyst surface, and scanning electron microscopy. Porous, sintered-nickel electrodes are shown to be utilized only to approximately 10%. On the time average, roughly 90% of the inner surface of these electrodes is gas-blanketed. Nanoporous, smooth Raney nickel coatings are divided by micron-scale cracks. The essential part of the catalytically active electrode surface of Raney nickel coatings is represented by the walls of nanopores whose diameter is around 2 nm. Tafel slopes of less than {minus}120 mV/dec, namely, {minus}50 to {minus}70 mV/dec, are measured at 50 {micro}m thick smooth Raney nickel coatings. These low Tafel slopes are explained by an increasing degree of nanopore utilization with increasing current density rising from less than 0.6 to {approximately}10% if the overpotential rises from {minus}30 to {minus}120 mV. The effect can be modeled for nanopores and is at variance with known micropore behavior under concentration polarization known for increased Tafel slopes. From pore modeling it follows also that in another type of Raney nickel coatings, the so-called composite coating composed of micrometer particles of Raney nickel, differentmore » from smooth Raney nickel coatings, the utilization of that part of particles which is contacted by the electrolyte is almost 100%. Since, as in sintered electrodes, only 10% of the particle surface are expected not to be gas-blanketed, the total utilization of composite coated nanoporous catalysts amounts to {approximately} 10%, independent of overpotential and current density.« less