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, si ntered-nickel electrodes and Raney nickel-coated electrodes was invest igated and compared by steady-state voltammetry, impedance spectroscop y, coulometric determination of catalyst surface, and scanning electro n microscopy. Porous, sintered-nickel electrodes are shown to be utili zed only to approximately 10%. On the time average, roughly 90% of the inner surface of these electrodes is On gas-blanketed. Nanoporous(a), smooth Raney nickel coatings are divided by micron-scale cracks.(b) T he essential part of the catalytically active electrode surface of Ran ey nickel coatings is represented by the walls of nanopores whose diam eter is around 2 nm. Tafel slopes of less than -120 mV/dec, namely, -5 0 to -70 mV/dec, are measured at 50 mu m thick smooth Raney nickel coa tings. These low Tafel slopes are explained by an increasing degree of nanopore utilization with increasing current density rising from less than 0.6 to similar to 10% if the overpotential rises from -30 to -12 0 mV, The effect can be modeled for nanopores and is at variance with known micropore behavior under concentration polarization known for in creased Tafel slopes. From pore modeling it follows also that in anoth er type of Raney nickel coatings, the so-called composite coating comp osed of micrometer particles of Raney nickel, different from smooth Ra ney nickel coatings the utilization oi that pad of particles which is contacted by the electrolyte is almost 100%. Since, as in sintered ele ctrodes, only 10% of the particle surface are expected not to be gas-b lanketed, the total utilization of composite coated nanoporous catalys ts amounts to similar to 10%, independent of overpotential and current density.