~ Pergamon Acta metall, mater. Vol.43, No. 8, pp. 3027 3033, 1995 ElsevierScience Ltd 0956-7151(95)00019-4 Copyright © 1995.Acta Metallurgica Inc. Printed in Great Britain.All rights reserved 0956-7151/95$9.50 + 0.00 THE INFLUENCE OF INTERCRYSTALLINE DEFECTS ON HYDROGEN ACTIVITY AND TRANSPORT IN NICKEL D. M. DOYLEIt, G. PALUMBO2, K. T. AUST1, A. M. EL-SHERIK3 and U. ERB3 ~Department of Metallurgy and Materials Science, University of Toronto, Toronto, Canada M5S IA4, 2Ontario Hydro Technologies, 800 Kipling Avenue, Toronto, Canada M8Z 5S4 and 3Department of Materials and Metallurgical Engineering, Queen's University, Kingston, Canada K7L 3N6 (Received 20 May 1994; in revised form 25 November 1994) A~tract--The effect of high intercrystalline content on hydrogen activity and transport in nickel has been studied. The electrocatalytic behavior of Ni has been determined in low concentration alkaline media with reference to the hydrogen evolution reaction (HER) by potentiostatic methods. The effect of structure on the Tafel parameters has been examined. Nanocrystalline Ni was found to exhibit enhanced electrocata- lytic activity relative to cold worked, fine grained and fully annealed Ni electrodes. A supplementary investigation into the transport behavior of hydrogen in Ni was studied in order to determine hydrogen diffusivities and apparent concentrations as a function of intercrystalline content. Electrolytic charging of hydrogen was performed on Ni bi-electrodes in an electrochemicaldouble cell. Detection of permeated hydrogen in Ni membranes of identical thickness is observed in the order: nanocrystalline, fine grained and single crystal structures. The apparent concentration of hydrogen determined from permeation transients is dramatically increased in nanocrystalline Ni. 1. INTRODUCTION The use of hydrogen as an energy source has gathered considerable interest because of its inexhaustible supply [i.e. from (alkaline) water electrolysis] and its nonpolluting nature (combustion in air produces water vapor). Hydrogen energy research programs generally focus on developing (1) active electro- catalysts, required to minimize cell overpotentials; and (2) safe, high capacity hydrogen storage facilities. Common methods used in improving the electro- catalytic performance of conventional Ni cathodes have been through selective alloying [1] or by increas- ing the working surface area of the electrode (i.e. porous "Raney Ni" electrodes [2]). Concurrent re- search on developing hydrogen storage systems have investigated solid metal hydrides as a viable alterna- tive to storage via high pressure gas containers or liquid storage at T < 20 K. Features desirable in materials considered candidates for metal hydrides are high hydrogen uptake and high reversibility (adsorption-desorption) characteristics [3]; criteria associated with enhanced hydrogen solubility and diffusivity. The recent development of nanocrystalline materials has led to interest in developing appli- cations where high intercrystalline content could be of technological use. One such application is the utilization of microstructural defects (i.e. grain boundaries) in the optimization of electrocatalysis. Huot and co-workers [4] observed enhanced hydro- tTo whom all correspondence should be addressed. gen evolution reaction kinetics (HER) when investi- gating the electrocatalytic behavior of a series of nanocrystalline Ni-Mo alloys in concentrated alka- line solutions at 70°C. The improved performance was attributed to three factors [4]: (1) the expansion of the Ni lattice caused by the diffusion of Mo into Ni; (2) the chemical effects associated with the incor- poration of Mo into the Ni lattice; and (3) the decrease in grain size. The authors speculated that the grain size was the most significant factor in the enhanced HER kinetics observed. Enhanced hydrogen solubility and diffusivity ob- served in nanocrystalline Pd has been attributed to the high intercrystalline content present. Mutchete and Kirchheim [5] reported hydrogen solubility in- creases of one to two orders of magnitude in a 5 nm Pd sample as compared to a single crystal sample. Furthermore, the diffusivity of hydrogen was found to be strongly dependent on hydrogen concentration and increased considerably as trapping (segregation) sites were filled at the interfaces. The two referred studies [4, 5] reveal that substan- tial deviations in hydrogen-metal interactions occur when materials are processed in nanocrystalline form. The present study reports results pertaining to the performance of 99.8% + nanocrystalline Ni with re- spect to (1) electrocatalytic activity; and (2) hydrogen transport behavior. The performance of nanocrys- talline Ni devoid of major alloying elements gives evidence pertaining to the contributing effect of inter- crystalline content on the enhanced electrocatalytic and transport behavior detected. As the intercrys- talline region of a metal is comprised of both grain AM 43/8~J 3027