The Dependence of Charge Transfer and Surface Diffusion Rates on the Structure and Stability of an Electrode Surface: Copper

Abstract

Experiments have been carried out to ascertain the effect on the parameters of the Cu deposition reaction of varying the surface configuration, of the substrate. Electrodeposited, He‐quenched, H2‐quenched, dissolved, oxide film and chemically reduced surfaces were used. The observations were made by means of cathodic transients. An examination was also made of the change of activity of an electrode surface for times in the range 10−3 sec‐10 min after production in situ of a fresh surface.The charge transfer process per real sq. cm., is about ten times higher on electrodes prepared by quenching from the liquid than from those of other methods. The velocity of surface diffusion is about ten times lower on the quenched electrodes than on other electrodes. The concentration of adions on the quenched electrodes is much smaller than those on other substrates. The double layer capacity is approximately the same on He‐quenched and electro‐deposited electrode but much higher on oxide film and chemically reduced surfaces. There is a rapid fall of the velocity of surface diffusion on exposure of a fresh surface produced by anodic dissolution to a solution and no fall for oxide film electrodes. When the He‐quenched electrodes were submitted to a successive series of cathodic pulses, such that Cu deposition was succeeded by H2 evolution in each pulse, io, Cd. I. and τ all increased linearly with number of pulses, τ increased about ¼ the common rate of increase of io and Cd. I.The rate‐determining step on electrodeposited, oxide film, dissolved and chemically reduced surfaces is shared at low current density by surface diffusion and transfer process: at high current density, it is predominantly by transfer process. On He‐ and H2‐quenched electrodes, surface diffusion control is predominant at low current density and transfer process at high current density. The change of rate constant for surface diffusion with time on exposure of a fresh surface to the solution is inconsistent with adsorption of impurities from solution and consistent with an equilibration of the adion concentration among the different planes on the surface. The dependence of io on substrate is probably a secondary effect of a larger concentration of adions on electrodeposited surfaces. Oxide film, chemically reduced, dissolved, and H2‐evolved electrodes all have similar surface configuration to that of the electrodeposited electrodes. The surface adion concentration arises from the dominance of 100 planes in the surfaces of these substrates. The remarkable effects of successive cathodic pulses arise from the increase in surface area attendant on the production of Cu in fine particle form by a secondary mechanism. The anomalous linear dependence of τ on the number of pulses is consistent with a horn‐like shape of the pores.