Determination of the Faradaic Impedance at Solid Electrodes and the Electrodeposition of Copper

Abstract

The techniques of pre-electrolytic purification of the solution and the maintenance of electrodes in glass sheaths until after immersion in the purified electrolytic solution have been applied for the determination of the properties of the ac impedance at the copper-solution interface. Measurements of double layer capacity and resistance at copper interfaces in H2SO4 and in CuSO4 solution over a considerable potential range have been made at frequencies from 10 to 2000 cps. Significant results can only be obtained with careful elimination of screening and unsymmetry effects in the cell, and by the use of spherical electrodes. The double layer impedance varies with frequency in potential regions where trivial causes such as the dissolution of Cu are not effective. The limiting double layer capacity at high frequency is about 70 μf per apparent sq cm of the surface. Pre-electrolytic purification led to an apparent halving of the transfer coefficient and a considerable increase in exchange current density. The experimentally evaluated coefficients ∂Rdi/∂ω—½ (where Rdi is the diffusional resistance) are about double those theoretically evaluated on the assumption that the true area of the electrode is equal to the apparent area. The values of the experimental coefficients ∂Rdi/∂ω—½ and ∂1/ωCdi/∂ω—½ (where Cdi is the diffusional capacitance) agree well. ∂ lni0/∂ lnaCu++=0.75 when i0 is the exchange current density. The variation of the double layer capacity with frequency is due to dielectric loss in the water molecules in the double layer. Application of the theory of dielectric loss taking into account the distribution of adsorption energies among the water molecules shows that the observed resistance and capacitance of the double layer in certain Hg-solution and Cu-solution systems can be reproduced with the assumption of relaxation times of about 10—8 and 10—7 sec, respectively. The effect is small on Hg, but much larger on solid metals where the heat of adsorption of water is greater than on Hg. The resistance and capacitance of the double layer on solid metals is intrinsically frequency dependent and this fact (and the existence of a significant double layer resistance) must be taken into account in the evaluation of Faradaic impedance. If the intrinsic frequency dependence of the double layer is taken into account, the Faradaic resistance and the diffusional capacitance behave theoretically; if a frequency-independent double layer impedance is utilized, irrational values of the Faradaic impedance and reaction resistance are obtained. Diffusion occurs to only about one-fifth of the true surface area.