The crystalline heterogeneity correction of the double-layer capacity for single-crystal electrodes

Abstract

It has been shown by the present author that the experimental double-layer capacity for single-crystal faces of silver should be corrected for the crystalline heterogeneity (CH), due to the superficial roughness, in order to have significant estimates of the inner-layer capacity [l-3]. In addition, it was proposed that the dependence of the capacity on the potential for CH was similar to that for a single-crystal sphere. [4], or to that for a simplified model of a polycrystalline electrode [3]. As the potential of zero charge (pzc) for the (110) face is close to the potential of the minimum of capacity (pmc) for the two representations of CH, involving consequently a similar capacitance behaviour for (110) and CH, the CH correction could therefore be made directly from the roughness factor [l]. In contrast, as the pzc for the (111) face is far from the pmc for CH, involving a different capacitance behaviour for (111) and CH, the roughness factor can be used only to estimate CH [3]. This CH correction was ignored by Hamelin et al. in recent papers [5,6]. The main argument advanced in ref. 5 is that the Parsons and Zobel (P-Z) plot [7] of C-i =f(C;i), where C and C, stand for the double-layer capacity and the diffuse-layer capacity, should lead to the same slope at the pzc and for slightly positive or negative charges, when CH is responsible only for the deviation of the P-Z slope from unity, namely in the absence of specific ionic adsorption. One can wonder at this proposition and ask oneself whether it is based on sound theoretical or experimental pieces of information. It would therefore appear necessary to consider the CH effect and its consequences from theoretical calculations and experimental capacitance data.