Evaluation of the surface roughness of microporous Ni±Zn±P electrodes by in situ methods R. KARIMI SHERVEDANI and A. LASIA DeÂpartement de Chimie, Universite de Sherbrooke, Sherbrooke, QueÂbec, J1K 2R1, Canada Received 3 September 1998; accepted in revised form 14 January 1999 Key words: CO oxidation, electrode capacitance, hydrogen evolution reaction, nickel based electrodes, surface roughness Abstract The surface roughness of porous Ni±Zn±P electrodes was studied in 1 M NaOH using in situ electrochemical techniques: ratio of the polarization current densities, electrochemical impedance spectroscopy, cyclic voltammetry, coulometric oxidation of the surface, and a new technique of a CO molecular probe. The obtained surface roughness was about 5:5  10 3 . Good agreement was observed between the results obtained by all these techniques. 1. Introduction Evaluation of the roughness factor, R, of the electrode is indispensable in electrocatalysis, particularly in studies of the reaction mechanisms and kinetics and the deter- mination of the intrinsic activities of electrode materials. In situ and ex situ methods [1, 2] may be applied. In situ methods are preferred because the surface properties may change upon drying the electrode material, however, there are no good techniques for the surface roughness determination. For very porous electrodes a precise value of the roughness factor is usually not necessary and a roughly estimated value is usually sucient. Various methods have been previously applied to determine the real surface area of the electrodes. For example, it has been determined from the double layer capacitance using cyclic voltammetry [1, 2] or EIS [3±6], from initial potential-decay curve slopes [4±6], from a comparison of the steady-state currents due to the HER at a constant overpotential [7, 8], from the charge of nickel oxidation using voltammetric [9, 10] or chron- opotentiometric [8] techniques, or from the current corresponding to a redox process in solution [11]. However, dierent techniques (i.e., more than two) are rarely compared in one paper. For some noble metals the charge of the hydrogen underpotential deposition (UPD) may be used for the determination of the surface area [12]. The use of the overpotentially deposited hydrogen (OPD) was also suggested in the literature [13] but this method cannot give correct results because the OPD hydrogen coverage is, in general, lower than one and it depends on the reaction rate constants [3, 14]. Moreover, during the desorption (discharge) process the Volmer and Heyrov- sky reactions proceed in opposite directions but only the net current may be recorded. In some cases the Tafel reaction may also proceed during the discharge decreas- ing the amount of adsorbed hydrogen and causing further errors. CO adsorbs on nickel surface and it may be oxidized to CO 2 [15±18]. It was suggested that such an oxidation is not quantitative in acidic solutions [17] and the coulometric results in alkaline solutions were not conclusive. This reaction could, in principle, be used for the determination of the total surface area. The purpose of the present paper is the comparison of several in situ methods for the determination of the real surface area or surface roughness factors of nickel based electrodes. For these studies we have chosen a recently developed Raney-type Ni±Zn±P electrode [19], which is active, stable and easily prepared by electroplating; it is also characterized by a large real surface area. The surface may be easily changed by changing the leachable zinc content. Several methods were tested: electrochem- ical impedance spectroscopy (EIS), coulometric oxida- tion of the surface, cyclic voltammetry (CV), ratio of the hydrogen evolution currents, and oxidation of CO adsorbed on the electrode surface. Journal of Applied Electrochemistry 29: 979±986, 1999. 979 Ó 1999 Kluwer Academic Publishers. Printed in the Netherlands.