Oxygen Reduction on Silver in 6.5M Caustic Soda Solution

The cathodic reduction of oxygen in 6.5M membrane-grade caustic soda solution has been studied experimentally at a silver rotating disk electrode at 25~ The results can be approximated by the parallel mechanism for oxygen reduction with catalytic decomposition of peroxide. Further analysis of this mechanism indicates that the sequential process with catalytic decomposition of peroxide predominates over the direct 4eprocess. Direct application of the sequential mechanism to the data indicates that the latter mechanism with catalytic decomposition of peroxide much more accurately reflects the experimental results. The relevant kinetic parameters are calculated on the basis of the mechanisms presented. Silver shows a reasonably high catalytic activity for oxygen reduction in alkaline electrolytes. Furthermore, it serves as an excellent peroxide decomposition catalyst (1, 2). For this reason, silver has attracted considerable interest as a potential oxygen reduction catalyst in fuel cells (3), metal-air batteries (4), and more recently as a replacement of hydrogen-generating cathodes in the chloralkali industry (5). Oxygen reduction on silver has been investigated by several investigators (6-13). The reduction process has been found to follow two reaction pathways: the direct 4epath as represented by reaction [1] 02 + 2H20 + 4e= 4 OH[1] and the 2epath (generally referred to as the sequential path) involving the formation of peroxide as an intermediate product (Eq. [2]) with further reduction of peroxide to OHby Eq. [3] at higher applied potentials 02 + H20 + 2e= HO2+ OH[2] *Electrochemical Society Active Member. HO2+ H20 + 2e= 3 OH[3] Both the direct 4epathway and the sequential pathway have been shown to occur simultaneously on silver (i i). The decomposRion of peroxide occurs by the catalytic reaction in Eq. [4] HO2= 1/2 O2 + OH[4] To enhance its catalytic activity, silver is used in high surface area forms such as porous electrodes where the silver catalyst is dispersed in the porous matrix. In the porous structure, silver acts as a catalyst for the oxygen reaction and at the same time suppresses the peroxide concentration via reaction [4] to a very low level, thereby extending the operating life of the electrode as well as improving the cell efficiency. Serious discrepancies exist in the kinetic parameters reported in the literature for oxygen reduction on silver. For example, Fabjan et al. (12) reported Tafel slopes of -45 to -50 mV/decade as compared to the value of -150 mV! decade reported by Fischer and Heitbam (11). Similarly, the reported exchange current densities show significant ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 129.252.69.176 Downloaded on 2014-10-20 to IP 2510 J. Electrochem. Soc.: E L E C T R O C H E M I C A L S C I E N C E A N D T E C H N O L O G Y October 1988 variations. Similar variations in the kinetic parameters for oxygen reduction on silver have been reported by other investigators as well (10). The discrepancies in the reported values have generally been attributed to the nature of the electrode surface, the different pretreatment of the electrodes, and the condition under which the data are col, lected (10). Another significant factor contributing to the discrepancies in the reported values that has been neglected until recently, is the fact that the classical techniques for electrochemical data analysis are generally inadequate for the analysis of such complex electrochemical reaction systems because of coupling and interaction of the component reactions or processes with each other. An alternative data analysis technique presented recently by the authors (14) is well suited for analyzing such complex reaction systems. To date, a significant proportion of the kinetic data in the literature on oxygen reduction on silver has been reported for potassium hydroxide as the electrolytic solution medium (7-13). Very limited data is available for oxygen reduction in caustic soda solutions, and where available, it has been reported for very low caustic concentrations (6, 15). With increasing application of membrane-cell technology to the production of caustic soda, and with the potential application of the oxygen (or air) cathode as a substitute for the hydrogen-generating cathodes currently in use in the chlor-alkali industry, it is important to understand the kinetics of oxygen reduction in strong caustic solutions. The objective of this work is to elucidate the reaction mechanism and to obtain the kinetic parameters for oxygen reduction on silver in 6.5M sodium hydroxide solution. This concentration is typical of the caustic soda produced from membrane-cells. Also, alkaline fuel cells and alkaline metal-air batteries normally operate with concentrated solutions of KOH (or NaOH) electrolyte [up to 45 weight percent (w/o)] (3). A concentrated solution of the electrolyte has the advantage of lowering the vapor pressure of the solution and thereby avoiding the operation of the cell at high pressures for water product control. Another advantage of using a concentrated solution of the electrolyte is that the ohmic potential drop in the solution can be reduced appreciably. Mechanistic Models for Oxygen Reduction The reduction process involves the transport of oxygen from the bulk solution to the electrode where it is adsorbed on sites at the electrode surface. The peroxide intermediate formed can, in addition to undergoing electrochemical and catalytic decomposition, desorb from the surface to be transported into the bulk solution. Depending on the experimental conditions and the nature of the electrode, the reduction process can occur by a simple reaction step or by a combination of two or more simple reactions as represented by Eq. [1]-[4]. Each simple reaction is made up of a number of elementary steps as postulated in Eq. [la]-[ld], Eq. [2a]-[2d], Eq. [3a]-[3d], and Eq. [4a]-[4c], respectively. The elementary steps represent the mechanistic process by which the simple reactions occur. M 9 9 O indicates the adsorption of a species O on a site M at the electrode surface 02 + 2H20 + 4e~ 4 OH 2 I O 2 + 2 M ~ 2 M . . . O 2(M 9 9 9 O + e---> MO-) 2(MO+ H 2 0 ~-MOH + OH-) 2(MOH + e~ OH+ M) [la]