P. Delahay (Editor), Advances in Electrochemistry and Electrochemical Engineering

The kinetics of reaction of scratched cadmium electrodes in alkaline solution have been measured. The anodic reaction rate of the freshly generated metal surface follows Tafel's law with a slope aE/a log i = 63 mV and a reaction order a log/D log (OH-) = 1.1. The equilibrium parameters are defined by Eo ~ = 18 mV (NHE) and io = 0.002 A cm -2. The parameters are interpreted in terms of simultaneous transfer of two electrons per cadmium atom oxidized from the bare metal surface.The behavior of cadmium electrodes in alkaline solution is still the subject of some debate (1, 2), despite its widespread use as a reversible battery electrode material. Even in acidic sulfate and perchlorate solutions in which the metal does not equilibrate with any of it~ oxides (3), there is no agreement regarding its mechanism of dissolution (4-10). Lorenz (4-6) originally showed that the reaction Cd ~ Cd 2+ -t-2e proceeds in a single step with both electrons being transferred simultaneously; this process has been cited (7) as typifying a two-electron transfer reaction. The work was, however, criticized by Heusler and Gaiser (8) for the limited potential range over which it was studied. By extending the potential range to more positive values, Heusler and Gaiser showed that the anodic behavior deviates from the 60 mV Tafel law given by Lorenz (4-6) and gave a Tafel slope of 108 mV where the anodic current density i is between ~60 and 500 mA-cm -s, The idea that both mechanisms operate in different potential ranges was supported by Hampson et al, (9,10).The mechanism of the initial stages of oxidation in alkaline solution also involves the question of whether each electron is transferred consecutively or whether both electrons are transferred simultaneously (1, 2).The former mechanism would involve the formation of a surface intermediate of Cd(I), probably as adsorbed CdOH, allowing film growth to occur by a solid-state mechanism which does not involve the formation of a dissolved intermediate, as proposed by Armstrong et al. (11)(12)(13). This is not in agreement with the dissolution-precipitation model (14-16) in which metal ions enter solution and are then precipitated to form the film. While there is general agreement that the steady-state film consists primarily of Cd(OH)2 (2), in both the ~ and -y structures (17, 18), the formation of CdO, either anhydrous or hydrated, cannot be ruled out (16,17). Indeed, it is claimed that a thin amorphotis film of CdO may well exist (17) and that this film provides the passivity of the Cd electrode (16). The dissolved ion in alkaline solutions has been identified as Cd(OH)~ 2-(11, 12) or Cd(OH)~-(t5). Species of this kind would probably result from dissolution of Cd(OH)~ rather than acting as dissolved ions prior to precipitation of Cd(OH)2 since it is improbable that three or four hydroxide ligands can react in one step. Thus, measurements of dissolved ion concentra-9 Electrochemical Society Active Member. tions in ring disk electrode studies (12, 16) probably detect dissolution of the films, Mi...

Section: Discussionmentioning

confidence: 99%

Reactivity of Scratched Cadmium Electrodes in Alkaline Solutions

Burstein

1983

“…--1 --e [4] where iid is the limiting diffusion current density of either the cathodic or the anodic process. Since the exchange current density, i ~ of an electrode reaction is a function of the activities of the reacting species, Delahay and Berzins (3) have introduced the following equation i ~ = zFk ~ aox 1-a ar ~ (n = z) [5] where k ~ is the heterogeneous rate constant for electrode reaction [1] and ai is the activity of the oxidized or reduced species. By defining the standard exchange current density, i ~176 as i oo = zFk o or zFk~ )…”

Section: Single Process Electrode Kineticsmentioning

confidence: 99%

“…[6] 1000 Spiro (4) has rewritten Eq. [5] as i ~ = i ~176 aox 1-a ar a (n = z) [7] Generally, Eq. [5] and [7] are written with concentrations substituted for activities.…”

Section: Single Process Electrode Kineticsmentioning

confidence: 99%

See 1 more Smart Citation

Theoretical Analysis of Mixed Potentials

Gray¹,

Cahill²

1969

“…The charge carrier distribution is one of the essential aspects for understanding the application mechanism of mixed ionic-electronic conductors ͑MIECs͒. There have been a number of theoretical treatments of this topic, such as Wagner's fundamental work 1,2 on mixed conduction in general, and some others on fuel cell application in particular. [3][4][5][6][7][8] The essential problem for analytically modeling mixed conduction is how to decouple ionic and electronic transports.…”

mentioning

confidence: 99%

Comparison of the Mobile Charge Distribution Models in Mixed Ionic-Electronic Conductors

Chen

2004

Different models of the steady-state ionic and electronic defect distributions in mixed ionic-electronic conductors under chemical and electrical potential gradients are compared in this paper. Simple analytical expressions without any dummy parameters are obtained under the local charge neutrality approximation and constant electrical field approximations, respectively. The accuracies of the approximate solutions are evaluated by numerically solving the Nernst-Plank-Poisson equation set using the Galerkinweighted residual finite element method. The numerical results show that the constant electrical field approximation introduces errors of less than 10% so long as the average ionic transference number is not smaller than 0.5; and that the local charge neutrality approximation introduces only very small errors ͑less than 3%͒ even when the average ionic transference number is as small as 0.001. Analytical criteria for the constant electrical field approximations to be valid are also given, and the effects of dopant concentration and the sample thickness are discussed. The analytical criteria are in agreement with the numerical results.