The Electrochemical Behavior of the Nickel Oxide Electrode: Part Ii. Quasi-Equilibrium Behavior

Abstract

The electrode potential of the nickel oxide electrocle has been determined as a function of water and solute activity in aqueous solutions of potassium hydroxide. The electrode, which was charged to a mean state of oxidation corresponding to 5 0 y 0 Ni I1 and 50yo Ni 111, was examined after long periods of time and by cathodic and anodic e.m.f. decay meas~irements after polarization in order to establish the quasi-reversible potentials by two methods. The results are discussed in terms of the stoichiometry of the potential-determining reaction, and distinction between mixed and true reversible potentials is made. INTRODUCTIONWe have previously examined in part I (1) the kinetic behavior of the "overcharged" nickel oxide electrode with regard to rates of self-discharge and oxygen evolution. In the present paper we present data on the quasi-equilibriunl potentials of the nickel oxide electrode in aqueous potassium hydroxide solutions. The behavior of the electrodes is referred to as "quasi-equilibrium" since the electrodes are continuously losing charge on open-circuit by the corrosion type of process described previously (1).This can affect the electrode in two ways: firstly to produce a continuous but slow decrease of the state of oxidation of the oxide and secondly to give rise to a n electrode potential which is more cathodic than the true equilibrium potential corresponding to the formal state of oxidation of the oxide. In fact, the potentials measured on opencircuit are analogous to mixed potentials observed in corrosion, since there is a slow anodic evolution of oxygen and a corresponding cathodic self-reduction of the oxide. This factor does not seem to have been realized in previous work (2, 3, 4). However, by suitable choice of conditions and inethods of study it is possible to determine the potential of the electrode under conditions very near to equilibrium. Thus, if the electrodes are left in dilute potassium hydroxide (0.001 M) for several days, the potential ceases to change by more than 0.1 111v per hour and the potential of the electrode may then be examined as a function of potassiunz hydroxide activity without further significant selfdischarge during the measurements. Also in the solutions of higher allcali concentration the electrode is lllore stable* since the open-circuit potential becomes less anodic to the reversible ox).gen potential and the rate of self-discharge (already allowed to become small in the 0.001 M solution) becomes negligible.Electrode potentials and stoichiometric reactions for the potential-determi~li~~g reaction a t the nickel oxide electrode have been discussed in earlier work by 1;orster (2) and Zedner (3) and more recently by Kornfeil (4). ' 4 potential of 0.40 v has been attributed to the nickel oxide electrode by Latimer ( 5 ) , but no eviclence that the potential-determining species (e.g. NiOz) are those written by Latimer (5) has been given; in fact there is no evidence that NiOz exists as such in the electrocl~emicall~ active material in the nickel

Redox and electrochemical water splitting catalytic properties of hydrated metal oxide modified electrodes

“…There is an assumption evident in the mathematical treatment outlined in eqn (79)(80)(81)(82)(83)(84)(85)(86) that the capacitance C is constant. 289 In fact, Conway and Bourgault [290][291][292] have shown that interpretations of OCP decay slopes in terms of rate-determining mechanisms are only valid under conditions where the electrode surface capacity is independent of potential. In situations where the surface capacity varies during self-discharge the observed decay slope can differ quite significantly from the steady-state Tafel slope.…”

Study on Electrode Potential of Zinc Nickel Single-Flow Battery during Charge

“…If the value of c ranges from 7.0 × 10 −4 to 14.7 × 10 −3 mol/cm 3 [21], the equilibrium potential curve of NiOOH electrode, as shown in Figure 3, can be obtained.…”

“…If the value of c ranges from 7.0 × 10 −4 to 14.7 × 10 −3 mol/cm 3 [21], the equilibrium potential curve of NiOOH electrode, as shown in Figure 3, can be obtained. From the positive half reaction, it can be known that the concentration of potassium hydroxide decreases during charge.…”

Electrochemistry of the Nickel Oxide Electrode: Part Iv. Electrochemical Kinetic Studies of Reversible Potentials as a Function of Degree of Oxidation

“…Niclrel hydroxide electrodes were prepared in sintered-nickel plaques by methods described in detail previously (1,3,5). Each electrode was charged and discharged three times before use, a t a 5-hour charging rate, in order to stabilize its electrochemical behavior (3,4).…”

“…A polarization decay method was employed (1,3,4) in which the reversible potential was approached both fro111 tlle anodic and the cathodic directions.…”