Wendy Pell

Self-discharge and potential recovery phenomena at thermally and electrochemically prepared RuO2 supercapacitor electrodes

Abstract Films of ruthenium oxide (RuO 2 ) exhibit large, almost constant capacitance, over a potential range of ~ 1.4 V in aqueous acid solutions. This behaviour has led to their development as supercapacitor materials giving many Farads per gram. Applications of electrochemical capacitors require minimum self-discharge rates. In the present paper, the self-discharge kinetics of charged RuO 2 electrodes are studied and a remarkable phenomenon of successive potential recovery after sequential discharge transients is reported. The self-discharge and potential-recovery behaviour is analysed in terms of a process of diffusion of oxidation state involving proton and electron hopping, treated in a model of the RuO 2 film structure having three regions between which redistribution of oxidation states in the oxide film takes place on self-discharge and recovery.

Diagnostic analyses for mechanisms of self-discharge of electrochemical capacitors and batteries

Abstract In the charged condition, electrochemical capacitors, like batteries, are in a state of high energy relative to that of the system in the discharged state. Hence there is a ‘driving force’, corresponding to the free energy of discharge, tending to spontaneously diminish the charge if some mechanism(s) of self-discharge exist. An ideally polarizable (chargeable) capacitor has no self-discharge or current-leakage pathway and hence can remain charged indefinitely. However, practical capacitors, like batteries, suffer appreciable self-discharge over periods of days or months so that this phenomenon is of major interest in evaluation of capacitor performance and choice of materials to minimize self-discharge. Several mechanisms of self-discharge are distinguished and the resulting forms of the change of potential on open-circuit with time or log time provide a means of identifying the type of self-discharge process that occurs. With RuO 2 , some remarkable potential-recovery effects arise following discharge.

Voltammetry at a de Levie brush electrode as a model for electrochemical supercapacitor behaviour

Electrochemical supercapacitors provide electrical energy storage systems complementary to batteries. Based on the double layer capacitance of high area porous electrode materials, e.g. carbon powders, felts, foams, aerogels or on the redox pseudocapacitance of oxide or polymer films, specific capacitances of the order of 50∼100 F g−1 are realizable. However, the porous nature of the electrode structures introduces a distribution of resistive and capacitative elements giving rise to electrical behaviour like that of a transmission line, as treated by de Levie, with a resulting complex power spectrum depending on charging or discharging rates. The present paper examines the cyclic voltammetry behaviour of de Levie type wire brush electrodes as models for porous electrodes, in comparison with that of single wire electrodes of the same metal. Comparisons are also made with constant current charging behaviour and with the electrochemical behaviour of specially made, 3 V, non aqueous solution, double layer capacitor modules, examined under similar conditions in relation to the current response profiles of a 5 RC element hardware model circuit. These approaches enable the effects of the distribution of R and C elements on charge acceptance and delivery at various rates to be quantitatively evaluated for various resistivities of the conducting electrolyte in pores.

Analysis of non-uniform charge/discharge and rate effects in porous carbon capacitors containing sub-optimal electrolyte concentrations

The effects of internal electrolyte resistance on the acceptance and delivery of charge by a series of 3 V, non-aqueous, double-layer capacitors have been investigated by cyclic voltammetry and dc charging/discharging procedures conducted at different rates. The data obtained enable Ragone plots to be constructed for the performance of the capacitor modules. A series of four capacitor modules have been constructed using 2000 m2 g−1 carbon and containing tetraethylammonium tetrafluoroborate in dry propylene carbonate as electrolyte at four concentrations: 1.0, 0.4, 0.2 and 0.08 M. The lower concentrations are chosen to introduce electrolyte starvation deliberately and thus to be able to investigate the effects of such conditions on the discharge/recharge behaviour of the capacitor modules. In dilute solutions, electrolyte starvation can become enhanced on charge by redistribution of cations and anions of the electrolyte due to adsorption of the ions at respective negative and positive surfaces of the capacitor electrode matrices, from the electrolyte bulk. The magnitude of such effects depends on the adsorption charge capacity in relation to the total electrolyte ion concentrations available in the device and to the potential attained at the termination of charging.

Stages in the development of thick cobalt oxide films exhibiting reversible redox behavior and pseudocapacitance

Thick hydrous oxide films can be grown on Co metal electrodes in aqueous NaOH under conditions of potential cycling in cyclic voltammetry. Such films develop reversible reduction/reoxidation and corresponding pseudocapacitance behavior after formation by about 2800 cycles over the potential range −0.2 or +0.1 to 1.56 V at a sweep-rate of 20 mV s−1. Equations expressing the origin of such pseudocapacitance, based on the Nernst equation for an electrochemical redox system, are derived and have similar forms to those for electrosorption pseudocapacitance. By recording voltammograms successively during a total history of 2800 cycles, the steady progression of behavior from completely irreversible oxide formation and reduction (like that at Pt or Au) to the development of a largely reversible redox oxide film is demonstrated. Progressive evaluation of the anodic and cathodic charges passed in each cycle, together with their charge–discharge imbalance, enables the total charge equivalent of the formed oxide to be calculated. This result shows that the anodic and cathodic charges which can be delivered reversibly after the thick film has been generated amount to only about 3% of the total formation charge. The reversible behavior of the developed film is therefore associated with an outer, near-surface region of the oxide, as has also been found with RuO2.

Analysis of power limitations at porous supercapacitor electrodes under cyclic voltammetry modulation and dc charge

A simple capacitor is perceived as being capable of discharge (or recharge) at high rates, limited only by a small equivalent series resistance. However, in the case of electrochemical capacitors, based on high specific area porous electrode materials, power limitations arise due to the complex-distribution of electrolyte internal resistance, coupled with double-layer or pseudo-capacitative elements. The present paper quantitatively examines both numerically and experimentally the effects of internal electrolyte resistance as evaluated by the current-response functions for porous capacitor electrodes, generated in cyclic voltammetry (CV) experiments and for voltage versus time responses in dc charge/discharge regimes.

Electrochemical efficiency in multiple discharge/recharge cycling of supercapacitors in hybrid EV applications

The use of large capacitance supercapacitors in series or series/parallel configurations has been envisaged and evaluated as a basis for load-levelling, and hence performance enhancement, in electric vehicles (EV) powered primarily by rechargeable batteries or fuel-cells. In this paper, charge/discharge efficiency in duty-cycles of the supercapacitor component are examined in relation to distributed resistance in porous double-layer and redox-type devices which restricts efficiency of discharge and recharge cycling, especially at high rates. Results of quantitative experiments on the responses of a five-element RC model hardware equivalent-circuit and a gold brush electrode are presented. Potential-recovery after discharge and potential-decay after recharge, and potential changes following load variations, enable efficiencies of charge delivery and acceptance to be evaluated.

Quantitative modeling of factors determining Ragone plots for batteries and electrochemical capacitors

Abstract Ragone plots relating power-density to achievable energy-density have been used for many years as an empirical basis for comparative performance evaluation of various battery systems. However, their theoretical basis, and the relative contributions of ohmic and activation (Tafel) polarization effects to the dependence of energy-density on operating power-density have been little studied. Quantitative modeling calculations for various selected values of ohmic resistance, and Tafel exchange-current densities and polarization slopes, b, provide rationalizations for the shape of Ragone diagrams and their positions in the log-log plane of energy-density versus power-density plots. Separate evaluations of the ohmic and Tafel polarization ‘components’ of the Ragone relations are of practical value in the evaluation of the battery performance. The Tafel value b is of special importance. Complications which arise in trying to represent electrochemical capacitor behaviour in terms of Ragone-type plots are analysed; they are important for the design and operation of hybrid capacitor/battery systems for electric-vehicle drive trains.

Capacitance of the double-layer at polycrystalline Pt electrodes bearing a surface-oxide film

Abstract Although double-layer charging corrections to results of charge-transient and cyclic voltammetry current responses are required in experiments conducted at oxidized Pt electrodes, little information is available on the interfacial double-layer capacitance at such surfaces. Problems have arisen in both electro-reflectance and nanogravimetry studies related to the double-layer charging correction. In the present work, the interfacial capacitance of an oxidized polycrystalline Pt surface was determined by means of impedance spectroscopy at a series of descending controlled potentials in order to avoid time-dependent effects that otherwise arise due to oxide film growth. The experimental impedance data for an oxidized polycrystalline Pt electrode in 0.5 M H 2 SO 4 or 0.5 M HClO 4 between 0.9 and 1.4 V (RHE) were best described by a CPE with a ϕ value greater than 0.93, and increasing with increasing potential. The double-layer capacitance decreased from 80 to 40 μF cm −2 between the potential limits of 0.9 and 1.4 V (RHE). Both ϕ and C were independent of added Cl − concentration within the concentration range 0–0.01 M. The variation in capacitance may be attributed to the diminishing contribution of anion adsorption pseudocapacitance as the potential is increased and/or residual pseudocapacitance due to a Pt 2+ /Pt 4+ redox couple of the film. The double-layer capacitance at unoxidized Pt metal was also determined in 0.5 M H 2 SO 4 (including 0, 0.001 M and 0.01 M Cl − ) and in 0.5 M HClO 4 (including 0, 0.001 M and 0.01 M Cl − ) at 100 mV intervals in the double-layer charging region on Pt.

Nanogravimetry study of the processes of anodic dissolution and oxide-film formation at a gold electrode in aq. HClO4 containing Br− ions by means of EQCN

The anodic dissolution of gold, coupled with anodic oxide formation, in aq. HClO4 in the presence of various concentrations of Br− ions was studied using cyclic voltammetry and chronoamperometry, complemented by nanogravimetry using an electrochemical quartz crystal nanobalance (EQCN). The results clearly indicate that gold initially dissolves through a 3e− oxidation process, qualitatively supported by UV–vis spectrometry. The kinetics of the Au dissolution process was evaluated using CV, EQCN, chronoamperometry, and rotating-electrode experiments. The linear relationship between mass change and inverse of square root of sweep-rate and between-anodic peak currents at ca. 1.16 V (RHE), and square root of sweep-rate indicated quantitatively that the dissolution process is diffusion-controlled.