New aspects of the electrochemical-catalytic (EC’) mechanism in square-wave voltammetry

A Hydrogen-Bromate Flow Battery as a Rechargeable Chemical Power Source

The hydrogen-bromate flow battery represents one of the promising variants for hybrid power sources. Its membrane-electrode assembly (MEA) combines a hydrogen gas diffusion anode and a porous flow-through cathode where bromate reduction takes place from its acidized aqueous solution: BrO3− + 6 H+ + 6 e− = Br− + 3 H2O (*). The process of electric current generation occurs on the basis of the overall reaction: 3 H2 + BrO3− = Br− + 3 H2O (**), which has been studied in previous publications. Until this work, it has been unknown whether this device is able to function as a rechargeable power source. This means that the bromide anion, Br−, should be electrooxidized into the bromate anion, BrO3−, in the course of the charging stage inside the same cell under strongly acidic conditions, while until now this process has only been carried out in neutral or alkaline solutions with specially designed anode materials. In this study, we have demonstrated that processes (*) and (**) can be performed in a cyclic manner, i.e., as a series of charge and discharge stages with the use of MEA: H2, Freidenberg H23C8 Pt-C/GP-IEM 103/Sigracet 39AA, HBr + H2SO4; square cross-section of 4 cm2 surface area, under an alternating galvanostatic mode at a current density of 75 mA/cm2. The coulombic, voltaic and energy efficiencies of the flow battery under a cyclic regime, as well as the absorption spectra of the catholyte, were measured during its operation. The total amount of Br-containing compounds penetrating through the membrane into the anode space was also determined.

Hydrogen-Chlorate Electric Power Source: Feasibility of the Device, Discharge Characteristics and Modes of Operation

A power source based on the current-generating reaction of aqueous chlorate-to-chloride reduction by molecular hydrogen would provide as much as 1150 Wh per 1 L of reagent storage (for a combination of 700 atm compressed hydrogen and saturated aqueous solution of lithium chlorate) at room temperature, but direct electroreduction of chlorate only proceeds with unacceptably high overvoltages, even for the most catalytically active electrodes. In the present study, we experimentally demonstrated that this process can be performed via redox-mediator catalysis by intermediate products of chlorate reduction, owing to their participation in homogeneous com- and disproportionation reactions. A series of current–voltage and discharge characteristics were measured for hydrogen-chlorate membrane–electrode assembly (MEA) cells at various concentrations of chlorate and sulfuric acid under operando spectrophotometric monitoring of the electrolyte composition during the discharge. We established that chlorine dioxide (ClO₂) is the key intermediate product; its fraction in the electrolyte solution increases progressively, up to its maximum, equal to 0.4–0.6 of the initial amount of chlorate anions, whereas the ClO₂ amount decreases gradually to a zero value in the later stage. In most discharge experiments, the Faradaic yield exceeded 90% (maximal value: 99%), providing approximately 48% chemical energy storage-to-electricity conversion efficiency at maximal power of the discharge (max value: 402 mW/cm²). These results support prospect of a hydrogen-chlorate flow current generator as a highly specific energy-capacity source for airless media.

Simulation-Based Approach to Determining Electron Transfer Rates Using Square-Wave Voltammetry

The efficiency with which square-wave voltammetry differentiates faradic and charging currents makes it a particularly sensitive electroanalytical approach, as evidenced by its ability to measure nanomolar or even picomolar concentrations of electroactive analytes. Because of the relative complexity of the potential sweep it uses, however, the extraction of detailed kinetic and mechanistic information from square-wave data remains challenging. In response, we demonstrate here a numerical approach by which square-wave data can be used to determine electron transfer rates. Specifically, we have developed a numerical approach in which we model the height and the shape of voltammograms collected over a range of square-wave frequencies and amplitudes to simulated voltammograms as functions of the heterogeneous rate constant and the electron transfer coefficient. As validation of the approach, we have used it to determine electron transfer kinetics in both freely diffusing and diffusionless surface-tethered species, obtaining electron transfer kinetics in all cases in good agreement with values derived using non-square-wave methods.

A mechanistic study of the EC′ mechanism – the split wave in cyclic voltammetry and square wave voltammetry

In this paper, a detailed investigation of electrochemical reactions coupled with homogenous chemical steps using cyclic voltammetry (CV) and square wave voltammetry (SWV) was carried out to study the electrocatalytic (EC′) mechanism.