Digital simulation of steady state and non-steady state voltammetric responses for electrochemical reactions occurring at an inlaid microdisk electrode

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.

Bromate anion reduction: novel autocatalytic (EC″) mechanism of electrochemical processes. Its implication for redox flow batteries of high energy and power densities

Recent theoretical studies of the bromate electroreduction from strongly acidic solution have been overviewed in view of very high redox-charge and energy densities of this process making it attractive for electric energy sources. Keeping in mind non-electroactivity of the bromate ion the possibility to ensure its rapid transformation via a redox-mediator cycle (EC′ mechanism) is analyzed. Alternative route via the bromine/bromide redox couple and the comproportionation reaction inside the solution phase is considered within the framework of several theoretical approaches based on the conventional Nernst layer model, or on its recently proposed advanced version (Generalized Nernst layer model), on the convective diffusion transport equations. This analysis has revealed that this process corresponds to a novel (EC″) electrochemical mechanism since the transformation of the principal oxidant (bromate) is carried out via autocatalytic redox cycle where the bromate consumption leads to progressive accumulation of the bromine/bromide redox couple catalyzing the process. As a result, even a tracer amount of its component, bromine, in the bulk solution leads under certain conditions to extremely high current densities which may even overcome the diffusion-limited one for bromate, i.e. be well over 1 A/cm2 for concentrated bromate solutions. This analysis allows one to expect that the hydrogen–bromate flow battery may generate very high values of both the current density and specific electric power, over 1 A/cm2 and 1 W/cm2.

ANALYTICAL SOLUTION FOR THE STEADY-STATE CHRONOAMPEROMETRIC CURRENT FOR AN EC′ REACTION AT SPHEROIDAL ULTRAMICROELECTRODES

The steady-state chronoamperometric current for an EC′ reactions at spheroidal ultramicroelectrodes is derived from the non-steady-state diffusion limited current. The polynomial expressions pertaining to two extreme limits of reaction rates are combined for all reaction rates. Starting with the result for spheroidal electrode, equations are obtained for the steady-state currents at disc, oblate, hemisphere and prolate electrodes. Tabular compilation of dimensionless current for disc electrodes are reported. A good agreement with previously available simulation results is noticed.