B. Fang

A study of the Ce(III)/Ce(IV) redox couple for redox flow battery application

Abstract In this study, the electrochemical behavior of the Ce(III)/Ce(IV) redox couple in sulfuric acid medium with various concentrations and the influence of the operating temperature were investigated. A change of the concentration of sulfuric acid mainly produced the following two results. (1) With an increase of the concentration of sulfuric acid the redox peak currents decreased. (2) The peak potential separation for the redox reactions increased with rising concentration of sulfuric acid from 0.1 to 2 M and then decreased with further increase of the concentration. Elevated temperature was electrochemically favorable for Ce(III)/Ce(IV) couple, which caused an increase of the peak currents for the redox reactions and a decrease of the peak potentials separation. Constant-current electrolysis shows that the current efficiency was 73% for the oxidation process of Ce(III) and 78% for the reduction process at 298 K, and could be improved by elevating the temperature. The open-circuit voltage of the Ce–V cell, after full charging, remained constant at 1.870±0.005 V for more than 48 h, and is about 29% higher than that of the all-vanadium batteries. The coulombic efficiency was approximately 87%, showing that self-discharge of the Ce–V battery was small. The preliminary exploration shows that the Ce(III)/Ce(IV) couple is electrochemically promising for redox flow battery (RFB) application.

Development of a novel redox flow battery for electricity storage system

A novel cylindrical battery which uses carbon fibres with high specific surface area as electrodes and a porous silica glass with high chemical stability as membrane has been fabricated. The results obtained from electrolysis of 0.5 M K3Fe(CN)6–0.5 M KCl and of 85 mM V(IV)–1 M H2SO4 indicate that the cell possesses excellent electrolytic efficiency. As a redox flow battery (RFB) its performance was investigated by employing all-vanadium sulfate electrolytes. The results of the cyclic voltammetry measurements indicate that at a glassy carbon electrode the electrochemical window for 2 M H2SO4 solution could reach 2.0 ∼ 2.4 V. Constant current charging–discharging tests indicate that the batteries could deliver a specific energy of 24 Wh L−1 at a current density of 55 mA cm−2. The open-circuit cell voltage, after full charging, remained constant at about 1.51 V for over 72 h, while the coulombic efficiency was over 91%, showing that there was negligible self-discharge due to active ions diffusion through the membrane during this period.

Mechanism for nucleation and growth of electrochemical deposition of palladium(II) on a platinum electrode in hydrochloric acid solution

Palladium(II) and chloride ions tend to form complexes in aqueous solution. Both theoretical and experimental (by UV spectrum) results indicate that there are four complexes formed in aqueous solution containing 3 mol/L hydrochloric acid and 20 mmol/L PdCl2. This work evaluates the kinetics of electrochemical deposition of palladium on a Platinum electrode. For this purpose, palladium electrodeposition was investigated by means of cyclic voltammetry (CV), potentiostatic current-time transients (CTTs) and Tafel curve. By CTTs curves, the regions corresponding to the charge transfer control, mixed control and diffusion control were identified. In the diffusion control region, palladium electrodeposition mechanism was characterized as progressive nucleation with three-dimensional (3D) growth under diffusion control; as for the mixed control region, an adsorption (IAds), ion transfer (IIT), and nucleation and growth (ING) model were proposed to analyze the current-time transients quantitatively, which could separate the IAds, IIT and ING perfectly.

Electrochemical behavior and electrowinning of palladium in nitric acid media

In this study, the electrochemical behavior of Pd(II) in nitric acid media was investigated using various electrochemical techniques. By analyzing the cyclic voltammogram of Pd(II) recorded at Pt electrode, a series of electrochemical reactions associated with palladium were recognized, indicating that Pd(II) undergoes a single step two-electrons irreversible process. Electroreduction reaction of Pd(II) and auto-catalytic reactions of nitrous acid are supposed to play a leading role in low and high concentrations of nitric acid, respectively. Stirring could facilitate the reduction of Pd(II) in relatively low nitric acid concentration (⩽ 3 mol/L). The value of charge transfer coefficient was determined to be 0.18 for the measurements at 298 K. The diffusion coefficient of Pd(II) increased from 1.89 × 10−8 cm2/s at 288 K to 4.23 × 10−8 cm2/s at 318 K, and the activation energy was calculated to be 21.5 kJ/mol. In electrowinning experiments, SEM images of palladium obtained by electrolysis reveal the dendrite growth in all cases, which is uniform all over the entire surface of Pt electrode. The recovery ratios of Pd at different nitric acid concentrations are high, and the faradic efficiency of electrolysis decreases with increasing the nitric acid concentration. When stirring was introduced during electrolysis, the electrodeposition rate of Pd increased substantially.