Yuehua Wen

Studies on Iron (Fe3+/Fe2+)-Complex/Bromine (Br2/Br-) Redox Flow Cell in Sodium Acetate Solution

bResearch Institute of Chemical Defense, Beijing 100083, China The formal potential of the FeIII/FeII couple shifts markedly in the negative direction by complexation with ethylenediamine tetraacetate EDTA, oxalate, and citrate. The potentials of the complexes with EDTA and oxalate are less pH-dependent than with citrate. But, the relatively high pH of around 6.0 is favorable electrochemically due to high corresponding currents. Complexation of FeIII/FeII couple can provide fast electrode kinetics except for the complex with citrate. But, the solubility of the complex with citrate is up to 0.8 M. Charge–discharge measurements were conducted with the iron-complex/Br2 redox cells. The results show that performance of the cells with 0.1 M FeIII/FeII-oxalate or FeIII/FeII-citrate is relatively poor due to slow kinetics for the FeIII/FeII-citrate and the unstability of the ferric form for the FeIII/FeII-oxalate, whereas performance of the iron-citrate/Br2 cell is improved considerably by increasing concentration of the FeIII-citrate complex. Also, energy efficiencies of up to approximately 80 and 70% could be obtained for the cell with 0.1 M FeIII/FeII-EDTA and 0.8 M FeIII/FeII-citrate, respectively. The preliminary study shows that novel Br2/iron-complex cells are technically feasible in redox flow batteries but need further investigation.

Novel organic redox flow batteries using soluble quinonoid compounds as positive materials

Novel organic redox flow battery is proposed based on 4, 5- dihydroxy-1, 3-benzenedisulfonate and 2, 5 dihydroxy-benzenedisulfonate as positive materials and PbSO4/Pb as negative electrode in aqueous H2SO4 solution with an ion-exchange membranes between the positive and negative electrolyte. 4, 5-dihydroxy-1, 3-benzenedisulfonate oxide and 2, 5-dihydroxybenzenedisulfonate oxide have good reversibility and relatively high potentials in acidic solutions although the two hydroquinones have different electrochemical mechanism. Hydroquinones are oxidized to quinones at positive electrode during charge and quinones are reduced to hydroquinones during discharge. Results obtained with a small laboratory cell show that high efficiencies can be achieved with an average coulombic efficiency of 99% and energy efficiency of 70% over 100 cycles. High performance obtained indicates that soluble quinones are promising positive materials of novel organic redox flow batteries.

Correlation of Capacitance with the Pore Structure for Nanoporous Glassy Carbon Electrodes

The correlation between the pore structure of monolithic nanoporous glassy carbons (NPGCs) and their double-layer capacitance in KOH-aqueous electrolyte was investigated. The results obtained show that, in general, capacitance increases with surface area. However, the capacitance for NPGCs with high conductivity strongly depends on pore structure. At a low current rate, the double-layer capacitance (DLC) comes mostly from the contribution of micropores with the properties of a metal conductor, while the contribution of external pores with the properties of a semiconductor is very limited. As current densities increase, micropores contribute less to the DLC due to the existence of too narrow micropores, while external pores contribute more to the DLC due to a decrease in the effect of semiconductor behaviors. In addition, it is determined that with the charge-discharge rate increasing, minimum pore sizes that contribute to the DLC increase but are still in the microporous range. It has also been observed that for samples with mesopore or wider micropore size distribution, resistance is low and capacitance becomes insensitive to the charge-discharge rate. Therefore, carbon materials with a wider micropore size distribution have higher surface areas and larger capacity than those with a mesoporosity, being more suitable electrodes for electrochemical capacitor applications.

Effects of zinc and manganese ions in aqueous electrolytes on structure and electrochemical performance of Na0.44MnO2 cathode material

The sodium manganese oxide, Na0.44MnO2, was synthesized by a solid-state reaction routine combined with a sol–gel process using Mn(CH3CO2)2·4H2O as the manganese source. Results show that the capacity and cycling stability of Na0.44MnO2 cathodes are enhanced significantly by using a hybrid aqueous electrolyte (Na2SO4, ZnSO4 and MnSO4). The energy storage mechanism of as-prepared Na0.44MnO2 in the hybrid aqueous electrolyte is associated with the insertion/extraction of zinc and sodium multi-ions with the help of synergistic effects between zinc and manganese ions and the quasi-reversible deposition–dissolution process of Mn2+ ions. The Na0.44MnO2 electrode displays both excellent storage properties with zinc, sodium and manganese ions (∼340 mA h g−1 at 100 mA g−1 after 150 cycles) and reversibility (∼100% coulombic efficiency during cycling). The excellent reversibility and good cycling properties indicate that the Na0.44MnO2 can be a promising material for energy storage devices by using a hybrid aqueous electrolyte.

Effect of extra Li content on the property of tetragonal Li7La3Zr2O12 solid electrolyte prepared by auto-consolidation method

For all-solid-state lithium ion battery, Li7La3Zr2O12 (LLZO) is a unique solid electrolyte possessing both high total ionic conductivity and stability against Li. As to conventional preparation methods, there is a stereotype that higher density always comes from higher pressure enforced upon the LLZO pellets. A different way called “auto-consolidation” was provided by us. Without the demand of pressing operations, the preparation process is greatly simplified. The surface tension of liquid Li2O at sintering temperature is the key factor for samples to consolidate. In this paper, the effect of extra Li content (0-25%) on the property of tetragonal LLZO was studied. Results show that only when the extra Li content reaches up to 10%, can dense tetragonal LLZO be gained. The optimized extra content value is about 15% with a relative density about 93% and a total conductivity of 5.6×10 S cm at 30 C, which is the highest one for tetragonal LLZO in reported issues, about two times higher than that prepared by hot-pressing method. At 30-100 C, the activation energies for total conductivity are about 0.30~0.42 eV atom, decreasing with the increasing of extra Li content, which is slightly lower than the previous reported values. This work suggests a simple and reliable route for the preparation of ceramic solid electrolytes.

A silica-based gel electrolyte system for improving the cycle performance of LiFePO4 batteries in an aqueous medium

LiFePO4 based aqueous lithium batteries using aqueous electrolytes suffer from poor cycling performance. This is mainly caused by the Fe dissolution and Li loss generated from the effects of water. In this paper, fumed silica based gel electrolytes were prepared and optimized for improving the cycling performance of the LiFePO4 electrodes owing to superior stability and comparable ionic conductivity. It was manifested that the Zn/LiFePO4 cells using this homogenous gel electrolyte showed stable charge/discharge voltage profiles and excellent cycling performance at room temperature. The dissolution of Fe and the loss of Li in this electrolyte is significantly suppressed. These superior performances could endow this gel electrolyte as a promising alternative to aqueous electrolyte systems in the LiFePO4 batteries at room temperatures.

Synthesis and Electrochemical Performance of a Benzoquinone-Based Polymer Anode for Aqueous Lithium-Ion Batteries

Using the tetrachloro-p-benzoquinone (TCQ) monomer, poly(benzoquinonyl sulfide) (PBQS) was synthesized by a simple polycondensation reaction. The influence of the molar ratio of S to Na2S on the electrochemical performance of a PBQS anode was assessed by changing the amount of S added. The results showed that the electrochemical performance of PBQS strongly depended on the molar ratio of S to Na2S. When the molar ratio of S to Na2S was 0.4, two Cl were replaced by S, and PBQS with a stable structure was obtained. The discharge capacity of PBQS exceeded 140 mAh∙g -1 . At the same time, PBQS displayed satisfactory rate capability and excellent cyclability. Conversely, when the molar ratio of S to Na2S was decreased to 0.25, Cl was not substituted completely and the polymerization degree was low. Upon increasing the molar ratio of S to Na2S to 0.7, an unstable S―S bond may form in the polymer. The above two factors degraded the electrode performance of these materials.

Uninterruptible flow power system consisting of a zinc-air cell and an organic electro-synthesis reactor

An uninterruptible flow power system (UFPS) is proposed, which is comprised of a positive electrode for catalyzing the organic electro-oxidation, a communal negative electrode, and an air electrode for catalyzing the oxygen reduction and flow electrolytes. During electrolysis of the electro-synthesis reactor, the electrical capacity passed through the communal negative electrode is extracted by the zinc-air cell in order to deliver energy. Under the operation condition of successive organic electro-synthesis, this electrochemical system can be applied as an uninterrupted power source. Thus, the UFPS can be used for the organic electro-synthesis and discharge as power supply simultaneously or asynchronously. This paper demonstrates that the uninterruptible flow power system is feasible by utilizing the equilibrium of zinc deposition/dissolution on a perforated nickel-plated steel electrode with a propanol-oxidation electrode and an air electrode. Such combined electrochemical systems are capable of uninterrupted and stable operation with an energy efficiency of around 66%.