Electrode Design for MnO2-Based Aqueous Electrochemical Capacitors: Influence of Porosity and Mass Loading

International audienceThe purpose of this study is to highlight the influence of some fabrication parameters, such as mass loading and porosity, which are not really elucidated and standardized during the realization of electrodes for supercapacitors, especially when using metal oxides as electrode materials. Electrode calendering, as one stage during the fabrication of electrodes, was carried out step-by-step on manganese dioxide electrodes to study the decreasing porosity effect on the electrochemical performance of a MnO2 symmetric device. One other crucial parameter, the mass loading, which has to be understood and well used for realistic supercapacitors, was investigated concurrently. Gravimetric, areal and volumetric capacitances are highlighted, varying the porosity for low-, medium- and large-mass loading. Low-loading leads to the best specific capacitances but is not credible for realistic supercapacitors, except for microdevices. Down 50% porosities after calendering, capacitances are increased and become stable faster, suggesting a faster wettability of the dense electrodes by the electrolyte, especially for high-mass loading. EIS experiments performed on electrodes without and with calendering lead to a significant decrease of the device’s time response, especially at high loading. A high-mass loading device seems to work as a power battery, whereas electrode calendaring, which allows decreasing the time response, leads to an electrical behavior closer to that expected for a supercapacitor

Intercalated Organic Redox-active Anions for Enhanced Capacity of Layered Double Hydroxides

International audienceA Layered Double Hydroxide (LDH) compound LDH ([Mg 2 Al(OH) 6 ] + x 2 H 2 O) intercalated with a redox active organic anion, Anthraquinone-2-sulfonate (AQS), has been envisioned as an electrode material for high power aqueous based battery. The purpose is to use this interlayer redox active molecule for the enhancement of the specific capacity at the LDH composite electrode, which should allow fast charge transfer at the negative electrode for high power storage applications. This is achieved by the reduction of AQS in charge and oxidation in discharge within a redox inactive LDH matrix. The first charge of this new material [Mg 2 Al(OH) 6 ] + [AQSO 3 ] − x 2 H 2 O leads to a capacity of 100 mAh g −1 at − 0.78 V vs Ag/AgCl (based on the weight of the active material) when operated in aqueous 1 M sodium acetate electrolyte. However, low cycling stability was observed, since a drastic loss in specific capacity occurs after the first charge. This study focuses at elucidating the mechanism behind this phenomenon via in situ UV/vis experiments. Subsequently, the dissolution of charged AQS anions into the electrolyte during the first charge of the anode has been identified and quantified. Such understanding of fading mechanism might lead to the design of improved LDH-based electrodes, which utilize redox active anions working in the positive potential range with enhanced cycling ability

Comparative performances of birnessite and cryptomelane MnO as electrode material in neutral aqueous lithium salt for supercapacitor application

International audienceNa-doped Birnessite-type manganese oxide (δ-MnO2) has been synthesized using the chemical method and characterized through X-ray diffraction and SEM, showing the lamellar structure and high crystal structure. A comparative study of the electrochemical performances of this material with those of the commercial Cryptomelane-type MnO2 has then been undertaken in ten neutral aqueous electrolytes for supercapacitor applications. Aqueous electrolytes, containing a lithium salt, LiX (where X = SO42-, NO3-, CH3CO2-, CH3SO3-, ClO4-, C7H15CO2-, TFSI-, Beti-, BOB-, or Lact-), have been first prepared under neutral pH conditions to reach the salt concentration, providing the maximum in conductivity. Their transport properties are then investigated through conductivities, viscosities, and self-diffusion coefficient measurements. Second, the thermal behaviors of these electrolytic aqueous solutions are then evaluated by using a differential scanning calorimeter from (213.15 to 473.15) K in order to access their liquid range temperatures. Cyclic voltammograms (CV) in three electrode configurations are thereafter investigated using Na Birnessite and Cryptomelane as working electrode material from (−0.05 to 1.5) V versus Ag/AgCl at various sweep rates from (2 to 100) mV*s-1. According to anion nature/structure and manganese oxide material type, different CV responses are observed, presenting a pure capacitive profile for Beti- or C6H13CO2- and an additional pseudocapacitive signal for the smallest anions, such as ClO4- and NO3-. The capacitances, energies, and efficiencies are finally calculated. These results indicate clearly that electrolytes based on a mineral lithium salt under neutral pH condition and high salt concentration (up to 5 mol*L-1) have better electrochemical performances than organic ones, up to 1.4 V with good material stability and capacity retention. The relationship between transport properties, electrostatic and steric hindrance considerations of hydrated ions, and their electrochemical performances is discussed in order to understand further the lithium intercalation-deintercalation processes in the lamellar or tunnel structure of investigated MnO2