Contribution of Cations and Anions of Aqueous Electrolytes to the Charge Stored at the Electric Electrolyte/Electrode Interface of Carbon-Based Supercapacitors

For their use in supercapacitors, aqueous electrolytes of acidic (H2SO4), neutral (Na2SO4, K2SO4), and basic (NaOH, KOH) nature are studied, using two microporous binder-free and self-standing carbon cloths as electrodes. The carbon cloths show similar porosities and specific surface areas but different contents in surface oxygen groups. The working potential window and the specific capacitance associated with the cations and anions are measured. From these parameters, the charges stored by the cations and anions at the electric electrolyte/electrode interface are deduced. The charge stored by the cations is higher than that stored by the anions for the three types of electrolytes. The differences between cations and anions are higher for the acidic and basic electrolyte than for the neutral electrolyte and also higher for the carbon cloth with the highest content in surface oxygen groups. The charge stored by the cations follows the sequence H3O+ > Na+ or K+ from the basic electrolytes > Na+ or K+ from the neutral electrolytes. The charge stored by the anions follows the sequence SO42– > HSO4– > OH–. The results here reported provide a better understanding on the electric double layer of carbon-based supercapacitors. Those results are also of interest for asymmetric and hybrid supercapacitors.Financial support from the projects of reference MAT2014-57687-R and FCT-M-ERA-NET/0004/2014, PCIN-2015-024 are gratefully acknowledged

Thermal performance evaluation of a passive building wall with CO2-filled transparent thermal insulation and paraffin-based PCM

Novel thermal insulation materials and wall configurations have the potential to play a major role in reducing energy demand and carbon emissions from the building sector. In this study, a passive heating wall system composed by a CO2-filled transparent thermal insulation (TTI) and an organic phase change material (PCM), and a passive cooling system composed by a Tromble Wall with nano-film and a CO2-filled TTI are proposed and evaluated. The aim is to present a detailed analytical model for rapidly calculating thermal performance of the proposed wall configurations. As case study, a 108 m2 south façade of a building located in Mexico has been used. Outputs suggest that as a passive heating measure, the system has the potential to supply heat in the order of 118 W, 126 W, 134 W, and 157 W, during the months of December, January, February, and March respectively. Additionally, thermal performance and air velocity simulations suggest that for the heating case, considering an outdoor and indoor temperature conditions of 0 °C and 21 °C respectively, the internal layer surface reaches a temperature of 9.2 °C; while for the cooling case, considering outdoor and indoor temperature conditions of 25 °C and 21 °C respectively, it reaches 22.5 °C with a maximum indoor air velocity of 0.5 m/s. Compared to other gases, CO2 could hold a greater potential due to its low thermal conductivity and capital costs. Large-scale implementation of such systems could make the building sector an interesting option as an artificial sink for carbon storage

Understanding the rate performance of microporous carbons in aqueous electrolytes

Variation of specific capacitance versus current density is studied for microporous carbons. Although literature states that capacitance retention is higher for macro/mesoporous than for microporous carbons, the results reported here show that high capacitance retention can be reached for microporous carbons in combination with aqueous electrolytes (2M H2SO4, 1M KOH and 6M KOH). Six carbon monoliths are studied; three pristine ones and those three heat-treated, so as to reduce their content of surface oxygen groups and develops porosity. The capacitance retention is analyzed based on five parameters: electronic conductivity, surface chemistry and porosity of the monoliths, ionic conductivity and type of electrolyte. The capacitance retention is higher for the monoliths working as negative (H3O+ and K+) electrodes than as positive (HSO4− and OH−) ones, being these results of interest for the use of carbon monoliths in asymmetric and hybrid supercapacitors. The highest capacitance retention is obtained by combining (i) monolith electronic conductivity of 11–14 Scm−1 and micropore size of 0.6–0.8 nm for H3O+, K+ and HSO4−, and of 0.85–0.95 nm for OH−; (ii) electrolyte ionic conductivity above 600 mScm−1 and 6M KOH electrolyte, since this electrolyte performs better than 2M H2SO4 and 1M KOH.Funding through the PID2019-104717RB-I00 project is acknowledged to Spanish MICINN

Electrochemical response of a high-power asymmetric supercapacitor based on tailored MnOx/Ni foam and carbon cloth in neutral and alkaline electrolytes

Tailored 3D Ni foams with smaller macropores and larger surface areas than commercial Ni foams are prepared by electrodeposition under the dynamic hydrogen bubble template on stainless steel substrates. These Ni foams are functionalized with electrodeposited manganese oxide (MnOx), resulting in MnOx/Ni foam composites. The electrochemical performance of the composites is studied in aqueous Na2SO4 and KOH electrolyte. Moreover, asymmetric cells, combining the MnOx/Ni foam composites as positive electrodes and carbon cloths as negative electrodes, are tested in the presence of the two electrolytes. Despite the significant number of papers dealing with asymmetric supercapacitors, there is still the need of understanding the electrolyte role for optimizing their electrochemical response. The cell potential window is broader in neutral electrolyte, 1.6 V, compared to the alkaline one, 1.2 V, but the cell capacitance is lower in the neutral electrolyte, 37 F g(-1), than in the alkaline one, 49 F g(-1). The energy density is similar for the two electrolytes, ca. 10 Wh kg(-1). The power density reaches 1-3.10(4) W kg(-1), which is among the highest values reported for asymmetric cells in aqueous electrolytes. The stability of the cells on cycling, floating and self-discharge are compared for the two electrolytes.info:eu-repo/semantics/publishedVersio