Prof. Dr. Volker Presser

The energy storage mechanism of electric double-layer capacitors is governed by ion electrosorption at the electrode surface. This process requires high surface area electrodes, typically highly porous carbons. In common organic electrolytes, bare ion sizes are below one nanometer but they are larger when we consider their solvation shell. In contrast, ionic liquid electrolytes are free of solvent molecules, but cation-anion coordination requires special consideration. By matching pore size and ion size, two seemingly conflicting views have emerged: either an increase in specific capacitance with smaller pore size or a constant capacitance contribution of all micro- and mesopores. In our work, we revisit this issue by using a comprehensive set of electrochemical data and a pore size incremental analysis to identify the influence of certain ranges in the pore size distribution to the ion electrosorption capacity. We see a difference in solvation of ions in organic electrolytes depending on the applied voltage and a cation-anion interaction of ionic liquids in nanometer sized pores.

In this study, we investigate two different activated carbons and four conductive additive materials, all produced in industrial scale from commercial suppliers. The two activated carbons differed in porosity: one with a narrow microporous pore size distribution, the other showed a broader micro-mesoporous pore structure. Electrochemical benchmarking was carried out in one molar tetraethylammonium tetrafluoroborate in acetonitrile. Comprehensive structural, chemical, and electrical characterization was carried out by varied techniques. This way, we correlate the electrochemical performance with composite electrode properties, such as surface area, pore volume, electrical conductivity, and mass loading for different admixtures of conductive additives to activated carbon. The electrochemical rate handling (from 0.1 A g−1 to 10 A g−1) and long-time stability testing via voltage floating (100 h at 2.7 V cell voltage) show the influence of functional surface groups on carbon materials and the role of percolation of additive particles.

Carbon beads with sub-micrometer diameter were produced with a self-emulsifying novolac-ethanol-water system. A physical activation with CO2 was carried out to create a high microporosity with a specific surface area varying from 771 (DFT) to 2237 m2/g (DFT) and a total pore volume from 0.28 to 1.71 cm3/g. The carbon particles conserve their spherical shape after the thermal treatments. The controllable porosity of the carbon spheres is attractive for the application in electrochemical double layer capacitors. The electrochemical characterization was carried out in aqueous 1 M Na2SO4 (127 F/g) and organic 1 M tetraethylammonium tetrafluoroborate in propylene carbonate (123 F/g). Furthermore, an aqueous redox electrolyte (6 M KI) was tested with the highly porous carbon and a specific energy of 33 W·h/kg (equivalent to 493 F/g) was obtained. In addition to a high specific capacitance, the carbon beads also provide an excellent rate performance at high current and potential in all tested electrolytes, which leads to a high specific power (>11 kW/kg) with an electrode thickness of ca. 200 ?m.

We demonstrate stable hybrid electrochemical energy storage performance of a redox-active electrolyte, namely potassium ferricyanide in aqueous media in a supercapacitor-like setup. Challenging issues associated with such a system are a large leakage current and high self-discharge, both stemming from ion redox shuttling through the separator. The latter is effectively eliminated when using an ion exchange membrane instead of a porous separator. Other critical factors toward the optimization of a redox-active electrolyte system, especially electrolyte concentration and volume of electrolyte, have been studied by electrochemical methods. Finally, excellent long-term stability is demonstrated up to 10 000 charge/discharge cycles at 1.2 and 1.8 V, with a broad maximum stability window of up to 1.8 V cell voltage as determined via cyclic voltammetry. An energy capacity of 28.3 Wh/kg or 11.4 Wh/L has been obtained from such cells, taking the nonlinearity of the charge–discharge profile into account. The power performance of our cell has been determined to be 7.1 kW/kg (ca. 2.9 kW/L or 1.2 kW/m2). These ratings are higher compared to the same cell operated in aqueous sodium sulfate. This hybrid electrochemical energy storage system is believed to find a strong foothold in future advanced energy storage applications.

This study examines the performance of porous carbon as quasi-reference electrode (QRE) in aqueous media and evaluates their suitability. The performance of activated carbon and carbon black as QRE was investigated in acidic (H2SO4) and neutral (Na2SO4, NaCl, Li2SO4) solutions and compared to platinum metal wire and Ag/AgCl reference electrode. In neutral and acidic electrolyte, the porous carbon based QREs exhibited a notable stability and reliability with low level of potential drift (1 mV per day) and potential deviation of less than 10 mV. These results can contribute to the further development in porous carbon based QREs leading to novel opportunities in electrochemical analysis.

We introduce a high performance hybrid electrochemical energy storage system based on an aqueous electrolyte containing tin sulfate (SnSO4) and vanadyl sulfate (VOSO4) with nanoporous activated carbon. The energy storage mechanism of this system benefits from the unique synergy of concurrent electric double-layer formation, reversible tin redox reactions, and three-step redox reactions of vanadium. The hybrid system showed excellent electrochemical properties such as a promising energy capacity (ca. 75 W h kg-1, 30 W h L-1) and a maximum power of up to 1.5 kW kg-1 (600 W L-1, 250 W m-2), exhibiting capacitor-like galvanostatic cycling stability and a low level of self-discharging rate.

The electrochemical flow capacitor (EFC) is a novel design for supercapacitor technologies. To avoid misinterpretation arising from non-flow analytical methods, we have investigated an EFC system under continuous flow conditions. Several different surfactants were introduced as modifiers to activated carbon in an aqueous electrolyte with sodium sulfate (Na2SO4). A significant reduction in viscosity was found by adding sodium lignosulfonate, and as a consequence, a maximum volumetric capacitance of 26 F cm−3 was achieved for the EFC system. A steady performance of the EFC system was observed for 200 h in terms of the specific capacitance (90±5 F g−1); however, degradation in the power performance was observed. Membrane fouling was confirmed to be the major contributor to the power degradation, and a cleaning process using water was developed to partially restore the initial performance (≈70 %).

Herein the application of a recently introduced new method of tracking in-situ the intercalation-induced deformations of supercapacitor and Li-battery electrodes is reviewed. The method is based on the use of multi-harmonic electrochemical quartz microbalance with dissipation monitoring, EQCM-D (in-situ hydrodynamic spectroscopy) which enables a permanent control of the electrodes' state-of-health by probing their mechanical properties. The potential-dependent frequency and resonance width changes are fitted to a chosen hydrodynamic admittance model allowing thus quantification of the electrode deformations under different charging conditions. Intercalation of different alkaline metal cations into layered MXene electrode serves as a readily understandable working example of quantifying such electrodes deformations. Further method developments including in-situ viscoelastic characterization of composite porous electrodes are envisaged in the near future.

Capacitive deionization (CDI) is an emerging technology for the energy-efficient removal of dissolved ions from aqueous solutions. Expanding this technology to non-aqueous media, we present an experimental characterization of a pair of porous carbon electrodes towards electrosorption of dissolved ions in propylene carbonate. We demonstrate that application of CDI technology for treatment of an organic solution with an electrochemical stability window beyond 1.2 V allows for a higher salt removal capacity and higher charge efficiency as compared to CDI applied for treatment of aqueous electrolytes. Further, we show that using conductivity measurements of the stream emerging from the CDI cell combined with an equilibrium electric double-layer structure model, we can gain insights into charge compensation mechanisms and ion distribution in carbon nanopores.