Rüdiger Kötz

Bio: Rüdiger Kötz is an academic researcher from Paul Scherrer Institute. The author has contributed to research in topics: Electrode & Electrolyte. The author has an hindex of 63, co-authored 195 publications receiving 17364 citations. Previous affiliations of Rüdiger Kötz include Fritz Haber Institute of the Max Planck Society & Brown, Boveri & Cie.

Principles and applications of electrochemical capacitors

Abstract: Electrochemical capacitors (EC) also called ‘supercapacitors’ or ‘ultracapacitors’ store the energy in the electric field of the electrochemical double-layer. Use of high surface-area electrodes result in extremely large capacitance. Single cell voltage of ECs is typically limited to 1–3 V depending on the electrolyte used. Small electrochemical capacitors for low-voltage electronic applications have been commercially available for many years. Different applications demanding large ECs with high voltage and improved energy and power density are under discussion. Fundamental principles, performance, characteristics, present and future applications of electrochemical capacitors are presented in this communication.

Developments and perspectives of oxide-based catalysts for the oxygen evolution reaction

Abstract: The growing need to store large amounts of energy produced from renewable sources has recently directed substantial R&D efforts towards water electrolysis technologies. Although the description of the electrochemical reaction of water electrolysis dates back to the late 18th century, improvements in terms of efficiency and stability are foreseen for a widespread market penetration of water electrolysers. Particular advances are required for the electrode materials catalysing the oxygen evolution reaction (OER) at the anode side, which has slow kinetics and thus is one of the major sources of the cell efficiency loss. In recent years, high-level theoretical tools and computational studies have led to significant progress in the atomic-level understanding of the OER and electrocatalyst behaviour. In parallel, several experimental studies have explored new catalytic materials with advanced properties and kinetics on a technical relevant level. This contribution summarises previous and the most recent theoretical predictions and experimental outcomes in the field of oxide-based catalysts for the OER, both operating in acidic and alkaline environments.

Capacitance limits of high surface area activated carbons for double layer capacitors

Abstract: A large specific surface area (SSA) of carbon materials used for electrochemical double layer capacitors (EDLC) is the most important parameter leading to a large gravimetric capacitance ( C g ). However, for a SSA determined with the differential functional theory (DFT) model above a value of 1200 m 2 /g the plot of C g versus S DFT exhibits a plateau. We suggest that this limitation of C g can be ascribed to a space constriction for charge accommodation inside the pore walls. As a consequence, the use of extremely high surface area carbons for EDLCs may be unprofitable.

Stabilization of RuO2 by IrO2 for anodic oxygen evolution in acid media

Abstract: Anodic evolution of oxygen on mixed oxides RuxIr1−xO2 has been investigated for x=0,0.3,0.5,0.8 and 1 using electrochemical as well as surface physical techniques. In terms of Tafel slope and corrosion rate the electrochemical behaviour of the mixed oxide is mainly determined by the iridium component for x <0.5. XPS results show that there is no change in surface composition during O2 evolution. Valence band spectra and cyclic voltammetry results suggest that band mixing occurs, giving rise to a shift of oxidation potentials. While the activity of the RuO2 component in the mixed oxide is lost the stability of the slightly activated IrO2 component is maintained. A model for the stabilization effect is proposed.

Temperature behavior and impedance fundamentals of supercapacitors

Abstract: Electrochemical impedance spectroscopy (EIS) is one of the most important analytical tools for characterization of electrochemical double-layer capacitors (EDLC). As an example, we have characterized a commercial capacitor (BCAP0350 Maxwell Technologies) by EIS, and we will discuss the typical performance of an EDLC compared to an ideal capacitor. EIS was used to determine internal resistance and capacitance of the same capacitor as a function of temperature and as a function of time during constant voltage tests. In addition, the effect of the electrolyte on the temperature behavior was investigated. While the capacitance is a very weak function of temperature, the ESR increases significantly with reduced temperature. Temperature effects are much more pronounced for propylene carbonate (PC) than for acetonitrile (AN)-based electrolytes. From the data obtained at various temperatures and voltages, we could determine acceleration factors for the degradation. On the basis of an Arrhenius plot of the leakage current measured during load life tests at capacitor voltages between 2.5 V and 3.0 V and temperatures between −40 °C and +70 °C, we determined acceleration factors for capacitor degradation of about 2 for a temperature increase of 10 °C and also a factor of about 2 for a potential increase of 0.1 V.

Materials for electrochemical capacitors

Abstract: Electrochemical capacitors, also called supercapacitors, store energy using either ion adsorption (electrochemical double layer capacitors) or fast surface redox reactions (pseudo-capacitors). They can complement or replace batteries in electrical energy storage and harvesting applications, when high power delivery or uptake is needed. A notable improvement in performance has been achieved through recent advances in understanding charge storage mechanisms and the development of advanced nanostructured materials. The discovery that ion desolvation occurs in pores smaller than the solvated ions has led to higher capacitance for electrochemical double layer capacitors using carbon electrodes with subnanometre pores, and opened the door to designing high-energy density devices using a variety of electrolytes. Combination of pseudo-capacitive nanomaterials, including oxides, nitrides and polymers, with the latest generation of nanostructured lithium electrodes has brought the energy density of electrochemical capacitors closer to that of batteries. The use of carbon nanotubes has further advanced micro-electrochemical capacitors, enabling flexible and adaptable devices to be made. Mathematical modelling and simulation will be the key to success in designing tomorrow's high-energy and high-power devices.

Solar Water Splitting Cells

Abstract: Energy harvested directly from sunlight offers a desirable approach toward fulfilling, with minimal environmental impact, the need for clean energy. Solar energy is a decentralized and inexhaustible natural resource, with the magnitude of the available solar power striking the earth’s surface at any one instant equal to 130 million 500 MW power plants.1 However, several important goals need to be met to fully utilize solar energy for the global energy demand. First, the means for solar energy conversion, storage, and distribution should be environmentally benign, i.e. protecting ecosystems instead of steadily weakening them. The next important goal is to provide a stable, constant energy flux. Due to the daily and seasonal variability in renewable energy sources such as sunlight, energy harvested from the sun needs to be efficiently converted into chemical fuel that can be stored, transported, and used upon demand. The biggest challenge is whether or not these goals can be met in a costeffective way on the terawatt scale.2

A review of electrode materials for electrochemical supercapacitors

Abstract: In this critical review, metal oxides-based materials for electrochemical supercapacitor (ES) electrodes are reviewed in detail together with a brief review of carbon materials and conducting polymers. Their advantages, disadvantages, and performance in ES electrodes are discussed through extensive analysis of the literature, and new trends in material development are also reviewed. Two important future research directions are indicated and summarized, based on results published in the literature: the development of composite and nanostructured ES materials to overcome the major challenge posed by the low energy density of ES (476 references).

Graphene-Based Ultracapacitors

Abstract: The surface area of a single graphene sheet is 2630 m2/g, substantially higher than values derived from BET surface area measurements of activated carbons used in current electrochemical double layer capacitors. Our group has pioneered a new carbon material that we call chemically modified graphene (CMG). CMG materials are made from 1-atom thick sheets of carbon, functionalized as needed, and here we demonstrate in an ultracapacitor cell their performance. Specific capacitances of 135 and 99 F/g in aqueous and organic electrolytes, respectively, have been measured. In addition, high electrical conductivity gives these materials consistently good performance over a wide range of voltage scan rates. These encouraging results illustrate the exciting potential for high performance, electrical energy storage devices based on this new class of carbon material.

Carbon-based materials as supercapacitor electrodes

Abstract: This tutorial review provides a brief summary of recent research progress on carbon-based electrode materials for supercapacitors, as well as the importance of electrolytes in the development of supercapacitor technology. The basic principles of supercapacitors, the characteristics and performances of various nanostructured carbon-based electrode materials are discussed. Aqueous and non-aqueous electrolyte solutions used in supercapacitors are compared. The trend on future development of high-power and high-energy supercapacitors is analyzed.