R. Kötz

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

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.

DIRECT IN SITU EVIDENCE FOR PROTON/ANION EXCHANGE IN POLYANILINE FILMS BY MEANS OF PROBE BEAM DEFLECTION

Probe beam deflection (PBD) was used to monitor the ion exchange between polymer film and bulk electrolyte during the redox reaction in polyaniline in aqueous electrolyte (HCl, HClO 4 , H 2 SO 4 ). The PBD results clearly show that protons as well as anions are exchanged during the first oxidation process

Oxygen reduction at Pt and Pt70Ni30 in H2SO4/CH3OH solution

The electrochemical oxygen reduction reaction (ORR) was studied at Pt and Pt alloyed with 30 atom% Ni in 1 M H2SO4 and in 1 M H2SO4/0.5 M CH3OH by means of rotating disc electrode. In pure sulphuric acid, the overpotential of ORR at 1 mA cm−2 is about 80 mV lower at Pt70Ni30 than at pure Pt. It was found that in methanol containing electrolyte solution the onset potential for oxygen reduction at PtNi is shifted to more positive potentials and the alloy catalyst has an 11 times higher limiting current density for oxygen reduction than Pt. Thus, PtNi as cathode catalyst should have a higher methanol tolerance for fuel cell applications. On the other hand, no significant differences in the methanol oxidation on both electrodes was found using cycling voltammetry, especially regarding the onset potential for methanol oxidation. During all the measurements no significant electrochemical activity loss was observed at Pt0.7Ni0.3. Ex-situ XPS investigations before and after the electrochemical experiments have revealed Pt enrichment in the first surface layers of the PtNi.

Interfacial Capacitance and Electronic Conductance of Activated Carbon Double-Layer Electrodes

We have measured double-layer capacitance and electronic conductance of an activated carbon electrode in an aprotic electrolyte solution, 1 mol/L in acetonitrile. Both quantities show a similar dependence on the electrode potential with distinct minima near the potential of zero charge. This correlation suggests that the capacitance like the conductance is governed substantially by the electronic properties of the solid, rather than by the properties of the solution side of the double layer. These findings can be explained by treating activated carbon as a metal with a finite density of electronic states at the Fermi level, and with hopping conduction between these states.

UV-irradiation induced modification of PDMS films investigated by XPS and spectroscopic ellipsometry

UV-irradiation (172 nm) induced changes of PDMS surfaces were investigated with X-ray photoelectron spectroscopy (XPS) and spectroscopic ellipsometry (SE). Both methods indicate the modification of the PDMS to a silicalike surface (SiO2). These conclusions could be drawn from the elemental composition determined by XPS and the binding energy shifts in the XPS spectra of the Si 2p and O 1s levels. Similarly the refractive index n determined with ellipsometry reaches a value close to the one of SiO2. Additionally, ellipsometry allows to monitor the decrease of the original film thickness with increasing UV-irradiation time. 2003 Elsevier Science B.V. All rights reserved.

Nanoscale Dimensional Changes and Optical Properties of Polyaniline Measured by In Situ Spectroscopic Ellipsometry

The thickness and optical properties of polyaniline films deposited on gold substrates by electropolymerization were determined using in situ spectroscopic ellipsometry. The polymer films were deposited by potential cycling in 0.1 M aniline/ 1 M H 2 SO 4 . For the simultaneous determination of optical properties and thickness, two approaches were used: (i) The evolution of the ellipsometric parameters 4 and ψ at a constant wavelength was evaluated assuming a homogeneous layer growth resulting in a parabolic growth curve with a film thickness of ca. 65 nm (reduced state) after 40 cycles

The complex electrochemistry of graphite electrodes in lithium-ion batteries

This paper discusses the interrelated phenomena of solid electrolyte interphase (SEI) formation and the irreversible charge consumption which occurs during the first cycle of a graphite electrode, as well as their relevance to the cycling stability of lithium-ion batteries. Thus, results from relevant characterization methods, namely, in situ mass spectrometry, in situ infrared spectroscopy, in situ Raman and video microscopy, in situ scanning probe microscopy, in situ quartz crystal microbalance, and differential scanning calorimetry were combined for a more thorough understanding of observations made in cycling experiments. From electrochemical cycling tests, we have learned that a high specific charge (∼360 Ah/kg of carbon), satisfactory cycle life of the graphite electrodes (1000 deep cycles), and an irreversible charge of <7% during SEI formation can only be obtained when water contamination of the cell is avoided. Under such conditions, a good-quality SEI film is formed on the carbon surface. We conclude that during SEI film formation, at first the carbonate solvent(s) are reduced, forming ethylene gas, organic radicals, oligomers, and polymers. Then a SEI film is precipitated on the surface via a nucleation and growth mechanism. The irreversible charge consumption due to SEI formation is proportional to the BET specific surface area of the graphite and rapidly increases with increasing water content in the cell.

Electrochemically modified glassy carbon for capacitor electrodes: Characterization of thick anodic layers by cyclic voltammetry, differential electrochemical mass spectrometry, spectroscopic ellipsometry, X-ray photoelectron spectroscopy, FTIR, and AFM

Glassy carbon (GC) electrodes were activated by electrochemical constant potential anodization in order to generate high‐surface area, high‐capacitance electrodes. After anodic oxidation in sulfuric acid the electrodes exhibited increased capacitance. After subsequent electrochemical reduction of the activated layer, a further significant increase in capacitance was observed. Growth, structure, and surface properties of the activated electrodes were monitored by cyclic voltammetry, differential electrochemical mass spectrometry, spectroscopic ellipsometry, X‐ray photoelectron spectroscopy (XPS), and atomic force microscopy (AFM). Two different types of glassy carbon obtained by pyrolysis at 1000°C and at 2200°C were compared. Differential electrochemical mass spectrometry reveals that is the main reaction product during oxidation, while and are detected during reduction. The values of surface layer capacitance and thickness determined by spectroscopic ellipsometry increase as linear functions of oxidation time. The resulting volumetric capacitance was at least . After oxidation, the presence of functional surface groups was demonstrated by XPS. The relative contributions of the different surface functionalities depend on the pyrolysis temperature of the GC. Reduction lowered the concentration of oxygen‐containing functional surface groups. The XPS results were qualitatively confirmed by Fourier transform infrared measurements carried out at the same samples. AFM measurements on glassy carbon showed that the film growth both into and out of the substrate, resulted in a raised surface after activation. A qualitative model for film growth is presented.

Sulfonated polyaniline films as cation insertion electrodes for battery applications. Part 1.—Structural and electrochemical characterization

Sulfonated polyaniline (SPAN) was synthesized by sulfonation of polyaniline (PANI) base with fuming sulfuric acid. Thin films were cast from polymer solutions in basic media. The polymer films were characterized by X-ray photoelectron spectroscopy (XPS), Fourier transform infrared (FTIR) spectroscopy, ultraviolet–visible–near-infrared spectroscopy, scanning electron microscopy (SEM) and cyclic voltammetry. XPS in combination with FTIR showed that the preparation procedure led to ca. 47% sulfonation of an otherwise unchanged polyaniline backbone. The NIR spectra of SPAN films showed a polaron band at higher energies than with polyaniline. This is in agreement with the lower conductivity of SPAN as compared with polyaniline. SEM micrographs of the SPAN films showed a compact globular morphology. Electrodes modified with thin SPAN films exhibited two redox steps, both in aqueous and in non-aqueous electrolytes. The specific charge stored in SPAN films was found to be ca. 37 A h kg–1 in aqueous solution (only the first redox step) and ca. 68 A h kg–1 in non-aqueous media (both redox steps). A practical SPAN–Li battery could have 50% more specific energy than a PANI–Li battery. The optical spectra of SPAN films exhibited bands at 310, 450 and 750 nm, the intensities of which changed during the redox process. The absorption coefficients of SPAN (emeraldine base state) solutions had values of a= 410 at 313 nm and a= 239 at 563 nm. The suitability of SPAN for use as a cation-insertion material for battery and electrochromic applications is discussed.

Probe beam deflection investigation of the charge storage reaction in anodic iridium and tungsten oxide films

Abstract The electrochromic charge storage reaction in anodic iridium oxide films (AIROF) was investigated in aqueous electrolytes using the probe beam deflection (PBD) technique in combination with cyclic voltammetry. In add electrolyte (1 M HClO4 for iridium and 1 M H2SO4 for tungsten) the sign of the PBD signal shows clearly that protons are ejected from the oxide film into the bulk electrolyte during oxidation. In basic electrolyte (1 M LiOH, 1 M NaOH) OH− ions are consumed during the oxidation reaction in the AIROF. The PBD signal for the main oxidation peak in the cyclic voltammogram was measured for iridium at different pHs. The change in sign of the beam deflection signal, indicative for one or the other reaction, occurs around pH 4 (±0.5). The coloration of the AIROF in acid was monitored by means of photothermal deflection spectroscopy (PDS) and the results are in good agreement with published reflectivity data.