G. Sundholm

Influence of the composition on the structure and electrochemical characteristics of the PEFC cathode

The influence the composition of the cathode has on its structure and electrochemical performance was investigated for a Nafion content spanning from 10 to 70 wt.%. The cathodes were formed on a Na ...

A novel polymer electrolyte fuel cell for laboratory investigations and in-situ contact resistance measurements

A novel polymer electrolyte membrane fuel cell and assembly was developed for laboratory investigations. In this cell a simultaneous measurement of clamping pressure and contact resistances is possible. In the study presented this paper, the cell was utilised in in-situ contact resistance measurements of unplated and plated stainless steel (type 316). These contact resistances were studied in situ as a function of time, clamping pressure, gas pressure and current density. Ex-situ measurements were used to validate the in-situ contact resistance measurements. The validity and error sources of the applied in-situ measurement method were studied using both computer simulations and experiments.

Electrochemical characterization of PVDF-based proton conducting membranes for fuel cells

The electrochemical characterization of proton-conducting membranes prepared by irradiation-induced grafting and subsequent sulfonation of PVDF films has been performed. In particular, measurements ...

Coupling between ionic defect structure and electronic conduction in passive films on iron, chromium and iron–chromium alloys

Abstract A quantitative kinetic model is presented for the steady-state passive films on Fe, Cr and Fe–Cr alloys. It emphasises the coupling between the ionic defect structure and electronic conduction. According to the model, the passive film can be represented as a heavily doped n-type semiconductor–insulator-p-type semiconductor junction. At low potentials in the passive state, the positive defects injected at the metal/film interface play the role of electron donors. At high positive potentials, the negative defects injected at the film/solution interface play the role of electron acceptors. At sufficiently high positive potentials the concentration of these ionic defects and corresponding electron holes reaches high enough values for the film to transform into a conductor. This enables transpassive dissolution of Cr and oxygen evolution on the film surface. Equations for the electronic conductivity of the passive film, as depending on the concentration of point defects, are derived. The proposed model is compared with experimental data obtained for pure Fe, pure Cr, Fe–12%Cr alloy and Fe–25%Cr alloy passivated in 0.1 M borate solution (pH 9.2) using rotating ring-disk voltammetry, photocurrent and impedance spectroscopy and in situ dc resistance measurements by the contact electric resistance (CER) technique.

Polypyrrole as a model membrane for drug delivery

Abstract The possibility of using the conductive polymer polypyrrole as an ion gate membrane for the controlled release of anionic drugs has been studied using three model substances with therapeutic activity: salicylate, naproxen and nicoside. In addition, the release of tosylate, used for the electrosynthesis of the membrane, was investigated. The electrochemical quartz crystal microbalance was used to determine the mass changes occurring in 150 nm thick membranes loaded with the anion as a result of cathodic polarisation. Thicker membranes, 1–6 μm, were grown on a large gold electrode. The release of drugs from these membranes as a result of negative potential steps was followed using HPLC. It was found that the stability of the membrane doped with the appropriate drug towards chloride exchange in a 0.1 M NaCl solution was very good. Less than 5% of the theoretical amount of anion in the membrane was spontaneously released. Negative potential step experiments showed that salicylate, naproxen and tosylate, but not nicoside, could be released from the membrane in a controllable way. These results show that controlled release of a drug from polypyrrole membranes can be achieved using simple electrochemical step or staircase signals. Thus tailor made parts for iontophoretic devices can be manufactured. The amount of drug which can be released is, however, small and the application may therefore be restricted to drugs with high therapeutic activity.

Investigation of Mass-Transport Limitations in the Solid Polymer Fuel Cell Cathode II. Experimental

In this work, we investigated the kinetics and mass-transport limitations of the oxygen reduction reaction in the solid polymer fuel cell. The information obtained from electrochemical experiments ...

Preparation of silica particles utilizing the sol-gel and the emulsion-gel processes

Abstract The preparation of silica particles by means of the sol-gel or emulsion-gel techniques has been studied. In the case of precipitation from homogeneous solutions, we have studied the whole solution phase in the system tetraethyl orthosilicate-ethanol-aqueous ammonia and have mapped the region for monodisperse particles. These particles have a size of 70–640 nm. Their size and monodispersity are controlled by the composition of the system. Utilizing the emulsion-gel technique it is possible to increase the particle size to some tens of micrometers, but the polydispersity increases. In the emulsion-gel process the best results were obtained for small water contents (below 15 wt.%) and low amounts of surfactant (less than 0.5 wt.%) and for considerably longer reaction times than for the conventional sol-gel process.

Structure and properties of sulfonated poly [(vinylidene fluoride)–g-styrene] norous membranes porous membranes

The possibility of developing polyfvinylidene fluoride (PVDF)-based membranes as proton conductors has been investigated. Using electron beam radiation grafting and functionalisation, the composition of the membranes can be controlled. Membranes have been prepared from porous films of PVDF, and their structure and properties have been studied with Raman spectroscopy, wide angle X-ray scattering (WAXS), small angle X-ray scattering (SAXS), swelling tests, and by impedance spectroscopy. The grafting reaction of styrene, initiated in the amorphous regions and at the surfaces of the crystallites in the partly crystalline PVDF matrix, is very efficient, and high degrees of grafting can be achieved. Grafts are formed both from C–H and C–F branch pointsl Sulfonation can be accomplished to ca. 100%, and occurs mainly at the para-position of the phenyl rings. The hydration number was found to be independent of the degree of grafting, degree of sulfonation and crystallinity. The overall crystallinity decreased in the structure with the degree of grafting and sulfonation, partly owing to the diluting effect of the grafts, partly owing to efficient penetration of the grafts in the crystallites. The conductivity increased with the content of sulfonic acid groups and, in particular, with decreasing crystallinity. In the case of fully sulfonated membranes, a levelling-out of the conductivity was found at a degree of grafting of around 200%. Conductivities up to 120 mS cm–1 at room temperature were achieved.

In-situ measurements of gas permeability in fuel cell membranes using a cylindrical microelectrode

A new method to study permeation of gases in proton conducting membranes using a cylindrical microelectrode is presented. The focus of this work was to develop an in-situ method to study transport ...

Electrochemical study of the passive behaviour of Ni–Cr alloys in a borate solution—a mixed-conduction model approach

This paper discusses the application of the recently introduced mixed-conduction model (MCM) to describe the electrochemical behaviour of anodic films formed on Ni–Cr alloys. The MCM emphasises the coupling between the spatial distribution of ionic point defects in the film and the electronic conductivity of the film. The basic concepts of the MCM are introduced, and equations for the concentration profiles of ionic defects, for the conductivity and for the ac response of the film are derived. The experimental part of this work consist of characterising the anodic behaviour of Ni–Cr alloys, pure Ni and pure Cr in 0.1 M Na2B4O7 solution (pH 9.3) at ambient temperature using rotating disc and cyclic voltammetry, contact electric resistance (CER) and electrochemical impedance spectroscopic (EIS) techniques. The experiments indicate that the film on a Ni–20%Cr alloy resembles that on pure Cr, whereas similarities can be found between the film on Ni–10%Cr alloy and pure Ni. The conductivity of the passive film formed on Ni–Cr alloys can be concluded to be based on the spatial and potential distribution of the valence states of Ni and Cr in the film. In spite of several simplifying assumptions used in the simulations, a qualitative agreement between the theory and experimental data was achieved.