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Concentration frequency response analysis (CFRA) applies partial pressure of a certain reactant and detects current or voltage response depending on the electric control applied. In this paper experimental and theoretical spectra of this novel frequency response analysis method of polymer electrolyte fuel cells are analyzed. It is shown that the dynamic contribution of oxygen transport in the channel and water sorption in Nafion could be separated from other ones if oxygen and water partial pressure are respectively used as inputs. This allows to study these phenomena separately and to avoid ambiguities encountered in EIS spectra where several dynamics are often overlapped and indistinguishable. The time required to the oxygen to diffuse in the channel and in the porous electrode was directly estimated through analysis of patterns in the CFRA spectra.

Proton exchange membrane fuel cells fed with H 2 /CO mixtures at the anode have a considerably lower performance than fuel cells fed with pure hydrogen. However, when operated in an autonomous oscillatory regime, the overall voltage loss decreases due to a self-cleaning mechanism. Another molecule, also widely used as feed in the fuel cell and susceptible to kinetic instabilities, is methanol. To the best of our knowledge, there are no reports on autonomous voltage oscillations in the direct methanol fuel cell (DMFC). The purpose of this work was to explore if such instabilities also occur in the DMFC system. Initially, half-cell experiments with a gas diffusion electrode were performed. Then, a DMFC was operated under current control and studied by means of electrochemical impedance spectroscopy. The half-cell measurements revealed that the induction period for oscillations depends on the mass transfer conditions, where on stagnant electrode the induction time was shorter than in the case of forced convection. The DMFC showed also autonomous voltage oscillations above a certain threshold current. The results obtained by electrochemical impedance spectroscopy give evidence of a negative differential resistance in the fuel cell, hitherto not described in the literature, which can be related to the appearance of oscillations during galvanostatic methanol electro-oxidation. These results open the possibility to evaluate the performance of low-temperature fuel cells fed with carbon-containing fuels under oscillatory operating conditions.

A B S T R A C T Glucose was selectively oxidized to gluconic acid in a membraneless, flow-through electroenzymatic reactor operated in the mode of co-generating chemicals and electrical energy. At the anode the enzyme glucose oxidase (GOx) in combination with the redox mediator tetrathiafulvalene (TTF) was used as catalyst, while the cathode was equipped with an enzyme cascade consisting of GOx and horseradish peroxidase (HRP). The influence of the electrode preparation procedure, the structural and the operating parameters on the reactor performance was investigated in detail. Under optimized conditions, an open circuit potential of 0.75 V, a current density of 0.6 mA cm 2 and a power density of 100 mA cm 2 were measured. The space time yield of gluconic acid achieved at a glucose conversion of 47% was 18.2 g h 1 cm 2 .

1-D model of a porous enzymatic electrode with direct electron transfer mechanism has been developed. As a model reaction, hydrogen peroxide reduction catalyzed by Horseradish Peroxidase has been chosen. The model description includes material and charge balances in different phases as well as detailed kinetics of bioelectrochemical hydrogen peroxide reduction. The model has been solved numerically and validated experimentally under steady state conditions. To investigate the influence of the electrode structure and the immobilization procedure, two types of enzymatic electrodes have been developed. In one procedure (Vulcan-PVDF) enzymes were entrapped into a porous conductive matrix, while in the second one (Vulcan-Gelatin) gelatin was used as a binder and enzymes were cross-linked. The performances of Vulcan-PVDF electrodes were significantly better than of Vulcan-Gelatin electrodes under all studied conditions. According to the model, the main reasons for this observation are higher number of active enzymes and higher diffusivity of hydrogen peroxide in the catalyst layer (CL) in case of Vulcan-PVDF procedure. The model pointed out that the major limitation in both studied systems is mass transfer limitation. Enzyme utilization in both systems is very low.

A novel process for the recycling of chlorine based on the gaseous HCl oxidation on gas diffusion electrodes has been investigated. The cathodic and anodic reactions have been analyzed separately as half-cell reactions in a cyclone flow cell and optimized regarding catalyst as well as ionomer loadings. For the anodic hydrogen chloride oxidation, an optimal value of ca. 60 wt% Nafion has been found, whereas for the cathodic oxygen reduction a value below 37 wt% has been obtained. This discrepancy was mainly attributed to the different absorption and transport properties of HCl in Nafion compared to oxygen. The main potential losses in the reactor were identified at the cathode.

The electrochemical conversion of hydrogen chloride to chlorine plays a significant role within major industrial processes like the polyurethane or polycarbonate production. Our recent studies demonstrated that the direct electrolysis of gaseous HCl in combination with novel strategies for product purification leads to significant exergetic savings (36–38%) compared to the Bayer UHDENORA state-of-the art process, which employs aqueous hydrochloric acid as a feedstock. Furthermore, we showed that despite of the improvements in the efficiency of the electrochemical reactor, it still has by far the greatest exergy demand of all process units. While the oxygen depolarized cathode (ODC) utilized in both process variants has been investigated in great detail in the scientific literature, the oxidation reaction of gaseous HCl (HClOR) has not yet received much attention and is hence the major subject of this work. In earlier experimental investigations of the HClOR, a limiting current was observed that has the potential to critically reduce the performance of the overall reactor. In the past, membrane dehydration and mass transfer resistances on the anode side were suggested as possible reasons. In order to shed light on this phenomenon, a dynamic, one dimensional agglomerate model of the gas phase HCl oxidation half-cell, considering detailed micro kinetics, as well as the mass transfer of HCl in the different cell parts, is developed. The modeling results suggest that neither mass transfer resistance nor membrane dehydration but a kinetic limitation is the underlying reason for the observed limiting behavior. This has a significant impact on the strategy for further reactor optimizations and harbors the potential of major energy savings in an industrial application of this highly relevant process.

Oxygen reduction reaction (ORR) was studied on a silver polycrystalline electrode in different NaOH concentrations with a help of rotating disc and rotating ring disc electrodes. Soluble reaction intermediate was detected at all alkaline concentrations, but its concentration increased with an increase of the level of impurities. ORR is not NaOH concentration dependent at low concentrations (0.1 and 1 M). In 11 M NaOH ORR onsets at more positive potentials in the region where underpotential silver oxide formation shows less reversible behaviour. The nonlinear frequency response analysis spectra show significant qualitative difference with NaOH concentration indicating a high capability of this method for kinetic mechanism investigations.

The application of electrochemical impedance spectroscopy (EIS) in theory and experiment for investigation of redox enzyme kinetics has been described. As a model system an enzyme horseradish peroxidase adsorbed on graphite electrode has been chosen. Three different mathematical models based on generalized mechanism of horseradish peroxidase catalyzed hydrogen peroxide reduction have been formulated and used for derivation of theoretical electrochemical impedances. In this way, mechanistic details related to bioelectrochemical reaction including all relevant kinetic parameters are obtained. The presented approach overcomes limitations of classical equivalent circuit approach, since it does not rely on phenomenological elements, but on particular reaction mechanism. We have shown that the EIS is more sensitive for parameter estimation and model discrimination than a steady state response.

An important parameter in normalizing activities of porous electrocatalysts is real surface area of the catalyst. In this study CO stripping voltammetry was applied for the determination of the surface area of PtRu catalysts in a membrane electrode assembly. The difficulties in applying this method are summarized and a practical solution for the “accurate” voltammetric CO charge determination is suggested. The influence of adsorption time, potential, sweeping range and sweep rate on voltammetric CO charge determination is studied. It is shown that the CO charge determined in this way is independent on the sweep rate applied.

The activities of three 30 wt.% PtRu/C catalysts of the same nominal composition (Pt:Ru, 1:1 a/o) were investigated with regard to methanol oxidation. The catalysts were synthesized by Bonnemann's colloidal precursor method using different metal salt precursors and reducing agents. The catalysts were characterized ex situ by energy dispersive X-ray analysis (EDX) and X-ray diffraction (XRD) and in situ CO stripping and cyclic voltammetry. The activity towards methanol electro-oxidation was checked in steady-state experiments at 22 and 60 °C. The experimental rate data can be described well by a kinetic model, which includes methanol adsorption on the Pt-sites, formation of C-containing adsorbed species, OH ads formation on the Ru sites and heterogeneous surface reaction between C-adsorbate and OH ads . The kinetic model parameters were identified from the experimental data and were used to explain the differences in catalytic activity.

Electrochemical oxidation of methanol on carbon supported Pt/Ru gas diffusion electrodes was investigated in a cyclone flow cell at room temperature using chronoamperometry, cyclic voltammetry and electrochemical impedance spectroscopy. The influence of the flow rate was checked. It was proved that the cyclone cell is suitable for the investigation of methanol electrooxidation and provides additional information on the mass transfer limitations in the electrode assembly. Chronoamperometric measurements showed slow, but constant current decay at all investigated potentials. Impedance measurements in water and methanol containing solutions were performed and the experimental data were fitted to an appropriate equivalent circuit.

Methanol oxidation was investigated on the following Pt catalysts: bare Pt electrode, Pt covered by nafion (R) film, and pt catalysts supported on high area carbons in the form of an ink. The reaction was carried out using slow linear potential sweep and potentiostatic steps. Active surface area of Pt particles was estimated from hydrogen desorption charge on cyclic voltammograms. The influence of Nafion film, Pt loading, type of carbon support, and methanol concentration were studied. No influence of Nafion (R) film on kinetic parameters of methanol oxidation was found. Tafel slope on smooth Pt was found to be about 90 mV dec(-1), while on supported Pt catalysts time depende(n)t values between 140 and 90 mV dec(-1) were estimated from the potentiostatic decays by using data between 0.5 and 5 min. Specific activity was slightly dependent on Pt loading with flat maximum at about 20%, Pt. Activity of Pt/SAB catalyst was two to three times higher than that of Pt/BP and Pt/XC-72 catalysts.