Tanja Vidaković-Koch

Electrochemical Oxidation of Carbon-Containing Fuels and Their Dynamics in Low-Temperature Fuel Cells

Fuel cells can convert the energy that is chemically stored in a compound into electrical energy with high efficiency. Hydrogen could be the first choice for chemical energy storage, but its utilization is limited due to storage and transport difficulties. Carbon-containing fuels store chemical energy with significantly higher energy density, which makes them excellent energy carriers. The electro-oxidation of carbon-containing fuels without prior reforming is a more challenging and complex process than anodic hydrogen oxidation. The current understanding of the direct electro-oxidation of carbon-containing fuels in low-temperature fuel cells is reviewed. Furthermore, this review covers various aspects of electro-oxidation for carbon-containing fuels in non-steady-state reaction conditions. Such dynamic investigations open possibilities to elucidate detailed reaction kinetics, to sense fuel concentration, or to diagnose the fuel-cell state during operation. Motivated by the challenge to decrease the consumption of fossil fuel, the production routes of the fuels from renewable resources also are reviewed.

Nonlinear Frequency Response of Electrochemical Methanol Oxidation Kinetics: A Theoretical Analysis

In this theoretical contribution, nonlinear frequency response analysis was applied for the investigation of electrochemical methanol oxidation. This technique expresses the input-output behavior of any weakly nonlinear system with the help of the Volterra series expansion and generalized Fourier transform into so-called higher order frequency response functions. These functions contain the systems nonlinear fingerprint. They can be derived analytically from a nonlinear model. These functions can be obtained experimentally from the measurement of higher harmonics induced by a high amplitude sinusoidal perturbation of the system of interest. Frequency response functions up to the second order have been derived analytically for four different model varieties describing the kinetics of the electrochemical methanol oxidation. The first-order frequency response function corresponds to the reciprocal value of the well-known electrochemical impedance, which represents the linear part of the frequency response. This function does not contain sufficient information for discrimination between the different kinetic models. In contrast, the symmetrical second-order frequency response functions H 2 (ω,ω) show differences in shape, which substantiate the availability of the theoretical prerequisites for model discrimination. A detailed parametric study for all four model variants has been performed. The results show that the basic features of the shapes of the H 2 (ω,ω) amplitude spectra corresponding to the four models remain unique. The ubiquitousness of the qualitative differences between the competing models, for the whole set of parameters chosen for our analysis, suggests that the aforementioned amplitude spectra contain sufficient information for an unequivocal model discrimination.

Electrochemical membrane reactors for sustainable chlorine recycling.

Polymer electrolyte membranes have found broad application in a number of processes, being fuel cells, due to energy concerns, the main focus of the scientific community worldwide. Relatively little attention has been paid to the use of these materials in electrochemical production and separation processes. In this review, we put emphasis upon the application of Nafion membranes in electrochemical membrane reactors for chlorine recycling. The performance of such electrochemical reactors can be influenced by a number of factors including the properties of the membrane, which play an important role in reactor optimization. This review discusses the role of Nafion as a membrane, as well as its importance in the catalyst layer for the formation of the so-called three-phase boundary. The influence of an equilibrated medium on the Nafion proton conductivity and Cl− crossover, as well as the influence of the catalyst ink dispersion medium on the Nafion/catalyst self-assembly and its importance for the formation of an ionic conducting network in the catalyst layer are summarized.

Dynamic and Steady State 1-D Model of Mediated Electron Transfer in a Porous Enzymatic Electrode

A 1-D mathematical model of a porous enzymatic electrode exhibiting the mediated electron transfer (MET) mechanism has been developed. As a model system, glucose oxidation catalyzed by immobilized glucose oxidase (GOx) in the presence of a co-immobilized tetrathiafulvalene (TTF) mediator in the porous electrode matrix has been selected. The balance equations for potential fields in the electron- and ion-conducting phases as well as concentration field have been formulated, solved numerically and validated experimentally under steady state conditions. The relevant kinetic parameters of the lumped reaction kinetics have been obtained by global optimization. The confidence intervals (CIs) of each parameter have been extracted from the respective likelihood. The parameter study has shown that the parameters related to mediator consumption/regeneration steps can be responsible for the shift of the reaction onset potential. Additionally, the model has shown that diffusion of the oxidized mediator out of the catalyst layer (CL) plays a significant role only at more positive potentials and low glucose concentrations. Only concentration profiles in different layers influence the electrode performance while other state fields like potential distributions in different phases have no impact on the performance. The concentration profiles reveal that all electrodes work through; the observed limiting currents are diffusion-reaction limiting. The normalized electrode activity decreases with an increase of enzyme loading. According to the model, the reason for this observation is glucose depletion along the CL at higher enzyme loadings. Comparison with experiments advices a decrease of enzyme utilization at higher enzyme loadings.

Glucose Electrooxidation for Biofuel Cell Applications

The kinetics of glucose electrooxidation on different catalysts has been studied at physiological conditions (pH 7 and 37 °C). Electrochemically activated rough gold, rough gold modified with a self-assembled monolayer (SAM) of cystamine and an enzymatic electrode based on a charge transfer complex (CTC) and glucose oxidase (GOx) have been tested. The influence of glucose concentration, electrode rotation rate and presence of oxygen has been investigated and the stability of the different catalysts has been evaluated. All parameters have been discussed in the context of the potential application of these catalysts in an implantable glucose/O 2 biofuel cell. Rough gold exhibits high activity with very low overpotential for glucose oxidation but its extreme instability and low oxygen tolerance make it inappropriate as potential anode in a biofuel cell. The CTC enzymatic electrode on the other side shows high activity for glucose oxidation, reasonably low overpotential and relatively high stability.

Toward Artificial Mitochondrion: Mimicking Oxidative Phosphorylation in Polymer and Hybrid Membranes

For energy supply to biomimetic constructs, a complex chemical energy-driven ATP-generating artificial system was built. The system was assembled with bottom-up detergent-mediated reconstitution of an ATP synthase and a terminal oxidase into two types of novel nanocontainers, built from either graft copolymer membranes or from hybrid graft copolymer/lipid membranes. The versatility and biocompatibility of the proposed nanocontainers was demonstrated through convenient system assembly and through high retained activity of both membrane-embedded enzymes. In the future, the nanocontainers might be used as a platform for the functional reconstitution of other complex membrane proteins and could considerably expedite the design of nanoreactors, biosensors, and artificial organelles.

Combined electrochemical and microscopic study of porous enzymatic electrodes with direct electron transfer mechanism

In the present work electrochemical and microscopic methods have been utilized to get more insight into the complex relationship between the preparation route, structure and activity of porous enzymatic electrodes. Enzymatic electrodes have been prepared following two procedures. In one procedure enzymes were physically entrapped into a porous conductive matrix stabilized by “inert” binder (Vulcan-PVDF), while in the second one (Vulcan-Gelatin) gelatin has been used as a binder and the electrodes were cross-linked. Vulcan-PVDF electrodes show exceptionally high activity (up to 1.2 mA cm−2) compared to Vulcan-Gelatin electrodes (0.3 mA cm−2) at nominally lower enzyme loading. The scanning electron microscopy cross-sections of these electrodes revealed similar thicknesses, but a higher level of Vulcan nanomaterial agglomeration, somewhat reduced porosity and formation of gelatin film on top in the case of Vulcan-Gelatin electrodes. Additionally, fluorescence microscopy studies provided evidence of a higher level of enzyme agglomeration in the case of cross-linking. Although the gelatin matrix and the reduced catalyst layer porosity might slow down hydrogen peroxide diffusion, Vulcan-Gelatin electrodes are less affected by mass transfer conditions than Vulcan-PVDF electrodes. A plausible cause of the Vulcan-Gelatin electrode inferior performance is a lower number of active enzymes (lower enzyme utilization) compared to the Vulcan-PVDF electrode caused by a higher level of enzyme agglomeration in former case.

AlGaN/GaN-based HEMTs for electrical stimulation of neuronal cell cultures

Unipolar source?drain voltage pulses of GaN/AlGaN-high electron mobility transistors (HEMTs) were used for stimulation of cultured neuronal networks obtained from embryonic rat cerebral cortex. The HEMT sensor was grown by metal organic vapour phase epitaxy on a 2?inch sapphire substrate consisting of 10 single HEMTs concentrically arranged around the wafer centre. Electrolytic reactions between the HEMT sensor surface and the culture medium were not detected using cyclic voltammetry.During voltage pulses and resulting neuronal excitation, capacitances were recharged giving indications of the contributions of the AlGaN and AlOx isolation layers between the two-dimensional electron gas channel and the neuron culture. The resulting threshold current for stimulation of neuron activity strongly depended on the culture and HEMT position on the sensor surface under consideration which was caused by different impedances of each neuron culture and position within the culture. The differences of culture impedances could be explained by variations of composition, thickness and conductivity of the culture areas.

Electron Transfer Between Enzymes and Electrodes

Efficient electron transfer between redox enzymes and electrocatalytic surfaces plays a significant role in development of novel energy conversion devices as well as novel reactors for production of commodities and fine chemicals. Major application examples are related to enzymatic fuel cells and electroenzymatic reactors, as well as enzymatic biosensors. The two former applications are still at the level of proof-of-concept, partly due to the low efficiency and obstacles to electron transfer between enzymes and electrodes. This chapter discusses the theoretical backgrounds of enzyme/electrode interactions, including the main mechanisms of electron transfer, as well as thermodynamic and kinetic aspects. Additionally, the main electrochemical methods of study are described for selected examples. Finally, some recent advancements in the preparation of enzyme-modified electrodes as well as electrodes for soluble co-factor regeneration are reviewed. Graphical Abstract.

Catalyst Layer Modeling

The overall performance of a fuel cell or an electrochemical reactor depends greatly on properties of catalyst layers, where electrochemical reactions take place. Optimization of these structures in the past was mainly guided by experimental methods. For substantial progress in this field, combination of experiments with modeling is highly desirable. In this chapter focus is on macroscale models, since at the moment they provide more straightforward relationship to experimentally measurable quantities. After introducing the physical structure of a catalyst layer, we discuss typical macroscale modeling approaches such as interface, porous, and agglomerate models. We show how governing equations for the state fields, like potential or concentration can be derived and which typical simplifications can be made. For derivations, a porous electrode model has been chosen as a reference case. We prove that the interface model is a simplification of a porous model, where all gradients can be neglected. Furthermore, we demonstrate that the agglomerate model is an extension of the porous model, where in addition to macroscale, additional length scale is considered. Finally some selected examples regarding different macroscale models have been shown. Interface model has low capability to describe the structure of the catalyst layer, but it can be utilized to resolve complex reaction mechanisms, providing reaction kinetic parameters for distributed models. It was shown that the agglomerate models, having more structural parameters of the catalyst layer, are more suitable for catalyst layer optimization than the porous models.