Theorie der Zyklischen Voltammetrie an Makroporösen Elektroden

Investigating the electrokinetic performance of novel electrode materials by means of diffusional cyclic voltammetry has emerged to the standard approach in electrochemistry. The straightforward implementation of the method in a three-electrode compartment provides scientists with a feasible ex-situ technique for assessing reaction kinetics in terms of potential-dependent redox currents. Providing that well-defined diffusion conditions are complied, i.e. the experiments are conducted at planar electrodes in a semi-infinite diffusion domain, characteristic features such as the separation, symmetry and magnitude of the redox peaks can be related unambiguously to the electrode kinetics. However, as soon as non-planar electrodes or electrodes with finite diffusion domains are employed an equivocation between the measured redox peaks and the intrinsic electrode kinetics emerges. Consequently, a quantitative interpretation of cyclic voltammetry data becomes exceptionally arduous. In particular porous structures like felts and foams, predominantly utilized as electrode materials in the field of battery research, exhibit an intimidating ambiguity of the polarographic current signal. Therefore, the majority of experimentalists restrict themself to a qualitative interpretation of cyclic voltammetry data in terms of arbitrarily chosen onset-potentials. Scientists who are still targeting to quantify the electrode kinetics usually aim to exploit alternative techniques such as electrochemical impedance spectroscopy. However, from a theoretical perspective this approach is not capable of solving the dilemma either since the experiments are subjected to the same diffusion complication, examined with a different potential perturbation only. Consequently, developing a theory of cyclic voltammetry for porous electrodes is inevitable to permit a quantitative analysis of experimental results. This thesis consists of the cumulative work on the theory of cyclic voltammetry at macroporous electrodes with emphasis on felt-like structures. It is demonstrated that linking the high sensitivity of cyclic voltammetry with a sophisticated mathematical diffusion model allows for an electrochemical and morphological characterization of porous electrodes simultaneously, promoting the so-called „electrochemists spectroscopy “ to the next level. All theoretical concepts are supported by experimental data acquired for the electrochemical redox-reactions of vanadium(II)/ vanadium(III) and oxovanadium(IV)/ dioxovanadium(V), relevant in the field of vanadium redox-flow battery research. In a first approximation, porous electrodes are treated as random arrays of microelectrodes in a finite diffusion space with a statistically fluctuating size. A systematic investigation of simulated and experimentally acquired cyclic voltammetry data for both, porous and non-porous electrodes, draws an enlightening picture on the complex interplay of electrode porosity and reaction kinetics. With this knowledge, precise values for the heterogeneous rate constant of the oxovanadium(IV)/ dioxovanadium(V) redox reaction are obtained. These values usually scatter over orders of magnitude in the recent literature, most likely due to an inconsequent interpretation of data. In another study, a strategy for real-space simulation of cyclic voltammetry at carbon felt electrodes is presented. For this purpose, in-situ micro X-ray computed tomography is exploited to construct a template of the three-dimensional diffusion domain inside a porous electrode. This renders any statistical assumptions obsolete. To perform the simulations, two self-reliant computational methods, namely digital simulation and convolutive modeling, are combined. The resulting method offers significant advantages with respect to computation time, programming effort and mathematical complexity. Since effects of electrochemical double-layer charging, nonlinear contributions of ohmic resistances, coupled chemical reactions and limited electron transfer kinetics can be accounted for readily, the novel approach covers an extraordinarily wide range of electrochemical situations. The exceptional endowment of simulating polarographic experiments at porous electrodes was finally implemented into an open source program named „Polarographica “. This software provides the experimentalists community with a straightforward way of interpreting cyclic voltammetry data of porous electrodes in terms of a fitting routine. Since many other electroanalytical techniques are supported in the environment of Polarographica as well, it will eventually lead to a more decent interpretation of cyclic voltammetry data, based on mathematical models instead of ambiguous current peaks and arbitrarily chosen onset-potentials