Browsing by Author "Kopljar, Dennis"

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Open Access
Advanced impedance analysis for performance degradation during low-temperature CO2 electroreduction
(2024) Chen, Qinhao; Kube, Alexander; Kopljar, Dennis; Friedrich, Kaspar Andreas
Electrochemical impedance spectroscopy (EIS) is a powerful tool commonly used to study electrochemical systems. Nevertheless, its application in CO2 electroreduction has been so far limited due to its complex reaction mechanism and environment. Although initial findings have demonstrated the viability of applying EIS analysis in CO2 electrolyzers, the assignment of individual processes in the impedance spectra remains ambiguous. Therefore, a more detailed investigation, especially focused on its application in evaluating degradation mechanisms, is essential. In this study, a stable gas diffusion electrode (GDE) system was developed for a comprehensive EIS and distribution of relaxation time (DRT) evaluation to assess key degradation mechanisms under accelerated stress conditions such as high current density and low operating temperature. Validated by post-mortem analysis and complementary methods, we demonstrate the viability of this approach for operando monitoring of CO2 electroreduction by assigning individual mechanistic processes in the GDE and linking them to performance degradation over time.
Open Access
Development and investigation of gas-diffusion electrodes for the electrochemical reduction of CO2
(2023) Kopljar, Dennis; Klemm, Elias (Prof. Dr.-Ing.)
An attractive solution to produce carbon-based chemicals is to utilize CO2 as raw material and renewable electricity as energy source for its activation such as in the reduction of CO2 (CO2RR) into formate salts. In the course of this work, carbon-based and PTFE-bound gas-diffusion electrodes (GDE) were developed with highly promising performance characteristics combining a newly developed simple and scalable dry deposition technique with a homogeneous precipitation method to obtain fine dispersion of the tin oxide electrocatalyst on the carbon substrate. Optimizing electrode properties yielded GDEs that allowed for operation at current densities >500 mA/cm² while maintaining FE towards formate >80%. While these are promising values and already in the range of what is targeted for technical application, long-term stability evidenced by a continuously increasing rise of hydrogen evolution over time is far from suitable from a technical point of view. By investigating the effect of the GDE properties - that are the composition (carbon type, PTFE content, electrocatalyst content), texture (adjusted via pore-forming agents and compacting pressure) as well as hydrophobicity (function of carbon substrate and adjusted by its oxidation) - and evaluating the time behavior, it could be shown that working at high current densities is determined by i) reactant and product transport, ii) wettability and iii) catalyst accessibility. Accordingly, an elaborate and systematic investigation of the relationship between said variables and the electrochemical performance was conducted to establish a starting point for a more targeted optimization of the GDE in future work. Ultimately, an analysis on the effect of the operating point on total electrolysis cost was performed to derive performance targets for formic acid production.
Open Access
Scalable fabrication of multi-layered Cu-based electrodes via solvent-free method for the selective electrochemical conversion of CO2 to C2+ products
(2024) Chen, Qinhao; Kube, Alexander; Rana, Bhawna; Biswas, Indro; Morawietz, Tobias; Kopljar, Dennis; Friedrich, Kaspar Andreas
In the research field of CO2 electroreduction, gas diffusion electrodes (GDEs) are predominantly manufactured through solvent-based processes. Meanwhile, the solvent-free method has gained heightened attention due to its potential to reduce operational and production expenses, while considering ecological aspects such as solvent evaporation, circulation, and waste treatment. Drawing from its successful applications in other fields, we have specifically developed a solvent-free manufacturing method to produce multi-layered Cu-based GDEs for CO2 electroreduction. The procedure is compatible with industrial production lines, specifically through a roll-to-roll process. By evaluating the interplay between production parameters and electrochemical performance of GDEs via various characterization methods, key factors, i.e., hydrophobicity, gas permeability, thickness, and pore size, were adjusted and applied to achieve a highly selective GDE towards C2+ products (alcohols and ethylene) at industrial relevant currents up to 300 mA cm-2 (ethylene ∼40%, ethanol ∼10%, n-propanol ∼15%).
Open Access
A segmented cell measuring technique for current distribution measurements in batteries, exemplified by the operando investigation of a Zn-air battery
(2021) Kube, Alexander; Meyer, Jens; Kopljar, Dennis; Wagner, Norbert; Friedrich, Kaspar Andreas
A transimpedance amplifier circuit as well as an instrumental amplifier circuit were used to measure current densities of a zinc-air battery with an integrated segmented current collector foil. Error calculation showed that the transimpedance amplifier is superior to the used instrumental amplifier, but both methods provide valuable and consistent results. They both showed comparable results with operando insight into the current distribution of the battery. The knowledge about those distributions is essential to avoid fast degradation of battery materials and irreversible capacity loss due to heterogeneous dissolution of the anode during discharge. In this work we showed that oxygen starvation as well as gas flow rate leads to large current gradients. It was also demonstrated that heterogeneous current distributions on cathode side induces also a heterogenous dissolution behavior on the anode, resulting in irreversible capacity loss.