Browsing by Author "Friedrich, K. Andreas (Prof. Dr.)"

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Open Access
Degradation study on solid oxide steam electrolysis
(2020) Hörlein, Michael Philipp; Friedrich, K. Andreas (Prof. Dr.)
Untersuchung der Degradation von Festoxidzellen im Elektrolysebetrieb von Wasserdampf anhand von Variationen der Betriebsbedingungen.
Open Access
Edge effects on the single cell level of polymer electrolyte fuel cells
(2018) Stahl, Peter; Friedrich, K. Andreas (Prof. Dr.)
Polymer electrolyte fuel cells (PEFC) are about to gain an important role in an energy supply based on renewable energy sources. In order to facilitate the market entry of PEFCs, various targets regarding lifetime, functionality and costs have to be fulfilled. An aspect of PEFCs which has not gathered much attention so far is the outer perimeter of the active cell area. The design of this region is substantially determined by the sealing concept and sealant manufacturing process and can have a considerable influence on the overall cell design. This work aims to illustrate the impact of the configuration of this specific area on operating conditions and degradation effects of the whole cell. The results enable the appropriate design of the sealing solution in order to mitigate unfavorable local operating conditions and degradation effects in the outer cell perimeter. In Chapter 3 state-of-the-art cell concepts are presented with respect to the design of the cell edge region and the impact on cell design, local operating conditions and manufacturing processes. General impact of the edge region design on the water transport in a cell is discussed in Chapter 5. Five cells with different sealing concepts were operated and the in-plane water distribution was analyzed by means of neutron radiography. It was shown that void volumes in the outer perimeter of a cell favor accumulation of liquid water there, as long as they are not fed by a direct gas flow. As water transport between edge region and flowfield is slow with time constants of > 1 h the removal of these water clusters is not possible with the applied cell operation protocols. Cells with a gas feed to the outer cell perimeter are in turn subjected to bypass flows around the flowfield. Particularly for flowfields with high flow resistances - e.g. with serpentine-shaped flowfield channels - this can lead to a significantly reduced stoichiometry in the flowfield and hence to decreased liquid water discharge. As a result the measured water content in the flowfield reached a maximum of twice the water content compared to a cell without a flowfield bypass. In general, local operating conditions were strongly influenced by the cell setup in the outer perimeter. Startup of PEFCs under freezing conditions is a requirement for many mobile applications. It can be challenging for cell design and operation as electrochemically produced water as well as residual water from a previous operation can freeze and block gas transport pathways, leading to cell failure. In Chapter 6 it is shown by a recently developed dual spectrum neutron radiography method that freezing of water over a limited fraction of a cell can occur while water in the rest of the cell remains liquid. At moderate temperatures of ≥ −5°C this partial freezing occurred simultaneously with the beginning of a cell voltage decline while the final cell failure could be assigned to the freezing of water over the entire cell area. Furthermore it was shown that residual water in the edge region and flowfield of a cell can have a negative influence on cold start capability. Residual water freezes as soon as the cell is cooled below 0°C and poses a nucleus for a fast phase transition of liquid product water to ice. Chapter 7 focuses on specific degradation mechanisms occurring in the outer cell perimeter. It is shown that if a catalyst coated membrane (CCM) is sandwiched between gaskets in its outer perimeter and between gas diffusion layers (GDL) in the flowfield, a gap between gasket and GDL can lead to accelerated mechanical deterioration of the membrane. Particularly under oscillating humidification conditions shrinking and swelling of the membrane can induce high local stresses in the membrane at the edges of GDL and gasket. As a result cell failure occurred after 10 000 cycles as cracks or pinholes in the membrane led to strong leakages. The experiment showed that a mechanically favorable integration of the CCM with the sealing setup is essential in order to mitigate membrane stress, especially for applications with lifetime requirements of more than 10 000 h. Manufacturing and assembling tolerances can lead to a lateral offset of the gaskets or single layers of a sub-gasket on both sides of a CCM. As a result the gas supply to the anode and cathode catalyst layer (CL) can be asymmetric in the outer perimeter of the active area. In Chapter 7 it is shown experimentally that particularly a local interruption of the anode gas supply can cause massive carbon corrosion of the cathode catalyst support. From 1 mm onwards under the covered area strong thinning of the cathode CL was seen, while the thickness of membrane and anode CL remained unchanged. The results were confirmed by numerical simulation. A specific characteristic of the discussed case was found to be the local electrical isolation between GDL and CL on the anode side by the introduced sub-gasket layer. Thereby a strong negative electrical potential gradient in the anode CL can emerge towards the outer cell perimeter, favoring a low local electrolyte potential since anode overpotentials remain small. As the electrical potential of the cathode CL does not exhibit pronounced potential gradients, the low electrolyte potential leads to high cathode electrode potentials and therefore to significant carbon corrosion rates. It is concluded that cells should intentionally exhibit a lateral offset of gaskets or sub-gasket layers on both sides of the CCM, so that with respect to the assembling tolerances local oxygen starvation occurs on the cathode side rather than hydrogen starvation on the anode side in every case.
Open Access
Einfluss von Platin-Abscheidungen auf die Membrandegradation in Polymerelektrolytbrennstoffzellen
(2016) Helmly, Stefan; Friedrich, K. Andreas (Prof. Dr.)
Open Access
Entwicklung und Integration neuartiger Komponenten für Polymerelektrolytmembran- (PEM) Elektrolyseure
(2018) Lettenmeier, Philipp; Friedrich, K. Andreas (Prof. Dr.)
Open Access
Fabrication and characterization of lithium-sulfur batteries
(2015) Canas, Natalia Andrea; Friedrich, K. Andreas (Prof. Dr.)
The lithium-sulfur battery is a promising system for the future generation of rechargeable batteries. Its main advantages are the high theoretical capacity (1675 Ah kgS-1), high energy density (2500 Wh kgS-1), and low cost of sulfur. So far, the commercial application of this battery has been hindered by the reduced cycle-life. The isolating properties of sulfur as well as the formation of polysulfides in a complex reaction mechanism, which is not completely understood, are mainly causes for battery degradation. This work is focused on the characterization of the Li-S battery by application of several characterization techniques under in situ and ex situ conditions. Using X-ray diffraction, the reaction of sulfur was monitored during discharge and charge, and the formation of nano-crystalline lithium sulfide as end product of discharge was identified for the first time in operando. The structural changes of sulfur and its partial amorphization were observed after charge and analyzed using the Rietveld method. Furthermore, electrochemical impedance spectroscopy was applied during cycling to measure the impedance characteristics of the cell. For this, an electrical equivalent circuit was designed to describe specific physical and electrochemical process. Thus, the resistance of the electrolyte, the charge transfer resistance in the electrodes, as well as the reaction and dissolution of isolating products were simulated and quantified. The polysulfides, as well as S8 and Li2S, were investigated in an organic electrolyte using UV-vis spectroscopy. Here, the species S62-; and S3•-; were identified and semi-quantified at several states of discharge. Further characterization methods, like scanning electron microscopy, atomic force microscopy, and thermal analysis coupled with mass spectroscopy were used to understand the degradation processes that caused morphological changes in the cathode. The output obtained through the application of the different characterization techniques was compared with a physico-chemical model in order to obtain a deeper knowledge in the reaction mechanisms occurring in the battery. Moreover, through further developments on the fabrication process of the battery, main factors influencing the battery capacity were identified. Thereby, the capacity of the battery was increased from 275 Ah kg-1 to 800 Ah kgS-1 (after 50 cycles, at a discharge rate of 0.18 C). This thesis provides new insights into the electrochemical and degradation processes of Li-S batteries and will hopefully contribute to enhance the energy density of future Li-S batteries.
Open Access
Investigation of sulfur poisoning of Ni-based anodes in solid oxide fuel cells
(2019) Riegraf, Matthias; Friedrich, K. Andreas (Prof. Dr.)
Open Access
Lattice Boltzmann simulation of liquid water transport in gas diffusion layers of proton exchange membrane fuel cells
(2024) Sarkezi-Selsky, Patrick; Friedrich, K. Andreas (Prof. Dr.)
Polymer electrolyte membrane fuel cells (PEMFCs) offer a compelling powertrain solution for the e-mobility sector and in particular for heavy-duty applications. Generating electrical energy by electrochemical reaction of hydrogen and oxygen to water, these fuel cells present (when using green hydrogen) a climate-friendly technology in support of reducing overall greenhouse gas emissions. During cell operation, the reactand gases are consumed at the electrodes and water is produced in the cathode catalyst layer (CCL). In dependence of the operating conditions, this product water can condensate and block as a liquid phase available transport paths for the reactand gases. In order to prevent advancing liquid water accumulation and therewith flooding of the cell, the product water has to be therefore removed efficiently. Thus, stable fuel cell (FC) operation is only possible with an appropriate water management. In this context, the porous gas diffusion layer (GDL) plays a pivotal role by ensuring both homogeneous reactand gas distribution to the electrodes and efficient water removal from the cathode catalyst layer. Consequently, optimal water management is only achievable with an appropriate design of the GDL material properties, which requires a profound understanding of the capillary transport phenomena on the pore scale. In long-term operation, the GDL is furthermore in general subject to different aging processes, due to the harsh conditions typically present in automotive applications. With proceeding material degradation, the water removal capabilities of the GDL can decrease over time, leading to a progressively deteriorated gas transport and increasing cell performance losses upon aging. In order to achieve the component durability needed to meet the lifetime requirements for long-term fuel cell operation, degradation phenomena and their repercussions on the water management therefore have to be fully understood. Owing to the complexity of multiphase transport mechanisms in porous media, however, conventional experimental testing can be expensive, time-consuming and with limited depth of detail. In recent years, pore-scale modeling (PSM) has therefore gained increasing popularity as a comparatively fast and inexpensive technique for investigation of porous media transport processes directly at the pore scale. In this work, liquid water transport through a carbon felt GDL was thoroughly investigated using multiphase PSM simulations and real microstructural data. At first, a 3D Color-Gradient Lattice-Boltzmann model was developed and validated against analytical references. GDL microstructures of a plain and an impregnated carbon fiber substrate of a Freudenberg H14 GDL were then reconstructed via segmentation of high-resolution X-ray micro-computed tomography (µCT) images. For the microstructure reconstruction of the impregnated GDL, an in-house algorithm was furthermore developed to distinguish a hydrophobic additive component (polytetrafluoroethylene (PTFE)) from the support material (carbon fibers). Subsequently, a first computational domain for the simulation of GDL liquid water transport was generated according to the experimental boundary conditions of a test bench for measuring capillary pressure-saturation (pc - S) relations. Here, special attention was paid to the GDL surface regions by explicit consideration of two semipermeable membranes according to the experimental setup. Starting from an initially dry GDL, liquid water intrusion was then simulated by gradually decreasing the gas pressure until full saturation was reached. Whereas the obtained pc - S relation is not unique but dependent on the wetting history of the GDL, drainage of liquid water was simulated as well, thereby recovering the complete capillary hysteresis. Parametric studies then showed that the obtained pc - S characteristics are significantly affected by boundary effects owing to the highly porous GDL surface regions. In addition, microstructure-resolved simulations require a high resolution of the porous medium in order to predict capillary behavior reasonably. Assuming uniform wettability with a carbon fiber contact angle of θ_CF = 65° for the plain fiber substrate, the simulated pc - S curves were furthermore in good agreement with the experimental reference. Considering mixed wettability for the impregnated fiber substrate, on the other hand, the simulations showed an expected shift of the pc - S characteristics towards higher capillary pressures owing to the hydrophobicity of the additive (PTFE) but could otherwise not reproduce the experimentally observed significant enlargement of the capillary hysteresis. This discrepancy was primarily related to a measurement artifact. After validation of the simulated capillary hystereses, novel pc - S relations were furthermore derived for both the intrusion and drainage of liquid water in the plain and impregnated H14 fiber substrate. On the other hand, the widely-used Leverett relation was found to be incapable to describe the capillary characteristics of the plain and impregnated carbon felt GDL appropriately. In order to investigate the impact of aging on the GDL water management, a second computational domain was generated, mimicking the operando conditions during fuel cell operation. For this purpose, the GDL microstructure reconstruction was virtually compressed corresponding to a typical cell assembly clamping pressure. In addition, a microporous layer (MPL) was considered as well by reconstruction of its macropores via segmentation of µCT images of an impregnated and MPL-coated Freudenberg H14 GDL. Generating a computational setup, the GDL/MPL assembly was then sandwiched between a liquid and a gas buffer zone representing the catalyst layer (CL) and the gas channel (GC) according to operando conditions. In order to investigate aging effects, the microstructure reconstructions were furthermore degraded virtually assuming PTFE loss from the GDL and increase of the MPL macroporosity as the main aging scenarios. Corresponding to stationary cell operation, liquid water transport through the partially aged GDL and MPL was then simulated with a constant inlet velocity. Once the liquid phase percolated through to the gas channel (i.e., at breakthrough), the simulations were halted and the invasion patterns were then characterized by means of local and overall saturation. In order to investigate mass transport limitations due to liquid water accumulation in the pore space, effective gas transport properties were furthermore determined for the partially saturated and aged GDLs and MPLs. Subsequent simulations then showed that with the MPL in the pristine state loss of PTFE had no measureable effect on the liquid water transport through the degraded GDL. This observation was related to the dominant impact of the MPL on the capillary transport by strongly restricting the liquid water invasion sites into the GDL. Upon aging of the MPL, on the other hand, significant degradation effects were observed, as indicated by rising breakthrough saturations and an increasingly disturbed gas transport. Once the MPL was already degraded, aging of the GDL was then found to impact the liquid water transport and consequently the effective gas transport as well.
Open Access
Physical modeling of PEMFC performance and chemical membrane degradation
(2019) Futter, Georg; Friedrich, K. Andreas (Prof. Dr.)
Open Access
Physical modelling of DMFC performance heterogeneities and the recovery of reversible cathode degradation
(2022) Fischer, Marie-Dominique; Friedrich, K. Andreas (Prof. Dr.)
Direct methanol fuel cells (DMFC), which are an alternative power source to batteries and diesel engines, exhibit a great potential for a locally heterogeneous cell performance. The DMFC anode is fed with a liquid methanol-water mixture while the cathode is supplied with air, which results in an even more complex fluid management in comparison with the structurally similar polymer electrolyte fuel cell (PEMFC) operated with hydrogen as fuel. The transfer of water and methanol from the anode through the polymer electrolyte membrane to the cathode side is an important factor for limits in the cell performance. The crossover of water from the anode to the cathode side, where water is also produced in the electrochemical reaction, increases the risk of liquid accumulation in the porous layers of the cathode and thus mass transport limitations in the cell. Methanol crossover leads to the formation of a mixed potential in the DMFC cathode, and the resulting high overpotential increase the development of oxide species on the platinum catalyst surface. These processes lead to a reduction of the cell performance, which is partially reversible. In this work, a physics-based DMFC model in 2D is developed in order to study the local cell performance along the channel with a focus on the two-phase flow as well as humidity-related properties of the ionomer. The model features a spatial resolution of the catalyst layers, which enables the examination of the local conditions' impact on the electrochemical reactions and on effects at the membrane interface. The model is verified against experimental data from a macro-segmented DMFC single cell for two different humidity levels in the cathode. The validation not only comprises the local cell performance, but also mass transport and the ohmic resistance of the membrane. Simulation results for the cell performance under varying operating conditions are shown in comparison with corresponding experimental data, proving the predictiveness of the model. The transient model is further used to study the processes inside the cell during the recovery of reversible degradation effects in the cathode. The formation of platinum oxide species during DMFC operation and their reduction during a refresh sequence including an OCV phase as well as a phase with air starvation is simulated and explored with respect to the local conditions inside the cathode catalyst layer. Moreover, the simultaneously occurring spontaneous evolution of hydrogen in the DMFC anode is examined. Several variations of the air stop sequence are simulated and evaluated with regard to their effectiveness in recovering the temporary performance losses within the DMFC cathode.
Open Access
Production and characterization of bifunctional highly structured oxygen electrodes for secondary zinc-air battery
(2022) Kube, Alexander; Friedrich, K. Andreas (Prof. Dr.)