Lattice Boltzmann simulation of liquid water transport in gas diffusion layers of proton exchange membrane fuel cells: Parametric studies on capillary hysteresis

Water management is crucial for reliable operation of Polymer Electrolyte Membrane Fuel Cells (PEMFC). Here, the gas diffusion layer (GDL) plays an essential role as it has to ensure efficient water removal from and oxygen transport to the catalyst layer. In this study water transport through porous carbon felt GDLs was simulated using a 3D Color-Gradient Lattice Boltzmann model. Simulations were carried out on microstructures of plain and impregnated fiber substrates of a Freudenberg H14. The GDL microstructures were reconstructed from high-resolution X-ray micro-computed tomography ($\mu$-CT). For the distinction of carbon fibers and polytetrafluoroethylene (PTFE) in the binarized microstructures an in-house algorithm was developed. The additive was specified heterogeneously in the GDL through-plane direction employing a PTFE loading profile as derived based on $\mu$-CT image data. In the in-plane direction the additive was furthermore defined in a realistic fashion near carbon fiber intersections. Prior to parametric studies on capillary behavior a sophisticated modeling approach for semipermeable membranes had to be developed to account for experimental boundary conditions. Capillary hysteresis was then investigated by simulation of intrusion and drainage curves and subsequent comparison to testbench data

Modeling of Powder Bed Dynamics in Thermochemical Heat Storage

Storing energy in the form of heat has been under long-standing investigation for prospective applications, such as the capturing of excess heat from industrial processes as well as storing energy in concentrated solar power plants. Investigated mechanisms for the heat storage include the adsorption in porous media, materials undergoing phase changes and thermochemical reactions. Among these, thermochemical heat storage provides a large energy capacity and next to perfect reversibility. More specifically, storage in the CaO/Ca(OH)2-System is investigated because of the low price and environmental friendliness of the reactants. In the project THEMSE, DLR is developing models and simulations as well as experimental characterization methods for thermochemical heat storage in the CaO/Ca(OH)2-System. In this talk, we shall give an overview over the project with a focus on the modeling activities. Special attention is given to the investigation of how the cycling of the material influences the heat and mass transport in the powder bed inside the reactor. This happens through mechanical and physical alteration of the powder bed, mainly through three mechanisms. First, the gas flow through the reactor exerts a force on the powder particles, compacting the powder bed. The resulting densification of the bed increases its flow resistance, while improving the heat transport. Second, the agglomeration of powder particles, where bonds between the particles form, turning the bed into a solid. The exact mechanism of the agglomeration is yet unknown, but it can be characterized by mechanical measurements. Third, the expansion of the powder particles through water uptake during the hydration stage, and the corresponding contraction during dehydration. To model the compaction and solidification of the powder bed during cycling, we present a mechanical model based on Drucker-Prager-Cap plasticity, which has been used previously for powder compaction, see e.g. [1]. The parameterization of the model, i.e., the plastic yield surface, is done via flow tester experiments. The changes in the powder bed during cycling are modeled by hardening mechanisms, i.e., a changing yield surface, corresponding to powder compaction and agglomeration, respectively. Then, the plastic model is coupled to a reactor scale model, simulating the heat and mass transport, as well as the thermochemical reaction using a model, similar to [2]. This enables the study of the powder bed dynamics under different boundary conditions during cycling, such as pressure drop, water vapor fraction and reactor geometry. Finally, an outlook will be given on the multi-scale modeling of the reactor. The geometrical micro-scale characterization of the material is done using micro computed tomography (µCT). From the µCT-Images, effective transport parameters, such as diffusivity and permeability are computed for different stages of agglomeration. These are then used in the reactor-scale model to produce predictions, which can be verified on the reactor-scale. [1] Wu, C.-Y & Ruddy, O.M. & Bentham, A.C. & Hancock, B.C. & Best, Serena & Elliott, James. (2005). Modelling the mechanical behaviour of pharmaceutical powders during compaction. Powder Technology. 152. 107-117. 10.1016/j.powtec.2005.01.010. [2] Nagel, Thomas & Shao, Haibing & Singh, Ashok & Watanabe, Norihiro & Roßkopf, Christian & Linder, Marc & Wörner, A & Kolditz, Olaf. (2013). Non-equilibrium thermochemical heat storage in porous media: Part 1 – Conceptual model. Energy. -. 10.1016/j.energy.2013.06.025

A Pathway towards Pt-free Cathodes in High-Temperature Proton Exchange Membrane Fuel Cells

The high temperature proton exchange membrane fuel cell (HT-PEMFC) has several advantages compared to its low temperature (LT) counterpart. The typical operation temperature of 160 °C enables an easier heat management, and the omission of humidification. However, due to the partial blocking of the cathodic and anodic Pt catalyst by phosphates from the phosphoric acid-doped membrane, higher catalyst loadings compared to LT-PEMFC of 0.86 mgPt cm-2 per electrode are commonly employed.[1] To increase the competitiveness of HT-PEMFCs implementation of Fe-N-Cs is a promising option for reduction of catalyst costs. In this study, we give an overview about the application of different Fe-N-C catalysts in Pt free HT-PEMFC cathodes.[2] Moreover, we show their application in hybrid PtNi/C+Fe N C cathodes.[1] The complete replacement of Pt catalyst by Fe N C in the cathode results in low performance[2] and a strong voltage decay within the first 60 hours of HT-PEMFC operation. In contrast, a hybrid MEA with reduced Pt-loading displayed a more comparable performance to commercial MEA (Celtec®-P1200) and constant voltage over 60 h. Furthermore, it was found that the typical activation procedure of HT-PEMFC MEAs (around 60 h constant load) is not sufficient for hybrid MEAs. There, a voltage increase over the first 240 h of operation was observed.[1] These results give the basis for further optimization of Pt-free Fe N C electrodes. Furthermore, the potential of hybrid MEAs for Pt-loading reduction in HT-PEMFC is pointed out

Effect of polytetrafluorethylene content in Fe‐N‐C‐based catalyst layers of gas diffusion electrodes for HT‐PEM fuel cell applications

Fe-N-C catalysts are a promising alternative to replace cost-intensive Pt-based catalysts in high temperature polymer electrolyte membrane fuel cell (HT-PEMFC) electrodes. However, the electrode fabrication needs to be adapted for this new class of catalysts. In this study, gas diffusion electrodes (GDEs) are fabricated using a commercial Fe-N-C catalyst and different polytetrafluorethylene (PTFE) binder ratios, varying from 10 to 50 wt % in the catalyst layer (CL). The oxygen reduction reaction performance is investigated under HT-PEMFC conditions (160 °C, conc. H3PO4 electrolyte) in a half-cell setup. The acidophilic character of the Fe-N-C catalyst leads to intrusion of phosphoric acid electrolyte into the CL. The strength of the acid penetration depends on the PTFE content, which is visible via the contact angles. The 10 wt % PTFE GDE is less capable to withdraw product water and electrolyte and results into the lowest half-cell performance. Higher PTFE contents counterbalance the acid drag into the CL and impede flooding. The power density at around 130 mA mgCatalyst−2 increases by 34 % from 10 to 50 wt % PTFE.DLR project LaBreNAFederal Ministry for Economic Affairs and Climate ActionProject HT-PEM 2.0German BundestagElectron and Light Microscopy Service Unit, Carl von Ossietzky University of Oldenbur

In situ Studien von Pt Nanopartikeln auf verschiedenen Trägern für korrosionsstabile PEM Brennstoffzellenkathoden

Wasserstoffbrennstoffzellen bieten als elektrochemische Energiewandler die Möglichkeit, Wasserstoff aus erneuerbaren Resourcen als saubere und emissionsarme Energiequelle für elektrische Verbraucher, wie Elektromotoren, zu nutzen. Um eine konkurrenzfähige Alternative zu heutigen Verbrennungsmotoren in der Automobilindustrie darstellen zu können, müssen aber noch verschiedenste Hindernisse überwunden werden. Im Blickfeld der Forschung sind dabei vor allem die Katalysatoren, die größtenteils aus Platin bestehen und an der Sauerstoffreduktionsseite von Niedertemperaturbrennstoffzellen zum Einsatz kommen. Neben der Maximierung der masse-basierten Aktivität dieser Katalysatoren, ist vor allem die Langlebigkeit entscheidend, um leistungsfähige Brennstoffzellen herzustellen. Dabei spielt der Katalysatorträger und sein chemisches und elektrochemisches Korrosionsverhalten unter Brennstoffzellbedingungen eine entscheidende Rolle. Hochoberflächige bis graphitisierte Kohlenstoffe sind dabei heute Stand der Technik mit ihren Vor- und Nachteilen. Ziel dieser Arbeit ist es, den Einfluss neuartiger oxidischer oder kohlenstoffbasierter Trägermaterialien auf das elektrochemische und morphologische Stabilitätsverhalten der katalytischen Pt Nanopartikel zu untersuchen. Solche Katalysatorträger, exemplarisch dargestellt an Metalloxiden und Heteroatom-modifizierten Kohlenstoffen, wurden mit Pt Nanopartikeln dekoriert und umfangreich auf ihre physikochemischen Eigenschaften hin untersucht. Dabei kamen verschiedenste in situ Analysemethoden zum Einsatz, um die Katalysatorkomponenten im simulierten Brennstoffzellenbetrieb zu evaluieren und mit den Ergebnissen zum fundamentalen Wissen über Degradationsprozesse beizutragen. Es konnte gezeigt werden, dass Indium Zinn Oxid (ITO) als Trägermaterial unter simulierten Betriebsbedingungen potentialbereich-abhängige Alterung zeigt. Den Platin Nanopartikeln konnte eine exzellente strukturelle und morphologische Stabilität nachgewiesen werden. Jedoch vergiftete Trägerdegradation die Platinoberfläche auf atomarer Ebene, was folglich zu erheblichen Aktivitätsverlusten führte. Der Modifizierung von hochoberflächigen Kohlenstoffen mit Stickstoff Heteroatomen konnte ein stabilisierender Effekt auf die Pt Nanopartikel zugewiesen werden. Der modifizierte Kohlenstoff erwies sich als höchst korrosionsresistent und der Katalysator zeigte eine außerordentliche morphologische und elektrochemische Stabilität in ausgedehnten Alterungsprotokollen. Der stabilisierende Effekt konnte auf die Einführung von pyrrolischen Stickstoffgruppen an der Oberfläche zurückgeführt werden. Des weiteren wurde die elektrochemische Pt Oxidation auf verschiedenen Trägern untersucht um Rückschlüsse über mögliche Degradationmechanismen auf Basis von Oxidationsvorgängen zu erhalten. Diese Arbeit zeigt die Wichtigkeit der Untersuchung von Pt-basierten Brennstoffzellen-katalysatoren unter simulierten Betriebsbedingungen und die Aufdeckung von Degradationsmechanismen und stabilisierenden Trägereffekten. Eine detaillierte Kenntnis auf Grundlage von tiefgreifenden Studien ist essentiell für die Weiterentwicklung effizienter Brennstoffzellen als Bestandteil einer emissionsärmeren Gesellschaft.Hydrogen fuel cells offer the possibility as devices for electrochemical energy conversion to utilize hydrogen from renewable sources as a clean and low-emission energy source for electrical motors. However, to be able to represent a competitive alternative to todays internal combustion engines in the automotive sector, several difficulties have to be overcome. Especially the catalysts, that are employed at the cathode side for the oxygen reduction reaction (ORR) in low temperature fuel cells and mostly consist of the scarce metal platinum, are studied extensively. Besides maximizing mass-based activities of these catalysts, the durability is of tremendous importance for the design of efficient fuel cells. Thereby, the catalyst support and its chemical and electrochemical corrosion behavior under operating conditions plays a crucial role for the overall the stability. Nowadays, the use of graphitized and high surface area carbons as supports offer advantages as well as disadvantages. This thesis aims at understanding the influence of alternative oxidic or carbonaceous catalyst supports on the electrochemical and morphological behavior of the catalytic Pt nanoparticles. The use of alternative catalyst support materials is exemplified by the choice of metal oxides and modified carbons, that were decorated with Pt nanoparticles and extensively studied with regard to their physicochemical properties. Different in situ methods are employed to monitor the catalyst/support components under simulated fuel cell operating conditions and to contribute to the scientific knowledge on fundamental degradation processes. It could be shown that indium tin oxide (ITO) used as support degrades depending on the applied potential range under simulated operating conditions. Pt nanoparticles were found to have an excellent structural and morphological stability. However, support degradation lead to a poisoning of the Pt surface on an atomic level and consequently to activity deterioration. Modification of high surface area carbon with nitrogen heteroatoms resulted in an increased corrosion resistance and was was further proven to have a stabilizing effect on the Pt nanoparticles. The catalyst showed extraordinary morphological and electrochemical stability in long-term stress tests. This was ascribed to surface modification in the form of introduction of pyrrolic N groups as most abundant surface species. Furthermore, the electrochemical oxidation of Pt on different supports was studied by various in situ methods to track the structural response of the Pt nanoparticles and to draw conclusions about possible degradation pathways as a consequence of oxidation processes. Together, this work illustrates the importance of investigating Pt-based fuel cell catalysts under simulated working conditions and revealing degradation mechanisms and support-related stabilizing effects. A detailed knowledge based on profound studies is essential for the ongoing development of efficient fuel cells as part of a clean energy society

PAFC, HT-PEMFC cathode

HT-PEMFC and PAFC cathodes are constituted similarly in terms of gas diffusion layers and applied catalysts. In general, Pt-alloy catalysts with loadings up to 1 mgPt cm−2 are commonly employed due to partial poisoning of the Pt catalyst through the adsorption of phosphates present from the phosphoric acid electrolyte in both FC types. This chapter gives an overview of the theoretical background of oxygen reduction reaction and discusses the state-of-the art cathodes with catalysts and supports. Relevant catalyst degradation pathways present in HT-PEMFC as well as PAFC are shown. The specification of gas diffusion electrodes as well as their fabrication and characterization are described followed by an overall trend of current research topics in HT-PEMFC toward reduction of Pt loading