Adsorption and Thermally Induced Reactions of Methanol on Bimetallic X/Ru(0001) Layers (X = Cu, Pt)

This thesis summarizes the results of thermally induced methanol (CH_3OH) reactions on bimetallic Ru(0001)-based catalyst surfaces under ultrahigh vacuum conditions. Specifically, the following clean and oxygen covered surfaces were used: Ru(0001), (sub-) monolayer Cu/Ru(0001), multilayer Cu/Ru(0001), Pt_n/Ru(0001) layers, and Pt_xRu_{1-x}/Ru(0001) surface alloys. The adsorption and reactions of methanol are of great technological relevance for the direct methanol fuel cell (DMFC). Thereby, it is desirable to influence chemical reactivity and selectivity of catalysts to convert methanol to CO_2 instead of CO which acts as a poison affecting a continued and stable operation. The stability of reaction intermediates and the height of the activation barriers of the various reaction steps critically depends on specific properties of the substrate material. Straightforward methods to design novel catalysts in a controlled way are the deposition of ultrathin metal films on a host material, the building of alloys or the addition of coadsorbates. In the experiments performed in this work methanol was added at 20 or 80 K to the catalyst surfaces and slowly annealed with 1 K/s to increasingly higher temperatures. Thereby, the surface species were identified using Fourier transform infrared spectroscopy, i.e. the observed vibrational modes were analyzed in detail. For an unambiguous assignment of the observed peaks isotopic labeling was applied using different isotopes of methanol, oxygen, carbon monoxide, and hydrogen. The desorbing species, on the other hand, were analyzed by temperature programmed desorption using a quadrupole mass spectrometer. The desorption temperature provides information about the binding strength of an adsorbate and about the dissociation temperature of stable surface species which decays into gaseous products. Moreover, isotopic labeling (18-O and 16-O) allows the discrimination of reactions involving surface oxygen, e.g the formation of desorbing water. Similarly, the CD_3OH isotope allows to distinguish whether hydrogen from the CD_3 or the OH group contributes to a certain reaction. For the quantitative analysis of the chemical composition of the surface and the adsorbates, X-ray photoelectron spectroscopy was applied. The experiments focus on the identification of fundamental reaction steps and stable intermediate species and, in the second step, on the variation of surface parameters, such as the sort and thickness of a deposited metal, the addition of coadsorbates, changing the adsorbate order and density or modifying the composition of the surface alloy. The reactions on the investigated surfaces can be subdivided into two major pathways; (i): a total dehydrogenation pathway leading to CO, and (ii): an oxidation pathway which produces gaseous CO_2. On the clean Ru(0001), Cu/Ru(0001) and Pt/Ru(0001) surfaces either the dehydrogenation pathways are observed or no reaction occurs at all. The CO_2 producing path, on the other hand, can be opened by the adsorption of oxygen. In parallel, the CO formation becomes reduced. In this context, the influence of oxygen on the yielded reaction products was investigated. Generally, it is found that only disordered and dilute oxygen promote methanol reactions; dense and ordered O-overlayers passivate the surface effectively. A significant drawback of adding oxygen is the reaction of the oxygen atoms with hydrogen from methanol dehydrogenation to gaseous water. As hydrogen is the energy provider in a DMFC the desorbing water represents an unwanted drain of H atoms from the surface. Interestingly, the surfaces which produce the highest amount of CO_2 are also most efficient with respect to the formation of water. As on oxygen covered Pt_xRu_{1-x}/Ru(0001) surface alloys the drain of H atoms is limited and they nonetheless exhibit CO_2 as a final product they represent a compromise regarding the ideal catalyst material for a DMFC. In particular, alloys with a Pt contents of 50 - 80% are found to be most suitable

PERFORMANCE AND DURABILITY CHARACTERISATION OF MEA‘S

Standard electrochemical characterization methods are discussed. Moreover, approach from the PEMTASTIC project on definition of durability testing protocols for fuel cells is presented

A Novel Accelerated Stress Test for a Representative Enhancement of Cathode Degradation in Direct Methanol Fuel Cells

Performance decay of direct methanol fuel cells hinders technology competitiveness. The cathode electrochemical surface area loss is known to be a major reason for performance loss and it is mainly affected by cathode potential and dynamics, locally influenced by water and methanol crossover. To mitigate such phenomenon, novel materials and components need to be developed and intensively tested in relevant operating conditions. Thus, the development of representative accelerated stress tests is crucial to reduce the necessary testing time to assess material stability. In the literature, the most diffused accelerated stress tests commonly enhance a specific degradation mechanism, each resulting in limited representativeness of the complex combination and interaction of mechanisms involved during real-life operation. This work proposes a novel accelerated stress test procedure permitting a quantifiable and predictable acceleration of cathode degradation, with the goal of being representative of the real device operation. The results obtained with a 200 h accelerated stress test are validated by comparing both in situ and post mortem measurements with those performed during a 1100 h operational test, demonstrating an acceleration factor equal to 6.25x and confirming the development of consistent cathode degradation

Reversible and Irreversible Degradation Phenomena in PEMFCs

The presentation is focused on reversible and irreversible degradation phenomena in polymer electrolyte membrane fuel cells (PEMFCs). Analytical methods for the determination of component degradation will be presented and a new systematic approach for the analysis of reversible and irreversible degradation rates in an operating fuel cell will be introduced. A detailed description of voltage loss rates and particularly of the discrimination between reversible and irreversible voltage losses will be given. A major motivation of the presented work is the lack of common description procedures and determination approaches of voltage losses in durability tests of fuel cell. This issue results in severe difficulties in the comparison of results obtained by different testing facilities or within different research projects especially if only one value for a degradation rate is reported. In order to systematically analyze voltage losses we have performed single cell durability measurements of several hundreds of hours in 25 cm2 lab-scale cells. Specific test protocols containing regular refresh procedures were used for this purpose (see Figure 1). This enables distinguishing between reversible and irreversible voltage losses. To test the refresh procedures and analyze their effect on cell performance, parameters such as the duration of e.g. a soak time step have been varied. Between these refresh steps the cells were typically operated for 50 to 150 h. Conventional 5-layer MEAs with PFSA membranes, carbon supported Pt-catalysts and hydrophobized carbon fiber substrates with micro porous layers as GDLs were used for this study. For in-situ diagnostics of the operated cells polarization curves, impedance spectra, and CVs were recorded in order to determine the impact of the refresh procedures on the cells. Ex-situ methods were used to determine the causes for the reversible and irreversible voltage losses. Using different methods, detailed information about the physical composition of the individual fuel cell components can be obtained in order to optimize them and increase cell durability. Depending on the examined component and the analytical objective infrared absorption spectroscopy (FTIR), Raman, and X-ray photoemission spectroscopy (XPS) can be used to analyze the degradation effects and the sources for reversible and irreversible voltage loss during fuel cell operation. An overview of the different methods and their application will be given. It will be shown, that a combination of complementary methods is necessary to gather a comprehensive view of the occurring processes and mechanisms. As an example, depth profiling techniques combined with XPS can be used to determine the composition changes inside the fuel cell electrodes

Durability Testing of Polymer Electrolyte Fuel Cells Under Stationary and Automotive Conditions

Our presentation focuses on durability testing and degradation of fuel cells. A major motivation of our work is the lack of common description procedures and determination approaches of voltage losses in durability tests of fuel cell for both stationary and automotive applications; this issue leads to severe difficulties in the comparison of results obtained by different institutions or within different projects, especially if only a single value for the degradation rate is reported. In this context, special attention is devoted to the discrimination between so called reversible and irreversible voltage losses. The first are permanent and determine the maximum lifetime of a fuel cell. The latter strongly depend on the chosen operation conditions and can be recovered by specific procedures. In order so systematically address voltage losses we have performed single cell durability measurements of several hundreds of hours in 25 cm2 lab-scale cells using different test protocols containing regular refresh procedures (soak time) allowing to distinguish between reversible and irreversible losses. Furthermore, operation strategies to minimize reversible degradation without using the time consuming refresh procedures are provided. To test the refresh procedures and analyze their effect on cell performance, parameters such as duration of the soak time steps have been varied. Between these refresh steps the cells were typically operated for 50 to 150 h. As samples conventional 5-layer membrane electrode assemblies were used with PFSA membranes, Pt-based catalysts and hydrophobized carbon fiber substrates with micro porous layers as GDLs. For in-situ diagnosis of the operated cells polarization curves, electrochemical impedance spectra, and cyclic voltammograms were recorded in order to determine the impact of the operation conditions and the refresh procedures on degradation. The interpretation of the degradation of the measured membrane electrode assemblies is supported by post-mortem analysis using physical characterization techniques. Additionally, we provide possible approaches to quantitatively determine irreversible voltage decay rates. For instance, voltage values before or after voltage recovery steps can be used to calculate the irreversible loss rate. The advantages and drawbacks of different approaches are discussed. One clear conclusion is that short time tests in the range of 100 hour are not conclusive since this time is too short to make a reliable discrimination between reversible and irreversible losses; also, the decay rate of reversible loss observed after each refresh step increases substantially upon long time operation independent on the type of the refresh procedure. In summary, in our presentation strategies for determination of fuel cell voltages loss rates are compared, evaluated and assessed according to their suitability to distinguish between reversible and irreversible degradation rates; a description of voltage loss rates is proposed. Moreover, operation strategies to minimize reversible degradation are provided. The research leading to these results has received funding from the European Union's Seventh Framework Programme (FP7/2007-2013) for Fuel Cell and Hydrogen Joint Technology Initiative under Grant No. 621216 (SecondAct) and No. 303452 (Impact)

Impact of cell degradation on transport and structural properties of the cathodic catalyst layer in a PEMFC

Over the last decades, catalysts and ionomers have been significantly improved to increase the efficiency and lower the PGM content in PEMFCs. This reduction of PGM electrode loadings has a significant impact on performance and degradation due to local transport challenges near the catalyst surface, which is often only attributed to oxygen diffusion limitations. But up to the present date, it is not proven that this limitation is not also caused by oxygen convection or proton and water transport. Thus, the origin and importance of different transport limitations are still under discussion. The presented study applied a 500 h dynamic degradation test to a low Pt-loaded MEA and analyzed the impact of the applied load cycling to the transport and structural properties of the cathodic catalyst layer. This includes electrochemical analysis of the catalyst layer properties and identification of reasons for the appearing performance losses and changes in the transport limitations. Additionally, local AFM measurements are applied to evaluate structural changes in different positions of the cell and to improve the understanding of ionomer/catalyst interaction in the catalyst layer and the resulting changes during load cycling. The combination of different techniques enabled the detailed understanding of the degradation mechanisms causing the performance decay and can provide useful guidelines to design future PEMFC electrodes with significantly improved performance and durability. The project FURTHER-FC has received funding from the Fuel Cells and Hydrogen 2 Joint Undertaking (now Clean Hydrogen Partnership) under Grant Agreement No 875025. This Joint Undertaking receives support from the European Union’s Horizon 2020 Research and Innovation program, Hydrogen Europe and Hydrogen Europe Research

Experimental and numerical study on catalyst layer of polymer electrolyte membrane fuel cell prepared with diverse drying methods

High manufacturing cost is a major challenge to commercialization of the polymer electrolyte membrane fuel cell (PEMFC) technology in high volume market. Catalyst layer (CL) of PEMFC should incorporate high effective porosity, electrochemically active surface-area, gas permeability, and favorable ionomer distribution. Drying of the CL is a very significant step of electrode fabrication, and determines most of the properties mentioned above, but is rarely a subject of investigation. From various possible drying processes of CL, freeze-drying shows some beneficial properties, such as higher porosity, better ionomer distribution, and reduces the mass transport resistance significantly by allowing more reactant gas into reactive interface. In this work, the influence of diverse drying techniques on the microstructure and performance is investigated. Complementarily, a transient 2D physical continuum-model is used to investigate the effect of the structural properties on cell performance of electrodes prepared with different drying methods. A sensitivity analysis has been also performed to determine the influence of individual parameters applied in the model. Both of the experimental and simulation results stress on the fact that the freeze-drying technique not only significantly enhances the oxygen transport properties through ionomer but also improves the porosity along with the tortuosity of the CL microstructure