HT-PEM Fuel Cells Research Articles

Impact of Contact Pressure Cycling on Non-Woven GDLs of HT-PEM Fuel Cells

The Gas Diffusion Layer (GDL) plays an important role in the performance of a fuel cell, but it also strongly controls the durability of the membrane-electrode-assembly (MEA). Mechanical damages like fiber intrusion at high contact pressure can lead to hydrogen crossover and electrical short circuit formation. In this paper a non-woven GDL is analyzed by electrochemical methods in a fuel cell test station under contact pressure cycling between 0.2 and 1.5 MPa. These results are compared ex-situ with the microscopic structural changes in the material investigated by micro computed tomography in combination with a specially designed compression tool. The optical measurements allow the identification of GDL defects. A clear correlation between the loss of internal resistance and hydrogen crossover current and the emerging of small cracks and fiber intrusions could be established.

Impact of Contact Pressure Cycling on Non-Woven GDLs of HT-PEM Fuel Cells

The Gas Diffusion Layer (GDL) plays an important and critical role within High Temperature Polymer Electrolyte Membrane fuel cells (HT-PEMFC). GDLs usually consist of woven (cloth) or non-woven (e.g. felt or paper) arrays of carbon fibers. The main tasks of a GDL are:Gas transport and distribution of reactants and reaction productsElectrical conductivityHeat transportationWater managementSupport of Membrane-Electrode-Assemblies (MEA). The influence of the GDL on the performance of a fuel cell is an extensively discussed topic nowadays (1, 2). The impact on the durability also needs attention. Physical damages, like for example fiber intrusion through the catalyst layer into the membrane, due to high contact pressures have been confirmed prior to this (3, 4). For this reason, ex-situ measurements with an ad hoc designed compression device (5) have been performed. In this work we will examine in more detail the dependency on compression and the behavior of non-woven carbon cloth GDL material. Therefore the compression device, used before, had to be optimized and rescaled furthermore. This allows a better geometrical resolution while utilizing the tool in micro-computed tomography (µ-CT). Results with the new device will be shown within this work.In addition to the ex-situ investigations of the GDL defects based on compression forces, in-situ electrochemical measurements of MEAs like polarization curves, electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV) and linear sweep voltammetry (LSV) will be carried out. Therefore, MEAs were physically stressed with specific contact pressure cycling tests. One cycle starts at 0.2 MPa up to 1.5 MPa (steps: 0.2, 0.5, 1.0 and 1.5 MPa) and will be repeated 3 times with conditioning or relaxation time over night for each contact pressure and electrochemical characterization during day time.Supplementary, GDLs will be analyzed separately with the help of the new compression device and the µ-CT. Focus of this work lies on the investigation of the porosity of the non-woven GDL material, which is utilized in the investigated MEAs. Within bipolar plates (BPP) with their various flow fields, the porosity of a GDL typically shows a non-uniform behavior (6). Due to the channel and land areas of the serpentine flow fields utilized in this study, the compression on the carbon fibers is locally different. Under the land areas the GDL will be squeezed and within the channels a dilation of the GDL material will be observed (4, 7). To analyze reversible and irreversible defects, the in-situ contact pressure cycling will be imitated completely ex-situ with pure non-woven GDL material. With help of the reconstructed and binary µ-CT data, the porosity of the GDL will be determined and illustrated within 3-dimensional models for each contact pressure (Figure 1). Figure 1: 3D-Model of an uncompressed non-woven GDL

Mass Transfer in the HT-PEM Fuel Cell Electrode

A two-dimensional, single phase flow model for high temperature proton exchange membrane fuel cell (HT-PEM fuel cell) is developed to study the mass transfer in its cathode; the influences of the cell temperature, the porosity and thickness of gas diffusion layer on its performance are analyzed. The good consistency between the simulation results and the experimental results proves the feasibility of this model. The research results show that, when the cell temperature increases, performance of the HT-PEM fuel cell improves, but both water and oxygen concentration distributions in its cathode hardly changes; the decrease of the gas diffusion layer thickness and the increase of the porosity of the gas diffusion layer reduce the concentration over-potential, which finally leads to the improvement of the performance of the HT-PEM fuel cell.

Performance and Durability of HT-PEM Fuel Cells

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Micro-Computed Tomography Imaging of HT-PEM Fuel Cells Under Contact Pressure Control

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Micro-Computed Tomography Imaging of HT-PEM Fuel Cells under Contact Pressure Control

In order to increase the efficiency of nowadays fuel cells it is important to understand degradation processes. This paper focuses on defects related to compression of the Membrane-Electrode-Assembly. A particular useful tool therefore is the micro-computed tomography, as it allows a non-destructive insight into the MEA. The results from this imaging method are compared to classical electrochemical characterizations procedures. MEAs with different Gas Diffusion Layers (cloth and paper) are investigated and their individual behavior upon compression is evaluated.

The Optimization Method of Gaskets for HTPEM Fuel Cells

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Cross-linked high temperature polymer electrolytes through oxadiazole bond formation and their applications in HT PEMfuel cells

Chemical cross-linking of linear, high molecular weight polymer electrolytes has been performed herein, using thermally stable cross-linking bonds. Aromatic copolymeric or terpolymeric polyethers combining polar main chain pyridine units with side cross-linkable carboxylic acid groups were employed. The selected cross-linking chemistry was that of oxadiazole ring formation due to the synthetic versatility and the exceptional thermal and chemical stability of the formed linkages. Membranes of increased robustness and thermal stability were obtained after cross-linking that moreover presented higher doping levels than their linear analogues. Selected cross-linked membranes were formulated into membrane electrode assemblies (MEAs) that were tested in a single cell at temperatures up to 220 °C for 350 hours showing stable operation and high ionic conductivities close to 10−1 S cm−1. These facts point to the superiority of the cross-linked materials and their great potential as PEMs for fuel cells operating well above 180 °C.

Distributed Combined Heat and Power from Biogas with Optimized Heat Integrated Wall Reactor Reformer Integrated with Hybrid Cooled Metal Stack HT PEM Fuel Cell

Very few reformers have properties that make them useful for biogas CHP systems with fuel cells, and most fuel cell designs are not performing well when integrated with reformers for biogas. The reasons are several: Since biogas typically contains 30 – 40% CO2, any further dilution reduces the fuel cell efficiency to an unacceptable level. For good fuel cell efficiency the reformate must have high hydrogen content and low CO, and such a matched combination of fuel cell and reformer has to be of low complexity and low cost. Most reformer designs are simply too complex, too bulky and too expensive. Metacon has developed a Hybrid Cooled Metal Stack Technology (HCMST), that when integrated with the Helbio developed HIWAR reformer gives excellent performance, compact design and low cost. By matching and optimizing these two main parts of the complete energy system, and at the same time integrating the temperature control, heat management and water management of the total system. systems up to 1 MW that can be built, without negative effects such as noise, pollution, vibration and heavy maintenance.

Vapor Delivery Systems for the Study of the Effects of Reformate Gas Impurities in HT-PEM Fuel Cells

The reforming of methanol can be an alternative source of hydrogen for fuel cells because it has many practical advantages over hydrogen, mainly due to the technological limitations related to the storage, supply, and distribution of the latter. However, despite the ease of methanol handling, impurities in the reformate gas produced from methanol steam reforming can affect the performance and durability of fuel cells. In this paper different vapor delivery systems, intended to assist in the study of the effects of some of the impurities, are described and compared with each other. A system based on a pump and electrically heated evaporator was found to be more suitable for the typical flow rates involved in the anode feed of an H3PO4/PBI based HT-PEMFC unit cell assembly. Test stations composed of vapor delivery systems and mass flow controllers for testing the effects of methanol slip, water vapor, CO, and CO2 are also illustrated.

Development of a Highly Integrated Micro Fuel Processor Based on Methanol Steam Reforming for a HT-PEM Fuel Cell with an Electric Power Output of 30 W

Within the scope of a German cooperative project a consortium with partners from industry and academia develops a Reformed Methanol Fuel Cell System (RMFC) on the basis of a HT-PEM fuel cell with an electric power output of 30 W. The fuel cell system is used as a hybrid system with an accumulator as an energy supply for golf-caddies. The Steam Reforming of Methanol (SRM) is applied for hydrogen production in a micro-structured reactor which is built on the basis of thin hydroformed metal sheets. An innovative heat exchanger design allows the integration of all relevant fuel processing steps (catalytic combustion, vaporization, reforming, heat exchange) into a single component and also the integration with the HT-PEM fuel cell as such. The fuel processor was tested with different types of catalysts (base metal, precious metal). It turns out that among the catalysts tested a precious metal catalyst has the best stability and performance.

Modelling of CO Poisoning and its Dynamics in HTPEM Fuel Cells

AbstractIn this work, a dynamic, 2‐dimensional, non‐isothermal model of a PBI‐based HTPEM fuel cell has been developed. The model consists of a five‐layer geometry with gas channels, gas diffusion layers (GDL) and membrane. The catalyst layers are taken into account as infinitesimal thin reaction layers between GDL and membrane. The overall cell behaviour is simulated considering conservation of mass, momentum, species, charge and energy. The model is focussed on CO poisoning of the anode in steady state as well as in dynamic operation. Therefore, a temperature and time‐dependent approach of adsorption/desorption of CO and H2 on the catalyst sites and the electrochemical reactions of the adsorbed species is applied. The temperature dependency of the fuel cell performance is investigated in a temperature range between 125 and 160 °C at pure hydrogen operation. CO poisoning of the anode is analysed with polarisation curves for different CO concentrations as well as the dynamic response during a CO pulse. The model results are validated by experimental data of in‐house measurements.

Modeling and Implementation of a 1 kW, Air Cooled HTPEM Fuel Cell in a Hybrid Electrical Vehicle

This work is a preliminary study of using the PBI-based, HTPEM fuel cell technology in automotive applications. This issue was investigated through computational modeling and an experimental investigation. A hybrid fuel cell system, consisting of a 1 kW stack and lead acid batteries, was implemented in a small electrical vehicle. A dynamic model was developed using Matlab-Simulink to describe the system characteristics, select operating conditions and to size system components. Preheating of the fuel cell stack with electrical resistors was investigated and found to be an unrealistic approach for automotive applications. A simple and reliable approach to temperature management in the stack was using the unpressurized, cathode air stream as coolant.

Experimental Evaluation of a Pt-based Heat Exchanger Methanol Reformer for a HTPEM Fuel Cell Stack

Fuel cell systems running on pure hydrogen can efficiently produce electricity and heat for various applications, stationary and mobile. Storage volume can be problematic for stationary fuel cell systems with high run-time demands, but it is especially a challenge when dealing with mobile and automotive applications. Using a liquid hydrocarbon as e.g. methanol as the hydrogen carrier and reforming it to a hydrogen rich gas can solve some of these storage issues. The work presented here examines the use of a heat exchanger methanol reformer for use with a HTPEM fuel cell stack. Initial experimental results and conclusions are presented for this reformer.