Redefinition of stack efficiency and optimization of stack performance for PEMFCs

In the first part of this work, we present a general redefinition of the PEMFC stack efficiency taking into account all power losses directly connected with the stack performance and the applied stack operating conditions. These are the stack fuel loss, the stack polarization- and reaction entropy- and enthalpy losses, and the theoretical losses for stack feed stream conditioning. In general, the latter includes humidification and pressurization (or pumping) of the reactants as well as pumping of the coolant. Furthermore, examples will be given which power losses are relevant for different applications and which are dominant and have to be considered. By the use of this new figure of merit, the efficiencies of two stacks can be compared to each other in a system-relevant way and can be determined for a given application scenario. In addition, the accompanying redefined balance-of-plant efficiency is a parameter characterizing uniquely the effectiveness of the BoP design and of the BoP components. In the second part of this work, a procedure for the optimization of the PEMFC stack performance will be highlighted. The performance of a stack at a given constant load strongly depends on the operating conditions characterized by 7 parameters. These are the stack temperature, the stoichiometric values of the reactants, the relative humidity of the reactants, and the pressures in both compartments of the stack. The effect of these parameters on the stack performance is non-linear and synergistic. A separate optimization of each single parameter is not meaningful. Therefore, a direct-search algorithm, the Nelder-Mead simplex, was used for the simultaneous optimization of all parameters

Development and Characterization of a LT-PEMFC Stack with an Extended Operating Temperature Range up to 120 °C

Nowadays, the operation temperature of a polymer electrolyte fuel cell (PEFC) stack for automotive application is about 80 °C. The presented work concerns the characterization of a 30-cell PEFC stack (2.5 kWel) developed at the German Aerospace Center, designed for operating temperatures up to 120 °C for a limited time span. Short-term operation of the stack at higher temperatures would contribute to the improvement of cooling system components with scaled-down dimensions. This concept helps to reduce the vehicle weight and thus to save fuel. In this contribution we present a proof-of-concept of the feasibility of short-term operation for excess temperature events. For this purpose, the stack behaviour was investigated through a series of 20 temperature cycles from 90 to 120 °C at galvanostatic conditions of 70 A (approx. 0.5 A·cm–2 and 1.5 kWel) and without adaption of the gas dew points. The stack power decreased by 21 ± 1 %, with a fully reversible performance recovery at the end of every thermal cycle (see Fig. 1). The higher irreversible degradation rate under these harsh conditions was attributed to the enhanced mechanical stress, which is also correlated to the cycling of the membrane humidity. Furthermore, the results of a long-term steady-state test of 1200 hours under automotive relevant conditions at 80 °C are presented. An end-of-life characterization of the individual cells helped to identify possible causes for performance losses due to catalyst, electrode and membrane degradation. A reduction of electrochemically active surface areas in some cells was ascribed to a platinum catalyst particle growth. Membrane degradation and carbon corrosion occurred additionally, evidenced by increased high frequency resistances and hydrogen crossover rates of the membranes. As result of carbon corrosion, the hydrophobicity of the carbon-based components decreased, causing water accumulation in individual cells

Summary of Test Procedures for PEMFC Stack Performance Tests

The general approach of the EU-funded project Stack-Test and the prepared testing procedures were presented. The different test modules and test programs were explained including differences to already existing standards. Parameters with high impact on the test results were identified and required control strategies for comparable testing were pointed out

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

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

INSIDE – In-situ Diagnostics in Water Electrolysers

In this joint R&D project supported by the EU Fuel Cell and Hydrogen Joint Undertaking, an electrochemical in-situ diagnostics tool for the monitoring of locally resolved current densities in polymer electrolyte membrane fuel cells, is adapted to three different water electrolysis technologies. The developed tools allow correlating performance issues and ageing processes with local anomalies. The corresponding mechanisms are investigated with ex-situ analytics. The patented segmented printed circuit board (PCB) for the monitoring of current density distributions in PEM based fuel cells is used and steadily improved at DLR. Applications are specific degradation mechanisms and optimisation of operation parameters. The real time technology allows, e. g., to observe and mitigate local deactivation of the fuel cell due to condensing water or irreversible local ageing. It has already been adapted for the use in Redox-Flow Battery systems and is ready for the next development step. In water electrolysis, the technological boundaries are different to that of fuel cells, but similarly, there is need for systematic optimisation by locally resolved in-situ analytics and, in particular for an on-line diagnostics tool. The challenges for the adaptation of the segmented board technology to chemical and physical environment are different for each of the three involved technologies: - Alkaline water electrolysis - Proton exchange membrane based water electrolysis - Anion exchange membrane based water electrolysis For each technology, pH and chemical ambience, pressure temperature, bubble formation, and typical range of current densities hold different requirements to layout and corrosion stability. The proof of concept has already been shown in PEM based electrolysis

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

Stack-Test Work Package 2: Functional and Performance Testing on PEMFC stacks

The EU-funded research project Stack-Test has developed different test procedures for the functional and performance characterization of PEMFC stacks. The test procedures are based on test modules related to the variation of one test input parameter and the determination of the influence on the stack performance. These test modules are suitable for all stack applications and have a general nature. The procedure for parameter variation, the critical parameters and the test parameters of interest for the performance characterization have been defined. The following test modules are available: - Humidity, temperature, and lambda sensitivity - Pressure sensitivity (exemplarily presented) - Fuel/oxidant composition - Freeze start - Continuous operation at constant load - Polarization curve - Impact of ambient conditions - Electrochemical methods - Dead end operation The objective and scope of the test modules will be presented and the nature of the test modules will be demonstrated using an exemplary module. Furthermore, application specific test programs have been developed in the project. These test programs are focusing automotive, stationary and portable applications. Different test modules are combined to a test program and parameter sets are suggested for each application. Five test programs have been created so far: - Stack performance assessment - Stack performance mapping - Stack tolerance to ambient conditions - Dead end performance - Stack performance characterizatio

Evaluation of reversible and irreversible degradation rates of polymer electrolyte membrane fuel cells tested in automotive conditions

This work provides single cell durability tests of membrane electrode assemblies in dynamic operation regularly interrupted by recovery procedures for the removal of reversible voltage losses. Degradation rates at different loads in one single test can be determined from these tests. Hence, it is possible to report degradation rates versus current density instead of a single degradation rate value. A clear discrimination between reversible and irreversible voltage loss rates is provided. The irreversible degradation rate can be described by a linear regression of voltage values after the recovery steps. Using voltage values before refresh is less adequate due to possible impacts of reversible effects. The reversible contribution to the voltage decay is dominated by an exponential decay after restart, eventually turning into a linear one. A linear-exponential function is proposed to fit the reversible voltage degradation. Due to this function, the Degradation behavior of an automotive fuel cell can be described correctly during the first hours after restart. The fit parameters decay constant, exponential amplitude and linear slope are evaluated. Eventually, the reasons for the voltage recovery during shutdown are analyzed showing that ionomer effects in the catalyst layer and/or membrane seem to be the key factor in this process

Identification of critical parameters for PEMFC stack performance characterization and control strategies for reliable and comparable stack benchmarking

This paper is focused on the identification of critical parameters and on the development of reliable methodologies to achieve comparable benchmark results. Possibilities for control sensor positioning and for parameter variation in sensitivity tests are discussed and recommended options for the control strategy are summarized. This ensures result comparability as well as stable test conditions. E.g., the stack temperature fluctuation is minimized to about 1 °C. The experiments demonstrate that reactants pressures differ up to 12 kPa if pressure control positions are varied, resulting in an average cell voltage deviation of 21 mV. Test parameters simulating different stack applications are summarized. The stack demonstrated comparable average cell voltage of 0.63 V for stationary and portable conditions. For automotive conditions, the voltage increased to 0.69 V, mainly caused by higher reactants pressures. A benchmarking concept is introduced using “steady-state” polarization curves. The occurring 20 mV hysteresis effect between the ascending and descending polarization curve can be corrected calculating the mean value of both voltages. This minimizes the influence of preceding load levels, current set points, and dwell times