Development of new test instruments and protocols for the diagnostic of fuel cell stacks

In the area of fuel cell research, most of the experimental techniques and equipments are still devoted to the analysis of single cells or very short stacks. However, the diagnosis of fuel cell stacks providing significant power levels is a critical aspect to be considered for the integration of fuel cell systems into real applications such as vehicles or stationary gensets. In this article, a new instrument developed in-lab is proposed in order to satisfy the requirements of electrochemical impedance studies to be led on large FC generators made of numerous individual cells. Moreover, new voltammetry protocols dedicated to PEMFC stack analysis are described. They enable for instance the study of membrane permeability and loss of platinum activity inside complete PEMFC assemblies. Keywords: PEMFC; Stack; Characterization; Electrochemical Impedance Spectroscopy; Cyclic Voltammetry; Linear Sweep Voltammetry

Study of temperature, air dew point temperature and reactant flow effects on PEMFC performances using electrochemical spectroscopy and voltammetry techniques

A single PEMFC has been operated by varying the assembly temperature, the air dew point temperature and the anode/cathode stoichiometry rates with the aim to identify the parameters and combinations of factors affecting the cell performance. Some of the experiments were conducted with low humidified reactants (relative humidity of 12%). The FC characterizations tests have been conducted using in-situ electrochemical methods based on load current and cell voltage signal analysis, namely: polarization curves, EIS measurements, cyclic and linear sweep voltammetries (CV and LSV). The impacts of the parameters on the global FC performances were observed using the polarization curves whereas EIS, CV and LSV test results were used to discriminate the different voltage loss sources. The test results suggest that some parameter sets allow maximal output voltages but can also induce material degradation. For instance, higher FC temperature and air flow values can induce significant electrical efficiency benefits, notably by increasing the reversible potential and the reaction kinetics. However, raising the cell temperature can also gradually dry the FC and increase the risk of membrane failure. LSV has also shown that elevated FC temperature and relative humidity can also accelerate the electrolyte degradation (i.e. slightly higher fuel crossover rate) and reduce the lifetime consequently. PEMFC; Characterization; Electrochemical Impedance Spectroscopy; Cyclic Voltammetry; Linear Sweep Voltammetr

APPROCHES EXPERIMENTALES ET ANALYSE PROBABILISTE POUR LE DIAGNOSTIC DE PILES A COMBUSTIBLE DE TYPE PEM.

The lifetime expectancy and the reliability of Fuel Cell generators (FC) equipped with polymer membranes are major topics for the marketing of this technology in the transport sector. However, the constraint of the lifetime could be overcome by preventive maintenance procedure which would allow detecting at which moment the fuel cell elements must be changed. The actual need of high-voltage acquisition systems for the characterization of FC power generators has led to the development of a high-voltage multiways impedance spectrometer. This new acquisition system includes 31 ways dedicated to the cell(s) voltage recording. Voltage measurement can be realized up to 1000 (ou 700 Vrms). Various tests have been performed on different FC stacks operated in normal and degrading conditions according to an appropriate experimental design. Some Bayesian networks have been used to analyze the experimental data collected through Electrochemical Impedance Spectroscopy (EIS) and to diagnose the FC stack behavior. This approach consists in discerning seven operating modes (including six failures modes) which are the most frequently encountered during FC lifetime. The Bayesian methodology has shown excellent performances and capabilities to ensure efficient FC diagnosis. A 98,5 % rate of good fault classification was reached in the experimental design applied. In addition to the work done on the EIS basis, two electroanalytical methods (linear sweep voltammetry and cyclic voltammetry) until now reserved to the study of single cells were adapted to the FC stack analysis. The linear sweep voltammetry has been used to detect any hydrogen permeability through the FC membranes. The fuel crossover is actually an excellent indicator of the FC state-of-health. Cyclic voltammetry was used to characterize the true electrochemical active area at the FC electrodes. Finally, the gas crossover trough the membrane(s) has also been highlighted using a home-made experimental method. This test procedure includes some pressure and electrical measurements to detect any failing cell(s) inside the FC assembly. This Ph.D work is a contribution to the PAN-H ANR DIAPASON project (FC diagnosis methods for vehicle and stationary applications without intrusive sensors).La durée de vie et la fiabilité des générateurs piles à combustible (PàC) à membranes polymère constituent des enjeux majeurs pour le développement de cette technologie, en particulier dans le secteur des transports. La contrainte de la durée de vie pourrait néanmoins être en partie contournée en optant pour des procédures de maintenance préventive qui permettraient de détecter à quel moment il convient de changer certains organes de la PàC, tels que le cœur de pile. Le besoin au FC LAB d'instruments de mesure pour la caractérisation de générateurs PàC de fortes puissances a conduit au développement d'un spectromètre d'impédance original, haute tension et multivoies. Ce nouveau système possédant 31 voies d'acquisition est capable de réaliser des mesures sur des générateurs électrochimiques dont les tensions peuvent atteindre 1000 (ou 700 Vrms). Différentes caractérisations de stacks basses températures (PEMFC) ont été réalisées en mettant en œuvre un plan d'expérience combinant des conditions normales et dégradantes. Les données issues des mesures par spectroscopie d'impédance ont été analysées par la méthode probabiliste des réseaux bayésiens en vue du diagnostic du générateur. L'approche consiste à discerner 7 modes de fonctionnements (incluant six modes de défaillances) qui sont les plus fréquemment rencontrés par la PàC. L'approche bayésienne a montré d'excellentes capacités pour assurer un diagnostic efficient de PàC. Un taux de bonne détection/ classification des modes de fonctionnement de 98,5% a notamment été obtenu sur le plan d'expérience préalablement défini. En complément des travaux menés sur l'impédance électrochimique, deux techniques électroanalytiques (voltammétries à balayage linéaire et cyclique) jusqu'alors réservées à l'étude de mono-cellules ont été adaptées pour l'analyse sur générateur multi-cellules. La voltammétrie à balayage linéaire a notamment permis d'analyser la perméabilité des membranes perfluorées qui équipent les PàC de type PEMFC. La mesure de la perméabilité de la membrane est d'ailleurs un excellent indicateur de l'état de santé de la PàC. La voltammétrie à balayage cyclique a été utilisée pour caractériser les surfaces réellement actives aux électrodes. Enfin, une méthode combinant mesures pressiométriques et mesures électriques a été développée dans le but d'identifier les cellules défaillantes (inétanchéités anode - cathode) dans un générateur multi-cellules. Les travaux de cette Thèse de Doctorat s'inscrivent dans le contexte du projet PAN-H ANR intitulé DIAPASON (Méthodes de diagnostic pour Piles à combustible pour les applications Automobile et Stationnaire sans instrumentation)

Electrochemical characterisation of fuel cell stack during cold start

International audienceFuel cell self start at negative temperature conditions is not an easy task due to the water produced by the reduction of oxygen at the cathode. This amount of water can turn into ice and block the reaction before the temperature inside the fuel cell reaches positive values. The mechanism of the physical process which leads to oxidant starvation phenomena when ice appears is not yet well identified. In order to obtain a deeper understanding of this problem, the article presents some experimental investigations conducted on a short fuel cell stack. These experiments simulate vehicle technology operated in cold start conditions not with the primary objective to reach a successful and rapid start-up but much rather to characterise and understand the cold start phenomena until starvation occurs. A number of polarisation curves, electrochemical spectroscopy and cyclic voltammetry measurements are done on the stack before, after and also during the cold starts experiments. It is observed that the process of drying and cooling down prior to cold start have a great impact on the electrochemical cathode area. The results obtained show the evolution of the stack behaviour during the low temperature operation with a slow production of frost. The consequence on the individual cells in terms of inhomogeneous degradation is highlighted

Development of a High Voltage Impedance Spectrometer for the Characterization and Diagnosis of Large PEFC Stacks

International audienc

EMC studies on grid integration of distributed energy resources ::a 10 kW, IEC-61850 compliant inverter design and preliminary test results

In this paper the main features of a transformerless 10 kW three-phase inverter, compliant to IEC-61850 communication standard, will be presented. It will be used for a research project concerning the study of grid harmonics pollution produced by high penetration of DER (Distributed Energy Resources). It will allow testing different harmonics mitigation and power injection control strategies

A Simplified Design Strategy for Multi-Resonant Current Control of a Grid-Connected Voltage Source Inverter with an LCL Filter

A distorted grid voltage or nonlinear behavior in the current control loop can cause low frequency current harmonics in a grid-connected voltage source inverter (VSI). Many efforts have been made to mitigate such phenomena, including hardware and/or control structure improvements. A well-known suitable strategy to reduce current harmonics in a selective manner is to apply a Proportional Multi-Resonant (PMR) current controller. Inverter-grid stability is another common issue when dealing with grid-connected VSI. Stability is influenced by the inverter impedance, which depends on the controller parameters. This paper presents a simplified tuning strategy for the PMR controller, taking into consideration the inverter-grid stability issue. The obtained controller was implemented and tested in a 10 kW three-phase inverter with a passively damped LCL filter. A significant reduction of current harmonics emission from the inverter up to 650 Hz was achieved without any hardware modification. The limits of PMR controllers to mitigate current harmonics were studied, and the influence of the grid impedance was verified

Development of a High Voltage Impedance Spectrometer for the Characterization and Diagnosis of Large PEFC Stacks

International audienc

Model-less/measurement-based computation of voltage sensitivities in unbalanced electrical distribution networks ::experimental validation

Thanks to its intrinsic linear properties, the application of voltage sensitivity coefficients theory for the development of fast and effective low voltage (LV) distribution grid control algorithms knows a strong interest among the scientific community. In particular, model-less methods for computation of sensitivity coefficients received lots of attention, as the accurate grid model (e.g., line parameters) is not always available in LV grids. In this paper, a model-less/measurement-based method for computation of voltage sensitivity coefficients in a real distribution grid under known operation scenario is evaluated and experimentally validated. The model-less method for computation of sensitivity coefficients has been implemented in a decentralized measurement and controldevice called GridEye. GridEye is a digital grid optimization tool to design, operate, monitor, analyze, and automate any power distribution grid. The tests are performed in the “ReIne” laboratory, a new flexible structure, with nominal power up to 100kVA, able to emulate different distribution network topologies, such as radial, meshed or partly meshed network

A New High Voltage Impedance Spectrometer for the Diagnostic of Fuel Cell Stacks

This paper presents a novel architecture of an impedance spectrometer dedicated to the characterization and diagnostics of large fuel cell stacks operated in galvanostatic mode. The validation tests are first performed on a single proton exchange membrane fuel cell (PEMFC). Then, experiments are carried out on a 20-cell PEMFC stack delivering more significant power levels. The proposed impedancemeter allows spectrum measurements on cells located in the middle of the stack, where common mode potentials are usually too high for commercial devices. Moreover, the impedances of different individual cells in the stack are acquired with a synchronous measurement reference (global stack impedance). This capability allows distinguishing any singular cell behavior or drift effect of operational parameters (e.g., stack temperature and polarization current)