Electrochemical pressure impedance spectroscopy for polymer electrolyte membrane fuel cells: Signal interpretation

Electrochemical pressure impedance spectroscopy (EPIS) is an emerging tool for the diagnosis of polymer electrolyte membrane fuel cells (PEMFC). It is based on analyzing the frequency response of the cell voltage with respect to an excitation of the gas-phase pressure. Several experimental studies in the past decade have shown the complexity of EPIS signals, and so far there is no agreement on the interpretation of EPIS features. The present study contributes to shed light into the physicochemical origin of EPIS features, by using a combination of pseudo-two-dimensional modeling and analytical interpretation. Using static simulations, the contributions of cathode equilibrium potential, cathode overpotential, and membrane resistance on the quasi-static EPIS response are quantified. Using model reduction, the EPIS responses of individual dynamic processes are predicted and compared to the response of the full model. We show that the EPIS signal of the PEMFC studied here is dominated by the humidifier. The signal is further analyzed by using transfer functions between various internal cell states and the outlet pressure excitation. We show that the EPIS response of the humidifier is caused by an oscillating oxygen molar fraction due to an oscillating mass flow rate

A paired-kidney allocation study found superior survival with HLA-DR compatible kidney transplants in the Eurotransplant Senior Program

The Eurotransplant Senior Program (ESP) has expedited the chance for elderly patients with kidney failure to receive a timely transplant. This current study evaluated survival parameters of kidneys donated after brain death with or without matching for HLA-DR antigens. This cohort study evaluated the period within ESP with paired allocation of 675 kidneys from donors 65 years and older to transplant candidates 65 years and older, the first kidney to 341 patients within the Eurotransplant Senior DR-compatible Program and 334 contralateral kidneys without (ESP) HLA-DR antigen matching. We used Kaplan-Meier estimates and competing risk analysis to assess all cause mortality and kidney graft failure, respectively. The log-rank test and Cox proportional hazards regression were used for comparisons. Within ESP, matching for HLA-DR antigens was associated with a significantly lower five-year risk of mortality (hazard ratio 0.71; 95% confidence interval 0.53-0.95) and significantly lower cause-specific hazards for kidney graft failure and return to dialysis at one year (0.55; 0.35-0.87) and five years (0.73; 0.53-0.99) post-transplant. Allocation based on HLA-DR matching resulted in longer cold ischemia (mean difference 1.00 hours; 95% confidence interval: 0.32-1.68) and kidney offers with a significantly shorter median dialysis vintage of 2.4 versus 4.1 yrs. in ESP without matching. Thus, our allocation based on HLA-DR matching improved five-year patient and kidney allograft survival. Hence, our paired allocation study suggests a superior outcome of HLA-DR matching in the context of old-for-old kidney transplantation.</p

Selectivity and Mechanism of Hydrogen Atom Transfer by an Isolable Imidoiron(III) Complex

This article discusses a mechanistic study of hydrogen atom transfer by an isolable iron (III) imido complex, LᴹᵉFeNAd (Lᴹᵉ = bulky β-diketiminate ligand, 2,4-bis(2,6-diisopropylphenylimido)pentyl; Ad = 1-adamantyl)

Electrochemical pressure impedance spectroscopy for studying mass transport processes in polymer electrolyte membrane fuel cells: A model-based analysis

Electrochemical pressure impedance spectroscopy (EPIS) has received the attention of researchers as a method to study mass transport processes in polymer electrolyte mem-brane fuel cells (PEMFC). It is based on analyzing the cell voltage response to a harmonic excitation of the gas phase pressure in the frequency domain. Several experiments with a single-cell fuel cell have shown that the spectra contain information in the frequency range typical for mass transport processes and are sensitive to specific operating condi-tions and structural fuel cell parameters. To further benefit from the observed features, it is essential to identify why they occur, which to date has not yet been accomplished. The aim of the present work, therefore, is to identify causal links between internal processes and the corresponding EPIS features. To this end, the study follows a model-based approach, which allows the analysis of inter-nal states that are not experimentally accessible. The PEMFC model is a pseudo-2D model, which connects the mass transport along the gas channel with the mass transport through the membrane electrode assembly. A modeling novelty is the consideration of the gas vol-ume inside the humidifier upstream the fuel cell inlet, which proves to be crucial for the reproduction of EPIS. The PEMFC model is parametrized to a 100 cm² single cell of the French project partner, who provided the experimental EPIS results reproduced and in-terpreted in the present study. The simulated EPIS results show a good agreement with the experiments at current den-sities ≤ 0.4 A cm–2, where they allow a further analysis of the observed features. At the lowest excitation frequency of 1 mHz, the dynamic cell voltage response approaches the static pressure-voltage response. In the simulated frequency range between 1 mHz – 100 Hz, the cell voltage oscillation is found to strongly correlate with the partial pressure oscillation of oxygen, whereas the influence of the water pressure is limited to the low frequency region. The two prominent EPIS features, namely the strong increase of the cell voltage oscillation and the increase of phase shift with frequency, can be traced back via the oxygen pressure to the oscillation of the inlet flow rate. The phenomenon of the oscillating inlet flow rate is a consequence of the pressure change of the gas phase inside the humidifier and in-creases with frequency. This important finding enables the interpretation of experimen-tally observed EPIS trends for a variation of operational and structural fuel cell parame-ters by tracing them back to the influence of the oscillating inlet flow rate. The separate simulation of the time-dependent processes of the PEMFC model through model reduction shows their individual influence on EPIS. The sluggish process of the wa-ter uptake by the membrane is visible below 0.1 Hz, while the charge and discharge of the double layer becomes visible above 1 Hz. The gas transport through the gas diffusion layer is only visible above 100 Hz. The simulation of the gas transport through the gas channel without consideration of the humidifier becomes visible above 1 Hz. With consideration of the humidifier the gas transport through the gas channel is visible throughout the fre-quency range. The strong similarity of the spectra considering the humidifier with the spectra of the full model setup shows the dominant influence of the humidifier on EPIS. A promising observation is the change in the amplitude relationship between the cell volt-age and the oxygen partial pressure oscillation as a function of the oxygen concentration in the catalyst layer. At a frequency where the influence of oxygen pressure on the cell voltage is dominant, for example at 1 Hz, the amplitude of the cell voltage oscillation could be used to indirectly measure the oxygen concentration in the catalyst layer.Die elektrochemische Druckimpedanzspektroskopie (EPIS) ist ein neues Verfahren, das aktuell zur Untersuchung von Stofftransportprozessen in Polymerelektrolytbrennstoff-zellen (PEMFC) erforscht wird. Es basiert auf der spektralen Analyse der Zellspannungs-antwort auf eine harmonische Anregung des Gasdrucks. Verschiedene Experimente mit Einzelzell-Brennstoffzellen haben gezeigt, dass die Spektren grundsätzlich Informationen in dem für Stofftransportprozesse typischen Frequenzbereich enthalten und sensitiv auf geänderte Betriebsbedingungen und strukturelle Veränderung der Brennstoffzelle rea-gieren. Um weiter von diesen Merkmalen der Spektren zu profitieren, ist es notwendig, ihre Ursache zu identifizieren, was bis jetzt noch nicht erreicht wurde. Das Ziel der vor-liegenden Dissertation ist es, Kausalzusammenhänge zwischen internen Prozessen und ihren zugehörigen Merkmalen im EPIS-Spektrum herzustellen. Hierzu wird ein modellgestützter Ansatz gewählt, der den Vorteil hat, interne Zustände analysieren zu können, die experimentell schwer zugänglich sind. Das verwendete PEMFC-Modell ist ein pseudo-2D-Modell, das den Transport entlang des Gaskanals und den Transport durch die Membran-Elektroden-Einheit (MEA) abbildet und miteinander verknüpft. Eine Neuheit gegenüber anderen in der Literatur verwendeten Modellen ist die Berücksichtigung des Gasvolumens in dem der Brennstoffzelle vorgeschalteten Gasbe-feuchter, welche sich als essenziell für die Reproduktion der EPIS-Experimente erweist. Das Brennstoffzellenmodell wird mithilfe der experimentellen Daten des französischen Projektpartner parametrisiert, welcher EPIS-Experimente mit einer 100 cm2 Einzelzell-Brennstoffzelle durchgeführt hat. Die besagten Daten wurden als Interpretationsgrund-lage in der vorliegenden Arbeit verwendet. Die EPIS-Simulationen weisen eine gute Übereinstimmung mit den Experimenten für Stromstärken ≤ 0.4 A cm–2 auf, wo sie eine weitere Analyse der EPIS-Merkmale ermögli-chen. Bei der niedrigsten Anregungsfrequenz von 1 mHz nähert sich die dynamische Zellspannungsantwort dem statischen Verhalten an. In dem übrigen Frequenzbereich zwischen 1 mHz – 100 Hz korreliert die Oszillation der Zellspannung stark mit der des Sauerstoffpartialdrucks, während der Einfluss des Wasserdrucks nur im niederfrequen-ten Bereich sichtbar ist. Die zwei auffälligsten EPIS-Merkmale, die Verstärkung der Zellspannungsoszillationen und die Zunahme der Phasenverschiebung mit der Frequenz, können über den Sauerstoff-partialdruck auf die Oszillation des Eingangsmassenstroms zurückgeführt werden. Das Phänomen des oszillierenden Eingangsmassenstroms wird durch die Druckänderung der Gasphase im Befeuchter erzeugt und verstärkt sich mit zunehmender Frequenz. Diese wichtige Erkenntnis ermöglicht die Interpretation von experimentell beobachteten Ver-änderungen des EPIS-Signals für geänderte Betriebsbedingungen und strukturelle Verän-derungen der Brennstoffzelle. Die separate Simulation der zeitabhängigen Brennstoffzellenprozesse anhand von Mo-dellreduktionen zeigt ihren individuellen Einfluss auf das EPIS-Signal. Der träge Prozess der Wasseraufnahme der Membran ist nur in dem Frequenzbereich unterhalb von 0.1 Hz sichtbar, während die Ladung und Entladung der elektrischen Doppelladungsschicht erst oberhalb von 1 Hz sichtbar wird. Der Gastransport durch die Gasdiffusionsschicht ist so-gar erst oberhalb von 100 Hz sichtbar. Ohne die Berücksichtigung des Befeuchters ist der Gastransport durch den Gaskanal oberhalb von 1 Hz sichtbar. Mit Berücksichtigung des Befeuchters hingegen ist der Prozess des Gastransportes durch den Gaskanal im gesam-ten Frequenzbereich zu sehen. Die starke Ähnlichkeit des Spektrums unter Berücksichti-gung des Befeuchters mit dem Spektrum des vollständigen Brennstoffzellenmodells zeigt den dominanten Einfluss des Befeuchters auf das EPIS-Signal. Eine vielversprechende Beobachtung ist die Änderung des Amplitudenverhältnis zwi-schen der Oszillation der Zellspannung und des Sauerstoffpartialdrucks in Abhängigkeit der Sauerstoffkonzentration in der Katalysatorschicht. In dem Frequenzbereich, in dem der Einfluss des Sauerstoffpartialdrucks auf die Zellspannungsoszillation dominiert, z.B. bei 1 Hz, könnte die Amplitude der Zellspannungsoszillation indirekt zur Messung der Sauerstoffkonzentration in der Katalysatorschicht genutzt werden

Model-based analysis of Electrochemical Pressure Impedance Spectroscopy (EPIS) for PEM Fuel Cells

Electrochemical impedance spectroscopy (EIS) is a widely-used diagnostic technique to characterize electrochemical processes. It is based on the dynamic analysis of two electrical observables, that is, current and voltage. Electrochemical cells with gaseous reactants or products, in particular fuel cells, offer an additional observable, that is, the gas pressure. The dynamic coupling of current or voltage with gas pressure gives rise to a number of additional impedance definitions, for which we have previously introduced the term electrochemical pressure impedance spectroscopy (EPIS) [1,2]. EPIS shows a particular sensitivity towards transport processes of gas-phase or dissolved species, in particular, diffusion coefficients and transport pathway lengths. It is as such complementary to standard EIS, which is mainly sensitive towards electrochemical processes. First EPIS experiments on PEM fuel cells have recently been shown [3]. We present a detailed modeling and simulation analysis of EPIS of a PEM fuel cell. We use a 1D+1D continuum model of a fuel/air channel pair with GDL and MEA. Backpressure is dynamically varied, and the resulting simulated oscillation in cell voltage is evaluated to yield the ▁Z_( V⁄p_ca ) EPIS signal. Results are obtained for different transport situations of the fuel cell, giving rise to very complex EPIS shapes in the Nyquist plot. This complexity shows the necessity of model-based interpretation of the complex EPIS shapes. Based on the simulation results, specific features in the EPIS spectra can be assigned to different transport domains (gas channel, GDL, membrane water transport)