The Effect of Voltage Cycling on Catalyst Degradation in Automotive PEM Fuel Cells
Dissertation
Authors
Kneer, Alexander Paul
Referee
Tillmetz, WernerStreb, Carsten
Faculties
Fakultät für NaturwissenschaftenInstitutions
Zentrum für Sonnenenergie- und Wasserstoff-Forschung Baden-Württemberg (ZSW)Institut für Anorganische Chemie I (Materialien und Katalyse)
External cooperations
Daimler AGCumulative dissertation containing articles
A. Kneer, J. Jankovic, D. Susac, A. Putz, N. Wagner, M. Sabharwal and
M. Secanell. “Correlation of Changes in Electrochemical and Structural Parameters
due to Voltage Cycling Induced Degradation in PEM Fuel Cells”.
Journal of The Electrochemical Society, 165(6), F3241–F3250 (2018). DOI: 10.1149/2.0271806jes
A. Kneer, N. Wagner, C. Sadeler, A.-C. Scherzer and D. Gerteisen. “Effect of Dwell Time and Scan Rate during Voltage Cycling on Catalyst Degradation in PEM Fuel Cells”. Journal of The Electrochemical Society, 165(10), F805–F812 (2018). DOI: 10.1149/2.0651810jes
A. Kneer and N. Wagner. "A Semi-Empirical Catalyst Degradation Model Based on Voltage Cycling under Automotive Operating Conditions in PEM Fuel Cells". Journal of The Electrochemical Society, 166(2), F120-F127 (2019). DOI: 10.1149/2.0641902jes
A. Kneer, N. Wagner, C. Sadeler, A.-C. Scherzer and D. Gerteisen. “Effect of Dwell Time and Scan Rate during Voltage Cycling on Catalyst Degradation in PEM Fuel Cells”. Journal of The Electrochemical Society, 165(10), F805–F812 (2018). DOI: 10.1149/2.0651810jes
A. Kneer and N. Wagner. "A Semi-Empirical Catalyst Degradation Model Based on Voltage Cycling under Automotive Operating Conditions in PEM Fuel Cells". Journal of The Electrochemical Society, 166(2), F120-F127 (2019). DOI: 10.1149/2.0641902jes
Abstract
Durability and cost are the two major remaining challenges for the commercialization
of fuel cells in passenger vehicles. The platinum catalyst is one of the most
expensive components in a fuel cell and degrades during normal operation due to
load cycling. While a reduction in platinum loading is beneficial for cost, it affects
catalyst durability negatively. Therefore, an understanding of the catalyst degradation
mechanism is required to increase fuel cell lifetime and lower cost.
In this dissertation, the conditions that promote catalyst degradation in an automotive
context and its consequences on catalyst layer microstructure are studied
experimentally.
In order to degrade the platinum catalyst during fuel cell operation square wave
voltage cycling in hydrogen-air atmosphere is carried out as an accelerated stress
test on small scale single cells. For characterization of aged samples and visualization
of the degradation mechanism electrochemical techniques as well as electron
microscopy are applied. The same state-of-the-art automotive membrane electrode
assembly is used in all experiments in order to obtain consistent results.
Voltage cycling results in the loss of electrochemically active surface area (ECSA)
of the platinum catalyst which is correlated to an increase in activation overpotential.
In addition, limiting current density measurements in degraded samples
show that the oxygen transport resistance scales proportionally to the inverse of the
platinum surface area. In the catalyst layer microstructure several changes due to
redistribution of platinum are detected: platinum particle growth, precipitation of
platinum in the membrane as well as the formation of a platinum depleted layer at
the membrane-cathode interface. Quantification of platinum loading in aged samples
reveals that only a small amount of platinum mass is lost from the catalyst layer,
indicating that Ostwald ripening is the main catalyst degradation mechanism.
The application of different potential cycles demonstrates that not only the number
of potential transients but also the dwell time at high potentials determines ECSA
loss, indicating that platinum likely dissolves during the cathodic potential transient.
Analysis of different stressors shows an exponential increase in ECSA loss with upper
potential limit and temperature as well as a linear increase with humidity and
dwell time at the upper potential limit. ECSA loss is modeled semi-empirically using
a first order kinetic rate model and coupled to an analytical performance model.
The model allows the estimation of voltage losses due to catalyst degradation up to
current densities of 2.0A/cm2 with good accuracy.
The findings in this work can be applied to derive an operating strategy optimized regarding
catalyst degradation in order to increase fuel cell lifetime under load cycling.
Date created
2018
Subject Headings
Polymer-Elektrolytmembran-Brennstoffzelle [GND]Katalysator [GND]
Degradation <Technik> [GND]
Platin [GND]
Proton exchange membrane fuel cells [LCSH]
Catalysts [LCSH]
Dissolution (Chemistry) [LCSH]
Platinum [LCSH]
Keywords
PEM Fuel Cell; Catalyst degradation; Voltage cycling; Platinum dissolutionDewey Decimal Group
DDC 540 / Chemistry & allied sciencesMetadata
Show full item recordCitation example
Kneer, Alexander Paul (2019): The Effect of Voltage Cycling on Catalyst Degradation in Automotive PEM Fuel Cells. Open Access Repositorium der Universität Ulm. Dissertation. http://dx.doi.org/10.18725/OPARU-13598