Cathode Catalyst Degradation Research Articles

Operando 3D Visualization of Migration and Degradation of a Platinum Cathode Catalyst in a Polymer Electrolyte Fuel Cell

AbstractThe three‐dimensional (3D) distribution and oxidation state of a Pt cathode catalyst in a practical membrane electrode assembly (MEA) were visualized in a practical polymer electrolyte fuel cell (PEFC) under fuel‐cell operating conditions. Operando 3D computed‐tomography imaging with X‐ray absorption near edge structure (XANES) spectroscopy (CT‐XANES) clearly revealed the heterogeneous migration and degradation of Pt cathode catalyst in an MEA during accelerated degradation test (ADT) of PEFC. The degradative Pt migration proceeded over the entire cathode catalyst layer and spread to MEA depth direction into the Nafion membrane.

Operando 3D Visualization of Migration and Degradation of a Platinum Cathode Catalyst in a Polymer Electrolyte Fuel Cell.

The three-dimensional (3D) distribution and oxidation state of a Pt cathode catalyst in a practical membrane electrode assembly (MEA) were visualized in a practical polymer electrolyte fuel cell (PEFC) under fuel-cell operating conditions. Operando 3D computed-tomography imaging with X-ray absorption near edge structure (XANES) spectroscopy (CT-XANES) clearly revealed the heterogeneous migration and degradation of Pt cathode catalyst in an MEA during accelerated degradation test (ADT) of PEFC. The degradative Pt migration proceeded over the entire cathode catalyst layer and spread to MEA depth direction into the Nafion membrane.

Impact of Polymer Electrolyte Membrane Properties on Transport, Performance and Durability

The structure and properties of the polymer electrolyte membrane has a major impact on the MEA performance and durability. Polymers with different chemical structures can propose different mechanisms for water and proton transport which lead to differences in performance and durability. For membranes with similar chemical structures, factors such as thickness and equivalent weight (EW) are crucial in transport and therefore could cause differences in the performance. The focus of this work was a closer look at the impact of these properties, and their changes due to degradation, on the performance and durability of the MEA and its components. The impact of membrane thickness and equivalent weight was studied by comparing the water content of different PFSA membranes as a function of relative humidity (RH). The data was then used to further discuss the conductivity and hydrogen permeability of the membranes in different regimes of temperature and RH. The impact of membrane chemical degradation on transport property was studied by comparing the membrane conductivity, thickness, hydrogen permeability and performance before and after membrane degradation. To study the impact of membrane properties on cathode catalyst performance and degradation, the cathode degradation test was applied on MEAs with and without membrane degradation, and the results were compared. The water content (Lambda) of all membranes showed a type II isotherm increase by increasing the relative humidity. Lambda was found to be independent of membrane thickness and equivalent weight, while, as expected, the absolute water content (g water/g membrane) showed a decrease at higher equivalent weights. Increasing the relative humidity and operational temperature showed an increase in membrane conductivity. Higher thickness or higher equivalent weight both resulted in higher membrane resistance; supporting a more general trend which is the linear increase of membrane conductivity with increase in the absolute water content. Cathode degradation testing showed an impact on the ECSA loss and performance. The effect was more intense for thinner or lower EW membranes suggesting that membrane water content was the contributing factor. After the membrane degradation test, losses in membrane thickness, membrane conductivity, performance and ECSA were observed. In addition, the hydrogen crossover increased due to the membrane thinning. The membranes with pre-existing degradation showed less ECSA loss but increased performance losses. This trend was explained by the combination of higher catalyst layer ionomer resistance and platinum depletion which lead to higher catalyst layer ionic losses related to a reaction distribution shift further into the catalyst layer. Acknowledgements: Funding was provided by Department of Energy EERE Hydrogen and Fuel Cell Technology Program (Project DE-EE0006375).

Impact of Membrane Properties and Membrane Degradation on Cathode Catalyst Layer Degradation

Understanding the effect of material properties and the structure of MEA components is critical to understanding MEA performance and degradation. Previous studies have shown that cathode catalyst layer degradation was observed to be influenced by water content within the MEA. Both relative humidity and membrane type were shown to have a substantial effect on the degradation rates1. As the membrane / ionomer plays a key role in the water management within an MEA, affecting water sorption, water crossover and phase changes, an improved understanding of the effects of membrane properties on cathode catalyst degradation is required. This study examines the effect of membrane properties, as well as the effects of membrane degradation on cathode catalyst layer degradation. In this work, six membranes, including dense membranes Nafion®211 and Nafion®212, experimental reinforced PFSA membranes with low and high equivalent weights (EW), and experimental reinforced partially fluorinated hydrocarbon membranes of high and low equivalent weights, were chosen for investigations of membrane property impacts on beginning of test (BOT) performance and cathode accelerated stress test (AST) durability. These membranes had EW values ranging from ~ 600 to 1100, while the dry thickness ranged from 13 to 50 microns. Performance variations at BOT for the different membranes may be due to a combination of membrane thickness, resistance, and hydrogen and water cross-over effects. After cathode AST cycling, ECSA loss and performance loss were increased for the thinner, lower EW membranes of each membrane family suggesting that membrane water content was the contributing factor. The effects of membrane degradation on cathode catalyst layer degradation for three down-selected membranes will also be discussed. Acknowledgements Funding was provided by Department of Energy EERE Hydrogen and Fuel Cell Technology Program (Project DE-EE0006375).

Mitigation of Cathode Catalyst Degradation during Air/Air Startup Cycling Via the Atmospheric Resistive Switching Mechanism of a Hydrogen Anode with a Pt Catalyst Supported on Ta-Doped TiO2

One of the most important issues for polymer electrolyte fuel cells (PEFCs) is the severe degradation of cathode catalysts during startup and shutdown. The degradation is brought about by the reverse current mechanism [1]. In the present study, we propose a new strategy for reducing the reverse current using unique characteristics of an “atmospheric resistive switching mechanism” (ARSM) of the metal oxide semiconductor support, such that the electrical resistivity changes depending on gas atmosphere (Fig. 1) [2]. 10 atom% Ta-doped TiO2 nanoparticles with a fused-aggregate structure were synthesized by a flame oxide-forming method [3]. The powder obtained was heat-treated at 850 °C in a 4% H2 atmosphere for 2 h to form Ti0.9Ta0.1O2–δ having a surface area of 14.7 m2 g−1. Pt nanoparticles were deposited on the Ti0.9Ta0.1O2–δ by the colloidal method. The powder obtained was heat-treated at 900 °C in a 4% H2 atmosphere for 2 h to produce 19.8 wt% Pt on the Ti0.9Ta0.1O2–δ support catalysts (Pt/Ti0.9Ta0.1O2–δ). We fabricated two membrane-electrode assemblies (MEAs), one using the Pt/Ti0.9Ta0.1O2–δ and the other a graphitized carbon black-supported platinum (Pt/GCB) (TEC10EA50E, Tanaka Kikinzoku Kogyo K. K. (TKK)) as the anode catalysts. Both of the cathode catalysts were Pt/GCB. The electrochemically active surface area (ECSA) values of Pt/Ti0.9Ta0.1O2–δ and Pt/GCB were 30.7 m2 gPt −1 and 49.3 m2 gPt −1, respectively. The resistance of the Pt/Ti0.9Ta0.1O2–δ anode cell was showed approximately one order of magnitude greater resistance in air than in hydrogen, which could be explained by the ARSM of Pt/Ti0.9Ta0.1O2–δ. The overpotentials of the hydrogen oxidation reaction (HOR) of both anodes were compared by the H2 pump method [4]. The HOR overpotential for the Pt/Ti0.9Ta0.1O2–δ anode was negligible, at the same level as that for the Pt/GCB anode, up to at least 1.5 A cm−2. The MEAs were subjected to repeated anode gas switching from air to H2, thus simulating air/air (for anode/cathode) startup cycling. Figure 2 shows the H2/air polarization curves measured at 65 °C with 100% relative humidity. Initially, the Pt/Ti0.9Ta0.1O2–δ anode cell showed slightly lower performance than the Pt/GCB anode cell due to the greater loss of oxygen mass transport in the cathode, which was probably caused by more accumulation of generated water than the latter cell, because back-diffusion of the generated water from the cathode toward the anode must be small due to the limited capability for water expulsion via the hydrophilic anode.. On the other hand, the performance of the Pt/Ti0.9Ta0.1O2–δ anode cell after 1000 cycles exceeded that of the Pt/GCB anode cell. The percentages of retention of the ECSA of cathode at 1000 cycles were 64.7% for the Pt/Ti0.9Ta0.1O2–δ anode cell and 42.4% for the Pt/GCB anode cell. The retention percentages of mass activity at 0.9 V were 51.4% for the Pt/Ti0.9Ta0.1O2–δ anode cell and 39.1% for the Pt/GCB anode cell. Therefore, the low performance degradation of the Pt/Ti0.9Ta0.1O2–δ anode cell at low current density could be attributed to the low ECSA degradation. At high current density (> 0.4 A cm−2), the Pt/Ti0.9Ta0.1O2–δ anode cell showed distinctively higher performance than the Pt/GCB anode cell after 1000 cycles. The thickness of the cathode catalyst layer in the tested MEA with Pt/Ti0.9Ta0.1O2–δ anode was nearly the same as that of the pristine MEA. On the other hand, thinning of the cathode catalyst layer of approximately 40% was observed in the tested MEA with the Pt/GCB anode (Fig. 2 insets). Such a difference indicates that the carbon corrosion of the cathode catalyst during air/air startup cycling was significantly suppressed by the use of the Pt/Ti0.9Ta0.1O2–δ anode. The decrease in the degradations is attributed to a reduction of the reverse current due to a low oxygen reduction reaction rate at the Pt/Ti0.9Ta0.1O2–δ anode, which showed a high resistivity in air atmosphere. These results demonstrate the effectiveness of the ARSM in mitigating the catalyst degradation and the oxygen mass-transport overpotential in the cathode during air/air startup cycling [5]. 1. C.A. Reiser, L. Bregoli, T.W. Patterson, J.S. Yi, J.D. Yang, M.L. Perry, T.D. Jarvi, Electrochem. Solid-State Lett. 8 (2005) A273. 2. Y. Senoo, K. Kakinuma, M. Uchida, H. Uchida, S. Deki, M. Watanabe, RSC Adv. 4 (2014) 32180. 3. K. Kakinuma, M. Uchida, T. Kamino, H. Uchida, M. Watanabe, Electrochim. Acta 56 (2011) 2881. 4. K.C. Neyerlin, W. Gu, J. Jorne, H.A. Gasteiger, J. Electrochem. Soc. 154 (2007) B631. 5. H. Shintani, Y. Kojima, K. Kakinuma, M. Watanabe, M. Uchida, submitted in J. Power Sources. Figure 1

Novel strategy to mitigate cathode catalyst degradation during air/air startup cycling via the atmospheric resistive switching mechanism of a hydrogen anode with a platinum catalyst supported on tantalum-doped titanium dioxide

We propose a new strategy for alleviating the reverse current phenomenon using a unique “atmospheric resistive switching mechanism” (ARSM) of a metal oxide semiconductor support, such that the electrical resistivity changes depending on the gas atmosphere. The membrane-electrode assembly (MEA) using Ta-doped TiO2-supported platinum (Pt/Ta–TiO2) as the anode catalyst showed approximately one order of magnitude greater resistance in air than in hydrogen. The overpotential of the hydrogen oxidation reaction was negligible up to at least 1.5 A cm−2. The losses of electrochemically active surface area and carbon corrosion of the cathode catalyst during air/air startup cycling were significantly suppressed by the use of the Pt/Ta–TiO2 anode. The decrease in the degradation is attributed to a reduction of the reverse current due to a low oxygen reduction reaction rate at the anode, which showed high resistivity in air. These results demonstrate the effectiveness of the ARSM in mitigating cathode catalyst degradation during air/air startup cycling.

In Situ Techniques to Study the Effects of Anode or Cathode Gas‐Exchange Cycles on the Deterioration of Pt/C Cathode Catalysts in PEFCs

AbstractRepeated, quick anode gas‐exchange (AGEX) or cathode gas‐exchange (CGEX) cycles were applied to polymer electrolyte fuel cell (PEFC) membrane electrode assemblies (MEAs) with Pt/C cathode catalysts to simulate start‐up/shut‐down conditions and to examine a possible countermeasure for their effects on MEA cathode catalyst degradation. The peak height of the white line during in situ time‐resolved X‐ray absorption near‐edge structure (XANES) spectra at the Pt LIII‐edge decreased with an increasing number of AGEX cycles and only in the top region of the MEA cathode layer (close to the gas inlet of the fuel cell), not in the whole MEA area. End‐of‐life analysis after AGEX by using SEM and TEM revealed isolated Pt nanoparticles in micro‐cracks and voids produced in the top region of the MEA cathode layer. The nanoparticle sizes remained almost unchanged for top, middle, and bottom regions of the MEA cathode layer. In the case of CGEX, a small increase in the Pt nanoparticle sizes in the top and bottom positions of the MEA cathode layer was observed. These findings support the role of carbon corrosion, leaving isolated, electrochemically inactive Pt nanoparticles as a consequence of the harsh reaction conditions caused by the reverse current decay mechanism during the start‐up/shut‐down phase, as in the case of AGEX cycles, to be the main cause for the cathode catalyst deterioration close to the anode gas inlet. On the other hand, the hydrogen gas feed to both the anode and cathode during start‐up and shut‐down processes seems to effectively suppress carbon corrosion and prevents a decrease in the performance.

MEA Pt/C Cathode Catalyst Degradation during Repeated Anode Gas Exchange Cycles: In-Situ XAFS, STEM-EDS and Electrochemical Analysis

The effects of repeated start-up/shut-down cycles on degradation of Pt/C cathode catalysts in polymer electrolyte fuel cells (PEFC) were investigated by repeated anode gas exchange (AGEX) cycles. The degradation of MEA Pt/C catalysts during the AGEX cycles was examined at three different areas of the MEAs by in-situ XAFS and STEM-EDS. The steady decreases of the electrochemical surface area and ORR performance by the AGEX processes were observed. In contrast to the aspects averaged over the whole MEA areas, in-situ XAFS can provide the position-dependent structural and electronic information on Pt nanoparticles at different positions in the MEA during the AGEX cycles. The white line peak intensity in in-situ XANES spectra at Pt LIII-edge decreased in the order for the MEA regions, top > bottom ≥ middle. STEM images showed that Pt particles dropped off from the carbon supporting particles in the cathode catalyst layer and large cracks and voids formed in the top area of the MEA. These aspects of the in-situ XANES spectra and STEM images are correlated with each other and the MEA degradation.

MEA Pt/C Cathode Catalyst Degradation during Repeated Anode Gas Exchange Cycles: In-Situ XAFS, STEM-EDS and Electrochemical Analysis

The start-up and shut-down processes in polymer electrolyte fuel cell (PEFC) operations involve anode gas (fuel) mixing with air, which may diffuse into the Pt/C anode from either the anode gas outlet or the Pt/C cathode through the Nafion membrane. This causes a transient, short-lived anode-condition, in which a part of the anode catalyst layer is exposed to hydrogen while another is exposed to oxygen (air). The resulting mixed gas-condition leads to oxygen reduction reaction (ORR) at the oxygen-covered area of the anode, while hydrogen oxidation reaction takes place at the hydrogen-covered area of the anode, by the so called reverse current decay mechanism./1/ The electrolyte potential (potential of the Nafion layer) is drawn to negative values in the air-covered regions to facilitate the ORR on the anode, thus causing the potential difference between the Nafion layer and cathode catalyst layer on the corresponding areas of the cathode to increase up to 1.6 V. At the high potentials not only dissolution of Pt from the cathode catalyst but also oxidative corrosion of the carbon support take place.We have measured the Pt/C cathode catalyst in PEFC during repeated fast anode-gas exchanges (AGEX) cycles by means of in-situ time-resolved XAFS, to study the structural and electronic transformations of Pt nanoparticles at the cathode caused by the AGEX cycles. The in situ XAFS and time-resolved XAFS techniques together with STEM-EDS provided new insights into the origin and mechanism for the Pt/C cathode catalyst degradation by the AGEX cycles. /1/ Carl A. Reiser et al.; Electrochem. Solid-State Lett.; 2005 (6) A273

Durability of Low-Temperature Fuel Cell Electrocatalysts

Durability of Low-Temperature Fuel Cell Electrocatalysts

Electrochemical Test Procedures for Accelerated Evaluation of Fuel Cell Cathode Catalyst Degradation

AbstractThe paper introduces an accelerated test procedure that was developed to evaluate the cathode catalyst stability. It is based on systematic analysis of square wave cycles (SWC) in comparison to triangular wave cycles (TWC). Two voltage windows were used: (i) 0.6–0.9 V to simulate the fuel cell drive cycles and (ii) 0.6–1.2 V to accelerate the test. When simulating the drive cycles over 6,000 h, an estimated loss of 40% in electrochemical surface area (ECSA) is found. Increasing the potential window, the testing time could be reduced to 20 h for TWC and for SWC to 4 h.

Cathode Catalyst Degradation in Proton Exchange Membrane Fuel Cells

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Fingerprints of Automotive Fuel Cell Cathode Catalyst Degradation

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Effects of Upper Potential Dwell Time, Transients and Relative Humidity on PEM Fuel Cell Cathode Catalyst Degradation

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Platinum-Alloy Cathode Catalyst Degradation in Proton Exchange Membrane Fuel Cells: Nanometer-Scale Compositional and Morphological Changes

Electrochemical measurements showed an Pt surface area loss and an specific activity loss for a membrane electrode assembly (MEA) cathode with acid-treated “” catalyst particles in a proton exchange membrane fuel cell after voltage cycling between 0.65 and vs reversible hydrogen electrode. Transmission electron microscopy, scanning transmission electron microscopy, associated X-ray energy dispersive spectroscopy, and high angle annular dark-field techniques were used to probe the microstructural changes of the MEA cathode and the compositional changes along the MEA cathode thickness and within individual nanoparticles before and after voltage cycling. Further Co dissolution from acid-treated particles that leads to an increased thickness of a Pt-enriched surface layer and the development of core/shell particles was largely responsible for the reduction in the specific activity of nanoparticle after potential cycling. The Pt weight loss associated with the formation of Pt crystallites near the cathode/membrane interface largely contributed to the measured electrochemical surface area loss, while particle growth of the particles via Ostwald ripening played a lesser role.

Fingerprint of automotive fuel cell cathode catalyst degradation: Pt band in PEMs

An advanced hydrogen-based fuel cell power-train system for the automotive industry must meet a number of demanding requirements to become commercially attractive. These include improvements in fuel economy, power efficiency and durability in order to provide a vehicle range equalling that of an internal combustion engine – all of which have to be achieved while trying to meet cost targets. This article highlights research that is being done at Automotive Fuel Cell Cooperation Corp to gain a fundamental understanding of cathode catalyst degradation. In particular, it discusses Pt dissolution and Pt band formation after in-situ membrane electrode assembly durability testing. Automotive data indicate that Pt dissolution is a significant degradation mode that currently limits the lifetime of membrane electrode assemblies.

Co Oxidation Accompanied with Degradation of Pt-Co Alloy Cathode Catalyst in Polymer Electrolyte Fuel Cell

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Durability studies on performance degradation of Pt/C catalysts of proton exchange membrane fuel cell

In the present paper, a proton exchange membrane fuel cell (PEMFC) using 20 wt.% Pt/C as anode and cathode catalysts, and ambient air at cathode was operated at a current density of 160 mA cm−2 for 2250 h. The measurement results showed that electrochemically active specific areas (SEAS) of both electrode catalysts calculated from CV curves after test evidently decreased. The decay rate of SEAS of anode catalyst was much lower than that of cathode one. X-ray diffraction (XRD), energy dispersive analysis of X-ray (EDAX), and X-ray photoelectron spectrometry (XPS) were employed to characterize the anode and cathode catalysts before and after the life test. The XRD results showed that their crystal structures were perfect, the particle size of new Pt/C catalyst was about 2.5 nm, however, the particle sizes of anode and cathode ones markedly increased, and were about 4.9 nm and 6.8 nm, respectively, after the life test. Furthermore, the size of cathode catalyst was much bigger than that of anode one after test. The Pt element was also found in Nafion® film as shown in EDAX result. The XPS results presented that the content of Pt oxidation states in cathode was much more than that in anode, and the corrosion of carbon support in cathode was also more severe than that in anode after the life test. The experimental results indicated that the increase of particle size of Pt/C catalyst was illustrated with the dissolution/redeposition mechanism. The degradation of cathode catalyst for oxygen electroreduction was one of the main factors affecting on the performance decay of PEMFC.