Electrochemically Active Surface Area Loss Research Articles

Influence Factors of Platinum Dissolution in Proton Exchange Membrane Fuel Cells: A Sensitivity Study

Polymer electrolyte fuel cells for heavy-duty applications require lifetimes beyond 30,000 h, which poses a durability challenge. In this study, we investigated the influence of various factors on loss of electrochemically active surface area (ECSA) in the cathode, which is a major limiting factor. We derive a parameter range from simulated drive cycles showing that the voltage ranges between 0.70 and 0.85 V and that the cells are in idle state at upper potential limit (UPL) most of the time. We evaluate the influence and interaction of UPL, lower potential limit (LPL), temperature, relative humidity, and cycle time on ECSA and performance at four different operating conditions after 10,000 potential cycles based on 25 experiments. The results indicate that UPL and the hold time at UPL have the strongest impact on degradation, while LPL has a small impact, which does not increase below the potential of full platinum reduction (0.55 V) or hold times longer than 2 s. Furthermore, the interaction of humidity with other factors becomes significant for long experiment times. In summary, the findings of this work can serve as guidelines for minimizing ECSA loss, e.g. by keeping the fuel cell in a benign operation regime via systems control.

Revealing the critical role of low voltage excursions in enhancing PEM fuel cell catalyst degradation by automotive hydrogen/air potential cycling experiments

The durability of Polymer Electrolyte Membrane Fuel Cells under dynamic operation still needs to be improved. To understand the automotive voltage cycling-induced catalyst degradation, the loss of the electrochemically active surface area (ECSA) is investigated through an experimental campaign on Membrane Electrode Assemblies. Ad-hoc hydrogen/air accelerated protocols were designed to evaluate the voltage profile impact in a range relevant for both automotive and heavy duty transport application (<0.90 V). Besides the well-known aging dependence on the upper potential limit, this work evidences the critical role of the short-stops, characterized by low voltage transients. Effort was spent in studying this procedure parameters (voltage level, duration, scan rate, humidification). The accelerated ECSA loss is due to Pt nanoparticles coarsening as proved by transmission electron microscopy and is suspected dominated by Pt cathodic dissolution, incentivized during excursions towards very low potentials (<0.4 V). These findings help the development of system mitigation strategies.

Numerical analysis of PEMFC stack performance degradation using an empirical approach

This paper proposes a performance degradation model based on empirical approaches, which can break down the power loss and quantify the influence of each aging phenomena from the overall performance degradation. With the presence of primary degradation mechanisms, the approach can be used to analyze the performance degradation of fuel cells under different degradation conditions. The aging parameters that represent hydrogen crossover, membrane resistance increase, and electrochemical active surface area (ECSA) loss are introduced to characterize the performance degradation. Based on the polarization behavior of proton exchange membrane fuel cells (PEMFCs), the variations of degradation stressors are determinated by fitting the aging parameters to the measuring polarization curves. The empirical approach is validated by a 150 h durability experiment and agrees well with all polarization curves. The results show that ECSA loss always maintains an apparent influence and hydrogen crossover accounts for 56% of the performance degradation in OCV.

Accelerated Stress Tests to Project PEM Fuel Cell Durability

One of the major degradation mechanisms limiting the long-term durability of proton exchange membrane fuel cells (PEMFCs) is the loss of platinum electrochemically active surface area (ECSA) of the carbon-supported platinum (Pt/C) cathode catalyst, caused by Pt dissolution that is followed by both Ostwald ripening of the Pt nanoparticles and loss of Pt into the ionomer phase [1]. The Pt ECSA loss is accelerated when subjecting PEMFCs to extended load-cycling inducing concomitant cycling of the cathode potential. To this end, accelerated stress tests (ASTs) can be conducted either by controlling the cell/stack current (“load-cycling” AST) under H2/air (anode/cathode) or the potential (“voltage-cycling” AST) under H2/N2 (anode/cathode).Most of the experiments studying the effect of load-cycling on catalyst durability have been based on voltage-cycling in a H2/N2 configuration, showing that Pt ECSA loss is aggravated with increasing upper potential limit (UPL), temperature, and relative humidity (RH) [2, 3, 4]. Comparing voltage-cycling induced degradation under H2/N2 versus H2/air, a recent study has found an essentially identical Pt ECSA loss, but a slightly higher H2/air performance decay when cycling under H2/N2 [4]. This study by General Motors Corporation indicates that Pt ECSA loss may not be a unique descriptor for H2/air performance loss, contrary to what was observed in recent work by Toyota Motor Corporation and Kyushu University [5] as well as in our own studies [6].In this talk, we will discuss the correlation between H2/air performance loss and Pt ECSA loss during voltage-cycling ASTs at different conditions (UPL, RH, and gas-feed) conducted in 5 cm2 active area single-cell PEMFCs, complemented by a voltage-loss analysis to deconvolute oxygen reduction reaction (ORR) activity losses and mass transport losses (due to oxygen mass transport and proton conduction in the cathode catalyst layer). We will also discuss whether the H2/air performance loss is governed by the Pt ECSA loss (independent of catalyst loading) or, as we had proposed previously, by the cathode electrode roughness factor (rf) loss (in cm2 Pt/cm2 cathode, i.e., the product of ECSA and Pt loading) [7]. As roughly 100,000 [8] or even more voltage-cycles are expected for heavy-duty applications, requiring very long measurement times, an approach to relate the degradation under harsh AST conditions with those under application-relevant conditions will be discussed. Experiments are conducted with cathode catalysts based on different carbon supports (Vulcan, Ketjenblack, or so-called accessible carbon supports [9, 10]), on catalysts with different initial Pt ECSAs (i.e., different Pt nanoparticle sizes) and with different initial Pt-loadings (i.e. different initial rf). Finally, exploratory experiments to evaluate the effect of start-up/shut-down on the correlation between electrode rf and H2/air performance decay will be discussed.

Towards a Realistic Prediction of Catalyst Durability from Liquid Half-Cell Tests

Half-cell measurements with rotating disc electrodes (RDE) in liquid electrolyte are a convenient and often used method to determine relevant electrochemical parameters of catalyst materials in early stage development, i.e. when small quantities and/or high number of samples need to be tested. The applicability of the obtained results to real-world systems, e.g. in form of membrane electrode assemblies (MEA) is, however, often challenging. [1] In the presented study, we focus on the reproducibility of voltage cycling-based accelerated stress testing (AST) of PEMFC cathode catalysts (Pt/C). The degradation, i.e. the extent of the different degradation mechanisms such as Ostwald ripening, nanoparticle coalescence, and loss of dissolved platinum into the electrolyte phase are strongly dependent on the environment and operating conditions, both of which differ significantly in RDE and MEA setups. [2-5] Nevertheless, the ability to make realistic predictions of catalyst durability from ASTs in early stage liquid half-cell measurements is highly desirable.To this end, we studied the degradation of a commercial 20%wt Pt/Vulcan catalyst (Tanaka) based on the loss of electrochemically active surface area (ECSA) throughout a typical voltage cycling AST with a square wave profile and potential limits of 0.6-1.0 VRHE. We systematically screened different parameters in the RDE, such as temperature, pH, rotation, electrolyte, and ionomer content, trying to determine the conditions that would yield a similar catalyst degradation rate in the RDE configuration compared to that in actual MEA measurements. Additionally, particle size distributions of the degraded catalysts were determined from TEM images to gain deeper insights into the changes in degradation mechanisms on the nanoscale. The excerpt of our results depicted in the attached figure show that the ECSA loss in the MEA could by far not be reached in typical RDE measurements carried out at room temperature (22°C). In accordance with Urchaga et al., we see that elevated temperature (80 °C) accelerates catalyst degradation in the liquid half-cell ASTs. [6] When we prepared catalyst films with Nafion® with an I/C ratio of 0.7, which is comparable to what we use in the MEA but at the same time beyond what is typically applied for RDE catalyst films, we observe an additional increase in ECSA loss in comparison to ionomer-free catalyst films cycled in 0.1 M HClO4. The high density of sulfonate groups in the ionomer lowers the local pH and they are known to adsorb on the Pt surface, which might be both factors for increased platinum dissolution-related degradation. ASTs in 1 M HClO4 and 0.05 M H2SO4 reveal that both lower pH and the presence of adsorbing anions accelerate the ECSA loss and influence the particle growth. We conclude, that high exposure of platinum particles to the sulfonate groups of the ionomer leads to faster degradation during potential cycling. We could show for Pt/Vulcan (as well as for other catalysts not shown in the attached figure) that the RDE AST can predict catalyst durability as accurately as am MEA, when relevant operating conditions (temperature) and electrode compositions (I/C ratio) are applied.[1] T. Lazaridis, B. M. Stühmeier, H. A. Gasteiger and H. A. El-sayed, Nat. Catal., 2022, 5, 363–373.[2] R. Sharma, S. M. Andersen, ACS Catal. 2018, 8, 3424–3434.[3] K. Ehelebe, D. Escalera-López, S. Cherevko, Curr. Opin. Electrochem. 2021, 100832.[4] H. Yu, M. J. Zachman, C. Li, L. Hu, N. N. Kariuki, R. Mukundan, J. Xie, K. C. Neyerlin, D. J. Myers, D. A. Cullen, ACS Appl. Mater. Interfaces 2022, DOI 10.1021/acsami.1c23281.[5] J. A. Gilbert, N. N. Kariuki, X. Wang, A. J. Kropf, K. Yu, D. J. Groom, P. J. Ferreira, D. Morgan, D. J. Myers, Electrochim. Acta 2015, 173, 223–234.[6] P. Urchaga, T. Kadyk, S. G. Rinaldo, A. O. Pistono, J. Hu, W. Lee, C. Richards, M. H. Eikerling, C. A. Rice, Electrochim. Acta 2015, 176, 1500–1510. Figure 1

Degradation of Pt-Based Cathode Catalysts Upon Voltage Cycling in Single-Cell PEM Fuel Cells Under Air or N2 at Different Relative Humidities

A major degradation mechanism of polymer electrolyte membrane fuel cells (PEMFCs) in transportation applications is the loss of the electrochemically active surface area (ECSA) of platinum cathode catalysts upon dynamic load cycling (resulting in cathode potential cycles). This is commonly investigated by accelerated stress tests (ASTs), cycling the cell voltage under H2/N2 (anode/cathode). Here we examine the degradation of membrane electrode assemblies with Vulcan carbon supported Pt catalysts over extended square-wave voltage cycles between 0.6-1.0 VRHE at 80 °C and 30%-100% RH under either H2/N2 or H2/Air; for the latter case, differential reactant flows were used, and the lower potential limit is controlled to correspond to the high-frequency resistance corrected cell voltage, assuring comparable aging conditions. Over the course of the ASTs, changes of the ECSA, the hydrogen crossover current, the proton conduction resistance and the oxygen transport resistance of the cathode electrode, as well as the differential-flow H2/O2 and H2/Air performance at 80 °C/100% RH were monitored. While the ECSA loss decreases with decreasing RH, it is independent of the gas feeds. Furthermore, the H2/Air performance loss only depends on the ECSA loss. ASTs under H2/N2 versus H2/Air only differ with regards to the chemical/mechanical degradation of the membrane.

A holistic physics-based observer for online flooding and ECSA loss detection in FCEV

This paper addresses the critical need for improving fuel cell electric vehicles (FCEVs) reliability to promote their commercialization. A physics-based observer that combines an Extended Kalman Filter (EKF) with a Sequential Probability Ratio Test (SPRT) has been employed to address two major failure modes in FCEV stacks: flooding and reduction of Electrochemical Active Surface Area (ECSA). Furthermore, the observer estimates the hidden states of a standard automotive airpath system to enable state-feedback control. Therefore, the proposed observer is a novel approach which holistically tackles three coupled problems (i.e., flooding, ECSA loss, and airpath state estimation) which are typically addressed individually. Additionally, the observer is computationally affordable and relies solely on typical vehicle sensors. The observer performance has been validated through simulations, testing various dynamic scenarios in the presence of flooding, ECSA loss failures, and sensors uncertainty. The estimation errors have been confined within a 3% error band for all test cases while the health indicators coherently responded in the presence of faults.

A one-dimensional model for Pt degradation and precipitation in proton exchange membrane fuel cell considering Pt nucleation, particle size growth, and band formation

Reducing Pt catalysts amount without sacrificing the performance and durability is critical for the commercialization of proton exchange membrane fuel cells (PEMFCs). Pt degradation in the cathode catalyst layer (CCL) decreases the electrochemically active surface area (ECSA), and Pt precipitation, especially Pt band formation, has a significant effect on proton exchange membrane (PEM) stability, resulting in severe PEMFC degradation during long-term operation. In this study, a one-dimensional model is developed focusing on Pt catalyst degradation and precipitation, including Ostwald-ripening, mass loss, precipitation in the PEM from nucleation, particle size growth, to Pt band formation. The model accuracy is comprehensively validated against the corresponding experimental data, including ECSA loss and particle size distribution (PSD) evolution in the CCL under different operating conditions (temperature, relative humidity, and loading mode), as well as the precipitated Pt size distribution characteristics and local potential in the PEM. Using this model, the Pt band formation process is comprehensively analyzed, and the effects of operating conditions on Pt degradation and Pt band formation are investigated in detail. It is found that the nucleation stage is brief, and then the significant inhomogeneity at the size growth stage causes Pt band formation. In addition, the high activation enthalpy of dissolution under low humidity can effectively inhibit Pt degradation, and Ostwald-ripening is the mean cause of Pt degradation, which is likely to occur during the rapid voltage change period.

The Effect of Ionomer to Carbon Ratio and Relative Humidity on Cathode Catalyst Degradation in PEM Fuel Cells

The effect of ionomer to carbon (I/C) weight ratio and relative humidity (RH) on cathode catalyst degradation was investigated by comprehensive in situ characterization. Membrane electrode assemblies (MEA) with I/C ratios of 0.5, 0.8 and 1.2 were subjected to an accelerated stress test performed at 40, 70 and 100% RH. The results show an increasing loss in electrochemical active surface area (ECSA) for both higher I/C ratios and RH during voltage cycling. To differentiate between ionomer and water connected ECSA, carbon monoxide stripping measurements were performed at varying RH. Before degradation, all MEAs show comparable total ECSA values, while higher I/C ratios lead to a larger fraction of ionomer connected ECSA. After degradation, ECSA measurements of the lowest I/C ratio showed a relatively higher loss of Pt in contact with ionomer than Pt in contact with water, while an opposite trend was observed for higher I/C ratios. H2/N2 impedance measurements showed drastically increasing protonic catalyst layer resistances for decreasing RH especially at low I/C ratios, which might hinder Pt2+ ion diffusion towards the membrane, hence decreasing the ECSA loss. Limiting current measurements show increasing molecular O2 diffusion resistances at end of test for samples with higher I/C ratios and higher ECSA loss.

Towards a Realistic Prediction of Catalyst Durability from Liquid Half-Cell Tests

Half-cell measurements with rotating disc electrodes (RDE) in liquid electrolyte are a powerful method to determine relevant electrochemical parameters of catalyst materials in early stage development, i.e. when small quantities and/or high number of samples need to be tested. The interpolation of the results to real systems, e.g. in form of membrane electrode assemblies (MEA) is, however, often challenging. [1] In the presented study, we focus on the reproducibility of voltage cycling-based accelerated stress testing (AST) of PEMFC cathode catalysts (Pt/C). The degradation, i.e. the extend of the different degradation mechanisms such as Ostwald ripening, nanoparticle coalescence and platinum dissolution are strongly dependent on the environment and operating conditions, which differ significantly in RDE and MEA setups. [2-5] Nevertheless, the ability to make realistic predictions of catalyst durability from ASTs in early stage liquid half-cell measurements is highly desirable.To this end, we studied the degradation of a commercial Pt/Vulcan catalyst (20 %, Tanaka) based on the loss of electrochemically active surface area (ECSA) throughout a typical load cycle AST with a square wave profile and potential limits of 0.6-1.0 V/RHE. We systematically screened different parameters in the RDE such as temperature, pH, rotation, electrolyte and ionomer content to find an agreement with the degradation rate we observe in complementary MEA measurements. Additionally, particle size distributions of the degraded catalysts were determined from TEM images to gain deeper insights into the changes in degradation mechanisms on the nanoscale. The excerpt of our results depicted in the attached figure show that the ECSA loss in the MEA could by far not be reached in typical RDE measurements carried out at room temperature (22°C). In accordance with Urchaga et al., we see that elevated temperature accelerates catalyst degradation in the liquid half-cell ASTs. [6] But even at equal temperature (80°C), the loss in active surface area is still much higher in the MEA with only 10 % remaining ECSA compared to 40 % in the RDE after 20k cycles, when the RDE catalyst films are prepared without ionomer. The severity is only slightly increased, when the RDE is being rotated at 1600 rpm. When we apply Nafion® with an I/C ratio of 0.7, which is comparable to what we use in the MEA but at the same time beyond what is typically applied for RDE catalyst films, we see a strong increase in ECSA loss measured in 0.1 M HClO4 electrolyte. The high density of sulfonate groups in the ionomer lowers the local pH and they are known to adsorb on the Pt surface, which might be both factors for increased platinum dissolution-related degradation. ASTs in 1 M HClO4 and 0.05 M H2SO4 reveal that both lower pH and the presence of adsorbing anions accelerate the ECSA loss. We conclude, that high exposure of platinum particles to the sulfonate groups of the ionomer leads to faster degradation during potential cycling. We could show for Pt/Vulcan and another commercial catalyst (not shown in figure) that the RDE AST predicts catalyst degradation accurately, when relevant operating conditions (i.e. temperature and I/C ratio) are applied.[1] T. Lazaridis, B. M. Stühmeier, H. A. Gasteiger and H. A. El-sayed, Nat. Catal., 2022, 5, 363–373. [2] R. Sharma, S. M. Andersen, ACS Catal. 2018, 8, 3424–3434. [3] K. Ehelebe, D. Escalera-López, S. Cherevko, Curr. Opin. Electrochem. 2021, 100832. [4] H. Yu, M. J. Zachman, C. Li, L. Hu, N. N. Kariuki, R. Mukundan, J. Xie, K. C. Neyerlin, D. J. Myers, D. A. Cullen, ACS Appl. Mater. Interfaces 2022, DOI 10.1021/acsami.1c23281. [5] J. A. Gilbert, N. N. Kariuki, X. Wang, A. J. Kropf, K. Yu, D. J. Groom, P. J. Ferreira, D. Morgan, D. J. Myers, Electrochim. Acta 2015, 173, 223–234. [6] P. Urchaga, T. Kadyk, S. G. Rinaldo, A. O. Pistono, J. Hu, W. Lee, C. Richards, M. H. Eikerling, C. A. Rice, Electrochim. Acta 2015, 176, 1500–1510. Figure 1

Investigating the acceleration factors of proton exchange membrane fuel cell degradation

In this work, a voltage square wave was used as accelerated stress test (AST) to assess the cathode catalyst durability in membrane electrode assemblies (MEAs) of the proton exchange membrane fuel cells (PEMFCs). The degradation of MEAs was investigated under square waves with dwell time of 3 s (SW-3 s) and 6 s (SW-6 s). AST conditions and the difference of impact between both ASTs conditions was calculated by using the degradation of electrochemical active surface area (ECSA) as an indicator. The platinum particle sizes of the cathode catalyst before and after the accelerated testing were characterized by transmission electron microscopy (TEM) and its correlation with ECSA was regarded. Finally, the loss of ECSA is used as an indicator to calculate the acceleration factor between AST and steady-state durability testing for 7500 h. The acceleration factor (fECSA) was estimated to be 150 by only regarding the ECSA.

Towards a realistic prediction of catalyst durability from liquid half-cell tests.

Liquid half-cell measurements provide a convenient laboratory method for determining relevant parameters of electro-catalysts applied in e.g. polymer electrolyte membrane fuel cells. While these measurements may be effective in certain contexts, their applicability to real-world systems, such as single-cells in a membrane electrode assembly (MEA) configuration, is not always clear. This is particularly true when assessing the stability of these systems through accelerated stress tests (ASTs). Due to different electrode compositions and operating conditions, nanoscale degradation proceeds differently. Nevertheless, given the high demands of MEA measurements in terms of time, testing equipment complexity, and amount of catalyst material, application-relevant predictions of catalyst durability from liquid half-cell tests are highly desirable. This study combines electrochemical and nanoparticle analysis based on transmission electron microscopy to conduct a typical voltage cycling AST for rotating disc electrode (RDE) measurements, showing that the loss of the electrochemically active surface area (ECSA) of the used Pt/Vulcan catalyst is strongly enhanced at 80 °C compared to room temperature, which goes along with increased nanoparticle coarsening. Additionally, a high ionomer/carbon mass ratio (I/C = 0.7) accelerates the ECSA loss, and further investigations of its influence suggest a combination of several factors, including the high local proton concentration and the presence of adsorbing anions. At the same temperature (80 °C) and I/C ratio (0.7), the ECSA loss vs. AST cycle number of the Pt/Vulcan catalyst is essentially identical for a voltage cycling AST conducted in either an RDE half-cell or an MEA configuration, suggesting that liquid electrolyte half-cell based ASTs can provide application-relevant results. Thus, our study points out a way for predicting the stability of electro-catalysts in MEAs based on RDE experiments that require less specialized equipment and only μg-quantities of catalysts.

Spatially Resolved Heterogeneous Electrocatalyst Degradation in Polymer Electrolyte Fuel Cells Subjected to Accelerated Aging Conditions

This work quantifies in-plane spatial heterogeneity (polymer electrolyte fuel cell cathode inlet vs outlet) in Pt particle size growth and distribution as a function of nitrogen (N2) flow rate during a square-wave accelerated stress test (AST). The average Pt particle sizes for membrane electrode assemblies (MEAs) subjected to N2 flow rates ranging from 4–16 sccm cm−2 are in the range 9–10.5 nm at the end-of-life (EOL) with similar electrochemically active surface area (ECSA) loss (∼65%). However, Pt particle size at EOL exhibits spatial heterogeneity: greater Pt particle size growth occurs near the flow field outlet than the inlet. The spatial heterogeneity for a fully-humidified N2 flow is believed to originate from non-uniform humidification (outlet is more humidified than the inlet) across the cell for a co-flow arrangement. A first-order rate model for ECSA loss predicts linear increase of the rate constant with N2 flow rate. The polarization losses of the aged MEAs over a wide range of operating conditions increase with N2 flow rate. From the results of this work, for holistically assessing durability of Pt catalysts in fuel cells at high humidity conditions, it is recommended to include purge gas flow rate as a stressor during an AST.

Platinum Dissolution Induced Catalyst Layer Degradation in Polymer-Electrolyte Fuel Cells for Heavy-Duty-Vehicle Applications

Interest in electrifying the transportation sector has intensified as the value in reducing carbon emissions is recognized globally. Polymer-electrolyte fuel cells (PEFCs) provide an option for electrifying transportation if hydrogen is produced electrolytically from water and without carbon emissions if the electricity is provided by renewable energy resources. Though research in fuel cell powered vehicles has been ongoing, hydrogen infrastructure and concerns about hydrogen storage safety limit the technology’s uptake in the passenger vehicle market. However, fuel cells applied to heavy-duty-vehicle (HDV) transportation may address these shortcomings. HDV provide predictable routes allowing for more simple hydrogen infrastructure and larger cargo allowance in HDV allows for more secure hydrogen storage and safety. Fuel-cell-powered-HDV require higher operating temperature, pressure, and potential to realize the power-demands of the vehicle, therefore, durability requirements increase, and a better understanding of degradation is a priority. A major cause of performance degradation is due to catalyst dissolution, migration, and reprecipitation. This causes a loss of electrochemically active surface area (ECSA) which reduces the activity of the catalyst and is particularly detrimental in the cathode where the limiting factor for performance is often sluggish oxygen reduction kinetics. This loss in ECSA is due to an overall broadening of the particle size distribution (PSD) within the catalyst layer and a net loss of catalyst to the formation of a Pt band in the membrane. In this work, a Pt catalyst degradation model is developed which considers particle size dependent Pt oxidation and dissolution kinetics, Pt migration, and reprecipitation of Pt within the cell. This work interrogates the particle size dependence of these processes to explain experimentally observed loss in ECSA and changes in PSD. The degradation model is coupled with a PEFC performance model through the ECSA loss to study how the HDV-relevant operating conditions impact the cathode catalyst degradation behavior observed in PEFCs.

Universal Correlation between the Roughness Factor and PEMFC Performance Losses in Voltage Cycling Based Accelerated Stress Tests

During operational lifetime, proton exchange membrane fuel cells (PEMFCs) suffer from high performance losses. For the year 2025, the U.S. Department of Energy (DoE) set the durability target for PEMFC light-duty transportation applications at 5,000 hours.1 To meet the automotive target, different aging protocols were established in order to simulate load cycle variations.2 It is well known that load or voltage cycling induces substantial catalyst degradation due to an increasing loss of electrochemically active surface area (ECSA), which is known to be a main driver for the resulting performance penalties.3 Thus, an increasing interest exists in developing accelerated stress test (AST) protocols that can sufficiently describe the degradation of the different components of a membrane electrode assembly (MEA) during automotive application using shorter measurement times.In this study, we report on a strategy to predict the performance degradation of an MEA, making use of a strongly accelerated voltage cycling based ASTs. For this, voltage cycling based ASTs were performed using 5 cm2 MEAs with a 0.1 mgPt cmMEA -2 loaded Pt/C catalyst (TEC10V20E, Tanaka) for both cathode and anode. Voltage cycling was done under H2/N2 (200/75 nccm) at 80 °C, 95% RH, and ambient pressure using square wave profiles with a constant lower potential limit (LPL) of 0.6 V and different upper potential limits (UPLs) of 0.85, 0.95, and 1.0 V, with LPL/UPL hold times of 1, 2, or 8 s. Full characterization of the MEA at beginning-of-life and after each set of voltage cycling intervals was performed by: i) measuring H2/O2 and H2/air polarization curves; ii) determining the ECSA by cyclic voltammetry and CO stripping; iii) conducting limiting current measurement to calculate the O2 transport resistance (R O2 total); and, iv) conducting electrochemical impedance spectroscopy (EIS) measurements under blocking conditions to quantify the proton conduction resistance in the cathode catalyst layer (R H+ cath).When the ECSA loss ranges between ≈20-85%, it was found that the ECSA decreases approximately linearly when plotted against the logarithm of the number of cycles (see fig. 1), exhibiting higher slopes for procedures with higher UPLs and longer hold times, in agreement with the results from previous studies.3, 4 In addition, a direct and AST protocol independent correlation between the loss in total available surface area, i.e., the roughness factor (rf ≡ ECSA × Pt-loading), and the individual loss contributions in H2/air performance curves, namely kinetic (mass/specific activity for the oxygen reduction reaction (ORR)) and O2 transport resistance contributions, was shown. This is due to the fact, that these individual voltage losses are not only highly affected by the cathode rf,3, 5 but that the Pt dissolution/redeposition mechanism seems to be identical for all of the here investigated voltage cycling ASTs, despite the largely varying UPLs and hold times. As a result, a universal correlation between the H2/air performance losses and the rf deterioration was found throughout all ASTs. The important corollary of this finding is that the data from quickly degrading voltage cycling ASTs (i.e., with high UPL) can be used to project the H2/air performance losses when cycling under less degrading conditions.

Dynamic modeling of Pt degradation and mitigation strategies in polymer electrolyte membrane fuel cells

The durability issue of polymer electrolyte membrane fuel cells must be resolved before their widespread commercialization in transportation. A central issue in fuel cell durability is the loss of Pt electrochemical active surface area (ECSA) during automotive conditions, which is induced primarily by the Pt mass loss and Pt particle growth mechanisms. A comprehensive Pt degradation model to study the evolution of the ECSA distribution, induced mechanisms and mitigation strategies is developed. After validation with experimental results, the importance of the Pt mass loss and particle growth mechanisms is studied. Results reveal that the Pt mass loss mechanism occurs primarily near the proton-exchange-membrane/catalyst-layer (PEM/CL) interface and becomes more critical with improved electrode potential. The ECSA loss by the particle growth (Ostwald ripening) mechanism is, for the most part, uniformly distributed along the thickness direction, whereas it is suppressed near the PEM/CL interface by the Pt mass loss mechanism. The model also studies possible mitigation strategies for ECSA loss by suppressing the mass loss mechanism. Both the graded PEM structure and reduced Pt 2+ ion diffusivity can help mitigate the ECSA loss process. • Modeling of Pt degradation in the membrane electrode assembly is developed. • The model distinguishes Pt ECSA loss by mass loss and particle growth types. • Pt mass loss mechanism distributes nonuniformly and increases with cell voltage. • Pt growth mechanism distributes uniformly and increases with voltage cycling. • The model proposes mitigation strategies for ECSA loss by reducing Pt mass loss.

Single step polyol synthesis of highly stable Pt/C/SiO2 catalysts for use in PEMFCs: effects of pH, temperature and W/EG ratio

Hybrid supports have been proposed as a new alternative to increase the stability of ORR catalysts used in PEMFCs for automotive applications since they are known to be stable under harsh conditions. In this work, Pt nanoparticles were deposited over C/SiO2, via single-step polyol method, to take advantage of the corrosion-resistance properties of silica nanoparticles. In fact, the synthesis parameters, namely, pH, temperature, and glycol concentration had a remarkable impact on the Pt size-distribution, crystallinity, and dispersion over the C/SiO2 supports. A maximum ORR activity and stability was obtained for Pt/C/SiO2 catalysts produced at 1:6 W/EG (v/v). The addition of SiO2 nanoparticles to the carbon structure showed their ability to effectively inhibit support corrosion and Pt nanoparticles detachment and/or growth, with the pH adjustments being critical for obtaining highly stable C/SiO2 supports. Pt/C/SiO2 synthetized under acidic conditions revealed the highest stability when subjected to accelerated stress tests (ASTs), losing only 30 % of the initial electrochemically active surface area (ECSA) of Pt after 4 000 cycles from 0.6 to 1 V (vs RHE), whereas the commercial Pt/C revealed &gt; 50 % of ECSA loss.

Durability of Platinum Overlayers formed By Self-Terminating Electrodeposition

In Proton Exchange Membrane Fuel Cell (PEMFC), the membrane electrode assembly (MEA), especially the electrode, is considered 'the heart' and is designed to accommodate the need for efficient transport of electrons, reactants, and heat as well as constraints imposed by the platinum cost. Advances in PEMFC electrode design over the years, which range from using Pt black films with a loading of 10 gpt/cm2 in the 1970s to present-day Pt/ PGM (Platinum Group Metal) nanoparticle coated carbon-black particles (Pt/C) that use about 0.3 mgpt/cm2, have led to significant cost-reduction and performance enhancement. However, durability problems associated with corrosion of carbon support and subsequent loss of active surface area during start-up or shut down cycles have led to renewed interest in carbon-free nanostructured electrodes that employ a thin coating of Platinum or PGM-based catalytic layer on mesostructured conductive support ((Debe 2012), (Liu et al. 2012)).In this context, we carried out systematic ex-situ investigations on the durability of platinum thin films comprising of atomic layers formed by Self-Terminating platinum electrodeposition (Liu et al. 2012). We characterized the sequential growth of platinum film using XPS and AFM measurements; and analyzed the electrocatalytic performance as a function of the number of self-terminating pulses. We found that eight pulses of electrodeposition, corresponding to ~10 nm thick platinum film, can meet stringent ex-situ durability targets set, by the Department of Energy (DOE), USA, for a potential electrocatalyst in Fuel Cell Vehicles (FCVs) application. Our results also show that platinum layers formed using four pulses (< 4 µg/cm2 loading) and even one pulse (< 1 µg/cm2 loading) are electrochemically active. For platinum samples formed using eight or more pulses, we observed ~ 20% loss in Electrochemically Active Surface Area (ECSA) during the first 3000 cycles and 10-15% ECSA loss in the remaining 27000 cycles, indicating stabilization of platinum activity over time. We will present the results of our studies on the minimum number of platinum pulses needed to form a complete overlayer of electrochemically active platinum and discuss the platinum loss during the ex-situ durability tests using ICP-OES and Cyclic Voltammetry measurements. These studies pave the way for an additive, roll-to-roll, and cost-effective manufacturing process of ultra-low platinum loaded MEAs for PEMFCs by building on our earlier results on printing silver nanostructures using a simple inkjet printer by Print-Expose-Develop technique (Parmar and Santhanam 2014) in conjunction with the ability to controllably form atomic layers of platinum on metallic substrates by Self-terminating electrodeposition. Figure 1

Studying Pt-based fuel cell electrode degradation with nanoscale X-ray computed tomography

Improvements in Pt-based catalysts and support durability are important to enable mass-market penetration of proton exchange membrane fuel cells (PEMFCs) in the automotive sector. This work focuses on a membrane electrode assembly (MEA) scale analysis of cathode catalyst layer (CCL) degradation processes occurring during catalyst accelerated stability testing (AST). We examined the degradation of the carbon support, decrease in electrochemically active surface area (ECSA), and formation of a platinum (Pt) band. We utilized a combined electrochemical and ex-situ nanoscale X-ray computed tomography (nano-CT) imaging technique to characterize these important degradation processes in state-of-the-art MEAs and examined the effects of AST cycling on two carbon supports, Vulcan (Vu) and a KetjenBlack high surface area carbon (HSC), and two Pt-based catalysts, pure Pt and PtCo. We found that neither support nor catalyst type completely suppressed the formation of a Pt band, but we did find that the PtCo catalyst and HSC support combination was able to limit the ECSA loss and Pt band formation better than the pure Pt and Vu combination. Our results also showed some loss of macroporosity in the HSC support after catalyst AST cycling, helping to explain the reduced performance seen with HSC-supported catalysts at high current densities.

Physical Modeling of Catalyst Degradation in Low Temperature Fuel Cells: Platinum Oxidation, Dissolution, Particle Growth and Platinum Band Formation

The loss of electrochemical active surface area (ECSA) at the cathode is one of the main causes of performance degradation in Polymer Electrolyte Membrane Fuel Cells (PEMFCs). In order to investigate the catalyst degradation and the influence of the operating conditions we develop a multiscale degradation model which includes the formation and reduction of platinum oxides, platinum dissolution, particle growth due to Ostwald ripening, platinum ion transport through the ionomer and platinum band formation in the membrane. This degradation model is coupled with a 2D PEMFC performance model and predictions regarding ion concentration, ECSA evolution and particle growth are validated with dedicated experiments and literature data. Degradation under several AST protocols and under steady state operation are compared and discussed. The importance of a spatially resolved catalyst degradation model is conveyed by the occurrence of a depletion zone in the catalyst layer close to the membrane due to the platinum migration into the membrane. By comparing the correlation between platinum mass loss in the catalyst layer and the ECSA loss we conclude that catalyst degradation under AST conditions with nitrogen is not representative for the degradation under normal operation.