Drive Cycle Operation Research Articles

Error estimation of Bernardi heat evaluation of Li-ion cells under drive cycle operation

Accurate estimation of heat generation is critical for EV batteries. Conventional means of heat estimation (i.e. Bernardi equation) works only for constant current discharge. However, fluctuating current condition is the normal mode of operation for cells in EV applications. The current study attempts a comprehensive investigation of heat generation and temperature evolution for a cylindrical lithium iron phosphate cell under standard drive cycles (FUDS, US06). An experimentally parameterized electrochemical-heat transfer model is developed. Experimental voltage curve of FUDS drive cycle is used to obtain unknown electrochemical parameters while its fidelity is tested against US06. It is observed that Bernardi method overpredicts heat generation rate by approximately 25% under drive cycle operation. It is noted that Bernardi method is an indirect method of heat estimation based majorly on electrical parameters. It is also observed that the reversible part of Bernardi heat is negligible under drive cycle operation. Moreover, SOC and temperature evolution are accurately estimated by the electrochemical-heat transfer model – errors lie within 7% and 2° C, respectively. Thus, the present study demonstrates the capability of the electrochemical-heat transfer model for estimating heat generation rate of Li-ion cells under drive cycle operation where Bernardi equation is shown to be an inadequate method.

Estimation of battery temperature during drive cycle operation by the time evolution of voltage and current

During the operation of electric vehicles, accurate temperature monitoring of lithium-ion batteries by battery management system is crucial to their safety. However, temperature sensors possess limitations as they necessitate regular calibration to avoid inaccuracies, which can be challenging to accomplish in certain scenarios. As an alternative, a general sensorless temperature estimation framework is developed in this study to either serve as a complementary approach alongside sensors or function independently to ensure reliable temperature measurement. The proposed framework relies solely on data regarding battery voltage and current measurements, and this information is employed to determine the parameters of the equivalent circuit model via the vector-type recursive least squares algorithm in the first step. In the second step, these parameters and the voltage/current data serve as the input to the deep neural network (DNN) to estimate the instantaneous battery surface temperature during dynamic driving cycles under various ambient temperatures and degradation scenarios. To reflect the effect of past battery operation on the current temperature, a newly defined current integral function is proposed, by which the input time length of the DNN model is suggested. Four combinations of the model input are studied, and the best input option yields an average estimation error of the battery temperature that falls below 0.5 °C.

Modelling Analysis of MEA Dynamic Operation Under Real-World Automotive Driving Cycle

In last years, Proton Exchange Membrane Fuel Cells (PEMFC) have received an increasing interest as a candidate for light and heavy-duty vehicle application1. To guarantee an efficient and durable operation of PEMFC, the understanding of local conditions, experienced by MEA, during dynamic operation is highly important. Effective numerical modelling can help to gain insights on the local heterogeneity linked to water and thermal management of the cell.A 1 + 1D multiphase dynamic non isothermal PEMFC model, implemented into MATLAB Simulink environment, was used to investigate experimental data obtained on a segmented cell hardware 2. Particular attention was paid to properly model the mass transport and kinetic related phenomena in catalyst layer(CL), as the local oxygen transport resistance through the ionomer thin film3 and the platinum oxide formation/reduction reaction, and their mutual interaction. Exploiting experimental data, obtained from tests on the segmented cell hardware, it was possible to give a robust validation to the 1+1D model, both at global and local level, thanks to the spatially-resolved information available. By simulating real-world driving cycle (DLC) protocol, developed within the ID-FAST project4, drying-up periods of ionomer contained in catalyst layers and in membrane, as well as flooding periods of porous layers and channels, were identified and analyzed during low-load and high-load operation.The performance model was coupled with a catalyst durability model, such to investigate the effect of ageing on dynamic performance of PEMFC. An innovative semi-empirical electrochemical surface area (ECSA) loss model was developed, based on the platinum dissolution mechanism, and validated initially on accelerated stress tests on different operating conditions5. The simulated load profile and local operating conditions in CLs were processed by a proper algorithm, to convert a driving cycle into a combination of elementary steps and to associate a loss factor for the ECSA to each of the identified elements, thus to estimate the retained local and global ECSA. Exploiting experimental database made up of 1000 operating hours performing ID-FAST DLC, the effect of local operating conditions on PEMFC degradation and performance was furtherly comprehended, focusing mainly on the effect of reactant humidification. The developed model framework succeeded in estimating performance loss during driving cycle operation, as visible in Figure 1(a-b). Simulated and experimental ECSA decay over time is shown in Figure 1(c), demonstrating good model prediction capability. The obtained results showed the potentiality of the model to capture complex two-phase and non-isothermal transport phenomena both in the along channel and through-MEA direction, consistently with a large set of experimental data.Experimental data on single cell were collected under ID-FAST project (Grant Agreement No 779565, Joint Undertaking, EU Horizon 2020).References D. A. Cullen et al., Nat. Energy, 6, 462–474 (2021) https://www.nature.com/articles/s41560-021-00775-z.E. Colombo, A. Baricci, A. Bisello, L. Guetaz, and A. Casalegno, J. Power Sources, 553, 232246 (2023) https://doi.org/10.1016/j.jpowsour.2022.232246.T. Suzuki, H. Yamada, K. Tsusaka, and Y. Morimoto, J. Electrochem. Soc., 165, F166–F172 (2018) https://iopscience.iop.org/article/10.1149/2.0471803jes.F. Wilhelm et al., ID-FAST -D4.3 – Analysis of coupling between mechanisms and definition of combined ASTs, p. 49, (2021) https://www.id-fast.eu/uploads/media/ID-FAST_D4-3_Analysis_of_coupling_between_mechanisms_and_definition_of_combined_ASTs_OK.pdf.A. Kneer and N. Wagner, J. Electrochem. Soc., 166, F120–F127 (2019) https://iopscience.iop.org/article/10.1149/2.0641902jes. Figure 1

State-of-the-Art 800 V Electric Drive Systems: Inverter–Machine Codesign for Energy Efficiency Optimization

This paper introduces a state-of-the-art inverter–machine codesign methodology for achieving an energy-efficient electric drive system. The methodology is applied in an optimal design case study with the context of the latest trend in the 800 V system. To reduce the voltage rating, a three-level active neutral point clamped topology is utilized in the inverter design, incorporating silicon carbide technology for high-speed operation. This combination allows for efficient power conversion with reduced losses. In the machine design, the increased number of poles that can be achieved in the design phase, along with the utilization of high-speed switching frequencies, enables the development of more efficient machines. Based on the design of the experiment method, by optimizing the parameters of both the inverter and machine simultaneously, using a cosimulation model, the 7.7% of total energy loss during drive cycle operation can be improved.

Investigation of the Influence of Selected Operating Modes on the Long-Term Stability of Electrocatalysts in the PEM Fuel Cell

The membrane electrode assembly (MEA) is the core component of the fuel cell stack. In it, the conversion of chemical energy into electrical energy takes place at the catalyst layers. The functional capability of the MEA is an indispensable prerequisite for the operation of the fuel cell stack and thus plays a decisive role in defining its stability. In addition to the selection of suitable materials, the mode of operation plays a major role in the durability of the MEA. It is known from the literature that electrochemical degradation reactions generally depend on operating parameters such as temperature, potential and dynamics. [1] Therefore, in order to derive a durability promoting mode of operation for the PEM Fuel Cell as well as to make lifetime predictions, it is necessary to identify and understand the processes occurring during the aging of the MEA.This work deals with the investigation of the influence of the operating parameters in the drive cycle as an important operating mode on the aging of the cathode catalyst layer in the MEA. According to the state of the art, nanoparticulate platinum supported on carbon (Pt/C) is usually used as the cathode catalyst. The degradation of Pt/C has been intensively studied in recent years. Platinum dissolution and Pt agglomeration are discussed as the main causes of Pt/C degradation during PEMFC operation. [2, 3]Studies have shown that the dissolution rate generally increases with potential and can accelerate under potentiodynamic conditions. [1] Dissolution of Pt from the catalyst, transport of Pt ions through the electrode, and precipitation of Pt in the membrane, possibly due to reduction of Pt ions by the H2 crossover from the anode can be associated with the loss of cathode electrochemical surface area (ECSA) and thus irreversible performance loss. [4]The primary objective of this work is to investigate the influence of different operating parameters such as temperature and potential range on the degradation rate of the cathode using an accelerated stress test (AST) which was developed to simulate an analogous to real drive cycle operation of a PEM fuel cell in a vehicle.A secondary objective is to evaluate the AST itself in order to determinate its capability of fuel cell lifetime prediction. For that the transferability of the observed degradation rates are to be evaluated on different integration levels such as single-cell PEM test bench and short-stack multi-cell test bench. As a measure to evaluate the transferability of the accelerated stress tests, the loss of electrochemical active surface area (ECSA) serves as an indirect measure of catalyst degradation and the resulting Pt particle size distribution which is determined by TEM serves as a direct measure. The derivation of acceleration factors for the lifetime tests of the different integration levels would allow an early estimation of the lifetime, this way test time and cost could be significantly saved in the evaluation of new materials.

Wheel Load Torque Emulation for Electric Propulsion Structure using Dual Induction Motors

Electric vehicle is an adaptation of conventional vehicle, with an integration of electrical motors. It seems to be one of the most promising technologies that can lead to significant improvements in vehicle performance and polluting emissions. However, for any vehicle in urban traffic requires regime changes, frequent acceleration, deceleration, and stopping phases, which lead to serious breakdowns. During the above phases, electric motors are continuously being exposed to thermal and mechanical effects.This paper highlights the possibility of representing the wheel load torque emulation of an electric propulsion structure using dual induction motors vector-controlled. The emulation of load torque acting on one of both electric motors placed at the rear wheels of electric vehicle (EV) structure is accomplished by a DC-generator coupled with an induction motor during vehicle drive cycle operation andunpredictable load profiles. Simulation results confirm widely the feasibility and the effectiveness of the proposed emulator scheme of induction motor-based vector-control in the electric vehicle application.

Predicting Platinum Dissolution and Performance Degradation under Drive Cycle Operation of Polymer Electrolyte Fuel Cells

A protocol is presented that allows for fuel cell performance degradation to be determined based on a vehicle drive cycle. Four stages are outlined beginning with the conversion of vehicle velocity data to a cell voltage profile. The amount of platinum dissolved in the system and oxide coverage on platinum particles are simultaneously calculated by considering several degradation mechanisms including Ostwald ripening and platinum particles loss to the membrane. The platinum loss is used to determine the Electrochemically Active Surface Area (ECSA) loss in the catalyst layer. The voltage loss due to platinum degradation is then determined from the ECSA data. The results show that longer times at higher upper potential limits lead to more platinum degradation and thus performance loss as expected. Accelerated Stress Test data is reproduced within the acceptable error. The model is applied to real-world data from a vehicle drive cycle showing that the model simplifications and assumptions outlined are reasonable and prove predictive capabilities. Although more experimental data would be beneficial to fully validate the model, the present work provides a complete, physics-based catalyst degradation model that can be integrated with performance models to predict durability and optimize future system designs and operating conditions. This paper is part of the JES Focus Issue on Proton Exchange Membrane Fuel Cell and Proton Exchange Membrane Water Electrolyzer Durability.

A hybrid power-unit based on a passive fuel cell/battery system for lightweight vehicles

Hybrid fuel cell/battery power-unit configurations are gaining growing attention; in fact, they offer advantages in terms of fuel cell downsizing, higher efficiency and fast fueling. In these configurations, the power-sharing is actively controlled employing power electronics, mainly boost converters, that affect the costs and performance of the system. In this work, a novel hybrid power-unit based on a passive fuel cell/battery system is proposed for a lightweight electric vehicle (5 kW nominal power). This configuration eliminates the need for costly power converters, and increases the overall performance. The hybrid power-unit consists of 5 fuel cell stacks, 6 battery modules and two resistors that allow safe fuel cell start-up and shut-down. The power profile requested by the electric motor is derived from an ARTEMIS drive cycle and tested by means of two electronic loads.The analysis on the power-unit behavior and performance has been carried out by means of a numerical model and experimental activities.The power-unit response has been analyzed focusing on: i) start-up operation; ii) drive cycle operation; iii) shut-down operation. During the whole test, the fuel cell produced about 51% of the total requested energy while the battery pack about 49%. The average stack efficiency during operation resulted to be 47%, while it dropped to 41% considering also start-up and shut-down.Results proved the technical feasibility as well as the safety of the system and showed that the developed power-unit has promising features for practical applications.

Carbon Corrosion in PEM Fuel Cells and the Development of Accelerated Stress Tests

Carbon corrosion is an important degradation mechanism that can impair PEMFC performance through the destruction of catalyst connectivity, collapse of the electrode pore structure, loss of hydrophobic character, and an increase of the catalyst particle size. In this study, carbon corrosion was quantified in situ by measurement of carbon dioxide in the fuel cell exhaust gases through non-dispersive infrared spectroscopy during simulated drive cycle operations consisting of potential cycling with varying upper and lower potential limits. These studies were conducted for three different types of carbon supports. A reduction in the catalyst layer thickness was observed during a simulated drive cycle operation with a concomitant decrease in catalyst layer porosity, which led to performance losses due to increased mass transport limitations. The observed thickness reduction was primarily due to compaction of the catalyst layer, with the actual mass of carbon oxidation (loss) contributing only a small fraction (< 20%). The dynamics of carbon corrosion are presented along with a model that simulates the transient and dynamic corrosion rates observed in our experiments. Accelerated carbon corrosion stress tests are presented and their effects are compared to those observed for the drive cycle test.

Analysis of Magnetizing Trajectories for Variable Flux PM Synchronous Machines Considering Voltage, High-Speed Capability, Torque Ripple, and Time Duration

This paper discusses various transient trajectories for increasing the magnetization state (MS) of variable flux permanent magnet synchronous machines (PMSMs), a class of PMSM where the magnets are demagnetized and remagnetized using the drive inverter during drive cycle operation. To manipulate MS, a magnetizing current pulse can be used, which may drive a large amount of flux linkage in the machine. As a result, above low-speed operation, a large voltage may be required from the inverter. This paper extends several existing methods for improved voltage properties, and proposes and experimentally evaluates a new method—a straight line stationary frame flux linkage trajectory—for higher speed capability. The presented trajectories, along with existing trajectories, are organized into families and compared regarding their required voltage, high-speed capability, torque ripple, and time duration.

Look Insight Advances in Nano X-Ray Computed Tomography for Fuel Cells

We have heard it over and over again – internal combustion engines (ICE) operated with fossil fuels can cause severe damage to our health and environment.1 Nevertheless, they are still very widespread and will not disappear until a compelling alternative is found. One possible alternative is the low temperature polymer electrolyte fuel cell (LT-PEFC). They are commonly operated with hydrogen and air and only emit water. However, before LT-PEFCs can replace ICEs on a large scale mainly two barriers need to be overcome. Those barriers are cost and lifetime. Hence, reducing the costs of a fuel cell and increasing its lifetime are the two most relevant research fields regarding fuel cell commercialization. Whereas the costs are mainly dictated by the raw material prices (e.g. platinum) and the manufacturing process the reasons for limited lifetime are numerous. PEFCs in general are affected by various degradation effects at different locations of the fuel cell.2,3 The membrane suffers for example from pinhole formation and membrane thinning. The hydrophobicity of the gas diffusion layer decreases over operation time which decreases its water transport ability. The bipolar plates are prone to surface corrosion which reduces their conductivity. The catalyst and catalyst layer also represent a major degradation area. Some specific degradation mechanisms are carbon corrosion, catalyst detachment and catalyst particle growth. All degradation effects are complex functions of the specific operational parameters, making it challenging to identify them individually. This in turn is necessary to gain insights into limitations in fuel cell lifetime and is the key to successful development of mitigation strategies to reach a desired lifetime of >5,000 hours for automotive drive cycle operation. There are different techniques available to identify and characterize the individual degradation mechanisms. Some common ones are polarization curve or electrochemical active surface area measurements which are based on electrochemistry. Furthermore, imaging techniques like secondary electron microscopy (SEM) are frequently used for degradation analysis. Another established imaging technique is X-ray tomography. The main advantages of X-ray tomography over SEM imaging are its ability to capture the fuel cell area of interest as a whole and that the tomography scan can be recorded nondestructively and in-operando.4 For some time, fuel cell X-ray tomography was mostly carried out at large scale synchrotron beamline facilities. Unfortunately, they tend to have only very limited access and therefore only few experiments can be conducted in a short time period. In recent development though, lab sized X-ray microscopes have gained interest in the community.5 Such microscopes have excellent availability and therefore enable the possibility to carry out long term experiments. The present contribution will utilize this particular advantage. Our unique approach is to retrace fuel cell degradation with X-ray tomography imaging (cf. Figure 1) over time and correlate it with individual failure mechanisms. In particular, electrode degradation will be discussed. MEAs, with e.g. different carbon/platinum ratios, were exposed to accelerated stress tests and imaged at beginning and end of life. With the aid of X-ray tomography, we are able to segment the MEA and subsequently locate and analyze the degradation effects at the electrode. In combination with electrochemical measurements, exclusive in depth insights of fuel cell degradation are provided. In addition, a novel approach for post X-ray tomography processing is discussed. In summary, the combined findings lead to an improved understanding of LT-PEFC degradation which is a necessary first step in order to investigate new mitigation strategies to increase the fuel cell lifetime. Figure 1. A membrane electrode assembly visualized by X-ray computed tomography recorded at beginning of life. 1. S. M. Platt et al., ‘Two-stroke scooters are a dominant source of air pollution in many cities’, Nat. Commun., 5, p. 3749, 2014.2. J. Wu et al., ‘A review of PEM fuel cell durability: Degradation mechanisms and mitigation strategies’, J. Power Sources, 184, pp. 104–119, 2008.3. T. Engl, L. Gubler, and T. J. Schmidt, ‘Think Different! Carbon Corrosion Mitigation Strategy in High Temperature PEFC: A Rapid Aging Study’, J. Electrochem. Soc., 162, pp. F291–F297, 2015.4. J. Eller et al., ‘Progress in In Situ X-Ray Tomographic Microscopy of Liquid Water in Gas Diffusion Layers of PEFC, J. Electrochem. Soc., 158, B963 (2011).5. M. Andisheh-Tadbir, F. P. Orfino, and E. Kjeang, ‘Three-dimensional phase segregation of micro-porous layers for fuel cells by nano-scale X-ray computed tomography’, J. Power Sources, 310, pp. 61–69 (2016) Funding for this research was provided by the Natural Sciences and Engineering Research Council of Canada, Canada Foundation for Innovation, British Columbia Knowledge Development Fund, and Ballard Power Systems through an Automotive Partnership Canada grant. Figure 1

Carbon Corrosion in PEM Fuel Cells during Drive Cycle Operation

One of the major contributors to degradation involves the electrocatalyst, including the corrosion of the carbons used as catalyst supports, which leads to changes in the catalyst layer structure. We have measured and quantified carbon corrosion during drive cycle operation and as a variation of the upper and lower potential limits used during drive cycle operation. The amount of carbon corrosion is exacerbated by the voltage cycling inherent in the drive cycle compared with constant potential operation. The potential gap between upper and lower potentials appears to be more important than the absolute operating potentials in the normal operating potential regime (0.40V to 0.95V) as changes in the measured carbon corrosion are similar when the upper potential was lower compared to raising the lower potential. Catalyst layer thinning was observed during the simulated drive cycle operation which had an associated decrease in catalyst layer porosity. This catalyst layer thinning is not due solely to carbon corrosion, although carbon corrosion likely plays a role; much of this thinning must be from compaction of the material in the catalyst layer. The decrease in catalyst layer porosity leads to additional performance losses due to mass transport losses.

Carbon Corrosion in PEM Fuel Cells during Drive Cycle Operation

PEM fuel cells (PEMFCs) show great promise to increase the fuel efficiency for transportation applications; however, for this application, they must show performance and durability with the requirements for transportation. For transportation applications, the fuel cell will be subjected to frequent power cycling. For example, the DOE/Fuel Cell Tech Team (FCTT) protocol for durability includes load cycling from 0.02 A/cm2 to 1.2 A/cm2 every 0.5 min. The cathode catalyst and catalyst layer have been shown as susceptible to degradation causing loss of performance due to both loss of kinetics for the oxygen reduction reaction and loss of mass transport. Catalyst support-carbon corrosion can result in thinning of the catalyst layer contributing to degradation in performance. To examine the effect of power cycling in situ on carbon corrosion and electrode degradation, we directly measured the catalyst support degradation by measuring CO2 in the cathode outlet by NDIR (Non-Dispersive Infra-Red) while operating a single-cell fuel cell. CO2 present in air was removed by a lime bed prior to introduction to the fuel cell. We operated with a modified DOE/FCTT durability protocol using controlled voltage, and varied the potential limits to explore the effects of the upper potential limit, lower potential limit, the potential step size and time at potential. The upper potential limit was varied from 0.95 to 0.55V; the lower potential limit from 0.40V to 0.80V, with times ranging from 0.5 min to 5 min. The corrosion of three different types of carbon were explored, high surface area (E), vulcan (V), and graphitized (EA). The catalyst support carbon corrosion occurs under normal fuel cell operating conditions and is exacerbated by the voltage cycling inherent in these steps in potential. A series of carbon corrosion spikes during potential cycling is shown in Figure 1 for E-type carbon, varying the upper potential from 0.95 V to 0.60V while keeping the lower potential constant at 0.40V. Sharp spikes in the carbon corrosion rate are observed during a step increase in cell potential with the magnitude of the spikes decreasing as the high cell potential is reduced from 0.95 V to 0.6 V. The carbon corrosion rate at high cell potential (0.95V) decreases with time at potential, indicating formation of passivating carbon surface oxides. Carbon corrosion was measured during the drive cycle measurements for all three types of carbon, with the relative carbon corrosion rates of E>V>EA. The series of step potential carbon corrosion spikes where the potential was varied for the lower potential from 0.40 V to 0.60V while keeping the upper potential constant at 0.95V shows similar results in terms of the carbon corrosion. The magnitude of the spikes decrease as the lower cell potential is raised. These results indicate that the size of the step in potential has a more significant impact on the carbon corrosion rate than does the absolute value of the potential for normal cathode operating potentials. The peak in CO2 evolution occurs when the cell potential increases from high power operation to low power near open circuit. This correlates with when CO2 evolution is observed during cyclic voltammograms, which occurs during the positive sweep at ~ 0.55 to 0.60 V. The evolution of this CO2 peak suggests that oxygen is adsorbed onto the carbon and/or CO is formed on the Pt surface. During long-term operation, a reduction in catalyst layer thickness is observed during drive cycle operation, which can be due to the loss of carbon through carbon corrosion or possibly due to compaction; both effects likely lead to a loss of void volume. This reduction in thickness includes a sharp decrease in catalyst layer thickness within the first 100 hours of operation (30%), eventually reaching ~50% of its thickness after 1000 hours. Most of this reduction in electrode thickness does not appear to be directly due to carbon corrosion as there is little evidence for carbon corrosion from microscopic analysis, especially during the early stages of operation, where the thickness reduction is substantially more than what should be due to carbon corrosion. These results show that carbon corrosion occurs during normal potential operation, and is exacerbated by potential variations during operation. To minimize the carbon corrosion, the size of the steps in potential should be minimized. For a more stable catalyst, another requirement is for the Pt particles to be stabilized on graphitized carbon supports. Acknowledgments Funding for this work is from DOE EERE FCTO, Technology Development Manager Nancy Garland Figure 1

Development of Accelerated Stress Tests for Polymer Electrolyte Membrane Fuel Cells

The durability of Polymer Electrolyte Membrane Fuel Cells (PEMFCs) is one of the key barriers to their commercialization in automotive applications.1 The development of accelerated stress tests (ASTs) is a critical step in the evaluation and improvement of the durability of PEMFCs. The availability of validated component ASTs can lead to the accelerated development of materials with improved durability. However the ASTs need to faithfully reproduce the degradation mechanisms of interest and not introduce new degradation modes while attempting to accelerate the degradation rates. Furthermore, the acceleration factors with respect to the intended application need to be determined accurately to provide manufacturers with a valuable tool that enables informed decision making regarding cost, performance and durability. For example the DOE – Fuel Cell Technologies program has set lifetime durability targets of 5500 hours for automotive fuel cells and the U.S. Drive Fuel Cell Technical Team (FCTT) has developed AST protocols for the various components.2,3 In this paper we will compare the degradation observed during the ASTs to the degradation observed during simulated drive cycle operation and report on the acceleration factors observed for various materials. We will also propose new ASTs that have greater acceleration factors than the reported FCTT ASTs while retaining the degradation mechanisms of interest. The FCTT has recommended potential cycling from 0.6 V to 1.0 V at 50 mV/sec as the electrocatalyst AST. This potential cycling is performed at 80 °C in 100% RH with H2 (anode) and N2 (cathode) and results in Pt particle growth and loss in electrochemical surface area (ECSA). This loss in ECSA and associated performance loss correlates well to the degradation observed in the field and during simulated drive cycle operation. Figure 1 compares the degradation observed during this AST (Old AST) to that observed during the wet portion of the FCTT: Drive cycle (current cycling from 0.02 to 1.2 A/cm2 at 80 °C and saturated conditions). It is seen that this AST has a 5X acceleration factor for 3 different MEAs that were evaluated. In order to further accelerate this AST, we modified the triangle wave to a square wave and lowered the upper potential to 0.95 V. This new AST consisted of a square wave with upper and lower potentials of 0.95 V and 0.6 V with 3 seconds duration, and was based on literature reports.4 This New AST (Fig. 1) demonstrated a 100X acceleration factor compared to the drive cycle which is a 20 times improvement over the old AST. Moreover, this new AST retained the Pt growth mechanism while minimizing carbon corrosion. The FCTT has 2 recommended ASTs for membranes with one focusing on chemical degradation (OCV hold at 90 °C and 30% RH) and another focusing on mechanical degradation (RH cycling in Air). Our previous results have indicated that both these ASTs are not representative of membrane failure in the field.5 In this paper we will present a new AST that combines the mechanical and chemical degradation and is representative of field failure. This AST consists of cycling the RH between saturated and dry conditions in a H2/Air atmosphere at 90 °C. The duration of the wet and dry cycles are 30s and 45s respectively. The fluoride release rate observed during this AST using DuPont XL® membranes, illustrated in Figure 2, is comparable to the chemical AST and higher than in the mechanical AST.

(Energy Technology Division Research Award) PEM Fuel Cell Electrode Layer Degradation

PEM fuel cells (PEMFCs) show great promise to increase the fuel efficiency for transportation applications; however, for this increase in efficiency to be implemented in a wide-spread manner, these systems need to be competitive with current technologies. As PEMFCs have evolved for transportation applications from prior applications (space and stationary), materials changes have been made to increase the power density and decrease the cost (including thinner, higher conductivity membranes and higher surface area/lower Pt loadings) that have negatively affected the durability. In addition, as these systems were developed for transportation, the operating conditions changed, including frequent start-up/shut-downs, power cycling, and higher temperature/lower relative humidity operation. While there has been significant progress in improving durability with lower cost materials, further improvements are needed to meet the commercialization targets. Past improvements have largely been made possible because of the fundamental understanding of the underlying degradation mechanisms. By investigating component and cell degradation modes and defining the fundamental degradation mechanisms of individual components and component interactions, new materials can be designed to improve durability. The primary degradation modes involve materials in the the electrode layer. The high cost of the noble metal(s) used as the catalyst makes this a crucial area requiring durability improvement. Other degradation modes include decreases in electrode kinetics and increases in resistance of both transport and conductivity. Although the electrode structure and durability thereof has been studied extensively, perfect solutions for commercialization do not yet exist. The primary degradation mechanisms within the cathode catalyst layer include catalyst nanoparticle stability, catalyst support degradation, porosity losses, and ionomer degradation. Catalyst support carbon corrosion occurs under normal fuel cell operating conditions and is exacerbated by the voltage cycling inherent in transportation drive cycles. An example of how the drive cycle potential cycling affects carbon corrosion, as measured by CO2 evolution, is shown in Figure 1. The peak in CO2 evolution occurs when the cell potential increases from high power operation to low power near open circuit. This correlates with when CO2 evolution is observed during cyclic voltammograms, which occurs during the positive sweep at ~ 0.55 to 0.60 V. The evolution of this CO2 peak suggests that oxygen is adsorbed onto the carbon and/or CO is formed on the Pt surface. Many materials and operating variables affect the amount of carbon corrosion, including the type of carbon support used, the Pt loading and dispersion, and operating conditions, such as relative humidity (RH) and temperature. Not surprisingly, high surface area carbons with a low extent of graphitization, exhibit the greatest amount of corrosion, followed by Vulcan carbon, which has some meso-graphitic structure, and the least amount of carbon corrosion occurs with graphitized, low surface area carbon (LSAC). Higher Pt loadings and poor Pt dispersions lead to higher rates of carbon corrosion, which in turn leads to loss of Pt surface area, thinning of the electrode layer, and increases in mass transport losses. However, as Figure 2 demonstrates, carbon corrosion is only a portion of the issues in play. Figure 3 shows the initial and final Pt particle sizes for the same carbon support materials. In this case, the Pt size (measured by XRD) is inversely correlated to the amount of carbon corrosion, with the greatest Pt size observed after shut-down/start-up ageing for graphitized LSAC carbon. Together with the other degradation mechanisms, a reduction in catalyst layer thickness is observed during drive cycle operation, which can be due to the loss of carbon through carbon corrosion or due to compaction; both effects likely lead to a loss of void volume. This reduction in thickness includes a sharp decrease in catalyst layer thickness within the first 100 hours of operation (30%), eventually reaching ~50% of its thickness after 1000 hours. Most of this reduction in electrode thickness does not appear to be directly due to carbon corrosion as there is little evidence for carbon corrosion from microscopic analysis, especially during the early stages of operation, where the thickness reduction is substantially more than what should be due to carbon corrosion. Acknowledgments Funding for this work is from DOE EERE FCTO, Technology Development Manager Nancy Garland I gratefully acknowledge all of my collaborators from Los Alamos, Ion Power, University of Nancy, Commissariat à l'énergie atomique et aux énergies alternatives, Argonne National Lab and Oak Ridge National Lab. Figure 1

Invited: Electrocatalyst Layer Degradation of PEM Fuel Cells

Fuel cells have the potential to replace the internal combustion engine in vehicles and provide power in stationary because they are energy-efficient, clean, and fuel-flexible. The durability of MEAs (Membrane Electrode Assemblies) of PEM (Polymer Electrolyte Membrane) fuel cells is a major barrier to the commercialization of these systems for transportation applications. Power transients, shut-down/start-up, temperature cycling, RH (relative humidity) cycling, are all operating conditions imposed by the rigid requirements for transportation applications. These operational transients have the effect of inducing a number of chemical and structural degradation mechanisms on electrocatalysts and MEAs, changing their performance. Past improvements have largely been made possible because of the fundamental understanding of the underlying degradation mechanisms. By investigating component and cell degradation modes; defining the fundamental degradation mechanisms of components and component interactions new materials can be designed to improve durability.One of the major degradation mechanisms involves the catalyst and catalyst layer, including corrosion of the catalyst support leading to structural changes in the catalyst layer. Cathode catalyst layer thinning is regularly observed with increased losses associated with both catalyst activity and to mass transport limitations which are coupled to loss of porosity in the catalyst layer. Thinning of the catalyst layer can be due to loss of carbon through carbon corrosion or due to compaction and loss of void volume. Carbon corrosion is induced by air/hydrogen (or hydrogen/air) fronts during shut-down/start-up which cause high potentials leading to carbon support corrosion. The degree of the effect is dependent on many factors including the types of carbon utilized as the catalyst support, water content, and the anode catalyst layer. An example of cathode catalyst layer is changing porosity during carbon corrosion accelerated stress testing, where the initial catalyst layer porosity is approximately 40%, decreasing to 33% over 10 hours, and is only about 7% after 40 hours, as measured by TEM. A reduction of the median pore diameter of the carbon particles in the electrode structure is also measured. Electrode structure degradation has been shown to occur in microscopically localized regions, with support corrosion leading to detachment and agglomeration of catalyst particles, while weakening of the carbon structure allows collapse of electrode pores and can severely limits gas transport. Post-analysis, after drive cycle operation, shows localized bands of carbon corrosion that are correlated increased Pt particle sizes and closer Pt-Pt interparticle spacings. Thinning of the catalyst layer can be due to loss of carbon through carbon corrosion or due to compaction and loss of void volume.Membrane degradation has been noted to affect both the membrane and the electrode structure. After testing, post-characterization detects electrode regions (channel area) where less Nafion (less fluorine signal) compared to the rest of the electrode surface. Other regions show bands where no electrode is bonded to the membrane surface. Characterization also shows regions (under the lands) where a part of the membrane remained attached to the catalyst layer. In this region, there was a delamination of the membrane from the PTFE reinforcement. Delamination of the membrane from the PTFE reinforcement has also been noted with a large amount of particles (analyzed as Si-O by EDS) present along the two reinforcement interfaces. At the cathode side, the membrane (between the reinforcement and the cathode electrode layer) has virtually been removed.The material design and fabrication methods of producing the catalyst layer can greatly affect the catalyst layer durability. Utilization of different electrode materials can stabilize the catalyst layer void volume, thus preventing a portion of the performance loss.

PEM Fuel Cell MEA Structure Degradation

The durability of MEAs (Membrane Electrode Assemblies) of PEM (Polymer Electrolyte Membrane) fuel cells is a major barrier to the commercialization of these systems for transportation applications [1]. Power transients, shut-down/start-up, temperature cycling, RH (relative humidity) cycling, are all operating conditions imposed by the rigid requirements for transportation applications. These operational transients have the effect of inducing a number of chemical and structural degradation mechanisms on MEAs changing their performance. One of the major degradation involves the electrocatalyst, including corrosion of the catalyst support leading to structural changes in the catalyst layer. These structural changes can cause changes in the PEM fuel cell performance due to loss of catalyst utilization, electronic/protonic conductivity, and changes in the mass transport of the gases and product water. A primary cause for changes in the structure of the catalyst layer is due to the corrosion of carbonaceous electrode supports leading to electrode collapse, reduced active catalyst area, and increased surface hydrophilicity. The carbons typically used as the support for Platinum catalysts in proton exchange membrane fuel cells (PEMFCs) are vulnerable to electrochemical corrosion, especially during potential excursions generated by startup/shutdown cycling and local anode starvation [2].Cathode catalyst layer thinning is regularly observed and increases losses associated with mass transport. This catalyst layer thinning is coupled with loss of porosity in the catalyst layer. The graphitic content of catalyst supports increases during the operation of fuel cells at high potentials which is likely due to a decrease of amorphous carbon due to corrosion. Support corrosion also leads to detachment and agglomeration of catalyst particles, while weakening of the carbon structure allows collapse of electrode pores and severely limits gas transport [3]. The electrode structure degradation has been shown to occur in microscopically localized regions. An example of the localized degradation is demonstrated in Figure 1 after testing utilizing a simulated Drive Cycle Test prescribed by DOE/Fuel Cell Tech. Team [4]. The fresh catalyst layer (Fig. 1a) shows a mostly graphitic carbon support with well distributed Pt nanoparticles. After Drive Cycle operation, TEM post-analysis shows localized bands of carbon corrosion which are correlated increased Pt particle sizes and closer Pt-Pt interparticle spacings (Figure 1b). Typically, these oxidized regions of carbon surround non-oxidized regions. Regions of non-oxidized catalyst retain their graphitic structure and are correlated with smaller Pt particle sizes.Thinning of the catalyst layer can be due to loss of carbon through carbon corrosion or due to compaction and loss of void volume. This loss of cathode layer porosity is shown in Figure 2, where Figure 2a is a digital TEM image of the fresh cathode catalyst layer, 2b is after operation at 1.2V for 10 hours, and 2c is after 40 hours operation at 1.2V. The initial catalyst layer porosity is approximately 40%, decreasing to 33% over 10 hours, and is only about 7% after 40 hours.The air/hydrogen (or hydrogen/air) fronts during shut-down/start-up have been shown to cause high potentials leading to carbon support corrosion and catalyst layer degradation. The anode catalyst layer ability to conduct the oxygen reduction reaction (ORR) plays a major role in MEA degradation during H2/Air start-stops. By limiting the anode participation in ORR during these phenomena, the degradation can be reduced. Other mitigation techniques can also be employed.The catalyst layer fabrication process using different solvents can have a dramatic effect on the performance/durability of fuel cell electrodes, although the catalyst ECSA durability is largely unaffected. Catalyst loading has a significant impact on performance of MEAs made via decal process using a more traditional water/alcohol/glycerol mixture in the ink. The fact that the ECSA shows similar decreases during the potential cycling indicates that other changes in performance are occurring. Further measurements show that the mass transport resistance is decreasing during the potential cycling. Enhanced performance and impedance behavior of ultra-low loading cathode with potential cycling have similarity with performance improvement with HelOx (or air flow increase). Thus, electrode structural changes occur during potential cycling which favor oxygen diffusion, and lower mass transport resistance.

Degradation of Gas Diffusion Layers in PEM Fuel Cells During Drive Cycle Operation

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Degradation of Gas Diffusion Layers in PEM Fuel Cells during Drive Cycle Operation

Polymer electrolyte membrane (PEM) fuel cells show voltage degradation when subjected to long-term drive cycle operation. While the kinetic losses in the fuel cells can be assigned to catalyst layer degradation, there is a significant mass transport loss of up to 100mV at 2 A/cm2 in 400 hours of operation. This mass transport loss can be attributed to the Gas Diffusion Layer (GDL) degradation especially when using low surface area carbon based catalyst materials. Ex situ ageing studies where the GDL is oxidized in a peroxide solution seem to simulate this degradation mechanism in terms of both the increase in graphitic carbon and surface-oxide species. The ex situ aged GDLs result in similar voltage losses when operated with fresh membrane-electrode assemblies (MEAs). AC impedance results confirm that these losses are due to mass transport limitations, probably arising from loss of hydrophobicity of the GDL material. To alleviate these mass transport losses, various combinations of materials have been examined; improved performance under drive cycle operation was obtained with a GDL material that had carbon nano-tubes in the MPL.

Lean Gasoline Engine Reductant Chemistry During Lean NOx Trap Regeneration

Lean Gasoline Engine Reductant Chemistry During Lean NOx Trap Regeneration