Cell Shutdown Research Articles

Fuel-Cell Performance and Stability during Liquid-Water Removal Cycles

Water management is crucial for achieving high-performance proton-exchange-membrane fuel cells (PEMFCs), as it helps keep the electrolyte hydrated while avoiding performance degradation and cell shutdown due to liquid-water accumulation. According to ex-situ measurements [1-3], accumulation of liquid water in gas-diffusion layers (GDLs) of PEMFCs is a transient process that can be accompanied with oscillations in capillary pressure and saturation. To understand how liquid-water accumulation and drainage impact PEMFC performance hysteresis and stability, a transient cell-level model that accounts for electrode structure and composition and is computationally efficient is needed. A number of volume-averaged models that describe the electrode structure through pore-size distribution have been developed in the past [4-7], but they are steady-state and thus cannot predict dynamic PEMFC performance. Existing transient models have also not been used to analyze cyclic liquid-water accumulation [8-10].In this work, a transient two-phase 2D PEMFC model is developed in the open-source fuel-cell modeling software OpenFCST [11] and applied to analyze PEMFC performance hysteresis and stability during liquid-water accumulation and drainage cycles. The model accounts for the electrode structure through a mixed-wettability pore-size-distribution framework and incorporates a novel dynamic boundary condition to describe the experimentally observed cyclic liquid-water accumulation. Results of the numerical simulations are compared to transient current-density and resistance data at two polarization scan rates and during voltage steps measured with an in-house single-channel cell at multiple operating conditions. This work demonstrates how high breakthrough pressure and rapid liquid-water removal from GDLs may cause highly unstable fuel-cell operation with strong hysteresis and oscillations in the polarization curve (such as those in the figure below) that closely resemble experimental reports [12]. Numerical simulations also reveal the existence of a scan rate that maximizes polarization hysteresis due to a match between the time scale of GDL flooding and of a complete voltage sweep. Fast-scan polarization sweeps are shown most suitable for detecting catalyst-layer flooding that depends on its wettability and occurs within single seconds in contrast to GDL flooding that takes hundreds of seconds. Overall, this work brings more attention to the transient analysis of fuel-cell performance under wet conditions.Figure: Polarization-curve oscillations caused by cyclic liquid-water removal from the cathode GDL. Similar fluctuations have been experimentally observed in [12]. Transient graphs show the dynamics of current density and cathode GDL saturation. Operating conditions are 60 °C, 90% RH, 1.5 atm; scan rate is 0.5 mV/s. References J. T. Gostick et al., J. Electrochem. Soc. 157.4 (2010) C. Quesnel et al., J. Phys. Chem. C 119.40 (2015) D. Ziegler, B.Sc. thesis, Hochschule Mannheim / University of Alberta (2020) A. Z. Weber et al., J. Electrochem. Soc. 151.10 (2004) M. Eikerling, J. Electrochem. Soc. 153.3 (2006) V. Mulone and K. Karan, Int. J. Hydrog. Energy 38.1 (2013) Zhou et al., J. Electrochem. Soc. 164.6 (2017) R. J. Balliet and J. Newman, J. Electrochem. Soc. 158.8 (2011) I. V. Zenyuk et al., J. Electrochem. Soc. 163.7 (2016) A. Goshtasbi et al., J. Electrochem. Soc. 166.7 (2019) M. Secanell et al. ECS Transactions 64.3 (2014) C. Ziegler and D. Gerteisen, J. Power Sources 188.1 (2009) Figure 1

Effects of Gas Diffusion Layer Substrates on PEFC Water Management: Part II. In Situ Liquid Water Removal via Evaporation

Desaturation of polymer electrolyte fuel cells (PEFCs) is a critical operation step for providing cell cold-start performance by minimizing residual water in the gas diffusion layers (GDLs), flow field (FF) channels, catalyst layers and membrane after cell shutdown. In this work, transient liquid water removal processes in the FF channels and GDLs are visualized and quantified by subsecond in situ X-ray tomographic microscopy (XTM), and correlated to high frequency resistance (HFR) measurements of the cell. Time-resolved desaturation profiles are analyzed for three commercially available GDLs with representative substrate dimensions. The influence of different substrates on the GDL desaturation behavior is investigated with a cluster connectivity analysis and saturation-dependent effective diffusivities are determined by numerical simulations. Characteristic drying phases are identified for the HFR curves and confirmed with XTM imaging results, providing fundamental understanding of the desaturation dynamics in the PEFCs and enabling the optimization of GDL substrates and gas purge protocols accordingly.

A chemical-mechanical ex-situ aging of perfluorosulfonic-acid membranes for fuel cells: Impact on the structure and the functional properties

Perfluorosulfonic acid (PFSA) membranes are key components for the operation of Polymer Electrolyte Membrane Fuel Cells (PEMFC). They undergo harsh conditions in terms of chemical environment and mechanical fatigue leading to irreversible degradations, up to their failure and to the Fuel Cell (FC) shutdown in worst cases. Through an ex-situ approach, this study aims to provide new insights about the impact of conjoint mechanical and chemical stresses on the structure and the functional properties of PFSA membranes. In this regard, various aging tests have been carried out using a custom-made device able to combine the application of a mechanical stress (sinusoidal profile) and the exposure to an environment containing free radicals ( i.e. Fenton's reagents). After about 20 h of accelerated aging test the analysis of the aged membranes permitted to demonstrate that the exposure to the combined stress significantly altered the morphology of Nafion™ XL membrane, with the appearance of bubbles close to the membrane surface and even delamination at the PFSA/reinforcement interface. On the other hand, and rather paradoxically, the water sorption and self-diffusion properties of the aged membranes as well as their cell performances remained satisfactory. • Nafion™ membranes were both exposed to chemical and mechanical stress simultaneously. • Coupling mechanical and chemical stresses induces important morphological changes. • Interfacial delamination of XL layers appeared after 20 h of conjoint stress. • Reinforcement layer in XL had little/lower effect on FER for severe solicitations.

Conjoint Effects of Chemical and Mechanical Stresses on Fuel Cell PFSA Membranes

During fuel cell operation, proton-exchange membranes (PEMs) are subjected to aggressive chemical environment and mechanical fatigue that alter their properties and may lead to performance loss or, in the worst cases, fuel cell shutdown due to membrane failure. Membrane degradation due to chemical and mechanical stresses remains one of the main factors limiting PEM fuel cell lifetime [1]. Chemical damages are mostly due to radical attacks leading to the scission of the polymer chains (backbone or/and side chains), while mechanical stress comes from repeated swelling/shrinkage cycles caused by the membrane water-uptake. Despite quantities of works focusing independently on chemical (most often) or mechanical (more rarely) degradations, only a few studies were published on the effect of both combined [2]–[4]. Among them, Kusoglu et al. demonstrated that a static compression affects its microstructure and accelerates the chemical decomposition of the polymer chains in Nafion® membranes [2].In this study, we investigate the impact of both chemical and mechanical degradations on the morphology and physicochemical properties of Nafion® membranes: chemical structure, as well as water transport and sorption. The membrane degradation process was induced by a custom-made device that mimics the operating conditions of the fuel cell by exposing the membrane simultaneously to a free radical environment and cyclic compression. The generation of free radical environment is based on the circulation of a continuous flow of hydrogen peroxide or Fenton’s solution (i.e. hydrogen peroxide and ferrous ions), while mechanical fatigue is induced by a cyclic compression to reproduce the swelling/shrinkage sequences entailed by the membrane water-uptake during transient fuel cell operation.Preliminary studies were necessary to determine the appropriate experimental conditions, both from a chemical and a mechanical point of view. Firstly, an investigation similar to the one of Frensch et al. was performed to determine the effect of Fenton’s reagent concentrations on membrane degradation [5]: fluoride emission measurements showed that a higher degradation rate was obtained at a low iron ions concentration. Moreover, at low iron concentration, the degradation rate depended also significantly on the hydrogen peroxide concentration. On the other hand, high hydrogen peroxide concentration (≈ 30 %vol) entailed strong membrane morphology modification with the appearance of many bubbles - with a diameter varying from some micrometers to several millimeters- at the surface of both membranes [6]. Such morphology evolutions cannot be considered as representative of the degradations occurring during fuel cell operation.The first mechanical and chemical degradation tests were carried out using a 0.0503 cm3.s-1 flow of either pure water or 3%vol diluted H2O2 through two thermostated (80°C) half-cells. The half-cells and the membrane were inserted between the clamps of an electromechanical testing machine (MTS load frame model 312.21) and the mechanical stress consisted in a sinusoidal constraint between 0 and 5 MPa at 0.1 Hz. As expected, the application of this sinusoidal constraint increased significantly the degradation rate (Figure): the fluoride emissions were 5 to 7 times higher than under chemical stress only. Indeed, Nafion® XL’s degradation rate increases from 11 µg/h/gNafion to 68 µg/h/gNafion when mechanical stress was applied in addition to chemical stress. Likewise, the values increased from 11 to 54 µg/h/gNafion with NR-211. Moreover, with the combination of cyclic compression and H2O2 solution exposure, the degradation rates approached the values obtained with a Fenton’s reagents exposure.[1] R. Borup et al., Chem. Rev., 107 (10), 3904–3951, 2007.[2] A. Kusoglu et al., ECS Electrochem. Lett., 3 (5), F33–F36, 2014.[3] S. velan Venkatesan et al., J. Electrochem. Soc., 163 (7), F637–F643, 2016.[4] V. M. Ehlinger et al., 166 (7), F3255–F3267, 2019.[5] S. H. Frensch et al., J. Power Sources, 420, 54–62, 2019.[6] S. Mu et al., J. Appl. Polym. Sci., 129 (3), 1586–1592, 2013. Figure 1

Numerical investigation of cold-start behavior of polymer electrolyte fuel cells in the presence of super-cooled water

In this study, a mathematical model has been developed to simulate the transient cold-start processes of polymer electrolyte fuel cells. The super-cooled water is assumed to exist within the cell. The non-equilibrium water transfer between the membrane and the catalyst layer is considered. The models of water freezing and ice melting in the catalyst layer and gas diffusion layer have been established. For the first time, the randomicity of the freezing process is captured by introducing a freezing probability function. Based on this model, the cold-start processes of a single polymer electrolyte fuel cell starting at various operating and initial conditions have been simulated numerically. The results indicate that the cold-start performance of the cell is determined by the water storage potential of the electrolyte in cathode catalyst layer. For each startup temperature and operating current load, there is a most appropriate initial membrane water content, which corresponds to the longest cell shutdown time. When the cold-start process is failed, the ice is mainly accumulated in the cathode catalyst layer. The ice distribution becomes more non-uniform as the cold-start temperature is lower.

Evaluation of Allylboronic Acid Pinacol Ester as Effective Shutdown Overcharge Additive for Lithium Ion Cells

Allylboronic acid pinacol ester (ABPE) was investigated as shutdown overcharge additive to increase the intrinsic safety of lithium ion cells during operation at elevated charge cutoff potentials up to 4.5 V vs. Li/Li+. It was demonstrated that the additive had no negative influence on the cycling performance of LiNi1/3Co1/3Mn1/3O2 (NMC-111)/graphite full cells operated in a standard operation voltage range between 2.5 V and 4.2 V. Electrochemical impedance spectroscopy was used to study the influence of the electrolyte additive on the impedance before and after overcharge and concomitant cell shutdown. Thereby, an immense increase in the charge transfer resistance after electrode shutdown was observed, thus, hinting to the formation of a lithium ion-insulating layer on the positive electrode surface. To further elucidate the working principle of the shutdown additive, surface investigations by means of scanning electron microscopy and X-ray photoelectron spectroscopy were carried out. The existence of a surface layer consisting mainly of polymeric species was proven. The presented results open up a new family of compounds for overcharge protection.

Ice Formation Processes in PEM Fuel Cell Catalyst Layers during Cold Startup Analyzed by Cryo-SEM

For further improvements in the startup ability below freezing and the durability of polymer electrolyte fuel cells, understanding the ice formation mechanism during cold startup is particularly significant. This study observes cross-sectional ice distributions in a catalyst layer (CL) during isothermal galvanostatic operation at −20°C using a cryo-scanning electron microscope. The effects of current density, cathode gas conditions, initial water content of the membrane, and cell temperature on the cold start characteristics and the ice formation process in the CL are evaluated. The observational results show that at higher current densities, the region with active ice formation moves from the membrane to the gas diffusion layer sides during the freezing period and vacant pores remain near the membrane even after cell shutdown, while the pores are completely filled with nearly-uniformly growing ice at lower current density operation. This is consistent with the experimental finding from the cold start characteristics that the estimated amount of ice accumulated in the cell until the shutdown decreases as the current density increases. Contrary to expectations, these changes are largely independent of cathode gas conditions, even with pure oxygen. Additional factors controlling the ice formation process are discussed based on the experimental results.

Mathematical Modeling of Salt Lens Formation Process in the Cathode Assembly of Aluminum Electrolysis Cells

In this paper, with the help of mathematical modeling we investigate growth under saline lens bottom blocks of the cathode aluminum cell. The penetration of the melt in the hearth of the cathode of the device is one of the main causes of premature shutdown of cells, because it leads to excessive heat loss and destruction of the hearth. The study was built mathematical model of unsteady filtration and heat the porous body with an incompressible viscous fluid. With the help of the developed mathematical model we studied the dynamics of growth of the lens. Research has identified the rate of melt penetration into the hearth of the cathode unit in the course of its life. The calculation results were compared with the field data.

A Comprehensive Review of Solutions and Strategies for Cold Start of Automotive Proton Exchange Membrane Fuel Cells

Proton exchange membrane fuel cell (PEMFC) can be a significant eco-friendly alternative power source for vehicles. However, under subfreezing conditions, cell degradation and irreversible performance decay can occur because of ice formation and repetitive thaw/freeze cycles. These problems have limited the further commercialization of PEMFC in cold weather countries. Thus, many improvements have been made to repair the freeze protection and rapid cold startup problems in PEMFC vehicles. In this paper, a comprehensive review dedicated to engineers of the recent research progress on the PEMFC cold start problems is presented. Systems and methods for fuel cell shutdown are summarized and classified into two categories: purge solution and material to avoid freezing. Regarding the system and solutions for PEMFC cold startup, different heating solutions are classified into two main groups depending on their heating sources and categorized as internal and external heating methods. This paper concludes with a detailed review of cold startup strategies based on an exhaustive survey of journal papers and patents.

Use of Glyceraldehyde 3-Phosphate Dehydrogenase (GAPDH) as Internal Control in Western-Blot Experiments?

to 1, 3-biphosphoglycerate during glycolysis, GAPDH is involved in several non-metabolic processes, including transcriptional activation and cell apoptosis [3]. Activated Akt2 may increase ovarian cancer cell survival via prevention of GAPDH-induced apoptosis. This effect of Akt2 is partially mediated by its phosphorylation of GAPDH at Thr-237, which results in the prevention of GAPDH nuclear translocation [3]. This study prove that GAPDH is S-nitrosylated by Nitric Oxide (NO) in response to cell stress, which result in, it to bind the protein Siah1, anubiquitin ligase. The complex moves into the nucleus where Siah1 targets nuclear proteins for degradation, thus beginning controlled cell shutdown [4].

Direct methanol fuel cells with streamline graded structure under ultra-low fuel stoichiometry condition

Operating a direct methanol fuel cell (DMFC) with ultra-low fuel stoichiometry is desirable to build a compact portable power system. Through 3D computational modeling with statistical analysis, severe non-uniformity of fuel concentration leading to high methanol crossover in the inlet region and poor fuel supply in the outlet region is predicted under ultra-low fuel stoichiometry condition, which causes large voltage loss or even cell shut-down. After identifying controlling parameters of methanol transport, this paper proposes streamline-graded structures (SGS) of the anode channel to mitigate anode fuel distribution non-uniformity and to boost cell performance and fuel efficiency together. Computational results show that streamline-graded structures achieve about 10% of voltage gain and 3% of fuel efficiency improvement compared to the conventional DMFC design by mitigating anode non-uniformity.

Performance degradation studies on an poly 2,5-benzimidazole high-temperature proton exchange membrane fuel cell using an accelerated degradation technique

In this work, the performance degradation of a poly 2,5-benzimidazole (ABPBI) based high-temperature proton exchange membrane fuel cell (HT-PEMFC) was examined using an accelerated degradation technique (ADT). Experiments using an ADT with 30 min intervals were performed by applying 1.5 V to a membrane electrode assembly (MEA) with hydrogen and nitrogen feeding to the anode and cathode, respectively, to simulate the high voltage generated during fuel cell shutdown and restart. The characterization of the MEAs was performed using in-situ and ex-situ electrochemical methods, such as polarization curves, AC impedance, and cyclic voltammetry (CV), and TEM imaging before and after the ADT experiments. The measured results demonstrated that the ADT testing could be used to dramatically reduce the duration of the degradation. The current output at 0.4 V decreased by 48% after performing ADT testing for 30 min. From the AC impedance, CV and RTGA measurements, the decline in cell performance was found to be primarily due to corrosion and thinning of the catalyst layer (or carbon support) during the first 30 min, leading to the dissolution and agglomeration of the platinum catalyst.

Endurance study of a solid polymer electrolyte direct ethanol fuel cell based on a Pt–Sn anode catalyst

The performance decay of a solid polymer electrolyte direct ethanol fuel cell (DEFC) based on a Pt3Sn1/C anode catalyst during an endurance test has been investigated. The effect of different cell shut-down procedures on the cycled behaviour of the DEFC has been studied. To get specific insights into the degradation mechanism, polarization and ac-impedance spectroscopy studies have been carried out. These analyses have been complemented by post-operation transmission electron microscopy and X-ray diffraction studies. The combination of these techniques has allowed to get information on recoverable and unrecoverable losses. This provides a basis for further improvement of DEFC components.

Current mapping of a proton exchange membrane fuel cell with a segmented current collector during the gas starvation and shutdown processes

Current distribution during the gas starvation and shutdown processes is investigated in a proton exchange membrane fuel cell with an active area of 184 cm2. The cell features a segmented cathode current collector. The response characteristics of the segmented single cell under different degrees of hydrogen and air starvation are explored. The current responses of the segment cells at different positions under a dummy load in the shutdown process are reported for various operating conditions, such as different dummy loads, cell temperatures, and gas humidities under no back pressure. The results show that applying a dummy load during the cell shutdown process can quickly reduce the cell potential and thereby avoid the performance degradation caused by high potentials. The currents of all the segment cells decrease with time, but the rate of decrease varies with the segment cell positions. The rate for the segment cells near the gas outlet is much higher than that of the segment cells near the gas inlet. The current of the segment cells decreases much more quickly at a lower gas humidity and high temperature. This study provides insights in the development of mitigation strategies for the degradation caused by starvation and shutdown process.

Factors determining the gas crossover through pinholes in polymer electrolyte fuel cell membranes

Membrane degradation in polymer electrolyte fuel cells often results in pinhole formation, which does not force an immediate fuel cell shutdown, as the performance is still good and the gas crossover is moderate for low membrane defect densities. However, the gas crossover increases locally at defects, which may accelerate chemical polymer decomposition and enlarge the defect. Hence, a fundamental understanding of the gas crossover through small pinholes is required to deduce strategies mitigating fast membrane degradation. Methods are developed to implement pinholes (10μm) artificially in polymer electrolyte membranes. The pinhole morphology and chemical environment are characterized by X-ray tomographic microscopy and FTIR spectro-microscopy. The gas crossover is measured in situ for different fuel cell operating conditions using a mass spectrometry based method. In saturated environment liquid water can seal pinholes and eliminate the crossover. This sealing effect depends on the pinhole size and the pressure gradient between anode and cathode. Increasing temperature or humidity reduces the gas crossover. Hydrogen, permeating through defects, does not oxidize quantitatively at the cathode catalyst layer, but permeates through the gas diffusion layer (GDL) into the gas channel. Then, the permeability of the GDL, in particular its micro-porous layer, limits the gas crossover through pinholes significantly.

Study of high voltage applied to the membrane electrode assemblies of proton exchange membrane fuel cells as an accelerated degradation technique

The durability of proton exchange membrane fuel cells has been extensively studied. The main aim of the present study was to elucidate membrane electrode assembly (MEA) degradation mechanisms using an accelerated degradation technique (ADT). An ADT experiment was performed by applying 1.5V to an MEA with hydrogen and nitrogen feeding to the anode and cathode, respectively, to simulate the high voltage generated during fuel cell shutdown and restart. This study adopted in situ as well as ex situ techniques, such as polarization curves, AC impedance, cyclic voltammetry (CV), linear sweep voltammetry (LSV), SEM and TEM to analyze the MEA before and after ADT experiments. Results show that the ADT could dramatically reduce the duration of the experiment while still observing MEA degradation. Current output at 0.4V decreased by 41% after performing ADT for 60min. LSV showed unchanging hydrogen crossover through the MEA, implying membrane degradation could be ignored during ADT evaluation. An increase in charge transfer resistance seen by AC impedance, shifts in the electrochemical surface area and double layering suggested that the main degradation mechanism during ADT evaluation is degradation of the carbon supporting the catalyst layer.

Evaluating cold-start behaviors of end and intermediate cells in a polymer electrolyte fuel cell (PEFC) stack

In a polymer electrolyte fuel cell (PEFC) stack, the end cells immediately adjacent to the end plates of a stack usually exhibit different cell behaviors compared to its intermediate cells. In this study, cold-start behaviors of end and intermediate cells in a fuel cell stack are numerically examined by using the three-dimensional cold-start PEFC model. The cold-start simulations show that the ice fraction in the end cell grows much faster than in the intermediate cell, indicating that the heat loss through stack end plate to the ambient is critical, determining the fate of cold-start whether cell shutdown or successful startup. As a consequence, under the assumed cold-start condition (start-up from −20°C and 100mA/cm2), the end cell experiences a sudden voltage drop and cold-start failure due to considerable ice filling in the cathode catalyst layer (CL) whereas a successful startup is achieved in the intermediate cell because its temperature increases faster than the rate of ice growth and finally exceeds freezing temperature.

Autonomic Shutdown of Lithium‐Ion Batteries Using Thermoresponsive Microspheres

AbstractAutonomic, thermally‐induced shutdown of Lithium‐ion (Li‐ion) batteries is demonstrated by incorporating thermoresponsive polymer microspheres (ca. 4 μm) onto battery anodes or separators. When the internal battery environment reaches a critical temperature, the microspheres melt and coat the anode/separator with a nonconductive barrier, halting Li‐ion transport and shutting down the cell permanently. Three functionalization schemes are shown to perform cell shutdown: 1) poly(ethylene) (PE) microspheres coated on the anode, 2) paraffin wax microspheres coated on the anode, and 3) PE microspheres coated on the separator. Charge and discharge capacity is measured for Li‐ion coin cells containing microsphere‐coated anodes or separators as a function of capsule coverage. For PE coated on the anode, the initial capacity of the battery is unaffected by the presence of the PE microspheres up to a coverage of 12 mg cm−2 (when cycled at 1C), and full shutdown (>98% loss of initial capacity) is achieved in cells containing greater than 3.5 mg cm−2. For paraffin microspheres coated on the anode and PE microspheres coated on the separator, shutdown is achieved in cells containing coverages greater than 2.9 and 13.7 mg cm−2, respectively. Scanning electron microscopy images of electrode surfaces from cells that have undergone autonomic shutdown provides evidence of melting, wetting, and resolidification of PE into the anode and polymer film formation at the anode/separator interface.

Understanding Membrane Failure in PEMFC: Comparison of Diagnostic Tools at Different Observation Scales

AbstractPolyelectrolyte membrane used in proton exchange membrane fuel cell (PEMFC) should act as a barrier to hydrogen and electrons. In the present work, fast and robust specific tools were developed both to characterize these properties and localize defects, such as pinholes or short‐circuits, within the membrane electrode assembly (MEA). The identification of defective cells without disassembling the Axane's stack was performed in a single step through a passive electrical technique so‐called “relaxometry” and by internal gas leak measurements. The flaws were further tracked inside MEA using Infrared Thermography in order to obtain their precise location. The latter, performed after stack disassembly, permitted to reveal the defects responsible for the Fuel Cell shutdown. A third step dealt with SEM examinations of the flaws in order to identify the origin and nature of the membrane failure.

Bid and Bim Collaborate during Induction of T Cell Death in Persistent Infection

Upon Ag encounter, naive T cells undergo extensive Ag-driven proliferation and can differentiate into effector cells. Up to 95% of these cells die leaving a small residual population of T cells that provide protective memory. In this study, we investigated the contribution of the BH3-only family protein Bid in the shutdown of T cell responses after acute and persistent infection. Influenza virus pathogenicity has been proposed to be mediated by a peptide encoded in the basic polymerase (PB1-RF2) acting through Bid. In our experiments, we found that after acute infection with influenza virus, mice lacking Bid had normal expansion and contraction of Ag-specific CD8(+) T cells. However, in chronic γ-herpesvirus infection, Bid-deficient virus-specific CD8(+) T cells expanded normally but failed to contract fully and were maintained at ∼2-fold higher levels. Previously, we have demonstrated that Bim plays a prominent role in T cell shutdown in persistent infection by cooperating with the death receptor Fas, which regulates apoptosis in response to repeated TCR signaling. Bid lies at the nexus of these two signaling pathways, thus we reasoned that Bid and Bim might cooperate in regulation of T cell shutdown in persistent infection. In this study, we observed that the combined loss of Bid and Bim synergistically enhanced the persistence of CD8(+) T cells during γ-herpesvirus infection. Thus, these data uncover a role for Bid in coordinating apoptotic signaling pathways to ensure appropriate shutdown of T cell immune responses in the setting of persistent Ag exposure.