Degradation Of Membrane Electrode Assembly Research Articles

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.

Diagnosis of membrane electrode assembly degradation with drive cycle test technique

One of important factors determining the lifetime of proton exchange membrane fuel cells (PEMFCs) is degradation and failure of membrane electrode assembly (MEA). The lack of effective mitigation methods is largely due to the currently limited understanding of the degradation mechanisms for fuel cell MEAs. This study adopted the accelerating degradation technique to analyze durability of MEA using drive cycle test protocol developed by Chinese NERC Fuel Cell & Hydrogen Technology to assess the long-term durability of fuel cells for vehicular application. During 900 h durability test of the MEA, the polarization curve, cyclic voltammetry (CV), linear sweep voltammetry (LSV) and electrochemical impedance spectroscopy (EIS) were performed as diagnostics during and on completion the test. The experimental results show that the performance degradation rate of the cell is about 70 μV h−1 at the operating current density of 500 mA cm−2, failure of the proton exchange membrane is the decisive factor leading to the failure of the MEA. And the damage of the micro-structural of catalytic layer, crucial for electrochemical reaction, is the decisive factor for the performance degradation.

Effects of anisotropic bending stiffness of gas diffusion layer on the MEA degradation of polymer electrolyte membrane fuel cells by wet/dry gas

The effects of anisotropic bending stiffness of a gas diffusion layer (GDL) on membrane electrode assembly (MEA) degradation were investigated. We prepared GDLs with a fiber direction perpendicular to the major flow (i.e., “90° GDL”) and with a fiber direction parallel to the major flow (i.e., “0° GDL”). To analyze the mechanical durability as a function of GDL anisotropy, we examined cell performances such as the I–V characteristics and impedances and the hydrogen crossover characteristics during wet/dry cycles. The results showed that the 90° GDL fuel cell is superior to the 0° GDL fuel cell in terms of higher I–V performance, lower resistance at high frequency, and lower hydrogen crossover through the MEA. Mechanical degradation of the 0° GDL was investigated using scanning electron microscopy (SEM).

PEMFC membrane electrode assembly degradation study based on its mechanical properties

Abstract The membrane electrode assembly determines the performance of the polymer electrolyte membrane fuel cell. Degradation of the membrane electrode assembly has been the subject of recent research with impedance measurements, cyclic voltammetry measurements, scanning electron microscopy, tunneling electron microscopy as the techniques currently in use for characterization. This paper considers the membrane electrode assembly as a laminate and uses the study of mechanical properties as a tool for identification of delamination. An attempt has also been made to compare the changes in the mechanical properties obtained due to delamination with the results obtained from impedance measurement, cyclic voltammetry studies and performance in the fuel cell. In this study mechanical properties such as stress, strain and modulus have been measured and the decrease in modulus as a result of delamination has been characterized. It has been found that study of the mechanical properties of the membrane electrode assembly compliments the other techniques and can be used in the post-mortem analysis of causes for degradation.

Perfluorinated Sulfonic Acid Membrane and Membrane Electrode Assembly Degradation Correlating Accelerated Stress Testing and Lifetime Testing

An important step in achieving fundamental understanding of fuel cell failure mechanisms and development of technology to mitigate these failures is accomplished by analysis of directed lifetime and failure test results. Several lifetime, accelerated stress, and drive cycle test protocols have been developed and carried out. The two major ASTs that have been developed to evaluate membrane degradation are 1) Open circuit voltage tests, which are designed to accelerate chemical degradation, and 2) Relative humidity cycling tests, which are designed to accelerate mechanical degradation. The results from these tests have been compared to field tests. The ultimate goal is to use the laboratory tests to predict data in the field. An overall predictive decay model is being developed through a combination of specific modeling and tests.

Degradation reduction of polymer electrolyte membranes using CeO2 as a free-radical scavenger in catalyst layer

Ceria nanoparticles were added to the electrodes of proton exchange membrane fuel cells as free-radical scavengers to minimize the degradation of membrane electrode assembly (MEA) components. Accelerated durability tests were performed at low humidity under open circuit voltage (OCV) conditions, and the results were compared with traditional MEAs without CeO2. Gas crossover was monitored during the durability test, and the MEAs were examined by SEM before and after the durability test. The results showed that adding CeO2 as free-radical scavengers to the electrode greatly improves the chemical stability of the membrane. The degradation rate of the MEA with CeO2 in the anode was similar to that of the MEA with CeO2 in the cathode. The fuel cell with CeO2 in the cathode showed better MEA performance that the fuel cell with CeO2 in the anode.

Long-term durability of radiation-grafted PFA-g-PSSA membranes for direct methanol fuel cells

Poly(tetrafluoroethylene-co-perfluoropropyl vinyl ether)-graft-poly(styrene sulfonic acid) (PFA-g-PSSA) membranes were prepared by γ-ray radiation-induced grafting of styrene onto 50μm PFA films and a subsequent sulfonation. The properties of the PFA-g-PSSA membranes such as ion exchange capacity, water uptake, proton conductivity, and methanol permeability were evaluated and compared with commercial Nafion 115 membrane. The characteristic properties and direct methanol fuel cell (DMFC) performances with PFA-g-PSSA membrane were better than that with the Nafion 115 membrane. The power density of the single cell with PFA-PSSA30, PFA-PSSA50, PFA-PSSA80 membranes having different degree of grafting (DOG), and Nafion 115 membranes was 113, 123, 118, and 111mWcm−2 at 0.4V, respectively. To study the long-term durability of the prepared membrane electrode assembly (MEA) with a PFA-PSSA50 membrane, the single cell was tested for 2066h in a galvanostatic mode under a constant load 150mAcm−2 and at a temperature of 60°C. The performance loss of the MEA after 2066h operation was about 26.8%. Before and after the long-term performance test, the degradation of MEA with PFA-g-PSSA membrane was investigated by polarization curve technique, electrochemical impedance spectroscopy, TEM, X-ray diffraction, and ion exchange capacity.

Degradation mechanisms and mitigation strategies of metal cations in recycled fuel for direct methanol fuel cell membrane electrode assembly

Some metal contaminants, such as Al3+, Ni+2, Fe2+ and Cr3+, are produced during reactions in heat exchangers, stacks, and other fuel/water management system components. Due to the gradual build-up of these contaminants generated in the system, direct methanol fuel cell (DMFC) membrane electrode assemblies (MEAs) deteriorate steadily with increasing operation time. Hence, this study systematically investigates the effects of metal cations by supplying various concentrations of metal solutions to the fuel stream at constant-current densities, with the aim of understanding the mechanism and influence of metal contamination on a DMFC MEA. Various electrochemical diagnostic techniques are used to determine the main cause of MEA degradation, including electrochemical impedance spectroscopy, electrode polarization, and methanol stripping voltammetry. In addition, the critical concentration of metal cations in methanol fuel is investigated for high DMFC MEA stability. Further, various novel methods for mitigating the influence of the metal contaminants on the performance of a DMFC are suggested and verified.

Degradation of membrane electrode assemblies utilizing PtRu catalysts under high potential conditions

We investigated the degradation of membrane electrode assemblies (MEAs) utilizing PtRu catalysts by way of an accelerated anode degradation test (anode potential cycling) that simulated localized fuel starvation. We predicted that anode potential would be high (approx. 1.4 V vs. RHE) when localized fuel starvation occurred during start-up of the stack. Although the dissolution of Ru in the anode increased with increasing upper potential limits in anode potential cycling that simulated localized fuel starvation, the Ru crossover from anode to cathode and Ru deposition in the cathode reached a maximum at an upper potential limit of 1.2 V. In addition, we investigated a method for restoring the performance of MEAs deteriorated by Ru deposition. With this method more than 90% of cathode performance (H2–O2 voltage) and 79% of H2–Air voltage can be recovered.

A polymer electrolyte fuel cell life test using accelerating degradation technique

Durability is one of the major barriers to proton exchange membrane fuel cells (PEMFCs) being accepted as a commercially viable product. It is therefore important to understand the degradation phenomena of MEA (membrane electrode assembly) in PEMFCs and analyze degradation mechanisms. This study adopted the accelerating degradation technique to analyze durability of PTFE/Nafion MEA using the Taiwan HS-150 fuel cell tester and MEA/Stack Durability Protocol developed by the Fuel Cell Technical Team (FCTT) of the FreedomCAR and Fuel Partnership. The experimental process for the MEA degradation contains dry/wet and load cycling. During durability test of the MEA, the polarization curve, cyclic voltammetry (CV), linear sweep voltammetry (LSV) and electrochemical impedance spectroscopy (EIS) are used for diagnosing state of the MEA. The experimental results show that failure of the proton exchange membrane is the decisive factor leading to the end-of-life of the MEA and the electrochemical reaction micro-structural damage of catalytic layer is the decisive factor of the performance degradation and stability deteriorates.

One-dimensional manganese oxide nanostructures as radical scavenger to improve membrane electrolyte assembly durability of proton exchange membrane fuel cells

One-dimensional manganese oxide (MnO2) scavengers are proposed to minimize the radical degradation of membrane electrode assembly (MEA) in fuel cells. The synthesized MnO2 nanotubes and MnO2 nanowires are ca. 5–8 nm in diameter and more than 100 nm in length, respectively. Water retention of the hydrophilic MnO2 reduces the humidity sensitivity of the fuel cells. At 75 RH% humidify levels of inlet gas at 60 °C, single cells assembled by catalyst layer with MnO2 nanotubes and nanowires as the cathodes have peak power densities of 599.3 and 536.8 mW cm−2, respectively. Radical scavenger MnO2 in the cathode catalyst is effective in minimizing the radical degradation in proton exchange membrane (PEM) fuel cells. After the OCV degradation, the hydrogen crossover of cell assembled by catalyst layer with MnO2 nanowires or MnO2 nanotubes in anode and traditional catalyst layer in cathode are about 5.3 and 4.9 mA cm−2, respectively. As a comparison, single cell with conventional catalyst layer in both anode and cathode, the hydrogen crossover through the MEA is increased to 14.7 mA cm−2.

Understanding the mechanism of membrane electrode assembly degradation by carbon corrosion by analyzing the microstructural changes in the cathode catalyst layers and polarization losses in proton exchange membrane fuel cell

Electrochemical corrosion of carbon in a membrane electrode assembly (MEA) is a critical issue to be resolved to satisfy the durability targets for commercialization of proton exchange membrane (PEM) fuel cells. In order to understand the dominant mechanisms of MEA degradation by carbon corrosion, accelerated stress tests (ASTs), which consisted of holding at 1.3V for 50h, were conducted in a single cell, and the microstructural changes of the cathode catalyst layers (CCLs) and polarization losses were analyzed. The results of carbon corrosion were an increase in CCL porosity and 50% loss in the surface area of the Pt, leading to a 70% loss of the initial MEA performance at 0.6V. Quantitative analyses of polarization losses for the MEA showed that oxygen diffusion was reduced significantly, while there were somewhat smaller increase of the kinetic and high frequency resistance (HFR) losses after carbon corrosion. Carbon oxide species that form on carbon particles during the carbon corrosion process make the surfaces of the CCLs more hydrophilic, and this is believed to diminish oxygen diffusion through the water-filled pores of the CCLs, leading to dramatic degradation of the fuel cell performance. However, another proposed mechanism of diffusion loss, which involves structural collapse and the loss of porosity in the CCLs due to carbon corrosion did not occur in the present study. These findings are anticipated to contribute to the development of more durable MEAs for PEM fuel cells.

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.

Application of TGA techniques to analyze the compositional and structural degradation of PEMFC MEAs

Thermogravimetric analysis (TGA) has been proposed as a new post-analysis tool of membrane electrode assembly (MEA) for polymer electrolyte membrane fuel (PEMFC). Analysis of catalyst layer (CL) samples with various Nafion ionomer contents quantitatively confirmed that the Nafion ionomer decomposed first at around 320 °C and the support carbon at around 410 °C. For the degradation analysis of MEAs, the amount variation of components, including platinum (Pt) and attached gas diffusion layer (GDL), and the degree of Pt agglomeration could be evaluated from weight changes and DTG peak shifts (the platinum activation effect), respectively. For an MEA degraded by start-up/shut-down cycling, Pt agglomeration and significant GDL attachment, as well as invariant Pt amount and slight weight decrease of Nafion and supporting carbon, could be analyzed by a single TGA measurement. Similar degradation analysis was also possible for stack-operated MEAs. This TGA analysis technique is expected to be effectively utilized as a preliminary diagnosis tool in a routine characterization of degraded PEMFC MEAs.

Operating Temperature Dependency on Performance Degradation of Direct Methanol Fuel Cells

AbstractThe effect of operating temperature on performance degradation of direct methanol fuel cell (DMFCs) is examined to disclose the main parameter of the degradation mechanism and the degradation pattern in the membrane electrode assemblies (MEAs). The DMFC MEA degradation phenomenon is explained through the use of various electrochemical/physicochemical tools, such as electrochemical impedance spectroscopy, electrode polarization, methanol stripping voltametry, field emission‐scanning electron microscopy, X‐ray diffraction, inductively coupled plasma‐atomic emission spectroscopy, and X‐ray photoelectron spectroscopy analysis. The operation of DMFC under high temperature accelerates the degradation process of the DMFC. The higher degradation rate under high temperature DMFC operation is mainly attributed to the formation of membrane pinhole with interfacial delamination and cathode degradation. A high operating temperature may result in more considerable thermal and mechanical stress of the polymeric membrane continuously due to frequent dry–wet cycling mode and substantial uneven distribution of water between the anode and the cathode during a long period of DMFC operation. On the other hand, the electrochemical surface area deterioration by Pt coarsening and ionomers loss is not directly related to the larger DMFC performance decay at high temperature.

Membrane electrode assembly degradation under idle conditions via unsymmetrical reactant relative humidity cycling

The objective of this study is to investigate membrane electrode assembly (MEA) failure modes under accelerated test conditions via reactant relative humidity (RH) cycling. In this study, the anode and cathode underwent RH cycling in an unsymmetrical manner and the cell was run at a minimal “idle” current during endurance testing. The cell performance was monitored periodically and the degradation curve showed a difference in the anode and cathode induced RH cycling modes. Anode RH cycling had a more pronounced effect on MEA degradation than cathode RH cycling. Electrochemical diagnostic testing methods such as AC impedance and H2 crossover measurements revealed the degradation phenomena in more detail. The fluoride release data of the anode RH cycling cell showed a sudden increase in fluoride rate within a short period of endurance testing. The infrared imaging results revealed thinning and hotspot pinholes in the membrane, and ionomer delamination from the PTFE reinforcement layer was identified by scanning electron microscopy. The anode RH cycling cell had a shorter lifespan than the cell with cathode RH cycling, highlighting the significance of anode humidification for MEA durability.

Characterization of MEA degradation for an open air cathode PEM fuel cell

As fuel cell technology matures and time scale to commercialization decreases, the need for a more comprehensive knowledge of materials’ aging mechanisms is essential to attain specified lifetime requirements for applications. In this work, the membrane electrode assembly (MEA) degradation of an eight-cell PEM low power stack was evaluated, during and after fuel cell aging in specified testing conditions of load-cycling that may compromise the durability of the catalyst. The stack degradation analysis comprised observation of catalytic layers, morphology and composition. Examination of the MEAs cross sections, in a joint SEM and TEM study, revealed thickness variation of catalytic layer (up to 47% for the cathode layers), and cracking, delamination, and catalyst migration were observed even though catalyst sintering and consequent loss of electrochemical active area seem to be predominant together with F loss from the ionomer used as binder in the catalytic layers.

Effect of open circuit voltage on degradation of a short proton exchange membrane fuel cell stack with bilayer membrane configurations

In the present work, membrane electrode assembly degradation of a 4-cell stack with specially designed twin catalyst coated membranes (twin-CCMs) was carried out for 1600h under open circuit voltage (OCV) conditions. Four types of membrane with various thicknesses were employed in the experiment. Each twin-CCM was composed of two membranes of the same type with a catalyst layer coated only on one side. The purpose of this special configuration was to facilitate postmortem analysis of the membrane after degradation due to the detachable membrane structure. By means of several in situ electrochemical measurements, the performance of the individual cells was analysed every 200h during the degradation process. Postmortem analyses such as scanning electron microscopy, atomic force microscopy, and infrared imaging were also conducted to identify the membrane degradation mechanisms after exposure to OCV. The results indicate that membrane degradation, which correlates with electrode degradation, is the most direct reason for the failure of the whole cell/stack during OCV operation, especially for cells with thinner membranes.

Degradation of a PEM fuel cell stack with Nafion® membranes of different thicknesses. Part II: Ex situ diagnosis

Part I of this study carried out membrane electrode assembly degradation of a four-cell stack with Nafion membranes of different thicknesses, including N117, N115, NR212, and NR211, for 1000h under idle conditions. Through on-line electrochemical measurements it was found that as degradation advanced, cells with thinner membranes experienced much more rapid performance degradation than those with thicker membranes, especially after 800h of operation, due to a dramatic increase in hydrogen crossover. In the present work we investigate the degradation mechanisms of this four-cell stack using several ex situ diagnostic tools, including scanning electron microscopy (SEM), infrared (IR) imaging, ion chromatography (IC), gas permeability measurement, contact angle measurement, and simulation. The results indicate that the drastic increase in hydrogen crossover is due to membrane thickness loss and pinhole formation.

The dependence of performance degradation of membrane electrode assembly on platinum loading in polymer electrolyte membrane fuel cell

The durability of membrane electrode assemblies (MEAs) with varying amounts of Pt loading on the cathode of polymer electrolyte membrane fuel cells was investigated using load cycling as an accelerated degradation test (ADT). The single-cell performance of the MEA as determined by the ADT declined by approximately 34, 48, and 78%, when cathode Pt loading in the MEA was reduced to 0.3, 0.2, and 0.1 mg cm −2, respectively. The increase in MEA performance declined at higher cathode Pt loading conditions, and the degradation rate of MEA performance was also diminished. To characterize the electrochemical and structural properties of the MEAs, cyclic voltammograms, electrochemical impedance spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and field-emission scanning electron microscopy were utilized before and after ADT.