Polymer Electrolyte Membrane Degradation Research Articles

Temperature Reduction as Operando Performance Recovery Procedure for Polymer Electrolyte Membrane Fuel Cells

To efficiently mitigate the reversible performance degradation of polymer electrolyte membrane fuel cells, it is crucial to thoroughly understand recovery effects. In this work, the effect of operando performance recovery by temperature reduction is evaluated. The results reveal that operando reduction in cell temperature from 80 °C to 45 °C yields a performance recovery of 60–70% in the current density range below 1 A cm−2 in a shorter time (1.5 h versus 10.5 h), as opposed to a known and more complex non-operando recovery procedure. Notably, the absolute recovered voltage is directly proportional to the total amount of liquid water produced during the temperature reduction. Thus, the recovery effect is likely attributed to a reorganization/rearrangement of the ionomer due to water condensation. Reduction in the charge transfer and mass transfer resistance is observed after the temperature reduction by electrochemical impedance spectroscopy (EIS) measurement. During non-operando temperature reduction (i.e., open circuit voltage (OCV) hold during recovery instead of load cycling) an even higher recovery efficiency of >80% was achieved.

Suppression of PEFC Membrane Degradation By Using SnO2 As Electrocatalyst Support

Introduction Long-term durability of polymer electrolyte fuel cells (PEFCs) is required for various applications such as automobiles. It has been suggested that certain ions dissolved from electrocatalyst materials accelerate radical formation and subsequent chemical degradation of polymer electrolyte membrane (PEM) by the Fenton reaction [1]. Membrane degradation increases hydrogen and oxygen permeability and decreases proton conductivity and mechanical strength [2]. These phenomena significantly affect the electrochemical performance and durability of PEFCs. Therefore, in case metal oxides are used as catalyst support, it is necessary to check the dissolution of metal ions and to evaluate their effects on membranes. Our research group has been studying Nb-doped SnO2 (Sn(Nb)O2), showing improved durability of the electrocatalysts [3]. Here in this study, we evaluate the effects of Sn(Nb)O2 on the chemical degradation of PEMs. Experimental A commercial Pt/C catalyst (TEC10E50E, Tanaka Kikinzoku Kogyo, Japan) was used as the anode catalyst, while three types of cathode catalysts were applied and compared: 1) the commercial Pt/C; 2) Pt/MC where Pt was directly deposited on mesoporous carbon (MC); and 3) Pt/Sn(Nb)O2/MC where Pt was deposited on the Sn(Nb)O2 decorated on the MC. The durability test at open circuit voltage (OCV) was performed for 100 h. Before and after the durability test, cell performance, hydrogen crossover current density, and electrochemical surface area (ECSA) were characterized. The degree of membrane degradation was evaluated by measuring fluorine emission rate (FER) by ion chromatography, and membrane thickness and elemental distribution by energy-dispersive x-ray spectroscopy (EDS) coupled with field-emission scanning electron microscopy (FESEM). Results and discussion It was found that the use of Pt/Sn(Nb)O2/MC led to better durability in the OCV durability test. The decrease in OCV using Pt/Sn(Nb)O2/MC was smaller than that using Pt/C and Pt/MC, less than half of that Pt/C, as shown in Figure 1. The membrane thickness with Pt/Sn(Nb)O2/MC after the durability test was the same as that before the durability test, indicating that SnO2 actually suppressed the membrane degradation as shown in Figure 2. EDS elemental analysis confirmed no Sn ion dissolution from the cathode side as shown in Figure 3, also suggesting that SnO2 at the cathode suppressed the membrane degradation. In this presentation, we will quantitatively analyze the membrane degradation in using SnO2 and discuss possible mechanisms. Acknowledgment This paper is based on results obtained from a project, JPNP20003, commissioned by the New Energy and Industrial Technology Development Organization (NEDO).

Numerical Simulations for in-Plane Distribution of Platinum Degradation in Dynamic Operating Conditions of Polymer Electrolyte Membrane Fuel Cells

Predicting degradation of polymer electrolyte membrane fuel cells (PEMFCs) is an important issue from the viewpoint of durability and cost. In fact, in technical roadmaps regarding fuel cell of each country, high targets of durability and performance are set for transport applications especially for heavy duty vehicles. One of important degradation process takes place in catalyst layer. PEMFC catalysts are typically platinum or platinum alloys and consist of nanoparticles to increase the electrochemically active surface area (ECSA). However, these particles are not stable in dynamic operating conditions of the transport applications. A typical degradation mechanism of the catalyst is the electrochemical Ostwald ripening mechanism, and it is known that the platinum particle size distribution shifts to larger particle diameter, resulting in lower ECSA and lower performance. Therefore, predicting platinum catalyst degradation is important to enhance durability of the products.Various numerical models for catalyst degradation of PEMFC have been proposed and used to explore degradation mechanisms. Many of these are zero or one-dimensional models, but factors that affect catalyst degradation, such as temperature, potential, and humidity, have non-uniform distributions in the active area of large-size (several hundred cm2) cells. Furthermore, these factors change unsteadily due to the dynamic operating conditions. In order to predict the degradation of cells, it is necessary to analyze the dynamic behavior of large-area cells, but there are few spatially resolved catalyst degradation predictions [1].In this analysis, transient analysis of a large-area cell for a dynamic operating conditions is performed to predict in-plane distribution of catalyst degradation. Furthermore, we will analyze changes in the power generation performance of the cells when these degradations progress. A three-dimensional numerical simulation model based on our original methodology [2] was used as the performance prediction model for the PEMFC. Catalyst degradation considers the mechanisms of platinum dissolution, oxidation, and platinum oxide dissolution [3]. The in-plane distribution of platinum degradation is predicted from the time-series results of in-plane distributions of temperature, potential, humidity, etc., obtained by the performance prediction model. The performance of the PEMFC at the time of progressive degradation is analyzed by reflecting the effect of ECSA decrease due to platinum degradation in the performance prediction model. Although this method considers only a part of the catalyst degradation process, a path is presented to analyze the distribution of catalyst degradation under dynamic operating conditions in large-area cells.

(Digital Presentation) Operando x-Ray Fluorescence Spectroscopic Study of in-Plane Cerium-Ion Radical Quencher Distribution in Polymer Electrolyte Membranes

One of the challenges for the widespread use of polymer electrolyte fuel cells (PEFCs) is the suppression of chemical degradation in polymer electrolyte membrane (PEM). This degradation is caused by radical species generated due to the reaction of hydrogen peroxide with impurities in the PEM during PEFC operation. One way to suppress the chemical degradation is to add cerium ions to the PEM as radical quencher, 1) which absorbs the radicals through a redox reaction, thereby suppressing PEM degradation caused by radicals. However, it is known that cerium ions move in the in-plane direction due to the humidity and potential distribution, where the depletion of cerium ions causes PEM degradation.2) Therefore, it is required to elucidate the mechanism of cerium ion transport phenomena in the in-plane direction under PEFC operating conditions. High-energy X-ray fluorescence spectroscopy enables operando measurements of cerium distribution by taking advantage of penetration into the fuel cell inside.3) In this study, we have developed operando measurement cells that can observe the distribution phenomenon of cerium ions by using X-ray fluorescence spectroscopy while controlling the humidity gradient in the in-plane direction. Using the developed method, we analyze the cerium ion transport phenomena under the humidity gradient in the membrane electrode assembly (MEA). For the operando X-ray fluorescence measurements, end-plates with an X-ray transmission window and two gas inlets were used to control two different humidity conditions in the PEM. X-ray was incident at 90 degrees from the MEA plane, and a germanium solid-state detector was positioned perpendicular to the incident X-rays to acquire Ce-Ka fluorescence X-ray from the MEA. The catalyst coated membranes composed of a perfluorosulfonic acid-based cation exchange membrane with cerium-ion substitution, coated with a platinum catalyst layer were used. Operando X-ray fluorescence measurements were performed at BL37XU at SPring-8 (Japan). X-rays with incident energy ranging from 40 to 65 keV were used to scan the cell position and detect Ce-Ka fluorescence X-ray. The cell temperature was set at 60°C. Cerium ions in the MEA were found to migrate from the high humidity side to the low humidity side within a range of millimeters in approximately one hour. The migrated cerium ions tended to diffuse into homogeneous state within in a few hours under the uniform humidity condition. Acknowledgments The authors would like to thank M. Toida (Toyota Motor Corporation) for providing samples and for in-depth discussions. References 1) Endoh, ECS Trans., 16, 1229 (2008).2) Y-H. Lai, K. Rahmoeller, J. Hurst, R. Kukreja, M. Atwan, A. Maslyn and C. Gittleman, Electrochem. Soc., 165, F3217 (2018).3) Y. Orikasa, A. Takezawa, K. Amemiya, Y. Tsuji, T. Asaoka, M. Ohki, O. Sekizawa, and K. Nitta, ECS Trans., 109, 109 (2022).

Manufacturing defects in slot die coated polymer electrolyte membrane for fuel cell application

Degradation of polymer electrolyte membrane (PEM) is among the major hurdles for the commercialization of the PEM fuel cells. All of the previous studies on degradation of membrane assumed they were pristine and void of defects that would contribute to the degradation of the cell, although, preexisting defects may seriously contribute to membrane degradation. In this research, manufacturing defects in PEM have been reported. A slot-die coating method using in house roll-to-roll manufacturing system has been used for the membrane manufacturing. Various micron sized defects such as cavities, cracks, non-uniform thickness, contamination, and air-bubble have been observed in the membrane. These defects may create spatial variation in the membrane properties, membrane electrode assembly imperfections and localized weak zones and can serve as initial points for membrane degradation. Further, pristine and defected membranes have been used in the fuel cell and the cell performance have been compared. The defected membrane was found to have a lower open circuit voltage due to higher hydrogen crossover. However, the polarization curve of the fuel cell with the defected membrane was found to be similar to the one with a pristine membrane. Overall, the present research evidence that defects can be present in membrane, however they may not alter the fuel cell performance initially. To understand the long term effect of the defects on fuel cell performance further studies are needed. A linkage between membrane defects and fuel cell performance is crucial in identifying the threshold of the manufacturing defects that are acceptable in the fuel cell assembly and the present study serves as the first step in this regard.

Shunt current protocol to eliminate reversible degradation of polymer electrolyte membrane fuel cells during constant load operations

The objective of this study is to determine the durability of polymer electrolyte membrane fuel cells (PEMFCs) in constant current operation incorporated with regular recovery protocol for eliminating reversible performance loss of membrane electrode assemblies. Effects of operation ‘shunt current protocol’ on PEMFC durability are studied through analyses of the main degradation mechanism based on results of electrochemical characterizations and post-mortem investigations. The voltage of the protected cell using the shunt current protocol is stably preserved under the applied current density for 700 h with less degradation (4.2% of decay ratio), while the performance of the unprotected cell steadily decreased with time (15.1% over 700 h). The substantial performance deterioration of the unprotected cell is mainly attributed to morphology deterioration of the cathode catalyst layer with oxidation of Pt catalysts and chemical degradation of ionomers caused by generation of excessive water from electrochemical oxygen reduction reactions under high-humidity operating conditions. In contrast, the shunt current protocol plays an important role in sustaining high oxygen activity at the cathode catalyst surface, detaching partially covered OH− species on Pt active sites from water oxidation during cell operation caused by periodically applied shunt current for a very short period of 1 s every 5 h. We hope to provide insight into the operation protocol to extend the lifetime of PEMFCs, minimizing conversion from recoverable performance loss to irreversible (permanent) degradation during operation.

Electrochemical characterization of evolving ionomer/electrocatalyst interactions throughout accelerated stress tests

The degradation of polymer electrolyte membrane fuel cells (PEMFCs) catalyst layers used for heavy-duty vehicles was examined using a catalyst-specific accelerated stress test (AST). High surface area carbon supported dispersed Pt (Pt/HSC), annealed Pt (a-Pt/HSC) and PtCo (PtCo/HSC) alloy catalysts were examined over the course of 90,000 cycles by measuring changes in mass activity, O2 transport resistance, electrochemical active surface area (ECSA), catalyst accessibility and ionomer-electrocatalyst interactions. Compared to a-Pt/HSC and Pt/HSC catalyst, the PtCo/HSC showed better initial mass activity, a larger initial mass transport loss, and faster degradation after the first 30k AST cycles, as a large portion of Co leached out during potential cycling. Pt/HSC showed higher initial performance relative to a-Pt/HSC but had faster degradation. STEM characterizations show that the ECSA losses are largely related to Pt dissolution resulting in either catalyst particle growth via the Ostwald ripening mechanism or redeposition in the membrane. Catalyst accessibility measurements showed decreased RH sensitivity for all three samples, while CO impedance measurements revealed a decrease in both Pt-water and carbon-water interactions. This implies that, Pt is either preferentially redepositing on the exterior of the carbon support, or that the ionomer is undergoing morphological changes enabling the enhanced intrusion of mesopores.

WO x /Pt(111) Prepared As PEFC Model Anode Catalyst: Surface Structure and Suppressed Hydrogen Peroxide Generation

Introduction To achieve prolonged lifetime of polymer electrolyte fuel cell (PEFC), hydrogen-peroxide (H2O2)-induced chemical degradation of polymer electrolyte membrane (PEM) is a crucial problem. In general, oxygen (O2) crossover from cathode to anode through the PEM yields H2O2 generation on Pt/C anode catalyst surface through 2-electron pathway of O2 reduction reaction. Thus, mitigation of H2O2 generation on Pt/C surface is a key to suppress the PEM degradation. Trogadas and Ramani reported tungsten trioxide (WO3) addition to Pt/C suppressed H2O2 generation.1 The results suggest that the cooperation of Pt nanoparticles and WO3 located near-by should be effective for suppression of H2O2 generation. However, influence of the oxidation states of tungsten oxide (WO x ) on H2O2 generation at the Pt surface is not fully resolved. Furthermore, it was reported that, although fully oxidized WO3 is stable in acid solution (pH < 2),2 sub-oxides of tungsten (WO x ) could dissolve in Pt-WO x electrochemical system.3 Thus, electrochemical dissolution behaviors of WO x located on Pt substrate surface should be investigated in detail. In this study, we prepare Pt(111) surface that modified by WO x having different oxidation states, and investigated the electrochemical H2O2 generations. Experimental Pt(111) single crystal substrate surface was cleaned by repeated cycles of Ar+ sputtering and annealing in an ultrahigh vacuum (UHV; ~10-8Pa). Subsequently, WO x was deposited on the cleaned Pt(111) via an arc-plasma deposition (APD) method using a W target under partial pressures of O2 (p(O2) = 1×10-1 or 1×10-3 Pa) at ca. 298 K. Deposition amounts of the WO x was estimated to be ca. 1.5 µg/(cm2 of substrate) by using a quartz crystal microbalance installed in the UHV chamber. Then, WO x -modified Pt(111) were annealed at 703 K for 10 min. in UHV. Hereafter, the fabricated samples are referred to as p(O2)-WO x /Pt(111). X-ray photoelectron spectroscopy (XPS) was conducted for the as-fabricated samples in situ in UHV.H2O2 generation amounts were evaluated by the tip generation/substrate collection mode of scanning electrochemical microscope (SECM)4 in O2-saturated 0.1 M HClO4. Generated H2O2 was detected by a Pt tip micro-electrode (diameter: ca. 20 µm, tip potential: 1.26 V vs. RHE) positioned ca. 50 µm at surface normal of the sample electrodes, with sweeping sample electrode potential (E S) at sweep rate of 2 mV/s in negative going direction. Thereafter, potential cycles (PCs: 0.05–1.0 V vs. RHE, sweep rate: 100 mV/s, 100 cycles) were applied to the sample surfaces; then, H2O2 generation was re-evaluated. Finally, the PCs-applied sample surfaces were re-introduced to the UHV chamber to perform XPS measurements. Results and Discussion XP spectra of W4f bands for the as-fabricated and PCs-applied sample surfaces are summarized in Figure. 1 (A). Considering peak areas of deconvoluted components (solid lines), the oxidation states of WO x as-fabricated can be judged mainly to be W6+ and Wx+ (x < 4) for 10-1 Pa-WO x /Pt(111) (a) and 10-3 Pa-WO x /Pt(111) (c), respectively.Figure. 1 (B) shows SECM-estimated H2O2 generations of the WO x /Pt(111) surfaces before (as-fabricated) and after the 100 PCs. As for the as-fabricated surfaces (solid lines), H2O2 detection current (i T) of the Pt tip micro-electrode normalized by respective sample electrode current (i S) is lower for 10-1 Pa-WO x /Pt(111) than 10-3 Pa-WO x /Pt(111), yet normalized i T/|i S|-values for both surfaces are smaller than that of clean Pt(111). The results show that surface modification of Pt(111) by the small amount of WO x , particularly with higher oxidation states, effectively suppressed H2O2 generations. However, the suppression effect degraded by the 100 PCs application for both the electrode surfaces (dotted lines). Compared of W4f bands for the PCs-applied surfaces (Figure. 1 (A) (b, d)) with corresponding as-fabricated ones (a, c), the bands shifted to negative- and positive-binding-energies for 10-1 Pa-WO x /Pt(111) and 10-3 Pa-WO x /Pt(111), respectively, indicating respective reduction and oxidation of the surface WO x by the PCs. In addition, W4f bands for both surfaces decreased in intensity by the PCs application, suggesting decrease in surface amount of the WO x , probably through electrochemical dissolution during the PCs. The PCs-induced changes in oxidation states and amounts of surface WO x should correlate to the degraded H2O2 generation suppressions. Acknowledgement This study was supported by the new energy and industrial technology development organization (NEDO) of Japan, JSPS KAKENHI Grant Number JP21H01645, and JST SPRING Grant Number JPMJSP2114. References P. Trogadas and V. Ramani, J. Electrochem. Soc., 155, B696–B703 (2008).M. Anik and K. Osseo-Asare, J. Electrochem. Soc., 149, B224 (2002).A. J. Martín, A. M. Chaparro, and L. Daza, J. Power Sources, 196, 4187–4192 (2011).C. M. Sánchez-Sánchez and A. J. Bard, Anal. Chem., 81, 8094–8100 (2009). Figure 1

A paradigm shift in CO tolerant catalyst design for fuel cells via introducing defect-controlled carbon molecular sieve layers

The CO poisoning effect leads to serious performance degradation of polymer electrolyte membrane fuel cells using reformed H2 as fuel. To address this problem, PtRu-based catalysts have been extensively studied over decades. Nevertheless, the catalyst stability issue due to Ru dissolution has always risen to the surface. Here, we propose a solution to simultaneously achieve high CO tolerance and stability of PtRu catalysts via introducing a defect-controlled carbon molecular sieve layer. The ultrathin carbon shell with controlled pore structures allows for the selective permeation of H2, providing exceptionally high CO tolerance. The molecular sieve effect is demonstrated even by Ru-free Pt nanoparticles with carbon shells. The carbon shell-encapsulated PtRu nanoparticles also show remarkably improved stability due to the protective carbon layer. This atomic-scale molecular sieve effect is expected to bring about a radical change in the design of CO tolerant catalyst structures that have been entirely dependent on metal composition.

Quantification of effects of performance recovery procedures for polymer electrolyte membrane fuel cells

To distinguish and mitigate the reversible and irreversible performance degradation of polymer electrolyte membrane fuel cells (PEMFCs), the evaluation and investigation of recovery procedures are of great importance. In this work, a methodology for quantification of recovery of reversible voltage losses is presented taken a durability test containing well-defined operation periods interrupted by two recovery procedures as an example. Each recovery procedure is characterized by polarization curves and electrochemical impedance spectroscopy (EIS). To evaluate the influence of the two used recovery procedures on the fuel cell performance, four factors are considered: (i) absolute recovered voltage, (ii) absolute non-recovered voltage, (iii) relative recovery related to previous operation period, and (iv) relative recovery related to the beginning of the test. The results show that voltage recovery quantified using polarization curves and voltage data logged during load cycling lead to the same result. Hence, to briefly assess the efficiency of a recovery procedure in a (narrow) current density range covered by the used load cycling profile, in-depth electrochemical characterization is not required. Moreover, EIS data were used to determine the main mechanisms leading to recovery. By this means, weaknesses of specific recovery procedures can be identified in order to propose specific improvements. • Methodology for quantification of reversible voltage losses versus current density. • Reversible losses using logged voltage data and polarization curves leads to similar results. • Analysis of effect of electrochemical measurements on the evolution of recovery. • Analyzed parameters demonstrate recovered performance losses as well as non-recovered losses.

Comparison of different performance recovery procedures for polymer electrolyte membrane fuel cells

To both extend system lifetime and enable reliable performance benchmarking, recovery procedures are of great importance to distinguish and mitigate the reversible and irreversible performance degradation of polymer electrolyte membrane fuel cells. In this work, three common recovery procedures available in the literature (JRC-based protocol, DOE-based protocol, and overnight rest) being part of a load cycling durability test are characterized by polarization curves and electrochemical impedance spectroscopy. Such direct comparison using same conditions and material has not been published so far. To compare the relative recovery of the three recovery procedures the recovery related to the last operation period, to the beginning of the whole test, and the non-recovered performance loss within each operation period are assessed. Moreover, the mechanisms leading to the various recovery effects are analyzed. Generally, with the contribution of gas purging to water removal, all three recovery procedures reduce greatly mass transfer resistance and recover most of the performance loss in the high current density range. At lower current density, the three procedures differ substantially. In the case of JRC-based protocol, the kinetic losses are recovered by the reduction of Pt oxides and structure change of ionomer by reducing the cathode potential and fuel cell temperature, respectively. The overnight rest results in similar recovery of the performance loss. The DOE-based protocol leads to relatively low recovery of losses in the kinetic region of the polarization curves. Additionally, the effect of the cell operating history is considered.

(PEFC&E 2nd Place Best Poster Winner) Molecular Analysis of Cerium Ion Transport Properties in Polymer Electrolyte Membrane

In order to improve the durability of Polymer electrolyte fuel cell (PEFC), to clarify the cause of deterioration and take measures is necessary. Chemical degradation of polymer electrolyte membrane (PEM) is considered as one of the causes of degradation of PEFC. Hydroxyl radical (OH ∙) is generated by decomposition of hydrogen peroxide generated by oxygen reduction reaction (ORR) during PEFC operation. This OH ∙ has high reactivity, and OH ∙ would degrade the polymer membrane. Based on this degradation mechanism, a method of adding a substance that inactivates OH ∙ before reacting with the polymer (radical scavenger) has been put into practical use. One of the most useful radical scavengers is cerium ions. Cerium ions in the PEM have been reported to migrate during operation and the distribution becomes non-uniform, and degradation occurs at points where there are only a few cerium ions in the membrane. Understanding the cerium ion transport mechanism in PEM is important for controlling cerium ion migration. Because investigating the cerium ion transport in operating PEFC is limited in experiments, nanoscale analysis by numerical simulation is useful to understand the properties of cerium ion transport. In this study, we analyze the distribution and transport properties of cerium ions in the membrane by modeling the PEM containing cerium ions using molecular dynamics (MD) simulations. Diffusion due to the concentration gradient, migration due to the ionomer potential gradient, and convective ion transport due to water crossover are considered as the transport factors of the cerium ions in the membrane. In the present study, the effect of water contents and water flux on the convection ion transport is analyzed. Simulation system is composed of polymer chains, cerium ions, and solvent molecules (water molecules and hydronium ions). Nafion® model was used as polymer and the equivalent weight (EW) was set at 1146. The potential based on the DREIDING force field was adopted for the molecular interaction of Nafion®, and particle mesh Ewald method was used to calculate Coulomb force.The cutoff distance of the LJ potential and Coulomb potential was set at r c=12 Å. The positive charge of the hydronium ion and the cerium ion was balanced with the negative charge of the sulfonic acid group in the system. The number of water molecules was determined by the water content λ which represents the number of water molecules per sulfonate group. The water content was set at λ = 4, 8, 12 and 16, which correspond to the water content in the bulk Nafion® membrane at approximately RH = 30%, 70%, 85% and 100%, respectively. Cerium ion concentration is equivalent to 1.2 wt% cerium ion relative to the polymer mass in the membrane, which is the typical concentration for Ce-Nafion composite membranes. Three-dimensional periodic boundary condition was applied to simulation box. The annealing process was applied in order to equilibrate the system. After annealing, the production run was performed for 15 ns using the NVT ensemble. The temperature in the system was 350 K. The time step was set at 1 fs, and the sampling interval was 10,000 steps. The simulation system is shown in Fig. 1. To evaluate the convection ion transport, an external force was applied to water molecules along the uniaxial direction to obtain flux of water. The carbon atom at the base of the side chain in Nafion® was fixed to prevent the flowing of Nafion®. The convection ion transport coefficient, K convection, is defined as the ratio of the mass flux of cerium ions and the mass flux of water molecules. The mass flux can be obtained by multiplying the concentration of X and the mean velocity of X. In addition, structural analysis will be performed to identify the factors that are dominant in cerium ion transportation. Figure 1

Boosting the Role of Ir in Mitigating Corrosion of Carbon Support by Alloying with Pt

Corrosion of carbon support is one of the most crucial causes of the degradation of polymer electrolyte membrane fuel cells (PEMFCs) utilizing carbon-supported platinum nanoparticles (Pt/C) as a ca...

Anode aging in polymer electrolyte membrane fuel Cells I: Anode monitoring by ElectroChemical impedance spectroscopy

Degradation of polymer electrolyte membrane fuel cells (PEMFC) is investigated through an accelerated stress test (AST) consisting of load-induced humidity cycling combined with open circuit voltage. This combined stressor-AST was designed to mimic real operating conditions. Commercially available membrane-electrode assemblies with an initial voltage of about 0.7 V at 0.5 A cm−2 showed a performance drop of about 900 μV h−1. Their operation was followed by monitoring various parameters such as polarization plots, electrode electrochemical surface area, hydrogen permeation and electrochemical impedance spectra. The results demonstrate that, although initially, the anode may be ignored to model the impedance data, this is no longer possible during the AST. Experimental data show that, beyond classical cathode and membrane degradations, the cell undergoes pronounced anode degradations, that significantly affect the cell performances. Local potential measurements excluded the anode degradation to be linked to electrode potential cycling, the latter remaining always between 0 and 0.2 V vs reference hydrogen electrode. Classical mechanisms of Pt/C degradation may thus not be at stake here, but rather mechanical destabilization of the anode microstructure under wet-dry cycling. The temperature elevation at high current density, known to entail local membrane dehydration may be an aggravating factor.

Tunable catalysis via insulator–metal transition

Suppressing the degradation of polymer electrolyte membrane fuel cells due to anode side-reactions during repetitive cell start-up/shut-down remains a formidable challenge. Now, a phase transition material of WO3 has been explored as a smart catalytic switch to enable highly selective electrocatalysis and improve fuel cell longevity.

Chemical stability enhancement of crown ether grafted sulfonated poly(arylene ether ketone) fuel cell membrane by cerium ion fixation

ABSTRACTIn operation of polymer electrolyte membrane fuel cell or direct methanol fuel cell, ·OH radicals are the major cause for the degradation of polymer electrolyte membrane. In order to enhance its antioxidation stability, cerium ion (Ce3+, CE), an ·OH radical quencher, is introduced to membrane, as it converts the ·OH radicals into inactive chemicals. In this study, aminoethyl‐15‐crown‐5 (CRE) is grafted on the sulfonated poly(arylene ether ketone) (SPAEK) to prevent the migration of CE ions from the membrane for long‐term antioxidation stability, as CRE forms a coordination complex with CE. The chemical and physical structures of the CRE grafted SPAEK are examined using proton nuclear magnetic resonance, energy dispersive X‐ray, and small‐angle X‐ray scattering spectroscopy. The physical properties of the CRE grafted SPAEK membrane are investigated and compared with those of the CRE blended and CE blended ones. While the grafting of CRE does not significantly affect the thermal and mechanical and water uptake behaviors of membranes, it leads to a significant improvement of antidegradation effect compared with other blend systems according to Fenton's test. The proton conductivity decreases with addition of CE but its effect is lessened by introduction of CRE. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019, 57, 101–109

PEM fuel cell degradation effects on the performance of a stand-alone solar energy system

After comparing fresh and degraded performances of Polymer Electrolyte Membrane (PEM) based components of a hydrogen cycle with the help of computational fluid dynamics simulations, recently established stand-alone solar energy system producing hydrogen for energy storage is investigated focusing on the effects of degradation of fuel cells on the overall performance of the system. A complete model of the system has been developed using TRNSYS, and a degraded PEM Fuel Cell Subsystem has been incorporated into the model. Then, the effects of the PEM fuel cell degradation on the overall performance of the energy system are estimated. After reviewing the simulation results, the model shows that the PEM Fuel Cell degradation has a substantial impact on the overall system performance causing a system down time of approximately one month in a typical simulation year. Consequently, the stand-alone system is not capable of operating continuously for a complete year when the PEM fuel cells are degraded. Furthermore, an economic analysis is performed for a project lifetime of 25 years and the Levelized Cost of Electricity (LCE) value of the degraded system is found to be 0.08 $/kWh higher than the newly established system. Nevertheless, LCE calculations that are repeated for declining PV panel costs show that the considered hybrid system may be an economically competitive alternative to conventional diesel generators, even when the degradation of PEM based components and their regular maintenance are considered.

Degradation of Polymer Electrolyte Membrane Fuel Cell By Siloxane in Biogas

We studied the degradation and durability of polymer electrolyte membrane fuel cell (PEMFC) at membrane-electrode-assembly (MEA) level by injection of octamethylcyclotetrasiloxane (D4) as a representative siloxane, which has been found in many industrial and personal products. Specifically, i) GC/MS analysis demonstrated that the ring-opening polymerization of D4 could result in the formation of various linear and cyclic siloxanes in both electrodes of MEA; ii) post-test analysis revealed that the transformed siloxanes were transported from the anode to the cathode via free-volumes in the polymer membrane; iii) RDE measurement and DFT calculation revealed that D4 was not directly responsible for the electrocatalytic activity of Pt; iv) electrochemical analysis demonstrated that the residual methyl groups of siloxane and various siloxanes did not hinder the proton transport in the polymer membrane; and v) siloxanes accumulated in the primary and secondary pores with the exception of an external surface of carbon, causing an increase in the oxygen reactant’s resistance and resulting in a decrease of the cell performance. In addition, we confirmed that injection of D4 did not affect the carbon corrosion adversely because the siloxane had little influence on water sorption in the catalyst layer.

Quantitative Evaluation of Chemical Degradation of Polymer Electrolyte Membrane for Fuel Cells and the Method of Lifetime Prediction

A fuel cell vehicle (FCV) is a vehicle that utilizes fuel cells as its power source. Development of fuel cells for FCV is always aimed at a more compactsize by means of higher power density and longer driving range through enhanced energy efficiency. Fuel cells also require endurance reliability comparable to internal combustion engines, and cost reductions will be required to encourage widespread consumer use. The power generating part of a fuel cell is called the membrane electrode assembly (MEA), which consists of a polymer electrolyte membrane (PEM) and electrodes with a catalyst. The performance of the fuel cell is mainly determined by the activity of the catalyst on each electrode and by the proton conductivity of the PEM. Because the performance, energy efficiency, durability, and cost of fuel cells are all related and involve trade-offs, optimization of the PEM design cannot be readily achieved. A particular issue that has faced optimization of the PEM design is the amount of time it takes to estimate PEM durability. The only method available has been to test it over a long period of time, and this is why FCV has had such an extended development period. In order to optimize the PEM design in less time, a new method capable of estimating durability without durability testing must be developed. To this aim, we first made it possible to quantitatively evaluate the rate of chemical degradation of PEM to estimate durability and predict its lifetime. Next, we developed a mathematical formula for the rate of chemical degradation of the PEM to predict its service life. Two possible reaction mechanisms have been proposed as the chemical degradation of the perfluorosulfonic acid (PFSA) membrane. One is the scission of the main chains of the polymer and the other is an unzipping reaction in which the end groups of the main chains progressively degrade. In order to quantitatively estimate the rate of chemical degradation of the PFSA membrane, we numerically modelized these two reaction mechanisms. When a scission of the main chain occurs, the location of the split becomes two new end groups, which suggests that the number of points of origin for an unzipping reaction increases exponentially. The fluoride release behavior was calculated based on this model. As a result, we confirmed that the experimental fluoride release data can be explained by these two reaction mechanisms of chemical degradation. In order to describe the fluoride release quantity F as a function of duration time t, exponential function Eq. (1) was formulated to closely match the experimental results. F(t) = a [exp(b･t) - 1] (1) One of the factors that influence the chemical degradation rate of the PEM is contamination by metal ions. Adding a radical quencher is a well-known method to assure durability of the PEM by mitigating chemical degradation. In order to estimate the acceleration of PEM degradation caused by metal ion contamination or the mitigation effects offered by a radical quencher, durability testing was carried out using different levels of metal ion contamination or radical quencher additives. The results of these tests show that fluoride release behavior can be represented by Eq. (1) by fixing coefficient b and varying only coefficient a. Coefficient ais therefore defined as the accelerating factor of chemical degradation. The accelerating factor can be obtained by making Eq. (1) fit the experimental fluoride release data, and it can be used as an indicator of the severity of chemical degradation. Introducing the concept of accelerating factor to represent the fluoride release rate made it possible to deal quantitatively with the effect of factors that accelerate or mitigate chemical degradation. Moreover, the size of acceleration or mitigation effect caused by multiple factors can be indicated with a single variable. We confirmed changes in accelerating factor during FCV operation with bench testing, which demonstrated the driving modes of an actual vehicle using a fuel cell stack with the same specifications as stacks installed in actual vehicles. The fluoride release rate was calculated from the accelerating factor envisioning actual FCV operation. Fluoride release behavior can be converted into membrane thickness loss. In this way, decrease in membrane thickness during FCV operation and the lifetime of the PEM can be predicted using calculations.

Degradation of polymer electrolyte membrane fuel cell by siloxane in biogas

We studied the degradation and durability of polymer electrolyte membrane fuel cell (PEMFC) at membrane-electrode-assembly (MEA) level by injection of octamethylcyclotetrasiloxane (D4) as a representative siloxane, which has been found in many industrial and personal products. Specifically, i) GC/MS analysis demonstrated that the ring-opening polymerization of D4 could result in the formation of various linear and cyclic siloxanes in both electrodes of MEA; ii) post-test analysis revealed that the transformed siloxanes were transported from the anode to the cathode via free-volumes in the polymer membrane; iii) RDE measurement and DFT calculation revealed that D4 was not directly responsible for the electrocatalytic activity of Pt; iv) electrochemical analysis demonstrated that the residual methyl groups of siloxane and various siloxanes did not hinder the proton transport in the polymer membrane; and v) siloxanes accumulated in the primary and secondary pores with the exception of an external surface of carbon, causing an increase in the oxygen reactant's resistance and resulting in a decrease of the cell performance. In addition, we confirmed that injection of D4 did not affect the carbon corrosion adversely because the siloxane had little influence on water sorption in the catalyst layer.