New Insights into Perfluorinated Sulfonic-Acid Ionomers

Self Cite

Water management is a serious concern for alkaline-exchange-membrane fuel cells (AEMFCs) because water is a reactant in the alkaline oxygen-reduction reaction and hydroxide conduction in alkaline-exchange membranes is highly hydration dependent. In this paper, we develop and use a multiphysics, multiphase model to explore water management in AEMFCs. We demonstrate that the low performance is mostly caused by extremely non-uniform distribution of water in the ionomer phase. A sensitivity analysis of design parameters including humidification strategies, membrane properties, and water transport resistance was undertaken to explore possible optimization strategies. Furthermore, the strategy and issues of reducing bicarbonate/carbonate buildup in the membrane-electrode assembly with CO 2 from air is demonstrated based on the model prediction. Overall, mathematical modeling is used to explore trends and strategies to overcome performance bottlenecks and help enable AEMFC commercialization. Among existing fuel-cell types, alkaline-exchange-membrane fuel cells (AEMFCs) or hydroxide-exchange-membrane fuel cells (HEMFCs) have intriguing features as compared to proton-exchangemembrane fuel cells (PEMFCs). Their advantage is mainly the possibility of using non-noble catalysts due to faster oxygenreduction-reaction (ORR) kinetics in alkaline media than in acidic media 1-4 as well as perhaps the use of less expensive hydrocarbon membranes. Disadvantages of AEMFCs include lower hydrogenoxidation-reaction (HOR) kinetics, more complicated water management, and lower intrinsic conductivity for hydroxide compared to proton transport. [5][6][7][8][9][10][11][12][13] In addition, CO 2 in the air reacts with hydroxide ions to form bicarbonate and carbonate ions, possibly hindering the terrestrial development of AEMFCs by removing hydroxide available for HOR and further limiting hydroxide conductivity. 14,15 Compared to conventional alkaline fuel cells (i.e., using a liquid electrolyte of potassium hydroxide), AEMFCs use a polymeric membrane and should have better tolerance for CO 2 because the precipitate K 2 CO 3 is absent in the AEMFC due to the lack of mobile cations in the membrane. 16 However, AEMFCs still suffer a decline in performance due to bicarbonate/carbonate formation in the membrane and catalyst layers (CLs) leading to additional ohmic and kinetic losses. [13][14][15] In an AEMFC, the following electrochemical reactions occur in the CLs as follows:Net :Hydrogen combines with hydroxide ions to produce water in the anode. Oxygen reacts with water to generate hydroxide ions in the cathode. Along with the transport of ions from cathode to anode, electro-osmosis moves water from cathode to anode. As shown in the reactions, AEMFCs are expected to have more complicated water management. Water production by the HOR and electro-osmosis may cause flooding in the anode. Since water is a stoichiometric reactant at the cathode, dehydration may occur and limit ORR kinetics as well as ionic conduction in the membrane and cathode CL, ...

Section: Mathematical Modelmentioning

confidence: 99%

“…Finally, possible existence of interfacial effects and "skin" resistances could be important if they exist in AEMs as they are believed to be in PEMs. 27,41,42 Figure 8. Spatial distribution of ionomer water content in the AEM and cCL at 0.6 V for different asymmetric RHs and dominating resistances.…”

Section: Variablementioning

confidence: 99%

Elucidating Performance Limitations in Alkaline-Exchange- Membrane Fuel Cells

Shiau

Zenyuk

Weber

2017

Self Cite

“…One can find the summary of many of these publications in review articles and book chapters. [1][2][3][4] Among these, many studies on the subject of membrane z E-mail: yeh-hung.lai@gm.com durability have relied on single-stressor tests that produce either mechanical or chemical stress under various test conditions. For material screening and development, the U.S. Department of Energy (DOE) has recommended two single-stressor accelerated stress tests (ASTs) to evaluate a membrane's chemical and mechanical robustness.…”

mentioning

confidence: 99%

Accelerated Stress Testing of Fuel Cell Membranes Subjected to Combined Mechanical/Chemical Stressors and Cerium Migration

Lai

Rahmoeller

Hurst

et al. 2018

A highly accelerated stress test (HAST) has been developed to generate local stressful conditions that are representative of those in automotive fuel cell stacks. Using a 50-cm 2 cell cycled between 0.05 and 1.2 A/cm 2 with a low inlet RH in the co-flow configuration, the HAST creates a distribution of combined mechanical/chemical stressors in the membrane with the maximum chemical stress occurring near the gas inlets and the maximum mechanical stress near the outlets. Conducting HASTs using a current distribution measurement tool and a shorting/crossover diagnostic method to track the state of health of a robust membrane containing both a mechanical support and a chemical stabilizing additive, the result shows that the membrane location with the most severe thinning coincides with that of the deepest membrane hydration cycling. Upon examination of the cerium redistribution patterns after the test, it was found that the severe humidity cycling generated by the HAST condition near the outlet region not only generated the highest membrane mechanical stress but also resulted in the strongest water flux, which may cause local depletion of the cerium added as chemical stabilizer. One of the key challenges facing the commercialization of automotive fuel cells is the development of membrane electrode assemblies (MEAs) that can meet durability targets. In proton exchange membrane (PEM) fuel cells, the PEM serves to conduct protons from the anode to the cathode of the fuel cell while simultaneously insulating electronic current from passing across the membrane as well as preventing crossover of the reactant gases, H 2 and O 2 . State-of-the-art PEM fuel cells for high power density operation utilize perfluorosulfonic acid (PFSA) membranes that are typically no more than 25 μm thick. To be viable for automotive applications, these membranes must survive 10 years in a vehicle and 8000 hours of operation including transient operation with start-stop and freeze-thaw cycles. Fuel cells cannot operate effectively when gas can permeate the membrane through microscopic pinholes or excessively reduced thickness. Ultimately, fuel cells can fail because such excessive crossover leaks develop and propagate within the polymer membranes. Fuel cells can also fail if electronic current passes through the membranes and causes the system to short. It is thus critical that these membranes are sufficiently robust to thinning, cracking and thermal decomposition over the range of conditions experienced during fuel cell operation. In automotive fuel cell systems, there are three primary root causes of membrane failure: (1) Chemical degradation: polymer decomposition caused by the direct attack on the polymer from radical species generated as byproducts or side reactions of the fuel cell electrochemical reactions; (2) Mechanical degradation: membrane fracture caused by cyclic fatigue stresses imposed on the membrane via hygrothermal fluctuations in a constrained cell; and (3) Thermal degradation: membrane degradation caused by ohmic heating thr...

“…Proton flux to the cathode is reduced not only due to proton site occupancy but also decreased proton mobility caused by cationic interaction. [14][15][16] Strong interactions between the cations and sulfonic acid sites may also induce ionic cross-linking throughout the ionomer network. In addition, the reduced hydrophilicity of metal cations in comparison to protons reduces the water content (λ) in the membrane.…”

mentioning

confidence: 99%

Electrode Edge Cobalt Cation Migration in an Operating Fuel Cell: An In Situ Micro-X-ray Fluorescence Study

Cai

Ziegelbauer

Baker

et al. 2018

PtCo-alloy cathode electrocatalysts release Co cations under operation, and the presence of these cations in the membrane electrode assembly (MEA) can result in large performance losses. It is unlikely that these cations are static, but change positions depending on operating conditions. A thorough accounting of these Co cation positions and concentrations has been impossible to obtain owing to the inability to monitor these processes in operando. Indeed, the environment (water and ion content, potential, and temperature) within a fuel cell varies widely from inlet to outlet, from anode to cathode, and from active to inactive area. Synchrotron micro-X-ray fluorescence (μ-XRF) was leveraged to directly monitor Co 2+ transport in an operating H 2 /air MEA for the first time. A Nafion membrane was exchanged to a known Co cation capacity, and standard Pt/C electrocatalysts were utilized for both electrodes. Co Kα 1 XRF maps revealed through-plane transient Co transport responses driven by cell potential and current density. Because of the cell design and imaging geometry, the distributions were strongly impacted by the MEA edge configuration. These findings will drive future imaging cell designs to allow for quantitative mapping of cation through-plane distributions during operation. In pursuit of vehicle electrification, considerable efforts have been devoted to elucidating the degradation mechanisms of proton exchange membrane (PEM) fuel cells. [1][2][3][4][5][6][7] While the majority of these studies are based around post-mortem analyses of fuel cell materials (e.g. changes in nanoparticle sizes and shape distributions), a full accounting of the losses that contribute to membrane-electrode assembly (MEA) performance degradation requires looking beyond wellresearched catalyst nanoparticle degradation processes.2,8 Indeed, the performance of a PEM fuel cell is affected by many internal and external factors, such as fuel cell design and assembly, material degradation, operational conditions, and impurities or contaminants.9 The above-mentioned factors interact, and they are closely related to the cation activities in the fuel cell. In the current PEM fuel cell system, in addition to protons, various other cations are introduced from a variety of sources. For example, cerium ions are intentionally introduced to the MEA to improve membrane durability by neutralizing radical species before they attack the ionomer. 10,11 In contrast, when Pt-alloy catalysts are used, cobalt, nickel, or other 3d transition metal cations can leach out during fuel cell operation.1,3,5 Finally, cell component corrosion or impurities in the reactant/fuel flows may serve as additional cation sources. 12,13 In general, these cations exhibit greater affinities for the sulfonic acid groups in the ionomers than protons. Proton flux to the cathode is reduced not only due to proton site occupancy but also decreased proton mobility caused by cationic interaction.14-16 Strong interactions between the cations and sulfonic acid sites may also induce io...