Anion Exchange Membrane Fuel Cells Performance Research Articles

Solvent Effects on the Catalyst Ink and Layer Microstructure for Anion Exchange Membrane Fuel Cells.

Understanding the complex solvent effects on the microstructures of ink and catalyst layer (CL) is crucial for the development of high-performance anion exchange membrane fuel cells (AEMFCs). Herein, we study the solvent effects within the binary solvent ink system composed of water, isopropyl alcohol (IPA), commercial anion exchange ionomer, and Pt/C catalyst. The results show that the Pt/C particles and ionomer tend to form large aggregates wrapped with a thick ionomer layer in IPA-rich ink and promote the formation of large mesopores within the CL. With the increase of the water content in the ink, Pt/C particles are more likely to bridge to each other through wrapped FAA to form a well-connected three-dimensional network. The CL fabricated using water-rich ink shows smaller pores, higher porosity, and a more homogeneous ionomer network without the formation of large aggregates. Based on these results, we propose that the properties of the solvent mixture, including dielectric constant (ε) and solubility parameter (δ), affect the coulomb interaction of charged particles and surface tension at interfaces, which in turn affects the microstructure of ink and CL. By leveraging the solvent effects, we optimize the CL microstructures and improve the performance of AEMFC. These results may guide the rational design and fabrication of AEMFCs.

Effects of fabrication parameters of membrane–electrode assembly for high-performance anion exchange membrane fuel cells

High-performance membrane–electrode assemblies (MEAs) are required for anion exchange membrane fuel cells (AEMFCs). In this study, we investigated different electrode coating methods and the effect of the ionomer/carbon (I/C) ratio to determine a suitable method for use with different anion exchange membranes (AEMs). The MEAs were manufactured by spray coating using catalyst-coated substrate (CCS) and catalyst-coated membrane (CCM) methods. By controlling the I/C ratio in the electrode, a reduction in the ohmic resistance was possible due to changes in the ionic path at the membrane–electrode interface, which affected the electrode morphology. In particular, for the CCS method, the ohmic resistance can be reduced by increasing the I/C ratio, whereas, for the CCM method, which yields a relatively low ohmic resistance, the I/C ratio has a greater effect on the electrode morphology than the resistance at the membrane/electrode interface, thus improving AEMFC performance. Finally, MEAs were fabricated from four commercially available AEMs using CCS and CCM, and the choice of method was found to be dependent on the properties of the AEMs. Overall, selecting a fabrication method optimized for the MEA requires understanding the electrode and AEM properties.

How can we design anion-exchange membranes to achieve longer fuel cell lifetime?

The substantial advancements and availability of new cost-effective materials have drawn significant attention to the anion-exchange membrane fuel cell (AEMFC) technology. The anion exchange membrane (AEM) is a core component in AEMFCs, essential for conducting hydroxide ions and controlling water transport between the fuel cell electrodes. In this study, we apply a numerical model to investigate the relationship and sensitivity of AEMFC performance and its stability to key AEM properties. Our findings show that membrane maximum hydration level (λmax) has the most significant impact on AEMFC lifetime, followed by membrane ion-exchange capacity, water diffusivity, and membrane thickness. We also demonstrate the significance of improving the stability of the functional group to AEMFC lifetime, while AEM hydroxide conductivity shows a negligible effect on AEMFC lifetime. Finally, we provide a simple algebraic functional relationship between key dimensionless parameters as well as a machine learning-based analysis of the relationship between AEM parameters and AEMFC lifetime. Through these analyses, we calculate the lifetime of selected membranes from the literature and compare them to the measured operation time. This study summarizes and highlights important AEM property targets suggested to improve AEM design to boost the performance stability of AEMFCs.

(Digital Presentation) Diagnostic Study of the Key Performance-Limiting Factors in Anion Exchange Membrane Fuel Cells

Anion exchange membrane fuel cells (AEMFC) have received significant attention for their promised low production cost, largely enabled by the facile kinetics of the oxygen reduction reaction in alkaline electrolytes and the broader selection of potential ionomers.1,2 The performance of AEMFCs has been improved significantly by utilizing AEM with high ion exchange capacity (IEC) and optimizing membrane electrode structures.3 However, some fundamental issues limiting the performance of AEMFCs need to be better understood, especially the reaction kinetics at the catalyst-ionomer interface and the transport process at the multi-phase boundaries in the membrane electrode assembly (MEA).4,5 In this talk, we will present a systematic diagnostic study of the critical performance-limiting factors in AEMFC. We will discuss the reaction kinetics at the Pt/ionomer interface and the oxygen transport characteristics of the ionomer layer based on electrochemical data. In addition, we correlate the electrode pore structure, reaction kinetics, and mass transfer behavior with the cell performance based on the analysis of a series of MEA with varied multi-phase boundary designs. Further, experimental data will be presented with decoupled polarization behavior of both the anode and cathode of working AEMFCs. The results reveal that the deteriorated mass transport at the anode and the restricted reaction kinetic at the cathode are the crucial factors limiting the performance of AEMFC, which need to be addressed with high priority. References Wang, Y-J., Qiao, J., Baker, R., Zhang J. Chem. Soc. Rev., 2013, 42, 5768.Chen, N., Hu, C., Lee, Y. M. et al., Angew. Chem. Int. Ed. 2021, 60, 7710–7718.Huang, G., Mandal, M., Mustain, W. E., Kohl P. A. et al., J. Electrochem. Soc. 2019, 166, F637–F644.Dekel, D. R., J. Power Sources 2018, 375, 158e169.Jiang, H., Luo, R., Li, Y., Chen, W. EcoMat. 2022, 4, e12199. Acknowledgement Financial support from the National Key R&D Program of China (No. 2021YFB4001203) is greatly acknowledged.

Migration and Precipitation of Platinum in Anion-Exchange Membrane Fuel Cells.

Despite the recent progress in increasing the power generation of Anion-exchange membrane fuel cells (AEMFCs), their durability is still far lower than that of Proton exchange membrane fuel cells (PEMFCs). Using the complementary techniques of X-ray micro-computed tomography (CT), Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray (EDX) spectroscopy, we have identified Pt ion migration as an important factor to explain the decay in performance of AEMFCs. In alkaline media Pt+2 ions are easily formed which then either undergo dissolution into the carbon support or migrate to the membrane. In contrast to PEMFCs, where hydrogen cross over reduces the ions forming a vertical "Pt line" within the membrane, the ions in the AEM are trapped by charged groups within the membrane, leading to disintegration of the membrane and failure. Diffusion of the metal components is still observed when the Pt/C of the cathode is substituted with a FeCo-N-C catalyst, but in this case the Fe and Co ions are not trapped within the membrane, but rather migrate into the anode, thereby increasing the stability of the membrane.

Migration and Precipitation of Platinum in Anion‐Exchange Membrane Fuel Cells

AbstractDespite the recent progress in increasing the power generation of Anion‐exchange membrane fuel cells (AEMFCs), their durability is still far lower than that of Proton exchange membrane fuel cells (PEMFCs). Using the complementary techniques of X‐ray micro‐computed tomography (CT), Scanning Electron Microscopy (SEM) and Energy Dispersive X‐ray (EDX) spectroscopy, we have identified Pt ion migration as an important factor to explain the decay in performance of AEMFCs. In alkaline media Pt+2 ions are easily formed which then either undergo dissolution into the carbon support or migrate to the membrane. In contrast to PEMFCs, where hydrogen cross over reduces the ions forming a vertical “Pt line” within the membrane, the ions in the AEM are trapped by charged groups within the membrane, leading to disintegration of the membrane and failure. Diffusion of the metal components is still observed when the Pt/C of the cathode is substituted with a FeCo−N−C catalyst, but in this case the Fe and Co ions are not trapped within the membrane, but rather migrate into the anode, thereby increasing the stability of the membrane.

Transition-Metal and Nitrogen-Doped Carbon Nanotube/Graphene Composites as Cathode Catalysts for Anion-Exchange Membrane Fuel Cells

Transition-metal and nitrogen-doped graphene-like material and carbon nanotube (M-N-Gra/CNT) composites are prepared, characterized, and used as cathode catalysts in anion-exchange membrane fuel cells (AEMFCs). Melamine as a nitrogen source and cheap iron and cobalt salts as metal precursors are used for doping via high-temperature pyrolysis. The success of doping is proven by several physicochemical analysis methods, and the catalyst materials possess rather similar textural properties. The initial assessment of the oxygen reduction reaction activity using the rotating disk electrode method shows that Fe-N-Gra/CNT, Co-N-Gra/CNT, and CoFe-N-Gra/CNT materials have very similar electrocatalytic performances in alkaline media as well as excellent short-term stability but a different yield of HO2– formation. The M-N-Gra/CNT materials as cathode catalysts together with the Aemion+ reinforced anion-exchange membrane exhibit very good AEMFC performance, especially CoFe-N-Gra/CNT, comparable to that of Pt/C, reaching a peak power density of 638 mW cm–2. Such an excellent fuel cell performance of the M-N-Gra/CNT catalyst materials is attributed to the presence of M–Nx sites, carbon-encapsulated transition-metal nanoparticles, and feasible nitrogen-containing moieties.

Recent advances in platinum-group-metal based electrocatalysts for alkaline hydrogen oxidation reaction

The reaction kinetics of hydrogen oxidation reactions (HOR) unfavorably decreases by 2~3 orders of magnitude under alkaline conditions, even on the most active platinum-group-metal (PGM) electrocatalysts. This sticky problem severely restricts the efficiency and commercialization of anion-exchange membrane fuel cells (AEMFCs). So far, no other material has HOR electrocatalytic performance comparable to PGM-based electrocatalysts. Forced by the scarce reserves and high prices of PGMs, it is significant to elaborately design and synthesize PGM-based electrocatalysts with ultimately atomic utilization and substantially improved alkaline HOR performance. In this review, we summarize recent advances in the structure engineering approaches to synthesis of advanced PGM-based nanocatalysts toward enhanced alkaline HOR performance. The generally acknowledged catalytic mechanisms with corresponding activity descriptors are reviewed firstly to deeply understand the discrepancies in the HOR kinetics of alkaline and acidic reactions. Then, several representative strategies are emphasized and discussed at length by changing the chemical and coordination environment and size/morphology of nanocatalysts. Meanwhile, the influence factors for the performance of AEMFC devices constructed by PGM-based anode catalysts are briefly highlighted. In conclusion, strategies for boosting the electrocatalytic performance and challenges on the roles of catalytic mechanism insights and practical AEMFC applications are finally outlined. We hope this review will guide the design and catalytic mechanism research of novel PGM-based alkaline HOR catalysts, thereby promoting their further development and application in AEMFC technologies.

Water transport in anion and proton exchange membranes

Water balance in anion exchange membrane fuel cells (AEMFCs) is crucial because water not only is produced in the anode but also functions as a reactant in the cathode. Therefore, accurate measurement of AEM water transport properties is important for AEM design to improve AEMFC performance and durability. Very few studies report water transport properties of AEMs; even in those limited studies, interfacial transport rates were either not considered in data analysis or not given as a function of water activity. In this work, the liquid–vapor permeation method was used to determine the water flux across the Aemion® AH1-HNN8-50-X, Fumapem® FAA-3-30/50, and Versogen™ PiperION-A40. Using three numerical models, the results were analyzed to understand whether diffusion or interfacial transport resistances were limiting, and the values were estimated. Our results indicate that interfacial transport is limiting; therefore, the interfacial exchange rate and its activation energy were determined. Water desorption rate of AH1-HNN8-50-X is similar to Nafion® N115 and N117 (10−5–10−4 cm/s), and the activation energy for this process is also similar at 53.4 kJ/mol. On the other hand, FAA-3-30/50 and PiperION-A40 exhibit two to three times faster desorption and a lower activation energy: 46.0, 41.8, and 46.8 kJ/mol, respectively.

Cobalt-, iron- and nitrogen-containing ordered mesoporous carbon-based catalysts for anion-exchange membrane fuel cell cathode

Cobalt-, iron- and nitrogen-doped ordered mesoporous carbon (OMC)-based electrocatalysts are prepared, characterized, and used as cathode catalysts in anion-exchange membrane fuel cell (AEMFC). The OMC material is synthesized using a green and simple route via soft-template method and without the usage of harsh chemicals. To study the effect of porous structure of carbon support on the electrocatalytic properties, the OMC material is also mixed either with carbide-derived carbon (CDC) or carbon nanotubes (CNTs). Doping of carbon nanomaterials is done via high-temperature pyrolysis in the presence of cobalt and iron acetate as well as 1,10-phenanthroline. The physico-chemical characterization shows that the preparation of OMC and subsequent doping of nanocarbons has been successful, and the catalysts contain single-atom M-Nx centres. The initial assessment employing the rotating disc electrode method indicates that all three doped catalyst materials exhibit very high electrocatalytic activity toward the oxygen reduction reaction (ORR) in alkaline media and good stability after 10,000 potential cycles. In AEMFC testing with AEMION+ anion exchange membrane, the prepared cathode catalysts show good performance with CoFe-N-OMC/CNT obtaining the highest peak power density of 336 mW cm–2. Slightly lower AEMFC performance observed for CoFe-N-OMC and CoFe-N-OMC/CDC cathodes indicates that it is influenced by the catalyst's porous structure. It can be concluded that the OMC-based materials are promising cathode catalysts for AEMFC application.

Influence of Gas Humidity and Flow Rate on the Water Movement Direction through an Anion Exchange Membrane Fuel Cell

Environmental concerns lead to a continually growing demand for clean energy generation. Thus, alternative energy sources or green fuels attract attention. The anion exchange membrane fuel cell (AEMFC) emerged as an excellent alternative due to its air CO2-free emissions, low-cost electrocatalyst used at the cathode, and industrial-scale membrane production development [1-3].Many efforts have been made to have AEMFC performances like those already shown by the proton exchange membrane fuel cell (PEMFC). Excellent results have been obtained for the lasted technology regarding power densities and operational cell life [4]. It should be recognized that due to the nature of the electrode reactions in AEMFC, Eqs. 1-2, the performance enhancement of the overall cell reaction, Eq. 3, depends not only on the selected catalysts and ionomers/membranes but also on the cell's water management. The complexity of water management relies on the simultaneous production and consumption of water in anode and cathode, respectively, where the production is two-fold higher than the consumption. This effect might lead to anode flooding during the hydrogen oxidation reaction (HRR), Eq. 1. In contrast, during oxygen reduction reaction (OER), Eq. 2, the risk of membrane drying is latent as long as a water imbalance arises between both anodic and cathodic sides. It has been confirmed quantitatively that water direction movement could go from anode to cathode in a hydrophilic membrane (diffusion transport mode); however, using a membrane with hydrophobic properties promotes the water to travel from cathode to anode (electro-osmotic drag, EOD mode). In this context, it is convenient to identify the water direction trend under variable selected operating conditions. In particular, achieving the anode-to-cathode water direction would improve the AEMFC performance by avoiding flooding on the anode by cathode hydration through water diffusion through the membrane. Therefore, this work tested the influence of symmetric and asymmetric flow rates and relative humidities within an AEMFC using a hydrophilic membrane. According to both modes of transport, the AEMFC performance was studied using polarization curves.The experimental procedure involved trapping water out of each half-cell during a chronoamperometry in steady-state conditions. Also, the water incoming was quantified independently for each selected operating condition in order to make a mass balance (Figure 1a). The variable operating conditions were the reactant flow rate, low (125 cm3 min-1) and high (250 cm3 min-1), and the reactant humidity level.In Figure 1b, the results demonstrated that the EOD predominance was promoted when low symmetric flow rates (125 cm3 min-1 for anode and cathode) were used in the AEMFC, suggesting membrane dehydration. On the contrary, the water diffusion dominance appears under high (250 cm3 min-1) symmetric and asymmetric flow rates between anode and cathode; under these conditions, the membrane dehydration might be suppressed by anode flooding alleviation. Once the flow conditions are given, no changes in water transport mode were detected for changes in the humidity level (Figure 1c-d). The AEMFC performance was affected by the water transport mechanism, giving lower peak power densities for EOD-dominated systems (35.2 mW cm-2 at 60°C, Figure 1e) than those obtained by diffusion (62.8 mW cm-2 at 60°C, Figure 1f). The most suitable operating condition is under water-diffusion at non-saturated streams (below 65%), where a decrease in performance is experienced as the humidifier temperature increases from 60°C to 80°C (supersaturated flow), with peak power densities of 62.8 and 3.7 mW cm-2, respectively.

Optimization of Ionomer Content in Membrane Electrode Assemblies and Its Impact on the Performance in Anion Exchange Membrane Fuel Cells

Anion exchange membrane fuel cells (AEMFCs) have recently attracted significant attention as low-cost alternative fuel cells to traditional proton exchange membrane fuel cells due to the possible use of platinum-group metal-free electrocatalysts [1]. Over the past decade, new materials dedicated to the alkaline medium, such as anion exchange membranes (AEMs) and anion exchange ionomers (AEIs), have been developed and studied in AEMFCs [2, 3]. However, only a few AEMs and AEIs are commercially available, and there are not ready to use catalyst coated membranes (CCMs) and/or gas diffusion electrodes (GDEs) with the wished AEMs or AEIs.In order to manufacture CCMs and/or GDEs on the basis of commercial materials, catalyst inks need to be prepared before testing them in AEMFC. It is well known that the composition of catalytic ink and the way to deposit it can influence the interaction between solvent, ionomer and catalyst particles during solvent evaporation and thus on the final structure and morphology of the catalyst layer. However, there are only a few papers dealing with catalyst layer compositions and structures for AEMFCs [4, 5], probably due to the recent development of alkaline fuel cells and new dedicated materials such as AEM and AEI.Since the ionomer/catalyst particle interface plays a crucial role in electrochemical reactions, it is essential to understand the impact of ionomer content on AEMFC performance as well as on water management. For this purpose, catalytic inks were prepared with different amounts of ionomer, ranging from 13 to 33 % in ratio. During this work, Pt / C (40 % in wt) catalyst as well as Aemion® membranes and ionomers were used. Different CCMs and GDEs were manufactured at 60 °C using a commercial ultrasonic spray coating bench. The morphology of the catalyst layers was characterized by scanning electron microscopy, and the thickness of the deposition was measured by a profilometer. Before testing in AEMFC, all prepared samples and membranes were converted to OH- form for 48 h in KOH 3M (the solution was changed every 12h). The performance of the prepared CCMs and GDEs was studied in a home-made AEMFC bench after an activation step. The results shown in Fig.1 highlight that: (i) the ionomer content in the catalyst layers affects the performance of the fuel cell, regardless of the coated support (membrane or GDL), (ii) concerning CCMs-based MEAs, the lower the ionomer content, the better the performance via the polarization curve, (iii) CCMs and GDEs-based MEAs do not behave similarly, (iv) GDEs-based MEAs show high OCV and high voltage for a given current density in comparison with CCMs-based MEAs, (v) concerning GDEs-based MEAs, the variation of the ionomer content in catalyst layer affects less the OCV value than the water management, and (vi) the water management of GDEs-based MEAs seems depend on the relative humidity of both gases and ionomer content in catalyst layers. This work is still under investigation. We will attempt to understand the relationship between the membrane/ionomer under different relative humidity and gas flow rates.[1] H. A. Firouzjaie and W. E. Mustain, “Catalytic Advantages, Challenges, and Priorities in Alkaline Membrane Fuel Cells,” ACS Catal., pp. 225–234, 2019, doi: 10.1021/acscatal.9b03892.[2] J. R. Varcoe et al., “Anion-exchange membranes in electrochemical energy systems †,” 2014, doi: 10.1039/c4ee01303d.[3] N. Chen and Y. M. Lee, “Anion exchange polyelectrolytes for membranes and ionomers,” Prog. Polym. Sci., vol. 113, p. 101345, Feb. 2021, doi: 10.1016/j.progpolymsci.2020.101345.[4] J. Hyun et al., “Tailoring catalyst layer structures for anion exchange membrane fuel cells by controlling the size of ionomer aggreates in dispersion,” Chem. Eng. J., vol. 427, no. August 2021, 2022, doi: 10.1016/j.cej.2021.131737.[5] P. Santori, A. Mondal, D. Dekel, and F. Jaouen, “The critical importance of ionomers on the electrochemical activity of platinum and platinum-free catalysts for anion-exchange membrane fuel cells,” R. Soc. Chem., vol. 2020, no. 7, pp. 3300–3307, doi: 10.1039/d0se00483aï. Figure 1

Which Properties Should Anion-Exchange Membranes Have to Achieve a Longer Fuel Cell Lifetime?

The substantial advancements and availability of new cost-effective materials have drawn significant attention to the technology of anion-exchange membrane fuel cells (AEMFCs). The anion exchange membrane (AEM) is a core component in AEMFCs, essential for conducting hydroxide ions and, more importantly, controlling water transport between the fuel cell electrodes. In this contribution, we apply the computational methodology outlined in Refs. [1,2] to explore the effects of various membrane properties on AEMFC performance and its stability. We specifically consider the following parameters: (A) water diffusivity, (B) membrane thickness, (C) ion exchange capacity (IEC), (D) maximum hydration level (λmax) corresponding to ionomer/water contact (maximum number of water molecules per functional group), (E) hydroxide conductivity, and (F) degradation rate constant of the AEM functional group.First, we describe our modeling approach, which includes a one-dimensional isothermal and time-dependent model of AEMFC operations. The model considers the chemical degradation process of the ionomeric materials in the cell, transport phenomena through the anode and cathode gas diffusion layers, anode and cathode catalyst layers, and the membrane. Finally, the electrochemical reactions, hydrogen oxidation in the anode and oxygen reduction in the cathode, are described by Butler-Volmer kinetics. We present our main findings, demonstrating that membrane water diffusivity and λmax have the most significant impact on AEMFC lifetime, followed by membrane thickness and IEC, attributed to enhanced water distribution across the cell. We also demonstrate the significance of improving functional group stability to performance stability, while AEM hydroxide conductivity shows a negligible effect on AEMFC lifetime. Finally, we perform dimensional analysis and provide an analytical, useful correlation to estimate the AEMFC lifetime of selected membranes from the literature and compare it to the measured operation time. In conclusion, this work highlights important AEM parameters aimed at improving AEM designs in order to enhance the performance stability of AEMFCs.[1] D.R. Dekel, I.G. Rasin, S. Brandon, Predicting performance stability of anion exchange membrane fuel cells, J. Power Sources. 420 (2019) 118–123. https://doi.org/10.1016/j.jpowsour.2019.02.069.[2] K. Yassin, I.G. Rasin, S. Brandon, D.R. Dekel, Elucidating the role of anion-exchange ionomer conductivity within the cathode catalytic layer of anion-exchange membrane fuel cells, J. Power Sources. 524 (2022) 231083. https://doi.org/10.1016/j.jpowsour.2022.231083.

Water Movement Direction through an Anion Exchange Membrane Fuel Cell Exerted By Gas Humidity and Flow Rate: A Simulation Study

The growing demand for clean energy generation has been urged due to the threat of climate change by greenhouse emissions (1 kg of CO2 per 1 kW h of generated electricity). In 2021, the electricity generation rose to 1530 TW h, 62% coming from fossil fuels [1]. To lower this significant fossil fuels usage, the development and use of alternative energy sources and green fuels have reached a formidable level of interest. Renewable solar and wind energy sources are currently available; however, those appear to be limited by natural fluctuations in energy production relegated to day- and night-time wheatear or geographical regions [2]. Electrochemical energy storage and conversion technologies can be employed to minimize production variations as an alternative to this limitation. The anion-exchange membrane fuel cell (AEMFC) is an excellent alternative for energy generation due to its air CO2-free emissions, low-cost electrocatalyst used at the cathode, and industrial-scale membrane production [3]. The design of a fuel cell involves several tightly interrelated factors, such as electrode and membrane materials, reactor geometry, and characterization of the reaction environment in terms of the momentum, mass and heat transport, electrode kinetics, and current distribution, to find the best operational conditions to reach a suitable energy efficiency [4]. In either case, the present work deals with designing a hybrid flow-field single-fuel cell. The monopolar anode plate is a multiple-serpentine flow field that benefits the water drainage. On the other hand, the cathode plate is an interdigitated leaf-type flow field that might ensure an efficient gas reactant distribution over the entire electrode geometric area.A two-phase flow CFD simulation is performed to analyze the influence of the flow field patterns on the mass transport processes in the electrodes. Moreover, the water movement direction through the membrane as a function of the operational conditions is presented. The commercial code COMSOL Multiphysics was used to solve the transport equations through the finite element method.In the experimental field, the water movement direction between the anodic and cathodic sides shows the capability to favor the electro-osmotic drag (EOD) or the diffusion water transport mechanism under different gas flow rates and humidities, as shown in Figure 1a. Accordingly, the results demonstrated that the EOD predominance was promoted when low symmetric flow rates (125 cm3 min-1 for anode and cathode) were used in the AEMFC. On the contrary, the water diffusion dominance appears under asymmetric flow rates between anode and cathode (125 and 250 cm3 min-1, respectively). The different tested relative humidities do not influence the water direction once the flow rates are given. The AEMFC performance was affected by the water transport mechanism, giving lower peak power densities for EOD-dominated systems (35.2 mW cm-2, in Figure 1b) than those obtained by diffusion (62.8 mW cm-2, in Figure 1c). The most suitable operating condition is for water diffusion dominance at non-saturated streams, where a decrease in performance is experienced as the flow becomes saturated (at higher humidifier temperature). A close agreement between simulation and experimental results was obtained regarding current-potential curves and water movement through the membrane dictated by electro-osmotic drag and water diffusion.

The effect of membrane thickness on AEMFC Performance: An integrated theoretical and experimental study

We present a comprehensive theoretical and experimental study of the effect of membrane thickness on the anion-exchange membrane (AEM) fuel cell (AEMFC) performance. AEMFC tests are carried out with several AEMs with thickness in the range of 5 – 50 µm and assembled with a PtRu anode, and two different cathode catalysts (Pt/C or FeNC). Dramatic improvements in cell performance are observed as the membrane thickness decreases, which is mainly attributed to reduced ohmic losses and enhanced water transport between the electrodes. The simulated cell performance obtained using our previously developed AEMFC model qualitatively and quantitatively explains the experimental results in the entire range of current densities (0–4 Acm−2). Simulated results show that thinner membranes enhance water transport between the electrodes, mitigating the anode flooding, and resulting in increased local hydration in the membrane and cathode catalytic layer. These high hydration values enhance the anionic conductivity of the ionomeric materials, thereby improving cell performance. Furthermore, the enhanced water transport towards the cathode electrode provides sufficient water to participate in the oxygen reduction reaction, thus reducing the activation losses. Simulation modeling allows for a thorough understanding of cell behavior and aids in the development of the next generation of advanced AEMFCs.

Water movement through an anion exchange membrane fuel cell (AEMFC): Influence of gas humidity and flow rate

The water movement between the anodic and cathodic sides of a fuel cell (AEMFCs) motivated the analysis of the effect exerted by the operational conditions on the water direction using a commercial anion exchange membrane (IONOMR®). This work shows the capability to favor the electro-osmotic drag (EOD) or the diffusion water transport mechanisms under different gas flow rates and humidities. Accordingly, the results demonstrated that the EOD predominance was promoted when low symmetric flow rates (125 cm3 min−1 for anode and cathode) were used in the AEMFC, suggesting membrane dehydration. On the contrary, the water diffusion dominance appears under asymmetric flow rates between anode and cathode (125 and 250 cm3 min−1, respectively); under these conditions, the membrane dehydration might be suppressed by anode flooding alleviation. The AEMFC performance was affected by the water transport mechanism, giving lower peak power densities for EOD-dominated systems (35.2 mW cm−2 at 60 °C) than those obtained by diffusion (62.8 mW cm−2 at 60 °C). The most suitable operating condition is under water-diffusion at non-saturated streams (below 65%), where a decrease in performance is experienced as the humidifier temperature increases from 60 °C to 80 °C (supersaturated flow), giving peak power densities of 62.8 and 3.7 mW cm−2, respectively.

Ionomer Distribution Strategy of Anion Exchange Membrane Fuel Cell Catalyst Layer in Terms of Interaction between Catalyst Slurry Components

Anion exchange membrane fuel cells (AEMFCs) have been intensively studied in recent years to replace proton exchange membrane fuel cells (PEMFCs). The acid-to-alkali transitions have the potential to lower overall system costs because it allows the non-precious metal catalysts and inexpensive metal stack hardware. Significant strides have been made in materials science in the last few decades, particularly the development of high IEC-containing anion exchange membrane (AEM) and ionomer (AEI) possessing high OH- conductivity, enabling comparable cell performance to that of PEMFC. In addition, the discovery of high HOR activity of PtRu by oxophilic and/or electronic effects, and reducing ionomer poisoning, provided an opportunity to further advance the cell performance of AEMFC. However, despite the remarkable development of materials, modest AEMFCs still have inadequate power performance (< 0.5 W cm-2) even using the precious catalysts, which stems from a lack of understanding of the catalyst layer (CL) design.CL consists of a catalyst and an ionomer, where the chemical energy of the fuel is converted into electrical energy in triple-phase-boundaries (TPBs). TPB is an electrochemically active site where catalyst (electrons), ionomer (H+ or OH-), and reactant gases were concurrent. In order to realize a high-performance fuel cell, the TPB should be high enough to utilize the capabilities of the catalyst, and the ideal state would be to maximize the contact area between the ionomer and the catalyst while minimizing the loss of CL porosity. Accordingly, the structure of the CL is one of the key factors that directly affect the performance of the fuel cell and is in great account.CL is fabricated from a slurry containing a catalyst, ionomer, and dispersing solvent, whose structure is determined by the complex interactions between slurry components. Therefore, fine-tuning of comprehensive interaction (catalyst/ionomer, catalyst/solvent, and ionomer/solvent) is the core technology in CL design, and an in-depth understanding of each interaction is also essential. In this regard, we recently presented the rational design of the CL by controlling AEI size, distribution via a solvent selection of the catalyst slurry. Specifically, the larger the solubility parameter (excluding the hydrogen bonding term) of the organic solvent mixed with water, the smaller the dispersed AEI size, leading to an even distribution of the AEI in the CL. This induces a high electrochemical surface area of the CL, making it possible to achieve high performance AEMFC from the low current region. However, nevertheless, we found that the distribution of AEI in the AEMFC catalyst layer was still not completely uniform through various types of electron microscopy analysis. In particular, when the morphology of the AEMFC CL was compared with the Nafion ionomer (commonly used in PEMFC field) as a reference, AEI caused lower porosity by clogging the pores of Pt/C nanoparticles. For this reason, we found that AEMFCs had lower performance than Nafion-based PEMFCs, despite their high ORR activity under alkaline conditions.Molecular dynamics (MD) and density functional theory (DFT) simulation analyzes show that AEI has a particularly low interaction with carbon compared to Nafion, and for this reason, we found that AEI aggregates with each other rather than evenly distributed on the Pt/C surface. We used QPC-TMA as AEI in this study and confirmed that commercialized AEI (FAA-3 and XA-9) also form low CL porosity and pore-closing characteristics. This result suggests that low interaction between AEI/carbon is a universal property of current levels of AEI. Therefore, when designing an ionomer to improve the performance of AEMFC, high interaction with carbon should be considered as another important variable.

Water limiting current measurements in anion exchange membrane fuel cells (AEMFCs); part 1: Water limiting current method development

Water management is critical in optimizing anion exchange membrane fuel cell (AEMFCs) performance and durability. The role of membranes on determining cell water balance in AEMFCs has received limited study. A more thorough understanding of membrane water transport parameters is necessary for thorough cell optimization. In this study, a novel method for probing limiting current based on water flux across an AEMFC is presented for the first time. The water limiting current has been investigated as a function of relative humidity of anode and cathode streams, cathode backpressure and cell operating temperature. From these studies, it is clearly shown that water diffusion through the membrane from the anode is the most critical mechanism for achieving high current density performance and durability. The developed method can act as a novel screening technique that focuses on the impacts of water transport parameters and helps identify promising membrane materials based on their water transport properties. A critical but underappreciated facet of AEMFC performance and durability.

Elucidating the role of anion-exchange ionomer conductivity within the cathode catalytic layer of anion-exchange membrane fuel cells

The anion-exchange ionomer (AEI) is a crucial component of anion-exchange membrane fuel cells (AEMFCs). In this study, computational analysis is employed to study the unexplored and critical effect of AEI hydroxide conductivity, within the cathode electrode, on AEMFC performance and its stability. The cathode is of particular importance due to its tendency to dry-out in a manner that may impact ionomer conductivity during AEMFC operation. Our modeling results clearly show that enhanced AEI hydroxide conductivity, within the cathode, significantly increases AEMFC performance. Less intuitive is its positive impact on cell stability. Superior conductivity is associated with high catalyst utilization and a reduced potential drop across the cathode. This enhances hydration levels in the cathode resulting in slower degradation kinetics. While the cathode reaction kinetics is considered as a major factor restricting cell performance, the transport of water and hydroxide through the cathode catalyst layer is extremely critical to ensure long-term AEMFC performance stability. In addition, a two-step degradation mechanism is observed where initially the voltage loss is controlled by the cathode AEI degradation, and only later membrane degradation becomes dominant. These insights are critical for understanding cell operation and for the achievement of further progress in AEMFC performance stability.

Impact of side-chains in poly(dibenzyl-co-terphenyl piperidinium) copolymers for anion exchange membrane fuel cells

Anion exchange membrane fuel cells (AEMFCs) have experienced rapid advancement in the past few years due to the discovery of high-performance AEMs and ionomers. Here, we report a series of side-chain-type poly(dibenzbeyl-co-terphenyl piperidinium) ( s -PDTP) copolymers as ionomers and membranes for AEMFC applications. The features of aryl ether-free PDTP polymer backbone and pendent ammonium groups endow both high alkaline stability and excellent micromorphology to s -PDTP copolymers. These ion-exchange-capacity (IEC)-controlled s -PDTP membranes possess high OH − conductivity >120 mS cm −1 at 80 °C, reasonable molecular weight (M w = 150–290 kDa), and excellent ex-situ durability in 1 M NaOH at 80 °C for 1000 h (∼90% conductivity remaining). For ionomer research, s -PDTP ionomers with moderate IEC (∼3.0 mmol g −1 ) and rational water uptake (∼200%) exhibit optimal AEMFC performance based on a PDTP membrane compared to high-IEC or low-IEC s -PDTP ionomers (>3.4 mmol g −1 or <2.4 mmol g −1 ). Based on Pt/C, s -PDTP-based AEMFCs reach a peak power density (PPD) of 1.47 W cm −2 in H 2 –O 2 and 1.04 W cm −2 in H 2 -air (CO 2 -free) at 80 °C. • IEC-controlled side-chain-type poly(aryl-co-aryl piperidinium)s is developed. • The effect of IECs on the performance of s-PDTP- x ionomers and AEMs is elucidated. • s-PDTP- x copolymers display excellent durability >1000 h in 1 M NaOH at 80 °C. • s-PDTP-based AEMFCs reach power densities of 1.04 W cm −2 in H 2 -air.