Anion Exchange Membrane Fuel Cells Performance Research Articles

Modeling Water Management and Carbon Dioxide Contamination Effects in Anion-Exchange Membrane Fuel Cells

Anion-exchange membrane fuel cells (AEMFCs) offer a potential advantage over their traditional proton-exchange membrane counterparts in that many non-precious metal catalysts are chemically compatible with the alkaline environment in an AEMFC [1]. Recently, high-power operation of AEMFCs has been demonstrated, including achieving 1 W/cm2 using silver metal cathode catalysts [2], [3]. The performance of AEMFCs, however, can be highly sensitive to small changes in relative humidity, as water is a reactant in the alkaline oxygen reduction reaction. Additionally, performance degradation due to carbon dioxide contamination remains a major issue. The absorption of carbon dioxide and subsequent conversion to carbonate anions reduces membrane conductivity and worsens fuel cell performance. In this work, we use our previously developed two-dimensional model [4] to illustrate and explore the effects of membrane and ionomer properties, relative humidity changes, and absorption of carbon dioxide on AEMFC performance. A 2+1D modeling approach is used, where an external stepping model is used to move along the gas channels. At each step, the 2D cross-section model [4] is used to simulate cell performance. Mass and energy balances are performed at each step to obtain the channel conditions, such as concentration, relative humidity, and temperature, for the subsequent step. The 2+1D model accounts for the performance changes due to concentration and relative humidity changes along the channel, especially at integral cell conditions. Finally, we show how performance limitations from carbon dioxide absorption, flooding, and dry-out could be alleviated through changes in operating parameters or ionomer properties. Acknowledgements The work was funded under the Fuel Cell Performance and Durability Consortium (FC-PAD), by the Fuel Cell Technologies Office (FCTO), Office of Energy Efficiency and Renewable Energy (EERE), of the U.S. Department of Energy under contract number DE-AC02-05CH11231. We thank Bryan Pivovar and the members of the Fuel Cells group of the National Renewable Energy Laboratory for helpful discussion and model validation data. [1] J. R. Varcoe et al., “Anion-exchange membranes in electrochemical energy systems,” Energy Environ. Sci., vol. 7, pp. 3135–3191, 2014. [2] D. R. Dekel, “A mini-review on cell performance in anion exchange membrane fuel cells,” J. Power Sources, p. submitted, 2017, 2017. [3] L. Wang and J. R. Varcoe, “The first anion-exchange membrane fuel cell to exceed 1 W cm-2 at 70 °C with a non-Pt cathode,” Chem. Commun., pp. 1–3, 2017. [4] H.-S. Shiau, I. V. Zenyuk, and A. Z. Weber, “Elucidating Performance Limitations in Alkaline-Exchange- Membrane Fuel Cells,” J. Electrochem. Soc., vol. 164, no. 11, pp. E3583–E3591, 2017.

Determining Electro-Osmotic Drag of Water in Anion Exchange Membrane Fuel Cells

In an effort to reduce the cost of proton exchange membrane fuel cells (PEMFCs), a significant amount of research into non-precious metal (NPM) oxygen reduction catalysts has been conducted in recent decades. Despite this effort, platinum remains the only material with suitable stability and activity for use in the harsh environment of PEMFCs. Anion exchange membrane fuel cells (AEMFCs) have been gaining increased attention in recent years, providing advantages of higher stability and activity of NPM catalysts in alkaline environments. Unfortunately, the relevant properties, e.g. conductivity and mechanical & chemical stability, of anion exchange membranes (AEMs) are inferior to their proton conducting counterparts. Many studies have been conducted which focus on improving membrane properties such as swelling ratio, conductivity, and stability. In comparison, relatively little attention has been paid to studying ion and water transport in anion exchange membranes. Previous work conducted in our group has indicated that the majority of AEMFC polarization arises from the anode. The performance is relatively insensitive to catalyst loading and initial results indicate that electrode flooding is the primary cause of poor performance. To improve the performance of AEMFCs and better design AEMs, it is therefore important to study the transport of water and ions within AEMs. One key parameter which has received very little attention in the literature is the electro-osmotic transport of water due to the conduction of anions from the cathode to the anode. In this work, we study the electro-osmotic drag (EOD) in anion exchange membranes. The cell design and concept for electro-osmotic drag coefficient (ε) measurements in this work follow from the work of Fuller and Newman as adapted by others1–3. Analysis is conducted on the membranes in pure hydroxide form and carbonate form. The conductivity, water uptake and water mobility of the membranes is also studied over a wide range of relative humidities and temperatures. Better understanding of the transport of water and ions within anion exchange membranes will facilitate the development of models for AEMFC behavior and offer insight for future AEM production. Figure 1: Effect of water activity on the EOD coefficient (ε) and water content (λ) in Tokuyama A201. T. F. Fuller and J. Newman, J. Electrochem. Soc., 139, 1332–1337 (1992).T. A. Zawodzinski, J. Davey, J. A. Valerio, and S. Gottesfeld, Electrochim. Acta, 40, 297–302 (1995).X. Wang, J. P. Mcclure, and P. S. Fedkiw, Electrochim. Acta, 79, 126–132 (2012) http://dx.doi.org/10.1016/j.electacta.2012.06.098. Figure 1

The Effect of Ambient Carbon Dioxide on Anion-Exchange Membrane Fuel Cells.

Over the past 10 years, there has been a surge of interest in anion-exchange membrane fuel cells (AEMFCs) as a potentially lower cost alternative to proton-exchange membrane fuel cells (PEMFCs). Recent work has shown that AEMFCs achieve nearly identical performance to that of state-of-the-art PEMFCs; however, much of that data has been collected while feeding CO2 -free air or pure oxygen to the cathode. Usually, removing CO2 from the oxidant is done to avoid the detrimental effect of CO2 on AEMFC performance, through carbonation, whereby CO2 reacts with the OH- anions to form HCO3- and CO32- . In spite of the crucial importance of this topic for the future development and commercialization of AEMFCs, unfortunately there have been very few investigations devoted to this phenomenon and its effects. Much of the data available is widely spread out and there currently does not exist a resource that researchers in the field, or those looking to enter the field, can use as a reference text that explains the complex influence of CO2 and HCO3- /CO32- on all aspects of AEMFC performance. The purpose of this Review is to summarize the experimental and theoretical work reported to date on the effect of ambient CO2 on AEMFCs. This systematic Review aims to create a single comprehensive account of what is known regarding how CO2 behaves in AEMFCs, to date, as well as identify the most important areas for future work in this field.

Analysis of alkaline exchange membrane fuel cells performance at different operating conditions using DC and AC methods

Membrane electrode assemblies (MEAs) for anion exchange membrane fuel cells (AEMFCs) were manufactured from commercial materials: Pt/C catalyst, A201 AEM and AS4 ionomer by using an industrial mass-production digital printing method. The MEA designs selected are close to those recommended by US Department of Energy, including low loading of platinum on the cathode side (0.2 mg cm−2). Polarization curves and electrochemical impedance spectroscopy (EIS) were applied for MEA evaluation in fuel cell conditions with variation of gas humidification and oxygen partial pressure (air vs oxygen). The typical impedance curves recorded at H2/O2 gas configuration consist of high- and medium-frequency arcs responsible for hydrogen oxidation and oxygen reduction, respectively. Operation with air as a cathode feed gas resulted in a decrease in AEMFC performance due to possible CO2 poisoning and mass transfer losses. At the same time, EIS demonstrated formation of a low frequency loop due to diffusion limitations. Despite the low loading of platinum on the cathode (0.2 mg cm−2), a peak power density of ∼330 mW cm−2 was achieved (at 50/50% of RH on anode and cathode), which is substantially higher performance than for AEMFC MEAs tested at similar conditions.

Anion exchange membrane fuel cells: Current status and remaining challenges

The anion exchange membrane fuel cell (AEMFC) is an attractive alternative to acidic proton exchange membrane fuel cells, which to date have required platinum-based catalysts, as well as acid-tolerant stack hardware. The AEMFC could use non-platinum-group metal catalysts and less expensive metal hardware thanks to the high pH of the electrolyte. Over the last decade, substantial progress has been made in improving the performance and durability of the AEMFC through the development of new materials and the optimization of system design and operation conditions. In this perspective article, we describe the current status of AEMFCs as having reached beginning of life performance very close to that of PEMFCs when using ultra-low loadings of Pt, while advancing towards operation on non-platinum-group metal catalysts alone. In the latter sections, we identify the remaining technical challenges, which require further research and development, focusing on the materials and operational factors that critically impact AEMFC performance and/or durability. These perspectives may provide useful insights for the development of next-generation of AEMFCs.

La and Al co-doped CaMnO3 perovskite oxides: From interplay of surface properties to anion exchange membrane fuel cell performance

This work reports the first account of perovskite oxide and carbon composite oxygen reduction reaction (ORR) catalysts integrated into anion exchange membrane fuel cells (AEMFCs). Perovskite oxides with a theoretical stoichiometry of Ca0.9La0.1Al0.1Mn0.9O3-δ are synthesized by an aerogel method and calcined at various temperatures, resulting in a set of materials with varied surface chemistry and surface area. Material composition is evaluated by X-ray diffraction, energy dispersive X-ray spectroscopy, and X-ray photoelectron spectroscopy. The perovskite oxide calcined at 800 °C shows the importance of balance between surface area, purity of the perovskite phase, and surface composition, resulting in the highest ORR mass activity when evaluated in rotating disk electrodes. Integration of this catalyst into AEMFCs reveals that the best AEMFC performance is obtained when using composites with 30:70 perovskite oxide:carbon composition. Doubling the loading leads to an increase in the power density from 30 to 76 mW cm−2. The AEMFC prepared with a composite based on perovskite oxide and N-carbon achieves a power density of 44 mW cm−2, demonstrating an ∼50% increase when compared to the highest performing composite with undoped carbon at the same loading.

A Model Based Analysis of Alkaline Membrane Fuel Cells

The implementation of reduced-cost catalyst materials in Anion Exchange Membrane Fuel Cells (AEMFCs) renders them an attractive alternative to Proton Exchange Membrane Fuel Cells (PEMFCs). An important challenge existing in the development of AEMFCs involves water management, which is especially problematic since in these systems (as compared to PEMFCs) water is produced in the anode and consumed in the cathode. We present a model-based analysis of AEMFC performance with an emphasis on water management. Using this approach it is possible to elucidate the relation between reduced performance and issues of hydration throughout the Membrane Electrode Assembly (MEA). We provide a brief overview of the model, validate our results against published experimental data and present results related to the impact of relative humidity, as well as the depletion of water vapor in the air flow stream, on system performance.

Anion-Exchange Membrane Fuel Cells with Improved CO2 Tolerance: Impact of Chemically Induced Bicarbonate Ion Consumption.

Over the last few decades, because of the significant development of anion exchange membranes, increasing efforts have been devoted the realization of anion exchange membrane fuel cells (AEMFCs) that operate with the supply of hydrogen generated on-site. In this paper, ammonia was selected as a hydrogen source, following which the effect of conceivable impurities, unreacted NH3 and atmospheric CO2, on the performance of AEMFCs was established. As expected, we show that these impurities worsen the performance of AEMFCs significantly. Furthermore, with the help of in situ attenuated total reflection infrared (ATR-IR) spectroscopy, it was revealed that the degradation of the cell performance was primarily due to the inhibition of the hydrogen oxidation reaction (HOR). This is attributed to the active site occupation by CO-related adspecies derived from (bi)carbonate adspecies. Interestingly, this degradation in the HOR activity is suppressed in the presence of both NH3 and HCO3- because of the bicarbonate ion consumption reaction induced by the existence of NH3. Further analysis using in situ ATR-IR and electrochemical methods revealed that the poisonous CO-related adspecies were completely removed under NH3-HCO3- conditions, accompanied by the improvement in HOR activity. Finally, a fuel cell test was conducted by using the practical AEMFC with the supply of NH3-contained H2 gas to the anode and ambient air to the cathode. The result confirmed the validity of this positive effect of NH3-HCO3- coexistence on CO2-tolerence of AEMFCs. The cell performance achieved nearly 95% of that without any impurity in the fuels. These results clearly show the impact of the chemically induced bicarbonate ion consumption reaction on the realization of highly CO2-tolerent AEMFCs.

Review of cell performance in anion exchange membrane fuel cells

Anion exchange membrane fuel cells (AEMFCs) have recently received increasing attention since in principle they allow for the use of non-precious metal catalysts, which dramatically reduces the cost per kilowatt of power in fuel cell devices. Until not long ago, the main barrier in the development of AEMFCs was the availability of highly conductive anion exchange membranes (AEMs); however, improvements on this front in the past decade show that newly developed AEMs have already reached high levels of conductivity, leading to satisfactory cell performance. In recent years, a growing number of research studies have reported AEMFC performance results. In the last three years, new records in performance were achieved. Most of the literature reporting cell performance is based on hydrogen-AEMFCs, although an increasing number of studies have also reported the use of fuels others than hydrogen - such as alcohols, non-alcohol C-based fuels, as well as N-based fuels. This article reviews the cell performance and performance stability achieved in AEMFCs through the years since the first reports in the early 2000s.

Importance of balancing membrane and electrode water in anion exchange membrane fuel cells

Anion exchange membrane fuel cells (AEMFCs) offer several potential advantages over proton exchange membrane fuel cells (PEMFCs), most notably to overcome the cost barrier that has slowed the growth and large scale implementation of fuel cells for transportation. However, limitations in performance have held back AEMFCs, specifically in the areas of stability, carbonation, and maximum achievable current and power densities. In order for AEMFCs to contend with PEMFCs for market viability, it is necessary to realize a competitive cell performance. This work demonstrates a new benchmark for a H2/O2 AEMFC with a peak power density of 1.4 W cm−2 at 60 °C. This was accomplished by taking a more precise look at balancing necessary membrane hydration while preventing electrode flooding, which somewhat surprisingly can occur both at the anode and the cathode. Specifically, radiation-grafted ETFE-based anion exchange membranes and anion exchange ionomer powder, functionalized with benchmark benzyltrimethylammonium groups, were utilized to examine the effects of the following parameters on AEMFC performance: feed gas flow rate, the use of hydrophobic vs. hydrophilic gas diffusion layers, and gas feed dew points.

Improving Performance in Alkaline Membrane Fuel Cells through Enhanced Water Management

Water management in anion exchange membrane fuel cells (AEMFCs) is more complex than proton exchange membrane fuel cells (PEMFCs). In the PEM system, water is generated at the cathode and otherwise only serves as the membrane proton transport medium. In the hydroxide AEMFC, water is generated at the anode, consumed at the cathode, and is greatly needed for transport of the larger hydroxide ion. The challenge this introduces is the need to provide adequate water to maintain the membrane humidity without flooding the catalyst or gas diffusion layers.1 One method being used to achieve the necessary high membrane water content is utilizing hydrophilic gas diffusion layers without microporous layers.2 Others have used gas diffusion layers with a hydrophobic coated microporous layer while raising the temperature of the humidifiers well above the cell temperature.3 Gas stream dew points above 100% relative humidity and back pressure are common approaches in increasing the membrane water content. However there is instability in the cell performance and conditions that generate high performance levels are not easily repeatable. The problem is the line between proper membrane hydration and flooded catalysts layers is very thin, and non-existent in some cases. In this study, the influence of the membrane, ionomer and gas diffusion layer as well as the flow rate and dew points of the anode and cathode gases on AEMFC performance were explored. Tokuyama A201 membranes and AS-4 ionomer were investigated alongside quaternary-ammonium-functionalized radiation-grafted ETFE alkaline anion-exchange membranes and ionomers.4 Using a hydrophilic gas diffusion layer without a microporous layer increased membrane hydration, but as expected increased the potential for flooding. Manipulating the dew points led to the counter-intuitive discovery that the cell performs better with the humidity higher at the anode than the cathode, despite water generation and electro-osmotic drag towards that electrode. In fact, removing too much water from the anode caused instability in the cell, while increasing the water at the anode decreased the membrane resistivity. Back diffusion likely plays an important role in membrane hydration and hydroxide transport through the membrane. A very high flow rate (1.0 L/min) also increased cell performance, despite being several orders of magnitude above the stoichiometric need. Combining all areas of improvement resulted in a very high performing AEMFC with a maximum current density of 2.2 A/cm2 (at 0.1 V) and max power density of 670 mW/cm2 (880 mW/cm2 iR-corrected) with a membrane resistivity of 75 mOhms*cm2 (Figure 1a). Only a minor drop in the current was observed using air at the cathode with the same 1.0 L/min flow rate as oxygen, giving max current density of 1.7 A/cm2 and a max power density of 580 mW/cm2 with a resistivity of 74 mOhms*cm2 (Figure 1b). This near identical behavior confirms that the amount of reactant present supplied by the higher flow rate is not necessary, but the volumetric flow rate is needed for water management. It is likely that the pressure drop along the single pass cell hardware allows the gas to “jump the bar” only at very high flowrates, which results in better water removal and limits cell flooding in the cell.

High performance porous polybenzimidazole membrane for alkaline fuel cells

In this study, a highly ion-conductive and durable porous polymer electrolyte membrane based on ion solvating polybenzimidazole (PBI) was developed for anion exchange membrane fuel cells (AEMFCs). The introduction of porosity can increase the attraction of electrolytic solutions (e.g., potassium hydroxide (KOH)) and ion solvation, which results in the enhancement of PBI's ionic conductivity. The morphology, thermo-physico-chemical properties, ionic conductivity, alkaline stability, and the AEMFC performance of KOH-doped PBI membranes with different porosities were characterized. The ionic conductivity and AEMFC performance of 70 wt.% porous PBI was about 2 times higher than that of the commercially available Fumapem® FAA. All KOH-doped porous PBI membranes maintained their ionic conductivity after accelerated alkaline stability testing over a period of 14 days, while the commercial FAA degraded just after 3 h. The excellent performance and good durability of KOH-doped porous PBI membrane makes it a promising candidate for AEMFCs.

Polydimethylsiloxane-Based Block Copolymer for Anion Exchange Membrane Fuel Cells (AEMFC)

The anion exchange membrane fuel cells (AEMFCs) known as hydroxide ion (OH-) exchange fuel cells have evoked considerable attention due to their faster oxygen reduction reaction, lower fuel crossover, reduced CO poisoning, a wide choice of fuels (e. g. hydrogen, methanol, ethanol, hydrazine), and to the possibility of Pt-free catalysts. In the development of AEMFCs, anion exchange membranes (AEMs) play a critical role to make a breakthrough in AEMFC performance. Especially, alkaline stability and ion conductivity are key properties of AEMs. Although various AEMs were developed, there are still required new AEM with higher ion conductivity and stability. One of the attractive candidates is siloxane-based polymer substituted with anion exchangeable functional groups. In this study, we introduce the property of PDMS copolymer with quaternary ammonium groups.

Influences of Structural Differences of Layered Double Hydroxides on Their Hydroxide Ion Conductivities

Anion-exchange membrane fuel cells (AEMFCs) have attracted much attention since they can potentially overcome the stumbling blocks of proton-exchange membrane fuel cells. There are many research approaches to improve the performance of AEMFCs, and one of them is to seek new ion conductors that effectively transport hydroxide ions at the ambient temperature. Ion conductors with not only high ion conductivities but also high robustness are strongly required to realize the high-efficiency and long-life performance of AEMFCs. Among such ion conductors, we have focused on layered double hydroxides (LDHs). The basic layer structure of LDHs is based on that of brucite [Mg(OH)2]. In the brucite lattice, magnesium ions are surrounded octahedrally with hydroxide ions, and these octahedral units form infinite plane of the 2D layers. LDHs are derived by the substitution of a fraction of divalent cations in brucite by trivalent cations, which lead to positive charges in hydroxide layers. These charges are balanced by intercalation of anions and water between the positively charged layers. The possibility of varying the kinds and relative proportions of the di- and trivalent cations as well as the interlayer anions gives rise to the large variety of LDHs with a general formula [M2+ 1−xM3+ x(OH)2]x+[An−]x/n·yH2O, where M2+ and M3+ are di- and trivalent metal cations respectively, and An− is an anion. This flexibility in composition of LDHs has induced an increase in interest for many applications, such as catalyst supports, ion-exchange and adsorption, drug-delivery system, etc. In addition, some research groups including our group recently reported that LDHs operated as an OH−conductor, which could be applied for alkaline fuel cells, metal-air secondary batteries, and alkaline secondary batteries. In this study, we focus on the nature of ion conductivities in LDHs. To understand the mechanism of ion conductivities in LDHs in detailed, we synthesized Mg-Al and Mg-Ga LDHs with different relative proportions of di- and trivalent metal cations, and evaluated the influence of the proportion of trivalent cation on their ion conductivities by electrochemical measurements. We chose Mg-Al and Mg-Ga LDHs since they are widely used in LDH studies. We shed light on the structural relationship, which could guide the design of improved or novel hydroxide-conductive materials. Using the co-precipitated method, we successfully synthesized Mg-Al-CO3 and Mg-Ga-CO3 LDHs with various proportions of trivalent cations. In the results of X-ray diffraction, as the Mg/Al ratio decreased from 4 to 2, the 003 diffraction peak positively shifted, which indicates that layer height between metal hydroxide layers were diminished. This decrease in the layer height was due to an increase in electrostatic attraction between positive and negative layers. In contrast, resulted LDHs were characterized in similar BET surface areas and mean diameter in spite of changing the proportion of trivalent cations. Both series of LDHs did not displayed a linear increase in ion conductivities as a function of the proportion of trivalent cations, but a sharp raise appeared only for Mg2+/Al3+ = 2 and Mg2+/Ga3+ = 3. Based on the results of electron-diffraction analysis, Mg-Al and Mg-Ga LDHs, showing the highest ion conductivities had the ordered honeycomb cation arrangement within some parts of (0001) hydroxide layers. Other LDHs had complete random cation distribution. The ordered honeycomb structure was a cation distribution that can elude direct contact of M3+−M3+ and diminish the distance of the nearest trivalent cations; “neither near nor far” for individual interlayer anions. These things for the ordered cation arrangements not only demonstrate a helpful strategy for improving ion conductivities of LDHs, but also present new insights into structural relationship between immobile cationic charge centers and mobile interlayer anions.

Activity, Selectivity, and Anion-Exchange Membrane Fuel Cell Performance of Virtually Metal-Free Nitrogen-Doped Carbon Nanotube Electrodes for Oxygen Reduction Reaction

Virtually metal-free, aligned nitrogen-doped carbon nanotubes (N-CNTs) are prepared by an alumina template technique and characterized using XRD, Raman spectroscopy, SEM, and XPS. Highly uniform and aligned nanotubes are identified from SEM. The surface nitrogen concentration in the N-CNTs is estimated to be ∼8.0 at.%. Electrocatalytic oxygen reduction reaction (ORR) activity in alkali media is investigated by rotating disk electrode (RDE) voltammetry and compared with commercial Pt/C (E-TEK). RDE measurements indicated the comparable ORR activity of nitrogen-doped carbon nanotubes with Pt/C. Kinetic analysis reveals that the ORR on nitrogen-doped carbon nanotubes follows the four-electron pathway leading to water. Moreover, during the ORR, nitrogen-doped carbon nanotubes exhibited substantially higher ethanol tolerance than Pt/C. A maximum power density of 37 and 62 mW/cm2 is observed with N-CNT and Pt/C cathodes, respectively, under anion-exchange membrane fuel cell conditions. Durability measurements performed under fuel cell operating conditions for 30 h indicate the good stability of the catalysts.

PTFE based composite anion exchange membranes: thermally induced in situ polymerization and direct hydrazine hydrate fuel cell application

Anion exchange membrane fuel cells (AEMFCs) are advantageous over proton exchange membrane fuel cells in terms of electrode reaction kinetics and electrocatalyst versatility. As one of the key materials for this type of fuel cell, AEM transports anions from cathode to anode during cell operation and governs the performance of AEMFC. In this work, we report on the fabrication of a novel type of PTFE based composite anion exchange membrane and its application in direct hydrazine hydrate fuel cells (DHFC). The membrane was prepared viain situ thermal polymerization of chloromethyl monomer in pre-treated PTFE matrix followed by quaternary amination and alkalization. Attributed to a high chloromethyl monomer uptake and a membrane structure featuring “hydrophobic matrix confined hydrophilic domain”, the fabricated membrane showed hydroxide conductivity up to 0.049 S cm−1 at room temperature and a maximum DHFC power density of 110 mW cm−2 at a voltage of 0.7 V. Such membranes are promising candidates for application in AEMFC.