Anion Exchange Membrane Fuel Cells Research Articles

Challenges and Strategies of Anion Exchange Membranes in Hydrogen-electricity Energy Conversion Devices.

Alkaline hydrogen-electricity energy conversion technologies, involving anion exchange membrane fuel cells (AEMFCs) and anion exchange membrane water electrolyzers (AEMWEs) are more appealing than the acidic counterparts due to the elimination of precious metal catalysts. However, the physicochemical properties of anion exchange membrane (AEMs), i.e., ionic conductivity, mechanical strength, stability, etc., are inferior to that of proton exchange membranes (PEMs), thus hindering these alkaline technologies from practical employment. To promote their development, we summarize the main challenges and the corresponding strategies of AEMs for the application of AEMFCs and AEMWEs in this review. The hydroxide transportation mechanism, ion exchange capacity, hydration and microscopic morphology that are relevant to the ionic conductivity are discussed firstly. Following the ionic conductivity, another obstacle, stability of AEMs is comprehensively described in terms of alkaline stability, mechanical stability and electrochemical stability. Upon integrating into the devices, water management, carbonation effect and membrane-electrode interface that are critical to the cell performance are highlighted as well. This review is anticipated to provide insights into the AEM design for hydrogen-electric energy conversion devices, thus accelerating the widespread commercialization of these promising technologies.

Fine adjustment of catalyst agglomerate for the controllable construction of Co/Fe–N–C catalyst layers

Adjusting catalyst/ionomer agglomerate is crucial to a high-performance catalyst layer (CL), particularly for non-precious metal catalysts. As a sacrificial component, the solvent plays a vital role during the CL formation process and resulting microstructure, determining the performance of the anion exchange membrane fuel cells (AEMFCs). Here, solvent mixtures of isopropanol (IPA) and tetrahydrofuran (THF) are studied for the controllable construction of the Co/Fe–N–C CL. The results indicate that the aggregate state of the ionomers and catalysts and the final CL microstructure strongly depend on the ratio of IPA and THF. The CL prepared with IPA: THF = 1:2 (v/v) shows a homogeneous distribution of Co/Fe–N–C catalysts, embedded in the ionomer matrix, due to the fast volatilization and low viscosity of the catalyst ink. The Co/Fe–N–C CL with high moisture adsorption behavior and retention ability results in a high performance AEMFC with peak power density of 545.3 mW cm−2 at 80 °C. The obtained results can guide the rational design of microstructure in non-precious metal CLs for high-performance AEMFCs.

Porous Iron‐Nitrogen‐Carbon Electrocatalysts for Anion Exchange Membrane Fuel Cells (AEMFC)

AbstractHigh‐performance platinum group metal‐free (PGM‐free) electrocatalysts were prepared from porous organic polymers (POPs) precursors with highly‐porous structures and adjustable surface area. A resin phenol‐melamine‐based POP and an iron salt were used to synthesize Fe−N−C catalysts with different iron contents (0.2–1.3 wt.%). Electrochemical and spectroscopical characterization allowed us to elucidate the effect of Fe content on the material's structure, surface chemistry, and electrocatalytic activity toward the oxygen reduction reaction (ORR). The increase of iron content led to a specific surface area decrease, preserving the morphological structure, with the formation of highly‐active catalytic sites, as indicated by X‐ray photoelectron spectroscopy (XPS) analysis. The rotating ring disk electrode experiments, performed at pH=13, confirmed the high ORR activity of both 0.5 Fe (E1/2=0.84 V) and 1.3 Fe (E1/2=0.83 V) catalysts, which were assembled at the cathode of a H2‐fed anion exchange membrane fuel cells (AEMFC) equipped with a FAA‐3‐50 membrane, evidencing promising performance (0.5 Fe, maximum power density, Max PD=69 mA cm−2 and 1.3 Fe, Max PD=87 mA cm−2) with further advancement prospects.

MOF-Based Co and Mn Embedded in Nitrogen-Doped Microporous Carbon as an Efficient Catalyst for Oxygen Reduction Reaction in Anion Exchange Membrane Fuel Cell

Developing a low-price, high catalytic activity, and strong durability electrocatalyst for alkaline oxygen reduction reaction (ORR) is significantly important for anion exchange membrane fuel cell (AEMFC). Herein, Co and Mn salts were added into ZIF-8 to obtain CoMn-ZIF-Ac-2. Co and Mn embedded in nitrogen-doped microporous carbon (CoMn-N-C-Ac-2-Ts, T = 700 , 800, 900, and 1000°C) were obtained by carbonizing CoMn-ZIF-Ac-2 at various temperature. The influence of various pyrolysis temperature and molar ratios between Co and Mn toward ORR catalytic activity was researched. For CoMn-N-C-Ac-2-Ts, CoMn-N-C-Ac-2-800 had the highest ORR catalytic activity with the half-wave potential of 0.875 V in 0.1 M KOH, only 5 mV lower than that of 20 wt % Pt/C. Besides, high 4e- selectivity, excellent stability (retaining 100% for 20 h at 0.6 V vs. RHE), and methanol tolerance are also exhibited. In addition, CoMn-N-C-Ac-2-800 exhibited better ORR activity than Mn-N-C-Ac-800 and Co-N-C-Ac-800, which was attributed to more Co-N4, more MnIII species, and higher surface area. Moreover, the AEMFC based on the CoMn-N-C-Ac-2-800 cathode catalyst obtained a maximal power density of 291 mW·cm-2, which was 78% of the P max achieved with 20 wt % Pt/C (375 mW·cm-2). The highest ORR performance for CoMn-N-C-Ac-2-800 was contributed by the highest defect degree, the most amounts of Co/Mn-N4 active site, the maximum BET surface area, and micropore structure.

Efficiency of Neat and Quaternized-Cellulose Nanofibril Fillers in Chitosan Membranes for Direct Ethanol Fuel Cells.

In this work, fully polysaccharide based membranes were presented as self-standing, solid polyelectrolytes for application in anion exchange membrane fuel cells (AEMFCs). For this purpose, cellulose nanofibrils (CNFs) were modified successfully with an organosilane reagent, resulting in quaternized CNFs (CNF (D)), as shown by Fourier Transform Infrared Spectroscopy (FTIR), Carbon-13 (C13) nuclear magnetic resonance (13C NMR), Thermogravimetric Analysis (TGA)/Differential Scanning Calorimetry (DSC), and ζ-potential measurements. Both the neat (CNF) and CNF(D) particles were incorporated in situ into the chitosan (CS) membrane during the solvent casting process, resulting in composite membranes that were studied extensively for morphology, potassium hydroxide (KOH) uptake and swelling ratio, ethanol (EtOH) permeability, mechanical properties, ionic conductivity, and cell performance. The results showed higher Young's modulus (119%), tensile strength (91%), ion exchange capacity (177%), and ionic conductivity (33%) of the CS-based membranes compared to the commercial Fumatech membrane. The addition of CNF filler improved the thermal stability of the CS membranes and reduced the overall mass loss. The CNF (D) filler provided the lowest (4.23 × 10-5 cm2 s-1) EtOH permeability of the respective membrane, which is in the same range as that of the commercial membrane (3.47 × 10-5 cm2s-1). The most significant improvement (~78%) in power density at 80 °C was observed for the CS membrane with neat CNF compared to the commercial Fumatech membrane (62.4 mW cm-2 vs. 35.1 mW cm-2). Fuel cell tests showed that all CS-based anion exchange membranes (AEMs) exhibited higher maximum power densities than the commercial AEMs at 25 °C and 60 °C with humidified or non-humidified oxygen, demonstrating their potential for low-temperature direct ethanol fuel cell (DEFC) applications.

2,6-Diaminopyridine-Based Polyurea as an ORR Electrocatalyst of an Anion Exchange Membrane Fuel Cell

In order to yield more Co(II), 2,6-diaminopyridine (DAP) was polymerized with 4,4-methylene diphenyl diisocyanates (MDI) in the presence of Co(II) to obtain a Co-complexed polyurea (Co-PUr). The obtained Co-PUr was calcined to become Co, N-doped carbon (Co-N-C) as the cathode catalyst of an anion exchange membrane fuel cell (AEMFC). High-resolution transmission electron microscopy (HR-TEM) of Co-N-C indicated many Co-Nx (Co covalent bonding with several nitrogen) units in the Co-N-C matrix. X-ray diffraction patterns showed that carbon and cobalt crystallized in the Co-N-C catalysts. The Raman spectra showed that the carbon matrix of Co-N-C became ordered with increased calcination temperature. The surface area (dominated by micropores) of Co-N-Cs also increased with the calcination temperature. The non-precious Co-N-C demonstrated comparable electrochemical properties (oxygen reduction reaction: ORR) to commercial precious Pt/C, such as high on-set and half-wave voltages, high limited reduction current density, and lower Tafel slope. The number of electrons transferred in the cathode was close to four, indicating complete ORR. The max. power density (Pmax) of the single cell with the Co-N-C cathode catalyst demonstrated a high value of 227.7 mWcm-2.

Operando EPR Study of Radical Formation in Anion-Exchange Membrane Fuel Cells

The alkaline operating environment of anion-exchange membrane (AEM) fuel cells (AEMFCs) makes it possible to employ a wide variety of catalysts, including platinum group metals (PGMs) as well as PGM-free materials. However, little is understood about radical formation during AEMFC operation with different catalysts and their implications. In this investigation, we utilized spin-trapping and electron-paramagnetic resonance measurements to measure and identify the radicals produced on selected PGM and PGM-free oxygen-reduction reaction catalysts. To the best of our knowledge, this is an original study exploring radical formation on different classes of catalysts in operando AEMFCs. This work highlights an unexplored radical degradation mechanism in AEMFCs and calls for innovative strategies in designing radical attack-resistant AEMs.

MATHEMATICAL MODELING AND SIMULATION OF CO2 REMOVAL FROM AN ALKALINE SOLUTION FOR FUEL CELLS APPLICATIONS

In order to reduce oil dependency and reduce CO2 emissions stabilizing the greenhouse effect on the planet, the search for new renewable energy sources has been intensified, with a particular interest in hydrogen based solutions. Hydrogen can be used in fuel cells, which have several applications. Fuel Cells are among the environmentally friendly energy conversion systems for the 21st century with simple components such as membrane, catalyst, rearrangeable configurations that allow them to accommodate space limitations, and their use of hydrogen and oxygen. There are many types of fuel cells that are distinguished by the electrolyte type and their operating temperature. Alkaline Membrane Fuel Cells (AMFCs) and Proton-Exchange Membrane Fuel Cells (PEMFCs) are major types that work in low temperatures and produce only H2O and electricity as part of the electrochemical reaction. AMFC is a fuel cell that has more affordable membranes, when compared to the PEMFC that uses a polymeric membrane with high cost, making applications more expensive. In AMFCs, the alkaline membrane used, is a simple filter paper saturated with KOH solution that allows ions to pass through the membrane, however, suffers CO2 poisoning when it gets in contact to the carbon dioxide present in the air, reacting in the KOH and capturing hydroxyl ions. The poisoning will generate chemical compounds that will interfere with the energy generation and efficiency of the fuel cell. The main cause of the decreasing performance of carbonate formation is the precipitation of large metal carbonate crystals such as K2CO3 and the formation of H2O in the membrane, decreasing KOH concentration. If not addressed, this issue will limit the use of AMFC to pure oxygen applications only, instead of the air itself, which restricts the applicability of the technology. This study presents a mathematical model of a purifier that reduces the concentration of CO2 present in the air, improving conditions to be used in AMFC for mobile applications as automotive vehicles and without the need to use pure oxygen.

Transition metal (Fe, Co, Mn, Cu) containing nitrogen-doped porous carbon as efficient oxygen reduction electrocatalysts for anion exchange membrane fuel cells

Delving into highly active and cost-efficient electrocatalysts for oxygen reduction reaction (ORR) is crucial for the large-scale application of polymer electrolyte fuel cells. Anion exchange membrane fuel cells (AEMFCs) are promising clean energy devices owing to their mild reaction conditions and the high probability of employing Pt-free catalysts for ORR. Developing the promising non-Pt ORR catalysts for AEMFC is still of great importance. Herein, we report the transition metal (Fe, Co, Mn, and Cu) impregnated melamine-phloroglucinol-formaldehyde (MPF) polymeric networks to derive metal-nitrogen-carbon (MNC) electrocatalysts via a robust synthesis route. The catalysts are screened through variable metal contents and different pyrolysis temperature optimizations by virtue of their ORR activity. The controlled synthesis method resulted to the prominent textural properties of the catalysts with efficient active centers to enhance the ORR performance. Amongst, iron-doped (MPF/Fe), and cobalt-doped (MPF/Co) catalysts are performing better in terms of half-wave potential (E1/2) values of 0.81 and 0.80 V vs RHE which is attributed to the highly active M−Nx sites and hierarchical porous structure of catalysts. Outstanding electrochemical stability in half-cell and high-power density in an AEMFC (up to 347 mW cm−2) made the present work drive to the development of highly efficient MNC catalysts for fuel cell applications.

A mini-review on alkaline stability of imidazolium cations and imidazolium-based anion exchange membranes

An important component of anion-exchange membrane fuel cells (AEMFCs) is an anion exchange membrane (AEM). AEMFCs have substantial potential to use nonprecious electrocatalysts at high pH and recently attracted much attention. Sufficient long-term stability in highly alkaline conditions and at elevated temperatures is one of the requirements of ideal AEMs. Due to high alkaline stability, imidazolium with various substitutes is an auspicious candidate as a stable cation for AEMs. Here, the current improvement of the chemical durability of imidazolium cations and corresponding AEMs is reviewed. And included the investigations which were done by both theoretical calculations and experimental studies. The strategy for enhancing the chemical durability of imidazolium groups and the mechanical properties of equivalent AEMs have been summarized. This study will offer an outline of some significant features for designing highly alkaline stable imidazolium cationic groups and their analogous AEMs in the future.

Alkaline Stability of Anion-Exchange Membranes.

Recently, the development of durable anion-exchange membrane fuel cells (AEMFCs) has increased in intensity due to their potential to use low-cost, sustainable components. However, the decomposition of the quaternary ammonium (QA) cationic groups in the anion-exchange membranes (AEMs) during cell operation is still a major challenge. Many different QA types and functionalized polymers have been proposed that achieve high AEM stabilities in strongly alkaline aqueous solutions. We previously developed an ex situ technique to measure AEM alkaline stabilities in an environment that simulates the low-hydration conditions in an operating AEMFC. However, this method required the AEMs to be soluble in DMSO solvent, so decomposition could be monitored using 1H nuclear magnetic resonance (NMR). We now report the extension of this ex situ protocol to spectroscopically measure the alkaline stability of insoluble AEMs. The stability ofradiation-grafted (RG) poly(ethylene-co-tetrafluoroethylene)-(ETFE)-based poly(vinylbenzyltrimethylammonium) (ETFE-TMA) and poly(vinylbenzyltriethylammonium) (ETFE-TEA) AEMs were studied using Raman spectroscopy alongside changes in their true OH- conductivities and ion-exchange capacities (IEC). A crosslinked polymer made from poly(styrene-co-vinylbenzyl chloride) random copolymer and N,N,N',N'-tetraethyl-1,3-propanediamine (TEPDA) was also studied. The results are consistent with our previous studies based on QA-type model small molecules and soluble poly(2,6-dimethylphenylene oxide) (PPO) polymers. Our work presents a reliable ex situ technique to measure the true alkaline stability of AEMs for fuel cells and water electrolyzers.

Multi-atom Pt and PtRu catalysts for high performance AEMFCs with ultra-low PGM content

To reduce the platinum group metal (PGM) loading in anion exchange membrane fuel cells (AEMFCs), it is important to transition to catalysts with very low PGM content, and eventually to catalysts that are completely PGM-free. In this work, four supported low-PGM Pt and PtRu catalysts were prepared using a new, simple, scalable technique: Controlled Surface Tension (CST) method. CST allows for a high density of very small multi-atom clusters. Catalysts were physically characterized using a wide array of techniques and tested for their ORR and HOR activity both ex-situ and integrated into operating AEMFCs. The PGM loading was reduced by a factor of 14 while achieving comparable performance to commercial catalysts. AEMFCs were also assembled with ultralow PGM loading (0.05 mgPGM cm-2), where PtRu anodes were paired with Fe–N–C cathodes to achieve a specific power of 25 W/mgPGM (40 W/mgPt).

Distribution of relaxation times as an accessible method to optimize the electrode structure of anion exchange membrane fuel cells

Recent significant progress in performance of anion exchange membrane fuel cells (AEMFCs) is proving this technology to be a meaningful low-cost alternative to commercial proton-conducting fuel cells. This progress has been largely driven by improved water management through electrode layer engineering. The plethora of membrane and ionomer chemistries makes the optimization of the electrode composition often unreproducible, leading to inaccurate performance comparisons. In this study we introduce the distribution of relaxation times (DRT) as an accessible and convenient tool to diagnose the limiting polarization processes in AEMFCs, which significantly simplifies the electrode optimization. To demonstrate the potential of DRT diagnosis, we fabricate cells with different anode electrode compositions simulating the effects of water transport limitations, such as anode flooding and cathode dry-out. The DRT separates and quantifies the contributions to the polarization resistance of the following processes: cathode and anode charge transfer, ion transport in the catalyst layer and gas diffusion. The anode catalyst layer optimization using information from the DRT has helped to increase the peak power density of the current cells from 550 to 690 mW/cm2. We foresee a large potential in using the DRT approach for the diagnosis and optimization of AEMFCs’ operational, structural, and compositional parameters.

Modeling direct ammonia anion-exchange membrane fuel cells

Ammonia has recently been proposed as a promising candidate fuel for anion-exchange membrane fuel cell (AEMFC) technology. Direct ammonia AEMFCs (DA-AEMFCs) are a carbon-free technology that combines the ammonia's high energy density with the fuel cells' high efficiency. However, two major challenges face this technology: ammonia crossover (due to ammonia's high solubility in water) and sluggish ammonia oxidation reaction (AOR). We have developed, applied, and presented a one-dimensional and transient model of a DA-AEMFC system to address these challenges. Excellent agreement is obtained between the experimentally-measured and computationally-simulated performance of DA-AEMFCs operating at 100 and 120 °C with KOH-free anode feed. As the current density increases, the initial cell performance analysis reveals a reduction in the parasitic AOR rate through the cathode. More intriguingly, the results demonstrate a positive impact of ammonia crossover on cell longevity. Crossover drives an AOR within the cathode, which has a detrimental effect on performance but comes with an associated benefit of water generation in this region. The resultant improvement in the cathode hydration reduces the degradation rate of ionomeric material, ultimately increasing cell lifetime. Further studies are required to determine the desired rate of ammonia crossover and its influence on the system's cost-effectiveness.

Influence of Ion-Exchange Capacity on the Solubility, Mechanical Properties, and Mass Transport of Anion-Exchange Ionomers for Alkaline Fuel Cells

We have studied the solubility, mechanical properties, and mass transport behavior of a series of methylpiperidinium-functionalized polyethylene (PEPM) ionomers with different ion-exchange capacities (IECs) for anion-exchange membrane fuel cells (AEMFCs). The influence of IEC on the ionomer viscoelasticity in water and the transport/permeability of charged and uncharged species were investigated by acoustic impedance tests and electrochemical measurements, respectively. We found that the solubility of the PEPM ionomers increased with increasing IEC, while the rigidity of the ionomer films in water was reduced due to the higher content of the hydrophilic cations in the ionomers. In electrochemical tests, N,N,N′,N′-tetramethyl-p-phenylenediamine (TMPD) and 1,4-benzoquinone (BQ) were used as redox probes with ionomers serving as inert layers coating the electrodes. Analysis of the cyclic voltammetric profiles indicated that the electrostatic effects from the ionomers exclude cations, such as TMPD+, from the ionomer-coated electrode and attract anions like BQ2– into the ionomer films. Such effects intensified with an increase in the IEC, which could be screened by increasing the ionic strength of the electrolyte. Rotating disk electrode experiments indicated that the increased IEC had a negative impact on the permeability of neutral species. In addition, MEA studies with symmetric and asymmetric ionomers provided preliminary results on the impact of the IEC on fuel cell performance. This work provides insights into the design of ionomers and membranes with deliberately controlled IEC for different applications in AEMFCs from the stand points of solubility, mechanical properties, and mass transport.

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.

Ionomer degradation in catalyst layers of anion exchange membrane fuel cells.

Anion exchange membrane fuel cells (AEMFCs) that operate at high pH, offer the advantage of enabling the use of abundant 3d-transition metal-based electrocatalysts. While they have shown remarkable improvement in performance, their long-term durability remains insufficient for practical applications with the alkaline polymer electrolytes (APEs) being the limiting factor. The stability of APEs is generally evaluated in concentrated alkaline solutions, which overlooks/oversimplifies the complex electrochemical environment of the catalyst layer in membrane electrode assembly (MEA) devices. Herein, we report a study of the degradation of the membrane and ionomer independently under realistic H2-air (CO2 free) fuel cell operation, using proton nuclear magnetic resonance (1H-NMR), cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and X-ray photoelectron spectroscopy (XPS). While the membrane degradation was minimal after the AEMFC stability test, the ionomer in the catalyst layers degraded approximately 20% to 30% with the cathode being more severely affected than the anode. The ionomer degradation decreased the catalyst utilization and significantly increased the ionic resistance, leading to significant performance degradation in the AEMFC stability test. These findings emphasize the importance of ionomer stability and the need to consider the electrochemical environments of MEAs when evaluating the stability of APEs.

Material and system development needs for widespread deployment of hydroxide exchange membrane fuel cells in light-duty vehicles

We have developed a comprehensive system model for hydroxide exchange membrane fuel cell (HEMFC)-based light-duty vehicles which allows us to determine the material and system developments needed to enable affordable HEMFC-based cars for the mass market.

Ru-doped WO3 enabling efficient hydrogen oxidation reaction in alkaline media.

Developing highly efficient hydrogen oxidation reaction (HOR) electrocatalysts in alkaline media is of great significance for anion exchange membrane fuel cells (AEMFCs). Herein, we report an efficient Ru-doped hexagonal tungsten trioxide (Ru-WO3) HOR electrocatalyst through a hydrothermal strategy. The prepared Ru-WO3 electrocatalyst exhibits enhanced HOR performance with a 6.1 times higher exchange current density and better durability than commercial Pt/C. Structural characterizations and theoretical calculations reveal that the uniformly distributed Ru was modulated by oxygen defects and the electron transfer from O to Ru modulated the H* adsorption of Ru sites. In addition, plenty of W sites can serve as hydroxyl adsorption sites to accelerate the HOR kinetics. This work not only provides an efficient HOR catalyst in alkaline media, but also promotes the fundamental understanding of the effect of modulation on the adsorption of H* and *OH in tungsten oxides with a relatively low oxidation state by Ru doping, broadening the range of HOR catalysts to Ru-doped metal oxides.

Conformational distortion of the ionomer backbone for reinforcing the catalyst layer under dynamic operation

The conformational distortion of anion exchange ionomers can strengthen the catalyst layer under dynamic conditions resulting in a robust fuel cell performance.