Anion Exchange Membrane Fuel Cells Research Articles

Four-arm star-shaped high-performance poly(aryl piperidine) anion exchange membranes for fuel cells

Anion exchange membrane fuel cells (AEMFCs) are a new generation of energy conversion technology that excels proton exchange membrane fuel cells in terms of oxygen reduction kinetics.

Various states of water species in an anion exchange membrane characterized by Raman spectroscopy under controlled temperature and humidity.

Anion exchange membrane fuel cells (AEMFCs) hold the key to future mass commercialisation of fuel cell technology, even though currently, AEMFCs perform less optimally than proton exchange membrane fuel cells (PEMFCs). Unlike PEMFCs, AEMFCs have demonstrated the capability to operate independently of Pt group metal-based catalysts. Water characterization inside the membrane is one factor that significantly influences the performance of AEMFCs. In this paper, different water species inside an anion exchange membrane (AEM), QPAF-4, developed at the University of Yamanashi, were studied for the first time using micro-Raman spectroscopy. Spectra of pure water, alkaline solutions, and calculations based on density functional theory were used to identify the water species in the AEM. The OH stretching band was deconvoluted into nine unique Gaussian bands. All the hydrogen-bonded OH species increased steadily with increasing humidity, while the CH and non-H-bonded OH remained relatively constant. These results confirm the viability of micro-Raman spectroscopy in studying the various water-related species in AEMs. The availability of this technique is an essential prerequisite in improving the ionic conductivity and effectively solving the persisting durability challenge facing AEMFCs, thus hastening the possibility of mass commercialisation of fuel cells.

Chemical States of Water in Anion Exchange Membrane for Fuel Cells Using Raman and Coherent Anti-Stokes Raman Spectroscopies

Purpose Low membrane durability remains a major challenge in anion exchange membrane fuel cells (AEMFCs). This is despite the AEMFCs’ proven potential to eliminate the dependency of Fuel cells on Pt catalysts and bring about the mass commercialization of the technology. This problem can be effectively addressed through combined optimization of the AEMFCs’ operating conditions during evaluation, followed by finetuning of the membrane chemistry to address any observed weaknesses. After synthesis, water distribution is the primary operation condition determining a given AEM’s performance and lifespan [1]. This is partly because water affects the ionic conductivity and, thus, maximum current density, which is known to play a significant role in the longevity of membranes. Therefore, establishing the water distribution during power generation for any given membrane is highly desirable. For this to happen, suitable and reliable techniques must exist to measure and compare the water distribution in situ and operando. In this paper, we used in-house assembled Raman spectroscopy and coherent anti-Stokes Raman scattering spectroscopy (CARS) systems to measure the water in an operational AEMFC for the first time. CARS comes with a high time resolution of 0.1 s, which is essential in the transient response study of FCs. Experimental Method QPAF-4 membrane film (Fig. 1) of the thickness of 30 µm and IEC of 2.0 meq g-1 was solution cast as reported [2]. The cell was then assembled, as reported [3]. An airtight transparent quartz window 200 μm thick was mounted at the cathode endplate for optical access to the QPAF-4 membrane through a 500 mm pinhole in the GDE. A thin Pt disk was placed on the opposite side of the membrane to reflect the signal to the spectrometer through the ×50 objective lens used. For Raman spectroscopy, a 632 nm, 1 mW laser with 5.5 min total exposure was used. For CARS, an 11 mW 785nm pump laser and 16 mW Stokes laser with 200 ms total exposure were used. The flow rate was 100 ml min-1 for all gases. The cell temperature was maintained at 60 °C. The relative humidity of the gas varied from 30% to 100% in steps of 10% RH. The system was allowed to stabilize for 3 hours at each humidity level before measurement. Results and discussion Figures 2 and 3 show the normalized in situ spectra recorded at the center of the membrane at different relative humidity values using CARS and Raman, respectively. The peak positions are comparable. However, the OH peak in CARS is taller relative to C=C than in Raman. CARS signal intensity is generally stronger than Raman [3]. Within the CARS peak, the purely OH portion of the signal is more prominent relative to the rest of the peak than in Raman. This is attributed to the fact that the Infrared laser associated with the stokes light (817-1144nm) of CARS is known to be more sensitive to OH vibration than the lower wavelength laser (632nm) used in Raman [4]. Conclusion We successfully developed a 785nm CARS system capable of operando water measurement in AEMFCs. The results from this technique are comparable to those from micro-Raman spectroscopy but with better signal intensity and time resolution. This increases the number of practical methods available to researchers interested in measuring water content in AEMFCs allowing for faster development of the technology. The water distribution results during power generation will be discussed at the conference. With CARS, higher temporal/time resolution is achieved, which is advantageous when cell response needs to be studied.

Effect of Asymmetrically Reinforced Membrane on Anion Exchange Membrane Fuel Cell

Anion Exchange Membrane Fuel Cells (AEMFCs) is promising for the generation of electric power from hydrogen, with the expected possible replacement of precious metal catalysts by Earth abundant ones.[1-2] Beyond the independent properties of core materials (AEM, anode and cathode catalysts), the water management during operation is key to achieving high power density, but also high longevity. Thin AEMs are critical to achieve high AEMFC power performance, as the back-diffusion of water from anode to cathode critically helps to mitigate anode flooding. [3] Due to the high swelling of hydrated AEMs, good mechanical durability of thin AEMs however typically requires the introduction of a reinforcement, often a porous polytetrafluoroethylene (PTFE) membrane filled with the anion exchange ionomer.Here, we investigated the structure of different commercial AEMs and their behavior in operating AEMFC. We identified that the position of the reinforced layer (PTFE or other) strongly differs from one type of AEM to another. In the case of strongly asymmetric through-plane position of the PTFE layer (Fig. 1a), the positioning of the PTFE reinforcement close to anode or to cathode has a significant impact on initial power performance (Fig. 1b). A better performance and improved stability was observed with the PTFE layer close to the anode, suggesting that the asymmetric structure can improve the water management in such configuration. The presentation will discuss the results obtained on different commercial AEMs, and whether the asymmetric structure of AEM can be combined with PTFE hydrophobized anodes for even improved performance. The effect of the reinforcement position on the water management was also studied operando by two techniques. With high energy X-ray diffraction technique on an AEMFC under current load, the water distribution profile across the MEA thickness could be obtained from phase deconvolution analysis. With the use of relative humidity sensors at the outlet of anode and cathode of a regular AEMFC setup, [4] the water balance at a given current load could be established and compared for the two different positionings of reinforced AEM.The results shed light on the importance of identifying the position of PTFE reinforcement in commercial or in-house AEM for AEMFC application, with potential implications also for AEM electrolyzers or water-CO2 co-electrolyzers.

Improved Activity and Durability of Carbon-Coated Pd and PdNi Anode Catalysts in AEMFC

Anion exchange membrane fuel cells (AEMFC) have received growing attention due to the strategic potential of using low content (or absence) of PGM catalysts in the electrodes.[1] Completely PGM-free cathode catalysts are well established in those conditions,[2] while the anode ones remain critical in terms of the beginning-of-life activity and long-term performance.[3] The widely Pt- and Pd-based catalysts degrade mainly by nanoparticles detachment and agglomeration in alkaline conditions.[4] These degradations may be slowed down by wrapping the nanoparticles in some carbon layers (called carbon-coated), that prevent also metal oxidation and leaching into the electrolyte in the RDE setup.[5] However, the performance in AEMFC of carbon-coated nanoparticle catalysts is still unclear.Here, two groups (previously characterized in RDE, XRD, IL-TEM, XPS and ICP-MS techniques[5]) of carbon-coated catalysts were investigated in 5 cm2 AEMFC: monometallic Pd (Pd@C/C) and bimetallic PdNi (PdNi@C/C) nanoparticles supported on carbon, and compared them to unprotected ones (Pd/C and PdNi/C). The four different anode catalysts were compared for a fixed Pd anode loading of 0.27 mgPd cm-2, and were combined with a ZIF-8 derived Fe-N-C cathode catalyst (previously developed Fe0.5-NH3). The Pd@C/C and PdNi@C/C reached superior initial performances (700 and 1300 mA cm-2) than the Pd/C and PdNi/C references (300 and 390 mA cm-2) at 0.6 V and Tcell = 60 °C. This translates into an initial activity trend of PdNi@C/C > PdNi/C > Pd@C/C > Pd/C. After 12 h durability test at a cell voltage of 0.3 V, the AEMFC with the PdNi@C/C based anode exhibits a dramatically higher retention of performance (95%) than the cells with Pd/C (55%) and PdNi/C (70%) anodes. These results evidence the robustness of bimetallic carbon-coated nanoparticle catalysts for AEMFC anodes. Figure 1

Hydrogen Crossover in Anion Exchange Membrane Fuel Cells

In recent years anion exchange membrane fuel cells (AEMFC) have started to draw more attention by potentially allowing catalysts based on more abundant materials, and by showing high performance [1]. One often overlooked parameter in these systems is membrane permeability of hydrogen, often referred to as hydrogen crossover, which directly affects fuel cell stability and efficiency. This undesirable effect creates mixed potential due to the permeated hydrogen reacting with oxygen on the cathode, and not generating useful electric energy, only water and heat [2]. The generated heat creates hotspots of temperatures enough to cause membrane defects or even destruction which further accelerates the degrading process [3].During fuel cell operation relative humidity (RH) and temperature varies in the system, therefore in this study hydrogen crossover is measured at different temperatures (between 50 and 70 ⁰C) and RH (between 30 and 100 %), on Aemion anion exchange membranes (AEMs) with and without reinforcement and with a variety of membrane thicknesses (25 and 50 μm in Figure 1. a). The hydrogen crossover is measured using a mass spectrometer (MS) at the inert gas exhaust. To avoid effects of other components pure membranes and gas diffusion layers (GDLs) were utilized without electrodes.It is shown that AEMs increase in permeability with temperature for all tested while the effect of RH depends on if they are reinforced or not (Figure 1. a). The non-reinforced membranes show a constant decrease in hydrogen crossover with increasing RH, while the reinforced show constant hydrogen crossover until full humidification where it sharply decreases. While the observed trends are different, the quantitative values between reinforced and non-reinforced membranes were relatively similar: at dry conditions, RHs lower than 50 %, crossover is 10 to 30 % lower for reinforced membranes, around 10 % higher at 70 % RH, and at full humidification a non-significant difference compared with non-reinforced (Figure 1. a). It is shown that an increase in the membrane thickness is not proportional to the decrease in hydrogen crossover, suggesting that interface resistance is not negligible. The calculated interface resistance is higher than the bulk resistance for both reinforced and non-reinforced membranes at all RHs (Figure 1. b). However, the bulk resistance is higher for reinforced membranes than for non-reinforced, while interface resistances are relatively similar. Interestingly, the ratio between the interface and the bulk resistance remains constant at all RHs. However, for the membranes without reinforcement the bulk resistance is 20 % of total crossover resistance, while for reinforced it is two times higher, 40 %. Scanning electron microscope (SEM) cross-section analysis shows that the reinforcement is a relatively thin layer of a different polymer in the membrane, which suggests that even a very thin reinforcement could affect membrane permeability properties. Understanding the effects of the reinforcement and their influence on fuel cells will allow for more durable and safer AEMFCs.Figure 1. H2 crossover measured using a MS at the inert gas side. a) At different temperatures, relative humidities and membrane thicknesses. b) H2 crossover resistance at the interface and in the bulk for membranes with and without reinforcement. Measurements are done at 70 °C and at flows 20 ml/min of both hydrogen and the inert gas (Ar).[1] D. R. Dekel, “Review of cell performance in anion exchange membrane fuel cells,” J. Power Sources, vol. 375, pp. 158–169, 2018, doi: 10.1016/j.jpowsour.2017.07.117.[2] S. S. Kocha, J. D. Yang, and J. S. Yi, “Characterization of Gas Crossover and Its Implications in PEM Fuel Cells,” 2006, doi: 10.1002/aic.10780.[3] Q. Tang, B. Li, D. Yang, P. Ming, C. Zhang, and Y. Wang, “Review of hydrogen crossover through the polymer electrolyte membrane,” Int. J. Hydrogen Energy, vol. 46, no. 42, pp. 22040–22061, 2021, doi: 10.1016/j.ijhydene.2021.04.050. Figure 1

In-Situ Analysis of Electrodes Using Fine-Tuned Poly(Terphenyl Piperidinium) Ionomers in Anion Exchange Membrane Fuel Cells

In the field of low temperature polymer electrolyte fuel cells, anion exchange membrane fuel cells (AEMFCs) have been considered a promising alternative by circumventing the obstacles to commercialization faced by the established PEMFC technology. The application of platinum group metal-free catalysts for the oxygen reduction reaction, the potential to utilize cheaper and more sustainable polymers than the industry benchmark Nafion®, and the generally less corrosive alkaline operation conditions, have all been identified as inherent advantages of the AEMFC technology, potentially enabling more cost-effective and environmental-friendly mass production of AEMFC system.The first major challenge in developing AEMFC systems was the lack of stable and conductive anion exchange polymers to be used as membrane and ionomer materials. This challenge was at least partly overcome by the successful development of a variety of next-generation polymers. Among those, poly(arylene piperidinium) show great potential through the combination an ether-free aromatic backbone for increased mechanical stability with a piperidinium-based cationic groups displaying excellent resistance to hydroxide attack in highly alkaline environment [1]. Furthermore, the crucial parameters ion exchange capacity (IEC), water uptake and dimensional swelling can be finely controlled via copolymerization and partial substitution of the cationic functional group in the repeating unit [2].In a previous study using a series of poly(terphenyl piperidinium) membranes with different IECs, we established the importance of these three parameters, in particular for the ionomer used in the catalyst layer of the electrode [3]. Furthermore, it was established that water balance of AEMFC systems proved to be one of the greatest challenges in reaching and maintaining high cell performance. The asymmetrical nature of the simultaneous electrochemical production and consumption of water at the anode and cathode, respectively, necessitates fine-tuning the ionomer properties, as well as selecting operational parameters for each electrode individually in order to strike an optimal water balance [4].In this study, we expand on our previous work by analyzing a matrix of electrode combination based on three poly(terphenyl piperidinium)ionomers with different IEC values. Furthermore, we complemented our analysis by evaluating the effects of substituting a fraction of regular dissolvable ionomer with insoluble non-conformal particles of cross-linked poly(terphenyl piperidinium). The beneficial effects of insoluble non-conformal ionomer particles in the catalyst layer has been previously shown by Varcoe and coworkers utilizing radion-grafted anion exchange membranes [5], and by Holdcroft and coworkers utilizing phenylated poly(phenylene) ionomers for PEMFCs [6].Different electrode combinations were evaluated while varying operational parameters such as gas flow rates, gas relative humidities and back pressure. The recording of observed cell performances were complemented by in-situ analysis of in-let/out-let gas relative humidities, H2/O2 electrochemical impedance spectroscopy for kinetic analysis and information on ohmic losses, H2/Ar electrochemical impedance spectroscopy to measure ionic conductivity, cyclic voltammetry to measure electrochemically active surface area as well as ex-situ characterization of the electrodes before/after the test run.The study was carried out with the aim to establish a correlation between the properties of individually tuned ionomers and operational parameters, and to investigate reasons for performance losses due to mass transport limitations, drop in ionomer conductivity, loss of active area, and decreased kinetics caused by phenyl adsorption.Finally, we demonstrate how the application of individually optimized electrodes using fine-tuned poly(terphenyl piperidinium) based ionomers in conjunction with suitable operating conditions will lead to a drastic increase in fuel cell performance compared to symmetrical non-optimized electrodes.A selection of experimental data is shown in Figure 1.[1] J. S. Olsson, T. H. Pham, P. Jannasch, Adv. Funct. Mater. 2017, 28, 1702758.[2] J. S. Olsson, T. H. Pham, P. Jannasch, Tuning poly(arylene piperidinium) anion-exchange membranes by copolymerization, partial quaternization and crosslinking, J. Membrane Sci., 2019, 578, 183-195.[3] T. Novalin, D. Pan, G. Lindbergh, C. Lagergren, P. Jannasch, R.W. Lindström, Electrochemical performance of poly(arylene piperidinium) membranes and ionomers in anion exchange membrane fuel cells, J. Power Sources, 2021, 507, 230287. [4] D. P. Leonard, S. Maurya , E. J. Park , S. Noh , C. Bae , E. D. Baca , C. Fujimoto and Y. S. Kim , J. Mater. Chem. A, 2020, 8 , 14135-14144.[5] L. Q. Wang , J. J. Brink , Y. Liu , A. M. Herring , J. Ponce-Gonzalez , D. K. Whelligan and J. R. Varcoe, Energy Environ. Sci., 2017, 10 , 2154-2167.[6] E. Balogun, S. Cassegrain, P. Mardle, M. Adamski, T. Saatkamp, and S. Holdcroft, ACS Energy Lett. 2022, 7, 2070-2078. Figure 1

Metal−Organic Framework CPO-27 Derivatives As Non-Precious Metal Catalyst for Anion Exchange Membrane Fuel Cell

Fuel cell stands as an environmentally friendly technology harnessing the power of hydrogen to generate clean water. In the range of fuel cell types, the anion exchange membrane fuel cell (AEMFC) stands out, effectively converting chemical energy to electricity without relying on platinum catalyst. Nonetheless, platinum catalyst is still used in addressing the sluggishness of the oxygen reduction reaction (ORR) during AEMFC operation. The scarcity and expense of platinum, however, hinder its widespread adoption. Furthermore, in the alkaline environment, platinum surface is covered with OH species, impeding active sites and diminishing electrochemical activity. Consequently, seeking an efficient, stable, and abundant platinum substitute is a primary target in the development of fuel cell.This study focuses on the development of the non-precious metal catalyst for the ORR in AEMFC. The catalyst, denoted as NiCo2-CPO-27/PCN, is derived from a carbon nanorod structure obtained from the metal-organic framework (CPO-27). Through the optimized process, the catalyst demonstrates the distinct carbon nanorod configuration. Additionally, the NiCo2-CPO-27/PCN catalyst exhibits enhanced ORR activity, achieving the electron transfer number of 3.99, which closely approaches the ideal four-electron transfer number. This improvement in activity is attributed to factors such as the specific structure, heteroatom contribution, and coordination arrangement of the catalyst.

From Synthesis to Application: Ni3 Fe N As Oxygen Reduction Electrocatalyst in Anion Exchange Membrane Fuel Cells

Catalysis has played a crucial role in advancing and commercializing energy conversion technologies. It is essential to identify abundant, active, and stable materials to enable the reliable and cost-efficient use of catalysts in renewable technologies such as fuel cells (FCs) and electrolyzers. Suitable candidates, like nonprecious metals, can be found in the transition metals' first row, where materials like bimetallics, metal oxides, and metal nitrides can be obtained. Recently, these materials have shown high activity toward the Oxygen Reduction Reaction (ORR) and the Oxygen Evolution Reaction (OER) in an alkaline medium, which can be related to good performance in FCs and electrolyzers. However, most of the studies do not go further than half-cell reactions. In this study, we explored the synthesis of a metal nitride, Ni3FeN, and its application as a cathode material in hydrogen fuel Anion Exchange Membrane Fuel Cells (AEMFC). A one-step ammonolysis synthesis was done to obtain Ni3FeN supported on carbon. Various metal loadings were synthesized with nanoparticles sizes of 7 ± 2 nm and 10 ± 4 nm for 40% and 60%, respectively, revealed with Transmission Electron Microscopy (TEM). High-resolution Scanning Transmission Electron Microscopy (STEM) images and Energy Dispersive X-ray (EDX) mapping analysis revealed a metal nitride core and a metal oxide shell of ca. 3 nm. The bulk electronic structure was further evaluated using X-ray Absorption Spectroscopy (XAS), where the EXAFS modeling suggests a bulk nitride electronic structure. Consequently, cyclic voltammetry (CV) was used to investigate the electrochemical properties and ORR performance of the Ni3FeN catalyst in 1 M KOH. The catalyst exhibited a stable electrochemical structure with a slight decrease in E1/2 of 10 mV (from 0.85 V to 0.84 V) during a 100 K ORR accelerated durability test (ADT). In addition, Levich and Rotating Ring Disk Electrode (RRDE) analyses showed that the number of electrons transferred during the ORR was consistent with a 4-electron pathway. Furthermore, the Ni3FeN catalyst was tested as the cathode in an AEMFC, obtaining a power density output of 400 mW/cm2, highlighting its potential for alkaline medium fuel cell applications.

Optimization of PdCu Alloys as Efficient and Durable HOR Electrocatalysts in Anion Exchange Membrane Fuel Cells

Anion-exchange membrane fuel cells (AEMFCs) allow for the use of Pt-free or precious group metal (PGM-free) catalysts and for a wider choice of cell and stack materials. Currently, there are still development challenges, such as the need to improve the efficiency of anode catalysts, due to the sluggish hydrogen oxidation reaction (HOR) in alkaline electrolytes. It has been demonstrated that the HOR rate can be improved using bifunctional materials, which have both an optimized hydrogen binding energy (HBE) and a favorable OHads binding energy (OHBE) [1,2]. In literature, experimental and theoretical studies demonstrate that HOR electrochemical activity of PdCu alloys strongly depends on their crystalline structure, which can be tuned through thermal treatment (300-600°C) modifying the Body-Centered Cubic (BCC) and Face-Centered Cubic (FCC) phase ratio [3-5]. On this basis, a FCC-phased PdCu(1:1) alloy supported on Vulcan XC-72 was synthesized via a metal-vapor method (MVS) and furtherly modified through thermal annealing (5 hours under N2 atmosphere) in order to obtain the appearance of an ordered BCC structure. The HOR performance and stability of these materials were tested for first time in a complete in AEMFC. The use of PdCu allows us to lower the effective Pd loading on the electrode to about 0.16 mg cm-2, obtaining in the case of PdCu/C-500°C (where FCC-BCC phases coexist) a maximum power density of 941 mW/cm2. The increased activity and stability of the PdCu/C-500°C was demonstrated through long stability test, where it exhibits an average of 685 mW/cm2 of power density over 15 hours of testing during 3 days in AEMFC fixed at a constant potential of 0.4 V and 80°C (H2 0.5 L/min, O2 1 L/min), 10% more of the FCC-phased PdCu. Moreover, acquiring a final scan of potential at the end of the experiments, it is clear that PdCu/C 500°C suffers of a minor drop of performances (only of 19%), respect with disordered FCC-phased PdCu which efficiency in terms of maximum power density drops of 30% (Figure 1). Figure 1

Non-Precious Hydrogen Oxidation Catalysts for Anion Exchange Membrane Fuel Cells

Green hydrogen as an energy carrier has the potential to dramatically reduce our greenhouse gas emissions and meet the net-zero emissions targets set by the Paris Agreement countries by 2050. This hydrogen can be produced by electrolysis using water and renewable electricity, and be converted into electricity on demand in a fuel cell. Proton exchange membrane fuel cells (PEMFC) are already commercialized, but due to their acidic environment, these devices require precious metal catalysts, in particular platinum, which is an obstacle to their large-scale deployment. In contrast, anion exchange membrane fuel cells (AEMFCs) operate at high pH, facilitating the use of cost-effective and sustainable precious metal free catalysts.To catalyze the hydrogen oxidation reaction (HOR) in alkaline media, nickel has shown interesting performances to replace precious metal catalysts at the anode of AEMFCs [1]. In order to improve the intrinsic activity of nickel-based materials and reach the performance of precious metals catalysts, two approaches are studied in the literature: optimization of intrinsic properties by alloying Ni with other earth abundant elements, or core@shell nanostructuration of Ni by a protective carbon shell. We are focused on this second approach through the mechanochemical synthesis of a nickel-based Metal-Organic Framework (MOF) from a Ni2+ salt and BTC (1,3,5-benzenetricarboxylic acid), followed by pyrolysis under different atmospheres (Figure 1.a). As already shown in the literature [2], there is a significant effect of the pyrolysis atmospheres (NH3, H2, N2, in various ratios) on the electrochemical performance of the catalysts (Figure 1.b). In this study, we employed ex situ and operando studies approaches to investigate the influence of these different atmospheres on the nanoparticles structuration and to relate them to the electrochemical performances obtained.Our Ni-based anode materials were characterized by X-ray diffraction, electron microscopy, nitrogen adsorption, and electrochemical techniques, including AEMFC tests. Furthermore, the transformation of the MOF into an active catalyst was followed with in-situ X-ray absorption spectroscopy.[1] Zhao, J. Chen, W. Sun, H. Pan, «Non-Platinum Group Metal Electrocatalysts toward Efficient Hydrogen Oxidation Reaction», Adv. Funct. Mater., vol. 31, no 20, 2021, p. 2010633.[2] Ni et al., « An efficient nickel hydrogen oxidation catalyst for hydroxide exchange membrane fuel cells », Nat. Mater., vol. 21, no 7, 2022, p. 804. Figure 1

Unveiling the Impact of Carbon Corrosion on the Degradation of Fe-N-C Catalyst Layers in Alkaline Media

One of the most promising candidates to replace platinum group metal catalysts for oxygen reduction reaction (ORR) in fuel cells (FCs) is the sub-class of iron-nitrogen-carbon (Fe-N-C) catalysts. However, Fe-N-C materials considerably suffer from varied degradation mechanisms. [1-2] Compared to FC operating conditions, start/stop events where the cathodes experience anodic potential may be more damaging due to the carbon corrosion phenomenon. [3] The correlation between carbon corrosion and Fe dissolution rates has been reported in aqueous model systems in acidic media. [3] However, different carbon corrosion mechanisms are reported for acidic and alkaline media. [4] While anion-exchange membrane FCs (AEMFCs) have gained increased attention, studies on the effects of carbon corrosion on realistic AEMFC Fe-N-C catalyst layers are still missing.In this work, using a gas diffusion electrode (GDE) half-cell coupled with inductively coupled plasma mass spectrometry (ICP-MS), [5] we observed that the rate of Fe loss significantly accelerates with rising potential (E > 1.0 VRHE), commonly experienced during the start/stop events. Increased temperature intensifies the rate of Fe leaching during carbon corrosion (see Figure 1), while the gas atmosphere (Ar or O2) shows a negligible influence. On the contrary, the subsequent Fe deposition and the drop of ORR activity depend on the presence of O2 and the varied temperature. Combining in situ and post-mortem analyses, we report how carbon corrosion in alkaline media degrades Fe-N-C catalyst layers in various atmospheres and at different temperatures. These insights can contribute to rational designs of AEMFCs' start/stop protocol and more robust Fe-N-C materials. Figure 1. Fe-N-C demetallation during anodic potential holds (1.0 - 1.5 VRHE) in O2-saturated alkaline (0.1 M NaOH) environment at 22 ± 2 and 62 ± 2 ℃. (A) Potential profile. (B) The Fe dissolution rate normalized to catalyst loading.

Electrochemistry of and Electrocatalysis by Low-Symmetry N4 Macrocycles

The introduction of efficient and affordable catalysts for three seemingly simple electrochemical reactions could improve very much the sustainability of our energy supply: reduction of protons to hydrogen gas (the hydrogen evolution reaction, HER), water oxidation, and selective reduction of oxygen to water (the oxygen reduction reaction, ORR).1–6 Platinum group metals (PGM) are outstanding catalysts for all these reactions, but being rare and expensive prevents their utility for large scale operations.7 Within the search for catalysts based on earth abundant 1st row transition metals, we now introduce two different molecular catalysts: Co(III) complexes of corroles, with very different in size meso-C substituents (C6F5 >> CF3 >> H), as well by newly synthesized and fully characterized Co(II), Cu(II) and Ni(II) monoazaporphyrins (MAzP) with meso-CF3 substituents. All the catalyst were adsorbed on carbon supports with very different porosity (Vulcan << BP2000). The corrole-modified electrodes were studied as ORR catalysts, revealing that the onset potential of the best performing cobalt corrole is approaching that of Pt and with low percentage formation of undesired hydrogen peroxide. Testing that complex as a cathode for an anion exchange membrane fuel cell (AEMFC) revealed high power density relative to other molecular catalysts and excellent stability over 12 hours.Metal complexes of a novel MAzP with CF3 on the three meso-C positions were studied as catalysts for HER in PBS buffer (pH=7.4), as well as for water oxidation and ORR at pH 13. The cobalt complex performed best in all processes. Compared to cobalt porphyrins with either three or four CF3 groups on the meso-C positions, Co-MAzP performed better for water oxidation. For HER catalysis, it has the same onset potential and current as the tetra-CF3 cobalt porphyrin and more current than the tris-CF3 analogue. In ORR catalysis, Co-MAzP performs with the lowest onset potential, but less H2O2 is formed via catalysis by the other porphyrins.(1) Meng, J.; Lei, H.; Li, X.; Qi, J.; Zhang, W.; Cao, R. Attaching Cobalt Corroles onto Carbon Nanotubes: Verification of Four-Electron Oxygen Reduction by Mononuclear Cobalt Complexes with Significantly Improved Efficiency. ACS catalysis. 2019, pp 4551–4560.(2) Beyene, B. B.; Hung, C.-H. Recent Progress on Metalloporphyrin-Based Hydrogen Evolution Catalysis. Coord. Chem. Rev. 2020, 410, 213234.(3) Mondal, B.; Chattopadhyay, S.; Dey, S.; Mahammed, A.; Mittra, K.; Rana, A.; Gross, Z.; Dey, A. Elucidation of Factors That Govern the 2e–/2H+ vs 4e–/4H+ Selectivity of Water Oxidation by a Cobalt Corrole. J. Am. Chem. Soc. 2020, 142 (50), 21040–21049.(4) Cui, X.; Cui, Y.; Chen, M.; Xiong, R.; Huang, Y.; Liu, X. Enhancing Electrochemical Hydrogen Evolution Performance of CoMoO4-Based Microrod Arrays in Neutral Media through Alkaline Activation. ACS Appl. Mater. Interfaces 2020, 12 (27), 30905–30914.(5) Fornaciari, J. C.; Weng, L.-C.; Alia, S. M.; Zhan, C.; Pham, T. A.; Bell, A. T.; Ogitsu, T.; Danilovic, N.; Weber, A. Z. Mechanistic Understanding of PH Effects on the Oxygen Evolution Reaction. Electrochim. Acta 2022, 405, 139810.(6) Yao, B.; He, Y.; Wang, S.; Sun, H.; Liu, X. Recent Advances in Porphyrin-Based Systems for Electrochemical Oxygen Evolution Reaction. International journal of molecular sciences. 2022, p 6036.(7) Mehta, V.; Cooper, J. S. Review and Analysis of PEM Fuel Cell Design and Manufacturing. Journal of power sources. 2003, pp 32–53.

(Invited) Development of Sustainable Electrocatalysts for Anion Exchange Membrane Fuel Cells and Electrolyzers

The electrocatalytic hydrogen oxidation (HOR) and evolution (HER) reactions under alkaline conditions are kinetically slow processes even when Pt group metals are employed. This presentation discusses our strategies to improve the activity of non platinum catalysts. For example, to improve the alkaline HOR activity of transition metal nanoparticles we look at engineering strong interactions with metal oxides incorporated with carbon-based catalyst supports.(1,2) Another strategy involves utilizing novel carbon materials such as onion like carbon (OLC). We have shown that the combination of CeO2 and OLC in Pd–CeO2/OLC induces surface defects and modifies the electronic structure of the Pd-OLC interface, significantly improving electrical conductivity due to enhanced charge redistribution. Such results were corroborated by density functional theory (DFT) calculations.(3) Using this strategy, peak power densities of up to 2 W cm-2 have been achieved when Pd-CeO2 is incorporated in AEMFCs.(4) Regarding the alkaline HER, mixed phase Ni-Mo and MoO3-x materials are known to have enhanced alkaline hydrogen evolution reaction (HER) activity. Here we describe the optimization of the preparation of an active HER catalyst composed of MoO2 surface nanorod arrays on nickel foam (see Figure). Large circular cathode electrodes (Area 80 cm2) were prepared and tested in a three cell stack AEM electrolyser.(5) M. V. Pagliaro, C. L. Wen, B. Sa, B. Y. Liu, M. Bellini, F. Bartoli, S. Sahoo, R. K. Singh, S. P. Alpay, H. A. Miller and D. R. Dekel, ACS Catal., 10894 (2022).M. V. Pagliaro, M. Bellini, F. Bartoli, J. Filippi, A. Marchionni, C. Castello, W. Oberhauser, L. Poggini, B. Cortigiani, L. Capozzoli, A. Lavacchi, H. A. Miller and F. Vizza, Inorg. Chim. Acta, 543 (2022).J. Ogada, A. K. Ipadeola, P. V. Mwonga, A. B. Haruna, F. Nichols, S. W. Chen, H. A. Miller, M. V. Pagliaro, F. Vizza, J. R. Varcoe, D. M. Meira, D. M. Wamwangi and K. I. Ozoemena, ACS Catal., 12, 10938 (2022).H. A. Miller, M. Bellini, D. R. Dekel, and F. Vizza, Electrochemistry Communications, 135, 107219 (2022).F. Bartoli, L. Capozzoli, T. Peruzzolo, M. Marelli, C. Evangelisti, k. Bouzek, J. Hnat, G. Serrano, L. Poggini, K. Stojanovski, V. Briega-Martos, S. Cherevko, H. A. Miller and F. Vizza, J. Mater. Chem. A, 11, 5789 (2023). Figure 1

(Invited) Enhanced Material Durability of Transition Metal-Antimony X-Ide Nanoparticles in Oxygen Reduction Electrocatalysis

Hydrogen fuel cells (FCs) are key energy devices that can accelerate our transition away from non-renewable, carbon-intensive fuels especially in the grid-scale storage and transportation sectors. FC cathodes perform the oxygen reduction reaction (ORR), and despite decades of progress towards higher performance electrocatalysts, the ORR remains kinetically sluggish and limits the efficiency of assembled FC devices. Modern, commercially available FC cathodes typically contain platinum (Pt)-based catalysts that can cause the overall FC to become prohibitively expensive for some applications so identifying alternate non-platinum group ORR catalyst formulations could increase deployment of FCs. Physics-based calculations with density functional theory (DFT) have predicted many first-row transition metal oxides (MOx, M = Mn, Fe, Cr, Ni, Co) as having favorable ORR performance trends. Additionally, there is experimental validation of nanoscale ternary and quaternary transition metal oxide systems with a mixture of transition metals used for catalyzing the ORR where relatively modest activity enhancements are reported. Despite this progress, an important limitation is present in the low material stability of non-precious transition metal oxides especially in acidic environments such as those encountered in proton exchange membrane FCs. Using synthetic material strategies to overcome such stability limitations in a variety of FC operating conditions will provide a deeper understanding of the relationship between structure, activity, and degradation.In this work, we employ two simultaneous strategies to work towards enhancing the activity and stability of transition metal oxides: (1) addition of antimony (Sb) as a ligand and (2) functionalization with nitrogen or sulfur. We utilized a previously reported colloidal synthesis for uniform ~ 50 nm oxide nanoparticles for manganese, nickel, cobalt, and iron to produce four sets of nanoparticles based on each of these metals. By using a colloidal synthesis that concurrently introduces Sb with the secondary metal and drying the particles afterwards, we incorporate Sb evenly throughout the polycrystalline structure of the of these nanoparticle catalysts. A high temperature surface modification reaction with reactive gas such as air, ammonia, or hydrogen sulfide was used to convert any remaining transition metal antimonate into an oxide, nitride, or sulfide, respectively. In total, twelve materials are used in this study: the oxide, nitride, and sulfide (collectively “X-ides”) of each of the four transition metal containing, antimony incorporated nanoparticles. We optimize this material synthesis and evaluate the nanoparticle catalyst’s ability to enhance electrochemical ORR activity in both acidic (pH 1) and alkaline (pH 13) conditions which are simulating operating conditions of proton-exchange membrane and anion-exchange membrane FCs, respectively. In addition to activity modulation, we measure the ORR selectivity change towards hydrogen peroxide formation for each material and note the role of nitrogen and sulfur modification in suppressing hydrogen peroxide formation in alkaline environments. In addition to electrochemical characterization, we employed a vast array of bulk and surface characterization techniques to understand catalyst dynamics and contextualize observed activity trends. Transmission electron microscopy (TEM) enabled determination of particle sizes and morphologies alongside electron diffraction patterns to validate the catalyst synthesis. X-ray diffraction (XRD) then provided further information about crystallinity that connected bulk-level insights to the local information from TEM. X-ray photoelectron spectroscopy (XPS) enabled determination of oxidation state changes in the transition metal and in Sb that serve as important descriptors for estimating the activity of these catalysts. We also employed in situ/operando techniques to probe catalyst dynamics and stability under various conditions. Using manganese K-edge synchrotron x-ray absorption spectroscopy (XAS) on the manganese antimony X-ides, we find significant changes to oxidation state and coordination of manganese in the nitride and sulfide as a function of potential indicating processes that displace nitrogen and sulfur in the lattice under certain conditions. Lastly, we use an in situ electrochemical flow cell coupled to inductively coupled plasma-mass spectrometry (ICP-MS) to measure corrosion as a function of ORR current density, potential sweeping, and electrolyte gas saturation to determine degradation patterns. Insights from all these techniques provide a fascinating perspective on what factors in a material and its environment are responsible for enhanced activity, selectivity, and stability. We perform DFT modeling of these systems to further elucidate material properties that are principally responsible for achieving key metrics in activity and stability especially as non-precious catalysts are evaluated for their performance in assembled device conditions. Combining electrochemical experiments, physics-based modeling, and material characterization to better understand structure-performance relationships is a promising path towards accelerating the process of materials discovery for eventual deployment in critical energy applications for the future.

Poly(arylene alkylene)s Carrying Quaternary Ammoniums via Flexible Phenylpropyl Spacers as Hydroxide Conducting Membranes

Anion exchange membranes (AEM)s are attracting growing research interests as central components in various alkaline energy conversion and storage devices such as AEM fuel cells and water electrolyzers.1,2 Compared to the devices operated under acidic conditions, the alkaline conditions open up for possibilities to use, e.g., nickel and iron catalysts instead of platinum. However, the insufficient hydroxide conductivity and long-term stability of AEMs at elevated temperatures are still hindering a widespread application of these devices.3 An effective strategy to improve AEM properties by structure design is to functionalize hetero-atom free polymer backbones with quaternary ammonium cations via flexible spacers.4-6 In the present work, we have synthesized a series of AEM materials by tethering three different quaternary ammonium cations to poly(arylene alkylene)s via a phenylpropyl spacer (Figure 1a). The Friedel-Crafts acylation and polyhydroxyalkylation employed in the monomer and polymer syntheses are both free of noble metals and have excellent yields. All the polymers prepared in this way formed robust membranes reaching high hydroxide conductivities – up to 175 mS/cm at 80 °C (Figure 1b). PpT-DMP, carrying dimethyl piperidinium cations, was found to have the highest alkaline stability with less than 7% ionic loss after treatment in 7 M aq. NaOH at 90 oC during 240 h. The hydroxide conductivity of PpT-DMP reached 104 mS/cm at an ion exchange capacity (IEC) of 1.79 mequiv./g., which is still quite high. Atom force microscope (AFM) phase imaging revealed a phase separated morphology of PpT-DMP, most probably involving ion-clustering (Figure 1c). In this presentation we will further discuss the synthesis of these polymers, and influence of the phenylpropyl spacer on AEM properties.[1] Xue, J.; Zhang, J.; Liu, X.; Huang, T.; Jiang, H.; Yin, Y.; Qin, Y.; Guiver, M. D. Toward alkaline-stable anion exchange membranes in fuel cells: cycloaliphatic quaternary ammonium-based anion conductors. Electrochem. Energy Rev. 2022, 5, 348-400.[2] Chatenet, M.; Pollet, B. G.; Dekel, D. R.; Dionigi, F.; Deseure, J.; Millet, P.; Braatz, R. D.; Bazant, M. Z.; Eikerling, M.; Staffell, I.; Balcombe, P.; Horn, Y. S.; Schafer, H. Water electrolysis: from textbook knowledge to the latest scientific strategies and industrial developments. Chem. Soc. Rev. 2022, 51, 4583-4762.[3] Varcoe, J. R.; Atanassov, P.; Dekel, D. R.; Herring, A. M.; Hickner, M. A.; Kohl, P. A.; Kucernak, A. R.; Mustain, W. E.; Nijmeijer, K. N.; Scott, K.; Xu, T.; Zhuang, L. Anion-exchange membranes in electrochemical energy systems. Energy Environ. Sci. 2014, 7, 3135-3197.[4] Dang, H. S.; Jannasch, P. Exploring different cationic alkyl side chain designs for enhanced alkaline stability and hydroxide ion conductivity of anion-exchange membranes. Macromolecules 2015, 48, 5742-5751.[5] Allushi, A.; Pham, T. P.; Olsson, J. S.; Jannasch, P. Ether-free polyfluorenes tethered with quinuclidinium cations as hydroxide exchange membranes. J. Mater. Chem. A 2019, 7, 27164-27174.[6] Lee, W. H.; Kim, Y. S.; Bae, C. Robust Hydroxide Ion Conducting Poly(biphenyl alkylene)s for Alkaline Fuel Cell Membranes. ACS Macro Lett. 2015, 4, 814-818. Figure 1. a) Poly(arylene alkylene)s tethered with different quaternary ammonium cations via phenylpropyl spacers, b) hydroxide conductivity of the AEMs in the fully hydrated state, and c) an AFM phase image of PpT-DMP. Figure 1

(Poster Award - Honorable Mention) Durable Polybenzimidazole Anion Exchange Membranes for Alkaline Water Electrolyzers

Water electrolysis under alkaline conditions allows the use of inexpensive non-platinum-group catalysts such as nickel.1, 2 Conventional alkaline electrolyzers usually employ a highly concentrated aq. KOH solution as electrolyte (5-7 M) with a porous diaphragm as separator. Alternatively, protolysable polymers such as polybenzimidazole (PBI) may be swollen with electrolyte and used as ion-solvating membranes.2, 3 However, the chemical stability of the polymers can become a serious issue under these harsh conditions. Operation under more dilute conditions, e.g., up to 2 M KOH, may bring the benefits of the alkaline conditions combined with a longer cell lifetime. Under these conditions, aliphatic cyclic quaternary ammonium (QA) cations, such as piperidinium, have shown excellent stability.In the present work, performed as part of the EU project “NEXTAEC”, we have designed and synthesized m-PBI grafted with mono- and bis-piperidinium functionalized side chains, respectively (Figure 1a-b). AEMs were prepared from polymers with varying ion exchange capacities and studied at 80 °C containing KOH solutions in the concentration range 0.5-2 M KOH. The membranes were characterized with respect to, e.g., electrolyte solution uptake, alkaline stability, and electrolyser performance (Figure 1c). The uptake showed a strong inverse correlation with KOH concentration, which also affected the electrochemical performance. The stability as evaluated by 1HNMR spectroscopy was excellent showing, in the worst case, less than 8 % ionic loss after 6 months storage in 2 M KOH at 80 °C. Li, D.; Park, E. J.; Zhu, W.; Shi, Q.; Zhou, Y.; Tian, H.; Lin, Y.; Serov, A.; Zulevi, B.; Baca, E. D.; Fujimoto, C.; Chung, H. T.; Kim, Y. S., Highly quaternized polystyrene ionomers for high performance anion exchange membrane water electrolysers. Nature Energy 2020, 5 (5), 378-385.Aili, D.; Kraglund, M. R.; Rajappan, S. C.; Serhiichuk, D.; Xia, Y.; Deimede, V.; Kallitsis, J.; Bae, C.; Jannasch, P.; Henkensmeier, D.; Jensen, J. O., Electrode Separators for the Next-Generation Alkaline Water Electrolyzers. ACS Energy Letters 2023, 8 (4),1900-1910.Aili, D.; Yang, J.; Jankova, K.; Henkensmeier, D.; Li, Q., From polybenzimidazoles to polybenzimidazoliums and polybenzimidazolides. Journal of Materials Chemistry A 2020, 8 (26), 12854-12886.Marino, M. G.; Kreuer, K. D., Alkaline stability of quaternary ammonium cations for alkaline fuel cell membranes and ionic liquids. ChemSusChem 2015, 8 (3), 513-23. Figure 1

Triptycene Branched Poly(aryl-co-aryl piperidinium) Electrolytes for Alkaline Anion Exchange Membrane Fuel Cells and Water Electrolyzers.

Alkaline polymer electrolytes (APEs) are essential materials for alkaline energy conversion devices such as anion exchange membrane fuel cells (AEMFCs) and water electrolyzers (AEMWEs). Here, we report a series of branched poly(aryl-co-aryl piperidinium) with different branching agents (triptycene: highly-rigid, three-dimensional structure; triphenylbenzene: planar, two-dimensional structure) for high-performance APEs. Among them, triptycene branched APEs showed excellent hydroxide conductivity (193.5 mS cm-1 @80 °C), alkaline stability, mechanical properties, and dimensional stability due to the formation of branched network structures, and increased free volume. AEMFCs based on triptycene-branched APEs reached promising peak power densities of 2.503 and 1.705 W cm-2 at 75/100 % and 30/30 % (anode/cathode) relative humidity, respectively. In addition, the fuel cells can run stably at a current density of 0.6 A cm-2 for 500 h with a low voltage decay rate of 46 μV h-1 . Importantly, the related AEMWE achieved unprecedented current densities of 16 A cm-2 and 14.17 A cm-2 (@2 V, 80 °C, 1 M NaOH) using precious and non-precious metal catalysts, respectively. Moreover, the AEMWE can be stably operated under 1.5 A cm-2 at 60 °C for 2000 h. The excellent results suggest that the triptycene-branched APEs are promising candidates for future AEMFC and AEMWE applications.

Triptycene Branched Poly(aryl‐co‐aryl piperidinium) Electrolytes for Alkaline Anion Exchange Membrane Fuel Cells and Water Electrolyzers

AbstractAlkaline polymer electrolytes (APEs) are essential materials for alkaline energy conversion devices such as anion exchange membrane fuel cells (AEMFCs) and water electrolyzers (AEMWEs). Here, we report a series of branched poly(aryl‐co‐aryl piperidinium) with different branching agents (triptycene: highly‐rigid, three‐dimensional structure; triphenylbenzene: planar, two‐dimensional structure) for high‐performance APEs. Among them, triptycene branched APEs showed excellent hydroxide conductivity (193.5 mS cm−1@80 °C), alkaline stability, mechanical properties, and dimensional stability due to the formation of branched network structures, and increased free volume. AEMFCs based on triptycene‐branched APEs reached promising peak power densities of 2.503 and 1.705 W cm−2 at 75/100 % and 30/30 % (anode/cathode) relative humidity, respectively. In addition, the fuel cells can run stably at a current density of 0.6 A cm−2 for 500 h with a low voltage decay rate of 46 μV h−1. Importantly, the related AEMWE achieved unprecedented current densities of 16 A cm−2 and 14.17 A cm−2 (@2 V, 80 °C, 1 M NaOH) using precious and non‐precious metal catalysts, respectively. Moreover, the AEMWE can be stably operated under 1.5 A cm−2 at 60 °C for 2000 h. The excellent results suggest that the triptycene‐branched APEs are promising candidates for future AEMFC and AEMWE applications.

Hydrogenated TiO2 Carbon Support for PtRu Anode Catalyst in High-Performance Anion-Exchange Membrane Fuel Cells.

The availability of durable, high-performance electrocatalysts for the hydrogen oxidation reaction (HOR) is currently a constraint for anion-exchange membrane fuel cells (AEMFCs). Herein, a rapid microwave-assisted synthesis method is used to develop a core-shell catalyst support based on a hydrogenated TiO2/carbon for PtRu nanoparticles (NPs). The hydrogenated TiO2 provides a strong metal-support interactionwith the PtRu NPs, which improves the catalyst's oxophilicity and HOR activity compared to commercial PtRu/C and enables greater size control of the catalyst NPs. The as-synthesized PtRu/TiO2/C-400 electrocatalyst exhibits respectable performance in an AEMFC operated at 80 °C, yielding the highest current density (up to 3× higher) within the catalytic region (compared at 0.80-0.90V) and voltage efficiency (68%@ 0.5 A cm-2) values in the compared literature. In addition, the cell demonstrates promising short-term voltage stability with a minor voltage decay of 1.5mV h-1. This "first-of-its-kind in alkaline" work may open further research avenues to develop rapid synthesis methods to prepare advanced core-shell metal-oxide/carbon supports for electrocatalysts for use in the next-generation of AEMFCs with potential applicability to the broader electrochemical systems research community.