Anion Exchange Membrane Fuel Cells Research Articles

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.

V-O Species-Doped Carbon Frameworks Loaded with Ru Nanoparticles as Highly Efficient and CO-Tolerant Catalysts for Alkaline Hydrogen Oxidation.

Efficient and CO-tolerant catalysts for alkaline hydrogen oxidation (HOR) are vital to the commercial application of anion exchange membrane fuel cells (AEMFCs). Herein, a robust Ru-based catalyst (Ru/VOC) with ultrasmall Ru nanoparticles supported on carbon frameworks with atomically dispersed V-O species is prepared elaborately. The catalyst exhibits a remarkable mass activity of 3.44 mA μgPGM, which is 31.3 times that of Ru/C and even 4.7 times higher than that of Pt/C. Moreover, the Ru/VOC anode can achieve a peak power density (PPD) of 1.194 W cm-2, much superior to that of Ru/C anode and even better than that of Pt/C anode. In addition, the catalyst also exhibits superior stability and exceptional CO tolerance. Experimental results and density functional theory (DFT) calculations demonstrate that V-O species are ideal OH- adsorption sites, which allow Ru to release more sites for hydrogen adsorption. Furthermore, the electron transfer from Ru nanoparticles to the carbon substrate regulates the electronic structure of Ru, reducing the hydrogen binding energy (HBE) and the CO adsorption energy on Ru, thus boosting the alkaline HOR performance and CO tolerance of the catalyst. This is the first report that oxophilic single atoms distributed on carbon frameworks serve as OH- adsorption sites for efficient hydrogen oxidation, opening up new guidance for the elaborate design of high-activity catalysts for the alkaline HOR.

Sub-3 nm Pt@Ru toward Outstanding Hydrogen Oxidation Reaction Performance in Alkaline Media.

Anion-exchange membrane fuel cells (AEMFCs) are promising alternative hydrogen conversion devices. However, the sluggish kinetics of the hydrogen oxidation reaction in alkaline media hinders further development of AEMFCs. As a synthesis method commonly used to prepare disordered PtRu alloys, the impregnation process is ingeniously designed herein to synthesize sub-3 nm Pt@Ru core-shell nanoparticles by sequentially reducing Pt and Ru at different annealing temperatures. This method avoids complex procedures and synthesis conditions for organic synthesis systems, and the atomic structure evolution of the synthesized core-shell nanoparticles can be tracked. The synthesized Pt@Ru electrocatalyst shows an ultrasmall average size of ∼2.5 nm and thereby a large electrochemical surface area (ECSA) of 166.66 m2 gPt+Ru-1. Exchange current densities (j0) normalized to the mass (Pt + Ru) and ECSA of this electrocatalyst are 8.0 and 5.8 times as high as those of commercial Pt/C, respectively. To the best of our knowledge, the achieved mass-normalized j0 measured by rotating disk electrodes is the highest reported so far. The membrane electrode assembly test of the Pt@Ru electrocatalyst shows a peak power density of 1.78 W cm-2 (0.152 mgPt+Ru cmanode-2), which is higher than that of commercial PtRu/C (1.62 W cm-2, 0.211 mgPt+Ru cmanode-2). The improvement of the intrinsic activity can be attributed to the electron transfer from the Ru shell to the Pt core, and the ultrafine particles further enhance the mass activity. This work reveals the feasibility of using simple impregnation to synthesize fine core-shell nanocatalysts and the importance of investigating the atomic structure of PtRu nanoparticles and other disordered alloys.

Synthesis of NiMoW ternary alloy to boost hydrogen oxidation reaction in alkaline medium

The unsatisfactory activities of nickel-based electrocatalysts in the alkaline hydrogen oxidation reaction (HOR) pose a crucial challenge in the development of hydroxide-exchange membrane fuel cells (HEMFCs). Generally, alloying Ni with Mo or W efficiently promotes HOR in alkaline media by reducing the hydrogen binding energy and increasing the hydroxyl binding energy. However, the alkaline HOR activities of binary alloys still do not meet industrial requirements. Therefore, herein, we report that the alkaline HOR activity of the Ni-based electrocatalyst can be enhanced by preparing a NiMoW ternary alloy (Ni76Mo78W) fabricated via a hydrothermal process, followed by annealing under a H2 atmosphere. This as-prepared nickel foam-supported Ni76Mo78W alloy exhibits excellent HOR catalytic activity (the current density reaches 6.6 mA cm−2 at 100 mV vs. RHE, approximately twice that of the Pt/C electrocatalyst), fast kinetics (exchange current density j0 of 3.12 mA cm−2), and ultrahigh stability (maintenance of the current density over 80 h). Detailed characterizations reveales that the charge transfer between the alloy components presumably tunes the electronic structure of the alloy and ultimately promotes the HOR.

A Highly-Efficient Boron Interstitially Inserted Ru Anode Catalyst for Anion Exchange Membrane Fuel Cells.

Developing high-performance electrocatalysts for alkaline hydrogen oxidation reaction (HOR) is crucial for the commercialization of anion exchange membrane fuel cells (AEMFCs). Here, boron interstitially inserted ruthenium (B-Ru/C) is synthesized and used as an anode catalyst for AEMFC, achieving a peak power density of 1.37W cm-2 , close to the state-of-the-art commercial PtRu catalyst. Unexpectedly, instead of the monotonous decline of HOR kinetics with pH as generally believed, an inflection point behavior in the pH-dependent HOR kinetics on B-Ru/C is observed, showing an anomalous behavior that the HOR activity under alkaline electrolyte surpasses acidic electrolyte. Experimental results and density functional theory calculations reveal that the upshifted d-band center of Ru after the intervention of interstitial boron can lead to enhanced adsorption ability of OH and H2 O, which together with the reduced energy barrier of water formation, contributes to the outstanding alkaline HOR performance with a mass activity of 1.716mA µgPGM -1 , which is 13.4-fold and 5.2-fold higher than that of Ru/C and commercial Pt/C, respectively.

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

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

Nitrogen and sulphur co-doped carbon-based composites as electrocatalysts for the anion-exchange membrane fuel cell cathode

Nitrogen and sulphur co-doped nanocarbon composites based on silicon carbide-derived carbon (SiCDC), carbon nanotubes (CNT) and/or mesoporous carbon (MPC) are prepared for oxygen reduction reaction (ORR) and anion-exchange membrane fuel cell (AEMFC) studies. Thiosemicarbazide is used as nitrogen and sulphur doping agent. The rotating ring-disc electrode (RRDE) measurements exhibit good electrocatalytic activity towards the ORR for all the prepared catalysts in alkaline solution. Obtained materials are rich in pyridinic-N and consist of thiophene-S, which could improve the ORR performance due to the synergistic effect of N and S. Textural properties, including specific surface area (SSA) and porosity parameters are different amongst the three studied N,S-doped nanocomposite materials, which also affect the outcome of AEMFC test results. In AEMFC testing using Aemion+ (AF2-HLE8-10-X, 10 μm) anion-exchange membrane, N,S-SiCDC/MPC (with SSAdft = 952 m2 g−1) shows the highest performance with the peak power density (Pmax) of 379 mW cm–2. This result is comparable with the performance of Pt/C catalyst in AEMFC (Pmax = 347 mW cm–2), making N,S-doped nanocomposite materials promising cathode catalyst for AEMFCs.

Tailoring the Chemisorption Manner of Fe d‐Band Center with La2O3 for Enhanced Oxygen Reduction in Anion Exchange Membrane Fuel Cells

AbstractEngineering the electronic configuration and intermediates adsorption behaviors of high‐performance non‐noble‐metal‐based catalysts for the sluggish oxygen reduction reaction (ORR) kinetics at the cathode is highly imperative for the development of anion exchange membrane fuel cells (AEMFCs), yet remains an enormous challenge. Herein, a rare‐earth metal oxide engineering tactic through the formation of Fe3O4/La2O3 heterostructures in N,O‐doped carbon nanospheres (Fe3O4/La2O3@N,O‐CNSs) for efficient oxygen reduction electrocatalysis is reported. The theoretical calculations reveal that the interfacial bonds formed by the La─O─Fe heterogeneous interface effectively optimize the electronic structure of the Fe d‐band center relative to the Fermi level, which results in a significant reduction of the reaction barriers of rate‐limiting steps during the ORR. The modulation in intermediates chemisorption enables Fe3O4/La2O3@N,O‐CNSs an outstanding ORR performance and improved stability, with a significantly higher half‐wave potential value (0.88 V). More impressively, this integrated catalyst delivers a remarkable power density of 148.7 mW cm−2 in practical AEMFC operating conditions, along with negligible performance degradation over 100 h using an H2‐air atmosphere, which is higher than commercial Pt/C‐coupled electrodes. The results presented here are believed to provide guidelines for fabricating high‐performance AEMFCs electrocatalysts in terms of heterointerface engineering and strong electronic coupling effect induced by rare‐earth oxides.

High-performance and scalable poly(terphenyl-furan piperidinium) membrane for anion exchange membrane fuel cell with 2 W cm−2 of peak power density

Anion exchange membrane (AEM) with high ionic conductivity and stable dimension is urgently needed to improve cost-effective anion exchange membrane fuel cells (AEMFC). In this study, a dibenzofuran-containing poly(terphenyl furan piperidinium) (QPTFP) membrane is successfully synthesized in large quantities with a yield exceeding 500 g for the purpose of large-scale AEM production. The introduction of non-rotatable dibenzofurans results in an acceptable swelling ratio of 25 % and high ionic conductivity of 155 mS cm−1 at 80 °C. It also exhibits a favorable tensile strength of 52.6 MPa and excellent alkaline stability of 2200 h at 80 °C in 1 M KOH. Furthermore, the AEMFC employing QPTFP membrane achieves remarkable peak power densities at 80 °C for H2–O2 (2 W cm−2) and H2-Air (1.2 W cm−2), and demonstrates excellent stability of over 90 h at 70 °C and a current density of 0.2 A cm−2.