Anion Exchange Membrane Fuel Cells Research Articles

OH- Conductivity and Water Uptake of Anion Exchange Thin Films Under Humidity Control

Anion exchange membrane fuel cells (AEMFCs) have attracted widespread attention because of the lower cost of using non-precious metal catalysts and high oxygen reduction reaction (ORR) kinetics in alkaline conditions.[1] The previous research focused on the water uptake and OH- conduction properties of anion exchange membranes (AEMs) as thick forms.[2–5] The fuel cell reaction occurs at the triple-phase interface where the junction of ion-conductive ionomer, catalyst, and fuel/oxidant. The ionomer plays an important role to deliver OH- ion between the thick membrane andto electrochemical catalysts in fuel cells. However, the investigation of OH- ion conduction and hydration properties of thin ionomers is important but not sufficient.This work demonstrated the relations between OH- conductivity and water uptake of anion exchange thin films for the first time.[6] The reported poly[(9,9-bis(6′-(N,N,N-trimethylammonium)-hexyl)-9H-fluorene)-alt-(1,4-benzene)] (PFB+)[6] was synthesized as a model ionomer. We established in situ methods for measuring OH- conductivity and water uptake of anion exchange thin films[7] because OH- ion easily exchanges for carbon dioxide in the air. The OH- conductivity of 273 nm-thick PFB+ thin film form at 25 °C under 95 % relative humidity (RH) is comparable to the reported OH- conductivity value of PFB+ bulk membrane. Reduced OH- conductivity and water uptake were observed in 30 nm-thick PFB+ film compared to thicker 273 nm-thick PFB+ film. This reduced OH- conductivity was caused by the decreased number of water molecules contained in thinner PFB+ films. Under the same number of water molecules contained, similar OH- conductivity results can be obtained for both 273 and 30 nm-thick films as shown in Figure. Results show a different trend compared to the case of the proton conductive thin films.[8] References [1] G. Merle, M. Wessling, K. Nijmeijer, J. Memb. Sci. 2011, 377, 1–35.[2] J. Y. Jeon, S. Park, J. Han, S. Maurya, A. D. Mohanty, D. Tian, N. Saikia, M. A. Hickner, C. Y. Ryu, M. E. Tuckerman, S. J. Paddison, Y. S. Kim, C. Bae, Macromolecules 2019, 52, 2139–2147.[3] J. Chen, M. Guan, K. Li, S. Tang, ACS Appl. Mater. Interfaces 2020, 12, 15138–15144.[4] U. Salma, Y. Nagao, Polym. Degrad. Stab. 2020, 179, 109299.[5] C. G. Arges, L. Zhang, ACS Appl. Energy Mater. 2018, 1, 2991–3012.[6] W. H. Lee, A. D. Mohanty, C. Bae, ACS Macro Lett. 2015, 4, 453–457.[7] F. Wang, D. Wang, Y. Nagao, ChemSusChem 2021, 14, 2694–2697.[8] Y. Nagao, Sci. Tech. Adv. Mater. 2020, 21, 79–91. Figure 1

Aerogel-Derived Fe-N-C Catalysts for Oxygen Electro-Reduction. Linking Their Pore Structure and PEMFC Performance

Iron-based catalysts are intensely investigated for application at the cathode of proton exchange membrane (PEM) and, more recently, of anion exchange membrane fuel cells (AEMFC). Besides optimising their kinetic activity toward the oxygen reduction reaction (ORR), the design and control of their pore structure plays a key role in achieving the highest possible performance in fuel cell. Fe-N-C catalysts derived from the metal-organic framework ZIF-8 are highly active but also mainly microporous, which can lead to mass-transport issues. The silica templating approach typically leads to slightly less active Fe-N-C catalysts, but featuring a combination of micropores and mesopores, which can facilitate mass-transport. Aerogel methods for the synthesis of Fe-N-C catalysts with a hierarchical pore size distribution has been hitherto under-investigated. In contrast to the silica templating method, it is more environmental friendly since no dissolution of a template is needed (no HF). However, highly active Fe-N-C catalysts prepared by aerogel method have not yet been reported in PEM system (even if promising results were recently obtained in rotating disk electrode [1-2]), and it is therefore also unclear if the hierarchical pore size distribution derived from this method is beneficial for improving the mass-transport properties of Fe-N-C cathodes.This presentation will first report the synthesis method and the key parameters investigated, such as the ratio of melamine to resorcinol, the nature and amount of the iron precursor, the addition of an additionnal N-rich ligand, on their ORR activity. The materials were characterized for their texture (N2 sorption, Hg porosimetry), Fe coordination (X-ray absorption and Mössbauer spectroscopy) and surface chemistry (XPS) and morphology. The most promising materials were investigated in PEMFC, for their initial power performance and durability. The results show that, by playing with these synthesis parameters, the aerogel method can yield highly active ORR catalysts (Figure 1), and can give a control on their hierarchical porosity, especially in tuning the mesoporous volume and the average pore width of mesopores. Most catalysts display an atomic dispersion of Fe, explaining their high ORR activity. The presentation will particularly discuss the link between the pore size distribution in this set of materials (micro, meso and macropores), and the mass-transport properties in PEMFC, under pure O2 or air at the cathode. Their durability will also be discussed, from accelerated stress tests performed in rotating disk electrode and/or PEMFC.Acknowledgments:This study was financially supported by the French National Research Agency under the project ANIMA (ANR-19-CE05-0039).

Transformation of the Active Moiety in Fe-N-C Electrocatalyst through Invasive P-Doping for Highly Efficient Oxygen Reduction Reaction

Iron- and nitrogen-doped carbon (Fe-N-C) materials have been suggested as the most promising replacement for Pt-based catalysts in oxygen reduction reaction (ORR) owing to the FeN4 active moiety. Based on the relationship between the oxygen binding energy and the catalytic activity, Fe-N-C has very strong oxygen binding energy so that hard to desorb the final reaction intermediate of *OH. Herein, we provide for the first time an effective method of tuning the active moiety using a phosphine gas-phase treatment for Fe-N-C. Combined analyses of experimental and computational results reveal that the conventional FeN4 moiety is transformed into FeN3PO through the P-doping post-treatment. Furthermore, we propose an ORR mechanism on the unique FeN3PO moiety based on a microkinetic model, in which *OH intermediates are considered. The invasive P-doping effects on the ORR performance are also validated in both anion exchange membrane fuel cell (AEMFC) and proton exchange membrane fuel cell (PEMFC). Figure 1

Carbon-Capped Monometallic and Bimetallic Catalysts for Hydrogen Oxidation Reaction (HOR) and Their Positive Features on Durability in Alkaline Conditions

Alkaline fuel cells (AFC) and anion exchange membrane fuel cells (AEMFC) have exhibited attractive advantages versus their acid counterparts, including the strategical possibility of using PGM-free catalysts in the electrode composition.[1,2] PGM-free catalysts remain a challenge regarding the hydrogen oxidation reaction (HOR) catalysis, due to the reduced initial and long-term performance so far.[3,4] PGM-based materials have the highest HOR performances,[5] but Pt- and Pd-based catalysts undergo detrimental metallic nanoparticles detachment from the carbon support and agglomeration (in minor extent) upon extended potential cycling in the alkaline environments.[6,7] In this work, we explore the effect of long-term durability on the nanoparticles wrapped by a carbon layer, labeled here as capped catalysts; two different types of carbon-capped catalysts are evaluated, such as monometallic (PdG2/C) and bimetallic (PdNiG2/C) against their commercial ones (Pd/C and PdNi/C from Premetek). All the catalysts were evaluated in RDE set-up, in couple with IL-TEM, XPS and ICP-MS techniques for before and after accelerated stress test (0.1 – 1.23 V vs. RHE – 3s each potential), up to 6000 cycles. The durability enhancement is achieved when: i) there is a carbon-cap and ii) there is a second metal (i.e. Ni) on the surface of Pd. Thus, in the case of carbon-capped bimetallic (PdNiG2/C), there is a double protection of the Pd nanoparticles (from the carbon-cap and the presence of nickel). In the same time, the HOR activity is high. In the end, the best compromise of durability follows the trend: PdNiG2/C > PdNi/C > PdG2/C > Pd/C. These positive features of the carbon-capped bimetallic material open the way to the design of robust catalysts for highly-durable anodes for alkaline fuel cell systems.[1] D. R. Dekel, J. Power Sources 2018, 375, 158–169.[2] E. Wagner, H. ‐J. Kohnke, Fuel Cells 2020, 1–12.[3] A. G. Oshchepkov, G. Braesch, A. Bonnefont, E. R. Savinova, M. Chatenet, ACS Catal. 2020, 10, 7043–7068.[4] W. E. Mustain, M. Chatenet, M. Page, Y. S. Kim, Energy Environ. Sci. 2020, 13, 2805–2838.[5] W. Sheng, M. Myint, J. G. Chen, Y. Yan, Energy Environ. Sci. 2013, 6, 1509–1512.[6] C. Lafforgue, F. Maillard, V. Martin, L. Dubau, M. Chatenet, ACS Catal. 2019, 9, 5613–5622.[7] A. Zadick, L. Dubau, N. Sergent, G. Berthomé, M. Chatenet, ACS Catal. 2015, 5, 4819–4824.

(Invited) Electrocatalysts Dissolution Assessment in Fuel Cell and Water Electrolysis Research

Durability and degradation are in the focus of modern electrocatalysis research. Before moving to real applications, e.g. fuel cells in transportation or water electrolyzers for production of green hydrogen, novel electrocatalytic materials must prove acceptable stability, but “how to test the stability of electrocatalysts”?In the relatively mature proton exchange membrane fuel cell (PEMFC) research, stability is evaluated using various accelerated stress tests (ASTs). Unfortunately, even for the most studied Pt/C electrocatalysts, degradation processes like carbon corrosion and Pt dissolution that occur during common ASTs are not easily distinguishable [1]. Moreover, advanced electrocatalysts such as different shape-controlled Pt alloy nanostructures, showing promising stability in ASTs performed in model aqueous systems, are often rendered useless when moved to real applications [2]. Catalysts free of platinum-group-metals, e.g. FeNC, demonstrate different degradation extents if tested in oxygen or argon [3]. Iridium oxides, the state of the art oxygen evolution reaction (OER) electrocatalysts, are prone to dissolution in aqueous media but much more stable in solid electrolyte based electrolyzers [4]. These examples demonstrate the need for rethinking current approaches to test electrocatalyst stability.This work highlights our recent results on using coupled electrochemical techniques and tuned gas diffusion electrode (GDE) and membrane electrode assembly (MEA) cells in fuel cell and water electrolysis research. It shows that by hyphenating GDE with inductively coupled plasma mass spectrometry (ICP-MS) it is possible to investigate dissolution of electrocatalysts, such as Pt/C for PEMFC and Fe-N-C for anion exchange membrane fuel cells (AEMFC), in-operando at conditions closely resembling those in real devices [5, 6]. As another representative example, the use of model MEAs to address the discrepancy of Ir dissolution in aqueous and solid polymer electrolytes is given [7]. Based on these examples, new strategies to test and understand electrocatalysts’ degradation are discussed.

Ternary PtRuTe alloy nanofibers as an efficient and durable electrocatalyst for hydrogen oxidation reaction in alkaline media

Sluggish kinetics of anodic hydrogen oxidation reaction (HOR) in alkaline media, which arises from the two orders of magnitude lower HOR activity in alkali than that in acid media for platinum group metals, hinders the commercial implementation of anion exchange membrane fuel cells (AEMFCs). Consequently, the development of platinum-based catalysts combined with high efficiency and durability is urgently required. Herein, we report a facile route for the synthesis of ternary PtRuTe alloy nanofibers with Pt atomic ratio of only 11% via a simple galvanic replacement reaction. We optimize the adsorption strength of platinum and ruthenium towards hydrogen and hydroxyl species by regulating the electron donation from tellurium to platinum and ruthenium. Hence, the obtained trimetallic alloy catalyst exhibits an impressive kinetic current density of 30.6 mA cm−2geo at 50 mV and an exchange current density of 0.426 mA cm−2metal, which shows 3.0- and 2.5-fold enhancement compared with the commercial Pt/C in alkaline electrolyte, respectively. Moreover, the catalyst also demonstrates excellent stability with merely 5% activity attenuation after 2000 potential cycles. This work offers new pathways to boost alkaline HOR by rationally designing multicomponent alloys.

Tunning the Activity of Silver Alloy Electrocatalysts for the Oxygen Reduction Reaction in Alkaline Media

A bottleneck to achieve efficient and stable electrocatalysts for Anion Exchange Membrane Fuel Cells (AEMFCs) is the relatively slow kinetics of the oxygen reduction reaction (ORR) taking place at the cathode. The rather mild alkaline conditions in the AEMFC enable the utilization of various PGM-free catalysts, which brings up new possibilities of finding abundant and inexpensive electrocatalysts without compromising the power density. From theory and previous experiments, it is known that Au and Ag bind oxygen weakly, whereas other transition metals (e.g., Pd, Cu, Fe, Ni) bind strongly to oxygenated species1. Therefore, a suitable geometric arrangement between Ag, an abundant metal with relatively high specific activity, and other metals may give rise to optimum binding of oxygen and thus a high activity. A good strategy to improve the activity of Ag-based catalysts could be a rational design of alloys that can enhance the intrinsic activity of Ag by both ensemble and electronic effects2–4. The former is based on the combination of active sites that facilitate the initial oxygen binding together with Ag active sites that help desorb hydroxide anions, whereas the latter is induced by the alloying metal (which binds oxygenated species strongly) on the electronic structure of the overlaying Ag that tunes its binding to oxygen5. Herein, we report the fabrication and characterization of Ag thin-films alloyed with Pd, Ni, Cu and Mo as a tool to investigate the ORR reaction in terms of ensemble and electronic effects and thus relate alloy composition and geometric arrangement with ORR activity. Ag thin-film model catalysts are fabricated by means of physical vapor deposition methods (PVD) with high control over the amount of material deposited. By varying alloy composition and evaluating electrochemical activity and stability, together with detailed physical characterization, the degree and extent of electronic effects are elucidated. In the same way, sputtering of the same amount of Ag on thin-films of different metals evidences the role of the alloying metal on tuning the activity of Ag through an adjustment of its d-band that changes the binding of oxygenated species, as well as through an ensemble effect that provides active sites for the first elementary steps of the reaction. These results provide insights for more tailored design of electrocatalysts in alkaline media by shedding new light on the mechanisms through which the ORR kinetics are improved in alkaline media. Nørskov, J. K. et al. Origin of the overpotential for oxygen reduction at a fuel-cell cathode. J. Phys. Chem. B 108, 17886–17892 (2004).Betancourt, L. E. et al. Enhancing ORR Performance of Bimetallic PdAg Electrocatalysts by Designing Interactions between Pd and Ag. ACS Appl. Energy Mater. 3, 2342–2349 (2020).Slanac, D. A., Hardin, W. G., Johnston, K. P. & Stevenson, K. J. Atomic ensemble and electronic effects in Ag-rich AgPd nanoalloy catalysts for oxygen reduction in alkaline media. J. Am. Chem. Soc. 134, 9812–9819 (2012).Zamora Zeledón, J. A. et al. Tuning the electronic structure of Ag-Pd alloys to enhance performance for alkaline oxygen reduction. Nat. Commun. 12, 1–9 (2021).Stamenkovic, V. et al. Changing the Activity of Electrocatalysts for Oxygen Reduction by Tuning the Surface Electronic Structure. Angew. Chemie 118, 2963–2967 (2006).

(Digital Presentation) Boosting Hydrogen Oxidation Reaction Activity of Nickel Based Catalyst By Reactive Spray Deposition Technology: Effects of Particle Size

Ni-Based materials are one of the most promising candidates for hydrogen oxidation reaction (HOR) in anion exchange membrane fuel cells (AEMFC). However, the performance for pure Ni is limited. In this work, Ni mixed CeO2 nanoparticles supported on carbon black were synthesized for the first time by Reactive Spray Deposition Technology (RSDT) and followed by an oxygen reduction process in a furnace. By controlling the reaction parameters during the RSDT fabrication process, the particle size of Ni/CeO2/C is set at 3 nm. The resultant 3 nm catalyst Ni/CeO2/C catalyst exhibits excellent mass activity (14.92 mA mg-1 Ni) compared to the catalyst without the addition of CeO2 (which is Ni/C 4.39mA mg-1 Ni) as well as the activity of most of the other non-precious metal-based materials for HOR. The excellent electrocatalytic performance indicates that Ni/CeO2/C can be considered as a potential non-precious metal at the anodes for AMEFCs.

(Invited) Oxygen Reduction Reaction in Alkaline Media: The Effect of Carbon Support Crystallinity, Conductivity, and Doping

The rapid development of energy consumption combined with growing environmental awareness is conducive to the wide-ranging search for ecological energy sources. In the recent decade, a significant amount of research has focused on the development of anion-exchange membrane (AEM) fuel cells (AEMFCs), as such approach exhibits faster oxygen reduction reaction (ORR) kinetics in the alkaline environment of the cell [1]. However, to become a serious alternative to the current mainstream acidic fuel cells, alkaline systems should overcome current limitations of performance and stability [2]. The design and synthesis of high-performance electrocatalysts facilitating the slow oxygen reduction reaction (ORR) is, therefore, a key issue in the context of the full commercialization of fuel cells. Transition metal oxides with spinel structure constitute an interesting group of potential catalysts that can replace the widely used materials based on Pt [3, 4]. However, due to the low electrical conductivity and the tendency to undesirable aggregation of spinel nanoparticles, it is important to support them on appropriate carbon carriers. Since the nature of the carbon carrier plays a significant role in the ORR activity, we aimed to comprehensively investigate the influence of the electric conductivity of carbon supports, the role of the surface functional oxygen-bearing groups, and doping with heteroatoms on the performance of the bare and cobalt-manganese spinels deposited on the functionalized carbon supports, acting as ORR electrocatalysts in the alkaline media [5].We investigated commercially available Vulcan XC-72, Printex85, MWCNT, amorphous carbon (C-am.), and carbons synthesized by structural nano-replication using the mesoporous molecular sieves MCM-48 (carbon denoted CMK-1) and the spherical silica calcinated at temperature range 650-1050 oC (SPH-650-1050). To investigate the effect of doping on the ORR catalytical activity the MWCNT was modified by nitrogen and sulfur heteroatoms. Additionally, the doped supports were then treated by plasma to introduce surface oxygen groups. On all investigated carbon supports the nanoparticles of manganese-cobalt spinel were dispersed. They were prepared at 150oC by a microwave-assisted hydrothermal route. Comprehensive physicochemical characterization of carbon support was performed using Raman and UV-Vis spectroscopies, XPS, TPD, XRD, TGA techniques, electron microscopy (TEM), and N2 adsorption (BET) (Figure 1). The relative electric conductivity measurements were performed using the contactless microwave technique. The electrochemical properties of the obtained materials were determined by cyclic voltammetry (CV), whereas the electrocatalytic activity of the obtained materials towards ORR was determined using the RDE and RRDE techniques.It was found that the nature of the carbon carrier plays a triple role in the ORR activity, determining dispersion and faceting of the spinel nanoparticles, as well as the extent of the undesired 2e– reduction pathway, controlled by the fraction of an amorphous component. In the case of SPH samples, it was shown that with the increasing carbonization temperature of the carbon support the electric conductivity increase in parallel to the crystallization extent. The electrocatalytic performance of the carbon carriers and the supported cobalt manganese spinels exhibits a volcano-type shape reaching the maximum for carbon materials obtained at 850°C. Such dependence results from two descriptors changing oppositely with the carbonization temperature: an increasing electric conductivity and a decreasing amount of the carbonyl and quinone groups in the sample. The electric conductivity multiplied by the amount of the carboxyl and quinone groups exhibits the same variation with the carbon calcination temperature as the number of electrons exchanged in ORR, confirming that optimal conjunction of these both properties plays a decisive role in this process. The dispersed spinel active phase favors the direct 4e− pathway of ORR and improves the value of the E onset potential distinctly. To the best of our knowledge, the role of electric conductivity and surface oxygen functionalities in the ORR activity over carbon materials and supported spinel electrocatalysts was disentangled for the first time in this study. Acknowledgments: This work was supported by the Polish National Science Centre (NCN) project OPUS-14, No. 2017/27/B/ST5/01004.

Quinuclidinium-piperidinium based dual hydroxide anion exchange membranes as highly conductive and stable electrolyte materials for alkaline fuel cell applications

Anion-exchange membrane fuel cells (AEMFCs) utilizing quaternary ammonium functionalized poly(arylene ether sulfone)s have been rapidly advanced in the clean energy research arena. However, it is highly desirable to integrate the benefits of quaternary ammonium cations in the polymeric membrane systems to achieve a high-power output together with substantial stabilities that significantly reduce the material costs. Herein, we report the tethering of two different quinuclidinium and piperidinium cations separated by a flexible –(CH2)4˗ spacer as a dual hydroxide conductor in the poly(arylene ether sulfone)s copolymer membrane (QP-PES) for enhancing the structural and physical properties as well as the ionic conductivities. The results proved that the tethering of dual hydroxide conductor in the PES backbone led to show maximum hydroxide conductivity of 88 mS cm−1 at 80 °C while enabling a hydrophilic/hydrophobic micro-phase separation due to the presence of flexible alkyl spacer. We also investigated the properties of a single hydroxide conductor exclusively comprising the quinuclidinium cation (Q-PES) and compared it with that of QP-PES. Interestingly, both the Q-PES and QP-PES membranes displayed superior thermal, mechanical, and dimensional stabilities. Most importantly, the QP-PES membrane demonstrated excellent alkaline stability over the period of 1000 h due to the existence of dual hydroxide conductor together with alkyl spacer units. Furthermore, the maximum power density of a H2/O2 single cell using QP-PES (92.3 mWcm−2) is higher than that of QP-PES (70.0 mWcm−2). The results provide a greater perception to design the high performance AEM materials.

Mixed Metal Phthalocyanine-Modified Carbon Nanotubes for Bifunctional Oxygen Reduction and Evolution Reaction

In this ever-expanding world, researchers are focused on changing how we consume energy. Progress has been made over time, but the devices such as low-temperature polymer electrolyte fuel cells and metal-air batteries that can replace the conventional fossil fuel engines still use noble metal catalysts,1 which hinders the mass production and long-term usage of the devices. Transition metal macrocyclic (MN4-type) catalysts are good alternatives for the noble metal catalysts due to their low-price and promising electrocatalytic activity.2 Thus, the aim of this study was to develop the bifunctional ORR/OER electrocatalysts using mixed transition metal phthalocyanine-modified multi-walled carbon nanotubes (MWCNT) by simple pyrolysis method.3 Among the prepared bifunctional catalysts, FeCoN-MWCNT showed the superior electrocatalytic ORR activity in alkaline media with half-wave potential of 0.86 V vs RHE and FeNiN-MWCNT catalyst exhibited the exceptional OER activity with E j=10 mA cm–2 of 1.58 V (see Fig. 1a, b). Physical characterization of the prepared catalysts was investigated with (scanning) electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy and Raman spectroscopy. Further, catalysts were tested in anion-exchange membrane fuel cells (AEMFCs) as cathode material and very good peak power density of 692 mW cm–2 was obtained for FeCoN-MWCNT (see Fig. 1c). FeNiN-MWCNT was tested in anion-exchange membrane electrolyzer (AEMEL) and showed comparable results with state-of-the-art RuO2 catalysts (Fig. 1d). Altogether, this study unveils the insight into potential ways of preparing the bifunctional ORR/OER electrocatalysts with transition metal macrocyclic-based compounds. References S. Hussain, H. Erikson, N. Kongi, A. Sarapuu, J. Solla-Gullón, G. Maia, A. M. Kannan, N. Alonso-Vante, K. Tammeveski, Int. J. Hydrogen Energy, 45, 31775-31797 (2020).A. Sarapuu, E. Kibena-Põldsepp, M. Borghei, K. Tammeveski, J. Mater. Chem. A, 6, 776–804 (2018).Y. Kumar, E. Kibena-Põldsepp, J. Kozlova, M. Rähn, A. Treshchalov A. Kikas, V. Kisand, J. Aruväli, A. Tamm, J. C. Douglin, S. J. Folkman, I. Gelmetti, F. A. Garcés-Pineda, J. R. Galán-Mascarós, D. R. Dekel, K. Tammeveski, ACS Appl. Mater. Interfaces, 13, 41507–41516 (2021). Figure 1

The Influence of Pre-Irradiation and Simultaneous Grafting Methods on the Physicochemical Properties of Polyethylene-Based Anion-Exchange Membranes and Ionomers

Controlling the synthesis parameters during radiation-induced grafting (RIG) is a key feature to design anion-exchange membranes (AEMs) with enhanced properties that lead to high performances and stability. The present study provides a systematic investigation of the effect of RIG methods on physicochemical properties of LDPE-based AEMs with similar degree of grafting (DoG) and ion exchange capacity (IEC). The LDPE films were grafted with 4-vinylbenzyl chloride using two methods: i) pre-irradiation (PIM), using electron-beam (EB), and ii) simultaneous (SM), using gamma-ray (γ), both in air atmosphere and with different absorbed doses. The AEMs grafted by SM were irradiated with 20, 25, and 30 kGy, while the ones grafted by PIM were irradiated with doses of 30, 70, and 100 kGy. Samples grafted by the two different methods have the same molecular structure, but distinct physicochemical properties due to markedly differences in the degree of crosslinking. The studies have shown that grafting step directly influences the internal structure and morphology of AEMs. Multi-technique characterization demonstrated that RIG method determines the mechanical properties, water transport, and the distribution of ionic groups in AEMs. These characteristics have direct impact on fuel cell performance, anion-conductivity, and AEM stability.Figure 1 shows the polarization curves of anion-exchange membrane fuel cells (AEMFCs) operating at 80 °C with AEMs grafted by PIM and SM. Comparing AEMFCs performances containing AEMs with similar IECs from the two different grafting methods, it is possible to notice that AEMs synthesized by PIM, in general, promote better fuel cell performances than the corresponding ones prepared by SM. The main differences in the polarization curves are evidenced in the ohmic and the mass transport limiting regions. This result suggests that water management is enhanced in AEMs synthesized by PIM compared to the SM-AEMs. A membrane that allows rapid back diffusion of liquid water from the anode to the cathode, should reduce the current losses due to mass transport limitations.The gel content results confirmed the presence of more crosslinking in membranes synthesized by PIM than in the ones synthesized by the SM. As a consequence of that, enhanced mechanical properties were found for PIM-AEMs, indicating polymer backbone reinforcement due to high degree of crosslinking. Small-angle X-ray scattering measurements have confirmed that the microstructure of PIM and SM AEMs are different and this feature is directly related to ion conduction and water transportation.Figure 1. AEMFCs performances at 80 °C with LDPE-based AEMs grafted by the pre-irradiation method and by the simultaneous method. H2 anode gas flow=0.8 L min-1, O2 cathode gas flow=0.5 L min-1, both supplied unpressurized with optimal dewpoint temperatures ranging from 77 to 80 °C. AEMs are referred according to the grafting method and absorbed dose. 30 and 70-EB-PIM refer to the samples irradiated with 30 and 70 kGy, respectively, in the EB accelerator and grafted by the pre-irradiation method. 20 and 25-γ-SM refer to AEMs irradiated with 20 and 25 kGy, respectively, by γ-rays and grafted by the simultaneous method. Figure 1

Hydrogen-powered Electrochemically-driven CO2 Removal from Air Containing 400 to 5000 ppm CO2

The performance of a hydrogen-powered, electrochemically-driven CO2 separator (EDCS) was demonstrated at cathode inlet CO2 concentrations from 400 ppm to 5,000 ppm. The impact of current density and CO2 concentration were evaluated to predict operating windows for various applications. The single-cell data was used to scale a 100 cm2, multi-cell stack using a shorted-membrane design for four applications: direct air capture (DAC), hydroxide exchange membrane fuel cell (HEMFC) air pretreatment, submarine life support, and space habitation. For DAC, a 339-cell EDCS stack (7.7 L, 17 kg) was projected to remove 1 tonne CO2 per year. The addition of the EDCS in HEMFC systems would result in nearly a 30% increase in volume, and therefore further improvements in performance would be necessary. A module containing five 338-cell EDCS stacks (38 L, 85 kg) in parallel can support a 150 person crew at 2.1% of the volume of the liquid amine system employed in submarines. For space habitation, a 109-cell EDCS stack (3.2 L, 10 kg) is adequate for 6 crewmembers, and is less than 1% the size and 5% the weight of the current CO2 removal system installed on the International Space Station.

Understanding Recoverable vs Unrecoverable Voltage Losses and Long-Term Degradation Mechanisms in Anion Exchange Membrane Fuel Cells

Understanding Recoverable vs Unrecoverable Voltage Losses and Long-Term Degradation Mechanisms in Anion Exchange Membrane Fuel Cells

Development of Non-Platinum Metal Catalysts for Anion Exchange Membrane Fuel Cells

The development of non-platinum metal catalysts used in anion exchange membrane fuel cells (AEMFCs) has been considered as a key to bring the fuel cell devices to practical applications. This paper presents the achievement in developing non-platinum metal catalysts including bimetallic palladium-nickel nanoparticles attached on Vulcan carbon particles (PdNi/C) and silver nanoparticles supported on multi-walled carbon nanotubes (Ag/MWCNT) employed in the anode and cathode electrodes of AEMFCs, respectively. These catalysts were synthesized by a wet impregnation method. The SEM analysis showed that the nanoparticle sizes of Pd, Ni, and Ag were less than 50 nm. The electrochemical characterizations of the synthesized PdNi/C and Ag/MWCNT were explored by cyclic voltammetry measurements. In addition, the prepared catalyst performance was evaluated using a single anion exchange membrane fuel cell (AEMFC). The measured results showed that the cell performance of a single H2/O2 AEMFC with PdNi/C and Ag/MWCNT in anode and cathode electrodes, respectively, exhibited a peak power density of about 463 mW·cm-1, which was lower about 24.1% than that of the single-cell containing Pt/C (~610 mW·cm-1) at both anode and cathode sides, indicating that the PdNi/C và Ag/MWCNT catalysts can be used to substitute Pt/C in AEMFCs for cost reduction.

AgNPs@Fe-N-C oxygen reduction catalysts for anion exchange membrane fuel cells

Anion exchange membrane fuel cells (AEMFCs) are hopeful clean energy equipment due to the mild reaction condition and the possibility of using Pt-free catalysts for cathodic oxygen reduction reaction (ORR). Developing excellent Pt-free ORR catalysts for AEMFC is still of great significance. Here, we construct a composite catalyst (AgNPs@Fe-N-C) of Ag nanoparticles (NPs) surrounded by the atomically dispersed Fe-N-C porous carbon. The AgNPs@Fe-N-C demonstrates outstanding ORR performance and enhanced durability, and its half-wave potential obviously exceeds AgNPs-N-C and Fe-N-C counterparts in alkaline condition. Impressively, the AgNPs@Fe-N-C-based AEMFC exhibits not only the ultrahigh peak power density of 0.848 W cm−2, but also long-term device durability of 200 h in H2/air (CO2-free) system, which is obviously better than that of the Fe-N-C-based AEMFC. Density functional theory (DFT) calculations further reveal the synergistic effect of Ag NPs and Fe-N-C for ORR. In short, this work provides a new route to develop high activity and robust Pt-free ORR catalysts for AEMFCs.

Controlled Doping of Electrocatalysts through Engineering Impurities.

Fuel cells recombine water from H2 and O2 thereby can power, for example, cars or houses with no direct carbon emission. In anion-exchange membrane fuel cells (AEMFCs), to reach high power densities, operating at high pH is an alternative to using large volumes of noble metals catalysts at the cathode, where the oxygen-reduction reaction occurs. However, the sluggish kinetics of the hydrogen-oxidation reaction (HOR) hinders upscaling despite promising catalysts. Here, the authors observe an unexpected ingress of B into Pd nanocatalysts synthesized by wet-chemistry, gaining control over this B-doping, and report on its influence on the HOR activity in alkaline conditions. They rationalize their findings using abinitio calculations of both H- and OH-adsorption on B-doped Pd. Using this "impurity engineering" approach, they thus design Pt-free catalysts as required in electrochemical energy conversion devices, for example, next generations of AEMFCs, that satisfy the economic and environmental constraints, that is, reasonable operating costs and long-term stability, to enable the "hydrogen economy."

Nanofiber-Based Oxygen Reduction Electrocatalysts with Improved Mass Transfer Kinetics in a Meso-Porous Structure and Enhanced Reaction Kinetics by Confined Fe and Fe3C Particles for Anion-Exchange Membrane Fuel Cells

The development of high-performance nonprecious metal catalysts for oxygen reduction reactions is critical for the commercialization of fuel cells. In this paper, we report a non-precious catalyst with high-performance, in which Fe and Fe3C is embedded in nitrogen-doped carbon nanofibers (MIL-N-CNFs) by co-electrospinning Fe-MIL and polyacrylonitrile (PAN) and pyrolyzing. The mass ratio of Fe-MIL to PAN in the precursors and the pyrolysis temperature were optimized to be 1.5 and treated at 800 °C, respectively. The optimized catalyst exhibited an onset potential of 0.950 V and a half-wave potential of 0.830 V in alkaline electrolytes, thanks to the improved mass transfer kinetics in a meso-porous structure and enhanced reaction kinetics by confined Fe and Fe3C particles. Additionally, the optimized catalyst showed a better methanol tolerance than the commercial 20 wt.% Pt/C, indicating a potential application in direct methanol fuel cells. Serving as the cathode in CCM, the anion-exchange membrane fuel cell reaches a power density of 192 mW cm−2 at 428 mA cm−2 and 80 °C.

High performance poly(carbazolyl aryl piperidinium) anion exchange membranes for alkaline fuel cells

Anion exchange membrane fuel cells (AEMFCs) is a potential substitute to the prevailing proton exchange membrane fuel cells (PEMFCs), since non-precious metal catalysts can be applied in the AEMFCs, leading to much lower cost of it. However, anion exchange membranes (AEMs), as a crucial component of the AEMFCs, has always been a challenge in the commercialization of AEMFCs, such as poor life-time and conductivity. Herein, a series of poly(carbazolyl aryl piperidinium) AEMs and ionomers are prepared with non-rotatable rigid carbazole group next to piperidinium ring on the polymer backbone for the first time, and the membranes were fabricated by mass production method of tape-casting. The membranes exhibit extraordinary comprehensive performance, for instance, high hydroxide conductivity up to 204.8 mS cm−1 at 90 °C, good mechanical strength of >50 MPa in wet state, low hydrogen permeability and excellent alkaline stability of only 3% decline after 2100 h in 1 M KOH at 80 °C. Fuel cell (H2–O2) based on the as-prepared membranes and ionomers shows outstanding peak power density of 1.72 W cm−2 at a quite low back pressure of 20 kPa, and only 2.6% decay was observed after 120 h.

Simulation of an interdigitated flow channel assembled in a proton exchange membrane Fuel Cell (PEMFC)

This short communication deals with designing an interdigitated flow channel fitted in a PEMFC. The flow plate created here through CFD simulations consists of interdigitated channels based on leaf veins with an active area of 50 cm2. CFD simulations of fluid flow, mass transport, and current distribution were conducted using the finite element method (FEM). Experimental and CFD simulations were carried out to characterize the operating conditions applied to the cathode of a PEMFC using different O2–H2O mixtures. Flow velocities ranged from 3 to 5 m/s, while the humidity and temperature were between 80 and 100% and 323–343 K, respectively. A typical pressure drop around 400 Pa was observed over the flow velocities studied in the flow channel. The linear flow velocities found in the gas diffusion layer (GDL) domain were between 2 × 10−7 to 0.98 m/s. The experimental polarization curve showed close agreement with CFD simulations for the current density interval of 0–0.5 A/cm2, where no limitations to mass transport affect the fuel cell responses. The current model and flow channels configuration will be on continuous development considering water transport through the membrane and future application on Anion Exchange Membrane Fuel Cells (AEMFC).