As substantial progress has been made in improving the performance of anion exchange membrane fuel cells (AEMFCs) over the last decade, the durability of AEMFCs has become the most critical ...requirement to deploy competitive energy conversion systems. Because of different operating environments from proton exchange membrane fuel cells, several AEMFC-specific component degradations have been identified as the limiting factors influencing the AEMFC durability. In this article, AEMFC durability protocol, the current status of AEMFC durability, and performance degradation mechanisms are reported based on the discussion during the US Department of Energy (DOE) Anion Exchange Membrane Workshop at Dallas, Texas, May 2019. With additional recent progress, we provide our perspectives on current technical challenges and future action to develop long-lasting AEMFCs.
This perspective provides information on durability challenges and future actions of anion exchange membrane fuel cells.
Single‐atom M‒N‒C catalysts have attracted tremendous attention for their application to electrocatalysis. Nitrogen‐coordinated mononuclear metal moieties (MNx moities) are bio‐inspired active sites ...that are analogous to various metal‐porphyrin cofactors. Given that the functions of metal‐porphyrin cofactors are highly dependent on the local coordination environments around the mononuclear active site, engineering MNx active sites in heterogeneous M‒N‒C catalysts would provide an additional degree of freedom for boosting their electrocatalytic activity. This work presents a local coordination structure modification of FeN4 moieties via morphological engineering of graphene support. Introducing highly wrinkled structure in graphene matrix induces nonplanar distortion of FeN4 moieties, resulting in the modification of electronic structure of mononuclear Fe. Electrochemical analysis combined with first‐principles calculations reveal that enhanced electrocatalytic lithium polysulfide conversion, especially the Li2S redox step, is attributed to the local structure modified FeN4 active sites, while increased specific surface area also contributes to improved performance at low C‐rates. Owing to the synergistic combination of atomic‐level modified FeN4 active sites and morphological advantage of graphene support, Fe‒N‒C catalysts with wrinkled graphene morphology show superior lithium–sulfur battery performance at both low and high C‐rates (particularly 915.9 mAh g−1 at 5 C) with promising cycling stability.
Atomic‐level engineering of MNx active sites is a desirable strategy to enhance and fine‐tune electrocatalytic performance of M‒N‒C catalysts. FeN4 active sites on wrinkled graphene support exhibits different structural and electronic properties compared to square‐planar FeN4 moieties. The synergistic combination of modified FeN4 active sites and morphological advantage of wrinkled graphene support improves the electrocatalytic performance for lithium–sulfur conversion chemistry.
Quaternized aryl ether-free polyaromatics are an important family of polymer electrolytes for alkaline membrane fuel cells (AMFCs) due to their outstanding alkaline stability. In this review, ...state-of-the-art quaternized aryl ether-free polyaromatics are discussed with respect to their synthesis and preparation. The mechanical and electrochemical properties and alkaline stability of the polyaromatics that impact AMFC performance and durability are discussed in comparison with aryl ether-containing polyaromatic and polyolefinic electrolytes. Their performance in membrane electrode assemblies (MEAs) is discussed with emphasis on the area specific resistance and phenyl group adsorption on hydrogen oxidation catalysts. The AMFC performance of MEAs employing state-of-the-art aryl ether-free polyaromatics is compared with those employing polyolefins and aryl ether-containing polyaromatics. Finally, the limitations and outlook of quaternized aryl ether-free polyaromatics are briefly summarized.
Modern electrochemical energy conversion devices require more advanced proton conductors for their broad applications. Phosphonated polymers have been proposed as anhydrous proton conductors for fuel ...cells. However, the anhydride formation of phosphonic acid functional groups lowers proton conductivity and this prevents the use of phosphonated polymers in fuel cell applications. Here, we report a poly(2,3,5,6-tetrafluorostyrene-4-phosphonic acid) that does not undergo anhydride formation and thus maintains protonic conductivity above 200 °C. We use the phosphonated polymer in fuel cell electrodes with an ion-pair coordinated membrane in a membrane electrode assembly. This synergistically integrated fuel cell reached peak power densities of 1,130 mW cm
at 160 °C and 1,740 mW cm
at 240 °C under H
/O
conditions, substantially outperforming polybenzimidazole- and metal phosphate-based fuel cells. Our result indicates a pathway towards using phosphonated polymers in high-performance fuel cells under hot and dry operating conditions.
Native extracellular matrix (ECM) can exhibit cyclic nanoscale stretching and shrinking of ligands to regulate complex cell–material interactions. Designing materials that allow cyclic control of ...changes in intrinsic ligand‐presenting nanostructures in situ can emulate ECM dynamicity to regulate cellular adhesion. Unprecedented remote control of rapid, cyclic, and mechanical stretching (“ON”) and shrinking (“OFF”) of cell‐adhesive RGD ligand‐presenting magnetic nanocoils on a material surface in five repeated cycles are reported, thereby independently increasing and decreasing ligand pitch in nanocoils, respectively, without modulating ligand‐presenting surface area per nanocoil. It is demonstrated that cyclic switching “ON” (ligand nanostretching) facilitates time‐regulated integrin ligation, focal adhesion, spreading, YAP/TAZ mechanosensing, and differentiation of viable stem cells, both in vitro and in vivo. Fluorescence resonance energy transfer (FRET) imaging reveals magnetic switching “ON” (stretching) and “OFF” (shrinking) of the nanocoils inside animals. Versatile tuning of physical dimensions and elements of nanocoils by regulating electrodeposition conditions is also demonstrated. The study sheds novel insight into designing materials with connected ligand nanostructures that exhibit nanocoil‐specific nano‐spaced declustering, which is ineffective in nanowires, to facilitate cell adhesion. This unprecedented, independent, remote, and cytocompatible control of ligand nanopitch is promising for regulating the mechanosensing‐mediated differentiation of stem cells in vivo.
Materials allowing unprecedented, remote, and cytocompatible control of in situ and time‐regulated nanoscale stretching and shrinking of ligand‐presenting magnetic nanocoils, that independently modulate the ligand pitch in the nanocoils, are presented. It is demonstrated that magnetic control of ligand nanostretching promotes cyclic adhesion and mechanotransduction of stem cells, both in vitro and in vivo, which facilitates their consequential differentiation.
Interest in the low-cost production of clean hydrogen is growing. Anion exchange membrane water electrolyzers (AEMWEs) are considered one of the most promising sustainable hydrogen production ...technologies because of their ability to split water using platinum group metal-free catalysts, less expensive anode flow fields, and bipolar plates. Critical to the realization of AEMWEs is understanding the durability-limiting factors that restrict the long-term use of these devices. This article presents both durability-limiting factors and mitigation strategies for AEMWEs under three operation modes,
i.e.
, pure water-fed (no liquid electrolyte), concentrated KOH-fed, and 1 wt% K
2
CO
3
-fed operating at a differential pressure of 100 psi. We examine extended-term behaviors of AEMWEs at the single-cell level and connect their behavior with the electrochemical, chemical, and mechanical instability of single-cell components. Finally, we discuss the pros and cons of AEMWEs under these operation modes and provide direction for long-lasting AEMWEs with highly efficient hydrogen production capabilities.
Understanding the durability-limiting factors of anion exchange membrane water electrolyzers operating under pure water-, KOH- and K
2
CO
3
-fed conditions.
The anion exchange membrane fuel cell (AEMFC) is an attractive alternative to acidic proton exchange membrane fuel cells, which to date have required platinum-based catalysts, as well as ...acid-tolerant stack hardware. The AEMFC could use non-platinum-group metal catalysts and less expensive metal hardware thanks to the high pH of the electrolyte. Over the last decade, substantial progress has been made in improving the performance and durability of the AEMFC through the development of new materials and the optimization of system design and operation conditions. In this perspective article, we describe the current status of AEMFCs as having reached beginning of life performance very close to that of PEMFCs when using ultra-low loadings of Pt, while advancing towards operation on non-platinum-group metal catalysts alone. In the latter sections, we identify the remaining technical challenges, which require further research and development, focusing on the materials and operational factors that critically impact AEMFC performance and/or durability. These perspectives may provide useful insights for the development of next-generation of AEMFCs.
•Reviewed technology progress over the last decade on alkaline membrane fuel cells.•Demonstrated the H2/CO2-free air performance that reaches 0.8 W cm−2 at 0.6 V.•Discussed key research challenges of fuel cell materials and system components.
We report in this article a detailed study on how to stabilize a first-row transition metal (M) in an intermetallic L10-MPt alloy nanoparticle (NP) structure and how to surround the L10-MPt with an ...atomic layer of Pt to enhance the electrocatalysis of Pt for oxygen reduction reaction (ORR) in fuel cell operation conditions. Using 8 nm FePt NPs as an example, we demonstrate that Fe can be stabilized more efficiently in a core/shell structured L10-FePt/Pt with a 5 Å Pt shell. The presence of Fe in the alloy core induces the desired compression of the thin Pt shell, especially the two atomic layers of Pt shell, further improving the ORR catalysis. This leads to much enhanced Pt catalysis for ORR in 0.1 M HClO4 solution (at both room temperature and 60 °C) and in the membrane electrode assembly (MEA) at 80 °C. The L10-FePt/Pt catalyst has a mass activity of 0.7 A/mgPt from the half-cell ORR test and shows no obvious mass activity loss after 30 000 potential cycles between 0.6 and 0.95 V at 80 °C in the MEA, meeting the DOE 2020 target (<40% loss in mass activity). We are extending the concept and preparing other L10-MPt/Pt NPs, such as L10-CoPt/Pt NPs, with reduced NP size as a highly efficient ORR catalyst for automotive fuel cell applications.
Galvanic replacement reactions provide a simple and versatile route for producing hollow nanostructures with controllable pore structures and compositions. However, these reactions have previously ...been limited to the chemical transformation of metallic nanostructures. We demonstrated galvanic replacement reactions in metal oxide nanocrystals as well. When manganese oxide (Mn₃O₄) nanocrystals were reacted with iron(ll) perchlorate, hollow box-shaped nanocrystals of Mn₃O₄/γ-Fw₂O₃ ("nanoboxes") were produced. These nanoboxes ultimately transformed into hollow cagelike nanocrystals of γ-Fe₂O₃ ("nanocages"). Because of their nonequilibrium compositions and hollow structures, these nanoboxes and nanocages exhibited good performance as anode materials for lithium ion batteries. The generality of this approach was demonstrated with other metal pairs, including Co₃O₄/SnO₂ and Mn₃O₄/SnO₂.
Developing materials with the capability of changing their innate features can help to unravel direct interactions between cells and ligand‐displaying features. This study demonstrates the grafting ...of magnetic nanohelices displaying cell‐adhesive Arg‐Gly‐Asp (RGD) ligand partly to a material surface. These enable nanoscale control of rapid winding (“W”) and unwinding (“UW”) of their nongrafted portion, such as directional changes in nanohelix unwinding (lower, middle, and upper directions) by changing the position of a permanent magnet while keeping the ligand‐conjugated nanohelix surface area constant. The unwinding (“UW”) setting cytocompatibility facilitates direct integrin recruitment onto the ligand‐conjugated nanohelix to mediate the development of paxillin adhesion assemblies of macrophages that stimulate M2 polarization using glass and silicon substrates for in vitro and in vivo settings, respectively, at a single cell level. Real time and in vivo imaging are demonstrated that nanohelices exhibit reversible unwinding, winding, and unwinding settings, which modulate time‐resolved adhesion and polarization of macrophages. It is envisaged that this remote, reversible, and cytocompatible control can help to elucidate molecular‐level cell–material interactions that modulate regenerative/anti‐inflammatory immune responses to implants.
The use of ligand‐presenting nanohelices is reported that are partly grafted to a material surface to enable magnetic field‐controlled unwinding and winding of their nongrafted portion. It is demonstrated that the unwinding of ligand‐conjugated nanohelix facilitates direct integrin recruitment onto the ligand‐conjugated nanohelix on a single cell level to mediate paxillin adhesion assembly that stimulates M2 polarization of macrophages.
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