ConspectusThe current research in the field of dynamic covalent chemistry includes the study of dynamic covalent reactions, catalysts, and their applications. Unlike noncovalent interactions utilized in supramolecular chemistry, the formation/breakage of covalent bonding has slower kinetics and usually requires the aid of a catalyst. Catalytic systems that enable efficient thermodynamic equilibrium are thus essential. In this Account, we describe the development of efficient catalysts for alkyne metathesis, and discuss the application of dynamic covalent reactions (mainly imine, olefin, and alkyne metathesis) in the development of organic functional materials.Alkyne metathesis is an emerging dynamic covalent reaction that offers robust and linear acetylene linkages. By introducing a podand motif into the catalyst ligand design, we have developed a series of highly active and robust alkyne metathesis catalysts, which, for the first time, enabled the one-step covalent assembly of ethynylene-linked functiona...

Dynamic Covalent Chemistry Approaches Toward Macrocycles, Molecular Cages, and Polymers

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.

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Durability challenges of anion exchange membrane fuel cells

Alkaline anion exchange membrane (AEM) electrolysers to produce hydrogen from water are still at an early stage of development, and their performance is far lower than that of systems based on proton exchange membranes. Here, we report an ammonium-enriched anion exchange ionomer that improves the performance of an AEM electrolyser to levels approaching that of state-of-the-art proton exchange membrane electrolysers. Using rotating-disk electrode experiments, we show that a high pH (>13) in the electrode binder is the critical factor for improving the activity of the hydrogen- and oxygen-evolution reactions in AEM electrolysers. Based on this observation, we prepared and tested several quaternized polystyrene electrode binders in an AEM electrolyser. Using the binder with the highest ionic concentration and a NiFe oxygen evolution catalyst, we demonstrated performance of 2.7 A cm−2 at 1.8 V without a corrosive circulating alkaline solution. The limited durability of the AEM electrolyser remains a challenge to be addressed in the future. Anion exchange membrane water electrolysers have potential cost advantages over proton exchange membrane electrolysers, but their performance has lagged behind. Here the authors investigate the cause of the poor performance of anion exchange membrane electrolysers and design ionomers that can overcome some of the challenges.

https://www.nature.com/articles/s41560-020-0577-x.pdf?proof=t

Highly quaternized polystyrene ionomers for high performance anion exchange membrane water electrolysers

Molecular Design of Single-Atom Catalysts for Oxygen Reduction Reaction

Alkaline-Stable Anion Exchange Membranes: A Review of Synthetic Approaches

One promising approach to reduce the cost of fuel cell systems is to develop hydroxide exchange membrane fuel cells (HEMFCs), which open up the possibility of platinum-group-metal-free catalysts and low-cost bipolar plates. However, scalable alkaline polyelectrolytes (hydroxide exchange membranes and hydroxide exchange ionomers), a key component of HEMFCs, with desired properties are currently unavailable, which presents a major barrier to the development of HEMFCs. Here we show hydroxide exchange membranes and hydroxide exchange ionomers based on poly(aryl piperidinium) (PAP) that simultaneously possess adequate ionic conductivity, chemical stability, mechanical robustness, gas separation and selective solubility. These properties originate from the combination of the piperidinium cation and the rigid ether-bond-free aryl backbone. A low-Pt membrane electrode assembly with a Ag-based cathode using PAP materials showed an excellent peak power density of 920 mW cm−2 and operated stably at a constant current density of 500 mA cm−2 for 300 h with H2/CO2-free air at 95 °C. A key challenge for hydroxide exchange membrane fuel cells is the development of membranes with both high ionic conductivity and mechanical strength. Here the authors report a high-performance family of poly(aryl piperidinium) membranes enabling promising durability and power density.

Poly(aryl piperidinium) membranes and ionomers for hydroxide exchange membrane fuel cells

We report the synthesis and characterization of a novel series of main-chain boron-containing conjugated polymers (CPs), boron “doped” polyacetylenes (BPAs), which represent the first examples of main-chain boron-containing CPs without aromatic moieties within the polymer backbone, potentially enhancing electronic delocalization along the main-chain. These polymers are characterized by NMR, absorption and emission spectroscopy, cyclic voltammetry, theoretical calculation, and fluoride titration experiments, providing detailed understanding of the effects from different cross-conjugated side-chains on the physical and electronic properties of this new class of boron-containing polymers.

https://par.nsf.gov/servlets/purl/10081792

Boron “Doped” Polyacetylenes

Novel polydiacetylenes (PDAs) bearing alkyl and phenyl substituents have been synthesized, for the first time, by solution polymerization using acyclic enediyne metathesis. The resulting polymers are soluble in common organic solvents and show distinct physical and photophysical properties both in solution and as thin films, caused by different steric and electronic effects from the side-groups. Bulk heterojunction solar cells employing these PDAs have been fabricated and evaluated.

Solution processable polydiacetylenes (PDAs) through acyclic enediyne metathesis polymerization

We report the synthesis and characterization of a unique class of conjugated polymers, polytriacetyelenes (PTAs), from a set of trans-enediyne (EDY) monomers bearing alkyl and aromatic substituents. Two different methods, i.e., Glaser-Hay coupling and Pd-catalyzed oxidative coupling, were employed depending on the nature of the side-chains. Our methodology is highly versatile in that it allows direct attachment of aromatic substituents to the PTA main-chains, allowing enhanced tunability in polymer electronic properties, which has not been achievable through previously reported methods. The newly synthesized PTAs displayed strong ability of quenching the fluorescence of poly(3-hexylthiophene) (P3HT), a prototypical conjugated polymer in organic solar cells, indicating possible applications of these PTAs in optoelectronic devices. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016, 54, 1391-1395

Polytriacetylenes bearing directly attached functional groups with tunable physical and electronic properties

https://iopscience.iop.org/article/10.1149/1945-7111/abc4bd/pdf

Keda Hu

Papers

Poly(aryl piperidinium) membranes and ionomers for hydroxide exchange membrane fuel cells

Boron “Doped” Polyacetylenes

Solution processable polydiacetylenes (PDAs) through acyclic enediyne metathesis polymerization

Polytriacetylenes bearing directly attached functional groups with tunable physical and electronic properties

Editors’ Choice—Uncovering the Role of Alkaline Pretreatment for Hydroxide Exchange Membrane Fuel Cells