Technological developments showed that fuel cells are effective sources of power for medium-to large-scale backup power and vehicular applications (e.g., automobiles and buses). [4] In particular, fuel-cellpowered electric vehicles have a longer driving range and shorter refueling time than rechargeable battery-powered vehicles. For example, Toyota has reported that its hybrid fuel cell vehicle can drive up to 830 km powered by 5 kg of H 2 at a pressure of less than 700 bar. [5,6] In this Perspective, we analyze progress in the development of fuel cells fabricated from polymer electrolyte membranes, which run at relatively low temperature (60-90 °C), start up fast, and are portable, and have thus drawn much attention for their potential commercial applications. [7] Proton exchange membrane fuel cells (PEMFCs) have proven successful for transportation and power generation applications, owing to their high power output, compact design, and lightweight cells. Their high performance is due to their use of Nafion, a sulfonated tetrafluoroethylene-based fluoropolymer-copolymer with outstanding chemical stability [8] and excellent proton (H + ) conductivity. [9] PEMFCs' high power output has been demonstrated in stationary, portable, and high-demand devices, and in consumer vehicles. [10][11][12] However, PEMFC membranes and catalysts are expensive, and this has hindered the broader adoption of PEMFC technology by the market. Specifically, PEMFC stacks (i.e., combinations of multiple PEMFCs) are expensive due to their requirement for platinum-group metal (PGM) catalysts and fluorocarbon membranes, which account for 40% and 11% of a cell's cost, respectively, and translate to high vehicle prices. [13,14] As a result, manufacturers are attempting to meet market demand for cheaper PEMFCs by lowering their content of PGM catalysts (or eschewing PGM catalysts entirely) and avoiding their use of fluorocarbon membranes. As an alternative to the use of H + in ion transport, the corresponding redox reactions of hydroxide (OH − ) ions have received attention. Fuel cells with OH − -transporting membranes are termed anion exchange membrane fuel cells (AEMFCs). The working principles of AEMFCs are slightly different from those of PEMFCs, but their overall redox bases are the same. In an AEMFC, water and oxygen are fed into the cathode for reduction to OH − , which then travel through the AEM to the anode to meet the fuel, H 2 , which is oxidized to generate electricity, with water as a byproduct. [15][16][17][18] In contrast to PEMFCs, AEMFCs use cationic Fuel cell technology is a clean way of generating energy and should enable carbon neutrality. However, although traditional proton exchange membrane fuel cells (PEMFCs) have made a commercial impact, their high cost hinders their wider adoption. Fuel cells that use anion exchange membranes (AEMs) are a promising alternative to PEMFCs, as they can achieve the same performance at a lower cost. In this Perspective, recent trends in the fabrication of polymer-based high-performance AEM...
Macrocycles with well‐defined cavities and the ability to undergo supramolecular interactions are classical materials that have played an essential role in materials science. However, one of the most substantial barriers limiting the utilization of macrocycles is their aggregation, which blocks the active regions. Among many attempted strategies to prevent such aggregation, installing macrocycles into covalent organic frameworks (COFs), which are porous and stable reticular networks, has emerged as an ideal solution. The resulting macrocycle‐based COFs (M‐COFs) preserve the macrocycles’ unique activities, enabling applications in various fields such as single‐atom catalysis, adsorption/separation, optoelectronics, phototherapy, and structural design of forming single‐layered or mechanically interlocked COFs. The resulting properties are unmatchable by any combination of macrocycles with other substrates, opening a new chapter in advanced materials. This review focuses on the latest progress in the concepts, synthesis, properties, and applications of M‐COFs, and presents an in‐depth outlook on the challenges and opportunities in this emerging field.
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