Electrochromic (EC) materials with a dark-to-transmissive switch have great applications in optical communications, infrared wavelength detectors for spacecraft, and infrared camouflage coatings. However, such electroactive materials with high stability and cyclability are rare. Considering the advantages of the donor-acceptor approach (wide-range tuneable band position) and porous two-dimensional (2D) covalent organic framework (COF, well-ordered crystalline framework with stable structure and high surface area), in this work we constructed an extended delocalised π-electron layered dark purple EC-COF-1 by reacting the donor N,N,N′,N′-tetrakis(p-aminophenyl)-p-benzenediamine (TPBD) with the acceptor 2,1,3-benzothiadiazole-4,7-dicarboxaldehyde (BTDD). A sandwiched device made of EC-COF-1 exhibits the two-band bleaching (370 nm and 574 nm) in the visible region and becomes transparent under the applied potential with an induced absorption centring at 1400 nm. This discovery of a stable dark-to-transmissive switch in COF might open another door for their application in many EC devices for various purposes.

Combining the chemistry of metal−organic frameworks (MOFs) and covalent organic frameworks (COFs) can bring new opportunities for the design of advanced materials with enhanced tunability and functionality. Herein, we constructed two COFs based on Ni−bis(dithiolene) units and imine bonds, representing a bridge between traditional MOFs and COFs. The Ni− bis(dithiolene)tetrabenzaldehyde as the 4-connected linker was initially synthesized, which was further linked by 4-connected tetra(aminophenyl)pyrene (TAP) or 3connected tris(aminophenyl)amine (TAA) linkers into two COFs, namely, Ni-TAP and Ni-TAA. Ni-TAP shows a two-dimensional sql network, while TAA is a twofold interpenetrated framework with an ffc topology. They both exhibit a high Brunauer−Emmett−Teller surface area (324 and 689 m 2 g −1 for Ni-TAP and Ni-TAA, respectively), a fairly good conductivity (1.57 × 10 −6 and 9.75 × 10 −5 S m −1 for Ni-TAP and Ni-TAA, respectively), and high chemical stability (a stable pH window of 1−14 for Ni-TAA). When applied in lithium metal batteries as an intermediate layer for guiding the uniform Li electrodeposition, Ni-TAP and Ni-TAA displayed impressive lithiophilicity and high Li-ion conductivity, enabling the achievement of smooth and dense Li deposition with a clear columnar morphology and stable Li plating/stripping behaviors with high Li utilization, which is anticipated to pave the way to upgrade Li metal anodes for application in high-energy-density battery systems.

The rational design of efficient and stable catalysts for the oxygen reduction reaction and oxygen evolution reaction (ORR/OER) is the key to improving Li-O 2 battery performance. Here, we report the construction of ORR/OER bifunctional cathode catalysts in a covalent organic framework (COF) platform by simultaneously incorporating Ni-bis(dithiolene) and Co-porphyrin units. The resulting bimetallic Ni/Co-COF exhibits high surface area, fairly good electrical conductivity, and excellent chemical stability. Li-O 2 batteries with the Ni/Co-COF–based cathode show a low discharge/charge potential gap (1.0 V) and stable cycling (200 cycles) at a current density of 500 mA g −1 , rivaling that of PtAu nanocrystals. Density functional theory computations and control experiments using nonmetal or single metal–based isostructural COFs reveal the critical role of Ni and Co sites in reducing the discharge/charge overpotentials and regulating the Li 2 O 2 deposition. This work highlights the advantage of bimetallic COFs in the rational design of efficient and stable Li-O 2 batteries.

Nitrate-containing industrial wastewater poses a serious threat to the global food security and public health safety. As compared to the traditional microbial denitrification, electrocatalytic nitrate reduction shows better sustainability with ultrahigh energy efficiency and the production of high-value ammonia (NH 3 ). However, nitratecontaining wastewater from most industrial processes, such as mining, metallurgy, and petrochemical engineering, is generally acidic, which contradicts the typical neutral/alkaline working conditions for both denitrifying bacteria and the state-of-the-art inorganic electrocatalysts, leading to the demand for pre-neutralization and the problematic hydrogen evaluation reaction (HER) competition and catalyst dissolution. Here, we report a series of Fe 2 M (M = Fe, Co, Ni, Zn) trinuclear cluster metal-organic frameworks (MOFs) that enable the highly efficient electrocatalytic nitrate reduction to ammonium under strong acidic conditions with excellent stability. In pH = 1 electrolyte, the Fe 2 Co-MOF demonstrates the NH 3 yield rate of 20653.5 μg h À 1 mg À 1 site with 90.55 % NH 3 -Faradaic efficiency (FE), 98.5 % NH 3 -selectivity and up to 75 hr of electrocatalytic stability. Additionally, successful nitrate reduction in high-acidic conditions directly produce the ammonium sulfate as nitrogen fertilizer, avoiding the subsequent aqueous ammonia extraction and preventing the ammonia spillage loss. This series of cluster-based MOF structures provide new insights into the design principles of high-performance nitrate reduction catalysts under environmentally-relevant wastewater conditions.