Petkov P, Heine T, Mannsfeld SCB, Feng X, Dong R. Surface-Modified Phthalocyanine-Based Two-Dimensional Conjugated Metal-Organic Framework Films for Polarity-Selective Chemiresistive Sensing

2D conjugated metal-organic frameworks (2D c-MOFs) are emerging as electroactive materials for chemiresistive sensors, but selective sensing with fast response/recovery is a challenge. Phthalocyanine-based Ni2 [MPc(NH)8 ] 2D c-MOF films are presented as active layers for polarity-selective chemiresisitors toward water and volatile organic compounds (VOCs). Surface-hydrophobic modification by grafting aliphatic alkyl chains on 2D c-MOF films decreases diffused analytes into the MOF backbone, resulting in a considerably accelerated recovery progress (from ca. 50 to ca. 10 s) during humidity sensing. Toward VOCs, the sensors deliver a polarity-selective response among alcohols but no signal for low-polarity aprotic hydrocarbons. The octadecyltrimethoxysilane-modified Ni2 [MPc(NH)8 ] based sensor displays high-performance methanol sensing with fast response (36 s)/recovery (13 s) and a detection limit as low as 10 ppm, surpassing reported room-temperature chemiresistors.

Successive Storage of Cations and Anions by Ligands of π-d-Conjugated Coordination Polymers Enabling Robust Sodium-Ion Batteries

The oxidation of π-d-conjugated coordination polymers (CCPs) accompanied with anion insertion has the merits of increasing the capacity and elevating the discharge voltages. However, previous reports on this mechanism either required more investigations or showed low capacity and poor cyclablity. Herein, triphenylene-catecholate-based two-dimensional CCPs are constructed by employing inactive transition-metal ions (Zn²⁺ ) as nodes, forming Zn-HHTP. Substantial characterizations and theoretical calculations indicate the successive storage of cations and anions by redox reactions of only ligands, leading to a high reversible capacity of ≈150 mAh g^-1 at 100 mA g^-1 and a remarkable capacity retention of 90 % after 1000 cycles. On the contrary, as a control experiment, the analogous CCPs (Cu-HHTP) with Cu²⁺ nodes, where both ligands and metal ions undergo redox reactions, accompanied by the storage of only Na⁺ cations, show a much poorer cyclability. These results highlight the importance of redox reactions of only ligands for long-term cycle life and the insight into the storage mechanisms deepens our understanding on CCPs for the further design of CCPs with high performance.

Two-Dimensional π-Conjugated Frameworks as a Model System to Unveil a Multielectron-Transfer-Based Energy Storage Mechanism

ConspectusAlthough electrochemical energy storage is commonplace in our society, further advancements in this technology are indispensable for the transition to a low-carbon society. Recent intensive research has expanded concepts in this field; however, finding one suitable material to obtain a high energy density accomplishing the criteria of next-generation batteries is still a conundrum. To solve this issue, material investigations based on big data combined with artificial intelligence are a present trend. On the contrary, this Account focuses on an alternative approach, i.e., fundamental research to shed light on key basic principles to design new electrode materials and new principles achieving multielectron transfer, which is a key to improve a specific capacity. In addition to the cation-redox mechanism, materials showing the multielectron-transfer mechanism based on cation-/anion-redox can enrich material choices with high theoretical energy densities. The challenge in this mechanism is that a rational design of electrode materials based on microscopic understanding of underlying electrode processes has not been fully achieved so far. This is a key bottleneck in machine-learning approaches as well because the reliability of outputs from an algorithm is dependent on the reliability of data from a corresponding microscopic electrode process. Therefore, uncovering fundamental mechanisms in electrochemical energy storage remains one of the primary goals for the present research. In our series of investigations, we developed concepts for replacing complex practical electrode materials, such as polyanion or Li-rich layered oxides, by simplified model systems based on two-dimensional (2D) π-conjugated frameworks, which are based on purely organic aromatic systems and metal-containing coordination polymers. These materials are relatively simple, but it is still possible to control their complexity of systems in order to mimic certain aspects of structure-property relations in practical electrode materials. In particular, recent studies have shown that we can tune electronic structures of 2D π-conjugated frameworks, which is a key feature to investigate electron-transfer mechanisms, along with the concept of the threefold correlation approach, i.e., the relations in chemical structures, electronic structures, and electrochemical reactions. In this Account, several model studies focusing on microscopic understandings of structure-electrochemical energy storage functions are presented in which we investigate how the structural periodicity and nature of the coordination environment affect their electronic properties and the electrochemical reactions. In particular, we investigate the effects of combinations of linkers and metal ions toward the mechanism of the electrochemical energy storage reaction. We identified few major factors determining the energy storage mechanism of 2D π-conjugated frameworks. Local configurations of coordinate covalent bonding and organic linkers interact with each other, and these effects provide unique electronic states. These electronic states are projections of intriguing electrochemical features in this materials system, such as cation/anion co-redox mechanism, anion-insertion mechanism, or inductive effect. This Account indicates that 2D π-conjugated frameworks can be applied as models to extract fundamental/microscopic principles in the complicated electrode processes, which is linked to practical electrode materials, such as oxides. Therefore, the approach shown here is a powerful tool to unveil microscopic electrochemical energy storage mechanisms, which is indispensable to advance clean energy technology and accelerate decarbonization.

Double-Layer Nitrogen-Rich Two-Dimensional Anionic Uranyl-Organic Framework for Cation Dye Capture and Catalytic Fixation of Carbon Dioxide

A novel two-dimensional double-layer anionic uranyl-organic framework, U-TBPCA {[NH₂(CH₃)₂][(UO₂)(TBPCA)], where H₃TBPCA = 4,4',4″-s-triazine-1,3,5-triyltripamino-methylene-cyclohexane-carboxylate}, with abundant active sites and stability was obtained by assembling UO₂(NO₃)₂·6H₂O and a triazine tricarboxylate linker, TBPCA^3-. Due to the flexibility of the ligand and diverse coordination modes between carboxyl groups and uranyl ions, U-TBPCA exhibits an intriguing topological structure and steric configuration. This double-layer anionic uranyl-organic framework is highly porous and can be used for selective adsorption of cationic dyes. Due to the presence of high-density metal ions and basic -NH- groups, U-TBPCA acts as an effective heterogeneous catalyst for the cycloaddition reaction of carbon dioxide with epoxy compounds. Moreover, the various modes of coordination between the tricarboxylic ligand and uranyl ion were studied by density functional theory calculations, and several simplified models were established to probe the influence of hydrogen bonding between carbon dioxide and U-TBPCA on the ability of U-TBPCA to bind carbon dioxide. This work should aid in improving our understanding of the coordination behavior of uranyl ion as well as the development and utilization of new actinide materials.