Covalent‐Organic Frameworks (COFs) as Proton Conductors

Abstract

202102300half (54%) of the total energy to run several processes, such as heating, refining, and distillation. [1][2] Besides the industrial sector, the fastest-growing transportation sector demands the consumption of 25% energy, whereas the residential and commercial sectors are also accountable for the consumption of 20% energy among the total delivered energy resources. [1] According to the World Energy Statistical Review report, such a high amount of required deliverable energy is acquired mostly from the nonrenewable energy source, i.e., burning of fossil fuels (more than 80%), among which 27%, 33.1%, and 24.2% are coming from coal, oil, and natural gas, respectively. [3,4] According to the Government of India 2018 energy statistics report, even though the production of coal and lignite has been increased 2.9% and 3.79% for the years 2007-08 and 2016-17, respectively, their consumption also increased tremendously in 2016-17 (5.29%) than 2007-08 (2.22%), which displays the emergency and necessity of other alternative energy resources development. [5] Moreover, due to the scarcity of such nonrenewable energy resources and several unavoidable disadvantages of fossil fuels enforce the researchers toward developing renewable and greener alternative energy resources not only from economic perspective but also in terms of effectivity, practicality, and reliability. Toward this direction, fuel cell (FC) system attracts the immense attention over the others due to its several superiorities such as high energy conversion efficiency, low to zero-emission, mild operating conditions, fuel flexibility along with high energy security, and extended durability. [6] FCs are considered as electrochemical power plants, which convert chemical energy to electrical energy with high thermodynamic efficiencies by the cost of particular type of fuels. [7,8] Among several types of FCs, those are typically differentiated by the class of electrolytes used (phosphoric acid (PA), oxide, alkaline, and proton exchange membrane (PEM) (for hydrogen or methanol)), the proton exchange membrane fuel cell (PEMFC) has received particular attention. The hydrogen and direct methanol fuel cells use a polymeric solid-state PEM as an electrolyte and operate at comparatively lower temperatures than most other varieties of FCs, typically from 258 to about 908 °C. [8] In a hydrogen FC, oxygen and hydrogen are supplied to the cathode and anode sides, respectively. While protons Proton conductivity is the paramount property of proton-conducting materials that are playing significant roles in diverse electrochemical devices with applications in proton exchange membranes (PEMs) for fuel cells (PEMFCs). Considering the scarcity of fossil fuels, the development of clean and green renewable energy resources is in-demand across the globe. Toward this direction, the development of solid-state proton conductors is of significant interest. The higher structural tunability, lower density, good crystallinity, accessible well-defined pores, excellent thermal and chemic...

Proton-Conducting Hydrogen-Bonded Organic Frameworks

“…The high proton conductivity value is directly associated with the extent of H-bond formation and their arrangements inside the pore channels (concentration of proton donor–acceptor species) for water-mediated proton transport. Porous crystalline solids such as MOFs ,− or COFs have been particularly of interest recently because the proton conduction properties can be finely adjusted by controlling porosity, crystallinity, and chemical functionality. In contrast to semicrystalline or amorphous polymers, the precise pore systems in crystalline solids make them ideal to study the probable proton-transport conduits and conduction mechanisms. ,,− In this context, HOFs are an emerging class of crystalline solids with unique properties that have attracted much attention in the past decade. − ,− The functional diversity of the organic building blocks or metal–organic building blocks offered in HOFs allows the accommodation of protonic guest solvent molecules (H 2 O, NH 3 ) in the interspace or channels of HOFs during the self-assembly process. − These solvent molecules establish well-defined H-bonded networks, with the functional groups containing proton donor–acceptor sites to create efficient proton-transfer pathways, and thus HOFs have become a newly developed proton-conducting platform having potential in the field of fuel cell technology.…”

“…In the past few decades, crystalline porous materials have been widely investigated by scientists due to their enormous scope of applications in scientific and technological fields like gas storage and separation, catalysis, drug delivery, chemical sensing, and so on, including applications in the domain of proton conduction . Metal–organic frameworks (MOFs) , and covalent organic frameworks (COFs) , have emerged as porous crystalline frameworks having a structure–function correlation that could be smartly fabricated from diverse types of molecular building units through formation of metal-ion coordination bonds and covalent bonds, respectively, affording high dimensionality, tunability, and variable functionality. , Excellent porosity, high stability (thermal/chemical), ease of surface functionalization, and controllable guest adaptability inside the pores of MOFs and COFs render them versatile platforms for proton conduction with ultrahigh superprotonic conductivity of 10 –1 S cm –1 and even higher. , …”

Covalent Organic Framework for Rechargeable Batteries: Mechanisms and Properties of Ionic Conduction

“…[69][70][71][72] Such rigid 1D nanochannels render COFs with great potentials as a novel conductor for efficient mass transfer. [23,[73][74][75] The transport of gas molecules, [76,77] liquid molecules, [78,79] protons, [80,81] and ions [31,43] in the nanochannels of COFs develop a novel research areas of mass transfer in nanoporous crystalline materials, and broadened the application regions, such as membrane separation, [82,83] proton exchange membrane fuel cells, [84,85] and solid-state batteries. [23] This review focuses on the ionic conduction behavior in the nanochannels of COFs and introduces its applications in rechargeable batteries.…”

High-density sulfonic acid-grafted covalent organic frameworks with efficient anhydrous proton conduction

“…1 H NMR spectra were recorded at room temperature (298 K, 101 kPa) in DMSO-d 6 , D 2 O and CDCl 3 as solvents using Bruker Advance (400 MHz) spectrometers. 13 C CP/MAS NMR spectra were recorded on a Bruker Advance 300 at 75 MHz. Powder X-ray diffraction (PXRD) data were collected from 2 to 35 at a 5 min À1 scanning speed with a Bruker D8 Advance diffractometer with a Cu Ka X-ray source (l ¼ 1.54056 Å) operated at 40 kV and 40 mA.…”

“…[7][8][9][10] The ordered proton transport channels, high thermal/chemical stabilities, tailored functionality and adjustable channel structures of COFs make them promising proton exchange membrane candidates. [11][12][13][14] The proton conductivity is mainly determined by proton concentration. In fact, most COF materials are proton insulators, and in order to increase the number of free migrating protons in COFs, there are two distinct strategies to ne-tune the framework and channel space: (i) decorating the framework with suitable acidic units (such as -SO 3 H), because the ability of those acidic moieties to readily release protons permits high proton conductivity; 15 (ii) encapsulating guest molecules with low pK a values (such as H 3 PO 4 , triazole, and imidazole) into the channels, [16][17][18][19][20][21] so that the embedded molecules form a network between the molecules and specic sites on the framework to construct proton-conduction pathways.…”