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...

Solid-state proton-conducting materials play essential roles in various electrochemical devices, including fuel cells as solid electrolytes. Recently, research on hydrogen-bonded organic frameworks (HOFs) has gained considerable momentum in diverse applications, as several of them show high stability with permanent microporosity. The inherent well-defined H-bonded networks in HOFs make them versatile platforms as solid-state proton conductors exhibiting conductivities as high as 10–1 S cm–1. In this Focus Review, we present the development of HOFs as proton conductors while briefing early reports on proton-conducting H-bonded organic systems. Reports on proton conductivity with other terminologies, such as supramolecular organic frameworks (SOFs), porous organic salts (POSs), or porous molecular crystals (PMCs), are also taken into consideration. All efforts have been made to organize and classify the proton-conducting HOFs with a deeper insight into the design principle and critical features in realizing such conduction properties. The advantages, potential challenges, and prospects of HOFs as proton conductors are discussed.

Great efforts have been made toward the separation of CO 2 from flue gas and biogas to mitigate environmental pollution and the demand for renewable fuels, respectively. Nonthermal-based separations, such as adsorption-based or membrane-separation technology employing porous materials, are considered to be more promising than traditional cryogenic and absorption-based systems. Due to several advantages of metal-organic frameworks (MOFs) over other conventional porous materials, reports on flue and biogas separation by MOFs are burgeoning (423 for adsorption and 56 for membrane-based separations until June 2021) and demand urgent summarization. This review presents a bird's eye view on such separations while organizing the developed strategies and considering several performance parameters, such as tradeoff between sorption capacity and separation selectivity in adsorption-based systems and permeability versus separation selectivity in membrane-based systems. In addition, the mechanisms involving such separations at the molecular level are presented. A critical discussion section offers more crucial insights into these materials from industrial deployment viewpoints. Finally, future recommendations are made for further developments of MOF materials as flue and biogas separators and thus toward solving both the challenging universal problems of global warming and energy scarcity simultaneously.

Trivalent metal ions (Cr 3+ , Al 3+ , and Fe 3+ ) constitute a major section of the environmental pollutants, and their excess accumulation has a detrimental effect on health, so their detection in trace quantity has been a hot topic of research. A highly scalable 3D porous Zn-based luminescent metal− organic framework (MOF) has been synthesized by exploiting the mixed ligand synthesis concept. The strategic selection of an aromatic π-conjugated organic linker and N-rich spacer containing the azine functionality as metal ion binding sites immobilized across the pore spaces, have made this MOF an ideal turn-on sensor for Al 3+ , Cr 3+ , and Fe 3+ ions with very high sensitivity, selectivity, and recyclability. An in-depth study revealed absorbance caused enhancement mechanism (ACE) responsible for such turn-on phenomena. In order to make the detection process straightforward, convenient, portable, and economically viable, we have fabricated MOF test paper strips (the MOF could be simply immobilized onto the paper strips) for naked eye visual detection under UV light, which, thus, manifests its potential as a real-time smart sensor for these trivalent ions.

The development of chemically stable metal–organic framework (MOF)-based luminescent platforms for toxic ion detection in an aqueous medium is highly challenging because most of the classical MOFs are prone to water degradation, and that is the reason why most of the MOF-based luminescent sensors use a nonaqueous medium for sensing. In this contribution, we report two new water-stable luminescent MOFs (Zn-MOF-1 and Zn-MOF-2), assembled from a mixed-ligand synthesis approach. Because of the presence of a hydrophobic trifluoromethyl group to the backbone and stronger metal–N coordination, these MOFs exhibit excellent stability not only in water but also in acidic/alkaline aqueous solutions (pH = 3–10). Here, we report a green sensing approach by exploiting the significant reduction in photoluminescence of these MOFs in the presence of toxic ions. Fe3+ and CrO4 2–/Cr2O7 2– ions could be traced with a detection limit (LOD) in the micromolar range (0.045 and 0.745/0.33 μM for Zn-MOF-1; 125.2 and 114.2/83.5 μM for Zn-MOF-2). The mechanistic study reveals that competitive absorption of the excitation energy coupled with fluorescent resonance energy transfer are responsible for the turn-off quenching. The anti-interference ability and recyclability along with the pH stability gave these MOFs high potential to be used as practical sensors toward FeIII and CrVI ions in water as a greenest medium.