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.

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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.

As efficient alternative renewable energy resources, proton-exchange membrane fuel cells (PEMFCs) have received immense attention where proton-conducting materials act as PEMs for efficient proton migration. The recent past has witnessed considerable progress in the use of metal–organic frameworks (MOFs) and coordination polymers (CPs) as solid-state proton conductors (SSPCs) due to their structural superiority and architectural diversity. In order to impart proton conductivity, a variety of intrinsic and extrinsic proton sources have been installed onto these frameworks; however, reports on proton conduction where metal-coordinated water molecules act as solely intrinsic proton sources by virtue of their enhanced acidity are largely unexplored. The key examples of proton conduction driven by coordinated water molecules discussed in this Perspective present some unique features of MOFs and CPs as SSPCs, displaying a wide range of proton conductivity. The scope of works with plausible design principles that are urgently required to be investigated is also highlighted.