Solid-State Proton Conduction Driven by Coordinated Water Molecules in Metal–Organic Frameworks and Coordination Polymers

Abstract

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.

Dual-Functional Eu–Metal–Organic Framework with Ratiometric Fluorescent Broad-Spectrum Sensing of Benzophenone-like Ultraviolet Filters and High Proton Conduction

“…In addition, proton-conducting materials as one of the most excellent heart materials in clean and renewable proton-exchange membrane fuel cells (PEMFCs) have been investigated extensively. − Effective hydrogen bonding networks are key to materials with attractive proton conductivity. − Up to now, two types of strategies have been proposed to regulate proton conductivity: (i) design and prepare a wide variety of building blocks including hydrophilic groups (such as −COOH, −OH, −SO 3 H, and −PO 3 H 2 ) to build proton transport routes; (ii) incorporating guest molecules (e.g., H 2 O, imidazole, triazole, etc.) into the host framework to facilitate the formation of efficient proton transfer pathways. − Combined with the above information, it is evident that the functional groups (−COOH and −SO 3 H) in the selected KHSBA ligand can not only improve the sensitivity of detection through weak interactions but also possess tremendous potential in proton conduction. Because the aforementioned functional groups can provide plentiful protons, they can also interact with water molecules to further form hydrogen-bonding networks that can efficiently transfer protons.…”

Metalo Hydrogen-Bonded Organic Frameworks Self-Assembled by Charge-Assisted Synthons for Ultrahigh Proton Conduction

“…The conductivities are 1.2 × 10 –4 S cm –1 for 1 and 4.2 × 10 –4 S cm –1 for 2 under 60% RH at 353 K (Table S11, SI). Such a scenario should be attributed to the high density of coordinated water molecules and carboxylic acid groups. − Recent studies point out that coordinated water molecules are liable to prone the proton transport through the H-bonded network due to improved acidity after metal coordination . In addition to this, the high density of protonated and deprotonated carboxylic acid groups, which are not coordinated to the metal centers, is also important and can enhance the proton conductivity by enhancing the acidity of the material .…”

Kinetic Impacts of Defect Sites in Metal–Organic Framework Catalysts under Varied Driving Forces

“…The aforementioned material properties of MOFs also confer a wide range of design approaches that can lead to their development as active and stable electrocatalysts, including alternatives to bulk noble metal electrocatalysts that are among the most active materials but are too costly for practical applications for reactions such as carbon dioxide reduction (CO 2 R), the oxygen evolution reaction (OER), and the oxygen reduction reaction (ORR) . The porous nature of MOFs can improve the delivery of molecules or ions to electrode surfaces to circumvent the effects of mass transport limitations on catalysis-relevant current densities, and the modularity of linker functional groups can promote inductive or second coordination sphere intermediate stabilization to overcome intermediate scaling relationships for multi-electron redox processes . In these cases, the MOF acts as the active catalytic phase or as an auxiliary component that can utilize specific material properties (e.g., porosity or functional group proximity) to facilitate catalysis .…”