Anhydrous Proton Conduction in Crystalline Porous Materials with a Wide Working Temperature Range

Anhydrous Solid-State Proton Conduction in Crystalline MOFs, COFs, HOFs, and POMs

Strategic design of solid-state proton-conducting electrolytes for application in anhydrous proton-exchange membrane fuel cells (PEMFCs) has gained burgeoning interest due to a spectrum of advantageous features, including higher CO tolerance and ease in the water management systems. Toward this direction, crystalline materials like metal-organic frameworks (MOFs), covalent organic frameworks (COFs), hydrogen-bonded organic frameworks (HOFs), and polyoxometalates (POMs) are emerging PEM materials, offering strategic structural engineering through crystallography, thus enabling ultrahigh anhydrous proton conductivity up to 10-2-10-1 S/cm. This Perspective highlights significant progress achieved thus far with such crystalline platforms in the domain of anhydrous proton conduction across a wide temperature window (sub-zero to above 100 °C). Based on their structural backgrounds, these platforms are categorized into four classes (viz. MOFs, COFs, HOFs, and POMs) with a detailed evolutionary timeline since their emergence early in 2009. Insightful discussions with a key focus on the strategies undertaken to attain anhydrous proton conductivity along with implementation in fuel cell technology through membrane electrode assembly are presented. A section on "Critical Analysis and Future Prospects" provides decisive key viewpoints on those overlooked issues with future endorsement (e.g., performance assessment with CO tolerance analysis and fuel cell test stand) for further development while comparing them with other anhydrous platforms from both academic and industrial perspectives.

Proton-electron coupling and mixed conductivity in a hydrogen-bonded coordination polymer

The fundamental understanding of coupled proton-electron transport in mixed protonic-electronic conductors (MPECs) remains unexplored in materials science, despite its potential significance within the broader context of mixed ionic-electronic conductors (MIECs) and the possibility of controlled diffusion of protons using hydrogen-bond networks. To address these limitations, we present a hydrogen-bonded coordination polymer Ni-BAND ({[Ni(bpy)(H2O)2(DMF)2](NO3)2·2DMF}n), which demonstrates high mixed protonic-electronic conductivity at room temperature. Through detailed analysis, we unravel the coupled transport mechanism, offering insights for the rational design of high-performance MPECs. We demonstrate the practical implications of this mechanism by examining the humidity-dependent synaptic plasticity of Ni-BAND, showcasing how MPECs can expand into traditional MIEC applications while leveraging their unique proton-mediated advantages.

Proton -conducting lanthanide metal-organic frameworks: a multifunctional platform

Lanthanide metal-organic frameworks (LMOFs) have established themselves as promising proton-conducting materials among all types of porous coordination polymers and covalent organic frameworks. The structural diversity of LMOFs and high oxophilicity with a high coordination number of lanthanide ions make LMOFs a standout material for proton conduction. In the last few years, ample research efforts have been devoted to designing and developing proton-conducting lanthanide metal-organic frameworks (PCLMOFs). Some of the PCLMOFs have shown great potential with proton conductivity comparable to that of commercially used perfluorosulfonic acid (PFSA) polymers for proton-exchange membranes (PEMs) in fuel cells. At present, it is apparent that PCLMOFs are becoming a potential platform to explore other functional properties (e.g. fluorescence sensing, gas adsorption, molecular magnetism, impedance sensing, ferroelectricity, and nonlinear optics). The intrinsic structural features of PCLMOFs inevitably bring the opportunity to introduce the multifunctional character of such materials. Therefore, any scope for additional functional properties must be investigated for this class of material. In this article, we concisely discuss the design strategy and structural features of some multifunctional PCLMOFs. Furthermore, multifunctional properties of some excellent PCLMOFs are reviewed. In addition, the prospect of PCLMOFs is briefly discussed in the context of real-world material applications.

Inherent Ultrahigh Proton Conductivity of Two Highly Stable COOH-Functionalized Hafnium-Based Metal-Organic Frameworks

Although there has been some recent interest in the proton conductivity (σ) of highly stable carboxyl metal-organic frameworks (MOFs) made of tetravalent metal ions, given their potential applications in fuel cells and electrochemical sensing, research on MOFs constructed by hafnium(IV) ions needs to be expanded significantly. Based on this, we used two common and easily prepared phenylpoly(carboxylic acid) ligands, 1,2,4-phenyltricarboxylic acid and 1,2,4,5-phenyltetracarboxylic acid, to react with hafnium tetrachloride, respectively, creating two porous hafnium(IV)-based MOFs, UiO-66-COOH-Hf (1) and UiO-66-(COOH)2-Hf (2), with the same structure as UiO-66-Hf but with different numbers of free carboxylic groups. A series of stability assays revealed that the two MOFs had excellent structural rigidity, including thermal and water stability. More crucially, alternating current impedance experiments demonstrate that the σ of the two MOFs varies positively with humidity and temperature, reaching up to 10-3 S·cm-1 (1: 2.83 × 10-3 S·cm-1 and 2: 4.35 × 10-3 S·cm-1) under the right conditions (98% relative humidity and 100 °C). The latter roughly doubles the proton conductivity of the former, which is due to the difference in the number of free carboxyl groups, as confirmed by the structural analysis and proton conduction mechanism investigation. The high intrinsic σ of the two MOFs lays a solid foundation for their future application and affords new inspiration for developing high-performance proton-conductive materials.

Prominent Intrinsic Proton Conduction in Two Robust Zr/Hf Metal-Organic Frameworks Assembled by Bithiophene Dicarboxylate

To date, developing crystalline proton-conductive metal-organic frameworks (MOFs) with an inherent excellent proton-conducting ability and structural stability has been a critical priority in addressing the technologies required for sustainable development and energy storage. Bearing this in mind, a multifunctional organic ligand, 3,4-dimethylthiophene[2,3-b]thiophene-2,5-dicarboxylic acid (H2DTD), was employed to generate two exceptionally stable three-dimensional porous Zr/Hf MOFs, [Zr6O4(OH)4(DTD)6]·5DMF·H2O (Zr-DTD) and [Hf6O4(OH)4(DTD)6]·4DMF·H2O (Hf-DTD), using solvothermal means. The presence of Zr6 or Hf6 nodes, strong Zr/Hf-O bonds, the electrical influence of the methyl group, and the steric effect of the thiophene unit all contribute to their structural stability throughout a wide pH range as well as in water. Their proton conductivity was fully examined at various relative humidities (RHs) and temperatures. Creating intricate and rich H-bonded networks between the guest water molecules, coordination solvent molecules, thiophene-S, -COOH, and -OH units within the framework assisted proton transfer. As a result, both MOFs manifest the maximum proton conductivity of 0.67 × 10-2 and 4.85 × 10-3 S·cm-1 under 98% RH/100 °C, making them the top-performing proton-conductive Zr/Hf-MOFs. Finally, by combining structural characteristics and activation energies, potential proton conduction pathways for the two MOFs were identified.

Four Lanthanide(III) Metal-Organic Frameworks Fabricated by Bithiophene Dicarboxylate for High Inherent Proton Conduction

Currently, it is still a challenge to directly achieve highly stable metal-organic frameworks (MOFs) with superior proton conductivity solely through the exquisite design of ligands and the attentive selection of metal nodes. Inspired by this, we are intrigued by a multifunctional dicarboxylate ligand including dithiophene groups, 3,4-dimethylthieno[2,3-b]thiophene-2,5-dicarboxylic acid (H2DTD), and lanthanide ions with distinct coordination topologies. Successfully, four isostructural three-dimensional lanthanide(III)-based MOFs, [Ln2(DTD)3(DEF)4]·DEF·6H2O [LnIII = TbIII (Tb-MOF), EuIII (Eu-MOF), SmIII (Sm-MOF), and DyIII (Dy-MOF)], were solvothermally prepared, in which the effective proton transport will be provided by the coordinated or free solvent molecules, the crystalline water molecules, and the framework components, as well as a large number of highly electronegative S and O atoms. As expected, the four Ln-MOFs demonstrated the highest proton conductivities (σ) being 0.54 × 10-3, 3.75 × 10-3, 1.28 × 10-3, and 1.92 × 10-3 S·cm-1 for the four MOFs, respectively, at 100 °C/98% relative humidity (RH). Excitingly, Dy-MOF demonstrated an extraordinary ultrahigh σ of 1 × 10-3 S·cm-1 at 30 °C/98% RH. Additionally, the plausible proton transport mechanisms were emphasized.

Recent Progress of Crystalline Porous Frameworks for Intermediate-Temperature Proton Conduction

Proton exchange membranes (PEMs), especially for work under intermediate temperatures (100-200 °C), have attracted great interest because of the high CO toleration and facial water management of the corresponding proton exchange membrane fuel cells (PEMFCs). Traditional polymer PEMs faced challenges of low stability and proton carrier leaking. Crystalline porous materials, such as metal-organic frameworks (MOFs) and covalent organic frameworks (COFs), are promising to overcome these issues contributed by nanometer-sized channels. Herein we summarized the recent development of MOF/COF-based intermediate-temperature proton conductors. The strategies of framework engineering and pore impregnation were introduced in detail for raising proton conductivity. The proton-conducting mechanism was described as well. This spotlight will provide new insight into the fabrication of MOF/COF proton conductors under intermediate-temperature and anhydrous conditions.

Proton conducting metal-organic frameworks with light response for multistate logic gates

The simulation of neurons receiving stimulation and transmitting signals by proton conduction has great potential applications in electrochemistry and biology. In this work, copper tetrakis(4-carboxyphenyl)porphyrin (Cu-TCPP), which is a proton conductive metal organic framework (MOF) with photothermal response, is adopted as the structural framework, with the in situ co-incorporation of polystyrene sulfonate (PSS) and sulfonated spiropyran (SSP) to prepare the composite membranes. The resultant PSS-SSP@Cu-TCPP thin-film membranes were used as the logic gates i.e., NO gate, NOR gate and NAND gate because of the photothermal effect of Cu-TCPP MOFs and the photoinduced conformational changes of SSP. This membrane exhibits the high proton conductivity of 1.37 × 10^-4 S cm^-1. Under the conditions of 55 °C and 95% relative humidity (RH), using 405 nm laser irradiation with 400 mW cm^-2 and 520 nm laser irradiation with 200 mW cm^-2 as inputs, the device can be adjusted between various steady states, and the value of the conductivity is regarded as the output with different thresholds in different logic gates. Before and after laser irradiation, the electrical conductivity changes dramatically, and the ON/OFF switching ratio reached 1068. The application of three logic gates is realized by constructing circuits with LED lights. Depending on the convenience of light and the easy measurement of conductivity, this kind of device with light source as input and electrical signal as output provides the possibility to realize the remote control of chemical sensors and complex logic gates devices.

Rare-earth squarate frameworks with topology

As a typical planar 4-connected ligand that possesses D_4h symmetry, the squarate ligand is expected to construct some interesting topologies. Here, we report that the assembly of the squarate ligand with rare-earth ions can produce a series of (4, 8)-connected frameworks with the "smallest" scu type topology. Among these compounds, the Tb based analogue not only possesses a good proton conductivity, but also exhibits luminescence responses toward MnO₄^- and Cr₂O₇^2-, making it a candidate for multifunctional materials.

Metal-Organic Framework-Derived N-Doped Porous Carbon for a Superprotonic Conductor at above 100 °C

The development of proton conductors capable of working at above 100 °C is of great significance for proton exchange membrane electrolysis cells (PEMECs) and proton exchange membrane fuel cells (PEMFCs) but remains to be an enormous challenge to date. In this work, we demonstrate for the first time that the N-doped porous carbon derived from metal-organic frameworks (MOFs) with great superiority can be exploited for high-performing proton conductors at above 100 °C. Through the pyrolysis of ZIF-8, the N-doped porous carbon (ZIF-8-C) featuring high chemical resistance to Fenton's reagent was readily prepared and then served as a robust host to accommodate H₃PO₄ molecules for proton transport. Upon impregnation with H₃PO₄, the resulting PA@ZIF-8-C exhibits low water swelling and high proton conduction of over 10^-2 S cm^-1 at a temperature above 100 °C, which is superior to many reported proton conductors. This work provides a new approach for the design of high-performing proton conductors at above 100 °C.

A Potential Roadmap to Integrated Metal Organic Framework Artificial Photosynthetic Arrays

Metal organic frameworks (MOFs), a class of coordination polymers, gained popularity in the late 1990s with the efforts of Omar Yaghi, Richard Robson, Susumu Kitagawa, and others. The intrinsic porosity of MOFs made them a clear platform for gas storage and separation. Indeed, these applications have dominated the vast literature in MOF synthesis, characterization, and applications. However, even in those early years, there were hints to more advanced applications in light-MOF interactions and catalysis. This perspective focuses on the combination of both light-MOF interactions and catalysis: MOF artificial photosynthetic assemblies. Light absorption, charge transport, H2O oxidation, and CO2 reduction have all been previously observed in MOFs; however, work toward a fully MOF-based approach to artificial photosynthesis remains out of reach. Discussed here are the current limitations with MOF-based approaches: diffusion through the framework, selectivity toward high value products, lack of integrated studies, and stability. These topics provide a roadmap for the future development of fully integrated MOF-based assemblies for artificial photosynthesis.

Switched Proton Conduction in Metal-Organic Frameworks

Stimuli-responsive materials can respond to external effects, and proton transport is widespread and plays a key role in living systems, making stimuli-responsive proton transport in artificial materials of particular interest to researchers due to its desirable application prospects. On the basis of the rapid growth of proton-conducting porous metal-organic frameworks (MOFs), switched proton-conducting MOFs have also begun to attract attention. MOFs have advantages in crystallinity, porosity, functionalization, and structural designability, and they can facilitate the fabrication of novel switchable proton conductors and promote an understanding of the comprehensive mechanisms. In this Perspective, we highlight the current progress in the rational design and fabrication of stimuli-responsive proton-conducting MOFs and their applications. The dynamic structural change of proton transfer pathways and the role of trigger molecules are discussed to elucidate the stimuli-responsive mechanisms. Subsequently, we also discuss the challenges and propose new research opportunities for further development.