Key Takeaways & Executive Findings
- •• Comprehensive summary of the effects of size, structure, and stability of MOFs on proton conduction. • Critical discussion of advanced strategies for constructing proton-conducting MOFs, including charged network construction, ligand functionalization, and defect engineering. • Thorough analysis of proton conduction mechanisms and behavior in MOFs, highlighting structure-property relationships. • Outlook on challenges and opportunities for developing novel proton-conducting MOFs for practical applications in fuel cells and energy devices.
Abstract
Proton-conducting materials have attracted considerable interest because of their extensive application in energy storage and conversion devices. Among them, metal–organic frameworks (MOFs) present tremendous development potential and possibilities for constructing novel advanced proton conductors due to their special advantages in crystallinity, designability, and porosity. In particular, several special design strategies for the structure of MOFs have opened new doors for the advancement of MOF proton conductors, such as charged network construction, ligand functionalization, metal-center manipulation, defective engineering, guest molecule incorporation, and pore-space manipulation. With the implementation of these strategies, proton-conducting MOFs have developed significantly and profoundly within the last decade. Therefore, in this review, we critically discuss and analyze the fundamental principles, design strategies, and implementation methods targeted at improving the proton conductivity of MOFs through representative examples. Besides, the structural features, the proton conduction mechanism and the behavior of MOFs are discussed thoroughly and meticulously. Future endeavors are also proposed to address the challenges of proton-conducting MOFs in practical research. We sincerely expect that this review will bring guidance and inspiration for the design of proton-conducting MOFs and further motivate the research enthusiasm for novel proton-conducting materials.
1. Introduction
The rising global demand for energy coupled with environmental concerns stemming from fossil fuel usage has driven a global expansion of renewable and sustainable energy alternatives [1–6]. Renewable energy-based hydrogen energy, including production, storage, and conversion, is broadly recognized as a promising alternative for future energy sources [7–9]. In this process, proton exchange membrane fuel cells (PEMFCs) can convert the chemical energy in hydrogen into electrical energy efficiently in a carbon emission-free process, have aroused considerable attention due to their environmentally friendly and high-efficiency properties [10–12]. Notably, the polymer electrolyte membranes/proton exchange membranes (PEMs) as the core of PEMFCs, are the key to realizing their high performance, safety, and durability [13, 14]. To advance the FC iteration, several materials, such as Nafion and its substitute polymers, porous organic/inorganic/carbon materials, and inorganic/polymer composites have been successively designed and explored for different application scenarios [15–18]. Proton-conducting materials are evolving rapidly in this context. Indeed, two general design targets emerged in the research of proton-conducting materials for FCs: (i) developing novel materials suitable for operation in humid conditions (<100 °C); (ii) developing efficient anhydrous proton conductors with performance independent of humidity conditions (>100 °C).
During the iterative updating process of new energy technologies, porous materials capable of storing energy carriers or facilitating rapid ion conduction for efficient energy storage and conversion have been extensively investigated and analyzed [19–26]. Indeed, desirable morphology, suitable surface area, and exceptional functionality are decisive features for ion conduction and substrate reaction kinetics in this process [27–30]. Compared with conventional inorganic porous materials, MOFs feature tunable topologies/pore sizes, flexible customizability, permanent porosity, remarkable surface area, and organic–inorganic hybrid nature, which render them significantly superior in proton conduction [31–35]. Concurrently, the development of synthetic methods has also provided better control over the fabrication of MOFs with hierarchical microstructures [36, 37]. Particularly, MOFs with an extensive number of candidate structures (>70,000) provide an unprecedented opportunity to further investigate the proton conduction mechanism and conduction behavior [38–44]. The proton-conducting MOFs were first presented in 1979 by Kanda et al., whereas the proton conduction mechanism of MOFs was unclear because of lacking crystallinity [45]. Until 2
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Liyu Zhu, Hongbin Yang, Ting Xu, Feng Shen, Chuanling Si (2024). Precision-Engineered Construction of Proton-Conducting Metal–Organic Frameworks. Nano-Micro Letters. https://doi.org/10.1007/s40820-024-01558-3
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Frequently Asked Questions
What are proton-conducting metal-organic frameworks (MOFs)?
Proton-conducting MOFs are a class of porous crystalline materials that facilitate the transport of protons (H+) through their structure. They combine the tunable porosity and designability of MOFs with the ability to conduct protons, making them promising for applications in fuel cells and other energy conversion devices.
What are the key strategies for enhancing proton conductivity in MOFs?
Key strategies include charged network construction, ligand functionalization, metal-center manipulation, defective engineering, guest molecule incorporation, and pore-space manipulation. These approaches aim to increase proton carrier concentration, create efficient proton transport pathways, and improve structural stability.
Why are MOFs considered advantageous for proton conduction compared to traditional materials?
MOFs offer tunable topologies and pore sizes, high surface area, permanent porosity, and organic-inorganic hybrid nature. These features allow for precise control over the proton conduction environment, leading to potentially higher conductivities and better understanding of conduction mechanisms.
What are the main challenges in developing proton-conducting MOFs for practical applications?
Challenges include improving long-term stability under operating conditions (e.g., humidity and temperature), achieving high proton conductivity at low humidity or anhydrous conditions, and scaling up synthesis. Additionally, integrating MOFs into device architectures remains a hurdle.
What is the significance of this review on proton-conducting MOFs?
This review provides a comprehensive and critical analysis of the fundamental principles, design strategies, and implementation methods for improving proton conductivity in MOFs. It summarizes recent advances, discusses mechanisms, and outlines future directions, serving as a valuable resource for researchers in the field.
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