Acidic Groups Functionalized Carbon Dots Capping Channels of a Proton Conductive Metal–Organic Framework by Coordination Bonds to Improve the Water-Retention Capacity and Boost Proton Conduction

Abstract

Crystalline porous materials, such as metal−organic frameworks (MOFs) and covalent organic frameworks (COFs), have been demonstrated to be versatile material platforms for the development of solid proton conductors. However, most crystalline porous proton conductors suffer from decreasing proton conductivity with increasing temperature due to releasing water molecules, and this disadvantage severely restricts their practical application in electrochemical devices. In this work, for the first time, hydrophilic carbon dots (CDs) were utilized to hybridize with high proton conductivity MOF-802, which is a model of MOF proton conductors, aiming to improve its water-retention capacity and thus enhance proton conduction. The resultant CDs@MOF-802 exhibits impregnable proton conduction with increasing temperature, and the proton conductivity reaches 10 −1 S cm −1 , much superior to that of MOF-802, making CDs@MOF-802 one of the most efficient MOF proton conductors reported so far. This study provides a new strategy to improve the water-retention capacity of porous proton conductors and further realize excellent proton conduction.

A multifunctional anionic metal–organic framework for high proton conductivity and photoreduction of CO2 induced by cation exchange

“…Previously reported achievements based on various MOFs are presented in Table S4, † where the proton conductivity of NH 4 + @In-MOF24 is used for comparing with these listed experimental results. Clearly, the performances of NH 4 + @In-MOF24 locate at a middle level, compared to these covered proton conductive MOFs, such as CDs@MOF-802 possessing a value of 1.13 × 10 −1 S cm −1 under 25 °C and 98% RH, 26 Im@MOF-808 with a value of 3.45 × 10 −2 S cm −1 at 60 °C and 99% RH, 27 MOF-801 with a value of 1.88 × 10 −3 S cm −1 at 25 °C and 98% RH 28 and Cd-MOF with a value of 1.15 × 10 −3 S cm −1 at 90 °C and 98% RH, 29 and so on.…”

Ionothermal synthesis of a highly crystalline zirconium phosphate proton conductor

“…It agrees with the fact that that water molecules adsorbed on the samples are critical for proton conductivity. 36–39 Improvement of proton conductivity of ZrP-3 was also observed along with the increasing temperature while maintaining the relative humidity. As shown in Fig.…”

A Potential Roadmap to Integrated Metal Organic Framework Artificial Photosynthetic Arrays

“…61,62 These structures can achieve high levels of conductivity (10 −1 S/cm), but come at the tradeoff of lower porosity and available surface area, since the MOF pore is loaded with the proton carriers. 63 Similar to proton transport, movement of electrons and counterions are critical for driving the redox reactions of water oxidation. In redox reactions, these two processes are normally described by the apparent diffusion coefficient (D app ), but they can be separated to better understand the limits of electrochemical charge transport in MOFs.…”

“…The Co analogue [Co 2 Cl 2 (BTC) 4/3 ]­(Me 2 NH 2 ) 2 demonstrates high proton conductivity (5.93 × 10 –4 S/cm) compared to the neutral framework (1.5 × 10 –5 S/cm). , Other node modifications involve the addition of proton carrier molecules such as imidazole or sulfamate to MOF nodes, providing H-bonding sites for structured solvent in the MOF pores. , Similarly, MOF linkers have be functionalized with −OH, −COOH, −NH 2 , −SH, or −SO 3 H groups resulting in proton conductivities on the order of 10 –3 –10 –1 S/cm (Figure b). , Since proton conductivity is directly related to the mobility of protons, functionalization with highly acidic functional groups (such as sulfonic acid), which are more fully dissociated, generally show increased conductivity over weaker acids. Finally, MOF conductivity can be increased through host–guest interactions by the incorporation of proton carrier molecules into the MOF pores, rather than bound to the ligand or node. , These structures can achieve high levels of conductivity (10 –1 S/cm), but come at the trade-off of lower porosity and available surface area, since the MOF pore is loaded with the proton carriers …”