Superprotonic Conductivity of MOF‐808 Achieved by Controlling the Binding Mode of Grafted Sulfamate

Abstract

A metal–organic framework (MOF) having superprotonic conductivity, MOF‐808, is prepared by modulating the binding mode of the sulfamate (SA) moieties grafted onto the metal clusters. The activation of the SA‐grafted MOF‐808 at 150 °C changes the binding mode of the grafted SA from monodentate to bridging bidentate, thus converting the neutral amido (‐S−NH2) moiety of the grafted SA to the more acidic cationic sulfiliminium (‐S=NH2+) moiety. Further, the acidic sulfiliminium moiety of MOF‐808‐4SA‐150 results in more efficient proton conduction than the amido moiety of MOF‐808‐4SA‐60. At 60 °C and 95 % relative humidity, MOF‐808‐4SA‐150 is found to have a proton conductivity of 7.89×10−2 S cm−1, which is more than 30‐times higher than that of MOF‐808‐4SA‐60. Moreover, this superprotonic conductivity is well maintained over 1000 cycles of conductivity measurements and for similar cyclic measurements each day for seven days.

Proton-Conductive Cerium-Based Metal–Organic Frameworks

“…The development of metal–organic frameworks (MOFs) , in the past two decades has rendered the use of such highly porous materials in a variety of applications including adsorption, , separation, , catalysis, − sensing, , and charge storage. − As most MOFs are highly porous but electrically insulating, , and hydrophilic functional groups facilitating proton transport can be incorporated within the entire frameworks, such materials have been widely served as proton conductors for potential applications in fuel cells and humidity sensors. − Among various MOFs, frameworks constructed from group IV metal-based nodes, such as zirconium-, hafnium-, and titanium-based MOFs, are chemically robust in aqueous solutions with a wide range of pH; − this characteristic further provides the structural stability during the use of such MOFs in the applications that require humid environments. Thus, proton conduction within zirconium-based MOFs (Zr-MOFs) and their derivatives has been extensively investigated recently. , For example, proton transport within one of the most commonly reported Zr-MOFs, UiO-66, has been investigated in several studies, − and it is known that the presence of hydrophilic functional groups on the linkers or the aqua/hydroxo groups on the defective nodes can significantly facilitate the proton conduction in UiO-66. − , Another common Zr-MOF, MOF-808, was thereafter used as the scaffold to design the highly proton-conductive MOFs, − as the six-connected nature of MOF-808 may allow the presence of six aqua/hydroxo pairs on each node after the complete removal of coordinated formate. As demonstrated recently, the proton conductivity (σ) of MOF-808 could be further enhanced after the incorporation of imidazole or sulfamate within the MOF pore.…”

“…Thus, proton conduction within zirconium-based MOFs (Zr-MOFs) and their derivatives has been extensively investigated recently. , For example, proton transport within one of the most commonly reported Zr-MOFs, UiO-66, has been investigated in several studies, − and it is known that the presence of hydrophilic functional groups on the linkers or the aqua/hydroxo groups on the defective nodes can significantly facilitate the proton conduction in UiO-66. − , Another common Zr-MOF, MOF-808, was thereafter used as the scaffold to design the highly proton-conductive MOFs, − as the six-connected nature of MOF-808 may allow the presence of six aqua/hydroxo pairs on each node after the complete removal of coordinated formate. As demonstrated recently, the proton conductivity (σ) of MOF-808 could be further enhanced after the incorporation of imidazole or sulfamate within the MOF pore.…”

Microwave-Assisted Rapid Synthesis of Nanoscale MOF-303 for Hydrogel Composites with Superior Proton Conduction at Ambient-Humidity Conditions

“…Proton-conducting MOFs have recently received an increasing amount of research interest as a consequence of their remarkable structural characteristics. − The ordered pores/channels in the frameworks of MOFs not only favor the transport of proton carriers but also allow the encapsulation of guest molecules as proton carriers (e.g., acids, ionic liquids, and heterocyclic molecules) to improve proton conduction. − Alternatively, the proton-conducting performance of MOFs can be significantly improved by grafting acidic functional groups (e.g., −SO 3 H, −PO 3 H 2 , and −COOH) in the frameworks. − Some MOFs exhibit ultrahigh proton conductivities of up to 10 –2 –10 –1 S cm –1 , ,,, which is comparable to that of Nafion. Notably, most MOFs exhibit superior proton conduction only at high-humidity conditions, since sufficient water molecules adsorbed in the pores/channels of MOFs play an important role in the formation of well-established hydrogen-bonding networks, serving as the proton-transfer pathway.…”

“…17−21 Alternatively, the proton-conducting performance of MOFs can be significantly improved by grafting acidic functional groups (e.g., −SO 3 H, −PO 3 H 2 , and −COOH) in the frameworks. 22−25 Some MOFs exhibit ultrahigh proton conductivities of up to 10 −2 −10 −1 S cm −1 , 18,23,24,26 which is comparable to that of Nafion. Notably, most MOFs exhibit superior proton conduction only at high-humidity conditions, since sufficient water molecules adsorbed in the pores/ channels of MOFs play an important role in the formation of well-established hydrogen-bonding networks, serving as the proton-transfer pathway.…”

Switched Proton Conduction in Metal–Organic Frameworks

“… 47 − 50 Despite the inherently poor proton conduction of MOFs, the strategy of loading protonic media molecules into pores to form hydrogen-bond networks acting as protonic transfer pathways has successfully induces a series of MOFs with high proton conductivity (>10 –3 S cm –1 ) over a wide operating temperature. 51 − 57 …”