Boosting Proton Conductivity in Highly Robust 3D Inorganic Cationic Extended Frameworks through Ion Exchange with Dihydrogen Phosphate Anions

Abstract

The limited long-term hydrolytic stability of rapidly emerging 3D-extended framework materials (MOFs, COFs, MOPs, etc.) is still one of major barriers for their practical applications as new solid-state electrolytes in fuel cells. To obtain hydrolytically stable materials, two H2 PO4 (-) -exchanged 3D inorganic cationic extended frameworks (CEFs) were successfully prepared by a facile anion-exchange method. Both anion-exchanged CEFs (YbO(OH)P and NDTBP) show significantly enhanced proton conductivity when compared with the original materials (YbO(OH)Cl and NDTB) with an increase of up to four orders-of-magnitude, reaching 2.36×10(-3) and 1.96×10(-2) S cm(-1) at 98 % RH and 85 °C for YbO(OH)P and NDTBP, respectively. These values are comparable to the most efficient proton-conducting MOFs. In addition, these two anion-exchanged materials are stable in boiling water, which originates from the strong electrostatic interaction between the H2 PO4 (-) anion and the cationic host framework, showing a clear advance over all the acid-impregnated materials (H2 SO4 @MIL-101, H3 PO4 @MIL-101, and H3 PO4 @Tp-Azo) as practical solid-state fuel-cell electrolytes. This work offers a new general and efficient approach to functionalize 3D-extended frameworks through an anion-exchange process and achieves water-stability with ultra-high proton conductivity above 10(-2) S cm(-1) .

Highly Water‐Stable Lanthanide–Oxalate MOFs with Remarkable Proton Conductivity and Tunable Luminescence

“…Although MOF‐based proton conductors have been widely investigated over the last decade, only a small number of these materials contained lanthanide elements and only a few showed high room‐temperature proton conductivity and the highest record obtained is 1.3 × 10 −3 S cm −1 (Table S4, Supporting Information) . To the best of our knowledge, MOF‐ 1 discussed in this work displays so far the best room‐temperature proton conductivity of 3.42 × 10 −3 S cm −1 and possesses an added benefit of remarkable water stability, which is due to the high coordination number of the lanthanide ions introduced in the framework.…”

Loading Acid–Base Pairs into Periodic Mesoporous Organosilica for High Anhydrous Proton Conductivity over a Wide Operating Temperature Window

“…The metal–organic frameworks (MOFs) − /covalent–organic frameworks (COFs) , based proton conductors as a new type of solid-state electrolyte candidate have drawn much attention. They possess competitive advantages, such as designable framework structures, − high surface areas, and controllable pore sizes, which can load conducting small molecules , and tune the concentration and movement of proton carriers in their inherent pores . There are two types of proton-conducting crystalline porous materials: (1) One is proton-conducting under humidity, which can impart or improve proton conductivity in excess of the 1 × 10 –3 S/cm benchmark under humidity conditions, but the humidification requirement also raises some critical issues, the high humidity reliance and narrow operating temperature window from 20 to 80 °C.…”

Phase Transformation Dynamics in Sulfate-Loaded Lanthanide Triphosphonates. Proton Conductivity and Application as Fillers in PEMFCs

“… 26 , 27 In addition, derivatizing CPs with strongly acidic groups has shown to be a highly efficient strategy for enhancing proton conductivity. 28 , 29 …”