Guest Molecule-Responsive Functional Calcium Phosphonate Frameworks for Tuned Proton Conductivity

Abstract

We report the synthesis, structural characterization, and functionality (framework interconversions together with proton conductivity) of an open-framework hybrid that combines Ca(2+) ions and the rigid polyfunctional ligand 5-(dihydroxyphosphoryl)isophthalic acid (PiPhtA). Ca2[(HO3PC6H3COOH)2]2[(HO3PC6H3(COO)2H)(H2O)2]·5H2O (Ca-PiPhtA-I) is obtained by slow crystallization at ambient conditions from acidic (pH ≈ 3) aqueous solutions. It possesses a high water content (both Ca coordinated and in the lattice), and importantly, it exhibits water-filled 1D channels. At 75 °C, Ca-PiPhtA-I is partially dehydrated and exhibits a crystalline diffraction pattern that can be indexed in a monoclinic cell with parameters close to the pristine phase. Rietveld refinement was carried out for the sample heated at 75 °C, Ca-PiPhtA-II, using synchrotron powder X-ray diffraction data, which revealed the molecular formula Ca2[(HO3PC6H3COOH)2]2[(HO3PC6H3(COO)2H)(H2O)2]. All connectivity modes of the "parent" Ca-PiPhtA-I framework are retained in Ca-PiPhtA-II. Upon Ca-PiPhtA-I exposure to ammonia vapors (28% aqueous NH3) a new derivative is obtained (Ca-PiPhtA-NH3) containing 7 NH3 and 16 H2O molecules according to elemental and thermal analyses. Ca-PiPhtA-NH3 exhibits a complex X-ray diffraction pattern with peaks at 15.3 and 13.0 Å that suggest partial breaking and transformation of the parent pillared structure. Although detailed structural identification of Ca-PiPhtA-NH3 was not possible, due in part to nonequilibrium adsorption conditions and the lack of crystallinity, FT-IR spectra and DTA-TG analysis indicate profound structural changes compared to the pristine Ca-PiPhtA-I. At 98% RH and T = 24 °C, proton conductivity, σ, for Ca-PiPhtA-I is 5.7 × 10(-4) S·cm(-1). It increases to 1.3 × 10(-3) S·cm(-1) upon activation by preheating the sample at 40 °C for 2 h followed by water equilibration at room temperature under controlled conditions. Ca-PiPhtA-NH3 exhibits the highest proton conductivity, 6.6 × 10(-3) S·cm(-1), measured at 98% RH and T = 24 °C. Activation energies (Ea) for proton transfer in the above-mentioned frameworks range between 0.23 and 0.4 eV, typical of a Grothuss mechanism of proton conduction. These results underline the importance of internal H-bonding networks that, in turn, determine conductivity properties of hybrid materials. It is highlighted that new proton transfer pathways may be created by means of cavity "derivatization" with selected guest molecules resulting in improved proton conductivity.

A Water Stable Magnesium MOF That Conducts Protons over 10–2 S cm–1

“…Above this temperature, a drop in conductivity is observed as is typical for many water-mediated proton conductors that undergo dehydration at elevated temperatures. 7,8,17 This is supported by impedance measurements performed under varying humidity conditions ( Figure S4). At 70°C, the conductivity val- Figure 2.…”

“…19 Cabeza et al recently reported an alkaline earth metal phosphonocarboxylate MOF, CaPiPhtA, which possesses a pillared layered structure in which monoprotonated phosphonic acid groups protrude into inter-layer spaces. 17 20 a MIL-101 framework whose pores were loaded with H 2 SO 4 , which exhibits a conductivity of 6.0×10 -2 S.cm -1 at 80°C and 20%RH.…”

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

“…The smaller p K a value of H 2 PO 4 − (7.2) than H 2 BO 3 − (12.74) results in more than one order of magnitude higher conductivity for NDTBP than NDTB. Such high proton conductivities of CEFs, especially for NDTBP, are comparable to the most efficient MOF counterparts, such as PCMOF‐3 (3.5×10 −5 S cm −1 at 25 °C and 98 % RH),11d ZPGly (1×10 −3 S cm −1 at 140 °C and 95 % RH),18 Ca‐PiPhtA‐NH 3 (6.6×10 −3 S cm −1 at 24 °C and 98 % RH),19 PCMOF2 1/2 (2.1×10 −2 S cm −1 at 85 °C and 90 % RH) ,[11b] PCMOF10 (3.55×10 −2 S cm −1 at 70 °C and 95 % RH),13 CPM‐103a (5.8×10 −2 S cm −1 at 22.5 °C and 98 % RH),14 and UiO‐66(SO 3 H) 2 (8.4×10 −2 S cm −1 at 80 °C and 90 % RH) 12…”

Nowe materiały i procesy w konwersji oraz przechowywaniu energii – wybrane zagadnienia

“…Chociaż prace badawcze nad obecnymi na rynku wodorowymi ogniwami paliwowymi nie są zakończone, to podjęcie badań nad rozwojem idei ogniwa paliwowego zasilanego węglem brunatnym jest interesujące [49]. Realizowane są również badania materiałowe nad nowymi krystalicznymi ciałami stałymi typu MOF, posiadającymi zdolność przewodzenia protonów w podwyższonej temperaturze [3]. Projektowanie i konstrukcja magnesów nadprzewodzących będących źródłem bardzo silnych pól magnetycznych jest również aktualnym wyzwaniem [36,47].…”