Studies on the Genesis and Proton Conductivity of Imidazole-Based Linear and Open-Framework Zinc Phosphites

Abstract

Three new zinc phosphites, [HIm]2[Zn3(HPO3)4] (1), [Zn2(HPO3)2Im2] (2), and [Zn­(HPO3)­Im] (3) (Im = imidazole), have been synthesized from the hydro/solvothermal reaction of zinc acetate, dimethyl phosphite, and imidazole by varying the temperature and solvent of the reaction medium. The structure of 1 is built from vertex-sharing of four HPO3-capped Zn3P3 units and adopts an open framework with 12-ring channels stabilized by HIm cations via N–H···O hydrogen bonds. For 2, the inorganic skeleton is comprised of alternating ZnO4 and HPO3 tetrahedra, while the coordinatively associated ZnN2O2 fragments occupy the 12-ring hexagonal channels. Compound 3 adopts a ladder-type one-dimensional structure and exhibits N–H···O hydrogen-bonding interactions to afford a supramolecular assembly. A plausible rationale on the genesis of 1–3 has been put forth by reacting the preformed inorganic zinc phosphites Zn­{OP­(O)­(OMe)­H}2 or [Zn2(HPO3)2(H2O)4]·H2O with imidazole as the structure-directing ligand. Alternating-current impedance measurements reveal that 1 and 3 exhibit proton conductivities on the order of 10–3–10–4 S cm–1 between 25 and 100 °C under 35 and 77% relative humidity in repeated impedance cycles (E a = 0.22–0.35 eV). On the contrary, the conduction property is completely impaired in 2 under similar conditions.

Developing a Unique Hydrogen-Bond Network in a Uranyl Coordination Framework for Fuel Cell Applications

“…Proton conductibility at 170 °C under strictly anhydrous conditions was evaluated for HUP-3 , which increased with temperature, reaching a maximum of 0.89 × 10 –3 S·cm –1 at 170 °C (Figure a). The conductivity values of HUP-3 reach a maximum at 170 °C, which is higher than those of the imidazole-included materials, as shown in Table S2. − Furthermore, HUP-3 also exhibits a high proton conductivity of 3.62 × 10 –3 S·cm –1 under the conditions of 25 °C and 100% RH (Figure S14). This further supports the conclusion that strong hydrogen-bonded networks composed of Brønsted acid–base pairs in HUP-3 can generate an easy and efficient proton transfer pathway by means of electrostatic interaction in the structure.…”

The proton conduction behavior of two 1D open-framework metal phosphates with similar crystal structures and different hydrogen bond networks

“…12 Our group has also carried out several studies on the preparation of highly conductive metal phosphates, such as (C 2 H 10 N 2 )[Mn 2 (HPO 4 ) 3 ](H 2 O) reached 1.64 × 10 −3 S cm −1 at 298 K and 99% RH, 13 However, owing to the required balance of positive and negative charges in open-framework metal phosphate, its structural framework is xed, which leads to its difficulty in increasing the proton conductivity by further modifying its structure using effective strategies, such as MOFs. [16][17][18][19][20][21][22][23][24] For example (1) inclusion of functional guest molecules into the cavity of MOFs, such as acid molecules, imidazoles, and triazoles; [25][26][27][28][29][30][31] (2) use of organic linkers bearing hydrophilic groups (including -COOH, -OH, -SO 3 H, and -PO 3 H 2 ) [32][33][34][35][36][37][38] as structural building units. Therefore, the most ways to improve the proton conductivity of open-framework metal phosphate are by optimizing the conductive environment at present, for example, increasing the temperature and relative humidity and changing the water to an ammonia environment.…”

“…However, it is also very important for proton-conducting materials to exhibit high proton conductivity in the mediumlow temperature and humidity regions to maintain high conductivity over a wide temperature and humidity range, while research on high-proton conductive materials under relatively medium-low temperature and low humidity is still relatively lacking. 28,35,[39][40][41][42] Herein, to further understand the proton conduction mechanism of open-framework metal phosphates and to prepare materials that can exhibit high conductivity even at medium-low temperature and low humidity, we synthesized two metal phosphate compounds [C 3 N 2 H 12 ][Zn(HPO 4 ) 2 ] (1) and [C 6 N 4 H 22 ] 0.5 [Zn(HPO 4 ) 2 ] (2) with similar crystal structures, both showing 1D ladder-like chain structure of the inorganic skeleton and the organic cations interspersed in the inorganic chains. The results demonstrate that the effective hydrogen bond grid plays a key role in proton transport.…”

Insight into the High Proton Conductivity of One-/Two-Dimensional Cadmium Phosphites

“…We have recently reported the synthesis and proton conduction behavior of imidazole-based open framework zinc phosphites, [H 2 Im] 2 [Zn 3 (HPO 3 ) 4 ], [Zn 2 (HPO 3 ) 2 (HIm) 2 ], and [Zn­(HPO 3 )­(HIm)] (HIm = imidazole). The proton conductivity on the order of 10 –3 to 10 –4 S cm –1 (25–100 °C, 80% RH) is attributed to the presence of H 2 Im/HIm as the intrinsic proton source . As part of our ongoing studies, we report herein the synthesis of one-/two-dimensional cadmium phosphites, [Cd­{OP­(O)­(OH)­H} 2 (4,4′-bipy)] ( 1 ) and [H 2 pip]­[Cd­(HPO 3 ) 2 (H 2 O)]·H 2 O ( 2 ), (bipy = bipyridine; pip = piperazine).…”

“…The proton conductivity on the order of 10 −3 to 10 −4 S cm −1 (25−100 °C, 80% RH) is attributed to the presence of H 2 Im/HIm as the intrinsic proton source. 8 As part of our ongoing studies, we report herein the synthesis of one-/two-dimensional cadmium phosphites, [Cd{OP(O)(OH)H} 2 (4,4′-bipy)] (1) and [H 2 pip][Cd(HPO 3 ) 2 (H 2 O)]•H 2 O (2), (bipy = bipyridine; pip = piperazine). X-ray structure analysis reveals strong hydrogen bonding interactions between the appended H 2 PO 3 groups (O---O = 2.55 Å) in 1 and coordinated water, lattice water, and phosphite group in 2.…”