High and Tunable Proton Conduction in Six 3D-Substituted Imidazole Dicarboxylate-Based Lanthanide–Organic Frameworks

Abstract

Six isostructural three-dimensional (3D) Ln(III)−organic frameworks, {[Ln 2 (HMIDC) III (2), Sm III (3), Nd III (4), Pr III (5), and Ce III (6)], have been fabricated by using a multifunctional ligand of 2methyl-1H-imidazole-4,5-dicarboxylic acid (H 3 MIDC). Ln-metal−organic frameworks (MOFs) 1−6 present 3D structures and possess abundant H-bonded networks between imidazole-N atoms and coordinated and free water molecules. All the six Ln-MOFs demonstrate humidity-and temperature-dependent proton conductivity (σ) having the optimal values of 2.01 × 10 −3 , 1.40 × 10 −3 , 0.93 × 10 −3 , 2.25 × 10 −4 , 1.11 × 10 −4 , and 0.96 × 10 −4 S•cm −1 for 1−6, respectively, at 100 °C/98% relative humidity, in the order of Ce III (6) < Pr III (5) < Nd III (4) < Sm III (3) < Eu III (2) < Gd III (1). In particular, the σ for 1 is 1 order of magnitude higher than that for 6, and it enhances systematically according to the decreasing order of the ionic radius, indicating that the lanthanidecontraction tactics can effectively regulate the proton conductivity while retaining the proton conduction routes. This will offer valuable guidance for the acquisition of new proton-conducting materials. In addition, the outstanding water stability and electrochemical stability of such Ln-MOFs will afford a solid material basis for future applications.

Dysprosium(III) Metal–Organic Framework Demonstrating Ratiometric Luminescent Detection of pH, Magnetism, and Proton Conduction

“…Nonetheless, Dy-MOF can still show good proton conductivity. A similar situation can be observed in some other carboxylate-based MOFs …”

“…Proton conduction is another important proton-related property of MOFs. Proton-conducting materials have attracted extensive attention as solid-state electrolytes for fuel cells. − In terms of proton conductors, the highly tunable crystal structure and variously modified pore surfaces of MOFs provide great opportunities to improve proton conduction and provide an ideal crystal model for insight into the mechanism of proton transfer. − For most proton conductors, the transport of protons is achieved by the hopping of protons in the hydrogen-bonding network . Zuo et al reported an In-based 2D conductive MOF with a high proton conductivity of 1.30 × 10 –2 S·cm –1 .…”

Synthesis and Proton Conductivity of Two Molybdate Polymers Based on [Mo8O26]4− Anions

“…The proton conductivity property was measured under different temperatures and variable relative humidity conditions. In general, the proton conductivity (σ) value increases with the temperature or humidity [28,29] …”

“…In general, the proton conductivity (σ) value increases with the temperature or humidity. [28,29] The AC impedance spectroscopy of the solid pelletized sample was used to investigate the proton conduction behaviors at various RH and temperatures. We use the pelletized sample to measure proton conductivity with the increase of RH at 30 °C (Figure 3a).…”

High Proton Conduction in Three Highly Water-Stable Hydrogen-Bonded Ferrocene-Based Phenyl Carboxylate Frameworks

“…Note that as the temperature or humidity increases, the diameter of the semicircle becomes smaller. This phenomenon belongs to the characteristic proton conduction behavior. − …”