Enhanced Proton Conductivity by Aliovalent Substitution of Cadmium for Indium in Dimethylaminium-Templated Metal Anilicates

• Published in: ACS applied materials & interfaces 2020-09, Vol.12 (37), p.41605-41612
• Main Authors: Gao, Hui, He, Yan-Bin, Hou, Juan-Juan, Zhai, Quan-Guo, Zhang, Xian-Ming
• Format: Article
• Language: English
• Subjects: Energy, Environmental, and Catalysis Applications
• ISSN: 1944-8244; 1944-8252
• DOI: 10.1021/acsami.0c13125

Metal–organic frameworks (MOFs) with excellent proton conducting ability are crucial to fuel cells, chemical sensors, and redox flow batteries, but achieving them remain a challenge because of the difficulty in simultaneous fulfillment of large number of proton carriers, high mobility of protons, and long-term durable proton conduction. To explore a simple, efficient, and general route toward highly proton-conducting MOFs, we propose herein an aliovalent substitution metal strategy for isostructural aminium-templated MOFs which benefit the acquisition of rich proton sources without modifying ligands or exchanging protic organic molecules. This idea is verified by 100-fold enhancement of conductivity in compounds (Me2NH2)2[Cd­(mdhbqdc)2] (Cd-BQ) and (Me2NH2) (Me2NH)­[In­(mdhbqdc)2] (In-BQ) (H2mdhbqdc = dimethyl 3,6-dihydroxy-2,5-benzoquinone-1,4-dicarboxylic acid) that feature three-dimensional diamond-like structures with two-dimensional intersected channels. Accompanied by the in situ formation of an anilicate ligand, a great number of −OH groups are grafted onto the inner wall of pores, which interact with neutral Me2NH and/or protonated Me2NH2 + cations via N–H···O hydrogen bonds. The high concentration of protons and dynamics of protic amines in the porous framework readily leads to a moderate conductivity of In-BQ (2.10 × 10–4 S cm–1, at 303 K under 95% RH) and an activation energy of 0.73 eV (95% RH). It should be noted that the aliovalent substitution of Cd­(II) for In­(III) results in the doubling of dimethylaminium proton carriers in Cd-BQ, indicating more frequent hopping and multiple proton-transfer pathways. This indication is supported by a very high protonic conductivity of 2.30 × 10–2 S cm–1 and a reduced activation energy of 0.48 eV under the same conditions. Molecular dynamics simulations visually elucidate the fact that compared with In-BQ, aliovalent-substituted Cd-BQ has shorter proton-migration distances, which in combination with more proton numbers results in more frequent hopping and sliding of protons, in agreement with the experimental results.