Hydrated metal ions as weak Brønsted acids show promoting effects on proton conduction. Issue 21 (11th May 2022)

• Record Type: Journal Article
• Authors: Huang, Xia-Lu; Chen, Yi-Qing; Wen, Ge-Hua; Bao, Song-Song; Zheng, Li-Min
• Is Part Of: CrystEngComm. Volume 24:Issue 21(2022)
• Page Start: 3886
• Page End: 3893
• Publication Date: 2022-05-11
• Subjects: Crystals -- Periodicals; Crystal growth -- Periodicals; Crystallography -- Periodicals; Cristaux -- Périodiques; Cristaux -- Croissance -- Périodiques; Cristallographie -- Périodiques; 548
• DOI: 10.1039/d2ce00430e
• Languages: English
• ISSNs: 1466-8033

Abstract

Abstract : The proton conductivity of a single crystal of Co–Co phosphonate along the c axis is 1.5 times higher than that of isostructural Ni–Co phosphonate, originating from the slight p K a difference of water molecules coordinated to Co 2+ or Ni 2+ centers. Abstract : It is well-known that hydrated metal ions can act as Brønsted acids, which tend to donate protons increasing the acidic proton concentration in materials, as well as the proton conductivity. However, it is challenging to relate proton conductive properties to the contribution to the inherent Lewis acidity of selected metal ions blocked by many factors such as multiple acidic proton sources, inconsistent proton pathways, or extrinsic grain boundaries. Herein, we assemble a metalloligand [Co III (notpH3 )] with Co 2+ or Ni 2+ ions by solvothermal reactions to obtain three Co–Co phosphonates [Co II Co III (notpH)(H2 O)3 ]· x H2 O (1 ), [Co II Co III (notpH)(H2 O)]·2H2 O (2 ), and [Co II 3 Co III 2 (notp)2 (H2 O)12 ]·2H2 O (3 ), and one Ni–Co phosphonate [Ni II 3 Co III 2 (notp)2 (H2 O)12 ]·2H2 O (4 ). Layered compounds 3 and 4 are isostructural, and the pentahydrated metal ions MO(H2 O)5 (M = Co 2+ in 3 and Ni 2+ in 4 ) act as a single proton source in the structures. Their proton conductive properties have been investigated using compacted pellets and single crystals. Co 2+ is a slightly stronger Lewis acid than Ni 2+, while the conductivities of single crystals of 3 and 4 along the c axis have a 1.5 times difference and are measured to be 1.37 × 10 −5 and 8.07 × 10 −6 S cm −1 at 25 °C and 95% RH, respectively. This work provides an excellent example of the effect of different hydrated metal ions on proton conduction.