Hydrated metal ions as weak Bronsted acids show promoting effects on proton conduction

It is well-known that hydrated metal ions can act as Bronsted 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(notpH(3))] with Co2+ or Ni2+ ions by solvothermal reactions to obtain three Co-Co phosphonates [(CoCoIII)-Co-II(notpH)(H2O)3]center dot xH(2)O (1), [(CoCoIII)-Co-II(notpH)(H2O)]center dot 2H(2)O (2), and [Co(II)3Co(III)2(notp)(2)(H2O)12]center dot 2H(2)O (3), and one Ni-Co phosphonate [Ni(II)3Co(III)2(notp)(2)(H2O)(12)]center dot 2H(2)O (4). Layered compounds 3 and 4 are isostructural, and the pentahydrated metal ions MO(H2O)5 (M = Co2+ in 3 and Ni2+ in 4) act as a single proton source in the structures. Their proton conductive properties have been investigated using compacted pellets and single crystals. Co2+ is a slightly stronger Lewis acid than Ni2+, 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 x 10(-5) and 8.07 x 10(-6) S cm(-1) at 25 degrees C and 95% RH, respectively. This work provides an excellent example of the effect of different hydrated metal ions on proton conduction.

Automated summary

It is well known that hydrated metal ions can act as Bronsted acids, increasing proton concentration and conductivity. However, relating proton conductive properties to the inherent Lewis acidity of metal ions is challenging. This study provides insights into the impact of different hydrated metal ions on proton conduction.