A sandwich-type POM containing mixed cations: synthesis, thermal performance and proton-conducting properties

Abstract

A new sandwich-type polyoxometalate, Na5H[N(CH3)4]2[Co(C3N2H4)2(H2O)4][Co4(H2O)2(PW9O34)2]·21H2O (1), has been synthesized. 1 is composed of a Weakley-type polyanion, [Co4(H2O)2(PW9O34)2]10−, four kinds of cations (five Na+, two [N(CH3)4]+, one [Co(C3N2H4)2(H2O)4]2+, and one H+), and 21 crystalline H2O molecules. The surface oxygen of the polyanion in 1, the crystalline water, and coordinated water molecules make an extended 3-D hydrogen-bonding network. Alternating current (AC) impedance experiments of 1 reveal good proton conductivity for 1 of 5.03 × 10− 4 S cm−1 at 25 °C under 98% relative humidity (RH). Activation energy of 1 calculated from Arrhenius plots is 0.358 eV, indicating Grotthuss mechanism is dominant in the proton transfer. Thermal decomposition behavior of 1 was examined by thermogravimetry/mass spectrometry (TG/MS) measurements.