Mn-modified phosphomolybdates by hydrothermal route for pseudocapacitor application

Abstract

In this study, hydrothermal method was used to synthesize Mn modified organic-inorganic phosphomolybdates, Product-1 and 2, as considered to be, [Mn x (Imi) y {PMo 12 O 40 }], [Mn x (Imi) y {P 2 Mn 2 Mo 12 O 50 }] with Product-3, which was considered to be mixed by-products, might have [Mn x (Imi) y {P 2 Mn 2 Mo 12 O 50 (Imi) 4 Na 4 (OH) 8 }(OH) z ] (majority) in its structure, (imi = imidazole). Product-1 was the main product, Product-2 and 3 were prepared from the by-product using only heating followed by concentrating technique. All products had high temperature resistance. According to XPS data, the percentages of Mo, Mn, C, O, P, N and Na were 3.91, 23.30, 13.54, 47.83, 7.9, 2.30, and 1.15%, 20.10, 2.39, 22.98, 41.53, 3.53, 6.83, and 2.6%, and 15.51, 1.06, 31.29, 43.56, 1.84, 5.58, and 0.58%, in Product-1, 2 and 3, respectively (by the no. of atoms), along with the average valency states of Mn were 2.26, 2.39, and 2.14, respectively. Product-1 was considered having polyoxo kegging clusters, Product-2 and 3 were considered having polyoxohexamolybdate units in their structures, as well as Product-2 was deliberately assumed to be the polymer or a derivative of Product-3. Cyclic voltammetry (CV) showed all three compounds having different reversible pseudo-capacitor character. At 2.0 Ag -1 current density, Product-1, 2 and 3@GCE showed specific discharge capacitance of 37.01, 380.45, and 635.37 Fg -1 , respectively. In galvanostatic charging-discharging (GCD), Product-1@GCE showed comparatively higher retention as compared to other electrodes.