Enhanced hydrogen adsorption in alkali metal based copper hexacyanoferrate Prussian blue analogue nanocubes

Abstract

Two Prussian blue analogues (PBAs) CuII[FeIII(CN)6)]2/3·15H2O (1), and KICuII[FeII(CN)6)]3/4·9H2O (2) are synthesised using a co-precipitation method at room temperature. The compounds are studied using XRD (x–ray diffraction), FESEM (Field Emission Scanning Electron Microscopy), IR (infrared), Raman spectroscopy, and TGA (thermogravimetric analysis). The surface morphology of the compounds is investigated using the FESEM technique, which confirms the formation of cube-type microstructure with an average size of ∼200 and ∼120 nm for compounds (1), and (2), respectively. The x-ray diffraction (XRD) results, derived from the Rietveld refinement method, reveal that both the compound crystallized in a pure single nanocrystalline phase with face centred cubic crystal structure of space group Fm3m. The lattice constants are found to be ∼10.04, and ∼10.08 Å for the compound (1), and (2) respectively. The IR and Raman spectroscopy show characteristic peaks, observed in the range of 1900–2200 cm−1, corresponds to the CN stretching frequency of CuII–CN–FeIII/II, confirming the formation of PBAs compounds. The hydrogen adsorption isotherm measurements have been used to investigate the molecular hydrogen adsorption into PBAs compounds at low temperature (77 K) conditions. The compounds (1) and (2) show hydrogen storage capacity of ∼1.12 and 1.63 wt %, respectively. The compound (2) shows an enhanced hydrogen capacity of ∼45% as compared to compound (1), which is due to the presence of unsaturated Cu/Fe metal ions, and the larger surface area of the nanocubes in the framework structure. In addition, the presence of K+ ions in the compound (2) also strongly favours to catalysing more H2 for their enhanced storage capacity.