Standard enthalpy of formation of NaxCoO2 system

Layered sodium cobalt oxide (NaxCoO2) materials have attracted lot of attention in recent times because of their large thermoelectric power. Information on thermodynamic properties is necessary to predict the stability of these compounds. The present study deals with determination of enthalpy of formation of sodium-cobalt oxides [NaxCoO2 (x = 0.1, 0.2...0.9)] using a solution calorimeter. The standard molar heat of dissolutions of NaxCoO2 (x = 0.1, 0.2 ... 0.9), NaCl(s), CoCl3(s) and H2O (l) in 0.150 dm(3) (1:1 ratio) aqueous HCl solution of concentration 5.96 mol dm(-3) were measured by an isoperibol solution calorimeter. Using the values of the measured enthalpy of dissolution and other auxiliary data, the standard molar enthalpies of formation (Delta H-f degrees(m) 298) of the compounds were determined. The values of Delta H-f degrees(m) 298 for the Na0.1CoO2(s), Na0.2CoO2(s), Na0.3CoO2(s), Na0.4CoO2(s), Na0.5CoO2(s), Na0.6CoO2(s), Na0.7CoO2(s), Na0.8CoO2(s) and Na0.9CoO2(s) compounds are found to be -(598.61 +/- 6.03), -(637.40 +/- 6.04), -(676.26 +/- 6.04), -(716.53 +/- 6.05), -(758.03 +/- 6.06), -(800.04 +/- 6.07), -(841.58 +/- 6.08), -(883.38 +/- 6.09), -(924.93 +/- 6.10) kJ mol(-1), respectively. (C) 2012 Elsevier B.V. All rights reserved.