Electrochemical determination of the Gibbs energy of formation of Na2Fe2O4 and Na3Fe5O9 employing Na-β-Al2O3 solid electrolyte

The Gibbs energies of formation of Na2Fe2O4 and Na3Fe5O9 from solid Na2O and α-Fe2O3 were measured using solid-state galvanic cells that used Na-β-Al2O3 as the solid electrolyte and a two-phase mixture of α-Al2O3 + Na-β-Al2O3 as the reference electrode. Results indicate that knowledge of the correct equilibrium phase relationship is essential to conduct meaningful and reliable thermodynamic measurements that use solid-state electrochemical techniques.