Governing principles of hydration of mixed proton conducting Co-based double perovskites

Proton ceramic electrochemical cells PCECs hold promise for efficient, sustainable production and use of hydrogen. The positive electrodes are mixed proton conducting perovskites that facilitate water splitting and oxygen reduction, but the factors that determine the protonation are poorly understood. Here, we establish the governing principles of protonation through a study of hydration of 45 double perovskites with the general formula <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msup><mml:mrow><mml:mi>A</mml:mi></mml:mrow><mml:mrow><mml:mi>I</mml:mi></mml:mrow></mml:msup><mml:msup><mml:mrow><mml:mi>A</mml:mi></mml:mrow><mml:mrow><mml:mi>II</mml:mi></mml:mrow></mml:msup><mml:msub><mml:mrow><mml:mi>Co</mml:mi></mml:mrow><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub><mml:msub><mml:mrow><mml:mi>O</mml:mi></mml:mrow><mml:mrow><mml:mn>6</mml:mn><mml:mo>-</mml:mo><mml:mi>δ</mml:mi></mml:mrow></mml:msub></mml:math>, having Ba or Ba+Sr on A^I and a mix of rare earths (Y and lanthanides Ln = La, Pr, Nd, Sm, Gd, Dy, Tb, Lu) on A^II. We show how hydration is coupled to the A-site basicity and disorder as well as population of electron holes in the Co-O bond (Co oxidation state), promoted by a closed or semi-closed Ln 4 f shell, i.e., Ln = La, Gd, Lu.