Intrinsic Effects of Ruddlesden-Popper-Based Bifunctional Catalysts for High-Temperature Oxygen Reduction and Evolution

Unveiling the intrinsic effects of Ruddlesden-Popper (RP) series A(n)(+1)B(n)O(3)(n)(+1) (A = La, B = Ni, Co, Mn, Cu, n = 1, 2 and 3) catalysts is essential in order to optimize the activity of oxygen reduction reaction (ORR) and evolution reaction (OER). Here, it is demonstrated that the oxygen vacancy is not the key point for RP to realize high ORR and OER activity at high temperature. Instead, interstitial O2- with high concentration and fast migration, and lattice oxygen with high activity are favorable for the high-temperature catalytic activity. Aliovalent cation doping is an effective strategy to modify the catalytic activity. For the RP catalysts, low-valence ion doping does not introduce oxygen vacancies, which suppresses the activity of lattice oxygen and decreases the interstitial O2- concentration; whereas high-valence ion doping enhances the interstitial O2- concentration and the lattice oxygen activity. The evaluations of six RP series (La2NiO4, La2CoO4, La3Co2O7, La4Ni3O10, La2MnO4, and La2CuO4 based) and twenty samples as oxygen electrodes for solid oxide fuel cells (SOFCs) and solid oxide electrolysis cells (SOECs) demonstrate that this finding is applicable to all the selected RP series.