Reversible perovskite electrocatalysts for oxygen reduction/oxygen evolution.

Area of Science:

• Materials Science
• Electrochemistry
• Catalysis

Background:

• Reversible fuel cells and rechargeable metal-air batteries require bifunctional electrocatalysts for both oxygen reduction (ORR) and oxygen evolution (OER).
• Transition metal perovskites offer tunable properties, but optimizing both ORR and OER activities simultaneously has been challenging due to anti-correlated performance in pseudo-binary phases.

Purpose of the Study:

• To identify a general strategy for optimizing oxide electrocatalysts with reversible activity for both ORR and OER.
• To investigate the role of A-site sub-stoichiometry and cation substitution in La-based perovskites on their bifunctional catalytic performance.

Main Methods:

• Synthesis and characterization of lanthanum-deficient La_1-xMn_yNi_zO_3-δ perovskites.
• Investigation of La_1-xCa_xMnO_3-δ perovskites with direct A-site substitution.
• Electrochemical evaluation of ORR and OER activity, correlating performance with the Mn³⁺/Mn⁴⁺ redox couple.

Main Results:

• Lanthanum A-site sub-stoichiometry in La_1-xMn_yNi_zO_3-δ perovskites leads to reversible ORR/OER activity.
• The reversible activity is linked to the emergence of the Mn³⁺/Mn⁴⁺ redox couple, indicating mixed-valent manganese species.
• Direct A-site substitution with Ca²⁺ in La_1-xCa_xMnO_3-δ also induces the Mn³⁺/Mn⁴⁺ couple and reversible bifunctional activity.

Conclusions:

• A-site cation substitution or sub-stoichiometry in perovskite oxides is a viable strategy to achieve bifunctional electrocatalysts for reversible oxygen reactions.
• The presence and accessibility of the Mn³⁺/Mn⁴⁺ redox couple is key to enabling reversible oxygen reduction and evolution reactions.
• This work provides a generalizable approach for designing efficient oxide electrocatalysts for energy storage and conversion devices.