Electrocatalysis of oxygen reduction and evolution (ORR and OER) have become of significant importance due to their critical role in the performance of electrochemical energy conversion and storage devices, such as fuel cells, electrolyzers, and metal air batteries. While efficient ORR and OER have been reported using noble-metal based catalysts, their commercialization is cost prohibitive. In this Perspective, we discuss the potential of nonprecious metal based, mixed electronic–ionic conducting oxides (i.e., perovskites, double perovskites, and Ruddlesden–Popper (R-P) oxides) for efficient oxygen electrocatalysis at high and low temperatures. The nonstoichiometry of oxygen in these materials provides key catalytic properties that facilitate efficient ORR/OER electrocatalysis. We discuss the importance of surface structure and composition as critical parameters to understand and tune the ORR/OER activity of these oxides. We argue that techniques facilitating controlled synthesis and characterization of the surface structures are key at achieving a correlation between structure and activity of these materials. We make the case for combinatorial approaches involving quantum chemical calculations combined with detailed characterization, controlled synthesis, and testing as effective ways for developing the fundamental knowledge at the molecular level required to guide the design of efficient nonstoichiometric, mixed metal oxides for oxygen electrocatalysis. We conclude by summarizing current advances and devising future directions in this area.

Recent advances in the use of nonstoichiometric mixed metal oxides belonging to the perovskite family as cost-effective catalysts for various oxygen-related heterogeneous thermochemical and electrochemical reactions have led to the need for the development of robust design criteria to tune their catalytic performance. The current paradigm for describing the electrocatalytic activity of these oxides relies on the averaged oxidation state of the transition metal in the structure, which often fails to systematically describe activity. In addition, the current design strategies mainly focus on activity and often overlook oxide stability. Therefore, the development of robust criteria that rely on measurable oxide properties and can shed light on the electrochemical activity and stability of these oxides still remains a challenge. Herein, we demonstrate an approach for correlating experimentally measurable oxide properties (i.e., oxide surface reducibility) with the oxide activity and stability. This is demonstrated through the use of electrochemical oxygen reduction reaction (ORR) as a probe reaction. We show that the oxide surface reducibility describes the transition metal−lattice oxygen bond strength and captures effects from both the oxide composition and crystal symmetry on the binding energetics of the oxygenated intermediates and consequently the ORR activity and stability. Comprehensive design strategies for efficient ORR on nonstoichiometric mixed metal oxides are devised. Such strategies have the potential to be extended to other oxygen-related catalytic reactions on these nonstoichiometric mixed metal oxides.

Oxygen electrocatalysis plays a critical role in the efficiency of important energy conversion and storage systems. While many efforts have focused on designing efficient electrocatalysts for these processes, optimal catalysts that are inexpensive, active, selective, and stable are still being searched. Nonstoichiometric, mixed-metal oxides present a promising group of electrocatalysts for these processes due to the versatility of the surface composition and fast oxygen conducting properties. Herein, we demonstrate, using a combination of theoretical and experimental studies, the ability to develop design principles that can be used to engineer oxygen electrocatalysis activity of layered, mixed ionic-electronic conducting Ruddlesden-Popper (R-P) oxides. We show that a density function theory (DFT) derived descriptor, O binding energy on a surface oxygen vacancy, can be effective in identifying efficient R-P oxide structures for oxygen reduction reaction (ORR). Using a controlled synthesis method, well-defined nanostructures of R-P oxides are obtained, which along with thermochemical and electrochemical activity studies are used to validate the design principles. This has led to the identification of a highly active ORR electrocatalyst, nanostructured Co-doped lanthanum nickelate oxide, which when incorporated in solid oxide fuel cell cathodes significantly enhances the performance at intermediate temperatures (∼550 °C), while maintaining long-term stability. The reported findings demonstrate the effectiveness of the developed design principles to engineer mixed ionic-electronic conducting oxides for efficient oxygen electrocatalysis, and the potential of nanostructured Co-doped lanthanum nickelate oxides as promising catalysts for oxygen electrocatalysis.

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Compositionally versatile, nonstoichiometric, mixed ionic–electronic conducting metal oxides of the form A n +1 B n O 3 n +1 ( n = 1 → ∞; A = rare-earth-/alkaline-earth-metal cation; B = transition-metal (TM) cation) remain a highly attractive class of electrocatalysts for catalyzing the energy-intensive oxygen evolution reaction (OER). The current design strategies for describing their OER activities are largely derived assuming a static, unchanged view of their surfaces, despite reports of dynamic structural changes to 3d TM-based perovskites during OER. Herein, through variations in the A- and B-site compositions of A n +1 B n O 3 n +1 oxides ( n = 1 (A 2 BO 4 ) or n = ∞ (ABO 3 ); A = La, Sr, Ca; B = Mn, Fe, Co, Ni), we show that, in the absence of electrolyte impurities, surface restructuring is universally the source of high OER activity in these oxides and is dependent on the initial oxide composition. Oxide surface restructuring is induced by irreversible A-site cation dissolution, resulting in in situ formation of a TM oxyhydroxide shell on top of the parent oxide core that serves as the active surface for OER. The rate of surface restructuring is found to depend on (i) composition of A-site cations, with alkaline-earth-metal cations dominating lanthanide cation dissolution, (ii) oxide crystal phase, with n = 1 A 2 BO 4 oxides exhibiting higher rates of A-site dissolution in comparison to n = ∞ ABO 3 perovskites, (iii) lattice strain in the oxide induced by mixed rare-earth- and alkaline-earth-metal cations in the A-site, and (iv) oxide reducibility. Among the in situ generated 3d TM oxyhydroxide structures from A n +1 B n O 3 n +1 oxides, Co-based structures are characterized by superior OER activity and stability, even in comparison to as-synthesized Co-oxyhydroxide, pointing to the generation of high active surface area structures through oxide restructuring. These insights are critical toward the development of revised design criteria to include surface dynamics for effectively describing the OER activity of nonstoichiometric mixed-metal oxides.