Design Strategies for Efficient Nonstoichiometric Mixed Metal Oxide Electrocatalysts: Correlating Measurable Oxide Properties to Electrocatalytic Performance

Abstract

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.

Dynamic Surface Reconstruction Unifies the Electrocatalytic Oxygen Evolution Performance of Nonstoichiometric Mixed Metal Oxides

“…The amount of Sr dissolution during pretreatment also correlated to the reducibility of the oxide, which is an indicator of the strength of the metal–oxygen bonds in the oxide. 37 We have previously reported that this class of nonstoichiometric, mixed ionic–electronic conducting metal oxides became more reducible (characterized by weaker metal–oxygen bonds) as the B-site varied from Mn to Ni across the 3d TM cations. 37 This suggested that an increase in oxide reducibility as the B-site varied from Mn to Ni in LSBO-75 oxides correlated with an increase in Sr dissolution, potentially due to the facile formation of oxygen defects in the structure, leading to destabilization of the Sr cations in the oxide.…”

“… 37 We have previously reported that this class of nonstoichiometric, mixed ionic–electronic conducting metal oxides became more reducible (characterized by weaker metal–oxygen bonds) as the B-site varied from Mn to Ni across the 3d TM cations. 37 This suggested that an increase in oxide reducibility as the B-site varied from Mn to Ni in LSBO-75 oxides correlated with an increase in Sr dissolution, potentially due to the facile formation of oxygen defects in the structure, leading to destabilization of the Sr cations in the oxide. B-site cation dissolution was also observed, but to a lesser extent, with Mn and Fe dissolution being significantly higher than that of Co and Ni cations from these LSBO-75 oxides during pretreatment ( Figure 2 c).…”

“…Conversely, a much higher slope of 193 ± 3 mV dec –1 was obtained for LSMnO-75 ( Figure 2 b), potentially due to the change in mechanism or strong adsorbate interactions with this oxide surface. 37 , 48 …”

“… 36 For instance, variations in the A-site composition can be used to tune the electronic structure and catalytic properties of TM cations in these oxides. 33 , 37 Although immense opportunities exist to tune the catalytic properties of these oxides for targeted reactions, their performance is still limited by lack of effective design criteria. Generally, the bulk electronic structure of the B-site cations in these oxides has been correlated to their electrocatalytic activity for the OER under the assumption that the overall initial state of the oxide surface remains unchanged under electrochemical conditions.…”

Multidimensional Nonstoichiometric Electrode Materials for Electrochemical Energy Conversion and Storage

“…[174,281,282] Based on the studies on a series of mixed metal oxides, Nikolla's group proposed a descriptor of the measurable oxide surface reducibility to correlate the structure and performance of ORR electrocatalysts. [72] The researchers described surface reducibility of the perovskite oxides with the reduction temperature, which can be measured by H 2 temperature-programmed reduction method. In addition, the reduction temperature reflects the surface oxygen vacancy formation energy and the TM-O bond strength relevant to the binding energetics Reproduced with permission.…”