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

Abstract

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.

Electrochemical preparation of nano/micron structure transition metal-based catalysts for the oxygen evolution reaction

“…Electrocatalyst surface restructuring is induced by irreversible metal-site cation dissolution, resulting in in situ formation of a TM oxyhydroxide shell on top of the parent electrocatalyst core that serves as the active surface for OER. 210 For instance, Zhou et al 211 reported a simple self-sacrificing strategy to obtain high-performance OER electrocatalysts FeCo–oxyhydroxide. A trimetallic selenide heterostructure (FeCoMo–Se) consisting of FeSe 2 , CoSe 2 and MoSe 2 is first one-step synthesized on a carbon cloth substrate.…”

Atomistic Insights into Activation and Degradation of La0.6Sr0.4CoO3−δ Electrocatalysts under Oxygen Evolution Conditions

“…Here, leaching of cations ,− as well as structural modifications such as amorphization ,,,,− and even complete decomposition of oxide catalysts , were observed and linked to changes in the catalysts’ activity. At the same time, recent studies have shown that near-surface phase transitions may play a key role in the catalytic process. ,, …”

“…At the same time, recent studies have shown that near-surface phase transitions may play a key role in the catalytic process. 8,12,28 Hence, it is apparent that surface phase transformations can trigger catalytic activity and are also involved in the aging of catalysts. The link between these processes however remains unresolved to date.…”

Eranda Nikolla Selected to Receive the 2022 ACS Catalysis Lectureship Award

“…This effect would be difficult to achieve through direct synthesis of the active surface and provides a new way of using the modular nature of nonstochiometric MMOCs to generate catalytically active and stable active sites. 6 Professor Nikolla's contributions will be celebrated in an award symposium prepared in her honor at the ACS National Meeting this August in Chicago, Illinois. Please join us in congratulating her, hopefully also in person, at the award symposium this August!…”

“…Prof. Nikolla has made significant contributions in the area of surface structure and nanostructure control in MMOCs as important electrocatalysts. − She has developed approaches for correlating experimentally measurable oxide properties (i.e., oxide surface reducibility and electric conductivity) to their activity and stability for oxygen reduction and evolution reactions (critical chemical transformations for energy generation in fuel cells and carbon-neutral H 2 generation from water electrolysis, respectively). Her work revealed that the transition metal–lattice oxygen bond strength captures effects from both the oxide composition and crystal symmetry on the binding energetics of the oxygenated intermediates and consequently on the measured electrochemical activity and stability .…”