Engineering Complex, Layered Metal Oxides: High-Performance Nickelate Oxide Nanostructures for Oxygen Exchange and Reduction

Electrocatalysis in Solid Oxide Fuel Cells and Electrolyzers

Interest in energy-to-X and X-to-energy (where X represents green hydrogen, carbon-based fuels, or ammonia) technologies has expanded the field of electrochemical conversion and storage. Solid oxide electrochemical cells (SOCs) are among the most promising technologies for these processes. Their unmatched conversion efficiencies result from favorable thermodynamics and kinetics at elevated operating temperatures (400-900 °C). These solid-state electrochemical systems exhibit flexibility in reversible operation between fuel cell and electrolysis modes and can efficiently utilize a variety of fuels. However, electrocatalytic materials at SOC electrodes remain nonoptimal for facilitating reversible operation and fuel flexibility. In this Review, we explore the diverse range of electrocatalytic materials utilized in oxygen-ion-conducting SOCs (O-SOCs) and proton-conducting SOCs (H-SOCs). We examine their electrochemical activity as a function of composition and structure across different electrochemical reactions to highlight characteristics that lead to optimal catalytic performance. Catalyst deactivation mechanisms under different operating conditions are discussed to assess the bottlenecks in performance. We conclude by providing guidelines for evaluating the electrochemical performance of electrode catalysts in SOCs and for designing effective catalysts to achieve flexibility in fuel usage and mode of operation.

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

Compositionally versatile, nonstoichiometric, mixed ionic-electronic conducting metal oxides of the form A _n+1B _n O_3n+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+1B _n O_3n+1 oxides (n = 1 (A₂BO₄) or n = ∞ (ABO₃); 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 A₂BO₄ oxides exhibiting higher rates of A-site dissolution in comparison to n = ∞ ABO₃ 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+1B _n O_3n+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.

Formation of Two-Dimensional Homologous Faults and Oxygen Electrocatalytic Activities in a Perovskite Nickelate

Atomic-scale direct probing of active sites and subsequent elucidation of the structure-activity relationship are important issues involving oxide-based electrocatalysts to achieve better electrochemical conversion efficiency. By generating Ruddlesden-Popper (RP) two-dimensional homologous faults via simple control of the cation nonstoichiometry in LaNiO₃ thin films, we demonstrate that strong tetragonal distortion of [NiO₆] octahedra is induced by more than 20% elongation of Ni-O bonds in the faults. In addition to direct visualization of the elongation by scanning transmission electron microscopy, we identify that the distorted [NiO₆] octahedra in the faults show considerably higher electrocatalytic activities than other surface sites during the electrochemical oxygen evolution reaction. This unequivocal evidence of the octahedral distortion and its impact on electrocatalysis in LaNiO₃ suggests that the formation of RP-type faults can provide an efficient way to control the octahedral geometry and thereby remarkably enhance the oxygen catalytic performance of perovskite oxides.

Porous Iron-Tungsten Carbide Electrocatalyst with High Activity and Stability toward Oxygen Reduction Reaction: From the Self-Assisted Synthetic Mechanism to Its Active-Species Probing

We synthesized a novel nonprecious metal electrocatalyst by pyrolysis of a colloid mixture consisting of a tungsten source and phenolic resin, with the simultaneous addition of ferric salt. The rationally designed electrocatalyst has a unique structure, with nanosized WC and Fe₃W₃C uniformly dispersed in a three-dimensional porous carbon framework. WC, which was thought difficult to produce, is successfully prepared at a relatively low temperature of about 750 °C at an inert atmosphere. XRD studies demonstrate the self-assisted effect of Fe, which accelerates the formation of WC, getting around the pathway of direct carbonaceous reduction of tungsten by carbon. The porous iron-tungsten carbide (Fe-W-C) nanocomposite as electrocatalyst shows excellent ORR activity with the onset and half-wave potentials of 0.864 and 0.727 V (vs RHE), respectively, which are close to those of Pt/C (0.976 and 0.820 V vs RHE). Electrochemical measurements show that Fe-W-C follows almost the effective four-electron-transfer pathway and would not be disturbed by methanol. The presence of a protective graphite shell outside the active carbide cores substantially improves the durability of the electrocatalyst. Both the removal of Fe species and the absence of W species would severely degrade the activity, while halide ions Cl^- and sulfur-containing species SCN^- can significantly suppress the ORR activity by the blocking of Fe species. These facts indicate that the ORR active species of Fe-W-C should be relevant to both W and Fe species.