An official website of the United States government
Abstract Atomic‐scale engineering of chromite spinels featuring redox‐active tetrahedral A‐sites and strong Cr–O covalency offers a promising route to superior platinum‐group‐metal‐free oxygen evolution reaction (OER) catalysts. However, comprehensive studies addressing how cation substitution influences surface chemistry and governs OER activity and durability in chromite spinels remain limited. Here, a systematic investigation of the multicationic chromite series NixFeyCr3−x−yO4is presented, identifying composition‐dependent Lewis acidity as a descriptor of superior OER performance. It is further demonstrated that tuning surface acidity directly controls dynamic reconstruction processes and lattice‐oxygen participation during spinel‐based electrocatalysis. Following activation, the optimized Ni0.8Fe0.3Cr1.9O4catalyst delivers a current density of 10 mA cm−2at an overpotential of 235 mV, surpassing RuO2, with excellent long‐term stability. Integrating microscopic and spectroscopic analysis with operando impedance spectroscopy, it shows that activation generates an oxyhydroxide overlayer and reveals a previously unrecognized link between surface Lewis acidity and the growth kinetics and activity of these shells. Density functional theory calculations indicate that Fe incorporation at octahedral sites raises the O 2p‐band center and lowers oxygen‐vacancy formation energy, promoting lattice‐oxygen activation and triggering reconstruction, yielding enhanced OER. This work integrates cation‐driven surface‐acidity modulation, acidity‐governed reconstruction, and OER activity enhancement into a unified predictive framework for designing earth‐abundant spinel‐based catalysts.
more »
« less
A Design Strategy for Highly Active Oxide Electrocatalysts by Incorporation of Oxygen‐Vacancies
Abstract Using both density functional theory (DFT+U) simulations and experiments, we show that the incorporation of an ordered array of oxygen‐vacancies in a perovskite oxide can lead to enhancement of the electrocatalytic activity for the oxygen‐evolution reaction (OER). As a benchmark, LaCoO3was investigated, where the incorporation of oxygen‐vacancies led to La3Co3O8(LaCoO2.67), featuring a structural transformation. DFT+U simulations demonstrated the effect of oxygen‐vacancies on lowering the potential required to achieve negative Gibbs Free Energy for all steps of the OER mechanism. This was also confirmed by experiments, where the vacancy‐ordered catalyst La3Co3O8(LaCoO2.67) showed a remarkable enhancement of electrocatalytic properties over the parent compound LaCoO3that lacked vacancies. We also synthesized and studied an intermediate system, with a smaller degree of oxygen‐vacancies, which showed intermediate electrocatalytic activity, lower than La3Co3O8and higher than LaCoO3, confirming the expected trend and the impact of oxygen‐vacancies. Furthermore, we employed additional DFT+U calculations to simulate a hypothetical material with the same formula as La3Co3O8but without the vacancy‐order. We found that the gap between centers of Codand Opbands, which is considered an OER descriptor, would be significantly greater for a hypothetical disordered material compared to an ordered system.
more »
« less
- Award ID(s):
- 1943085
- PAR ID:
- 10539536
- Publisher / Repository:
- Wiley Blackwell (John Wiley & Sons)
- Date Published:
- Journal Name:
- Small
- ISSN:
- 1613-6810
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
null (Ed.)Controlled energy transfer has been found to be one of the most effective ways of designing tunable and white photoluminescent phosphors. Utilizing host emission to achieve the same would lead to a new dimension in the design strategy for novel luminescent materials in solid state lighting and display devices. In this work, we have achieved controlled energy transfer by suppressing the host to dopant energy transfer in La 2 Hf 2 O 7 :Eu 3+ nanoparticles (NPs) by co-doping with uranium ions. Uranium acts as a barrier between the oxygen vacancies of the La 2 Hf 2 O 7 host and Eu 3+ doping ions to increase their separation and reduce the non-radiative energy transfer between them. Density functional theory (DFT) calculations of defect formation energy showed that the Eu 3+ dopant occupies the La 3+ site and the uranium ion occupies the Hf 4+ site. Co-doping the La 2 Hf 2 O 7 :Eu 3+ NPs with uranium ions creates negatively charged lanthanum and hafnium vacancies making the system highly electron rich. Formation of cation vacancies is expected to compensate the excess charge in the U and Eu co-doped La 2 Hf 2 O 7 NPs suppressing the formation of oxygen vacancies. This work shows how one can utilize the full color gamut in the La 2 Hf 2 O 7 :Eu 3+ ,U 6+ NPs with blue, green and red emissions from the host, uranium and europium, respectively, to produce near perfect white light emission.more » « less
-
Abstract Multifunctional materials that are capable of facilitating multiple electrocatalytic processes are highly desirable. This work reports the observation of bifunctional electrocatalytic properties for water‐splitting in layered oxides, featuring 2‐dimensional layers of octahedrally coordinated transition metals separated by alkaline‐earth or rare‐earth metals. Remarkably, these materials are able to catalyze both half‐reactions of water‐splitting,i. e., oxygen‐evolution reaction (OER) and hydrogen‐evolution reaction (HER). Electrical charge‐transport studies of SrLaFe1‐xCoxO4‐δin a wide range of temperatures, 25 to 800 °C, indicate semiconducting behavior for all three compounds, where there is a systematic increase in electrical conductivity as a function of temperature. The end member of the series, SrLaCoO4‐δ, exhibits the highest electrical charge transport and best electrocatalytic activity toward both OER and HER. This catalyst also features the highest degree of polyhedral distortion as well as the presence of oxygen‐vacancies. In addition, the transition metals in this material have a favorable electronic configuration for enhanced electrocatalytic activity.more » « less
-
Emergent and robust ferromagnetic-insulating state in highly strained ferroelastic LaCoO3 thin filmsAbstract Transition metal oxides are promising candidates for the next generation of spintronic devices due to their fascinating properties that can be effectively engineered by strain, defects, and microstructure. An excellent example can be found in ferroelastic LaCoO 3 with paramagnetism in bulk. In contrast, unexpected ferromagnetism is observed in tensile-strained LaCoO 3 films, however, its origin remains controversial. Here we simultaneously reveal the formation of ordered oxygen vacancies and previously unreported long-range suppression of CoO 6 octahedral rotations throughout LaCoO 3 films. Supported by density functional theory calculations, we find that the strong modification of Co 3 d -O 2 p hybridization associated with the increase of both Co-O-Co bond angle and Co-O bond length weakens the crystal-field splitting and facilitates an ordered high-spin state of Co ions, inducing an emergent ferromagnetic-insulating state. Our work provides unique insights into underlying mechanisms driving the ferromagnetic-insulating state in tensile-strained ferroelastic LaCoO 3 films while suggesting potential applications toward low-power spintronic devices.more » « less
-
Abstract Developing low‐cost, high‐performance electro‐catalysts is essential for large‐scale application of electrochemical energy devices. In this article, reported are the findings in understanding and controlling oxygen defects in PrBa0.5Sr0.5Co1.5Fe0.5O5+δ(PBSCF) for significantly enhancing the rate of oxygen evolution reaction (OER) are reported. Utilizing surface‐sensitive characterization techniques and first‐principle calculations, it is found that excessive oxygen vacancies promote OH−affiliation and lower the theoretical energy for the formation of O* on the surface, thus greatly facilitating the OER kinetics. On the other hand, however, oxygen vacancies also increase the energy band gap and lower the O 2pband center of PBSCF, which may hinder OER kinetics. Still, careful tuning of these competing effects has resulted in enhanced OER activity for PBSCF with oxygen defects. This work also demonstrates that oxygen defects generated by different techniques have very different characteristics, resulting in different impacts on the activity of electrodes. In particular, PBSCF nanotubes after electrochemical reduction exhibit outstanding OER activity compared with the recently reported perovskite‐based catalysts.more » « less
