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Electrocatalytic site activity enhancement via orbital overlap in A2MnRuO7(A = Dy3+, Ho3+, and Er3+) pyrochlore nanostructures

Electrocatalytic site activity enhancement via orbital overlap in A2MnRuO7(A = Dy3+, Ho3+, and Er3+) pyrochlore nanostructures
Electrocatalytic site activity enhancement via orbital overlap in A2MnRuO7(A = Dy3+, Ho3+, and Er3+) pyrochlore nanostructures

Oxygen electrocatalysis at transition metal oxides is one of the key challenges underpinning electrochemical energy conversion systems, involving a delicate interplay of the bulk electronic structure and surface coordination of the active sites. In this work, we investigate for the first time the structure-activity relationship of A2RuMnO7 (A = Dy3+, Ho3+, and Er3+) nanoparticles, demonstrating how orbital mixing of Ru, Mn, and O promotes high density of states at the appropriate energy range for oxygen electrocatalysis. The bulk structure and surface composition of these multicomponent pyrochlores are investigated by high-resolution transmission electron microscopy, X-ray diffraction, X-ray absorption spectroscopy, X-ray emission spectroscopy (XES), and X-ray photoemission spectroscopy (XPS). The materials exhibit high phase purity (cubic fcc with a space group Fd3¯ m) in which variations in M-O bonds length are less than 1% upon replacing the A-site lanthanide. XES and XPS show that the mean oxidation state at the Mn-site as well as the nanoparticle surface composition was slightly affected by the lanthanide. The pyrochlore nanoparticles are significantly more active than the binary RuO2 and MnO2 toward the 4-electron oxygen reduction reaction in alkaline solutions. Interestingly, normalization of kinetic parameters by the number density of electroactive sites concludes that Dy2RuMnO7 shows twice higher activity than benchmark materials such as LaMnO3. Analysis of the electrochemical profiles supported by density functional theory calculations reveals that the origin of the enhanced catalytic activity is linked to the mixing of Ru and Mn d-orbitals and O p-orbitals at the conduction band which strongly overlap with the formal redox energy of O2 in solution. The activity enhancement strongly manifests in the case of Dy2RuMnO7 where the Ru/Mn ratio is closer to 1 in comparison with the Ho3+ and Er3+ analogs. These electronic effects are discussed in the context of the Gerischer formalism for electron transfer at the semiconductor/electrolyte junctions.

DFT + U, DyRuMnO, ErRuMnO, HoRuMnO, orbital overlap, oxygen electrocatalysis, pyrochlore oxides, X-ray absorption/emission
2574-0962
176-185
Celorrio, V.
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Tiwari, D.
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Calvillo, L.
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Leach, A.
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Huang, H.
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Granozzi, G.
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Alonso, J. A.
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Aguadero, A.
d0665a8f-70ce-4c7e-8c45-98fac118f69f
Pinacca, R. M.
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Russell, A. E.
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Fermin, D. J.
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Celorrio, V.
5ebb7fb5-a74c-4872-9795-5830dc915d0b
Tiwari, D.
435fcc21-5942-41e3-b4a1-95db83f850b1
Calvillo, L.
05ff21ed-97ce-4ea4-9b73-f4c181be59b0
Leach, A.
eb2fee87-ab58-4a28-9319-529de90d1708
Huang, H.
132a8eda-b800-4fa7-9583-6b4306f30247
Granozzi, G.
e1561b71-a42d-4c72-9028-3fb07183fceb
Alonso, J. A.
5451cf7f-5c81-4efc-9750-d2faa4b265d3
Aguadero, A.
d0665a8f-70ce-4c7e-8c45-98fac118f69f
Pinacca, R. M.
ec77ecf5-160a-4331-b9a4-64003c434ac8
Russell, A. E.
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Fermin, D. J.
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Celorrio, V., Tiwari, D., Calvillo, L., Leach, A., Huang, H., Granozzi, G., Alonso, J. A., Aguadero, A., Pinacca, R. M., Russell, A. E. and Fermin, D. J. (2021) Electrocatalytic site activity enhancement via orbital overlap in A2MnRuO7(A = Dy3+, Ho3+, and Er3+) pyrochlore nanostructures. ACS Applied Energy Materials, 4 (1), 176-185. (doi:10.1021/acsaem.0c02060).

Record type: Article

Abstract

Oxygen electrocatalysis at transition metal oxides is one of the key challenges underpinning electrochemical energy conversion systems, involving a delicate interplay of the bulk electronic structure and surface coordination of the active sites. In this work, we investigate for the first time the structure-activity relationship of A2RuMnO7 (A = Dy3+, Ho3+, and Er3+) nanoparticles, demonstrating how orbital mixing of Ru, Mn, and O promotes high density of states at the appropriate energy range for oxygen electrocatalysis. The bulk structure and surface composition of these multicomponent pyrochlores are investigated by high-resolution transmission electron microscopy, X-ray diffraction, X-ray absorption spectroscopy, X-ray emission spectroscopy (XES), and X-ray photoemission spectroscopy (XPS). The materials exhibit high phase purity (cubic fcc with a space group Fd3¯ m) in which variations in M-O bonds length are less than 1% upon replacing the A-site lanthanide. XES and XPS show that the mean oxidation state at the Mn-site as well as the nanoparticle surface composition was slightly affected by the lanthanide. The pyrochlore nanoparticles are significantly more active than the binary RuO2 and MnO2 toward the 4-electron oxygen reduction reaction in alkaline solutions. Interestingly, normalization of kinetic parameters by the number density of electroactive sites concludes that Dy2RuMnO7 shows twice higher activity than benchmark materials such as LaMnO3. Analysis of the electrochemical profiles supported by density functional theory calculations reveals that the origin of the enhanced catalytic activity is linked to the mixing of Ru and Mn d-orbitals and O p-orbitals at the conduction band which strongly overlap with the formal redox energy of O2 in solution. The activity enhancement strongly manifests in the case of Dy2RuMnO7 where the Ru/Mn ratio is closer to 1 in comparison with the Ho3+ and Er3+ analogs. These electronic effects are discussed in the context of the Gerischer formalism for electron transfer at the semiconductor/electrolyte junctions.

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Celorrio_ae_2020_020603_R2_Manuscript_Editor_VC - Accepted Manuscript
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Accepted/In Press date: 21 December 2020
e-pub ahead of print date: 9 January 2021
Keywords: DFT + U, DyRuMnO, ErRuMnO, HoRuMnO, orbital overlap, oxygen electrocatalysis, pyrochlore oxides, X-ray absorption/emission

Identifiers

Local EPrints ID: 447153
URI: http://eprints.soton.ac.uk/id/eprint/447153
ISSN: 2574-0962
PURE UUID: 951ca27e-2f70-4195-8606-0c52fe1f1adb
ORCID for A. E. Russell: ORCID iD orcid.org/0000-0002-8382-6443

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Date deposited: 04 Mar 2021 17:38
Last modified: 10 Jun 2022 04:03

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Contributors

Author: V. Celorrio
Author: D. Tiwari
Author: L. Calvillo
Author: A. Leach
Author: H. Huang
Author: G. Granozzi
Author: J. A. Alonso
Author: A. Aguadero
Author: R. M. Pinacca
Author: A. E. Russell ORCID iD
Author: D. J. Fermin

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