Role of Oxygen Vacancy Ordering and Channel Formation in Tuning Intercalation Pseudocapacitance in Mo Single-Ion-Implanted CeO2- x Nanoflakes

Xiaoran Zheng; Sajjad S Mofarah; Alan Cen; Claudio Cazorla; Enamul Haque; Ewing Y Chen; Armand J Atanacio; Madhura Manohar; Corey Vutukuri; Joel Luke Abraham; Pramod Koshy; Charles C Sorrell

doi:10.1021/acsami.1c14484

Role of Oxygen Vacancy Ordering and Channel Formation in Tuning Intercalation Pseudocapacitance in Mo Single-Ion-Implanted CeO_{2- x} Nanoflakes

ACS Appl Mater Interfaces. 2021 Dec 22;13(50):59820-59833. doi: 10.1021/acsami.1c14484. Epub 2021 Dec 7.

Affiliations

¹ School of Materials Science and Engineering, University of New South Wales, Sydney, New South Wales 2052, Australia.
² EH Solid State Physics Laboratory, Gaffargaon, Mymensingh 2233, Bangladesh.
³ Centre for Accelerator Science, Australian Nuclear Science and Technology Organisation, New Illawarra Road, Lucas Heights, New South Wales 2234, Australia.

PMID: 34875170
DOI: 10.1021/acsami.1c14484

Abstract

Metal oxide pseudocapacitors are limited by low electrical and ionic conductivities. The present work integrates defect engineering and architectural design to exhibit, for the first time, intercalation pseudocapacitance in CeO_2-x. An engineered chronoamperometric electrochemical deposition is used to synthesize 2D CeO_2-x nanoflakes as thin as ∼12 nm. Through simultaneous regulation of intrinsic and extrinsic defect concentrations, charge transfer and charge-discharge kinetics with redox and intercalation capacitances together are optimized, where reduction increases the gravimetric capacitance by 77% to 583 F g^-1, exceeding the theoretical capacitance (562 F g^-1). Mo ion implantation and reduction processes increase the specific capacitance by 133%, while the capacitance retention increases from 89 to 95%. The role of ion-implanted Mo⁶⁺ is critical through its interstitial solid solubility, which is not to alter the energy band diagram but to facilitate the generation of electrons and to establish the midgap states for color centers, which facilitate electron transfer across the band gap, thus enhancing n-type semiconductivity. Critically, density functional theory simulations reveal, for the first time, that the reduction causes the formation of ordered oxygen vacancies that provide an atomic channel for ion intercalation. These channels enable intercalation pseudocapacitance but also increase electrical and ionic conductivities. In addition, the associated increased active site density enhances the redox such that the 10% of the Ce³⁺ available for redox (surface only) increases to 35% by oxygen vacancy channels. These findings are critical for any oxide system used for energy storage systems, as they offer both architectural design and structural engineering of materials to maximize the capacitance performance by achieving accumulative surface redox and intercalation-based redox reactions during the charge/discharge process.

Keywords: ceria 2D nanoflakes; defect engineering and architectural design; ion implantation; oxygen vacancy ordering and channel formation; surface and intercalation pseudocapacitance.