Ex-situ charge-discharge XANES study and device performance of a highly stable sodium-ion capacitor from cerium oxide-cerium titanate-reduced graphene oxide composite

Abstract

A CeO2-x-Ce2Ti3O8.7-reduced graphene oxide composite was synthesized by a two-step method consisting of an alkaline treatment of TiO2 nanoparticles, followed by hydrothermal reaction of Ce(NO3)(3) and graphene oxide. Transmission electron microscopy and X-ray diffraction revealed CeO2 nanoparticles directly decorating the GO sheet edges, while cerium titanate nanoparticles mostly intercalate between rGO sheets. The electrochemical characterization confirmed Faradaic and non-Faradaic responses. Ex-situ X-ray absorption near edge spectroscopy performed in pristine, single charged, single discharged, and 10 cycle charge-discharged CTG electrodes, demonstrated that reversible sodiation proceeds by Na intercalation within the Ce voids at the Ce titanate, Na reaction with CeO2-x, and Na adsorption at the rGO oxygen moieties. Cycling stability was attributed to the reversibility of the Ce4+/Ce3+ redox reaction, Na mobility aided by oxygen vacancies, and rGO’s role in buffering the volume change, while allowing rapid charge transfer through s and pi states. Coin cell device exhibited a maximum energy of 50 Wh kg(-1) and power density of 645 Wkg(-1) at 4.8 Ag-1, with 97% charge retention after 5000 cycles at 2 Ag-1, making the CTG system promising for large, reversible sodium ion storage.