Ceria Boosting on In Situ Nitrogen-Doped Graphene Oxide for Efficient Bifunctional ORR/OER Activity

Abstract

In the present work, a highly efficient and excellent electrocatalyst material for bifunctional oxygen reduction/evolution reaction (ORR/OER) was synthesized using the microwave-assisted hydrothermal method. In brief, ultrafine hexagonal cerium oxide (CeO₂) nanoparticles were tailored on the layered surface of in situ nitrogen-doped graphene oxide (NGO) sheets. The nanocomposites exhibited a high anodic onset potential of 0.925 V vs. RHE for ORR activity and 1.2 V for OER activity with a very high current density in 0.5 M KOH. The influence of oxygen cluster on Ce³⁺/Ce⁴⁺ ion decoration on outward/inward in situ nitrogen-coupled GO enhanced the physicochemical properties of composites and in turn increased electron transferability. The microwave-assisted hydrothermal coupling technique provides a higher density, active sites on CeO₂@NGO composites, and oxygen deficiency structures in ultrafine Ce-O particles and boosts higher charge transferability in the composites. It is believed that the physical states of Ce-N- C, Ce-C=O, and a higher amount of oxygen participation with ceria increase the density of composites that in turn increases the efficiency. N-doped graphene oxide promotes high current conduction and rapid electron transferability while reducing the external transport resistance in oxygen electrocatalysis by sufficient mass transfer through in-built channels. This study may provide insights into the knowledge of Ce-enabled bifunctional activity to guide the design of a robust catalyst for electrochemical performance.

Keywords: OER; ORR (oxygen reduction reaction); electrocatalyst; heterostructures; microwave.

SCHEME 1

Schematic illustration of hexagonal/isometric ceria NP growth/embedded on the surface of nitrogen-doped graphene oxide sheets.

FIGURE 1

(A) XRD patterns of NGO, 0.25-CeO₂@NGO, 0.5-CeO₂@NGO, and 1-CeO₂@NGO. (B) FTIR spectra of NGO, 0.25-CeO₂@NGO, and 1-CeO₂@NGO. (C) Raman spectra for NGO, 1-CeO₂@NGO, and inset image of pure CeO₂.

FIGURE 2

(A–C) FESEM images, (D–F) TEM, and an inset image of SAED patterns for NGO, CeO₂, and CeO₂-NGO.

FIGURE 3

(A) XPS survey scan of 0.25-CeO₂@NGO and 1-CeO₂@NGO. (B–E) High-resolution detailed scan of Ce 3d, O1S, C1S, and N 1S, respectively, for 0.25-CeO₂@NGO and 1-CeO₂@NGO

FIGURE 4

(A) CV in oxygen and nitrogen-saturated conditions, CV in 0.5 M KOH, scanning rate 100 mVS⁻¹ for NGO, 0.25@CeO₂-NGO, 1@CeO₂-NGO, and RuO₂. (B) LSV graph of 0.25@CeO2-NGO with different rpm values at 5 mVS⁻¹ (inset image Tafel slope for 0.25@CeO_2-NGO). (C) K-L plot for 0.25@CeO2-NGO. (D) Polarization curves of pure CeO₂, NGO,0.25@CeO₂-NGO, 0.50@CeO₂-NGO, 1@CeO₂-NGO, RuO₂, and Pt/C at 2000 rpm. E) Tafel slope for CeO2, NGO, 0.25@CeO₂-NGO, 0.50@CeO2-NGO, 1@CeO2-NGO, RuO₂, and Pt/C at 1,600 rpm. (F) Stability test for RuO₂, Pt/C, and 1-CeO₂@NGO at 2000 rpm, and (G) OER studies of pure CeO₂, NGO,0.25@CeO₂-NGO, 0.50@CeO₂-NGO, and 1@CeO₂-NGO. (H) Chronoamperometric study for 1@CeO₂-NGO at 0.9 V. (I) EIS spectra of pure CeO2, NGO, and 1-CeO₂@NGO.