Engineering Rare Earth-Assisted Cobalt Oxide Gels Toward Superior Energy Storage in Asymmetric Supercapacitors

Abstract

The rational design of transition metal oxides with tailored electronic structures and defect chemistries is critical for advancing high-performance supercapacitors. Herein, we report the engineering of cobalt oxide (Co3O4) gels through controlled sol-gel synthesis and rare earth (RE) incorporation using neodymium (Nd), gadolinium (Gd), and dual neodymium/gadolinium (Nd/Gd) doping. X-ray diffraction (XRD) confirmed the preservation of the cubic spinel structure with systematic peak shifts and broadening, evidencing lattice strain, oxygen vacancy generation, and defect enrichment. Field-emission scanning electron microscopy (FE-SEM) analyses revealed distinct morphological evolution from compact nanoparticle assemblies in pristine Co3O4 to highly porous, interconnected frameworks in Nd/Gd-Co3O4 (Nd/Gd-Co). X-ray photoelectron spectroscopy (XPS) verified the stable incorporation of RE ions, accompanied by electronic interaction with the Co-O matrix and enhanced oxygen defect states. Electrochemical measurements demonstrated that the Nd/Gd-Co electrode achieved a remarkable areal capacitance of 25 F/cm2 at 8 mA/cm2, superior ionic diffusion coefficients, and the lowest equivalent series resistance (0.26 Omega) among all samples. Long-term cycling confirmed 84.35% capacitance retention with 94.46% coulombic efficiency after 12,000 cycles. Furthermore, the asymmetric pouch-type supercapacitor (APSD) constructed with Nd/Gd-Co as the positive electrode and activated carbon as the negative electrode delivered a wide operational window of 1.5 V, an areal capacitance of 140 mF/cm2, an energy density of 0.044 mWh/cm2, and 89.44% retention after 7000 cycles. These findings establish Nd/Gd-Co gels as robust and scalable electrode materials and demonstrate that RE co-doping is an effective strategy for bridging high energy density with long-term electrochemical stability in asymmetric supercapacitors.