NiCo₂O₄/graphene quantum dots (GQDs) for use in efficient electrochemical energy devices: An electrochemical and X-ray absorption spectroscopic investigation

NiCo₂O₄/graphene quantum dots (GQDs) were synthesized by the incorporation of GQDs into NiCo₂O₄ as electrode materials for use in electrochemical energy devices. The electronic structure and the electrochemical performance of NiCo₂O₄/GQDs were investigated by X-ray absorption spectroscopy (XAS) and electrochemical measurements. The rich redox active sites were revealed on the NiCo₂O₄/GQDs, which were formed by the strong C[sbnd]O[sbnd]Ni/Co interaction between NiCo₂O₄ and GQDs. The synergetic double-layer capacitive and pseudo-capacitive properties were suggested to provide favorable electrochemical characteristics. In/ex situ XAS elucidates local electronic and chemical environments and provides insights into the energy storage/conversion mechanisms. Analytical results demonstrate that the NiCo₂O₄/GQDs have strong oxygen-carbon bridges and have more Ni active sites than does NiCo₂O₄, favoring metal-to-ligand charge transfer at the interface between NiCo₂O₄ and GQDs, critically influencing electrochemical performance. This work contributes importantly to the field by elucidating an energy storage/conversion mechanism for electrochemical energy devices.