CO₂ Adsorption by Carbon Quantum Dots/Metal Ferrites (M = Co²⁺, Ni²⁺, and Zn²⁺): Electrochemical and Theoretical Studies

In this study, we investigated the adsorption of CO₂ by carbon quantum dot-based ferrites (MFe₂O₄, M = Co²⁺, Ni²⁺, and Zn²⁺) in the context of industrial CO₂ emissions and global warming. The ferrites have been characterized using various analytical techniques [X-ray powder diffraction, FTIR, thermogravimetric analysis (TGA), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS)], showing cubic spinel for CoFe₂O₄, reverse cubic spinel for NiFe₂O₄, and typical spinel for ZnFe₂O₄. A TGA study revealed a significant weight loss around 740-780 °C, indicating structural change occurred with increasing temperature. SEM and TEM images displayed spherical particles with sizes ranging from 10 to 50 nm. XPS confirmed the presence of C, O, and Fe atoms with specific cations (Co²⁺, Ni²⁺, and Zn²⁺). Electrochemical impedance Nyquist diagrams suggest a linear relationship between Z″ (ohm) and Z' (ohm) at low frequencies, but the semicircular loop obtained was significantly increased at higher frequencies. This suggests that the charge transfer resistance (<i>R</i> _CT) at the electrode boundaries (interface) is much higher than at low frequencies, indicating the resistance per area was 1853 Ω cm² for carbon paste electrodes (CPE)/CoFe₂O₄ and it decreased to 1652 Ω cm² for CPE/NiFe₂O₄ and 1672 Ω cm² for CPE/ZnFe₂O₄. However, improved electron transfer with lower resistance was seen due to the composite nature of the samples (CQDs@MFe₂O₄), revealing a lower resistance (1163 Ω cm²) for CQD@MFe₂O₄-CO₂ as compared to 1567 Ω cm² for MFe₂O₄. Thus, the adsorption of CO₂ was studied electrochemically, and interaction between ferrates with CO₂ was enhanced by the presence of CQDs in the samples. This is consistent with the adsorption of CO₂ with the samples as it follows the Langmuir pseudo-second-order kinetics (<i>k</i> = 4.9, <i>qe</i> = 121.93 for CQD@CoFe₂O₄, <i>k</i> = 2.9, <i>qe</i> = 156.52 for CQD@NiFe₂O₄, and <i>k</i> = 3.0, <i>qe</i> = 141.71 for CQD@ZnFe₂O₄), and the data show that the adsorption efficiency has been decreased by around 1.0% after 7-8 cycles. Lastly, density functional theory analysis demonstrated the interaction of CO₂ on the surface of the ferrites, deforming the CO₂ linearity, which leads to a subsequent C-O interaction to form carbonate.