1D/2D Heterostructure of Gadolinium/2-Methylimidazole Metal-Organic Framework and Graphitic Carbon Nitride Nanosheet for Bifunctional Electrocatalytic Oxygen Evolution and Reduction Reactions

The electrochemical energy generation, conversion, and storage devices are very promising for developing sustainable clean technologies. Fuel cells and metal-air batteries are emerging as powerful energy conversion devices for future energy requirements. Economically feasible electrocatalysts can be engineered to meet these requirements. In this work, a bifunctional electrocatalyst 1D-gadolinium-<i>2</i><i>-</i>methylimidazole (1D-Gd-2-mim) metal-organic framework (MOF) functionalized 2D-graphitic carbon nitride (2D-g-C₃N₄) heterostructure (1D-Gd-2-mim/2D-g-C₃N₄) is demonstrated for electrocatalytic oxygen evolution reaction (OER) and oxygen reduction reaction (ORR). The electrocatalyst is synthesized through mixing and coprecipitation methods. The synthesized 1D-Gd-2-mim/2D-g-C₃N₄is characterized using diffraction (X-ray diffraction (XRD)), spectroscopic (Fourier-transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS)), microscopic (field Emission scanning electron microscopy and corresponding energy dispersive X-ray spectroscopy (FESEM-EDS), high-resolution transmission electron microscopy (HR-TEM), atomic force microscopy (AFM)), thermal (combined thermogravimetry and differential scanning calorimetry (TG-DSC)), surface and pore measurements (Brunauer–Emmet–Teller and Barret–Joyner–Halenda (BET-BJH)) techniques and compared with pristine 2D-g-C₃N₄. XRD and FTIR studies assure the formation of the heterostructure. The synthesized sample shows a remarkably higher specific surface area (322.4 m² g^-1) than the pristine 2D-g-C₃N₄ (69.12 m² g^-1) and 1D-Gd-2-mim MOF (93.5 m² g^-1). The FESEM and HR-TEM micrographs also confirm the (1D/2D) heterostructure formation where the growth of porous nanorod structured 1D-Gd-2-mim MOFs on 2D-g-C₃N₄ nanosheets is evident. High-angle annular dark-field-scanning transmission electron microscopic (HAADF-STEM) analysis of 1D-Gd-2-mim/2D-g-C₃N₄ confirms that all the elements C, N, Gd, and O are uniformly distributed in the nanoporous microstructure. A considerable weight loss (32%) is recorded within a temperature range of (0-900 °C) for 1D-Gd-2-mim/2D-g-C₃N₄, whereas 2D-g-C₃N₄is fully dissociated after 610 °C, as confirmed by the TG-DSC study. Further, the ORR activity of the synthesized materials is confirmed through the CV plot in the O₂ and N₂ saturated 0.1 M KOH in the potential window of 0.2 to 1.2 V (vs. RHE). Although the ORR onset and ORR peak potential do not improve remarkably over the addition of La-2-mim and Ce-2-mim MOFs into 2D-g-C₃N₄, the limiting current density increases remarkably, confirming the improvement of ORR activity. The 1D-Gd-2-mim/2D-g-C₃N₄shows a remarkable positive shift of the ORR onset and ORR peak potential along with enhanced limiting current density, which makes it the best ORR performing catalyst among all the synthesized materials. 1D-Gd-2-mim/2D-g-C₃N₄exhibits higher stability and methanol tolerance capacity than the state-of-the-art catalyst Pt/C. Furthermore, an LSV study using RDE with various rotation speeds reveals that the catalyst follows a four-electron reduction pathway for ORR. Also, the electrocatalyst shows its superiority over the pristine 2D-g-C₃N₄ and state-of-the-art catalyst RuO₂ by depicting the lowest onset potential (1.25 V vs. RHE), overpotential (59 mV), Rct (7 Ω), Taffel’s slope (17 mV decay^-1), and higher R_f (1.74), as obtained through several OER electrochemical characterizations. The catalyst is also stable from a morphological and OER performance perspective. So, the electrocatalyst executes the bifunctional activity very proficiently, which is highly demanding for fuel cell and metal-air batteries technology. 1D-Gd-2-mim/2D-g-C₃N₄ is a potential candidate as an alternative to the high cost and scares Pt/C and RuO₂ in energy conversions and storage applications.