Rare earth-based functionalized materials have promising potential application in the fields of luminescence, magnetism, and catalysis. Especially, rare earth-based functional materials have been widely used in electrocatalysis field in the past few years. Transition metal-based electrocatalytic materials exhibit good electrocatalytic performance, whereas the catalytic activity and stability cannot meet the current energy demand. When the transition metal electrocatalytic materials doped or composited by cerium, the cerium could improve the electron transfer of transition metals, optimize the binding energy of intermediates and active sites, promote the adsorption and
conversion of intermediates, and significantly improve the catalytic activity. Therefore, it is of great significance to rationally design rare earth-based functional materials to
develop new efficient and cost-effective rare earth-based functional electrocatalysts.
The reported the rare earth-based functional materials with electrocatalytic properties are mainly inorganic materials. Therefore, how to expand rare earth-based functional electrocatalytic materials has become the focus of this field. In this work, based on the characteristics that rare earth cerium ions have high affinity for carboxylic acid ligands, high coordination numbers, and various coordination modes, while
transition metal ions tend to coordinate with nitrogen atoms, the "synergistic coordination of rare earth cerium and transition metal ions" was developed to construct and regulate structure and morphology, and obtain a rare earth cerium-based electrocatalytic material with a new type of coordination hybrid and a unique heterostructure, exhibiting obviously improved performance. The works are as follows:
1. Generally, coordination polymer materials are easily oxidized to metal oxides/hydroxides in an electrochemical oxidative environment, and the skeleton structure is prone to collapse. Pyridine carboxylic acid ligands show high selective
coordination to metal ions, especially, aromatic pyridine carboxylic acid ligands produce strong coordination ability with metals. Therefore, in the work, the 2,3-pyridine dicarboxylic acid ligands with multiple coordination sites of containing
oxygen and nitrogen atoms was used to coordinate with rare earth cerium and transition metal atoms through the "synergistic coordination" under solvothermal reaction, and
obtain the rare earth cerium-heterometallic coordination polymer with a spherical shape. Due to the difference in properties for different metals, rare earth cerium and transition
metal nickel or cobalt are coordinated with ligands to obtain hollow nanospheres, while transition metal iron is introduced, resulting in the production of solid nanosphere coordination polymers. Due to the presence of aromatic rings in the structure of the cerium-based heterometallic coordination polymer, and the aromatic structure is beneficial to the electron and energy transfer, and the rare earth ion and the oxygen
atom on the carboxyl group will produce a strong bonding effect, the combination of these factors together is conducive to the stability of the framework structure. Therefore, the prepared cerium-based heterometallic complex polymers exhibit good catalytic activity and stability.
2. Given the fact that two-dimensional materials usually exhibit advantages that are different from bulk materials, and rare earth-based inorganic-organic hybrid materials combine the advantages of inorganic rare earth and organic components. Therefore, how to synthesize ultrathin CeO2-based inorganic-organic hybrid electrocatalytic materials is still a problem to be solved. In the work, oleic acid-stabilized ultrathin CeO2 nanosheets were used as precursors to synthesize inorganic-organic hybrid materials with terephthalic acid coordinated on the surface of ultrathin CeO2 nanosheets by ligand exchange. Surface-coordinated terephthalic acid terminal carboxyl groups anchor transition metal ions to obtain ultrathin CeO2-based inorganic-organic hybrid heterostructures (CeO2@NiFe-MOFs), in which monolayer NiFe-MOFs are coordinated to both the top and bottom surface of CeO2 nanosheets via joining carboxylic acid groups. Compared with oleic acid-stabilized ultrathin CeO2, CeO2 nanosheets decorated with NiFe-MOFs present different electronic, optical properties, bandgap, as well as local strain due to the re-coordination and anchoring of transition metal ions on its surface, demonstrating the coordinative binding interactions in the ultrathin two-dimensional heterostructures constructed based on coordination. Since the monolayer NiFe-MOFs expose more active sites on the surface, which is conducive to the contact with the electrolyte, and the conjugation of terephthalic acid effectively stabilizes the structure and promotes the electron transfer between the NiOOH active material and CeO2. Therefore, the ultrathin CeO2@NiFe-MOFs inorganic-organic hybrid heterostructure exhibits enhanced OER catalytic activity and structural stability.
3. The various functional nanomaterials can be obtained by the thermal decomposition of coordination polymers. In the work, the chelating organic ligands containing oxygen and nitrogen was selected, and by adjusting the reaction conditions, rare earth ions (Ce3+) and transition metal nickel ions (Ni2+) were introduced into the same framework to construct rare earth-based heterometallic coordination polymers with specific morphology (NiCe-CPs). The concentration of alkali in the reaction system plays an important role for the formation of specific morphology, the
coordination polymer with flower-like structure was synthesized under the condition of very low alkali concentration, while at relatively high alkali concentration, a nano-spherical coordination polymer is obtained. The method has also been found to be universal for synthesizing other rare earth-based heterometallic coordination polymers with flower-like structure. Crystalline/amorphous (a-NiO/c-NiCeOx) functional nanomaterials were obtained by thermal decomposition of NiCe-CPs. High-resolution transmission electron microscope characterization proves that there are crystalline phase (c-NiCeOx) and amorphous phase NiO in the a-NiO/c-NiCeOx structure; Raman spectroscopy also proves that there are more amorphous NiO on the surface of the structure. a-NiO/c-NiCeOx exhibited an overpotential of 240 mV at current density of 10 mA cm-2 . The DFT method demonstrated that amorphous NiO in the a-NiO/c-NiCeOx heterostructure can significantly improve the charge transfer efficiency and lower the reaction energy barrier, thereby enhancing the catalytic performance.