Doped carbon nanomaterials have polarized electron cloud distribution, excellent electron transport properties, high specific surface area and stable structure. Particularly, for precious metal-free N-doped carbon nanomaterials (M-N/C, M represents Fe, Co and Cu, etc.), it still retains some characteristics of pure carbon nanomaterials. Moreover, both the electronic/chemical structures of M and N could be rationally tuned to improve the performance of M-N/C by changing preparation methods and the structure of precursors/ligands. Such a class of M-N/C are even similar to some precious metals such as Pt/Pd and M-N complexes in catalysis. Accordingly, it would be a promising candidate for replacement of noble metals and could be applied in many fields ranging from energy conversion to heterogeneous organic catalysis. However, it is difficult to systematically study and establish the structure-activity relationships between the structure of precursors/ligands and the structure and activity of M-N/C via a conversional pyrolysis and low temperature stratagy, hinding the development and broad application of M-N/C. It is mainly ascribed to the fact: (a) the high temperature pyrolysis system is relatively complex; (b) M-N/C, prepared by the low temperature strategy, has poor activity, limiting the practical application in the catalytic process. The goal of this dissertation is to design and prepare M-N/C with high performance at the molecular level by high temperature and low temperature, respectively, under the premise of simplifying the preparation system and study their related catalytic activity in energy conversion and biomimetic catalysis. Moreover, the physical/chemical structure of M-N/C is modulated by tuning the structure of precursors/ligands. Accordingly, The relationship between the structure of precursors/ligands and the structure/activity of M-N/C is systematically studied and the related catalytic mechanism is further disclosed. The detailed works are summaried as follows:
(1) A class of high efficient Fe-N/C electrocatalysts derived from versatile imidazolium-based ionic liquids (ILs) were developed. Without any supports, templates, or multi-step of prolysis, the as-prepared Fe-N/C catalysts exhibited superior ORR activities than the state-of-the-art Pt/C in alkaline electrolytes. More interstingly, due to the versatile configuration of the imidazolium-based ILs precursors, a diverse range of catalyst structures was sucessfully modulated. Based on this, it was revealed unambiguously that electric conductivity, type/amount of N-dopants, and “effective porosity” (not conventional total surface area) jointly determined electrocatalytic activities. A pivotal radar chart was further proposed to successfully predict the ORR activity merely by structures of Fe-N/C catalysts.
(2) The as-prepared Fe-N/C showed an intrinsic cytochrome P450 oxidase-like activity in oxidation of various organic substrates using O2 at room temperature in aqueous solution. Moreover, the Fe-N/C surpassed natural oxidase by high activity and stability, and ease of separation and recyclability even in organic solvents, high temperature and extreme pH. The Fe-N center was proved to be the active site that reductively activated O2 by spontaneous generation of ROS species. Based on these findings, the Fe-N/C was further successfully exampled to kill live and proliferative lung cancer cells under normal physiological conditions. It would open up a promising platform of Fe-N/C in biomimetic catalysis ranging from simulation of oxidase metabolism to industrial oxidation processes, and to disease treatment.
(3) Inspired by structure-property relationships of laccases (a kind of macromolecular biological catalysts), we report a facile molecular assembly of Cu(3,3’-diaminobenzidine) polymeric complex on carbon black via Cu-N complexing and π-π interaction as a highly efficient bifunctional electrocatalyst for ORR and hydrazine oxidation reaction (HOR), two half reactions for hydrazine fuel cells. Similar to the function of the Cys-His group in natural laccases, the 3,3’-diaminobenzidine ligand in the proposed polymeric catalyst synergistically adjusted the electronic structure of Cu-N complex center and mediated a multiple-electron transfer cooperatively with carbon black via a long-range π-π interaction, owing to its electron-reservation and π-conjugated properties. This work may provide a new way to design highly efficient biomimetic noble-metal free electrocatalysts with well-defined and tunable structures.
(4) Three kinds of Cu-based bifunctional ORR and HOR electrochemical catalysts with different structures were designed and synthesized by tuning π-conjugation and coordination groups of ligands. The relationships between the structure of ligands and the structure/activity of catalysts were systematically studied. The results showed that reducing the π-conjugation of ligands and increasing the number of coordinated amino groups of ligands were not only beneficial to both expose the active sites and improve the hydrophilicity of catalysts, but also to optimize the electronic structure of Cu in catalysts for adsorption of catalatic intermediates. Both of these would lead to improve the activity of Cu-based catalysts in the ORR/HOR process.