Synthesis And Application Research On Multi-scale Electrocatalytic Materials Derived From Phthalocyanine Compounds

As a fundamental reaction in natural organisms, oxygen reduction reaction (ORR) also plays an important role in the application of energy conversion and storage (e.g., fuel cell technologies and metal-air batteries).However, ORR, which is a cathodic reaction of the four basic half-reactions, is regarded as a multi-step electrochemical reaction with sluggish kinetics, high overpotential, complicated process and intermediates. Meanwhile, as the state-of-the-art electrocatalysts for ORR, the platinum group metals (PGMs)materials are still non-substitutable due to their ultra-high performance over a wide pH range, but the high cost and scarcity of PGMs have greatly hindered the large-scale application of these energy conversion devices. Therefore, the development of high-performance, low-cost and designable non-PGMs materials for ORR has become one of the crucial issues in improvement of the related energy conversion and electrochemical application, as well as in the promotion of environmental protection.In this paper, we utilize the phthalocyanine compounds as the electrocatalytic molecules or precursors to prepare the multi-scale (i.e.,molecule-scale, atom-scale and nano-scale) non-PGMs electrocatalysts by supporting on carbon carriers and undergoing pyrolysis treatment. The prepared methods contain the modification of phthalocyanine molecules and carbon supports, the adjustment of the molecular assembly processes, and the optimization of pyrolysis treatment. In addition, the main catalyst development strategies are based on improving the intrinsic activity of single site, enhancing the synergistic effects, increasing the number of active sites and enriching the porous structure. Finally, the electrocatalytic performance and mechanism of the as-prepared electrocatalysts are investigated by electrochemical experiment and theoretical calculation to provide informative conclusions to further improve the catalytic performances of based multi-scale electrocatalysts derived from phthalocyanine compounds, and promote the development of TM-N-C electrocatalysts for various renewable-energy devices.(1) A series of transition-metal phthalocyanine (TMPc, TM: Fe, Co, Ni and Cu) is supported on graphitized carbon black (TMPc/GCB) as ORR electrocatalysts, via the ?-? interaction self-assembly. The catalytic activity,both in terms of the onset potential and half-wave potential follows the trend of FePc/GCB > CoPc/GCB > CuPc/GCB > NiPc/GCB. However, the catalytic stability follows the decreasing order of NiPc/GCB > CoPc/GCB >FePc/GCB > CuPc/GCB. To better elucidate the ORR catalytic mechanism for TMPc/GCB, we employed density functional theory (DFT) calculations and drew the following results: (?) the -O2 adsorption is the major step to determine the ORR catalytic activity; (?) the way of -O2 adsorption on TMPc is the key point to affect the Tafel slope; (?) the -H2O2 desorption determine the transfer electron number; and (?) the -OH desorption and the central metal atom removal leads to the damage for the catalytic stability. Moreover, the high-coordination environment for Fe atoms and low-coordination environment for Co atoms in the TM-N-C complexes are in favor of improving the ORR performance.(2) To overcome the obstacle of rapidly declined activity of FePc molecules, it is essential to enhance the catalytic stability of FePc-based molecular electrocatalysts. Herein, a superior FePc-based and molecule-scale electrocatalyst is prepared with low-cost graphitized carbon black as carbon supports, and tuning the Fe coordination environment in FePc, via the delocalized ? coordination from unsubstituted phthalocyanine. Owing to the excellent ORR catalytic activity, the remarkable stability and the outstanding selectivity, the novel molecule-scale electrocatalyst is promising substitutes for PGM materials for ORR. Furthermore, to better elucidate the origin of the ORR performance, the DFT calculation indicates that the unique structure of unsubstituted phthalocyanine tethered iron phthalocyanine (Pc-FePc) plays a key role in improving of both the catalytic activity and stability.(3) As a promising candidate to replace PGM materials, single Fe atom catalysts with N-rich coordination environment may be a high-performance and cost-effective alternative for ORR. Herein, a general strategy to prepare a single Fe atom catalyst dispersed on hierarchical porous carbon has been developed with the aid of confinement effect of micropores and unsubstituted phthalocyanine complexes as barrier to expand isolation of Fe atoms during the high-temperature pyrolysis treatment. The resultant single Fe atom catalyst exhibits the well-fine atomic dispersion of Fe atoms coordinated with N-ligands supporting on 3D hierarchical porous carbon. Our strategy simultaneously optimizes the coordination environment of Fe active sites and increases the concentration of active sites. Accordingly, the as-prepared single atom catalysts outperforms the commercial Pt/C electrode in a high ORR activity and long-term electrochemical stability in alkaline/acidic electrolyte,along with a good fuel selectivity, promising for a potential non-platinum group metal catalysts for large-scale applications of fuel cell technologies.(4) In this work, a facile and interchangeable cross-linking process has been developed to synthesize the polyphthalocyanine with different cross-linking degree in using pyromellitic dianhydride. After the pyrolysis treatment, the cross-linked polyphthalocyanine generated into the N-doped carbon encased metal nanoparticles materials. During the pyrolysis treatment,the cross-linked structure for polyphthalocyanine can effectively restrain the aggregation of the metal core and significantly increase the amount of active N species in the pyrolyzed carbon shell. Different from the pyrolyzed pure phthalocyanine, the pyrolyzed cross-linked polyphthalocyanine electrocatalyst shows excellent electroactivity via 4-electron pathway along with a remarkable stability and good methanol tolerance. In addition, the unraveling of cross-linking degree effect also gives the guidance for future design of more efficient non-precious metal catalysts for oxygen reduction and other electrochemical application.(5) The Fe/Co polyphthalocyanine undergoes high-temperature pyrolysis,acid-washing and pyrolysis treatment again. The as-prepared electrocatalyst based on the bimetal (Fe and Co) nitrogen-codoped 3D porous carbon networks possess a few-layer graphene-like texture with hierarchically porous in meso/micro multimodal pore size distribution, and perform as an efficient electrocatalyst with a high ORR activity, good methanol tolerance and stability comparable to and even better than the commercial Pt/C. Owing to the highly generic nature characteristic of the thermal treatment/pyrolysis processes, the methodology developed in this study is highly general, which can be applied to the development of various high-performance TM-N-C catalysts containing one or more different metals.(6) In order to improve the ORR performance of the TM-N-C complexes,we develop a series of multi-scale electrocatalysts derived from phthalocyanine compounds in their catalytic activity and stability over a wide pH range and drew the following results: (?) the high-coordination environment for Fe atoms and low-coordination environment for Co atoms in the TM-N-C complexes are both in favor of improving the intrinsic ORR activity of single site; (?) bimetal (Fe and Co) nitrogen-codoped carbon possesses electronic effects and synergistic effects to improve the ORR performance; (?) increasing surface area, enriching surface chemical structure,and enhancing the cross-linking degree in precursors can effectively increase the number and dispersion of active sites; (?) hierarchically porous structure can not only enhance the mass transfer through mesopores and macropores,but stabilize the single-atom active sites in its microporous structure.