Studied On Preparation And Performance Of Porphyrin-and Phthalocyanine-based Covalent Organic Polymer Oxygen Reduction Catalysts

The development and utilization of conventional energy sources have led to the growing problems of environmental pollution and energy crisis,and therefore new energy sources need to be developed to replace conventional energy sources.Metal air battery（MAB）is a clean energy conversion device that directly converts chemical energy into electrical energy and is not limited by the Carnot cycle,but the slow kinetic process of oxygen reduction reaction（ORR）on its cathode seriously affects the performance of the battery.Therefore,there is an urgent need to develop ORR catalysts with high activity,excellent durability and low cost to enhance the battery performance.The porphyrin and phthalocyanine transition metal macrocyclic complexes have excellent ORR catalytic properties and well-defined site structures,which are favorable for theoretical studies of catalytic reactions.Porphyrins and phthalocyanines have their own intermolecularπ-πstacking,which leads to a lower utilization of the catalytic active site.The poor electrical conductivity of porphyrins and phthalocyanines also limits the catalytic performance of these catalysts.Pyrolysis is a common means to enhance the conductivity of catalysts,but porphyrins and phthalocyanines have poor thermal stability,are easily decomposed during heat treatment and are difficult to control the process.In addition,the conductivity of porphyrins and phthalocyanines can be enhanced by using conductive carriers,such as carbon nanotubes,two-dimensional graphene and activated carbon,but the specific surface area and structural stability of these carriers still need to be improved.Therefore,the following studies are conducted to address the above issues.Meso-5,10,15,20-tetrakis（phenyl）porphyrin（TPPH₂）is sequentially sulfonated and ligated to obtain water-soluble meso-5,10,15,20-tetrakis（4-sulfophenyl）porphyrin iron（Ts PPFe）.Ts PPFe is used as the transition metal and sulfur source,polyacrylamide（PAM）as the nitrogen and carbon source,and sodium chloride as the template.The above precursors are dissolved in deionized water to form a homogeneous aqueous solution,and the Fe,N,and S co-doped 3D graphene（Ts PPFe/3D-NG）is prepared by freeze-drying and pyrolysis.During the pyrolysis,the molten sodium chloride acts as a template and also protects the precursor,which increases the doping amount of transition metal atoms and the active site density of the catalyst.Ts PPFe has a well-defined M-N₄ site structure and hydrogen bonding interactions with PAM,which prevents the loss during the pyrolysis and facilitates the uniform distribution of active sites.In addition,the introduction of sulfur atoms can change the d-orbital energy level of the metal at the center of the M-N_X site,thus realizing the modulation of the activity of the catalytic site.Under alkaline conditions,the half-wave potential(E_1/2)of Ts PPFe/3D-NG#2-800 reaches 0.868 V vs RHE with an electron transfer number of 3.97（@0.760 V vs RHE）,which is better than that of Fe S/3D-G#2-800(E_1/2=0.851 V vs RHE)and 20 wt%Pt/C(E_1/2=0.850 V vs RHE).Density flooding theory（DFT）calculations of the reaction mechanism are performed using VASP software,and the modulation of the catalytic active site by sulfur doping is further investigated.The application of Ts PPFe/3D-NG#2-800 as a cathode catalyst for zinc-air batteries resulted in a peak power density of 318 m W cm^-2,a specific capacity of 816 m Ah g^-1,and an excellent charge-discharge cycle durability（293 h）.In order to further enhance the catalytic performance and the structural controllability of its catalytic active sites,a one-step solid-phase method is used to synthesize conjugated polymerized cobalt phthalocyanine（PPc Co）linked by benzene rings as catalyst precursors.PPc Co has a large conjugated structure and excellent thermal stability,so that PPc Co is not easily decomposed during heat treatment.The 3D-graphene（3D-G）is prepared by high temperature pyrolysis using hard template method with coal pitch as carbon source and magnesium oxide nanoparticles as template as catalyst carrier.The 3D-G has excellent electrical conductivity,large specific surface area,porous structure and excellent structural stability.Theπ-πinteractions exist between PPc Co and 3D-G,which enable PPc Co to be uniformly solidified on the surface of 3D-G.The PPc Co/3D-G catalyst is obtained.In addition,theπ-πinteraction is also beneficial to enhance the catalytic activity and durability of the catalyst.Under alkaline conditions,the E_1/2 of PPc Co/3D-G reached 0.860 V vs RHE with an electron transfer number of 3.95（@0.740 V vs RHE）,which is better than that of Pc Co/3D-G(E_1/2=0.840 V vs RHE)and 20 wt%Pt/C(E_1/2=0.850 V vs RHE).The molecular orbital energy levels,energy band structures,and density of states of PPc Co are calculated using Gaussian16 and Material Studio,and the calculated results are consistent with the experiments.The application of PPc Co/3D-G to the cathode catalyst of Zn-air batteries resulted in a peak power density of 347 m W cm^-2,a specific capacity of 776 m Ah g^-1,and excellent charge/discharge cycle durability（350 h）.To further investigate the effect of catalyst precursor structure and carrier on catalytic activity,bimetallic polymerized phthalocyanine iron-cobalt（PPc Fe Co）is synthesized by a one-step solid-phase method in order to study the synergistic catalytic effect of bimetals.The transition metals iron and cobalt affect each other’s electron cloud density and d-orbital energy level in the electron leaving domain system of the conjugated polymerized phthalocyanine,thus changing the adsorption energy of the active site to oxygen molecules and reaction intermediates.The 3D-G is prepared by high-temperature pyrolysis using a hard-template method with coal pitch as the carbon source and magnesium oxide nanoparticles and zinc oxide as templates as carriers.The E_1/2 of PPc Fe Co/3D-G under alkaline conditions reached 0.890 V vs RHE with an electron transfer number of 3.98（@0.768 V vs RHE）,which is superior to that of PPc Co/3D-G(E_1/2=0.860 V vs RHE).The mechanism of the ORR reaction catalyzed by PPc Fe Co,PPc Co,and PPc Fe was calculated by DFT using VASP software,and the synergistic effect of Fe-N₄ and Co-N₄ was investigated.The application of PPc Fe Co/3D-G as a cathode catalyst for Zn-air batteries results in a peak power density of 222 m W cm^-2,a specific capacity of 817 m Ah g^-1,and an excellent charge/discharge cycle durability（120 h）.