Preparation And Applications Of Heteroatom Doped Porous Carbon Based Electrocatalytic Materials

Application of fossil fuels causes the serious problems such as environmental pollution,ecological damage,and resource shortage.The development of green and sustainable energy storage and conversion technologies is highly desired.Among these new energy storage technologies,metal-air batteries,especially zinc-air batteries（ZABs）are expected to serve as ideal power sources in the future due to their excellent safety,high energy density,high conversion efficiency,and good sustainability.Currently,the catalysts in air electrodes are still dominated by precious metals such as platinum,ruthenium,and iridium.However,the high cost,low abundance,single catalytic activity,and poor stability of precious metals severely hindered their large-scale application as catalysts in ZABs.Thus,it is highly desired to develop non-precious catalysts.Among the noble metal-free substitutes,carbon-based catalyst including transition metal-containing carbon or heteroatom-doped carbon electrocatalysts have been widely investigated as bifunctional electrocatalysts on the air cathode of ZABs due to their low cost,high catalytic activity,and stability.In this thesis,highly-efficient carbon-based bifunctional electrocatalysts were developed by rational design and regulation of the structure,morphology,chemical composition of the porous organic frameworks that serve as the templates for the preparation of carbon based electrocatalysts.The main research contents and achievements are listed as follows:（1）An overview of the basic principle and development history of ZABs,the catalytic mechanism and types of ORR/OER bifunctional catalysts,the preparation methods of air electrodes,and the application of novel porous framework materials（MOFs,COFs）in the field of electrocatalysis.The elaboration in detail of the main problems and solutions of current carbon-based catalysts as well as the significance,innovation,and research content of this thesis.（2）Carbon-based materials have been extensively used as bifunctional electrocatalysts due to their good electrical conductivity.However,the catalytic activity towards ORR and OER of the pure carbon materials is low.The construction of metal-containing carbon or heteroatom-doped carbon is an effective way to solve the catalytic inertness of pristine carbons and further enhance their catalytic properties.Besides,many previous studies revealed that the morphology and structure of catalysts could substantially influence the key properties of carbon-based catalysts.Considering the complementary advantages of COFs and MOFs in structures and functions,herein,we have successfully constructed a core-shell COF@MOF hybrid by epitaxial growth of ZIF-67 nanocrystals on the surface of imidazole-functionalized COF（denoted as COF-SM@ZIF-67）.The subsequent pyrolysis of COF-SM@ZIF-67 hybrid in nitrogen atmosphere gave rise to a hollow core-shell carbon-based composite,which composed of COF-derived hollow N,S co-doped inner core and ZIF-67 derived Co/NSC outer shell（H-NSC@Co/NSC）.The synergistic interaction between core and shell and the unique hollow nanostructure endowed the catalyst with enhanced catalytic activity and good diffusion kinetics.As a result,the resulting H-NSC@Co/NSC catalyst exhibits excellent bifunctional catalytic activity with a small OER-ORR potential gap of 0.75 V in alkaline solution,which is superior to the most reported bifunctional electrocatalysts.Additionally,the assembled ZABs using H-NSC@Co/NSC delivers a high peak power density of 204.3 mW cm^-2,a small charge discharge voltage gap,and excellent cycle stability,outperforming that of benchmark Pt+Ru O₂ catalysts.DFT calculations reveals that the electronic structure of the carbon frameworks on the Co/NSC shell could be effectively modulated by the embedded Co nanoparticles（NPs）,facilitating the adsorption of oxygen intermediates,and leading to improved catalytic activity.Meanwhile,the in-situ generated Co OOH is highly active for OER.This work demonstrated that the performance optimization of carbon-based materials could be achieved by rational regulation of their morphology and electronic structure.（3）Despite great electrocatalytic activity of meal-containing carbon-based catalysts,some intrinsic deficiencies,such as structural instability and susceptible dissolution,and agglomeration under a strong alkaline environment,have impeded their practical applications of the metal-containing carbon catalysts.Alternatively,the metal-free carbons can supply better p H-universal catalytic activity and stability.However,pristine carbon is intrinsically inactive to oxygen electrocatalysis due to its poor adsorption/activation of O₂ and ORR intermediates.Herein,we developed a N-doped porous carbon electrocatalyst（NPC）via pyrolyzing N-rich COF（Triazole-COF）precursor to improve the catalytic activity of the carbon catalysts.Triazole-COF is used as a self-sacrificial template to construct heteroatom-doped carbon and synthesized via the conventional condensation of 1,3,5-benzenetricarboxaldehyde and 3,5-diamino-1,2,4-triazole.The obtained NPC catalyst shows significantly enhanced catalytic activity for both ORR and OER in comparison to the pure carbon catalyst.This work provides an important guideline for improvement of OER/ORR activity of metal-free carbon catalysts.（4）On the basis of the above chapter,we developed heteroatom fluorine（F）on N doped carbon-based catalysts and investigated the heteroatom doping effects on the improvement of catalytic activity.Compared to the N doping,the electronegative property of F atom is larger（4.0）than carbon（2.5）,which can cause charge polarization on the neutral carbon skeleton,and lead to the formation of rich-edge defects,thus greatly enhancing the electrocatalytic activity.Usually,introduction of heteroatoms in carbon catalysts can be achieved by precisely predesign the organic units to obtain desired organic building blocks and heteroatom electrocatalytic active sites.COFs is considered as an ideal precursor for the preparation of heteratom-doped carbon catalysts,due to the structural advantages of COFs（including large specific surface area and high porosity,as well as tunable structure）.In this chapter,F,N co-doped porous carbon catalysts（F-NPC）were successfully prepared as an efficient metal-free bifunctional electrocatalyst by one-step pyrolysis of F,N-enriched COF（denoted as F-COF）.Comparing with the traditional fabrication method of F,N co-doped carbon catalysts that largely rely on the physical mixture of the foreign F,N sources with carbon precursors followed by the subsequent,thermal treatment,the in-situ introduction of F,N heteroatoms in COFs could result in the uniform distribution of active sites in the heteroatom-doped carbon catalysts.The synthesized F-NPC catalysts exhibit excellent ORR catalytic performance in alkaline environments with a half wave potential of 0.86V and a small OER-ORR potential gap of 0.79 V,which is higher than the single N-doped porous carbon（NPC）.The enhanced electrocatalytic performance demonstrates the effective effect of F-doping on the improvement of the electrocatalytic performance.DFT calculations reveals that F doping can cause spin density changes in a wide range,which is conducive to OER/ORR bifunctional catalytic activity.Moreover,the assembled ZABs with F-NPC catalyst exhibit excellent power density and cyclic stability.（5）It has been reported that the simultaneous incorporation of heteroatoms with different electronegativity into the carbon matrix can create a unique electronic structure to induce abundant active sites through the synergistic effect between them.In this chapter,we developed ternary（N,S,F）doped carbon catalysts and investigated the synergetic effects of heteroatom doping on the catalytic performance.Herein,a self-template carbonization strategy was proposed for converting the predesigned N,S,and F enriched novel porous polymer（TFS-MA）into ternary heteroatoms-doped porous carbon（termed as TFS-NC）.It was found that the TFS-NC-900 pyrolyzed at 900℃shows a superior ORR half-wave potential of 0.88 V and a small OER-ORR potential gap of 0.76 V to that of the counterpart with Pt/C+Ru O₂ catalysts（0.85 V@0.81 V）.When assembled into liquid ZABs with TFS-NC-900 catalyst,a high peak power density of 230.5 mA cm^-2 was achieved.Notably,the charge-discharge voltage gap can be maintained at about 1 V at a high current density of 20 mA cm^-2.The excellent catalytic performance of the synthesized catalyst is attributed to the strong synergistic interaction of N,S and F doping,which induces more active sites to promote ORR/OER catalytic activity,the large specific surface area and the unique porous structure.