Preparation And Electrocatalytic Properties Of Metalloporphyrin-based Carbon Composites

With the development of human society,it becomes an urgent issue to seek the clean renewable energy storage and conversion devices owe to the significant increase in energy demand.The advent of fuel cells has given people hope.As the cathode reaction of the battery,the oxygen reduction reaction（ORR）has a large overpotential,which seriously affects the efficiency of the battery.Therefore,it desires to find a cathode catalyst with high catalytic activity,high stability,low cost and simple preparation method.Based on this background,mushrooms are selected as the carbon source caused by its feature with wide resources,low cost and easy availability,and metalloporphyrin macrocyclic compounds serve as the active precursors in this thesis.A serious of metal-based biomass-derived hierarchical porous carbon materials was prepared through different construction strategies as the high performance catalysts for ORR.Meanwhile some of them are disclosed to exhibit good activity to hydrogen evolution reaction（HER）and oxygen evolution reaction（OER）.The main research content of this thesis is as follows:1.A Br-regulated transition metal active-site anchoring and exposure strategy in biomassderived carbon nanosheets for obtaining robust ORR/HER electrocatalysts at all p H valuesA novel Br-regulated“Transition Metal Active-site Anchoring and Exposure”（TMAAE）strategy is reported,for the first time,to fabricate a hierarchical porous Co/N-doped bio-carbon bifunctional electrocatalyst（Co TBr PP@bio-C）for an all-p H ORR and HER by using a simple one-pot co-pyrolysis over a mushroom（MR）template with adsorbed Br-substituted porphyrinato cobalt（Co TBr PP）.Introduction of Co TBr PP onto the MR template not only leads to a larger specific area of 604 m² g^-1 in Co TBr PP@bio-C relative to that of Co TPP@bio-C（Co TPP=Br-free Co TBr PP）,98 m² g^-1,but more importantly also regulates the oriented distribution of pores precisely which promotes effective exposure of multiple active sites depending on the departure of Br-species attached to the Co-N₄macrocycle periphery.Impressively,Co TBr PP@bio-C with a Co loading down to 0.77 at%exhibits higher all-p H ORR and HER performance compared to Co TPP@bio-C with a Co loading of 3.5 at%.An efficient TMAAE makes Co TBr PP@bio-C possess higher onset and half-wave potentials of 0.93 and 0.85 V for the ORR,and a smaller Tafel slope of 80 m V dec^-1for the HER in an alkaline medium.Co TBr PP@bio-C also had a superior all-p H longterm stability and outstanding methanol tolerance,surpassing commercial Pt/C and most non-preciousmetal catalysts reported to date.Furthermore,Zn-air batteries assembled with Co TBr PP@bio-C exhibited a higher peak power density of 100 m W cm^-2 and excellent durability than Pt/C.2.In situ construction of Co/N/C-based heterojunction on biomass-derived hierarchical porous carbon with stable active sites using a Co-N protective strategy for high-efficiency ORR,OER and HER trifunctional electrocatalystsThe facile designs and fabrication of noble metal-free electrocatalysts are highly required to achieve multifunctional catalytic activity with excellent stability in Zn-air batteries,fuel cells and water splitting systems.Herein,a heterostructure engineering is applied to construct the high performance Co,Ncontaining carbon-based multifunctional electrocatalysts with the feature of isotype(i.e.n-n type Co₂N_0.67-BHPC)and anisotype（i.e.p-n type Co₂O₃-BHPC）heterojunctions for ORR,OER and HER.The n-n type Co₂N_0.67-BHPC,in which biomass（e.g.mushroom）-derived hierarchical porous carbon（BHPC）incorporated with nonstoichiometric active species Co₂N_0.67,is fabricated by using an in situ protective strategy of macrocyclic central Co-N₄ from Co TPP（5,10,15,20-tetrakis（phenyl）porphyrinato cobalt）precursor through the intermolecular p-p interactions between Co TPP and its metal-free analogue H₂TPP.Meanwhile,an unprotected strategy of macrocyclic central Co-N₄ from Co TPP can afford the anisotype Co₂O₃-BHPC p-n heterojunction.The as-prepared n-n type Co₂N_0.67-BHPC heterojunction exhibited a higher density of Co-based active sites with outstanding stability and more efficient charge transfer at the isotype heterojunction interface in comparison with p-n type Co₂O₃-BHPC heterojunction.Consequently,for ORR,Co₂N_0.67-BHPC exhibits the more positive onset and half-wave potentials of 0.93 and 0.86 V vs.RHE,respectively,superior to those of the commercial 20 wt%Pt/C and most of Co-based catalysts reported so far.To drive a current density of 10 m A cm^-2,Co₂N_0.67-BHPC also shows the lower overpotentials of 0.34 and 0.21 V vs.RHE for OER and HER,respectively.Furthermore,the Zn-air battery equipped with Co₂N_0.67-BHPC displays higher maximum power density(109 m W cm^-2)and charge-discharge cycle stability.Interestingly,the anisotype heterojunction Co₂O₃-BHPC as trifunctional electrocatalyst reveals evidently photoelectrochemical enhancement compared with the photostable Co₂N_0.67-BHPC.That is to say,isotype heterojunction material(n-n type Co₂N_0.67-BHPC)is equipped with better electrocatalytic performance than anisotype one（p-n type Co₂O₃-BHPC）,but the opposite is true in photoelectrochemical catalysis.Meanwhile,the possible mechanism is proposed based on the energy band structures of the Co₂N_0.67-BHPC and Co₂O₃-BHPC and the cocatalyst effects.The present work provides much more possibilities to tune the electrocatalytic and photoelectrochemical properties of catalysts through a facile combination of heterostructure engineering protocol and macrocyclic central metal protective strategy.3.In-situ protection combined with self-activation strategy for the construction of cobalt carbide and nitride on porous carbon catalyst for high-efficiency ORR/OER/HERBy means of in-situ protection of Co-N₄ core in Co Br TPP combined with self-activation effect of Co TBr PP,a porous carbon material Co CN@bio-C containing Co₂N_0.67 and Co₂C active nanocrystallines is prepared by a one-step co-pyrolysis over the MR template with adsorbed a mixture of Co TBr PP and H₂TPP.The Co CN@bio-C exhibits excellent ORR performance under alkaline condition(E_onset=0.93 V,E_1/2=0.86 V,J_L=5.51 m A cm^-2),even exceeded the commercial 20 wt%Pt/C.In the stability measurement,after 10 hours of operation,the current density is only reduced by 12%.And this material can also be operated under neutral and acidic conditions for ORR.In addition,OER and HER activities are disclosed under alkaline condition and all p H conditions,respectively.The present work provides a way for simple preparation of stable non-noble metal carbon-based catalytic materials.4.Phosphating reaction to construct a bimetallic phosphide nanocrystalline catalyst with low platinum content for ORR and HERAfter loading platinum species on the porous carbon containing the C₃N₄ phase from Co₂O₃-BHPC,phosphating treatment was carried out.The resultant Co/Pt/P@Bio-C material is found to contain cobalt phosphates and platinum phosphates.The resultant Co/Pt/P@bio-C possesses outstanding ORR performance(E_onset=0.94 V,E_1/2=0.86 V,J_L=5.68 m A cm^-2)surpassing 20 wt%Pt/C under alkaline conditions.It is also found that this catalyst also displays excellent HER activity with small overpotential of 46 m V at current density of 10m A cm^-2 under the same alkaline condition.In addition,the Co/Pt/P@bio-C also possesses high stability,which will be more beneficial for practical applications.This system provides a design idea for the preparation of stable and efficient catalyst with low platinum content.