Synthesis Of Carbon Materials Modified By Transition Metal Species And Study On Their Electrocatalytic Performance

Zinc-air batteries(ZABs)are known as one of the most promising green energy sources in the 21st century due to their rich raw materials,low cost,high energy density,stable performance,etc.However,as an electrocatalysts for oxygen reduction reaction(ORR)and oxygen evolution reaction(OER)at the cathode,precious metal nanomaterials have hindered the practical applications of ZABs due to their high cost and low durability.Recently,the researchers proposed that the transition metal-loaded heteroatom doped carbon nanomaterials(M-N-C,M=Fe,Co,Ni,Cu,etc.)can meet the requirements of efficient oxygen electrocatalytic reaction,which is widely considered as a candidate of bifunctional electrocatalysts.However,the activity of the currently reported catalysts are still not satisfactory.Therefore,it is critical to improve the ORR and OER activity and durability of M-N-C catalysts such that they may be used as the bifunctional oxygen catalysts in ZABs.The surface binding energy of the M-N-C catalysts have a great relationship with the size of the transition metal species.The surface binding energy of large-sized nanoparticles(NP)based catalysts are very high and not conducive to the desorption of reaction products,which will affect the timely release of active sites,and further decrease the turnover efficiency during the catalytic process.In contrast,the single-atom catalysts(SACs)has a weak adsorption capacity for the reactants,which will increase the barrier during the adsorption process,and reduce the electrocatalytic reaction rate.Therefore,to maximally improve the electrocatalytic performance of the M-N-C catalysts,the adsorption rate of reactants and desorption rate of products should be well balanced in the surface of the catalysts.The latest research has found that the transition metal species formed by two or more atoms(multi-atom nanoclusters ?5nm)have stronger adsorption energy for reactants than SACs,and the reaction products are easier to desorb than NP catalysts,combining the merits of SA catalysts and traditional NP catalysts.Furthermore,multi-atom nanoclusters catalysts can provide sufficient electron transfer capability for the multi-electron process during OER or ORR process,which is a kinetically sluggish process involving more energy consumption due to its multistep electron transfer reaction pathway.In addition,the strong chemical interactions between neighboring atom-atom in the mlti-atom nanoclusters catalysts can efficiently stabilize the individual species and prevent agglomeration,further improve the stability of active sites.Therefore,the design and preparation of heteroatom doped carbon catalysts with homogeneously distributed Fe multi-atoms nanoclusters as electrocatalysts,will be an effective strategy to further promote their ORR activity and durability.Moreover,the Fe-N-C catalysts have a strong binding energy while the Cu-N-C catalysts have weak adsorption of reactants.Therefore,it is necessary to prepare Fe-Cu bimetallic nanoclusters catalysts which can achieve a balance between the adsorption of reactants and the desorption of reaction products to further improve the ORR activity and stability of the catalysts.In addition,it is critical importance to explore bifunctional catalysts with efficient OER/ORR activity for the development of Zinc-air battery.However,the adsorption ability of Ni SACs for OER reactants is too weak,which will increase the barrier during the adsorption process and reduce the OER reaction rate.Furthermore,it is difficult for Ni SACs to become a bifunctional catalysts with efficient OER/ORR activity.It has been reported that Fe atoms can upgrade the d-band center of Ni atoms,and can balance the adsorption of reactants and the desorption of products.Therefore,it is necessary to prepare Fe-Ni alloy nanoclusters catalysts with excellent ORR and OER bifunctional catalytic activity and stability,which is of positive significance to better expand the application of non-precious metal catalysts in zinc-air batteries.Based on the current issues mentioned above,a series of new transition metal species(Fe-single-atom-nanoclusters(Fe-SA-NC),Fe-Cu alloy nanoclusters(Fe-Cu ANC)and Fe-Ni alloy nanoclusters(Fe-Ni ANCs))modified three-dimensional N,S co-doped porous carbon catalysts can be prepared due to the chelation ability between tannic acid(TA)and metal ions(Fe3+,Cu2+,Ni2+)as well as interaction between metal ions and the imino group of polyaniline(PANI)chain for the first time.Then,their OER and ORR performances were investigated in alkaline media,respectively.Finally,the performance of a homemade rechargeable Zn-air battery assembled with these catalysts were also investigated.The detailed research content is as follows:Firstly,three-dimensional N,S co-doped carbon with high density(5.6 wt%) Fe-SA-NCs of homogeneous dispersion(N,S co-doped CPANI-TA-Fe Fe-SA-NC catalysts)is successfully prepared due to the controlled chelation between TA and Fe3+ions and the interaction between Fe3+ions and PANI chains.Compared with Fe-SA,the active site Fe-SA-NCs consists of several N4-Fe-O2-Fe-N4 moiety.The resulting N,S co-doped CPANI-TA-Fe Fe-SA-NC catalysts exhibit a excellent ORR performance(E1/2=0.923 V,Eonset=1.09 V,jk=12 mA cm-2 at 0.9 V),better methanol tolerance and better long-term durability in alkaline media.Additionally,the performance of the zinc-air battery prepared with N,S co-doped CPANI-TA-Fe Fe-SA-NC catalysts as the air electrode exhibited a high performance,which are better than those of the state-of-the-art commercial Pt/C catalysts.Secondly,three-dimensional N,S co-doped carbon with high density Fe-Cu ANCs of homogeneous dispersion(Fe-Cu ANC@NSC)is successfully prepared due to the controlled chelation between TA and Fe3+,Cu2+ions and the interaction between Fe3+,Cu2+ions and PANI chains.Among them,the active site Fe-Cu ANCs consists of several N4-Fe-Ox-Cu-N4 moiety.The resulting Fe-Cu NC@NSC catalysts perform better ORR catalytic activity,the half-wave potential is(0.951 V vs Pt/C is 0.89 V),the onset potential is(1.1 V vs Pt/C is 0.98 V),and the kinetic current density at 0.9 V is 38.4 mA cm-2.In addition,the stability of Fe-Cu NC@NSC catalyst is higher than that of Fe-SA-NC catalysts and commercial Pt/C catalysts.Finally,three-dimensional N,S co-doped carbon aerogel with high density Fe-Ni ANCs(Fe-Ni ANC@NSCA catalysts)are successfully prepared due to the difference chelation between TA and Fe3+,Ni2+ions and the nearly same interaction between Fe3+,Ni2+ions and PANI chains.When the molar ratio of TA:Fe:Ni was 1:2:5,the active site Fe-Ni ANCs consisting of N4-Fe-Ox-Ni-N4 moiety.The resulting Fe-Ni ANC@NSCA catalysts show excellent performance in ORR(E1/2=0.891 V)and OER(260 mV@10 mA cm-2)in alkaline media as bifunctional catalysts,which are better than the state-of-the-art the commercial Pt/C catalysts and RuO2 catalysts.Moreover,a Zn-air battery assembled with the Fe-Ni ANC@NSCA catalysts also shows a remarkable performance and exceptionally high stability.In summary,in this thesis,a series of new transition metal species(Fe-SA-NC,Fe-Cu ANC and Fe-Ni ANC)modified three-dimensional N,S co-doped porous carbon catalysts can be prepared,and the OER and ORR performances were also investigated.Finally,a homemade rechargeable Zn-air battery assembled with these catalysts as an air cathode shows a remarkable performance and exceptionally high stability.