Preparation And Performance Study Of Metal-polymer Derived Fe-N-C Electrocatalysts For Oxygen Reduction Reaction

Proton exchange membrane fuel cell（PEMFC）is an ideal energy conversion technology,which has the advantages of cleanliness,high energy density and high conversion efficiency.The development of low-cost,high-activity platinum group metal（PGM）-free oxygen reduction reaction catalysts to replace expensive and resource-limited platinum-based catalysts is the key to the commercialization of PEMFC.Among the PGM-free catalysts,metal-nitrogen-carbon catalysts（M-N-C）show good prospects for development.However,the M-N-C catalysts still suffer from insufficient activity and unsatisfactory stability under acidic conditions,which are mainly related to the low intrinsic activity,low density and demetallization of the M-N_x/C sites,as well as the oxidative corrosion of the catalyst carbon matrix under high temperature conditions.In this paper,the metal-polymer supramolecular structure design coordination strategy is used as the basis for the preparation of Si/Fe-N-C and AA/SA-Fe-N catalysts with high ORR performance by combining silicon doping and the introduction of acrylic acid（AA）to design metal-polymer double cross-linking network,respectively.The mechanism of the enhanced ORR activity and stability of Fe-N-C catalysts is investigated by a series of physical and electrochemical characterizations.The main research contents and experimental results are as follows:（1）The Si/Fe-N-C catalysts with high density and uniformly dispersed Fe-N_x/C sites are prepared using sodium alginate（SA）as the precursor,combined with metal-polymer supramolecular design coordination and Si doping strategy.Firstly,the coordination between Fe³⁺and the oxygen-containing functional group in SA forms an"egg-box"structure with three-dimensional space,which makes Fe³⁺uniformly dispersed in the precursor and prevents the agglomeration of Fe atoms during the pyrolysis process;then the hydrogen bonding between tetraethyl silicate and SA is used to introduce Si,which is conducive to the activation of O₂ and the desorption of H₂O in the catalytic process,and can enhance the graphitization of the carbon matrix of the catalyst.The 0.5-Si/Fe-NC-800 catalyst with high activity and stability is obtained by subsequent pyrolysis and acid washing,and the half-wave potential(E_1/2)reached 0.817V（vs.Reversible hydrogen electrode,RHE）in 0.5 M H₂SO₄,which is 32 m V higher than the Fe-NC-800 catalyst without Si doping;the 0.5-Si/Fe-N-C-800 catalyst exhibited excellent stability,as evidenced by a loss of only 20 m V in E_1/2 after 5000potential cycles from 0.6 to 1.0 V in O₂-saturated 0.5 M H₂SO₄ solution at 60°C.The loss in E_1/2 of Fe-N-C-800 catalyst reaches 40 m V.The excellent ORR activity of the Si/Fe-N-C catalyst is mainly due to the homogeneous dispersion and high density of Fe-N_x/C active sites in the carbon matrix;the stability enhancement is mainly due to the Si doping which enhances the graphitization of the carbon matrix of the catalyst.（2）The porous carbon materials（AA/SA-Fe-N）with uniformly dispersed Fe-N_x/C active sites are prepared as efficient ORR catalysts by using SA and AA as precursors in combination with a metal-polymer double cross-linking network strategy.Firstly,the AA monomer polymerizes to form a polyacrylic acid（PAA）network,SA chain is physically interspersed into the PAA network.Subsequently,Fe Cl₃ is added to the PAA/SA solution,and Fe³⁺chelates with three carboxyl groups in PAA/SA to form an"egg-box"structure.SA chain is physically interspersed in the PAA network,which in turn makes Fe³⁺more uniformly dispersed in the double cross-linked network and increases the distance between metal ions,effectively alleviating agglomeration during the pyrolysis,and the density of Fe-N_x/C active sites is effectively enhanced.To study the effect of particle agglomeration on the catalyst activity during precursor drying,we use evaporative solvent drying and freeze-drying to treat the precursors,and the catalysts obtained after pyrolysis are e-AA/SA-Fe-N and f-AA/SA-Fe-N,respectively.In 0.5 M H₂SO₄,the E_1/2 of e-AA/SA-Fe-N-1 is 0.796 V（vs.RHE）,which is 11 m V higher than Fe-N-C-800 without AA;the E_1/2 of f-AA/SA-Fe-N-1 reached 0.83 V（vs.RHE）.It is shown that the high activity of the f-AA/SA-Fe-N originated from the uniformly dispersed Fe-N_x/C sites and the extensive pore structure in the catalyst.