Preparation Of Doped Carbon-based Catalysts And Their Electrocatalytic Performance For Oxygen Reduction Reaction

With the development of society,there is an increasing demand for energy nowadays.However,the limited reserves of fossil fuels as well as the environmental pollution problems stemming from the fossil fuel use have greatly affected the subsistence and development of human beings.Therefore,developing renewable energy sources has become imperative.Fuel cell as a new kind of energy storage and conversion device has attracted more and more attention because of its high energy conversion efficiency,environmentally friendly characteristics,and high stability.Currently,one of key issues precluding the commercialization of fuel cell is the sluggish kinetics of its cathodic oxygen reduction reaction?ORR?requiring noble metal Pt,which has a high activity for ORR,to facilitate the reaction.Nevertheless,the high cost of Pt greatly hinders the commercial application of fuel cells.Therefore,developing low-coat ORR catalysts has great significance.In this paper,several low-cost and high-performance ORR catalysts were prepared and the main experimental results are as follows:?1?Nitrogen-doped carbon?N-C?materials are regarded as promising low-cost catalyst for ORR.However,the commonly used route for N-C synthesis,viz.,the high-temperature pyrolysis of N-and C-containing precursors,usually results in a great loss of N-containing species that are related with the active sites for ORR.Herein,wrinkled sheet-like N-C materials were synthesized by using embedded g-C₃N₄ as template and N source.The N-C sample synthesized at 900°C with a mass ratio of glucose to g-C₃N₄ being 4:1 exhibits a positive half-wave potential?0.823 V vs.reversable hydrogen electrode?RHE??,good long-term stability and dominant 4 e^-pathway for ORR in alkaline media,which can be attributed to its large specific surface area,high porosity,and large fraction of pyridinic and graphitic N.Notably,when a small amount of Fe was doped into the N-C sample,its ORR performance can be greatly improved and outperforms the commercial Pt/C catalyst in terms of ORR activity,long-term stability and methanol tolerance.This work offers a promising route for the synthesis of advanced porous N-C materials for ORR.?2?High-temperature pyrolysis of C-,N-and Fe-containing precursors is the most commonly used route to prepare Fe-N-C catalyst,which is regarded as the most promising low-cost catalyst for ORR.However,the simple pyrolysis usually results in a great loss of N-containing species and a poor dispersion of Fe-N_x moieties,which are generally viewed as the dominant catalytic active sites for ORR.In this work,Fe-N-C catalyst is prepared by using glucose as C source and urea-derived g-C₃N₄ as ligand for Fe coordination,N source and self-sacrificing template.The catalyst prepared at pyrolysis temperature of 1100°C?Fe-N-C-1100?has finely dispersed Fe-N_x species,high surface area(730 m² g^-1)and extremely large pore volume(2.56 cm³ g^-1),and accordingly exhibits good ORR performance in alkaline media,as evidenced by its more positive half-wave potential?0.894 V vs.RHE?,higher stability,stronger methanol tolerance,and higher peak power density when used as cathode catalyst in Zn-air battery,which are all superior to those of the commercial Pt/C catalyst.This work will bring a general method for the design and development of advanced porous Fe-N-C catalysts for ORR.?3?High-temperature pyrolysis of Fe-,N-,and C-containing precursors usually produces Fe-N-C samples with limited porosity and poor dispersion of Fe N_x moieties,which are regarded as the dominant active sites for ORR.Herein,highly porous Fe-N-C nanomaterials with highly dispersed Fe N_x active sites coupled with large specific surface area and high pore volume are synthesized by using g-C₃N₄ coordinated Fe matrix embedded in glucose-derived carbon as precursor.The Fe-N-C-575 sample synthesized with g-C₃N₄ obtained at 575°C being the precursor exhibits a specific surface area of1012.2 m² g^-1,a pore volume of 3.028 cm³ g^-1,and a half-wave potential(E_1/2)of 0.900 V vs.RHE in alkaline media,and after acid washing and second annealing the obtained Fe-N-C-575-A sample exhibits an increased pore volume(3.642 cm³ g^-1)and correspondingly a more positive E_1/2?0.908 V vs.RHE?,which is much higher than that of the commercial Pt/C catalyst(E_1/2=0.864 V vs.RHE).This study demonstrates a facile strategy to prepare highly porous Fe-N-C nanomaterials decorated with highly dispersed active sites for promising applications in ORR catalysis.