Pt-low-loading Or Pt-free Catalyts For The Oxygen Reduction Reaction In Fuel Cells

Fuel cells(FCs)are considered as the most promsing new energy conversion device due to their high energy density,high efficient and environmental friendly,which can be broadly applicated in military,hospital,portable power source and power station.However,the high cost,low Pt utilization and poor durability of Pt-based electocatalyst have been recently regarded as the most severely issues to be resolved before the commercialization of FCs.Moreover,more Pt loading at the cathode is required due to the sluggish of oxygen reduction reaction(O RR)in FCs,which unevitably increased the cost of the whole FCs system.Thus,to address the issues,it is urgently need to develop non-Pt oxygen reduction catalysts to replace the currently expensive Pt elctrocatalysts,or explore a strategy to reduce the loading of Pt and increase the stability of Pt in the Pt-based electrocatalysts.Based on the above mentioned,an efficient non-platinum catalyst was developed basd on the structure engineering and two kinds of low-Pt catalysts with highly activity and durablity for ORR were obtianed in this thesis.Firstly,we report that the electrocatalytic performance of a spinel for the catalysis of ORR can be significantly promoted by reversing its crystalline structure from the normal to the inverse.A serial of Co-Fe based spinel structures are tuned from its normal to the inverse and then back to its normal by adjusting the iron content in the synthesis.Their structures are verified by XRD,TEM,HRTEM and XPS,the results confirm that {Co}[Co2]O4 and {Co}[Fe2]O4 are spinels with a normal structure,whereas,{Co}[FeCo]O4 possess an inverse structure.The electrochemical test were carried out to investigated their differences catalytic ativities for ORR,and the results show that electrocatalytic activities for ORR are maximized for inverse {Co}[FeCo]O4 and decrease successively for normal {Co}[Co2]O4 and{Co}[Fe2]O4.Combined with the DFT method,we reveal the structure-activity relationship between spinel structure reversal and ORR.The DFT results presents that the higher ORR activity of the inverse spinel {Co}[FeCo]O4 originates from the broken electronic balance and charge polarization occurring between the octahedral atoms,which influences the O-O bond activation.We define these changes caused by different atoms at the octahedral site in the spinel as a dissimilarity effect.Contrastly,{Co}[Co2]O4 and{Co}[Fe2]O4 present a relative poor activity for ORR without such a dissimilarity effect.Secondly,to resolve the the low utilization rate of Pt catalyst,we designed a three-dimensional N-doped mesoporous carbon material(N-MCF)as Pt catslyst support by using the metal organic framework ZIF-8 as the precursor.The structure,morphology,and surface functional groups of N-MCF and Pt/N-MCF are detected by XRD,SEM,TEM,and FTIR,respectively.The results show that the N-MCF support contains a large amount of amorphous carbon.Due to the instability of amorphous carbon,the surface of the N-MCF is easily oxidized to an oxygen-containing functional group.Therefore,when the Pt nanoparticles are supported on the N-MCF,these surface oxygen-containing functional groups will anchor the Pt nanoparticles evenly on the surface of N-MCF,and thus the growth of the Pt nanoparticles is limited.The nitrogen desorption isothermal analysis and electrochemical test demonstrates that Pt nanoparticles are mainly anchored on the inner wall of mesoporous in the N-MCF,which greatly improves the utilization of Pt.Hence Pt/N-MCF exhibits an excellent ORR activity.The accelerated aging test showed that the ECSA of Pt/N-MCF was lost by 18% and the half-wave potential decreased by 8mV after 2000 cycles of cyclic voltammetry.Whereas,the ECSA was lost by 26% and half-wave potential decreased by 20 mV for the comerical JM Pt/C after 2000 cycles.The XPS analysis results show that there are two interaction forces between N-MCF and Pt,one is the induced by the oxygen-containing functional groups located on the surface of N-MCF,another is the electron interaction between the N-MCF and Pt.The latter one will change the electronic structure of Pt,and then tune the adsorption behavior of the oxygen-containing intermediate species on the Pt surface,and thus the oxidation potential of Pt is promoted.Therefore,Pt/N-MCF catalyst possesses good ORR performance and durability.Finally,to address the poor durability of Pt catalyst,we implanted Pt nanoparticles into ZIF-8 by applying the growth mechanism of ZIF-8 and acquired Pt@ZIF-8 with core-shell structure.After sintering of Pt@ZIF-8 at high temperature,we obtained a high-efficiency ORR Pt Zn@N-MCF catalyst with Pt Zn intermetallic nanoparticles uniformly confined in a three dimensional N-doped mesoporous carbon framework(N-MCF).The XRD,SEM,TEM and HRTEM results show that Pt nanoparticles can be evenly embedded into the ZIF-8 by using its growth mechanism.After high temperature,the Pt and Zn2+ originated from ZIF-8 formed the Pt Zn intermetallic nanoparticles,which can be uniformly confined in the N-MCF.The XPS analysis results show that Pt nanoparticles embedded in the ZIF-8 can promote the nitrogen doping during the high temperature process,and the Pt Zn nanoparticles in the Pt Zn@N-MCF catalyst are mainly embedded into the inner of the N-MCF,rather than on the surface of the N-MCF.The nitrogen desorption isothermal analysis results show that Pt Zn@N-MCF owns rich mesoporous and the mesoporous structure is interpenetrated,which is beneficial to form the effective three-phase.The electrochemical test exhibits that the Pt Zn@N-MCF catalyst has a mach higher ECSA,mass specific activity and area specific activity compared to the commercial JM Pt catalyst.The accelerated durability test shows that the onset and half-wave potential of the Pt Zn@N-MCF catalyst has almost no recession after 2000 cycles of cyclic voltammetry,whereas,the onset and half-wave potential of JM Pt/C are decreased by 9 and 20 mV,respectively.The TEM results of after aging PtZn@N-MCF results show that the N-MCF with three-dimensional structure can effectively prevent the precipitation,agglomeration and ripening growth of Pt,and thus prevent the loss of ECSA of Pt.The results of the single cell test show that PtZn@N-MCF achieves a higher power density compared to commercial JM Pt/C at a lower Pt loading no matter when Pt Zn@N-MCF act as a cathodic catalyst or as anode catalyst.Therefore,Pt Zn@N-MCF is a very promising ORR catalyst.