Support Functionalized Catalysts For Oxygen Reduction Reaction In A Hybrid PEM/AEM Fuel Cell

Fuel cells can transfer chemical energy to electrical energy in a clean way.Because of its high efficiency and portability,fuel cell is attracting interests from all over the world.Even though fuel cells are already used in some cars nowadays,the high cost and short life span still constrain their commercialization.Thus,the development in exploration of low cost,high activity and stability fuel cell catalysts for oxygen reduction reaction is crucial now.In this research,a hybrid PEM/AEM fuel cell is used to investigate the catalytic activity of platinum catalysts and non-noble catalysts to ORR.As a hybrid fuel cell,the cathodic reaction is in alkaline media while the anodic reaction is in acidic media.In this case,the dissociation of platinum catalysts decreased and at the same time the usage of non-noble catalysts on the cathode is possible.Functionalized carbon nanotubes are made by sonication in mixed acid and ammonium hydroxide.Two types of platinum catalysts were made via chemical reduction method.HRTEM is used to observe the morphology of the platinum nanoparticles on different functionalized carbon nanotubes,including the particle size and distribution.It is found that the functional groups on the carbon nanotubes of the catalysts could affect the size of platinum particles.The particle size of Pt/CNTON is the smallest among the three platinum catalysts which is 3.71 nm,while that of Pt/C is the biggest.XPS was used to investigate the surface properties of functionalized carbon nanotubes and catalysts supported on functionalized carbon nanotubes.The correlation between functionalized carbon nanotubes and the morphology of platinum nanoparticles can also be established by this characterization method.The XPS showed that on the surface of Pt/CNTON has more carbonyl groups and less carboxyl groups compared to the other two platinum catalysts.The fuel cell tests showed that in a H2/O2 PEIM/AEM fuel cell,the catalytic activity of Pt/CNTON to ORR is the highest among all the platinum catalysts while that of Pt/C is the lowest.Interestingly,in a methanol PEM/AEM fuel cell,the catalytic activity of Pt/CNTON to ORR is poorest,while Pt/CNTOX performed best.The difference in performance is suggested to be caused by methanol crossover.The nitrogen-containg functional groups on the Pt/CNTON is electron donor,so it will attack the proton of methanol.This accelerates the oxidation of methanol on the cathode.The side reaction not only takes the active site of ORR but can also poison the platinum catalysts from its intermediate products.Different iron phthalocyanine catalysts were made by heat treatment which are P-FePc-py-CNT,P-FePc-CNTN and P-FePc-CNT.Diazonium reaction was used to add axial pyridinic groups on the surface of carbon nanotubes.Melamine was also used as a nitrogen source to synthesize functionalized carbon nanotubes by heat treatment.The core level spectra of Fe 2p of P-FePc-py-CNT through XPS showed that Fe 2p2/3 upshifted compared to P-FePc-CNTN and P-FePc-CNT.Additionally,Raman results showed that compared with P-FePc-CNT,more defects were on the surface of P-FePc-py-CNT.It is evident that the axial ligands still exist after the heat treatment.The electrochemical activity was investigated in a H2/O2 PEM/AEM fuel cell.It was found that all the pyrolyzed iron phthalocyanine catalysts performed significantly better than non-pyrolyzed iron phthalocyanine catalysts(FePc-py-CNT).Accordingly,the structure of iron phthalocynine catalysts is decisive factor to the catalytic activity to ORR.Among all the pyrolyzed iron phthalocyanine catalysts,P-FePc-py-CNT performed best.This could result from the axial ligand fixed the Fe which can accelerate the electron tranfer between the support and the active sites of the catalysts.The cobalt based non-noble catalysts were synthesized and investigated in a hybrid PEM/AEM fuel cell.An ultrasonic method and a tetrahydrofuran-refluxing method were used to synthesize heat-treated catalysts CoPc-CNTS and CoPc-CNTR,respectively.Based on the XPS CoPc-CNTS has most pyridinic groups and the second most pyrrolic groups on the surface,while CoPc-CNTR has most pyridinic groups and the second most graphitic nitrogen groups.The fuel cell tests in a H2/O2 hybrid PEM/AEM fuel cell showed that CoPc-CNTS performed better than CoPc-CNTR.Therefore,the synergistic effect of pyridinic groups and pyrrolic groups accelerates the catalytic activity of CoPc-CNTS to ORR.Two types of cobalt oxides catalysts were also synthesized.The XRD and XPS proved that the cobalt oxides catalysts are Co3O4/CNT and CoO/CNT.However,neither of the cobalt oxides catalysts has high catalytic activity to ORR in a methanol hybrid PEM/AEM fuel cell.Among all the cobalt based catalysts pyrolyzed cobalt pthalocyanine catalysts performed better than Co3O4/CNT in a H2/O2 hybrid PEM/AEM fuel cell.However,P-FePc-py-CNT performed the best among all the non-noble catalysts in this research.In this research,the catalytic activity of platinum catalysts to ORR was improved,and high catalytic activity non-noble catalysts to ORR were synthesized.The correlation between performance and surface properties of ORR catalysts was established.The results are of importance for the exploration of fuel cell catalysts with high ORR performance and stability.