TY - BOOK
T1 - Systematic Investigations of the Oxygen Reduction Reaction on Pt Based Catalysts Comparing Transient and Steady State Performance
AU - Deng, Yujia
PY - 2016
Y1 - 2016
N2 - This thesis investigates the electro reduction reaction of oxygen (ORR), which is the cathodicreaction in fuel cells, on Pt-based catalysts. The aim is to scrutinize different factors that caninfluence the ORR.The influence of oxygenated species on the ORR on polycrystalline Pt is compared attransient and steady state conditions. At steady state conditions the dominant potential rangefor the adsorption of the different oxygenated species is elucidated by employing anelectrochemical stripping method. The results indicate that below 0.6 VRHE, the surface of Ptis free from oxygenated species. Above 0.6 VRHE, OHad begins to form on the surface and at0.95 VRHE a monolayer completes. Above 0.95 VRHE, OHad can be oxidized to Oad and at 1.1VRHE a monolayer of Oad is formed. When the potential increases further above 1.1 VRHE, Pt-oxidestarts to form. The ORR can only proceed on free Pt sites. Above 0.95 VRHE, as thesurface is taken up entirely by oxygenated species, the reaction stops. Interestingly thehydrogen oxidation reaction (HOR) can proceed until 1.1 VRHE, although the reaction alsocan only occur on Pt free sites. It is therefore suggested that H2 can reduce some of theoxygenated species thus making the surface active even at high potential.The influence of anions on the ORR is investigated as well. Under transient conditions, thecatalytic behavior of Pt in H2SO4 and H3PO4 electrolyte solutions, respectively, are similar.That is, the specific anion adsorption can block the reactive sites for the ORR, thus leading toa decrease in activity as compared to that in HClO4 electrolyte solution. At steady state theORR activity is inhibited in all the three acid electrolyte solutions as compared to transientconditions. The ORR can reach its diffusion limited current in both HClO4 and H2SO4electrolyte solutions. However, in H3PO4, the ORR rates decreases so significant that nodiffusion limited current is observed. This is a considerable difference to the behavior inH2SO4 electrolyte solution. Furthermore the ORR activity is time dependent in H3PO4 andcontinues to decrease when the electrode potential is held. A combination of a site blockingmechanism and an increase in viscosity at the polarized electrode surface is proposed toexplain this extraordinary behavior in H3PO4.Atomic layer deposition of Pt onto an Au substrate is employed as model catalyst toinvestigate how ligand, ensemble, and strain effects affect the catalytic performance.Combining the experimental data with DFT calculations, it is seen that for one or two layersof Pt on the Au substrate, the ligand and ensemble effects play a dominant role, thusenhancing the catalytic activity. With increasing number of Pt layers, the strain effectbecomes the predominant factor, thus decreasing the catalytic activity.The last part of this thesis focuses on combining steady state tests and the design of catalysts.Shaped-controlled synthesized tetrahexahedral (THH) Pt nanoparticles are employed asmodel catalysts and their catalytic activity towards ORR is tested at both steady state andtransient conditions. Polycrystalline Pt and carbon supported Pt nanoparticles (Pt/C) serve asreference. The excellent performance of THH Pt nanoparticles at steady state is linked totheir resistance to oxide formation
AB - This thesis investigates the electro reduction reaction of oxygen (ORR), which is the cathodicreaction in fuel cells, on Pt-based catalysts. The aim is to scrutinize different factors that caninfluence the ORR.The influence of oxygenated species on the ORR on polycrystalline Pt is compared attransient and steady state conditions. At steady state conditions the dominant potential rangefor the adsorption of the different oxygenated species is elucidated by employing anelectrochemical stripping method. The results indicate that below 0.6 VRHE, the surface of Ptis free from oxygenated species. Above 0.6 VRHE, OHad begins to form on the surface and at0.95 VRHE a monolayer completes. Above 0.95 VRHE, OHad can be oxidized to Oad and at 1.1VRHE a monolayer of Oad is formed. When the potential increases further above 1.1 VRHE, Pt-oxidestarts to form. The ORR can only proceed on free Pt sites. Above 0.95 VRHE, as thesurface is taken up entirely by oxygenated species, the reaction stops. Interestingly thehydrogen oxidation reaction (HOR) can proceed until 1.1 VRHE, although the reaction alsocan only occur on Pt free sites. It is therefore suggested that H2 can reduce some of theoxygenated species thus making the surface active even at high potential.The influence of anions on the ORR is investigated as well. Under transient conditions, thecatalytic behavior of Pt in H2SO4 and H3PO4 electrolyte solutions, respectively, are similar.That is, the specific anion adsorption can block the reactive sites for the ORR, thus leading toa decrease in activity as compared to that in HClO4 electrolyte solution. At steady state theORR activity is inhibited in all the three acid electrolyte solutions as compared to transientconditions. The ORR can reach its diffusion limited current in both HClO4 and H2SO4electrolyte solutions. However, in H3PO4, the ORR rates decreases so significant that nodiffusion limited current is observed. This is a considerable difference to the behavior inH2SO4 electrolyte solution. Furthermore the ORR activity is time dependent in H3PO4 andcontinues to decrease when the electrode potential is held. A combination of a site blockingmechanism and an increase in viscosity at the polarized electrode surface is proposed toexplain this extraordinary behavior in H3PO4.Atomic layer deposition of Pt onto an Au substrate is employed as model catalyst toinvestigate how ligand, ensemble, and strain effects affect the catalytic performance.Combining the experimental data with DFT calculations, it is seen that for one or two layersof Pt on the Au substrate, the ligand and ensemble effects play a dominant role, thusenhancing the catalytic activity. With increasing number of Pt layers, the strain effectbecomes the predominant factor, thus decreasing the catalytic activity.The last part of this thesis focuses on combining steady state tests and the design of catalysts.Shaped-controlled synthesized tetrahexahedral (THH) Pt nanoparticles are employed asmodel catalysts and their catalytic activity towards ORR is tested at both steady state andtransient conditions. Polycrystalline Pt and carbon supported Pt nanoparticles (Pt/C) serve asreference. The excellent performance of THH Pt nanoparticles at steady state is linked totheir resistance to oxide formation
UR - http://rex.kb.dk/KGL:KGL:KGL01009243397
M3 - Ph.D. thesis
BT - Systematic Investigations of the Oxygen Reduction Reaction on Pt Based Catalysts Comparing Transient and Steady State Performance
PB - Department of Chemistry, Faculty of Science, University of Copenhagen
ER -