Mechanistic Understanding And Fine-tuning Of Pt/C Electrocatalysts For Oxygen Reduction Reaction

Fuel cells have emerged as a highly efficient and clean technology to enable the conversion of chemical energy into electrical energy.The whole electrochemical process is limited by the sluggish kinetics of cathodic oxygen reduction reaction(ORR).Although commercial Pt catalysts demonstrate relatively high catalytic activity for this reaction,their wide applications are limited by the high Pt loading and low catalyst stability.Moreover,the air pollutions such as the particulate matter less than 2.5 μm(PM2.5)also significantly affect the catalytic performance for the practical application of fuel cells.Considering the typical structure-sensitive characteristic of Pt/C catalyzed ORR system,it is highly desirable to understand the effects of the sizes of Pt nanoparticles together with their distributions,surface chemistry of carbon supports,catalyst surface-interface structure,and foreign environments on the catalytic performance.In this dissertation,the catalytic activity is expressed as the number of active sites(N)over Pt/C catalyst and turnover frequency per site(TOF),i.e.,r＝Nactive site×TOFactive site.This aims to elucidate the underlying nature of the structure-sensitive reaction,further establishing the regulation law of catalytic performance based on mechanistic understanding and fine-tuning of the active sites.The insights revealed here could shed new light on the development of cost-effective and stable Pt/C ORR nanocatalysts.The main results are summarized as follows:(1)Identification of active sites.A methodology by combining experiments,density functional theory(DFT)calculations with model calculations is developed to clarify the size-dependent ORR activity and selectivity over the Pt/CNT catalysts.It is found that the Pt nanoparticles supported on CNT possess a well-defined truncated octahedron shape in most cases and similar electronic properties.The observed size-insensitive TOFactive site based on the number of Pt(111)atoms suggests the Pt(111)surface as the dominant active sites.Moreover,the Pt(111)surface is also suggested as the dominant active sites for the formation of H2O2.The difference in the H2O2 selectivity mainly originates from the difference in the Pt binding energy,and the defect-rich CNT supported Pt catalyst with the higher Pt binding energy facilitates the oxygen reduction to form H2O.(2)Engineering of active sites.Three differently sized CNT-supported Pt nanoparticles(Pt/CNT)are prepared by both atomic layer deposition(ALD)and impregnation methods.The performances of the catalysts toward the ORR in the acidic media are comparatively studied to probe the effects of the sizes of the Pt nanoparticles together with their distributions,electronic properties,and local environments.The ALD-Pt/CNT catalysts show much higher ORR activity and selectivity than the impregnation-Pt/CNT catalysts.This outstanding ORR performance is ascribed to the well-controlled Pt particle sizes and distributions,the desirable Pt0 4f binding energy,and the Cl-free Pt surfaces based on the electrocatalytic measurements,catalyst characterizations and model calculations.(3)Interfacial effects.A fabrication strategy is proposed by employing high-surface-area and defect-rich vulcan carbon(VC)as the support to immobilize Pt-CoO ORR electrocatalyst using ALD method.The sequential growth of CoO onto the Pt/VC catalyst elaborately constructs the unique Pt-CoO interface and facilitates the electron transfer from CoO to Pt,and the as-obtained Pt-CoO/VC catalysts exhibit superior ORR activity,selectivity and stability as well as higher Pt utilization efficiency than the commercial 20 wt%Pt/C catalyst.Additionally,volcano-type relationships are established between the Pt particle size together with Pt0 4f binding energy and catalytic activity/selectivity,which could be valuable for the fine-tuning of Pt/C ORR electrocatalysts.(4)Ions effects and deactivation mechanism.The underlying nature of the effects of representative water-soluble ions in simulated PM2.5 pollutions on the Pt/C-catalyzed ORR are studied in an acid media by combining simulated experiments,Tafel kinetics with DFT calculations.The Cl-ion is discriminated as the major obstacle for the O2 adsorption and reduction,which are correlated with both the catalyst active site blocking and negative electronic property.The adsorbed Cl-also leads to the change in the rate-determining step from the formation of OOH*to the protonation of O*,since it has an excessively weakening ability for the adsorption and protonation of the oxygen-containing reaction intermediates.To improve the anti-poisoning ability of Pt/C catalyst based on the poisoning mechanism proposed,a high-efficiency and stable Pt/C nanocatalyst is synthesized by ALD method as a promising candidate of ORR electrocatalysts for the practical applications of fuel cells.