S-Space College of Engineering/Engineering Practice School (공과대학/대학원) Dept. of Chemical and Biological Engineering (화학생물공학부) Theses (Ph.D. / Sc.D._화학생물공학부)
Electrochemistry in Oxygen Reductions for Pt-Based Nanoparticles Prepared by Chemical Methods
화학적 방법으로 준비된 백금 기반 나노입자에 대한 산소환원의 전기화학
- 공과대학 화학생물공학부(에너지환경 화학융합기술전공)
- Issue Date
- 서울대학교 대학원
- electrochemistry; electronic structure; fuel cell; nanoparticle; oxygen reduction; platinum
- 학위논문 (박사)-- 서울대학교 대학원 : 화학생물공학부 에너지환경 화학융합기술 전공, 2013. 8. 성영은.
- The proton exchange membrane fuel cell (PEMFC) would be potentially suitable as a sustainable power source to substitute fossil fuel for automobiles and portable devices. For commercialization of the PEMFC, the oxygen reduction reaction (ORR) is one of the most significant problems to be overcome due to a considerable high overpotential for Pt, known as the highest electrocatalytic activity of the ORR. According to the theory, the overpotential of the ORR for Pt(111) is originated from the adsorbed oxygen containing species tend to be so stable at near equilibrium potential. As a result, there are many attempts to weaken the adsorption strength onto Pt by modifying the surface d-band structure.
For a practical application, the preparation of Pt-based nanoparticles supported on carbon has been intensively investigated for several decades. To obtain nanoparticles, the nuclei formed from the precursor should be grown slowly. If the growth rates cannot be controlled, the dissolved particles will diffuse toward larger particles and deposit to minimize the surface free energy through a process known as Ostwald ripening. Once Ostwald ripening occurs, the particle size distribution increases. The synthetic environments including, organic solvents, capping, and reducing agents can be stabilized on the surface of the nanoparticles by lowering the rate of growth through either steric or electrostatic stabilization. However, the capped organic species were strongly adsorbed on the surface of Pt-based nanoparticles resulting in tuning the electrochemical reactions. Considering electrochemistry in the ORR is significantly influenced on the electronic structures, the synthetic environment must be primarily important factors, that is, the preparation stage, as-prepared state, and post-treatment process due to alteration of electronic structure. In this thesis, we investigated the relationship between electrochemistry in oxygen reductions and electronic structure for Pt-based nano-catalysts in accordance with these factors.
Concerning the Pt-based nano-catalysts in the preparation stage, the Pt1Fex (x = 1, 2, and 3) nanoparticles were studied. The aim of this part is primarily on understanding of the effect of the surface composition for the electrocatalytic activity. To clarify this correlation, we compared two types of nanoparticles with the different surface composition, the Fe-rich and the Pt-Fe mixed surface. We synthesized highly dispersed carbon supported Pt1Fex nanoparticles with the Fe-rich surface (~ 2 nm) through a preferential interaction of a capping agent and metal species, i.e., Fe-OOC. Because of the phase separation of Pt and Fe species through the preferential interaction of the capping molecule, the electrocatalytic properties of nanoparticles were not significantly changed despite the various Pt/Fe ratios. Otherwise, nanoparticles with the Pt-Fe mixed surface, prepared by the difference of segregation energy, demonstrated that the electrochemical characteristic were significantly altered due to the interaction between Pt and Fe species on the surface of electrocatalysts. In particular to the ORR, the Pt1Fe2 nanoparticle with the Pt-Fe mixed surface showed the highest enhancement for the specific activity compared to Pt, resulting from the development of the stable fcc PtFe phase. This result indicates the electrocatalytic activity of the bimetallic nanoparticles decisively is determined the surface composition rather than the bulk composition.
Next, we scrutinized the as-prepared state of nano-catalysts. Capping organic molecules including oleylamine, strongly adsorbed onto Pt nanoparticles during preparation steps, are considered as an undesirable species for the oxygen reduction reaction due to decreasing electrochemical active sites. However, we found the small amount of oleylamine modified platinum nanoparticles showed the significant enhancement of the electrochemical activity oxygen reduction reaction, even with the loss of electrochemical active surface area. The enhancement was correlated with downshift of frontier d-band structure of platinum and the retardation of competitively adsorbed species. These results suggest that a capping organic molecule modified electrode can be a strategy to design an advanced electrocatalyst by modification of electronic structures.
Finally, we examined how to affect the post-treatment process, i.e., thermal annealing after the preparation of PtNi nanoparticles into the two variation, temperature and atmosphere. Firstly, we explored that the temperature of thermal annealing is how to influence the ORR behaviors. The reconstruction of PtNi nanoparticles, that is, intermetallic ordering and surface reorientation into the (111) facet, were developed by the heat treatment at the different thermal energy without severe agglomeration. The enhancement of electrocatalytic activity was correlated with this structural change. Also, electrochemically stable structure was formed after the reconstruction resulting in increasing intermetallic interaction between Pt and Ni. This result explained how to enhance the ORR properties by the thermal annealing process and proposed the way to design the advanced electrocatalysts. Secondly, the purpose of this part is mainly on tuning the oxygen reduction activity of Pt2Ni1 nanoparticles induced by the heat treatment atmosphere. From the observation of the X-ray spectroscopic measurement, it is revealed that the electronic structures were varied with the heat treatment condition. The argon treated electrocatalyst demonstrated the highest catalytic activity in the half-cell measurement owing to the appropriate electronic interaction between Pt and Ni. This result suggested that the alteration of electronic structures induced by the heat treatment atmosphere was decisively influenced to the ORR activity particularly in the presence of the specific adsorption.