Dynamics at the electrified solid/liquid interface: self-organization and dissolution at platinum electrocatalysts

Author(s)

Jéssica Alves Nogueira

• Document type: Doctoral Thesis
• Press: São Carlos.
• Institution: Universidade de São Paulo (USP). Instituto de Química de São Carlos (IQSC/BT)
• Defense date: 2020-04-16

Examining board members

Hamilton Brandão Varela de Albuquerque; Joelma Perez

• Advisor: Hamilton Brandão Varela de Albuquerque

Abstract

Some aspects of the dynamics at the electrified solid/liquid interface are explored in this thesis. In particular, the studies are divided into two main parts: the self-organized variations in potential and the degradation of platinum electrocatalysts. Chapters I and II deal with autonomous oscillatory kinetics in fuel cells based on the electro-oxidation of small organic molecules. By means of an external reference electrode, we observed oscillations at the cathode, which start simultaneously with the dynamics in the voltage of a direct formic acid and methanol fuel cells. We attribute this phenomenon to two coupling processes: through the concentration of protons and through the system control mode (galvanostatic). To evaluate the temperature effect, the cathode interference was minimized by using it as a dynamic hydrogen electrode. We studied four molecules: CH3OH, HCOOH, CH3CH2OH and CH3OCH3, and only the electro-oxidation of the latter did not exhibit oscillatory kinetics. The relationship between oscillatory frequency and temperature during the methanol and formic acid electro-oxidation followed a conventional Arrhenius dependence, whereas with ethanol there was temperature compensation. The atypical behavior of ethanol was addressed taking into account its main catalytic poisons: adsorbed CO and acetate. The absence of oscillations during dimethyl ether electro-oxidation is probably due to its weak interaction with Pt surface. Chapters III, IV and V explore Pt stability by means of inductively coupled plasma mass spectrometry (ICP-MS) and by scanning tunneling microscopy (STM). First, we evaluated how the composition of acid electrolytes affects the dissolution of the basal planes of Pt. Although specific anion adsorption, (H)SO4-, CH3COO- and Cl-, decreases the oxide formation, it does not imply less dissolution. There is a balance between the anion driving force towards Pt complex formation and its influence on irreversible oxide formation. We explored the CO electrochemical annealing process by monitoring Pt dissolution during bulk CO electrooxidation. We proposed that the increase in Pt dissolution triggered by CO is due to several phenomena occurring simultaneously: an unstable surface due to the reduction of oxides; CO readsorption; and dynamic phase transitions at the adsorbed CO adlayer. Since CO is also an intermediate during the electro-oxidation of CH3OH and HCOOH, we evaluated the effect of these reactions on Pt stability. In both cases there is an increase in dissolution, attributed to its main intermediates: adsorbed CO and formate. Finally, we overlapped the two parts of this work by evaluating oscillations and stability. In the potential window where the oscillatory electro-oxidation of CH3OH and HCOOH take place, there is no Pt dissolution: there is the selfcleaning process of the electrode without one of its main degradation mechanisms. (AU)

• FAPESP's process: 15/09295-9 - Dynamic behavior in low-temperature fuel cells based on oxidation of small organic molecules
• Grantee: Jéssica Alves Nogueira
• Support Opportunities: Scholarships in Brazil - Doctorate