"Understanding and Tuning Ionic Transport and Electrochemical Reactions on Air Electrodes of Solid Oxide Fuel Cells at the Nanoscale"

Hung-Sen Kang

Mechanical Engineering

University of California, Merced

Abstract

In this dissertation, I present two topics: (1) nanoscale engineering effects on cathodes for intermediate temperature solid oxide fuel cells (SOFC); (2) ionic diffusion and oxygen exchange at the surface and

through the bulk of oxygen ion-conducting media at the nanoscale via in situ atomic force microscopy

study.

Topic I: The impact of various infiltrates on the kinetics and rate-limiting step of oxygen reduction

reaction (ORR) is examined with LaNi0.6Fe0.4O3-δ (LNF) as the cathode backbone of SOFC. Multiple

materials were infiltrated on the backbone. The dependencies of electrode polarization resistance on the precursor concentration, temperature, and oxygen partial pressure are presented, and related

discussions are made to interpret the differences in ORR kinetics and the rate-determining step for ORR.

Topic II: The ionic transport in solid oxides is understood as a hopping process of point defects (i.e. oxygen vacancies). The kinetics is highly dependent upon lattice environment surrounding the moving ionic species. In this study, I report a direct quasi in situ observation of ionic transport in SrTiO3 (STO) and Y2O3-stabilized ZrO2 (YSZ)–the most widely used cathode material in intermediate temperature SOFC–by use of Kelvin probe force microscopy (KPFM) and conductive atomic force microscopy (CAFM), which enables space-resolved study of ionic diffusion. In addition, the impact of mechanical and electrical stimuli on the apex geometry of gold-coated tips and electrical conduction properties at the tip-substrate contact is discussed.

Biography

Hung-Sen Kang is a Ph.D. candidate in the Department of Mechanical Engineering. He joined Prof. Min Hwan Lee’s (E2C Lab) summer of 2016, and has since been working under his supervision to study nanoscale phenomena on solid oxide fuel cell electrodes. He has been using the atomic force microscope(AFM) to observe oxygen reduction and evolution reactions (ORR/OER) at the nano-scale particularly observing the impact of grain boundaries on ORR/OER kinetics and related ionic transport. He obtained his Master’s degree in Applied Physics from National Taiwan University and Bachelor’s degree in Physics from National Sun Yat-sen University in Taiwan.