Date of Award

6-30-2016

Document Type

Open Access Dissertation

Department

Chemical Engineering

First Advisor

Branko N. Popov

Abstract

With the limited fossil fuel reserve and increased power demand, polymer electrolyte membrane fuel cells (PEMFC) have been considered to be a promising alternative to the current energy consumption mode due to its high energy conversion, high efficiency, and zero emissions. However, high cost, poor stability, and sluggish kinetic for oxygen reduction reaction (ORR) of Pt/C cathode catalysts are obstacles for the commercialization of PEMFC for automotive application. The observed poor stability is attributed to a corrosion of carbon supports due to low pH, high temperature, and high anodic potentials (1.0-1.5 V) at the cathode interface during start-up/shutdown conditions. Electrochemical oxidation of carbon results in carbon loss leading to Pt detachment/sintering and subsequent loss of electrochemical surface area (ECSA). Another contributing factor is Pt and/or alloying element dissolution and particle sintering in operating conditions (0.6-1.0 V).

In this study, a support material, a Pt catalyst and a compressive Pt lattice catalyst were optimized to develop an active and stable cathode catalyst for PEMFC. A carbon composite catalyst (CCC) was developed from high surface area carbon black (HSACB), which has unique ORR activity and stability compared to those of HSACB. By using CCC support for Pt/C catalysts, the support stability was improved significantly. Also, transition metals embedded in CCC structure were used to synthesize the compressive Pt catalyst by using USC’s novel method. The catalyst indicated improved activity when compared with pristine Pt catalyst.

To further enhance activity and stability, a novel activated carbon composite support (ACCS) was developed by optimizing surface area, pore-size distribution, as well as the degree of graphitization and the hydrophobicity. Pt deposition on the ACCS was optimized using a modified polyol process developed in our laboratory in order to control Pt particle size and Pt particle distribution. Fuel cell performance and stability of Pt/ACCS were evaluated using accelerated stress test (AST) protocols recommended by the US Fuel Cell Tech Team for both the catalyst and the support. The Pt/ACCS catalyst showed improved activity and excellent support stability at 1.0-1.5 V over those of commercial catalysts due to the controlled Pt particles and optimized properties of ACCS. Also, a compressive Pt catalyst (Pt*/ACCS) was developed to further increase activity and stability at 0.6-1.0 V. Pt*/ACCS was prepared using the in-house developed procedure in which Co diffuses into the Pt/ACCS catalyst followed by controlled heattreatment. The pyrolysis temperature and Pt/Co ratio were optimized to initiate formation of compressive Pt catalyst. A protective coating method was used to inhibit particle growth during heat treatment which maintains the catalyst particle size in the range between 3 and 5 nm. Pt*/ACCS showed enhanced catalyst stability at 0.6-1.0 V over that of Pt/ACCS while keeping good performance and good support stability. The good stability of Pt*/ACCS is attributed to a potential shift of Pt oxide formation to a more positive direction which results in less Pt dissolution due to less reduction of Pt oxide when the catalyst is cycled in cathode direction from 1.0 to 0.6 V.

Rights

© 2016, Taekeun Kim

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