Electrochemistry of proton-exchange-membrane electrolyte fuel cell (PEMFC) electrodes

Jia, Nengyou (1999) Electrochemistry of proton-exchange-membrane electrolyte fuel cell (PEMFC) electrodes. Masters thesis, Memorial University of Newfoundland.

[English] PDF (Migrated (PDF/A Conversion) from original format: (application/pdf)) - Accepted Version Available under License - The author retains copyright ownership and moral rights in this thesis. Neither the thesis nor substantial extracts from it may be printed or otherwise reproduced without the author's permission. Download (15MB) [English] PDF - Accepted Version Available under License - The author retains copyright ownership and moral rights in this thesis. Neither the thesis nor substantial extracts from it may be printed or otherwise reproduced without the author's permission. (Original Version)

Abstract

The purpose of this research is mainly focused on four areas of the electrochemistry of PEMFCs: (1) developing a fast method to screen catalysts, (2) modifying carbon-supported Pt catalysts to improve their performance; (3) demonstrating a new method to accurately measure the catalyst active surface area by cyclic voltammetry; (3) reducing MeOH crossover in direct methanol fuel cells by modifying the electrolyte membrane. -- In order to achieve the above goals, a new electrochemical apparatus was designed to study the oxygen reduction reaction (ORR) on gas diffusion electrodes. Different kinds of electrode & membrane assemblies (MEA) were made at various platinum loadings and different catalysts (10 % and 20 % Pt on Vulcan XC-72R) using the hot bonding method. Several electrochemical techniques (cyclic voltammetry, transient and steady state polarization, and ac impedance) have been used to evaluate the key parameters that determine the oxygen reduction performance of the electrodes, such as active catalyst surface area, Tafel slope, and resistance. Polarization results indicate that a self-written program for collecting current provides useful data for both transient and steady state polarizations. The transient (10s) polarization can fully represent the electrode performance rapidly and reliably. AC impedance is a useful technique for measurement of the cell performance. -- Commercial carbon-supported Pt catalysts were modified by chemical oxidation (HNO₃, H₂SO₄, H₃PO₄, HCIO₄, NaCIO, H₂O₂, or (NH₄)₂S₂O₈) to enhance the catalytic activity for oxygen reduction reaction. Catalysts modified by either acidic or non-acidic oxidants can significantly improve ORR performance. The catalysts treated by acids, especially by HNO₃, increase the number of carbon surface functional groups such as - OH and - COOH allowing protons more easy access to the catalyst surface. The improved ionic conductivity on the catalyst surface leads to a higher performance based on the triple contact mechanism for the ORR in the catalyst layer. -- Furthermore, a technique using CO₂ electroreduction/reoxidation was evaluated to accurately measure the active surface area of carbon-supported catalysts as an alternative to the conventional use of hydrogen adsorption. The reduced CO₂ oxidation charge can be used to compare the active surface areas of different catalysts although charge should be used due to the complexity of the products of CO₂ reduction. This method retains the resolution advantage of the CO stripping method, and avoids the overestimation of the active surface area. Moreover, peak and onset potentials for reduced CO₂ oxidation are shown to be representative parameters for explaining catalyst tolerance to MeOH and CO, especially the oxidation onset potentials. -- Finally, membranes modified with conducting polymers by different polymerization methods such as Fe³⁺, (NH₄)₂S₂O₈, H₂O₂, and O₂ with UV irradiation were evaluated in terms of ionic conductivity and permeability to methanol. Electrochemical measurements showed that the modified membranes can significantly reduce methanol crossover. The inhibition of methanol crossover is realized by a lower methanol diffusion coefficient in the membrane as measured by chronoamperometry. The stability of modified membrane needs to be evaluated under practical operating conditions of direct methanol fuel cells.

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