Introduction

1.1 Introduction to fuel cells

Because of the expected shortage of fossil fuels and the urgent need of environment friendly energies, such as renewable energy resources, fuel cells receive extensive attention in recent decades. The working principle behind fuel cells strongly relies on chemical interactions between the fuels and the electrodes, and shares some common characteristics with primary batteries. Fuel cells are powerful energy sources combining numerous advantages of both conventional electricity generation engines and batteries, such as a long operation lifetime for the former and being clean and portable for the latter. Therefore, they are thought to be promising renewable energy resources to reduce the demand of traditional petrochemical energies, which will be eventually exhausted and are a major cause resulting in environmental pollution and the greenhouse effect.

In a simple fuel cell, the fuel electrochemical reaction is split into two half reactions:

H₂ ↹ 2H⁺ + 2e^- (1-1)

2

1

The electrons transferred from the fuel are forced to flow through an external circuit by spatially separating these reactions, thus constituting an electric current. Spatial separation is performed by utilizing an electrolyte, which is a medium that allows ions to flow but not electrons. A full fuel cell must have two electrodes separated by an electrolyte. The two electrochemical half reactions tale place in the two electrodes, respectively. In 1839, the first fuel cell, invented by William Grove, probably looked like an extremely simple H2-O2 fuel cell, as shown in fig. 1-1 [2]. This fuel cell consisted of two electrodes dipped into an aqueous acid electrolyte, such as H₂SO₄ or HClO₄. At the anode electrode, bubbling hydrogen gas is split into H⁺ and electrons following Equation

2

(1-1). The electrons flow from anode to cathode through external circulation that connects the two electrodes. Furthermore, the H⁺ can flow through the electrolyte, but the electrons cannot. When the electrons reach the cathode electrode, they recombine with H⁺ and bubbling oxygen gas to produce water following Equation (1-2). The flowing electrons will provide power to the load, such as a light bulb, and the simple fuel cell will produce electricity.

Fuel cells are classified into five major types which are based on the kinds of the electrolyte: Phosphoric acid fuel cells (PAFC); Polymer electrolyte membrane fuel cell (PEMFC); Alkaline fuel cell (AFC); Molten carbonate fuel cell (MCFC); Solid oxide fuel cell (SOFC). They all operate at different temperature regimens, incorporate different materials, and often differ in their fuel tolerance and performance characteristics, as shown in table 1-1 [2]. PEMFCs utilize a thin polymer membrane where protons can be saturated as the electrolyte and platinum-based materials are the prevailing catalysts for cathode and anode. Pure hydrogen and oxygen are used as the fuel in anode and cathode electrode, respectively. However, liquid fuels such as methanol, ethanol or formic acid are also considered because difficulties in the aspeck of storage and transportation is the extreme

Figure 1-1 A simple H2-O2 fuel cell.

3

weakness of hydrogen. In addition, direct methanol fuel cells (DMFCs) use methanol of high energy density as the fuel, and methanol has been extensively considered as renewable power source and is an attractive candidate for low-power portable fuel cell applications.

The advantages of DMFCs comparing with several kinds of fuel cells are the operation at low temperatures, easy fuel storage, and the reduced volume and weight in equipment.

Even if there are many advantages, DMFCs used Pt as the catalysts in the practical commercialization is also impeded by many difficulties such as methanol crossover, the high cost of the Pt catalyst, the inaction of the oxygen reduction reaction and the low efficiency of the electroactivity of MOR due to CO poison.

Table 1-1 Description of major fuel cell types [2]

PEMFC PAFC AFC MCFC SOFC

Catalyst Platinum Platinum Platinum Nickel Perovskites (ceramic)

Cell electrocatalytic activity of anode with a minimized Pt loading. Researchers try to reduce the size and optimize the distribution of Pt nanoparticles to minimize the use of t he preious Pt catalyst and increase concurrently the electroactivity surface area (ESA) for methanol oxidation on the catalyst. Another major approach to improving the MOR electroactivity

4

is to increase the resistance of Pt catalysts against CO poisoning, which results from catalytic site blocking by carbonaceous byproducts due to incomplete methanol oxidation.

On the other hand, to improve the CO tolerance, Pt-based binary or ternary alloys commonly used as catalyst in considerable studies, in particular Pt-Ru, are to enhance CO electro-oxidation via the bi-functional mechanism, which governs the electro-oxidation reaction of the carbonaceous byproducts with neighboring OH adspecies, and/or the so-called electronic effect [3-10]. A fast removal rate of the carbonaceous adspecies can continuously create free adsorption sites for methanol molecules, thus resulting in a high reaction rate of methanol electro-oxidation. The removal rate speedily of the carbonaceous adspecies can be continuously created the free adsorption sites for methanol molecule which results in the high reaction rate of methanol electro -oxidation. More transition metals and metal oxides as a promoter or the catalyst support are, more the CO tolerance of the Pt catalyst can be gained effectively which is based on a similar principle [3-22].

Figure 1-2 schematically illustrates the likely reaction steps of CO oxidation on the Pt nanoparticles via the bi-functional mechanism. Dissociative adsorption of water

Figure 1-2 Schematic diagram illustrating the bi-functional mechanism of the Pt nanoparticle and the metal oxide support on CO oxidation.

5

molecules on the metal oxide support creates OH surface groups. The groups adjacent to Pt nanoparticles may readily oxidize CO groups bonded on the Pt surface. In the thesis, we used TiO2, PdO and NiO nanostructured thin films as the support of Pt nanoparticles and studied the effect of the oxide supports on the enhancement of the electrocatalytic activity of the Pt catalyst toward MOR [20, 23]. We ascribed the observed electroactivity enhancement to the synergism of the electronic effect and the bi-functional mechanism, which was a result in electronic and chemical interactions between the hydrous oxides and the Pt nanoparticles.

6