Chapter 1 Introduction
1.2 Motivation
Recently, substantial attention has been paid to the development of fuel cells with solid polymeric electrolytes that operate on liquid fuel, in particular, methanol, in which methanol directly oxidizes on the anode, i.e. the so-called direct methanol fuel cells (DMFCs).
Although the technology of such fuel cells was developed to an extent, some fundamental problems remain unclear and only their solution can result in wide-scale commercialization of DMFC, especially as the efficient current sources for portable devices. The most important problem is the development of efficient catalysts, which would provide the long-term (for several thousand hours) operation of fuel cells without sacrificing their characteristics.
However, their sluggish electrochemical reactions result in inevitable high Pt-catalyst loading and low catalyst utilization, in addition to the well-known methanol crossover problem. It is essential to improve Pt-catalyst utilization in DMFCs, because the high cost of Pt-based catalyst understandably dominates the material cost of membrane electrode assembly (MEA) for low temperature fuel cells. There are two way to overcome this problem. First, the synthesis of ordered nanostructure materials with high surface area as a supports of Pt or Pt alloy catalysts. Secondly, direct synthesis of nanostructure materials (Pt or Pt alloy catalysts) with high surface area. Moreover, an appropriate synthetic route ultimately determines the success or failure of nanostructured materials synthesis, because the physical properties and applications of nanostructured materials are heavily dependent upon their synthetic method.
As a result, there have been tremendous efforts toward the development of new synthetic methodologies for several decades.
In this study, we have proposed very simple methods for fabrication and synthesis of nanostructured materials for supporting nanoparticles Pt or Pt alloy catalysts. Well-ordered arrays of Si nanocones (SNCs) were fabricated using the combination of anodic aluminum oxide (AAO) templating and dry etching techniques. The self-organized nanodot array of
titanium oxide (TiOX) in use is prepared from TiN/Al film on the silicon substrate by electrochemical anodization. This novel method can not only reveal highly ordered nanostructures but also overcome lithography limitation. TiOX nanodots were then used as nanomasks to etch TiN layers and the underlying layers in an inductively coupled plasma reactive ion etch (ICP-RIE) system. The ICP-RIE is a plasma-based dry etching technique characterized by a combination of physical sputtering and chemical activity of reactive species. Owing to the well-controlled etching depth and profile for nanostructures, TiOx
nanodots were used as nanomasks to fabricate well ordered SNC arrays. Then Pt nanoparticles were electrochemically deposited on the SNCs for fuel cell electrode. An amorphous carbon coated Si nanocones (ACNCs) were used to obtain well-dispersed Pt nanoparticles with high mass activity, where bipolar pulse electrodeposition was used to deposit Pt nanoparticles.
In order to increase the performance of DMFCs electrode, we synthesized the graphitic carbon (g-C) with a high surface area on the Si substrate and thus were used as a support of Pt and Pt-Ru alloy catalysts.
We have also developed new simple and efficient techniques for direct synthesis of Pt nanostructures. In this regards, we have synthesized the two-dimensional (2D) continuous Pt island networks and three-dimensional (3D) Pt nanoflowers for DMFCs electrodes. In addition, we have synthesized the shape-controlled Pt nanoparticles by fasten silicon and study there electrochemical performance for DMFCs application.
Chapter 2
Literature Review 2.1 Fuel cells
2.1.1 Introduction
The first fuel cell was demonstrated by Sir William Grove in 1839. Namely, a fuel cell is an electrochemical cell that directly converts chemical energy into electric energy. Hence, a fuel cell is a converter which enables the energy conversion via the electrochemical reaction, like the well-known electrochemical batteries. However, fuel cells are unique in that they consume reactants, which must be replenished, while batteries store electrical energy chemically in a closed system. Additionally, while the electrodes within a battery react and change as a battery is charged or discharged, a fuel cell's electrodes are catalytic and relatively stable. But its commercialization has been limited by high cost, material limitations, and low operational efficiencies. The first successful application of a fuel cell was demonstrated by NASA in the Gemini and Apollo space programs as a way to deliver potable water to the astronaut crew [27]. Today research has focused on developing fuel cells for stationary, automotive, portable, and military power applications. Fuel cells are attractive because they provide an innovative alternative to current power sources with higher efficiencies, renewable fuels, and a lower environmental cost.
2.1.2 Principles of fuel cells
A schematic representation of a various types of fuel cells is shown in Fig. 2-1. The main active components of a fuel cell are fuel electrode (anode), oxidant electrode (cathode), and electrolyte sandwiched between them. Figure 2-1 shows the basic operational principle of a fuel cell with the reactant/product gases and the ion conduction flow directions. In a typical
fuel cell, fuels are fed continuously to the anode (negative electrode) and an oxidant (i.e., pure oxygen or air) is fed continuously to the cathode (positive electrode); the electrochemical.
Reactions take place at the electrodes. The fuel is oxidized at the anode to gives up electrons which travel through the external load to provide the power while ions migrate through the electrolyte from one electrode to the other. On the cathode, the oxidant combines with the ions and incoming electrons by a reduction reaction to complete the process. Usually, the basic physical structure of an electrode consists of an electrolyte layer in contact with a porous anode and cathode on each side. Triple phase boundaries are established among the reactants, electrolyte, and catalyst in the region of the porous electrode. The nature of this interface plays a critical role in the electrochemical performance of a fuel cell, particularly in those fuel cells with liquid electrolytes. In such fuel cells, the reactant gases diffuse through a thin electrolyte film that wets portions of the porous electrode and react electrochemically on their respective electrode surface. In order to maximize the efficiency of fuel cell, a delicate balance must be maintained among the electrode, electrolyte, and gaseous phases in the porous electrode structure. The electrolyte in fuel cell not only transports dissolved reactants to the electrode, but also conducts ionic charge between the electrodes and thereby completes the cell electric circuit. In addition, it also provides a physical barrier to prevent the fuel and oxidant gas streams from directly mixing.
The simplest and most common reaction encountered fuel cell reaction is:
H2 + ½ O2 → H2O (2.1) From a thermodynamic point of view, the maximum-electric work obtained from the above reaction corresponds to the free-energy change (available energy in an isothermal process) of the reaction. Gibbs-free energy is more useful than the change in Helmholtz-free energy, since it is more practical to carry out chemical reactions at a constant temperature and pressure rather than constant temperature and volume.
The above reaction is spontaneous and is also thermodynamically favored because the free
energy of the products is less than that of the reactants. The standard free energy change of the fuel cell reaction is represented by the equation:
ΔG = ─ nFEr (2.2) where ∆G is the free energy change, n is the number of moles of electrons involved, Er is the reversible potential, and F is Faraday’s constant. If the reactants and the products are in their standard states i.e., at a temperature of 25○C and 1 atm pressure, the equation can be rewritten as:
∆G = ─ nFE○r (2.3)
Accordingly, the reversible cell voltage (E○r) of a fuel cell can be calculated from:
E○r = ─∆G○/nF (2.4) For the reaction (2.1), ∆G○ is –229 kJ/mol, n = 2, F = 96,500 C/eq and, hence, the calculated value of E○r is ~1.29 V.
Water is the product of the fuel cell reaction and can be produced either as liquid water or steam. The higher-heating value (HHV) corresponds to the released heat when water is produced as liquid water and the lower-heating value (LHV) when water is produced as steam.
The difference in the HHV and LHV is the heat required to vaporize the product water.
2.1.3 Types of fuel cells
There are a number of different fuel cells being investigated, which are classified based on the electrolyte used. Each type of fuel cell has intended applications based on power output limitations, operating temperature, and size of the power system. There are presently five major fuel cell types at varying stages of development and commercialization: (1) alkaline fuel cell (AFC), (2) phosphoric acid fuel cell (PAFC), (3) solid oxide fuel cells (SOFC), (4) molten carbonate fuel cells (MCFC), and (5) polymer electrolyte membrane fuel cells (PEMFC). Figure 2-1 shows the operational characteristics of each fuel cell family. A brief
description of different electrolyte cells is as follows.
[1] Alkaline fuel cell (AFC)
The AFCs typically utilize KOH as the electrolyte with the concentration of 35-50 wt%
and have the highest electrical efficiency of all fuel cells but suffer economically from the necessity for ultra pure gases for its fuel. The electrolyte is retained in a matrix (usually asbestos), and a wide spectrum of electrocatalysts can be used (e.g., Ni, Ag, metal oxides, spinals, and noble metals). The operating temperature for AFCs is below 100○C, but higher temperatures are desirable for improved hydrogen oxidation kinetics. Furthermore, the AFCs are among the first fuel cells to have been studied and taken into development for practical applications, and they are the first type of fuel cells to have reached successful routine applications, mainly in space programs such as space shuttle missions. However, almost all of the AFC development activities have come to an end now.
Figure 2-1 Simplistic representation of a various types of fuel cells.
The major challenge is that alkaline electrolytes, like potassium or sodium hydroxide, do not reject carbon dioxide, even the 300-350 ppm of carbon dioxide in atmospheric air is not tolerated, whereas terrestrial applications almost invariably require the use of atmospheric air as oxidant due to technical and economic considerations. The expected power output of an AFC is in the range of tens of MW [27].
[2] Phosphoric acid fuel cell (PAFC)
The PAFCs are commercially the most advanced system due to simple construction and the thermal and chemical stability of the phosphoric acid electrolyte at an operating temperature in the range of 150-200○C. Phosphoric acid concentrated up to 100% is used as the electrolyte in PAFC. The matrix universally used to retain the acid is silicon carbide, and the electrocatalyst in both the anode and cathode is the platinum catalyst. The stability of concentrated phosphoric acid is relatively high compared to other common acids;
consequently, PAFC is capable of operating at the high end of the temperature range (100 to 220○C) among the acid-type fuel cells. It is mainly used for stationary power ranging from dispersed power to on-site generation plants. Power outputs of 0.2-20 MW are able to supply shopping malls and hospitals with electricity, heat and hot water and are commonly used as primary or backup power for these sites [27].There are several disadvantages associated with the PAFC design. These include the need to use the expensive noble metal, platinum, as electrodes. Furthermore the electrodes are susceptible to CO poisoning and the electrolyte in the fuel cells is a corrosive liquid which, is consumed during operation [28].
[3] Solid Oxide Fuel Cell (SOFC)
The electrolyte in this fuel cell is a solid, nonporous metal oxide, usually Y2O3- stabilized ZrO2. The fuel cell operates at 800-1000○C where ionic conduction by oxygen ions takes place. The high-temperature operation results in fast electrochemical kinetics and no need for
noble-metal catalysts. Typically, the anode is Co-ZrO2 or Ni-ZrO2 cermet, and the cathode is Sr-doped LaMnO3. The fuel may be gaseous hydrogen, H2/CO mixture, or hydrocarbons because internal in-situ reforming of hydrocarbons with water vapor can occur at high temperatures.
The SOFC has been investigated for applications ranging from industrial and home generators, telecommunication systems, and in hybrid electric vehicles. The SOFC provides high-quality waste heat that can be utilized for cogeneration applications or combined cycle operation for additional electric power generation. The operating condition of SOFC is also compatible with the coal gasification process, which makes the systems highly efficient when using coal as the primary fuel. It has been estimated that the chemical to electrical energy conversion efficiency is 50–60%, even though some estimates go as high as 70%–80%. Also, nitrogen oxides are not produced, and the amount of carbon dioxide released per kWh is around 50% less than for power sources based on combustion because of the high efficiency.
There are several advantages to using SOFC systems for practical power generation as compared with the other types of fuel cell. SOFCs have a solid electrolyte, which eliminates the corrosion and liquid management problems of the PAFCs [27].
[4] Molten carbonate fuel cell (MCFC)
The MCFCs uses liquid lithium potassium or lithium sodium carbonate stabilized in a matrix as the electrolyte for the system, which is supported on Al2O3 fibers for mechanical strength. The fuel cell operates at 600 to 800○C where the alkali carbonates form a highly conductive molten salt, with carbonate ions providing ionic conduction. At the high operating temperatures in MCFCs, Ni (anode) and nickel oxide (cathode) can promote reaction;
therefore, noble metals are not required.
MCFCs are aimed at stationary application such as the distributed power plants for industrial and commercial applications. The MCFCs system can attain efficiencies of up to
50 %, or up to 70 % with the combination with other power generators. MCFCs can operate on a wide range of different fuels and are not prone to CO or CO2 contamination, as is the case for low temperature cells.
[5] Polymer electrolyte membrane fuel cell (PEMFC)
The PEMFCs uses a proton-conducting polymer membrane as the electrolyte at an operating temperature of 80-105○C. The most commonly used reactants for this system are hydrogen and methanol. Due to its low operating temperature, hydrogen and methanol fuel cells are popular for use in automotive and portable electronic applications. The standard membrane is a perfluorinated sulfonic acid membrane developed by DuPont and trademarked as Nafion®. The platinum catalyst is widely employed as the catalyst in both the anode and cathode electrode.
Moreover, the PEMFC has fast-start capability and yields the highest output power density among all types of the fuel cells. Because of the solid membrane as the electrolyte, there is no corrosive fluid spillage hazard, and there is lower sensitivity to orientation. It has no volatile electrolyte and has minimal corrosion concerns. It has truly zero pollutant emissions with potable liquid product water when hydrogen is used as fuel. As a result, the PEMFC is particularly suited for vehicular power applications, although it is also being considered for stationary power applications to a lesser degree [29]. The fuel – gas hydrogen can be produced by steam reforming or partial oxidation of hydrocarbons; however, reforming these fuels produces impurities such as carbon monoxide, to which the Pt-based electro-catalyst have very low tolerance. Even a trace amount of CO drastically reduces the performance levels, although CO poisoning effect is reversible and does not cause permanent damages to the PEMFC system [30]. Furthermore, the performance reduction due to CO poisoning takes a long time (on the order of two hours) to reach steady state. This transient effect may have profound implication for transportation applications. Therefore, the PEMFC
requires the use of a fuel virtually free of CO (must be less than a few ppm). Purifying the fuel stream through a water gas shift reaction and preferential oxidation adds both extra equipments and costs to a fuel cell system. Therefore, a fuel cell researcher’s dream is to directly electrooxidize an organic fuel, rather than process it to hydrogen. Methanol is one alternative because it is a common, widely used and inexpensive substance that can be easily achieved from natural gas. Methanol offers other advantages over gas hydrogen, including liquid state at room temperature, ease to fuel-feeding, distribute and store. Hence, direct methanol fuel cell (DMFC) can be regarded as a subcategory of PEMFC, in which liquid methanol is directly converted into the electrical energy without the further use of reformer. In this thesis we shall focus our attention on the DMFCs and described in more detail below.
2.2 Direct methanol fuel cell (DMFCs)
2.2.1 Introduction
DMFCs using polymer electrolyte membranes are promising candidates for transportation applications and portable power sources such as replacing batteries. By eliminating reformer, DMFC offer simple system design and potentially higher overall efficiency than the reformate-fed fuel cells. Significant advances in H2/air polymer electrolyte fuel cells have been reported including the low electrocatalyst consumption and high power density.
However, fuel processor-fuel cell stack system on board the vehicle presents problems of packaging, complexity, and an overall system efficiency significantly lower than that of the fuel cell itself. Moreover, methanol is the liquid fuel that has substantial electroactivity and can be directly oxidized to CO2 and water on catalytically active anodes in DMFC. In addition to high efficiency and environmental compatibility, liquid methanol is inexpensive, widely available and can be handled and distributed to consumers to such an extent that the present supply networks of gasoline can be used for methanol without difficulty. There are several
conditions, which fuel cells must meet in order to become a real alternative to the internal combustion engines. One of the most important requirements is system size, because of the need to generate sufficient power within limited space on car board. This requirement is not met by liquid electrolyte fuel cells as they have considerably lower power density than solid electrolyte fuel cells. Highly efficient molten carbonate and solid oxide electrolytes, operating at temperatures in the range 700 to 1000○C require extended, power consuming periods to reach working temperature. Therefore, they cannot start rapidly and respond quickly to the change in power demand of the particular vehicle. Using a gaseous fuel (commonly hydrogen) is not suitable for small light duty electric cars due to difficulties with fuel distribution and safe handling, and on-vehicle space and weight constraints. Currently methanol and also other organic fuels are steam reformed to hydrogen rich gas before entry into the anode area of a cell. This fuel feed usually contains traces of carbon monoxide, which acts as catalyst poison and needs to be purified. Purification of the fuel feed with water gas shift reaction will reduce the overall system efficiency and increase the weight, volume and start-up time of the device.
From the point of view of system simplicity and convenience in operation the direct methanol fuel cell where methanol fuel is supplied directly to the anode is a most attractive technological solution for automotive application. The specific advantages in comparison to other fuel cell types such as high energy efficiency (weight and volume) stationary electrolyte, hence no corrosive liquids, self starting at ambient temperature, long-term experience, up to several 10000 h, stability, etc. make the PEM-DMFC the most promising transportation power source.
2.2.2 Principle of DMFC operation
A schematic drawing of a DMFC is given in Fig. 2-2, which demonstrates the principle of operation of a DMFC. The DMFC works by oxidizing the liquid methanol to CO2 and water.
This eliminates the need for an external hydrogen supply. A proton conducting solid membrane, used both as electrolyte and separator between anode and cathode, is sandwiched between porous structures (i.e. carbon). The latter serve as current collectors and at the same time as a support for catalyst particles. Before catalyst deposition the current collectors are
This eliminates the need for an external hydrogen supply. A proton conducting solid membrane, used both as electrolyte and separator between anode and cathode, is sandwiched between porous structures (i.e. carbon). The latter serve as current collectors and at the same time as a support for catalyst particles. Before catalyst deposition the current collectors are