Essay: Fuel cells

1.1 Introduction
The observed increase in human numbers requires the establishing of huge factories to meet consumer needs such as furniture, vehicles, electrical appliances, and others. The operation of such factories by standard fuels increases the ratio of pollution due to their residual toxic gases that are the result of fuel arson. From this, a need for producing energy sources is emerged; environmentally friendly inherently and relatively low cost. Fuel cells were the best solution of this issue and there for they are called green energy cells[1].
Fuel cell is a low pollution power generator. It works as an electrochemical device because it converts the chemical energy stored in gas fuel into electrical energy[2,3]. The converting process occurs directly due to chemical reactions and without combustion of the fuel. A simple fuel cell consists of an anode and a cathode separated by an electrolyte. The gas fuel inters the cell from the anode side is hydrogen while oxidant flows from the cathode [4,5]. The hydrogen undergoes an oxidation reaction to produce electrons and cations. These electrons leave the anode and form an electric current which can be employed in any external load before reaching the cathode. Whereas the hydrogen cations diffuse through the electrolyte toward the cathode, recombine with the electrons to reproduce hydrogen gas atoms which in their roll react with the oxygen and form water. The only waste products of fuel cell are water and heat[6 ].
Fuel cell similar to a simple battery in operation mechanism where the chemical reaction produces electricity; However it differs from battery in many other points such as life time, energy storage, and by ‘ products. Fuel cell works continuously as long as it is fed by fuel [7], unlike battery in which the depletion of its chemical reactors decides the battery life time and stops its work. Also a fuel cell is only an energy convertor it does not storage this energy as battery does [4].
There are several types of power generators which use traditional fuel. All of them depends on the same principle of work that is chemical to mechanical energy transformation [8].Conventional generators or what are known as heat engine burn or oxidize the fuel with air as a first step of operation [9]. By this step, fuel chemical energy is turned into thermal energy. This thermal energy expands the engine gases by increasing their temperature and pressure. Then the disturbed gases rotate crankshaft in thermal to mechanical energy conversion step. Finally the crankshaft performs work to provide electricity where the mechanical energy is now converted to electricity[9]. Thus heat engines generate electricity passing through three stages. As the number of stages increases, the conversion efficiency declines [10]. Unlike fuel cell which convert chemical energy to electric energy directly.
Despite their ability to supply high power, heat engines have numbers of disadvantages such as CO2 emission, voice pollution, low conversion efficiency, environmental warming, and high cost arising from lack of fuel and damage to some mechanical parts. For this fuel cell has not only come to be the source of clean energy but also to replenish the advantage of thermal engine, simple and rechargeable batteries. The simple structure with few changeable parts of fuel cells makes them have long ‘ lasting and noiseless operation[11]. Also in contrast to batteries, fuel cells have an infinite time of operation and higher energy density thus larger storage capacity [10].
All these advantages of fuel cell formed an attraction to researchers for the purpose of developing these cells for commercial uses. Their efforts aim to raise the output power, enhance the conversion efficiency, and reducing the cost more. The present work represents an attempt to increase the efficiency of the fuel cell through employing the field of nanotechnology. The details will be given in the next chapters of this thesis.
1.2 Fuel Cells’ Principle of operation.
All fuel cell’s designs are based around a central structure using two electrodes separated by electrolyte. The electrodes are a negatively charged anode and positively charged cathode. Some other components such as gas diffusion layer may be added for enhancing the cell performance. The electrolyte is either solid or liquid and is the very important member due to its functions in separating the two electrodes from being mixed, transporting the charged particles between the electrodes, and working as a filter to allow only the positive ions to pass through the cathode [12]. A catalyst is usually added to both electrodes in order to speed up the electrochemical reaction [13] and reduce the energy barrier for the reaction[14]. A scheme of fuel cell components is illustrated in Figure (1. 1).
Figure 1.1: The Structure of Fuel Cell (FC)[14].
In theory, any material have the ability to chemical oxidation can be used as fuel for a fuel cell, But for most applications hydrogen is the main choice because of its high reactivity with a suitable catalyst, the possibility of being produced from wide range of energy sources, and its high energy density. [3].Hydrogen gas (H2) is fed to the anode during the operation of the cell and spreads through the diffusing layer. The catalyst of the anode oxidizes the hydrogen atoms and the results of the oxidization process are free electrons and positive ions (H+ cations) as described by equation (1-1)[10].
(1-1)
The released free electrons migrate through the catalytic metal in an external circuit forming a usable current before reaching the cathode [15], whereas the hydrogen cations are transported through the electrolyte membrane toward the cathode. At the same time, oxygen gas is supplied to the cathode and spreads through the diffusing layer. The oxygen atoms, the hydrogen ions, and the exhausted electrons recombine (reduction reaction)[16] at the cathode to form water and releasing heat as the only residuals. The oxygen reduction and the total reaction of the fuel cell can be explained in equation (1-2) and (1-3) respectively[17] [15].
(1-2)
(1-3)
Oxygen is commonly used as oxidant agent because its ability to be reduced and is economically available in air. A schematic representation of oxidization ‘reduction processes of fuel cell is shown in Figure (1.2).
Figure 1.2: A schematic Representation of Oxidization ‘ Reduction Processes at FC Electrodes[17].
The voltage produced by a single cell is approximately one volt [13] which is not enough for any possible use. Thus there are two ways to achieve power requirements of real word applications; either by increasing the active area of the cell, or by combining a number of cells unit in series manner. The combination of several cells is known as a fuel cell stack [6]and the out pout power of one stack is given by [14].
(1-4)
where: V and I are the stack total voltage and current respectively, n is the number of cells, v, i, and a are the voltage, the current, and the area of an individual cell in the stack.
1.3 Fuel Cells’ Performance and Polarization Curve
The moving of the conduction electrons through the load circuit and the diffusing of the ions through the electrolyte leads to reduction in output voltage of the cell. This voltage loss is called overvoltage. The overvoltage causes variation in the cell performance with operation conditions and makes it differ from load to other[18]. The main sources of voltage losses in fuel cell are:
1. 3. 1 Activation losses
The activation losses are caused by the slowness of the reaction that takes place in the surface of the electrodes. A certain amount of voltage is also needed to transport the electrons to or from the electrodes, thus some of the useful energy is lost. This electron transport is referred to as the exchange current density, which depends on temperature and gas pressure[19].To reduce the activation losses an increase in reaction rate is wanted. This can be achieved by increasing the temperature, use more effective catalysts, increasing the active surface of the catalysts, increasing the amount of reactant or increasing the pressure.
1. 3. 2 Fuel crossover and internal currents
Although a fuel cell electrolyte is designed to conduct positive ions, some electrons pass through as well. Since these electrons are not conducted through the external circuit, they are not useful and give rise to internal currents. There is also an amount of fuel that diffuses from the anode through the electrolyte to the cathode, where it reacts directly with the oxygen without producing any current to the external circuit[20]. The amount of wasted fuel is known as fuel crossover. The fuel crossover can be reduced by using thicker membranes; this will however reduce the ionic conductivity of the membrane[13].
1.3. 3 Ohmic losses
The Ohmic losses are caused by the resistance to the transport of electrons through the electrodes and the different interconnections, and also to the passage of ions through the electrolyte. To reduce the Ohmic losses it is important to use electrodes with high conductivity and to use thin membranes[21].
1 .3.4 Concentration losses
These losses are caused by the diffusion of ions through the electrolyte which produces an increase of the concentration gradient. The relation between the voltage of the cell and the current density is a approximately linear until a limit value, above of which the losses grow quickly[14]. The concentration losses are certainly important when mixed gases are used as fuel and oxidant, instead of pure gases, for example reformer gas instead of H2 and air instead of O2[22]. To reduce the losses due to mass transport, it is important to keep the area around the electrodes clean from contaminants that will block the pathways.
A characteristic graph of voltage and power versus current for a set of operating condition is known as polarization curve[23]. This curve gives an indication of the typical contribution of the four voltage losses as shown in Figure (1 . 3).
Figure 1. 3: Polarization Curve of Fuel Cell [13].
1 .4 Types of Fuel Cells
Fuel cell types are generally classified according to the nature of the electrolyte they use. Each type requires particular materials and fuels and is suitable for different applications.
1.4.1 Alkaline Fuel Cells (AFCs)
Alkaline fuel cells (AFCs) were one of the first fuel cell technologies developed, and they were the first type widely used in the U.S. space program to produce electrical energy and water on-board spacecrafts [24]. AFCs generally use a solution of potassium hydroxide (KOH) in water as their electrolyte. They operate at 100-250 ”C with efficiency of 60% [24, 25]. They are, however, the cheapest type of fuel cell to manufacture so it is possible that they could be used in small stationary power generation units. AFCs are extremely sensitive to carbon monoxide and other impurities that would poison the catalyst. The designs of AFCs are similar to that of a PEM cell but with an aqueous solution or stabilized matrix of potassium hydroxide as the electrolyte. The electrochemistry is somewhat different in that hydroxyl ions (OH-) migrate from the cathode to the anode where they react with hydrogen to produce water and electrons as shown in Figure (1.4) and equation (1-5) [26]. These electrons are used to power an external circuit then return to the cathode where they react with oxygen and water to produce more hydroxyl ions as shown in equation (1-6) [27].
The reactions in AFCs can be expressed by the following equations:
Anode Reaction: H2 + 2OH 2H2O + 2e- (1-5)
Cathode Reaction: (1 )/(2 )O2 + H2O + 2e- 2OH- (1-6)
Figure 1.4: Schematic representation of alkaline fuel cell [27].
1.4.2 Phosphoric Acid Fuel Cells (PAFCs)
The phosphoric acid fuel cells are currently the most commercially advanced fuel cell technology. PAFCs use liquid phosphoric acid as an electrolyte with a platinum catalyst [24]. PAFCs work slightly at higher temperatures than PEM or alkaline fuel cells around 150-200o C making them more tolerant to reforming impurities [25]. PAFCs have been used for stationary power generation, but also used to power buses. There are currently a number of working units installed around the world providing power to hospitals, schools and small power stations. The anode and cathode reactions are the same as those in the PEM fuel cell with the cathode reaction occurring at a faster rate due to the higher operating temperature [26, 27]. A schematic diagram of a representative Phosphoric Acid Fuel Cell is shown in Figure (1.5)and their reaction in equation (1-7) and (1-8). The reactions in PAFCs can be expressed by the following equations:
Anode Reactions: H2 2H+ + 2e- (1-7)
Cathode Reaction: 1/2O2 +2H++ 2e- H2O (1-8)
Figure 1.5: Schematic representation of phosphoric acid fuel cell [27].
1.4.3 Molten Carbonate Fuel Cells (MCFCs)
Molten carbonate fuel cells (MCFCs) are currently being developed for natural gas and coal-based power plant for electrical utility, industrial and military applications [26]. These cells use molten carbonate salts mixture as the electrolyte. When heated to a temperature of around 650 ”C these salts melt and generate carbonate ions which flow from the cathode to the anode where they combine with hydrogen to give water, carbon dioxide and electrons; as shown in Figure (1.6) and reaction (1-9) [25,26]. These electrons are routed through an external circuit back to the cathode, generating power on the way as illustrated in equation (1-10) [27]. The reactions in MCFCs can be expressed by the following equations:
Anode Reaction: CO3-2 + H2 H2O + CO2 + 2e- (1-9)
Cathode Reaction: CO2+ 1/2O2 + 2e- CO32- (1-10)
At high temperature the cells provides fuel flexibility. They can use hydrogen, simple hydrocarbons and simple alcohols, to generate hydrogen within the fuel cell structure. At the elevated temperatures there is only sulfur released. These fuel cells can work at up to 65% efficiency and this could potentially rise to 85% if the waste heat is utilized [28].
Figure 1.6: Schematic representation of molten carbonate fuel cell [27].
1.4.4. Solid Oxide Fuel Cells (SOFCs)
Solid oxide fuel cells (SOFCs) use a hard, non-porous ceramic compound as electrolyte and operating at high temperature around 1000 ”C [24,25]. The high temperature means that this cell are resistant to poisoning by carbon monoxide as this is readily oxidized to carbon dioxide. SOFCs are sulfur-resistant fuel cell type. Energy is generated by the migration of oxygen anions from the cathode to the anode to oxidize the fuel gas, which is typically a mixture of hydrogen and carbon monoxide as shown in equation (1-11) and (1-12) [26]. The electrons generated at the anode move via an external circuit back to the cathode where they reduce the incoming oxygen, thereby completing the cycle as shown in Figure (1.7) and their overall reaction in equation (1-13) [28].
The reactions in SOFCs can be expressed by the following equations:
Anode Reactions: H2 + O-2 H2O + 2e- (1-11)
CO + O2 CO2 + 2e- (1-12)
Cathode Reaction: 1/2O2 + 2e- O-2 (1-13)
SOFCs have the efficiencies of 50-60% that are expected to be used for generating electricity and heat in industry and potentially for providing auxiliary power in vehicles [27].
Figure 1. 7: Schematic representation of solid oxide fuel cell [27].
1.4.5. Solid polymer fuel cells (SPFC)
1.4.5.1. Direct methanol fuel cells (DMFCs)
DMFCs also use solid polymer as an electrolyte but differ from PEMFCs because they use liquid methanol fuel rather than hydrogen. DMFCs operate at slightly higher temperatures than PEMs 50-120o C and achieve around 40% efficiency [25, 29]. DMFCs are directed toward small mobile power applications such as laptops and cell phones, using replaceable methanol cartridges at power ranges of 1-50 W [26]. It does not have many of the fuel storage problems typical of some fuel cells because methanol has a higher energy density than hydrogen. Methanol is also easier to transport and supply to the public using our current infrastructure because it is a liquid [27]. The liquid methanol (CH3OH) is oxidized in the presence of water at the anode generating CO2, hydrogen ions and the electrons that travel through the external circuit as the electric output of the fuel cell. The hydrogen ions travel through the electrolyte and react with oxygen from the air and the electrons from the external circuit to form water at the anode completing the circuit. A schematic diagram of a representative DMFCs is shown in Figure (1.8) and their reaction in equation (1-14) and (1-15).
The reactions in DMFCs can be expressed by the following equations:
Anode Reaction: CH3OH + H2O CO2 + 6H+ + 6e- (1-14)
Cathode Reaction: 3O2 + 6 H+ + 6e- 3H2O (1-15)
Major drawbacks of the DMFCs are poor performance of the anode where more efficient methanol electro-oxidation catalysts are needed [29].
Figure 1.8: Schematic representation of direct methanol fuel cells [29].
1.4.5.2. Proton exchange membrane fuel cells (PEMFCs)
The proton exchange membrane fuel cells (PEMFCs) are an energy conversion device by electrochemically convert energy of fuels such as hydrogen and methanol to electricity. It has attracted much attention as clean energy generation technologies with high power density, high efficiency, and low greenhouse gas emissions for various applications such as portable electronic devices, transportation and residential power generation [6]. PEMFCs have solid polymer as an electrolyte [30]. Improvements in the performance can be identified by evaluating the polarization curve. PEMFCs have quick starts, with full power available in a minutes or less, low weight and volume with good power to weight ratio at low temperature operation that makes them suitable used in automobiles [31]. PEMFCs usually operate at low temperatures 60-100o C, which makes them also suitable for portable applications [24]. PEMFCs offer efficient operation up to 50% electrical efficiency for the fuel cell itself and over 85% total efficiency when waste heat is captured for small-scale space and water heating. Their performances are influence by many parameters such as operating temperature, pressure and relative humidity. The protons permeate through the polymer electrolyte membrane to the cathode as shown in Figure (1.9) and equation (1-16). The electrons travel along an external load circuit to the cathode, thus creating the current output of the fuel cell. Meanwhile, a stream of oxygen is delivered to the cathode as shown in Figure (1.9). At the cathode side oxygen molecules react with the protons permeating through the polymer electrolyte membrane and the electrons arriving through the external circuit to form water molecules as shown in equation (1-17) [24, 25, 26, 27].
The reaction in PEMFCs can be expressed by the following equations:
Anode Reactions: H2 2H+ + 2e- (1-16)
Cathode Reaction: 1/2O2 +2H++ 2e- H2O (1-17)
The prime requirement of fuel cell membranes are high proton conductivity, low methanol/ water permeability, good mechanical properties and thermal stability [32,33,34]. PEMFCs face several challenges because of platinum catalysts are expensive and also subject to CO poisoning from hydrocarbon fuels, so catalyst improvements, non-precious metal catalysts and other alternatives are under investigation. Membranes more resistant to chemical impurities are also being developed. The attractiveness of this PEMFCs system has increased significantly with improvements in many areas.
Figure 1.9: Schematic representation of polymer electrolyte membrane fuel cell [27].
1.5 Low Temperature Fuel Cells
The low temperature fuel cells that are mostly used in automotive and portable electronic applications are the proton exchange membrane fuel cells (PEMFCs) and the direct methanol fuel cells (DMFCs). The DMFCs directly consumes liquid fuel (methanol) while the PEMFCs are fuelled with hydrogen [35]. Both of the cells consist of six major parts: end plates, current collectors, graphite flow channel blocks, gaskets, gas diffusion layers and a membrane electrode assembly (MEA) [36]. These components are shown in Figure (1.10). The major problem of the DMFCs is the low performance at low temperature as their produced power density is only one third of that of PEMFC’s [37, 38]. Because the PEMFC uses a solid polymer electrolyte, it results in excellent resistance to gas crossover and a simpler design that requires less maintenance [25], and eliminates the corrosion and safety concerns associated with liquid electrolyte fuel cells[28].
Figure 1.10 : Low Temperature fuel cell component [38].
1.6 Proton Conductivity Mechanisms
The proton is unique, it is the only ion which possesses no electronic shell. It therefore strongly interacts with the electron density of its environment. In the case of metals, the proton interacts with the electron density of the conduction band, and is considered to be a hydrogen atom with a protonic or hydridic character. Metals are also unique, they allow the proton to have a high coordination number, typically four or six at a tetrahedral or octahedral site. In non-metallic compounds, the proton interacts strongly with the electron density of only one or two nearest neighbors. Proton transfer phenomena follow two principal mechanisms namely the vehicle mechanism and the structural diffusion (Grotthuss mechanism) where the proton remains shielded by electron density along its entire diffusion path, so that in effect the momentary existence of a free proton is not seen [39]. In this mechanism the proton diffuses through the medium together with a ‘vehicle’ (for example, with H2O as H3O+). The counter diffusion of unprotonated vehicles (H2O) allows the net transport of protons [40]. In the other principal mechanism, the vehicles show pronounced local dynamics but reside on their sites. The protons are transferred from one vehicle to the other by hydrogen bonds (proton hopping). The Nafion proton transport and conductivity are strongly correlated to the water content. The water content in Nafion is specified by the quantity, which indicates number of water molecules per sulfonate group [41]. At high hydration level, structure diffusion dominates at the centre of hydrophilic domains and it results in a high diffusion coefficient of protons that approaches bulk water. At intermediate and lower hydration levels, the increased acid concentration favours the vehicle mechanism. The presence of sulfonate groups will also retard the diffusion of H3O+ ions by electrostatic interactions.
1.7 Applications of Fuel Cells
Fuel cell has a low emission level, high efficiency and low maintenance requirement. It can use in stationary and portable power generation as well as transportation.
1.7.1. Stationary Power Generation
Fuel cells are considered for stationary power generation (1 – 500 kW) mainly due to the high conversion efficiencies. The PEMFC offers rapid startup, which may be of paramount importance for auxiliary power supply systems. It allows on-site power generation, where the energy is actually required. The advantage in stationary systems is that there are not so strong weight and space constraints, which facilitates fuel cell system integration issues. Because of this, and the fact that cost targets for stationary fuel cell power plants are higher than for the other applications stationary power is currently viewed as the market where PEMFC systems will become competitive and commercial in the near future [42,43]. It is usually used as a backup power in buildings such as hotels, hospitals, industrial facilities or stand-by generators, factories, banks and shopping centres. It can be used to produce electricity and hot water in rural areas[44]. The plants are fuelled primarily with natural gas, and operation of complete, self-contained, stationary plants has been demonstrated using PEMFC, AFC, PAFC, MCFC, SOFC technology [45]. Using fuel cell as power generation will provide each housing development or apartment complex with its own power. This will remove the environmental pollution.
1.7.2. Portable Applications
Portable fuel cells has extend the duration of grid independent operation with the production of less noise and higher quality of energy production [46]. It appears to be the most promising candidate for battery replacement for portable applications such as cellular phones, laptop, computers and video cameras, so that they can be functions in days or weeks without the need to plug a device into an electrical grid or use batteries, also safer and more environmentally friendly compared to batteries when it comes to recycling [45, 47]. It is very useful in areas where there’s no electricity, it can be used to power telecommunications satellites and also provide power to computer chips. The Analytic Power Corporation reported on portable hydrogen fuelled PEMFC units, ranging from 50 to 500 W, for applications such as powering microclimate cooling systems [48]. It can safely produce power for biological applications, such as hearing aids and pacemakers [49]. These systems can be life savers in natural emergencies such as hurricanes, earthquakes, fires and ice storms. Portable fuel cells can provide reliable, high quality power in non-emergency situations as well, such as remote construction sites or even as a cost-effective solution for lighted trade show displays. In emergency response, military, law enforcement, transportation safety, and surveillance markets, fuel cells bring the benefits of longer run time and smaller, lighter, quieter systems.
1.7.3. Automotive Applications
PEM fuel cells have still remained the technology of choice for transportation systems. For automotive applications, primarily passenger cars and busses are the main issue and also the most challenging sector for mobile industries. The interest in fuel cells for vehicle propulsion is based on the demand for non-polluted environments and on the call for efficient energy use, resulting in less carbon dioxide emissions [50-51]. The car manufacturers like Toyota and Ford have chosen the methanol fuel cell as its storage and tank refilling is easy [52], while others such as Opel and General Motors have preferred to use pure hydrogen as there is enough storage space on the rooftop of the bus [53]. Since 1994, Daimler-Benz working in collaboration with Ballard built a series of PEMFC powered cars which fuelled with hydrogen. Daimler-Benz around 1997 introduced a methanol fuelled car with a 640 km range and also Ballard in that same year provided 205 kW PEMFC units for a small fleet of hydrogen-fuelled buses for demonstrations in Chicago, Illinois, and Vancouver, British Columbia [54].
1.8 Membranes
Membrane has become a separation technology over the past years and has been a competitive for conventional techniques. Their separation process is based on the presence of semi permeable membranes. Separation in synthetic membranes has put the terminology separation in a wider context. A range of separation of the chemicals/ mass transfer type have developed around the use of membranes including distillation, extraction, absorption, adsorption, and stripping, as well as separations of the physical type such as filtration. This work will focus only on the Fuel cell membrane which classified according to their performance.
1.8.1 Nafion Membranes
Nafion membrane is the most widely used membranes in H2-PEMFCs with the high proton conductivity at 80”C and 100% relative humidity, good thermal and chemical stability [35,55,56,57]. It made of a perfluorinated material composed of carbon-fluorine backbone chains and perfluoro side chains with sulfonic acid ion-exchange groups which gives it high chemical resistance as shown in Figure (1.11) [58]. This Nafion membrane functions as both separator and an electrolyte in the fuel cell. Nafion membranes are now produced at various thicknesses. The researcher of Nafion membranes in the past several decades has shown that the membrane performance and intrinsic properties are dependent on its chemical identity and the thermal history of the polymer (drying, exposure to temperature, and membrane pretreatment) [59].
The development of Nafion began in the early 1960s when the DuPont Company’s plastics exploration research group was expanding to fluorine technology that had previously resulted in the development of Teflon fluorocarbon resins and Viton fluoroelastomers. The group was studying new monomers for copolymerization with tetrafluoroethylene (TFE). They also developed a general method to synthesize perfluorinated vinyl ethers from perfluorinated acid fluorides, which resulted in vinyl ether-monomers, the starting point for Nafion as well as other novel commercial polymers. One of the acid fluorides studied, based on the reaction product of TFE and sulphur trioxide (SO3-), led to an unusual TFE copolymer containing branches with pendant sulfonic acid groups (SO3H) [60]. The structure of Nafion perfluorinated membrane is given in Figure (1.11).
Figure 1.11: Chemical structure of Nafion perfluorinated membrane.
1.9 Catalyst
In a PEMFC the ongoing reactions are taking place in presence of a heterogeneous catalyst, which has to show certain requirements for giving high performance:
– electrical conductivity
– good interplay with the ionomer
– accessible for reactant gases
– stable while in contact with reactant gases, products and membrane electrolyte
– reactions on both the anode and the cathode side have to occur as close aspossible to the thermodynamic potential
As mentioned before, the most commonly used catalyst for the reactions in a PEMFC is platinum group. It shows all the required characteristics above and is highly active and stable.
Since the reaction takes place at the surface of the catalyst its active surface area has to be as high as possible. One way of increasing the active surface area of the platinum catalyst is reducing the particle size.
1.9.1 Nanostructured Materails (NSMs)
Nanoscale is a magical point on the dimensional scale: Structures in nanoscale (called nanostructures) are considered at the borderline of the smallest of human-made devices and the largest molecules of living systems. Their ability to control and manipulate nanostructures will make it possible to exploit new physical, biological and chemical properties of systems that are intermediate in size, between single atoms, molecules and bulk materials [68]. Nanostructured materials (NSMs) as a subject of nanotechnology are low-dimension materials comprising building units of submicron or nanoscale at least in one direction , which have scale dimension in the range of 1-100 nm (1 nm = 10-9m, one nanometer is about 60,000 times smaller than a human hair in diameter), have received tremendous attention in recent years and exhibiting size effects [69]. With the emergence of nanotechnology and nanoscience the investigation and application of nanostructured materials are growing rapidly [70]. Materials can be placed into broad categories according to their size into :- Macroscopic matter which is visible with the naked eye, atoms and most molecules are microscopic matter with dimensions < 1nm, and mesoscopic particles, such as bacteria and cells that have dimensions on the order of micron(s), can be observed with optical microscopes. Nanoscoping materials are filling the gap between the microscopic and the mesoscopic which is another class of matter as well [71]. Colloids solutions can contain particles ranging in size from 1 to 100nm . Colloids may form from any combination of liquid, solid or gas, except for gas and gas. Gases always dissolve in each other to form true solutions. Colloidal dispersions of insoluble materials are called sols. Hence NSMs can be classified according to the order of dimensionality, as zero-, one-, two and three dimensional nanostructures [72]. 1.9.1.1 Zero-dimensional (0-D) nanostructures Zero-dimensional or isotropic nanomaterials are materials where in all the dimensions are measured within the nanoscale. Also named as nanoparticles, with all possible morphologies, such as spheres, cubes these nanoparticles include single crystal, polycrystalline and amorphous particles. If the nanoparticles are single crystalline, nanoparticles are often referred to as nanocrystals. When the NPs have dimension is sufficiently small and quantum confinement effects are observed, the common term used to describe such nanoparticles is quantum dots [73,74] . As shown in Fig. (1.12) A. [75]. 1.9.1.2 One-dimensional (1-D) nanostructures The one-dimension nanomaterials have one dimension that is outside the nanoscale. This type has been called by a variety of names such as: whiskers, fibers or fibrils, nanowires and nanorods. In many cases one-dimensional systems take into account carbon-based, metal-based or even oxide-based systems. Nanotubues and nanocables are also considered one-dimensional structures if the extension over one dimension is predominant over the other types [76,77,78]. As shown in Fig. (1.12) B. 1.9.1.3 Two-dimensional (2-D) nanostructures Two-dimensional nanomaterials are materials in which two of the dimensions are not confined to the nanoscale. They are another important nanostructure, they include many shapes such as nanofilms, nanolayers, nanocoatings and nanodiscs, thus they have been a subject of intensive study for almost a century [76,77,78]. As shown in Fig. (1.12) C. 1.9.1.4 Three dimensions (3-D) nanostructures Three-dimensional nanomaterials, also known as bulk nanomaterials, are relatively difficult to classify. However, it is true to say that bulk nanomaterials are materials that are not confined to the nanoscale in any dimension. These materials are thus characterized by having three arbitrarily dimensions above 100 nm [76,77,78]. As shown in Fig.(1.12)D. Figure (1.12) : Various kinds of nanomaterials. (A) 0D spheres and clusters. (B) 1D nanofibers, wires, and rods. (C) 2D films, plates, an networks. (D) 3D nanomaterials [74]. 1.10 Properties of Nanostructured Materials Nanomaterials display optical, thermal, mechanical and magnetic properties that differ significantly not only from those of molecular units but also from those of macroscopic system: 1.10.1 Optical Properties The electronic structure of any material plays an important role on optical response of this material. In nanomaterials the electronic structure is closely related to its chemical composition, arrangement, and physical dimensions. The effect of reduced dimensionality on electronic structure has the most profound effect on the energies of valance band (the highest occupied molecular orbital) and the conduction band (the lowest unoccupied molecular orbital), therefore the transitions between these two states determine the optical emission and absorption of nanomaterials in particular, semiconductors and metals, show large changes in optical properties, such as color, as a function of particle size, i.e red shift attributed to increase in particle size [78,79]. 1.10.2 Thermal Properties One of the most important thermal properties of nanomatarials is melting point, the melting point of a nanomaterials decreases sharply as the particle size reaches critical diameter usually < 5 nm. The changes in melting point occur because nanoscale materials have a much larger surface-to-volume ratio than bulk materials [80], therefore they tend to be able to move easier at the lower temperature. Thus the large increases in surface energy and the change in interatomic spacing have a clear effect on thermal properties [81]. Figure (1.13) shows the melting point decreases dramaticallyas a function of particle size gets below 5 nm [82]. Figure (1.13): Melting point decreases dramatically as the particle size of gold particle gets below 5 nm [81]. The temperature of reaction is one of a significant effect on the growth of nanoparticles. The mean particle size decreases with decrease intemperature under otherwise the same conditions, this may be due to an increase in the nucleation rate and a concomitant decrease in the crystal growth rate as the temperature is decreased resulting in formation of larger number of nuclei hence smaller particle size. Also the reaction time of the heating is play a crucial role in tuning the size of the particles. The mean the particles size increased as the reaction time increased [83]. 1.10.3 Mechanical Properties The important factors that determine the mechanical properties of nanostructured materials are the grain boundary structure, boundary angle, boundary sliding and movement of dislocations. The large amount of grain boundaries in bulk materials made of nanoparticles allows grain boundary sliding reaches to high plasticity. In plastic deformation, grain size reduction can yield improvements in strength, hardness and reduce ductility due to new grain boundaries, which act as effective barriers to dislocation motion [84]. Mechanical properties of nanomaterials may reach the theoretical strength, which are one or two orders of magnitude higher than that of single crystals in the bulk form. The enhancement in mechanical strength is simply due to the reduced probability of defects [85]. 1.10.4 Magnetic Properties As other properties, the magnetic properties of nanomaterials differ from those of bulk material. Different local environment for the surface atoms in their magnetic coupling/ interaction with neighboring atoms occur because of large surface to volume ratio and this leading to the mixed volume and surface magnetic characteristic [85], therefore ferromagnetism of bulk materials disappears and transfers to super Para magnetism in the nanometer scale due to the huge surface energy [84]. 1.11 Nanostructured Noble Metals Noble metal nanoparticles have received great interest owing to their unique properties and potential wide applications. Because of their structure, which is intermediate between that of molecules and of bulk material, they enable to bridge the gap between molecular chemistry and surface science as shown in figure (1.14) [86]. Figure (1.14) : Representation of atomic, nano and bulk gold nanoparticle is a bridge between atom and bulk [86]. Noble metal nanoparticles such as platinum, gold, silver, rhodium and palladium possess close-lying energy bands which allow electrons to move in a nearly free fashion [87]. These noble metal nanoparticles are having their application in the fields of catalysis, sensing, biomedicine cancer therapy due their unique size- and shape-dependent optoelectronic properties, electronics, optics and low temperature fuel cells [88]. The use of noble metal nanoparticles (NPs) as catalysts provides new opportunities for the remarkable enhancement in the chemical process industry as they offer an efficient combination of homogenous and heterogeneous catalyst advantages. These properties have been of interest for centuries, and scientific research on metal nanoparticles dates back to Michael Faraday [89].[84]. [86]. [91]. 1.12 The properties of Pd bulk: The study of palladium-based materials is hugely important and valuable. As a consequence, nanoparticles of palladium have been heavily studied in a wide range of applications including catalysis, gas sensors, and hydrogen permselective membrane. Nanostructured palladium, because of their high activities toward oxygen reduction reactions, have attracted much attention in recent years as cathode catalysts for fuel cell applications [90]. Table (1.1) represents some properties of Pd bulk. Table (1.1): The properties of Pd bulk 1.14 Synthesis Approaches of Nanostructured Materials There are two opposite, but complementary approaches mechanism to synthesis nanomaterials, Top-down approach and Bottom-up approach, both these two approaches play very important roles in nanomaterials properties depending on their size, shape, and chemical composition [92]. 1.14.1 Top-Down Approach (Dispersion Method) The top-down method is one where material is removed from the bulk material, leaving only the desired nanostructures, i.e. it is based on miniaturizing techniques, such as machining, templating or lithographic techniques [93]. Figure (1.16) shows a schematic representation of the 'top-down' approach of nanomaterials. Figure (1.16): Schematic representation of the 'top-down' approach of nanomaterials [94]. This method usually starts from patterns generated at large scale (generally at microscale) and then they are reduced to nanoscale. The main advantage of this method is that the parts are both patterned and build in place, so that no need to further assembly steps [93]. 1.14.2 Bottom-Up Approach (Reduction Method) The bottom up method or self-assembly, is one where the atoms are produced from reduction of ions, are assembled to generate nanostructures, this technique uses chemical or physical forces operating at the nanoscale to assemble basic units into large structures .The chemical growth of nanometer-sized materials often implies colloidal systems and it frequently passes through phase from solution ,it refers to the build-up of a material from the bottom: atom-by-atom, molecule-by-molecule, or cluster-by-cluster. Colloidal dispersions used in the synthesis of nanoparticles is a good example of a bottom-up approach [95,96]. The size of the nanostructures, which can be obtained with a bottom-up approach, spans the full nanoscale. An advantage of the bottom-up approach is the better possibilities to obtain nanostructures with less defects and more homogeneous chemical compositions. This is due the mechanisms utilized in the synthesis of nanostructures reducing the Gibbs free energy, so that the produced nanostructures are in a state closer to a thermodynamic equilibrium [97]. Figure (1.17) shows a schematic representation of the 'bottom-up' approach of nanomaterials. Figure (1.17): Schematic representation of the 'bottom-up' ap roach of nanomaterials [94]. Reduction Methods (bottom up method) is the method starting from atoms, include chemical [98], electrochemical [99], sono-chemical [100], thermal and photochemical reduction [101], Polyol method [102]'. etc, have been used to generate nanoparticles. Bottom up synthesis techniques usually employ an agent to stop growth of the particle at the nanoscale. Capping materials, such as a surfactant or polymer are used to prevent aggregation and precipitation of the metal nanoparticles out of solution. Choice of the reduction technique, time, and capping material determine the size and shape of the nanoparticles generated. Spheres, rods, cubes, disks, wires, tubes, branched, triangular prisms and tetrahedral nanoparticles have been generated in gold, silver, palladium and platinum with various reduction techniques and capping materials [96, 94]. When the particle size decreases less than either the Bohr radius or deBroglie wavelength of bulk materials used, the valance and conduction bands break into quantized energy level and electronic transition becomes discrete this leads to an increase in the band gap energy [103, 104, 105], i.e. the quantum confinement leads to a collapse of the continuous energy bands of a bulk material into the discrete structure of energy states [106]. 1.15 Polyol Method A powerful method for synthesizing metal nanoparticles from 'bottom-up' approach is a Polyol method compared to other methods, widely used and can be easily applied in open-air environments [102]. Polyol process employing the polymer poly(vinyl pyrrolidone) (PVP) as a surface-capping agent. Metal salt, the precursor for reduction, is dissolved and reduced at different temperatures by an alcohol (typically ethylene glycol because of its good reactivity for the fabrication of a wide verity of metal nanomaterials). For the synthesis of colloids using the Polyol process, shape control is achieved by controlling reaction kinetics. Reaction parameters such as temperature and reaction time are adjusted to promote nucleation and growth to form nanoparticles. The reaction proceeds with continuous injections of metal salt into hot ethylene glycol. Typical PVP is generally added at the start of the reaction or continuously during metal reduction. The role of the polymer is twofold: it acts as a stabilizing agent, preventing aggregation of metal particles and retaining a uniform colloidal dispersion [107,108]. 1.16 Electrode The electrode of a PEMFC is the region where the actual electrochemical reactions take place. To give high performance the active surface area has to be maximized both per mass of the catalyst and per area of the electrode. Further the restraints on the transport rates of the reactant gases to the catalyst, which is distributed on the electrode material, have to be as low as possible. Obviously the overall performance of a good electrode has to be constant with respect to time and the cell's operation conditions, i.e. the materials have to be extremely stable andreliable. The key factor of achieving these goals is a perfect catalyst distribution over the electrode. The catalyst particles have to assure for ionic conduction for hydronium ions, hydrophobic pathways for gas transport, hydrophilic pathways for the removal of liquid water, and most importantly for good contact to the external electrical circuit, where electrons are transported from the anode to the cathode in order to complete the cell reaction. To be able to match these requirements one has to select specific materials for the different parts of a MEA. 1.16.1 Laser Ablation Laser ablation is the removal of matter from the target (i. Atoms), atoms are heavy and inertial so that laser light preferentially interacts with electrons in the material, while atoms are heated later due to energy exchange processes [109]. It is a complex physical phenomenon which involves optical penetration of laser in the material and the transport of energy inside the material [110]. 'To distinguish photoablation or ablative photodecomposition from thermal interaction. In the case of photoablation, the energy of a single UV photon is sufficient to dissociate the former bound molecule. In thermal interactions, the situation is completely different. The photon energy is not high enough for the molecule to reach a repulsive state. The molecule is promoted only to a vibrational state within the ground level or to a rather low electronic state including any of its vibrational states. By means of nonradiative relaxation, the absorbed energy then dissipates to heat, and the molecule returns to its ground state. Hence, the crucial parameter for differentiating these two mechanisms ' photoablation and thermal interaction ' is the photon energy or laser wavelength. Only if h'' > 3.6 eV, or in other measures ” < 350 nm, is the single photon dissociation of C'C bonds enabled. Of course, several photons with h'' < 3.5 eV can be absorbed. Then, these photons add up in energy and may thus lead to a dissociated state, as well. However, during the time needed for such a multiphoton absorption process, other tissue areas become vibrationally excited, hence leading to a global increase in temperature and an observable thermal effect (usually either vaporization or melting). If this effect is associated with ablation, the whole process is called thermal decomposition and has to be distinguished from pure ablative photodecomposition. These statements hold true, unless ultrashort pulses with pulse duration shorter than 100 psec are used at pulse energies high enough to induce localized microplasma. Then, even VIS- and IR-lasers can interact nonthermally' [111]. A power density exceeding 1011 W/cm2 in solids and fluids (or 1014 W/cm2 in air) a phenomenon called optical breakdown occurs. A bright plasma spark is clearly visible which is pointing toward the laser source. If several laser pulses are applied, a typical sparking noise at the repetition rate of the pulses is heard. The plasma-induced ablation provides very clean and well-defined removal of tissue. In particular, the important feature of optical breakdown is that it energy deposition is possible not only in pigmented tissue but also in weakly absorbing media. This is due to the increased absorption coefficient of the induced plasma[111] . The plasma mediated interaction expected either in Q-switched pulses in the nanosecond range or mode locked laser pulses in the picosecond or femtosecond range which can induce localized microplasma. In Q-switched pulses, the initial process for the generation of free electrons is supposed to be thermionic emission, i.e. the release of electrons due to thermal ionization as shown in Fig. (1.18). In mode locked pulses, multi-photon ionization may occur due to the high electric field induced by the intense laser beam [112]. Initiation of ionization with subsequent electron avalanche is schematically presented in Fig (1.18)[111]. Fig 1.18 :Initiation of ionization with subsequent electron avalanche[111] 1.16.2 Laser Ablation In Nanosecond Regime The situation in the ns regime is very different being completely dominated by hydrodynamics. A low density plasma corona is formed in front of the target surface. The laser interacts with the plasma and it is absorbed in the plasma corona at the critical surface. Energy is then transported inwards by electronic thermal conduction (and / or radiation and / or suprathermal electrons)[113]. Matter is constantly removed from the bulk at the ablation surface. The plasma expands in vacuum and is fed by laser ablation. Beyond the ablation surface a shock is formed as a consequence of momentum conservation: the material is compressed by such ablation (shock) pressure. Shock front and ablation front moves inward in the material that causes photodisruption. The ablated depth can be difficult to be measured because in this regime often the shock is also a very effective factor producing further ablation of material[112]. During photodisruption, the tissue is split by mechanical forces. Whereas plasma-induced ablation is spatially confined to the breakdown region. For pulse durations in the nanosecond range, the spatial extent of the mechanical effects is already of the order of millimetres even at the very threshold of breakdown. Shock wave and cavitation effects propagate into adjacent tissue, thus limiting the localizability of the interaction zone. Actually, purely plasma induced ablation is not observed for nanosecond pulses, because the threshold energy density of optical breakdown is higher compared to picosecond pulses[111,112] . 1 ' Literature Survey. Over the past decades, many experiments and attempts have been conducted to generate clean, efficient, and low cost power. The first successful trial in fuel cell history was done in 1950th by National Aeronautics and Space Administration, (NASA).Researchers inNASA developed the firstprototype of Proton Exchange Membrane fuel cell (PEMFC). In 1955, Willard Thomas improved this prototype by using a sulphonated polystyrene ion-exchange membrane as the electrolyte and, three years later, L''onard Niedrach developed a suitable techniquefor deposition the platinum on the membrane.Researchers continue to improve fuel cell technology in order to enhance its performance and reduce its cost. Achieving commercially viable product required examining different catalysts and electrolytes.Most of the studies focused on platinum as it is a high active and stabile catalyst. In 1995 Wilson et al [1] produced Low platinum loading catalyst layers for polymer electrolyte fuel cells (PEFCs) consist of a thin film of highly inter-mixed ionomer and catalyst that is applied to the electrolyte membrane. High performances are achieved with loadings as low as 0.12 mg Pt cm'2 at the cathode and even lower loadings are required at the anode. They discover that when the thermoplastic form of the solubilized ionomer is used in the membrane catalyzation process, the reproducibility is greatly improved and the long-term performance losses are quite low. In 2001 Genies et al [2] prepared Series of sequenced sulfonatednaphthalenic polyimides with improved solubility by polycondensation in m-cresol. The preparation of SPIs series with different ion-exchange capacities and sulfonated polyimide block length was also considered which allows us to study structure properties relationships in terms of water swelling and proton conductivity. In 2003 Kumar and Reddy [3] attempt to improve the performance of the polymer electrolyte membrane fuel cell (PEMFC) through optimization of the channel dimensions and shape in the flow-field of bipolar/end plates. Single-path serpentine flow-field design was used for studying the effect of channel dimensions on the hydrogen consumption at the anode. Simulations were done ranging from 0.5 to 4 mm for different channel width, land width and channel depth. Optimum values for each of the dimensions (channel width, land width and channel depth) were obtained. In 2008 Kongkachuichag and Pimprom[4] synthesized Nafion / Zeolite composite membrane. They used two types of Zeolites; Analcime and Faujasite. Zeolites were known by their ability to exchange ion and can be hydrated by water thus they increased the proton conductivity of the membrane by 6.8 times of pure nafion at 80Co. They also determined some of the physic ' chemical properties of their composite such as H2 permeability, H+ conductivity, ion ' exchange capacity, and water uptake. In 2009 Gasdaet. al. [5] used magnetron sputtering to deposit Platinum nanoparticles onto gas diffusion layer (GDL). They varied the deposition angle during sputtering and studied its effect on Platinum utilization. Their work showed that layers deposited at normal incidence ('' = 0o) are continuous. In contrast, oblique incidence angle ('' = 87o) caused a porous coating of platinum. They conclude that the mass transport efficiency increased when oblique angle deposition was used and they attributedthatto the increasing in porosity. In addition, this method was a good technique to control the catalyst nanostructure. In 2010 Lee et.al. [ 6] enhanced the performance and the lifetime of fuel cell by in situ monitoring and controlling of its operation temperature, output voltage, and fuel flow distribution. They developed flexible and multi ' functional micro sensor made of stainless steel foil substrate of 40 ''m thickens. Their sensors provided information about the interior working of the fuel cell in a short response time and with more efficient than that of other invasive measuring methods. In 2011 Rusnaeniet. al. [7] prepared Pt ' Ni/ C alloy nanocatalysts with various atomic ratio. They employed polyol method to synthesize the nanocatalyst and used different techniques like X ' ray diffraction, cyclic voltammetry, and scanning electron microscopy to characterize the alloy. An enhancement of the oxygen reduction reaction at the fuel cell electrode was investigated. In 2012 Jung et. al. [8] solved the problem of carbon corrosion. Carbon was usually used in PEFC to spurt catalyst. After long ' term operation or many repeated on/off cycles, fuel starts to deplete causes electrochemical corrosion of the carbon. This structure distortion leads to migration of platinum particles and reduction of the cell performance. Jung and his group used spray method to prepare platinum catalyst on carbon nanofibers (Pt/CNFs). The results showed higher carbon corrosion resistance of Pt/CNF compared with other commercial catalysts. In 2013 Khantimerovet. al. [9] growth carbon nanotube on substrate made of porous pellets of stainless steel powder. Then they directly deposited metallic particles of thermally evaporated Pt and Ag on the carbon nanotube base. This composite was used to combine the fuel cell catalytic electrode, gas diffusion layer, and current collector. By this method they simplified the design of the fuel cell. In 2014 Liu et. al. [10] proposed a new approach to use an alternative cathode. They replaced the conventional oxygen cathode by photoregenerative solution based cathode. The solution was electrochemically reduced at the cathode to produce ions. These ions guided to external tank and exposed to photocatalytic oxidation process to reproduce . The mediated redox reaction insures that photoenergy continuously converted to electricity at the cathode of PEM fuel cell. Their design increased the current density by 5 ' 7 orders of magnitude higher than others. In 2015 Xia et. al. [11] introduced for the first time a new design which based on mimicking enzymes functionally. The new designhad nano-architectural electrode with platinum nano-particles specified at the boundaries of transport channels to conduct both electrons and ions. The pathways were constructed with vertically aligned nanowire arrays by nafionpolypyrrole (NfnPPy) grown directly on a gas diffusion layer of PEM fuel cells. In this configuration, the polypyrrole (PPy) acts as the electronic conductor, and the nafionionomer act as the ionic conductor. The interspaces between the nanowires acting as continuous channels for reactant and product transport. The platinum nanoparticles formed the enzyme-like active sites foroxygen reduction reactions. In 2016 Han et. al. [12] proposed a three ' dimensional numerical model to simulate the transporting dynamics of the water in the porous layers of PEM fuel cell. In PEM fuel cell, water is necessary for fully hydration the polymer membrane in order to raise its ionic conductivity. The amount of water should be managed to avoid water rafting in the pores of gas diffusion layers and catalyst layers. The excessive water prevents hydrogen and oxygen from reaching the reaction sites. Han model compares water (gas or liquid) particle size with pores size. The results showed that water particles tend to gather in relatively large pores. This model enabled researchers pretest their designs of fuel cells and choose the suitable pores sizes before manufacturing. 1.18 The aim of this work: The present work aims to increase the efficiency of the PEMFC by increase the active area using nanocatalyst synthesized by polyole method. micro channels for gas diffusion on the surface of the electrodes etched by nanosecond laser irradiation. For this purpose, nanosecond laser irradiation at 1064nm at 6ns pulse durations will focus at the surface of the electrodes in order to reduce the channel size and increase hydrogen gas flow rate to gerents the separation of each electron of any interference hydrogen atom . The present work aims to increase the efficiency of the PEMFC by increase the active area using nanocatalyst synthesized by polyole method. To increase hydrogen gas flow rate and gerents the separation of each electron of any interference hydrogen atom, micro channels etched on the surface of the electrodes by nanosecond laser irradiation at 1064nm at 6ns pulse durations will focus at the surface of the electrodes in order to reduce the channel size In addition the particles have to be dispersed on an inert material, matching the same requirements as platinum itself. Carbon black is the most commonly used support. It shows good electrical and thermal conductivity, low thermal expansion and a large porosity to ensure for reactant gas and liquid water transport [66,67]. Hence the ratio of Pt/C will play a significant role in the catalyst utilization. For the experiments in this study a catalyst by Tanaka with Pt/C of 46.2 wt-% and 48.9 wt-% of Pt was used. 1.16.3 Ablation With Gaussian Laser Pulses The spatial distribution of the beam profile is responsible for significant difference in ablation mechanism across the irradiated region and for heterogeneous sample[115] . The radial fluence profile of a Gaussian beam is given by the well known equation[ 114,112]: F_((r) )=F_'' e^((2r^2)/(''_''^2 ))(1.15) Where F(r) is the fluence at the radial distance r, F0 is the maximum fluence and ''0 is the radius of the beam waist. Assuming that a defined material-dependent ablation threshold exists and that material perforation occurs when this threshold is exceeded equation [1.16] can be rearranged to[112,114] (1.16) Where rth represents the radius of the ablation site at the ablation threshold Fth, which implicates that at the rims of the damage zone the threshold fluence, is reached. A logarithmic dependence on the ratio of the fluencies F0/Fth can also be stated for the etch depth per pulse L [112,114]. (1.17) Here L is the depth of ablated material, ' is the density of the material (g/cm3), ' the evaporation heat (J/g) and l is the characteristics length for the transport of energy inside the target material. Fig (1.19) demonstrates all these correlations. Fig 1.19: Ablation with a Gaussian beam profile demonstrating the dependence of the damage radius rth on the threshold fluence Fth. D is the diameter of the ablated cavity, d is the etch depth[114] 1.16.4 Particle Generation Mechanisms Induced By Laser Abalation Collected particles generated by laser ablation show that the size of particles varies from nanometres to several microns and the shape of the particles changes from spherical single particles to being totally irregular when they form agglomerates. Several mechanisms like phase explosion and melt splashing may be present permitting multiple size distributions to result from just one laser pulse. Nevertheless repetitive pulses further change the particle surface topography and increase the probability that larger particles are produced[116,117]. The generation of particles in laser ablation processes is highly dependent on the laser parameters like wavelength[115] , fluence....etc. and the material properties. The magnitude of the laser energy changes the mass removal mechanism from pure vaporization to liquid ejection and eventually even phase explosion when very high energy is involved. Ambient parameters including the gas properties and pressure affect the formation of particles condensed from the vapour phase. The mechanical properties of the material may influence whether large scale of exfoliation or palliation would occur even under the same laser conditions[114,116] . One of the most important parameters in particle generation is the laser energy; some mechanisms like vaporization are dominant at low laser fluence but become less important as hydrodynamic effects or phase explosion prevail when laser fluence increases[115] . Temperature and pressure plume pushes the ambient gas away and forms the shockwave. A contact surface exists between the vapour plume and the ambient gas[115,116]. As the time advances, the vapour plume and the shock wave expand outward into the ambient gas. At ~ 50 nanoseconds after laser pulse, particles ejected from the surface can be observed. These particles ejected from the surface are caused by spallation. However at high laser fluence, very large particles (~10 microns) were observed which erupted perpendicular from the surface at about 1 microsecond after the laser pulse. These big particles were suspected to result from the phase explosion phenomenon[114,116] and their relative round shape confirmed that they were formed in the liquid phase[115,116] . The process lasted until about 10 microseconds after the laser pulse. At later time (20 -30 microseconds after the laser pulse) very big particles estimated as large as 20-30 microns were generated; the irregular shape of the particles can be easily discerned which identified themselves as solid exfoliation products[114]. In simple words the particle size distribution is composed of an abundant nano-sized fraction, produced by vapour condensation, while the micro-sized fractions are formed during melt explosion[117]. 1.16.5 Absorption Mechanism The absorption of a light as a function of depth can be determined from the absorption coefficient, which can be derived from material dielectric function and conductivity. the specific mechanisms by which the absorption occurs will depend on the type of material. In general, photons will couple into the available electronic or vibrational states in the material depending on the photon energy. In insulators and semiconductors, the absorption of laser light predominantly occurs through resonant excitations such as transitions of valence band electrons to the conduction band (interband transitions) or within bands (intersubband transitions) [118]. These excited electronic states can then transfer energy to lattice phonons. Photons with energy below the material's band gap will not be absorbed (unless there are other impurity or defect states to couple to or if there is multi-photon absorption). Such energies typically correspond to light frequencies below vacuum ultraviolet for insulators and below the visible to infrared spectrum for semiconductors. However, resonant coupling to high-frequency optical phonons in the near-infrared region is possible in some cases [119]. In metals, optical absorption is dominated by the free electrons through such mechanisms as inverse bremsstrahlung. Energy is subsequently transferred to lattice phonons by collisions. An important parameter relating the electron density of a metal Ne to its optical properties is the plasma frequency which is given by[120]: (1.18) Where me is a mass of electron and is the permittivity of free space. Reflectivity and absorptance for light frequencies below the plasma frequency are high because electrons in the metal screen the electric field of the light. However, above the plasma frequency, reflectivity and absorptance drop off drastically because the electrons cannot respond fast enough to screen it [121]. The time it takes for the excited electronic states to transfer energy to phonons and thermalize depends on the specific material and the specific mechanisms within the materials. For most metals, this thermalization time is on the order of 10-12'10-10 s, whereas in non-metals, there is significantly more variation in the absorption mechanisms and the thermalization time can be as long as 10-6 s[118]. When the laser-induced excitation rate is low in comparison to the thermalization rate, the details of the transient electronically excited states are not significant. Rather, one can consider the absorbed laser energy as being directly transformed into heat. Such processes are called photothermal (pyrolytic) and the material response can be treated in a purely thermal way. For instance, laser processing of metals or semiconductors with laser pulse times that are slow (>ns) is typically characterized by photothermal mechanisms. When the laser induced excitation rate is high in comparison to the thermalization rate, large excitations can build up in the intermediary states. These excitation energies can be sufficient to directly break bonds (photo-decomposition). This type of non-thermal material modification is typically referred to as photochemical (photolytic) processing.
1.8.2 Limitations of Nafion membranes
The performance of the PEM improves with increasing temperature until 90o C, which reflects a decrease in the internal resistance of the cell. This decrease is due largely to the decrease in ohmic resistance of the electrolyte [61]. Nafion membranes do not perform well in DMFCs because of its high methanol crossover through the membrane from anode to cathode, high in production cost and also having a complicated and time-consuming manufacturing process [62]. Methanol crossover has two bad effects to the cells. Firstly, the chemical energy of methanol is lost when it crosses through the membrane, thus severely lowering the efficiency of fuel utilization [63]. Secondly, it is oxidized by the cathodic electro-catalyst, which depolarizes the electrode and subsequently increases the amount of air or oxygen. Since none of the energy from this oxidation is extracted as electricity, it all ends up as waste heat that increases the cooling load on the cell [64]. Nafion membrane performs very well in a saturated environment; its proton conductivity has a strong dependence on water content but if it is not properly hydrated, proton conduction becomes slow [65]. At higher temperatures, the PEM will dehydrate and lose proton conductivity, and may result in irreversible mechanical damage. However, higher working temperatures are favourable for the kinetics of Pt catalyst and may improve its tolerance to contaminants [61]. Nafion membranes has osmotic swelling problem and is also potentially dissolved in methanol solution when increasing methanol concentration and temperature [58].The efforts to increase membrane working temperature and decrease the relative humidity are currently conducted on the laboratory scale, one of which is to incorporate nanoparticles.