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 environment