Advantages And Disadvantages Of Solid Oxide Fuel Cells Engineering Essay

vantages And Disadvantages Of Solid Oxide Fuel Cells Engineering EssaySolid oxide discharge cells (SOFCs) ar a class of device which make conversion of electrochemical force out to electricity with negligible pollution1. SOFCs have two major configurations flat two-dimensional and tubular and the SOFCs system consists of a galvanic pile that is made of many unit cells. Each unit cell is represent of two porous electrodes, a unhurt ceramic electrolyte and interconnects. Unlike otherwise fuel cells, the SOFCs demeanor oxygen ions from the cathode to the anode through the electrolyte, and hydrogen or carbon monoxide reacts with the oxygen ions in the anode2. The materials of anode and cathode have different requirements the anode should survive a very reducing graduate(prenominal) temperature environment whilst the cathode has to survive a very oxidising high temperature environment3.Among all the important fuel cells under development, the solid oxide fuel cells operate at the highest operating temperature, typically between 600 and 10004. So the SOFCs has also been called the third-generation fuel cell technology because it was expected to be put into performance astray after the commercialisation of Phosphoric Acid Fuel Cells (PAFCs) (the first generation) and Molten Carbonate Fuel Cells (MCFCs) (the second generation)2. The solid oxide fuel cell is composed of all solid components with the electrolyte acting as an oxide ion conductor and operating at high temperature (1000) in order to ensure adequate noodle and electronic conductivity for the cell components5.1.1.1 SOFC Advantages and DisadvantagesSOFCs have a offspring of advantages due to their solid materials and high operating temperature.Since all the components argon solid, as a result, there is no need for electrolyte loss maintenance and also electrode corrosion is eliminated6.Since SOFCs argon operated at high temperature, expensive catalysts such as platinum or ruthenium are totally avoided2, 6.Also because of high-temperature operation, the SOFC has a bring out ability to tolerate the presence of impurities as a result of life increasing6.Costs are reduced for privileged reforming of natural gas6.Due to high-quality waste heat for cogeneration applications and low energizing losses, the efficiency for electricity production is greater than 50and even possible to reach 652, 6.Releasing negligible pollution is also a commendable reason why SOFCs are popular today5.However, there are also some disadvantages in existence for deteriorating the performance of SOFCs.SOFCs operate high temperature, so the materials utilize as components are thermally challenged5.The relatively high cost and complex fabrication are also significant problems that need to be solved6.1.1.2 SOFC ApplicationsDue to the advantages mentioned above, SOFCs are being considered for a wide range of applications, such as working as actor systems for trains, ships and vehicles supplying galvan ising power for residential or industrial utility2, 7.1.1.3 SOFC Components and ConfigurationsA SOFC system is composed of fuel cell stacks, which consist of many unit cells. there are two major configurations, tubular and platelike, being pursued, described generally as follows.Tubular unit cell is shown in Figure 18, 9. The schematic illustrates the fit current flow direction and components.According to X. Li2, due to easy stacking consideration, recently more and more tubular cells have the structure of cathode inside and anode outside the electrolyte layer.The planar unit cell has a flat structure with a bipolar arrangement, as shown in Figure 210.Seung-Bok Lee at el.11 reported that since the more potent current collection by planner interconnects, planar SOFCs have superiority in power density. On the contrary, the thermal and mechanical properties of tubular SOFCs are better than that of planner SOFCs.Table 12 lists a comparison of the two different SOFC cell configuratio nsTable 1 A comparison of the two different SOFC cell configurations2AdvantageDisadvantageEase of manufacturingEdge current collectionTubularNo need for gas-tight cell sealingLow-power densityLess thermal quip due to thermal expansion mismatchHigh materials costLower fabrication costHigh temperature gas-tight sealingPlanarEase in flow arrangementHigh assembly effort and costHigher power densityStricter requirement on thermal expansion matchAn SOFC stack consist of many unit cells, which are connected by interconnects. Figure 312 illustrates image of planar SOFC stack.1.1.3.1 CathodeThe typical material for the cathode is strontium-doped lanthanum manganite (La1-xSrxMnO3, x=0.10-0.15), because of its swell electrochemical activity for oxygen reduction, high electronic conductivity, good stability2, 4.Other materials, like platinum and other noble metals have also been considered as candidates of the SOFC cathode due to the highly oxidising environment. However, considering the high cost of platinum, it is not best choice to use this metal as the cathode.1.1.3.2 AnodeThough as for the cathode, precious metals like platinum can be used for the SOFC anode, the most widely used material is a nickel-zirconium oxide cermet, i.e. a mixture of nickel and yttria-stabilised zirconia (YSZ) skeleton2. About 20-40 porosity in the anode structure is good for mass expatriation of reactant and product gases1, 2. Nickel plays the role as the electrocatalyst for anode reaction and also can conduct the electrons produced at the anode whilst the yttria-stabilised zirconia is used for conducting oxygen ions2.1.1.3.3 Electrolytethither are a number of materials that can be used for the SOFC electrolyte. Among them, yttria stabilised zirconia (YSZ), i.e. zirconia doped with around 8 mol yttria and gadolinia-doped ceria (GDC) is the most widely used materials suitable for the SOFC electrolyte. GDC has very good ionic conductivity, only when it also shows a high electronic conducti vity5. Compared with GDC, YSZ is stable in either reducing or oxidising environments and has a good conductivity to transmit ions, especially at sufficiently high temperature. But unlike GDC, YSZ shows little or no capability to conduct electrons. Each time two yttria ions (Y3+) replace two zirconia ions (Zr4+) in the zirconia crystal lattice, three oxide ions (O2-) replace four O2- ions, which make one O2- range become vacant, as shown in Figure 45.The vacancies are determined by the amount of yttria doped. So it seems superficially that the more yttria doped, the better the conductivity. But there is an velocity limit for the amount of doped yttria, which is shown in Figure 55. The peak conductivity appears at yttria concentration of 6% to 8 mol%.Very thick-skulled YSZ has a very low gas permeability, which does not allow the reactant gases to mix. However, since YSZ has a low ionic conductivity, in order to ensure the ohmic loss and match with other components, it has to be ma de close 20-50 m thick 1, 2.1.1.3.4 InterconnectsInterconnects are used to connect the neighbouring cells. Materials which act as interconnect must have properties of high electronic conductivity1. Ceramics are usually used for the interconnect since the operating temperature is around 1000. Mg-doped lanthanum chromite, LaCr1-xMgxO3 (x = 0.02-0.01) shows advantages because its electronic conductivity typically increases with temperature2. However, although noble metals have good electronic conductivity, their high price limits their becoming a candidate for the interconnect 2, 4.1.1.5 Electrochemical ConversionThe air is carried to the cathode and the oxygen reacts with electrons from the external circuit yielding oxide ions2, 4Cathode O2 + 2e- O2- (1)The electrolyte does not permit the oxygen pass through it, save the oxide ions migrate from the electrolyte to the anode. At the anode hydrogen or carbon monoxide reacts with oxygen ions to produce water or carbon dioxide2, 4Anode H2 +O2- H2O + 2e- (2)CO + O2- CO2 + 2e- (3)This releases electrons to move through the external circuit to the cathode, thus generating an electric current.So the overall cell reaction occurring is2, 4H2 + O2 H2O +Waste Heat + electric automobile Energy (4)CO + O2 CO2 +Waste Heat + Electric Energy (5)The electrochemical conversion is shown in Figure 613.1.2 Electrolyte Materials1.2.1 ZirconiaZirconia is a egg white ceramic, with the properties of high temperature, wear and corrosion resistance, high melting take down and low coefficient of thermal expansion. Historically, the application of zirconia has been in refractory and ceramic paints2. However, with the development of good technologies, due to its stabilised and excellent properties mentioned above, it can be used as electrical conductivity material in the solid oxide fuel cells, wear split and sensors.Zirconia can exist in three different crystal structures monoclinic, tetragonal and cubic. At room temperature, it na turally exists as the form of the monoclinic crystal clear structure. When the temperature reaches around 1100, the crystal form changes to tetragonal, and then to cubic at about 237014. Pure zirconia is never used because of its unstable properties, so many dopants are added to stabilise the high temperature forms and hence avoid the damaging tetragonal to monoclinic transformation, e.g. MgO, CaO, Ce2O3, and Y2O3. Of these, yttria is the most common dopant, yielding yttria stabilised zirconia (YSZ).1.2.2 Yttria Stabilised Zirconia (YSZ) and the marrow of Different Yttria ContentsYSZ is considered to be an important electrolyte material for solid oxide fuel cells. The proportion of yttria in YSZ is still under research, but is often around 8 mol%. This yields a cubic fluorite-structure YSZ, which displays good thermal stability, good ionic conductivity at high temperature and a thermal expansion compatibility with electrode materials15. However, it is mechanically light(a) as a result of the high fraction of vacancies present in the structure.Different amount of yttria in zirconia has different effect on the properties of YSZ, including ionic conductivity, toughness, fracture strength etc16. 8 mol% yttria stabilised zirconia (8YSZ) has a cubic structure with properties of high ionic conductivity, good chemical stability but its low mechanical strength, limits the fabrication17, 18. However, for 3-7 mol% Y2O3, both cubic and tetragonal phases exist in the microstructure. Table 219 lists comparison of phases for different yttria concentration in zirconia.Table 2 Phase variation for different concentration of yttria in zirconia19%Yphase2YSZTetragonal with some monoclinic3YSZPure tetragonal4.5YSZCubic and tetragonal6YSZ and highercubicIf the YSZ has a great volume fraction of metastable tetragonal phase, which will provide good mechanical properties (strength and toughness) to the ceramic16. For example, 3 mol% yttria stabilised zirconia (3YSZ) has an excellen t mechanical properties of high flexural strength and good fracture toughness. M. Ghatee et al.16 also demonstrated that 3YSZ shows higher electrical conductivity than 8YSZ at T550C. That is because the activation heartiness of electrical conductivity for 3YSZ is lower than 8YSZ at all temperatures. And the strength of the material is determined by grain size and flaw size16.1.2.3 Nanostructured ZirconiaNanostructured ceramics are expected the come particle size is less than 20nm20. And recently, nanotechnology have drawn much attention because of the good mechanical properties, i.e. increasing of hardness, strength, of the materials in nano-size. It is reported that the electrical conductivity of nanostructured YSZ is about 2-3 orders of the magnitude larger than that of microcrystalline YSZ15.Since nanostructured YSZ has many advantages, the development of nanocrystalline YSZ electrolyte grows rapidly. Y. subgenus Chen et al.15, has synthesised nanocrystalline YSZ electrolyte vi a the plasma spray technique.1.3 Characterisation of YSZ1.3.1 Ionic ConductivityConductivity is a measurement of whether charges transport well or not. Ionic conductivity is derived fromion mobility rate, which is determined by carrier concentration c and carrier mobility u, which is shown in Equation 1 5.(1) 5where is the charge number of the carrier,is Faradays constant.1.3.1.1 AC Impedance SpectroscopyElectrochemical ohmic resistance spectroscopy (EIS) is a widely used technique for differentiating different losses, i.e. anode activation losses, ohmic losses and cathode activation losses. Impedance, Z, a judgement of the capacity of a system to resist current flow relates to variation of time and frequency. It is given by the following Equation 25Z = (2) 5Where V(t) is time-dependent voltage = V0 cos()i(t) is time-dependent current = i0 cos()V0 and i0 are the amplitudes of voltage and currentis radial frequencyis phase shiftIt often uses sinusoidal voltage perturbation, V = V0co s(), dominating responded current, i = i0cos(), to measure impedance. So according to Equation 2, impedance Z is written by Equation 35Z = = Z0 (3)5Ionic conductivity is often investigated by impedance spectroscopy. Temperature and frequency are important factors which should be controlled accurately21. Measurements are often processed using platinum electrodes, in air. The YSZ electrolytes are coated with platinum paste on both sides. Two platinum wires which adhere to each side of the YSZ electrolyte were connected to the frequency response analyser. And the measurements are carried out under the temperature range of 200-1000C21, 22.1.3.1.2 4-Probe Method4-point investigation method is used to measure the electrical impedance of YSZ. The configuration of the 4-point investigating shown in Figure 723, is composed of four independent electrical terminals, the two probe (A and B) are used to provide current whilst the potential drop is measured by the inner terminals (C and D)23, 24 .Figure 7 Principle of 4-point probe technique23And the face concussion should be ensured when the measurement was made25. According to H. Kokabi23, before measurement, the following two assumptions must be processedThe area of measurement is uniformThe diameter of the contact point is far less than the distance between two probes.1.3.1.3 Sintered Density and Grain Size Effect on Ionic ConductivityAccording to X.J. Chen et al.21, ionic conductivity can be divided to two parts intragranular conductivity and intergranular conductivity. The former one is related to density, while the later one depends on the grain size and grain demarcation line. Intragranular conductivity increases with increasing density, and intergranular conductivity increases with the sintering temperature till 1350, then drop down21.It is reported that high densities and small grain sizes can cleanse the electrical and mechanical properties of YSZ26. In the case of the porosity, 10%, can has great reduction fo r conductivity because the pores hinder the conduction way between grains26. On the contrary, the fully dense YSZ has a maximum conductivity.Han et al.27 said that the grain boundary motion induces grain growth, which is driven by two processes grain boundary diffusion and grain boundary migration. They both make densification increase, but the latter one gives rapid grain growth22. So if dense sintering with little grain growth needs to be achieved, hindering grain boundary migration, whilst keeping grain boundary diffusion active, is a good method. The activation energy for grain boundary migration, which is the least energy to ensure migration occurring, is higher than that for grain boundary diffusion. So as D. Mland22 suggests, it is better keeping the sintering temperature to no more than 1300C, which means that grain boundary migration is inhibited, but grain boundary diffusion is active.