Advantages And Disadvantages Of Solid Oxide Fuel Cells Engineering Essay

Solid oxide fuel cells (SOFCs) are a class of device which make conversion of electrochemical fuel to electricity with negligible pollution[1]. SOFCs have two major configurations: flat planar and tubular and the SOFCs system consists of a stack that is made of many unit cells. Each unit cell is composed of two porous electrodes, a solid ceramic electrolyte and interconnects. Unlike other fuel cells, the SOFCs conduct oxygen ions from the cathode to the anode through the electrolyte, and hydrogen or carbon monoxide reacts with the oxygen ions in the anode[2]. The materials of anode and cathode have different requirements; the anode should withstand a very reducing high temperature environment whilst the cathode has to survive a very oxidising high temperature environment[3].

Among all the important fuel cells under development, the solid oxide fuel cells operate at the highest operating temperature, typically between 600 and 1000℃[4]. So the SOFCs has also been called “the third-generation fuel cell technology” because it was expected to be put into application widely 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 ionic and electronic conductivity for the cell components[5].

1.1.1 SOFC Advantages and Disadvantages

SOFCs have a number of advantages due to their solid materials and high operating temperature.

Since all the components are solid, as a result, there is no need for electrolyte loss maintenance and also electrode corrosion is eliminated[6].

Since SOFCs are operated at high temperature, expensive catalysts such as platinum or ruthenium are totally avoided[2, 6].

Also because of high-temperature operation, the SOFC has a better ability to tolerate the presence of impurities as a result of life increasing[6].

Costs are reduced for internal reforming of natural gas[6].

Due to high-quality waste heat for cogeneration applications and low activation losses, the efficiency for electricity production is greater than 50¹ªand even possible to reach 65¹ª[2, 6].

Releasing negligible pollution is also a commendable reason why SOFCs are popular today[5].

However, there are also some disadvantages in existence for deteriorating the performance of SOFCs.

SOFCs operate high temperature, so the materials used as components are thermally challenged[5].

The relatively high cost and complex fabrication are also significant problems that need to be solved[6].

1.1.2 SOFC Applications

Due to the advantages mentioned above, SOFCs are being considered for a wide range of applications, such as working as power systems for trains, ships and vehicles; supplying electrical power for residential or industrial utility[2, 7].

1.1.3 SOFC Components and Configurations

A SOFC system is composed of fuel cell stacks, which consist of many unit cells. There are two major configurations, tubular and planar, being pursued, described generally as follows.

Tubular unit cell is shown in Figure 1[8, 9]. The schematic illustrates the corresponding current flow direction and components.

According to X. Li[2], 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 2[10].

Seung-Bok Lee at el.[11] reported that since the more effective 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 1[2] lists a comparison of the two different SOFC cell configurations

Table 1 A comparison of the two different SOFC cell configurations[2]

Advantage

Disadvantage

Ease of manufacturing

Edge current collection

Tubular

No need for gas-tight cell sealing

Low-power density

Less thermal cracking due to thermal expansion mismatch

High materials cost

Lower fabrication cost

High temperature gas-tight sealing

Planar

Ease in flow arrangement

High assembly effort and cost

Higher power density

Stricter requirement on thermal expansion match

An SOFC stack consist of many unit cells, which are connected by interconnects. Figure 3[12] illustrates image of planar SOFC stack.

1.1.3.1 Cathode

The typical material for the cathode is strontium-doped lanthanum manganite (La1-xSrxMnO3, x=0.10-0.15), because of its good electrochemical activity for oxygen reduction, high electronic conductivity, good stability[2, 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 Anode

Though as for the cathode, precious metals like platinum can be used for the SOFC anode, the most widely used material is a nickel-zirconia cermet, i.e. a mixture of nickel and yttria-stabilised zirconia (YSZ) skeleton[2]. About 20¼…-40¼… porosity in the anode structure is good for mass transport of reactant and product gases[1, 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 ions[2].

1.1.3.3 Electrolyte

There 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 mol¼… yttria and gadolinia-doped ceria (GDC) is the most widely used materials suitable for the SOFC electrolyte. GDC has very good ionic conductivity, but it also shows a high electronic conductivity[5]. 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- site become vacant, as shown in Figure 4[5].

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 upper limit for the amount of doped yttria, which is shown in Figure 5[5]. The peak conductivity appears at yttria concentration of 6% to 8 mol%.

Very dense 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 made about 20-50 μm thick [1, 2].

1.1.3.4 Interconnects

Interconnects are used to connect the neighbouring cells. Materials which act as interconnect must have properties of high electronic conductivity[1]. 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 temperature[2]. 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 Conversion

The air is carried to the cathode and the oxygen reacts with electrons from the external circuit yielding oxide ions[2, 4]:

Cathode: O2 + 2e- → O2- (1)

The electrolyte does not permit the oxygen pass through it, but 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 dioxide[2, 4]:

Anode: 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 is[2, 4]:

H2 + O2 → H2O +Waste Heat + Electric Energy (4)

CO + O2 → CO2 +Waste Heat + Electric Energy (5)

The electrochemical conversion is shown in Figure 6[13].

1.2 Electrolyte Materials

1.2.1 Zirconia

Zirconia is a white ceramic, with the properties of high temperature, wear and corrosion resistance, high melting point and low coefficient of thermal expansion. Historically, the application of zirconia has been in refractory and ceramic paints[2]. However, with the development of advanced 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 parts and sensors.

Zirconia can exist in three different crystal structures: monoclinic, tetragonal and cubic. At room temperature, it naturally exists as the form of the monoclinic crystalline structure. When the temperature reaches around 1100℃, the crystal form changes to tetragonal, and then to cubic at about 2370℃[14]. Pure zirconia is never used because of its unstable properties, so many dopants are added to stabilise the higher 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 Effect of Different Yttria Contents

YSZ 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 materials[15]. However, it is mechanically weak 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 etc[16]. 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 fabrication[17, 18]. However, for 3-7 mol% Y2O3, both cubic and tetragonal phases exist in the microstructure. Table 2[19] lists comparison of phases for different yttria concentration in zirconia.

Table 2 Phase variation for different concentration of yttria in zirconia[19]

%Y

phase

2YSZ

Tetragonal with some monoclinic

3YSZ

Pure tetragonal

4.5YSZ

Cubic and tetragonal

6YSZ and higher

cubic

If the YSZ has a great volume fraction of metastable tetragonal phase, which will provide good mechanical properties (strength and toughness) to the ceramic[16]. For example, 3 mol% yttria stabilised zirconia (3YSZ) has an excellent 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 T<550°C; though this reverses when the temperature is T>550°C. That is because the activation energy 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 size[16].

1.2.3 Nanostructured Zirconia

Nanostructured ceramics are expected the average particle size is less than 20nm[20]. 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 YSZ[15].

Since nanostructured YSZ has many advantages, the development of nanocrystalline YSZ electrolyte grows rapidly. Y. Chen et al.[15], has synthesised nanocrystalline YSZ electrolyte via the plasma spray technique.

1.3 Characterisation of YSZ

1.3.1 Ionic Conductivity

Conductivity is a measurement of whether charges transport well or not. Ionic conductivity is derived from ion mobility rate, which is determined by carrier concentration c and carrier mobility u, which is shown in Equation 1 [5].

(1) [5]

where is the charge number of the carrier,

is Faraday’s constant.

1.3.1.1 AC Impedance Spectroscopy

Electrochemical impedance 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 2[5]:

Z = (2) [5]

Where: 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 current

is radial frequency

is phase shift

It often uses sinusoidal voltage perturbation, V = V0cos(), dominating responded current, i = i0cos(), to measure impedance. So according to Equation 2, impedance Z is written by Equation 3[5]:

Z = = Z0 (3)[5]

Ionic conductivity is often investigated by impedance spectroscopy. Temperature and frequency are important factors which should be controlled accurately[21]. 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 200-1000°C[21, 22].

1.3.1.2 4-Probe Method

4-point probe method is used to measure the electrical impedance of YSZ. The configuration of the 4-point probe shown in Figure 7[23], 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 technique[23]

And the face contact should be ensured when the measurement was made[25]. According to H. Kokabi[23], before measurement, the following two assumptions must be processed:

The area of measurement is uniform;

The 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 Conductivity

According 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 boundary. Intragranular conductivity increases with increasing density, and intergranular conductivity increases with the sintering temperature till 1350℃, then drop down[21].

It is reported that high densities and small grain sizes can improve the electrical and mechanical properties of YSZ[26]. In the case of the porosity, >10%, can has great reduction for conductivity because the pores hinder the conduction way between grains[26]. 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 growth[22]. 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. Mæland[22] suggests, it is better keeping the sintering temperature to no more than 1300°C, which means that grain boundary migration is inhibited, but grain boundary diffusion is active.