Solid oxide fuel cells ( SOFCs ) are a category of device which make transition of electrochemical fuel to electricity with negligible pollution [ 1 ] . SOFCs have two major constellations: level 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 behavior O ions from the cathode to the anode through the electrolyte, and H or C monoxide reacts with the O ions in the anode [ 2 ] . The stuffs of anode and cathode have different demands ; the anode should defy a really cut downing high temperature environment whilst the cathode has to last a really oxidising high temperature environment [ 3 ] .
Among all the of import fuel cells under development, the solid oxide fuel cells operate at the highest operating temperature, typically between 600 and 1000a„? [ 4 ] . So the SOFCs has besides been called “ the third-generation fuel cell engineering ” because it was expected to be put into application widely after the commercialization of Phosphoric Acid Fuel Cells ( PAFCs ) ( the first coevals ) and Molten Carbonate Fuel Cells ( MCFCs ) ( the 2nd coevals ) [ 2 ] . The solid oxide fuel cell is composed of all solid constituents with the electrolyte moving as an oxide ion music director and operating at high temperature ( ~1000a„? ) in order to guarantee equal Attic and electronic conduction for the cell constituents [ 5 ] .
1.1.1 SOFC Advantages and Disadvantages
SOFCs have a figure of advantages due to their solid stuffs and high operating temperature.
Since all the constituents are solid, as a consequence, there is no demand for electrolyte loss care and besides electrode corrosion is eliminated [ 6 ] .
Since SOFCs are operated at high temperature, expensive accelerators such as Pt or Ru are wholly avoided [ 2, 6 ] .
Besides because of high-temperature operation, the SOFC has a better ability to digest the presence of drosss as a consequence of life increasing [ 6 ] .
Costss are reduced for internal reforming of natural gas [ 6 ] .
Due to high-quality waste heat for cogeneration applications and low activation losingss, the efficiency for electricity production is greater than 50i??and even possible to make 65i?? [ 2, 6 ] .
Let go ofing negligible pollution is besides a applaudable ground why SOFCs are popular today [ 5 ] .
However, there are besides some disadvantages in being for deteriorating the public presentation of SOFCs.
SOFCs operate high temperature, so the stuffs used as constituents are thermally challenged [ 5 ] .
The comparatively high cost and complex fiction are besides important jobs that need to be solved [ 6 ] .
1.1.2 SOFC Applications
Due to the advantages mentioned above, SOFCs are being considered for a broad scope of applications, such as working as power systems for trains, ships and vehicles ; providing electrical power for residential or industrial public-service corporation [ 2, 7 ] .
1.1.3 SOFC Components and Configurations
A SOFC system is composed of fuel cell tonss, which consist of many unit cells. There are two major constellations, cannular and planar, being pursued, described by and large as follows.
Tubular unit cell is shown in Figure 1 [ 8, 9 ] . The conventional illustrates the corresponding current flow way and constituents.
Harmonizing to X. Li [ 2 ] , due to easy stacking consideration, late more and more cannular cells have the construction of cathode inside and anode outside the electrolyte bed.
The planar unit cell has a level construction with a bipolar agreement, as shown in Figure 2 [ 10 ] .
Seung-Bok Lee at EL. [ 11 ] reported that since the more effectual current aggregation by contriver interconnects, planar SOFCs have high quality in power denseness. On the contrary, the thermal and mechanical belongingss of cannular SOFCs are better than that of contriver SOFCs.
Table 1 [ 2 ] lists a comparing of the two different SOFC cell constellations
Table 1 A comparing of the two different SOFC cell constellations [ 2 ]
Ease of fabrication
Edge current aggregation
No demand for airtight cell sealing
Less thermic snap due to thermic enlargement mismatch
High stuffs cost
Lower fiction cost
High temperature airtight waterproofing
Ease in flow agreement
High assembly attempt and cost
Higher power denseness
Stricter demand on thermic enlargement lucifer
An SOFC stack consist of many unit cells, which are connected by interconnects. Figure 3 [ 12 ] illustrates image of two-dimensional SOFC stack.
The typical stuff for the cathode is strontium-doped La manganite ( La1-xSrxMnO3, x=0.10-0.15 ) , because of its good electrochemical activity for O decrease, high electronic conduction, good stableness [ 2, 4 ] .Other stuffs, like Pt and other baronial metals have besides been considered as campaigners of the SOFC cathode due to the extremely oxidising environment. However, sing the high cost of Pt, it is non best pick to utilize this metal as the cathode.
Though every bit for the cathode, cherished metals like Pt can be used for the SOFC anode, the most widely used stuff is a nickel-zirconia cermet, i.e. a mixture of Ni and yttria-stabilised zirconium oxide ( YSZ ) skeleton [ 2 ] . About 20i?…-40i?… porousness in the anode construction is good for mass conveyance of reactant and merchandise gases [ 1, 2 ] . Nickel plays the function as the electrocatalyst for anode reaction and besides can carry on the negatrons produced at the anode whilst the yttria-stabilised zirconium oxide is used for carry oning oxygen ions [ 2 ] .
There are a figure of stuffs that can be used for the SOFC electrolyte. Among them, yttria stabilised zirconium oxide ( YSZ ) , i.e. zirconium oxide doped with around 8 moli?… yttria and gadolinia-doped ceria ( GDC ) is the most widely used stuffs suited for the SOFC electrolyte. GDC has really good Attic conduction, but it besides shows a high electronic conduction [ 5 ] . Compared with GDC, YSZ is stable in either reduction or oxidizing environments and has a good conduction to convey ions, particularly at sufficiently high temperature. But unlike GDC, YSZ shows little or no capableness to carry on negatrons. Each clip two yttria ions ( Y3+ ) replace two zirconium oxide ions ( Zr4+ ) in the zirconium oxide 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 sum of yttria doped. So it seems superficially that the more yttria doped, the better the conduction. But there is an upper bound for the sum of doped yttria, which is shown in Figure 5 [ 5 ] . The peak conduction appears at yttria concentration of 6 % to 8 mol % .
Very heavy YSZ has a really low gas permeableness, which does non let the reactant gases to blend. However, since YSZ has a low ionic conduction, in order to guarantee the ohmic loss and lucifer with other constituents, it has to be made about 20-50 I?m midst [ 1, 2 ] .
Interconnects are used to link the neighbouring cells. Materials which act as interconnect must hold belongingss of high electronic conduction [ 1 ] . Ceramicss are normally used for the interconnect since the operating temperature is about 1000a„? . Mg-doped La chromite, LaCr1-xMgxO3 ( ten = 0.02-0.01 ) shows advantages because its electronic conduction typically increases with temperature [ 2 ] . However, although baronial metals have good electronic conduction, their high monetary value bounds their going a campaigner for the interconnect [ 2, 4 ] .
1.1.5 Electrochemical Conversion
The air is carried to the cathode and the O reacts with negatrons from the external circuit giving oxide ions [ 2, 4 ] :
Cathode: O2 + 2e- a†’ O2- ( 1 )
The electrolyte does non allow the O base on balls through it, but the oxide ions migrate from the electrolyte to the anode. At the anode H or C monoxide reacts with O ions to bring forth H2O or C dioxide [ 2, 4 ] :
Anode: H2 +O2- a†’ H2O + 2e- ( 2 )
CO + O2- a†’ CO2 + 2e- ( 3 )
This releases negatrons to travel through the external circuit to the cathode, therefore bring forthing an electric current.
So the overall cell reaction happening is [ 2, 4 ] :
H2 + O2 a†’ H2O +Waste Heat + Electric Energy ( 4 )
CO + O2 a†’ CO2 +Waste Heat + Electric Energy ( 5 )
The electrochemical transition is shown in Figure 6 [ 13 ] .
1.2 Electrolyte Materials
Zirconia is a white ceramic, with the belongingss of high temperature, wear and corrosion opposition, high thaw point and low coefficient of thermic enlargement. Historically, the application of zirconium oxide has been in furnace lining and ceramic pigments [ 2 ] . However, with the development of advanced engineerings, due to its stabilised and first-class belongingss mentioned above, it can be used as electrical conduction stuff in the solid oxide fuel cells, wear parts and detectors.
Zirconia can be in three different crystal constructions: monoclinic, tetragonal and three-dimensional. At room temperature, it of course exists as the signifier of the monoclinic crystalline construction. When the temperature reaches around 1100a„? , the crystal signifier alterations to tetragonal, and so to cubic at approximately 2370a„? [ 14 ] . Pure zirconium oxide is ne’er used because of its unstable belongingss, so many dopants are added to brace the higher temperature signifiers and hence avoid the detrimental tetragonal to monoclinic transmutation, e.g. MgO, CaO, Ce2O3, and Y2O3. Of these, yttria is the most common dopant, giving yttria stabilised zirconium oxide ( YSZ ) .
1.2.2 Yttria Stabilised Zirconia ( YSZ ) and the Effect of Different Yttria Contentss
YSZ is considered to be an of import electrolyte stuff for solid oxide fuel cells. The proportion of yttria in YSZ is still under research, but is frequently about 8 mol % . This yields a three-dimensional fluorite-structure YSZ, which displays good thermic stableness, good ionic conduction at high temperature and a thermic enlargement compatibility with electrode stuffs [ 15 ] . However, it is automatically weak as a consequence of the high fraction of vacancies present in the construction.
Different sum of yttria in zirconium oxide has different consequence on the belongingss of YSZ, including ionic conduction, stamina, break strength etc [ 16 ] . 8 mol % yttria stabilised zirconia ( 8YSZ ) has a three-dimensional construction with belongingss of high ionic conduction, good chemical stableness but its low mechanical strength, limits the fiction [ 17, 18 ] . However, for 3-7 mol % Y2O3, both three-dimensional and tetragonal stages exist in the microstructure. Table 2 [ 19 ] lists comparing of stages for different yttria concentration in zirconium oxide.
Table 2 Phase fluctuation for different concentration of yttria in zirconia [ 19 ]
Tetragonal with some monoclinic
Cubic and tetragonal
6YSZ and higher
If the YSZ has a great volume fraction of metastable tetragonal stage, which will supply good mechanical belongingss ( strength and stamina ) to the ceramic [ 16 ] . For illustration, 3 mol % yttria stabilised zirconia ( 3YSZ ) has an first-class mechanical belongingss of high flexural strength and good break stamina. M. Ghatee et Al. [ 16 ] besides demonstrated that 3YSZ shows higher electrical conduction than 8YSZ at T & lt ; 550A°C ; though this reverses when the temperature is T & gt ; 550A°C. That is because the activation energy of electrical conduction for 3YSZ is lower than 8YSZ at all temperatures. And the strength of the stuff is determined by grain size and defect size [ 16 ] .
1.2.3 Nanostructured Zirconia
Nanostructured ceramics are expected the mean atom size is less than 20nm [ 20 ] . And late, nanotechnology have drawn much attending because of the good mechanical belongingss, i.e. increasing of hardness, strength, of the stuffs in nano-size. It is reported that the electrical conduction 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 quickly. Y. Chen et Al. [ 15 ] , has synthesised nanocrystalline YSZ electrolyte via the plasma spray technique.
1.3 Word picture of YSZ
1.3.1 Ionic Conduction
Conductivity is a measuring of whether charges transport good or non. Ionic conduction is derived fromA ion mobility rate, which is determined by bearer concentration degree Celsius and bearer mobility U, which is shown in Equation 1 [ 5 ] .
( 1 ) [ 5 ]
where is the charge figure of the bearer,
is Faraday ‘s invariable.
184.108.40.206 AC Impedance Spectroscopy
Electrochemical electric resistance spectrometry ( EIS ) is a widely used technique for distinguishing different losingss, i.e. anode activation losingss, ohmic losingss and cathode activation losingss. Impedance, Z, a opinion of the capacity of a system to defy current flow relates to fluctuation of clip and frequence. It is given by the undermentioned Equation 2 [ 5 ] :
Z = ( 2 ) [ 5 ]
Where: V ( T ) is time-dependent electromotive force = V0 cos ( )
I ( T ) is time-dependent current = i0 cos ( )
V0 and i0 are the amplitudes of electromotive force and current
is radial frequence
is phase displacement
It frequently uses sinusoidal electromotive force disturbance, V = V0cos ( ) , ruling responded current, one = i0cos ( ) , to mensurate electric resistance. So harmonizing to Equation 2, electric resistance Z is written by Equation 3 [ 5 ] :
Z = = Z0 ( 3 ) [ 5 ]
Ionic conduction is frequently investigated by electric resistance spectrometry. Temperature and frequence are of import factors which should be controlled accurately [ 21 ] . Measurements are frequently processed utilizing Pt electrodes, in air. The YSZ electrolytes are coated with Pt paste on both sides. Two Pt wires which adhere to each side of the YSZ electrolyte were connected to the frequence response analyzer. And the measurings are carried out under the temperature scope of 200-1000A°C [ 21, 22 ] .
220.127.116.11 4-Probe Method
4-point investigation method is used to mensurate the electrical electric resistance of YSZ. The constellation of the 4-point investigation shown in Figure 7 [ 23 ] , is composed of four independent electrical terminuss, the two investigation ( A and B ) are used to supply current whilst the possible bead is measured by the inner terminuss ( C and D ) [ 23, 24 ] .
Figure 7 Principle of 4-point investigation technique [ 23 ]
And the face contact should be ensured when the measuring was made [ 25 ] . Harmonizing to H. Kokabi [ 23 ] , before measuring, the following two premises must be processed:
The country of measuring is unvarying ;
The diameter of the contact point is far less than the distance between two investigations.
18.104.22.168 Sintered Density and Grain Size Effect on Ionic Conductivity
Harmonizing to X.J. Chen et Al. [ 21 ] , ionic conduction can be divided to two parts: intragranular conduction and intergranular conduction. The former one is related to denseness, while the ulterior one depends on the grain size and grain boundary. Intragranular conduction additions with increasing denseness, and intergranular conduction additions with the sintering temperature boulder clay 1350a„? , so drop down [ 21 ] .
It is reported that high densenesss and little grain sizes can better the electrical and mechanical belongingss of YSZ [ 26 ] . In the instance of the porousness, & gt ; 10 % , can has great decrease for conduction because the pores hinder the conductivity manner between grains [ 26 ] . On the contrary, the to the full heavy YSZ has a maximal conduction.
Han et Al. [ 27 ] said that the grain boundary gesture induces grain growing, which is driven by two procedures: grain boundary diffusion and grain boundary migration. They both make compaction addition, but the latter one gives rapid grain growing [ 22 ] . So if heavy sintering with small grain growing needs to be achieved, impeding grain boundary migration, whilst maintaining grain boundary diffusion active, is a good method. The activation energy for grain boundary migration, which is the least energy to guarantee migration happening, is higher than that for grain boundary diffusion. So as D. M?land [ 22 ] suggests, it is better maintaining the sintering temperature to no more than 1300A°C, which means that grain boundary migration is inhibited, but grain boundary diffusion is active.