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    C - FUTURE SOURCE OF SUSTA...

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    SOFC - FUTURE SOURCE OF SUSTAINABLE POWER

    Dr J D Bapat *

    bstract

    he state-of-the-art on the solid oxide fuel cell (SOFC) is presented, covering the basic principles underlying its working, thermodynami

    hemical aspects, utilization of fuels and available options, cell design configurations, power generation and the materials used for electrolyt

    ectrodes. The major advantages of SOFC are multi-fuel capacity, non-requirement of expensive catalysts, high quality exhaust heat usefu

    ogeneration and wide applications. The major disadvantages are high temperature operation, limited unit cell size due brittle ceramic compo

    nd high capital cost-to-output ratio. The conduction of oxygen ions through the electrolyte takes place due to the oxygen vacancies in the c

    ructure generated due to the defect reactions. The maximum thermodynamic or the open circuit potential of the SOFC is given by the N

    quation. The actual potential is less than that predicted on thermodynamic considerations, on account of the irreversible losses occurring due

    sistance to the transport of gaseous species through the porous electrodes (concentration overpotential), activation energy barriers fo

    ectrochemical reaction (activation overpotential), resistance to the transport of ions or electrons through the electrolyte or electrodes (

    verpotential), leakage electronic current through the electrolyte (leakge overpotential) and the interfacial resistances at the material bound

    nterface overpotential). All the overpotentials are the functions of current density. The mathematical correlations commonly used to estimate

    verpotentials are given. The trends reveal that increasing working temperature results in extended operating current range and reduced irreve

    sses, but reduced open circuit potential also. The cathode contributes most to the cell irreversible losses. The four factors, commonly us

    valuate the performance of fuel cells, namely fuel utilisation factor, air ratio, power output and fuel efficiency are defined. The two ways to

    el have been discussed, namely the reforming and the direct oxidation of hydrocarbons. The direct oxidation yields maximum open cotential but the process is still under development. The internal steam reforming of fuels is preferred over external reforming, on accou

    gnificant reduction in losses and capital, operating costs. However there are drawbacks, namely possibility of carbon deposition at the

    eteriorating the cell operating performance and life and rapid endothermic cooling leading to large temperature gradients across the cell an

    terconnect. The carbon deposition can be prevented by maintaining the steam-to-fuel ratio above the limiting value calculated on the ba

    ermodynamic equilibrium considerations. However such steam-dilution of fuel leads to the reduction in its chemical potential. The probl

    rge temperature gradients is tackled, to some extent, by partial pre-reforming of fuels or by reducing the anodes catalytic activity to

    forming reaction. The oxidative reforming, in which a mixture of hydrocarbon fuel and stoichiometrically deficient quantity of oxygen is f

    e anode, is being studied for use in small size SOFC. The Nernst potential reduces with fuel utilisation. It is possible to improve th

    ficiency maintaining the fuel utilisation at low levels and increasing the fuel recycle ratio. However this measure increases the co

    omplexity of the fuel cell system. The alternative simpler approach is the multistage oxidation of fuel, in which partially utilised fuel from

    ack is fed to the next stack. The direct oxidation of hydrocarbons at high temperature, on the conventional Ni-based anodes suffers fromawback of carbon formation. Alternate anode materials are being tried to tackle this phenomenon. The fuels which could be used in S

    esides hydrocarbons, are natural gas and other hydrocarbons, methanol, ethanol and biogas. The selection of the most appropriate fuel for S

    r the given application is a multi-criteria task involving both qualitative and quantitative parameters. Based on the overall considerations, va

    el options could be arranged in the following order of preference: methane >ethanol>biogas>gasoline. The two principal design configuratio

    OFC are planar and tubular designs. The operation at elevated pressure yields higher cell power output. In a megawatt capacity plant, such S

    n successfully replace the combustor in a gas/steam turbine power generation system and achieve fuel efficiency up to 70%. The individual S

    e arranged in stack. The interconnect connects the cathode of one cell to the anode of the other, while protecting it from the reducing atmosp

    power generation unit may consist of groups of fuel cell stacks of a particular size and output, held in containment vessels. The tubular SO

    ore useful for stationary power generation, whereas the planar design for auxiliary power units, due to relatively higher power density

    quirements for the selection of materials are given. The cathode is porous and possesses high electronic and small ionic conductivity and

    talytic activity towards oxygen molecule dissociation and reduction. The ABO3 oxide system of perovskite structure satisfies most

    quirements. The commonly used SOFC cathode material is a mixture of strontium substituted lanthanum manganite (LSM) and the

    bstituted zirconia (YSZ). The anode materials are also porous and require high stability in reducing atmosphere. The transition metals best f

    quirement, as they have high catalytic activity towards hydrocarbon reforming or oxidation. The Ni-YSZ cermets are commonly used as an

    he Cu-ceria anodes have been reported to effectively tackle the problem of carbon deposition. The YSZ is commonly used electrolyte mate

    esent. The advantages in reducing the operating temperature of SOFC are stated. The mixed oxide electrolytes made from ceria doped with

    adolinia and samaria show high oxygen ion conductivity at low temperatures, but suffers from the drawback of Ce4+ reduction to Ce3+ a

    xygen partial pressures, obtaining at the anode. Promising results have also been obtained on ceria based multi-phase (heterogeneous) mate

    he doped lanthanum gallate is another promising electrolyte material under investigation. At operating temperatures below 700 0C, it is possi

    e stainless steel, which is comparatively inexpensive and readily available material, as the interconnect. The SOFC is the means of pollution

    stainable power generation of the future but its success will depend upon the effective dealing of certain aspects like reducing capital co

    https://docs.google.com/Doc?id=dfb2ks7_6hmxnfhv5&hl=enhttps://docs.google.com/Doc?id=dfb2ks7_6hmxnfhv5&hl=en
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    utput ratio, developing low cost large volume production techniques, developing materials and techniques for low temperature, mult

    peration.

    ey words: fuel cells, power generation, electrolytes, electrodes, hydrocarbon reforming

    Paper presented at the 3rd National Symposium on Forefronts of Engineering, Theme: Power for Sustainable Development, AISSMS Colle

    ngineering, 25-26 September 2008, Pune, India

    / Introduction:

    he rapid growth of industrialisation and urbanisation in the twentieth century have set apace the demand for electric power in the industrial as

    domestic sectors. The sustainable development requires that the power generation should be done making minimum use of the non-renewab

    atural resources and the environmental pollution, in all forms, should be minimised. The generation of power using fossil fuels, namely coal

    esel and that using nuclear fission, entails huge environmental pollution besides depletion of natural resources. The combustion of fossil fuel

    sults in emission of carbondioxide, which is a green house gas. The International Energy Agency estimated that in the year 1996, there were

    34x106 vehicles on road which collectively emitted some 3.7x109 tons of carbondioxide [1}; the figure must have gone up by at least 20 % by

    ow. The generation of thermal power requires the conversion of thermal energy, produced from combustion of fuel, to work. The Carnot effic

    efines the absolute limit of efficiency of any heat engine operating on thermodynamic principles. However, it is an unachievable value. The C

    ficiency relates to a heat engine that operates both internally and externally reversibly, and while the former is theoretically possible, the latternnot be achieved, if the engine produces power output. In practice, the achievable efficiency of a heat engine is just 35 to 45 %, in most case

    Whilst the alternative, renewable, energy sources such as solar panels and wind farms are useful as they are pollution free, there are limitations

    eir applicability, such as storage of solar power, portability, sustained availability of wind, and so on. Under this situation, the fuel cells appe

    e a promising means of power generation, for the future.

    fuel cell is an electrochemical device that converts the free energy of an electrochemical reaction directly into electrical energy (and heat). T

    ee energy is provided by the electrochemical oxidation of fuel, such as hydrogen, a mixture of hydrogen and carbon monoxide or a hydrocarb

    ke methane, methanol, butane or even gasoline and diesel. The fuel cell harvests the electrochemical energy by controlling the path and the m

    y which such a reaction occurs. The device is simple and contains no moving parts and only four basic functional units, namely cathode,

    ectrolyte, anode and the interconnecting device. In some aspects, fuel cell is similar to an electrolytic cell. Like an electrolytic cell, fuel cell h

    ositive and negative electrodes separated by electrolyte. However unlike the former, the reactants are not stored in the cell but continuously fe

    e cell and the products of the reaction are continuously removed. Thus fuel cell operation is a steady flow process and continues as long as the

    eady supply of reactants is assured. The fuel cells convert the energy in the fuel directly into electricity and are not subject to the second law o

    ermodynamics, in the same way as the heat engines are. They operate at higher efficiency at low loads than the high loads, which makes them

    ore suitable for vehicular application, in comparison with the internal combustion engines [2].

    he name of the fuel cell generally refers to the type of electrolyte used in the cell. The prominent fuel cell technologies are alkaline fuel cell

    AFC), phosphoric acid fuel cell (PAFC), proton exchange membrane fuel cell (PEMFC), molten carbonate fuel cell (MCFC), and solid oxide

    ll (SOFC). The Table-1 draws a brief comparison between these technologies [3,4]. The present paper reviews the latest developments in the

    OFC technology.

    / Working of SOFC:

    he SOFC is constructed with two porous electrodes which sandwich an electrolyte. Air (oxidant) flows along the cathode, which is therefore

    r electrode. The gaseous fuel flows along the anode, which is therefore called the fuel electrode. The oxygen diffuses through the porous cath

    nd comes in contact with the cathode-electrolyte interface. The oxygen molecule catalytically acquires four electrons from the cathode and ge

    duced into two oxygen ions (O=). These ions are transported to the anode-electrolyte interface, through the ion-conducting electrolyte. The f

    ffuses through the porous anode. Both, the fuel and the oxygen ions meet at anode-electrolyte interface. They react catalytically at the gas-an

    ectrolyte three phase boundaries (TPB) giving off water, carbon dioxide, heat and most importantly electrons. The products, i.e. water and ca

    oxide, are then transported back to the fuel channel through the porous anode. The electrons transport back to the cathode through the extern

    rcuit and the process continues. The SOFC has the following advantages

    High efficiency (> 60%) and low performance degradation (< 0.1%/1000 h) achievable.

    Multi fuel capacity: methane, propane, methanol, gasoline, diesel can be used

    Environment friendly: No or negligible NOx , SOx , VOC, particulates

    Modular construction, noiseless operation

    Multiple applications: stationary, transportation and military

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    Moderate power density (w/m2)

    Reasonable tolerance to gas phase poisons, S or CO

    Expensive catalysts like platinum or ruthenium are not needed

    High quality exhaust heat (800-900 0C) useful for cogeneration

    Combined cycle operation possible

    he device also has the following drawbacks:

    High electrolyte resistivity and electrode polarisation

    Formation of low conducting phases due to solid state reactions at cathode/electrolyte interface

    Brittle ceramic components, which limit the maximum cell size to 0.2 m 2, in comparison to PAFC and MCFC having a standard cell size o

    ound 0.5 m2. The smaller size of SOFC puts a limit on the scale-up of SOFC based power plants to megawatt capacity.

    High capital cost-to-output ratio.

    Higher Temperature of Operation

    / Thermodynamic and electrochemical aspects:he study of thermodynamic and electrochemical aspects reveals useful information such as the open circuit potential, thermodynamic efficien

    reversible losses in the fuel cell and various performance related factors.

    1/ Open circuit potential of a reversible cell:s explained earlier, fuel cell is an electrochemical device in which the energy released during the reaction, appears at least in part, directly as

    ectrical energy. The reactants are not stored but fed to it continuously and the products of reaction are also continuously withdrawn. Thus, it

    perates as a continuous flow system, so long as the reactants are supplied, producing a steady electric current.

    he fuel electrode (anode) is fed with fuel such as hydrogen, methane, propane, methanol, ethanol, etc., The air electrode ( cathode) is continuo

    d with the oxidant i.e. air. The solid electrolyte, through which the oxygen ions migrate from cathode to anode, has high oxygen ion conducti

    ue to high concentration of oxygen vacancies. The oxygen vacancies are produced by doping a semiconductor like zirconia (ZrO 2) by soluble

    iovalent, rare earth oxide impurity like yttria (Y2O3). The electrolyte is known as yttria stabilised zirconia (YSZ). The oxygen vacancies are

    enerated through the following defect reaction, written in the Krger-Vink notations

    -- --- (1)

    s per the Eqn. (1), one oxygen vacancy is created for every mole of dopant Y 2O3. The cathode operates in an oxidising environment at abou000 0C and participates in the following reduction process.

    O2 (g) + 2e- O= --- ---- - (2)

    he fuel (hydrogen, for example) is transported through porous anode to the anode/ electrolyte interface, where it gets oxidised at the TPB

    = + H2 (g) H2O (g) + 2e---- ----- (3)

    he electrons generated flow back to cathode through the external circuit. The overall oxidation reaction can be written as follows.

    2 (g) + O2 (g) H2O (g) --- ---- (4)

    he decrease in free energy in a chemical reaction [like reaction (4)] at constant pressure and temperature, known as the Gibbs free energy

    GP,T), represents the maximum electrical work (WE) that can be obtained from a fuel cell.

    GP,T = WE -- -- -- (5)

    the electromotive force (emf) or the open circuit potential of a reversible cell is E volts and the process involves passage of N Faradays of

    ectricity

    GP,T = NFE -- -- -- (6)

    = -GP,T / NF -- -- -- (7)

    he value of N in an electrochemical oxidation reaction is equal to four times the number of oxygen molecules required to oxidize one molecu

    el.

    onsidering fuel cell as a steady flow process, operating reversibly and isothermally, the First Law of Thermodynamics takes the form

    H = TS + WE -- -- (8)

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    WE = H - TS = -GP,T = NFE -- -- (9)

    2/ Thermodynamic efficiency:he thermodynamic or the maximum efficiency obtainable in a fuel cell is defined as

    th = (thermodynamically feasible maximum energy generation) / (total input energy) -- (10)

    -GP,T /H = NFE / H -- -- (11)

    3/ Nernst potential:he Eqn. (7) gives the open circuit potential of a reversible cell. Consider the following chemical reaction taking place in a reversible cell

    aA + bB + -- -- -- = lL + mM + -- --- - - (12)

    the reactants and the products are considered at their standard state , then the open circuit potential (E 0) is given by

    E0 = - G0/ NF -- -- - - (13)

    Where G0 is the free energy change at standard state. Similarly the free energy change for the chemical reaction is given by

    --- -- (14)

    ased on Eqn.(7) and (13), the open circuit potential of the reversible cell may be derived from Eqn. (14) as

    (15)

    Where a are the activities of the reactants and the products. In practice, the activity, as an approximation, can be expressed in terms of the

    oncentration or the partial pressure of the constituent in gas phase reactions. Thus the open circuit potential for the oxidation reaction (4) may

    ritten as

    -- -- (16)

    he Eqn. (16) is the famous Nernst equation and the open circuit potential is also called the Nernst potential.

    4/ Cell operating potential:he actual potential in an electrochemical cell is less than that predicted on thermodynamic considerations, on account of the following irrever

    sses or the so called overpotentials.

    Concentration overpotential: This loss is due to the resistance to the transport of gas phase species through the porous electrodes i.e. tnode (Ec-an) and the cathode (Ec-ct). The loss becomes important when the fuel utilisation and the current density in the cell are high [5].

    ) Activation overpotential: This loss is due to the activation energy barrier for the electrochemical reaction at the electrode-electrolyte

    terface, on the anode (Ev-an) and the cathode (Ev-ct) side. The modeling of activation overpotential requires an understanding of the element

    ermal and electrochemical reaction mechanisms and the microstructure of the electrode. The value of the activation overpotential is usually sm

    high temperatures, as obtained in the SOFC [6]. The loss becomes important when the electrochemical reaction is controlled by slow electro

    netics. In case of SOFCs using composite electrodes with mixed ionic and electronic conduction, such as Ni -YSZ for anode and lanthanum-

    rontium-manganite or LSM-YSZ for cathode, the electrochemical reaction zone can be extended from the electrode-electrolyte interface into

    ectrode resulting in the reduction in the activation overpotential. The micro structural properties of composite electrode, such as grain size an

    olume fraction of components will affect the exchange current density.

    Ohmic overpotential: This loss (Eo) is due to the resistance to the transport of oxygen ions through the electrolyte or the electronic flow

    rough the electrode. The electronic conductivity of electrodes is high in SOFC, due to high metal loading. Thus the ohmic overpotential in SO

    typically due to the ion transfer resistance through the electrolyte. The loss can be reduced by decreasing the electrolyte thickness and enhanc

    ionic conductivity.

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    ) Leakage overpotential: This loss (El) is due to the leakage electronic current through the electrolyte

    Interface overpotential: This loss (En) is due to the contact resistance at the material boundaries, created due the discontinuities.

    he cell operating potential (Ecell )may be expressed taking into account these irreversible losses

    Ecell = E - Ec-an - Ec-ct - Ev-an - Ev-ct - Eo - El En -- -- (17)

    he overpotentials mentioned in Eqn. (17) can be evaluated using correlations reported in the literature [7, 8, 9]. The correlations commonly u

    estimate these overpotentials are given Table 2. All the overpotentials are functions of current density. The trends given by these correlation

    veal that increasing working temperature of fuel cell brings about the advantage of increased limiting current density, resulting in the extendeperating range and reduced irreversible losses. The overall result is improved efficiency and power density of the fuel cell. However the draw

    reduced open circuit voltage (E) or Nernst potential, associated with the increased cell temperature [8] (Eqn. (9)) . The cell potential at any p

    space in the cell (Ecell-L) is determined by the local current density (iL) and the internal local area specific resistance (RL), which includes t

    verpotentials around the operating point

    Ecell-L = EL - iL. RL -- -- (18)

    ll the three terms on the right hand side of the Eqn. (18) vary spatially in the cell. Assuming uniformity in the cell thickness, the variation in t

    urrent density is caused by the variation in the local Nernst potential caused by the reactants (fuel) depletion and the change in the material

    sistivity caused by the local temperature variations in the cell. Thus the actual cell potential can be obtained by integrating the local cell poten

    ver the active cell area

    -- -- (19)

    5/ Performance indicators:he four factors, defined as follows, are commonly used to evaluate the performance of the fuel cells.

    Fuel utilization factor (u): It refers to the fraction of total electrical energy available in the inlet fuel that is actually used to produceectricity in the fuel cell.

    ----- --

    ) Air ratio ( :( The air ratio indicates the excess air supplied to the fuel cell, in relation to that required stoichiometrically

    --- - -- (21)

    Power output (PSOFC):

    PSOFC = (average current density)x(active anode surface area for electrochemical reactions)x

    (output potential)

    ----- --- (22)

    ) Fuel efficiency ( f): The fuel efficiency represents the fraction of total thermal energy in the inlet fuel stream, that is converted into

    ectrical energy

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    ----- --- (23)

    he net chemical energy consumed in the fuel cell is converted either to the electrical energy (power) or to the heat on account of the entropy

    hange during the electrochemical reaction and the irreversible losses due to the overpotentials.

    / Fuels utilisation and options:1/ Fuels utilisation:the fuel cell, gaseous fuel is fed at the anode and the oxidant at the cathode. There are two ways to utilize the fuel, namely (a) reforming

    ydrocarbon fuels by steam and (b) direct oxidation of hydrocarbons. However the technology of direct oxidation is under development and in

    ses, fuels are steam reformed outside or inside (internal reforming) the fuel cell. The process suffers from the drawback of the loss of chemic

    otential on account of the conversion of hydrocarbons to lower hydrocarbons, H 2, CO, CO2 [11]. The Nernst potential is highest for the direc

    xidation of hydrocarbons. It is also a function of fuel utilisation. The fuel fed at the anode inlet gets gradually converted to the oxidation prod

    hus the concentration or the partial pressure of the fuel (reactants) gets reduced as it flows towards the exit and consequently the Nernst poten

    so reduced as can be seen in Eqns. 14 to 16. The authors, Zhu and Kee [7], calculated the Nernst potentials presuming that the oxidiser is pur

    e. no depletion of oxygen in air. The two methods of fuel utilisation are briefly discussed, highlighting the current trends, in Sections 4.1.1 an

    1.2

    1.1/ Internal steam reforming:he reforming process converts the hydrocarbon fuel into a mixture of lower hydrocarbons, hydrogen, carbonmonoxide and carbondioxide. Th

    verall reaction of steam reforming may be represented as:

    ----- --- (24)

    Where m is the steam reforming factor or steam-to-fuel molar ratio. The internal steam reforming requires anode material that has good cata

    forming properties as well as those required for the oxidation of fuel in the fuel cell. At the SOFC temperature, the anode material is also acti

    wards shift reaction and the reaction, given as follows, is generally in equilibrium.

    CO + H2O H2 + CO2 --- -- - (25)

    comparison to the external reforming the internal reforming is preferred for the following reasons [12]:

    ) The catalytic steam reforming of hydrocarbons is carried out between 750-900 0C. The high operating temperature of SOFC allows for the

    ternal reforming of gaseous fuels

    ) The heat released from the fuel oxidation provides endothermic heat required for the reforming process.

    ) A part of the steam required for the reforming reaction can be obtained from the steam generated by fuel cell reaction. This is more so with

    node off-gas recycle

    ) The reforming reaction can proceed beyond the thermodynamic equilibrium, as the products hydrogen, carbonmonoxide, are continuously

    moved from the reaction site by the fuel cell reactions

    ) The excess air load required for cooling at the anode is reduced due to endothermic nature of the reforming reaction. In a typical case of

    ethane reforming, the cooling load is reduced by 40-50 %.

    ) Significant reduction in capital and operating costs in comparison to external reforming

    ) Capacity to offer faster response to load variations

    he internal steam reforming on the Ni-YSZ based anodes has the following limitations

    ) There is a risk of carbon formation on the anode, according to the Boudouard reaction

    --- - -- (26)

    he carbon formation deteriorates the cell operating performance and life, hence it should be prevented. The criterion for carbon formation in

    forming reaction is given by the Eqn. 27, based on the equilibrium concsiderations [12].

    -- -- - (27)

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    he Eqn. (27), giving the limiting carbon activity, states that the carbon formation is thermodynamically favored when a c 1. The limiting ste

    -fuel ratios, corresponding to the limiting carbon activity, above which carbon formation is thermodynamically impossible, for gasoline, etha

    ethane and methanol fuels, have been calculated by Douvertzides et.al. [13]. Higher steam-to-fuel ratios lower the electrical efficiency of the

    ll due to steam dilution of the fuel. Hence there is a search for advanced catalysts, which will allow internal reforming at low steam-to-fuel

    tios.

    ) Excessively large temperature gradients can develop across the cell and the interconnect, due to endothermic cooling. The reforming is usua

    omplete within a small distance on the anode leading to strong cooling effect and large temperature gradients.

    here are two common approaches to tackle the problem, mentioned as follows:

    i. Partial pre-reforming of fuels: With increased levels of pre-reforming, some of the advantages of full internal reforming of fuel aresacrificed.

    ii. Reduction of reforming rate on the anode: This is achieved by partial poisoning of active sites. Ahmed and Foger [12] report that both tnumber and the nature of active sites are changed by the incorporation of a basic compound in Ni/YSZ anode formation and as a result,

    much lower reforming rates are obtained. Similar results have also been reported by Finnerty et.al. and Marina et.al, using Mo and Ce

    dopants [14]

    oth the techniques mentioned as above need to be optimised. Hiei et.al. [15] report about the oxidative reforming of methane, in which a mixt

    methane and stoichiometrically deficient quantity of oxygen is fed to anode. The methane gets partially oxidized in the first step

    CH4 + O2 CO + 2H2 (exothermic) - -- (28)

    ubsequently complete oxidation of methane takes place, leading to generation of electric power

    CO + O2 CO2 (exothermic) -- --- (29)

    H2 + O2 H2O (exothermic) -- --- (30)

    his is followed by the reforming of CH 4 with CO2 and H2O producing gaseous mixture of CO and H2 (synthesis gas) which can be used for

    nthesis of methanol

    CH4 + H2O CO + 3H2 (endothermic) -- -- (31)

    CH4 + CO2 2CO + 2H2 (endothermic) -- -- (32)

    he reaction heat is smoothly removed and consequently thermal stresses are reduced. The feeding of oxygen (air) for the purpose of oxidative

    forming is easier in comparison to that of steam required for steam reforming. The authors claim that high electric power and the synthesis ga

    onsisting of CO and H2 in the molar ratio of 2:1 was obtained using La0.9Sr0.1Ga0.8Mg0.2O3, Ni, and La0.6Sr0.4CoO3 as the electrolyte, the

    node and the cathode respectively. However, oxidiser (oxygen) in fuel channel does not take part in the current generation; only fuel oxidation

    ith oxidiser from the cathode can produce power. Thus any oxidiser in fuel channel must reduce the cell potential because of its diluting effec

    he small size SOFCs (typically of 2.5 mm outside diameter, 50 mm length and 150 m thickness) are currently being studied for applications

    portable power units, domestic heat and power generators and automotive power units. Using butane as fuel Sammes et.al. [14] observed tha

    eam reforming is inadequate for the operation of small size SOFC system, irrespective of the presence of dopants on the anode, instead the

    xidative reforming using butane, air mixture proved promising.

    can be seen in Eqn. (14) to (16), the Nernst potential reduces with fuel utilisation. Thus it is possible to improve the cell efficiency by keepin

    onversion at low level (thereby ensuring practically uniform potential from inlet to exit ) and operating the cell with a high fuel recycle ratio,

    ombined with the removal of reaction products ( H2O and CO2 ) external to the cell. Thus the fuel recycling my increase the total system

    ficiency of a single-stage system but with considerable cost in the capital and complexity. The alternative and simpler approach is the multist

    xidation of fuel. The Eqn. (11) assumes cent percent fuel utilisation and it also does not account for the variation in G with fuel utilisation.

    ewriting the Eqn. (11), accounting for the variation ofG with fuel utilisation

    -- -- (33)

    he integral in Eqn. (33) may be viewed as the summation over infinite number of reversible cells fed in series. Elangovan et.al. [16] examined

    ultistage oxidation concept on a configuration consisting of multiple stacks with sequential fuel flow. In that system, the fuel exhaust from on

    ack, containing partially utilised fuel, was fed to the next stack. The planar SOFC design and the co-flow arrangement of fuel and air streams

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    onsidered. The analysis and the experimental verification of the multistage oxidation concept showed that the stack efficiency increases due to

    maller variation in the Nernst potential across the cell area. The system reliability is increased due to reduced temperature gradients. The high

    ower density was also observed. The multistage system eliminates the complexity of the fuel recycling system.

    1.2 / Direct oxidation of hydrocarbons:fuel cells, the direct oxidation may be defined as the oxidation of hydrocarbons (to H 2O, CO2) at the anode in the absence of oxygen suppl

    e fuel stream. Theoretically, direct oxidation of any hydrocarbon fuel is possible in fuel cell, as the species that migrate through the electrolyt

    xygen anions. The elimination of reforming step will reduce the complexity of the fuel cell system. However, there is a thermodynamic drivin

    rce towards carbon formation, with dry hydrocarbons at high temperatures.

    CH4 C + 2H2 --- - - (34)

    he direct oxidation of methane and the other hydrocarbons is reported in the literature. Using conventional Ni-based anodes, Murray et.al. [1

    ported excellent power densities and stable operation for dry CH4 fuel between 500-7000C, because carbon formation is not thermodynami

    vored at these temperatures. The use of conducting oxides as anodes, like ceria and bismuth oxide, has been reported to solve the problem, as

    o not catalyse C-C bond formation in the same manner as the nickel does [18]. However the power densities achieved were too low for practi

    e. One limitation of the direct oxidation is that the free radical chemistry can lead to the carbon formation in the anode compartment even in

    bsence of catalyst. Gorte [19] observed this phenomenon while using butane as fuel at 1070 K, when tars were formed on the sides of alumina

    he typical electrochemical reaction for direct oxidation, in case of methane and n-butane could be written as follows

    H4 + 4O= CO2 + 2H2O + 8e- -- -- (35)

    C4H10 + 13O= 4CO2 + 5H2O + 26e

    - -- -- (36)

    budula et.al.[20] studied the direct oxidation of methane at low concentrations (4-9 %) at 1000 0C, considering Ni/YSZ cermet anode as a

    ompletely mixed reactor. The threshold current, Itsh, below which the carbon deposition is favored, is given by the following Equation.

    --- - - (37)

    he value of threshold current as obtained by the experiments conducted by the authors and as given by the Eqn. (37), will be more when, (a) t

    node thickness increases, as the area of tpb also increases in proportion, (b) the diameter of NiO particles decreases, (c) the methane concentra

    creases, (d) the temperature increases. Thus to keep the rate of carbon formation or the threshold current at the minimum, there is a need toptimize the thickness of anode, YSZ electrolyte (to minimise ohmic overpotential), anode structure (NiO particle size) and the operating

    mperature of SOFC.

    2 / Fuel options:he research conducted in the area so far indicates that, besides hydrogen, the natural gas (methane) and other hydrocarbons (butane, gasoline,

    ethanol, ethanol, biogas can be used as fuels in the fuel cells. The direct oxidation is welcome but the process is under development; hence in

    ost cases, steam reforming of fuels is carried out. The steam reforming produces H2 and CO. It is hydrogen which primarily contributes towa

    e power generation, whereas CO is consumed in the shift reaction producing additional hydrogen.

    he methane and the other hydrocarbons are mineral fuels. Their deposits are limited and their oxidation contributes to the generation of green

    as (CO2), hence they do not provide a long term solution to the energy problem. The utilization of other renewable, environment friendly fuel

    ch as methanol, ethanol or biogas is drawing considerable attention of the researchers. The ethanol is considered a very promising and reliablel option because it can be alternatively produced biochemically from the biomass.

    he Eqn. (24) gives the overall reaction of steam reforming of a fuel represented by CxHyOz. Douvertzides et.al. [13] analyzed the performanc

    ethane, methanol, ethanol and gasoline fuels for the generation of electric power in SOFC, in terms of the electromotive force (emf) and the

    stems efficiency. The limiting reforming factor, m (steam-to-fuel ratio), varies as per the boundary of carbonisation for different fuels and

    mperatures. The authors have made the following observations, based on their analysis.

    1. Higher the carbon content of the fuel higher is the reforming factor (m), at a given temperature.2. The limiting value of m, as given by the boundary of carbonization, decreases as the temperature increases.3. The behavior of methane and ethanol is similar for T >1100 K; for example, under that condition they have an identical value of steam

    fuel ratio as well as the maximum average emf generated.

    4. As the temperature increases, the equilibrium concentration of H2 and CO produced after steam reforming of hydrocarbons, also increa

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    5. The optimum T and m for the operation of SOFC correspond to their values at the carbonization boundary, as these conditions of reformyield the maximum possible fuel-to-oxidant ratio.

    6. The maximum average emf obtainable, for the four fuels studied, decreased with the temperature and so did the overall efficiency of thcell. However the worst absolute value of efficiency for any fuel and temperature is still above 80 %

    7. The actual fuel cell efficiency was correlated with the maximum (thermodynamic) efficiency as follows:

    ---- - -- (38)

    -- -- (39)

    he expression in Eqn. (38) shows that the fuel cell efficiency is half of the maximum efficiency,

    hen it runs at full power (pr = 1), as the conditions are far from reversibility.

    8. The selection of most appropriate fuel for SOFC is a multicriteria task involving both qualitative and quantitative parameters. Based onevaluation, the authors arranged the fuel options in the following sequence.

    methane>ethanol>methanol>gasoline

    he choice of the fuel and its processing also depends upon the type of application. Sammes et.al. [14] found that for small size SOFC , the so

    lled micro-SOFC working on butane fuel, the steam reforming is inadequate. Herle et.al. [21] observed that the use of biogas as the SOFC fu

    ill encourage wide scale use of SOFC units in 5-500 kW range at farms, landfill sites, sewage treatment plants, organic solid waste digesters

    ganic liquid effluent treatment sites. The use of fuels like methanol or ethanol will be more advantageous for the SOFC applications in the

    ectric motor vehicles.

    / Cell design configurations and power generation:

    he two principal design configurations of SOFC, namely the planar and the tubular designs, are discussed in this Section. The typical concept

    rangements, showing the basic building blocks, of the stationary and the auxiliary power units based on SOFC, is also presented. The details

    e design and the operation of SOFC based power plants have been kept out of the purview of this paper.

    1/ Cell design configurations:OFC are designed in planar and tubular configurations of SOFC [22]. In the planar design, the cell components are configured in thin, flat pla

    he interconnection has ribs on both sides and serves as bipolar gas separator, connecting the anode and the cathode of the adjoining cells. In th

    bular design, the components are arranged in the form of a hollow tube, with the cell i.e. the electrolyte and the anode constructed in layers ar

    e tubular cathode. Air flows through the inside of the tube and the fuel flows around the exterior. The Table 3 shows typical materials andbrication processes for SOFC [22, 23]

    he Table 4 compares the two configurations in terms of the achievable power density, requirement of the high temperature seals, the

    anufacturing cost and the limitations. The tubular SOFCs have also been operated at high pressures up to 15 atm. The operation at elevated

    essures yields higher cell power at any current density due to increased Nernst potential and reduced cathode overpotential. The voltage outp

    rectly proportional to the partial pressure of oxygen at the cathode and that increases with the pressure. The SOFCs can be successfully used

    placement for combustors in gas turbine power generation systems, with pressurised operation. Such hybrid, pressurised SOFC- gas turbine

    ower systems, in multi-megawatt size, are expected to reach efficiency up to 70 % and thus result in reduced fuel consumption and capital cos

    nit power output [24]. The concept of mass customisation, initiated by the Solid State Energy Conversion Alliance (SECA) of the United State

    epartment of Energy, has enabled to bring in sizeable cost reductions in planar design SOFCs [25]. The concept involves the development of

    W size core planar SOFC modules which can be mass produced and the combined to obtain power for different applications in stationary,

    ansportation and military sectors. The anode supported planar SOFC design use electrolyte at a very low thickness of 5-20 m, which decreas ohmic overpotential and makes it suitable for low temperature operation. It is possible to obtain very high power densities in such cells, as

    quired by the mass customisation concept.

    2/ Power generation:he individual SOFCs are arranged in stacks, for the purpose of power generation. The cells need to be connected together and a mechanism f

    ollection of electrical current need to be provided. In a typical case, the tubular cells are arranged in rectangular arrays called bundles. The

    terconnect connects the cathode of one cell to the anode of the other, while protecting it from the reducing atmosphere of the anode. The

    dividual cells in the bundle are connected in both series as well as parallel arrangements. These bundles or the modular groups of the individu

    lls are used to construct a stack. The stacks have the necessary mechanical arrangements and electrical connections to support, separate, insu

    nd service the bundles of individual cells. The Siemens Westinghouse fabricated a 100 kW power generation system, using this scheme. The

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    OFC stack in the system consisted of 1152 cells, each of 2.2 cm diameter and 150 cm active cell length, arranged in 12 bundle-rows. Each row

    omprised of four cell bundles and each bundle contained 24 cells (12x4x24 = 1152). The system was successfully operated for 2 years in the

    etherlands on desulphurised natural gas without any significant performance degradation [24]. The fuel cell power generating unit consists of

    umber of stacks of a particular size and output. In a modular construction power plant, groups of stacks are held in containment vessels. The

    utput, size and the shape of the vessels must be such as to give maximum reliability and maintainability to the power station. Another advanta

    e containment vessel is that if the SOFCs are to operate at elevated pressure, the vessel can be pressurised, thereby reducing the mechanical a

    ermal loading on the fuel cell components [3]. The APU is used in the transport and military applications. As mentioned in Table 4, the tub

    OFC is more useful for stationary power generation due to lower power densities, whereas the planar design finds greater application in APU

    high power densities. The APU requires fast heat up and ability to be thermally cycled.

    / Materials:

    mong the key features of SOFC are the solid-state materials of construction and high temperature (800-1000 0C) operation. The SOFC can b

    bricated in thin layers and configured in different shapes, as all components are in solid state. This feature permits compact, lightweight cell

    esigns and additional performance improvement. The high operating temperature makes it suitable for internal reforming or direct oxidation o

    ydrocarbon fuels as well as for integration with gas/steam turbines to form highly efficient, hybrid power generation systems, as discussed ear

    he materials for different cell components are selected based on the following criteria:

    1. Suitable electronic and/or ionic conducting properties required to perform intended cell functions2. Adequate chemical and structural stability at high temperatures during cell operation as well as during cell fabrication.3. Minimal reactivity and inter-diffusion among the cell components.4. Matching thermal expansion among different cell components

    5. Ability to be formed into films with desired microstructure and good adherence to each otherthe past few years, significant developments have taken place in the area of materials science and technology for SOFC. These development

    clude, (i) cathodes with improved microstructure to reduce the overpotential losses and high electronic conductivity, (ii) anodes for direct

    xidation of hydrocarbon fuels, (iii) electrolytes for operation at reduced temperature, (iv) interconnect or bipolar plate (in planar design) for h

    mperature operation and (v) fabrication and processing techniques to make thin film components with required microstructure [10].

    he SOFCs commonly operate at temperatures around 1000 0C. These temperatures lead to materials constraints, high cost of manufacture and

    oblems of long term stability. Lowering the operating temperature makes the fabrication and operation of SOFC more cost effective. On the

    and, decreasing fuel cell operating temperature limits the performance of the electrodes, affecting the cell power generation capacity. The cath

    olarization resistance rapidly increases with the reduction in operating temperature. The La0.9Sr0.1MnO3 cathode, for instance, exhibits an ar

    ased polarisation resistance of less than 1.0 ohm.cm2 at 1000 0C which increases to above 2000 ohm.cm2 at 500 0C. [26]. The rate of interna

    forming has also been found low at reduced temperatures.

    the proceeding Sections, a brief review will be taken of the current developments in the materials for the SOFC components. The details of tbrication and processing have been kept out of the purview of this paper.

    1/ Cathode materials:

    he cathode must meet all the general requirements and in addition it must possess high electronic conductivity (> 100 ohm-1.cm-1), small ion

    onductivity (0.1 ohm-1.cm-1) and sufficient porosity to facilitate transport of molecular oxygen from the gas phase to the electrode-electrolyt

    terface. It must also possess high catalytic activity towards oxygen molecule dissociation and oxygen reduction. In the tubular design, the cat

    ontributes over 90 % of the cells weight and therefore provides structural support to the cell.

    he ABO3 oxide system of perovskite structure satisfies most of these requirements; where A is a rare earth element (mostly La in SOFC cath

    nd B is a transition metal (Fe, Ni, Co, Mn). The most commonly used cathode material is lanthanum manganite, LaMnO3, which is a p-type

    erovskite. The alkaline earth cations (Sr2+, Ca2+) partially substitute the rare earth element, to enhance its conductivity. Such cathode materia

    ferred to as La1-xSrxMnO3 or LSM and behaves as a mixed (electronic + ionic) conductor. It has been found that the conductivity is highest

    0.5. In general, the SOFC cathodes use a mixture of LSM and YSZ . The ionic conducting YSZ is added to LSM to extend the triple phase

    oundary, where oxygen reaction takes place.

    he LSM has adequate functionality at intermediate (about 700 0C) temperatures. Recently various approaches have been reported to engineer

    thode microstructure to improve performance at reduced temperatures. Some of the approaches reported by Minh [10] are (a) catalysed YSZ

    ectrolyte interface with a small amount of dopant such as Mn and Ce, (b) use of highly dispersed (nanometer size) noble metal particles on L

    rface, (c) structured electrolyte surface covered with a thin microporous LSM film.

    he fabrication of LSM depends upon the cell design. In the tubular SOFC, for instance, the cell is constructed extruding the cathode tube and

    uilding the rest of the cell around it. In a typical planar SOFC, cathode is designed as a bottom supporting layer, fabricated by tape casting usi

    anoscale particles, followed by sintering.

    2/ Anode materials:

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    he anode (fuel electrode) must meet most of the requirements as that of the cathode, like high electrical conductivity, compatibility of therma

    xpansion and porosity. In addition, anode materials require high stability in reducing atmosphere (CO, H 2, CH4) and the surroundings contain

    ace levels of higher hydrocarbons, particulate matter and sulphur compounds. The transition metals are the best because of their high catalytic

    tivity.

    he most commonly used anode for hydrogen oxidation is porous cermet structure consisting of Ni metal powder and YSZ particles. The YSZ

    ramic phase mainly has mechanical function, namely supporting the Ni particles, preventing them from sintering together and closing the po

    ving electrode a thermal expansion coefficient close to that of YSZ electrolyte. The microstructure of cermets, i.e. particle size and distributio

    oth the phases, optimum surface area, connectivity of Ni particles, porosity and the triple phase contact between gas, electrode and electrolyte

    n important role in determining the performance and the long-term stability of anode in the fuel cell operating environment. The gas phase in

    ores accounts for about 50% of the volume. Thus the Ni-YSZ structure has a large internal pore surface area. This makes it possible to mainta

    rge external current densities on the one hand, while keeping the internal (local) current densities and the resultant overpotentials low, on the o

    he slurry of Ni is applied over the cell and then YSZ is deposited by electrochemical vapor deposition. Alternatively, Ni-YSZ slurry is applie

    ntered. More recently, NiO-YSZ slurry has been used wherein NiO gets reduced to particulate Ni in the firing process. According to the recen

    search finding, using freeze drying approach, it is possible to develop excellent porous structure [27].

    s discussed earlier (Section-4 ), the hydrocarbon fuel systems depend upon reforming- external or internal. When fuel is reformed by steam, t

    gh equilibrium conversions require high temperatures, for example the equilibrium conversion of methane at methane/steam ratio of 1.0 at 1

    essure is 37 % at 600 0C, 68 % at 700 0C, 87 % at 800 0C [28]. Even if the reformation reaction is carried out externally, the operating

    mperature at the anode will have to be maintained at higher levels to prevent the methanation (Reaction-31 in the reverse direction) as the ni

    also one among the best methanation catalysts [29].

    he nickel and most other reforming metals are also excellent catalysts for C-C bond formation and therefore tend to catalyse the formation of

    rbon deposits. When the internal reforming of fuel is carried out in SOFC, the problem of carbon deposition is encountered at the anode

    Reaction-26), especially when the reforming factor m (steam/fuel ratio) is below the thermodynamic minimum, as indicated by the carbonisa

    oundary. Therefore more steam is added to fuel only to keep m above the carbonisation threshold.

    he direct oxidation of methane and other hydrocarbons, without the addition of steam or oxygen, has been achieved by number of groups [19]

    en that the C-C bond formation is not thermodynamically favored in case of dry methane, between 500-700 0C, using conventional nickel b

    nodes. However, this thermodynamic window of stability is different for different hydrocarbons and for some it does not exist. Some workers

    ccessfully approached the problem of carbon formation by using conducting ceramic oxides as anodes, since they do not favor the formation

    bond in the same way as the nickel based anodes do. Some of the ceramic oxides which have been tried are ceria and bismuth oxides,

    a0.8Ca0.2CrO3, Sm-doped ceria. However the power densities with this type of anodes are always low due to their limited electronic conduct

    comparison to metals.

    he nickel has good tolerance towards small levels of sulphur in the fuel, however degradation is observed when it was exposed to gaseous sul

    ompounds during the fuel cell operation [30].Therefore alternative anode materials are being investigated, which will prevent C-C bond formafer higher microstructural stability and higher tolerance to impurities, such as sulphur in gas phase.

    ark et.al. [31] report that that the problem of carbon deposition can be effectively tackled by replacing Ni-YSZ anode by Cu-Ceria anode. Th

    opper is an excellent electronic conductor but a poor catalyst for C-C bond formation, therefore carbon formation is avoided by the Cu-cermet

    he ceria has high activity towards hydrocarbon oxidation and high ionic conductivity. The tubular SOFC used in the experiment had 12.5 mm

    ameter, cathode containing LSM and YSZ mixed in 1:1 proportion , 60 m thick YSZ electrolyte and anode made from 40 % Cu, 20 % CeO

    m/m) held in place by YSZ matrix formed from zircon fibres. The authors recommend the use of thin film electrolytes to obtain higher power

    ensities at low operating temperatures. Minh [10] reports that using the new anode material, direct oxidation of methane, ethane, butane, deca

    utene, toluene and synthetic diesel is possible, without carbon formation and peak power densities above 200 mW/cm2 have been achieved ab

    00 0C. The stability of Cu to sintering after long time exposure to water vapor and CO 2, needs to be further investigated.

    3/ Electrolyte materials:t present SOFCs mostly use yttria stabilised zirconia (YSZ) as electrolyte material. The conventional design SOFCs typically employ electrol

    icker than 50 m and operate at temperatures of about 1000 0C to minimise the electrolyte ohmic losses. The ionic conductivity of YSZ at 10

    ole percent yttria is the maximum at 0.1 ohm-1cm-1, at 1000 0C and the electronic transference number is low (

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    he high temperature operation of SOFC results in several materials related problems mentioned as follows [32]

    1. Development of poor conducting phases due to interfacial diffusion. The lanthanum and strontium, for instance, from cathode, diffuse iYSZ electrolyte forming poor conducting SrZrO3 and La2Zr2O7 at the cathode side, especially when (La, Sr)CoO3 cathode is employe

    2. Mechanical stresses due to differential thermal expansion of cell components, destroying good surface contact.3. Corrosion of compnenets4. High cost of materials that withstand high temperatures on long term.

    hus decreasing operating temperature of SOFC is important both from technical as well as commercial angles. The need is felt to develop oxi

    n electrolytes with high ionic conductivity at low temperatures; two promising cases are discussed below.

    3.1/ Doped Ceria:he pure ceria (CeO2) has a cubic fluorite structure and negligible ionic conductivity However it shows high oxygen ion conductivity and low

    tivation energy for conduction, when doped with aliovalent cations such as yttrium (Y), gadolinium (Gd) or samarium (Sm) by forming oxid

    ased mixed electrolytes such as yttria doped ceria or YDC (CeO 2-Y2O3) [33], gadolinia doped ceriaor GDC (CeO2-Gd2O3) [34], samaria d

    ria or SDC (CeO2-Sm2O3). It has been reported that ceria based, fluorite type, gadolinium doped solid solutions exhibit highest oxide ion

    onductivity when the amount of Gadolinium oxide is 20 mol percent. The doped ceria however tends to undergo reduction (Ce4+ Ce3+) at t

    w oxygen partial pressures (fuel side) with consequent introduction of electronic defects (electronic conductivity). This reduction tendency

    creases with temperature. Therefore, for efficient operation, doped CeO2 is used at 5000C or at lower temperatures. Another approach is to

    with more stable ionic conducting compound such as YSZ, on the fuel side [10].

    hu et.al. report results on innovative nano- and hybrid-conducting ceria based composite materials [35]. These composite, multi-phase, materi

    ere prepared by (a) mixing cationic doped ceria with salts like chlorides, fluorides, hydroxides or carbonates, (b) mixing ceria with other rare

    rth oxide different from ceria, like lanthanum oxide (La 2O3). The conventional doped ceria (YDC, SDC, GDC, etc.) is a single phase materi

    olid solution) with strict restrictions on dopant kinds and dopant levels. Whereas, ceria based composites are two or multi-phase (heterogeneo

    aterials. The amount mixed is larger than the normal doping levels, as it is not controlled by the solubility of the compound mixed. It has bee

    und that the significant increase in the ionic conductivity and diffusivity is achieved due to the interfacial conducting path between the two ph

    he high defect concentration existing in the nano-structured host oxide phase can provide a large number of active sites for ion conduction as

    that for gas-solid catalysis. The authors further report that the ceria composites exhibit dual conduction, oxygen ions from ceria and proton

    om salt . The phenomenon was actually seen during the experiments, when water formation was observed on both sides of the electrode. This

    onduction enhances the overall material conductivity and may also promote the electrode reaction and the kinetics at TPB, resulting in high cu

    utputs. The conductivity in the region of 0.01 to 1.0 ahm-1.cm-1 was obtained between 300 to 700 0C, using these materials. The cell could

    nction at temperatures as low as 200 0C. The two-cell stack studied by the authors showed expected performance for several months. The ne

    ndings will promote further research in the development of low temperature SOFC.

    3.2/ Lanthanum gallate:ecently a new family of perovskites (ABO3 structure), based on LaGaO3, have shown potential for the use as electrolyte in SOFC [36]. In the

    aterials, lanthanum can be partially replaced by Sr, Co or Ba or some other rare earth element such as Nd or Sm , at A-site. Whereas gallium

    te can be partially substituted by Mg, In, Al or Zn. It has been shown that for Sr substitution of at least 10-20 % at A-site and Mg substitutio

    etween 10-20 % at B-site, the materials have relatively high ionic conductivity in both oxidising and reducing atmospheres, over wide range

    xygen partial pressure and TEC comparable with the other fuel cell components. The conductivity of La 0.9Sr0.1Ga0.8Mg0.2O2.85 (LSGM) is

    ur times that of YSZ at 800 0C, as an example. The conductivity of this composition can be further enhanced by doping the material with a s

    mount of Co or Fe. The SOFC based on LaGaO3 has been fabricated and tested using various cathode and anode materials. However the new

    ectrolyte is unstable in fuel (reducing) environment [30]. The depletion of Ga takes place under reducing atmospheres. The addition of Sr at

    te accelerates the depletion further. Thangadurai and Weppner [32] report that the instability of LSGM structure, under reducing atmosphere,e effectively dealt by applying an interlayer of CeO2 doped with 40 mol percent La2O3 between anode and the LSGM based electrolyte. How

    a diffusion at the cathode/electrolyte interface is still a potential problem, which needs further investigation. The pervoskites based on lanthan

    allate are very sensitive to small change on stoichiometry and lead to formation of impurity phases like SrGaO 3, La4SrO7, LaSrGaO4,

    aSrGa3O7. It is essential to control Sr/Mg ratio in order to obtain single phase materials.

    t lower operating temperatures (700-800 0C), it is possible to use very thin film electrolyte (up to 10 m), to reduce its ohmic resistance and

    crease ionic conductivity. However, the denseness of film need to be maintained, to avoid short-circuiting of gases. In general, the oxygen io

    onductivity increases in the following order YSZ < doped ceria < LSGM [30].

    SOFC, a solid oxide electrolyte is sandwiched between the electrodes. The interfacial resistances exist on account of the materials characteris

    well as the discontinuities remaining during the fabrication processes. The research is being carried out to develop SOFC with only one mate

    ith homogeneous composition, to effectively deal with the problem of interfacial resistances. Such a material becomes excess electron condu

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    the anode and hole conducting at the cathode, but remains predominantly ion conducting at the intermediate activities of the elctroactive

    omponents. The solid oxides with perovskite ABO3 structure are considered a promising material for the purpose, in view of its large flexibil

    r different substitutions both at A and B sites [32,37]

    4/ Interconnect materials:power generation, the cells need to be connected together and a mechanism for collection of electrons giving rise to electrical current need to

    ovided; hence the requirement of interconnects. The interconnect connects the anode of one SOFC to the cathode of the other. Thus it will be

    xposed simultaneously to the reducing environment at the anode and the oxidising environment at the cathode. The high operating temperatur

    e cell combined with the severe environment means that the interconnect must satisfy the most stringent materials requirements of all the cell

    omponents, given as follows [38].

    1. High electronic conductivity (> 1.0 ohm-1cm-1) with small variation within the oxygen partial pressure range form air to fuel gas2. Chemical and phase stability in oxidising and reducing conditions and during fabrication3. Matching TEC with cell components4. Physical and electrochemical gas tightness

    he interconnect material of choice for SOFC, using YSZ electrolyte and operating at 1000 0C, is LaCrO3 doped at A-site with rare earth elem

    Ca, Mg, Sr etc.) to improve its conductivity. Ca-doped yttrium chromite (YCrO3) is also being considered as it has better thermal expansion

    ompatibility, especially in reducing atmosphere. The interconnect is applied to the anode by plasma spraying and the entire cell is co-fired.

    he thermal conductivity of the conventional interconnect materials is extremely low

    5 Wm-1k-1), which imposes severe thermal gradients in the fuel cell stack. Moreover the materials expand differentially on heating in oxidi

    reducing environments, leading to stresses and warpage of interconnect bipolar plates (planar design) . The materials are brittle and the costs

    sociated with fabrication, forming and machining are relatively high [30].

    he metals or alloys are preferred as interconnects due to their relatively low cost, easy machinability and high thermal as well as electrical

    onductivity. However most metals have TEC much higher than other cell components and they also corrode rapidly at SOFC operating

    mperature. Thus only high temperature oxidation resistant alloys with built-in protection mechanism are suitable. At operating temperature o

    00-1000 0C nickel base alloys like Inconel-600, below 800 0C ferritic steels and below 700 0C it becomes possible to use stainless steel, whi

    omparatively inexpensive and readily available.

    5/ Seal materials [30]:he tubular SOFC does not require a seal. However seals are an integral part of planar design, as they are required to stop mixing of air and fue

    he seals are exposed to oxidising and reducing environment. The seals must remain leak-proof and insulating over the lifetime of the stack (>

    0,000 h). They must withstand thermal cycles and pressures above atmospheric. The commonly used materials are cement, glass or glass-cera

    omposites.

    / Future of SOFC:he SOFC is the means of pollution free and sustainable power generation of the future. However improvements in certain areas, briefed as foll

    eed to take place before SOFC finds a wide application on commercial scale

    1. Reducing capital cost-to-output ratio2. Low cost, large volume production techniques3. Broadening the application range in stationary, transportation and military sectors4. Performance improvement in terms of electrolyte resistivity and electrode polarisation5. Low temperature operation.6. Stability of materials7. Internal reforming and direct oxidation of fuels.

    8. Capacity to handle multiple fuels.9. Fast start-up.

    10. Thin film and cost effective fabrication processes.11. Brittleness of ceramic components.

    / Conclusions:

    he following broad conclusions can be drawn on the basis of foregoing discussion.

    1/ The fuel cell is an electrochemical device that converts the free energy of an electrochemical reaction directly into electrical energy (and h

    he free energy is provided by the electrochemical oxidation of fuels which may be hydrogen, carbon monoxide or hydrocarbons. The operati

    steady flow process and continues as long as the continuous supply of fuel and the oxidant is assured. As the free energy of he chemical react

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    rectly converted to the electrical energy, the operation of fuel cell is not subject to the second law of thermodynamics. The name of fuel cell

    enerally refers to the type of electrolyte used in the cell.

    2/ In SOFC, the solid oxide electrolyte is sandwiched between two porous electrodes. The oxygen, supplied at the cathode, gets reduced to ox

    ns and diffuses through the electrolyte to the anode-electrolyte interface. The gaseous fuel, supplied at the anode, also diffuses to the anode-

    ectrolyte interface and catalytically reacts with oxygen ions, giving off water, carbon monoxide, heat and electrons. The gaseous products ar

    ansported back to the fuel channel through the porous anode and the electrons flow back to the cathode through the external circuit.

    3/ The SOFC has advantages like high efficiency with low performance degradation, multi-fuel capacity, multiple applications, moderate pow

    ensity, reasonable tolerance to gas phase poisons , high quality exhaust heat useful for cogeneration of power. It also as certain drawbacks like

    gh electrolyte resistivity an electrode polarisation, high temperature of operation, brittleness of ceramic components and high capital cost-to-utput ratio

    4/ The thermodynamic maximum or the open circuit potential of the oxidation reaction occurring in the fuel cell is given by the Nernst Equati

    owever the operating potential is less than the open circuit potential of the cell on account of the irreversible losses called overpotentials . The

    sses are due to the resistance to the transport of gases through porous electrodes (concentration overpotential), activation energy barriers to th

    ectrochemical reactions that occur at the electrode-electrolyte interface (activation overpotential), resistance to the transfer of ions through the

    ectrolyte (ohmic overpotential), leakage electronic current through the electrolyte (leakage overpotential) and due to the contact resistance at

    aterials boundaries (interface overpotential). Generally the cathode contributes maximum to the cell potential losses. The high working

    mperature of SOFC (about 1000 0C) reduces the open circuit potential but reduces the irreversible losses also. The cell operation at elevated

    essure yields increased Nernst potential and reduced cathode overpotential.

    5/ The four factors commonly used to evaluate SOFC performance are , (i) fuel utilisation factor, (ii) air ratio, (iii) power output and (iv) fuel

    ficiency.

    6/ The two ways to utilise the hydrocarbon fuel in SOFC are the steam reforming (internal or external), and the direct oxidation. The reformi

    ocess converts hydrocarbon fuels into a mixture of lower hydrocarbons, hydrogen, carbon monoxide and carbon dioxide. The chemical poten

    the fuel is partially lost in this process. The steam-to-fuel molar ratio is known as the steam reforming factor , m. In comparison to the

    xternal, the internal steam reforming of fuel is considered more energy efficient and saves the capital as well as the operating costs. However

    ffers from the drawback of carbon formation on the Ni-YSZ based anodes, leading to the deterioration in the performance and the lifetime of

    node material. However above a certain limiting value of m, the carbon formation is thermodynamically impossible. Therefore excess steam

    dded to the fuel to maintain m above that limiting value, which lowers the electrical efficiency on account of the fuel dilution. The reforming

    ctor at any given temperature increases with the carbon content of the fuel. The optimum temperature and m for the SOFC operation corres

    their values at the carbonisation boundary, referring to the limiting value of m. There is a search for the advanced catalysts which will allo

    ternal steam reforming of hydrocarbons at low m.

    7/ The kinetic rate of endothermic steam reforming reaction on the Ni-YSZ anode-catalyst is usually high and, as a result, the reaction gets

    ompleted over a short distance in the fuel channel. This leads to the excessively sharp temperature gradients, on account of the endothermic ef

    he problem can be tackled by (i) partial pre-reforming of fuels, (ii) reduction in the reforming rate by modifying the anode catalytic activity a

    i) oxidative reforming. The oxidative reforming is preferred for the portable power units using SOFC.

    8/ Thermodynamically it can be seen that the Nernst potential gets reduced with the fuel utilisation. One way to tackle the problem is to ope

    e cell with high recycle ratio combined with external removal of reaction products, maintaining the conversion per cycle at low level. Howev

    creasing the systems efficiency by this method entails considerable cost in the capital and the complexity. The alternative simpler approach is

    ulti-stage oxidation of fuel. In this approach, the partially oxidised fuel from one fuel cell stack is fed to the next stack.

    9/ The direct oxidation of fuel in the fuel cell may be described as the oxidation of hydrocarbons , to H2O and CO2, in the absence of steam

    xygen supplied in the fuel stream. Thermodynamically, any fuel can be directly oxidised in the SOFC, as the species that migrate through the

    ectrolyte are oxygen ions. However , with dry hydrocarbons at high temperature, there is also a thermodynamic driving force towards the

    rmation of carbon, at the anode. In order to keep the carbon formation at the minimum, the thickness of anode (Ni-YSZ) and electrolyte (YSZ

    node microstructure and the operating temperature of the cell need optimisation.

    10/ The research conducted in the area shows that besides hydrogen, natural gas (methane), hydrocarbons (butane, gasoline, etc.), methanol,

    hanol, biogas can be used as fuels in the SOFC. Based on the overall considerations, the available fuel options could be arranged in the follow

    der of preference: methane> ethanol > biogas > methanol > gasoline > others. The ethanol and the biogas could be considered as the promisi

    els for the future, as they can be produced from the biomass.

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    11/ The two design configuration of SOFC are the planar and the tubular designs. Whereas, in the planar design, the cell components are

    onfigured in thin flat plates, in tubular design, they are arranged in a hollow cylindrical shape. The SOFCs are suitable for stationary, large si

    ybrid (in combination with gas or steam turbine) power generation plants due to their operation at elevated temperature and pressure. Such po

    ants can reach efficiency up to 70 %. The SOFC finds application in the transportation and the military sectors also.

    12/ The individual cells are arranged in stacks for the purpose of power generation. In the stacks, the cells need to be connected together and

    echanism for the collection of electrical current needs to be provided. The fuel cell power generating unit consists of number of such stacks o

    articular size and output and the groups of stacks may be held in containment vessels.

    13/ The SOFC components can be fabricated in thin layers and configured in different shapes, as they are all in the solid state. The selection

    aterials for different cell components is based on certain criteria like, (i) electronic and ionic conduction properties, (ii) chemical and structura

    ability at high temperatures, (iii) minimal reactivity and inter-diffusion among the cell components, (iv) matching thermal expansion, (v) abil

    e formed in the desired microstructure and good adherence to each other.

    14/ The high temperature of operation of SOFC (around 1000 0C) leads to materials constraints, high cost of manufacture and the problems o

    ng-term stability. Lowering the temperature makes SOFC fabrication and operation more cost effective; the limitations on the polarisation

    sistance, internal reforming and the power generating capacity due to lower operating temperature, notwithstanding

    15/ The material used for cathode must possess high electronic conductivity (> 100 ohm -1cm-1), small ionic conductivity (~ 0.1 ohm -1cm-1

    dequate porosity to transport molecular oxygen from the gas phase to the electrode-electrolyte interface and high catalytic activity towards ox

    ssociation and reduction. The most commonly used cathode material for SOFC is strontium doped lanthanum manganite (LSM), suitably mix

    ith YSZ

    16/ The anode materials must satisfy all the requirements as that of the cathode materials. In addition, the anode requires high stability in

    rroundings containing high concentration of reducing agents, trace levels of hydrocarbons, particulate matter and sulphur compounds. The N

    SZ cermet is the commonly used anode. The macrostructure, particle size and surface area, connectivity of Ni particles, porosity and the three

    hase boundary play an important role in determining the performance and the long-term stability of anode in the fuel cell operating environm

    he new anode material, Cu-ceria cermet , effectively solves the problem of carbon formation at the anode, during the direct oxidation of

    ydrocarbons.

    17/ The YSZ containing around 10 mole percent yttria is the most commonly used electrolyte material in the SOFC. The two new electrolyteaterials, which showed promising results at low temperature operation are (i) ceria based electrolytes, such as ceria doped with yttrium,

    adolinium or samarium; multiphase materials prepared from cationic doped ceria mixed with salts or mixing ceria with other rare earth oxide

    fferent from ceria, like lanthanum oxide and (ii)lanthanum gallate. At lower operating temperature, the ohmic resistance of electrolytes can be

    duced by reducing its thickness. In general, ionic conductivity increases in the following order: YSZ < doped ceria < LSGM

    18/ The interconnect needs to satisfy the most stringent requirements of all the SOFC components, as it is exposed to both oxidising as well a

    ducing surroundings, simultaneously, at high temperature. The important requirements are, high electronic conductivity in the oxygen partia

    essure range obtained at anode and cathode sides, chemical and phase stability, matching TEC and gas tightness. The commonly used

    terconnect materials are lanthanum or yttrium chromite, doped with Co, Mg, or Sr. The reduction in the operating temperature of SOFC (belo

    00 0C) enables the use of comparatively inexpensive and readily available stainless steel as the interconnecting material.

    19/ In order to eliminate the interfacial resistances and other related problems like formation of non-conducting compounds, attempts are bein

    ade to develop SOFC with only one material with homogeneous composition, which will become excess electron conducting (anode), hole

    onducting (cathode) and ion conducting (electrolyte), under the appropriate conditions of surroundings, temperature and pressure.

    20/ The SOFC technology will have to overcome the challenges posed by the problems in the areas of operating parameters, materials and th

    anufacturing processes and costs, before it is accepted as the alternate means of power generation, on the commercial scale.

    omenclature:ymbols: activity of the reactants or products

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    : activity of carbon

    an, Act : pre-exponential factor in the rate equation for reactions

    anode ad cathode, A.L-2

    : constant, ohm-1.K.L

    CH4] : concentration of methane, mol.mol-1, dimensionless

    GP,T : Gibbs free energy, Joule, ML2T-2

    G(U) : Gibbs free energy at fuel utilisation U, Joule, ML2T-2

    WE : electrical work, Joule, ML2T-2

    s : diffusion factor, C/s.cm2.atm

    g : free gas diffusivity, L2T-1

    : open circuit potential of reversible cell, Volt

    cell : cell operating potential, Volt

    cell-L : cell potential at any point in space in the cell, Volt

    L : open circuit cell potential at any point in space in the cell, Volt

    c-an, Ec-ct : concentration overpotentials at anode and cathode respectively, Volt

    v-an , Ev-ct : activation overpotentials at anode and cathode respectively, Volt

    o : ohmic oveptential, Volt

    : leakage overpotential, Volt

    -op : leakage overpotential at open circuit, Volt

    n : interface overpotential, Volt

    : exponent

    : number of unit charges transferred in the reaction, dimensionless

    : Faraday constant, 96487 C/mol or 96487 J/V.mol

    H : enthalpy change in the process, Joule, ML2T-2

    average current density, A/cm2

    : local current density, A/cm2

    max : maximum current density, A/cm2

    current, A

    sh : threshold current, A: tortuosity factor, dimensionless

    c : equilibrium constant for the Boudouard reaction

    b : rate constant for carbon formation, mol.T-1

    : fuel reforming factor or steam/fuel molar ratio, dimenionless

    : number of moles ofith species produced in the reforming reaction

    : partial pressure of the gaseous constituents, atm, MLT -2

    : power ratio, PSOFC/ Pth, dimensionless

    SOFC : actual power output of SOFC, W, ML2T-3

    h : maximum (thermodynamic) power output of SOFC, W, ML2T-3

    : universal gas constant, 8.314 J/mol.K or 82.06 cm3.atm/gmol.K

    an, Rct : activation resistance at anode and cathode, ohm.L2

    L : local area specific resistance, ohm.L2

    o : ohmic resistance of electrolyte, Ohm

    pb : area of three phase boundary per unit anode volume, L2.L-3

    : temperature, K

    f, Ta : temperature in fuel and air channel, K

    : fuel utilisation factor, dimensionless

    : fuel utilisation, mole/mole (dimensionless)

    an , Uct : activation energy for anode and cathode reaction, J/mol

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    in : activation energy for ionic transport, J/mol

    : electrode thickness, L

    l : electrolyte thickness, L

    void fraction, dimensionless

    : air ratio

    h : thermodynamic efficiency of a fuel cell, dimensionless

    f: actual fuel efficiency of a fuel cell, dimensionless

    in : ionic conductivity of electrolyte, Ohm-1.L-1

    uperscripts:: standard state

    ubscripts:

    air channel

    fuel channel

    : ionic

    T : pressure, temperature

    : electrical

    electrode

    : electrolyte

    : thermodynamic

    concentration

    n, ct : anode, cathode

    : activation (overpotential)

    : ohmic (overpotential)

    leakage (overpotential), local

    : interface (overpotential)

    diffusing species

    OFC : solid oxide fuel cell

    h threshold

    b : three phase boundary

    y, z : number of atoms in the molecular formula

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