Advanced supporting anodes for Solid Oxide Fuel Cells


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Advanced supporting anodes for Solid Oxide Fuel Cells
Maarten Verbraeken M. Sc. Thesis
Faculty of Science and Technology Inorganic Materials Science MESA+, EMPA Dübendorf Enschede, March 2005

Advanced supporting anodes for Solid Oxide Fuel Cells
M. Sc. Thesis By
Maarten Verbraeken Enschede, March 2005
Graduation Committee Prof. Dr. ing. D.H.A. Blank (chairman) Dr. B.A. Boukamp (supervisor, IMS) Dr. P. Holtappels (supervisor, EMPA) Dr. H.J.M. Bouwmeester Dr. B.L. Mojet

Advanced supporting anodes for Solid Oxide Fuel Cells
Summary
Three different ceramic materials for nickel-cermet anodes have been used to prepare and characterise supporting anodes for Solid Oxide Fuel Cells. Symmetrical cells with nominally identical supporting anodes were prepared and electrochemically tested. The three different cermets were nickel/yttria stabilised zirconia (YSZ), nickel/gadolinium doped ceria (CGO) and nickel/titania and yttria doped zirconia (YZT). The Ni/YSZ anodes contained different ratios of fine and coarse YSZ. Their polarisation resistances varied from 2.0 – 7.4 Ωcm2; the resistance increased with an increasing amount of coarse YSZ. The anodes suffered a lot from degradation, which makes a good analysis and comparison of the measurement data hard. Therefore it is hard to ascribe the impedances to certain electrochemical processes. The lowest polarisation resistance was found for the Ni/CGO anodes, with a polarisation resistance of 1.1 Ωcm2. In the first place, the fine microstructure plays an important role for the relatively low impedance. Secondly, the stability of this anode in contrast to the other materials is thought to be due to the high purity powder that was used. From the electrochemical characterisation, it is believed that an adsorption process and oxidation/reduction of the ceria cause the main impedances. The last material, Ni/YZT, performed a bit worse than the Ni/YSZ anodes. The impedances of these Ni/YZT anodes were fitted with a Finite Length Fractal Gerischer, resulting in consistent fit data. This Gerischer describes a diffusion process coupled to a side reaction, which limits the amount of diffusing species. A proper electrochemical explanation has yet to be formulated, but the mixed ionic and electronic conductivity of YZT is almost certainly involved. In any case, the total polarisation impedances for this material amounted 5.9 – 10 Ωcm2. Like the Ni/YSZ anodes, the impedance decreased with an increasing amount of fine YZT.

Advanced supporting anodes for Solid Oxide Fuel Cells
Contents
1. Introduction ..................................................................................................................................... 6 1.1. Fuel cell characteristics............................................................................................................... 7 1.2. State-of-the-art SOFC ................................................................................................................. 8 1.2.1. Electrolyte ............................................................................................................................ 8 1.2.2. Cathode ............................................................................................................................... 9 1.2.3. Anode................................................................................................................................... 9 1.3. Objectives ................................................................................................................................. 10
2. Theoretical background ................................................................................................................ 11 2.1. The anode ................................................................................................................................. 11 2.1.1. Hydrogen oxidation ............................................................................................................ 11 2.1.2. Polarisation ........................................................................................................................ 12 2.1.3. Anode structure.................................................................................................................. 14 2.1.4. Alternative materials: mixed ionic-electronic conductors (MIEC) ...................................... 15 2.1.5. Coke formation................................................................................................................... 16 2.2. Impedance spectroscopy .......................................................................................................... 17 2.2.1. Cell design ......................................................................................................................... 17 2.2.2. Equivalent circuits .............................................................................................................. 19
3. General set-up considerations...................................................................................................... 20 3.1. Electrochemical set-up.............................................................................................................. 20 3.1.1. Gas tightness of the set-up/position of the sample in the furnace..................................... 20
4. General cell preparation ............................................................................................................... 22 4.1. Experimental ............................................................................................................................. 22 4.1.1. Anode substrates ............................................................................................................... 22 4.1.2. Functional anode layers..................................................................................................... 22 4.1.3. Electrolyte layers................................................................................................................ 23 4.1.4. Symmetrical cells ............................................................................................................... 24 4.2. Results ...................................................................................................................................... 25 4.2.1. Symmetrical cells – dilatometer tests ................................................................................ 25 4.2.2. Microstructure .................................................................................................................... 28
5. Ni/YSZ functional anodes ............................................................................................................. 30 5.1. Theoretical background............................................................................................................. 30 5.2. Experimental ............................................................................................................................. 32 5.2.1. Materials ............................................................................................................................ 32 5.2.2. Slurry preparation .............................................................................................................. 33 5.2.3. Impedance spectroscopy................................................................................................... 33 5.3. Results & discussion ................................................................................................................. 34 5.3.1. Microstructure .................................................................................................................... 34 5.3.2. Electrochemical characterisation ....................................................................................... 35 5.4. Concluding remarks .................................................................................................................. 41 5.4.1. Effect of microstructure on electrochemical performance ................................................. 42
6. Ni/CGO anodes ............................................................................................................................ 43 6.1. Ceria.......................................................................................................................................... 43 6.2. Experimental ............................................................................................................................. 44 6.2.1. Materials ............................................................................................................................ 44 6.2.2. Slurry preparation .............................................................................................................. 45 6.2.3. Impedance spectroscopy................................................................................................... 45 6.3. Results ...................................................................................................................................... 45 6.3.1. Microstructure .................................................................................................................... 45 6.3.2. Electrochemical measurements......................................................................................... 46 6.4. Discussion................................................................................................................................. 49 6.4.1. Microstructure .................................................................................................................... 49 6.4.2. Electrochemical measurements......................................................................................... 49
7. Ni/YZT anodes.............................................................................................................................. 51 7.1. TiO2 doped YSZ – YZT ............................................................................................................. 51 7.1.1. YZT-cermets ...................................................................................................................... 52 7.2. Experimental ............................................................................................................................. 53

Advanced supporting anodes for Solid Oxide Fuel Cells
7.2.1. Materials ............................................................................................................................ 53 7.2.2. Slurry preparation .............................................................................................................. 55 7.2.3. Impedance spectroscopy................................................................................................... 55 7.3. Results ...................................................................................................................................... 55 7.3.1. Microstructure .................................................................................................................... 55 7.3.2. Electrochemical characterisation ....................................................................................... 56 7.4. Discussion................................................................................................................................. 66 7.4.1. Increased porosity in the anode supports.......................................................................... 67 7.4.2. Set-up change.................................................................................................................... 67 8. Conclusions .................................................................................................................................. 68 9. Danke, danke,............................................................................................................................... 69 10. Literature....................................................................................................................................... 70

Advanced supporting anodes for Solid Oxide Fuel Cells
1. Introduction
Fuel cells are of great interest nowadays for their high efficiencies of converting chemical energy into electrical energy. Like combustion engines, fuel cells use some sort of chemical fuel as its energy source. However, in the fuel cell the chemical energy is converted directly into electrical energy. In other words, the intrinsically inefficient conversion steps in the combustion process are surpassed. Efficiencies are hence not restricted by the Carnot cycle and could theoretically reach values approaching 100%. Besides the high efficiencies, fuel cells are of interest because of their low emissions and zero noise production. A fuel cell primarily consists of three components: an anode, a cathode and an electrolyte. A schematic representation is depicted in Figure 1-1.

Figure 1-1: Schematic representation of a fuel cell with oxide ion conducting electrolyte.

The electrochemical reactions occur at the electrodes. The fuel is fed to the anode side, whereas the oxidant (often air or oxygen) is fed to the cathode. There exists an electrochemical potential for the chemicals to react; a driving force is thus created. However, the dense electrolyte prevents the fuel and the oxygen from reacting directly with each other. On the other hand, it does allow ion transport. Accordingly, half-cell reactions occur at the electrodes, producing ions that can migrate through the electrolyte. For example, when an electrolyte conducts oxide ions, oxygen will be reduced at the cathode to produce O2- ions, which in turn react with the fuel at the anode. The anode releases electrons that are consumed again at the cathode. The half-cell reactions that occur are the following:

Cathode: Anode:

1 O + 2e− → O2−
22
H2 + O2− → H2O + 2e−

(1.1) (1.2)

Analogous electrode reactions occur for proton conducting electrolytes:

Cathode:

1 2

O2

+ 2H +

+ 2e−



H2O

Anode:

H2 → 2H + + 2e−

(1.3) (1.4)

As the electrolyte should be a pure ion conducting material, the electron current that balances the ion flux, flows through an external circuit. This balance creates the electrical power.
Since the fabrication of the first fuel cell in 1839 by Sir Grove, a number of fuel cell types have been developed1. The distinction of the different types of fuel cells is based on their electrolyte and the ion that is able to migrate through it. Table 1-1 lists the five most important types, along with their mobile

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Advanced supporting anodes for Solid Oxide Fuel Cells

ion, temperature of operation, fuel and electrolyte. Since ion conduction is a thermally promoted process whose magnitude is strongly determined by the material used, operational temperatures vary strong from one fuel cell to the other2.

Table 1-1: Fuel cell types and selected features

Type
PEM: polymer electrolyte membrane AFC: alkali fuel cell

Temperature (ºC) 70 – 110
100 – 250

PAFC: phosphoric acid fuel cell 150 – 250

MCFC: molten carbonate fuel cell 500 – 700

SOFC: solid oxide fuel cell

600 – 1000

Fuel H2, CH3OH H2 H2 H2, hydrocarbons, CO H2, hydrocarbons, CO

Electrolyte Sulfonated polymers Aqueous KOH H3PO4 (Na,K)2CO3 (Zr,Y)O2-δ

Mobile ion (H2O)nH+
OH-
H+
CO32-
O2-

This work emphasises on the SOFC, one of the most promising fuel cell types. Its advantages are the solid electrolyte (instead of liquid, corrosive electrolyte materials), the possibility of using hydrocarbons as fuel and the good mechanical properties of the ceramic materials. A disadvantage is the high operating temperature, which restricts its use to stationary power production (i.e. power plants), since heating and cooling cycles take too long for the use in mobile applications (automotives, etc.).

1.1. Fuel cell characteristics
The performance of a fuel cell is measured as the voltage output as a function of current drawn from the cell. Figure 1-2 shows such an I-V curve along with a power density curve. The measured voltage, E, can be written as:

E = Eeq − E L − ηact − ηiR − ηconc

(1.5)

In equation (1.5) Eeq is the equilibrium voltage as calculated from the Nernst equation, EL is the voltage loss due to leaks in the electrolyte, ηact is the activation overpotential due to slow electrode reactions, ηiR is the overpotential due to ohmic losses in the entire cell and ηconc is the overpotential caused by slow gas diffusion processes in the electrodes. The Nernst equation for the half-cell reactions in (1.1)
and (1.2) reads:

1

E

=

E0

+

RT

ln

p p2 O2 H2

eq

nF p

H2O

(1.6)

E0 is the standard potential difference (T = 293 K) between the two half-cell reactions: E0 = E02 – E01 = -1.23 V, where E0 is related to the standard Gibbs energy by E0 = -∆G/nF. Further, R is the gas constant, T the absolute temperature, n the amount of electrons involved (in this case n=2) and F is Faraday’s constant3.
From equation (1.5) it becomes clear that apart from a dense electrolyte layer, three electrode processes play an important role in fuel cell performance. By choosing the right electrode materials and tailoring their microstructures, polarisation resistances due to slow electrochemical reactions, diffusion and low conductivity can be minimised.

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Advanced supporting anodes for Solid Oxide Fuel Cells

V (mV) P (mW/cm2)

100

Electrolyte leak

Theoretical EMF

1000

Peak power density

Slow reactions

(activation polarisation)

80

800

60

600

Total loss

Ohmic resistance

40

400

200
0 0

20 Slow mass diffusion (Concentration polarisation)

0

50

100

150

200

250

300

i (mA/cm2)

Figure 1-2: Schematic I-V curve and power density curve

1.2. State-of-the-art SOFC
Basically, two SOFC designs are used: the electrolyte-supported and electrode supported cell. The first design consists of a thick electrolyte that has both sides coated with a thin electrode. The second uses an electrode as the support layer. State-of-the-art SOFCs use the latter design. Its advantage is the smaller ohmic resistance as compared to the electrolyte-supported design. The ohmic resistance in the electrolyte is caused by the low total conductivity, which is inherent in ‘pure’ ionic conductors. The ohmic loss can be reduced by decreasing the thickness of the electrolyte. Anode supported SOFCs have an electrolyte with a thickness of 10 – 30 µm. On the other hand, in the electrode-supported design, attention must be paid to gas transport through the thick electrode. A porous microstructure is necessary to promote gas diffusion. Figure 1-3 shows both an electrolyte-supported cell and an electrode (anode) supported fuel cell4.

50 µm 150 µm 50 µm

Cathode Electrolyte
Anode

50 – 100 µm 10 – 30 µm
300 2000 µm

Figure 1-3: Design of an electrolyte supported SOFC (left) and an anode supported SOFC (right)

1.2.1. Electrolyte
The electrolyte material of the SOFC is in most cases 8 mol% yttria stabilised zirconia (8-YSZ). This material is preferred for its high oxygen ion conductivity, mechanical strength and stability. Doping ZrO2 with Y2O3 has two functions. First, zirconia is transformed from the monoclinic phase into the otherwise only at elevated temperatures stable cubic phase (fluorite structure). And second, the doping with the Y3+ ions creates oxygen vacancies in the zirconia lattice, which is beneficial for the oxygen ion conductivity. The highest oxygen ion conductivity is obtained when doping with 8 – 10 mol% Y2O3; higher levels of dopant cause the positive oxygen vacancies and negative yttria ions to combine, lowering the concentration of free oxygen vacancies. Other dopants can be used instead of yttria as well. A good example is scandia (Sc2O3), which has a comparable stability compared to yttria stabilised zirconia, but higher ionic conductivity5.

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Advanced supporting anodes for Solid Oxide Fuel Cells