sections 9 3 to 11dcdcdcdc

8/14/2019 Sections 9 3 to 11dcdcdcdc

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10. Medium temperature fuel cells (200 - 650 C)


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Enhanced cathode kinetics.

Heat recovery.

Simpler operation.

Enhanced CO tolerance.

Matching with hydrogen supply system. This will facilitateintegration with the fuel cell stack and hence smaller,lighter, more efficient and simpler systems.

Advantages of operating at elevated temperature

Medium temperature fuel cells


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10.1 The phosphoric acid fuel cell (PAFC)


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Was the only commercially available fuel cell (> 200 fuel cell systems havebeen installed all over the world)

Runs on H 2, methane, natural gas, sulfur free petrol + air/O 2

Generate electricity at > 40% efficiency ( ca . 85% if the steam produced isused for cogeneration; cff ca . 35% for the utility power grid in the USA)

Graphite felt+low Pt loading, concentrated phosphoric acid(polyphosphoric acid) electrolyte absorbed in SiC

Operating temperatures 150 - 220 C

High O 2 solubility

CO tolerant ca . 1 2 % due to higher operating T

PAFCs had outputs up to 200 kW (11 MW units had also been tested).

Combined Heat and Power operation.

The PAFC (2)


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100% phosphoric acid electrolyte immobilised in aca . 0.2 mm- thick, PTFE -bonded, particulatesilicon carbide matrix.

Pure phosphoric acid freezes at 42 C, and so thePAFC stack must be maintained above thistemperature if potentially destructive freeze/thawstress is to be avoided.

Conductivity of the acid falls below 150 C and itsvolatility is too high & it tends to decompose attemperatures above 220 C, hence the operatingtemperature is ca . 200 C.

The electrodes of the PAFC comprise small ( ca . 2nm) Pt particles deposited on porous carbon, atthe same loadings as in the PEMFC, andsupported on porous carbon paper or graphite feltGDL s.

The PAFC (4)


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Multicomponent bipolar plate: a = anode, e = electrolyte and c = cathode

PAFC stacks externally manifolded, utilise boiling water as the coolant for 100 kW

systems, and air for lower power systems, typically with every 5 th cell cooled, and anoperating temperature up to 220 C.

PAFC employ multicomponent bipolar plates.

The PAFC (5)


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Applications

Stationary supply

Large vehicles

400kW Stationary Fuel Cell

The PAFC (6)


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PAFC Problems 2000 per kW; fuel cost 0.50 per kWh with reformer

H 2S in reformate poisons anode catalyst

Need desulfurizer, water separator, heat exchanger andreformer- complex (especially wr t heat management) & heavy

system hence mainly stationary applications, although alsobuses.

Oxidation of carbon support, agglomeration of Pt particles,

flooding of electrodes and loss of acid-eg

. reliability, lifetimeand maintenance costs

Corrosive electrolyte

Low power density

The PAFC (7)


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10.2 The molten carbonate fuel cell


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Anode Reaction:CO 32- + H 2 H 2O + CO 2 + 2e -

Electrolyte: molten lithiumcarbonate + potassium carbonate

Cathode Reaction:CO 2+ O 2 + 2e - CO 32-

The MCFC (1)

Genesis in work of Broers and Ketaarin 1950 s: materials largelyunchanged since mid- 1970 s.

Operational T = 650 C

H 2 + O 2 + CO 2(Cathode) H 2O + CO 2(Anode)

CO 2 would cancel in overall cellreaction, concealing its role, so:

Moving ions


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Electrolyte: molten mixture of Li 2CO 3+K 2CO 3 or Li 2CO 3+Na 2CO 3, ca . 60wt.%, in ceramic matrix 40 wt.% LiAlO 2, the latter being in the form offibres 1 mm in diameter (formed by tape casting).

Anode Ni/Cr/Al alloy, cathode NiO; both porous.

Relative pore sizes of the anode, cathode and LiAlO 2 matrix areemployed to create 3-phase region.

The MCFC (2)


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Electrolyte management of this nature is critical in, and unique, to theMCFC.

The MCFC (3)


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The MCFC (5)

Thin stainless steel bipolar plates, with both internal and external manifoldingbeing employed . Efficiencies ca. 60%, BUT at lower current densities (150mA cm -2 at ca. 0.8V) achievable with other fuel cells.


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The increased temperature of operation reduces the maximum cell voltagefor thermodynamic reasons from 1.23 V to 1.02 V, which contributes to the

relatively poor performance of MCFC s at higher current densities.

However, it does have a number of advantages:

Noble metal catalysts no longer necessary. At the anode CO can beemployed as a fuel:

CO + CO 32- 2CO 2 + 2e

-

or:

CO + H 2O CO 2 + H 2

followed by oxidation of the H 2. Reaction is catalysed by Ni alloyanode catalyst.

The MCFC (6)

(water-gas shift reaction)


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Indirect Internal Reforming (IIR).

A wider range of feedstocks is accessible.

The MCFC (7)


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Majority of voltage loss is due to resistance of electrolyte (thickness d):V = 0.533d

Thus, the cell voltage of a MCFC with a 0.02 mm thickness of electrolyte wouldbe 96 mV higher than that having a 0.2 mm thickness.

MCFC thickest electrolyte HERE.

The MCFC (8)


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The MCFC (9)

Problems:

Cost.

Durability.

System complexity.


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11. High temperature fuel cells: the SOFC (1000 C)

SOFC (1)


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Genesis of SOFC s in discovery of solid oxide electrolytes by Nernst in1899. Haber filed the first patent on a fuel cell with a solid electrolyte in1905.

The SOFC is a completely solid state device utilising an ion conductingceramic as the electrolyte & mixed metal oxide-based anodes and cathodeswith an operating temperature of typically 1000 C. The high temperaturemeans that the theoretical open circuit voltage is only 0.92 V but kineticlosses are no longer a problem, and the most significant loss in SOFC s isdue to Ohmic losses in the electrodes, electrolyte and interconnects

between the cells.

SOFC (1)

SOFC (2)


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Pioneered by Westinghouse Power Corp (now Siemens WestinghousePower Corp).

Tubular design still the most commonly employed.

At the anode:

H2 + O 2-

H 2O + 2e -

CO + O 2- CO 2 + 2e-

At the cathode:

O 2 + 4e - 2O 2-

Overall:

H 2 + CO + 2O2- H 2O + CO 2

CO now a fuel rather than a poison

SOFC (2)

Moving ion

SOFC (3)


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The specific advantages of the SOFC, largely due to the highoperational temperature, are:

It is simpler in concept than any of the other fuel cells.

CO is a fuel rather than a poison.

The SOFC is the most sulfur-tolerant of the fuel cells, although

sulfur is usually removed from the fuel stream prior toreformation.

Precious metal catalysts not required.

Internal reforming is possible. A wide range of fuels can be employed.

Reduced Ohmic losses (due to enhanced conductivities)through electrode and electrolyte materials.

SOFC (3)

SOFC (4)


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SOFC (4)

Overall system efficiencies (with CHP or cogeneration) of 85% have beenquoted, lifetimes of 20,000 hours without performance loss (target 40,000hours) and power densities up to 2 W cm -2 reported; to what extent the formeris a fact or aspiration is unclear. NOTE: In order to drive a gas turbine, thefuel cell must be pressurised, and this has only recently been demonstrated

over a prolonged period using a SOFC.

SOFC (5)


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Anode Cathode

SOFC (5)

The most commonly employed electrolyte is Zirconia, ZrO 2, stabilised by ca . 8 10% Yttria, Y 2O 3, YSZ. Anionic conduction of O

2- species starts at temperaturesabove 800 C with a conductivity of 0.02 -1 cm -1 at 800 C rising to 0.1 -1 cm -1 at1000 C. YSZ is relatively stable at high temperatures in both oxidising andreducing atmospheres and can be made very thin (25 50 mm) to minimise Ohmicloss.


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SOFC (7)


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Anode

Catalysts limited to Ni, Co, Pt and Au.

Typically, NiO mixed with YSZ to formCermet (CERamic+METal).

Direct Internal Reforming possible intheory.

SOFC (7)

SOFC (8)


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Cathode must be:

Active wrt oxygen reduction.

Stable.

Highly electronically conducting.

Porous to O 2.

Unreactive wrt other components.

La (1-x) Sr xMnO 3 (0.10 < x < 0.25) most commonly employed.

SOFC (8)


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SOFC (10)


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(a) SOFC bundle of 24 tubular cells. (b)& (c) Schematic showing cross section ofsingle tubular cell. (d) Cross section of stack

consisting of Bundles .

(a)

(b)

(c)(d)

SOFC (10)

2 cm dia. YSZ 1-2 mm thick

SOFC (11)


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In the common tubular SOFC design, interconnects:

Link the anode & cathode of adjacent cells.

Must be stable at high temperatures in oxidising & reducingenvironments.

Must not react with the electrode materials.

Must be impermeable to the fuel and air.

Must be a thermal match with YSZ electrolyte.

Most SOFC s use Sr, Ca or Mg doped LaCrO 3.

SOFC (11)

SOFC (12)


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(BIG) problems with tubular design:

Cost per kWh not known.

High Ohmic loss /low power densities.

Coatings made via electrochemical vapour deposition.

LaCrO 3 (interconnects) difficult to process (expensive) due to

chromia evaporation at high temperature leading to poordensification.

Hence planar designs

Utilise significantly shorter conduction paths hence higher powerdensities.

Interconnects bipolar plates providing both electronic connection andfuel/oxidant distribution.

BUT sealing!

SOFC (12)

SOFC (13)


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Planar solid oxide fuel cell (SOFC)

electrolyteanode

porous metallic substrateFe-26Cr-(Mo, Ti, Mn, Y 2O 3) alloy

cathodeCathode current collector

bipolar plate

bipolar plate

Fuel channel

Air channel

SOFC (13)

SOFC (14)


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But- sealing is a MAJOR problem in planar SOFC s: Glass ceramics only sealing option: very brittle and prone to

thermal stress failure.

Glass seals are also prone to failure under the tension necessaryto facilitate sealing.

Difficult to manufacture thin enough.

Difficult to manufacture large areas, severely restricted, eg. toca . 10 cm x 10 cm.

SOFC (14)


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SOFC (17)


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SFC-200 systemSiemens-Westinghouse design

SOFC (17)

sections 9 3 to 11dcdcdcdc

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