High Temperature Fuel Cells

Chapters 3, 8, 9, 10

• The temperature (in the cell and of the exhaust gas) is

high enough to facilitate extraction of hydrogen

Introduction

• The electrochemical reactions proceed more quickly at

high temperatures, and noble metal catalysts are often not needed

Solid Oxide Fuel Cells (SOFCs), Intermediate Temperature Solid Oxide Fuel Cells(ITSOFCs) will

be presented in this lecture.

• High temperature fuel cells create opportunities for

combined heat and power (CHP) system

• High temperature exhaust gas can be used to

operate gas turbines in a bottoming cycle

• Fuel utilization: how much hydrogen fuel must be

supplied and used to prevent zero voltage at the exit of the stack

Challenges

• Fuel reforming is very often required in the system (if

fossil fuels are used or if pure hydrogen is not used). How to integrate into the fuel cell system?

• The integration of a fuel cell and a heat engine can lead to

high system efficiency

• Stack exhaust gases can be used to preheat the fuel

and oxidant by using proper Heat Exchangers. How to reach an optimal thermal balance?

• Reforming reactions are usually carried out above 500 OC,

and the required heat can be supplied by the fuel cell exhaust heat

Fuel Reforming

• For hydrocarbon fuels, i.e., CxHy, the production of

hydrogen usually involves a steam reforming reaction, which is endothermic (heat needs to be supplied):

• Sulphur or its compounds should be removed prior to

the reformer and stack

( / 2)x yC H x x y xCO+ = + +2 2H O H

• The stack is sufficiently hot to allow the reforming

reactions to occur in the stack itself, so-called

internal reforming

Reactions in a conventional SOFC

Reactions:

The product water appears at the

anode side.

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

CO+H2O -- CO2+H2 (exothermic)

H2+O2- -- H2O+2e- (exothermic)

2CO+O2- -- 2CO2+4e- (exothermic)

0.5O2+2e- -- O2- (exothermic)

• Hydrogen is utilized by the cell and its concentration is

reduced while the other components simply pass through. If the hydrogen partial pressure is reduced from P1 to P2,

Fuel Utilization

• Hydrogen is supplied as part of a mixture or becomes

part of a mixture due to internal reforming

• Moreover the voltage drop is greater for high

temperature cells as RT becomes large.

2

1

ln( )2

PRTV

F P =

• The exhaust gas is

used in a boiler to increase the steam temperature for a steam turbine

Bottoming Cycles

• There are two ways to use the waste heat to enable

that heat engines reach a high energy conversion efficiency

• The exit gas from

the fuel cell may power a gas turbine (GT)

Illustration of an SOFC and a Steam Turbine Combined System

SOFC-Gas Turbine Hybrid Power Plant

High Temp Heat

Exchangers

SOFC system and Components

Thermal

Management

Components

• Limited by cost of

materials to 650 oC

• Significant

improvements are

required in both

reliability and cost

• New Concept Multi-

function HEX

• The first stage of a system design is to establish the basic

requirements of the chemical processes to produce a system configuration and identify the hot and cold streams and then find the pinch temperature

Pinch Analysis and System Design

• Pinch analysis is a method applied to energy systems for

finding the optimum arrangement of heat exchangers and other units, to maximize the energy recovery (MER)

• Other considerations in the system design: choice of

materials for the components and corresponding mechanical layout

• Each external manifold must have an insulating gasket to

form a seal with the edges of the stack. The zirconia felt has a small amount of elasticity to ensure a good seal

External versus Internal Manifolds

• External manifolds are simple, enabling a low-pressure

drop in the manifold, and good gas flow distribution between cells, but two streams flow at right angles (cross-flow) and cause uneven temperature distribution

• Internal manifolds are much more flexible in the

direction of gas flows (gas distribution among the cells through channels or ducts within the stack), e.g., co- or counter-flows.

Specific Issues Relevant to SOFCs

Why should we think about SOFCs?

• Usage of Solid State Electrolyte

• Corrosion problems are Reduced

• Water Management is Eliminated

• Very Thin Layers and Cell Components Possible

• Fuel Flexibility is High

• Precious Metal Catalysts are not needed

• Internal Reforming Possible and Combined Heat

and Power Cycles can be achieved

Material Issues

• Electrolyte materials: Initially zirconia (ZrO2) was

used but currently zirconia stablilized with addition of a small percentage of yttria (Y2O3) is more common.

• Anode: Zirconia cermet, i.e., a mixture of ceramic

and metal (e.g., nickel).

•Cathode: Oxides or ceramics, e.g., pervoskite

Cell Components

• Electrolytes and interconnects must be chemically,

morphologically, and dimensionally stable for both oxidizing and reducing conditions

➢ The components must be chemically stable in order

to limit chemical interactions with other cell components

➢ No components may exhibit any significant

changes in volume between room temperature and the fabrication temperature

Conductivities or Resistivities

• The resistivities of typical cell components at

1,000°C are 10 ohm-cm (ionic) for the electrolyte (8-10 mol % Y2O3 doped ZrO2), 1ohm-cm (electronic) for the cell interconnect (doped LaCrO3), 0.01 ohm-cm (electronic) for the cathode (doped LaMnO3), and 3 x 10-6 ohm-cm (electronic) for the anode (Ni-ZrO2 cermet).

• The solid oxide electrolyte is the least conductive cell component, followed by the cell interconnect.

• Two different design configurations are being developed

for solid oxide fuel cells due to a) consideration of how to seal the anode and cathode compartments, b) ease of manufacturing, and c) minimizing losses due to electric resistance.

• The two principal types are tubular and planar.

• The tubular SOFC has been developed gradually since the late 1950s. Operating temperature is between 900-1,000°C. Long tubes have relatively high electrical resistance but are simple to seal. Some tubular designs eliminate the need for seals and allow for thermal expansion. Several tubular units are operating in practical applications, with a huge amount hours of demonstrated operation.

•The planar one is built up by flat, thin ceramic plates. It operates at about 800°C or even less. Ultra-thin electrode and electrolyte sheets provide low electrical resistance and enable high efficiency. The operation at a temperature lower than that for the tubular SOFC makes it possible to use less exotic construction materials, thus saving cost.

Structures of SOFCs, Tubular and Planar

Tubular (left) and planar SOFC technology

Tubular SOFC Designs

• Tubular SOFCs: the cathode tube is fabricated first

with a porosity of 30 - 40% to permit rapid transport of the reactant and product gases to the cathode-electrolyte interface where the electrochemical reactions occur. The electrolyte is applied to the cathode tubes by electrochemical vapor deposition (EVD). By this technique, the appropriate metal chloride vapor is introduced on one side of the tube surface, and O2/H2O is introduced on the other side.

Cross Section of a Tubular Cell

A typical SOFC stack

Siemens cylindrical-tubular

SOFC technologySeal-less, high

internal ohmic losses

Siemens Westinghouse Tubular Cell Performance

at 1,000oC (2.2 cm diameter,150 cm active length)

High power density SOFC (Siemens)

Conventional new design

• In the planar configuration, the anode, electrolyte, and cathode form thin, flat layers being sintered together, and then separated by bipolar plates similar to the design of other fuel cell types. The plates can be either rectangular, square, circular, or segmented in series and can be manifolded externally or internally. Many planar designs use metallic bipolar plates and operate at a lower temperature than the all-ceramic tubular design.

• The planar SOFCs can be categorized on the basis of the supporting component of the cathode-electrolyte-anode structure. Two approaches are: electrode-supported or electrolyte-supported cells.

Planar SOFC Designs

Planar SOFC with a Cross-Flow Stack Configuration

Planar Solid Oxide Fuel Cell Stack

• Problems? A recent development has been to use a bipolar interconnect made of ferritic stainless steel. This cannot be used in the manufacturing of high-temperature fuel cells, because most steels oxidize quite readily at temperatures above 800°C. To limit corrosion on the air side, the operating temperature of the planar SOFC must be maintained below 800°C. However, the conductivity of the electrolyte decreases with falling temperature.

Planar SOFCs

• Reduced Temperature SOFCs?

Dropping the Operating Temperature to Below 800oC ─ Intermediate Temperature SOFCs (ITSOFCs).

• Two options for mitigating decreased performance. The well-established YSZ (yttria-stabilized-zirconia) electrolyte can be used at temperatures as low as 700°C when the thickness is about 15 micrometers.

• At even lower temperatures, better conducting electrolytes such as lanthanum gallate, scandium doped zirconia, or gadolinium doped ceria (GDC) can be used. Also, at significantly-reduced operating temperature, the lanthanum manganite cathode material becomes kinetic rate limiting so it must be modified or replaced.

ITSOFC design options

• This design has a thick anode, which acts as a supporting structure. The electrolyte and cathode are on the other hand very thin. • Such stacks operate in a temperature range between 700 – 800oC. Every individual cell is sandwiched or held between metal interconnecting plates that act as air and fuel flow channels as well as electric connection between cells in a stack. •In this concept the metals are more durable than ceramics. By using metallic bipolar plates as the main load-bearing component of the stack, the fracture resistance and thermal stress tolerance might be improved.

Anode Supported ITSOFCs

• Option 1: For Conventional YSZMaterials, Thinner

Electrolyte and Electrode-Supported Structure to be

Employed.

Intermediate Temperature ITSOFCs

• Critical Issues Concern: The Reduced Temperature

Causes Lower Electrolyte Ionic Conductivity and

Electrode Catalytic Activity; ASRelectrolyte< 0.15 Ωcm2.

Temperature

(oC)

Conductivity

(Scm-1)

Thickness

(μm)Structures

950 0.1 150Self-

Supported

700 0.01 15Electrode-

Supported

Films of oxide electrolytes can be reliably produced using cheap, conventional ceramic fabrication routes at thicknesses down to 15 μm. The specific conductivity of the electrolyte must exceed 10-2 S/cm. This is achieved at 500 °C for the electrolyte Ce0.9Gd0.1O1.95, and at 700 °C for the electrolyte (ZrO2)0.9(Y2O3)0.1.

ITSOFCs

• Option 2: New Ceramic Materials to be Employed for

Electrolyte and Electrodes

MaterialsTempera-

ture (oC)

Conductivity

(Scm-1)

Thickness

(μm)Structures

YSZ 700 0.01 15Thin

Electrolyte

Samarium

Doped

Ceria and

Li2SO4

400-700 0.01-0.4 500Thick

Electrolyte

Performance of ITSOFC at Reduced Temperatures

Projected Performance of Seal-less Planar Design with Current Seal-less Cylindrical Design

• Fuel Cell Applications require many heat exchangers

with different operating conditions.

Challenges Facing Fuel Cell Systems

• For ITSOFCs and HTSOFCs, a major design challenge is to

select cost effective materials. Due to operating conditions

and operating environment, the corrosion and durability

issues may need to be identified and resolved. Fouling and

its effect on heat exchanger performance needs to be

investigated.

• Balance of heat transfer versus

pressure drop is an important issue.

Summary

• High temperature fuel cells, i.e., SOFCs and MCFCs, have

several advantages compared to low temperature ones like PEMFC. The electrochemical reactions proceed more quickly, which is reflected in lower activation voltage losses, and then noble metal catalysts are not required. Opportunities for CHP exist. Mainly stationary applications.

• MCFC has been well developed and demonstrated, but a

major problem is the degradation of the cell components over long operation periods

• There is a trend to reduce the SOFC operating temperature

to below 800 oC due to material problems