high temperature steam electrolysis - · pdf file · 2004-09-17st andrews centre...

St Andrews Centre for Advanced MaterialsSTACAM

High Temperature Steam Electrolysis

John Irvine, Kelcey Eccleston,Angela Kruth, Alan Feighery, Fran Jones, Paul

Connor and Cristian Savaniu

University of St Andrews


Contents

• Hydrogen and its Production • High Temperature Electrolysis• St Andrews work

– Oxide Electrolyser– Reversible fuel cell concept– Solid oxide proton conductors

• Conclusion


Hydrogen Fuel

• Fuel cell vehicles, power generation– Eliminates polluting emissions at point of use

• Secondary energy carrier – Complete fuel cycle must be considered– Primary energy production: fossil fuel,

hydroelectric, solar, wind– Secondary energy production: electricity


The Opportunity• Scotland has 25% of Europe’s Renewable

Capacity• Wind, Wave and Tidal• Possible Net Exporter - New “Oil” Economy

• But not in short term– Capacity is remote– Need to Distribute Energy

• Hydrogen?


Hydrogen Production Methods

• Steam Reforming– Most of world hydrogen production– Produces CO2

• Photoelectrochemical, Biomass• Electrolysis

– Alkaline– Polymer Membrane– Solid Oxide


Hydrogen Production by Electrolysis

• Separating hydrogen and oxygen in water by electric current

• Two electrodes, cathode and anode, separated by an ion-conducting electrolyte

• Different types of electrolysis characterised by the type of electrolyte: Alkaline, Polymer Membrane, Solid Oxide


Disadvantages of Polymer Membrane and Alkaline Electrolysis• Must use precious metal catalysts• Not amenable to high pressure operation• Poor long term stability• Easily contaminated:

– Alkaline electrolyte absorbs CO2

– Must use ultrapure water or polymer membrane accumulate cations.


High Temperature Electrolysis• Heat provides part of total

energy required for electrolysis• ∆H lower in vapour phase than

liquid phase• Higher electrical efficiency• High temperature favours

reaction, reduces overpotential – Thus practical total efficiency

should exceed low temperature• Oxygen ion electrolyte: 750-

1000 ºC• Protonic electrolyte: 450-750 ºC


Solid Oxide Electrolysis

• Reverse operation of SOFC

• Solid electrolyte• No need for ultrapure

water• Can use less expensive

electrode materials• High pressure operation

possible

H2O

H2

O2

e-

O=

H2O

O2

H2

e-

H+


Solid Oxide Electrolysis

• High efficiency with respect to electricity• 80% of production cost from electricity• 2x cost of steam reforming, at present• Economic in regions with large renewable

energy sources– Egypt--hydroelectric– Iceland--geothermal– Scotland--wind, wave?


System


HexisTM Principle: Cell StackA HexisTM stack segment consists of a fuel cell and a current collector. Approximately 50 segments form a stack. Current Collector

Cell

Fuel Cell

Cell Stack

Area 100 cm2

Current/cell approx. 30 amp Voltage/cell approx. 0.7 voltPower/cell approx. 21 wattelConnected in series 1050 wattel


• The current collectors are responsible for gas distribution, heat exchange and making the electrical contact between the segments

• The cell stack also functions as a burner

HexisTM stack segment Advantage of the HexisTM principle

A high degree of integration is the key to low manufacturing costs

Current collectorand internal heatexchanger

CathodeElectrolyteAnode

Air

AfterburningFuel

Cell


Siemens Westinghouse Tubular


Component Materials• Electrolyte:

– Gastight, thin, not electrically conductive– Oxygen ion conductors: Yttria-stabilised zirconia (YSZ)– Proton conductors: Ba-doped cerates (produce dry H2)

• Cathode:– Porous, stable in reducing atmospheres– Transition metal cermets: Ni-YSZ

• Anode:– Porous, stable in oxidizing atmospheres– Pt (high cost)– Electronic conducting mixed oxides: (La,Sr)MnO3,

(La,Sr)CoO3


Component Material Preparation

• YSZ electrolyte– 2.5-cm diameter pressed pellets (2 cm after

sintering)– Tape cast electrolyte cut to 2cm diameter– Sintered 1500 C, 10 hrs

• (La0.8Sr0.2)CoO3 anode– Sol gel powder preparation – Screen printing on YSZ disks


Sintered LSC Layer on YSZ


LSC on YSZ with Ceria Interlayer


Electrolysis Test SetupReference electrode

Counter electrode

O2 outDry Ar in

Gold ringsTest cell

Furnace

H2O + H2 outWet Ar inWorking electrode


Faraday efficiency

0

0.2

0.4

0.6

0.8

1

1.2

0 0.01 0.02 0.03 0.04 0.05 0.06

I (A)

Eff

icie

ncy

865

920

Changes in pO2 with applied current for 865 C and 920 C

0.00E+00

2.00E-04

4.00E-04

6.00E-04

8.00E-04

1.00E-03

1.20E-03

1.40E-03

1.60E-03

0 0.01 0.02 0.03 0.04 0.05 0.06

I (A)

pO

2 (

atm

)865920

0.1ccmin-1


0

0.2

0.4

0.6

0.8

1

0.0 0.2 0.4 0.6 0.8 1.0 1.2

I/A

V/V

0.0

0.1

0.2

0.3

0.4

0.5

0.6

P/W

Reversible fuel cell


BC10Y electrolyte discPt electrodes

Alumina tubes

OCV thermocouple

I/V electrolysis

OCV pO2

wet Ar gas

dry ArgaspO2 sensor

Au seals

Electrolysis cell: Pt, steam/Ar | BaCe0.9Y0.1O2.95 | Pt, 0.5%H2O/Arelectrolyte thickness ~ 2mm


I/mA

log

(pH

2/atm

)

-0.5

0

0.5

1

1.5

1 2 3

U/V

I/mA

3% steam

47% steam

-15

-10

-5

00.5 1 1.5 2

Observed variation of hydrogen partial pressure in dry gas with applied current at 618 ˚C and 3% steam feed.

I-V curves for cell in galvanodynamicmode at 618 ˚C at two different steam feeds.


Summary• Low Temp Electrolysers

– Work– Available– Expensive

• High Temp Electrolysers– Still need development– promise efficiency and cost gains

• Solid Proton conducting Oxides– offer useful temperature compromise– Stability an issue for best materials so far

• HT Electrolyser LT Fuel Cell– Chance to gain a little back from thermodynamics if theoretical voltages are approached


Acknowledgements

ORS

Scottish Enterprise

EPSRC

EU HIT Proton Network

high temperature steam electrolysis - · pdf file · 2004-09-17st andrews centre...

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