Alkaline Electrolysis Cells: Materials, Properties and Challenges - PowerPoint Presentation

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Alkaline Electrolysis Cells: Materials, Properties and Challenges

Mogens B. Mogensen Technical University of Denmark, DTU Risø CampusDK-4000 RoskildeDenmarkmomo@dtu.dk2nd Joint European Summer School on Fuel Cell and Hydrogen Technology,Crete, September 25th, 2012

Acknowledgements to colleagues at DTU Energy Conversion

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Outline

Principles in alkaline water electrolysisCommercial statusMaterials in alkaline electrolysersElectrolyteAnodeCathodeSeparators, sealings, containmentsMaterials of electrolysers under developmentProperties commercial systems – examplesChallenges – optimization of system

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26 September 2012

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Principle -reactionsThe electrolyte is usually ca. 30 wt% KOH in water

Cathode (negative electrode) reaction: 2 H2O + 2 e-  H2 + 2 OH-Anode (positive electrode) reaction: 2 OH-  ½ O2 + H2O + 2 e-Total: H2

O  H2

+ ½ O2Very simple reaction, which may be carried out in practise at a temperature as low as 60 CEven so, it shows up that systems are not that simple3

26 September 2012

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Principle designs

426 September 2012Different architectures of electrolyzers with: (a) immersion electrodes, (b) porous electrodes in the “zero-gap” configuration, (c) electrodes comprisingGas Diffusion Layers (GDL) separating the gas compartments and the re-circulating electrolyte compartment. From: S. Marini et al., Electrochimica Acta, 82 (2012) 384

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Process flow diagram of a modern electrolyzer

526 September 2012

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History of industrial water electrolysis

626 September 2012Year

Event

1800

Discovery of electrolytic splitting of water

1902

More than 400 industrial electrolyzers in operation

1939

First large electrolysis plant with capacity 10,000 m

3

H

2

h

-1

1948

First pressurized electrolyzer by

Zdansky

/

Lonza

1966

First solid polymer electrolyte system (General Electric)

1972

Development of solid oxide water

electrolysis started

1978

Development of advanced alkaline electrolysis

started

From: W. Kreuter and H. Hofmann, Int. J. Hydrogen Energy,

23

, (1998) 661

Slide7

Commercial alkaline electrolyzers

Norsk Hydro set up a small installation in Notodden in 1927, Norway, for test purposes, later followed by a large industrial installation in Rjukan, Norway where the energy was supplied by Vemork power station – the largest hydro power station in the world at that time. The Norwegian company still exist with new owners, and today its name is NELSeveral other companies that produce alkaline electrolysers existCompanies selling “big” systems, > 50 Nm3 h-1:NEL (NO)Hydrogenics (BE, CA)Linde (DE)ELT (DE)iht (CH)Teledyn (USA)

Many more companies sell smaller systems

726 September 2012

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The alkaline electrolyser is commercial available

826 September 2012Hans Jörg Fell, CTO

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Photo of NEL on-site electrolyser

926 September 2012NEL does not tell the details of what is on their photos. As far as I can figure out by studying their homepage, http://www.nel-hydrogen.com/home/, and various presentations, this is an atmospheric pressure, ca. 2.2 MW, unit, which seems to be one of NEL’s units from which the bigger systems are built. The nominal production capacity is 500 Nm3 H

2 h-1

.The photo is from a presentation by NEL CTO Hans Jörg Fell, “Alkaline electrolysis for distributed and central hydrogen production”, International Water Electrolysis Symposium, Copenhagen, 10-11 May. 2012.

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Hydrogenics alkaline electrolyser cell stack

1026 September 2012Photo from: Raymond Schmidt, Global Market Strategist, Hydrogenics, “Electrolysis for grid balancing”, International Water Electrolysis Symposium, Copenhagen, 10-11 May. 2012.HySTAT® 10 – 10

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Hydrogenics Alkaline system

From Hydrogenics’ homepage:HySTAT® 10 – 1010 Nm3H2 h-1, 5.4 kWh/Nm3 H2

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Problem - faradaic efficiency

Faradaic efficiency, F is defined as the percentage of the current (not energy) that is used to produce H2The Faradaic efficiency is not 100 % (but may come close) for alkaline electrolysisWhy is F < 100 %? Which processes?12

26 September 2012

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Materials in alkaline electrolysers

From the previous information it should be clear that a big number of components and even bigger number of materials are involvedIt is not possible to cover all of them in this presentation, and honestly, I do not know which materials the commercial companies are using; I even do not know exactly which components each of them use.Companies simple do not inform publicly about what they are doing.No noble metals or other expensive materials!Therefore, we can only know about what has been published from measurements in the laboratories1326 September 2012

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ElectrolyteThe most used electrolyte for alkaline electrolysers is concentrated aqueous solution of KOH

Often 30 – 35 wt% KOH is used as this has the highest conductivity1426 September 2012

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Conductivity of aqueous KOH ≤ 100 °C

Conductivity of aqueous solutions of

KOH

Data from: R.J

. Gilliam, J.W.

Graydon

, D.W. Kirk, S.J. Thorpe,

Int. J Hydrogen Energy

,

32

(2007) 359-364.

Figure By Frank Allebrod, DTU Energy Conversion.

0

10

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40

50

60

70

80

90

100

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30

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50

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0.6

0.8

1

1.2

1.4

1.6

1.8

Temperature [°C]

KOH concentration [wt%]

Conductivity [S∙cm

-1

]

The electrical conductivity of KOH increases with increasing temperature

For each temperature the conductivity goes through a maximum with increasing concentrations

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Questions

Why does the conductivity decrease above a certain concentration?How are conductivity of electrolytes measured?What size of conductivity is needed in electrochemical cells?

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Alkaline Electrolysis

Is there an electrolyte conductivity issue related to the simple immersed electrode configuration?

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EMF - the equilibrium voltage

Dependence of the reversible cell voltage Erev to the system pressure and the temperature for pure water at 1 bar (dash dot) and for aqueous KOH with a concentration of 45 wt% at a pressure of 1 bar (full line), 10 bar (dashed), 25 bar (o) and 50 bar (+).

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Thermodynamic diagram

F. Allebrod

HHV

LHV

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Phase Transition (aq <-> aqueous + gaseous) lines of KOH

The figure shows the phase transition lines between the aqueous and the aqueous + gaseous phase of KOH The area above each line shows the gaseous + aqueous phase, the area below shows the aqueous phase The temperature and pressure has to be set to values below the lines during operation

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Measured conductivity of aqueous KOH

Frank Allebrod et al., Internat. J. Hydrogen Energy, (2012), doi:10.1016/j.ijhydene.2012.02.088.

0

100

200

300

0

0.5

1

1.5

2

2.5

3

3.5

35 wt% KOH

Conductivity [S x cm

-

1

]

0

100

200

300

0

0.5

1

1.5

2

2.5

3

3.5

45wt% KOH

Temperature [°C]

0

100

200

300

0

0.5

1

1.5

2

2.5

3

3.5

55 wt% KOH

measured

literature values

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Characteristics of Porous Structure for Immobilization of Liquid KOH Solution

10

-3

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1.2

1.4

Pore size [10

-6

m]

Log differential intrusion [

mL∙g

-1

]

Porosimetry analysis showed pore-sizes around 60 nm, as shown in the figure

The total porosity of the porous structure is about 51%

Frank Allebrod et al.,

Internat. J. Hydrogen Energy

, (2012), doi:10.1016/j.ijhydene.2012.02.088.

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Measured aqueous immobilzed conductivity of KOH

Frank Allebrod et al., Internat. J. Hydrogen Energy, (2012), doi:10.1016/j.ijhydene.2012.02.088.

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Ratio of conductivity of immobilized KOH to the conductivity of aqueous KOH

The figure shows the ratio of the conductivity of immobilized KOH, σim

, to the conductivity of aqueous KOH, σ

aq, for three the different concentrationsThe porosity of the porous structure is ca. 60%. This, and the tortuosity of the pellets explain the loss in conductivity of the electrolyte. Frank Allebrod et al.,

Internat. J. Hydrogen Energy

, (2012), doi:10.1016/j.ijhydene.2012.02.088.

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Comparison to commercial electrolytes/ diaphragms

A conductivity of immobilized KOH of 0.25 S∙cm-1 and 0.84 S∙cm-1 for 45 wt% was achieved at 80 C and 200 C, respectivelyKOH immobilized in Zirfon, a commercially available diaphragm, was reported as 0.6 S∙cm-1 at 80 C by Vermeiren et al. Unknown porosity. Zirfon is not stable at 200 C.

P. Vermeiren, J.P. Moreels

, A. Claes, H. Beckers, Electrode diaphragm electrode assembly for alkaline water electrolysers, Int J Hydrogen Energy. 34 (2009) 9305-9315

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Conventional cathode

Raney nickel has been the most popular cathode materials since it was invented in 1948, but simple Ni sheets and Ni-coated steel have also been used due to low costThe name “Raney nickel” covers now a group of Ni alloys of Ni-Zn and Ni-Al. When the Raney Ni is treated in concentrated KOH then the Zn and the Al will be dissolved and a nano-porous Ni sponge is left behind. Due to the high surface area and the good electrocatalytic properties of Ni this forms a very good cathode = H2 evoælution electrode2626 September 2012

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Cathodes recent developmentsThe composition of one of the best cathodes was: 70%

Mo-doped Raney Ni (A7000), 10% MoO3, 5% Cu, 5% graphite and 10% PTFE. It is known that 1%, or less, of Mo in Raney Ni improves stability and HER activity of this catalyst, and that even coarse mixtures of Ni alloys and molybdenites may perform better than the individual catalysts in HER

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Anodes

Ni and Ag-coated NiActivated Nickel electrodes (Ni-Co, Mo, Pt)

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Separator, sealing, containments

Separator was originally made from asbestos, but this is now forbidden due to health riskNow, other materials like Zirfon (ZrO2 with polymer binder) or NiO have been used. The sealing may be PTFE or similar stable polymers - metal-ceramic-metal layers for higher temperatureContainment is probably Ni-coated steel in most casesIt is difficult to get info about what industry actually use

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Improved components for advanced alkaline water electrolysis

[6] J. Divisek, P. Malinowski, J. Mergel, H. Schmitz, Improved components for advanced alkaline water electrolysis, Int J Hydrogen Energy. 13 (1988) 141-150.

Divisek

et al. developed and tested a zero gap cell with cell voltages around 1.55 V at 400 mA/cm2 at 100°CWhen considering the values of (Ucell - UIR free) as a function of time, a voltage loss of about 75 mV is measured. He states that approximately half of this value is ascribed to the diaphragm, the rest being caused through a gas-bubble effect and electrolyte resistivity.

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Perovskite-type oxides of cobalt -electrocatalytic

surface properties in relation to oxygen evolution S.K. Tiwari, P. Chartier, R.N. Singh, Preparation of perovskite-type oxides of cobalt by the malic acid aided process and their

electrocatalytic

surface properties in relation to oxygen evolution, J. Electrochem. Soc. 142 (1995) 148-153.

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Volcano plot

J.O. Bockris, T. Otagawa, ELECTROCATALYSIS OF OXYGEN EVOLUTION ON PEROVSKITES. J. Electrochem. Soc. 131 (1984) 290-302.

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Tentative conclusion on anode materials

After reviewing a number of paper about catalysts for the OER in alkaline media, it was found that Co3O4, Raney-Nickel, RuO2 and IrO2 are most active.Never the less, it has also been shown that the difference to perovskite-type materials are rather small. It is likely that, towards higher current densities, other attributes like cell design, diffusion and gas bubbles are more important

Slide34

New developmentsIt is well known that there will be important advantages if the operation temperature could be raised from the 60 – 120

C to say 200 – 300 CWhich advantages could you think of?Naturally, there would also be disadvantages.Which could you think of?Inspiration may be taken from the alkaline fuel cell3426 September 2012

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From: J.O. Jensen – at Joint European Summer School, Crete, September 17-28, 2012

Alkaline fuel cell (AFC)Electrolyte: Aqueous KOH (ca. 30 w%)

Slide36

New material structure Recently, F.

Bidault, D.J.L. Brett, P.H. Middleton, N. Abson and N.P. Brandon published a paper “A new application for nickel foam in alkaline fuel cells”, in Int. J. Hydrogen Energy, 34 (2009) 67993626 September 2012Scanning electron microscope image showing the

open structure of the nickel foam.Ni foam cost only ca 1/3 of the cost of Ni mesh per m

2

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Ni – foam AFC

3726 September 2012Polarization curves of nickel foam and nickel mesh in an aerated 32 wt% KOH

Slide38

Pressure and temperatureIt is well established that both pressure and temperature increase the electrode kinetics – for both oxygen and hydrogen electrode

So what to do if we should really optimize?As high temperature and pressure as possible!Materials put limits, however!3826 September 2012

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New Alkaline Electrolyser

Electrolyte: -aqueous KOH immobilized in a porous structureGas diffusion electrodes: - porous

Nickel, Raney-Nickel

High temperature and pressure alkaline electrolysisF. Allebrod, C. Chatzichristodoulou, M. Mogensen, submitted paper and filed patent application

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High Temperature and Pressure Alkaline (HT-AEC)

Cyclic voltage sweep on a cell with nickel-based gas diffusion electrodes. Current densities of 1.0 A·cm-2 at 1.5V and 1.9 A·cm-2 at 1.75V. 3.7 MPa and 241 ºC. Calculated EMF 1.2 V. 1 cm2 button cell.Conductivity of aqueous 45 wt% KOH immobilized in nano-porous structure reached 0.84 S·cm

-1

at 200 ºCF. Allebrod, C. Chatzichristodoulou M. Mogensen, submitted paper and filed patent application

40

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SEM analysis of the used foam and the Cell surface

The electrolysis cells are pressed out of Nickel and Inconel foams with a pore size of 200-450 µm as deliveredAfter the press and sintering production method the pore size is reduced to 50 -150 µmF. Allebrod et al.

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Cell 68 for 6 h at 250 C

The current density of the cell has been measured at 1.5, 1.625 and 1.75 V

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Performance of commercial Alkaline Electrolysers

Laboratory results are fine, in particular for natural and technical scienceCommercial field results are better for economy considerations4326 September 2012

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NEL statements on perforamnce

Main challengesCapital costOperational cost + power infrastructureFlexible operation = control system Some advantagesWell proven, reliable, robust technologyLife time > 10 yearsHigh H2 purity (99.9 ±0.1 %, < 1ppm of O2 and H2O, < 5 ppm N2)Immediate Start-up from stand-byQuick response time (<1 s)Broad range of operational range (10 – 100 %) Energy consumption4.1 - 4.45 kWh/Nm

3 H2

4426 September 2012

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Capital costs - ENEL

4526 September 2012

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Operational costs -ENEL

Electricity: 98%Remaining 2% :Cooling water make-upOperation & maintenanceElectrolyte charge + make-upRaw waterPurificationEmergency (N2, diesel back-up)4626 September 2012

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Hydrogenics HySTAT® - Best Cell Stack Performance

Alkaline electrolysis: 30% vol. KOHPressure: 10 barg to 30 bar (barg =  gauge pressure, i.e., pressure in bars above ambient pressure)Conversion efficiency: 4.44 kWh/Nm3 H2 (HHV: 80%, LHV: 68%)Lifetime: 60,000 hours (6.8 years)Hydrogen purity: 99.9 %< 1000 ppm O2 in H212 ppm N2H2O saturated

From: Raymond Schmidt, Global Market Strategist, Hydrogenics, “Electrolysis for grid balancing”, International Water Electrolysis Symposium, Copenhagen, 10-11 May. 2012.

4726 September 2012

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ChallengesAt the end it is all about economy

This is first of all electricity cost and next, production rate, efficiency plus investment costThis could easily be an economic lesson of its own, but as this school is not about economy let us spend the remaining time for discussion of efficiency4826 September 2012

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Please find the “round trip” efficiency of H2

– O2 Alkaline Electrolyser – Fuel CellFrom electricity out of the grid back into the grid?Let us discuss and find the numbers (Google) together. First which numbers do you need?Are there other relevant efficiencies?49

26 September 2012