1. Alkaline Electrolysis Cells: Materials, Properties and Challenges
Acknowledgements to colleagues at DTU Energy Conversion
Mogens B. Mogensen
Technical University of Denmark,
DTU Risø Campus
DK-4000 Roskilde
Denmark
2nd Joint European Summer School on Fuel Cell and Hydrogen Technology,
Crete, September 25th, 2012

2. Outline Principles in alkaline water electrolysis Commercial status
Materials in alkaline electrolysers
Electrolyte
Anode
Cathode
Separators, sealings, containments
Materials of electrolysers under development
Properties commercial systems – examples
Challenges – optimization of system
Add Presentation Title in Footer via ”Insert”; ”Header & Footer”
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3. Principle -reactions
The 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: H2O  H2 + ½ O2
Very simple reaction, which may be carried out in practise at a temperature as low as 60 C
Even so, it shows up that systems are not that simple
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4. Principle designs
Different architectures of electrolyzers with: (a) immersion electrodes, (b) porous electrodes in the “zero-gap” configuration, (c) electrodes comprising
Gas 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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5. Process flow diagram of a modern electrolyzer
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6. History of industrial water electrolysis
Year
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 m3 H2 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
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7. 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 NEL
Several other companies that produce alkaline electrolysers exist
Companies 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
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8. The alkaline electrolyser is commercial available
Hans Jörg Fell, CTO
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9. Photo of NEL on-site electrolyser
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, May
NEL does not tell the details of what is on their photos. As far as I can figure out by studying their homepage, 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 H2 h-1.
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10. Hydrogenics alkaline electrolyser cell stack
Photo from: Raymond Schmidt, Global Market Strategist, Hydrogenics, “Electrolysis for grid balancing”, International Water Electrolysis Symposium, Copenhagen, May
HySTAT® 10 – 10
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11. Hydrogenics Alkaline system
From Hydrogenics’ homepage:
HySTAT® 10 – 10
10 Nm3H2 h-1, 5.4 kWh/Nm3 H2

12. Problem - faradaic efficiency
Faradaic efficiency, F is defined as the percentage of the current (not energy) that is used to produce H2
The Faradaic efficiency is not 100 % (but may come close) for alkaline electrolysis
Why is F < 100 %? Which processes?
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13. 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 involved
It 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 laboratories
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14. Electrolyte
The most used electrolyte for alkaline electrolysers is concentrated aqueous solution of KOH
Often 30 – 35 wt% KOH is used as this has the highest conductivity
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15. Conductivity of aqueous KOH ≤ 100 °C10 20 30 40 50 60 70 80 90
100
0.2
0.4
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
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) Figure By Frank Allebrod, DTU Energy Conversion.

16. 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?

17. Alkaline Electrolysis
Is there an electrolyte conductivity issue related to the simple immersed electrode configuration?

18. 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 (+).

19. Thermodynamic diagram
HHV
LHV
F. Allebrod

20. 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

21. Measured conductivity of aqueous KOH
100
200
300
0.5 1
1.5 2
2.5 3
3.5
35 wt% KOH
Conductivity [S x cm - ]
45wt% KOH
Temperature [°C]
55 wt% KOH
measured
literature values
Frank Allebrod et al., Internat. J. Hydrogen Energy, (2012), doi: /j.ijhydene

22. Characteristics of Porous Structure for Immobilization of Liquid KOH Solution10 -3 -2 -1 1
0.2
0.4
0.6
0.8
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: /j.ijhydene

23. Measured aqueous immobilzed conductivity of KOH
Frank Allebrod et al., Internat. J. Hydrogen Energy, (2012), doi: /j.ijhydene

24. 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, σaq, for three the different concentrations
The 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: /j.ijhydene

25. 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, respectively
KOH 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)

26. 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 cost
The 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 electrode
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27. Cathodes recent developments
The 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

28. Anodes
Ni and Ag-coated Ni
Activated Nickel electrodes (Ni-Co, Mo, Pt)

29. Separator, sealing, containments
Separator was originally made from asbestos, but this is now forbidden due to health risk
Now, 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 temperature
Containment is probably Ni-coated steel in most cases
It is difficult to get info about what industry actually use

30. Improved components for advanced alkaline water electrolysis
Divisek et al. developed and tested a zero gap cell with cell voltages around 1.55 V at 400 mA/cm2 at 100°C
When 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.
[6] J. Divisek, P. Malinowski, J. Mergel, H. Schmitz, Improved components for advanced alkaline water electrolysis, Int J Hydrogen Energy. 13 (1988)

31. 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)

32. Volcano plot
J.O. Bockris, T. Otagawa, ELECTROCATALYSIS OF OXYGEN EVOLUTION ON PEROVSKITES. J. Electrochem. Soc. 131 (1984)

33. 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

34. New developments
It 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 C
Which advantages could you think of?
Naturally, there would also be disadvantages.
Which could you think of?
Inspiration may be taken from the alkaline fuel cell
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35. Alkaline fuel cell (AFC)
Electrolyte:
Aqueous KOH (ca. 30 w%)
From: J.O. Jensen – at Joint European Summer School, Crete, September 17-28, 2012

36. 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) 6799
Scanning 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 m2
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37. Ni – foam AFC
Polarization curves of nickel foam and nickel mesh in an aerated 32 wt% KOH
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38. Pressure and temperature
It 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!
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39. New Alkaline Electrolyser
High temperature and pressure alkaline electrolysis
Electrolyte:
-aqueous KOH immobilized in a porous structure
Gas diffusion electrodes:
- porous Nickel, Raney-Nickel
F. Allebrod, C. Chatzichristodoulou, M. Mogensen, submitted paper and filed patent application

40. High Temperature and Pressure Alkaline (HT-AEC)
Conductivity of aqueous 45 wt% KOH immobilized in nano-porous structure reached 0.84 S·cm-1 at 200 ºC
F. Allebrod, C. Chatzichristodoulou M. Mogensen, submitted paper and filed patent application
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.

41. 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 µm as delivered
After the press and sintering production method the pore size is reduced to µm
F. Allebrod et al.

42. Cell 68 for 6 h at 250 C
The current density of the cell has been measured at 1.5, and 1.75 V

43. Performance of commercial Alkaline Electrolysers
Laboratory results are fine, in particular for natural and technical science
Commercial field results are better for economy considerations
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44. NEL statements on perforamnce
Main challenges
Capital cost
Operational cost + power infrastructure
Flexible operation = control system
Some advantages
Well proven, reliable, robust technology
Life time > 10 years
High H2 purity (99.9 ±0.1 %, < 1ppm of O2 and H2O, < 5 ppm N2)
Immediate Start-up from stand-by
Quick response time (<1 s)
Broad range of operational range (10 – 100 %)
Energy consumption
kWh/Nm3 H2
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45. Capital costs - ENEL
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46. Operational costs -ENEL
Electricity: 98%
Remaining 2% :
Cooling water make-up
Operation & maintenance
Electrolyte charge + make-up
Raw water
Purification
Emergency (N2, diesel back-up)
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47. Hydrogenics HySTAT® - Best Cell Stack Performance
Alkaline electrolysis: 30% vol. KOH
Pressure: 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 H2
12 ppm N2
H2O saturated
From: Raymond Schmidt, Global Market Strategist, Hydrogenics, “Electrolysis for grid balancing”, International Water Electrolysis Symposium, Copenhagen, May
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48. Challenges At the end it is all about economy
This is first of all electricity cost and next, production rate, efficiency plus investment cost
This 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 efficiency
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49. Please find the “round trip” efficiency of H2 – O2 Alkaline Electrolyser – Fuel Cell
From 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?
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