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Feasibility of alkaline water electrolysis with cation-selective membrane
M. Paidar, K. Vazač, M. Roubalík, K. Bouzek
Department of Inorganic Technology, Unniversity of Chemistry and Technology Prague
Technická 5, Prague, Czech Republic ([email protected])
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Water electrolysis process
• Direct method of producing hydrogen
• Inexhaustible storage material: WATER
• Highest hydrogen quality
• Zero emmision clean process
• High operating costs high cost of hydrogen little extended process (cca 5 %)
• Water electrolysis
– Alkaline
– PEM (Acidic)
– High-temperature
– Electrolysis of brine (hydrogen as byproduct)
[1] www.instructables.com/id/Separate-Hydrogen-and-Oxygen-from-Water-Through-El
Schema of water electrolysis [1]
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Water electrolysis in energy storage
h-tec unavoidable step in hydrogen production from alternative sources process flexibility - required
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Industrial alkaline water electrolysis
• Electrolyte: 25 – 35 % (wt.) KOH
• Temperature: 70 – 90 °C
• Anode: Nickel
• Cathode: Nickel, Steel
• Separating partition: Diaphragm – asbestos, ceramic, polymer, composite
• Operating prassure: 1 – 30 Bar
• Advantage: Low investment cost, simple construction – well matured process
• Disadvantage: Higher power consumption (U = 1.8 – 2.1 V), larger dimensions (compared to PEM electrolysis), asbestos diaphragm (thickness, electrochemistry properties)
[1] http://www.nel-hydrogen.com
NEL hydrogen electrolyzer [1]
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Alkaline water electrolysis with Nafion®
Replacement of asbestos diaphragm by Nafion® membrane
• Compared to industry electrolysis
– Reduced inter-electrode distance
– Reduced device dimensions
– High membrane cost
• Compared to PEM electrolysis
– Low cost catalysts
– Lower mobility of K+ / Na+ ions (vs H+) higher cell voltage, lower efficiency
• Additional benefits
– Allows to provide additional process to hydrogen production – e.g.: caustic concentration from diluted solutions
– Possibility to raise temperature (> 150 °C) reduction of the cell voltage
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Experimental Electrolyzer • Membrane: Nafion® N 117 (20 cm2)
• Anode: Ni plate, Ni expanded mesh
• Cathode: Ni (Fe) plate, Ni (Fe) expanded mesh
• Cell arrangement: space between electrodes and membrane – spacer (1 mm)
– expanded metal (zero-gap) Experimental conditions • Electrolytes
– KOH vs. NaOH
– different concentrations (5 to 25 % wt.)
– different temperatures (25, 50 and 73 °C)
– volume: 0.5 dm3
• Current density (125 to 500 mA/cm2)
• Mode of operation
– Load curves – linear voltametry
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Electrolyzer construction
Current feeders
Electrolyte inlet
Electrolyte outlet
Heating
Ni smooth plates electrodes
Teflon frames
Nafion® membrane
Spacers used
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Influence of spacer
Electrolysis of 15 % KOH, 73 °C, 120 ml/min electrolyte flow.
0
125
250
375
500
1.5 2 2.5 3 U [V]
j [mA/cm2]Ni smooth plate electrodes
+ spacer
Prevent sticking the membrane to the electrodes + better electrolytes distribution
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Ni plate with flow channels
Ni expanded mesh
Zero-gap electrode
Expanded metal - Zero-gap arrangement
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Electrolysis of 15 % KOH, 73 °C, 120 ml/min electrolyte flow.
Zero-gap = minimal inter-electrode distance + improved construction = better electrolyte distribution and bubbles dissipation Improvement about 35 % (measured at 2.2 V)
Zero-gap arrangement
0
125
250
375
500
1.5 2 2.5 3 U [V]
j [mA/cm2]Ni smooth plate electrodes
+ spacer
zero-gap
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Influence of operating temperature
• Energy parameters strongly influenced by temperature
– space for another improvement
0
250
500
1.5 2.5 3.5U [V]
j [mA/cm2]
25 °C50 °C73 °C
Electrolysis of 10 % NaOH, different temperature, 120 mL/min.
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Influence of operating parameters
• NaOH better than KOH
• 15 – 20 % (wt.) NaOH best results
0
250
500
1.5 2 2.5 3U [V]
j [mA/cm2]15% KOH20% KOH15% NaOH20% NaOH
Electrolysis of KOH vs. NaOH, 73 °C, 120 mL/min.
0
250
500
1.5 2 2.5U [V]
j [mA/cm2]
5% NaOH10% NaOH15% NaOH20% NaOH25% NaOH
Electrolysis of NaOH, 73 °C, 120 mL/min.
KOH vs. NaOH Concentration
Ullmann encyclopedia: a-NaOH(40 °C), b-NaOH(60 °C), c-NaOH(80 °C), d-KOH(60 °C), e-KOH(80 °C)
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Cell conductivity
• decrease of conductivity due to membrane conductivity loss (hydration)
• similar behavior for NaOH
membrane N117 in KOH
0
0,02
0,04
0,06
0,08
0,1
10 15 20 25 30
KOH / wt.%
cell
cond
uctiv
ity [S
/cm
]50 °C
75 °C
83 °C
92 °C
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Steel cathode
Steel has lower hydrogen overvoltage than nickel
Improvement about 25 % (measured at 2.2 V)
Electrolysis of 15 % KOH, 73 °C, 120 ml/min electrolyte flow.
0
125
250
375
500
1.5 2 2.5 3 U [V]
j [mA/cm2]Ni smooth plate electrodes+ spacerzero-gapSteel cathode
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Comparison with Industry
Results comparable with industry alkaline water electrolyzers
• Space for another improvement (temperature, catalyst)
0
100
200
300
1,5 1,75 2 2,25U [V]
j [mA/cm2]
Alkaline water electrolysis with NafionOronzio De Nora (Italy)Nors Hydro (Norway)Electrolyzer Corp. (Canada)Teledyne Energy System (USA)BBC Ag (Switzerland)Lurgi (Zdanski-Lonza) (Germany)
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Conclusions • Alkaline water electrolysis with Nafion® membrane is feasible.
• Zero-gap arrangement – best results from tested construction type.
• Results comparable with industrial alkaline water electrolysis.
• Low electrolyte concentration - more flexible operation
• Optimal operating parameters:
– hydroxide concentration: 15 – 20 % (wt.)
– NaOH better than KOH
• Next possibility for another improvement in the future:
– higher temperature (120 – 130 °C)
– using catalyst
– cell design (thinner membrane)
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Thank you for attention