1

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 (paidarm@vscht.cz)

2

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]

3

Water electrolysis in energy storage

h-tec unavoidable step in hydrogen production from alternative sources process flexibility - required

4

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]

5

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

6

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

7

Electrolyzer construction

Current feeders

Electrolyte inlet

Electrolyte outlet

Heating

Ni smooth plates electrodes

Teflon frames

Nafion® membrane

Spacers used

8

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

9

Ni plate with flow channels

Ni expanded mesh

Zero-gap electrode

Expanded metal - Zero-gap arrangement

10

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

11

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.

12

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)

13

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

14

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

15

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)

16

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)

17

Thank you for attention