NTNU - Materials Technology Sustainable Electrolysis · 2-KF, 1144 K Changes of cathode potential...

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NTNU - Materials TechnologySustainable Electrolysis

Geir Martin HaarbergNTNU, Trondheim, Norway

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Norwegian University ofScience and Technology (NTNU)

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NTNU key figures (2005)

• 52 departments in 7 faculties• 58 000 student applications

– of which 9000 had NTNU as their first choice

• 20 000 registered students• 3000 degrees awarded• 220 PhD degrees awarded

• 4320 employees• 2600 empl. in education and research; 555

professors• 555 000 m2 owned and rented premises

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Norwegian University ofScience and Technology (NTNU)

Typical study programs

5 years MSc

3 – 4 years PhD

2 years International MSc

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Department of Materials TechnologyElectrochemistry Group

CorrosionCorrosion protection (offshore)Surface treatment (aluminium alloys)

Energy conversionFuel cells (PEM, direct methanol)Water electrolysis (PEM)

ElectrolysisMolten salts electrowinningAqueous solutions electrowinningSustainable electrolysis

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Department of Materials Technology

ElectrolysisMolten salts electrowinningAqueous solution electrowinning

Research projects in electrolysisOxygen supply by water electrolysisKinetics for oxygen evolution in copper electrowinningTi production by deoxidation of TiO2 in molten CaCl2Al production by deoxidation of Al2O3 in molten CaCl2Impurities in electrowinning of aluminiumElectrowinning of iron Electrorefining of silicon in molten salts

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Sustainable electrolysisSustainable development can be achieved by using renewable energy sources for the production of new and advanced materials,metals and chemicals.

Electrolysis can provide efficient use of energy and alternativeways of industrial production with less impact on the environment.

Topics

Aluminium electrowinning – Fundamental electrochemical studies

Anode processes in aqueous solutions – Oxygen evolution for electrowinning

Iron electrowinning – New process with no CO2 emissions

Silicon electrorefining – Solar grade Si by refining of metallurgical Si

Electrolytic titanium production – Develop new industrial process

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Solar cell silicon

Silica (SiO2) Silicon metal ingot for solar cell

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Silicon

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Production of Silicon

MG-Si by carbothermal reduction of silica at ~1900 oC:

SiO2 + C → Si + CO2

Energy requirement: ~12 kWh/kg Si

High purity silicon (Poly-Si) by the Siemens process at ~1150 oC

2 HSiCl3 → Si + 2 HCl + SiCl4Energy requirement: ~145 kWh/kg Si

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Solar cell silicon by electrodeoxidation of SiO2

Ito et al. produce silicon from SiO2 in a molten CaCl2 electrolyte at 850 oC.

SiO2 is a good insulator. Therefore Ito uses a ”SiO2 metal contacting electrode”, in which a Mo wire is directly in contact with SiO2.

Cathode reaction :SiO2 + 4e- (through Mo or Si) → Si + 2O2-

SiO2 contacting electrode

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ScanA/SUN:SOLSILC feedstock

ScanA/SUN:SOLSILC feedstock

SINTEF/Fesil:Recycled Si

WP1: Cleaning & Refining DMR

SINTEF: Small scale purificationFesil: Pilot scale purification

Pillar:Cz

crystallisation

SINTEF:Bridgman crystallisation

(small scale)

WP3: Electrochemical refining

NTNU, SINTEF: Electrochemical

refining

Fesil:MG-Si

production

SINTEF:Bridgman crystallisation

(small scale)

SINTEF: Modelling

Deutsche Solar:Bridgman crystallisation

(large scale)

Deutsche Solar: n-type purificationin pilot equipment

Deutsche Solar:Highly doped n-

type waste

WP2: Cleaning & Refining HDN

WP4: Material Characterisation

SINTEF: SIMS, LECO analysis UKON: lifetimeNTNU: GD-MS, PVScan (particle analysis) UMIB: PL, EBICECN: ICP-AES, IR, lifetime analysis

P-type cell process:UKON: high efficiency baseline, ECN: industrial

baseline, Isofoton: industrial pilot

Characterisation: UKON: lifetime, IV/SPR, IR thermography, UMIB: PL, EBIC, ECN: lifetime,

IV/SPR, FTIR, CoRe

N-type cell process:UKON: high efficiency baseline, ECN: industrial

baseline, Isofoton: industrial pilot

Increased yield:ECN: RPECVD, belt furnace gett., UKON:

mechanical stability, MIRHP, tube furnace gett,

WP5: Cell optimisation

WP6:Modules&Recycling

Isofoton: demo module, n-type module recycling, ECN: LCA

WP

7: Integration

& exp

loitation

ScanA/SUN:SOLSILC feedstock

ScanA/SUN:SOLSILC feedstock

SINTEF/Fesil:Recycled Si

WP1: Cleaning & Refining DMR

SINTEF: Small scale purificationFesil: Pilot scale purification

Pillar:Cz

crystallisation

SINTEF:Bridgman crystallisation

(small scale)

WP3: Electrochemical refining

NTNU, SINTEF: Electrochemical

refining

Fesil:MG-Si

production

SINTEF:Bridgman crystallisation

(small scale)

SINTEF: Modelling

Deutsche Solar:Bridgman crystallisation

(large scale)

Deutsche Solar: n-type purificationin pilot equipment

Deutsche Solar:Highly doped n-

type waste

WP2: Cleaning & Refining HDN

WP4: Material Characterisation

SINTEF: SIMS, LECO analysis UKON: lifetimeNTNU: GD-MS, PVScan (particle analysis) UMIB: PL, EBICECN: ICP-AES, IR, lifetime analysis

P-type cell process:UKON: high efficiency baseline, ECN: industrial

baseline, Isofoton: industrial pilot

Characterisation: UKON: lifetime, IV/SPR, IR thermography, UMIB: PL, EBIC, ECN: lifetime,

IV/SPR, FTIR, CoRe

N-type cell process:UKON: high efficiency baseline, ECN: industrial

baseline, Isofoton: industrial pilot

Increased yield:ECN: RPECVD, belt furnace gett., UKON:

mechanical stability, MIRHP, tube furnace gett,

WP5: Cell optimisation

WP6:Modules&Recycling

Isofoton: demo module, n-type module recycling, ECN: LCA

WP

7: Integration

& exp

loitation

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Electrochemical refining of Si - principlesSource: MG-Si

Potentially low cost

Chloride/fluoride electrolyte

Solid deposit, 800°C

Efficient removal of elements less noble than Si (B,P,Ca) –

will not deposit at the cathode

Efficient removal of elements more noble than Si –

will not dissolve anodically

MG-Si

+

SoG-Si

-

Si4+ Si

alloy substrate

anode cathode

CaCl2

Si (with impurities) → Si4+ + 4e- (anode)

Si4+ +4e- → Si (without impurities) (cathode)

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Solar grade silicon - Experimental

Si counter electrode

W reference electrode

W working electrode

Glassy carbon crucible

Gold film furnace

Electrolyte:CaCl2 + NaCl + CaOat 850 °C

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-0.80

-0.60

-0.40

-0.20

0.00

0.20

0.40

-1.0 -0.5 0.0 0.5

E/ VW

i/ A

cm-2

Pure meltAfter Si addition

Solar grade silicon - Voltammetry

Si can both be deposited and dissolved in the melt

Sweep rate: 200 mVs-1

Electrolyte:85 mol % CaCl25 mol % NaCl

10 mol % CaO

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Voltammetry

Cyclic voltammetry, sweep rate 2 Vs-1 at 800 °C. A): Voltammogram in 65 mol% CaCl2, 35 mol% NaCl, 5 mol% CaO. B): Voltammograms in 62.7 mol % CaCl2, 33.7 mol % NaCl, 4.8 mol % CaO and 3.7mol % SiO2.

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Conclusion

Silicon was electrodeposited successfully.

MG-Si powder dissolved in the melt.

Anode passivation is a problem.

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Lake Biwa -A Water Electrolysis Model

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Restoration of Lake Biwa by deep water electrolysis to supply oxygen

Lake Biwa Seminar, June 27

Main objective: Supply dissolved oxygen by electrolysis

Other aspects: Silicon solar cells- Produce electricity for water electrolysisCapture and handling of hydrogen- Produce additional electricity by on-shore fuel cellsDevelop a general method for oxygen supply in lakes

Background datapH 7 - 9

Dissolved oxygen 2 – 12 mg/l

Suspended solids ~1mg/l

Dissolved total nitrogen ~0.2 – 0.4 mg/l

Dissolved total phosphorus ~0.002 – 0.05 mg/l

Dissolved chloride ~10 mg/l

Spec el conductivity ~135 μS/cm at 25oC

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The Lake Biwa Physical Model

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Electrowinning

Annual production of metals – million tonnes

Aluminium 23Copper 13Zinc 9Nickel 1Magnesium 0.5Cobalt 0.03

Iron 1000

Titanium 0.1

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Letter to Nature – 21 September 2000

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FFC possibilities

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Electrodeposition of iron frommolten salt electrolytes

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Fe ULCOS

ULCOS (Ultra Low CO2 Steelmaking)Purpose: Develop a new process for iron production with reduced CO2 emissions

Iron smelting by carbon reduction of Fe2O3 CO2 emissions

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Electrowinning of iron?

Possibilities Problems/challenges

Aqueous solutions Low current efficiency Low current density? Large space required

Molten salts Low Fe2O3 solubility? No inert anode?

Molten oxides High temperature, corrosive electrolyte Electronic conduction No inert anode

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CV’s of Mo in molten CaCl2-CaF2-Fe2O3 (80-20-0.5 mol%), 827 °C

Reversible cathode reaction

Fe (III) + 3 e- → Fe (s)

Controlled by diffusion Fe(III) towards cathode

DFe(III)= 3.0×10-5 cm2s-1

-0.3 -0.2 -0.1 0.0 0.1 0.2-0.06

-0.04

-0.02

0.00

0.02

0.04

0.06

0.08

0.10

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Cyclic voltammetry in molten CaCl2-CaF2

ip c/[Ac

m-2]

E[V] vs Fe reference

0.05V/s 0.1V/s 0.2V/s 0.3V/s 0.4V/s 0.5V/s

0.20 0.25 0.30 0.35 0.40 0.45 0.50-0.035

-0.030

-0.025

-0.020

-0.015

-0.010

-0.005

0.000

i pc /Acm

-2

cFe2O3/mol%

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9-0.060

-0.055

-0.050

-0.045

-0.040

-0.035

-0.030

-0.025

-0.020

-0.015

-0.010

-0.005

ic p/Acm

-2

v1/2/(V/s)1/2

Cathodic peak current density vs square root of sweep rate

Peak current density vs content of Fe2O3

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Reversible cathode reaction

Cyclic voltammetry in molten NaCl - FeCl3, 890oC

Fe (III) + 3 e- → Fe (s)

Controlled by diffusion Fe(III) towards cathode

DFe(III)= 1.4×10-5 cm2s-1

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Bulk electrolysis0.85 Acm-2, molten CaCl2-KF, 1144 K

Changes of cathode potential and cell voltage during galvanostaticelectrolysis at 0.85 Acm-2 in CaCl2-KF-Fe2O3 (1.5 mol% Fe2O3 added) melts at 871 oC

W.E. Fe rod cathode with rotation (260 rpm)

C.E. Magnetite (Fe3O4) anode

R.E. Pt wire

–0.8V is the cathodic limit potential of this melt

1.5 mol% Fe2O3 addition

-0.8

-0.7

-0.6

-0.5

-0.4

-0.3

-0.2

0

0.5

1

1.5

0 500 1000 1500 2000 2500

Cat

hode

pot

entia

l / V

vs.

Pt

Cel

l vol

tage

/ V

Time / sec

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0

500

1000

1500

2000

2500

3000

3500

4000

20 30 40 50 60 70 80

2θ (Cu-Kα )

Inte

nsity

/ cps

0

500

1000

1500

2000

2500

3000

3500

4000

20 30 40 50 60 70 80

2θ (Cu-Kα )

Inte

nsity

/ cps

FeFeO

CaF2

Pure iron

Small amount of impurities

Electrolyte → CaF2

Rinsing the deposit with distilled water → FeO

XRD pattern of the deposit obtained after galvanostatic electrolysis at 0.85 Acm-2 in CaCl2-KF-Fe2O3 (1.5 mol% Fe2O3 added) melt at 1144 K

Galvanostatic electrolysis 0.85 Acm-2 in CaCl2-KF at 1144 K

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Electrowinning of Iron from Molten SaltsEnergy and heat

½Fe2O3(diss) = Fe (s) + ¾ O2 (g)

800ºC: ΔGo = 271 114 J/mol, ΔHo = 403 622 J/mol

Erev = -0.947 V, Eiso = -1.395 V

Current density: 0.5 A/cm2

Cell voltage: 2.2 V Current efficiency: 0.90Energy consumption: 3.5 kWh/kg Fe

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Pure iron can be deposited from molten saltsFe(III) species are stable in mixed fluoride/chloride melts

High current efficiency (> 90 %)

High current density ( 0.85 Acm-2, in CaCl2-KF, rotating cathode )

Conclusions - Fe molten salts

Oxygen evolving anode materials show promising behaviour