NTNU - Materials Technology Sustainable Electrolysis

Geir Martin Haarberg NTNU, Trondheim, Norway

1 2 Norwegian University of Science and Technology (NTNU)

3 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

4 5 6 7 8 9 10 11 12 13 14 Norwegian University of Science and Technology (NTNU)

Typical study programs

5 years MSc

3 – 4 years PhD

2 years International MSc

15 Department of Materials Technology Electrochemistry Group

Corrosion Corrosion protection (offshore) Surface treatment (aluminium alloys)

Energy conversion Fuel cells (PEM, direct methanol) Water electrolysis (PEM)

Electrolysis Molten salts electrowinning Aqueous solutions electrowinning Sustainable electrolysis

16 Department of Materials Technology

Electrolysis Molten salts electrowinning Aqueous solution electrowinning

Research projects in electrolysis Oxygen supply by water electrolysis Kinetics for oxygen evolution in copper electrowinning

Ti production by deoxidation of TiO2 in molten CaCl2

Al production by deoxidation of Al2O3 in molten CaCl2 Impurities in electrowinning of aluminium Electrowinning of iron Electrorefining of silicon in molten salts

17 Sustainable electrolysis

Sustainable 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 alternative ways 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

18 Solar cell silicon

Silica (SiO2) Silicon metal ingot for 19 solar cell Silicon

20 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 + SiCl4 Energy requirement: ~145 kWh/kg Si

21 Solar cell silicon by electrodeoxidation of SiO2

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

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 : - 2- SiO2 + 4e (through Mo or Si) → Si + 2O

SiO2 contacting electrode 22 WP2: Cleaning & WP1: Cleaning & Refining DMR WP3: Electrochemical Refining HDN refining Deutsche Solar: ScanA/SUN: Fesil: ScanA/SUN: SINTEF/Fesil: Highly doped n- SOLSILC MG-Si SOLSILC feedstock Recycled Si type waste feedstock production

Deutsche Solar: NTNU, SINTEF:

SINTEF: Small sc ale purification SINTEF: WP7: Integration & exploitation n-type purification Electrochemical Fesil: Pilot scale purification Modelling in pilot equipment refining

Deutsche Solar: Pillar: SINTEF: SINTEF: Bridgman crystallisation Cz Bridgman crystallisation Bridgman crystallisation (large scale) crystallisation (small scale) (small scale)

WP4: Material SINTEF: SIMS, LECO analysis UKON: lifetime Characterisation NTNU: GD-MS, PVScan (particle analysis) UMIB: PL, EBIC ECN: ICP-AES, IR, lifetime analysis

WP5: Cell P-type cell process: N-type cell process: optimisation UKON: high efficiency baseline, ECN: industrial UKON: high efficiency baseline, ECN: industrial baseline, Isofoton: industrial pilot baseline, Isofoton: industrial pilot Characterisation : UKON: lifetime, IV/SPR, IR Increased yield: thermography, UMIB: PL, EBIC, ECN: lifetime, ECN: RPECVD, belt furnace gett., UKON: IV/SPR, FTIR, CoRe mechanical stability, MIRH P, tube furnace gett,

WP6:Modules& Isofoton: demo module, n-type module Recycling recycling, ECN: LCA 23 Electrochemical refining of Si - principles

Source: MG-Si anode cathode + - Potentially low cost alloy substrate Chloride/fluoride electrolyte

Solid deposit, 800°C

MG SoG- 4+ Efficient removal of elements less -Si Si Si Si noble than Si (B,P,Ca) – CaCl will not deposit at the cathode 2

Efficient removal of elements more 4+ - Si (with impurities) → Si + 4e (anode) noble than Si –

will not dissolve anodically 4+ - Si +4e → Si (without impurities) (cathode)

24 Solar grade silicon - Experimental

Electrolyte:

CaCl2 + NaCl + CaO at 850 °C

Gold film furnace

W working electrode Si counter electrode W reference Glassy carbon electrode crucible

25 Solar grade silicon - Voltammetry

0.40 Sweep rate: 200 mVs-1

0.20 Electrolyte:

0.00 85 mol % CaCl2 5 mol % NaCl -2 -0.20 10 mol % CaO i/ Acm -0.40 Pure melt After Si addition -0.60

-0.80 -1.0 -0.5 0.0 0.5 E/ V W Si can both be deposited and dissolved in the melt

26 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.

27 Conclusion

Silicon was electrodeposited successfully.

MG-Si powder dissolved in the melt.

Anode passivation is a problem.

28 Lake Biwa - A Water Electrolysis Model

29 Restoration of Lake Biwa by deep water electrolysis to supply oxygen

Main objective: Supply dissolved oxygen by electrolysis

Other aspects: Silicon solar cells - Produce electricity for water electrolysis Capture and handling of hydrogen - Produce additional electricity by on-shore fuel cells Develop a general method for oxygen supply in lakes Background data pH 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 30 o Spec el conductivity ~135 μS/cm at 25 C Lake Biwa Seminar, June 27 The Lake Biwa Physical Model

31 Electrowinning

Annual production of metals – million tonnes

Aluminium 23 Copper 13 Zinc 9 Nickel 1 Magnesium 0.5 Cobalt 0.03 Titanium 0.1 Iron 1000

32 Letter to Nature – 21 September 2000

33 FFC possibilities

34 Electrodeposition of iron from molten salt electrolytes

35 Fe ULCOS

Iron smelting by carbon reduction of Fe2O3 CO2 emissions

ULCOS (Ultra Low CO2 Steelmaking)

Purpose: Develop a new process for iron production with reduced CO2 emissions

36 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 37 Cyclic voltammetry in molten CaCl2-CaF2 0.12 -0.005

0.10 0.05V/s -0.010 0.1V/s -0.015 0.08 0.2V/s 0.3V/s -0.020 0.06 0.4V/s -0.025 ] 0.5V/s -2 0.04 -0.030 -2

/[Acm 0.02 c -0.035 p i /Acm p c 0.00 i -0.040

-0.02 -0.045 -0.050 -0.04 -0.055 -0.06 -0.060 -0.3 -0.2 -0.1 0.0 0.1 0.2 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 E[V] vs Fe reference v1/2/(V/s)1/2 CV’s of Mo in molten CaCl2-CaF2-Fe2O3 (80-20-0.5 mol%), 827 °C Cathodic peak current density vs square root of sweep rate 0.000

-0.005

-0.010

-0.015

-2 Reversible cathode reaction

-0.020 /Acm c p i Fe (III) + 3 e- → Fe (s) -0.025 -0.030 Controlled by diffusion Fe(III) towards cathode -0.035 0.20 0.25 0.30 0.35 0.40 0.45 0.50 c /mol% -5 2 -1 38 Fe2O3 DFe(III)=3.0×10 cm s

Peak current density vs content of Fe2O3 o Cyclic voltammetry in molten NaCl - FeCl3, 890 C

Reversible cathode reaction

Fe (III) + 3 e- → Fe (s)

Controlled by diffusion Fe(III) towards cathode

-5 2 -1 39 DFe(III)=1.4×10 cm s Bulk electrolysis -2 0.85 Acm , molten CaCl2-KF, 1144 K

1.5 -0.2 W.E. Fe rod cathode -0.3 with rotation (260 rpm) -0.4 1 C.E. Magnetite (Fe3O4) anode -0.5 R.E. Pt wire -0.6 0.5 Cell voltage / V / voltage Cell

-0.7 1.5 mol% Fe2O3 addition Cathode potential/ V vs. Pt -0.8 0 –0.8V is the cathodic limit 0 500 1000 1500 2000 2500 potential of this melt Time / sec

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

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

4000

3500 Fe Pure iron 3000 FeO

2500 CaF2

2000 Small amount of impurities 1500 IntensityIntensity / / cps cps

1000 Electrolyte → CaF2 500 20 30 40 50 60 70 80 0 Rinsing the deposit with distilled water → FeO 2θ (Cu-K α )

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

41 Electrowinning of Iron from Molten Salts Energy 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: 0.90 Energy consumption: 3.5 kWh/kg Fe

42 Conclusions - Fe molten salts

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

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

High current efficiency (> 90 %)

Oxygen evolving anode materials show promising behaviour

43