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 scale purification SINTEF: exploitation & Integration WP7: 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, MIRHP, 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: 85 mol % CaCl 0.00 2 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/ VW 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 Fe 3500 Pure iron FeO 3000 CaF2 2500 2000 Small amount of impurities IntensityIntensity / / cps cps 1500 1000 Electrolyte → CaF2 500 0 Rinsing the deposit with 20 30 40 50 60 70 80 distilled water → FeO 2θ (Cu-Kα ) 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.