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Hydrogen production by electrolysis
Ann Cornell, Department of Chemical Engineering, KTH [email protected]
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Sources for hydrogen International Energy Agency. Technology Roadmap – Hydrogen and Fuel Cells, 2015
Natural gas 48%
Oil refineries 30%
Coal 18%
Electrolysis 4%
Natural gas 37%
Coal 33%
Electrolysis 22%
Biomass 8%
Today Prediction year 2050
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Electrolysis for H2-production
Only large scale alternative for fossil-free production Expensive method Pure gases produced Water electrolysis: Total reaction: H2O → H2 + ½ O2
Acidic conditions Alkaline conditions Anode H2O → ½ O2 + 2H+ + 2e- 2OH- → ½ O2 + 2H2O + 2e-
Cathode 2H+ + 2e- → H2 2H2O + 2e- → H2 + 2OH-
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Commercial techniques water electrolysis
Technology Alkaline water electrolysis
SPE (Solid polymer
electrolyte) electrolysis
Process Aqueous electrolysis ”Reversed PEFC”
Feed 80% KOH, 80oC Pure H2O, 80oC
Charge carriers OH- H+
Industrial use Well developed
Large scale
High current densities Differential pressure Expensive catalysts
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Alkaline water electrolysis
2OH- → ½ O2 + 2H2O + 2e-
2H2O + 2e- → H2 + 2OH-
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Alkaline water electrolyser
Anode: 4OH- → O2 + 2H2O + 4e- Cathode: 4H2O + 4e- → 2H2 + 4OH- Electrolyte: 25% KOH 80oC
By courtesy of StatoilHydro
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Inside a water electrolyser
Bipolar technology Electrodes of coated mild steel
By courtesy of StatoilHydro
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Solid polymer electrolyte electrolysis
H2O → ½ O2 + 2H+ + 2e-
2H+ + 2e- → H2
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Alkaline membrane cells
Less noble catalysts can be used than under acidic conditions (e.g. nickel) Development of alkaline membranes Still not commercially available
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∆G = ∆H -T∆S ∆G = - zFE G, Gibbs free energy J/mole H, enthalpy J/mole T, absolute temperature K S, entropy J/mole,K z, moles electrons/moles substance F, Faradays constant 96500 As/mole E, equilibrium cell voltage
Steam electrolysis at 1000oC: • low equilibrium cell voltage (0.91 V, compare 1.23 V) • low overpotentials and IR drops
H2O → H2 + ½ O2
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Technologies for water electrolysis
Technology Alkaline water electrolysis
SPE (Solid polymer
electrolyte) electrolysis
SOEC (Solid oxide electrolysis cell)
Process Aqueous electrolysis ”Reversed PEFC” ”Reversed SOFC”
Feed 80% KOH, 80oC Pure H2O, 100oC Steam, 800-900oC
Charge carriers OH-, K+ H+ O2-
Industrial use Well developed
Large scale
High current densities Differential pressure Expensive catalysts
Not yet commercial Pilot scale
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High temperature steam electrolysis (SOEC)
Mix of H2 and steam
2O2- → O2 + 4e- 2H2O + 4e- → H2 + 2O2-
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Electrolytic hydrogen production from other processes than water electrolysis…
Why oxygen evolution as anode reaction? • Oxygen often not used • High Eeq
• Slow kinetics Other anode reaction? Anode reaction Red → Ox + ne-
e- e-
anions
cations
Ecell
Icell
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Anode reaction 2Cl- → Cl2 + 2e-
Chlor-alkali 2Cl- + 2H2O → Cl2 + H2+ 2OH- ~60 million tonnes/year Cl2
produced world wide Many uses for the products
Chlorate NaCl + 3H2O → NaClO3 + 3H2 About 4 million tonnes/year NaClO3 produced world wide Main use in the bleaching of chemical pulp In some plants the hydrogen formed is not used at all
European plants (Eurochlor) These processes produce close to 2 Mtonnes H2/year
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Hydrogen from chlorate production
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Anode reaction oxidation of alcohols, sugars etc organic compounds
Equilibrium cell voltage Oxygen evolution (water electrolysis) Eeq = 1.23 V at 25oC Alcohol oxidation corresponding Eeq ~ 0.1- 0.2 V Electricity need directly proportional to the cell voltage (Ecell*I*t) Hydrogen can be produced at significantly lower electricity consumption compared to in water electrolysis!
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Thermo electrochemical cycles
Part thermal energy, part electricity Example sulfur-hydrogen cycle Anode: SO2 + 2H2O → H2SO4 + 2H+ + 2e-
Overall: SO2 + 2H2O → H2SO4 + H2 Eeq= 0.17 V (compare 1.23 V)
H2SO4 catalytically decomposed back to SO2
H2SO4 → SO2 + H2O + ½O2
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Summary
• Hydrogen an important energy carrier in a future fossile free society • Only large scale production alternative today produced without
emission of greenhouse gases is electrolysis • Oxygen produced often not used – other useful anodic products in
aqueous based electrolytic processes, for example chlor-alkali • Possibility to considerably reduce the electrical energy need if chosing
certain anode reactions • Future development: steam electrolysis, alkaline membrane
electrolysis, alternative anode reactions, hybrid thermal/electrochemical cycles, improvement of existing techniques. (Also much reseach on photo chemical electrolysis.)