Laurea in Scienza dei Materiali Materiali Inorganici Funzionali. Electrolyzers

Transcription

1 Laurea in Scienza dei Materiali Materiali Inorganici Funzionali Electrolyzers Prof. Dr. Antonella Glisenti -- Dip. Scienze Chimiche -- Università degli Studi di di Padova

2 H 2 by Electrolysis High purity Simple process No pollution Versatile source/sustainable energy Hydrogen by water electrolysis: an energy carrier to bridge generation of power source (regionalism, intermittence, unstorability) and renewable energy and energy demand Hydrogen by water electrolysis only for special applications (high purity) Supplementary and not alternative technology with respect to hydrocarbon sources

3 Theoretical energy consumption for production of 1m 3 H 2 Because of overpotentials no gas evolution until V; practical cell voltages: V Theoretical energy consumption for production of 1m 3 H 2 Energy efficiency

4 Practical cell voltage Ea = anode potential for O 2 evolution Ec = cathode potential for H2 evolution i = current density R = total ohmic resistance U = theoretical decomposition voltage Acid Electrolyte Alkaline Electrolyte

5 Overpotentials Theoretical decomposition voltage (i.e V) = water electrolysis is reversible kinetically (H2 and O2 evolution reactions hardly proceed) Additional electrodic overpotentials (η) are indispensable to overcome energy barrier and increase the rate of electrolysis A = overpotential at 1 A/cm 2 INTRINSIC PROPERTIES OF ELECTRODE, MATERIALS

6 Re Ohmic voltage drop total ohmic resistance Electrolyte resistance Membrane resistance Bubble resistance Circuit resistance > Electrolyte concentrations (max 20-30% NaOH or KOH Corrosion) Additives in electrolytes Ionic liquid electrolytes Rm Optimization of production process Rc Optimization of wire connections

7 Bubble effect Average rising velocity Magnetic field Ultrasonic field 0 T 0.5 T 1.0 T 5 T Bubble surface coverage Super gravity field

8 Electrolysis: Traditional vs. FCs Carbon assisted water electrolysis

9 ALKALINE ELECTROLYZER Electrolyte = 30 wt% KOH or NaOH Cathode = Ni with a catalytic coating, Pt Anode = Ni or Co coated with metal oxides (Mn, W, Ru) Anode: 4OH - O 2 + 2H 2 O + 4 e - Cathode: 4H 2 O + 4 e - 2H 2 + 4OH - Total: 2H 2 O 2H 2 + O 2 The liquid electrolyte is not consumed in the reaction, but must be replenished because of losses during H 2 recovery. Current density = macm -2 Efficiencies = 50 60%

10 PROTON EXCHANGE MEMBRANE ELECTROLYZER Electrolyte = H-Conducting polymer (Nafion) Cathode, Anode = Pt black, Ir, Ru, and Rh Anode: 2H 2 O O 2 + 4H e - Cathode: 4H e - 2H 2 Total: 2H 2 O 2H 2 + O 2 Current density = 1600 macm -2 Efficiencies = 55 70%

11 Solid Oxide Electrolysis Cell SOEC History First demonstration by Nicholson and Carlisle in s Faraday clarified the principles and in 1934 he introduced the word electrolysis Oerlikon Engineering Company: first commercial use. During the same period, Nernst developed the high-temperature electrolyte YSZ, 1951, the first commercially available high pressure electrolyser (30 bar) presented by Lurgi. Nowadays low temperature electrolysis technology is available with at least 13 manufactures (3 using alkaline electrolysers and 10 using polymer membranes). SOEC technology is still under development. 1980s studies curried out by Donitz and Erdle: the first SOEC results within the HotElly project from Dornier System GmbH

12 Solid Oxide Electrolysis Cell - SOEC The electrochemical reactions that take part in an SOEC are the inverse reactions to those that take part in an SOFC. Cell polarization is the opposite and anode and cathode interchange their roles Water = reactant supplied to the cathode Oxygen ions = cathode anode H 2 = produced in the cathode side

13 Thermodynamics of SOECs Water decomposition is endothermic Electric energy demand and decomposition voltage decrease with the increase of temperature. 25 C and 1 atm. Electric energy demand (ΔG) = 474 kj/mol Theoretical decomposition voltage (U θ ) = 1.23 V. Theoretical energy saving at 900 C is up to 23%, compared to that at 25 C. Overpotentials and ohmic voltage drop. are also decreased at high temperature. At 900 C and 1 atm. Electric energy demand (ΔG) = 366 kj/mol Theoretical decomposition voltage (U θ ) = 0,95 V

14 Materials for SOECs YSZ BASED Cell ASR (Ω cm 2 ) HT ceramics 0.2-1,4 Ni/YSZ YSZ La manganite 0.15 Current density A/cm 2 Scandia Stabilized Zirconia (ScSZ) CERIA BASED Cell Idaho/Ceramatec LaGaO 3 -based (ASR 0.6 Ω cm 2 at 800 C) ASR (Ω cm 2 ) 0.33 (850 C) Imperial college 0.99 (900 C) -0,45 Current density A/cm 2

15 Proton conductors based SOECs pure hydrogen is produced, and the fuel is not diluted with water vapor as in the case of using an oxide ion conductor problems associated with chemical stability and also with the integration with other cell components SrCe 1-x M x O 3 (M = Yb, Mg, Sc, Y, In, Zn, Nd, Sm, Dy; x = ) A/cm2 900 C SrCe 1-x Yb x O 3 SrZr 1-x Yb x O 3 Ba-Ce-Zr-Y-O

16 CO 2 and H 2 O co-electrolysis problems associated Ni-based (Ni-YSZ) cells: Ni synthering Coke

17 Experimental setup

18 Cu impregnated LSCM LaSrCrMnO 3 LSCM = p-type semiconductor > Sr > ion conductivity > Sr > Mn(IV) < electron conductivity Low electron conductivity under reducing conditions Cu = electron conductivity

19 LSCM LSCM-Cu Difficult to calculate the theoretical potential is CO, CO2 are present > CO2 > current density < Ohmic R after Cu deposition > Polarization R - diffusion

20 SOECs for Digester Gas Upgrading Biomasses Dry (< 50%) Wet (> 50%) Pyrolysis Anaerobic Digestion Raw digester gas = CH 4, CO 2, H 2 O Energy density is quite low compared to other gaseous fuels, and depends on the CH 4 fraction (varies between 16 and 23 MJ/Nm 3 ) Membranes Water scrubbing Pressure swing adsorption Upgrade After transformation: 95 97% vol. CH % vol. CO 2

21 SOECs for Digester Gas Upgrading 1. Poisoning of the catalyst with Sulfur 2. Methanation

22 Methane consumption tuning: Pressure (formed molecules) Temperature (endothermic) Sulfur

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24 Pression Swing Adsorption Water scrubbing Membranes

25 Reforming of biogas < 703 C > 527 C > 642 C Methane pyrolysis: Without catalysts: 900 C only 15% methane decomposed at 1000 C With catalysts (Nibased):% methane decomposition at C

26 Reforming of Biogas with a porous GDC electrolyte SOEC CO 2 decomposition to CO + ½ O 2 : Occurs at the boundary between Ni and supporting oxide (zirconia-alumina)

27 Reforming of Biogas with a porous GDC electrolyte SOEC

28 Reforming of Biogas with a porous GDC electrolyte SOEC > CH 4 /CO 2 > H 2

29 Water gas shift reaction with SOECs Different cathodes = Different reactions SOECs as reactors

30 Oxygen induced delamination

31 SOECs for Energy storage

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33 Constant electrolysis voltage from 1.33 V to 1.73 V (from 96% to 75% electricaltochemical energy efficiency) during 420 h Reversible test: cell voltage stable at 1.33 V Reversibility vs Stability the cell operated only in electrolysis mode experienced rapid and continuous degradation the cell tested with reversible cycling did not noticeably degrade

34 Deterioration causes Deterioration due to high anodic overpotential of the oxygen electrode = very high internal oxygen pressure (more than 100 bar) at the oxygenelectrode/electrolyte interface

35 Deterioration and solutions Micrographs of the deteriorated oxygenelectrode/electrolyte interface in the cell operated continuously in electrolysis mode High internal P O2 = precipitation of O 2 bubbles in closed cavities (mainly in grain boundaries) 1. High performance Oxygen electrodes 2. Reversible SOECs Nanosized pores Separation of YSZ grains Delamination of the interface Reversibility avoids the formation of bubbles or enables reversal of their growth

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37 Bibliography 1. Renewable and sustainable energy reviews 29 (2014) Meng Ni, Michael K.H. Leung, Dennis Y.C. Leung, K. Sumathy; Renewable and Sustainable Energy Reviews 11 (2007) A review and recent developments in photocatalytic water-splitting using TiO 2 for hydrogen production 3. R. Xing et al J. Power Sources 274 (2015) ; 4. M.A. Laguna-Bercero; J. Power Sources 203 (2012) V. Menon et al. J. Power Sources 274 (2015) C. Graves et al. Nature Materials 14 (2015) C.H. Wendel et al. J. Power Sources 276 (2015) Y. Hirata et al. J. Ceramic Soc. Jpn 119 (2011) G. Lorenzi et al. Energy&Fuels 29 (2015)