Presentation is loading. Please wait.

Presentation is loading. Please wait.

Ionic ceramic conductors. Solid Oxide Fuell Cells (SOFCs)

Similar presentations


Presentation on theme: "Ionic ceramic conductors. Solid Oxide Fuell Cells (SOFCs)"— Presentation transcript:

1 Ionic ceramic conductors. Solid Oxide Fuell Cells (SOFCs)

2 Fuel cells (FCs): electrochemical devices for the direct conversion of chemical energy in electricity by redox reactions at the electrodes. Differently from batteries, FCs are open systems wich allow continuous supply of the reactants (oxygen/air at cathode, hydrogen/hydrocarbons at anode). First application: power generation in space (Gemini & Apollo missions). Current applications (still under development): - Miniaturized power generation for portable electronic devices (notebooks, tablets, mobile phones, military applications.) - Small to average size cogeneration systems (hot water + electricity). Large scale power generation and car engines: no longer a target. Advantages: better conversion efficiency (60%; >90% in cogeneration) in comparison to combustion engines and gas turbines (25%): lower environmental impact. Steady power. Fully clean energy production using H 2 as fuel: still a dream. Drawbacks: still suffer of reliability issues and short operation time (target: 40000 h/5y). Fuel cells. Generalities Replacement of combustion engines requires hybrid electrochemical devices

3 Alkaline Polymeric electrolyte membrane Direct methanol Phosphoric acid Molten carbonate Solid oxide Fuel cells. Existing technologies Anode: negative electrode associated with fuel (H 2 ) oxidation and release of electrons into the external circuit (porous). Cathode: positive electrode associated with reduction of the oxidant (O 2 ) that gains electrons from the external circuit (porous). Electrolyte: Material that provides pure ionic conductivity and physically keep separated fuel and oxidant (dense).

4 Solid Oxide Fuel Cells (SOFCs). Principles n: number of electrons per mol of product F: Faraday constant (charge of 1 equiv. of electrons) E: cell reaction voltage (OCV: open circuit voltage) (electromotive force of the cell reaction) E = 1.23 V in standard conditions E  1.0 V using air and typical reforming gas (25% H 2 ) Nernst’s equation:  G=-nFE The basic reaction in SOFC is:  G° = -236 kJ/mol (Gibbs’ free energy) (net useful energy available) fueloxidantexhaust If hydrocarbons are used as fuel they must be converted to hydrogen by a reforming reaction. SOFCs can be directly feeded with hydrocarbons. Reforming of hydrocarbons is promoted at the anodic size of SOFCs using a suitable catalyst due to the high operation temperature. Electrode reactions Anode Cathode

5 SOFCs. Polarization phenomena  G=-nFE Equilibrium conditions. Only describes the maximum available energy/voltage (OCV) In practice, when the current flows through the circuit, there is a voltage drop due the polarization of the electrodes  :  = E OCV – E T = 0.3-0.4 V  E T = 0.6 – 0.7 V Polarization is determined by irreversibilities (losses) and kinetic limitations. Three effects: Activation polarization: kinetics of electrochemical redox reactions at the electrolyte/electrode interface; Ohmic polarization: resistance of cell components and resistance due to contacts problems;  = RI Concentration polarization: arises from limited mass transport capabilities (electrolyte). Typical operating conditions: 0.7 V, 500 mA cm -2 Power = V I = 0.35 W cm -2 Stack of 29 cells, 10x10 cm 2 :  1kW

6 SOFCs and electrolytes. Two different approaches Oxide-ion conducting electrolyte. Most research and pilot modules are focused on this approach. Proton conducting electrolyte. Lower working temperature but problems of chemical stability and durability still to be solved.

7 SOFCs. Architecture and material requirements Planar design Tubular design Requirements for SOFC materials Very high operation temperatures: 800 (today)-1000°C (1990s). Severe requirements for materials: - Chemically stable in oxidizing and reducing atmospheres; - Absence of interface reaction/diffusion (chemical compatibility); - Similar thermal expansion coefficients; - Dimensional stability in the presence of chemical gradients;  Resistance to thermal cycling and stresses

8 Anode supported cellCathode supported cell Interconnects supported cell Porous substrate (metal foam) supported cell SOFCs. Different SOFC architectures

9 SOFCs. From single cells to stacks Examples of planar SOFC stacks

10 SOFCs. Tubular SOFCs Siemens Westinghouse 100-kW SOFC–CHP power system Elements of a micro-tubular SOFC

11 SOFCs. Materials Advanced SOFC concept Functional layer: optimized microstructure for long TPB Support layer: coarse porosity and mechanical resistance Present research mainly focused on lowering the working temperature below 800°C to improve reliability, increase life time (target: 40000 h) and reduce costs. Lower temperatures determine: >Slow down of the kinetic processes; >Increase electrode polarization and polarization resistance; LSM: 1  cm 2 (1000°C)  1000  cm 2 (500°C) >Increase electrolyte resistance; >Reduction of cell voltage, Efficient low-temperature SOFCs require optimization of materials and new combinations of electrolyte and electrode materials for: Rapid ion transport (thin electrolytes, new electrolytes); Fast reactions at the electrodes (new cathode materials, optimized microstructure); Efficient electrocatalysis of oxygen reduction and fuel oxidation

12 Kinetic processes at the anode Three-phase percolating composite gas-Ni-YSZ. The hydrogen oxidation reaction occurs at the triple phase boundary (TPB) gas – Ni – YSZ and involves many elementary steps: > Hydrogen adsorption > Surface diffusion > Charge transfert > Water desorption The reaction kinetics is limited by the length of the TPB. TPB length is increased by the use of cermets. Microstructure optimization (small grains, high number of small pores leads to higher performance but increased sensitivity to carbon deposition. With pure Ni or noble metal electrodes, hydrogen oxidation only occurs at the metal/YSZ interface rather than in the whole anode volume. SOFCs. Materials

13 Kinetic processes: (1) Gas diffusion; (2) O 2 adsorption and dissociation; (3) O reduction (4) Solid-state diffusion; (5) Incorporation in the electrolyte at the interface or TPB; Electrode resistance. Determined by microstructure (tortuosity, porosity, surface area) Surface exchange velocity. Determined by electrode reaction kinetics. Oxygen diffusion coefficient SOFCs. Materials Kinetic processes at the cathode Good electron conductor Poor oxygen conductor Good electron conductor Good oxygen conductor

14 ComponentFunctionRequirementsMaterials Cathode p(O 2 ) = 0.2-1 atm Gas transport Current pick-up Long TPB Porosity Mixed conductivity Catalytic activity for oxygen surface exchange High electrocatalytic activity Sr x La 1-x MnO 3 (LSM) For T < 800°C: Sr x La 1-x Co x Fe 1-x O 3 (LSCF) Sr x La 1-x FeO 3 (LSF) also mixed with YSZ ElectrolyteOxygen ion/proton transport Electronic insulator High density (gas tightness) Pure ionic conductor Mechanical stability Oxide-ion conductors: Y x Zr 1-x O 2-  (YSZ) Gd x Ce 1-x O 2-  (GDC) La 1-x Sr x Ga 1-y Mg y O 3 (LSGM) Compatible with LSM Proton conductors: BaY x Ce 1-x O 3, BaY x Zr 1-x O 3 Anode p(O 2 ) = 10 -15 -10 -20 atmGas transport Current pick-up Electrocatalytic activity for H 2 oxidation Long TPB Porosity Electronic conductivity Redox stability Tolerance to S and C poisoning High electrocatalytic activity Ni-YSZ cermets InterconnectCurrent collector Gas distribution High electronic conductivity Resistant to oxidation/corrosion Stainless steels Fe-Cr alloys Fe-Al alloys Overeview of materials and requirements for SOFCs components SOFCs

15 SOFCs. Materials 1 mm Supporting Ni-YSZ anode with graded porosity Electrolyte-supported SOFC Examples of cathodes Thin electrolyte layer on a anode-supported cell

16 Proton conductors YSZ: Y x Zr 1-x O 2-  Good oxygen conductivity; High stability and good mechanical properties; Compatible with Ni/NiO electrodes; Reactivity with La-containing perovskites (formation of resistive La 2 Zr 2 O 7 ); GDC: Gd x Ce 1-x O 2-  Highest conductivity at low temperature; Good chemical compatibility with new cobalt-containing cathodes (La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3 ). Electronic conductivity in reducing atmosphere for T > 500°C. LSGM: La 1-x Sr x Ga 1-y Mg y O 3 Higher oxygen conductivity than YSZ Better compatibility with La-containing perovskites; Reactivity with Ni/NiO electrodes. Instability in moist H 2. Y:BaZrO 3 High bulk conductivity, resistive grain boundaries; Y:BaCeO 3 Good conductivity, thermodynamic instability in the presence of CO 2 SOFCs. Electrolytes Minimum working temperature for electrolytes (thickness: 10  m; S = 10 -2 Scm -1 ) YSZ: 700 °C; GDC (CGO) and LSGM: 550°C Y:BaZrO 3 : 400°C; Y:BaCeO 3 : 550°C 1000K700K Oxide-ion conductors

17 Use of some electrolytes with high conductivity is limited by phase transitions. The conductive phase is the high-temperature disordered modification. The high temperature phase can be stabilized by appropriate dopants but problems related to instability in reducing conditions and reactivity with electrodes remain. SOFCs. Electrolytes Pure electrolytes with order-disorder transition 1000K 625K 1670K Doped electrolytes 1000K 625K 1670K Oxide-ion conductors Ordering phase transitions

18 A B Oxygen diffusion in perovskites (LaBO 3, B=Fe, Cr, Ni, Mn ) B SOFCs. Electrolytes Saddle point configuration

19 SOFCs. Electrolytes Fluorite structure Zr O Optimal compositions: YSZ – Y x Zr 1-x O 2-  x  0.16 (8 mol.% Y 2 O 3 ) SSZ – Sc x Zr 1-x O 2-  x  0.2 (8-12 mol% Sc 2 O 3 ) (highest conductivity, low defect association energy) Monoclinic Tetragonal Cubic Y2O3Y2O3 Zr 3 Y 4 O 12 Y x Zr 1-x O 2-  Oxide-ion conductors

20 SOFCs. Electrolytes Oxide-ion conductors Real vs. simulated lattice Oxygen column occupancy Grain boundary oxygen vacancy segregation in YSZ Column intensity ratio Calculated gb potential barrier: 0.5-1.2 V

21 SOFCs. Electrolytes Oxide-ion conductors Grain boundary oxygen vacancy segregation in YSZ EELS analysis Small angle tilt boundary Column intensity

22 SOFCs. Electrolytes Y x Ce 1-x O 2-δ Conductivity (S cm -1 ) Y 2 O 3 mol. % M x Zr 1-x O 2-δ Conductivity (S cm -1 ) M 2 O 3 mol. % Ca x Ce 1-x O 2-δ Conductivity (S cm -1 ) x10 2 CaO mol. % Oxide-ion conductors

23 Interaction between dopant ions and charge compensating defects with cluster formation is determined by coulombic attraction. The biding energy is strongly modified by lattice relaxation and lattice polarization. For binary oxides with fluorite structure: Divalent dopant Electrical conductivity Z: numero di cariche; e: carica dell’elettrone;  : mobilità c: concentrazione For a single charge carrier type: CaseActivation energy, E a Free vacancies HmHm  H m +  H A2 /2  H m +  H A1  H m : enthalpy of migration  H A : binding energy In doped ceria:  H m :  0.6 eV  H 2 : 0.4-0.6 eV  H 1 :  0.25 eV Influence of defect associates on E a of conductivity of fluorite oxides Dilute range (x <0.08): Defect associations takes place al lower T. E a is constant (2+ dopants) or decreases (3+ dopants) Concentrated range (x > 0.08): Defect association even at high T. E a increases with x Trivalent dopant Prevails at high T and low dopant conc.  SOFCs. Electrolytes Oxide-ion conductors

24 SOFCs. Electrolytes Ceria-based electrolytes (Gd x Ce 1-x O 2-δ, Gd x Ce 1-x O 2-δ x  0.1). Best electrolytes at 500-600°C Electronic conductivity at low p(O 2 ) (< 10 -15 atm at 700°C) Extrinsic vacancies Intrinsic vacancies 600°C 700°C Oxide-ion conductors

25 Proton conductors Formation of protonic defects SOFCs. Electrolytes S: effective acceptor concentration = = water solubility limit Normalized hydration isobars Proton conductors

26 SOFCs. Electrolytes Mobility of protonic defects Two-step transport process: (1) Rotational diffusion of the proton (2) Transfer of the proton to an neighbouring oxide ion by transient formation of an hydrogen bond Transient state Proton mobility strongly sensitive to: O-O distance; B-O bond; Crystallographic distortions; Acceptor dopant Migration activation hentalpies: 0.4 – 0.6 eV Proton conductors

27 SOFCs. Electrolytes Proton conductors Effect of grain boundaries on ionic conductivity BaZr 0.8 Y 0.2 O 3-δ (BZY) 350°C 450°C 550°C wet 5%H 2 Comparison of ceramics and epitaxial thin films wet 5%H 2 Bulk conductivities of best oxide- ion and proton conductors

28 MgO substrate. Film orientation: (100) Al 2 O 3 substrate. Film orientation: (111) SOFCs. Electrolytes Proton conductors Effect of grain boundaries on ionic conductivity Epitaxial polycrystalline BZY thin films on different substrates

29


Download ppt "Ionic ceramic conductors. Solid Oxide Fuell Cells (SOFCs)"

Similar presentations


Ads by Google