2 Fuel cells. Generalities 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 H2 as fuel: still a dream.Drawbacks: still suffer of reliability issues and short operation time (target: h/5y).Replacement of combustion engines requires hybrid electrochemical devices
3 Fuel cells. Existing technologies AlkalinePolymeric electrolyte membraneDirect methanolPhosphoric acidMolten carbonateSolid oxideAnode: negative electrode associated with fuel (H2) oxidation and release of electrons into the external circuit (porous).Cathode: positive electrode associated with reduction of the oxidant (O2) 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 The basic reaction in SOFC is:G° = -236 kJ/mol (Gibbs’ free energy)(net useful energy available)fueloxidantexhaustn: number of electrons per mol of productF: 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 conditionsE 1.0 V using air and typical reforming gas (25% H2)Nernst’s equation: G=-nFEIf 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 : = EOCV – ET = V ET = 0.6 – 0.7 VPolarization 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; = RIConcentration polarization: arises from limited mass transport capabilities (electrolyte).Typical operating conditions:0.7 V, 500 mA cm-2Power = V I = 0.35 W cm-2Stack of 29 cells, 10x10 cm2: 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 designPlanar designRequirements for SOFC materialsVery 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
9 SOFCs. From single cells to stacks Examples of planar SOFC stacks
10 SOFCs. Tubular SOFCs Elements of a micro-tubular SOFC Siemens Westinghouse100-kW SOFC–CHP power systemElements of a micro-tubular SOFC
11 SOFCs. MaterialsPresent research mainly focused on lowering the working temperature below 800°C to improve reliability, increase life time (target: h) and reduce costs.Lower temperatures determine:>Slow down of the kinetic processes;>Increase electrode polarization and polarization resistance;LSM: 1 cm2 (1000°C) 1000 cm2 (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 oxidationAdvanced SOFC conceptFunctional layer: optimized microstructure for long TPBSupport layer: coarse porosity and mechanical resistance
12 SOFCs. Materials 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 desorptionThe 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.
13 SOFCs. Materials Kinetic processes: Kinetic processes at the cathode Good electron conductorPoor oxygen conductorGood oxygen conductorKinetic processes:(1) Gas diffusion;(2) O2 adsorption and dissociation;(3) O reduction(4) Solid-state diffusion;(5) Incorporation in the electrolyte at the interface or TPB;Oxygen diffusioncoefficientElectrode resistance. Determined by microstructure (tortuosity, porosity, surface area)Surface exchange velocity. Determined by electrode reaction kinetics.
14 SOFCs Overeview of materials and requirements for SOFCs components FunctionRequirementsMaterialsCathodep(O2) =0.2-1 atmGas transportCurrent pick-upLong TPBPorosityMixed conductivityCatalytic activity for oxygen surface exchangeHigh electrocatalytic activitySrxLa1-xMnO3 (LSM)For T < 800°C:SrxLa1-xCoxFe1-xO3 (LSCF)SrxLa1-xFeO3 (LSF)also mixed with YSZElectrolyteOxygen ion/proton transportElectronic insulatorHigh density (gas tightness)Pure ionic conductorMechanical stabilityOxide-ion conductors:YxZr1-xO2- (YSZ)GdxCe1-xO2- (GDC)La1-xSrxGa1-yMgyO3 (LSGM)Compatible with LSMProton conductors:BaYxCe1-xO3, BaYxZr1-xO3AnodeatmElectrocatalytic activity for H2 oxidationLong TPBElectronic conductivityRedox stabilityTolerance to S and C poisoningNi-YSZ cermetsInterconnectCurrent collectorGas distributionHigh electronic conductivityResistant to oxidation/corrosionStainless steelsFe-Cr alloysFe-Al alloys
15 SOFCs. Materials Examples of cathodes Thin electrolyte layer on a anode-supported cell1 mmSupporting Ni-YSZ anode with graded porosityElectrolyte-supported SOFCExamples of cathodes
16 YSZ: 700 °C; GDC (CGO) and LSGM: 550°C SOFCs. ElectrolytesMinimum working temperature for electrolytes (thickness: 10 m; S = 10-2 Scm-1)YSZ: 700 °C; GDC (CGO) and LSGM: 550°CY:BaZrO3: 400°C; Y:BaCeO3: 550°C1000K700KOxide-ion conductorsYSZ: YxZr1-xO2-Good oxygen conductivity;High stability and good mechanical properties;Compatible with Ni/NiO electrodes;Reactivity with La-containing perovskites (formation of resistive La2Zr2O7);GDC: GdxCe1-xO2-Highest conductivity at low temperature;Good chemical compatibility with new cobalt-containing cathodes (La0.6Sr0.4Co0.2Fe0.8O3).Electronic conductivity in reducing atmosphere for T > 500°C.LSGM: La1-xSrxGa1-yMgyO3Higher oxygen conductivity than YSZBetter compatibility with La-containing perovskites;Reactivity with Ni/NiO electrodes. Instability in moist H2.Y:BaZrO3High bulk conductivity, resistive grain boundaries;Y:BaCeO3Good conductivity, thermodynamic instability in the presence of CO2Proton conductors
17 Ordering phase transitions SOFCs. ElectrolytesOxide-ion conductorsOrdering phase transitionsUse 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.Pure electrolytes with order-disorder transition1000K625K1670KDoped electrolytes1000K625K1670K
19 Optimal compositions: YSZ – YxZr1-xO2- x 0.16 (8 mol.% Y2O3) SOFCs. ElectrolytesMonoclinicTetragonalCubicY2O3Zr3Y4O12YxZr1-xO2-Oxide-ion conductorsFluorite structureZrOOptimal compositions:YSZ – YxZr1-xO2- x 0.16 (8 mol.% Y2O3)SSZ – ScxZr1-xO2- x 0.2 (8-12 mol% Sc2O3)(highest conductivity, low defect association energy)
20 SOFCs. ElectrolytesOxide-ion conductorsGrain boundary oxygen vacancy segregation in YSZReal vs. simulated latticeOxygen column occupancyCalculatedgb potentialbarrier: VColumn intensity ratio
21 Small angle tilt boundary SOFCs. ElectrolytesOxide-ion conductorsGrain boundary oxygen vacancy segregation in YSZEELS analysisSmall angle tilt boundaryColumn intensity
22 SOFCs. Electrolytes CaxCe1-xO2-δ Oxide-ion conductors MxZr1-xO2-δ Conductivity (S cm-1) x102CaO mol. %Oxide-ion conductorsMxZr1-xO2-δConductivity (S cm-1)M2O3 mol. %YxCe1-xO2-δConductivity (S cm-1)Y2O3 mol. %
23 SOFCs. Electrolytes Oxide-ion conductors 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 dopantTrivalent dopantPrevails at high T and low dopant conc.Z: numero di cariche; e: carica dell’elettrone; : mobilitàc: concentrazioneElectrical conductivityFor a single charge carrier type:Influence of defect associates on Ea of conductivity of fluorite oxidesDilute range (x <0.08):Defect associations takes place al lower T.Ea is constant (2+ dopants) or decreases (3+ dopants)Concentrated range (x > 0.08):Defect association even at high T.Ea increases with xCaseActivation energy, EaFree vacanciesHmHm + HA2/2Hm + HA1In doped ceria:Hm : 0.6 eVH2 : eVH1 : 0.25 eVHm : enthalpy of migrationHA : binding energy
24 Electronic conductivity at low p(O2) (< 10-15 atm at 700°C) SOFCs. ElectrolytesOxide-ion conductorsCeria-based electrolytes (GdxCe1-xO2-δ, GdxCe1-xO2-δ x 0.1). Best electrolytes at °CElectronic conductivity at low p(O2) (< atm at 700°C)Extrinsic vacanciesIntrinsic vacancies600°C700°C
25 Formation of protonic defects Proton conductors SOFCs. ElectrolytesFormation of protonic defectsProton conductorsProton conductorsS: effective acceptor concentration = = water solubility limitNormalized hydration isobars25
26 Mobility of protonic defects SOFCs. ElectrolytesProton conductorsMobility of protonic defectsTwo-step transport process:(1) Rotational diffusion of the proton(2) Transfer of the proton to an neighbouring oxide ionby transient formation of an hydrogen bondTransient stateProton mobility strongly sensitive to:O-O distance;B-O bond;Crystallographic distortions;Acceptor dopantMigration activation hentalpies: 0.4 – 0.6 eV26
27 Effect of grain boundaries on ionic conductivity SOFCs. ElectrolytesProton conductorsEffect of grain boundaries on ionic conductivityComparison of ceramics and epitaxial thin filmswet 5%H2Bulk conductivities of best oxide-ion and proton conductorsBaZr0.8Y0.2O3-δ (BZY)350°C450°C550°Cwet 5%H2
28 SOFCs. ElectrolytesProton conductorsEffect of grain boundaries on ionic conductivityEpitaxial polycrystalline BZY thin films on different substratesMgO substrate. Film orientation: (100)Al2O3 substrate. Film orientation: (111)