Advanced Materials for Energy Conversion Solid Cells Electrolyte PDF

ADVANCED MATERIALS FOR ENERGY CONVERSION SOLID CELLS ELECTROLYTE ELECTROLYTE An electrolyte is the heart of a SOFC unit, which conducts oxide ions from the cathode to the anode where it reacts with hydrogen ions to produce H 2O or with hydrocarbons to form H2O and CO2, and thus completes...

ADVANCED MATERIALS FOR ENERGY CONVERSION SOLID CELLS ELECTROLYTE ELECTROLYTE An electrolyte is the heart of a SOFC unit, which conducts oxide ions from the cathode to the anode where it reacts with hydrogen ions to produce H 2O or with hydrocarbons to form H2O and CO2, and thus completes the overall electrochemical reaction. The oxide ion conduction occurs via the oxygen vacancy hopping mechanism, which is a thermally activated process. Their crystal structure must possess large interionic open space that allows high level of point defect disorder, and low migration enthalpy.  SOLID ELECTROLYTES The main requirements for an electrolyte to work efficiently: (i) Sufficiently high oxide ion conductivity (0.1 S/cm [ siemens/cm] at operating temperature. (ii) Low electronic transference number (400 MPa). The electrolytes are generally oxygen ion conductors, in which current flow occurs by the movement of oxygen ions through the crystal lattice. This movement is a result of thermally activated hopping of the oxygen ion, moving from one crystal lattice site to its neighbor site. To achieve the movement, the crystal must contain unoccupied sites equivalent to those occupied by the lattice oxygen ions.  SOLID ELECTROLYTES Examples of candidate crystal structures: ZrO2- and CeO2-based oxides with fluorite structure LaGaO3-based perovskite OXIDES Simple oxides (AO2) ZrO2 CeO2 Mixed oxides (ABO3) LaGaO3  ZIRCONIA (ZRO2) Pure zirconia (ZrO2) is poor ionic conductor in nature. At room temperature, it is present in monoclinic phase. When its temperature is raised to about 1100 ◦C, it undergoes transformation to reversible martensitic phase to tetragonal structure. It persists the tetragonal phase between temperature 1180 °C to 2370 °C On further elevation of temperature it gets converted to cubic fluorite phase Cubic tetragonal monoclinic  DOPING Divalent or trivalent Cations (Y3+, Ca2+, Mg2+ and Sc3+ ), on introducing in the ZrO2 crystal lattice, produce significant concentrations of charge carriers, Oxygen vacancies, which are the mobile species, are formed to maintain the charge neutrality.  OXYGEN ION CONDUCTIVITY The ionic conductivity, σ, can be calculated by: e: charge μ: mobility of oxygen vacancy n: number of mobile oxygen ion vacancies The activation energy for conduction determines how differently the doped fluorite oxides conduct, and the pair binding energy contributes significantly to this activation energy. The dopant size affects the pair binding energy, which reaches a minimum when Rdopant = Rhost.  ZIRCONIA ELECTROLYTES  The zirconia-based oxides are the most widely used electrolytes for SOFCs operated at high (800–1000°C) and intermediate (600–800°C) temperatures.  The zirconia is abundant and relatively low in cost.  Most of the SOFC systems currently are being developed employ YSZ or scandia-stabilized zirconia (ScSZ) electrolytes. Kröger–Vink notation 8.0 mol% Y2O3 (8YSZ) The higher conductivity of ScSZ is attributed to the smaller mismatch in size between Zr4+ and Sc3+ ions.  KRÖGER–VINK NOTATION M corresponds to the species. atoms – e.g., Zr, Y, O, Sc, vacancies – V or v (since V is also the symbol for vanadium) interstitials – i electrons – e electron holes – h S indicates the lattice site that the species occupies. C corresponds to the electronic charge of the species relative to the site that it occupies. ×: null charge : single positive charge ′: single negative charge  ZIRCONIA ELECTROLYTES Depending on the concentration, the structure is either fully or partially stabilized resulting in cubic and tetragonal structure respectively The phase transformation occurring from metastable tetragonal phase to monoclinic, induces stress resulting into ~4% volume expansion. Compatibility of Cathode materials with YSZ LSCF (cathode material) creates a secondary phase like SrZrO3 or La2Zr2O7 via inter-diffusion when used directly with a zirconia-based electrolyte.  CERIA ELECTROLYTES  Pure stoichiometric ceria is not a good oxygen ion conductor. The oxygen ion conductivity can be introduced dramatically by low valance doping.  The conductivities of ceria with trivalent dopants are much higher than those with bivalent dopants  Gadolinia- and samaria-doped ceria (GDC & SDC) are widely considered to be the electrolytes for low SOFCs.  CERIA ELECTROLYTES  Unlike zirconia, pure stoichiometric ceria forms the fluorite structure over the whole temperature range from room temperature to the melting point, 2,400°C.  This fluorite structure is built on the basis of a Ce4+ cation face centered cubic (FCC) packing with oxygen ions located in the tetrahedral sites of the structure.  To obtain high oxygen ion conduction properties part of the Ce4+ must be substituted by another cation with a lower valence state, such as Gd3+ or Sm3+.  The concentration of the vacancies is given by the electrical neutrality condition.  GDC & SDC  Since the radius of Gd3+ is the closest to rc, GDC solid solutions have been recognized to be leading electrolytes for use in low-temperature SOFCs.  The controversy in the peak conductivity might be due to the microstructure difference caused by the different fabrication processes as well as to an amorphous glassy phase in the grain boundaries caused by impurities.  GDC & SDC  Doped ceria electrolytes easily develop electronic conduction at high temperatures and low oxygen partial pressures, which is a constraint for their use as an electrolyte material for SOFCs.  The electronic conduction is due to the reduction of Ce4+ to Ce3+ under low- oxygen pressure conditions.  reducing the operating temperature can suppress the electronic conduction. This might be the most important reason that doped ceria–based SOFCs are usually considered to operate at temperatures below 600°C.  ELECTROLYTES Area Specific Resistance (ASR) The three dominant polarisations are ohmic polarisation (ohmic loss), concentration polarisation and activation polarisation. RS: ohmic resistance RP: electrode polarisation resistance (both concentration and activation) ρ: resistivity l: thickness R: any contact resistances SOFC CELL DESIGNS  LaGaO3-BASED ELECTROLYTES  Solid solutions based on the perovskite-type oxide LaGaO3—in particular, the oxide doped with Sr and Mg (LSGM)—exhibit a superior ionic conductivity higher than the conventional stabilized zirconia electrolytes  The solid solubility of Sr into La sites is poor and secondary phases of SrGaO3 or La4SrO7 are formed when Sr content becomes higher than 10 mol%.  The conductivity is further increased by increasing the amount of doped Mg, which can attain a maximum of 20 mol% Mg doped on Ga sites.  MIXED OXIDES Oxide groups consisting of two or more different cations are called complex or mixed oxides, and many types of crystal structures are known. The most typical structure of a mixed oxide consists simply of two or more different cations with different oxidation states, ionic radii, and coordination numbers. One of the most well known and important complex oxide structures is perovskite structure. Perovskite (ABO3) Cations: An+ and Bm+ Anion: O2- Coordination Number (CN): A-site: 12 & B-site: 6 Oxidation states: n+m: 1+5, 2+4 & 3+3 Na1+Nb5+(O2-)3 Ba2+Zr4+(O2-)3 La3+Al3+(O2-)3 NaNbO3 BaZrO3 LaAlO3 PEROVSKITE STRUCTURE where A is either an (alkaline) earth or a rare earth metal and B is a transition metal with partly filled d-orbitals form the perovskite crystal family. IDEAL PEROVSKITE STRUCTURE K1+Ta5+(O2-)3 Na1+Ta5+(O2-)3 Na1+Nb5+(O2-)3 Ba2+Mn4+(O2-)3 Ba2+Zr4+(O2-)3 Sr2+Ti4+(O2-)3 IDEAL PEROVSKITE STRUCTURE Ideal, cubic ABO3 perovskite structure Goldschmidt Tolerance factor (t): Perovskite cubic structure (spac𝑒 𝑔𝑟𝑜𝑢𝑝: 𝑃𝑚3ത 𝑚, No. : 221) symmetry) is usually observed for materials, for which the tolerance factor is close to 1 (0.9≤t≤1.05).  TOLERANCE FACTOR t In perovskite-type compounds, the value of t lies between approximately 0.80 and 1.10.  ELECTROLYTE EFFICIENCY  Electrolyte efficiency was given by a function of fuel utilization and internal resistance When the thickness of the electrolyte is too low, chemical leakage of oxygen due to electronic conduction became significant, and the extra fuel consumption resulted in decreased electrolyte efficiency. On the other hand, with increasing electrolyte thickness, the internal resistance of the cell increased, also resulting in decreased electrolyte efficiency. The benefit of LSGM is that its highest efficiency with a thickness near 5 μm is achieved around 450°C. Thus LSGM appears to be the best electrolyte discovered so far for low-temperature operation. SOFC ELECTROLYTES FACTORS INFLUENCING LIFETIME Chemical stability of the electrolyte itself leading to a gradual decrease in its conductivity and hence increase in its contribution to the cell ASR. Chemical interactions between the electrolyte and the materials it is in contact with leading to reaction zones at interfaces that have low ionic conductivity and increase the cell ASR. Mechanical instability of the electrolyte arising from thermal gradients, thermal cycles, or redox cycles cause fracture of the electrolyte leading to direct combustion of fuel or even catastrophic failure.