357 lines
13 KiB
Text
357 lines
13 KiB
Text
#########################################################################
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#
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# This is a an example input file that defines models for phases and
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# interfaces that could be used, for example, to simulate a solid
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# oxide fuel cell. Note, however, that reaction rate coefficients and
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# species thermochemistry ARE NOT REAL VALUES - they are chosen only
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# for the purposes of this example.
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#
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#########################################################################
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# since Cantera input files are actually executable Python scripts,
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# we can put any valid Python statements in the input file. Here we
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# import the value of R from Cantera.
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from Cantera import GasConstant
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# These units will be used by default for any quantities entered
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# without units. Quantities with compound units (e.g. concentration)
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# will be constructed from these - the units of concentration will be
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# mol/cm^3, etc.
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units(length = "cm", time = "s", quantity = "mol", act_energy = "kJ/mol")
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# Turn on mechanism validation to detect unbalanced reactions, if any
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validate()
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#------------------------------------------------------------------
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#
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# parameters
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#
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#------------------------------------------------------------------
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# a few numeric parameters are collected here to allow easy modification.
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# this temperature is used to initialize objects. But since
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# scripts/programs usually set the temperature, it is not really
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# necessary.
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tc = 800.0 # temperature in C
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tt = tc + 273.15 # temperature in K
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# these values are defined here only so they may be easily changed to
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# assess the effects of the oxide thermochemistry. For work at a
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# single temperature, all that we really need is g = h -
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# Ts. Therefore, it is somewhat arbitrary to assign separately
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# enthalpies and entropies (but this is what the input format
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# requires).
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hox = (-170.0, 'kJ/mol') # enthalpy of an oxygen ion
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sox = (50.0, 'J/K/mol') # entropy of an oxygen ion
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hhydrox = (-220.0, 'kJ/mol') # enthalpy of a surface hydroxyl group
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shydrox = (87.0, 'J/mol/K') # entropy of a surface hydroxyl group
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####################### BULK PHASES ####################################
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# First we'll define the bulk (i.e. 3D) phases - a gas, a metal, and
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# an oxide.
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#------------------------------------------------------------------
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#
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# Gas phase.
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#
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#------------------------------------------------------------------
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# The gas contains only the minimum number of species needed to model
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# operation on hydrogen. The species definitions are imported from
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# gri30.cti. The initial composition is set to hydrogen + 5% water, but
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# usually this is reset in the program importing this definition.
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#
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ideal_gas(name = "gas",
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elements = " H O N",
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species = "gri30: H2 H2O N2 O2",
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transport = "Mix",
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initial_state = state( temperature = tt,
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pressure = OneAtm,
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mole_fractions = 'H2:0.95, H2O:0.05'))
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#------------------------------------------------------------------
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#
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# Bulk solid metal phase.
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#
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#------------------------------------------------------------------
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#
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# This phase will be used for the electrodes. All we need is
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# a source/sink for electrons, so we define this phase as only
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# containing electrons. Note that the 'metal' entry type requires
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# specifying a density, but it is not used in this simulation and
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# therefore is arbitrary.
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#
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metal(name = "metal",
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elements = "E",
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species = "electron",
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density = (9.0, 'kg/m3'),
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initial_state = state( temperature =tt,
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mole_fractions = 'electron:1.0'))
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# The electron is set to have zero enthalpy and entropy. Therefore,
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# the chemical potential of the electron is zero, and the
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# electrochemical potential is simply -F * phi, where phi is the
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# electric potential of the metal. Note that this simple model is
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# adequate only because all we require is a reservoir for electrons;
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# if we wanted to do anything more complex, like carry out energy or
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# charge balances on the metal, then we would require a more complex
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# model. Note that there is no work function for this metal.
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species( name = "electron", atoms = "E:1",
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thermo = const_cp(h0 = (0.0, 'kcal/mol')))
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# Note: the "const_cp" species thermo model is used throughout this
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# file (with the exception of the gaseous species, which use NASA
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# polynomials imported from gri30.cti). The const_cp model assumes a
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# constant specific heat, which by default is zero. Parameters that
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# can be specified are cp0, t0, h0, and s0. If omitted, t0 = 300 K, h0
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# = 0, and s0 = 0. The thermo properties are computed as follows: h =
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# h0 + cp0*(t - t0), s = s0 + cp0*ln(t/t0). For work at a single
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# temperature, it is sufficient to specify only h0.
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#-------------------------------------------------------------------
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#
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# Bulk solid oxide electrolyte
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#
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#--------------------------------------------------------------------
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# Here too, we create a very simple model for the bulk phase. We only
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# consider the oxygen sublattice. The only species we define are a
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# lattice oxygen, and an oxygen vacancy. Again, the density is a
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# required input, but is not used here, so may be set arbitrarily.
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incompressible_solid(name = "oxide_bulk",
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elements = "O E",
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species = "Ox VO**",
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density = (0.7, 'g/cm3'),
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initial_state = state( temperature = tt,
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pressure = OneAtm,
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mole_fractions = "Ox:0.95 VO**:0.05")
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)
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# The vacancy will be modeled as truly vacant - it contains no atoms,
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# has no charge, and has zero enthalpy and entropy. This is different
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# from the usual convention in which the vacancy properties are are
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# expressed relative to the perfect crystal lattice. For example, in
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# the usual convention, an oxygen vacancy has charge +2. But the
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# convention we will use is that an oxygen ion has charge -2, and a
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# vacancy has charge 0. It all works out the same, as long as we are
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# consistent.
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# A bulk lattice vacancy
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species( name = "VO**", atoms = "",
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thermo = const_cp(h0 = (0.0, 'kJ/mol')))
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# A bulk lattice oxygen
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species( name = "Ox", atoms = "O:1 E:2",
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thermo = const_cp(h0 = hox, s0 = sox))
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####################### SURFACE PHASES ####################################
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#--------------------------------------------------
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#
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# Metal surface
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#
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#--------------------------------------------------
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# The surface of a bulk phase must be treated like a separate phase, with its
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# own set of species. Here we define the model for the metal surface.
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# We allow the following species:
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# (m) - an empty metal site
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# H(m) - a chemisorbed H atom
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# O(m) - a chemisorbed O atom
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# OH(m) - a chemisorbed hydroxl
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# H2O(m) - a physisorbed water molecule
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# Notes:
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# 1. The site density is in mol/cm2, since no units are specified and
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# 'mol' and 'cm' were specified in the units directive above as the
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# units for quantity and length, respectively.
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# 2. The 'reactions' field specifies that all reaction entries in this file
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# that have ID strings beginning with "metal-" are reactions belonging
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# to this surface mechanism.
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ideal_interface(name = "metal_surface",
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elements = "H O",
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species = " (m) H(m) O(m) OH(m) H2O(m) ",
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site_density = 2.60e-9,
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phases = 'gas',
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reactions = ["metal-*"],
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initial_state = state( temperature = 973.0,
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coverages = '(m):0.5 H(m):0.5') )
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species( name = "(m)", atoms = "",
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thermo = const_cp(h0 = (0.0, 'kJ/mol'),
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s0 = (0.0, 'J/mol/K')))
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species( name = "H(m)", atoms = "H:1",
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thermo = const_cp(h0 = (-35.0, 'kJ/mol'),
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s0 = (37.0, 'J/mol/K')))
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species( name = "O(m)", atoms = "O:1",
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thermo = const_cp(h0 = (-220.0, 'kJ/mol'),
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s0 = (37.0, 'J/mol/K')))
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species( name = "OH(m)", atoms = "O:1, H:1",
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thermo = const_cp(h0 = (-198.0, 'kJ/mol'),
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s0 = (102.0, 'J/mol/K')))
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species( name = "H2O(m)", atoms = "H:2, O:1",
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thermo = const_cp(h0 = (-281.0, 'kJ/mol'),
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s0 = (123.0, 'J/mol/K')))
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# Surface reactions on the metal. We assume three dissociative
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# adsorption reactions, and three reactions on the surface
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# among adsorbates. All reactions are treated as reversible.
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surface_reaction( "H2 + (m) + (m) <=> H(m) + H(m)",
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stick(0.1, 0, 0), id = 'metal-rxn1')
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surface_reaction( "O2 + (m) + (m) <=> O(m) + O(m)",
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stick(0.1, 0, 0), id = 'metal-rxn2')
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surface_reaction( "H2O + (m) <=> H2O(m)",
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stick(1.0, 0, 0), id = 'metal-rxn3')
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surface_reaction( "H(m) + O(m) <=> OH(m) + (m)",
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[5.00000E+22, 0, 100.0], id = 'metal-rxn4')
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surface_reaction( "H(m) + OH(m) <=> H2O(m) + (m)",
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[5.00000E+20, 0, 40.0], id = 'metal-rxn5')
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surface_reaction( "OH(m) + OH(m) <=> H2O(m) + O(m)",
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[5.00000E+21, 0, 100.0], id = 'metal-rxn6')
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#--------------------------------------------------------
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#
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# Oxide surface.
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#
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#--------------------------------------------------------
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#H
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# On the oxide surface, we consider four species:
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# 1. (ox) - a surface vacancy
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# 2. O''(ox) - a surface oxygen with charge -2
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# 3. OH'(ox) - a surface hydroxyl with charge -1
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# 4. H2O(ox) - physisorbed neutral water
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ideal_interface(name = "oxide_surface",
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elements = "O H E",
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species = "(ox) O''(ox) OH'(ox) H2O(ox)",
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site_density = 2.0e-9,
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phases = 'gas oxide_bulk',
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reactions = 'oxide-*',
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initial_state = state( temperature = tt,
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coverages = "O''(ox):2.0, (ox):0.0") )
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# Note: hox, sox, hhydrox, and shydrox are defined near the top of
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# this file.
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# An oxygen ion at the surface, with charge = -2
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species( name = "O''(ox)", atoms = "O:1 E:2",
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thermo = const_cp(h0 = hox,
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s0 = sox))
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# An OH at the surface, with charge = -1
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species( name = "OH'(ox)", atoms = "O:1 H:1 E:1",
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thermo = const_cp(h0 = hhydrox,
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s0 = shydrox))
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# A surface vacancy in the oxygen sublattice
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species( name = "(ox)", atoms = "",
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thermo = const_cp(h0 = (0.0, 'kJ/mol'),
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s0 = (0.0,'J/mol/K')))
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species( name = "H2O(ox)", atoms = "H:2, O:1",
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thermo = const_cp(h0 = (-265.0, 'kJ/mol'),
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s0 = (98.0,'J/mol/K')))
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# This reaction represents the exchange of a surface oxygen vacancy and
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# a subsurface vacancy. The concentration of subsurface vacancies is
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# fixed by the doping level. If this reaction is given a large rate,
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# then the surface vacancies will stay in equilibrium with the bulk
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# vacancies.
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surface_reaction("(ox) + Ox <=> VO** + O''(ox)",
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[5.0e8, 0.0, 0.0], id = "oxide-vac")
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# Desorption of physisorbed water. This is made fast.
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surface_reaction("H2O(ox) <=> H2O + (ox)",
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[1.0e14, 0.0, (0.0, 'kJ/mol')], id = "oxide-water")
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# chemisorption of water as surface hydroxyls. In reality, this
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# reaction would surely be activated and have a lower pre-exponential
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surface_reaction("H2O(ox) + O''(ox) <=> OH'(ox) + OH'(ox)",
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[1.0e14, 0.0, (0.0, 'kJ/mol')], id = "oxide-oh")
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####################### TRIPLE PHASE BOUNDARY #########################
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# The triple phase boundary between the metal, oxide, and gas. A
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# single species is specified, but it is not used, since all reactions
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# only involve species on either side of the tpb. Note that the site
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# density is in mol/cm. But since no reactions involve TPB species,
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# this parameter is unused.
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edge(name = "tpb",
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elements = "H O",
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species = "(tpb)",
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site_density = 5.0e-17,
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reactions = "edge-*",
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phases = 'metal metal_surface oxide_surface',
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initial_state = state( temperature = tt,
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coverages = '(tpb):1.0 ') )
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# dummy species
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species( name = "(tpb)", atoms = "")
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# Here we define two charge transfer reactions. Both reactions are
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# reversible, and can be used to model either anodes or cathodes
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# (although real anodes and cathodes would usually have different
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# reaction mechanisms, except in a symmetric cell).
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# in this reaction, a proton from the metal crosses the TPB to the
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# oxide surface to make a hydroxyl and deliver an electron to the
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# metal.
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edge_reaction("H(m) + O''(ox) <=> (m) + electron + OH'(ox)",
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[5.0e13, 0.0, 120.0], beta = 0.5, id="edge-f2")
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# in this reaction, an oxygen on the metal surface plus 2 electrons
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# from the metal bulk fill a surface vacancy in the oxide lattice.
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edge_reaction("O(m) + (ox) + 2 electron <=> (m) + O''(ox)",
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[5.0e13, 0.0, 120.0], beta = 0.5, id="edge-f3")
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# this reaction is commented out, but you can explore its effects by
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# uncommenting it. Be careful, if you are not solving for the OH'
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# concentration that the system does not become overdetermined
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# (i.e. impossible for all reactions to be simultaneously in
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# equilibrium). If this happens, the wrong OCVs will result.
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#edge_reaction("H(m) + OH'(ox) <=> H2O(ox) + (m) + electron",
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# [5.0e13, 0.0, 120.0], beta = 0.5, id="edge-f")
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