ABSTRACT
This paper reports the model development for a dual-channel protonic-ceramic fuel cell (PCFC) operating on ammonia fuel. The model considers the coupled interactions of several physical and chemical processes, including three-dimensional heat conduction within the bipolar plates and the membrane-electrode assembly (MEA), one-dimensional flow within the fuel and air channels, detailed heterogeneous catalytic reactions within the porous composite anode structure, Butler–Volmer representation of the charge-transfer chemistry, and Nernst–Planck transport of three charged defects (protons, oxygen vacancies, and small polarons) within the dense electrolyte membrane. The membrane-electrode assembly is composed of a Ni-BCZYYb () anode, a BCZYYb electrolyte membrane, and a BCFZY (
) cathode. Chemical and physical parameters for the MEA model are established using previously published button-cell data. One aspect of the study is to investigate the partial ammonia decomposition upstream of the fuel cell. Such fuel cracking increases the H
content of the fuel entering the PCFC, which may have benefits. However, endothermic ammonia pyrolysis within the composite anode structure assists with thermal control of the cell. The dual-channel model can be considered as the unit cell of a full fuel-cell stack.
Acknowledgment(s)
This work was supported by the Advanced Research Projects Agency-Energy (ARPA-E) through the REFUEL program (Award No. DE-AR0000808). We gratefully acknowledge insightful discussions on protonic ceramics and ammonia chemistry with our colleagues Profs. Sandrine Ricote, Ryan O’Hayre, Neal Sullivan, Robert Braun and Colin Wolden.
Disclosure statement
No potential conflict of interest was reported by the author(s).
Nomenclature
Channel cross-section flow ara
Permeability
Pre-factor of the Arrhenius form of the
reaction
Solid-phase specific heat capacity
Channel hydraulic diameter
Diffusion coefficients of defects
Pre-factors for defect diffusion coefficients
Effective Knudsen diffusion coefficients
Effective binary diffusion coefficients
Total internal energy of gas mixture
Specific internal energy of gas mixture
Cell potential
Activation energy of defect diffusion coefficients
Standard electric-potential difference
Equilibrium electric-potential difference
Equilibrium electric-potential difference in the anode
Equilibrium electric-potential difference in the cathode
Faraday constant
Channel flow friction factor
Graetz number
Heat enthalpy of the gas-phase mixtures
Convective heat-transfer coefficient
Species heat enthalpy
Charge-transfer rate in Butler-Volmer equation
Charge-flux due to defect concentration gradients
Current density
Current density in the electronic-conducting phase
Current density in the ionic-conducting phase
Exchange current density
Prefactor for exchange current density
Exchange current-density factor
Gas-phase species mass flux
Species mole flux
Gas-phase species mole flux from MEA
Backward rate constant of
-th reaction
Backward rate constant of
incorporation
Backward rate constant of
incorporation
Backward rate constant of
incorporation
Backward rate constant of trap reaction
Concentration-based Equilibrium constant
Forward rate constant of
-th reaction
Forward rate constant
Forward rate constant of
incorporation
Forward rate constant of
incorporation
Forward rate constant of
incorporation
Forward rate constant of trap reaction
Number of gas-phase species
Equilibrium constant of
adsorption
Equilibrium constant of
adsorption
Equilibrium constant of
adsorption
Pressure-based Equilibrium constant
Number of surface-adsorbed species
Anode thickness
Cathode thickness
Electrolyte thickness
Nusselt number
Pressure
Channel hydrodynamic perimeter
partial pressure
reference pressure
partial pressure
reference pressure
Channel interface perimeter with MEA
partial pressure
reference pressure
Prandtl number
Convective heat flux of channel flow
Heat fluxes of MEA-channel species transport
Heat fluxes of MEA-fuel channel species transport
Heat fluxes of MEA-air channel species transport
Volumetric heat generation rate from MEA
Convective heat flux of channel walls
Rate of progress of
incorporation
Rate of progress of
incorporation
Rate of progress of
incorporation
Rate of progress of trap reaction
Universal gas constant
Species production rate
Ni particle radius
BCZYYb particle radius
BCFZY particle radius
Reynolds number
Defect production rates
Production rate of
Production rate of
Production rate of
Production rate of
Production rate of
Gas-phase temperature
Time
Fuel channel flow temperature
Air channel flow temperature
MEA temperature
MEA thickness
Solid-phase temperature
Channel flow velocity
Lattice molar volume
Mean molecular weight
Species molecular weight
Spatial coordinate
Species mole fractions
Species mole concentrations
Total gas-phase mole concentrations
Gas-phase species mass fractions
Number of charges of
th defect
Greek letters
Channel aspect ratio
Anodic symmetric factor
Cathodic symmetric factor
Surface site density
Standard enthalpies of defect reactions
Standard entropies of defect reactions
Activation overpotential
Activation overpotential within the anode
Activation overpotential within the cathode
Site fractions of surface-adsorbed species
Gas-phase heat conductivity
Solid-phase heat conductivity
Species chemical potentials
Species standard-state chemical potentials
standard-state chemical potentials
standard-state chemical potentials
standard-state chemical potentials
Solid-phase density
Channel wall shear stress
Gas-phase viscosity
Gas-phase mass density
Solid-phase mass density
Electric conductivity in the electronic-conducting phase
Electric conductivity in the ionic-conducting phase
Electric potential in the electronic-conducting phase
Electric potential in the ionic-conducting phase
Equilibrium electric potential in the electronic-conducting phase
Equilibrium electric potential in the ionic-conducting phase
Porosity
Volume fraction of phase
Ni volume fraction
Electric potential at the anode-current collector interface
Electric potential at the cathode-current collector interface