Boundary Conditions for CV(Constant Voltage) and CC(Constant Current)

[hokonkim]

Hello, Communities.

I have questions about using Boundary Conditions, specifically for CV(Constant Voltage) and CC(Constant Current).

First, some codes for BCs are written below and these are done for CV.
It works as I want.

[BCs]
  [./WaterVaporPartialPressure_top]
    type = DirichletBC
    variable = p_H2O
    boundary = 'SF_PORE_WITH_ZMAX' # cylinder geometry
    value = 0.90 # (atm)
  [../]

  [./potentialNi_top]
    type = FunctionDirichletBC
    variable = phi_Ni
    boundary = 'SF_NI_WITH_ZMAX' # cylinder geometry
    function = 'funcPotentialNi' # (V)
  [../]

  [./potentialYSZ_bottom]
    type = DirichletBC # Toy cell, Half cell
    variable = phi_YSZ
    boundary = 'SF_YSZ_WITH_ZMIN' # cylinder geometry, Half cell
    value = 0 # (V)
  [../]
[]

Second, another codes for BCs are written below and these are done for CC.

[Functions]
  [./funcCurrentDensity]
    type = ParsedFunction
    expression    = 'j'
    symbol_names  = 'j'
    symbol_values = '0.2'
    
    # Think about the direction of phi_YSZ at Boundary.
    # This is reverse direction to that of oxygen ion.
  [../]

  [./funcTimeStepper]
    type = PiecewiseLinear
    x = '0.0  5.0 10.0' # time_t
    y = '1.0  1.0  1.0' # time_dt
  [../]
[]

[BCs]
  [./WaterVaporPartialPressure_top]
    type = DirichletBC
    variable = p_H2O
    boundary = 'SF_PORE_WITH_ZMAX' # cylinder geometry
    value = 0.90 # (atm)
  [../]

  [./potentialNi_top]
    type = DirichletBC
    variable = phi_Ni
    boundary = 'SF_NI_WITH_ZMAX' # cylinder geometry
    value = -0.808 # (V)
  [../]

  [./currentdensityYSZ_bottom]
    type = FunctionNeumannBC
    variable = phi_YSZ
    boundary = 'SF_YSZ_WITH_ZMIN' # cylinder geometry
    function = 'funcCurrentDensity' # (A/cm^2)
  [../]
[]

The codes for CC don't work as I want.
I think there would be a problem in flows of variables (phi_Ni and phi_YSZ).
This is a reaction for Solid Oxide Electrolysis Cells (SOECs), so principally the potential should be negative.
In CC mode, I set the boundary at top (-0.808 V) with DirichletBC and the boundary at bottom (0.2 A/cm^2) with NeumannBC.
As far as I understand, these configuration of boundary is correct for CC mode.
But the problem is the flow of variables having negative values, perhaps?
Even I tried both value (0.2 or -0.2), I was not able to get any flow under this condition.
I checked the syntax and tried many ways as much as I know, but still I'm stuck at this point.
What do I miss here?
Please give me any tips to solve this issue.

Thank you for your help :)

FYI, I attached the input files for CV and CC.
SOEC_cathode.zip

[GiudGiud]

@hokonkim which module are you using?
Which example did you start from?

[hokonkim]

@GiudGiud
I’m using base modules and some functions from phase field module.
I didn’t use a specific example; I built my input file from scratch, based on the documentation.

[mangerij]

Does phi_Ni and phi_YSZ represent the electrostatic potential in some way? It is very hard to help unless there are more details here - as far as I understand, constant voltage means the electrostatic potential is short-circuited in such a way that phi is the same on opposing boundaries. Constant current would mean you have a constant electric field so phi should be different from the opposing boundaries assuming your current is calculated with j = sigma * E where E is computed from -grad phi and is constant. But this is based on my limited understanding if what you are trying to do.

[hokonkim]

Thank you for your reply. Yes, phi_Ni and phi_YSZ represent the electrostatic potential in my model.
In the CV case, I applied DirichletBC at both top and bottom boundaries and it worked as expected.
In the CC case, I tried DirichletBC on the top boundary and NeumannBC with a current density on the bottom boundary, but I couldn’t get any current flow. I wonder if I’m misunderstanding how to apply the NeumannBC correctly.