[jorgensd]

Two extensions of the Butler–Volmer kinetics in sumfem.electrochemistry, the first part of the full-cell work (B3).

Changes

• Other species of a reaction. A reaction can now produce or consume species besides its redox couple, e.g. VO²⁺ + H₂O ⇌ VO₂⁺ + 2H⁺ + e⁻, which produces two protons and consumes one water molecule.
  • ElectrodeReaction takes others: concentrations, each with the moles produced per mole oxidized (negative for one consumed). The reaction's rate enters the residual of each species that has a field.
  • ButlerVolmer and MassTransferButlerVolmer take the same by ion name, as stoichiometry, e.g. {"H": 2, "H2O": -1}.
  • An ion that follows from electroneutrality has no field to produce or consume, so naming one raises a ValueError.
• PistonVelocityButlerVolmer. Butler–Volmer kinetics limited by diffusion between the fibres, with the mass-transfer coefficient (piston velocity) D/δ of each ion of the couple, where δ is the mean distance between fibres (parameter <name>.fiber_distance). Unlike MassTransferButlerVolmer, it does not depend on the flow.
• Shared surface balance. The surface concentrations at which mass transfer balances the reaction are computed by one private function, _balanced_surface, used by both mass-transfer-limited kinetics. MassTransferButlerVolmer's results are unchanged.
• Docs. PistonVelocityButlerVolmer is exported from sumfem.electrochemistry and added to the package docstring and the API page.

Tests

• test_positive_couple_with_protons_water_and_piston_velocities: the positive vanadium couple, with piston velocities, two protons produced and one water consumed, against the same residual written in plain UFL, on intervals, triangles, quadrilaterals, tetrahedra and hexahedra. Both sides add a mass term for the proton and water fields, which otherwise appear only through the reaction and would leave their Jacobian blocks empty.
• test_the_stoichiometry_needs_ions_with_fields: a stoichiometry that names the ion given by electroneutrality is rejected.