The 3D companion shows a whole stack of cells with CO₂/O₂ particles flowing past it at a cosmetic rate. This 2D simulator instead zooms into one cell's cross-section — cathode | electrolyte | anode — and computes something the 3D view never derives: the actual drift velocity of the O²⁻ ions that carry the current across the ceramic membrane, from the same electrochemistry:
Cathode: 2 CO2 + 4 e- → 2 CO + 2 O2-
Anode: 2 O2- → O2 + 4 e-
ASR(T) = ASR0 · exp[Ea/R · (1/T − 1/T0)]
I = (V − V_N) / ASR(T) · A_cell (Ohmic model, V_N ≈ 1.0 V)
Faraday's law gives O₂ output from stack current exactly as in the 3D model. This 2D view adds the ion-transport step underneath that: the same current is carried across the membrane by O²⁻ ions, each with charge 2e⁻, so the ionic flux and drift velocity follow directly from current density J = I / A_cell and an assumed mobile-ion concentration n (a modeled order-of-magnitude value for the oxygen-vacancy population of a YSZ electrolyte, not a specific measured cell):
J_ion = J / (2F) [mol O2- /(cm2 s)]
v_drift = J_ion / n n ≈ 4×10⁻³ mol/cm³ (assumed)
crossing time = membrane thickness / v_drift (150 µm assumed)
At typical operating current this drift works out to only a few µm/s — crossing a 150 µm membrane for real would take on the order of a minute, so the animated ions on screen are sped up for visibility (their speed still scales exactly linearly with the computed drift velocity, which is also reported numerically alongside the real crossing time).
Fixed relative to the 3D model: the CO₂-supply current cap in the 3D sim was computed as molPerSec·2F/2 — an extra stray /2 that silently halves the true supply-limited current (2 electrons cross per CO₂ split, so the cap should be molPerSec·2F). This 2D model uses the corrected cap; conversion approaches 100% only once the corrected cap actually binds.
- Cell voltage — raise it past VN to push more ohmic current, more ion drift, and more O₂.
- Stack size — series cells share the same current (only power scales with cell count); MOXIE flew 10 cells.
- CO₂ inlet flow — hard ceiling on ion/O₂ production once the corrected supply cap binds (watch conversion climb toward 100%).
- Temperature — higher T lowers ASR (Arrhenius law), raising current and ion drift at the same voltage.