Zinc-Air Battery: Oxygen Reduction Catalyst Kinetics

Cathode Conditions

Catalyst quality 50%

Discharge current density 10 mA/cm²

O₂ partial pressure 0.21 atm

Electrolyte temperature 25 °C

Live Readouts

Cell voltage--

Current delivered--

4e⁻ Faradaic efficiency--

O₂ diffusing to catalyst

Catalyst nanoparticles

4e⁻ pathway → OH⁻ (efficient)

2e⁻ pathway → HO₂⁻ (lossy)

How it works

A zinc-air cell discharges by oxidizing zinc at the anode while air diffuses into a porous gas-diffusion cathode and is electrochemically reduced on a catalyst. The oxygen reduction reaction (ORR) can follow two competing pathways:

4-electron (efficient):  O2 + 2H2O + 4e- -> 4OH-
2-electron (lossy):     O2 + H2O + 2e-  -> HO2- + OH-

A poor catalyst (bare carbon) favors the 2-electron peroxide route, which wastes charge and corrodes the electrode; a good bifunctional catalyst (Pt/C, MnOx, spinel oxides) pushes selectivity toward the full 4-electron route. Activation losses follow Tafel kinetics from the Butler-Volmer equation:

eta_act = (R T / (alpha F)) * ln(i / i0)
i_lim   = k * pO2 * (T / 298)      (diffusion-limited current)
eta_conc = -(R T / (4F)) * ln(1 - i / i_lim)
V_cell  = E0 - eta_act - eta_conc - eta_Zn - i * ASR

Raising catalyst quality raises the exchange current density i0 (lower activation loss) and shifts selectivity toward the 4-electron path. Raising O2 partial pressure or temperature raises the diffusion-limited current i_lim, pushing back the mass-transport penalty that appears as the delivered current approaches it. E0 ≈ 1.65 V is the standard Zn-air open-circuit voltage; ASR is the cell's area-specific ohmic resistance.

O₂ OH⁻ HO₂⁻

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