Superior Fe-doped α -MnO ₂ bifunctional catalyst for zinc–air batteries: comparative analysis of cycle stability and performance in transition metal-doped α -MnO ₂ systems

Abstract Transition-metal-doped α -MnO ₂ catalysts (Ni, Cu, Fe at 10 at%) were synthesized by a facile hydrothermal method and evaluated as bifunctional electrocatalysts for rechargeable zinc–air batteries (ZABs). Fe–MnO ₂ exhibited the best bifunctional activity, with the most positive oxygen reduction reaction (ORR) onset potential (0.895 V vs RHE), lowest oxygen evolution reaction (OER) overpotential (510 mV at 10 mA cm ⁻² ), and smallest ORR/OER potential gap (Δ E = 0.964 V). Electrochemically active surface area (ECSA) normalization revealed that Fe–MnO ₂ achieves an optimal balance between intrinsic per-site activity and active-site density, surpassing both undoped α -MnO ₂ (high intrinsic activity but limited ECSA) and Cu–/Ni–MnO ₂ (high ECSA but low intrinsic activity). In full ZAB cells at 10 mA cm ⁻² , Fe–MnO ₂ cycled stably for 900 cycles with a charge–discharge voltage gap of only 0.81 → 0.97 V, comparable to undoped α -MnO ₂ (0.75 → 1.05 V) and substantially smaller than those of Cu–MnO ₂ (→1.32 V) and Ni–MnO ₂ (→1.58 V). Under more demanding conditions (50 mA cm ⁻² , 200 cycles), Fe–MnO ₂ sustained stable cycling (1.23 → 1.78 V), whereas undoped α -MnO ₂ degraded rapidly beyond ∼50 cycles (1.42 → 2.39 V), attributed to Jahn–Teller-driven phase transformation. Post-cycling x-ray absorption spectroscopy linear combination fitting confirmed preserved MnO ₂ -like character in Fe–MnO ₂ , whereas α -MnO ₂ , Cu–MnO ₂ , and Ni–MnO ₂ showed phase conversion (∼80% → ∼ 60%, ∼70% → ∼ 20%, and ∼80% → ∼ 60%, respectively, after 10 mA cm ⁻² cycling). Density functional theory calculations revealed that interstitial Fe doping reduces Mn ³⁺ e _g orbital splitting by 46%, suppressing Jahn–Teller distortion, and shifts the binding energies of *OOH, *O, and *OH toward the optimal range, establishing Fe–MnO ₂ as a promising earth-abundant bifunctional catalyst for practical ZABs.