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

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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.