Electrocatalytic Hydrazine Oxidation Modulated Through Transition‐Metal‐Substitution in Manganese Ferrites

ABSTRACT Transition‐metal‐substituted manganese ferrites, Mn _0.95 M _0.05 Fe ₂ O ₄ (M: Co, Cu, and Zn), with a single‐phase cubic spinel structure, were synthesized using the combustion method. A high specific surface area (SA _BET ) of 107 m ² g ⁻¹ , along with pore size distribution (1.7 and 3.4 nm), was obtained for Mn _0.95 Zn _0.05 Fe ₂ O ₄ . All the synthesized ferrites exhibited significant hydrazine oxidation (HzOR) activity, and mass‑loading optimization revealed that their performance correlated with increasing SA _BET in the following order: Mn _0.95 Zn _0.05 Fe ₂ O ₄ > Mn _0.95 Co _0.05 Fe ₂ O ₄ > Mn _0.95 Cu _0.05 Fe ₂ O ₄ > MnFe ₂ O ₄ . Importantly, Mn _0.95 Zn _0.05 Fe ₂ O ₄ demonstrated fast kinetics for HzOR with a low Tafel slope of 193 mV dec ⁻¹ and an electron transfer coefficient of 0.31. Further, it delivers a high current density of 115 mA cm ⁻² with mass loading of 0.96 mg for HzOR. The Mn _0.95 Zn _0.05 Fe ₂ O ₄ electrocatalyst exhibits the lowest onset potential of 0.65 V vs. RHE compared to other synthesized electrocatalysts. The double‐layer capacitance (C _dl ) of 0.49 mF cm ⁻² corroborates a higher SA _BET and porous microstructure of Mn _0.95 Zn _0.05 Fe ₂ O ₄ . DFT study reveals that the potential‑determining step (*N ₂ H ₃ → *N ₂ H ₂ ) has the lowest free‑energy barrier with a low limiting potential of 0.19 V for Mn _0.95 Zn _0.05 Fe ₂ O ₄ . The low barrier arises from Zn substitution, which moderates N ₂ H ₃ adsorption, stabilizes intermediates, and enhances HzOR activity.