2,5-Diformylfuran (DFF) is an important oxidation product of 5-hydroxymethylfurfural (HMF) with significant industrial relevance; however, achieving high selectivity using non-noble metal electrocatalysts remains challenging. Herein, hydrogen peroxide (H2O2) was introduced during the electrodeposition of NiMn-C to induce in situ oxidative reconstruction. This treatment induces morphological roughening, generates oxygen vacancies, and alters the Mn oxidation state, collectively modifying the structural and electronic properties of the catalyst. The resulting materials were characterized by FTIR, XRD, FE-SEM, and XPS and subsequently evaluated for HMF electrooxidation. The H2O2-treated NiMn-C catalyst exhibits enhanced activity and maintains high DFF selectivity (90.7%) at 1.65 V vs RHE, producing nearly twice the DFF yield compared to the untreated NiMn-F sample. These findings demonstrate that oxidative-agent-driven structural and electronic modulation provides an effective strategy for improving the performance of non-noble metal electrocatalysts for biomass oxidation. © 2026 The Authors. Published by American Chemical Society.