The development of renewable, low-cost, and high-efficiency cathode materials for oxygen reduction to H2O2 is desirable for use in electro-Fenton systems for the oxidative treatment of wastewater. In the present study, we report the synthesis of a renewable biopolymer with lignin (lig) interpenetrated into a polypyrrole (PPy) framework via a simple one-step electropolymerization method. This allows the formation of PPy/lig composites that are uniformly coated on a graphite felt (GF) substrate, having large surface area, increased mechanical stability, and abundant C=O (including quinone-type) groups. These features improved the electrocatalytic activity of the PPy/lig-GF cathode for oxygen reduction to H2O2 when an optimal cathodic potential (-0.5 V vs. SCE) was applied, as evidenced by the observed higher H2O2 electro-generation yield and efficiency compared to the raw GF and PPy/ClO4--GF cathodes. Electro-Fenton systems equipped with the GF cathodes (either modified or unmodified) for the decolorization and mineralization of AO7 (an azo dye) followed a pseudo-first-order kinetic model. The PPy/lig-GF cathode achieved substantially higher rate constants and degradation efficiencies. In addition, the incorporation of lignin significantly improved the cycling stability of the cathode material. (c) 2017 Elsevier Ltd. All rights reserved.