Carbon-based cathodes degradation during electro-Fenton treatment at pilot scale: Changes in H₂O₂ electrogeneration

Autopsy of carbon-PTFE cathodes was performed by addressing their degradation in a commercial plate-and-frame cell equipped with a Nb-BDD anode. Cell is arranged within an electrochemical pilot plant designed for treating wastewaters by electrochemical Fenton-like processes, thus an efficient electrocatalytic production of H₂O₂ is necessary to guarantee Fenton's reaction. Significant decrease in H₂O₂ electrogeneration occurred during pilot plant operation, hindering the efficient performance of Fenton-like processes. Two cathodes were studied, first was operated at pH 3 and second at neutral pH by using EDDS as complexing agent to maintain iron in solution. Electrogenerated H₂O₂ decreased from 43 mg L^-1 to 16 mg L^-1 in the first cathode after 50 h of operation and from 49 mg L^-1 to 24 mg L^-1 in the second one after 26 h of operation. Both were cleaned with 30% (v/v) solution of HCl/water for 24 h and H₂O₂ production was recovered only in the second cathode (able to generate 39 mg L^-1). Autopsy of the cathodes was tackled by scanning electron microscopy (SEM) and X-ray energy dispersive (EDX), evidencing a strong degradation of first cathode surface and iron oxide inlays in second one due to the decomposition of Fe³⁺:EDDS and consequent iron precipitation at neutral pH.