ORIGINAL RESEARCH Synthesis and Application of MnO 2 /Exfoliated Graphite Electrodes for Enhanced Photoelectrochemical Degradation of Methylene Blue and Congo Red Dyes in Water Onoyivwe Monday Ama 1 & Khotso Khoele 2,3 & William Wilson Anku 4 & Suprakas Sinha Ray 2,5 & Peter Ogbemudia Osifo 1 & David Jacobus Delport 3 # Springer Science+Business Media, LLC, part of Springer Nature 2020 Abstract Exfoliated graphite (EG), manganese dioxide (MnO 2 ), and EG/MnO 2 -based electrodes were used in this work for the degradation of organic pollutants in wastewater under visible light irradiation. Methylene blue and Congo red dyes were used for the degradation. The synthesis of the nanocomposite electrodes was carried out through the co-precipitation technique. The elec- trodes were engaged for degradation of the dyes under electrochemical oxidation, photolysis, and photoelectrochemical methods. The characterization techniques utilized encompass transmission electron microscopy (TEM), ultraviolet-visible (UV) analysis, scanning electron microscopy (SEM), energy dispersive X-ray (EDX) spectroscopy, Fourier transformed infrared (FTIR) spec- troscopy, thermogravimetric analysis (TGA), and X-ray diffraction (XRD). The EG/MnO 2 photoanode was applied in the photoelectrochemical degradation of 0.1 × 10–4 M methylene blue and Congo red dyes in 0.1 M Na 2 SO 4 under visible light irradiation. The XRD analysis revealed that the MnO 2 exist as α-MnO 2 . SEM morphologies showed a satisfying dispersion of MnO 2 on EG. The EG/MnO 2 composite absorbed a noticeable amount of light in the visible light region compared with the pure EG and MnO 2 . The photoelectrochemical degradation process resulted in enhanced degradation efficiency of Congo red (97.6%) and methylene blue (98.7%) within 60 min and was observed to be higher than those of photolysis and electrochemical oxidation processes. Keywords Exfoliated graphite . Manganese dioxide . Photocatalysis . Dyes . Congo red . Photoelectrochemical degradation Introduction Although synthesis and processing of dyes are significant in textiles, paper printing, leathers, ceramics, cosmetics, inks, and food processing, portions of these dyes which are non- biodegradable and toxic to human and aquatic life find their way into the wastewater. In fact, about 15% of dyes are lost annually to the wastewater [1]. This occurrence is hazardous to human health, and unfriendly to the environment [ 2]. Hence, it is a matter of urgency to remove dyes from wastewater. Among the variety of wastewater treatment methods, ad- vanced oxidation processes (AOPs) are the most effective to remove dyes from wastewater. Within their utilization, AOPs disintegrate organic pollutants into CO 2 and H 2 O with no secondary harmful waste as a by-product [3]. Furthermore, application of AOPs on wastewater treatment occurs at fast reaction rates with non-selective oxidation of contaminants which significantly leads to a simultaneous treatment of mul- tiple contaminants. Moreover, AOP utilization for wastewater treatment is non-toxic, energy-efficient, and cost-effective [4]. Nonetheless, due to the high voltage needed on AOP applica- tion, lower mass transfer of organic pollutants from bulk * Onoyivwe Monday Ama onoyivwe4real@gmail.com 1 Department of Chemical Engineering, Vaal University of Technology, Private Mail Bag X021, Vanderbijlpark 1900, South Africa 2 DST-CSIR National Center for Nanostructured Materials, Council for Scientific and Industrial Research, Pretoria 0001, South Africa 3 Department of Chemical, Metallurgical and Materials Engineering, Tshwane University of Technology, Pretoria, South Africa 4 CSIR-Water Research Institute, P. O. Box M. 32, Accra, Ghana 5 Department of Chemical Science, University of Johannesburg, Doornfontein, Johannesburg 2028, South Africa Electrocatalysis https://doi.org/10.1007/s12678-020-00601-2