Study of The Performance of A Cylindrical Flow-Through Electro-Fenton Reactor Using Different Arrangements of Carbon Felt Electrodes: Degradation of Amoxicillin In Aqueous Solution

In this work, a cylindrical flow-through electro-Fenton reactor integrated by graphite felt electrodes and Fe(II) loaded resin was evaluated for the production of the Fenton reaction mixture and for the degradation of amoxicillin (AMX) containing aqueous solutions. First, the influence of several factors such as treatment time, current intensity, flow rate and electrode position were investigated for the electrogeneration of H2O2 and the energetic consumption by means of a factorial design methodology using a 24 factorial matrix. Electric current and treatment time were found to be the pivotal parameters influencing the H2O2 production with respective contributions of 40.2% and 26.9%. The flow rate had low influence on the responses, however, 500 mL min-1 (with an average residence time of 1.09 min obtained in the residence time distribution analysis) allowed to obtain a better performance due to the high mass transport to and from the electrodes. As expected, polarization was also found to play an important role, since for cathode-to-anode flow direction, lower H2O2 concentrations were determined when compared with anode-to-cathode flow arrangement, indicating that part of the H2O2 produced in the cathode could be destroyed at the anode. A fluorescence study of hydroxyl radical production on the other hand, showed that higher yields were obtained using an anode-to-cathode flow direction (up to 3.88 µM), when compared with experiments carried out using a cathode-to-anode flow direction (3.11 µM). The removal of a commercial formulation of the antibiotic amoxicillin (AMX) was evaluated in terms of total organic carbon, achieving up to 57.9 % and 38.63% of the pollutant mineralization using synthetic and real sanitary wastewater spiked, respectively. Finally, the efficiency of the process on the inactivation of fecal coliforms in sanitary wastewater samples was assessed, reducing 90% of the bacterium after 5 min of electrolysis.

Characteristics of Graphite Felt Electrodes Treated by Atmospheric Pressure Plasma Jets for an All-Vanadium Redox Flow Battery

In an all-vanadium redox flow battery (VRFB), redox reaction occurs on the fiber surface of the graphite felts. Therefore, the VRFB performance highly depends on the characteristics of the graphite felts. Although atmospheric pressure plasma jets (APPJs) have been applied for surface modification of graphite felt electrode in VRFBs for the enhancement of electrochemical reactivity, the influence of APPJ plasma reactivity and working temperature (by changing the flow rate) on the VRFB performance is still unknown. In this work, the performance of the graphite felts with different APPJ plasma reactivity and working temperatures, changed by varying the flow rates (the conditions are denoted as APPJ temperatures hereafter), was analyzed and compared with those treated with sulfuric acid. X-ray photoelectron spectroscopy (XPS) indicated that the APPJ treatment led to an increase in O-/N-containing functional groups on the GF surface to ~21.0% as compared to ~15.0% for untreated GF and 18.0% for H2SO4-treated GF. Scanning electron microscopy (SEM) indicated that the surface morphology of graphite felt electrodes was still smooth, and no visible changes were detected after oxidation in the sulfuric acid or after APPJ treatment. The polarization measurements indicated that the APPJ treatment increased the limiting current densities from 0.56 A·cm−2 for the GFs treated by H2SO4 to 0.64, 0.68, and 0.64 A·cm−2, respectively, for the GFs APPJ-treated at 450, 550, and 650 °C, as well as reduced the activation overpotential when compared with the H2SO4-treated electrode. The electrochemical charge/discharge measurements showed that the APPJ treatment temperature of 550 °C gave the highest energy efficiency of 83.5% as compared to 72.0% with the H2SO4 treatment.

Origin of the catalytic activity at graphite electrodes in vanadium flow batteries

For many electrochemical devices that use carbon-based materials such as electrolyzer, supercapacitors, and batteries, oxygen functional groups (OFGs) are considered essential to facilitate electron transfer. Researchers implement surface-active OFGs to improve the electrocatalytic properties of graphite felt electrodes in vanadium flow batteries. Herein, we show that graphitic defects and not OFGs are responsible for lowering the activation energy barrier and thus enhance the charge transfer properties. This is proven by a thermal deoxygenation procedure, in which specific OFGs are removed before electrochemical cycling. The electronic and microstructural changes associated with deoxygenation are studied by quasi in situ X-ray photoelectron and Raman spectroscopy. The removal of oxygen groups at basal and edge planes improves the activity by introducing new active edge sites and carbon vacancies. OFGs hinder the charge transfer at the graphite‒electrolyte interface. This is further proven by modifying the sp2 plane of graphite felt electrodes with oxygen-containing pyrene derivatives. The electrochemical evolution of OFGs and graphitic defects are studied during polarization and long-term cycling conditions. The hypothesis of increased activity caused by OFGs was refuted and hydrogenated graphitic edge sites were identified as the true reason for this increase.

Rapid wet-chemical oxidative activation of graphite felt electrodes for vanadium redox flow batteries

Schematic diagram of the K-GF fabrication process. Step 1: deposition of MnOx layers onto the P-GF electrode surface using acidified KMnO4 solutions. Step 2: removal of MnOx layers using an acidified H2O2 solution to produce the K-GF electrode.