Carbon-Based Air-Cathodes for Hydrogen Peroxide Production in Microbial Fuel Cells

Abstract

Harnessing the chemical energy in wastewater while treating it is a key goal of new treatment technologies. Microbial fuel cells (MFC), a type of microbial electrochemical technology, could be one potential treatment technology with this goal. MFCs can help recover the chemical energy in wastewater organics directly in the form of electricity or as valuable chemicals such as hydrogen peroxide (H2O2). In MFC, anode-respiring bacteria (ARB) oxidize organic matter and respire electrons to an anode. At the cathode, the electrons combine with oxygen to result in the oxygen reduction reaction (ORR), which can proceed by the 4-e- reduction pathway on precious metal catalysts (e.g. platinum) to produce water, or via the 2-e- pathway on carbon catalysts to produce H2O2. The cathode catalyst may likely be a key factor affecting the efficiency of the reduction reaction, especially in relation to producing H2O2. In this study, we examine the production efficiency of H2O2 using three different carbon-based catalysts: 1) Vulcan carbon, 2) graphene oxide, and 3) MXene (Ti3C2). To prepare the cathodes, 1.5 mg/cm2 of carbon-based ink was coated on CeTech carbon cloth with a carbon-containing hydrophobic microporous layer. Three different graphene oxide powders were produced using modified Hummers method from pure graphite powder with mesh sizes of: 1) -325 (GO-325), and 2) +200 (GO+200), and (3) graphene nanoplatelets. MXene was produced by exfoliating Ti3AlC2 in hydrofluoric acid. The prepared cathodes were then placed inside an electrochemical cell with the gas diffusion layer (GDL) open to air and the coated catalyst layer facing the electrolyte solution inside the cathode chamber. The anode (plain carbon cloth) and the cathode chambers were separated with a Chemours Nafion cation-exchange membrane. For each experiment, the anode and the cathode chambers were filled with 100mM of phosphate buffer solution to serve as the electrolyte. An Ag/AgCl reference electrode was used to measure/control anode and cathode potentials. The cathode chamber electrolyte was recirculated at a rate of 25 mL/min to allow well-mixed conditions. A constant current density of 1 mA/cm2 was applied using chronopotentiometry (CP), and the anode and cathode potentials were measured over time. The H2O2 concentration in the cathode solution was measured every half an hour for a total of four hours using Titanium(IV) oxysulfate-sulfuric acid solution as the reagent. The cathodic Coulombic efficiency (CCE) for H2O2 production ([CH2O2actual/ CH2O2theoretical]*100%) was then determined for each sampling time point. Results show that adding a carbon catalyst layer to the CeTech carbon cloth increases the initial CCE from 20% to 50%. After an hour of operation, CCE dropped from 50% to 23% and 14% for Vulcan carbon and GO-325 mesh, respectively. Whereas, the CCE remained mostly stable for MXene and GO+200 during the entire four hours of operation. Here, no H2O2 was produced with GO- nanoplatelets as the carbon catalyst layer. We are currently analyzing our data to correlate catalyst selection with H2O2 degradation due to local pH changes, and electrochemical decomposition of H2O2 on the cathode surface. To better understand the morphology of the catalyst layer, scanning electron microscopy (SEM) and a laboratory X-ray CT system will also be conducted. Results from these characterization studies will also be presented. Figure 1