Reactivity, Stability, and Thermodynamic Feasibility of H 2 O 2 /H 2 O at Graphite Cathode: Application of Quantum Chemical Calculations in MFCs Anam Asghar, a,b Abdul Aziz Abdul Raman , a Wan Mohd Ashri Wan Daud, a and Anantharaj Ramalingam c a Department of Chemical Engineering, Faculty of Engineering, University of Malaya, Kuala Lumpur 50603, Malaysia; azizraman@um.edu.my (for correspondence) b Department of Chemical Engineering, University of Engineering and Technology, G.T. Road, Lahore 54890, Pakistan c Department of Chemical Engineering, SSN College of Engineering, Chennai, Tamil Nadu 603110, India Published online 00 Month 2017 in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/ep.12806 A microbial fuel cell (MFC) is a sustainable technology which commonly uses graphite as cathode for the production of hydrogen peroxide. Besides, water formation through four-electron oxygen reduction mechanism is a commonly observed product. Determining the selectivity of H 2 O 2 /H 2 O reaction through experimental means is time consuming because of the slow kinetics of oxygen reduction reaction. Therefore, quantum chemical approaches are essential to comprehend the molecular nature of this process. Thus, den- sity functional theory (DFT) was employed and quantum chemical calculations were performed to predict the chemi- cal reactivity, stability, and thermodynamic properties of molecules participating in oxygen reduction reaction at graphite cathode. The calculations showed that graphene with higher value of “highest occupied molecular orbital” (HOMO), i.e., 24.544 eV has a higher tendency to donate electron for oxygen reduction reaction Furthermore, with an aim of predicting the most favorable conditions for H 2 O 2 production, two different points, i.e., at the edge and middle of graphene plane were investigated. Calculated values showed that oxygen adsorption with the lowest energy requirement of 43.638 kcal/mol is energetically favorable at the edge of graphene plane. Nevertheless, oxygen complexes (O 2 *, HOO*, and HO*) characterized by high HOMO values 24.96, 24.37, and 24.34 eV are highly polarizable in the middle of the graphene plane. Furthermore, thermodynamic feasibility analysis showed that oxygen reduction required for hydrogen peroxide production had lower DG values of 290.94 (edge) and 298.44 (middle) kcal/mole than that of water synthesis (i.e., DG 5248.37(edge), 248.97 (middle) kcal/mole) at two-electron reduction step. Therefore, it was concluded that H 2 O 2 which followed the lowest energy path- way would be more thermodynamically feasible compared to water synthesis. VC 2017 American Institute of Chemical Engineers Environ Prog, 00: 000–000, 2017 Keywords: microbial fuel cells, density functional theory, graphene, hydrogen peroxide, thermodynamic feasibility INTRODUCTION Fenton oxidation, based on the generation of hydroxyl radical (HO • ) are efficient methods for recalcitrant wastewater treatment (Eq. (1)) [1,2]. However, its industrial applications are limited due to high consumption of Fenton reagents (H 2 O 2 , Fe 12 ), excess sludge formation and cost of the chemicals. It is estimated that commercial-grade hydrogen peroxide (H 2 O 2 ) costs $300–590 per ton [3,4] and is classified as oxidizing substance under “division 5.1” by United Nations [5]. Therefore, it accentuates the impor- tance of in situ H 2 O 2 production for Fenton oxidation. Fe 12 1 H 2 O 2 ! Fe 13 1 HO • 1 HO 2 (1) Since the last decade, there has been a growing interest on in situ production of H 2 O 2 (thus in situ Fenton oxidation) in microbial fuel cells (MFCs) in view of its potential for both wastewater treat- ment and power production [6]. Like a typical electrochemical fuel cell, a MFC consists of an anode and cathode compartments separated by a proton exchange membrane (PEM). Organic con- taminants in wastewater are oxidized by microbes in the anode compartment, resulting in the formation of electrons and protons. Protons and electrons enter the cathode compartment through the separator membrane and external circuit respectively. Oxygen adsorbed on cathode is then reduced to produce H 2 O 2 (Eq. (2a,b)) through a two-electron oxygen reduction reaction [7]. Oxy- gen reduction reaction (ORR) in the cathode chamber may follow a four-electron reduction pathway which results in the formation of water. Water formation is a frequently observed reaction that follows the steps provided as (Eq. 3a–c) [8]: O 2 1 H 1 1 e 2 5 HOO  (2a) HOO  1 H 1 1 e 2 5 H 2 O 2 (2b) O 2 1 H 1 1 e 2 5 HOO  (3a) HOO  1 H 1 1 e 2 5 HO  1 HO 2 (3b) 2HO  1 2H 1 1 2e 2 5 2H 2 O (3c) Synthesis of H 2 O 2 has been witnessed experimentally by a few authors by replacing expensive Pt cathodes with simple VC 2017 American Institute of Chemical Engineers Environmental Progress & Sustainable Energy (Vol.00, No.00) DOI 10.1002/ep Month 2017 1