Density functional theory study of oxygen reduction reaction (ORR) products with graphene

Abstract

The 2D crystal lattice graphene has attracted tremendous research interest due to its exceptional properties that provide interesting opportunities for many applications, including energy storage technologies. Graphene has revealed remarkable potential in electrochemical energy capacity and conversion, particularly in rechargeable metal-air batteries. However currently metal air batteries (MABs) are faced with challenges such as anode issues (corrosion, dendrite formation at the metal anode and passivation) [1]. In this study, first-principle density functional theory was employed to investigate reaction mechanisms between graphene and oxygen reduction reaction (ORR) products XxOy, where X= Li, Na, Mg and K with x,y = 1 or 2, specifically for energy storage application of the 2D graphene in an effort to address the energy crisis. The reaction mechanisms of a single atom-, double atom-, and molecules (XO-, X2O-,XO2- and X2O2-) adsorbed onto graphene was investigated. For single-atom adsorption, the three adsorption sites, i.e., top, hollow and bridge sites were considered. The calculated adsorption energies revealed that single Li atom adsorbs stronger on graphene than all the alkaline metals and oxygen, with the hollow site being the most preferred site. For double atom adsorption, the calculated adsorption energies revealed that Na2 has a strong interaction with graphene layer and as a result the hollow site being the most preferred adsorption site. Furthermore, the order of stability was found to be Na2 >K2 > Li2 > O2 > Mg2. The ORR product NaO2 was found to be the most favoured reaction product with a calculated adsorption energy of -4.209 eV, followed by KO2 with the adsorption energy of -2.808 eV The results show a stronger interaction between oxygen atom and carbon atoms, which could potentially suggest a formation of C-O bond. In addition, the study revealed that adsorbates molecules move away from graphene layer together with two neighboring carbon atoms (possibly forming CO, CO2, LiCO3, NaCO2, KCO3 and MgCO3). The electronic properties of all the systems predicted metallic behaviour along the Fermi level due to no energy band gap between the valence and conduction bands. Thus, the electronic properties of XO-, X2O-,XO2-and X2O2- adsorbed graphene systems indicated that the typical electronic model of pristine graphene remains a conductor even after adsorption. Overall Na atom adsorption as a double and reaction product was the most stable system when adsorbed onto graphene implying that graphene could be considered as a better alternative anode electrode for sodium batteries, particularly sodium air batteries (SIBs)