Yamini Sudha SISTLA1, Ashi SHARMA1
1Shiv Nadar Institute of Eminence, Gautam Buddha Nagar, India
Electrochemical production of hydrogen via electrolysis of water is a clean and economically viable route compared to other technologies such as steam reforming. The total efficiency of the electrolysis process significantly depends on the nature of electrolyte as well as cathode and anode materials and catalyst. The electrolyte is responsible for diffusion of water (H2O) molecules to the positively charged anode where oxidation of H2O produces hydrogen ions (H+) and diffuse back the H+ ions to cathode to produce hydrogen. Recently, ionic liquids (IL) gained attention as potential electrolytes due to their tunability, wide electrochemical window, chemical stability and ionic conductivity. The present work focuses on studying the diffusion of H2O in an IL-H2O electrolyte mixture (30 vol% of H2O) confined between two graphite electrodes by using molecular dynamic (MD) simulations. Cations such as 1-butyl-3-methylimidazolium ([bmim]+) and 1-methyl-1-propylpyrrolidinum ([mppy]+) and anions such as tetrafluoroborate ([BF4]?), nitrogen trioxide ([NO3]?), hexafluorophosphate ([PF6]?), and bis(fluorosylfonyl)imide ([FSI]?) were studied. Molecular simulations were performed on a layer model built by confining the IL+H2O mixture between graphite cathode and anode. The graphite electrode walls had dimensions as 128 Å x 43 Å x 21 Å. Cathode was applied a negative charge of –0.01 e/Å2 and anode was applied a positive charge of +0.01 e/Å2. Relative concentrations of the cation and anion of IL and H2O molecules between the graphite electrodes were analysed. The concentration profiles indicate that the concentration of H2O at anode was higher than at cathode as the H2O molecules preferably diffuse towards anode. Results also designate that the effect of anion of IL was more significant than cation in directing the diffusion of H2O towards anode. Based on the cation-anion interaction strength, the anions preferably diffused towards positively charged anode and cations are more concentrated at negatively charged cathode. This indicates that, the stronger H2O-anion interactions resulted in diffusion of H2O towards anode. The concentration of H2O at anode increased with increase in H2O-anion interaction strength. The radial distribution function (RDF) between the H2O-cation and H2O-anion also confirms the strong H2O-anion interactions and weak H2O-cation interactions. The RDF indicated that the strongest interaction exists between H of H2O and F of [BF4]– / [PF6] – and H of H2O and O of [FSI]– and [TF2N]– and [NO3]–. The effect of cation followed the order as [bmim] > [mppyr] and the effect of anion as [BF4]– > [PF6] – > [TF2N]– > [FSI]– > [NO3]–.