DEVELOPMENT OF NON-NOBLE ELECTROCATALYSTS FOR QUINONE BASED ORGANIC REDOX FLOW BATTERIES

Abstract

The surge in global energy generation from renewable sources has triggered the need for large scale electrical energy storage technologies. These storage technologies act like a bridge between generation and end usage. Recently, the vanadium redox flow battery (VRFB) technology has taken the lead in the market; however, the high cost of vanadium electrolyte raises concern over commercial usage. In this work, the organic flow battery systems have been proposed as an alternative to VRFB as they are simple, economical, and abundantly available. The quinone-based redox chemistry was explored and proposed two battery systems, i.e., (i) hydrogen-1,4 p-Benzoquinone redox flow battery (H2-BQ RFB) and (ii) hydroquinone benzoquinone redox flow battery (HQ-BQ RFB). Carbon-based electrodes are extensively used as electrode materials due to their comprehensive properties, but they possess poor hydrophilic nature and low electrochemical activity. It is essential to modify carbon-based materials before employing them in battery applications. Numerous techniques are proposed to refine carbon based materials. The modification by incorporating catalysts is one of the best ways to improve redox kinetics. Metal oxides are widely preferred as electrocatalysts to modify carbon due to their low cost, tunability, and high activity. The Tungsten trioxide (WO3) and Titanium dioxide (TiO2) have already proved their suitability in energy systems with exceptional electrochemical activity. Hence, these materials are proposed for the first time for the H2-BQ RFB and HQ-BQ RFB. The present work focuses on developing non-noble Tungsten trioxide-doped carbon (WO3/C) and TiO2 supported on carbon nanoparticles (TiO2@CNP) electrocatalyst for proposed organic redox flow batteries. The catalysts are characterized by field emission scanning electron microscope (FESEM) for topography, X-ray diffraction (XRD) for examining crystallinity, and Fourier transform infrared spectroscopy (FTIR) for identifying the functional bonds. TiO2@CNP and WO3/C electrocatalysts are tested in positive half-cell of H2-BQ RFB and single-cell HQ-BQ RFB, respectively. The electrochemical activity of the electrocatalysts is evaluated by cyclic voltammetry (CV). The voltammograms of TiO2@CNP-CP in positive half cell of H2-BQ RFB, TiO2@CNP-CP in both half cells of HQ-BQ RFB and WO3/C of in both half cells of HQ-BQ RFB were recorded. These show high anodic and cathodic peak currents, low charge transfer resistance, and relatively high electro-kinetic reversibility. Enhanced electrochemical active surface area (ECSA) and turnover frequency (TOF) revealed higher electrode kinetics than the pristine carbon paper. ECSA of WO3/C-CP and TiO2@CNP-CP in vi positive half-cell of HQ-BQ RFB are 10 cm2 and 8.33 cm2 respectively while in negative half cell of HQ-BQ RFB are 4.8 cm2 and 2.25 cm2. The higher ECSA implies that the WO3/C-CP provides more active sites for the reactions to take place in the battery system. The calculated number of active sites of WO3/C-CP in the positive half-cell and the negative half-cell of HQ BQ RFB are 4.3 × 10−6 mol and 2.6 × 10−6 mol, respectively. The corresponding TOF of WO3/C-CP in the positive and negative half-cells of HQ-BQ RFB are 0.19 s−1 and 0.079 s-1, respectively. These values are observed to be higher than that of CP, TiO2@CNP-CP, and possess improved catalytic activity. The TiO2@CNP electrocatalyst was tested in an H2-BQ RFB positive half-cell by galvanostatic charge-discharge and obtained energy efficiency of up to 73%. The charge–discharge test on single cell HQ-BQ RFB was conducted using pristine carbon paper (CP), TiO2@CNP-CP, and WO3/C coated carbon paper (WO3/C-CP). The columbic efficiency (CE), voltage efficiency (VE), and energy efficiency (EE) of the RFB using WO3/C-CP are around 90%, 75%, and 70%, respectively, which are significantly higher than the CP and TiO2@CNP-CP. The developed non-noble WO3/C and TiO2@CNP electrocatalysts proved their suitability in proposed H2-BQ RFB and HQ-BQ RFB systems by providing higher active sites and reaction kinetics.