Step Toward Eco-Friendly Green Source of Non-Aqueous Electrolytes for Redox Flow Battery

Abstract

The efficient utilization of energy from non-exhaustive resources without adverse effects on the environment is a way forward for the sustainable development of society. The intermittent nature of renewable energy from non-exhaustive resources such as solar, wind, etc., requires a large-scale and advanced energy storage system to eliminate the impact of intermittency on the power grid. Redox flow battery (RFB) technology is one of the promising candidates for large-scale grid systems. Present commercial RFB technology is based on the redox reactions of electroactive metals, which are constrained by material abundance, cost, and environmental issues. These issues are prompting scientists to look into organic-based technologies as potential metal-free battery alternatives. Organic-based materials offer structural diversity of organic molecules to adjust the electrolyte materials' thermodynamics and redox characteristics, in addition to low cost and scalability. This opens the door to the development of green and sustainable energy storage devices.Despite development over the last decade, organic compound exploitation for RFB remains limited. Organics with limited ionic conductivity and poorly defined conducting channels have poor voltage efficiency and cyclability. In aqueous flow batteries, the cell potential is also limited by the electrolysis of water. The scientific community, on the other hand, is addressing this issue by introducing the non-aqueous RFB (NAqRFB).Quinones are the most redox-active moieties found in natural organic materials, and they play a critical role in biological electron transport. They are a class of molecules that are formed from aromatic compounds like benzene or naphthalene and have a fully conjugated cyclic diketone structure with a redox reaction based on the rearrangement of the conjugated double bond. 1,4-naphthoquinone (NQ) is investigated as a possible negolyte candidate. NQ as metal-free organic negolyte was coupled with metal acetylacetonate-based posolytes, M(acac), where M = Cr, V, Mn. Both types of redox species (metal & metal-free) have good solubility in the solvent - Dimethylacetamide (DMA) - and ionic conductivity was enhanced using the supporting electrolyte of TEABF4. Cyclic voltammetry (CV) analysis using glassy carbon working electrode (GCE) highlighted that NQ offers reversibility; the high value of negative redox potential -1.75 V vs. Ag/Ag+ enables higher cell potential and posses no irreversible side reactions.The electrochemical stability of the negolyte was further tested using the symmetrical cell having an electrolyte of 0.06 M concentration of NQ as redox species. The electrolyte flow from the reservoir, cathode, and anode in a series arrangement. The redox species are reduced at the cathode and then oxidized at the anode, which helps maintain the state-of-charge (SOC) at 50 %. The cell was operated for 50 cycles at the C-rate = 1C, i.e., 2.75 mA⋅cm-2 at the cutoff potentials of ±0.3 V. The energy efficiency of that symmetric cell was 99.7 %, and discharge capacity retention was 99.967%. These results are validated from the reported study by Yu Ding, 2016. This strengthen the motivation of using the NQ as metal-free organic negolyte for single NAqRFB-cell. The insertion of the mentioned metals (Cr, V, Mn) at the posolyte while keeping NQ ion at the negolyte ensured the generation of novel chemistries for NAqRFBs, in detail NQ||Cr, NQ||V, and NQ||Mn, with cell potential differences of 3.25 V, 2.2 V, and 2.45 V, respectively.Detailed characterization was performed using polarization, EIS, and charge-discharge cycling (CDC) – Table 1. Post-mortem CVs of the electrolyte using GCE highlight the stability of the electrolyte after intensive CDC. The RFB assembled with NQ||Cr offered the highest capacity and energy density. However, it was faced with lower energy efficiency. The post-mortem analyses on the posolyte signify that there was crossover through the membrane, which ended up with a contaminated electrolyte (mixture with negolyte chemistry) that affected the extended capacity of the cell. The RFB composed of NQ||V offers the 2nd highest capacity and energy density than others and is two times more than reported V||V NAqRFB. Reinforced membranes permit to decrease the permeability of the membrane toward smaller ions of NQ as compared to V2+ redox ions. Furthermore, the NQ||Mn battery demonstrated a stable performance up to 25 cycles having higher efficiencies and with a decent energy density of ca. 60 mWh.g-1 . NQ has high solubility of 1.5 M in DMA and future work at higher capacities is in progress for in-depth study covering other aspects. The study may ascertain the feasibility of NQ-based RFB to be the eco-friendly green alternate battery chemistry. Figure 1