Study on viologen derivatives as anolyte for redox flow battery

Abstract

With the shift of energy system from fossil fuels to renewable energy, energy storage techniques are in charge of the critical role to mitigating the intermittency of renewable energy resources. Vanadium utilizing aqueous redox flow batteries (VRFBs) with their high power density, scalability (up to MW and MWh), long life time and safety feature are suitable option for large scale energy storage system. However, the corrosive electrolyte solution (e.g., concentrated sulfuric acid) and the high cost vanadium limit its widespread application. Therefore, organic redox molecules have emerges as new generation of RFBs for green energy storage to replace VRFB. In the thesis, two viologen derivatives are reported as anolyte with two different approaches utilizing structural characteristic of viologen in order to improve the performance of organic RFB. First utilizing approach is to simply introduce new counter anion (NO3-) to improve the electrochemical properties of methyl viologen. Methyl viologen with nitrate counter anion showed improved coulombic reversibility (average peak current ratio: 0.8) and diffusion coefficient (D= 5.13 x 10-5) when compared with commonly used chloride counter anion((D= 1.95 x 10-5, average peak current ratio: 0.8), which may attributed to weak ion association between methyl viologen and nitrate. These results give the expectation of fast charge-discharge performance for organic aqueous redox flow batteries The second utilizing approach is to introduce the asymmetric diakylation and bulky delocalized anion for viologen moiety to liquefy the redox active molecules which has great potential to improve energy density of RFB with high molarity of redox active materials. Among the new series of asymmetric dialkylated viologen compounds, [C4C6bpy][TFSI]2 showed not only the lowest melting point of 18 ℃ compared to previously reported dialkylated viologen but also reversible two electron utilization redox reaction with excellent electrochemical stability for 200 continuously recorded CV cycles. Using proof of concept non-flow cell test to roughly validate the [C4C6bpy][TFSI]2 as anolyte molecules, two well defined voltage plateaus were clearly observed and stable average coulombic efficiency (91 %)and energy efficiency (69 %) were exhibited, suggesting[C4C6bpy][TFSI]2 is promising anolyte as the solo or co-solvents for high energy non-aqueous RFB applications.