Aqueous Organic Redox Flow Batteries - Electrochemical Studies of Quinonoid Compounds

Licentiate thesis, 2019

Society faces a large challenge in transitioning to sustainably generating electricity from renewable energy sources such as wind and solar. One of the biggest obstacles to this transition is the intermittence of the sources, leading to a mismatch of supply and demand, a problem most commonly solved by installing large‐scale energy storage.

A promising energy storage technology that has the potential to surpass the currently used systems in performance, cost and environmental benignity are aqueous organic redox flow batteries (AqORFB). In this work, the performance of the archetypical AqORFB candidate, 9,10‐anthraquinone‐2,7‐disulfonic acid (AQDS) is investigated by the novel combination of rotating disk electrode voltammetry and diffusion NMR. At a concentration of 1 mM, AQDS shows near ideal electrochemistry, involving two electrons, but at higher concentrations, the redox kinetics and the accessible charge/discharge capacity suffer. The former problem is examined by employing the scheme of squares model to explore the redox mechanism for AQDS at different pH values and is seen to come from rate‐limitations in the protonation step. At higher concentrations, the relative concentration of protons to AQDS is lower, resulting in a larger impact on the rate‐limiting step on the apparent rate constant. The latter is explained by self‐association of the molecule into an electrochemically inactive dimer, and this process limits the capacity that can be collected from the system with bulk electrolysis. At a concentration of 1 M AQDS in 1 M H2SO4, only 27% of the molecules occur as redox‐accessible monomers.

Furthermore, in pursuit of an attractive molecular candidate for AqORFBs, a naphthalene diimide molecule containing a solubilizing quaternary amine as a sidechain is synthesized and characterized both chemically and electrochemically. It is seen to show excellent behavior for AqORFBs by having a two‐electron reduction at a suitable potential, a cheap and environmentally friendly synthesis route as well as showing exceptional stability during cycling with bulk electrolysis. The collection of cyclic voltammograms at a concentration of 1 mM over a range of different pH values revealed some interesting electrochemical behavior. Firstly, the reduction of only one of the two electrons shows any pH dependence, and only at pHs between 0 and 3. At pH 0, the two electrons share the same reduction potential, whereas at higher pH, they are separated by about 0.4 V. Secondly, at higher concentrations, this separation increases further, reaching 0.6 V at 50 mM, possible allowing for the molecule to be used as both anodic and cathodic material in a symmetric flow battery. Thirdly, the Potential ‐ pH dependence between pH 0 and 3 exhibited a two electron‐three proton relationship, possibly indicating self‐association. Utilizing the same combination of diffusion NMR and rotating disk electrode voltammetry as for AQDS, the naphthalene‐based molecule was found to dimerize similarly to AQDS, but to a larger extent. The capacity accessible for reduction using bulk electrolysis was close to the theoretical value, however, indicating no negative effect of the dimerization.