Redox flow batteries (RFBs) hold great potential for large-scale renewable energy storage as they offer distinctive merits of decoupled energy and power, flexible scalability, great safety, and long cycle life. In recent years, researchers are making increasing efforts in developing organic RFBs via designing organic redox-active species which are considered to be highly chemically and physically tunable, abundant, and sustainable. However, the wide application of organic RFBs is hindered by issues including cross-contamination of electrolyte, low battery energy density, and high material cost. The primary objective of this thesis is to address these challenges via exploring and developing redox-active materials for cross-contamination eliminated, high energy density, and cost-effective...[ Read more ]

Redox flow batteries (RFBs) hold great potential for large-scale renewable energy storage as they offer distinctive merits of decoupled energy and power, flexible scalability, great safety, and long cycle life. In recent years, researchers are making increasing efforts in developing organic RFBs via designing organic redox-active species which are considered to be highly chemically and physically tunable, abundant, and sustainable. However, the wide application of organic RFBs is hindered by issues including cross-contamination of electrolyte, low battery energy density, and high material cost. The primary objective of this thesis is to address these challenges via exploring and developing redox-active materials for cross-contamination eliminated, high energy density, and cost-effective organic RFBs.

We begin with addressing the cross-contamination issue in aqueous organic RFBs by designing the bipolar organic salt [(bpy-(CH₂)₃NMe₃)]I₂ (2.3), which serves as catholyte, anolyte, and charge carrier in the battery test. Such a compound can display superior electrochemical properties with a high theoretical energy density of 32.5 Wh L^-1 and good cycling stability with no observable chemical decomposition over 290 cycles. We have also designed the dialkoxybenzene-based artificial bipolar compound (2,5-dimethyl-1,4-dialkoxybenzene/viologen, 3.5) for non-aqueous organic RFB. This compound exhibits an enhanced solubility of 0.66 M in MeCN and delivers capacity retention rate of 99.5 % per cycle within 35 cycles in the charge/discharge cycling test.

We then report a highly water-soluble 4-carboxylic-2,2,6,6-tetramethylpiperidin-N-oxyl (4-CO₂Na-TEMPO, 4.3) molecule to optimize the energy density of TEMPO-based RFBs. When paired with 1,10-bis(3-sulfonatopropyl)-4,4'-bipyridinium (SPr)₂V (anolyte), the resultant RFB operating through a cation-exchange membrane achieved an open circuit voltage of 1.19 V and a high energy density of 14.7 Wh L^-1. In the long-term cycling study, this flow battery features stable capacity retention rate of 99.94% per cycle over 400 cycles with nearly 100% CE.

Organic RFBs utilizing non-aqueous solvents with wider electrochemical window (up to 5 V) can offer a new pathway to realize high energy density. We explore new classes of compounds including nitrobenzenes and boron-based compounds as anolytes in non-aqueous RFBs. The cost-effective nitrobenzene (NB, 5.1) and its derivatives are well-demonstrated to be promising anolytes for non-aqueous RFBs. Notably, NB shows a low redox potential of -1.47 V and extremely high solubility of 6.5 M in salt-containing acetonitrile solution. The NB/DBMMB RFB, which has a high theoretical energy density of 195 Wh L^-1, is successfully operated for more than 100 cycles with a capacity retention of 99.5% per cycle. In-depth electrolyte analysis reveals that azobenzene is a major decomposed species in the cycling test. Besides, we develop a RFB based on a boron-based tert-butyl diketonate (tBuBF₂, 6.6), which shows a high solubility of 2.0 M in MeCN and a low reversible redox couple at -1.83 V. The assembled tBuBF₂-based RFB exhibits an open circuit cell voltage of 2.57 V and shows good capacity retention over 80 cycles.

Finally, to maximize the energy density of non-aqueous RFBs, we design and develop a series of [M(tpy-4OMe)₂]²⁺ (M = Mn, Fe, Co, Ni, Cr) metal coordination complexes. These terpyridine-based complexes are designed to enable multi-electron redox reactions, higher redox potential, and enhanced solubility. [Ni(tpy-4OMe)₂](TFSI)₂ exhibits a remarkably high solubility of 0.72 M in MeCN, cathodic redox potential as high as 1.2 V versus Ag/Ag⁺, and low multiple electron transfers redox reaction at the range of -1.6 to -2.0 V. The fabricated symmetric flow battery delivers an extremely high cell voltage of 2.8 V and shows good capacity retention rate of 99.6% per cycle over 100 charge-discharge cycles.

Keywords: Organic redox flow battery; Bipolar; Cross-contamination; Energy density; Metal coordination complex.