Redox Active Polymers for Non-Aqueous Redox Flow Batteries: Validation of the Size-Exclusion Approach

Abstract

Non-aqueous redox flow batteries (NRFBs) are emerging technologies that promise higher energy densities than aqueous counterparts. Unfortunately, cell resistance and redox component crossover observed when using ion-exchange membranes (IEMs) hinders NRFB development. The size exclusion approach for polymer-based NRFBs addresses these issues by using macromolecular design to mitigate crossover. Here, we highlight the benefits of this approach using inexpensive nano-porous separators (PS) (Celgard and Daramic). We evaluated these along with an IEM (Fumasep) in a flow cell configuration using a classical redox couple of viologen and ferrocene, both in monomer and polymer forms. High Coulombic efficiencies above 98% and access up to 80% of capacity were observed for the polymer cells. These displayed better performance with PS than with the IEM, which exhibited lower energy efficiencies from higher overpotentials. The monomer equivalent cells with PS resulted in lower efficiencies and rapid decrease in depth of discharge. Post-cycling analysis by ultramicroelectrode voltammetry showed that the small molecules freely crossed PS and to a lesser degree through the IEM. Therefore, here we demonstrate that the combination of redox active polymers and simple PS enables a potential next-generation NRFB system that provides a competitive alternative to the use of IEMs in NRFBs. Renewable energy sources, such as wind and solar, are emerging inputs in the electrical grid.1 However, these sources are inherently intermittent, thus strategies designed to decouple energy production from its use are highly desirable. Redox flow batteries (RFBs) promise to address these challenges by offering convenient charge storage in fluid form with straightforward scalability and deployment, and long-term durability.2-4 In particular, non-aqueous redox flow batteries (NRFBs) promise higher energy densities than aqueous RFBs owing to a wider electrochemical window. 5,6 NRFBs provide a wide range of molecular and electrolyte designs with attractive redox potentials, solubilities and chemical tunability.6-11 Yet, their wide-scale adoption has been hampered by the lack of chemically-suitable ion-exchange membranes (IEMs) that decrease areal resistance while simultaneously preventing material crossover between compartments. 6,8,12,13 Most commercially developed RFBs use IEMs, 14 such as perfluorinated Nafion, 12 due to its robustness, stability and suitable performance in aqueous environments. IEMs are typically the membrane of choice since these are non-porous sheets of crosslinked polyelectrolytes capable of exchanging ions at its interface with low levels of solution mixing.12 Crossover in those cases is typically ascribed to varying levels of ion selectivity 12,14 in the membranes. However, recent reports suggest low conductivity of these membranes in typical non-aqueous battery solvents (e.g. propylene carbonate, acetonitrile, etc.).13,15 A possible solution is to explore membrane chemistries that provide highly-conductive IEMs tailored to the...

Exploring the Role of Electrode Microstructure on the Performance of Non-Aqueous Redox Flow Batteries

“…For all experiments performed here, ohmic losses remain the largest contribution to total cell resistance as expected given the moderate ionic conductivity of non-aqueous electrolytes. 5,33,84 We anticipate that kinetic contributions will be minor for all the electrodes tested as the TEMPO/TEMPO + redox couple has a high rate constant (1.0 × 10 −1 > k 0 > 2.3 × 10 −2 cm s −1 ) 34,85 and is an outer sphere electron transfer reaction, therefore insensitive to electrode surface chemistry. 86 The magnitude of this contribution is difficult to estimate by inspection of the Nyquist plot in Figure 4c as the higher frequency kinetic and lower frequency mass transfer responses are convoluted.…”

Biredox Eutectic Electrolytes Derived from Organic Redox‐Active Molecules: High‐Energy Storage Systems

“…The discharge capacities were maintained at 1.21, 1.16, 1.06, and 0.93 Ah L −1 at current densities of 40, 50, 60, and 80 mA cm −2 with Coulombic efficiencies (CE) of 89, 91, 92, and 94 %, respectively (Figure b). The low CE could be attributed to the porous structure of the Daramic separator which has poor ion selectivity . The capacity maintained about 72 % of the initial value after 500 cycles at 60 mA cm −2 with corresponding Coulombic and energy efficiencies of about 93 and 51 %, respectively (Figure c).…”

“…Porous separators are able to sustain higher current density, but their ion selectivity is poor because of the large pore size, leading to the crossover of redox materials . Several ways have been employed to mitigate crossover, such as employing a thicker separator, polymer‐based redox species, mixed electrolytes, and applying higher operating current densities –. However, it is still challenging but highly desirable to develop concentrated and stable nonaqueous electrolytes which not only deliver high capacity but also retain good cycling stability, especially at high current density.…”

Impact of Charge Transport Dynamics and Conditioning on Cycling Efficiency within Single Redox Active Colloids

“…[7,[11][12][13][14] With billions of redox sites per particle, RACs attain high capacity per molecule and high structural tunability while exhibiting molecular-like electrochemical properties. [15] RACs act as isolated reservoirs for charge, with a capacity determined by the particle size and the internal concentration of redox sites. [15] RACs act as isolated reservoirs for charge, with a capacity determined by the particle size and the internal concentration of redox sites.…”

“…[12] In SE-RFBs, RAC size is leveraged towards decreasing species crossover across nanoporous separators while alleviating the poor conductivity displayed by nonaqueous solvents. [15] RACs act as isolated reservoirs for charge, with a capacity determined by the particle size and the internal concentration of redox sites. However, the benefits and limitations of using a flowable particle for energy storage have not been fully elucidated.…”