Towards Low Resistance Nonaqueous Redox Flow Batteries

Abstract

Nonaqueous redox flow batteries (NAqRFBs) are a promising, but nascent, concept for cost-effective grid-scale energy storage. While most studies report new active molecules and proof-of-concept prototypes, few discuss cell design. The direct translation of aqueous RFB design principles to nonaqueous systems is hampered by a lack of materials-specific knowledge, especially concerning the increased viscosities and decreased conductivities associated with nonaqueous electrolytes. To guide NAqRFB reactor design, recent techno-economic analyses have established an area specific resistance (ASR) target of <5 cm 2 . Here, we employ a state-of-the-art vanadium flow cell architecture, modified for compatibility with nonaqueous electrolytes, and a model ferrocene-based redox couple to investigate the feasibility of achieving this target ASR. We identify and minimize sources of resistive loss for various active species concentrations, electrolyte compositions, flow rates, separators, and electrode thicknesses via polarization and impedance spectroscopy, culminating in the demonstration of a cell ASR of ca. 1.7 cm 2 . Further, we validate performance scalability using dynamically similar cells with a ten-fold difference in active areas. This work demonstrates that, with appropriate cell engineering, low resistance nonaqueous reactors can be realized, providing promise for the cost-competitiveness of future NAqRFBs. Grid-scale energy storage has emerged as a critical technology for alleviating the intermittency of renewables, 1 improving the efficiency of the existing grid infrastructure, and offering frequency or voltage regulation.2 Redox flow batteries (RFBs) are particularly attractive storage devices for energy-intensive grid applications.2,3 In a typical RFB, charge-storing active species are dissolved in liquid electrolytes, which are housed in large and inexpensive tanks. The electrolytes are pumped through a power-generating electrochemical reactor, where the active species undergo reduction or oxidation on the surfaces of porous electrodes to charge or discharge the battery. [3][4][5][6] This unique architecture offers several advantages over conventional battery systems, including decoupled power and energy scaling, long operational lifetimes, easy maintenance, simplified manufacturing, and high active-to-inactive materials ratio (particularly at long storage durations). Despite many attractive features, state-of-the-art RFBs are currently too expensive for widespread adoption.7 While the majority of RFB electrolytes are aqueous, 6 transitioning to nonaqueous chemistries could enable higher cell potentials via wider electrolyte stability windows, subsequently increasing the electrolyte energy density and reducing chemical costs. 6,[8][9][10][11] Furthermore, the use of nonaqueous solvents enables a broader palette of potentially inexpensive active materials, which could significantly lower RFB costs. 7,12 In contrast to their aqueous counterparts, nonaqueous redox flow batteries (NAqRFBs) are a nascent technology,...

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

“…0.62 -1.4 cm 2 , which is in general agreement with our prior work. 33 The range in these values is attributable to varying contact resistance contributions between the different electrodes and the graphite flow field, which may be due in part to the differences in the degree of mechanical compression (Table S1). 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.…”

“…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.…”

“…The mass transport in RFBs is an area of growing interest and is increasingly recognized as a performance-limiting factor [28][29][30][31] for both aqueous 32 and non-aqueous systems. 29,33 Moreover, many emerging redox chemistries, particularly those based on organic couples, [34][35][36] have high kinetic rate constants (k 0 > 10 −3 cm s −1 ), and thus reduced activation overpotentials which may obviate the need for modifications of the electrode surface, at least for the purpose of enhancing redox reaction rates. To date, most of the published literature on this topic has focused on understanding and optimizing flow field design, [28][29][30][37][38][39] though there have been some efforts to describe mass transport within given electrode materials 29,40 as well as to engineer electrode structure post-process.…”

Performance Optimization of Differential pH Quinone-Bromide Redox Flow Battery

“…By decreasing the number of the electrodes from 10 to 5, the porosity/compression remains constant but the contact resistance is decreased as seen by the relatively small decrease of R ei . [27][28][29][30][31][32][33] Further reduction to 3 carbon papers, increases R ei slightly which is somewhat counterintuitive. However the increase was very small (0.03 cm 2 ) and we anticipate that the higher compression leads to a lower solution uptake in the membrane and hence a lower ion conductivity.…”

Repurposing the Industrial Dye Methylene Blue as an Active Component for Redox Flow Batteries

“…A unique aspect of RFBs is the spatial separation of the electrodes from the reservoirs serving to decouple energy (stored capacity) and power (energy released per unit time). A moderate library of electrolyte materials including complexes, [28][29][30][31][32][33][34][35][36] organic compounds, [36][37][38][39][40][41] and redox active polymers [42][43][44] has been explored as charge-carriers in RFBs; however, the broad-scale implementation of these devices is hindered by the lack of suitable charge-carriers.…”