With commercial development of redox flow batteries (RFBs) underway and given the importance of capital cost, we analyze RFB supporting electrolyte charge carrier, membrane, open-circuit voltage (OCV), cell performance, and active species size as possible major cost drivers. Physics-based and techno-economic modeling includes a porous electrode model and accounts for chemical and material selection and amounts, stack and tank sizing, and flow-dependent pumping power.¹ The 1D porous electrode model, validated in a prior study,² couples mass transfer, charge transfer, and ohmic transport to estimate the electrode contribution to full cell area specific resistance (ASR).

Primary findings for a 5-h discharge of an aqueous RFB are that present costs are in general $10 ~ 100/kWh lower with 1.4 V versus 1.0 V OCV, size selective separators (SSS) versus ion exchange membranes (IEM), as shown in Figure 1a. Large active species size alone would increase costs slightly due to higher viscosity and thus higher pumping cost, reduced mass transfer, and higher mass-based electrolyte cost versus lower molecular weight actives. However, large actives enable SSS use, and the slight incremental cost is more than offset by cost reductions from the high conductivity and inherent low cost of SSS membranes. In the future, the costs of different scenarios converge assuming significant cost reductions in the stack, membranes, and unit cost less materials. In Figure 1b, multiple options, led by H⁺/IEM with small active species and OCV > 1.3 V, can meet a Joint Center for Energy Storage Research target adjusted to $100/kWh for 5-h storage.

Implications of the work here are: 1) recommendations on the path to enable low cost aqueous RFBs, and 2) a modeling tool and opportunity to quantify cost implications of additional factors held constant in the present study. One recommendation is the further exploration of IEMs using H⁺ or Na⁺ and SSS, as these membranes have relatively favorable likelihoods to enable system cost targets. We also suggest development work towards redox couples and conditions for high OCV, which enhances electrolyte energy content and stack power. Subsequently, engineering effort and adoption of RFBs is needed to enable significant reduction in cost through high volume manufacturing of stacks, membranes, and overall RFB systems. Lastly, one can use the present model to explore the effect of various additional parameters on cost. Such parameters include: the cell components (e.g., electrode properties, membrane thickness) affecting ASR, the operation (e.g., flow rate, V-I operating point) affecting ASR, and the application (e.g., discharge duration) affecting power-related cost (e.g., stack) versus energy-related cost (e.g., electrolyte volume, tank size).

Acknowledgments

This work was supported by the Joint Center for Energy Storage Research (JCESR), an Energy Innovation Hub funded by the United States Department of Energy. JDM acknowledges additional support from the NSF Graduate Research Fellowship.

References:

1. J. D. Milshtein, R. M. Darling, J. Drake, M. L. Perry, and F. R. Brushett, J. Electrochem. Soc., 164, A3883 (2017).
2. J. D. Milshtein, K. M. Tenny, J. L. Barton, J. Drake, R. M. Darling, and F. R. Brushett, J. Electrochem. Soc., 164, E3265 (2017).