Air-Breathing Aqueous Sulfur Flow Battery for breakthrough ultralow cost energy storage

Joule Journal – Air-Breathing Aqueous Sulfur Flow Battery for Ultralow-Cost Long-Duration Electrical Storage

•Chemical cost analyzed for 40 rechargeable couples developed over the past 60 years
•Aqueous sulfur/sodium/air system identified with ultralow chemical cost of ∼US$1/kWh
•Air-breathing flow battery architecture demonstrated at laboratory scale
•Techno-economic analysis shows installed cost is comparable with PHS and CAES

Above – Curves for the present air-breathing aqueous sulfur flow battery approach using Na and Li chemistry are shown in green and gray, respectively. The chemical costs for Na and Li are shown as dashed lines. Curves of constant power cost show that the power stack dominates the system cost at short storage durations, whereas at long duration the cost asymptotically approaches the energy cost due to chemical constituents plus storage tank and related costs. 5 M concentrations of both Na and S are assumed, with cycling of the sulfur over the speciation range S22− to S42− corresponding to 25% of theoretical capacity. The peak power density of the stack ranges from 4.3 mW/cm2 at US$4,000/kW and US$2,000/kW to 28.6 mW/cm2 at US$150/kW. The projected costs for air-breathing aqueous sulfur compare favorably with those for pumped hydroelectric storage (PHS) and underground compressed air energy storage (CAES), while also having several-fold higher energy density and being free of the locational constraints faced by each.

Context and Scale
Wind and solar generation can displace carbon-intensive electricity if their intermittent output is cost-effectively re-shaped using electrical storage to meet user demand. Reductions in the cost of storage have lagged those for generation, with pumped hydroelectric storage (PHS) remaining today the lowest-cost and only form of electrical storage deployed at multi-gigawatt hour scale. Here, we propose and demonstrate an inherently scalable storage approach that uses sulfur, a virtually unlimited byproduct of fossil fuel production, and air, as the reactive components. Combined with sodium as an intermediary working species, the chemical cost of storage is the lowest of known batteries. While the electrical stacks extracting power can and should be improved, even at current performance, techno-economic analysis shows projected costs that are competitive with PHS, and of special interest for the long-duration storage that will be increasingly important as renewables penetration grows.

The intermittency of renewable electricity generation has created a pressing global need for low-cost, highly scalable energy storage. Although pumped hydroelectric storage (PHS) and underground compressed air energy storage (CAES) have the lowest costs today (∼US$100/kWh installed cost), each faces geographical and environmental constraints that may limit further deployment. Here, we demonstrate an ambient-temperature aqueous rechargeable flow battery that uses low-cost polysulfide anolytes in conjunction with lithium or sodium counter-ions, and an air- or oxygen-breathing cathode. The solution energy density, at 30–145 Wh/L depending on concentration and sulfur speciation range, exceeds current solution-based flow batteries, and the cost of active materials per stored energy is exceptionally low, ∼US$1/kWh when using sodium polysulfide. The projected storage economics parallel those for PHS and CAES but can be realized at higher energy density and with minimal locational constraints.

The Na chemical cost is US$1.7/kWh, and after accounting for tank cost (US$0.15/L) and other costs, the resulting energy cost for Na chemistry is a factor of 4–6 higher than the chemical cost alone. Thus, as with PHS and CAES, the energy-storing fluids have lower cost than the structures used to contain them.

The largest contributions to power cost come from the membrane and catalyst. Our current experiments use ceramic membranes, for which the US DOE has projected a cost, at high production volumes, of less than US$10/m2.39 In the cost model, we conservatively assume costs of US$10/m2 and US$100/m2. Alternative low-cost membranes such as polymer-ceramic composites40 could also be developed for this application. The total PGM catalyst loading is assumed to be 0.05 mg/cm2; the experimentally validated value in Figure 7 is 0.03 mg/cm2 for the Pt black alone.

This work demonstrates a new electrochemical storage approach that uses an aqueous polysulfide anolyte in conjunction with an air-breathing catholyte to reach exceptionally low chemical cost of storage (∼US$1/kWh) while providing moderately high energy density (29–121 Wh/L at the solution level). The chemical cost of stored energy is one of, if not the, lowest among known rechargeable batteries. Implemented in a flow battery architecture, this approach could offer the cost/performance characteristics of PHS and CAES, today the lowest-cost and most widely scaled energy storage technologies, while being free of geographical and environmental constraints and having up to a 1,000 times higher energy density at system level. Techno-economic modeling shows that at the current stage of development, stack power cost is the limiting cost factor. A modest reduction in stack resistance over current laboratory results would allow power cost of US$1,000– 2,000/kW to be reached, competitive with PHS and CAES. This is achievable while using ceramic membranes and PGM catalysts as in the current experiments. Nonetheless, the scalability and cost of the flow battery could be significantly improved through development of low-cost low-resistance membranes, such as ceramic/polymer composite membranes, and non-PGM OER and ORR catalysts. With further development, a new ultralow-cost electrochemical storage option may become available to support the growth of intermittent renewable generation and decarbonization of the world’s energy systems.