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Will organics take the lead in long-duration grid batteries?

This article is contributed by Penghui Ding

  • Aqueous organic redox flow batteries (AORFBs) are a newcomer to the field of grid storage, needed for the rapid expansion of renewable energy.
  • AORFBs have the potential to be low-cost, and the tunable nature of redox active organic molecules leaves a lot of room for optimization.
  • Further efforts are needed in terms of understanding the degradation mechanisms of organics and proving feasibility with large-scale pilot installations.

Realizing a green grid from renewable resources such as solar and wind requires new developments in advanced electrochemical energy storage (EES) technology. Taking America as an example, the Biden-Harris administration hopes to reach 100 percent carbon-neutral electricity, including an enormous amount of renewable generation, by 2035 [1]. The currently existing electrical grids were not designed for the kind of intermittent electricity generated by sunlight and wind, which can vary greatly depending on the time and location, but it may be possible to minimize the effect of variation on the grid using low-cost and efficient EES. While lithium-ion batteries (LIBs) dominate the EES market, they suffer from fire hazards, short cycle life, and rising lithium prices [2]. Redox flow batteries (RFBs), on the other hand, stand out from other EES technologies in terms of their long cycle life, low risk of fire, and decoupled energy and power. They may even be economically viable for long-duration stationary energy storage, as described in the United States Department of Energy’s (DOE) Energy Storage Market Report 2020 [3]. Of all the RFB chemistries reported, vanadium redox flow batteries (VRFBs) are the most mature technology currently deployed on the grid scale [4]. Nevertheless, there is one major barrier to adoption for VRFBs: the price (>$400/kWh) is much higher than the roughly $100/kWh target for commercialization. Consequently, almost a decade after their initial development, aqueous organic redox flow batteries (AORFBs) are once again drawing more attention.

Schematic of an AORFB [5]. Copyright 2020 John Wiley & Sons.

AORFBs use the same components as VRFBs, and may even be able to take advantage of existing facilities for VRFBs without significant alteration. AORFBs consist primarily of negolyte (anolyte) and posolyte (catholyte) tanks, electrodes and separators, and pumps. The organic active species with low and high potentials are called the negolyte and posolyte, respectively. The electrodes are industrially available carbon materials, such as carbon paper and carbon felts, and the separators used are typically Nafion and other cheaper porous membranes, such as biomass-based dialysis membranes. Organic or organometallic electroactive solutions are stored in each tank and external pumps keep the aqueous solution circulating continually. Redox reactions take place when negolyte and posolyte reach the electrodes inside the electrochemical cell, and the difference in the standard reduction potential between these reversible redox reactions defines the overall cell voltage.

The two key attributes that distinguish AORFBs from VRFBs are low electrolyte cost and precise tunability of the electrolyte structure through synthetic chemistry. Electrolyte cost is probably the most important factor in the attempt to reach commercial deployment. The cost of current VRFBs ranges between $400 and $600/kWh depending on power capacity and energy duration, over 4 times higher than the target of $100/kWh set by the DOE [6]. Techno-economic analysis estimates that the price of organic redox-active materials could go as low as $0.90/kg if produced in sufficiently large quantities, compared to the much higher price of V2O5, currently at $23.3/kg [7]. The underlying vanadium chemistry for VRFBs is nearly fixed, while AORFBs chemistries still have a lot of room for optimization in terms of the solubility, reduction potential, and the number of electrons transferred, all of which can improve the energy density. Active species permeation (crossover), which induces capacity decay and lower coulombic efficiency, still exists in VRFBs even when expensive Nafion separators are used [8]. With AORFBs, on the other hand, the molecular size of the organic electrolyte materials can be carefully tuned to decrease crossover via size exclusion (making the size of the organic active molecules larger than that of the separator pores), potentially even allowing for the use of much cheaper separators given an organic electrolyte of the proper molecular size.

AORFBs have received considerable attention since the pioneering work of Professors Michael J. Aziz and Roy G. Gordon of Harvard University in 2014 [9, 10]. The authors used the kinetically fast quinone-based molecules, anthraquinone-2,7-disulfonic acid (AQDS) and 2,6-dihydroxyanthraquinone (DHAQ) as negolytes in acidic and basic environments, respectively. Besides quinone-based electrolytes, other organic redox-active molecules such as nitroxide radical derivatives, viologen derivatives, organometallic materials, and heterocyclic aromatics have been reported by different groups.

These organic systems are generally believed to be unstable compared with vanadium chemistries owing to the inevitable degradation of the organic electrolytes. Over the decade-long course of operation at the industrial scale, a capacity decay of >0.1%/day would require frequent electrolyte replacement and repeated stopping and starting of the RFB system, which is not only expensive but potentially detrimental to grid stability. Therefore, it is of utmost importance to elucidate the intrinsic material degradation mechanisms and optimize the trade-off between material price and stability for commercial AORFBs.

The solubility of these unmodified organic materials is usually lower than 1 M in aqueous systems, especially in non-corrosive pH-neutral solutions, thus limiting the energy density. To address the limited-solubility problem, it is necessary to revisit the basic yet complicated solvation chemistry — solute-solute, solvent-solvent, and solute-solvent interactions. Although the aqueous environment brings about several advantages, such as low risk of thermal run-away, high electrolyte conductivity, and minimal supporting electrolyte costs, the cell voltage becomes limited by water splitting and negolytes and posolytes need to be designed with redox potentials at the edges of the electrochemical stability window. Anticatalytic electrode materials can be also tuned to decrease parasitic reaction rates, especially those of the kinetically preferred hydrogen evolution reaction. Developing organic molecules with multi-electron transfer processes might be another strategy to enhance the otherwise low energy density of AORFBs. However, most multi-electron transfer processes are kinetically sluggish and require catalytic electrodes. The use of additional anticatalytic or catalytic electrodes would push up the costs of AORFBs, especially a commercial system with thousands of stacks. Electrolyte composition is another factor affecting the stability window. For instance, AORFBs in pH-neutral solutions allow for water splitting to be inhibited more compared with the oft-used acidic solutions. Finally, the environmental footprints of these organic materials need to be carefully assessed. Given that a grid-scale AORFB might need several kilotons of materials per year, the organic materials need to be synthesized according to the principles of green chemistry and the toxicity of these organic materials after long-term operation needs to be assessed.

Although AORFBs were invented less than 10 years ago, several startups across the globe have already started to commercialize this technology. Most of the start-ups from the early days (2013–2015) of AORFBs were founded in Europe and only recently have the US and China started to test AORFBs at the industrial scale. All startups below claim 15+ years of calendar life and 10,000 cycles, longer than other battery systems. Most use neutral solutions to avoid the need for extra supporting electrolytes and to decrease corrosion. Since AORFBs are still relatively new, most of these companies use the organic electrolytes of their associated university labs [11]. Quinone derivatives are the most commonly employed organic materials, probably owing to their mature industrial manufacturing process. Most planned or completed AORFB projects have much less power or energy capacity compared to their vanadium counterparts. However, one AORFB project with a price tag of 158 million USD (1 billion CNY), announced in Suqian, China just last December, has promised the capability to store electricity at the GWh scale, over twice the capacity of the largest existing VRFB facility in Dalian, China. [12]. If successful, the Suqian project would be the perfect demonstration of the potential of AORFBs.

Existing AORFB startups

Although still considered a very new player in EES, AORFBs are advancing rapidly in both fundamental science and pilot demonstrations. Based on the many benefits described above, they will help enable the transition to a clean energy future.

*The company name is translated from Chinese (宿迁时代储能科技有限公司), since an official English name could not be found.

Penghui Ding is currently a third-year PhD student in Applied Physics with a focus in electrochemistry at the Laboratory of Organic Electronics at Linköping University, Sweden. His current interests are two-electron oxygen reduction to hydrogen peroxide and aqueous organic redox flow batteries, motivated by his love of quinone molecules and the Koutecký-Levich equation. When he is not in the lab, he enjoys running, reading books, and finding delicious food.

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