Organic Flow Batteries: Electrochemical Processes for Energy Technology

Skip to these sections:

  1. The Energy Challenge
  2. Flow Batteries: Massive Electrochemical Energy Storage for the Grid
  3. A Metal-Free Organic-Inorganic Aqueous Flow Battery
  4. An Alkaline Quinone Flow Battery
  5. A Vitamin Inspired Redox Flow Battery
  6. A Long-Lifetime, Neutral pH Flow Battery
  7. Fun Facts about Quinones!
  8. TV, Radio and Print Media Coverage
  9. Recommended Readings

The Energy Challenge

Finding the energy to power human civilization without ruining the environment is the greatest challenge facing humanity this century. World energy demand is projected to double by mid-century and triple by the end of the century. The overwhelming majority of this power is provided by the combustion of fossil fuels. Their combustion emits CO2 into the atmosphere. This must be ramped down and replaced by carbon-free sources of energy, such as wind, solar, biomass, and maybe nuclear.

Flow Batteries: Massive Electrochemical Energy Storage for the Grid

The ability to inexpensively store large amounts of electrical energy is of increasing importance with the growing fraction of electric generation from intermittent renewable sources such as wind and solar. As this fraction increases, problems associated with the mismatch between power supply from wind and solar and grid demand (Fig. 1) become more severe. We analyzed the requirements that storing photovoltaic or wind power place on a storage device in order to provide power when it is needed and the wind isn't blowing and the sun isn't shining [Ref. mja212]. We did this for a simplified, prototypical storage device in which the instantaneous losses are proportional to the instantaneous current (e.g. Ohmic losses). In this case, any device can be characterized by a single scalar figure of merit – its peak discharge power capability. It then shows how much storage power and storage energy is needed per MW of wind or solar power, for any specified round-trip system efficiency.

Fig 1. A: Electrical power calculated using the performance curve of a commercial wind turbine from real wind speed data, showing a windy week, an intermediate week, and a calm week. B: Solar PV power output, showing sunny days, cloudy days, and -- of course -- dark nights. C: Demand on the electrical grid. The problem: A and B don't look like C. What would it take to solve this problem with storage? [mja212]

 

Our analysis shows that traditional (solid-electrode) batteries are grossly inadequate for this role. They need to be able to discharge at full power for tens of hours whereas they are typically drained after tens of minutes. Much more promising are flow batteries, in which the power capability and the energy storage capability can be sized independently, thus enabling tens of hours, or even days, of discharge at peak power. Our current research is aimed at developing breakthrough chemistries and materials for flow batteries, which could be used for massive electrical energy storage -- up to grid-scale. In a flow battery, the energy is stored in fluids held externally in arbitrarily large, inexpensive storage tanks. These fluids pass by the electrodes where they undergo an electrochemical redox reaction before flowing back out to the tanks. The electrodes, whose size is designed to match the required power delivery capacity, don't participate in the reaction, although they may catalyze it.

A Metal-Free Organic-Inorganic Aqueous Flow Battery

 

Fig. 2. A flow battery.  (a) Schematic of a discharging metal-free organic flow battery. Reactions are reversed during charging. (b) Energy-storing chemicals (hydroquinone/quinone and bromine/hydrobromic acid) are held in containers (background) and are pumped through the energy conversion hardware (foreground), converting chemical energy to electrical energy and vice versa. In order to store more energy, the energy conversion hardware does not need to change: only the container size and the amount of chemicals needs to increase.

The opportunities are immense in organic-based energy storage systems. In particular, a class of molecules known as quinones (Fig. 2(a)) provides a number of benefits when used in a battery system due to their special properties. Quinones undergo rapid and reversible two-electron, two-proton redox reactions in aqueous solution (Fig. 3). A big advantage of organic molecules is the opportunity to tune their physical properties including solubilities and redox potentials with constituent substitutions such as the –R1 and –R2 groups in Fig. 3(b); the –R1 groups can enhance the solubility and the –R2 groups can shift the redox potential to a more negative voltage.

Fig. 3. Cyclic voltammetry of quinone oxidation and reduction. (a) Schematic of cyclic voltammetry measurement setup. (b) A plot of the cycle, starting at the right, swinging to low potential and then back again to high. The presence of various R­2 groups (dashed voltammogram) shifts the reduction potential to more negative voltage.

As described in a publication appearing in Nature in January 2014, we have successfully implemented a quinone in a flow battery using the molecule 9,10-anthraquinone-2,7-disulphonic acid (AQDS) on one side and bromine on the other side [Ref. mja240] (Fig. 2). In our more recent reports, the cell is capable of reaching galvanic power densities of 1.0 W/cm2: this is a world-class power density and we haven't even fully optimize it yet!  The cell has been cycled about 800 times (so far) without any sign of degradation, indicating the robustness of the molecules. Current efforts are geared towards evaluating cycle life, increasing the cell voltage, and synthesizing other quinones with desirable properties for use in an all-quinone flow battery.

An Alkaline Quinone Flow Battery

Fig. 4. (a) Schematic of an alkaline quinone flow battery for renewable energy storage. (b) Cyclic voltammogram showing reversible redox reactions of 2,6-DHAQ negolyte and non-toxic ferrocyanide posolyte indicating an open-circuit voltage of 1.2 V.

Most recently, we have reported exciting results in Science [Ref. mja255] describing an alkaline quinone flow battery with non-toxic electrolytes, which makes it ideally suited for cost-effective storage for homes and small businesses. It uses 2,6-dihydroxyanthraquinone (2,6-DHAQ) in the negolyte opposite a food additive, ferrocyanide, in the posolyte. This halogen-free chemistry also increases our open-circuit potential by almost 50% over previous quinone-based flow batteries.

A Vitamin Inspired Redox Flow Battery

Vitamin-Inspired Battery

Fig. 5. (a) Schematic of ACA-Fe(CN)6 cell showing processes occurring during discharge. Gray arrows show electron flow, white arrows show reactant flow, and blue arrow shows counter-ion flow(b) Photo of battery (right foreground), negative electrolyte composed of ACA in alkaline solution immersed under water to keep air out (left foreground), and pumps (background).

We have introduced a new class of redox-active organic molecules—aza-aromatics—into aqueous flow batteries for safe, low-cost, large-scale electricity storage. In this class of molecules nitrogen atoms, rather than the oxygen atoms in quinones, are involved in the delocalized orbitals that give off and pick up electrons in oxidation and reduction. The prototype, published in Nature Energy, is alloxazine 7/8 carboxylic acid (ACA). We started with the organic micronutrient riboflavin (vitamin B2) and gave it just a couple of tweaks that make it more stable. ACA has a very high solubility in alkaline solution. It performs well as a negative electrolyte opposite a conventional ferrocyanide positive electrolyte and gives current efficiencies and capacity retentions >99.7% and >99.98%/cycle, respectively, over hundreds of cycles. This novel storage chemistry further highlights the great potential of organic-based redox flow batteries for large-scale energy storage.  Whether aza-aromatics turn out to be more competitive than the quinones (on which our research continues) likely depends on currently-unanswered questions about long-term stability (calendar life) and large-scale manufacturing cost.

A Long-Lifetime, Neutral pH Flow Battery

Figure 6. (a) Cycling of a BTMAP-Vi/BTMAP-Fc pH 7 cell at 50 mA cm−2 in the presence of an excess of reduced BTMAP-Vi such that the posolyte is capacity-limiting. Blue traces, left axis: Evolution of the cell charge and discharge capacity as functions of cycle number. Red trace, right axis: Coulombic efficiency of the cell for each charge−discharge cycle. The red line is the best fit line to the discharge capacity of the cell between cycles 1 and 500. (b) Representative voltage vs time traces of selected cycles. Inset: Magnification of the potentiostatic regions of each charge−discharge cycle, showing the evolution of the potentiostatic capacity of the cell with cycle number. Inset traces are vertically offset from each other for clarity.

Continuing on the development of organic and organometallic flow batteries, we have reported a new set of active molecules that perform well at high concentrations simply dissolved in water creating a neutral pH electrolyte solution. This work, published in ACS Energy Letters introduces viologen and ferrocene monomers with bis(trimethylammoniopropyl) (called BiTMAP) functionalization for increased solubility and stability while minimizing crossover. In contrast to acidic and basic chemistries, pH neutral electrolytes enable a greater variety of inexpensive, chemically compatible components, which greatly enhances both their safety and opportunity for decreased equipment costs. Furthermore, by selecting compatible ion-selective membranes, this battery 99.9989% of its storage capacity per cycle. We believe this to be the highest retention rate per cycle and per day for any flow battery work ever published – non-aqueous or aqueous, organic or inorganic, monomer or polymer, acidic, alkaline, or neutral. The combination of a neutral pH electrolyte with high capacity retention extrapolating to many thousands of cycles represents an exciting opportunity for enabling mega-scale energy storage.

Fun Facts about Quinones!

  1. Hydroquinone cream is used to bleach dark spots, moles, etc off of skin.
  2. Rhein (1,8-dihydroxy-3-carboxy-anthraquinone) is obtained from Rhubarb and used as a laxative and an antibacterial agent.
  3. Plastoquinone shuttles electrons as part of photosystem II, and is found in all green plants.
  4. Blatellaquinone is a sex pheromone female cockroaches use to attract males.

 

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TV, Radio, Print Media Coverage

Recommended Readings

mja240: B. Huskinson, M.P. Marshak, C. Suh, S. Er, M.R. Gerhardt, C.J. Galvin, X. Chen, A. Aspuru-Guzik, R.G. Gordon and M.J. Aziz, "A metal-free organic-inorganic aqueous flow battery", Nature 505, 195 (2014). This paper started the field of organic aqueous flow batteries. We introduced a new approach to inexpensive, large-scale, stationary electrical energy storage by exploiting the favorable properties of a family of redox-active organic molecules known as quinones. These chemicals can exhibit energy and power performance comparable to vanadium, which is the basis of the incumbent flow-battery technology, but have the potential to be much less expensive at mass-production scale. We demonstrate a quinone – hydrobromic acid flow battery with high power density and plenty of headroom for further performance improvements. Click here to see the paper.

mja255: K. Lin, Q. Chen, M.R. Gerhardt, L. Tong, S.B. Kim, L. Eisenach, A.W. Valle, D. Hardee, R.G. Gordon, M.J. Aziz and M.P. Marshak, "Alkaline Quinone Flow Battery", Science 349, 1529 (2015). In this paper we describe the first high-performance, non-flammable, non-toxic, non-corrosive, and low-cost chemicals for flow batteries and report the performance of a flow battery employing them. We modified the quinone structure to make it highly soluble in an alkaline solutions which is safer than acidic solutions. We paired the modified quinone, at the negative terminal of the battery, with a food additive, ferrocyanide, at the positive terminal. This chemistry appears to be particularly well suited for storage of rooftop PV-generated electricity in homes and businesses. Click here to see the paper.

mja266: E.S. Beh, D. De Porcellinis, R.L. Gracia, K.T. Xia, R.G. Gordon  and M.J. Aziz, "A Neutral pH Aqueous Organic/Organometallic Redox Flow Battery with Extremely High Capacity Retention", ACS Energy Letters 2, 639 (2017). In this paper we report a new aqueous flow battery chemistry operating in neutral pH conditions, thereby promising very inexpensive containment and reducing the need for highly corrosion-resistant battery components. This battery retains 99.9989% of its capacity per charge-discharge cycle and 99.967% of its capacity per day. These figures represent the highest capacity retention rates for any flow battery work ever published. The positive terminal utilizes a functionalized ferrocene molecule and the negative terminal utilizes a functionalized viologen molecule. Although we were not the first to introduce ferrocenes or viologens into aqueous flow batteries, we determined the decomposition mechanisms limiting the molecular lifetime of shorter-lived cousins and designed functionalizations that suppressed those mechanisms. Although the open-circuit voltage of 0.75 V is too low for economical deployment, this work showed that it is possible to engineer molecules for long lifetime in a flow battery which, to be practical, must operate for a decade or more. Click here to see the paper.

mja279: M.-A. Goulet & M.J. Aziz, “Flow Battery Molecular Reactant Stability Determined by Symmetric Cell Cycling Methods”, J. Electrochem. Soc. 165, A1466 (2018). In this paper we introduce a new flow battery testing protocol that permits high-precision measurements of very low capacity fade rates. We use it test several organic and organometallic redox couples and to show that the capacity fade rate of organic aqueous flow batteries is dominated by the calendar life of the molecules and not by the number of charge-discharge cycles the molecules have undergone. This work shows that the greatest challenge facing organic aqueous flow batteries is engineering the organic molecules for decadal calendar life. Click here to see the paper.

mja280: D.G. Kwabi, K. Lin, Y. Ji, E.F. Kerr, M.-A. Goulet, D. DePorcellinis, D.P. Tabor, D.A. Pollack, A. Aspuru-Guzik, R.G. Gordon, and M.J. Aziz, “Alkaline Quinone Flow Battery with Long Lifetime at pH 12” Joule 2, 1907 (2018). In this paper we report an engineered flow battery chemistry that appears to be the first to meet all the technical requirements for practical deployment: long calendar life, open-circuit voltage above 1.0 V, and respectable energy and power densities. The capacity retention rate is second only to that reported by Beh et al. in publication #266 (A Neutral pH Aqueous Organic/Organometallic Redox Flow Battery with Extremely High Capacity Retention). Other highlights include a low-corrosivity, weak alkaline electrolyte a very inexpensive, non-fluorinated membrane that will greatly reduce the cost per kW of power generating capacity. Click here to see the paper.