ESS Inc. designs, builds and deploys the most environmentally sustainable, lowest
Iron flow batteries. Long-duration energy storage (LDES) is the linchpin of the
The Iron Redox Flow Battery (IRFB), also known as Iron Salt Battery (ISB), stores and releases energy through the electrochemical reaction of iron salt. This type of battery belongs to the class of redox-flow batteries (RFB), which are alternative solutions to Lithium-Ion Batteries (LIB) for stationary applications. The IRFB can achieve up to 70% round trip energy efficiency. In comparison, other long duration storage technologies such as pumped hydro energy storage provide around 80% round trip energy efficiency [1].
The single cells are then stacked and electrically connected in series via bipolar plates, forming a battery stack.[3]
The energy storage is based on the electrochemical reaction of iron. During charge, iron(II) oxidizes to iron(III) in the positive half-cell (Reaction 1) while in the negative half-cell iron(II) is reduced to iron(0) (Reaction 2). The latter reaction is also called the plating reaction, as iron(0) is deposited on the negative electrode. During discharge, the plated iron(0) is dissolved into the electrolyte forming iron(II), while iron(III) reduces to iron(II) in the positive half-cell.[1]
Unwanted side reactions lead to coulombic efficiency and capacity loss because charge is irreversibly lost.
The acidic iron electrolyte can oxidize when it is in contact with air, therefore, mitigating measures need to be taken (e.g., operating under inert atmosphere) to prevent air oxidation (Reaction 4).[1][4]
Air oxidation: 4 Fe2+(aq) + O2 + 4 H+ → 4 Fe3+(aq)+ 2 H2O (4)
Further, Fe3+ can migrate through the separator and react with the plated Fe0 on the negative side forming Fe2+. This migration especially takes place when using a microporous separator (Reaction 5).[2]
Crossover reaction: Fe3+(aq)+ Fe0(s)⇌ 2 Fe2+(aq) (5)
During charge, hydrogen will evolve, as the standard potential of the hydrogen evolution reaction (HER) lies between the standard potential of Fe2+/Fe3+ and of Fe2+/Fe0. The acidic protons H+ in solution react to form hydrogen gas (Reaction 7) whilst iron(II) oxidises in the positive half-cell (Reaction 6). The HER is pH dependent. At lower pH values, the concentration of H+ is high, which increases the kinetics of the side reaction. Over time, the pH increases on the negative side. At a pH ≥ ~4, insoluble iron hydroxide forms and deposits onto the separator. This leads to increased resistance of ionic transfer, reduced coulombic and voltaic efficiency and ultimately cell failure.[2]
Positive half-cell: Fe2+(aq) → Fe3+(aq) + e− E0 = +0.77 V (6)
HER at negative half-cell: H+ + e− → ½ H2(g) E0 = 0.00 V (7)
Adding ascorbic acid to the electrolyte can reduce hydrogen evolution. Ascorbic acid enhances the coulombic efficiency by increasing the pH near the electrode, which improves iron deposition kinetics. Operating at 60°C with a pH of around 3 can achieve a high coulombic efficiency of 97.9%.[5][3]
The IRFB needs to operate at pH values below 3.5. The iron(III) salt precipitates at pH > 3.5 forming insoluble Fe(OH)3 which is also referred to as rust. However, at low pH values more hydrogen will evolve during charge on the negative side.[6] The coulombic efficiency can be increased by higher pH values.[5]
Hruska et al. studied the temperature effect on the performance of the IRFB. The voltaic efficiency increases at higher temperature due to higher electrolyte conductivity and a decrease in electrode polarisation.[1] Additionally, higher temperatures of ~60 °C improve the iron deposition kinetics in comparison to the hydrogen evolution reaction, thus increasing the coulombic efficiency.[7]
The IRFB can also operate at lower temperatures (~ 5 °C), however, the reaction kinetics are reduced, leading to lower voltaic efficiency.[4]
The base electrolyte consists of iron(II) salts which are dissolved in water. SO42- or Cl− are possible counter ions. Iron(II) chloride is often the preferred choice as the conductivity is higher than iron(II) sulphate. By increasing the ionic conductivity of the electrolyte, the voltaic efficiency, and thus the overall energy efficiency, can be increased. NH4Cl, (NH4)2SO4, KCl,[1] Na2SO4 and NaCl[8] are possible supporting additives.
Further additives were investigated to minimise rust precipitation. Complexing the iron salt with ligands can hinder the precipitation of Fe(OH)3 as the ligands stabilise the iron salt. Possible additives which were looked into are citrate, DMSO, glycerol, malic acid, malonic acid and xylitol.[7]
Buffer additives (e.g., ascorbic acid) help to maintain a constant pH during hydrogen production.[7] Additionally, these additives adsorb onto the active sites of the electrode, blocking these sites for the H+ adsorption and increase the overpotential for the hydrogen evolution reaction.[4][7]
One main challenge is to reduce the hydrogen evolution reaction. One method is through co-deposition of a different metal (e.g., cadmium), which can hinder the HER, and improve the coulombic efficiency during iron deposition.[7]
There have been different approaches to solving the issue with the HER. Additives in the electrolyte can reduce the production of hydrogen (see chapter Electrolyte), however, additives cannot fully eliminate the HER. Therefore, alternative solutions are proposed in literature.
The counter reaction of HER can be achieved in a chemical or electrochemical manner. Chemical solutions are trickle-bed reactors[9] or in-tank hydrogen-ferric ion recombination systems.[10] An electrochemical approach is coupling a hydrogen-iron fuel cell to the IRFB. This can bring the IRFB back to the original state of health.[2][11]
The trickle-bed reactor is a chemical reactor with a packed bed containing a catalyst (e.g. Platin). This type of rebalancing system is coupled to the IRFB. The electrolyte from the IRFB is flushed into the packed bed from the top of the reactor whilst hydrogen gas is forwarded from the bottom. At the three-phase-boundary (catalyst, hydrogen gas, electrolyte) the chemical reaction between excess iron(III) and hydrogen takes place forming iron(II) and H+. The excess gas removed from the trickle-bed reactor and the electrolyte is then pumped back into the IRFB.[9]
The in-tank rebalancing system is also based on the chemical reaction of iron(III) and H2, but takes place in the positive tank of the IRFB. Hydrogen produced within the negative half-cell is forwarded from the negative to the positive tank. A felt is positioned perpendicular to the liquid level into the positive electrolyte. The upper part is coated with a catalytic layer (e.g. Platinum). Through capillary effect, the positive electrolyte flows through the felt to the catalytic layer. Here, at the three-phase boundary (catalyst, H2, Fe3+) the chemical reaction takes place forming H+ and Fe2+.[10]
A different option is to couple the IRFB with a hydrogen-iron fuel cell. The produced hydrogen from the IRFB is forwarded to the negative side of the rebalancing fuel cell system whilst the electrolyte of the IRFB is pumped to the positive side. On the negative side, the hydrogen reacts to acidic protons (H+) at a catalytic layer (e.g., Platinum, Palladium). On the positive side, the excess Fe3+ is reduced to Fe2+ (Reaction 8).[12]
Rebalancing Reaction: 2 Fe3+(aq) + ½ H2 (g) → 2 Fe2+(aq) + H+ (8)
The advantage of redox-flow batteries in general is the separate scalability of power and energy, which makes them good candidates for stationary energy storage systems.[2] This is because the power is only dependent on the stack size while the capacity is only dependent on the electrolyte volume.[4]
As the electrolyte is based on water, it is non-flammable. All electrolyte components are non-toxic and abundantly available. The reactants in both half-cells are soluble salts of the same species and only differ in their oxidation state (Fe0, Fe2+, Fe3+). This means that unwanted membrane crossover of the active species does not lead to irreversible reactant loss,[1] but can be rebalanced using either a trickle-bed reactor or a fuel cell.[2][9] Iron chloride is cheaply and widely available as it is a by-product of steel production.[13]
The IRFB is stable within different temperature ranges, therefore, the stationary energy storage can be used in regions with higher temperature without the need of a thermal management system. [5] The battery efficiency would even benefit from higher temperatures. Other battery types (e.g. Vanadium-Redox-Flow Batteries (VRFB)) cannot perform at higher temperatures. For instance, toxic Vanadium pentoxide (V2O5) in VRFBs precipitates at ~ 40 °C.[14]
Overall, the components are low in cost (2 $/kg iron) and abundantly available. All the other parts (e.g. membrane, bipolar plate, monopolar plate, frames, gaskets, pumps) are widely available on the market and associated costs can be expected to decrease as production of these batteries scales up.
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