Sealed Recombinant Flow Batteries

Flow battery systems were developed in the late 1970s, and have received renewed interest for grid-scale energy storage [1]. No flow batteries, though, have achieved mature commercial status, and this is partly because of poor stability and lifetime. Many flow battery systems have complications that results from electrolyte imbalances that result from undesired side reactions.

Commercial lead-acid batteries have well-developed recombination systems that help extend battery lifetime and reduce maintenance [2]. This has not been the case for flow batteries though. For flow (and hybrid flow) batteries, there have been many proposed systems to deal with the electrolyte imbalances, but they are typically complicated open systems that require the continuous supply of rebalancing reactants such as hydrogen or chlorine, and which allow gas venting or the formation of other waste products. Typically, the reaction is carried out in a fuel cell or trickle-bed reactor, and it requires careful monitoring of state-of-charge, as well as a control system. Here, we show the behavior of all-iron flow battery systems [3] designed to be sealed and valve-regulated just like modern lead acid batteries. All-iron flow batteries charge and discharge according to Equation 1, and the undesired side reaction at the negative electrode is given by Equation 2.

3Fe²⁺ + 4e  ↔ Fe⁰ + 2Fe³⁺                                                                                     (1)

2H⁺ + 2e → H₂                                                                                                (2)

The recombination reaction in the all-iron battery, which is the same reaction originally used by NASA’s rebalancing fuel cell, is given by Equation 3.

H₂ + 2Fe³⁺ → 2Fe²⁺ + 2H⁺                                                                                      (3)

This reaction is spontaneous (E0 = 0.77 V), and occurs readily at room temperature over a catalyst. In the all-iron flow battery system, recombination can also function as a pH control system, mitigating problems associated with the formation of iron hydroxide precipitates such as those given by Equations 4 and 5.

Fe²⁺ + 2OH⁻   →  Fe(OH)₂,                                                                                      (4)

Fe³⁺ + 3OH⁻   →  Fe(OH)₃.                                                                                      (5)

In this study, we show that recombination in a sealed system can be used to keep the electrolyte balanced in terms of both iron and hydrogen species, without the need for an external reactants or control systems (see Figure 1).  The pressure in the electrolyte reservoir and the pH of the electrolytes rise during charge, and fall during discharge as the recombination occurs. These results represent a major step forward toward sealed recombinant flow batteries and shed new light on flow battery electrolyte dynamics.

Figure 1:  System dynamics in a recombinant all-iron hybrid flow battery during charge/discharge cycling.  System pressure – blue curve, Negative electrolyte pH – green curve, Positive electrolyte pH – red curve.