|Schematic of reactant flow within the HBLFB. During discharge, liquid bromine is reduced to hydrobromic acid along the lower solid graphite electrode, and hydrogen is oxidized at the upper porous electrode. Credit: Braff et al. Click to enlarge.|
MIT researchers have engineered a new rechargeable, membrane-less hydrogen bromine laminar flow battery with high power density. The membrane-less design enables power densities of 0.795 W cm−2 at room temperature and atmospheric pressure, with a round-trip voltage efficiency of 92% at 25% of peak power.
That is about three times as much power per square centimeter as other membrane-less system—a power density that is an order of magnitude higher than that of many lithium-ion batteries and other commercial and experimental energy-storage systems. A paper on the work is published in Nature Communications.
Low-cost energy storage remains a critical unmet need for a wide range of applications, including grid scale frequency regulation, load following, contingency reserves and peak shaving, as well as portable power systems. For applications that require the storage of large quantities of energy economically and efficiently, flow batteries have received renewed attention. A wide variety of solutions have been proposed, including zinc-bromine and vanadium redox cells. This includes recent efforts to incorporate novel concepts such as organic electrolytes for greater voltage stability and semisolid reactants for higher reactant energy density or chemistries to reduce reactant cost.
One such permutation is the hydrogen bromine flow battery. The rapid and reversible reaction kinetics of both the bromine reduction reaction and the hydrogen oxidation reaction minimize activation losses, while the low cost ($1.39 kg-1) and abundance (243,000 metric tons produced per year in the United States alone) of bromine distinguishes it from many other battery chemistries. However, theoretical investigations of such systems have revealed that the perfluorosulfonic acid membranes typically used suffer from low conductivity in the absence of sufficient hydration. In the presence of hydrobromic acid, this membrane behaviour is the dominant limitation on overall performance.
...In this work, we present a membrane-less hydrogen bromine laminar flow battery (HBLFB) with reversible reactions and a peak power density of 0.795 W cm-2 at room temperature and atmospheric pressure. The cell uses a membrane-less design similar to previous work, but with several critical differences that allow it to triple the highest previously reported power density for a membrane-less electrochemical cell and also enable recharging.—Braff et al.
Laminar flow batteries—which rely on diffusion to separate reactants—eliminate the need for an ion-exchange membrane. In such a device, two liquids are pumped through a channel, undergoing electrochemical reactions between two electrodes to store or release energy. Under the right conditions, the solutions stream through in parallel, with very little mixing. The flow naturally separates the liquids, without requiring the costly membrane.
However, the authors note, although the laminar flow design has been explored for a variety of chemistries, none of those systems have achieved power densities as high as their membrane-based counterparts—largely because the chemistries already work well with existing, optimized membrane technologies.
Two new characteristics of the MIT team’s hydrogen bromine laminar flow battery (HBLFB) enable the high-power-density storage and discharge of energy at high efficiency:
The use of gaseous hydrogen fuel and aqueous bromine oxidant. This allows for high concentrations of both reactants at their respective electrodes, greatly expanding the mass-transfer capacity of the system.
Both reactions have fast, reversible kinetics, with no phase change at the liquid electrode, eliminating bubble formation as a design limitation.
Although the chemical reaction between hydrogen and bromine is promising for energy storage, earlier fuel-cell designs based on hydrogen and bromine have had mixed results, as hydrobromic acid tends to eat away at a battery’s membrane, effectively slowing the energy-storing reaction and reducing the battery’s lifetime. Without a membrane, however, that is not an issue.
This technology has as much promise as anything else being explored for storage, if not more. Contrary to previous opinions that membrane-less systems are purely academic, this system could potentially have a large practical impact.—Cullen Buie, assistant professor of mechanical engineering at MIT, co-author
The prototype HBLFB has a small channel between two electrodes. Through the channel, the researchers pumped liquid bromine over a graphite cathode and hydrobromic acid under a porous anode. At the same time, the researchers flowed hydrogen gas across the anode. The resulting reactions between hydrogen and bromine produced energy in the form of free electrons that can be discharged or released.
The researchers were also able to reverse the chemical reaction within the channel to capture electrons and store energy—a first for any membrane-less design.
The team operated the flow battery at room temperature over a range of flow rates and reactant concentrations and found the maximum power density of 0.795 W cm−2.
In addition to conducting the experiments, the researchers developed a mathematical model to describe the chemical reactions in a hydrogen-bromine system. Their predictions from the model agreed with their experimental results—an outcome that co-author Martin Bazant, a professor of chemical engineering, sees as promising for the design of future iterations.
The researchers estimate that the membrane-less flow battery may be able to cost as little as $100/kWh—a goal that the US Department of Energy has estimated would be economically attractive to utility companies.
This work represents a major advance of the state-of-the-art in flow batteries. To the best of the authors’ knowledge, the data presented here represent the highest power density ever observed in a laminar flow electrochemical cell by a factor of three, as well as some of the first recharging data for a membrane-less laminar flow electrochemical cell.
Although previous work has identified the appropriate scaling laws, the result presented here represents the first exact analytical solution for limiting current density applied to a laminar flow electrochemical cell, and serves as a guide for future designs. The HBLFB rivals the performance of the best membrane-based systems available today without the need for costly ion-exchange membranes, high-pressure reactants or high-temperature operation. This system has the potential to have a key role in addressing the rapidly growing need for low-cost, large-scale energy storage and high- efficiency portable power systems.—Braff et al.
William A. Braff, Martin Z. Bazant, and Cullen R. Buie. (2013). Membrane-less hydrogen bromine flow battery. Nature Communications. doi: 10.1038/ncomms3346