The Elestor solution
The choice of hydrogen and iron
Elestor is powered by a mission to build a storage system with the lowest possible storage costs per MWh. With this as our cornerstone criteria, and it is one that can only be met with inexpensive chemistries if we are to take full advantage of the typical flow battery features, our behind the scene R&D wizards have explored a variety of chemistry combinations.
In theory, there are many different chemistries that could be used to design a flow battery, but to date we have only come across two that could work in the real world. One relies on hydrogen and bromine as active materials, and this solution remains great on paper, but we have come to the conclusion that there is another, even better so-called redox coupling, namely hydrogen-iron, which is better suited to real-world applications and the present geopolitical environment.
Like bromine, iron is obviously abundantly available all over the world; indeed, arguably slightly more so than bromine, which is extracted from sea water and thus not abundant far from the sea. The importance of selecting a redox coupling made up of two materials that are not restricted by geographical availability has become even more important during the last couple of years. Given the ongoing geopolitical turbulence, it is more important than ever that we create a resilient and self-reliant energy storage solution that is independent from and cannot be dominated by a small group of suppliers, or indeed by a single or a small number of countries. Iron, hydrogen and bromine all have these qualities, unlike other materials such as lithium, cobalt and vanadium, which are presently more popular with makers of battery solutions in spite of their scarcity.
By going for abundant rather than scarce materials, we have by definition chosen very low-priced active materials. Not only presently, but for decades to come, indeed, even when large volumes will be required for high-volume production of hydrogen-iron flow batteries.
Another advantage of selecting a hydrogen and iron-sulphur redox couple, is that these enable a high power density [W/m2] as well as a high energy density [kWh/m3], both contributing to the reduction of storage costs per MWh.
Sure, this combination’s power and energy density is not quite as impressive as that of a hydrogen and bromine redox couple, but it still beats all other rival flow battery chemistries, while at the same time it comes with several other advantages.
The heart of all Elestor’s storage systems is the cell stack. This stack consists of a number of individual electrochemical cells, as shown above, connected in series.
Each membrane in this stack is in contact with the electrolyte circuit, an aqueous iron-based solution (FeSO4).
On the other side, each membrane is in contact with a hydrogen (H2) gas circuit. Both active materials circulate in a closed loop along their own respective side of the cell. The electrolyte (aqueous FeSO4 solution) and hydrogen (H2) circuits are separated by a proton-conductive membrane.
Long-term performance of hydrogen-bromine flow batteries using single-layered and multi-layered wire-electrospun SPEEK/PFSA/PVDF membranes
Sanaz Abbasiab, Yohanes Antonius Hugob, Zandrie Bornemanac, Wiebrand Koutb and Kitty Nijmeijer*ac
aMembrane Materials and Processes, Department of Chemical Engineering and Chemistry, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands. E-mail: D.C.Nijmijer@tue.nl
bElestor BV P.O. Box 882, 6800 AW Arnhem, The Netherlands
cDutch Institute for Fundamental Energy Research (DIFFER), P.O. Box 6336, 5600 HH Eindhoven, The Netherlands
Abstract
Sulfonated poly (ether ketone) (SPEEK), perfluorosulfonic acid (PFSA), and polyvinylidene fluoride (PVDF) were wire-electrospun. Subsequently, multiple electrospun layers in different arrangements were hot-pressed into sustainable membranes for use in hydrogen-bromine flow batteries (HBFBs). The relationship between the electrospun layer composition and arrangement, membrane properties, and battery performance was explored. Wire-electrospinning and hot-pressing improved SPEEK and PFSA/PVDF compatibility, yielding dense membranes. Higher SPEEK contents lead to rougher morphologies, while the insulating nature of PVDF decreases the ion exchange capacity (IEC) and HBr uptake compared to commercial PFSA. The multi-layer assembly negatively impacted the membrane transport properties compared to the single-layer arrangement. Although wire-electrospinning improves the polymer dispersion and fixed charge density, SPEEK-rich regions of the blend membranes lack the high selectivity of PFSA, thus reducing the ionic conductivity. This is especially clear in the multi-layer membranes with accumulated SPEEK in the intermediate layer in the through-plane direction. Following initial property comparisons, thinner wire-electrospun SPEEK membranes were prepared with area resistance in the PFSA-comparable range. Among the wire-electrospun SPEEK/PFSA/PVDF membranes, the single-layered membrane with 8 wt% SPEEK (SPF1-8; 62 μm) displayed stable HBFB performance at 200 mA cm−2 over 100 cycles (64 cm2 active area). Based on the ex-situ measurements and cell performance results, a total of ∼10.5 wt% SPEEK is suggested as the limit for both single and multi-layered wire-electrospun membranes, combined with a maximum membrane thickness of ∼50 μm. This ensures robust HBFB performance, positioning wire-electrospun SPEEK/PFSA/PVDF membranes as a PFSA alternative in energy storage.
Wire based electrospun composite short side chain perfluorosulfonic acid/ polyvinylidene fluoride membranes for hydrogen-bromine flow batteries
Yohanes Antonius Hugo a, b, Wiebrand Kout b, Antoni Forner-Cuenca a, Zandrie Borneman a, c, Kitty Nijmeijer a, c, *
a Membrane Materials and Processes, Department of Chemical Engineering and Chemistry, Eindhoven University of Technology, PO Box 513, 5600MB Eindhoven, the Netherlands
b Elestor B.V., 6827 AV Arnhem, the Netherlands
c Dutch Institute for Fundamental Energy Research (DIFFER), P.O. Box 6336, 5600 HH Eindhoven, the Netherlands
⁎ Corresponding author. E-mail address: d.c.nijmeijer@tue.nl (K. Nijmeijer).
Abstract
A main component of a hydrogen-bromine flow battery (HBFB) is the ion exchange membrane. Available membranes have a trade-off between the major requirements: high proton conductivity, low bromine species crossover, and high mechanical and chemical stability. To overcome this, electrospinning of a highly proton conductive polymer (short side chain perfluorosulfonic acid (SSC PFSA)) and a hydrophobic inert polymer (polyvinylidene fluoride (PVDF)) was used to electrospin composite polymer fiber mats. Piles of multiple mats were hot pressed resulting in dense ion exchange membranes. Membranes with three different SSC PFSA/PVDF ratios were prepared, characterized, and subjected to short and long term (1500 h) HBFB testing. The electrospun membranes have performances very comparable to those of commercial membranes. For the SSC PFSA/PVDF electrospun membrane, a higher SSC PFSA loading gives a higher membrane proton conductivity compared to a lower loading, but at the expense of a higher bromine species crossover. The SSC PFSA/PVDF (50/50 wt%) membrane shows a coulombic efficiency of 98%, a voltaic efficiency of 80% and an initial available capacity of 105 Ah L− 1 at a current density of 150 mA cm− 2, which equals that of the current benchmark long side chain PFSA membrane. This performance is constant over 200 cycles during 2 months of continuous HBFB operation.
High selectivity-conductivity reinforced perfluorosulfonic acid membranes for hydrogen-bromine flow batteries.
Yohanes Hugo1, 2, Wiebrand Kout1, Friso Sikkema1, Zandrie Borneman2, Kitty Nijmeijer2
1Elestor B.V., 6812 AR Arnhem, the Netherlands
2Membrane Materials and Processes, Eindhoven University of Technology, Department of Chemical Engineering and Chemistry, PO box 513, 5600 MB Eindhoven, The Netherlands
Abstract
Reinforced proton exchange membrane (PEM) were developed to increase the mechanical strength of thin membranes (≤30 µm) for PEM fuel cell applications. In hydrogen-bromine flow batteries (HBFBs), Br2 and Br– crossover through the membrane may affect the lifetime of HBFBs as a result of dissolution or passivation of the platinum catalyst. One study suggested that the reinforcement reduces the bulk Br2 and Br– transport and x-y (in-plane) swelling [1].