Flow batteries have been widely deployed in large-scale energy-storage applications in recent years, but the low energy density and high cost of the incumbent vanadium electrolyte limit the long-term return on investment of such systems. A new organic redox flow-battery electrolyte — made from earth-abundant organic materials — could lower raw-material costs and improve energy density for flow batteries.
Kodiaq Technologies (Cambridge, U.K.; www.kodiaqtechnologies.com), building upon foundational technology developed in the Melville Laboratory at the Yusuf Hamied Department of Chemistry at the University of Cambridge (www.cam.ac.uk), has developed an organic electrolyte material that reportedly exhibits 50–70% higher energy density than traditional vanadium electrolytes, while enabling operation at much milder conditions.
The organic electrolyte depends on a novel molecular structure developed at Cambridge. Oren Scherman, Kodiaq’s chief scientific officer and co-founder, and professor of supramolecular and polymer chemistry at University of Cambridge (www.cam.ac.uk) explains: “The pyridinium-based materials have two pyridine subunits that exist in a permanently charged state. Typically, they have an aromatic linking core that allows us to control the number of electrons that the electrolyte can incorporate and retain in a stable manner. The charged pyridinium structure has flanking units that control the solubility of the electrolyte.”
Being able to control the electrolyte’s solubility, as well as the number of electrons it can accept, significantly improves the energy density that the electrolyte can achieve. To ensure the electrolyte has the necessary functionality for flow-battery applications, Kodiaq employed a supramolecular concept Scherman likens to a ‘molecular handshake.’
He says: “When a conventional organic electrolyte molecule with unpaired electrons comes into contact with another electrolyte molecule, they can form irreversible covalent bonds and ultimately precipitate from solution, and those electrolytes would no longer be capable of carrying electrons.
“However, in our chemistry, we can bring reduced electrolytes together in a very well-defined manner, and hold them together in the ‘handshake’ through reversible supramolecular interactions, without forming covalent bonds, until we want the electrons released.”
This ‘handshake’ mechanism also serves as a protective mechanism for the electrolytes, resulting in extremely high resiliency.
Scherman adds: “When you want to pull those electrons back out of the electrolyte in the battery and export them, the ‘handshake’ comes apart, and you now release those electrons. The other advantage is that by having this kind of association of single molecules, we protect the electrolytes from oxygen sensitivity.”
In fact, oxygen sensitivity is a key differentiator of the Kodiaq organic electrolyte technology when compared to vanadium flow batteries, points out Kodiaq’s chairman and co-founder David Fyfe.
He says: “In vanadium flow batteries, it’s absolutely essential to exclude oxygen, and so the whole system is nitrogen blanketed. One of the main ways oxygen can be introduced in the system is to raise the voltage of the battery too high and start splitting water by electrolysis. We believe we can raise the voltage of these cells higher than where current technologies can operate, therefore requiring fewer cells.”
Furthermore, the Kodiaq electrolytes operate at or around neutral pH, reducing the need for expensive materials of construction.
The company is currently working with a U.K. contract-manufacturing organization to scale up the synthesis process for its electrolytes.
Fyfe says: “We’ve already measured the comparative energy density against vanadium and find it increased by 70%. Vanadium is a one-electron transfer system with limited energy density, and what we have is a two-electron transfer system. Later this year, we will raise additional funding to be able to investigate increasing the number of electrons the ‘handshake’ can retain — and therefore increase energy density yet further.”