Welcome to the Flowbat Project!

The challenges ahead

ZnFe chemistry has the makings of a flow battery king—but a handful of hidden traps have kept it from claiming the throne.

A zinc battery is always one dendrite away from a blackout

The ZnFe flow battery relies on the plating of zinc metal on the electrode to store electricity. Under standard operating conditions and cell design, the zinc does not plate evenly, and can eventually result in long ‘tendrils’ of deposited zinc metal, resulting in battery failure caused by piercing of the separator membrane. Flowbat has conducted experiments to visualise the occurrence of this phenomenon, by plating zinc from zincate solution onto a carbon felt electrode – see right figure.

Hydrogen formation – a problem for energy storage?

When charging batteries, energy is lost when unwanted chemical reactions compete with the desired redox reaction. During charging of the ZnFe flow battery, zinc ions plate onto the electrode as zinc metal, which depletes its concentration around the electrode. This phenomenon facilitates the competing reaction of water electrolysis, forming hydrogen and oxygen gas, which bubbles out. The energy used to drive this competing reaction is energy that the battery cannot store – see left image (right electrode is hydrogen, left electrode is oxygen). So, the higher the rate of these reactions, the lower the charging efficiency, or the amount of energy lost between charging and discharging.”

If there’s a space, liquid will find it’s way through

Those with a plumbing or mechanical engineering background will be familiar with the saying that “if there’s a gap, water will get through”. The conventional flow battery design involves sandwiching the stack components between bulky steel plates, secured by heavy duty bolts and sealed with rubber seals, and chamfers on membrane compartments. For the purposes of small-scale R&D, this design is sufficient. However, having experienced it personally, this “sandwich” design is very cumbersome to work with and can have severe leakage issues – see right image (zinc has plated on the outside of the battery stack).

What does it all mean for the ZnFe battery?

The three points discussed above are the Achille’s Heel of ZnFe flow batteries. If not addressed, they will both reduce the charging efficiency of the battery and shorten its lifespan significantly. Every time the battery is charged, the operator must hope that the membranes in the ZnFe cell will not be pierced. Even if that did not happen, hydrogen gas will be seen bubbling out of the stack, upsetting the pressure balances and forcing the electrolytes out of any sliver of space. 

In short, the ZnFe battery, designed in the standard way, would not last very long at all, and so has remained in R&D limbo for many years, despite possessing so much potential.

Is it doomed for the technology? Will there be a comeback story?

The answer is – surprisingly – a no, followed by a YES! Flow battery technology faces many challenges, but there’s also real hope. That classic “sandwich” stack design? It’s not the only way. The flow channels? They don’t have to remain the same. Even the charging method doesn’t need to be just like lithium-ion batteries.

We can rethink the stack entirely; no more relying on hopes and dreams for perfect seals! But, it takes deep design expertise. The current layout exists simply because it’s straightforward and efficient for small-scale R&D, good enough to pump out some academic publications, not because it’s the only way of designing flow batteries. The design of truly usable flow battery stacks requires a multidisciplinary approach, relying on insights from fluid dynamics, materials science, electrochemistry, and even computational chemistry. And charging them? That in itself is a control engineering and power electronics challenge.

At Flowbat, we believe the path forward lies in bold redesigns. With the right team, there’s massive room for improvement, and we’re ready to reshape how the world sees flow batteries once and for all.