I sip slowly from the information firehose that is my cornucopia of industry newsletters, RSS feeds, and Google Scholar updates on new publications in the flow battery landscape. There’s not enough time to give everything a proper read, and often if I see something useful I just file it away in Zotero for it to collect digital dust. I want to try to more regularly share my initial takes as these developments cross my radar, so that I can discuss them more widely with folks, write more, and understand these developments more deeply.
“Flowdown”… get it … “The Flow Battery Lowdown” … anyways…
CMBlu’s flow battery has solids in the tanks
That sounds like someone has been struggling with adequate practical energy density/active material solubility. As reported by Brian Martucci at Utility Dive [1], quoting the company’s CEO:
The battery’s active chemistry is a “hydrocarbon polymer chain that is conductive,” Damato said.
. . .
The second twist is the presence of “stationary solids in the energy storage tank,” which no other flow battery has, Damato said. The solids dramatically increase energy density by allowing “more surface area on the chemicals themselves and more ions per liter inside the tank,” ultimately achieving per-square-foot energy densities on par with Tesla’s lithium-ion Megapack battery, he said.
I am skeptical of the “solid-booster”/“redox-targeting” approach, but if it works, it works. I haven’t dug into CMBlu’s technology before, I assumed they were licensing the alkaline quinone/ferricyanide technology from the Aziz Group at Harvard, but haven’t verified this. A look at their patent literature would be instructive. Maybe they truly are all-organic and don’t use ferrocyanide.
The company’s website has some nice pictures of their stack, too (following photos from their website): [2]
Flow batteries are already complicated systems, and adding more custom materials to the mix and managing dissolution kinetics and matching of redox potentials just sounds like a headache and less elegant system. To get into the details, one would have to dive into their patents, seems like [3] would be a good place to start.
As always, I wish them the best technical success, if they can crack long-duration energy storage with sustainable supply chains, that’s great.
A big stack for an academic lab: hybrid asymmetric Zinc-Ferrocyanide RFB paper
From the supporting information of [4]:
The ASZIFB cell stack.
A hundred-watt level cell stack was assembled to study the stability and practicality of the alkalescent electrolyte. The cell stack is composed of ten single cells pressed together. The effective electrode area of each single cell is 1000 cm2. The single cell consists of two carbon felt electrodes, a homemade carbon-plastic bipolar plate, PVC frames and gaskets, with the electrodes separated on both sides by the membrane. The ~60 L of 0.6 mol L-1 EDTA+ 0.6 mol L-1 ZnBr2 at pH=12 and ~90 L of 0.8 mol L-1 Fe(CN)64- at pH=12 are used as anolyte and catholyte, respectively. The stack is charged at 40 mA cm-2 for 2 hours and discharged at 40 mA cm-2 with a cut-off voltage of 8 V
1000 cm2 for an academic study is a big cell. No details on the stack beyond this though. Also, homemade bipolar plates! Pretty cool.
From their abstract:
Using a chelating agent to rearrange ferri/ferro-cyanide ion-solvent interactions and improve salt dissociation, we increased the solubility of ferri/ferro-cyanide to 1.7 mol L-1 and prevented zinc dendrites. Our battery has an energy density of ~74 Wh L-1 at 60 C and remains stable for 1800 cycles (1800 hours) at 0 C and for >1400 cycles (2300 hours) at 25 C. An alkalescent zinc-ferricyanide cell stack built using this alkalescent electrolyte stably delivers 608 W of power for ~40 days.
That’s not shabby. The ferri/ferrocyanide couple can have some weird behavior on long timescales, if I understand correctly in part due to shedding of cyanide ligands. A kicker for this system (for me) is it’s asymmetric nature. It requires a high-performance ion-exchange membrane to keep the positive and negative sides separated for a very long time. These membranes are never perfect and always allow some extend of crossover, which leads to irreversible capacity loss and sometimes additional issues.
I think a more robust solution can be found in symmetric systems, or systems with identical electrolyte compositions in the positive and negative tanks in the discharged state. This allows one to use a porous separator, like the super-cheap ones in lead-acid or Li-ion batteries—even just using paper. This type of approach is what we are trying to do at https://fbrc.dev.
References
Citation
@online{pollard_smith2024,
author = {Pollard Smith, Kirk},
title = {The {Flowdown} \#1},
date = {2024-05-04},
url = {https://dualpower.supply/posts/flowdown-1/},
langid = {en}
}