Flow batteries and cars: a complicated love story
We go deep into the feasibility of flow-battery-powered cars
Gaël Mourouga is a PhD candidate at ETH Zürich working on organic redox-flow batteries. He’s been closely involved with a few startups in the field of flow batteries, including Jena Batteries GmbH in Germany, KemiWatt in France, PinFlow in Czech Republic and Elestor BV in the Netherlands.
Flow batteries can be filled up like gasoline tanks, but their low energy and power density make them tricky to use for mobility.
We look into some folks who are trying anyway: but things don’t look good vs onboard Li-ion.
Don’t rule out flow batts just yet, as fast charging adds a lot of load to the grid.
This is where flow batteries may shine to alleviate grid load by providing stationary storage for chargers.
EV mass adoption and the charging challenge
According to a 2020 study by Castrol (a British oil company) the tipping point to achieve mass adoption for electric vehicles (EVs) will be to achieve 470 km (291 miles) of range for a cost under $36’000 while achieving charging times under 31 minutes. Battery energy density has made a lot of progress over the past decade, and costs are getting under $30’000 so the only remaining difficulty essentially comes down to fast charging: is it possible for EVs to reach in 31 minutes a state-of-charge sufficient to last for 470 km (291 miles)?
An idea that seems to resurface from time to time to overcome this difficulty is to replace the common Lithium-ion batteries used in EVs with Redox-flow batteries (RFBs): in RFBs, active materials are dissolved in a liquid electrolyte rather than intercalated in solid electrodes, so that it is possible to fill up the battery with fresh electrolyte, making the whole process more akin to fueling up a tank rather than plugging a battery in a power outlet.
In that regard, flow batteries have been dubbed a “game-changer for electric vehicles” or a “crazy dream not so crazy anymore” by various news outlets in the past. Unfortunately, these articles provide very little detail about how a flow battery vehicle would actually work. As a flow battery researcher, I thought I could provide an in-depth, quantitative analysis of the idea.
Let us assume that:
The average energy consumption per unit distance E_d of a car is 140 Wh per kilometer
The theoretical energy density of gasoline e_g is 10 kWh per liter
The efficiency η of an average internal combustion engine is around 20%
The average Li-ion cell has a nominal voltage U_b of 3.7 V and capacity A_b of 4 Ah, which should correspond to a theoretical energy density e_b of roughly 400 Wh per liter.
Calculation of vehicle range
Internal combustion engine
In an ICE vehicle, traction is provided by the combustion of gasoline in a combustion chamber, where mechanical work is extracted from thermal work at a given efficiency η. The range of the vehicle D depends on volume V of the tanks where gasoline is stored according to the relationship:
where e_g is the theoretical energy density of gasoline and E_d is the energy consumption of a vehicle per unit distance. Therefore, from our base assumptions above, with a 55 L tank, you should be able to drive for about 785 km before needing to fill up, in line with the 760 km (490 miles) minimum range advertised by most ICE vehicles.
Li-ion battery vehicle
An EV is a bit different, as mechanical work is produced by the conversion of electric work through a magnetic field. Very roughly speaking, the total electric work depends on the combination of voltage and current output of the battery, where voltage adds up proportionally to the number of cells in series and current to the number of cells in parallel. Therefore, the more cells in series the higher the voltage, and the more cells in parallel the higher the capacity and therefore the range.
As an example, the Tesla Model S battery is made up of 16 modules in series, each module is made of 6 groups in series, and each group is 74 cells in parallel. Taking our base assumptions on Li-ion cells, this should add up to a total voltage U_tot of 355.2 V and a total capacity A_tot of 269 Ah. The range of an EV is given by a formula similar to that of the ICE vehicle
Taking an efficiency η of 90% into account for an EV yields a calculated range of 614 km (381 miles), still lower than ICE vehicles but above the minimum range forecasted by Castrol.
However, this range assumes a 100% state-of-charge (SoC) of the battery, which can only be reached after about one hour of charging. A 31-minute charge would yield about 75% SoC, which converts to about 460 km: just below the tipping point predicted by Castrol.
What if we were able to refill these batteries with a liquid? Let’s take a look at where things might get complicated.
Flow battery vehicle
To start things off, a flow battery vehicle would be conceptually in-between an ICE vehicle and a Li-ion battery vehicle: electrolyte tanks would contain the electrolyte responsible for the range, just like gasoline in an ICE vehicle, which would be pumped to a power component (a porous carbon electrode) to produce electrical work. Then, an electric engine would convert electrical work from the power component to mechanical work, just like in a Li-ion vehicle. Since we use an electric engine, we need to place some cells in series in order to produce sufficient voltage, and some in parallel to produce sufficient current.
This is where the serious constraints start: if our chemistry is aqueous, we are limited to a cell nominal voltage U_c of about 1.4 V, otherwise we start electrolysing the water at the electrodes. That means we need to place about 270 cells in series, whereas the Model S only needed about 100. The calculation of the cells in parallel is a little bit less straightforward (I’d need assumptions on the current densities, operating currents, etc…), but overall it is safe to assume that the power component of our flow battery should take up at least as much space and weight as all the Li-ion cells, especially factoring in the tubing and pumping system.
But we haven’t accounted for the tanks yet! We just accounted for the power component (the “engine”), but we also need to provide range to our flow battery vehicle. The energy density of the electrolyte is given by the following calculation:
Where n is the number of exchanged electrons in the reaction, F is Faraday’s constant, c is the concentration of active species in our electrolyte and U_c is the cell voltage.
For a typical Vanadium flow battery, the cell voltage U_c is about 1.23 V, the concentration c in the limiting reservoir is around 3 mol/L and the reactions exhibit a one-electron transfer (n=1), so we would get a mere theoretical energy density e_e of 100 Wh/L.
Remember that we need to take the efficiency of the conversion η into account: for flow batteries, since we need to pump liquid around it is usually lower than for Li-ion, around 75% which translates to 75 Wh/L practical energy density (and that’s a rather optimistic calculation compared to other reported values).
To put things in perspective, Lithium-ion cells provide about 360 Wh/L practical energy density, while gasoline provides 2 kWh/L practical energy density (from my base assumptions).
To reach 470 km (291 miles) we would need two reservoirs of at least 877 L each.
Except, the car would weigh two additional tons compared to my base assumption for a car consumption, so the consumption per km would be much higher, so the required tank volume would also go up etc…
In conclusion, it is safe to say that with my assumptions above, it seems impossible to design a working flow battery vehicle.
So, what was the idea behind the European project funded on the topic? And how do companies advertising flow battery vehicles overcome the limitations outlined above?
Flow battery vehicles in practice
In order to learn more, I contacted Jens Noack, an engineer at the Fraunhofer ICT and associate professor at UNSW, a good friend of Maria Skyllas-Kazacos, the inventor of the Vanadium flow battery.
The main idea back then was to get rid of the negolyte, the reservoir that is usually limiting in terms of capacity. Basically, we wanted to design a fuel cell where you replace the hydrogen tank with a liquid vanadium tank.
Indeed, that would make a lot of sense, as it would get rid of two large tanks (one to store the fresh electrolyte and another to discard the used electrolyte) and result in significant volume gains. It also may help rising the current density, and therefore power density, of individual cells.
According to our preliminary calculations, it was possible to reach an energy density of about 170 Wh/L, so we wanted to see how far we could go in practice, maybe paving the way to future improvements in flow battery chemistries which may allow applications for short-distance mobility, such as buses. Pushing the limits of energy and power density is always interesting from an academic perspective, as you may figure out tricks that can prove useful in other applications. Ultimately, we got to about 80 Wh/L if I recall correctly, which was about enough to power a golf cart.”
This kind of work actually prompted a lot of optimism with respect to hybrid technologies, where a liquid stored in a tank reacts with a gas, either stored in a gas tank like in the technology from Elestor BV in the Netherlands or open to the outside air, as in the case of the Zinc-air technology that was investigated in my own project, FlowCamp.
But still, the values reached by these systems are very, very far away from allowing the design of flow battery EVs. Most flow battery companies advertising high energy densities target the market of behind-the-meter stationary storage.
So how are companies advertising flow-battery vehicles managing to overcome these problems?
Let’s start off with IFbattery, a company launched in 2019 from Purdue University, which was covered by Green car reports under the title “flow battery for EVs claim 300 mile range”, or other Chinese news outlets.
So, how does it work?
It uses a water, ethanol, and salts and either an aluminum or zinc anode. The electrical charge in the liquid cathode-electrolyte are depleted with use (as in any flow battery). The chemical reaction produces electricity and hydrogen. The hydrogen is then collected in low-pressure tanks and fed into a small fuel cell to make additional electricity for the motor
I am a bit perplexed by the affirmation that it is possible to produce both electricity and hydrogen in an energetically profitable way: when I typically operate flow batteries in my lab, I want to avoid the voltage to go above 1.7 V, otherwise, the water will be split into hydrogen and oxygen (the water electrolysis I was mentioning earlier), which will create gas bubbles, rising the ohmic resistance of the system. As a consequence, all the energy we put in the system is converted to heat rather than chemical energy.
Redox Dual-flow batteries allowing both energy storage and hydrogen production do exist, but they imply the use of separate redox-mediated electrolysis reservoirs where hydrogen and oxygen evolution occur in a controlled manner. The redox mediators must exhibit specific properties and are usually far more complex than simple aluminum or zinc anodes as quoted above.
This system generates hydrogen as you need it, so you can store safe hydrogen at pressures of 20 or 30 PSI instead of 10,000
The main reason why fuel cell vehicles store hydrogen at high pressures is to get a fuel of high energy density. Generating hydrogen as you need it means you have to carry the water to produce the hydrogen which... sounds very inefficient.
One of the big weaknesses of most batteries is the breakdown of the membrane. We don't have a membrane
That surprises me even more, as the main problem of membraneless flow batteries is the trade-off power versus lifetime: the goal of the membrane is to separate efficiently the electrodes, while allowing ions to pass to enable the electrochemical reactions. When you get rid of the membrane, you are either severely limited by the rate at which you can charge the battery, or you have high degradation through crossover and mixing of active species.
With a flow rate of 10 mL/min, the current density could reach 3 mA/cm2
which was on the higher end of the current densities reported in the review. To put this figure in perspective, the batteries I cycle in my lab are typically operated at 100 mA/cm2 for about 1V nominal voltage, which results in a low power density of about 100 mW/cm2, unsuited for portable applications. Membraneless flow batteries would be at least one (if not two or three) orders of magnitude lower, not to mention degradation problems. This makes the following sentence from IFBattery surprising:
We are at the point now where we can generate a lot of power. More power than you would ever guess could come out of a battery like this
Perhaps these claims are better summed up by the user u/demultiplexer on Reddit
IFBattery's founder is, still, a professor at Purdue. He's likely started IFBattery because he thought there would be money in stepping into commodity flow batteries. This is a VERY common thing for academics to do […] That single press release is essentially their exit […] The press release screams this, with over the top claims and aspirational lines like being able to revolutionize battery (and strangely hybrid) vehicles. Sure. […] This is a dead-end technology for mobile applications and the entire flow battery industry knows it. That's why it's not getting funded, nor bought by established flow battery companies.
I wish I could access more content from the Purdue researchers themselves, but their publications seem to be under some kind of embargo.
Another company, NanoFlowCell, based in Liechtenstein, claims to have achieved performant flow battery vehicles. This time, the system does not appear to be a hybrid gas/liquid, but rather fully liquid as the theoretical system I imagined previously.
They were recently at the Geneva Motor Show and were covered by a 2021 article in autoevolution, so let’s look at the claims:
It's all about providing an electric voltage of 48 V to animate four electric motors able to develop, all together, some 136 HP (4 x 25 kW). Quantino’s maximum speed is 200 kph (124 mph) and its range should reach about 1000 km - which sounds like a "goodnight" song even for the most fuel-efficient turbodiesels. As the potential of the electrolytic liquid inside the fuel cell is decreasing, two tanks (positively and negatively charged liquid) are providing fresh resources. This feeding process was solved by gravitational means. The neutralized liquids are ejected in two other tanks.
The sentence “The feeding process was solved by gravitational means” sounds surprising: as I mentioned previously gas bubbles are to be avoided at all costs in order to prevent the ohmic resistance of the power component to rise, which dissipates a lot of energy through heat. That’s why we typically pump liquids from bottom to up, to avoid gas bubbles. Plus, the fact that neutralized liquids are ejected in two other tanks means you need a total of 4 tanks. That sounds like a large volume for a car.
The advertised range of 1000 km sounds good, indeed, but how exactly did they achieve a suitable energy density?
I managed to find a blog article with a bit more information on their electrolyte, the bi-ion
The composition of this molecule (electrolyte) and its concentration within the solution permits an energy density that is exceptionally high for electrolyte solutions (> 600 Wh/l)
As per my previous formula:
For an aqueous liquid, U_c would be limited to 1.4 V (remember the hydrogen evolution I mentioned before), so we need an unheard-of concentration of 22 mol/L for a one-electron transfer.
Maybe their technology is non-aqueous? In which case they could have a higher operating voltage (no more hydrogen evolution due to the absence of H2O in the system), say 4V, which would bring down the required concentration to about 7 mol/L, which is (to the best of my knowledge) still unheard of in the field of non-aqueous chemistries. Perhaps they also have more than one-electron transfer? In which case there still remain many problems to overcome: viscosity, lifetime, conductivity, oxygen sensitivity (hermetically sealing a car sounds difficult) etc… which all contribute to bringing down the efficiency of the system, and therefore the “effective” energy density.
Even assuming that the advertised value holds at 100% efficiency, with the advertised range of 1000 km we would still require 234 L of bi-ion in each reservoir. Times four, to account for the reservoirs that host the discarded fuel.
You can have a look at the Concept view of the NanoFlowCell Quant F. Very nice looking, but where do you fit the >1000 L of tanks, pumping system and power components?
According to Jens Noack from Fraunhofer ICT
I had a friend who actually visited them at the Geneva convention and went inside the car. She said there were no visible pumps or tubing, no extra space for the reservoirs. Nothing that would look out of place in a regular car.
Is there any future for flow batteries in electric mobility?
Probably not in cars themselves, but a few companies are targeting the market of EV charging stations, which sounds much more promising.
Charging EVs with high-power charging stations will add a considerable strain on the electric grid. Remember that the electric grid can be seen as an oscillating system at a nominal frequency of 50 Hz (60 Hz in the US), where deviations are caused by differences between production and consumption, at any time. Any local deviation of more than 0.2 Hz will cause a black-out, to protect high-voltage transformation systems. This has already started to become a real problem in Texas, with Tesla asking owners to limit charging during heatwaves in order to alleviate loads on the grid.
Vehicle-to-Grid (V2G) may be a long-term solution to this problem, but there’s also a very simple and efficient short-term solution: making sure that EVs are not taking their load from the grid itself, whenever possible (especially during fast charging). That is made possible by using a big battery at the charging station, which charges during off-peak hours and covers the need of commuting vehicles when they need it.
Flow batteries, in that application, present a few advantages:
they are longer-lasting than their lithium-ion counterparts, so a charging station operator may need to change the battery after 15/20 years rather than 8/12 years
Energy and power density are not so much an issue in that case (provided the whole system is cheap), since you can store large volumes of electrolyte underground, just like gas stations currently store gasoline.
It is possible to imagine having an “emergency” tank if the battery runs out during, say, a holiday period with a lot of commuters. These “emergency” tanks can be transported with trucks, rather than charged through the grid, just like gasoline is currently brought to refuelling stations.
Also, if lithium is used in cars and cell phones, maybe it would be nice to use other materials in stationary applications?
So, in conclusion, I think the love story between flow batteries and cars is not over yet.
I think we are on the verge of a major decentralization of the power sector, where the electric grid will act as a connector between a network of decentralized renewable power plants coupled with storage facilities, larger baseload power plants, prosumers and consumers. EV charging stations equipped with stationary storage technologies are, in my opinion, very likely to be part of this future.
🌞 Thanks for reading!
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