This Flow Battery Aims To Kill Natural Gas, Not Just Coal

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The goal of knocking fossil energy off the power generation perch is a bit wobbly these days, as concerns rise over new demands from the surging data center industry among others. The missing link is a low cost, long lasting, go-anywhere energy storage system that can keep the clean kilowatts flowing regardless of what the sun and the wind are doing on any particular day. Flow batteries fit the bill. Some flow battery systems are already on the market, but the real decarbonization magic will happen when costs come down and flow batteries cross  the bridge to widespread adoption.

Flow Batteries Need Better Membranes

One big advantage of flow batteries over their lithium-ion counterparts is scalability. The hardware consists mainly of two tanks that store two types of specialized liquids, along with some pumps and piping. Currently, the transition metal vanadium is the operative ingredient of choice for both tanks (see more vanadium background here).

The tanks can be sized up or down depending on need (here and here are a couple of examples). Innovators are also working on a miniaturized device that can power an electric vehicle. Another project of interest is taking place in the US, where the startup Quino Energy is demonstrating how installation costs can be reduced by repurposing existing oil tanks and related infrastructure.

Tanks aside, the key to a flow battery is a membrane that prevents unwanted transfers between the two liquids. The problem is that the cost of the membrane is also a key factor keeping the cost of flow batteries higher than they could be. According to some estimates, the membrane in a conventional flow battery can account for up to 40% of the overall cost of the equipment.

An sPEEK Membrane For Low Cost Flow Batteries

It’s difficult to engineer a new membrane that costs significantly less than the current crop because the list of demands is a long one. “The ideal membrane should have good ionic exchange capacity; high ionic conductivity, low water uptake, swelling ratio, area electrical resistance and vanadium and other poly-halide ions permeability; and good chemical stability, as well as low cost,” a team of researchers based at the University of Hong Kong observed back in 2019.

That’s a pretty comprehensive list, but there’s more. They left out the thing about using toxic substances. The search for a better membrane is complicated by a concurrent effort aimed at reducing if not eliminate the toxic substances used in conventional vanadium flow batteries. That requires developing new liquid solutions, and that’s where the new research comes in.

A multi-institutional team based at Imperial College in London and the Dalian Institute of Chemical Physics in China has been developing a new membrane for flow batteries that deploy alternative electrolytes such as aqueous organic or zinc-iron formulas, which have the potential to combine low toxicity with high energy density and a long lifecycle.

The Imperial Colege/DICP team is focusing its efforts on a class of membranes called sPEEK (short for sulfonated poly(ether ether ketone), described as “low-cost and scalable alternatives with better environmental profiles” than conventional membranes.

The sPEEK membranes are also very good at shuttling the necessary ions from one liquid to the other, and they can be fabricated with relatively inexpensive roll-to-roll manufacturing systems.

Solving The sPEEK Problem

There being no such thing as a free lunch, sPEEK membranes have some shortcomings. For one thing, they are not so good at keeping ions in their proper place. As described by the communications department at Imperial College, “a key challenge of sPEEK membranes is their performance, which is limited by a trade-off between ionic conductivity and selectivity.”

To engineer new, more effective sPEEK membranes, the researchers took advantage of a new material with microscopic-sized pores. “By tailoring the sulfonation process and incorporating microporous architecture, the membranes achieve remarkable performance metrics, particularly enhanced ion conductivity that overcomes the trade-off between ion conductivity and selectivity,” explains lead author Dr. Toby Wong, of the Department of Chemical Engineering

“A three-dimensional contorted monomer, known as triptycene, is incorporated into the backbone of these sulfonated PEEK membranes,” Imperial College added in a press release last week. “These new membranes feature highly interconnected water channels that allow fast and selective transport of both cations and hydroxide ions, enabling high efficiency and reduced energy loss during battery operation.”

There’s much more to it the new membrane than that. You can get all the details in the journal Joule under the title, “Sulfonated poly(ether-ether-ketone) membranes with intrinsic microporosity enable efficient redox flow batteries for energy storage.”

Next Steps For The Flow Battery Of The Future

The team has successfully tested their new membrane on different kinds of electrolytes, including aqueous organic redox flow batteries and alkaline zinc-iron flow batteries.

“The battery can be charged at high current densities of up to 500 mA/cm² with high energy efficiency, outperforming most membranes reported in the literature. This capability significantly enhances the practicality and scalability of redox flow battery systems for real-world applications,” Imperial College reported.

Next steps include additional performance tweaks and investigating ways to scale up production, deploying the roll-to-roll membrane fabrication technology demonstrated by researchers at DICP. Future plans also include constructing a similar manufacturing facility in the UK. Interested in joining the team? Contact team leader Professor Qilei Song at Imperial College.

Meanwhile, that mention of zinc-iron flow batteries calls to mind the US startup Zinc Air, first profiled by CleanTechnica editor Zachary Shahan all that way back in 2012. The Montana-based company launched in 2009 and acquired the rights to zinc-air technology from the US Department of Energy’s Lawrence Berkeley National Laboratory in 2010 before turning attention to zinc-iron technology for grid scale energy storage.

If you have heard anything from the company in recent years, drop a note in the comment thread. Other firms to take up the flow battery torch including the US startup ESS (Energy Storage Systems). Among other projects, ESS is providing its iron-based flow battery to the Nigerian firm Sapele Power, for use at a power plant. The goal is to cut the widespread use of diesel backup generators in Nigeria, by improving efficiency at the power plant.

As described by ESS, the project is significant because it is one of the first utility scale energy storage projects in Nigera and the entire sub-Sahara region. It also demonstrates how public-private collaboration can help accelerate the energy transition globally. ESS developed its iron-based battery with support from the US Department of Energy, and the ESS-Sapele deal was backed by EXIM, the Export-Import Bank, which is the US government’s official credit agency for facilitating export transactions that support jobs in the US.

“This agreement represents the largest battery storage system export to Africa financed by the Export-Import Bank of the United States of America to date,” ESS also noted in a press release last May.

For the record, the US is one among 38 nations with official credit agencies that belong to the Organization for Economic Cooperation and Development. OECD countries have financed many a fossil energy project through their credit agencies over the years. During the Obama administration, the group finally agreed that its members would stop financing new overseas coal projects.

Oil and gas are next for consideration on the no-go list, as reported by our friends over at Grist. That will be much more difficult than cutting off coal, which had a much smaller share of financing to begin with. However, EXIM has already demonstrated its willingness to throw oil under the bus. Exhibit A is a new $527 million loan enabling Guyana to take hundreds of local, inefficient oil-fueled generators offline and replace them with an energy efficient combined-cycle gas power plant.

The ESS project in Nigeria is just one indication that gas will have to work harder to fend off flow batteries and other new, non-fossil energy technologies for a share of credit agency dollars. In the case of EXIM that also means nuclear energy is in the mix, despite long lead times, high costs, and security risks. If you have any thoughts about that, drop a note in the comment thread.

Image (cropped): A membrane makeover for flow batteries is expected to cut costs and improve the environmental footprint, leading to widespread adoption of sustainable energy storage (courtesy of Imperial College London).