A recipe for an affordable, safe, and scalable flow battery

Whether we are willing to admit it or not, we are all rather dependent on batteries. We mostly think about small batteries at the individual level—in our cell phones, watches, or laptops. But batteries can be used in technologies that support multiple people, like the hybrid car. As scientists develop batteries with larger energy storage capacity, we can begin to expand our outlook to even grander applications.

If batteries could be designed to safely store large amounts of energy, they could be integrated into the existing power grid. This type of integration could be used to store energy harvested from renewable sources such as solar and wind, and to smooth out sudden surges in demand. Recent advancements in a technology called redox-flow batteries (RFB) hold out a promise for scalable energy storage.

RFBs are composed of organic materials that are able to transport electrons (redox-active). Almost all RFBs are composed of two pools of liquid electrolyte separated by a membrane, which allows some ions to cross between the two liquids. In these systems, electrons then flow from the negatively charged liquid (anolyte) through to the positive charged liquid (catholyte). These electrons can either charge the system or be extracted for use. Since the key components of the batteries are liquid, things can be scaled up simply by making the holding tanks larger.

A new, affordable battery platform

In order for this energy storage technology to work, the membrane must meet a high list of demands. The corrosive nature of the electrolyte requires a membrane with very high chemical stability. To commercialize the systems, the membrane needs to be inexpensive. All the materials used in the system must also be easy to manufacture on a large scale.

The quality of the membrane is also of the utmost importance to the RFB performance. It must allow solely for the flow of electrons and must retain the active chemicals. This is critical to prevent short-circuits and a self-discharge. The high cost of membranes that meet these criteria poses a large obstacle to the commercialization of many RFBs. One particularly common membrane, Nafion, accounts for 40 percent of the cost of an entire cell.

A team of scientists developed a battery system that combines a water-based electrolyte with an organic reactive material and a low-cost membrane. Their design includes an organic polymer as the redox-active species, simple salt (NaCl) as the electrolyte, and a regular dialysis membrane (the team used a cellulose-based dialysis membrane). These membranes are very inexpensive and are typically used at lab-scale to separate polymers

The team used polymers that had been designed for optimal redox-activity. The anolyte contains a simple organic chemical (a 4,4’ bipyridine derivative) which is able to donate electrons. The catholyte contains a redox active material (a TEMPO radical) that is able to receive electrons. The catholyte and anolyte behave very similarly to water—due to the low viscosity, only a small amount of energy is required to pump the liquids around the system. This allows for more efficient transportation of electrons. They were also able to obtain good cell performance through optimization of the membrane’s permeability to salt.

The scientists were interested in testing the selectivity of the membrane—it needs to allow electrons to pass, but not other materials. So they determined whether any of the components of the catholyte or anolyte were able to cross the membrane. First, they performed 10,000 charge-discharge cycles of an RFB test cell. Then, they took samples from both the anolyte and catholyte solutions. They found that only trace amounts of the polymers passed through.

Validating the system

The team built additional batteries and examined the properties of the system. They found that the RFB can provide an open-circuit voltage of 1.1 V. That means that the system can safely be charged and discharged within a voltage window of 0.8- 1.35 V.

These cells were also able to retain a high percentage of their initial capacity, exhibiting energy efficiencies between 75 percent and 80 percent. Incremented charge-discharge studies were also performed to understand the voltage stability. They found the voltage to be stable between 10 to 90 percent state of charge. Even after 10,000 cycles in a static unpumped cell, 80 percent of the initial capacity was retained.

Overall, this system eliminates many of the hazards associated with earlier flow batteries while providing an affordable alternative. Additionally, the redox-active materials actually exhibit a color change upon electron transfer, which acts as a visual indication of the charge state of the battery. This proof-of-principle study could pave the way for a new class of safer batteries that could scale up to work on the grid.

Nature, 2015. DOI: 10.1038/nature15746  (About DOIs).

Listing image by Andy Armstrong via Flickr