(Left) Cell configuration; (right) comparison of theoretical specific energies of state-of-the-art aqueous flow batteries.
(Left) Cell configuration; (right) comparison of theoretical specific energies of state-of-the-art aqueous flow batteries.

Flow batteries, where two active electro-chemical components are separated by a membrane though which ion exchange occurs, can be used as fuel cells or as rechargeable batteries. But although such batteries can be readily scaled-up and have exceptional longevity, making them ideal for storing renewable energy, power output tends to be low.

Now researchers from the University of Hong Kong and Yale University have devised a rechargeable vanadium-hydrogen (V-H2) flow battery with one of the highest practical open circuit and discharge voltages reported so far [Weng et al., Materials Today Energy 10 (2018) 126-131].

“We have developed a scalable pH differential V-H2 flow battery with very high cell voltage and specific energy compared with existing rechargeable flow batteries,” says first author of the study, Guo-Ming Weng.

The flow battery is similar to others of the type in that it consists of a positive acidic VO2+/VO2+ component and a negative alkaline hydrogen component separated by a commonly used bipolar membrane. During charging, the V-H2 flow battery stores energy in the form of hydrogen gas (H2), VO2+, and water split into protons and hydroxide ions. When the battery is discharged, H2 is consumed on the negative side of the cell, VO2+ is reduced to VO2+ on the positive side, and protons and hydroxide ions combine to form water.

“This is the first pH differential V-H2 flow battery with high cell voltage of 1.8 V,” points out Kwong-Yu Chan.

The flow battery also has a high specific energy and utilizes neutralization energy. In fact, the researchers claim the novel V-H2 flow battery has five times the theoretical specific energy storage of conventional all-vanadium redox flow batteries.

“The present prototype demonstrated very limited current density and power density,” admits Chi-Ying Vanessa Li.

But the researchers are confident that improvements to the highly ion-selective membranes or tailor-made separators would allow the potential benefits of V-H2 flow batteries to be realized.

This type of flow battery could have multiple advantages, they believe. It is one of the simplest options for storing intermittent energy generated by solar, wind, and other renewable sources, while offering almost unlimited capacity simply by adding more and/or larger tanks of electro-active material. The system can be almost instantaneously recharged by replacing the electro-active materials and can be matched to the energy and power requirements of a particular application.

“We believe this V-H2 system is cheaper than all-vanadium redox flow batteries, since a less costly vanadium-based electrolyte is used and the alkaline hydrogen gas electrode is a mature electrochemical component,” adds Chan. “Any successful discovery could lead to better energy storage technologies.”