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A Breakthrough for Redox Flow Batteries

August 1, 2014


The ability to supplement fossil-fuel -based electric power with large quantities of energy from renewable sources will require massive energystorage devices. One promising technology is the redox flow battery (RFB). Redox flow batteries reversibly convert electrical energy to chemical energy. The key difference between these batteries and traditional rechargeable batteries is that traditional batteries store electricity internally in their redox pairs fixed on the two electrodes, whereas RFBs store their energy externally in two flowing electrolytes. The energy capacity (a function of the volume of electrolyte) and power (a function of the size of the electrochemical cell stacks) are decoupled in the flow battery, providing design flexibility and economical scalability - all of which make the RFB attractive for grid-scale electricity storage.

"The most significant advantage of the flow battery over the normal battery is the separation of power and energy," says Yushan Yan, a professor of chemical and biomolecular engineering at the Univ. of Delaware. "Now you can design the power and the energy separately," Yan says. "If you want to have a battery last longer, you only need to get a bigger tank, you don't touch the battery stack itself," he explains. "If you want to have a more powerful battery, you make the stack bigger and you don't have to touch the tank."

Although RFBs offer many advantages over other electrochemical technologies for grid-scale storage, their performance has been constrained by their architecture.

A typical RFB consists of two electrodes separated by an ionically selective membrane (e.g., anionexchange membrane), and two liquid-filled tanks, one containing a negative electrolyte (e.g., Cr3+/Cr2+) and the other a positive electrolyte (e.g., Fe37Fe2+). When the battery is charging, the electrolytes flow into the electrochemical ceil, where Cr3+ cations take electrons from the negative electrode and are reduced to Cr2+ cations, and Fe2* cations are oxidized to Fe3+ cations and give electrons to the positive electrode. The CP anions move across the membrane from the negative electrolyte to the positive electrolyte, and the electrons move through the external circuit from the positive electrode to the negative electrode against the cell voltage. In this case, the membrane must be anion-selective and only allow negative ions to pass through.

Yan and his team identified a key limitation of this design: the use of one membrane, which limits the redox pairs that can be used. "For example, if you have an anion-exchange membrane, all of the four species need to be positive," Yan explains. "If you have a cation-exchange membrane, all of the four species need to be negative," he continues. "What we realized was we don't have to use one membrane; we can use two membranes," he says. "By having two membranes, you can select any redox pairs you wish from the electrochemical potential table. This gives you a tremendous amount of freedom you never had before."

Yan and his colleagues are the first to use two membranes - one anionexchange membrane (AEM) and one cation-exchange membrane (CEM) - in a redox flow battery. An example of this design is a positive electrode with a +/+ redox pair electrolyte on one side separated by an anion-exchange membrane, and a negative electrode with a 7- redox pair electrolyte on the other side separated by a cation-exchange membrane. In the middle of the two membranes is a neutral salt solution.

Based on this new design concept, the researchers created two batteries that could not be made with one membrane: a super-high-voltage RFB and a very inexpensive RFB. For the highvoltage battery, an anionic redox pair of Zn(OH)427Zn in a base is combined with a cationic redox pair of Ce4+/Ce3+ in an acid. When the RFB is being charged, Zn(OH)42~ anions in the negative electrolyte take electrons from the negative electrode and are reduced to Zn metal; Ce3+ cations in the positive electrolyte give electrons to the positive electrode and are oxidized to Ce4t cations; Na+ cations move across the cation-exchange membrane from the middle electrolyte to the negative electrolyte, and C104" anions move across the anion-exchange membrane from the middle electrolyte to the positive electrolyte. This RFB achieved a cell voltage of 2.96 V, which exceeds the highest voltage for a single-membrane RFB of 2 V.

For the ultra-low-cost RFB, Yan and his team combined two of the most abundant and inexpensive elements found on earth: iron and sulfur. This battery has a S427S22" anion redox pair and an Fe3+/Fe2+ cation redox pair. Its voltage of 1.22 V is similar to that of the expensive, all-vanadium singlemembrane RFB ( 1.25 V).

The results of Yan's work are encouraging for the use of RFBs for grid-scale storage. However, challenges exist, including the durability of the electrodes in a super-high-voltage cell and the additional resistance from the second membrane. The team is working on these challenges as well as evaluating different redox pairs.

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Source: Chemical Engineering Progress

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