PNNL team finds that new electrolyte mix increases energy storage capacity of vanadium redox batteries by 70%
Researchers at the Department of Energy’s Pacific Northwest National Laboratory (PNNL) have found that adding hydrochloric acid to the sulfuric acid electrolyte typically used in vanadium redox flow batteries increased the batteries’ energy storage capacity by 70% and expanded the temperature range in which they operate. A paper on their work appears in the journal Advanced Energy Materials.
Although considered a promising large-scale energy storage device, the vanadium redox battery’s (VRB) use has been limited by its inability to work well in a wide range of temperatures and its high capital cost and life-cycle cost. A VRB, for example, has operating cost about $500/kWh or higher—“obviously” still too high for broad market penetration, PNNL researchers noted in an earlier paper. (Earlier post.)
Our small adjustments greatly improve the vanadium redox battery. And with just a little more work, the battery could potentially increase the use of wind, solar and other renewable power sources across the electric grid.—lead author and PNNL chemist Liyu Li
A redox flow battery (RFB) stores electrical energy typically in two soluble redox couples contained in external electrolyte tanks sized in accordance with application requirements. Liquid electrolytes are pumped from storage tanks to flow-through electrodes where chemical energy is converted to electrical energy (discharge) or vice versa (charge). The all-vanadium RFB uses one tank with the positively-charged vanadium ion V5+ floating in its electrolyte, while the other tank holds an electrolyte full of a different vanadium ion, V2+. When energy is needed, pumps move the ion-saturated electrolyte from both tanks into the stack, where a chemical reaction causes the ions to change their charge, creating electricity.
To charge the battery, electricity is sent to the vanadium battery’s stack. This causes another reaction that restores the original charge of vanadium ions. The electrical energy is converted into chemical energy stored in the vanadium ions. The electrolytes with their respective ions are pumped back into to their tanks, where they wait until electricity is needed and the cycle is started again.
The battery’s capacity to generate electricity is limited by how many ions it can pack into the electrolyte. Vanadium batteries conventionally use pure sulfuric acid for their electrolyte. But sulfuric acid can only absorb so many vanadium ions.
Another drawback is that sulfuric acid-based vanadium batteries only work between about 50 and 104 °F (10 to 40 °C). Below that temperature range, the ion-infused sulfuric acid crystallizes. The larger concern, however, is the battery overheating, which causes an unwanted solid to form and renders the battery useless. To regulate the temperature, air conditioners or circulating cooling water are used, which causes up to 20% energy loss and significantly increasing the battery’s operating cost, the researchers noted.
To improve the battery’s performance, Li and his colleagues began searching for a new electrolyte. They tried a pure hydrochloric acid electrolyte, but found it caused one of the vanadium ions to form an unwanted solid. Next, they experimented with various mixtures of both hydrochloric and sulfuric acids. PNNL scientists found the ideal balance when they mixed 6 parts hydrochloric acid with 2.5 parts sulfuric acid. They verified the electrolyte and ion molecules present in the solution with a nuclear magnetic resonance instrument and the Chinook supercomputer at EMSL, DOE’s Environmental Molecular Sciences Laboratory at PNNL.
Tests showed that the new electrolyte mixture could hold 70% more vanadium ions, making the battery’s electricity capacity 70% higher. The discovery means that smaller tanks can be used to generate the same amount of power as larger tanks filled with the old electrolyte.
The new mixture allowed the battery to work in both warmer and colder temperatures, between 23 and 122 °F (-5 to 50 °C), greatly reducing the need for costly cooling systems. At room temperature, a battery with the new electrolyte mixture maintained an 87% energy efficiency rate for 20 days, which is about the same efficiency of the old solution.
The results are promising, but more research is needed, the authors noted; furthermore, the battery’s stack and overall physical structure could be improved to increase power generation and decrease cost.
This research was supported by DOE’s Office of Electricity Delivery and Energy Reliability and internal PNNL funding.
Liyu Li, Soowhan Kim, Wei Wang, M. Vijaayakumar, Zimin Nie, Baowei Chen, Jianlu Zhang, Guanguang Xia, Jianzhi Hu, Gordon Graff, Jun Liu, Zhenguo Yang (2011) A Stable Vanadium Redox-Flow Battery with High Energy Density for Large-Scale Energy Storage. Advanced Energy Materials. doi: 10.1002/aenm.201100008