Scientists at the University of Colorado Boulder have devised redox flow battery electrolyte that could double the performance capabilities of traditional variants of the technology.
The team used chromium and organic binding agents to achieve ‘exceptional voltage and high efficiencies’ of 100% current efficiency and an 80% round-trip energy efficiency. The results were published in the journal Joule.
The results from two flow batteries were collected from a 3.5-watt test cell. The technology achieved more than 75 cycles over 24 hours.
Researchers Michael Marshak and Brian Robb (pictured right and left respectively) and undergraduate researcher Jason Farrell used CrPDTA as the negative electrolyte. The material consists of chromium ions coordinated to an organic chelate material called PDTA (1,3-PropyleneDiamineTetraAcetate), which is related to the chelate EDTA.
“We describe two flow batteries because we use the negative CrPDTA electrolyte with two different positive electrolytes that have already been demonstrated,” Marshak told BEST Battery Briefing.
“One is ferricyanide, Fe(CN)6, and gives a cell voltage of 1.62V; the other is bromine and gives a cell voltage of 2.13V. We believe the latter cell voltage is a record for an all-aqueous non-hybrid flow battery.”
Marshak said the research followed a report he had read about a flow battery using CrEDTA, which had reported a 7% energy efficiency.
“I wanted to know why,” he said. “By studying the coordination of the chelates to the metal ions at a molecular level, it allowed us to understand how to fix the losses in efficiency. We report a quantitative (100%) current efficiency, and an 80% round-trip energy efficiency.”
PDTA creates a ‘shield’ around the chromium electron, preventing water from hampering the reactant and allowing one of the battery cells to disperse 2.13 volts, claim the scientists.
Both “chrome alum” and PDTA is a spin-off of non-toxic EDTA, an agent used in hand soap, food preservatives and municipal water treatments.
Robb, lead author of the new study and a doctoral student in the Department of Chemical and Biological Engineering (CHBE), said: “Some people have taken this approach before, but hadn’t paid enough attention to the binding agents.”
“You need to tailor the chelate for the metal ion and we did a lot of work finding the right one that would bind them tightly.”
“We got this to work at the relatively neutral pH of 9, unlike other batteries that use highly corrosive acid that’s difficult to work with and difficult to dispose of responsibly. This is more akin to laundry detergent.”
Marshak and Robb have filed a patent on the innovation with assistance from Venture Partners at CU Boulder. They plan to scale up the technology in the laboratory in order to cycle the battery for even longer periods of time.
BEST insight by technical editor Dr Mike McDonagh:
Flow batteries using chromium and iron were developed by NASA in the mid 70’s. They relied on the di and tri-valent states of both metals to provide the required redox reactions for electron transfer. It gave low voltages (1.18V) and was not very efficient.
A later development using all chromium with two solutions of penta-valent and tri-valent states achieved with EDTA as a chelating mechanism. The Cr(V)-EDTA/Cr(III)-EDTA redox couple gave better voltages but was highly inefficient, less than 7%.
This was probably due to the poor chelation of Cr, which had a large over-potential and allowed reactions with water in the electrolyte. The use of PDTA gives better chelation with no side reactions with water and the highest voltage of the flow battery couples (vanadium etc.).
Chelates are used extensively in water treatment to remove heavy metals such as chromium. EDTA is often used to treat people with heavy metal poisoning. Chelates are part of our biology and are present in human blood.
Both EDTA and PDTA chelators are forms of acetic acid: Ethylene Diamine Tetra Acetic acid and Propylene Diamine Tetra Acetic acid.
