University of Michigan researchers find pathway to better battery chemistry

Researchers at the University of Michigan have learned why flow batteries, using the metal cerium in a sulfuric acid electrolyte, fall short on voltage, a discovery that could pave the way for better battery chemistry in the future.

According to a study published in the journal JACS Au, scientists found that, while cerium can store energy at a relatively high voltage and low cost, it struggles to make electric charges transfer to and from an electrode efficiently due to water and sulfate molecules impeding the transfer. 

Researchers used X-ray absorption methods to observe the cerium ions' bond and association with sulfates and water, followed by computer simulations. The process identified the electrolytes that have fast reaction rates and high efficiency, paving the way for improved battery chemistry, according to a release on Michigan's website.

Flow batteries could be a solution for storing intermittent sources of renewable electricity, such as solar and wind power. They can store large quantities of energy by keeping the chemical potential in liquid form, using two electrolytes that flow through porous electrodes to charge and discharge. 

The challenge with cerium lies, however, in the slow transfer of electric charges to and from the electrode, researchers explained.

"We find that when cerium is short three electrons, it is only surrounded by water molecules, while when it gives up that fourth electron, sulfate or bisulfate ions are hanging off the cerium ion," said Nirala Singh, assistant professor of chemical engineering and corresponding author of the study. 

"As a result of this, when we oxidize cerium by taking away that electron, or reduce it by giving the electron back, both an electron transfer has to occur and the molecules around it have to rearrange," she added.

The researchers discovered that the reaction is asymmetrical, meaning the oxidation and reduction behave differently. Because of this, the Marcus theory, the traditional theory for predicting the rate of electron transfer, isn't enough. Instead, researchers can use Marcus theory to figure out the electron transfer piece and then add in the effects of rearrangement in a two-step process.

"Through this study we have a better understanding of how cerium ions behave in acidic electrolytes during charge transfer," said Cailin Buchanan, a study co-author. "This understanding will help us and future researchers to design more efficient cerium-based batteries that have less voltage loss during charging and discharging."

The two-step method will enable identifying electrolytes that have fast reaction rates and high efficiencies, ultimately aiming to use electrolytes that won't store different amounts of energy in the complexes around the oxidized or reduced cerium ion. 

This discovery could improve other chemical processes that rely on cerium, such as the manufacturing of carbon-based products and wastewater decontamination. 

The research was supported by the University of Michigan and the Dow Sustainability Fellows Program, the release said.