First direct measurement of elusive Donnan potential

Scientific achievement

At the Advanced Light Source (ALS), researchers performed the first direct measurement of the Donnan electrical potential, which arises from an imbalance of charges at membrane-solution interfaces.

Significance and impact

Considered unmeasurable for over a century, the Donnan potential is relevant to a wide range of fields, from cell biology to energy storage and water desalination.

A breakthrough with great potential

The Donnan electrical potential arises from an imbalance of charges at the interface of a charged membrane and a liquid, and for more than a century it stubbornly eluded direct measurement. Many researchers had even written off such a measurement as impossible. Now, using ambient-pressure x-ray photoelectrion spectroscopy (APXPS) at the ALS, scientists directly measured the Donnan potential for the first time.

The ability to probe the characteristics of this potential at membrane-solution interfaces could yield new insights in biology, energy science, and materials science. For example, the Donnan potential plays a critical role in biological functions ranging from muscle contractions to neural signaling. Energy storage and water purification using ion exchange membranes (IEMs) are also important applications involving the Donnan potential.

Read more on the ALS website

Image: Left: Schematic of the x-ray experiment. Right: The presence of fixed ions inside a membrane generates an electrochemical potential gradient (the Donnan potential) that leads to more counter-ions (with charge opposite that of the fixed ions) diffusing from the solution to the membrane relative to co-ions (which have the same charge as the fixed ions).

Modelling electrochemical potential for better Li-batteries

To understand the electrochemical potential of lithium-ion batteries, it’s important to decipher the chemical processes at electrode interfaces occurring during device activity. Using HIPPIE beamline, a research group investigated and modelled the influence of electrochemical potential differences in operando in these batteries.

“With our experiments at HIPPIE, we had the opportunity to look at battery materials and interface reactions under operating conditions exploring the capabilities of the electrochemical setup at the end station,” said Julia Maibach, study author and professor at the Institute for Applied Materials – Energy Storage Systems at Karlsruhe Institute of Technology (KIT) in Germany. “We were among the first users testing the electrochemical set up including the glove box for inert sample transfer.”

Why study electrochemical potential difference in batteries? This phenomenon drives the transfer of charged particles to different phases in redox reactions at battery electrode-electrolyte interfaces. In simple terms, the difference enables the chemical reaction necessary for Li-ion battery function.

Read more on the MAX IV website

Image: Research group studies gold and copper model electrodes at MAX IV’s HIPPIE beamline with Ambient Pressure Photoelectron Spectroscopy (APPES) during lithiation

Credit: MAX IV Laboratory