Electric double layer unlocks molecular switch behind battery and hydrogen reactions

From smartphone charging to hydrogen production, the fundamental principles of energy technology have been revealed. Korean researchers have, for the first time, identified how molecular structures change within the ultra-small space called the “electric double layer.” The study, published in the journal Nature Communications, opens a new path to simultaneously improve efficiency and performance in battery, hydrogen, and carbon-neutral technologies by reducing energy loss and selectively inducing desired reactions.

Unraveling the electric double layer

A research team led by Professor Hyungjun Kim from the Department of Chemistry, in collaboration with Professor Chang Hyuck Choi from POSTECH and Professor Seung-Jae Shin from UNIST, has identified structural phase transitions (phenomena in which the state or arrangement of matter changes) occurring within the electric double layer.

In particular, they revealed at the molecular level the cause of the phenomenon in which the pattern of electrical storage capacity (capacitance) changes from a camel shape to a bell shape depending on electrolyte concentration.

Electrochemical reactions occur within the ultra-small space called the electric double layer, where the electrode and electrolyte meet. In the field of electrochemistry, it has long been known that as electrolyte concentration increases, the capacitance curve changes from a camel shape with two peaks to a bell shape with a single peak, but the underlying cause had remained unexplained at the molecular level.

What happens at each electrode

Through atomically precise simulations and experiments, the research team discovered that two key changes occur depending on the voltage applied to the electrode.

At the cathode, water molecules collectively realign in a uniform direction, while at the anode, anions (negatively charged particles) accumulate densely on the surface, forming a two-dimensional structure in a phenomenon known as condensation.

These two processes each create peaks in the capacitance curve, and as electrolyte concentration increases, they merge into one, causing the curve to transition from a camel to a bell shape.

In simple terms, on one side, water molecules line up in an orderly fashion, while on the other side, ions gather densely. As the concentration increases, these two phenomena merge into one, and the graph changes from two peaks to a single peak.

Mapping invisible electrochemical changes

In particular, the research team presented, for the first time in the world, a phase diagram that shows at a glance how the structure of the electric double layer changes depending on electrode potential (the voltage applied to the electrode) and electrolyte concentration. They also experimentally validated these theoretical predictions in real time using infrared spectroscopy (ATR-SEIRAS, an experimental technique that observes molecular movements in real time).

In simple terms, they created a map that shows how structures change under different conditions and verified through experiments that the map is accurate.

Professor Hyungjun Kim stated, “This study is meaningful in that it provides the first understanding of the otherwise invisible, microscopic electrochemical reaction environment and opens the way to design it. If we can precisely control phase transitions in the electric double layer, we will be able to accurately enhance the performance of energy technologies, such as increasing battery charging speed or maximizing hydrogen production efficiency.”