KAIST

<(From Left) Professor Hyungjun Kim, Ph.D candidate Dong Hyun Kim, Ph.D candidate Minho M. Kim, Ph.D candidate Junsic Cho, Professor Chang Hyuck Choi, Professor Seung-Jae Shin>

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” (a thin interface where the electrode and electrolyte meet; the electrode is a material through which electricity flows, and the electrolyte is a liquid through which ions move), where electrochemical reactions occur. This study 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.

KAIST (President Kwang Hyung Lee) announced on the 3rd of May that a research team led by Professor Hyungjun Kim from the Department of Chemistry, in collaboration with Professor Chang Hyuck Choi from POSTECH (President Sung Keun Kim) and Professor Seung-Jae Shin from UNIST (President Jong Rae Park), 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.

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.

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).

<Transition from a ‘camel-shaped’ to a ‘bell-shaped’ curve caused by changes in the electric double-layer structure>

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,” adding, “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.”

This study, with Minho Kim, a doctoral student in the Department of Chemistry at KAIST, and Dong Hyun Kim and Junsic Cho, doctoral students from the Department of Chemistry at POSTECH, as co-first authors, was published on March 7 in the international journal Nature Communications.
 ※ Paper title: “Electric double layer structure in concentrated aqueous solution,”
 DOI: 10.1038/s41467-026-70322-5

This research was supported by the Samsung Future Technology Development Program, the InnoCore program of UNIST Hydro*Studio, and the National Research Foundation of Korea (NRF) through the Top-Tier Research Institution Collaboration Platform and Joint Research Support Program, as well as the Nano and Materials Technology Development Program.