A molecular “Tetris” strategy could help solve one of green chemistry’s most stubborn problems: producing ammonia without wasting most of the reaction’s energy on hydrogen.
Researchers led by Professor Yousung Jung at Seoul National University have proposed a catalyst-design principle that selectively suppresses hydrogen evolution while preserving the electrochemical reaction that converts nitrogen into ammonia. Their approach relies not on removing protons from the reaction environment, but on controlling how easily those protons can physically reach the electrode surface. The concept could offer a new route toward cleaner ammonia production powered by renewable electricity.
Ammonia is increasingly viewed as a practical carrier for hydrogen energy. It stores a large amount of hydrogen, remains liquid at room temperature under moderate conditions, and is easier to transport than gaseous hydrogen. Today, most ammonia is manufactured through the Haber–Bosch process, which requires high temperatures and pressures and consumes substantial amounts of energy. Because the process is commonly powered by fossil fuels, it also produces significant carbon dioxide emissions.
Electrochemical nitrogen reduction has emerged as a possible alternative. In principle, the process uses nitrogen, water and electricity to produce ammonia, potentially allowing production facilities to operate near wind or solar farms rather than relying on massive centralized chemical plants. The central obstacle, however, is that the electrode preferentially produces hydrogen instead of ammonia. This competing hydrogen evolution reaction consumes electrons and protons that would otherwise contribute to nitrogen reduction, sharply reducing ammonia yields.
Previous strategies have often attempted to control hydrogen evolution by changing the proton concentration or acidity of the electrolyte. That solution creates a fundamental trade-off: protons are needed not only to generate hydrogen but also to convert nitrogen into ammonia. Reducing their availability can therefore suppress both reactions. Jung’s team instead focused on the reaction interface—the narrow region where the electrolyte, catalyst and electrode meet—and asked whether the two reactions could be separated geometrically.
The researchers designed a reaction environment in which proton donors encounter greater steric hindrance as they approach the electrode surface. Steric hindrance occurs when the size and arrangement of molecules make it physically difficult for another molecule to reach a reactive site. In this case, the researchers effectively create a molecular gate around the electrode. Proton donors can still exist in the surrounding solution, but their paths toward the surface become restricted, making the initial step of hydrogen production less favorable.
That initial step is known as the Volmer reaction. During the Volmer reaction, a proton receives an electron and forms an adsorbed hydrogen atom on the electrode. Two such hydrogen atoms can subsequently combine to form molecular hydrogen, or an adsorbed hydrogen atom can react with another proton and electron. By making it harder for proton donors to reach the electrode, the new strategy raises the energy barrier for the Volmer reaction and slows the entire hydrogen evolution pathway.
The geometry of nitrogen reduction is different. According to the researchers, nitrogen molecules protrude outward from the catalyst environment, allowing protons to interact with nitrogen rather than needing to reach the electrode surface directly. This difference means that the same steric barrier that obstructs hydrogen evolution has a much smaller effect on nitrogen reduction. The result is a form of molecular selectivity based on access and positioning rather than solely on chemical composition.
The team used microkinetic modeling to examine how these competing pathways respond to changes in steric hindrance and applied voltage. Microkinetic models describe the rates of individual elementary reactions by tracking intermediates and solving differential equations for the overall reaction network. The simulations indicated that increasing steric hindrance can maintain high Faradaic efficiency across a broad voltage range. Faradaic efficiency measures the fraction of supplied electrical charge that produces the desired product—in this case, ammonia rather than hydrogen.
The findings could address a major limitation in electrochemical ammonia research, where Faradaic efficiencies have often remained near 70 percent or lower. The researchers report that their design principle could raise the value toward nearly 100 percent under modeled conditions, although future catalyst development and experimental validation will be essential. The strategy may also be transferable to other electrochemical systems in which a desired reaction competes with an unwanted one, including carbon dioxide reduction.
The study, published in the Journal of the American Chemical Society, presents the reaction interface itself as a programmable component of catalyst design. Rather than treating the electrolyte as a passive medium, the approach uses molecular shape to control which reactants can reach specific locations. Jung’s team plans to identify highly active catalyst materials that can incorporate this principle and to test the concept across additional reactions. If successful, the work could help bring renewable-powered ammonia production closer to practical use while offering a broader blueprint for controlling chemical reactions at the atomic scale.
Subject of Research: Electrochemical nitrogen reduction and selective suppression of the hydrogen evolution reaction
Article Title: Selective Suppression of Hydrogen Evolution in Electrochemical Nitrogen Reduction through Steric Control of Proton Donors
News Publication Date: July 22
Web References: https://doi.org/10.1021/jacs.6c07080
References: Journal of the American Chemical Society, DOI: 10.1021/jacs.6c07080
Image Credits: Seoul National University College of Engineering
Keywords
Electrochemical ammonia production, nitrogen reduction reaction, hydrogen evolution reaction, steric hindrance, catalyst design, green hydrogen, renewable energy, Faradaic efficiency, microkinetic modeling, Seoul National University
