Science & Technology (Commonwealth Union) – Hydrogen in its pure form is considered extremely valuable as it is often found in nature in combinations such as water with two Hydrogens and one oxygen.
Ammonia (NH₃) is considered a promising medium for transporting hydrogen because it can be stored and moved relatively easily in liquid form. Hydrogen is widely used in fuel cells, semiconductor production, chemical manufacturing, and a range of other industrial applications. However, conventional ammonia cracking requires very high temperatures to split the compound into hydrogen and nitrogen. The resulting mixture must then undergo additional purification to obtain hydrogen of sufficient quality for many applications.
Researchers at MIT have developed an electrochemical technique that encourages hydrogen to be released from ammonia while simultaneously separating and concentrating it into a highly purified stream.
In a newly published study, the team demonstrated that its technology can produce streams of hydrogen with both high concentration and high purity.
“We have shown the ability to use electrochemistry to drive thermodynamically uphill and kinetically difficult dehydrogenation reactions,” explained Yogesh Surendranath, who is the Professor of Science and a professor of chemistry and chemical engineering. “In this case, we studied the conversion of ammonia and a liquid organic molecule because of their importance as possible hydrogen carriers for a hydrogen economy. But the concepts we learned here could in principle be translated further, and we’re actively working on translating it to other important dehydrogenation reactions.”
Surendranath serves as the study’s corresponding author. The research, that appeared recently in Nature, was led by MIT postdoctoral researcher Rui Zeng, who is now a professor of materials science and engineering at the Harbin Institute of Technology in Shenzhen, China.
Hydrogen plays an important role in semiconductor production and chemical manufacturing. It is also used as an energy carrier in fuel cells, where hydrogen reacts with oxygen to produce electricity without combustion. However, broader adoption of hydrogen will depend on developing more practical methods for storing and transporting it.
Transporting hydrogen in its gaseous form can be challenging because it generally requires either high-pressure compression or liquefaction. A potential alternative is to chemically bind hydrogen within compounds that are already liquids or can be easily converted into liquid form, allowing the hydrogen to be released when and where it is required.
Ammonia is considered a promising hydrogen-storage and transport medium because it is already manufactured and shipped over long distances on a large scale. However, extracting hydrogen from ammonia presents significant technical challenges. The process, called “cracking,” typically requires temperatures above 500 degrees Celsius to achieve high reaction rates and efficient conversion. Once the reaction occurs, the resulting hydrogen must also be separated from nitrogen and any ammonia that remains unreacted.
Surendranath indicated that they wanted to seek out if they had the ability to utilize the electrical inputs to drive what would otherwise be a not desirable dehydrogenation reaction, and simultaneously carry it out in a way that would divide the hydrogen from the hydrogen carrier, so that it end up very pure and could be utilized directly in a fuel cell or other application that would need a high purity hydrogen stream.
At the heart of the researchers’ new system is a palladium-based membrane that separates hydrogen and is connected to a hydrogen-producing electrode by a molten hydroxide electrolyte. The membrane allows hydrogen to pass through selectively while blocking the other substances present in the reaction mixture.
In the process, ammonia is initially broken down through dehydrogenation using a catalyst made from ruthenium and cesium. The hydrogen produced then moves toward the separation membrane, with the membrane’s other side exposed to the molten hydroxide electrolyte.
An electrochemical potential across the membrane effectively acts like a “vacuum” for hydrogen, creating a powerful force that drives hydrogen through the membrane. As it crosses, hydrogen is separated into protons and electrons. The protons move through the molten electrolyte, while the electrons flow through an external electrical circuit. They eventually meet again at a second electrode, where they combine to produce hydrogen gas.
