Today, over 80% of the world’s hydrogen is produced by reforming natural gas, specifically methane (CH4). This process is very resource intensive, consuming more than 205 billion cubic metres of natural gas per year.
Despite its high consumption, it remains cost-effective, with production costs ranging from $1 to $3 per kilogram of hydrogen. This is significantly cheaper than producing hydrogen by electrolysis, costing between $5 and $10 per kilogram. While this form of hydrogen is suitable for many industrial applications, it’s considered too impure for others, such as powering fuel cells for electricity generation or manufacturing components for the electronics and semiconductor industries.
Fuel cells, particularly those in hydrogen-powered vehicles, require a platinum catalyst to operate efficiently. However, these catalysts only work optimally with high-purity hydrogen.
If hydrogen is obtained from methane, however, the incompleteness of the chemical conversion – which is all but guaranteed – will lead to the presence of impurities in the gas. These impurities are a serious obstacle and genuinely hazardous, as they can “poison” many catalysts that are regularly found in the chemical sector.
This issue is especially pressing today when the use of fuel cells that consume hydrogen to produce electricity is rising all the time. Fuel cells are seen as an important part of the strategies not only of car companies like Toyota and Hyundai but also of aviation industry players working on hydrogen aircraft for short-haul flights. They are slightly more energy efficient than turbines used in aviation, so the combination of fuel cells with propellers for short-haul aircraft may be a more promising solution than simply burning hydrogen in flight.
Purification with cold… or palladium?
There are, of course, techniques to minimize unwanted impurities in hydrogen production. One effective method is to cool the gas produced. Hydrogen remains in its gaseous form down to -250°C, while almost all other impurities condense into liquids. This phase difference makes it possible to separate the impurities from the hydrogen. This cooling process is particularly useful in the production of ammonia, a key ingredient in fertilizers. Another strategy is to use sorbents, materials that capture and remove the polluting impurities from the gas.
The purity of the resulting product after using this method is still limited. To obtain truly pure hydrogen, you need a method that reliably cuts off everything except the gas needed by the consumer. This can be achieved using a membrane that only allows hydrogen molecules to pass through.
There is a metal with exactly this property: palladium. It is essentially “transparent” to hydrogen molecules. Until recently, membranes based on it were relatively simple, using just a thin layer of palladium on a substrate made from a different material. The main physical requirement of the substrate was its ability to affix itself tightly to the palladium over many cycles.
New approaches
Recently, some development teams have revolutionized the way we purify hydrogen by redesigning separation membranes. They’ve introduced a three-layer structure, similar to a sandwich, with palladium as the outer layers and a vanadium alloy as the middle layer. When hydrogen encounters the first palladium layer, it ‘dissolves’ and breaks down into individual atoms, which are very mobile in vanadium that allows them to pass through the membrane with ease. When they reach the final palladium layer, these hydrogen atoms recombine to form molecular hydrogen and are released from the membrane surface. This innovative approach results in the production of exceptionally pure hydrogen.
The key difference between the sandwich and alternative approaches is not only the reliable “adhesion” of palladium in the membrane design, although this is important. The other advantage is the increased durability of the system as a whole. There is, however, another even more significant factor: membranes of this type produce several times more hydrogen with the same energy input per time unit than previously existing designs. Laboratory tests on one of these next-generation installations are now being finalized before testing in mock industrial systems on a larger scale commences.
The hydrogen produced using this process has a purity is 99.9999 %. The product can be used effectively in the most demanding chemical production processes as well as hydrogen-powered vehicles. At the same time, the cost of purification will be significantly lower than that of previous methods, which are less productive than the sandwich structure based on palladium and vanadium.
Alexander Livshits, Professor of Bonch-Bruevich University
