Wider use of hydrogen fuel cells could be one step closer thanks to a new engineered material which acts as a filter, separating hydrogen out from a mixture of gases. This could lead to a more efficient way of purifying hydrogen for use in fuel cells.

Hydrogen fuel cells offer a promising clean, alternative source of energy to fossil fuels, which produce no emissions other than water. They can be used to power vehicles and heat buildings, so could provide clear benefits in terms of tackling climate change and air pollution.

However, there are several barriers to overcome before a 'hydrogen economy' can start to become reality. For example, the storage and transport of hydrogen for fuel cells is problematic. Another major hurdle is the production of hydrogen on an industrial scale, which is itself an energy intensive process that, with today's technologies, typically relies on fossil fuels.

This research presents a new, more efficient method of producing hydrogen. Currently, hydrogen production typically results in a mixture of gases, including carbon monoxide, carbon dioxide and methane, from which the hydrogen has to be separated. The researchers devised a way to separate hydrogen using an engineered material.

The structure of the material has a regular array of pores, about 3 nanometres in diameter, and resembles a honeycomb. It offers a very large surface area for the gases which are passed through the material. As the gases flow through the material, they are selectively adsorbed onto the inner surface of the pores, allowing them to be separated: some gases travel faster through the material than others. The elements making up the material interact with the gases, strongly adsorbing carbon dioxide, methane and carbon monoxide and leaving the hydrogen to pass through first. These elements in the pore walls are 'soft' and have a greater attraction for other 'soft' molecules. The hydrogen molecule is small and 'hard' and therefore passes quickly through the material.

The researchers suggest both the composition of the material's framework and the nature of the gases are important in the separation process. By increasing the 'soft' nature of the framework, it becomes more selective. One form of the material, composed of germanium, lead and tellurium, appeared to be four times better at separating hydrogen from carbon dioxide than current processes. The researchers suggest this material could be designed as a membrane to separate hydrogen from a mixture of gases.

The gases adsorbed in the pores can be recovered and the membrane cleaned for reuse. There are other advantages to the material. For example, it can be used between the temperatures of 0�C and room temperature, and is not likely to suffer from sulfur contamination, unlike many other separation materials.

Source: Armatas, G.S. and Kanatzidis, M.G. (2009). Mesoporous germanium-rich chalcogenido frameworks with highly polarizable surfaces and relevance to gas separation. Nature Materials. 8: 217-222.

Contact: m-kanatzidis@northwestern.edu