Future in fuel cells

Transportation may currently belong to fossil fuels and biofuels, with electric cars coming up fast behind, but the future will very likely belong to hydrogen-powered fuel cells. But that doesn’t necessarily mean that biomass will no longer have a role to play, as two recent studies make clear.

This is because the rise of hydrogen-powered fuel cells will obviously require the production of a lot of hydrogen. At the moment, the main route to producing hydrogen at industrial scales, mainly for use in the production of ammonia, is the steam reforming of methane, in which methane is reacted with steam to form hydrogen and carbon monoxide. The carbon monoxide can then be reacted with steam at slightly lower temperatures to form more hydrogen and carbon dioxide.

Because this process requires lots of energy and produces carbon dioxide as a byproduct, it’s a far from ideal way for producing hydrogen at the massive scales required for the widespread adoption of hydrogen-powered fuel cells. The ideal way would involve splitting water via electrolysis, with all the electric power coming from renewable sources such as solar and wind, in which case the whole process wouldn’t use any fossil fuels or produce any carbon dioxide. Unfortunately, the technologies required to do this efficiently at large scales are not yet available.

A slightly less ideal option, but still much better than steam-reforming methane, is to generate the hydrogen from plant biomass, which contains a lot of hydrogen in the form of hydrocarbons. Two research groups have now developed catalysis-based methods for doing this, although both methods require the biomass to be converted into something else first.

For Eric McFarland and his team at the University of California, Santa Barbara, US, that something else is methane, because McFarland and his team were actually looking to develop a greener alternative to steam reforming. Most of the methane currently used to produce hydrogen is derived from natural gas, but it can also be produced from biomass by methane-producing microbes, as regularly happens at landfills.

As McFarland and his team reported at the end of last year in a paper in Science, their greener alternative involves splitting methane (CH₄) directly into carbon and hydrogen. Previous research has shown that metals such as nickel and platinum can catalyze this splitting reaction, but in the process they tend to get covered in carbon, which deactivates them. Alternatively, molten forms of these metals can also catalyze the reaction, in which case the carbon can simply be scraped off the top of the molten metal, but these metals have high melting temperatures, making the process very energy intensive.

So McFarland and his team decided to try dissolving the catalytic metals in molten forms of metals such as bismuth, lead and indium, which melt at lower temperatures and can thus act as solvents. They tried bubbling methane through various combinations of metals, finding that nickel in molten bismuth worked best, able to convert 95% of methane into just hydrogen and carbon.

By contrast, for a team from China and Germany led by Yang Li at Xi’an Jiaotong University that something else is formic acid. Li and his team were specifically looking for a way to convert biomass directly into hydrogen, and they came up with a one-pot, two-step method for doing just that. This involves first converting the biomass to formic acid by mixing it with sodium metavanadate in sulfuric acid with dimethyl sulfoxide at 160°C. Next, they add sodium hydroxide to this solution to neutralize the acid and then add a solution containing a specially-prepared iridium catalyst bearing an imidazoline moiety, which can convert the formic acid to hydrogen.

As they report in a paper in Nature Catalysis, this method could release hydrogen from a wide range of different types of biomass and organic waste, including wheat straw, corn straw, cardboard and newspaper, with yields of up to 95%. Furthermore, it only produced methane and carbon monoxide as byproducts at trace concentrations, meaning the hydrogen would be pure enough to be used directly in a fuel cell.

The views represented here are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd. or of the SCI.