How X-ray absorption spectroscopy is making hydrogen fuel more efficient

Hydrogen fuel could be a promising alternative to fossil fuels and other polluting forms of energy. When consumed in a fuel cell, the only waste product is water, making it a much cleaner process than burning fossil fuels. However, producing hydrogen fuel is energy-inefficient and can emit greenhouse gases, depending on the method.

Hydrogen production: inefficient and costly

Hydrogen is most often produced from steam reforming, in which steam reacts with hydrocarbon fuels to produce hydrogen. Greenhouse gas emissions from this process are estimated to be 9.35 kg of CO2/kg H2, which is equivalent to the emissions of an average passenger vehicle driving approximately 23 miles for every kilogram of hydrogen produced. Approximately 95% of all hydrogen is produced from steam reforming of natural gas.

Another hydrogen production process involves separating water into oxygen and hydrogen by electrolysis, which uses electricity to split water molecules. Unlike steam reforming, this process can be carbon-free, depending on the electricity source. While this method is promising, it is inefficient, expensive and requires a lot of electricity. Electrolysis costs $4.95/kg of hydrogen produced compared to $4.13/kg of hydrogen produced for steam reforming. Electricity accounts for up to 78% of the cost. Electrolysis needs to become more efficient to encourage widespread adoption.

Producing hydrogen by biomass oxidation

Now, a team at Canadian Light Source (CLS) may have found a better way. Their electrolysis process combines water with hydroxymethylfurfural (HMF), an organic compound derived from the breakdown of plant materials such as pulp and paper residue. The HMF is then oxidized at the anode, producing hydrogen at the cathode. “At the same energy input, we can double the production of hydrogen,” says Hamed Heidarpour, a Ph.D. student in Ali Seifitokaldani’s Electrocatalysis Lab at McGill University in Montreal, and an author on the paper.

The team also developed a chromium-coated copper catalyst to make the reaction even more energy-efficient. They used X-ray absorption spectroscopy (XAS) to study the catalyst at the atomic scale, showing that the chromium kept the copper in its metallic state, allowing the catalyst to work better and longer. The researchers published their findings in the Chemical Engineering Journal.

Studying catalysts at the atomic scale

XAS uses synchrotron radiation to provide information about the electronic, structural and magnetic properties of materials. In this process, a monochromatic X-ray beam is directed at the sample. Then, photon energy is gradually increased and scanned through one of the absorption edges of the elements contained within the sample, when the photon energy matches the binding energy of a core electron in an atom. When this happens, photons eject core electrons from atoms in the sample, and a large increase in absorption occurs.

The photon energy continues to increase past the absorption edge, and a series of oscillations is measured as a function of energy. The oscillations can be used to determine the atomic number and the distance and coordination number of the atoms surrounding the element. Each element has a unique X-ray absorption spectrum, which allows researchers to determine the elements present in the sample as well as its geometry, electron density, oxidation state and other properties.

XAS is element-specific and works on crystalline or amorphous materials. It provides local structural information that is vital for studying catalysts, battery materials, materials under operating conditions and more.