Synthesising catalysts for energy storage

Materials World magazine
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31 Jan 2019
Using the inner shell spectroscopy (ISS) beamline at NSLS-II, to analyse nanomaterials that work as catalysts for fuel cells. Credit: Brookhaven National Laboratory.

Research into catalyst materials for hydrogen fuel cells could significantly reduce the cost of viable renewable energy. Rhiannon Garth Jones reports. 

The variable nature of renewable power sources, such as solar and wind, is often cited as a major obstacle to the uptake of widespread renewable energy. Affordable, reliable energy storage would remove this barrier. Already, large-scale battery installations are now increasingly being added to electricity grids around the world. In 2017, more than 1GW of energy storage capacity was added globally. 

Hydrogen fuel cells are a particularly promising technology in this field. They currently rely on platinum catalysts to convert stored chemical energy into electricity, which makes them too expensive for commercial viability. National Synchrotron Light Source II (NSLS-II), USA, scientist Adrian Hunt says, ‘Finding a cheap and effective catalyst for hydrogen fuel cells is basically the holy grail for making this technology more feasible’.

He and fellow scientists at NSLS-II have collaborated with researchers at various institutions, including the University of Akron (UoA), USA, to work on a synthesised, cheaper catalyst – Pt3Ni. The reduced platinum content of the synthesised catalyst should make it cheaper to produce on a commercial scale.

Initially, a research team at UoA developed an innovative way of synthesising catalysts from a combination of platinum and nickel that form octahedral-shaped nanoparticles. The team realised this catalyst was one of the most efficient replacements for pure platinum but didn't fully understand why the nanoparticles were octahedral-shaped, leading to their collaboration with other institutions.

Understanding the growth pathway of faceted alloy nanoparticles at the atomic level is crucial to developing better hydrogen fuel cells, but modern characterisation tools struggle to deal with the complexity of the growth process.

University of Akron Principal Investigator of the Catalysis Lab, Zhenmeng Peng, explains, 'Understanding how the faceted catalyst is formed plays a key role in establishing its structure-property correlation and designing a better catalyst. The obtained knowledge should provide useful guidance in design and synthesis of platinum alloy nanoparticles with facet control.'

To further their understanding of the growth process in the formation of Pt3Ni, the team used the ultrabright X-rays at NSLS-II and the advanced capabilities of NSLS-II’s in situ and Operando Soft X-ray Spectroscopy beamline. The ambient-pressure X-ray photoelectron spectroscopy technique allowed them to study the surface composition and chemical state of the metals in the nanoparticles during the growth reaction.

The work demonstrates that octahedral Pt3Ni is initiated by platinum nuclei. Their continuous reduction simultaneously catalyses the nickel reduction, resulting in a mixed alloy formation with moderate elemental segregation. Carbon monoxide molecules modulate the facet formation and induce nickel segregation to the surface, which causes the nanoparticle shape to evolve from a spherical cluster to an octahedron.

The findings in the paper, Deconvolution of octahedral Pt3Ni nanoparticle growth pathway from in situ characterizations, published in Nature Communications, highlight the importance of advanced in situ techniques in researching catalyst preparation and morphology control. They also demonstrate the significant role the surface-enriched nickel plays in controlling the octahedra development, which is crucial to the alloy’s performance. 

Peng says the group will continue their research efforts to develop new faceted platinum alloy nanoparticle catalysts that are active and durable in fuel cell reactions. He identifies the 'long-term durability of the nanoparticle catalyst materials, which determines lifetime of fuel cell operation, is equally important and yet to be overcome' as the next main challenge for researchers.