Hydrogen fuel cells are a technology that promises to produce clean and renewable energy, but the cost and activity of their cathode materials is a major challenge for commercialization. Many fuel cells need expensive platinum-based catalysts – substances that start and accelerate chemical reactions – to help convert renewable fuels into electrical energy. To make hydrogen fuel cells commercially feasible, scientists are looking for more affordable catalysts that provide the same efficiency as pure platinum.
"Like batteries, hydrogen fuel cells convert stored chemical energy into electricity. The difference is that you use rechargeable fuel so that, in principle, & # 39; will last forever, "said Adrian Hunt, a scientist at National Synchrotron Light Source II (NSLS-II), US Department of Energy (DOE) Office of User Science Facilities at DOE's Brookhaven National Laboratory." Finding a catalyst that is cheap and effective for hydrogen fuel cells it is basically the holy grail to make this technology more feasible. "
Taking part in the search for fuel cell cathode materials around the world, researchers at the University of Akron developed a new method for synthesizing catalysts from metal combinations – platinum and nickel – which form octahedral nanoparticles (eight sides). While scientists have identified this catalyst as one of the most efficient substitutes for pure platinum, they have not fully understood why it grows in the octahedral form. To better understand the growth process, researchers at the University of Akron collaborate with various institutions, including Brookhaven and NSLS-II.
"Understanding how faceted catalysts are formed plays a key role in building structure-property correlations and designing better catalysts," said Zhenmeng Peng, principal investigator of the catalysis laboratory at Akron University. "The growth process for the nickel-platinum system is quite sophisticated, so we are working with several experienced groups to overcome the challenges. Advanced techniques at Brookhaven National Lab are very helpful to study the topic of this research. "
Using ultrabright X-rays in NSLS-II and the advanced capabilities of NSLS-II's In situ and Operando Soft X-ray Spectroscopy (IOS) beamline, IOS, the researchers revealed the chemical characterization of the catalyst growth path in real time. Their findings are published on Nature Communication.
"We used a research technique called ambient-pressure x-ray photoelectron spectroscopy (AP-XPS) to study the surface composition and chemical state of metals in nanoparticles during growth reactions," said Iradwikanari Waluyo, principal scientist at IOS and a paper co-correspondent writer research. "In this technique, we direct the x-ray to the sample, which causes electrons to be released. By analyzing the energy of these electrons, we can distinguish the chemical elements in the sample, as well as their chemical and oxidation states. "
Hunt, who is also a writer on paper, added, "This is similar to how sunlight interacts with our clothes. Sunlight is about yellow, but once you touch someone's shirt, you can tell if the shirt is blue, red or green. "
Instead of color, scientists identify chemical information on the surface of the catalyst and compare it to the interior. They found that, during the growth reaction, platinum metal was formed first and became the nucleus of nanoparticles. Then, when the reaction reaches a slightly higher temperature, platinum helps form metallic nickel, which then segregates to the surface of the nanoparticles. At the final stage of growth, the surface becomes approximately the same mixture of the two metals. This interesting synergistic effect between platinum and nickel plays an important role in the development of octahedral nanoparticles, as well as their reactivity.
"The nice thing about this finding is that nickel is a cheap material, while platinum is expensive," Hunt said. "So, if nickel on the surface of the nanoparticles catalyzes the reaction, and these nanoparticles are still more active than platinum by themselves, then hopefully, with further research, we can know the minimum amount of platinum to add and still get high activity, creating catalysts that are more cost-effective . "
This finding depends on iOS's advanced capabilities, where researchers are able to run experiments on gas pressure higher than what is usually possible in conventional XPS experiments.
"On iOS, we can follow changes in the composition and chemical conditions of nanoparticles in real time during real growth conditions," Waluyo said.
Additional x-ray studies and electron imaging were completed at Advanced Photon Source (APS) at DOE's Argonne National Laboratory – DOE's other Office of Science Users Facility – and University of California-Irvine, respectively, completing work at NSLS- II.
"This fundamental work highlights the important role of separate nickel in forming octahedral-shaped catalysts. "We have achieved more insight into the control of the shape of catalyst nanoparticles," Peng said. "Our next step is to study the catalytic properties of faceted nanoparticles to understand structure-property correlation."