Hydrogen fuel cells show great potential as renewable and eco-friendly energy sources for land, air, and sea transportation.
But typical catalysts used to drive chemical processes in hydrogen fuel cells are too costly and inefficient to support a large-scale commercial transition away from present technologies.
In new interdisciplinary study published in ACS Catalysis, Northeastern scientists have identified a new class of catalysts that, because of their particular non-noble-metal nature, could replace the platinum-based standard that has blocked hydrogen from developing in the fuel sector.
“We are swiftly migrating to electric forms of transportation, and as I see it, batteries are merely a transitionary phase,” says Sanjeev Mukerjee, a distinguished professor of chemistry and chemical biology at Northeastern, who is a co-author of the paper. “It’s not the ultimate answer to replacing fossil fuels.”
It’s in hydrogen, or “hydrogen carriers”—larger molecules in which hydrogen is merely one part—that the answer rests, he argues. The most prevalent element in the universe, hydrogen works as an energy carrier and may be extracted from water, fossil fuels or biomass and used as fuel. Hydrogen fuel cells turn hydrogen into energy; and unlike the internal combustion engine, which produces harmful and carcinogenic chemical byproducts, hydrogen fuel cells only produce water—actual consumable water—as a result of the chemical reaction.
“The main obstacle right now is, one: infrastructure for the fuel, i.e., hydrogen or a hydrogen carrier; and number two is the high expense of catalysts, because the current state-of-the-art demands noble metals,” Mukerjee explains. “So there are parallel initiatives to both lessen the noble metal loading and find more sustainable catalysts using materials that are quite common on earth.”
Catalysts are employed in hydrogen fuel cells to speed up the energy conversion process, called the oxygen reduction reaction. A sustainable catalyst is one that is made of “earth-abundant elements” and one that, when oxygen is brought into the chemical reaction, does not form carbon, says Arun Bansil, university distinguished professor of physics at Northeastern and co-author of the study.
As it pertains, Northeastern researchers have been looking at a certain type of catalysts, namely so-called “nitrogen-coordinated iron catalysts,” as potentially sustainable candidates. A nitrogen-coordinated iron catalyst is molecularly characterized as an iron atom surrounded by four nitrogen atoms. The nitrogen atoms are called “ligands,” or molecules that attach to a central metal atom to form a bigger complex.
Bansil states, “This structure is well-known.” “What we have demonstrated very conclusively in this paper is that the addition of a fifth ligand—that is, four nitrogens plus another one—can result in a much more stable and robust electrocatalyst, thereby establishing a new paradigm or pathway for the rational design of this class of catalysts for fuel cell applications.”
The fifth ligand, according to Bansil, also increases the catalyst’s endurance. When oxygen is added to this structure, it appears that this fifth ligand is able to retain the iron in the plane of the iron-nitrogen molecule.
If the fifth ligand is not there, Bansil notes, the iron is displaced from the plane of the iron-nitrogen in many of these complexes when the oxygen is put in, so making the catalyst “less durable.”
Researchers employed X-ray emission spectroscopy and Mössbauer spectroscopy, tools used in computational chemistry, to observe these effects.
“It’s not enough to know that something looks to be working better; you must also understand why it’s working better,” he argues. “Because then we are in a position to generate superior materials through a reasonable design approach.”
Qingying Jia, a staff scientist at Northeastern, and Bernardo Barbiellini, a computational and theoretical physicist from Lappeenranta University of Technology who is visiting Northeastern, contributed in the research.
Mukerjee explains that the development represents a number of “firsts” in the sector.
“The computational technique has helped us discover the catalytic sites as they evolve during preparation, and it has also provided a picture of which of these [catalysts] are the most stable,” he explains.
