Iron to catalyze hydrogen fuel cell future

Researchers from Northeastern University in Boston have identified a novel class of catalysts that, because of their particular non-noble-metal nature, could offer a lower-cost alternative to the platinum-based standard that has prevented hydrogen from advancing in the fuel sector.

In the most basic terms, a fuel cell transforms hydrogen and atmospheric oxygen into electricity, water vapor, and a little heat. Using platinum as a catalyst, hydrogen is split and fed into one side of the fuel cell that then breaks into protons and electrons, while oxygen supplied through air introduced to the other side is split into ions.

This offers an intriguing alternative to fossil fuel-burning cars and even the battery electric vehicles that are currently replacing them.

"We are quickly transitioning to electric modes of transportation, and as I see it, batteries are only a transitionary phase," said Sanjeev Mukerjee, a distinguished professor of chemistry and chemical biology at Northeastern and co-author of the study. "It's not the ultimate answer to replacing fossil fuels."

However, platinum's high price, currently around US$900 per ounce, pushes up the costs of fuel cells.

"The biggest bottleneck right now is, one: infrastructure for the fuel, i.e., hydrogen or a hydrogen carrier; and number two is the high cost of catalysts, because the current state-of-the-art requires noble metals," said Mukerjee. "So there are dual efforts to both lower the noble metal loading and find more sustainable catalysts using elements that are very abundant on Earth."

Many believe the answer is hydrogen, or "hydrogen carriers," – larger molecules in which hydrogen is just one part.

Easily the most abundant element in the universe, hydrogen acts as an energy carrier and can be separated from water, fossil fuels or biomass and harnessed as fuel. And instead of toxic or carcinogenic chemical byproducts, hydrogen fuel cells only produce water.

The catalyst in fuel cells is used to speed up the energy conversion process, called the oxygen reduction reaction. A sustainable catalyst would be one that is made of "Earth-abundant materials" and one that, when oxygen is introduced into the chemical reaction, does not produce carbon.

As it relates, Northeastern scientists have been looking for a specific class of catalysts, namely so-called "nitrogen-coordinated iron catalysts," as potentially sustainable candidates.

A nitrogen-coordinated iron catalyst is molecularly defined as an iron atom surrounded by four nitrogen atoms. The nitrogen atoms are called "ligands," or molecules that bind to a central metal atom to form a larger complex.

"This is a well-known structure," said Arun Bansil, university distinguished professor of physics at Northeastern and co-author of the study. "What we have shown very conclusively in this paper is that by adding a fifth ligand-that is, four nitrogens plus another one-that can lead to a much more stable and robust electrocatalyst, thereby opening up a new paradigm or pathway for the rational design of this class of catalysts for applications for fuel cells."

Bansil further states that the fifth ligand also improves the durability of the catalyst, "it appears that this fifth ligand manages to keep the iron in the plane of the iron-nitrogen when oxygen is added into this structure," he explained.

If the fifth ligand is not present, Bansil added, "the iron is dislodged from the plane of the iron-nitrogen in many of these complexes when the oxygen is put in, thereby making the catalyst 'less durable.'"

Researchers used X-ray emission spectroscopy and Mössbauer spectroscopy, techniques used in computational chemistry, to observe these effects.

"It's not enough to just know that something seems to be working better-it's important to know why it is working better," continued Bansil. "Because then we are in a position to develop improved materials through a rational design process."

This advancement represents several "firsts" in the field, said Murkerjee.

"The computational approach has helped us identify the catalytic sites as they evolve during preparation, and it also helped provide a picture of which these [catalysts] are more stable," finished Murkerjee.