June 5, 2017

Electrocatalyst nanostructures key to improved fuel cells, electrolyzers

WEST LAFAYETTE, Ind. - Purdue University scientists’ simulations have unraveled the mystery of a new electrocatalyst that may solve a significant problem associated with fuel cells and electrolyzers.

Fuel cells, which use chemical reactions to produce energy, and electrolyzers, which convert energy into hydrogen or other gases, use electrocatalysts to promote chemical reactions. Electrocatalysts that can activate such reactions tend to be unstable because they can corrode in the highly acidic or basic water solutions that are used in fuel cells or electrolyzers.

A team led by Jeffrey Greeley, an associate professor of chemical engineering, has identified the structure for an electrocatalyst made of nickel nanoislands deposited on platinum that is both active and stable. This design created properties in the nickel that Greeley said were unexpected but highly beneficial.

“The reactions led to very stable structures that we would not predict by just looking at the properties of nickel,” Greeley said. “It turned out to be quite a surprise.”

Greeley’s team and collaborators working at Argonne National Laboratory had noticed that nickel placed on a platinum substrate showed potential as an electrocatalyst. Greeley’s lab then went to work to figure out how an electrocatalyst with this composition could be both active and stable.

Greeley’s team simulated different thicknesses and diameters of nickel on platinum as well as voltages and pH levels in the cells. Placing nickel only one or two atomic layers in thickness and one to two nanometers in diameter created the conditions they wanted.

“They’re like little islands of nickel sitting on a sea of platinum,” Greeley said.

The ultra-thin layer of nickel is key, Greeley said, because it’s at the point where the two metals come together that all the electrochemical activity occurs. And since there are only one or two atomic layers of nickel, almost all of it is reacting with the platinum. That not only creates the catalysis needed, but changes the nickel in a way that keeps it from oxidizing, providing the stability.

Collaborators at Argonne then analyzed the nickel-platinum structure and confirmed the properties Greeley and his team expected the electrocatalyst to have.

Next, Greeley plans to test similar structures with different metals, such as replacing platinum with gold or the nickel with cobalt, as well as modifying pH and voltages. He believes other more stable and active combinations may be found using his computational analysis.

The U.S. Department of Energy supported the research. The research was published in May by the journal Nature Energy.

Writer: Brian Wallheimer: 765-532-0233, brian.wallheimer@gmail.com 

Contact: Jim Bush, 765-494-2077, jsbush@purdue.edu         

Source: Jeffrey Greeley, 765-494-1282, jgreeley@purdue.edu 


ABSTRACT

Stabilization of ultrathin (hydroxy)oxide films on transition metal substrates for electrochemical energy conversion

Zhenhua Zeng1, Kee-Chul Chang2, Joseph Kubal1, Nenad M. Markovic2 and Jeffrey Greeley1

1 School of Chemical Engineering, Purdue University, West Lafayette, Indiana

2 Materials Science Division, Argonne National Laboratory, Argonne, Illinois

Design of cost-effective electrocatalysts with enhanced stability and activity is of paramount importance for the next generation of energy conversion systems, including fuel cells and electrolysers. However, electrocatalytic materials generally improve one of these properties at the expense of the other. Here, using density functional theory calculations and electrochemical surface science measurements, we explore atomic-level features of ultrathin (hydroxy)oxide films on transition metal substrates and demonstrate that these films exhibit both excellent stability and activity for electrocatalytic applications. The films adopt structures with stabilities that significantly exceed bulk Pourbaix limits, including stoichiometries not found in bulk and properties that are tunable by controlling voltage, film composition, and substrate identity. Using nickel (hydroxy)oxide/Pt(111) as an example, we further show how the films enhance activity for hydrogen evolution through a bifunctional effect. The results suggest design principles for this class of electrocatalysts with simultaneously enhanced stability and activity for energy conversion.


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