Cobalt atoms on graphene: a low-cost catalyst for producing hydrogen from water

Rice University catalyst may lead to clean, inexpensive hydrogen production for fuel cells
October 23, 2015

A new catalyst just 15 microns thick has proven nearly as effective as platinum-based catalysts but at a much lower cost, according to scientists at Rice University. The catalyst is made of nitrogen-doped graphene with individual cobalt atoms that activate the process. (credit: Tour Group/Rice University)

Graphene doped with nitrogen and augmented with cobalt atoms has proven to be an effective, durable catalyst for the production of hydrogen from water, according to scientists at Rice University.

The Rice University lab of chemist James Tour and colleagues has developed a robust, solid-state catalyst that shows promise to replace expensive platinum for hydrogen generation. (Catalysts can split water into its constituent hydrogen and oxygen atoms, a process required for fuel cells.)

The latest discovery, detailed in Nature Communications, is a significant step toward lower-cost catalysts for energy production, according to the researchers.

Disordered graphitic carbon doped with nitrogen and augmented with cobalt atoms serves as an efficient, robust catalyst for hydrogen separation from water. The material discovered at Rice University could challenge more expensive platinum-based catalysts. (credit: Tour Group/Rice University)

Cost-effective replacement for platinum

“What’s unique about this paper is that we show … the use of atoms,” Tour said, instead of the conventional use of metal particles or nanoparticles. “The particles doing this chemistry are as small as you can possibly get.”

Even particles on the nanoscale work only at the surface, he explained. “There are so many atoms inside the nanoparticle that never do anything. But in our process, the atoms driving catalysis have no metal atoms next to them. We’re getting away with very little cobalt to make a catalyst that nearly matches the best platinum catalysts.” He said that in comparison tests, the new material nearly matched platinum’s efficiency to begin reacting at a low onset voltage (the amount of electricity it needs to begin separating water into hydrogen and oxygen).

The researchers discovered that heat-treating graphene oxide and small amounts of cobalt salts in a gaseous environment forced individual cobalt atoms to bind to the material. Electron microscope images showed cobalt atoms widely dispersed throughout the samples. They also tested nitrogen-doped graphene on its own and found it lacked the ability to kick the catalytic process into gear. But adding cobalt in very small amounts significantly increased its ability to split acidic or basic water.

The new catalyst is mixed as a solution and can be reduced to a paper-like material or used as a surface coating. Tour said single-atom catalysts have been realized in liquids, but rarely on a surface. “This way we can build electrodes out of it,” he said. “It should be easy to integrate into devices.”

Cobalt atoms shine in an electron microscope image of a new catalyst for hydrogen production invented at Rice University. The widely separated cobalt atoms are bound to a sheet of nitrogen-doped graphene. (credit: Tour Group/Rice University)

“This is an extremely high-performance material,” Tour added. He noted platinum-carbon catalysts still boast the lowest onset voltage. “No question, they’re the best. But this is very close to it and much easier to produce and hundreds of times less expensive.”

Atom-thick graphene is the ideal substrate, Tour said, because of its high surface area, stability in harsh operating conditions, and high conductivity. Samples of the new catalyst showed a negligible decrease in activity after 10 hours of accelerated degradation studies in the lab.

Rice colleagues at the Chinese Academy of Sciences, the University of Texas at San Antonio, and the University of Houston were also involved in the research.

Rice University | H2 evolution

Abstract of Atomic cobalt on nitrogen-doped graphene for hydrogen generation

Reduction of water to hydrogen through electrocatalysis holds great promise for clean energy, but its large-scale application relies on the development of inexpensive and efficient catalysts to replace precious platinum catalysts. Here we report an electrocatalyst for hydrogen generation based on very small amounts of cobalt dispersed as individual atoms on nitrogen-doped graphene. This catalyst is robust and highly active in aqueous media with very low overpotentials (30 mV). A variety of analytical techniques and electrochemical measurements suggest that the catalytically active sites are associated with the metal centres coordinated to nitrogen. This unusual atomic constitution of supported metals is suggestive of a new approach to preparing extremely efficient single-atom catalysts.