Testing the mettle of metal free: X-rayed catalysts

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  • Published: Mar 1, 2018
  • Author: David Bradley
  • Channels: X-ray Spectrometry
thumbnail image: Testing the mettle of metal free: X-rayed catalysts

Metallic nature

Study of graphene catalysts finds trace of manganese, suggests better ultrathin fuel-cell components Credit: Tour Group

A study of graphene catalysts by researchers at Rice University and their colleagues has revealed traces of manganese, which could explain some of the anomalous behaviour of these materials and suggest new ways to make ultrathin fuel-cell components.

Graphene has become the darling of the materials science world offering a wide range of properties, both chemical and physical, that might lend themselves to novel developments in electrochemistry including fuel cell catalysts. Indeed, there is hope that the all-carbon material might be developed to supplant expensive noble metals, including platinum, in such devices. The graphene component would carry out the catalytic function of the metal in the oxygen reduction reaction (ORR) and so allowing stored chemical energy to be converted into electrical energy.

However, something has puzzled scientists testing graphene as a cathode material in fuel cells. Graphene is purportedly just an atom-thick layer of graphite, a sheet of nothing but carbon atoms hooked together with covalent bonds in the familiar tessellating array of hexagons. It is not naturally metallic and so its properties which are ostensibly metallic in nature in this context are baffling, to say the least.


Researchers had used X-ray diffraction and X-ray photoelectron spectroscopy (XPS) to study graphene, but these had not shown the contaminants. Now, James Tour of Rice University and colleagues there and at and the University of Texas at San Antonio and , the Chinese Academy of Sciences, Beijing, have used inductively coupled plasma mass spectrometry (ICP-MS) to reveal that trace amounts of manganese in the graphite precursors used to make graphene may well underpin the properties of the material to some degree. They suggest that under certain conditions, the metal contaminants can lead to activation of the oxygen reduction reaction in a manner that is not available to metal-free graphene. At first sight, this might appear to be a problem for graphene science and its quest to usurp the noble metals. However, if manganese or other inexpensive trace metals must play a role, then this work sheds a spotlight on this role and could lead to new ways to make ultrathin catalysts based on graphene that deliberately, rather than inadvertently, incorporate such trace metals with a view to improving them.

"Labs have reported 'metal-free' graphene catalysts, and the evidence they've gathered could easily be interpreted to show that," Tour explains. "In fact, the tools they were using simply weren’t sensitive enough to show the manganese atoms." Tour and his colleagues have now spotted the interlopers in their nitrogen-doped graphene samples reduced from graphene oxide and acid washed. The tests revealed fewer and fewer manganese atoms with each acid wash, until no manganese could be detected after the sixth wash. Moreover, by the fifth acid wash, the catalytic activity of the graphene sheet was entirely different suggesting that it was the presence of the metallic contaminant that had given rise to the particular catalytic activity observed.

Catalytically speaking

It turns out that twice-washed nitrogen-graphene was the most effective of the materials, catalytically speaking. These samples tended to incorporate single atoms of manganese into the graphene structure, which facilitated full reduction of oxygen through a four-electron process in which four electrons are transferred to oxygen atoms, usually from hydrogen.

"In a four-electron process, oxygen is reduced to water or hydroxide," team member Ruquan Ye explains. "However, peroxide is formed in a two-electron process, which results in a lower diffusion-limited current density and generates hazardous reactive oxygen species." Without metal, this reaction over graphene is far less efficient.

Tour adds that it is perhaps time to now test other materials with this sensitive technique to discover whether claims of their being metal-free are substantiated and to allow what might amount to essentially fake news about such materials to be overwritten. "Single-atom catalysts can hide among graphene, and their activity is profound," Tour says. "So what has sometimes been attributed to the graphene was really the single metal buried into the graphene surface. Graphene is good in its own right, but in these cases, it was being made to look even better by these single metal-atom stowaways."

Related Links

Carbon 2018, online: "Manganese deception on graphene and implications in catalysis"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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