These three transmission electron microscope images of nitrogen-doped graphene show the relative presence of manganese atoms on graphene. The top image shows many manganese atoms (white) remaining on graphene that has been washed once; the center image shows a few manganese atoms on twice-washed graphene; and the bottom image shows no manganese atoms on graphene washed six times. Image: Tour Group/Rice University.Detective work by chemists at Rice University has identified a previously unsuspected deception in graphene catalysts.
Graphene has been widely tested as a replacement for expensive platinum in applications like fuel cells, where the material can catalyze the oxygen reduction reaction (ORR) essential for turning chemical energy into electrical energy. But because graphene, the atom-thick form of graphite, isn't naturally metallic, researchers have been baffled by its catalytic activity when used as a cathode.
Wonder no more, says Rice chemist James Tour and his team, who have discovered that trace quantities of manganese contamination from graphite precursors or reactants hide in the graphene lattice. Under the right conditions, those metal bits can activate the ORR. Tour said their discovery also provides insight into how ultrathin catalysts like graphene can be improved. They report this finding in a paper in Carbon.
Because the contrast between carbon and manganese atoms is so slight, trace atoms of the contaminants can't be seen with traditional characterization techniques like X-ray diffraction and X-ray photoelectron spectroscopy (XPS).
"Labs have reported 'metal-free' graphene catalysts, and the evidence they've gathered could easily be interpreted to show that," Tour said. "In fact, the tools they were using simply weren't sensitive enough to show the manganese atoms." A more sensitive tool – inductively coupled plasma mass spectrometry (ICP-MS) – clearly revealed the interlopers among samples made by the Rice lab.
Nitrogen-doped graphene test samples were produced by reducing graphene oxide and then washed in acid between one and six times. With each wash, the ICP-MS scan showed fewer manganese atoms and detected none in graphene samples washed six times. By the fifth wash, the catalytic activity completely changed and showed the former activity had been due to those residual metal atoms. The lab reported that no manganese atoms were observed in any of the same samples using conventional analytical tools, including XPS and transmission electron microscopy.
The researchers characterized the samples' ORR activity and found twice-washed nitrogen-graphene was most effective. 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, with the electrons usually derived from hydrogen.
"In a four-electron process, oxygen is reduced to water or hydroxide," explained Rice graduate student Ruquan Ye, the paper's lead author. "However, peroxide is formed in a two-electron process, which results in a lower diffusion-limited current density and generates hazardous reactive oxygen species." Ye said that without any metal, the ORR in graphene is far less efficient.
According to Tour, these results should lead to investigations of the role of trace metals in other materials thought to be metal-free.
"Single-atom catalysts can hide among graphene, and their activity is profound," he said. "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."
This story is adapted from material from Rice University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.