Researchers Show How to Enhance Nanomaterials for Fuel-Cell Cathodes

According to researchers of Rice University, nitrogen-doped carbon nanotubes or modified graphene nanoribbons may be ideal replacements for platinum, used for rapid oxygen reduction. This is the main reaction in fuel cells, which converts chemical energy into electricity.

The findings are from computer simulations by Rice researchers who aimed to find out how carbon nanomaterials can be enhanced for fuel-cell cathodes. Their study exposes the atom-level mechanisms by which doped nanomaterials catalyze oxygen reduction reactions (ORR).

The research has been published in the Royal Society of Chemistry journal Nanoscale.

Theoretical physicist Boris Yakobson and his Rice colleagues are among many seeking a way to accelerate ORR for fuel cells, which were discovered in the 19^th century but not extensively used until the latter part of the 20^th. They have since powered a wide variety of transportation modes, ranging from cars to spacecraft.

The Rice team, including lead author and former postdoctoral associate Xiaolong Zou and graduate student Luqing Wang, used computer simulations to find out why graphene nanoribbons and carbon nanotubes modified with nitrogen and/or boron (long considered as a substitute for expensive platinum) are so sluggish and how they can be transformed.

Doping, or chemically modifying, conductive nanotubes or nanoribbons alters their chemical bonding features. They can subsequently be used as cathodes in proton-exchange membrane fuel cells. In a simple fuel cell, anodes draw in hydrogen fuel and divide it into electrons and protons. While the negative electrons flow out as functional current, the positive protons are drawn to the cathode, where they recombine with returning electrons and oxygen to yield water.

The models displayed that thinner carbon nanotubes with a comparatively high concentration of nitrogen would work best, as oxygen atoms easily bond to the carbon atom close to the nitrogen. Nanotubes have a plus point over nanoribbons because of their curvature, which distorts chemical bonds around their circumference and results in easier binding, the researchers discovered.

The complicated bit is creating a catalyst that is neither too strong nor too weak as it bonds with oxygen. The curve of the nanotube provides a way to adjust the nanotubes’ binding energy, according to the researchers, who established that “ultrathin” nanotubes with a radius between 7 and 10 Å would be best. (An angstrom is one ten-billionth of a meter; for comparison, a standard atom is approximately 1 Å in diameter.)

They also demonstrated that co-doping graphene nanoribbons with nitrogen and boron improves the oxygen-absorbing abilities of ribbons with zigzag edges. In this situation, oxygen finds a double-bonding opportunity. First, they attach straightaway to positively charged boron-doped sites. Second, they are drawn by carbon atoms with high spin charge, which interacts with the oxygen atoms’ spin-polarized electron orbitals. While the spin effect improves adsorption, the binding energy remains feeble, also attaining a balance that allows for good catalytic performance.

The researchers demonstrated that the same catalytic principles were true, but to lesser effect, for nanoribbons with armchair edges.

“While doped nanotubes show good promise, the best performance can probably be achieved at the nanoribbon zigzag edges where nitrogen substitution can expose the so-called pyridinic nitrogen, which has known catalytic activity,” Yakobson said.

“If arranged in a foam-like configuration, such material can approach the efficiency of platinum,” Wang said. “If price is a consideration, it would certainly be competitive.”

Zou is currently an assistant professor at Tsinghua-Berkeley Shenzhen Institute in Shenzhen City, China. Yakobson is the Karl F. Hasselmann Professor of Materials Science and NanoEngineering and a professor of chemistry.

The Robert Welch Foundation, the Army Research Office, the Development and Reform Commission of Shenzhen Municipality, the Youth 1000-Talent Program of China, and Tsinghua-Berkeley Shenzhen Institute supported the research.