Rice researchers show how to optimize nanomaterials as replacements for platinum in fuel-cell cathodes
Nitrogen-doped carbon nanotubes or modified graphene nanoribbons may be suitable replacements for platinum for fast oxygen reduction, the key reaction in fuel cells that transform chemical energy into electricity, according to Rice University researchers.
The findings are from computer simulations by Rice scientists who set out to see how carbon nanomaterials can be improved for fuel-cell cathodes. Their study, published in the RSC journal Nanoscale, reveals the atom-level mechanisms by which doped nanomaterials catalyze oxygen reduction reactions (ORR).
The sluggish kinetics of the cathodic oxygen reduction reaction (ORR) is the major bottleneck limiting large-scale applications of the proton-exchange membrane fuel cell. Pt-based catalysts, the most efficient catalysts for ORR with relatively low overpotential and high current density, still suffer from multiple shortcomings, including their rarity, instability under working electrochemical conditions, susceptibility to the crossover effect, and deactivation by CO poisoning. Therefore, intense efforts have been devoted to the development of various advanced non-precious metal or even metal-free catalysts to compete with Pt. Especially, carbon nanostructures have received considerable attention due to their superb mechanical and electrical properties, as well as their abundant π electrons which are beneficial for electron-demanding reduction reaction.
Recent work on ORR activities of heteroatom (such as B and N)-doped C materials has led to the understanding that the breakage of neutral π network by heteroatom doping is crucial for enhancing their catalytic performance. … Here, we carry out systematic first-principles study of the heteroatom (B, N) doping effects on the ORR activities of C materials, paying special attention to the influences of different local environments by investigating dopants in various systems, including pure graphene matrix, edges and carbon nanotubes (CNTs).—Zou et al.
Theoretical physicist Boris Yakobson and his Rice colleagues are among many looking for a way to speed up ORR for fuel cells. The Rice researchers, including lead author and former postdoctoral associate Xiaolong Zou and graduate student Luqing Wang, used computer simulations to discover why graphene nanoribbons and carbon nanotubes modified with nitrogen and/or boron, long studied as a substitute for expensive platinum, are so sluggish and how they can be improved.
Doping, or chemically modifying, conductive nanotubes or nanoribbons changes their chemical bonding characteristics. They can then be used as cathodes in proton-exchange membrane fuel cells. In a simple fuel cell, anodes draw in hydrogen fuel and separate it into protons and electrons. While the negative electrons flow out as usable current, the positive protons are drawn to the cathode, where they recombine with returning electrons and oxygen to produce water.
The models showed that thinner carbon nanotubes with a relatively high concentration of nitrogen would perform best, as oxygen atoms readily bond to the carbon atom nearest the nitrogen. Nanotubes have an advantage over nanoribbons because of their curvature, which distorts chemical bonds around their circumference and leads to easier binding, the researchers found.
The tricky bit is making a catalyst that is neither too strong nor too weak as it bonds with oxygen. The curve of the nanotube provides a way to tune the nanotubes’ binding energy, according to the researchers, who determined that “ultrathin” nanotubes with a radius between 7 and 10 angstroms would be ideal. (An angstrom is one ten-billionth of a meter; for comparison, a typical atom is about 1 angstrom in diameter.)
They also showed co-doping graphene nanoribbons with nitrogen and boron enhances the oxygen-absorbing abilities of ribbons with zigzag edges.
In this case, oxygen finds a double-bonding opportunity. First, they attach directly to positively charged boron-doped sites. Second, they’re drawn by carbon atoms with high spin charge, which interacts with the oxygen atoms’ spin-polarized electron orbitals. While the spin effect enhances adsorption, the binding energy remains weak, also achieving a balance that allows for good catalytic performance.
The researchers showed the same catalytic principles held 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.—Boris Yakobson
If arranged in a foam-like configuration, such material can approach the efficiency of platinum. If price is a consideration, it would certainly be competitive.—Luqing Wang
Zou is now 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 research was supported by 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.
Xiaolong Zou, Luqing Wang and Boris I. Yakobson (2017) “Mechanisms of the oxygen reduction reaction on B- and/or N-doped carbon nanomaterials with curvature and edge effects” Nanoscale doi: 10.1039/C7NR08061A