Scientists at Rice University in the United States are exploring a way to improve the cost-effectiveness of fuel cells by optimizing the nanomaterials of the cathode, and to explain the atomic-scale mechanisms that catalyze the oxygen reduction reaction (ORR) of doped nanomaterials. Nitrogen-doped carbon nanotubes (CNTs) or modified graphene nanoribbons can be a viable alternative to platinum in rapid oxygen reduction, converting chemical energy into electrical energy, which is the main reaction of fuel cells.

In order to obtain the best performance of the oxygen reduction reaction, different carbon materials obtained by different doping methods are applied. In the figure gray is carbon, pink is boron, blue is nitrogen and white is hydrogen.

Because of their good electrical conductivity and mechanical properties, high performance, well-designed carbon materials are key to oxygen reduction reactions. As researcher Xiaolong Zou said in Materials Today, “Developing efficient catalysts for cathodic oxygen reduction reactions is critical for the large-scale application of proton exchange membrane fuel cells.” According to Nanoscale Magazine [Zou et Al. Nanoscale (2017) DOI: 10.1039/C7NR08061A] It can be seen that through the use of computer simulations, the team investigated why graphene nanoribbons and nitrogen/boron-doped carbon nanotubes react too slowly, and how to improve them.

Conductive nanotubes or doped nanoribbons change the properties of their chemical bonds, which facilitate their use as cathodes in proton exchange membrane fuel cells. In a standard fuel cell, the anode is fed with hydrogen fuel and then separated into protons and electrons. When the negative electrons flow out into usable current, protons are drawn into the cathode and combine with electrons and oxygen to produce water.

It has been found that nitrogen-doped ultrathin carbon nanotubes can function most effectively due to the interaction between dopants and the deformation of chemical bonds. Nanotubes are better than nanobelts in this respect because their curvature distorts the edges of the chemical bonds making them easier to bond. They found that ultrathin nanotubes with a radius of between 7 and 10 angstroms are ideal.

The development of efficient catalysts in cathodic oxygen reduction reactions is critical for the large-scale application of proton exchange membrane fuel cells. ——Xiaolong Zou

It has also been demonstrated that graphene nanoribbons with rich edges, nitrogen and boron doped, show comparable performance to oxygen-absorbing nanotubes. Here oxygen provides the opportunity to form double bonds because they can be directly attached to a positively charged boron doping site. As Boris Yakobson said: “Although doped nanotubes show good prospects, the replacement of nitrogen at the serrated edge of the nanoribbon can expose the so-called pyridine nitrogen (which has known catalytic activity) and therefore may achieve optimal performance.”

Now, the team hopes to develop new methods to study nanoscale electrochemical processes in real time and to better perform the interaction between dopants and defective carbon materials to improve performance.

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