(Nanowerk Spotlight) The key electrode reactions for renewable and high-capacity energy systems like fuel cells and metal-air batteries are multi-electron processes called oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). The performance of these reactions is is substantially affected by the activity of the catalyst used for the electrode material.
In spite of high catalytic activity, conventional noble metal catalyst materials like platinum, ruthenium, and iridium all suffer from their high cost and poor stability. As a result, scientists are seeking for substitute catalysts from non-noble metal and even non-metal materials. One solution could be found in nanocarbon materials, which afford much improved reactivity and catalytic performance.
"The incorporation of nitrogen atoms into the carbon frameworks can effectively modulate the electronic structure of the surrounding carbon atoms and tune the local charge density distribution, which results in the improvement of the chemical reactivity and subsequently promotes the catalytic performance," Zhang explains to Nanowerk. "However, in most bulk-doped nitrogen-doped carbon nanotubes the nitrogen atoms distribute uniformly, i.e. also on the inner – and thus barely accessible – nanotube walls."
By contrast, the active sites rendered by the surface enriched dopant atoms on the carbon nanocables are accessible and effective to catalyze the oxygen involved electrochemical reactions. Therefore, the as-obtained [email protected] nanocables afforded higher ORR/OER current compared with the routine bulk doped nitrogen-doped carbon nanotubes (NCNT).
N-doped coaxial carbon nanocables with active sites effectively exposed at the surface for oxygen reduction and evolution reaction. (Image: Department of Chemical Engineering, Tsinghua University)
To fabricate the [email protected] coaxial nanocables, Gui-Li Tian, a graduate student and the first author of the paper, developed a facile non-liquid phase method.
"A thin N-containing turbostratic – a crystal structure in which basal planes have slipped out of alignment – carbon layer can epitaxial grow on the outer walls of pristine CNTs by CVD of N-containing compounds, resulting in the coaxial nanocables constituted by the cylindrical CNT walls and the wrinkled N-doped layers," explains Tian. "The dopant N atoms are enriched at the surface of the as-fabricated nanocables. And the inner walls remained intact as expected, leading to a high electrical conductivity of 3.3 S cm-1."
As the researchers points out, combining both the merits of surface-enriched dopant N atoms and continuous inner walls, [email protected] possesses superior electrocatalytic activity.
"Compared with the routine bulk-doped NCNTs at similar doping level, the [email protected] catalyst afforded higher current density and lower overpotential both for oxygen reduction and evolution reaction," adds Wei. "Not only are the active surface sites induced by the doping atoms more accessible to reactants, the polarity and hydrophily of the carbon material are also improved which facilitated mass transfer at the interface between electrode material and electrolyte."
He also notes that, in addition, high electrical conductivity attributed to the intact inner walls benefit rapid charge transfer from the N-doped layers into the CNT scaffolds. "As a result, [email protected] affords superior electrocatalytic performance in comparison with routine NCNTs."
Su points out that, in addition to superior catalysts for oxygen electrochemistry, [email protected] coaxial nanocables are also a good platform towards full exposure of active sites for robust interfaces in high performance composites, as well as efficient catalysts and/or metal nanoparticle supports for selective oxidation reaction and biosensors, etc.
Since the surface hetero-junction nanostructures are not limited to CNTs, the researchers foresee a new branch of chemistry evolving in the area of full exposure of active sites through the 3D heterogeneous systems.