The oxygen reduction reaction (ORR) within a fuel cell is a fundamental process for determining its performance. Currently, platinum-based catalysts are used to achieve a high electrocatalytic activity during the ORR, but the low stability and scarcity of platinum has prompted Researchers to find alternative materials which are not limited in their applications.
A team of Researchers from South Korea have now developed nitrogen-doped nano-onions (NNO) for use as effective metal-free catalysts in the ORR of fuel cells.
With the high cost and limited applications of platinum catalysts, alternatives are being sourced to replace such catalysts in fuel cells. The materials in question are required to have a high electrocatalytic ability to be able to perform the fundamental oxygen reduction reaction (ORR). Alternatives, to date, include alloys, nitrides and carbon-based materials which are either doped or complexed with metal ions.
In addition, nitrogen doping of materials have proven to be an effective process for increasing a materials electrocatalytic ability and its ORR efficiency. Up until now, many of the carbon-based materials that have been doped with nitrogen have not provided ORR efficiencies shown in platinum-based catalysts, nor have they been easy to synthesize. As such, a new non-metal catalyst with high electrocatalytic activity toward the ORR has been required.
In a different approach, the Researchers chose to fabricate carbon nano-onions and dope them with nitrogen atoms in an effort to achieve an efficient non-metal-based catalyst.
As one of many interesting carbon architectures out there today, a carbon nano-onion is essentially a graphitized nano-diamond. A nano-diamond is a carbon material that possesses a spherical nanocarbon structure, large surface area and excellent mechanical and electrical properties.
Carbon nano-onions have been shown to have an abundance of active sites for the ORR, due to its large surface area, however, only a few studies have previously been undertaken as it is a new area of carbon material research. The effect of doping on carbon non-onions has never been previously investigated.
The Researchers synthesized carbon nano-diamonds through dry controlled dry detonation methods, followed by annealing and cooling under nitrogen. The Researchers then produced oxidized nano-onions, which was followed by a modified Hummers’ method, mixing with urea and pyrolysis methods to yield nitrogen nano-onions. The Researchers controlled the concentration and molar ratio, of nitrogen-containing groups on the nano-onion by controlling the oxygen content of the oxidized nano-onions.
The Researchers characterized the materials using a combination of Raman spectroscopy (LabRAM HR800, Horiba Ltd), X-ray diffraction (XRD, D8-Advance, Bruker-AXS), high-resolution transmission electron microscopy (HR-TEM, JEM-2100F, JEOL), X-ray photoelectron spectroscopy (XPS, (K-Alpha+, Thermo Fisher Scientific), field emission scanning electron microscopy/energy dispersive X-ray spectroscopy (FE-SEM/EDS, Sigma, Carl Zeiss) and Brunauer–Emmett–Teller (BET, 3Flex, Micromeritics) analyses. Electrochemical measurements were taken using a rotating disc electrode (RDE, RRDE-3A, ALS Co) and a potentiostat (DY2322, Digi-Ivy).
The nano-onions doped with nitrogen showed a much higher onset potential than the nitrogen-free nano-onions. On top of this, the OOR activity of these nano-onions could be increased by increasing the number of active sites, i.e. nitrogen groups, in the nano-onion. The nitrogen-doped nano-onion was also found to possess a greater long-term stability and resistance against methanol crossover in the ORR than is found with current platinum catalysts.
The concentration of both the oxygen-containing groups, and therefore nitrogen containing groups were found to be dependent upon the reaction time. It was found that after 6 hours, no further oxygen groups were added to the nano-onion.
The absolute amount of nitrogen-containing active sites in the nitrogen-doped nano-onions for ORR increased in the order NNO-24h<NNO-1h<NNO-12h<NNO-3h<NNO-6h. The electrocatalytic activity of the nano-onions improved as the number of active sites was increased. It was found that the NNO-6h, which contains the largest number of active sites in this study, showcased the best performance with respect to electrocatalytic activity towards the ORR.
Whilst there is still some work to do, the comparable, and greater properties compared to current platinum catalysts makes them a promising candidate for future fuel cells. In particular, their excellent long-term stability and high resistance to methanol crossover shows that they will be an effective catalyst for the ORR, which in turn will increase its commercial viability.
“Fabrication of nitrogendoped nano-onions and their electrocatalytic activity toward the oxygen reduction reaction”- Choi E. Y., and Kim C. K., Scientific Reports, 2017, DOI:10.1038/s41598-017-04597-6