In this review, we start with a brief introduction of heteroatom doping for the development of carbon-based ORR electrocatalysts. Afterward, we present some typical examples of ORR from the aspect of constructing various defects as active sites. Furthermore, we comprehensively and emphatically sum up the recent advancements of carbon-based nanomaterials toward ORR from the combination of various defects and heteroatom dopants. Finally, we point out the current issues and challenges in the construction of highly efficient carbon-based materials for electrocatalysis and other electrochemical applications. This review not only provides an overview on the development of carbon-based metal-free ORR electrocatalysts, but also reveals the relationship between the defects/dopants and the electrocatalytic ORR performance.
As mentioned above, both defect and heteroatom doping can lead to the change of the local electronic structure of carbon materials. The altered electron structures with the positive charge and/or higher charge and spin densities are beneficial for the chemisorption of oxygen molecular and the oxygen-related intermediates [2, 96], thereby contributing to the improvement in electrocatalytic ORR activity. Therefore, connecting together with various defects and heteroatom dopants or designing heteroatom dopants at specific defective sites is the most effective strategy to modify electron structures and obtain optimal electrocatalytic ORR performance for metal-free carbon-based electrocatalysts.
Colavita Law And Society Pdf Free
As described in the previous sections, the electrocatalytic ORR activities of the carbon nanomaterials are highly associated with both defects and heteroatom dopants. Herein, in order to investigate the general rule of the various modified nanocarbon ORR electrocatalysts, we made an ORR activities comparison based on the differences between their ORR onset/half-wave potentials (ΔE0 and ΔE1/2, the two key parameters for ORR) and the related performance of commercial Pt/C catalysts, as roughly derived from their ORR polarization curve in the literature, since Pt-based materials (in most cases of the commercial Pt/C catalyst, 20%) are considered as the current state-of-the-art ORR catalyst, which is usually employed as the benchmark and the comparison object toward ORR. As exhibited in Fig. 10a, b, the non-modified pristine carbon nanomaterials, such as graphite, graphite nanoplates, graphene, fullerene, and CNTs, have little or even no activity toward ORR. After heteroatom doping or defect introducing, their ORR activities were improved. Most heteroatom-doped carbon materials describe higher ORR activity than defect-induced ones, while only a few are opposite. So it is difficult to predict which strategy is more facility for enhancing the ORR activity for the nanocarbons. Remarkably and undeniably, when combined with heteroatom doping and defect inducing, they usually reveal the best ORR performance, though there are a few examples to the contrary. Thus, the overall trend of electrocatalytic ORR activity of various functionalized carbon nanomaterials is approximately in accordance with Fig. 11, in which the defect- and doping-co-engineered nanocarbons reveal the highest ORR activity, the individual defect-enriched or heteroatom-doped carbons take the second place, and pristine carbons are the lowest among them. Table 1 presents the part of recent progress on advanced carbon-based metal-free ORR electrocatalysts by co-engineering of various defects and heteroatom dopants. Interestingly, their ORR performance can be comparable to or even outperformed than that of the state-of-the-art Pt/C catalyst, further confirming the synergistic promotion effect between defects and dopants.
Applications. The electrocatalytic ORR activity of metal-free carbon-based nanomaterials in alkaline conditions is usually higher than that in acidic conditions, which include the quite different absorption intermediates and rate-determining steps in acidic and alkaline media. The current carbon-based ORR electrocatalysts are hence normally used in alkaline solutions, but not in acidic electrolytes. Since most of the practical fuel cells are more promising and competitive in acidic conditions (e.g., high efficiency and stability, non-effect of carbon dioxide), exploring and synthesizing more effective acidic carbon-based ORR electrocatalysts is of great significance. 2ff7e9595c
Comentarios