2. C ARBON N ANOTUBES /H ETEROATOM -D OPED C ARBON C ORE –S HEATH
2.3. R ESULTS AND D ISCUSSION
2.3.3. ORR Activity in Half-Cell Configurations
Figure 2.6. (a) Raman spectra of CNT, acid-treated CNT, and CNT/HDC catalysts carbonized at different temperatures. (b) N2 adsorption-desorption isotherms of acid-treated CNT and CNT/HDC carbonized at different temperatures.
Table 2.3. Summary of textural properties of the CNT/HDC nanostructures, HDC, and the acid- treated CNT.
Sample BET surface area (m2 g−1) Total pore volume (cm3 g−1)
CNT/HDC-1000 325 0.53
CNT/HDC-900 293 0.43
CNT/HDC-800 315 0.42
HDC 489 0.39
Acid-treated CNT 428 1.19
and half-wave potentials of the CNT/HDC nanostructures were significantly shifted to positive potentials with well-defined plateaus, indicating the synergistic effect of hybridization between CNT cores and HDC sheath layers. The highest ORR activity was achieved with the CNT/HDC-1000, followed by CNT/HDC-900 and CNT/HDC-800. The most active CNT/HDC-1000 showed onset and half-wave potentials at 0.92 and 0.82 V, respectively, and had a kinetic current density of 8.3 mA cm−2 at 0.8 V. Such a high activity of CNT/HDC nanostructures compared favorably with that of a benchmark Pt/C catalyst, which showed onset and half-wave potentials at 0.98 and 0.85 V, respectively. We compared the ORR activity of CNT/HDC nanostructure catalysts with N-doped CNTs. The N-doped CNTs were prepared by using ammonia or urea as an N source, and the resulting catalysts were denoted as N-CNT-NH3 and N-CNT-Urea, respectively. Figure 2.7b shows the ORR activity of the N-CNT catalysts, along with those of undoped CNTs and the CNT/HDC-1000. The two N-CNT catalysts showed better ORR activity than undoped CNT, as consistent with the previous results10–14; however, their activity is inferior to that of the CNT/HDC-1000. The CNT/HDC-1000 is one of the best-performing ORR catalyst in alkaline electrolytes when compared to reported doped carbons.
Figure 2.7. (a,b) ORR polarization curves of (a) CNT/HDC-X, CNT, HDC, the physical mixture of the CNT and HDC (CNT + HDC), and a commercial Pt/C, and (b) CNT/HDC-1000, N-CNT-NH3, N- CNT-Urea, CNT, and the Pt/C measured in O2-saturated 0.1 M KOH at an electrode rotation speed of 1,600 rpm.
The ORR kinetics was evaluated based on the Tafel plots and the 4-electron selectivity measurements. The Tafel slopes (Figure 2.8a) for the CNT/HDC catalysts ranged from 65 to 68 mV dec−1, and were comparable to that of Pt/C (62 mV dec−1), indicating that the ORR kinetics of the CNT/HDC catalysts is similar to that of Pt/C. Figures 2.8b and 2.8c clearly reveal that the number of electrons transferred by the CNT/HDC catalysts was higher than those by the other samples and
similar to that of Pt/C catalyst, approaching 4 in the high potential region. More intrinsic kinetic insight could be gained from the exchange current densities of these catalysts for the ORR (Figure 2.8d). Notably, the exchange current density of the CNT/HDC-1000 is the same order of magnitude as that of Pt/C. In contrast, the CNTs, HDC, and their mixture showed one or two orders of magnitude lower exchange current densities than those of CNT/HDC catalysts and Pt/C.
Figure 2.8. (a) ORR Tafel plots and (b) electron transfer number (n) of CNT/HDC-X, CNT, HDC, the physical mixture of the CNT and HDC (CNT + HDC), and the Pt/C. (c,d) Bar graph comparing (c) the electron transfer number at 0.20 V (vs RHE) and (d) the exchange current density (j0) of the catalysts.
Previous routes to doped carbon-based ORR catalysts required the judicious selection of precursor and experimental conditions for CVD or the unavoidable use of toxic gases. In contrast, in our approach to CNT/HDC catalysts, the formation of HDC sheath layers relies on a simple solution process, followed by annealing in mild atmosphere, which is more amenable to large-scale preparation. Furthermore, the choice of ILs can allow for the facile control of type and quantity of
heteroatoms in the HDC layers. We extended the IL-derived synthetic methods for preparing CNT/HDC catalysts with other four different ILs, which contains B, N, P, and F heteroatoms, demonstrating the universal applicability of IL-coating synthetic strategy for heteroatom-doped carbon electrocatalysts. The resulting catalysts also exhibited excellent ORR activity in an alkaline solution (Figure 2.9). The information about ILs used is provided in Section 2.2.1.
Figure 2.9. ORR polarization curves of CNT/HDC catalysts derived from different ILs, CNT, and Pt/C measured in O2-saturated 0.1 M KOH at an electrode rotation of 1,600 rpm.
As demonstrated in the ORR activity and kinetics data, the CNT/HDC catalysts show high electrocatalytic activity for the ORR, surpassing those of doped CNTs as well as previous catalysts. In the CNT/HDC nanostructures, the CNT cores could enable efficient transport of electrons, while the thin HDC sheath layers with numerous heteroatoms provides catalytically active sites. Particularly, the presence of multiple dopants (N, S, and F) in the sheath layers could further enhance ORR activity, in accordance with recent reports demonstrating enhanced ORR activity in dual-doped carbon structures. We also note that the highest ORR activity of the CNT/HDC-1000 catalyst could originate from the increased ratios of graphitic nitrogen atoms as well as their enhanced electrical conductivity.
We next investigated the durability of the most active catalyst, CNT/HDC-1000, with Pt/C during 10,000 potential cycles between 0.6 and 1.0 V (vs RHE) at a scan rate of 50 mV s−1. The changes in the current density percentages for the ORR at 0.85 V with cycling (Figure 2.10a) clearly show the superior durability of the CNT/HDC-1000 relative to the Pt/C catalyst. The initial current density of the CNT/HDC-1000 composites was minimally decreased (4.5% after cycling), whereas that of Pt/C declined dramatically by 32%. The CNT/HDC-1000 catalyst also showed superior tolerance against poison molecule such as methanol, compared to the Pt/C (Figure 2.10b).
Figure 2.10. (a) The current changes at 0.85 V (vs RHE) versus the number of the potential cycling, and (b) ORR polarization curves measured in the presence (dotted lines) and the absence (solid lines) of 0.5 M methanol (MeOH) in 0.1 M KOH.