2.1. Characterization of the BC/MB and BC/PMB
As presented in Figure 1
, the N/S-CNF were prepared by three steps. First, the MB was absorbed on the surface of BC at 100 °C, driven by the electrostatic interaction or hydrogen bond. Secondly, the chemical oxidative polymerization of MB was initiated by (NH4
and the formed PMB enwrapped evenly the nanofibers of BC [34
]. Finally, the obtained BC/PMB hybrid was carbonized to form the N/S-CNF.
To identify the formation of PMB on the surface of BC, the samples of BC, BC/MB and BC/PMB were characterized. Scan electron microscopy (SEM) image (Figure 2
a) shows that the BC consists of the intertwining nanofibers with a dimension of about 100 nm. After adsorbing MB, some MB particle aggregations are deposited on the surface of BC (Figure 2
b). However, after polymerization, these aggregations disappear and some smooth joints gumming the nanofiber together are observed from the BC/PMB (Figure 2
c), suggesting the dissolution/reprecipitation process happened during the chemical oxidation polymerization of MB. The EDS results show that N, S and Cl are detected in the BC/MB and BC/PMB, and the contents of N and S in these two samples are similar (Figure 2
b,c and Figure S1
). However, the content of Cl in the BC/PMB is much lower than that of the BC/MB. This result indicates that, the MB cations are adsorbed on the surface of BC by static electric attractive, after in situ oxidation polymerization, Cl−
dissolved into solution and the electroneutral PMB was formed. From the FTIR spectra of BC, PMB, and BC /PMB in Figure 2
d, the typical peaks belonging to the PMB and BC, which are in agreement with the reported [35
], are observed. The peak at 1600 cm−1
assigned to the stretching vibration of the –C=N group of the PMB is detected from the BC/PMB, demonstrating that the PMB has successfully loaded on the BC. In addition, the survey spectra of X-ray photoelectron spectroscopy (XPS) (Figure 2
e) further prove the presence of S and N in both the BC/MB and BC/PMB. Similar to the results of EDS, the peak (198.9 eV) ascribed to Cl−
is detected from the BC/MB while not the BC/PMB. The XPS fine spectra of N1s in Figure 2
f further demonstrates the formation of PMB in the BC/PMB due to the appearance of PMB characteristic peak at 400.1 eV [39
]. Furthermore, the peaks of pyridinic N (399.7 eV) and protonated amine N (401.6 eV) of the PMB shift to low energy direction, indicating the PMB tightly enwrap the nanofibers of BC, which results in the electron of the skeleton carbon atoms in the BC shifting toward the N atom of PMB due to the difference in electronegativity between them. All these results reveal that the hybrid of BC/PMB has been successfully prepared. Further study (Figure S2
) reveals that, without the BC, the prepared PMB are blocks with irregular morphology, testifying the important role of BC in inhibiting the agglomeration of PMB.
Thermogravimetric analysis (TGA) was further carried out to verify the thermal decomposition behavior of the BC, MB, PMB, BC/MB and BC/PMB, and the corresponding TG curves are shown in Figure 3
. The MB exhibits the first mass loss step below 250 °C with a mass loss of about 13%, and the second one centers at 250–400 °C with a mass loss of about 26%. With further increasing the temperature, the mass loss increases slowly, and the carbon product at 800 °C is about 52%. Compared with the MB, PMB shows much better thermal stability, and the onset decomposition temperature is up to ~250 °C, but the carbon yield has no change. The BC has a sharp mass loss in the range of 250–375 °C, and a low carbon yield of 11%. For the BC/MB, the thermal decomposition behavior in the low temperature (<250 °C) is similar to that of MB, and the carbon yield is ~40%. No surprise, the BC/PMB displays higher thermal stability below 250 °C, and the carbon yield is close to that of BC/MB and of 37%. Based on these results, it can be suspected that the high decomposition temperature of PMB should be helpful for its decomposition products to participate in the carbonization reaction, and thus promote the formation of N/S co-doped carbon materials.
2.2. Characterization of the Carbonization Products of BC, BC/MB and BC/PMB
After carbonizing the BC, BC/MB and BC/PMB, the resultant products are named by C-BC, C-BC/MB and N/S-CNF, and their morphologies were investigated by SEM and TFM. Compared the images of BC (Figure 2
a) and C-BC (Figure 4
a,c), no obvious change is observed, except that the nanofibers are fluffier and thinner in the case of C-BC. However, compared with the BC/MB (Figure 2
b), the C-BC/MB (Figure 4
b) exhibits quite different morphology, which is similar to that of C-BC, seeming that MB particles have been completely decomposed. Nevertheless, the corresponding transmission electron microscopy (TEM) image (Figure 4
e) shows that, except for the nanofibers derived from the BC, there are some isolated nanoparticles with a dimension of about 30 nm. Based on the different Z-contrast between S and C elements, these nanoparticles should be the sulfur-rich materials, which are proved by the element mapping results of energy dispersive spectrometer (EDS) (Figure S3
). Obviously, these nanoparticles are the carbonized products coming from the incomplete decomposition of MB, due to the absence of S in the BC. Considering the similar morphology of the carbon nanofibers in cases of C-BC and C-BC/MB, it is deduced that the adsorbed MB has little influence on the decomposition of BC, probably due to the weak interface interaction between them, just as shown the TG curve in Figure 3
. It is interesting to note that, in contrast to the C-BC and C-BC/MB, the N/S-CNF derived from the BC/PMB, consists of some short and wide nanobelts (Figure 4
c), and their diameter and length (Figure S4
) are about 70 and 400 nm, respectively. This result demonstrates the great influence of PMB on the carbonization of BC. TEM image in Figure 4
e shows that the contrast of the nanobelts is uniform, indicating the absence of sulfur-rich domains in the N/S-CNF. In addition, elemental mappings from energy dispersive spectroscopy (EDS) (Figure 4
h,i) confirm the nitrogen and sulfur are successfully incorporated and uniformly distribute in the carbon matrix. Moreover, EDS results (insets in Figure 4
b,c) show that the contents of N and S are slightly higher in the case of N/S-CNF than that of C-BC/MB, further proving that PMB as N/S source is helpful for the incorporation of N/S into the carbon framework. No Cl was found by EDS, probably because of the formation of Cl-containing gas.
To gain more insight, the porous structure of N/S-CNF was characterized. As shown in Figure 5
a, with respect to the C-BC and C-BC/MB, the N/S-CNF shows a type I isotherm due to the accomplishment of the predominant adsorption of N2
below the relative pressure (P/P0
) of 0.02, implying the presence of micro-pores. In addition, a hysteresis loop at P/P0
from 0.40 to 1.0 is also observed, which is a characteristic of mesoporous materials. The pore size distribution (PSD) and surface area were calculated with the slit/cylinder model of quenched solid density functional theory using the adsorption branch. The PSD curve further confirms that both the micro-pore centering at 1.0 nm and the meso-pores with various sizes in 2–35 nm coexist in the N/S-CNF. The calculated surface area is 729 g·cm−2
. The C-BC has similar PSD but smaller surface area, while the C-BC/MB belongs to the mesoporous material and has the smallest surface area. These results suggest that the MB or its decomposition product destroyed the micro-pores of C-BC, but the PMB promoted to form more micro-pores in the C-BC, probably by chemical etching the carbon matrix. The hierarchical porous structure and high surface area of N/S-CNF are beneficial to the exposure of more active sites and the diffusion of reactants.
To elucidate the crystallinity of N/S-CNF, X-ray powder diffraction (XRD) and Raman spectroscopic investigation were conducted. The XRD patterns of the N/S-CNF and the control samples show a broad peak at approximately 2θ = 24° and a very weak peak at 2θ = 42° (Figure 5
b), which are the characteristics of graphitic carbon materials with low graphitization degree [40
]. Raman spectra (Figure 5
c) further reveal that both amorphous and crystalline carbon coexist in these samples. The intensity ratios of the D to G bands (ID
) are nearly same for these samples, reflecting their similar graphitization degree [41
To further confirm the chemical state of N and S, X-ray photo electron spectroscopy (XPS) measurement was carried out. As shown the XPS survey spectra in Figure 5
d, N and S are detected from the N/S-CNF and C-BC/MB, confirming that the N and S atoms have been successfully introduced into the carbon matrix, which is in agreement with the EDS results. However, the contents of N and S in the N/S-CNF are 3.2% and 0.8%, respectively, which are higher than that (2.6% and 0.4%) of the C-BC/MB. Generally, It is believed that the N and S content play a key role for the improved ORR catalytic activity [13
], and thus the N/S-CNF should have better catalysis performance than that of C-BC/MB. In addition, it is reported that the N and S species proportion also is crucial for the ORR catalytic activity of electrocatalyst [43
]. Form the high-resolution N1s spectra (Figure 5
e) of the N/S-CNF and C-BC/MB, four peaks can be deconvoluted into, which are assigned to the pyridinic N (398.7 eV), pyrrolic N (399.8 eV), graphitic N (401.2 eV) and (403.2eV), respectively. Usually, it is accepted that the pyridinic N and graphitic N are active species for the ORR [43
]. Compared with the C-BC/MP, the N/S-CNF has a higher amount (77.3 at. %) of the two species and thus should display better catalysis performance for the ORR. In addition, the high-resolution S2p spectra (Figure 5
f) show the peaks of P1/2
at binding energy of 163.5 and 164.3 eV which attribute to C–S–C bonds, and the ones at 166.0–170 eV are associated with the oxidized-S species that are chemically inactive for the ORR [30
]. Similar to the case of N, the N/S-CNF also possesses higher amount (74.4%) of the active S species than that (64%) of C-BC/MP. Therefore, combining all the above results of SEM, PSD, XRD, XPS, and so on, a conclusion can be drawn that the PMB is more suitable as N and S source to prepare efficient elecrocatalyst for the ORR than the MB.
2.3. Electrocatalytic Activity of the N/S-CNF for the ORR
The electrocatalytic activity of the N/S-CNF for the ORR was evaluated by cyclic voltammetry (CV) measurements in 0.1 M KOH electrolyte. Being compared with the smooth CV curve obtained from the N2
-saturated electrolyte (Figure 6
a), the CV curve in O2
-saturated electrolyte shows a cathodic peak at 0.78 V, implying the electrocatalytic activity of N/S-CNF for the ORR in alkaline media. The ORR performance of the N/S-CNF was further measured with the rotating disk electrode (RDE) using linear sweep voltammetry (LSV) technique. As control subjects, the ORR activities of the C-BC, C-BC/MB and Pt/C electrocatalyst were also tested under the same experimental conditions (Figure 6
b). Unsurprisingly, the C-BC shows the worst catalytic activity due to lacking active centres derived from N/S doping. With the benefit of N/S doping, the C-BC/MB exhibits a significant performance boost over the C-BC. Especially, with the aid of the large surface area and high amount of active N and S species, the N/S-CNF shows the best catalytic performance for the ORR, featuring with a comparable E1/2
value (0.80 V) to the commercial 20 wt % Pt/C (0.83 V). In addition, the E1/2
value (0.80 V) is also more positive than some of the reported N,S-co-doped carbon-based electrocatalysts (Table 1
). Especially, the E1/2
(0.80 V) in this work is 170 mV higher than that reported N-S-CNF-800 (MB) [31
], which was prepared through BC physically absorbing MB and followed the carbonization process. We think the novel preparation method should be responsible for the improved catalytic activity. Firstly, based on the adsorption kinetic of MB on the cellulose [44
] a harsh adsorption condition, heating the saturated solution of MB containing the dried BC at 100 °C for 4.5 h with autoclave, was employed to increase the adsorption amount of MB. Secondly, the N/S-CNF was obtained by carbonizing the BC/PMB hybrid that derived from the in situ polymerization of MB on the BC surface, but not by directly carbonizing the BC/MB hybrid as mentioned in the literature [31
]. Obviously, compared with the small molecule compound, N,S-containing polymer is much more suitable as N/S source for the synthesis of carbon-based electrocatalyst with high ORR catalytic activity. To clarify the influence of synthesis parameters on the catalytic activity of N/S-CNF, a series of samples were prepared. The corresponding LSV curves for the ORR (Figure S5
) reveal that the catalytic activity of N/S-CNF is sensitive to the synthesis condition, and the optimized experimental parameters are critical for achieving the N/S-CNF with high catalytic activity.
The catalysis kinetic of the N/S-CNF for the ORR was further investigated. It is normal that the limiting diffusion current increases with the rotation speed due to the thinned diffusion layer (Figure 6
c). The transferred electron number (n) per oxygen molecule involved in the ORR process was calculated with Koutecky–Levich equation, which was to be ca. 3.89 in the potential range of 0.4 to 0.6 V, demonstrating an approximate four-electron pathway. To elucidate the electron transfer mechanism, the hydrogen peroxide yields were measured with rotating ring-disk electrode (RRDE). As shown in Figure 6
f, the ring current originating from the oxidation of hydrogen peroxide ions (HO2−
) is low. The calculated percentage of HO2−
is below 17% over the potential range from 0.2 to 0.8 V, which corresponds to a transfer number of ~3.88. This is agreement with the results obtained from the Koutecky–Levich plots, again illuminating a nearly 4e−
pathway for the ORR catalyzed by the N/S-CNF.
Subsequently, the chronoamperometric responses is used to evaluate the electrocatalytic activity and stability of the N/S-CNF. As shown in Figure 6
e, after a brief transient period, the oxygen reduction current at the N/S-CNF electrode remains stable for the long time (6000 s) of polarization, while the current at the Pt/C electrode reduced to about 77% during the same test period, implying the excellent durability of the N/S-CNF.
Methanol-tolerance is an important benchmark for the electrocatalyst used in fuel cells, the catalysis performance of the N/S-CNF in KOH electrolyte containing methanol was investigated by chronoamperometry. As displayed in Figure 6
f, after adjusting the concentration of methanol to 1 M in the O2
-saturated 0.1 M KOH electrolyte, the ORR current for Pt/C electrocatalyst shows a drastic surge and cannot be recovered to the initial level. Conversely, the current level of the N/S-CNF remains virtually unchanged, indicating its excellent methanol-tolerance.