The polymer electrolyte membrane fuel cell (PEMFC) has been widely regarded as the next-generation energy technology because of advantages related to zero emission, high efficiency and energy density [4]. Currently, the commercialization of PEMFCs is economically unviable because of high catalyst costs resulting from the requirement of large quantities of the noble metal Pt [5]. It is, therefore, highly desirable to develop non-precious metal catalysts [10, 11], or alternatively, metal-free carbonaceous catalysts [12].
Among the alternatives, the nanostructured nitrogen-doped carbon is of particular interest, which shows promise in energy storage and conversion applications [13]. However, challenges remain to further increase the activity and stability of carbon catalysts. Enormous efforts have been devoted to optimizing a controllable synthesis. Among the conditions, the precursor is believed to play a key role in determining the composition, structure and resulting final properties of the final carbon catalyst. For example, diaminobenzene [14], aminoglucose [15], ethylenediamine [16], polyaniline [17], and phenanthroline [20, 21] have been explored, which are reported to yield different activities.
It should be pointed out that the electrocatalytic activity cannot easily be directly compared as both the synthesis and evaluation were conducted by different research groups under differing conditions. As such, the precursor effect needs to be explicitly investigated as to how it influences composition, structure and electrocatalysis for oxygen reduction reactions (ORRs). In our previous work, nitrogen-doped ordered mesoporous carbons (NOMCs) were synthesized by a modified nanocasting method, which exhibited high specific surface areas, uniform pore structures and excellent electrocatalytic activity [22]. In this work, individual NOMCs are synthesized using three precursors, aniline, pyrrole, and phenanthroline, each having varying nitrogen compositional structures. The effect of the precursor on the resultant NOMC is extensively investigated by nitrogen adsorption-desorption measurements, scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), cyclic voltammetry (CV), and rotating-ring-disk electrode (RRDE) methods.
NOMCs were synthesized via a nanocasting method using SBA-15 as the template [23]. First, 3.2 g SBA-15 was dispersed in an ethanol solution (20.0 mL ethanol + 20.0 mL deionized water). Second, 4.0 g phenanthroline was dissolved in 10 mL ethanol and mixed with a FeCl2 aqueous solution. The molar ratio of iron to phenanthroline was 1:3 to ensure complete coordination. Thereafter, the above two solutions were mixed and sonicated for 6 h. The resulting powder was filtered, washed and subjected to pyrolysis at high temperatures (700, 800, 900, 1000 ℃) for 3 h under an Ar atmosphere. Finally, the NOMC catalysts were obtained by removing the silica template and Fe species. The template was removed by refluxing the powders in 10 mol/L NaOH at 120 ℃ for 24 h, and the iron species was leached by boiling the powders in 0.10 mol/L HClO4 at 80 ℃ for 24 h. The samples were referred to as C-Phen-x. Here, x refers to the pyrolysis temperature, viz. 700, 800, 900, and 1000 ℃.
The pyrrole- and aniline-derived carbon catalysts, C-PY-900 and C-PA-900, were synthesized as described in our previous work [20, 24, 25]. Pyrolysis was conducted at 900 ℃ for 3 h under an Ar atmosphere.
X-ray diffraction (XRD) measurements were performed using a Bruker D8 ADVANCE diffractometer employing a Cu Kα radiation source operating at 40 kV at a scan rate of 10°/min. XPS (Physical Electronics PHI 5600) measurements were carried out with a multi-technique system using an Al monochromatic X-ray source at a power of 350 W. Transmission electron microscopy (TEM) images were taken on a FEI Tecnai G2 F20 S-TWIN operating at 200 kV. Nitrogen adsorption-desorption isotherms were measured at -196 ℃ using a Micromeritics TriStar II 3020 analyzer. Total surface area was determined by the Brunauer-Emmett-Teller (BET) method, the microporous (MP) surface area was obtained via the t-plot method, and the pore size distribution was analyzed using the Barrett-Joyner-Halenda (BJH) method.
The electrochemical behavior of the catalysts was characterized by CV and linear sweeping voltammetry (LSV) methods using a three-electrode cell with a Zennium electrochemical work station (Zahner) at room temperature (25 ℃). A gold wire and a double junction Ag/AgCl reference electrode (PINE) were used as the counter and reference electrodes, respectively. The working electrode was a RRDE (glassy carbon disk: 5.0 mm in diameter, platinum ring: 6.5 mm inner diameter and 7.5 mm outer diameter). The thin-film electrode on the disk was prepared as follows: 10 mg of the catalyst was dispersed in 1.0 mL Nafion/ethanol (0.84 wt% Nafion) by sonication for 2 h. Thereafter, 10 μL of the dispersion was transferred by pipette onto the glassy carbon disk, yielding a catalyst loading of 0.50 mg/cm2. For comparison, we also measured the ORR electrocatalytic activity of a commercial 40 wt% Pt/C catalyst (HiSPEC4000, Johnson Matthey) having a metal loading of 20 μg/cm2.
A KOH electrolyte solution (0.10 mol/L) was first bubbled with Ar for 1 h. Thereafter, the CV test was conducted at 20 mV/s across the potential range of 0-1.23 V (vs. reversible hydrogen electrode, RHE) for 20 cycles. If unspecified, the LSV curve was collected by scanning the disk potential from 1.2-0 V at 5 mV/s in the oxygen-saturated electrolyte solution under 1600 r/min, from which the ORR polarization curve was extracted by subtracting the capacitive current. During the collection, the potential of the ring was set as 0.5 V (vs. RHE) to determine the yield of hydrogen peroxide, respectively.
The electron transfer number (n) and hydrogen peroxide yield in the ORR was calculated from the following equations:
where id is the disk current, ir is the ring current, and N is the collection efficiency (= 20.50%).
Fig. 1 presents the nitrogen adsorption-desorption isotherms of the synthesized C-PA-900, C-PY-900, and C-Phen-900. It can be observed that all the curves display a type-IV isotherm, indicating their mesoporous structure. The textural parameters are listed in Table 1. For C-PA-900, C-PY-900, and C-Phen-900, the total BET specific surface areas are 569, 765 and 746 m2/g, respectively; in comparison, the MP specific surface areas are 41, 49 and 7 m2/g, respectively. Thus, it is inferred that the specific surface area is attributed to the mesopores having diameters in the range of 3.7-5.9 nm, which ensures accessibility to the electroactive species in liquid electrolytes. In comparing the three materials, even though the specific surface areas are sufficiently high to achieve acceptable electrocatalytic activity, as seen below, the textural property data strongly indicate that the precursor influences the pore structure—thought to originate from the difference in the thermodecomposition of the three macromolecules.
Fig. 2 shows the TEM micrographs of the three NOMC materials. All samples exhibit highly ordered mesoporous channels—the electron density of which is the inverse replica of the SBA-15 template. The pores run parallel and are uniform in diameter (3-6 nm). The results are consistent with the above physical adsorption analysis.
The XRD patterns are shown in Fig. 3. Two wide diffraction peaks are observed centered at 2θ = 25.2° and 43.7°, which are indexed to the (002) and (100) planes, respectively [19, 20]. The results indicate that all three NOMCs are amorphous in nature and the precursor does not yield noticeable effects on the crystalline structure.
XPS analysis allowed elemental information on the NOMC surface to be obtained. Quantitative results are listed in Table 2. The nitrogen content is 3.13 at%, 3.32 at% and 3.33 at% for C-PA-900, C-PY-900, and C-Phen-900, respectively, suggesting that the precursor has a slight influence on the doped nitrogen content. In comparison, the pyrolysis temperature shows a significant influence in determining the surface composition (Table 3). The nitrogen content shows a dramatic decrease from 7.44 at% to 2.12 at% as a function of increasing pyrolysis temperature from 700 to 1000 ℃. Conversely, in addition to nitrogen content, it is acknowledged that the coordination of the nitrogen dopant is equally important for electrocatalysis [20].
Accordingly, structural information on the nitrogen dopant is resolved by fitting the N 1s spectra into three peaks residing at 398.4 ± 0.2, 401.0 ± 0.1, and 401.5-404 eV, which correspond to pyridinic-, graphitic- and oxide-nitrogen (Fig. 4) [1], respectively. First, the shapes of the curve are similar for the three NOMCs, and the graphitic-nitrogen is the dominant component. Quantitative analysis (Table 4) shows that the content of each component differs, indicating that the precursor influences the nitrogen doping. For example, the edge-type nitrogen, viz. pyridinic-nitrogen, increases in the order C-PA-900 < C-PY-900 < C-Phen-900. The edge-type nitrogen is acknowledged to be a highly effective dopant for electrocatalysis [2]. Therefore, this result may yield a positive effect on the electrocatalytic activity, as discussed below.
In our previous work, the ORR active sites are claimed to be the nitrogen-activated carbon [3]. As such, curve fitting of the high-resolution C 1s peak is performed (Fig. 5 and Table 5). It is observed that the nitrogen-activated carbon composition is 15.60% for C-PA-900, 19.87% for C-PY-900, and 23.04% for C-Phen-900; therefore, it can be expected that the ORR electrocatalytic activity follows the order: C-PA-900 < C-PY-900 < C-Phen-900.
Fig. 6 shows the cyclic voltammograms of the three NOMCs. The curves are similar in shape, featuring a large capacitive current, which originates from the high specific surface area (vide supra). A broad electrochemically reversible wave is observed in the potential range of 0-0.8 V, which has been attributed to the adsorption-desorption of hydroxyl ions. Specifically, C-Phen-900 shows the highest pseudocapacitive current and a unique redox couple at 0.6 V, indicating the presence of more electrochemically active functional groups on the surface [4].
Fig. 7 shows the ORR polarization curves and the H2O2 yield of the NOMC catalysts in an O2-saturated 0.10 mol/L KOH solution. First, Fig. 7(a) reveals that the three catalysts yield equal or even enhanced ORR activity compared with the commercial Pt catalyst. C-Phen-900 shows a half wave potential (E1/2) of 0.89 V, which is higher than C-PY-900 (0.87 V), Pt catalyst (0.84 V), and C-PA-900 (0.83 V). Fig. 7(b) shows that the electron transfer number is approximately 4 (C-PA-900: 3.3-3.9, C-PY-900: 3.5-4.0, C-Phen-900: 3.6-4.0), indicating a reaction highly selective to the complete reduction of oxygen. The high electrocatalytic activity of these materials can first be attributed to the uniform mesoporous structure associated with the high specific surface area (vide supra). Second, it is observed that the precursor significantly influences the electrochemical behavior, and the electrocatalytic activity follows the order: C-PA-900 < C-PY-900 < C-Phen-900. This result correlates well to the above-mentioned compositional change of the nitrogen-activated carbon, which confirms our claim on the chemical nature of the active sites. Finally, it is noted that C-Phen-900 shows a higher limited current density. The reason remains unclear; however, this may result from the well-defined ordered mesoporous structure (Fig. 2). In summary, the above findings indicate that both the NOMC itself and the electrochemical properties can be tailored by varying the precursor.
In this work, the NOMC catalysts are synthesized through a nanocasting method employing three carbon precursors: polyaniline, polypyrrole, and phenanthroline. The synthesized NOMC catalysts yield superior ORR electrocatalytic activity and selectivity. In addition to the hard template, the textural properties of the NOMCs closely relate to the carbon precursor used, which significantly influences both the electrocatalysis and mass transfer. Second, the content of the edge-type nitrogen is dependent on the carbon precursor, and is observed to be highest when using phenanthroline as the precursor. The composition of the nitrogen-activated carbon atoms follows the order: C-PA-900 < C-PY-900 < C-Phen-900, and furthermore, the electrocatalytic activity follows the same order, confirming the claim that the nitrogen-activated carbon atoms are the active sites. This work not only yields superior non-noble metal electrocatalysts for fuel cell applications, but also offers further experimental design to tailor the carbon type during pyrolysis-based protocols.
2016,
Vol. 37 Issue (9): 1562-1568 
PDF
2016,
Vol. 37 Issue (9): 1562-1568 PDF