Polymer electrolyte fuel cells (PEFCs) are one of the most promising candidates for clean electricity generation technologies; they have attracted increasing attention because of the limited supply of fossil fuel resources and environmental pollution caused by traditional energy production methods [1]. The catalytic species at the PEFC cathode typically includes Pt, which has a low natural abundance resulting in a significant restriction of the ability to meet global market demand; thus, non-precious metal catalyst alternatives are highly desired. To date, Fe-Nx doped carbons (Fe-N/C) are considered to be the most promising potential alternatives to Pt and its alloys as ORR catalysts at the cathodes of PEFCs [1-7].
Therefore, Fe-N/C catalysts with superior catalytic activities are being actively pursued through a variety of strategies; these include: (1) adjusting the type and amount of Fe compound [8-11] and N source [11-13]; (2) altering the synthetic conditions, e.g., tuning the temperature [9, 10, 14] and pressure [15] of pyrolysis methods as well as investigating non-pyrolyzing routes such as axial ligand tethering [16], covalent grafting [17], and ball-milling [18]; (3) forming nanostructures, such as core-shell nanoparticles [18], ordered mesoporous carbons [19], arrays [20], thin graphene sheets [14, 21, 22], and amorphous carbon spheres [23]; (4) building composites including binary materials, such as graphene/carbon nanotubes [9], graphene/carbon black [24, 25], graphene nanoribbons/carbon nanotubes [26], and carbon nanoparticles/carbon nanofibers [27], and ternary hybrids, such as carbon nanospheres/carbon nanotubes/graphene sheets [10]; (5) introducing post-processing, such as acid-washing [14, 19-21], pore-forming [28], and reheating [28, 29]; and (6) designing different typical Fe phases [29-31] to regulate the generation of Fe-nitrides, Fe-carbides, and Fe-nanoparticles. All these efforts are aimed at enhancing the ORR catalytic activities of the compounds.
Concurrently, significant progress has been made in identifying the detailed compositions of the Fe-based species through spectroscopic studies. A growing body of work suggests that the mode of Fe-Nx coordination plays a key role in promoting the electrocatalytic performance for ORR [7-36]. Moreover, it should be noted that Fe and Fe carbides have recently been identified as possible active species [37-42]. For example, adjacent Fe-carbides particles can boost the ORR activity of Fe-Nx sites [41, 42], and Fe nanoparticles encapsulated in CNTs can modify the electronic structure of the carbon atoms [31], thereby indirectly participating in electrocatalysis. However, the extent of the role of Fe remains controversial, although it is clear that their ORR activity is inferior to that of the Fe-Nx sites [29-31]. Further, in-depth studies focused on the Fe-Nx species have revealed that Fe-N4, and/or other Fe-nitrides, may be the primary contributors to the electrocatalytic performance of Fe-N/C catalysts. This information provides direction for the development of highly active ORR electrocatalysts. However, there is yet to be a rational strategy for the preferential induction of dense growth of highly active Fe-Nx sites.
In this work, we suggest the following effective solution for inducing the formation of Fe-Nx active sites: Introducing a carbon template into the precursor mixture containing melamine and Fe salts results in preferential generation of active Fe-Nx sites rather than inactive Fe and Fe-carbide compounds. This was confirmed through X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and Mössbauer spectroscopy characterization. This method enhances the intrinsic ORR activities of the active species.
In brief, the carbon catalysts are synthesized via heat-treatment of the precursor mixtures. To generate the binary FeN carbon nanotube/carbon nanosphere (CNT/CS) catalyst, we first dispersed 4 g of melamine and 20 mg of FeSO4∙7H2O into 50 mL of deionized water with vigorous ultrasonication. After addition of the CS template (typically 50 mg), the Fe salts and melamine molecules preferentially absorb onto the surface of the CS agglomerations. After thoroughly blending the precursor mixture, it was dried in a rotary evaporator at 60 ℃ and then heated at 900 ℃ for 1 h in a tube furnace under ultrapure argon. Using suitable analogous procedures, FeN-CNT and N-CS were also obtained.
The 57Fe Mössbauer spectra were recorded at room temperature using an MS 500 instrument (Oxford Instruments, U.K.), and the data were fitted using Reciol software. XPS measurements were performed on a Kratos Ltd. XSAM-800 spectrometer with Mg Kα radiation, and the data were fitted via Gaussian/Lorentzian fitting using XPSPEAK41 software. XRD patterns of the catalysts were obtained using an XRD-6000 X-ray diffractometer (Shimadzu) with a Cu Kα radiation source operating at 40 kV and 30 mA. Scanning electron microscopy (SEM) images were obtained using a field-emission S-4800 microscope (HITACHI). Transmission electron microscopy (TEM) images were obtained using a JEM-2100F microscope. N2 adsorption isotherms of the catalysts were recorded using an ASAP2020 surface area and porosity analyzer (Micromeritics, USA).
Electrochemical experiments were performed using a CHI 440 electrochemical workstation and rotator in a standard three-electrode cell using a Pt foil counter electrode. The ORR tests were performed in O2-saturated electrolytes at a potential scan rate of 5 mV/s, and CV curves were scanned in Ar-saturated electrolytes at a potential scan rate of 100 mV/s. The catalyst loadings were 0.6 and 0.1 mg/cm2 for the prepared catalysts and 20 wt% Pt/C, respectively. To calibrate the reference electrodes to the reversible hydrogen electrode (RHE) scale, the RHE zero potential was estimated from the value at which the current crosses zero by measuring the polarization curves of the hydrogen electrode reactions on the Pt/C-loaded electrode in electrolytes saturated with H2. The calibration values were E(RHE) = E(reference) + xV, where x equals 0.915 and 0.32 in alkaline and acid solutions, respectively.
A rotating ring-disk electrode (RRDE) was used as the working electrode. The electron transfer numbers (n) and peroxide yields were calculated based on the following equations:
where ID and IR represent the disk and ring currents, respectively, and N is the current collection efficiency of the Pt ring (0.25 in our system).
The specific activities (SAs), mass activities (MAs), and turnover frequencies (TOFs) of the catalysts were calculated using the following equations:
respectively, where jk, ECSA, mcatalyst, CFe, XPS, AFe-Nx, Mössbauer, and NA represent the kinetic current density, electrochemical surface area, catalyst loading, Fe atomic percent in the catalyst (determined by XPS), area percent of Fe-Nx species (determined from the Mössbauer spectra), Avogadro's constant (6.02 × 1023), and atomic weight of carbon (12.01 amu).
Preparation of the Fe-Nx doped carbon catalysts begins with thermal annealing of the melamine in the presence of Fe salts. In a typical procedure, as is schematically illustrated in Scheme 1, melamine was mixed with the Fe salts and Vulcan XC-72 CSs to form a homogeneous mixture. Through the adsorption interactions, the Fe salts and N-rich melamine effectively disperse on the surface of the CS template, thus facilitating uniform nucleation and growth of the carbon nanotubes and Fe-Nx species via pyrogenation. Concurrently, the nitrogen species (e.g., NH3) formed during decomposition of the melamine precursor attacks the CS agglomerations to produce N (and Fe-Nx) doping.
The morphology of the resultant FeN-CNT/CS catalyst is shown in Fig. 1(a) and (b). As is evident from the SEM results (Fig. 1(a)), the nanotubes, which were uniformly ~30–40 nm diametral, were interspersed among the CS agglomerations, resulting in a picture reminiscent of earthworms loosening the soil. This structure should produce a porous carbon, which is necessary for good dispersion of the Fe-Nx species and the activity of the catalytic reaction. Furthermore, the TEM results (Fig. 1(b)) show that the bamboo-like nanotubes possess the characteristic peapod-like structures of the bean-like Fe-based nanoparticles embedded within them. Additionally, the dark area on the surface of the CS in Fig. 1(b) is the nucleation zone for the Fe-based nanoparticles, which could correspond to the headwaters or roots of the carbon nanotubes. However, removal of the CS template resulted in an accumulation of the different types of nanotubes, i.e., thick/thin, long/short, and traditional/bamboo-like nanotubes, as is evident from the SEM picture in Fig. 1(c). Many studies have reported on the synthetic methods and possible growth mechanisms of carbon nanotubes [43-45]. The primary growth models that have been proposed include tip-growth and base-growth mechanisms. The variety of types of nanotubes in FeN-CNT suggests that both growth models occur; this is very similar to the results reported by Wu et al. [44]. The irregular morphology might be also ascribed to the presence of a variety of Fe-based nanoparticle catalysts. Introduction of the CS template results in uniform adsorption of Fe salts and N-rich melamine on its surface, which leads to uniform nucleation of Fe-based nanoparticles. Accordingly, uniform growth of nanotubes occurred in FeN-CNT/CS. Although the growth mechanism is not completely clear, it is evident that the presence of N causes the formation of bamboo-type morphologies [45].
Only mixing melamine with CS in the absence of Fe salts, N-doping occurred but the topography of N-CS kept the same as CS (Fig. 1(d)). From these results, it is evident that there is an effective synergy between the nanotubes and CS in FeN-CNT/CS, suggesting that the CS template is important for the formation of Fe-Nx active sites and promotion of ORR performance.
The ORR activities of the catalysts were evaluated using rotating-disk electrode (RDE) and RRDE measurements in alkaline media. As shown in Fig. 2(a), FeN-CNT/CS, which has a half-wave potential of 0.78 V, clearly demonstrates a higher ORR activity than FeN-CNT and N-CS. Although the onset potentials of FeN-CNT and FeN-CNT/CS are similar, FeN-CNT delivered a lower current density in the fuel cell operating range (~0.6–0.8 V), which confirms the importance of the CS template in improving the ORR activity. Accordingly, we investigated the effects of CS addition on the structure and activity of catalysts: Slightly adjusting the CS ratio in the precursor did not significantly affect the morphology nor electrochemical activity of the formed binary composites. However, reducing the amount of CS reduces the yield, while adding excess CS template generates N-CS (or even CS) with an aggregated morphology and dramatically decreased the ORR activity.
For the ORR, a four-electron pathway, which occurs at Pt-based catalysts, is desirable. From Fig. 2(b), we determined that the Koutecky-Levich (K-L) plots of the two catalysts had very similar slopes, indicating that FeN-CNT/CS also favors a four-electron pathway. Furthermore, the disk and ring currents of FeN-CNT/CS were obtained using RRDE measurements (Fig. 2(c)). From these values, we calculated an electron transfer number of ~4 (i.e., > 3.98) and a very low H2O2 yield (< 1%) over the entire potential region (Fig. 2(d)); these results indicate a high selectivity for the four-electron reduction of oxygen. In addition, FeN-CNT/CS showed superior durability than commercial Pt/C, as evidenced by the chronoamperometric curves at the potential of 0.7 V (vs. RHE) shown in Fig. 2(e). Moreover, the methanol tolerance of FeN-CNT/CS was investigated in a 0.1 mol/L KOH solution containing methanol. From the results shown in Fig. 2(f), it is evident that the ORR polarization curves at the FeN-CNT/CS electrode were almost equivalent in the presence and absence of methanol. This differs significantly from the results for the Pt/C electrode, which showed a dramatic decrease in ORR activity after the addition of methanol. The merits of avoiding the "crossover" issue make this catalyst very promising for direct methanol fuel cell applications.
N2 adsorption/desorption isotherms and the Barrett-Joyner-Halenda (BJH) pore-size distribution (Fig. S1) are key to elucidating the structures of the formed nanocarbons. The pore sizes of the samples containing nanotubes (i.e., FeN-CNT/CS and FeN-CNT) are in the range of ~0–100 nm; in comparison, the pore sizes of N-CS are in the range of ~50–1000 nm. Therefore, the Brunauer-Emmett-Teller (BET) surface areas (BSAs) and total pore volumes (TPVs) of FeN-CNT/CS and FeN-CNT can be attributed to the mesoscale pores. Table 1 shows that the BSA and TPV values of FeN-CNT/CS are 177 m2/g and 0.41 cm3/g, respectively, which facilitate the dispersion of the Fe-Nx species. Both the single materials provided lower BSA and TPV values: 68 m2/g and 0.21 cm3/g, respectively, for FeN-CNT and 160 m2/g and 0.36 cm3/g, respectively, for N-CS. From these results, it is reasonable to propose that there is an effective synergy between the nanotubes and CS in FeN-CNT/CS that does not occur in the absence of either component. This supports that the CS template promotes both the formation of Fe-Nx active sites and ORR performance.
For non-precious metal electrocatalysts, it is well-known that the types of Fe-Nx species and doping density play a crucial role in enhancing the ORR activity. In the following sections, we will discuss, in detail, the inductive effect of the CS template on the growth of Fe-Nx active sites using XRD, XPS, and 57Fe Mössbauer spectroscopy to analyze the elemental and structural compositions of the prepared catalysts. As shown in Fig. 3, the XRD patterns can be used to compare the compositions of the three catalysts. Clearly, the C(002) characteristic peak of FeN-CNT/CS combines with those of FeN-CNT and N-CS, which indicates successful in situ growth of nanotubes on the CS template, as seen in Fig. 1(b). Additionally, it is evident that the conspicuous metallic Fe and Fe-carbides are present in FeN-CNT. Given that the type of Fe-Nx species formed, especially non-crystalline Fe-N4, is not evident in the XRD pattern, it is likely that it is the dominant Fe-Nx species in FeN-CNT/CS. Thus, adding the CS template to the melamine/Fe-salt mixture promoted Fe-Nx coordination.
Furthermore, XPS characterizations were performed to analyze the bonding configurations of N and Fe. From the high resolution XPS spectra of N 1s and Fe 2p shown in Fig. 4, the N 1s spectra can be deconvoluted into pyridinic-N (~398.3 eV), pyrrolic-N (~400.2 eV), quaternary-N (~401.4 eV), and "N–O" (~403.1 eV) and the Fe 2p spectra can be fitted with zero-valent iron (Fe0) and oxidation iron (Fe2+/3+). It is worth mentioning that the pyridinic-N peak in XPS may also include contributions from Fe-Nx because of their negligible bonding energy difference [46]. As summarized in Table 1, the total surface N contents of FeN-CNT/CS, FeN-CNT, and N-CS were 1.1, 4.5, and 0.7 at%, respectively. Although the N content of FeN-CNT is more than four times that of FeN-CNT/CS, the ORR activity of FeN-CNT is inferior. Moreover, the N content of N-CS is similar to that of FeN-CNT/CS but it has the lowest ORR performance. Regardless, introducing Fe resulted in an immediate increase in the ORR activity even at low Fe contents of 0.39 and 1.44 at% for FeN-CNT/CS and FeN-CNT, respectively. These results confirm that the formed Fe-Nx species are highly active for ORR, which keeps consistence with that of the previous reports [7-36]. Interestingly, the addition of the CS template affects the oxidation state of Fe: The proportion of Fe0 in FeN-CNT/CS (0.097 at%) is significantly lower than that in FeN-CNT (~0.66 at%). That is, the CS template induces the formation of oxidized Fe2+/3+, which can further couple with N to form the various Fe-Nx species.
To identify the compositions of the Fe-Nx species, 57Fe Mössbauer spectra were obtained. As can be clearly seen in Fig. 5, the Mössbauer spectra feature singlet, doublet, and sextet signals; the assignments are given in Table 2. Generally, singlets indicate superparamagnetic Fe and Sextet1 corresponds to α-Fe. Together, these signals comprise ~70% of the Fe in FeN-CNT but only ~25% in FeN-CNT/CS. Additionally, the Fe-carbides in FeN-CNT, as represented by Sextet3, are not evident in FeN-CNT/CS. Surprisingly, the proportion of the total Fe-Nx species (Fe-N4 and other Fe-nitrides) represented by the two doublets and Sextet2 is much higher in FeN-CNT/CS (~75%) than in FeN-CNT (~15%). All the results suggest that the addition of the CS template to the melamine/Fe-salt mixture greatly reduces the content of metallic Fe and Fe-carbides by inducing the growth of Fe-Nx species including Fe-N4 and other Fe-nitrides. Finally, in FeN-CNT/CS, the ORR active Fe-Nx sites are dominant.
The SA can be used to elucidate the differences in electrocatalyst activities. We estimated the electrochemical surface area (ECSA) from the CV curves (Fig. S2(a)) in acid media; the results were 10.8, 18.2, and 4.8 C/g for FeN-CNT/CS, FeN-CNT, and N-CS, respectively, as shown in Table S1. FeN-CNT/CS shows a lower ECSA than FeN-CNT, although FeN-CNT/CS has a higher BSA (177 m2/g) than FeN-CNT (68 m2/g) (Table 1). These results suggest that the high N-doping of 4.48 at% in FeN-CNT is due to its high ECSA. Furthermore, we calculated the kinetic current density (jk) from the polarization curves acquired in acid (Fig. S2(b)) and alkaline (Fig. 2(a)) solutions. By normalizing the jk, which represents the electrochemical activity, with the ECSA, we obtained SA values for FeN-CNT/CS, FeN-CNT, and N-CS (Table S1) of 0.448, 0.313, and 0.19 A/C, respectively, at 0.7 V (vs. RHE) in the acid media and 1.562, 0.639, and 0.152 A/C, respectively, at 0.75 V (vs. RHE) in the alkaline media. FeN-CNT/CS shows the highest SA, suggesting that the growth of Fe-Nx species that is induced by the CS template plays a vital role in enhancing its catalytic activity.
Next, we calculated the MAs of the catalysts by normalizing the jk at 0.75 V (vs. RHE) with the catalyst mass in the alkaline media. As shown in Fig. 6(a), FeN-CNT/CS has the highest MA even though FeN-CNT contains the most N, while that of N-CS is negligible; this indicates that the ORR activity is primarily derived from the highly active Fe-Nx sites. By combining the MA with the Mössbauer spectra and XPS results, the TOFs were calculated to determine the intrinsic activity per active site [42]. Although we propose that the doublets in Mössbauer spectra represent the ORR active sites, TOFs were calculated assuming both the doublets (D1+D2) and all the possible Fe-Nx species (D1+D2+S2) were active sites. The results were 33.04 × 10−20 and 11.03 × 10−20 A/site for FeN-CNT/CS and 15.93 × 10−20 and 10.18 × 10−20 A/site for FeN-CNT, as displayed in Fig. 6(b). Both methods of calculation revealed that the TOF of FeN-CNT/CS is higher than that of FeN-CNT, implying that the CS template significantly promotes the activities of the Fe-Nx sites. The results show that the introduction of the CS template into the melamine/Fe-salt mixture not only promotes the conversion of the inactive Fe and Fe-carbide sites into active Fe-Nx sites but also essentially enhances the catalytic activities of the Fe-Nx sites.
In summary, we confirmed the effectiveness of introducing a carbon template into the melamine/Fe-salt mixture to preferentially create catalytically active Fe-Nx sites for ORR. Characterization of the compounds provides evidence that the template facilitates the uniform nucleation of Fe-Nx species and the growth of nanotubes on the skin of carbons. The introduction of the carbon template significantly suppresses the formation of inactive Fe-based particles (i.e., metallic Fe and Fe-carbides) and changes the coupling of Fe and N to induce the growth of ORR-active Fe-Nx species including Fe-N4 and other Fe-nitrides. In addition, their intrinsic activities were significantly enhanced. We believe that this work provides an important strategy for promoting the dense-doping of highly active Fe-Nx sites.
2018,
Vol. 39 Issue (8): 1427-1435 
PDF
2018,
Vol. 39 Issue (8): 1427-1435 PDF