Proton exchange membrane fuel cells (PEMFCs) are promising clean energy generation devices. One of the major impediments to their commercialization is the sluggish kinetics of the oxygen reduction reaction (ORR). To date, the ternary Fe/N/C system has been extensively developed as the most promising ORR catalyst among the non-platinum candidates. Initially,Fe/N/C was obtained by pyrolyzing Fe-N4 or Co-N4 macrocycles adsorbed on a carbon support in an inert atmosphere [1]. More recently, several methods have been developed to improve the catalyst’s ORR activity and stability, as well as to lower the cost. As an example, ball milling of precursors and two-step pyrolysis in Ar and NH3 atmospheres have been found to greatly boost the ORR activity of the Fe/N/C catalyst, providing a high ORR activity of 4.30 A/g at 0.9 V during rotating disk electrode (RDE) trials [2, 3]. Fe/N/C has also been obtained by coating a heteroatom polymer on various precursors through pyrolysis-acid leaching-pyrolysis. The obtained Fe/N/C exhibited a high ORR activity of approximately 6.7 A/g at 0.80 V [4]. Further improvements have been made by adding various procedures to the pyrolysis-acid leaching process, resulting in an ORR activity of 21.0 A/g at 0.8 V [5]. Alternatively,S-doped Fe/N/C synthesized using a Fe(SCN)3 precursor has been shown to increase the ORR activity to 23.0 A/g at 0.8 V [6].
Along with this significant progress in the preparation of Fe/N/C catalysts with high ORR activity, explorations of their applications in PEMFCs have been attempted by several groups [2, 4, 6]. Fe/N/C catalysts are low cost, but appropriate levels of fuel cell performance require extremely high Fe/N/C loadings in the PEMFC (approximately 4 mg/cm2), which results in poor mass transfer. Typically, the thickness of the Fe/N/C cathode is on the order of 100 mm, a value that is about 10 times that of a Pt/C-based catalyst layer. A design incorporating array channels/pores on the meso- or macro-scale has been reported to facilitate mass transfer. As well, the synthesis of metal/N/C catalysts using a foaming agent [7, 8] or mesoporous silica templates [9, 10] to control the pore structure has been described, and these approaches have led to some improvements in mass transfer.
Herein, we report a non-template method for the preparation of a mesoporous Fe/N/C catalyst, using 2- aminobenzimidazole (2-ABI) as the nitrogen source. In this method, the catalyst (denoted as Fe/N/C-ABI) is synthesized through polymerizing 2-ABI on KJ600 carbon black and FeCl3, followed by pyrolysis. This technique was found to generate abundant mesopores that facilitate mass transfer and improve the PEMFC performance.
A schematic showing the synthesis of Fe/N/C-ABI is provided in Fig. 1. As a first step,KJ600 carbon black was pretreated in concentrated nitric acid for 5 h at 80 ℃ to remove any residual metals. The acid-pretreated KJ600 carbon black (0.25 g) and 2-ABI (1.33 g) were subsequently dispersed in 50 mL water by sonication and magnetic stirring. Following this, 10 mL of a NaClO solution (8%) was added dropwise with magnetic stirring. The homogeneously dispersed suspension was transferred into a 100 mL autoclave and the 2-ABI was hydrothermally polymerized at 160 ℃ for 36 h, such that poly-2-ABI was coating onto the carbon nanoparticles. Following this, the ABI/KJ600 precipitate was filtered, washed with deionized water, and dried overnight at 80 ℃. Iron was added by ultrasonic dispersion of a FeCl3 solution (1 mol/L, 1 mL) with the as-prepared ABI/KJ600 powder (0.3 g) in 100 mL water, followed by removal of the solvent and drying at 80 ℃ for 12 h. The first heat treatment was applied at 950 ℃ in an Ar atmosphere for 1 h. The pyrolyzed powder was then acid leached in 1 mol/L HCl at 80 ℃ for 8 h. The sample was subjected to a second heat treatment at 950 ℃ in Ar for 3 h. This synthesis was similar to our previous method, in which poly-m-phenylenediamine (PmPDA) was used as the nitrogen precursor [11]. The Fe/N/C-PmPDA catalyst has few mesopores, so it was used as a control material in this study.
Rotating ring-disk electrode (RRDE,Pine Inc.) tests were performed in a three-electrode electrochemical cell within a 30 ℃ water bath using a CHI bi-potentiostat (CHI-760E). A RRDE with a Pt ring and a glassy carbon disk (GC,φ = 5.61 mm, geometric area = 0.2475 cm2) was employed as the working electrode, while a graphite plate and a reversible hydrogen electrode (RHE) were used as the counter electrode and reference electrode, respectively. To deposit the catalyst onto the GC disk electrode, a 6.0 mg catalyst sample was ultrasonically dispersed in a combination of 0.5 mL water, 0.5 mL ethanol and 50 L of a Nafion solution (5 wt%) over 1 h to form a homogeneous catalyst ink. Subsequently, 25 L of the ink was dropped onto the GC disk of the RRDE, resulting in a catalyst loading of 0.6 mg/cm2. The ORR performance tests were conducted in an O2-saturated 0.1 mol/L H2SO4 electrolyte. The working electrode was subjected to potential cycling between 1.0 and 0.2 V (RHE) at a scan rate of 10 mV/s with a rotation rate of 900 r/min. The background capacitive current was recorded in N2-saturated electrolyte under the same test conditions. This background current was subtracted from the current recorded in the O2-saturated solution to determine the net ORR current. To correct for the effects of mass transfer, the kinetic current (ik) was calculated using the Koutecky-Levich equation:
where i is the current obtained from polarization curves,iL is the limited diffusion current, and ik is the kinetic current. The ik term was normalized by the catalyst loading on the electrode surface to obtain the mass activity.
Membrane electrode assemblies (MEAs) were prepared using the hot-pressing method. The cathode ink was prepared by ultrasonic mixing of the desired amounts of Fe/N/C-ABI catalyst powder, deionized water, and a Nafion solution (5 wt%) in an ice bath over 1 h. The ink was directly deposited onto a gas diffusion layer (GDL,PTFE-pretreated Toray 060 carbon paper) at a Fe/N/C-ABI catalyst loading of 4.0 mg/cm2. Commercial Pt/C (40%) was used as the anode catalyst at a loading of 0.4 mgPt/cm2. The MEA was fabricated by hot-pressing the as-prepared cathode together with an anode, a Nafion membrane (NRE 211), and a gasket at 135 ℃ and 3 MPa for 2 min. The active area of the MEA was 1.0 cm × 1.0 cm. Polarization curves were obtained at 80 ℃ using a Model 850e fuel cell test system (Scribner Associates,Inc.) in conjunction with a back pressure of 1 bar. The H2 and O2 flow rates were 0.3 L/min at 100% RH during the polarization curve measurements.
Fig. 2(a) presents the data obtained from X-ray photoelectron spectroscopy (XPS) of Fe/N/C-ABI. The catalyst evidently contained C,N,O, and Fe, at mass proportions of 93.5%, 1.79%, 4.32%, and 0.39%, respectively. The ratio between C and N was confirmed by CHNS elemental analysis, with a value of 49.6 from the elemental analysis compared to 52.2 from XPS. The much lower N and Fe contents in the catalyst compared to the precursor can be attributed to losses during the synthetic procedure. The high resolution N 1s spectrum in Fig. 2(b) can be resolved into five peaks, assigned to cyano-N, pyridic-N,Fe-N, pyrrolic-N, and graphitic N-O, at 398.1, 398.8, 399.8, 400.7, and 402.3 eV, respectively, with relative proportions of 4.6%, 7.0%, 6.1%, 25.0%, and 57.2%.
Fig. 2(c) presents a transmission electron microscopy (TEM) image of the Fe/N/C-ABI. It shows a carbon nanoparticle less than 100 nm in diameter with a hollow-shell structure. The thickness of the graphite shell is approximately 10 nm while the hollow region is 10 to 25 nm. In addition, no crystalline metal or metal carbide phases, which would show up as regions of high contrast in the TEM image, are observed.
Fig. 2(d) presents the Ar adsorption-desorption isotherms obtained from the Fe/N/C-ABI, which evidently conform to a type IV isotherm. The hysteresis in the adsorption and desorption isotherms indicates the presence of mesopores, and this is confirmed by the mesopore distribution (obtained via the Barret-Joyner-Halenda (BJH) method) shown in Fig. 2(e). The most common mesopore size is about 20 nm, which is consistent with the TEM observation (Fig. 2(c)). In this work,Fe/N/C-PmPDA previously synthesized by our group [11] was used as a comparison. The Fe/N/C-PmPDA produced isotherms quite different from those of the Fe/N/C-ABI, and few mesopores were observed in this material. Nevertheless, the two catalysts had similar Brunauer-Emmett-Teller (BET) surface areas: 662 vs. 656 m2/g for Fe/N/C-ABI and Fe/N/C-PmPDA, respectively.
Fig. 3(a) compares the ORR polarization curves obtained from the Fe/N/C-ABI and Fe/N/C-PmPDA catalysts in an O2-saturated 0.1 mol/L H2SO4 solution. The catalytic activity of the Fe/N/C-PmPDA was slightly higher than that of the Fe/N/C-ABI, with similar half-wave potentials (0.807 vs. 0.820 V). Mass activity was used to quantitatively compare the intrinsic activities of these materials. The currents of the Fe/N/C-ABI and Fe/N/C-PmPDA at 0.80 V were 2.55 and 2.64 mA/cm2, and the limited diffusion currents were 4.71 and 3.97 mA/cm2, respectively. The catalyst loading on the RDE was 0.6 mg/cm2 and, according to Eq. (1), the mass activity values of the Fe/N/C-ABI and Fe/N/C-PmPDA were 9.21 and 13.4 A/g at 0.8 V, respectively (Fig. 3(b)). Note that the porous distribution did not influence the kinetic mass activity in the RDE tests because this activity is controlled by electrochemical kinetics and not by mass transfer.
We also fabricated MEAs with Fe/N/C-ABI and Fe/N/C-PmPDA as the cathode catalysts. Fig. 4 presents the polarization curves and power densities of the PEMFCs. Consistent with the RDE data, similar catalytic activities were observed at low current density (< 0.2 A/cm2), with slight higher activity exhibited by the Fe/N/C-PmPDA. In this region, the fuel cell performance is controlled by electrochemical kinetics. In contrast, at higher current density values (> 0.2 A/cm2), the PEMFC with Fe/N/C-ABI showed much better performance. The peak power densities were 0.710 and 0.616 W/cm2 for the Fe/N/C-ABI and Fe/N/C-PmPDA devices, respectively.
The voltage can be seen to have varied with current density in an almost linear manner at high current densities. In this region, the mass transfer is more important than the electrochemical kinetics, and thus dominates the cell performance. Therefore, the better performance of the PEMFC with Fe/N/C-ABI at high current density values indicates better mass transfer than the Fe/N/C-PmPDA device. This can be explained by the unique hollow-shell structure of the Fe/N/C-ABI. Generally, much of the water produced by the ORR is in the liquid state, and this liquid water will flood parts of the cathode, dramatically decreasing O2 transfer. The hollow shells of the Fe/N/C-ABI nanoparticulates likely act as buffer zones for water formation and vaporization, and so facilitate the mass transfer of oxygen gas. The loading of the Fe/N/C required to maximize PEMFC performance is much higher than that required when using state-of-the-art Pt/C, and so mass transfer normally becomes a bottleneck. In this work, however, the unique mesoporous structure of the catalyst was found to facilitate mass transfer, providing an ideal solution to the mass transfer issue associated with Fe/N/C applications in PEMFCs.
A Fe/N/C catalyst having a unique hollow-shell structure with abundant mesopores was synthesized without using a template. The hollow shell structure facilitated mass transfer when this material was employed in a PEMFC, and resulted in a high power density output of 0.71 W/cm2. This study demonstrates a new strategy for the preparation of Fe/N/C catalysts with improved mass transfer in PEMFCs.
2016,
Vol. 37 Issue (7): 1103-1108 
PDF (986 KB)
2016,
Vol. 37 Issue (7): 1103-1108 PDF (986 KB)