ARTICLE pubs.acs.org/Langmuir
Ordered Mesoporous Carbon Nitrides with Graphitic Frameworks as Metal-Free, Highly Durable, Methanol-Tolerant Oxygen Reduction Catalysts in an Acidic Medium Kyungjung Kwon,† Young Jin Sa,‡ Jae Yeong Cheon,‡ and Sang Hoon Joo*,‡ † ‡
Department of Energy & Mineral Resources Engineering, Sejong University, Seoul 143-747, Republic of Korea School of Nano-Bioscience and Chemical Engineering and KIER-UNIST Advanced Center for Energy, Ulsan National Institute of Science and Engineering (UNIST), UNIST-gil 50, Ulsan 689-798, Republic of Korea.
bS Supporting Information ABSTRACT: Developments of high-performance cost-effective electrocatalyts that can replace Pt catalysts have been a central theme in polymer electrolyte membrane fuel cells (PEMFCs) and direct methanol fuel cells (DMFCs). In this direction, nitrogen-doped carbon nanostructures free of metallic components have attracted particular attention. Here we show that directing graphitic carbon nitride frameworks into mesoporous architecture can generate a highly promising metal-free electrocatalyst for an oxygen reduction reaction (ORR) in an acidic medium. The ordered mesoporous carbon nitride (OMCN) was synthesized with a nanocasting strategy using ordered mesoporous silica as a template. A variety of characterizations revealed that the OMCN is constructed with graphitic carbon nitride frameworks and ordered arrays of uniform mesopores. The OMCN showed significantly enhanced electrocatalytic activity for ORR compared to bulk carbon nitride and ordered mesoporous carbon in terms of the current density and onset potential. A high surface area and an increased density of catalytically active nitrogen groups in the OMCN appear to contribute concomitantly to the enhanced performance of the OMCN. Furthermore, the OMCN exhibited superior durability and methanol tolerance to a Pt/C catalyst, suggesting its widespread utilization as an electrocatalyst for PEMFCs and DMFCs.
’ INTRODUCTION Fuel cells employing proton exchange membranes (PEMs), including PEM fuel cells (PEMFCs) and direct methanol fuel cells (DMFCs), have been considered to be a promising power source of alternative energy because of their cleanliness and high efficiency, the reusability of exhaust heat, and their flexibility for mobile, transportation, and stationary applications.1,2 Current research efforts in PEMFCs and DMFCs are focused on the development of cost-effective, durable, high-performance fuel cell components to realize their potential for widespread commercialization fully. For the cathode side of electrodes, platinum nanoparticles supported on porous carbons (Pt/C) are commonly the catalyst material of choice.3 However, even Pt/C catalysts show sluggish kinetics for the oxygen reduction reaction (ORR). Furthermore, the prohibitively high cost and limited supply of Pt and its susceptibility to methanol and agglomeration during fuel cell operation have been raised as major issues to be addressed. As such, the exploration of new ORR electrocatalysts beyond Pt-based catalysts has recently been of prime importance. Efforts toward this aim have been geared to develop new electrocatalysts that have a minimized amount of Pt or Pt-free compositions.4 8 In this context, nitrogen-doped carbon nanostructures that are, in most cases, free of metallic components have attracted particular attention.9 12 A variety of N-doped carbon nanostructures r 2011 American Chemical Society
including carbon nanotubes,13 16 graphene,17 19 ordered mesoporous carbon (OMC),20 22 and carbon nitrides23 25 have shown promising catalytic activity for ORR. The nitrogen groups have been demonstrated to play a critical role in ORR activity. The incorporation of nitrogen moieties into the nanostructured carbons has been primarily achieved by in situ doping using N-containing precursors or by postdoping treatments, which are performed under high temperature or in highly corrosive environments. In a basic medium, some examples of N-doped nanostructured carbons have shown promising ORR activity and durability that are comparable with those of Pt/C catalysts.13,21 However, only a limited number of N-doped carbon nanostructures have been reported to be applicable in an acidic medium where PEMFCs and DMFCs are employed.15,16,20 Among the aforementioned N-doped carbons, carbon nitrides are a class of material that is highly enriched in nitrogen atoms.26 In particular, carbon nitride with a graphitic framework structure (g-C3N4) is regarded as the most stable allotrope and exhibits excellent chemical and thermal stability. Accordingly, the successful applications of graphitic carbon nitrides as metal-free catalysts for organic reactions26 28 and hydrogen evolution29,30 Received: October 21, 2011 Revised: November 27, 2011 Published: November 28, 2011 991
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as well as ORR23 25 have been reported. For ORR application, Lyth and co-workers showed that a bulk graphitic carbon nitride exhibits electrocatalytic activity for ORR in an acidic medium; however, the current value was only marginal because of the low surface area of bulk carbon nitride.23 They later demonstrated that the blending of the bulk carbon nitride with carbon black followed by heat treatment significantly enhances the ORR activity.24 M€ullen and co-worker prepared graphene carbon nitride composite nanosheets that exhibit excellent electrocatalytic activity and durability for ORR in a basic medium.25 In this work, we demonstrate that directing graphitic carbon nitride frameworks into the mesoporous architecture can yield a highly promising metal-free electrocatalyst for ORR in an acidic medium. The ordered mesoporous carbon nitride (OMCN) with uniform mesopores and a high surface area was synthesized with a nanocasting strategy using hexagonally ordered mesoporous silica as a template. In an ORR experiment performed in an acidic medium, the OMCN showed significantly enhanced electrocatalytic activity compared to bulk carbon nitride and ordered mesoporous carbon (OMC). Significantly, OMCN exhibited superior durability and methanol tolerance to a commercial Pt/C catalyst. These results suggest the widespread utilization of OMCN as an electrocatalyst for PEMFCs and DMFCs.
Figure 1. (a) TEM and (b) SEM images of SBA-15 and (c) TEM and (d) SEM images of OMCN. analyzed by a nitrogen adsorption experiment at 196 °C using a BEL Belsorp-Max machine. The surface area and pore size distribution of the samples were calculated by using the Brunauer Emmett Teller (BET) equation and the Barrett Joyner Halenda (BJH) method, respectively. The composition of the samples was analyzed by a Thermo Scientific Flash 2000 elemental analyzer. X-ray photoelectron spectroscopy (XPS) spectra were recorded with a monochromatic Al Kα X-ray source using a Thermo Fisher K-Alpha instrument. Electrochemical Measurements. Electrochemical characterizations of the samples were performed by using a rotating disk electrode system (ALS RRDE-3A) with a standard three-electrode cell. A glassy carbon rotating disk electrode was used as a working electrode. The counter and reference electrodes were a Pt foil and Ag/AgCl, respectively. For the thin-film working electrode, a catalyst suspension of 2 mg cm 3 was produced by ultrasonically dispersing the catalyst in water. A 5 μL aliquot of the ultrasonically redispersed suspension was pipetted onto the glassy carbon substrate and dried under ambient conditions. Then 5 μL of the Nafion solution (0.05 wt %) was applied to the substrate to fix the catalyst. The catalyst (including supports) loading of the thin-film working electrode was 124 μg cm 2 for all catalysts. More details on the preparation of the catalyst ink and electrode can be found in our previous report.34 The potentials in this article were reported on the basis of the reversible hydrogen electrode (RHE). Cyclic voltammetry (CV) was conducted at a scan rate of 50 mV s 1 in a nitrogensaturated 0.1 M HClO4 electrolyte. Linear scan voltammetry for the ORR activity measurement was performed at a scan rate of 10 mV s 1 in an oxygen-saturated electrolyte from 1 to 0.4 V.
’ EXPERIMENTAL SECTION Synthesis of SBA-15 Mesoporous Silica. Pluronic P123 (8.0 g, Mw = 5800, Aldrich), deionized (DI) water (251.4 g), and 35 wt % HCl (48.6 g, Samchun) were added to a 500 mL polypropylene bottle, and the mixture was stirred at 35 °C. After the complete dissolution of P123, 17.0 g of tetraethyl orthosilicate (98%, Aldrich) was added to the solution, which was stirred for 5 min and aged at 35 °C without stirring for 24 h. The reaction mixture was then transferred to a Teflon-lined autoclave and held at 150 °C for 24 h. White precipitates were filtered and washed with DI water twice and then dried in a 60 °C oven for 1 day. Finally, the sample was calcined in 550 °C for 5 h in air. Synthesis of OMCN, Bulk Carbon Nitride, and OMC. OMCN was prepared using SBA-15 mesoporous silica and cyanamide as a template and a precursor, respectively.30 32 The syntheses were performed by varying the concentration of cyanamide and the mode of impregnation. In an optimized procedure for OMCN, 2.4 g of cyanamide dissolved in 3.0 g of water was used to impregnate 1.0 g of SBA-15 by vigorous stirring at room temperature for 1 h. The mixture was centrifuged at 4000 rpm for 10 min to remove the excess water and dried in a 60 °C oven. After complete drying, the sample was heated to 550 °C for 4 h and maintained for 4 h under a flow of Ar. The composite was stirred with a 4 M (7.4 wt %) HF (50 wt %, J. T. Baker) solution for 30 min to etch the SBA-15 template, filtered, and washed with ethanol twice. This process was repeated again, and the sample was finally dried in a 60 °C oven. Bulk carbon nitride was prepared by the same procedure with OMCN except that the synthesis was performed in the absence of a SBA-15 mesoporous silica template. The hexagonally ordered OMC was prepared using SBA-15 mesoporous silica and sucrose as a template and a carbon source, respectively, following the reported procedure.33 Characterization Methods. The morphology of the samples was analyzed by scanning electron microscopy (SEM) using a FEI Quanta 200 microscope operating at 15 kV. The internal pore structure of the samples was observed by transmission electron microscope (TEM) using a JEOL JEM-2100F microscope at an accelerating voltage of 200 kV. X-ray diffraction (XRD) patterns of the samples were taken with a Rigaku D/Max 2500 V/PC X-ray diffractometer equipped with a Cu Kα source at 40 kV and 200 mA. The porous structure of the samples was
’ RESULTS AND DISCUSSION OMCN with a hexagonal mesostructure was synthesized with a nanocasting method using hexagonally ordered SBA-15 mesoporous silica and cyanamide as a template and a precursor, respectively. The synthesis of OMCN was performed by changing the experimental factors including the concentration of the precursor and the mode of impregnation. A variety of characterizations reveal that the final material after HF treatment is a negative replica of SBA-15 silica with a graphitic carbon nitride composition. Figure 1 shows TEM and SEM images of the SBA-15 template 992
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Figure 3. CV of OMCN in nitrogen- and oxygen-saturated HClO4 electrolytes.
a type IV isotherm indicative of the existence of mesopores. The pore size deduced from the maximum in the pore size distribution curve (Figure 2b inset) was 4.8 nm, and the BET surface area was 190 m2 g 1. The chemical composition of OMCN analyzed by CHNS elemental analysis revealed a C/N ratio of 0.70. The deviation from the ideal C/N value (0.75) of single-crystalline graphitic carbon nitride (C3N4)26 is presumably due to the lower degree of condensation35 and numerous terminal defect sites exposed on the walls of carbon nitride frameworks. Overall, the OMCN is composed of periodic arrays of highly graphitic carbon nitride frameworks and contains a significant number of nitrogen moieties that are expected to be catalytically active sites for ORR. The electrocatalytic applicability of OMCN was investigated with CV and ORR activity measurements. Figure 3 compares the CV of OMCN in the nitrogen- and oxygen-saturated electrolytes at a scan rate of 50 mV s 1. In the nitrogen-saturated electrolyte, the CV of OMCN shows a capacitive shape where there are no remarkable redox peaks except a broad redox peak at around 0.6 V. This redox peak was attributed to the oxygen groups of surface functionalities on the carbon nitride whose CV was similarly observed by Lyth et al.23 In the presence of oxygen, there is an additional reduction current starting to flow at ca. 1 V with deviation from the CV of the nitrogen-saturated electrolyte, which indicates the reactivity of OMCN with oxygen. To scrutinize the origin of the reduction current in the presence of oxygen, the rotational speed of the rotating disk electrode was changed and its corresponding reduction currents were monitored at a scan rate of 10 mV s 1 as shown in Figure 4a. The reduction currents increase as the supply of oxygen increases, indicating that the reduction currents could be interpreted as ORR currents. In addition, it is noteworthy that the ORR onset potentials in between 0.9 and 1 V coincide with the starting potential of the oxygen-inducing current mentioned in Figure 3. A Koutecky Levich plot was obtained at various potentials where the ORR currents show increasing behavior with the speed of rotation (Figure 4b). Although there is a slight lack of linearity at certain potentials, a linear relationship between the inverse current and the inverse square root of the rotational speed is generally observed. The linear relationship indicates that the ORR follows first-order kinetics with respect to the oxygen concentration.36
Figure 2. (a) Low-angle XRD patterns of SBA-15 and OMCN (inset: wide-angle XRD of OMCN) and (b) nitrogen adsorption desorption isotherms of OMCN (inset: corresponding pore size distribution from the adsorption branch).
and OMCN replica. The TEM image of the SBA-15 template viewed along the channels (Figure 1a) shows the hexagonal arrays of uniform mesopores, which was typically observed in images of SBA-15 silica. The TEM image of OMCN in Figure 1c clearly displays the hexagonal arrays of carbon nitride nanorods with mesopores existing between the nanorods. Thus, the pores and silica frameworks of the SBA-15 template were faithfully replicated into the carbon nitride frameworks and pores of the OMCN replica, respectively. The SEM images of the SBA-15 template and OMCN replica (Figure 1b,d) indicate that the short rodlike, spherical morphology of SBA-15 was maintained after the replication because the SEM image of the OMCN exhibits a shape similar to that of SBA-15. Figure 2a shows the small-angle XRD pattern of SBA-15 and OMCN. The XRD of OMCN clearly displays three distinct peaks that are commensurate with a hexagonal mesostructure, which indicates that the hexagonal mesostructure of the SBA-15 template was preserved after the replication. Notably, the change in the unit cell parameter of OMCN from that of SBA-15 was very small, revealing that only marginal shrinkage of the carbon nitride frameworks took place during the replication. In the wide-angle XRD pattern (Figure 2a inset), OMCN exhibits a peak centered at 26.4° that corresponds to the graphitic interlayer (002) peak. The porous nature of OMCN was accessed with nitrogen adsorption desorption isotherms (Figure 2b) that clearly show 993
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Figure 6. N 1s XPS spectrum and its deconvolution of OMCN.
Figure 4. (a) ORR activity of OMCN with rotational speed and (b) a Koutecky Levich plot of OMCN.
Figure 5. ORR activity of OMCN, bulk carbon nitride, OMC, and glassy carbon.
Figure 7. CV (numbers in the legend indicate cycle numbers) of (a) OMCN and (b) Pt/C during the ADT.
The ORR current of OMCN at 900 rpm was compared to those of bulk carbon nitride, OMC, and glassy carbon as shown in Figure 5. OMCN shows the highest ORR in the investigated potential range among the catalysts. OMC has a higher reduction current, particularly in the potential range around 0.9 V; however, this originates from a double-layer charging current due to
the large surface area (BET surface area: 1237 m2 g 1) of OMC rather than from the oxygen reduction current. In comparing the OMCN and bulk carbon nitride, the superior ORR activity of OMCN can be ascribed to the larger surface area of OMCN (190 m2 g 1 vs 12.6 m2 g 1 of bulk carbon nitride) that consequently increased the site density of catalytically active nitrogen 994
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Figure 8. ORR activity of (a) OMCN and (b) Pt/C before and after ADT and in the presence of MeOH.
groups. The chemical nature of the nitrogen group in OMCN was probed by XPS. The N 1s XPS spectrum of the OMCN sample in Figure 6 can be deconvoluted into three peaks at 398.4, 399.9, and 404.3 eV, which correspond to pyridinic, pyrrolic, and pyridinic oxide species, respectively.10 Note the predominant contribution from pyridinic moieties that have been previously reported as catalytically active species for ORR.13,16,23 Hence, the increased number of catalytically active pyridinic species through the mesostructure formation in OMCN should be responsible for the enhanced ORR activity. The electrochemical stability in corrosive acidic media is also an important factor in designing high-performance electrocatalysts. The durability of OMCN and Pt/C (20 wt % Pt, JohnsonMatthey) in an acidic environment was determined by comparing changes in the ORR activity before and after an accelerated durability test (ADT).37 The potential was continuously cycled from 0.05 to 1.4 V in the ADT where both the surface oxidation/ reduction of Pt nanoparticles and carbon support corrosion occur as in an actual fuel cell electrode.38,39 Figure 7a,b shows the CVs of OMCN and Pt/C during the ADT, respectively. Pt/C shows significant degradation behavior during the ADT, as evidenced by a decrease in the hydrogen desorption/adsorption current with cycling. After the 200 cycles of ADT, the Pt/C catalyst underwent a substantial loss of electrochemical surface area from 94 to 69 m2 g 1. The reduction in the electrochemical surface area is mostly caused by the Ostwald ripening phenomenon where Pt dissolution and redeposition occur during potential cycling in the investigated potential range resulting in an irreversible
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change in the Pt particle size.39 By contrast, no significant change occurred in the CV of OMCN that does not inherently undergo a similar dissolution/redeposition process to Pt, indicating the superior structural durability of OMCN with respect to the Pt/C catalyst in an acidic medium. Figure 8a,b compares the ORR activities of OMCN and Pt/C after the ADT and methanol additions at 900 rpm in the oxygensaturated electrolyte. As expected from the degraded behavior of Pt/C in Figure 7b, Pt/C also shows a reduction in ORR activity in terms of a negative shift (a few tens of millivolts degradation in the half-wave potential) in the ORR current after the ADT (Figure 8b). By contrast, OMCN maintains its ORR activity after the ADT similarly to Figure 7a. When the effect of the presence of 0.5 M methanol in the electrolyte is considered, a superior methanol tolerance of OMCN to Pt/C is evident. Because Pt/C is reactive with methanol in addition to oxygen during the ORR process, a mixed potential forms at which methanol and oxygen simultaneously react at the surfaces of Pt particles.40,41 This mixed potential is in fact formed in an actual cathode in DMFCs, which is caused by the crossover of methanol through a polymer membrane, leading to a net loss of cell voltage. Therefore, it is expected that the use of this methanol-tolerant, durable OMCN as an ORR catalyst in the cathode of DMFCs has a competitive edge over the state-of-the-art Pt/C. It is noteworthy that the intrinsic ORR activity of OMCN is inferior to that of Pt/C in terms of the on-set potential and diffusion current value, presumably due to the low electrical conductivity of OMCN. Thus, increasing the conductivity of OMCN may lead to enhanced electrocatalytic performance for ORR, which can be achieved by hybridizing OMCN with highly conductive carbons such as carbon black or graphene.24,25
’ CONCLUSIONS We have presented the first example of the use of OMCN as a metal-free, highly efficient, durable, methanol-tolerant electrocatalyst for ORR in an acidic medium. OMCN with graphitic frameworks was prepared with a versatile nanocasting method using hexagonal mesoporous silica as a template. The high surface area and increased density of catalytically active nitrogen moieties combined with well-defined mesoporosity in OMCN enabled significantly enhanced catalytic activity for ORR compared to that of the bulk carbon nitride and OMC. Importantly, OMCN exhibited superior durability and methanol tolerance to a Pt/C catalyst, suggesting its widespread utilization as an electrocatalyst for PEMFCs and DMFCs. The potential of OMCN catalysts can be further extended to other important electrochemical and gas-phase reactions. Furthermore, the use of OMCN as a catalyst support42 for metallic nanoparticles can potentially give rise to bifunctional catalysts, the investigation of which is currently underway. ’ ASSOCIATED CONTENT
bS
Supporting Information. N2 adsorption desorption isotherms and corresponding pore size distributions of SBA-15, bulk carbon nitride, and OMC. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. 995
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’ ACKNOWLEDGMENT This work was supported by the Basic Science Research Program through the National Research Foundation (NRF) of Korea funded by the Ministry of Education, Science and Technology (KRF-2011-0005713) and the Inter-Metropolitan Cooperation Development funded by the Presidential Committee on Regional Development. S.H.J is a TJ Park Junior Faculty Fellow supported by the TJ Park Science Foundation. K.K. was supported by the Korea Sanhak Foundation.
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