Transition Metal Ion-Chelating Ordered Mesoporous Carbons as

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Transition Metal Ion-Chelating Ordered Mesoporous Carbons as Noble Metal-Free Fuel Cell Catalysts Johanna K. Dombrovskis,† Hu Y. Jeong,‡ Kjell Fossum,† Osamu Terasaki,§ and Anders E. C. Palmqvist*,† †

Applied Surface Chemistry, Department of Chemical and Biological Engineering, Chalmers University of Technology, SE-412 96 Göteborg, Sweden, ‡ UNIST Central Research Facilities (UCRF) and School of Mechanical & Advanced Materials Engineering, Ulsan National Institute of Science and Technology (UNIST), Ulsan 689-798, Republic of Korea, § Graduate School of EEWS, WCU Project, KAIST, Deajeon 305-701, Republic of Korea S Supporting Information *

ABSTRACT: A new concept for noble metal-free polymer electrolyte membrane fuel cell catalysts has been developed. The catalysts consist of chelated transition metal ions incorporated in a nitrogen-functionalized ordered mesoporous carbon matrix, which is evidenced by a combination of X-ray absorption fine structure analysis and high-resolution transmission electron microscopy. The ordered mesoporous carbon matrix of the catalyst offers an exceptionally high specific surface area and allows conceptually for a high degree of tuning, enabling controlled variability of, e.g., pore size and curvature and thickness of the pore walls of the catalysts. Single cell fuel cell tests of membrane electrode assemblies prepared with a cathode made of iron- or cobalt-based versions of the catalyst show high power densities, reaching up to one-third of a commercial Pt/C catalyst at 0.6 V. KEYWORDS: fuel cell, transition metals, mesoporous materials, EXAFS spectroscopy, electrocatalysis

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INTRODUCTION Large scale application of proton exchange membrane fuel cells (PEMFCs) requires low cost alternatives to platinum catalysts. Especially the cathode currently needs much platinum due to the large activation loss for the 4-electron transfer reaction converting oxygen molecules to water. Alternative catalysts should consist of abundant elements, exhibit sufficient activity, stability, and efficient mass transport properties. This is a phenomenally challenging set of demands. In 1964, it was first demonstrated that noble metal-free Co-phtalocyanine can catalyze the oxygen reduction reaction (ORR),1 and shortly thereafter other porphyrin-type compounds were found active.2 These compounds suffered from low stability, which was subsequently shown possible to partly rectify by heat treatments. During the 1980s, preparation techniques were developed for noble metal-free catalysts from low cost reagents instead of expensive porphyrins or enzyme-like compounds.3−7 Recently, highly active8 and stable9 iron- and cobalt-containing nitrogen-modified carbons were presented, and properties are now approaching that of platinum-based catalysts. However, to reach the necessary additional areal activity of the catalysts, it is necessary to utilize the volume of the electrode layer at maximum efficiency. For this the catalyst must have as high a specific surface area as possible. In this context, templated materials, such as ordered mesoporous carbon (OMC), are of great interest. Typical OMC materials can reach specific surface areas above 1000 m2/g, and their mesoscopic pore structure can be fine-tuned in terms of pore size, pore volume, pore wall thickness, and atomic as well as © 2013 American Chemical Society

mesoscopic structure and degree of hydrophilicity. All these properties are of importance in tuning the performance of electrocatalysts, and OMC materials thus provide unique opportunities as fuel cell catalyst supports. There are a few reported studies of incorporating platinum nanoparticles in OMC materials for fuel cell catalysts,10,11 and, recently, ordered mesoporous carbon nitrides were found active as metal-free ORR catalysts.12 An appealing concept for a noble metal-free electrocatalyst is to combine the active local structure of chelated transition metal ions with a high surface area OMC matrix. Here, we target this concept by simultaneously including a nitrogencontaining carbon source and transition metal salts during the formation of ordered mesoporous carbons. Very encouragingly the cobalt- and iron-based versions of the catalyst show high ORR activities as cathodes in a fuel cell. Through the use of extended X-ray absorption fine structure (EXAFS) spectroscopy, high resolution transmission electron microscopy (HRTEM), and scanning transmission electron microscopy (STEM) with a high angle annular dark field (HAADF) detector, we show unambiguously that the transition metal ions are chelated to the nitrogen-functionalized carbon and not present as metallic, oxide, or carbide particles. Received: October 17, 2012 Revised: February 22, 2013 Published: February 27, 2013 856

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(99.996%, from AGA) at a flow rate of 50 mL/min, and the cathode was supplied with an overstoichiometric flow rate of 25 mL/min of pure O2 (99.998%, AGA). The gases were 100% humidified, and the cell was operated without backpressure. The fuel cell data presented here were recorded once the cell had reached stable conditions, which was typically achieved after 60−70 potential sweep cycles between OCP and −0.4 V vs RHE. Further details of the catalyst preparation and physical and electrochemical characterization of the catalysts are given in the Supporting Information.

EXPERIMENTAL SECTION

Cubic mesoporous silica was prepared and used as hard template to synthesize the catalysts following a modified route for the preparation of OMC. For a typical sample the silica was impregnated with furfuryl amine saturated with a metal salt. This was followed by a heat treatment at 100 °C in air for 2 h and a second carbon-/metalprecursor impregnation and heat treatment at 160 °C for 2 h in air in order to polymerize the carbon precursor. Finally the samples were pyrolyzed at high temperature (between 800 and 1100 °C) in inert atmosphere, after which the silica template was removed by HFleaching. One sample (catalyst 4) was prepared using different heat treatments, and a reference sample (catalyst 7) was prepared using a different procedure as described below. The series of functionalized OMCs studied is summarized in Table 1.

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RESULTS AND DISCUSSION The physical characterization of the materials presented in the Supporting Information shows the expected mesostructure, surface area, and pore properties within the typical ranges for ordered mesoporous carbons. The transmission electron microscopy images shown in Figure 1 illustrate very high

Table 1. Description of Prepared Catalysts catalyst

composition and preparation details

1 2 3 4

ordered mesoporous carbon (OMC) (pyrolysis at 950 °C) N-functionalized OMC (pyrolysis at 950 °C) Co/N-functionalized OMC (pyrolysis at 950 °C) Co/N-functionalized OMC (not polymerized prior to pyrolysis at 950 °C) Fe/N-functionalized OMC (pyrolysis at 950 °C) Fe/N-functionalized OMC (pyrolysis at 1100 °C followed by acid leaching) OMC impregnated by Fe-porphine (followed by pyrolysis at 800 °C)

5 6 7

The introduction of nitrogen in the material was done to promote formation of functional groups for chelating iron and cobalt ions and achieved using furfuryl amine as precursor. Two reference materials were prepared free from metals, where catalyst 1 was nitrogen-free and synthesized using furfuryl alcohol, while for catalyst 2 furfuryl amine without metal salt was used as precursor. Two Co-containing catalysts were synthesized. One with polymerization steps at 100 and 160 °C between the impregnation steps as described above (catalyst 3), while the other was not polymerized at 100 or 160 °C but instead pyrolyzed twice at 800 °C before the final pyrolysis at 950 °C (catalyst 4). The Fe-containing catalyst 5 was prepared as catalyst 3 but using Fe salt. Catalyst 6 was prepared as catalyst 5 but with pyrolysis at 1100 °C followed by an acid-leaching procedure. Further details on the synthesis procedure are described in the Supporting Information. In addition to these catalysts prepared from low cost precursors, a reference material (catalyst 7) was prepared using a ready-made porphine as precursor. This catalyst was mainly used as an EXAFS reference material to establish the local structure of the transmission metals in the catalysts. Thus, catalyst 7 was prepared by impregnating an OMC, of the same type as catalyst 1, with 5,10,15,20-tetrakis(4methoxyphenyl)-21H,23H-porphine iron(III) chloride dissolved in chloroform for 2 h. The amount of iron-porphine used here would theoretically allow for the iron concentration in catalyst 7 to be twice as high as in catalysts 5 and 6. The impregnation was followed by centrifuging and drying at 100 °C, after which the material was pyrolyzed for 2 h in inert atmosphere at 800 °C. Some iron is lost during these synthesis steps resulting in an iron content of catalyst 7 that is somewhat lower than in catalysts 5 and 6. The catalysts in Table 1 were evaluated as cathode catalysts using 2.25 cm2 electrodes in a single cell PEMFC test cell (Fuel Cell Technologies Inc.). All of the prepared Fe- and Co-containing catalysts have an average metal loading of 0.9 ± 0.2 wt.% according to Energy Dispersive X-ray Spectroscopy analysis. This results in a typical metal loading on the prepared cathode gas diffusion electrodes (GDEs) of 0.06 ± 0.015 mg metal/cm2. The catalyst material loading on the GDEs was 6.5 mg/cm2 including the weight of the OMC support. At the anode a commercial 10 wt.-% Pt electrode from Electrochem Inc. with a metal loading of 0.5 mg/cm2 was used. The same material was used as cathode in the platinum reference measurement. During operation the anode side of the cell was fuelled with 10% H2/Ar

Figure 1. TEM images of catalysts 3 (a-b), 4 (c-d), and 5 (e-f).

degrees of mesoorder in catalysts 3 and 5 and less in catalyst 4 in agreement with gas adsorption and Small Angle X-ray Scattering (SAXS) analyses. In the HRTEM images all three catalysts show the presence of graphitic-like sheets in the pore walls. The prepared materials were evaluated as cathodes in membrane electrode assemblies (MEAs) in a PEM fuel cell. Figure 2a shows the polarization curves of the catalysts compared to a MEA based on a commercial Pt/C-based cathode. The OMC catalyst 1 did not show any catalytic 857

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It was also noted that Fe-containing catalysts were superior to Co-containing catalysts with regards to stability. While both Fe- and Co-containing catalysts exhibited similar behavior between room temperature and 50 °C, Fe-containing catalysts improved further with increased temperature and reached a maximum activity at 70 °C, which remained stable at 80 and 90 °C, the highest tested temperatures. Co-containing catalysts on the other hand showed signs of degradation once the fuel cell operating temperature exceeded 50 °C. This effect is small for catalyst 3 and more pronounced for catalyst 4, which was synthesized without the polymerization step. Figure 2b shows the Tafel plots of the catalysts compared to a MEA based on a commercial Pt/C-based cathode. The general trends between the different catalysts seen in the polarization plots are visible here as well but additional details, specifically in the high potential region, can be seen. While catalysts 6 and 7 exhibit a similar behavior as the Pt/C reference catalyst having a Tafel slope close to −120 mV/dec in the potential range between 0.9 to 0.75 V vs RHE, the transition metal catalysts without acid washing (catalysts 3, 4, and 5) exhibit a distinctly different behavior. The Tafel plots of these catalysts feature steep slopes of more than −600 mV/dec in the potential region of 1 to 0.8 V (catalyst 5) and 1 to 0.7 V (catalysts 3 and 4), respectively. Only at lower potentials, when mass-transport losses dominate, these catalysts adopt a behavior similar to catalysts 6 or 7. The origin of the steep Tafel slopes of catalysts 3, 4, and 5 is not yet fully understood, but we note that catalyst 6 exhibited a similar steep slope prior to it being exposed to the acid leaching procedure, see details in the Supporting Information. The mechanistic effect of the acid leaching is the topic of a more detailed future study. Understanding the nature of the active site in transition metal-containing nitrogen-modified carbons has been a topic of some research over the years and remains a matter of debate. Some authors consider active sites to be based on transition metal ions chelated to a number of nitrogen atoms anchored to an aromatic carbon structure,13−26 and others suggest the active site to be related to nonmetallic heteroatoms, such as nitrogen, present in the carbon matrix.20,27−29 While we found some catalytic activity of the nitrogen-modified transition metal-free catalyst 2, significantly higher activity was obtained with all of the transition metal-containing catalysts. It is hence of interest to characterize the nature of these transition metals within the catalysts. In none of the materials studied using HRTEM could transition metal particles be found. Instead, the Fe and Co present in catalysts 3−5 were found to be evenly distributed throughout the materials and possibly atomically dispersed as shown by Energy Dispersive X-ray Spectroscopy (EDX) and STEM Electron Energy Loss Spectroscopy (EELS) elemental mapping presented in the Supporting Information. The oxidation state and local structure of the transition metals incorporated in the nitrogen-functionalized OMC catalysts were determined using Fe- and Co K-edge EXAFS shown in Figures 3 and 4, respectively. From the white line intensities of the normalized absorption plots and from the position of the absorption edge energy of the materials shown in Figure 3a, it is obvious that Fe in catalysts 5 and 6 are in an oxidized state more similar to that in the Fe-porphine than in the metal. The radial distance distribution plot of Fe in catalysts 5 and 6 presented in Figure 3b shows unambiguously that the local structure of Fe is different from that in Fe metal and Fe2O3 and

Figure 2. Single cell PEMFC measurements showing data for membrane electrode assemblies (MEAs) made from commercial Pt/ C anodes and cathodes consisting of catalysts: 1: ordered mesoporous carbon (OMC) (light magenta), 2: nitrogen-functionalized OMC (magenta), 3: cobalt- and nitrogen-functionalized OMC (blue), 4: cobalt- and nitrogen-functionalized OMC prepared without prepolymerization (light green), 5: iron- and nitrogen-functionalized OMC (red), 6: iron- and nitrogen-functionalized OMC exposed to acid leaching (brown), and 7: OMC impregnated by 5,10,15,20-tetrakis(4methoxyphenyl)-21H,23H-porphine iron(III) chloride followed by pyrolyzation (dotted green). A MEA prepared from a commercial Pt/ C cathode catalyst from Electrochem Inc. with a Pt-loading of 0.5 mg/ cm2 (black) was measured under the same conditions as the OMC catalysts. Tests were done at 100% relative humidity and 70 °C for the Pt- and Fe-containing catalysts and at 50 °C for the Co-containing catalysts. The data are corrected to 1 bar H2 using the Nernst equation. Graph a) shows the potentiodynamic behavior of the catalysts, while graph b) shows the Tafel behavior.

activity for the reaction within the potential range studied, whereas the N-functionalized OMC catalyst 2 showed some activity at potentials below 0.6 V. Introducing transition metals to the N-functionalized OMC significantly improved the catalytic activity for both the Co/N- and Fe/N-functionalized catalysts. In general, the Fe-containing materials showed higher activities than Co-containing materials with an onset of oxygen reduction around 0.8 V. The highest activity was obtained with catalyst 6, a Fe/N-functionalized OMC pyrolized at 1100 °C and subsequently subjected to acid-leaching. It showed a current density at 0.6 V close to one-third of that obtained using the commercial Pt/C cathode in the same measurement setup. 858

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Figure 3. a) Fe K-edge EXAFS profiles and b) radial distance distribution functions of the local atomic structure of iron in Fe-foil (black), Fe2O3 iron oxide (orange), 5,10,15,20-tetrakis(4-methoxyphenyl)-21H,23H-porphine iron(III) chloride (solid dark green), OMC impregnated by 5,10,15,20-tetrakis(4-methoxyphenyl)21H,23H-porphine iron(III) chloride followed by pyrolyzation (catalyst 7 - dotted dark green) and iron- and nitrogen-functionalized OMCs (catalyst 5 - red and catalyst 6 - brown).

Figure 4. a) Co K-edge EXAFS profiles and b) radial distance distribution functions of the local atomic structure of cobalt in Co foil (black), 5,10,15,20-tetrakis(4-methoxyphenyl)-21H,23H-porphine cobalt(II) (dark green) and cobalt- and nitrogen-functionalized OMCs (catalyst 4 - light green and catalyst 3 - blue).

has been shown to appear at a similar energy to that of Fe metal and with similar white line intensity, distinct from what we find for catalysts 5 and 6. Second, Fe3C exhibits its first coordination peak at a radial distance similar to that of the Fe metal, where we find only Fourier transformation ripples in the spectra of our catalysts.27,30 Clearly, the details of the preparation of transition metal-containing nitrogen-functionalized carbon catalysts determine the exact nature, local structure, and properties of their active sites. The methodology presented here results in nitrogen-functionalized OMCs chelating iron ions, via nitrogen or possibly oxygen bridges as judged from the short radial distance of the first coordination shell. Similar local structures with FeN4 or FeN2+231,32 coordination have been proposed for other transition metal-based catalysts prepared using different synthesis approaches.23,33,34 From the Co K-edge absorption spectra shown in Figure 4a, it is evident that the majority of cobalt in catalysts 3 and 4 is in

instead more similar to that in the Fe-porphine. Two different measurements of the 5,10,15,20-tetrakis(4-methoxyphenyl)21H,23H-porphine iron(III) chloride are shown in Figure 3. The solid green line shows the pure porphine measured as received, while the dotted green line shows an OMC impregnated by the porphine followed by pyrolyzation (catalyst 7). The first shell bond distance in catalysts 5 and 6 corresponds very well with the first shell bond distance of catalyst 7, and catalyst 6 even shares signals at larger distances with catalyst 7. We note that the local structure of Fe in catalysts 5, 6, and 7 is significantly different from that reported for Fe-containing CNx materials prepared using other methods resulting in oxidation states and local structures of Fe that resemble that of Fe metal or Fe3C.27,28 Based on the EXAFS analysis we can completely rule out the presence of Fe3C in catalysts 5 and 6. First, the K-edge of Fe3C 859

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Chemistry of Materials an oxidation state similar to the Co-porphine. However, the two OMC materials and the porphine have somewhat different electronic environments and local structures as judged from differences in the appearance of the characteristic kink in the absorption edge.35 A similar behavior was previously observed by Ziegelbauer et al. between Co-octaethyl-porphyrin (CoOEP) exhibiting a kink and pyrolyzed Co-tetra-methylphenylporphyrin (Co-TMPP) material, not exhibiting a kink.19 The local atomic structures of cobalt in the catalysts are shown in Figure 4b. In catalyst 3 it is distinctly different from that of Co metal, and for this catalyst we conclude that the transition metal ion must be chelated to the nitrogenfunctionalized carbon. Catalyst 4, however, while exhibiting in large parts a local atomic structure similar to catalyst 3, also seems to have minor contributions of additional binding distances. As these contributions appear at distances similar to those characteristic for Co metal they could be attributed to Co−Co bindings. These Co−Co contributions only make up a minor part of the distribution function of catalyst 4. Thus, as such, catalyst 4 mainly exhibits chelated cobalt ions. However, we cannot rule out that the few species representing the Co− Co bond distance may be responsible for the lower thermal stability of catalyst 4 observed during fuel cell measurements. The difference between catalysts 3 and 4 evidence the great importance of the different synthesis steps for the active site structure formation, namely the importance of the polymerization steps, which appear imperative for a complete chelation of the transition metal ions to be achieved.

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CONCLUSION In summary, we have shown that electrocatalysts consisting of Fe- or Co-chelating ordered mesoporous carbons can be prepared and exhibit high activity as cathodes in PEMFC. This is the first time transition metal ions are evidenced to be incorporated as chelated ions in a tailor-made high surface area nitrogen-functionalized carbon. The large possibilities to tailor the physicochemical properties of ordered mesoporous carbons are expected to be advantageous in further optimizing and refining the concept of transition metal-chelated electrocatalysts in general and noble metal-free PEMFC cathode catalysts in particular. We also conclude that the details of the preparation of transition metal-containing nitrogen-functionalized carbon catalysts are of great importance and have a large impact and in fact determine the exact nature, local structure, and properties of the active sites in these catalyst materials. This is of importance for future research on and discussions of transition metal-containing nitrogen-functionalized carbons. ASSOCIATED CONTENT

S Supporting Information *

Experimental methods, Supplementary Figures X1 to X8 and Table X1, and references. This material is available free of charge via the Internet at http://pubs.acs.org.

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ACKNOWLEDGMENTS

The Swedish Research Council (VR) is gratefully acknowledged for project funding and a Senior Researcher grant for AECP. The Swedish Foundation for Internationalization of Research (STINT) is acknowledged for support of a Swedish/ Korean research network. TEM work was supported by the year of 2012 Research Fund of the UNIST. Support from the WCU programme, Korea (R-31-2008-000-10055-0) is also acknowledged (O.T.). Portions of this research were carried out at beamline I811, MAX-lab synchrotron radiation source, Lund University, Sweden. Funding for the beamline I811 project was kindly provided by The Swedish Research Council and The Knut och Alice Wallenbergs Stiftelse.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. 860

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