N-Doped Carbon Hybrid

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Simple Fabrication of Titanium Dioxide/N-Doped Carbon Hybrid Material as Non-Precious Metal Electrocatalyst for the Oxygen Reduction Reaction Shangbin Jin, Cuiling Li, Lok Kumar Shrestha, Yusuke Yamauchi, Katsuhiko Ariga, and Jonathan P Hill ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 08 May 2017 Downloaded from http://pubs.acs.org on May 10, 2017

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ACS Applied Materials & Interfaces

Simple Fabrication of Titanium Dioxide/N-Doped Carbon Hybrid Material as Non-Precious Metal Electrocatalyst for the Oxygen Reduction Reaction Shangbin Jin,1,2,‡,† Cuiling Li,3,‡ Lok Kumar Shrestha,1 Yusuke Yamauchi,3,4 Katsuhiko Ariga1 and Jonathan P. Hill*,1 1. Supermolecules Group, International Center for Materials Nanoarchitechtonics, National Institute for Materials Science, Namiki 1-1, Tsukuba, Ibaraki 305-0044, Japan. 2. School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, 1037 Luoyu Road, Wuhan, 430074, China. 3. Mesocale Materials Chemistry Group, International Center for Material Nanoarchitechtonics, National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan. 4. Australian Institute for Innovative Materials (AIIM), University of Wollongong, Squires Way, North Wollongong, NSW 2500, Australia. Corresponding author: Jonathan P. Hill E-mail: [email protected]

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KEYWORDS Porous coordination polymer; Catechol porphyrin; Titania; Anatase; Thermolysis; Oxygen reduction reaction; N-doped carbon.

ABSTRACT We report a new approach for the fabrication of hybrid titanium dioxide/carbon materials derived from a porous titanium coordination polymer composed of a catechol-substituted porphyrin and Ti4+. Titanium dioxide nanocrystals were formed distributed in a nitrogen-doped carboniferous matrix after the thermolysis of the coordination polymer. The identity of the titanium dioxide phase, i.e. anatase or rutile, could be controlled by varying the thermolysis temperature. Electrochemical performances of the composites were explored with results demonstrating that the hybrid materials are promising cathodic materials for the oxygen reduction reaction.


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Introduction The preparation of materials that can be used for efficient energy conversion processes is becoming an increasingly urgent issue in the search for sustainable sources to address the global energy challenge.1 Fuel cell technology is useful for the local conversion of chemical energy to electricity, and can be considered one of the most promising solutions to energy generation problems. However, fuel cell operation involves the oxygen reduction reaction (ORR), which is a limiting step due to its slow rate.2 For this reason, investigations have focused on the enhancement of fuel cell performance by the identification of an effective catalyst for ORR. Platinum on carbon (Pt/C) and other Pt-containing materials have been reported as being some of the most active electrocatalytic materials for ORR3 although the high cost and rarity of platinum makes large scale practical application inconvenient, which in turn drives the search for cost-effective replacement materials.4,5 Various hybrids of metallic nanoparticles with carbon materials have been demonstrated as promising alternative materials for ORR, while other particulates such as metal oxides have also been considered for this application based on the preparation of hybrid composites with carbonaceous materials.6−14 These include composites containing TiO2.15−18 The composite approach has also been applied as a means for improving the effective ORR activities of Pt-containing materials.19−22 Porous coordination polymers (PCPs) are a class of materials formed from organic linkers connecting transition metal cations. They can be of crystalline (i.e., metal organic framework, MOF23) or ‘soft’ crystalline morphologies.24 PCP materials are promising for the preparation of hybrid materials of metal/metal oxides with carbon materials because of the intimate mixing of the precursor components.25−30 Also, the structures of thermolytic and other degradation products are


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known to be influenced by the structures of the crystalline precursors.31,32 More specifically, the advantages represented by PCPs for the preparation of hybrid materials are due to 1) uniform distribution of metal cations in the frameworks facilitating the formation of nanoparticles in the resulting carbon matrix, and 2) the high porosities of PCPs, which can induce a porous structure in composites obtained by thermolysis. Both of these features can contribute to charge diffusion in thermolytic products making them useful for electrochemical applications. With these potential advantages in mind, we have recently developed a series of porous coordination polymers derived from a catechol-substituted porphyrin that can coordinate various transition metal cations (Scheme 1), and from which highly electrochemically-active hybrid materials could be easily obtained by thermolysis.33 Although there have been some reports where porphyrins incorporated in porous organic polymers were used as carbonization precursors to prepare metal and N-doped carbon materials for oxygen reduction reactions, most of these have involved oxides of cobalt, iron or manganese to achieve excellent oxygen reduction activity. For example, Hou and coworkers reported the preparation of two-dimensional (2D) cobalt/nitrogen co-doped porous carbon nanosheets for use as catalysts in the ORR (under basic or acidic conditions) by using a cobalt porphyrin based 2D conjugated microporous polymer as a precursor.34 Brueller and coworkers similarly reported ORR materials based on thermolyzed porphyrin conjugated polymers but produced bimetallic N-doped carbons.35 Several other reports have utilized cobalt and iron complexes with porphyrin as precursors36-38 but reports of the use of complexes with titanium as precursor are uncommon.


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Scheme 1. Thermolytic preparation of TiO2/npC materials from a porous coordination polymer derived from catechol-substituted porphyrin and titanium(IV) tetraisopropoxide. Titanium is a relatively inexpensive and abundant element in the Earth’s crust. Titanium dioxide (TiO2) is also an inexpensive and low toxicity material, which has been extensively investigated as a semiconductor material for photocatalytic applications.39 Despite recent advances in the application of TiO2 for photocatalysis, it has not been well investigated for electrochemical applications because of its low conductivity and poor activity. For these reasons, there are only a few reports involving the study of the electrochemical properties TiO2 with respect to the oxygen reduction reaction.40−42 Recently, defect-containing TiO2 was reported to be an efficient catalyst for ORR, although it was necessary to combine this material with carbon nanoparticles in order to improve the overall conductivity of the material.43 In this work, we report the fabrication of a novel TiO2/nanoporous carbon hybrid material, which was easily prepared by the thermolysis of an amorphous titanium(IV) porous coordination polymer, and contains nanoscale TiO2 crystals strongly coupled with the carbon matrix (Scheme 1). The electrochemical properties for the oxygen reduction reaction were investigated with good performance being revealed. The results demonstrate that materials prepared using such an approach are promising electrode materials for application.


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Experimental Section Synthesis of the titanium(IV) porous coordination polymer (TiCP-PCP). The catecholsubstituted porphyrin was prepared as reported previously.33 This was then reacted using conditions similar to those applied by Benedict and Coppens.44 Thus, titanium tetraisopropoxide (0.8 mmol, 227 mg) was added to a solution of catechol porphyrin ligand (0.4 mmol, 296 mg) dry toluene (20 mL) and the mixture was refluxed for 48 h. The resulted precipitate was collected by filtration, washed with ethanol and tetrahydrofuran, followed by Soxhlet extraction with tetrahydrofuran for two days (to remove unreacted materials). TiCP-PCP was obtained as dark purple powder in 99% yield based on the starting porphyrin. Synthesis of titanium dioxide/nanoporous carbon TiO2/npC composites. TiCP–PCP was converted to hybrid materials by thermolysis of the sample at the designated temperature under flowing nitrogen gas. The temperature was gradually increased from room temperature to the designated temperature during a 2 h period and then maintained for 3 hours before cooling to room temperature. Characterization of materials. FT-IR spectra were recorded using a Nicolet 4700 FT-IR instrument, ThermoElectron Corporation. The morphologies were observed by scanning electron microscopy (SEM) using a Hitachi S-4800 field effect SEM operating at 5 kV. High resolution transmission electron microscopy (HR-TEM; JEOL JEM-2100F operated at 200 kV) was also used to observe the morphologies. Samples for HR-TEM observations were prepared by dropping the samples as a suspension in ethanol onto standard carbon-coated copper grids followed by drying overnight under reduced pressure. Powder X-ray diffraction patterns were measured using a Rigaku Ultima III diffractometer with Cu Ka radiation (λ = 1.5406 Å). Samples were mounted on a glass plate. Raman scattering spectra were recorded using a Jobin-Yvon T64000 Raman


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spectrometer. Samples were prepared on a clean silicon wafer and excited using a green laser of wavelength 514.5 nm at 0.01 mW power. Nitrogen adsorption-desorption isotherms were recorded on an automatic adsorption instrument (Quantachrome Instruments, Autosorb-iQ2, USA) at liquid nitrogen temperature 77.35 K. The pore size distribution curves were obtained by the non-localized density functional theory (NLDFT) method. For each measurement, about 20 mg of sample was degassed for 20 h at 100 C prior to the measurements. X-ray photoelectron spectroscopy (XPS) was carried out on a Theta Probe spectrometer (ThermoElectron Co., Germany) using monochromated Al Ka radiation (photon energy 15 keV, maximum energy resolution 0.47eV, maximum space resolution 15 mm). Electrochemical Measurements. All the electrochemical measurements were performed using a CHI 842B electrochemical analyzer (CHI Instruments, USA). A conventional three-electrode cell was used, including an Ag/AgCl (saturated KCl) reference electrode, a platinum wire as a counter electrode, and a modified rotating ring-disk electrode (RRDE, 4 mm in diameter) as a working electrode. The working electrode was prepared as follows. 5 mg of catalyst was dispersed in the mixture of water/ethanol (v/v = 3/1, 950 μl) and Nafion (5 wt%, 50 μl) under ultra-sonication for 30 min to get a uniform suspension of the catalysts. Then, 5 μL of the above suspension was dropped on the disk electrode surface of the RRDE and dried at room temperature. The electrochemical measurements were performed in 0.1 M KOH saturated with O2 at a potential range between -1.0 V and 0.2 V (vs. Ag/AgCl) with a scan rate of 10 mV s-1 and a typical rotating speed of 1600 rpm. The potentials reported in present work were referenced to the reversible hydrogen electrode (RHE) through RHE calibration, and in 0.1 M KOH, E(RHE) = E(Ag/AgCl) + 0.96 V.


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Results and Discussion The titanium(IV) porous coordination polymer (TiCP-PCP) was prepared by refluxing titanium tetraisopropoxide with the catechol porphyrin ligand in toluene.45 The resulted precipitate was collected by filtration, washed with ethanol and tetrahydrofuran, followed by Soxhlet extraction with tetrahydrofuran for two days to remove unreacted starting materials. The FT-IR spectrum of TiCP-PCP (Fig. S1) indicates that there are isopropyl moieties contained in the polymer. This is either due to trapping of isopropanol in the network structure or, more likely, the presence of a small amount of terminating alkoxide at the periphery of the network structure. C=C stretching vibrations in the starting porphyrin CP appear at 1598 cm-1 and 1514 cm-1 and are shifted to 1579 cm-1 and 1473 cm-1 in TiCP-PCP, which we attribute to the formation of coordination bonds in the polymer. The resulting TiCP-PCP material was amorphous as revealed by the lack of reflections in powder X-ray diffraction (Fig. 1a). XPS analysis revealed Ti-O bonding in the TiCPPCP polymer due to network formation through catechol oxygen atoms. However, Ti2p XPS peaks attributable to Ti-N were not observed (Fig. S2) indicating that Ti2+/4+ does not complex the porphyrin during the polymer synthesis. Also, two types of nitrogen atom observable in TiCP-PCP (Fig. S3) are consistent with the porphyrin remaining in a free base state. This is due to the reaction conditions employed – it is known that the porphyrin macrocycle remains in its free base state without being metallated and more aggressive reaction conditions are required for Ti2+/4+ insertion.46 In fact, this suggests that our method ought also to be useful for preparation of heterometallic porous coordination polymers provided that reagents for porphyrin metalation can diffuse effectively through the network without compromising the Ti-catecholate complex linkers. CP-MAS

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C NMR spectrum of TiCP-PCP (Fig. S4) revealed the presence of two groups of

aromatic carbon atom with minor peaks suggesting low levels of aliphatic impurity (due to


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isopropoxide) and free carbonyl, probably uncoordinated 1,2-benzoquinone moieties. Finally, Ti4+ can react with catechol to give a variety of coordination stoichiometries with catechol:Ti4+ = 2:1 or 3:1 having been commonly observed.47,48,49 Based on the non-crystalline structure obtained here (previous compounds prepared by a similar method were microcrystalline materials33), we assume that the disorder implies the presence of both connectivities. Titanium dioxide/nanoporous carbon hybrid materials (TiO2/npC) were prepared by thermolysis of TiCP-PCP at different temperatures (600 – 900 oC at 100o intervals). Samples are denoted as TiCP-PCP-600, TiCP-PCP-700, TiCP-PCP-800 and TiCP-PCP-900. Powder X-ray diffraction patterns (Fig. 1a) were measured revealing the formation of titanium dioxide in the hybrid materials. The identity of the titanium dioxide formed depended on the thermolysis temperature with anatase and rutile phases being clearly identified. Diffraction peaks at 25.2, 37.6, 48.1, 55.0, and 63.1° are ascribed to anatase phase while peaks due to rutile crystal appear at 27.7, 36.3, 41.4, 44.1, 54.6, 57.0, 64,4, 69.2°. At pyrolysis temperatures of 700 oC and below only anatase was obtained, while for temperatures of 800 oC and above the rutile phase forms together with anatase phase in the composites. This is consistent with the reported temperature range for the anatase-rutile transformation.50 Raman spectra (Fig. S5) also indicate the presence of TiO2 in the samples although laser sintering of the samples lead to their decomposition (in the case of TiCP-PCP) and anatase-rutile transformation in the samples obtained at lower temperatures of pyrolysis.51,52


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Figure 1. (a) Powder X-ray diffraction patterns of materials thermolyzed at 600 °C (red), 700 °C (green), 800 °C (blue) and 900 °C (orange). Peak indexing for anatase (red) and rutile (black) are given in the 700 °C and 900 °C patterns, respectively. (b) XPS survey scan for the material obtained by thermolysis at 700 °C. (c) XPS analysis scan for Ti(2p) (d) XPS analysis scan for N(1s) regions. (e) XPS survey scan for the material obtained by thermolysis at 800 °C. (f) XPS analysis scan for Ti(2p) (g) XPS analysis scan for N(1s) regions. X-ray photoelectron spectroscopy (XPS) was used to estimate the compositions of the samples (Fig. 1b-d). XPS spectra in the Ti region confirm that TiO2 found in TiCP-PCP-700 is anatase phase52,53 (Fig. 1c). The peak position (459.3 eV) is matched with Ti4+ of anatase phase. With further increasing the temperature up to 900 °C, the peak top was shifted in lower binding energy, due to the increased rutile content.54 Nitrogen is present in measurable quantities as pyridinic nitrogen atoms denoted by the peak at binding energy 398.6 eV together with pyrrolic N at 400.7 eV (see Fig. 1d). For TiCP-PCP-800, pXRD (Fig. 1a) and XPS (Fig. 1e-g) analyses both indicate the presence of an increasing proportion of rutile over anatase (see also Figs. S2, S3 in Supporting Information).


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The thermolysis of metalloporphyrins might more reasonably be expected to yield simple amorphous carbon/metal oxide composites since oxidation at the metalloporphine core, including intramolecular processes, ought to lead to the metal oxo species during decomposition with concomitant release of ligating nitrogen as oxides with only a small proportion ultimately being incorporated as a dopant in the amorphous carbon products. In this case, since the porphyrin core is not metallated due to its low reactivity towards titanium alkoxides, we propose that the pyrrolic and pyrroline N-atoms are thus more likely to be incorporated into the amorphous carbon matrix leading to the level of N-doping found here. Nitrogen doping of carbon is a particularly important feature since its presence in pyridinic form can promote ORR processes. N-doping levels of 3 at% found here for TiCP-PCP-800 (TiCP-PCP-600, TiCP-PCP-700 contain around 5 at%) are known to be effective for ORR.55 Important trends can be found in the XPS data for samples heated at different temperatures. Notably, carbon content increases for thermolyzed samples. C contents for the materials prepared at 600, 700 and 800 oC are 72 – 76 at% C with a sudden increase in C content to 83 at% on heating at 900 oC. At this point, oxygen content also decreases significantly probably signaling not only increasing graphitization but also the formation of titanium oxycarbide, which has been reported to occur around 900 oC.56 Half the observed oxygen is associated with titania so that there exists a significant quantity of other oxygen species, probably at graphitic edge sites. In the sample obtained by thermolysis at 700 oC, titania (anatase) content is estimated from XPS data at 5 mol%.


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Figure 2. (a) Nitrogen sorption isotherms and (b) pore size distributions of TiCP-PCP (black) and TiCP-PCP-800 (blue) samples. The hybrid TiO2/npC materials remain highly porous even after thermolysis. Porosity of TiCPPCP was investigated by nitrogen sorption measurements at 77 K and a relatively high surface area of 315 m2 g-1 was found indicating the PCP type form of this compound (Fig. 2a, black). The pore size distribution, calculated based on a density functional theory method, indicates a pore size around 2.7 nm (Fig. 2b, black). Nitrogen sorption measurements on the hybrid TiO2/npC materials indicate a highly porous structure even after carbonization (Fig. 2 and Figs. S6-S8). The highest BET surface area (521 m2 g-1) was found for the sample thermolyzed at 800 oC. The porosities of the other materials prepared at 600, 700 and 900 oC were also measured and the surface areas were calculated as being 444 m2 g-1, 343 m2 g-1, and 356 m2 g-1, respectively, consistently greater than the starting material TiCP-PCP. Porosity prior to thermolysis is due to the coordination polymer network structure of TiCP-PCP. This is limited by the crystallinity in the starting compound. Thermolysis results in formation of amorphous carbon, which is known to have a high intrinsic porosity. Porosity of the amorphous carbons obtained by this kind of method is usually also affected by the identity of the starting material. Typical porous amorphous carbons can have surface areas greater than 1000 m2/g or so, although the surface area of our TiCP-PCP increases


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on thermolysis, the measured surface areas are lower than often obtained for amorphous carbon samples.

Figure 3. (a) Low resolution and (b) high resolution TEM images of TiCP-PCP-800. (c) and (d) EDX mapping of N and Ti in TiCP-PCP-800. Morphologies of the materials were observed by scanning electron microscopy (see Fig. S9-S10). TiCP-PCP displays a sphere-like morphology (Fig. S9), which is not significantly affected by thermolysis. Formation of TiO2 nanocrystals could be clearly observed by TEM and reveal that the composites formed at lower temperatures have TiO2 nanoparticles of smaller sizes (Fig. 3 and Figs. S11-S13). TiCP-PCP-800 contains TiO2 nanocrystals having diameters around 10 nm. Having characterized the products of thermolysis as nitrogen-containing TiO2/npC hybrid materials and motivated by their specific structures, we investigated their potential application in the oxygen reduction reaction (ORR) by evaluating their electrocatalytic properties. Commercially


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available Pt/C (normally 20% supported on carbon, abbreviated as Pt/C-20%) was also studied for comparison. Well defined cathodic ORR peaks were observed in typical cyclic voltammetric (CV) curves obtained in O2-saturated 0.1 M KOH solution, in a potential range from 0.76 V to 0.51 V (Fig. S10). When compared to the precursor sample without thermal treatment (TiCP-PCP), the samples after carbonization showed a clear positively-shifted peak potential, revealing that ORR activity is enhanced following thermolytic treatment (Fig. S14). Of the samples studied, TiCPPCP-800 gave the most right-shifted peak potential (0.76 V), which is 250 mV, 90 mV, 30 mV, and 50 mV more positive than that of TiCP-PCP, TiCP-PCP-600, TiCP-PCP-700, and TiCP-PCP900, respectively. This improved ORR activity may be ascribed to the structural changes occurring during carbonization and the differing effects of the TiO2 crystallinity toward ORR. The dependence of ORR activity of the catalysts on thermolysis temperature was also investigated with a rotating ring-disk electrode (RRDE) in O2-saturated 0.1 M KOH solution (Fig. 4). Typical polarization curves for the samples are shown in Fig. 4a and Fig. S15. It is apparent that the thermolyzed materials exhibit superior ORR activity, in sharp contrast to the weak response of the precursor material TiCP-PCP, a feature that can be attributed to improvements in conductivity of the material on carbonization of the supports (Fig. S16). This also leads to the formation of a large number of active sites. ORR performance initially improves by increasing the thermolysis temperature from 600 °C to 800 °C and then decreases in the sample prepared at 900 °C, indicating that pyrolysis at 800 °C is the optimum temperature in this system for ORR activity.


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Figure 4. (a) Polarization curves of TiCP-PCP samples in O2-saturated 0.1 M KOH at rotation speed 1600 rpm with sweep rate of 10 mV s-1. (b) Polarization curves of TiCP-PCP-800 obtained at different rotation speeds, and (c) their corresponding Koutecky-Levich plots. (d) Durability evaluation from the chronoamperometric responses of the TiCP-PCP-800 and commercially available Pt/C-20% catalyst in a long term of 2 h. To investigate the pathways of ORR (i.e., electron number transferred) catalyzed by TiCP-PCP800, the Koutecky-Levich (K-L) equation was applied and the result shows how the inverse current density (j-1) varies as a function of the inverse of the square root of the rotation speed (ω -1/2) at different potential values. Fig. 4c shows the linear K-L plots at different potentials, which are derived from the polarization curves at various rotation speeds (shown in Fig. 4b). The n values of TiCP-PCP-800 calculated by using the K-L equation vary slightly from 3.5 to 3.9, suggesting an approximate four-electron reaction pathway. The TiCP-PCP-700 sample, which has a similar


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composition and N content to TiCP-PCP-800, gave a similar electron transfer number (Fig. S17). The remaining samples, however, showed lower n values indicating a greater contribution from the two-electron pathway in ORR. This feature is most likely due to the higher content of pyridinic N in TiCP-PCP-800 and TiCP-PCP-700 samples. Pyridinic N atoms can effectively donate electron density to adjacent carbon atoms, facilitating the adsorption of O2 molecules and reducing the energy required for cleavage of O-O bonds of dioxygen.57,58 In addition, the metal oxides, herein TiO2, may also enhance the ORR activity by improving the wettability of the catalyst layer due to self-humidification.59 A survey of the available literature further indicates that our titanium dioxide/nanoporous carbon hybrid materials exhibit excellent activity when compared with materials of similar composition (Table 1). Table 1. Comparison of electrochemical parameters for various ORR-active materials.

a

Material

Onset Potentiala

Cathodic ORR peaka

n

Ref.

{001} facet TiO2 single crystal

0.71

--

0.44-2.41

43

Fe-N/C

0.92

0.86

3.06

8

All-C electrocatalysts

--

0.74

3.94

60

Nanoporous N-doped C fibers

--

0.80

3.7

61

N-doped mesoporous C spheres

0.85

0.75

3.4

62

N & S co-doped biocarbon

0.76

--

2.7-3.82

63

N-doped TiO2/npC hybrid

0.76

0.87

3.5-3.9

This work

V vs. RHE


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Stability of the catalysts is critical for practical applications so that we also studied chronoamperometry of TiCP-PCP-800, which is an effective approach for evaluating stability, over a time period of approx. 2 h. Commercially available Pt/C-20% was also studied under the same conditions for comparison. The resulting data (Fig. 4d) clearly shows that TiCP-PCP-800 exhibits a higher current retention over the entire time period compared to Pt/C-20%. A careful calculation reveals that the TiCP-PCP-800 retains 94% of its initial activity, which is larger than that exhibited by Pt/C-20% (87%) (Fig. 4d). The superior stability of TiCP-PCP-800 is supported by this better retention of ORR activity during the long-term stability measurements (Fig. S18). These results clearly indicate that TiCP-PCP-800 possesses superior catalytic activity and durability for ORR processes while at the same time, of course, not containing any noble metals.64,65

Conclusions In summary, we have developed a thermolytic method for the preparation of N-doped titanium dioxide/nanoporous carbon (TiO2/npC) hybrid materials from an amorphous titanium porous coordination polymer. The advantages of this method include ease of preparation, simple thermal control over the anatase/rutile ratio and the use of N-containing ligand which ultimately leads to incorporation of nitrogen atoms in the amorphous carbon matrix. On the other hand, since the composition of the starting material is largely invariable, it is difficult to modify the compositions of the thermolysis products beyond by thermolysis temperature variation. Despite this latter disadvantage, we were still able to some extent optimize nitrogen doping and anatase phase content simply by varying the thermolysis temperature. Optimum composition is 5 at% N-doping with anatase phase content of 5 mol% according to XPS data. The materials have a morphology of titanium dioxide nanocrystal particles contained within a nanoporous carbon matrix. Thermolysis


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temperature affects the identity of the TiO2 phase obtained as well as the particle size of the titanium dioxides in the porous carbon matrix with anatase being favoured at lower temperatures while rutile is increasingly prevalent above 700 °C. The TiO2/npC composites are high performance electrode materials for the oxygen reduction reaction suggesting that this type of material is promising for electrochemical applications. Our data suggest that the anatase phase is important for the ORR activity since materials lacking well defined anatase components either due to insufficiently high thermolysis temperature (i.e. TiCP-PCP-600) or high temperature transformation to rutile (i.e. TiCP-PCP-900) also exhibit attenuated performance in ORR. However, this feature is complicated by reduction in nitrogen content as the pyrolysis content is increased. Thus, it is important to carefully select the pyrolysis temperature for the optimization of ORR properties of the composites for the respective component contents of amorphous carbon, N-dopant and anatase. Overall, therefore, the compositions of our materials favor the ORR processes because of (a) presence of anatase phase, (b) bulk composition of amorphous carbon for improved conductivity, and (c) N-doping. All of (a)-(c) are known to improve the properties of amorphous C for the ORR and may operate synergistically. With these points in mind, we are now attempting to further develop the synthesis of similar TiO2/npC composites with the aim of optimizing the anatase N-doped-carbon compositions of the materials perhaps by utilizing N-rich ligands as carbon/nitrogen sources.66−67 ASSOCIATED CONTENT Supporting Information The supporting information is available free of charge on the ACS Publications website at DOI:


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Raman spectra, N2 sorption-desorption isotherms, additional SEM and TEM images, electrochemical measurements (PDF)

Notes The authors declare no competing financial interests.

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected]

Author Contributions ‡These authors contributed equally. The manuscript is written through contributions of all the authors. All the authors have approved its submission. ACKNOWLEDGMENT This work was partly supported by World Premier International Research Center Initiative (WPI Initiative), MEXT, Japan.

REFERENCES (1) Gielen, D.; Boshell, F.; Saygin, D. Climate and Energy Challenges for Materials Science. Nat. Mater. 2016, 15, 117–120.


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