mesoporous Titanium Nitride Structure and Its

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Fabrication of Macro/mesoporous Titanium Nitride Structure and Its Application as Catalyst Support for Proton Exchange Membrane Fuel Cell Yi-Min Chi, Mrinalini Mishra, Tzu-Kang Chin, Wei-szu Liu, and Tsong-Pyng Perng ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b01426 • Publication Date (Web): 06 Dec 2018 Downloaded from http://pubs.acs.org on December 10, 2018

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ACS Applied Energy Materials

Fabrication of Macro/Mesoporous Titanium Nitride Structure and Its Application as Catalyst Support for Proton Exchange Membrane Fuel Cell Yi-Min Chi1, Mrinalini Mishra1, Tzu-Kang Chin1, Wei-Szu Liu1, and Tsong-Pyng Perng1,* 1Department

of Materials Science and Engineering, National Tsing Hua University Hsinchu 300, Taiwan

Abstract A macro/mesoporous structure of titanium nitride (TiN) was fabricated on carbon paper via solgel method in combination with phase separation and nitridation. The surface area and pore size of TiN were tuned by different concentrations of polyvinylpyrrolidone (PVP) in the sol. Pt nanoparticles were then uniformly deposited on TiN by atomic layer deposition to form a Pt@TiN@carbon paper electrode for application in proton exchange membrane fuel cell. The performance of membrane electrode assembly (MEA) increased with increasing the content of PVP when the homemade electrode was used as the anode or cathode due to the increased surface area of TiN and the controllable amount of Pt loading. The MEAs prepared by using homemade electrodes as either anode or cathode showed higher catalyst-mass specific power densities than commercial E-Tek electrode. Furthermore, the MEAs using homemade electrodes for both anode and cathode also showed higher catalyst-mass specific power density than those of MEAs using commercial E-Tek electrodes and other previously reported studies.

Keywords: titanium nitride, platinum, macro/mesoporous structure, atomic layer deposition, proton exchange membrane fuel cell

*Corresponding Author: E-mail: [email protected], Tel: (886)-3-5742634, Fax: (886)-35723857 1 ACS Paragon Plus Environment

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1. Introduction Fuel cells are considered as one of the most promising green energy devices for their quietness, near-zero emissions, and high energy conversion efficiency. They can transform chemical energy to electrical energy without producing harmful greenhouse gases (e.g., CO2, NOx, SOx, etc.) [1]. Among all types of fuel cells, the proton exchange membrane fuel cell (PEMFC) has drawn much attention due to its low operating temperature and high energy conversion efficiency. However, the degradation of traditional carbon support during operation poses a setback on its widespread commercialization. Titanium nitride, with high electrical conductivity (4000 Sm-1 as opposed to 1190 Sm-1 for carbon black) and corrosion resistance, is a potential material to replace carbon black as catalyst support for Pt. In addition, TiN is remarkably good for electrochemical oxidation resistance to completely alleviate CO poisoning effect [1]. Density functional theory (DFT) calculations have also shown the suitability of TiN as a catalyst support [2]. Mostly, (nano-)particulate [3-7] nanofibers [8], scaffoldlike nanotubes [9], and nanorod arrays [10] of TiN have been investigated as potential catalyst support for direct methanol fuel cell (DMFC) and PEMFC. Mesoporous TiN fabricated from Zn2TiO4 by solid-solid phase separation bearing a meagre surface area of 28 m2/g has also been investigated [11]. Block co-polymers were also used to synthesize mesoporous TiN, and its electrochemical stability was tested to show that TiN remains conductive in acid electrolyte up to 1.4 V vs. RHE after 200 cycles [12]. Some TiN-based catalyst supports have also shown excellent cyclic stability and durability [13-15]. These reports have commonly focused on the oxygen reduction reaction (ORR) activity and electrochemical stability test to validate and quantify potential of TiN as a catalyst 2 ACS Paragon Plus Environment

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support, whereas membrane electrode assembly (MEA) performance of single cells prepared by utilizing the as-fabricated TiN nanostructure has seldom been demonstrated [4,10]. Possibility of using TiN based material directly as the catalyst has also been demonstrated. Layer-by-layer TiN/TiCN was studied as a replacement of Pt in the cathode of a H2-O2 alkaline fuel cell and a maximum power density of 82 mW/cm2 was demonstrated while using Pt/C as the anode [15]. A catalyst support with hierarchical macro/mesoporous structure with interconnected macroporous channels would enable transport of fuel to mesoporous internal regions for reaction [16]. Previously, several oxides such as SiO2 [17], TiO2 [18], Al2O3 [19], and ZrO2 [20] with macropores, mesostructure, and high surface area have been prepared by a cost-effective technique of polymerization-induced phase separation during sol-gel synthesis. Further, the growth of the catalyst, usually Pt, is also dependent on the substrate condition and electrode structure. It is affected by the electrocatalyst synthesis technology as well. Owing to the high cost of Pt, development of PEMFC with low Pt loading and high Pt utilization is imperative. There are numerous methods to synthesize low Pt loading catalysts for PEMFC [21,22], such as electroless deposition [23], thin film process [24], electrodeposition [25], sputtering [26-28], and atomic layer deposition (ALD) [29,30]. ALD ensures better conformality and step coverage on high aspect-ratio surface than deposition methods like sol-gel, physical vapor deposition (PVD), and chemical vapor deposition (CVD) [31]. It can achieve precise control of particle size and uniformity. The advantages of ALD in fabrication of Pt catalyst on carbon cloth and carbon nanotubes for application in PEMFC have been demonstrated previously by our group [29,30]. Recently, we have attempted to fabricate TiN inverse 3 ACS Paragon Plus Environment


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opals as a support and deposit Pt by ALD to form electrodes for application in PEMFC. A maximum power density of 71 mW/cm2 using homemade electrodes for both anode and cathode, each with a Pt loading of only 18.14 μg/cm2, i.e., equivalent to a Pt specific power density of 1957 mW/mgPt, was achieved [32]. ALD was used to deposit Pt nanoparticles on carbon cloths and carbon nanotubes for applications in PEMFCs In this work, we have further fabricated a TiN hierarchical macro/mesoporous structure with high surface area for use as the catalyst support for PEMFC. The TiN structure was formed by polymerization-induced phase separation in a sol-gel coating on carbon paper during heat treatment in ammonia atmosphere. Subsequently, ALD was employed to deposit Pt nanoparticles on the TiN support. To the best of our knowledge, this study is the first approach to obtain TiN by such a method for application in PEMFC. 2. Experimental A schematic process for fabrication of Pt@TiN macro/mesoporous structure is shown in Fig. S1. The solution contained titanium isoproxide (TTIP), polyvinylpyrrolidone (PVP, CAS number 900-39-8, Aldrich), and N-methyl-2-pyrrolidone (NMP, C5H9NO, CAS number 872-50-4, ECHO). PVP with a molecular weight of 29,000 and NMP, a kind of polar solvent, were used to induce the phase separation. The amount of PVP was controlled at 0 wt% (no PVP), 10 wt%, 20 wt%, 30 wt%, 40 wt%, or 50 wt% of that of TTIP, and the content of NMP was twice of that of TTIP. Carbon paper (SIGRACT® GDL 24 BC) was chosen as the substrate. The TTIP solution was 4 ACS Paragon Plus Environment

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dropped and spin coated uniformly on the carbon paper at 600 rpm. This was followed by direct heating in NH3 at 800 oC in a furnace with a heating rate of 20 oC /min and a holding time of 2 h. After nitridation, the samples were cooled down in a continuous flow of NH3 gas. In

the

ALD

process

of

Pt,

the

Pt

precursor

and

co-reactant

were

(methylcyclopentadienyl)trimethyl platinum (IV) (MeCpPtMe3) and oxygen gas, respectively. The temperature of substrate in the chamber was kept at 280 oC. The TiN@carbon paper was alternatively exposed to the precursors of MeCpPtMe3 with 0.5 s pulse time and O2 with 2 s pulse time. The setup and process of ALD have been presented previously [29,30]. X-ray diffraction (XRD) analysis was performed using the Shimadzu XRD-6000 X-ray diffractometer with Cu 𝐾𝛼 radiation at 30 kV and 20 mA and the Rigaku TTRAX III X-ray diffractometer with Cu 𝐾𝛼 radiation at 50 kV and 300 mA. The surface morphologies of the samples were examined by a cold field emission scanning electron microscope (SEM, Hitachi SU-8010). The Pt nanoparticles fabricated by ALD were examined by a high resolution transmission electron microscope (HRTEM, JEOL HRTEM-3000F). The specific surface area and pore size of TiN were determined by a Brunauer-Emmett-Teller (BET, Micromeritics ASAP2020) gas adsorption analyzer with N2 physisorption. The amounts of Pt loading on TiN were measured by a Perkin Elmer SCIEX 5000 inductively coupled plasma mass spectrometer (ICP-MS). The Pt@TiN@carbon paper and commercial E-Tek were used as the electrodes in an area of 2 × 2 cm2, with a proper amount of 5% Nafion® solution (Aldrich Chemicals) dropped on the electrodes. The MEA was prepared by hot pressing two electrodes with a Nafion® 115 membrane in the middle. 5 ACS Paragon Plus Environment


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The hot pressing was conducted under a pressure of 600 psi at 140 oC for 90 s. The performance of the MEA was measured by a single cell test station (PEMSCT-150, JNP Tech. Co.) consisting of a DC electronic load (Agilent, N3302A), two mass flow controllers (Brooks, 5850 E Series) for controlling the flow of O2 and H2 and two water reservoirs to keep the cell at a humidity of RH 40%. The measurement was conducted at 60 °C from 1 V to 0.05 V in a duration of 300 s with 50 sccm of O2 and H2 and the polarization curves were obtained by a Labview program. 3. Results and Discussion 3.1. Formation and characterization of TiN porous structure The XRD patterns (Fig. 1) reveal that all of the samples have transformed into TiN through nitridation at 800 oC for 2 h. Fig. 2 shows the morphologies of TiN with various PVP contents by SEM. It is seen that the gel skeleton is composed of numerous aggregated TiN nanoparticles, and the channels become smaller as the amount of PVP increases, creating a hierarchical macro/mesoporous structure. The inset in Fig. 2 (c) is a higher magnification to show the aggregation of TiN nanoparticles. As the content of PVP further increases, the TiN particles become smaller and less distinguishable. Figs. 3 and 4 show the N2 adsorption-desorption isotherms and the pore size distribution based on the DFT calculation, respectively. The hysteresis of the isotherm of 0 wt% PVP corresponding to type II could be attributed to nonporous or macroporous structure. The hysteresis loop becomes smaller after adding PVP and corresponds to type IV, which is attributed to the slit-type mesopores composed of interstices of aggregated particles [33]. Furthermore, the pore size distribution shifts to 6 ACS Paragon Plus Environment

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smaller range after adding PVP. The BET specific surface areas are summarized in Table 1, which indicates that the specific surface area increases with the content of PVP. The specific surface area of the hierarchical macro/mesoporous TiN in this study is much higher than previously reported mesoporous TiN for DMFC [11] and at par with mesoporous TiN fabricated by using block copolymers [12]. Higher surface area indicates more reactive sites for catalyst to deposit on the structure. The formation of TiN macro/mesoporous structure may take place by consecutive hydrolysis of TTIP and phase separation followed by nitridation. There are three processes for conversion of TTIP into TiO2, as shown below: (1) hydrolysis

≡ Ti - OC3H7 + H2O → ≡ Ti -OH + C3

H7OH (2) water condensation

≡ Ti -OH + ≡ Ti -OH → ≡ Ti -O - Ti ≡ + HOH

(3) alcohol condensation

≡ Ti -OH + ≡ Ti -OC3H7 → ≡ Ti -O - Ti

≡ + C3H7OH In the present case, although there was no water added into the solution, TiO2 particles were produced after coating on the carbon paper because the moisture in the atmosphere can help the hydrolysis of TTIP and induce the water condensation reaction [34]. Then the reduction of miscibility between solvent and polymer chains that contain adsorbed TiO2 particles drives the separation of the solution to solvent-rich and TiO2-rich phases. Therefore, various morphologies of TiN were obtained with varied PVP contents after nitridation. 7 ACS Paragon Plus Environment


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3.2. Morphological and structural characterization of Pt catalyst prepared by ALD Fig. 5 presents the SEM micrographs of Pt catalyst deposited on TiN porous structure by ALD. The Pt nanoparticles were fabricated successfully and dispersed uniformly on TiN. The average particle sizes of Pt deposited by 50 cycles, 100 cycles, and 200 cycles of ALD are determined to be 2.95 nm, 5.38 nm, and 10.73 nm, respectively, as also shown in Fig. 5. From the XRD patterns shown in Fig. S2, the intensity of Pt peaks increases as the number of ALD cycles increases, and the amount of Pt is also higher. The Pt peaks for 50 cycles of ALD are relatively weak due to the small amount and particle size of Pt. The presence and morphology of Pt nanoparticles deposited by 50 cycles of ALD were examined by HRTEM, as shown in Fig. S3, where the d spacing in the lattice image is 2.2 Å, corresponding to the (111) plane. The average particle sizes of Pt nanoparticles obtained by the Scherrer’s equation are estimated to be 2.2 nm, 6.2 nm, and 9.9 nm for 50 cycles, 100 cycles, and 200 cycles, respectively, which are quite close to those observed from the SEM micrographs. The growth rate of the Pt nanoparticles is calculated to be approximately 0.52 Å/cycle from both of the XRD and SEM analyses, and therefore the particle size of Pt can be well controlled by the cycle number of ALD. Table 1 also summarizes the loadings of Pt nanoparticles deposited on TiN under different conditions. If correlated with the BET result, the Pt loading increases with increasing the surface area. The relation between the Pt loading and the ALD cycle number for the TiN support in which PVP is 50 wt% of TTIP is shown in Fig. S4. It is seen that the loading is linearly proportional to the ALD cycle number. This phenomenon can be attributed to the self-limiting reaction occurring on the surface during the ALD process. 8 ACS Paragon Plus Environment

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3.3. Performance of membrane electrode assemblies 3.3.1. Effect of TiN porous structure for a single electrode Figs. 6 (a) and (b) show the performances of MEAs using Pt@TiN@carbon paper electrodes as the anode and cathode, respectively, and commercial E-Tek as the opposite electrodes. The Pt catalyst in all samples was prepared with 100 cycles of ALD, and the TiN supports were prepared with various contents of PVP. The performance of MEA using E-Tek as both anode and cathode is also included for comparison. It is observed that the performance of MEA increases with increasing the PVP content for both anode and cathode. The MEA prepared with the catalyst support in which PVP is 50 wt% of TTIP shows the best result, that could be attributed to the higher amount of Pt loading and more regions for oxidation or reduction reaction to take place due to its high specific surface area (160.1 m2/g). For TiN, it is a relatively denser material than carbon. The introduction of a porous structure can increase the surface area-to-volume ratio and diminish the number of buried non-functional precious Pt. Achieving higher surface area for a dense material facilitates high Pt particle density on the support, thereby improving the catalytic activity [6]. Note that although all of the MEAs using homemade electrodes show lower power densities than using commercial E-Tek, the catalyst-mass specific power densities based on Pt loadings at the homemade electrodes are, in effect, all higher than that of E-Tek. The crucial factor affecting the activity is mass loading, but the Pt loadings of all samples are much smaller than that of E-Tek. The effect of Pt loading will be discussed in detail in the following section. 3.3.2. Effect of Pt particle size and loading for a single electrode 9 ACS Paragon Plus Environment


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To examine the effect of Pt nanoparticle size, the TiN structure prepared by 50 wt% of PVP was chosen as the catalyst support. Figs. 7 (a) and (b) show the performances of MEAs using Pt@TiN@carbon paper electrodes prepared with various ALD cycles of Pt as the anode and cathode, respectively, and E-Tek as the opposite electrodes. In the case of anode, the performance becomes better upon decreasing the cycle number of ALD. The result agrees with other studies which indicate that Pt particle size of 2-3 nm shows the best catalytic activity [35,36]. The power density of the electrode prepared with 50 cycles of Pt is even higher than that of E-Tek. Conversely, an opposite behavior is observed when homemade electrodes were used as the cathode. Nonetheless, the catalystmass specific power densities at 0.6 V in terms of the Pt loading on anode or cathode for the homemade electrodes are all better than that of E- Tek electrode. For instance, the values for the electrodes prepared by 50 cycles of Pt are 62 and 3 times higher than that of E-Tek when used as the anode and cathode, respectively. Table 2 summarizes the catalyst-mass specific power densities for different electrodes. In all cases, the catalyst-mass specific power density increases with decreasing the cycle number of ALD, which may be attributed to the higher catalytic activity of smaller Pt nanoparticles. Therefore, both the loading and particle size of Pt are the key factors affecting the performance. 3.3.3. Pt@TiN@carbon paper as both anode and cathode Fig. 8 shows the performances of MEAs prepared by using Pt@TiN@carbon paper for both anode and cathode. The Pt catalyst prepared for anode was fixed at 50 cycles of ALD, and that for cathode was prepared with 50 cycles and 200 cycles. The performance of the MEA using commercial 10 ACS Paragon Plus Environment

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E-Tek for both anode and cathode is included for comparison. It is seen that the performance of MEA using 200 cycles of ALD as the cathode is better than that of MEA using 50 cycles of ALD. Both the MEAs prepared from homemade electrodes show lower performance than that using commercial ETek electrode as both anode and cathode, presumably because the Pt loadings of homemade electrodes are much lower than that of commercial E-Tek electrode. Nevertheless, the catalyst-mass specific power densities (based on the combined Pt loading of homemade anode and cathode) for the MEAs fabricated from homemade electrodes are consistently higher. The MEA using Pt nanoparticles deposited by 50 cycles of ALD for anode and 200 cycles of ALD for cathode exhibits the best catalystmass specific power density, 3.6 times higher than that of E-Tek electrodes. The same MEA, i.e., Pt-50/Pt-200 in Fig. 8, was subjected to 1000 cycles of load-cycling test. The variation of normalized power density at 0.6 V is shown in Fig. S5 (a). It is seen that the power density gradually decreases. The degradation is approximately 20 % after 600 cycles and 40 % after 1000 cycles. When it underwent potential-cycling test between 0.6 V and 0.8 V, the variation of current is shown in Fig. S5 (b). The degradation after 600 cycles was approximately 40 %. The XRD patterns of the anode and cathode in the MEA from the load-cycling test were taken, as shown in Fig. S6. Since the whole MEA was used as the sample for analysis, the diffraction could only be taken from the backside of anode and cathode. Therefore, the signals of TiN and Pt are weaker than those of Fig. S2. Nevertheless, the peak positions of both TiN and Pt remain unchanged, and no TiO2 is not observed. From the XRD analysis, it confirms the stability of the TiN support, but the cycling tests indicate that there is room for improving the durability of the MEA. 11 ACS Paragon Plus Environment


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Table 3 shows the comparison of MEA performances for various noncarbonaceous catalyst supports such as titanium oxide [37,38], titanium diboride [39], tungsten oxide [40], tungsten carbide [41], tin oxide [42], and titanium nitride [4,10,32]. These materials show several properties suitable for replacing the traditional carbonaceous support and improving the catalytic activity. It is inappropriate to compare the performances due to different operating conditions, MEA materials, support structures, and intrinsic properties. However, the MEA performance of single cell fabricated with TiN nanostructures as catalyst support has seldom been reported [4,10]. Hence, this table helps us to evaluate the performance of our material. Furthermore, this work aims to demonstrate MEA performance using homemade TiN macro/mesoporous electrodes as both cathode and anode simultaneously for PEMFC. The catalyst-mass specific power density of the MEA in our study is much higher than the work by Ottakam Thotiyl et al. [4]. They showed MEA performance using homemade electrodes as both cathode and anode simultaneously for DMFC. Recently, Jiang et al. [10] have reported a high catalyst-mass specific power density (5837.1 mW/mg) for PtPdCo@TiN vertical nanorod arrays as the cathode. However, they have considered only the loading of Pt for calculating the specific power density of the cathode. Upon including the loading of Pd and Co, the catalyst-mass specific power density would reduce to 3762 mW/mg. As they have used Pt@C with a Pt loading of 0.2 mg/cm2 as the anode, the catalyst-mass specific power density of their MEA could be calculated to be 1285.4 mW/mg, which is lower than the result of this work, 2739.6 mW/mg. As stated earlier, the MEAs constructed with homemade electrodes of Pt@TiN inverse opals were studied in our group, and a maximum Pt specific power density of 1957 mW/mg was obtained [32]. 12 ACS Paragon Plus Environment

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This value is also lower than the present result. The higher catalyst-mass specific power density of the present work can be attributed to highly uniform size and deposition of Pt nanoparticles prepared by ALD on the supports with a unique slit-like mesoporous structure fabricated by polymerizationinduced phase separation during heat treatment in ammonia atmosphere. Additionally, Pt nanoparticles deposited by ALD on TiN support show good adherence and reduced usage of the catalyst. Therefore, the homemade MEAs prepared with TiN hierarchical macro/mesoporous structure as the support and low loading of Pt as the catalyst can be considered as a potential candidate for future hydrogen-based fuel cell development. 4. Conclusion A TiN macro/mesoporous structure with high surface area was successfully fabricated by solgel method accompanied by phase separation and nitridation at 800 oC for 2 h. The surface area increased with increasing the PVP content. Pt nanoparticles were deposited uniformly on TiN by ALD, and the Pt loading increased with increasing the cycle number of ALD. Therefore, the particle size and loading of Pt could be controlled by adjusting the cycle number of ALD. In addition, the Pt nanoparticles prepared by 50 cycles of ALD showed the best catalytic activity. The performance of MEAs increased with increasing the content of PVP when the Pt@TiN@carbon paper was used as the anode or cathode due to the increased surface area of TiN and the controllable amount of Pt loading. The MEAs prepared by using homemade electrodes as either anode or cathode showed higher catalyst-mass specific power densities than commercial E-Tek electrodes. Furthermore, the MEAs using homemade electrodes for both anode and cathode also showed higher catalyst-mass 13 ACS Paragon Plus Environment


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specific power density than those of MEAs using commercial E-Tek electrodes and other previously reported studies. Hence, TiN hierarchical macro/mesoporous structure deposited with Pt nanoparticles by ALD could be a promising catalyst for PEMFC. Acknowledgement This work was supported by the Ministry of Science and Technology, Taiwan under Contract Nos. NSC 101-2221-E-007-059-MY3 and MOST 104-2221-E-007-110-MY3. We are grateful to the assistance for the BET measurement by Jing-Yuan Liu in Prof. Ruey-An Doong’s Lab in the Department of Biomedical Engineering and Environmental Sciences, National Tsing Hua University, Hsinchu, Taiwan. References [1]

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[18] Konishi, J.; Fujita, K.; Nakanishi, K.; Hirao, K. Monolithic TiO2 with controlled multiscale porosity via a template-free sol−gel process accompanied by phase separation. Chem. Mater. 2006, 18, 6069-6074. [19] Tokudome, Y,; Fujita, K.; Nakanishi, K.; Miura, K.; Hirao, K. Synthesis of monolithic Al2O3 with well-defined macropores and mesostructured skeletons via the sol−gel process accompanied by phase separation. Chem. Mater. 2007, 19, 3393-3398. [20] Konishi, J.; Fujita, K.; Oiwa, S.; Nakanishi, K.; Hirao, K. Crystalline ZrO2 monoliths with welldefined macropores and mesostructured skeletons prepared by combining the alkoxy-derived sol–gel process accompanied by phase separation and the solvothermal process. Chem. Mater. 2008, 20, 2165-2173. [21] Wee, J. H.; Lee, K. Y.; Kim, S. H. Fabrication methods for low-Pt-loading electrocatalysts in proton exchange membrane fuel cell systems. J. Power Sources 2007, 165, 667-677. [22] Esmaeilifar, A.; Rowshanzamir, S.; Eikani, M. H.; Ghazanfari, E. Synthesis methods of low-Ptloading electrocatalysts for proton exchange membrane fuel cell systems. Energy 2010, 35, 3941-3947. [23] Beard, K. D.; Schaal, M. T.; Van Zee, J. W.; Monnier, J. R. Preparation of highly dispersed PEM fuel cell catalysts using electroless deposition methods. Appl. Catal. 2007, 72, 262-271. [24] Xiong, L.; Manthiram, A. High performance membrane-electrode assemblies with ultra-low Pt loading for proton exchange membrane fuel cells. Electrochim. Acta 2005, 50, 3200-3204. [25] Kim, S. S.; Nah, Y. C.; Noh, Y. Y.; Jo, J.; Kim, D. Y. Electrodeposited Pt for cost-efficient and 17 ACS Paragon Plus Environment


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flexible dye-sensitized solar cells. Electrochim. Acta 2006, 51, 3814-3819. [26] Hirano, S.; Kim, J.; Srinivasan, S. High performance proton exchange membrane fuel cells with sputter-deposited Pt layer electrodes. Electrochim. Acta 1997, 42, 1587-1593. [27] Alvisi, M.; Galtieri, G.; Giorgi, L.; Giorgi, R.; Serra, E.; Signore, M. A. Sputter deposition of Pt nanoclusters and thin films on PEM fuel cell electrodes. Surf. Coat. Technol. 2005, 200, 1325-1329. [28] Kim, H. T.; Lee, J. K.; Kim, J. Platinum-sputtered electrode based on blend of carbon nanotubes and carbon black for polymer electrolyte fuel cell. J. Power Sources 2008, 180, 191-194. [29] Liu, C.; Wang, C. C.; Kei, C. C.; Hsueh, Y. C.; Perng, T. P. Atomic layer deposition of platinum nanoparticles on carbon nanotubes for application in proton-exchange membrane fuel cells. Small 2009, 13, 1535-1538. [30] Hsueh, Y. C.; Wang, C. C.; Kei, C. C.; Lin, Y. H.; Liu, C.; Perng, T. P. Fabrication of catalyst by atomic layer deposition for high specific power density proton exchange membrane fuel cells. J. Catal. 2012, 294, 63-68. [31] He, W. ALD: atomic layer deposition - precise and conformal coating for better performance. In: Nee, A. Y. C. Editor. Handbook of manufacturing engineering and technology Vol. 5, London: Springer-Verlag 2015, 2964-2966. [32] Liu, Y. R.; Hsueh, Y. C.; Perng, T. P. Fabrication of TiN inverse opal structure and Pt nanoparticles by atomic layer deposition for proton exchange membrane fuel cell. Int. J. Hydrogen Energy 2017, 42, 10175-10183. 18 ACS Paragon Plus Environment

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[33] Sing, K. S. W.; Everett, D. H.; Haul, R. A. W.; Moscou, L.; Pierotti, R. A.; Rouquérol, J.; Siemieniewska, T. Reporting physisorption data for gas/solid systems with special reference to the determination of surface area and porosity. Pure Appl. Chem. 1985, 57, 603-619. [34] Slooff, L. H.; Kroon, J. M.; Loos, J.; Koetse, M. M.; Sweelssem, J. Influence of the relative humidity on the performance of polymer/TiO2 photovoltaic cells. Adv. Funct. Mater. 2005, 15, 689-694. [35] Maillard, F.; Martin, M.; Gloaguen, F.; Léger, J. M. Oxygen electroreduction on carbonsupported platinum catalysts. Particle-size effect on the tolerance to methanol competition. Electrochim. Acta 2002, 47, 3431-3440. [36] Shao, M.; Peles, A.; Shoemaker, K. Electrocatalysis on platinum nanoparticles: particle size effect on oxygen reduction reaction activity. Nano Lett. 2011, 11, 3714-3719. [37] Huang, S. Y.; Ganesan, P.; Park, S.; Popov, B. N. Development of a titanium dioxide-supported platinum catalyst with ultrahigh stability for polymer electrolyte membrane fuel cell applications. J. Am. Chem. Soc. 2009, 131, 13898-13899. [38] Rajalakshmi, N.; Lakshmi, N.; Dhathathreyan, K. S. Nano titanium oxide catalyst support for proton exchange membrane fuel cells. Int. J. Hydrogen Energy 2008, 33, 7521-7526. [39] Roth, C.; Bleith, P.; Schwöbel, C. A.; Kaserer, S.; Eichler, J. Importance of fuel cell tests for stability assessment-suitability of titanium diboride as an alternative support material. Energies 2014, 7, 3642-3652. [40] Saha, M. S.; Banis, M. N.; Zhang, Y.; Li, R.; Sun, X.; Cai, M.; Wagner, F. T. Tungsten oxide 19 ACS Paragon Plus Environment


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nanowires grown on carbon paper as Pt electrocatalyst support for high performance proton exchange membrane fuel cells. J. Power Sources 2009, 192, 330-335. [41] Meng, H.; Shen, P. K.; Wei, Z.; Jiang, S. P. Improved performance of direct methanol fuel cells with tungsten carbide promoted Pt/C composite cathode electrocatalyst. Electrochem. SolidState Lett. 2006, 9, A368-372. [42] Dou, M.; Hou, M.; Liang, D.; Lu, W.; Shao, Z.; Yi, B. SnO2 nanocluster supported Pt catalyst with high stability for proton exchange membrane fuel cells. Electrochim. Acta 2013, 92, 468473.

20 ACS Paragon Plus Environment

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Table 1 Summary of BET specific surface areas of the TiN macro/mesoporous structure and loadings of Pt nanoparticles deposited by different cycles of ALD

Content of

SBET

PVP

(m2/g)

(wt%)

Pt loading ( μg/cm2) 50

100

200

cycles

cycles

cycles

0

30.9

--

9.83

--

10

81.0

5.69

11.08

--

20

101.4

--

11.92

--

30

120.8

6.45

12.57

--

40

123.7

--

13.26

--

50

160.1

9.19

17.49

39.26



Page 22 of 33

Table 2 Comparison of catalyst-mass specific power densities at 0.6 V using homemade and ETek electrodes Electrode

PMEA

Anode

Cathode

Pt-200

E-Tek

Pt-100

(W/cm2)

Pt loading (mg/cm2)

PsA

PsC

PsMEA

(W/mg)

(W/mg)

(W/mg)

@ 0.6V

@ 0.6V

@ 0.6V

Anode

Cathode

0.234

0.039

0.5

6.000

--

--

E-Tek

0.318

0.017

0.5

18.706

--

--

Pt-50

E-Tek

0.373

0.009

0.5

41.444

--

--

E-Tek

Pt-200

0.062

0.5

0.039

--

1.590

--

E-Tek

Pt-100

0.032

0.5

0.017

--

1.882

--

E-Tek

Pt-50

0.018

0.5

0.009

--

2.000

Pt-50

Pt-200

0.057

0.009

0.039

--

--

1.188

Pt-50

Pt-50

0.015

0.009

0.009

--

--

0.833

E-Tek

E-Tek

0.334

0.5

0.5

0.668

0.668

0.334

@ 0.6V

P PMEA: Power density of MEA PsA: Catalyst-mass specific power density of the anode PsC: Catalyst-mass specific power density of the cathode PsMEA: Catalyst-mass specific power density of the MEA


Page 23 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46


Table 3. Comparison of performances of homemade electrodes and other electrodes using different noncarbonaceous materials as the support Support material

Advantages of support

Pt particle size

Types of fuel cell, Working temp.

Type of electrolyte

Electrodes (anode & cathode)

Max. PMEA (mW/cm2)

Max. PsMEA (mW/cm2)

Ref.

Titanium oxide

‧Extremely good corrosion resistance ‧High stability ‧High durability

1. 6.2 nm

1. PEMFC, 75 ℃

1. N-112

[37]

2. PEMFC, 60 ℃

2. N-1135

1. ~940 (cathode) 2. ~165 (anode + cathode)

1. 1044.4

2. 2-30 nm

1. 0.5 mg/cm2 LT140EW & 0.4 mg/cm2 Pt@TiO2 2. 0.25 mg/cm2 Pt@TiO2 & 0.5 mg/cm2 Pt@TiO2

2. 220

[38]

Titanium diboride

‧Good conductivity ‧Excellent thermal stability ‧Extremely good corrosion resistance

5 nm

PEMFC, 70 ℃

NTM

0.8 mg/cm2 Pt@C & 0.8 mg/cm2 Pt@TiB2

17 (cathode)

106.3

[39]

Tungsten oxide

‧Excellent CO tolerance

2-4 nm

PEMFC, 80 ℃

N-112

0.5 mg/cm2 Pt@C & 1.7 mg/cm2 Pt@W18O49

~680 (cathode)

390.1

[40]

Tungsten carbide

‧High electrical Conductivity (103 Sm-1) ‧Platinum-like catalyst behavior ‧High stability ‧Excellent CO tolerance

--

DMFC, 90 ℃

N-117

3 mg/cm2 PtRu@C & 1 mg/cm2 Pt@W2C

~200 (cathode)

50

[41]

Tin oxide

‧Good electrochemical stability ‧High corrosion resistance

1-3 nm

PEMFC, 60 ℃

N-212

0.2 mg/cm2 Pt@SnO2 & 0.4 mg/cm2 Pt@C

~900 (anode)

1500

[42]

Titanium nitride

‧High electrical conductivity (4000 Sm-1) ‧Outstanding corrosion resistance ‧Exceptional stability ‧Good adhesion for catalyst

1. 250-400 nm

1. DMFC, 70 ℃

1. N-117

2. 5 nm thick PtPdCo film

2. PEMFC, 65 ℃

2. N-212

3. ave. 10.5 nm

3. PEMFC, 60 ℃

3. N-115

4. 2-11 nm

4. PEMFC, 60 ℃

4. N-115

1. 33 mg/cm2 Pt@TiN & 2 mg/cm2 Pt@TiN 2. 0.2 mg/cm2 Pt@C & 0.1038 mg/cm2 PtPdCo@TiN 3. 0.018 mg/cm2 Pt@TiN & 0.018 mg/cm2 Pt@TiN 4. 0.009 mg/cm2 Pt@TiN & 0.039 mg/cm2 Pt@TiN

N: Nafion


1. 13.5 (anode + cathode) 2. 390.5 (cathode)

1. 4.1

[4]

2. 1285.4

[10]

3. 71 (anode + cathode) 4. 131.5 (anode + cathode)

3. 1957

[32]

4. 2739.6

This work


Page 24 of 33

List of figures Fig. 1 XRD patterns of TiN hierarchical macro/mesoporous structures prepared with various amounts of PVP and annealed at 800 oC in NH3 for 2 h. Fig. 2 SEM images of TiN hierarchical macro/mesoporous structure prepared with various contents of PVP. (a) 0 wt%, (b) 10 wt%, (c) 30 wt%, and (d) 50 wt%. The inset in (c) is a higher magnification to show the aggregation of TiN nanoparticles. Fig. 3 Nitrogen adsorption-desorption isotherms of TiN macro/mesoporous structure prepared with (a) 0 wt% and (b) 50 wt% of PVP. Fig. 4 DFT pore size distributions of TiN macro/mesoporous structure prepared with (a) 0 wt% and (b) 50 wt% of PVP. Fig. 5 SEM images and size distributions of Pt nanoparticles deposited by (a) 50, (b) 100, and (c) 200 cycles of ALD on TiN support. Fig. 6 Performances of MEAs using homemade electrodes prepared with various PVP contents as (a) anode and (b) cathode. The Pt catalyst was prepared by 100 cycles of ALD. The opposite electrodes are commercial E-Tek. The performance of MEA using E-Tek for both anode and cathode is included for comparison. Fig. 7 Performances of MEAs using homemade electrodes prepared with various ALD cycles Pt as (a) anode and (b) cathode. The PVP content to prepare TiN support was 50 wt%. The opposite electrodes are commercial E-Tek. The performance of MEA using E-Tek for both anode and cathode is included for comparison. Fig. 8 Performances of MEAs using homemade electrodes for both anode and cathode, as compared with that of commercial E-Tek. The PVP content to prepare TiN support was 50 wt%.


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Fig. 1 XRD patterns of TiN hierarchical macro/mesoporous structures prepared with various amounts of PVP and annealed at 800 oC in NH3 for 2 h.



Fig. 2 SEM images of TiN hierarchical macro/mesoporous structure prepared with various contents of PVP. (a) 0 wt%, (b) 10 wt%, (c) 30 wt%, and (d) 50 wt%. The inset in (c) is a higher magnification to show the aggregation of TiN nanoparticles.


Page 26 of 33

80

(a) 0 wt% 60 40 Adsorption Desorption

20 0

0.0

0.2

0.4

0.6

0.8

1.0

Relative Pressure (P/P0)

Quantity Adsorbed (cm3/g STP)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Quantity Adsorbed (cm3/g STP)

Page 27 of 33

100

(b) 50 wt%

80 60 40 Adsorption Desorption

20 0

0.0

0.2

0.4

0.6

0.8

1.0

Relative Pressure (P/P0)

Fig. 3 Nitrogen adsorption-desorption isotherms of TiN macro/mesoporous structure prepared with (a) 0 wt% and (b) 50 wt% of PVP.


0.12

(a)

0.10

0 wt%

0.08 0.06 0.04 0.02 0.00 1

10

100

dV/dlog(W) pore volume (cm3/g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

dV/dlog(W) pore volume (cm3/g)


Page 28 of 33

0.12

(b)

0.10

50 wt%

0.08 0.06 0.04 0.02 0.00 1

10

100

Pore width (nm)

Pore width (nm)

Fig. 4 DFT pore size distributions of TiN macro/mesoporous structure prepared with (a) 0 wt% and (b) 50 wt% of PVP.


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Fig. 5 SEM images and size distributions of Pt nanoparticles deposited by (a) 50, (b) 100, and (c) 200 cycles of ALD on TiN support.



Page 30 of 33

Fig. 6 Performances of MEAs using homemade electrodes prepared with various PVP contents as (a) anode and (b) cathode. The Pt catalyst was prepared by 100 cycles of ALD. The opposite electrodes are commercial E-Tek. The performance of MEA using E-Tek for both anode and cathode is included for comparison.


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Fig. 7 Performances of MEAs using homemade electrodes prepared with various ALD cycles Pt as (a) anode and (b) cathode. The PVP content to prepare TiN support was 50 wt%. The opposite electrodes are commercial E-Tek. The performance of MEA using E-Tek for both anode and cathode is included for comparison.



Fig. 8 Performances of MEAs using homemade electrodes for both anode and cathode, as compared with that of commercial E-Tek. The PVP content to prepare TiN support was 50 wt%.


Page 32 of 33

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