OxideâCarbon Nanofibrous Composite Support for a Highly Active

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An Oxide-Carbon Nanofibrous Composite Support for a Highly Active and Stable Polymer Electrolyte Membrane Fuel Cell Catalyst Yukwon Jeon, Yunseong Ji, yong il cho, Chanmin Lee, Dae-hwan Park, and Yong-Gun Shul ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b02040 • Publication Date (Web): 02 Jul 2018 Downloaded from http://pubs.acs.org on July 4, 2018

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An Oxide-Carbon Nanofibrous Composite Support for a Highly Active and Stable Polymer Electrolyte Membrane Fuel Cell Catalyst Yukwon Jeona,b‡, Yunseong Jia‡, Yong Il Choa‡, Chanmin Leea, Dae-Hwan Parkc, and Yong-Gun Shula,* a

Department of Chemical and Biomolecular Engineering, Yonsei University, Yonsei-ro 50,

Seodaemun-gu, Seoul, 03722, Republic of Korea b

c

School of Chemistry, St Andrews University, KY16 9ST, Fife, United Kingdom

Department of Nano Materials Science and Engineering, Kyungnam University, Changwon,

Gyeongsangnamdo 51767, Republic of Korea

KEYWORDS PEMFC cathode catalyst; Oxide-carbon composite support; Nanofibrous structure; Electronic configuration; Electrochemical stability

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ABSTRACT Well-designed electronic configuration and structural properties of electrocatalyst alter the activity, stability and mass transport for enhanced catalytic reactions. We introduce a nanofibrous oxide-carbon composite by an in situ method of carbon nanofiber (CNF) growth by highly dispersed Ni nanoparticles that are exsoluted from a NiTiO3 surface. The nanofibrous feature has basically a 3D web structure with improved mass transfer properties at the electrode. Also, the design of CNF/TiO2 support allows complex properties for excellent stability and activity from the TiO2 oxide support and high electric conductivity through the connected CNF, respectively. Developed CNF/TiO2-Pt nanofibrous catalyst displays an exemplary oxygen reduction reaction (ORR) activity with significant improvement of the electrochemical surface area (ECSA). Moreover, exceptional resistance to carbon corrosion and Pt dissolution is proved by durability test protocols based on DOE. These results are well-reflected to the single-cell tests with an even better performance at the kinetic zone compared to the commercial Pt/C under different operation conditions. CNF/TiO2-Pt displays enhanced active state due to the strong synergetic interactions, which decreases the Pt d-band vacancy by an electron transfer from the oxide-carbon support. A distinct reaction mechanism is also proposed, and eventually demonstrates a promising example of an ORR electrocatalyst design.


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Polymer electrolyte membrane fuel cells (PEMFCs) are potentially considered as an environmental energy conversion device with high power efficiency and portable feature for power generation and transportation applications.1,2 At this juncture for commercialization to the electrical power market, significant issues such as the development of alternative materials have to be still addressed. Notably, cathode catalysts are required with an effective oxygen reduction reaction (ORR) property by a chemically active phase to activate the O2 and eject in the formula of H2O.1,2 In the current field, platinum (Pt) based catalysts have been used with carbon supports that have sufficient electrical conductivity, high surface area, and suitable pore structure.3 Nevertheless, these carbon materials are deteriorated by corrosion from a highly corrosive environment at the PEMFC operation conditions. The absence of carbon support can result in the decrease of the electrical conductivity and accelerate the agglomeration/sintering of the Pt nanoparticles (NPs) via Ostwald ripening. These may considerably reduce the catalyst electrochemical surface area (ECSA), which hinders the actual working life of the fuel cell devices.4–6 Thus, stable materials for support need to be explored under real conditions. Among many candidates, metal oxides are attractive supports for electrode catalysts, because of their excellent mechanical strength and chemical stability in highly acidic or basic conditions. It is also expected that oxide supports inhibit the dissolution of Pt. Several groups have developed different oxides (MxOy, M = Ti, W, Nb, Ta, Ce) for electrocatalysts as an alternative system.7,8 K. Sasaki insisted that titanium dioxide (TiO2) is one of the most appropriate candidates through a theoretical calculation.7 Especially, TiO2 has been suggested by their economical price for commercial availability and strong metal-support interaction (SMSI) effect for enhanced performance under the acidic condition.7,8 For example, when Pt NPs loaded on the TiO2 nanofiber support, the electron density of Pt is increased by the additional charge transfer of electrons from


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the TiO2. Due to this spillover effect, relatively enhanced ORR activity has been reported and compared with the commercial Pt deposited carbon catalyst.9–12 However, there are still issues regarding the utilization of TiO2 as a support in the real electrode system. Firstly, TiO2 has a low specific surface area that reduces the mass transfer of reactants to react at catalyst surfaces. Furthermore, pure TiO2 reveals low electrical conductivity since it is a semiconductor with a high band gap, which makes the surface absorbed hydroxyl (OH) groups challenging to obtain electrons for the ORR. Therefore, we need further works to build an effective structure with active and electrical conductive phases when we design an electrode in PEMFC. To produce an operative electrode, a nanofibrous structure has been received attention to the field of electrochemical systems, for instance, fuel-cells and batteries.13 Especially, electrospun nanofiber nonwoven matrix provides a uniform and controllable web structure. Nanofibrous supports have been used due to their high aspect ratio that allows appropriate 3D open configuration electrode structure, providing higher surface area and easily accessible pores for better reactant/product diffusion.11–14 Recently, oxide based supports are designed as a composite with carbon sources due to its lack of the electric conductivity. Carbon nanofiber (CNF) and carbon nanotube (CNT) have been studied as the carbon sources due to their connective structure for high electron conductivity and the favorable morphology.11,12,14 For example, our previous study presented a TiO2 and carbon nanotube (CNT) mixed composite as durable support material with strong metal interaction by an electron transfer, which are essential properties for a high fuel cell performance.12 Here in, we are introducing a nanofibrous structured electrode that is composed of CNF/TiO2 nanofibers through an in situ CNF growth on the electro-spun TiO2 nanofibers. CNFs were finely grown by an in situ process from the highly distributed Ni on the TiO2 nanofiber surface to boost


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up the electrical conductivity. Usually, Ni on the supports is prepared by deposition techniques, but it is difficult to control the particle sizes and distribution.15,16 Exsolution method, which exclusively reduces the cations to metals, has been presented to grow nano-size particles directly from a perovskite (ABO3) backbone.17 A Ni exsolution process is designed to pull out the more reducible Ni from a single phase of the NiTiO3 perovskite structure. After Pt deposition, the contacts with TiO2 oxide and CNFs enhance the Pt electronic structure to an active Pt for successful catalytic activity. Moreover, exceptional stability is proved by carbon corrosion and Pt dissolution tests from the designed durability test protocols based on DOE (Department of Energy) and FCCJ (Fuel Cell Commercialization Conference of Japan).18,19 Through this developed CNF/TiO2-Pt nanofibers, we attempted to replace the commercial Pt/C as a real PEMFC catalyst.

RESULT AND DISCUSSION Synthesis of nanofibrous CNF/TiO2-Pt catalyst. We introduced an in situ method of CNF growth on the Ni/Ti based nanofibrous oxide composite (CNF/TiO2) through the following procedure in Figure 1. From Figure 1(a), the electrospun PVP/Ni/Ti nanofibrous web with an average fiber diameter of 2-3 µm was uniformly prepared by electrospinning process under an optimized condition. The thermal treatment process in air atmosphere was followed to eliminate the guide polymer/solvent from the electrospun nanofibers and synthesize a NiTiO3/TiO2 oxide nanofiber at the same time, as shown in Figure 1(b). The nanofibers were shrunk more than 5 times to an average diameter of 500 nm (inset of Figure 1(b)) with a well-defined and highly connected nanofibrous structure. Due to the different formation speeds between Ni and Ti based oxide phases, we successfully designed a core-shell like structure of NiTiO3/TiO2 oxide nanofiber by a Ni to Ti molar ratio with a value of less than 1. The (R) and (A) are denoted as rutile and


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anatase phase, respectively. As shown in Figure S1(a), the EDS mapping reveals that the Ni component was primarily positioned at the shell of the composite nanofiber due to the oxidation at relatively lower temperature, while the Ti element was entirely distributed. The total Ti and Ni molar ratio of 7:3 were confirmed by the EDS analysis in Figure S1(b) as we expected. Then, based on the proposed Ni exsolution process and color change of the nanofibers in Figure 1(c), we found that the Ni NPs were released from the NiTiO3 perovskite single phase surface during the reduction process in the H2 atmosphere. Finally, as from Figure 1(d), CNFs were grown from these highly distributed Ni NPs by a CVD method under CH4 conditions and high temperature treatment. Thus, it was possible to obtain the expected CNF/TiO2 nanofibers with a diameter of a 1500 nm in average from the counted frequency in the inset of Figure 1(d). Furthermore, uniform CNFs with a diameter of 40 nm were observed in the magnified SEM of Figure 1(d), which is resulted from the uniformly produced Ni NPs by the introduced in situ exsolution method. The carbon amount was around 57 wt% confirmed from the weight loss at 600-700 oC by the TGA analysis in Figure S2(a), which is basically enough to provide an electric conductivity for the use in PEMFC. It was also found from the N2 adsorption-desorption analysis in Figure S2(b) that the surface area of the CNF/TiO2 nanofibers dramatically increases 2 times than the Ni-TiO2 nanofibers from the values from 51.8 m2/g to 107.8 m2/g. This increment of surface area is a considerable advantage to use as an electrode catalyst for enhanced catalytic activity and mass transfer. To design an active cathode catalyst, Pt NPs were deposited on the developed CNF/TiO2 nanofibrous composite support by a microwave-assisted polyol process. Finally, Figure 1(e) displays the perfect CNF/TiO2-Pt nanofibrous structure with a well-distributed CNF around the TiO2 nanofiber without morphological changes during the Pt deposition. From Figure S3, it was


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proved that the carbon fibers were wounded the TiO2 nanofiber and the Pt was deposited well on our developed support with a similar Pt amount (approximately 40.0 wt.%) to the commercial Pt/C. Morphological changes. Figure 2 demonstrates the HR-TEM and XRD analysis for each step of the CNF/TiO2-Pt nanofibrous composite. Basically, the HR-TEM image in Figure 2(a) shows a complete synthesis flow of the nanofibrous composite structure. We found that the surface before the reduction is a single phase of NiTiO3 with a typical inter-planar distance of 2.7 Å corresponding to the (104) plane.20 After the reduction process, we were able to produce well-deposited spherical Ni NPs strongly stocked on the TiO2 nanofiber surface with an average size of 10 nm. From the interplanar distances, it was evidently seen that the particles are composed of Ni metal with the value of 2.1 Å in agreement at the (111) plane.21 In contrast, the rest of the part changed to a TiO2 anatase phase with an interplanar distance of 3.3 Å which corresponds to the (110) plane.22 HRTEM images in Figure 2(a) support the morphological feature of CNF/TiO2 nanofibrous composite. The oxide nanofiber and CNF were well distinguishable due to the different contrast of each material. The nanofibrous composite displays good connectivity, and each diameter was well agreed with results from the SEM images. For the nanofibrous CNF/TiO2-Pt, the Pt NPs were loaded well on the surface of the TiO2 surface with an average size of 3.8 nm, where an appropriate Pt size for a good ORR activity is commonly known to be 3-5 nm.23 From the magnified image in the inset of CNF/TiO2-Pt, spherical Pt NPs were distinguished with a lattice spacing of 2.2 Å corresponding to the (111) planes for Pt metal.24 From observing the Pt NPs on CNF/TiO2 surface, HR-TEM images clearly displayed that the Pt NPs were mainly distributed well on the semiconducting oxide site regions at the surface of the TiO2 nanofiber, which is maybe owing to a strong interaction between the TiO2 oxide and Pt metal surface energy.25,26


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Structural changes. From the XRD analysis in Figure 2(b), the structural change can be seen after the calcination and reduction process of the electrospun nanofibers. As we designed for the NiTiO3/TiO2 composite nanofiber with a Ni/Ti molar ratio less than 1, diffraction peaks of NiTiO3 perovskite were mainly revealed at 2θ positions of 24.2o, 33.1o, 35.5 o, 38.9o, 41.3o, 49.5o, 54.2o and 55.6o that are indexed from the JCPDS card no. 33-0960.20,27 Meanwhile, some overlapping peaks at positions of 27.5o, 35.5o, 41.3o, 54.2o, and 55.6o were risen from the rutile TiO2(R) positioned in the core.22 No additional peaks for the single metallic phase of Ni were detected, indicating principally a single phase of NiTiO3 perovskite and rutile TiO2 at the surface. When the sample was reduced at H2 atmosphere, we can anticipate that Ni NPs were exsoluted from the NiTiO3 skin to the nanofiber surfaces from the appearance of reflections for Ni metallic phase at 44.5o and 51.8o, correspond to (111) and (200) crystal faces in JCPDS card no. 04-0850.21 Due to the low temperature treatment of 400 oC, the remained Ti oxide at the shell changed to anatase TiO2(A) structure with a peak position of 25.3o, 48.2o, 54.2o, and 55.6o, resulting to the core-shell composite nanofiber with a rutile-anatase TiO2 phase.22 This mixed structure changed again after the carbon nanofibers growth on the TiO2 nanofiber surface. Interestingly, the mixed TiO2 phase shifted to a single phase of rutile TiO2 which is maybe due to the high temperature treatment of 900 oC. XRD pattern also displays the formation of broad carbon peaks at 26.5o and 43.5o corresponding to the (002) and (200) planes of graphitic carbon. After the Pt deposition process, a change of TiO2 rutile phase and CNF structure was not observed. However, major diffraction peaks of Pt metal were detected at 39.5o (111) and 46.1o (200) (JCPDS card no. 04-0802).12 These indicate that the CNF/TiO2-Pt nanofiber composite was successfully formed. From the size calculation by the Scherrer’s equation, the Pt NPs were consistent in sizes with the TEM results. Interestingly, the Pt(111) diffraction peak of CNF/TiO2-Pt positioned at 39.4o while the original


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peak for Pt/C was at 39.9o.28 Through the Bragg’s law, an increment in Pt’s d-space was revealed by the decrement of θ, which provides an evidence on a strain of the Pt NPs due to the interaction among the ternary system. The Ni metal NPs exsolution on the TiO2 nanofiber skin and change in the TiO2 phase can be explained by XANES and EXAFS analysis in Figure 3 for more insights to the geometric structure and identify the oxidation states in the developed composite. As indicated in Figure 3(a), the XANES spectra for Ni K-edge indicate different Ni oxidation states from the shape of the white lines, which is more obviously seen in the inset. The spectrum of NiTiO3 displayed a steep edge and high white-line at about 8350 eV, which is consistent of the Ni oxide phase.29 As the exsolution process proceeded, the absorption edge became bigger in spectral weight at the lower energy. Moreover, the height of the white-line decreased and changed the same as the spectral feature of the Ni foil as a metallic phase. The Ni L-edge and Ni XPS result in Figure S4(a)-(b), respectively, reveals also the change to the Ni metal chemical structure from the shift to the lower adsorption energy and the formation of an additional peak at the lower binding energy. The fine metallic local structure was directly observed in Figure 3(b), corresponding to Fourier transforms (FT) of k2weighted EXAFS oscillations for the Ni K-edge. Excellent data quality of EXAFS oscillations was obtained with a high signal/noise ratio as proved in Figure S4(c)-(d) by the k2-weighted Ni Kedge EXAFS spectra. Ni contained oxide nanofibers illustrates a Ni-O coordination structure as we expected. On the other hand, as seen from the EXAFS oscillation of Ni foil, the peak at the position of 2.3 Å represents the first Ni-Ni coordination shell contribution. The similarity of the FT features of the Ni exsoluted TiO2 denotes a highly metallic Ni phase by having an fcc structure of bulk nickel. Moreover, the lower magnitude of the exsoluted Ni than the Ni foil specifies a smaller particle size, which explains perfect Ni NPs exsolution from the NiTiO3 nanofiber single


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phase skin with also a high dispersion. In Figure 3(c), evidence for the transformation of different Ti phases was received from the XAS spectra for Ti L II, III-edge during the synthesis of CNF/TiO2 nanofibers. The main spectrum features are attributable to the dipole transition from core 2p to unoccupied state of 3d (2p→3d).30 In L III-edge, we could see that the XANES spectra of eg show a mixed phase like as the desired NiTiO3 structure located at the surface. This phase changed to a phase with mainly anatase TiO2 after the reduction process from the larger intensity at the value of 461.0 eV.31 However, a peak shift to 461.9 eV was revealed that is consistent with that of rutile Ti phase, as also confirmed in the XRD patterns. Meanwhile, it is known that a rutile TiO2 is more stable than the other phases,32 which is likely to be effective under an acidic atmosphere for the use as a PEMFC catalyst support. In Figure 4, the Ti and carbon structure of the final CNF/TiO2 was also supported by the Raman analysis. It shows a rutile TiO2 due to the precisely same positions of the commercial one, consistent well with XRD results. Additionally, the result reveals also two conspicuous carbon peaks at approximately 1357.7 cm-1 and 1586.3 cm-1 for D-band and G-band, respectively, similar to the commercial carbon black. It is known that G-band peak comes from the graphite crystal planes whereas D-band originates from the lattice distortion at the sp2-hybridized carbon resulting in several types of defects (for example, topological defects, vacancy, and impurity).32,33Therefore, the intensity ratio of ID/IG is generally used to investigate the graphitization and defect degree. While the commercial carbon black displays an ID/IG of 1.04, the ID/IG for the prepared CNF/TiO2 was 0.85. Furthermore, a sharp G-band peak was revealed for the CNF/TiO2 nanofibrous composite. Therefore, we could insist that the CNFs grown by an in situ process possess a high graphitization sp2-carbon structure compared to the commercial carbon black, which may provide a better electric conductivity.


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The conductivity was then simply but directly investigated by the electric conductivity measurements as design in Figure S5(a). The electrical conductivity was collected by the 4 probe system with Pt wire under constant pressure. As listed in Figure S5(b), around 2 times higher conductivity was revealed for the Ni exsoluted TiO2 nanofibers than the original TiO2 even though both shows almost no conductivity. The electrical property was then boosted up after growing carbon nanofiber on the TiO2 surface to the value of 43.4 S/m, which was even higher than the commercial carbon black (33.4 S/m). This enhanced property was maybe the result from the formation of highly graphitized carbon nanofibers, and it provides evidence to possibly efficient use as PEMFC cathode catalysts. Half cell measurements. To investigate the catalytic activity changes, NiTiO3/TiO2, Ni-TiO2, CNF/TiO2, CNF/TiO2-Pt nanofibrous catalysts were tested and compared with Pt/C commercial. Polarization curves were performed in Figure 5(a) for the ORR measurements. It was easily expectable that the oxide catalyst of NiTiO3/TiO2 showed almost no ORR activity due to a lack of the electronic conductivity. From the Ni exsolution from NiTiO3 perovskite, a slight increase in activity was seen may be due to small electronic conductivity from the distributed Ni metal NPs. And, much further increase was revealed from the CNF grown on the TiO2 nanofiber surface by the aforementioned enhanced electron transfer. Finally, the Pt deposited CNF/TiO2 nanofibrous catalyst obtained an outstanding ORR activity with a comparison to the conventional Pt/C catalyst. The shapes of polarization curves were similar, which means a well-made catalyst was developed like as the commercial one. Interestingly, the limiting current densities were even higher with the value of 4.8 mA/cm2 (Pt/C: 4.4 mA/cm2) due to a greater mass transfer effect by the nanofibrous composite structure. As from the inset in Figure 5(a), the CNF/TiO2-Pt catalyst has more positive onset potential (0.99 V) than the Pt/C (0.97 V). Especially, the current density (j) at the potential


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of 0.85 V was 2.08 mA/cm2 that was improved by around 20% over the commercial Pt/C (1.86 mA/cm2). This reveals a higher ORR activity for the CNF/TiO2-Pt catalyst originated from the noted advantages such as the nanofibrous configuration for enhanced electron/ion pathway and synergy effect by the contacts among Pt, TiO2, and CNFs. This enhanced catalytic property can also be confirmed by the cyclic voltammetry (CV) curves in Figure 5(b). These were measured in N2 purged 0.5 M H2SO4 solution by 50 mV/s at room temperature. From the result of CV tests, the adsorption and desorption peaks of hydrogen and oxygen species are clearly observed without any additional oxidation or reduction peaks. This reveals that the CNF/TiO2 support is electrochemically inactive under PEMFC operating conditions. In general, the activity of the electrocatalyst is often decided by integration of the hydrogen desorption peak, which represents the electrochemical surface area (ECSA). By equation (1), ECSA values can be calculated regarding the Pt in the electrode. (1) Here, QH is the charge of the hydrogen adsorption/desorption and m indicates the Pt loading.5,12 QH is assumed as 210 mC/cm2, when the hydrogen desorption occurs on the Pt surface. As from the calculations, the specific ECSA for our CNF/TiO2-Pt nanofibrous catalyst with the value of 115.3 m2/g was much higher than that of the Pt/C commercial (68.4 m2/g, comparable with other works summarized in Table S1), indicating superior catalytic activity. Surprisingly, the synthesized catalyst exhibited not only a great ORR activity but also exceptional durability than the commercial catalyst under H2SO4. As in Figure 6(a), two different protocols of PCT (potential cycling test) and AST (accelerated stability test) were applied to evaluate the stability of the developed CNF/TiO2-Pt nanofibrous catalyst. Both protocols are designed based on the DOE and FCCJ durability test protocols, which ours are equivalent or even


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severer.18,19 The PCT protocol was proposed to estimate the stability of the support by a rapid carbon corrosion in a potential range from 1.0 V to 1.6 V. Correspondingly, AST protocol was used to evaluate the dissolution resistivity of Pt on the support at the potential range from 0.6 V to 1.0 V. LSV and CV were recorded after each protocol as in Figure S6-S7 to compare the stability from the initial ECSA and j (at 0.85 V) values, respectively, for the CNF/TiO2-Pt nanofibrous and commercial Pt/C catalysts. After the 6,000 PCT cycles in Figure S6, we could easily see that the shape of the LSV and CV did not change much for the CNF/TiO2-Pt nanofibrous catalyst while a huge degradation was observed for the Pt/C commercial catalyst. This result was confirmed by the change of the calculated numeric values of ECSA and j which are summarized in Figure 6(b) before and after the PCTs. The ECSA for the CNF/TiO2-Pt nanofibrous catalyst was only reduced by 96 % of the initial value, while Pt/C catalyst exhibited 26 %. The similar result was obtained for the j value, where 95 % and 47 % of each original values were revealed for the developed and commercial catalysts, respectively. This result discloses that the enhanced catalyst stability of the CNF/TiO2Pt nanofiber is originated from the corrosion resistivity of TiO2 nature under such conditions of 1.0-1.6 V and H2SO4. Even though CNF/TiO2-Pt catalyst has potentially corrosive carbon species, CNFs were found to be a highly graphitized carbon allotrope by the Raman analysis in Figure 4, providing greater resistance to corrosion. The great durability of our synthesized catalyst was also observed from the AST. Accordingly, we calculated the changes in ECSA and j values during 30,000 cycles from the CV and LSV analysis in Figure S7, respectively, and presented in Figure 6(c). The ECSA of Pt/C catalyst rapidly decreased after the first 10,000 cycles by almost 50 %, and then gradually dropped to 37 % of its initial ECSA value. In comparison with the CNF/TiO2Pt nanofibrous catalyst, slight and gradual ECSA decrement was revealed with a total ECSA loss


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of less than 20 %. Likewise, the similar tendency on the degradation of the j values was revealed with the values of 21.3 % and 76.6 %, from each initial values for CNF/TiO2-Pt nanofibrous and Pt/C commercial catalysts, respectively. Additionally, in Figure S8, alteration of Pt particle size distributions after each stability tests is presented, statistically calculated from the HR-TEM images. As we expected, the Pt particles on the CNF/TiO2-Pt nanofibrous catalyst after AST grew slightly more than after the PCT. However, the average growth after the protocols (3.8 nm 4.2-5.4 nm) was much less compared to the Pt NPs size growth for commercial Pt/C (4.2 nm 7.9-8.1 nm). These results clearly suggest that the advantageous bonding structure originated from the strong SMSI effect between Pt and TiO2, not only affects to the great catalytic activity but also alleviates the Pt dissolution and growth that will be efficient to the real PEMFC operation. PEMFC single cell performance. To perform a single cell test of the nanofibrous electrode, membrane electrode assembly (MEA) was fabricated with our CNF/TiO2-Pt nanofiber catalyst. For comparison, MEAs with Pt-TiO2, Pt-Ni-TiO2 nanofibers and Pt/C commercial catalysts were prepared by the same Pt loading to focus on the effect of the TiO2 nanofiber and CNF. From the SEM cross-sections of the cathode layer composed by the CNF/TiO2-Pt nanofibrous catalyst in Figure 7(a), the cathode was finely stacked by nanofibrous webs, providing a distinctive pore structure. The feature of the CNFs grown on the TiO2 nanofiber surface was evidently observed where CNFs could be the electrical channel in the MEA for an effective electron transfer in the cathode whereas pure TiO2 cannot.12,34 The thickness of cathode catalyst layer was approximately 2 times thinner than the Pt/C catalyst based cathode layer (Figure S9), owing to the higher density of nanofibrous feature, which also can be a key point for a short mass transfer in the electrode.


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Figure 7(b)-(c) shows the I-V, CV, and impedance (Figure S9) results, respectively, for the single cell test (Tcell=75oC, RH 100%). As we expected, Figure 7(b) shows that CNF/TiO2-Pt nanofiber catalyst achieved much better performance than the TiO2-Pt and Ni-TiO2-Pt nanofiber catalysts. Basically, from Table 1, OCVs show a big difference between these with the value of 0.99 and 0.85 V for CNF/TiO2-Pt and Pt-TiO2 nanofiber catalysts, respectively, which provide us direct evidence for lack of electrical conductivity on pure TiO2. A small increase was observed for the Ni-TiO2-Pt nanofiber catalysts due to the Ni metal phase as we mentioned before. This variance was also seen for the lower area of the H2 desorption peak at the CV curves and much larger charge transfer resistance. Figure 7(c) displays increased ECSA values from the 13.01 m2/g for TiO2-Pt nanofiber catalyst to maximum 44.97 m2/g for CNF/TiO2-Pt nanofiber. Moreover, we were able to see from Table 1, according to the impedance analysis in Figure S10, the ohmic resistance at 0.6 V decreases almost 6 times from 1.11 Ω/cm2 to 0.16 Ω/cm2 for the TiO2-Pt and CNF/TiO2-Pt nanofiber catalysts, respectively. We could describe that connected CNF provides appropriate electrical pathways for reducing the resistance of cathode layer that increases the number of triple boundary phase (TBP). From these enhancements, the developed CNF/TiO2-Pt nanofibrous catalysts revealed a high current density of 1588 mA/cm2 at 0.6 V with a maximum power density of 995 mW/cm2. Even more, this value was higher than the Pt/C catalyst based single cell with a maximum power density of 953 mW/cm2 at same loading of Pt. Meanwhile, it is widely known that the I-V curve can be divided into three zones according to the primary reason for overpotential.12 In Figure 7(b), the kinetic zone at OCV–0.75 V, ohmic polarization zone at 0.75–0.40 V and mass transfer controlled zone at 0.40–0.20 V are shown. In the kinetic zone, the activation overpotential is generally determined by the catalyst activity for the ORR. As summarized in Table 1, there was a huge difference in performances at 0.9 V from


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the kinetic zone between the CNF/TiO2-Pt nanofiber (282.5 A/gPt) and Pt/C commercial (107.5 A/gPt) catalysts. Since the ECSA value is also related to the catalytic activity, the ECSA for the CNF/TiO2-Pt nanofiber catalyst was much higher with a value of 44.97 m2/g than the Pt/C commercial that has the ECSA of 33.32 m2/g. These results were previously expected from the enhanced activity in the half-cell results (Figure 5). Interestingly, the nanofibrous structure also provided comparable current density to Pt/C commercial catalyst in the mass-transfer-controlled zone with a similar charge transfer resistance (Figure S10). In this zone, diffusion limitation is commonly occurred by lack of the reactant (e.g., O2) on the Pt surface, which can be typically observed for the oxide material due to its small surface area. Therefore, the performance in this zone shows the effectiveness of transportation. In this point of view, the nanofibrous structure can provide a higher surface area and even a 3D open pore configuration for better mass transfer at the electrode. As a result, we achieved better performance for the CNF/TiO2-Pt nanofibrous catalyst than Pt/C at ohmic polarization zone that may be summed up with both properties at the kinetic and mass transfer zones. This superiority in performance was even more obvious at low relative humidity (RH 50%) condition or circumstance with supplying air instead of oxygen in the cathode. As well known, the performance decreases at both conditions, especially 70 % by using air. Interestingly, in Figure S11(a), the CNF/TiO2-Pt nanofibrous catalyst reveals much more distinguish performance than the commercial Pt/C at RH 50%. Furthermore, the performance difference in Figure S11(b) was even bigger from the single cell tests with air. These directly indicate an enhanced physical structure for water capturing and better ORR activity of our developed catalyst, which provides us a definite evidence for a promising cathode catalyst system at the real PEMFC.


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Expected catalytic reaction mechanism. Chemical interaction of each material (TiO2, CNF, and Pt) after Pt deposition is important to investigate the great catalytic activity and stability. In Figure 8, we carried out an X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS) analysis for TiO2, TiO2-Pt, and CNF/TiO2-Pt. XPS peaks for Ti 2p are shown in Figure 8(a) with two consequence peaks of Ti 2p1/2 and Ti 2p3/2, where each position and shapes matched with a perfect TiO2 structure as also proved by the XRD results. However, it was found that the peak moved to positive binding energy by the Pt deposition on the pure TiO2 support, and even higher value for the developed CNF/TiO2-Pt. As shown in the inset in Figure 8(a), a similar trend was revealed with the similar result of XPS O 1s. This obvious change in the binding energy to a higher value can signify a strong bonding between Pt, CNF, and TiO2 by the SMSI effect10,32,35,36 More evidence for this interactive structure can be detected by XAS analysis, surrounding X-ray absorption near edge structure (XANES) for Ti L-edge (2p→3d) in Figure 8(b). All XAS spectra display the same positions for TiO2 and CNF/TiO2-Pt as an original rutile TiO2 structure.36 The only difference is the peak intensity of the absorption edge that is generally explained by the d-electron vacancies. We can clearly see an increase of the intensity for the CNF/TiO2-Pt compare to the TiO2 and even the TiO2-Pt. This suggests an increase of the Ti vacancy amount, which leads to a larger number of Ti 3d holes owing to an electron contribution to the Pt species from the TiO2 support.28,36 The same tendency was revealed at O K-edge XAS spectra from the inset in Figure 8(b), which directly evidenced the SMSI effect by the electron donation to Pt. This SMSI with TiO2 and CNF can adjust the electronic structure of the dispersed Pt species on the CNF/TiO2 composite support by the catalytic activation with better charge transfer for an efficient electrocatalytic system.10,12,37,38


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It was also possible to understand the geometric and electronic Pt structure for CNF/TiO2-Pt by the XAS (Pt LIII-edge) and XPS (Pt 4f) spectra. The Pt LIII-edge XANES results were exhibited in Figure 8(c) with the spectra of CNF/TiO2-Pt, TiO2-Pt, Pt/C, and Pt foil. Generally, the intensity of the white line nearby 11569 eV links to a transition from 2p3/2 to 5d, which is influenced by the Pt shape/size and electronic structure (e.g., d-band vacancy).28,29,36 It was found that the intensities progressively reduced with the order of Pt foil, commercial Pt/C, TiO2-Pt, and CNF/TiO2-Pt. The reduced intensities mostly indicate a change of the Pt d-band vacancy since Pt particle sizes for all samples were similar in this study. Comparing Pt/C and TiO2-Pt, the smaller intensity for TiO2-Pt may reveal more electrons transferred at TiPt than CPt. This results in a decrease of the Pt dband vacancy and hence means a high electron density at the Pt atoms due to the SMSI effect.9,10,34– 36,39

Interestingly, the intensity of the white-line for the CNF/TiO2-Pt catalyst further decreases

maybe from donated electrons by the contact with CNFs. This is quite understandable because the carbon contact with other substrates is generally known to induce chemical states alteration through an electron transport.10,36,37 Therefore, a further decrease in the Pt d-band vacancies is anticipated with a lowest unfilled d-states that can also mean a change to a highly metallic Pt phase at the CNF/TiO2-Pt catalyst. This enhanced catalyst feature consequences to a reduction of the intermediates adsorptive strength for more effective catalytic ORR activity.12 The local Pt atom structure in TiO2-Pt and CNF/TiO2-Pt catalysts was investigated in Figure 8(d) by the extended X-ray absorption fine structure (EXAFS). From Figure S12, excellent data quality of the k2-weighted Pt LIII-edge EXAFS oscillations was found from the good signal to noise ratio. From the reference Pt foil and Pt/C, peaks in Figure 8(d) positioned at around 1.5 Å and 2.6 Å represent the contribution of the first Pt-O and Pt-Pt coordination shells to the EXAFS oscillations, respectively.29 The similar FT features of Pt-Pt bond proved that the Pt possesses fcc


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structure of the bulk Pt. Furthermore, an individual Pt-O shell (indexed as Pt-O and/or Pt-OH bond contributions) was detected at the Pt/C commercial, which is strongly related to the decline of ORR activity. On the other hand, the Pt local structure changed when the Pt is deposited on the TiO2 based supports. Even though the length of the first shell coordination of Pt-Pt slightly decreases to the value of around 2.3 Å from the coordination with the oxide, there was no peak separated peak of Pt-O due to the SMSI effect that may provide a much stable and active structure for the ORR. Even, the first Pt shell of CNF/TiO2-Pt was moved to the metallic phase of Pt-Pt by the interaction with the additional CNF. The XPS spectra also support the Pt local structure in the inset of Figure 8(d). The first peak positions of the Pt 4f spectra were slightly different with the values of 71.58 eV (Pt/C), 70.98 eV (TiO2-Pt), and 71.18 eV (CNF/TiO2-Pt), respectively. The most notable thing is the shift to a higher value for CNF/TiO2-Pt, indicating fewer contributions of Pt-O and Pt-OH bonds with a highly metallic phase when CNFs were contacted with the Pt on the TiO2 nanofiber. Consequently, the Pt configuration may have an enhanced active and stable phase to apply as a PEMFC cathode catalyst. From all these findings, an improved catalytic structure can be proposed by the prepared CNF/TiO2-Pt. From the illustration in Figure 7(e), the chemical structure of Pt NPs was changed in our CNF/TiO2-Pt catalyst system, through interaction with the TiO2 surface and the contact of the CNF. As we mentioned above, the Pt d-state can be filled to a relatively high metallic Pt by the electron donation from TiO2 surface and additionally from the touched CNFs. This distinctive catalytic structure is anticipated to increase the numbers of the active Pt atoms which will advance the ORR activity for the PEMFC operation.27–39,40 Therefore, it was possible to highlight that our CNF/TiO2-Pt catalyst can increase not only the physical property but also the catalytic property at


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the same time, suggesting an approach in designing active and durable cathode catalysts for the real PEMFC system.

CONCLUSIONS Here, we demonstrate a successful synthesis of the nanofibrous composite with TiO2 and carbon nanofibers as a cathode support material to replace the carbon-based catalysts that are corrosive in the PEMFC operating condition. The fine in situ production of Ni NPs was performed from an exsolution process of the NiTiO3 single phase as proved by the XANES/EXSAFT/XPS tools. The uniformly grown CNF on the TiO2 nanofibers were then finely produced for better electrical pathway and confirmed by physical characterizations such as SEM and TEM. After the Pt NPs were deposited, it was found that there is an enhanced electrochemical structure of the Pt atoms on the TiO2 surface compared to the commercial Pt/C, which decreased the adsorptive strength of intermediates by the change to a highly metallic Pt phase, resulting in an outstanding ORR activity. Even exceptional stability was received apart from carbon corrosion by the oxide support connected with highly graphitized CNF and Pt dissolution by the SMSI effect. Furthermore, these enhanced properties were conveyed to the single cell operation with an excellent performance, which could facilitate implementation of PEMFC commercialization.

EXPERIMENTAL METHODS Electrospinning for Ni/Ti nanofiber. Nanofibers were synthesized through electrospinning method by a spinnable solution of stoichiometric Ni/Ti. Polyvinylpyrrolidine (PVP, Mw 1,300,000, Aldrich), a viscoelasticity enhancer and guide polymer, was dissolved in ethanol (Duksan) by a PVP to EtOH ratio of 1:10 in weight. Then, the mixture of Titanium isopropoxide (C12H20O4Ti,


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Aldrich), nickel (II) acetate tetrahydrate 98% (Ni(OCOCH3)2·4H2O, Aldrich) and acetic acid (protecting agent) was further blended to make a NiTiO3 precursor solution. It was stirred for 3 hours under room temperature to attain complete dissolution and mixing and charged in a syringe. With a 30-gauge plastic nozzle tip, the syringe was set up with a distance of 15 cm between the tip and collector, which is in a chamber with controlled conditions of 20 oC and RH 30%. The flow rate was 30 µm·min-1, and 15 kV was applied. Synthesis of CNF/TiO2 nanofiber. The obtained electro-spun nanofiber web was heated at 600 °C (1 °C·min-1) for 6 hours in an air atmosphere to eliminate nitrate and polymeric ingredients. To exsolute the Ni NPs from the Ni/Ti oxide nanofibers, a reduction process was carried out at 400 °C by 1 °C·min-1 for 6 hours in H2 atmosphere. Then, CNFs were grown from the Ni at the conditions of 680 °C (5 °C·min-1), 8 hours, and CH4 atmosphere.15,16 Finally, we could obtain a CNF/TiO2 nanofibers by a stabilizing process at 900 °C (1 °C·min-1) for 6 hours in Ar atmosphere. Preparation of CNF/TiO2-Pt nanofiber. Pt NPs were loaded on the CNF/TiO2 nanofibers in the means of microwave polyol method.12 The CNF/TiO2 nanofibers were dispersed in Ethylene glycol (EG, Duksan). Then, chloroplatinic acid hexahydrate (H2PtCl6·6H2O, Kojima Chemicals) solution was added under controlled pH of 11 by the addition of 1M NaOH (Duksan) solution. After homogeneously stirred for 1 hour, the mixture was taken to Teflon vessel and employed in a microwave oven. The mixture solution was reacted at the power of 800 W for 3 min to obtain Pt NPs on the sample surface. Finally, the CNF/TiO2-Pt nanofibrous catalyst was achieved after filtration, washing and drying process for 8 hours at 80 oC vacuum oven. Characterizations. The morphological characterizations were displayed by scanning electron microscope (SEM, JEOL JSM-6701F). The chemical compositions were detected by the energy dispersive X-ray spectroscopy (EDX) with an accelerating voltage (15 kV). The particle sizes and


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lattice spacing were evaluated by the high resolution transmission electron microscopy (HR-TEM, JEOL, JEM-ARM 200F) which is operated at 200 kV. Powder X-ray diffraction (XRD) patterns were measured by X-ray diffractometer (Miniflex AD11605, Rigaku) with Cu Kα source (λ = 1.5405 nm) at 30 mA, 30 kV, and room temperature. Data were collected from 20o to 60o in the scale of 2θ (2o·min-1). To determine the carbon amount, thermogravimetric analysis (TGA) was performed with a heating temperature range of 30 oC-900 oC by 5 oC·min-1 under air flow. The Brunauer-Emmett-Teller (BET) surface area was obtained by using an N2 adsorption-desorption distribution graph at 77 K (Micromeritics ASAP 2010). To see the chemical states of Ti/Ni/O/Pt, X-ray photoelectron spectroscopy (XPS, Thermo VG.) with an Al monochromated X-ray source (Al Kα, 1486.7 eV) and a hemispherical energy analyzer was used. All binding energies were calibrated at C 1s, 284.8 eV. Moreover, the X-ray absorption spectroscopy (XAS) results were also collected for each element at Pohang Accelerator Laboratory (PAL). Measurements were made with solid samples at room temperature. Pt LIII-edge and Ni K-edge spectra were measured in a transmission mode with a separated He filled IC Spec ionization chamber for an incident transmitted beam at 10C beamline (Multipole-wiggler and Ge 13-element detector). For more information, Ti L-edge, Ni L-edge, and O K-edge spectra were also obtained from the 10D beamline (Bending magnet, Phoibos 150). Electrochemical measurments. To evaluate the electrochemical properties, cyclic voltammetry (CV) and linear sweep voltammetry (LSV) were measured at room temperature in 0.5 M H2SO4. These are used by setting on the Rotate Assembly Instructions with a three-electrode system from Pine Instruments. A standard calomel reference electrode (SCE, 0.24V vs. RHE) and a Pt wire as a counter electrode were used, respectively. Ethanol, Nafion solution (5 wt.% in water and lower aliphatic alcohols, 1100 EW), and catalyst powder were mixed as a catalyst slurry. In the case of


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LSV for ORR measurements, the slurry was loaded to a glassy carbon electrode (ϕ=5mm) and dried in air at room temperature. LSVs were recorded at the range of 0.1-1.1 VRHE with the rate of 10 mV/s in the O2 saturated atmosphere and 1600 rpm. CVs were conducted at a range of 0.05-1.2 V versus the SHE at 50 mV/s in the N2 saturated atmosphere. To estimate the electrochemical stability, additional studies were verified by two different test protocols according to DOE and FCCJ durability evaluation protocols. 

Potential cycling test (PCT) to evaluate carbon corrosion: 1.0-1.6 V vs. SHE, scan rate 100 mV s-1, room temperature, 6,000 cycles.



Accelerated stability test (AST) to evaluate Pt dissolution: 0.6-1.0 V vs. SHE, scan rate 100 mV s-1, room temperature, 30,000 cycles. The electrochemical changes were measured by following abovementioned LSV and CV

processes after PCT, and after every 1,000, 3,000, 10,000, 20,000 and 30,000 AST cycles. This result of the cycling potential windows was adjusted from many previous studies that conducted acceleration testing methods.6,12 For comparisons, we tested the Pt/C commercial (Pt 40 wt.%, HISPEC 4000, Johnson Matthey). After corrosion and dissolution tests, used catalysts were collected from the RDE by sonicating, and then post-characterized by HR-TEM. Single cell performance test. The developed catalysts were applied to the cathode and Pt/C commercial (40 wt.% of Pt, HISPEC 4000, Johnson Matthey) was prepared for the anode. Each catalyst was mixed with Nafion solution and isopropyl alcohol (Aldrich) to produce a catalyst slurry. The slurries were prepared by 5 min for stirring and 3 min for sonicating, and this process was repeated at least four times. Then, we sprayed the catalyst inks on each side of the Nafion 212 membrane (1 cm2) by same Pt loadings of 0.4 mg/cm2. The prepared MEAs were assembled with the gas diffusion layer (GDL, SGL 10BC) for both cathode and anode sides.12,41,42 The initial


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PEMFC single cell measurements were operated under same conditions of 75 °C and 100 % RH using vaporized steam to humidify. The stoichiometry of the gas flows was H2/O2=1/1.5 with no back-pressure. Before the tests, activation steps were applied to hydrate and stimulate the catalytic activity. The polarization (I-V) plots were obtained by DC electrode load (6060B, Hewlett Packard). CVs were carried out by changing the O2 gas to N2 gas. All MEAs were measured at a potential range of 0V-1.2V vs. RHE by 50 mV/sec. We used an impedance spectroscopy (VSP, BioLogic) to collect the contact and interfacial resistances data at a frequency from 0.1 Hz to 10,000 Hz. For further applications, more practical tests were performed basically at the fixed condition by only changing to low relative humidity (RH 50%) condition from lowering the humidifier temperature (THumidifier = 59.3 oC) and an air oxidant condition in the cathode to decrease the oxygen partial pressure in the reactant gas.


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FIGURES

Figure 1. Schematic illustration of synthesis procedure of the CNF/TiO2-Pt nanofibrous catalyst with each pictures and SEM images from the left for (a) electrospun Ni/Ti/PVP, (b) NiTiO3/TiO2(R), (c) Ni-TiO2(A)/TiO2(R), (d) CNF/TiO2(R) and (e) CNF/TiO2-Pt nanofibers (inset: each diameter distributions), including the proposed exsolution process of Ni NPs from NiTiO3 perovskite skin where (R) and (A) are denoted as rutile and anatase phase, respectively.


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Figure 2. (a) HR-TEM images (inset: magnified image with lattice spacing) and (b) XRD patterns for NiTiO3/TiO2(R), Ni-TiO2(A)/TiO2(R), CNF/TiO2, and CNF/TiO2-Pt nanofibers.


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Figure 3. Ni K-edge (a) XANES and (b) EXSAFT, (c) Ti2p L-edge XANES for Ni foil and NiTiO3/TiO2(R), Ni-TiO2(A)/TiO2(R), CNF/TiO2 nanofibers.


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Figure 4. Raman spectra for CNF/TiO2, TiO2 and commercial carbon black at the region of carbon and titanium.


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Figure 5. (a) Polarization curves in 0.5M H2SO4 for NiTiO3/TiO2, Ni-TiO2(A)/TiO2(R), CNF/TiO2(R), CNF/TiO2-Pt, Pt/C commercial, (b) Cyclic voltammetry in 0.5M H2SO4 for Pt/C and CNF/TiO2-Pt catalysts.


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Figure 6. (a) Two different protocols for carbon corrosion and Pt dissolution tests, as well as the comparison of ECSAs and current density (j) at 0.85 V before and after (b) 6,000 PCT cycles and (c) every 1,000, 3,000, 10,000, 20,000 and 30,000 AST cycles for Pt/C and CNF/TiO2-Pt catalysts.


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Figure 7. (a) Cross-section SEM image of the CNF/TiO2-Pt catalyst based MEA (inset: magnification of nanofibrous cathode layer), and single cell tests at Tcell=75 oC, RH 100 % through (b) I-V plots and (c) CVs analysis for the nanofibrous TiO2–Pt, Ni-TiO2–Pt, CNF/TiO2-Pt and Pt/C commercial catalysts.


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Figure 8. (a) XPS results of Ti 2p (inset: O 1s) and (b) XAS results of Ti L-edge (inset: O K-edge) for TiO2, TiO2-Pt, CNF/TiO2-Pt, as well as (c) normalized XANES spectra of Pt LIII edge, (d) FT k2-weighted EXAFS oscillations for Pt LIII-edge (inset: XPS spectra of Pt 4f) for Pt foil, Pt/C commercial, TiO2-Pt, CNF/TiO2-Pt samples, and (e) schematic illustration of the Pt chemical state and catalytic reaction mechanism of the CNF/TiO2-Pt catalyst.


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TABLES Table 1. Summary of important values from the I-V, CV, and impedance results of the single cell test (Tcell=75 oC and RH 100 %) for the TiO2–Pt, CNF/TiO2-Pt nanofibrous catalysts and Pt/C commercial.

OCV

Current density at 0.9 V

Maximum power density

ECSA

Charge transfer resistance

V

A/gpt

mW/cm2

m2/g

Ω/cm2

TiO2-Pt nanofiber

0.85

10

106

13.01

1.11

CNF/TiO2-Pt nanofiber

0.99

282.5

995

44.97

0.16

Pt/C commercial

0.98

107.5

953

33.32

0.11


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ASSOCIATED CONTENT The Supporting Information is available free of charge from the ACS Nano website with Figures S1−S12 and Table S1. The authors declare no competing financial interest. AUTHOR INFORMATION Corresponding Author *Yong-Gun Shul. E-mail: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ‡These authors contributed equally.

ACKNOWLEDGMENTS This work was funded by the Ministry of Science, ICT & Future Planning the Technology through the Development Program to Solve Climate Changes of the National Research Foundation (NRF) (NRF-2015M1A2A2056833). This work was also supported by the Technology innovation industrial Program funded by the Ministry of Trade, Industry and energy (MOTIE), Republic of Korea (grant number 10052823).

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BRIEFS Graphic Table of Contents.

Outstanding activity/stability is achieved by an oxide-carbon nanofibrous composite catalyst through synergetic effects between Pt and CNF grown TiO 2 nanofiber. The nanostructured design of CNF/TiO2 composite support tunes catalytic properties by modifying the physical and chemical structure, as well as the electronic states for improved stability and activity. The involved reaction mechanisms are also briefly introduced and shows an outlook in the design of enhanced cathode catalysts for ORR electrocatalysts eventually.


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OxideâCarbon Nanofibrous Composite Support for a Highly Active

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