A highly durable N-enriched titanium nanotube suboxide fuel cell catalyst support

Journal Pre-proof A highly durable N-enriched titanium nanotube suboxide fuel cell catalyst support Reza Alipour Moghadam Esfahani, Holly M. Fruehwald, Nadia O. Laschuk, Mason T. Sullivan, Jacquelyn G. Egan, Iraklii I. Ebralidze, Olena V. Zenkina, E. Bradley Easton

PII:

S0926-3373(19)31018-5

DOI:

https://doi.org/10.1016/j.apcatb.2019.118272

Reference:

APCATB 118272

To appear in:

Applied Catalysis B: Environmental

Received Date:

22 June 2019

Revised Date:

23 August 2019

Accepted Date:

7 October 2019

Please cite this article as: Alipour Moghadam Esfahani R, Fruehwald HM, Laschuk NO, Sullivan MT, Egan JG, Ebralidze II, Zenkina OV, Bradley Easton E, A highly durable N-enriched titanium nanotube suboxide fuel cell catalyst support, Applied Catalysis B: Environmental (2019), doi: https://doi.org/10.1016/j.apcatb.2019.118272

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A highly durable N-enriched titanium nanotube suboxide fuel cell catalyst support Reza Alipour Moghadam Esfahani, Holly M. Fruehwald, Nadia O. Laschuk, Mason T. Sullivan, Jacquelyn G. Egan, Iraklii I. Ebralidze, Olena V. Zenkina†, E. Bradley Easton* Electrochemical Materials Lab, Faculty of Science, Ontario Tech University1, 2000 Simcoe Street N, Oshawa, ON, Canada

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*Corresponding author: E-mail: [email protected]; † Email: [email protected]

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Graphical abstract

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Highlights

Molecularly defined approach for catalyst design and optimization

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Remarkable corrosion-resistance N-enriched metal suboxide catalyst support

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Highly stable and durable catalyst based on Pt supported N-enriched metal suboxide

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N-enriched ligand enhanced Pt nano particles stability

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N-enriched ligand extends the stability and durability of catalyst in high potential range

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Abstract

Reduced bandgap metals oxides materials are consider an alternative to carbon black supports

for fuel cell catalysts. Here we report a unique N-enriched titanium nanotube suboxide doped with molybdenum (TNTS-Mo) fuel cell catalyst support. The support was prepared by the surface

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Formerly known as University of Ontario Institute of Technology

modification of a TNTS-Mo with a terpyridine (TPY) ligand, yielding TPY/TNTS-Mo. We have deposited Pt onto this support and evaluated its activity and stability. The Pt/TPY/TNTS-Mo catalyst was subjected stringent accelerated stress tests (AST) designed to induce catalyst degradation similar to automotive operation and startup-shutdown conditions. The Pt/TPY/TNTSMo was compared with Pt/TNTS-Mo and a benchmark carbon supported Pt catalyst (Pt/C). These stability protocols confirmed that TPY modification greatly enhances the stability and durability of the Pt nanoparticles via strong metal support interaction (SMSI). In addition, the covalently

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attached TPY layer stabilizes the doped metal oxide supports itself, preventing changes in its surface state and composition at high potentials.

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Keywords

N-enriched titanium nanotube suboxide; Corrosion-resistant support; Oxygen reduction reaction;

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1. Introduction

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fuel cells; Stability and durability.

Proton exchange membrane fuel cells (PEMFCs) are an appealing clean energy technology

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due to their high theoretical efficient [1-3]. Despite the significant advances in PEMFCs technology over the past decades, the high cost of component materials and low stability of electrodes are major barriers delaying their large-scale commercialization in both automotive and

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stationary power applications [4-5]. Currently, fuel cells technology employs Pt-based catalysts in both the anode and cathode catalyst layers. Typically, these Pt catalysts are in the form of nanoparticles dispersed on a support material to increase surface area. However, the nature of the support materials onto which the Pt nano particles (NPs) are embedded can exert a significant influence on Pt electroactivity and durability [5-8]. Carbon black materials are the most common

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support of choice due high electronic conductivity, low cost, and high specific surface area [9-10]. However, the stability of carbon supported Pt catalysts is a key issue since carbon is prone to corrosion under the harsh conditions encountered during real-world fuel cell deployment [5-6]. Start-up/shutdown conditions exacerbates carbon corrosion [11-13], which can severely reduce performance and operational lifetime of a fuel cell electrodes. Furthermore, the aggregation and dissolution of Pt NPs occurs readily on carbon supports which causes a significant decay in performance [14].

In order to overcome the issues associated with the carbon corrosion and weak stability of Pt NPs supported on carbon, many research efforts have focused on enhancing the stability of traditional Pt/C catalysts by endowing Pt NPs with new morphologies [15-19], integrating and combining the Pt with transition or nobles metals [20-27], combining the carbon support with metal oxides [28-31], and by nitrogen doping of carbon support through nitrogen-containing polymer in order to stabilize the Pt NPs [32-34]. Currently, there is a great demand to develop viable alternatives to carbon supports. Such

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supports must be electrically conductive, have high corrosion-resistance, and display strong electronic interactions with catalytic Pt NPs. Metal oxides such as TiO2, NbOx, WOx, and MoOx have been considered as a potential alternative support material due to their chemical inertness and

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strong electronic interaction with Pt NPs [35-39]. However, most metal oxides are highly resistive [7], which introduced high ohmic losses, making their deployment in fuel cell devices impractical.

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Because of this, there is a great desire to design new the search for metal oxide-based support materials that have sufficient electronic conductivity for use in real-world fuel cell devices.

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Titanium suboxides (TixO2x-1) are a promising class of support materials since they possess high thermal and oxidative stability, and acceptable electronic conductivity, which could promote the

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Pt catalytic activity towards the oxygen reduction reaction (ORR) via an electronic interaction [40] and enhance Pt NPs stability via strong metallic support interaction (SMSI) [41]. Often these materials are synthesized from TiO2 by high temperature heat treatment with a dopant that creates

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oxygen vacancies, which enhances the electronic conductivity of the material. This dopant is typically embedded the titanium suboxide lattice or remains as an inert oxide phase. Furthermore, the synthetic methods also enable the support material’s morphology to be tailored into nano-tubes structures which can further boost performance. Esfahani et al. recently reported a titanium nanotube suboxide doped with molybdenum (TNTS-Mo) as a promising fuel cell catalyst support [42].

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The reported Pt/TNTS-Mo catalyst showed enhanced ORR activity and excellent durability compare to Pt/C. Cheng et al. [43] used Nb-doped-TiO2 (NTO) as supping material for Pt electrocatalyst. They reported this catalyst retained 78% of its initial electrochemical active surface area (ECSA) compare to 57.6% retained by Pt/C catalyst and Pt/NTO catalyst displayed 21% higher ORR mass activity compared to Pt/C. They attributed this enhancement in stability and electroactivity to the SMSI effect between NTO support and Pt. Anwar et al. have reported a

tantalum doped titanium support material for Pt electrocatalyst [44] with enhanced activity and durability compared to Pt/C. To the best of our knowledge, there have been no reports of introducing N-functional groups onto a metal suboxide support surface. In this study, we have examined introducing a molecularlydefined N-enriched ligand to the surface of metal suboxide, with the goal of understanding if the N-doping of the surface will further enhance performance and/or stability of catalyst. Recently, Zenkina’s group has [45-46] demonstrated that this rational approach can be used to systematically

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create electroactive surfaces with molecularly defined nitrogen sites available for binding. Here, we have employed [2,2':6',2''-terpyridin]-4'-ylphosphonic acid (TPY) and covalently attached it to the TNTS-Mo surface as shown in Figure 1. This unique N-enriched metal suboxide nano-tube

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support (TPY/TNTS-Mo) was subsequently platinized to form the Pt/TPY/TNTS-Mo catalyst. We examined its ORR activity and stability under both load cycling and startup/shutdown fuel cell

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protocols and compared it to that of Pt/TNTS-Mo and a benchmark Pt/C catalyst. The Pt/TPY/TNTS-Mo catalyst showed enhanced activity and stability, which was due to the combined

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stabilizing effects on the Pt nanoparticles from both the TPY ligand and the TNTS-Mo support.

Fig. 1. Synthesis procedure of TPY/TNTS-Mo support and Pt/TPY/TNTS-Mo catalyst

2. Experimental Section 2.1 Chemicals

The following reagents were purchased from Sigma-Aldrich: titanium (IV) oxide 99.8% anatase, sodium hydroxide (NaOH) 98 wt%, hydrochloric acid (HCl) 37 wt%, platinum (IV) chloride (H2PtCl6·xH2O) 99.9%, sulphuric acid (H2SO4) 95-98 wt%, Nafion® perfluorinated resin solution 5 wt%, acetone (CH3-COCH3) 99.5 wt%, 2-propanol (C3H8O) 99.5 wt%, ammonium molybdate (H24Mo7N6O24.4H2O), dichloromethane (CH2Cl2) 99.5 wt%, diethyl ether (CH3CH2)2O 99 wt%, ethyl acetate (CH3COOC2H5) 99.5 wt%, n-hexane (CH3(CH2)4CH3) 99 wt%, toluene (C6H5CH3) 99.5 wt%, ammonium acetate (CH3CO2NH4) 97 wt%, pyridine (C5H5N) 99 wt%,

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triphenylphosphine ((C6H5)3P) 98.5 wt%, diethyl phosphite ((C2H5O)2P(O)H) 98 wt%, ethyl 2picolinate (C8H9NO2) 99 wt%, trifluoromethanesulfonic anhydride ((CF3SO2)2O) 99 wt%. Triethylamine was purchased from Caledon Laboratories Ltd. A commercial Johnson Matthey

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platinum catalyst 20 wt% on carbon black (HiSPEC 3000) was purchased from Alfa Aesar. Nitrogen and oxygen gases were supplied in cylinders by PRAXAIR with 99.999% purity. All

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aqueous solutions were prepared using ultrapure water obtained from a Millipore Milli-Q system

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with resistivity >18 mΩ cm-1.

Synthesis of the TPY ligand

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2.2 Synthesis of the TPY/TNTS-Mo support

[2,2':6',2''-terpyridin]-4'-ylphosphonic acid (hereafter denoted as TPY) was synthesized

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following the published procedure from Thompson [47]. Details of our synthetic process can be found in the supplementary information. Synthesis of TNTS-Mo

The synthetic procedure for TNTS-Mo is reported in detail elsewhere [42]. Briefly, TNT into was dispersed in a solution of ultrapure water and ethanol for 5h. Ammonium molybdate was

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added to the solution such that the final Mo content would be 10 wt%. The solution was then sonicated and stirred for 5 h. The solid was then collected and dried, after which it was heat-treated at 850 ℃ for 8 h under a reducing atmosphere. Synthesis of TPY/TNTS-Mo The synthesis of TPY/TNTS-Mo was performed through the one step autoclave process. 300 mg of TNTS-Mo support was added to 70 ml ultrapure water and sonicated for 30 min. The support mixture was mechanically stirred for 1 h, after which 15 wt% of dissolved TPY in water was added

dropwise to the support mixture and left to stir for 2 h, then the mixture was transferred to the autoclave reactor and heat-treated at 120 ℃ for 24 h. Resulting mixture was stirred at room temperature for 2 h, and then centrifuged and washed by ultrapure water. The obtained TPY/TNTS-Mo dried at 100℃ under N2 atmosphere. The sample had a TPY loading of 10 wt% as confirmed by thermogravimetric analysis.

2.3 Synthesis of the Pt/TPY/TNTS-Mo catalyst

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The synthesis of a Pt/TPY/TNTS-Mo catalyst was performed through an adding of 200 mg of the TPY/TNTS-Mo support to 100 ml ultrapure water. The mixture was left stirring for 1 h at room temperature. A solution of H2PtCl6·xH2O (110 mg) was dissolved in ultrapure water (10 ml), added

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dropwise to the solution containing TPY/TNTS-Mo, after 2 h the solution was purged by H2 gas for 1h and then sealed and left stirring for 24 h. The obtained solution was filtered, washed and

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dried at 80 ℃ under N2 purging. The obtained sample of Pt/TPY/TNTS-Mo was heat-treated at 150 ℃ (heating rate of 5 ℃ min-1) for 4 h under a reducing atmosphere (H2 : N2 10 : 90 vol%). The

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obtained support had a Pt loading of 15 wt% confirmed by inductively coupled plasma – optical emission spectroscopy analysis.

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2.4 Physical characterization of the catalysts

X-ray diffraction patterns were acquired using a Rigaku Ultima IV X-ray diffractometer system, which employs a Cu Kα X-ray source (λ = 0.15418 nm). X-ray photoelectron spectroscopy

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(XPS) was performed using a Thermo Scientific K-Alpha Angle-Resolved system equipped with a monochromatic Al Kα (1486.7 eV) X-ray source and a 180° double focusing hemispherical analyzer. Transmission electron microscopy (TEM) images of the TNT, TPY/TNTS-Mo support and the Pt/TPY/TNTS-Mo catalyst were acquired using a Zeiss Libra 200MC Transmission Electron Microscopy (TEM) system operating at 200 kV. Scanning Electron Microscopy (SEM)

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images were obtained using a Hitachi FlexSEM 1000 system equipped with an energy dispersive X-ray analyzer.

2.5 Electrochemical characterization of the catalysts The electrochemical assessment of catalysts was performed by depositing the samples ink onto the surface of a polished glassy carbon or gold rotating disk electrode (Pine Instruments). The method of preparing the inks fully described elsewhere [41]. Briefly, inks were prepared by

dispersing each support/catalyst in a solution containing ultrapure water and isopropanol alcohol (50-50 vol%), followed by adding Nafion® at an ionomer-to catalyst ratio of 0.22. After mixing, 4 µL of ink was deposited onto the surface of electrode (0.196 cm2) and the casted working (Glassy carbon or Gold) allowed to dry under blowing hot air (50 ℃) and rotation (300 rpm) for 10 minutes. This process is similar to that reported by Garsany and coworkers [48] and results in reproducible high-quality films as confirmed by the Pt/C activity on par with similar studies in the literature [49]. The ink-coated electrode served as the working electrode and was placed in a solution of 0.5

electrode. All potentials reported here were corrected to the RHE scale.

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M or 0.1M H2SO4 along with a Hg/HgSO4 reference electrode and a graphite rod as a counter

Electrochemical experiments were performed using either a Solartron 1470E multichannel

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potentiostat paired with Solartron 1260 frequency response analyzer or a Pine WaveDriver 20 bipotentiostat. Cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and

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accelerated stress tests (ASTs) were performed in N2-saturated 0.5 M H2SO4 solution. Impedance spectra were collected over a frequency range of 100 kHz to 0.1 Hz at a DC bias potential of 0.8

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and 0.425 VRHE for support and catalysts, respectively. These potentials were chosen to minimize effects from pseudocapacitance, enabling the EIS data to be analyzed using a form a transmission

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line model developed by Pickup and Easton group [50-51]. In terms of durability testing, EIS was used to determine the total catalyst layer resistance (R, where = Relectronic+ Rionic) and the lowfrequency limiting capacitance (Cdl). The manner in which these varying over the course of an

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AST enables diagnosis of the dominant degradation pathways [50, 52]. The ORR activity was assessed using linear sweep voltammetry (LSV) using a rotating disk electrode in O2-saturated 0.5 M H2SO4 solution at 1200 rpm.

The ex situ electrochemical stability of the supports and catalysts were evaluated using ASTs that involved repeated cycling of the working electrode at different potential range according to

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protocols developed by US Department of Energy [11, 53-54]. In this study, we used three different AST protocols:

(I): the stability of support was assessed using a triangular wave form between 0.05-1.35 VRHE

at a scan rate of 200 mV/s for 5,000 cycles. (II): the stability of catalyst under load cycling was assessed using a triangular wave form between 0.6-1 VRHE at a scan rate of 500 mV/s for 10,000 cycles.

(III): the stability of catalyst according to startup-shutdown was assessed using a triangular wave form between 1-1.5 VRHE at a scan rate of 500 mV/s for 5,000 cycles. These protocols exacerbate the corrosion of the support, the impact of catalyst degradation due to dissolution/Ostwald ripening, and the impact of support corrosion (under startup-shutdown potential) on Pt NPs agglomeration/dissolution and consequently catalyst deactivation rate. The catalyst condition was periodically monitored by CV and EIS during the ASTs and ORR activity of each catalyst was assessed before and after the ASTs. Moreover, in order to validate the obtained

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results, each test was repeated three times for each sample.

3. Results and discussion

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3.1. Materials Characterization

Detailed structural analysis was performed on Pt/TPY/TNTS-Mo and Pt/TNTS-Mo catalysts.

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Fig. 2 shows the XRD pattern obtained for TNTS-Mo, and TPY/TNTS-Mo supports and Pt/TNTSMo and Pt/ TPY/TNTS-Mo catalysts. The Mo-doped TNT (TNTS-Mo) exhibited prevailing Ti6O

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phase (ICDD card no. 01-072-1471, main characteristic reflection at 2θ: 39.5°, {111}) and Ti3O5 phase (ICDD card no. 01-072-2101) with Cmcm orthorhombic structure (main characteristic

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reflection at 2θ: 25.5°, {110}). Molybdenum exists as MoO2 (ICDD card no. 01-078-1073) and MoO3 (ICDD card 00-047-1320) in lattice structure of Ti suboxide. for TPY/TNTS-Mo supports after deposition of TPY over the surface of TNTS-Mo the main peaks referring to Ti6O and Ti3O5

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slightly shifted to higher degree. Platinum was exhibited in the expected face-centered cubic (fcc) structure. However, compared to the standard card of Pt (ICDD card 01-087-0640), all corresponding Pt Bragg reflections of Pt/TPY/TNTS-Mo and Pt/TNTS-Mo catalysts were shifted to higher angles. This was attributed to a decrease in the lattice parameter brought about by the strong electronic interaction between Pt and the TPY/TNTS-Mo and TNTS-Mo supports, which

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is commonly referred to as the SMSI [55-57]. Compared to Pt/TNTS-Mo the shift of corresponding Pt peaks for Pt/TPY/TNTS-Mo are slightly lower due to presence of TPY in surface of TNTS-Mo. The size of Pt NPs crystallites over the TPY/TNTS-Mo supports was determined from the width of the 81.55° {311} and 86.05° {222} peaks using the Scherrer–Debye equation, resulting in a mean Pt NPs crystallite size of 4 nm. Additional detailed characterization of TNT, TNTS-Mo support and Pt/TNTS-Mo catalyst is reported elsewhere [42].

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Fig. 2. X-ray diffraction pattern obtained for support and catalysts. (a) Pt/TNTS-Mo catalyst, (b) Pt/TPY/TNTS-Mo catalyst.

XPS is a powerful tool for the analysis of the composition and oxidation state of elements on the surface materials. In order to track the changes that may take place on the surface of the

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Pt/TPY/TNTS-Mo catalyst, the XPS analysis was examined before and after the AST (Protocol III) and the results are shown in Fig. 3. The major 2p1/2 and 2p3/2 peaks of Ti are located at the binding energy of 464.7 eV and 458.9 eV, respectively (Fig. 3a1). It is reported that pure TiO2 exhibited the double peaks of Ti 2p1/2 and 2p3/2 at binding energies of 464.2 eV and 458.5 eV, respectively. While the suboxide form of TiO2 (Magneli phase) has corresponding binding

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energies of 464.7 eV and 459.0 eV [58]. The shift in the Ti 2p levels of Ti3O5 suboxide compared to TiO2 is a result of the oxygen vacancies/defects in the lattice of TNTS-Mo support. We observe similar shift of the major Ti peaks here. Moreover, the presence of minor peaks at 456.5 and 462.2 eV, which are characteristic to Ti3+ 2p3/2 and 2p1/2, respectively, confirms the presence of oxygen vacancies at the surface [58-60]. In fact, oxygen vacancies are created when Mo is incorporated in the TiO2 lattice, resulting in a change in energy gap between the conduction and valence bands [61-63]. Fig. 3b1 shows a single N1s peak at 399.9 eV in freshly prepared Pt/TPY/TNTS-Mo

catalyst, which is characteristic to aromatic nitrogen [64]. Deconvolution of the Mo3d spectrum using a Powell peak-fitting algorithm yields peaks affiliated with Mo6+, Mo4+ and Mo0 (Fig. 3c1) in 3.3: 1.0: 1.3 ratio, which after stability test under Protocol III, the amount of Mo4+ slightly decreases and the ratio Mo6+: Mo4+: Mo0 converted to 3.5: 0.5: 1.5 (Fig. 3c2). Molybdenum was primarily found as surface oxide (MoOx), along with trace amounts of metallic Mo [65]. Pt displayed spin-orbit splitting peaks at the binding energies of 71.6 eV and 74.9 eV, referring to 4f7/2 and 4f5/2, respectively. These high-intensity peaks were attributed to metallic platinum (Pt0) (Fig. 3d1) [41, 66-67]. The observed binding energy of 71.6 eV for Pt0 4f7/2 reveals 0.6 eV positive

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shift towards higher binding energy compared to the 4f7/2 conventional value of Pt0 [68]. This shift to higher binding energy corresponds to induced positive charge on the deposited Pt NPs due to

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the strong banding with TPY/TNTS-Mo support which positively influenced the d-band state of Pt NPs [67, 69]. This may also be due to the small size of Pt NPs which have a greater content of

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atoms near the surface that have reduced in polarizability in the metal environment [70-71]. Also, the Pt0: Ti4+ ratio was determined using relative XPS sensitivity factors as determined by Wagner

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[72] to give 1.0: 1.7 before and 1.0: 1.9 after the treatment according the Protocol III, suggesting

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high stability of Pt NPs over the TPY/TNTS support.

Fig. 3. XPS spectra of the Pt/TPY/TNTS-Mo catalyst before and after stability test (Protocol III).

The morphology of the materials at all stages of synthesis were tracked by TEM. The TNT with diameters of about 10–15 nm and lengths 300-500 nm (Fig. 4a) were transformed into a ropelike structure of TNTS-Mo through Mo-doping and heat treatment process that was retained after surface functionalized with TPY (Fig. 4b). The addition of H2PtCl6 likely results in the formation of surface-anchored Pt-TPY complex. Upon the reduction of Pt, surface-anchored Pt-TPY complexes work as seeding spots for Pt NPs (Fig. 4c) [73]. The STEM-EDX elements mapping of the resulting Pt/TPY/TNTS-Mo material (Fig. 4d) shows the Pt NPs are well localized on the

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surface of support.

Fig. 4. TEM image corresponding to (a) the TNT, (b) the TPY/TNTS-Mo support, (c) Pt/TPY/TNTS-Mo catalyst, (d) HRTEM of Pt/TPY/TNTS-Mo catalyst, and (e) STEM-EDX elements mapping of Pt/TPY/TNTS-Mo catalyst.

3.2. Electrochemical Testing

3.2.1 Support stability AST Protocol (I): Support Stability Screening The stability of support is an important characteristic that has an enormous impact on the stability of catalyst active sites [74]. Thus, the durability of the metal oxide-based supports were first evaluated before platinization and compared to stability of commercial carbon Vulcan support. The stability of the TPY/TNTS-Mo, TNTS-Mo, and carbon Vulcan supports were assessed via AST protocol (I) which involved 5000 potential cycling in the range of 0.05-1.35

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VRHE. Fig. 5a illustrates the variation in the CV profile of the TNTS-Mo support. The CV curve shows reversible redox peaks between 0.05 to 0.75 VRHE at beginning of the test. These peaks are attributed to the faradaic process due to adsorption/desorption of H+ into MoOy lattice, resulting

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in the formation of hydrogen molybdenum bronzes. This well-known process by which MoOy absorbs hydrogen at room temperature [75-76] is shown in Eq. 1:

(Eq. 1)

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HxMoOy: xH++ xe−+ MoOy↔ HxMoOy.

Over the course of AST the peaks current density associated with hydrogen molybdenum

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bronzes were significantly decreased and process became more irreversible due to the formation of passive film and formation of MoOx(OH)y. Figs. 5b and 5c displays the impedance responses

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obtained from the TNTS-Mo support at various stages of AST, shown as Nyquist and capacitance plots, respectively. A short Warburg region was observed for TNTS-Mo which confirms the good electronic conductivity of this support [77-78]. The Warburg region slightly increased over the

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course of stability test assigned to negligible decrease in conductivity of TNTS-Mo support. The capacitance plot showed kind of decrease in limiting capacitance over the course of 5000 cycles due to formation of MoOx(OH)y on surface of the support, while the conductivity essentially remained unchanged.

Fig. 5d shows the variation in the CV profile of the TPY/TNTS-Mo support. The TPY/TNTS-

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Mo support showed high stability with almost no changes in the peaks associated with the adsorption/desorption of H+ onto MoOy (Fig 5d). The impedance responses are shown in Figures 5e and 5f as Nyquist and capacitance plots. Each of these show the expected shape for a fuel cell catalyst layer in the absence of oxygen in the double region. The length of the Warburg region in the Nyquist plots are indicative of total catalyst layer resistance, R. R can be calculated by projecting the real component of the Warburg length onto the real axis (Warburg length = R/3]. The short Warburg lengths here indicate high ionic and electronic conductivity in the layers.

Furthermore, since these remained unchanged over the course of the AST, indicating no change in either the electronic or ionic conductivity (Fig. 5e) [50-51]. Capacitance plots (Fig 5f) which plots the series capacitance (-1/Z”) against the Z’, are often more useful as they better visualization both resistive and capacitive features of the layer. These show a steep initial rise in capacitance through the high-to-mid frequency regions, which is indicative of the Warburg region. The capacitance levels off at limiting values in the low frequency regions, with the value at 0.1 Hz being take as Cdl [74]. The capacitance plots are virtually unchanged after 5000 cycles for

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TPY/TNTS-Mo support (Fig. 5d). Moreover, the TPY/TNTS-Mo support showed a considerably higher capacity compared to the TNTS-Mo support, which is consistent with the capacitive

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properties of N-containing surfaces [79-80]. The fact that the limiting capacitance was unchanged during the AST indicates that the TPY ligand is strongly bound to the surface.

Figure S2a shows the change in the CV obtained for Vulcan carbon over course of the stability

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test. A strong oxidation current was observed for carbon support, which upon cycling the quinone/hydroquinone redox couple (0.55–0.7 VRHE) increased significantly after 500 cycles. The

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carbon layer shows a short Warburg region, which remained stable throughout the stability test,

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while its limiting capacitance exhibited a ca. 21% decay after 5000 cycles (Figs. S2b and S2c).

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Fig. 5. Variation in the (a) CV response, (b) Nyquist plot and (c) capacitance plot obtained for the TNTS-Mo support subjected to AST protocol (I). Variation in the (d) CV response, (e) Nyquist plot and (f) capacitance plot obtained for the TPY/TNTS-Mo support subjected to AST protocol (I). CVs were recorded in N2-purged 0.5 M H2SO4 at 25 ℃ at a scan rate of 50 mV s-1. EIS measurements were made at a DC bias potential of 0.8 VRHE.

3.2.2 Electrochemical characterization of catalysts

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Figure 6a compares the CVs obtained for Pt/TPY/TNTS-Mo, Pt/TNTS-Mo and Pt/C catalysts.

All catalysts exhibit the classic CV pattern of polycrystalline Pt, where H+ ad/desorption dominate the potential below 0.35 VRHE and Pt surface oxide formation and stripping features are located at potential above 0.5 VRHE. An earlier onset of oxide reduction on the reductive scan was observed for Pt/TPY/TNTS-Mo compared to both Pt/TNTS-Mo and Pt/C. Likewise, the formation of oxide on Pt/TPY/TNTS-Mo appeared at more positive potentials on the anodic sweep. The ECSA of each catalyst was specified by integrating the charge associated with HUPD (210 µC cm-2Pt) [81].

Table 1 lists the ECSA measured for each catalyst, along with a summary of their key electrochemical properties. The comparable and high ECSAH2 value of Pt/TPY/TNTS-Mo confirms that Pt NPs are homogeneously dispersed over the TPY/TNTS-Mo support and Pt active sites are highly accessible [82]. ORR activity of Pt/TPY/TNTS-Mo, Pt/TNTS-Mo and Pt/C catalysts presented in Fig. 6b. Both Pt/TPY/TNTS-Mo and Pt/TNTS-Mo catalysts exhibited high onset potential of 0.98 VRHE for O2 reduction with excellent ORR activity, producing 1.20 and 1.05 mA cm-2 at 0.9 VRHE, respectively. These activities compare favorably with metal oxide supported

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catalysts in the literature [44, 57, 69, 83-84] and are considerably higher than that reported for Pt/C (see Table S1 for ORR activities of Pt/C catalysts reported in the literature). On the other hand, the Pt/C catalyst exhibited onset potential of 0.97 VRHE and current density 0.68 mA cm-2. The

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enhanced activity of the metal-oxide supported catalysts over the carbon supported catalyst is attributed to the SMSI present in both metal oxide-based supports [68, 85-87]. Furthermore, the

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difference in activity between the Pt/TPY/TNTS-Mo and Pt/TNTS-Mo catalysts can only be due to the interaction between the Pt NPs and the N-containing ligands on the surface. The fact that

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activity was further enhanced when TPY was present indicates that the interaction between the N-

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based ligand and the Pt NPs is acting in concert with the SMSI. Table 1. Summary of the key electrochemical properties of the catalysts. Avg Pt particle size (nm) 4

wt.% Pt

Pt/TNTS-Mo Pt/C

Catalyst

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ECSA [m2 g-1] 76.4 ± 2.6

I @ 0.9 [mA cm-2] 1.20

im @ 0.9 [mA mgPt-1] 120

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15

0.010

69.7 ± 3.4

1.05

105

3

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0.012

81.2 ± 2.4

0.68

56.7

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Pt/TPY/TNTS-Mo

Pt loading [mg cm-2] 0.010

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Fig. 6. (a) Comparison of the initial CVs (20 mV s-1) obtained for each catalyst in N2-purged 0.5 M H2SO4 at 25 ℃. (b) Comparison of the initial ORR activity of each catalyst recorded. Measurements were made in O2-saturated 0.5 M H2SO4 at 25 ℃ at a scan rate of 5 mV s-1, using a rotation rate of 1200 rpm.

3.2.3 Catalyst stability In this study, we evaluated catalyst stability by using two different AST protocols. The CV and EIS response were assessed before and after the AST, as was the ORR activity. AST Protocol (II): Load Cycle Testing

Under stability protocol (II), all catalysts were exposed to a stress test that mimics fuel cell load cycling, which involved using a triangular wave form between 0.6 VRHE (close to the maximum power; approaching full load condition under fuel cell) and 1.0 VRHE (close to the open circuit voltage; approaching no-load condition under fuel cell) at scan rate of 500 mV/s for 10,000 cycles. This US Department of Energy (DoE) based protocol is commonly used to evaluate the stability of Pt NPs against dissolution/agglomeration [11, 55, 57]. The impact of this stability protocol on catalysts, shows the significance of SMSI effect on stabilization of Pt NPs over the

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supporting materials by avoiding involvement of support corrosion in catalyst deactivation mechanism.

The CV response for the Pt/TPY/TNTS-Mo catalyst before and after AST protocol (II) is

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shown in Fig. 7a. The CV shapes were virtually unchanged, with only a 2.8% decay in ECSA. The EIS responses (Figs. 7b and 7c) were practically identical before and after the AST, which means

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that the electronic and ionic conductivities of catalyst layers remained constant throughout the test for Pt/TPY/TNTS-Mo catalyst. Furthermore, there was only a miniscule change in ORR activity,

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with only a 4.2% decay in specific catalytic activity (SCA) (Fig. 7d).

The CV response obtained for Pt/TNTS-Mo before and after AST protocol (II) is shown in

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Fig. 7e, which shows a slight change in features and a 4.3% decay in ECSA. The EIS responses (Fig. 7f, 7g) showed only minor change over the course of the AST and slight decay in ORR activity was observed after the AST (Fig. 7h), amounting to a 14.4% decline in SCA.

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The CV response obtained for Pt/C before and after AST protocol (II) is shown in Fig. 7i, which showed a more prominent (18.6%) decay in ECSA compared to the other catalysts. The EIS response (Fig. 7j and Fig 7k) changed slightly over the course of the test. The Nyquist plot shows a slight increase in resistance of catalyst layer and its limiting capacitance exhibited decreased which is the EIS signature of Pt NP dissolution/agglomeration degradation pathway [87]. The ORR

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activity of Pt/C (Fig 7l) exhibited a significant decline after the AST that is attributed to loss Pt active sites due to Pt NPs agglomeration/dissolution (Fig. 7l).

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Fig. 7. Variation in the CVs, Nyquist, capacitance plots obtained for each catalyst before and after the AST (Protocol II). CVs were obtained in N2-purged 0.1 M H2SO4 at 25 ℃ at a scan rate of 50 mV s-1. EIS measurements were made at a DC bias potential of 0.425 VRHE in N2-purged 0.1 M H2SO4 at 25 ℃. ORR activity was measure in O2-saturated 0.5 M H2SO4 at 25 ℃ at a scan rate of 5 mV s-1, using a rotation rate of 1200 rpm.

A summary of the changes in key electrochemical properties of each catalyst during the AST

protocol (II) durability test are reported in (Fig. 8). It is clear that the presence of TPY on the support surface has a small but beneficial influence on catalyst stability (Fig. 8a). The detail EIS response over the course of stability test showed stable trend in Cdl for Pt/TPY/TNTS-Mo and Pt/TNTS-Mo catalysts, while a slight decline in Cdl was observed for Pt/C catalyst due to the decrease in Pt surface area (Fig. 8b). Likewise, R (Fig. 8c) of Pt/TPY/TNTS-Mo and Pt/TNTS-

Mo catalysts, exhibited slight decrease due to better wettability of catalyst layer with progress of the test and remained stable throughout the test, while increase trend was observed for R of Pt/C. This EIS profile is consistent with the expected profile for Pt particle size growth in the absence of support corrosion [87]. The US DoE targets state that electrocatalysts shouldn’t loss more than 40% of their initial activity after testing under load cycling protocol (0.6 - 0.95 VRHE) [88-92]. Pt/TPY/TNTS-Mo, Pt/TNTS-Mo and Pt/C showed declines of 4.2%, 14.4% and 38.3% of their initial ORR activity

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(i900), respectively. This results indicating Pt NPs are strongly anchored onto the TPY/TNTS-Mo support via metal-ligand coordinative interactions and SMSI effect. The Pt/TNTS-Mo was less

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stable than Pt/ TPY/TNTS-Mo, losing 14.4% of its initial activity. This is still decent stability, owing to the SMSI between Pt NPs and the TNTS-Mo support. Pt/C was the least durable, with 38.35% decline in activity after this AST. Moreover, the half-wave potential (E1/2) was measured

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for all electrocatalysts before and after the AST (Fig. 8a). Both Pt/TPY/TNTS-Mo, Pt/TNTS-Mo showing no change in E1/2 after stability test, while E1/2 for Pt/C catalyst exhibited decay from 0.84

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VRHE to 0.80 VRHE.

To put this in context, we have compared our results with those reported for other metal oxide-

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containing catalysts reported in the literature tested by a similar load cycling protocol. Schmies et al. [57] reported an 8% decline in ECSA and 54% decay in ORR mass activity Pt supported on indium tin oxide (Pt/ITO) that was load cycled for 5000 cycles. Song et al. [55] reported an ECSA

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loss of 7.5% and an ORR mass activity loss of 9% after 10,000 cycles for their best TaOx-Pt/C load cycling stability test. Similarly, Gao et al. [49] reported a 4% decline in ECSA for a Pt-Ta2O5CNT catalyst cycled for 10,000 cycles (they did not report the ORR activity decline). Based on this, we can conclude that Pt/TPY/TNTS-Mo is one of the most stable catalysts towards load

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cycling reported in the literature to date.

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AST Protocol (III:) Startup/Shutdown Cycle Testing

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Fig. 8. Variation in (a) ECSA, ORR mass activity (im) and half-wave potential (E1/2) measured for each catalyst before and after load cycling AST (Protocol II). Variation in (b) Cdl and (c) R with cycle number for each catalyst layer during the AST (Protocol II).

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In stability protocol (III), all catalysts were exposed to a higher potential stress test that mimics cell startup/shutdown conditions, which employed a triangular wave form between 1-1.5 VRHE at

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a scan rate of 500 mV/s for 5,000 cycles. Using this protocol, the stability of TPY/TNTS-Mo, TNTS-Mo and carbon supports against corrosion and stability of Pt NPs over the supports against dissolution/agglomeration were assessed.

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The CV response for the Pt/TPY/TNTS-Mo catalyst before and after AST protocol (III) is shown in Fig. 9a, where a 12.9% decay in ECSA was observed. The EIS response (Fig. 9b and Fig. 9c) remained stable over the course of the AST, indicating that no significant change to the support surface occurred. Moreover, the ORR activity showed only a 21% decline in SCA after the completion of the AST (Figs. 9d).

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The CV response for the Pt/TNTS-Mo catalyst before and after AST protocol (III) is shown in

Fig. 9e. A significant decay (47.7%) in ECSA was observed for Pt/TNTS-Mo over the course of AST. EIS (Fig. 9f, 9g) revealed a slight increase in resistance of catalyst layer and a small decline in limiting capacitance, consistent with significant agglomeration/dissolution of Pt NPs. This decay lead to a decline in ORR activity (Fig. 9h), with a 47.2% reduction in ORR activity. The CV response for Pt/C catalyst before and after AST protocol (III) is shown in Fig. 9i. Much like the Pt/TNTS-Mo, a substantial decline in ECSA was observed. EIS analysis (Fig. 9j,

9k) revealed that the Warburg length increased over the course of the AST, while the limiting capacitance decreased substantially from its initial value. As expected, this kind of decay resulted

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in a significant decline in ORR activity (Fig. 9i-9l), with a 76.4% decline in SCA observed.

Fig. 9. Variation in the CVs, Nyquist, capacitance plots obtained for each catalyst before and after the AST (Protocol III). CVs were obtained in N2-purged 0.1 M H2SO4 at 25 ℃ at a scan rate of 50 mV s-1. EIS measurements were made at a DC bias potential of 0.425 VRHE in N2purged 0.1 M H2SO4 at 25 ℃. ORR activity was measure in O2-saturated 0.5 M H2SO4 at 25 ℃ at a scan rate of 5 mV s-1, using a rotation rate of 1200 rpm.

A summary of the changes in key electrochemical properties of each catalyst during the AST protocol (III) durability test are reported in (Fig. 10). From this test it is clear that the

Pt/TPY/TNTS-Mo has far superior durability. Furthermore, the margin of superiority over the Pt/TNTS-Mo catalyst is considerably larger in this test, indicating that the presence of TPY has a much larger influence on catalyst stability at higher potentials. The detail EIS results indicate minimal fluctuation in both R and Cdl of Pt/TPY/TNTS-Mo catalyst over the 5,000 cycles, indicating that the high corrosion-resistance of this support is maintained at the higher potential. The Pt/TNTS-Mo exhibited a decay in Cdl over the course of stability test, attributed to Pt NPs sintering/agglomeration, while Rinitially decrease (due to

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improving wetting of the catalyst layer) after which it remained stable. Over the course of stability, the Pt/C catalyst shows different trend, with its Cdl initially increased over the first 1000 cycles,

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which was attributed to the oxidative formation of pseudo-capacitive groups on the carbon surface [55]. Continued cycling of Pt/C resulted in a decline Cdl. Furthermore, EIS revealed that for Pt/C R increased steadily throughout the AST (Fig. 10 b and c). This EIS response is characteristic of

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carbon support corrosion [51].

The most stable catalyst reported here was Pt/TPY/TNTS-Mo, which showed a 12.9% decline

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in ECSA after 5000 cycles under startup/shutdown conditions. This is considerably less than the 35% and 25.5% ECSA decays reported for Pt/ITO [57] and TaOx-Pt/C [55], respectively.

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Furthermore, Pt/TPY/TNTS-Mo showed slight lower ECSA losses compared to that reported by He et al. for Pt/Nb-TiO2 [43]. These finding indicate the enhanced stability and durability of

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Pt/TPY/TNTS-Mo catalyst against support corrosion and Pt NPs agglomeration.

Fig. 10. Variation in (a) ECSA, ORR mass activity (im) and half-wave potential (E1/2) measured for each catalyst before and after the startup/shutdown AST (Protocol III). Variation in (b) C dl and (c) R with cycle number for each catalyst layer during the AST (Protocol III).

In order to investigate this further, XPS and SEM-EDX post analysis were performed on Pt/TPY/TNTS-Mo catalysts after the AST protocol (III). The XPS post-analysis shows there is no changes on chemical states of Ti (Fig. 3a2). The N1s peak shifts to 401.2 eV which this value is characteristic to N+ and also might be related to Pt to N+ charge transfer that has been previously observed for Pt NPs stabilized by a poly(diallyldimethylammonium chloride), a polymer consisting of positively charged nitrogen atoms (Fig. 3b2) [33]. Interestingly, Mo characterization after the stability test reveal the amount of Mo4+ decreased while the portion of Mo0 increased. This phenomenon may be attributed to the electrochemical surface cleaning of the supporting

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material during the stability test which Mo4+ undergoes reduction (Fig. 3c2). Also, no changes on chemical states of Pt and no trace of Pt oxide Pt2+ or Pt4+ formation observed after post-analysis

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on Pt/TPY/TNTS-Mo catalyst (Fig. 3d2). This is consistent with the results from SEM-EDX

remained stable over the course of the AST.

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Pt

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250 μm

Ti

Mo

Pt

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(b)

Mo

Ti

(a)

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analysis of the Pt/TPY/TNTS-Mo (Figure 11), confirming that the catalyst and catalyst support

250 μm

Fig. 11. SEM-EDX analysis of Pt/TPY/TNTS-Mo. (a) fresh (b) after stability test (startupshutdown protocol). Based on this and the electrochemical tests, the presence of TPY on the surface enhanced

stability by two main modes. First, the presence of the covalently attached layer stabilized the

doped metal oxide supports itself, preventing changes in its surface state and composition at high potentials. Second, the nitrogen functional groups provide “anchor points” for the Pt nanoparticles that enable a SMSI that prevent particle size growth over a wide range of potentials. Under both AST protocols the obtained results for Pt/TPY/TNTS-Mo meet the DOE targets regarding the electrocatalyst activity and stability [88-92].

4. Conclusion

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In this study, we have demonstrated a very unique case of rational design of an ORR catalyst with enhanced performance and durability, which was achieved by a surface modification that that stabilized both the support and the catalytically active Pt NPs. The catalyst was endowed with

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strong metal-ligand coordinative interactions between Pt metal centers and TPY moieties, which itself is chemically embedded onto the surface of a metal oxide support. We have demonstrated

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that TNTS-Mo support modified by TPY has enhanced ORR activity, with exceptional stability and durability. This remarkable performance of the Pt/TPY/TNTS-Mo catalyst results from the

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ability of TPY/TNTS-Mo support to hinder the segregation of Pt NPs, which indicates that the Pt NPs are strongly anchored onto the TPY/TNTS-Mo support by TPY. This anchoring effect, along

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with the SMSI with the metal oxide support, work in combination to significantly enhance both the activity and durability of the Pt catalyst. Furthermore, the stability protocols that were employed in this study, the load cycling and the startup-shutdown, indicated that this strategy was

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successful over the broad range of potential transients that are observed during fuel cell full-load and startup and shutdown. The developed TPY/TNTS-Mo support material provides multiple advantages and superior characteristics under stability protocols.

5. Acknowledgements

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This work was supported by the Natural Sciences and Engineering Research Council of Canada

(NSERC) through the Discovery Grants Program (RGPIN-2015-003652, RGPIN-2016-05823) of Canada and Ontario Tech University. The authors acknowledge equipment support from the Canada Foundation for Innovation. We thank Dr Lei Zhang (University of Waterloo, WATLab) for acquiring the TEM images

List of Figure Captions Fig. 1. Synthesis procedure of TPY/TNTS-Mo support and Pt/TPY/TNTS-Mo catalyst. Fig. 2. Fig. 2. X-ray diffraction pattern obtained for support and catalysts. (a) Pt/TNTS-Mo catalyst, (b) Pt/TPY/TNTS-Mo catalyst.

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Fig. 3. XPS spectra of the Pt/TPY/TNTS-Mo catalyst before and after stability test (Protocol III). Fig. 4. TEM image corresponding to (a) the TNT, (b) the TPY/TNTS-Mo support, (c)

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Pt/TPY/TNTS-Mo catalyst, (d) HRTEM of Pt/TPY/TNTS-Mo catalyst, and (e) STEM-EDX

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elements mapping of Pt/TPY/TNTS-Mo catalyst.

Fig. 5. Variation in the (a) CV response, (b) Nyquist plot and (c) capacitance plot obtained for the

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TNTS-Mo support subjected to AST protocol (I). Variation in the (d) CV response, (e) Nyquist plot and (f) capacitance plot obtained for the TPY/TNTS-Mo support subjected to AST protocol

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(I). CVs were recorded in N2-purged 0.5 M H2SO4 at 25 ℃ at a scan rate of 50 mV s-1. EIS

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measurements were made at a DC bias potential of 0.8 VRHE. Fig. 6. (a) Comparison of the initial CVs (20 mV s-1) obtained for each catalyst in N2-purged 0.5 M H2SO4 at 25 ℃. (b) Comparison of the initial ORR activity of each catalyst recorded. Measurements were made in O2-saturated 0.5 M H2SO4 at 25 ℃ at a scan rate of 5 mV s-1, using a

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rotation rate of 1200 rpm.

Fig. 7. Variation in the CVs, Nyquist, capacitance plots obtained for each catalyst before and after the AST (Protocol II). CVs were obtained in N2-purged 0.1 M H2SO4 at 25 ℃ at a scan rate of 50 mV s-1. EIS measurements were made at a DC bias potential of 0.425 VRHE in N2-purged 0.1 M H2SO4 at 25 ℃. ORR activity was measure in O2-saturated 0.5 M H2SO4 at 25 ℃ at a scan rate of

5 mV s-1, using a rotation rate of 1200 rpm.Fig. 8. (a) ECSA and ORR mass activity (im) before and after stability test via Protocol (II), (b and c) R and Cdl of catalysts obtained at DC bias potential of 0.425 VRHE. Fig. 8. Variation in (a) ECSA, ORR mass activity (im) and half-wave potential (E1/2) measured for each catalyst before and after load cycling AST (Protocol II). Variation in (b) Cdl and (c) R with

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cycle number for each catalyst layer during the AST (Protocol II).

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Fig. 9. Variation in the CVs, Nyquist, capacitance plots obtained for each catalyst before and after the AST (Protocol III). CVs were obtained in N2-purged 0.1 M H2SO4 at 25 ℃ at a scan rate of 50

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mV s-1. EIS measurements were made at a DC bias potential of 0.425 VRHE in N2-purged 0.1 M

5 mV s-1, using a rotation rate of 1200 rpm.

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H2SO4 at 25 ℃. ORR activity was measure in O2-saturated 0.5 M H2SO4 at 25 ℃ at a scan rate of

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Fig. 10. Variation in (a) ECSA, ORR mass activity (im) and half-wave potential (E1/2) measured for each catalyst before and after the startup/shutdown AST (Protocol III). Variation in (b) Cdl and

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(c) R with cycle number for each catalyst layer during the AST (Protocol III). Fig. 11. SEM-EDX analysis of Pt/TPY/TNTS-Mo. (a) fresh (b) after stability test (startupshutdown protocol).

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List of Table Captions

Table 1. Summary of the key electrochemical properties of the catalysts.

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