Advanced Nanoelectrocatalyst of Pt Nanoparticles Supported on

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Advanced nanoelectrocatalyst of Pt nanoparticles supported on robust Ti0.7Ir0.3O2 as a promising catalyst for fuel cells Tai Thien Huynh, Hau Quoc Pham, At Van Nguyen, Long Giang Bach, and Van Thi Thanh Ho Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b05486 • Publication Date (Web): 12 Dec 2018 Downloaded from http://pubs.acs.org on December 15, 2018

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Industrial & Engineering Chemistry Research

Advanced nanoelectrocatalyst of Pt nanoparticles supported on robust Ti0.7Ir0.3O2 as a promising catalyst for fuel cells Tai Thien Huynh1,2, Hau Quoc Pham2,3, At Van Nguyen2, Long Giang Bach3, Van Thi Thanh Ho*1 1

Hochiminh City University of Natural Resources and Environment (HCMUNRE), Vietnam 2

3

Ho Chi Minh City University of Technology (HCMUT), Vietnam

NTT Hi-Tech Institute, Nguyen Tat Thanh University, Ho Chi Minh City, Vietnam

*Corresponding author: [email protected]

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

Abstract Developing robust catalysts is still challenging for further commercialization of lowtemperature fuel cell technologies. In this study, we introduce Platinum nanocatalyst loading on Ti0.7Ir0.3O2 to assemble a robust Pt/Ti0.7Ir0.3O2 catalyst toward methanol oxidation reaction (MOR) in direct methanol fuel cell systems. These observational results demonstrated the Pt/Ti0.7Ir0.3O2 catalyst is a promising anodic electrocatalyst due to the superior electrocatalytic activity and durability towards methanol electrooxidation reaction (MOR), which exhibited from onset potential ~0.1 V, high current density ~21.69 mA/cm2, versus that the state-of-the-art Pt/C (E-TEK) catalyst. We also demonstrated that the electronic transfer from Ti0.7Ir0.3O2 catalyst support to Pt NPs resulting in the modified surface electronic structure of Pt NPs, which could interpret for the high activity and durability of the Pt/Ti0.7Ir0.3O2 catalyst. In addition to this approach could supply a promising potential catalyst for fuel cell technology and other applications such as solar cells, biosensors, and water splitting.

Keywords: fuel cells, Ti0.7Ir0.3O2, Iridium doped TiO2, non-carbon catalyst supports. *E-mail: [email protected], Tel: +84-283-9916416, Fax: +84-283-9916416


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1. INTRODUCTION The low-temperature fuel cell systems have been gaining drastically attention because of its high energy production efficiency and low greenhouse gas emissions1-3, which can solve to the serious consequences of burning fossil fuel. In fuel cell technology, electrocatalyst plays an important role at anode and cathode which directly impacts to the fuel cell performance. Notwithstanding, the currently carbon-supported Platinum catalyst, which was widely utilized in fuel cell technologies, has still faced some restriction namely the poor durability due to the corroded carbon leading to the sintering/detachment and agglomeration of Pt nanocatalysts4-7; the sluggish kinetics of anodic oxidation8 and oxygen reduction reaction (ORR)9-11, the CO poisoning of active sites of Platinum catalyst at even low CO concentration (< 5 ppm)12,

13

causing the

significant performance deterioration in long-terms operating coditions of fuel cells. Up to now, developing robust electrocatalysts is still challenging for further commercialization of fuel cell technology. One of the most efficient approaches to solving these problems is to use non-carbon materials, which have been emerged as promising alternative catalyst support for carbon materials due to the high corrosion resistance in electrochemical media of fuel cells and thus enhancing the electrocatalytic activity and the stability of Pt-based catalysts14-16. Among metal oxides, Titanium dioxide (TiO2) material has been gained considerable attention in fuel cell application owing to non-toxic, affordability and superior electrochemical stability17,

18

. Furthermore, the strong metal-support interaction (so-called “SMSI”)

between TiO2 support and Pt nanocatalyst resulting in the enhancement of electrocatalytic activity and durability of this electrocatalyst, which was proven in the previous studies19,

20

. Notwithstanding, the intrinsic low electrical conductivity of

Titanium dioxide (TiO2) was a major hinder have to solve for its further application in fuel cell technologies. To tackle this issue, doping strategy with metal cation into Titanium dioxide material was known as one of the most effective ways for improving both the electronic conductivity of TiO2-based materials and electrochemical activity of the Pt-based electrocatalyst.18, 21-24



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To be the best of our knowledge, there is a limited number of researches on designing Iridium-modified Titanium dioxide materials as a catalyst support for fuel cell application. In this work, we introduce the combination between Platinum nanocatalyst and Iridium-modified Titanium dioxide, which were successfully synthesized

by

means

of

one-pot

hydrothermal

synthesis

without

using

surfactants/stabilizers or further heat treatment in our previous studies25, 26, to assemble robust Pt/Ti0.7Ir0.3O2 catalyst toward methanol oxidation reaction (MOR) in direct methanol fuel cells (DMFCs). These experimental outcomes demonstrated that the Iridium-modified Titanium dioxide supported Platinum catalyst is a promising anodic electrocatalyst for DMFCs. For instance, an electrochemical surface area (ECSA) was found to be around 96.98 m2/gPt, which is ~1.1-fold higher than the commercial carbon-supported Platinum catalyst. For the methanol oxidation reaction (MOR), the Pt/Ti0.7Ir0.3O2 catalyst possessed the onset potential was negatively shifted about 50 mV, high current density (~ 21.69 mA/cm2) and high If/Ib ratio (~1.72) versus that the commercial Pt/C (E-TEK) catalysts. We also demonstrated that the electronic transmission from Ti0.7Ir0.3O2 catalyst support to Pt NPs, which could interpret for these aforementioned enhancements of the robust Pt/Ti0.7Ir0.3O2 catalyst. 2. EXPERIMENT DETAILS 2.1. Chemical All reagents were commercially procured and used without further purification. Iridium trichloride hydrate (IrCl3.xH2O, 99.9%, 52 wt% Iridium), Ethylene glycol (EG, 99.8%) and Sodium borohydride (NaBH4, 98%), as well as Chloroplatinic acid hydrate (H2PtCl6.xH2O), were obtained from Sigma-Aldrich. Titanium tetrachloride (TiCl4, 99.5 %) and Hydrochloric acid (HCl, 37%) were purchased from Aladdin and Merck, respectively. Purified water was used as solvent throughout the experiments. 2.2. Synthesis of Ti0.7Ir0.3O2 nanoparticles In a typical experiment, the single-step hydrothermal method was performed to synthesize Ti0.7Ir0.3O2 nanoparticles at pH = 1 and 210oC for 8 hours without utilizing surfactants or stabilizers as described in our previous work.25, 26 2.3. Synthesis of the 20 wt. % Pt/Ti0.7Ir0.3O2 catalysts 4 ACS Paragon Plus Environment

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The Platinum nano-forms were immobilized over the Ti0.7Ir0.3O2 through the modified chemical reduction route at the room temperature without utilizing surfactant or stabilizers. First, 3 mL of H2PtCl6 was added into a mixture consist of 0.5 mL EG and 25 mL H2O under stirring 5 minutes at room temperature. Next, a pH value of this solution was adapted to 11 via a NaOH solution and then 110 mg Ti0.7Ir0.3O2 powder was ultrasonically dispersed into the obtained solution until homogenous suspension. Following that, NaBH4 acting as a reducing agent was added drop-wise into the asprepared suspension combined with vigorous stirring at ambient temperature for 2 hours. Finally, the obtained Pt/Ti0.7Ir0.3O2 catalyst was copiously rinsed with purified water and then dried at 80oC in the oven overnight. (Scheme 1)

Scheme 1. Procedure to assemble the 20 wt. % Pt/Ti0.7Ir0.3O2 electrocatalyst 2.4. Material Characterization The structure information of these synthesized specimens was recorded by means of X-ray diffraction (XRD) analysis operated on D2 PHASER–Brucker (Germany) using Cu Kα radiation at an electric potential of 30 kV. The morphology and particle size of Ti0.7Ir0.3O2 and Pt nano-forms loading on the support was recorded by means of TEM and HR-TEM images. To examine the elemental composition in the samples, the Xray fluorescence (XRF) measurements were implemented. The electronic conductivity of catalyst support was measured by the standard four-point technique on the MWP-6 instrument (Jandel, British). To perform this test, specimens were compressed under a pressure of around 300 MPa to make into a pellet with 10 mm in a diameter and 5 ACS Paragon Plus Environment


thickness about 1 mm. The Brunauer– Emmett–Teller (BET) measurement was performed on a NOVA 1000e device to examine the specific surface area of support nanoparticles. Before BET test, specimens were dried/degassed at 250oC for 3 hours to completely remove molecules adsorbed in the oxygen-containing oxides. Next, the Xray photoelectron spectra (XPS) of specimens were recorded on PHI 5000 VersaProbe (Ulvac-PHI) instrument. 2.5. Electrode Preparation and Electrochemical Measurements The electrochemical properties of this electrocatalyst were investigated on EC-LAB Electrochemistry instrument (Bio-Logic SAS) with a three-electrode comprising of a silver chloride electrode (Ag/AgCl), and 5 mm glassy carbon disk as well as Pt gauze with respect to the reference electrode, working electrode, and the counter electrode. All potential windows in this work were reported with a normal hydrogen electrode (NHE) scale. The catalyst ink preparation as followed: the catalyst powder was ultrasonicated in a solution comprising 0.5 % Nafion and ethanol absolute for 30 min. Before coating catalyst, the surface of the glassy carbon disk was processed with 0.5 µm BAS and then copiously rinsed with with ethanol and purified water. Prior to each test, the catalyst electrode was activated with 50 cycles at a sweep rate of 50 mV/s. The the cyclic voltammogram in nitrogen-purged 0.5 M H2SO4 aqueous solutions with a sweep rate of 25 mV/s to calculate the ECSA value of these catalysts. Furthermore, the methanol electro-oxidation activity of the catalyst was recorded in nitrogen-purged 10 v/v% CH3OH/0.5 M H2SO4 aqueous solution at a sweep rate of 25 mV/s. The durability of these catalysts was investigated via the chronoamperometry method in nitrogen-purged 10 v/v% CH3OH/0.5 M H2SO4 aqueous solution at an immobilized potential of 0.7 V for 1 hour. Furthermore, the oxygen reduction activity of these catalysts was tested at the potential window of 0 – 1.1 V in oxygen-purged 0.5 M H2SO4 aqueous solution at 25oC at a rotation rate of 1600 rpm at a sweep rate of 1 mV/s. 3. RESULTS AND DISCUSSION 3.1. Properties of Ti0.7Ir0.3O2 nanoparticles To explore the lattice structure information of the as-prepared Iridium-modified Titanium dioxide support, the X-ray diffraction (XRD) analysis was implemented. As 6 ACS Paragon Plus Environment

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shown in the XRD profile (Figure 1 (a)), the structure of the as-synthesized Ti0.7Ir0.3O2 NPs were found to be a mixture of brookite-TiO2 structure (JCPDS 291360) and anatase-TiO2 structure (JCPDS 84-1286). Additionally, no typically diffraction peaks of IrO2 (JCPDS 15-0870) located at 2positions 28.05o; 34.71o and 40.06o corresponding to the (110); (101) and (200) facets or the segregated diffraction peaks of Iridium and Titanium dioxide were detected. This suggested that the Iridium successfully incorporated into TiO2 lattices and thus the formation of a solid solution with a mixture of anatase structure and brookite structure. Figure 1 (b, c) shown that the morphology of this catalyst support consisted of sphere shape with 20-25 nm in the diameter and the rod-like architecture with the length about 45 nm.

Figure 1. (a) XRD profile and (b, c) TEM images of Ti0.7Ir0.3O2 NPs The characterization of Ti0.7Ir0.3O2 NPs was further recorded by means of the X-ray photoelectron spectroscopy (XPS) analysis, which was shown in Figure 2 (a). The Ti 2p1/2 and Ti 2p3/2 peaks of Ti 2p spectra (see Figure 2 (b)) were observed around 464.54 eV and 458.72 eV, respectively, which was positively shifted compared to that of the Ti 2p peaks of Titanium dioxide material (Ti 2p1/2 at 464.0 eV and 458.4 eV for Ti 2p3/2)27. Furthermore, Figure 2 (c) displayed the Ir 4f5/2 and Ir 4f7/2 peaks of Ir 4f spectra were detected about 63.22 eV and 60.33 eV, respectively; which negatively shifted versus that the two binding states of Iridium in the IrO2 single crystal (64.7 eV 7 ACS Paragon Plus Environment


for Ir 4f5/2 and Ir 4f7/2 at 71.7 eV)28,

29

. These results indicated that the Iridium

successfully incorporated into Titanium dioxide (TiO2) lattices.

Figure 2. (a) XPS spectroscopy of Ti0.7Ir0.3O2 NPs; (b) XPS spectroscopy of Ti 2p and (c) XPS spectroscopy of Ir 4f in Ti0.7Ir0.3O2 nanoparticles The X-ray fluorescence and Energy-dispersive X-ray measurements were utilized for measuring the elemental composition of Ti0.7Ir0.3O2 NPs. As results are shown in Figure 3 (a, b) indicated that the elemental composition of Ti and Ir was closely agreed with the desired ratio (Ti: Ir = 70: 30). Furthermore, the distribution of Ti and Ir element was found to be relatively uniform via an elemental mapping analysis (see Figure 3 (c-e)). These suggested that the elemental composition of binary metallic oxide could be effortlessly controlled by adjusting the starting precursors.


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Figure 3. (a) XRF spectroscopy; (b) EDX spectroscopy and (c-e) elemental mapping of Ti, Ir, O of Ti0.7Ir0.3O2 nanoparticles Furthermore, the surface area of the as-prepared Ti0.7Ir0.3O2 NPs was investigated by means of a Brunauer–Emmett–Teller (BET) measurement. As can been seen in Figure 4 (a), the as-synthesized Ti0.7Ir0.3O2 NPs possessed the moderate surface area (98.03 m2/g), which larger than that of non-carbon support namely W-doped TiO2 30, ITO

31

and Ti0.7Nb0.3O232 as well as Ti0.7Ru0.3O233 along with Ti0.7Ta0.3O2

34 .

The high

surface area of Ti0.7Ir0.3O2 versus that other non-carbon supports attributable to the facile and simple hydrothermal route, one-step utilizing inorganic precursors, which could avoid the influence of grafted organic macromolecules

35

, as well as without

using the surfactant/stabilizers or further heat treatments. These outcomes indicated that the Ti0.7Ir0.3O2 NPs was a promising support for the deposition of Pt nanocatalysts.



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Figure 4. Comparison (a) surface area and (b) electronic conductivity between the Ti0.7Ir0.3O2 and other non-carbon materials Besides the surface area, the electronic conductivity was considered as one of the main requirements of support for Pt-based electrocatalysts which facilitate for electron transportation during the electrochemical reaction. The standard four-point probe technique was performed for measuring the electronic conductivity of the as-prepared Ti0.7Ir0.3O2 support. As experimental results indicated that the Ti0.7Ir0.3O2 NPs possessed the high electronic conductivity (0.03 S/cm) in comparison with other noncarbon supports namely Ti0.7W0.3O2 Ti0.7Mo0.3O2

24

36

, Ti0.7Nb0.3O2

32

, Ta0.08Nb0.2Ti0.7O2

as well as pure TiO2 (1.37x10-7 S/cm)

24

37

and

. The high electrical

conductivity of the as-prepared Ti0.7Ir0.3O2 support attributable to the Iridium metal, which was considered as conductive metal (2.12x105 S/cm)

38

, successfully

incorporated into TiO2 lattice resulting in the appearance of “aliovalent ions” effect 39, 40

. These results suggested that the Ti0.7Ir0.3O2 was a suitable catalyst support. 3.2. Characterization of Pt/Ti0.7Ir0.3O2 catalysts The structure information of the Platinum nano-forms anchored on Iridium-

modified Titanium dioxide support was recorded via the X-ray diffraction (XRD) measurement. As the XRD profile was shown in Figure 1 (a) shows demonstrated that the as-prepared catalyst with three typical diffraction peak of the face-centered cubic Platinum (JDCPS 04-0802) located at 2positions ~39.76o; 46.24o and 67.45o with respect to the crystal (111); (200) and (220) facets. Importantly, the strongest peak emerges at 2 positions around 39.76o showed that the formation of Pt NPs has a high orientation toward (111) facet, which was evidenced as active facet towards methanol 10 ACS Paragon Plus Environment

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electro-oxidation reaction because of a low poisoning rate in the low potential range41. Interestingly, no signal of segregated phase between Iridium and Titanium dioxide was detected after long reaction time in the strong reduction media of NaBH4 agents, implying that the as-prepared Ti0.7Ir0.3O2 support possessed the high chemical stability.

Figure 5. (a, b) TEM images, (c) HR-TEM image and (d) SEM image and (e) EDX spectroscopy and (f-h) elemental mapping of the 20 wt. % Pt/Ti0.7Ir0.3O2 catalyst The particle size and the general morphology of Platinum nano-forms loading on the as-prepared Ti0.7Ir0.3O2 support were recorded by the TEM images. As shown in Figure 5 (a, b) demonstrated that the morphology of Pt NPs was observed to be spherical-like shape about 3 nm in a diameter and less particle agglomeration which could anticipate the high electrochemical surface area (ECSA) of the as-prepared Pt/Ti0.7Ir0.3O2 catalysts. The small size and less agglomeration of Pt NPs attributable to the modified chemical reduction approach using a mixture of NaBH4 and Ethylene glycol, which exhibited the suitable dispersion of Ethylene glycol and the strong 11 ACS Paragon Plus Environment


reduction of NaBH4 agent42-44. Additionally, the high-resolution transmission electron microscopy (HR-TEM) measurement was also implemented for further investigating the orientation toward (111) facets of Pt nanocatalysts. The HR-TEM image (Figure 5 (c)) shows the fringe with an interplanar spacing of ~2.3 Å corresponded to the (111) facets of face-centered cubic Platinum structure, demonstrating that the Pt NPs was formed according to the orientation of (111) facets, which agreed with the XRD profile (see Figure 1 (a)). Furthermore, the Pt NPs loading was found to be about 19.81 wt. %, which close agreed with the desired ratio (20 wt. % Pt NPs) and relatively distributed on the support (Figure 5 (f)). Interestingly, the distribution of Ti and Ir after (Figure 3 (c, d)) and before (Figure 5 (g, h)) the deposition of Pt NPs was negligibly changed, implying that the as-prepared Ti0.7Ir0.3O2 support possessed the high stable structure. These experimental results indicated that the modified reduction reaction utilizing a mixture of NaBH4 and EG are the suitable route for preparing the 20 wt. % Pt/Ti0.7Ir0.3O2 electrocatalyst in this work.

Figure 6. The cyclic voltammograms of 20 wt. % Pt/Ti0.7Ir0.3O2 and 20 wt. % Pt/C (E-TEK) catalysts in N2-purged 0.5 M H2SO4 solution at a sweep rate of 25 mV/s at room temperature; inset: the estimated ECSA value of electrocatalysts The electrocatalytic activity of Iridium-modified Titanium dioxide supported Platnium catalyst was investigated and compared with the commerical Pt/C (E-TEK) catalyst by means of the cyclic voltammetry (CV) measurements in nitrogen-purged 0.5 M H2SO4 aqueous solution at a sweep rate of 25 mV/s. The ECSA value was 12 ACS Paragon Plus Environment

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calculated by integrating the area of hydrogen underpotential deposition (HUPD) region from cyclic voltammogram (CV) curves, which was shown in Figure 6. As an estimated result (Figure 6; inset) indicated that the as-prepared Pt/Ti0.7Ir0.3O2 possessed the high ECSA value to be around 96.98 m2/gPt, which ~1.1-folds larger than the common Pt/C (E-TEK). Besides the small diameter and good distribution of the Pt nano-forms on Iridium-modified Titanium dioxide, the high ECSA value of the as-prepared Pt/Ti0.7Ir0.3O2 attributable to the strong metal-support interaction (SMSI) between Pt NPs and TiO2-based materials, which was proven as a beneficial effect for the activity and selectivity for hydrogenation 20, 45.

Figure 7. Cyclic voltammograms of (a) 20 wt. % Pt/Ti0.7Ir0.3O2 and (b) commercial 20 wt. % Pt/C (E-TEK) in N2-purged 0.5 M H2SO4 at a scan rate of 25 mV/s after potential cycling over 2000 cycles



The potential cycling over 2000 cycles at a sweep rate of 25 mV/s in nitrogenpurged 0.5 M H2SO4 aqueous solutions was implemented for examining the electrocatalytic stability of catalysts. Figure 7 showed that the cyclic voltammograms curves of these catalysts after 2000 cycling test indicated that the ECSA value of the 20 wt. % Pt/Ti0.7Ir0.3O2 catalyst was kept around 91.41% (from 96.98 m2/gPt to 88.64 m2/gPt), which more stable than the commercial Pt/C (E-TEK) electrocatalyst (remained 82.80% of initial value, from 89.04 m2/gPt to 71.85 m2/gPt) (see Figure 7 (c)). The high electrochemical durability of the Pt/Ti0.7Ir0.3O2 catalysts could be interpreted due to the high corrosion resistance of the TiO2-based materials versus that the carbon materials in acidic media46.


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Figure 8. TEM images of (a, b) the 20 wt. % Pt/Ti0.7Ir0.3O2 and (c, d) the 20 wt. % Pt/C (E-TEK) catalyst before and after 2000 cyclic voltammetry (CV) cycles The changes of size and distribution of Platinum nano-forms loading on the Iridium-modified Titanium dioxide support after 2000 cycling test was investigated by means of the TEM images. As can be seen in Figure 8, the Pt/Ti0.7Ir0.3O2 catalyst exhibited the negligible change in the morphology and particle size after 2000 cycling test. Meanwhile, the significant agglomeration of Pt NPs supported on carbon was clearly observed which could be ascribed to the corroded carbon support. This result demonstrated that the as-prepared Pt/Ti0.7Ir0.3O2 electrocatalyst possessed the superior durability versus that the common Pt/C (E-TEK) electrocatalyst.

Figure 9. Cyclic voltammograms of 20 wt. % Pt/Ti0.7Ir0.3O2 and 20 wt. % common commercial Pt/C (E-TEK) catalysts in N2-purged 10 v/v% CH3OH/0.5 M H2SO4 solution at a scan rate of 25 mV/s; inset: the onset potential of electrocatalysts The electrocatalytic activity towards methanol oxidation reaction (MOR) of the electrocatalysts was appraised via the cyclic voltammetry measurement in nitrogenpurged 10 v/v% CH3OH/0.5 M H2SO4 aqueous solution at a sweep rate of 25 mV/s. As can be seen Figure 9, it is worth noting that (1) the onset potential of the asprepared Pt/Ti0.7Ir0.3O2 possess about ~0.1 V vs. NHE and 4-folds lower than that of the Pt/C (E-TEK) (~0.4 V vs. NHE) (Figure 9; inset) which exhibited the good 15 ACS Paragon Plus Environment


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methanol oxidation of the as-synthesized Pt/Ti0.7Ir0.3O2 electrocatalyst; (2) the methanol electro-oxidation current density (If) of the as-prepared Iridium-modified Titanium dioxide supported Platinum electrocatalyst was found up to be to ~21.69 mA/cm2 and ~1.5-fold higher than that of carbon-supported Platinum catalyst (E-TEK) (~14.70 mA/cm2), implying the high methanol oxidation capability of Pt/Ti0.7Ir0.3O2 in comparison with Pt/C. Last but not least, the If/Ib ratio, which displays the electrode’s efficiency in destroying CO-like residue species, of the Platinum nano-forms loading on Iridium-modified Titanium dioxide support was found to be around 1.72 and ~1.87folds higher than that of the commercial Platinum nano-forms loading on carbon support (~0.92). These results proved that the as-prepared Pt/Ti0.7Ir0.3O2 possess the high methanol oxidation activity versus that the Pt/C (E-TEK) catalyst. (Table 1) Table 1. The electrochemical properties of 20 wt. % Pt/Ti0.7Ir0.3O2 and commercial 20 wt. % Pt/C (E-TEK) in this work ECSAa (m2/gPt) Catalyst

Methanol oxidationb

Initial cycle

2000 cycle

Pt/Ti0.7Ir0.3O2

96.98

Pt/C (E-TEK)

89.04

Onset potential Current density

If/Ib ratio

(V)

(mA/cm2)

84.64

0.1

21.69

1.72

71.85

0.4

14.70

0.92

a

Calculated from the CV curves in N2-purged 0.5 M H2SO4 solution

b

Calculated from the CV curves in N2-purged 10 v/v% CH3OH/0.5 M H2SO4 solution The stability of these electrochemical catalysts toward methanol oxidation reaction

(MOR) was measured and shown in Figure 10 (a, b) via the 2000 cycling test in methanol acidic solution at a sweep rate of 25 mV/s. These experimental results indicated that our catalyst possessed the superior durability versus that the commercial catalyst. For example, the as-prepared Pt/Ti0.7Ir0.3O2 electrocatalyst exhibited the decay of the methanol electro-oxidation current density to be around 8.34% (from ~21.69 mA/cm2 to ~19.88 mA/cm2), which ~2.8-times lower than that of the commercial Pt/C (E-TEK) electrocatalysts (about 33.80%, from 14.70 mA/cm2 to 9.72 mA/cm2) (see Figure 10 (c)). This demonstrated that the Platinum nano-forms anchored on Iridium-


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modified Titanium dioxide (TiO2) possessed the high stability versus that the commercial carbon-supported Platinum catalyst (E-TEK).

Figure 10. Cyclic voltammograms of (a) the 20 wt. % Pt/Ti0.7Ir0.3O2 catalyst and (b) the commercial 20 wt. % Pt/C (E-TEK) catalyst in N2-purged 10 v/v% CH3OH/0.5 M H2SO4 solution at a scan rate of 25 mV/s after 2000 cyclic voltammetry (CV) cycles The durability of the as-prepared Pt/Ti0.7Ir0.3O2 catalyst and the commercial Pt/C catalyst was further examined via chronoamperometry (CA) measurement in methanol acidic solution at the immobilized potential of 0.7 V for 120 min. Figure 11 revealed that the initial methanol electro-oxidation current density of the 20 wt. % Pt/Ti0.7Ir0.3O2 catalyst was found to be about 41.35 mA/cm2 and ~1.06-fold higher than the Pt/C (ETEK) (~38.98 mA/cm2), implying the good methanol electro-oxidation of Pt/Ti0.7Ir0.3O2 catalyst. After the test, the Iridium-modified Titanium dioxide supported Platinum electrocatalyst possessed the remained current density to be about ~22.66 17 ACS Paragon Plus Environment


mA/cm2 with respect to the dropping rate of ~0.31 mA/cm2.min, which ~2.0-folds lower than the dropping rate (~0.60 mA/cm2.min) of the common carbon-supported Platinum catalyst (E-TEK). This evidenced that the as-synthesized Pt/Ti0.7Ir0.3O2 catalyst that the superior durability versus that the common Pt/C (E-TEK) catalyst in methanol acidic media.

Figure 11. Chronoamperometry curves of 20 wt. % Pt/Ti0.7Ir0.3O2 and commercial 20 wt. % Pt/C (E-TEK) catalysts in N2-purged 10 v/v% CH3OH/0.5 M H2SO4 solution at the oxidation potential of 0.7 V for 120 min The X-ray photoelectron spectra were recorded and shown in Figure 12 in order to further confirm the characterization of the Platinum nano-forms anchored Iridiummodified Titanium dioxide. The Pt 4f5/2 and Pt 4f7/2 peaks of Pt 4f spectra (see Figure 12 (b)) were observed at 74.06 eV and 70.86 eV, respectively; which was negatively shifted versus that the Pt 4f peaks of zero-valent Pt (74.6 eV for Pt 4f5/2 and Pt 4f7/2 at 71.3 eV)47. Furthermore, no change of the position of Ti 2p1/2 and Ti 2p3/2 peaks of Ti 2p spectra (Figure 12 (c)) was observed after the anchorage of Platinum nanocatalyst. These results demonstrated that the electronic transmission from Iridium-modified Titanium dioxide to Pt nanocatalysts resulting in the modified surface electronic 18 ACS Paragon Plus Environment

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structure of Platinum nanoparticles. This electronic mechanism makes the weak adsorption of carbonaceous intermediate species on the active sites of the catalysts leading in the enhancement both the electrocatalytic activity and the durability of Iridium-modified Titanium dioxide supported Platinum catalysts20, 23, 24. Furthermore, the TiO2-based conducting oxide catalyst support possessed the much higher corrosion resistance and electrochemical stability22, 46 versus that the carbon support, leading to the electrochemical durability of the 20 wt. % Pt/Ti0.7Ir0.3O2 catalyst was significantly improved compared to that of the commercial Pt/C (E-TEK) catalyst.

Figure 12. XPS spectroscopy of (a) Pt/Ti0.7Ir0.3O2 catalyst; (b) Pt 4f and (c) Ti 2p of Pt/Ti0.7Ir0.3O2 catalyst CONCLUSION In summary, we introduce the Platinum nanocatalyst anchored on Iridium-modified Titanium dioxide, which was prepared via a modified chemical reduction route without using surfactant or stabilizers, as a robust electrocatalyst toward methanol electro-oxidation reaction (MOR). For example, the electrochemical surface area (ECSA) of Platinum nano-forms deposited on Iridium-modified Titanium dioxide was around 96.89 m2/gPt and ~1.1-fold higher than that of the Platinum nanocatalyst loading on a carbon support (89.04 m2/gPt). For methanol electro-oxidation reaction (MOR), this electrocatalyst possessed the high methanol oxidation current density (~21.69 mA/cm2) and the If/Ib ratio (~1.72) versus that the commercial catalysts. 19 ACS Paragon Plus Environment


Furthermore, after 2000 cycling test shown the decay of the methanol electrooxidation current density was observed to be around 8.34%, which ~2.8-times lower than that of the commercial Pt/C (E-TEK) electrocatalysts. We also demonstrated that the electronic transmission from Iridium-modified Titanium dioxide to Platinum nanocatalyst leading to the modified surface electronic structure of Pt NPs which could interpret for the unique characterization of Iridium-modified Titanium dioxide supported Platinum catalyst. Acknowledgment This study was supported by the Youth Innovative Science and Technology Incubation Program, managed by Youth Promotion Science and Technology Center, Hochiminh Communist Youth Union, HCMC, Vietnam (Project No.17/2017/HĐKHCN-VƯ). References (1). Liu, Y.; Chen, L.; Cheng, T.; Guo, H.; Sun, B.; Wang, Y., Preparation and application in assembling high-performance fuel cell catalysts of colloidal PtCu alloy nanoclusters. J. Power Sources 2018, 395, 66-76. (2). Martínez-Huerta, M. V.; Lázaro, M. J., Electrocatalysts for low temperature fuel cells. Catal. Today 2017, 285, 3-12. (3). Ruiz-Camacho, B.; Baltazar Vera, J. C.; Medina-Ramírez, A.; Fuentes-Ramírez, R.; Carreño-Aguilera, G., EIS analysis of oxygen reduction reaction of Pt supported on different substrates. Int. J. Hydrogen Energy 2017, 42, 30364-30373. (4). Du, L.; Shao, Y.; Sun, J.; Yin, G.; Liu, J.; Wang, Y., Advanced catalyst supports for PEM fuel cell cathodes. Nano Energy 2016, 29, 314-322. (5). Li, L.; Hu, L.; Li, J.; Wei, Z., Enhanced stability of Pt nanoparticle electrocatalysts for fuel cells. Nano Res. 2015, 8, 418-440. (6). Zheng, Y.; Zhang, J.; Zhan, H.; Sun, D.; Dang, D.; Tian, X. L., Porous and three dimensional titanium nitride supported platinum as an electrocatalyst for oxygen reduction reaction. Electrochem. Commun. 2018, 91, 31-35. (7). Nan, H.; Dang, D.; Tian, X. L., Structural engineering of robust titanium nitride as effective platinum support for the oxygen reduction reaction. J. Mater. Chem. A 2018, 6, 6065-6073. (8). Mateos-Santiago, J.; Hernández-Pichardo, M. L.; Lartundo-Rojas, L.; Manzo-Robledo, A., Methanol Electro-Oxidation on Pt–Carbon Vulcan Catalyst Modified with WOx Nanostructures: An Approach to the Reaction Sequence Using DEMS. Ind. Eng. Chem. Res. 2016, 56, 161-167. (9). Jung, W. S.; Popov, B. N., Effect of Pretreatment on Durability of fct-Structured Pt-Based Alloy Catalyst for the Oxygen Reduction Reaction under Operating Conditions in Polymer Electrolyte Membrane Fuel Cells. ACS Sustainable Chem. Eng. 2017, 5, 9809-9817. (10). Jia, Q.; Caldwell, K.; Strickland, K.; Ziegelbauer, J. M.; Liu, Z.; Yu, Z.; Ramaker, D. E.; Mukerjee, S., Improved Oxygen Reduction Activity and Durability of Dealloyed PtCox Catalysts for Proton Exchange Membrane Fuel Cells: Strain, Ligand, and Particle Size Effects. ACS Catal 2015, 5, 176-186. (11). Shao, M.; Peles, A.; Shoemaker, K., Electrocatalysis on platinum nanoparticles: particle size effect on oxygen reduction reaction activity. Nano Lett. 2011, 11, 3714-9. (12). Alcaide, F.; Álvarez, G.; Tsiouvaras, N.; Peña, M. A.; Fierro, J. L. G.; Martínez-Huerta, M. V., Electrooxidation of H2/CO on carbon-supported PtRu-MoOx nanoparticles for polymer electrolyte fuel cells. Int. J. Hydrogen Energy 2011, 36, 14590-14598. (13). Maillard, F.; Peyrelade, E.; Soldo-Olivier, Y.; Chatenet, M.; Chaînet, E.; Faure, R., Is carbonsupported Pt-WOx composite a CO-tolerant material? Electrochim. Acta 2007, 52, 1958-1967. (14). Samad, S.; Loh, K. S.; Wong, W. Y.; Lee, T. K.; Sunarso, J.; Chong, S. T.; Wan Daud, W. R., Carbon and non-carbon support materials for platinum-based catalysts in fuel cells. Int. J. Hydrogen Energy 2018, 43, 7823-7854.


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Advanced Nanoelectrocatalyst of Pt Nanoparticles Supported on

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