Fabrication of Highly Stable and Efficient PtCu Alloy Nanoparticles on

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Fabrication of highly stable and efficient PtCu alloy nanoparticles on highly porous carbon for direct methanol fuel cells Inayat Ali Khan, Yuhong Qian, Amin Badshah, Dan Zhao, and Muhammad Arif Nadeem ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b06068 • Publication Date (Web): 28 Jul 2016 Downloaded from http://pubs.acs.org on August 2, 2016

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ACS Applied Materials & Interfaces

Fabrication of Highly Stable and Efficient PtCu Alloy Nanoparticles on Highly Porous Carbon for Direct Methanol Fuel Cells

Inayat Ali Khan,1,2 Yuhong Qian,2 Amin Badshah,1 Dan Zhao2*, and Muhammad Arif Nadeem,1* 1

Catalysis and Nanomaterials Lab 27, Department of Chemistry, Quaid-i-Azam University,

Islamabad 45320, Pakistan 2

Department of Chemical & Biomolecular Engineering, National University of Singapore, 4

Engineering Drive 4, 117585 Singapore

*

Corresponding Authors:

D.Z. E-mail: [email protected], Telephone: (+65) 6516 4679 M.A.N. E-mail: [email protected], Telephone: (+92) 5190642062

1 ACS Paragon Plus Environment


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Abstract Boosting the durability of Pt nanoparticles by controlling the composition and morphology is extremely important for fuel cells commercialization. We deposit the Pt-Cu alloy nanoparticles over high surface area carbon in different metallic molar ratios and optimize the conditions to achieve desired material. The novel bimetallic electro-catalyst {Pt-Cu/PC-950 (15:15%)} offers exceptional electro-catalytic activity when tested for both oxygen reduction reaction and methanol oxidation reactions. A high mass activity of 0.043 mA/µgPt (based on Pt mass) is recorded for ORR. An outstanding longevity of this electro-catalyst is noticed when compared to 20 wt.% Pt loaded either on PC-950 or commercial carbon. The high surface area carbon support offers enhanced activity and prevents the nanoparticles from agglomeration, migration and dissolution as evident by TEM analysis. KEYWORDS: fuel cells, bimetallic nanoparticles, energy storage, methanol oxidation, durability


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INTRODUCTION Polymer electrolyte membrane fuel cells (PEMFCs) can be used in flex-fuel vehicles and are also portable energy devices.1, 2 Methanol is not only wieldy and a cheap source of fuel but also has higher theoretical energy density3 than hydrogen. Therefore, direct-methanol fuel cells (DMFCs) could be fascinating moveable gadgets. However, a promising catalyst is needed for materialistic approach which can used at cathode for oxygen reduction reaction (ORR) as well as at anode for methanol oxidation reaction (MOR). Now a days, Pt supported on variety of carbon materials is marked as most efficient catalysts for fuel cells reactions.1, 2, 4 Yet, the solidity of such catalysts is related with the problems like Pt depositing, deterioration of carbon support, CO poisoning, and severe operating conditions.5-8 Hence, exploration of novel catalytic materials with enhanced electrochemical activity and stability is challenging target. To address the above mentioned issues, in recent years, extensive efforts have been dedicated. For example, a Pt shell or monolayer on a non-platinum group metals (non-PGM) core could effectively decrease the Pt cost with improved utilization.9-11 Alloying Pt with nonprecious metals (first row transition elements) is an unconventional approach to amalgamate the non-PGM catalysts.12-17 It is now well understood that such non-precious metals can play instrumental role in modifying the Pt d-band center to achieve desired catalytic performance.18, 19 For instance, alloyed Pt-Cu hierarchical trigonal pyramid nano-frames have been effectively used for formic acid electro-oxidation.20 Zhang et. al.,21 prepared octahedral alloy Pt2CuNi nanoparticles to achieve outstanding ORR and improved catalytic stability. Similarly, Zhu et. al.,22 have fabricated PtCoCuNi quaternary alloys, which exhibited superb ORR and MOR activities in acidic media.



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The carbon as a support material can significantly play its role towards effective utilization of Pt nanoparticles. Ideally, carbon support should provide maximum surface to metallic nano-particles and the pathway for facile conduction of electrons.1,

2, 23

Enormous

studies have been attempted to achieve the targeted goals,24-28 for example, improving the carbon support with other material like polymers,29 with organic functional groups30, 31 or with metal oxides.32-36 All these systems presented good ORR activity, but most often suffered with less stability and low electronic conduction. More importantly, modifying the support matrix have not suppressed the Pt NPs dissolution and migration. Ergo, up to the minute plans are needed to develop efficient catalysts. Herein, we present a coherent strategy to integrate Pt-Cu alloyed nanoparticles with highly porous carbon which is purposefully derived from MOF-5. This system showed resistance to particles dissolution and detachment, during accelerated durability tests. Blending the Cu metal in Pt nanoparticles results in modified lattice parameters, electronic structure, and contributes to the overall improvement of the catalyst as compared to commercial and literature catalytic systems. EXPERIMENTAL SECTION MOF-5 synthesis and its carbonization. Details of synthesis and carbonization of MOF-5 is available in our previous report.37 In short, vacuum dried MOF-5 was slowly ramped to 950 °C and kept at this temperature for 9 h under inert atmosphere. The obtained carbon yield was 65% and the sample was named as PC 950. Synthesis of Catalysts. The catalysts were also fabricated according to the our previously published work.37 Briefly, calculated amount of H2PtCl6·6H2O and Cu(NO3)2·3H2O were added ethylene glycol (10 mL). After stirring for ca. 15 min, an aqueous suspension of carbon was 4 ACS Paragon Plus Environment

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added into above mixture, followed by heating to reflux at 180 °C for 4 h. The solution was cooled to room temperature, filtered to obtain black product and dried in a vacuum oven. Following catalysts were synthesized with different wt. % of Pt and Cu, (1) Pt-Cu/PC-950 (15%:15%), (2) Pt-Cu/PC-950 (10%:20%), (3) Pt-Cu/PC-950 (5%:25%), (4) Cu/PC-950 and (5) Pt/PC-950 (20%). RESULTS AND DISCUSSIONS Figure 1 depicts the powder XRD analysis of synthesized catalysts. For Pt/PC 950 (20%) catalyst, all the appeared peaks (2θ = 40.2°, 47°, 67°, and 82°) are associated with the relevant crystal planes of fcc Pt, curve b. The peaks established in Pt-Cu/PC-950 (15%:15%) are shifted towards the higher angle of pure Pt peaks and towards lower angle of pure Cu. These X-ray reflections of the synthesized catalyst are located in between the standard patterns of Pt and Cu and this behavior indicates the alloy formation.38 The PXRD patterns of all the synthesized catalysts with different metallic ratios are presented in the Figure S2. The catalysts with 5% and 10% of Pt loading have peaks which are relevant to Pt only, indicating the strong interaction between Pt and Copper and perfect alloy formation. The peaks of Cu/PC 950 (30%) are assigned to the CuO and Cu2O reflections, Figure S2. A broad peak at ca. 2θ = 26° is signature of carbon support. The high intensity peak (111) was used to calculate the lattice parameter (LP) (equation 1), the crystallite size (Dcryst) (equation 2) and the extent of Cu alloying by Vegrad’s law (equation 3) and the data is presented in Table 1.

√

= ……… (1) .

= ………… (2) 5 ACS Paragon Plus Environment


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= 1

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! "# ……………… (3)

Where, a represents the lattice parameter in nm, λ is the wavelength (0.15406 nm Cu Kα), θ111 is half the 2θ angl e at the max imu m, Dcryst is the crystallite size (nm), β represents the full width at half maximum, X is the proportion of Pt atoms. aPt = 0.39242 (nm) and aCu = 0.36148 (nm), are the standard values for Pt and Cu lattice contact parameters, respectively.39

Figure 1- A comparison of powder XRD patterns of (a) Pt/C (20%), (b) Pt/PC-950 (20%) and (c) Pt-Cu/PC-950 (15%:15%). The lattice parameter of Pt-Cu/PC-950 (15%:15%) (0.371 nm) is smaller than Pt/PC-950 (20%) (0.388 nm) and Pt/C (20%) (0.392), Table 1. During alloy formation, replacement of Pt by Cu atoms (smaller atomic radius) leads to the reduced lattice.38 The calculated crystallite size of PtCu nanoparticles is also smaller than that of pure Pt loaded either on PC 950 carbon or 6 ACS Paragon Plus Environment

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commercial carbon, Table 1. It can be concluded from XRD analysis that using the same synthetic protocol of ethylene glycol reduction, the particle size can be decreased by incorporating a secondary metal with smaller atomic radii like Cu and Ni37. A good agreement is noticed in calculated and the experimental lattice constant (d111 in nm) of PtCu nanoparticles. This is pinpointed on the regression line stand on Vegrad’s law,

which recommends the

formation of fully miscible solid solution. Calculated from Vegrad’s law,

the fraction of

Cu11.31% in the Pt-Cu/PC 950 (15:15%) catalyst is in close agreement with the bulk Cu composition (Pt:Cu 13:12%) as determine by ICP-OES analysis, Table 1.

Table 1- A comparison of XRD, TEM and ICP-OES data of catalysts Catalysts

Pt/PC-950

d111

LP

FWHM

Dcrys

TEM

(nm)

(nm)

(radian)

(nm)

(nm)

20.092

0.224

0.388

1.063

7.985

4.7

21.086

0.214

0.371

1.887

4.524

3.2

θ111

at. %a

Pt18

at. %b

-

Wt.% from ICP-OES Pt

Cu

Total

17.96

-

17.96

(20%) Pt-Cu/PC-950

Pt13Cu12 Pt13Cu11 13.12 12.01 25.13

(15%:15%) a b

Calculated from ICP-OES Calculated from Vegrad’s rule

The elemental composition of the synthesized samples were determined by XPS analysis. The spectra (Figure S2), show that the Pt-Cu/PC 950 (15:15%) catalyst consist of Pt, Cu, C and O while Pt/PC 950 (20%) catalyst is dominated by presence of Pt, C and O species. The presence of O is originated from the surface organic functionalities of the carbon support. The C 1s spectrum of (Figure 1a) consists of C- sp3 peak (284.5 eV). The Cu 2p spectrum (Figure 1b) displays two peaks, one for Cu 2p3/2 (932.2 eV) and another for Cu 2p1/2 (952.2 eV). These peaks could be attributed to typical Cu0 and/or Cu1+. The CuO is also verified by the appearance of



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small peak at 934.2 eV for Cu 2p3/2, a reshuffled peak at 944.2 eV, and a 954.8 eV peak for Cu 2p1/2. Copper LMM Auger spectra can be used to discriminate between the Cu0 and Cu1+ species, (Figure S3-verticle doted green line).40, 41 The presence of Copper LMM Auger kinetic energy for Pt-Cu/PC-950 (15%:15%) shows the presence of metallic copper.42 From XPS analysis, the presence of copper oxides may derive from the surface oxidation of metallic copper upon exposing the sample in air.43 For Pt containing catalysts, the HR-XPS spectra of Pt unveil strong peaks emerged from Pt 4f7/2 and Pt 4f5/2. These peaks can be further divided into two doublets/triplets indicating the existence of elemental Pt and surface oxides of Pt (Figure 2c & 2d).42 For, bimetallic catalyst, the strong peaks at 71.1 eV and 74.5 eV are assigned to Pt, while the small peaks at 72.0 eV, 75.6 eV, 73.0 eV and 77.3 eV are assigned to the surface oxides of Pt.42 Similarly, in case of Pt/PC-950, the peaks at 71.3 eV and 74.6 eV are emerged due to Pt while peaks at 74.0 eV, 75.8eV and 73.0 eV can be allocated to the oxides of platinum, Figure 2d. It has been observed that the Pt 4f binding energies is shifted negatively up to ca. 0.2 eV in the PtCu catalyst compared to Pt/PC 950 (20%) catalyst, Figure 2d, and in the commercial catalyst, Figure S4. Such behavior is expected from the electron interaction of Pt with the Cu atoms, owing to their different electronegativity values.44, 45 The electron interaction will result in d-band structure modification of Pt which is favorable to ORR and catalyst stability.46, 47 It has also been observed by different research groups that a carbon support with surface organic functional groups can also interact with Pt nanoparticles and improves catalytic activity.44,

45

Herein, the negative shift in binding energy of Pt also implies on the fact that PC 950 high surface area carbon electronically interact with metal nanoparticles that has resulted in enhanced stability of Pt-Cu/PC 950 (15:15%) catalyst. The percent composition of the Pt-Cu/PC 950 (15:15%) catalyst was found as; 84.83 wt.% of carbon, 6.33 wt.% of oxygen, 4.31 wt.% of


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platinum, and 3.74 wt.% of copper. In comparison to the ICP-OES results, the percent Pt:Cu loading from XPS analysis appeared slightly low due to the fact of the XPS surface sensitivity. Markovic and coworker48 worked on Pt alloying with different metals in order to enhance the ORR activity. They have explored that by tuning the Pt band via alloying causes the change in binding of oxygen with Pt, a factor which played its role in ORR. The report further revealed that a Pt3Ni combination was found exceptionally 10 times more active than Pt catalyst due to upgradation in its electronic and morphological structure. 49 Figure 2- HR-XPS spectra of (a) carbon, (b) Cu 2p and (c) Pt 4f in Pt-Cu/PC 950 (15:15%) and (d) Pt 4f in Pt/PC 950 (20%). 9 ACS Paragon Plus Environment


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Figure 3 represents the gas adsorption analysis data of carbon and the synthesized catalysts. A range of different pore regions in pure carbon is evident from a type-IV adsorption isotherm, Figure 3a. At very low P/Po, gas adsorption occur abruptly due to presence of micropores, followed by a slow increase making a low angle. This behavior along with hysteresis loop, suggests the existence of mesopores. Finally, abrupt increase gas adsorption again near ca. 1 P/Po confirms the existence of macropores.50, 51 This diversity of pore region makes the PC950 as archetypal for capturing metal nanoparticles52 and also presents a good platform for the inside diffusion of molecular oxygen. A high surface area of 1453 m2 g–1 with a pore volume of 2.01 cm3 g–1 was calculated from the data, respectively. The gas adsorption volume has significantly dropped down in case of Pt/PC-950 (20%) (curve b) and Pt-Cu/PC-950 (15%:15%) (curve c) catalysts (Figure 3a). This behavior can be explain as; the metal nanoparticles are incorporated inside carbon matrix and blocked the

micropores.52 The Pt/PC-950 (20%) has BET area of 419 m2 g–1 with 0.384 cm3 g–1 of pore volume, similarly, for Pt-Cu/PC-950 (15%:15%), the recorded values are 231 m2 g–1 and 0.333 cm3 g–1, respectively. Figure 3b represents the pore size distribution (PSD) plots, which further


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justifies the gas adsorption/desorption behavior of the carbon and catalyst. The intensity of the peak positioned at 4 (Å) is notably decreased with the addition of metal nanoparticles, Figure 3b. It is apparent that the metal particles has blocked the micropores as a results, a reduction in BET area and pore volume is observed. Figure 3- (a) Gas adsorption isotherms; (b) pore size distribution curves of pure carbon and the synthesized catalysts.

TEM analyses were performed to elucidate the morphology and particles size distribution and the images are presented in Figure 4 and Figure S5. The TEM measurements has revealed the widespread uniformity of metallic particle size and evenly distributed on carbon support which is very significant for catalysis, Figure 4. The Pt nanoparticles are aggregated and

irregularly distributed as is obvious in Figure S5 for Pt/PC 950 (20%) catalyst. There is close agreement between particle sizes calculated from XRD with the observed microscopic analyses; 3.2 nm for Pt-Cu/PC-950 (15%:15%) and 4.7 nm for Pt/PC-950 (20%). Comparatively, the particles size and degree of agglomeration of nanoparticles is high for catalyst without copper 11 ACS Paragon Plus Environment


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content. This means that the particles size of the catalyst is highly dependent on the second alloying metal. In this case, Cu has smaller atomic size as compare to Pt, which upon alloying leads to a decrease in the size of the resultant particles. Figure 4- TEM images of Pt-Cu/PC-950 (15%:15%) (a-h).

Electrochemical testing was started by starting cyclic voltammetry (CV) experiment by depositing the catalysts on carbon desk. During CV cycling of the alloyed PtCu catalyst redox peaks at specific potential values corresponding to the de-alloying and re-alloying of Cu have been observed, Figure 5. First few CV cycles confirms the presence of Pt with the appearance of small hydrogen adsorption/desorption peaks (0.05-0.3 V). The anodic peak A1 can be assigned to Cu dissolution from alloyed catalyst53 while the peak A2 and corresponding cathodic peak C2 for the Pt oxides formation/reduction. Similarly, the cathodic peak C1 corresponds to re-alloying of the Cu metal. The reversible redox behavior of Cu metal presents rearrangement of the alloying metal and the subsequent stability of the catalyst surface. Strasser et al.,

18

reported

different Pt–Cu alloys and revealed that ORR activity can be enhanced by decreasing the bonding strength between oxygenated species and catalyst surface. This goal could be achieved by tuning the band structure through alloying. Furthermore, they have noticed that intensity of the Had peak was gradually increased while it decreased in case of redox peaks of copper, which means a stable catalyst with Pt bright surface was developed. Electrochemical active surface area (EASA) was calculated from a stable and well defined Pt-CV, Figure S7. After 250 cycles, the CV displayed two distinctive potential regions; one for Had/des and the another for the oxides formation/reduction of Pt. The EASA was obtained by using the H2 desorption charges; a value of 210 µC cm–2 was earmarked for the adsorption of a H2 monolayer on a Pt fresh surface. The EASA of Pt-Cu/PC-950 (15%:15%) (63.22 m2 g–1) is 12 ACS Paragon Plus Environment

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16.5% larger than Pt/PC 950 (20%) catalyst, which is assigned to smaller particles size and rearrangement of

the

electrochemical

activation. The synthesized

catalysts

have

presented high background

for

electric double-layer due to

porous nature of

the carbon support which is

significant

charge accumulation and

current

to

alloyed

metal

after

mass transport.

Figure 5- 250 cycles CV profile of Pt-Cu/PC 950 (15:15%) showing redox peaks of metals at their specific potential.

The electro-catalytic activities of the synthesized and the commercial catalysts during ORR were recorded in oxygen saturated acidic solution (0.1 M) at ca. 25 °C, Figure 6. The ORR polarization curves were recorded at 1600 rpm, Figure 6a. Each curve displays a diffusion limiting current region (below 0.55 V) and the mixed kinetic-diffusion control region (from 0.55



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V to 0.95 V) , Figure S8. The observed onset potential (Eonset) values are 0.941V, 0.900 V and 0.911 V for the synthesized Pt-Cu, Pt only and the commercial catalyst respectively. The Eonset of Pt-Cu alloy is shifted positively by 30 mV in comparison to 20 wt.% Pt loaded on either PC 950 or commercial carbon. Interestingly, the Eonset for different ratio Pt-Cu catalysts (10%:20%) and (5%:25%) are shifted negatively when compared with Pt/C (20%). Polarization curves of materials without Pt show almost inert behavior toward ORR, Figure S9. Following order of electro-catalytic activities is derived from E1/2; Pt-Cu/PC-950 (15%:15%) > Pt/PC-950 (20%) ≈ Pt/C (20%). Based on E1/2, the Pt-Cu alloyed catalyst have manifested the inflated ORR activity which is attributed to the sufficient tuning Pt band structure by alloying with Cu, support by porous carbon via providing extra surface area and well dispersion of nanoparticles. Small particle size and uniform distribution has resulted in proper utilization of the catalyst while change in d-band center has affected the strength of bond between catalyst active surface site and the adsorbed intermediates produced during ORR. After estimation of activities using the E1/2 values, the mass activities were calculated to find out more accurate ordering of ORR active catalysts. For this purpose, the kinetic current density (jk) was recorded at 0.9 V vs. RHE. In our experiments, the diffusion limiting current density values are also lower than the ideal value (5.8 mA cm–2 at 1600 rpm) with the 1.26×10–3 M dissolved O2 in the electrolyte. The highest mass activity (0.043 mA/µg_Pt) is obtained for Pt-Cu/PC-950 (15%:15%) as compared to Pt/PC-950 (0.010 mA/µg_Pt) and commercial Pt/C (0.017 mA/µg_Pt). The porous carbon supported Pt-Cu alloyed material is considered as the best catalyst, whose specific mass activity is 2.6 and 3.2 times larger than 20 wt.% Pt supported on commercial carbon and PC-950, respectively. The Tafel plots for all the tested catalysts are shown in Figure 6c. From these plots the order kinetic activities is found as; Pt-Cu/PC-950 (15%:15%) > Pt/PC-950 (20%) > Pt/C (20%). This order


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clearly demonstrates the superior activity of alloyed Pt-Cu catalyst over non-alloyed materials towards ORR. For kinetic studies, polarization curves were recorded in the range of 625-2500 rpm in Oxygen rich HClO4 (0.1 M). The Koutecky-Levich sketches are shown in Figure 6d; a linear fitting corresponding to 1st order kinetics is observed for dissolved oxygen. The calculated electron transfer number ca. 3.89 is matched with the expected value (4).

Table 2- The Electrochemical properties and mass activity values of all catalysts before and after ADTs. EASA (m2 g–1)

Eonset (V)

Before ADT

After ADTa

Before ADT

After ADT

Before ADT

After ADT

Before ADT

After ADTb

Pt/C (20%)

81.5

53.1

0.910

0.76

0.773

0.65

0.017

-

Pt/PC-950 (20%)

51.1

49.2

0.900

0.82

0.780

0.69

0.010

-

Pt-Cu/PC-950 (15%:15%)

63.2

65.3

0.941

0.941

0.797

0.79

0.043

0.043

Catalysts

E1/2 (V)

Mass activity (mA/µgPt) at 0.9 V

a

The EASA of the catalysts after ADT was calculated from the 500th cycle of CV As the LSV profiles shifted more negatively beyond that of 0.9 V so, after ADTs mass activities of 20 wt.% Pt loaded catalysts were not calculated b



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Figure 6- LSV polarization plots in O2-saturated HClO4 (0.1M) solution at 1600 rpm and at 10 mV s–1 (a), mass activities (b), Tafel plots (c), and Koutecky-Levich plots at 0.68 V (d) of the synthesized and commercial catalysts.

The durability of catalysts was checked via accelerated durability tests (ADTs) between 0.25 V to 1.0 V (vs. Ag/AgCl) with a scan rate of 50 mV s–1 for 500 cycles. The supply of oxygen was constantly flowed and the LSV curve was instantaneously recorded at 1600 rpm, Figure 7. The observed values are shown in Table 2. As indicated by the ORR polarization curves in Figure 7a, bimetallic Pt-Cu/PC-950 (15%:15%) have almost seen unchanged and the Eonset and E1/2 values are given in Table 2. However, Pt/PC 950 (20%) and Pt/C 950 (20%) underwent 28% and 35% loss in EASA after ADTs, which can also be observed by a negative


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shift of ~120 mV E1/2 value, Figure 7b, c. This extraordinary stability of the Pt-Cu alloyed particles as compared to the 20 wt.% Pt loaded on PC 950 or commercial carbon black can be attributed to the firmness between carbon support and catalytic particles which has retained the particles as active monodispersed for longer time.



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For practical implementation of the catalysts, we have also tested the synthesized materials for MOR. For this purpose, CV experiments were carried in 1M methanol solution acidified with 0.1 M HClO4, at a scan rate of 25 mV s–1, Figure 8a. During forward scan of CV, an increase in current between 0.75–1.5 V (vs. RHE), similarly, in reverse scan, between 0.5–0.7 V was observed. This behavior indicates that the catalysts are active for MOR. 54 The forward scan oxidation peak is attributed to methanol oxidation (dehydrogenation: CH3OH → COad + 4H++ 4e–) forming catalyst surface adsorbed carbonaceous intermediates.55 The COad can make the catalyst inactive or less active, however, oxidation of COad into CO2 regenerates the catalyst surface which is confirmed by oxidation peak during reverse scan. The enhanced current density (Ja), at If in forward scan of potential, in case of Pt-Cu alloyed nanocatalyst confirms its best


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candidature for MOR among the options. The observed Ja values and the catalyst activity decreasing-order toward MOR is; Pt-Cu/PC 950 (15:15%) (28.3 mA cm–2) > Pt/PC 950 (20%) (23.7 mA cm–2) > Pt/C (20%) (21.0 mA cm–2). Chronoamperometric measurements were performed for 3000 s at 1.0 V (vs. RHE) to find out the catalysts stability towards MOR, Figure S11. In graph, all curves present initial sharp decay in current followed by no further decrease in current. This initial current decay is due to the carbonaceous intermediates, which renders

catalyst activity. The current vs time curves show that the Pt-Cu alloyed catalyst have a high current during the whole time period of testing, showing its superiority over other materials under investigation.



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Figure 7. LSV polarization curves before and after ADTs (a-c) and combined LSV polarization curves after ADTs (d).

Methanol tolerance level of catalysts was evaluated by chronoamperometric tests at 0.45 V (vs. Ag/AgCl) and 1600 rpm in 0.1 M HClO4/ 1M CH3OH solution for a period of 42000, Figure S12a. Both the catalysts show a rapid decay in current in the initial of the current-time response experiment. The Pt/C (20%) benchmark catalyst retains only 6% of its initial current which presents its lower methanol tolerance and low ORR selectivity. Alloying Pt with Cu in 1:1 ratio (15%:15 wt.%) comparatively improves the catalyst tolerance level toward methanol and better selectivity toward ORR, as the catalyst evidently retains about 35% of its initial current in acidic methanol solution. Relative methanol tolerance of the catalysts were also evaluated by running LSV in O2-saturated acidic CH3OH solution. Polarization curve at 1600 rpm, Figure S12b, shows current density reversal with minimum current density at 0.6 V and 0.76 V(vs. RHE) for Pt-Cu and Pt/C, respectively. The current reversal presents a competition phenomenon between MOR and ORR at catalysts surface. The Pt-Cu catalyst is clearly found to be less affected when compared with Pt/C (20%). The better methanol tolerance of the Pt-Cu/PC-950 (15%:15%) catalyst can be attributed to its alloying nature that can enhance the catalyst long term stability toward CO poisoning generated during methanol oxidation and good selectivity towards ORR in the presence of methanol solution. The frequency response analyses of the catalysts were studied via electrochemical impedance spectroscopy. Figure 8b shows Nyquist plots of the catalysts deposited on the electrode surface at 1.00 V (vs. RHE) using three electrode assembly with similar electrolyte composition that is used for MOR studies. All the curves consists of a semicircle (arc) at real axis, Figure S13a, b. The diameter of the arc is important and usually if the arc is small a less


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charge transfer resistance is considered and vice versa. The impedance measurements depict that Rct (charge transfer resistance) is low for Pt-Cu/PC-950 (15%:15%) catalyst combination suggesting fast reaction rate with low resistivity. The Relec (electrolyte resistance) values are 0.09, 0.06 and 0.02 Ohm cm2 for Pt-Cu, Pt only/PC-950, and Pt only/C respectively, presenting different electrolyte concentration gradient toward the electrode surface at high alternating current (AC) frequency.56 Similarly, Rct values are 3.4, 15.9, and 47.0, also suggest the less resistance in charge transfer for Pt-Cu/PC-950 catalyst which makes it more active than other set catalysts under investigation. 57

Figure 8. CV curves of methanol oxidation (a) and Nyquist plots of the catalysts; a-Pt-Cu/PC 950 (15:15%), b- Pt/PC 950 (20%), c- Pt/C (20%), the inset is the magnified EIS at high frequency region (b).

The different degradation behavior of the synthesized and commercial catalysts were further investigated by checking of the morphologies after ADTs with TEM techniques. Figure S14a, b shows that Pt-Cu alloyed nanoparticles are well dispersed on support surface before and after ADTs. No severe gathering and enlargement of the catalyst particles were observed in TEM observation, indicating the effect of Cu alloying and electron structure modification of Pt that 21 ACS Paragon Plus Environment


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impart good stability to Pt on PC 950 support during electrochemical evaluation. The alloying effect and porous high surface area carbon support have prevented Pt nanoparticles from dissolving, migration and augmentation. These results accounted for the excellent stability of PtCu/PC 950 (15:15%) in catalyzing ORR and MOR. However, in case of Pt/PC 950 (20%) and Pt/C (20%) catalysts, Figure S14(c-f), the nano-particles are converted into irregular larger particles. This was consistent with drastic decrease in EASA (28% and 35% loss) after ADTs. Finally, it can be concluded that for long-term usage of Pt based catalysts in fuel cell technology, high surface area carbon support and perfect alloying to change the d-band center of the catalyst are simultaneously necessary as has been evidently investigated in the current studies. CONCLUSIONS Highly porous carbon obtained by heat treatment of MOF-5 was used as a support for 30 wt.% Pt-Cu bimetallic and 20 wt.% Pt to comparatively investigate the support and alloying effect toward ORR and MOR for fuel cells application. X-ray and microscopic analyses have revealed well dispersed alloyed nanoparticles of Pt-Cu on carbon support. During electrochemical investigations, a mass activity of 0.043 mA/µgPt for ORR and enhanced current density of 28.3 mA cm–2 for MOR was found for Pt-Cu/PC 950 (15:15%) catalyst. For the longterm usage during ADTs and time-current response, the Pt-Cu alloyed catalyst have presented excellent stability and durability. The combined effect of high surface area support and the copper alloying not only boost the Pt catalyst activity but also effectively prevent the nanoparticles from cessation and aggrandizement. ACKNOWLEDGEMENTS This work is supported by Higher Education Commission (HEC) of Pakistan (No. 202704/NRPU/R&D/HEC/12), and National University of Singapore (CENGas R-261-508-00122 ACS Paragon Plus Environment

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646), Singapore Ministry of Education (MOE AcRF Tier 1 R-279-000-410-112, AcRF Tier 2 R279-000-429-112). I. A. Khan acknowledges the IRSIP scholarship of Higher Education Commission (HEC) of Pakistan for financial support while carrying out the research work in National University of Singapore. ASSOCIATED CONTENTS Supporting information is available including PXRD, XPS, TEM, CV, LSV, Chronoamperometry, EIS data, details of characterization tools and electrochemical measurements. REFERENCES (1) Fang, B.; Chaudhari, N.; Kim, M.; Kim, J.; Yu, J. Homogeneous Deposition of Platinum Nanoparticles on Carbon Black for Proton Exchange Membrane Fuel Cell. J. Am. Chem. Soc. 2009, 131, 15330–15338. (2) Fang, B.; Kim, J.; Kim, M.; Yu, J. Ordered Hierarchical Nanostructured Carbon as a Highly Efficient Cathode Catalyst Support in Proton Exchange Membrane Fuel Cell. Chem. Mater. 2009, 21, 789–796. (3) Abdullah, S.; Kamarudin, S. K.; Hasran, U. A.; Masdar, M. S.; Daud. W. R. W. Modeling and Simulation of a Direct Ethanol Fuel Cell: An Overview. J. Power Sources 2014, 262, 401–406. (4) Singh, B.; Murad, L.; Laffir, F.; Dickinson, C.; Dempsey. E. Pt Based Nanocomposites (Mono/Bi/Tri-Metallic) Decorated Using Different Carbon Supports for Methanol ElectroOxidation in Acidic and Basic Media. Nanoscale 2011, 3, 3334–3349.



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Fabrication of Highly Stable and Efficient PtCu Alloy Nanoparticles on

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