C for High-Efficiency Cathode of Fuel Cells with Superhigh

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C: Surfaces, Interfaces, Porous Materials, and Catalysis 0.61

Pt Ni/C for High Efficient Cathode of Fuel Cells With Super-High Platinum Utilization Jing Liu, Yuping Li, Zhemin Wu, Mingbo Ruan, Ping Song, Luhua Jiang, Yong Wang, Gongquan Sun, and Weilin Xu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b03966 • Publication Date (Web): 11 Jun 2018 Downloaded from http://pubs.acs.org on June 12, 2018

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The Journal of Physical Chemistry

Pt0.61Ni/C for High Efficient Cathode of Fuel Cells with Super-high Platinum Utilization Jing Liu,a,b Yuping Li,c Zhemin Wu,d Mingbo Ruan,a Ping Song,a Luhua Jiang,c,e Yong Wang,d Gongquan Sun, c and Weilin Xu* a a

State Key Laboratory of Electroanalytical Chemistry &Jilin Province Key Laboratory of Low

Carbon Chemical Power, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, 5625 Renmin Street, Changchun, 130022, P. R. China; b

c

University of Chinese Academy of Sciences, Beijing, 100049, P. R. China;

Division of Fuel Cell & Battery, Dalian National Laboratory for Clean Energy, Dalian Institute

of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, P. R. China; d

Center of Electron Microscopy and State Key Laboratory of Silicon Materials, School of

Materials Science and Engineering, Zhejiang University, Hangzhou 310027, P. R. China. e

College of Materials Science and Engineering, Qingdao University of Science and Technology,

Qingdao, 266042, P. R. China. Corresponding Author *Email: [email protected] (W.X.)

1 Environment ACS Paragon Plus

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ABSTRACT Exploring advanced electrocatalysts to accelerate the sluggish oxygen reduction reaction (ORR) in polymer electrolyte membrane fuel cells (PEMFCs) is a promising route to alleviate the current challenges of fossil fuel exhaustion and environment pollution. Herein, a carbonsupported highly dispersed PtNi nanocatalyst (Pt0.61Ni/C) with low-platinum content of 2.76 wt% was prepared simply based on galvanic replacement for high efficient ORR process. It presents a mass activity of about 5 times of the conventional Pt-based catalyst at 0.9 V (vs.RHE) and a remarkable durability and methanol tolerance. The acidic fuel cell with such cathode catalyst (Pt0.61Ni/C) presents a striking performance with maximum power density up to 1.1W cm-2 at 80 o

C. Due to the extremely low platinum loading in the whole fuel cell, its Pt utilization (0.093 gPt

kW-1) is the highest reported ever in H2/O2 fuel cells. Such impressive performance of Pt0.61Ni/C makes the obtained Pt0.61Ni/C catalyst a very promising alternative to conventional Pt-based catalysts for their large-scale application in future.


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INTRODUCTION Benefiting from the low carbon emission and high energy efficient, polymer electrolyte membrane fuel cells (PEMFCs) have been studied extensively in recent years for their potential applications in many different fields. Due to the high cost and low stability of traditional Pt/Cbased cathode, the large-scale application of fuel cells is still a big challenge.1 A high efficient electrocatalyst for ORR in cathode, one of the major components in PEMFCs, is one of keys for the overall PEMFC performance.2,3 The costly traditional electrocatalysts with high Pt loadings are currently used extensively as efficient cathode electrocatalysts, however, the large consumption of precious Pt limits their large-scale application.4,5 How to maximize the utilization of Pt has been the subject of fundamental and technological significance for the past decades.6-8 One of the most promising methods is to replace Pt in part with a nonprecious metal (M).9-13 Such composites include PtM nanoparticles loaded on carbon,12,14 PtM core−shell structures with the shell of Pt,15-19 and etc. Among these, Pt-Ni system (such as Pt3Ni) has attracted lots of attention due to their high ORR performance.20 However, many of these catalysts still suffer from their complicated synthesis and the large consumption of Pt in the H2/O2 fuel cells due to their high Pt loadings. Actually, by now, very few fuel cell results have been reported based on such type of Pt-Ni cathode.21 In this work, in order to further reduce the Pt consumption in such type of Pt-Ni cathodes for ORR in fuel cells, we prepared a carbon supported highly dispersed PtNi nanocatalyst (Pt0.61Ni/C) by a facile galvanic replacement reaction based on as-prepared Ni-loaded carbon (Ni/C) (Scheme 1). The Pt loading of 2.76 wt% in Pt0.61Ni/C is much lower than previously reported PtNi catalysts.9,12 The as-synthetic Pt0.61Ni/C showed a superior high ORR performance in activity, durability and tolerance to poisons. More significantly, the acidic fuel cell test with



Pt0.61Ni/C as cathode delivered a highest platinum utilization reported ever (0.093 gPt kW-1) in H2/O2 fuel cells.

Scheme 1. Schematic illustration of the formation of PtxNi nanoparticles under the synthetic condition.

EXPERIMENT SECTION Materials Anhydrous ethanol (ETOH, 99.7%), n-Hexane (C6H14, 98%), sodium hydroxide (NaOH, 96%) and anhydrous sodium sulfate (Na2SO4, 99%) were purchased from Beijing Chemical Works. Nickel chloride hexahydrate (NiCl2·6H2O, 98%) was purchased from Xilong Chemical Co., Ltd. Sodium oleate (NaOA, 99%) was purchased from Adamas Reagent Co., Ltd. Chloroplatinic acid (H2PtCl6·6H2O, 37%) was purchased from Shanghai Reagent Factory. Concentrated perchloric acid (HClO4, 70%) was purchased from Tianjin Cheng Cheng Chemical Co., Ltd. Amorphous carbon (C, black pearls (BP)-2000) was purchased from Asian-Pacific Specialty Chemicals Kuala Lumpur. Nafion solution (5 wt %) were obtained from Sigma-Aldrich. All the chemicals were used as delivered without further treatment. Ultrapure water with the specific resistance of 18.23 MΩ·cm was obtained by reversed osmosis followed by ion-exchange and filtration.


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Rotating ring-disk electrode of glassy carbon (RRDE, 4 mm in diameter) was purchased from CH Instruments, Inc, USA. Preparation of nanoscale Ni nanoparticles on C supports (Ni/C). The Ni/C samples were prepared using an improved method based on previous report.22 In a typical procedure, 0.5 mmol of NiCl2·6H2O and 1 mmol of NaOA were added to the solution containing 5 mL of deionized water, 10 mL of absolute ethanol and 15 mL of hexane. The mixed solution was stirred at 70 °C for 30 min until the nickel ions were transferred from the lower aqueous solution to the upper organic phase. Then 1 mmol of NaOH was added to the solution and the mixture solution was stirred for 4 h. After cooling down to room temperature, the upper organic phase was separated and further added to the solution containing of 300 mL hexane and 1.347g BP-2000. After that, the mixed solution was further stirred at 70 °C for 4 h and then dried by rotary evaporation at 50 °C. The obtained powder was mixed with 5g of Na2SO4 and ground in a mortar for 30 min to form a fine powder. Therefore, the mixture was heated to 500 °C at a heating rate of 5 °C·min−1 under hydrogen (H2) atmosphere and then kept at that temperature for 3 h. After cooling down, the product was washed with deionized water and dried under vacuum condition at 50 °C. Then the Ni/C samples were obtained. Preparation of nanoscale Pt-Ni nanoparticles on C supports (Pt-Ni/C). The Pt-Ni/C samples were prepared by the method of galvanic replacement. In a typical procedure, 50 mg of Ni/C was added to 50 ml of deionized water and mixed in ultrasonic cleaning machine for 1hour. The mixed solution was stirred under ambient temperature along with adding a certain amount of chloroplatinic acid solution dropwisely and further stirred for several hours until the end of the reaction. And then, the solution was filtered. The resulted solid was washed with deionized water and dried under vacuum condition at 50 °C. Therefore, the



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mixture was heated to 900 °C at a heating rate of 5 °C·min−1 under argon (Ar) atmosphere and then kept at that temperature for 1 h. After cooling down, the Pt-Ni/C samples were obtained. Physical characterization. The morphology and dimensions of as-prepared samples were obtained using transmission electron microscopy (TEM) obtained on a JEM-2100F microscopy with an accelerating voltage of 200 kV. Subangstrom resolution HAADF-STEM images were obtained on a FEI TITAN Chemi STEM equipped with a CEOS (Heidelburg, Ger) probe corrector, operating at 200 kV. Xray

diffraction

(XRD)

spectrum

was

obtained

from

Bruker

D8 ADVANCE X-ray

Diffractometer with using Cu Kα radiation (λ = 0.15418 nm). X-ray photoelectron spectroscopic (XPS) measurements were performed on a AXIS Ultra DLD (Kratos company) using a monochromic Al X-ray source. The Brunauer-Emmett-Teller (BET) surface areas and pore volumes were obtained from 77 K N2 sorption isotherms using ASAP 2020 instrument. The final contents of Pt and Ni in the catalysts were obtained from inductively coupled plasma mass spectrometry (ICP-MS) (ICAP-6000, Thermo Fisher Scientific). Electrochemical characterization The activity for the oxygen reduction reaction (ORR) was evaluated by voltamperometry on the PtNi-loaded carbon material electrodes. Fabrication of the working electrodes was done by pasting catalyst inks on a glassy carbon rotating disk electrode (4 mm in diameter). Its apparent surface area (0.1256 cm2) was used to normalize the ORR activity of the catalysts. The carbon ink was formed by mixing 5 mg of doped carbon materials catalysts, 50 µL of a 5 wt % Nafion solution in alcohol, and 950 µL of ethanol in a plastic vial under ultra-sonication. A 10-µL aliquot of the carbon ink was dropped on the surface of the glassy carbon rotating disk electrode, yielding an approximate catalyst loading of 0.39 mg·cm-2. For comparison, a commercially


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available platinum/carbon catalyst, nominally 20 wt. % on carbon black from E-TEK was used. The platinum based ink was obtained by mixing 1 mg catalyst, 50 µL of a 5 wt % Nafion solution in alcohol, 950 µL of ethanol. Then, a 15 µL aliquot of the platinum ink was dropped on the glassy carbon rotating disk electrode, yielding an approximate loading of 0.12 mg cm-2 or 24 ugPt cm-2. The electrochemical performance was conducted in 0.1 M HClO4; the counter and the reference electrodes were a carbon rod and a SCE electrode, respectively. The potential of the electrode was controlled by an EG&G (model CHI750e) potentiostat/galvanostat system. Oxygen reduction reaction (ORR) measurements were conducted in oxygen saturated 0.1 M HClO4 solution which was purged with oxygen during the measurement. The scan rate for ORR measurement was 5 mV s-1. The ORR polarization curves were collected at 1600 rpm. Long-term operation stability of Pt0.61Ni/BP were performed at room temperature in oxygen saturated 0.1 M HClO4 solutions by applying cyclic potential sweeps between 0.5 and 1.1 V versus reversible hydrogen electrode (RHE) at a sweep rate of 100 mVs-1 for 10,000 cycles. For commercial Pt/C catalyst, the long-term operation stability was evaluated similarly in oxygen saturated 0.1 M HClO4 solution for 10,000 cycles. For the calculation of yields of H2O2 on different catalysts, based on both ring and disk currents from rotating ring disk electrode (RRDE), the percentage of HO2- generated from ORR and the electron transfer number (n) were estimated by the following equations: 23 HO2− % = 200 ×

n = 4×

iR N iD + iR N

iD iD + iR N

(1)

(2)

Where iD is the disk current density, is the ring current density and N is the current collection efficiency of the Pt ring disk. N is 0.37 from the reduction of K3Fe [CN]6.



All the current densities have already been normalized to the electrode surface area. Single cell tests. The membrane electrode assembly (MEA) includes cathode gas diffusion layer (GDL), cathode catalyst layer, anode GDL, anode catalyst layer and proton exchange membranes. The GDL on both electrodes was PTFE-treated carbon paper (Toray TGP-H-060) covered with 0.4 mg cm-2 carbon powder containing 40wt% PTFE. The cathode catalyst Pt0.61Ni/C was mixed with Nafion solution (DuPont, 5 wt %) and ethanol with a mass ratio of 1:20:30 to obtain a uniform ink, which was then brushed onto the cathode GDL to obtain the cathode. The loading of Pt0.61Ni/C on the electrode was 3 mg cm-2. The anode catalyst layer was prepared by a catalyst-coated-membrane procedure. Specifically, 20 wt% Pt/C (Johnson Matthey), Nafion solution and absolute ethanol with a mass ratio of 1:5:200 were mixed uniformly to obtain catalyst ink, which was directly sprayed on one side of the Nafion212 membrane until the Pt loading is 0.02 mg cm-2 to obtain the anode catalyst layer after dried. Then the MEA components was stacked up in the order of anode GDL, Nafion membrane with the anodic catalyst layer facing down and cathode GDE with the cathodic catalyst layer facing down, and then placed on a hot plate at 130 ºC for 120s without pressure. The obtained MEA was sandwiched between two Au-plating stainless steel bi-polar plates embedded graphite plates with flow fields. The active area of the MEA is 1cm2. The single cell was evaluated on a fuel cell test station (Green Light, Inc.) at a cell temperature of 80 oC. The anode supply was pure H2 and the flow rate was 100 sccm with 100% humidity. The cathode supply was pure O2 /Air and the flow rate was 200 mL min-1 with 100% humidity. For the H2/O2 cell, the discharging curve was recorded at different back-pressure of 0.5/1.5/2/2.5 bar. For the H2/Air cell, the discharging curve was recorded at the


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back-pressure of 0.5 bar. The fuel cell with commercial Pt/C as both cathode (0.08 mgPt cm-2) and anode (0.02 mgPt cm-2) was prepared and tested in the same way as described above.

RESULTS AND DISCUSSION In a typical synthesis of PtxNi/C samples, Ni/C with Ni content of 3 wt.% served as the reaction initiating materials (Figure S1).22 To obtain PtxNi/C samples with different Pt contents, different amount of Pt precursor [PtCl4]2- was dropped into the ink made from Ni/C in water and stirred overnight to replace some Ni atoms on Ni nanoparticle surface with Pt atoms through a galvanic replacement reaction. The obtained samples were then washed thoroughly with water by filtration to remove replaced Ni ions and further sintered at 900 oC under argon (Ar) atmosphere (see Experimental Section for more details). According to the atomic ratio of Pt/Ni determined by ICP-OES (Table S1), the synthesized materials with different Pt/Ni atomic ratio were named as Ni/C, Pt0.13Ni/C, Pt0.34Ni/C, Pt0.52Ni/C, Pt0.61Ni/C, Pt0.72Ni/C. To characterize their ORR performance, rotating disk electrode (RDE) tests of these samples were performed in HClO4 solution with oxygen saturated, for comparison, commercial Pt/C was also tested as reference (Figure S2a). Interestingly, Figure 1a shows that the ORR activities indicated by the values of half-wave potentials (E1/2) present a volcano-shaped dependence on the atomic ratio of Pt/Ni, which probably could be attributed to the fact that the Ni atoms can moderate the electronic structure of Pt and then lead to a largest improvement of performance at an optimal composition of alloy.

24

The optimal atomic ratio of Pt/Ni is 0.61 with best E1/2 (0.857 V),

slightly better than the commercial Pt/C (0.820 V) with Pt content of 20 wt. % (Table S1). To better understand the Pt utilization, the kinetic currents on the ORR polarization curves were normalized to the Pt mass. As shown in Figure S2b, the Pt utilization of 0.096 A/mgPt for



Pt0.61Ni/C is about 5 times of commercial Pt/C (0.018 A mgPt) at 0.9 V and about 2 times of Pt/C at the limiting-diffusion current area of 0.3V (Figure S3). The high performance or Pt utilization of Pt0.61Ni/C may be attributed to the fact that transition metal Ni can moderate the Pt electronic structure and then lead to an improved activity of Pt for ORR.10

Figure 1. Electrochemical and physical characterization of different catalysts. (a) The atomic ratio of Pt/Ni dependence of the half-wave potential (E1/2) (the red dot represents the E1/2 of commercial Pt/C (b,c) Typical transmission electron microscopy (TEM) images of Pt0.61Ni/C (Inset in c represents a part of an individual particle). Elemental mapping images of Pt (d), Ni (e) and PtNi (f) on a single particle shown in (c) in Pt0.61Ni/C catalyst.

In order to deeply understand the structure/morphology of the obtained Pt0.61Ni/C catalyst, the high-angle annular dark-field scanning TEM (HAADF-STEM) images of the Pt0.61Ni/C catalyst 10 Environment ACS Paragon Plus

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were obtained. As shown in Figure 1b, the metal nanoparticles with size about 3.94 nm are dispersed uniformly on carbon support (Figure S4). The lattice fringes on small metal nanoparticles show a measured inner-planar distance of 0.216 nm, consistent with the wellknown reported lattice distance of PtNi alloy;25 while on the surface of some larger metal nanoparticles, we can see the coexistence of Ni metal, Pt metal and PtNi alloy simultaneously as shown in Figure S5. Such difference observed indicates a stochastic incomplete replacement of surface Ni by Pt. The elemental mapping images of a small PtNi nanoparticle shown in Figure 1d-f indicate the inhomogeneous Pt distribution on the surface of a single Ni nanoparticle. Furthermore, X-ray diffraction (XRD) spectra were used to investigate the as-prepared PtNibased catalyst. As shown in Figure 2, compared with standard XRD spectra of Pt or Ni nanoparticles on carbon, it can be seen that the peaks for both Pt and Ni in the PtxNi/C samples all shifted positively to higher angles. Such positive shifts confirm the formation of PtNi alloy which has been proved to be effective for improving the ORR electrocatalytic activity of Pt.26-28 The composition (contents of Pt and Ni) of Pt-Ni alloy was further calculated based on Bragg’s law (SI) and Vegard’s law (SI). The calculating results shown in Table S2 indicate that the Pt-Ni alloy phase exists in all of the samples of PtxNi/C but with different Pt/Ni ratio in Pt-Ni alloy phase. The Pt composition in the alloy phase increased from Pt0.13Ni/C to Pt0.72Ni/C with the increase of the amount of Pt precursor ([PtCl4]2- ) during the replacement process. However, the ORR activities of these PtxNi/C catalysts present a volcano-shaped dependence on the atomic ratio of Pt/Ni (Figure 1a). This volcano plot of Pt/Ni ratio can be attributed to the optimal Pt/Ni ratio (~5:1) existed in the Pt-Ni alloy phase of the Pt0.61Ni/C catalyst (Table S2) which could make the Pt0.61Ni/C catalyst possess the highest electrocatalytic ORR performance because of the modest existence of Ni atoms in the Pt-Ni alloy phase could preferably regulate Pt electronic



structure leading to an optimal activity of PtxNi/C catalyst which is consistent with the previous reports.12,24,29,30

Figure 2. X-ray diffraction spectra for different catalysts: E-TEK-Pt/C, Ni/C, Pt0.13Ni/C, Pt0.34Ni/C, Pt0.56Ni/C, Pt0.61Ni/C and Pt0.72Ni/C catalyst.

The Pt0.61Ni nanoparticles dispersed on carbon support were further characterized with X-ray photoelectron spectroscopy (XPS). For comparison, the spectra for Pt/C and Ni/C were also presented. Figure 3a shows that most of the Ni atoms (79.4%) on Ni/C catalyst are metal Ni (0) while with small amount of them in oxide as NiO or Ni(OH)2.21 The observed Ni oxide could be due to the partial oxidation of surface Ni atoms. After the replacement of surface Ni by Pt, interestingly, as shown in Figure 3b, part of the surface Ni atoms were oxidized into NiO or Ni(OH)2. It should be noted here, the detection of Ni oxide (Fig. 3b) on Pt0.61Ni/C from XPS spectrum is apparently different the XRD results (Fig. 2) which show the existence of mainly Ni(0). Such difference could be due to the difference of detection mechanisms between these two methods. Generally, XPS analysis mainly distinguishes different state and species on the surface of samples,31,32 while XRD mainly distinguishes the state and species of atoms in the whole 12 Environment ACS Paragon Plus

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samples.33,34 Such apparent difference indicates, i) the majority of Ni on the catalyst is in metallic state (Ni(0)) indicated by XRD results (Fig. 2), ii) part of the surface Ni atoms were oxidized by oxygen in air indicated by XPS analysis (Fig. 3b). While the oxidation of surface Ni by oxygen is inevitable since the catalyst works always in oxygen atmosphere for ORR, but such oxidation should not affect much the catalytic properties of PtNi alloy for ORR due to the fact that such oxide can be reduced to Ni(0) when the electrode potential is negative for ORR process. According to previous knowledge, the surface oxidized Ni can promote the adsorption of O2 in the nearby Pt sites and then significantly enhance the ORR activity of Pt0.61Ni/C.35 Additionally, as for the Pt XPS spectrum in Pt0.61Ni/C, compared with the bulk Pt in commercial Pt/C as shown in Table S3, Figure 3c and Figure 3d, the Pt 4f peak position of Pt0.61Ni/C shows a slight shift to higher binding energy due to the strong interaction between Ni and Pt in PtNi alloy. Interestingly, compared with commercial Pt/C, most of the Pt atoms on the Pt0.61Ni/C are metal (0) (80.2%). Such high content of Pt (0) indicates that the alloying of Ni with Pt can promote the observed high ORR performance later on due to the well-known knowledge that the Pt (0) usually possesses higher ORR catalytic activity than Pt oxides.21,36



Figure 3. High resolution XPS spectra of Ni for Ni/C (a) and Pt0.61Ni/C (b); High resolution XPS spectra of Pt for Pt0.61Ni/C (c) and commercial Pt/C (d).

The relative yields of water (H2O) (through a 4e- pathway) and hydrogen peroxide (H2O2) (through a 2e- pathway) were also quantified to further confirm the ORR efficiency of Pt0.61Ni/C.27 Based on the currents on both disk and ring from rotating ring disk electrode (RRDE) (Figure S6), the electron transfer number (n) and the H2O2 yields were obtained at different potentials. As shown in Figure 4a, compared with commercial Pt/C, the Pt0.61Ni/C always presents larger n (~3.97) and lower HO2- yield (approximately to 1.35%), indicating that a high efficient 4e-ORR process on Pt0.61Ni/C.


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Figure 4. (a)The electron transfer number (n) (left) and the H2O2 yield (right) on the catalysts of Pt0.61Ni/C and commercial Pt/C. (b) Durability test of Pt0.61Ni/C for ORR. (c) The response of Pt0.61Ni/C and commercial Pt/C to methanol (0.5 M). All these tests shown in (a,b,c) were done in O2-saturated 0.1 M HClO4. (d) The polarization curves of acidic H2/O2 fuel cell with Pt0.61Ni/C as cathode (3 mgPt0.61Ni/C cm-2 or 0.083 mgPt cm-2) and Nafion212 as the membrane (anode: 0.02 mgPt cm-2, back pressure: 2.5 bar, 80°C, H2 and O2 in 100% RH).

Furthermore, according to

the accelerated durability test protocol proposed by U.S.

Department of Energy, the ORR durabilities of both Pt0.61Ni/C and commercial Pt/C were further evaluated by multiple CV cycling the catalysts between 0.5 and 1.1 VRHE at a sweep rate of 100 mV s-1 in O2-saturated HClO4.37,38 As shown in Figure S7, the E1/2 of the state-of-art commercial Pt/C shifts to negative potential by 37 mV after 10,000 CV cycles, indicating a severely attenuation of Pt occurred on Pt/C. As for the Pt0.61Ni/C, however, it exhibited a much better



durability (Figure 4b) indicated by a small negative shift (~ 7 mV) of E1/2 after 10,000 cycles of CV. The remarkable durability of Pt0.61Ni/C could be ascribed to the anchoring effect of Ni to Pt on carbon.39 Such remarkable durability was further confirmed by the TEM and XRD analysis (Figure S8) of Pt0.61Ni/C catalyst after 10,000 cycles of CV. Interestingly, in contrast to the large response of traditional commercial Pt/C to methanol (Figure 4c), the Pt0.61Ni/C is almost inert to methanol, indicating that PtNi alloy is a methanol-tolerant ORR catalyst to some extent.26 Such tolerance to methanol makes it a promising alternative for the traditional cathode Pt/C in direct methanol fuel cells. In an attempt to further confirm the high electrocatalytic activity of Pt0.61Ni/C for ORR, the acidic H2/O2 single cell measurement with Pt0.61Ni/C as cathode catalyst was also performed (Experimental Section). As illustrated in Figure 4d and Figure S9, the acidic H2/O2 PEMFC with Pt0.61Ni/C as cathode (83 ugPt cm-2) tested at the backpressure of 2.5 bar presents the highest performance with the mass activity of 0.38 A mgPt at 0.9V and maximum power density up to 1.10 W cm-2 at 80 oC, corresponding to a highest Pt utilization (0.093gPt kW-1) ever (Table S4) in H2/O2 fuel cells. Interestingly, as shown in Figure S9, even at very low back pressure of 0.5 bar, the Pt utilization of 0.136gPt kW-1 is still comparable with previous best result (Table S4). Additionally, the acidic H2/air PEMFC with Pt0.61Ni/C as cathode (83 ugPt cm-2) also shows a considerable performance with the current density of 0.27A cm-2 or mass activity of 2.62 A mgPt at 0.6 V and maximum power density of 0.22 W cm-2 at the back pressure of 0.5 bar and 80 oC (Figure S10).

CONCLUSION


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In summary, based on a simple galvanic replacement reaction, we prepared a carbon supported PtNi nanocatalyst (Pt0.61Ni/C) with ultralow Pt loading (2.76 wt.%) for high efficient ORR process. The catalyst exhibits a high mass activity, remarkable durability and methanol tolerance. More significantly, the acidic single-cell with Pt0.61Ni/C as cathode catalyst delivers power density up to 1.1W cm-2 and a highest platinum utilization ever (0.093 gPt kW-1) at 80oC. These results indicate that the obtained Pt0.61Ni/C catalyst is one of the most feasible substitutes to conventional Pt-based catalysts for fuel cells in future.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.xxx. Electrochemical measurement data and single cell test data of catalysts; TEM images of catalysts, including histograms of particle sizes. AUTHOR INFORMATION Corresponding Author *Email: [email protected] (W. X.) ORCID Weilin Xu: 0000-0001-7140-8060 Notes The authors declare no competing financial interest.



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ACKNOWLEDGMENTS Work was supported by National Basic Research Program of China (973 Program, 2014CB932700), National Natural Science Foundation of China (U1601211, 21433003, 21733004,

and

21633008),

and

Natural

Science

Foundation

of

Jilin

Province

(Nos.20160519005JH, 20160520137JH).

REFERENCES (1) Jaouen, F.; Proietti, E.; Lefèvre, M.; Chenitz, R.; Dodelet, J.-P.; Wu, G.; Chung, H. T.; Johnston, C. M.; Zelenay, P. Recent Advances in Non-precious Metal Catalysis for Oxygenreduction Reaction in Polymer Electrolyte Fuel Cells. Energy Environ. Sci. 2011, 4, 114-130. (2)Carrette, L.; Friedrich, K. A.; Stimming, U. Fuel Cells: Principles, Types, Fuels, and Applications. ChemPhysChem 2000, 1, 162-193. (3) Haile, S. M. Fuel Cell Materials and Components. Acta Mater. 2003, 51, 5981-6000. (4) Steele, B. C. H.; Heinzel, A. Materials for Fuel-cell Technologies. Nature 2001, 414, 345352. (5) Debe, M. K. Electrocatalyst Approaches and Challenges for Automotive Fuel Cells. Nature 2012, 486, 43-51. (6) Lepró, X.; Terrés, E.; Vega-Cantú, Y.; Rodríguez-Macías, F. J.; Muramatsu, H.; Kim, Y. A.; Hayahsi, T.; Endo, M.; Torres R, M.; Terrones, M. Efficient Anchorage of Pt Clusters on Ndoped Carbon Nanotubes and Their Catalytic Activity. Chem.Phys. Lett. 2008, 463, 124-129.


Page 19 of 24 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


(7) Chen, Y.; Wang, J.; Liu, H.; Li, R.; Sun, X.; Ye, S.; Knights, S. Enhanced Stability of Pt Electrocatalysts by Nitrogen Doping in CNTs for PEM Fuel Cells. Electrochem. Commun. 2009, 11, 2071-2076. (8) Bu, L.; Zhang, N.; Guo, S.; Zhang, X.; Li, J.; Yao, J.; Wu, T.; Lu, G.; Ma, J.-Y.; Su, D.; Huang, X. Biaxially Strained PtPb/Pt Core/Shell Nanoplate Boosts Oxygen Reduction Catalysis. Science 2016, 354, 1410-1414. (9) Huang, X.; Zhao, Z.; Cao, L.; Chen, Y.; Zhu, E.; Lin, Z.; Li, M.; Yan, A.; Zettl, A.; Wang, Y. M.; Duan, X.; Mueller, T.; Huang, Y. High-performance Transition Metal–doped Pt3Ni Octahedral for Oxygen Reduction Reaction. Science 2015, 348, 1230-1234. (10) Zhao, X.; Chen, S.; Fang, Z.; Ding, J.; Sang, W.; Wang, Y.; Zhao, J.; Peng, Z.; Zeng, J. Octahedral [email protected] Core–Shell Nanocrystals with Ultrathin PtNi Alloy Shells as Active Catalysts for Oxygen Reduction Reaction. J. Am. Chem. Soc. 2015, 137, 2804-2807. (11) Baldizzone, C.; Mezzavilla, S.; Carvalho, H. W. P.; Meier, J. C.; Schuppert, A. K.; Heggen, M.; Galeano, C.; Grunwaldt, J.-D.; Schüth, F.; Mayrhofer, K. J. J. Confined-Space Alloying of Nanoparticles for the Synthesis of Efficient PtNi Fuel-Cell Catalysts. Angew. Chem. Int. Ed. 2014, 53, 14250-14254. (12) Stamenkovic, V. R.; Fowler, B.; Mun, B. S.; Wang, G.; Ross, P. N.; Lucas, C. A.; Markovic, N. M. Improved Oxygen Reduction Activity on Pt3Ni(111) via Increased Surface Site Availability. Science 2007, 315, 493-497.



(13) Fu, S.; Zhu, C.; Song, J.; Engelhard, M. H.; He, Y.; Du, D.; Wang, C.; Lin, Y. Threedimensional PtNi Hollow Nanochains as An Enhanced Electrocatalyst for the Oxygen Reduction Reaction. J. Mater. Chem. A 2016, 4, 8755-8761. (14) Choi, J.; Lee, Y.; Kim, J.; Lee, H. Enhancing Stability of Octahedral PtNi Nanoparticles for Oxygen Reduction Reaction by Halide Treatment. J. Power Sources 2016, 307, 883-890. (15) Shen, L.-L.; Zhang, G.-R.; Miao, S.; Liu, J.; Xu, B.-Q. Core–Shell Nanostructured Au@NimPt2 Electrocatalysts with Enhanced Activity and Durability for Oxygen Reduction Reaction. ACS Catal. 2016,6, 1680-1690. (16) Wang, G.; Huang, B.; Xiao, L.; Ren, Z.; Chen, H.; Wang, D.; Abruña, H. D.; Lu, J.; Zhuang, L. Pt Skin on AuCu Intermetallic Substrate: A Strategy to Maximize Pt Utilization for Fuel Cells. J. Am. Chem. Soc. 2014, 136, 9643-9649. (17) An, W.; Liu, P. Rationalization of Au Concentration and Distribution in AuNi@Pt Core– Shell Nanoparticles for Oxygen Reduction Reaction. ACS Catal. 2015, 5, 6328-6336. (18) Oezaslan, M.; Hasché, F.; Strasser, P. Pt-Based Core–Shell Catalyst Architectures for Oxygen Fuel Cell Electrodes. J. Phys. Chem. Lett. 2013, 4, 3273-3291. (19) Gan, L.; Cui, C.; Rudi, S.; Strasser, P. Core–Shell and Nanoporous Particle Architectures and Their Effect on the Activity and Stability of Pt ORR Electrocatalysts. Top. Catal. 2014, 57, 236-244. (20) Zhu, J.; Xiao, M.; Li, K.; Liu, C.; Zhao, X.; Xing, W. Active Pt3Ni (111) Surface of Pt3Ni Icosahedron for Oxygen Reduction. ACS Appl. Mater. Interfaces 2016, 8, 30066-30071.


Page 20 of 24

Page 21 of 24 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


(21) Du, S.; Lu, Y.; Malladi, S. K.; Xu, Q.; Steinberger-Wilckens, R. A Simple Approach for PtNi-MWCNT Hybrid Nanostructures as High Performance Electrocatalysts for the Oxygen Reduction Reaction. J. Mater. Chem. A 2014, 2, 692-698. (22) Wu, G.; Liang, X.; Zhang, H.; Zhang, L.; Yue, F.; Wang, J.; Su, X. Highly Stable and Sub-3 nm Ni Nanoparticles Coated with Carbon Nanosheets as a Highly Active Heterogeneous Hydrogenation Catalyst. Catal. Commun. 2016, 79, 63-67. (23) Tan, Y.; Xu, C.; Chen, G.; Fang, X.; Zheng, N.; Xie, Q. Facile Synthesis of ManganeseOxide-Containing Mesoporous Nitrogen-Doped Carbon for Efficient Oxygen Reduction. Adv. Funct. Mater. 2012, 22 (21), 4584-4591. (24) Mezzavilla, S.; Baldizzone, C.; Swertz, A.; Hodnik, N.; Pizzutilo, E.; Polymeros, G.; Keeley, G.; Knossalla, J.; Heggen, M.; Mayrhofer, K. J. J.; Schüth, F. Structure–Activity– Stability Relationships for Space-Confined PtxNiy Nanoparticles in the Oxygen Reduction Reaction. ACS Catal. 2016, 6, 8058-8068. (25) Zhang, C.; Hwang, S. Y.; Trout, A.; Peng, Z. Solid-State Chemistry-Enabled Scalable Production of Octahedral Pt–Ni Alloy Electrocatalyst for Oxygen Reduction Reaction. J. Am. Chem. Soc. 2014, 136, 7805-7808. (26) Zou, L.; Fan, J.; Zhou, Y.; Wang, C.; Li, J.; Zou, Z.; Yang, H. Conversion of PtNi Alloy from Disordered to Ordered for Enhanced Activity and Durability in Methanol-tolerant Oxygen Reduction Reactions. Nano Res. 2015, 8, 2777-2788.



(27) Yang, H.; Vogel, W.; Lamy, C.; Alonso-Vante, N. Structure and Electrocatalytic Activity of Carbon-Supported Pt−Ni Alloy Nanoparticles Toward the Oxygen Reduction Reaction. J. Phys. Chem. B 2004, 108, 11024-11034. (28) Gao, H.; Liao, S.; Zeng, J.; Xie, Y.; Dang, D. Preparation and Characterization of Core– shell Structured Catalysts using PtxPdy as Active Shell and Nano-sized Ru as Core for Potential Direct Formic Acid Fuel Cell Application. Electrochimica Acta 2011, 56, 2024-2030. (29) Beermann, V.; Goclya, M.; Kuhl, S.; Padgett, E.; Schmies, H.; Goerlin, M.; Erini, N.; Shviro, M.; Heggen, M.; Dunin-Borkowski, R.; Muller, D.; Strasser, P. Tuning the Electrocatalytic Oxygen Reduction Reaction Activity and Stability of Shape-Controlled Pt–Ni Nanoparticles by Thermal Annealing−Elucidating the Surface Atomic Structural and Compositional Changes. J.Am. Chem. Soc. 2017, 139, 16536-16547. (30) Dubau, L.; Asset, T.; Chattot, R.; Bonnaud, C.; Vanpeene, V.; Nelayah, J.; Maillard, F. Tuning the Performance and the Stability of Porous Hollow PtNi/C Nanostructures for the Oxygen Reduction Reaction. ACS Catal. 2015, 5, 5333−5341. (31) Kim, K. S.; Winograd, N. X-Ray Photoelectron Spectroscopic Studies of RutheniumOxygen Surfaces. J. Catal. 1974, 35, 66-72. (32) Kim, K. S.; Gossmann, A. F.; Winograd, N. X-Ray Photoelectron Spectroscopic Studies of Palladium Oxides and the Palladium-Oxygen Electrode. Anal. Chem.1974, 46, 197-200. (33) Chipera, S. J.; Bish, D. L. Baseline Studies of the Clay Minerals Society Source Clays: Powder X-ray Diffraction Analyses. Clay. Clay Miner. 2001, 49, 398-409 (34) Borchert, H.; Shevchenko, E. V.; Robert, A.; Mekis, I.; Kornowski, A.; Grubel, G.; Weller,


Page 22 of 24

Page 23 of 24 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


H. Determination of Nanocrystal Sizes: A Comparison of TEM, SAXS, and XRD Studies of Highly Monodisperse CoPt3 Particles. Langmuir, 2005, 21, 1931–1936. (35) Ramos-Sanchez, G.; Praserthdam, S.; Godinez-Salomon, F.; Barker, C.; Moerbe, M.; Calderon, H. A.; Lartundo, L. A.; Leyva, M. A.; Solorza-Feria, O.; Balbuena, P. B. Challenges of Modelling Real Nanoparticles: Ni@Pt electrocatalysts for the Oxygen Reduction Reaction. Phys. Chem. Chem. Phys.2015, 17, 28286-28297. (36) Jiang, Q.; Jiang, L.; Hou, H.; Qi, J.; Wang, S.; Sun, G. Promoting Effect of Ni in PtNi Bimetallic Electrocatalysts for the Methanol Oxidation Reaction in Alkaline Media: Experimental and Density Functional Theory Studies. J. Phys. Chem. C 2010, 114, 1971419722. (37) Gong, K.; Du, F.; Xia, Z.; Durstock, M.; Dai, L. Nitrogen-Doped Carbon Nanotube Arrays with High Electrocatalytic Activity for Oxygen Reduction. Science 2009, 323, 760-764. (38) Sun, X.; Song, P.; Zhang, Y.; Liu, C.; Xu, W.; Xing, W. A Class of High Performance Metal-Free Oxygen Reduction Electrocatalysts based on Cheap Carbon Blacks. Sci. Rep. 2013, 3,2505. (39) Colón-Mercado, H. R.; Kim, H.; Popov, B. N. Durability study of Pt3Ni1 catalysts as cathode in PEM fuel cells. Electrochem. Commun. 2004, 6, 795-799.



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C for High-Efficiency Cathode of Fuel Cells with Superhigh

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