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PtRu alloy nanoparticles supported on nanoporous gold as an efficient anode catalyst for direct methanol fuel cell
Accepted Manuscript PtRu alloy nanoparticles supported on nanoporous gold as an efficient anode catalyst for direct methanol fuel cell Miaomiao Tian, Shuai Shi, Yongli Shen, Huiming Yin PII:
S0013-4686(18)32287-4
DOI:
10.1016/j.electacta.2018.10.048
Reference:
EA 32845
To appear in:
Electrochimica Acta
Received Date: 25 September 2018 Revised Date:
8 October 2018
Accepted Date: 8 October 2018
Please cite this article as: M. Tian, S. Shi, Y. Shen, H. Yin, PtRu alloy nanoparticles supported on nanoporous gold as an efficient anode catalyst for direct methanol fuel cell, Electrochimica Acta (2018), doi: https://doi.org/10.1016/j.electacta.2018.10.048. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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PtRu alloy nanoparticles supported on nanoporous gold as an efficient anode catalyst for direct methanol fuel cell
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Miaomiao Tian1, Shuai Shi1, Yongli Shen, Huiming Yin* Tianjin Key Laboratory of Advanced Functional Porous Materials and Institute for
Tianjin 300384, China. * Corresponding author. E-mail address: [email protected] These authors contributed equally to this work.
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1
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New Energy Materials and Low Carbon Technologies, Tianjin University of Technology,
Abstract
An efficient and CO-tolerant anode catalyst for direct methanol fuel cell is fabricated by depositing PtRu alloy nanoparticles (~3 nm) on nanoporous gold film (NPG-PtRu).
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Those catalysts with various Pt/Ru molar ratios are prepared by using linear scanning voltammetry and their electrocatalytic activities are measured for methanol oxidation reaction (MOR) and CO oxidation. Owing to the notably reduced CO adsorption energy, NPG-PtRu catalyst with Pt/Ru molar ratio of ~2:1 exhibits close
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CO-tolerance but 3-fold enhanced MOR activity in contrast to commercial JM-PtRu/C, and thus is chosen as the anode in direct methanol fuel cell. The membrane
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electrode assembly with anode of NPG-PtRu (0.2 mgPt cm-2 and 0.3 mgAu cm-2) shows a maximum power density of 96 mW cm-2, about 6-fold enhancement of Pt efficiency than that of commercial PtRu/C (1 mgPt cm-2, 78 mW cm-2) under similar conditions. Keywords: nanoporous gold; PtRu; methanol oxidation; fuel cell 1. Introduction In recent years, polymer electrolyte membrane fuel cells have been viewed as the promising energy conversion devices for developing sustainable and clean energy. Direct methanol fuel cell (DMFC), since the high energy density and easy handling of the liquid methanol fuel, will extremely reduce the complexity and cost of the energy
ACCEPTED MANUSCRIPT conversion system. Therefore, DMFC offers certain specific advantages over proton exchange membrane fuel cells (PEMFC) for portable, transportation, and stationary power sources [1-5]. However, different from the facile hydrogen oxidation reaction with a two-electron transfer in the anode of hydrogen-feed PEMFC, methanol
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oxidation reaction (MOR) exhibits a slow oxidation kinetics and releases various partial oxidation intermediates such as COHad and COad. These intermediates will strongly adsorb on the surface of the most effective metal catalyst i.e. Pt, thus block the oxidation process and reduce the performance of DMFC [2-5]. Therefore, there
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are two major challenges for the commercialization of DMFC: one is the sluggish methanol oxidation kinetics even on some state-of-the-art anode catalysts, and the
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other is the methanol crossover through the membrane, which not only depresses cathode performance, but also reduces fuel efficiency. Currently, exploring methanol-tolerant cathode catalysts and methanol-preventing membranes is the main strategy to solve the performance depression by methanol crossover. According to the sluggish kinetics of MOR, nanoscale catalysts with suitable structures,
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compositions, i.e. hollow or core-shell structures, binary or ternary alloy, as well as optimized supports i.e. metal oxides, carbon-based materials, have been abundantly investigated for anodic catalysts in DMFC, especially Pt-based catalysts towards acceptable fuel cell performance for practical application [1, 6-12].
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Among Pt-based bimetallic catalysts that have been investigated for MOR, Pt-Ru alloy was proved to be the most practical electro-catalysts for the DMFC application
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with reasonable activities and stability [2-6, 13-20]. According to a well-described mechanism, the two primary processes that comprise the overall MOR are the initial dehydrogenation step producing intermediates like CO (equation (1)), and the CO removal step including water activation and CO oxidation (equation (2) and (3)). Pt + CH3OH → Pt-CO + 4 H+ + 4 e
(1)
Pt + H2O → Pt-OH + H+ + e
(2)
Pt-CO + Pt-OH → 2 Pt + CO2 + H+ + e
(3)
In methanol electro-oxidation, the active sites of Pt will be occupied by the intermediates e.g. CO from equation (1), because equation (2) producing the
ACCEPTED MANUSCRIPT necessary oxidative groups i.e. OHad for removing adsorbed CO should react at a high potential on Pt (from 0.75 V). Therefore, additive of Ru, which can provide oxygenated species at lower potentials (equation (4), from 0.35 V) will undoubtedly enhance the resistance to CO-poisoning (equation (5)) and
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improve the electro-catalytic activities for MOR [21]. Including the bifunctional mechanism mentioned above, electronic effect from Ru could also enhance the
methanol dehydrogenation step and weaken the adsorption strength of COad on
Pt sites [2]. Therefore, rationally designing the PtRu alloy structure is highly
Ru + H2O → Ru-OH + H+ + e
(4) (5)
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Pt-CO + Ru-OH → Pt + Ru + CO2 + H+ + e
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desired to maximize the role of Ru for high CO-tolerance and MOR activity.
As well as the decreased removing potential ascribed to oxidic species, reducing the binding energy between Pt and CO will further improve the resistance to CO poisoning. Yang et al reported that Au nanoparticles adsorbed onto the Pt-Ru alloy particles could weaken the adsorption of CO and thus promote the catalytic activity
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and durability of PtRu/C [22]. In addition, gold clusters modified on PtRu alloy could also reduce the dissolution of the more oxophilic Ru and thus improve the activity and stability towards MOR [22-26].
Nanoporous metals e.g. nanoporous gold (NPG) films prepared by dealloying
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method have exhibited excellent electrocatalytic activities owing to the unique nanoscale bi-continuous pore/ligament structure [27,28]. The continuous pores
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ensure smooth channels for mass transfer while the connected metal ligaments provide abundant active sites for surface reactions and accelerated electron conduction [29-32]. On the other hand, unlike the nanoparticles, the curved surfaces of the dealloyed nanoporous metals provide a high density of steps and kinks which are active for chemical reactions, high thermal stability [28]. And what’s more, the facilely modified metal ligaments and the strong metal bond interact make nanoporous metal films ideal substrates to construct electrocatalysts with enhanced durability. In this paper, NPG film was used as the substrate, and PtRu alloy nanoparticles
ACCEPTED MANUSCRIPT with various Pt/Ru molar ratios were coated on the substrate (NPG-PtRu) for high CO-tolerance and MOR activity. A comparative evaluation of the structure, composition and electrochemical performance of the NPG-PtRu catalysts is discussed together with commercial PtRu/C. And furthermore, the optimized NPG-PtRu catalyst
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with the highest catalytic activity and strong resistance to CO poisoning was applied as the anode catalyst in DMFC and the output power density was measured relative to commercial PtRu/C. 2. Experimental
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2.1. Chemicals
Chloroplatinic acid (H2PtCl6), ruthenium trichloride (RuCl3), nitric acid (HNO3),
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perchloric acid (HClO4), sulphuric acid (H2SO4), and methanol (CH3OH) were purchased from Sinopharm Chemical Reagent Co., Ltd. Commercial Pt/C (60%, Johnson Matthey), PtRu/C (60 wt%, Johnson Matthey) and membrane Nafion 115 (DuPont) were purchased from Alfa Aesar Co., Ltd. Au-Ag alloy films were purchased from Sepp Leaf Products, Inc. All chemicals were used as received without further
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purification.
2.2. Synthesis of NPG-Pt and NPG-PtRu
NPG substrate with ligament size ~30 nm was prepared by dealloying 100-nm-thick, 12-Karat Au-Ag alloy films in concentrated HNO3 (65 wt.%) for 30 min at 30 °C [33,34].
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The obtained NPG substrates were thoroughly rinsed and preserved in ultrapure water (18.23 MΩ cm) for further use. Plating solutions with various Pt/Ru molar
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ratios of 4/0.25, 4/2, 4/4 and 4/7 were prepared by dissolving H2PtCl6 and RuCl3 in 0.1 M HClO4 and the concentration of platinum was fixed to be 4 mM. Electro-deposition of PtRu alloy or Pt nanoparticles were prepared using an electrochemical workstation (VersaSTAT MC, Princeton) with a standard three-electrode system. A graphite plate was used as the counter electrode, and a saturated calomel electrode (SCE) was used as the reference electrode. As the working electrode, NPG film floating on water was attached on a glassy carbon electrode (φ = 4 mm) by immersing the electrode in water. 2 μL 0.5 wt% solution Nafion-ethanol was dropped on the glassy carbon and the surface area was fixed by erasing the NPG substrate outside the glassy carbon. By
ACCEPTED MANUSCRIPT using the electrochemical cell containing the three-electrode system, PtRu alloy or Pt nanoparticles were deposited on NPG by using negatively potential sweeping from 0.6 to 0 V for 20 cycles with a scan rate of 50 mV s-1. All solutions used in this section were deaerated with ultrapure N2 for approximately 30 min. All the potentials listed
2.3. Physicochemical characterization
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in this paper were normalized to reversible hydrogen electrode (RHE).
The structure and morphology of NPG-PtRu were observed from the high-resolution transmission electron microscope (HRTEM) and high-angle annular
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dark field (HAADF) images (Talos F200X, FEI). The elemental mapping was obtained using the energy dispersive X-ray spectroscopy (EDX) on a FEI Talos F200X. The
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composition and actual precious metal loading of NPG-PtRu was determined by inductively coupled plasma atomic emission spectroscopy (ICP-AES) (IRIS Advantage). All X-Ray diffraction (XRD) measurements were performed with a SmartLab diffractometer (Rigaku Co.) using a Cu Kα (λ= 1.5405 Å) radiation source. The X-ray photoelectron spectroscopy (XPS) spectra of the catalysts were acquired with an
120 W X-ray source.
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ESALAB 250Xi (Thermo Scientific) spectrometer fitted with an Mg K (h = 1253.6 eV)
2.4. Electrochemical characterization
All electrochemical measurements were performed with an electrochemical
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workstation (CHI 760D, CH Instruments, Inc.). SCE was used as the reference electrodes and a graphite plate was used as a counter electrode. A glassy carbon thin
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film electrode (diameter φ = 4 mm) was used as a working electrode. The NPG-based electrodes were prepared by lift-coating the glassy carbon with the free-standing nanoporous films. Commercial PtRu/C catalyst ink was prepared by ultrasonically dispersing 1 mg of PtRu/C (60 wt.%) catalyst into 3 mL Nafion alcohol solution (0.5 wt.%) for 10 min. Then, 12.5 μL of the ink was drop-coated onto a glassy carbon electrode, resulting in a PtRu loading of ~20 μg cm-2. The coating ink was dried for 20 min in air to form a uniform thin film on the electrode surface. CO stripping test was carried out in 0.5 M H2SO4. The solution was firstly bubbled with ultra-pure CO for 20 min to remove any other impurity gas. Then working
ACCEPTED MANUSCRIPT electrode was immersed into the electrolyte holding the potential at 0.2 V with CO bubbling for 15 min and following N2 bubbling for 30 min to ensure the single atomic layer adsorption of CO molecular. Finally, cyclic voltammetry (CV) curves were performed with an initial potential of 0 V between 0 and 1.2 V at a scanning rate of
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20 mV s-1. Catalytic activity towards methanol oxidation was measured in N2-saturated mixture solution of 0.5 M H2SO4 and 0.5 M CH3OH by using linear scanning voltammetry (LSV) at 10 mV s-1 between 0 and 0.75 V and using CV technique at 50
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mV s-1 between 0 and 1.2 V. Durability of the catalysts were tested by using the chronoamperometry (CA) technique in mixture solution of 0.5 M H2SO4 and 0.5 M
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CH3OH at 0.6 V. All the solutions used in this section were N2-saturated prior to utilization.
2.5. Fabrication of MEA and single cell performance test
In this paper, Nafion 115 was used as the proton exchange membrane and successively pretreated in 3 vol.% H2O2, ultra-pure water, 0.5 M H2SO4 and ultra-pure
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water, at 80 oC for 1 h in each step. Then the membrane was restored in ultra-pure water before utilization. Hydrophobic treated carbon paper (TGP-H-060, Toray) sprayed with 1 mg cm-2 carbon powder was used as the diffusion layer and the homemade NPG-PtRu film was attached on the surface and used as the anode
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catalysts. In comparison, PtRu/C catalyst (Johnson Matthey) was sprayed on the same diffusion layer with the Pt loading of 1 mg cm-2. For all MEAs in this study,
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commercial Pt/C (Johnson Matthey) was used as the cathode at a standard Pt loading of 4 mg cm-2. Hot-pressing was conducted at 130 oC and 100 kg cm-2 for 180 s. As-prepared MEAs were separately assembled into single fuel cells using high
purity graphite plates as flow and current collecting plates. Methanol solution (1 M) was supplied to the anode by a peristaltic pump with the flow rate of 2 mL min-1, while oxygen was fed to the cathode by a mass flow controller with the flow rate of 100 SCCM. The cell temperature was controlled through a temperature controller, and the steady state polarization curves were recorded by using automatic electric load. Electrochemical impedance spectra (EIS) of the MEAs using NPG-Pt2Ru1 and
ACCEPTED MANUSCRIPT PtRu/C as anodes were separately measured with cell voltages of 400 and 440 mV using an electrochemical workstation (VersaSTAT MC, Princeton) in a frequency range from 0.1 Hz to 105 Hz at 30 oC. The amplitude of the AC voltage was 5 mV. 3. Results and Discussion
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3.1. Physicochemical characterization of NPG-PtRu Typical morphology of the NPG substrate is exhibited in Figure S1 which features a smooth and bi-continuous ligament/pore network averaging ~30 nm. To explore the morphology, structure, composition and surface state of NPG-PtRu, typical NPG-PtRu
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sample prepared in mixture solution of 4 mM H2PtCl6, 4 mM RuCl3 and 0.1 M HClO4 was used as the target sample. The composition of this sample was measured to be
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NPG-Pt1.7Ru1 by using ICP-AES. The subscripts of NPG-PtRu represent the molar ratios of Pt/Ru. Electro-deposition of metal alloys is influenced by many factors including the solution composition, deposition process, the substrate and etc [35,36]. According to the preparation method in this study and the complexity of alloy plating, several possible reasons will be given for the inconsistent Pt/Ru ratio between the
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plating solution and the product NPG-PtRu. One possible reason is the electro-deposition method i.e. sweeping from 0.6 to 0 V. During this process, metal with relatively high reduction potential will be deposited preferentially. Other possible reasons may result from the substrate NPG, which possesses the same
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crystal structure as Pt and a close lattice parameter to Pt. The nucleation rate of Pt should be faster than that of Ru on NPG. More information about the morphology
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and crystal structure of the NPG-PtRu sample will be demonstrated using electron microscopy and spectral technology. Figure 1a-d give the HRTEM and HAADF images of NPG-Pt1.7Ru1. Figure 1a-c reveal
that the bi-continuous structure with pore and ligament dimensions averaging ~30 nm is still maintained but the ligament is completely covered with well dispersed nanoparticles ~3 nm. Atomic resolution HAADF images in Figure 1d and Figure S2 illustrate the typical atomic structure of the surface region in detail. Together with the inserted FFT pattern, an epitaxially growth mode is revealed since the particle and substrate are together in one single-crystalline region, like the previous literature
ACCEPTED MANUSCRIPT indicated for NPG-Pt by chemical plating NPG with Pt [29]. Furthermore, the EDS elemental mapping images in Figure 2 indicates that the NPG substrate is coated with a thin layer composed of Pt and Ru. Integrating the figures above, highly dispersed PtRu alloy nanoparticles were uniformly plated on the surface of NPG substrate by
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using electrochemical plating. The nanoparticles have a very narrow size distribution ~3 nm.
XRD patterns of NPG-Pt, NPG-Pt1.7Ru1 and commercial PtRu/C are shown in Figure 3. From Figure 3, broaden diffraction peaks (marked with black dotted lines) of
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face-centred cubic (FCC) Pt are obtained both for PtRu/C and NPG-Pt1.7Ru1 catalyst. Intensive diffraction peaks (marked with red dotted lines) of FCC Au are also
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observed for the NPG substrate in NPG-Pt1.7Ru1. Ascribed to none distinct diffraction peaks related to metal Ru or RuO2 as well as the increased 2θ values of FCC Pt, PtRu alloys have been formed by incorporating Ru into FCC Pt structure [14]. Furthermore, the alloying degrees of the PtRu nanoparticles in NPG-Pt1.7Ru1 and PtRu/C were calculated by using the lattice parameter from XRD patterns and applying the
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calculation method reported in previous literatures [37,38]. Pt (1 1 1) peaks were chosen to calculate the lattice parameter and the peak positions(θmax) were obtained through curve fitting. The lattice parameter(a) was calculated using the following
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equation:
=
√3 2
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while the alloying degree of PtRu nanoparticles (Ru atomic fraction, xRu) was calculated through the following equation proposed by Antolini et al [39]: = 0 − 0.124
The lattice parameters of NPG-PtRu and PtRu/C are separately calculated to be 3.91 and 3.86 Å and the corresponding alloying degrees of PtRu alloys are 8 % and 48% in NPG-PtRu and PtRu/C. The possible reasons for the low alloying degree of NPG-Pt1.7Ru1 include two factors. One is obvious the lower Ru proportion than PtRu/C [40], and the other is the lack of thermal treatment which is commonly used
ACCEPTED MANUSCRIPT for carbon supported PtRu alloy catalysts [39]. As indicated in Figure 4, each XPS spectrum was fitted by considering two resolved doublets. Owing to the large amount of internal Au element in NPG substrate, binding energies of Au 4f7/2 in pristine NPG (84.2 eV), NPG-Pt (84.1 eV) and NPG-PtRu
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(84 ev) are close to each other as indicated in Figure 4a. According to Pt element, Pt 4f7/2 peaks of NPG-Pt (70.8 eV) and NPG-Pt1.7Ru1 (70.6 eV) both exhibit a negative shift in contrast to that of PtRu/C (71.3 eV), while Ru 3p spectra present close binding energies between NPG-Pt1.7Ru1 and PtRu/C. The XPS spectra of PtRu/C in this
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paper is consistent with the data previously reported [14]. Associating with the XPS spectra of Au4f, Pt4f and Ru3p, it is clear that NPG substrate could obviously reduce
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the binding energy of Pt which will weaken the adsorption energy of CO on Pt as previous literatures reported [14,41,42]. Therefore, the XPS data indicate an improved CO-tolerance on NPG-Pt which will be further enhanced by the adding of Ru as revealed before [2]. In addition, previous study demonstrated that the dissolution of Ru from the PtRu electrocatalyst could be reduced by depositing the
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Au clusters onto the PtRu nanoparticles. So, NPG substrate in NPG-PtRu may also enhance the durability of the supported PtRu alloy nanoparticles [23]. 3.2. Electrochemical characterization
Firstly, the interaction between NPG substrate and Pt was evaluated by using the
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CO stripping tests relative to commercial Pt/C. NPG-Pt were fabricated by using the same electro-deposition method as NPG-PtRu. As shown in Figure 5, the absence of
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hydrogen desorption peaks on the CO stripping curves demonstrates the monolayer adsorption of CO on the Pt surface. On NPG-Pt, the peak current potential of CO oxidation reaction (0.83 V) exhibits an obviously negative shift ~150 mV in contrast with Pt/C (0.68 V). This result indicates that the adsorption energy of CO is notably weakened ascribed to the synergetic effect between the substrate NPG and Pt which is consistent with the results from XPS (Figure 4b). Based on the reduced CO adsorption on NPG-Pt, NPG-PtRu samples with various nPt/nRu molar ratios were fabricated and the electrochemical properties were subsequently measured for MOR. CV curves in black solution (0.5 M H2SO4) are
ACCEPTED MANUSCRIPT exhibited in Figure S3, which indicate that typical hydrogen adsorption-desorption peaks of polycrystalline Pt gradually disappear along with the increasing Ru proportions. As CO poisoning resistance plays a key role for MOR, CO stripping measurements were carried out to evaluate the performance for CO-tolerance. The
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striping curves are indicated in Figure 6 while the data are calculated and summarized in Table 1.
According to the onset potentials for CO oxidation reaction, the values evidently shift from ~0.68 to ~0.52 V on NPG-Pt and NPG-Pt4Ru1. Then, the onset potentials
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gradually shift to ~0.42 V along with the increasing Ru proportions. These results indicate that Ru element enhanced OH adsorption could successfully reduce the
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removal potential of CO as reported before. While in comparation with PtRu/C (0.46 V), the NPG-based electrocatalyst with equal nPt/nRu (NPG-Pt1Ru1) exhibits more negative onset potential. These results confirm the weakened CO adsorption as revealed in Figure 4 and 5. According to the peak current potentials for CO oxidation reaction, the peak current potentials shift from 0.59 to 0.56, 0.53 and 0.52 V with
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nPt/nRu varying from 4/1 to 2/1, 1.7/1 and 1/1 which indicates the enhanced CO-tolerance along with the increased Ru contents. Unfortunately, while in contrast to PtRu/C, even the NPG-PtRu sample with equal Pt/Ru molar ratio (NPG-Pt1Ru1) exhibits a slightly higher peak current potential (10 mV). The value of onset potential
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represents the reaction thermodynamics of CO oxidation while the width of the CO oxidation peak represents the reaction kinetics [43,44]. Experimental data from
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Figure 6 demonstrate that the adsorbed CO molecular is easier but slower to be removed on NPG-PtRu than on PtRu/C with equal Pt/Ru molar ration. Therefore, NPG-PtRu catalysts exhibit slightly weak CO-tolerance relative to PtRu/C. Considering the possible effect of Ru on the hydrogen adsorption-desorption on Pt
atoms, electrochemically active surface areas (ECSAs) of Pt were calculated and listed in Table 1 because the Coulombic charge required for a monolayer of COad oxidation is 420 µC cm−2 [1]. The ECSA was used to calculate the specific activity later for methanol oxidation. In addition, the compositions of home-made NPG-PtRu samples were measured by using ICP-AES as exhibited in Table 1. The results indicate that
ACCEPTED MANUSCRIPT nPt/nRu of the epitaxially growing PtRu nanoparticles are not equal to the nPt/nRu in the plating solutions. The probable reason is the preferred reduction of Pt than Ru during the negatively potential sweeping in the preparation process. The electrochemical activities and stabilities of NPG-PtRu catalysts towards MOR
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are investigated and compared with commercial PtRu/C as indicated in Figure 7. Excluding the possible influences of mass transfer, the stationary LSV curves in Figure 7a indicate that the peak current densities of MOR gradually decreased with the increasing Ru contents among NPG-PtRu catalysts. Considering the reaction
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mechanism of MOR illustrated in equations (1-5), the reverse correlation between MOR activity and Ru proportion reveal that Ru is adverse for the adsorption or
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dissociation of methanol molecular on the electrode surface of NPG-PtRu catalysts. In Figure 7b, the current densities are normalized to ECSAs, and the peak current densities of MOR gradually decreased with the increasing Ru contents among NPG-PtRu and PtRu/C. This result indicates that although addition of Ru will cause the CO oxidation reaction at lower potentials, excessively added Ru will slow down
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the MOR, consistent with the result revealed in Figure 7a and previous literatures [22-24]. Towards anode catalysts for DMFC, current densities at 0.6 V commonly represent their catalytic activities for MOR. Among NPG-PtRu and PtRu/C catalysts, the specific current density at 0.6 V i.e. the intrinsic activity of NPG-Pt2Ru1 (0.44 mA
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cm-2) is the highest, about 2.5 times higher than that of PtRu/C (0.18 mA cm-2), while the mass activity of NPG-Pt2Ru1 is ~3 times higher than that of PtRu/C by normalizing
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the currents at 0.6 V to Pt loadings (Figure 7c). Therefore, under similar resistance to CO poisoning as indicated in Figure 6, NPG-Pt2Ru1 performs better catalytic activity than commercial PtRu/C. To further reveal the stability of the electrocatalysts, CA curves of the relatively
higher MOR catalyst NPG-Pt2Ru1 were measured in comparation with commercial PtRu/C at 0.6 V for 7200 s (Figure 7d). Clearly, the initial current density decay is ascribed to the poisoning caused by carbonaceous intermediate species. After the durability test, the current density of NPG-Pt2Ru1 is also about 2.5 times higher than PtRu/C in keeping with the initial value indicated in Figure 7b. Consequently,
ACCEPTED MANUSCRIPT NPG-Pt2Ru1 presents a relatively higher catalytic activity and equal activity stability in contrast to commercial PtRu/C. 3.3. Fuel cell performance In line with the specific and mass activities in Figure 7b-c, NPG-Pt2Ru1 with Pt
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loading of 0.2 mg cm-2 is chosen to be the anode catalyst for DMFC in this case while the commercial PtRu/C (60%) is used with Pt loading of 1 mg cm-2 in comparison. The single cell polarization curves and power density curves of these two catalysts are shown in Figures 8a and S4. As shown in these figures, the single cell open circuit
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voltages (OCV) using NPG-Pt2Ru1 all are lower than that of PtRu/C at various operating temperatures. Owing to the equal test process i.e. the same electrolyte
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membrane, cathode catalysts, hot pressing parameters and operating conditions, OCVs could undoubtedly reflect the performance of the anode materials and the results imply a relatively higher overpotential of NPG-Pt2Ru1 in this case. Overpotential for MOR is mainly ascribed to the integral effect of specific activity and active sites. Owing to the close onset potentials on NPG-Pt2Ru1 and PtRu/C (Figure
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7b), the mainly reason for the relatively lower OCV of NPG-Pt2Ru1 is the obviously reduced Pt loading. In addition, the OCVs increase with the raising operation temperatures as expected.
From the polarization curves in Figure 8a, we can see that voltage drop of the
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single cell using NPG-Pt2Ru1 as the anode (from 0.68 to 0.56 V) is smaller than that of PtRu/C (from 0.8 to 0.58 V) during the activation polarization region, which
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demonstrates the faster reaction kinetics of NPG-Pt2Ru1 as revealed in Figure 7b. Furthermore, during the ohmic polarization region the voltage of NPG-Pt2Ru1 also decreased slower than that of PtRu/C which reveals a smaller ohmic resistance of the single cell using NPG-Pt2Ru1 as the anode. According to the power density curves in Figure 8a, the maximal power density of NPG-Pt2Ru1 is 96 mW cm-2 while that of PtRu/C is 78 mW cm-2 at 80 oC. Considering the Pt loadings, NPG-Pt2Ru1 exhibits an outstanding Pt efficiency (480 W gPt-1), ~6-fold higher than that of PtRu/C (78 W gPt-1). Figure 7c indicates that the mass activity of NPG-Pt2Ru1 is 3 times higher than that of PtRu/C, so an extra 2 times enhancement of the Pt efficiency is obtained while NPG-
ACCEPTED MANUSCRIPT Pt2Ru1 is used as the anode in DMFC. In addition, even if we take the Au loading (0.3 mg cm-2) of NPG-Pt2Ru1 into account and the noble metal efficiency is calculated to be 192 W g-1, about 2.5 times enhancement than PtRu/C. To further analyze the reasons for the enhanced Pt efficiency of the single cell
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using NPG-Pt2Ru1 as the anode, Nyquist diagrams under the potentials near each OCV at 30 oC are measured and shown in Figure 8b. Potentials at which the output current densities ~10 mA cm-2 are chosen for the EIS tests and the values are 400 and 440 mV separately for MEAs with NPG-Pt2Ru1 and PtRu/C as anode catalysts. The
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values of the intersection point on Zre axis represent the ohmic resistance (RΩ) of the single cells while the arc at high frequency is related to the charge transfer resistance
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(Rct), and the arc at low frequency is ascribed to the diffusion resistance (Rd) [42,45]. It is clearly that RΩ of DMFC with NPG-Pt2Ru1 (320 mΩ) as the anode catalyst is evidently smaller than that with PtRu/C (670 mΩ) as the anode catalyst which is in good agreement with polarization curves in Figure 8a. The reduced Rct of DMFC with NPG-Pt2Ru1 as anode catalyst in contrast with PtRu/C indicates that the rates of MOR
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on NPG-Pt2Ru1 catalyst are much faster than that on PtRu/C. This result is also consistent with the CV curves in Figure 7c and the polarization curves in Figure 8a. Considering the possible reason for the obviously reduced RΩ and Rct of NPG-Pt2Ru1, the ultra-thin NPG film with bi-continuous pore/ligament structure plays a key role,
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which provide rapid electron conduction, enhanced catalytic activity and good durability. Furthermore, owing to the nanoscale pore ~30 nm but ultra-thin catalyst
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layer (~400 nm) using NPG-Pt2Ru1 anode relative to the micron pore but thick catalyst layer (~50 μm) using PtRu/C anode, the Rd of MEA with NPG-Pt2Ru1 anode is a little larger than that with PtRu/C anode because methanol diffusion through NPG substrate is very difficult with large concentration polarization. Therefore, developing porous metal film substrate with high specific surface area as well as fluent mass transfer is a perspective way to improve power density of DMFC with low Pt loadings. 4. Conclusions In this study, a highly efficient and CO-tolerant anode catalyst for DMFC was fabricated by epitaxially growing PtRu alloy nanoparticles on nanoporous gold film
ACCEPTED MANUSCRIPT (NPG-PtRu). Ascribed to the spectral analysis and performance tests, NPG substrate play dual functions in anode catalyst NPG-PtRu of DMFC. One is the synergetic effect between NPG substrate and PtRu nanoparticles which will reduce the binding energy of Pt i.e. CO adsorption energy. According to NPG-PtRu catalysts, the resistance to CO
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poisoning increases while the catalytic activity for MOR decreases with the enlarged proportion of Ru. Therefore, NPG-PtRu catalyst with Pt/Ru molar ratio of ~2:1 (NPG-Pt2Ru1) exhibits the optimal property with 3-fold catalytic activity and equal CO-tolerance in comparison with commercial PtRu/C. The other is the nanoscale but
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bulk in nature structure characteristics of NPG film which will provide large specific surface area and rapid electron conduction in contrast with conventional powder
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supports i.e. carbon. So, excessive performance improvement is obtained by using NPG-PtRu as the anode in DMFC owing to the reduced ohmic resistance of the single fuel cell. The membrane electrode assembly with anode of NPG-Pt2Ru1 (0.2 mgPt cm-2 and 0.3 mgAu cm-2) shows a maximum power density of 96 mW cm -2, about 6-fold enhancement of Pt efficiency than that of commercial PtRu/C (1 mgPt cm-2, 78 mW
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cm-2) under similar conditions. Therefore, this study reveals that changing the surface structure of PtRu alloy by choosing suitable substrates is a perspective way for high-activity and strong CO-tolerance anode catalyst in DMFC, and furthermore, the structural advantages of nanoporous metal film make it well adapted as anode
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materials in DMFC.
Acknowledgements
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This work was financially supported by the National Natural Science Foundation of China (51671145 and 21506148). The authors acknowledge useful discussions with Dr. Xizhen Liu and Prof. Yi Ding. References
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Figures and captions
Figure 1 HRTEM images of NPG-Pt1.7Ru1 (a-c) with various magnifications; (d) Atomically resolved HAADF images of NPG-Pt1.7Ru1. The inset picture in Figure 1c is the selective diffraction pattern
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while the inset one in Figure 1d is the fast Fourier transform (FFT) pattern.
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Figure 2 (a) HAADF image of NPG-Pt1.7Ru1. (b-d) Elemental mapping images of (a) using the EDS
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signals of Au, Pt and Ru, respectively. (e) Overlay of images a-c.
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Figure 3 XRD patterns of NPG-PtRu and commercial JM-PtRu/C.
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Figure 4 XPS spectra of NPG-Pt, NPG-Pt1.7Ru1 and commercial PtRu/C: Au 4f (a), Pt 4f (b), Ru 3p
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(c).
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Figure 5 CO stripping curves of NPG-Pt and Pt/C in 0.5 M H2SO4 at a scanning rate of 20 mV S-1.
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s-1.
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Figure 6 CO stripping curves of NPG-PtRu and PtRu/C in 0.5 M H2SO4 at a scanning rate of 20 mV
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Figure 7 (a) LSV curves in 0.5 M H2SO4 + 0.5 M CH3OH at a scanning rate of 10 mV s-1; (b) and (c)
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CV curves in 0.5 M H2SO4 + 0.5 M CH3OH at a scan rate of 20 mV s-1; (d) CA curves of NPG-Pt2Ru1 and PtRu/C holding at 0.6 V. Current densities in (a), (b) and (d) are normalized to the ECSAs,
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while current densities in (c) are normalized to Pt loadings, respectively.
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C and (b) EIS curves of NPG-Pt2Ru1 and PtRu/C based MEAs.
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o
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Figure 8 Performance of single DMFCs with different catalysts. (a) Single-cell performances at 80
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ECSAPt (CO) cm2
NPG-Pt4Ru1
53
0.52
0.59
3.8
57.1
NPG-Pt2Ru1
39
0.48
0.56
3.6
73.5
NPG-Pt1.7Ru1
31
0.47
0.53
3.2
82.1
NPG-Pt1Ru1
20
0.42
0.52
2.2
87.6
PtRu/C
13
0.46
0.51
1.0
61.2
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Ep(CO) V
SA m2 g-1
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mPt μg cm-2
S0013-4686(18)32287-4
DOI:
10.1016/j.electacta.2018.10.048
Reference:
EA 32845
To appear in:
Electrochimica Acta
Received Date: 25 September 2018 Revised Date:
8 October 2018
Accepted Date: 8 October 2018
Please cite this article as: M. Tian, S. Shi, Y. Shen, H. Yin, PtRu alloy nanoparticles supported on nanoporous gold as an efficient anode catalyst for direct methanol fuel cell, Electrochimica Acta (2018), doi: https://doi.org/10.1016/j.electacta.2018.10.048. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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PtRu alloy nanoparticles supported on nanoporous gold as an efficient anode catalyst for direct methanol fuel cell
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Miaomiao Tian1, Shuai Shi1, Yongli Shen, Huiming Yin* Tianjin Key Laboratory of Advanced Functional Porous Materials and Institute for
Tianjin 300384, China. * Corresponding author. E-mail address: [email protected] These authors contributed equally to this work.
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1
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New Energy Materials and Low Carbon Technologies, Tianjin University of Technology,
Abstract
An efficient and CO-tolerant anode catalyst for direct methanol fuel cell is fabricated by depositing PtRu alloy nanoparticles (~3 nm) on nanoporous gold film (NPG-PtRu).
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Those catalysts with various Pt/Ru molar ratios are prepared by using linear scanning voltammetry and their electrocatalytic activities are measured for methanol oxidation reaction (MOR) and CO oxidation. Owing to the notably reduced CO adsorption energy, NPG-PtRu catalyst with Pt/Ru molar ratio of ~2:1 exhibits close
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CO-tolerance but 3-fold enhanced MOR activity in contrast to commercial JM-PtRu/C, and thus is chosen as the anode in direct methanol fuel cell. The membrane
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electrode assembly with anode of NPG-PtRu (0.2 mgPt cm-2 and 0.3 mgAu cm-2) shows a maximum power density of 96 mW cm-2, about 6-fold enhancement of Pt efficiency than that of commercial PtRu/C (1 mgPt cm-2, 78 mW cm-2) under similar conditions. Keywords: nanoporous gold; PtRu; methanol oxidation; fuel cell 1. Introduction In recent years, polymer electrolyte membrane fuel cells have been viewed as the promising energy conversion devices for developing sustainable and clean energy. Direct methanol fuel cell (DMFC), since the high energy density and easy handling of the liquid methanol fuel, will extremely reduce the complexity and cost of the energy
ACCEPTED MANUSCRIPT conversion system. Therefore, DMFC offers certain specific advantages over proton exchange membrane fuel cells (PEMFC) for portable, transportation, and stationary power sources [1-5]. However, different from the facile hydrogen oxidation reaction with a two-electron transfer in the anode of hydrogen-feed PEMFC, methanol
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oxidation reaction (MOR) exhibits a slow oxidation kinetics and releases various partial oxidation intermediates such as COHad and COad. These intermediates will strongly adsorb on the surface of the most effective metal catalyst i.e. Pt, thus block the oxidation process and reduce the performance of DMFC [2-5]. Therefore, there
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are two major challenges for the commercialization of DMFC: one is the sluggish methanol oxidation kinetics even on some state-of-the-art anode catalysts, and the
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other is the methanol crossover through the membrane, which not only depresses cathode performance, but also reduces fuel efficiency. Currently, exploring methanol-tolerant cathode catalysts and methanol-preventing membranes is the main strategy to solve the performance depression by methanol crossover. According to the sluggish kinetics of MOR, nanoscale catalysts with suitable structures,
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compositions, i.e. hollow or core-shell structures, binary or ternary alloy, as well as optimized supports i.e. metal oxides, carbon-based materials, have been abundantly investigated for anodic catalysts in DMFC, especially Pt-based catalysts towards acceptable fuel cell performance for practical application [1, 6-12].
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Among Pt-based bimetallic catalysts that have been investigated for MOR, Pt-Ru alloy was proved to be the most practical electro-catalysts for the DMFC application
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with reasonable activities and stability [2-6, 13-20]. According to a well-described mechanism, the two primary processes that comprise the overall MOR are the initial dehydrogenation step producing intermediates like CO (equation (1)), and the CO removal step including water activation and CO oxidation (equation (2) and (3)). Pt + CH3OH → Pt-CO + 4 H+ + 4 e
(1)
Pt + H2O → Pt-OH + H+ + e
(2)
Pt-CO + Pt-OH → 2 Pt + CO2 + H+ + e
(3)
In methanol electro-oxidation, the active sites of Pt will be occupied by the intermediates e.g. CO from equation (1), because equation (2) producing the
ACCEPTED MANUSCRIPT necessary oxidative groups i.e. OHad for removing adsorbed CO should react at a high potential on Pt (from 0.75 V). Therefore, additive of Ru, which can provide oxygenated species at lower potentials (equation (4), from 0.35 V) will undoubtedly enhance the resistance to CO-poisoning (equation (5)) and
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improve the electro-catalytic activities for MOR [21]. Including the bifunctional mechanism mentioned above, electronic effect from Ru could also enhance the
methanol dehydrogenation step and weaken the adsorption strength of COad on
Pt sites [2]. Therefore, rationally designing the PtRu alloy structure is highly
Ru + H2O → Ru-OH + H+ + e
(4) (5)
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Pt-CO + Ru-OH → Pt + Ru + CO2 + H+ + e
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desired to maximize the role of Ru for high CO-tolerance and MOR activity.
As well as the decreased removing potential ascribed to oxidic species, reducing the binding energy between Pt and CO will further improve the resistance to CO poisoning. Yang et al reported that Au nanoparticles adsorbed onto the Pt-Ru alloy particles could weaken the adsorption of CO and thus promote the catalytic activity
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and durability of PtRu/C [22]. In addition, gold clusters modified on PtRu alloy could also reduce the dissolution of the more oxophilic Ru and thus improve the activity and stability towards MOR [22-26].
Nanoporous metals e.g. nanoporous gold (NPG) films prepared by dealloying
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method have exhibited excellent electrocatalytic activities owing to the unique nanoscale bi-continuous pore/ligament structure [27,28]. The continuous pores
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ensure smooth channels for mass transfer while the connected metal ligaments provide abundant active sites for surface reactions and accelerated electron conduction [29-32]. On the other hand, unlike the nanoparticles, the curved surfaces of the dealloyed nanoporous metals provide a high density of steps and kinks which are active for chemical reactions, high thermal stability [28]. And what’s more, the facilely modified metal ligaments and the strong metal bond interact make nanoporous metal films ideal substrates to construct electrocatalysts with enhanced durability. In this paper, NPG film was used as the substrate, and PtRu alloy nanoparticles
ACCEPTED MANUSCRIPT with various Pt/Ru molar ratios were coated on the substrate (NPG-PtRu) for high CO-tolerance and MOR activity. A comparative evaluation of the structure, composition and electrochemical performance of the NPG-PtRu catalysts is discussed together with commercial PtRu/C. And furthermore, the optimized NPG-PtRu catalyst
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with the highest catalytic activity and strong resistance to CO poisoning was applied as the anode catalyst in DMFC and the output power density was measured relative to commercial PtRu/C. 2. Experimental
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2.1. Chemicals
Chloroplatinic acid (H2PtCl6), ruthenium trichloride (RuCl3), nitric acid (HNO3),
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perchloric acid (HClO4), sulphuric acid (H2SO4), and methanol (CH3OH) were purchased from Sinopharm Chemical Reagent Co., Ltd. Commercial Pt/C (60%, Johnson Matthey), PtRu/C (60 wt%, Johnson Matthey) and membrane Nafion 115 (DuPont) were purchased from Alfa Aesar Co., Ltd. Au-Ag alloy films were purchased from Sepp Leaf Products, Inc. All chemicals were used as received without further
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purification.
2.2. Synthesis of NPG-Pt and NPG-PtRu
NPG substrate with ligament size ~30 nm was prepared by dealloying 100-nm-thick, 12-Karat Au-Ag alloy films in concentrated HNO3 (65 wt.%) for 30 min at 30 °C [33,34].
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The obtained NPG substrates were thoroughly rinsed and preserved in ultrapure water (18.23 MΩ cm) for further use. Plating solutions with various Pt/Ru molar
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ratios of 4/0.25, 4/2, 4/4 and 4/7 were prepared by dissolving H2PtCl6 and RuCl3 in 0.1 M HClO4 and the concentration of platinum was fixed to be 4 mM. Electro-deposition of PtRu alloy or Pt nanoparticles were prepared using an electrochemical workstation (VersaSTAT MC, Princeton) with a standard three-electrode system. A graphite plate was used as the counter electrode, and a saturated calomel electrode (SCE) was used as the reference electrode. As the working electrode, NPG film floating on water was attached on a glassy carbon electrode (φ = 4 mm) by immersing the electrode in water. 2 μL 0.5 wt% solution Nafion-ethanol was dropped on the glassy carbon and the surface area was fixed by erasing the NPG substrate outside the glassy carbon. By
ACCEPTED MANUSCRIPT using the electrochemical cell containing the three-electrode system, PtRu alloy or Pt nanoparticles were deposited on NPG by using negatively potential sweeping from 0.6 to 0 V for 20 cycles with a scan rate of 50 mV s-1. All solutions used in this section were deaerated with ultrapure N2 for approximately 30 min. All the potentials listed
2.3. Physicochemical characterization
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in this paper were normalized to reversible hydrogen electrode (RHE).
The structure and morphology of NPG-PtRu were observed from the high-resolution transmission electron microscope (HRTEM) and high-angle annular
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dark field (HAADF) images (Talos F200X, FEI). The elemental mapping was obtained using the energy dispersive X-ray spectroscopy (EDX) on a FEI Talos F200X. The
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composition and actual precious metal loading of NPG-PtRu was determined by inductively coupled plasma atomic emission spectroscopy (ICP-AES) (IRIS Advantage). All X-Ray diffraction (XRD) measurements were performed with a SmartLab diffractometer (Rigaku Co.) using a Cu Kα (λ= 1.5405 Å) radiation source. The X-ray photoelectron spectroscopy (XPS) spectra of the catalysts were acquired with an
120 W X-ray source.
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ESALAB 250Xi (Thermo Scientific) spectrometer fitted with an Mg K (h = 1253.6 eV)
2.4. Electrochemical characterization
All electrochemical measurements were performed with an electrochemical
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workstation (CHI 760D, CH Instruments, Inc.). SCE was used as the reference electrodes and a graphite plate was used as a counter electrode. A glassy carbon thin
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film electrode (diameter φ = 4 mm) was used as a working electrode. The NPG-based electrodes were prepared by lift-coating the glassy carbon with the free-standing nanoporous films. Commercial PtRu/C catalyst ink was prepared by ultrasonically dispersing 1 mg of PtRu/C (60 wt.%) catalyst into 3 mL Nafion alcohol solution (0.5 wt.%) for 10 min. Then, 12.5 μL of the ink was drop-coated onto a glassy carbon electrode, resulting in a PtRu loading of ~20 μg cm-2. The coating ink was dried for 20 min in air to form a uniform thin film on the electrode surface. CO stripping test was carried out in 0.5 M H2SO4. The solution was firstly bubbled with ultra-pure CO for 20 min to remove any other impurity gas. Then working
ACCEPTED MANUSCRIPT electrode was immersed into the electrolyte holding the potential at 0.2 V with CO bubbling for 15 min and following N2 bubbling for 30 min to ensure the single atomic layer adsorption of CO molecular. Finally, cyclic voltammetry (CV) curves were performed with an initial potential of 0 V between 0 and 1.2 V at a scanning rate of
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20 mV s-1. Catalytic activity towards methanol oxidation was measured in N2-saturated mixture solution of 0.5 M H2SO4 and 0.5 M CH3OH by using linear scanning voltammetry (LSV) at 10 mV s-1 between 0 and 0.75 V and using CV technique at 50
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mV s-1 between 0 and 1.2 V. Durability of the catalysts were tested by using the chronoamperometry (CA) technique in mixture solution of 0.5 M H2SO4 and 0.5 M
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CH3OH at 0.6 V. All the solutions used in this section were N2-saturated prior to utilization.
2.5. Fabrication of MEA and single cell performance test
In this paper, Nafion 115 was used as the proton exchange membrane and successively pretreated in 3 vol.% H2O2, ultra-pure water, 0.5 M H2SO4 and ultra-pure
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water, at 80 oC for 1 h in each step. Then the membrane was restored in ultra-pure water before utilization. Hydrophobic treated carbon paper (TGP-H-060, Toray) sprayed with 1 mg cm-2 carbon powder was used as the diffusion layer and the homemade NPG-PtRu film was attached on the surface and used as the anode
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catalysts. In comparison, PtRu/C catalyst (Johnson Matthey) was sprayed on the same diffusion layer with the Pt loading of 1 mg cm-2. For all MEAs in this study,
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commercial Pt/C (Johnson Matthey) was used as the cathode at a standard Pt loading of 4 mg cm-2. Hot-pressing was conducted at 130 oC and 100 kg cm-2 for 180 s. As-prepared MEAs were separately assembled into single fuel cells using high
purity graphite plates as flow and current collecting plates. Methanol solution (1 M) was supplied to the anode by a peristaltic pump with the flow rate of 2 mL min-1, while oxygen was fed to the cathode by a mass flow controller with the flow rate of 100 SCCM. The cell temperature was controlled through a temperature controller, and the steady state polarization curves were recorded by using automatic electric load. Electrochemical impedance spectra (EIS) of the MEAs using NPG-Pt2Ru1 and
ACCEPTED MANUSCRIPT PtRu/C as anodes were separately measured with cell voltages of 400 and 440 mV using an electrochemical workstation (VersaSTAT MC, Princeton) in a frequency range from 0.1 Hz to 105 Hz at 30 oC. The amplitude of the AC voltage was 5 mV. 3. Results and Discussion
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3.1. Physicochemical characterization of NPG-PtRu Typical morphology of the NPG substrate is exhibited in Figure S1 which features a smooth and bi-continuous ligament/pore network averaging ~30 nm. To explore the morphology, structure, composition and surface state of NPG-PtRu, typical NPG-PtRu
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sample prepared in mixture solution of 4 mM H2PtCl6, 4 mM RuCl3 and 0.1 M HClO4 was used as the target sample. The composition of this sample was measured to be
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NPG-Pt1.7Ru1 by using ICP-AES. The subscripts of NPG-PtRu represent the molar ratios of Pt/Ru. Electro-deposition of metal alloys is influenced by many factors including the solution composition, deposition process, the substrate and etc [35,36]. According to the preparation method in this study and the complexity of alloy plating, several possible reasons will be given for the inconsistent Pt/Ru ratio between the
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plating solution and the product NPG-PtRu. One possible reason is the electro-deposition method i.e. sweeping from 0.6 to 0 V. During this process, metal with relatively high reduction potential will be deposited preferentially. Other possible reasons may result from the substrate NPG, which possesses the same
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crystal structure as Pt and a close lattice parameter to Pt. The nucleation rate of Pt should be faster than that of Ru on NPG. More information about the morphology
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and crystal structure of the NPG-PtRu sample will be demonstrated using electron microscopy and spectral technology. Figure 1a-d give the HRTEM and HAADF images of NPG-Pt1.7Ru1. Figure 1a-c reveal
that the bi-continuous structure with pore and ligament dimensions averaging ~30 nm is still maintained but the ligament is completely covered with well dispersed nanoparticles ~3 nm. Atomic resolution HAADF images in Figure 1d and Figure S2 illustrate the typical atomic structure of the surface region in detail. Together with the inserted FFT pattern, an epitaxially growth mode is revealed since the particle and substrate are together in one single-crystalline region, like the previous literature
ACCEPTED MANUSCRIPT indicated for NPG-Pt by chemical plating NPG with Pt [29]. Furthermore, the EDS elemental mapping images in Figure 2 indicates that the NPG substrate is coated with a thin layer composed of Pt and Ru. Integrating the figures above, highly dispersed PtRu alloy nanoparticles were uniformly plated on the surface of NPG substrate by
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using electrochemical plating. The nanoparticles have a very narrow size distribution ~3 nm.
XRD patterns of NPG-Pt, NPG-Pt1.7Ru1 and commercial PtRu/C are shown in Figure 3. From Figure 3, broaden diffraction peaks (marked with black dotted lines) of
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face-centred cubic (FCC) Pt are obtained both for PtRu/C and NPG-Pt1.7Ru1 catalyst. Intensive diffraction peaks (marked with red dotted lines) of FCC Au are also
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observed for the NPG substrate in NPG-Pt1.7Ru1. Ascribed to none distinct diffraction peaks related to metal Ru or RuO2 as well as the increased 2θ values of FCC Pt, PtRu alloys have been formed by incorporating Ru into FCC Pt structure [14]. Furthermore, the alloying degrees of the PtRu nanoparticles in NPG-Pt1.7Ru1 and PtRu/C were calculated by using the lattice parameter from XRD patterns and applying the
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calculation method reported in previous literatures [37,38]. Pt (1 1 1) peaks were chosen to calculate the lattice parameter and the peak positions(θmax) were obtained through curve fitting. The lattice parameter(a) was calculated using the following
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equation:
=
√3 2
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while the alloying degree of PtRu nanoparticles (Ru atomic fraction, xRu) was calculated through the following equation proposed by Antolini et al [39]: = 0 − 0.124
The lattice parameters of NPG-PtRu and PtRu/C are separately calculated to be 3.91 and 3.86 Å and the corresponding alloying degrees of PtRu alloys are 8 % and 48% in NPG-PtRu and PtRu/C. The possible reasons for the low alloying degree of NPG-Pt1.7Ru1 include two factors. One is obvious the lower Ru proportion than PtRu/C [40], and the other is the lack of thermal treatment which is commonly used
ACCEPTED MANUSCRIPT for carbon supported PtRu alloy catalysts [39]. As indicated in Figure 4, each XPS spectrum was fitted by considering two resolved doublets. Owing to the large amount of internal Au element in NPG substrate, binding energies of Au 4f7/2 in pristine NPG (84.2 eV), NPG-Pt (84.1 eV) and NPG-PtRu
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(84 ev) are close to each other as indicated in Figure 4a. According to Pt element, Pt 4f7/2 peaks of NPG-Pt (70.8 eV) and NPG-Pt1.7Ru1 (70.6 eV) both exhibit a negative shift in contrast to that of PtRu/C (71.3 eV), while Ru 3p spectra present close binding energies between NPG-Pt1.7Ru1 and PtRu/C. The XPS spectra of PtRu/C in this
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paper is consistent with the data previously reported [14]. Associating with the XPS spectra of Au4f, Pt4f and Ru3p, it is clear that NPG substrate could obviously reduce
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the binding energy of Pt which will weaken the adsorption energy of CO on Pt as previous literatures reported [14,41,42]. Therefore, the XPS data indicate an improved CO-tolerance on NPG-Pt which will be further enhanced by the adding of Ru as revealed before [2]. In addition, previous study demonstrated that the dissolution of Ru from the PtRu electrocatalyst could be reduced by depositing the
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Au clusters onto the PtRu nanoparticles. So, NPG substrate in NPG-PtRu may also enhance the durability of the supported PtRu alloy nanoparticles [23]. 3.2. Electrochemical characterization
Firstly, the interaction between NPG substrate and Pt was evaluated by using the
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CO stripping tests relative to commercial Pt/C. NPG-Pt were fabricated by using the same electro-deposition method as NPG-PtRu. As shown in Figure 5, the absence of
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hydrogen desorption peaks on the CO stripping curves demonstrates the monolayer adsorption of CO on the Pt surface. On NPG-Pt, the peak current potential of CO oxidation reaction (0.83 V) exhibits an obviously negative shift ~150 mV in contrast with Pt/C (0.68 V). This result indicates that the adsorption energy of CO is notably weakened ascribed to the synergetic effect between the substrate NPG and Pt which is consistent with the results from XPS (Figure 4b). Based on the reduced CO adsorption on NPG-Pt, NPG-PtRu samples with various nPt/nRu molar ratios were fabricated and the electrochemical properties were subsequently measured for MOR. CV curves in black solution (0.5 M H2SO4) are
ACCEPTED MANUSCRIPT exhibited in Figure S3, which indicate that typical hydrogen adsorption-desorption peaks of polycrystalline Pt gradually disappear along with the increasing Ru proportions. As CO poisoning resistance plays a key role for MOR, CO stripping measurements were carried out to evaluate the performance for CO-tolerance. The
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striping curves are indicated in Figure 6 while the data are calculated and summarized in Table 1.
According to the onset potentials for CO oxidation reaction, the values evidently shift from ~0.68 to ~0.52 V on NPG-Pt and NPG-Pt4Ru1. Then, the onset potentials
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gradually shift to ~0.42 V along with the increasing Ru proportions. These results indicate that Ru element enhanced OH adsorption could successfully reduce the
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removal potential of CO as reported before. While in comparation with PtRu/C (0.46 V), the NPG-based electrocatalyst with equal nPt/nRu (NPG-Pt1Ru1) exhibits more negative onset potential. These results confirm the weakened CO adsorption as revealed in Figure 4 and 5. According to the peak current potentials for CO oxidation reaction, the peak current potentials shift from 0.59 to 0.56, 0.53 and 0.52 V with
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nPt/nRu varying from 4/1 to 2/1, 1.7/1 and 1/1 which indicates the enhanced CO-tolerance along with the increased Ru contents. Unfortunately, while in contrast to PtRu/C, even the NPG-PtRu sample with equal Pt/Ru molar ratio (NPG-Pt1Ru1) exhibits a slightly higher peak current potential (10 mV). The value of onset potential
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represents the reaction thermodynamics of CO oxidation while the width of the CO oxidation peak represents the reaction kinetics [43,44]. Experimental data from
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Figure 6 demonstrate that the adsorbed CO molecular is easier but slower to be removed on NPG-PtRu than on PtRu/C with equal Pt/Ru molar ration. Therefore, NPG-PtRu catalysts exhibit slightly weak CO-tolerance relative to PtRu/C. Considering the possible effect of Ru on the hydrogen adsorption-desorption on Pt
atoms, electrochemically active surface areas (ECSAs) of Pt were calculated and listed in Table 1 because the Coulombic charge required for a monolayer of COad oxidation is 420 µC cm−2 [1]. The ECSA was used to calculate the specific activity later for methanol oxidation. In addition, the compositions of home-made NPG-PtRu samples were measured by using ICP-AES as exhibited in Table 1. The results indicate that
ACCEPTED MANUSCRIPT nPt/nRu of the epitaxially growing PtRu nanoparticles are not equal to the nPt/nRu in the plating solutions. The probable reason is the preferred reduction of Pt than Ru during the negatively potential sweeping in the preparation process. The electrochemical activities and stabilities of NPG-PtRu catalysts towards MOR
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are investigated and compared with commercial PtRu/C as indicated in Figure 7. Excluding the possible influences of mass transfer, the stationary LSV curves in Figure 7a indicate that the peak current densities of MOR gradually decreased with the increasing Ru contents among NPG-PtRu catalysts. Considering the reaction
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mechanism of MOR illustrated in equations (1-5), the reverse correlation between MOR activity and Ru proportion reveal that Ru is adverse for the adsorption or
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dissociation of methanol molecular on the electrode surface of NPG-PtRu catalysts. In Figure 7b, the current densities are normalized to ECSAs, and the peak current densities of MOR gradually decreased with the increasing Ru contents among NPG-PtRu and PtRu/C. This result indicates that although addition of Ru will cause the CO oxidation reaction at lower potentials, excessively added Ru will slow down
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the MOR, consistent with the result revealed in Figure 7a and previous literatures [22-24]. Towards anode catalysts for DMFC, current densities at 0.6 V commonly represent their catalytic activities for MOR. Among NPG-PtRu and PtRu/C catalysts, the specific current density at 0.6 V i.e. the intrinsic activity of NPG-Pt2Ru1 (0.44 mA
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cm-2) is the highest, about 2.5 times higher than that of PtRu/C (0.18 mA cm-2), while the mass activity of NPG-Pt2Ru1 is ~3 times higher than that of PtRu/C by normalizing
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the currents at 0.6 V to Pt loadings (Figure 7c). Therefore, under similar resistance to CO poisoning as indicated in Figure 6, NPG-Pt2Ru1 performs better catalytic activity than commercial PtRu/C. To further reveal the stability of the electrocatalysts, CA curves of the relatively
higher MOR catalyst NPG-Pt2Ru1 were measured in comparation with commercial PtRu/C at 0.6 V for 7200 s (Figure 7d). Clearly, the initial current density decay is ascribed to the poisoning caused by carbonaceous intermediate species. After the durability test, the current density of NPG-Pt2Ru1 is also about 2.5 times higher than PtRu/C in keeping with the initial value indicated in Figure 7b. Consequently,
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loading of 0.2 mg cm-2 is chosen to be the anode catalyst for DMFC in this case while the commercial PtRu/C (60%) is used with Pt loading of 1 mg cm-2 in comparison. The single cell polarization curves and power density curves of these two catalysts are shown in Figures 8a and S4. As shown in these figures, the single cell open circuit
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voltages (OCV) using NPG-Pt2Ru1 all are lower than that of PtRu/C at various operating temperatures. Owing to the equal test process i.e. the same electrolyte
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membrane, cathode catalysts, hot pressing parameters and operating conditions, OCVs could undoubtedly reflect the performance of the anode materials and the results imply a relatively higher overpotential of NPG-Pt2Ru1 in this case. Overpotential for MOR is mainly ascribed to the integral effect of specific activity and active sites. Owing to the close onset potentials on NPG-Pt2Ru1 and PtRu/C (Figure
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7b), the mainly reason for the relatively lower OCV of NPG-Pt2Ru1 is the obviously reduced Pt loading. In addition, the OCVs increase with the raising operation temperatures as expected.
From the polarization curves in Figure 8a, we can see that voltage drop of the
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single cell using NPG-Pt2Ru1 as the anode (from 0.68 to 0.56 V) is smaller than that of PtRu/C (from 0.8 to 0.58 V) during the activation polarization region, which
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demonstrates the faster reaction kinetics of NPG-Pt2Ru1 as revealed in Figure 7b. Furthermore, during the ohmic polarization region the voltage of NPG-Pt2Ru1 also decreased slower than that of PtRu/C which reveals a smaller ohmic resistance of the single cell using NPG-Pt2Ru1 as the anode. According to the power density curves in Figure 8a, the maximal power density of NPG-Pt2Ru1 is 96 mW cm-2 while that of PtRu/C is 78 mW cm-2 at 80 oC. Considering the Pt loadings, NPG-Pt2Ru1 exhibits an outstanding Pt efficiency (480 W gPt-1), ~6-fold higher than that of PtRu/C (78 W gPt-1). Figure 7c indicates that the mass activity of NPG-Pt2Ru1 is 3 times higher than that of PtRu/C, so an extra 2 times enhancement of the Pt efficiency is obtained while NPG-
ACCEPTED MANUSCRIPT Pt2Ru1 is used as the anode in DMFC. In addition, even if we take the Au loading (0.3 mg cm-2) of NPG-Pt2Ru1 into account and the noble metal efficiency is calculated to be 192 W g-1, about 2.5 times enhancement than PtRu/C. To further analyze the reasons for the enhanced Pt efficiency of the single cell
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using NPG-Pt2Ru1 as the anode, Nyquist diagrams under the potentials near each OCV at 30 oC are measured and shown in Figure 8b. Potentials at which the output current densities ~10 mA cm-2 are chosen for the EIS tests and the values are 400 and 440 mV separately for MEAs with NPG-Pt2Ru1 and PtRu/C as anode catalysts. The
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values of the intersection point on Zre axis represent the ohmic resistance (RΩ) of the single cells while the arc at high frequency is related to the charge transfer resistance
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(Rct), and the arc at low frequency is ascribed to the diffusion resistance (Rd) [42,45]. It is clearly that RΩ of DMFC with NPG-Pt2Ru1 (320 mΩ) as the anode catalyst is evidently smaller than that with PtRu/C (670 mΩ) as the anode catalyst which is in good agreement with polarization curves in Figure 8a. The reduced Rct of DMFC with NPG-Pt2Ru1 as anode catalyst in contrast with PtRu/C indicates that the rates of MOR
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on NPG-Pt2Ru1 catalyst are much faster than that on PtRu/C. This result is also consistent with the CV curves in Figure 7c and the polarization curves in Figure 8a. Considering the possible reason for the obviously reduced RΩ and Rct of NPG-Pt2Ru1, the ultra-thin NPG film with bi-continuous pore/ligament structure plays a key role,
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which provide rapid electron conduction, enhanced catalytic activity and good durability. Furthermore, owing to the nanoscale pore ~30 nm but ultra-thin catalyst
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layer (~400 nm) using NPG-Pt2Ru1 anode relative to the micron pore but thick catalyst layer (~50 μm) using PtRu/C anode, the Rd of MEA with NPG-Pt2Ru1 anode is a little larger than that with PtRu/C anode because methanol diffusion through NPG substrate is very difficult with large concentration polarization. Therefore, developing porous metal film substrate with high specific surface area as well as fluent mass transfer is a perspective way to improve power density of DMFC with low Pt loadings. 4. Conclusions In this study, a highly efficient and CO-tolerant anode catalyst for DMFC was fabricated by epitaxially growing PtRu alloy nanoparticles on nanoporous gold film
ACCEPTED MANUSCRIPT (NPG-PtRu). Ascribed to the spectral analysis and performance tests, NPG substrate play dual functions in anode catalyst NPG-PtRu of DMFC. One is the synergetic effect between NPG substrate and PtRu nanoparticles which will reduce the binding energy of Pt i.e. CO adsorption energy. According to NPG-PtRu catalysts, the resistance to CO
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poisoning increases while the catalytic activity for MOR decreases with the enlarged proportion of Ru. Therefore, NPG-PtRu catalyst with Pt/Ru molar ratio of ~2:1 (NPG-Pt2Ru1) exhibits the optimal property with 3-fold catalytic activity and equal CO-tolerance in comparison with commercial PtRu/C. The other is the nanoscale but
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bulk in nature structure characteristics of NPG film which will provide large specific surface area and rapid electron conduction in contrast with conventional powder
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supports i.e. carbon. So, excessive performance improvement is obtained by using NPG-PtRu as the anode in DMFC owing to the reduced ohmic resistance of the single fuel cell. The membrane electrode assembly with anode of NPG-Pt2Ru1 (0.2 mgPt cm-2 and 0.3 mgAu cm-2) shows a maximum power density of 96 mW cm -2, about 6-fold enhancement of Pt efficiency than that of commercial PtRu/C (1 mgPt cm-2, 78 mW
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cm-2) under similar conditions. Therefore, this study reveals that changing the surface structure of PtRu alloy by choosing suitable substrates is a perspective way for high-activity and strong CO-tolerance anode catalyst in DMFC, and furthermore, the structural advantages of nanoporous metal film make it well adapted as anode
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materials in DMFC.
Acknowledgements
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This work was financially supported by the National Natural Science Foundation of China (51671145 and 21506148). The authors acknowledge useful discussions with Dr. Xizhen Liu and Prof. Yi Ding. References
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Figures and captions
Figure 1 HRTEM images of NPG-Pt1.7Ru1 (a-c) with various magnifications; (d) Atomically resolved HAADF images of NPG-Pt1.7Ru1. The inset picture in Figure 1c is the selective diffraction pattern
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while the inset one in Figure 1d is the fast Fourier transform (FFT) pattern.
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Figure 2 (a) HAADF image of NPG-Pt1.7Ru1. (b-d) Elemental mapping images of (a) using the EDS
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signals of Au, Pt and Ru, respectively. (e) Overlay of images a-c.
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Figure 3 XRD patterns of NPG-PtRu and commercial JM-PtRu/C.
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Figure 4 XPS spectra of NPG-Pt, NPG-Pt1.7Ru1 and commercial PtRu/C: Au 4f (a), Pt 4f (b), Ru 3p
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(c).
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Figure 5 CO stripping curves of NPG-Pt and Pt/C in 0.5 M H2SO4 at a scanning rate of 20 mV S-1.
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s-1.
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Figure 6 CO stripping curves of NPG-PtRu and PtRu/C in 0.5 M H2SO4 at a scanning rate of 20 mV
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Figure 7 (a) LSV curves in 0.5 M H2SO4 + 0.5 M CH3OH at a scanning rate of 10 mV s-1; (b) and (c)
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CV curves in 0.5 M H2SO4 + 0.5 M CH3OH at a scan rate of 20 mV s-1; (d) CA curves of NPG-Pt2Ru1 and PtRu/C holding at 0.6 V. Current densities in (a), (b) and (d) are normalized to the ECSAs,
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while current densities in (c) are normalized to Pt loadings, respectively.
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C and (b) EIS curves of NPG-Pt2Ru1 and PtRu/C based MEAs.
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Figure 8 Performance of single DMFCs with different catalysts. (a) Single-cell performances at 80
ACCEPTED MANUSCRIPT Table 1 Experimental data from the ICP-AES and CO-stripping Eon(CO) V
ECSAPt (CO) cm2
NPG-Pt4Ru1
53
0.52
0.59
3.8
57.1
NPG-Pt2Ru1
39
0.48
0.56
3.6
73.5
NPG-Pt1.7Ru1
31
0.47
0.53
3.2
82.1
NPG-Pt1Ru1
20
0.42
0.52
2.2
87.6
PtRu/C
13
0.46
0.51
1.0
61.2
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Ep(CO) V
SA m2 g-1
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mPt μg cm-2