High-performance quaternary PtRuIrNi electrocatalysts with hierarchical nanostructured carbon support

Journal of Catalysis 306 (2013) 133–145

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High-performance quaternary PtRuIrNi electrocatalysts with hierarchical nanostructured carbon support Jung Ho Kim, Seon Young Kwon, Dhrubajyoti Bhattacharjya, Geun Seok Chai, Jong-Sung Yu ⇑ Department of Advanced Materials Chemistry, Korea University, 2511 Sejong-ro, Sejong 339-700, Republic of Korea

a r t i c l e

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Article history: Received 27 March 2013 Revised 4 June 2013 Accepted 7 June 2013 Available online 27 July 2013 Keywords: PtRuIrNi Quaternary catalysts Combinatorial analysis DMFC Methanol oxidation reaction Hierarchical nanostructured carbon

a b s t r a c t The PtRuIrNi quaternary system is explored by a robotic dispenser and combinatorial optical screening method, and some active ternary and quaternary electrocatalysts for methanol oxidation in direct methanol fuel cells are discovered. When combinatorial screening is employed, pH change allows one to differentiate active catalysts using fluorescent acid–base indicators. A quaternary catalyst with Pt34Ru30Ir13Ni23 composition is found to be the most active, demonstrating superior electrochemical catalytic activity and stability for methanol oxidation compared to commercial Pt50–Ru50 binary catalyst. The effect of carbon support on methanol oxidation is also investigated using hollow mesoporous carbon (HMC), mesoporous CMK-3, and commercial Vulcan XC-72 as supporting materials for the quaternary composition. The newly developed PtRuIrNi/VC, PtRuIrNi/CMK-3, and PtRuIrNi/HMC catalysts exhibit an enhancement in activity of ca. 26–50% and demonstrate better electrochemical stability toward methanol oxidation than the Vulcan carbon-supported Pt50–Ru50 binary alloy catalyst. About half of the total improvement in catalytic activity is attributed to a new active quaternary catalyst composition, whose activity can be explained by a bifunctional mechanism, electronic effect, and stability effect occurring from addition of Ni and Ir. An additional boost amounting to the other half of the total enhancement is obtained from a unique hierarchical nanostructured carbon support with high surface area and pore volume, which favors not only homogeneous dispersion of small catalyst nanoparticles, but also fast mass transport in the catalyst layer. Ó 2013 Elsevier Inc. All rights reserved.

1. Introduction Fuel cells are being actively considered for automotive and stationary power applications, as they offer high efficiency with little or no pollution. For transportation applications, both proton exchange membrane fuel cells (PEMFCs) and direct methanol fuel cells (DMFCs) are being actively pursued, due to their high fuel efficiency and low operation temperature. DMFCs are one of the most promising technologies for energy generation, which directly convert the chemical energy of CH3OH fuel and O2 into electrical energy [1–6]. Pt-based binary catalyst compositions, such as PtRu, PtMo, PtCo and PtNi, have been studied as active electrocatalysts for the methanol oxidation reaction (MOR) [7–10]. Despite many investigations, active and stable anode catalysts for methanol oxidation are still in high demand for enhanced DMFC performance. Among various technical issues, the enhancement of catalytic activity and the stability of electrodes represent the most important issues in fuel cell technology [5,10–12]. So far, most of the investigations on Pt-based alloy catalysts have been mainly limited to binary systems because of the difficulty and time consumption ⇑ Corresponding author. Fax: +82 44 860 1331. E-mail address: [email protected] (J.-S. Yu). 0021-9517/$ - see front matter Ó 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jcat.2013.06.005

in finding new active catalyst compositions beyond binary compositions by conventional ‘‘one-at-a-time’’ methods in a very wide range of compositional possibility. The combinatorial approach is the most plausible solution to counter this problem and can allow efficient discovery of the optimum compositions among many different combinations. In principle, the combinatorial method involves preparation and screening of combinatorial libraries for discovery of a specific type of activity [13]. A combinatorial material library consists of many different samples with various compositions that are synthesized quickly, simultaneously, and in parallel. High-throughput screening is used to assess single or multiple properties of a large number of samples in the combinatorial libraries automatically and rapidly. Even though combinatorial methods have been intensively explored for last two decades in the discovery of interesting new organic and biological molecules [14], new functional materials [15–18], sensors [19], and catalysts [20–23], it is still challenging to successfully apply this technique to a specific system, in particular to the discovery of efficient electrocatalysts. Several quaternary and ternary anode catalyst compositions for DMFCs identified by combinational methods were reported to have better electrocatalytic activity than the best known PtRu binary composition [20–23]. It is certain that presence of additional ele-

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ments causes strong modifications of the chemisorptive properties and electrocatalytic performance. Reddington et al. [21,22] has pioneered this field by successfully identifying active electrocatalyst regions from hundreds of samples based on Pt, Ru, Os, Ir, and Rh in combinatorial quaternary arrays by monitoring acid–base fluorescence signals. Sullivan et al. [24] reported an automated electrochemical analysis system in a common electrolyte to measure proton concentration and electrochemical current at each electrode of a 64-electrode array. Lin and Smotkin [25] investigated DMFC anode electrocatalysts through a high-throughput screening device. Jiang and Chu [26] proposed a different analysis method that can measure the electrochemical properties of various electrodes using a movable electrolyte probe system. Different Pt-based ternary systems have been explored by different groups using a combinational synthesis method [27–31]. The oxidation of H2/CO on the PtRuW/C catalyst was found to be better than on the other catalysts tested [31]. Recently, novel quaternary catalysts such as PtRuMoW [23], PtRuNiZr [32], PtRuCoW [33], and PtRuFeSe [34] have also been newly reported and found to exhibit superior electrochemical catalytic activity toward methanol electro-oxidation compared to the commercial PtRu binary catalyst. Methanol-tolerant non-noble-metal quaternary and ternary RuSnWSe, RuSnMoSe, and RuMoSe cathode electrocatalysts were also investigated for oxygen reduction in DMFC [29]. On the other hand, rapid developments have been made toward porous carbon support with control over microporous and mesoporous structures. Carbon materials including carbon black [35– 37], carbon nanotubes [38–42], carbon nanofibers [43–45], carbon nanohorns [46], hollow mesoporous carbon [47], and ordered hierarchical nanostructured carbon [48,49] have been most widely studied as catalyst supports for low-temperature fuel cells because of their unique structure and properties, such as large surface area and well-developed pore structure, along with high electrical conductivity. Recently, carbon-supported ternary PtRuIr/C [50,51] and PtRuNi/C [52,53] catalysts have been developed for DMFC. In addition, Sun et al. [54] prepared ternary PtRuNi catalysts supported on N-doped carbon nanotubes and evaluated their electroactivity for methanol oxidation. In this paper, we report new Pt-based active PtRuIrNi ternary and quaternary alloy electrocatalysts using a combinatorial approach. The choice of Ni and Ir is based mainly on the consideration of the lowering of electronic binding energy in Pt–Ru by alloying with Ni and the improvement of the stability of the alloy catalyst by alloying with Ir [50–53]. It is well documented that Ni is present in various oxidative states such as NiO, Ni(OH)2, and NiOOH, in addition to metallic Ni [53]. The enhanced activity of Pt–Ru–Ni compared to Pt–Ru was attributed to the electronic effect as a result of the modification of the electronic properties of Pt by Ni, along with abundant surface oxides as the oxygen donor required to remove COads [52,53]. On the other hand, it is reported that the addition of Ir improves the stability of Pt–Ru alloy catalyst and reduces the loss of Ru, and iridium dioxide (IrO2) has good activity toward oxygen evolution reaction [51,55]. Eventually, the quaternary Pt–Ru–Ni–Ir system can have the characteristics of both Ni and Ir in the well-known Pt–Ru system. Thus, the quaternary system is expected not only to improve methanol oxidation by providing an oxygen source for CO oxidation at lower potentials, but also to increase the stability of the quaternary alloy systems during methanol oxidation. The newly developed PtRuIrNi quaternary anode electrocatalysts show superior performance for methanol oxidation in terms of activity and stability, with better resistivity against CO surface poisoning than the commercial PtRu alloy catalyst. The best PtRuIrNi compositions were identified, and the corresponding catalysts were then synthesized in bulk and tested separately. In addition, hierarchical nanostructured carbon materials were also prepared and explored as DMFC anode supports. A

novel hollow mesoporous carbon (HMC) support further enhances the catalytic activity, which is evidenced by better fuel cell performance than those of other common carbon supports such as CMK3 and Vulcan XC-72 carbon (VC) under identical DMFC anode operation conditions. 2. Experimental 2.1. Preparation of anode catalysts and combinatorial analysis Aqueous solutions of 0.5 M metal salt were prepared from each of H2PtCl66H2O, RuCl3xH2O, K2IrCl6, and NiCl26H2O (Aldrich). Quaternary electrode arrays with a large number of diverse chemical compositions were fabricated onto Teflon-coated Toray carbon paper by a robotic dispensing system (Cartesian Technologies, PixSys 3200), which uses modified dispensing software to deliver metal salt solutions with a resolution of 88 nL/spot. The preparation method is described in detail elsewhere [13,33]. Each composition spot was painted with various amounts of each metal solution, and the final quaternary array contains 220 different catalyst compositions at 11% resolution, each of which has the same total number of moles of metals (1.5 lM per spot). The array were then dried at 80 °C for 2 h under vacuum and then reduced by adding 5 wt% NaBH4 solution (10-fold molar excess) to each spot. The spot diameter in these arrays was approximately 2 mm. The distance between spots was adjusted to 7 mm in order to avoid mixing up adjacent spots [33]. The electrochemical analysis via optical screening was carried out by clamping the arrays into a homemade reactor cell containing an indicator solution consisting of 6.0 M CH3OH, 0.5 M NaClO4, 30 mM Ni(ClO4)2, and 100 lM 3-pyridin-2-yl-(4,5,6)triazolo-[1,5-a] pyridine (PTP, Ni2+ complex, pKa = 1.5) in water [13,21,22,33]. It is intended to perform a combinatorial screening test under experimental conditions as similar as possible to real fuel cell operation conditions. Because PEM fuel cells operate under highly acidic conditions, the Ni–PTP complex was selected as a fluorescent indicator that turns on at acidic pH values. The pH was adjusted to 3.0 using HClO4. The cell was connected to a potentiostat (EG&G 362) with Pt as a counter electrode and Ag/ AgCl (in saturated KCl) as a reference electrode. Each spot in the array was used as a working electrode. An UV lamp (360 nm) was held above the surface of the electrode array, and the observed fluorescence was noted. The most active catalyst compositions produce the greatest local pH change because of protons generated during methanol oxidation, glowing under UV irradiation at the lowest overpotential. The working electrode potential was initially set to +200 mV vs. Ag/AgCl and was then stepped up to increasingly anodic potentials in 50 mV increments. The electrode was held at each potential for about 5 min. This process was repeated until a steady visible fluorescence signal was observed over the catalyst array, which was triggered by protons generated in an electrochemical anode half-cell reaction:

CH3 OH þ H2 O ! CO2 þ 6e þ 6Hþ Typically, fluorescence is preferred over absorbance as a detection method because of its greater sensitivity. One of the advantages of this screening method is its simplicity, since it requires only aqueous indicator solutions and a handheld ultraviolet (UV) lamp, and also is an intrinsically parallel technique. One drawback is that the method is indirect and thus does not allow one to measure current directly. However, fluorescence images can be taken at a number of potentials to give a qualitative I–V characteristic for the spots in a catalyst array, where MOR activity on each spot can be expressed in the fluorescence image, which can be seen

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by the naked eye [13,21]. Optical screening is also performed on the basis of complete oxidation of methanol to CO2. Under current optical screening conditions, only the compositions inducing significant pH variation based on complete oxidation of methanol are detected for the comparison of electrocatalyst performance. Compositions causing incomplete oxidation will not decrease the pH significantly and thus will not be detected in the optical screening. 2.2. Fabrication of nanostructured carbon supports 2.2.1. Hollow mesoporous carbon (HMC) HMC was synthesized by replication through nanocasting of solid core/mesoporous shell (SCMS) silica [56]. In a typical synthesis process, 5 mL of aqueous ammonia (32 wt%) was added into a solution containing 125 mL of ethanol and 10 mL of deionized water. After ca. 15 min of stirring at room temperature, 4 mL of tetraethyl orthosilicate (TEOS, 98%, ACROS) was added to the mixture and stirred for ca. 6 h to yield uniform silica spheres. A solution containing a mixture of 1.7 mL of TEOS and 0.7 mL of n-octadecyltrimethoxysilane (C18-TMS, 90% tech., Aldrich) was added into the colloidal solution containing the silica spheres and kept for 1 h. An octadecyl-group-incorporated silica shell was then created on each of the pre-prepared solid silica spheres. The nanocomposite was retrieved by centrifugation, dried at room temperature, and further calcined at 550 °C for 6 h under oxygen to produce the final octadecyl-group-free SCMS silica. Aluminum was incorporated into the silica framework through an impregnation method to produce acidic points on the surface of the SCMS silica, which will catalyze polymerization of phenol and paraformaldehyde as follows. A total of 1.0 g of SCMS silica was added to an aqueous solution containing 0.27 g of AlCl36H2O in 0.3 mL of water, and the resulting slurry was stirred for 30 min [56]. The slurry was then dried overnight in air at 80 °C. Finally, the Al-impregnated SCMS silica was calcined at 550 °C for 5 h in air to yield SCMS aluminosilicate. The typical synthesis route for HMC with a core size of ca. 130 nm and shell thickness of ca. 30 nm is as follows. A quantity of 0.374 g of phenol was incorporated into the mesopores of 1.0 g of the SCMS aluminosilicate template by heating it at 100 °C for 6 h under vacuum. The resulting phenol-incorporating SCMS template was further reacted with paraformaldehyde (0.238 g) under vacuum at 130 °C for 6 h to yield a phenol-resin/SCMS aluminosilicate composite. The composite was calcined at 1 °C/min to 160 °C and held for 5 h under a nitrogen flow. The temperature was then ramped at 5 °C/min to 1000 °C and held for 5 h to carbonize the cross-linked phenolic resin inside the mesopores of the SCMS structure. The SCMS silica template was dissolved using 2.0 M NaOH, and the remaining black slurry was washed with ethanol–water solution several times. The as-prepared HMC was dried overnight at 80 °C. Compared with the parent SCMS silica, a slight shrinkage (10%) in core size and shell thickness of the HMC is observed. 2.2.2. CMK-3 CMK-3, the most representative ordered mesoporous carbon, was fabricated by replication through nanocasting of SBA-15 silica instead of SCMS silica using the same process [57]. In this work, rod-type CMK-3 ca. 750 nm in length and 200 nm in diameter was fabricated by replication from corresponding rod-type SBA15 as a template. Aluminum was also incorporated into the silicate framework through the same impregnation method as in the SCMS silica to produce acidic sites on the surface of the SBA-15 silica. Detailed information about synthesis of the rod-type CMK-3 can be seen in our earlier work [57,58].

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2.3. Preparation of various carbon-supported anode catalyst electrodes Carbon-supported 60 wt% metal alloy anode catalysts were synthesized at room temperature through an impregnation method using metal salt solutions (H2PtCl6, RuCl3, K2IrCl6, and NiCl2), each of different nanostructured carbons—in this case, HMC, CMK-3, and Vulcan XC-72 carbon (VC) as supports and NaBH4 as a reducing agent, according to a previously reported process [2–4]. The catalyst inks were prepared by dispersing each of various nanostructured carbon-supported metal alloy catalysts into a mixture of an appropriate amount of deionized water and the required amount of 5 wt% Nafion ionomer (Aldrich). The Nafion ionomer content in the catalyst layers was set to 25 wt%. Appropriate amounts of the catalyst inks were painted uniformly on Teflonized carbon paper (TGPH-090) and dried at 70 °C for 30 min. The catalyst loadings at anode and cathode based on metal were 2.5 and 5.0 mg/cm2 (unsupported Pt from Johnson Matthey (J.M.)), respectively. The membrane electrode assembly (MEA) was fabricated by hot-pressing the anode and cathode catalyst layers on either side of a pretreated Nafion 115 membrane (Du Pont) under a pressure of 800 psi at 130 °C for 3 min. 2.4. Characterization of anode catalysts The catalysts were characterized by transmission electron microscopy (TEM), inductively coupled plasma (ICP), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and N2 adsorption and desorption analysis. The XRD patterns of supported catalysts were obtained using a Rigaku 1200 diffractometer operated with Cu Ka radiation (k = 0.154 nm) at a scan rate of 2°/min (2h) at 40 kV and 20 mA. High-resolution transmission electron microscopy (HR-TEM) images of the samples were collected using a JEOL FE-2010 microscope operated at 200 kV. The chemical compositions of the quaternary anode electrocatalysts were determined by ICP analysis to be PtvRuxIryNiz, where v, x, y, and z represent mole%. XPS measurements were obtained with a Kratos XSAM 800 PCI spectrometer with an Mg Ka line source. Unsupported powder catalyst samples were mounted on carbon tape, and spectra were obtained with 40 eV pass energy and at a 15° takeoff angle from the normal surface. Binding energies were referenced to a graphite standard (C1s = 284.5 eV). N2 adsorption and desorption isotherms were measured at 77 K on a Micromeritics ASAP-2010 gas adsorption analyzer after the carbon was degassed at 423 K to 20 mTorr for 4 h. The specific surface areas were determined from nitrogen adsorption using the Brunauer–Emmett–Teller (BET) equation. The total pore volumes were determined from the amounts of gas adsorbed at a relative pressure of 0.99. Micropore (pore size <2 nm) volumes of the porous carbons were calculated from the analysis of the adsorption isotherms using the Horvath–Kawazoe (HK) method. The pore size distribution (PSD) was derived from analysis of the adsorption branch using the Barrett–Joyner–Halenda (BJH) method. 2.5. Electrocatalytic activity toward methanol oxidation of various anode catalysts Different electrochemical methods such as cyclic voltammetry (CV), CO stripping voltammetry, chronoamperometry (CA), and the fuel cell constant-current polarization performance test were used to analyze the electrocatalytic activity of various Pt-based anode catalysts toward methanol oxidation. All electrochemical experiments were conducted in a single cell with MEA configuration using a WMPG-1000 potentiometer (WonA-Tech). A single-cell configuration was used for CV measurements at 30 °C with a 1.0 M aqueous CH3OH solution feed to the anode serving as working electrode. The Pt black cathode was fed with

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humidified H2 gas at 50 mL/min (zero back pressure). Methanol solution was fed to the working electrode by a Masterflex liquid micro-pump at a rate of 1.0 mL/min. The baseline curve was obtained at the same condition with H2O only fed to the anode without methanol. CO gas was adsorbed onto the working electrode by flowing 20% CO in N2 at 50 mL/min (zero back pressure) over the working electrode for 20 min, while keeping the electrode potential at 0.1 V versus the dynamic hydrogen electrode (DHE). The gas was then switched for 2 min to Ar at a flow rate of 50 mL/ min, without changing the electrode potential, to remove unadsorbed CO from the working electrode. CV and CO stripping voltammogram curves were recorded from 0.0 to +1.2 V at a scan rate of 20 mV/s. CA measurements were carried out in a N2 degassed mixed solution of 0.5 M H2SO4 and 1.0 M CH3OH at a potential of 0.45 V (vs. Ag/AgCl) at ambient temperature. The working electrodes were precleaned prior to the CA measurements by running several CV cycles in 0.5 M H2SO4 from 0.2 to +1.0 V (vs. Ag/ AgCl) at a scan rate of 20 mV/s until a reproducible response was reached. The catalyst loading on the working electrode was 2.0 mg/cm2 (metal base) with an effective geometric area of 1.0 cm2. The single-cell test fixture was composed of two copper end plates and two graphite plates with rib-channel patterns allowing the passage of methanol to the anode and oxygen gas to the cathode. For constant-current polarization measurements, a 2.0 M methanol solution was supplied to the anode at a rate of 1.0 mL/ min, while dry O2 was fed to the cathode at a rate of 300 mL/min using a flow meter. The polarization measurements were con-

ducted in a single cell of active area 2 cm2 using a WMPG-1000 potentiometer. 3. Results and discussion The optical screening method used in the anode experiment was described in detail in our earlier work [13]. Briefly, a fluorescent indicator was used to project the H+ ions generated in the diffusion layer above different composition spots in a large array, as shown in Fig. 1a. The electrode arrays were tested in a solution of CH3OH and NaClO4 electrolyte in the pH range where PTP indicator is in nonfluorescent (deprotonated) form with Ni(ClO4)2. The catalyst regions that fluoresced due to methanol oxidation at the lowest overpotential were noted. The selected active composition zone was further focused with better resolution by another screening to locate the lead compositions in the active zone. Individual samples of these active compositions were then prepared and tested for fuel cell performance. The four-component space can be represented as a tetrahedron, which unfolds into a two-dimensional (2D) map to give a smooth variation in composition across the array [13,21,33]. Each corner of the 3D tetrahedral quaternary array represents a single component spot among the various quaternary composition spots under study. Each of the four triangle faces of the tetrahedron represents a three-component phase diagram including a single component at each corner and binary components on the edge. Quaternary components are located at two inner shells among three concentric shells for the resulting 220-spot array in the quaternary system

Fig. 1. (a) Optical screening mechanism for Ni2+ complex of PTP fluorescence indicator, (b) 2D optical screening results with fluorescence images (bright spots indicate active compositions for MOR in the presence of methanol), and (c) refolded tetrahedron activity map of 220 different quaternary composition spots at 11% resolution for Pt–Ru–Ir– Ni system, illustrating the active MOR spots marked with large blue balls. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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at 11% resolution, in which case, there are 10 different compositions along each binary edge. Fig. 1b shows the 2D PtRuIrNi quaternary array of 220 different composition spots fabricated onto Toray carbon paper by a robotic dispensing system from Cartesian Technologies. The image in white light (left) and the fluorescence image at low potential (center) identifying the most active region (marked by a red circle) of the composition space are seen in Fig. 1b. The fluorescence image at high overpotential (right) also indicates that the initial active zone spots glow more intensely due to higher electrocatalytic activity at higher potential, along with many other new glowing spots (marked by green colors) in the array. Regions of the array that fluoresce are noted with the potential at which they fluoresce. Usually the screening analysis was repeated twice to make sure of consistent results. Fig. 1c shows a refolded 3D tetrahedron quaternary map with the active MOR spots marked with large blue balls, which were identified by optical screening in the 2D PtRuIrNi composition diagram. Interestingly, PtRuNi and PtRuIr ternary phases have several active spots, while PtNiIr and RuNiIr phases show almost no active spots. In particular, many active spots are on or near the PtRuNi ternary phase. The binary active spots are found only in the PtRu system, as seen in the refolded quaternary map in Fig. 1c. This active zone was again fabricated with the boundaries of the active zone as the corner points of a higher-resolution 220-spot

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tetrahedron and screened again as a ‘‘zoom’’ array. Some boundary compositions are indicated in Fig. 1c. Such processes pinpoint the best compositions, and the corresponding catalysts are then synthesized in bulk and tested separately. Several compositions were individually tested, among which three new best compositions, Pt34Ru30Ir13Ni23, Pt42Ru34Ir12Ni12, and Pt45Ru30Ni25, were confirmed and analyzed further in comparison with a state-of-theart commercial binary Pt50Ru50 catalyst. The X-ray diffraction (XRD) patterns for the newly discovered ternary and quaternary catalysts in this work are shown in Fig. 2a and are also compared with that of a commercial J.M. PtRu catalyst. The XRD patterns of all the new Pt-based catalysts confirm the face-centered cubic (fcc) structure. The diffraction peak observed at 2h  25° in all the XRD patterns is due to the (0 0 2) plane of the Vulcan XC-72 carbon (VC) support. No evidence of elemental Ru, Ir, or Ni was observed in the XRD patterns, suggesting uniform Pt alloy formation [59]. The average crystallite sizes of the supported 60 wt% electrocatalysts were calculated from the (2 2 0) diffraction peak in the Pt fcc lattice using the Scherrer equation [60] and found to be ca. 3.1 nm for Pt34Ru30Ir13Ni23, 3.1 nm for Pt42Ru34Ir12Ni12, 3.6 nm for Pt45Ru30Ni25 and 3.5 nm for the commercial binary Pt50Ru50. In general, when metal particles are assumed to be dispersed uniformly onto a carbon support, smaller sizes of the catalyst particles correspond to higher active surface

Fig. 2. (a) Powder XRD patterns and (b) cyclic voltammograms in 1.0 M CH3OH with a scan rate of 20 mV/s for newly developed VC-supported PtRuIrNi (60 wt%) and commercial PtRu (60 wt%) catalysts. (c) Chronoamperograms obtained in a mixed solution of 0.5 M H2SO4 and 1.0 M CH3OH at 0.45 V (vs. Ag/AgCl) at ambient temperature and (d) CO stripping voltammograms for Pt34Ru30Ir13Ni23, Pt45Ru30Ni25, and Pt50Ru50 catalysts. Potentials were obtained against the dynamic hydrogen electrode.

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area and thus higher activity. Therefore, on the basis of the results obtained from XRD, the new quaternary alloy catalysts are expected to be more favorable for methanol oxidation in DMFC than the commercial binary catalyst. Cyclic voltammograms of the anode catalysts in 1.0 M CH3OH at 30 °C are shown in Fig. 2b. Newly developed ternary and quaternary anode catalysts give lower onset potentials (280–298 mV) for methanol oxidation than commercial J.M. binary catalysts (320 mV), as seen in the figure, showing the superior catalytic activity of the PtRuNi ternary and PtRuIrNi quaternary catalysts vs. the PtRu binary catalyst for methanol oxidation. In addition,

all the newly discovered anode catalysts exhibit much higher current density for methanol oxidation (vs. Ag/AgCl) than the commercial PtRu catalyst. In fact, the newly developed catalysts demonstrate a 10–30% improvement in catalytic activity toward methanol oxidation compared to the commercial catalyst. Particularly, the quaternary Pt34Ru30Ir13Ni23 composition revealed the best methanol oxidation activity among the new anode catalysts. Considering that homemade binary PtRu/VC catalyst always shows less methanol oxidation activity (ca. 10–20% less activity) than the corresponding commercial state-of-the-art binary PtRu/VC catalyst [3], we certainly believe that the new quaternary Pt34Ru30Ir13Ni23

Fig. 3. XPS survey scan (a) for Pt34Ru30Ir13Ni23/VC and narrow-range XPS spectra and deconvoluted curves for (b) Pt4f, (c) Ir4f, (d) Ni2p, and (e) Ru3p.

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catalyst in fact possesses superior catalytic activity for methanol oxidation compared to the commercial binary. In addition, the Pt34Ru30Ir13Ni23/VC catalyst also outperforms the new ternary Pt45Ru30Ni25/VC, which still showed better activity (20% better activity) than the commercial binary under the same conditions. With Pt as the most common DMFC anode electrocatalyst, it has been shown that CO resulting from methanol decomposition is a strong poison for the Pt catalyst [61]. To address the poisoning by COads, modification of Pt with foreign metals (M) (e.g., Ru, Ni, Co, Ir, and Sn) is necessary. CA measurements can provide important information regarding reactant diffusion kinetics and electrode electrochemical stability, because they show the current profiles of the methanol oxidation process as a function of time at a fixed potential setting. Fig. 2c shows different CA patterns of methanol oxidation for newly developed ternary and quaternary anode catalysts and commercial binary PtRu catalyst tested in a mixed solution of 0.5 M H2SO4 and 1.0 M CH3OH. The Pt34Ru30Ir13Ni23 catalyst exhibited similar initial current density, but much higher current density after 1 h of methanol oxidation compared to the new ternary Pt45Ru30Ni25 (TEM image shown in Fig. S1 of the supporting information (SI)) and commercial PtRu catalyst, which is consistent with CV results, and also showed good stability, as indicated by the slow decrease in the current response with the increasing time period during the measurements. After 1 h, the Pt34Ru30Ir13Ni23 catalyst maintained 79% of the initial oxidation current density, while the PtRuNi and commercial PtRu catalyst maintained only 65% and 60%, respectively, of the initial current density. The result argues that PtRuIrNi is ‘‘less poisoned by CO’’ than the PtRuNi and PtRu catalysts, especially indicating that the addition of Ir leads to enhanced electrode stability for CO and other poisoning species.

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The CO oxidation activities of the catalysts were also investigated using CO stripping voltammetry and are shown in Fig. 2d. The CO oxidation by Pt34Ru30Ir13Ni23 occurs at a much lower potential than for Pt45Ru30Ni25 and commercial PtRu. This indicates that the quaternary catalyst with additional Ni and Ir components has a superb ability to promote the electro-oxidation of absorbed CO at lower potential. The CO stripping peak potentials were 575 and 600 mV for PtRuIrNi and PtRuNi catalysts, respectively, compared to 660 mV for the J.M. binary catalyst, indicating that the oxidation of COads on Pt surfaces occurs more easily in the PtRuIrNi. Therefore, addition of a small amount of Ni and Ir helps to effectively clean the CO-poisoned Pt surfaces and to maintain reaction sites for further methanol electroxidation. Based on the results of CV, CA, and CO stripping voltammograms of Fig. 2, addition of Ir is suggested to further increase MOR kinetics and improve the electrode stability against CO poisoning. Fig. 3 displays an XPS survey scan and narrow-range XPS spectra and deconvoluted curves for Pt, Ir, Ni, and Ru in PtRuIrNi/VC catalyst. The survey scan spectrum clearly indicates the presence of Pt, Ru, Ir, and Ni species in PtRuIrNi/VC. The composition determined from mass% by XPS is similar to one for Pt34Ru30Ir13Ni23 determined by ICP, indicating that the composition is homogeneous on the surface and inner part of the quaternary catalyst. The Pt4f XPS spectrum in Fig. 3b shows two intense peaks at 71.5 and 74.6 eV attributable to Pt4f7/2 and Pt4f5/2, respectively, with a theoretical peak area ratio of 1:1, which can be deconvoluted into two pairs of doublets. The intense doublets centered at binding energies of 71.2 and 74.5 eV reveal the presence of metallic Pt(0), whereas the less intense doublets centered at 73.0 and 76.1 eV reveal the presence of a small amount of bivalent Pt(II), probably in the form of Pt(OH)2 or PtO. Fig. S2 in the SI shows

Fig. 4. TEM images for (a) HMC, (b) CMK-3, and (c) VC as support and (d) nitrogen adsorption–desorption isotherms obtained at 196 °C and pore size distribution for HMC, CMK-3, and VC.

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Table 1 Structural parameters for HMC, CMK-3, and commercial carbon black VC. Sample

SBET (m2/g)

Vtotal (cm3/g)

Vmeso (cm3/g)

Pore (nm)

HMC CMK-3 VC

1680 1120 235

1.72 1.15 0.31

1.13 0.70 0.17

3.4 4.1 —

the corresponding core-level Pt4f XPS spectrum of commercial binary PtRu catalyst with similar deconvolution into two pairs of doublets. The relative contents of Pt(0) and Pt(II) in the quaternary PtRuIrNi catalyst are 64% and 36%, comparable to those of the commercial binary PtRu catalyst. The Ir4f spectrum in Fig. 3c also reveals two intense peaks, which can be deconvoluted into two pairs of doublets corresponding to different oxidation states. Comparison of the observed binding energies with those reported in the literature suggests that the two species are metallic Ir (at 61.7 and 64.7 eV) and IrO2 (at 62.8 and 65.8 eV) [55]. The deconvolution results suggest that 85% of the iridium is present as the metal state and 15% as the oxidized state. The Ni2p spectrum appears to be very complicated, with an overlap of several peaks, as seen in Fig. 3d. High-binding-energy satellite peaks called ‘‘shake-up’’ at 860.4 and 876.7 eV are also found adjacent to the main peaks, which are generally caused by multielectron excitation. After these ‘‘shake-up’’ peaks are taken into account, the Ni2p spectra are deconvoluted into four different peaks at 852.6, 853.6, 854.2, and 856.2 eV, which correspond to metallic Ni (22.1%), NiO (15.4%), Ni(OH)2 (49.6%), and NiOOH (12.9%), respectively, with Ni(OH)2 as a major component among

Ni species, which is consistent with earlier work [53]. Deconvolution of the Ru3p region indicates that the Ru3p3/2 spectrum is composed of two peaks at 463.3 and 464.1 eV, which correspond to metallic Ru and RuO2, with 79.2% of the Ru as metallic Ru and 20.8% as RuO2, as shown in Fig. 3e. Fig. S3 in the Supporting Information shows the XPS spectra of the wide survey scan and narrow scan Ni2p and Ru3p spectra of ternary PtRuNi/VC catalyst. The Ru3p spectrum consists of 81.4% metallic ruthenium (463.1 eV) and 18.6% RuO2 (464.0 eV), respectively. The Ni2p XPS peaks at the binding energies of 852.7, 853.7, 854.2, and 856.4 eV correspond to Ni0, NiO, Ni(OH)2, and NiOOH, respectively. The Ni2p XPS spectrum comprises 22.3% metallic Ni, 13.6% NiO, 54.6% Ni(OH)2, and 9.5% NiOOH, which are similar to those of quaternary Pt34Ru30Ir13Ni23. The presence of additional elements presents strong modifications of chemisorptive properties and electrocatalytic performances of Pt or Pt–Ru electrocatalysts. To increase the CO tolerance of Pt, oxophilic metals such as Ru and Ni are incorporated into Pt either individually or collectively to form Pt-alloy electrocatalysts, since the oxophilic metals provide adsorption sites of hydroxyl groups, which can oxidize the COads to CO2 at a much lower potential than on Pt [52,62,63]. This bifunctional mechanism is well adapted for our Pt-based quaternary alloy catalysts in MOR. In addition, the modified electronic structure of Pt with additional M can alter the electronic properties of Pt and decrease the CO coverage of the surface by changing the adsorption energy of the CO or other adsorbed intermediates. The electronic effect would make the formation of the adsorbed CO on Pt less favorable, while the formation of reactive intermediates could be preferred

Fig. 5. TEM images for (a) commercial Pt50Ru50/VC (J.M.), (b) Pt34Ru30Ir13Ni23/VC, (c) Pt34Ru30Ir13Ni23/CMK-3, and (d) Pt34Ru30Ir13Ni23/HMC with respective histograms of the particle size distribution for the 60 wt% Pt-based catalysts as measured from 400 particles in the TEM micrograph; and (e) X-ray diffraction patterns of various nanostructured carbon-supported 60 wt% Pt-based catalysts.

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[64–66]. The electron is likely to transfer from nickel to platinum in PtRuIrNi according to electronegativity order, i.e., 1.91, 2.2, 2.2, and 2.28 for Ni, Ru, Ir, and Pt, respectively. This electron transfer may contribute to the decay of the Pt–CO bond and enhance the MOR. Such bifunctional and electronic effects, occurring mainly from Ru and Ni, will result in a high forward current density of the alloy catalysts, providing improved tolerance to CO poisoning during MOR [67–69]. On the other hand, since the Pt34Ru30Ir13Ni23/VC catalyst outperforms the Pt–Ru–Ni ternary phase compositions, including the new ternary Pt45Ru30Ni25/VC, the addition of Ir is also considered beneficial for MOR performance. The Ir was reported to make stronger M–C (and M–H) bonds than the Pt and Ru [59]. Hence, it is reasonable to assume that the Ir could also contribute positively to the cleavage of C–H bonds in the methanol molecule, resulting in favorable methanol dissociative adsorption and consequently improved activity for methanol electro-oxidation, as seen by the CV and CA profiles in Fig. 2b and c, respectively, in agreement with previous work [59]. In addition, Ir is also known to assist in enhancing the COads oxidation activity and stabilize catalysts from anodic corrosion [70]. Since iridium dioxide (IrO2) has good activity in the oxygen evolution reaction [51,55], the IrO2 observed by XPS in our quaternary catalyst is also believed to contribute to enhancing the COads oxidation activity. It was also reported that OH groups could be stabilized at the metallic Ir surface, thus assisting in the oxidation of CO or other adsorbed intermediates [50]. Thus, the Ir, in addition to Ni, for our new quaternary catalyst is considered to further increase MOR kinetics and provide much improved electrode stability against CO and other poisoning species. Different nanostructured carbon materials were also prepared and explored as DMFC anode supports. Figs. 4a–c show TEM images for HMC, CMK-3, and VC used as supports. Fig. 4d shows nitrogen adsorption–desorption isotherms for various employed carbon supports and their pore size distribution curves. The VC reveals an isotherm typical of microporous material. The resulting isotherms for CMK-3 and HMC can be classified as type IV with H2 and H3 hysteresis, respectively, according to IUPAC nomenclature, and suggest a narrow PSD. The pore sizes were estimated from the PSD maximum as 3.4 nm for HMC and 4.1 nm for CMK3. VC reveals a wide range of PSD from micropores to macropores. Table 1 summarizes the surface parameters for the HMC, CMK-3, and VC as obtained from the N2 adsorption–desorption isotherms at 196 °C. The HMC capsules exhibited a high surface area of 1680 m2/g and a total pore volume of 1.72 cm3/g, which can be attributed mainly to the presence of mesopores in the shell. The isotherms also indicate the presence of some micropores in the framework, as seen from high N2 absorption at low relative pressure. The HMC is generated as an individual uniform spherical particle with particle size ca. 190 nm, consisting of a hollow core ca. 130 nm in diameter and a shell thickness of 30 nm as observed from the TEM image of Fig. 4a. In addition, the HMC possesses a unique 3D interconnected hierarchical nanoarchitecture consisting of a hollow macropore core, a mesoporous shell, and interconnected large interstitial spaces between the packed spherical carbon particles, which can guarantee fast mass transport, whereas CMK-3 reveals only uniform mesopores along with some micropores in the framework, as shown in Fig. 4b and d. These unique structural and surface properties can make HMC a more ideal candidate as a catalyst support, compared to others. Fig. 5 shows TEM images and histograms of 60 wt% Pt34Ru30Ir13Ni23 quaternary anode catalysts supported by various nanostructured carbons, namely VC, CMK-3, and HMC, along with those for the commercial J.M. 60 wt% PtRu/VC catalyst. Homogeneous dispersion of small nanoparticles (NPs) can be clearly seen for Pt34Ru30Ir13Ni23 supported on HMC, with hollow core and

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Fig. 6. (a) Cyclic voltammograms in 1.0 M CH3OH for various carbon-supported Pt34Ru30Ir13Ni23 quaternary anode catalysts compared with commercial binary catalyst with a scan rate of 20 mV s1, and (b) specific activity and mass activity for methanol oxidation determined on the basis of CV data for different catalysts.

mesoporous shell morphology. Interestingly, the catalyst NPs are distributed along the mesopore channels for rod-shaped CMK-3supported Pt34Ru30Ir13Ni23 catalyst. The commercial J.M. PtRu/VC with 60 wt% loading contains homogenously dispersed PtRu NPs, most of which are spherically shaped with sizes ca. 3.5 nm, but some agglomeration is also observed (Fig. 5a). Interestingly, when compared to the commercial binary catalyst, the newly discovered quaternary PtRuIrNi alloy NPs are found to be dispersed homogenously without much agglomeration as smaller, spherical, and more uniform NPs on the surface of different carbon supports. In particular, the HMC shows a uniform size distribution of small metal NPs with much less agglomeration. Fig. 5e shows XRD patterns of various nanostructured carbonsupported Pt34Ru30Ir13Ni23 quaternary and commercial Pt50–Ru50 catalysts. The XRD patterns of all the Pt-based catalysts again confirmed the face-centered cubic (fcc) structure. No other phase was detected in the XRD pattern, indicating the formation of Pt alloy. The average particle sizes of the Pt34Ru30Ir13Ni23 quaternary alloy NPs loaded on VC, CMK-3, and HMC supports, calculated from the (2 2 0) diffraction peak in the Pt fcc lattice using the Scherrer equation, were determined to be ca. 3.1, 2.9, and 2.8 nm, respectively, which are smaller than that (3.5 nm) of commercial PtRu/ VC. This is in good agreement with the TEM images shown in Fig. 5. Smaller crystallite size and more homogeneous dispersion of the PtRuIrNi alloy NPs on the HMC are assumed to enhance electrocatalytic activity toward methanol oxidation and correspond-

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ingly improve the fuel cell performance compared to that of the other carbon-supported ones. The catalytic activities of the as-synthesized catalysts for methanol electro-oxidation were also evaluated by CV measurements. Fig. 6a shows typical CVs for methanol electro-oxidation by the Pt34Ru30Ir13Ni23 catalyst NPs supported on HMC, CMK-3, and VC, respectively, as well as the state-of-the-art commercial Pt50–Ru50 catalyst. All the carbon-supported quaternary Pt34Ru30Ir13Ni23 catalysts show lower onset potentials (223, 235, and 280 mV for PtRuIrNi/HMC, PRuIrNi/CMK-3, and PRuIrNi/VC catalysts, respectively) compared to that (320 mV) of commercial PtRu/VC for methanol oxidation. In addition, all supported PtRuIrNi catalysts exhibited higher current densities than that of commercial PtRu/ VC. It is also interesting to notice that the Pt34Ru30Ir13Ni23 quaternary anode catalysts revealed different catalytic activities depending on the type of catalyst supports employed. It can be seen from Fig. 6a that the maximum oxidation current densities in the forward sweep are 17.2, 16.0, and 16.2 mA/cm2 for PtRuIrNi/HMC, PtRuIrNi/CMK-3, and PtRuIrNi/VC, respectively, which are much higher than the 12.6 mA/cm2 of commercial J.M. PtRu/VC binary catalysts, demonstrating that not only the components and composition of the catalyst, but also the supporting material, contributes a lot to catalytic activity to reactions in fuel cells. The ratio of current densities associated with the anodic peaks in the forward (If) and reverse (Ib) scans is an important parameter, which has been utilized to assess the CO tolerance of the catalysts [71,72]. A high If/Ib ratio implies very efficient oxidation of methanol to carbon dioxide and effective removal of poisoning species from the surface of the catalyst during the anodic scan. The PtRuIrNi/CMK-3 and PtRuIrNi/HMC catalysts show high If/Ib ratios of 3.5 and 3.2, respectively, suggesting an improved tolerance to poisoning from carbonaceous species and hence increased stability compared to commercial PtRu/VC (J.M.). This improvement in

nanostructured carbon-supported catalysts over VC-supported catalysts can be attributed not only to the high mesoporosity of the nanostructured carbon, favoring efficient mass transfer, but also to the smaller dimensions of catalyst NPs dispersed on the nanostructured carbon support, significantly improving the contact area between catalyst and reagent. Fig. 6b reveals the specific activity and mass activity for methanol oxidation of different catalysts. Both the mass activity and the specific activity are good indicators of the catalytic performance of an electrocatalyst. Since the newly developed quaternary and ternary compositions have smaller amounts of expensive Pt for an identical 60 wt% loading amount than the commercial binary Pt50Ru50/VC, the merits of the new electrocatalysts can be more significantly expressed in terms of the mass activity of Pt. As shown in Fig. 6b, the Pt34Ru30Ir13Ni23/HMC quaternary catalyst exhibits a mass activity of 41.8 mA/mg Pt, which is ca. 3.5 times that of the Pt50Ru50/VC (J.M.) catalyst (12.0 mA/mg Pt) and 2.4 times that of the Pt45Ru30Ni25/VC ternary catalyst (17.8 mA/mg Pt). It can be emphasized that addition of Ir and Ni not only decreases Pt and Ru content significantly, but also greatly increases the performance of the new quaternary catalysts. To gain further insight into the elemental distribution in the Pt34Ru30Ir13Ni23/HMC catalyst, we carried out elemental analysis by scanning transmission electron microscopy (STEM). Fig. 7 shows an STEM micrograph of the Pt34Ru30Ir13Ni23/HMC catalyst, in which the hollow core–mesoporous shell morphology can be seen clearly. More definite compositional information is provided by a high-angle annular dark-field STEM energy-dispersive spectrometer ((HAADF-STEM)-EDS) attached in the TEM for elemental mapping analysis. It can be seen in Fig. 7 that the distribution of Pt, Ru, Ir, and Ni elements is completely overlapped. On the basis of the above results, it can be concluded that the Pt34Ru30Ir13Ni23/ HMC catalyst forms a definitely quaternary alloy structure, as also

Fig. 7. Elemental analyses of a typical Pt34Ru30Ir13Ni23/HMC catalyst with individual elemental maps of Pt, Ru, Ir, and Ni by STEM and overlay map of the elements. Scale bar: 30 nm.

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seen by XRD spectra in Fig. 5e. The spatial distribution of elements in the Pt34Ru30Ir13Ni23/HMC catalyst may not be uniform and seems to thin out toward the surface of the particle. However, if we look at Fig. 5, showing the full image of HMC, we can clearly observe that the spatial distribution of the catalyst on the full HMC support is rather homogenous without much agglomeration. The polarization behavior of DMFC using various carbon-supported Pt34Ru30Ir13Ni23 catalysts and commercial PtRu/VC catalyst as anodes for methanol oxidation at two different temperatures is shown in Fig. 8. The open-circuit voltages (OCVs) for the PtRuIrNi catalyst supported on HMC, CMK-3, and VC and for commercial PtRu catalyst were 0.71, 0.68, 0.67, and 0.63 V, respectively, at

Fig. 8. The polarization curves for a direct methanol fuel cell using a commercial PtRu (60 wt%) catalyst or various carbon-supported PtRuIrNi (60 wt%) catalysts as anodes, determined (a) at 30 °C and (b) at 70 °C, and (c) chronoamperograms obtained in a mixed solution of 0.5 M H2SO4 and 1.0 M CH3OH at 0.45 V (vs. Ag/ AgCl) at ambient temperature for various carbon-supported catalysts.

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30 °C (Fig. 8a). The obtained current density at 0.4 V (where the polarization is mainly affected by the activity of the catalyst) for the PtRuIrNi/HMC catalyst was 165 mA/cm2, which corresponds to a 161% enhancement as compared to the value of 63 mA/cm2 for commercial PtRu/VC catalyst under the same test conditions. The maximum power densities were 82 mW/cm2 for PtRuIrNi/ HMC, 73 mW/cm2 for PtRuIrNi/CMK-3, 68 mW/cm2 for PtRuIrNi/ VC, and 48 mW/cm2 for commercial PtRu/VC catalysts. These values also indicate that the PtRuIrNi/HMC catalysts exhibited ca. 70% higher power density than the commercial catalyst. The results obtained at 70 °C also show similar trends, as shown in Fig. 8b. At 70 °C, the maximum power density was 197 mW/cm2 for PtRuIrNi/HMC, 180 mW/cm2 for PtRuIrNi/CMK-3, and 166 mW/cm2 for PtRuIrNi/VC, which corresponds to an enhancement of ca. 26–50% compared to the commercial one (132 mW/ cm2). Since all the catalysts were tested under identical conditions, the improvement in OCV and maximum power density values can be fully ascribed to both newly developed quaternary alloy composition and corresponding nanostructured supporting material with high surface area and pore volume, giving rise to better MOR activity and fuel cell performance. The reactant diffusion kinetics and electrode electrochemical stability were also tested by CA measurements, which display the current profiles of the methanol oxidation process as a function of time at a constant potential. Fig. 8c shows different CA patterns for the various anode catalysts. All the carbon-supported PtRuIrNi catalysts exhibited not only much higher initial and final current densities but also slower current decreases for methanol oxidation compared to the commercial PtRu/VC catalyst, clearly demonstrating better electrocatalytic activity and stability toward methanol oxidation. After 1 h of testing, the PtRuIrNi/HMC, PtRuIrNi/CMK3, and PtRuIrNi/VC catalysts maintained 80%, 80%, and 79%, respectively, of the initial oxidation current densities, i.e., activities, while the commercial PtRu/VC catalyst maintained only 60% of the initial current density. This high stability toward methanol oxidation, electrode stability, and resistivity to CO poisoning of the PtRuIrNi quaternary catalysts is also ascribed to the presence of additional metals (Ir and Ni) and the efficient supporting effect of the nanostructured carbon support. The superior electrocatalytic performance demonstrated by the newly discovered quaternary Pt34Ru30Ir13Ni23 catalyst compared with the commercial PtRu binary catalyst can be attributed to the addition of Ni and Ir to Pt–Ru, which helps to further enhance the COads oxidation and improve electrode stability. Similar efficient multicomponent Pt-based alloy catalysts such as ternary PtRuOs [73] and quaternary PtRuWSn [74], PtRuMoW [27], and PtRuCoW [33] have also been reported in methanol oxidation. However, they still contain a high Pt content of more than 40%, whereas our active PtRuIrNi quaternary catalysts have Pt content less than 40%. The considerable improvement in the electrocatalytic activity and fuel cell performance of the supported quaternary catalyst is also ascribed to the HMC support with unique structural properties. The larger specific surface area and pore volume of the HMC favor a better dispersion of the supported catalyst NPs with a smaller particle size, as evident from the TEM images and XRD patterns, resulting in more active reaction sites for the oxidation of methanol. In addition, the 3D well-combined hierarchical nanoporous structure with the mesopores in the shell open to the outer surface and to the inner hollow macroporous core provides an open highway network around the active catalyst for efficient mass transport. Furthermore, the 3D interconnected large interstitial spaces between the packed spherical carbon particles, unique in this system, are open to the mesoporous channels, serving as primary fast pathways for the delivery of the reactants and products. In contrast, randomly distributed pores with varying sizes in the VC with

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low surface area, along with significant microporosity, may result in much less efficient mass transport. The mass transport is also less efficient for the CMK-3, mainly that with single pore structure. Fig. S4 in the Supplementary Information shows an individual elemental map of Pt, Ru, Ir, and Ni in a typical Pt34Ru30Ir13Ni23/CMK-3 catalyst by STEM. It can be seen in the STEM micrograph that the distribution of Pt, Ru, Ir, and Ni elements is also homogenous and completely overlapped, as much as in the Pt34Ru30Ir13Ni23/HMC catalyst in Fig. 7. Less activity of Pt34Ru30Ir13Ni23/CMK-3 catalyst than of Pt34Ru30Ir13Ni23/HMC is attributed to the transport-limited kinetics associated with the former catalyst, as the supporting carbon of the former possesses mainly long mesoporous channels, into which the transfer of electrolyte and oxygen molecules is more restricted than that into the hierarchical pores of the HMC with mesopores in combination with macropores. 4. Conclusions In this work, PtRuIrNi quaternary alloy electrocatalysts were investigated for DMFC, and new highly active quaternary PtRuIrNi compositions were identified by a modified combinatorial optical screening method. The newly developed PtRuIrNi catalysts demonstrated not only higher catalytic MOR activity and better fuel cell performance, but also higher electrode stability and resistivity to CO poisoning than those of commercial J.M. binary PtRu catalyst. In particular, Pt34Ru30Ir13Ni23 composition was found to be most active. Not only a bifunctional mechanism and an electronic effect, but also stabilization effects of added Ni and Ir metals were the most probable reasons for overall enhancement of catalytic activity and stability in the new quaternary PtRuIrNi system. In addition, hierarchical nanostructured HMC was explored for the first time as a support for the PtRuIrNi anode catalyst. The superb structural characteristics of the HMC have further increased the catalytic activity for methanol oxidation, enabling the HMC to be an ideal catalyst support for a low-temperature fuel cell. The HMC-supported Pt34Ru30Ir13Ni23 demonstrated ca. 50% higher catalytic activity than the commercial binary catalyst. About half of the total enhancement in the catalytic activity can be attributed to new quaternary catalyst composition, while the other half of the total enhancement is ascribed to the unique 3D interconnected hierarchical nanostructured carbon support with high surface area and pore volume. Acknowledgments This work was supported by an NRF Grant (NRF 2010-0029245) funded by the Ministry of Education, Science, and Technology through the National Research Foundation of Korea. Special thanks are given to the Korean Basic Science Institute at Jeonju, Chuncheon, and Daejeon for SEM, TEM, XPS, and XRD measurements. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jcat.2013.06.005. References [1] [2] [3] [4] [5]

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