Cobalt-Modified Palladium Bimetallic Catalyst: A Multifunctional

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Cobalt-Modified Palladium Bimetallic Catalyst: A Multifunctional Electrocatalyst with Enhanced Efficiency and Stability Toward the Oxidation of Ethanol and Formate in Alkaline Medium Sasidharan Sankar, Gopinathan M. Anilkumar, Takanori Tamaki, and Takeo Yamaguchi ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00790 • Publication Date (Web): 01 Aug 2018 Downloaded from http://pubs.acs.org on August 7, 2018

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ACS Applied Energy Materials

Cobalt-Modified Palladium Bimetallic Catalyst: A Multifunctional Electrocatalyst with Enhanced Efficiency and Stability Toward the Oxidation of Ethanol and Formate in Alkaline Medium Sasidharan Sankar a†, b‡, Gopinathan M. Anilkumarc§, b‡, Takanori Tamaki a†, b‡, Takeo Yamaguchia†, b‡* a†

Laboratory for Chemistry and Life Sciences, Tokyo Institute of Technology, R1-17, 4259 Nagatsuta, Midori-ku, Yokohama 226-850 *E-mail: [email protected] b‡

c§

Core Research for Evolutionary Science and Technology, Japan Science and Technology Agency (JST-CREST), Japan 102-0076

R&D Centre, Noritake Co., Ltd., 300 Higashiyama, Miyochi-cho, Miyoshi 470-0293, Japan

ABSTRACT In recent years, there has been an ever-increasing demand for liquid-fueled energy carriers, including direct ethanol fuel cells and direct formate fuel cells. Current research on liquid fuel cells is focused on the exploration of new, durable and highly efficient non-platinum-based electrocatalysts with long-term stability. In this research, bimetallic palladium-cobalt (Pd-Co) nanoalloys were investigated as bifunctional anode catalysts with enhanced efficiency and stability toward the direct electrooxidation of ethanol and formate in an alkaline medium. X-ray diffraction,

transmission

electron

microscopy–energy-dispersive

spectroscopy,

thermogravimetric analysis, and X-ray photoelectron spectroscopy were employed for the indepth characterization of the nanocatalysts synthesized by a modified polyol reduction process in triethylene

glycol

medium.

Electrochemical

analysis

via

cyclic

voltammetry

and

chronoamperometry revealed improved electrocatalytic activity and excellent stability in an alkaline medium toward the anodic oxidation of liquid fuels: ethanol (ethanol oxidation reaction, EOR) and formate (formate oxidation reaction, FOR). Excellent mass activity values as high as 3.7 Amg−1 and 3.2 Amg−1 were observed for the as-made Pd2.3Co/C and PdCo/C electrocatalyst samples, respectively, during EOR. In addition, enhanced FOR activity and high durability were observed for the Pd2.3Co/C catalyst compared to that of Pd/C prepared in similar manner. The superior electrochemical activities for the prepared catalysts compared to monometallic Pd/C is related to the reduced poisoning on the catalyst surface induced by the synergistic effect of the asprepared bimetallic catalysts and altered Pd electronic structure by the incorporation of Co. 1 ACS Paragon Plus Environment

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Keywords: Alkaline Fuel Cell, Liquid Fuel, Anodic Oxidation, Ethanol, Formate, Bifunctional Catalyst

1. Introduction Fuel cells hold the key to solving the demand for efficient future power sources due to several factors, including their high efficacy, their environmentally benign nature and the abundance of fuel resources (1). Platinum has been in the limelight for several years as the electrocatalyst for fuel cell use and has been investigated at various scales for plentiful anodic and cathodic reactions (2). However, because of the lack of specificity and other issues related to Pt catalysts, highperforming nonplatinum electrocatalysts have more recently been increasingly sought and have attracted immense attention for the effective commercialization of fuel cells (3-5). In addition, the effect of strong CO binding on the Pt surface, which displaces the surface-bound hydrogen, drastically affects the catalytic efficiency with regards to liquid fuels such as ethanol and methanol. As an evolving energy technology, liquid fuel cells, including direct ethanol fuel cells (DEFCs) and direct formate fuel cells (DFFCs), still need to be substantially improved with respect to system design using newer electrocatalysts with high efficiency and durability. In this environment, palladium (Pd) and Pd-based nanoalloy catalysts have conclusively gained attraction and momentum as effective catalysts because of their durability and effectiveness in the successful electrooxidation of liquid fuels, including alcohol(s) and formate, which are sources of clean energy (6-12). Although Pd-based electrocatalysts have exhibited advantages over several Pt-based catalysts, issues related to the improvement of oxidation kinetics and the reduction in the surface adsorption of poisoning species need to be efficiently addressed. Efficient strategies adopted for improving the catalytic performance of Pd over the years include alloying with transition metals and doping

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with various nonmetals (13). More recently, several researchers have attempted to prepare bimetallic, Pd–transition metal (Pd–TM) alloys, including Pd–Ni, Pd–Sn, Pd–Cu, Pd–Au, Pd-Bi and Pd–transition metal composites for the enhancement of the catalytic efficiency in electrochemical reactions such as the oxygen reduction reaction and hydrogen evolution reaction (14-20). The oxophilicity induced by the TMs in these bimetallic alloys, such as Co and Ni, is crucial, as it enhances the electrochemical activity. The incorporation of these oxophilic TMs during the formation of metallic nanoalloys could not only improve the cost-effectiveness but also enhance the electrocatalytic properties of the material through surface modification. These oxophilic metal additions can improve the CO tolerance, and the alteration of the electronic structure can enhance the catalytic activity of noble metal catalysts such as Pd. However, these modified catalysts exhibit different catalytic performances and durability depending on both the synthesis procedures adopted and the engineered morphological characteristics. The incorporation of cobalt and its corresponding alloy formation have been reported to effectively increase the electrocatalytic property of the parent metals, including palladium (21), via a reduction in the adsorption of intermediate species on the catalyst surface, though it depends on the Pd surface enrichment level. Few studies have reported the use of Pd–Co bimetallic alloy catalysts for the oxidation of liquid fuels in alkaline medium, particularly for the direct electrooxidation of formate and ethanol. Recently, Zheye et al. (22) reported the preparation of Pd–Co bimetallic particles encapsulated in N-doped porous carbon nanocapsules after hightemperature carbonization for EOR and FOR at high pH but at low mass activity. Bai et al. (8), through a solvothermal technique, could prepare large-sized hollow Pd–Co nanospheres (77–89 nm) over multiwalled carbon nanotubes (MCWNTs) and applied them for the EOR in alkaline medium.



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Liquid fuels, such as ethanol and formate, constitute a large pool of renewable liquid fuel resources, which are considerably safer and better than pure hydrogen in terms of production, safety, and distribution (11, 23-24). Apart from safety and transportation constraints, the physical properties of hydrogen, including the lightweight nature and its energy density (8.4 MJ/L as liquid hydrogen and 4.6 MJ/L as compressed hydrogen at 70 MPa), limit the usage as fuel. As an alternative fuel resource to hydrogen, ethanol, with an energy density of 23 MJ/L, is an ambient temperature fuel with advantages over hydrogen such as easier handling and storage. Formate (HCOO−) is considerably less toxic and environmentally benign compared to other liquid fuels such as methanol and liquid ammonia (11). Furthermore, HCOO− can be regenerated by the electroreduction of CO2, making formate a viable fuel for liquid fuel cell applications (25). This fact permits the possibility of regenerating formate after the FOR, thereby ensuring its reusability. Recently, formate fuel cells (DFFCs) have also been identified as an excellent choice in the liquid fuel cell category (26). Alkaline fuel cell technology is known to positively address many of the issues encountered in acidic media; for instance, it can further ensure more rapid electrooxidation of formate ions (27), which enhances the DFFC performance. In this work, we report a wet chemical approach, i.e., a versatile modified polyol reduction involving a solid-solution alloy processing of Pd–Co nanoalloys as catalysts for alkaline fuel cells. The robust Pd–Co nanoalloys obtained over carbon supports were tested for their electrocatalytic performances as direct liquid fuel cell anode catalysts (for EOR and FOR) in an alkaline medium. Pd-based alloy electrocatalysts based on a solid-solution process have been previously reported by Kusada et al. (28) and Kobayashi et al. (29) for processing Pd–Ru alloy nanoparticles, wherein the Pd was found to mimic the electronic properties of Rh. Herein, a further modified chemical reduction technique is utilized for synthesizing Pd-based bimetallic


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catalysts with an oxophilic TM to improve both efficiency and stability at high pH for direct ethanol fuel cell (DEFC) and direct formate fuel cell (DFFC) applications.

2. Materials and Methods Palladium nanoparticles and their carbon-supported Co nanoalloys (i.e., Pd/C and Pd–Co/C, respectively) were prepared from the corresponding metal precursor salts (K2PdCl4, Furuya Chemicals, Japan, and CoCl2·6H2O, Wako Chemicals, Japan), and Vulcan XC72 carbon (Cabot, USA) was used as the carbon support. Commercial Pd/C (10% Pd, Wako, Japan) was used as the reference. Triethylene glycol (TEG) was purchased from TCI Chemicals, Japan, and polyvinylpyrrolidone (K30) was purchased from Wako Chemicals. All chemicals (analytical grade) were used as received. Millipore water with a resistivity of 18.2 MΩ cm (Merck Millipore 3x Essential, Germany) was used for the entire process and analysis. 2.1 Synthesis of Pd/C & Pd-Co/C catalysts The polyol reduction method (30), with considerable modifications, was employed to prepare varying catalyst compositions of Pd and Pd–Co nanoalloys using different molar ratios (i.e., PdCo, Pd2Co, Pd2.3Co, and Pd3Co). In a typical synthesis of PdCo/C, 0.346 g of K2PdCl4 was dissolved in 15 mL of Millipore water, and 0.238 g of CoCl2·6H2O was added, followed by the addition of ~0.056 g of PVP (K30). The mixed solution was subjected to ultrasonication for 30 min. Vulcan carbon (0.2 g) was added to the aqueous mixed solution and subjected to additional sonication for 30 min. TEG (0.2 L) in a glass beaker was heated to 200 °C over a mantle heater. The Pd and PdCo content ratios with respect to the carbon content were ~1:2 and ~1:1, respectively. The aqueous carbon-metal precursor slurry was then sprayed into the hot TEG solution, maintaining the temperature at 200–205 °C. Carbon-supported metal particles were separated by centrifugal washing and dried overnight at 80 °C in an air oven. 5 ACS Paragon Plus Environment


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2.2 Catalyst Characterization The thermal decomposition characteristics of the as-prepared catalysts were analyzed under N2 using a Pyris 1 thermogravimetric analyzer (Perkin Elmer, USA) at a heating rate of 2.5 °C/min. The metal weight ratios in the prepared catalysts were determined using inductively coupled plasma - mass spectrometry (ICP-MS) using 4 vol% aqua reagent. XRD patterns were recorded on an Ultima IV (Rigaku, Japan) system with a Cu Kα (λ = 1.5406 Å) X-ray source operating at 40 kV and 40 mA and at a scan rate of 1°/min. High-resolution transmission electron microscopy (HR-TEM) images (micrographs were captured using a TOPCON EM-002BF-J system operating at an accelerating voltage of 200 kV with a twin EDS facility) were recorded to examine the morphology and elemental distribution of the catalysts. The surface composition of the catalysts was analyzed on a Quantum 2000 (ULVAC-PHI Inc., Japan) X-ray photoelectron spectrometer fitted with a twinanode X-ray source using Al Kα radiation (hv = 1486.58 eV). The spectra curves were fitted with the Gauss-Lorentz wave shape function and the background was revised using "Shirley".

2.3 Electrochemical Measurements The detailed electrochemical behavior of the catalysts toward EOR and FOR was examined by cyclic voltammetry (CV) and chronoamperometry (CA) using an HZ-7000 electrochemical measurement system and a dynamic electrode HR-301 (HD, Hokuto Denko). EOR and FOR evaluated by CV were carried out using a three-electrode system with Hg/HgO and a Pt wire as the reference and counter electrodes, respectively. A glassy carbon electrode (GCE) with a surface area of 0.196 cm2 was used as the working electrode. The current density was normalized with the surface area of the GCE. CVs were recorded at a scan rate of 50 mV/s in a N2-saturated 1 M KOH solution. After measuring the CV profile of a blank alkaline solution, liquid fuels (CH3CH2OH and HCOOK) were added in the range of 0.1 M–1 M to analyze the fuel oxidation characteristics. The catalyst ink was prepared by 6 ACS Paragon Plus Environment

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mixing ‘x’ mg of catalyst with 20 µL of Nafion (5%) and 5 mL of IPA (25%), followed by ultrasonication in an ice bath for ~2 h. Approximately 10 µL of the catalyst ink was drop-cast on the polished GCE surface and dried overnight. The amount of ‘x’ was varied so that the catalyst loading on the GCE surface was maintained at 15 µg/cm2. The cyclic stability of catalysts for EOR was investigated by carrying out 250 CV cycles in N2-saturated KOH solution at a scan rate of 50 mV/s.

3. Results and Discussion Table 1 show the residual metal content (wt %) for the different catalyst compositions, which ranged from 30 - 46%, after TGA under a N2 atmosphere (Figure S1). ICP measurements were carried out to determine the metal composition variation among the catalysts with varying Pd loading (table S1), which showed a Co content in the range of 6 to 9 wt% for the prepared catalysts. Figure 1 shows the powder XRD patterns obtained for Pd/C, carbon-supported Pd–Co catalysts and commercial Pd/C. The XRD patterns observed for the Pd–Co catalysts corresponded to the characteristic fcc structures of Pd, which are indexed in the figure (JCPDS Card No. 461043). The diffraction peaks at ∼40° 2θ and ∼45° 2θ correspond to the (111) and (200) planes of Pd, respectively. A definite but marginal shift toward higher angles of 2θ (table 1) was observed for these peaks of the as-prepared Pd–Co catalysts compared with those of Pd/C, which is related to the alteration in the Pd lattice caused by the incorporation of Co. The average crystallite size of the as-formed nanoparticles was calculated by using the Debye–Scherrer equation (Table 1) with all the composites showing an average particle size of approximately 5 nm and with Pd/C showing a size of approximately 4 nm, which is in agreement with the electron microscopy results. The lattice parameters for Pd in Pd/C, PdCo/C and Pd2.3Co/C alloy catalysts were



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determined to be 0.391 nm, 0.386 nm and 0.381 nm, respectively. Furthermore, the absence of metallic Co and CoO peaks in the XRD patterns indicates the formation of Pd–Co alloys (31).

Figure 1. (a) XRD patterns of Pd/C and Pd–Co/C catalysts and (b) magnified spectra of Pd/C and PdCo/C and Pd2.3Co/C showing the variation in the 2θ value

Table 1. Structural and compositional features of Pd/C and Pd–Co/C nanocatalysts with different molar ratios (*obtained from supplier specifications) Catalyst Composition

Angle - 2θ (Pd-111 plane)

PdCo/C

Metal Content from TGA (wt%) 36

ECSA from CV (m2/g)

40.17

Average Particle Size from XRD (nm) 4.5

Pd2Co/C

43

40.24

4.8

11

Pd2.3Co /C

30

40.13

4.7

15

Pd3Co/C

46

40.19

6.0

12

Pd/C

36

39.98

4.0

20

Pd/C (Commercial)

10*

40.03

2.2

45

24


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Figure 2 shows the electron micrographs (TEM) and EDS mapping images of the Pd2.3Co/C nanoalloy catalyst. Near-spherical crystalline particles were observed over the carbon support, with an average size of ~5 nm. When compared with that of the monometallic Pd particles (Pd/C, ~4 nm), the size of the bimetallic alloy nanoparticles had slightly increased (Supporting Information, Figure S2 (a)). The inset HR-TEM images reveals the lattice fringes for the obtained nanoparticles corresponding to the fcc facets of Pd with a “d” spacing of ~0.23 nm; this value is in good agreement with the XRD data for the (111) plane. The micrographs indicate the uniform nature of the alloy particles formed by the employed technique. The lower-magnification TEM micrograph shows the uniform distribution of nanoparticles (Figure S2 (b)), and the EDS mapping of the catalyst confirmed the relatively good distribution of Co along Pd over the carbon support. The EDS spectra of Pd2.3Co/C further confirmed the successful incorporation (Figure S2 (c)) of Co with Pd particles through the solid solution processing.



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Figure 2. TEM images of the Pd2.3Co/C catalyst and the EDS mapping showing the distribution of elemental composition in the Pd2.3Co/C

XPS analysis was employed to examine the elemental surface composition of the catalysts, and the bimetallic samples showed peaks representing the binding energies for Pd, C, O, and Co (Figure S3). Figure 3a shows the deconvoluted XPS data fitted for the Pd 3d region of the PdCo/C and Pd2.3Co/C catalysts. The binding energy (core peak) for metallic Pd (Pd 3d) on the carbon support (Pd/C) was observed at 335.3 eV (32), while that for the bimetallic PdCo/C catalyst was observed with an energy shift of 0.4 eV to a higher energy of 335.7 eV; this result is in agreement with that reported for the formation of a nanoalloy (Pd–Co) (33-34). As in the case of the Pd2.3Co/C composition, the binding energy for Pd 3d was observed at a value greater than 10 ACS Paragon Plus Environment

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0.1 eV, i.e., at 335.8 eV, confirming the chemical interaction between Pd and Co. Unlike the binding energy reported for metallic cobalt (778.1–778.3 eV) (35), the core peak for Co2p was changed and is now observed at 783 eV (Figure 3b), indicating the successful formation of the Pd–Co alloy. The XPS data of the as-prepared catalysts are in agreement with the XRD data; hence, the modified polyol reduction process adopted effectively promotes alloying of Co with Pd surface.

Figure 3. (a) Pd (3d) region in PdCo/C and Pd2.3Co/C (b) Co (2p) region in Pd2.3Co/C

3.1 Electrochemical Behavior Figure 4 shows the CV profiles obtained for Pd/C and the various PdCo/C catalysts in N2saturated 1 M KOH measured at −0.9–0.2 V vs Hg/HgO at room temperature. Characteristic peaks for Pd-containing catalysts in an alkaline medium were observed in the voltammograms. The peak position marked “I” corresponded to the oxidation of the adsorbed hydrogen (Hads, Figure 4a) (36). The peak denoted as II was observed at approximately −0.3 V, which probably corresponds to the activation of Pd water, as has been reported previously (37); however, this observation was not noted in the CV profile of Pd2.3Co/C, where only a peak corresponding to



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OH− was observed. Peak III corresponded to the Pd oxide formation, which is reduced at approximately −0.2 V of the reverse scan (Peak IV). The slight difference in the oxide peak positions for the catalysts is possibly related to the different dispersion conditions of Pd on the surface. The CV characteristics of commercial Pd/C in 1 M KOH in comparison with bimetallic catalyst and in-house Pd/C are provided as Supporting Information, Figure S4. Table 1 summarizes the electrochemical surface area (ECSA) values for the various catalysts as calculated from the CV data. The ECSA was calculated from the PdO peak (at approximately −0.2 V) in the reverse scan based on the area of the reduction peak of PdO. A higher ECSA for commercial Pd/C was observed because of its considerably smaller particle size compared with that of the Pd/C prepared herein. The PdCo catalyst exhibited an increase in ECSA; however, lower ECSA was observed with increased Pd loading among the catalysts, probably because of the size increase caused by alloy formation.

Figure 4. Cyclic voltammograms of PdCo/C, Pd2Co/C, Pd2.3Co/C, and Pd/C catalysts in N2saturated 1 M KOH at 50 mV s−1


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3.2 Liquid Fuel Oxidation Activity of Pd and Pd-Co Nanoalloys 3.2.1 Ethanol Oxidation Reaction (EOR) Ethanol oxidation using Pd is more efficient and feasible in an alkaline medium than in an acidic medium due to the presence of OH− species, which can immediately remove hydrogen, leading to the continuous electrooxidation of ethanol (38). Figure 5a shows the CV profiles obtained for EOR using the prepared catalysts. Table 2 summarizes the CV data, including the onset potential, the current density at −0.4 V, and the obtained peak current density. The peak current density, or the maximum current density, which has been frequently used as an index to assess the catalyst performance in previously reported studies, is calculated for comparison with other catalysts. However, when considering the practical fuel-cell operation in combination with the oxygen reducing cathode, the potential at the peak current density (Ep) corresponds to a low voltage, almost Pd2Co > PdCo/C > Pd/C (commercial) > Pd/C (in-house) > Pd3Co. The Pd2.3Co/C catalyst also exhibited a maximum peak current density of 56 mA/cm2 at a peak potential of −0.17 V and a more negative onset potential of −0.59 V compared to the prepared and commercial Pd/C. The obtained current density for the Pd2.3Co/C catalyst was two times that of the commercial Pd/C. The more negative onset potential indicates better kinetics during the ethanol oxidation reaction. This shows that the addition of Co has certainly promoted the Pd activity and enhanced the kinetics of ethanol oxidation in an alkaline medium. As the ethanol concentration increased, Pd2.3Co/C exhibited increased fuel oxidation ability (Supporting Information, figure S5) in an alkaline medium.



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Table 2. Cyclic voltammetry data obtained for the ethanol oxidation reaction

Onset Potential (Eonset) (V)

Current Density at -0.4 V vs Hg/HgO (mA/cm2)

Maximum Current Density (at Ep) (mA/cm2)

If/Ib

PdCo/C

−0.5

2.85

48

1.03

Pd2Co/C

−0.51

3.55

44

1.02

Pd2.3Co/C

−0.59

4.95

56

1.02

Pd3Co/C

−0.46

1.77

27

0.99

Pd/C

−0.47

2.32

33

0.96

Pd/C (Commercial)

−0.5

3.1

27

0.66

The excellent EOR ability of the Pd–Co catalysts is possibly related to the superior tolerance granted to the catalyst’s surface by the addition of Co toward poisoning intermediate species. In alkaline medium, ethanol is oxidized to acetic acid (which exists as an acetate ion) on the Pdcatalyst during EOR (35, 38), and the rate-determining step in EOR over a Pd-catalyst is reported to be the removal of adsorbed acyl ((COCH3) ads) by the adsorbed hydroxyl (OHads), where the rapid dissociative adsorption of ethanol proceeds (39). Figure 5b shows the forward and backward scans for the EOR profile: The backward scan peak (Ib) was clearly less intense than the forward scan peaks (If) for Pd2.3Co/C and PdCo/C, while the scans for Pd/C showed an If/Ib that was considerably lower (Table 2). A lower If/Ib ratio implies more poisoning species on the surface, thereby leading to poor electroactivity (40), while a large ratio means good tolerance of the anode with regard to the poisoning species. As an oxophilic metal, Co generates OHads, which transforms poisoning species to CO2, thereby improving the ethanol oxidation by generating more 14 ACS Paragon Plus Environment

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active Pd sites. Thus, the addition of Co has definitely enhanced the oxophilicity to form oxygenated species, and the synergistic effect leads to enhanced tolerance against poisoning species on the Pd2.3Co/C catalyst surface. Figure 5c summarizes the mass activity values, which are key features for assessing the applicability of metals as catalysts. The mass activities at peak potentials for the Pd2.3Co/C, in-house Pd/C, and commercial Pd/C catalysts were calculated as 3.7 Amg−1, 2.2 Amg−1, and 1.8 Amg−1, respectively. This value obtained for Pd2.3Co/C is highest among recently reported Pd-based electrocatalysts, including Pd–Au/C (42), Pd–Ag (43), Pd–Ni (44), Pd–Bi (45), and Pd-Cu (46), which demonstrates an excellent anode EOR catalyst activity in alkaline medium. Table 3 summarizes the comparison of the catalytic efficiency in terms of current density with those of some recently reported Pd-based EOR electrocatalysts with similar and varying catalyst loadings over GCE. Thus, Pd2.3Co/C demonstrated excellent potential as an EOR anode electrocatalyst, with a lower onset potential, enhanced peak current density, and higher mass activity than monometallic Pd/C.



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Figure 5. Electrochemical data for the ethanol oxidation reaction (EOR) over Pd/C and Pd-Co (with varying molar ratios) a) CVs with 1 M KOH + 1 M C2H5OH b) forward and backward scans obtained during EOR (1 M KOH + 1 M C2H5OH) c) mass activities of the catalyst samples at the peak potential and at −0.4 V d) chronoamperogram curves recorded at −0.25 V vs. Hg/HgO in 1 M KOH

To elucidate the stability of the catalysts over longer cycles, CV tests were conducted for 250 cycles in 1 M KOH, and changes in the current densities of Pd2.3Co/C and Pd/C catalysts (Supporting Information, S6) in EOR were measured over time. The EOR activity of Pd2.3Co/C clearly increased until the 70th cycle and started decreasing extremely gradually compared to the rapid decrease in the activity of Pd/C from the 50th cycle (Figure S6 (a,b)). After the completion of 250 cycles, Pd2.3Co/C retained >75% of the original activity, compared with that of Pd/C ( Pd2Co > PdCo/C > Pd/C (in-house) > Pd3Co. Furthermore, Pd2.3Co/C exhibited a good mass activity of 2.7 mA/mg−1 compared with the 1.5 mA/mg−1 for Pd/C (Figure 6b); this value is ~1.8 times higher. The enhanced current density for these compositions underlines the excellent FOR kinetics and enhanced efficiency in alkaline medium obtained for the Co-modified catalysts. Figure S6 (Supporting Information) shows the concentration dependence of FOR in alkaline medium for Pd2.3Co/C and Pd/C. The formate oxidation reaction is reported to be related to the release of carbonate ion (CO32−) during the anodic oxidation of formate using Pd-based catalysts; HCOO- + 3OH- CO32- + 2H2O + 2e(54-55).

Figure 6. Electrochemical data on formate oxidation (FOR) by Pd/C & Pd-Co samples a) CVs with 1 M KOH + 1 M HCOOK b) FOR mass activity for the catalysts at peak potential and -0.4 V

Similar to that observed in EOR, the reduced adsorption of intermediate species on the catalyst surface via the adjustment of the adsorption energy during FOR by the bimetallic Pd–Co catalyst



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increased the stability and efficiency for liquid fuel oxidation. As hydrogen is well known to diffuse easily to the Pd surface, the enhancement of FOR kinetics is possible via the weakening of the bond between Pd and adsorbed hydrogen (Hads). The addition of oxophilic Co to Pd led to the alteration of the surface and electronic structure, as can be seen from the XRD and surface analysis, which possibly decreased the adsorption of hydrogen on the catalyst surface, leading to the creation of additional Pd sites. This in turn further enhanced the rate of FOR in the alkaline medium. Table 4. Cyclic voltammetry data for the formate oxidation reaction Onset Potential (Eonset) (V)

Current Density at -0.4 V vs Hg/HgO (mA/cm2) 5.26

Maximum Current Density (at Ep) (mA/cm2)

PdCo/C

−0.63

34

Pd2Co/C

−0.62

5.65

28

Pd2.3Co/C

−0.69

7.70

38

Pd3Co/C

−0.59

3.63

28

Pd/C

−0.62

5.24

22

Figure 7 shows the chronoamperograms for bimetallic Pd2.3Co, PdCo and monometallic Pd catalysts recorded at −0.46 V vs Hg/HgO in a N2-saturated alkaline medium to analyze the catalyst durability. Initially, the current densities of both catalysts may have decreased due to the formation of surface intermediate species. After 3000 s, Pd2.3Co/C, PdCo/C and Pd/C catalysts exhibited mass activities of 156 mA/mg, 105 mA/mg and 95 mA/mg, respectively. CA tests revealed the possibility of the reduced adsorption of poisoning species on the bimetallic catalyst surface facilitated by the Co adjacent to Pd sites, which enhanced their stability in an alkaline


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medium toward FOR. The above results obtained during FOR demonstrate the potential for applying Pd–Co bimetallic catalysts obtained using a modified polyol reduction method for DFFCs instead of monometallic palladium catalysts.

Figure 7. Chronoamperograms recorded at −0.46 V vs Hg/HgO in I M KOH + 1 M HCOOK

4. CONCLUSIONS Bimetallic palladium–cobalt nanocatalysts with dual functionality were synthesized by versatile solid-solution processing, which can be scaled up for commercial fuel-cell applications. X-ray diffraction and X-ray photon correlation spectroscopy revealed the successful alloying of Pd–Co on the carbon support. TEM micrographs with EDS elemental mapping revealed the distribution of Co with Pd particles. Compared to the other prepared catalysts, Pd2.3Co/C exhibited excellent electrocatalytic activity and durability for the oxidation of ethanol and formate in alkaline medium, followed by PdCo/C. The electrocatalytic efficiency was superior to that of commercial



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monometallic Pd/C and recently reported Pd-based catalysts. The EOR characteristics for Pd2.3Co/C were two times better than those of commercial Pd/C with a good current density, low onset potential, excellent mass activity, and enhanced tolerance to surface poisoning, caused by the synergistic effect from Co incorporation and the electronic structure alteration of Pd in the bimetallic Pd–Co catalysts. Excellent FOR kinetics in alkaline medium for Pd2.3Co/C establishes the nanoparticles as a bifunctional electrocatalyst. This study demonstrates that the addition of an oxophilic metal such as Co to noble metals (e.g., Pd) and the formation of bimetallic nanocatalysts via versatile solid solution reduction leads to the improved electrocatalytic efficiency and durability toward the oxidation of renewable fuel sources, such as ethanol and formate in alkaline media, which is attractive for creating cost-effective nanocatalysts for the future commercialization of DEFC and DFFCs. Acknowledgments The authors acknowledge the financial assistance from the Japan Science and Technology (JST), Core Research for Evolutionary Science and Technology, Japan Science and Technology Agency (JST-CREST, JPMJCR1543). Acknowledgments are due to Mr. Masaaki Ito of the R&D Center, Noritake Company Limited, Japan for TEM and XPS analysis. Supporting Information Supporting information available Additional analysis, further characterization details and electrochemical properties of the asprepared nanocatalysts are provided.


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Cobalt-Modified Palladium Bimetallic Catalyst: A Multifunctional

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