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Pd nanoparticles supported on PDDA-functionalized carbon black with enhanced ORR activity in alkaline medium
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Pd nanoparticles supported on PDDAfunctionalized carbon black with enhanced ORR activity in alkaline medium Yan Hong Xue a,*, Lan Zhang a, Wei Jiang Zhou a, Siew Hwa Chan a,b,** a
Energy Research Institute, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore School of Mechanical and Aerospace Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore
b
article info
abstract
Article history:
Pd/C catalyst with small particle size, high dispersion and high wt.% of metal was in situ
Received 23 October 2013
synthesized by a simple aqueous phase reduction method. Poly(diallyldimethylammonium
Received in revised form
chloride) PDDA-functionalized carbon black was used as a support material for the in situ
20 March 2014
deposition of Pd nanoparticles by means of electrostatic attraction. The catalysts were
Accepted 22 March 2014
characterized by transmission electron microscopy, X-ray diffractometry and X-ray
Available online 21 April 2014
photoelectron spectroscopy, cyclic voltammetry and rotating disc electrode test. The results indicated that Pd nanoparticles with an average size of 2.09 nm were uniformly
Keywords:
dispersed onto the carbon black with a metal weight percentage of w30 wt.%. The prepared
Palladium
Pd/C catalyst has showed remarkably larger electrochemical surface area and higher and
PDDA
more stable ORR activity as compared to commercial Pd/C catalyst and commercial Pt/C
Fuel cell
catalyst in alkaline media, which was believed to be a promising alternative to Pt-based
Oxygen reduction reaction
catalyst used in alkaline fuel cell.
Alkaline
Copyright ª 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Fuel cells have been widely recognized as one of the cleanest and most efficient alternatives to conventional heat engines for generating electricity with near zero pollutant emission, high efficiency and low noise [1e8]. Today, most of the lowtemperature PEM fuel cells use hydrogen exclusively as the fuel, but hydrogen is still expensive at the moment and
difficult to be stored, not to mention transport and distribution issues as compared to the logistic hydrocarbon fuels. Thus, direct methanol fuel cells (DMFCs) have attracted more and more attention, especially for small portable power application, due to the use of liquid methanol with acceptable gravimetric and volumetric energy density [9,10]. However, the full potential of DMFCs has not yet been realized due to the methanol crossover through the polyelectrolyte membrane, sluggish kinetics of oxygen reduction reaction (ORR) and
* Corresponding author. Tel.: þ65 67905591; fax: þ65 67905591. ** Corresponding author. Energy Research Institute, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore. Tel.: þ65 67906957; fax: þ65 67954634. E-mail addresses: [email protected], [email protected] (Y.H. Xue), [email protected] (S.H. Chan). http://dx.doi.org/10.1016/j.ijhydene.2014.03.165 0360-3199/Copyright ª 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
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methanol oxidation reaction (MOR), high cost and particularly sensitive to CO poisoning of platinum (Pt) and Pt alloys catalyst. The slow kinetics of ORR at the cathode results in a large overpotential and low current density. Unlike in acidic media, the kinetics of both methanol oxidation reaction and oxygen reduction reaction in alkaline media is much faster [11,12], and non-precious metal catalyst may be used to reduce the cost of the fuel cell [13,14]. Moreover, fuel crossover is reduced because it is opposite to electro-osmotic drag direction and CO positioning is less an issue in alkaline environment [15]. Palladium (Pd) exhibits roughly half the price of Pt and is 200 times higher in reserve than Pt (0.6 ppb vs. 0.003 ppb) in the earth’s crust, which makes it more attractive for largescale fuel cell applications. Although Pd electrode exhibits no electrocatalytic activity for alcohol oxidation (AO) in acidic media, it has demonstrated competitive alcohol oxidation activity and slightly better ability to break CeC bond of ethanol in alkaline media as compared to traditional Pt-based catalysts [16e21]. Besides, it has recently been demonstrated that the activity of Pd nanoparticles toward ORR surpasses that of Pt nanoparticles in alkaline environment [22e25]. Therefore, the high activity for certain reactions and good resistance to CO poisoning make Pd a good candidate for DMFC application. However, palladium nanoparticles with small size and finely metal dispersion on carbon support are difficult to be achieved, especially when with high metal loading, because metallic particles tend to sinter together due to weak interactions between the metal and the carbon. Mamlouk et al. have prepared a series of carbon supported Pd nanoparticles using different reducing agent, including ethylene glycol, formaldehyde and sodium borohydride. The average sizes of all prepared Pd nanoparticles with different reducing agents are more than 5 nm [26]. Pd nanoparticles with a smaller size and a high metal loading are desirable for good catalytic activity. Hence, it is imperative to develop a convenient and efficient synthesis method to prepare highly dispersed nano Pd catalyst with a high metal weight percentage for “tough” reactions such as ORR and MOR. Self-assembly method is a well-known technique in the preparation of well-ordered nano-structures [27]. Using this method, Jiang et al. have reported the application of polycation, poly(diallyldimethylammonium chloride) PDDA, to stabilize Pt nanoparticles. The PDDA stabilized Pt can not only be self-assembled onto the Nafion electrolyte
membrane, forming an electrochemically active monolayer on the Nafion membrane surface to block the methanol permeability, but also can serve as an excellent homogeneous catalyst, showing interesting catalytic properties [28e30]. In the present study, carbon supported Pd catalyst with a high wt.% and finely dispersed nano size was prepared in the presence of PDDA by the self-assembly method. The whole reduction reaction was carried out in an aqueous solution, which is simple, easy to be scaled up and environmentally friendly. The positive charged groups of PDDA were expected to act as anchor sites and enable the subsequent in situ formation of Pd particles. The prepared catalyst was characterized by XRD, TEM and XPS, and the electrochemical activity was also examined.
Experimental Poly diallylmethylammonium chloride (35 wt.%), palladium (II) chloride (5 wt.% in 10 wt.% HCl solution), ethanol and Nafion solution (5 wt.%) were purchased from Sigma Aldrich. All the chemicals were used as received without any purification. Commercial 20% Pd/C catalyst from Alfa Aesar (denoted as AA Pd/C) and 40% Pt/C from Johnson Matthey (denoted as JM Pt/C) were used as reference.
Synthesis of Pd/C catalyst As shown in Scheme 1, Pd/C catalysts were prepared by reducing the metallic ions with alcohol in the presence of ionic polymers poly(diallylmethylammonium chloride) PDDA. 42.6 mg of carbon black (Vulcan XC-72R from Cabot, refluxed in 2 M HCl and 5 M HNO3 separately before use) was ultrasonically dispersed in 80 ml, 0.002 mol/L of PDDA solution and then transferred into a three-neck flask under intensive stirring. 8 ml, 0.02 mol/L of PdCl2 (10 wt.% HCl) solution was added and vigorously stirred for another 30 min. 60 ml of EtOH was subsequently added into the solution. The pH of the solution was then adjusted to 8.5 by adding 0.5 M of NaOH solution drop by drop. At last, the solution was refluxed at 85 C in a temperature-controlled oil bath for 5 h. After the sample was cooled down to room temperature, the prepared catalyst was filtrated, fully washed with DI-water and then dried at 80 C.
Scheme 1 e The schematic Pd/C preparation.
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Characterization The zeta potentials were measured using a zeta potential analyzer (Malvern, Nano Zetasizer). The as-synthesized catalyst samples were examined by X-ray diffraction (XRD), Bruker GADDS diffractometer using a CuKa source ˚ ) operating at 40 kV and 40 mA. Transmission (l ¼ 1.54056 A electron microscopy (TEM) attached with energy dispersive spectrometry (EDS) was performed on a JEOL JEM 2010 microscope operating at 200 kV accelerating voltage. X-ray photoelectron spectroscopic (XPS) analysis of the samples was carried out on a VG ESCALAB MKII and the narrow scan Pd3d XPS spectra were de-convoluted by XPSPEAK (version 4.1) after correction by adventitious carbon (graphite C1s level at 284.5 eV).
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Results and discussion The zeta potential is the potential difference between the dispersion medium and the stationary layer of fluid attached to the dispersed particle. It is widely used for quantification of the magnitude of the electrical charge at the double layer [31]. Herein we performed the zeta potential measurement to confirm the successful functionalization of carbon black by PDDA. The PDDAeC sample was
Electrochemical measurements All the electrochemical measurements were carried out at room temperature using Solartron electrochemical workstation with a three-electrode system, in which Pt wire and Ag/AgCl, KCl (4 M) served as the counter and reference electrodes, respectively. To prepare the working electrode, 10 mg of Pd/C catalyst was dispersed in a solution containing 10 ml of ethanol and 0.1 ml, 5 wt.% of Nafion solution by fully ultrasonic treatment. The work electrode of commercial AA Pd/C and JM Pt/C was prepared in the same way for comparison. 10 mL of the above catalyst ink was then transferred onto a polished glassy carbon electrode with a micro-syringe. CO stripping voltammograms were measured in 0.1 M HClO4. First, CO was purged into the HClO4 solution for 10 min to allow the complete adsorption of CO onto the surface of the catalyst when the working electrode was kept at 0.1 mV vs. RHE electrode. Then excess CO in the electrolyte was purged out with N2 for 15 min. The working electrode was swept between 0 and 1.4 V in N2-purged, 0.1 M HClO4 mentioned above to get the CO stripping voltammograms or between 0.6 V and 0.8 V in 0.1 M NaOH solution to get the CV curves. The RDE tests were conducted in O2-saturated, 0.1 M NaOH solution with a scan rate of 5 mV s1. All potentials in the study were made reference to the reversible hydrogen electrode (RHE).
Fig. 1 e Zeta potential of acid treated carbon black, PDDAeC, home made Pd/C and commercial AA Pd/C.
Fig. 2 e (a) TEM image of as-prepared Pd/C; (b) Particles distribution got from TEM; (c) EDS spectra recorded from TEM.
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prepared by dispersing the acid pretreated carbon black in PDDA solution, followed by filtered, washed and dried. All the samples were suspended in DI water in very low concentrations and the measurements were conducted immediately after the ultrasonication dispersion. The change of zeta potential can only be ascribed to the surface properties of different samples. From Fig. 1, it can be found that the zeta potential of acid treated carbon black was negative, which was due to the introduction of carboxyl and hydroxyl negative functional groups on the surface of carbon black during the acid pretreatment. However, the zeta potential of carbon black after PDDA modification changed to be positive, which confirmed that the positively charged PDDA molecules were successfully wrapped or adsorbed on the carbon black sphere surface. The cationic charges in the PDDA chains subsequently served as anchor sites to form complexes with PdCl4 2 by electrostatic attraction, which can facilitate a uniform deposition of Pd nanoparticles during in situ reduction. The zeta potential after Pd reduction on the PDDA functionalized carbon black decreased to be a lower positive value, which indicated the Pd particles were negative charged, but the positive charge of PDDA was dominate in the as-prepared Pd/C catalyst. The zeta potential of commercial AA Pd/C was a more negative value
comparing to the carbon black, which further confirmed the Pd particles were negative charged. Fig. 2a shows the TEM image of the as-prepared Pd/C catalyst. One can see that the particles were well dispersed onto carbon black sphere. There was no agglomeration observed from TEM image, indicating that PDDA, as anchor sites, successfully improve the conglomeration of the Pd particles during the subsequent in situ formation of Pd particles. By counting more than 200 particles, the Pd particle size distribution based on the TEM image was shown in Fig. 2b. The results showed that the size distribution was rather narrow. The average particle size was 2.09 nm, which was much smaller than that of commercial AA Pd/C (5 nm and above) and JM Pt/C catalyst (3.0 nm and above). Fig. 2c showed the EDS spectrum recorded from the TEM, revealing that the asgrown Pd particles on carbon black. Further quantitative analysis result (table inset in Fig. 2c) showed that the weight ratio of Pd and C was nearly 3:7 (30 wt.% of Pd loading). The result was consistent with our initial weight ratio of Pd precursor to carbon described in the Experimental section above. More EDS spectra recorded from other areas also exhibited similar results, indicating the homogeneous distribution of Pd on carbon black. The X-ray powder diffraction (XRD) pattern of the Pd/C catalyst is shown in Fig. 3a. The peak indexed to the
Fig. 3 e (a) XRD pattern of as-prepared Pd/C; (b) XPS survey spectrum of as-prepared Pd/C and PDDAeC; (c) N 1s spectrum of as-prepared Pd/C and PDDAeC; (d) Pd 3d spectrum of as-prepared Pd/C.
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(111) of face-centered cubic (fcc) Pd diffractions (JCPDS, Card No. 05-0681) could be found at 2q values of w40 . However, the characteristic (200), (220), (311) peaks at 47 , 68 and 82 were indiscernible, especially the characteristic (200) peak at 47 C, which may be due to the particles was too small, and its high distribution in carbon black. XPS was used to determine the surface composition and the surface oxidation states of the catalytic metals. Fig. 3b and c showed the survey spectrum and the N1s spectra of PDDAeC and as-prepared Pd/C catalyst respectively, the peaks at w402.2 eV for PDDAeC and homemade Pd/C catalyst were attributed to the charged nitrogen(Nþ), which further confirmed the successfully functionization of carbon black by the positive charged PDDA [31]. Fig. 3d showed the characteristic of Pd 3d XPS spectrum. It can be seen that 3d5/2 and 3d3/2 doublet peaks appeared at 341.0 and 336.2 eV. The two pairs of peaks indicated the existence of two different Pd oxidation states on the surface. The relatively lower binding energy set of double peaks (336.2 eV and 340.0 eV) was due to metallic Pd and the other set of double peaks (337.8 eV and 342.6 eV) belonged to the þ2 oxidation state of Pd. These values were in good agreement with the results previously reported [32,33].
COad stripping voltammogram is a powerful technique for estimating the electrochemical surface area (ECSA) of catalyst on a rotating disk electrode. Fig. 4a showed COestripping CV curves of the prepared Pd/C catalyst and JM 40% Pt/C catalyst recorded at room temperature in 0.1 M of HClO4 solutions at a sweep rate of 20 mV s1. The COad stripping voltammograms of home-made Pd/C, AA Pd/C and JM Pt/C displayed that the stripping peaks of COad appeared at 0.89 V, 0.92 V and 0.8 V, which belong to the oxidation of COad on Pd and Pt respectively. There was a 30 mV negative shift in the oxidation peak between home-made Pd/C and commercial AA Pd/C, indicating a better CO oxidation ability of as-prepared Pd/C catalyst. The adsorption area of COad on the CV can be used to estimate the electrochemically active surface area (ECSA) of the electrocatalysts according to the following equation: ECSA ¼
QCO ; 0:42 ½M
(1)
in which [M] represents the metal content (mg), QCO (mC) is the charge related to CO absorption, which was determined by integrating the charge of the electro-oxidation peak of adsorbed COad, and 0.42 (mC cm2) is the charge required to oxidize a monolayer of CO on Pd or Pt. The calculated value of ECSA for Pd/C was 175.0 m2 g1, which was much higher than 74.0 m2/g and 91.1 m2/g obtained from commercial AA’s Pd/C and JM’s Pt/C, respectively. These results revealed that the ECSA of as-prepared Pd/C was more than twice of commercial AA Pd/C and almost twice of commercial Pt/C catalyst. These results coincided with the particles size obtained from the TEM image. Moreover, the electrocatalytic activities of asobtained catalysts were also studied in alkaline medium. As shown in Fig. 4b, CV curves of both as-prepared Pd/C and commercial AA Pd/C showed a flat anodic peak (E z 0.5 V) but a strong cathodic peak (E z 0.2 V) for the formation and reduction of palladium oxide, respectively. Oxidation of the palladium surface started at ca. 0.1 V. The oxidation peak of as-prepared Pd/C was more intense than AA Pd/C, indicating more extensive oxidation, which could be due to more oxidation sites of home-made Pd/C catalyst because of its smaller Pd particles with larger areas available for reaction. Fig. 4b also showed that the palladium oxide reduction peak position has been shifted for the as-prepared Pd/C as compared to commercial Pd/C, which is believed to be due to the size of Pd particles. Catalyst with smaller particles possessed lower reduction potential [26]. Similar voltammograms were reported for Pd supported on carbon microsphere and graphene [33,34]. The cathodic peak of Pd/C appears to be more negative than that of Pt/C. Similar phenomenon was found by Xu et al. [35]. The ECSA can also be determined from the charge of reduction of Pd or Pt oxide according to the following equation [11]: ECSA ¼
Fig. 4 e (a) CO stripping voltammograms of home-made Pd/ C and commercial AA Pd/C and JM Pt/C catalysts in 0.1 M of HClO4 solutions (b) CV curves home-made Pd/C and commercial AA Pd/C and JM Pt/C catalysts of N2-purged, 0.1 M of NaOH.
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QO 0:42 ½M
(2)
in which [M] represents the metal content (mg), QO (mC) is the charge for reduction of Pd or Pt oxide, which is formed due to the adsorption of a monolayer of oxygen, and 0.42 (mC cm2) is the charge required to reduce a monolayer of O2 on Pd or Pt. The calculated ECSAs of as-prepared Pd/C, AA Pd/C and JM Pt/ C were 179.4 m2/g, 79.2 m2/g and 93.0 m2/g, respectively. The
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ECSA results showed similar trend with that of CO absorption but with different values, which may be due to some factors such as dissolution of adsorbed CO into Pd bulk, different adhesive strength of CO and oxygen, etc. Fig. 5 showed the polarization curves of Pd/C and Pt/C catalyst recorded at room temperature in O2-saturated, 0.1 M NaOH solution using a glassy carbon rotating disk electrode at room temperature with a sweep rate of 5 mV s1 and a rotation speed of 1600 rpm. It can be found that the onset potential of the ORR of the home-made Pd/C was nearly 50 mV more positive than that of commercial Pd/C. The half-wave potential of an ORR polarization curve, E1/2, is often used to evaluate the electrocatalytic activity of a catalyst as shown in Fig. 5, the half-wave potential of as-prepared Pd/C showed a marked positive shift as compared to JM Pt/C and AA Pd/C. The more positive onset potential and half-wave potential of asprepared Pd/C indicated a much more favorable catalytic activity toward ORR in alkaline condition. To further investigate the kinetics of ORR of home-made Pd/C during the ORR process, the rotating disk electrode measurement was conducted in an O2-saturated, 0.1 M of NaOH solution at a sweep rate of 5 mV s1 and at different rotating speeds. As can be seen in Fig. 6a, the diffusion current increased with the increase in rotating speed from 100 to 2500 rpm, and was proportional to the square root of the rotating speed. The plot in Fig. 6b is known as KouteckyeLevich plot and is based on the following equations [36]: 1 1 1 1 1 ¼ þ ¼ þ ; I Id IK Bu1=2 IK
(3)
B ¼ 0:62nFCO ðDO Þ2=3 v1=6 ;
(4)
IK ¼ nFkCO ;
(5)
electrolyte, and u is expressed in rotations per minute. The corresponding KouteckyeLevich plots (I1 vs. u1/2) at the potential of 0.4 V and 0.1 V were shown in Fig. 6b. The number of transferred electrons (n) per oxygen molecule during the ORR process can be determined from the KouteckyeLevich equations. The electron transfer numbers were
where I is the experimentally measured current, IK and Id are the mass-transport-free kinetic and diffusion-limiting current, respectively; F is the Faraday constant, Co is the concentration of oxygen dissolved, Do is the diffusion coefficient of oxygen in the solution, v is the kinematic viscosity of the
Fig. 5 e RDE polarization curves of Pd/C and commercial AA Pd/C and JM Pt/C catalysts in O2-saturated, 0.1 M of NaOH solution at a sweep rate of 5 mV sL1.
Fig. 6 e (a) RDE polarization curves of home-made Pd/C recorded in O2-saturated, 0.1 M of NaOH solution at different rotating speeds; (b)KouteckyeLevich plot of IL1 vs. uL1/2 at potentials of L0.4 and L0.1 V; (c) Currentetime (iet) chronoamperometric responses for ORR at homemade Pd/C and commercial JM Pt/C, AA Pd/C electrodes in an O2-saturated 0.1 M NaOH solution at 0.1 V vs. RHE.
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calculated to be 4.00 and 3.75 at the potential of 0.4 and 0.1 V, respectively, indicating that home-made Pd/C was mainly reduced by the four-electron pathway. The similar results were obtained from literature [11,37]. Finally, the durability of as-prepared Pd/C and commercial Pt/C, Pd/C during ORR was evaluated via the chronoamperometric method at 0.10 V vs. RHE in an O2-saturated 0.1 M of NaOH solution. The results were shown in Fig. 6c. Impressively, the current loss at the home-made Pd/C showed a much slower decay than that at the JM Pt/C and AA Pd/C electrode. About 18% loss of the current occurred for the home-made Pd/C catalyst after 2000 s, while the corresponding current loss at the commercial JM Pt/C electrode under the same condition was as high as 68% and 41% at AA Pd/C electrode. This result indicated that the home-made Pd/C catalyst was more stable than the commercial Pt/C and Pd/C electrodes.
Conclusion A convenient, efficient and environmentally friendly synthesis approach was proposed to prepare highly dispersed small nano-size Pd catalyst with a high metal weight percentage. The Pd nanoparticles were in situ synthesized and homogeneously dispersed on carbon black in the presence of PDDA. The home-made Pd/C catalyst presented smaller size, much higher electrochemically surface area, higher activity toward ORR and more stable in alkaline environment as compared to commercial Pd/C catalyst, or even commercial Pt/C catalyst, indicating a great potential to be applied in alkaline fuel cell or many other applications, which involve the oxygen reduction reactions such as metal-air battery, oxygen sensor and electro-synthesis of hydrogen peroxide, etc.
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Pd nanoparticles supported on PDDAfunctionalized carbon black with enhanced ORR activity in alkaline medium Yan Hong Xue a,*, Lan Zhang a, Wei Jiang Zhou a, Siew Hwa Chan a,b,** a
Energy Research Institute, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore School of Mechanical and Aerospace Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore
b
article info
abstract
Article history:
Pd/C catalyst with small particle size, high dispersion and high wt.% of metal was in situ
Received 23 October 2013
synthesized by a simple aqueous phase reduction method. Poly(diallyldimethylammonium
Received in revised form
chloride) PDDA-functionalized carbon black was used as a support material for the in situ
20 March 2014
deposition of Pd nanoparticles by means of electrostatic attraction. The catalysts were
Accepted 22 March 2014
characterized by transmission electron microscopy, X-ray diffractometry and X-ray
Available online 21 April 2014
photoelectron spectroscopy, cyclic voltammetry and rotating disc electrode test. The results indicated that Pd nanoparticles with an average size of 2.09 nm were uniformly
Keywords:
dispersed onto the carbon black with a metal weight percentage of w30 wt.%. The prepared
Palladium
Pd/C catalyst has showed remarkably larger electrochemical surface area and higher and
PDDA
more stable ORR activity as compared to commercial Pd/C catalyst and commercial Pt/C
Fuel cell
catalyst in alkaline media, which was believed to be a promising alternative to Pt-based
Oxygen reduction reaction
catalyst used in alkaline fuel cell.
Alkaline
Copyright ª 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Fuel cells have been widely recognized as one of the cleanest and most efficient alternatives to conventional heat engines for generating electricity with near zero pollutant emission, high efficiency and low noise [1e8]. Today, most of the lowtemperature PEM fuel cells use hydrogen exclusively as the fuel, but hydrogen is still expensive at the moment and
difficult to be stored, not to mention transport and distribution issues as compared to the logistic hydrocarbon fuels. Thus, direct methanol fuel cells (DMFCs) have attracted more and more attention, especially for small portable power application, due to the use of liquid methanol with acceptable gravimetric and volumetric energy density [9,10]. However, the full potential of DMFCs has not yet been realized due to the methanol crossover through the polyelectrolyte membrane, sluggish kinetics of oxygen reduction reaction (ORR) and
* Corresponding author. Tel.: þ65 67905591; fax: þ65 67905591. ** Corresponding author. Energy Research Institute, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore. Tel.: þ65 67906957; fax: þ65 67954634. E-mail addresses: [email protected], [email protected] (Y.H. Xue), [email protected] (S.H. Chan). http://dx.doi.org/10.1016/j.ijhydene.2014.03.165 0360-3199/Copyright ª 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
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methanol oxidation reaction (MOR), high cost and particularly sensitive to CO poisoning of platinum (Pt) and Pt alloys catalyst. The slow kinetics of ORR at the cathode results in a large overpotential and low current density. Unlike in acidic media, the kinetics of both methanol oxidation reaction and oxygen reduction reaction in alkaline media is much faster [11,12], and non-precious metal catalyst may be used to reduce the cost of the fuel cell [13,14]. Moreover, fuel crossover is reduced because it is opposite to electro-osmotic drag direction and CO positioning is less an issue in alkaline environment [15]. Palladium (Pd) exhibits roughly half the price of Pt and is 200 times higher in reserve than Pt (0.6 ppb vs. 0.003 ppb) in the earth’s crust, which makes it more attractive for largescale fuel cell applications. Although Pd electrode exhibits no electrocatalytic activity for alcohol oxidation (AO) in acidic media, it has demonstrated competitive alcohol oxidation activity and slightly better ability to break CeC bond of ethanol in alkaline media as compared to traditional Pt-based catalysts [16e21]. Besides, it has recently been demonstrated that the activity of Pd nanoparticles toward ORR surpasses that of Pt nanoparticles in alkaline environment [22e25]. Therefore, the high activity for certain reactions and good resistance to CO poisoning make Pd a good candidate for DMFC application. However, palladium nanoparticles with small size and finely metal dispersion on carbon support are difficult to be achieved, especially when with high metal loading, because metallic particles tend to sinter together due to weak interactions between the metal and the carbon. Mamlouk et al. have prepared a series of carbon supported Pd nanoparticles using different reducing agent, including ethylene glycol, formaldehyde and sodium borohydride. The average sizes of all prepared Pd nanoparticles with different reducing agents are more than 5 nm [26]. Pd nanoparticles with a smaller size and a high metal loading are desirable for good catalytic activity. Hence, it is imperative to develop a convenient and efficient synthesis method to prepare highly dispersed nano Pd catalyst with a high metal weight percentage for “tough” reactions such as ORR and MOR. Self-assembly method is a well-known technique in the preparation of well-ordered nano-structures [27]. Using this method, Jiang et al. have reported the application of polycation, poly(diallyldimethylammonium chloride) PDDA, to stabilize Pt nanoparticles. The PDDA stabilized Pt can not only be self-assembled onto the Nafion electrolyte
membrane, forming an electrochemically active monolayer on the Nafion membrane surface to block the methanol permeability, but also can serve as an excellent homogeneous catalyst, showing interesting catalytic properties [28e30]. In the present study, carbon supported Pd catalyst with a high wt.% and finely dispersed nano size was prepared in the presence of PDDA by the self-assembly method. The whole reduction reaction was carried out in an aqueous solution, which is simple, easy to be scaled up and environmentally friendly. The positive charged groups of PDDA were expected to act as anchor sites and enable the subsequent in situ formation of Pd particles. The prepared catalyst was characterized by XRD, TEM and XPS, and the electrochemical activity was also examined.
Experimental Poly diallylmethylammonium chloride (35 wt.%), palladium (II) chloride (5 wt.% in 10 wt.% HCl solution), ethanol and Nafion solution (5 wt.%) were purchased from Sigma Aldrich. All the chemicals were used as received without any purification. Commercial 20% Pd/C catalyst from Alfa Aesar (denoted as AA Pd/C) and 40% Pt/C from Johnson Matthey (denoted as JM Pt/C) were used as reference.
Synthesis of Pd/C catalyst As shown in Scheme 1, Pd/C catalysts were prepared by reducing the metallic ions with alcohol in the presence of ionic polymers poly(diallylmethylammonium chloride) PDDA. 42.6 mg of carbon black (Vulcan XC-72R from Cabot, refluxed in 2 M HCl and 5 M HNO3 separately before use) was ultrasonically dispersed in 80 ml, 0.002 mol/L of PDDA solution and then transferred into a three-neck flask under intensive stirring. 8 ml, 0.02 mol/L of PdCl2 (10 wt.% HCl) solution was added and vigorously stirred for another 30 min. 60 ml of EtOH was subsequently added into the solution. The pH of the solution was then adjusted to 8.5 by adding 0.5 M of NaOH solution drop by drop. At last, the solution was refluxed at 85 C in a temperature-controlled oil bath for 5 h. After the sample was cooled down to room temperature, the prepared catalyst was filtrated, fully washed with DI-water and then dried at 80 C.
Scheme 1 e The schematic Pd/C preparation.
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Characterization The zeta potentials were measured using a zeta potential analyzer (Malvern, Nano Zetasizer). The as-synthesized catalyst samples were examined by X-ray diffraction (XRD), Bruker GADDS diffractometer using a CuKa source ˚ ) operating at 40 kV and 40 mA. Transmission (l ¼ 1.54056 A electron microscopy (TEM) attached with energy dispersive spectrometry (EDS) was performed on a JEOL JEM 2010 microscope operating at 200 kV accelerating voltage. X-ray photoelectron spectroscopic (XPS) analysis of the samples was carried out on a VG ESCALAB MKII and the narrow scan Pd3d XPS spectra were de-convoluted by XPSPEAK (version 4.1) after correction by adventitious carbon (graphite C1s level at 284.5 eV).
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Results and discussion The zeta potential is the potential difference between the dispersion medium and the stationary layer of fluid attached to the dispersed particle. It is widely used for quantification of the magnitude of the electrical charge at the double layer [31]. Herein we performed the zeta potential measurement to confirm the successful functionalization of carbon black by PDDA. The PDDAeC sample was
Electrochemical measurements All the electrochemical measurements were carried out at room temperature using Solartron electrochemical workstation with a three-electrode system, in which Pt wire and Ag/AgCl, KCl (4 M) served as the counter and reference electrodes, respectively. To prepare the working electrode, 10 mg of Pd/C catalyst was dispersed in a solution containing 10 ml of ethanol and 0.1 ml, 5 wt.% of Nafion solution by fully ultrasonic treatment. The work electrode of commercial AA Pd/C and JM Pt/C was prepared in the same way for comparison. 10 mL of the above catalyst ink was then transferred onto a polished glassy carbon electrode with a micro-syringe. CO stripping voltammograms were measured in 0.1 M HClO4. First, CO was purged into the HClO4 solution for 10 min to allow the complete adsorption of CO onto the surface of the catalyst when the working electrode was kept at 0.1 mV vs. RHE electrode. Then excess CO in the electrolyte was purged out with N2 for 15 min. The working electrode was swept between 0 and 1.4 V in N2-purged, 0.1 M HClO4 mentioned above to get the CO stripping voltammograms or between 0.6 V and 0.8 V in 0.1 M NaOH solution to get the CV curves. The RDE tests were conducted in O2-saturated, 0.1 M NaOH solution with a scan rate of 5 mV s1. All potentials in the study were made reference to the reversible hydrogen electrode (RHE).
Fig. 1 e Zeta potential of acid treated carbon black, PDDAeC, home made Pd/C and commercial AA Pd/C.
Fig. 2 e (a) TEM image of as-prepared Pd/C; (b) Particles distribution got from TEM; (c) EDS spectra recorded from TEM.
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prepared by dispersing the acid pretreated carbon black in PDDA solution, followed by filtered, washed and dried. All the samples were suspended in DI water in very low concentrations and the measurements were conducted immediately after the ultrasonication dispersion. The change of zeta potential can only be ascribed to the surface properties of different samples. From Fig. 1, it can be found that the zeta potential of acid treated carbon black was negative, which was due to the introduction of carboxyl and hydroxyl negative functional groups on the surface of carbon black during the acid pretreatment. However, the zeta potential of carbon black after PDDA modification changed to be positive, which confirmed that the positively charged PDDA molecules were successfully wrapped or adsorbed on the carbon black sphere surface. The cationic charges in the PDDA chains subsequently served as anchor sites to form complexes with PdCl4 2 by electrostatic attraction, which can facilitate a uniform deposition of Pd nanoparticles during in situ reduction. The zeta potential after Pd reduction on the PDDA functionalized carbon black decreased to be a lower positive value, which indicated the Pd particles were negative charged, but the positive charge of PDDA was dominate in the as-prepared Pd/C catalyst. The zeta potential of commercial AA Pd/C was a more negative value
comparing to the carbon black, which further confirmed the Pd particles were negative charged. Fig. 2a shows the TEM image of the as-prepared Pd/C catalyst. One can see that the particles were well dispersed onto carbon black sphere. There was no agglomeration observed from TEM image, indicating that PDDA, as anchor sites, successfully improve the conglomeration of the Pd particles during the subsequent in situ formation of Pd particles. By counting more than 200 particles, the Pd particle size distribution based on the TEM image was shown in Fig. 2b. The results showed that the size distribution was rather narrow. The average particle size was 2.09 nm, which was much smaller than that of commercial AA Pd/C (5 nm and above) and JM Pt/C catalyst (3.0 nm and above). Fig. 2c showed the EDS spectrum recorded from the TEM, revealing that the asgrown Pd particles on carbon black. Further quantitative analysis result (table inset in Fig. 2c) showed that the weight ratio of Pd and C was nearly 3:7 (30 wt.% of Pd loading). The result was consistent with our initial weight ratio of Pd precursor to carbon described in the Experimental section above. More EDS spectra recorded from other areas also exhibited similar results, indicating the homogeneous distribution of Pd on carbon black. The X-ray powder diffraction (XRD) pattern of the Pd/C catalyst is shown in Fig. 3a. The peak indexed to the
Fig. 3 e (a) XRD pattern of as-prepared Pd/C; (b) XPS survey spectrum of as-prepared Pd/C and PDDAeC; (c) N 1s spectrum of as-prepared Pd/C and PDDAeC; (d) Pd 3d spectrum of as-prepared Pd/C.
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(111) of face-centered cubic (fcc) Pd diffractions (JCPDS, Card No. 05-0681) could be found at 2q values of w40 . However, the characteristic (200), (220), (311) peaks at 47 , 68 and 82 were indiscernible, especially the characteristic (200) peak at 47 C, which may be due to the particles was too small, and its high distribution in carbon black. XPS was used to determine the surface composition and the surface oxidation states of the catalytic metals. Fig. 3b and c showed the survey spectrum and the N1s spectra of PDDAeC and as-prepared Pd/C catalyst respectively, the peaks at w402.2 eV for PDDAeC and homemade Pd/C catalyst were attributed to the charged nitrogen(Nþ), which further confirmed the successfully functionization of carbon black by the positive charged PDDA [31]. Fig. 3d showed the characteristic of Pd 3d XPS spectrum. It can be seen that 3d5/2 and 3d3/2 doublet peaks appeared at 341.0 and 336.2 eV. The two pairs of peaks indicated the existence of two different Pd oxidation states on the surface. The relatively lower binding energy set of double peaks (336.2 eV and 340.0 eV) was due to metallic Pd and the other set of double peaks (337.8 eV and 342.6 eV) belonged to the þ2 oxidation state of Pd. These values were in good agreement with the results previously reported [32,33].
COad stripping voltammogram is a powerful technique for estimating the electrochemical surface area (ECSA) of catalyst on a rotating disk electrode. Fig. 4a showed COestripping CV curves of the prepared Pd/C catalyst and JM 40% Pt/C catalyst recorded at room temperature in 0.1 M of HClO4 solutions at a sweep rate of 20 mV s1. The COad stripping voltammograms of home-made Pd/C, AA Pd/C and JM Pt/C displayed that the stripping peaks of COad appeared at 0.89 V, 0.92 V and 0.8 V, which belong to the oxidation of COad on Pd and Pt respectively. There was a 30 mV negative shift in the oxidation peak between home-made Pd/C and commercial AA Pd/C, indicating a better CO oxidation ability of as-prepared Pd/C catalyst. The adsorption area of COad on the CV can be used to estimate the electrochemically active surface area (ECSA) of the electrocatalysts according to the following equation: ECSA ¼
QCO ; 0:42 ½M
(1)
in which [M] represents the metal content (mg), QCO (mC) is the charge related to CO absorption, which was determined by integrating the charge of the electro-oxidation peak of adsorbed COad, and 0.42 (mC cm2) is the charge required to oxidize a monolayer of CO on Pd or Pt. The calculated value of ECSA for Pd/C was 175.0 m2 g1, which was much higher than 74.0 m2/g and 91.1 m2/g obtained from commercial AA’s Pd/C and JM’s Pt/C, respectively. These results revealed that the ECSA of as-prepared Pd/C was more than twice of commercial AA Pd/C and almost twice of commercial Pt/C catalyst. These results coincided with the particles size obtained from the TEM image. Moreover, the electrocatalytic activities of asobtained catalysts were also studied in alkaline medium. As shown in Fig. 4b, CV curves of both as-prepared Pd/C and commercial AA Pd/C showed a flat anodic peak (E z 0.5 V) but a strong cathodic peak (E z 0.2 V) for the formation and reduction of palladium oxide, respectively. Oxidation of the palladium surface started at ca. 0.1 V. The oxidation peak of as-prepared Pd/C was more intense than AA Pd/C, indicating more extensive oxidation, which could be due to more oxidation sites of home-made Pd/C catalyst because of its smaller Pd particles with larger areas available for reaction. Fig. 4b also showed that the palladium oxide reduction peak position has been shifted for the as-prepared Pd/C as compared to commercial Pd/C, which is believed to be due to the size of Pd particles. Catalyst with smaller particles possessed lower reduction potential [26]. Similar voltammograms were reported for Pd supported on carbon microsphere and graphene [33,34]. The cathodic peak of Pd/C appears to be more negative than that of Pt/C. Similar phenomenon was found by Xu et al. [35]. The ECSA can also be determined from the charge of reduction of Pd or Pt oxide according to the following equation [11]: ECSA ¼
Fig. 4 e (a) CO stripping voltammograms of home-made Pd/ C and commercial AA Pd/C and JM Pt/C catalysts in 0.1 M of HClO4 solutions (b) CV curves home-made Pd/C and commercial AA Pd/C and JM Pt/C catalysts of N2-purged, 0.1 M of NaOH.
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QO 0:42 ½M
(2)
in which [M] represents the metal content (mg), QO (mC) is the charge for reduction of Pd or Pt oxide, which is formed due to the adsorption of a monolayer of oxygen, and 0.42 (mC cm2) is the charge required to reduce a monolayer of O2 on Pd or Pt. The calculated ECSAs of as-prepared Pd/C, AA Pd/C and JM Pt/ C were 179.4 m2/g, 79.2 m2/g and 93.0 m2/g, respectively. The
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ECSA results showed similar trend with that of CO absorption but with different values, which may be due to some factors such as dissolution of adsorbed CO into Pd bulk, different adhesive strength of CO and oxygen, etc. Fig. 5 showed the polarization curves of Pd/C and Pt/C catalyst recorded at room temperature in O2-saturated, 0.1 M NaOH solution using a glassy carbon rotating disk electrode at room temperature with a sweep rate of 5 mV s1 and a rotation speed of 1600 rpm. It can be found that the onset potential of the ORR of the home-made Pd/C was nearly 50 mV more positive than that of commercial Pd/C. The half-wave potential of an ORR polarization curve, E1/2, is often used to evaluate the electrocatalytic activity of a catalyst as shown in Fig. 5, the half-wave potential of as-prepared Pd/C showed a marked positive shift as compared to JM Pt/C and AA Pd/C. The more positive onset potential and half-wave potential of asprepared Pd/C indicated a much more favorable catalytic activity toward ORR in alkaline condition. To further investigate the kinetics of ORR of home-made Pd/C during the ORR process, the rotating disk electrode measurement was conducted in an O2-saturated, 0.1 M of NaOH solution at a sweep rate of 5 mV s1 and at different rotating speeds. As can be seen in Fig. 6a, the diffusion current increased with the increase in rotating speed from 100 to 2500 rpm, and was proportional to the square root of the rotating speed. The plot in Fig. 6b is known as KouteckyeLevich plot and is based on the following equations [36]: 1 1 1 1 1 ¼ þ ¼ þ ; I Id IK Bu1=2 IK
(3)
B ¼ 0:62nFCO ðDO Þ2=3 v1=6 ;
(4)
IK ¼ nFkCO ;
(5)
electrolyte, and u is expressed in rotations per minute. The corresponding KouteckyeLevich plots (I1 vs. u1/2) at the potential of 0.4 V and 0.1 V were shown in Fig. 6b. The number of transferred electrons (n) per oxygen molecule during the ORR process can be determined from the KouteckyeLevich equations. The electron transfer numbers were
where I is the experimentally measured current, IK and Id are the mass-transport-free kinetic and diffusion-limiting current, respectively; F is the Faraday constant, Co is the concentration of oxygen dissolved, Do is the diffusion coefficient of oxygen in the solution, v is the kinematic viscosity of the
Fig. 5 e RDE polarization curves of Pd/C and commercial AA Pd/C and JM Pt/C catalysts in O2-saturated, 0.1 M of NaOH solution at a sweep rate of 5 mV sL1.
Fig. 6 e (a) RDE polarization curves of home-made Pd/C recorded in O2-saturated, 0.1 M of NaOH solution at different rotating speeds; (b)KouteckyeLevich plot of IL1 vs. uL1/2 at potentials of L0.4 and L0.1 V; (c) Currentetime (iet) chronoamperometric responses for ORR at homemade Pd/C and commercial JM Pt/C, AA Pd/C electrodes in an O2-saturated 0.1 M NaOH solution at 0.1 V vs. RHE.
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calculated to be 4.00 and 3.75 at the potential of 0.4 and 0.1 V, respectively, indicating that home-made Pd/C was mainly reduced by the four-electron pathway. The similar results were obtained from literature [11,37]. Finally, the durability of as-prepared Pd/C and commercial Pt/C, Pd/C during ORR was evaluated via the chronoamperometric method at 0.10 V vs. RHE in an O2-saturated 0.1 M of NaOH solution. The results were shown in Fig. 6c. Impressively, the current loss at the home-made Pd/C showed a much slower decay than that at the JM Pt/C and AA Pd/C electrode. About 18% loss of the current occurred for the home-made Pd/C catalyst after 2000 s, while the corresponding current loss at the commercial JM Pt/C electrode under the same condition was as high as 68% and 41% at AA Pd/C electrode. This result indicated that the home-made Pd/C catalyst was more stable than the commercial Pt/C and Pd/C electrodes.
Conclusion A convenient, efficient and environmentally friendly synthesis approach was proposed to prepare highly dispersed small nano-size Pd catalyst with a high metal weight percentage. The Pd nanoparticles were in situ synthesized and homogeneously dispersed on carbon black in the presence of PDDA. The home-made Pd/C catalyst presented smaller size, much higher electrochemically surface area, higher activity toward ORR and more stable in alkaline environment as compared to commercial Pd/C catalyst, or even commercial Pt/C catalyst, indicating a great potential to be applied in alkaline fuel cell or many other applications, which involve the oxygen reduction reactions such as metal-air battery, oxygen sensor and electro-synthesis of hydrogen peroxide, etc.
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