Article Cite This: ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX
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Palladium Deposition on Nickel Microparticles by a Galvanic Replacement Reaction for Electrocatalytic Oxidation of Ethanol Yusuke Kobayashi,† Zhiwei Cai,‡ Gang Chang,*,‡ Yunbin He,*,‡ and Munetaka Oyama*,† †
Department of Material Chemistry, Graduate School of Engineering, Kyoto University, Nishikyo-ku, Kyoto 615-8520, Japan Hubei Collaborative Innovation Center for Advanced Organic Chemical Materials, Ministry-of-Education Key Laboratory for the Green Preparation and Application of Functional Materials, Hubei Key Laboratory of Polymer Materials, School of Materials Science and Engineering, Hubei University, No. 368 Youyi Avenue, Wuchang, Wuhan 430062, China
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‡
ABSTRACT: Palladium was deposited or modified on nickel microparticles (NiMPs) via a galvanic replacement reaction between tetrachloropalladate (PdCl42−) and NiMPs by simply dispersing NiMPs in an aqueous solution of 1.0 mM PdCl42−. Because Pd modified NiMPs (Pd/NiMPs) reasonably exhibit characteristic electrochemical responses coming from Pd for ethanol oxidation in alkaline media, Pd deposition on NiMPs could be evaluated by measuring cyclic voltammograms of 1.0 M ethanol in 1.0 M NaOH aqueous solution with a Pd/NiMPs modified glassy carbon (GC) electrode. For three kinds of commercially available NiMPs and one kind of Ni nanoparticles (NiNPs), we performed the Pd deposition and observed the differences of the electrocatalytic responses for ethanol oxidation. SEM and EDS analyses were also performed to discuss the relationships between the Pd deposition and electrocatalytic responses. Consequently, it was found that compared with spherical NiMPs, irregular-shaped NiMPs and/or NiMPs having rugged surfaces were suitable to the modification of Pd by capturing PdCl42− in the reaction solution effectively. The maximum current of the ethanol oxidation with Pd/NiMPs per the Pd amount was over 1000 mA mg(Pd)−1, assuming that all PdCl42− were captured by the galvanic replacement reaction. The present results show that NiMPs have substantial potential as supporting materials of electrocatalysts for ethanol oxidation. KEYWORDS: palladium deposition, nickel microparticles, electrocatalysis, ethanol oxidation, nickel-supported electrocatalyst
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transition metal carbides, and transition metal nitrides.5 In the cases of Pt-based electrocatalysts, detailed considerations have been given for the roles of supporting materials,6 while unsupported electrocatalysts have been attracting attention.7 The use of pure metals as supporting materials would be scarce, except the application of Ni foam that has special characteristics as an anode material utilizing its unique structure.2,8 Recently, Niu et al. reported an approach using the spontaneous galvanic replacement reaction of Ni foam with Pd precursors (PdCl42− or PdCl62−) and applied the Pd modified Ni foam to methanol oxidation.9 For this system, the effects of Ag incorporation on the formation of Pd deposits on
INTRODUCTION Direct alcohol fuel cells have been attracting considerable interest in recent years as summarized in several reviews.1−5 In particular, palladium-based3,4 or platinum-free5 electrocatalysts in alkaline solutions, mainly for ethanol oxidation, would be one of the active topics. In the review articles, electrocatalysts have been naturally focused on and described. However, considering the performances of the electrocatalysts, supporting materials would be a critical factor to construct the anode and cathode materials, and therefore the choice or the effects of the supporting materials have been noticed and described in some reviews.3−5 The majority of the supporting materials would be carbon-based supports including carbon blacks, carbon nanotubes, carbon microspheres, carbon nanofibers, and graphenes, as summarized in ref 5 by Ozoemena. In his review, the other category is non-carbon supports, which include conducting polymers, metal oxides and hydroxides, © XXXX American Chemical Society
Received: June 7, 2019 Accepted: July 15, 2019
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DOI: 10.1021/acsaem.9b01132 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX
Article
ACS Applied Energy Materials
We purchased three kinds of NiMPs which have been sold as Ni powder from Nilaco Co. (Japan). As descriptions of Ni powder products of Nilaco Co., the three NiMPs are as follows: (1) 18 MW cm. Preparation of Pd Modified Ni Microparticles (Pd/NiMPs). On the basis of our previous modification of Pd on Ni wire,13 we performed and refined the preparation of Pd/NiMPs by dispersing NiMPs in an aqueous solution of 1.0 mM K2PdCl4. While some points of the preparation are described later, in principle, 50 mL of 1.0 mM K2PdCl4 aqueous solution in a 200 mL conical flask was heated at 60 °C in a thermostat bath having a shaking function, and then, 100 mg of NiMPs was dispersed in the solution. After shaking for 1 h at 60 °C, the temperature was lowered by adding ca. 100 mL of pure water, and after leaving over 20 min, the supernatant solution was removed by decantation. The prepared Pd/NiMPs were washed with pure water three times by repeating the leaving and decantation process. Finally, Pd/NiMPs were washed with acetone; the supernatant acetone was removed by a pipet and dried at 50 °C overnight. Thusprepared dried Pd/NiMPs were used for modifying a GC disk electrode. Preparation of a Pd/NiMPs Modified GC Electrode. To prepare the modifier solution, 10 mg of dried Pd/NiMPs was added into a mixed solution of 100 μL of 5% Nafion solution and 900 μL of ethanol and sonicated for 10 min with hand-shaking to disperse Pd/ NiMPs well. Then, 5.0 μL of the modifier solution containing Pd/ NiMPs was dropped on the GC disk surface (diameter 3.0 mm) by using a microsyringe. After being air-dried for 1 h, the thus-fabricated Pd/NiMPs modified GC (Pd/NiMPs/GC) electrode was used as a working electrode for recording CVs of 1.0 M ethanol in 1.0 M NaOH aqueous solutions. We tried different amounts of Pd/NiMPs for dispersion, but the present concentration [(10 mg of Pd/NiMPs)/1.0 mL of solution] was good to obtain relatively stable electrochemical responses. Also, we tried to drop 10 μL of the dispersed solution, but the modified Pd/ NiMPs could not be accumulated in the GC surface area. Therefore, we fixed the dropped amount as 5.0 μL. To evaluate the current magnitude, the current value of CVs was represented after dividing by a geometrical surface area of the GC disk electrode (diameter 3.0 mm), though the GC surface was covered in part with Pd/NiMPs (the covered area was roughly 30−40% of the GC surface). In the above procedures, however, the amount of Pd/ NiMPs loaded on the GC surface can be estimated from the dropped volume as 0.05 mg [(10 mg/1000 μL) × 5.0 μL] regardless of the partial loading of Pd/NiMPs. From this value, the maximum amount of Pd can be also estimated with assuming that all Pd in the initial reaction solution was captured. That is, the ratio was 50 mL of 1.0 mM K2PdCl4:100 mg of NiMPs, which equals 5.0 mg of Pd:100 mg of NiMPs. Therefore, at most, [5.0/(5.0 + 100)]% of 0.05 mg (i.e., 2.4 μg) would be regarded as the amount of Pd on the GC surface. Using this value, we discuss the current magnitude per Pd to evaluate performances of Pd/NiMPs.
Ni foam were also studied to reveal the promotion of the electrocatalytic oxidation of methanol.10 Although Ni foam would be a unique and interesting material in fuel cell applications, we considered that Ni nanoparticles (NiNPs) or Ni microparticles (NiMPs) should be candidates of the supporting materials for electrocatalysts. It is because some characteristics similar to carbon-based supports are recognized for NiNPs or NiMPs, such as enough electrical conductivity and less expensive cost. Furthermore, since the galvanic replacement reactions are possible between Ni and Pd precursors,9−13 it is expected that the Pd deposition or modification on NiNPs or NiMPs should be possible by simply treating them in an aqueous solution of Pd precursors. In addition, some synergetic effects have been reported for PdNi bimetallic electrocatalysts14−18 or Ni@Pd core−shell nanoparticles;19,20 additional effects may be expected for the combination between Pd and Ni over the combinations with carbon materials. On the other hand, we were interested in nanostructuremaking of noble metals via galvanic replacement reactions.13,21 As the start of our trials, the modification of Au nanostructures on Ni wire via the galvanic replacement reaction between AuCl4− and Ni wire was studied for electroanalytical applications.21 Then, the Pd deposition of Ni wire was explored, assuming the electrocatalytic applications of alcohols in alkaline media for fuel cell applications.13 Although the reproducibility of Pd deposition of Ni wire was not necessarily good, we found that the pretreatment with 1.0 M HCl solution and the increased temperature of 50 °C were effective in improving the Pd modification.13 We used Ni wire because it would be suitable to explore the fundamentals of the galvanic replacement reactions to form Au or Pd nanostructures. However, the same principle should be applicable to any Ni based materials regardless of their shapes and sizes. Thus, in the course of our research progress, we were interested in the modification of Pd on NiNPs or NiMPs, considering their potentials as supporting materials for ethanol oxidation in alkaline media. In the present work, we report the Pd deposition or modification on NiMPs via the galvanic replacement reactions between PdCl42− and NiMPs. After loading Pd modified NiMPs (Pd/NiMPs) on the surface of glassy carbon (GC) electrodes, cyclic voltammograms (CVs) of ethanol oxidation in alkaline aqueous solutions were recorded to evaluate the Pd deposition on NiMPs. In addition, the Pd deposition was evaluated by performing SEM and EDS analyses of Pd/NiMPs. Through the observations, we would like to propose possibilities of NiMPs as supporting materials for the ethanol oxidation by exploring the Pd deposition for three kinds of commercially available NiMPs prepared with the same manner systematically.
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EXPERIMENTAL SECTION
Apparatus and Materials. Scanning electron microscopic (SEM) images were obtained with the Sigma 500 instrument (Carl Zeiss Microscopy). EDS analysis was performed on the XFlash 6130 instrument (Bruker) coupled with FE-SEM. Cyclic voltammograms (CVs) were recorded by using a potentiostat (PGSTAT 128N, Metrohm Autolab). A glassy carbon (GC) disk electrode (3.0 mm diameter, BAS Inc.) was used as a base electrode to load Pd/NiMPs on the GC surface. Platinum wire and an Ag|AgCl (3.0 M NaCl) electrode (BAS Inc.) were employed as the counter and reference electrodes, respectively.
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RESULTS AND DISCUSSION
SEM Images of NiMPs before Treating with PdCl42−. At first, we show typical SEM images of NiMPs and NiNPs examined in the present work. For NiMPs-1, we observed mainly NiMPs whose sizes are in the range of 1−4 μm, and the B
DOI: 10.1021/acsaem.9b01132 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX
Article
ACS Applied Energy Materials
amount of Pd is important to evaluate the performance as electrocatalysts for ethanol oxidation. Therefore, although denser modification of Pd around NiMPs would be possible with the increase in [PdCl42−] in the reaction solution, it would have less meaning for evaluating the potentials as supporting materials for Pd. Therefore, in the present work, we fixed the amount of NiMPs as 100 mg, the concentration of K2PdCl4 as 1.0 mM, and the volume of the solution as 50 mL for all the modifications. As other conditions, to promote the reactions significantly, we performed the Pd deposition treatment at 60 °C with shaking for 1 h using a thermostat bath having a shaking function. The actual procedures to prepare dry Pd/NiMPs including washing were described in the Experimental Section. In our previous work, we pretreated Ni wire with 1.0 M HCl to improve the reproducibility of Pd deposition.13 Thus, at first, we tried to pretreat NiMPs with 1.0 M HCl. However, some dissolution of NiMPs was recognized in 1.0 M HCl, so that eventually we used NiMPs as received in the present work. CVs of Ethanol Oxidation Recorded with Pd/NiMPs/ GC Electrodes. After modifying or loading prepared Pd/ NiMPs on GC disk electrodes, we recorded the CVs of 1.0 M ethanol in 1.0 M NaOH aqueous solutions. Figures 2, 3, and 4
surfaces are uneven or rugged as shown in the right image of Figure 1A. The shapes of NiMPs-2 are more spherical
Figure 1. Typical FE-SEM images of (A) NiMPs-1, (B) NiMPs-2, (C) NiMPs-3, and (D) NiNPs-1: (left) low-magnification images; (right) high-magnification images. For the detailed sources of NiMPs and NiNPs, see the Experimental Section.
compared with those of NiMPs-1 having smoother surfaces as shown in Figure 1B; the size was
