DOI: 10.1002/cctc.201500206
Communications
Determining the Effect of Plasma Pre-Treatment on Antimony Tin Oxide to Support Pt@Pd and the Oxygen Reduction Reaction Activity Xiaoteng Liu,*[a] Kui Zhang,[a] Jianwei Lu,[b] Kun Luo,[c] Jinlong Gong,[b] Vinod K. Puthiyapura,[a] and Keith Scott*[a] This article reveals the effect of plasma pre-treatment on antimony tin oxide (ATO) nanoparticles. The effect is to allow Pt@Pd to be deposited homogeneously on the ATO surface with high dispersion and narrow particle size distribution. The Pt@Pd core–shell catalyst was prepared using the polyol method and shows a dramatic improvement towards ORR activity and durability.
The durability and the high cost of cathode catalysts are key issues which hinder the commercialization of proton exchange membrane fuel cells (PEMFCs). One of the most common types of PEMFC are highly porous carbon black supported nanosized Pt catalysts.[1] Pt sintering and dissolution, and carbon corrosion are the main factors which can affect the fuel cell’s performance.[2] Pt particles intimately attached to carbon can also accelerate the carbon corrosion rate, which leads to a significant loss of the fuel cell performance, especially on the cathode side, as it operates at highest potential.[3] Conductive oxide materials that can tolerate a strong acidic and oxidative environment have been considered as alternative substrate material to carbon. ATO nanopowder is one of these candidates, owing to its excellent electrical conductivity property, and has been used in various applications including conducting coating and as a transparent solar battery electrode.[4] ATO displays high stability in acid and peroxide environments at temperatures up to 200 8C.[5] The commercially available ATO nanoparticles used in our experiments have an average particle size less than 50 nm and a specific surface area of 47 m2 g¢1.[6] [a] Dr. X. Liu, Dr. K. Zhang, Dr. V. K. Puthiyapura, Prof. K. Scott School of Chemical Engineering and Advanced Materials Merz Court, Newcastle University Newcastle upon Tyne, NE1 7RU (U.K.) E-mail:
[email protected] [email protected] [b] J. Lu, Prof. J. Gong Key Laboratory for Green Chemical Technology of Ministry of Education School of Chemical Engineering and Technology, Tianjin University Tianjin 300072 (P.R. China) [c] Prof. K. Luo Key Laboratory of New Processing Technology for Nonferrous Metals & Materials, Ministry of Education College of Materials Science and Engineering Guilin University of Technology 12 Jiangan Road, Guilin 541004 (P.R. China) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cctc.201500206.
ChemCatChem 2015, 7, 1543 – 1546
Dielectric barrier discharges (DBDs) plasma was used for the pre-treatment of ATO. This technique produces non-equilibrium plasma to generate energetic electrons.[7] Through electron-impact ionization, dissociation, and excitation of the source gases, active radicals, and ionic, excited atomic and molecular species are generated, and reacted with ATO. The source gas we used in our experiment is air at room temperature and ambient pressure. The reaction mechanism of plasma effect to the ATO surface is still under investigation, however, the effect of the pre-treatment is clear after analysis of the results. The polyol method has been widely used for catalyst preparation and ethylene glycol (EG) is a popular reducing agent to deposit Pt and/or Pd nanoparticles on suitable supports.[8] Researchers have deposited Pt particles with different shapes and sizes using this method by controlling the preparation conditions, such as pH value and temperature.[8, 9] In this work, the Pt@Pd core shell structure catalyst was prepared by depositing Pd on ATO first, then depositing Pt on Pd particles to form shell. The advantages of core–shell structure have been extensively discussed previously.[10] In our experiments, Pt and Pd cannot be deposited on to the untreated ATO support using the same preparation conditions. When using untreated ATO the ATO particles and the Pd mixed and aggregated. When treated ATO was employed, however, Pd homogenously settled on the ATO with intimate contact during the reduction process, Pt then selectively attached on Pd rather than the substrate when the precursor was introduced. The structure of the core–shell clearly benefited from increased Pt, which was not found deposited on ATO. In this case Pd acted as the seed to ‘grow’ the Pt shell. The activity and durability were measured using various half-cell electrochemical methods. Shown in Figure 1 are the XRD patterns of untreated ATO and plasma-treated ATO. After treatment, increased intensity of all peaks was observed. The mean particle sizes for untreated and plasma-treated ATO are 51.2 nm and 59.8 nm, respectively, were calculated by using the Debye–Scherrer equation. During the treatment, high energy oxygen and nitrogen radicals hit the ATO particles which resulted in an increase in particle size. The nitrogen BET surface area of untreated and plasma-treated ATO are 46.8 m2 g¢1 and 52.3 m2 g¢1, respectively. A dramatic increase of surface area on bigger particles reveals a roughened surface of ATO after treatment. Another advantage of plasma treatment, explained by Savastenko et al.,[11] is that plasma treatment can leave more adhesive surface which can trap introduced mole-
1543
Ó 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Communications
Figure 1. The XRD patterns of untreated ATO and plasma-treated ATO.
cules, in our case this creates a more intimate contact between the depositing Pd with the ATO surface. The surface of the catalyst was analyzed by XPS for any possible nitrogen-containing compounds, however, both XPS spectra showed the same profile and no nitrogen was found. Shown in Figure 2 are the TEM images of a) Pd on untreated ATO, b) Pd on H2SO4 pre-treated ATO, c) Pt@Pd on plasma pretreated ATO and d) particle size distribution of Pt@Pd/ATO catalyst. The H2SO4 pre-treatment was completed by boiling ATO in 1 m H2SO4 for 10 h. The added Pt and Pd particles mixed with the acid treated ATO rather than deposit on it with intimate contact, and there is a broad particle size distribution (Figure 2a). The TEM analysis of the plasma pre-treated ATO sample reveals that the Pt@Pd particles are homogeneously deposited with an average particle size of 4.96 nm and very narrow particle size distribution. Such improvement in control resulted from the increased surface area and better intimate contact between catalyst and the substrate.
The catalyst is proposed to have a Pt¢Pd core–shell structure, which is supported by analysis of the EDX line scan profile shown in Figure 3. The EDX line scan revealed a high concentration of Pd as the core, and that Pt is located at the edge of Pd to form the shell. The EDX mapping profile in Figure 4 clearly displays the core–shell structure. Such structure has been previously demonstrated to have improved durability when used as cathode catalyst for fuel cells.[12] No Pt (red) was found individually deposited on ATO, this indicates that our preparation of forming core–shell structure was successful. Less Pt intensity was observed on the bottom right side of the particle where the boundary between the Pd and ATO is. This is further evidence that Pt only attached on Pd to form a shell. Color overlapping of Pd (green) and Pt (red) causes the yellow dots. Shown in Figure 5 a are the initial and 20 000th cyclic voltammograms (CV) of Pt@Pd/ATO and commercial 20 % Pt/C (HiSPEC4000, mean particle size is 4.5 nm) as comparison. In
Figure 3. TEM Dark image and EDX line scan profile of Pt@Pd/ATO.
Figure 2. TEM image of a) Pd on untreated ATO, b) Pd on H2SO4 pre-treated ATO, c) Pt@Pd on plasma pre-treated ATO and d) particle size distribution of Pt@Pd/ATO catalyst.
ChemCatChem 2015, 7, 1543 – 1546
www.chemcatchem.org
Figure 4. TEM Dark image and EDX mapping scan profile of Pt@Pd/ATO.
1544
Ó 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Communications lecular oxygen. In addition, the calculated n value of Pt@Pd/ATO is 3.92, which is indicative of a four-electron reduction of O2. These kinetic factors provide strong evidence that Pt@Pd/ATO is a suitable catalyst for ORR in terms of the catalytic activity. In comparison with Pt/C, in Figure 5 d, Pt@Pd/ATO has 0.06 V higher ORR onset potential than Pt/C. The increased activity of Pt@Pd/ATO is attributed to the homogeneous distribution of the catalysts and the core–shell structure, which can greatly increase fuel cell performance when the catalyst is used at the cathode. The catalysts were cycled between 0.4 to 1.1 V vs. RHE 1000 times under the same conditions with a constant O2 Figure 5. The electrochemical evaluation of Pt@Pd/ATO, a) CV, b) ESA, c) RDE analysis, d) ORR durability result. flow for the durability evaluation. The initial scan and the terms of hydrogen adsorption/desorption peaks in H underpo1000th are shown as solid and dash curves respectively. Both tential deposition region (UPD), the Pt@Pd/ATO showed typical catalysts have a decrease on current between 0.4–0.9 V vs. Pt@Pd alloy profiles,[13] which are similar in profile to Pt/C cataRHE, the decrease of Pt/C is more intense than Pt@Pd/ATO. The normalized activity calculated using the current at 0.7 V vs. lyst on the initial scan. The PtPd-oxide started to form at RHE are shown in the inserted diagram. It is clear that Pt@Pd/ 0.72 V vs. RHE on the forward sweep for Pt/C, and 0.80 V vs. ATO is more stable as the activity decreased 10.8 % which is RHE for the Pt@Pd/ATO catalyst. much less than 26.9 % of Pt/C. Again, the improved durability That the oxide reduction also occurred with the backward of Pt@Pd/ATO is attributed to the core–shell structure, the sweep indicated that the Pt@Pd/ATO has a higher onset potenhigh dispersion, and the ATO substrate, which is very stable in tial than Pt/C. These results indicate that our catalyst has comthis oxidative environment. parable activity to the commercial Pt/C catalyst and that the In conclusion, the effect of plasma treatment on ATO has Pt@Pd core–shell nanoparticles also have intimate contact with been shown to be positive in terms of supported core–shell the plasma-treated ATO support. The electrochemical surface preparation and the resulting ORR activity. The plasma treatarea (ESA) is obtained by analysis of the hydrogen adsorption ment likely creates a roughened surface, as high energy and desorption peak between 0.05 V and 0.4 V vs. RHE on the oxygen and nitrogen radical impact the surface of ATO. Our forward scans. The trend of ESA with increasing number of analysis techniques (XRD, XPS, and TEM), however, could not scans is shown in Figure 5 b. The decease of ESA is caused by give any direct evidence for this roughening. The PdPt core– sintering of the metal nanoparticles and the degradation of shell structure has been successfully prepared and the catalyst the substrate material. The superior stability of Pt@Pd/ATO ORR activity demonstrated. The greatly improved surface area over the Pt/C, was attributed to the plasma treatment and the of Pt and the increased number of active sites for ORR resulted consequent core–shell structure. Core–shell structures are in a great improvement of ORR activity. The durability of this known to increase the durability and in this case the Pt rich catalyst greatly benefited from the core–shell structure and esouter shell protects the catalyst from degradation during elecpecially the stable oxidative resistant ATO substrate. Overall, trochemical tests. The stability of the Pt outer shell stops the Pt@Pd/ATO is a promising ORR catalyst with excellent activity dissolution of the core material, because it is strongly bound and durability. to the core atoms. Shown in Figure 5 c is rotating disk electrode (RDE) linear scan response of Pt@Pd/ATO at 400, 900, 1600 and 2500 rpm rotating rate recorded at a scan rate of 5 mV s¢1 in O2 saturated Experimental Section 0.5 m H2SO4 electrolyte, along with the appropriate Koutecky– Levich (K–L) plot. The limiting current is gradually increased Plasma Pre-treatment for ATO: The plasma treatment for ATO is with increasing rotation speed with a parallel profile, K–L plots briefly described as follows. ATO powder was loaded between two at different potentials showed a linear dependence at all poparallel steel sheets inserted into a homemade acrylic device for tentials, which indicates first-order kinetics with respect to moplasma pre-treatment. 20 kHz, 3000 V current was applied onto the ChemCatChem 2015, 7, 1543 – 1546
www.chemcatchem.org
1545
Ó 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Communications steel sheets to generate active radicals under air at room temperature. Catalyst Preparation: ATO powder was pre-treated under such condition for 10 min. The treated ATO was dispersed in ethylene glycol (EG) in a three neck round bottom flask with stirring. Ammonium tetrachloropalladate(II) [(NH4)2PdCl4] was dissolved in 15 mL de-ionized water and added drop-wise to the reactor. After 1 hour mixing, the pH of this mixture was adjusted to 11 by carefully adding of 1 m NaOH solution. The reactor was slowly heated to 75 8C to reflux for 6 h under nitrogen atmosphere to allow the deposition of Pd to be completed. Then, the reactor was cooled to room temperature, further EG was added in the reactor and mixed for 30 min. Chloroplatinic acid hexahydrate (H2PtCl6·6 H2O) was dissolved in de-ionized water, and titrated into the above solution. The pH value was again adjusted to 11 using 1 molar NaOH, and the content was heated to 75 8C for 6 h reflux under nitrogen atmosphere. Then the solution was cooled to room temperature and followed by stirring 12 h. A mass ratio of Pt/Pd = 1:2 supported on ATO sample with 20 % total metal loading can be obtained using above calculation. Characterization and Evaluation: Physical examination was performed using nitrogen Brunauer–Emmett–Teller (BET), X-ray diffraction (XRD), and XPS. Micrographs were taken by using a FEI Tecnai TF20 FEGTEM Field emission gun, transmission electron microscopy (TEM) fitted with HAADF detector, and Oxford Instruments INCA 350 energy-dispersive X-ray (EDX) analysis system/80 mm X-Max SDD detector and Gatan Orius SC600A CCD camera. Half cell electrochemical evaluation experiments were performed using an AutoLab PGSTAT30 potentiostat/galvanostat with GPES software. Cyclic voltammetry, oxygen reduction linear voltammetry were performed to study the catalytic activity. Half cell electrochemical evaluations were conducted using a BASi RRDE-3A package which consists of a rotating disk electrode (RRDE) apparatus, a glass cell vial (100 mL), an Ag/AgCl reference electrode, a 7.5 cm long Pt wire with 0.5 mm diameter and glassy carbon RDE tip with a surface area of 0.1256 cm2. The potential reported in this study are referred to the reference hydrogen electrode (RHE). The loading of catalyst deposited on the GC disk was 0.5 mg cm¢2. The ink was prepared by ultrasonically mixing the catalysts and 5.0 wt % Nafion ionomer in ethanol. A required amount of the catalyst slurry was carefully dropped on glassy carbon surface and allowed to dry at room temperature for 15 min to obtain a uniform catalyst film. The electrolytes were 0.5 m H2SO4 solution saturated with N2 for the CV and potential cycling ADT test. ORR profile was analyzed using the
ChemCatChem 2015, 7, 1543 – 1546
www.chemcatchem.org
same electrode in O2 saturated 0.5 m H2SO4, linear scan voltammetry (LSV) was recorded from 1.1 to 0.4 V vs. RHE against the increasing rotation speeds-400, 900, 1600 and 2500 rpm. All half cell experiment was performed at room temperature.
Acknowledgements We would like to thank EPSRC (ref number EP/I037024/1) for the financial support. Keywords: antimony tin oxide · durability · fuel cells · oxygen reduction reaction · plasma [1] M. S. Wilson, F. H. Garzon, K. E. Sickafus, S. Gottesfeld, J. Electrochem. Soc. 1993, 140, 2872. [2] E. Antolini, E. R. Gonzalez, Solid State Ionics 2009, 180, 746. [3] L. M. Roen, C. H. Paik, T. D. Jarvi, Electrochem. Solid-State Lett. 2004, 7, A19; D. A. Stevens, M. T. Hicks, G. M. Haugen, J. R. Dahn, J. Electrochem. Soc. 2005, 152, A2309; P. J. Ferreira, G. J. la O, Y. Shao-Horn, D. Morgan, R. Makharia, S. Kocha, H. A. Gasteiger, J. Electrochem. Soc. 2005, 152, A2256; Y. Shao, G. Yin, Y. Gao, J. Power Sources 2007, 171, 558. [4] E. Shanthi, V. Dutta, A. Banerjee, K. L. Chopra, J. Appl. Phys. 1980, 51, 6243; V. K. Puthiyapura, M. Mamlouk, S. Pasupathi, B. G. Pollet, K. Scott, J. Power Sources 2014, 269, 451. [5] F. Vicent, E. Morallûn, C. Quijada, J. L. Vzquez, A. Aldaz, F. Cases, J. Appl. Electrochem. 1998, 28, 607. [6] Commercially available from Sigma–Aldrich. [7] K. Zhang, B. Eliasson, U. Kogelschatz, Ind. Eng. Chem. Res. 2002, 41, 1462; K. Zhang, U. Kogelschatz, B. Eliasson, Energy Fuels 2001, 15, 395. [8] C.-C. Chou, C.-H. Liu, B.-H. Chen, Energy 2014, 70, 231. [9] S. Harish, S. Baranton, C. Coutanceau, J. Joseph, J. Power Sources 2012, 214, 33; N. V. Long, T. Asaka, T. Matsubara, M. Nogami, Acta Mater. 2011, 59, 2901. [10] E. H. Yu, X. Wang, X. T. Liu, L. Li in Catalysts for Alcohol-Fuelled Direct Oxidation Fuel Cells, The Royal Society of Chemistry, 2012, 227; X. Liu, E. H. Yu, K. Scott, Appl. Catal. B 2015, 162, 593. [11] N. A. Savastenko, V. Brìser, M. Brìser, K. Anklam, S. Kutschera, H. Steffen, A. Schmuhl, J. Power Sources 2007, 165, 24. [12] G. Zhang, Z.-G. Shao, W. Lu, F. Xie, H. Xiao, X. Qin, B. Yi, Appl. Catal. B 2013, 132 – 133, 183. [13] P. Hong, F. Luo, S. Liao, J. Zeng, Int. J. Hydrogen Energy 2011, 36, 8518; P. P. Lopes, E. A. Ticianelli, H. Varela, J. Power Sources 2011, 196, 84; M. Łukaszewski, T. Ke˛dra, A. Czerwin´ski, Electrochem. Commun. 2009, 11, 978. Received: March 1, 2015 Published online on April 21, 2015
1546
Ó 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
