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Thermal Treatment of PtNiCo Electrocatalysts: Effects of Nanoscale Strain and Structure on the Activity and Stability for the Oxygen Reduction Reaction Bridgid N. Wanjala, Rameshwori Loukrakpam, Jin Luo, Peter N. Njoki, Derrick Mott, and Chuan-Jian Zhong* Department of Chemistry, State UniVersity of New York at Binghamton, Binghamton, New York 13902
Minhua Shao* and Lesia Protsailo UTC Power, 195 GoVernor’s Highway, South Windsor, Connecticut 06074
Tetsuo Kawamura Toyota Motor Engineering & Manufacturing North America, Incorporated, 1588 Woodridge AVenue, Ann Arbor, Michigan 48105 ReceiVed: July 22, 2010; ReVised Manuscript ReceiVed: August 30, 2010
The ability to control the nanoscale size, composition, phase, and facet of multimetallic catalysts is important for advancing the design and preparation of advanced catalysts. This report describes the results of an investigation of the thermal treatment temperature on nanoengineered platinum-nickel-cobalt catalysts for oxygen reduction reaction, focusing on understanding the effects of lattice strain and surface properties on activity and stability. The thermal treatment temperatures ranged from 400 to 926 °C. The catalysts were characterized by microscopic, spectroscopic, and electrochemical techniques for establishing the correlation between the electrocatalytic properties and the catalyst structures. The composition, size, and phase properties of the trimetallic nanoparticles were controllable by our synthesis and processing approach. The increase in the thermal treatment temperature of the carbon-supported catalysts was shown to lead to a gradual shrinkage of the lattice constants of the alloys and an enhanced population of facets on the nanoparticle catalysts. A combination of the lattice shrinkage and the surface enrichment of nanocrystal facets on the nanoparticle catalysts as a result of the increased temperature was shown to play a major role in enhancing the electrocatalytic activity for catalysts. Detailed analyses of the oxidation states, atomic distributions, and interatomic distances revealed a certain degree of changes in Co enrichment and surface Co oxides as a function of the thermal treatment temperature. These findings provided important insights into the correlation between the electrocatalytic activity/stability and the nanostructural parameters (lattice strain, surface oxidation state, and distribution) of the nanoengineered trimetallic catalysts. Introduction One of the major challenges for the commercialization of proton exchange membrane fuel cells (PEMFCs) is the development of a catalyst system that is robust, active, and cost effective. Alloying Pt with other transition metals is one approach to address this challenge.1-14 Studies have shown that the overpotential for oxygen reduction reaction (ORR) at the cathode currently accounts for about 70% of the energy losses.15 Different supported and unsupported Pt-group electrocatalysts with binary PtM composition where M ) Sc, Y, Au, Co, Ni, V, Fe, Cr, Pd, W, Ag, Ti, and Mn or ternary composition PtM1M2 where M1M2 ) V, Fe, Ni, Cr, Co, and Cu1-14,16-33 have been prepared using different techniques. Examples include nanoengineered synthesis approaches1-4,6-8,18,19 and related techniques,20 coprecipitation,22,23,27,34 incipient wetness method,21 electrochemical synthesis,33 sputtering,35-37 codeposition,38,39 electrodeposition,14 etc. In the studies of Pt-based catalysts with bimetallic and trimetallic compositions, the enhanced catalytic activity in these systems has been attributed to numerous factors including the bifunctional nature of the catalyst surface,3,8,39 * To whom correspondence should be addressed. E-mail: cjzhong@ binghamton.edu;
[email protected].
lattice shrinking where there is a change in the Pt-Pt interatomic distance,8,21,41 number of Pt nearest neighbors, d-band center shift,42 Pt metal content on the particle surface,41 and Pt skin effect/segregation.9-11,25,37,43-46 For example, annealed polycrystalline Pt3Co nanocrystals with a Pt skin have been found to exhibit higher catalytic activity than the bimetallic Pt3M surfaces where M ) Fe, Ni, Co, and Ti in a study conducted by Stamenkovic et al.9,47 High-temperature annealing is also reported to enrich Pt on extended Pt alloy systems by promoting surface segregation of Pt.48,49 Size39,50-52 and shape/morphology effects,26,53 catalyst composition and particularly surface composition, oxidation state of Pt, and the second metal have been reported to have some effects on the activity of electrocatalysts.27 The effect of cobalt dissolution in the PtCo/C system was shown to neither detrimentally reduce the cell voltage nor dramatically affect the membrane conductance.31 The study on PtNiCo/C electrocatalyst systems was reported for methanol oxidation reaction (MOR), whereby it was prepared by coprecipitation method.23 The catalyst has also been studied for ORR in a phosphoric acid fuel cell (PAFC).54 It has also been reported as a catalyst for PEMFCs whereby it was prepared by the incipient wetness technique in a study conducted by Seo et al.21
10.1021/jp106843k 2010 American Chemical Society Published on Web 09/23/2010
Thermal Treatment of PtNiCo Electrocatalysts Ternary catalysts,55 e.g., trimetallic nanoparticle catalysts PtVFe and PtNiFe, have been found to exhibit catalytic activities in fuel cell reactions, which are 4-5 times higher than that of pure Pt catalysts.1,6 However, an overview of the literature indicates that little is available for understanding how the detailed nanoscale phase and strain properties of the catalysts correlate with the electrocatalytic properties in terms of thermal treatment temperatures of the nanoengineered catalysts. The strain effect of the interatomic distances on the catalytic activity is considered to be an important aspect for the mechanistic understanding of the ORR electrocatalysis of Ptbased alloy nanoparticles.56 Alloying Pt with elements of smaller atomic radius such as Co and Ni, as described in this work, should reduce the interatomic distances near the Pt sites and thus enhance the electrocatalytic activity. Recent computational studies have also demonstrated the lowering of the d-band center of Pt through compressive strain and electronic effect induced by the transition metals. While the effects of treatment temperature on the electrocatalytic activity and stability have been reported previously,57 the understanding of how lattice shrinking and nanocrystal structures can be manipulated by thermal temperature and how they affect the electrocatalytic activity and stability has received little attention. In general, heat treatment is considered to induce changes in particle size, morphology, dispersion of the metal on the support, alloying degree, active surface site formation, catalytic stability, and surface electronic properties, with the optimum treatment temperature being strongly dependent on the individual catalyst.57 There is an increasing need for an in-depth understanding of the correlation between the nanoscale phase and faceting structures and the electrocatalytic properties. In this report, we describe the results of an investigation of the thermal treatment on lattice shrinking and nanocrystal faceting properties for PtNiCo/C catalysts, which are nanoengineered in terms of size and composition. To the best of our knowledge, there is no prior report of such a detailed study of the PtNiCo nanocatalysts in terms of the lattice parameter, nanocrystal faceting, surface oxidation states, and atomic distributions as a function of the thermal treatment temperature in enhancing the electrocatalytic activity for ORR, which is the focus of the present work. Experimental Section Chemicals. Platinum(II) acetylacetonate (Pt(acac)2, 97%) and nickel(II) acetylacetonate (Ni(acac)2, anhydrous, >95%) were purchased from Alfar Aesar, Cobalt(III) acetylacetonate (Co(acac)3, 99.95%) was purchased from Strem Chemicals. 1,2Hexadecanediol (CH3-(CH2)13-CH(-OH)-CH2-OH, 90%), octyl ether ([CH3(CH2)7]2O, 99%), oleylamine (CH3(CH2)7CHd CH(CH2)8NH2, 70%), oleic acid (CH3(CH2)7CHdCH(CH2)7COOH, 99+%), and Nafion solution (5 wt %) were purchased from Aldrich. Optima-grade perchloric acid was purchased from Fisher Scientific. Other solvents such as ethanol and hexane were purchased from Fisher. All chemicals were used as received. Synthesis. The general synthesis of PtNiCo nanoparticles involved the use of three metal precursors, PtII(acac)2, NiII(acac)2, and CoIII(acac)3, in controlled molar ratios. These metal precursors were dissolved in an octyl ether solvent. A mixture of oleylamine and oleic acid was also dissolved in solution and used as capping agents. 1,2-Hexadecanediol was used as a reducing agent for reduction of the Pt, Ni, and Co precursors. The general reaction for the synthesis of the (oleylamine/oleic acid)-capped PtNiCo nanoparticles involves a combination of thermal decomposition and reduction reactions. The composition
J. Phys. Chem. C, Vol. 114, No. 41, 2010 17581 of the Pt0n1Ni0n2Co0n3 nanoparticles, where n1, n2, and n3 represent the atomic percentages of each metal, is controlled by the feeding ratio of the metal precursors. The nanoparticle product can be collected by precipitation method. Possible byproducts include CH2-(CH2)13-CH(-OH)-CH(dO) (aldehyde), CH2-(CH2)13-CH(-OH)-COOH (carboxylic acid), acac, and CO, which were soluble in the solvent and discarded after precipitating out the nanoparticles. PtNiCo nanoparticles of different compositions were synthesized by manipulating the relative concentrations of metal precursors such as platinum(II) acetylacetonate, nickel(II) acetylacetonate, and cobalt(III) acetylacetonate and capping agents such as oleylamine and oleic acid. In a typical procedure for the synthesis of Pt36Ni14Co49, for example, 4.2 g of 1,2hexadecanediol (4.88 mmol), 1.5735 g of cobalt acetylacetonate (Co(acac)3, 1.44 mmol), 0.5919 g of nickel acetylacetonate (Ni(acac)2, 0.73 mmol), 1.5735 g of platinum acetylacetonate (Pt(acac)2, 0.95 mmol), 3 mL of oleylamine (6.4 mmol), 3 mL of oleic acid (9.4 mmol), and 450 mL of octyl ether were added to a 3-neck 1 L flask under stirring. The solution was purged with N2 and heated to 105 °C. The solution appeared to have dark green color. At this temperature, N2 purging was stopped and the solution was kept under N2. The mixture was heated to 280 °C and refluxed for 40 min. The solution appeared black in color. After the reaction mixture was allowed to cool to room temperature, the solution was transferred to a large flask under ambient environment. The product was precipitated by adding ethanol (∼1000 mL). The yellow-brown supernatant was discarded. The black precipitate was completely dried under nitrogen and dispersed in a known amount of hexane (∼100 mL). The trimetallic composition was based on the linear correlation between the trimetallic feed ratios and the nanoparticle product composition for a series of trimetallic ratios. The details will be described in another synthesis-focused report. Catalyst Preparation. Catalyst preparation included the assembly of PtNiCo nanoparticles on carbon black and thermal treatment. The assembly was accomplished by a process of loading the nanoparticles onto carbon black materials through interactions between the capping shells and the carbon surface. A typical procedure included the following steps. First, 480 mg of carbon black (Ketjen Black) was suspended in 400 mL of hexane. After sonicating for ∼3 h, ∼320 mg of Pt36Ni15Co49 was added into the suspension. The suspension was sonicated for 5 min, followed by stirring for ∼15 h. The suspension was evaporated slowly for ∼8 h by purging N2 while stirring. The powder was collected and dried under N2. Thermal treatment involved removal of organic shells and annealing of the alloy nanoparticles. All samples were treated in a tube furnace using a quartz tube. The PtNiCo nanoparticles supported on carbon (PtNiCo/C) were first heated at 260 °C in N2 for 60 min for removing the organic shells and then treated at various temperatures in the range between 400 and 926 °C in 15% H2/85% N2 for 120 min during the calcination process. Measurements and Instrumentation. The catalysts were characterized using several techniques. Transmission Electron Microscopy (TEM). TEM was performed on a Hitachi H-7000 electron microscope (100 kV) to obtain the particle size and its distribution. For TEM measurements, nanoparticle samples were diluted in hexane solution and drop cast onto a carbon-coated copper grid followed by solvent evaporation in air at room temperature. Direct Current Plasma-Atomic Emission Spectroscopy (DCP-AES). DCP-AES was used to analyze the composition, which was performed using an ARL Fisons SS-7 Direct Current
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Plasma-Atomic Emission Spectrometer. The nanoparticle samples were dissolved in concentrated aqua regia and then diluted to concentrations in the range of 1-50 ppm for analysis. Calibration curves were made from dissolved standards with concentrations from 0 to 50 ppm in the same acid matrix as the unknowns. Detection limits, based on three standard deviations of the background intensity, are 20, 2, and 5 ppb for Pt, Ni, and Co. Standards and unknowns were analyzed 10 times each for 3 s counts. Instrument reproducibility, for concentrations greater than 100 times the detection limit, results in
