Article pubs.acs.org/JPCC
Palladium Nanoparticles/Defective Graphene Composites as Oxygen Reduction Electrocatalysts: A First-Principles Study Xin Liu,†,‡,* Lin Li,‡ Changgong Meng,‡ and Yu Han†,* †
Advanced Membranes and Porous Materials Center, King Abdullah University of Science and Technology, Thuwal, 23955-6900, Kingdom of Saudi Arabia ‡ School of Chemistry, Dalian University of Technology, Dalian, 116024, P. R. China S Supporting Information *
ABSTRACT: The impact of graphene substrate−Pd nanoparticle interaction on the O, OH, and OOH adsorption that is directly related to the electrocatalytic performance of these composites in oxygen reduction reaction (ORR) has been investigated by first-principles-based calculations. The calculated binding energy of a Pd13 nanoparticle on a single vacancy graphene is as high as −6.10 eV, owing to the hybridization between the dsp states of the Pd particles with the sp2 dangling bonds at the defect sites. The strong interaction results in the averaged d-band center of the deposited Pd nanoparticles shifted away from the Fermi level from −1.02 to −1.45 eV. Doping the single vacancy graphene with B or N will further tune the average d-band center and also the activity of the composite toward O, OH, and OOH adsorption. The adsorption energies of O, OH, and OOH are reduced from −4.78, −4.38, and −1.56 eV on the freestanding Pd13 nanoparticle to −4.57, −2.66, and −1.39 eV on Pd13/single vacancy graphene composites, showing that the defective graphene substrate will not only stabilize the Pd nanoparticles but also reduce the adsorption energies of the O-containing species to the Pd particle, and so as the poisoning of the ORR active sites. (PEMFCs).17,18 The superior performance of Pd nanoparticle/ graphene composites can be understood at least partially in terms of the well-known substrate−metal interaction, which would not only stabilize the catalyst but also change the catalytic activity. Careful selection of the substrate allows the electronic states in the TM particle to be finely tuned, making the catalyst highly reactive or completely inert.19−24 The durability of the electrode catalyst is another factor that determines the energy conversion performance of the fuel cell.25 The durability of the deposited Pd nanoparticles is controlled by the stability of the particles against sintering. Jin et al. directly observed the migration and aggregation of Pd nanoparticles on the graphene sheets by STEM, and they found that above 800 °C the Pd nanoparticles began to migrate, coalesce, and aggregate to form larger particles.26 This particle growth process can also be accelerated by the working environment, such as pH value of the solution, and the working temperature. Chen et al. showed that while catalyzing the formic acid and ethanol oxidation in acid and alkaline media Ostwald ripening obviously occurred when the reaction temperature goes to above 50 °C, leading to a decrease in the number of the nuclei and an increase in the particle size distribution of monodispersed Pd nanoparticles on the graphene substrate.15 In principle, this stability issue could be
1. INTRODUCTION Fuel cells are considered as an important Eco-begin power source nowadays with high energy conversion efficiency and low pollution. The sluggish kinetic rates of both the oxidation reaction at the anode and the oxygen reduction reaction (ORR) at the cathode are the main challenges that hinder the energy conversion efficiency of fuel cells. A considerable effort has been focused on developing new electrocatalytic materials with both high catalytic activity and durability.1,2 A fundamental understanding of the ORR mechanism and its relationship with the catalyst active site structures and composition is vital to design new catalysts with high active site density, superior ORR activities, and long durability.1 Among the transition metals (TMs), Pd is regarded as the most Pt-like metal, and recent investigations have validated that the ORR activity can be effectively enhanced by altering the local chemical environment of Pd reaction centers, via alloying with other elements or forming a strong interaction with the substrate materials. The promotion mechanism was then presented with respect to the synergistic effect between Pd and transition metals,3−7 the modified electronic structure of the Pd d-band,8−10 and the decrease in O adsorption strength on the Pd catalysts.11−13 Comparing with Pt, Pd-based electrocatalysts are also endowed with excellent performance, as they are able to overcome CO poisoning through the direct pathway,14 especially when used as cathode materials in direct formic acid fuel cells (DFAFCs),15,16 direct alcohol fuel cells (DAFCs),15,17,18 and proton exchange membrane fuel cells © 2011 American Chemical Society
Received: October 9, 2011 Revised: December 20, 2011 Published: December 30, 2011 2710
dx.doi.org/10.1021/jp2096983 | J. Phys. Chem. C 2012, 116, 2710−2719
The Journal of Physical Chemistry C
Article
(PBE) functional within the formulation of gradient approximation (GGA) was used to handle the exchange and correlations.41 A kinetic energy cutoff of 30 Ry was used for the graphene substrate and the Pd nanoparticle/graphene composites. The integration of the Brillouin zone was conducted with a 2 × 2 × 1 Monkhorst−Pack grid centered at Γ-point.42 For the structures reported in this work, all atoms were fully relaxed until the forces were reduced below 1 × 10−2 eV/Å. The electronic structure of the pristine graphene (PG), defective graphene substrates, and Pd nanoparticle/graphene composites was explored within the c(4 × 8) supercell (an orthorhombic supercell of 19.73 × 17.09 × 16.00 Å3) with respect to the two-atom unit cell of graphene. For the single vacancy graphene (SVG) and B-doped and N-doped single vacancy graphene (denoted as BVG and NVG hereafter), the vacancy sites are at the center of the cells. The geometry optimization of defective graphene was carried out using the same condition as for the nanoparticle−graphene systems. The bulk lattice parameters of Pd are obtained as a = 3.96 Å, comparable with the experimental values (a = 3.82 Å).43 On the basis of this, the geometry of the Pd13 particle was further optimized in a 25.10 × 25.20 × 25.30 Å3 orthorhombic cell. The Brillouin zone integration was carried out only for the Γ point. The calculated Pd13 particle is of icosahedral symmetry with a total spin of 7.47 μB. which is in reasonable agreement with previous theoretical results of 8.0 μB, and the 0.53 μB difference is due to the smearing used.44 The binding energy Eb is calculated as the energy difference between the Pd13/graphene composite and the separated graphene substrate plus the freestanding Pd13 nanoparticle, following eq 1.
overcome by appropriate treatment of the substrate materials, so that suitable interaction can be formed with the Pd nanoparticles, and making the composites environmentally tolerant.25 As a unique two-dimensional carbon material, graphene has been predicted to be an excellent substrate material for dispersion of a TM nanoparticle catalyst, for its large surface area, outstanding electronic and thermal conductivity, as well as the high mechanical strength and potential low production cost.27−29 On pristine graphene, the stability of the composites suffers from the low surface energy of graphene. However, there are abundant types of defects, such as vacancies, stacking faults, domain boundaries, etc., on the as-synthesized graphene (such as reduced graphene oxide), providing the toolbox for the enhanced TM nanoparticle−substrate interaction.30,31 Besides these, doping with heteroatoms of nitrogen, boron, sulfur, or phosphorus can also effectively modify the physical, chemical, and electronic properties of graphene. Especially, doping of graphene with nitrogen or boron can significantly improve catalyst performance and durability through the optimization of catalyst− substrate interactions.32,33 Among various types of Ndoped structures, the pyridinic N changes the valence band structure of graphene most efficiently, by raising the density of π states near the Fermi level (EF) and the reduction of work function, and the improvement in durability of TM/graphene composites can be directly related to nitrogen levels and specifically to the amount of pyridinic nitrogen.34 However, the potential contributions of the graphene defects and doping to the stability and catalytic activity of the TM/graphene composites are rarely explored theoretically. To aid the design of new Pd/graphene composite catalysts, theoretical calculations should be used to understand the trends in activity and locate new promising candidates for further investigation. The use of modern techniques to synthesis TM/ graphene composites with precise size and composition makes it easier to compare directly experiment with theory and better understand the relationship between the structure of nanoparticles and their catalytic function. In this work, the impact of graphene substrate−Pd nanoparticle interaction on the stability of the nanocomposites and the O, OH, and OOH adsorption on Pd/graphene nanocomposites, which are directly related to the electrocatalytic performance of these composites in oxygen reduction reaction, has been investigated by first-principlesbased calculations, with a focus on the promotion effect of the defective graphene as the substrate. We show that the benefit of using the defective graphene as the substrate will be 3-fold, as it provides anchoring points for the TM deposition, prohibits the segregation of Pd nanoparticle via the Ostwald ripening mechanism, and gives rise to the ORR activity of the Pd/ graphene composites by weakening the adsorption of Ocontaining intermediates. The paper is organized as follows: the background of this work is briefly introduced in Section 1, and the theoretical methods and the computational details are described in Section 2. The results are presented and discussed in Section 3 and concluded in Section 4.
Eb = EPd13/Graphene − (EGraphene + EPd13)
(1)
For the study concerning adsorption of O, OH, and OOH, the adsorption energy is calculated as the energy difference between the species absorbed Pd/graphene composite and the gas phase species plus the bare composite, following eq 2. Both the composites and the O-containing species were fully optimized until the forces were reduced below 1 × 10−2 eV/Å.
Ead = ESpecies + Pd13/Graphene − (ESpecies + EPd13/Graphene)
(2)
The contribution of dispersive interaction to the Ead and Eb was also evaluated by the DFT-D2 method45 but found to be less significant (
