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Multi-Layered Platinum Nanotube for Oxygen Reduction in a Fuel Cell Cathode: Origin of Activity and Product Selectivity AKHIL S NAIR, Arup Mahata, and Biswarup Pathak ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00641 • Publication Date (Web): 10 Jul 2018 Downloaded from http://pubs.acs.org on July 12, 2018
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ACS Applied Energy Materials
Multi-Layered Platinum Nanotube for Oxygen Reduction in a Fuel Cell Cathode: Origin of Activity and Product Selectivity Akhil S Nair,† Arup Mahata, † Biswarup Pathak,†,#,* †
Discipline of Chemistry, Indian Institute of Technology Indore, Simrol, Indore 453552, India
#
Discipline of Metallurgy Engineering and Materials Science, Indian Institute of Technology
Indore, Simrol, Indore 453552, India Email:
[email protected] The practical usages of proton exchange membrane fuel cells from the economical perspective is closely related with the development of catalysts with reduced platinum loading for improved oxygen reduction reaction (ORR) activity. For this, a multi-layered platinum nanotube enclosed by (111) and (100) facets has been modelled and studied for ORR activity using the density functional theory calculations. The stability of the nanotube is verified through energetic, thermal, and dynamic stability calculations. Activation barrier analysis shows that the rate determining steps (O2 dissociation and OH formation) are highly improved over the nanotube surface. We find that four-electron reduction pathway (for H2O formation) is favored over twoelectron reduction (for H2O2 formation) for the nanotube catalyst, which ensures excellent product selectivity (H2O vs. H2O2). The excellent catalytic activity and product selectivity of the nanotube can be attributed towards the favorable adsorption energies of ORR intermediates, as the adsorption energies of key ORR intermediates are reported to be excellent descriptors for ORR activity. Therefore, the platinum nanotube can be a potential electrode material for fuel cell and other related applications.
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Keywords: Fuel Cell, Oxygen Reduction Reaction, nanotube, adsorption, overpotential, coverage
1. Introduction Proton exchange membrane (PEM) fuel cells have become an inevitable presence in the renewable energy scenario for decades. Apart from the general versatile aspects of fuel cells, they are featured with low temperature working condition, reduced emission, good energy density, high efficiency and significant technical and cost advantages compared to competing other energy sources.1-4 Hydrogen evolution reaction (HER) and oxygen reduction reaction (ORR) are the two important reactions taking place within a PEM fuel cell at anode and cathode respectively. The fast kinetics associated with HER marks ORR as the critical process of fuel cell.5 Conventionally, platinum supported by carbon black has been used as catalyst in both the electrodes of fuel cell, but the slow and sluggish nature of ORR serves as a limiting cause for the expected efficiency of fuel cell.6,7 A significant amount of research has been carried out for the designing of novel, efficient catalysts for ORR. Developments within the platinum based as well as platinum free materials have been put forwarded.8 In case of non-platinum based materials, other transition metal based catalysts,9-11 bimetallic alloys,12-14 inorganic materials like graphene, carbon nanotubes15-17 and so on have been addressed with significant catalytic activity. Despite of the immense diversity in platinum free materials, the lack of stability to sustain the activity at extremely harsh reaction conditions within the fuel cell invariantly prompts researchers to be actively focused on developing platinum based catalysts.18 Since platinum being an expensive metal, the major challenge in the catalyst designing for ORR is reducing platinum loading without compromising the catalytic efficiency.
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The vibrant search for efficient catalyst options have led researchers to come up with platinum based materials differing in structure and activity. The dependence of catalytic activity on the size and structure of the catalyst has paved the way to the synthesis of stabilized platinum nanoparticles including nanoclusters, 19-21 nanoshells, 22-23 nanofilms24 and so on. In fact, the facet controlled synthesis of nanocrystals has been verified to possess sound improvement in catalytic performance. Recently, Xue Wang et al.25 have synthesized Pt-based icosahedral nanocage enclosed by multiple twin defects and exposed (111) facets with great enhancement in both specific and mass activity as well as remarkable catalytic durability. Among these structurally diverse set of nanocatalysts, one dimensional metal nanostructures such as nanowires, nanorods and nanotubes have gained noticeable research interest for the efficient catalytic performance arising from their morphological characteristics.26-30 The unique anisotropic structure as well as low defect density associated with 1D nanostructures enable them to perform as potential catalysts for ORR.31 Although, significant research has been done in the area of platinum nanowires and nanorods, similar extent of studies have not been explored for the platinum nanotube based structures. The nanotubes are prone to considerable interest because of their high surface area, stiff structure and preferential ordering.32 The choice of tubular structures also reduces the platinum agglomeration problem and provides much higher specific activity as compared to other nanostructures.33 S. M. Alia and coworkers34 have synthesized porous Pt nanotubes with an outer diameter of 60 nm with greater percentage of surface area and 3.1 times activity as compared to the Pt/C commercial catalyst. An array of Pt nanotubes with greater surface specific activity than the Pt/C catalyst towards ORR has been investigated by S. Galbiati et al.35 These studies illustrate the fact that there are plethora of chances for developing nanotubes with reduced platinum loading and improved catalytic efficiency.
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Inspired by the previous studies, we have modelled a multilayered platinum nanotube (Pt-NT) enclosed by (111) and (100) facets to investigate the ORR activity. The dynamic and thermal stability of the nanotube are measured by phonon and molecular dynamics calculations. We try to understand the trend in adsorption behavior of various intermediates of ORR including OH, O and OOH on the nanotube surface as they are key intermediates and reported to be good descriptors for ORR activity. Then the mechanism of ORR occurring on the platinum nanotube (111) facet is investigated by analyzing the energy changes associated with the ORR elementary steps. We have studied both the four-electron reduction reaction (for H2O formation) and twoelectron reduction (for H2O2 formation) pathways to have a comprehensive picture of product selectivity. We have also tried to address the overpotential problem which is crucial in determining the catalyst activity in electrochemical conditions. The results obtained are compared with the previously reported single-layered Pt66-nanocage (Pt-NC) since it possesses a void inside analogous to the nanotube structure.36 Furthermore, we have compared our calculated data with the previously reported periodic Pt (111) surface to understand them and find ways to improve them. 2
Model and Computational Details
First-principles calculations are performed using the Vienna Ab initio Simulation Package (VASP).37 The exchange–correlation potential is described by using the generalized gradient approximation of Perdew–Burke–Ernzerhof (GGA-PBE).38 The projector augmented wave (PAW) method39 is employed to treat interactions between ion cores and valence electrons. A plane wave with a kinetic energy cutoff of 470 eV is used to expand the electronic wave functions. A 29×29×5 Å cubic supercell is used to model the Pt-NT. A set of 1×1×3 Monkhorst Pack40 k-point meshes are used to perform the Brillouin Zone calculations. The periodic Pt (111) 4 ACS Paragon Plus Environment
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surface is modelled by using a slab model with a 3×3 supercell of 5 atomic layers. During optimization, the bottom three layers are fixed and top two layers are relaxed. A vacuum of 12 Ǻ is used to avoid the interactions between periodic images. The Pt (111) surface calculations are done with a set of 3×3×1 k-point meshes. The convergence criteria for total energy and force during optimization are taken as 10-4 eV and
