The Influence of Particle Shape and Size on the Activity of Platinum Nanoparticles for Oxygen Reduction Reaction: A Density Functional Theory Study

Tuning the chemical composition of binary alloy nanoparticles to prevent their dissolution

The dissolution of nanoparticles under corrosive environments represents one of the main issues in electrochemical processes. Here, a model for alloying and protecting nanoparticles from corrosion with an anti-corrosive element (e.g. Au) is proposed based on the hypothesis that under-coordinated atoms are the first atoms to dissolve. The model considers the dissolution of atoms with coordination number ≤6 on A-B nanoparticles with different sizes, shapes, chemical compositions, and exposed crystallographic orientations. The results revealed that the nanoparticle's size and chemical composition play a key role in the dissolution, suggesting that a certain composition of an element with corrosive resistance could be used to protect nanoparticles. DFT simulations were performed to support our model on the dissolution of four types of atoms commonly found on the surface of Au0.20Pd0.80 binary alloys - terrace, edge, kink, and ad atoms. The simulations suggest that the less coordinated ad and kink Pd atoms on Au0.20Pd0.80 alloys are dissolved in a potential window between 0.26-0.56 V, while the rest of the Pd and Au atoms are protected. Furthermore, to show that a corrosion-resistant element can indeed protect nanoparticles, we experimentally investigated the electrochemical dissolution of immobilized Pd, Au0.20Pd0.80, and Au0.40Pd0.60 nanoparticles in a harsh environment. In line with the dissolution model, the experimental results show that an Au molar fraction of the nanoparticle of 0.20, i.e., Au0.20Pd0.80 binary alloy, is a good compromise between maximizing the active surface area (Pd atoms) and corrosion protection by the inactive Au.

Synthesis of Pt Double-Walled Nanoframes with Well-Defined and Controllable Facets

In this paper, we demonstrate the synthesis of morphologically complex nanoframes wherein a mixture of frames and thin solid planes, which we refer to as walled-nanoframes, are present in a single particle. By applying multiple chemical steps including shape evolution of Au nanocrystals and controlling chemical potential of solution for selective deposition, we successfully designed a variety of Pt nanoframes including Pt cuboctahedral nanoframes and Pt single-walled nanoframes. The rationale for on-demand chemical steps with well-faceted Au overgrowth allowed for the synthesis of double-walled nanoframes where two Pt single-walled nanoframes are concentrically overlapped in a single entity with a clearly discernible gap between the two nanoframes. Given the coexistence of an open structure of nanoframe and thin plates within one entity, the double-walled nanoframes showed a dramatic increase in catalytic activity toward the methanol oxidation reaction, acting as high-surface area, carbon-free, and volume-compact nanocatalysts.

A density functional theory (DFT) study on reduced partition function ratios of oxygen species adsorbed on a Pt19 cluster and oxygen isotope effects

A density functional theory (DFT) computation on oxygen species adsorbed on platinum (Pt) catalyst surfaces has been carried out to elucidate oxygen isotope fractionation observed at the cathode of a polymer electrolyte membrane fuel cell (PEMFC). The Pt(111) catalyst surface was modelled by a Pt₁₉ cluster, and O, OH, OHH, OO, OOH, OHOH and HOHOH were assumed to be the oxygen species adsorbed on the Pt(111) surface. The oxygen isotope reduced partition function ratios (RPFRs) of the adsorbed species were calculated using the vibrational frequencies obtained by normal mode analyses performed on the optimized structures. Various oxygen isotope exchange equilibria among the adsorbed oxygen species and oxygen and water molecules in the gas phase were examined using their RPFRs. Experimental observation that the lighter ¹⁶O is enriched in water molecules exhausted from the cathode is explainable in a satisfactory manner by assuming oxygen isotope exchange equilibria of O₂ molecule with O, OH, OO and OOH adsorbed on the Pt(111) surface that appear in the first half of the conversion reaction from O₂ to H₂O and those of H₂O molecule with the adsorbed oxygen species, OHH, OHOH and HOHOH, formed in the latter half of the conversion reaction.

Shape control of size-selected naked platinum nanocrystals

Controlled growth of far-from-equilibrium-shaped nanoparticles with size selection is essential for the exploration of their unique physical and chemical properties. Shape control by wet-chemistry preparation methods produces surfactant-covered surfaces with limited understanding due to the complexity of the processes involved. Here, we report the controlled production and transformation of octahedra to tetrahedra of size-selected platinum nanocrystals with clean surfaces in an inert gas environment. Molecular dynamics simulations of the growth reveal the key symmetry-breaking atomic mechanism for this autocatalytic shape transformation, confirming the experimental conditions required. In-situ heating experiments demonstrate the relative stability of both octahedral and tetrahedral Pt nanocrystals at least up to 700 °C and that the extended surface diffusion at higher temperature transforms the nanocrystals into equilibrium shape.

Controlling the Surface Functionalization of Ultrasmall Gold Nanoparticles by Sequence-Defined Macromolecules

Ultrasmall gold nanoparticles (diameter about 2 nm) were surface-functionalized with cysteine-carrying precision macromolecules. These consisted of sequence-defined oligo(amidoamine)s (OAAs) with either two or six cysteine molecules for binding to the gold surface and either with or without a PEG chain (3400 Da). They were characterized by 1 H NMR spectroscopy, 1 H NMR diffusion-ordered spectroscopy (DOSY), small-angle X-ray scattering (SAXS), and high-resolution transmission electron microscopy. The number of precision macromolecules per nanoparticle was determined after fluorescent labeling by UV spectroscopy and also by quantitative 1 H NMR spectroscopy. Each nanoparticle carried between 40 and 100 OAA ligands, depending on the number of cysteine units per OAA. The footprint of each ligand was about 0.074 nm2 per cysteine molecule. OAAs are well suited to stabilize ultrasmall gold nanoparticles by selective surface conjugation and can be used to selectively cover their surface. The presence of the PEG chain considerably increased the hydrodynamic diameter of both dissolved macromolecules and macromolecule-conjugated gold nanoparticles.

Polyhedral Effects on the Mass Activity of Platinum Nanoclusters

We use a coordination-based kinetics model to look at the kinetics of the turnover frequency (TOF) for the oxygen reduction reaction (ORR) for platinum nanoclusters. Clusters of octahedral, cuboctahedral, cubic, and icosahedral shape and size demonstrate the validity of the coordination-based approach. The Gibbs adsorption energy is computed using an empirical energy model based on density functional theory (DFT), statistical mechanics, and thermodynamics. We calculate the coordination and size dependence of the Gibbs adsorption energy and apply it to the analysis of the TOF. The platinum ORR follows a Langmuir–Hinshelwood mechanism, and we model the kinetics using a thermodynamic approach. Our modeling indicates that the coordination, shape, and the Gibbs energy of adsorption all are important factors in replicating an experimental TOF. We investigate the effects of size and shape of some platinum polyhedra on the oxygen reduction reaction (ORR) and the effect on the mass activity. The data are modeled quantitatively using lognormal distributions. We provide guidance on how to account for the effects of different distributions due to shape when determining the TOF.

First-principles investigation of electrochemical dissolution of Pt nanoparticles and kinetic simulation

Dissolution is the primary route of Pt nanoparticle degradation in electrochemical devices, e.g., fuel cells. Investigation of potential-dependent dissolution kinetics of Pt nanoparticles is crucial to optimize the nanoparticle size and operating conditions for better performance. A mean-field kinetic theory under the steady-state approximation, combined with atomistic thermodynamics and Wulff construction, was developed to study the interplay between oxygen chemisorption, electrode potential, and particle size on the dissolution of Pt nanoparticles. We found that although oxygen chemisorption from electrode potential-induced water splitting can stabilize Pt nanoparticles through decreasing the surface energy and increasing the redox potential, the electrode potential plays a more decisive role in facilitating the dissolution of Pt nanoparticles. In comparison with the minor effect of oxygen chemisorption, an increase in the particle size, though reducing the dispersion, has a more significant effect on the suppression of the dissolution. These theoretical understandings on the effects of electrode potential and particle size on the dissolution are crucial for optimizing the nanoparticle size under oxidative operating conditions.

Fast identification of optimal pure platinum nanoparticle shapes and sizes for efficient oxygen electroreduction

Recent advances in experimental synthesis of nanostructures have shown that the interplay between nanoparticle shapes and sizes is crucial to achieve catalysts with high mass activity toward oxygen electroreduction. This is particularly important for proton-exchange membrane fuel cells (PEMFCs), in which expensive and scarce Pt electrocatalysts are used. In this work, we propose a theoretical approach for oxygen electroreduction on PEMFCs to identify not only the size of optimal nanoparticles, but also their shapes. Remarkably, high mass activities up to 4.28 A mg_Pt ^-1 are predicted for rod-like nanostructures. Furthermore, we examine nanostructure size effects to guide chemical routes for experimental synthesis of the identified electrocatalysts. Our fast theoretical evaluation of thousands of different nanostructures aids in the search for active catalysts, as substantially enhanced mass activities over commercial Pt/C are predicted for pure Pt electrocatalysts, thus unveiling great potential to reduce the Pt loading in PEMFCs.

Revealing the nature of active sites in electrocatalysis

Heterogeneous electrocatalysis plays a central role in the development of sustainable, carbon-neutral pathways for energy provision and the production of various chemicals. It determines the overall efficiency of electrochemical devices that involve catalysis at the electrode/electrolyte interface. In this perspective, we discuss key aspects for the identification of active centers at the surface of electrocatalysts and important factors that influence them. The role of the surface structure, nanoparticle shape/size and the electrolyte composition in the resulting catalytic performance is of particular interest in this work. We highlight challenges that from our point of view need to be tackled, and provide guidelines for the design of "real life" electrocatalysts for renewable energy provision systems as well as for the production of industrially important compounds.

Shape-controlled metal nanoparticles for electrocatalytic applications

The application of shape-controlled metal nanoparticles is profoundly impacting the field of electrocatalysis. On the one hand, their use has remarkably enhanced the electrocatalytic activity of many different reactions of interest. On the other hand, their usage is deeply contributing to a correct understanding of the correlations between shape/surface structure and electrochemical reactivity at the nanoscale. However, from the point of view of an electrochemist, there are a number of questions that must be fully satisfied before the evaluation of the shaped metal nanoparticles as electrocatalysts including (i) surface cleaning, (ii) surface structure characterization, and (iii) correlations between particle shape and surface structure. In this chapter, we will cover all these aspects. Initially, we will collect and discuss about the different practical protocols and procedures for obtaining clean shaped metal nanoparticles. This is an indispensable requirement for the establishment of correct correlations between shape/surface structure and electrochemical reactivity. Next, we will also report how some easy-to-do electrochemical experiments including their subsequent analyses can enormously contribute to a detailed characterization of the surface structure of the shaped metal nanoparticles. At this point, we will remark that the key point determining the resulting electrocatalytic activity is the surface structure of the nanoparticles (obviously, the atomic composition is also extremely relevant) but not the particle shape. Finally, we will summarize some of the most significant advances/results on the use of these shaped metal nanoparticles in electrocatalysis covering a wide range of electrocatalytic reactions including fuel cell-related reactions (electrooxidation of formic acid, methanol and ethanol and oxygen reduction) and also CO2 electroreduction.

Graphical Abstract:

Defects on carbons for electrocatalytic oxygen reduction

The exploration of highly active and durable cathodic oxygen reduction reaction (ORR) catalysts with economical production costs is still the bottleneck to realize the large-scale commercialization of fuel cells. In recent years, remarkable progress has been achieved in fabricating effective non-precious metal based ORR catalysts. In particular, modified carbon materials have aroused extensive research interest because of their excellent performance and low cost. In this review, we present an overview on recent advancements in developing defective carbon based materials for catalyzing the ORR. In particular, three general kinds of defective carbon electrocatalysts will be summarized. They are non-metal induced defective carbons (modified by heteroatoms), intrinsic defective carbons (defects created by a physical or chemical method), and atomic metal species induced/coordinated defective carbons (metal-macrocycle complexes with different coordination environments). The common configurations of various defective carbons will be discussed, with typical examples on recently developed both metal-free and precious/non-precious metal species coordinated carbons. Finally, the future research directions of the defective carbon materials are proposed. The newly established defect promoted catalysis mechanism will be beneficial for the design and fabrication of highly effective electrocatalysts for practical energy storage and conversion applications.

Emerging Materials in Heterogeneous Electrocatalysis Involving Oxygen for Energy Harvesting

Water-based renewable energy cycle involved in water splitting, fuel cells, and metal-air batteries has been gaining increasing attention for sustainable generation and storage of energy. The major challenges in these technologies arise due to the poor kinetics of the oxygen reduction reaction (ORR) and the oxygen evolution reactions (OER), besides the high cost of the catalysts. Attempts to address these issues have led to the development of many novel and inexpensive catalysts as well as newer mechanistic insights, particularly so in the last three-four years when more catalysts have been investigated than ever before. With the growing emphasis on bifunctionality, that is, materials that can facilitate both reduction and evolution of oxygen, this review is intended to discuss all major families of ORR, OER, and bifunctional catalysts such as metals, alloys, oxides, other chalcogenides, pnictides, and metal-free materials developed during this period in a single platform, while also directing the readers to specific and detailed review articles dealing with each family. In addition, each section highlights the latest theoretical and experimental insights that may further improve ORR/OER performances. The bifunctional catalysts being sufficiently new, no consensus appears to have emerged about the efficiencies. Therefore, a statistical analysis of their performances by considering nearly all literature reports that have appeared in this period is presented. The current challenges in rational design of these catalysts as well as probable strategies to improve their performances are presented.

Oxygen Reduction Reaction: Rapid Prediction of Mass Activity of Nanostructured Platinum Electrocatalysts

Tailored Pt nanoparticle catalysts are promising candidates to accelerate the oxygen reduction reaction (ORR) in fuel cells. However, the search for active nanoparticle catalysts is hindered by the laborious effort of experimental synthesis and measurements. On the other hand, density functional theory-based approaches are still time-consuming and often not efficient. In this study, we introduce a computational model which enables rapid catalytic activity calculation of unstrained pure Pt nanoparticle electrocatalysts. Regarding particle size effects on Pt nanoparticles, experimental catalytic mass activities from previous studies are accurately reproduced by our computational model. Moreover, beyond available experiments, our computational model identifies potential enhancement in mass activity up to 190% over the experimentally detected maximum. Importantly, the rapid activity calculation enabled by our computational model may pave the way for extensive nanoparticle screening to expedite the search for improved electrocatalysts.

DFT calculation of oxygen adsorption on platinum nanoparticles: coverage and size effects

DFT calculations are used to simultaneously explore the effects of nanoparticle size and coverage for O adsorption on Pt nanoparticles.

Ethanol, O, and CO adsorption on Pt nanoparticles: effects of nanoparticle size and graphene support

DFT calculations reveal aspects of size and support effects for Pt nanoparticles on graphene interacting with O, CO and ethanol.

Ultrathin two-dimensional metals with fully exposed (111) facets

Large-size ultrathin two-dimensional (2D) metals with a fully exposed (111) surface have been synthesized by a heat-pressing process.

Why conclusions from platinum model surfaces do not necessarily lead to enhanced nanoparticle catalysts for the oxygen reduction reaction

Experiments on model surfaces commonly help in identifying the structural sensitivity of catalytic reactions. Nevertheless, their conclusions do not frequently lead to devising superior "real-world" catalysts. For instance, this is true for single-crystal platinum electrodes and the oxygen reduction reaction (ORR), an important reaction for sustainable energy conversion. Pt(111) is substantially enhanced by steps, reaching a maximum at short terrace lengths of 3-4 atoms. Conversely, regular platinum nanoparticles with similar undercoordinated defects are less active than Pt(111) and their activity increases alongside the terrace-to-defect ratio. We show here that a model to design ORR active sites on extended surfaces can also be used to solve this apparent contradiction and provide accurate design rules for nanoparticles. Essentially, only surfaces and nanostructures with concave defects can surpass the activity of Pt(111), whereas convex defects are inactive. Importantly, only the latter are present in regular nanoparticles, which is why we design various concave nanoparticles with high activities.

Effect of graphene support on large Pt nanoparticles

Large scale DFT calculations of Pt nanoparticles supported on graphene explore the non-trivial interplay of size and support effects.

Design of electrocatalysts for oxygen- and hydrogen-involving energy conversion reactions

A fundamental change has been achieved in understanding surface electrochemistry due to the profound knowledge of the nature of electrocatalytic processes accumulated over the past several decades and to the recent technological advances in spectroscopy and high resolution imaging. Nowadays one can preferably design electrocatalysts based on the deep theoretical knowledge of electronic structures, via computer-guided engineering of the surface and (electro)chemical properties of materials, followed by the synthesis of practical materials with high performance for specific reactions. This review provides insights into both theoretical and experimental electrochemistry toward a better understanding of a series of key clean energy conversion reactions including oxygen reduction reaction (ORR), oxygen evolution reaction (OER), and hydrogen evolution reaction (HER). The emphasis of this review is on the origin of the electrocatalytic activity of nanostructured catalysts toward the aforementioned reactions by correlating the apparent electrode performance with their intrinsic electrochemical properties. Also, a rational design of electrocatalysts is proposed starting from the most fundamental aspects of the electronic structure engineering to a more practical level of nanotechnological fabrication.

High-yield synthesis of sub-10 nm Pt nanotetrahedra with bare ⟨111⟩ facets for efficient electrocatalytic applications

Unlike other shapes, the design of tetrahedral Pt nanocrystals (Pt-NTd), which have the largest number of Pt(111) surface atoms and highest catalytic activities toward the electron transfer reactions, has widely been considered a synthetic challenge due to their thermodynamic instability. Here, we show that, by inducing their nucleation on functionalized carbon, Pt NTds can be obtained with tunable sizes and high yields. The carbon support anchors the nanocrystals early and prevents their oriented attachment leading to nanowire formation. Therein, an in situ generated amine is crucial for stabilization of Pt-NTds, which can later be removed to expose the Pt(111) facets for higher catalytic efficiency. The bare nanocrystals exhibit much improved stability and electrocatalytic activity characteristic of Pt(111) toward oxygen reduction reaction (ORR) and methanol and formic acid oxidation reactions. For example, ∼90% of their activity was retained after 5000 potential cycles, while the ORR onset potential was recorded to be very high, 1.01 V vs reversible hydrogen electrode (RHE).

First principles investigation of the activity of thin film Pt, Pd and Au surface alloys for oxygen reduction

Several binary and ternary thin film surface alloys made of Pt, Pd and Au are promising catalysts for the oxygen reduction reaction.

Oxygen reduction reaction activity and structural stability of Pt–Au nanoparticles prepared by arc-plasma deposition

Sequential arc-plasma deposited Pt–Au alloy nanoparticles show superior electrochemical structural durability compared with arc-plasma deposited Pt nanoparticles before and after electrochemical potential cycles.