Electrocatalysis of Oxygen Reduction Reaction on Shape-Controlled Pt and Pd Nanoparticles-Importance of Surface Cleanliness and Reconstruction

Intentional Formation of Persistent Surface Redox Mediators by Adsorption of Polyconjugated Carbonyl Complexes to Pd Nanoparticles

Adsorbing polyconjugated carbonyl and aromatic species to Pd nanoparticles forms persistent intermediates that mediate reactions between hydrogen and oxygen-derived species. These surface redox mediators form in situ and increase selectivities toward H2O2 formation (∼65-85%) compared to unmodified Pd nanoparticles (∼45%). Infrared spectroscopy, temperature-programmed oxidation measurements, and ab initio calculations show that these species adsorb irreversibly to Pd surfaces and persist over extended periods of catalysis. Combined rates and kinetic isotope effect measurements and simulations suggest that carbonyl groups of bound organics react heterolytically with hydrogen to form partially hydrogenated oxygenated complexes. Subsequently, these organic species transfer proton-electron pairs to O2-derived surface species via pathways that favor H2O2 over H2O formation on Pd nanoparticles. Computational and experimental measurements show redox pathways mediated by partially hydrogenated carbonyl species form H2O2 with lower barriers than competing processes while also obstructing O-O bond dissociation during H2O formation. For example, adsorption and hydrogenation of hexaketocyclohexane on Pd forms species that react with oxygen with high H2O2 selectivities (85 ± 8%) for 130 h on stream in flowing water without additional promoters or cosolvents. These paths resemble the anthraquinone auto-oxidation process (AAOP) used for industrial H2O2 production. These surface-bound species form partially hydrogenated intermediates that mediate H2O2 formation with high rates and selectivities, comparable to AAOP but on a single catalytic nanoparticle in pure water without organic solvents or multiunit reaction-separation chains. The molecular insights developed herein provide strategies to avoid organic solvents in selective processes and circumvent their associated process costs and environmental impacts.

Enhanced durability of Pd/CeO2-C via metal-support interaction for oxygen reduction reaction

It is a challenge to improve the long-term durability of Pd-based electrocatalysts for oxygen reduction reaction (ORR) in fuel cells. Herein, Pd/CeO2-C-T (T= 800 °C, 900 °C and 1000 °C) hybrid catalysts with metal-support interaction are prepared from Ce-based metal organic framework precursor. Abundant tiny CeO2nanoclusters are produced to form nanorod structures with uniformly distributed carbon through a calcination process. Meanwhile, both carbon and CeO2nanoclusters have good contact with the following deposited surfactant-free Pd nanoclusters. Benefited from the large specific surface area, good conductivity and structure integrity, Pd/CeO2-C-900 exhibits the best electrocatalytic ORR performance: onset potential of 0.968 V and half-wave potential of 0.857 V, outperforming those obtained on Pd/C counterpart. In addition, the half-wave potential only shifts 7 mV after 6000 cycles of accelerated durability testing, demonstrating robust durability.

Investigation of Effect of Platinum Nanoparticle Shape on Oxygen Transport in PEMFC Catalyst Layer Using Molecular Dynamics Simulation

For the widespread adoption of polymer electrolyte membrane fuel cells, it is compelling to investigate the influence of the Pt nanoparticle shapes on the electrocatalytic activity. In this study, a catalyst layer was modeled by incorporating four types of Pt nanoparticles: tetrahedron, cube, octahedron, and truncated octahedron, to investigate the relationship between the shapes of the nanoparticles and their impact on the oxygen transport properties using molecular dynamics simulations. The results of our study reveal that the free volume, which has a substantial impact on the oxygen transport properties, exhibited higher values in the sequence of the tetrahedron, cube, octahedron, and truncated octahedron model. The difference in free volume following the formation of less dense ionomers was also related to the surface adsorption of Pt nanoparticles. Consequently, this led to an improved facilitation of oxygen transport. To clarify the dependence of the oxygen transport on the shape of the Pt nanoparticles in detail, we analyzed the structural properties of different Pt shapes by dividing the Pt nanoparticle regions into corners, edges, and facets. Examination of the structural properties showed that the structure of the ionomer depended not only on the shape of the Pt nanoparticles but also on the number of corners and edges in the upper and side regions of the Pt nanoparticles.

Ternary PdCoP nanoparticles with nanopore structures: synergic boosting of electrocatalytic activity for ethanol oxidation

PdCoP nanoparticles (PdCoP NPs) with nanopore structures were synthesized by a facile one-pot solvothermal approach. Due to their unique geometric structures and the electronic and synergistic effects among multiple components, the optimized PdCoP NPs (PdCoP-NPs-1) show superior mass activity (5.97 A mg_Pd^-1) for the ethanol oxidation reaction under alkaline conditions.

Recent Progress on Revealing 3D Structure of Electrocatalysts Using Advanced 3D Electron Tomography: A Mini Review

Electrocatalysis plays a key role in clean energy innovation. In order to design more efficient, durable and selective electrocatalysts, a thorough understanding of the unique link between 3D structures and properties is essential yet challenging. Advanced 3D electron tomography offers an effective approach to reveal 3D structures by transmission electron microscopy. This mini-review summarizes recent progress on revealing 3D structures of electrocatalysts using 3D electron tomography. 3D electron tomography at nanoscale and atomic scale are discussed, respectively, where morphology, composition, porous structure, surface crystallography and atomic distribution can be revealed and correlated to the performance of electrocatalysts. (Quasi) in-situ 3D electron tomography is further discussed with particular focus on its impact on electrocatalysts' durability investigation and post-treatment. Finally, perspectives on future developments of 3D electron tomography for eletrocatalysis is discussed.

Electrochemistry as a Complementary Technique for Revealing the Influence of Reducing Agent Concentration on AgNPs

The synthesis process of AgNPs has been attracting a lot of attention in the fields of biosensors/sensors, diagnostics, and therapeutic applications. An attempt to understand the effect of different concentrations of reducing agents on the synthetic design process has been made. In this paper, we gather information on voltammetry studies and relate it with UV-vis and scanning electron microscopy (SEM) analyses. Given the kinetics, localized surface plasmon absorption (LSPR) band, and narrow size distribution of these methods, it was possible to compare the obtained measurements and clearly distinguish sizes and aggregation. AgNPs measured by SEM showed a statistically significant reduction of the nanoparticle sizes from 65 to 37.5 nm as the reducing agent increased. Well-matched d-spacing data calculated from selected area electron diffraction (SAED) patterns and X-ray diffraction (XRD) were obtained for all of the samples. The UV-vis studies showed that the SPR bands shift toward the blue region as the reducing agent concentration is increased, indicating a decrease in particle sizes. It is worth emphasizing that cyclic voltammetry (CV) and differential pulse voltammetry (DPV) coincide well with SEM on the aggregation of AgNPs at higher concentrations. A 10 mM reducing agent concentration resulted in uniform outcomes for producing AgNPs with the smallest size in terms of full width at half-maximum (FWHM) in all of the methods used in this study, while UV-vis band gaps increase with increasing reducing agent concentration. In agreement with all of the methods investigated, the results suggested that the best concentration of the reducing agents is 10 mM for a target application. These findings suggest the usefulness of voltammetry as a complementary method that can be used as a qualitative guide to identify the size and aggregation of NPs.

Using Palladium and Gold Palladium Nanoparticles Decorated with Molybdenum Oxide for Versatile Hydrogen Peroxide Electroproduction on Graphene Nanoribbons

Electrocatalytic production of H₂O₂ via a two-electron oxygen reduction reaction (ORR-2e^-) is regarded as a highly promising decentralized and environmentally friendly mechanism for the production of this important chemical commodity. However, the underlying challenges related to the development of catalytic materials that contain zero or low content of noble metals and that are relatively more active, selective, and resistant for long-term use have become a huge obstacle for the electroproduction of H₂O₂ on commercial and industrial scales. The present study reports the synthesis and characterization of low metal-loaded (≤6.4 wt %) catalysts and their efficiency in H₂O₂ electroproduction. The catalysts were constructed using gold palladium molybdenum oxide (AuPdMoO_x) and palladium molybdenum oxide (PdMoO_x) nanoparticles supported on graphene nanoribbons. Based on the application of a rotating ring-disk electrode, we conducted a thorough comparative analysis of the electrocatalytic performance of the catalysts in the ORR under acidic and alkaline media. The proposed catalysts exhibited high catalytic activity (ca. 0.08 mA g_noble metal^-1 in an acidic medium and ca. 6.6 mA g_noble metal^-1 in an alkaline medium), good selectivity (over 80%), and improved long-term stability toward ORR-2e^-. The results obtained showed that the enhanced ORR activity presented by the catalysts, which occurred preferentially via the two-electron pathway, was promoted by a combination of factors including geometry, Pd content, interparticle distance, and site-blocking effects, while the electrochemical stability of the catalysts may have been enhanced by the presence of MoO_x.

Noble Metal-Based Multimetallic Nanoparticles for Electrocatalytic Applications

Noble metal-based multimetallic nanoparticles (NMMNs) have attracted great attention for their multifunctional and synergistic effects, which offer numerous catalytic applications. Combined experimental and theoretical studies have enabled formulation of various design principles for tuning the electrocatalytic performance through controlling size, composition, morphology, and crystal structure of the nanoparticles. Despite significant advancements in the field, the chemical synthesis of NMMNs with ideal characteristics for catalysis, including high activity, stability, product-selectivity, and scalability is still challenging. This review provides an overview on structure-based classification and the general synthesis of NMMN electrocatalysts. Furthermore, postsynthetic treatments, such as the removal of surfactants to optimize the activity, and utilization of NMMNs onto suitable support for practical electrocatalytic applications are highlighted. In the end, future direction and challenges associated with the electrocatalysis of NMMNs are covered.

Strategies to enhance the electrochemical performances of Pt-based intermetallic catalysts

The need for improving the energy conversion efficiency of proton exchange membrane fuel cells (PEMFCs) has motivated the development of advanced electrocatalysts with desirable activity and durability. Pt-Based intermetallic compounds, featuring atomically ordered structures, have long been considered to be very promising alternatives to widely employed Pt and Pt alloy (solid solutions) catalysts. To facilitate the practical application of Pt-based intermetallics in PEMFCs, effective strategies have been developed to further improve their catalytic activity and durability over the last decade. This feature article overviews the recent advances on the strategies for enhancing the electrochemical performances of Pt-based intermetallic catalysts, which include size control, surface engineering, and composition tuning. Thermodynamic and kinetic perspectives on the formation of the intermetallic phases are summarized to better design the synthesis conditions and realize the size control. After this, the size-control approaches (e.g. coating protection, matrix protection) are illustrated and discussed. We highlight the positive effect of surface engineering and discuss the recently developed methods for surface engineering. Finally, we discuss the thermodynamic feasibility of composition tuning and recent work based on composition-tunable intermetallic electrocatalysts.

MOF-derived PtCo/Co3O4 nanocomposites in carbonaceous matrices as high-performance ORR electrocatalysts synthesized via laser ablation techniques

ZIF-67-derived carbon-based bimetallic nanocomposites with reduced Pt-loading via laser ablation synthesis in solution (LASiS) as a superior electrocatalyst for oxygen reduction reaction (ORR).

Enhanced oxygen reduction activity of bimetallic Pd–Ag alloy-supported on mesoporous cerium oxide electrocatalysts in alkaline media

Currently, the rational design and fabrication of Pt-free electrocatalysts towards the oxygen reduction reaction for extensive applications in fuel cells is a challenging task.