Direct Observation of Dynamic Bond Evolution in Single-Atom Pt/C3 N4 Catalysts

Single-atom catalysts are promising platforms for heterogeneous catalysis, especially for clean energy conversion, storage, and utilization. Although great efforts have been made to examine the bonding and oxidation state of single-atom catalysts before and/or after catalytic reactions, when information about dynamic evolution is not sufficient, the underlying mechanisms are often overlooked. Herein, we report the direct observation of the charge transfer and bond evolution of a single-atom Pt/C₃ N₄ catalyst in photocatalytic water splitting by synchronous illumination X-ray photoelectron spectroscopy. Specifically, under light excitation, we observed Pt-N bond cleavage to form a Pt⁰ species and the corresponding C=N bond reconstruction; these features could not be detected on the metallic platinum-decorated C₃ N₄ catalyst. As expected, H₂ production activity (14.7 mmol h^-1  g^-1 ) was enhanced significantly with the single-atom Pt/C₃ N₄ catalyst as compared to metallic Pt-C₃ N₄ (0.74 mmol h^-1  g^-1 ).

Atomic-Level Construction of Tensile-Strained PdFe Alloy Surface toward Highly Efficient Oxygen Reduction Electrocatalysis

Exploring the high-performance non-Pt electrocatalysts for oxygen reduction reaction (ORR), the bottleneck process in fuel cells, is desirable but challenging. Here, we report the Pd@PdFe core-shell icosahedra as an active and durable electrocatalyst toward ORR in alkaline conditions, which feature a three-atomic-layer tensile-strained PdFe overlayer on Pd icosahedra. Our optimized catalyst shows 2.8-fold enhancement in mass activity and 6.9-fold enhancement in specific activity than commercial Pt/C catalyst toward ORR, representing one of the best non-Pt electrocatalysts. Moreover, the boosted ORR catalysis is strongly supported by the assembled fuel cell performance using Pd@PdFe core-shell icosahedra as the cathode electrocatalyst. The density functional theory calculations reveal that the synergistic coupling of tensile strain and alloy effects enables the optimum binding strength for intermediates, thus causing the maximum activity. The present work suggests the coupling between multiple surface modulations endows larger room for the rational design of remarkable catalysts.

Continuous syntheses of carbon-supported Pd and Pd@Pt core-shell nanoparticles using a flow-type single-mode microwave reactor

Continuous syntheses of carbon-supported Pd@Pt core-shell nanoparticles were performed using microwave-assisted flow reaction in polyol to synthesize carbon-supported core Pd with subsequent direct coating of a Pt shell. By optimizing the amount of NaOH, almost all Pt precursors contributed to shell formation without specific chemicals.

Template Regeneration in Galvanic Replacement: A Route to Highly Diverse Hollow Nanostructures

The ability to produce a diverse spectrum of hollow nanostructures is central to the advances in many current and emerging areas of technology. Herein, we report a general method to craft hollow nanostructures with highly tunable physical and chemical parameters. The key strategy is to regenerate the nanoscale sacrificial templates in a galvanic replacement reaction through site-selective overgrowth. As examples, we demonstrate the syntheses of nanocages and nanotubes made of silver, gold, palladium, and/or platinum with well-controlled wall thicknesses and elemental distributions. Using the nanocages of silver and gold as models, we demonstrate they possess intriguing plasmonic properties and offer superior performance in biosensing applications. This study provides a powerful platform to customize hollow nanostructures with desired properties and therefore is expected to enable a variety of fundamental studies and technologically important applications.

Single Atom on the 2D Matrix: An Emerging Electrocatalyst for Energy Applications

The electrochemical energy conversions play an essential role in the production of sustainable and renewable energy. However, the performance is not up to the mark due to the absence of highly efficient and stable electrocatalysts. Recently, both 2D-matrix and single-atom catalysts (SACs) are two intense research topics in the field of electrocatalysis due to the high activity and stability and to maximize the utilization efficiency. Engineering the materials from 3D to 2D and modification from nanoparticles to single atoms have created a significant enhancement in the electrocatalytic activity. Hybridizing both the 2D matrix and SACs (2DM@SACs) creates a new electronic state in the materials, and that bequeaths with enhancing potentials toward the electrocatalytic activity. The strong covalent interaction between the 2D matrix and SACs tunes the intrinsic activity of the electrocatalysts. In this mini-review, we have discussed the different synthesis methods of 2DM@SACs with a focus on their electrochemical energy applications such as hydrogen evolution, oxygen evolution, oxygen reduction, and carbon dioxide reduction. This mini-review appraises the contribution to the rational proposal for the synthesis of perfect 2DM@SAC catalysts with their electrochemical properties toward energy conversion applications.

Covalent organic framework-supported platinum nanoparticles as efficient electrocatalysts for water reduction

The hydrogen evolution reaction (HER) is one of the most effective and sustainable ways to produce hydrogen gas as an alternative clean fuel. The rate of this electrocatalytic reaction is highly dependent on the properties (dispersity and stability) of electrocatalysts. Herein, we developed well-dispersed and highly stable platinum nanoparticles (PtNPs) supported on a covalent organic framework (COF-bpyTPP), which exhibit excellent catalytic activities toward HER as well as the hydride reduction reaction. The nanoparticles have an average size of 2.95 nm and show superior catalytic performance compared to the commercially available Pt/C under the same alkaline conditions, producing 13 times more hydrogen with a far more positive onset potential (-0.13 V vs.-0.63 V) and ca. 100% faradaic efficiency. The reaction rate of the hydride reduction of 4-nitrophenol was also 10 times faster in the case of PtNPs@COF compared to the commercial Pt/C under the same loading and conditions. More importantly, the PtNPs@COF are highly stable under the aqueous reactions conditions and can be reused without showing noticeable aggregation and activity degradation.

Recover the activity of sintered supported catalysts by nitrogen-doped carbon atomization

The sintering of supported metal nanoparticles is a major route to the deactivation of industrial heterogeneous catalysts, which largely increase the cost and decrease the productivity. Here, we discover that supported palladium/gold/platinum nanoparticles distributed at the interface of oxide supports and nitrogen-doped carbon shells would undergo an unexpected nitrogen-doped carbon atomization process against the sintering at high temperatures, during which the nanoparticles can be transformed into more active atomic species. The in situ transmission electron microscopy images reveal the abundant nitrogen defects in carbon shells provide atomic diffusion sites for the mobile atomistic palladium species detached from the palladium nanoparticles. More important, the catalytic activity of sintered and deactivated palladium catalyst can be recovered by this unique N-doped carbon atomization process. Our findings open up a window to preparation of sintering-resistant single atoms catalysts and regeneration of deactivated industrial catalysts.

Visualization Analysis of Pt and Co Species in Degraded Pt3Co/C Electrocatalyst Layers of a Polymer Electrolyte Fuel Cell Using a Same-View Nano-XAFS/STEM-EDS Combination Technique

In order to obtain a suitable design policy for the development of a next-generation polymer electrolyte fuel cell, we performed a visualization analysis of Pt and Co species following aging and degradation processes in membrane-electrode assembly (MEA), using a same-view. Nano-X-ray absorption fine structure (XAFS)/Scanning transmission electron microscope (STEM)-energy dispersive X-ray spectroscopy (EDS) technique that we developed to elucidate durability factors and degradation mechanisms of a MEA Pt₃Co/C cathode electrocatalyst with higher activity and durability than a MEA Pt/C. In the MEA Pt₃Co/C, after 5000 ADT-rec (rectangle accelerated durability test) cycles, unlike the MEA Pt/C, there was no oxidation of Pt. In contrast, Co oxidized and dissolved over a wide range of the cathode layer (∼70% of the initial Co amount). The larger the size of the cracks and pores in the MEA Pt/C and the smaller the ratio of Pt/ionomer of cracks and pores, the faster the rate of catalyst degradation. In contrast, there was no correlation between the size or Co/ionomer ratio of the cracks and pores and the Co dissolution of the MEA Pt₃Co/C. It was shown that Co dissolved in the electrolyte region had an octahedral Co²⁺-O₆ structure, based on a 150 nm × 150 nm nano-XAFS analysis. It was also shown that its existence suppressed the oxidation and dissolution of Pt. The MEA Pt₃Co/C after 10,000 ADT-rec cycles had many cracks and pores in the cathode electrocatalyst layer, and about 90% of Co had been dissolved and removed from the cathode layer. We discovered a metallic Pt-Co alloy band in the electrolyte region of 300-400 nm from the cathode edge and square planar Pt²⁺-O₄ species and octahedral Co²⁺-O₆ species in the area between the cathode edge and the Pt-Co band. The transition of Pt and Co chemical species in the Pt₃Co/C cathode electrocatalyst in the MEA during the degradation process, as well as a fuel cell deterioration suppression process by Co were visualized for the first time at the nano scale using the same-view nano-XAFS/STEM-EDS combination technique that can measure the MEA under a humid N₂ atmosphere while maintaining the working environment for a fuel cell.

Probing the active sites of site-specific nitrogen doping in metal-free graphdiyne for electrochemical oxygen reduction reactions

The development of highly active and low-cost catalysts for electrochemical reactions is one of the most attractive topics in the renewable energy technology. Herein, the site-specific nitrogen doping of graphdiyne (GDY) including grap-N, sp-N(I) and sp-N(II) GDY is systematically investigated as metal-free oxygen reduction electrocatalysts via density functional theory (DFT). Our results indicate that the doped nitrogen atom can significantly improve the oxygen (O₂) adsorption activity of GDY through activating its neighboring carbon atoms. The free-energy landscape is employed to describe the electrochemical oxygen reduction reaction (ORR) in both O₂ dissociation and association mechanisms. It is revealed that the association mechanism can provide higher ORR onset potential than dissociation mechanism on most of the substrates. Especially, sp-N(II) GDY exhibits the highest ORR electrocatalytic activity through increasing the theoretical onset potential to 0.76 V. This work provides an atomic-level insight for the electrochemical ORR mechanism on metal-free N-doped GDY.

Integration mesoporous surface and hollow cavity into PtPdRh nano-octahedra for enhanced oxygen reduction electrocatalysis

Design and synthesis of Pt-based nanocrystals with controlled structural diversity and complexity can potentially bring about multifunctional properties. In this work, we present a facile two-step strategy for the construction of the PtPdRh mesoporous octahedral nanocages (PtPdRh MONCs). This unique nanoarchitectonics rationally integrates multiple advantages (i.e. the octahedral shape, hollow cavity and mesoporous surface) into one catalyst, which facilitates the efficient utilization of noble metal atoms at both of the interior and exterior surfaces. As expected, the resultant PtPdRh MONCs could effectively catalyze the oxygen reduction reaction (ORR) under acidic conditions. The demonstrated ORR activity and catalytic durability are superior to the commercial Pt/C catalyst. The present study would provide a general guidance for architectural and compositional engineering of noble metal nanocrystals with desired functionalities and properties.

Origin of High Activity and Durability of Twisty Nanowire Alloy Catalysts under Oxygen Reduction and Fuel Cell Operating Conditions

The ability to control the surface composition and morphology of alloy catalysts is critical for achieving high activity and durability of catalysts for oxygen reduction reaction (ORR) and fuel cells. This report describes an efficient surfactant-free synthesis route for producing a twisty nanowire (TNW) shaped platinum-iron (PtFe) alloy catalyst (denoted as PtFe TNWs) with controllable bimetallic compositions. PtFe TNWs with an optimal initial composition of ∼24% Pt are shown to exhibit the highest mass activity (3.4 A/mg_Pt, ∼20 times higher than that of commercial Pt catalyst) and the highest durability (<2% loss of activity after 40 000 cycles and <30% loss after 120 000 cycles) among all PtFe-based nanocatalysts under ORR or fuel cell operating conditions reported so far. Using ex situ and in situ synchrotron X-ray diffraction coupled with atomic pair distribution function (PDF) analysis and 3D modeling, the PtFe TNWs are shown to exhibit mixed face-centered cubic (fcc)-body-centered cubic (bcc) alloy structure and a significant lattice strain. A striking finding is that the activity strongly depends on the composition of the as-synthesized catalysts and this dependence remains unchanged despite the evolution of the composition of the different catalysts and their lattice constants under ORR or fuel cell operating conditions. Notably, dealloying under fuel cell operating condition starts at phase-segregated domain sites leading to a final fcc alloy structure with subtle differences in surface morphology. Due to a subsequent realloying and the morphology of TNWs, the surface lattice strain observed with the as-synthesized catalysts is largely preserved. This strain and the particular facets exhibited by the TNWs are believed to be responsible for the observed activity and durability enhancements. These findings provide new insights into the correlation between the structure, activity, and durability of nanoalloy catalysts and are expected to energize the ongoing effort to develop highly active and durable low-Pt-content nanowire catalysts by controlling their alloy structure and morphology.

Nanomaterials in electrochemical cytosensors

Nano-electrochemical cytosensors have attracted intensive attention and achieved huge progress in the biomedical field owing to their stability, rapidity, accuracy, and low-cost properties.

Bonding–antibonding state transition induces multiple electron modulations toward oxygen reduction reaction electrocatalysis

B doping induces the transformation from the bonding state to the antibonding state of an Ni–N bond, resulting in enhanced ORR activity.

Degradation of the transition metal@Pt core–shell nanoparticle catalyst: a DFT study

Electrocatalysts in acidic media face the issues of inactivation and degradation with complex thermodynamic processes.

Customizable Ligand Exchange for Tailored Surface Property of Noble Metal Nanocrystals

It is highly desirable, while still challenging, to obtain noble metal nanocrystals with custom capping ligands, because their colloidal synthesis relies on specific capping ligands for the shape control while conventional ligand exchange processes suffer from "the strong replaces the weak" limitation, which greatly hinders their applications. Herein, we report a general and effective ligand exchange approach that can replace the native capping ligands of noble metal nanocrystals with virtually any type of ligands, producing flexibly tailored surface properties. The key is to use diethylamine with conveniently switchable binding affinity to the metal surface as an intermediate ligand. As a strong ligand, it in its original form can effectively remove the native ligands; while protonated, it loses its binding affinity and facilitates the adsorption of new ligands, especially weak ones, onto the metal surface. By this means, the irreversible order in the conventional ligand exchange processes could be overcome. The efficacy of the strategy is demonstrated by mutual exchange of the capping ligands among cetyltrimethylammonium, citrate, polyvinylpyrrolidone, and oleylamine. This novel strategy significantly expands our ability to manipulate the surface property of noble metal nanocrystals and extends their applicability to a wide range of fields, particularly biomedical applications.

Sub-3 nm Intermetallic Ordered Pt3In Clusters for Oxygen Reduction Reaction

Industrial applications of Pt-based oxygen-reduction-reaction (ORR) catalysts are limited by high cost and low stability. Here, facile large-scale synthesis of sub-3-nm ordered Pt3In clusters on commercial carbon black as ORR catalyst that alleviates both these shortcomings is reported. As-prepared Pt3In/C exhibits a mass activity of 0.71 mA mg-1 and a specific area activity of 0.91 mA cm-2 at 0.9 V vs reversible hydrogen electrode, which are 4.1 and 2.7 times the corresponding values of commercial Pt/C catalysts. The as-prepared ordered Pt3In/C catalyst is also remarkably stable with negligible activity and structural decay after 20 000 accelerated electrochemical durability cycles, due to its ordered structure. Density-functional-theory calculations demonstrate that ordered-Pt3In is more energetically favorable for ORR than the commercial Pt/C catalysts because ∆G O is closer to the peak of the volcano plot after ordered incorporation of indium atoms.

Visualization and understanding of the degradation behaviors of a PEFC Pt/C cathode electrocatalyst using a multi-analysis system combining time-resolved quick XAFS, three-dimensional XAFS-CT, and same-view nano-XAFS/STEM-EDS techniques

We developed a multi-analysis system that can measure in situ time-resolved quick XAFS and in situ three-dimensional XAFS-CT in the same area of a cathode electrocatalyst layer in a membrane-electrode assembly of a polymer electrolyte fuel cell.

Heterogeneous assembly of Pt-clusters on hierarchically structured CoOx@SnPd2@SnO2 quaternary nanocatalysts manifesting oxygen reduction reaction performance

Atomic Pt clusters in the heterogeneous interface of CoO_x@SnPd₂@SnO₂ possess high heteroatomic intermixing facilities, oxygen splitting and hydration reactions resulting in high performance ORR.

Multinary PtPdNiP truncated octahedral mesoporous nanocages for enhanced methanol oxidation electrocatalysis

Multinary PtPdNiP TOMNs have been synthesized for the electrocatalytic methanol oxidation reaction with a superior electrocatalytic performance.

A dinuclear cobalt cluster as electrocatalyst for oxygen reduction reaction

Dinuclear metal clusters as metalloenzymes execute efficient catalytic activities in biological systems. Enlightened by this, a dinuclear {CoII 2} cluster was selected to survey its ORR (Oxygen Reduction Reaction) catalytic activities. The crystalline {CoII 2} possesses defined structure and potential catalytic active centers of {CoN4O2} sites, which was identified by X-ray single crystal diffraction, Raman and XPS. The appropriate supramolecular porosity combining abundant pyridinic-N and triazole-N sites of {CoII 2} catalyst synergistically benefit the ORR performance. As a result, this non-noble metal catalyst presents a nice ORR electrocatalytic activity and abides by a nearly 4-electron reduction pathway. Thus, this unpyrolyzed crystalline catalyst clearly provide precise active sites and the whole defined structural information, which can help researcher to design and fabricate efficient ORR catalysts to improve their activities. Considering the visible crystal structure, a single cobalt center-mediated catalytic mechanism was also proposed to elucidate the ORR process.

The importance of nanoscale confinement to electrocatalytic performance

Electrocatalytic nanoparticles that mimic the three-dimensional geometric architecture of enzymes where the reaction occurs down a substrate channel isolated from bulk solution, referred to herein as nanozymes, were used to explore the impact of nano-confinement on electrocatalytic reactions. Surfactant covered Pt-Ni nanozyme nanoparticles, with Ni etched from the nanoparticles, possess a nanoscale channel in which the active sites for electrocatalysis of oxygen reduction are located. Different particle compositions and etching parameters allowed synthesis of nanoparticles with different average substrate channel diameters that have varying amounts of nano-confinement. The results showed that in the kinetically limited regime at low overpotentials, the smaller the substrate channels the higher the specific activity of the electrocatalyst. This is attributed to higher concentrations of protons, relative to bulk solution, required to balance the potential inside the nano-confined channel. However, at higher overpotentials where limitation by mass transport of oxygen becomes important, the nanozymes with larger substrate channels showed higher electrocatalytic activity. A reaction-diffusion model revealed that the higher electrocatalytic activity at low overpotentials with smaller substrate channels can be explained by the higher concentration of protons. The model suggests that the dominant mode of mass transport to achieve these high concentrations is by migration, exemplifying how nano-confinement can be used to enhance reaction rates. Experimental and theoretical data show that under mass transport limiting potentials, the nano-confinement has no effect and the reaction only occurs at the entrance of the substrate channel at the nanoparticle surface.

Hollow PdAg-CeO2 heterodimer nanocrystals as highly structured heterogeneous catalysts

In the present work, hollow PdAg-CeO₂ heterodimer nanocrystals (NCs) were prepared and tested as catalysts for the selective hydrogenation of alkynes. These nanostructures combine for the first time the beneficial effect of alloying Pd with Ag in a single NC hollow domain with the formation of active sites at the interface with the CeO₂ counterpart in an additive manner. The PdAg-CeO₂ NCs display excellent alkene selectivity for aliphatic alkynes. For the specific case of hydrogenation of internal alkynes such as 4-octyne, very low over-hydrogenation and isomerization products were observed over a full conversion regime, even after prolonged reaction times. These catalytic properties were remarkably superior in comparison to standard catalysts. The promotion of Ag on the moderation of the reactivity of the Pd phase, in combination with the creation of interfacial sites with the CeO₂ moiety in the same nanostructure, is pointed as the responsible of such a remarkable catalytic performance.

Hollow PtFe Alloy Nanoparticles Derived from Pt-Fe3 O4 Dimers through a Silica-Protection Reduction Strategy as Efficient Oxygen Reduction Electrocatalysts

The development of efficient and stable electrocatalysts for the oxygen reduction reaction (ORR) is critical for the large-scale production of fuel cells. Platinum (Pt) nanoparticle catalysts show excellent performance for ORR, though the high cost of Pt is a limiting factor that directly impacts fuel cell production costs. Alloying Pt with other transition metals is an effective strategy to reduce Pt utilization whilst maintaining good ORR performance. In this work, novel hollow PtFe alloy catalysts were successfully synthesized by high-temperature pyrolysis of SiO₂ -coated Pt-Fe₃ O₄ nanoparticle dimers supported on carbon at 900 °C, followed by SiO₂ shell removal and partial dealloying of the PtFe nanoparticles formed using HF. The obtained hollow PtFe nanoparticle catalysts (denoted herein as PtFe-900) showed a 2.3-fold enhancement in ORR mass activity compared to PtFe nanoparticles synthesized without SiO₂ protection, and a remarkable 7.8-fold enhancement relative to a commercial Pt/C catalyst. Further, after 10 000 potential cycles, the ORR mass activity of PtFe-900 remained very high (90.9 % of the initial mass activity). The outstanding ORR performance of PtFe-900 can be attributed to the modification of Pt lattice and electronic structure by alloying with Fe at high temperature under the protection of the SiO₂ coating. This work guides the development of improved, highly dispersed Pt-based alloy nanoparticle catalysts for ORR and fuel cell applications.

All-metallic nanorattles consisting of a Pt core and a mesoporous PtPd shell for enhanced electrocatalysis

The fabrication of nanorattles with controllable compositions and structures is very important for catalytic applications. Herein, we propose a facile method for synthesis of very unique all-metallic nanorattle consisting of a Pt core and a mesoporous PtPd shell (named Pt@mPtPd). Owing to its spatially and locally separated active inner Pt core and mesoporous PtPd shell, the Pt@mPtPd nanorattle shows the enhanced performance for oxygen reduction reaction. The newly designed Pt@mPtPd nanorattle is quite different from traditional nanorattles with porous carbon and silica shell in its catalytically functional mesoporous metallic shell. The proposed facile method is highly valuable for the design of all-metallic nanorattle with controllable compositions and desired functions.

Reactive nanotemplates for synthesis of highly efficient electrocatalysts: beyond simple morphology transfer

Efficient electrocatalysts for energy conversion in general, and fuel cell operation and water electrolysis in particular, are pivotal for carbon-free hydrogen production. While the requirements of successful electrocatalysts include a high number density of catalytically active sites, high surface-to-volume ratio, inherently high catalytic activity, and robustness of the catalyst surface structure under harsh operating conditions, it is extremely difficult to synthesize nanocatalysts that could possess all these structural characteristics. Nanotemplate-mediated synthesis, namely, the coating or filling of a template with a desired material phase followed by the removal of the template, has captured the interest of researchers because of the ease of creating hollow-structured nanocatalysts with a high surface to volume ratio. Recent studies, however, have revealed that nanotemplates could be more than just passive supports because they greatly affect catalytic performance by creating an unusual synergy between the substrate and catalyst and by providing dopants to the actual catalyst phase owing to their reactive nature. In this review, we discuss the most notable recent advances in the nanotemplate-based synthesis of electrocatalysts as well as the unusual effects of nanotemplates on the performance of nanocatalysts. We also provide an outlook for this fledgling field so that future research efforts could be focused on the development of practically useful electrocatalysts that could shape the future of energy technologies.

Leaching- and sintering-resistant hollow or structurally ordered intermetallic PtFe alloy catalysts for oxygen reduction reactions

Carbon supported Pt-based alloy materials that have been developed for proton exchange membrane fuel cells (PEMFCs) are vulnerable to deactivation due to the loss of non-noble metal components (leaching) or detachment, migration and aggregation of active nanoparticles (sintering). Until now some methods have been developed to inhibit leaching or sintering individually. However, a route able to avoid leaching and sintering simultaneously is still lacking. Herein, we develop a thermally driven interfacial diffusion alloying route that allows for the direct evolution of solid Pt nanoparticles (NPs) supported on carbon (Pt/C) into a Pt-skin-like hollow PtFe alloy or a structurally ordered intermetallic PtFe alloy, together with in situ encapsulation of PtFe alloy NPs with a thin layer porous nitrogen-doped carbon (NC) shell. The robust NC shells not only effectively prevent Pt-based NPs from detachment, migration, and aggregation during accelerated durability tests but also allow smoother access of electrolyte to the Pt surface, thus allowing the catalysts to well preserve their high catalytic activity. The well-defined shape and atomic arrangement of PtFe alloy NPs exhibit over 600% increase in mass activity and specific activity when compared with that of the pristine Pt/C catalyst. Stability tests confirm that the ordered PtFe alloy is more electrochemically stable than the disordered hollow PtFe alloy and Pt/C catalysts due to its ordered atomic arrangement and the robust NC shell.

Synthesis of low- and high-index faceted metal (Pt, Pd, Ru, Ir, Rh) nanoparticles for improved activity and stability in electrocatalysis

Driven by the quest for future energy solution, faceted metal nanoparticles are being pursued as the next generation electrocatalysts for renewable energy applications. Thanks to recent advancement in solution phase synthesis, different low- and high-index facets on metal nanocrystals become accessible and are tested for specific electrocatalytic reactions. This minireview summarises the key approaches to prepare nanocrystals containing the most catalytically active platinum group metals (Pt, Pd, Ru, Ir and Rh) exposed with low- and high-index facets using solution phase synthesis. Electrocatalytic studies related to the different facets are highlighted to emphasise the importance of exposing facets for catalysing these reactions, namely oxygen reduction reaction (ORR), hydrogen oxidation reaction (HOR), alcohol oxidation including methanol (MOR) and ethanol oxidation reactions (EOR), formic acid oxidation reaction (FAOR), oxygen evolution reaction (OER), and hydrogen evolution reaction (HER). The future outlook discusses the challenges and opportunities for making electrocatalysts that are even more active and stable.

Facile Synthesis of Quaternary Structurally Ordered L12-Pt(Fe, Co, Ni)3 Nanoparticles with Low Content of Platinum as Efficient Oxygen Reduction Reaction Electrocatalysts

Synthesis of electrocatalysts for oxygen reduction reaction (ORR) with not only prominent electrocatalytic performance but also a low amount of Pt is the urgent challenge in the popularization of fuel cells. In this work, through a facile synthetic strategy of spray dehydration on a solid surface and annealing process, we demonstrate the first manufacture of quaternary structurally ordered PtM₃ (M = transition metal) intermetallic nanoparticles (NPs), Pt(Fe, Co, Ni)₃, in order to lower the content of Pt. The atomic contents of Pt, Fe, Co, and Ni are equal and the chemical structure of Pt(Fe, Co, Ni)₃ is a cubic L1₂-ordered structure. L1₂-Pt(Fe, Co, Ni)₃/C electrocatalysts exhibit enhanced electrocatalytic performance toward ORR with mass activity (MA) 6.6 times higher than the commercial Pt/C and a minimal loss of 17% in MA and 1.5% loss in specific activity (SA) after 10 000 potential cycles at 0.9 V. Furthermore, the stability behavior is confirmed to be attributed to the coaction of particle sizes and the ordering effect. Compared with traditional Pt-based electrocatalysts in the stoichiometric forms of Pt₃M and PtM, L1₂-Pt(Fe, Co, Ni)₃ intermetallic NPs exhibit excellent performance and higher cost effectiveness. Moreover, this work also proposes a facile and effective synthetic strategy for manufacturing multicomponent Pt-based electrocatalysts for ORR.

Three-dimensional open nano-netcage electrocatalysts for efficient pH-universal overall water splitting

High-efficiency water electrolysis is the key to sustainable energy. Here we report a highly active and durable RuIrO_x (x ≥ 0) nano-netcage catalyst formed during electrochemical testing by in-situ etching to remove amphoteric ZnO from RuIrZnO_x hollow nanobox. The dispersing-etching-holing strategy endowed the porous nano-netcage with a high exposure of active sites as well as a three-dimensional accessibility for substrate molecules, thereby drastically boosting the electrochemical surface area (ECSA). The nano-netcage catalyst achieved not only ultralow overpotentials at 10 mA cm⁻² for hydrogen evolution reaction (HER; 12 mV, pH = 0; 13 mV, pH = 14), but also high-performance overall water electrolysis over a broad pH range (0 ~ 14), with a potential of mere 1.45 V (pH = 0) or 1.47 V (pH = 14) at 10 mA cm⁻². With this universal applicability of our electrocatalyst, a variety of readily available electrolytes (even including waste water and sea water) could potentially be directly used for hydrogen production.

Water electrolysis is considered a key reaction for future sustainable fuel generation. Here, authors report a three-dimensional RuIrOx nano-netcage catalyst that shows high activities and efficiencies for pH-universal overall water splitting.

Size, Composition, and Support-Doping Effects on Oxygen Reduction Activity of Platinum-Alloy and on Non-platinum Metal-Decorated-Graphene Nanocatalysts

Recent investigations reported in the open literature concerning the functionalization of graphene as a support material for transition metal nanoparticle catalysts have examined isolated systems for their potential Oxygen Reduction Reaction (ORR) activity. In this work we present results which characterize the ability to use functionalized graphene (via dopants B, N) to upshift and downshift the adsorption energy of mono-atomic oxygen, O* (the ORR activity descriptor on ORR Volcano Plots), for various compositions of 4-atom, 7-atom, and 19-atom sub-nanometer binary alloy/intermetallic transition metal nanoparticle catalysts on graphene (TMNP-MDG). Our results show several important and interesting features: (1) that the combination of geometric and electronic effects makes development of simple linear mixing rules for size/composition difficult; (2) that the transition from 4- to 7- to 19-atom TMNP on MDG has pronounced effects on ORR activity for all compositions; (3) that the use of B and N as dopants to modulate the graphene-TMNP electronic structure interaction can cause shifts in the oxygen adsorption energy of 0.5 eV or more; (4) that it might be possible to make specific doped-graphene-Ni_xCu_y TMNP systems which fall close to the Volcano Peak for ORR. Our results point to systems which should be investigated experimentally and may improve the viability of future fuel cell or other ORR applications, and provide new paths for future investigations of more detail for TMNP-MDG screening.

Atomically Dispersed Pt on Screw-like Pd/Au Core-shell Nanowires for Enhanced Electrocatalysis

Engineering noble metal nanostructures at the atomic level can significantly optimize their electrocatalytic performance and remarkably reduce their usage. We report the synthesis of atomically dispersed Pt on screw-like Pd/Au nanowires by using ultrafine Pd nanowires as seeds. Au can selectively grow on the surface of Pd nanowires by an island growth pattern to fabricate surface defect sites to load atomically dispersed Pt, which can be confirmed by X-ray absorption fine structure measurements and aberration corrected HRTEM images. The nanowires with 2.74 at % Pt exhibit superior HER properties in acidic solution with an overpotential of 20.6 mV at 10 mA cm^-2 and enhanced alkaline ORR performance with a mass activity over 15 times greater than the commercial platinum/carbon (Pt/C) catalysts.

Intermetallic Pd3Pb ultrathin nanoplate-constructed flowers with low-coordinated edge sites boost oxygen reduction performance

Although tremendous efforts have been devoted to exploring non-Pt based electrocatalysts toward the oxygen reduction reaction (ORR), achievements in both catalytic activity and durability are still far from satisfactory. Here, we report a facile approach for the synthesis of intermetallic Pd3Pb ultrathin nanoplate-constructed flowers. Such highly opened hierarchical nanostructures with an ordered phase and low-coordinated edge sites exhibited a substantially enhanced activity toward the ORR. Especially, the intermetallic Pd3Pb nanoflowers achieved a record-breaking mass activity (1.14 mA μgPd-1) in an alkaline solution at 0.9 V vs. a reversible hydrogen electrode among the reported Pd-based ORR electrocatalysts to date, which was 1.8, 3.9 and 11.4 times higher than those of intermetallic Pd3Pb nanocubes, Pd3Pb dendrites and commercial Pt/C, respectively. More importantly, the intermetallic Pd3Pb nanoflowers also showed a higher durability with only 23.7% loss in mass activity after 10 000 cycles compared to the commercial Pt/C (35% loss in mass activity) due to their chemically stable intermetallic structures.

Recent Advances on Controlled Synthesis and Engineering of Hollow Alloyed Nanotubes for Electrocatalysis

The past decade has witnessed great progress in the synthesis and electrocatalytic applications of 1D hollow alloy nanotubes with controllable compositions and fine structures. Hollow nanotubes have been explored as promising electrocatalysts in the fuel cell reactions due to their well-controlled surface structure, size, porosity, and compositions. In addition, owing to the self-supporting ability of 1D structure, hollow nanotubes are capable of avoiding catalyst aggregation and carbon corrosion during the catalytic process, which are two other issues for the widely investigated carbon-supported nanoparticle catalysts. It is currently a great challenge to achieve high activity and stability at a relatively low cost to realize commercialization of these catalysts. An overview of the structural and compositional properties of 1D hollow alloy nanotubes, which provide a large number of accessible active sites, void spaces for electrolytes/reactants impregnation, and structural stability for suppressing aggregation, is presented. The latest advances on several strategies such as hard template and self-templating methods for controllable synthesis of hollow alloyed nanotubes with controllable structures and compositions are then summarized. Benefiting from the advantages of the unique properties and facile synthesis approaches, the capability of 1D hollow nanotubes is then highlighted by discussing examples of their applications in fuel-cell-related electrocatalysis. Finally, the remaining challenges and potential solutions in the field are summarized to provide some useful clues for the future development of 1D hollow alloy nanotube materials.

Functionalization of Hollow Nanomaterials for Catalytic Applications: Nanoreactor Construction

Hollow nanomaterials have attracted a broad interest in multidisciplinary research due to their unique structure and preeminent properties. Owing to the high specific surface area, well-defined active site, delimited void space, and tunable mass transfer rate, hollow nanostructures can serve as excellent catalysts, supports, and reactors for a variety of catalytic applications, including photocatalysis, electrocatalysis, heterogeneous catalysis, homogeneous catalysis, etc. Based on state-of-the-art synthetic methods and characterization techniques, researchers focus on the purposeful functionalization of hollow nanomaterials for catalytic mechanism studies and intricate catalytic reactions. Herein, an overview of current reports with respect to the catalysis of functionalized hollow nanomaterials is given, and they are classified into five types of versatile strategies with a top-down perspective, including textual and composition modification, encapsulation, multishelled construction, anchored single atomic site, and surface molecular engineering. In the detailed case studies, the design and construction of hierarchical hollow catalysts are discussed. Moreover, since hollow structure offers more than two types of spatial-delimited sites, complicated catalytic reactions are elaborated. In summary, functionalized hollow nanomaterials provide an ideal model for the rational design and development of efficient catalysts.

Catalytic System Based on Sub-2 nm Pt Particles and Its Extraordinary Activity and Durability for Oxygen Reduction

Carbon-supported Pt nanoparticles are used as catalysts for a variety of reactions including the oxygen reduction reaction (ORR) key to proton-exchange membrane fuel cells, but their catalytic performance has long been plagued by detachment and sintering. Here we report the in situ growth of sub-2 nm Pt particles on a commercial carbon support via the galvanic reaction between a Pt(II) precursor and a uniform film of amorphous Se predeposited on the support. The residual Se could serve as a linker to strongly anchor the Pt nanoparticles to the carbon surface, leading to a catalytic system with extraordinary activity and durability toward ORR. Even after 20 000 cycles of accelerated durability test, the sub-2 nm Pt particles were still dispersed well on the carbon support and maintained a mass activity more than three-times as high as the pristine value of a commercial Pt/C catalyst.