Impact of Transition Metal Carbide and Nitride Supports on the Electronic Structure of Thin Platinum Overlayers

Boosting the Hydrogen Evolution Reaction Performance of P-Doped PtTe2 Nanocages via Spontaneous Defects Formation

PtTe2 , a member of the noble metal dichalcogenides (NMDs), has aroused great interest in exploring its behavior in the hydrogen evolution reaction (HER) due to the unique type-II topological semimetallic nature. In this work, a simple template-free hydrothermal method to obtain the phosphorus-doped (P-doped) PtTe2 nanocages with abundant amorphous and crystalline interface (A/C-P-PtTe2 ) is developed. Revealed by density functional theory calculations, the atomic Te vacancies can spontaneously form on the basal planes of PtTe2 by the P doping, which results in the unsaturated Pt atoms exposed as the active sites in the amorphous layer for HER. Owing to the defective structure, the A/C-P-PtTe2 catalysts have the fast Tafel step determined kinetics in HER, which contributes to an ultralow overpotential (η = 28 mV at 10 mA cm-2 ) and a small Tafel slope of 37 mV dec-1 . More importantly, benefiting from the inner stable crystalline P-PtTe2 nanosheets, limited decay of the performance is observed after chronopotentiometry test. This work reveals the important role of the inherent relationship between structure and activity in PtTe2 for HER, which may bring another enlightenment for the design of efficient catalysts based on NMDs in the near future.

Microwave Synthesis of Pt Clusters on Black TiO2 with Abundant Oxygen Vacancies for Efficient Acidic Electrocatalytic Hydrogen Evolution

Oxygen vacancies-enriched black TiO₂ is one promising support for enhancing hydrogen evolution reaction (HER). Herein, oxygen vacancies enriched black TiO₂ supported sub-nanometer Pt clusters (Pt/TiO₂ -O_V ) with metal support interactions is designed through solvent-free microwave and following low-temperature electroless approach for the first time. High-temperature and strong reductants are not required and then can avoid the aggregation of decorated Pt species. Experimental and theoretical calculation verify that the created oxygen vacancies and Pt clusters exhibit synergistic effects for optimizing the reaction kinetics. Based on it, Pt/TiO₂ -O_V presents remarkable electrocatalytic performance with 18 mV to achieve 10 mA cm^-2 coupled with small Tafel slope of 12 mV dec^-1 . This work provides quick synthetic strategy for preparing black titanium dioxide based nanomaterials.

Direct identification of the charge state in a single platinum nanoparticle on titanium oxide

A goal in the characterization of supported metal catalysts is to achieve particle-by-particle analysis of the charge state strongly correlated with the catalytic activity. Here, we demonstrate the direct identification of the charge state of individual platinum nanoparticles (NPs) supported on titanium dioxide using ultrahigh sensitivity and precision electron holography. Sophisticated phase-shift analysis for the part of the NPs protruding into the vacuum visualized slight potential changes around individual platinum NPs. The analysis revealed the number (only one to six electrons) and sense (positive or negative) of the charge per platinum NP. The underlying mechanism of platinum charging is explained by the work function differences between platinum and titanium dioxide (depending on the orientation relationship and lattice distortion) and by first-principles calculations in terms of the charge transfer processes.

Crystalline Lattice-Confined Atomic Pt in Metal Carbides to Match Electronic Structures and Hydrogen Evolution Behaviors of Platinum

Platinum-based catalysts occupy a pivotal position in diverse catalytic applications in hydrogen chemistry and electrochemistry, for instance, the hydrogen evolution reactions (HER). While adsorbed Pt atoms on supports often cause severe mismatching on electronic structures and HER behaviors from metallic Pt due to the different energy level distribution of electron orbitals. Here, the design of crystalline lattice-confined atomic Pt in metal carbides using the Pt-centered polyoxometalate frameworks with strong PtO-metal covalent bonds is reported. Remarkably, the lattice-confined atomic Pt in the tungsten carbides (Pt_doped @WC_x , both Pt and W have atomic radii of 1.3 Å) exhibit near-zero valence states and similar electronic structures as metallic Pt, thus delivering matched energy level distributions of the Pt 5d_z 2 and H 1s orbitals and similar acidic hydrogen evolution behaviors. In alkaline conditions, the Pt_doped @WC_x exhibits 40 times greater mass activity (49.5 A mg_Pt ^-1 at η = 150 mV) than the Pt@C because of the favorable water dissociation and H* transport. These findings offer a universal pathway to construct urgently needed atomic-scale catalysts for broad catalytic reactions.

Advanced Pt-Based Core-Shell Electrocatalysts for Fuel Cell Cathodes

ConspectusProton-exchange membrane fuel cells (PEMFCs) are highly efficient energy storage and conversion devices. Thus, the platinum group metal (PGM)-based catalysts which are the dominant choice for the PEMFCs have received extensive interest during the past couple of decades. However, the drawbacks in the existing PGM-based catalysts (i.e., high cost, slow kinetics, poor stability, etc.) still limit their applications in fuel cells. The Pt-based core-shell catalysts potentially alleviate these issues through the low Pt loading with the associated low cost and the high corrosion resistance and further improve the oxygen reduction reaction's (ORR's) activity and stability. This Account focuses on the synthetic strategies, catalytic mechanisms, factors influencing enhanced ORR performance, and applications in PEMFCs for the Pt-based core-shell catalysts. We first highlight the synthetic strategies for Pt-based core-shell catalysts including the galvanic displacement of an underpotentially deposited non-noble metal monolayer, thermal annealing, and dealloying methods, which can be scaled-up to meet the requirements of fuel cell operations. Subsequently, catalytic mechanisms such as the self-healing mechanism in the Pt monolayer on Pd core catalysts, the pinning effect of nitrogen (N) dopants in N-doped PtNi core-shell catalysts, and the ligand effect of the ordered intermetallic structure in L1₀-Pt/CoPt core-shell catalysts and their synergistic effects in N-doped L1₀-PtNi catalysts are described in detail. The core-shell structure in the Pt-based catalysts have two main effects for enhanced ORR performance: (i) the interaction between Pt shells and core substrates can tune the electronic state of the surface Pt, thus boosting the ORR activity and stability, and (ii) the outer Pt shell with modest thickness can enhance the oxidation and dissolution resistance of the core, resulting in improved durability. We then review the recent attempts to optimize the ORR performance of the Pt-based core-shell catalysts by considering the shape, composition, surface orientation, and shell thickness. The factors influencing the ORR performance can be grouped into two categories: the effect of the core and the effect of the shell. In the former, PtM core-shell catalysts which use different non-PGM element cores (M) are summarized, and in the latter, Pt-based core-shell catalysts with different shell structures and compositions are described. The modifications of the core and/or shell structure can not only optimize the intermediate-binding energetics on the Pt surface through tuning the strain of the surface Pt, which increases the intrinsic activity and stability, but also offer a significantly decreased catalyst cost. Finally, we discuss the membrane electrode assembly performance of Pt-based core-shell catalysts in fuel cell cathodes and evaluate their potential in real PEMFCs for light-duty and heavy-duty vehicle applications. Even though some challenges to the activity and lifetime in the fuel cells remain, the Pt-based core-shell catalysts are expected to be promising for many practical PEMFC applications.

Electronic Modulation of Pt Nanoparticles on Ni3N-Mo2C by Support-Induced Strategy for Accelerating Hydrogen Oxidation and Evolution

Electrochemical energy conversion and storage through hydrogen has revolutionized sustainable energy systems using fuel cells and electrolyzers. Regrettably, the sluggish alkaline hydrogen oxidation reaction (HOR) hampers advances in fuel cells. Herein, we report a Pt/Ni₃N-Mo₂C bifunctional electrocatalyst toward HOR and hydrogen evolution reaction (HER). The Pt/Ni₃N-Mo₂C exhibits remarkable HOR/HER performance in alkaline media. The mass activity at 50 mV and exchange current density of HOR are 5.1 and 1.5 times that of commercial Pt/C, respectively. Moreover, it possesses an impressive HER activity with an overpotential of 11 mV @ 10 mA cm^-2, which is lower than that of Pt/C and most reported electrocatalysts under the same conditions. Density functional theory (DFT) calculations combined with experimental results reveal that Pt/Ni₃N-Mo₂C not only possesses an optimal balance between hydrogen binding energy (HBE) and OH^- adsorption but also facilitates water adsorption and dissociation on the catalyst surface, which contribute to the excellent HOR/HER performance. Thus, this work may guide bifunctional HOR/HER catalyst design in the conversion and transport of energy.

Hydrogen-Mediated Thin Pt Layer Formation on Ni3N Nanoparticles for the Oxygen Reduction Reaction

A simple wet-chemical route for the preparation of core-shell-structured catalysts was developed to achieve high oxygen reduction reaction (ORR) activity with a low Pt loading amount. Nickel nitride (Ni₃N) nanoparticles were used as earth-abundant metal-based cores to support thin Pt layers. To realize the site-selective formation of Pt layers on the Ni₃N core, hydrogen molecules (H₂) were used as a mild reducing agent. As H₂ oxidation is catalyzed by the surface of Ni₃N, the redox reaction between H₂ and Pt(IV) in solution was facilitated on the Ni₃N surface, which resulted in the selective deposition of Pt on Ni₃N. The controlled Pt formation led to a subnanometer (0.5-1 nm)-thick Pt shell on the Ni₃N core. By adopting the core-shell structure, higher ORR activity than the commercial Pt/C was achieved. Electrochemical measurements showed that the thin Pt layer on Ni₃N nanoparticle exhibits 5 times higher mass activity and specific activity than that of commercial Pt/C. Furthermore, it is expected that the proposed simple wet-chemical method can be utilized to prepare various transition-metal-based core-shell nanocatalysts for a wide range of energy conversion reactions.

Titanium Nitride-Supported Platinum with Metal-Support Interaction for Boosting Photocatalytic H2 Evolution of Indium Sulfide

Metal-support interaction strongly influences the catalytic properties of metal-based catalysts. Here, titanium nitride (TiN) nanospheres are shown to be an outstanding support, for tuning the electronic property of platinum (Pt) nanoparticles and adjusting the morphology of indium sulfide (In₂S₃) active components, forming flower-like core-shell nanostructures (TiN-Pt@In₂S₃). The strong metal-support interaction between Pt and TiN through the formation of Pt-Ti bonds favors the migration of charge carriers and leads to the easy reducibility of TiN-Pt, thus improving the photocatalytic atom efficiency of Pt. The TiN-Pt@In₂S₃ composite shows reduction of Pt loading by 70% compared to the optimal Pt-based system. In addition, the optimal TiN-Pt@In₂S₃ composite exhibits a H₂ evolution rate 4 times that of a Pt reference. This increase outperforms all other supports reported thus far.

Surface activation by electron scavenger metal nanorod adsorption on TiH2, TiC, TiN, and Ti2O3

Metal/oxide support perimeter sites are known to provide unique properties because the nearby metal changes the local environment on the support surface. In particular, the electron scavenger effect reduces the energy necessary for surface anion desorption, and thereby contributes to activation of the (reverse) Mars-van Krevelen mechanism. This study investigated the possibility of such activation in hydrides, carbides, nitrides, and sulfides. The work functions (WFs) of known hydrides, carbides, nitrides, oxides, and sulfides with group 3, 4, or 5 cations (Sc, Y, La, Ti, Zr, Hf, V, Nb, and Ta) were calculated. The WFs of most hydrides, carbides, and nitrides are smaller than the WF of Ag, implying that the electron scavenger effect may occur when late transition metal nanoparticles are adsorbed on the surface. The WF of oxides and sulfides decreases when reduced. The surface anion vacancy formation energy correlates well with the bulk formation energy in carbides and nitrides, while almost no correlation is found in hydrides because of the small range of surface hydrogen vacancy formation energy values. The electron scavenger effect is explicitly observed in nanorods adsorbed on TiH2 and Ti2O3; the surface vacancy formation energy decreases at anion sites near the nanorod, and charge transfer to the nanorod happens when an anion is removed at such sites. Activation of hydrides, carbides, and nitrides by nanorod adsorption and screening support materials through WF calculation are expected to open up a new category of supported catalysts.

Core-shell nanostructured electrocatalysts for water splitting

As the cornerstone of the hydrogen economy, water electrolysis consisting of the hydrogen and oxygen evolution reactions (HER and OER) greatly needs cost-efficient electrocatalysts that can decrease the dynamic overpotential and save on energy consumption. Over past years, observable progress has been made by constructing core-shell structures free from or with few noble-metals. They afford particular merits, e.g., a highly-exposed active surface, modulated electronic configurations, strain effects, interfacial synergy, or reinforced stability, to promote the kinetics and electrocatalytic performance of the HER, OER and overall water splitting. So far, a large variety of inorganics (carbon and transition-metal related components) have been introduced into core-shell electrocatalysts. Herein, representative efforts and progress are summarized with a clear classification of core and shell components, to access comprehensive insights into electrochemical processes that proceed on surfaces or interfaces. Finally, a perspective on the future development of core-shell electrocatalysts is offered. The overall aim is to shed some light on the exploration of emerging materials for energy conversion and storage.

ZnO twin-rods decorated with Pt nanoparticles for butanone detection

ZnO twin-rods were synthesized using a combination of the ultrasonic spray nozzle and microwave-assisted hydrothermal methods. The VOC detection test revealed that the decoration with 2% of Pt provides a more sensitive and selective butanone sensor.