Modification of the surface electronic and chemical properties of Pt(111) by subsurface 3d transition metals

Reversing the Interfacial Electric Field in Metal Phosphide Heterojunction by Fe-Doping for Large-Current Oxygen Evolution Reaction

Developing non-precious-metal electrocatalysts that can operate with a low overpotential at a high current density for industrial application is challenging. Heterogeneous bimetallic phosphides have attracted much interest. Despite high hydrogen evolution reaction (HER) performance, the ordinary oxygen evolution reaction (OER) performance hinders their practical use. Herein, it is shown that Fe-doping reverses and enlarges the interfacial electrical field at the heterojunction, turning the H intermediate favorable binding sites for HER into O intermediate favorable sites for OER. Specifically, the self-supported heterojunction catalysts on nickel foam (CoP@Ni2P/NF and Fe-CoP@Fe-Ni2P/NF) are readily synthesized. They only require the overpotentials of 266 and 274 mV to drive a large current density of 1000 mA cm-2 (j1000) for HER and OER, respectively. Furthermore, a water splitting cell equipped with these electrodes only requires a voltage of 1.724 V to drive j1000 with excellent durability, demonstrating the potential of industrial application. This work offers new insights on interfacial engineering for heterojunction catalysts.

Strain Engineering of Unconventional Crystal-Phase Noble Metal Nanocatalysts

The crystal phase, alongside the composition, morphology, architecture, facet, size, and dimensionality, has been recognized as a critical factor influencing the properties of noble metal nanomaterials in various applications. In particular, unconventional crystal phases can potentially enable fascinating properties in noble metal nanomaterials. Recent years have witnessed notable advances in the phase engineering of nanomaterials (PEN). Within the accessible strategies for phase engineering, the effect of strain cannot be ignored because strain can act not only as the driving force of phase transition but also as the origin of the diverse physicochemical properties of the unconventional crystal phase. In this review, we highlight the development of unconventional crystal-phase noble metal nanomaterials within strain engineering. We begin with a short introduction of the unconventional crystal phase and strain effect in noble metal nanomaterials. Next, the correlations of the structure and performance of strain-engineered unconventional crystal-phase noble metal nanomaterials in electrocatalysis are highlighted, as well as the phase transitions of noble metal nanomaterials induced by the strain effect. Lastly, the challenges and opportunities within this rapidly developing field (i.e., the strain engineering of unconventional crystal-phase noble metal nanocatalysts) are discussed.

Effects of Zr dopants on properties of PtNi nanoparticles for ORR catalysis: A DFT modeling

Pt-based alloys, such as Pt3Ni, are among the best electrocatalysts for oxygen reduction reaction (ORR) in polymer electrolyte membrane fuel cells. Doping of PtNi alloys with Zr was shown to enhance the durability of the operating ORR catalysts. Rationalizing these observations is hindered by the absence of atomic-level data for these tri-metallic materials, even when not exposed to the fuel cell operation conditions. This study aims at understanding structure-property relations in Zr-doped PtNi nanoparticles as a key to their ORR function. In particular, we calculated, using a method based on density functional theory, the most stable chemical orderings of pristine and Zr-doped Pt3Ni particles containing over 400 atoms. We thus clarify (i) preferential location and charge states of Zr atoms in the Pt3Ni NPs; (ii) effect of doping Zr atoms on the stability of the Pt skin of the Pt3Ni NPs; (iii) charge redistribution induced by Zr dopants; (iv) layer-by-layer atomic ordering in the Pt3Ni/Zr NPs with the increasing Zr content; and (v) effect of Zr atoms on the adsorption energies of O and OH species as indicators of the ORR activity.

Synthesis of PtNi Nanoparticles to Accelerate the Oxygen Reduction Reaction

We report the synthesis of core-shell Ni-Pt nanoparticles (NPs) with varying degrees of crystallographic facets and surface layers rich in Pt via a seed-mediated thermolytic approach. Mixtures of different surfactants used during synthesis resulted in preferential surface passivation, which in turn dictated the size, chemical composition, and geometric evolution of these PtNi NPs. Electrochemical investigations of these pristine core-shell Ni-Pt structures in the oxygen reduction reaction (ORR) show that their catalytic functionalities outperform the commercial Pt/C reference catalyst. The enhanced electrocatalytic ORR performances of these Pt-based PtNi NPs are correlated with the weakened oxygen binding strength or surface-adsorbed hydroxyl (OH) species on active Pt surface sites induced by the downshift of the d-band center as a result of compressive strain effects. Our studies offer a robust synthetic approach for the development of core-shell nanostructures for enhanced ORR catalysis.

Correlation between d Electrons and the Sweet Spot for the Hydrogen Evolution Reaction: Is Platinum Always the Best Electrocatalyst?

Herein, two Laves intermetallic series, ZrCo1.75M0.25 and NbCo1.75M0.25 (M = Fe, Co, Ni, Ru, Rh, Pd, Os, Ir, and Pt), were synthesized, and their hydrogen evolution reaction (HER) activities were examined to reveal the influence of d electrons to the corresponding HER activities. Owing to the different electronegativity between Zr and Nb (χZr = 1.33; χNb = 1.60), Co and/or M elements receive more electrons in ZrCo1.75M0.25 than that of the Nb one. This leads to the overall weak H adsorption energy (ΔGHad) of ZrCo1.75M0.25 series compared to that of NbCo1.75M0.25 and rationalizes well the superior HER activity of the Rh member compared to that of the Pt one in the ZrCo1.75M0.25 series. Under industrial conditions (333 K, 6.0 M KOH), ZrCo1.75Rh0.25 only requires an overpotential of 110 mV to reach the current density of 500 mA/cm2 and can be operated at high current density over 400 h. This work demonstrates that with a proper combination between elements in intermetallic phases, one can manipulate d electrons of the active metal to be closer to the sweet spot (ΔGHad = 0). The Pt member may no longer exhibit the best HER activity in series, and all elements exhibit the potential to outperform the Pt member in the HER with careful control of the d electron population.

Unraveling a volcanic relationship of Co/N/C@PtxCo catalysts toward oxygen electro-reduction

The cathodic oxygen reduction reaction (ORR) has been continuously attracting worldwide interest due to the increasing popularity of proton exchange membrane (PEM) fuel cells. So far, various Pt-group metal (PGM) or PGM-free catalysts have been developed to facilitate the ORR. However, there is still a gap to achieve the expected goals as proposed by the U.S. Department of Energy (DoE). Recently, PGM-free@PGM hybrid catalysts, such as the M/N/C@PtM catalyst, have achieved the milestones of oxygen reduction, as reviewed in our recent work. It is, nevertheless, still challenging to unravel the underlying structure-property relationships. Here, by applying different Pt/Co ratios, a series of Co/N/C@PtxCo catalysts are synthesized. Interestingly, the ORR activity and stability are not linear with the Pt content, but show a volcano-like curve with increased Pt usage. This relationship has been deeply unraveled to be closely related to the contents of pyrrolic N, pyridinic N, and graphitized carbon in catalysts. This work provides guidelines to rationally design the coupled PGM-free@PGM catalysts toward the ORR by appropriate surface engineering.

Boosting Bifunctional Catalysis by Integrating Active Faceted Intermetallic Nanocrystals and Strained Pt-Ir Functional Shells

PtIr-based nanostructures are fascinating materials for application in bifunctional oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) catalysis. However, the fabrication of PtIr nanocatalysts with clear geometric features and structural configurations, which are crucial for enhancing the bifunctionality, remains challenging. Herein, PtCo@PtIr nanoparticles are precisely designed and fabricated with a quasi-octahedral PtCo nanocrystal as a highly atomically ordered core and an ultrathin PtIr atomic layer as a compressively strained shell. Owing to their geometric and core-shell features, the PtCo@PtIr nanoparticles deliver approximately six and eight times higher mass and specific activities, respectively, as an ORR catalyst than a commercial Pt/C catalyst. The half-wave potential of PtCo@PtIr exhibits a negligible decrease by 9 mV after 10 000 cycles, indicating extraordinary ORR durability because of the ordered arrangement of Pt and Co atoms. When evaluated using the ORR-OER dual reaction upon the introduction of Ir, PtCo@PtIr exhibits a small ORR-OER overpotential gap of 679 mV, demonstrating its great potential as a bifunctional electrocatalyst for fabricating fuel cells. The findings pave the way for designing precise intermetallic core-shell nanocrystals as highly functional catalysts.

Pt Single-Atoms on Structurally-Integrated 3D N-Doped Carbon Tubes Grid for Ampere-Level Current Density Hydrogen Evolution

To date, the excellent mass-catalytic activities of Pt single-atoms catalysts (Pt-SACs) toward hydrogen evolution reaction (HER) are categorically confirmed; however, their high current density performance remains a challenge for practical applications. Here, a binder-free approach is exemplified to fabricate self-standing superhydrophilic-superaerphobic Pt-SACs cathodes by directly anchoring Pt-SAs via Pt-Nx C4-x coordination bonds to the structurally-integrated 3D nitrogen-doped carbon tubes (N-CTs) array grid (denoted as Pt@N-CTs). The 3D Pt@N-CTs cathode with optimal Pt-SACs loading is capable of operating at a high current density of 1000 mA cm-2 with an ultralow overpotential of 157.9 mV with remarkable long-term stability over 11 days at 500 mA cm-2 . The 3D super-wettable free-standing Pt@N-CTs possess interconnected vertical and lateral N-CTs with hierarchical-sized open channels, which facilitates the mass transfer. The binder-free immobilization adding to the large surface area and 3D-interconnected open channels endow Pt@N-CTs cathodes with high accessible active sites, electrical conductivity, and structural stability that maximize the utilization efficiency of Pt-SAs to achieve ampere-level current density HER at low overpotentials.

Multifunctional molybdenum-tuning porous nickel-cobalt bimetallic phosphide nanoarrays for efficient water splitting and energy-saving hydrogen production

The sluggish kinetics of the hydrogen evolution reaction (HER) and substantial barriers in the oxygen evolution reaction (OER) significantly impede its application in hydrogen production. To address this issue and enhance energy efficiency in hydrogen generation, we explored a high-activity alkaline HER catalyst while concurrently coupling it with the urea oxidation reaction (UOR). In this work, we designed and synthesized porous molybdenum (Mo)-modulated nickel-cobalt bimetallic phosphide nanoarrays (M0.3NCP@NF). This multifunctional self-supported electrocatalyst demonstrates superior performance in HER, OER, and UOR. The introduction of Mo, in the form of CoMoO4 nanoparticles, promotes interfacial electron transfer to reduce the electron density around the cations in phosphides, enhancing the kinetics and intrinsic activity. Furthermore, the morphological changes induced by Mo accelerate both electron and mass transfer processes. Density functional theory and operando electrochemical impedance spectroscopy indicate that Mo introduction optimizes the interaction with HER intermediate H*, facilitating the conversion to a high-valent active intermediate for OER and accelerating UOR kinetics. Benefiting from dual optimization of morphology and structure, the as-prepared M0.3NCP@NF electrocatalyst demonstrates outstanding HER, OER, and UOR performances. Notably, a full urea electrolysis device powered by M0.3NCP@NF operates with a cell voltage of only 1.53 V to achieve a current density of 100 mA cm-2. which is 240 mV lower than that of conventional water electrolysis, demonstrating the competitive potential of our approach for efficient and energy-saving hydrogen production, along with simultaneous urea wastewater remediation.

Single-Component Hydrophilic Terpolymer Thin Film Systems for Imparting Surface Chemical Versatility on Various Substrates

We demonstrate a single-component hydrophilic photocrosslinkable copolymer system that incorporates all critical functionalities into one chain. This design allows for the creation of uniform functional organic coatings on a variety of substrates. The copolymers were composed of a poly(ethylene oxide)-containing monomer, a monomer that can release a primary amine upon UV light, and a monomer with reactive epoxide or cyclic dithiocarbonate with a primary amine. These copolymers are easily incorporated into the solution-casting process using polar solvents. Furthermore, the resulting coating can be readily stabilized through UV light-induced crosslinking, providing an advantage for controlling the surface properties of various substrates. The photocrosslinking capability further enables us to photolithographically define stable polymer domains in a desirable region. The resulting copolymer coatings were chemically versatile in immobilizing complex molecules by (i) post-crosslinking functionalization with the reactive groups on the surface and (ii) the formation of a composite coating by mixing varying amounts of a protein of interest, i.e., fish skin gelatin, which can form a uniform dual crosslinked network. The number of functionalization sites in a thin film could be controlled by tuning the composition of the copolymers. In photocrosslinking and subsequent functionalizations, we assessed the reactivity of the epoxide and cyclic dithiocarbonate with the generated primary amine. Moreover, the orthogonality of the possible reactions of the presented reactive functionalities in the crosslinked thin films with complex molecules is assessed. The resulting copolymer coatings were further utilized to define a hydrophobic surface or an active surface for the adhesion of biological objects.

Lattice Strain and Composition Effects on the Methanol Oxidation Performance of Platinum-Ruthenium-Nickel Ternary Nanocatalysts

Ternary Pt-based structures are a positive progress in addressing the disadvantages of monometallic and bimetallic Pt-based alloys for the electrochemical oxidation process of simple alcohols, which is a vital half-reaction in fuel cell technologies. We herein report a facile NaBH4-assisted ethylene glycol reduction process for fabricating a series of nanosized PtRuNi ternary alloys to explore the relationship between physicochemical properties and electrocatalytic behaviors for the acidic methanol oxidation reaction (MOR). Owing to a balance between lattice strain and synergistic effects, the Pt60Ru20Ni20/C electrocatalyst shows the highest MOR efficiency with the mass activity/specific activity of 844.48 mA mgMetal-1/1.93 mA cm-2, being a 1.94 and 2.38 times increase compared to those of the PtRu catalyst, respectively. Also, the Pt60Ru20Ni20/C catalyst possesses superior CO-tolerance and durability in strongly acidic electrolytes. This work suggests that optimizing the surface strain and electronic effects can boost the overall MOR efficiency of multicomponent Pt-based materials, which can help to further develop next-generation catalysts for energy conversion-related technologies.

Role of Transition Metals in Pt Alloy Catalysts for the Oxygen Reduction Reaction

In pursuit of higher activity and stability of electrocatalysts toward the oxygen reduction reaction, it has become standard practice to alloy platinum in various structural configurations. Transition metals have been extensively studied for their ability to tune catalyst functionality through strain, ligand, and ensemble effects. The origin of these effects and potential for synergistic application in practical materials have been the subject of many theoretical and experimental analyses in recent years. Here, a comprehensive overview of these phenomena is provided regarding the impact on reaction mechanisms and kinetics through combined experimental and theoretical approaches. Experimental approaches to electrocatalysis are discussed.

Tuning Binding Strength of Multiple Intermediates towards Efficient pH-universal Electrocatalytic Hydrogen Evolution by Mo8 O26 -NbNx Oy Heterocatalysts

Developing efficient and robust hydrogen evolution reaction (HER) catalysts for scalable and sustainable hydrogen production through electrochemical water splitting is strategic and challenging. Herein, heterogeneous Mo8 O26 -NbNx Oy supported on N-doped graphene (defined as Mo8 O26 -NbNx Oy /NG) is synthesized by controllable hydrothermal reaction and nitridation process. The O-exposed Mo8 O26 clusters covalently confined on NbNx Oy nanodomains provide a distinctive interface configuration and appropriate electronic structure, where fully exposed multiple active sites give excellent HER performance beyond commercial Pt/C catalyst in pH-universal electrolytes. Theoretical studies reveal that the Mo8 O26 -NbNx Oy interface with electronic reconstruction affords near-optimal hydrogen adsorption energy and enhanced initial H2 O adsorption. Furthermore, the terminal O atoms in Mo8 O26 clusters cooperate with Nb atoms to promote the initial H2 O adsorption, and subsequently reduce the H2 O dissociation energy, accelerating the entire HER kinetics.

Bromine-Enhanced Generation and Epoxidation of Ethylene in Tandem CO2 Electrolysis Towards Ethylene Oxide

The indirect electro-epoxidation of ethylene (C2 H4 ), produced from CO2 electroreduction (CO2 R), holds immense promise for CO2 upcycling to valuable ethylene oxide (EO). However, this process currently has a mediocre Faradaic efficiency (FE) due to sluggish formation and rapid dissociation of active species, as well as reductive deactivation of Cu-based electrocatalysts during the conversion of C2 H4 to EO and CO2 to C2 H4 , respectively. Herein, we report a bromine-induced dual-enhancement strategy designed to concurrently promote both C2 H4 -to-EO and CO2 -to-C2 H4 conversions, thereby improving EO generation, using single-atom Pt on N-doped CNTs (Pt1 /NCNT) and Br- -bearing porous Cu2 O as anode and cathode electrocatalysts, respectively. Physicochemical characterizations including synchrotron X-ray absorption, operando infrared spectroscopy, and quasi in situ Raman spectroscopy/electron paramagnetic resonance with theoretical calculations reveal that the favorable Br2 /HBrO generation over Pt1 /NCNT with optimal intermediate binding facilitates C2 H4 -to-EO conversion with a high FE of 92.2 %, and concomitantly, the Br- with strong nucleophilicity protects active Cu+ species in Cu2 O effectively for improved CO2 -to-C2 H4 conversion with a FE of 66.9 % at 800 mA cm-2 , superior to those in the traditional chloride-mediated case. Consequently, a single-pass FE as high as 41.1 % for CO2 -to-EO conversion can be achieved in a tandem system.

Monatomic reactions with single vacancy monolayer h-BN: DFT studies

Hexagonal boron nitride (h-BN) has been widely utilized in various strategic applications. Fine-tuning properties of BN towards the desired application often involves ad-atom adsorption of modifying its geometries through creating surface defects. This work utilizes accurate DFT computations to investigate adsorption of selected 1st and 2nd row elements (H, Li, C, O, Al, Si, P, S) of the periodic table on various structural geometries of BN. The underlying aim is to assess the change in key electronic properties upon the adsorption process. In addition to the pristine BN, B and N vacancies were comprehensively considered and a large array of properties (i.e., atomic charges, adsorption energies, density of states) were computed and contrasted among the eight elements. For instance, we found that the band gap to vary between 0.33 eV (in case of Li) and 4.14 eV (in case of P). Likewise, we have illustrated that magnetic contribution to differ substantially depending on the adatom adsorbents. Results from this work has also lays a theoretical foundation for the use of decorated and defected BN as a chemical sensor for CO gases.

Electron Injection via Interfacial Atomic Au Clusters Substantially Enhance the Visible-Light-Driven Photocatalytic H2 Production of the PF3T Enclosed TiO2 Nanocomposite

A hybrid composite of organic-inorganic semiconductor nanomaterials with atomic Au clusters at the interface decoration (denoted as PF3T@Au-TiO2 ) is developed for visible-light-driven H2 production via direct water splitting. With a strong electron coupling between the terthiophene groups, Au atoms and the oxygen atoms at the heterogeneous interface, significant electron injection from the PF3T to TiO2 occurs leading to a quantum leap in the H2 production yield (18 578 µmol g-1 h-1 ) by ≈39% as compared to that of the composite without Au decoration (PF3T@TiO2 , 11 321 µmol g-1 h-1 ). Compared to the pure PF3T, such a result is 43-fold improved and is the best performance among all the existing hybrid materials in similar configurations. With robust process control via industrially applicable methods, it is anticipated that the findings and proposed methodologies can accelerate the development of high-performance eco-friendly photocatalytic hydrogen production technologies.

Coordination engineering of atomically dispersed zirconium on graphene for the oxygen reduction reaction

We study the effect of boron and sulfur doping on graphene with atomically dispersed zirconium as an electrocatalyst for the oxygen reduction reaction (ORR) by using density functional theory (DFT). The use of Zr as a metal center offers a highly stable catalyst due to the high electronegativity difference between Zr and its ligand. The origin of the ORR activity improvement has been investigated thoroughly. Here, we proposed a novel geometric descriptor for an atomically dispersed zirconium on a nitrogen-doped graphene catalyst with an axial oxygen ligand, which is the fractional coordination number of the Zr atom. We found that the fractional coordination number can successfully describe the shift of the dz2 band center in the doped compound, which is related to the binding energy of the Zr to the O ligand. We also found that the oxygen ligand is mobile during the adsorption process of ORR intermediates, and hence it is imperative for the axial oxygen ligand to bind neither too strongly nor too weakly to the Zr atom. The coordination engineering strategy can successfully enhance the ORR activity, shifting the ORR overpotential from 0.75 V and 0.92 V to 0.33 V and 0.32 V. This study provides new insights into the origin of ORR activity by connecting the novel geometric descriptor to the electronic structure and finally it is connected to the ORR activity.

Atomically Dispersed Mg-N-C Material Supported Highly Crystalline Pt3Mg Nanoalloys for Efficient Oxygen Reduction Reaction

Single-atom or atomically dispersed metal materials have emerged as highly efficient catalysts, but their potential as excellent supports has rarely been reported. In this work, we prepared Mg-N-C materials derived from annealing of a Mg-based metal-organic framework (MOF). By introducing Pt, Mg-N-C not only serves as a platform for anchoring Pt nanoparticles but also facilitates the integration of Mg into the Pt face-centered cubic lattice, resulting in the formation of highly crystalline Pt3Mg nanoalloys via the metal-support interfacial interaction. Synchrotron radiation-based X-ray absorption spectroscopy (XAS) enables us to study the interfacial interaction and the surface electronic structure of this intricate system. The formation of Pt3Mg nanoalloys induces a downshift of the Pt d-band (gaining d-charge), as revealed by the decrease in the Pt L3-edge white-line (WL) area under the curve. This downshift can weaken the binding of oxygen reduction reaction (ORR) intermediates, hence improving the ORR performance.

Asymmetrically Coordinated CoB1 N3 Moieties for Selective Generation of High-Valence Co-Oxo Species via Coupled Electron-Proton Transfer in Fenton-like Reactions

High-valence metal species generated in peroxymonosulfate (PMS)-based Fenton-like processes are promising candidates for selective degradation of contaminants in water, the formation of which necessitates the cleavage of OH and OO bonds as well as efficient electron transfer. However, the high dissociation energy of OH bond makes its cleavage quite challenging, largely hampering the selective generation of reactive oxygen species. Herein, an asymmetrical configuration characterized by a single cobalt atom coordinated with boron and nitrogen (CoB₁ N₃ ) is established to offer a strong local electric field, upon which the cleavage of OH bond is thermodynamically favored via a promoted coupled electron-proton transfer process, which serves an essential step to further allow OO bond cleavage and efficient electron transfer. Accordingly, the selective formation of Co(IV)O in a single-atom Co/PMS system enables highly efficient removal performance toward various organic pollutants. The proposed strategy also holds true in other heteroatom doping systems to configure asymmetric coordination, thus paving alternative pathways for specific reactive species conversion by rationalized design of catalysts at atomic level toward environmental applications and more.

Self-Optimized Ligand Effect of Single-Atom Modifier in Ternary Pt-Based Alloy for Efficient Hydrogen Oxidation

Modifying the atomic and electronic structure of platinum-based alloy to enhance its activity and anti-CO poisoning ability is a vital issue in hydrogen oxidation reaction (HOR). However, the role of foreign modifier metal and the underlying ligand effect is not fully understood. Here, we propose that the ligand effect of single-atom Cu can dynamically modulate the d-band center of Pt-based alloy for boosting HOR performance. By in situ X-ray absorption spectroscopy, our research has identified that the potential-driven structural rearrangement into high-coordination Cu-Pt/Pd intensifies the ligand effect in Pt-Cu-Pd, leading to enhanced HOR performance. Thereby, modulating the d-band structure leads to near-optimal hydrogen/hydroxyl binding energies and reduced CO adsorption energies for promoting the HOR kinetics and the CO-tolerant capability. Accordingly, PtPdCu₁/C exhibits excellent CO tolerance even at 1,000 ppm impurity.

Computational Insights and Design of Promising Ultrathin PdM Bimetallenes for Oxygen Reduction Electrocatalysis

Bimetallic Palladium-based catalysts as an alternative of Pt-free electrocatalysts play a vital role in electrocatalysis. The doping of transition metal (M) into the ultrathin Pd nanosheets is new promising strategy to regulate the reactivity and durability of surface Pd sites. In this work, an in-depth investigation of the origin oxygen reduction reaction (ORR) activity and stability over 2D ultrathin PdM bimetallenes is presented. The M doping can greatly modify the reactivity of the Pd site by changing the local Fermi softness (S_F(r) ). All PdM bimetallenes follow the dissociative 4e^- pathway, and a thorough screening identified several promising alternatives (PdTa, PdHf, PdZr, and PdNb) with much lower ORR overpotential than the pure Pd(111) metallene. The Pd-O bond length and the Fermi softness of surface Pd atoms are effective descriptors of the adsorption of O* key intermediates. The hetero-metal induced ligand effect plays the key role for the activity improvement, which modifies the electronic properties and surface reactivity of Pd by the Pd-M orbital hybridization and result in the decline of bonding filling between Pd 3d_z ² and O* 2p orbital. The computational insight provides useful guideline for future experimental realizations of bimetallic Pd-based nanoalloys in ORR and other electrocatalytic reactions.

Multifunctional Pt3Rh-Co3O4 alloy nanoparticles with Pt-enriched surface and induced synergistic effect for improved performance in ORR, OER, and HER

Engineering high-performance electrocatalysts to improve the kinetics of parallel electrochemical reactions in low-temperature fuel cells, water splitting, and metal-air battery applications is important and inevitable. In this study, by employing a chemical co-reduction method, we developed multifunctional Pt₃Rh-Co₃O₄ alloy with uniformly distributed ultrafine nanoparticles (2-3 nm), supported on carbon. The presence of Co₃O₄ and the incorporation of Rh led to a strong electronic and ligand effect in the Pt lattice environment, which caused the d-band center of Pt to shift. This shift improved the electrocatalytic performance of Pt₃Rh-Co₃O₄ alloy. When Pt₃Rh-Co₃O₄/C was used to catalyze the oxygen reduction reaction (E_1/2: 0.75 V), oxygen evolution reaction (η₁₀: 290 mV), and hydrogen evolution reaction (η₁₀: 55 mV), it showed greater endurance (mass activity loss of only 7%-17%) than Pt-Co₃O₄/C and Pt/C catalysts up to 5000 potential cycles in perchloric acid. Overall, the as-prepared Pt₃Rh-Co₃O₄/C showed high multifunctional electrocatalytic potency, as demonstrated by typical electrochemical studies, and its physicochemical properties endorse their extended performance for a wide range of energy storage and conversion applications.

Atomic Scaled Depth Correlation to the Oxygen Reduction Reaction Performance of Single Atom Ni Alloy to the NiO2 Supported Pd Nanocrystal

This study demonstrates the intercalation of single-atom Ni (NiSA ) substantially reduces the reaction activity of Ni oxide supported Pd nanoparticle (NiO2 /Pd) in the oxygen reduction reaction (ORR). The results indicate the transition states kinetically consolidate the adsorption energy for the chemisorbed O and OH species on the ORR activity. Notably, the NiO2 /Ni1 /Pd performs the optimum ORR behavior with the lowest barrier of 0.49 eV and moderate second-step barrier of 0.30 eV consequently confirming its utmost ORR performance. Through the stepwise cross-level demonstrations, a structure-Eads -ΔE correspondence for the proposed NiO2 /Nin /Pd systems is established. Most importantly, such a correspondence reveals that the electronic structure of heterogeneous catalysts can be significantly differed by the segregation of atomic clusters in different dimensions and locations. Besides, the doping-depth effect exploration of the NiSA in the NiO2 /Pd structure intrinsically elucidates that the Ni atom doping in the subsurface induces the most fruitful NiSA /PdML synergy combining the electronic and strain effects to optimize the ORR, whereas this desired synergy diminishes at high Pd coverages. Overall, the results not only rationalize the variation in the redox properties but most importantly provides a precision evaluation of the process window for optimizing the configuration and composition of bimetallic catalysts in practical experiments.

Alloying Matters for Ordering: Synthesis of Highly Ordered PtCo Intermetallic Catalysts for Fuel Cells

Porous carbon-supported atomically ordered intermetallic compounds (IMCs) are promising electrocatalysts in boosting oxygen reduction reaction (ORR) for fuel cell applications. However, the formation mechanism of IMC structures under high temperatures is poorly understood, which hampers the synthesis of highly ordered IMC catalysts with promoted ORR performance. Here, we employ high-temperature X-ray diffraction and energy-dispersive spectroscopic elemental mapping techniques to study the formation process of IMCs, by taking PtCo for example, in an industry-relevant impregnation synthesis. We find that high-temperature annealing is crucial in promoting the formation of alloy particles with a stoichiometric Co/Pt ratio, which in turn is the precondition for transforming the disordered alloys to ordered intermetallic structures at a relatively low temperature. Based on the findings, we accordingly synthesize highly ordered L1₀-type PtCo catalysts with a remarkable ORR performance in fuel cells.

[2,1,3]-Benzothiadiazole-Spaced Co-Porphyrin-Based Covalent Organic Frameworks for O2 Reduction

Designing N-coordinated porous single-atom catalysts (SACs) for the oxygen reduction reaction (ORR) is a promising approach to achieve enhanced energy conversion due to maximized atom utilization and higher activity. Here, we report two Co(II)-porphyrin/ [2,1,3]-benzothiadiazole (BTD)-based covalent organic frameworks (COFs; Co@rhm-PorBTD and Co@sql-PorBTD), which are efficient SAC systems for O₂ electrocatalysis (ORR). Experimental results demonstrate that these two COFs outperform the mass activity (at 0.85 V) of commercial Pt/C (20%) by 5.8 times (Co@rhm-PorBTD) and 1.3 times (Co@sql-PorBTD), respectively. The specific activities of Co@rhm-PorBTD and Co@sql-PorBTD were found to be 10 times and 2.5 times larger than that of Pt/C, respectively. These COFs also exhibit larger power density and recycling stability in Zn-air batteries compared with a Pt/C-based air cathode. A theoretical analysis demonstrates that the combination of Co-porphyrin with two different BTD ligands affords two crystalline porous electrocatalysts having different d-band center positions, which leads to reactivity differences toward alkaline ORR. The strategy, design, and electrochemical performance of these two COFs offer a pyrolysis-free bottom-up approach that avoids the creation of random atomic sites, significant metal aggregation, or unpredictable structural features.

Selective dealloying of chemically disordered Pt-Ni bimetallic nanoparticles for the oxygen reduction reaction

Changes in electronic and compositional structures of Pt-Ni electrocatalysts with 44% of Ni fraction with repeated chemical dealloying have been studied. By comparing the Pt-enriched surfaces formed using hydroquinone and sulfuric acid as a leaching agent, we found that hydroquinone generated Pt-enriched surfaces exhibit the highest oxygen reduction reaction (ORR) activity after repeating the treatment twice. In particular, it was found that while sulfuric acid causes an uncontrollable dissolution of Ni clusters, the unique selectivity of hydroquinone allows the preferential dissolution of Ni atoms alloyed with Pt. Despite its wide usage in the field, the results show that traditional acid leaching is unsuitable for Pt-Ni alloys with a high Ni content and an incomplete alloying level. We finally proved that the unique and lasting selectivity of hydroquinone enables an incompletely alloyed Pt-Ni catalyst to obtain a highly ORR active Pt shell region without an extensive loss of Ni.

Surface and Interface Engineering for the Catalysts of Electrocatalytic CO2 Reduction

The massive use of fossil fuels releases a great amount of CO₂ , which substantially contributes to the global warming. For the global goal of putting CO₂ emission under control, effective utilization of CO₂ is particularly meaningful. Electrocatalytic CO₂ reduction reaction (eCO₂ RR) has great potential in CO₂ utilization, because it can convert CO₂ into valuable carbon-containing chemicals and feedstock using renewable electricity. The catalyst design for eCO₂ RR is a key challenge to achieving efficient conversion of CO₂ to fuels and useful chemicals. For a typical heterogeneous catalyst, surface and interface engineering is an effective approach to enhance reaction activity. Herein, the development and research progress in CO₂ catalysts with focus on surface and interface engineering are reviewed. First, the fundaments of eCO₂ RR is briefly discussed from the reaction mechanism to performance evaluation methods, introducing the role of the surface and interface engineering of electrocatalyst in eCO₂ RR. Then, several routes to optimize the surface and interface of CO₂ electrocatalysts, including morphology, dopants, atomic vacancies, grain boundaries, surface modification, etc., are reviewed and representative examples are given. At the end of this review, we share our personal views in future research of eCO₂ RR.

Adsorption Energy in Oxygen Electrocatalysis

Adsorption energy (AE) of reactive intermediate is currently the most important descriptor for electrochemical reactions (e.g., water electrolysis, hydrogen fuel cell, electrochemical nitrogen fixation, electrochemical carbon dioxide reduction, etc.), which can bridge the gap between catalyst's structure and activity. Tracing the history and evolution of AE can help to understand electrocatalysis and design optimal electrocatalysts. Focusing on oxygen electrocatalysis, this review aims to provide a comprehensive introduction on how AE is selected as the activity descriptor, the intrinsic and empirical relationships related to AE, how AE links the structure and electrocatalytic performance, the approaches to obtain AE, the strategies to improve catalytic activity by modulating AE, the extrinsic influences on AE from the environment, and the methods in circumventing linear scaling relations of AE. An outlook is provided at the end with emphasis on possible future investigation related to the obstacles existing between adsorption energy and electrocatalytic performance.

Electronic structure and catalytic activity of exsolved Ni on Pd core-shell nanoparticles

This study reports first principles calculations performed to study the electronic structure and catalytic activity of exsolved Ni on Pd core-shell catalysts reported in recent experimental literature. The modification in the electronic and geometric properties of the Ni/Pd bimetallic system as successive layers of Ni are added on top of Pd is systematically investigated using the d-band model as well as the adsorption of O and CO on the surface of these core-shell structures. The results show that the adsorption of O and CO is more favourable on Ni/Pd core-shell catalysts compared to the pure Ni surface. As the dissociation of the O₂ molecule into atomic oxygen and CO oxidation are key steps in metal-catalysed oxidation reactions, we have examined the energetics of O₂ dissociation and CO oxidation reaction over the (111) faces of Ni as well as Ni/Pd structures. Our results suggest that both adsorption and dissociation are easier on Ni/Pd surfaces compared to a simple Ni surface. Unlike O₂ dissociation, we find that CO oxidation is unfavourable on Ni/Pd in comparison to Ni. The energetics of both reactions follow Brønsted-Evans-Polanyi relationships where the activation energy is linearly related to the reaction energy for all surfaces studied here. We found that a single monolayer of Ni on Pd, due to the synergistic effect of geometric and electronic factors, is the most active among the surfaces studied here towards the adsorption and dissociation of O₂. Both adsorption and dissociation become less favourable with an increase in the thickness of the Ni shell in these core-shell catalysts. A close analysis of the results indicates that both strain and ligand effects are active in the improved catalytic activity seen in Ni on Pd catalysts. Quite understandably, the ligand effect is only seen for the single monolayer of Ni on Pd and fades off as we go to two monolayers of Ni. The results reported here help us understand the connections between the electronic structure and catalytic activity of Ni/Pd core-shell nanoparticles, and these insights are expected to be useful in the development of core-shell catalysts.

Phase diagrams guide synthesis of highly ordered intermetallic electrocatalysts: separating alloying and ordering stages

Supported platinum intermetallic compound catalysts have attracted considerable attention owing to their remarkable activities and durability for the oxygen reduction reaction in proton-exchange membrane fuel cells. However, the synthesis of highly ordered intermetallic compound catalysts remains a challenge owing to the limited understanding of their formation mechanism under high-temperature conditions. In this study, we perform in-situ high-temperature X-ray diffraction studies to investigate the structural evolution in the impregnation synthesis of carbon-supported intermetallic catalysts. We identify the phase-transition-temperature (T_PT)-dependent evolution process that involve concurrent (for alloys with high T_PT) or separate (for alloys with low T_PT) alloying/ordering stages. Accordingly, we realize the synthesis of highly ordered intermetallic catalysts by adopting a separate annealing protocol with a high-temperature alloying stage and a low-temperature ordering stage, which display a high mass activity of 0.96 A mg_Pt^-1 at 0.9 V in H₂-O₂ fuel cells and a remarkable durability.

Review on Magnetism in Catalysis: From Theory to PEMFC Applications of 3d Metal Pt-Based Alloys

The relationship between magnetism and catalysis has been an important topic since the mid-20th century. At present time, the scientific community is well aware that a full comprehension of this relationship is required to face modern challenges, such as the need for clean energy technology. The successful use of (para-)magnetic materials has already been corroborated in catalytic processes, such as hydrogenation, Fenton reaction and ammonia synthesis. These catalysts typically contain transition metals from the first to the third row and are affected by the presence of an external magnetic field. Nowadays, it appears that the most promising approach to reach the goal of a more sustainable future is via ferromagnetic conducting catalysts containing open-shell metals (i.e., Fe, Co and Ni) with extra stabilization coming from the presence of an external magnetic field. However, understanding how intrinsic and extrinsic magnetic features are related to catalysis is still a complex task, especially when catalytic performances are improved by these magnetic phenomena. In the present review, we introduce the relationship between magnetism and catalysis and outline its importance in the production of clean energy, by describing the representative case of 3d metal Pt-based alloys, which are extensively investigated and exploited in PEM fuel cells.

Orbital Occupancy and Spin Polarization: From Mechanistic Study to Rational Design of Transition Metal-Based Electrocatalysts toward Energy Applications

Over the past few decades, development of electrocatalysts for energy applications has extensively transitioned from trial-and-error methodologies to more rational and directed designs at the atomic levels via either nanogeometric optimization or modulating electronic properties of active sites. Regarding the modulation of electronic properties, nonprecious transition metal-based materials have been attracting large interest due to the capability of versatile tuning d-electron configurations expressed through the flexible orbital occupancy and various possible degrees of spin polarization. Herein, recent advances in tailoring electronic properties of the transition-metal atoms for intrinsically enhanced electrocatalytic performances are reviewed. We start with discussions on how orbital occupancy and spin polarization can govern the essential atomic level processes, including the transport of electron charge and spin in bulk, reactive species adsorption on the catalytic surface, and the electron transfer between catalytic centers and adsorbed species as well as reaction mechanisms. Subsequently, different techniques currently adopted in tuning electronic structures are discussed with particular emphasis on theoretical rationale and recent practical achievements. We also highlight the promises of the recently established computational design approaches in developing electrocatalysts for energy applications. Lastly, the discussion is concluded with perspectives on current challenges and future opportunities. We hope this review will present the beauty of the structure-activity relationships in catalysis sciences and contribute to advance the rational development of electrocatalysts for energy conversion applications.

Ab Initio Investigation of the Adsorption and Dissociation of O2 on Cu-Skin Cu3Au(111) Surface

Surface adsorption and dissociation processes can have a decisive impact on the catalytic properties of metal alloys. We have used density functional theory to investigate the adsorption and dissociation of O2 on Cu-skin Cu3Au(111) surface. The calculated results show that the b-f(h)-b adsorption configuration is the most energetically favorable on the Cu-skin Cu3Au(111) surface. For O2 dissociation, there are two thermodynamically favorable dissociation paths. One path is from b-f-b to two O atoms in hcp sites, and the other path is from b-h-b to two O atoms in fcc sites. Moreover, the stability of O2 adsorption is higher and the dissociation energy barrier of the adsorbed O2 is lower as compared to those on the Cu(111) surface. This theoretical work provides valuable guidance for the practical application of Cu-Au alloys as highly efficient CO oxidation catalysts.

Catalysis of Alloys: Classification, Principles, and Design for a Variety of Materials and Reactions

Alloying has long been used as a promising methodology to improve the catalytic performance of metallic materials. In recent years, the field of alloy catalysis has made remarkable progress with the emergence of a variety of novel alloy materials and their functions. Therefore, a comprehensive disciplinary framework for catalytic chemistry of alloys that provides a cross-sectional understanding of the broad research field is in high demand. In this review, we provide a comprehensive classification of various alloy materials based on metallurgy, thermodynamics, and inorganic chemistry and summarize the roles of alloying in catalysis and its principles with a brief introduction of the historical background of this research field. Furthermore, we explain how each type of alloy can be used as a catalyst material and how to design a functional catalyst for the target reaction by introducing representative case studies. This review includes two approaches, namely, from materials and reactions, to provide a better understanding of the catalytic chemistry of alloys. Our review offers a perspective on this research field and can be used encyclopedically according to the readers' individual interests.

Intermetallic PtFe Electrocatalysts for the Oxygen Reduction Reaction: Ordering Degree-Dependent Performance

Platinum-based atomically ordered alloys (i.e., intermetallic compounds) have distinct advantages over disordered solid solution counterparts in boosting the cathodic oxygen-reduction reaction (ORR) in proton-exchange-membrane fuel cells. Nevertheless, the pivotal role of ordering degree of intermetallic catalysts in promoting ORR performance has been ignored heavily so far, probably owing to the lack of synthetic routes for controlling the ordering degree, especially for preparing highly ordered intermetallic catalysts. Herein, a family of intermetallic PtFe catalysts with similar particle size of 3-4 nm but varied ordering degree in a wide range of 10-70% are prepared. After constructing the PtFe/Pt core/shell structure with around 3 Pt-layer skin, a positive correlation between the ordering degree of the intermetallic catalysts and their ORR activity and durability is identified. Notably, the highly ordered PtFe/Pt catalyst exhibits a high mass activity of 0.92 A mg_Pt ^-1 at 0.9 V_iR-corrected as cathode catalyst in H₂ -O₂ fuel cell, with only 24% loss after accelerated durability tests. The ordering degree-dependent performance can be ascribed to the compressive strain effect induced by the intermetallic PtFe core with smaller lattice parameters, and the more thermodynamically stable intermetallic structure compared to disordered alloys.