Near-surface alloys for hydrogen fuel cell applications

Electrocatalysis in Solid Oxide Fuel Cells and Electrolyzers

Interest in energy-to-X and X-to-energy (where X represents green hydrogen, carbon-based fuels, or ammonia) technologies has expanded the field of electrochemical conversion and storage. Solid oxide electrochemical cells (SOCs) are among the most promising technologies for these processes. Their unmatched conversion efficiencies result from favorable thermodynamics and kinetics at elevated operating temperatures (400-900 °C). These solid-state electrochemical systems exhibit flexibility in reversible operation between fuel cell and electrolysis modes and can efficiently utilize a variety of fuels. However, electrocatalytic materials at SOC electrodes remain nonoptimal for facilitating reversible operation and fuel flexibility. In this Review, we explore the diverse range of electrocatalytic materials utilized in oxygen-ion-conducting SOCs (O-SOCs) and proton-conducting SOCs (H-SOCs). We examine their electrochemical activity as a function of composition and structure across different electrochemical reactions to highlight characteristics that lead to optimal catalytic performance. Catalyst deactivation mechanisms under different operating conditions are discussed to assess the bottlenecks in performance. We conclude by providing guidelines for evaluating the electrochemical performance of electrode catalysts in SOCs and for designing effective catalysts to achieve flexibility in fuel usage and mode of operation.

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.

Electrocatalysis in Alkaline Media and Alkaline Membrane-Based Energy Technologies

Hydrogen energy-based electrochemical energy conversion technologies offer the promise of enabling a transition of the global energy landscape from fossil fuels to renewable energy. Here, we present a comprehensive review of the fundamentals of electrocatalysis in alkaline media and applications in alkaline-based energy technologies, particularly alkaline fuel cells and water electrolyzers. Anion exchange (alkaline) membrane fuel cells (AEMFCs) enable the use of nonprecious electrocatalysts for the sluggish oxygen reduction reaction (ORR), relative to proton exchange membrane fuel cells (PEMFCs), which require Pt-based electrocatalysts. However, the hydrogen oxidation reaction (HOR) kinetics is significantly slower in alkaline media than in acidic media. Understanding these phenomena requires applying theoretical and experimental methods to unravel molecular-level thermodynamics and kinetics of hydrogen and oxygen electrocatalysis and, particularly, the proton-coupled electron transfer (PCET) process that takes place in a proton-deficient alkaline media. Extensive electrochemical and spectroscopic studies, on single-crystal Pt and metal oxides, have contributed to the development of activity descriptors, as well as the identification of the nature of active sites, and the rate-determining steps of the HOR and ORR. Among these, the structure and reactivity of interfacial water serve as key potential and pH-dependent kinetic factors that are helping elucidate the origins of the HOR and ORR activity differences in acids and bases. Additionally, deliberately modulating and controlling catalyst-support interactions have provided valuable insights for enhancing catalyst accessibility and durability during operation. The design and synthesis of highly conductive and durable alkaline membranes/ionomers have enabled AEMFCs to reach initial performance metrics equal to or higher than those of PEMFCs. We emphasize the importance of using membrane electrode assemblies (MEAs) to integrate the often separately pursued/optimized electrocatalyst/support and membranes/ionomer components. Operando/in situ methods, at multiscales, and ab initio simulations provide a mechanistic understanding of electron, ion, and mass transport at catalyst/ionomer/membrane interfaces and the necessary guidance to achieve fuel cell operation in air over thousands of hours. We hope that this Review will serve as a roadmap for advancing the scientific understanding of the fundamental factors governing electrochemical energy conversion in alkaline media with the ultimate goal of achieving ultralow Pt or precious-metal-free high-performance and durable alkaline fuel cells and related technologies.

Platinum Nanoparticles Modified Copper/Titanium Electrodes as Electrocatalysts for Borohydride Oxidation

In this study, sodium borohydride oxidation has been investigated on the platinum nanoparticles modified copper/titanium catalysts (PtNPsCu/Ti), which were fabricated by employing the electroless copper plating and galvanic displacement technique. ICP-OES, XRD, FESEM, and EDX have been used to characterize PtNPsCu/Ti catalysts’ composition, structure, and surface morphology. The oxidation of sodium borohydride was examined on the PtNPsCu/Ti catalysts using cyclic voltammetry and chrono-techniques.

Computational Methods in Heterogeneous Catalysis

The unprecedented ability of computations to probe atomic-level details of catalytic systems holds immense promise for the fundamentals-based bottom-up design of novel heterogeneous catalysts, which are at the heart of the chemical and energy sectors of industry. Here, we critically analyze recent advances in computational heterogeneous catalysis. First, we will survey the progress in electronic structure methods and atomistic catalyst models employed, which have enabled the catalysis community to build increasingly intricate, realistic, and accurate models of the active sites of supported transition-metal catalysts. We then review developments in microkinetic modeling, specifically mean-field microkinetic models and kinetic Monte Carlo simulations, which bridge the gap between nanoscale computational insights and macroscale experimental kinetics data with increasing fidelity. We finally review the advancements in theoretical methods for accelerating catalyst design and discovery. Throughout the review, we provide ample examples of applications, discuss remaining challenges, and provide our outlook for the near future.

Adsorbate-Induced Segregation of Cobalt from PtCo Nanoparticles: Modeling Au Doping and Core AuCo Alloying for the Improvement of Fuel Cell Cathode Catalysts

Platinum, when used as a cathode material for the oxygen reduction reaction, suffers from high overpotential and possible dissolution, in addition to the scarcity of the metal and resulting cost. Although the introduction of cobalt has been reported to improve reaction kinetics and decrease the precious metal loading, surface segregation or complete leakage of Co atoms causes degradation of the membrane electrode assembly, and either of these scenarios of structural rearrangement eventually decreases catalytic power. Ternary PtCo alloys with noble metals could possibly maintain activity with a higher dissolution potential. First-principles-based theoretical methods are utilized to identify the critical factors affecting segregation in Pt-Co binary and Pt-Co-Au ternary nanoparticles in the presence of oxidizing species. With a decreasing share of Pt, surface segregation of Co atoms was already found to become thermodynamically viable in the PtCo systems at low oxygen concentrations, which is assigned to high charge transfer between species. While the introduction of gold as a dopant caused structural changes that favor segregation of Co, creation of CoAu alloy core is calculated to significantly suppress Co leakage through modification of the electronic properties. The theoretical framework of geometrically different ternary systems provides a new route for the rational design of oxygen reduction catalysts.

Dynamic Restructuring Induced Oxygen Activation on AgCu Near-Surface Alloys

Recent studies have shown that the addition of Cu to Ag catalysts improves their epoxidation performance by increasing the overall selectivity of the bimetallic catalyst. We have prepared AgCu near-surface alloys and used scanning tunneling microscopy to gain an atomistic picture of O₂ dissociation on the bimetallic system. These data reveal a higher dissociative sticking probability for O₂ on AgCu than on Ag(111), and density functional theory (DFT) confirms that the O₂ dissociation barrier is 0.17 eV lower on the alloy. Surprisingly, we find that, after a slow initial uptake of O₂, the sticking probability increases exponentially. Further DFT calculations indicate that surface oxygen reverses the segregation energy for AgCu, stabilizing Cu atoms in the Ag layer. These single Cu atoms in the Ag surface are found to significantly lower the O₂ dissociation barrier. Together, these results explain nonlinear effects in the activation of O₂ on this catalytically relevant surface alloy.

Facile synthesis of ternary PtPdCu alloy hexapods as highly efficient electrocatalysts for methanol oxidation

Developing an efficient Pt-based multimetallic electrocatalyst with well-defined shapes for methanol oxidation reaction (MOR) is critical, however, it still remains challenging. Here we report a one-pot approach for the synthesis of ternary PtPdCu alloy hexapods with different compositions. Their MOR activities increased in the sequence Pt₂PdCu₄ < Pt₃PdCu₄ < Pt₅PdCu₅, and were substantially higher than that of commercial Pt/C. Specifically, the Pt₅PdCu₅ hexapods exhibited the highest mass (0.97 mA μg_Pt ^-1) and specific (7.39 mA cm^-2) activities towards MOR, and were 5.4 and 19.4 times higher than those of commercial Pt/C (0.18 mA μg_Pt ^-1 and 0.38 mA cm^-2), respectively. This enhancement could be probably attributed to the bifunctional mechanism and ligand effect through the addition of Cu and Pd as well as the unique dendritic structure. The better tolerance for CO poisoning also endowed the PtPdCu hexapods with superior durability relative to commercial Pt/C.

Surface alloys in nanochemistry: catalysis and synthesis

A surface alloy is a surface-confined mixture of metals that are immiscible in bulk. It has unique chemical properties that can be utilized in various catalytic and synthetic reactions.

Predicting aggregation energy for single atom bimetallic catalysts on clean and O* adsorbed surfaces through machine learning models

Machine learning models are successfully developed for simultaneous prediction of stability and adsorption energy at single-atom bimetallic sites.

Influence of Van der Waals Interactions on the Solvation Energies of Adsorbates at Pt-Based Electrocatalysts

Solvation can significantly modify the adsorption energy of species at surfaces, thereby influencing the performance of electrocatalysts and liquid-phase catalysts. Thus, it is important to understand adsorbate solvation at the nanoscale. Here we evaluate the effect of van der Waals (vdW) interactions described by different approaches on the solvation energy of *OH adsorbed on near-surface alloys (NSAs) of Pt. Our results show that the studied functionals can be divided into two groups, each with rather similar average *OH solvation energies: (1) PBE and PW91; and (2) vdW functionals, RPBE, PBE-D3 and RPBE-D3. On average, *OH solvation energies are less negative by ∼0.14 eV in group (2) compared to (1), and the values for a given alloy can be extrapolated from one functional to another within the same group. Depending on the desired level of accuracy, these concrete observations and our tabulated values can be used to rapidly incorporate solvation into models for electrocatalysis and liquid-phase catalysis.

Ir-Ni Bimetallic OER Catalysts Prepared by Controlled Ni Electrodeposition on Irpoly and Ir(111)

The alteration of electrocatalytic surfaces with adatoms lead to structural and electronic modifications promoting adsorption, desorption, and reactive processes. This study explores the potentiostatic electrodeposition process of Ni onto polycrystalline Ir (Irpoly) and assesses the electrocatalytic properties of the resulting bimetallic surfaces. The electrodeposition resulted in bimetallic Ni overlayer (OL) structures and in combination with controlled thermal post-deposition annealing in bimetallic near-surface alloys (NSA). The catalytic oxygen evolution reaction (OER) activity of these two different Ni-modified catalysts is assessed and compared to a pristine, unmodified Irpoly. An overlayer of Ni on Irpoly showed superior performance in both acidic and alkaline milieu. The reductive annealing of the OL produced a NSA of Ni, which demonstrated enhanced stability in an acidic environment. The remarkable activity and stability improvement of Ir by Ni modification makes both systems efficient electrocatalysts for water oxidation. The roughness factor of Irpoly is also reported. With the amount of deposited Ni determined by inductively coupled plasma mass spectrometry (ICP-MS) and a degree of coverage (monolayer) in the dependence of deposition potential is established. The density functional theory (DFT) assisted evaluation of H adsorption on Irpoly enables determination of the preferred Ni deposition sites on the three low-index surfaces (111), (110), and (100).

Lonely Atoms with Special Gifts: Breaking Linear Scaling Relationships in Heterogeneous Catalysis with Single-Atom Alloys

We discuss a simple yet effective strategy for escaping traditional linear scaling relations in heterogeneous catalysis with highly dilute bimetallic alloys known as single-atom alloys (SAAs). These systems, in which a reactive metal is atomically dispersed in a less reactive host, were first demonstrated with the techniques of surface science to be active and selective for hydrogenation reactions. Informed by these early results, PdCu and PtCu SAA nanoparticle hydrogenation catalysts were shown to work under industrially relevant conditions. To efficiently survey the many potential metal combinations and reactions, simulation is crucial for making predictions about reactivity and guiding experimental focus on the most promising candidate materials. This recent work reveals that the high surface chemical heterogeneity of SAAs can result in significant deviations from Brønsted-Evans-Polanyi scaling relationships for many key reaction steps. These recent insights into SAAs and their ability to break linear scaling relations motivate discovery of novel alloy catalysts.

Finite Size Effects in Submonolayer Catalysts Investigated by CO Electrosorption on PtsML/Pd(100)

A combination of scanning tunneling microscopy, subtractively normalized interfacial Fourier transform infrared spectroscopy (SNIFTIRS), and density functional theory (DFT) is used to quantify the local strain in 2D Pt clusters on the 100 facet of Pd and its effect on CO chemisorption. Good agreement between SNIFTIRS experiments and DFT simulations provide strong evidence that, in the absence of coherent strain between Pt and Pd, finite size effects introduce local compressive strain, which alters the chemisorption properties of the surface. Though this effect has been widely neglected in prior studies, our results suggest that accurate control over cluster sizes in submonolayer catalyst systems can be an effective approach to fine-tune their catalytic properties.

A Review on Bimetallic Nickel-Based Catalysts for CO2 Reforming of Methane

In recent years, CO₂ reforming of methane (dry reforming of methane, DRM) has become an attractive research area because it converts two major greenhouse gasses into syngas (CO and H₂ ), which can be directly used as fuel or feedstock for the chemical industry. Ni-based catalysts have been extensively used for DRM because of its low cost and good activity. A major concern with Ni-based catalysts in DRM is severe carbon deposition leading to catalyst deactivation, and a lot of effort has been put into the design and synthesis of stable Ni catalysts with high carbon resistance. One effective and practical strategy is to introduce a second metal to obtain bimetallic Ni-based catalysts. The synergistic effect between Ni and the second metal has been shown to increase the carbon resistance of the catalyst significantly. In this review, a detailed discussion on the development of bimetallic Ni-based catalysts for DRM including nickel alloyed with noble metals (Pt, Ru, Ir etc.) and transition metals (Co, Fe, Cu) is presented. Special emphasis has been provided on the underlying principles that lead to synergistic effects and enhance catalyst performance. Finally, an outlook is presented for the future development of Ni-based bimetallic catalysts.

Strain-induced Stranski-Krastanov growth of Pd@Pt core-shell hexapods and octapods as electrocatalysts for methanol oxidation

Bimetallic nanocrystals with a branched shape have received great interest as catalysts due to their unique structures and fascinating properties. However, the conventional synthetic approaches based on the island growth mode often lead to the dendritic nanostructures with inhomogeneous and uncontrolled branches. Here precise control over the number of branches has been realized in the deposition of Pt on Pd seeds through the Stranski-Krastanov growth mechanism. Based on such a growth mode, Pd@Pt core-shell hexapods and octapods have been generated by a seeded growth with Pd octahedra and cubes as the seeds, respectively. We found that Pt atoms are initially deposited on the side faces of Pd seeds through a layer-by-layer epitaxial growth in the presence of oleylamine (OAm), leading to a local strain focused at their corners. These strain-concentrated sites promote the subsequent island growth of Pt atoms at the corners of the Pd seeds, resulting in the Pd@Pt core-shell hexapods or octapods. Both the Pd@Pt core-shell hexapods and octapods exhibit the substantially enhanced catalytic properties in terms of activity and stability towards a methanol oxidation reaction (MOR) relative to the commercial Pt/C. Specifically, the Pd@Pt core-shell hexapods show the highest specific (1.97 mA cm^-2) activity and mass activity (0.52 mA μg_Pt^-1) for the MOR, which are 5.8 and 2.6 times higher than those of the commercial Pt/C, respectively. This enhancement can probably be attributed to their unique structures and the synergistic effect between Pt and Pd.

Catalytic activity of Pd-doped Cu nanoparticles for hydrogenation as a single-atom-alloy catalyst

The concept of single atom alloy that comes from extended surfaces can be effectively applied to nanoscales.

Stability of Pt near surface alloys under electrochemical conditions: a model study

The stability is one of the key requirements for commercializing the fuel cell electrocatalysts in automotive applications.

Design of Pt-shell nanoparticles with alloy cores for the oxygen reduction reaction

We report that the oxygen binding energy of alloy-core@Pt nanoparticles can be linearly tuned by varying the alloy-core composition. Using this tuning mechanism, we are able to predict optimal compositions for different alloy-core@Pt nanoparticles. Subsequent electrochemical measurements of ORR activities of AuPd@Pt dendrimer-encapsulated nanoparticles (DENs) are in a good agreement with the theoretical prediction that the peak of activity is achieved for a 28% Au/72% Pd alloy core supporting a Pt shell. Importantly, these findings represent an unusual case of first-principles theory leading to nearly perfect agreement with experimental results.

Synthesis of bimetallic platinum nanoparticles for biosensors

The use of magnetic nanomaterials in biosensing applications is growing as a consequence of their remarkable properties; but controlling the composition and shape of metallic nanoalloys is problematic when more than one precursor is required for wet chemistry synthesis. We have developed a successful simultaneous reduction method for preparation of near-spherical platinum-based nanoalloys containing magnetic solutes. We avoided particular difficulties in preparing platinum nanoalloys containing Ni, Co and Fe by the identification of appropriate synthesis temperatures and chemistry. We used transmission electron microscopy (TEM) to show that our particles have a narrow size distribution, uniform size and morphology, and good crystallinity in the as-synthesized condition. Energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD) confirms the coexistence of Pt with the magnetic solute in a face-centered cubic (FCC) solid solution.

AgAu bimetallic Janus nanoparticles and their electrocatalytic activity for oxygen reduction in alkaline media

Bimetallic AgAu Janus nanoparticles were prepared by galvanic exchange reactions of 1-hexanethiolate-passivated silver (AgC6) nanoparticles with gold(I)-mercaptopropanediol complex. The AgC6 nanoparticles were deposited onto a solid substrate surface by the Langmuir-Blodgett method such that the galvanic exchange reactions were limited to the top face of the nanoparticles that was in direct contact with the gold(I) complex solution. The resulting nanoparticles exhibited an asymmetrical distribution not only of the organic capping ligands on the nanoparticle surface but also of the metal elements in the nanoparticle cores, in contrast to the bulk-exchange counterparts where these distributions were homogeneous within the nanoparticles, as manifested in contact angle, UV-vis, XPS, and TEM measurements. More interestingly, despite a minimal loading of Au onto the Ag nanoparticles, the bimetallic AgAu nanoparticles exhibited enhanced electrocatalytic activity in oxygen reduction reactions, as compared to the monometal AgC6 nanoparticles. Additionally, the electrocatalytic performance of the Janus nanoparticles was markedly better than the bulk-exchange ones, suggesting that the segregated distribution of the polar ligands from the apolar ones might further facilitate charge transfer from Ag to Au in the nanoparticle cores, leading to additional improvement of the adsorption and reduction of oxygen.