Engineering Ru@Pt Core-Shell Catalysts for Enhanced Electrochemical Oxygen Reduction Mass Activity and Stability

Origin and Formation Mechanism of Carbon Shell-Encapsulated Metal Nanoparticles for Powerful Fuel Cell Durability

Proton exchange membrane fuel cells (PEMFCs) face technical issues of performance degradation due to catalyst dissolution and agglomeration in real-world operations. To address these challenges, intensive research has been recently conducted to introduce additional structural units on the catalyst surface. Among various concepts for surface modification, carbon shell encapsulation is known to be a promising strategy since the carbon shell can act as a protective layer for metal nanoparticles. As an interesting approach to form carbon shells on catalyst surfaces, the precursor ligand-induced formation is preferred due to its facile synthesis and tunable control over the carbon shell porosity. However, the origin of the carbon source and the carbon shell formation mechanism have not been studied in depth yet. Herein, this study aims to investigate carbon sources through the use of different precursors and the introduction of new methodologies related to the ligand exchange phenomenon. Subsequently, we provide new insights into the carbon shell formation mechanism using X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD). Finally, the thermal stability and electrochemical durability of carbon shells are thoroughly investigated through in situ transmission electron microscopy (in situ TEM) and accelerated durability tests.

Insights into the performance and degradation of Ru@Pt core-shell catalysts for fuel cells by advanced (scanning) transmission electron microscopy

Ru@Pt core-shell nanoparticles are currently being explored as carbon monoxide tolerant anode catalysts for proton exchange membrane fuel cells. However, little is known about their degradation under fuel cell conditions. In the present work, two types of Ru@Pt nanoparticles with nominal shell thicknesses of 1 (Ru@1Pt) and 2 (Ru@2Pt) Pt monolayers are studied as synthesized and after accelerated stress tests. These stress tests were designed to imitate the degradation occurring under fuel cell operating conditions. Our advanced (scanning) transmission electron microscopy characterization explains the superior initial electrochemical performance of Ru@1Pt. Moreover, the 3D reconstruction of the Pt shell by electron tomography reveals an incomplete shell for both samples, which results in a less stable Ru metal being exposed to an electrolyte. The degree of coverage of the Ru cores provides insights into the higher stability of Ru@2Pt during the accelerated stress tests. Our results explain how to maximize the initial performance of Ru@Pt-type catalysts, without compromising their stability under fuel cell conditions.

Catalysts for Oxygen Reduction Reaction in the Polymer Electrolyte Membrane Fuel Cells: A Brief Review

This mini-review presents a short account of materials with exceptional activity towards oxygen reduction reaction. Two main classes of catalytic materials are described, namely platinum group metal (PGM) catalyst and Non-precious metal catalyst. The classes are discussed in terms of possible application in low-temperature hydrogen fuel cells with proton exchange membrane and further commercialization of these devices. A short description of perspective approaches is provided and challenging issues associated with developed catalytic materials are discussed.

Electrochemical sandwich-type immunosensor for the detection of PSA based on a trimetallic AgAuPt nanocomposite synthesized using the galvanic replacement reaction

A sandwich-type electrochemical immunoassay was introduced for the determination of the prostate-specific antigen (PSA) biomarker. A direct and simple galvanic replacement reaction was performed between the Ag framework and metallic salts of tetrachloroauric(iii) acid trihydrate and chloroplatinic acid to produce a trimetallic composite of AgAuPt. The trimetallic composite of AgAuPt was applied to the preparation of the capture layer of the immunoassay for stabilizing the primary Ab at the surface of the prepared composite. The immunoassay detection layer was also prepared using a labeled antibody containing a bimetallic composite of AgPt as a label. The various procedures in the immunoassay fabrication were monitored step by step using cyclic voltammetry and electrochemical impedance spectroscopy. Also, the electrochemical determination of PSA was performed using differential pulse voltammetry in the presence of the ferrocene redox probe and H₂O₂. Furthermore, the effective parameters in the fabrication of the immunoassay included the drop volume of the AgAuPt trimetallic composite and the incubation time for the immobilization of biomolecules (i.e., Ab₁, BSA, PSA, and labeled Ab₂), and the concentration of H₂O₂ were optimized during the determination of PSA. Then, the determination of PSA was performed under optimized conditions. It could be seen that there was a linear relation between the PSA concentration and DPV responses in the concentration range of 50 pg mL^-1 to 500 ng mL^-1 and the limit of detection (LOD) for the proposed immunoassay was calculated as 17.0 pg mL^-1. In the following investigation, the cross-reactivity of the proposed immunoassay was studied in the presence of BSA, CEA, IgG, and human hepatitis surface antigen, in which the results showed a negligible change in the performance of the immunoassay.

Highly efficient bifunctional oxygen reduction/evolution activity of a non-precious nanocomposite derived from a tetrazine-COF

We report a novel s-tetrazine based covalent organic framework (TZA-COF) and its hybrid nanocomposites with reduced graphene oxide (TZA-COF-rGO) and Co metal to illustrate novel structure-activity relationships in this class of compounds for electrocatalytic oxygen reduction reaction (ORR). The Co-impregnated hybrid composites (TZA-COF-rGO-Co) were further annealed to yield Co-encapsulated nitrogen doped graphitic carbon (Co@NC-600), which exhibited excellent ORR activity comparable to that of the state-of-the art Pt/C in terms of onset potential, E1/2 (half-wave potential), 4e- reduction selectivity and methanol tolerance. Sequential mechanistic analyses of activity enhancement and electron transfer pathways for the ORR, at different stages of controlled catalyst engineering, elucidated the crucial role of active sites and overall catalyst nature in tuning the ORR mechanism. Co@NC-600 also exhibited high oxygen evolution reaction (OER) activity under alkaline conditions which makes it one of the most efficient non-precious metal bifunctional catalysts, capable of catalyzing complex 4e- reduction processes like the ORR and OER.

O2 activation by core-shell Ru13@Pt42 particles in comparison with Pt55 particles: a DFT study

The reaction of O₂ with a Ru₁₃@Pt₄₂ core-shell particle consisting of a Ru₁₃ core and a Pt₄₂ shell was theoretically investigated in comparison with Pt₅₅. The O₂ binding energy with Pt₅₅ is larger than that with Ru₁₃@Pt₄₂, and O-O bond cleavage occurs more easily with a smaller activation barrier (E _a) on Pt₅₅ than on Ru₁₃@Pt₄₂. Protonation to the Pt₄₂ surface followed by one-electron reduction leads to the formation of an H atom on the surface with considerable exothermicity. The H atom reacts with the adsorbed O₂ molecule to afford an OOH species with a larger E _a value on Pt₅₅ than on Ru₁₃@Pt₄₂. An OOH species is also formed by protonation of the adsorbed O₂ molecule, followed by one-electron reduction, with a large exothermicity in both Pt₅₅ and Ru₁₃@Pt₄₂. O-OH bond cleavage occurs with a smaller E _a on Pt₅₅ than on Ru₁₃@Pt₄₂. The lower reactivity of Ru₁₃@Pt₄₂ than that of Pt₅₅ on the O-O and O-OH bond cleavages arises from the presence of lower energy in the d-valence band-top and d-band center in Ru₁₃@Pt₄₂ than in Pt₅₅. The smaller E _a for OOH formation on Ru₁₃@Pt₄₂ than on Pt₅₅ arises from weaker Ru₁₃@Pt₄₂-O₂ and Ru₁₃@Pt₄₂-H bonds than the Pt₅₅-O₂ and Pt₅₅-H bonds, respectively. The low-energy d-valence band-top is responsible for the weak Ru₁₃@Pt₄₂-O and Ru₁₃@Pt₄₂-OH bonds. Thus, the low-energy d-valence band-top and d-band center are important properties of the Ru₁₃@Pt₄₂ particle.

Bimetallic Cu/Pt Oxygen Reduction Reaction Catalyst for Fuel Cells Cathode Materials

The oxygen reduction reaction (ORR) is a key process for the operation of fuel cells. To accelerate the sluggish kinetics of ORR, a wide range of catalysts have been proposed and tested. In this work, a nano-dispersed copper-impregnated platinum catalyst prepared by electrodeposition of platinum on a poly[Cu(Salen)] template followed by polymer destruction is described. In addition to the high activity of the thus prepared catalyst in the oxygen reduction reaction surpassing that of both polycrystalline platinum catalyst and the commercial carbon-platinum catalyst (“E-TEK”), it showed remarkable tolerance to the presence of methanol in solution.

PtPd Hybrid Composite Catalysts as Cathodes for Proton Exchange Membrane Fuel Cells

In this work, PtPd hybrid cathodic catalysts were prepared for a proton exchange membrane fuel cell (PEMFC) application by two different strategies. The first strategy was the physical mixing of bimetallic PtPd onto partially reduced graphene oxide (PtPd/rGO) and PtPd onto multi-walled carbon nanotubes (PtPd/MWCNT); (PtPd/rGO) + (PtPd/MWCNT). The second strategy was physical mixing of both carbonaceous supports before the PtPd deposition to form PtPd/(rGO:MWCNT). Our experimental results revealed that the PtPd nanomaterial prepared over a mixture of both carbonaceous supports had better oxygen reduction reaction (ORR) and PEMFC performances than the individually prepared catalysts. The insertion of MWCNT between rGO sheets prevented their stacking. This promoted the diffusion of oxygen molecules through the interlayer spacing, enhancing the ORR’s electrocatalytic activity. The durability test demonstrated that the hybrid supporting material dramatically improved the catalyst’s stability even after 3000 reaction cycles. This highlighted an increase greater than 100% for hybrid nanocomposites in their electrocatalytic activity as compared with the PtPd/rGO nanocomposite.

Electrochemical Analysis for Demonstrating CO Tolerance of Catalysts in Polymer Electrolyte Membrane Fuel Cells

Since trace amounts of CO in H2 gas produced by steam reforming of methane causes severe poisoning of Pt-based catalysts in polymer electrolyte membrane fuel cells (PEMFCs), research has been mainly devoted to exploring CO-tolerant catalysts. To test the electrochemical property of CO-tolerant catalysts, chronoamperometry is widely used under a CO/H2 mixture gas atmosphere as an essential method. However, in most cases of catalysts with high CO tolerance, the conventional chronoamperometry has difficulty in showing the apparent performance difference. In this study, we propose a facile and precise test protocol to evaluate the CO tolerance via a combination of short-term chronoamperometry and a hydrogen oxidation reaction (HOR) test. The degree of CO poisoning is systematically controlled by changing the CO adsorption time. The HOR polarization curve is then measured and compared with that measured without CO adsorption. When the electrochemical properties of PtRu alloy catalysts with different atomic ratios of Pt to Ru are investigated, contrary to conventional chronoamperometry, these catalysts exhibit significant differences in their CO tolerance at certain CO adsorption times. The present work will facilitate the development of catalysts with extremely high CO tolerance and provide insights into the improvement of electrochemical methods.

Precious Metal-Free Nickel Nitride Catalyst for the Oxygen Reduction Reaction

With promising activity and stability for the oxygen reduction reaction (ORR), transition metal nitrides are an interesting class of non-platinum group catalysts for polymer electrolyte membrane fuel cells. Here, we report an active thin-film nickel nitride catalyst synthesized through a reactive sputtering method. In rotating disk electrode testing in a 0.1 M HClO₄ electrolyte, the crystalline nickel nitride film achieved high activity and selectivity to four-electron ORR. It also exhibited good stability during 10 and 40 h chronoamperometry measurements in acid and alkaline electrolyte, respectively. A combined experiment-theory approach, with detailed ex situ materials characterization and density functional theory calculations, provides insight into the structure of the catalyst and its surface during catalysis. Design strategies for activity and stability improvement through alloying and nanostructuring are discussed.