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Group VI transition metal chalcogenides are the subject of increasing research interest for various electrochemical applications such as low-temperature water electrolysis, batteries, and supercapacitors due to their high activity, chemical stability, and the strong correlation between structure and electrochemical properties. Particularly appealing is their utilization as electrocatalysts for the synthesis of energy vectors and value-added chemicals such as C-based chemicals from the CO2 reduction reaction (CO2R) or ammonia from the nitrogen fixation reaction (NRR). This review discusses the role of structural and electronic properties of transition metal chalcogenides in enhancing selectivity and activity toward these two key reduction reactions. First, we discuss the morphological and electronic structure of these compounds, outlining design strategies to control and fine-tune them. Then, we discuss the role of the active sites and the strategies developed to enhance the activity of transition metal chalcogenide-based catalysts in the framework of CO2R and NRR against the parasitic hydrogen evolution reaction (HER); leveraging on the design rules applied for HER applications, we discuss their future perspective for the applications in CO2R and NRR. For these two reactions, we comprehensively review recent progress in unveiling reaction mechanisms at different sites and the most effective strategies for fabricating catalysts that, by exploiting the structural and electronic peculiarities of transition metal chalcogenides, can outperform many metallic compounds. Transition metal chalcogenides outperform state-of-the-art catalysts for CO2 to CO reduction in ionic liquids due to the favorable CO2 adsorption on the metal edge sites, whereas the basal sites, due to their conformation, represent an appealing design space for reduction of CO2 to complex carbon products. For the NRR instead, the resemblance of transition metal chalcogenides to the active centers of nitrogenase enzymes represents a powerful nature-mimicking approach for the design of catalysts with enhanced performance, although strategies to hinder the HER must be integrated in the catalytic architecture.
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As a key precursor for nitrogenous compounds and fertilizer, ammonia affects our lives in numerous ways. Rapid and sensitive detection of ammonia is essential, both in environmental monitoring and in process control for industrial production. Here we report a novel and nonperturbative method that allows rapid detection of ammonia at low concentrations, based on the all-optical detection of surface-enhanced Raman signals. We show that this simple and affordable approach enables ammonia probing at selected regions of interest with high spatial resolution, making in situ and operando observations possible.
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We investigated the oxygen reduction reaction (ORR) mechanism on Pt nanoparticles (NPs) dispersed on several carbon blacks with various physicochemical properties (i. e. specific surface ranging from 80 to 900â m2 g-1 , different graphitization degree, etc.). Using the kinetic isotope effect (KIE) along with various electrochemical characterizations, we determined that the rate determining step (RDS) of the ORR is a proton-independent step when the density of Pt NPs on the surface of the carbon support is high. Upon decrease of the density of Pt NPs on the surface, the RDS of the ORR starts involving a proton, as denoted by an increase of the KIE >1. This underlined the critical role played by the carbon support in the oxygen reduction reaction electrocatalysis by Pt supported on high surface area carbon.
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The front cover artwork is provided by the groups of Prof. Atanassov and Prof. Zenyuk (University of California Irvine, USA). The image shows rate-determining step of oxygen reduction reaction on platinum nanoparticle supported by carbon, which requires electron transfer but no proton. Read the full text of the Article at 10.1002/cphc.201901091.
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Kinetic isotope effect (KIE) was used to study the rate-determining step for oxygen reduction reaction (ORR) on dispersed Pt/C electrocatalyst and polycrystalline Pt (Pt-poly). KIE is defined as the ratio of the kinetic current measured in protonated electrolyte versus deuterated electrolyte, with KIE values larger than one indicating proton participation in the rate-determining step. The effect of poisoning anions on the platinum rate determining step is investigated by assessing the KIE in perchloric (non-poisoning) and sulfuric acid-based electrolytes. The kinetics currents were calculated using the Koutechy-Levich and Tafel analysis. A KIE of 1 was observed for Pt/C (with a 40â wt.% Pt loading) and Pt-poly, thus indicating that, on 40 wt. % Pt/C and Pt-poly, the rate determining step is proton independent.
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Shifting syngas (an H2 /CO mixture) production away from fossil-fuel-dependent processes (e.g., steam methane reforming and coal gasification) is mandatory, as syngas is of interest as both a fuel and as a value-added chemical precursor. With appropriate electrocatalysts, such as silver-based and metal-nitrogen-carbon (M-N-C) materials, the electrochemical CO2 reduction reaction (CO2 RR) allows for the production of CO alongside H2 (from the hydrogen evolution reaction), and thus leads to syngas generation. In this Minireview, the application of M-N-C electrocatalysts for syngas generation is discussed. The mechanisms leading to different faradaic selectivities for CO are reviewed as a function of the nature of the metal, by using both computational and experimental approaches. The role played by the metal-free moieties in the M-N-C electrocatalysts is underlined. Since M-N-C electrocatalysts only recently entered the CO2 RR field (as opposed to Cu-, Ag-, or Au-based nanostructures), they have been mainly characterized in static liquid environments, in which the reaction rate is significantly hampered by CO2 -dissolution/diffusion limitations. Therefore, the design of CO2 RR electrolyzers for M-N-C electrocatalysts is addressed, and designs such as zero-gap electrolyzers with anionic membranes and humidified CO2 gas feed at the cathode are highlighted.
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The morphology of electrode materials is often overlooked when comparing different carbon-based electrocatalysts for carbon dioxide reduction. To investigate the role of morphological attributes, we studied polymer-derived, interconnected, N-doped carbon structures with uniformly sized meso or macropores, differing only in the pore size. We found that the carbon dioxide reduction selectivity (versus the hydrogen evolution reaction) increased around three times just by introducing the porosity into the carbon structure (with an optimal pore size of 27 nm). We attribute this change to alterations in the wetting and CO2 adsorption properties of the carbon catalysts. These insights offer a new platform to advance CO2 reduction performance by only morphological engineering of the electrocatalyst.
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Tuning the surface structure at the atomic level is of primary importance to simultaneously meet the electrocatalytic performance and stability criteria required for the development of low-temperature proton-exchange membrane fuel cells (PEMFCs). However, transposing the knowledge acquired on extended, model surfaces to practical nanomaterials remains highly challenging. Here, we propose 'surface distortion' as a novel structural descriptor, which is able to reconciliate and unify seemingly opposing notions and contradictory experimental observations in regards to the electrocatalytic oxygen reduction reaction (ORR) reactivity. Beyond its unifying character, we show that surface distortion is pivotal to rationalize the electrocatalytic properties of state-of-the-art of PtNi/C nanocatalysts with distinct atomic composition, size, shape and degree of surface defectiveness under a simulated PEMFC cathode environment. Our study brings fundamental and practical insights into the role of surface defects in electrocatalysis and highlights strategies to design more durable ORR nanocatalysts.
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Due to their interesting electrocatalytic properties for the oxygen reduction reaction (ORR), hollow Pt-alloy nanoparticles (NPs) supported on high-surface-area carbon attract growing interest. However, the suitable synthesis methods and associated mechanisms of formation, the reasons for their enhanced specific activity for the ORR, and the nature of adequate alloying elements and carbon supports for this type of nanocatalysts remain open questions. This Review aims at shedding light on these topics with a special emphasis on hollow PtNi NPs supported onto Vulcan C (PtNi/C). We first show how hollow Pt-alloy/C NPs can be synthesized by a mechanism involving galvanic replacement and the nanoscale Kirkendall effect. Nickel, cobalt, copper, zinc, and iron (Ni, Co, Cu, Zn, and Fe, respectively) were tested for the formation of Pt-alloy/C hollow nanostructures. Our results indicate that metals with standard potential -0.4
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The oxygen reduction reaction (ORR) activity of Pt-alloy electrocatalysts depends on (i) the strain/ligand effects induced by the non-noble metal (3d-transition metal or a rare-earth element) alloyed to Pt, (ii) the orientation of the catalytic surfaces, and (iii) the density of structural defects (SDs) (e.g., vacancies, voids, interconnections). These SDs influence the "generalized" coordination number of Pt atoms, the Pt-alloy lattice parameter, and thus the adsorption strength of the ORR intermediates (O*, OH*, OOH*). Here, we discuss a set of parameters derived from COads stripping measurements and the Rietveld refinement of X-ray diffraction (XRD) patterns, aiming to show how the leaching of the non-noble metal and the density of SDs influence the ORR activity of porous hollow PtNi/C nanoparticles (PH-PtNi/C NPs). PH-PtNi/C NPs were aged at T = 353 K in an Ar-saturated 0.1 M HClO4 electrolyte during 20 000 potential cycles between E = 0.6 and 1.0 V versus the reversible hydrogen electrode, with an intermediate characterization after 5000 cycles. The losses in the ORR specific activity were attributed to the dissolution of Ni atoms (modifying strain/ligand effects) and to the increase of the crystallite size (dXRD), resulting in a diminution of the density of grain boundaries. In agreement with the Gibbs-Thompson equation, the electrocatalysts that presented larger crystallites (dXRD > 3 nm) were far more stable than the ones with the smallest crystallites (dXRD < 2 nm). We also observed that performing intermediate characterizations (in an O2-saturated electrolyte) results in activity losses for the ORR.
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Determining the formation and growth mechanism of bimetallic nanoparticles (NPs) with atomic detail is fundamental to synthesize efficient "catalysts by design". However, an understanding of the elementary steps which take place during their synthesis remains elusive. Herein, we have exploited scanning transmission electron microscopy coupled to energy-dispersive X-ray spectroscopy, operando wide angle and small-angle X-ray scattering, and electrochemistry to unveil the formation and growth mechanism of hollow PtNi/C NPs. Such NPs, composed of a PtNi shell surrounding a nanoscale void, catalyze efficiently and sustainably the oxygen reduction reaction (ORR) in an acidic electrolyte. Our step-by-step study reveals that (i) Ni-rich/C NPs form first, before being embedded in a NixByOz shell, (ii) the combined action of galvanic displacement and the nanoscale Kirkendall effect then results in the sequential formation of Ni-rich core@Pt-rich/C shell and ultimately hollow PtNi/C NPs. The electrocatalytic properties for the ORR and the stability of the different synthesis intermediates were tested and structure-activity-stability relationships established both in acidic and alkaline electrolytes. Beyond its interest for the ORR electrocatalysis, this study also presents a methodology that is capable to unravel the formation and growth mechanism of various nanomaterials including preferentially shaped metal NPs, core@shell NPs, onion-like NPs, Janus NPs, or a combination of several of these structures.
