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Ammonia, a key feedstock used in various industries, has been considered a sustainable fuel and energy storage option. However, NH3 production via the conventional Haber-Bosch process is costly, energy-intensive, and significantly contributing to a massive carbon footprint. An electrochemical synthetic pathway for nitrogen fixation has recently gained considerable attention as NH3 can be produced through a green process without generating harmful pollutants. This review discusses the recent progress and challenges associated with the two relevant electrochemical pathways: direct and indirect nitrogen reduction reactions. The detailed mechanisms of these reactions and highlight the recent efforts to improve the catalytic performances are discussed. Finally, various promising research strategies and remaining tasks are presented to highlight future opportunities in the electrochemical nitrogen reduction reaction.
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Metastable phases-kinetically favoured structures-are ubiquitous in nature1,2. Rather than forming thermodynamically stable ground-state structures, crystals grown from high-energy precursors often initially adopt metastable structures depending on the initial conditions, such as temperature, pressure or crystal size1,3,4. As the crystals grow further, they typically undergo a series of transformations from metastable phases to lower-energy and ultimately energetically stable phases1,3,4. Metastable phases sometimes exhibit superior physicochemical properties and, hence, the discovery and synthesis of new metastable phases are promising avenues for innovations in materials science1,5. However, the search for metastable materials has mainly been heuristic, performed on the basis of experiences, intuition or even speculative predictions, namely 'rules of thumb'. This limitation necessitates the advent of a new paradigm to discover new metastable phases based on rational design. Such a design rule is embodied in the discovery of a metastable hexagonal close-packed (hcp) palladium hydride (PdHx) synthesized in a liquid cell transmission electron microscope. The metastable hcp structure is stabilized through a unique interplay between the precursor concentrations in the solution: a sufficient supply of hydrogen (H) favours the hcp structure on the subnanometre scale, and an insufficient supply of Pd inhibits further growth and subsequent transition towards the thermodynamically stable face-centred cubic structure. These findings provide thermodynamic insights into metastability engineering strategies that can be deployed to discover new metastable phases.
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Alloy structures with high catalytic surface areas and tunable surface energies can lead to high catalytic selectivity and activities. Herein, the synthesis of sponge-like Pd3 Pb multiframes (Pd3 Pb MFs) is reported by using the thermodynamically driven phase segregation, which exhibit high selectivity (93%) for the conversion of furfural to furfuryl alcohol (FOL) under mild conditions. The excellent catalytic performance of the Pd3 Pb MF catalysts is attributed to the high surface area and optimized surface energy of the catalyst, which is associated with the introduction of Pb to Pd. Density functional theory calculations show that the binding energy of FOL to the surface energy-tuned Pd3 Pb MF is sufficiently lowered to prevent side reactions such as over-hydrogenation of FOL.
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Postmodification of nanocrystals through cation exchange has been very successful in diversifying nanomaterial compositions while retaining the structural motif. Copper compound nanoparticles are particularly useful as templates because of inherent defects serving as effective cation diffusion routes and excellent cation mobility. Therefore, the development of shape-controlled multianion systems, such as copper phosphosulfide, can potentially lead to the formation of diverse metal phosphosulfide nanomaterials that have otherwise inaccessible compositions and structures. However, there is, to the best of our knowledge, no report on the shape-controlled synthesis of copper phosphosulfide nanoparticles because the introduction of the second anion to the metal compound might destroy the nanoparticle morphology and crystallinity due to the required high energy for anion diffusion and mixing. Herein, we report that it is feasible to transfer the structural motif of copper sulfide to copper phosphosulfide using tris(diethylamino)phosphine. The anion-mixed copper phosphosulfide in the form of a hollow toroid could provide a pathway to previously inaccessible phases and morphologies. We verified the versatility of a copper phosphosulfide hollow toroid as a cation-exchange template by the successful synthesis of cobalt, nickel, indium, and cadmium phosphosulfides as well as bimetallic cobalt-nickel phosphosulfide (Co2-xNixP1-ySy) with a retained structural motif.
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Liquid metals are emerging as fluidic inorganic materials in various research fields. Micro- and nanoparticles of Ga and its alloys have received particular attention in the last decade due to their non toxicity and accessibility in ambient conditions as well as their interesting chemical, physical, mechanical, and electrical properties. Unique features such as a fluidic nature and self-passivating oxide skin make Ga-based liquid metal particles (LMPs) distinguishable from conventional inorganic particles in the context of synthesis and applications. Here, recent advances in the bottom-up and top-down synthetic methods of Ga-based LMPs, their physicochemical properties, and their applications are summarized. Finally, the current status of the LMPs is highlighted and perspectives on future directions are also provided.
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While the realization of clean and sustainable energy conversion systems primarily requires the development of highly efficient catalysts, one of the main issues had been designing the structure of the catalysts to fulfill minimum cost as well as maximum performance. Until now, noble metal-based nanocatalysts had shown outstanding performances toward the oxygen reduction reaction (ORR), oxygen evolution reaction (OER), and hydrogen evolution reaction (HER). However, the scarcity and high cost of them impeded their practical use. Recently, hollow nanostructures including nanocages and nanoframes had emerged as a burgeoning class of promising electrocatalysts. The hollow nanostructures could expose a high proportion of active surfaces while saving the amounts of expensive noble metals. In this review, we introduced recent advances in the synthetic methodologies for generating noble metal-based hollow nanostructures based on thermodynamic and kinetic approaches. We summarized electrocatalytic applications of hollow nanostructures toward the ORR, OER, and HER. We next provided strategies that could endow structural robustness to the flimsy structural nature of hollow structures. Finally, we concluded this review with perspectives to facilitate the development of hollow nanostructure-based catalysts for energy applications.
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Highly active and durable electrocatalysts for the hydrogen evolution reaction (HER) may play a pivotal role in commercial success of electrolytic water splitting technology. Among various material classes, binary metal sulphides show a great promise as HER catalysts because of their tunable energy levels conducive to a high catalytic activity and high robustness under harsh operating conditions. On the other hand, facet-controlled nanoparticles with controlled surface energies have gained great recent popularity as active and selective catalysts. However, binary metal sulphide nanoparticles with well-defined facets and high surface areas are very rare. Herein we report the synthesis of a facet-controlled hollow Rh3Pb2S2 nanocage as a new catalytic material and its excellent activity (overpotential: 87.3 mV at 10 mA cm-2) and robustness toward HER under harsh acidic conditions.
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Hollow nanostructures such as nanocages and nanoframes can serve as advanced catalysts with their enlarged active surface areas, and hence they have been of widespread interest. Despite the recent progress in the synthesis of this class of nanomaterials, hollow nanostructures with tunable compositions and controlled morphologies have rarely been reported. Here, we report a facile synthetic route to a series of compositionally tunable, hollow mixed metal sulphide (CoxNiySz) octahedral nanocages. The sulfidation of CoO octahedral nanoparticles generates CoO@CoxSy core-shell octahedra, and the in situ etching of the CoO core and annealing yield Co9S8 (pentlandite) octahedral nanocages (ONC). The addition of a Ni precursor during the etching/annealing process of CoO@CoxSy core-shell octahedra progressively yields hollow ONC structures of Co9-xNixS8, Ni9S8, Ni9S8/ß-NiS, and Ni3S2/ß-NiS via cation exchange reactions. Mixed cobalt/nickel sulphide, Co9-xNixS8 ONC, shows superior oxygen evolution reaction activity to monometallic sulphide ONC structures, demonstrating the synergy between different metal species.
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Highly efficient and low-cost electrocatalysts are essential for water spitting via electrolysis in an economically viable fashion. However, the best catalytic performance is found with noble metal-based electrocatalysts, which presents a formidable obstacle for the commercial success of electrolytic water splitting-based H2 production due to their relatively high cost and scarcity. Therefore, the development of alternative inexpensive earth-abundant electrode materials with excellent electrocatalytic properties is of great urgency. In general, efficient electrocatalysts must possess several key characteristics such as low overpotential, good electrocatalytic activity, high stability, and low production costs. Direct synthesis of nanostructured catalysts on a conducting substrate may potentially improve the performance of the resultant electrocatalysts because of their high catalytic surface areas and the synergistic effect between the electrocatalyst and the conductive substrate. In this regard, three dimensional (3D) nickel foams have been advantageously utilized as electrode substrates as they offer a large active surface area and a highly conductive continuous porous 3D network. In this review, we discuss the most recent developments in nanostructured materials directly synthesized on 3D nickel foam as potential electrode candidates for electrochemical water electrolysis, namely, the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER). We also provide perspectives and outlooks for catalysts grown directly on 3D conducting substrates for future sustainable energy technologies.
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Multimetallic nanocatalysts with a controlled structure can provide enhanced catalytic activity and durability by exploiting electronic, geometric, and strain effects. Herein, we report the synthesis of a novel ternary nanocatalyst based on Mo doped PtNi dendritic nanowires (Mo-PtNi DNW) and its bifunctional application in the methanol oxidation reaction (MOR) at the anode and the oxygen reduction reaction (ORR) at the cathode for direct methanol fuel cells. An unprecedented Mo-PtNi DNW structure can combine multiple structural attributes of the 1D nanowire morphology and dendritic surfaces. In the MOR, Mo-PtNi DNW exhibits superior activity to Pt/C and Mo doped Pt dendritic nanowires (Mo-Pt DNW), and excellent durability. Furthermore, Mo-PtNi DNW demonstrates excellent activity and durability for the ORR. This work highlights the important role of compositional and structural control in nanocatalysts for boosting catalytic performances.
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Spectroscopic properties (i.e., peak positions and widths) of vibrational probes are sensitively dependent on their local environments in liquids. Such spectroscopic sensitivities can be utilized for studying the structures and dynamics of a variety of molecular systems. Here, we have studied the ion pairing equilibrium and dynamics of SeCN(-) ion pairs with Li(+) and Mg(2+) cations in N,N-dimethylformamide (DMF). SeCN(-) ion is an excellent vibrational probe for studying ion dynamics in electrolyte solutions, not only because the vibrational lifetime of the CN stretch is substantially long but also because the CN stretch frequency is very sensitive to its local environment. When SeCN(-) ion forms contact ion pairs (CIPs) with Li(+) (Mg(2+)) ion in DMF solutions, the CN stretch frequency is found to be significantly blue-shifted such that free SeCN(-) ion is spectrally well distinguished from Li-SeCN CIP and Mg-SeCN(+) CIP. This fact allows us to study the ion pairing equilibrium between SeCN(-) ion and metal ions as well as the dynamics of metal-SeCN(-) ion pairs. Ion pairing equilibrium between SeCN(-) ion and Li(+) (or Mg(2+)) was studied by temperature-dependent Fourier transform infrared (FTIR) spectroscopy. The formation of CIPs in DMF was found to be entropically favored. Time-resolved IR pump-probe spectroscopy was used to study the vibrational population relaxation and orientational relaxation dynamics. Vibrational lifetimes of free SeCN(-) ion, Li-SeCN CIP, and Mg-SeCN(+) CIP were determined to be 83.6, 72.3, and 55.6 ps, respectively. Orientational relaxation dynamics were found to get slower in the order free SeCN(-) ion, Li-SeCN CIP, and Mg-SeCN(+) CIP. The orientational anisotropy decays of the CIPs, which were well fit by a biexponential function, were explained by two orientational relaxation processes, that is, a restricted (tethered) orientational relaxation of SeCN(-) within the CIPs followed by the overall orientational diffusion of the whole CIPs. The orientational relaxation time constants of Li-SeCN CIP and Mg-SeCN(+) CIP in DMF were twice different but the orientational diffusion radii calculated by the Debye-Stokes-Einstein equation were found to be almost identical within experimental error. The biexponential decay of the orientational anisotropy was analyzed by the wobbling-in-a-cone model. As a vibrational probe, SeCN(-) ion and SeCN group can be potentially used for measuring the molecular dynamics on a relatively long time scale because of their long lifetimes.
