Liquid Metal Interfacial Growth and Exfoliation to Form Mesoporous Metallic Nanosheets for Alkaline Methanol Electroreforming

Directly Printable Flexible Optoelectronics through Surface Atomic Modification of Liquid Metals at Room Temperature

Flexible optoelectronics have fully demonstrated their transformative roles in various fields, but their fabrication and application have been limited by complex processes. Liquid metals (LMs) are promising to be ideal raw materials for making flexible optoelectronics due to their extraordinary fluidity and printability. Herein, we propose a painting-modifying strategy based on solution processability for directly printing out fluorescent flexible optoelectronics from LMs via surface modification. The LMs of eGaIn, which were obtained by the mixing of gallium with indium metal spheres, were used as ink to paint high-finesse patterns on flexible substrates. Through introducing surface modification of LMs, the gallium atom on the surface of the LMs was directly transformed into the composite fluorescent functional layers of GaO(OH) and GaN after being modified with an ammonia aqueous solution. Owing to painting, this strategy is not limited by any curved surfaces, shapes, or facilities and has excellent adaptability. Particularly, the fluorescent layers were obtained through a spontaneous, instantaneous, and solution-processable process that is environmentally friendly, easy to administrate, recyclable, and adjustable. The present finding breaks through the limitations of LMs in making flexible optoelectronics and provides strategies for addressing severe challenges facing existing materials and flexible optoelectronics. This method is expected to be very useful for fabricating flexible lights, transformable displays, intelligent anticounterfeiting devices, skin-inspired optoelectronics, and chameleon-biomimetic soft robots in the coming time.

2D PtRhPb Mesoporous Nanosheets with Surface-Clean Active Sites for Complete Ethanol Oxidation Electrocatalysis

The development of active and selective metal electrocatalysts for complete ethanol oxidation reaction (EOR) into desired C1 products is extremely promising for practical application of direct ethanol fuel cells. Despite some encouraging achievements, their activity and selectivity remain unsatisfactory. In this work, it is reported that 2D PtRhPb mesoporous nanosheets (MNSs) with anisotropic structure and surface-clean metal site perform perfectly for complete EOR electrocatalysis in both three-electrode and two-electrode systems. Different to the traditional routes, a selective etching strategy is developed to produce surface-clean mesopores while retaining parent anisotropy quasi-single-crystalline structure without the mesopore-forming surfactants. This method also allows the general synthesis of surface-clean mesoporous metals with other compositions and structures. When being performed for alkaline EOR electrocatalysis, the best PtRhPb MNSs deliver remarkably high activity (7.8 A mg-1) and superior C1 product selectivity (70% of Faradaic efficiency), both of which are much better than reported electrocatalysts. High performance is assigned to multiple structural and compositional synergies that not only stabilized key OHads intermediate by surface-clean mesopores but also separated the chemisorption of two carbons in ethanol by adjacent Pt and Rh sites, which facilitate the oxidation cleavage of stable C─C bond for complete EOR electrocatalysis.

Unlocking Efficient Hydrogen Production: Nucleophilic Oxidation Reactions Coupled with Water Splitting

Electrocatalytic water splitting driven by sustainable energy is a clean and promising water-chemical fuel conversion technology for the production of high-purity green hydrogen. However, the sluggish kinetics of anodic oxygen evolution reaction (OER) pose challenges for large-scale hydrogen production, limiting its efficiency and safety. Recently, the anodic OER has been replaced by a nucleophilic oxidation reaction (NOR) with biomass as the substrate and coupled with a hydrogen evolution reaction (HER), which has attracted great interest. Anode NOR offers faster kinetics, generates high-value products, and reduces energy consumption. By coupling NOR with hydrogen evolution reaction, hydrogen production efficiency can be enhanced while yielding high-value oxidation products or degrading pollutants. Therefore, NOR-coupled HER hydrogen production is another new green electrolytic hydrogen production strategy after electrolytic water hydrogen production, which is of great significance for realizing sustainable energy development and global decarbonization. This review explores the potential of nucleophilic oxidation reactions as an alternative to OER and delves into NOR mechanisms, guiding future research in NOR-coupled hydrogen production. It assesses different NOR-coupled production methods, analyzing reaction pathways and catalyst effects. Furthermore, it evaluates the role of electrolyzers in industrialized NOR-coupled hydrogen production and discusses future prospects and challenges. This comprehensive review aims to advance efficient and economical large-scale hydrogen production.

Bifunctional Electrocatalysts for Overall and Hybrid Water Splitting

Electrocatalytic water splitting driven by renewable electricity has been recognized as a promising approach for green hydrogen production. Different from conventional strategies in developing electrocatalysts for the two half-reactions of water splitting (e.g., the hydrogen and oxygen evolution reactions, HER and OER) separately, there has been a growing interest in designing and developing bifunctional electrocatalysts, which are able to catalyze both the HER and OER. In addition, considering the high overpotentials required for OER while limited value of the produced oxygen, there is another rapidly growing interest in exploring alternative oxidation reactions to replace OER for hybrid water splitting toward energy-efficient hydrogen generation. This Review begins with an introduction on the fundamental aspects of water splitting, followed by a thorough discussion on various physicochemical characterization techniques that are frequently employed in probing the active sites, with an emphasis on the reconstruction of bifunctional electrocatalysts during redox electrolysis. The design, synthesis, and performance of diverse bifunctional electrocatalysts based on noble metals, nonprecious metals, and metal-free nanocarbons, for overall water splitting in acidic and alkaline electrolytes, are thoroughly summarized and compared. Next, their application toward hybrid water splitting is also presented, wherein the alternative anodic reactions include sacrificing agents oxidation, pollutants oxidative degradation, and organics oxidative upgrading. Finally, a concise statement on the current challenges and future opportunities of bifunctional electrocatalysts for both overall and hybrid water splitting is presented in the hope of guiding future endeavors in the quest for energy-efficient and sustainable green hydrogen production.

Functional micro-/nanostructured gallium-based liquid metal for biochemical sensing and imaging applications

In recent years, liquid metals (LMs) have garnered increasing attention for their expanded applicability, and wide application potential in various research fields. Among them, gallium (Ga)-based LMs exhibit remarkable analytical performance in electrical and optical sensors, thanks to their excellent conductivity, large surface area, biocompatibility, small bandgap, and high elasticity. This review comprehensively summarizes the latest advancements in functional micro-/nanostructured Ga-based LMs for biochemical sensing and imaging applications. Firstly, the electrical, optical, and biocompatible features of Ga-based LM micro-/nanoparticles are briefly discussed, along with the manufacturing and functionalization processes. Subsequently, we demonstrate the utilization of Ga-based LMs in biochemical sensing techniques, encompassing electrochemistry, electrochemiluminescence, optical sensing techniques, and various biomedical imaging. Lastly, we present an insightful perspective on promising research directions and remaining challenges in LM-based biochemical sensing and imaging applications.

Synthesis and Prospects of Holey Two-dimensional Platinum-group Metals in Electrocatalysis

Advanced electrocatalysts can enable the widespread implementation of clean energy technologies. This paper reviews an emerging class of electrocatalytic materials comprising holey two-dimensional free-standing Pt-group metal (h-2D-PGM) nanosheets, which are categorically challenging to synthesize but inherently rich in all the qualities necessary to counter the kinetic and thermodynamic challenges of an electrochemical conversion process with high catalytic efficiency and stability. Although the 2D anisotropic growth of typical nonlayered metal crystals has succeeded and partly improved their atom-utilization efficiency, regularly distributed in-planar porosity can further optimize three critical factors that govern efficient electrocatalysis process: mass diffusion, electron transfer, and surface reactivity. However, producing such advanced morphological features within h-2D-PGMs is difficult unless they are specially engineered using approaches such as templating or kinetic ramification during 2D growth or controlled etching of preformed 2D-PGM solids. Therefore, this review highlighting the successful fabrication of various porous PGM nanosheets and their electrocatalytic benefits involving smart nanoscale features could inspire next-generation scientific and technological innovations toward securing a sustainable energy future.

Two-dimensional mesoporous metals: a new era for designing functional electrocatalysts

Two-dimensional (2D) mesoporous metals contribute a unique class of electrocatalyst materials for electrochemical applications. The penetrated mesopores of 2D mesoporous metals expose abundant accessible undercoordinated metal sites, while their 2D nanostructures accelerate the transport of electrons and reactants. Therefore, 2D mesoporous metals have exhibited add-in structural functions with great potential in electrocatalysis that not only enhance electrocatalytic activity and stability but also optimize electrocatalytic selectivity. In this Perspective, we summarize recent progress in the design, synthesis, and electrocatalytic performance of 2D mesoporous metals. Four main strategies for synthesizing 2D mesoporous metals, named the CO (and CO container) induced route, halide ion-oriented route, interfacial growth route, and metal oxide atomic reconstruction route, are presented in detail. Moreover, electrocatalytic applications in several important reactions are summarized to highlight the add-in structural functions of 2D mesoporous metals in enhancing electrochemical activity, stability, and selectivity. Finally, current challenges and future directions are discussed in this area. This Perspective offers some important insights into both fundamental investigations and practical applications of novel high-performance functional electrocatalysts.

Liposome-Templated Green Synthesis of Mesoporous Metal Nanostructures with Universal Composition for Biomedical Application

Porous noble metal nanoparticles have received particular attention recently for their unique optical, thermal, and catalytic functions in biomedicine. However, limited progress has been made to synthesize such porous metallic nanostructures with large mesopores (≥25 nm). Here, a green yet facile synthesis strategy using biocompatible liposomes as templates to mediate the formation of mesoporous metallic nanostructures in a controllable fashion is reported. Various monodispersed nanostructures with well-defined mesoporous shape and large mesopores (≈ 40 nm) are successfully synthesized from mono- (Au, Pd, and Pt), bi- (AuPd, AuPt, AuRh, PtRh, and PdPt), and tri-noble metals (AuPdRh, AuPtRh, and AuPdPt). Along with a successful demonstration of its effectiveness in synthesis of various mesoporous nanostructures, the possible mechanism of liposome-guided formation of such nanostructures via time sectioning of the synthesis process (monitoring time-resolved growth of mesoporous structures) and computational quantum molecular modeling (analyzing chemical interaction energy between metallic cations and liposomes at the enthalpy level) is also revealed. These mesoporous metallic nanostructures exhibit a strong photothermal effect in the near-infrared region, effective catalytic activities in hydrogen peroxide decomposition reaction, and high drug loading capacity. Thus, the liposome-templated method provides an inspiring and robust avenue to synthesize mesoporous noble metal-based nanostructures for versatile biomedical applications.

Understanding and Expanding the Prospects for Electrosynthesis with Liquid Metal Electrodes

ConspectusThis Account describes and summarizes the latest work from our laboratory on developing and maturing strategies based on low-temperature liquid metals as reaction environments for materials synthesis. The electrochemical liquid-liquid-solid (ec-LLS) crystal growth concept is a hybrid method that combines electrodeposition and melt crystal growth. Using liquid metals as both electrodes and solvents for the purpose of producing inorganic crystals and materials, a simple and environmentally friendly process is possible. The impetus is to address the key deficiency in the inorganic crystalline materials that are the basis of modern optoelectronics and renewable energy capture/conversion systems. Specifically, existing methods for synthesizing crystalline inorganic materials for these purposes are largely energy- and resource-intensive, with a substantial impact on the environment when scaled. A long-term goal of our work with ec-LLS is to realize a materials synthetic process that is matured without requiring intensive resources or negatively impacting the environment. To this end, the factors that both limit and govern ec-LLS processes must be identified and understood. To date, questions regarding the factors that affect crystal nucleation and growth, form factors, and overall composition remain.Previous work established concretely ec-LLS as a versatile method for synthesizing and producing crystalline semiconductors at low temperatures as either particles, nanowires, or microwires. Subsequent experiments have focused on two tiers. First, the microscopic details of the liquid metal and its interfaces that dictate materials synthesis and crystal growth must be identified. Second, strategies that widen the attainable material form factors to facilitate device architectures must be realized. Hence, this Account describes results aimed at answering three questions: (1) What are the consequences of reaching supersaturation by an electrochemical rather than a thermal driving force for crystal growth in ec-LLS? (2) Can the location of nucleation and subsequent crystal growth be controlled? (3) Does the atomic structure of the liquid metal affect product formation in ec-LLS? The science described herein illustrates the value of in situ methods spanning transmission electron microscopy, X-ray diffraction, and X-ray reflectance for revealing the role that liquid metal composition and structure can play in ec-LLS. Additionally, we summarize work that shows for the first time that it is possible to produce both single-crystalline epitaxial films and complex intermetallic compounds through ec-LLS by tuning the cell design, electrochemical excitation waveform, and composition of the liquid metal electrodes.The cumulative findings described here substantially enrich our understanding of the ec-LLS concept while simultaneously motivating further questions moving forward. Is it possible to attain complete control over the crystalline quality and composition of ec-LLS products? Can the materials produced by ec-LLS provide tailored functional properties for targeted applications? Can the ec-LLS strategy be further refined to allow material synthesis and deposition at precise locations with deterministically chosen form factors? What synthetic pathways are accessible when even more sophisticated electrochemical waveforms and cell designs are used? Our hope is that this Account will spur additional researchers to help answer such questions.

A self-healing electrocatalytic system via electrohydrodynamics induced evolution in liquid metal

Catalytic deterioration during electrocatalytic processes is inevitable for conventional composite electrodes, which are prepared by depositing catalysts onto a rigid current collector. In contrast, metals that are liquid at near room temperature, liquid metals (LMs), are potential electrodes that are uniquely flexible and maneuverable, and whose fluidity may allow them to be more adaptive than rigid substrates. Here we demonstrate a self-healing electrocatalytic system for CO₂ electroreduction using bismuth-containing Ga-based LM electrodes. Bi₂O₃ dispersed in the LM matrix experiences a series of electrohydrodynamic-induced structural changes when exposed to a tunable potential and finally transforms into catalytic bismuth, whose morphology can be controlled by the applied potential. The electrohydrodynamically-induced evolved electrode shows considerable electrocatalytic activity for CO₂ reduction to formate. After deterioration of the electrocatalytic performance, the catalyst can be healed via simple mechanical stirring followed by in situ regeneration by applying a reducing potential. With this procedure, the electrode's original structure and catalytic activity are both recovered.

Design and Synthesis of Cobalt-Based Hollow Nanoparticles through the Liquid Metal Template

Co-based compounds have attracted much attention due to their competitive catalytic activities. To enhance their intrinsic electrocatalytic activity, morphology engineering is one of the effective strategies. Hollow structures have fascinating properties due to their low density and high loading capacity. In this work, we introduce a Ga-based liquid alloy as a reactive template for the synthesis of varying Co-based hollow nanoparticles. The fluidity character of the Ga-based liquid alloy facilitates the large-scale production of nanoparticles via a top-down shearing process. The pre-installed active species (here is Zn) in the liquid alloy serve as a sacrificial source to quantitatively reduce Co²⁺ ions and form Co-based compounds. Well-structured Ga/CoOOH core-shell nanospheres are thus successfully prepared, and more varied Co-based hollow nanoparticles can be obtained by post-treatment and reaction. Hollow structures can offer enhanced interfacial area and increased active sites, benefiting the catalytic performance. Among the prepared Co-based catalysts, CoSe₂ hollow nanoparticles exhibited the best oxygen evolution reaction (OER) activity with an overpotential of 340 mV at the current density of 10 mA/cm². This work provides a novel strategy for the rational design and simple preparation of hollow nanoparticles.

Highly Curved, Quasi-Single-Crystalline Mesoporous Metal Nanoplates Promote CC Bond Cleavage in Ethanol Oxidation Electrocatalysis

The ability to manipulate metal nanocrystals with well-defined morphologies and structures is greatly important in material chemistry, catalysis chemistry, nanoscience, and nanotechnology. Although 2D metals serve as interesting platforms, further manipulating them in solution with highly penetrated mesopores and ideal crystallinity remains a huge challenge. Here, an easy yet powerful synthesis strategy for manipulating the mesoporous structure and crystallinity of 2D metals in a controlled manner with cetyltrimethylammonium chloride as the mesopore-forming surfactant and extra iodine-ion as the structure/facet-selective agent is reported. This strategy allows for preparing an unprecedented type of 2D quasi-single-crystalline mesoporous nanoplates (SMPs) with highly curved morphology and controlled metal composition. The products, for example, PdCu SMPs, feature abundant undercoordinated sites, optimized electronic structures, excellent electron/mass transfers, and confined mesopore environments. Curved PdCu SMPs exhibit remarkable electrocatalytic activity of 6.09 A mg_Pd ^-1 and stability for ethanol oxidation reaction (EOR) compared with its counterpart catalysts and commercial Pd/C. More importantly, PdCu SMPs are highly selective for EOR electrocatalysis that dramatically promotes C-C bond cleavage with a superior C1 pathway selectivity as high as 72.1%.

Surface Engineering of Defective and Porous Ir Metallene with Polyallylamine for Hydrogen Evolution Electrocatalysis

The design of defects and porous structures into metallene with functional surfaces is highly desired to improve its permeability, surface area, and active sites, but remains a great challenge. In this work, polyallylamine-encapsulated Ir metallene with defects and porous structure (Ir@PAH metallene) is easily fabricated by a one-step wet chemical reduction method. The Ir@PAH metallene exhibits excellent hydrogen evolution reaction (HER) performance with an overpotential of only 14 mV at 10 mA cm^-2 , a low Tafel slope of 31.2 mV dec^-1 , and almost no activity decay after stability test. The abundant defects and pores as well as several-atomic-layer nanosheet structures of Ir@PAH metallene provide a large specific surface area, high conductivity, and efficient mass transport/diffusion. In addition, surface-functionalized PAH molecules can modulate the electronic structure through strong Ir-N interaction and act as proton carriers to capture hydrogen ions, which is very beneficial for the HER in acidic media. This work provides a useful strategy for the synthesis of the defective and porous metallene with functionalized surfaces for various catalytic applications.