Metal Aerogels Research Articles

High-Performance Bismuth-Doped Nickel Aerogel Electrocatalyst for the Methanol Oxidation Reaction.

Low-cost, non-noble-metal electrocatalysts are required for direct methanol fuel cells, but their development has been hindered by limited activity, high onset potential, low conductivity, and poor durability. A surface electronic structure tuning strategy is presented, which involves doping of a foreign oxophilic post-transition metal onto transition metal aerogels to achieve a non-noble-metal aerogel Ni97 Bi3 with unprecedented electrocatalytic activity and durability in methanol oxidation. Trace amounts of Bi are atomically dispersed on the surface of the Ni97 Bi3 aerogel, which leads to an optimum shift of the d-band center of Ni, large compressive strain of Bi, and greatly increased conductivity of the aerogel. The electrocatalyst is endowed with abundant active sites, efficient electron and mass transfer, resistance to CO poisoning, and outstanding performance in methanol oxidation. This work sheds light on the design of high-performance non-noble-metal electrocatalysts.

Self-supporting hierarchical PdCu aerogels for enhanced catalytic reduction of 4-nitrophenol

This work reports a new kind of self-assembled PdCu monolithic aerogels via a mild reduction process, which exhibits highly efficient catalytic reduction activity towards 4-nitrophenol. The enhanced catalytic reduction performance can be contributed the following unique features of PdCu aerogels: 1) the interconnected channels and three-dimensional network provide a platform for accelerating mass transfer during catalysis; 2) metallic aerogels combined with stretching ultrathin nanowires has a large surface area and good crystallinity affording sufficient reactive sites and high atom utilization; 3) the introduction of nonprecious Cu not only drastically cuts down the cost but also attains the excellent catalytic activity due to the bimetallic intrinsic synergetic effect; 4) the self-supporting feature is good for improving the durability of the catalyst. This study pushes a new avenue to develop robust catalysts for heterogeneous catalytic reactions.

Metallic Sandwiched-Aerogel Hybrids Enabling Flexible and Stretchable Intelligent Sensor.

Flexible strain sensors have been widely investigated with their rapid development in human-machine interfaces, soft robots, and medical care monitoring. Here, we report a new in situ catalytic strategy toward the fabrication of metallic aerogel hybrids, which are composed of vanadium nitride (VN) nanosheets decorated with well-defined vertically aligned carbon nanotube arrays (VN/CNTs) for the first time. In this architecture, the two-dimensional VN nanosheets as the main bone structure are favorable for the flexible devices due to their excellent structural compatibility during the repetitive deforming process. In addition, the sandwiched aerogel hybrids form highly conductive 3D network, affording outstanding sensitivity for the strain-responsive behaviors. Further, the VN/CNTs-based flexible strain sensors are successfully fabricated, showing a high gauge factor of 386 within a small strain of 10%, fast response, and extraordinary durability. The monitoring of physical signals and an actual real-time human-machine controlling system based on the sensors are also presented.

Unveiling reductant chemistry in fabricating noble metal aerogels for superior oxygen evolution\xa0and ethanol oxidation

Amongst various porous materials, noble metal aerogels attract wide attention due to their concurrently featured catalytic properties and large surface areas. However, insufficient understanding and investigation of key factors (e.g. reductants and ligands) in the fabrication process limits on-target design, impeding material diversity and available applications. Herein, unveiling multiple roles of reductants, we develop an efficient method, i.e. the excessive-reductant-directed gelation strategy. It enables to integrate ligand chemistry for creating gold aerogels with a record-high specific surface area (59.8 m2 g−1), and to expand the composition to all common noble metals. Moreover, we demonstrate impressive electrocatalytic performance of these aerogels for the ethanol oxidation and oxygen evolution reaction, and discover an unconventional organic-ligand-enhancing effect. The present work not only enriches the composition and structural diversity of noble metal aerogels, but also opens up new dimensions for devising efficient electrocatalysts for broad material systems.

3D Nitrogen-Doped Graphene Encapsulated Metallic Nickel-Iron Alloy Nanoparticles for Efficient Bifunctional Oxygen Electrocatalysis.

It is extremely desirable to explore high-efficient, affordable and robust oxygen electrocatalysts toward rechargeable Zn-air batteries (ZABs). A 3D porous nitrogen-doped graphene encapsulated metallic Ni3 Fe alloy nanoparticles aerogel (Ni3 Fe-GA1 ) was constructed through a facile hydrothermal assembly and calcination process. Benefiting from 3D porous configuration with great accessibility, high electrical conductivity, abundant active sites, optimal nitrogen content and strong electronic interactions at the Ni3 Fe/N-doped graphene heterointerface, the obtained aerogel showed outstanding catalytic performance toward the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR). Specifically, it exhibited an overpotential of 239 mV to attain 10 mA cm-2 for OER, simultaneously providing a positive onset potential of 0.93 V within a half-wave potential of 0.8 V for ORR. Accordingly, when employed in the aqueous ZABs, Ni3 Fe-GA1 achieved higher power density and superior reversibility than Pt/C-IrO2 catalyst, making it a potential candidate for rechargeable ZABs.

Engineering Multimetallic Aerogels for pH‐Universal HER and ORR Electrocatalysis

AbstractThe advent of noble metal aerogels (NMAs), that feature the high catalytic activity of noble metals and unique structural attributes of aerogels, has stimulated research on a new class of outstanding electrocatalysts. However, limited by the available compositions, the explored electrocatalytic reactions on NMAs are highly restricted and certain important electrochemical processes have not been investigated. Here, an effective gelation approach is demonstrated by using a strong salting‐out agent (i.e., NH4F), thereby expanding the composition of NMAs to various multimetallic systems and providing a platform to investigate composition‐dependent electrocatalytic performance of NMAs. Combining structural features of aerogels and optimized chemical compositions, the Au–Pt and Au–Rh aerogel catalysts manifest remarkable pH‐universal (pH = 0–14) performance surpassing commercial Pt/C and many other nanoparticle (NP)‐based catalysts in the electrocatalytic oxygen reduction reaction, hydrogen evolution reaction, and water splitting, displaying enormous potential for the electrochemical hydrogen production, fuel cells, etc.

Disturbance-Promoted Unconventional and Rapid Fabrication of Self-Healable Noble Metal Gels for (Photo-)Electrocatalysis

SummaryAs an emerging class of porous materials, noble metal aerogels (NMAs) have drawn tremendous attention and displayed unprecedented potential in diverse fields. However, the development of NMAs is impeded by the fabrication methods because of their time- and cost-consuming procedures, limited generality, and elusive understanding of the formation mechanisms. Here, by revealing the self-healing behavior of noble metal gels and applying it in the gelation process at a disturbing environment, an unconventional and conceptually new strategy, i.e., a disturbance-promoted gelation method, is developed by introducing an external force field. It overcomes the diffusion limitation in the gelation process, thus producing monolithic gels within 1–10 min at room temperature, 2–4 orders of magnitude faster than for most reported methods. Moreover, versatile NMAs are acquired by using this method, and their superior (photo-)electrocatalytic properties are demonstrated for the first time in light of combined catalytic and optic properties.

Promoting the Electrocatalytic Performance of Noble Metal Aerogels by Ligand-Directed Modulation.

Noble metal aerogels (NMAs) are an emerging class of porous materials. Embracing nano‐sized highly‐active noble metals and porous structures, they display unprecedented performance in diverse electrocatalytic processes. However, various impurities, particularly organic ligands, are often involved in the synthesis and remain in the corresponding products, hindering the investigation of the intrinsic electrocatalytic properties of NMAs. Here, starting from laser‐generated inorganic‐salt‐stabilized metal nanoparticles, various impurity‐free NMAs (Au, Pd, and Au‐Pd aerogels) were fabricated. In this light, we demonstrate not only the intrinsic electrocatalytic properties of NMAs, but also the prominent roles played by ligands in tuning electrocatalysis through modulating the electron density of catalysts. These findings may offer a new dimension to engineer and optimize the electrocatalytic performance for various NMAs and beyond.

Promoting the Electrocatalytic Performance of Noble Metal Aerogels by Ligand‐Directed Modulation

AbstractNoble metal aerogels (NMAs) are an emerging class of porous materials. Embracing nano‐sized highly‐active noble metals and porous structures, they display unprecedented performance in diverse electrocatalytic processes. However, various impurities, particularly organic ligands, are often involved in the synthesis and remain in the corresponding products, hindering the investigation of the intrinsic electrocatalytic properties of NMAs. Here, starting from laser‐generated inorganic‐salt‐stabilized metal nanoparticles, various impurity‐free NMAs (Au, Pd, and Au‐Pd aerogels) were fabricated. In this light, we demonstrate not only the intrinsic electrocatalytic properties of NMAs, but also the prominent roles played by ligands in tuning electrocatalysis through modulating the electron density of catalysts. These findings may offer a new dimension to engineer and optimize the electrocatalytic performance for various NMAs and beyond.

Room temperature ionic liquids to tailor resorcinol – Formaldehyde polymer gels

Abstract Synthesis of resorcinol – formaldehyde polymer gels was studied in various room temperature ionic liquid (RTIL) – water mixtures. Low temperature nitrogen adsorption observations reveal that the size of both the anions and of the cations influences the pore morphology of the aerogel. The 1-ethyl-3-methylimidazolium ethyl sulphate ionic liquid (IL) on its own is also able to catalyse the synthesis, yielding a totally metal free carbon aerogel precursor. The effect of water content on the gel structure is striking, thereby also offering a means of systematically adjusting the apparent surface area, the width of the pore entrance and the volume of the pores detectable by nitrogen. Here, the nature of the gel synthesis itself depends sensitively on the intrinsic pH, which is influenced by the anion of the IL. Comparison of these findings with published molecular dynamics simulations shows that the morphology of the gels is correlated with the structure of the mixed water − IL solvent, which also tunes the concentration of the IL catalyst.

2020 roadmap on pore materials for energy and environmental applications

Porous materials have attracted great attention in energy and environment applications, such as metal organic frameworks (MOFs), metal aerogels, carbon aerogels, porous metal oxides. These materials could be also hybridized with other materials into functional composites with superior properties. The high specific area of porous materials offer them the advantage as hosts to conduct catalytic and electrochemical reactions. On one hand, catalytic reactions include photocatalytic, photoelectrocatalytic and electrocatalytic reactions over some gases. On the other hand, they can be used as electrodes in various batteries, such as alkaline metal ion batteries and electrochemical capacitors. So far, both catalysis and batteries are extremely attractive topics. There are also many obstacles to overcome in the exploration of these porous materials. The research related to porous materials for energy and environment applications is at extremely active stage, and this has motivated us to contribute with a roadmap on ‘porous materials for energy and environment applications’.

A Cobalt Sunrise: Thermites Based on LiClO4-Filled Co(0) Aerogels Prepared from Polymer-Cross-Linked Cobaltia Xerogel Powders.

A new route to metallic aerogels that bypasses the use of supercritical fluids and handling fragile wet-gel and aerogel precursors is exemplified by the carbothermal synthesis of monolithic Co(0) aerogels from compressed cobaltia xerogel powders coated conformally (cross-linked) at the primary particle level with a carbonizable polyurea. Residual carbon is removed, and carbon-free samples are obtained by high-temperature treatment of as-prepared Co(0) aerogels under a flowing stream of H2O/H2 that prevents oxidation of the Co(0) network. The durability of Co(0) aerogels is demonstrated under harsh processing conditions in their application as thermites. For this, Co(0) aerogel discs are infiltrated with LiClO4 from a melt and are ignited at about 1100 °C with an electric resistor. As Co(0) "burns" to CoO, temperature exceeds 1500 °C, and the heat released (-552 ± 2 kcal mol-1) is near to both the theoretical value (-58.47 kcal mol-1) and that from well-known pressed-pellet iron/perchlorate thermites (-66.6 kcal mol-1). The advantage of nanostructured thermites based on Co(0) aerogels is the efficiency (100%) by which the metal is consumed during its reaction with LiClO4 filling the pores.

Three-dimensional inorganic polymer of Pd aerogel as a highly active support-less anode catalyst toward formic acid oxidation

Metallic aerogels as a novel class of three-dimensional inorganic polymers have attracted a lot of attentions in academic researches. In this manuscript, an efficient way for the production of Pd aerogel is presented. The proposed way includes advantages such as one-pot synthesis, being fast, simple and surfactant free. The Pd aerogel is successfully synthesized by the reduction of H2PdCl4 with sodium borohydride followed by supercritical drying with liquid carbon dioxide. The extraordinary 3D network with the large porosity of Pd aerogel is confirmed by using TEM, FESEM, XRD, EDS, FT-IR and nitrogen isotherm techniques. The extraordinary 3D polymer is used as a support-less anode catalyst for the oxidation of formic acid. Based on the obtained results, the mass activity of 3D nanonetwork was estimated to be 1791.4 mAmg−1Pd, which was 2.67 times higher compared to Pd/C (668.5 mAmg−1Pd) nanocatalyst. Moreover, the Pd/C indicated a positive shift in onset potential, lower ECSA, greater Ea, greater RCT as well as weaker stability in comparison to the resultant aerogel. The exceptional catalytic behavior of 3D nanonetwork toward oxidation of formic acid was seriously affected by unique properties such as large porosity, high surface area as well as self-supporting characteristic of 3D nanonetwork.

Thermal enhancement and shape stabilization of a phase-change energy-storage material via copper nanowire aerogel

Low thermal conductivity (k) has limited the applications of phase change materials (PCMs). To enhance k of paraffin PCM, copper nanowire (CuNW) aerogels (CuNWAs) are fabricated and infiltrated with paraffin, inducing a ∼30% k-enhancement and good electrical conductivity σ with 2.0 wt% CuNW. The composite shows interesting behavior of shape stabilization: it permanently shrinks after each thermal cycle but is fully shape-stabilized after ∼50 cycles, showing a stable >130% k-enhancement with 6.3 wt% CuNW. To investigate the dominant factors for the k- and σ-enhancements, the filler-filler and filler-matrix interfacial bondings are studied through pre-sintering of CuNWAs. The pre-sintering induces two effects: (1) building the CuNW-CuNW bonding and a >∼200% σ-enhancement, and (2) weakening the CuNW-paraffin bonding and a ∼4% k-reduction. This filler-matrix interfacial weakening is confirmed by nanoindentation technique and computational modeling. The pre-sintering also enhances the shape-stabilization against thermal cycling. In summary, a metallic aerogel is first-ever employed to fabricate shape-stabilized electrically-conductive thermal-energy storage materials (TESMs) with >130% k-enhancement. The pre-sintered-CuNWA-paraffin composite shows an impressive σ (=14 S/m). This study also demonstrated that the fundamental dominant factors for σ- and k-enhancements are the filler-filler and filler-matrix interfacial bondings, respectively (not conjointly). This provides important insights into designing TESMs with high electronic and/or thermal conductivities, and also high durability.

Controlled organization of building blocks to prepare three-dimensional architecture of Pd–Ag aerogel as a high active electrocatalyst toward formic acid oxidation

Among self-assembled architectures, noble metal aerogels have emerged as state-of-the-art catalysts, and these unique architectures play a vital role in widespread applications. Until now, profuse ways have been introduced for the development and synthesis of noble metal aerogels, which suffer from long gelation time as well as complexity. In this work, a facile and efficient method is presented to create the Pd–Ag aerogel. This route offers advantages such as a surfactant-free route, short gelation time and a simple strategy. The Pd–Ag aerogel was prepared under controlled condition by using a galvanic displacement of sacrificial Ag nanoparticles with H2PdCl4 followed by supercritical CO2 drying. Porous three-dimensional architecture of Pd–Ag aerogel is confirmed by the TEM, FESEM, XRD, ICP-AES and N2 isotherm. The resultant aerogel displays exceptional catalytic activity and durability toward electro-oxidation of formic acid on account of two vital reasons: 1) the unique architecture of resultant aerogel reflects exceptional advantages such as meso-and macroporous properties, and 2) the combination of Ag with Pd, leading to the acceleration in the oxidation of formic acid (synergistic effects).

Synthesis Method for Cellulose Nanofiber Biotemplated Palladium Composite Aerogels.

Here, a method to synthesize cellulose nanofiber biotemplated palladium composite aerogels is presented. Noble metal aerogel synthesis methods often result in fragile aerogels with poor shape control. The use of carboxymethylated cellulose nanofibers (CNFs) to form a covalently bonded hydrogel allows for the reduction of metal ions such as palladium on the CNFs with control over both nanostructure and macroscopic aerogel monolith shape after supercritical drying. Crosslinking the carboxymethylated cellulose nanofibers is achieved using 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide hydrochloride (EDC) in the presence of ethylenediamine. The CNF hydrogels maintain their shape throughout synthesis steps including covalent crosslinking, equilibration with precursor ions, metal reduction with high concentration reducing agent, rinsing in water, ethanol solvent exchange, and CO2 supercritical drying. Varying the precursor palladium ion concentration allows for control over the metal content in the final aerogel composite through a direct ion chemical reduction rather than relying on the relatively slow coalescence of pre-formed nanoparticles used in other sol-gel techniques. With diffusion as the basis to introduce and remove chemical species into and out of the hydrogel, this method is suitable for smaller bulk geometries and thin films. Characterization of the cellulose nanofiber-palladium composite aerogels with scanning electron microscopy, X-ray diffractometry, thermal gravimetric analysis, nitrogen gas adsorption, electrochemical impedance spectroscopy, and cyclic voltammetry indicates a high surface area, metallized palladium porous structure.

Specific ion effects directed noble metal aerogels: Versatile manipulation for electrocatalysis and beyond.

Noble metal foams (NMFs) are a new class of functional materials featuring properties of both noble metals and monolithic porous materials, providing impressive prospects in diverse fields. Among reported synthetic methods, the sol-gel approach manifests overwhelming advantages for versatile synthesis of nanostructured NMFs (i.e., noble metal aerogels) under mild conditions. However, limited gelation methods and elusive formation mechanisms retard structure/composition manipulation, hampering on-demand design for practical applications. Here, highly tunable NMFs are fabricated by activating specific ion effects, enabling various single/alloy aerogels with adjustable composition (Au, Ag, Pd, and Pt), ligament sizes (3.1 to 142.0 nm), and special morphologies. Their superior performance in programmable self-propulsion devices and electrocatalytic alcohol oxidation is also demonstrated. This study provides a conceptually new approach to fabricate and manipulate NMFs and an overall framework for understanding the gelation mechanism, paving the way for on-target design of NMFs and investigating structure-performance relationships for versatile applications.

Shape engineering of palladium aerogels assembled by nanosheets to achieve a high performance electrocatalyst

Three-dimensional noble metal aerogels are unique solid materials with ultralow densities, large open pores, and high surface areas. Despite several research works on noble metal aerogels assembled by nanochains, the investigation of noble metal aerogels assembled by nanosheets has not been reported to date. In this work, palladium aerogels assembled by nanosheets have been synthesized in four various carboxylic acid (RCOOH) solvents with different alkyl groups (R= H– (formic acid, FA), CH3– (acetic acid, AA), CH3CH2– (propionic acid, PA), and CH3CH2CH2– (butyric acid, BA)). It has been found that the R groups play a crucial role in the morphology of palladium aerogels so that the Pd aerogels synthesized in acetic acid and propionic acid solvents show better morphology than that of palladium aerogels synthesized in formic acid and butyric acid solvents. The electrocatalytic activity and durability of all aerogels are evaluated using cyclic voltammetry (CV) and chronoamperometry (CA) experiments toward ethanol oxidation. Based on CV and CA data, Pd aerogels synthesized in AA and PA show high catalytic activity and durability due to their unique structures.

A versatile ethanolic approach to metal aerogels (Pt, Pd, Au, Ag, Cu and Co)

We present a universal ethanolic one-step approach to noble/less noble metal aerogels at room temperature and −70 °C.

Air cathode of zinc-air batteries: a highly efficient and durable aerogel catalyst for oxygen reduction.

In this communication, N-doped carbon shell encapsulated Co3O4@Co nanoparticles (NPs) were assembled on N-doped reduced graphene oxide to form non-precious metal aerogels (Co3O4@Co/N-r-GO) for oxygen reduction. In an alkaline medium, the aerogels exhibit comparable oxygen reduction reaction (ORR) performances to the commercial Pt/C with respect to half-wave potential, current density and durability. Zn-air batteries (ZABs) assembled with Co3O4@Co/N-r-GO as the air cathode display high average voltages of ∼1.38 and ∼1.3 V at a current density of 1 and 5 mA cm-2, respectively, and show a specific capacity of ∼800 mA h g-1. These results demonstrate that Co3O4@Co/N-r-GO materials are highly active and durable ORR catalysts for ZABs. The unique structure of the composite aerogel, i.e. the carbon shell encapsulated Co3O4@Co NPs and the 3D interconnected macroporous N-r-GO matrix, is responsible for its high ORR activity and stability.