Metal Aerogels Research Articles

Aerogels for sustainable CO2 electroreduction to value-added chemicals

Carbon dioxide electrochemical reduction (CO2ER) affords an appealing pathway for transforming discarded CO2 to fuels and economic chemicals. Various nanocatalysts have been used for CO2ER, of which porous catalysts have attracted widespread attentions because of their large electrochemically active surface area, large number of pores for molecule transportation, and high local pH. Aerogels (including carbon-based aerogels and metallic aerogels), as a new class of porous catalysts, have been applied to CO2ER in recent years because of their high electrical conductivity (to reduce overpotential), three-dimensional porous structure and intrinsic hydrophobicity (to inhibit parasitic hydrogen evolution reaction, HER). In this article, we reviewed latest progresses toward aerogels for CO2ER, including (1) synthesis strategies of carbon-based aerogels and metallic aerogels; (2) innovations in aerogels design, such as heteroatom doping and metal incorporation in carbon-based aerogel, creating grain boundaries, regulating Cu0-Cu+ interfaces, and optimizing synergistic effect in metal aerogels; and (3) structural properties of aerogel catalysts to enhance CO2ER performance. Finally, we discuss the challenges, possible solutions and future directions for further development of aerogels in CO2ER.

Controllable synthesis of Pt-rich shell/Mx core [M = Ni, Fe, Co] metal aerogels as efficient electrocatalysts for hydrogen evolution reaction

Water electrolysis for hydrogen production offers a solution to the fossil fuel crisis, relying heavily on electrocatalysts for the hydrogen evolution reaction (HER). Fortunately, metal aerogels (MAs) with three-dimensional network structures reveal great potential as HER electrocatalysts. However, their simple synthesis and widespread application still present significant challenges. Here, PtNix MAs electrocatalysts, by controlling the Ni/Pt feeding ratio and using NaBH4 reduction, are prepared via a simple one-step method. Notably, PtNi4 MAs with Pt-rich shell/Ni core structure exhibit excellent HER property, showing a low overpotential of 16.3 mV at 10 mA cm−2 and a mass activity density of 2.28 A mgPt-1 at 70 mV. The outstanding HER properties of PtNi4 MAs are ascribed to the synergistic catalytic effect of Pt and Ni, the core–shell structure of the sample, and the abundant three-dimensional network porosity. Density functional theory (DFT) demonstrates that PtNi4 MAs’ core–shell structure have the lowest dissociation energy of H2O and ΔGH* on the catalyst surface. Furthermore, the replacement of non-precious metal (Ni/Fe/Co) does not affect the formation of the Pt-rich shell structure. Opposite the substitution of reducing agent species affects the formation of Pt-rich shell structure. This work can offer valuable insights for the future sustainable energy applications of MAs materials.

Liquid Metal Aerogel With Janus Architecture for Selective Direction Recognition and High‐Efficiency Moisture Energy Harvesting

AbstractLiquid metal (LM) micro‐nano droplets show the superiority in reducing the high surface tension applicable in conductive inks for flexible electronics. However, the dynamic surface makes LM droplets susceptible to be oxidized, facing challenges in storage and applications. Herein, a bifunctional groups cross‐linking strategy is adopted to encapsulate LM droplets into a robust shell. During sonicating LM in carboxymethyl chitosan (CMCS) solution with bifunctional groups (i.e., carboxyl and amino groups), a robust interface shell is constructed by anchoring effect, thereby providing high chemical stability (>7d). When directionally freezing‐drying LM dispersions, aerogel with Janus architecture is produced driven by the gravity of LM droplets. Due to the unique heterogeneous structure, the resultant aerogel exhibits a selective multifunctional response toward directional bending. In addition, owing to the gradient distribution of CMCS within aerogel, moisture electricity generator can be built with a high output voltage of 460 mV. Thus, this bifunctional groups cross‐linking strategy not only improves the chemical stability of LM micro‐nano droplets, but also renders a new method for producing multifunctional aerogel with energy harvesting and selective directional recognition, and applicable in smart sensors and power supplying devices.

Direct Synthesis of Pd2+‐Rich Palladene Aerogels as Bifunctional Electrocatalysts for Formic Acid Oxidation Reaction and Oxygen Reduction Reaction

AbstractIn this work, we developed a direct strategy to fabricate Palladene (i. e. Palladium metallene) aerogels and propose a temperature‐dependent growth mechanism. Besides the typical three‐dimensional networks and wrinkled surface morphologies, the as‐prepared aerogel is endowed with abundant Pd2+. The as‐prepared aerogel exhibits an excellent mass activity in the formic acid oxidation reaction and a good half‐wave potential in the oxygen reduction reaction in comparison with Pd/C and a Pd aerogel. This work expands the range of metal aerogels from the perspective of the building block units and demonstrates a direct approach to fabricate highly promising bifunctional electrocatalysts for fuel cells.

Au enables PdAu aerogel efficient catalysts for oxygen reduction and C2 alcohol oxidation

Metallic aerogels (MAs) with self-supporting three-dimensional (3D) porous structures constitute a potential platform for the construction of robust catalysts. In this work, we successfully constructed 3D PdAu MAs with the optimized d-band electron of Pd and geometrical lattice expansion using a maneuverable in-situ seed-mediated strategy at room temperature. The introduction of Au accelerated the kinetics of Pd4Au3 MAs toward oxygen reduction reaction (ORR), ethylene glycol oxidation reaction (EGOR), and ethanol oxidation reaction (EOR) in alkaline electrolyte, boosting the activity and stability of Pd4Au3 MAs. The mass activities of Pd4Au3 MAs/C in ORR, EGOR and EOR were 1.74, 8.57, and 9.54 A mgPd−1, respectively, which were remarkably better than those of referenced Pd MAs/C, commercial Pt/C and Pd/C. The rotating ring-disk electrode measurement illustrated that Pd4Au3 MAs/C achieved a 4-electron transfer process in ORR from O2 to OH−. In-situ Fourier transform infrared spectroscopy revealed that Pd4Au3 MAs/C enabled the cleavage of the C–C bonds, thus accomplishing the C1-complete oxidation of 10-electron ethylene glycol and 12-electron ethanol. This study provides an efficient strategy to fabricate binary non-Pt alloy aerogels as high-performance multifunctional electrocatalysts for ORR, EGOR, and EOR.

Anchored silver-palladium aerogel on carbon cloth via diazonium chemistry for electrochemical reduction of CO2

Electrochemical CO2 reduction reaction (eCO2RR) is a promising route for removal of CO2 from the atmosphere and transforming it into value-added products. The development of new and efficient catalysts plays a vital role in this regard. Here a combination of carbon grafting and bimetallic metal aerogel is presented as a catalyst for eCO2RR. The grafted surface plays an essential function in assembling the catalyst on the carbon cloth (CC) surface as a supported catalyst. Herein, we describe the use of para-aminothiophenol (p-TP) as a grafting agent combined with diazonium chemistry to fix a metallic aerogel catalyst on CC as support. The silver, palladium and bi-metallic silver-palladium aerogels (Ag-p-TP, Pd-p-TP, and AgPd-p-TP) anchored onto the surface of CC yielding the corresponding surface-bound materials were used for testing the electrochemical reduction of CO2. The mono-metallic Ag carbon cloth (Ag-p-TP/CC) exhibited the ability in reducing CO2 to CO with moderate Faradaic efficiency (FE) of 74 % in 0.1 M, NaHCO3 electrolyte at pH = 7.5. The bimetallic AgPd-p-TP-CC further enhanced the Faradaic efficiency for CO formation to 88.7 %, with an unexpectedly low onset potential of −0.09 V vs. RHE. Surprisingly, formate was identified to be the liquid product when Pd-p-TP-CC and AgPd-p-TP-CC were used, with maximal FEformate of 19.0 % and 25.7 % at −1.0 V vs. RHE, respectively. In contrast, CC lacking the chemical grafting that allows immobilization of the metal aerogel (Ag-Pd/CC) exhibits lower FEco% for eCO2RR, highlighting the importance of this chemical modification of the carbon cloth surface. Diazonium chemistry coupled with metal aerogels is a new approach to generating surface-bound catalysts for the effective reduction of CO2.

Crystal-Phase-Engineered High-Entropy Alloy Aerogels for Enhanced Ethylamine Electrosynthesis from Acetonitrile.

Crystal-phase engineering that promotes the rearrangement of active atoms to form new structural frameworks achieves excellent result in the field of electrocatalysis and optimizes the performance of various electrochemical reactions. Herein, for the first time, it is found that the different components in metallic aerogels will affect the crystal-phase transformation, especially in high-entropy alloy aerogels (HEAAs), whose crystal-phase transformation during annealing is more difficult than medium-entropy alloy aerogels (MEAAs), but they still show better electrochemical performance. Specifically, PdPtCuCoNi HEAAs with the parent phase of face-centered cubic (FCC) PdCu possess excellent 89.24% of selectivity, 746.82mmol h-1 g-1 cat. of yield rate, and 90.75% of Faraday efficiency for ethylamine during acetonitrile reduction reaction (ARR); while, maintaining stability under 50h of long-term testing and ten consecutive electrolysis cycles. The structure-activity relationship indicates that crystal-phase regulation from amorphous state to FCC phase promotes the atomic rearrangement in HEAAs, thereby optimizing the electronic structure and enhancing the adsorption strength of reaction intermediates, improving the catalytic performance. This study provides a new paradigm for developing novel ARR electrocatalysts and also expands the potential of crystal-phase engineering in other application areas.

Unveiling synergy of strain and ligand effects in metallic aerogel for electrocatalytic polyethylene terephthalate upcycling

Recently, there has been a notable surge in interest regarding reclaiming valuable chemicals from waste plastics. However, the energy-intensive conventional thermal catalysis does not align with the concept of sustainable development. Herein, we report a sustainable electrocatalytic approach allowing the selective synthesis of glycolic acid (GA) from waste polyethylene terephthalate (PET) over a Pd67Ag33 alloy catalyst under ambient conditions. Notably, Pd67Ag33 delivers a high mass activity of 9.7 A mgPd-1 for ethylene glycol oxidation reaction (EGOR) and GA Faradaic efficiency of 92.7 %, representing the most active catalyst for selective GA synthesis. In situ experiments and computational simulations uncover that ligand effect induced by Ag incorporation enhances the GA selectivity by facilitating carbonyl intermediates desorption, while the lattice mismatch-triggered tensile strain optimizes the adsorption of *OH species to boost reaction kinetics. This work unveils the synergistic of strain and ligand effect in alloy catalyst and provides guidance for the design of future catalysts for PET upcycling. We further investigate the versatility of Pd67Ag33 catalyst on CO2 reduction reaction (CO2RR) and assemble EGOR//CO2RR integrated electrolyzer, presenting a pioneering demonstration for reforming waste carbon resource (i.e., PET and CO2) into high-value chemicals.

Confinement-enhanced electrochemiluminescence by Ru(dcbpy)32+-functionalized γ-CD-MOF@COF-LZU1 porous hybrid material as micro-reactor for CYFRA 21-1 detection

The improvement of electrochemiluminescence (ECL) performance relies on the electron transfer efficiency between luminophore and coreactant. An ultrasensitive ECL micro-reactor with confinement-enhanced performance was prepared by using the covalent organic framework-LZU1-functionalized metal-organic framework (MOF@COF-LZU1) as a platform to assemble enormous N,N-dibutyl-2-hydroxyethylamine (DBAE) and tris(4,4′-dicarboxylic acid-2,2′-bipyridyl) ruthenium(II) [Ru(dcbpy)32+] into its pore channels. Compared to individual substances of γ-CD-MOF and COF-LZU1, the synergistic effects can conduce to the enhancement of the intensity, durability and sensitivity of the micro-reactor. Besides, COF-LZU1 can provide a mild environment to accommodate a certain amount of DBAE by concentrating them from the aqueous solution into its hydrophobic cavities and boost the oxidation efficiency of DBAE to generate more DBAE●+ and profited the survival of DBAE●, leading to an improved reaction efficiency with the Ru(dcbpy)32+ intermediate. Thanks to the confinement-enhanced strategy, engineered as high-functioning luminescent materials, Ru@γ-CD-MOF@COF-LZU1 micro-reactors decorated with Au NPs can facilitate electron transfer and capture primary antibodies (Ab1). Moreover, Au–Pd–Pt noble metal aerogels (NMAs) functionalized MoS2 NFs (Au–Pd–Pt NMAs@MoS2 NFs) were chosen as base material due to its large specific surface areas, high porosity, and excellent electrical conductivity. Based on above merits, the sensor demonstrated a sensitive response to CYFRA 21-1 detection in a linear concentration gradient from 10 fg/mL to 50 ng/mL with a detection limit of 0.0055 pg/mL (S/N = 3). The COF-LZU1 decorated ECL micro-reactors were constructed based on the signal amplification strategies to realize accurate CYFRA 21-1 detection.

Confining N-Doped Carbon Dots into PtNi Aerogels Skeleton for Robust Electrocatalytic Methanol Oxidation and Oxygen Reduction.

Noble metallic aerogels with the self-supported hierarchical structure and remarkable activity are promising for methanol fuel cells, but are limited by the severe poisoning and degradation of active sites during electrocatalysis. Herein, the highly stable electrocatalyst of N-doped carbon dots-PtNi (NCDs-PtNi) aerogels is proposed by confining NCDs with alloyed PtNi for methanol oxidation and oxygen reduction reactions. Comprehensive electrocatalytic measurements and theoretical investigations suggest the improvement in structure stability and regulation in electronic structure for better electrocatalytic durability when confining NCDs with PtNi aerogels. Notably, the NCDs-PtNi aerogels perform 12-fold higher activity than that of Pt/C and maintain 52% of their initial activity after 5000 cycles toward acidic methanol oxidation. The enhanced stability and activity of NCDs-PtNi aerogels are also evident for oxygen reduction reactions in different electrolytes. These results highlight the effectiveness of stabilizing metallic aerogels with NCDs, offering a feasible pathway to develop robust electrocatalysts for fuel cells.

Defect-rich Pt-Ru metallic aerogels for highly efficient ethanol oxidation reaction

Direct ethanol fuel cells (DEFCs) are a new type of green and efficient energy conversion devices, but the slow anodic reaction kinetics limit their application. Herein, a series of Pt-Ru metallic aerogels (MAs) with controllable composition, rich surface defects, and large specific surface area were prepared for ethanol oxidation reaction (EOR) by freeze–thaw method. Pt6Ru MAs exhibit the highest mass activity (1.264 A mgPt−1) toward EOR among all the catalysts we have prepared, which is 2.0, 1.8, and 3.7 times higher than those of Pt MAs (0.632 A mgPt−1), the commercial Pt/C (0.696 A mgPt−1), Pt6Ru NPs (0.339 A mgPt−1). 1H nuclear magnetic resonance spectroscopy confirms that only acetic acid was the liquid product of Pt6Ru MAs during EOR catalysis. Furthermore, Pt6Ru MAs show little attenuation and satisfactory reactivation abilities in stability assessment, demonstrating their good stability. This proves that the introduction of Ru makes feasible adjustments to the electronic structure of Pt in MAs, improving the electrocatalysis activity of Pt6Ru MAs in EOR. Mechanistic investigations reveal that the optimal electronic structure is the intrinsic reason for the excellent EOR performance of Pt6Ru MAs. The findings open a new way to develop high-performance Pt-based materials electrocatalysts.

Controlled Synthesis of Noble Metal Aerogels and Their Applications in Electrocatalysis and Surface-Enhanced Raman Scattering

Controlled Synthesis of Noble Metal Aerogels and Their Applications in Electrocatalysis and Surface-Enhanced Raman Scattering

Tuning the surface electronic structure of noble metal aerogels to promote the electrocatalytic oxygen reduction

The sluggish kinetics of the oxygen reduction reaction (ORR) is the bottleneck for various electrochemical energy conversion devices. Regulating the electronic structure of electrocatalysts by ligands has received particular attention in deriving valid ORR electrocatalysts. Here, the surface electronic structure of Pt-based noble metal aerogels (NMAs) was modulated by various organic ligands, among which the electron-withdrawing ligand of 4-methylphenylene effectively boosted the ORR electrocatalysis. Theoretical calculations suggested the smaller energy barrier for the transformation of O* to OH* and downshift the d-band center of Pt due to the interaction between 4-methylphenylene and the surface metals, thus enhancing the ORR intrinsic activity. Both Pt3Ni and PtPd aerogels with 4-methylphenylene decoration performed significant enhancement in ORR activity and durability in different media. Remarkably, the 4-methylphenylene modified PtPd aerogel exhibited the higher half-wave potential of 0.952 V and the mass activity of 10.2 times of commercial Pt/C. This work explained the effect of electronic structure on ORR electrocatalytic properties and would promote functionalized NMAs as efficient ORR electrocatalysts.

Lattice-Strained Metallic Aerogels as Efficient and Anti-Poisoning Electrocatalysts for Oxygen Reduction Reaction.

Lattice strain engineering optimizes the interaction between the catalytic surface and adsorbed molecules. This is done by adjusting the electron and geometric structure of the metal site to achieve high electrochemical performance, but, to date, it has been rarely reported on anti-poisoned oxygen reduction reaction (ORR). Herein, lattice-strained Pd@PdBiCo quasi core-shell metallic aerogels (MAs) were designed by "one-pot and two-step" method for anti-poisoned ORR. Pd@PdBiCo MAs/C maintain their original activity (1.034 A mgPd -1 ) in electrolytes with CH3 OH and CO at 0.85 V vs. reversible hydrogen electrode (RHE), outperforming the commercial Pd/C (0.156 A mgPd -1 ), Pd MAs/C (0.351 A mgPd -1 ), and PdBiCo MAs/C (0.227 A mgPd -1 ). Moreover, Pd@PdBiCo MAs/C also show high stability and anti-poisoning with negligible activity decay after 8000 cycles in 0.1 m KOH+0.3 m CH3 OH. These results of X-ray photoelectron spectroscopy, CO stripping, and diffuses reflectance FTIR spectroscopy reveal that the tensile strain and strong interaction between different elements of Pd@PdBiCo MAs/C effectively optimize the electronic structure to promote O2 adsorption and activation, while suppressing CH3 OH oxidation and CO adsorption, leading to high ORR activity and anti-poisoning property. This work inspires the rational design of MAs in fuel cells and beyond.

D-d Orbital coupling induced by crystal-phase engineering assists acetonitrile electroreduction to ethylamine

The d-d orbital coupling induced by crystal-phase engineering can effectively adjust the electronic structure of electrocatalysts, thus showing significant catalytic performance, while it has been rarely explored in electrochemical acetonitrile reduction reaction (ARR) to date. Herein, we successfully realize the structural transformation of PdCu metallic aerogels (MAs) from face-centered cubic (FCC) to body-centered cubic (BCC) through annealing treatment. Specifically, the BCC PdCu MAs exhibit excellent ARR performance with high ethylamine selectivity of 90.91%, Faradaic efficiency of 88.60%, yield rate of 316.0 mmol h−1 g−1Pd+Cu and long-term stability for consecutive electrolysis within 20 h at −0.55 V vs. reversible hydrogen electrode, outperforming than those of FCC PdCu MAs. Under the membrane electrode assembly system, BCC PdCu MAs also demonstrate excellent ethylamine yield rate of 389.5 mmol h−1 g−1Pd+Cu. Density functional theory calculation reveals that the d-d orbital coupling in BCC PdCu MAs results in an evident correlation effect for the interaction of Pd and Cu sites, which boosts up the Cu sites electronic activities to enhance ARR performance. Our work opens a new route to develop efficient ARR electrocatalysts from the perspective of crystalline structure transformation.

Modulating Pd eg orbital occupancy in Pd-Au metallic aerogels for efficient carbon dioxide reduction

The electronic structure of electrocatalysts plays a critical role in energy conversion, whereas for an efficient catalyst, it is challenging to modulate the orbitals. Herein, we present a new strategy to modulate the eg orbital occupancy of Pd by constructing composition-controllable Pd-Au metallic aerogels (MAs), optimizing the d-band center of Pd to achieve excellent performance for electrochemical carbon dioxide reduction reaction (CO2RR). Specifically, Pd1Au2 MAs achieve almost 100% Faraday efficiency (FE) of CO in the range of −0.40 to −0.80 V vs. reversible hydrogen electrode (RHE), as well as the long-term stability, being one of the best Pd-based materials for CO2RR. The X-ray photoelectron spectroscopy (XPS) results and density functional theory (DFT) calculations demonstrate that the introduction of Au modulates the Pd eg orbital occupancy, which significantly weakens *CO adsorption on Pd, reduces the CO2RR energy barrier and consequently improves the electrocatalytic activity and stability for long-term applications. Our work highlights a new strategy for designing efficient electrocatalysts for CO2RR and beyond.

Surface engineering of Pt aerogels by metal phthalocyanine to enhance the electrocatalytic property for oxygen reduction reaction

Noble metal aerogels (NMAs) have been widely investigated as electrocatalysts for sustainable energy conversion, while enhancing the catalytic activity and stability of NMAs is conducive to reduce cost and expand their application. Herein, modifying Pt aerogel by metal phthalocyanine (MPc) molecules (Pt aer-MPc) was proposed to improve the oxygen reduction reaction (ORR) performance. Typically, the Pt aer-FePc showed significant improvement in half-wave potential, mass, and specific activity compared to commercial Pt/C. Theoretical calculations revealed that the synergy between FePc molecules and Pt aerogel constructs a new interface to facilitate the reduction of oxygen and weaken the bind energies of oxygen intermediates, thus leading to improved ORR performance. Similar promotion in ORR catalytic property was also observed for Pt nanoparticles and commercial Pt/C with the modification of FePc. Therefore, this work not only provides a universal engineering method of MPc decoration to enhance ORR performance of Pt-based electrocatalysts, but also opens a new pathway for designing advanced ORR electrocatalysts with functional molecules toward critical energy-related applications.

Simple Synthesis of AuPd Aerogel as a Self-Supporting Electrocatalysts with Excellent Performance

Direct ethanol fuel cell (DEFC) has the advantages of low pollution emissions, easy to carry, and high energy density. However, the development of fuel cells is limited due to the low catalytic activity and poor stability of electrode catalyst materials. Precious metal aerogels not only have three-dimensional self-supporting network structures but also have excellent properties of precious metals, which have very good catalytic efficiency for ethanol. In this study, bimetallic aerogels with varying Au/Pd ratios were prepared, and their impact on the electrocatalytic performance of ethanol was investigated.

In situ growth of Ag aerogels mediating effective electrocatalytic CO2 reduction and Zn-CO2 batteries

Metal aerogels have outshined themselves in electrocatalysis, however, the brittle property has constrained their intrinsic activity by the traditional drop-coating method of electrode preparation. Herein, a rapid in-situ growth strategy to obtain porous Ag aerogels on conductive carbon cloth (Ag As/CC) working as free-standing electrode was designed for electrocatalytic CO2 reduction to CO. The preserved porous network structure of Ag aerogels with hydrophobic surface effectively inhibits the competing reaction of hydrogen evolution. The CO Faraday efficiency of 99.6%, mass activityCO of 23.33 mA mg−1, and Energy efficiencyCO of 59.85% were achieved at −1.0 V vs. RHE. Ag As/CC exhibits quick charge response ability with relaxation time constant of 0.04 s, and high mass transfer with Tafel slope of 59 mV dec−1. It also worked as the cathode to drive Zn-CO2 rechargeable battery with a peak power density of 2.16mW cm−2, to achieve electrocatalytic CO2 fixation and energy storage.

Porous Thin Films with Large Specific Surface Area on the Basis of Spray-Coated Metal Aerogels

The preparation of aerogels in the form of thin films deposited on a substrate (e.g., as an electrode material) is an attractive but challenging task. Such thin films are expected to combine the highly porous gel nanostructure, good mechanical stability, and a remarkably high specific surface area with efficient charge-carrier transport via the interconnected nanoparticle chains. All together, they make them attractive for the fabrication of (electro)catalysts or sensors for microfluidics, flexible electrodes for microelectronics, soft electrodes compatible with a stress-sensitive bioenvironment, etc. In this work, thin aerogel films of colloidal Pd and PtNi building blocks were prepared from the corresponding wet gels using a spray technique. The films were fabricated on substrates of around 1.5 cm2. During the modification of the substrates, the film thickness can be adjusted by controlling the ink content (flow rate, gas pressure, and temperature) and the number of spray cycles. In this way, both the electrical (down to Rs = ∼0.2 kOhm/sq) and optical parameters of the thin porous films can be tuned. Comparative studies of the pore and surface structures of the films have confirmed their integrity and the highly porous structure typical for aerogels. In perspective, spray-coated aerogels can be easily adapted to industrial applications because upscaling is possible and expensive equipment such as supercritical or freeze-dryers is not required. It is important to note that the spray-coating technique is applicable through a mask or can be converted to inkjet printing, which allows for the desired micropatterning of aerogel thin films.