ConspectusMetallic and catalytically active materials with high surface area and large porosity are a long-desired goal in both industry and academia. In this Account, we summarize the strategies for making a variety of self-supported noble metal aerogels consisting of extended metal backbone nanonetworks. We discuss their outstanding physical and chemical properties, including their three-dimensional network structure, the simple control over their composition, their large specific surface area, and their hierarchical porosity. Additionally, we show some initial results on their excellent performance as electrocatalysts combining both high catalytic activity and high durability for fuel cell reactions such as ethanol oxidation and the oxygen reduction reaction (ORR). Finally, we give some hints on the future challenges in the research area of metal aerogels. We believe that metal aerogels are a new, promising class of electrocatalysts for polymer electrolyte fuel cells (PEFCs) and will also open great opportunities for other electrochemical energy systems, catalysis, and sensors.The commercialization of PEFCs encounters three critical obstacles, viz., high cost, insufficient activity, and inadequate long-term durability. Besides others, the sluggish kinetics of the ORR and alcohol oxidation and insufficient catalyst stability are important reasons for these obstacles. Various approaches have been taken to overcome these obstacles, e.g., by controlling the catalyst particle size in an optimized range, forming multimetallic catalysts, controlling the surface compositions, shaping the catalysts into nanocrystals, and designing supportless catalysts with extended surfaces such as nanostructured thin films, nanotubes, and porous nanostructures. These efforts have produced plenty of excellent electrocatalysts, but the development of multisynergetic functional catalysts exhibiting low cost, high activity, and high durability still faces great challenges.In this Account, we demonstrate that the sol–gel process represents a powerful “bottom-up” strategy for creating nanostructured materials that tackles the problems mentioned above. Aerogels are unique solid materials with ultralow densities, large open pores, and ultimately high inner surface areas. They magnify the specific properties of nanomaterials to the macroscale via self-assembly, which endow them with superior properties. Despite numerous investigations of metal oxide aerogels, the investigation of metal aerogels is in the early stage. Recently, aerogels including Fe, Co, Ni, Sn, and Cu have been obtained by nanosmelting of hybrid polymer–metal oxide aerogels. We report here exclusively on mono-, bi- and multimetallic noble metal aerogels consisting of Ag, Au, Pt, and Pd and their application as electrocatalysts.

Essentially, the term aerogel describes a special geometric structure of matter. It is neither limited to any material nor to any synthesis procedure. Hence, the possible variety of materials and therefore the multitude of their applications are almost unbounded. In fact, the same applies for nanoparticles. These are also just defined by their geometrical properties. In the past few decades nano-sized materials have been intensively studied and possible applications appeared in nearly all areas of natural sciences. To date a large variety of metal, semiconductor, oxide, and other nanoparticles are available from colloidal synthesis. However, for many applications of these materials an assembly into macroscopic structures is needed. Here we present a comprehensive picture of the developments that enabled the fusion of the colloidal nanoparticle and the aerogel world. This became possible by the controlled destabilization of pre-formed nanoparticles, which leads to their assembly into three-dimensional macroscopic networks. This revolutionary approach makes it possible to use precisely controlled nanoparticles as building blocks for macroscopic porous structures with programmable properties.

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Novel self-assembled architectures are currently of great interest for nanochemistry and nanotechnology. Aerogels, a unique class of inorganic polymers with low densities, large open pores, and high inner surface areas, are well known for their superior physical and chemical properties, which are linked to their combination of specific properties of the nanomaterials magnified by self-assembly on the macroscale. Up to now, a wealth of research has been carried out on oxidebased aerogels, with the traditional silica, alumina, and titania aerogels amongst the most widely studied systems. These materials have found attractive applications, for example as thermal insulators and components of electrochemical devices. [1] A great deal of research has also been conducted on hybrid aerogels of metal nanoparticles (e.g. platinum) supported on aerogels (silica, titania, alumina, carbon, etc.), which combine both the catalytic properties of the metal nanoparticles and the highly porous structures of the aerogels. [2] Recently, the extension of sol-gel methods to the preparation of chalcogenide aerogels was realized. [3] Most recently, Leventis et al. has developed metal aerogels (including Fe, Co, Ni, Cu) by a carbothermal method. [4] Our group has developed nonsupported monometallic (Pt, Au, and Ag) and bimetallic (PtAg and AuAg) aerogels, [5] and composite aerogels of both semiconductor and metal nanoparticles have been successfully realized. [6] These aerogels formed from chalcogenide semiconductor or metal nanocrystals constitute an evolving class of materials with enormous potential on account of their optical and catalytic properties; [3][4][5][6] however, applications of these materials are still to be explored in width and depth. [3e] In addition, it is of great interest to develop new nanostructured metallic aerogels with high porosity, large surface area, and high activity by simple strategies. Previously, other important strategies have been developed to generate porous metal nanostructures, for example, by templating, dealloying, electrodepositing approaches; [7] recently, Krishna et al. have also reported a rapid synthesis of high-surface-area noble metal nanosponges by simply mixing the precursors and reducing agent. [8] Cyclodextrins (CDs), a class of readily available, watersoluble, and nontoxic cyclic oligosaccharides with a hydrophobic inner cavity and a hydrophilic exterior, are of great importance in studies of host-guest interactions, molecular recognition, and drug delivery. [9] They have also drawn attention in the area of metal nanomaterials, mainly because they can enhance water-solubility properties and control particle size of nanoparticles. In contrast, not much attention has been paid to improving the catalytic activity of the metal nanoparticles by utilizing the host-guest interactions between cyclodextrin and the target molecules. [10] Herein, we report a facile method to prepare nanostructured Pd aerogels modified by a-, b-, or g-cyclodextrins (Pd CD ). When potassium tetrachloropalladate...

One plausible approach to endow aerogels with specific properties while preserving their other attributes is to fine-tune the building blocks. However, the preparation of metallic aerogels with designated properties, for example catalytically beneficial morphologies and transition-metal doping, still remains a challenge. Here, we report on the first aerogel electrocatalyst composed entirely of alloyed PdNi hollow nanospheres (HNSs) with controllable chemical composition and shell thickness. The combination of transition-metal doping, hollow building blocks, and the three-dimensional network structure make the PdNi HNS aerogels promising electrocatalysts for ethanol oxidation. The mass activity of the Pd83 Ni17 HNS aerogel is 5.6-fold higher than that of the commercial Pd/C catalyst. This work expands the exploitation of the electrocatalysis properties of aerogels through the morphology and composition control of its building blocks.