High‐Performance Electrocatalysis on Palladium Aerogels

Abstract

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...

Enhanced methanol oxidation and oxygen reduction reactions on palladium-decorated FeCo@Fe/C core–shell nanocatalysts in alkaline medium

“…The ratio of the forward anodic peak current density (I f ) to the reverse anodic peak current density (I b ) i.e., (I f /I b ) is indicative of the ability of the catalyst to tolerate the accumulation of carbonaceous species. [39][40][41][42][43] The I f /I b ratios were estimated as 4.2 and 2.7 for FeCo@Fe@Pd/C and Pd/C, respectively, indicating excellent MOR on the core-shell platform. EIS experiments were performed at À0.2 V to provide further insights into the activities of the two catalysts (Fig.…”

Nanostructuring Noble Metals as Unsupported Electrocatalysts for Polymer Electrolyte Fuel Cells

“…[37] As a proof-of-concept, metallic aerogels with compositions of pure Pd and Au, as well as alloyed Pd x Pt y , Pt x Ni y and Pt x Cu y have been synthesized and evaluated for electrocatalysis, as it will be further discussed in section 3. [25,26,[38][39][40] Linking aerogel research and nanotechnology, hierarchical aerogels with primary and secondary pore structures, built from shapeengineered NPs, have also been reported in the form of either hollow shell or dendritic particles. [41][42][43] As illustrated in Figure 1, the general synthesis of noble metal aerogels via the sol-gel process consists of three steps: i) the synthesis of nano-sized building blocks (NBBs); ii) the gelation of the NBBs into hydrogels (plus shape-control of the hydrogel) and iii) the supercritical drying of the hydrogel to yield an aerogel.…”

“…Citrate and cyclodextrin are frequently utilized as the stabilizers to narrow the size distribution of the metal NPs. [44] The relatively weak coordination interaction between the stabilizer and metal NPs is critical for the gelation step (from B1 to H1 in Figure 1), which can be realized via an spontaneous gelation process [25,26,38] or via controlled destabilizations (by heating [45] or adding salt [46] and cross-linkers [47] Besides the porous and interconnected 3D network structure, hierarchical aerogels also possess backbones with locally modified morphologies instead of solid nanochains. As shown in Figure 1, this sub-structure can be obtained either by tailoring the hydrogel via strategy II (B1-H1-H2-A2) [43] or engineering the NBBs via strategy III (B1-B2-H3-A3).…”

“…The nanowirelike backbones are derived from the fusion and connection of the particulate NBBs (mostly noble metal NPs) during the gelation process. Depending on the elemental composition and distribution, these aerogels can be divided into three types: monometallic aerogels (Pd, Pt, Au, Ag), [38,39,46,47] bimetallic alloy aerogels (Pd x Pt y , Pt x Ni, Pt x Cu, etc), [25,26,49] and mixed multi/bimetallic aerogels (Au-Pd, Pd-Pt, Au-Ag-Pt, etc.). [45,50] Figure 2 shows the representative scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images of the wire-based aerogels.…”

One-step synthesis of size-tunable Ag nanoparticles incorporated in electrospun PVA/cyclodextrin nanofibers

“…Therefore, CD are used in a number of industrial areas such as pharmaceuticals, food, chromatography, cosmetics and textiles (Chung, Guo, Priestley, & Kwak, 2011;Harada, Kobayashi, Takashima, Hashidzume, & Yamaguchi, 2010;Hedges, 1998;Szejtli, 1998). In addition to that, being a non-toxic and naturally occurring material, CD are also very promising candidates for use as reducing and stabilizing agent for the formation of metal NP (Alvarez, Liu, Román, & Kaifer, 2000;Huang, Meng, & Qi, 2009;Kochkar, Aouine, Ghorbel, & Berhault, 2011;Liu et al, 2012;Ng et al, 2008).…”