Cryoaerogels and Cryohydrogels as Efficient Electrocatalysts

Abstract

Additive‐free cryoaerogel coatings from noble metal nanoparticles are prepared and electrochemically investigated. By using liquid nitrogen or isopentane as cooling medium, two different superstructures are created for each type of noble metal nanoparticle. These materials (made from the same amount of particles) have superior morphological and catalytic properties as compared to simply immobilized, densely packed nanoparticles. The morphology of all materials is investigated with scanning electron microscopy (SEM). Electrochemically active surface areas (ECSAs) are calculated from cyclic voltammetry measurements. The catalytic activity is studied for the ethanol oxidation reaction (EOR). Both are found to be increased for superstructured materials prepared by cryoaerogelation. Furthermore, cryoaerogels with cellular to dendritic structure that arise from freezing with isopentane show the best catalytic performance and highest ECSA. Moreover, as a new class of materials, cryohydrogels are created for the first time by thawing flash‐frozen nanoparticle solutions. Structure and morphology of these materials match with the corresponding types of cryoaerogels and are confirmed via SEM. Even the catalytic activity in EOR is in accordance with the results from cryoaerogel coatings. As a proof of concept, this approach offers a novel platform towards the easier and faster production of cryogelated materials for wet‐chemical applications.

Noble-Metal Nanorod Cryoaerogels with Electrocatalytically Active Surface Sites

“…Additionally, we have also demonstrated recently that cryoaerogelation is a promising route to fabricate gel coatings for electrochemical applications. − Cryoaerogelation is a novel approach practically including two main steps: (i) the flash freezing of the highly concentrated and colloidally stable aqueous nanoparticle solution and (ii) the subsequent freeze drying of the evolved structure. In the freezing step, the formation of the ice crystals (under the control of their growth via the freezing medium) confines the nanoparticles (NPs) into the crystal boundaries forming sheetlike structures. , The ice template is then removed using lyophilization to preserve the structure prepared in the frozen state.…”

“…Therefore, metal nanoparticle gel structures mainly consist of small, spherical particles. The use of cryoaerogels as electrocatalysts with multimodal porosity is still in its infancy: first, we have demonstrated recently the enhanced electrocatalytic activity of cryoaerogels and cryohydrogels consisting of small Au, Pd, and Pt nanoparticles over their dried nanoparticle films . As another alternative, nanoporous gold and silver have also been developed and their advantageous electrocatalytic properties in ethanol oxidation reaction (EOR) and glucose sensing were proven. − Nanorods (NRs) are underrepresented in these studies, due to their larger size, more complex preparation route, and most importantly the scarcity of synthesis routes providing high enough nanorod concentrations.…”

“…After the upscaled wet-chemical synthesis of the NRs, their surfaces are grafted by a conductive polymer (poly­(3,4-ethylenedioxythiophene) poly­(styrenesulfonate), PEDOT:PSS) to enhance their stability and to replace the nonconductive original ligands. The cryoaerogel coatings are prepared via flash freezing (in cooled isopentane at 113 K) and subsequent freeze drying (under 0.06 mbar) of the concentrated NR solutions deposited onto 3-(mercaptopropyl)­trimethoxysilane (MPTMS)-coated indium tin oxide (ITO) glasses. , The steps of the electrode fabrication are schematically depicted in Scheme . It is demonstrated that the cryoaerogel coatings have a stable, dendritic, open-pore structure ensuring the high contact area between the nanorods and the electrolyte pointing out their superior properties compared to the drop-cast and ambient dried NR films.…”

Pd‐Doped Cellulose Carbon Aerogels for Energy Storage Applications

“…[ 11 ] Several carbon materials, such as single walled carbon nanotube, [ 12 ] graphene hydrogel, [ 13 ] carbon nano‐onions, [ 14 ] multi‐walled carbon nanotube (MWCNT), [ 15 ] and reduced graphene oxide [ 16 ] have shown an enhancement between 200% and 600% in their specific capacitances by means of doping with Ag, Au, or Pt. Palladium is of great interest as dopant due to its high affinity for hydrogen and catalytic activity, [ 17 ] besides it is the most abundant noble metal on the Earth and therefore the least expensive among them. Consequently, it has been widely evaluated as an additive in carbonaceous materials [ 18 ] for catalysis [ 19–23 ] and hydrogen storage.…”

“…Therefore, the present paper analyzes the electrochemical and physicochemical properties of Pd‐doped cellulose carbon aerogels as a novel carbon‐metal hybrid material obtained through cryogelation, which is a versatile tool for preparing aerogels from different colloidal nanoparticles. [ 17,36–39 ] In this work, a different strategy compared to other previously reported syntheses of metal‐doped carbon aerogels is used, [ 9 ] via dispersing the previously synthesized PdNPs in the cellulose solution, which is further processed to obtain Pd‐doped carbon aerogels. Thus, Pd nanoparticles are synthesized and then they are dispersed in the cellulose dissolution prior to the cross‐linking and carbonization process.…”