N2 adsorption analysis is an established method for the analysis of micro- and mesoporosity of open-porous solids. Thus, all researchers developing open-porous materials such as aerogels apply this commercially available technique. However, due to their extraordinary properties, i.e., huge specific mesopore volumes combined with low mechanical stiffness, aerogels question this method. A prerequisite for N2 adsorption analysis is the determination of a well-equilibrated isotherm. This study investigates experimental parameters and discusses their impact on the resulting isotherms. The findings yield recommendations for the reliable determination of adsorption and desorption isotherms of aerogels. In the case of aerogels, a second factor affecting the shape of the isotherms comes into play, i.e., the superposition of the isotherm and deformation effects resulting from capillary pressure upon mesopore filling. This yields erroneous bimodal pore size distributions (PSD) with their mean values differing from the true ones by up to a factor of 2, thus strongly compromising the strategies for targeted materials synthesis for a given application. This paper uses silica aerogel as a model system and provides, for the first time, an approach for the correction of deformation artifacts upon N2 adsorption analysis from the recorded isotherm only, and thus allows reliable determination of PSDs for compliant mesoporous materials.

Adsorption of water vapor in nanoporous carbons is rather complex due to an interplay between their pore structure and surface chemistry. Deciphering the mechanism of adsorption requires the knowledge of the spatial distribution and the filling fraction of the adsorbed water. Sending an ultrasonic wave through a nanoporous sample allows to retrieve a wealth of information, such as properties and spatial distribution of fluids confined in the pores. Here, we studied the adsorption of vapor water on monolithic carbon xerogel, a porous material with a bimodal pore size distribution consisting of micropores(1 nm) and mesopores (8 nm). A novel adsorption-ultrasonicexperimental setup was employed to record the ultrasonic waveforms propagated through the water-sorbing xerogel sample while measuring its water sorption isotherm. Analysis of the elastic moduli evolution suggested that confined water shows nearly bulk-like properties in mesopores, while in micropores its modulus noticeably differs from that of bulk water. Furthermore, the observed increase in ultrasonic attenuation during micropore filling indicated the spatial heterogeneity of the water-filled pore space within the overall sample volume. This study demonstrates the utilization of nondestructive ultrasonic testing to probe both the fluid adsorption mechanism in a nanoporous medium and the properties of the adsorbed phase.

This study explores the impact of pore volume distribution on the structural, thermal, and mechanical properties of spinodal phase-separated silica gels synthesized with poly(ethylene oxide) as a phase-separating agent. By systematically varying gelation temperatures between 20 and 60 °C, we investigate how reaction kinetics influence the resulting pore architecture, thermal conductivity, and elasticity. Nitrogen sorption, mercury intrusion porosimetry, and SEM analysis reveal a transformation from a bimodal pore structure at low temperatures, featuring interconnected macropores, to a predominantly mesoporous network with loss of bimodality. This shift in the diameter of the macropores significantly impacts the thermal insulation properties of the gels as thermal conductivity decreases from 68 to 27 mW (m·K)−1 due to reduced macroporosity, enhanced mesoporosity, and the Knudsen effect. Mechanical testing revealed a substantial decline in Young’s modulus with increasing gelation temperature. These changes are attributed to the interplay of mesoscale structural differences and density variations, driven by increasing gelation temperatures. While higher temperatures lead to reduced strut thickness and the loss of interconnected macropores, the substantial decline in Young’s modulus highlights the critical role of mesoscale structural integrity in maintaining mechanical stability. The findings underscore the importance of an optimized pore volume distribution in tailoring pore structure and performance characteristics, providing a pathway for optimizing silica gels for applications in thermal insulation, filtration, and catalysis.

Water adsorption in nanoporous carbons is a complex process due to the interplay between pore structure and surface chemistry. Sending an ultrasound wave through a nanoporous sample allows for the retrieval of a wealth of information, including the properties and spatial distribution of nanoconfined fluid. We used a novel adsorption-ultrasonic experimental setup to analyze the characteristics of ultrasound propagation through a xerogel sample while measuring its sorption isotherm by controlling relative humidity. The adsorption followed a type V isotherm, characteristic of weakly interacting carbon micropores and mesopores. Analysis of the elastic moduli revealed that confined water in mesopores deviates from bulk-like behavior. Additionally, the increase in ultrasonic attenuation during micropore filling suggested spatial heterogeneity in the water-filled pore space. This study demonstrates the utilization of non-destructive ultrasonic testing to probe both the fluid adsorption mechanism and the properties of an adsorbed phase in nanoporous materials.

In order to decouple structural parameters of silica aerogels like particle size, pore size and fractal dimension on the one hand from aerogel properties such as aerogel density, thermal and mechanical characteristics on the other hand, the structural properties were varied in a wide range. It has been a challenging task to find synthesis parameters still resulting in gels, but also covering a wide property space. For this goal, three synthesis routes, based on the classical tetraalkoxysilane route, were chosen. The structural properties of the silica aerogels produced cover more than two orders of magnitude in particle and pore sizes, whereas the variation of density and porosity is limited by the Si-content of the silica source. Due to physical limitations, not all combinations of pore size correlated to an aerogel density are possible, leading to a gap for small densities and small pores as well as for high densities and large pores. For increasing particle sizes, the structure generation mechanism seems to alter from particle generation and subsequent cluster formation to phase separation. Along with that, the mechanical stiffness drops down for larger structures (pores and particles). For the mechanical and thermal properties, only the solid thermal conductivity scales roughly with the Young’s modulus, thus giving the opportunity of decoupling mechanical and thermal conductivity at ambient pressure from each other.

Measurements of the thermal conductivity were performed as a function of gas pressure from 10–1 hPa up to 105 hPa on several bimodal silica xerogels. The xerogels exhibit a mesopore and a macropore phase. The measurements were done using a hot-wire apparatus, which can do automated, gas pressure-dependent measurements of the thermal conductivity from 10–3 up to 105 hPa. Results were fitted with a bimodal gas pressure-dependent thermal conductivity model to gain information on the thermal conductivity of the materials, its various contributions and on structural parameters such as the two main pore sizes, the macro- and mesoporosities. The pore sizes and porosities were compared to values gained from mercury porosimetry and nitrogen adsorption measurements. The porosities from the thermal conductivity measurements are in very good agreement to the other measuring methods. The macropore sizes from the thermal conductivity measurements are mostly in agreement within the given uncertainty range and the mesopore sizes show a good estimate of the order of magnitude of the pores.

Aerogels are an exciting class of materials with record-breaking properties including, in some cases, ultra-low thermal conductivities. The last decade has seen a veritable explosion in aerogel research and industry R&D, leading to the synthesis of aerogels from a variety of materials for a rapidly expanding range of applications. However, both from the research side, and certainly from a market perspective, thermal insulation remains the dominant application. Unfortunately, continued progress in this area suffers from the proliferation of incorrect thermal conductivity data, with values that often are far outside of what is possible within the physical limitations. This loss of credibility in reported thermal conductivity data poses difficulties in comparing the thermal performance of different types of aerogels and other thermal superinsulators, may set back further scientific progress, and hinder technology transfer to industry and society. Here, we have compiled 519 thermal conductivity results from 87 research papers, encompassing silica, other inorganic, biopolymer and synthetic polymer aerogels, to highlight the extent of the problem. Thermal conductivity data outside of what is physically possible are common, even in high profile journals and from the world’s best universities and institutes. Both steady-state and transient methods can provide accurate thermal conductivity data with proper instrumentation, suitable sample materials and experienced users, but nearly all implausible data derive from transient methods, and hot disk measurements in particular, indicating that under unfavorable circumstances, and in the context of aerogel research, transient methods are more prone to return unreliable data. Guidelines on how to acquire reliable thermal conductivity data are provided. This paper is a call to authors, reviewers, editors and readers to exercise caution and skepticism when they report, publish or interpret thermal conductivity data. Graphical Abstract

Material development processes are highly iterative and driven by the experience and intuition of the researcher. This can lead to time consuming procedures. Data-driven approaches such as Machine Learning can support decision processes with trained and validated models to predict certain output parameter. In a multifaceted process chain of material synthesis of electrochemical materials and their characterization, Machine Learning has a huge potential to shorten development processes. Based on this, the contribution presents a novel approach to utilize data derived from Small-Angle X-ray Scattering (SAXS) of SiO2 matrix materials for battery anodes with Neural Networks. Here, we use SAXS as an intermediate, high-throughput method to characterize sol–gel based porous materials. A multi-step-method is presented where a Feed Forward Net is connected to a pretrained autoencoder to reliably map parameters of the material synthesis to the SAXS curve of the resulting material. In addition, a direct comparison shows that the prediction error of Neural Networks can be greatly reduced by training each output variable with a separate independent Neural Network.

Determining reliable structural parameters for an aerogel by applying suitable characterization techniques is a key factor in terms of understanding the different synthesis steps and their impact on the resulting aerogel. Combining structural parameters with the physical properties of the material allows optimization for specific applications. It is only the profound knowledge of the structure–properties relationships that provides access to the full potential of this type of material. This chapter presents different characterization methods commonly used and discusses their potential and limitations. Furthermore, more recent developments and new approaches are introduced.

Techniques are presented that allow to introduce anisotropy into the structure of aerogels and tune properties such as mechanical strength, thermal conductivity, hydrophobicity, and porosity. These hybrid aerogels can be roughly divided into three classes. The first class is functionally graded aerogels. Functionally graded aerogels can be fabricated by creating density, catalyst, or polymerization gradients within aerogel monoliths. The resulting composites present mechanical strength gradients which have been used, for example, in the Space Dust Project. Functionally graded aerogels also make the materials promising for thermal insulation with enhanced mechanical properties, and aerogels with hydrophobicity gradients are potentially suitable for waste remediation. The second class is aerogels where nanoparticles are synthesized inside the pores to tune optical absorption, emission, and index of refraction. The third class is aerogels where selected regions are reinforced or otherwise modified by photolithographic techniques. For example, photolithographic techniques can be used to reinforce selected aerogel regions by polymer crosslinking. The character of the modification can be adapted, etc.

Gas adsorption in zeolites leads to adsorption-induced deformation, which can significantly affect the adsorption and diffusive properties of the system. In this study, we conducted both experimental investigations and molecular simulations to understand the deformation of zeolites 13X and 4A during carbon dioxide adsorption at 273 K. To measure the sample's adsorption isotherm and strain simultaneously, we used a commercial sorption instrument with a custom-made sample holder equipped with a dilatometer. Our experimental data showed that while the zeolites 13X and 4A exhibited similar adsorption isotherms, their strain isotherms differed significantly. To gain more insight into the adsorption process and adsorption-induced deformation of these zeolites, we employed coupled Monte Carlo and molecular dynamics simulations with atomistically detailed models of the frameworks. Our modeling results were consistent with the experimental data and helped us identify the reasons behind the different deformation behaviors of the considered structures. Our study also revealed the sensitivity of the strain isotherm of zeolites to pore size and other structural and energetic features, suggesting that measuring adsorption-induced deformation could serve as a complementary method for material characterization and provide guidelines for related technical applications.

In this study, we present a detailed comparison between a conventional supercritical drying process and an evaporative drying technique for hierarchically organized porous silica gel monoliths. These gels are based on a model system synthesized by the aqueous sol–gel processing of an ethylene-glycol-modified silane, resulting in a cellular, macroporous, strut-based network comprising anisotropic, periodically arranged mesopores formed by microporous amorphous silica. The effect of the two drying procedures on the pore properties (specific surface area, pore volume, and pore widths) and on the shrinkage of the monolith is evaluated through a comprehensive characterization by using nitrogen physisorption, electron microscopy, and small-angle X-ray scattering. It can clearly be demonstrated that for the hierarchically organized porous solids, the evaporative drying procedure can compete without the need for surface modification with the commonly applied supercritical drying in terms of the material and textural properties, such as specific surface area and pore volume. The thus obtained materials deliver a high specific surface area and exhibit overall comparable or even improved pore characteristics to monoliths prepared by supercritical drying. Additionally, the pore properties can be tailored to some extent by adjusting the drying conditions, such as temperature.

High-performance thermal insulations (HPI) have outstanding thermal properties minimizing heat transfer for a particular application. The term "high performance" is a relative assessment: whether an insulating property for a material or material system, i.e. the thermal conductivity or thermal transmittance, is significantly lower than for conventional insulating materials or systems depends on the use case and the associated conditions. Fields of applications for thermal insulations include energy, construction, industry and transport sector, as well as space and aviation industries. The total effective thermal conductivity of evacuated HPI depends on the contributions of the heat transfer via the solid skeleton of the porous insulation material and radiative heat exchange. At a given operating temperature, the effective thermal conductivity results in a minimum that depends on the density and the infrared-optically extinction properties of the insulation material. Examples for HPI with outstanding low thermal conductivity values are presented and compared with the theoretical predictions based on a percolation model for the solid thermal conductivity of the skeleton and on a diffusion model for the radiative heat transfer.

The specific surface area is key for various application fields of porous materials. Its reliable and fast determination is therefore crucial for materials development and product quality management. Surface area assessment is usually based on physical adsorption using the Brunauer-Emmett and Teller (BET) theory. However, the BET method/gas adsorption exhibits a number of limitations and challenges including (i) time consuming sample preparation and measurement time, and (ii) reliable surface areas only possible for non-porous and meso-/ macroporous materials to obtain reliable surface areas. In addition, the accuracy of surface area data obtained from adsorption depends on the proper choice of adsorptive/probe. Within this context, this work evaluates in a rigorous way in-depth Small Angle X-ray Scattering (SAXS) as an alternative and complimentary approach for reliable and fast surface area assessment. To our knowledge, this work can be considered the first systematic study where the surface areas from SAXS are compared and validated with true benchmark surface area data. We utilize silica-based nanoparticles as well as a well-defined mesoporous controlled pore glass for systematic SAXS and adsorption studies (argon and nitrogen at 87 K and 77 K, respectively). Owing to the lack of micro- and narrow mesopores of these model materials, the BET method based on argon 87 K adsorption can be applied to determine benchmark surface areas. Indeed, excellent agreement was found between surface areas derived from argon 87 K adsorption and SAXS. In fact, we demonstrate that the determination of specific surface area can be brought with SAXS to a new level, where parameters such as size of the probing adsorptive, its orientation and thus its effective cross-sectional area (when adsorbed on the surface) are no longer affecting the value of the specific surface area determined. Furthermore, SAXS was shown to be significantly faster than gas adsorption. For the silica materials used, the study shows that SAXS does not require degassing and - along with analysis times of only few minutes per sample - provides an accurate and extremely fast, high-throughput approach. This fundamental study can be considered a major step in enabling SAXS for reliable surface area assessment for applications both in nanoporous materials development and quality control, thus boosting SAXS for surface area determination in general, but in particular also for materials, where the usage of gas adsorption is restricted or not possible at all.

The impact of synthesis parameters and structural properties, respectively, on mechanical properties of porous materials on different structural levels provides valuable information for designing materials for specific applications. Within this study, we apply two nonstandard approaches for determining the mechanical properties of the mesoporous backbone phase in a series of disordered SiO2-based monolithic materials possessing hierarchical meso-macroporosity, that is, deformation upon mercury porosimetry and in situ dilatometry during nitrogen adsorption analysis. By using ordered porous model materials, the latter method has been recently proven to provide reliable mechanical moduli. This concept was now applied to a SiO2 monolith developed for high-performance liquid chromatography exhibiting disordered hierarchical meso- and macroporosity, as well as a series of analogue phenyl-modified meso-macroporous SiO2 monoliths with up to 36.1 at% organic modification. The phenyl group was introduced by adding phenyltrimethoxysilane to the sol-gel mixture. The study aimed at investigating in detail the impact of the organic modification on the morphology of the porous solid and the resulting mechanical properties. The study shows that both Hg porosimetry and in situ dilatometry performed during N2 adsorption at 77 K provide similar and reasonable moduli of compression for the mesoporous backbone of the silica materials investigated. These data were compared with moduli of the macroscopic sample as determined from sound velocity measurements by describing the fully connected macroporous backbone with a foam model. The comparison reveals an otherwise overseen side effect of the organic modification of the silica framework: in contrast to the pure reference SiO2 meso-macroporous monoliths, the hybrid material is composed of a more particulate morphology on the mesoscale, that is, mesoporous particles and corresponding necks between them are formed, which results in significant softening of the porous solid on the macroscale.

Supercapacitors play a major role in the field of energy storage. However, the production of highly efficient and durable supercapacitors using sustainable materials remains a challenge. Herein, we report on a green approach for the production of thin film binder electrodes for electric double layer capacitors (EDLCs) based on onion-like carbon. The active material obtained by high temperature treated detonation nanodiamond offers a specific surface area of 527 m(2) g(-1) after an additional activation step. The key to facilitate the electrode manufacturing for this active material was the introduction of an agglomeration step of the carbon nano-onions which qualified them for the usage of plant-based carboxymethyl cellulose (CMC) as binder. The new types of electrodes were successfully tested in a supercapacitor full cell setup and benchmarked against electrodes relying on conventional fluorinated polymer (PVDF) as binder. The green electrodes provide about 22% higher specific capacitance of 56 F g(-1) at a similar durability over more than 5000 cycles, combining sustainable production with excellent performance.

Vanadium redox flow batteries (VRFB) typically show performance degradation due to V2+/V3+-reaction activity deterioration on their carbon anodes. The reason for the deterioration has not yet been identified. Employing different graphite rotating-disk electrodes, we demonstrate that V2+ inhibits the hydrogen evolution reaction (HER). Our findings suggest that V2+ adsorbs at cathodic potentials and deactivates the HER. Analogously, the V2+-adsorption deteriorates the V2+/V3+-activity, causing the VRFB performance degradation. With graphite felts in symmetric and full-cells, we demonstrate that the slowly degrading anode activity reflects an inhibiting V2+-coverage formation. As the rate of degradation decelerates, the coverage approaches equilibrium. We restored the V2+/V3+-activity by reversing the electrode polarity, representing V2+-desorption. At anodic potentials, the adsorbed V2+ liberates the surface, which we also confirmed in half-cells. Here, in vanadium-free sulfuric acid, the HER and V2+/V3+-activity recovered already below -0.26 V vs. NHE due to the Nernst shift. Our findings facilitate the understanding of several features of VRFB electrode behavior. Moreover, they promote operational strategies upon which the VRFB performance can be entirely restored and considerably increased.

Executive Summary: In the context of global climate control policies, improving the energy efficiency of existing buildings represents a great challenge, worldwide as well as at the European level. Reducing the energy consumption of buildings is nowadays preferably achieved by increasing the thermal resistance of the insulation layer in the building envelope. The AEROCOINs project, in order to contribute to the future reduction of energy consumption by decreasing heating and cooling demands of existing-buildings, has been working on the clever combination of sol-gel science and nanotechnology aiming to advance on the design and development of

Graphite felt (GF) electrodes of vanadium redox flow batteries show enhanced performance when thermally treated before their assembly. Thermal treatment works by simultaneously increasing electrode wettability, kinetic activity, and total surface area (TSA). This study examines these performance determining yet inseparable effects, especially considering the electrodes’ long-term operation. We exposed GF electrodes to 5 min plasma treatment, ensuring equal wettability, and thermally treated them in air at 400 °C for different durations. We then linked the resulting GF surface structure with the electrode performance, monitored with a high temporal resolution, and controlled electrolyte conditions. The performance, expressed in charge-transfer resistances and voltage efficiencies, correlated accurately with the thermal treatment times. According to XPS, against expectation, the thermal treatment decreased the number of surface oxygen functionalities. Instead, SEM and krypton adsorption revealed that the surface had become rougher, and the TSA increased. Upon corrosion, the surface presumably exposed more carbon edge sites being catalytically active, explaining the improved performance. Therefore, compared with the commonly suggested surface oxygen enrichment, increasing the GF surface roughness and TSA may be the more promising strategy to enhance and stabilize the long-term VRF electrode performance.

Due to their special combination of nanostructure and high porosity, aerogels are key materials for high performance thermal insulation. However, measuring reliable thermal conductivity values, which are essential for material’s optimization and as product parameter, is a challenging task in the case of aerogels. Experimentally derived thermal conductivity values for aerogels are so far more or less influenced by the experimental set-up and the experimental conditions and have to be carefully discussed. Here we present results of an intercomparison on thermal conductivity measurements performed for a polyurethane aerogel produced on a pilot scale as stiff panels by BASF. Prior to the intercomparison, the material was checked for reproducibility in production and homogeneity. The dependence of thermal conductivity on atmospheric pressure and temperature was also determined. We discuss the results submitted by 12 participants with respect to the different experimental techniques applied and identify experimental parameters with severe impact on the resulting thermal conductivities. The derived mean values for the thermal conductivity at 20 °C, 40 °C and 60 °C were related with relative uncertainties in the range from 0.6 % to 0.9 %. The dependence of the derived thermal conductivity values on the geographical location of the participating laboratory and the atmospheric weather conditions could be clearly observed and the precision of the results could be significantly improved by correcting for these effects. The values had to be partly corrected up to 2.5 %.