Ultralight nanofiber aerogels (NFAs) or nanofiber sponges are a truly three-dimensional derivative of the intrinsically flat electrospun nanofiber mats or membranes (NFMs). Here we investigated the potential of such materials for particle or aerosol filtration because particle filtration is a major application of NFMs. Ultralight NFAs were synthesized from electrospun nanofibers using a solid-templating technique. These materials had a tunable hierarchical cellular open-pore structure. We observed high filtration efficiencies of up to 99.999% at the most penetrating particle size. By tailoring the porosity of the NFAs through the processing parameters, we were able to adjust the number of permeated particles by a factor of 1000 and the pressure drop by a factor of 9. These NFAs acted as a deep-bed filter, and they were capable of handling high dust loadings without any indication of performance loss or an increase in the pressure drop. When the face velocity was increased from 0.75 to 6 cm s, the filtration efficiency remained high within a factor of 1.1-10. Both characteristics were in contrast to the behavior of two commercial NFM particle filters, which showed significant increases in the pressure drop with the filtration time as well as a susceptibility against high face velocities by a factor of 105.

Freeze-casted nanofiber based sponges or aerogels exhibit a hierarchical porous structure. Pore formation is only partially understood. Therefore, we studied the underlying solid templating mechanism. We were able to tailor the secondary pore size between 9.5 and 123 µm while retaining the smaller primary pores known from electrospun nanofiber membranes. To understand the effect of microstructure on the sponges' bulk properties, mass flow through the pores and interaction with the sponges' internal surface were investigated. By solely altering the sponges' microstructure we indeed found tunability in permeability by a factor 7 and in filtration efficiency by a factor of 220. Hence, pore architecture of nanofiber based sponges is a key element for their performance. The selected pullulan/PVA polymer blends and aqueous electrospinning conditions are benign and allow the facile adaptation of these ultralight highly porous sponges for a large number of applications.

COMMUNICATION (1 of 7)often suffer from poor mechanical stability, [10] the scientific resources devoted to developing electrospun 3D materials are increasing each year. [6b] The unique properties of 3D materials from electrospun nanofibers enable their application as scaffolds in tissue engineering, [11] framework for heterogeneous catalysis, [12] drug release composites, [11c,13] personal safety equipment, [14] sensors, [15] or electrodes. [16] It has been shown recently, that colloidal dispersions of short electrospun nanofibers prepared by cutting electrospun membranes open a complementary and controlled approach in nonwoven nanofiber processing. [17] This change in paradigm-to separate fiber formation and fiber processing-elegantly overcomes the main drawback of electrospinning, since fiber processing is no longer coupled to the intrinsically lamellar fiber deposition, but separated into more versatile liquid handling process. Using short electrospun nanofiber dispersions, Ding and co-workers [5a] and Greiner and co-workers [18] pioneered the preparation of ultralight 3D aerogels or sponges by freeze casting. These nanofiberbased 3D materials are either referred to as aerogels [5a] or sponges. [18] They show high porosities like silica-based aerogels, but the solid scaffold is preformed by short nanofibers. [19] OurThe ORCID identification number(s) for the author(s) of this article can be found under http://dx.doi.org/10.1002/admi.201700065. Water PurificationElectrospinning allows the fabrication of nanofibers from biopolymers, synthetic polymers, and even metals. In terms of applications electrospinning has outperformed alternative technologies for nanofiber production such as melt blowing, [1] selfassembly, [2] phase separation, [3] solution blow spinning, [4] and nanofiber drawing. [3a] Electrospun nanofibers feature a large specific surface area, ultrahigh aspect ratio, extreme flexibility, and the electrospinning process can easily be scaled up. One limitation in electrospinning is the anisotropic lamellar deposition character due to the layer-by-layer manufacturing process. [5] Different ways to fabricate porous 3D structures from electrospun nanofibers have been exploited, [6] such as self-assembly, [7] cool drum spinning, [8] or gas expansion. [9] Even though those 3D scaffolds lack the possibility to add scalable pores and

Nanofiber production by electrospinning has made great progress over the past two decades. Recently the research area was revolutionized by a novel post-processing approach. By cutting the endless and intertwined nanofibers into short pieces, it is now possible to reassemble them into interconnected 3D structures. Such highly porous structures are built from dispersed short nanofibers by freeze-casting. This solid templating process controls the structures' ultimate properties and architecture in terms of primary and secondary pores below 5 µm and between 10 and 300 µm, respectively. The objective of this review is to provide insight into this young field of research, in particular highlighting the processing steps, materials and current applications, from scaffolds for tissue engineering, acoustics, sensors and catalyst supports to filtration.

Electrospun nanofiber membranes are frequently used in adsorption processes thanks to their high specific surface area, tailored surface functionality, and fiber uniformity. However, they are still facing challenges such as low mechanical stability and unfavorable mass transport properties. In this study, an ultra-light and robust 3D nanofiber aerogel (NFA) or nanofiber sponge with tunable porosity and flexibility was synthesized from short pullulan/polyvinyl alcohol/polyacrylic acid nanofibers using a freeze casting process followed by thermal crosslinking. We demonstrate time the application of such NFAs in batch and continuous adsorption systems and compare their performance with flat nanofiber membranes (NFM). The NFAs proved to be promising adsorbents for cationic dyes due to their high adsorption capacity (383 mg/g) and their reusability. Langmuir isotherm was a suitable model for describing the adsorption process. The endothermic system followed a pseudo second order kinetic model and intra-fiber adsorption is found to be involved in the adsorption process. Dye adsorption by 3D NFAs was four times faster than for the respective flat NFMs and when used in a continuous process as a deep-bed filter, the pressure drop through the NFA was reduced by a factor of 40 while maintaining equal adsorption performance as for the NFM.