**3.2 Gas sensors**

*Nanofibers - Synthesis, Properties and Applications*

conductivity to the growing cells in bone regeneration.

Tissue engineering scaffolds are often made of biodegradable polymeric materials. The biodegradable polymeric scaffolds are not widely used in regeneration of load-bearing bones due to their limited mechanical strength. There have been many efforts invested to enhance the mechanical properties of the scaffolds, i.e., CNFs, reinforced by hydroxyapatite (HA), have provided mechanical support and osteo-

Wu et al. [36] electrospun a precursor mixture of Ca(NO3)2·4H2O and (C2H5O)3PO with a polymer additive, then applied thermal treatment at 600°C for 1 h and prepared HA (Ca10(PO4)6(OH)2) fibers. The pure fibers obtained were 10–30 μm in diameter and up to 10 mm in length. The HA grain size was ~1 μm [36]. Kim and Kim produced HA nanofibers and also their fluoridated forms for dental restoration applications to stimulate bone cell responses and provide protection against the formation of dental caries [44]. Xiaoshu and Shivkumar [35] electrospun nanofibers from polyvinyl alcohol (PVA) solution containing calcium phosphate-based sol and then calcined them at 600°C for 6 h to obtain an inorganic, fibrous network, which was suggested for use in the tissue engineering and drug delivery [35]. Franco et al. used phosphorus pentoxide (P2O5) and calcium nitrate tetrahydrate (Ca (NO3)2.4H2O) as precursors of phosphorus and calcium and polyvinylpyrrolidone (PVP) as the polymer to electrospin CNFs [43]. Wang et al. [45] electrospun pure titanium dioxide (TiO2) nanofiber meshes with different surface microroughness and nanofiber diameters and investigated the osteoblast differentiation on these meshes by analyzing the cell number, differentiation markers and local factor production for MG63 cells seeded on TiO2 meshes. Cells with similar morphology were observed to grow throughout the entire surfaces. While the cell number was found to be sensitive to surface microroughness, the cell differentiation and local factor production were observed to be regulated by both surface roughness and nanofiber diameter. These results indicated that the TiO2 nanofiber meshes could be used to create an osteogenic environment without using exogenous factors [45]. Aly et al. [46] developed wollastonite glass ceramic composites reinforced by electrospun TiO2 nanofibers for use in hard tissue engineering applications. The composite material exhibited greater densification and better mechanical characteristics in comparison to pure wollastonite. The composites having 0, 10, 20 and 30 wt.% metal oxide nanofibers were sintered at 900, 1100 and 1250°C. While the compressive strength, bulk density, and microhardness increased, the water adsorption capacity and porosity decreased with the increase in the TiO2 nanofiber content. When the wollastonite and wollastonite/TiO2 nanofibers were soaked in simulated body fluid, bone-like apatite was formed on their surface. The characteristics of wollastonite were improved with incorporation of TiO2 nanofibers while its in-vitro bioactivity was preserved. The developed composite was suggested for use as a bone substitute in high load bearing sites [46]. Nagarajan et al. [47] produced boron nitride-reinforced gelatin nanofibers as a new class of two-dimensional biocompatible nanomaterials, showing enhanced mechanical properties, stability to the glutaraldehyde cross-linking, high bioactivity in forming bone-like HA, and biodegradability. Depending on the analysis of osteoblast gene expression and the measurement of alkaline phosphatase activity, they were proven to be suitable for

**3.1 Tissue engineering applications**

**62**

bone tissue engineering applications [47].

Apart from the use of CNFs in bone tissue engineering applications, Du et al. [42] recently fabricated highly aligned, zirconia-based, shape memory nanofiber yarns and springs by electrospinning for artificial muscle applications at elevated temperatures. The nanofiber yarns displayed a recoverable strain of up to ~5% and short recovery time (0.16 s) at actuation temperatures of 328–388°C. When heated

Sensors, which are used to monitor and quantify volatiles related to environmental monitoring, analyze food quality, and diagnose illnesses, have attracted great interest in the recent years. Sensors designed are required to display high selectivity, low power consumption, fast response/recovery rate, low detection limit and a low humidity dependence [31].

The ceramic nanofibers have been extensively studied for gas sensing applications due to their advantages such as good directional carrier transport, high surface energy, large surface-to-volume ratio, high chemical stability, great sensing performance. Ceramics are inherently resistant to aggressive physical and corrosive chemical circumstances and they offer significantly minimized hysteresis with increased relaxation time, which improves the stability, performance, and response time of pressure sensors [87]. Many researchers showed the applicability of electrospun ceramic nanofibers in gas sensing applications for detection of many different gases such as acetone [34, 49, 88–97], ethanol [98–101], formaldehyde [102], ammonia [103–107], hydrogen sulfide [108], nitrogen dioxide [109–111], acetic acid [112], carbon monoxide [113, 114], hydrogen [115], and toluene [116]. The sensing properties of metal oxides depend on their shape, size, size distribution, surface area, structure, phase, the grain size, crystallinity, the presence of crystal lattice defects, the type of the charge carriers, and the oxidizing or reducing nature of the target gas [31, 117–120]. Besides, the composition of the sensor is another important factor. The most effective methods for improving response time and sensitivity are (i) doping with different metals such as Ce, Cu, Pr, La, Pd, Mn [91, 94, 106, 112, 114, 121], (ii) formation of composites by coupling of two or more oxide metals [118, 119, 122–124], and (iii) addition of graphene [125–127]. For gas sensors made up of ceramic nanofibers, the composition and the nanofiber configuration are other important characteristics that can be controlled to improve the sensor performance.

Liu et al. [32] prepared polycrystalline CeO2 nanofibers by combination of electrospinning and calcination. The average diameter of the nanofibers was measured as 376 ± 55 nm. They displayed good morphological and structural stability at high temperatures (800–1000°C) and showed reversible, sensitive, and reproducible response when used for real-time oxygen (O2) and carbon monoxide (CO) monitoring at 800°C and 1000°C, respectively [32]. Tong et al. [49] prepared acetone sensors based on LnFeO3 (Ln = La, Nd, and Sm) nanofibers produced by electrospinning and investigated the effect of lanthanide on acetone sensing properties of the nanofibers at different temperatures and acetone concentrations. The results indicated that the lanthanides significantly affected the sensing properties of LnFeO3. When exposed to 100 ppm acetone at 140°C, the SmFeO3 sensor exhibited the largest sensing response (Response = 9.98). The response and recovery times for the SmFeO3 sensor were about 17 and 16 s, respectively [49]. Ma et al. [34]prepared hollow perovskite praseodymium ferrite (PrFeO3) nanofibers via electrospinning followed by calcination. The samples had a large specific surface area (33.74 m2 g−1) with mesoporous characteristics. They showed good selectivity, long-time stability at 180°C, and high response value. While the response time of the sensor to 10 ppm acetone was about 4 s, the recovery time was measured as 4 s. [34]. Teli and Nadathur [48] prepared

reusable, reversible dye doped nanofibrous silica and silica/PVP, and silica/PMMA membranes by combination of sol–gel and electrospinning and demonstrated their use as an effective optical gas sensor. A durable, sensitive, reversible, and visually detectable response to HCl and NH3 was observed when the silica composite NF membranes was doped with a pH sensitive dye Bromothymol blue. Eliminating the need for electronic instrumentation, the regeneration of the doped NF membranes' color under mild thermal treatment and their thermal stability, permitted their repeated use for naked-eye sensing. The magnitude of color change was affected by the presence of copolymer in the NF membrane structure due to the copolymer's effects on the fiber diameter, surface area, porosity, and polarity of nanofiber membranes. The reliable and reproducible visual sensor performance along with its flexible and porous nature offered many advantages for different applications such as detecting volatiles relevant to environmental safety, tracing stability to spoilage in fresh food storage, air quality monitoring. Besides, they could be used to detect biogenic amines, or volatiles released by biological materials, to monitor plant life or human health, and to confirm the safety of work environments in chemical and nuclear plants [48]. Han et al. [33] fabricated perovskite samarium ferrite (SmFeO3) nanofibers by electrospinning route and calcination process for use as a sensor for ethylene glycol, which is on one hand one of the most significant raw materials, and on the other hand one of toxic pollutants for animals, humans, and environments. The roughness of SmFeO3 nanofibers contributed to gas sensing properties by increasing the contact area between gases and material surface. They displayed high response value (18.19) to 100 ppm EG at 240°C, fast response/recovery times, good stability and selectivity [33].

#### **3.3 Water remediation applications**

Water resources being contaminated by many different types of pollutants such as heavy metal ions, organic dyes, pesticides, bacterial pathogens, etc. generates a major challenge worldwide. Many of these contaminants are skin sensitizers and mutagens responsible for different types of cancers in human beings. On the other hand, bacterial pathogens are responsible for several severe health problems. It has become very critical to develop efficient water purification technologies. Among many different methods such as electrochemical treatment, ozonation, membrane filtration, flocculation, ceramic nanofibers have gained significant interest because of their potential in water remediation due to their adsorption properties and photocatalytic activity. The major advantages of nanofibers in water remediation applications are their high aspect ratio, enhanced surface area, higher surface activity, higher porosity, continuous structure, easy handling and retrieving compared to nanoparticles [53].

Many attempts have been devoted to the use of ceramic nanofibers in the removal of contaminants from water sources. Fe2O3 [50], ZnO/TiO2 [51], TiO2 [52], CuO/ZnO [53], ZnO/SnO2 [54], ZnO [55], NiO [83], and many other nanofibers have been developed and proved to be successful in water remediation applications.

Nalbandian et al. [50] produced α-Fe2O3 nanofibers via electrospinning and investigated their applicability in chromate removal from water. Adsorption capacity of the nanofibers increased as the average nanofiber diameter decreased. Based on (CrO4 2−) adsorption isotherms at pH 6, the nanofibers with 23 nm average diameter exhibited an adsorption capacity of 90.9 mg g−1 [50]. Malval et al. [53] fabricated CuO-ZnO composite nanofibers and explored their adsorption capacity and antibacterial properties. The composite ceramic nanofibers displayed excellent adsorption capacity for congo red dye. Depending on the adsorption isotherms and kinetic studies composite nanofibers performed better than their single

**65**

**3.4 Batteries**

*Recent Advances in Applications of Ceramic Nanofibers DOI: http://dx.doi.org/10.5772/intechopen.97118*

for use in water treatment and purification processes [53].

blue (MB, 92.4%), and tetracycline (TC, 99.5%) [51].

Some polymer-ceramic composites have been suggested for the removal of organic pollutants from water. Allabashi et al. [56] impregnated Al2O3, SiC and TiO2 ceramic nanofibers by triethoxysilylated derivatives of poly (propylene imine) dendrimer, polyethylene imine and polyglycerol hyperbranched polymers and β-cyclodextrin and subsequently sol–gel reaction led to their polymerization and chemical bond formation with the ceramic substrates. The resulting organic–inorganic filters were found to remove the polycyclic aromatic hydrocarbons (up to 99%), of monocyclic aromatic hydrocarbons (up to 93%), trihalogen methanes (up to 81%), pesticides (up to 43%) and methyl-tert-butyl ether (up to 46%) [56].

Electrospun ceramic nanofibers, with their extremely high surface area and fast charge-transfer channels along its 1D nanostructure, have been considered as ideal materials for batteries. The use of lithium-ion batteries (LIBs) has attracted great interest for use in the smart micro-electronic devices and electric vehicles because of their potential for high energy density and power. However, their energy

counterparts. The ceramic nanofibers were also very active inhibitors against the growth of *S. aureus* and *GFP-E. Coli*. With their excellent adsorption capacity and adequate antibacterial properties, the composite ceramic nanofibers were suggested

Nalbandian et al. [52] produced titanium dioxide (TiO2) nanofibers and tailored their structure and composition to optimize their photocatalytic treatment efficiency. Nanofibers with controlled diameter (30–210 nm), crystal structure (anatase, rutile, mixed phases), and grain size (20–50 nm) were manufactured. Besides, composite nanofibers with either surface-deposited or bulk-integrated Au nanoparticle cocatalysts were developed. When the nanofibers' reactivity against some model pollutants such as phenol and emerging pollutants such as pharmaceuticals, personal care products were analyzed, optimized TiO2 nanofibers displayed superior performance than the traditional nanoparticulate photocatalysts. The photoactivity increased by 5 to 10 fold after Au deposition onto the surface of the nanofibers independent of the solution concentration which was attributed to the improved charge separation [52]. Malval et al. [83] produced nickel oxide (NiO) nanofibers for use as photocatalysts. The NiO nanofibers were tested for their photoactivity against model dye congo red (CR). The concentration of the catalyst was observed to be a significant factor. Due to their non-aggregating nature in aqueous solution, NiO nanofibers performed better than NiO nanoparticles. Additionally, reusability and stability were other advantages NiO nanofibers provided [83]. Pascariu et al. [54] fabricated ZnO–SnO2 nanofibers by electrospinning technique combined with calcination at 600°C. The composite ceramic nanofibers showed photocatalytic activity against Rhodamine B (RhB) dye and the highest efficiency was obtained for nanofibers with Sn/Zn molar ratio of 0.030 [54]. Pant et al. [55] produced fly ash incorporated zinc oxide nanofibers and investigated their ability to remove methylene blue (MB) from the water. Adding fly ash, which is a waste material from thermal power plants to ZnO nanofibers resulted in increased adsorption and photocatalytic removal of MB from water. Huh et al. [51] prepared TiO2-coated yttria-stabilized zirconia/silica nanofiber (an) by coating TiO2 on the surface of YSZ/silica NF using a sol–gel process. The coating layer improved the separation ability of the membrane as well as the photocatalytic degradation ability. With its smaller pore size, TiO2-coated YSZ/silica NF membrane rejected over 99.6% of the 0.5 μm polymeric particles. Furthermore, the TiO2-coated YSZ/silica NF membrane showed excellent adsorption/degradation of humic acid (HA, 88.2%), methylene

*Nanofibers - Synthesis, Properties and Applications*

stability and selectivity [33].

to nanoparticles [53].

Based on (CrO4

**3.3 Water remediation applications**

reusable, reversible dye doped nanofibrous silica and silica/PVP, and silica/PMMA membranes by combination of sol–gel and electrospinning and demonstrated their use as an effective optical gas sensor. A durable, sensitive, reversible, and visually detectable response to HCl and NH3 was observed when the silica composite NF membranes was doped with a pH sensitive dye Bromothymol blue. Eliminating the need for electronic instrumentation, the regeneration of the doped NF membranes' color under mild thermal treatment and their thermal stability, permitted their repeated use for naked-eye sensing. The magnitude of color change was affected by the presence of copolymer in the NF membrane structure due to the copolymer's effects on the fiber diameter, surface area, porosity, and polarity of nanofiber membranes. The reliable and reproducible visual sensor performance along with its flexible and porous nature offered many advantages for different applications such as detecting volatiles relevant to environmental safety, tracing stability to spoilage in fresh food storage, air quality monitoring. Besides, they could be used to detect biogenic amines, or volatiles released by biological materials, to monitor plant life or human health, and to confirm the safety of work environments in chemical and nuclear plants [48]. Han et al. [33] fabricated perovskite samarium ferrite (SmFeO3) nanofibers by electrospinning route and calcination process for use as a sensor for ethylene glycol, which is on one hand one of the most significant raw materials, and on the other hand one of toxic pollutants for animals, humans, and environments. The roughness of SmFeO3 nanofibers contributed to gas sensing properties by increasing the contact area between gases and material surface. They displayed high response value (18.19) to 100 ppm EG at 240°C, fast response/recovery times, good

Water resources being contaminated by many different types of pollutants such as heavy metal ions, organic dyes, pesticides, bacterial pathogens, etc. generates a major challenge worldwide. Many of these contaminants are skin sensitizers and mutagens responsible for different types of cancers in human beings. On the other hand, bacterial pathogens are responsible for several severe health problems. It has become very critical to develop efficient water purification technologies. Among many different methods such as electrochemical treatment, ozonation, membrane filtration, flocculation, ceramic nanofibers have gained significant interest because of their potential in water remediation due to their adsorption properties and photocatalytic activity. The major advantages of nanofibers in water remediation applications are their high aspect ratio, enhanced surface area, higher surface activity, higher porosity, continuous structure, easy handling and retrieving compared

Many attempts have been devoted to the use of ceramic nanofibers in the removal of contaminants from water sources. Fe2O3 [50], ZnO/TiO2 [51], TiO2 [52], CuO/ZnO [53], ZnO/SnO2 [54], ZnO [55], NiO [83], and many other nanofibers have been developed and proved to be successful in water remediation applications. Nalbandian et al. [50] produced α-Fe2O3 nanofibers via electrospinning and investigated their applicability in chromate removal from water. Adsorption capacity of the nanofibers increased as the average nanofiber diameter decreased.

diameter exhibited an adsorption capacity of 90.9 mg g−1 [50]. Malval et al. [53] fabricated CuO-ZnO composite nanofibers and explored their adsorption capacity and antibacterial properties. The composite ceramic nanofibers displayed excellent adsorption capacity for congo red dye. Depending on the adsorption isotherms and kinetic studies composite nanofibers performed better than their single

2−) adsorption isotherms at pH 6, the nanofibers with 23 nm average

**64**

counterparts. The ceramic nanofibers were also very active inhibitors against the growth of *S. aureus* and *GFP-E. Coli*. With their excellent adsorption capacity and adequate antibacterial properties, the composite ceramic nanofibers were suggested for use in water treatment and purification processes [53].

Nalbandian et al. [52] produced titanium dioxide (TiO2) nanofibers and tailored their structure and composition to optimize their photocatalytic treatment efficiency. Nanofibers with controlled diameter (30–210 nm), crystal structure (anatase, rutile, mixed phases), and grain size (20–50 nm) were manufactured. Besides, composite nanofibers with either surface-deposited or bulk-integrated Au nanoparticle cocatalysts were developed. When the nanofibers' reactivity against some model pollutants such as phenol and emerging pollutants such as pharmaceuticals, personal care products were analyzed, optimized TiO2 nanofibers displayed superior performance than the traditional nanoparticulate photocatalysts. The photoactivity increased by 5 to 10 fold after Au deposition onto the surface of the nanofibers independent of the solution concentration which was attributed to the improved charge separation [52]. Malval et al. [83] produced nickel oxide (NiO) nanofibers for use as photocatalysts. The NiO nanofibers were tested for their photoactivity against model dye congo red (CR). The concentration of the catalyst was observed to be a significant factor. Due to their non-aggregating nature in aqueous solution, NiO nanofibers performed better than NiO nanoparticles. Additionally, reusability and stability were other advantages NiO nanofibers provided [83]. Pascariu et al. [54] fabricated ZnO–SnO2 nanofibers by electrospinning technique combined with calcination at 600°C. The composite ceramic nanofibers showed photocatalytic activity against Rhodamine B (RhB) dye and the highest efficiency was obtained for nanofibers with Sn/Zn molar ratio of 0.030 [54]. Pant et al. [55] produced fly ash incorporated zinc oxide nanofibers and investigated their ability to remove methylene blue (MB) from the water. Adding fly ash, which is a waste material from thermal power plants to ZnO nanofibers resulted in increased adsorption and photocatalytic removal of MB from water. Huh et al. [51] prepared TiO2-coated yttria-stabilized zirconia/silica nanofiber (an) by coating TiO2 on the surface of YSZ/silica NF using a sol–gel process. The coating layer improved the separation ability of the membrane as well as the photocatalytic degradation ability. With its smaller pore size, TiO2-coated YSZ/silica NF membrane rejected over 99.6% of the 0.5 μm polymeric particles. Furthermore, the TiO2-coated YSZ/silica NF membrane showed excellent adsorption/degradation of humic acid (HA, 88.2%), methylene blue (MB, 92.4%), and tetracycline (TC, 99.5%) [51].

Some polymer-ceramic composites have been suggested for the removal of organic pollutants from water. Allabashi et al. [56] impregnated Al2O3, SiC and TiO2 ceramic nanofibers by triethoxysilylated derivatives of poly (propylene imine) dendrimer, polyethylene imine and polyglycerol hyperbranched polymers and β-cyclodextrin and subsequently sol–gel reaction led to their polymerization and chemical bond formation with the ceramic substrates. The resulting organic–inorganic filters were found to remove the polycyclic aromatic hydrocarbons (up to 99%), of monocyclic aromatic hydrocarbons (up to 93%), trihalogen methanes (up to 81%), pesticides (up to 43%) and methyl-tert-butyl ether (up to 46%) [56].

### **3.4 Batteries**

Electrospun ceramic nanofibers, with their extremely high surface area and fast charge-transfer channels along its 1D nanostructure, have been considered as ideal materials for batteries. The use of lithium-ion batteries (LIBs) has attracted great interest for use in the smart micro-electronic devices and electric vehicles because of their potential for high energy density and power. However, their energy density is usually restricted by the working potentials and specific capacity of the electrodes. There is great effort to develop alternative electrodes with higher specific capacity that will be able to replace commercially available graphite anode. Due to their high theoretical specific capacity, transition metal oxides have attracted considerable attention.

Recently, Zaidi et al. [57] prepared a separator film by electrospinning to develop high-performance LIBs. The film had a hybrid morphology of SiO2 nanofibers (SNFs) and Al2O3 nanoparticles (ANPs). It had a polymer-free composition, smooth surface, good thermal stability with high electrolyte uptake (876%) and high porosity (79%). Compared to some commercial products, higher ionic conductivity, lower bulk resistance at elevated temperature (120°C), lower interfacial resistance with lithium metal, and a wider electrochemical window was obtained. When full cells were fabricated, the specific capacity of the full cell with the SNF-ANP separator film was measured as 165 mAh g−1; the cell was stable for 100 charge/discharge cycles and exhibited a capacity retention of 99.9% at room temperature [57]. Jing et al. [59] used electrospun ZrO2 membrane as separator in lithium or sodium batteries. The membranes displayed remarkable mechanical flexibility, excellent electrolyte wettability, and infiltration, ample porosity, high electrochemical inertness, and outstanding heat and flame-resistance. The separator could withstand high current densities and showed longer cycling life than the state-of-the-art separators [59].

Different types of CNFs were also developed as electrodes for batteries. Gangaja et al. [60] electrospun CuO nanofibers for use as LIB anode. The nanofibers were made up of CuO nanoparticles, which formed a good inter-connected network. Fabricated half cells maintained specific capacity of 310 mAh g−1 at 1°C rate for 100 cycles and stabilized capacity of about 120 mAh g−1 at 5°C rate for 1000 cycles. While SEI layer content remained the same, its thickness increased at the end of 10th charge according to the ex situ x-ray photoelectron spectroscopy [60].

Incorporation of carbon materials into ceramic nanofibers has been used as an effective strategy to improve electrical conductivity and act as a buffer to suppress the volume variation of the anodes. For example, addition of graphene into Co3O4 anodes has improved the electrochemical performance because of the high capacity of Co3O4 together with the high surface area, excellent flexibility of graphene, and good electrical conductivity [128, 129]. Hu et al. [61] synthesized porous Co3O4@ rGO nanofiber anode materials via electrospinning, post thermal and hydrothermal treatments. The interconnected structure of Co3O4 nanocrystals and presence of reduced graphene oxide networks were effective in accommodating volume change, deterring aggregation of Co3O4 fibers and facilitating rapid Li<sup>+</sup> ion transport during charge/discharge cycling. The developed anode material exhibited high reversible capacity, good rate capability, and excellent cycling stability when it was tested in half and full cells. The full cell constructed from a LiMn2O4 cathode and a Co3O4@ rGO anode displayed a stable capacity with operation voltage of ~2.0 V, which was promising for the electronic devices working at low voltages [61].

CNFs with their high ionic conductivity are promising candidates for use as electrolytes in all-solid-state batteries, which are among the most promising technologies to replace conventional lithium-ion batteries [130]. Cui et al. [58] prepared an electrolyte using polyethylene oxide–lithium (bis trifluoromethyl) sulfate–succinonitrile (PLS) and frameworks of three-dimensional SiO2 nanofibers (3D SiO2 NFs), which had a conductivity of 9.32 × 10−5 S/cm at 30°C. While the Li/ LiFePO4 cells assembled with PLS and 3D SiO2 NFs (PLS/3D SiO2 NFs) delivered an original specific capacity of 167.9 mAh g−1, they only suffered 3.28% capacity degradation after 100 cycles. The solid lithium batteries based on composite electrolytes offered high safety at elevated temperatures since the cells automatically shut down

**67**

*Recent Advances in Applications of Ceramic Nanofibers DOI: http://dx.doi.org/10.5772/intechopen.97118*

full cells delivered impressive electrochemical properties [63].

(5.3 V vs. Li/Li+

coupling reactions [67].

**3.5 Catalyst supports and catalysts**

with the decomposition of PLS above 400°C [58]. Yang et al. [62] developed a solidstate ceramic/polymer composite electrolyte by embedding a three-dimensional (3D) electrospun aluminum-doped Li0.33La0.557TiO3 (LLTO) nanofiber network in a polyvinylidene fluoride-hexafluoropropylene (PVDF-HFP) matrix and reported an enhanced interfacial Li-ion transport along the nanofiber/polymer interface. The chemical interaction between the nanofibers and the polymer was further enhanced by coating lithium phosphate onto the LLTO nanofiber surface, which also promoted the lithium-ion transport along the polymer/nanofiber interface, improved the ionic conductivity and electrochemical cycling stability of the nanofiber/polymer composite. The full cell consisted of a lithium metal anode, a LiFePO4-based cathode and the composite electrolyte in between exhibited excellent cycling performance and rate capability [62]. Zhang et al. [63] embedded Li7La3Zr2O12 (LLZO) nanofibers into a poly (vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP)/ ionic liquid (IL) matrix to construct a solid polymer electrolyte (SPE). The developed SPE, containing 15 wt. % LLZO nanofibers, exhibited an improved ionic conductivity (6.5 × 10−3 S cm−1) at room temperature, a wide electrochemical window

), excellent flexibility and mechanical strength. SPE-based half and

Ceramic nanofibers with their unique properties have been extensively explored as either catalyst supports or catalysts for various types of heterogeneous reactions, such as sinter-resistant catalysts [68], photocatalytic reaction [71], electrocatalytic reaction [69], hydrogenation reaction [70], oxidation reaction [66], and Suzuki

Fu et al. [68] prepared γ-Al2O3 nanofibers with a loofah-like surface using single-needle electrospinning for use as Pt supports. After sintering at 500°C, the Al2O3 nanofiber supported Pt catalysts were employed in catalytic reduction of p-nitrophenol and 4-times higher reaction rate constant (6.8 s−1 mg−1) was observed compared to Pt nanocrystals. The high performance of the Al2O3 nanofiber supported Pt catalysts was attributed to the special surface structure and the strong metal–support interactions between Pt and γ-Al2O3 [68]. There are many studies on the use of ceramic nanofibers as photocatalysts in water remediation studies, as already explained previously. Other than water remediation studies, ceramic nanofibers are also employed in the photocatalytic H2 evolution from water splitting, which is a promising renewable energy generation process. Using electrospinning method, Wang et al. [71] fabricated MgTiO3 nanofibers and compared their photocatalytic H2 generation ability with the MgTiO3 nanoparticles and P25. The MgTiO3 nanofibers showed high efficiency and stability in photocatalytic H2 generation under ultraviolet light. Attributed mainly to their large specific surface area, special 1D structure, unique mesh morphology, and pure phase, photoelectrochemical measurements showed that the MgTiO3 nanofibers facilitated the transport and separation of the photoinduced charge carriers [71]. Ceramic nanofibers are also utilized in electrocatalysis to speed up the charge transfer reaction between electrodes and electrolytes. Hosseini et al. [69] fabricated CuO/NiO composite nanofibers and investigated their photocatalytic performance as anode catalyst for hydrazine oxidation in alkaline media. The best catalytic performance was observed when the proportion of Cu(OAc)2:Ni(OAc)2 was 25:75 in polymeric solution [69]. Liu et al. [70] electrospun mesoporous CeO2-based ultrathin nanofibers in fibril-intube configuration. The fibril-in-tube configuration was achieved by choosing two metal precursors with different decomposition rates. Al(acac)3, which rapidly led to the growth kinetics varied along the radial direction of nanofibers by releasing

*Nanofibers - Synthesis, Properties and Applications*

considerable attention.

state-of-the-art separators [59].

density is usually restricted by the working potentials and specific capacity of the electrodes. There is great effort to develop alternative electrodes with higher specific capacity that will be able to replace commercially available graphite anode. Due to their high theoretical specific capacity, transition metal oxides have attracted

Recently, Zaidi et al. [57] prepared a separator film by electrospinning to develop high-performance LIBs. The film had a hybrid morphology of SiO2 nanofibers (SNFs) and Al2O3 nanoparticles (ANPs). It had a polymer-free composition, smooth surface, good thermal stability with high electrolyte uptake (876%) and high porosity (79%). Compared to some commercial products, higher ionic conductivity, lower bulk resistance at elevated temperature (120°C), lower interfacial resistance with lithium metal, and a wider electrochemical window was obtained. When full cells were fabricated, the specific capacity of the full cell with the SNF-ANP separator film was measured as 165 mAh g−1; the cell was stable for 100 charge/discharge cycles and exhibited a capacity retention of 99.9% at room temperature [57]. Jing et al. [59] used electrospun ZrO2 membrane as separator in lithium or sodium batteries. The membranes displayed remarkable mechanical flexibility, excellent electrolyte wettability, and infiltration, ample porosity, high electrochemical inertness, and outstanding heat and flame-resistance. The separator could withstand high current densities and showed longer cycling life than the

Different types of CNFs were also developed as electrodes for batteries. Gangaja et al. [60] electrospun CuO nanofibers for use as LIB anode. The nanofibers were made up of CuO nanoparticles, which formed a good inter-connected network. Fabricated half cells maintained specific capacity of 310 mAh g−1 at 1°C rate for 100 cycles and stabilized capacity of about 120 mAh g−1 at 5°C rate for 1000 cycles. While SEI layer content remained the same, its thickness increased at the end of 10th charge according to the ex situ x-ray photoelectron spectroscopy [60].

Incorporation of carbon materials into ceramic nanofibers has been used as an effective strategy to improve electrical conductivity and act as a buffer to suppress the volume variation of the anodes. For example, addition of graphene into Co3O4 anodes has improved the electrochemical performance because of the high capacity of Co3O4 together with the high surface area, excellent flexibility of graphene, and good electrical conductivity [128, 129]. Hu et al. [61] synthesized porous Co3O4@ rGO nanofiber anode materials via electrospinning, post thermal and hydrothermal treatments. The interconnected structure of Co3O4 nanocrystals and presence of reduced graphene oxide networks were effective in accommodating volume change,

charge/discharge cycling. The developed anode material exhibited high reversible capacity, good rate capability, and excellent cycling stability when it was tested in half and full cells. The full cell constructed from a LiMn2O4 cathode and a Co3O4@ rGO anode displayed a stable capacity with operation voltage of ~2.0 V, which was

CNFs with their high ionic conductivity are promising candidates for use as electrolytes in all-solid-state batteries, which are among the most promising technologies to replace conventional lithium-ion batteries [130]. Cui et al. [58] prepared an electrolyte using polyethylene oxide–lithium (bis trifluoromethyl) sulfate–succinonitrile (PLS) and frameworks of three-dimensional SiO2 nanofibers (3D SiO2 NFs), which had a conductivity of 9.32 × 10−5 S/cm at 30°C. While the Li/ LiFePO4 cells assembled with PLS and 3D SiO2 NFs (PLS/3D SiO2 NFs) delivered an original specific capacity of 167.9 mAh g−1, they only suffered 3.28% capacity degradation after 100 cycles. The solid lithium batteries based on composite electrolytes offered high safety at elevated temperatures since the cells automatically shut down

ion transport during

deterring aggregation of Co3O4 fibers and facilitating rapid Li<sup>+</sup>

promising for the electronic devices working at low voltages [61].

**66**

with the decomposition of PLS above 400°C [58]. Yang et al. [62] developed a solidstate ceramic/polymer composite electrolyte by embedding a three-dimensional (3D) electrospun aluminum-doped Li0.33La0.557TiO3 (LLTO) nanofiber network in a polyvinylidene fluoride-hexafluoropropylene (PVDF-HFP) matrix and reported an enhanced interfacial Li-ion transport along the nanofiber/polymer interface. The chemical interaction between the nanofibers and the polymer was further enhanced by coating lithium phosphate onto the LLTO nanofiber surface, which also promoted the lithium-ion transport along the polymer/nanofiber interface, improved the ionic conductivity and electrochemical cycling stability of the nanofiber/polymer composite. The full cell consisted of a lithium metal anode, a LiFePO4-based cathode and the composite electrolyte in between exhibited excellent cycling performance and rate capability [62]. Zhang et al. [63] embedded Li7La3Zr2O12 (LLZO) nanofibers into a poly (vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP)/ ionic liquid (IL) matrix to construct a solid polymer electrolyte (SPE). The developed SPE, containing 15 wt. % LLZO nanofibers, exhibited an improved ionic conductivity (6.5 × 10−3 S cm−1) at room temperature, a wide electrochemical window (5.3 V vs. Li/Li+ ), excellent flexibility and mechanical strength. SPE-based half and full cells delivered impressive electrochemical properties [63].
