A review on advancement and future perspective of 3D hierarchical porous aerogels based on electrospun polymer nanofibers for electrochemical energy storage application

Highlights

• The implementation of electrospinning as an advanced technique for energy applications.

• The use of electrospun polymer nanofibers as a scaffolding structure for aerogels in energy applications.

• The development of electrospun polymeric nanofiber aerogels and also a brief discussion on the obstacle and potential of electrospun polymer nanofibers aerogel for energy storage applications.

Abstract

Nanotechnology provides innovative approaches and prospects for maintaining renewable resources and future ecosystems. Nanofibrous morphological materials are desirable in solving various energy and environmental problems. Electrospinning can effectively generate nanofibers, which is a simple and inexpensive technique. Three-dimensional (3D), highly compressible and robust aerogels derived from 1D electrospun polymer nanofibers will have wide technical implications for areas ranging from bioengineering, electrical devices and energy storage; however, aerogels derived from these electrospun polymer nanofibers have proved too challenging to develop. In this review, new strategies for the development of 3D structured aerogels with a hierarchical cellular structure and super elasticity for fibrous, isotropically bonded elastic reconstructed 1D electrospun polymer nanofibers were reported by combining electrospun nanofibers and various fibrous freeze-shaping techniques. The contents of this review article are arranged in the following way. The first section will present aerogel manufacturing technology with a focus on how the electrospinning technology can be used to make a significant impact as aerogel reinforcement. The analysis of nanocellulose-derived aerogels and chitins will then be addressed with a focus on nanofiber network aerogels and potential properties in energy storage. The applications of the electrospun nanofibers and electrospun carbon nanofibers aerogels will be defined after that. There will be examples of energy converters, secondary battery electrodes, and supercapacitors made of lightweight carbon nanofiber aerogels. Finally, it will raise opportunities for potential research.

Introduction

With increasing requirements for high-performance and environmentally friendly electrochemical energy storage devices, lithium-ion batteries and supercapacitors have considerable potential for a variety of power electronic applications. Because of their high-power density, long cycle life, and quick charge/discharge methods, supercapacitors have been extensively studied. Several attempts to improve the energy density of supercapacitors have been made for practical applications. Theoretically, the energy density could be increased by raising the basic capacitance of electrode materials or extending the operating voltage window by assembling asymmetric supercapacitors (ASCs). The overall electrochemical performance of ASCs is strongly determined by the anode and cathode material micro/nanostructures and their electrochemical activities and kinetics [1].

Meanwhile, for lithium-ion batteries (LIBs) which is considered as the primary power source for portable electronic devices, the ever-increasing need for high power and/or high energy, especially for emerging large-scale applications such as electric cars. Various research has prompted efforts to develop new high-performance electrode materials for LIBs of the next generation [2].

Electrode materials and, subsequently, battery and supercapacitor performance have advanced rapidly with the advancement of material design methods, synthesis processes, and characterization methodologies. Standard electrodes are on the milli or microscales in size for energy storage products. More and more nanostructured materials have recently been investigated for electrochemical energy storage applications, including 0D nanoparticles, 1D nanowires and nanotubes, 2D nanosheets and nanoflakes, and core-shell structured nanomaterials.

The electrode materials of LIBs have been investigated as powders or particles of metal phosphides, metal oxides, and oxysalts with diameters ranging from several to tens of nanometers (low dimensional materials). The Li-ion diffusion path can be reduced due to its nanoscale particle size. And it is possible to lower the inner tension induced by Li insertion/desertion. Consequently, it is possible to demand higher rate performance and cyclability. For these nanoparticles, though, one issue is their comparatively poor conductivity. One-dimensional (1D) nanostructures, including, among other morphologies, nanowires, nanorods, nanoribbons, nanotubes and nanofibers, are known as one of the most promising materials for energy-related applications to solve this issue [3].

The hierarchical pore structure is one of the most significant aspects of high-performance energy storage. The hierarchical porous structure is based on the various activities of electrolytes in pores of different sizes. The transport length of ions inside a porous particle can be minimized by the electrolyte in macropores, which preserves their bulk phase behaviour. Electrolyte ions are less likely to smash into large mesopore pore walls, thus decreasing ion transport resistance [4]. The pore aspect ratio can be synergistically reduced by macropores and mesopores, while the high electric potential in micropores can efficiently trap ions and increase the density of charge storage. The combination of macro-/meso-/micropores will also lead to good-performance electrode materials with low resistance, short transport distance of the ion and high storage density of the charge [5].

Typical porous materials for LIB electrodes are divided into porous 3D and 1D materials. 3D Porous Structure: 3D microporous materials are made of well-interconnected cores and walls with a thickness of tens of nanometres, and these materials can be readily used for improving the rate performance of LIBs as the duration of solid-state diffusion is much shorter and the charge-transfer rate will also benefit from their relatively large surface area. Meanwhile, 3D microporous materials have a large specific energy and specific strength, and the pore structure of electrode materials is closely related to the processes of transporting ions and electrons, and is therefore most important for improving supercapacitor efficiency [6].

In addition, porous electrodes for high-performance supercapacitors should comply with the following requirements: sufficient volume of pore to store electrolytes; suitable electrolyte ion channels to be easily accessed and transferred easily to or from the entire electrode material surface; and ample chemically active capacitance enhancement sites [7]. The precise mechanism of pores 'ion transport is very complex, associated with pores' tortuosity, connectivity, size distribution and form distribution, functional community of surface oxygen, electrolyte nature, and solid-liquid interface. Three parameters, including pore aspect ratio, pore regularity, and surface functional group population, should primarily be considered among these variables [8].

Due to their intrinsic advantages such as high porosity, light weight, good mechanical stability and excellent electrical conductivity, aerogels, especially carbon-based aerogels (CA), have recently attracted intense interest, thus realizing their exciting applications in elastic conductors1, water treatment, catalyst support, energy storage and conversion [9]. More interestingly, the introduction of one-dimensional (1D) nanofibrous building blocks into three-dimensional (3D) aerogels can significantly reduce the density in all aspects and improve the properties of aerogels. Natural structures such as spider webs and bone tissues both indicate that 3D networks consisting of 1D building blocks have excellent structural integrity and low density at the same time. Most aerogels are created by extracting the gel liquid through critical point drying (CPD) to keep the gel network intact [10]. Through a sol-gel process, the gel networks are assembled, where nanoparticle suspensions are formed and then the nanoparticles are crosslinked by a suitable gelling agent precursor into 3D network branches. Therefore, if gels can be assembled by crosslinking the material itself without gel precursors, there are a large number of interesting possibilities for creating a wide range of aerogels [11].

A significant approach is given by 3D hierarchic porous materials. In addition, an intriguing alternative 3D assembly method is provided by spinning long lengths of fiber electrodes and their assembly by electrospinning technique. Owing to its high mechanical power, flexible wettability, strong durability, wide surface area, and component variety, the recently produced electrospun nanofiber-based aerogels are promising materials for applications. A summary of recent research advances in the synthesis, properties and applications of aerogel-derived electrospun polymer nanofibers will therefore be addressed in this review. At the end of the article, viewpoints on the challenges (perspectives) and possibilities of electrospun polymer nanofibers derived from energy-use aerogels will be discussed.