Bacterial cellulose/MXene hybrid aerogels for photodriven shape-stabilized composite phase change materials

Highlights

• MXene is employed as the photothermal material for solar thermal energy storage.

• BC/MXene aerogels are introduced to improve the shape stability of composite PCMs.

• The energy storage capacity of composite PCMs is higher than that of pure PEG.

• Solar thermal energy storage associated with PCMs is realized.

Abstract

The development of solar thermal energy storage technologies associated with phase change materials (PCMs) can greatly improve the utilization efficiency of solar energy, which is of significance to alleviate the global energy shortage. A large number of photodriven shape-stabilized composite PCMs has been exploited in recent decades, while the majorities of studies have demonstrated that the properties of composite PCMs are improved at the expense of energy storage density owing to that the introduced functional materials are substituted for a considerable portion of phase change matrix. Here, three-dimensional (3D) porous skeletons composed of bacterial cellulose (BC) as the supporting material and MXene as the photothermal material are introduced into polyethylene glycol (PEG) to fabricate composite PCMs with excellent comprehensive properties. The obtained composite PCMs show excellent shape stability, efficient photothermal conversion ability and extremely high energy storage capacity, which sheds substantial light on the efficient utilization of solar energy.

Introduction

As the main energy source of human society, fossil fuels such as coal, oil and natural gas have been irreversibly exhausted after hundreds of years of over-exploitation and huge consumption. At the same time, extensive use of fossil fuels has led to greenhouse gas emissions and environmental degradation [[1], [2], [3], [4], [5], [6]]. With the growth of population and the acceleration of industrialization, the demand for energy in the world is rapidly increasing, and the energy supply is facing greater pressure than ever. Energy shortage and environmental pollution have evolved into unavoidable issues for human beings. In order to alleviate these issues, more and more attention has been paid to the effective utilization of solar energy, wind energy, marine energy, bio-energy and other green renewable energy sources [7]. There is no doubt that the utilization of solar energy in various fields has become a hotspot nowadays [[8], [9], [10]]. Solar energy is the most abundant and accessible energy on the earth, but its intermittence and discontinuity restrict its applications [11,12]. Fortunately, thermal energy storage (TES) technology can alleviate the contradiction between supply and demand of energy in time and space [13]. Additionally, most energy is utilized in the form of thermal energy, or through the link of thermal energy. Therefore, there are more and more studies focusing on converting solar energy into thermal energy and storing it through TES technology [14,15]. It is called solar thermal energy storage (STES) technology, which improve the utilization efficiency of solar energy [[16], [17], [18], [19]]. Advanced latent heat storage technology, based on phase change materials (PCMs), can store lots of heat with small temperature fluctuation during the phase change transition [20,21]. Organic solid-liquid PCMs have attracted significant attention owing to their small or negligible supercooling, high energy storage density, desirable thermal stability, etc. [22,23] However, the majorities of organic solid-liquid PCMs suffer from some crucial issues in practical applications, such as liquid leakage and low photothermal conversion capacity [[24], [25], [26]].

For the leakage problem, the most effective strategy is to fabricate shape-stabilized composite PCMs with microencapsulation technology [27,28], polymer supporting materials [29,30], nanomaterial supporting materials [31,32], porous supporting materials [[33], [34], [35], [36]], solid-solid PCMs [[37], [38], [39]], etc. [40] It has been proved that introducing photothermal conversion materials such as noble metal nanoparticles, carbon-based materials, semiconductor materials and organic dyes can enhance the photoabsorption ability of composites [[41], [42], [43], [44], [45]]. Photothermal conversion technology is widely used in the fields of photothermal therapy [46,47], seawater desalination [48,49], photothermal energy storage [22,50], and so on [51,52]. Combined with PCMs, it can contribute to achieving the conversion and storage of light to heat. Wang et al. [26] employed single-walled carbon nanotubes as photon capturers and molecular heaters to prepare the sunlight-driven and form-stable composite PCMs. The photothermal conversion efficiency was up to 91.3%, while the latent heat reduced to about 100 J/g from 197.2 J/g. Previously, we made the best of the photoabsorption of polydopamine and developed a novel photodriven composite PCMs with polydopamine-modified boron nitride [53]. Also, its thermal storage capacity decreased to 122.7 J/g from 174.5 J/g.

The high porosity of three-dimensional (3D) porous skeleton is conducive to incorporating PCMs with a high mass fraction, which ensures composite PCMs have high thermal storage capacity [54]. At the same time, the 3D supporting skeleton and porous structure are also beneficial to obtain shape-stabilized composite PCMs. Therefore, 3D porous skeletons with photoabsorption capability are promising candidates for composite PCMs in the field of solar energy systems [55]. Qi et al. [56] prepared hierarchical graphene foam (HGF) and corresponding composite PCMs by chemical vapor deposition (CVD) and vacuum impregnation, respectively. The sunlight-driven and shape-stabilized composite PCMs maintained a relatively high energy storage density, up to 153.1 J/g, which is only slightly lower than that of paraffin wax (160.9 J/g). Wu et al. [57] prepared polyamine-modified melamine foam (MF@PDA) as a supporting framework with photothermal conversion properties, and combined the framework with paraffin to obtain MF@PDA/PW PCMs. MF@PDA/PW PCMs can achieve the photothermal conversion efficiency of 80.8% and maintain the latent heat of about 140 J/g.

With these strategies, the shape stability and photothermal conversion ability are improved, but the reduction of the energy storage capacity of composite PCMs is almost inevitable. However, high energy storage capacity is a main feature of organic solid-liquid PCMs. Therefore, it is still a great challenge in the fabrication of composite PCMs with balanced comprehensive performance and energy storage density. More and more researchers have pay attention to this problem [[58], [59], [60]]. In this work, a 3D porous scaffold composed of bacterial cellulose (BC) and MXene was introduced into polyethylene glycol (PEG) to prepare photodriven shape-stabilized composite PCMs. BC with a large aspect ratio can construct a 3D porous supporting skeleton at very low concentrations. MXene with excellent photoabsorption can be embedded in the 3D BC skeleton to improve the photothermal conversion ability. The obtained composite PCMs exhibit excellent shape stability and photothermal conversion ability, and more importantly, show even higher energy storage capacity than pure PEG.