Bio-inspired three-dimensional carbon network with enhanced mass-transfer ability for supercapacitors

Abstract

Governing mass-transfer process, the interface of electrode/electrolyte greatly affects the performance of electrochemical energy storage applications. Inspired by the spider web, a 3D carbon network (3DCN) with bionic surface is fabricated by using the zeolitic imidazolate frameworks (ZIF) as the precursors. In this strategy, 3D carbon network serves as the matrix for the formation of ZIF-8 nanoparticles. A following calcination enables these nanoparticles fuse into continuous network and introduce “spindle-knot” structure on the surface of carbon matrix. The further examines reveal that this specially designed nanostructure enables the liquid to transfer in a fast way. Powered by the "spider web" pattern, the obtained material (S-3DCN) shows great potential in the electrolyte-based applications. As the electrode in supercapacitor, S-3DCN shows a specific capacitance of 395 F g⁻¹ at the current density of 1 A g⁻¹ and delivers energy much faster than other similar carbon materials. Assembled in a solid-state supercapacitor device, S-3DCN achieves the high energy density for practical applications.

Introduction

Supercapacitor, as an important energy storage application, plays promising roles in the high power delivery or energy harvesting [1,2]. A typical supercapacitor is empowered by surface ion-adsorption or surface redox reactions [1,2]. Both of these processes consist of two basic steps: charge transport and mass transfer [[3], [4], [5]]. Charge transport largely relies on the intrinsic features of electrode materials. As an example, carbon has been widely used in supercapacitor due to its high electron conductivity and chemical stability, and can deliver charges in an effective and stable way [6]. Given the materials with similar charge transport ability, the dominating factor turns to the mass transfer, which depends on the accessibility between electrode and electrolyte. Based on such guide, various electrode materials tend to have high specific surface area and abundant transfer channels, such as activated carbon with hierarchical pores, metal oxides with well-designed nanostructures and metal-organic framework with fancy microarchitectures [7,8]. However, an important factor is often neglected: the surface morphology of electrodes. This factor largely influences the interface of electrode/electrolyte, thus affects the mass-transfer process happened in these regions.

To design a better surface of electrode, the nature is inspirational [[9], [10], [11], [12]]. The special functionalities of organisms are not only controlled by the material properties, but also determined by their micro/nano-architectures [[13], [14], [15]]. For example, spider web applies its special microstructure to achieve a superior wettability on liquid droplets. Previous studies revealed that the ability of spider web derives from their special “spindle-knot” structures [11]. Until now, researchers have mimicked the structure of spider webs at the micrometer level by using solution immersion and electrospinning [16]. Through these methods, the obtained bionic materials have the spindle-knot of 0.2–100 μm [[17], [18], [19], [20], [21]], which have been proved to be effective in water collection and fog harvesting [11,19]. Yet, the advantages of spider web have not been taken by energy storage applications. The major issue is the length scale: the electrochemical reactions often occur in the nanometer regions. It is unknown whether the bionic surface can remain its ability below micrometer.

To answer this question, it is critical to fabricate the bionic surface in nanometer-scale. For this purpose, there are several candidate strategies, such as physical/chemical vapor depositions, laser-etching and molecular self-assembly [[22], [23], [24], [25], [26]]. However, few of them have been successfully applied in fabricating spider web nanopartterns, let alone these methods are subject to high cost, complex process and impossible mass-production. To solve these questions, there may be opportunities in some state-of-the-art materials. Recently, zeolitic imidazolate frameworks (ZIFs) emerged from various kinds of nanomaterials and attracted increasing attention [27]. ZIFs are topologically isomorphic with zeolites, which are composed of tetrahedrally-coordinated transition metal ions connected by imidazolate linkers [28]. Among various ZIFs, ZIF-8 containing high content of N (∼34 wt %) has acted as an ideal precursor to synthesize nitrogen-doped porous carbon, which enjoys the merits of large specific surface area and large amount of chemical active sites [29]. Also, the ZIF-derived carbon can tune the size and connected mode according to the production situation. Yet, these ZIF-8 derived carbon presented as the isolated particles, which are unfavorable for the fast electrolyte diffusion and electron transport [[29], [30], [31]]. Also, introducing the carbon matrix is a common method to achieve hierarchical porous structure [[32], [33], [34]]. Researchers have successfully combining ZIFs with 1D or 2D carbon materials, further approving its fabrication flexibility [[35], [36], [37]]. Comparing with 1D or 2D carbon, 3D carbon networks (3DCN) possess the interconnected structure and enjoy the most openness, which not only serves as the electron transfer pathway but also enhances the mass-transfer ability [38,39]. More importantly, a well-designed carbon network can provide an ideal platform for combing with other nanomaterials [[38], [39], [40]]. The merits of ZIF and 3DCN fired up new hope on the fabrication of spider-web bionic nanomaterials.

Herein, we develop a strategy to fabricate a bio-inspired carbon hybrid with enhanced mass-transfer ability, relying on a 3DCN and ZIF-8. In our approach, the 3DCN produced by salt-template method serves as the matrix for the ZIF-8 formation. Then, a calcination process enables ZIF-8 particles gradually to fuse into a continuous network-like, which is closely attached to 3DCN. Therein, ZIF-8 particles act as the “spider” to introduce a “spider web” pattern on the surface of 3DCN. After etching out the metal elements of ZIF, a spider-web-like carbon pattern is formed on the surface of 3D carbon networks. Such a modified surface morphology endows the carbon hybrid (S-3DCN) with enhanced mass-transfer ability and fully takes the advantages of ZIF-derived carbon. As a result, S-3DCN shows great potential in the electrolyte-based energy storage applications. Especially, this bio-inspired material can deliver the energy much faster than untreated samples.