Boron and nitrogen co-doped sodium alginate-based porous carbons for durable and fast Zn-ion hybrid capacitors

Abstract

In recent years, zinc-ion hybrid capacitors (ZIHCs) have attracted increasing attention due to their environmental friendliness and excellent electrochemical properties. However, their performance is mainly limited by the electrochemical performance of the cathode, so it is necessary to develop an advanced cathode material. N, B co-doped sodium alginate-based porous carbon (NBSPC) was prepared by one-step co-carbonization using sodium alginate as the matrix and NH4B5O8 as the N and B source. This N, B co-doping strategy improves the pore structure of the carbon materials and increases the number of surface functional groups, greatly improving the capacitive behavior of the raw materials and thus improving their electrochemical performance. When used as the cathode in ZIHCs, the NBSPC had an excellent rate performance (85.4 mA h g−1 even at ultra-high current density of 40 A g−1) and good cycling stability (15 000 cycles at 20 A g−1 with a capacity retention rate of 94.5%).

1. Introduction

In the past two decades, energy storage technology has developed rapidly, resulting in the increasingly excellent performance of electric vehicles and portable mobile devices [1–3]. This is all due to the rapid development of batteries and supercapacitors. Among numerous electrochemical energy storage devices (EESDs), lithium-ion batteries (LIBs) have occupied most of the commercial market owing to their superior energy density [4–6]. However, the lack of lithium resources, safety issues of organic electrolytes and environmental pollution limit the further development of LIBs [7–10]. Therefore, it is necessary to develop a novel high-performance aqueous electrolyte EESD. As an emerging EESD with aqueous electrolyte, Zn-ion hybrid capacitor (ZIHC) has higher energy density and similar power density compared to aqueous supercapacitors. Thus, it has attracted many scholars to conduct research in recent years.

In terms of device structure, a typical ZIHC is mainly composed of a zinc metal anode and an advanced material cathode [9,11]. Therefore, the performance of a ZIHC mainly depends on the performance of the cathode material [12]. Porous carbon is an ideal material for the ZIHC cathode because of the strong structural designability, low-cost and environmentally friendliness [13,14]. At present, high-quality porous carbon electrode materials can be prepared through both the hard template and soft template methods [15]. However, the complicated removal process of hard templates and the high price of soft template agents limit the large-scale preparation of porous carbons [16]. Consequently, it is imperative to explore a method for synthesize porous carbons without adding templates. In addition, modification of porous carbon materials is also a key step to improve their electrochemical performance in ZIHCs [17]. The more commonly used modification method is to introduce defects and active groups by doping heteroatoms into the carbon matrix to improve the zinc storage capacity and rate performance of porous carbons [18]. For instance, Chen et al. prepared a N-doped biomass porous carbon with one step carbonization, which exhibits 140 mAh g−1 at 0.2 A g−1 and 86.2 mAh g−1 at 6.4 A g−1 in a ZIHC [9]. Qiu et al. synthesized O, S co-doped porous carbon nanosheets, which shows a high capacity (194 mAh g−1 at 0.5 A g−1) in ZIHC [19]. Yang and his co-workers developed a N/P co-doped hierarchical porous carbon for ZIHCs, which also exhibits good electrochemical performance [16]. There are many types of heteroatoms (such as N, S, O and B), and there are also some wonderful synergistic effects between different heteroatoms, which have a great impact on the zinc storage performance of carbon materials [20]. Therefore, it’s necessary to develop a high-performance heteroatom-doped porous carbon.

In this work, the N, B co-doped sodium alginate-based porous carbons (NBSPCs) are prepared by one-step co-carbonization (Scheme 1). Due to the self-template of sodium alginate (SA) [2] and the large amount of gas generated by the decomposition of ammonium borate, NBSPCs have an improved hierarchical porous structure. In addition, this co-carbonization method can efficiently doped N and B into the carbon skeleton. This N, B co-doped strategy can improve the wettability and electric double layer electrochemical behavior of the materials. When used as the cathode in zinc-ion hybrid capacitor (ZIHCs), the optimized NBSPC shows excellent rate performance.