Elsevier

Waste Management

Volume 106, 1 April 2020, Pages 250-260
Waste Management

Porous carbon derived from herbal plant waste for supercapacitor electrodes with ultrahigh specific capacitance and excellent energy density

https://doi.org/10.1016/j.wasman.2020.03.032Get rights and content

Highlights

  • Traditional Chinese medicine waste is converted into porous carbon materials.

  • The prepare process is green and sustainable.

  • The optimized porous carbon with 45.97 at% and 0.49 at% self-doped O and N.

  • The optimized porous carbon exhibited high performance in supercapacitors.

Abstract

Here in this work, porous carbon is prepared from waste of a traditional Chinese medicine Salvia miltiorrhiza flowers. Structures of the porous carbons are regulated by simply regulating of activation temperatures and dosages of activator. The optimized porous carbon owns a high specific surface area of 1715.3 m2 g−1 and total pore volume of 0.6392 cm3 g−1, together with a unique hierarchical architecture and ultrahigh content of 45.97 at% self-doped O and 0.49 at% of N. When used as electrode materials for supercapacitors, the prepared porous carbon exhibited excellent specific capacitance and energy density as well as fantastic cycle stability. Under a current density of 0.5 A/g, the electrode based on this material showed high specific capacitance of 530 F/g, with fantastic rate performance of 258 F/g at 20 A/g and excellent cycle stability of 91% capacitance retention for 10,000 cycles at 10 A/g in a three-electrode system in 6 M KOH. In assembled supercapacitors, the SF-PC700-3 based electrode worked under potential of 1 V and exhibited 222 F/g of specific capacitance at a current density of 0.5 A/g, and even when the current density was increased up to 30 A/g, the specific capacitance can still as high as 168 F/g, verified the excellent performance of SF-PC700-3. Symmetric supercapacitors in Na2SO4 and TEABF4/AN electrolyte showed voltage ranges of 1.8 V and 3 V respectively, and high energy density of 22.2 Wh Kg−1 at 448. W Kg−1 and 40.6 Wh Kg−1 at 755.8 W Kg−1 are obtained.

Introduction

At present, the development of mobile phones, portable computers, electric cars and other electric devices has put forward higher requirements for energy storage devices, in which, supercapacitors, which go hand in hand with lithium-ion batteries, play an important role (Simon et al., 2007, Winter and Brodd, 2004). Compared with lithium-ion batteries, supercapacitors have the characteristics of fast charging and discharging speed, high power density, good electrical conductivity and safe operation, thus they are considered as the future replacement of lithium-ion batteries (Yang et al., 2019). However, the drawback is that supercapacitors usually supply low energy density, which greatly limits their application in many fields (Wu et al., 2016, Wu et al., 2016, Wang et al., 2018), so they are a long way from really replacing lithium-ion batteries. According to the energy density formula of E = 1/2 CV2 (Pan et al., 2018), in which C and V represent for specific capacitance and operation voltage respectively, thus there are two ways to improve the energy density: enhancing specific capacitance or widening operation voltage. The operation voltage mainly depended on the electrolyte, KOH electrolyte typically provides up to 1.2 V, Na2SO4 up to 1.8 V, and organic electrolytes up to 3 V or more, but generally no more than 4 V to date (Li et al., 2018). Once the electrolyte is determined, the energy density mainly depends on the specific capacitance of the electrode material. Moreover, the theoretical voltage of an electrolyte can be reached or not also depends on whether the electrode material used can withstand such voltage. To sum up, developing excellent electrode materials is the main way to develop high energy density supercapacitors (Wang et al., 2012).

Based on the energy storage mechanism, supercapacitors are divided into two categories of double-layer capacitors and pseudo-capacitors (Wu et al., 2017, Chen et al., 2017). For pseudo-capacior, the electrode materials commonly used are transition metal oxides, etc., which realize the charging and discharging process through redox reactions (Zhou et al., 2015). This kind of supercapacitors usually exhibit ultra-high specific capacitance, but with defects such as poor electrical conductivity and poor cycling performance, and the preparation of materials is often complex and costly, thus limit their applications (Ren et al., 2014). However, the storage mechanism of double-layer supercapacitors is more physical, which uses the adsorption and desorption of charges on the surface of electrode materials to realize charging and discharging (Zhang et al., 2018a, Zhang et al., 2018b, Zhang et al., 2018c). This kind of supercapacitors use mostly porous carbon as the electrode material, which has the advantages of simple and cheap material preparation, good electrical conductivity and strong circulation tolerance, so it has attracted much attention in recent years. Porous carbon used for EDLC supercapacitors come in various forms as carbon nanotubes (Ma et al., 2015), carbon fibers (Huang et al., 2016) activated carbons (Zhang et al., 2018a, Zhang et al., 2018b, Zhang et al., 2018c), carbon aerogels (Zu et al., 2015) and graphene (Zhu et al., 2011a, Zhu et al., 2011b, Qin et al., 2014, Tan et al., 2013).

The past decade has witnessed the excellent performance of such porous carbon materials in double-layer supercapacitors (Phys, 2007, Ling et al., 2016, Wang et al., 2017). According to a large number of previous studies, porous carbon has to satisfy the following characteristics to perform well in supercapacitors. First, it should have a very large specific surface area to accommodate enough charges to produce a high specific capacitance. Second, there should be channels for the rapid passage of electrons to achieve rapid charge and discharge (Kondrat et al., 2012). Finally, it is very important to have a certain amount of hetero-atoms, such as N, O, S, P, etc., which can smooth the electrochemical performance by triggering partial pseudocapacitance or promoting the infiltration of electrolyte to the electrode material (Zhao et al., 2012). It is also worth mentioning that in order to achieve large-scale preparation and industrialization, the preparation process must be simple, cheap and environmentally friendly.

To fulfill these requirements, porous carbons derived from biomass recent years gained tremendous attentions (Ganesh et al., 2017, Jin et al., 2018, Mitravinda et al., 2018, Phiri et al., 2019., Zhu et al., 2018), because this kind of porous carbon has almost all the elements needed to prepare high-performance supercapacitors. Biomass is cheap, easy to get and renewable, and it can often obtain porous carbon with high specific surface area and abundant pores through simple carbonization and activation steps (Sevilla et al., 2018). In addition, compared with precursors such as coal and asphalt, biomass contains more active substances, which can provide a large number of N, O, S and other hetero-atoms themselves, don't need the extra doping process (Yuan et al., 2015). It is just due to these excellent characteristics, that a surprising number of studies have been published covering virtually every conceivable biomass used to prepare porous carbon for supercapacitors. These biomass include bamboo (Zhang et al., 2018a, Zhang et al., 2018b, Zhang et al., 2018c), almond shells (Wu et al., 2016, Wu et al., 2016), waxberry (Dong et al., 2018), sisal (Li et al., 2019), Chinese Date (Zhang et al., 2019a, Zhang et al., 2019b) etc., and some uncommon biomass, such as elm (Chen et al., 2016), and cicada slough (Jia et al., 2019).

China is the largest producer and consumer of Chinese herbal medicines, and a large amount of waste is generated in the production and process of Chinese herbal medicines every year, including non-medicinal parts and residues, thus, how to deal with these wastes is one of the thorny issues faced by related companies and farmers. Conventional disposal methods are incineration or landfill, which can cause environmental pollution, on the other hand, these so-called wastes are mainly composed of cellulose, lignin and hemicellulose, and contains more active ingredients compared with the conventional biomass, which can provide more hetero-atoms, so is ideal for the preparation of high performance supercapacitor electrode materials. Salvia miltiorrhiza is a traditional Chinese herbal medicine, mainly produced in China and mostly used in traditional Chinese medicine prescriptions. Here in this report, we successfully turned waste of a traditional Chinese medicine Salvia miltiorrhiza flowers into high specific surface area porous carbon with ultrahigh self-doped O and abundant pores. Scheme 1 concisely presents the prepare process of porous carbon from Salvia miltiorrhiza flowers. The root is the medicinal part of Salvia miltiorrhiza (Wei et al., 2017), however, roots account for less than a fifth of the biomass weight of salvia miltiorrhiza, and a large amount of above ground parts, including the flowers, stems and leaves, are produced during the growth and processing of roots. These nontraditional medicinal parts contain active ingredients similar to those found in roots, but in far smaller amounts, and are therefore treated mainly as waste (Zeng et al., 2017). Danshinone, salvianolic acid and other components contained in these parts contain extremely high content of carbon and oxygen elements, which is an ideal natural heteroatoms donor for preparing self-doped carbon materials. However, no relevant research reports have been found so far, take these parts as waste management, on one hand, cause the waste of resources, on the other hand, cause pollution to the environment.

Generally, the preparation of porous carbon from biomass consists of two steps, carbonization and activation (Zhu et al., 2019). During the carbonization process, biochar is obtained by calcining of the precursors under nitrogen atmosphere, and then porous carbon is obtained by producing pores under the action of activators KOH and ZnCl2. Direct calcination of biomass usually results in only biochar, and other volatile components generated during decomposition of biomass are directly discharged into the environment, which is a pollution to the environment. Moreover, most of these volatile components are phenols, acids and other components. Wood vinegar and wood tar obtained from condensation and recycling of these components have many applications in the field of agriculture (Xu et al., 2017). However, so far, relevant studies have reported direct calcining of biomass without any steps to recover volatile components. To produce proper pore structures, there are many kinds of activators, including water vapor, KOH, ZnCl2, CO2 and so on. Among these activators, KOH is the best activation method, so it is also the most widely used activation method (Hu et al., 2016). However, KOH is also the most corrosive, limiting its use in large-scale industrial production.

Therefore, in this work, we adopt a comprehensive and green approach to prepare porous carbon from salvia miltiorrhiza flowers and apply it to supercapacitors. This work not only provides an ideal raw material for the development of supercapacitors, but also presents a new idea for the high value development and utilization of traditional Chinese medicine waste.

Section snippets

Materials

The flowers of Salvia miltiorrhiza Bge were collected in Xianyang Shaanxi. NaHCO3, and HCl were purchased from Shaanxi Lebo Biochemical Technology co., Ltd. These chemicals are all in analytic grade and were used without any purification process. Ultra-pure water was prepared in our lab by a Merck Millipore water purification system and was used throughout the whole process.

Material preparation

The cleaned and dried flowers of Salvia miltiorrhiza Bge were added to a dry distillation kettle with a condensing system,

Results and discussion

Fig. 1 is the FESEM results of morphology of the prepared samples SF-BC and SF-PC700-3. Before activation, the dry-distilled product SF-BC without obvious pores, they are scattered carbon blocks with irregular shapes (Fig. 1a), and the surface of these carbon blocks are smooth, and the overall structure is tight and solid. However, the participation of NaHCO3 totally changed both the surface and inner structure of SF-BC. We can clearly see that the morphology of SF-BC is completely changed with

Conclusions

Here in this work, by dry distillation combined with one step NaHCO3 activation, porous carbon is prepared from waste of a traditional Chinese medicine Salvia miltiorrhiza flowers. Structures of the porous carbons are tunable by simply regulating of activation temperatures and dosages of activator NaHCO3. The optimized porous carbon owns a high specific surface area of 1715.3 m2 g−1 and total pore volume of 0.6392 cm3 g−1, together with a unique hierarchical architecture and ultrahigh content

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

We thank the finical support from Shaanxi Provincial Key Industries Innovation Chain Project (2018ZDCXL-SF-01-06). The author Dr. Zhang Yanlei especially want to thank his parents for babysitting his two children, when he was preparing works of this paper. Dr. Zhang Yanlei also would like to express his thanks to his wife, Zhang Xinshao, for giving birth to their two lovely sons.

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