NaCl-assisted pyrolysis to construct low metal content multiple-doped 3D porous carbon as oxygen reduction electrocatalysts for Zn-air battery
Graphical Abstract
Introduction
The continuous growth of human population, the non-renewable and increasing consumption of mineral materials, and the growing attention to environmental issues urgently require us to develop clean and renewable resources [1], [2]. Among the methods that have been explored, energy conversion through electrocatalysis seems to be an effective approach [3]. Zn-air batteries and fuel cells are recognized as the highest potential technologies for sustainable energy development and conversion because of their environmental benefits and high energy intensity and safe technologies [4], [5]. Oxygen reduction catalysts are essential as air cathodes in Zn-air batteries. However, the slow reaction kinetics and complex electron transfer process cause its catalytic efficiency and lifetime to be reduced [6]. Currently, the precious metal Pt and its alloys are still the best ORR catalysts in commercialization. However, the issues of costliness, poor durability and low methanol resistance have seriously affected the large-scale application of Pt and other alloy catalysts. Carbon-based materials doped with non-precious metals (Fe, Co, Ni, Cu etc.) and heteroatoms (N, S, B etc.) are gradually attracting the attention of researchers [7]. For example, in recent decades, materials such as carbon-based nanomaterials [8], [9], [10], two-dimensional metallic materials [11], transition metal sulfur compounds [12], [13], [14], [15], [16] and transition metal phosphides [17], have been more widely studied, have made excellent research progress. Among them, Fe ligand N-doped carbon catalysts (FeNx/CN) have superior catalytic activity, fine methanol resistance and durability, and are expected to be low-cost catalysts to replace precious metal catalysts [18].
In recent years, molecular electrocatalysis has also received attention due to its well-defined structure and systematic modulation, which facilitates the understanding of its catalytic mechanism and rational design [19]. Due to the multiphase nature of the conditions under which the reaction occurs, the catalytic performance of ORR catalysts is determined by their intrinsic activity and their mass/electron transport properties [20]. The electrochemical properties of the material are strongly influenced by the electronic structure [21]. The doped heteroatoms can be effectively used to change the intrinsic electronic properties of the carbon substrate during pyrolysis due to variations in the size of atoms and their ability to attract electrons [22], [23]. It has been shown that doped N atoms can evoke a redistribution of electrons, generating sites for reactions to occur and enhancing the adsorption and catalytic capacity for oxygen [24], [25]. Four N types (pyridine-N, graphite-N, pyrrole-N and oxide-N) are commonly found in N-doped carbon-based materials. Graphite-N and pyridine-N were found to be effective in improving ORR performance. Most studies have shown that graphite-N increases the limiting current density and pyridine-N affects the onset potential [26], [27]. In addition, the doping of S atoms can increase the density of positive charge and electron rotate density of carbon atoms, thus improving the oxygen reduction reaction of carbon materials [28], [29]. There are other heteroatoms that also have good effects on the design of materials, such as the introduction of highly electronegative B can change the local electron cloud density, which can boost the accumulation of C positive charge and improve the catalytic activity [30]. Therefore, the design of multiple heteroatoms introduced simultaneously into the catalyst can effectively improve the ORR catalytic performance of the material [31], [32]. Wu et al. prepared Fe, N and S co-doped catalysts using SiO2 as a template, where the Fe content was only 1.0 wt% with high ORR activity, however, the removal of the template was troublesome and dangerous and not suitable for large-scale production [33]. The effect of different Fe ligands on the material were further explored, and the S source was doped separately during the preparation process, adding to the tediousness of the preparation process [34]. Chae et al. chose graphene and carbon nanotubes as a mixed carbon source and introduced divalent iron and thiourea to prepare FeS/N,S:CNT-GR, which achieved catalytic performance comparable to that of Pt/C. However, the material showed nanoclusters that could not be removed by acid washing at the end of pyrolysis, which affected the further improvement of performance [35]. The above study demonstrates that Fe, S and N doping can be used as effective ORR catalysts, but the design of a reasonable scheme needs further thought.
In testing catalyst activity or assembled battery, oxygen must first diffuse into the active site before the reaction can take place. However, a thick catalyst layer can severely impede the mass transfer and ion transport, making Zn-air battery impossible to achieve the desired goals in practice. Oxygen reaching the active site is affected by a number of resistances, and these factors can reduce the activity of the catalyst. Microporous catalysts have a large specific surface area and dense active sites. The macroporous-dominated catalysts can effectively improve the morphology of catalysts and significantly enhance the catalytic performance of catalytic materials [36]. The formation of a three-dimensional (3D) porous structure facilitates sufficient contact between the catalyst and the electrolyte, improves the utilization of active sites [37]. Currently, the template method can effectively produce 3D porous carbon, but it is more complicated in the selection and dislodging of template agents and even pollutes the environment, which limits the mass production of 3D porous materials [38]. In contrast, water-soluble templating agent is simpler, safe and environmentally friendly. It is effective as an internal templating agent to prevent the agglomeration of metals and the loss of nitrogen sources [39], [40].
In this work, NaCl was used as a water-soluble template medium to construct 3D porous carbon, and the various materials was mixed homogeneously by ball mill. It was noteworthy that no solvent was used in the process, reduced the tediousness of the experiment and the pollution to the environment. During the pyrolysis process, sodium chloride completed the construction of the hollow layered structure and prevented the agglomeration of metals to some extent. This facilitated the rapid transfer of electrons and the full exposure of active sites. After the removal of NaCl, Fe, S and N sources were uniformly doped into the material, the synergistic effect of traces Fe, S and N could enhance the ORR activity of the material further.
Section snippets
Synthesis of Fe/S-CN three-dimensional porous carbon
In a typical procedure, 4000 mg NaCl, 60 mg FeCl3·6H2O and 84.8 mg NH4SCN were added in an agate grinding flask and ground with a ball mill at 220 rpm for 30 min. Then, 500 mg 2-methylimidazole (C4H6N2) and 500 mg Zn(NO3)2 were mixed to the agate grinding jar and continued to grind for 1 h. The mixture was subjected to a tube furnace and raised from room temperature to 350 °C at a rate of 3 °C/min and retained for 1 h. After that the temperature was further elevated from 350 °C to 900 °C by the
Results and discussion
The templating agent and various materials were thoroughly mixed by ball milling method. After that, the multi-doped three-dimensional (3D) porous carbon (Fe/S-CN) was prepared by pyrolysis and acid washing treatment. The synthesis procedure of typical Fe/S-CN is shown in Fig. 1. During the ball milling process, FeCl3·6H2O and NH4SCN undergo a complexation reaction. This prevents the iron ions agglomeration during the pyrolysis process. NaCl would be ground into smaller powders more easily
Conclusions
In summary, a facile NaCl-assisted pyrolytic method was established to construct non-precious metal (Fe) and S, N heteroatoms co-doping 3D porous carbon (Fe/S-CN) as oxygen reduction electrocatalyst for Zn-air battery. The loading of Fe in the synthesized electrocatalyst was about 0.589 wt%, and no undesirable Fe nanoparticles were formed. Due to the S, N heterogenic doping, abundant defects, and excellent interfacial contacts between the carbon nanosheets, the Fe/S-CN catalyst exhibits
CRediT authorship contribution statement
Shang Wu: Conceptualization, Methodology, Software, Data curation, Writing – review & editing, Funding acquisition. HuanLei Zhao: Conceptualization, Methodology, Writing – original draft preparation, Formal analysis. Xin Xu: Validation, Investigation. Chaoyang Liu: Data Curation, Writing – review & editing, Funding acquisition. Penghui Zhang: Formal analysis, Visualization. Shuaishuai Fu: Investigation, Formal analysis. Qiong Su: Supervision, Writing – review & editing, Funding acquisition.
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.
Acknowledgements
This work was financially supported by the National Natural Science Foundation of China (No. 21962017, 22165025, 21968032), the Fundamental Research Funds for the Central Universities (No. 31920220072, 31920220159, 31920220044), Science and Technology Plan project of Gansu Province (No. 20YF8FA045), Outstanding Graduate Student “Innovation Star” Project (No.2022CXZX-210), Chemistry innovation team of the Northwest Minzu University (No. 1110130139, 1110130141). We also thank Key laboratory for
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