Robust template-activator cooperated pyrolysis enabling hierarchically porous honeycombed defective carbon as highly-efficient metal-free bifunctional electrocatalyst for Zn-air batteries
Graphical abstract
Introduction
Facing the increasing crisis of energy shortage and environmental issues, it is essential to develop alternative energy storage and conversion systems with high energy densities [[1], [2], [3], [4], [5], [6]]. Among these energy systems, the rechargeable Zn-air batteries (RZABs) have drawn extensive attention due to their eco-friendly, intrinsically safe, low cost and the attractive theoretical energy density [[7], [8], [9]]. However, the practical application of RZABs are largely limited by the inferior round-trip efficiency and poor cycling stability, which are mainly caused by the inherently sluggish kinetics in oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) during the discharge and charge process [10,11]. The noble-metal based electrocatalysts, such as Pt/C, IrO2 and RuO2, display high catalytic activity to accelerate the ORR or OER, but their limited availability, insufficient stability and unsatisfied bifunctional catalytic capacity pose a great challenge for a large-scale commercialization [12,13]. Hence, exploring precious metal-free alternatives with cost-effective, efficient and corrosion-resistant bifunctional electrocatalysts are in urgent demand [[14], [15], [16]].
Compared with the traditional metal-based catalysts, the heteroatom (e.g., N, P, B, S and O) doped carbon materials are particularly attractive and considered as one of the most promising candidates due to their competitively catalytic activity, satisfactory cost, environmental acceptability and superior electric conductivity, as well as a combination of mechanical strength and lightness [[17], [18], [19], [20]]. Both theoretical calculations and detailed experiments have proof that the enhanced catalytic activity of doped carbon materials is ascribed to the rich active sites originated from the heteroatom doping [21]. For N or B doping, the higher electronegativity of N (3.04) or the lower electronegativity of B (2.04) can break the electroneutrality and spin distribution of adjacent C atoms and create positively charged sites (C+ or B+) favorable for O2 adsorption [22]. To O doping, the O atoms can be directly bonded to sp2-hybridized carbon, and then induce intrinsic electronic and band structure modulation, resulting in improved surface area of carbon matrix with abundant active sites [26]. Besides, the S doping can adjust the spin density redistribution rather than charge redistribution, and then the C atoms with high spin densities can serve as active sites [23,24]. And the P doping can create a defect-induced active surface for oxygen adsorption, leading to a decreased reaction free energy barrier [25]. However, the single-heteroatom doping carbon material is limited by the unsatisfied bifunctional catalytic capacity, and the S or P doping is more difficult because their large atomic radius. Recent studies show that the dual doped carbon with nitrogen and other heteroatoms can significantly improve the bifunctional catalysis activity with the synergistic coupling effect between the different dopants [[26], [27], [28]].At present, pyrolysis of rich heteroatoms-containing precursors has been widely used for fabrication of doped carbon materials [21,29]. Particularly the biomass conversion strategy is becoming one of the major trends because of the unique merits including the high abundance, environmentally friendly, low cost and scale production [[30], [31], [32]]. For example, gelatin, with the thermo sensitive gels characteristics, abundant surface hydrogen bonding and a high N content, can be as an ideal N, O-codoped carbon precursor and has gained considerable attention in energy storage and conversion, especially in supercapacitors [33] and lithium ion battery [34].
Besides, the oxygen catalytic activity of doped carbon is also limited by the surface area, which determines the available active sites. Therefore, the porosity of carbon matrix with high surface area is also essential for the electrocatalytic processes [35,36]. Constructing a hierarchically porous structure with micropores, mesopores and macrospores in the carbon matrix is an eff ;ective strategy to reduce the resistance against electron transfer, accelerate the O2 and electrolyte diffusion, and supply abundant active sites for oxygen conversion. As such, the overall electrocatalytic activity is remarkably improved [20]. The hard templates, such as colloid or mesoporous silica, are often used to prepared porous carbon [37]. However, the removal of silica template requires corrosive HF or concentrated alkali solution, making this method complicated and environmentally unfavorable [38]. Recently, the nano-CaCO3 template has become an appropriate choice due to its unique characters including low cost, eco-friendly, well-tunable particle size from 10 to 100 nm and convenient removal of calcium by rare hydrochloric acid [39,40]. Additionally, the nano-CaCO3 can act as catalyst to form the graphitic structure during the pyrolysis process, helping to improve the conductivity of resulted carbon [41]. Although the release of CO2 can form partial micropore during the CaCO3 decomposition, another activating agent is further needed to form highly-developed microporous structure. KOH [42] and ZnCl2 [43] as the most common activating agents show drawbacks of corrosion, massive use, and long calcination time. Other agents like Na2CO3 or K2CO3 [44] always display weak activation capability, and the product possesses insufficient microporous structure. However, some gas-producing reagents with low dosage and short calcination time, such as KHCO3, have been used as activating agents for activated carbonization of raw biomass [45,46] and drawn a lot of attention. The KHCO3 could release the H2O and CO2 gas to promote the expansion of biomass and then form the microporous structure below 300 °C. After that, the remaining product K2CO3 (fusion point, 891 °C) would fuse at 900 °C and intensify the chemical etching effect to further generate porous structure. Coincidentally, the nano-CaCO3 could also decomposed at 900 °C (Fig. S1) and might display part catalytic graphitization effect.
Therefore, with the synergistic effects of hard template (nano-CaCO3) and activating agent (KHCO3), a porous N, O-codoped carbon material with macro-meso-microporous and partially graphitic structure can be synthesized on a large scale via one-step pyrolysis. In such way, the dosage and calcination time can be effectively reduced. The resulted carbon material with a moderate amount of N and O in the bulk, an optimized pore network, a large surface area and a graphitic-amorphous carbon structure will possess a higher conductivity, a faster O2 and electrolyte diffusion, rich catalytic sites, and an improved electrochemical stability [47]. Unsurprisingly, such kind of carbon material will work as a highly efficient metal-free electrocatalysts and display satisfactory performance in the RZABs. To the best of our knowledge, such template-activator assisted strategy to construct hierarchical porous N, O-codoped carbon and its application in RZABs have not been reported.
Based on above assumption, in this work, we introduce a hard template-activator assisted pyrolysis route to synthesize a 3D honeycombed hierarchical porous N, O-codoped carbon (denoted as HHPC) from the mixed gel of gelatin, nano-CaCO3 and KHCO3. The gelatin is used as the carbon precursor, the nano-CaCO3 is used as the hard template, and KHCO3 is used as the activator. After pyrolysis, the product is leached by with rare HCl solution to remove calcium and potassium. This method is simple, efficient, environmentally friendly and readily scalable. Benefiting from the unique macro-meso-microporous framework, N, O-codoped nature and graphitic-amorphous hybrid structure, the HHPC possesses a short O2 and electrolyte diffusion paths, rich catalytic sites and excellent conductivity as well as electrochemical stability. As a metal-free bifunctional electrocatalyst, the HHPC is expected to present the remarkable catalytic activity toward ORR/OER and superior stability in the RZABs.
Section snippets
Material preparation
Typically, 10 g of gelatin and 0.5 mL triton X-100 was dispersed into the 100 mL DI water and stirred for 2 h at 60 °C to form a homogeneous aqueous dispersion. Then, 5 g of nano-CaCO3 (40∼50 nm) and 5 g KHCO3 were added into the solution and kept stirring for 1 h. The resulted uniform suspension solution was transferred to a refrigerator and dried at −51 °C in a vacuum freeze dryer. After that, the obtained white xerogel was transferred to a tube furnace and calcined at 900 °C for 2 h in Ar
Results and discussion
In a typical procedure for preparing HHPC (Fig. 1), by virtue of the emulsification of Triton X-100, a certain mass ratio of gelatin, nano-CaCO3 and KHCO3 were dispersed into ID water to form a homogeneous solution by continuous stirring. Within the cold condition, nano-CaCO3 and KHCO3 frozen in the hydrogel of gelatin. During the subsequent pyrolysis process, the KHCO3 firstly works as the activator to form the micropores, and then the remaining product K2CO3 combined with nano-CaCO3 starts
Conclusions
In summary, with the template-activator assistance, 3D honeycomb-like hierarchically porous N, O-codoped carbon has been prepared successfully via a simple, efficient, environmentally friendly and readily scalable one-step pyrolysis. The Ca in nano-CaCO3 helped to improve the partially graphitic phase, generating graphitic-amorphous hybrid structure, improving the conductivity of HHPC. Owing to the chemical etching effect of KHCO3 and the catalytic graphitization effect of nano-CaCO3, the
CRediT authorship contribution statement
Xiao Xiao: Data curation, Writing - original draft. Xinhai Li: Methodology, Resources, Supervision. Zhixing Wang: Data curation, Funding acquisition. Guochun Yan: Funding acquisition, Investigation, Software. Huajun Guo: Funding acquisition. Qiyang Hu: Resources. Lingjun Li: Resources. Yong Liu: Validation. Jiexi Wang: Conceptualization, Funding acquisition, Project administration, Writing - review & editing.
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.
Acknowledgment
This work was financially supported by the National Nature Science Foundation of China (51874360, 51704332, 51674295, 51674296), the Postdoctoral Science Foundation of China (BX201700290, 2018M630911) and the Program of Huxiang Young Talents (2019RS2002).
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