Materials Today Energy
Biomass-derived highly dispersed Co/Co9S8 nanoparticles encapsulated in S, N-co-doped hierarchically porous carbon as an efficient catalyst for hybrid Na–CO2 batteries
Graphical abstract
Introduction
In recent years, the excessive consumption of non-renewable energy has caused a deteriorative energy crisis and ‵greenhouse effect′, which has greatly stimulated the development of next-generation sustainable energy storage and conversion systems [1,2]. Rechargeable Na–CO2 batteries, as new electrochemical energy storage and conversion devices, have attracted extensive research attention on account of their high energy density, low cost of sodium resources, and the effective utilization of CO2. Na–CO2 batteries are not only of great significance for maximizing the use of electricity generated by surplus fossil fuels, nuclear energy, and renewable energy, but also show great potential for meeting the growing energy, chemical, and environmental needs of daily life and industry [3,4].
In Na–CO2 batteries, the involved electrochemical reaction on the three-phase interface (electrolyte, catalytic cathode, and CO2.) is: 4Na+ + 3CO2 + 4e− ⇋ 2Na2CO3 + C (2.35 V vs. Na/Na+) [[5], [6], [7]]. Until now, the development of Na–CO2 batteries is still in infancy because of the sluggish kinetics of CO2 reduction reaction (CRR, Na2CO3 formation during discharge process) and CO2 evolution reaction (CER, Na2CO3 decomposition during charge process) at the cathode. Several issues are related to the seriously sluggish kinetics. For example, insufficient catalytic activity cannot effectively accelerate the CRR and CER, which lead to high charge/discharge overpotentials and poor round-trip efficiency. Slow electron transfer and mass (e.g. Na+ and CO2) diffusion restrain the reaction rate, resulting in poor battery rate performance. Furthermore, during discharge, the gradual accumulation of discharge product (Na2CO3) is insulating and insoluble in the non-aqueous electrolytes, which not only clogs porous channels of Na+ and CO2 diffusion but also reduces the conductivity, causing low discharge capacity and poor cycle reversibility. Recently, hybrid Na–CO2 batteries, which could overcome the influence of insoluble discharge products in non-aqueous electrolytes to some extent by using aqueous electrolytes, had been developed as described in our previous work, corresponding reactions are as follows [8]:Anode: 4Na ⇋ 4Na+ + 4e−Cathode: 4Na+ + 3CO2 + 4e− ⇋ 2Na2CO3 + COverall: 4Na+3CO2 ⇋ 2Na2CO3 + C EƟ = 2.35 VNa2CO3+CO2 + H2O→2NaHCO3
However, they still suffer from an unsatisfactory electrochemical performance at large current densities. To construct high-performance hybrid Na–CO2 batteries, therefore, an efficient and robust cathode catalyst which is highly conductive that accelerates electron transfer, and with an optimized porous architecture that boosts Na+ and CO2 transfer, furthermore with excellent electrocatalytic activities toward CRR and CER is required.
Recent extensive studies revealed the advances made in research relating to carbon materials in energy conversion and storage devices because of their high conductivity, lightweight, abundance, easy-to-control pore structure, and high surface area. For instance, nanocarbon [9,10], carbon nanotubes [11], and graphenes [12], have been successfully introduced into CO2 batteries to increase discharge capacity and extend cycle-life. Besides, doping heteroatoms (such as S and N) into the carbon framework can lead to uneven charge distribution and enforce the nearby carbon atoms positively charged, which is conducive to CO2 conversion [13,14]. The Li–CO2 battery adopts B, N co-doped graphene cathode, which dramatically reduces the overpotential and improves the cycling performance up to 200 cycles [15]. However, most of the carbon materials currently available in the market are not easy to be applied due to their complicated preparation methods and high cost. Biomass resource-derived carbon is regarded as one of the most prospective candidate materials, as they are economical, green, and easy to implement involving large-scale application. Recently, biomass-derived carbon doped with heteroatom as electrocatalysts has been extensively documented within metal-air/O2 batteries exploiting the advantage due to its hierarchical micro/mesopore structure [[16], [17], [18]]. Microporous structure plays a critical role in CO2 adsorption, and further introduction of heteroatom is beneficial to enhance CO2 adsorption affinity [19]. Moreover, the porous conductive structure not only offers a ‵highway′ for accelerating electron transfer and mass transfer to enhance reaction kinetics but also maintains sufficient mechanical strength to eliminate volume changes caused by the accumulation of insoluble discharge products. Consequently, the development of economic and green biomass-derived heteroatom-doped hierarchically porous carbon for rechargeable hybrid Na–CO2 batteries is a desirable strategy.
Although various porous carbon materials can promote charge and mass transfer, their CO2 reduction reaction (CRR) and CO2 evolution reaction (CER) activities are generally weak because they cannot effectively catalyze discharge product formation and decomposition. To overcome this challenge, a large number of carbon-encapsulated non-precious metal active species have been explored. For instance, embedding Ni or nickel oxide (NiO) nanoparticles [[20], [21], [22]], copper (Cu) nanoparticles [23], manganese oxide (MnO) nanoparticles [24] into carbon framework have been reported to provide catalytically active sites for decomposing lithium carbonate (Li2CO3). Particularly, cobalt active substances (such as metallic cobalt and its oxides or sulfides) have been reported to be one of the most potentials in related energy technologies, owing to their high electrical conductivity and high catalytic activity and stability advantages [[25], [26], [27], [28]]. Qiao and co-workers found that Co atoms anchored on graphene oxide could serve as an efficient Li–CO2 batteries electrocatalyst, with 100 cycles [29]. The metallic active species and carbon defects in these materials synergistically enhance the electrocatalytic reaction process [30]. Besides, heteroatom-modified carbon materials can effectively prevent the dissolution and aggregation of nanoparticles, thereby enhancing the stability of the materials in the synthesis process and electrocatalytic reaction [29,30]. Therefore, such materials with cobalt active species can be expected to catalyze the formation and decomposition of sodium carbonate (Na2CO3), and thereby boost the performance of Na–CO2 batteries.
In this study, we report an effective and rational protocol for the synthesis of Co/Co9S8 active nanoparticles anchored on biomass-derived hierarchically porous S, N-co-doped carbon as an effective and efficient active material for CO2 reduction and carbonate decomposition reactions in rechargeable hybrid Na–CO2 batteries via a micromesopore confinement synthetic strategy. As we know, this is the first report on the biomass-derived highly active electrocatalysts including S, N-co-doped carbon-encapsulated cobalt active species for Na–CO2 batteries. This structural design provides significant advantages: (1) the nucleation of Co/Co9S8 active nanoparticles is dominated by the spatial confinement effect of micropores and mesopores in conductive carbon skeleton, which not only inhibits overgrowth and agglomeration of Co/Co9S8 nanoparticles but also facilitates the exposure of Co/Co9S8 catalytic active sites and accelerates electron transfer; (2) countless interconnected microporous and mesoporous channels ensure a high surface-to-volume ratio, which not only offers a ‵highway′ for CO2 and Na+ diffusion as well as rapid electrolyte penetration in triphase interface reactions but also provides sufficient space for the accumulation of discharge products; (3) the synergistic coupling effect between the chemical bond derived from S, N dopants and the defect-rich carbon interface strengthen the catalytic activity and prevent the agglomeration and separation of Co/Co9S8 nanoparticles, thus improving the stability. As a result, the Co/Co9S8@SNHC catalyst exhibits excellent activities towards CO2 reduction and carbonate decomposition reactions in hybrid Na–CO2 batteries, and batteries thereby demonstrate high capacity, low polarization, and long-term cyclability.
Section snippets
Reagents
Areca catechu (Ac) was purchased from local markets. Cobalt (II) acetate tetrahydrate (Co(CH3COO)2·4H2O), potassium hydroxide (KOH), ammonium sulfide solution ((NH4)2S), polyvinylpyrrolidone (PVP), ethylene glycol (EG) were brought from Aladdin Reagent (Shanghai, China). All the chemical materials used in this work were Analytical Reagents (ARs) and could be used without any further purification.
Preparation of biomass-derived hierarchically porous carbon (HC)
The HC was synthesized by the alkali activation method. Specifically, first, the Areca catechu was
Characterization of Co/Co9S8@SNHC
Fig. 1a illustrates a simple synthesis strategy to achieve the fabrication of Co/Co9S8 nanoparticles encapsulated in heteroatom-doped hierarchically porous defective biomass carbon, involving alkaline activation and S, N heteroatom-doping and metal-doping, followed by calcination in an argon atmosphere. Ammonium sulfide was served as the nitrogen source and sulfur source. The SEM images of as-synthesized materials show that Co and Co9S8 nanoparticles are anchored on hierarchically porous carbon
Conclusion
In this work, an excellent catalyst consisting of highly dispersed Co/Co9S8 nanoparticles anchored on biomass-processed hierarchically porous S, N-co-doped carbon was accomplished via micromesopore confinement method. This nanocomposite material showed the superiorities of simple synthesis, adjustable structure, large surface area, and abundant defective sites. Benefiting from the effective synergetic interaction between effective catalytic active sites (Co/Co9S8, C–N, C–S bonds) and defect-rich
Author contributions
The manuscript was written through the contributions of all authors. All authors have given approval to the final version of the manuscript.
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
This work was financially supported by the National Natural Science Foundation of China (No. 51704136, 11765010, 51974378), the Natural Science Foundation of Hunan Province (2020JJ4735), the Scientific and Technological Breakthrough and Major Achievements Transformation of Strategic Emerging Industries of Hunan Province (2018GK4001), and the Hunan Key Laboratory for Rare Earth Functional Materials (2017TP1031).
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