In Situ Coupling Strategy for Anchoring Monodisperse Co9S8 Nanoparticles on S and N Dual-Doped Graphene as a Bifunctional Electrocatalyst for Rechargeable Zn–Air Battery
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An effective in situ coupling strategy is proposed to construct Co9S8 nanoparticles/doped graphene.
Cobalt porphyrin derivative is employed as both coupling and heteroatom-doped agents.
The bifunctional oxygen electrocatalyst finds application in rechargeable all-solid-state Zn–air batteries.
KeywordsIn situ coupling strategy Porphyrin derivate Doped graphene Metal sulfide Bifunctional electrocatalyst Rechargeable Zn–air battery
Rechargeable Zn–air battery (ZAB), as one of the most promising power technologies, has attracted significant research interest due to its environment-friendliness, low cost, and high theoretical energy density [1, 2, 3, 4]. However, the large voltage gap and poor cycle life have severely hindered its practical application . Therefore, durable bifunctional electrocatalysts for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are urgently required to accelerate the recharge rate and overall electrochemical reactions of ZAB [6, 7]. To date, Pt-based materials have been considered state-of-the-art ORR catalysts, while Ir/Ru-based catalysts are considered efficient for OER . However, the prohibitive cost, poor durability, and single function for ORR or OER of these precious-metal-based catalysts are major foundational barriers . An ideal solution to the bottleneck problem is to replace commercial Pt- and Ir/Ru-based catalysts with the highly efficient and durable bifunctional electrocatalysts based on naturally abundant elements . Currently, transition metal sulfides (TMSs) [10, 11], especially Co9S8 [12, 13], have gained considerable attention due to their nature abundance, environment-friendliness, good durability, and high catalytic activity for both ORR and OER. Unfortunately, their low electronic conductivity has degraded their practical performance. Therefore, it is necessary to employ a highly conductive carbon matrix to anchor the rationally designed TMS nanoparticles.
To this end, graphene has been recognized as an effective matrix due to its high conductivity, chemical stability, and extraordinary specific surface area [14, 15]. Further, doping graphene with heteroatoms (such as N and S) can improve conductivity and provide additional electrocatalytic active sites [16, 17, 18]. Therefore, the incorporation of nanostructured TMSs into doped graphene has been intensively studied [12, 13]. However, simple incorporation may result in aggregation of the nanoparticles, thereby hampering exposure of active sites and leading to low catalytic activities. Furthermore, the weak anchors between nanoparticles and graphene cause nanoparticle leaching, resulting in poor durability. Therefore, incorporating N4-metallomacrocycles into carbon matrix seems to be a promising approach. On the one hand, the N4-metallomacrocycles can act as the coupling agent to anchor nanoparticles , thus accomplishing in situ anchoring of small and homogeneously distributed nanoparticles. On the other hand, the tunable structure of N4-metallomacrocycles with various heteroatom-containing functional groups endows them with additional functions. These functional groups can be employed as interfacial linkers to link graphene or graphene oxide via aromatic π–π interactions and reciprocal electrostatic interactions , thus realizing heteroatom-doped graphene. Moreover, it is universally accepted that heat-treated N4-metallomacrocycles can display high catalytic activity and chemical stability, with Me-N4 acting as the catalytic centers for ORR . However, direct synthesis of TMSs through this strategy remains challenging because their synthesis needs additional sulfuration reactions with sulfur or S-containing compounds, which in turn suffer from the shortcomings of using toxic precursors, sophisticated process, and/or the release of poisonous gases. Therefore, it is highly desirable to achieve the function-oriented design of N4-metallomacrocycles with S-containing functional groups, which could couple and anchor TMSs nanoparticles on doped graphene in situ as a high-performance bifunctional electrocatalyst for ORR and OER, even ZAB.
In this paper, for the first time, we report a function-oriented design of N4-metallomacrocycle derivatives to synthesize Co9S8/S and N dual-doped graphene composite (Co9S8/NSG). As a proof-of-concept demonstration, we used cobalt(II) 5,10,15,20-tetra-(4-sulfonatophenyl) porphyrin (TSPPCo) as not only the coupling agent to form and anchor Co9S8 on the graphene in situ, but also the heteroatom-doped agent to form S and N dual-doped graphene in situ. Benefiting from the function-oriented design and unique structure, the Co9S8/NSG exhibits high catalytic activity and outstanding stability for ORR and OER. To investigate its practical applications, a homemade all-solid-state ZAB is built based on our bifunctional electrocatalysts, which displays high performance and excellent long cycle life.
2 Experimental Section
2.1 Synthesis of Catalyst
Graphene oxide solution (4 g, 2.5 wt%), TSPPCo (0.05, 0.1, and 0.15 g), and 10 mL water were added to a 50-mL Teflon-lined autoclave and stored at 180 °C for 24 h. After cooling to room temperature, it was freeze-dried under vacuum, followed by calcination at 600, 700, and 800 °C for 2 h in N2, respectively. The obtained products were labeled as Co9S8/NSG-600, Co9S8/NSG-700, and Co9S8/NSG-800, respectively. Moreover, GO with different loading contents of TSPPCo (0.05, 0.1, and 0.15 g) were denoted as Co9S8/NSG-700-0.5, Co9S8/NSG-700, and Co9S8/NSG-700-1.5, respectively. Co9S8/C-700 was synthesized by a method similar to that used for Co9S8/NSG without the presence of GO, and NSG-700 was obtained by leaching the pyrolyzed product in HCl aqueous solution (0.1 M) for 8 h to remove Co9S8.
2.2 Electrochemical Measurements
All the electrochemical measurements of the electrocatalysts for ORR/OER were taken on a CS350 electrochemical workstation in the corresponding electrolytic solution using a standard three-electrode cell, in which a rotating disk electrode of diameter 5.0 mm (RDE, Pine Research Instrument, USA) served as the working electrode, Pt-foil as the counter electrode, and saturated calomel electrode (SCE) as the reference electrode.
To evaluate the ORR and OER performances, cyclic voltammetry (CV) was performed in N2- or O2-saturated solution with a scan rate of 50 mV s−1. Linear sweep voltammetry (LSV) measurements for ORR were taken at different speeds from 400 to 1600 rpm in an O2-saturated solution with a sweep rate of 10 mV s−1 without using iR-correction. LSV measurements for OER were also taken using the same three-electrode cell in O2-saturated 1 M KOH solution with a scan rate of 5 mV s−1 with iR-correction. Before all the electrochemical characterizations, the continuous sweep of the corresponding voltage range was measured until a steady voltammogram curve was obtained.
The durability tests of the ORR/OER electrocatalysts were both performed using chronoamperometric (i − t) measurement in O2-saturated corresponding solutions at a rotation rate of 1600 rpm, while 10 vol% methanol was added for demonstrating methanol tolerance during ORR.
2.3 Zn–Air Battery Assembly and Measurements
The air–electrode used for ZAB was composed of carbon paper as the catalyst-loaded layer (1 mg cm−2) facing the water side and the gas diffusion layer facing the air side. A zinc plate was used as the anode, while 6 M KOH containing 0.2 M Zn(Ac)2 was used as the electrolyte for ZAB. The effective area of the catalyst-loaded layer and zinc plate is controlled to 1 cm2.
The homemade all-solid-state ZAB was also fabricated using zinc foil as anode and the catalyst-loaded carbon paper as the air-electrode; however, a solid polymer electrolyte is used as a separator for the battery. The solid polymer electrolyte was prepared by the following steps. First, polyvinyl alcohol powder (4.5 g) was dissolved in 0.1 M KOH (40 mL) containing 0.02 M Zn(Ac)2 and then stirred at 90 °C for 2 h. The solution was then poured into a culture dish and dried at 55 °C to form a solid polymer film.
All the electrochemical tests of ZAB were conducted on the CS350 electrochemical workstation in ambient air. The galvanodynamic charge–discharge profiles were obtained via LSV (5 mV s−1). The cycling curves were obtained using 400 s for each cycle.
3 Results and Discussion
It is worth mentioning that the outstanding electrochemical performance and stability of Co9S8/NSG-700 could be attributed to the unique characteristics, which could be elaborated as follows. On the one hand, the graphene matrix composed of nanosheets can provide large surface area, thus increasing the exposure and adsorption at more active sites on the catalyst surface. Moreover, the S and N dual-doped graphene can endow the catalyst with high conductivity and additional electrocatalytic active sites. On the other hand, the abundant active sites, including N, S, Co–N, and Co9S8, derived from the TSPPCo precursor, could promote the ORR/OER activity, and the strong binding interaction derived from the in situ coupling and anchoring of Co9S8 on the graphene could prevent the leaching and aggregation of the Co9S8 nanoparticles. As a result, benefitting from the advantageous properties of large surface area, high conductivity, and tight coupling, Co9S8/NSG displayed high ORR/OER activity and good stability.
In summary, the novel and effective strategy of using N4-metallomacrocycles, with S-containing functional groups, as both the single-source precursor and the coupling agent, is applied to the in situ formation and anchoring of Co9S8 nanocrystals on the doped graphene. It is worth mentioning that Co9S8 can be synthesized via this strategy without using additional sulfur or S-containing compounds, thus avoiding the requirement of toxic precursors, sophisticated process, and/or the release of poisonous gases. More importantly, owing to the enhanced conductivity of the S and N dual-doped graphene, the ultrafine Co9S8 nanocrystals, and in situ coupling interaction, the as-obtained Co9S8/NSG-700 displayed significant catalytic activity and stability for ORR/OER. Furthermore, as the air-electrode catalyst for ZAB, even all-solid-state ZAB, Co9S8/NSG-700 exhibited good performance and good stability. Therefore, we believe that the function-oriented design of N4-metallomacrocycles, with S-containing functional groups, is versatile and effective for the synthesis of other electrocatalysts for wider practical applications.
This work was financially supported by the National Natural Science Foundation of China (Grant No. 21404014) and the Science & Technology Department of Jilin Province (No. 20170101177JC).
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