In situ grown MnCo2O4@NiCo2O4 layered core-shell plexiform array on carbon paper for high efficiency counter electrode materials of dye-sensitized solar cells
Introduction
The ever-growing environmental concerns originating with the present energy demands have motivated the researchers to develop alternative technologies for energy conversion [1,2]. As a clean and renewable resource, solar energy has aroused great interest in scientific research. Dye-sensitized solar cells (DSSCs) have been considered as an improvement clear energy conversion device because of its high power efficient, simple preparation and low-cost features [[3], [4], [5], [6]]. In general, DSSCs can be divided into three parts: photoanode, redox electrolyte and counter electrode (CE). Among them, CE can collect the electrons from peripheral circuit and catalyze reduction of redox couple (I3−/I−) in electrolytes, which plays a key role in electrochemistry properties of DSSCs [7,8]. Platinum (Pt) is commonly used as CE due to its high conductive and reduction characteristics. However, the expensive price and the limited resource hindering its commercial development, so find a low-cost and high catalytic activity CE to replace Pt is imminent.
Generally, DSSCs can be satisfied with power conversation efficiency (PCE) by designing a high-performance electrode material. As a good CE in DSSCs, it should include at least two properties: good conductive ability to transfer as many as possible electrons and excellent catalytic reduction characteristic to reduce the electrolyte further ensures DSSCs have a good stability. Carbon materials have been regarded as a promising candidate because of their high conductivity, large surface areas and high stability in many electrochemical fields [[9], [10], [11], [12]]. In addition to carbon materials, ultrathin 2D materials have attracted increasing attention over the past very few years [[13], [14], [15]]. Recently, in situ grown layered core-shell heterostructures with multicomponent (usually a combination of two or more materials) have been demonstrated to have more extraordinary electrochemical properties [16]. Therefore, a large number of core-shell materials are composed of various compounds (including sulfides, oxides, layered double hydroxides (LDH), etc.) and further applied in many fields [[17], [18], [19], [20], [21], [22], [23]]. For instance, Zhao et al. synthesized MnCo2O4@Ni(OH)2 core-shell nanoflowers by two-step hydrothermal method and further applied on supercapacitor exhibits an excellent specific capacitance [24]. Zhou et al. decorated NiFe-LDH nanosheets on V-doped Ni3S2 nanorod arrays to obtain a core-shell electrocatalyst for efficient water oxidation [25]. Miao et al. generated a Co3O4@MnO2 core-shell hybrid material used in DSSCs with the power conversion efficiency of 7.08% [26]. In particular, array structures can give composites interesting advantages, such as more exposed active centers, rapid electrolyte penetration, and avoidance of aggregation behavior to ensure the electron transport fast. Inspired by the above, we choose MnCo2O4 nanoneedle array as a core. As another typical bimetallic oxide, the diffraction peaks of NiCo2O4 are similar to that of MnCo2O4, which can provide a good synergistic effect to promote the redox of I3− in electrolyte and the special nanosheets structure can enhance the specific surface area immensely. However, to our knowledge, the core-shell arrays of MnCo2O4 and NiCo2O4 supported by CP have not been reported as CE materials for DSSCs.
In this work, a hierarchical core-shell material by combining of MnCo2O4 nanoneedle arrays and various size of NiCo2O4 nanosheets on carbon paper was successfully synthesized via a two-step solvothermal method and further coated on Ti mesh as CE of DSSCs. Compared with traditional FTO substrate Ti mesh has higher electron transfer efficiency because its low impedance and microporous structure. Finally, via a series electrochemical measurements, the power conversion efficiency (PCE) of MnCo2O4@NiCo2O4/CP Ti mesh CE can reach to 9.58% with the open-circuit voltage (Voc) of 0.802V, short-circuit current (Jsc) of 15.62 mA/cm2 and fill factor (FF) of 0.76, which is much higher than traditional Pt FTO CE (8.00%). These results not only reveal that the MnCo2O4@NiCo2O4/CP Ti mesh CE have an outstanding catalytic performance can be a novel low-cost and high efficiency.
candidate for Pt CE but also provide a new avenue for exploring carbon paper supported layered core-shell structures for other electrochemical energy-related applications. The fabrication process of MnCo2O4@NiCo2O4/CP Ti mesh CE is shown in Scheme 1.
Section snippets
Synthesis of MnCo-LDH on carbon paper substrate
The carbon paper (CP) was heat-treated at 500°С for 2 h in air to enhance its wettability before using [27]. Typically, 251 mg Mn(NO3)2 and 584 mg Co(NO3)2·6H2O, 200 mg urea and 37 mg NH4F were dissolved with 60 mL distilled (DI) water and 15 mL absolute ethanol to form a homogeneous solution under ultrasound for 10 min. Then a piece of pre-processing CP (3 cm × 6 cm) was put in a 100 mL Teflon-sealed autoclave with the mixed solution and heated at 120°С for 12 h and then the MnCo-LDH precursor
Results and discussion
The morphology of as-prepared sample for each step is characterized by SEM using electron beam energy of 15 KV, spot size of 3 under immersion mode. Fig. 1a shows a typical SEM image of MnCo2O4/CP composite. It can be seen clearly that MnCo2O4 nanoarray grow on the CP uniformly whose nanobelts are about 100 nm in diameter and 5 μm in length. The vertical growth pattern of MnCo2O4 provides an advantage for the subsequent growth of NiCo2O4. As shown in Fig. 1b, the CP supported MnCo2O4@NiCo2O4
Conclusion
In summary, the MnCo2O4@NiCo2O4/CP core-shell structure CE was successfully prepared on the Ti mesh via a two-step solvothermal method. Such unique core-shell structure drives rapid Iodine ion diffusion to deliver potential prospect in the DSSCs application by utilizing the combined effect of both materials (MnCo2O4 core and NiCo2O4 shell), which improved the redox ability for the reduction of I3− to I−. And the use of Ti mesh can enhance the diffusion rate of I3−/I−, reduce the charge transfer
CRediT authorship contribution statement
Zixiang Li: Conceptualization, Writing - original draft, Supervision. Shuang'an Liu: Methodology, Investigation. Lidong Li: Investigation, Formal analysis. Wankun Qi: Visualization, Data curation. Weidong Lai: Supervision, Funding acquisition. Ling Li: Supervision, Funding acquisition. Xiaohui Zhao: Funding acquisition, Project administration, Validation. Yucang Zhang: Funding acquisition, Project administration, Validation. Wenming Zhang: Conceptualization, 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.
Acknowledgement
We gratefully acknowledge the financial support from the following sources: National Natural Science Foundation of China (NSFC) (Grants 51772073, 51607054, 21672051, 51762013), Hebei province Outstanding Youth Fund (A2017201082, A2018201019), Key Project of Hebei Natural Science Foundation (E2020201030), The Second Batch of Young Talent of Hebei Province (NO.70280016160250, NO.70280011808), The Central Government Guide Local Funding Projects for Scientific and Technological Developemnt (
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