Dual-template synthesis of N-doped macro/mesoporous carbon with an open-pore structure as a metal-free catalyst for dye-sensitized solar cells
Graphical abstract
N-doped macro/mesoporous carbon with an open-pore structure (macro/meso-NC) has been developed as an efficient, metal-free catalyst toward tri-iodide reduction for low-cost dye-sensitized solar cells.
Introduction
Dye-sensitized solar cells (DSSCs) are considered as one of the most promising photovoltaic devices due to their high conversion efficiency, simple assembly, and green feature [1], [2], [3]. The prototypical structure of DSSCs is composed of the photosensitized dye, TiO2 photoanode, I−/ redox electrolyte, and Pt counter electrode. In a light-electricity conversion process, the dye adsorbed onto TiO2 surface is firstly excited under the illumination. Once the excited electron is injected into the conduction band of the TiO2, the dye becomes oxidized and then receives another electron through the oxidation of I− into . On the other hand, the injected electron is delivered to the counter electrode and followed by the reduction of into I− at the interface between the counter electrode and the electrolyte. Accordingly, the role played by the counter electrode in DSSCs is to provide a function with not only great conductivity to deliver the electron but also high catalytic ability for the regeneration of into I− to complete the light-electricity conversion process.
The most common material used for counter electrodes is platinum (Pt) due to its high electrical conductivity and great catalytic ability to meet the requirements. However, with the target of expanding solar generation up to the terawatt scale, the Pt suffers from the noble and rare nature as well as the poor stability in the electrolyte, which make it challenging to achieve the large-scale deployment of DSSCs [4]. Thus, the emphasis of DSSC studies has been placed on the emergence of Pt-free counter electrodes. In particular, carbonaceous material has been one of the most promising candidates for Pt-free counter electrodes due to their advantages of low cost, high electrical conductivity, high catalytic activity, and good corrosion resistance [5], [6].
Several carbonaceous materials have been explored, such as activated carbon [7], carbon black [8], graphite [9], graphene [10], mesoporous carbon [11], carbon nanotube [12], bio-derived carbon [13], etc. The catalytic activity of carbonaceous materials strongly relies on their microstructure and morphology which determines the amount of active sites and the diffusion property of redox species within the catalyst layer. Moreover, it has recently been reported that N-doping in carbon promotes the catalytic activity toward reduction. Zou et al. [14] demonstrated N-doped graphene as an efficient metal-free catalyst for DSSCs. It was found that N-doping remarkably improves the catalytic activity of graphene in DSSCs. Moreover, pyridinic and quaternary N possesses higher activity than pyrrolic N due to the weaker adsorption of redox species on pyridinic and quaternary N. The stronger adsorption property of pyrrolic N makes the transformation of I* (absorbed) into I− (solution) difficult for the iodine reduction reaction [14]. Since graphene is a two-dimensional (2D) material with limited electrolyte diffusion property, it is desirable to develop 3D N-doped porous carbon with high surface area and good electrolyte penetration.
To achieve this goal, Yu et al. [11] synthesized N-doped hollow carbon spheres with mesoporous walls as a metal-free cathode for DSSCs. While N-doping enhances the charge transfer and electrode-electrolyte interactions, the large surface area and hierarchical porosity decrease internal resistance and improve electrolyte diffusion. In another work, Lee et al. [15] synthesized a porous carbon with confined large mesopores. They found that the large-sized mesopores ∼22 nm are beneficial for lowering the charge-transfer resistance in DSSCs. Although these metal-free catalysts overcome some of previously mentioned problems, the access of redox species to the inner surface area of these confined pores is still challenging. Therefore, strategies are needed to “open” these confined pores and let the electrolyte and redox species have ready access to all the active sites. In addition, the developed catalysts should possess interconnected 3D structure to improve the electrical conductivity.
In this communication, we present a dual-template synthesis of N-doped macro/mesoporous carbon (macro/meso-NC) with 3D interconnected open-pore structure as a metal-free catalyst for DSSCs. 12 nm SiO2 nanoparticles (SiO2-12, Ludox® HS40 silica colloid solution, Sigma Aldrich) act as the mesopore template, which contribute to the high surface area. 400 nm SiO2 monolith (SiO2-400, synthesis details are present in the experimental section) acts as the macropore template, which create the open-pore structure in the catalyst particles. The synthesized material possesses interconnected 3D hierarchically porous structure with a large surface area of 728 m2 g−1 and rich N content on the carbon surface (5.3 wt. %). Cyanamide is blended with resol polymer solution, which act as, respectively, the nitrogen and carbon sources. As a comparison, undoped macro/mesoporous carbon (macro/meso-C) is synthesized by the same procedure without the cyanamide nitrogen source. It is found that the macro/meso-NC exhibits a larger current toward reduction than both macro/meso-C and Pt, confirming that the high catalytic activity arises from the synergistic effect between the hierarchical open-pore structure as well as N doping. The assembled DSSCs with metal-free macro/meso-NC exhibit a high conversion efficiency of 7.27%, proving as a promising alternative catalyst for low-cost DSSCs.
Section snippets
Materials synthesis
The dual templates for the synthesis of N-doped macro/mesoporous carbon (macro/meso-NC) were Ludox® HS40 silica colloid solution (SiO2-12, Sigma Aldrich) and 400 nm SiO2 sphere monolith template (SiO2-400). The monodispersed 400 nm SiO2 spheres were synthesized by a modified Stöber method [16]. The reaction was started by mixing 24.8 mL of deionized (DI) water, 61.8 mL of ethanol, and 9.0 mL of concentrated ammonia (28.9%, Fisher Scientific) at room temperature. The obtained solution was then
Results and discussion
Macro/meso-NC was synthesized by a dual-template method as shown in Fig. 1a. The carbon precursor is the resol polymer as described in the experimental section. Cyanimide acts as the nitrogen source, which forms a transparent and yellowish mixture with resol. After further blending with Ludox® HS40 silica colloid solution (∼12 nm in diameter, SiO2-12), the mixture is absorbed by a SiO2 monolith template with a particle size of ∼400 nm (SiO2-400). The detailed synthesis procedure of SiO2-400
Conclusions
In summary, by using dual templates, we have successfully synthesized N-doped macro/mesoporous carbon (macro/meso-NC) with an open-pore structure as a metal-free catalyst to facilitate reduction at the counter electrode in DSSCs. The macro/meso-NC shows much better Voc and conversion efficiency than the undoped counterpart (macro/meso-C) and even slightly better than the expensive Pt catalyst. The superior electrochemical performance arises from the synergistic effect of both the 3D
Acknowledgments
Financial support by the Welch Foundation grant number F-1254 is gratefully acknowledged. The authors thank Dr. Guang He for his suggestions on the synthesis of mesoporous carbon.
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