Elsevier

Solar Energy

Volume 167, June 2018, Pages 95-101
Solar Energy

Electrocatalytic porous nanocomposite of graphite nanoplatelets anchored with exfoliated activated carbon filler as counter electrode for dye sensitized solar cells

https://doi.org/10.1016/j.solener.2018.04.002Get rights and content

Highlights

  • The defect rich morphology of the exfoliated activated carbon filler is designed with graphite nanoplatelets.

  • Prepared design showed excellent electrocatalytic activity and electron pathways.

  • Prepared composite show a high conversion efficiency of 8.478%, comparable to Pt.

Abstract

A unique graphite nanoplatelet (GnP) composite synthesized with a synchronized distribution of exfoliated activated carbon (AC) filler is proposed for promising Pt-free dye sensitized solar cells. The defect rich morphology of the exfoliated activated carbon filler is designed with graphite nanoplatelets to enhance its electrocatalytic activity and electron pathways, and for this purpose different percentages of AC fillers were incorporated into the GnP matrix. The proposed GnP/AC composite shows a more defect-rich morphology synchronized with high electronic conductivity which greatly enhances the electrocatalytic activity and electron transfer mobility (RCT of 2.19 Ω). A DSSC fabricated with the proposed GnP/AC composite exhibited a high conversion efficiency rate of 8.478%, similar to that of the Pt electrocatalyst. The high catalytic activity of GnP/AC is attributed to the synergistic effect of the high electron affinity of GnP and the structural distortion caused by the AC filler material. This high-performance catalyst can be a promising material for efficient energy storage and harvesting applications.

Introduction

Dye-sensitized solar cells (DSSCs) are among the most promising energy-conversion devices capable of addressing the ever-increasing challenges related to energy harvesting due to their high efficiency and robustness (O'regan and Grätzel, 1991, Grätzel, 2001, Grätzel, 2004). AC has been investigated as a promising filler material for carbon composite structures for cathode materials for DSSCs (Arbab et al., 2016a, Arbab et al., 2016b, Memon et al., 2017). Activated carbon has a high surface area and maintains a multi-edge defect-rich porous morphology to endure the generation of active sites for charge storage and electrochemical reactions (Wang et al., 2015, Saha et al., 2014). AC is perhaps the most explored class of porous carbons, and it have been traditionally employed as a replacement for the Pt catalyst (Yoon et al., 2013). Moreover, AC and Pt have similar electron affinities and band-gap energies (Arbab et al., 2016b). AC has much higher ion diffusion than that of the Pt catalyst (Gong et al., 2009, Arbab et al., 2015). Furthermore, the high porosity and surface area of activated carbon are likely higher than those Pt and other types of carbon used as cathodes (Lee et al., 2008). However, due to the more defect-rich morphology and the poor chemical stability of activated carbon, the injection of tri-iodide ions from the electrolyte to the cathode can be blocked (Lee et al., 2012). Such defect-rich aggregates are detrimental to the performance of carbon-based DSSCs because they fill the electron pathways of activated carbon and reduce the electrolyte/cathode interference mechanism. Therefore, they can be effective filler materials for use in carbon composite structures.

Various strategies have been devised to fabricated different types of composite structures with activated carbon fillers so as to enhance the catalytic characteristics of the carbon matrix. To date, activated charcoal has been exfoliated and used as a filler material for various composite structures. Examples include titania (Li et al., 2010), zinc oxide (Xin et al., 2011) and metal oxide materials (Liao et al., 2013, Salunkhe et al., 2015, Zheng et al., 2014) created by various techniques, such as sol-gel synthesis, hydrothermal/solvothermal chemical deposition, ultra-sonication and electrochemical deposition. The photovoltaic performance rates of these types of composite cathodes range from 5 to 7% (Joshi et al., 2009, Yen et al., 2011, Lin et al., 2011). The rationale for different types of composites along with activated carbon as a filler material has brought considerable change to the electrocatalytic properties of cathodes and subsequently the photovoltaic performance capabilities of DSSCs. A matrix of thin forms of graphite nanostructures such as GnP can provide a direct conduction pathway for the rapid collection of tri-iodide ions at the collector cathode. The induction of GnP fills the porous channels of activated carbon and can enhance electronic stability and ion diffusion capacity of composite structures (Sahito et al., 2017). GnP, a thin form of graphite, has recently received widespread attention due to its electronic conductivity and morphology (Sahito et al., 2016). The thickness of GnP close to that of graphene is suitable for the development of functional and structural nanocomposites. Therefore, a new form of carbon electrocatalyst synchronized with GnP and activated must be developed to fulfil the need for optimized surface area and fast electron transport outcomes.

In this paper, we report the fabrication of a GnP matrix/AC filler composite structure for promising Pt-free DSSCs. The synthesis technique involves the functionalization of a graphite material to incorporate the effective oxygen moieties required for the reduction reaction followed by the addition of activated charcoal processed by ultra-sonication and an acidic dispersion method. The sonication technology shakes the GnP dimensions and fills the gaps of the charcoal vacancies. In this way, a high surface area with high electronic conductivity can be achieved. The proposed system of carbon cathode materials showed higher electrocatalytic activity and rapid electronic transport with very low charge transfer resistance values (RCT = 2.19 Ω). The photovoltaic performance of the proposed system of the GnP/AC composite is 8.478%. This unique combination of defect-rich AC with the fine dimensions of GnP can provide new approaches towards the fabrication of functionalized carbon composites for DSSCs. Different morphologies of composites were fabricated by altering the percentage of the AC filler. AC filler in amounts of 0–120% was incorporated into the GnP composite cathode material, and the electrochemical behavior and photovoltaic performance outcomes were investigated. Comparative studies with a Pt electrode were also done to ascertain the performance capabilities of the GnP/AC composite.

Section snippets

Materials

Activated carbon powder (charcoal with a 100 mesh size, Sigma Aldrich Co.) was selected for the formation of the conductive matrix. GnP powder (carbon content >99.5, C-300, 300 m2/g, xGnP Co.) was selected as the conductive network. Nitric acid (60%, Matsunoen Chemicals) was used for the oxidative functionalization of the carbon content. For the formation of the carbon paste, polymer carboxymethyl cellulose (a sodium salt of MW 250,000 g, Sigma Aldrich Co.) was used as a binding agent.

Morphological and structural characteristics

Scanning electron microscope (FE-SEM) images illustrate the 3D complex of the carbon composite structure, as shown in Fig. 2. Series A is the category of surface images and series B is the cross-cut category (0, 30, 60, and 120% of activated carbon). The pure GnP (0%) electrode system showed a lower defect-rich density, as after functionalization defects were only located at the edges of the GnP. After exfoliation of the activated carbon with different portions (30, 60, and 120%) more porous

Conclusion

Highly efficient metal-free DSSCs were successfully fabricated using a unique composition of a GnP matrix exfoliated with an AC filler. The composite structure demonstrated the best electrochemical characteristics with high electronic conductivity and good efficient electron transport. The high defect-rich morphology and good bonding strength of the composite synthesized with 60% of AC as a filler material are beneficial with a large volume electrolyte and can reduce the charge mobility within

Acknowledgement

This work was supported by the Ministry of Trade, Industry and Energy (10049639) and by the Korea Research Fellowship Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science and ICT (2017H1D3A1A01055133), Republic of Korea.

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