Jacobs Journal of Materials Science

Morphological, Structural and Electrochemical Evaluation of Nano-Sized Carbon Supported MoS2 as Platinum Free Counter Electrode Materials for DSSC

*Asanda Bede
Department Of Chemistry, Fort Hare Institute Of Technology (FHIT), University Of Fort Hare, Private Bag X1314, Alice, 5700, South Africa

*Corresponding Author:
Asanda Bede
Department Of Chemistry, Fort Hare Institute Of Technology (FHIT), University Of Fort Hare, Private Bag X1314, Alice, 5700, South Africa
Email:abede@ufh.ac.za

Published on: 2019-11-04

Abstract

It has been reported that the morphology, crystalline phase composition and electrochemical properties of counter electrode materials such as MoS2 and carbon supported MoS2 composite nanomaterials are of considerable importance because they govern the efficiency of many photon assisted chemical and physical reactions in dye sensitized solar cells (DSSCs). The efficiency of DSSCs with composite counter electrode materials is reliant on the stability of the photochemistry reactions which can be optimized by appropriate doping with the relevant materials. Moreover, the surface area morphology, distribution of the nanomaterials, and stability of the electrostatic bonds between the MoS2 with the carbon support play a significant role in the performance of the DSSCs. This work evaluates the effect of different mole ratios of the MoS2 and carbon supported MoS2 composite nanomaterials on the morphological, structural and electrochemical properties of the composite counter electrode materials. MoS2 nanoflakes have been synthesized by a hydrothermal technique using sodium orthovanadate (Na2MoO4) as precursor. In this work carbon supported MoS2 NFs were prepared by mixing different mole ratios of MoS2 NFs with MWCNTs and polyvinylidene in N-methyl-2-pyrrolidinone. The morphological, structural and electrochemical properties of the composite counter electrode materials were investigated using SEM, XRD FTIR, TEM, RS and CV. SEM analysis revealed the presence of large MoS2 nanoflakes (NFs). SEM analysis has also revealed a significant change in the surface morphology of the carbon supported MoS2 composite nanostructures as the ratio of composition is varied. Structural properties through HRTEM analysis revealed a d-spacing of 0.65 nm for the (002) lattice plane belonging to a trigonal crystalline phase of MoS2 . HRTEM analysis has also revealed a d-spacing of 0.291 nm corresponding to the 002 plane of MWCNTs. Raman spectroscopy revealed characteristic vibrational frequencies and symmetries at 264.6 cm-1 (Eg), 354.2 cm-1 (Ag) belonging to the trigonal phase of MoS2 (1T-MoS2). FTIR analysis showed a narrow peak at 457.6 cm-1 due to Mo-S vibration bands. This observation confirms the successful synthesis of MoS2 nanostructures. Cyclic voltammetry (CV), charge-discharge (CD) and electrochemical impedance spectroscopy (EIS) measurements confirmed that the MoS2 /MWCNT composite with ratio 6:3:1 performed at the optimum level which is attributed to its larger reduction rate compared to pristine MoS2 NFs and other carbon supported MoS2 NFs. Calculated reduction current densities for the carbon supported MoS2 NFs is in the order 3:6:1>1:8:1>6:3:1>8:1:1 indicating that the composite with ratio of 3:6:1 had the highest reduction rate than the rest of the other materials. Consequently, this composite also attained the least ΔEpp than the rest of the other counter electrodes.

Keywords

Molybdenum disulphide; nanoflakes; MWCNTs; CV; EIS; platinum free Counter electrode materials; composite counter electrode materials

Introduction

The severity of the energy crisis and environmental challenges has raised global concerns. In order to curb this problem several solutions have been offered including the use of renewable energy. Renewables have been touted as the solution since they offer cleaner energy as well as being plentiful, particularly solar energy. Despite decades long domination by silicon based solar cells, the photovoltaic industry has not produced anything significant to rival the substantially cheaper ways of energy generation by fossil fuels. To address this problem and generate cheaper energy from solar, research, fabrication and use of thin film technologies has taken precedence. Amongst the thin film technologies is the dye-sensitized solar cell (DSSCs) which has garnered significant consideration because of its cheaper fabrication techniques as well as its eco-friendly mechanism of operation [1]. The sun which is the most plentiful source of energy offers a cleaner source, is abundantly accessible thus can help solve the global warming problem and the energy crisis [2, 3]. The numerous advantages associated with DSSCs makes them very attractive thus they are a promising economical substitute to the silicon-based solar cells in various aspects such as low-cost, cheaper synthesis method, simplicity of building amalgamation and environmental friendliness. A typical dye sensitized solar cell (DSSC) is comprised of four parts, namely: the mesoporous semiconductor photo anode, electrolyte, dye sensitizer and counter electrode. The counter electrode (CE) plays a vital role in the reduction of the triiodide in the electrolyte thereby leading to a greater sunlight to electricity conversion efficiency for the DSSC. The key role for the CE is to collect electrons from the external circuit and catalyze the reduction reaction of triiodide to iodide (I-/I3-) in the electrolyte [3]. Platinum (Pt) being an excellent redox catalyst was the counter electrode of choice in the first assembled dye sensitized solar cell. The wider use of platinum and its scarcity makes it exorbitantly expensive thus negatively impacting on the commercial viability of the DSSC [4, 5]. These restrictions promoted several studies to advance a substitute material in terms of low-cost and concurrently maintain the effectiveness of the DSSCs.