The synthesis and modification of multi-walled carbon nanotubes (MWCNTs), and their use in dyesensitized solar cells (DSSCs)

Abstract

The global community has invested immensely in the research and development of solar, wind, nuclear technologies for energy generation and storage due to the adverse consequences of (i.e., environmental) using fossil fuels for energy generation. Solar energy remains the most viable sources of sustainable and renewable energy and it has led to an explosion in the development of solar technologies such as photovoltaic cells (PVCs). The 3rd generation PVCs, dye Sensitized Solar Cells (DSSCs), have received a lot of attention due to their ease of fabrication, chemical versatility, bidirectional optical absorption energy and structural tunability, low cost and environmental friendliness. The Pt counter electrode (Pt-CE) of the device and alternatives to it have been extensively investigated. Despite having ‘good’ chemical stability, excellent catalytic activity low sheet resistance and high conductivity, it has relatively high production costs and is susceptible to poisoning by the electrolyte. Multi-walled carbon nanotubes (MWCNTs) and their corresponding heteroatom doped MWCNTs have comparable and in some cases superior electrical and electrochemical properties to that of the Pt electrode. This work was aimed at investigating the synthesis, application of pristine and potentially heteroatom (N, B, P) doped MWCNTs and their corresponding thin films as alternatives to the Pt-CE in DSSCs.

MWCNTs were fabricated using a bimetallic Fe/Co supported on CaCO3 catalyst under different reaction conditions using the chemical vapor deposition (CVD) method. Pristine MWCNTs were fabricated in two studies: (i) The influence of the fabrication method of the bimetallic catalyst. Two catalysts were synthesized under microwave irradiation at constant temperature (Catalyst 1) and constant power (Catalyst 2), and one under thermal heating (Catalyst 3). All three catalysts produced MWCNTs at 700℃ with different properties. (ii) Catalyst 1 was utilized to investigate the effects of temperature (700, 750, 800℃) in the production of MWCNTs.

The attempted synthesis of heteroatom doped MWCNTs using Catalyst 1 was performed using two different boron sources (trimethyl borate and tri-isopropyl borate) at: (i) different reaction times (15, 30, 60 min) at 800℃. Trimethyl borate successfully produced MWCNTs with boron in their lattice structure at a reaction time of 15 (0.31%) and 60 min (0.99%). Tri-isopropyl borate failed to add boron to the lattice structure of the obtained MWCNTs at all reaction times. (ii) different reaction temperatures (700, 750, 800℃). A general decrease in yield (%) with an ii increase in temperature was observed for both sets of samples. Trimethyl borate produced boron containing MWCNTs (0.99%) at 800℃ only, while tr-isopropyl borate produced them at 700℃ (0.39%) and 750℃ (0.29%).

The influence of carbon source and carrier gas flow rates on the heteroatom doping of MWCNTs using tri-isopropyl borate as a boron source and Catalyst 3 were investigated. Two factors were considered for this study: the individual flow rates (either carbon source or carrier gas) and the total flow rates (sum of carrier and carbon source gas flow rates). A combination of the lowest carbon source flow rate (50 sccm) and highest carrier gas rate (280 sccm) produced MWCNTs with the highest % yield (36.9%), thermal stability and surface area (41.2 m2 /g). Lastly, the study investigated the influence of varying the total flow rate (290-370 sccm). The total flow rate was found to have significant influence on the yield, crystallinity, thermal stability, and surface area

The synthesis of boron-nitrogen co-doped MWCNTs was explored using two synthetic approaches; (i) Nitrogen-doped MWCNTs were synthesized using Catalyst 2 and acetonitrile as a nitrogen source at 800℃, followed by the treatment of the produced N-doped MWCNTs with tri-isopropyl borate to substitute boron into the MWCNTs lattice structure. Nitrogen doped MWCNTs were successfully synthesized with a nitrogen content of 0.97%, after treatment with tri-isopropyl borate, the sample showed the presence of nitrogen (0.25%) and boron (0.79%). (ii) MWCNTs that showed the highest thermal stability in the reaction temperature study (700℃ from tri-isopropyl borate and 750℃ from trimethyl borate) were treated with acetonitrile to incorporate nitrogen into the lattice structure. In both synthetic approaches, nitrogen was successfully added to the lattice structure of both the tri-isopropyl (0.25%) and trimethyl borate (2.47%) derived samples, but neither sample had boron in the lattice structure.

Two synthesis methods were used in the attempted synthesis of phosphorus doped MWCNTs; (i) a triphenyl phosphine solution in toluene was bubbled over Catalyst 2 at 800℃. This method did not incorporate phosphorus in the structure. (ii) two post doping methods were employed; the 1st method involved the mixing and milling of pristine MWCNTs and solid triphenyl phosphine, the resulting mixture was heated under nitrogen at 800℃. As observed from XPS, this method succeeded in the addition of phosphorus (0.29%) to the lattice structure. The 2nd approach involved the treatment of pristine MWCNTs with a triphenyl phosphine solution iii prepared from toluene at 800℃. The obtained sample had a low phosphorus atomic % of 0.07%.

A comprehensive investigation into the dispersion of MWCNTs in water, ethyl acetate, ethanol (EtOH), toluene, chloroform (CHCl3), dichloromethane (DCM) and hexane at different volumes (10, 15, 20 ml) was carried out using UV-vis analysis to identify the solvent with highest dispersive ability. Water and ethyl acetate were found to have the highest dispersive ability, and hexane reported the lowest dispersive ability. The approximation of the band gap energies of the dispersions revealed that dispersion of MWCNTs is reliant on their aspect ratio, particularly average diameter. The influence of different mass (4, 8, 16, 24 mg) of surfactants sodium dodecyl sulfate (SDS) and Dodecyl benzene sulfonic acid (DBSA) on the concentration and stability of ethyl acetate and water dispersions (10 ml) were also investigated. Pristine and heteroatom doped MWCNTs thin films were prepared using a drop-casting method. Their electrochemical performance using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) as CE in the reduction of the iodide tri-iodide electrolyte was evaluated relative to that of Pt. The Pt electrode reported two reduction peaks with current densities of -0.65 and -1.27 mA/cm2 . The pristine (-0.01, -0.11 mA/cm2 ) and nitrogen doped (-0.05, -0.58 mA/cm2 ) MWCNTs thin films had poorer electrocatalytic activity than Pt. However, the addition of nitrogen to pristine MWCNTs improved their electrocatalytic activity as evident in the respective current densities at the reduction peaks. This was also observed in the addition of other heteroatoms boron, oxygen, and phosphorus, which all reported greater catalytic activity than pristine MWCNTs. However, none of the phosphorus containing material surpassed the performance of Pt. One boron (0.31%) doped and nitrogen (2.47%) doped MWCNTs surpassed the performance of Pt with current densities of -1.42 and 1.47 mA/cm2 , respectively. In addition, oxygen (14%) containing MWCNT prepared using trimethyl borate also had better performance (-1.47 mA/cm2 ) than Pt. The characteristics of pristine MWCNTs were found to be reliant on the catalyst synthesis method, with catalysts produced under microwave irradiation producing better quality MWCNTs. The methods employed in the synthesis of boron doped MWCNTs were inefficient in the incorporation of boron to the lattice structure. However, they were proficient at modifying the surface of the material. Pronounced differences in the MWCNTs synthesized were found to be correlated to the individual and total flow rates of the gases used. The ex-situ addition of both nitrogen and boron to the lattice structure was found to be successful when the iv addition of nitrogen preceded that of boron. Although the concentration of phosphorus was low, triphenyl phosphine was found to be a promising phosphorous source in the synthesis of phosphorous doped MWCNTs. The dispersion of MWCNTs was found to be dependent on the average diameter of MWCNTs, with different solvents, surfactant and combination thereof having their dispersive ability governed polarity of the solvents, and the average diameter of MWCNTs. Given their superior electrochemical performance, modified MWCNTs (B, N, and O) thin films demonstrated their potential as replacements of Pt as a counter electrode in DSSCs.