Dye-Sensitized Solar Cells (DSSCs) are a promising alternative to Si solar cells, with efficiencies of 14% under sunlight and 28.9% under ambient light.
DSSCs’ components can separately be optimized. One of the major challenges is replacing Pt-counter-electrode (CE), due to its scarcity and easy corrosion. We propose to use carbon nanotube fibers (CNTf) as self-standing CEs.
CNTfs present superior catalytic, electrochemical, and conductivity properties (compared to other metals), joined to the exceptional mechanical ones.
However, they present impurities and defects at either microscopic or macroscopic scales, being not totally clear where CNTs’ catalytic activity comes from. With CNTf- CE efficiencies as high as 10.0% were obtained. When using CNT-composites, based on CNT and transition metal compounds, conductive polymers and/or carbon materials, an 8% of efficiency is reached.
In addition to Pt replacement, there is an interest in developing new electrolytes and redox pairs. I3-/I- is the most used redox pair, reaching efficiencies of 11%. Nevertheless, it corrodes Pt and absorbs visible light. Thus, new electrolytes, based on halogens, pseudohalogens, organic radicals, Cu(L)3I/II and Co(L)3II/III complexes, holding the actual record efficiency, are under constant development. The versatility of the used ligand on the redox complex allows the change of the electrolytes’ solvents, with an increasing tendency towards water. As the solvents’ leakage is one of the problems for long stability devices, quasi-solid and solid-state electrolytes are also developed. It is of interest to study the performance of CNT-based electrodes combined with emerging electrolytes/complexes, both to improve our understanding of CNT electrochemistry and to obtain higher efficiencies.
In this work, CNTf-CE have been tested with three new electrolytes:
standard electrolyte, aqueous electrolyte and Co electrolyte. The procedure followed consisted on the study of their catalytic activity with Pt and CNTf symmetric cells. The final step includes the assembling of a complete DSSC.
Standard electrolyte, based on I3-/I- and organic solvents, gives an efficiency of 7% using Pt-CE, increased to 8% when using CNTf-CE. Changing the solvent by water (aqueous electrolyte), gives efficiencies of 2% using Pt-CE. Using CNTf requires a previous functionalization of the material, obtaining similar efficiencies. The study of Co electrolyte gives interesting results, as its stability and limit current density seems to be intimately related to functionalization degree of CNTf, while the redox potential is related to metal impurities. These results shed light to catalyst and functionalization influence on electrolyte’s stability and their performance in DSSCs.