Graphite and platinum's catalytic selectivity for disulfide/thiolate (T2/T−) and triiodide/iodide (I3−/I−)

Abstract

Dye-sensitized solar cells (DSSCs) using pure graphite sheet as the substrate and catalytic material were first fabricated based on a disulfide/thiolate (T 2 /T À ) redox couple. It can present up to 4.79% of the power-conversion efficiency (PCE), compared with the 2.33% of triiodide/iodide (I 3 À /I À ) redox couple at the same experimental condition under AM 1.5 illumination (100 mW cm À2 ). In contrast, when FTO/ Pt is used as the counter electrode, PCE values are 3.97% and 7.13% based on the T 2 /T À and I 3 À /I À redox couples, respectively. With respect to the above dramatic difference, cyclic voltammetry (CV) and electrochemical impedance spectra (EIS) are applied to study the catalytic behavior at electrolyte (T 2 / T À and I 3 À /I À )/counter electrode (graphite sheet and FTO/Pt) interface. In particular, EIS indicated that the charge transfer resistance (R CT ) (1.1 U cm 2 ) of the graphite sheet for T 2 /T À is less than one-ninth of its R CT (10.2 U cm 2 ) for I 3 À /I À . However, the R CT of FTO/Pt (7.7 U cm 2 ) for T 2 /T À is more than twice the R CT (3.0 U cm 2 ) of I 3 À /I À . This demonstrates the high catalytic selectivity of graphite and platinum (Pt) for the two redox couples. These results show the great potential of carbon materials to replace Pt by using this novel electron-transfer mediator in DSSCs.

High Electrocatalytic Activity of Vertically Aligned Single-Walled Carbon Nanotubes towards Sulfide Redox Shuttles

“…Significant improvement in charge transfer at the counter electrode/electrolyte interface not only reduces internal resistances, but also attenuates the recombination rates and concentration gradients in the electrolyte, which have been proved to affect J sc strongly23. The obtained FF and PCE is of great advantage when compared to that with other catalyst materials, such as tungsten carbide embedded ordered mesoporous carbon8, as well as graphite9. Efforts to further enhance the PCE are currently in progress in our laboratory, which include optimizing the photoanode structure and employment of other sensitizers with strong hydrophobicity to inhibit charge recombination between the photoelectrode and electrolyte.…”

“…However, conventional planar platinum counter electrode showed poor catalytic activity towards this iodine-free redox couple, thus resulting in a poor fill factor (FF) and relatively moderate PCE. The phenomenon of a low FF with sulfur-functionalized redox mediators most likely results from the slow catalytic reduction of the oxidized species at the platinum counter electrode689. The slow electron transfer thus leads to a high interfacial charge transfer resistance limiting the FF of the device10.…”

Graphene-based cathodes for liquid-junction dye sensitized solar cells: Electrocatalytic and mass transport effects

“…The T 2 /T -redox couple is a member of larger family of similar organosulfur compounds based on disulphide, polysulfides, thiourea, cysteine etc. which were also tested in DSCs [20,22,[24][25][26][27]. Since the redox potential of T 2 /T -is close to that of I 3 -/I -, there is almost no improvement in V OC and Φ sol as compared to the reference I-mediated DSCs.…”

Functionalized graphene and other two-dimensional materials for photovoltaic devices: device design and processing

“…354,355 Graphene-based CEs have been confirmed to have superior catalytic activities towards Co(bpy) 3 3+/2+ redox couples. 0.56 V vs. SHE has been explored for DSSCs, which showed PCEs up to 12.3%.…”