Graphene Nanomaterials As Efficient Metal-free Counter Electrodes For Dye-Sensitized Solar Cells

Dye-sensitized solar cells?DSSCs?have emerged as promising alternatives for traditional silicon-based solar cells owing to their relatively high power conversion efficiency,cost-effectiveness and environmentally benign fabrication processes.As a crucial component of DSSCs,the counter electrode?CE?plays a pivotal role in regulating DSSCs performance by promoting the electron transfer from the external circuit and catalyzing reduction of the redox couple?I₃^-/I^-?.The key requirements for ideal CEs include high catalytic activity and excellent charge-transfer properties.Traditionally,platinum?Pt?is known to be the preferred catalyst in DSSC.However,the high cost and risk of Pt corrosion by the redox species have significantly impeded the large-scale commercialization of DSSCs.Therefore,it is imperative to search for cost-effective,earth-abundant and anticorrosive substitute materials.To address these challenges,metal-free graphene nanocarbon materials have been intensively investigated because of their excellent electrical conductivity,tunable catalytic activities,high resistance against electrolyte corrosion and easy availability.Nevertheless,the progress is not satisfactory by considering that the resultant DSSCs exhibit comparatively poor photovoltaic performance when comparing with those using Pt-based CEs.The reasons for that lie in the incapability of pure graphene materials in catalyzing the reduction of I₃^-due to its low intrinsic catalytic activity.In this thesis,a range of efficient graphene-based materials have been prepared and applied as CEs for DSSCs.N₂ plasma treatment has been employed to modify the chemical vapor deposition?CVD?grown graphene film,and the prepared N-doped graphene film?NG?is used as transparent CEs in bifacial DSSCs.Here,the focus is to create more catalytically active edges and introduce nitrogen atoms into carbon lattice to enhance the electrocatalytic activities for the reduction of I₃^-.The structural,chemical compositions of the synthesized materials and electrocatalytic activities are systematically investigated.The result indicates that the decrease in charge-transfer resistance and the increase in energy conversion efficiency of DSSCs with N-doped graphene CEs are related to the N₂plasma treatment time.The N-G-40 CEs show a?of 3.12%,which is nearly 3 times higher efficiency than the pristine-G CEs in DSSCs.Moreover,the bifacial DSSCs employing N-G films as CEs exhibit much higher?_rear/?_front ratio and better long-term stability than the case using Pt as CEs.Hybridized three-dimensional graphene conductive network with a uniform deposition of?-Fe₂O₃ nanoparticles?NPs?has been prepared by a simple hydrothermal self-assembly approach.The obtained three-dimesional?3D??-Fe₂O₃/GFs with interpenetrating structure not only efficiently increase the electrode-electrolyte contact area but also provide multidimensional pathways to facilitate the transport of electrons in the bulk electrode.Owing to the high electrocatalytic activity of?-Fe₂O₃ NPs and the unique 3D graphene frameworks structure,the?-Fe₂O₃/GFs composites are found to exhibit excellent electrocatalytic activity for the reduction of I₃^-and remarkable electrochemical stability in the I^-/I₃^-based electrolyte.The DSSCs fabricated with the?-Fe₂O₃/GFs CEs show a higher energy conversion efficiency of 7.45%in comparison to 7.29%for the DSSCs with Pt CEs.This strategy can be widely used for developing low-cost and highly efficient CEs materials for DSSCs by incorporating graphene frameworks,especially for those materials with excellent electrocatalytic activity but poor conductivity.3D nanomesh graphene frameworks?NGFs?are synthesized via a MgO-template CVD approach.The 3D NGFs structure possesses high surface curvature,as well as high surface area and tunnelling effect for facile electrolyte infiltration into the mesopores.The high-surface-area NGFs associated with the enriched surface edge defects make it very efficient towards I₃^-reduction even without any Pt catalyst.More interestingly,by virtue of the interpenetrating graphene frameworks,the NGFs exhibit excellent electron conductivity,thus leading to facile charge transfer.Owing to these unique features of NGFs,when being applied as CEs for DSSCs,the higher PCE of7.32%can be achieved,which is comparable to that with Pt CEs?7.28%?.More importantly,the NGFs CEs display better electrochemical stability than Pt CEs.Our present results indicate that the NGFs are low-cost metal-free electrocatalysts that can be used as a promising candidate for the replacement of expensive and scarce Pt CE in DSSC.An edge-enhanced modification strategy has been proposed to fabricate nitrogen doped holey graphene?NHG?by rationally employing N₂ plasma treatment onto exposed edge sites of holey graphene.The as synthesized NHG exhibits a highly conductive and unique holey scaffold with large surface area,along with abundant edge-induced topological defects and nitrogen dopants.Benefiting from such unique features,NHG exhibits outstanding electrocatalytic activity and high electrochemical stability for the I^-/I₃^-redox reaction.Furthermore,density functional theory calculations are performed to further elucidate the underlying mechanism behind this encouraging performance,in particular the effect of edge-induced topological defects.The DSSCs based on NHG CEs display a power conversion efficiency of 9.07%,which is even superior to that of Pt?8.19%?.These results strongly indicate possibilities for large scale fabrication of low-cost and metal-free NHG materials for DSSCs with I-complex redox couple.