Preparation And Properties Of Graphene-Based Counter Electrodes For Dye-Sensitized Solar Cells

The excessive exploitation and unilization of fossil fuels result in severe eneironmental problems and enrgy crisis.As the outstanding representatives of the third generation solar cells,dye-sensitized solar cells(DSSCs)possess low cost,easy processibility,and environmental friendliness,thus triggering great interests of researchers.Counter electrode(CE)is one of the crucial components in DSSC,which can collect the photo-generated electrons from the external circuit and catalyze the I3-reduction.Generally,the noble platinum(Pt)has been widely emplyeed as the CEs in DSSCs because of its highly active response to the generation reaction of I-.Nevertheless,Pt is resource-limited,costly,and unstable in electrochemical environments,significantly inhibiting the scalable applications of DSSCs.Therefore,it is imperative to develop cost-effective,high-efficiency,and stable CE materials to supersede the Pt.As a new star of carbon family,graphene has garnered great attention in multiple energy applications due to its intriguing physicochemical properties including high charge transfer capacity and robust electrochemical stability.However,the basal-plane of graphene is electrochemically inert compared with that of the edge-plane,which largely hinders the bulky electrochemical performance of graphene.In this context,heteroatom-doping techinique and functionalization for graphene surface have been adopted to engineer highly efficient graphene-based CEs for DSSCs,and corresponding electrocatalytic performance has been evaluated.The main results are highlighted as follows:(1)Chemically unzipping carbon nanotubes followed with high-temperature nitrogen-doping process has been utilized to construct nitrogen-doped graphene nanoribbons(N-GNRs).The structural characterizations revealed that three N configurations coexist in the N-GNRs framework,namely,pyridinic N,pyrrolic N,and quaternary N.When capitalizing on the N-GNRs as CEs for DSSCs,the charge transfer resistance(Rct)at the CE/electrolyte interface is 0.23 ? cm2,which is smaller than that of the Pt CE(1.13 S CIcm2),indicating the superior catalytic activity of the N-GNRs.The DSSC with the N-GNRs CE delivers a high power conversion efficiency(PCE)of 8.57%,which is superb to the device with Pt CE.The N-GNRs CE manifests robust electrochemical stability.Density functional theory(DFT)calculations first reveal that the N types,in particular the quaternary N,can remarkably reduce the ionization energy(Ei)of the N-GNRs,thus decreasing the Rct,facilitating the transfer of electrolyte,and accelerating the cathodic reduction reaction.(2)Sulfur-doped porous graphene(SPG)has been developed via ball milling followed with high temperature annealing technique,of which the precursors are graphene oxide(GO)and sulfur powder.The sulfur powder functions as the pore-forming agent and the dopants.The SPG exhibits rich edges,fully activated basal-planes,and interconnected porous structure.Investigations on the reaction kinetics over the SPG CE uncover that the porous structure of the SPG benefits the fast diffusion of electrolyte.The DSSCs based on the SPG CE show high photovoltaic performance with a high PCE of 8.67%,being superior to that of device with Pt CE(7.88%).Continuous electrochemical impedance spectra(EIS)of the SPG dummy cell exhibit negligiable change in Rct,indicative of the robust stability of SPG in harsh chemical environments.The roles of the S configurations within the SPG backbone in the catalytic activity have been unraveled in terms of the DFT calculations.The S species in SPG can decrease the Ei of graphene,thus rapidly transferring the electrons to the electrolyte and facilitating the I3-reduction.Finally,the influence of annealing temperature for the SPG on its activity was studied,and it is found that the SPG prepared at 900 ? demonstrates the optimal electrocatalytic activity and the highest photovoltaic performance.(3)Selenium-doped graphene(SeG)with rich active sites has been fabricated through the ball milling followed with the high-temperature doping process.The defect(i.e.,oxygen-containing functional groups,Se species,and holey defects)density of SeG can be tuned by adjusting the annealing temperature.The higher the temperature is,the larger the defect density of SeG is.Series of electrochemical characterizations demonstrate that the SeG exhibits outstanding electrochemical bevaviors to the I-regeneration.Compared to DSSC with the Pt CE,the device with the SeG CE delivers a higher PCE of 8.42%.Theoretical simulations discover that the Se species in the SeG can lower the Ei of grapene,thus promoting the charge transfer and bebefiting the cathodic reduction.The influence of the defects and the defect density of SeG on its activity has been first scrutinized at last.(4)Highly-dispersed and edge-enriched MoS2 nanosheets have uniformly grown on graphene film prepared by chemical vapor deposition via hrdrothermal reaction.DFT calculations unveil the growth mechanism of MoS2 nanosheet on the graphene surface.Highly-dispersed MoS2 nanosheets can also grow on other carbon substrats such as the graphite paper and the carbon nanofibers.When applied as electrocatalyst to the I3-reduction in DSSCs,the hybrids(MoS2-G)composed of the MoS2 nanosheets and the graphene show the suppressed electrochemical polarization,and facilitate the mass transfer in electrolyte.Benefiting from the synergetic effect between the electrocatalytically active MoS2 nanosheets and the graphene with high electronic conductivity,the MoS2-G film manifests Pt-like activity.Continuous cyclic voltammetric tests indicate that the MoS2-G film has robust electrochemical stability.The DSSC with MoS2-G composite achieve a PCE of 7.1%,which is 96%of the device with Pt CE.