Preparation and Photoelectric Conversion Properties of Manganese Oxides and Their Carbon-based Composites
Among the various types of solar cells,dye-sensitized solar cells(DSSCs)are believed to be one of the most promising devices,owing to their low cost,high photoelectric conversion efficiency,facile methods of fabrication and eco-friendly nature.Counter electrode(CE)collects the electron from an external circuit and helps in the regeneration of dye by oxidation-reduction of electrolyte,which significantly effects the overall performance of PV devices.The noble metal Platinum(Pt)and its derivatives have been widely used as the CE materials duing to the excellent electrocatalytic activity and high conductivity.However,the limited reserves of Pt associated with a high price hinder its potential large-scale commercialization.Moreover,Pt has poor corrosion resistance in iodine-containing electrolyte,which shortens the long-term service time of the DSSCs devices.Therefore,it is imperative to develop cost-effective and corrosion resistant counter electrode materials with catalytic properties comparable to those of Pt.In recent years,ABO3 perovskite metal oxides have attracted much attention for their excellent electrical and magnetic properties and have been applied extensively in many fields.Among them,LaMnO3 has received much attention owing to its high catalytic-activity.Substituted perovskites derived from LaMnO3,for example,partial replacement of La3+ by alkaline earth ions such as Ba2+,Sr2+ and Ca2+ has also been extensively researched.Due to the mixed valence of Mn3+/Mn4+ions,eg electrons can easily hops between the two type ions,leading to good conductivity and electrochemical catalytic reduction characteristics.This kind of manganites also have attracted consistent attention for their catalytic abilities of oxygen reduction reaction(ORR)/oxygen evolution reaction(OER)in metal-air batteries and fuel cell systems.Therefore,the main work of this paper is to prepare several pure phase perovskite manganese oxides and their carbon-based composites,and investigate their photoelectric properties in DSSCs as Pt-free cathodes materials.The main content of this paper are as follows.(1)The pure phase La1-xCaxMnO3(0.1≤x ≤0.4)(LCMO)series perovskite oxide nanoparticles were synthesized via a sol-gel method.The electrochemical properties and power conversion efficiency(PCE)of the DSSCs with these LCMO CEs were investigated,emphasizing the influence of catalytic performance of the CE,owing to the partial substitution of La by Ca in the film of LaMnO3 perovskite.The results indicate that:The performance of the DSSCs increases with the increase in x,the highest energy conversion efficiency when x=0.3.La0.7Ca0.3Mn03 perovskite oxide nanoparticles CE exhibited perfect electrical conductivity and electro-catalytic activity for oxidation and reduction of I-/3-.The DSSCs with La0.7Ca0.3MnO3 nanoparticles as CEs indicates considerable PCE of 6.46%,which reaches 91.2%of the level of Pt CE(7.08%).(2)In view of the agglomeration of La0.7Ca0.3Mn03(LCMO)nanoparticles,reduced graphene oxide(rGO)was introduced to synthesize LCMO@rGO nanocomposites.rGO nanosheets in composites have dual functions.One is to prevent the agglomeration of LCMO nanoparticles and make them well dispersed on rGO nanosheets,thus more active sites were exposed.Second,it can be used as a conductive substrate with superior performance for rapid charge transfer.LCMO@rGO nanocomposites combine these synergistic factors.Compared with LCMO nanoparticles and rGO,the catalytic activity of LCMO@rGO nanocomposites for I3-ions is higher and the charge transfer rate is faster.The DSSC equipped with the LCMO@rGO nanocomposites as the CEs achieved a PCE of 7.45%,which is much higher than that obtained with the pure LCMO as the CEs(6.46%),and reaches the level of Pt electrode(7.43%).(3)Graphene nanosheets with perfect planes possess excellent conductivity,but have a limited number of the active sites towards the reduction I3-in redox species.The introduction of heterogeneous atoms into rGO materials can provide an ideal electronic framework for the catalytic process.LCMO@N-rGO composites were prepared by compounding LCMO nanoparticles with nitrogen-doped rGO.The results of electrochemical test show that,LCMO@N-rGO nanocomposite shows an excellent electro-catalytic activity in the reaction of I3-ions to I一 ions and lower charge-transfer resistance(Rct).The PCE of DSSCs with LCMO@N-rGO nanocomposites as the CEs reaches 7.89%,which is much higher than that obtained with the pristine LCMO as the CEs(6.46%),and exceeds the level of Pt CEs(7.43%)under the same experimental conditions.(4)The morphology and crystal structure of compounds are the key factors determining their catalytic activity and durability.Hollow porous La0.7Sr0.3MnO3(LSMO)microspheres were synthesized via a solventthermal method and compounded with rGO to obtain LSMO@rGO composite,which was used as counter electrodes materials of DSSCs.The results of electrochemical tests show that LSMO@rGO composite have remarkable electrocatalytic properties in iodine electrolyte.The DSSCs with LSMO@rGO composite as CEs indicates considerable PCE of 8.05%,which is much higher than that obtained with the pristine LSMO microspheres as the CEs(6.25%),and exceeds the level of Pt CEs(7.52%).It is a promising alternative to Pt electrode.