Design, Preparation And Application Of Molybdenum Carbide Composites In DSSCs

With the rapid development of the modern world economy and the continuous progress of industrial production,the demand of human society for energy is increasing.However,coal,oil,natural gas and other traditional non-renewable fossil energy is gradually depleted,humanity is facing a serious energy crisis.In addition,the consumption of a tremendous amount of traditional fossil energy also causes great damage to the ecological environment that human beings rely on for survival,which seriously affects the sustainable development of human society.Therefore,to change the existing energy structure and develop sustainable new energy has become an urgent problem that human beings must face.And among the existing wind energy,hydrogen energy,tidal energy,geothermal energy,solar energy and other various kinds of new energy,solar energy has the unique advantages of safety,reliability,pollution-free,renewable,low cost and direct utilization.And it is also the most abundant resources in the global world,which can completely meet the needs of the future development of human society,thus has got the special attention all over the world.Solar cell is a kind of photovoltaic device that can directly convert solar energy into electricity.And as the third generation of solar cells,dye-sensitized solar cells?DSSCs?is characterized by high theoretical photoelectric conversion efficiency,low cost,environmental friendliness,polychromatic transparency,simple preparation method,and it can also has high efficiency in low sunlight.Since its introduction in the 1960s,it has been continuously developed.The counter electrode?CE?is one of the most important components of DSSCs,and the best conventional CE material is platinum?Pt?,which is also the recognized reference electrode in the field of CE material.But Pt only has limited reserves on earth and is very expensive,making it unsuitable for mass production and commercial applications.Moreover,Pt is easily corroded in iodide/triiodide electrolyte,which seriously affected the safety and life span of DSSCs.Therefore,the development of CE materials with low cost,high performance and high stability to replace Pt will be of great significance to the research of DSSCs.In DSSCs,good CE materials require good electrical conductivity and excellent electrocatalytic performance.And molybdenum carbides?Mo₂C and MoC?have good conductivity,and as one of the transition metal carbides,the electrocatalytic properties of molybdenum carbides and Pt metals are very similar.However,as molybdenum carbides is easy to aggregate,resulting in a small specific surface area,and also easy to fall off the FTO conductive substrate,they are not suitable for the use as a catalyst,thus is rarely used in DSSCs.Even applied in DSSCs,the power conversion efficiency of molybdenum carbide CE is very low,far from that of standard Pt CE,and the stability is also much worse than that of Pt CE.Due to the unique properties of molybdenum carbides,it has great potential to be used in DSSCs,therefore it would be a good choice to optimize the photovoltaic performances of molybdenum carbides and compound them with other substances.In this paper,Mo₂C was synthesized by a simple hydrothermal method which came from the previous paper,and the experimental conditions were optimized.Moreover,on this basis,the molybdenum carbide?MoC?composite material was synthesized indirectly and applied in DSSCs,with the specific contents as follows:?1?Firstly,according to the previous paper,MoO₃ precursors were synthesized with?NH₄?₆Mo₇O₂₄·4H₂O,H₂O₂ and concentrated HNO₃ as reaction materials and deionized water as solvent,then Mo₂C was synthesized by carbonizing MoO₃ precursors with CO?NH₂?₂ as carbon source.Finally,by adjusting the mass ratio of CO?NH₂?₂ and MoO₃precursors,the photovoltaic performances of Mo₂C were optimized.The experimental results show that the photovoltaic performances of Mo₂C are greatly improved.When the mass ratio of CO?NH₂?₂ to MoO₃ precursor is 8:1,the synthesized Mo₂C-1/8 not only achieves a power conversion efficiency of 6.49%,which is 85.06%that of Pt,but also has good electrochemical stability.Although the power conversion efficiency of Mo₂C-1/8 did not exceed that of Pt,synthesizing its related composites to further improve it and making it reach or even exceed Pt is also much promising.?2?MoO₃ precursors were synthesized with?NH₄?₆Mo₇O₂₄·4H₂O,H₂O₂ and concentrated HNO₃ as reaction materials and deionized water as solvent.After that,the MoO₃and MnO₂ precursor mixture was synthesized from MoO₃ precursors and KMnO₄.At last,MoC/MnO composites were synthesized by carbonizing the precursors mixture with CO?NH₂?₂ as carbon source.X-ray powder diffraction?XRD?and X-ray photoelectron spectroscopy?XPS?characterization proves the existence of MoC and MnO,indicating that CO?NH₂?₂ only carbonized MoO₃ precursors,but MnO₂ was not carbonized,and the addition of KMnO₄ changed the synthesis route.Scanning electron microscopy?SEM?and transmission electron microscopy?TEM?show that the composites are bilayer,with MnO distributed on the surface of the massive MoC.Electrochemical impedance spectroscopy?EIS?shows that composites CEs have small charge-transfer resistance(R_ct).Tafer polarization curves display that composites CEs have high exchange current density?J₀?and high limit diffusion current density(J_lim).The cyclic voltammetry?CV?results demonstrate that the composites CEs have small peak-to-peak separation(?E_pp)and a large cathodic peak current density.The incident photon-to-current conversion efficiency?IPCE?curves indicate that composites DSSCs have high external quantum efficiency.The above results all indicate that composites CEs have excellent electrocatalytic activity,even better than Pt.The current density-voltage?J-V?curves give the power conversion efficiency?PCE?of the composite DSSCs,and the results show that the power conversion efficiency of the optimal composite DSSC reaches 8.00%,which is indeed higher than that of DSSC based on Pt CE.Moreover,the electrochemical stability test shows that the electrochemical stability of the optimal composite DSSC was also better than that of Pt DSSC.