Morphology-Controllable Preparation and Electrocatalytic Performance of Mo-lybdenum Oxide Nanohybrids as Catalysts for Triiodide Reduction Reaction
The energy crisis and environmental pollution have promoted the exploration of clean and sustainable solar energy.Dye-sensitized solar cells(DSSCs),as a device for efficiently using solar energy to create electrical energy,has received extensive atten-tion as they are easy to make,low cost,and high energy conversion efficiency.The counter electrode(CE)of a DSSC is a critical com-ponent in determining the power conversion efficiency(PCE)of solar cells.Therefore,developing CE catalysts with high catalytic ac-tivity is of great significance.Among different CE catalysts,molybdenum-based oxides have an advantage of high conductivity and are often used in DSSCs.In particular,two-phase molybdenum oxide nanohybrids are of great significance for improving the photovoltaic and catalytic performance of DSSCs.In this work,the molybdenum oxide nanohybrids(MoO2,MoO3 and MoO2@MoO3)with different morphologies were controllably synthesized by a two-step hydrothermal method.Firstly,under magnetic stirring,molybdenum chloride was dissolved in deionized water.Secondly,the uniformly mixed solution was then placed in a stainless-steel autoclave and subjected to hydrothermal treatment at 200 ℃ for 12 h.Subsequently,the hydrothermal product was washed with deionized water and ethanol,and the precipitate was collected by using a centrifuge.Finally,the indigo blue precipitate was put into a tube furnace filled with N2 and annealed at 500 ℃ for 1 h to obtain molybdenum-based oxides.Different amounts of Ni(NO3)2·6H2O were added to adjust the mor-phology of the nanohybrids.When the amount of added Ni(NO3)2·6H2O was 0,0.006 and 0.004 mol,the nanohybrids were MoO2,MoO3 and MoO2@MoO3,respectively.The structure and morphology of different nanohybrids were characterized by X-ray diffractome-ter(XRD),X-ray photoelectron spectrometer(XPS),and scanning electron microscope(SEM).The XRD pattern showed that MoO2 and MoO3 contained monoclinic and orthorhombic crystals,respectively,while MoO2@MoO3 had both of monoclinic and orthorhombic phases.No other obvious impurity peaks were observed.XPS spectra showed that only the characteristic peaks of Mo and O existed in the MoO2,MoO3 and MoO2@MoO3,and there were no characteristic peaks of Ni.The XPS spectra of Mo 3d in MoO2@MoO3,MoO2 and MoO3 showed that there were two different molybdenum-based oxides in the MoO2@MoO3 catalyst.It could be seen from the SEM imag-es that nanohybrids had different morphologies,sphere(MoO2),ribbon(MoO3),and ribbon loaded with sphere(MoO2@MoO3).Ben-efitting from the randomly oriented three-dimensional pore structure formed by the disordered arrangement of the ribbon structure and spherical nanoparticles,MoO2@MoO3 was conducive to the diffusion of electrons from the external circuit through the catalyst material into the electrolyte,thereby accelerating the reduction reaction of I3-.Photovoltaic performance test under stable light intensity of 100 mW·cm-2(AM 1.5G).The photocurrent density-voltage curve is obtained through the photovoltaic performance test.The PCE of MoO2,MoO3 and MoO2@MoO3 catalysts were 7.16％,6.70％and 7.28％,respectively.The corresponding short-circuit currents were 17.15,15.50 and 18.09 mA·cm2,respectively;the open circuit voltages were 0.69,0.66 and 0.67 V,respectively;the fill factors were 0.60,0.66 and 0.60,respectively.The Cyclic voltammetry test was carried out under a standard three-electrode system.Cyclic voltammetry was used to evaluate the catalytic activity of the molybdenum-based oxide CE catalyst in DSSCs.MoO2@MoO3 exhibited anexcellent catalytic activity when the peak current of the oxidation-reduction current density peak and the peak-to-peak separation of the three molybdenum oxide catalysts were compared.At the same time,the cyclic voltammetry test could evaluate the electrochemical stability of the three molybdenum oxide catalysts.Through 100 continuous cyclic voltammetry tests,MoO2,MoO3 and MoO2@MoO3 were shown to have excellent electrochemical stability and were suitable for use as catalyst materials for large-scale industrial applica-tions.The electrochemical impedance test of the CE was conducted at test frequencies in the range from 100 mHz to approximately 1 MHz,the charge transfer resistance of MoO2,MoO3 and MoO2@MoO3 in I3-/I-electrolyte system was 85.09,16.33 and 3.66 Ω·cm2,respec-tively.The series resistances were 11.92,8.65 and 9.18 Ω·cm2,respectively.The Tafel polarization test was developed for the CE in the voltage range of-0.8～0.8V and the scan rate of 50 mV·s-1.The intersection of the cathode branch and y-axis was the limiting diffu-sion current density that reflected the diffusion rate of I3-in the electrolyte.The ability to reduce I,-of catalysts decreased in the order of MoO2@MoO3＞MoO2＞MoO,.From the analysis,the significant improvement in the catalytic performance of MoO2@MoO3 was mainly due to the following three reasons:(1)Electrostatic interaction between differently charged MoO3 and MoO2 produced close chemical and electronic coupling.(2)The three-dimensional network structure formed by the nano-particles and the ribbon structure in the two-phase molybdenum oxide catalyst provided more channels for the transmission of electrons,which was beneficial to the transport of electrons in the electrolyte.(3)The two-phase molybdenum oxide catalyst provided more active sites than the single-phase molybde-num oxide catalysts that promoted the redox process of iodide ions in the electrolyte.
molybdenum oxidecounter electrode(CE)dye-sensitized solar cell(DSSC)catalytic activitynanohybrid
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