Nickel Phosphide/Carbon Counter Electrodes For Dye-Sensitized Solar Cells

Dye-sensitized solar cells （DSSCs） are promising photovoltaic devices due totheir low-cost, easy fabrication and relatively high energy conversion efficiency. Asan important component of DSSC, counter electrodes collect electrons from externalcircuit and catalyze the reduction of triiodide to iodide. The conventional Pt counterelectrode has shown superior electrocatalytic activity, high electrical conductivity andgood stability. However, the large-scale manufacturing of DSSCs with Pt counterelectrode may be limited because Pt is one of the scarcest and most expensivematerials available in the world. Therefore, it is highly significant to develop newlow-cost and Pt-free counter electrode materials with a relatively high conversionefficiency for DSSCs.Firstly, in this work, individual Ni₂P and Ni₁₂P₅are prepared, respectively, bythe hydrothermal reaction of the red phosphorus and nickel chloride. The effect ofpressure, concentration, and the reaction time on the phase stuctrure of obtainedproducts are analyzed in detail. It is demonstrated that Ni₁₂P₅can be obtained withincreasing the pressure, extending the reaction time or reducing the concentrationof the reactants. On the contrary, Ni₂P phase is formed. Based on above-mentionedresults, the nickel phosphide-embedded graphene composite is subsequentlyprepared and used as counter electrodes for DSSCs. Photovoltaic performance ofthe solar cells assembled by different counter electrode materials andelectrochemical activity are investigated. It is shown that Ni₁₂P₅particles areembedded into the graphene layers to form sandwich-like composite. With suchunique structure, the Ni₁₂P₅-graphene composite shows the optimizedelectrochemical feature, including the lower charge-transfer resistance anddiffusion impedance due to synergistic efect of the high electrocatalytic activityof Ni₁₂P₅nanocrystallites, high electrical conductivity of the graphene, and fastmass-transfer process of the electrolyte species in the graphene. Therefore, DSSCswith the Ni₁₂P₅-graphene counter-electrode presents comparable performance（5.7%） to the device with conventional Pt counter electrode （6.1%） under thesame condition. In addition, after20consecutive cycles scan, curve shape and peak current density remain constant in cyclic voltammograms （CVs）, displayingthe excellent electrochemical stability of the Ni₁₂P₅-graphene counter-electrode inthe I-/I₃^-system.Secondly, based on carbon nanotubes （CNTs） as a support, the three composites（Ni-P/CNTs, Ni/Ni₃P/CNTs, and Ni₃N/Ni₃P/CNTs） are synthesized by electrolessplating method and investigated as counte electrodes for DSSC with an emphasis onthe electrocatalytic reduction of triiodide to iodide. It is demonstrated that the threecomposites show superior electrocatalytic activity to the reduction of triiodide toiodide. Among all composites, both Ni-P/CNTs and Ni/Ni₃P/CNTs counter electrodesshow a lower charge-transfer resistance due to the dense coverage of the metal nickelor Ni-P alloy with good electrical conductivity on the surface of CNTs. In the nitridedsample, a lower absorption impedance and diffusion impedance are measured due tothe formation of porous Ni₃N and Ni₃P nanocrystallites on the surface of CNTs,which is beneficial to the electrolyte diffusion in the counter electrode. Meanwhile,nickel nitride shows a higher electrocatalytic activity than the metal nickel. Therefore,after comparison, Ni₃N/Ni₃P/CNTs composite presents the best electrocatalyticactivity among the three counter electrode materials, and there are optimalphotovoltaic parameters in the DSSC assembled from the Ni₃N/Ni₃P/CNTs counterelectrode. In particular, the conversion efficiency of6.3%for DSSC withNi₃N/Ni₃P/CNTs is obtained, very close to the value of DSSC cell （6.8%） usingPt/FTO counter electrode under the same condition.Finally, Ni₂P/CNTs composites are assembled by CNTs and Ni₂P throughimpregmation-solid phase and electroless plating-hydrothermal methods, respectively.Subsequently, the electrochemical and photovoltaic performance of Ni₂P/CNTscomposites as counter electrodes for DSSCs are investigated. The obtained resultsshow that the loaded amount of Ni₂P nanocrystallites on the surface of CNTs is lowerin Ni₂P/CNTs composite materials prepared by impregmation-solid phase method. Inaddition, the dispersion of Ni₂P nanocrystallites on the surface of CNTs is notuniform, leading to5.5%conversion efciency of the DSSC with Ni₂P/CNTs ascounter electrode. In the case of Ni₂P/CNTs composite prepared by electrolessplating-hydrothermal methods, Ni₂P nanocrystallites （about10-20nm） were well dispersed on the surface on CNTs. No agglomeration of Ni₂P nanocrystallites isobserved. Therefore, the Ni₂P/CNTs counter electrode exhibites a low charge-transferresistance, adsorption impedance, and diffusion impedance. Correspondingly, theDSSC with this composite as counter electrode shows the better photovoltaicperformance with high conversion efciency of6.7%.In summary, to explore low-cost, Pt-free and highly efficient counterelectrode materials for DSSCs, several nickel phosphides/carbon composites areprepared, in which the electrocatalytic activity, electrical conductivity, andelectrolyte diffusion are integrated. The structure-activity relationship of thesecomposites are further investigated. Based on the results obtained in this work,some new research ideas can be proposed for exploring novel counter electrodematerials and improving photovoaltic performance of DSSCs.