【作者】 窦衍叶；

【导师】 高学平；

【作者基本信息】 南开大学 ， 无机化学， 2012， 博士

【摘要】 染料敏化太阳能电池（DSSCs）以成本低、易制作和相对高的光电转换效率而成为最有前途的光伏装置。作为DSSCs的重要组成部分，对电极主要起着从外电路收集电子以及催化I₃^-还原为I-的反应。传统的Pt对电极具有优越的电催化活性、高的导电性和较好的稳定性。但世界上的铂资源缺乏，价格昂贵，难适合于规模化应用。因此，研究新型廉价、非铂并且高效的DSSCs对电极材料非常重要。论文设计如下：第一，通过水热法，制备出纯相Ni₂P与Ni₁₂P₅，研究了体系压力，反应物浓度和反应时间对反应产物相结构的影响。研究结果表明，增加体系压力、延长反应时间和降低反应物浓度，产物倾向于生成Ni₁₂P₅。反之，则得到Ni₂P。在此基础上，制备出Ni₁₂P₅-graphene复合物，并将该复合材料作为DSSCs对电极研究了其光电性能和电化学作用机制。结果显示， Ni₁₂P₅-graphene复合物中Ni₁₂P₅嵌入到石墨烯片层中，构成“三明治”结构，这种特殊的结构利于电解液的扩散，这一点可从交流阻抗谱中较小的扩散阻抗得以证实。以Ni₁₂P₅-graphene复合物作为DSSCs对电极时，Ni₁₂P₅的高催化活性、石墨烯的高导电性与电解液的高效扩散性三者形成协同效应，使电池的光电转化效率提高至5.7%，在相同条件下，与Pt对电极所组装DSSCs的效率（6.1%）相近。同时，循环伏安测试结果显示，所合成的材料具有长期的循环稳定性，循环20周后，循环伏安曲线中的氧化峰与还原峰的峰电流密度基本保持稳定。第二，采用化学镀法制备了三种类型基于碳纳米管（CNTs）负载的复合材料（Ni-P/CNTs、Ni/Ni₃P/CNTs和Ni₃N/Ni₃P/CNTs），并将其作为DSSCs对电极，研究了其光电性能和电化学作用机制。研究结果显示，三种复合材料对I₃^-的还原反应均具有较好的电催化活性。其中，Ni-P/CNTs、Ni/Ni₃P/CNT两种复合材料中因为含有致密且导电性较好的金属镍，而使材料的电荷转移电阻较小。在氮化后的Ni₃N/Ni₃P/CNTs材料中，在碳纳米管表面形成颗粒状的Ni₃N与Ni₃P，为电解液的扩散提供较大空隙，加快了电解质的扩散。因此，Ni₃N/Ni₃P/CNTs材料对电极具有较小的吸附阻抗和扩散阻抗，且氮化镍较金属镍具有较高的电催化活性。综合对比显示，三种材料中Ni₃N/Ni₃P/CNTs对电极的DSSCs光电性能最优，其光电转化效率达6.3%，在相同条件下，接近于由Pt对电极组装的DSSC的效率（6.8%）。第三，采用浸渍-固相合成和化学镀-水热两种合成方法制备了Ni₂P/CNTs复合物，并将其作为DSSCs对电极，研究了其电化学作用机制和光电性能。研究发现，浸渍-固相合成法制备的Ni₂P/CNTs复合物负载量较小，且Ni₂P分散并不是很均匀，光电转化效率仅有5.5%。而化学镀-溶剂热合成的材料中Ni₂P纳米晶（约为10-20nm）在碳纳米管上负载均匀，没有出现纯相Ni₂P的团聚现象，表现出最低的电荷转移电阻、较低的吸附阻抗和扩散阻抗。因此，采用该对电极的DSSCs表现出较优的光电性能，光电转化效率达6.7%。总之，为研究低成本、非铂和高效的DSSCs对电极材料，本工作制备了几类磷化物/碳复合材料，深入研究了这些材料的构效关系。研究显示，这些材料集较高的电催化活性，良好的导电性和快速的离子扩散性于一体，展示了良好的光电特性。该研究结果为探索研究新型对电极材料提供了新思路。更多还原

【Abstract】 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.更多还原