Pt-free Catalytic Materials Synthesis And Their Application In Dye-sensitized Solar Cell

In recent years, platinum (Pt)-free catalytic materials have attracted considerable attentions, because they are expected to replace the noble metal (Pt) in the fields of energy conversion, energy storage, and organic synthesis. Building on the current research on the Pt-free catalytic materials, this research designed and synthesized a class of Pt-free catalytic materials containing ordered mesoporous carbon, transition metal carbides, nitrides, oxides, sulfides, phosphides, etc. Then the prepared Pt-free catalytic materials were used as low cost counter electrodes (CEs) in dye-sensitized solar cell (DSC) to fabricate photovoltaic devices and high energy conversion efficiencies were obtained.DSC has attracted much attention due to its simple production procedure, environmental friendliness, high power conversion efficiency, and relatively low cost. Generally, a DSC comprises three components, a photoanode (a mesoporous semiconductor oxide film that absorbed a dye), a CE, and an electrolyte. TiO2is widely used as the semiconductor; bipyridyl carboxylate ruthenium is used as the dye; the electrolyte commonly contains I3-/I-redox couples. Pt is generally used as the CE catalytic material. However, Pt is an expensive noble metal with limited reserves. Moreover, Pt can be corroded in I3-/I-electrolyte, influencing the stability of the DSC. Therefore, it is imperative to develop Pt-free catalytic materials that show the merits of high activity, abundant reserves, low cost, and strong erosion resistance to cut down the DSC cost as well as to improve its stability.First, ordered mesoporous carbon (Com) was synthesized and introduced into the DSC as CE catalytic material. Com showed a Pt-like catalytic behavior. Next, catalytic activities of nine kinds of carbon materials were compared systematically under the same conditions. These carbon materials comprised Com, activated carbon, carbon black, carbon dye, carbon nanotube, carbon fiber, and so on. The experimental results revealed that Com and carbon dye performed best amongst these carbon materials. The ordered mesoporous structure of Com is the favorable to the diffusion of the electrolyte and this is the key issue for obtaining high catalytic activity. Moreover, we found that adding TiO2senmiconductor nanoparticles can improve the stability of the carbon CE. The effects of carbon film thickness on the catalytic activity were also investigated.Second, a class of Pt-free catalytic materials containing the carbides, nitrides, and oxides of early transition metals (Cr, Mo, W, V, Nb, Ti, and Zr) were synthesized via a simple chemical or physical method. The variety, crystallinity, particle size, and morphology of the desired products can be controlled by the start materials quantity, reaction temperature, and reaction time. The relationship of the variety and microstructure of the catalytic materials with the catalytic activity has been systematically evaluated. The results demonstrated that, in the process of fabricating DSC, different materials showed major differences in catalytic activities. The prepared Mo2C, WC, TiC, Cr2O3, Mo2N, W2N, TiN, VN, CrN, and V2O3showed high catalytic activities; hence, they can be expected to replace the expensive Pt. In addition, for the same materials, the one with small particle size and large specific surface area exhibited a higher catalytic activity. Furthermore, the range of CE catalytic materials was expanded from carbides, nitrides, and oxides to sulfides and phosphides. The prepared MoS2and WS2performed very well while the prepared MoP and NisP4achieved relatively lower catalytic activities. Moreover, the experimental results proved that the in situ synthesized supported catalytic materials (mesoporous carbon supported MoC, Mo2C, WC, WO2, and VC) performed better than Pt, indicating that the strategy to disperse a catalytic material into a supportor was an effective path to improve the catalytic activity.Finally, the catalytic mechanisms of the catalytic materials for the regeneration of both the conventional redox couples (I3-/I-) and the new organic redox couples (T2/T-) were investigated by electrochemical impedance spectroscopy, cyclic voltammetry, and Tafel-polarization curve experiments. Some of the Pt-free catalytic materials (TiC, Cr3C2, WO2, MoS2, WS2, etc.) behave better than Pt in T2/T-system demonstrating that Pt-free catalytic materials are suitable for both T2/T-and I3-/I-redox couples.In a word, a series of low cost Pt-free catalytic materials with high efficiency were designed and synthesiezd for the DSC. As a result, we were able to greatly expand the range of CE catalytic materisls. By contributing to the reduction of the cost of production for this device, this research improves the competitiveness of this organic thin film solar cell amongst various photovoltaic devices and promotes its industrialization.