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New application of perovskite in solar cell: as electrocatalyst for photocathode enabling better efficiency of dye-sensitized solar cells
1  Department of Chemical Engineering Curtin University


Dye-sensitized solar cells (DSSCs), typically composed of a dye-sensitized TiO2 photoanode, an I/I3-based electrolyte and a photocathode, also called counter electrode (CE), convert solar energy into electricity directly, which attracted considerable attention due to the high theoretical power conversion efficiency (PCE) and environmental friendliness. CEs are equally important to other components and determine in part the efficiency and cost of DSSCs. Ideal CEs should possess high electrocatalytic activity for I3 reduction reaction (IRR) to I, superior charge transfer capability and stability. On the contrary to intensive research on other components, CEs were often overlooked in the past and platinum (Pt) is state-of-the-art CE in DSSCs, which however suffers from its scarcity and poor durability (decomposing to PtI4 in electrolyte). Until recently, alternative Pt-free CEs have been actively sought to heighten the cost competitiveness of DSSCs. To date, several kinds of carbons, e.g., carbon black (CB), carbon nanotubes (CNTs) and graphene, were investigated as CEs for DSSCs due to their high conductivity and large surface areas. Nonetheless, the performance of DSSCs with carbon-based CEs was usually much inferior to Pt. Functionalization, doping and morphology control were exploited as effective approaches to improve the activity of carbons. However, the complex synthesis processes and the poor stability of the functional groups may impede their practical applications. Alternatively, metal sulphides, selenides and oxides are also explored as Pt-free CEs in DSSCs. Among them, although metal sulphides and selenides displayed high activity for IRR, the inefficient charge transfer rate of metal sulphides, selenides originated from their band structure and the complex synthesis procedures largely impede their further applications. On the other hand, metal oxides are of particular interest due to high activity, low cost, robust stability, abundant in variety and facile in preparation. Oxygen vacancies and low metal valences in metal oxides play important roles in IRR and oxygen vacancies-contained SnO2 and WO3 were proved to be active CEs for DSSCs. However, due to the simple atomic environment, the oxygen vacancy in simple oxides is very limited, restricting further performance enhancement. Complex oxides such as perovskites (ABO3) are more attractive for their better chemical flexibilities. More oxygen vacancies can be generated by tailoring the compositions of the perovskite oxides. In addition, perovskites with easily produced redox couples in B-site are expected to have high IRR activity. However, up to now, few reports on the application of perovskites as CEs in DSSCs are available, due likely to their relatively low conductivity and small surface areas. In this study, for the first time, we report chlorine (Cl)-doped perovskite oxide with an orthorhombic structure that shows high activity and durability for IRR in DSSCs. By optimization of the Cl doping amount, LaFeO2.942-δCl0.058 showed much higher IRR activity relative to its parent compound, LaFeO3 (LF), due to the increased concentration of oxygen vacancies (active sites), enriched lower B-site metal valance and the reduction of the bond energies of iodine with the electrocatalyst. DSSC with LaFeO2.942-δCl0.058 CE shows a PCE of 8.20%, outperforming Pt CE (7.11%) and being much superior to that of LF (3.10%). By applying hierarchical TiO2 microsphere as an advanced photoanode and LaFeO2.942-δCl0.058 as CE, DSSC demonstrated a highly attractive PCE of 10.2% under simulated sunlight irradiation (AM 1.5G), which was 25% higher than a reference DSSC with Pt CE (8.11%). Furthermore, LaFeO2.942-δCl0.058 showed a much superior stability to Pt and the pristine LF for IRR and DSSC with LaFeO2.942-δCl0.058 CE operated very stably for 28 days. In addition, in this study, we report the development of strongly coupled LaNiO3 perovskite/carbon composites as new Pt-free CEs for DSSCs showing excellent electrocatalytic activity and stability for IRR. To prepare such composites, a facile mechano-chemical method based on high-energy ball milling was applied, where abundant oxygen vacancies, rich mixed B-site metal valences and a strong coupling effect between carbon and LaNiO3 were created. DSSC with LaNiO3/CB CE exhibited comparable PCE to that of Pt CE (reaching 98%). Hybriding LaNiO3 with multi-walled carbon nanotubes (MWCNTs) obtained a CE with even better performance than Pt or LaNiO3/CB CEs due to enhanced electron transfer capability and improved surface area; a more attractive PCE of 9.81% was achieved by DSSC with hierarchical TiO2 microsphere photoanode. Furthermore, LaNiO3/carbon composites afforded superior stability to Pt, making them highly promising CEs for DSSCs. The PCEs and the performance enhancements comparing with Pt that obtained by the perovskite-based photocathodes in this study were superior to most of the state-of-the-art highly efficient Pt-free photocathodes in DSSCs. The present work provides two simple and efficient strategies to develop perovskite-based highly-efficient and stable photocathodes for Pt replacement in DSSCs. This study also highlights the extended applications of perovskites in other I3/I-mediated PVs or electrochromic, batteries.