MFe2O4(M=Co, Mn)@MG as an Efficient Counter Electrode for Dye-sensitized Solar Cells

Abstract

Dye-sensitized solar cells (DSSCs) are considered as low-cost and eco-friendly alternatives for conventional silicon solar cells. Dye-sensitized metal oxide photoelectrode, electrolyte and counter electrode (CE) are the three major components of a DSSC. An ideal CE material should have superior electrocatalytic activity and distinguished conductivity, excellent chemical stability, and low cost. Noble metal Pt CE has an efficient capacity for collecting electrons from external circuits and facilitating the catalytic reduction of I3 - to I-. However, its high cost seriously restricts mass production and commercialization of DSSCs. Therefore, carbon composite materials, inorganic compounds, conducting polymers and their derivatives have been widely explored as CEs in DSSCs.Recently, multiple transition metals and their derivative have been investigated as the CE materials for DSSCs. Theoretically, binary or ternary transition metals display intrinsic electrocatalytic activity toward the reduction of I3 - with rapid charge-transfer ability and cost-effectiveness. Thus, transition-metal ferric acid MFe2O4 (M=Mn, Co) composited with modified graphene (MG) is a novel strategy to achieve an advanced CE material with remarkable electrochemical performance. As we all know, graphene microstructure has a lamellar structure. Through a process of catalytic reduction, a curly structure modified graphene not only displays a large specific surface area to facilitate charge transfer and promote catalytic performance during photoelectric conversion, but also the conjugated structure with continuous sp2 hybridized carbon centers can induce electronic mobility.Ferrite is a unique hierarchical nanostructure material mainly composed of transition metal (i.e., Co, Mn, Ni), iron (Fe), and oxygen (O). Transition-metal ions M2+ (M2+ =Mn2+, Co2+) have excellent electrocatalytic capability, and active Fe atoms can synergize with carbon atoms in graphene to significantly improve catalytic activity. This work offers a strategy to apply MFe2O4(M=Mn, Co)@MG composite as the CE of DSSCs to optimize catalytic sites and improve catalytic performance. The device based on the CoFe2O4@MG CE achieves a PCE of 7.78%, and the device based on MnFe2O4@MG CE has a PCE of 5.55%. Moreover, the device was prepared by a simple process with an excellent stability in the air atmosphere without encapsulation (maintaining 93.3% of an initial PCE after 30 days), which can facilitate large-scale production in the future.