Graphene Electrocatalysts for Fiber Dye-Sensitized Solar Cells
In this chapter, I will try to explain how to reasonably design a functional fiber electrode and systematically demonstrate a series of graphene-based fiber counter electrodes, including electrocatalysts, conducting additives as well as fiber substrates.
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Graphene Electrocatalysts for Fiber Dye-Sensitized Solar Cells
Dye-sensitized solar cells (DSSCs) have been drawing widespread attention for their unique properties, such as low cost, high efficiency, simple fabrication processes, and environmental friendliness [1, 2]. Typically, a DSSC consists of a dye-sensitized nanocrystalline TiO2 photoanode, an I =I 3 electrolyte, and a Pt/FTO counter electrode (CE) that reduces iodine to iodide to maintain a steady power output. Traditionally, noble metal Pt is utilized as electrocatalyst for CE because of its high conductivity and catalytic activity. However, Pt is expensive and scarce, and its stability in corrosive electrolyte is also inferior. Thus, alternatives to Pt electrocatalyst have been extensively studied, such as inorganic metal compounds [3, 4], conducting polymers [3, 4], and carbon materials [5–7]. Among them, carbon materials including carbon black [5], carbon nanotubes [6, 7], and nanocarbon [7, 8] are low cost, abundant, and stable in corrosive electrolyte. Usually, in order to overcome their inferior intrinsic catalytic activity, carbon-based electrocatalysts are made into thick porous films with high surface area, which meanwhile lead to high resistance and are detrimental to device performance [9]. Graphene, a two-dimensional atomically thick carbon nanomaterial, has been widely researched in energy conversion/storage fields for its excellent properties of light, transparency, high conductivity, and large specific surface area. However, pristine graphene, which lacks catalytic active sites, is hardly suitable for DSSC electrocatalyst [10, 11]. In 2010, Roy-Mayhew et al. [12] discovered that functionalizing graphene with oxygen-containing sites contributes to high catalytic activity of graphene toward I =I 3 , showing comparable performance to Pt catalyst. And graphene performs even better toward cobalt-based or sulfur-based redox couples [13]. The highest conversion efficiency of DSSCs utilizing commercial graphene nanoplatelets has reached 9.5% [14, 15]. Although oxygen-doping process improves catalytic activity, the conductivity of graphene is always tremendously decreased due to the destruction of conductive carbon framework [12]. To overcome the contradiction between the conductivity and catalytic activity by oxygen doping is still challenging. Recent studies show that nitrogen-doped graphene (NrG) is a © Springer Nature Singapore Pte Ltd. 2017 S. Hou, Fiber Solar Cells, Springer Theses, DOI 10.1007/978-981-10-2864-9_5
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5 Graphene Electrocatalysts for Fiber Dye-Sensitized Solar Cells
potentially efficient electrocatalyst for DSSC. Compared with oxygen-doped graphene, the nitrogen-doping process could improve the electrochemical activity of graphene without significant decrease in conductivity [16, 17], which helps to reduce the total internal resistance and thus improves the photovoltaic performance. Nevertheless, the N-doping level of NrG in previous studies is low [17, 18], and the contribution of large amount of abundant residual oxygen-c
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