Edge-Fluorinated Graphene Nanoplatelets As Electrocatalysts for Solar Cells and Lithium Ion Batteries

Abstract

Carbon materials, such as fullerenes, carbon nanotubes, nanodiamonds, graphene, graphite, and amorphous carbon, have been playing a significant role in the development of alternative clean and sustainable energy sources due to their unique and technically important properties. Specifically, graphene, a one-atom-thick graphitic layer comprising sp2 hybridized carbon atoms, has demonstrated many peculiar properties, including superior electrical conductivity, large surface area (≈2630 m2 g−1), excellent mechanical property, and high thermal/chemical stability. These unusual properties make graphene as one of the promising electrocatalysts and electrode materials for electrochemical energy conversion and storage. In this regard, the oxidative corrosion (e.g., C + O2 → CO2) of carbon-based electrodes is one of the most critical drawbacks leading to a short lifetime of energy conversion and storage devices. The typical half lifetime (50% charge capacity retention) of mobile phone battery is less than one year, and also the common pitfall for the commercialization of electric vehicle is short battery lifetime. Therefore, it is important to develop carbon-based electrode materials with strong chemical bonds and enhanced charge polarization to reduce the over-potentials for energy-related electrochemical reactions, and hence less susceptible to the oxidative corrosion. In this study, we have, for the first time, been able to introduce fluorine at the edges of graphene nanoplatelets (GnPs) by the ball-milling process under well-optimized conditions. Since handling fluorine requires extreme care (Caution!!! High concentration of fluorine gas cannot be handled in ordinary laboratory condition), we optimized and performed the “direct” fluorination by using diluted fluorine gas (20 vol%) in argon to safely realize edge-fluorination under the normal laboratory conditions. The resultant edge-selectively fluorinated GnPs (FGnPs) were demonstrated to display excellent electrocatalytic performance and electrode stability (or cycle life) in dye-sensitized solar cells (DSSCs) and lithium ion batteries (LIBs). Therefore, the scalable and cost-effective ball-milling technology could lead to mass production of FGnPs at low cost to satisfy commercial needs for energy conversion and storage applications. Edge-selectively fluorinated graphene nanoplatelets (FGnPs) are prepared by mechanochemically driven reaction between fluorine gas (20 vol% in argon) and activated carbon species from graphitic C-C bonds unzipped by high-speed stainless steel balls with a high kinetic energy. The fluorination at edges of the unzipped graphene nanoplatelets (GnPs) is confirmed by various analytical techniques while the content of fluorine in FGnPs is determined to be 3.0 and 3.4 at% by X-ray photoelectron spectroscopy and energy-dispersive X-ray spectroscopy, respectively. Because of the large difference in electronegativity between carbon (χ = 2.55) and fluorine (χ = 3.98) and the strong C-F bond, the edge-fluorination of GnPs can provide the maximized charge polarization with an enhanced chemical stability. Thus, electrodes based on the resultant FGnPs demonstrate superb electrochemical performance with excellent stability/cycle life in DSSCs (FF: 71.5%; J sc: 14.44 mA cm−2; V oc: 970 mV; PCE: 10.01%) and LIBs (650.3 mA h g−1 at 0.5 C, charge retention of 76.6% after 500 cycles).