carbon materials can also possess high surface area, such as carbon fiber paper (CFP), making them desirable in many electrochemical, fuel cell, [1] electrolyzer, [2] supercapacitor, [3] sensing, [4] water desalination, [5] and wastewater treatment [6] applications. Another advantage is the excellent biocompatibility of CFP for in vitro and in vivo studies, [7] for biosensing applications, [8] tissue engineering, [9] and in regenerative medicine and cancer treatment. [10] We focus here on CFP because only macroscopic carbon materials can serve as scaffolds for these applications; carbon nanotubes and nanosized graphene flakes require additional supports. Globally scalable clean energy [11] and water purification [12] technologies as well as biocompatible systems must operate in aqueous media and, therefore, require carbon support materials that are hydrophilic for long periods of time.One obstacle that inhibits widespread use of macroscopic carbon materials in aqueous systems is their hydrophobicity. Hydrophilic carbon surfaces are needed to take advantage of the high internal surface area of CFP in aqueous media. Any hydrophilicity-imparting treatment must leave the carbon fiber network structurally intact, so that specifically designed carbon network architectures and pore sizes, which critically affect mass transport characteristics, are not altered during the process.Previously reported methods to increase hydrophilicity of CFP are plasma or chemical etching techniques, sometimes in combination with heat treatments. Ammonia, [6a] nitrogen, [13] oxygen [5,14] or air [15] plasma etching protocols have been reported. Such plasma etches work reasonably well on a laboratory scale, where electrode sizes are on the order of square centimeters or less; yet, plasma methods lead to significant embrittlement of CFP, precluding the production of structurally stable, large-area CFP sheets that are needed for industrial-scale electrolyzers and other applications. Ozone treatment also oxidizes carbon surfaces for improved hydrophilicity. [16] An additional disadvantage is that plasma and ozone generators require large capital expense, especially for large area substrates. Thermal oxidation by a treatment that took more than 16 h also increased hydrophilicity, [17] but that lasted only for 2 d in air when we followed the reported procedure; significant damage to the CFP mesostructures was also observed (Figure S1, Supporting This study reports the selective hydroxylation of macroscopic carbon surfaces that renders initially hydrophobic carbon fiber paper hydrophilic for more than one year (62 weeks) so far. This long time of sustained hydrophilicity is unprecedented and transforms the utility of macroscopic carbon materials. Quantification of surface oxygenates of a systematic series of 13 chemical treatments reveals that surface hydroxyls are predictors of long-lasting hydrophilicity. The rapid, mild, acid-free, transition-metal-free treatment does not leave surface residues, inhibits overoxidation of graphitic carb...

We report the selective hydroxylation of carbon surfaces that rendered initially hydrophobic carbon fiber paper hydrophilic for more than five months. This long time of sustained hydrophilicity is unprecedented. Carbon fiber paper is an inexpensive, electrically conductive, high surface area material that is additionally nontoxic, biocompatible, robust, and scalable. But its hydrophobicity prevents widespread use in aqueous applications. Inhibition of overoxidation of carbon beyond hydroxylation is especially challenging because the first oxidation step is thermodynamically most difficult and subsequent oxidations are much easier. We achieved selectivity for less oxidized hydroxyls over carboxyls by an environmentally friendly, solution-processable, acid-free carbon surface functionalization treatment that is rapid, amenable to large scale applications, and did not damage carbon fibers and their network architectures. The development of this mild, green chemistry carbon surface treatment that provides selectivity for surface hydroxyls transforms the utility of carbon materials.

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