Selective Hydroxylation of Carbon Fiber Paper for Long‐Lasting Hydrophilicity by a Green Chemistry Process

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 carbon beyond hydroxyls, and introduces the highest edge density on carbon surfaces, without altering the mesostructure of carbon fiber network architectures and without lowering the flexural strength of carbon fiber paper. Graphitic edges are required for enhanced stability of surface oxygenates that retain a threshold amount of adsorbed water, which is necessary for keeping carbon fiber paper hydrophilic. Application of hydrophilic carbon fiber paper as high‐surface‐area working electrode supports for alkaline water oxidation catalyzed by [NiFe]‐layered double hydroxide nanosheets demonstrates a factor of 60 higher mass activity compared to catalysis on flat basal‐plane highly ordered pyrolytic graphite electrodes. The mild, solution‐processable, and rapid carbon surface functionalization process solves the long‐standing challenge of making initially hydrophobic carbon fiber paper hydrophilic for long periods of time without destroying the mesostructure of carbon fiber network architectures. It does so by selective generation of surface hydroxyls bound to graphitic edge sites to keep a threshold amount of adsorbed water in place.