Electrical Field-Induced Hydrogen Generation at Electrochemically Grown Catalyst-Free Carbon Fiber Microstructures

Simple cathodic treatments, conducted in an aqueous acidic solution and at hydrogen evolving potentials, are shown to form truncated cone-like microstructures on the surface of carbon fiber paper (CFP) electrodes. An effective ∼32-fold increase of effective surface area was determined for CFPs after consecutive cyclic voltammetry (CV) and chronopotentiometry (CHR) treatments as compared to untreated CFP. Moreover, this is accompanied, as determined by XPS and EIS techniques, by changes in surface chemistry toward a more dominating sp2 nature of the C-C bonds and a significant (∼270-fold) decrease of charge transfer resistance. Electrochemical observations, as well as numerical simulations, indicate that high electrical fields near the microstructures, arising from an applied potential on treated electrodes, increase local H+ concentrations, which can reach values in the order of ∼40- to ∼70-fold that of the bulk H+ concentration. This is shown to have a significant impact on the performance of the CFP-treated electrodes toward the important energy-related hydrogen evolution reaction (HER); the current density at -0.70 V vs RHE for a CV-treated CFP electrode can reach a value 70-fold higher than that of an untreated one (10.5 and 0.15 mA cm-2, respectively). An additional 10-fold current density increase related to effective surface increase can be achieved by a consecutive CHR treatment. Long-term stability was determined for such treated electrodes during H2 generation for 20 h in 1 M H2SO4 at a constant applied current density of 5.0 mA cm-2. It is anticipated that further development of the present concepts may lead to possible future use of such simply obtained structured electrodes in electrochemical energy conversion/storage devices.