New insights on electrochemical hydrogen storage in nanoporous carbons by in situ Raman spectroscopy S. Leyva-Garcı ´a a , E. Morallo ´n b , D. Cazorla-Amoro ´s a , F. Be ´guin c , D. Lozano-Castello ´ a, * a Departamento de Quı ´mica Inorga ´nica and Instituto Universitario de Materiales, Apdo. 99, 03080 Alicante, Spain b Departamento de Quı ´mica Fı ´sica and Instituto Universitario de Materiales, Apdo. 99, 03080 Alicante, Spain c ICTE, Poznan University of Technology, Piotrowo 3, 60-965 Poznan, Poland ARTICLE INFO Article history: Received 24 September 2013 Accepted 13 December 2013 Available online 20 December 2013 ABSTRACT In situ Raman spectroscopy was exploited to analyze the interaction between carbon and hydrogen during electrochemical hydrogen storage at cathodic conditions. Two different activated carbons were used and characterized by different electrochemical techniques in two electrolytes (6 M KOH and 0.5 M Na 2 SO 4 ). The in situ Raman spectra collected showed that, in addition to the D and G bands associated to the graphitic carbons, two bands appear simultaneously at about 1110 and 1500 cm 1 under cathodic conditions, and then they disappear when the potential increases to more positive values. This indicates that carbon–hydrogen bonds are formed reversibly in both electrolytes during cathodic condi- tions. Comparing the two activated carbons, it was confirmed that, in both electrolytes, the hydrogenation of carbon atoms is produced more easily for the sample with lower amount of surface oxygen groups. In KOH medium, for the two samples, the formation of carbon–hydrogen bonds proceeds at more positive potential with respect to the thermo- dynamic potential value for hydrogen evolution. Furthermore, changes in the shape of the D band (due to an intensity increase of the D1 band) during the formation of carbon–hydro- gen bonds suggest that hydrogenation of the carbon atoms increases the number of edge planes. Ó 2013 Elsevier Ltd. All rights reserved. 1. Introduction Carbon materials are widely studied as electrodes for energy storage devices (e.g. supercapacitors [1–5]), because of their high conductivity, low cost and versatility of structure/tex- ture. A relevant electrochemical application of carbon materi- als is hydrogen storage by electro-reduction of water in alkaline and neutral media. This process entails storing elec- trical charge through water reduction and weak chemical bonding of hydrogen. Different carbon materials have been studied for this application, such as activated carbons [6– 12], carbon nanotubes (CNTs) [13–17], and others [18,19]. The values of electrochemical hydrogen storage capacity published with CNTs show a large dispersion [13–15,17], what can be justified by the different purity degree of CNTs, which always contain some amorphous material and metal catalyst used in the synthesis [13,20]. Taking into account theoretical calculations and experimental results, storage of a relatively high amount of hydrogen in CNTs is quite unlikely [13,20]. Porous carbons have been studied for this application, giving more effective electrochemical hydrogen storage and greater reproducibility [7]. In the literature, several studies discussing the role of porosity and unsaturated carbon atoms in electrochemical 0008-6223/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.carbon.2013.12.042 * Corresponding author. E-mail address: d.lozano@ua.es (D. Lozano-Castello ´ ). CARBON 69 (2014) 401 – 408 Available at www.sciencedirect.com ScienceDirect journal homepage: www.elsevier.com/locate/carbon