Se-doped g-C3N4 was synthesized from sonicated aqueous suspensions of melamine cyanurate and SeO2. The different thermal condensation temperatures in the 500-650 ºC range were found to influence the photophysical properties and hydrogen evolution rates. H2 evolution increased dramatically by two orders of magnitude when Pt co-catalyst (1 wt.%) was incorporated, reaching an HER of 75 µmol H2/h·gcatThe authors gratefully acknowledge the support of Dr. Manuel Avella (Microscopy Unit, Parque Científico, Universidad de Valladolid) and Dr. Francisco M. Sánchez-Arévalo (IIM-UNAM) with the SEM and TEM analysis and the DR spectra measurements, respectively. P. Martín-Ramos would like to thank Iberdrola Foundation for its financial support. UVa group acknowledges financial support of Junta de Castilla y León through project VA036A12-2. ICP-CSIC group acknowledges the research program supported by the Secretaría de Estado de Investigación, Desarrollo e Innovación (MINECO, Spain) under the project CTQ2013-48669-PPeer reviewe

Currently, according to conventional charge–discharge profiles, energy consumed in charging Capacitive Deionization (CDI) systems is always a function of different parameters (current used for charging, capacitance and current employed for discharging) making it difficult to separate the effect of these parameters on CDI performance and energy efficiency. Thus, energy efficiencies are strongly influenced by the current in the preceding charge or discharge stage of the process. We find consistently that this phenomenon, which to our knowledge has not been addressed in previous CDI communications, is much more intense when different currents are applied for each of the charging and discharging cycles. The investigation reported here provides a mechanistic analysis of the operational aspects of CDI and develops a new procedure that allows for a precise evaluation of performance and energy efficiency. Furthermore, the model developed here allows one to separate charge and discharge cycles, and therefore contributes to the possibility of defining an operational mode for real-world devices in which effective separation of deionization and regeneration steps needs to be implemented. This method of analysis could be useful not only for CDI but also for other electrochemical systems such as in secondary batteries and supercapacitors where charge and discharge are typically employed.