Electrolysis of urea and urine for solar hydrogen

Abstract

Hybrid photovoltaic (PV) arrays-electrochemical systems have been envisaged as an alternative technology that co-manages an energy–water nexus, yet improving the technical viability and applicability of the hybrids remains a challenge. With this in mind, we studied the hybrid electrochemical system for urea and urine treatment and simultaneous H_2 production using BiO_x–TiO_2 anode and stainless steel cathode couples with different electrolytes (NaCl vs. LiClO_4 vs. Na_2SO_4). In the presence of NaCl, urea electrolysis was found to enhance the cathodic H_2 production by a maximum of ca. 20% at low urea concentrations, yet reduce the H_2 production at high urea concentrations as compared to water electrolysis. Varying degrees of the synergistic H_2 production were attributed to competitive reactions between active chlorine species and urea/urea intermediates for protons and electrons. The synergistic effect by the urea electrolysis disappeared when LiClO_4 and Na_2SO_4 were used as electrolytes indicating the crucial role of chlorine species in the hybrid reactions. In addition, the electrolysis of actual urine was found to successfully operate along with simultaneous generation of H_2 even in the absence of externally added electrolytes. This electrolyteless-hybrid electrolysis resulted from the large amounts of chloride and other ionic species originally included in the actual urine sample. A detailed reaction mechanism of urea electrolysis and its application potential in terms of solar hydrogen were discussed.