Electrolysis of urea and urine for solar hydrogen

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 H₂ production using BiO_x-TiO₂ anode and stainless steel cathode couples with different electrolytes (NaCl vs. LiClO₄ vs. Na₂SO ₄). In the presence of NaCl, urea electrolysis was found to enhance the cathodic H₂ production by a maximum of ca. 20% at low urea concentrations, yet reduce the H₂ production at high urea concentrations as compared to water electrolysis. Varying degrees of the synergistic H₂ 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 LiClO₄ and Na₂SO₄ 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 H₂ 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.