Insight into pH-dependent synergistic mechanism of the enhanced photoelectro-chlorine conversion of ammonia (NH4+/NH3) to N2 coupled with H2 evolution on Pt-decorated black TiO2 nanotube arrays

Achieving efficient NH 4 + -N degradation into harmless N 2 and simultaneous harvesting hydrogen energy is of great significance for protecting aquatic ecosystems and alleviating energy shortages. Herein, a photoelectrochemical-chlorine (PEC-Cl) system was developed aiming to selective converting ammonia to N 2 with simultaneous H 2 evolution in a wide pH range of 3–13. The Pt decorated black TiO 2 nanotube arrays (Pt/BTNAs) with sufficient oxygen vacancies defects and the enhanced PEC properties were synthesized as the bifunctional electrode. The parameters optimization illustrated high N 2 selectivity (88 %) and H 2 yield rate (67.5 μmol cm -2 h -1 ) at pH 9. The ClO • derived from the cross reactions between active chlorine and HO • plays a predominant role in oxidizing ammonia. More importantly, the Gibbs free energy calculation indicated that Pt decorating can also significantly promote the direct electrochemical ammonia oxidation reaction (AOR) on Pt/BTNAs anode. Under alkaline conditions (pH 9–13), the efficient removal and conversion of ammonia into N 2 in the PEC-Cl system was achieved primarily through the direct anodic AOR, with N 2 selectivity (89 %-93 %) and H 2 yield rate (60.9–68.6 μmol cm -2 h -1 ) comparable to those of the ClO • -mediated indirect AOR. This study presents a flexible method for treating high and medium-concentration NH 4 + -N and simultaneously producing H 2 through the synergistic PEC-Cl and direct electrochemical AOR processes, across a wide pH range.