Improved electrolysis of liquid ammonia for hydrogen generation via ammonium salt electrolyte and Pt/Rh/Ir electrocatalysts

Abstract

Liquid ammonia electrolysis is conducted out at ambient temperature using different ammonium salt electrolytes with and without the reference electrode. It is determined that electrolytes containing NH4+ increase the current density and reduce the solution resistance. Moreover, ammonium salts of NH4Br, NH4NO3 and NH4Cl preserve >80% current efficiency and a 0.6–0.8 mL min−1 average rate of H2 generation during chronopotentiometry tests at 120 mA cm−2 for 3 h. A minimum electrolysis voltage (Emin) of ca. 1.2 V is required for the NH4Cl electrolyte when using Pt-foil as the anode and cathode. To decrease the overpotential of the anode reaction, five types of electrocatalysts were prepared and characterized: Pt-black, Rh, Pt–Ir, Rh–Pt and Ph–Pt–Ir alloys. Trimetallic Rh–Pt–Ir and bimetallic Pt–Ir, Rh–Pt (1:1) alloy electrodes exhibit a combined benefit of better activity and minimized deactivation. The ternary Rh–Pt–Ir alloy anode shows the best electrocatalytic activity with the lowest Emin of ca. 0.47 V and highest current density of 46.9 mA cm−2 at 2.0 V. In comparison with the Pt-foil anode, the Emin is reduced by two-thirds, and the current density is increased two-fold.