Electrochemical oxidation of ammonia (NH4+/NH3) ON synthesized nickel-cobalt oxide catalyst

Abstract

Ni–C and Co/Ni–C electro-catalysts were prepared using solvothermal reaction method and used to oxidize ammonia in aqueous solution. Ni and Co were consistently dispersed in carbon generating spherical and porous structure (pore size~ 20–50 nm). The sphericity and porosity of the electro-catalysts increased by increasing the percentage of Co in the composite. The XRD pattern confirmed the formation of Co/Ni–C composite with a reasonable presence of metal hydroxide (Ni(OH)2 & Co(OH)2). The addition of Co to Ni–C enhanced ammonia oxidation reaction (AOR) by decreasing the oxidation peaks in the range of 0.23–0.56 V, increasing the reversible production of metal-hydroxides and generating power (Pgen) to sustain the reaction. The reported Pgen were 0.55, 0.85, 1.1 and 0.0.93 mW/cm2 for Ni–C, Co10/Ni–C, Co20/Ni–C and Co40/Ni–C. A concentration of Co higher than 40% decreased cell potential (Vcell) and reduced the percentage removal of ammonia (%REAmm). A %REAmm of 97 and 99% were achieved by Ni–C and Co20/Ni–C, respectively. The mechanism of AOR follows direct electron transfer at low initial ammonia concentrations and changed to oxidation mechanism at higher concentrations. AOR is more effective at alkaline and high temperature with a high possible chance to be used as fuel in electrochemical cell to generate electricity as well as treating wastewater contaminated with ammonia.