In-situ phase transformation of silver oxide nanoparticles encapsulated in reduced graphene oxide (Ag/rGO) for direct electrochemical ammonia oxidation

Abstract

Silver nanoparticle intercalated in graphene oxide substrate (Ag/GO400 and Ag/rGO400) was synthesized by electroplating and thermal methods for electrochemical oxidation of ammonia in dilute aqueous solutions. Electrochemical characterization, including voltammetry and Raman spectra, identified the solid-state phase transformation of silver, i.e., Ag(0)⇌ Ag(I)⇌ Ag(II), which were electron mediators for the conversion of ammonia to other nitrogen compounds, specifically dinitrogen. The electrochemical-chemical processes (EC) controlled the catalytic yields of N2 and NO3− in the applied potential regions of Ag2O and AgO formation, respectively. The intercalation of highly dispersed Ag nanoparticles/nanoclusters in lamellar sheets of reduced graphene oxide (rGO) enhanced the electrochemical reactivity, leading to the improved faradaic efficiency and N2 selectivity (SN2). GO reduction to rGO decreased surface oxygen content, thereby lowering the yield of nitrite and nitrate oxyanions on Ag/rGO400 than Ag and Ag/GO400 electrodes. Ag/rGO400 exhibited an ammonia removal efficiency of 97% and SN2 of 80% at + 1.0 V (vs. Hg/HgO, pH 11). The Ag/rGO400 anode, equipped with Cu(111)/Ni cathode in an asymmetric and undivided electrolyzer, was applied to treat real wastewater containing around 150 mg-N/L NH3. At an optimal anode to cathode number ratio of 2 to 2, SN2 was>90% in continuous flow mode, which confirmed the stability of the synthesized electrodes.