Characteristics of Copper and Tin-Based Rhodium Bimetallic Electrodes for Highly Selective Ammonia Yield in Electrochemical Denitrification

Electrochemical nitrate reduction as to recover valuable ammonia (NH4+/NH3) from nitrogen-contaminated waters is a promising alternative to the Haber–Bosch synthesis. Characteristics of two bimetallic electrodes, RhxCu1–x/Ni and RhxSn1–x/Ni, was investigated on the formation of ammonia from the nitrate reduction reaction (NO3–RR) in neutral media. Results of voltammetry showed that in bimetallic electrodes, Rh promoted the step of NO2– to NH4+ at a potential region which the faradaic current overlapped that of redox transformation of the supported metals, i.e., Cu(0)/Cu(I) and Sn(0)/Sn(II). Rh as the adsorption site controlled the rate-limiting NO3– to the NO2– step, thereby enhancing the current response of nitrate reduction. The loading percentage of Rh critically affected the conversion of intermediates NO2– and N2 to ammonia on Cu and Sn metals, respectively. Both the selectivity and faradaic efficiency achieved above 98% at the best Rh to Cu ratio of 1:4, i.e., Rh0.2Cu0.8/Ni at −1.4 V (vs Ag/AgCl). The specific ammonia yield rate of 3.5 mg-N h–1 g–1 in 0.05 M of KNO3 solution was obtained. However, at higher applied current density, N2 primarily formed on the Sn site competed the formation of NH4+ on the Rh site of the Rh0.5Sn0.5/Ni electrode.