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, Rh_xCu_1-x/Ni and Rh_xSn_1-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., Rh_0.2Cu_0.8/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 Rh_0.5Sn_0.5/Ni electrode.