Pathways of ammonia electrooxidation on nickel hydroxide anodes and an alternative route towards recycled fertilizers

Abstract

Ni(OH)2-catalyzed electrochemical oxidation of ammonia can be used for the synthesis S- or P-containing NH4NO3-based fertilizers with up to 72% faradaic efficiency and up to 98% ammonia removal efficiency.

Nickel‐Catalyzed Urea Electrolysis: From Nitrite and Cyanate as Major Products to Nitrogen Evolution

“…The cleavage of the bond between this second nitrogen atom and the carbon atom in the urea molecule leads to the formation of NCO − and its release into the bulk of the solution, while the *NH species remaining on the surface undergo further oxidation steps (Figure 2a). These subsequent oxidation steps are indistinguishable from the later steps of ammonia oxidation to nitrite and nitrate, which we recently described in detail [15, 17] . We note that N 2 could also be formed through this pathway, however, the overoxidation pathways are more energetically favourable [15, 17] …”

“…[15,17] We note that N 2 could also be formed through this pathway, however, the overoxidation pathways are more energetically favourable. [15,17] In contrast to the NO x À + NCO À formation mechanism described above, N 2 formation results from both N centres in urea undergoing oxidation, which requires both of them to bind to the catalytic active sites enabling electron transfer from both N centres to the anode. To this end, a triangular intermediate with nitrogen-nitrogen-carbon heterocycle (diaziridine-3-one) was previously proposed, stemming from the experimental evidence that both N atoms in N 2 predominantly originate from the same urea molecule.…”

Favorable Electrocatalytic Ammonia Oxidation Reaction Thermodynamics on the β-NiOOH(0001) Surface Computed by Density Functional Theory

“…In experimental works, the onset of dinitrogen formation ranges from 0.84 to 0.87 V SHE at pH = 11, 27−29 the tail end of which coincides with our calculated limiting potential for dinitrogen formation on β-Ni(OH) 2 . If we consider that the applied potential to form β-NiOOH already exceeds the calculated limiting potential for dinitrogen formation, this may explain why the onset of dinitrogen formation is so close to the formation of the β-NiOOH phase, with a gap of only 0.05 V SHE at pH = 11 in the work of Shih et al 28 and considered coincident with the transition in the work of Medvedev et al 30 It should also be noted that β-NiOOH may also be prepared via chemical means, making it theoretically possible to observe lower AOR onset potentials if chemically synthesized β-NiOOH were to be employed and stabilized as an electrocatalyst. 74,75 With respect to nitrite formation, an interesting experimental phenomenon which requires further investigation is at play; namely, depending on the electrolyte conditions, nitrite is either not present or formed in appreciable quantities.…”

“…27,28 In the case of Jiang et al 29 , studying AOR in solutions of KOH, appreciable nitrite was detected, which was further confirmed in work by Medvedev et al in NaOH electrolyte, suggesting that purely alkaline electrolytes change the progress of AOR. 30 While understanding the influence of electrolyte on the formation of nitrite is outside of the scope of this work, it is worth comparing our calculation of limiting potential to experimental works where nitrite formation is significant. In these cases, nitrite formation also coincides with the β-Ni(OH) 2 to β-NiOOH transition, and in the case of Medvedev et al, nitrite supersedes dinitrogen as the dominant AOR product at low potentials.…”

“…In particular, oxidized Ni surfaces, likely involving a mixture of Ni(OH) 2 , NiOOH, and NiO 2 , have shown potential due to its low cost, resistance to poisoning, and the formation of dinitrogen, nitrite, and nitrate as products. 27,28 Reports are divided on the relative amount of nitrite produced, with some studies reporting little to no nitrite formation 27,28 and others reporting that nitrite formation is comparable to nitrate formation, 29,30 with evidence that the proportion of nitrite formed is influenced by the electrolyte used. 30 The most promising feature of the Ni(OH) 2 /NiOOH system is a resistance to poisoning and consistent currents for hours of operation.…”