Enhancing valence-band charge localization via zinc doping into MnWO4 to promote selective ammonia electrooxidation

Abstract

The electrochemical ammonia oxidation reaction (AOR) to selectively produce nitrate/nitrite under ambient conditions can be a potential alternative to the Ostwald process. However, the potential bottlenecks of the AOR are sluggish electrokinetics and the involvement of multiple steps, which lead to various parallel reactions and side products. Herein, 5–14% zinc(II) ions are doped into a Wolframite-type MnWO₄ nanostructure, Zn_xMn_1−xWO₄, which significantly localizes the valence-band state and makes the manganese centres more electropositive, leading to the production of an anodic material for the AOR. Among all the tested variants of Zn_xMn_1−xWO₄ and control materials like α-MnO₂, MnWO₄, and ZnWO₄, only 17 ± 3 nm Zn_0.1Mn_0.9WO₄ nanoflakes show a moderate onset potential (0.53 V vs. Hg/HgO) for the AOR in alkaline and near-neutral electrolytes. Electrokinetic study via Tafel analysis and hydrodynamic electrochemistry, and the corresponding Koutecký–Levich analysis indicate that the first 2e⁻ oxidation of adsorbed ammonia to hydroxylamine is the rate-limiting step, with a rate constant of 4.915 ± 3 × 10⁻³ cm s⁻¹. Product analyses following the chronoamperometric AOR study highlight that nitrate predominantly forms in a high yield rate of 160 (±25) µmol h⁻¹ cm⁻² and a faradaic efficiency of 70% ± 10% at 0.8–1.2 V (vs. Hg/HgO). An in situ spectrophotometric kinetics study infers the formed [NO₂]⁻ as the intermediate, while accumulation of [NO₃]⁻ follows pseudo-first-order kinetics with a rate constant of 4.20 × 10⁻⁵ s⁻¹. D₂O labelling, control studies, like electrochemical hydroxylamine and hydrazine oxidation under similar conditions, subsequent product identification and in situ infrared analysis of the electrolyte further emphasize hydroxylamine and nitrite as the AOR intermediates. Zn_0.1Mn_0.9WO₄/NF electrode is further used as an anode to fabricate an NH₃ electrolyzer to produce H₂ with a rate of 1.33 mL h⁻¹ and 94% FE, and the performance of this NH₃ electrolyzer is nearly three times higher than the water splitting electrolyzer. Thus, doping of an atomically dilute quantity of zinc into the MnWO₄ lattice tunes the valence band to enhance AOR selectivity. The in-depth electrokinetic studies presented herein establish the AOR pathway and the nitrate selectivity, which will further help in interpreting the AOR mechanism on various earth-abundant metal catalysts.