Hierarchical Nanospheres with Polycrystalline Ir&Cu and Amorphous Cu₂O toward Energy‐Efficient Nitrate Electrolysis to Ammonia

Abstract

Electrochemical reduction reaction of nitrate (NITRR) provides a sustainable route toward the green synthesis of ammonia. Nevertheless, it remains challenging to achieve high‐performance electrocatalysts for NITRR especially at low overpotentials. In this work, hierarchical nanospheres consisting of polycrystalline Iridium&copper (Ir&Cu) and amorphous Cu2O (CuxIryOz NS) have been fabricated. The optimal species Cu0.86Ir0.14Oz delivers excellent catalytic performance with a desirable NH3 yield rate (YR) up to 0.423 mmol h−1 cm−2 (or 4.8 mg h−1 mgcat−1) and a high NH3 Faradaic efficiency (FE) over 90% at a low overpotential of 0.69 V (or 0 VRHE), where hydrogen evolution reaction (HER) is almost negligible. The electrolyzer toward NITRR and hydrazine oxidation (HzOR) is constructed for the first time with an electrode pair of Cu0.86Ir0.14Oz//Cu0.86Ir0.14Oz, yielding a high energy efficiency (EE) up to 87%. Density functional theory (DFT) calculations demonstrate that the dispersed Ir atom provides active site that not only promotes the NO3− adsorption but also modulates the H adsorption/desorption to facilitate the proton supply for the hydrogenation of *N, hence boosting the NITRR. This work thus points to the importance of both morphological/structural and compositional engineering for achieving the highly efficient catalysts toward NITRR.

Electrochemical Nitrate Reduction: Ammonia Synthesis and the Beyond

“…[ 146 ] synthesized a RhCu nanocube modified by polyallylamine (denoted as PA‐RhCu), and the nanocube presented a concave and twisted surface, indicating the possible existence of high‐index facet (Figure 6d), and this structure can expose more active sites with low‐coordinated structures unlike well‐crystallized materials with ordered atom arrangement. [ 147–150 ] RhCu exhibited a larger electrochemically active surface area than Rh (72.8 versus 21.6 m 2 g −1 ) according to the CO stripping experiment (Figure 6e). Correspondingly, PA‐RhCu showed far higher NH 3 FE (93.7%) with a small overpotential (Figure 6f), and it is worth mentioning that nitrate reduction was tested under 0.1 m HClO 4 for this material, which is different from the mainstream test in neutral or alkaline solution considering the dissolution of metal in acid environment.…”

Recent Advances in Electrocatalysts for Efficient Nitrate Reduction to Ammonia

“…[ 144 ] The Ir incorporation was also effective to enhance the nitrate reduction and ammonia selectivity on amorphous copper oxides. [ 141 ] The improved catalytic activity should be attributed to the accelerated electron transfer and enhanced reaction kinetics, while the improvement in selectivity should be contributed by the larger electronegativity of doped Ir sites and correspondingly enhanced the adsorption of reactants, including NO 3 − and H + .…”

“…[139,140] The incorporation of other elements is effective to tune the electronic property for various applications. [141][142][143] Xu and coworkers reported that Pd-incorporated Cu 2 O octahedra had an improved ammonia yield of 925.11 μg h −1 mg cat −1 at -1.3 V versus RHE with an FE of 96.56% [143] because of the formation of the Cu-Pd dimers and oxygen vacancies. Du et al reported the Zn-doped CuO nanosheets with a greatly improved ammonia yield.…”

Carbon Nanocage Confining CuCo Bimetallic Interface with Low Nitrate Adsorption Energy for Highly Efficient Electrochemical Ammonia Synthesis

“…Electrocatalytic ammonia synthesis via the nitrate reduction reaction (NITRR) is an eight-electron transfer process involving a series of side products including hydrogen, nitrogen, nitrite, and other compounds, leading to a lower Faraday efficiency of the ammonia synthesis at high current densities. − To improve the NITRR, noble metal-based catalysts, such as Pd, Ru, Ir, Au, and Pt, have received special attention due to their high conductivity, low-coordination surface, excellent durability, and appropriate adsorption energy for various reactants. − However, the scarcity and high price of precious metal catalysts are limiting factors hindering the industrial application of catalytic NITRR processes.…”