A Petal-like Structured NiCuOOH-NF Electrode by a Sonochemical Combined with the Electrochemical Method for Ammonia Oxidation Reaction

Abstract

Direct electrochemical oxidation, as an economical and efficient method, has recently received increasing attention for ammonia-nitrogen wastewater treatment. Developing a low-cost, efficient catalytic electrode is the key to solve the problem of sluggish ammonia oxidation reaction (AOR) kinetics. In this study, a three-dimensional (3D) Ni foam electrode coated with NiCuOOH petal-like cluster structures was prepared using a simple sonochemical method combined with a surface electrochemical reconstruction strategy. This structure has a large surface area and abundant NiCuOOH active sites, giving a good premise for extraordinary electrocatalytic activity of AOR. The results show that the maximum current density for AOR reaches 97.8 mA cm⁻² at 0.60 V vs. saturated calomel electrode (SCE). Additionally, 96.53% of NH₄⁺-N removal efficiency and 63.12% of TN removal efficiency were acquired in the electrolysis system based on the NiCuOOH-NF electrode, as well as a good stability for at least 24 h. It is a promising flow-through anode for the clean treatment of ammonia-nitrogen wastewater.

1. Introduction

The current global imbalance in the nitrogen cycle is becoming increasingly serious due to the growing indiscriminate discharge of nitrogenous pollutants [1,2]. Ammonia-nitrogen (NH₄⁺-N) originates from a wide range of sources, such as domestic sewage, industrial wastewater, and farmland drainage. The excessive discharge of such wastewater will lead to a series of environmental problems such as eutrophication and black odor of water bodies, which seriously endangers human health and ecosystem stability [3,4,5]. Traditional treatment technologies such as biological nitrification/denitrification [6,7], breakpoint chlorination [8,9] and air stripping [10] have been widely used. Comparably, electrochemical oxidation technology is a promising candidate owing to the advantages of simple operation, cost-effectiveness and good tolerance to toxic pollutants [11,12]. Concerning thermodynamics, ammonia is oxidized to nitrogen gas at −0.77 V vs. standard hydrogen electrode (SHE). Dynamically, this reaction occurs very slowly, which obliges researchers to search for effective catalytic electrodes to lower the reaction energy barrier [13,14]. So far, Pt and Pt-based materials are regarded as efficient catalysts because of their low overpotential toward ammonia oxidation reaction (AOR) [15,16,17,18]. However, the poison problem of N_ads (Nitrogen adsorption species) to Pt and the high costs result in researchers developing non-noble metal catalysts, such as Ni, Cu and their alloys [19,20,21,22].

Theoretically, Cu, as a dopant of Ni, would be one of the best active materials for the AOR according to the first-row principles [23,24]. Experimentally, it has been proven that Cu incorporation into Ni can boost the catalytic activity for AOR due to the synergistic effect between Cu and Ni [25,26]. In our previous study, NiCu alloy nanoparticles grown on carbon-nanotubes (CNT) were prepared and the high activity against AOR was corroborated [21]. To further design highly active NiCu electrodes, three-dimensional (3D) nanostructured electrodes have attracted extensive attention because of their advantages over conventional 1D, 2D materials [27]. For instance, 3D nanostructured electrodes have rich active sites, high electron collection efficiency, and a fast ion migration rate [28,29,30,31]. Surface reconstruction can induce conformational and morphological changes in the surface of the metal catalyst and re-establish a new stable surface, which greatly increases the active surface area of the catalytic electrode [32,33,34,35]. Consequently, 3D porous electrodes induced by surface reconstruction enables the improvement of the slow kinetics of AOR [36,37].

Ni foam has a natural 3D Ni skeleton, which is widely used as the substrate for preparing a 3D electrode in assembling an electrochemical device [38,39]. In this study, NiCu pre-catalyst coated on Ni foam electrode was prepared by means of galvanic replacement reaction (GRR) between Ni foam and Cu²⁺ via a facile sonochemical method, in which Cu was as the dopant into Ni foam skeleton. After activation under alkaline conditions, NiCu precatalytic electrode undergo substantial reconstruction to form petal array-like NiCu mixed-metal oxyhydroxides, which are proven to be the “real” AOR catalysts. The structure can rapidly diffuse ions and provide abundant active sites. In contrast to the interior, this “real” structure on the electrode surface is mainly responsible for extraordinary catalytic activity. Furthermore, this 3D flow-through porous anode is conducive to ion diffusion, and it is a promising candidate to solve the electrolyte concentration boundary layer effect in ammonia-nitrogen wastewater.

Herein, a 3D nickel foam electrode coated with NiCuOOH petal-like cluster structures was prepared using a facile strategy combining sonochemical methods with electrochemical reconstructions, which achieve efficient catalytic performance for AOR. The physical characterization results confirm the unique 3D petal-like cluster structure and the composition of the electrode, which provides an abundance of active sites and ion transport channels. As shown in electrochemical tests, the NiCuOOH-NF electrode needs a potential of 0.6 V to drive a current density of 97.8 mA cm⁻². Furthermore, 96.53% of NH₄⁺-N removal efficiency after 8 h were acquired in the electrolysis system based on the NiCuOOH-NF electrode, as well as a good stability for at least 24 h. This will boost the clean treatment of ammonia-nitrogen wastewater as a promising flow-through anode.

2. Materials and Methods

2.1. Materials

Cupric chloride (CuCl₂) and Anhydrous ethanol (C₂H₆O) were bought from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China) Sodium hydroxide (NaOH) and ammonium chloride (NH₄Cl) were purchased from Xilong Science Co., Ltd., Shantou, China. Additionally, all chemicals were analytical grade and utilized directly. Ni foam (NF) was bought from Shanghai Hesen Electric Co., Ltd. (Shanghai, China).

2.2. Electrode Fabrication

Ni foam with a geometric area of 2 cm⁻² was pretreated with ethanol and deionized water. Then, a batch of Ni foam substrates were immersed in each 1% CuCl₂ solution and subsequently underwent an ultrasonic process in an ultrasonic bath (Ningbo Aide, China) at 40 kHz and 60 °C. Respectively, the setting time was 0, 5, 10, 15, 20 and 30 min to screen the optimal electrode. All electrodes are rinsed with deionized water and then dried at a constant temperature of 70 °C. After that, a three-electrode system consisted of the above electrodes as the working electrodes, with Pt wire as the auxiliary electrode and SCE as the reference electrode. The electrochemical activation process was achieved with the aid of cyclic voltammetry (CV) in 2.0 M NaOH, with the potential interval of 0–0.7 V vs. SCE, 20 cycles and a scan rate of 50 mV s⁻¹. The catalytic-ammonia electrodes obtained were, respectively, denoted as NiCuOOH-NF-0, NiCuOOH-NF-5, NiCuOOH-NF-10, NiCuOOH-NF-15, NiCuOOH-NF-20, and NiCuOOH-NF-30.

2.3. Physical Characterization

The morphology of the catalyst was analyzed by a Zeiss Gemini 300 field emission scanning electron microscope (SEM) of Bruker company at 3 kV working voltage. In Esprit 2.0 system, energy dispersive X-ray spectroscopy (EDX) was selected to investigate the content and distribution of elements. The valence composition analysis of elements was carried out in ESCALAB 250Xi XPS of Thermo Scientific company using an Al Kα Light source (1486.6 eV).

2.4. Electrochemical Measurements

The electrochemical performance was tested with a three-electrode system on the electrochemical workstation (CHI 604E, Shanghai Chenhua, Shanghai, China). The above electrodes, SCE and Pt wires, respectively, served as working electrode, reference electrode and counter electrode. CV and Linear Sweep Voltammetry (LSV) were measured in the range of 0 V~0.7 V vs. SCE with a scan rate of 20 mV s⁻¹. The chronoamperometry (CA) technique was performed at 0.65 V vs. SCE. The electrolyte solution was 2 M NaOH or 2 M NaOH + 0.4 M NH₄Cl.

2.5. Electrochemical Measurements

The ammonia-nitrogen removal experiments of real wastewater were performed for 8 h in an electrolysis system at the applied potential of 0.60 V vs. SCE. Inside, the real wastewater refers to a landfill leachate with the initial index concentrations of 616 mg/L NH₄⁺-N, 150 mg/L nitrite nitrogen (NO₃⁻-N) and 204 mg/L nitrate nitrogen (NO₂⁻-N). The NiCuOOH-NF electrode and Pt wire, respectively, served as anode and cathode of the electrolysis system. The wastewater was replaced with a new one and the experiment was repeated three times to check the long-term durability of the electrode. Wherein, concentrations of NH₄⁺-N, NO₂⁻-N and NO₃⁻-N in solution were measured by Nessler reagent spectrophotometry, diazo coupling spectrophotometry and Ultraviolet spectrophotometry, respectively. Total nitrogen (TN) was determined by Alkaline potassium persulfate digestion UV Spectrophotometry. All measurements were supplied on the Ultraviolet visible spectrophotometer (T6 new century, Beijing Puxi General Instrument Co., Ltd. in Beijing, China).

3. Results and Discussion

3.1. The Preparation Principle of Electrode

The NiCuOOH-NF electrode was prepared by the galvanic replacement reaction (GRR) between nickel foam and Cu²⁺ by a sonochemical method, followed by electrochemical activation. On the thermodynamics, a lower reduction potential of −0.23 V (Ni²⁺/Ni) less than that of Cu²⁺/Cu (0.337 V) makes GRR occur between Cu and Ni when Ni foam was immersed in a CuCl₂ solution [38], shown in Equation (1). The Cu²⁺ ions were reduced to solid Cu on Ni foam.

$$\mathrm{Ni}\left(\mathrm{s}\right)+{\mathrm{Cu}}^{2+}\to { \mathrm{Ni}}^{2+}\left(\mathrm{aq}\right)+\mathrm{Cu}\left(\mathrm{s}\right)$$

(1)

When chloride ions coexist with Cu atoms in a solution, a series of subsequent reactions occur, as shown in Equations (2)–(6). Firstly, with the start of GRR, Cl⁻ ions were absorbed on the Cu surface to yield (CuCl⁻)_ad. Then, this product was combined with excess chloride ions to produce ${\mathrm{CuCl}}_{\mathrm{n}}^{1-\mathrm{n}}$ complex. At last, Cu₂O was produced by the hydrolysis of ${\mathrm{CuCl}}_{\mathrm{n}}^{1-\mathrm{n}}$. Meanwhile, Ni²⁺ ions resulting from GR were combined with OH⁻ to generate NiO or Ni(OH)₂. The ultrasonic process can accelerate the ${\mathrm{CuCl}}_{\mathrm{n}}^{1-\mathrm{n}}$ complex formation, and boost GRR between Ni and Cu²⁺, creating NiCu composites on the surface of Ni foam.

$$\mathrm{Cu}+{\mathrm{Cl}}^{-} \to {\left({\mathrm{CuCl}}^{-}\right)}_{\mathrm{ad}}$$

(2)

$${\left({\mathrm{CuCl}}^{-}\right)}_{\mathrm{ad}}+\left(\mathrm{n}-1\right){\mathrm{Cl}}^{-}-\mathrm{e} \to { \mathrm{CuCl}}_{\mathrm{n}}^{\mathrm{n}-1}$$

(3)

$${\mathrm{Cu}}_{\mathrm{n}}^{\mathrm{n}-1}+2{ \mathrm{OH}}^{-}\to \mathrm{Cu}{\left(\mathrm{OH}\right)}_2+{\mathrm{n} \mathrm{Cl}}^{-}$$

(4)

$$\mathrm{Cu}{\left(\mathrm{OH}\right)}_2 \to { \mathrm{Cu}}_2\mathrm{O} +{\mathrm{H}}_2\mathrm{O} +2{\mathrm{OH}}^{-}$$

(5)

$${\mathrm{Ni}}^{2+}+2{\mathrm{OH}}^{-} \to \mathrm{NiO} +{\mathrm{H}}_2\mathrm{O}$$

(6)

After the formation of NiCu composites, the reaction process of electrochemical reconstruction in the three-electrode system is shown in Equations (7) and (8). The NiCu mixed metal oxyhydroxides are recognized to be the “real” AOR catalysts.

$$\mathrm{NiO}+{\mathrm{OH}}^{-}\leftrightarrow \mathrm{NiOOH} +{\mathrm{e}}^{-}$$

(7)

$${\mathrm{Cu}}_2\mathrm{O} +2{\mathrm{OH}}^{-} \to 2 \mathrm{CuO} +{\mathrm{H}}_2\mathrm{O}$$

(8)

3.2. Characterization of Electrode

The structure and morphology on the electrodes’ surface were observed by SEM. Figure 1a shows the SEM image of NiCu-NF electrode, on which some uniform tetrahedrons are very visible. It can be observed that the electrode after electrochemical activation is needle-like, as shown in Figure 1c, and adheres to the Ni foam. In localized regions, the needle-like NiCuOOH-NF clusters together, forming the petal-like cluster structure in Figure 1b. This is due to the pooling of needle-like NiCuOOH growing on the surface of Ni foam during the process of electrochemical activation. Intuitively, there is a clear change in the morphology on the electrode surface after electrochemical activation, exhibiting the prefiguring abundant active sites of the needles shape. Figure 1d shows the SEM-EDX element mapping image of the NiCuOOH-NF electrode in the randomly selected region. The three elements Ni, Cu and O are uniformly distributed/highly dispersed on the surface of the nickel foam substrate. Combined with the elemental analysis

Table 1. Elemental distribution of NiCu-NF electrode and NiCuOOH-NF electrode.

Electrode	Elements	Atomic %	Electrode	Elements	Atomic %
NiCu-NF	O	20.7	NiCuOOH-NF	O	62.8
	Cl	7.9		Cl	0.54
	Ni	39.4		Ni	20.8
	Cu	28.6		Cu	15.9

, it can be seen that the NiCuOOH-NF electrode has significantly more O content, indicating that more metal hydroxyl oxides are generated.

XPS spectrum elucidates the change in elemental valence on the catalytic electrode. In Figure 2a, the Ni(III) (861.5 eV) is significantly enhanced after electrochemical reconstruction by comparing the NiCuOOH-NF electrode with NiCu-NF electrode. In Figure 2b, the Cu 2p 3/2 spectral peaks located at 933.2 eV and 935.8 eV belong to Cu(0) and Cu(II), respectively, and the intensity of the Cu(II) spectral peak in the reconstructed electrode is also enhanced. These are due to the transitions from Ni(II) to Ni(III) and from Cu(0) to Cu(II) induced by electrooxidation. Moreover, the O 1s peaks located at 530.1, 531.3 and 533.0 eV in Figure 2c correspond to metal oxygen species (M-O, M for Ni, Cu), hydroxyl oxygen species (M-OH) and H₂O, respectively. The intensity of M-OH species is evidently higher in the reconstructed electrode than before, which confirms that the metal hydroxyl oxide active species are produced after electrooxidation.

3.3. Electrochemical Activity Analysis

To investigate the electrochemical activity of the electrode, CV tests to AOR on NiCuOOH-NF electrode were examined. From the CV results in 2 M NaOH and 2 M NaOH + 0.4 M NH₄⁺ in Figure 3a, it can be observed that a pair of redox peaks appear in the interval of 0.2 to 0.5 V vs. SCE in 2.0 M NaOH, which corresponds to the redox electron pairs for NiCu(II) and NiCu(III). When 0.4 M NH₄⁺ was added into the 2.0 M NaOH, the AOR starts up from the potential of 0.2 V, and produces an increase in the oxidation peak centered at 0.35 V. These suggest AOR mainly occurs after the oxidation of NiCu(II) to NiCu(III). Another anodic peak appeared at the positive potential of 0.5 to 0.6 V, and the maximum current response was 97.8 mA cm⁻². Compared with the bare nickel foam electrode (Figure 3b), the NiCuOOH-NF electrode obtained more than 10 times of AOR current density, exhibiting excellent catalytic activity. This is because the latter has an active layer on the substrate. NiCu(II) combined with hydroxyl ions to produce NiCu(III) hydroxides (Equation (9)), which are considered as the active substances on the NiCuOOH-NF electrode.

$$\mathrm{NiCu}{\left(\mathrm{OH}\right)}_2+{ \mathrm{OH}}^{-} \to \mathrm{NiCu}\left(\mathrm{OOH}\right)+{\mathrm{H}}_2\mathrm{O} +{\mathrm{e}}^{-}$$

(9)

Additionally, AOR does not seem to refer to the reduction of NiCu(III) to NiCu(II) because the reverse peak does not decrease on all obtained electrodes. Thus, the direct electron transfer from ammonia to the electrode can be inferred.

The effect of sonication time on the ammonia catalytic activity of electrodes was investigated by CV tests with sonication times of 5 min, 10 min, 15 min, 20 min, and 30 min, respectively. Consequently, the current response to AOR increased with ultrasonic time from 5 to 20 min (Figure 4a–d). As shown in Figure 4f, the comparison of different electrode with 2 M NaOH solution can be analyzed; among them, the consecutive redox peaks represent the interconversion between the active species. In general, larger redox peaks means greater pseudo-capacitance from the reversible conversion between oxidized and reduced components, owing to the more abundant NiCuOOH sites on the electrode surface. For the NiCuOOH-NF-20 electrode, the maximum current density of 97.8 mA cm⁻² at 0.6 V was obtained. This extraordinary performance can be attributed to the formation of extensively activated NiCuOOH sites on the NiCuOOH-NF-20 electrode surface. However, as the time was extended to 30 min, compared with the case without ammonia, after deducting the contribution of alkaline OER, the actual AOR current response collected on the surface of NiCuOOH-NF-30 electrode decreased significantly (Figure 4e). In brief, this means that 20 min is a suitable ultrasonic time condition, which is conducive to the generation and accurate expression of rich active substances used to catalyze AOR.

To briefly evaluate the stability of NiCuOOH-NF electrode, the i-t curves were conducted as shown in Figure 5. Obviously, the current density collected in the electrolyte with ammonia can reach 79 mA cm⁻² excluding the initial polarization section, which remained stable for at least 1000 s. This is because the 3D petal-like cluster structure formed by electrooxidation effectively prevents the aggregation of nanoparticles, allowing a rapid diffusion of ions and providing abundant active sites for catalytic AOR. Considering the active contribution of oxygen evolution reaction (OER) in alkaline solution and in order to deduct this effect, the i-t curve in the electrolyte of 2 M NaOH was also acquired. It can be noticed that very low current density is generated, indicating that the effect of water electrolysis can be neglected through the AOR process on the NiCuOOH-NF electrode. Consequently, the current observed in the electrolyte of 2 M NaOH + 0.4 M NH₄⁺ is mainly generated by the ammonia electrooxidation. Furthermore, copper is usually prone to dissolution behavior at pH = 5–13 of the solution, forming a blue solution containing complex Cu²⁺-ammonia ions. This was not observed during the experiments, i.e., copper, one of the active metals on the electrode surface, did not dissolve. This indicates that the electrode has a good structural stability.

3.4. The Influence Factors on AOR

Further studies confirmed the transport properties of ammonia on NiCuOOH-NF electrodes. Figure 6 exhibits that CV curves of AOR in 2 M NaOH + 0.4 M NH₄⁺ at different scan rates. As observed in Figure 6a, the increase in current density and the positive shift in anode peak potential with increasing scan rate implies that there is a kinetic limitation to the redox reaction. However, in Figure 6b, the current at the anode peak is well linear with the square root of the scan rate (v^1/2) and the diffusion and migration of the electrolyte at the anode is limited at higher scan rates, resulting in low electrocatalytic utilization of the active substances. Signifying that the electrocatalytic oxidation of ammonia on the as-obtained electrodes is a diffusion-controlled behavior. Notably, the Tafel slope of 183 mV dec⁻¹ at 5 mV s⁻¹ is lower that of 263 mV dec⁻¹ at 50 mV s⁻¹, exhibiting the electron transfer rate on the electrode surface is faster with less scanning speed (Figure 6c). This may be because the relevant electron transfer on the electrode surface cannot be converted into electrical signals in time under a higher scanning speed.

Figure 7 shows the CV curves obtained on the NiCuOOH-NF electrode in a 2.0 M NaOH solution with different NH₄⁺ concentrations. The AOR peak current density increases with the NH₄⁺ concentration from 0.1 M to 0.5 M and then begins to decrease at 0.8 M. The variation curve of peak current density with NH₄⁺ concentration displays that AOR has a good linear relationship with NH₄⁺ concentration from 0.1 to 0.4. Yet, when maintaining a low level of NH₄⁺, the reaction is mainly controlled by ammonia diffusion, and the active center provided on the electrode is sufficient to realize AOR. As the NH₄⁺ level becomes higher, the current density starts to decrease. This may be attributed to that a high NH₃ concentration can over-saturate the active site, thus inhibiting the catalytic activity of AOR. In addition, surprisingly, the CV results of NH₄⁺ concentration overlapped in the range from the onset potential to 0.45 V, which verifies the above view again. It also implies that the effect of ammonia concentration on AOR is negligible under low potential.

As shown in Figure 8, the effect of OH⁻ on AOR for the as-obtained NiCuOOH-NF was investigated by experiments in 0.4 M NH₄⁺ with different NaOH levels. It can be observed that with NaOH level increases from 0.3 M to 2.0 M, the peak current density of AOR accelerates significantly and the peak shifts negatively. The phenomenon is attributed two reasons. From Nernst equation, when OH⁻ concentration increases, it would lead to a negative potential and make it easier to proceed AOR on thermodynamics. In addition, high OH⁻ concentration implies fast ion transport and the acceleration of the electrochemical reaction kinetics of AOR. Thus, the addition of hydroxide ions can boost the reaction rate of AOR.

3.5. Application in Real Wastewater

Accordingly, the experiment using the real wastewater with 616 mg/L NH₄⁺-N was conducted at 0.60 V vs. SCE for 8 h to examine the practical application performance of NiCuOOH-NF. The variation curve of current density with the degradation time is shown in Figure 9a, where the termination current density after 8 h is about 0.17 mA cm⁻². Figure 9b exhibits the variation in different nitrogen species concentrations in wastewater with degradation time. Obviously, a removal efficiency of 96.53% of NH₄⁺-N has been achieved after 8 h degradation (Figure 9c). To emphasize this, the initial concentrations of NO₃⁻-N of 150 mg/L and NO₂⁻-N of 204 mg/L coexisted in the real wastewater. It is of note that NO₃⁻-N and NO₂⁻-N were also removed to a certain extent, with the former reaching a removal efficiency of 35.65%, higher than the latter (13.00%). This is because the anodic electrochemical AOR process has good N₂ selectivity, inferred from synthetic wastewater with a little NO₃⁻ and NO₂⁻ formation. Meanwhile, it is reasonable to deduce that part of pristine nitrate is converted to nitrite and part is directly reduced to N₂ in the cathodic electrochemical reduction process. As a result, the removal efficiency of TN in wastewater also reached 63.12%.

Finally, multi-cycles of ammonia-nitrogen removal experiments (per cycle for 8 h) were run to further assess the durability of the NiCuOOH-NF (Figure 9d). Alertly, as the number of usages increase, the removal efficiency of NH₄⁺-N was weakened slightly. The high ammonia removal efficiency above 95% in the three uses can be seen, indicating that the NiCuOOH-NF electrode has long-term durability for at least 24 h in real wastewater. Thus, NiCuOOH-NF holds considerable potential in the field of direct electrochemical oxidation for the treatment of ammonia-nitrogen wastewater. Additionally, NiCuOOH-NF exhibited the high efficiency of 96.53% in ammonia-nitrogen removal, which was more superior than that of previously published electrodes, such as Pt/C of 66%, Cu_0.5Co_0.5/NF of 72% and NiCuFe of 90% (

Table 2. The removal efficiency of NH₄⁺-N with different electrodes.

Electrodes	Electrolytes	Anode Potential	Removal Efficiency (%)	Ref.
NiCuOOH-NF	0.20 M NaOH + 616 mg/L NH4+-N	0.60 V vs. SCE	96.53	This work
Ni0.8Cu0.2 oxyhydroxide	0.1 M KOH + 10 mM NH4+	1.53 V vs. RHE	72	[25]
Cu0.5Co0.5/NF	10 mM Na2SO4 + 0.032 M NH3	1.10 V vs. Ag/AgCl	72	[40]
NiCuFe	0.5 M NaOH + 0.055 M NH4Cl	0.55 V vs. SCE	90	[41]
Pt/C	1 M KOH + 0.2 M NH4OH	-	66	[42]

).

4. Conclusions

In summary, a NiCuOOH-NF electrode with 3D petal-like cluster structures was prepared on Ni-foam by the sonochemical method combined with electrochemical reconstruction. The excellent electrochemical performance of the as-obtained electrode can be summed up in the following aspects. Firstly, the surface functionalization of the heteroatoms greatly accelerates the galvanic exchange reaction. Secondly, the petal-like cluster structure formed in situ on Ni foam can provide a huge contact area between the electrode and the electrolyte, exposing more active sites. Thirdly, this unique 3D structure facilitates electrolyte immersion into the electrode material and provides many shorter ion channels, which have an efficient ion transport capacity. The electrochemical test results showed that the NiCuOOH-NF electrode exhibited high activity with a maximum current density of 97.8 mA cm⁻² at 0.6 V, as well as good stability in the i-t curve test in ammonia oxidation reaction. Consequently, 96.53% of NH₄⁺-N removal efficiency and 63.12% of TN removal efficiency were acquired in the electrolysis system based on the NiCuOOH-NF electrode, as well as a good stability for at least 24 h, examining its excellent catalytic activity and practical application performance. These results indicate that this study provides a novel preparation strategy of the practical non-noble metal nanomaterial-based electrode for AOR, and the electrode effectively solves the problem of slow ammonia oxidation kinetics.