Electrochemical Ammonia Oxidation in Water Treatment: A Comprehensive Review on Mechanisms, Catalysts, and Implementation Challenges

Abstract

The discharge of ammonia-rich wastewater poses significant threats to water quality and ecosystem health, driving the need for efficient and sustainable treatment technologies. The electrochemical ammonia oxidation reaction (eAOR) has emerged as a promising alternative to conventional biological and physicochemical methods, offering advantages such as in situ oxidant generation, tunable product selectivity, and applicability under challenging water matrices. This comprehensive review systematically examines the mechanisms, catalyst design, and environmental factors influencing eAOR performance. Two primary pathways are detailed: direct eAOR, involving stepwise dehydrogenation of NH₃ on the electrode surface, and indirect eAOR, mediated by electrogenerated reactive chlorine species (RCS). The mechanisms—including the Oswin-Salomon and Gerischer-Mauerer pathways for direct oxidation, as well as breakpoint chlorination and radical-mediated routes for indirect oxidation—are critically discussed alongside experimental and theoretical evidence. Recent advances in electrocatalyst development are highlighted, covering noble metals, non-noble transition metal oxides, alloys, and hybrid materials, with an emphasis on enhancing activity, selectivity toward N₂, and durability. Key operational parameters such as pH, chloride concentration, and coexisting ions are analyzed for their impact on reaction kinetics and byproduct formation. Finally, the review identifies current challenges—including catalyst poisoning, toxic byproduct generation, and scalability—and outlines future research directions aimed at advancing eAOR toward energy-efficient, resource-recovering water treatment systems.

1. Introduction

The escalating discharge of ammonia-laden wastewater from industrial sectors—including chemical synthesis, fertilizer production, and semiconductor manufacturing—has intensified pressure on global water security [1]. As a pervasive aquatic pollutant, ammonia nitrogen (NH₃/NH₄⁺) degrades water quality by driving eutrophication and depleting dissolved oxygen, while also serving as a precursor for the formation of carcinogenic byproducts (e.g., nitrosamines) through environmental transformations [2]. With regulatory frameworks such as the EU Water Framework Directive and US EPA effluent guidelines imposing stringent ammonia discharge limits (<1–2 mg-N L⁻¹), the development of robust, energy-efficient treatment technologies has become imperative. Conventional biological nitrification-denitrification processes, though widely adopted, exhibit critical limitations when treating wastewater with extreme salinity (>3% NaCl), low carbon-to-nitrogen ratios (C/N < 2), or inhibitory compounds (e.g., heavy metals) [3,4]. Physicochemical methods, including breakpoint chlorination and air stripping, face operational bottlenecks such as hazardous chlorine gas handling, excessive chemical consumption, and secondary contamination risks from chlorinated byproducts or ammonia-laden air emissions [5,6].

Electrochemical ammonia oxidation reaction (eAOR) has emerged as a disruptive technology that synergizes the merits of biological and physicochemical approaches while circumventing their inherent drawbacks. Unlike conventional methods requiring external oxidant dosing or microbial acclimatization, eAOR enables in situ generation of reactive species through interfacial electron transfer, achieving tunable selectivity toward harmless nitrogen gas (N₂) or nitrate (NO₃⁻) based on operational parameters [7,8]. The process operates via two distinct pathways: direct electrochemical oxidation, where NH₃ undergoes stepwise dehydrogenation and oxidation at the anode surface [9,10,11], and indirect oxidation, mediated by electrochemically generated reactive chlorine species (RCS) such as free chlorine (HClO/ClO⁻) or chlorine radicals (Cl•) [12]. Direct eAOR excels in low-chloride environments but demands electrocatalysts with high intrinsic activity to overcome sluggish reaction kinetics and intermediate poisoning [13]. Indirect eAOR, leveraging chloride ions (Cl⁻) ubiquitously present in industrial effluents, converts Cl⁻ to oxidizing agents via the chlorine evolution reaction (CER), enabling efficient ammonia degradation even in complex matrices. However, both pathways contend with competing side reactions—notably the oxygen evolution reaction (OER)—and undesirable byproduct formation (e.g., chloramines, perchlorates), necessitating strategic optimization of catalysts and operating conditions [14].

Recent advancements in electrocatalyst engineering have revitalized interest in eAOR. For direct oxidation, research focuses on tailoring electronic structures to enhance NH₃ adsorption and weaken N≡N bond formation barriers, while indirect oxidation catalysts prioritize selective CER activation over OER. Despite progress, fundamental challenges persist at the nexus of materials science and process engineering. These include reconciling the trade-off between catalyst durability and activity under harsh electrochemical conditions, suppressing toxic byproduct accumulation across variable water chemistries, and scaling laboratory breakthroughs to industrial systems with fluctuating contaminant loads.

This review systematically synthesizes mechanistic insights, material innovations, and operational considerations shaping the evolution of electrochemical ammonia oxidation. In this review, we first give the systematic comparison and integration of direct and indirect eAOR pathways with a focus on N₂ selectivity; (2) the critical analysis of catalyst design strategies across noble and non-noble metals; and (3) the synthesis of operational challenges and future research directions aimed at practical application. Accordingly, providing a critical knowledge base for the rational design of efficient and scalable electrochemical reactors for ammonia removal and resource recovery in real-world water treatment.

2. Direct eAOR

Direct eAOR is highly dependent on the solution pH [15]. Under acidic conditions, ammonia primarily exists in the form of ammonium ions (NH₄⁺), which are prone to indirect oxidation by external oxidants such as hypochlorous acid (HOCl). This indirect oxidation route tends to suffer from low efficiency and sluggish kinetics. In contrast, alkaline electrolytes facilitate the direct electrochemical oxidation of NH₃, where hydroxide ions (OH⁻) participate as reactants. As a result, most investigations into direct eAOR have been conducted under alkaline aqueous environments, as shown in Figure 1a.

In alkaline media, the anodic and cathodic half-reactions, along with the overall electrochemical process, are represented by the following equations:

Anode: 2NH₃ + 6OH⁻ → N₂ + 6H₂O + 6e⁻ (E⁰ = −0.77 V vs. SHE)

Cathode: 6H₂O + 6e⁻ → 3H₂ + 6OH⁻ (E⁰ = −0.83 V vs. SHE)

Overall: 2NH₃ → N₂ + 3H₂ (E_cell = 0.06 V)

The direct eAOR process involves a six-electron transfer, indicating its complex, multi-step nature. Consequently, several mechanistic studies have been conducted to identify the key steps and intermediates involved in this oxidation pathway. Two major mechanisms have been proposed over the years: the Oswin-Salomon (O-S) mechanism and the Gerischer–Mauerer (G-M) mechanism.

The O-S mechanism was initially proposed by Oswin and Salomon in 1963, suggesting that ammonia undergoes stepwise dehydrogenation on the catalyst surface, ultimately forming adsorbed nitrogen atoms (N_ads), which couple to produce N₂. Their analysis, based on Tafel slope comparisons using Langmuir and Temkin adsorption models, indicated that the conversion of *NH₂ (* denoted as adsorption intermediate) to *NH is the rate-determining step (RDS) at low current densities [18]. The reaction sequence is as follows:

NH₃ (aq) → *NH₃

*NH₃ + OH⁻ → *NH₂ + H₂O + e⁻

*NH₂ + OH⁻ → *NH + H₂O + e⁻

*NH + OH⁻ → *N + H₂O + e⁻

2*N → N₂

On the other hand, the G-M mechanism, introduced by Gerischer and Mauerer in 1970, proposes that N–N coupling occurs at the intermediate stage rather than after full dehydrogenation. In this pathway, *NH₂ and *NH species can dimerize to form N₂H_x+y-type intermediates (e.g., hydrazine), which subsequently undergo electrochemical oxidation to generate N₂ [19]. The reaction proceeds as follows:

NH₃ (aq) → *NH₃

*NH₃ + OH⁻ → *NH₂ + H₂O + e⁻

*NH₂ + OH⁻ → *NH + H₂O + e⁻

*NH_x + *NH_y → *N₂H_x+y

*N₂H_x+y + (x + y)OH⁻ → N₂ + (x + y)H₂O + (x + y)e⁻

*NH_ads + OH⁻ → *N + H₂O + e⁻

One notable feature of the G-M pathway is that strongly adsorbed nitrogen atoms (*N) can poison the electrode surface by occupying active sites, thereby diminishing the long-term catalytic performance. To evaluate and distinguish between these two mechanisms, density functional theory (DFT) calculations have been carried out for various metal surfaces [16]. On platinum Pt(100), for instance, the O-S mechanism suggests that the deprotonation of *NH₂ to *NH is the potential-determining step (PDS), requiring a free energy of 0.87 eV. The subsequent *N dimerization to form N₂ is kinetically favorable with an activation barrier of 0.53 eV and is thermodynamically downhill (−1.69 eV). In contrast, the G-M mechanism identifies the deprotonation of HNNH* to NNH* as the RDS, with a lower onset potential of 0.29 V (Figure 1b). For Ir(100), the onset potential under the O-S mechanism is 0.47 V, and the barrier for N* dimerization increases to 1.24 eV. The G-M mechanism for Ir(100) involves NNH* deprotonation as the PDS with an onset potential of 0.46 V (Figure 1c).

Although both mechanisms involve proton-coupled electron transfer (PCET) steps and appear pH-independent on the reversible hydrogen electrode (RHE) scale, experimental studies by Katsounaros et al. [20] introduced a revised viewpoint. They observed that increasing the pH of the electrolyte reduces the onset potential for Pt(100) and suppresses the formation of *NO. They proposed that OH⁻ ions (not *OH) directly participate in a decoupled proton–electron transfer step, forming negatively charged intermediates that exhibit pH sensitivity:

*N_xH_y + OH⁻ ⇌ *(N_xH_y−1)⁻ + H₂O

*(N_xH_y−1)⁻ ⇌ *N_xH_y−1 + e⁻

This modification implies that the electrochemical step does not occur concurrently with proton removal, leading to distinct mechanistic behavior compared to classical PCET assumptions.

To validate the proposed mechanisms, several experimental techniques have been employed, including surface-enhanced Raman spectroscopy (SERS), differential electrochemical mass spectrometry (DEMS), and rotating ring-disk electrode analysis. For instance, De Vooys et al. [21] used SERS to identify *N species on gold and palladium electrodes, supporting the existence of poisoning intermediates consistent with the G-M model. Hydrazine (N₂H₄), one of the key intermediates, has been detected using aggregation-induced emission, further confirming its role in the N₂ formation pathway (Figure 1d) [17].

Moreover, a reaction involving azide ions (N₃⁻) has also been proposed as a potential intermediate route:

NH₃ + N₂H₄ + 7OH⁻ ⇌ N₃⁻ + 7H₂O + 6e⁻

Pt + N₃⁻ ⇌ PtN₃⁻

PtN₃⁻ + 2H₂O + e⁻ ⇌ PtNH₂ + 2OH⁻ + N₂

The presence of N₃⁻ was confirmed by SERS, offering the first experimental evidence for its involvement in eAOR [22].

Further analysis by Vidal-Iglesias et al. [23] using DEMS revealed that ammonia oxidation on different Pt single crystal facets ((100), (110), and (111)) produces various products including N₂, NO, and N₂O, with Pt(100) showing the highest activity. Wasmus et al. [24] and Gootzen [25] et al. also demonstrated that nitrogen gas can be produced at potentials below 0.7 V vs. RHE, while nitrogen oxides dominate at higher anodic potentials (>0.8 V). Under such conditions, further oxidation can lead to the formation of nitrite (NO₂⁻) and nitrate (NO₃⁻) via [26,27]:

2NO₂ + 2OH⁻ → NO₃⁻ + NO₂⁻ + H₂O

NO + NO₂ + 2OH⁻ → 2NO₂⁻ + H₂O

In conclusion, Figure 2 integrates the O-S and G-M mechanisms with supporting experimental evidence, offering a comprehensive picture of the reaction landscape for electrochemical ammonia oxidation.

3. Indirect eAOR via RCS

Indirect eAOR exploits the in situ generation of RCS from chloride ions, which are ubiquitous in many industrial and municipal wastewaters. Unlike direct eAOR, where NH₃ is oxidized through surface-mediated dehydrogenation, the indirect pathway relies on chlorine-derived oxidants that can rapidly and non-selectively attack ammonia, leading to the formation of nitrogen gas or oxidized nitrogen products. Two main mechanistic routes dominate this process: the well-established breakpoint chlorination pathway and radical-driven pathways where Cl• and ClO• species play decisive roles. While the indirect ammonia oxidation benefits from high reactivity and effectiveness in saline matrices, it introduces challenges associated with chloramine accumulation, halogenated byproducts, and the balance between molecular chlorine and radical pathways [28]. Understanding these dual routes is essential for developing selective and environmentally benign indirect eAOR systems.

3.1. Breakpoint Chlorination Oxidation Mechanism

Breakpoint chlorination has long been recognized as a classical pathway for chemical ammonia removal and is also operative in electrochemical chlorine-mediated oxidation systems. In this process, chloride ions (Cl⁻) are oxidized at the anode to generate free chlorine species such as Cl₂, HOCl, and ClO⁻. These active chlorine species subsequently react with ammonia to form chloramines (NH₂Cl, NHCl₂, and NCl₃), which undergo further oxidation and decomposition, ultimately leading to the production of nitrogen gas (N₂) [29]. The reaction sequence is often divided into three main stages: (i) rapid chloramine formation, (ii) stepwise conversion and oxidation of chloramines, and (iii) final destruction of residual chloramines at the so-called “breakpoint,” yielding nitrogenous products (Figure 3a).

The stoichiometric requirement for complete conversion of ammonia to nitrogen gas is typically achieved when the molar ratio of chlorine to nitrogen (Cl/N) exceeds 1.6 [5,30,31]. At this ratio, chloramines are no longer stable intermediates and are further oxidized to nitrogen gas or nitrate [32]. Despite its efficiency, the breakpoint chlorination pathway suffers from several drawbacks: (i) the demand for high concentrations of active chlorine, (ii) the accumulation of intermediate chloramines, which may exert ecological and toxicological risks, and (iii) the potential formation of halogenated byproducts such as trihalomethanes and haloacetic acids. In electrochemical systems, these challenges are partially mitigated by the in situ generation of active chlorine, yet they remain significant hurdles for practical water treatment applications.

3.2. Chlorine Radical Oxidation Mechanism

Beyond molecular chlorine species, chlorine radicals (Cl• and ClO•) generated under anodic polarization have been shown to play a decisive role in indirect electrochemical ammonia oxidation [33,34,35]. These highly reactive transient species possess higher oxidation potentials and exhibit faster reaction kinetics than HOCl or ClO⁻ [33]. Experimental and computational studies suggest that Cl• can abstract hydrogen atoms directly from NH3, forming amidogen radicals (•NH₂), which subsequently couple or react with chlorine-derived oxidants to yield N₂. Similarly, ClO• can act through single-electron transfer pathways to facilitate ammonia dehydrogenation and radical chain propagation [12,36,37,38] (Figure 3b).

A key distinction between the breakpoint chlorination and chlorine radical mechanisms lies in the onset conditions and product distributions. Whereas breakpoint chlorination generally requires bulk Cl/N ratios >1.6, chlorine radical pathways can initiate ammonia oxidation even at much lower Cl/N ratios, often within the diffusion boundary layer of the anode, where radical concentrations are transiently high [39]. This helps explain why chloramines are sometimes undetectable during electrolysis, as the reactions occur within micrometer-scale regions before intermediates can diffuse into the bulk solution [37]. Nevertheless, the high reactivity of chlorine radicals can also trigger side reactions with coexisting organic matter or inorganic anions, increasing the risk of halogenated disinfection byproducts (DBPs) [40]. Balancing efficiency with selectivity, therefore, remains a critical challenge.

4. Fundamentals of the Design of Catalysts in eAOR

The efficiency and selectivity of eAOR are intrinsically governed by the properties of the electrocatalyst. In direct eAOR, catalysts must overcome sluggish multi-electron transfer kinetics, suppress electrode poisoning by nitrogen intermediates, and steer product distribution toward N₂ rather than undesired NOₓ species. Noble metals such as Pt and Ir exhibit excellent intrinsic activity but suffer from high costs and stability issues, motivating the development of alloyed systems and earth-abundant transition metal alternatives. In contrast, indirect eAOR catalysts focus on promoting the CER over competing OER, stabilizing reactive chlorine species, and resisting chlorine-induced corrosion. Both pathways increasingly rely on advanced design concepts such as single-atom dispersion, heterostructure engineering, and operando-guided tuning of electronic structures. These principles form the foundation for translating laboratory insights into scalable catalytic systems capable of addressing the dual goals of pollutant removal and sustainable nitrogen management. To this end,

Table 1. Performance comparison of metal-based catalysts for direct eAOR.

Anode Material	Electrolyte	Condition (mV s−1)	Onset Potential	Peak Potential	Peak Current Density	References
Pt	0.05 mol L−1 KOH + 0.1 mol L−1 NH3	50	0.4 V vs. RHE	0.7 V vs. RHE	–	[22]
Pt/SiO2-CNT-COOH	1.0 mol L−1 KOH + 0.1 mol L−1 NH3	5	0.484 V vs. RHE	–	77.3 A g−1	[41]
PtIrNi1/SiO2-CNT-COOH	1.0 mol L−1 KOH + 0.1 mol L−1 NH3	5	0.399 V vs. RHE	–	124 A g−1	[41]
NiO-TiO2	100 mmol L−1 NaNO3 + 200 mmol L−11 NH4+	100	0.5 V vs. Hg/HgO	~0.9 V	–	[8]
NiCo oxyhydroxide/ Ni foam	0.1 mol L−1 Na2SO4 + 50 mmol L−1 NH3	10	–	–	–	[13]
Ni(OH)2/NiOOH	0.1 mol L−1 Na2SO4 + 0.003 mol L−1 NH3	100	0.65 V vs. Hg/HgO	0.9 V	–	[42]
Ni/NiOOH	0.01 mol L−1 Na2SO4 + 50 ppm NH3	10	0.6 V vs. Hg/HgO	1.0 V	–	[43]
Ni0.8Cu0.2LHs	0.5 mol L−1 NaOH + 55 mmol L−1 NH4Cl	25	0.4 V vs. Ag/AgCl	~0.8 V	–	[44]
NiCu/CP electrode	1 mol L−1 NaOH + 55 mmol L−1 NH4Cl	25	0.47 V vs. Ag/AgCl	~0.7 V	–	[45]
Ni0.8Cu0.2 hydroxide/ oxyhydroxide	0.1 mol L−1 KOH + 1 mmol L−1 NH4+	10	1.4 V vs. RHE	–	–	[44]
NiCo2N	NH3 saturated 0.1 mol L−1 KPF6	–	0.55 V vs. NHE	–	–	[46]
(Mn,Fe,Co,Ni,Cu)3O4 oxides	NH3 saturated 0.1 mol L−1 KPF6	–	0.7 V vs. NHE	–	–	[47]

provides a performance comparison of representative metal-based catalysts.

4.1. Noble Metal-Based Catalysts for Direct eAOR

Noble metals have long been regarded as highly active electrocatalysts due to their favorable electronic structures, which enable effective adsorption and activation of reaction intermediates. In the context of direct eAOR, noble metal-based catalysts—especially those involving Pt, Ir, and their alloys—exhibit relatively high activity toward N₂ formation, attributed to their strong affinity for adsorbed nitrogen species [48]. In contrast, coinage metals such as Cu, Ag, and Au exhibit weak binding energy for nitrogen species, rendering them catalytically inactive for NH₃ dehydrogenation [49].

De Vooys et al. [50] systematically evaluated the performance of noble metals and demonstrated that 5d metals (Pt, Ir) possess relatively high steady-state activity for selective N₂ production, whereas 4d metals (Ru, Rh, Pd) show limited catalytic ability toward this reaction. The coinage metals, though thermodynamically capable, are kinetically hindered and generally fail to initiate the dehydrogenation of ammonia. This difference is rooted in their respective binding energies toward nitrogen species, which dictate the desorption or further transformation of NH_x intermediates on the catalyst surface.

Among noble metals, Pt has been widely recognized as one of the most efficient catalysts for direct eAOR. sts reveal that the Pt surface is prone to deactivation at this potential due to the accumulation of poisoning intermediates, which block active sites and hinder continuous catalytic turnover.

To gain deeper mechanistic insights, Ir-based catalysts have been investigated through in situ surface-sensitive techniques. Wei et al. [51] employed attenuated total reflection-surface enhanced infrared absorption spectroscopy (ATR-SEIRAS), a powerful tool capable of probing interfacial species at the molecular level [52,53]. The cyclic voltammetry (CV) profiles of Ir electrodes show two prominent oxidation peaks at approximately 0.2 V and 0.62 V vs. RHE (Figure 4a).

In situ spectral analysis revealed that the low-potential peak (0.2 V) corresponds to the initial dehydrogenation of NH₃, generating *H and NHₓ intermediates. These species can dimerize on the Ir surface to form *N₂H_x+y. The higher-potential peak (0.6 V) is attributed to the further oxidation of these dimerized intermediates to form molecular N₂ (Figure 4b). At potentials beyond 0.55 V vs. RHE, a competing pathway emerges, wherein *NH_x is partially oxidized to NO₂⁻ or NO₃⁻, reducing the N₂ selectivity. The proposed eAOR mechanism on Ir surfaces involves dual potential-dependent routes: (1) the N₂ formation pathway through NH₃ dehydrogenation and N–N coupling at lower potentials, and (2) the oxygenated byproduct formation at higher potentials (Figure 4c).

Palladium (Pd), another 4d metal with strong *N adsorption, has also been evaluated for direct eAOR. However, both theoretical predictions and experimental data suggest low overall activity on Pd surfaces. CV measurements in 1 mol L⁻¹ KOH (with and without 0.1 mol L⁻¹ NH₃) show weak anodic features (Figure 4d), consistent with its limited catalytic performance. Surprisingly, DEMS detected an N₂ signal at m/z = 28 (Figure 4e), suggesting that N₂ may still be formed Via surface-mediated reactions. Furthermore, when Pd electrodes were examined in a non-aqueous NH₃-saturated acetonitrile solution, a noticeable increase in anodic current and N₂ generation was observed. [55] These results suggest that surface oxygenated species may play a dual role: they can deactivate active Pd sites but may also facilitate partial NH₃ oxidation under specific conditions, such as the surface coverage of oxygen.

To address the activity–stability trade-offs of single noble metals, alloying strategies have emerged as a promising approach. Silva et al. [54] reported the synthesis of IrRh nanoparticles supported on carbon substrates to optimize catalytic performance while reducing the reliance on expensive Ir. Ir, although catalytically competent and relatively resistant to poisoning of adsorbed nitrogen species, suffers from scarcity and cost issues. By forming IrRh alloys with tunable compositions (Figure 4f), the authors were able to adjust the electronic properties of the active sites. Electrochemical evaluation in 1 mol L⁻¹ KOH + 2 mol L⁻¹ NH₄OH revealed that certain Ir/Rh ratios led to a pronounced enhancement in peak current density compared to monometallic Ir/C catalysts (Figure 4g). This performance enhancement is likely due to synergistic effects between Ir and Rh atoms, which modulate the binding strength for nitrogen species and facilitate the desorption of final products.

In summary, noble metal-based catalysts, particularly Pt and Ir, exhibit superior catalytic activity toward direct eAOR owing to their favorable energetics for nitrogen species and their ability to mediate multi-electron transfer processes. However, issues such as intermediate poisoning, limited selectivity, and high material cost constrain their large-scale application. Strategies including alloy engineering (e.g., IrRh) and the utilization of in situ diagnostic techniques (e.g., ATR-SEIRAS, DEMS) provide valuable insights and design directions for developing next-generation direct eAOR catalysts with enhanced activity, selectivity, and stability.

4.2. Non-Noble Metal in Direct eAOR

Direct eAOR holds significant promise for environmental remediation and energy applications, but the widespread use of noble metal catalysts such as Pt is severely limited by their high cost and susceptibility to rapid deactivation, especially due to poisoning by nitrogen intermediates [41]. Consequently, the development of low-cost, stable, and efficient non-noble metal electrocatalysts has become a critical research frontier. Among these, nickel (Ni)-based catalysts have attracted considerable attention due to their earth abundance, low cost, chemical stability, and versatility in surface modification, making them highly promising candidates for eAOR (Figure 5a) [43,56,57,58,59].

Despite the advantageous properties of Ni, pristine Ni metal exhibits inherently low catalytic activity for ammonia oxidation in alkaline media. This low activity primarily arises from the spontaneous formation of a surface nickel oxide (NiO) or hydroxide layer, which adversely affects ammonia adsorption and activation, as explained by valence bond orbital theory. The presence of NiO reduces the binding affinity for NH₃ molecules, thus limiting their effective oxidation. Moreover, the oxidation potential of Ni overlaps with that of NH₃, further hindering direct catalytic activity [60,61]. Initial experimental studies, such as those employing Raney Ni electrodes, confirmed these intrinsic limitations of pure Ni under typical eAOR conditions [62].

Recent mechanistic studies have demonstrated that the true active catalytic species are not the metallic Ni itself but rather the electrochemically generated nickel hydroxides and oxyhydroxides, specifically Ni(OH)₂ and NiOOH, which form on the electrode surface under anodic polarization (Figure 5b). The electrochemical cycling induces a phase transformation sequence: metallic Ni → NiO_x → α-Ni(OH)₂ → β-Ni(OH)₂ → β-NiOOH (Figure 5c). These oxyhydroxide phases possess favorable electronic and structural properties that facilitate the adsorption and oxidation of ammonia, promoting its conversion into nitrogen (N₂) and oxidized nitrogen species such as nitrite (NO₂⁻) and nitrate (NO₃⁻).

The catalytic activity enhancement is attributed to the redox cycling between Ni(II) and Ni(III) states in NiOOH, which mediates electron transfer processes and enables the formation of key reaction intermediates such as NH_x monolayers. Electrochemical characterization, including CV, confirms that the formation of NH_x intermediates occurs prominently within the potential range of 0.6–0.8 V vs. Hg/HgO, where N₂ selectivity is maximized, whereas higher potentials tend to favor the generation of NO₂⁻ and NO₃⁻ (Figure 5d). Advanced spectroscopic methods such as X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy have corroborated the presence and dynamic transformation of these nickel oxyhydroxide species during the reaction (Figure 5e).

The efficiency and selectivity of Ni-based catalysts for eAOR are highly sensitive to reaction parameters such as pH, temperature, and applied current density [42,43]. Experimental evidence indicates that alkaline conditions (pH > 9.5) favor ammonia oxidation with enhanced N₂ selectivity due to the predominance of un-ionized NH₃ species (Figure 5f). Elevated temperatures generally promote higher N₂ selectivity while reducing nitrite and nitrate yields, likely due to thermodynamic and kinetic effects on the oxygen evolution and nitrogen evolution reactions. Moreover, lower current densities tend to favor N₂ formation, whereas higher current densities increase NO₃⁻ selectivity but suppress N₂ generation. These trends reflect the complex interplay between electrochemical potential, surface adsorbates, and competing reaction pathways.

To overcome the intrinsic activity limitations of pure Ni catalysts, alloying with other transition metals has emerged as an effective strategy to tailor the electronic structure, surface adsorption properties, and morphology of Ni-based catalysts. Among various candidates, copper (Cu) has been identified as an optimal alloying element due to its complementary electronic effects and catalytic behavior [63]. NiCu alloys, particularly in the form of layered double hydroxides (LDHs) or double hydroxides (DHs), have been synthesized with diverse morphologies such as nanoflowers, nanonets, and nanotyres [64,65,66,67,68]. The incorporation of Cu enhances catalytic activity by facilitating charge transfer, increasing the electrochemically active surface area (ECSA), and modulating the adsorption energies of reaction intermediates. The synergistic effect arises because NiOOH serves as a conductive backbone enabling rapid electron transport, while Cu(OH)₂ sites modify the reaction pathways, favoring the formation of intermediates such as *NO₂ and ultimately leading to higher selectivity toward NO₂⁻ and NO₃⁻ products.

While NiCu alloys are desirable in producing NO₂⁻ and NO₃⁻, their ability to selectively generate N₂ remains limited [44,45,69,70]. To address this, the incorporation of a third element, such as cobalt (Co), has been explored. Co exhibits strong bonding with nitrogen but weak interaction with hydrazine-like intermediates, facilitating the dimerization of NH_x species into N₂ rather than partial oxidation to NO_x species [71]. NiCo and NiCoCu ternary alloys, including layered double hydroxides and metal nitrides (e.g., NiCo₂N), have demonstrated remarkable improvements in N₂ selectivity, often exceeding 60–90%. These materials balance electronic structure to optimize NH₃ adsorption, NH_x radical formation, and final N₂ desorption [13,46,72]. DFT calculations and advanced spectroscopic characterizations reveal that moderate contraction of the d-band center enhances metal-NH₃ bonding and stabilizes key cation radical intermediates essential for N₂ formation, while suppressing side reactions that lead to soluble metal-amine complexes [73,74].

Furthermore, high-entropy alloys and spinel oxides containing multiple transition metals (Mn, Fe, Co, Ni, Cu) have shown promise due to their unique electronic structures, which provide abundant active sites and optimal binding energies for efficient and selective ammonia oxidation to N₂ [47].

4.3. Fundamentals of the Design of Catalysts in Indirect eAOR

The design of electrocatalysts for indirect eAOR must address two often competing objectives: maximizing the generation of RCS for efficient ammonia degradation, while minimizing energy consumption and the formation of toxic byproducts. Unlike direct eAOR, which relies on the intrinsic activity of the catalyst surface toward NH₃ adsorption and dehydrogenation, indirect eAOR is mediated by chlorine-containing oxidants electrogenerated at the anode. This distinction necessitates a unique set of material design principles, emphasizing stability under harsh oxidative conditions, selective chlorine evolution, and resistance to corrosion. To evaluate the efficiency of different anodic materials in meeting these design criteria, Table 2 summarizes the performance of various anodes for ammonia oxidation reported in different studies.

4.3.1. Noble Metal and Metal Oxide Anodes

Active chlorine and chlorine radicals are produced at high anodic potentials, requiring materials with high oxidation resistance. Noble metal-based anodes, such as Pt, Ir, Ru, and their oxides, have been widely adopted due to their excellent conductivity, catalytic activity, and corrosion stability [75,76,77,78]. For instance, Ti/RuO₂ and Ti/IrO₂–RuO₂ are among the most commonly used dimensionally stable anodes (DSAs) in chloride-mediated eAOR systems. Zhang et al. [79] demonstrated that a Ti/IrO₂–RuO₂ anode could remove 50 mg L⁻¹ of ammonia within 30 min (Figure 6a). Similarly, PbO₂/Ti anodes have shown promise, achieving over 85% removal of both COD and ammonia within 6 h [36]. Recent studies suggest that Ti-, Ru-, and Ir-based anodes may exhibit higher active chlorine production efficiencies compared to Pt or Pb anodes [80]. Novel non-noble metal anodes, such as WO₃–Sb/SnO₂ and 3D Co₃O₄ nanowires, have also been developed, demonstrating the complete degradation of ammonia and phenolic compounds within 45–60 min (Figure 6b) [81,82]. To improve the electron transfer performance in indirect eAOR, Xu et al. [83] used the MXene substrate to support the Co₃O₄ catalysts, and the obtained Co₃O₄/MXene anode exhibited 97.8% ammonium removal within 5 h. In addition, the Co₃O₄/MXene electrode exhibited exceptional stability and achieved desirable degradation efficiency for actual tannery wastewater, with the desirable Faradaic Efficiency (24.12%) and energy consumption (85 kWh/kg-N). Wu et al. [84] demonstrated that micro-doping Co₃O₄ with Sn and Bi effectively enhances its CER performance by modulating the electronic structure. This synergistic doping creates electron-deficient Co(III) sites, enriches surface Cl⁻, and ultimately leads to superior catalytic activity, selectivity, and long-term stability.

4.3.2. Non-Metal and Diamond-Based Electrodes

Non-metal anodes, particularly boron-doped diamond (BDD), offer a wide potential window (up to 4.5 V), high oxygen evolution potential, and exceptional electrochemical stability. These properties make BDD highly suitable for indirect eAOR, especially in complex water matrices. Luu [85] reported that a Ti/BDD anode achieved over 80% ammonia removal from landfill leachate (initial concentration 1006–1197 mg L⁻¹) after 8 h of treatment (Figure 6c). Although BDD may show slightly lower ammonia removal efficiency compared to Ti/RuO₂, it often outperforms metal oxides in COD removal. Díaz et al. [86] demonstrated complete removal of 8 mg L⁻¹ ammonia within 45 min using a BDD anode. Other non-metal materials like graphite have been less widely adopted due to higher wear rates and lower efficiency in chlorine production [87].

4.3.3. Membrane Electrodes and Hybrid Materials

Membrane electrodes have gained attention for their large surface area and efficient contact with wastewater, enabling enhanced mass transfer and faster ammonia oxidation rates. These electrodes function similarly to filter membranes and are often used in electrochemical filtration systems. Sun et al. [80] developed a RuO₂/TiO₂ nanowire (NW) membrane anode that achieved 99.88% ammonia oxidation within 3 h under optimized conditions (200 mg L⁻¹ NH₃, 2000 mg L⁻¹ Cl⁻, 20 mA cm⁻², pH 9) (Figure 6d). The electrode also exhibited remarkable stability over 100 h of continuous operation. Pan et al. [88] constructed an electrochemical membrane reactor (EMR) featuring a coal-based carbon membrane as a flow-through anode for ammonia nitrogen wastewater decontamination. Under optimized conditions, the system achieved complete ammonia nitrogen removal with a low energy consumption of 0.0380 kWh/g-NH₄-N, demonstrating significant potential for practical application. To balance corrosion resistance and plasticity, recent studies have explored hybrid structures incorporating noble metals into low-cost substrates. For example, Chi et al. [89] electrodeposited RuO₂ nanospheres on lignin-derived carbon, achieving a low resistance of 0.91 Ω and a high current density of 129 mA cm⁻² at 1.2 V. Li et al. [90] fabricated a Ti/Ir-coated ceramic membrane anode that removed 93.5% of ammonia and 69.4% of dissolved organic nitrogen within 60 min, demonstrating stability over 14 days of continuous operation.

In summary, the indirect eAOR catalysts must (i) favor the chlorine evolution reaction (CER) over the OER, (ii) anode materials must withstand prolonged exposure to corrosive RCS, (iii) the balance between molecular chlorine (HOCl/ClO⁻) and radicals (Cl•, ClO•) should be considered for modulate the ammonia oxidation efficiency and byproduct formation, (iv) high-surface-area structures (e.g., nanowires, nanospheres, porous membranes) are important for enhancing mass transport and active site availability, improving overall catalytic performance. That is, the design of catalysts for indirect eAOR requires a multidisciplinary approach that integrates materials science, electrochemistry, and environmental engineering. Future efforts should focus on developing low-cost, high-efficiency, and durable anodes that can operate effectively across diverse water quality conditions while minimizing energy input and secondary pollution.

Table 2. Different anodes for ammonia oxidation and their effect on ammonia oxidation in different studies.

Anode Material	Rate Constant or Removal Efficiency	Concentration of Ammonia	Concentration of Chlorine	Current Density/Potential	References
Ti/IrO2–RuO2	100%/25 min	50 mg L−1	20 mM	5 mA cm−2	[79]
PbO2/Ti	0.306 h−1	78.2 mg L−1	40 mM	37.5 mA cm−2	[36]
WO3-Sb/SnO2	3.516 h−1	30 mg L−1	50 mM	2.0 V (VS Ag/AgCl)	[82]
3D Co3O4 NWs	4.836 h−1	56 mg L−1	50 mM	1.8 V (VS Ag/AgCl)	[81]
Co3O4/MXene	0.6059 h−1	200 mg L−1	50 mM	20 mA cm−2	[83]
Sn, Bi co-doped Co3O4	0.0055 min−1	100 mg L−1	600 mM	10 mA cm−2	[84]
Ti/BDD	>80%/480 min	1006–1197 mg L−1	1880–2700 mg L−1	17–83 mA cm−2	[85]
BDD	100%/45 min	8 mg L−1	26167 mg L−1	50 A m−2	[86]
RuO2/TiO2 NW membrane	99.876%/180 min	200 mg L−1	2000 mg L−1	20 mA cm−2	[80]
coal-based carbon membrane	100%/120 min	30 mg L−1	100 mM	2.8 V	[88]
Ceramic membrane	93.5%/60 min	8.8 mg L−1	1781.9 mg L−1	25 mA cm−2	[90]

Table 2. Different anodes for ammonia oxidation and their effect on ammonia oxidation in different studies.

Anode Material	Rate Constant or Removal Efficiency	Concentration of Ammonia	Concentration of Chlorine	Current Density/Potential	References
Ti/IrO2–RuO2	100%/25 min	50 mg L−1	20 mM	5 mA cm−2	[79]
PbO2/Ti	0.306 h−1	78.2 mg L−1	40 mM	37.5 mA cm−2	[36]
WO3-Sb/SnO2	3.516 h−1	30 mg L−1	50 mM	2.0 V (VS Ag/AgCl)	[82]
3D Co3O4 NWs	4.836 h−1	56 mg L−1	50 mM	1.8 V (VS Ag/AgCl)	[81]
Co3O4/MXene	0.6059 h−1	200 mg L−1	50 mM	20 mA cm−2	[83]
Sn, Bi co-doped Co3O4	0.0055 min−1	100 mg L−1	600 mM	10 mA cm−2	[84]
Ti/BDD	>80%/480 min	1006–1197 mg L−1	1880–2700 mg L−1	17–83 mA cm−2	[85]
BDD	100%/45 min	8 mg L−1	26167 mg L−1	50 A m−2	[86]
RuO2/TiO2 NW membrane	99.876%/180 min	200 mg L−1	2000 mg L−1	20 mA cm−2	[80]
coal-based carbon membrane	100%/120 min	30 mg L−1	100 mM	2.8 V	[88]
Ceramic membrane	93.5%/60 min	8.8 mg L−1	1781.9 mg L−1	25 mA cm−2	[90]

5. Key Environmental Factors in Practical Water Treatment

The practical application of electrochemical ammonia oxidation (eAOR) is governed not only by catalyst properties but also by environmental parameters that dictate the kinetics, selectivity, and byproduct profiles of both direct eAOR and indirect, chlorine-mediated eAOR. Understanding these environmental influences is essential to bridging the gap between laboratory-scale studies and real-world water treatment systems.

5.1. pH Dependence and Speciation Effects

In direct pathways, ammonia oxidation strongly depends on the acid–base equilibrium of ammonia (pKa ≈ 9.25). At alkaline pH, NH₃ is the dominant species, which adsorbs readily onto electrode surfaces and undergoes dehydrogenation according to the Oswin-Salomon or Gerischer–Mauerer mechanisms [18,19]. Alkaline conditions also favor OH−-assisted proton–electron transfer steps, lowering the onset potential for ammonia oxidation and improving selectivity toward N₂. Conversely, in acidic or neutral environments, ammonia exists mainly as NH₄⁺, which adsorbs poorly on metal surfaces, suppressing direct eAOR efficiency.

For chlorine-mediated oxidation, pH controls both the speciation of chlorine (HOCl vs. ClO⁻) and ammonia (NH₃ vs. NH₄⁺). HOCl, predominant at near-neutral pH, is a much stronger oxidant than ClO⁻ and react sefficiently with NH₄⁺ to form chloramines [91]. Acidic conditions, therefore, favor breakpoint chlorination. In contrast, alkaline solutions promote the formation of ClO• radicals and direct NH₃ attack by RCS, enhancing radical-driven pathways [92]. However, too high alkalinity lowers the reactivity of active chlorine and shifts selectivity toward nitrate formation [93]. Hence, optimal performance often requires pH tuning to balance reactivity, selectivity, and byproduct control.

5.2. Formation of Nitrogenous and Halogenated Byproducts

The principal challenge for direct oxidation is the generation of nitrogen oxides (NO₂⁻, NO₃⁻, N₂O) at elevated anodic potentials. While noble metal catalysts such as Pt and Ir can selectively yield N₂ at moderate potentials, higher bias leads to overoxidation of intermediates and accumulation of undesired NO_x species [51,94]. These not only reduce nitrogen selectivity but also pose environmental risks. Maintaining potential windows and designing catalysts that stabilize N–N coupling intermediates are critical for minimizing NO_x formation.

In chlorine-mediated pathways, the most concerning byproducts are chloramines (NH₂Cl, NHCl₂, NCl₃), oxychlorine species (ClO₃⁻, ClO₄⁻), and halogenated organics (e.g., trihalomethanes, haloacetic acids). Their formation is influenced by chlorine dosage (Cl/N ratio), current density, NOM content, and electrolysis duration. For example, exceeding the breakpoint Cl/N ratio significantly increases DBP generation [95]. Advanced catalysts that favor radical oxidation over molecular chlorine pathways and integrated treatment trains (e.g., adsorption, biological polishing) are being explored to mitigate these risks.

5.3. Background Matrix Effects and Coexisting Constituents

Inorganic ions such as sulfate, carbonate, and phosphate can influence direct eAOR by modifying the electrolyte conductivity, buffering capacity, and electrode surface chemistry. Carbonate/bicarbonate ions may alter local pH near the electrode, indirectly affecting NH₃ adsorption. Transition metal ions or heavy metals present in industrial effluents can poison the active sites of catalysts, further reducing their stability and efficiency.

In chlorine-mediated systems, background constituents play a more pronounced role because many species can scavenge reactive chlorine. Bicarbonate (HCO₃⁻) reacts with chlorine radicals to form CO₃^•−, which has much lower reactivity toward ammonia [96]. Nitrite (NO₂⁻) competes strongly with NH₃ for HOCl, reducing ammonia oxidation rates while forming secondary nitrogenous DBPs [97]. Natural organic matter (NOM) is a particularly strong competitor, consuming HOCl and radicals and acting as a precursor to halogenated DBPs [98]. Consequently, in natural waters with high NOM or alkalinity, chlorine-mediated eAOR efficiency is often lower than in synthetic laboratory solutions. In real water treatment scenarios, the selection between direct and indirect eAOR—or a hybrid strategy—should be guided by the specific water matrix. Direct eAOR is more suitable for chloride-deficient streams (e.g., inland industrial effluents), whereas indirect eAOR is advantageous in saline or chloride-rich wastewaters (e.g., aquaculture, coking wastewater, seawater-based systems). However, both are strongly conditioned by environmental parameters such as pH, coexisting ions, and NOM. The integration of pretreatment (e.g., NOM removal), operational control (pH and potential optimization), and post-treatment (adsorption or biological polishing) is necessary to ensure high ammonia removal efficiency with minimal byproduct risks.

6. Conclusions and Outlook

Electrochemical ammonia oxidation (eAOR) has emerged as a versatile and promising strategy for nitrogen removal and transformation in water treatment, benefiting from its flexibility, environmental compatibility, and capacity for integration with renewable electricity. Both direct eAOR and indirect, chlorine-mediated eAOR play pivotal roles depending on water chemistry, catalyst design, and treatment goals. Direct eAOR operates primarily via surface adsorption and stepwise dehydrogenation of NH₃ under alkaline conditions, and Pt and Ir offer high intrinsic activity but suffer from cost and poisoning, while Ni (in its NiOOH form) and NiCo alloys are promising non-noble alternatives due to their optimal d-band center for facilitating N–N coupling. This pathway offers the advantage of avoiding halogenated byproducts but is challenged by sluggish kinetics, poisoning of electrode surfaces by *N intermediates, and competing side reactions that yield NO₂⁻, NO₃⁻, or N₂O. Recent advances in catalyst design—including noble metal alloys, transition metal oxides, and single-atom catalysts—have improved activity and nitrogen selectivity, yet durability and cost remain pressing concerns.

Indirect eAOR, mediated by free chlorine and reactive chlorine species (RCS), leverages the abundance of chloride in many wastewaters. And the crucial properties are high CER selectivity and corrosion resistance. RuO₂ and IrO₂-based DSAs are benchmarks, while non-noble options like Co₃O₄ and doped diamond (BDD) are emerging for their stability and radical-generation capabilities. Through mechanisms of breakpoint chlorination and radical oxidation, indirect pathways provide rapid and effective ammonia degradation, even at low concentrations. However, their reliance on reactive chlorine introduces environmental challenges: chloramine accumulation, oxychlorine formation (ClO₃⁻, ClO₄⁻), and halogenated organic byproducts (e.g., THMs, HAAs). Catalyst innovations that tune the balance between free chlorine and chlorine radicals, suppress side reactions, and enhance corrosion resistance are central to improving the sustainability of this approach.

Looking forward, several key research directions can be identified:

Mechanistic Elucidation and In Situ Characterization. Despite progress, the detailed interplay between direct electron transfer, free chlorine, and chlorine radicals remains insufficiently understood. Advanced operando spectroscopies (e.g., Raman, X-ray absorption, EPR) and computational studies are needed to capture short-lived intermediates and map out the competition between nitrogen–nitrogen coupling and overoxidation pathways.

Catalyst Development for Activity, Selectivity, and Stability. Rationally designed catalysts that stabilize key intermediates (*NH₂, *N–N) in direct eAOR and precisely control radical flux in indirect eAOR are critical. Strategies such as single-atom anchoring, defect engineering, and heterostructure interfaces hold promise for achieving both efficiency and longevity, while minimizing noble metal dependence.

Byproduct Minimization and Process Integration. Addressing the generation of NO_x (in direct eAOR) and halogenated DBPs (in indirect eAOR) remains a major bottleneck. Coupling eAOR with downstream adsorption, ion exchange, advanced oxidation, or biological polishing could provide synergistic mitigation. Integrated processes must be tailored to the water matrix to ensure compliance with water quality standards. In addition, the future of eAOR lies not only in pollutant removal but also in resource recovery. Coupling ammonia oxidation with value-added product generation could transform eAOR from a remediation strategy into a sustainable resource management platform, such as two prominent examples of co-production of hydrogen and the recovery of nitrate.

System-Level and Engineering Considerations. Beyond electrode materials, the scalability of eAOR depends on reactor design, energy efficiency, and operational robustness. Three-dimensional porous electrodes, flow-through architectures, and modular electrochemical cells can improve mass transfer and reduce energy input. Life-cycle assessments and techno-economic analyses will be essential to evaluate the feasibility of real-world deployment.

Toward Sustainable and Resource-Oriented Treatment. The future of eAOR lies not only in pollutant removal but also in resource recovery. Coupling ammonia oxidation with hydrogen production, nitrogen fixation to value-added chemicals, or integration into circular nitrogen cycles could transform eAOR from a remediation strategy into a sustainable resource management platform.

In conclusion, electrochemical ammonia oxidation offers a powerful toolbox for addressing nitrogen pollution under diverse water treatment scenarios. The complementary roles of direct and indirect eAOR—one offering halogen-free selectivity, the other leveraging chloride-mediated reactivity—underscore the need for adaptive and context-specific approaches. Through advances in mechanistic understanding, materials innovation, process integration, and system-level optimization, eAOR has the potential to become a cornerstone technology for sustainable nitrogen management in the era of global water challenges.