Opportunities and difficulties are associated with the electrochemical conversion of nitrate to ammonia. Obtaining high selectivity for the desired NH₃ product was the main obstacle. Numerous competing routes are frequently present in electrochemical reactions, which may result in the generation of undesirable by-products. Researchers must design and optimize catalyst materials and electrode topologies to increase selectivity, boost overall efficiency, and ensure ammonia production with minimal side reactions [41] . Energy efficiency is another factor to be considered in electrochemical processes. Electrochemical cells require significant electrical energy to reduce nitrate [42] .

Thus, it is critical to investigate ways to use less energy and to investigate renewable energy sources. Integrating renewable energy sources such as solar or wind energy significantly increases sustainability [43] . Another important factor in the electrochemical reduction process is scalability. Despite encouraging outcomes in laboratory settings, transferring the technology to large-scale applications requires overcoming engineering difficulties [44] . The stability and durability of catalysts and electrodes are essential, and the cost of the materials used in electrochemical cells and catalysts is a significant consideration for practical applications [45] .

The electrochemical reduction of nitrate to ammonia depends on the regulatory and policy considerations. This method requires extensive regulatory frameworks, water quality, ammonia production, and water treatment clearances for large-scale use [46] . Reducing regulatory barriers and promoting sustainable and innovative technologies require the engagement of decision makers and stakeholders. Researchers, enterprises, and water management authorities must collaborate to achieve scientific breakthroughs [47] . Researchers can learn about water treatment facilities and agricultural needs through collaborations. Collaboration makes electrochemical systems compatible with contemporary infrastructure and operating needs.

The electrochemical reduction of nitrate to ammonia requires ongoing research and innovation [48] . Figure 2 shows a flowchart outlining the strategies and steps to improve the efficiency and selectivity of catalysts in electrochemical reduction processes for sustainable nitrate conversion. The flowchart shows the important elements for optimizing the reduction of nitrates, including catalyst design, operational parameters, and product selectivity. This guide will be useful for researchers and engineers seeking to develop more effective and eco-friendly nitrate conversion technologies.

Catalysts convert nitrate to ammonia electrochemically, promoting selective synthesis and inhibiting unwanted by-products. The choice of catalyst material greatly influences the performance and reaction kinetics, with high activity and specific surface area accelerating reaction efficiency [49] . Catalysts with certain surface features and active sites enhance selectivity toward the target product while preventing unintended consequences and increasing energy efficiency. This makes the process economically and environmentally viable [50] .

The electrochemical reduction process runs continuously for a long time because of the capacity of the catalysts to maintain stability and endurance. For scalability and practical application of this technology in large-scale applications such as water treatment facilities and ammonia production for agriculture, it is essential to produce durable and long-lasting catalyst materials [51] . Research and development in catalyst design are crucial for maximizing the effectiveness and selectivity of electrochemical reduction of nitrate to ammonia. Customizing the catalyst quality, investigating new materials, and comprehending the underlying catalytic mechanisms are required to realize the full potential of this technology. Collaboration among researchers in electrochemistry, materials science, and catalysis is essential to enhance catalyst discovery and advance this sustainable and transformative process [52] . The electrochemical reduction of nitrate to ammonia using catalysts has the potential to be an effective and environmentally acceptable method for reducing nitrate pollution and promoting sustainable agriculture [53] .

Catalysts are also incredibly adaptable and tailored to electrochemical processes and water compositions. Researchers have modified the reaction conditions based on the type of wastewater or contaminated water being treated because different catalyst materials demonstrate variable catalytic activity and selectivity degrees. This is significant because contaminants or interfering compounds in real-world water sources affect the effectiveness and selectivity of nitrate-reduction reactions [54] . Catalysts are also created to maximize the reaction at pH values and electrode potentials, improving electrochemical process control and efficiency. The electrochemical reduction of nitrate is customized to satisfy the requirements of various water treatment scenarios, agricultural practices, and energy requirements [55] .

Catalysts help reduce the energy required for electrochemical reduction processes [56] . With the aid of catalysts, the nitrate reduction reaction occurs at lower applied potentials, which lowers the energy required to operate the electrochemical cell. This is crucial for large-scale applications because it directly affects the economically viable and environmentally sustainable potential of the process [57] . During nitrate reduction, the catalysts also aid in creating a stable intermediate, hydrazine (N₂H₄). N₂H₄ stores nitrogen atoms that are subsequently transformed into NH₃ and is essential for NH₃ synthesis. With increased hydrazine generation and stability, catalysts increase NH₃ yields and boost the overall effectiveness of the electrochemical reduction process [58] .

The electrochemical reduction of nitrate to ammonia is more efficient and selective when metal catalysts are used [67] . Many different metal catalysts have been studied, and it is clear that they exhibit unique characteristics and catalytic capabilities [68] . Because of their high activity and stability in electrocatalytic reactions, noble metal catalysts such as platinum (Pt) and palladium (Pd) are ideal for increasing nitrate reduction to ammonia while limiting the generation of undesirable by-products. Because of how long they last, they are excellent stimuli for real-world use [69] . Moreover, complexes of transition metals have shown vigorous catalytic activity in electrochemical nitrate reduction. These metals included copper (Cu), nickel (Ni), iron (Fe), and cobalt (Co). These metal complexes are appealing for efficient and selective nitrate reduction because of their ability to achieve remarkable selectivity for ammonia generation [70] .

Nitrate is converted to NH₃ using metal-oxide catalysts including copper oxide (CuO), silver oxide (Ag₂O), and bismuth oxide (Bi₂O₃) [71] . The one-of-a-kind surface characteristics of these catalysts facilitate electron transport and boost the catalytic efficiency. In addition, metal nanoparticles and nanocatalysts are of interest because of their high surface areas and unusual characteristics [72] . Metal oxides integrated into carbon-based materials or metal nanoparticles supported on conductive substrates are two examples of nanomaterials that can boost catalytic efficiency and selectivity [73] .

Considerations such as reaction conditions, desired product selectivity, and electrochemical system performance are key when deciding on a metal catalyst. Researchers are constantly investigating and optimizing metal catalysts to develop more effective, cost-effective, and sustainable techniques for electrochemical reduction of nitrate to ammonia. Taking advantage of the catalytic capabilities of metals, this technique has the potential to reduce nitrate pollution and increase the use of sustainable materials in NH₃ manufacturing [74] . Owing to their malleable nature, metallic catalysts have considerable potential for the conversion of nitrate to ammonia [75] . Increases in the reaction kinetics and selectivity have resulted from the use of novel metal catalysts featuring highly defined surface structures and active sites [76] .

Metal catalysts can boost the performance of many electrochemical cell designs and electrode materials. Increasing the NH₃ production selectivity and decreasing unwanted reactions can be achieved by modifying the catalyst loading, shape, and composition [77] . However, issues such as catalyst stability, long-term performance, and possible poisoning effects must be resolved to make this technology helpful [78] . Scientists have consistently created novel catalyst designs and employed cutting-edge characterization techniques to better understand catalytic mechanisms and increase catalyst stability over time [79] . Electrochemical nitrate reduction using metal catalysts has the potential to significantly improve water pollution remediation and ammonia production processes while also being environmentally friendly. Responsible resource management and improved solutions to environmental concerns have been promoted by studying and optimizing metal catalysts [80] .

The electrochemical reduction of nitrate to ammonia, in which copper catalysts play a pivotal role, is just one example of why Cucatalysts have attracted significant attention [81] . It has been demonstrated that copper catalysts can effectively accelerate this crucial reaction. Their unique properties have made them a favorite among researchers and entrepreneurs [82] . Cu is an abundant and cost-effective metal, making it a practical choice for large-scale applications. Its wide availability contributes to the economic viability of Cu catalysts for the electrochemical reduction of nitrate to ammonia [83] .

The electrocatalytic activity of copper is relatively high, and it is this activity that drives the nitrate to ammonia reaction. Efficient electron transport facilitates nitrate ion conversion at the cathode of the electrochemical cell, which is responsible for this ability [84] . The catalytic performance was enhanced by modifying the surface characteristics of the Cu catalysts. The surface area, crystallinity, and oxidation state are the characteristics that researchers manipulate to maximize the efficiency and selectivity of the catalyst in the nitrate reduction process [85] . The low price and high availability of Cu makes it a viable option for large-scale projects. Cu catalysts for electrochemical nitrate reduction are feasible because of their widespread availability [86] .

Cu catalysts are highly stable during the reduction process, enabling their employment in a nonstop extended manner [87] . Although the pH, temperature, and competing species influence stability, scientists are working to find solutions [80] . The electrochemical reduction of nitrate to ammonia using copper catalysts is not unique to the water-purification industry [88] . Catalysts based on Cu have demonstrated promise in related processes, such as the electrocatalytic conversion of nitrogen-containing compounds, indicating their adaptability to sustainably manage the nitrogen cycle [89] . Studies have been conducted to enhance the long-term stability and endurance of Cu catalysts, making them more suitable for large-scale industrial applications [90] .

Cu catalysts play a crucial role in the electrochemical reduction of nitrate. Table 1 provides a concise summary of their critical characteristics and attributes to help researchers and practitioners understand and compare the various Cu catalysts for sustainable nitrate conversion applications. This table includes essential information regarding the catalyst composition, morphology, surface area, selectivity, and specific enhancements or modifications that contribute to their effectiveness in the nitrate reduction process.

Gold (Au) metal is a catalyst for the electrochemical reduction of nitrate to ammonia owing to its unique properties and performance. Despite its noble nature, its catalytic activity makes it desirable [98] . Au catalysts improve electron transport and reduce nitrate to ammonia, thereby increasing the NH₃ yields. Researchers have regulated the size and shape of gold nanoparticles (AuNPs) for nitrate reduction for catalytic activity [99] . The electrochemical stability of Au catalysts ensures long-term performance [100] . Changing the support materials or ligands increases Au catalyst selectivity for certain reactions. They also improve NH₃ production while reducing unwanted by-products in the electrochemical reduction of nitrate, proving their selectivity in various catalytic processes [93] [101] .

Au catalysts are energy-efficient and easy to integrate at neutral pH and low temperature. Au-based bimetallic or alloyed catalysts exhibit synergistic effects that increase catalytic activity and selectivity, allowing for improved electrochemical nitrate reduction to ammonia [102] . Figure 4 shows a visual representation of the different properties and characteristics of the Au catalysts. The diagram shows various aspects of Au catalysts, such as their remarkable stability, catalytic activity, selectivity, and distinctive surface properties. This is a useful resource for understanding the complex nature of Au catalysts and their importance in catalysis and different industrial applications.

Investigations of the use of Au catalysts in ecologically benign chemical transformations have been conducted to increase awareness of the importance of these processes [103] . In keeping with green chemistry principles, they are useful for water purification and ammonia generation by lowering the nitrate levels [104] . Despite their outstanding effectiveness, the high prices of Au catalysts limit their widespread use. Optimized gold catalyst architectures are the focus of current research, as they identify cheaper alternatives that maintain the catalytic performance [105] . Catalytic operations involving Au metal catalysts include the electrochemical reduction of nitrate to ammonia and synthesis of organic compounds. Their unique qualities make them useful in many applications, including cleaning polluted environments, converting energy, and creating sensors. Au catalysts allow for the creation of environmentally friendly technologies that assist multiple sectors without negatively affecting the natural world [106] [107] .

Graphene, a two-dimensional carbon allotrope comprised of a hexagonal lattice of carbon atoms, has proven an interesting and flexible catalyst in recent years [108] . Due to its electrical, thermal, and mechanical properties, large surface area, and high conductivity, graphene is a promising catalytic material [109] . Because of its unique structure, graphene provides many active sites for surface reactions, making it an efficient catalyst. Functionalization and doping boost its selectivity and catalytic capabilities. Graphene’s adaptability and catalytic power could revolutionise energy conversion, storage, environmental cleaning, and chemical synthesis [110] . Graphene is ideal for catalytic applications due to its properties. The catalyst’s enormous surface area and two-dimensional structure allow more active sites to speed up catalytic processes [111] .

The remarkable electrical conductivity of graphene allows for charge transfer, thereby improving the reaction speed and effectiveness. The mechanical strength of graphene makes it stable and durable in demanding catalytic environments, enabling its long-term catalytic performance. Functionalized and doped graphene can be customized for specific reactions to optimize catalytic activity and selectivity. Owing to its unique features, graphene has revolutionized several catalytic industries [112] . Graphene-based catalysts use the unique properties of graphene to improve the catalytic performance in numerous applications [113] . Table 2 provides key information on the graphene-based catalysts that enhance their performance. It discusses graphene oxide (GO) and its reduced form, reduced graphene oxide (rGO), the impact of modification on the selectivity and activity, the advantages of nanocomposites, and the catalytic influence of heteroatom doping.

Graphene catalyst design requires structural and content analysis. A catalyst’s surface area and flaws determine its catalytic ability [114] . Surface area and flaws affect active site density and catalyst reactivity. Shape and composition affect graphene catalyst performance. Recent attention has focused on graphene’s electrochemical potential [111] . Due to its hexagonal lattice of sp² hybridised carbon atoms, graphene conducts electricity well. Electrochemical applications include graphene’s excellent conductivity and fast electron transfer [115] . Many active sites in its two-dimensional structure and high surface area improve electrode-electrolyte interactions and provide effective charge storage and transmission in supercapacitors and batteries.

Owing to its mechanical strength and chemical stability, graphene lasts longer under electrochemical conditions [116] . The doping, functionalization, and

stacking of graphene can alter its properties. The unique features of graphene make it a viable electrochemical material for energy storage, sensors, and catalysis [117] . Figure 5 shows the linear sweep voltammetry (LSV) curves of the graphene and CoO NC/graphene electrodes that reduced nitrate. The electrochemical nitrate reduction started at 0 V vs. the reversible hydrogen electrode (RHE) at both electrodes. Based on the current density studies, the CoO NC/graphene electrode was more active in the nitrate reduction reaction than the graphene electrode.

The synergistic effects of graphene and nitrate electroreduction catalysts improved the reduction efficiency and selectivity. Owing to its high electrical conductivity and large surface area, graphene enhances the catalytic properties of several nitrate reduction catalysts by promoting electron transport and nitrate adsorption [119] . In addition to effective electron flow, the graphene matrix has active sites for nitrate adsorption via several interactions. This interaction boosts catalytic activity by introducing additional nitrate species near the catalyst [120] . Graphene increases the stability and lifetime of nitrate-reduction catalysts. The strength and chemical resistance of graphene improve its electrochemical catalytic endurance. Tailoring the reaction environment by altering the composition and structure of graphene with a catalyst material may accelerate the reduction routes and reduce by-products [121] . In electrochemistry, graphene and nitrate electroreduction catalysts work effectively together, despite their physical differences. They boost catalytic efficiency and increase the conversion rates and selectivity for nitrogen gas and ammonium [122] .

The reduction process was more efficient when graphene and nitrate electroreduction catalysts were used together. The high electrical conductivity and large surface area of graphene make it a suitable catalytic substrate [123] . Rapid electron transit from the electrode to the catalytic sites accelerates the reduction kinetics and reduces energy losses during electroreduction. The graphene matrix attaches to and stabilizes the catalytically active sites [124] . Graphene disperses and immobilizes nitrate electroreduction catalysts, preventing aggregation of the active species. Immobilization provides additional active sites for nitrate adsorption and reduction [125] . Graphene-enhanced nitrate electroreduction catalysts sustainably produce ammonia and control nitrogen [126] .

Catalysts made from Cu, Ag, Au, CuO-SnO₂ composites, and Pt-M alloys are commonly used in various fields for their effectiveness in reducing nitrate and producing nitrogen gas (N₂), carbon dioxide (CO₂), and water (H₂O), respectively. Cu-based catalysts are susceptible to corrosion, whereas silver-based catalysts are sensitive to sulfur compounds. Gold-based catalysts exhibit exceptional stabilities and selectivities. CuO-SnO₂ composites exhibit high water resistance and are used in environmental remediation and catalytic oxidation processes. Pt-M alloy catalysts have high efficiency and selectivity for water (H₂O) formation, making them commonly used in fuel cells for oxygen reduction (Table 3).

The development of new catalyst materials for the electrochemical reduction of nitrate to ammonia offers promising prospects in various fields of study. In sophisticated wastewater treatment systems, catalysts can be used to degrade nitrate contaminants and generate ammonia. Water pollution and resource depletion can be addressed permanently using this method. NH₃ can be used as a nitrogen fertilizer in agriculture, increasing crop yields while decreasing the negative effects of conventional nitrogen fertilizers on the environment [83] . When paired with renewable energy sources such as sunlight and wind, these catalysts might power an electrochemical system that stores and uses NH₃ as a carbon-neutral energy source. Furthermore, the manufacturing and distribution of NH₃ can be altered by introducing decentralized ammonia production units fuelled by locally accessible renewable energy sources. It positively affects the local economy, environment, and energy independence [135] .

There are several potential directions for future research in this field. To achieve this goal, in-depth studies on the catalytic mechanisms of novel materials such as MOFs, two-dimensional materials, and catalysts found naturally on Earth are necessary. Designing multifunctional catalysts that can facilitate nitrate reduction while minimizing by-product generation and withstanding severe working conditions is an interesting problem [136] . Real-world applications also require an in-depth understanding of the catalyst stability over time and under changing reaction conditions. Accelerating catalyst discovery and optimization through the prediction of catalytic behavior and the identification of interesting candidates from a broad pool of prospective materials is possible through the integration of machine learning and computational approaches. Finally, it is crucial to determine the overall sustainability of innovative catalyst materials by considering their environmental effects and life cycle assessments in the context of their application pathways [137] .