Investigating the influence of a thin copper film coated on nickel plates through physical vapor deposition for electrocatalytic nitrate reduction

Abstract

The removal of nitrate (NO₃⁻) from water and its subsequent valorization for various applications are crucial due to environmental, health, and economic considerations. A promising method for its removal is the process of electrocatalytic reduction of nitrate. Copper/nickel (Cu/Ni) composite electrodes have demonstrated potential for this process in aqueous solution, however, the effect of thin Cu film coated on Ni using physical vapor deposition (PVD) has not been investigated for NO₃⁻ removal. Here, the PVD technique was employed to deposit a thin film of Cu onto a Ni plate to form Cu-Ni composite electrodes of varying Cu thicknesses (25–100 nm), enabling the investigation of the influence of the Cu film thickness on NO₃⁻ reduction. Electrodes prepared using PVD were utilized for electrocatalytic nitrate reduction (NO₃RR) for the first time. The Cu-Ni electrodes were analyzed using X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) to examine the deposited Cu film which is critical for NO₃⁻ reduction and ammonium (NH₄⁺) selectivity. The Cu film was found to be uniformly distributed on the Ni plate without any additional contamination. Cyclic voltammetry was performed to obtain the information on electron transfer between the Cu-Ni electrode and the nitrogen (N₂) species on the surface. NO₃⁻ was primarily reduced to NH₄⁺, with no significant difference in the NO₃⁻ conversion rate observed as a function of the Cu thickness. As the Cu thickness increased, the current density decreased. This study also investigated the effect of stirring on NO₃⁻ reduction, considering potential applications where rotation or stirring is not feasible such as in some batteries. The findings of this investigation indicate that thin film coated electrodes fabricated using the PVD method exhibit capability for NO₃⁻ elimination through electrocatalytic reduction processes.

1 Introduction

Nitrate (NO₃⁻) is one of the most common toxic pollutants in groundwater (Li et al., 2015). NO₃⁻ contamination of groundwater is becoming increasingly serious in both developed and developing countries (Sancho et al., 2016; Jia et al., 2020; Abba et al., 2023). This problem is caused by a variety of activities, including agriculture, industry, sewage, septic tanks, and landfills, leading to an increase in NO₃⁻ levels in water sources (Lockhart et al., 2013; Abascal et al., 2022). The maximum amount of NO₃⁻ permitted in drinking water is 50 mg/L in Europe and 44.43 mg/L in the United States (Shen et al., 2009). Methemoglobinemia or “blue baby syndrome” can result from NO₃⁻ exposure above these levels and poses substantial health hazards, especially for young children and expectant mothers (Knobeloch et al., 2000). Additionally, NO₃⁻ poisoning of water, used for agriculture has an impact on both ecosystems and human health (Della Rocca et al., 2007). Water bodies are also affected by excessive NO₃⁻ contamination, resulting in eutrophication, algal blooms, and disruption of the delicate balance of aquatic life (Moffat, 1998). Thus, to maintain the water quality, public health, and ecological integrity of agricultural watersheds, effective NO₃⁻ removal technologies are essential (Tomer et al., 2013).

Various methods have been developed to remove NO₃⁻ from water, including reverse osmosis (Ahn et al., 2008), ion exchange (Leaković et al., 2000), electrodialysis (El Midaoui et al., 2002), photocatalytic reduction (Varapragasam et al., 2021), and biological denitrification (Park and Yoo, 2009). In recent times, electrochemical methods, particularly electrocatalytic nitrate reduction (NO₃RR), have gained recognition as a viable solution for the efficient elimination of low-concentration NO₃⁻ from water sources (de Groot and Koper, 2004). Although, still it is in the developmental stage, this technology holds immense promise. When it is fully realized, it can offer numerous advantages over conventional approaches, including environmental sustainability, compatibility, cost-effective energy consumption, high efficiency, satisfactory engineering compatibility, controllable operating conditions, selectivity to desired product, and potential integration with renewable energy sources (Lange et al., 2013; Jia et al., 2020; Wang Y. et al., 2021). Two routes are involved in the NO₃RR (Wang H. et al., 2023): The indirect autocatalytic reduction pathway and the direct electrocatalytic reduction pathway. Indirect autocatalytic reduction occurs when NO₃⁻ is not involved in electron transfer processes (de Groot and Koper, 2004; Wang Y. et al., 2021). This indirect autocatalytic reduction pathway usually occurs only at high NO₃⁻ concentrations and in strongly acidic media (Lange et al., 2013). Direct reductive chemical pathways are possible at low concentrations of NO₃⁻. In the NO₃⁻ reduction process, nitrogen (N₂) and ammonia (NH₃) are the main products, following reactions (Equations 1–5):

From the Pourbaix diagram shown in Figure 1, N₂ and NH₃ are the most thermodynamically stable forms of N₂ under standard conditions (Guo et al., 2019).

FIGURE 1

NO₃⁻ is more stable in alkaline solution. In alkaline solution, the reduction of NO₃⁻ will yield a series of products (e.g., dinitrogen tetraoxide (N₂O₄), hydrazine (N₂H₄), nitric oxide (NO), and hydroxylamine (NH₂OH)) which are not the primary products in NO₃⁻ reduction and may decompose into other species. The existing form of ammonia (NH₃) depends on the pH of the solution, where at pH ≥ 9.25, it exists in its molecular form, and at pH < 9.25, ionic NH₄⁺ is the major form. For N₂H₄, the case is similar, where the pH boundary is 6.07.

NO₃RR process involves the use of electrocatalysts, such as non-noble metals (e.g., Cu, Ni, Co, and Fe) or carbon-based materials, to reduce NO₃⁻ to N₂ gas or NH₃ (Liang et al., 2022; Zhang et al., 2022). The choice of electrocatalyst affects the reaction kinetics, selectivity, and efficiency (Xu et al., 2023). Copper (Cu) and Cu-based materials are considered the most promising (Hu et al., 2021; He L. et al., 2022) because of their low cost, abundant availability (Feng et al., 2022), high activity (Hu et al., 2021; Hong et al., 2022; Barrera et al., 2023), and excellent performance in producing NH₃ as the main electrolysis product (Abdallah et al., 2014; Wang et al., 2020; Karamad et al., 2023; Li X. et al., 2023). Cu is an active monometallic electrocatalyst for the NO₃RR in acidic and alkaline electrolytes. It also exhibit good electrocatalytic activity in both acidic and alkaline media (Hasnat et al., 2015).

NO₃⁻ reduction on Cu is a pH-dependent reaction forming NO and NH₄⁺ when an acidic electrolyte is used and NO₂⁻ and NH₂OH when the electrolyte is alkaline (Pérez-Gallent et al., 2017). Metallic Cu effectively attracts and retains oxygen (O) atoms from NO₃⁻. This interaction weakens the N-O bonds in NO₃⁻, creating a low energy point in the bond’s profile. Consequently, this facilitates the conversion of NO₃⁻ into nitrite (NO₂⁻) by easing the detachment of one oxygen atom from N₂ (Liu et al., 2019). In the process of NO₃⁻ reduction, the movement of charge is typically slow due to the high energy present in the lowest unoccupied π molecular (LUMO) orbital of NO₃⁻. This makes it difficult to inject charge into this orbital. However, the d-orbital energy levels of Cu-based materials are similar to the LUMO * of NO₃⁻ (Hao et al., 2021) allowing them to facilitate the electrochemical reduction of NO₃⁻(aq) and transfer electrons more easily to the adsorbed NO₃⁻ (Reyter, 2014; Rezaei-Sameti and Zarei, 2018; Beltrame et al., 2021a; Wei M. et al., 2024). Various studies have shown that Cu electrodes are used for NO₃⁻ reduction in a single chamber in acidic (Burke and Sharna, 2007; Lima et al., 2012), in neutral (Gao et al., 2018), and in alkaline media (Paidar et al., 1999; Bouzek et al., 2001; Badea, 2009), as well as in dual chambers in acidic (Beltrame et al., 2021b), and alkaline media (Cattarin, 1992; Badea, 2009; Rajmohan and Chetty, 2014). In alkaline media, Cu electrodes produce fewer oxides of N₂ as byproducts, and are less corrosive than in acidic media (Badea, 2009). However, despite its advantages, there are some disadvantages with pure Cu catalysts, such as oxidative dissolution or irreversible surface poisoning, both of which lead to undesired catalyst degradation (Dima et al., 2003; Hou et al., 2018), as well as undesirable formation of unwanted by-products (Reyter et al., 2006; Abdallah et al., 2014), such as NO₂⁻ (Roy et al., 2016; He W. et al., 2022).

To overcome the disadvantages of Cu electrodes, various strategies have been developed including (i) engineering Cu into nanoscale or single reaction site (Zhao et al., 2023), (ii) doping Cu with other elements (Pd, P, Ni, etc.), and (iii) depositing Cu on a metal oxide support (Liu Z. et al., 2023). Depositing Cu on a metal oxide support is the most preferred strategy, as it is simple to synthesize and provides considerable benefits from strong metal-support interactions (Smiljanić et al., 2022). There are various thin surface film deposition techniques (Jilani et al., 2017; Abegunde et al., 2019) for depositing or coating Cu such as electrochemical deposition (Epron et al., 2001; Epron et al., 2002; Ramos et al., 2001; Welch et al., 2005; Molodkina et al., 2010; Couto et al., 2011; Couto et al., 2017; Chen and Chang, 2012; Liu and Zou, 2014; Rajmohan and Chetty, 2014; Alam et al., 2015; Mattarozzi et al., 2017; Hou et al., 2018; Lei et al., 2018; Gao et al., 2019; Yin et al., 2019; Zhao et al., 2022; Zurita and Garcia, 2022; Wang C. et al., 2023; Zurita and García, 2023), photo-electrodeposition (Couto et al., 2012; Ribeiro et al., 2014), potentiostatic deposition (Roy et al., 2016; Chen et al., 2023; Lim et al., 2023), electro-crystallization (Hyusein and Tsakova, 2023) and dipping method (Hwang, 2012).

Using the above deposition techniques Cu has been deposited on many substrates such as palladium (Lim et al., 2023), graphene oxide (GO), modified graphite felt (Wang et al., 2022), polydopamine-derived nitrogen-doped hollow carbon spheres (Liu Y. et al., 2023), carbon nanotubes (Rajmohan and Chetty, 2017), Cu foil (Rajmohan and Chetty, 2014), and zinc oxide (Feng et al., 2024). Nickel (Ni) is also utilized as a substrate for catalyst preparation but mostly by forming an alloy with Cu. This approach is employed because Ni produces a homogeneous, stable, and highly active catalyst with Cu, exhibiting high chemical stability and good electron conductivity (Hou et al., 2018). Additionally, its site demonstrates strong adsorption for NO₂⁻ (He L. et al., 2022). The combination of Cu and Ni compensates for the low activity of pure Cu in mediating electron transfer between intermediates (Kobune et al., 2020; Shih et al., 2020; Bai et al., 2023), and reduces the overpotential while improving the stability of the NO₃RR reaction (Bai et al., 2023). However, Cu deposition on Ni using electrochemical deposition, results in uneven film thicknesses due to variation in applied current densities (Goranova et al., 2016), electrolyte composition (Dejang et al., 2025), and temperature (Abdullah, 2017).

The film thickness and electrolyte concentration govern NO₃RR activity (Roy et al., 2016; Guo et al., 2023). The thickness of Cu film affects the activity and selectivity for NO₃⁻ reduction (Chen et al., 2004; Shih et al., 2020). So, here PVD technique is adopted as it can provide coatings with high precision for various film thicknesses. In addition, PVD has several advantages over other coating techniques, such as high-purity coatings (Takahashi, 1998; Yanguas-Gil and Yanguas-Gil, 2017), low processing temperature (Shah et al., 2018), high deposition rates, better adhesion, denser microstructure, controllable material properties, the ability to use a larger choice of materials (Savale, 2016; Morgan et al., 2019), reduced production cost, improved productivity (Mubarak et al., 2005), and better quality films (O’Sullivan et al., 2002). Table 1 presents the state-of-the-art techniques for electrode preparation utilized in NO₃⁻ reduction, delineating the chemicals employed, electrolyte used, duration, and efficacy in NO₃⁻ reduction. It is significant to note that all previous investigations exclusively utilized Ni foam, with no studies exploring Ni plate as an alternative substrate for NO₃⁻ reduction. The Ni plate is chosen over Ni foam because it provides a more robust and stable structure compared to Ni foam, which helps maintain the catalyst’s integrity during prolonged electrochemical operations. This enhanced stability reduces the likelihood of structural breakdown often seen in the more fragile foam configuration (Kabiraz et al., 2024). Also, when Cu is deposited on Ni using electrochemical deposition, it shows poor stability owing to the oxidation and detachment of the deposited Cu layer from the substrates during the course of NO₃⁻ electroreduction (Hou et al., 2018). Furthermore, Ni foam is prone to corrosion, resulting in the release of free Ni ions. This process alters the catalyst’s composition and hinders the subsequent analysis of related catalytic mechanisms (Bu et al., 2021). Moreover, the utilization of PVD for electrode preparation has not been previously investigated in this field of research, as noted in (Yue et al., 2024).

Therefore the primary objective of this research is to investigate the influence of thin Cu film coated on Ni plate using PVD technique for NO₃RR. Previous investigations that employed Cu coating for electrode preparation have deposited thin Cu films varying from 1 nm to 390 nm on various substrates that were used as electrodes for diverse applications except NO₃⁻ reduction (Raaif and Mohamed, 2017; Gonzalez-Gallardo et al., 2024; Johnston et al., 2004; Löffler and Siewert, 2004; Nobili et al., 2009; Wu et al., 2010; Ince et al., 2012; Kang et al., 2013; Salazar et al., 2015; 2016; Sun et al., 2015; Ganchev et al., 2021). Therefore, in this study thicknesses of 25, 50, and 100 nm were selected to encompass this range. Subsequently, the effect of Cu film on the NO₃RR was examined. Additionally, the effect of stirring on the NO₃⁻ reduction reaction was investigated. Further, the obtained Cu-Ni electrodes were analyzed using scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS).

2 Experimental methods

2.1 Chemicals and materials

All chemicals used in this study were of analytical grade and purchased from Sigma Aldrich. Potassium nitrate (KNO₃) and sodium sulphate (Na₂SO₄) were used to prepare the anolyte and catholyte. Pure Cu powder with a particle size <425 μm, density of Cu = 8.930, Z-ratio = 0.437, and 99.5% purity was utilized for the deposition on Ni plates. These Ni plates served as substrates for the Cu films coating, which functioned as the cathodes in subsequent analysis. The Ni plates (purity = 99.96%, thickness = 0.2 mm) were purchased from Haoxuan Metal Materials Ltd. Platinum mesh was used as anode. All solutions were prepared using milliQ water (water obtained from a Millipore system). pH of the solution was measured using a Multi9420 InoLab IDS multimeter. Spectrophotometer (model number: DR3900) was employed for determining the concentration of NO₃⁻, NO₂⁻, and NH₄⁺. The LCK 340, LCK 342, and LCK (303 and304) Hach kits were utilized for measuring NO₃⁻, NO₂⁻, and NH₄⁺, respectively.

2.2 Synthesis of thin films of Cu on Ni plate using physical vapor deposition

First, the Ni plate was sanded and polished with sandpaper of grit size 400–2000. To remove impurities, the Ni sheet (of purity >99.8%) was ultrasonically degreased and cleaned in acetone and ethanol for 15 min. They were then immersed in a 1 mol.L⁻¹ aqueous hydrochloric acid (HCl) solution for 5 min and washed with milliQ water. 25, 50, and 100 nm thickness of the Cu was deposited on both sides of Ni plate using the PVD (VCM 600-SP3, rack-type vacuum evaporator) method by applying a current of ∼80 A. The operation details of the PVD are: substrate temperature = 1,600–1,800°C, evaporation rate = 0.5 Å/s, current intensity = 150 Amp, base pressure = 2.8 × 10⁻⁷ mbar, and vacuum of 5.0 × 10⁻⁶ mbar was achieved. The PVD involves the condensation of vaporized Cu atoms onto a Ni substrate under vacuum conditions, resulting in the formation of a uniform and cohesive layer (Rossnagel, 2003). The Cu–Ni sheet of 2 cm × 1 cm was dried in air at room temperature. Figure 2 shows a schematic of the PVD equipment and the synthesis of Cu-Ni electrodes.

FIGURE 2

2.3 Electrochemical NO₃⁻ reduction

The NO₃⁻ electrochemical reduction experiments were carried out using a three-electrode system in a 300 mL dual-chamber H-type reactor, as shown in Supplementary Figure S1 [Supplementary Material (SI)]. The anolyte was 3.5 mM Na₂SO₄, whereas the catholyte consisted of 2.5 mM KNO₃ and 0.5 g/L of Na₂SO₄ solution. 300 mL of catholyte was placed in the cathode chamber, which was then sparged with N₂ gas to remove oxygen from the solution to create anaerobic conditions. A Pt mesh (1 cm × 1 cm) was used as the counter (anode) electrode. A Cu-Ni plate (2 cm × 1 cm) was cut from a large Cu-Ni plate and used as the working electrode (cathode). The surface area of the working electrode was 4 cm². Ag/AgCl (3 M KCl) was used as the reference electrode. A titanium wire was connected to the Cu-Ni plate to form an external connection with the potentiostat (Admiral instrument’s Squidstat Prime). A proton exchange membrane (Nafion 117) was used to separate anode and cathode chambers. A gas bag was attached to the anode and cathode chamber to collect any produced gases like H₂. All electrochemical experiments were performed by applying a constant current of −8.5 mA for 6 h. Approximately 10 mL of the solution was removed between two sampling points to determine the concentrations of NO₃⁻, NO₂⁻, and NH₄⁺ ions and was replaced with fresh catholyte solution. NO₃⁻, NO₂⁻, and NH₄⁺ in the solutions were measured using standard Hach kits with a UV-visible spectrophotometer (DR-3900, Lange). Cyclic voltammetry (CV) was performed at a potential scan rate of 1 mV/s under two conditions: with stirring at 500 rotation per minute (rpm) and without stirring.

The conversion rate [C (NO₃⁻)%] of NO₃⁻ was calculated using Equation 6

The selectivity [S ()%] of can be calculated based on Equation 7

The concentration of gaseous compounds was calculated using mass balance Equation 8where, is initial the concentration of , V is the volume of the electrolyte in the cathode compartment (L), where subscript ‘0’ represents the initial condition whereas ‘t’ represents the condition after time ‘t’.

2.4 Electrode surface characterization

The morphologies of the samples were examined using field-emission scanning electron microscope (FE-SEM, JEOL JSM 6500F), which was equipped with an EDX detector. The surface chemistry of the samples was analyzed using a PHI-TFA XPS spectrometer from Physical Electronic Inc., which featured an Al Kα X-ray monochromatic source (hv = 1486.7 eV). The pass energy for the survey was set at 89.45 eV, and a vacuum of approximately 10^–9 mbar was maintained during the XPS analysis (Cornet et al., 2024). Data was analyzed using Multipak version 8.0 software.

3 Results and discussions

3.1 Characterization

Supplementary Figure S2 showcases SEM images of Cu-Ni electrodes, which were fabricated through PVD for the 25 nm thickness at 370x (Supplementary Figure S2A), and 4,000x (Supplementary Figure S2B) magnifications. Supplementary Figure S3 shows the elemental EDX mapping and spectrum of 25 nm Cu-Ni electrode. The elemental EDX mapping of pristine Cu-Ni electrode demonstrates the complete coverage of the Ni plate with Cu, a finding that is consistent with the XPS results.

Supplementary Figures S4–S6 (shown in SI) depict the XPS of Ni plate, Cu-Ni electrode with 25 nm Cu coating (pristine) and (spent) after electrochemical reduction of NO₃⁻, respectively. The detailed explanation about the XPS analysis given in SI. Supplementary Figure S7 depicts a comparison of XPS spectra of Ni 2p of Ni plate without coating (Supplementary Figure S7A), 25 nm Cu-Ni electrode (pristine) (Supplementary Figure S7B), and 25 nm Cu-Ni electrode (spent) (Supplementary Figure S7C). A noticeable difference in peaks can be observed for all three electrodes. The Ni plate without a coating exhibits the highest peaks, corresponding to Ni⁰, Ni²⁺, and Ni³⁺, whereas no peaks were detected for the Cu-Ni electrode (pristine). For the Cu-Ni electrode (spent), Ni peaks are barely visible, indicating that some Cu has been removed, exposing the Ni surface during NO₃RR facilitating the NO₃⁻ reduction. A schematic diagram depicting the mechanism of Cu removal from the cathode during NO₃⁻ reduction is illustrated in Supplementary Figure S8.

3.2 Electrochemical measurements

3.2.1 Cyclic voltammetry (CV) in H-type reactor

To assess the electrocatalytic performance of Cu deposition of different thicknesses on Ni plates, CV experiments were conducted. Figure 3 depicts the CV curves obtained for various concentrations of KNO₃ and Na₂SO₄ electrolyte solutions in the range of −1.8 to −0.4 V (versus Ag/AgCl (3 M KCl) reference electrode) at a scan rate of 1 mV/s.

FIGURE 3

From the cyclic voltammograms (CVs) shown in Figure 3, an onset potential at ca. −0.603 ± 0.015 V (black curve), −0.617 ± 0.015 V (green curve), and −0.687 ± 0.031 V (red curve) was clearly visible for the experiments in which KNO₃ + Na₂SO₄ was fed and stirring was absent. No visible peak corresponding to NO₃⁻ reduction was detected under stirred conditions. The absence of a NO₃RR peak was anticipated, as mass transport is not limited under stirring which facilitates faster replenishment of NO₃⁻ at the cathode surface than it is consumed, and products are also removed faster from the surface. This is in contrast to the situation when stirring is ceased, resulting in the formation of a peak, in the −0.6 to −1 V range with a maximum at around −0.9 V vs. Ag/AgCl (where no or limited H₂ production was observed). For 5.9 mM KNO₃ + 3.5 mM Na₂SO_4, concentration in stirring conditions (red curves) the potential of the reductive peak shifted to slightly more negative potentials, ca. −1.03 ± 0.15 V vs Ag/AgCl. For the experiments that contained only for 3.5 mM Na₂SO₄ (pink curves), no peak was observed, and an onset in reductive current was only observed at ca. −0.85 ± 0.03 V (with stirring) and −0.88 ± 0.01 V (without stirring), is either corresponds to proton (H⁺) and/or water reduction to H₂. The current densities at the NO₃RR peak in without stirring condition were ca. 1.48 ± 0.59 (black curve), 1.36 ± 0.68 (green curve), and 2.33 ± 0.11 mA cm^-2 (red curve). Therefore, the presence of 5.9 mM KNO₃ (red curves) partially alleviated mass transport limitations (without stirring), resulting in a higher NO₃RR peak compared to 2.5 mM KNO₃ (black curves). No significant differences in peak height and onset were observed at varying Cu layer thicknesses without stirring, except for a slightly higher maximum peak current at 5.9 mM KNO₃ (red curve) and 100 nm Cu film thickness. With 25 and 100 nm thick Cu film and under stirring, a higher reductive current was recorded with 5.9 mM KNO₃ than with 2.5 mM KNO₃ in the −0.6 to −1 V range, which would indicate a higher rate of NO₃⁻ reduction, though this was not observed for 50 nm Cu film. As the Cu film thickness on the Ni plate is enhanced, a decrease in the current density is observed, as illustrated in Supplementary Figure S9. This could be due to the evolution of microstructure and increased surface roughness (Lin et al., 2017), structural deterioration (Nguyen et al., 2024), and increased resistivity caused by variations in bonding mechanisms and interface voids in thicker films which impedes current flow (Lu T.-F. et al., 2024). Further investigation is necessary to elucidate these observations.

3.3 NO₃RR using 25 nm, 50 nm, and 100 nm Cu-Ni plates

The performance of Cu-Ni electrodes of varying thicknesses for electrochemical NO₃⁻ reduction and product formation (NO₂⁻, NH₄⁺, and gaseous compounds (GC)) with time is shown in Figure 4.

FIGURE 4

Figures 4A1,B1,C1 shows that the concentration of NO₃⁻ decreased with reaction time. The concentration of NO₂⁻, NH₄⁺, and GC increased with time for all electrode as seen from Figures 4A2,B2,C2 and Figures 4A3,B3,C3), respectively, while the concentration of gaseous compounds (GC) remained relatively stable (Figures 4A4,B4,C4). The thickness of the Cu film on the Ni plate does not appear to have a substantial effect on NO₃⁻ conversion under the conditions that were tested. This can be attributed to the use of sodium sulfate (Na₂SO₄), which was employed as the electrolyte in NO₃⁻ reduction experiments. Because, it closely replicates the neutral, unbuffered environment of real-world NO₃⁻ contaminated water (Costa et al., 2024). However, the presence of SO₄^2- ions in the solution may hinder the adsorption of NO₃⁻ on the Cu active sites (De Vooys et al., 2000). Moreover, high concentration of OH⁻ (aq) can cause a poisoning effect on the electrode causing a decrease in NO₃⁻ reduction (Wang et al., 2007) as can be seen from Figure 5.

FIGURE 5

Figure 5 depicts the changes in the pH of the anolyte and catholyte solutions. Initially, the pH range of both solutions was approximately 5.5–6.5, for all experiments carried out with 25, 50, and 100 nm Cu-Ni electrodes. During the first hour of the experiment, the pH of the anolyte suddenly dropped and became acidic, while the pH of the catholyte rose rapidly and became alkaline. Thereafter, the pH of both solutions changed very slowly until the end of the experiment, indicating that NO₃⁻ reduction on the Cu-Ni electrode was possible in alkaline conditions as well (Beltrame et al., 2020). This behavior is attributed to the reactions that occur at the electrodes where OH⁻ (aq) formation in the cathodic compartment (reactions [1]-[5]) increases the pH over time and H⁺ (aq) formation in the anodic compartment where water electrolysis occurs decreases the pH according to Equation 9

The low NO₃⁻ reduction activity of Cu can be attributed to its unfavorable electronic state and the slow proton transfer at its surface, which is particularly significant in neutral or alkaline environment (Liu Z. et al., 2023). The NO₃⁻ reduction activity can be increased by alloying Cu with Ni, which transform the unfavorable electronic state into a favorable one, resulting in an increased production of atomic hydrogen. Additionally, the three-dimensional porous structure of the alloy enhances active sites, accelerates reaction kinetics, and boosts electrocatalytic activity (Ma et al., 2024). The Cu-Ni alloy also changes the adsorption energies of intermediates like NO₃⁻, NO₂⁻, and NH₂, improving efficiency, selectivity, and reducing toxic NO₂⁻ buildup (Li R. et al., 2023; Ma et al., 2024). Compared to monometallic Cu, the Cu-Ni alloy exhibits higher initial currents and reduced current decay over time, making it more effective for NO₃⁻ reduction (Wang et al., 2020). In acidic media, Cu-Ni alloy electrodes outperform pure Cu or Ni electrodes, minimizing undesired side reactions like hydrogen evolution, and offering superior corrosion resistance and catalytic performance, essential for the long-term stability and efficiency of the NO₃⁻ reduction process (Lou et al., 2024). The Cu-Ni alloy’s ability to reduce energy barriers for intermediate steps is crucial for efficient NO₃⁻ reduction in water treatment applications, and it performs well across various NO₃⁻ concentrations and in simulated wastewater, demonstrating its robustness and broad applicability in NO₃⁻ reduction (Li R. et al., 2023; Wei J. et al., 2024). Here, it has been demonstrated that PVD can effectively be used to synthesize catalysts for NO₃⁻ reduction but that further optimization of electrodes by coating Cu coatings as thin films along with Ni addition must be done to reach higher conversion efficiency, conversion rates, selectivity and product concentration. Table 2 shows the NO₃⁻ conversion rate of Cu-coated Ni electrodes, prepared on different Ni structures using various coating techniques. It also shows the NO₃⁻ conversion rates for each study showing that these electrodes provides promising NO₃RR performance. As evidenced by Table 2 and from the literature (Meng et al., 2023), the conversion rate of NO₃⁻ is influenced by various factors, including the method of catalyst preparation, the structure of the Ni substrate, the surface area of the electrode, the amount of NO₃⁻ used, initial pH and the duration of the experiments conducted. In this study, only pure Cu was deposited on the Ni plate using PVD and ca.10.5% efficiency has been achieved. Use of PVD technique for electrode preparation is still promising because it can deposit up to 750 k atoms/min, making it suitable for rapid coating deposition, resulting in uniform film deposition which is confirmed from SEM analysis also. The evaporation process in PVD results in lower absorbed gas within the film, contributing to coatings’ purity and quality which is confirmed from Supplementary Figure S3 where the XPS of pristine 25 nm Cu-Ni electrode shows that no other metal impurities are present other than Cu, Ni, and oxygen. It is particularly versatile for industrial applications requiring thick films where surface morphology is not the primary quality requirement (Baptista et al., 2018).

4 Future work and scope

In future work, reaction mechanisms must be investigated further, e.g., by employing advanced techniques such as in-situ spectroscopic methods (e.g., FTIR, Raman) to identify the reaction intermediates and elucidate the rate-determining steps involved in NO₃⁻ reduction. Further, systematic studies must be conducted to comprehend the synergistic effects between Cu and Ni, including the role of interfacial properties and electronic interactions in enhancing catalytic activity, and utilize computational modelling techniques (e.g., DFT) to gain insights into the adsorption and reaction pathways of NO₃⁻ on the Cu-Ni surface (Wang Z. et al., 2021). Furthermore, rotating disk electrode experiments should be conducted on electrodes prepared using the PVD method to determine the kinetic current density and elucidate the electrocatalytic effect of Cu thin layers on Ni in the NO₃⁻ reduction reaction. The findings of this study, particularly the influence of Cu layer thickness on the NO₃RR performance, can inform the design of advanced electrodes for environmental remediation. Moreover, these electrodes could be employed in biosensors (Salazar et al., 2016), for energy storage as in supercapacitor (Madito et al., 2020), in microelectronic applications (Zhang et al., 2020), high-salt wastewater treatment (Tan et al., 2022), and in batteries (Pan et al., 2019; Liu et al., 2022; Lu J. et al., 2024). To further enhance the significance of this work, future studies should explore the durability and long-term performance of Cu-Ni electrodes under continuous operation and real-world conditions, investigate the synergistic effects of varying Cu deposition techniques, such as electrodeposition, alongside PVD, extend the approach to other electroactive species (e.g., NO₂⁻, NH₃), to develop multi-functional electrocatalysts for broader environmental applications.

5 Conclusion

This study investigated the influence of the Cu layer thickness on NO₃⁻ electrochemical reduction using Cu-Ni composite electrodes produced by Physical Vapor Deposition (PVD). Scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) analyses confirmed the uniform distribution of thin Cu film on the Ni plate, with small pits which played a critical role in NO₃⁻ reduction and NH₄⁺ selectivity. Similar NO₃⁻ conversion and product formation rates were obtained on all electrodes regardless of the thickness of the Cu layer. Based on the experimental results, it can be concluded that the NO₃⁻ removal efficiency with respect to electrode preparation techniques achieved by the PVD method is approximately 10% for varying thicknesses of Cu on Ni plate after 6 h, which is lower compared to other electrode preparation techniques. Furthermore, a decrease in current density was observed with an increase in the thickness of Cu on the Ni plate. Noting the importance of both Cu and Ni, present in a Cu-Ni alloy, it is essential to conduct further investigation in order to deposit pure thin Cu and Ni films combinedly on a Ni plate through PVD for efficient NO₃RR.

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