Front. Mater. Frontiers in Materials Front. Mater. 2296-8016 Frontiers Media S.A. 1527753 10.3389/fmats.2025.1527753 Materials Original Research Investigating the influence of a thin copper film coated on nickel plates through physical vapor deposition for electrocatalytic nitrate reduction Meshram et al. 10.3389/fmats.2025.1527753 Meshram Sumit Maya Moreshwar 1 * Gonugunta Prasad 2 Taheri Peyman 2 Jourdin Ludovic 3 Pande Saket 1 1 Department of Water Management, Faculty of Civil Engineering and Geosciences, Delft University of Technology, Delft, Netherlands 2 Department of Materials Science and Engineering (MSE), Faculty of Mechanical Engineering, Delft University of Technology, Delft, Netherlands 3 Department of Biotechnology, Faculty of Applied Sciences, Delft University of Technology, Delft, Netherlands

Edited by: Ahmed Eid Hassan, Al-Azhar University, Egypt

Reviewed by: Manisha Das, Japan Society for the Promotion of Science (JSPS), Japan

Dipayan Pal, University of California, San Diego, United States

 *Correspondence: Sumit Maya Moreshwar Meshram, s.m.meshram@tudelft.nl 26 05 2025 2025 12 1527753 13 11 2024 21 04 2025 Copyright © 2025 Meshram, Gonugunta, Taheri, Jourdin and Pande. 2025 Meshram, Gonugunta, Taheri, Jourdin and Pande

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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.

 physical vapor deposition electrocatalytic nitrate reduction copper-nickel electrode thickness XPS SEM section-at-acceptance Thin Solid Films 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): NO 3 − + H 2 O + 2   e −   →   NO 2 − + 2   O H − NO 2 − + 2 H 2 O + 3 e −   →   1 2 N 2 + 4   O H − NO 2 − + 5 H 2 O + 6 e −   →   1 2 N H 3 + 7   O H − NO 2 − + 4 H 2 O + 6 e −   →   N H 2 O H + 5 O H − NO 2 − + H 2 O + 2 e −   →   N O + 2 O H −

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).

Pourbaix diagram of the N2-H2O system including N2, NH3, N2H4, and NO3 −.

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).

State-of-the-art electrode preparation techniques utilized for nitrate reduction employing Cu coated on Ni.

Sr.No Electrode Electrode preparation technique Chemical composition Electrolyte Duration (hour) Nitrate reduction efficiency References
1 NiCu@N-C/Ni Foam Co-assembly and carbothermic reduction 3.57 mM NO3 − 50 mM SO4 2- 8 98.63% He et al. (2022a)
2 Cu0.66Ni0.33 Cu0.50Ni0.50 Cu0.33Ni0.66 Electrodeposition 0.01 mol/L NaNO3 0.1 mol/L Na2SO4 4 83.87% Bai et al. (2023)
3 Cu – Ni Foam Cold plasma jet printing 50 mg/L NaNO3 0.05 mol Na2SO4 3 88.9% Yue et al. (2024)
4 Cu3P-Ni2P/CP-x Vapor-phase hydrothermal method 200 ppm NaNO3-N 0.5 M Na2SO4 2 — Jin et al. (2024)
5 Cu and NiO rod on Ni foam Electrodeposition 0.1M KNO3-N 0.1M PBS 1 94% Liu et al. (2023a)
6 Cu/CoP/Ni Foam Hydrothermal 15 mg/L NO3-N 0.05 M Na2SO4 and 0–2000 mg/L Cl− 3.5 100% Yang et al. (2022)
7 Cu on Ni Foam Electrodeposition 200 ppm NO3-N 1M KOH 2 95.05% Li et al. (2021)
8 CuNi alloy on mesoporous carbon Evaporation-induced self-assembly 30 mg/L NO3 −-N 0.1M Na2SO4 7 90% Yao et al. (2021)
9 Cu(OH)2-Cu/NiFoam One-step hydrothermal method 50 mg/L NO3 −-N 50 mM Na2SO4 1.5 91.5% Liang et al. (2023)
10 Cu nanoparticles on Ni plate Physical vapour deposition 2.5 mM KNO3 0.5 g/L of Na2SO4 6 10% This study

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.

Schematic of the PVD equipment and Cu deposition process on a Ni plate.

 2.3 Electrochemical NO<sub>3</sub> <sup>−</sup> 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 C NO 3 − ] % = C 0 NO 3 − − N ] − C t NO 3 − − N ]   C 0 NO 3 − − N × 100 %

The selectivity [S ( NH 4 + )%] of NH 4 + can be calculated based on Equation 7 S NH 4 − ] % = C t NH 4 − − N ]   C 0 NO 3 − − N ] − C t NO 3 − − N × 100 %

The concentration of gaseous compounds was calculated using mass balance Equation 8 C   G a s e o u s   c o m p o u n d s = C 0 NO 3 − − N ] − C n i t r a t e   r e m a i n i n g   i n   s o l u t i o n + C n i t r i t e + C a m m o n i u m where, C 0 NO 3 − − N is initial the concentration of NO 3 − − N   mg / l , 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.

Cyclic voltammetry was recorded for Cu-Ni electrodes with varying Cu film thicknesses: (A1,A2) at 25 nm, (B1,B2) at 50 nm, and (C1,C2) at 100 nm, under N2 sparging and with different concentrations of KNO3 and Na2SO4 electrolyte. Additionally, (A1,B1, and C1) represent cyclic voltammetry measurements with stirring at 500 rpm, while (A2,B2, and C2) represent measurements without stirring, with a scan rate of 1 mV/s and an initial pH of 6.5.

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 Na2SO4, 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 NO<sub>3</sub>RR 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.

The concentrations (N-mg/L) of nitrate reduction reaction products were measured using a Cu-Ni electrode with varying Cu film thicknesses: (A1–A4) at 25 nm, (B1–B4) at 50 nm, and (C1–C4) at 100 nm. The experimental conditions included 2.5 mM KNO3, 3.5 mM Na2SO4, an uncontrolled pH starting at 6.5, an applied current of −8.5 mA, and a duration of 6 h. The notations (A1,B1, and C1)], (A2,B2, and C2), (A3,B3, and C3), and (A4,B4, and C4) correspond to the concentrations of NO3 −, NO2 −, NH4 +, and GC, respectively.

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.

Changes in pH during NO3 − reduction for (A) 25 nm Cu-Ni, (B) 50 nm Cu-Ni, and (C) 100 nm Cu-Ni at an applied current of −8.5 mA in 2.5 mM KNO3 and 3.5 mM Na2SO4.

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 Reaction at anode :    H 2 O   →   1 2 O 2 + 2   H + + 2   e −

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).

NO3RR conversion rate with respect to preparation technique of Cu coating on Ni.

Preparation technique Ni structure Surface area of working electrode (cm2) Initial pH Electrolyte composition Nitrate conversion rate Ref
Electrochemical deposition Ni foils (0.2 mm thick) 15 NA 45 mg/LNO3 −-N+0.1 M Na2SO4 29% after 48 h Hou et al. (2018)
Electrodeless plating Ni foam (sheet thickness = 2 mm) 80 12.5 50 mg/L NO3 −-N+0.1 M Na2SO4 For 4 h with Cu-Ni electrodes prepared using electrodeless Cu plating on Ni at different duration)a) Cu = 10%b) Cu/Ni/5 min = 60%c) Cu/Ni/10 min = 96%d) Cu/Ni/20 min = 92%e) Cu/Ni/40 min = 80%f) Cu/Ni/60 min = 50% Shih et al. (2020)
Chemical deposition Ni sponges 15 6.0–6.5 600 mg/L NaNO3 + 1,400 mg/L Na2SO4 44% ± 5% after 6 h Beltrame et al. (2020)
Potentiostatic deposition Polycrystalline Ni electrode 0.03 NA 5 mM NaNO3 + 0.1 M NaOH Not mentioned BADEA and BADEA (2003)
Physical vapor deposition Ni plate 5.2 5.5–6.5 2.5 mM KNO3 +3.5 mM Na2SO4 For 6 h with Cu-Ni electrode prepared using various thickness of Cu deposited on Ni by PVDa) Cu/Ni/25 nm = 10.9%b) Cu/Ni/50 nm = 10.5%c) Cu/Ni/100 nm = 10.0% This study
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.

 Data availability statement

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

 Author contributions

SM: Conceptualization, Data curation, Funding acquisition, Investigation, Methodology, Visualization, and Writing – original draft. PG: Formal Analysis, Investigation, Software, and Writing – review and editing. PT: Resources, and Writing – review and editing. LJ: Conceptualization, Formal Analysis, Methodology, Supervision, and Writing – review and editing. SP: Conceptualization, Methodology, Supervision, and Writing – review and editing.

 Funding

The author(s) declare that financial support was received for the research and/or publication of this article. This work was supported by the Ministry of Social Justice and Special Assistance Department, Government of Maharashtra, India.

 Conflict of interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

The author(s) declared that they were an editorial board member of Frontiers, at the time of submission. This had no impact on the peer review process and the final decision.

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 Supplementary material

The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fmats.2025.1527753/full#supplementary-material

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