Low-cost electrochemical nitrate reduction using recycled Cu-Ni foam electrodes

Introduction

Nitrate contamination remains a persistent water-quality problem in agricultural catchments, shallow groundwater, and some industrial effluents. Biological denitrification is effective when carbon dosing, hydraulic residence time, and microbial conditions can be controlled, but small or intermittent nitrate streams are often less suited to biological treatment. Electrochemical nitrate reduction is attractive because it can be switched on demand, paired with renewable electricity, and operated without chemical reductant addition. The field has grown from water-treatment studies toward broader interest in electrocatalytic nitrate-to-ammonia conversion [1,2,3].

Copper is one of the most studied non-noble catalysts for nitrate electroreduction. Experiments in alkaline media established that copper can reduce nitrate but that product distribution depends strongly on potential, electrolyte, surface state, and competing hydrogen evolution [4]. CuNi alloy electrodes have also been studied for nitrate and nitrite reduction, showing that nickel can change the balance between nitrate activation and hydrogenation steps [5]. Modified copper electrodes, including carbon-supported variants, provide further evidence that conductivity, surface area, and local copper chemistry interact strongly [6]. More recent mechanistic and theoretical work argues that copper binds key nitrogen-oxygen intermediates in a useful range for ammonia formation, while still being sensitive to surface oxidation and local pH [7,8,9].

From an environmental-engineering perspective, high ammonia selectivity is not automatically a benefit. Ammonium produced from nitrate must either be recovered, stripped, nitrified-denitrified, or otherwise managed. Nitrite accumulation is also undesirable because nitrite is more acutely toxic than nitrate. Early electrochemical water-treatment studies already highlighted the difficulty of converting nitrate without creating secondary treatment needs [16,17]. Reviews of electrochemical nitrate removal therefore emphasise product accounting, energy consumption, electrode durability, and realistic water matrices rather than nitrate disappearance alone [10,11,12]. This framing is important for low-cost electrodes, where metal leaching and passivation may dominate performance over many cycles.

This study evaluates recycled Cu-Ni foam as a pragmatic cathode platform. The nickel foam provides high surface area and conductivity, while electrodeposited copper supplies nitrate-active sites. The word recycled is used literally: the nickel foam came from offcuts and rejected sheets from battery-electrode production, and the copper plating bath was prepared from recovered copper sulfate solution after impurity polishing. The objective is not to set a catalytic record. It is to test whether a low-cost electrode can remove nitrate reliably enough for polishing applications and to identify the trade-offs in selectivity, leaching, and energy use.

Electrode preparation and characterisation

Open-cell nickel foam offcuts with nominal pore density of 95 pores per inch and thickness of 1.6 mm were cut into 25 mm by 50 mm coupons. The offcuts contained residual binder and carbonate deposits from battery-electrode coating trials. Cleaning used alkaline detergent, ultrasonic rinsing, dilute citric acid, and a final cathodic activation step in 0.1 M NaOH. Control experiments showed that acid cleaning alone left carbonaceous residues that increased charge-transfer resistance and nickel leaching.

Copper was electrodeposited from a recovered CuSO4 plating bath after filtration, activated-carbon polishing, and inductively coupled plasma screening for Pb, Zn, and Fe. The target copper loading was 1.8 mg cm^-2 geometric area, corresponding to approximately 17 wt% of the cleaned foam coupon. Deposition was performed galvanostatically at 12 mA cm^-2 for 18 min, followed by rinsing and reduction at -0.55 V versus reversible hydrogen electrode. SEM-EDS showed patchy but connected copper coverage over the nickel struts. XPS before electrolysis indicated mixed Cu(0)/Cu(I) surface species, which is expected for air-exposed copper and consistent with the sensitivity of copper nitrate-reduction activity to surface state [8,9].

Electrodes are labelled Ni-foam, Cu/Ni-low, Cu/Ni-mid, and Cu/Ni-high according to copper loading. The mid-loading electrodes were used for most tests because the low-loading electrodes exposed too much nickel and the high-loading electrodes partially clogged pores, increasing pressure drop in flow-through mode. The geometric surface area is reported for engineering comparison, while electrochemical surface area from double-layer capacitance is reported in the supplementary data with the usual caution that capacitance is not a universal surface-area metric for rough bimetallic foams.

Electrochemical tests and analytical methods

Batch tests used a divided H-cell with an anion-exchange membrane, graphite felt anode, Ag/AgCl reference electrode, and 80 mL catholyte. Potentials were converted to the reversible hydrogen electrode scale after pH correction. The baseline electrolyte contained 50 mg N L^-1 nitrate as NaNO3 in 0.05 M Na2SO4 at pH 8.2. Potential screens were conducted from -0.52 to -0.92 V versus reversible hydrogen electrode. Flow-through tests used a 3 mm gap compression cell with the foam cathode perpendicular to the flow path, a nickel mesh current collector, and recirculating anolyte separated by an anion-exchange membrane.

Nitrate and nitrite were measured by ion chromatography. Ammonium was measured by indophenol colorimetry and cross-checked by ion chromatography for selected samples. Dissolved N2 and N2O were not measured online; gaseous products were estimated by nitrogen mass balance and therefore reported as inferred. This is a limitation, because nitrate electroreduction studies can overstate selectivity if nitrogen-containing products are not fully accounted for [2,11]. Copper and nickel leaching were measured by ICP-OES after filtering samples through 0.22 micrometre membranes.

Three water matrices were tested: synthetic groundwater with bicarbonate, calcium, and magnesium; agricultural drainage water filtered to 0.45 micrometre; and chloride-amended synthetic water representing brackish intrusion. The drainage water contained 42-96 mg N L^-1 nitrate, 6-18 mg L^-1 dissolved organic carbon, and 1.1-2.7 mM bicarbonate. Energy consumption was calculated from cell voltage and charge passed, excluding pumping energy in batch tests and including measured pump power in flow-through tests. The economic analysis used electrode-preparation consumables, copper recovery chemicals, electricity, and amortised power-supply cost, but not labour.

Batch nitrate reduction performance

Bare recycled nickel foam removed nitrate slowly and produced substantial nickel leaching during the first electrolysis cycle. At -0.72 V, nitrate removal after 3 h was 31 +/- 4%, nitrite selectivity was 19 +/- 3%, and nickel concentration reached 0.19 mg L^-1. Copper deposition improved both activity and leaching. The Cu/Ni-mid electrode removed 93 +/- 2% nitrate-N from 50 mg N L^-1 feed in 3 h, with 74 +/- 4% selectivity to ammonium, 8 +/- 2% to nitrite, and 12 +/- 5% inferred gaseous or unmeasured nitrogen products. Copper leaching after conditioning was below 34 micro g L^-1, and nickel was below 18 micro g L^-1.

Potential strongly affected product distribution. At -0.52 V, nitrate removal was only 38% after 3 h but nitrite accumulation was limited. At -0.82 V, nitrate removal was complete within 2 h, but hydrogen evolution reduced current efficiency and ammonium selectivity. At -0.92 V, gas evolution physically blocked parts of the foam and caused unstable current. The best compromise for water-treatment operation was therefore not the most negative potential. This agrees with the broader nitrate electroreduction literature, where nitrate conversion, ammonia selectivity, and hydrogen evolution must be optimised together [1,2,3].

The mid-loading electrode outperformed both low and high copper loading. Low copper loading left nickel-rich regions that promoted nitrite accumulation and leaching. High copper loading improved initial nitrate conversion but increased diffusion limitations inside the foam and lost 15% of electrochemically active area after six cycles, probably due to weakly attached copper islands. The result supports a practical design rule: copper coverage should be continuous enough to shield nickel and activate nitrate, but not so thick that pore transport and mechanical adhesion degrade.

Flow-through operation and water-matrix effects

In flow-through operation, the Cu/Ni-mid electrode achieved lower conversion per pass but better current utilisation than in batch mode. At 2.1 min hydraulic residence time, 50 mg N L^-1 nitrate feed, and -0.72 V, nitrate removal was 68 +/- 5%, ammonium selectivity was 69 +/- 6%, and nitrite in the effluent was 2.8 +/- 0.9 mg N L^-1. Energy consumption was 0.46 kWh mol^-1 nitrate removed when pump power was included. Increasing residence time to 4.8 min raised removal to 86%, but energy per mole removed increased because cell voltage rose as gas accumulated within the foam.

Synthetic groundwater reduced nitrate conversion by 11% relative to Na2SO4 electrolyte. Bicarbonate buffered local pH and calcium carbonate deposits formed near the outlet side of the foam after long runs. Agricultural drainage water reduced conversion by 18-23%, depending on dissolved organic carbon and conductivity. Organic matter adsorbed onto copper-rich regions and increased charge-transfer resistance after approximately 14 h of cumulative operation. A 20 min cathodic cleaning pulse in 0.05 M Na2SO4 recovered 82% of the original activity, but repeated cleaning accelerated copper roughening.

Chloride changed selectivity more than conversion. In 10 mM chloride, nitrate removal decreased by only 6%, but nitrite selectivity increased from 8% to 16% in batch tests and from 6% to 13% in flow-through tests. We did not detect free chlorine under cathodic conditions, but chloride may alter surface oxide, local conductivity, or intermediate adsorption. Because nitrite accumulation is unacceptable for drinking-water treatment, chloride-rich waters would require either lower loading, longer residence time, or downstream nitrite polishing.

Durability was evaluated over 24 flow-through cycles using agricultural drainage water. Nitrate removal declined from 64% in cycle 1 to 51% in cycle 24. SEM after cycling showed carbonate scaling and partial copper smoothing on upstream struts. ICP-OES showed cumulative copper loss equivalent to 1.6% of the plated copper inventory. The electrode was therefore stable enough for polishing tests but not yet a maintenance-free treatment module.

Comparison with catalyst literature

The recycled Cu/Ni foam is less selective than many advanced catalysts reported for nitrate-to-ammonia synthesis, including copper molecular solids, structured copper foams, and bimetallic three-dimensional catalysts [7,13,14,15]. That comparison is useful but should not be read as failure. Those studies often use carefully prepared catalysts, controlled electrolytes, and performance metrics centred on ammonia production. The present electrode is designed for low-cost nitrate removal in variable water matrices. Its relevant metrics are nitrate conversion per pass, nitrite control, metal leaching, energy use, and compatibility with downstream treatment.

The Cu/Ni result nevertheless agrees with mechanistic expectations. Copper-rich surfaces provide nitrate activation and hydrogenation pathways [4,8,9], while nickel modifies hydrogen availability and can promote nitrite reduction when not exposed excessively [5]. Too much exposed nickel, however, increased leaching and nitrite accumulation in our tests. The foam geometry also matters. Three-dimensional electrodes increase geometric area and reduce current density per active site, but they introduce internal mass-transfer gradients and gas blockage. This is why the mid-loading electrode outperformed the high-loading electrode despite having less copper.

For treatment trains, the most attractive use case is nitrate polishing before ammonium capture or biological post-treatment. If ammonium is recovered, the process becomes a nitrogen-conversion step rather than complete denitrification. If ammonium is not recovered, the process merely shifts the contaminant form. Coupling electrochemical nitrate reduction to zeolite exchange, membrane contactors, breakpoint chlorination, or nitrification-denitrification should therefore be evaluated before field deployment.

Cost and sustainability considerations

The material cost of the recycled Cu/Ni-mid cathode was estimated at USD 7.40 per square metre of projected electrode area, excluding labour and quality control. New commercial nickel foam purchased at laboratory scale would raise that cost by a factor of six to eight. The recovered copper sulfate bath contributed little to cost after impurity polishing, but bath screening is essential because trace lead or zinc would be unacceptable in water-treatment electrodes. Electrode recycling is therefore only beneficial if feedstock control and leaching tests are built into the process.

Energy consumption was comparable to other low-temperature electrochemical polishing processes but higher than biological denitrification in carbon-rich wastewater. The process is therefore best matched to small, nitrate-rich, low-carbon streams where biological treatment is difficult or where intermittent operation is valuable. The cell also produces an ammonium-rich effluent that must be handled. A fair life-cycle comparison would need to include ammonium recovery value, electrode lifetime, membrane replacement, and avoided chemical dosing. Those data are beyond the scope of this bench study.

The main environmental risk is not nitrate conversion failure but secondary contamination. Nickel leaching from uncoated or damaged foam exceeded guidance levels in some control tests, and copper leaching was highest during the first conditioning cycle. A practical module would need preconditioning, effluent metal monitoring, and a conservative end-of-life criterion based on leaching as well as nitrate removal.

Conclusion

Recycled nickel foam coated with electrodeposited copper can serve as a low-cost cathode for electrochemical nitrate reduction in contaminated water. The best Cu/Ni foam removed nitrate efficiently in synthetic electrolyte, retained useful activity in agricultural drainage water, and kept copper and nickel leaching low after conditioning. Performance declined in bicarbonate-rich and organic-rich matrices, and chloride increased nitrite accumulation. The electrode is therefore promising for nitrate polishing but not sufficient as a standalone drinking-water technology.

The study highlights the value of water-treatment metrics for nitrate electroreduction: product distribution, nitrite accumulation, leaching, energy use, and durability matter as much as nitrate disappearance. Future work should test longer flow-through operation, integrate ammonium capture or biological post-treatment, and compare recovered-metal electrodes against commercial foams under the same water matrices.

Data and code availability

Ion chromatography exports, ammonium calibration files, ICP-OES leaching data, electrochemical time series, SEM-EDS maps, XPS spectra, residence-time distributions, and nitrogen mass-balance scripts are included in the supplementary archive. Data analysis was performed with Python 3.10, pandas 1.5, NumPy 1.24, SciPy 1.10, and statsmodels 0.13.

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