Electrochemical Nitrate Reduction System and Method of Inducing Electrochemical Reduction of Nitrate

Description

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to and the benefit of Korean Patent Application No. 10-2024-0193090 filed with the Korean Intellectual Property Office on Dec. 20, 2024, and Korean Patent Application No. 10-2025-0112657 filed with the Korean Intellectual Property Office on Aug. 13, 2025, the entire contents of which are incorporated herein by reference.

BACKGROUND

1. Field

This relates to an electrochemical nitrate reduction system and a method of inducing electrochemical reduction of nitrate.

2. Description of the Related Art

Research on highly efficient methods of reducing nitrates due to the growing demand for ammonia (NH₃ or NH₄⁺), which is used as a fertilizer feedstock and a hydrogen carrier, as well as efforts to remove a nitrate (NO₃₋), which is a toxic chemical, from various wastewaters is actively underway. Among them, an electrochemical reduction of the nitrates, which may be processed in a short period of operation without requiring a large installation area, is economical.

The electrochemical nitrate reduction reaction involves several hydrogenation and deoxygenation steps, wherein each step competes with a hydrogen evolution reaction (HER). Due to this competition, the nitrate reduction becomes very inefficient. In order to increase the efficiency of the nitrate reduction, an alkali or neutral aqueous solvent may be suggested but has problems of rather lowering the efficiency due to the slow H₂O dissociation rate, which limits hydrogen transfer.

SUMMARY

An electrochemical nitrate reduction system capable of reducing nitrate with high efficiency and a reduction method are provided, and their operating principles are presented.

In an embodiment, an electrochemical nitrate reduction system includes an electrolyte solution including an acidic aqueous solvent and a nitrate having a concentration of greater than or equal to about 6 m, and an electrode.

In another embodiment, a method of inducing electrochemical reduction of nitrate includes preparing an electrochemical device, which includes an electrolyte solution including nitrate having a concentration of greater than or equal to about 6 m and an acidic aqueous solvent and an electrode, applying a potential to the electrode to suppress a hydrogen evolution reaction and induce reduction of nitrate.

The electrolyte solution according to an embodiment may enable an electrochemical reduction reaction of the nitrate, suppress a hydrogen evolution reaction that is a competitive reaction, and maximize the efficiency of nitrate reduction.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cyclic voltammetry (CV) graph from a tip and substrate electrode associated with the H⁺/H₂ redox reaction at a scan rate of 0.02 V/s in the electrolyte of Comparative Example 1 through the tip generation/substrate collection (TG/SC) mode of a scanning electrochemical microscope (SECM) with a substrate voltage (E_s) of 0 V. In the cyclic voltammetry graph, the horizontal axis is the potential (E_T) of the tip electrode and the vertical axis is the current (i).

FIG. 2 is a cyclic voltammetry graph associated with the H⁺/H₂ redox reaction at a scan rate of 0.02 V/s in the electrolyte of Example 1 through the TG/SC mode of SECM with E_s=0 V.

FIG. 3 is a cyclic voltammetry graph associated with the H⁺/H₂ redox reaction at a scan rate of 0.02 V/s in the electrolyte of Example 2 through the TG/SC mode of SECM with E_s=0 V.

FIG. 4 is an UV-vis absorption spectrum of nitrite (NO₂₋) and ammonia (NH₃) from the aliquots after potentiostatic electrolysis through the nitrate reduction reaction from the electrochemical cell using the electrolyte solution of Example 2 as a catholyte.

FIG. 5 is a cyclic voltammetry graph associated with H⁺/H₂ redox reaction in the electrolyte solution of Example 2 at 0.001 V/s through the TG/SC mode of SECM.

FIG. 6 is a cyclic voltammetry graph (CV_NO3-) showing the voltage current of nitrate reduction in the electrolyte solution of Example 2 with a scan rate of 0.02 V/s is.

FIG. 7 is a graph showing the ratio (i_T,min/i_T,max) of the tip electrode minimum current to the tip electrode maximum current according to the concentration of LiNO₃ and the ratio (i_T,min/i_T,max) according to the concentration of LiTFSI.

FIG. 8 is a ΔR_ct plot showing the change in estimated charge transfer resistance (R_ct) according to the concentration of LiNO₃ and ΔR_ct plot according to the concentration of LiTFSI.

FIG. 9 is a graph showing the faradaic efficiency (FE) of NO₂₋ and NH₃ according to the concentration of LiNO₃.

FIG. 10 is a graph showing ΔR_ct according to the concentration of NaNO₃.

FIG. 11 is a fractional diagram showing the peak area ratios in infrared (IR) spectroscopy of network water, intermediate water, and multimer water according to the concentration of LiTFSI.

FIG. 12 is a fractional diagram showing the IR peak area ratio of network water, intermediate water, and multimer water according to the concentration of LiNO₃.

FIG. 13 is a graph showing the radial distribution functions (RDFs) of Li⁺ to H₂O at various concentrations of LiNO₃.

FIG. 14 is a graph showing the RDFs of Li⁺ to H₃O⁺ at various concentrations of LiNO₃.

FIG. 15 is a molecular dynamics snapshot representing the solvation structure of 9 m LiNO₃ acid electrolyte solution.

FIG. 16 is a graph showing the coordination numbers of Li⁺ with H₃O⁺ as a function of the concentration of LiNO₃.

FIG. 17 is a schematic illustrations of the electrical double layer during H⁺ reduction occurs on a Pt electrode in an acidic LiNO₃-based hydronium-in-salt electrolyte (HISE).

FIG. 18 is a graph showing the average collection efficiency (N_eff, i_S/i_T) according to the tip electrode potential (E_T) when HNO₃ was added as an acidic raw material to the electrolyte solution of Example 2.

FIG. 19 is a graph showing N_eff according to E_T for the case where HNO₃ was not added to the electrolyte solution of Example 2.

DETAILED DESCRIPTION

Hereinafter, embodiments of the present invention are described in detail so that those of ordinary skill in the art can easily implement the present invention. However, this disclosure may be embodied in many different forms and is not construed as limited to the example embodiments set forth herein.

Hereinafter, “combination thereof” refers to a mixture, a laminate, a composite, a copolymer, an alloy, a blend, or a reaction product of constituents.

Herein, it should be understood that terms such as “comprises,” “includes,” or “have” are intended to designate the presence of an embodied feature, number, step, element, or a combination thereof, and it does not preclude the possibility of the presence or addition of one or more other features, number, step, element, or a combination thereof.

Herein, “or” is not to be construed as an exclusive meaning, for example, “A or B” is construed to include A, B, A+B, and the like.

In an embodiment, a electrochemical nitrate reduction system includes an electrolyte solution including an acidic aqueous solvent and a nitrate having a concentration of greater than or equal to about 6 m, and an electrode.

The nitrate concentration in the electrolyte solution means the number of moles of the nitrate per weight of the acidic aqueous solvent, and the unit, m, means a molal concentration. When an electrolyte solution satisfying the nitrate concentration of greater than or equal to about 6 m is applied, the competing reaction, the hydrogen evolution reaction (2H⁺+2e⁻>H₂), may be suppressed, while inducing the electrochemical nitrate reduction reaction with efficiency.

The nitrate concentration in the electrolyte solution may be, for example, greater than or equal to about 6 m about 9 m, for example, about 6 m to about 9 m, about 6.5 m to about 8.5 m, about 6 m to about 8 m, about 6 m to about 7 m, about 7 m to about 9 m, or about 7 m to about 8 m but is not limited thereto. If the electrolyte concentration is less than about 6 m, the efficiency of the nitrate reduction reaction may sharply drop. In addition, if the electrolyte concentration is greater than about 9 m, hydronium ions (H₃O⁺) may not be dissolved in the solvent, thereby not forming an acidic aqueous solvent, which may resultantly fail in inducing the nitrate reduction reaction or deteriorate the efficiency

Types of the nitrates are not particularly limited but may include LiNO₃, NaNO₃, or a combination thereof, for example, LiNO₃.

In the acidic aqueous solvent, the hydronium ions (H₃O⁺) may be at a concentration of about 5 mM to about 100 mM, for example, about 5 mM to about 80 mM, about 5 mM to about 60 mM, about 5 mM to about 50 mM, or about 10 mM to about 100 mM. A unit of mM stands for millimolar concentration. If the concentration of the hydronium ions in the solvent is within the ranges, the nitrate reduction reaction may proceed with high efficiency.

A source of the hydronium ions in the acidic aqueous solvent is not particularly limited but may be, for example, HNO₃. In other words, the acidic aqueous solvent may include HNO₃.

The electrode is not particularly limited but may include Ag, Co, Cu, Fe, Ni, Pt, Ti, or a combination thereof. For example, the electrode may include Pt. The Pt electrode is an electrode that is advantageous for the hydrogen evolution reaction but disadvantageous for the nitrate reduction reaction, and if applying the electrolyte solution according to an embodiment, even when the Pt electrode is used, the nitrate reduction reaction may be induced with high efficiency.

According to the analysis described below, the electrolyte solution forms not a water-in-salt but a hydronium-in-salt as a solvation structure, which may suppress the hydrogen evolution reaction but effectively proceed the nitrate reduction reaction.

The hydronium-in-salt may be formed by a strong interaction of H₃O⁺ and Li⁺NO₃₋ (or Na⁺NO₃₋). The hydronium-in-salt may include H₃O⁺-in-Li⁺NO₃₋ or H₃O⁺-in-Na⁺NO₃₋.

The reduction of nitrate according to an embodiment may include the following Chemical Equation 1 and/or Chemical Equation 2.

NO₃₋+2H⁺+2e⁻→NO₂₋+H₂O  [Chemical Equation 1]

NO₃₋+9H⁺+8e⁻→NH₃+3H₂O  [Chemical Equation 2]

According to an embodiment, nitrite (NO₂₋) and/or ammonia (NH₃) are produced by the reduction reaction of nitrate.

In another embodiment, an electrochemical method includes preparing an electrochemical device, which includes an electrolyte solution including nitrate having a concentration of greater than or equal to about 6 m and an acidic aqueous solvent and an electrode, applying a potential to the electrode to suppress a hydrogen evolution reaction and induce reduction of nitrate.

The applying of the potential may be referred to as potentiostatic bulk electrolysis.

The potential may be a negative potential range in which a hydrogen evolution reaction (HER) occurs at the applied electrode, for example, about −0.1 V to about −2.0 V, or about −0.2 V to about −1.5 V, or about −0.4 V to about −1.2 V relative to the point of zero charge (PZC).

Hereinafter, specific examples according to an embodiment are described.

Comparative Example 1

An electrolyte solution according to Comparative Example 1 was prepared by dissolving 5 mM of HNO₃ and 0.1 m of LiNO₃ in distilled water.

After adjusting a distance between the tip and Pt substrate ultra-microelectrodes (UMEs; d_T-S) to 3.5 μm in a scanning electrochemical microscope (SECM), the electrolyte solution of Comparative Example 1 was injected thereinto to perform a cyclic voltammetry (CV) analysis by setting a substrate potential (E_s) to 0 V and scanning a tip potential (E_T) from 0 V to −0.8 V at a scan rate of 0.02 V/s in a tip generation (TG)/substrate collection (SC) mode, and the results are shown in FIG. 1.

Referring to a cyclic current-voltage graph of FIG. 1, no side reactions occurred as H⁺/H₂ redox cycles progressed, and no current decrease due to H₂ bubbles was observed. Accordingly, this confirmed that LiNO₃ was electrically inactive at low concentrations.

Example 1

An electrolyte solution according to Example 1 was prepared by dissolving 15 mM of HNO₃ and 6 m of LiNO₃ in distilled water. The electrolyte solution of Example 1 was subjected to the cyclic current voltammetry analysis in the same manner as in Comparative Example 1, and the results are shown in FIG. 2.

Example 2

An electrolyte solution according to Example 2 was prepared by dissolving 5 mM of HNO₃ and 9 m of LiNO₃ in distilled water. The electrolyte solution of Example 2 was subjected to the cyclic current voltammetry analysis in the same manner as in Comparative Example 1, and the results are shown in FIG. 3.

Referring to FIGS. 2 and 3, the electrolyte solutions at greater than or equal to about 6 m of LiNO₃ exhibited voltage and current drops in both forward and backward directions. Accordingly, in Examples 1 and 2, H⁺ hopping was expected to be significantly interrupted.

For comparison, an electrolyte solution according to Comparative Example 2 was prepared by dissolving 5 mM of HClO₄ and 9 m of LiTFSI in distilled water and then, subjected to the cyclic current voltammetry analysis in the same manner as in Comparative Example 1. However, in this case, no voltage and current drops occurred. An electrolyte solution according to Comparative Example 3 was prepared by dissolving 1 mM of [Fe(CN)₆]³⁻ and 9 m of LiNO₃ in distilled water and then, subjected to the cyclic current voltammetry analysis according to [Fe(CN)₆]³⁻/[Fe(CN)₆]⁴⁻ redox reaction in the same manner as in Comparative Example 1. However, in this case, no graphs like FIGS. 2 and 3 appeared.

It was confirmed that the voltage and current drops as shown in FIGS. 2 and 3 appeared only in the acidic electrolyte solutions having a nitrate concentration of greater than or equal to about 6 m. It was understood that a hydrogen evolution reaction was suppressed by irreversibly generated by-products. Herein, an electrochemical nitrate reduction was understood to be the most likely side reaction. Hereinafter, this is explained through UV-vis spectroscopic titration.

A three electrode cell was manufactured by using Pt as an operation electrode | the electrolyte solution of Example 2∥ the electrolyte solution of Example 2 as a catholyte | a carbon felt as a counter electrode. This cell was subjected to potentiostatic bulk electrolysis at an operation electrode potential of −0.7 V to obtain a reduction potential of 2 C, inducing nitrate reduction. Subsequently, a catholyte sample was collected therefrom and then, mixed with colorants respectively detecting NO₂₋ and NH₃ to perform the UV-vis spectral analysis, and the results are shown in FIG. 4.

Referring to FIG. 4, it was confirmed that nitrite (NO₂₋) and ammonia (NH₃) were formed by the nitrate reduction reaction. When measured from the UV-vis absorption peaks, NO₂₋ was at a concentration of 13.1%, NH₃ was at a concentration of 50.7%, and in addition, NO₂₋ exhibited faradaic efficiency of ±2.1%, and NH₃ exhibited faradaic efficiency of ±5.9%.

In the previous SCEM, the electrolyte solution of Example 2 was subjected to CVs at the scan rate of 0.02 V/s, but this time, the CVs was performed at a scan rate of 0.001 V/s, and the results are shown in FIG. 5. Referring to FIG. 5, as the tip potential (E_T) shifted toward a negative direction from the forward direction by greater than or equal to −0.4 V, significant voltage and current drops occurred. These voltage and current drops were induced by the nitrate reduction reaction, which was understood to be controlled by an adsorption process on the tip electrode surface.

A current (i_T,NO3-RR) due to the nitrate reduction reaction may be calculated by subtracting a current (i_T,HER) due to the hydrogen evolution reaction from a total current (i_T). However, a current (i_S) in the substrate, which is a flux of H₂ generated at the tip electrode, and the current (i_T,HER) in the hydrogen evolution reaction have the same absolute value. Accordingly, an absolute value of a current (i_T,NO3-RR) due to the nitrate reduction reaction may be obtained by subtracting the substrate current (i_S) from the absolute value of the tip current (i_T), wherein the absolute value of the current (i_T,NO3-RR) was used to derive a voltage current CV_NO3- of the nitrate reduction at the 0.02 V/s, as shown in FIG. 6.

Referring to FIG. 6, NO₃₋ started to be electrically reduced at about −0.3 V and stopped at −0.4 V, where the voltage and current drops started. This indicates that the hydrogen evolution reaction was significantly suppressed, while the nitrate reduction reaction was maximized.

Subsequently, the effect of nitrate concentrations was analyzed.

In addition to Comparative Example 1 and Examples 1 and 2, the electrolyte solution of Comparative Example 4 was prepared by dissolving 5 mM of HNO₃ and 3 m of LiNO₃ in distilled water, the electrolyte solution of Example 3 was prepared by dissolving 5 mM of HNO₃ and 7 m of LiNO₃ in distilled water, and the electrolyte solution of Example 4 was prepared by dissolving 5 mM of HNO₃ and 8 m of LiNO₃ in distilled water. Comparative Example 1 was at a nitrate concentration of 0.1 m, Comparative Example 4 was at a nitrate concentration of 3 m, Example 1 was at a nitrate concentration of 6 m, Example 3 was at a nitrate concentration of 7 m, Example 4 was at a nitrate concentration of 8 m, and Example 2 was at a nitrate concentration of 9 m.

Comparative Examples 1 and 4 and Examples 1 to 4 at different nitrate concentration were subjected to TG/SC mode SCEM at a speed of 0.02 V/s to measure CVs. The CVs results were used to derive a ratio (i_T,min/i_T,max) of a minimum current at a tip electrode to a maximum current at the tip electrode, and the i_T,min/i_T,max ratios according to the nitrate concentrations are shown in FIG. 7.

For comparison, each electrolyte solution was prepared by dissolving 5 mM of HClO₄ in distilled water and LiTFSI at different concentrations of 0.1 m, 3 m, 6 m, 7 m, 8 m, and 9 m and then, measured with respect to CVs, and i_T,min/i_T,max ratios according to the LiTFSI concentrations are shown in FIG. 7.

Referring to FIG. 7, the acidic LiTFSI electrolyte solutions exhibited no i_T,min/i_T,max difference according to the concentrations, but the acidic nitrate electrolyte solutions clearly exhibited i_T,min/i_T,max differences according to the concentrations based on the concentration of 6 m. In the case of the nitrate, the slope was −0.025 m⁻¹ until the concentration reached 6 m but sharply decreased slope to −0.11 m⁻¹ after the concentration reached 6 m. It was confirmed that within the range of the nitrate concentration of greater than or equal to about 6 m, reactivity of NO₃₋ significantly increased.

Subsequently, electrochemical impedance spectroscopy (EIS) was performed to measure estimated charge transfer resistance (R_ct) of Comparative Examples 1 and 4 and Examples 1 to 4, which was used to calculate a R_ct difference (ΔR_ct) between where a potential of −0.8 V was applied for 9 minutes and where not applied, and then, ΔR_ct according to nitrate concentrations was shown in FIG. 8. For comparison, ΔR_ct according to LiTFSI concentrations was shown in FIG. 8.

Referring to FIG. 8, LiTFSI exhibited no ΔR_ct change according to the concentrations, but nitrate exhibited sharply increasing ΔR_ct from the concentration of greater than or equal to about 6 m.

Comparative Examples 1 and 4 and Examples 1 to 4 having different nitrate concentrations were subjected to potentiostatic bulk electrolysis in the aforementioned method to calculate each faradaic efficiency of NO₂₋ and NH₃, nitrate reduction products, and the results are shown in FIG. 9.

The faradaic efficiency was calculated according to Equations 1 and 2.

FE_NO2-=2FVC_NO2-/Q_tot  [Equation 1]

FE_NH3=8FVC_NH3/Q_tot[Equation 2]

In Equations 1 and 2, FE_NO2- is faradaic efficiency of NO₂₋, FE_NH3 is faradaic efficiency of NH₃, F is a faraday constant, V is an electrolyte solution volume, C_NO2- is a concentration of NO₂₋ in an electrolyte solution after electrolysis, C_NH3 is a concentration of NH₃ in the electrolyte solution after the electrolysis, and Q_tot is a total charge of the electrolysis.

Referring to FIG. 9, when nitrate was at a concentration of 0.1 m, less than 2% of charges were involved in the nitrate reduction reaction, which confirmed that the hydrogen evolution reaction was dominant. Even if the nitrate concentration was increased to 3 m, total faradaic efficiency only increased to 8% or so. On the other hand, when the nitrate concentration was greater than or equal to about 6 m, total faradaic efficiency sharply increased even to 64% at the nitrate concentration of 9 m.

Referring to the SCEM, EIS analysis, and UV-vis absorption spectroscopy titration results as functions of nitrate concentrations, the nitrate concentration of greater than or equal to about 6 m had a critical significance in the acidic nitrate electrolyte solutions. It was understood that solvation structural changes in the acidic electrolyte solution activated the nitrate reduction reaction but suppressed the hydrogen evolution reaction.

NaNO₃ instead of LiNO₃ was used at various concentrations of 0.1 m, 3 m, 6 m, and 9 m to prepare electrolyte solutions, which were subjected to SECM CVs, EIS, and UV-vis appropriate analysis in the same methods as above. Among them, ΔR_ct changes according to the NaNO₃ concentrations are shown in FIG. 10. In the overall analysis including FIG. 10, NaNO₃, like LiNO₃, exhibited that the nitrate reduction reaction was rapidly activated at the concentration of greater than or equal to about 6 m, but the hydrogen evolution reaction was suppressed. In the case of KNO₃, which had solubility of about 4 in water at 25° C., it was difficult to directly compare.

Hereinafter, in order to determine a solvation structure that enables the nitrate reduction reaction at the concentration of greater than or equal to about 6 m, a water network was analyzed through hydrogen bonding in the electrolyte solutions.

Electrolyte solutions were prepared by dissolving 5 mM of HClO₄ in distilled water and also, LiTFSI at each concentration of 0.1 m, 3 m, 6 m, 7 m, 8 m, and 9 m and then, subjected to infrared (IR) spectroscopy to analyze an OH stretching mode of H₂O. In the IR spectra, peaks appeared at 3300 cm⁻¹, 3471 cm⁻¹, and 3593 cm⁻¹, which corresponded to network water, intermediate water, and multimer water in order. In the network water, H₂O had a coordination number of less than or equal to 4, but as the network water changed into the multimer water, a hydrogen bond between water molecules became significantly weaker. The three peaks were separated to calculate a ratio of each peak to a total area of the three peaks, and each peak area ratio according to the LiTFSI concentrations is shown in FIG. 11.

Referring to FIG. 11, H₂O largely existed as the network water at 0.1 m, which means that water molecules smoothly interact through the hydrogen bond. On the other hand, at 9 m, as the peak area ratio of the network water significantly decreased, but the ratio of the multimer water increased. Accordingly, the 9 m LiTFSI acidic electrolyte solution, in which the hydrogen bond between water molecules were significantly disrupted, was confirmed to be converted from conventional salt-in-water electrolyte (SIWE) to water-in-salt electrolyte (WISE).

Subsequently, other electrolyte solutions prepared by dissolving 5 mM of HNO₃ in distilled water and also, LiNO₃ at different concentrations of 0.1 m, 3 m, 6 m, 7 m, 8 m, and 9 m were prepared and then, subjected to IR spectroscopy analysis. In their IR spectra, peaks of network water (3300 cm⁻¹), intermediate water (3471 cm⁻¹), and multimer water (3593 cm⁻¹) were separated to calculate a ratio of each peak area to a total area of the three peaks, which is shown as a peak area ratio according to the nitrate concentrations in FIG. 12.

Referring to FIG. 12, unlike FIG. 11, as the nitrate concentration was increased, the ratio of the network water was slightly decreased, while the intermediate water was slightly decreased, but the multimer water exhibited a low ratio at all the concentrations. This shows that even when the nitrate concentration was increased to 9 m in the acidic aqueous solution, the hydrogen bond between the water molecules was not weakened, which means that WISE was not formed.

Hereinafter, formation of hydronium-in-salt electrolyte (HISE) is explained through molecule dynamic simulation.

Electrolyte solutions were prepared by dissolving 5 mM of HNO₃ in distilled water and also, LiNO₃ at different concentrations of 0.1 m, 3 m, 6 m, 7 m, 8 m, and 9 m to analyze a radial distribution function (RDFs) between Li⁺ and other types (H₂O, NO₃₋, Li⁺, and H₃O⁺). FIG. 13 is a graph showing RDFs of Li⁺ and H₂O according to the nitrate concentrations, and FIG. 14 is a graph showing RDFs of Li⁺ and H₃O⁺ according to the nitrate concentrations.

Referring to FIG. 13, even when the nitrate concentration was increased, H₂O with a distance of less than about 2 Å with Lit exhibited almost no change in density, which indicates that H₂O and Lit had weak the interactions even at the high concentrations. Referring to FIG. 14, the higher density of H₃O⁺ with a distance of about 4 Å or less or about 6 Å with Lit, the higher nitrate concentration. The higher nitrate concentration, the higher density of H₃O⁺, which was understood that H₃O⁺ had more interactions with Lit as well as NO₃₋.

FIG. 15 is a molecule dynamic snapshot showing the solvation structure of the 9 m LiNO₃ acidic electrolyte solution. In FIG. 15, each type represents H₂O (red), NO₃₋ (yellow), Li⁺ (pink), and H₃O⁺ (green). Referring to FIG. 15, the lower density of H₂O, the higher density of NO₃₋ increased in a solvation shell of H₃O⁺, and also, Li⁺ appeared. Accordingly, in the acidic high-concentration nitrate electrolyte, H₃O⁺ was solvated by Li⁺NO₃₋ to form HISE.

FIG. 16 is a graph showing a coordination number of H₃O⁺ to Lit according to the nitrate concentrations. Referring to FIG. 16, the coordination number sharply increased, as the concentration was increased from 0.1 m to 6 m, but then, stopped increasing at the 6 m. This result indicates that as the nitrate concentration was increased, the solvation structure changed from SIWE to HISE, and the change was almost completed at about 6 m. This is consistent with the experimental results described above, which shows that the concentration of greater than or equal to about 6 m has a critical significance in the nitrate reduction reaction.

Hereinafter, mechanism of the nitrate reduction reaction in HISE is explained.

FIG. 17 is a schematic view showing an electrical double layer where nitrate reduction may occur competitively with H⁺ on the Pt electrode surface in HISE of acidic LiNO₃. In HISE, ions are not individually hydrated but form a complex solvation structure. Within the ion cluster of HISE, a distance between H₃O⁺ and NO₃₋ is reduced, lowering an activation barrier of proton-coupled electron transfers (PCETs) from H₃O⁺ to NO₃₋, which may result in generating reactivity intermediates. This is distinct from SIWE, where H₃O⁺ and NO₃₋ are completely separated by a thick hydration shell. The ion cluster of HISE may access to IHP due to the overall charge cancellation by the cation-anion bond structure. In other words, the ion cluster composed of [Li⁺+H₃O⁺+NO₃₋] may be adsorbed on IHP, and the nitrate reduction reaction may occur through PCET.

In order to check a difference in the nitrate reduction reaction depending on the presence or absence of acidity, each electrolyte solution was prepared by adding or not adding 5 mM of HNO₃ to a 9 m LiNO₃ aqueous solution. These two electrolyte solutions were subjected to cyclic voltammetry analysis to −1.2 V at a scan rate of 1 V/s in SECM TG/SC mode. FIG. 18 is a graph showing collection efficiency (N_eff′, i_S/i_T) according to a tip electrode potential (E_T) when HNO₃ was added. FIG. 19 is a graph showing N_eff according to E_T when HNO₃ was not added. Referring to FIG. 19, N_eff was about 80%, which was more than twice of that in FIG. 18, indicating that a hydrogen evolution reaction was dominant. This result indicates that when there is no H⁺ (e.g., H₃O⁺ near a Pt electrode) in an electrolyte solution, even if the high nitrate concentration was high, the nitrate reduction reaction did not occur effectively because the hydrogen evolution reaction was not suppressed.

While this invention has been described in connection with what is presently considered to be practical example embodiments, it is to be understood that the invention is not limited to the disclosed embodiments. On the contrary, it is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.