Electrochemical nitrate reduction in acid enables high-efficiency ammonia synthesis and high-voltage pollutes-based fuel cells

Ammonia (NH₃) is crucial for fertilizers, pharmaceuticals, and chemical industries. Its role as an energy and hydrogen carrier is also gaining importance in the context of "hydrogen energy" and "carbon neutrality." Electrochemical nitrate reduction reaction (NO₃⁻ RR) for NH₃ synthesis is attractive due to nitrate's availability and abundance, especially in wastewater. Converting NO₃⁻ to NH₃ offers benefits for wastewater treatment and resource recovery. NO₃⁻ RR shows superior NH₃ yield efficiency and lower energy consumption compared to nitrogen reduction and the Haber-Bosch process. Most research focuses on NO₃⁻ RR in alkaline/neutral media, achieving high Faradaic efficiencies (FE) towards NH₃. However, nitrite (NO₂⁻) is often a byproduct, and large overpotentials are needed. In situ generated NH₃ is gaseous at the electrode surface due to increased local pH and requires acid adsorption for capture. The reaction in alkaline/neutral media involves nine proton-coupled electron transfers, potentially leading to large overpotentials and sluggish kinetics. In contrast, direct nitrate reduction under acidic conditions avoids NH₃ volatilization, allows direct ammonium fertilizer/salt production, and provides abundant protons for continuous hydrogenation, potentially enhancing the NO₃⁻ conversion rate and energy efficiency. However, this acidic NO₃⁻ RR approach is rarely explored, mainly due to the competitive hydrogen evolution reaction (HER). Most reported NO₃⁻ RR electrocatalysts at pH ≥ 7 are based on late transition metals, unstable in acidic conditions. This paper explores Fe phthalocyanine/TiO₂ (FePc/TiO₂) as a stable and active electrocatalyst for energy-efficient NO₃⁻ RR in acid (pH = 1), achieving impressive NH₃ yield and FE.

Extensive research explores selective electrocatalysts for NO₃⁻ RR in alkaline/neutral media, achieving high Faradaic efficiencies (>90%) towards NH₃. However, nitrite (NO₂⁻) is a common byproduct, requiring large overpotentials for optimal performance. The in-situ generation of gaseous NH₃ at the electrode surface due to increased local pH (above 10) necessitates acid adsorption for capture. The reaction in alkaline/neutral media involves nine proton-coupled electron transfers, which can lead to high overpotentials and slow reaction kinetics. In contrast, acidic conditions offer advantages like avoiding NH₃ volatilization and enabling the direct production of ammonium fertilizers/salts. The readily available protons in acidic media may enhance the NO₃⁻ conversion rate and improve energy efficiency. Despite the potential benefits, acidic NO₃⁻ RR for NH₃ production remains largely unexplored. The competitive HER poses a significant challenge in acidic media. Existing catalysts, primarily based on late transition metals, are often unstable in acidic conditions and show reduced NH₃ FE due to enhanced HER. This study addresses these limitations by exploring the application of FePc/TiO₂ as an electrocatalyst for NO₃⁻ RR in acidic environments.

The study investigated the NO₃⁻ RR performance of TiO₂ under acidic, neutral, and alkaline conditions using linear sweep voltammetry (LSV). Tafel slopes and Nyquist plots were analyzed to assess reaction kinetics. Activation energies (Ea) were calculated from temperature-dependent reaction kinetics. Liquid products (NH₃, N₂H₄, NO₂⁻) were detected using UV-Vis spectrophotometry, and NH₃ FE was calculated at different potentials and pH values. The pH-dependent influence on reaction pathways was investigated through free energy plots, analyzing the role of H⁺ and the effect of pH on the free energies of reaction intermediates. FePc/TiO₂ hybrid catalysts were synthesized via a wet chemical process, and their morphology and composition were characterized using SEM, EDS, XRD, Raman, and FTIR spectroscopy. The electrochemical performance of FePc/TiO₂ was evaluated for NO₃⁻ RR in terms of NH₃ yield rate and FE at different potentials and NO₃⁻ concentrations. ¹⁵NO₃⁻ and ¹⁴NO₃⁻ were used as feedstock to determine the nitrogen source and the amount of NH₃ produced, analyzed using ¹H NMR. Long-term stability was assessed through prolonged electrolysis (24 hours). Post-test characterizations (SEM, XRD, EDS, FTIR, XPS, ICP-MS) were conducted to examine the catalyst's structural and compositional changes. SCN⁻ intoxication experiments were performed to identify the active sites. In-situ FTIR was employed to detect reaction intermediates during electrolysis. Density functional theory (DFT) calculations using VASP with the PBE functional were performed to understand the NO₃⁻ reduction reaction mechanism on TiO₂ and FePc/TiO₂ surfaces. Charge density difference and density of states analyses were conducted to examine charge transfer and orbital overlap. Gibbs free energy profiles were calculated to identify the rate-limiting step and compare the catalytic activity of TiO₂ and FePc/TiO₂. An alkaline-acid hybrid Zn-NO₃ battery (AAHZNB) was assembled using FePc/TiO₂ as the cathode and a Zn plate as the anode, separated by a bipolar membrane. The battery's performance was evaluated in terms of open-circuit voltage (OCV), power density, NH₃ yield, and FE.

TiO₂ showed higher NH₃ FE and yield rate in acidic media (pH 1) compared to neutral and alkaline media. The activation energy for NO₃⁻ RR was significantly lower in acidic media. FePc/TiO₂ exhibited enhanced NO₃⁻ RR performance compared to TiO₂ and FePc alone, achieving a NH₃ yield rate of 17.4 mg h⁻¹ cm⁻² and a NH₃ FE of 90.6% at -0.65V. ¹H NMR confirmed the conversion of ¹⁵NO₃⁻ and ¹⁴NO₃⁻ to NH₃, consistent with UV-Vis results. The catalyst showed good long-term stability with minimal decrease in NH₃ FE over 24 hours. XPS analysis revealed the presence of absorbed NO₃⁻ and surface NH₃ species, along with changes in Ti and Fe oxidation states during electrolysis. SCN⁻ intoxication experiments indicated that Fe centers are the active sites. In-situ FTIR spectroscopy identified reaction intermediates, suggesting a possible reaction pathway: NO₃⁻ → *NO₃ → *NO₂ → *NO → *NOH → *NH₂OH → *NH₃. DFT calculations confirmed that FePc prefers to lie on the TiO₂ surface, forming a chemical bond with lattice oxygen. Significant charge transfer and robust Fe-O orbital overlap were observed. The rate-limiting step for NO₃⁻ reduction on FePc/TiO₂ was the formation of the NOH* intermediate (0.74 eV), compared to NH₃ desorption on TiO₂ (1.29 eV). FePc/TiO₂ showed high resistance to HER. The alkaline-acid hybrid Zn-NO₃ battery (AAHZNB) using FePc/TiO₂ achieved a high OCV (1.99 V) and power density (91.4 mW cm⁻²), surpassing previously reported Zn-NO₃ batteries. The AAHZNB also demonstrated a high NH₃ yield and FE during discharge.

The findings demonstrate the viability of acidic NO₃⁻ RR for efficient NH₃ synthesis and energy conversion. The FePc/TiO₂ catalyst combines the high selectivity of TiO₂ for NO₃⁻ RR with the enhanced activity of FePc, achieving high NH₃ yield and FE. The use of acidic conditions enhances the reaction kinetics and improves energy efficiency. The superior performance of the AAHZNB highlights the potential for integrating NH₃ synthesis with energy generation. The high OCV and power density of the AAHZNB are attributed to the combination of acidic NO₃⁻ RR and alkaline Zn oxidation, separated by a bipolar membrane. The results contribute to the development of sustainable technologies for wastewater treatment, NH₃ production, and energy storage.

This work successfully demonstrates the potential of acidic electrochemical nitrate reduction for high-efficiency ammonia synthesis and energy generation. The FePc/TiO₂ catalyst shows superior performance compared to existing catalysts, achieving high ammonia yield and Faradaic efficiency in acidic media. The developed alkaline-acid hybrid Zn-nitrate battery exhibits exceptional open-circuit voltage and power density. Future research could explore the optimization of catalyst design and battery architecture for further performance enhancement and scalability. Investigation of the long-term durability and cost-effectiveness of the system under industrial conditions is also crucial for practical applications.

The study primarily focused on laboratory-scale experiments. The long-term stability, although shown for 24 hours, requires further evaluation under more prolonged and demanding operational conditions. The cost-effectiveness of the FePc/TiO₂ catalyst and the scalability of the AAHZNB need to be investigated for industrial applications. The partial dissolution of the FePc/TiO₂ electrode in acidic conditions over prolonged electrolysis, while limited, could affect the long-term catalytic activity. Further research is needed to fully understand and mitigate this aspect.