Ni-loaded Co-NC catalysts for promoting electrocatalytic nitrate reduction to ammonia†

Fang Zhao^a, Yidi Liu^a, Chengjie Li^b, Zhen Yuan^a, Qianqian Hua^a, Liguo Gao^a, Xuefeng Ren*^a, Peixia Yang*^c and Anmin Liu*^a
aSchool of Chemical Engineering, Ocean and Life Sciences, Dalian University of Technology, Panjin 124221, China. E-mail: renxuefeng@dlut.edu.cn; liuanmin@dlut.edu.cn
^bShandong Engineering Research Center of Green and High-value Marine Fine Chemical, Weifang University of Science and Technology, Weifang 262700, China
^cMIIT Key Laboratory of Critical Materials Technology for New Energy Conversion and Storage, State Key Laboratory of Urban Water Resource and Environment, School of Chemistry and Chemical Engineering, Harbin Institute of Technology, Harbin 150001, China. E-mail: yangpeixia@hit.edu.cn

First published on 20th August 2024

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1. Introduction

Ammonia is an essential component of artificial fertilizers and a perfect hydrogen-rich, carbon-free energy source.¹ Currently, ammonia synthesis relies heavily on the Haber–Bosch process, which involves hydrogen derived from steam reforming of hydrocarbons, carried out at high temperatures and pressures.^2–4 Over 300 million tons of CO₂ emissions are produced annually from energy consumption, which makes up 2% of the world's total energy consumption. In recent years, inspired by natural microbial nitrogen fixation, the electrochemical nitrogen reduction reaction (NRR) to NH₃ has attracted great interest.^5,6 However, nitrogen's industrial utilization is hindered by its minimal water solubility, the stable NN bond (dissociation energy 941 kJ mol⁻¹) and the intense competition for hydrogen evolution reactions.^7,8 These factors lead to low NH₃ and low current densities. Compared with the NRR, the interesting method for synthesizing NH₃ under ambient conditions is the electrochemical nitrate reduction reaction (NO₃RR), which is thermodynamically more viable than the NRR because of the lower NO bond dissociation energy of 204 kJ mol⁻¹.⁹ In addition, residual NO₃⁻ in wastewater removed from wastewater treatment plants is an essential source of nitrogen.

Nonetheless, ammonia yield and selectivity are still constrained by competition from a variety of by-products (NO₂⁻, NO₂, NO, N₂O, N₂, NH₂OH, NH₂NH₂).¹⁰ The NO₃RR is an eight-electron, nine-proton reaction mechanism.¹¹ More seriously, nitrites are a prevalent by-product of the NO₃RR and are more toxic and carcinogenic than nitrates.¹² Among the reported NO₃RR catalysts, several types of catalysts are included, such as non-metallic, noble metal, and transition metal.⁹ Although catalysts based on precious metals have demonstrated potent activity in the NO₃RR, their broad applicability is constrained by the high cost. Transition metals are widely used in electrocatalysis due to their abundant reserves, metallic nature, and environmental friendliness.

Metal–organic framework (MOF) derived materials combine the advantages of organic and inorganic materials and have received increasing attention for their higher porosity, good electrical conductivity, and more metal–ligand sites.^13,14 Metal nanomaterials produced by pyrolysis of MOFs can retain the precursor's characteristics. Metal cations are linked to ligand molecules in MOFs, and the arrangement and localization of ligands are determined by the coordination configuration.^15,16 A zeolitic imidazolate framework (ZIF) is an ideal carrier for the construction of catalyst metal sites by pyrolytically converting other elements, such as nitrogen, into graphitic nitrogen and pyridine nitrogen.^17,18 However, the NO₃RR electrocatalytic process involves complex 8e⁻ and 9H* transfer mechanisms.¹⁹ Bimetallic composite materials have more advantages over monometallic compounds, such as stronger metal-to-metal interactions, enhanced charge transfer capabilities, and optimized electronic structures, showing more excellent catalytic activity and stability.^20,21 Co-based catalysts have been reported for the NO₃RR,^22–24 such as CuCo₂O₄,²⁵ PP–Co,²⁶ Co₃O₄/CC,²⁷ Cu–Co₃O₄.²⁸ Metal Co has excellent stability at the electrochemical reduction potential, which facilitates the dissociation of H₂O and provides the required electrons. Nonetheless, electron rich metal Co will hinder the electrostatic adsorption of NO₃⁻ and the formation of *NH_x substances.²⁹ Chu et al.³⁰ dispersed the Co single atoms on the C₃N₄ substrate, and the monatomic Co site can effectively activate NO₂⁻ and optimize the formation energy of key NOH intermediates. In additions, Li et al.³¹ prepared copper–nickel nanoparticles (CuNi/TM) on a titanium mesh via a simple and tunable electrodeposition method. The CuNi/TM catalyst showed excellent NO₃RR performance. The metal Ni on the catalyst surface provides H adsorption sites where hydrogen molecules are adsorbed to form H-Ni bonds.^32,33 Reduces the adsorption energies of the critical intermediates *NO₃, *NO₂, and *NH₂.³⁴ Consequently, the Ni–Co bimetallic catalysts can further improve the catalytic activity by coordinating the adsorption strength of the dual active sites with the intermediates and overcoming the restriction of the single active site.^35,36

Herein, we synthesized CoZn@ZIF precursors in aqueous solution under ambient conditions, loaded metal Ni onto the surface of the precursors by a simple impregnation method, and subsequently successfully prepared bimetallic NiCo-NC catalysts by pyrolytic carbonization. Compared to Co-NC catalysts, they have the ability to accelerate the NO₃RR eight-electron reaction and efficiently promote the synthesis of NH₃. NiCo-NC has a Faraday efficiency of 92.75% and 96.65% with yields of 5.01 mg cm⁻² h⁻¹ and 10.12 mg cm⁻² h⁻¹ at −0.6 V and −1.1 V, respectively. Meanwhile, ¹⁵N isotope labeling experiments further verified the source of N in NH₃ from NO₃⁻.

2. Results and discussion

2.1. Structure and morphology

The precursor CoZn@ZIF was synthesized at ambient temperature, impregnated under ultrasonic conditions to adsorb Ni²⁺, and pyrolytically carbonized to prepare Ni-loaded Co-NC materials with the dodecahedral structure of ZIF-8. The morphology of the catalysts during the synthesis process was investigated by scanning electron microscopy (SEM). As shown in Fig. 1, CoZn@ZIF impregnated with Ni²⁺ showed a slightly rough surface, and after pyrolysis at high temperatures, exhibited a slight collapse of the carbon skeleton, and had metal particles on the surface. The size of the Ni-coated Co-NC was around 1.5 μm. For comparison, we synthesized Ni-coated Co-NC materials with different Ni loadings under equivalent conditions (Fig. S1†). The morphology of bimetallic NiCo-NC was further investigated by transmission electron microscopy (TEM) and energy-dispersive X-ray spectroscopy (EDS), as shown in Fig. 2. The catalyst possesses a dodecahedral framework of ZIF-8, and the surface nanoparticles are in the range of 50 nm in diameter. The high-resolution transmission electron microscope (HRTEM) image of NiCo-NC shows a lattice spacing of 0.345 nm, which corresponds to the (002) plane of C. EDS mapping (Fig. 2c) analysis showed that the elements Ni, Co, N, and C were uniformly distributed throughout the amorphous porous structure, indicating that Ni and Co are highly dispersed coexisting substances.

The synthesized catalysts were analyzed for the crystal structure and chemical state by X-ray polycrystalline diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). As shown in Fig. 3a, the metallic peaks of Zn are negligible. The diffraction peaks at 2θ = 44.32°, 51.48° and 75.78° correspond to the (111), (200), and (220) planes of NiCo metal, respectively.^37–39 The diffraction peak near 2θ = 25° corresponds to the low crystallization (002) plane of graphitic carbon, which corresponds to the HRTEM image. In addition, the XRD patterns of precursors NiCoZn@ZIF and CoZn@ZIF are nearly similar to the pattern of ZIF-8, indicating that the absorption of Ni²⁺ ions did not destroy the ZIF crystal structure (Fig. S3a†). XPS was performed for the study of the surface composition and valence states of Co-NC and NiCo-NC. As shown in Fig. 3b, the elements determined in NiCo-NC are C, N, Co, and Ni. The C 1s orbital spectra of NiCo-NC catalysts are shown in Fig. 3c; the obvious characteristic peaks were found at binding energies of 284.8 eV, 285.5 eV, 286.8 eV, 288.65 eV, and 291.0 eV, which can be attributed to C–C, CN, and CC, and very low-content C–O and CO peaks, respectively, indicating high graphitization. The C 1s spectra of Co-NC are given in the ESI† (Fig. S3c). As shown in Fig. 3d, the N 1s peaks at binding energies of 398.8 eV, 399.65 eV, and 401.2 eV were attributed to pyridine N, M–N, graphitic N and some oxygenated moieties (oxidized N and pyridine-N-oxide), respectively, which indicated that ZIF imidazole during pyrolysis was successfully converted to graphitic carbon. Furthermore, the catalytic efficiency is enhanced by pyridine N and graphite N, which work together to synergize the electron distribution of the active sites off-domain, providing enough anchoring for the monometallic sites of the M–N–C catalysts.⁴⁰ The Co 2p peaks of the NiCo-NC catalysts at binding energies of 778.55 eV and 795.0 eV correspond to Co³⁺ species in the 2p_3/2 and 2p_1/2 orbitals, respectively, and the peaks at 780.35 eV and 796.7 eV correspond to Co²⁺ species in the 2p_3/2 and 2p_1/2 orbitals. Similar results have also been observed from Co-NC. The Ni 2p spectrum in Fig. 3f demonstrates Ni²⁺ 2p_3/2 and 2p_1/2 peaks at 854.4 eV and 871.5 eV, respectively.⁴¹

2.2. Electrocatalytic performance of catalysts in the NO₃RR

Initially, the NO₃RR catalytic activity and onset potentials of NiCo-NC catalysts were investigated by linear sweep voltammetry (LSV), which indicates a notable variation in current densities. As shown in Fig. 4, the current density gradually increased with the increase of applied potential. When NO₃⁻ was added to the solution, the current density changed significantly. At an applied potential of −1.0 V, the current density was as high as 117 mA cm⁻², indicating the occurrence of the NO₃RR on the NiCo-NC catalyst. LSV comparisons of Co-NC catalysts with different Ni loadings are shown in Fig. S3.† With the addition of Ni, the current density gradually increased. When the addition of Ni was 0.4 mmol, the current density decreased to 105 mA cm⁻². The maximum current density was achieved when the Ni loading during the synthesis of the material was 0.3 mmol. Therefore, all subsequent tests were based on NiCo-NC catalysts with a Ni loading of 0.3 mmol. Subsequently, we measured the current density variations of NiCo-NC catalysts in an electrolyte of 0.1 M KOH & KNO₃ at different potentials and performed electrochemical tests for 2 h under an Ar atmosphere. As shown in Fig. 4b, the current density increases gradually with the increase of applied potential, and the current density curves remain stable at different potentials without large fluctuations, indicating that NiCo-NC has excellent electrochemical stability in the catalytic NO₃RR process. The content of NH₄⁺ in the electrolyte was determined using indophenol blue colorimetry (Fig. 4c). NiCo-NC obtained 92.75% FE and 5.01 mg cm⁻² h⁻¹ ammonia yield at −0.6 V; 96.65% FE and 10.12 mg cm⁻² h⁻¹ ammonia yield at −1.1 V. The ammonia yield and FE were maintained at a high level in the −0.6–−1.1 V interval of the applied potential. The FE of the by-product NO₂⁻ showed completely opposite properties (Fig. 4d), with both the by-product nitrite yield and the FE as a whole remaining at a low level. The NiCo-NC catalyst could effectively inhibit the accumulation of by-products and promote the reduction of NO₃⁻ to NH₃.

The long-term stability and cycling stability of the catalyst are important factors in evaluating the performance of the catalyst. The NiCo-NC catalyst was subjected to five cycle stability tests at −0.6 V and −1.1 V and 24 h cycle stability test. As shown in Fig. 5, NiCo-NC performed remarkable cycling stability at a potential of −0.6 V, where both NH₄⁺ and FE were well maintained at a potential of −1.1 V. There was no discernible drop in current density throughout the 24 h long term current test (Fig. 5d), suggesting that the NiCo-NC catalyst has outstanding durability.

Based on the above analysis, we further explored the performance differences between NiCo-NC and Co-NC. As shown in Fig. 5e, the ammonia yields of Co-NC at applied potentials of −0.6 V and −1.1 V were 3.85 mg cm⁻² h⁻¹ and 8.32 mg cm⁻² h⁻¹, and the FEs were 82.4% and 86.4%; which were much lower than the ammonia yields and the FEs of NiCo-NC. The superiority of NiCo-NC over Co-NC for the NO₃RR was further confirmed. To demonstrate that the N in NH₄⁺ is derived from NO₃⁻, we designed and performed a comparative test in a blank 0.1 M KOH solution (Fig. 6). Negligible NH₃ eliminates contamination from catalysts, electrolytes, or other external environments that might contain N. Furthermore, ¹⁵N and ¹⁴N isotopic labeling experiments were performed to demonstrate the N source of the product NH₃ detected by ¹H NMR spectroscopy. After electrocatalytic K¹⁵NO₃ reduction, ¹H NMR of the electrolyte showed typical ¹⁵NH₄⁺ double peaks and no typical ¹⁴NH₄⁺ triple peaks, confirming that the NH₄⁺ formed was exclusively from the electroreduction of NO₃⁻ (Fig. 6c).

3. Conclusion

In summary, Ni-coated Co-NC catalysts were successfully prepared by pyrolytic carbonization through a simple impregnation method by loading metal Ni onto CoZn@ZIF precursors. Ni-coated Co-NC was experimentally demonstrated to be an efficient electrocatalyst for ammonia synthesis. Ammonia yields of 5.01 mg cm⁻² h⁻¹ and 10.12 mg cm⁻² h⁻¹ were achieved at applied potentials of −0.6 V and −1.1 V, with FEs of 92.75% and 96.65%, respectively. ¹⁵N isotope labeling experiments further verified that the source of N in NH₄⁺ is derived from NO₃⁻.

Data availability

The data used to support the paper are included within the article.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

Support from the National Natural Science Foundation of China (21902021, 21908017), the Fundamental Research Funds for the Central Universities (DUT22LK09), the Open Foundation of Key Laboratory of Industrial Ecology and Environmental Engineering, MOE (KLIEEE-20-01, KLIEEE-21-02), the Foundation of State Key Laboratory of High-efficiency Utilization of Coal and Green Chemical Engineering (2022-K70), and the Hefei Advanced Computing Center for this work is gratefully acknowledged.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4cy00942h

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