Bi/ZIF-8 catalysts: the important role of ZIF-8 for enhanced electrochemical N₂-to-NH₃ conversion using a neutral electrolyte†

Pengju Guo , Fengxiang Yin * and Jiahui Liang
Jiangsu Key Laboratory of Advanced Catalytic Materials and Technology, School of Petrochemical Engineering, Changzhou University, Changzhou 213164, China. E-mail: yinfx@cczu.edu.cn; Tel: +86-519-86330253

First published on 7th July 2025

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1. Introduction

Ammonia (NH₃), as a basic chemical raw material, is not only widely used in agriculture and industry but is also an excellent hydrogen energy carrier.^1,2 At present, industrial ammonia synthesis mostly uses the Haber–Bosch method, which releases large amounts of greenhouse gases and consumes high energy.^3,4 The electrochemical nitrogen reduction reaction (NRR) to synthesize ammonia using water and nitrogen as raw materials under mild conditions is a green ammonia synthesis route with great application prospects.⁵ However, the most currently reported NH₃ yields and faradaic efficiencies (FEs) during the NRR process are relatively low, mainly owing to the weak N₂ adsorption nature of the catalyst and stable NN bonds, resulting in a high activation energy barrier of the N₂ molecule and occurrence of the inevitable HER.⁶ Therefore, it is very important to design an efficient NRR catalyst with a reasonable structure and enhanced adsorption/activation ability for N₂ that could weaken the NN bond strength and significantly increase the NH₃ yield and FE.

In recent years, transition metal-based NRR catalysts, such as transition metal compounds, transition metal-based alloy materials, and transition metal-based single-atom materials,^7–10 have been developed to enhance the adsorption of N₂ molecules and promote the cleavage of NN bonds, thereby reducing the activation energy barrier. However, the d-orbital electrons of these materials are prone to form metal–H bonds and lead to occurrence of the HER,¹¹ which reduces the FE of NRR process. Many studies have shown that most main group metals have poor hydrogen adsorption capacity.¹² In particular, bismuth can not only effectively weaken the combination of charge and adsorbed proton owing to its limited surface electron accessibility,^13,14 but also effectively activate N₂ molecules owing to its unique p-electron structure.^15,16 Various Bi-based materials, including Bi nanosheets,¹³ Bi nanodendrites¹⁷ and PdBi alloys,¹⁸ have been explored as NRR catalysts in recent years. All these research results have proven that Bi-based catalysts can effectively inhibit the HER and enhance the activation of N₂ owing to the presence of a large number of Bi active sites. However, many Bi-based catalysts still encounter the problem of weak N₂ adsorption capacity.^19–21 Therefore, improving the adsorption capacity of Bi-based catalysts for N₂ is expected to synthesize high-performance NRR catalysts.

It is worth noting that metal–organic framework (MOF) materials usually have excellent N₂ adsorption capacity and have been significantly used in NRR in recent years.^22,23 For example, some pure MOFs, such as ZIF-8,²⁴ MIL-100 (Al)²⁵ and HKUST-1,²⁶ have been used as NRR electrocatalysts. In particular, ZIF-8 combined with active materials, such as Au/ZIF-8,²⁴ WS₂/ZIF-8,²⁷ ZIF-8@Ti₃C₂ (ref. 28) and CeO₂/ZIF-8,²⁹ have shown efficient NRR performance. Among these, AuCu/ZIF-8 catalyst has been reported to achieve an excellent NH₃ yield (63.9 μg⁻¹ mg_cat.⁻¹) and FE (14.2%).³⁰ Thus, the above studies have showed that ZIF-8 combined with metal active materials can exhibit higher NH₃ yield and FE. This is mainly owing to the high porosity and high specific surface area of ZIF-8, which could enhance the adsorption and mass transfer diffusion of N₂-/N-related intermediates;^31,32 however, the crucial role of ZIF-8 in improving NRR performance has not been thoroughly and systematically studied yet. Herein, Bi/ZIF-8 series catalysts were successfully synthesized via a simple one-step reduction method. These catalysts significantly improved the NRR performance (NH₃ yield and FE) compared with Bi NPs. Among them, 8%Bi/ZIF-8 exhibited the highest activity and selectivity, with an NH₃ yield of 34.53 μg h⁻¹ mg_cat.⁻¹ and FE of 23.27%, which were much higher than those of the latest reported Ti–Bi₂WO₆ (NH₃ yield: 23.14 μg h⁻¹ mg_cat.⁻¹, FE: 11.44%)³³ and Bi₂MoO₆ (NH₃ yield: 20.46 μg h⁻¹ mg_cat.⁻¹, FE: 8.17%).³⁴ The important role of ZIF-8 in improving the NRR performance of Bi/ZIF-8 series catalysts was studied in-depth and systematically via experiments and DFT calculations.

2. Experimental section

2.1. Preparation of Bi/ZIF-8 catalysts

ZIF-8 was synthesized via a static precipitation method, and the specific preparation process is provided in the ESI.† BiCl₃ (0.2 mmol) and PVP (0.05 g) were dissolved in ethylene glycol (100 mL). Subsequently, it was ultrasonically treated for 30 minutes to obtain a clear solution and stirred for 30 minutes. Then, ZIF-8 (0.5 g) was added to the above solution and stirred for 12 h to obtain a suspension. Following this, 3 mL NaBH₄/ethylene glycol (0.1 mol L⁻¹) was slowly added to the suspension and reacted in a reactor at 120 °C for 2 h. Finally, the precipitate was washed and filtered several times and vacuum-dried at 50 °C to obtain 8%Bi/ZIF-8. With the other synthesis conditions unchanged, 4%Bi/ZIF-8 or 12%Bi/ZIF-8 series catalysts were prepared by adjusting the content of BiCl₃, and Bi nanoparticles (Bi NPs) do not need the addition of ZIF-8 during the preparation process. Among these, the amounts of BiCl₃ added during the preparation of 4%Bi/ZIF-8 and 12%Bi/ZIF-8 were 0.1 mmol and 0.3 mmol, respectively. All samples were stored under Ar gas protection for further use. In this work, Bi/ZIF-8 is the general term used for the three catalysts, namely, 4%Bi/ZIF-8, 8%Bi/ZIF-8 and 12%Bi/ZIF-8.

3. Results and discussion

3.1. Structural characteristics of catalysts

As shown in Fig. 1a, ZIF-8 was first synthesized via a liquid phase method, and then, most of the Bi nanoparticles were uniformly loaded onto the ZIF-8 via a polyol reduction method to obtain Bi/ZIF-8 series catalysts using PVP as the dispersant. ZIF-8 was successfully synthesized (CCDC # 864311) (Fig. 1b), and exhibited an obvious polyhedral structure, which was consistent with the literature reports (Fig. 1c).^35–37 The prepared Bi NPs showed strong diffraction peaks (PDF # 85-1329) (Fig. 1b),¹⁴ exhibited a certain degree of agglomeration, and the average particle size was about 28.37 nm (Fig. 1d). HRTEM results showed an obvious 0.227 nm lattice fringe (Fig. 1e), which corresponded to the (110) crystal face of Bi (PDF # 85-1329).

The prepared Bi/ZIF-8 catalysts all show obvious ZIF-8 and Bi diffraction peaks (Fig. 2a), indicating that ZIF-8 is preserved during the synthesis process of Bi/ZIF-8 catalysts. The diffraction peak intensity of Bi gradually increases as the amount of Bi precursor increases. Among them, most of the Bi nanoparticles in 8%Bi/ZIF-8 are uniformly dispersed on the ZIF-8 surface (Fig. 2b), and the majority of particle sizes are distributed between 12.3 and 16.7 nm (Fig. 2c), with an average particle size of about 13.58 nm. Compared with Bi NPs (Fig. 1d), Bi nanoparticles supported on ZIF-8 have better dispersibility and a smaller average particle size, which indicates that the introduction of ZIF-8 can fully expose more of the Bi active sites. Fig. 2d shows that the Bi nanoparticles in 8%Bi/ZIF-8 exhibit a 0.319 nm lattice fringe, which is attributed to the (012) crystal face of Bi. Elemental mappings of 8%Bi/ZIF-8 (Fig. 2f–i) show that the C, N, Zn and Bi elements are uniformly distributed.

Fig. 3a shows the N₂ adsorption–desorption isotherms of ZIF-8, Bi NPs and Bi/ZIF-8 series catalysts. The isotherm curves of Bi NPs, ZIF-8 and Bi/ZIF-8 series catalysts belonged to the type I isotherm, indicating that they possess a typical microporous structure.^38,39 Bi NPs exhibited the smallest specific surface area (BET SSA: 134.61 m² g⁻¹). After the introduction of ZIF-8 (BET SSA: 812.37 m² g⁻¹), the BET SSA of Bi/ZIF-8 series catalysts were greatly improved. Among them, the BET SSA of 8%Bi/ZIF-8 was 687.42 m² g⁻¹, which was higher than that of 4%Bi/ZIF-8 (556.74 m² g⁻¹) and 12%Bi/ZIF-8 (483.72 m² g⁻¹). The high specific surface area of ZIF-8 can well disperse Bi nanoparticles and inhibit Bi nanoparticle agglomeration, thereby exposing more Bi active sites. Table S1† shows that the introduction of ZIF-8 in Bi results in a higher pore volume and mean pore diameter of Bi/ZIF-8 catalysts compared with Bi NPs, which facilitates the full contact of catalysts and electrolyte and accelerates the mass transfer diffusion of N-related intermediates during the NRR process. Therefore, the introduction of ZIF-8 with an abundant and ordered porous structure improves the NRR performance of Bi/ZIF-8 catalysts.

In ZIF-8 and 8%Bi/ZIF-8, C 1s can be deconvoluted into two peaks, which are attributed to the C–N peak (285.8 eV) and C–C/C–H peak (284.6 eV) (Fig. S1a†).^40,41 Similarly, the N 1s spectra show obvious N–C and NC bonds, corresponding to binding energies of 398.9 and 400.3 eV, respectively (Fig. S1b†).^40,42 Compared with ZIF-8, the binding energy of C 1s and N 1s in 8%Bi/ZIF-8 is negatively shifted, indicating that the introduction of bismuth leads to the redistribution of surface charges.

The Zn 2p spectra exhibited two strong peaks at 1021.4 and 1044.5 eV (Table S2†), which were attributed to Zn 2p_3/2 and Zn 2p_1/2, respectively (Fig. 3c).^40,42 The Zn 2p binding energy of 8%Bi/ZIF-8 showed a positive shift compared with Zn 2p binding energy of ZIF-8 (Table S2†). The introduction of Bi caused the decrease in the electron cloud density of Zn elements on the 8%Bi/ZIF-8 catalyst surface. Meanwhile, the introduction of NaBH₄ degraded a part of the pore structure of ZIF-8, thus exposing more Zn elements. An effective exposure to Zn led to an effective contact with highly dispersed Bi sites. As for the Bi 4f spectra of Bi NPs, the peaks at 164.2 and 158.9 eV belonged to Bi³⁺, and the peaks at 162.1 and 156.8 eV belonged to the metal Bi⁰ (Fig. 3d).^43,44 However, Bi 4f of 8%Bi/ZIF-8 only exhibited a Bi⁰ peak and showed an obvious negative shift compared with Bi NPs. This was mainly attributed to the fact that the introduction of ZIF-8 enabled full contact of highly dispersed Bi with highly exposed Zn, causing a part of the charge transfer from Zn to Bi, thereby promoting the formation of electron-rich Bi on the 8%Bi/ZIF-8 catalyst surface.

3.2. NRR performance of the catalysts

Electrochemical tests were carried out using an H-type electrolytic cell. For the preliminary evaluation of the NRR activity, LSV tests were conducted on an 8%Bi/ZIF-8 catalyst in N₂-/Ar-filled environments (Fig. 4a). Results showed that their current densities in the N₂-filled state were higher than those in the Ar-filled state when the applied voltage was lower than −0.3 V, indicating that the catalyst exhibited NRR activity. To further investigate the NH₃ yield and FE of the catalysts, i–t tests were performed (Fig. S2†). For all the catalysts, the current densities gradually increased with increasing applied voltages because an increase in the applied voltage promoted more charges to pass through the electrode surface. After conducting the i–t test for 3 h, a part of the electrolyte in the cathode chamber was collected, and the NH₃ yield was calculated via the indophenol blue method (Fig. S3 and S4†). Fig. 4b–c and S5–S7† indicate that both NH₃ yields and FEs of all the catalysts first increase and then decrease as the applied voltages gradually decrease from −0.3 to −0.7 V, and reach the highest value at −0.5 V (Tables S3 and S4†). Both proton transfer and charge transfer were enhanced when an initial voltage was applied, which facilitated the adsorption and activation of N₂. Therefore, both NH₃ yield and FE were improved. However, when the applied voltage became more negative, the kinetics of the HER process with two-electron transfer was greatly enhanced, exceeding the slow NRR process with six-electron transfer, and the HER gradually became dominant. Subsequently, the catalyst surface was covered with more hydrogen, which hindered the adsorption and activation of N₂. Therefore, the NH₃ yield and FE ultimately decreased to varying degrees.

Fig. 4d and Table 1 show that the NH₃ yields and FEs of Bi/ZIF-8 are much higher than those of Bi NPs and ZIF-8 at −0.5 V vs. RHE, indicating that the introduction of ZIF-8 can significantly improve the NRR catalytic activity of Bi/ZIF-8 catalysts. In the Bi/ZIF-8 series catalysts, the NH₃ yield and FE show the trend of first increasing and then decreasing with the gradual increase of Bi. Among them, the 8%Bi/ZIF-8 catalyst has the best performance, and its NH₃ yield and FE are 34.53 μg h⁻¹ mg_cat.⁻¹ and 23.27%, respectively, which are superior to most Bi-based NRR catalysts (Table S5†), such as OVs-BiVO₄ (NH₃ yield: 8.61 μg h⁻¹ mg_cat.⁻¹; FE: 10.03%),⁴⁵ Au nanoparticles on Bi nanosheets (NH₃ yield: 20.39 μg h⁻¹ mg_cat.⁻¹; FE: 15.53%),⁴⁶ 2D mosaic bismuth nanosheets (NH₃ yield: 13.23 μg h⁻¹ mg_cat.⁻¹; FE: 10.46%).¹³

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Fig. S8† and 4e show that the highest N₂H₄ yield is 0.04 μg h⁻¹ mg_cat.⁻¹, indicating that the 8%Bi/ZIF-8 catalyst exhibited good selectivity for the NRR. To prove that the produced NH₃ comes from the introduced N₂, a series of control experiments were performed (Fig. S9a and b†), including (i) 8%Bi/ZIF-8 tested in the N₂-filled state at OCP; (ii) 8%Bi/ZIF-8 tested in the Ar-filled state at −0.5 V; (iii) blank carbon paper (CP) tested in the N₂-filled state at −0.5 V and (iv) electrolyte (0.1 M Na₂SO₄) placed in air for 24 h. Fig. 4f shows that the highest NH₃ yield is only 0.08 μg h⁻¹ mg_cat.⁻¹ under the above control experimental conditions, which proves that the detected NH₃ was from the N₂ introduced during the NRR process. Fig. S9c† shows that the current density of 8%Bi/ZIF-8 remains stable at −0.5 V vs. RHE during the 24 h i–t test. Fig. 5a and S9d† show that during ten consecutive cycle tests, current densities and the corresponding absorbances remain stable, and the NH₃ yield and FE of 8%Bi/ZIF-8 have almost no decline, and its NH₃ and FE losses are only 1.62% and 1.93%, respectively (Fig. 5b). These results show that the 8%Bi/ZIF-8 catalyst possess excellent electrochemical stability.

3.3. Study on the crucial role of ZIF-8 on enhanced NRR performance

From the above results, the NH₃ yield and FE of Bi/ZIF-8 catalysts are much higher than those of Bi NPs, indicating that ZIF-8 plays a very important role in improving the NRR performance of Bi/ZIF-8 catalysts. Generally speaking, the electrochemical active surface area (ECSA) and conductivity of catalysts in electrochemical reactions have significant effects on the catalytic performance. Double-layer capacitance (C_dl) tests were performed to evaluate ECSAs of the catalysts (Fig. S10†). Fig. 6a shows that Bi/ZIF-8 catalysts have higher ECSAs than Bi NPs and ZIF-8, which shows that the introduction of ZIF-8 has a positive effect on improving ECSA. In Bi/ZIF-8 catalysts, the ECSAs first increase and then decrease with the gradual increases of Bi content. Electrochemical impedance (EIS) tests were performed to evaluate the impact of charge transfer on FE. Fig. 6b shows that Bi/ZIF-8 catalysts exhibit higher charge transfer efficiency than Bi NPs. In Bi/ZIF-8 catalysts, the high specific surface area of ZIF-8 can expose more Bi active sites and Zn elements, thereby allowing efficient charge transfer between the highly dispersed Bi sites and Zn, which can appropriately regulate the charge transfer rate and weaken the combination of charge and adsorbed proton, thus inhibiting HER.

Fig. S11† shows that the N₂-TPD results of Bi NPs, ZIF-8 and 8%Bi/ZIF-8. Bi NPs have N₂ desorption peaks at 267 °C. Meanwhile, 8%Bi/ZIF-8 also has N₂ desorption peaks at 309 and 370 °C. Compared with Bi NPs, 8%Bi/ZIF-8 has a higher N₂ desorption temperature and larger N₂ adsorption area, which further indicates that the introduction of ZIF-8 can enhance the N₂ adsorption capacity of 8%Bi/ZIF-8.

The previous experiments confirmed that the NRR process of 8%Bi/ZIF-8 and Bi NPs follows the same alternating pathway, but 8%Bi/ZIF-8 exhibited a better N₂ adsorption capacity than Bi NPs. Therefore, DFT simulation calculations were performed to further study the effect of ZIF-8 on the N₂ adsorption capacity of the catalysts. Fig. S12† shows the optimal structural models of ZIF-8, Bi (110), and Bi/ZIF-8. The N₂ adsorption Gibbs free energy of Bi/ZIF-8 (*N₂, −0.31 eV) was significantly lower than that of Bi (110) (*N₂, 0.13 eV) (Fig. 7), which indicated that the introduction of ZIF-8 was beneficial for improving the N₂ adsorption capacity of Bi/ZIF-8. Meanwhile, Fig. S13† shows that the N–N bond lengths of *NN increase from 1.47316 Å (ZIF-8) and 1.51524 Å (Bi (110)) to 1.58463 Å (8%Bi/ZIF-8). Similarly, Bi/ZIF-8 exhibited longer N–N bond lengths thanZIF-8 and Bi (110) (Fig. S11†), which indicated that Bi/ZIF-8 better adsorbed and activated N_xH_y, thereby effectively weakening NN and activating N₂. As shown in Fig. 7, the thermodynamic rate-determining step (RDS) of ZIF-8, Bi (110) and Bi/ZIF-8 was the first hydrogenation process (*NN → *NN–H). After the first protonation of N₂, the free energy values of the alternating pathway intermediates (*NHNH, *NHNH₂ and *NH₂NH₂) were lower than those of the distal pathway intermediates (*NNH₂, *N and *NH) (Fig. 7), indicating that ZIF-8, Bi (110) and Bi/ZIF-8 catalysts followed the alternating pathway. Meanwhile, 8%Bi/ZIF-8 exhibited a lower RDS (0.16 eV) than Bi (110) (0.47 eV) and ZIF-8 (0.64 eV). From the above results, it is clear that the introduction of ZIF-8 promotes Bi/ZIF-8 catalyst to better adsorb N₂ and cleave NN bonds, thereby reducing the RDS.

Generally, the N₂ adsorption/activation ability of the active sites on the catalyst surface is closely related to the electronic structure of the active species. Fig. 8a and b show that the p-band center of Bi in Bi/ZIF-8 (−0.138 eV) is closer to the Fermi level compared with Bi (110) (p-band center: −1.157 eV), which is mainly attributed to the fact that highly dispersed Bi in Bi/ZIF-8 can effectively contact with highly exposed Zn in ZIF-8, resulting in a partial charge transfer from Zn to Bi, thereby promoting the formation of electron-rich Bi on the Bi/ZIF-8 surface. This proper adjustment of the p-band center effectively enhanced the adsorption capacity for N₂- and N-related intermediates on Bi active sites in Bi/ZIF-8 series catalysts. Moreover, the projected state density (PDOS, Fig. 8c and d) of *NN showed that the formation of electron-rich Bi on the Bi/ZIF-8 surface further enhanced the p-electron feedback ability of Bi active sites and improved the Bi 6p-N₂ π* interaction, thereby transferring more occupied Bi 6p orbital electrons to the π* antibonding orbitals of N₂ (Fig. 9a). This process effectively weakened the strength of NN bonds, thus reducing the RDS (*NN → *NNH, 0.16 eV). Specifically, the Bi 6p state and N₂ π* orbitals of Bi/ZIF-8 overlapped below and above the Fermi level (Fig. 8d), and the p–π* interactions led to the increased feedback of the occupied Bi 6p orbital electrons to the N₂ π* orbital. This p-electron feedback effectively cleaved the NN bonds and strengthened the metal–nitrogen bonds. Compared with Bi/ZIF-8, the overlap of 6p states and π* orbitals of N₂ on Bi (110) below/above the Fermi level was significantly lower (Fig. 8c), indicating that Bi/ZIF-8 exhibited a stronger N₂ activation ability than Bi (110), which further proved that the effective charge transfer between Bi sites and Zn in Bi/ZIF-8 can enhance the p-electron feedback ability of Bi active sites and weaken NN bonds, thereby reducing the RDS.

Meanwhile, HER calculations were performed to further study the effect of ZIF-8 on the NRR selectivity of Bi/ZIF-8 catalysts. Fig. 9b shows that the ΔGH* values of ZIF-8, Bi (110) and Bi/ZIF-8 are 0.04, 0.20 and 0.35 eV, respectively, indicating that 8%Bi/ZIF-8 better suppress the proton adsorption, thereby limiting the HER process. Moreover, ZIF-8, Bi (110) and Bi/ZIF-8 were located in the N₂ dominant region (ΔGH* > ΔGN₂*) (Fig. 9c). Especially for Bi/ZIF-8, this indicated that it is more prone to the desired NRR process than the HER side reactions.

According to the above analysis results, ZIF-8 played a very important role in enhancing the NRR performance of Bi/ZIF-8 series catalysts. The introduction of ZIF-8 promoted the Bi/ZIF-8 series catalysts to better adsorb/activate N₂ molecules, thereby reducing the RDS (Fig. 9d), significantly increasing the hydrogen adsorption free energy and inhibiting the hydrogen adsorption, which improved the selectivity for NRR (Fig. 9d). Therefore, the NH₃ yield and FE of Bi/ZIF-8 catalysts were much higher than those of Bi NPs.

4. Conclusions

In summary, the important role of ZIF-8 in improving the NRR performance of Bi/ZIF-8 series catalysts were systematically and deeply studied through experiments and DFT calculations. On the one hand, introduction of ZIF-8 enhanced the N₂ adsorption ability of the catalysts and promoted the exposure of Bi active sites. On the other hand, ZIF-8 promoted effective contact between highly dispersed Bi and highly exposed Zn, thereby facilitating the formation of electron-rich Bi. The electron-rich Bi further improved its p-band center and p-electron feedback capabilities, thereby enhancing the catalyst's adsorption and activation capacity for N₂ and reducing RDS. In addition, the introduction of ZIF-8 increased the proton adsorption free energy, thereby suppressing the HER. Therefore, ZIF-8 plays a very important role in improving the NRR performance of Bi/ZIF-8 series catalysts. After optimization, the 8%Bi/ZIF-8 catalyst obtained the highest NH₃ yield (34.53 μg h⁻¹ mg_cat.⁻¹) and FE (23.27%). This work opens up a new avenue for developing rational bismuth-based NRR electrocatalysts.

Data availability

Data will be made available upon request.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work is supported by the National Natural Science Foundation of China (22078027) and International Joint Lab of Jiangsu Education Department. Special thanks to the support from Advanced Catalysis and Green Manufacturing Collaborative Innovation Center, Changzhou University (ACGM2016-06-02), Projects Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD) and Postgraduate Research & Practice Innovation Program of Jiangsu Province.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5ta02526e

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