Copper Doped Zeolitic Imidazole Frameworks (ZIF-8): A New Generation of Single-Atom Catalyst for Oxygen Reduction Reaction in Alkaline Media

Synthesis of CTAB-caped Cu NPs

Synthesis of ZIF-8

Preparation of Cu-ZIF-8

The structural features of ZIF-8 and Cu doped ZIF-8 (Cu/ZIF-8) catalysts were explored by the X-ray diffraction (XRD) are shown in Fig. 2a. The synthesized Cu doped ZIF-8 morphology is like ZIF-8, suggesting the formation of a body-centered cubic crystal lattice symmetry. Also, the deposition of Cu nanoparticles to the reaction mixture of ZIF-8 crystals did not affect the frame structure of the ZIF-8 materials, and the structural integrity is maintained due to the close ionic size between Cu²⁺ (0.71 Å) and Zn²⁺ (0.74 Å) ions. Cu-NC and N/C show high absorption peaks of 23.4° and 43.5°, respectively corresponding to amorphous carbon and graphite carbon (Fig. 1b). For Cu-NC, the other two peaks centered at 43.2° and 50.4° correspond to metallic copper (111) and (200), suggesting the presence of zero-valent copper (Cu⁰). Due to the formation of graphite structure and amorphous carbon during pyrolysis, at 23° and 44°, respectively, there are two large peaks of Cu-NC and N–C, corresponding to carbon diffraction (002) and (101) (Fig. 1b). In particular, the two peaks at 43.4°, 50.4°, and 74.3° respectively correspond to diffraction Cu (111), (200) and (220), suggesting the presence of pure copper. The XRD analysis reveals that the copper addition has no apparent effect on the ZIF-8 structure. The X-ray diffraction spectrum does not show an apparent copper peak, which may be due to the relatively low copper content.

Zoom In Zoom Out Reset image size

The adsorption-desorption isotherm of N₂ at 77 K was also measured to investigate the porosity characteristics of the sample (Fig. S1 is available online at stacks.iop.org/JES/167/155504/mmedia, Table I). Table I summarizes the characteristic parameters of samples, including an area of the BET surface and the total volume of the pores. The I-type isotherms of crystals with BET surface are 1437, 1230, 1177, 954, 765, 653, 632, and 621 m² g⁻¹, these are for ZIF-8, NC-900, Cu/ZIF-8, Cu/NC-500, Cu/NC-600, Cu/NC-700, Cu/NC-800, and Cu/NC-900, respectively, indicating microporous structure (Table I). The adsorption-desorption hysteresis curve of N₂ near P/P₀ = 1 is consistent with the porosity between particles, further demonstrating the dual microporosity and mesoporous existence of Cu/ZIF-8 crystal (Fig. S1). The BET specific surface area and pore volume of Cu-NC-900 nanocrystals are lower than other nanocrystals. Finally, the pore size distribution is centered at 5.63 nm, which is in accordance with the ZIF-8 structure model. Figure S1 reveals that the Cu-NC and N–C micropores are less than the Cu/ZIF-8 and ZIF-8 micropores, which could be caused by the collapse of certain structures.⁴² The heavy zinc evaporation and carbon gasification are mainly attributed to the N–C and Cu-NC mesopores and micropores, and the damage to the cubic structure is mainly attributed to certain macropores. Cu-NC has more mesopores and macropores relative to the matrix, whereas N–C has fewer mesopores and macropores.⁴¹ The particle size distribution (PSD) reveals that Cu-NC is a kind of hierarchical porous carbon with micropores, mesopores, and macropores in Fig. S1 and Table II. Three-dimensional porous hierarchical structure, combined with the synergistic effect of different pore sizes, may reduce resistance to mass transfer and reveal more ORR active sites.

SEM images of samples were measured to compare the morphology (Figs. 2c and S3–S9). These images clearly show that all samples show a well-defined dodecahedral structure with truncated rhomboid, which is a typical ZIF-8 morphology as reported. These findings suggest that the ZIF-8 surface morphology and skeleton structure did not change during Cu nanoparticle doping. EDX spectrum analyzes the composition of the elements of the samples, and the findings are shown in Figs. 2d and S3–S9. These EDX results agree well with the copper content in theoretical calculation. EDX showed that after carbonization, the accurate content of copper increased. It is because zinc evaporates during pyrolysis, and some small molecules such as water and nitrogen compounds are released. The copper content increased as a result. Furthermore, the mapping of the elements is shown in Figs. 2e and S3–S9. The element mappings show the presence of carbon, nitrogen, and copper in the samples. The element mapping images show uniform copper, carbon, and nitrogen dispersion. This indicates copper was successfully incorporated into the material. The transmission electron microscope (TEM) image of Cu-NC-800 (Fig. 2a) shows a porous structure, which increases the exposure of the active center and the speed of mass transfer. The high-resolution transmission electron microscope (HRTEM) image reveals a 0.285 nm d-spacing, which belongs to the (111) crystal surface of Cu(N₃)₂ (Fig. 2b). As shown in Fig. 2, the metal active sites are evenly distributed on Cu-N/C, which after high temperature pyrolysis shows the loose and porous characteristics, while the composite metal nanoparticles are coated in the carbon layer or at the top of the carbon, so the ingenious structure is conducive to improving the stability of the catalyst while avoiding the active sites directly exposed in the electrolyte.^43–45 At the same time, the carbon layer is coated with active metal material, which avoids the direct contact between the active site and the electrolyte and improves the stability of the catalyst.

Zoom In Zoom Out Reset image size

Figure S9a displays the FT-IR spectra of the ZIF-8 and the copper doped ZIF-8. A sharp peak at 423 cm⁻¹ is caused by the Zn-N stretching mode, suggesting that the linker's zinc and the nitrogen atom are bound to form a porous ZIF coordination structure. The C–H vibration is assigned to the tensile frequency from 1100 to 1300 cm⁻¹, while the C–N stretching vibration is assigned to the peaks at 1466 and 1470 cm⁻¹. Additionally, the 1229 and 1231 cm⁻¹ peaks confirmed C–N vibration in the imidazole ring. Cu-N bonding vibration of Cu doped ZIF-8 samples is within the 550–620 cm⁻¹ range, which is consistent with the previous studies.

ZIF-8 and Cu/ZIF-8 samples are just slightly different in the thermogravimetric analysis (TGA) (Fig. S9b). The TGA curve indicates only a very low weight loss of less than 0.5% from 20 to 200 °C, which corresponds to the removal from the cavity of guest molecules (methanol or Hmim and/or CO₂). After guest-free Cu/Zn (MIM)₂ crystal was formed at 350 °C, a long platform was observed, which indicated that the three-dimensional network of all samples had good thermal stability, like that found in the literature. Both curves indicate a step that can be assigned to the MIM linker decomposition above 350 °C (the start of exothermic decomposition).^5,46 A sharp weight loss of 63%–64% is observed as the temperature increases from 400 °C to 500 °C, which is very consistent with the theoretical weight loss of 64%. The residue was XRD-doped zinc oxide doped by wurtzite copper. It should be noted that the thermal stability of Cu/ZIF-8 crystal obtained in this study is greater than that of copper-based MOFs (such as Cu (BDC)) used for catalytic use, which was decomposed at 200 °C. It should be noted that, during the heating, the zinc oxide content is still very low, and the connector is completely burnt out, suggesting that zinc oxide (ZnO) is the final calcined component of ZIF-8 nanocrystals.

X-ray photoelectron spectroscopy (XPS) further analyzed the elementary structure and chemical state of the electrocatalysts Cu-NC-800. The XPS measurement continuum (Fig. S10) shows that Cu-NC-800 is composed of elements C, Cu, N, and O. The high-resolution N 1 s spectrum can be transformed into four peaks at 398.2 eV, 400.2 eV, 401.0 eV, and 403.3 eV, respectively, attributed to pyridine-N, pyrrole-N, graphite-N, and oxidation-N. The Cu⁰ is related to the main peak with a binding energy of 933.0 eV in the high-resolution Cu2p range. Cu (II) can be allocated with the shoulder peak at 934.6 eV and the satellite peak at 945.0 eV. Coordination with N atoms to form Cu (II)-N bond is suggested. Considering that Cu-NC 's ORR behavior is higher than N/C's, this means that Cu-NC's N content is rich and can play its role since the essence of nitrogen is not essential to ORR. The pyridine-N interaction with Cu (II) forms Cu (II)-N, while graphite-N affects the electronic and geometric carbon structure. It is reported that the Cu²⁺ center-coordinated MOF with nitrogen atoms such as pyrimidine and imidazole is an active catalyst for the aerobic oxidation of alkanes without any free radical initiator under mild reaction.^2,7 We are continuing to research and find the high stability of ZIF-8 in this way. The goal of this work is to dope Cu nanoparticles in the ZIF-8 system and convert the inactive ZIF-8 into the active ZIF-8 to promote ORR without any free radical initiator/co-catalyst. The introduction of another metal Cu can increase the proportion of O–C–O/N–O and Cu oxygen bond, which greatly promotes its initial potential as ORR electrocatalyst.

Firstly, cyclic voltammetry (CV) was used to calculate ORR activity in 0.1 M KOH electrolyte saturated with oxygen or nitrogen. If the electrolyte is saturated with O₂, Cu-NC has an apparent cathodic reduction peak, while N₂ has no noticeable oxidation-reduction peak, suggesting Cu-NC has ORR activity. The results of the Cu-NC electrocatalyst were reported with the linear sweep voltammetry (LSV) to evaluate the ORR operation (Figs. 3a, 3b, 3d). From all prepared catalysts, Cu-NC-800 exhibited the onset potential (E_o 0.99 V) and the halfwave potential (E_1/2 0.85 V), superior than that of all prepared ZIF-8 (E_o 0.92 V, E_1/2 0.79 V), NC-900 (E_o 0.92 V, E_1/2 0.80 V), Cu/ZIF-8 (E_o 0.93 V, E_1/2 0.81 V), Cu-NC-500 (E_o 0.95 V, E_1/2 0.82 V), Cu-NC-600 (E_o 0.97 V, E_1/2 0.83 V), Cu-NC-700 (E_o 0.98 V, E_1/2 0.84 V), Cu-NC-900 (E_o 0.98 V, E_1/2 0.83 V), and commercial Pt/C (E_o 0.97 V, E_1/2 0.84 V). Cu/ZIF-8 and Cu-N/C displayed the better ORR activity compared to ZIF-8 and NC-900, suggesting that Cu species introduction led to ORR activity. Furthermore, the optimization experiment shows that the synthesis of Cu-NC at 800 °C is the optimal parameter for achieving the optimum balance between Cu(II)-N and Cu⁰ NPs. The excellent catalytic efficiency can be due to the following factors: the porous structure is conducive to the transfer of electron and exposure of rich active centers; the transport of electrolyte and gas diffusion; nitrogen doping in carbon matrix also promotes catalyst activity significantly, and the incorporation of Cu nanoparticles is the key to improving catalytic efficiency.^4,16,24

Zoom In Zoom Out Reset image size

A change of the LSV polarization curve with rotation speed was recorded to understand better the ORR mechanism (Figs. 3c, 3e). The measured transferring electron number (n) of ZIF-8 (2.7), NC-900 (3.3), Cu/ZIF-8 (3.4), Cu-NC-500 (3.4), Cu-NC-600 (3.5), Cu-NC-700 (3.9), Cu-NC-900 (4.1), Cu-NC-900 (3.3), and commercial Pt/C (3.8), respectively according to the slope of the Koutecky-Levich (K-L) curve (Figs. 3c, 3e, Table II). Results show that nearly all catalysts are conducive to the creation of nearly four-electron channels, which is the most powerful ORR process.^10,21,47 This enhanced activity may be due to the presence of high content of graphite nitrogen can improve the conductivity of carbon materials.^12,42

The stability of catalyst is also very important in its practical application. Therefore, we test the stability of catalyst by chronoamperometry and accelerated ageing method. Firstly, the current density curves of Cu/NC-800 catalysts were measured in 0.612 V fixed potential by chronoamperometry and 0.1 M KOH electrolyte saturated with O₂ at 1600 rpm. Under the same conditions, the chronoamperometric curves of Pt/C (20 wt%) and N/C-900 catalyst with the second-best performance were compared. As shown in Fig. 4, that the current density of Cu-N/C-800 catalyst remained at 98.5% after 10 h long-term test at a constant potential, followed by the stability of N/C-900 catalyst under the same test conditions, the current density decreased to 74.6% of the initial current density, while the current density decreased to 83.3% of the initial current density at Pt/C (20 wt%). Using the accelerated aging method, in oxygen saturated 0.1 M KOH electrolyte, at a sweep rate of 50 mV s⁻¹, CV cycle scanning was carried out in the potential range of −0.8 ∼ 0.2 V (vs SCE) for 1000 cycles, and the ORR performance curves in oxygen saturated 0.1 M KOH electrolyte before and after scanning were recorded respectively. Results, as shown in Fig.3e, after 1000 cycles of CV aging, the ORR properties of Cu-N/C-800 in 0.1 M KOH electrolyte saturated with O₂ remained unchanged. However, after 1000 cycles of CV aging, the current density and half-wave potential of Pt/C (20 wt%) decreased significantly, which also reflected the excellent stability of Cu-N/C-800 catalyst and exceeded the commercial Pt/C (20 wt%). In commercial Pt/C catalyst, aggregation and migration of Pt nanoparticles are important factors for the degradation of Pt nanoparticles.^10,15,39 The strong covalent bond between carbon and nitrogen, and the protection of copper's carbon matrix, will prevent the degradation of the Cu-NC catalyst. The results suggest high stability for the porous carbon Cu-NC. The high-performance electrochemical activity of Cu-NC is mainly due to three factors. Secondly, Cu-NC has distributed active sites with adequate uniformity. Many researchers suppose that pyridine nitrogen and graphite nitrogen are ideal for electrochemical behavior. It has now been found that oxygen molecules are readily adsorbed on carbon atoms alongside pyridine nitrogen, as these carbon atoms are the active centers of ORR.^11,23 Therefore, we assume the contribution of graphite nitrogen to the electrochemical activity of the two materials may be similar. Hence, we conclude that Cu-NC's high activity is attributable to the higher pyridine nitrogen content than N-C-900, and due to the sacrifice of the Cu-ZIF-8 prototype, the active center is evenly distributed in the material. Second, Cu-NC has an acceptable porous layered structure, and Cu manages its high large surface area. Further ORR active sites are exposed due to the proper structure, and the resistance to mass transfer is significantly reduced. Cu-NC has adequate ORR active site access, short channel length and good mass transfer capacity, and has high electrochemical activity.^9,14 Second, the copper inlay increases the conductivity of Cu-NC, which is useful for operating ORR.

Zoom In Zoom Out Reset image size