Highly efficient and active Co–N–C catalysts for oxygen reduction and Zn–air batteries

Abstract

In this study, the Lewis doping approach of polyaniline (PANI) was employed to fabricate cobait–nitrogen–carbon (Co–N–C) oxygen electrocatalysts for Zn–air batteries, aiming to enhance the active spots of Co–N–C. This resulting Co–N–C catalysts exhibited well-defined nanofiber networks, and the Brunauer-Emmett-Teller (BET) analysis confirmed their substantial specific surface area. Electrochemical experiments demonstrated that the Co–N–C catalysts achieved the half-wave potential (vs. RHE) of 0.85 V in alkaline medium, overcoming Pt/C and iron–nitrogen–carbon (Fe–N–C) counterparts in extended cycle testing with only a 25 mV change in a half-wave potential after 5000 cycles. Remarkably, the highest power density measured in the zinc (Zn)-air battery reached 227 mW/cm², a significant improvement over the performance of 101 mW/cm² of the platinum on activated carbon (Pt/C) catalyst. These findings highlight the advantageous stability enhancement associated with the utilization of Co in the Co–N–C catalysts.

Graphical abstract

Keywords

oxygen reduction reaction (ORR) / oxygen evolution reaction (OER) / non-noble metal catalysts / Co–N–C catalysts / Zn–air battery

1 Introduction

In rechargeable Zn–air battery, the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) play a decisive role in enhancing performance. Because of the multistage and proton-coupled electron transfer (PCET) processes of the reversible oxygen reactions, the dynamics of OER and ORR are sluggish with the air cathode. Consequently, extensive research efforts have focused on exploring efficient electrocatalysts to shorten the overpotential and expedite reaction kinetics [1–3]. Currently, benchmark catalysts used for ORR or OER are predominantly noble metal catalysts (e.g., Pt, Ir, RuO₂), which are costly, scarce in supply, and exhibit poor bifunctional catalytic properties. These limitations significantly hinder their application in Zn–air batteries [4–7]. Hence, it has become urgent to explore inexpensive non-precious metal catalysts. Among these, conversion metal–nitrogen–carbon (M–N–C) materials, because of their easy accessibility, low economic expenditure, and well activity resulting from the cooperative co-doping effect between metals and nitrogen dopants [8–10], are considered the most hopeful substitutes for bifunctional precious metal catalysts [11–13].

The catalytic activity of various M–N–C catalyst systems was systematically evaluated. The results showed that Fe > Co > Cu > Mn > Ni was a volcanic activity order, with Fe–N–C having the highest oxygen electrochemical catalytic activity [14–16]. However, Fe-based catalysts tend to undergo demetalization on account of the Fenton effect. This existence of dissolved Fe ions in Fe–N–C catalysts catalyzes the generation of free radicals from H₂O₂, leading to a degradation in durability and a poor stability under acidic conditions. Hence, it is crucial to substitute Fe with metal atomic sites that do not generate H₂O₂ radicals. Co–N–C catalysts are highly effective in improving the stability of oxygen reduction, as they are not affected by the Fenton reaction [17,18]. As a result, Co may emerge as the most hopeful transition metal to proxy Fe. Furthermore, Co–N–C materials have emerged as hopeful bifunctional electrocatalysts owing to the existence of efficient active middles with appropriate binding energies, strong Co-N connecting effects, and collaborative impacts between N-doped carbon and nanocrystals [19]. Despite ongoing debates surrounding the actual active spots for both ORR and OER, it has been verified that M–N and pyridine-N play a non-negligible part in achieving remarkable electrocatalytic performance [20,21]. Consequently, a viable strategy to improve electrocatalytic performance is to decrease the grain size of the metal varieties and improve the dispersion of N to produce more metal-carrier interfaces, thereby facilitating the generation of a greater number of M–N species [22]. In single-atom or atomic cluster catalysts, single or multiple metal atoms are evenly distributed on a carrier, thereby enhancing the density of active sites and achieving a strong catalytic activity toward the reaction [23]. By adding different levels of Zn dopants, Han et al. [24] successfully complexed nanoparticles, atomic groups, and monoatomic Co catalysts on N-doped porous carbon. Bian et al. [25] developed a two-step calcination method to prepare cobalt nanoparticles uniformly dispersed on encapsulated crystalline oxide nanofibers and to regulate the oxygen vacancies in the encapsulated crystalline nanofibers. Electrochemical results demonstrate that, compared to other catalysts and the precious metal catalysts platinum on activated carbon (Pt/C) + RuO₂, the single-atom Co catalyst exhibits an excellent bifunctional ORR/OER activity, durability, and invertibility in a Zn–air battery.

In this study, morphology-controlled polyaniline (PANI) is utilized as an N–C source, while Co serves as the metal source to compound Co–N–C catalysts for bifunctional ORR/OER. The scheme describes the synthesis route of Co–N–C catalyst employing PANI nanofiber as a N precursor. The method can provide a uniform distribution of N atoms that serve as Lewis anchor sites with Co atoms [26]. By employing the Lewis doping strategy and utilizing electrostatic adsorption between metal precursors and PANI, confinement effects, and thermal treatment techniques, the N doping efficiency and active site density have been significantly enhanced. As expected, electrochemical experiments certified that the Co–N–C catalyst was made from using the method exhibited a half-wave potential (vs. RHE) of 0.85 V in an alkaline environment, along with a superior stability in extended cycle testing compared to Pt/C and Fe–N–C, with only a 25 mV forward shift in the half-wave potential after 5000 cycles. Furthermore, this maximum power density tested in the Zn–air cell reached 227 mW/cm², significantly surpassing the 101 mW/cm² achieved by the merchant Pt/C catalyst.

2 Experimental

2.1 Synthesis of PANI

The flowchart of the arrangement process is shown in Fig.1. PANI was synthesized without the use of a template. Reduced pressure-distilled aniline (0.91 mL) was hemolyzed in 50 mL of soft water and mixed magnetically at room temperature continuing 30 min until the mixture reached pH = 5. Then, 2 mL of 1 mol/L HCl was measured and 50 mL of 2.5 °C pre-cooled aqueous ammonium persulfate solution (0.25 mol/L) was added. The reaction was afforded to proceed for 24 h. The PANI generated from the reaction was separated by vacuum filtration, rinsed many times for ethanol and pure water, then dried in a vacuum oven at 80 °C for 24 h.

2.2 Synthesis of Co–N–C@MA-900 catalysts

PANI conversion to deprotonated polyaniline (EB-PANI) was achieved through deprotonation of 1 mol/L NH₃·H₂O. A sample weighing 600 mg was added to 150 mL of 1 mol/L NH₃·H₂O at room temperature for magnetic stirring for 12 h. The resulting mixture was then scoured multiple times for deionized water until the color of the liquor remained unchanged. Subsequently, it was dehydrated in a vacuum drying oven at 80 °C for 24 h to obtain EB-PANI.

To synthesize deprotonated cobalt-doped polyaniline and melamine mixtures (EB-PANI@MA-Co). 500 mg of EB-PANI and 50 mg of CoCl₂·6H₂O were dissolved in 150 mL of anhydrous acetonitrile. The resulting mixture was placed in a sonication cell and took out after half an hour. The solution was sealed and stirred for 12 h at room temperature on a magnetic stirrer. After being washed with anhydrous acetonitrile through filtration, it was placed in the vacuum drying oven at 80 °C and dried for 12 h. Then, the dried sample was mixed with an equal amount of melamine and ground in a mortar for 30 min to obtain a uniformly mixed powder, EB-PANI@MA-Co.

The EB-PANI@MA-Co obtained was subjected to a tube furnace in an N₂ environment, heated at a rate of 5 °C/min, and held at 900 °C for 3 h. The resulting carbonized sample was then heated and stirred at 80 °C in 0.5 mol/L H₂SO₄ solution, washed and filtered multiple times, and dried under vacuum at 80 °C for 12 h. Finally, the vacuum-dried sample was placed in a tube furnace and heated at a rate of 5 °C/min, and held at 900 °C for 3 h under an N₂ environment to obtain the final Co–N–C@MA-900 catalyst.

Similarly, samples were also synthesized at carbonization temperatures of 800, 850, 950, and 1000 °C to research the effect of carbonization temperature on oxygen reduction.

2.3 Material characterization

The surface morphology of PANI was observed using the FEI Scios 2 HiVac scanning electron microscope. The material structure and composition were studied using a transmission electron microscope (TEM) by collecting information from transmission electrons and analyzing the resulting micrographs. The modalism of the material was examined and analyzed using FEI Talos F200x microscope from the USA. The specific surface area and pore diameter were investigated using an American Micromeritics ASAP 2460 analytical instrument. The surface area at the adsorption isotherm was determined using the multi-point Brunauer–Emmett–Teller (BET) method. Desorption isotherms were used to determine the mean pore diameter with the Barrett–Joyner–Halenda (BJH) method. Chemical elements and valence states were identified by X-ray photoelectron spectroscopy (XPS) using the Thermo Scientific K-Alpha instrument from the USA, and X-ray diffraction (XRD) data were obtained using a Rigaku SmartLab SE X-ray diffractometer from Japan.

2.4 Electrochemical characterization

The oxygen reduction property of the catalyst materials was evaluated using a CHI760 Shanghai Chenhua electrochemical workstation and rotating disc electrodes from the PINE Company. A three-electrode system was used, with glassy carbon electrodes (5 mm diameter) serving as the working electrode, Ag/AgCl as the reference electrode, and platinum as the counter electrode. To prepare the catalyst ink solution, 6 mg of catalyst powder, 900 μL of ethanol, 1000 μL of purified water, and 100 μL of 5% Nafion solution were homogeneously mixed. The catalyst solution was then ultrasonicated in an ultrasonic bath to achieve a uniformly dispersed catalyst solution. Several volumes of the catalyst ink were dropwise added onto the polished and cleaned glassy carbon electrode. After drying on a heating table, the catalyst was uniformly and completely coated on the electrode surface. The oxygen reduction performance of the catalyst was tested under alkaline conditions using the 0.1 mol/L KOH aqueous solution as the electrolyte. Linear voltammetry was used to characterize the catalytic performance. Prior to the test, high purity oxygen was bubbled through the electrolyte for 30 min to saturate the solution with oxygen. The linear voltammetry scan (vs. reversible hydrogen electrode (RHE)) ranged from 0.15 to 1.15 V at a sweep speed of 5 mV/s. This rotating disk electrode was tested at 1600 r/min. The Ag/AgCl electrode potentials obtained were converted to reversible hydrogen potentials [27,28].

(1)E_{\mathrm{R}\mathrm{H}\mathrm{E}}=E_{\mathrm{A}\mathrm{g}/\mathrm{A}\mathrm{g}\mathrm{C}\mathrm{l}}+0.0592\times {\mathrm{p}\mathrm{H}}_{\mathrm{i}\mathrm{n}\mathrm{v}\mathrm{o}\mathrm{l}\mathrm{t}\mathrm{s}}.

Using the Koutecky–Levich (K–L) equation for rotating ring-disk electrode (RRDE) values obtained at different rotational speeds from 225 to 2500 r/min, the number of electron transfers (n) should be determined [29].

(2)\frac{1}{J}=\frac{1}{J_{\mathrm{k}}}+\frac{1}{J_{\mathrm{l}}}=\frac{1}{J_{\mathrm{k}}}+\frac{1}{B{\omega }^{1/2}},

(3)B=0.62nF{C}_0{D_0}^{2/3}{\mu }^{-1/2},

where J is the measured current density, J_k is the dynamic current density, J_l is the diffusion-limited current density, B is the reciprocal of the slope, ω is the angular velocity of the disk, n and F are the electron transfer number and Faraday’s constant (96485 C/mol). C₀ (1.2 × 10⁻⁶ mol·cm³) and D₀ (1.9 × 10⁻⁵ cm²/s) represent the initial oxygen concentration and the diffusion coefficient of oxygen at 0.1 mol/L KOH. Additionally, the amount to transferred electrons and the yield of hydrogen peroxide is determined using [30]

(4){\mathrm{H}}_2{\mathrm{O}}_2\left(\mathrm{\%}\right)=200\times {\displaystyle \frac{{\displaystyle \frac{i_{\mathrm{r}}}{N}}}{i_{\mathrm{d}}+{\displaystyle \frac{i_{\mathrm{r}}}{N}}}},

(5)n=4\times {\displaystyle \frac{i_{\mathrm{d}}}{i_{\mathrm{d}}+{\displaystyle \frac{i_{\mathrm{r}}}{N}}}},

where N (N = 0.3) represents the Pt ring current collecting effciency, and i_r, i_d, and n refer to the ring, disk, and electfon transfer number, respectively. To accurately capture the characteristics of Zn–air batteries, the prepared catalysts were assembled on gas diffusion layer electrodes, with Zn foil serving as the metal electrode, and the electrolyte consisted of the mixed solution of 6 mol/L KOH +0.2 mol/L ZnCl₂. A comparison experiment was also conducted with the benchmark catalyst (20 wt.% Pt/C) designated as the oxygen electrocatalyst. A catalyst loading for the catalyst solution (same as used for electrochemical measurements) was set at 0.1 mg/cm² and coated onto hydrophobic carbon paper.

3 Results and discussion

3.1 Structure characterizations

PANI exhibits a rich nanostructure [31], making it an ideal precursor for designing synthetic catalysts that meet the requirements for a high specific surface area. In this study, a morphology of the Co–N–C@MA-900 catalyst, procured through carbonization at 900 °C, was characterized, and the scanning electron microscopy (SEM) results are presented in Fig.2. The Co–N–C@MA-900 catalyst is constituted of nanofibers with a diameter range of about 80–120 nm. The rough surface facilitates the attachment of more catalytic sites, thereby enhancing the catalytic activity for the ORR [32].

The interlaced nanofiber structure is clearly depicted in the TEM image, with no apparent particle aggregation observed, thereby facilitating the mass transfer process during the ORR. Fig.3(a) reveals the TEM image of Co–N–C@MA-900, revealing a sparse fiber network with a mean diameter of approximately 100 nm, which provides a large specific surface area for catalytic reactions. Additionally, Fig.3(b) displays high-resolution electron micrographs demonstrating a homogeneous carbon substrate, with a grid spacing of 0.2635 nm corresponding to the Co(111) crystal plane and a lattice stripe with a spacing of 0.4962 nm corresponding to C(002) observed at the edge of the sample. The Co–N center exhibits a high stability under the electron beam, indicating a strong interaction between the surrounding carbon and the Co–N [33]. Furthermore, Fig.3(c) presents the EDS information, illustrating the even distribution of the three elements, Co (yellow), C (red), and N (green) within the structure. The well-dispersed atomic Co sites synergize with N, creating an environment favorable for catalytic reactions. Table S1 shows the mass ratios and atomic ratios of Co, N, and C. The atomic ratios of Co, N, and C in the catalyst are 1.20%, 1.49% and 97.31%, respectively, and the atomic ratios of Co are very small. However, the catalytic activity and stability of the material are very good, which shows the superiority of this catalyst.

In this study, the XRD was used to characterize the structural features and composition of the materials. It can be observed from Fig.4 that the catalyst samples obtained by carbonization at different temperatures were analyzed. The peaks at 2θ around 26° can be attributed to amorphous carbon, while the characteristic peaks of Co₂N are observed at 44° and 76°. The XRD spectra of the samples carbonized at different temperatures exhibit a similar appearance, with the intensity of the diffraction peaks corresponding to Co₂N increasing with the carbonization temperature. This indicates an improvement in the crystallinity of Co₂N at higher carbonization temperatures. However, the carbonization temperatures of 800 and 850 °C were not sufficient to form the characteristic Co₂N peaks [34]. The intensity of this diffraction peak decreases when the temperature reaches 1000 °C, suggesting a reduction in crystallinity due to increased graphitization at higher temperatures [35]. This phenomenon is likely related to the abundance of N-edge and O-edge sites. It has been shown that the presence of N and O near C atoms in N-doped carbon materials enhances the catalytic activity in the ORR, where these adjacent active sites facilitate the four-electron pathway for the reduction of O₂ to H₂O [36,37].

Not only does the collaborative interaction between Co-containing and N-containing groups in the carbon matrix affect the properties, but the specific surface area and pore distribution of the products also influence the catalytic activity in the ORR. To gain further insights into the structural characteristics of the Co–N–C@MA-900 catalyst synthesized via the Lewis doping method and to assess the exposure of active sites, a specific surface area measurement was conducted. From Fig.5, it can be concluded that the BET test reveals the high specific surface area of 767.0908 m²/g for the sample, which exhibits abundant pore structures with a mean pore diameter of 7.4960 nm. The sorption and desorption curves of the sample exhibits type IV isotherms, with noticeable H1-type hysteresis loops in the medium-high pressure region, indicating the presence of mesopores. The drooping tail in the curves further confirms the existence of macropores. These results highlight the advantages of nanofibrous N-rich precursors in the catalyst structure. The large specific surface area provides an optimal reaction environment for catalytic processes, while the rich pore structure enhances the accessibility of active points [38].

To understand the impact of functionalized nitrogen radicals on ORR catalysis, XPS was used in this study to thoroughly characterize the N–C binding state on the catalyst surface. The XPS spectral analysis results are presented in Fig.6(a) for Co–N–C@MA-900. The spectra reveal the presence of Co, N, and C elements. Fig.6(b) displays the fitting of the N 1s spectra, indicating the presence of pyridine N, pyrrole N, and graphitic N with binding energies of 398.4, 400.1, and 401.2 eV, respectively. The findings demonstrate the abundance of nitrogen species, with respective percentages of 38.07%, 36.88%, and 25.05%. Notably, it has been shown that a higher content of pyridine N doping can enhance the ORR reaction kinetics and improve durability [39]. The XPS spectrum of Co 2p is illustrated in Fig.6(c), exhibiting peaks at 778, 783, 787, and 796 eV, corresponding to Co 2p3/2, C–N_x, the satellite peak of Co, and Co 2p1/2, respectively. The presence of divalent cobalt signifies the successful binding of cobalt to the carbon framework, forming the active site Co-N_x. The C 1s XPS spectra in Fig.6(d) show the C–C (283 eV) bond and the C–N (286 eV) bond in all the synthesized samples. Quantum mechanical calculations [40,41] and laboratory studies have shown that N dopants, especially pyridine and graphitic N [42,43], are promising dopants as they enhance π-bonds and improve the nature and stability of the electron donor-acceptor. N atoms have easy access to specific sites on the carbon skeleton, as the C–N bond formed serves as an effective active site because of the positive charge on the C atom resulting from electron affinity between the C atom and neighboring N atoms [44].

3.2 Electrocatalytic performances

As can be seen from Fig. S1, the half-wave potential for the ORR is approximately 30 mV higher when alkali-treated EB-PANI is used as the precursor compared to ES-PANI. This observation confirms the superiority of the Lewis doping strategy in the synthesis of non-precious metal catalysts.

To examine the effect of carbonization temperature on oxygen reduction activity, the oxygen reduction performance of catalysts subjected to different temperatures was evaluated. Figure S2 shows that the linear sweep voltammetry (LSV) and cyclic voltammetry (CV) curves measured in alkaline media exhibit a maximum half-wave potential at 900 °C, approximately 0.85 V vs. RHE. Therefore, catalysts synthesized at 900 °C were chosen for the ORRs in alkaline medium.

Figure S3 demonstrates the effect of transition metal doping on the ORR. By varying the amount of Co incorporation, different forms of N doping can be controlled. Furthermore, Co doping can influence the nitrogen content and alter the formation environment of the active site. At the standard hydrogen electrode, Fig. S3 shows that the ORR half-wave potential reaches a maximum of 0.85 V, when the Co doping amount is 10% of the C and N precursor mass. Conversely, the O reduction performance is diminished at Co doping amounts of 15%, 5%, and 20%, with the poorest performance observed at 20%. The modulation of Co content proves to be advantageous in controlling the various forms of N doping, which also influences the graphitization and N content. Excessive metal content may lead to cluster formation, thereby hindering the catalytic effect of active sites. Therefore, a Co doping level of 10% was ultimately selected for characterization tests, aiming to investigate the properties of Co-N-C catalysts in oxygen reduction. Fig.7 demonstrates the ORR performance. It can be observed that the Co–N–C catalysts, evaluated using rotating disc electrodes in the 0.1 mol/L KOH solution, exhibit slightly lower ORR activities (E_1/2 (vs. RHE) = 0.83 V) compared to the Fe–N–C catalysts synthesized using the same method. However, they still demonstrate a favorable performance compared to C–N compounds formed through direct carbonization of PANI. As shown in Fig.7(a), the ORR half-wave potential of C-PANI at the standard hydrogen electrode potential is 0.68 V, while the ORR half-wave potential of the carbonitride formed by the carbonization of PANI deprotonated in alkaline solution is 0.72 V, which is 0.11 V lower than that of the Co–N–C catalyst. The half-wave potential of the Co–N–C catalyst is only 0.03 V lower compared with that of the commercial Pt/C catalyst. Moreover, the Co–N–C catalyst exhibits a superior durability, as depicted in Fig.7(b), where the half-wave potential changes by only 25 mV after 5000 cycles. By contrast, the Fe–N–C catalyst prepared using the same method shows a poorer durability of approximately four times, and even the commercial Pt/C catalyst experiences a 30 mV shift in the oxygen reduction site under the same test conditions. This improved that the durability of Co–N–C catalysts can be attributed to the lower activity of Co ions in the Fenton reaction, resulting in a reduced rate of radical formation and attack on the catalyst, as well as the enhanced resistance of Co–N–C to demetalization. The oxygen evolution test results, as shown in Fig.7(c), indicate an oxygen evolution potential (vs. RHE) of 1.7 V for Co–N–C at a current density of 10 mA/cm², outperforming the oxygen evolution potential of merchant Pt/C tested under identical conditions. Furthermore, Fig.7(d) demonstrates that the ORR process yields less than 7% of the reduction product H₂O₂, with an average electron transfer number of approximately 4, indicating that this process follows the faster four-electron pathway for the reduction process [45]. Fig.7(e) tests the cyclic voltammetry curves at 10–60 mV/s sweep speeds. The electrical double-layer capacitor (Cdl) value of 87.83 mF/cm² was calculated from the fitting of Fig.7(f), which illustrates that the catalysts prepared using the Lewis strategy have a large specific surface area.

Fig.8 illustrates the test results of the catalyst for practical application in an alkaline environment. As depicted in Fig.8(a), the maximum power density tested for the Zn–air battery is significantly higher at 227 mW/cm² compared to the commercial platinum carbon catalyst, which achieved only 101 mW/cm². In contrast, the highest watt density of the undoped metal carbon and nitrogen compound was only 89 mW/cm². This results strongly indicate that the M–N–C structure formed by Co metal plays a significant role in the catalytic system. Figure S4 compares some of the similar work in recent years, the Co–N–C@MA-900 catalyst material has a significant advantage in terms of maximum power density [46–53]. Fig.8(b) demonstrates a constant-current discharge at 20 mA/cm² for 11 h, surpassing the 10-h performance of the commercial platinum carbon. In Fig.8(c), the single charge/discharge test results reveal that the polarization voltage of Co–N–C ranges from 1.2 to 2.2 V, which is lower than the polarization range of 1.0–2.5 V observed for commercial platinum carbon. This lower degree of polarization suggests an excellent electrocatalytic performance. Fig.8(d) showcases the long cycle charge/discharge test of the Zn–air battery, which exhibits a duration of 18 h with a voltage polarization range of approximately 0.5–2.7 V. While the test results from the catalyst in practical applications indicate excellent oxygen reduction catalytic properties in an alkaline environment, the performance in the long cycle test of the full battery is not satisfactory. Furthermore, the dual function performance of the same type of Co-based catalyst reported in the literature is not outstanding.

4 Conclusions

In this study, the Lewis doping strategy was employed and the electrostatic adsorption between metal precursors and PANI was utilized, along with confinement effects and thermal treatment techniques, to significantly enhance the nitrogen doping efficiency and active site density. The Co–N–C catalysts exhibited a half-wave potential (vs. RHE) of 0.85 V in alkaline media and demonstrated a higher stability than Pt/C and Fe–N–C in long cycle tests, with only a 25 mV shift in half-wave potential after 5000 cycles. These findings align with simulation results reported in the literature, which indicate that Co exhibits a better tolerance to the Fenton reaction than Fe. In the Zn–air battery, the Co–N–C catalyst achieved an impressive maximum power density of 227 mW/cm², surpassing the 101 mW/cm² of commercial Pt/C. Additionally, constant current discharge for the current density of 10 mW/cm² for 11 h showed a better performance than commercial Pt/C. In summary, the series of test results highlight the excellent oxygen electrocatalysis performance of the material in alkaline environments. Recent advancements in the development of efficient M–N–C catalysts hold great potential for the future progress of ORR/OER catalysts.

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