English

• In pursuit of mitigating environmental pollution and addressing the energy crisis, it is imperative to develop a sustainable energy storage and conversion system to cope with the depletion of fossil fuels and escalating environmental challenges. The rechargeable Zn-air battery (ZAB) is considered the next generation of energy storage and conversion devices, owing to its remarkable attributes such as high energy density (1086 Wh/kg), cost-effectiveness, safety, and environmental friendliness [1-5]. However, the widespread commercialization of ZAB is prevented by the sluggish kinetics associated with the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), both of which involve a four-electron transfer process at the air cathode [6-10].

  At present, precious metal materials, such as ORR-active Pt/C and OER-active IrO₂/RuO₂ are commonly used as cathode catalysts to enhance the redox reactions of oxygen at the air cathode. Nevertheless, the high cost, scarcity, and limited durability of these catalysts pose significant obstacles to their widespread adoption in rechargeable Zn-air batteries [11-14]. Hence, it is imperative to engineer affordable, highly efficient, and enduring non-noble metal-based bifunctional catalysts as alternatives to Pt, Ru, and Ir-based catalysts. In recent years, many resources have been used to investigate a wide range of inexpensive bifunctional electrocatalysts, including transition metals (TMs) [15-17] and their nitrides [18]. These bimetallic (or polymetallic) nano-alloys of TMs have become one of the most intriguing alternatives for ORR and OER due to their adjustable redox characteristics and intermetallic synergy. Especially, Co_xFe_y alloys can serve as bifunctional catalysts for oxygen reactions, where Fe species exhibit high ORR electrocatalytic activity while Co species demonstrate excellent OER electrocatalytic activity [19-22].

  Considerable effort has been dedicated to the study of carbon materials doped with heteroatoms, such as N, P, and S, as efficient catalysts. Research indicates that doped heteroatoms alter spin charge densities, leading to a redistribution of charge within the carbon matrix [23-29]. The introduction of transition metal species into N-doped carbon materials with excellent electrical conductivity and large surface areas has proven to be an effective approach for enhancing electrocatalytic activity [30-33]. Chelation of metal ions with N atoms results in the formation of M-N_x species. These economic transition metal nitrides, characterized by usual metallic characteristics and a large number of M-N covalent bonds, demonstrate potential ORR performance owing to their efficient electron transport and excellent oxygen adsorption [23]. Therefore, transition metal nitrogen-doped carbon materials hold great potential for applications in oxygen electrocatalysis.

  Due to the high cost and complex preparation processes associated with traditional carbon materials, there has been a growing interest in biomass materials characterized by low cost, abundant reserves, and recyclability [34,35]. In recent years, lignin-derived carbon materials have gained increasing attention owing to their outstanding performance and broad application prospects. Historically, lignin has been treated as a waste product in the pulping and paper industry, resulting in the inefficient use of resources and environmental pollution. Lignin, with its high carbon content, abundance of reactive functional groups, and unique three-dimensional spatial structure, emerges as an ideal raw material for carbon sourcing [36-42]. Previous studies have consistently validated the exceptional properties of lignin-based carbon materials play pivotal roles in processes like the oxygen reduction reaction, evolution of oxygen and hydrogen [43-51]. This positions them as one of the most promising categories of biomass-derived carbon materials. Nevertheless, during prolonged cycling in electrochemical cells, lignin-derived compounds may aggregate metal particles, posing a threat to electrochemical stability. This instability may be a factor in the deterioration of the catalyst, which could decrease long-term performance. Considering these factors, we presented a simple strategy to build Co/Fe carbon-supported composites encapsulated in oxidative ammonolysis modified [52] lignin-derived N-doped biochar as effective bifunctional catalysts (CoFe-Co_xN@NOALC). One-step in-situ carbonization and nitrogen doping enhanced its potential for industrial application prospects. The CoFe alloy and Co_5.47N evenly dispersed within the carbon layer, forming a heterojunction. The smaller nano-particle size made the material have more active sites. As a result, the CoFe-Co_xN@NOALC exhibited good ORR activity with an onset potential of 0.98 V and a positive half-wave potential of 0.82 V in a 0.1 mol/L KOH solution. In a practical demonstration, the rechargeable ZAB using CoFe-Co_xN@NOALC as the air cathode exhibited a high peak power density of 154 mW/cm² compared to the ZAB assembled with the Pt/C and RuO₂ catalysts. After 1500 h cyclic charge-discharge test, the charge-discharge efficiency of the battery only decreased by 11.4%, which revealed excellent long-cycle stability. This study will provide an effective and feasible strategy for electrocatalysis of lignin-derived carbon-based materials.

  The schematic diagram (Fig. 1) illustrated methods to prepare CoFe-Co_xN@NOALC. Amide groups were incorporated to enhance the metal ion chelation capability and water solubility of lignin. The modified lignin and metal ions were homogeneously stirred and blended in an aqueous solution, leading to the formation of Co/Fe-OAL compounds through pH adjustment. The catalyst was obtained by carbonizing a mixture of Co/Fe-OAL compounds and melamine. The modified lignin served as an organic framework for chelating and dispersing metal ions, additionally providing a carbon source during the carbonization process.

  Figure 1

  

  Figure 1.  Synthesis schematic diagram of CoFe-Co_xN@NOALC.

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  XRD analysis was employed to characterize the crystal structure of the catalysts prepared. Fig. 2a illustrated the presence of both CoFe alloy and Co_5.47N. In particular, characteristic peaks of CoFe alloy were observed at approximately 44.8° and 65.6° positions (indexed to CoFe PDF #44-1433), corresponding to the (110) and (200) crystal planes of CoFe, respectively. Similarly, characteristic peaks of Co_5.47N were identified at around 43.8° and 51.1° (referenced to Co_5.47N PDF #41-0943), corresponding to the (111) and (200) crystal planes of Co_5.47N. Meanwhile, the crystal phase composition of different catalysts was compared. Unmodified alkali lignin had weak coordination and chelating ability to metal ions, which made it more inclined to form CoFe alloys in the process of crystal phase formation, which was not conducive to the formation of Co_5.47N. However, amide groups in OAL coordinated well with metal ions, and during the formation process, some amide groups chelated with Co tend to form Co_5.47N. Furthermore, it became apparent that the half-peak width of the CoFe alloy catalyst, synthesized from modified materials, was comparatively broader. This indicated that the particle size of the alloy was smaller.

  Figure 2

  

  Figure 2.  Comparison of different catalyst. (a) X-ray diffraction pattern. (b) Raman spectra and XPS spectra of CoFe-Co_xN@NOALC (c) C 1s, (d) N 1s, (e) Fe 2p, (f) Co 2p.

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  The Raman spectra in Fig. 2b revealed a D band at ≈1340 cm⁻¹, indicating disordered carbon, and a G band at ≈1588 cm⁻¹, characteristic of graphite carbon. The ratio of I_D/I_G could be employed to assess the degree of defects in carbon materials [18]. The I_D/I_G values of CoFe-Co_xN@NOALC, CoFe@NALC and CoFe@OALC were 1.03, 0.99, and 0.91 respectively, indicating that the inclusion of nitrogen resource and metal augmented the defect level of carbon materials. The distribution of pore sizes and surface area of CoFe-Co_xN@NOALC, determined through nitrogen adsorption/desorption analysis, were illustrated in Fig. S2 (Supporting information). The CoFe-Co_xN@NOALC exhibited a type Ⅳ isothermal absorption curve with a H4-type hysteresis loop, this indicated that the pore size distribution was a mixture of micropores and mesopores. The corresponding BET surface area of CoFe-Co_xN@NOALC was 490.1 m²/g.

  XPS analysis was used to determine the valence states and chemical makeup of CoFe-Co_xN@NOALC. The XPS full spectrum (Fig. S3 in Supporting information) exhibited the presence of Co, Fe, O, N, and C elements. In Fig. 2c, within CoFe-Co_xN@NOALC, the C 1s spectrum confirmed the presence of C-C (284.8 eV), C-N (285.6 eV), C=O (288.5 eV), and π-π* (291 eV) bonds. The N 1s spectrum (Fig. 2d) displayed peaks at 398.5, 399.4, 400.6, and 402.8 eV, corresponding to pyridine-N (21.6%), Co-N (24.8%), pyrrole-N (37.3%), and graphitic-N (16.3%). The Fe 2p spectrum in Fig. 2e exhibited peaks at 709.8 and 719.6 eV attributed to metallic CoFe alloy, while peaks centered at 712.6 and 725.2 eV were assigned to ionic state peaks, and those at 714.9 and 732.5 eV were designated as shakeup satellite peaks [18]. This observation aligned with the phase composition as determined by X-ray diffraction analysis. Similarly, Fig. 2f depicted the Co 2p spectrum, revealing two pairs of peaks: 778.7 and 793.7 eV attributed to Co atoms in CoFe alloy, and 780.5 and 795.9 eV corresponding to Co-N species. Peaks at binding energies of 784.5 and 801.9 eV were assigned to shakeup satellites [18,53].

  Figs. 3a-c presented TEM, HR-TEM images, and the SAED pattern of CoFe-Co_xN@NOALC, revealing the uniform encapsulation of CoFe alloy nanoparticles within the graphitic carbon layer. For our previous work, although we improved the metal chelating ability of lignin through modification, it obviously showed that there was a serious phenomenon of metal particle agglomeration [54]. The agglomeration of metal particles would affect the exposure of active sites, making the materials unable to effectively played a catalytic role. The modified lignin optimized in this work also solved this problem. A lattice distance of 0.202 nm, suggesting that the CoFe alloy's (110) diffraction crystal plane, closely approximated the lattice spacing of the Co_5.47N (111) diffraction crystal plane, which measured 0.207 nm, suggesting the presence of a distinct composite structure. Fig. 3d displayed the element-mapping images of CoFe-Co_xN@NOALC, wherein the main components matched the desired catalyst, confirming that nitrogen and transition metals were successfully doped into the catalyst.

  Figure 3

  

  Figure 3.  Morphological investigations of CoFe-Co_xN@NOALC. (a) TEM image. (b) HR-TEM image, (c) SAED pattern, and (d) HAADF-STEM image and the corresponding EDS mapping of C, N, Co, Fe.

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  The ORR and OER electrocatalytic performances were evaluated in 0.1 mol/L KOH. In Fig. 4a, CoFe-Co_xN@NOALC exhibited the excellent ORR activity with an onset potential (E_onset) of 0.98 V and a half-wave potential (E_1/2) of 0.82 V, surpassing CoFe@NALC (E_onset of 0.95 V, E_1/2 of 0.81 V), and CoFe@OALC (E_onset of 0.88 V, E_1/2 of 0.78 V). Close to the performance of Pt/C (E_onset of 1.0 V, E_1/2 of 0.85 V). As expected, the NOALC exhibited inferior catalytic activity (E_onset of 0.75 V, E_1/2 of 0.49 V) in contrast to those electrocatalysts based on metals. The improvement of performance was gained from the more effective CoFe-Co_xN heterojunction sites. By analyzing the kinetic current density (J_K), this was further confirmed. The CoFe-Co_xN@NOALC presented a higher J_K of 9.6 mA/cm² at 0.8 V, this exceeded the CoFe@NALC (4.7 mA/cm²). The CoFe-Co_xN@NOALC manifested superior electrocatalytic stability with less activity decay in current density (Initial 89%) following 40000 s constant potential at 0.6 V and E_1/2 (≈12 mV) following 3000 continuous potential cycles (Figs. 4c and d). By contrast, an activity decay in current density (Initial 84%) and negative shift in E_1/2 (≈40 mV) were observed for commercial Pt/C. A 4-electron pathway was determined for CoFe-Co_xN@NOALC from the slopes of Koutecky-Levich (K-L) plots and electron transfer result (Figs. 4e and f). The H₂O₂ yield of CoFe-Co_xN@NOALC was below 10% over the potential range from 0.2 V to 0.7 V. Similarly, the CoFe-Co_xN@NOALC sample exhibited excellent OER activity (Fig. 4g) with a low overpotential of 472 mV to reach a current density of 10 mA/cm², compared to those of CoFe@NALC (622 mV), CoFe@OALC (529 mV), close to commercial RuO₂ (462 mV). The Tafel slopes of ORR and OER of different catalysts were showed in Figs. 4b and h, respectively. Fig. S5 (Supporting information) showed CVs of the prepared catalysts at different scan rates, illustrating the larger C_dl of CoFe-Co_xN@NOALC (calculated from the slope) compared to RuO₂. CoFe-Co_xN@NOALC exhibited the smallest ΔE value (0.882 V) compared to those of CoFe@NALC (1.042V), CoFe@OALC (0.979 V), and close to Pt/C + RuO₂ (0.842 V) in Fig. 4i.

  Figure 4

  

  Figure 4.  Bifunctional ORR and OER activity of CoFe-Co_xN@NOALC. (a) ORR polarization curves (rotation rate: 1600 rpm; scan rate 5 mV/s). (b) Tafel plots for ORR. (c) Chronoamperometric responses of CoFe-Co_xN@NOALC and Pt/C catalysts at a fixed potential. (d) ORR polarization curves (before and after durability tests). (e) ORR polarization cures at different rotation speeds (400-1600 rpm). The inset shows the corresponding Koutecky-Levich plot at various potentials. (f) Electron transfer number (n) and peroxide yield (%). (g) OER polarization curves. (h) Tafel plots for OER. (i) Overall polarization curves within the ORR and OER potential window.

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  As the cathode material in a Zn-air battery, the practical application (Fig. 5a) of CoFe-Co_xN@NOALC electrocatalyst was assessed and contrasted with a Pt/C + RuO₂ electrocatalyst (mass ratio = 1:1). The CoFe-Co_xN@NOALC-based battery (Fig. 5b) reached an open-circuit voltage of 1.46 V, which was comparable to the Pt/C + RuO₂ (1.47 V) battery. Additionally, CoFe-Co_xN@NOALC had a better charge-discharge voltage gap at large current density (Fig. 5c). Based on CoFe-Co_xN@NOALC, the Zn-air battery had the highest peak power density of 154 mW/cm² (Fig. 5d), surpassing the performance of Pt/C + RuO₂ (142 mW/cm²), CoFe@NALC (108 mW/cm²), CoFe@OALC (147 mW/cm²), NOALC (52 mW/cm²) and other recently reported catalysts (Table S4 in Supporting information). In Fig. 5e, at discharge currents of 10 mA/cm², the Zn-air battery based on CoFe-Co_xN@NOALC exhibited a specific capacity of 770 mAh/g, indicating exceptional capability compared with comparison of CoFe@NALC (696 mAh/g) and CoFe@OALC (655 mAh/g) showed in Fig. S8 (Supporting information). The Pt/C + RuO₂ based battery only delivered a low specific capacity of 713 mAh/g. The discharge curves depicted in Fig. 5f illustrated that the CoFe-Co_xN@NOALC-based battery exhibited superior oxygen reduction reaction (ORR) stability and rate performance. Above all, the round-trip efficiency of CoFe-Co_xN@NOALC-based battery only decayed by 11.4%, after 1500 h (4500 cycles) of charge and discharge tests (Fig. 5g), emphasizing excellent operating stability and long-term rechargeability. In the physical demonstration, only one battery could light up the small LED digital display screen (Fig. 5h).

  Figure 5

  

  Figure 5.  Aqueous ZAB performance of CoFe-Co_xN@NOALC. (a) ZAB model diagram. (b) Open circuit voltage. (c) Discharge and charge polarization curves. (d) Discharge polarization curves and power density curves. (e) Specific capacity at 10 mA/cm². (f) Discharge curves at different current densities. (g) Charging/discharging cycling curves of ZABs at 10 mA/cm². (h) The photograph showing an LED panel powered by one aqueous ZAB.

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  In conclusion, the in-situ amide group introduction enhanced the capability of lignin to chelate metal ions, which improved the degree of metal alloy particle dispersion on lignin carbon and successfully reduced the particle size. The assembled ZABs exhibited superior power density of 154 mW/cm², high specific capacity of 770 mAh/g, and enhanced cycle durability more than 1500 h. CoFe-Co_xN@NOALC exhibited excellent electrocatalytic performance for bifunctional oxygen reactions and remarkable electrochemical stability. These results provided new strategies for the design of non-noble metal electrocatalysts towards future energy storage and conversion applications.

  Declaration of competing interest

  The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

  CRediT authorship contribution statement

  Jinhui Zhang: Writing – original draft, Methodology, Investigation, Formal analysis, Data curation. Jianglin Liu: Methodology, Formal analysis. Jie Ran: Methodology, Formal analysis. Xuliang Lin: Writing – review & editing, Supervision, Resources, Project administration, Funding acquisition, Conceptualization. Huan Wang: Conceptualization, Supervision, Writing – review & editing. Xueqing Qiu: Writing – review & editing, Supervision, Resources, Project administration, Funding acquisition, Conceptualization.

  Acknowledgment

  This work was sponsored by the National Natural Science Foundation of China (Nos. U23A6005 and 22178069).

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2024.110403.

  
  
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• 

  Figure 1  Synthesis schematic diagram of CoFe-Co_xN@NOALC.

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  Figure 2  Comparison of different catalyst. (a) X-ray diffraction pattern. (b) Raman spectra and XPS spectra of CoFe-Co_xN@NOALC (c) C 1s, (d) N 1s, (e) Fe 2p, (f) Co 2p.

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  Figure 3  Morphological investigations of CoFe-Co_xN@NOALC. (a) TEM image. (b) HR-TEM image, (c) SAED pattern, and (d) HAADF-STEM image and the corresponding EDS mapping of C, N, Co, Fe.

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  Figure 4  Bifunctional ORR and OER activity of CoFe-Co_xN@NOALC. (a) ORR polarization curves (rotation rate: 1600 rpm; scan rate 5 mV/s). (b) Tafel plots for ORR. (c) Chronoamperometric responses of CoFe-Co_xN@NOALC and Pt/C catalysts at a fixed potential. (d) ORR polarization curves (before and after durability tests). (e) ORR polarization cures at different rotation speeds (400-1600 rpm). The inset shows the corresponding Koutecky-Levich plot at various potentials. (f) Electron transfer number (n) and peroxide yield (%). (g) OER polarization curves. (h) Tafel plots for OER. (i) Overall polarization curves within the ORR and OER potential window.

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  Figure 5  Aqueous ZAB performance of CoFe-Co_xN@NOALC. (a) ZAB model diagram. (b) Open circuit voltage. (c) Discharge and charge polarization curves. (d) Discharge polarization curves and power density curves. (e) Specific capacity at 10 mA/cm². (f) Discharge curves at different current densities. (g) Charging/discharging cycling curves of ZABs at 10 mA/cm². (h) The photograph showing an LED panel powered by one aqueous ZAB.

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