Superior oxygen electrocatalysts derived from predesigned covalent organic polymers for zinc–air flow batteries

Abstract

Covalent organic polymers (COPs) as emerging porous materials with ultrahigh hydrothermal stability and well-defined and adjustable architectures have aroused great interest in the electrochemical field. Here, we reported a rational design approach for the preparation of a bifunctional electrocatalyst with the assistance of a predesigned bimetallic covalent organic polymer. With the predesigned nitrogen position and structural features of COP materials, the obtained CCOP_TDP–FeNi–SiO₂ catalyst affords a remarkable bifunctional performance with a positive half-wave potential (0.89 V vs. reversible hydrogen electrode: RHE, superior to the benchmark Pt/C) for ORR activity, and a low overpotential (0.31 V better than the benchmark IrO₂) at 10 mA cm⁻² for OER activity in alkaline solution. The potential gap between E_1/2 and E_j=10 reaches 0.650 V, in line with that observed in the current state-of-the-art bifunctional oxygen electrode materials. Moreover, a homemade rechargeable Zn–air flow battery using the CCOP_TDP–FeNi–SiO₂ catalyst as an air cathode exhibits an almost twofold power density (112.8 vs. 64.8 mW cm⁻²) and a lower charge–discharge voltage gap, compared with a commercialized noble Pt/C + IrO₂/C-driven Zn–air flow battery. More importantly, the CCOP_TDP–FeNi–SiO₂-driven battery maintains a better cycling stability compared to a noble metal-driven battery without performance decay. Accordingly, this work will open up new ways for fabricating practical oxygen electrodes for, but not limited to, metal air based battery applications.

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Introduction

Rechargeable zinc–air flow batteries are a promising power technology for the future large-scale energy storage and release of electrical energy fields, owing to their high theoretical specific energy, low cost, safety, and economic viability.^1–3 However, like most metal–air batteries, one of the main challenges for zinc–air flow batteries that needs to be overcome is the sluggish rates of the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER).^4–6 Therefore, it is highly desired to develop an ideal bifunctional catalyst to make the obtained zinc–air flow batteries have excellent ORR/OER catalytic performances.^7–9

Recently, metallic (e.g., Fe, Co, and Ni) alloy catalysts have attracted intensive attention as low-cost and efficient oxygen electrocatalysts,¹⁰ which can exhibit superior activity in comparison to their individual entities.^11,12 Among the various bimetallic alloys, FeNi alloy exhibits a great application prospect because iron or nickel shows high intrinsic activity for either the ORR or the OER,^13,14 providing additional synergistic properties during the electrocatalysis and enhancing the electrocatalytic performance.¹⁵ Furthermore, it is believed that combining alloy nanoparticles and heteroatom doped carbon materials is a logical strategy to enhance the catalytic activity of the alloy, through increasing the number of active sites and providing efficient electron conduction channels.^16,17 Wu et al. reported a new class of highly active and durable FeCoNi alloy particles attached onto nitrogen-doped graphene tube bifunctional electrocatalysts, which exhibited good stability across a wide potential window in alkaline media.¹⁸ Yang et al. synthesized a hybrid catalyst with NiCo alloy nanoparticles decorated on N-doped carbon nanofibers by a facile electrospinning method.¹⁹ However, most existing methods for synthesizing alloy-decorated carbon materials involve the mixing of the sources of transition metals, nitrogen, and carbon, followed by thermal treatment.^15,20 In these processes, the metal precursors and N source are often randomly distributed or mixed on the carbon supports, which is difficult to control and form ordered electrocatalytically active sites. If realized, however, the order control of metallic active sites should provide us with a powerful means to tailor the structure–property relationship for the creation of a high-performance bifunctional electrocatalyst.

Recently, covalent organic polymers (COPs) as a new exciting type of porous material with a high specific surface area, excellent chemical stability and porosity have aroused great interest because of their enormous potential to catalyze the oxygen reduction reaction.^21–23 Particularly, COPs not only possess a well-defined and precisely controllable capacity, such as robust tailoring of heteroatom incorporation and location of active sites, but also provide carbon and nitrogen atoms in the ligands, along with the flexibility to dope active transition metals into the frameworks.^24,25 Our previous studies have designed and synthesized a large class of COPs with precisely controlled locations of N atoms and hole sizes as efficient metal-free electrocatalysts for ORR.²⁶ Therefore, COPs are known to be good precursors for carbon materials with the possibilities and potential as highly efficient energy electrocatalysts for clean and renewable energy technologies.^27,28

Herein, we report an in situ one-step method to prepare an efficient and durable bifunctional electrocatalyst, i.e., a metallic FeNi alloy encased in nitrogen-doped carbon based on predesigned covalent organic polymers towards both the ORR and OER. This novel FeNi carbon-based catalyst was synthesized via pyrolysis of Fe/Ni co-doped nitrogen-rich COPs with the assistance of silica (SiO₂), which could effectively prevent FeNi alloy nanoparticle (NP) aggregation during the carbonization process. Meanwhile, FeNi alloy NPs are formed and embedded into the porous graphitic carbon layers during the pyrolysis process, preventing the corrosion of FeNi alloy by the harsh environment and promoting the stability of the catalyst.²⁹ The obtained bifunctional electrocatalyst, named CCOP_TDP–FeNi–SiO₂, displays an outstanding bifunctional performance and stability for ORR (half-wave potential of 0.89 V vs. RHE) and OER (overpotential of 0.31 V at 10 mA cm⁻²) under alkaline conditions, and is even superior to the commercial Pt/C and IrO₂ catalysts, respectively. A zinc–air flow battery constructed using the CCOP_TDP–FeNi–SiO₂ catalyst as an air-cathode exhibits a high power density of 112.8 mW cm⁻² and a low voltage gap between charge and discharge.

Experimental

Bifunctional catalyst synthesis

In a glovebox, bis(1,5-cyclooctadiene)nickel(0) ([Ni(cod)₂], 2.000 g), 2,2′-bipyridyl (1.138 g) and 1,5-cyclooctadiene (cod, 0.889 mL) were added to DMF (115 mL). Then the mixture was stirred at 80 °C for 30 min until completely dissolved. Then, meso-tetra (p-bromophenyl) porphine (0.433 g) and 3,8-dibromophenanthroline (0.315 g) were subsequently added to the resulting solution. Thereafter, the reaction vessel was heated at 85 °C for 10 h. After cooling to room temperature, concentrated HCl was added to the deep purple suspension. After filtration, the residual COP_TDP was washed with CHCl₃, THF, and deionized water, respectively, and dried in a vacuum oven at 80 °C overnight.

COP_TDP (0.100 g), FeCl₃ (0.100 g) and NiCl₂ (0.026 g) were added to DMF (10 mL) in a dried reaction bottle, respectively, and then put in an ultrasonic environment. After the ultrasound treatment, the mixture was stirred and refluxed at 120 °C for 12 h, and then cooled to room temperature. The resulting precipitate COP_TDP–FeNi was then washed with ethanol and dried in a vacuum oven at 80 °C for 12 h. COP_TDP–FeNi (0.116 g) was mixed with tetraethyl orthosilicate (TEOS, 0.29 mL) in a mortar, followed by mixing with formic acid (0.29 mL).³⁰ The COP_TDP–FeNi–SiO₂ composite was dried in a 60 °C oven overnight.

The COP_TDP–FeNi–SiO₂ composite was loaded into a tube furnace and heated under an Ar atmosphere. The COP_TDP–FeNi–SiO₂ powder was heated to 900 °C and maintained at that temperature for 3 h. The resulting CCOP_TDP–FeNi–SiO₂ composite was mixed with 10% aqueous HF solution and stirred for 12 h to etch the silica, followed by filtering and washing with deionized water several times. Then, the product was dried in a vacuum oven at 80 °C for 12 h. Besides, CCOP_TDP–FeNi and CCOP_TDP were synthesized by direct pyrolysis of COP_TDP–FeNi and COP_TDP at 900 °C, respectively.

Physical characterization

The morphologies and structures of catalysts were analysed on an S4700 SEM instrument and a high-resolution transmission electron microscope (HRTEM, 2100F). The crystalline structures of catalysts were recorded on a D/MAX 2000 X-ray diffractometer using a Cu Kα line (λ = 1.54178 Å) as the incident beam with the 2θ scan from 5° to 90° at a step of 0.02°. X-ray photoelectron spectroscopy (XPS) analysis was carried out using an ESCALAB 250 operated at 150 W and 200 eV with monochromated Al Kα radiation. Solid-state NMR spectra were obtained on a Bruker AV300 spectrometer operating at 75.5 MHz for ¹³C NMR. BET data were obtained on an ASAP 2460.

Electrochemical measurements

All electrochemical measurements were carried out at room temperature in a three electrode cell connected to an electrochemical analyzer (CHI 760E). A Pt wire and a saturated calomel electrode (SCE) were used as counter and reference electrodes, respectively. A rotating Pt-ring-disk electrode (RRDE, 5.61 mm diameter, PINE Instrument Inc.) was polished with alumina slurry (CH Instrument Inc.) in turn and subsequently rinsed with deionized water and dried in air. 5 mg of the as-prepared samples or Pt/C catalyst or IrO₂/C catalyst were ultrasonically dispersed in 1 mL of anhydrous ethanol and 50 μL of Nafion solution (0.5 wt%) for about 30 min to form a homogeneous ink. Then, 20 μL of the as-prepared catalyst ink was added dropwise on a RRDE giving a loading of 0.386 mg cm⁻² and drying at room temperature, while the Pt/C or IrO₂/C catalyst electrode was loaded with 0.193 mg cm⁻². The potential, measured against a SCE electrode, was converted to the potential vs. RHE according to E_RHE = E_SCE + 0.059PH + 0.24.

Electrochemically active surface area (ECSA) measurements

The electrochemical double layer capacitance (C_dl) of the resulting catalysts was determined to estimate the ECSA of the catalysts within a potential window without faradaic response. Typically, a series of CV curves were obtained at various scan rates (25, 50, 75, 100, 150 and 200 mV s⁻¹) within the potential range of 0.95–1.15 V (versus RHE) and 1.15–1.35 V (versus RHE) for ORR and OER in 1 M KOH, respectively. Thus, the gravimetric double-layer capacitance C_dl (F g⁻¹) at a given scan rate (v) and mass of catalyst deposited on the electrode (m) can be related to I_capacitive as shown below.

The ECSA was calculated as follows

where C_s (C_s = 0.2 F m⁻²) is the general specific capacitance for an atomically smooth planar surface under homogeneous electrolytic conditions.

Zinc–air flow battery test

Zn plates (5 cm × 6 cm) were used as the anode. Bifunctional air electrodes were prepared by loading 3 mg CCOP_TDP–FeNi–SiO₂ hybrid ink on carbon paper plates (1 cm × 1 cm). Air cathodes were also used as the gas diffusion layer and the current collector in assembled batteries. The void between two electrodes was filled with an electrolyte containing 8 M KOH and 0.5 M ZnO to assemble one Zn–air flow battery. Reference bifunctional air electrodes were prepared by loading commercial Pt/C and IrO₂ catalysts (1 : 1 by weight ratio of Pt/Ir) at a similar mass loading (3 mg cm⁻²) on carbon paper plates. Zn–air flow battery performance was tested on a battery testing station (LAND).

Results and discussion

The synthetic strategy of the CCOP_TDP–FeNi–SiO₂ catalyst is schematically illustrated in Fig. 1. Briefly, COP_TDP was synthesized with meso-tetra (p-bromophenyl) porphine and 3,8-dibromophenanthroline as monomers by the polymerization reaction. Then, Fe/Ni co-doped COP_TDP composites were carbonized with the assistance of SiO₂, converting into porous carbon material decorated FeNi alloy NPs. The solid state ¹³C CP/MAS NMR spectra of the as-synthesized COP_TDP display four obvious characteristic peaks at 111.9, 126.3, 130.5 and 140.0 cm⁻¹ attributed to the monomers (Fig. 2a), indicating that the skeletons of monomers are efficiently preserved in the COP_TDP structure. The detailed structural characteristics of the CCOP_TDP–FeNi–SiO₂ hybrid were firstly investigated by X-ray diffraction (XRD). The relatively sharp peak at 26° corresponds to the (002) plane of graphitic carbon, while the others at 43.5°, 50.7°, and 74.7° are attributed to the diffraction from the (111), (200), and (220) planes of FeNi alloy (JCPDS 47-1417), respectively (Fig. 2b).¹⁵ X-ray photoelectron spectroscopy (XPS) measurements were conducted to investigate the surface chemical composition of the CCOP_TDP–FeNi–SiO₂ catalyst. The XPS full survey spectrum revealed the presence of C (86.9 at%), N (2.2 at%), and O (10.9 at%) (Fig. S1a†). There is almost no Fe and Ni content because Fe and Ni species may be encapsulated by porous graphitic carbon layers which cannot be detected.³¹ This result was confirmed from the following HRTEM result. The high-resolution N 1s spectrum (Fig. S1b†) can be deconvoluted into several peaks at 398.7, 400.1 and 400.8, which can be assigned to pyridinic N (29.2%), pyrrolic N (27.5%) and graphitic N (43.3%), respectively. The high content of pyridinic and graphitic N should be beneficial for ORR and OER.^32,33 Nitrogen adsorption–desorption isotherms were obtained to investigate the Brunauer–Emmett–Teller (BET) surface area and pore structure of the CCOP_TDP–FeNi–SiO₂ catalyst (Fig. 2c). The N₂ adsorption isotherm obtained for CCOP_TDP–FeNi–SiO₂ can be identified as a type-IV isotherm.³⁴ The result indicates that CCOP_TDP–FeNi–SiO₂ has a big BET surface area of 435.5 m² g⁻¹. As displayed in the inset of Fig. 2c, the CCOP_TDP–FeNi–SiO₂ catalyst is mainly based on mesopores (most distributed at 3.8 and 23.7 nm) and macropores, which not only are more favorable for mass transport as compared to micropores that may be inaccessible to the electrolyte during the electrochemical reactions, but also provide enough active sites during the electrochemical reactions.^35,36

Fig. 1 The schematic illustration of the synthesis of CCOP_TDP–FeNi–SiO₂. For clarity, gray, blue, yellow and purple spheres refer to carbon, nitrogen, iron and nickel atoms, respectively.

Fig. 2 (a) ¹³C CP/MAS spectra of COP_TDP and its peak assignments. (b) XRD patterns of CCOP_TDP–FeNi–SiO₂ and CCOP_TDP–FeNi catalysts. (c) N₂ isotherms of CCOP_TDP–FeNi–SiO₂. Inset: the pore size distribution curves of CCOP_TDP–FeNi–SiO₂. (d, e and f) The HRTEM images for CCOP_TDP–FeNi–SiO₂. (g) High-angle annular dark-field (HAADF)-STEM image of CCOP_TDP–FeNi–SiO₂ and the corresponding element maps of Fe and Ni for CCOP_TDP–FeNi–SiO₂. (h) The energy dispersive X-Ray (EDX) spectra of CCOP_TDP–FeNi–SiO₂. Inset: the detection part of the HAADF-STEM image.

The morphology and microstructure were investigated by field scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM). The SEM image of the COP_TDP presents a hierarchical structure, which can provide more active site exposure (Fig. S2a†). Fig. 2d presents the overview HRTEM image of CCOP_TDP–FeNi–SiO₂. As can be seen, CCOP_TDP–FeNi–SiO₂ reaches a high degree of carbonization, presenting a graphene-analogue layer structure. Moreover, FeNi alloy NPs are dispersed on the surface of the CCOP_TDP support with an average size of ∼25 nm (Fig. S2b† and Fig. 2e), which is attributed to the SiO₂ protective shell effectively preventing the reunion of the FeNi alloy NPs in carbonization. Consistent with the XRD results, the lattice fringes with spacings of 0.21 and 0.34 nm are attributed to the (111) lattice plane of FeNi alloy NPs and graphitic carbon layers, respectively (Fig. 2f). Furthermore, FeNi alloy NPs are embedded in about 12 porous graphitic carbon layers with a thickness of about 4 nm, which not only prevents FeNi NPs from direct contact with the electrolyte to improve catalyst stability, but also prevents inner FeNi alloy NPs from dissolution and agglomeration under harsh conditions.³⁷ As discussed above, FeNi alloy NPs in CCOP_TDP–FeNi–SiO₂ were encased in the porous graphite shells, which is mostly in accordance with the XPS result (Fig. S1†). High-angle annular dark-field (HAADF)-STEM was performed to confirm the distribution of FeNi NPs in detail (Fig. 2g and Fig. S3†). It can be clearly seen that FeNi NPs of CCOP_TDP–FeNi–SiO₂ are uniformly distributed over CCOP_TDP–FeNi–SiO₂, while there is an obvious reunion and large particle size (∼60 nm) of FeNi NPs on CCOP_TDP–FeNi (Fig. S2c and Fig. S4†), which also results in a high metal content and low carbon content of the CCOP_TDP–FeNi catalyst (Tables S2 and S3†). The corresponding energy dispersive X-ray spectroscopy (EDS) map further verified the distribution of Fe, Ni, C, and N elements that match well with the HRTEM image of the CCOP_TDP–FeNi–SiO₂ (Fig. 2g and Fig. S3†). From the energy dispersive X-ray (EDX) spectroscopy results (Fig. 2h), it can be found that the encapsulated NPs are composed of homogeneously dispersed Fe and Ni elements, which supports the formation of metallic FeNi alloy.

To demonstrate the multifunctional electrochemical activities of the CCOP_TDP–FeNi–SiO₂ catalyst, the ORR performance was first investigated in O₂-saturated 1 M KOH electrolyte. As a comparison, CCOP_TDP–FeNi, CCOP_TDP and Pt/C (20 wt% Pt) were also measured under the same conditions. The linear sweep voltammetric (LSV) curves in Fig. 3a confirm that the CCOP_TDP–FeNi–SiO₂ catalyst exhibits the best ORR activity with an onset potential of 0.990 V versus reversible hydrogen electrode (vs. RHE) and a half-wave potential (E_1/2) of 0.890 V vs. RHE, among the ranks of the highest bifunctional performance of non-precious metal modified carbon-based catalysts (Table S1†). The measured E_1/2 of CCOP_TDP–FeNi–SiO₂ is 16 mV more positive than that of Pt/C (0.874 V) and much surpasses that of CCOP_TDP–FeNi (0.824 V) and CCOP_TDP (0.806 V) (Fig. 3b). This may be attributed to the synergetic effect between the N-doped carbon material and alloy NPs improving catalytic activities. Meanwhile, the CCOP_TDP–FeNi–SiO₂ catalyst still shows the calculated kinetic current density (J_K) value of 3.03 mA cm⁻² equal to that of Pt/C (2.94 mA cm⁻²) at 0.87 V, which is much higher than those of CCOP_TDP–FeNi (0.42 mA cm⁻²) and CCOP_TDP (0.20 mA cm⁻²) catalysts (Fig. 3b, Fig. S5–S8†). The superior catalytic activity of CCOP_TDP–FeNi–SiO₂ toward the ORR is further verified from the Tafel plots obtained from the polarization curves. The CCOP_TDP–FeNi–SiO₂ catalyst has a Tafel slope of 46 mV dec⁻¹, much lower than that of CCOP_TDP–FeNi (54 mV dec⁻¹), CCOP_TDP (59 mV dec⁻¹), and even Pt/C (57 mV dec⁻¹), suggesting the favorable ORR kinetics in the CCOP_TDP–FeNi–SiO₂ electrocatalyst (Fig. 3c). This also indicates that the transfer of the first electron is probably the rate-determining step in the ORR catalyzed by CCOP_TDP–FeNi–SiO₂. The transferred number of electrons during the ORR measured by ring currents signaling efficient O₂ activation on the CCOP_TDP–FeNi–SiO₂ catalyst presents a four-electron reduction path (Fig. S9†). The CCOP_TDP–FeNi–SiO₂ catalyst also shows excellent stability for ORR, as confirmed by cyclic voltammetry (CV) tests with cycling the potential between 0.7 V and 1 V vs. RHE at a scan rate of 100 mV s⁻¹. Remarkably, there is almost no shift occurring over the onset potential and half-wave potential of LSV for CCOP_TDP–FeNi–SiO₂ after 5000 cycles (Fig. 3d), confirming the high stability of the prepared CCOP_TDP–FeNi–SiO₂.

Fig. 3 (a) ORR LSV curves of CCOP_TDP–FeNi–SiO₂, CCOP_TDP–FeNi, CCOP_TDP and Pt/C in O₂-saturated 1 M KOH with a scan rate of 5 mV s⁻¹ and a rotating speed of 1600 rpm. (b) The comparison of E_1/2 and J_K of different catalysts. (c) Corresponding Tafel plots derived from the ORR LSV curves of different catalysts. (d) ORR LSV curves of CCOP_TDP–FeNi–SiO₂ before and after 5000 potential cycles ranging from 0.65 V and 0.95 V (vs. RHE) in O₂-saturated 1 M KOH. (e) OER LSV curves of CCOP_TDP–FeNi–SiO₂, CCOP_TDP–FeNi, CCOP_TDP and IrO₂/C in O₂-saturated 1 M KOH with a scan rate of 5 mV s⁻¹ and a rotating speed of 1600 rpm. (f) Corresponding Tafel plots derived from the OER LSV curves of different catalysts. (g) Nyquist plots of the electrochemical impedance spectra of different catalysts. (h) Corresponding electrocatalytic surface area (ECSA) for ORR and OER of different catalysts. (i) The overall polarization curves of all samples in the whole ORR and OER region in 1 M KOH solution.

In addition, the CCOP_TDP–FeNi–SiO₂ catalyst also exhibits remarkable activity and stability for the OER. The oxygen evolution activity of the CCOP_TDP–FeNi–SiO₂ catalyst was evaluated in O₂-saturated 1 M KOH solution with IR-correction, and the corresponding LSV curves are shown in Fig. 3e. The CCOP_TDP–FeNi–SiO₂ catalyst reaches a current density of 10 mA cm⁻² at an overpotential of 310 mV, which is smaller than that of IrO₂ (315 mV), CCOP_TDP–FeNi (330 mV) and CCOP_TDP (470 mV). Meanwhile, the CCOP_TDP–FeNi–SiO₂ catalyst exhibits a comparable Tafel slope (57 mV dec⁻¹) to that of the IrO₂ catalyst (55 mV dec⁻¹), but much lower than that of CCOP_TDP–FeNi (60 mV dec⁻¹) and CCOP_TDP (107 mV dec⁻¹), indicating an excellent OER kinetic process of this catalyst (Fig. 3f). Moreover, a continuous potential cycling test shows that there is no variation in the polarization curves, suggesting the high durability of CCOP_TDP–FeNi–SiO₂ in a long-term OER under 1 M KOH alkaline conditions (Fig. S10†). In addition, charge transport is also a crucial factor for the kinetics of the OER. Electrochemical impedance spectroscopy was performed to elucidate the superior OER activity of the CCOP_TDP–FeNi–SiO₂ catalyst. The smaller semicircle in the medium frequency region reveals the lower charge transfer resistance for the catalyst.³⁸ As shown in Fig. 3g, the charge transfer resistance of the CCOP_TDP–FeNi–SiO₂ catalyst is obviously lower than that of CCOP_TDP, suggesting a faster charge transfer process.

The apparent electrochemical surface area (ECSA) is more relevant to electrochemical activity; the ECSA of CCOP_TDP–FeNi–SiO₂, CCOP_TDP–FeNi, and CCOP_TDP is compared by obtaining cyclic voltammetry (CV) curves in 1.0 M KOH in the absence of O₂ (Fig. S11 and S12†).²² The ECSAs of CCOP_TDP–FeNi–SiO₂ for ORR and OER are confirmed to be 126.3 m² g⁻¹ and 114.2 m² g⁻¹, respectively (Fig. 3h), higher than those of CCOP_TDP (112.1 m² g⁻¹, 104.7 m² g⁻¹) and CCOP_TDP–FeNi (46.2 m² g⁻¹, 49.4 m² g⁻¹). These results further confirm that CCOP_TDP–FeNi–SiO₂ has the best bifunctional electrocatalytic activity.³⁹ The bifunctional oxygen electrode activity of the CCOP_TDP–FeNi–SiO₂ catalyst can be judged from the oxygen electrode activity parameter ΔE (ΔE = E_{j=10, OER} – E_{1/2, ORR}) in 1 M KOH, which is an important metric to evaluate the bifunctional activity towards the ORR/OER.⁴⁰ Generally, a smaller ΔE value indicates better bifunctional catalytic activity. Notably, the CCOP_TDP–FeNi–SiO₂ catalyst exhibits a low ΔE value of 0.650 V, much lower than that of CCOP_TDP–FeNi (0.736 V) and CCOP_TDP (0.794 V) (Fig. 3i), demonstrating that CCOP_TDP–FeNi–SiO₂ is an effective bifunctional electrocatalyst for ORR and OER.

Based on the excellent bifunctional catalytic performance of the CCOP_TDP–FeNi–SiO₂ catalyst, a zinc–air flow battery was constructed employing CCOP_TDP–FeNi–SiO₂ catalyst loaded carbon paper as the air cathode and Zn foil as the anode measured in 8.0 M KOH and 0.5 M ZnO electrolyte (Fig. 4a). For the control experiment, a mixed Pt/C + IrO₂ (1 : 1 by weight) as the cathode was also tested as a reference. As shown in Fig. 4b, the three zinc–air flow batteries constructed with the CCOP_TDP–FeNi–SiO₂ catalyst as the air-cathode can easily power up a light-emitting diode (LED, 3.7 V) panel displaying “BUCT”, demonstrating its promising application in Zn–air flow batteries. Furthermore, the discharge maximum power density of the Zn–air flow battery with the CCOP_TDP–FeNi–SiO₂ electrode is 112.8 mW cm⁻², which is higher than that of the Pt/C + IrO₂/C air electrode (64.8 mW cm⁻²) under the same conditions (Fig. 4c). The corresponding discharge and charge polarization curves of CCOP_TDP–FeNi–SiO₂ and Pt/C + IrO₂/C are presented in Fig. 4c. Compared with the Pt/C + IrO₂/C-driven Zn–air flow battery, the CCOP_TDP–FeNi–SiO₂ cathode shows a lower charge–discharge voltage gap of 0.79 V, indicating a better round-trip efficiency. The stability of the CCOP_TDP–FeNi–SiO₂ cathode was first performed by battery charging and discharging cycles at a current density of 5 mA cm⁻² (Fig. 4d). It is clearly seen that the CCOP_TDP–FeNi–SiO₂ catalyst has a better cycling performance without almost voltage gap change after 80 hours like Pt/C under the same conditions. Furthermore, the battery containing the CCOP_TDP–FeNi–SiO₂ cathode could withstand a high current density of 20 mA cm⁻², maintaining a low voltage gap of 1.24 V (Fig. 4e). Impressively, the final potential difference of the battery containing the CCOP_TDP–FeNi–SiO₂ cathode performed at the current density of 10 mA cm⁻² after 90 hours was almost the same as the initial potential (Fig. S13†). Based on the above results, the CCOP_TDP–FeNi–SiO₂ catalyst has the potential to replace the precious metal catalyst applied to the cathode catalyst of the zinc–air flow battery.

Fig. 4 (a) Schematic illustration of the rechargeable Zn–air flow battery. (b) Photographs of the Zn–air flow battery with an LED panel (3.7 V) powered by three-series Zn–air flow batteries in series with CCOP_TDP–FeNi–SiO₂ as the air-cathode. (c) Charge and discharge polarization curves and the corresponding power density plots of the primary zinc–air flow battery with the CCOP_TDP–FeNi–SiO₂ catalyst and Pt/C + IrO₂/C as the air-cathodes, respectively. (d) Charge–discharge cycling performance of a rechargeable Zn–air flow battery based on CCOP_TDP–FeNi–SiO₂ and Pt/C + IrO₂/C electrocatalysts at a current density of 5 mA cm⁻². (e) Charge–discharge cycling curves of CCOP_TDP–FeNi–SiO₂ at different current densities of 5 mA cm⁻², 10 mA cm⁻², and 20 mA cm⁻², respectively.

Conclusions

In summary, a rational design approach is reported to develop CCOP_TDP–FeNi–SiO₂ as a novel and excellent OER/ORR bifunctional electrocatalyst. Attributed to the synergetic effects between the N-doped carbon material and alloy NPs, the as-obtained CCOP_TDP–FeNi–SiO₂ catalyst shows excellent ORR activity with a positive half-wave potential of 0.89 V versus RHE and superior OER activity with a low overpotential of 310 mV at 10 mA cm⁻². The potential gap between E_1/2 and E_j=10 reach as small as 0.650 V. Meanwhile, FeNi alloy NPs are embedded into the graphitic carbon layer during the pyrolysis process, promoting the stability of the catalyst. A rechargeable zinc–air battery constructed with a CCOP_TDP–FeNi–SiO₂ cathode showed a high performance with a small voltage gap between charge and discharge of 0.79 V (at 5 mA cm⁻²) and a good cycling life. Therefore, this work reveals a new pathway for designing and constructing multifunctional electrocatalysts for metal–air batteries.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Key Research and Development Program of China (2017YFA0206500), the NSF of China (21676020; 51502012; 21620102007, 21606015), the Beijing Natural Science Foundation (17L20060, 2162032), the Young Elite Scientists Sponsorship Program by the CAST (2017QNRC001), the Start-up Fund for Talent Introduction of the Beijing University of Chemical Technology (buctrc201420; buctrc201714), the Talent Cultivation and Open Project (OIC-201801007) of the State Key Laboratory of Organic–Inorganic Composites, the Distinguished Scientist Program at BUCT (buctylkxj02) and the “111” project of China (B14004).

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available: Experimental setup including synthesis and characterization details. XPS spectra, SEM images, and LSV. See DOI: 10.1039/c8nr08330d

‡ These authors contributed equally.

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