Performance of iron-air battery with iron nanoparticle-encapsulated C–N composite electrode

Abstract

Highly efficient and stable iron electrodes are of great significant to the development of iron-air battery (IAB). In this paper, iron nanoparticle-encapsulated C–N composite (NanoFe@CN) was synthesized by pyrolysis using polyaniline as the C–N source. Electrochemical performance of the NanoFe@CN in different electrolytes (alkaline, neutral, and quasi-neutral) was investigated via cyclic voltammetry (CV). The IAB was assembled with NanoFe@CN as the anode and IrO₂ + Pt/C as the cathode. The effects of different discharging/charging current densities and electrolytes on the battery performance were also studied. Neutral K₂SO₄ electrolyte can effectively suppress the passivation of iron electrode, and the battery showed a good cycling stability during 180 charging/discharging cycles. Compared to the pure nano-iron (NanoFe) battery, the NanoFe@CN battery has a more stable cycling stability either in KOH or NH₄Cl + KCl electrolyte.

Graphical abstract

Keywords

energy storage and conversion / metallic composites / nanocomposites / iron-air battery / iron anode

1 Introduction

Metal-air batteries (MABs) have been paid extensive attention due to their high theoretical energy density and unique cathodic process where oxygen rich in resources is reduced to water. Among the MABs, the iron-air battery (IAB) reveals more promising application prospects because of the more abundant and low-cost iron resources [1,2]. Besides, IAB presents the following distinctive characteristics: First, no iron dendrite is formed during the charging process of the iron negative electrode. Next, the iron negative electrode can release a high theoretical specific capacity of 960 mAh/g. Finally, IAB is safer and environmentally friendly [3–6]. Compared to other MABs like Zn-air and Li-air batteries, however, development on the study of IAB severely lags behind due to the easy passivation of the iron anode caused by the formation of the compact iron oxides (or hydroxides) during the anodic oxidation [7,8]. In addition, the alkaline electrolyte is highly corrosive, and the carbonates can be easily formed and deposited on the air electrode due to the adsorption of CO₂, resulting in a serious decrease in the performance of air electrode. So far, there are only a few reports concerning the study of iron electrode and IAB. Generally, the electrolyte containing additive such as Bi₂S₃, Bi₂O₃, FeS, and Na₂S [9–14] can improve the stability of the iron electrode in alkaline media. Additionally, iron oxides particles can be loaded on conductive carbon materials to fabricate iron electrodes [15], which exhibit a higher specific capacity and a higher cyclic stability [12,13]. Polyhedral Fe₂O₃-based iron electrode present a good cyclic stability after 15 charging/discharging cycles [16]. Micro-morphological textures of Fe₂O₃ particles greatly influence their cyclic stability [7,17]. Taken overall, the iron electrodes reported in current literatures do not exhibit a high stability and the corresponding battery displays a low charging/discharging current and voltage [18]. Although solid electrolyte can improve the stability of the iron electrode [19], low discharging current density significantly limits its practical application. The electrochemical behaviors of the different iron electrodes reported were generally investigated in a conventional three-electrode system, and the iron electrodes presented a poorer cycle stability. Moreover, almost all the investigations were focused on the alkaline electrolyte and few studies were involved to other electrolytes. In acidic electrolytes, more H⁺ leads to a more severe hydrogen evolution reaction, which results in a lower charging and discharging efficiency of the iron electrode [20].

Development of iron matrix composites and exploration of different electrolyte solutions are effective means to improve the performance of IAB. In this work, a C–N composite layer coated iron electrode (NanoFe@CN) was synthesized by using polyaniline as C–N sources. Nitrogen-doped carbon network structure can immobilize metal catalytic sites and reduces hydrogen precipitation reaction potentials. IAB was assembled using the NanoFe@CN as the iron electrode and the IrO₂ + Pt/C as the catalyst of air electrode. Neutral solution 0.5 mol/L K₂SO₄ and quasi-neutral solution 4 mol/L NH₄Cl + 1 mol/L KCl were applied to the IAB to explore the influence of electrolyte on the battery performance. The results reveal that compared with the pure iron powder, the prepared NanoFe@CN is a highly efficient iron electrode of IAB and the corresponding battery can have a stable charging/discharging cycle.

2 Experimental section

2.1 Chemical reagents

The Fe(NO₃)₃·9H₂O (≥ 98%) and polyaniline were purchased from Tianjin Damao Chemical Reagent Factory, China. The NaOH (≥ 98%), acetylene black, N-methyl-2-pyrrolidone (98%), and IrO₂ (99.9% metals basis, Ir ≥ 84.5%) were obtained from the chemical reagent official website of Aladdin. The polyvinylidene difluoride was purchased from Arkema Co., Ltd., France. The Nafion solution (5%, Dupont) was purchased from Sinopharm Chemical Reagent Co., Ltd., China. The anhydrous ethanol, NH₄Cl (≥ 99.5%), and KCl (99.5%) were obtained from West Long Chemical Plant, China. The KOH (≥ 85%) was purchased from Tianjin Yongda Chemical Reagent Co., Ltd., China. The carbon paper (thickness = 0.3 mm) was purchased from Shanghai Jose Electric Co., Ltd., China. The commercial Pt/C (40%, Johnson Matthey Crop) was obtained from Shanghai Qunyi Energy Equipment Co., Ltd., China. The nickel foam (thickness = 1.5 mm) was obtained from Suzhou Keshenghe Metal Material Co., Ltd., China. The iron powder (99.9% metals basis, 100 nm) was obtained from Shanghai Meryer Biochemical Technology Co., Ltd., China. All chemical reagents were used without further treatment. An ultrapure water generator was used to produce experimental water (18.2 mΩ·cm)

2.2 Synthesis of NanoFe@CN

22.0 g of Fe(NO₃)₃·9H₂O were dissolved in 36 mL of ultrapure water, followed by the addition of 3 g of polyaniline and sonication for 30 min. 1 mol/L of NaOH was added dropwise to the solution to adjust the pH of the solution to 5–6. After stirring for 20 min, the solution was extracted and washed with water. Then the solution was dried in an oven at 60 °C for 24 h, and the solid obtained was protected by nitrogen in a tube furnace at 300 and 800 °C for 1 and 2 h, respectively. The obtained powder was ground to be the sample, which was labeled as NanoFe@CN.

2.3 Electrochemical measurements

The cyclic voltammetric curves of the catalysts were measured with an electrochemical workstation (AutoLabPGSTAT30/FRA) using a three-electrode system. The auxiliary electrode was a platinum foil, the reference electrode was a silver/silver chloride electrode, and the working electrode was a glassy carbon coated with catalyst ink which was prepared by mixing and sonicating 5 mg of catalyst powder, 950 μL of anhydrous ethanol, and 50 μL of Nafion solution to form a uniform ink-like dispersion. 20 μL of ink was transferred to the surface of the glassy carbon electrode (0.071 cm²) which was then dried at room temperature with a catalyst loading of 1.4 mg/cm². As a comparison, the working electrode loaded with pure iron powder (NanoFe) was also prepared in the same way. The potentials were converted to reversible hydrogen electrode (RHE) potentials according to the equation in Ref. [8]:

E_{\mathrm{R}\mathrm{H}\mathrm{E}/\mathrm{V}}=E_{\mathrm{A}\mathrm{g}/\mathrm{A}\mathrm{g}\mathrm{C}\mathrm{l}} +0.197+0.0591\times \mathrm{p}\mathrm{H}.

2.4 Electrode preparation and testing of rechargeable IABs

2.4.1 NanoFe@CN Fe electrode

First, 40 mg of NanoFe@CN, 10 mg of N-methyl-2-pyrrolidone, 5 mg of acetylene black, and 60 μL of polyvinylidene difluoride solution were mixed evenly. Then, the mixture was evenly applied to the nickel foam with a smearing area of 0.5 cm². After drying overnight in the oven at 60 °C, it was pressed under 100 MPa for 2 min. The NanoFe electrode was also fabricated using the similar process as that of the NanoFe@CN Fe electrode.

2.4.2 Air electrode

2.5 mg of Pt/C and 2.5 mg of IrO₂ were dispersed in 1000 μL of anhydrous ethanol solution containing 50 μL of Nafion. After ultrasonic treatment, 600 μL of the solution was placed on carbon paper. The smear area was the same as that of the vent area of the battery, and the load was 0.42 mg/cm².

As shown in Fig.1, the catalyst NanoFe@CN was synthesized by a facile pyrolysis, and then coated on the Ni foam. Afterwards, it was pressed to obtain the iron electrode. Corresponding IAB was assembled with the Ni foam-supported NanoFe@CN as the iron electrode and carbon paper-supported Pt/C + IrO₂ as the air electrode. The battery was measured on a Neware high-performance battery testing system (CT-4008-5V50 mA-164, NEWARE, China) at room temperature.

3 Results and discussion

The scanning electron microscopic (SEM) images of the catalyst are shown in Fig.2(a) and 2(b). It can be observed the pleated carbon nanosheets have many metal nanoparticles. The particle size of iron metal particles in NanoFe@CN ranges from ca. 120 nm to 1 μm, which was significantly larger compared to that of NanoFe (100 nm). This would be attributed to the sintering of iron metal nanoparticles during high temperature calcination. Meanwhile, most of the metal nanoparticles are encapsulated by the carbon layer, which fixes the catalytic sites to avoid further aggregation and hydrolysis of iron nanoparticles during discharge/charge testing, and thus improves the durability of the catalyst activity during long charge/discharge cycles. It is found from the energy-dispersive spectrometry (EDS) shown in Fig.2(c) that the energy peak of Fe appears at 0.70, 6.39, and 7.07 keV for catalyst NanoFe@CN, which indicates the presence of Fe element. The weight percentages of C, N, O, and Fe obtained by EDS are shown in Tab.1, where the prepared catalyst NanoFe@CN has 45.90% of Fe and 40.07% of C. The control sample NanoFe contains 95.53% of Fe. The X-ray diffraction (XRD) measurement shows that the NanoFe@CN has two obvious diffraction peaks at 44.7° and 65.0° which belong to the (110) and (200) crystal planes of Fe (PDF#06-0696), respectively [21]. The diffraction peak at 44.9° corresponds to the (031) crystal plane of FeC (PDF#06-0686). The (211) crystal plane at 82.3° is attributed to FeC and Fe. Five diffraction peaks at 35.6°, 43.9°, 50.7°, 59.3°, and 63.7° are attributed to Fe₂O₃ (PDF#21-1141), which proves that the particles loaded on the carbon nanosheets are metallic Fe nanoparticles and Fe₂O₃ particles. The presence of the metallic iron nanoparticles facilitates the highly efficient discharge/charge of the iron electrode.

The X-ray photoelectron spectroscopy (XPS) analysis was used to further investigate the elemental composition and valence state of the catalyst NanoFe@CN. The XPS spectrum ofFig.3(a) shows the presence of C, N, O, and Fe elements, which indicates the successful doping of Fe. The Fe 2p XPS spectrum (Fig.3(b)) can be divided into two spin orbitals, Fe 2p_3/2 and Fe 2p_1/2, with the satellite peaks located at 717.2 and 732.7 eV [22,23], where Fe 2p_3/2 is divided into Fe³⁺ (712.8 eV), Fe²⁺ (710.7 eV), and Fe⁰ (708.0 eV). Similarly, the three peaks of Fe 2p_1/2 at 726.5, 723.9, and 720.4 eV, are attributed to Fe³⁺, Fe²⁺, and Fe⁰, respectively [21,24]. The co-existence of the three different valence states of Fe is beneficial to improve the catalytic performance. The presence of Fe³⁺ indicates the possible formation of FeN active groups in the catalyst, which can reduce the activity of iron and improve the stability of the catalyst. Among them, the Fe contents for metallic Fe and other Fe species (Fe oxides + FeN + FeC) account for 5.1% and 94.9%, respectively. Metallic iron presents a lower content XPS, which can be ascribed to the fact that metallic iron on the surface of the sample can be easily oxidized while XPS is only a surface-detecting technique [25]. Fig.3(c) shows the C 1s spectrum, which deconvolutes into four peaks at 283.9, 284.8, 285.8, and 286.8 eV, corresponding to M–C_x, C–C, C–N, and C–O [26]. The weaker satellite at 283.9 eV indicates that a small amount of iron carbide is formed in NanoFe@CN, and the presence of C–N indicates that N is doped into the carbon network structure. The C–O group enhances the hydrophilicity of the catalyst, allowing its faster and fuller contact with the electrolyte solution [27]. The deconvolution of the N 1s spectrum shows five peaks of M–N_x (397.8 eV), pyridine N (398.8 eV), pyrrole N (400.3 eV), graphite N (401.3 eV), and oxidized N (403.1 eV). The presence of M–N_x further indicates the formation of FeN groups, and the presence of pyridinic N and graphite N indicates the successful doping of N, which is likely to improve the electrocatalytic efficiency of the catalyst [28].

Cyclic voltammetry (CV) was used to observe the electrochemical behavior of the iron electrodes in different electrolyte solutions. The usual equation for the first step of charging and discharging IABs in alkaline electrolytes is [1,13,29]

\mathrm{F}\mathrm{e}+2\mathrm{O}{\mathrm{H}}^{-}\leftrightarrows \mathrm{F}\mathrm{e}(\mathrm{O}\mathrm{H}{)}_2+ 2{\mathrm{e}}^{-}

(1)E(\mathrm{v}\mathrm{s}.\mathrm{R}\mathrm{H}\mathrm{E})=-0.93\mathrm{V}.

Černý & Micka [30] suggested that the oxidation of Fe to [Fe(OH)]_ad would occur before the oxidation of Fe to Fe(II), corresponding to the reaction equation

(2)\mathrm{F}\mathrm{e}+2\mathrm{O}{\mathrm{H}}^{-}\leftrightarrows [\mathrm{F}\mathrm{e}(\mathrm{O}\mathrm{H}){]}_{\mathrm{a}\mathrm{d}}+{\mathrm{e}}^{-}

(3)[\mathrm{F}\mathrm{e}(\mathrm{O}\mathrm{H}){]}_{\mathrm{a}\mathrm{d}}+\mathrm{O}{\mathrm{H}}^{-}\leftrightarrows \mathrm{F}\mathrm{e}(\mathrm{O}\mathrm{H}{)}_2+{\mathrm{e}}^{-}

The second step for the oxidation of iron electrode in alkaline electrolyte is

\mathrm{F}\mathrm{e}(\mathrm{O}\mathrm{H}{)}_2+\mathrm{O} {\mathrm{H}}^{-} \leftrightarrows \mathrm{F}\mathrm{e}\mathrm{O}\mathrm{O}\mathrm{H}+{\mathrm{H}}_2\mathrm{O}+{\mathrm{e}}^{-}

(4)E(\mathrm{v}\mathrm{s}.\mathrm{R}\mathrm{H}\mathrm{E})=-0.61\mathrm{V}.

and/or

3\mathrm{F}\mathrm{e}(\mathrm{O}\mathrm{H}{)}_2+2\mathrm{O}{\mathrm{H}}^{-}\leftrightarrows \mathrm{F}{\mathrm{e}}_3{\mathrm{O}}_4+4{\mathrm{H}}_2\mathrm{O}+2 {\mathrm{e}}^{-}

(5)E(\mathrm{v}\mathrm{s}.\mathrm{R}\mathrm{H}\mathrm{E})=-0.71\mathrm{V}.

In addition, the following hydrogen evolution reaction (HER) will occur at the iron electrode in the alkaline electrolyte

2{\mathrm{H}}_2\mathrm{O}+2 {\mathrm{e}}^{-} \leftrightarrows {\mathrm{H}}_2+2\mathrm{O}{\mathrm{H}}^{-}

(6)E(\mathrm{v}\mathrm{s}.\mathrm{R}\mathrm{H}\mathrm{E})=-0.88\mathrm{V}.

Fig.4(a) shows the CV curve of NanoFe@CN in 6 mol/L KOH electrolyte, where the positive scanning oxidation peak p₀ at −0.07 V can be attributed to the oxidation of Fe to [Fe(OH)]_ads (Eq. (2)). The oxidation peaks p₁ (0.21 V) and p₂ (0.63 V) correspond to the oxidation of Fe/Fe(II) and Fe(II)/Fe(III), respectively. The negative scanning reduction peaks n₂ (−0.16 V) and n₁ (−0.30 V) correspond to Fe(III)/Fe(II) and Fe(II)/Fe, respectively. As shown in Fig.4(b), only one reduction peak n₁ was observed in the reduction process. The onset potential of the reduction peak was −0.10 V smaller than the n₂ peak potential of NanoFe@CN, which indicates that the reduction peak n₁ of NanoFe may cover the reduction process from Fe(III) to Fe(II). Therefore, the n₁ reduction peak of NanoFe contains both Fe(III)/Fe(II) and Fe(II)/Fe reduction processes. It can be observed that the hydrogen evolution potential of NanoFe@CN is −0.46 V lower than that of NanoFe at −0.36 V. This indicates that the prepared carbon coated iron catalyst NanoFe@CN can inhibit the hydrogen precipitation reaction to some extent.

The CVs in 0.5 mol/L K₂SO₄ electrolyte are shown in Fig.4(c) and Fig.4(d). The oxidation peak of the catalyst in the 0.5 mol/L K₂SO₄ electrolyte lacks the a₀ peak compared to the 6 mol/L KOH electrolyte, which may be ascribed to the disappearance of the adsorbed hydroxy iron species due to the lower OH⁻ concentration in the solution. This indicates that the formation of intermediate [Fe(OH)]_ads can be avoided in the 0.5 mol/L K₂SO₄ electrolyte to optimize the reaction step. In addition, compared with the 6 mol/L KOH electrolyte, the 0.5 mol/L K₂SO₄ electrolyte contains more H⁺, which leads to the advancement of HER. The p₁ potential of the oxidation peak of NanoFe@CN (−0.17 V) is 250 mV lower than that of NanoFe (0.08 V), which reveals that the kinetic process for the oxidation of NanoFe@CN is enhanced. For the NanoFe@CN, the p₂ peak current in 0.5 mol/L K₂SO₄ solution is greater than the peak current in 6 mol/L KOH. The p₂ process corresponds to Eqs. (4) and (5), and the decrease in current is related to the fact that the Fe(OH)₂ generated during the reduction process is not fully converted to Fe in time, because the generated Fe(OH)₂ inhibits the further reaction of the iron on the electrode [7]. On the other hand, the reduction peak n₂ current of NanoFe@CN in the 0.5 mol/L K₂SO₄ solution is larger than the current in 6 mol/L KOH, which indicates more conversion of Fe(III) to Fe(II). These results show that 0.5 mol/L K₂SO₄ electrolyte can effectively inhibit the passivation of iron electrode.

The quasi-neutral 1 mol/L KCl + 4 mol/L NH₄Cl electrolyte is weakly acidic, which leads to a severe hydrogen precipitation reaction, and from Fig.4(c), it can be found that the hydrogen precipitation potential of catalyst NanoFe@CN shifts to −0.40 V. Usually, the reaction equation of iron anode in acidic electrolyte is [31]

\mathrm{F}\mathrm{e}\leftrightarrows \mathrm{F}{\mathrm{e}}^{2+}+2{\mathrm{e}}^{-}

(7)E(\mathrm{v}\mathrm{s}.\mathrm{R}\mathrm{H}\mathrm{E})=-0.49 \mathrm{V}.

Similar to the CVs in the neutral electrolyte, the CVs in weakly acidic solution only present two oxidation processes, Fe/Fe(II) and Fe(II)/Fe(III), and the reduction process also involves Fe(III)/Fe(II) and Fe(II)/Fe. In the 1 mol/L KCl + 4 mol/L NH₄Cl electrolyte, the increase of the oxidation peak p₁ current of the catalysts NanoFe@CN and NanoFe may be due to the p₁ process involving Eqs. (1) and (7), where Fe is converted to Fe(OH)₂ and Fe²⁺. In K₂SO₄ and 1 mol/L KCl + 4 mol/L NH₄Cl solutions, the sample NanoFe@CN has more negative potentials than NanoFe for the anodic oxidation processes of Fe/Fe(II) and Fe(II)/Fe(III), which indicates that NanoFe@CN enhances the oxidation kinetics. Meanwhile, in the reduction processes of Fe(III)/Fe(II) and Fe(II)/Fe, the sample NanoFe@CN has a more positive potential than NanoFe, which indicates that NanoFe@CN also accelerates the reduction kinetics.

IAB was constructed by using the NanoFe@CN as the iron electrode and carbon paper-supported Pt/C + IrO₂ as the air electrode. For comparison, pure iron nanoparticles with ca. 100 nm diameter were also used to fabricate the NanoFe iron electrode. The charging/discharging current density was set to 0.2, 1, and 2 mA/cm². One cycle consists of charging for 10 min and discharging for 10 min, with a 5 min rest between charging and discharging.

When 6 mol/L KOH electrolyte and a 0.2 mA/cm² current density are applied as shown in Fig.5(a), the discharge voltage of NanoFe@CN drops slowly to zero after 45 cycles, while that of the NanoFe rapidly drops to zero after 12 cycles. This may be attributed to the passivation of the iron electrode in the alkaline electrolyte, resulting in a dramatic decrease in battery performance. For the higher current densities than 0.2 mA/cm² in 6 mol/L KOH, the battery demonstrates a worse performance. The charge/discharge performance of NanoFe@CN at 6 mol/L KOH is superior to that of NanoFe, indicating that the structure of C−N wrapped iron nanoparticles can inhibit the passivation of anode iron to a certain extent, which is beneficial to improve the battery stability. In neutral 0.5 mol/L K₂SO₄ electrolyte (Fig.5(b)–Fig.5(d)), NanoFe exhibits low discharging voltages and performs only 34 cycles at 0.2 mA/cm² (Fig.5(b)), and at higher current densities (1 and 2 mA/cm²), the NanoFe battery even does not work at all. Based on the reduction peak n₁ (Fe(III)/Fe reduction process) of NanoFe at smaller currents in the CV test (Fig.4(d)), it is inferred that this may be due to the enrichment of Fe(OH)₂ generated during the reduction of NanoFe on the iron electrode, which prevents the iron electrode from continuing to work. However, the NanoFe@CN can last for more than 180 charging/discharging cycles at 0.2, 1, and 2 mA/cm². In addition, it displays stable and higher discharging voltages. The voltage efficiency of the NanoFe@CN battery at 0.2, 1, and 2 mA/cm² is 48.4%, 25.1%, and 22.2%, respectively. Considering the same time and current values set for the charging and discharging processes, the voltage efficiency is also equivalent to the Faraday efficiency. This indicates that the neutral 0.5 mol/L K₂SO₄ electrolyte can effectively inhibit the passivation and the hydrogen precipitation reaction of the iron electrode. Interestingly, at a current density of 0.2 mA/cm², the discharge voltage of the cell started to increase after 120 cycles, and the overall performance improved with the increase in the number of cycles. This could be attributed to the fact that the prepared iron electrode exposed more Fe, causing an increase in charge and discharge voltage. At a higher current density of 5 mA/cm², the voltage polarization becomes more prominent in the first 11 cycles, and gradually decreases and stabilizes in the following cycles. However, the battery presents a low discharge voltage of 0.13 V, and the voltage efficiency is only 5.42%. At a current density of 10 mA/cm², the voltage gap between discharge and charge increases rapidly, which eventually leads to the cell stopping.

In quasi-neutral 1 mol/L KCl + 4 mol/L NH₄Cl electrolyte and at 0.2 mA/cm², it can be observed that the NanoFe@CN battery is relatively stable before 101 cycles, with a voltage difference of 0.56 V between the charging and discharging platforms and a voltage efficiency of 61.6%. After 101 cycles, the discharging voltage shows a trend of decreasing, while the NanoFe always displays much lower discharge voltages than the NanoFe@CN. At a current density of 0.2 mA/cm², the NanoFe@CN starts with a low charging voltage, then it gradually increases and stabilizes at 33 cycles with the operation of the battery. The same phenomenon exists at a current density of 1 mA/cm², but the operation reaches stability after the 10th cycle. However, at a higher current density of 2 mA/cm², this phenomenon disappears. This may be due to the existence of two different pathways for the oxidation/reduction of Fe/Fe(II) in 1 mol/L KCl + 4 mol/L NH₄Cl electrolyte (Eqs. (1) and (7)) and the large difference in their redox potentials, where Fe is oxidized at the discharging process and restored at the charging process mainly at a lower redox potential of Eq. (7) at the beginning of the battery operation phase. This phenomenon is more obvious at low current densities. At a higher current density (1 and 2 mA/cm²) as indicated in Fig.4(b) and 4(c), the NanoFe@CN displays a relatively higher and stable discharging voltage, while the NanoFe presents a much low discharging voltage and stability. At a current density of 1 mA/cm², the voltage efficiency of the NanoFe@CN cell was 41.7% during the stable operation state, and the voltage efficiency started to decrease after 107 cycles. At a current density of 2 mA/cm², the NanoFe@CN cell maintained a voltage efficiency of 37.7% at the beginning, and the charging voltage decreased for the first time at the 25th cycle, which is probably due to the fact that the process of oxidation is mainly dominated by Eq. (7), then leading to a decrease in the battery charging voltage. The voltage efficiency of 35.0% was then maintained to continue stable operation, and at the 80th cycle, the passivation started to occur. By observing the color of the electrolyte before and after the battery test (Fig.6(d)), it can be found that a reddish-brown precipitate occurs at the bottom of the cell after the test is completed, while the electrolyte solution appears light green. This indicates the presence of Fe(OH)₂ in the solution and the formation of Fe₂O₃ precipitate. The charging and discharging performance of the battery at a higher current density was further tested. At 5 mA/cm², the battery maintains a relatively stable charging and discharging voltage, but its discharging voltage displays lower values. At the 100th cycle, the charging voltage is 1.87 V and the discharging voltage is 0.12 V. After 160 cycles, the discharging voltage is only 0.04 V, which results in a low voltage efficiency of 2.23%. At a higher current density of 10 mA/cm², the battery releases a low initial discharge voltage of only 0.07 V and ran for only 14 cycles.

4 Conclusions

In this paper, an iron nanoparticles-embedded C−N composite (NanoFe@CN) was synthesized by using a simple pyrolysis method derived from iron nitrate and polyaniline. The oxidation and reduction process of NanoFe@CN in different electrolytes (alkaline, neutral, and quasi-neutral) was investigated, and the results showed that the 0.5 mol/L K₂SO₄ electrolyte could optimize the reaction steps and inhibit the passivation of the iron-containing catalyst. A rechargeable IAB was assembled with NanoFe@CN as the anode. For the catalyst NanoFe@CN, its rational structure of the carbon layer encapsulation with nitrogen-doping fixes and protects the iron nanoparticles, leading to its high stability whether in alkaline or neutral electrolyte. Compared with the NanoFe battery, the NanoFe@CN battery presents a high discharging/charging cycle stability and a high discharging voltage both in alkaline (KOH), quasi-neutral (KCl + NH₄Cl), and neutral (K₂SO₄) electrolytes. Especially in 0.5 mol/L K₂SO₄ electrolyte, NanoFe@CN maintains a stable voltage efficiency after 180 cycles of charging and discharging, which further proves that 0.5 mol/L K₂SO₄ electrolyte can reduce the effect of hydrogen precipitation reaction and inhibit the passivation of iron electrode. These results show that iron nanoparticle coated with C–N composite is a suitable anode material of IAB. This study provides a valuable experimental idea for the development of IAB anode.

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