Mn₃O₄ nanoparticle-decorated hollow mesoporous carbon spheres as an efficient catalyst for oxygen reduction reaction in Zn–air batteries

Abstract

Hybrids of Mn₃O₄ nanoparticles and hollow carbon spheres prepared from templated pyrolysis of polydopamine were assembled via a straightforward sonication procedure. The resulting hybrids exhibit excellent catalytic activity toward the oxygen reduction reaction (ORR) in prototype Zn–air batteries. Impressively, these catalysts exhibit higher discharge potential and exceptional stability when compared to commercial Pt–Ru catalysts while simultaneously showing comparable onset potential and maximum current density.

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Introduction

Society's seemingly insatiable demand for energy and the associated environmental impact of fossil fuel use have led to calls for a global transition to renewable energy sources that include wind and solar power.¹ Despite maturing technologies and declining cost, the intrinsic intermittency of renewable energy production remains a challenge for large scale commercialization and implementation.^1,2 One approach toward mitigating this important issue is employing energy storage units that can provide better alignment of energy supply and demand timing.^3,4 Clearly, developing efficient and cost-effective cyclable energy storage technologies, like batteries, is of nascent importance.⁵

Among the many technologies that have appeared, metal–air batteries have attracted substantial attention in part because of their comparatively high energy density.^6–10 Aqueous Zn–air batteries are particularly attractive for stationary energy storage because they are based upon earth abundant, non-toxic elements (i.e., Zn) and have a high theoretical energy density (1086 kW kg⁻¹) which is four times that of current Li-ion batteries.¹¹ Despite these advantages, further advances are required if Zn–air batteries are to realize their full potential. The most substantial benefits will come from the development of efficient precious metal-free catalysts that facilitate reactions at the air electrode.¹²

The oxygen reduction and evolution reactions (i.e., ORR and OER, respectively) occur at the air electrode and are crucial to battery performance (Fig. 1). When the battery is discharging, the ORR reaction is active and oxygen is reduced to form zincate anions at the Zn electrode. Upon charging, this process is reversed and oxygen is regenerated via the OER. While ORR is thermodynamically favourable under operational conditions, both the ORR and OER are kinetically hindered.¹³ As a result, electrocatalysts are essential to realizing functional/cyclable batteries. Conventional ORR and OER catalysts rely on precious metals and their oxides.¹⁴ The high cost of these metals/metal oxides potentially limits the economic viability of their large-scale application.

Fig. 1 A general schematic of a Zn–air battery and its associated processes.

Precious-metal free systems are being aggressively explored in efforts to find alternative, efficient catalysts. Of late, a variety of carbon nanomaterials (e.g., N-doped carbon nanotubes (CNTs), carbon nanoribbons, and graphene nanosheets) have shown excellent ORR and/or OER catalytic activity.^15–18 Transition metal oxide nanoparticles (e.g., MnO_x, CoO_x) also exhibit promising performance.^19,20 There have even been reports of synergistic effects when these materials are combined, however its chemical origin was not identified.²¹ Previously we demonstrated that N-doped hollow mesoporous carbon (HMC) nanospheres for their catalytic activity.²²

Here, we report a new precious-metal free hybrid that combines the favourable ORR properties of N-doped HMCs with those of earth abundant Mn₃O₄ nanoparticles. The hybrid exhibits enhanced catalytic ORR performance when compared with the individual components, as well as benchmark Pt–RuO₂ catalysts. Further adding to the appeal of this new catalyst, its stability exceeds that of Pt–RuO₂ standards when incorporated into prototype Zn–air batteries.

Experimental section

Chemicals

Dopamine hydrochloride, pluronic F127, tris(hydroxymethyl)amino-methane (Tris), tetraethoxysilane (TEOS) (≥99%), 2-propanol, and anhydrous ethanol (≤0.005% water) were obtained from Sigma-Aldrich. Ammonium hydroxide (NH₄OH; 28–30%) was acquired from Caledon Laboratory Chemicals. Hydrofluoric acid (HF, electronic grade, 48–50%) and sodium hydroxide (NaOH, pellets) were purchased from Fisher Scientific. Manganese acetate tetrahydrate (Mn(CH₃COO)₂·4H₂O, powder) was obtained from Matheson Colman & Bell. Pt–Ru powder (30% Pt and 15% Ru on carbon black) and Nafion solution (D521, 5% w/w in water) were purchased from Alfa Aesar. Teflon-coated porous carbon paper (SGL 39BC gas diffusion layer; GDL) was acquired from the Fuel Cell Store. All reagents were used as received unless otherwise specified.

Synthesis of Stöber SiO₂ nanoparticles (SiO₂ NPs)

Stöber SiO₂ NPs were prepared using a modified literature procedure.²³ Briefly, TEOS (3.57 mL, equiv. 16.1 mmol) was added to a mixture of deionized water (80 mL) and anhydrous ethanol (156 mL), followed by the addition of NH₄OH (8.0 mL). The reaction mixture was stirred at 500 rpm for 3 h at room temperature (25 °C). Subsequently, the SiO₂ NPs were isolated upon centrifugation at 12 000 rpm for 30 min. They were then re-dispersed in anhydrous ethanol and purified by centrifugation. The dispersion/isolation process was repeated twice. The purified SiO₂ NPs were dried in ambient conditions (in air at 25 °C) for 16 h, after which they were transferred to a sealed glass vial and stored at room temperature (25 °C) and under ambient light until needed.

Synthesis of SiO₂@C nanoparticles (NPs)

The SiO₂ NPs (0.700 g) were dispersed in 187 mL deionized water with exposure to a bath sonicator for 10 min. Triblock copolymer pluronic F127 (0.350 g), Tris (0.210 g), and dopamine hydrochloride (0.700 g) were added as powders to the SiO₂ NP suspension. The mixture was magnetically stirred at 500 rpm for 24 h at room temperature (25 °C). The resulting polydopamine coated SiO₂ NPs (SiO₂@PDA) were isolated via centrifugation at 12 000 rpm for 30 min. Subsequently, the SiO₂@PDA were re-dispersed in DI water with sonication and recovered via centrifugation. This process was repeated twice, replacing water with anhydrous ethanol. The purified SiO₂@PDA was dried in air at 25 °C for 16 h. The SiO₂@PDA was then transferred to a quartz boat, placed in a standard tube furnace and carbonized under flowing Ar atmosphere. The furnace was heated to 400 °C (heating rate of 1 °C min⁻¹) and maintained at 400 °C for 2 h before the heating to 800 °C (heating rate of 5 °C min⁻¹) for 3 h. Finally, the furnace was cooled to 25 °C and SiO₂@C NPs were obtained as a black solid.

Synthesis of hollow mesoporous carbon spheres (HMCs)

The as-synthesized SiO₂@C (0.600 g) was dispersed in anhydrous ethanol (6.0 mL) and DI water (6.0 mL) via magnetic stirring (10 min at 500 rpm) followed by sonication (10 min) in a polyethylene terephthalate (PET) beaker. HF (49%, 6.0 mL) was added to the mixture, which was magnetically stirred at 25 °C for 1 h. The HF dissolved the SiO₂ core without damaging the carbon shell, yielding hollow mesoporous carbon spheres (HMCs). The HMCs were collected via vacuum filtration using a Teflon funnel and filter flask and washed with 60 mL of DI water and anhydrous ethanol (Caution! HF must be handled with extreme care and in accordance with local regulations).

Synthesis of Mn₃O₄@HMC

The hybrid material was synthesized according to a modified literature procedure.²¹ The purified HMCs (50 mg) were dispersed in anhydrous ethanol (10.0 mL) and magnetically stirred for 10 min at 500 rpm. Subsequently, Mn(CH₃CO₂)₂·4H₂O (250 mg) and NaOH (100 mg) were added to the HMC ethanol suspension. The mixture was transferred to a sealed glass reaction vessel and exposed to a bath sonicator for 2.5 h. The water bath was replenished with cold water and the sonication was continued for an additional 2.5 h. The final product (Mn₃O₄@HMC) was isolated via centrifugation at 12 000 rpm for 30 min. It was then re-suspended in anhydrous ethanol and centrifuged. The process was repeated three times. The centrifuged Mn₃O₄@HMC was again re-dispersed in anhydrous ethanol and filtered via vacuum filtration.

Synthesis of freestanding Mn₃O₄

Freestanding Mn₃O₄ NPs were synthesized for comparison. The preparation was identical to that for Mn₃O₄@HMC; however, no HMC was added to the reaction mixture. Mn(CH₃CO₂)₂·4H₂O (250 mg) and NaOH (100 mg) were added to ethanol (10.0 mL), followed by sonication (5 h) in a bath sonicator.

Material characterization

Fourier-transform infrared (FTIR) spectra were obtained with a Nicolet 8700 (Thermo Fischer, USA) FT-IR spectrometer. FTIR samples were prepared by mixing the material of interest (∼1 mg) and potassium bromide fine powder (KBr, 150 mg). The powder mixture was then pressed into a pellet using a hand press (Specac Ltd).

Scanning electron microscopy (SEM) was performed using a Zeiss Sigma 300 VP-FESEM (accelerating voltage of 5–20 kV) equipped with secondary and backscattered electron detectors and an in-lens detector. Samples were prepared by placing a drop of ethanol suspension of the material of interest onto an Al stub and was subsequently air dried for 1 h at room temperature (25 °C). Transmission electron microscopy (TEM) was performed with a JEOL JEM-ARM200CF TEM/STEM (accelerating voltage of 200 kV) equipped with energy dispersive X-ray (EDX) spectrometer. High resolution TEM (HRTEM) images were processed using Gatan Digital Micrograph software (Version 3.22.1461.0) and ImageJ (Version 1.52R). Selected area electron diffraction (SAED) was performed on a JEOL 2010 TEM. TEM samples were prepared by dispersing the purified sample in anhydrous ethanol, which was then drop cast onto a holy/lacey carbon coated Cu grid (Electron Microscopy Inc.) and dried under vacuum for at least 16 h. At least 300 nanoparticles were used for determination of thickness and diameter of the purified materials. EDX maps of several representative regions were also obtained.

X-ray photoelectron spectroscopy (XPS) was performed with a Kratos Axis 165 Ultra X-ray spectrometer operating in energy spectrum mode under ultra-high vacuum. A monochromatic Al K source (λ = 8.34 Å) was used as the X-ray source. The samples were prepared by drop-casting an ethanol suspension of material of interest onto a Cu foil that was air dried for at least 16 h at room temperature (25 °C). The take-off angle was 90°. CasaXPS software (VAMAS) was used to analyse the obtained data. In general, a Shirley-type background was subtracted.²⁴ The spectra were calibrated by setting the deconvoluted adventitious C 1s peak to 284.8 eV.

Powder X-ray diffraction (PXRD) was conducted by placing purified samples on a zero-background Si wafer. PXRD patterns were collected using a Rigaku Ultima IV XRD system equipped with a Cu Kα radiation source.

Nitrogen adsorption–desorption isotherms were obtained with a Quantachrome Autosorb-iQ-XR system at −196 °C. Before measurements, samples were outgassed at 150 °C under vacuum for 3 h. Data was analysed using Brunauer–Emmett–Teller (BET) theory. The BET specific surface area was extrapolated from the linear region of the BET graph and the total pore volume was obtained from the data point at around P/P₀ = 0.992.

Electrochemical testing

The electrochemical performance of candidate catalysts was evaluated using linear sweep voltammetry (LSV). Measurements were performed in O₂ saturated 1 M KOH with a potentiostat (VSP-100) using a three-electrode cell and at a scan rate of 5 mV s⁻¹. The KOH solution was purged with O₂ gas and was stirred constantly during testing. The working electrode was prepared as follows. An ethanol suspension was prepared by dispersing 50 mg of sample of interest in 15 mL of anhydrous ethanol and 1 mL of 5% Nafion. A pre-cut piece of GDL (circular, diameter = 4.5 cm) was soaked and sonicated in the ethanol suspension for 20 min, before it was dried in air at 25 °C for 15 min. Subsequently, the ethanol suspension (3 mL) was passed through the GDL via vacuum filtration, producing an impregnated GDL with a mass loading of ∼2 mg cm⁻². For comparison, a Pt–Ru sample was prepared by spray coating an ink consisting of Pt–Ru/C (50 mg), deionized water (1.00 mL), ethanol (2.00 mL), and 5% Nafion (0.100 mL) onto the GDL. The impregnated or sprayed GDL samples were used as the working electrode. A platinum wire and Hg/HgO (0.098 V vs. SHE) were used as counter electrode and reference electrode, respectively. The reported current densities were normalized to the exposed surface area of the working electrode. All reported potentials were IR compensated (R_u = 2–4 Ω).

Prototype Zn–air battery assembly

Prototype Zn–air battery testing was performed in both vertical and horizontal home-made cells. The vertical cell had a two-electrode setup whereas the horizontal cell had a tri-electrode configuration (decoupled electrodes for ORR and OER).²⁵ In both configurations, the electrolyte consisted of 6 M KOH and 0.25 M ZnO. In the vertical cell, zinc sheet (8 g) and catalyst loaded GDL were used as zinc and air electrodes. Discharge rate test and power density curves were obtained using this configuration. The horizontal cell consisted of zinc sheet (8 g), catalyst loaded GDL, and Ni foam as zinc, ORR, and OER electrodes respectively. Electrochemical impedance spectra (EIS) and discharge and charge cycling were performed using this setup. For comparison, Pt–Ru spray-coated GDL was used as the air electrode in the vertical cell and ORR electrode in the horizontal cell. Discharge and charge cycling were performed at 20 mA cm⁻² and 30 min per cycle.

Rotating disk electrode (RDE) testing

Cyclic voltammetry (CV) and linear sweep voltammetry (LSV) were also performed using a glassy carbon rotating disk electrode (RDE) (Pine instruments Co). An ink was prepared by combining 4 mg of the Mn₃O₄@HMC, 0.250 mL Nafion solution, and 0.750 mL 2-propanol. The ink was sonicated for 1 h in a bath ultrasonicator filled with ice-water. The dispersed ink was immediately drop-cast (0.005 mL aliquot) onto a polished and cleaned RDE electrode. The electrode was then dried under a heat lamp (20 W) for 1 h (mass loading 0.1 mg cm⁻²). A three-electrode cell was used, with the catalyst coated RDE, Pt coil, and Hg/HgO as working, counter, and reference electrodes, respectively. 0.1 M KOH aqueous solution was used as the electrolyte. CV and LSV were performed at a scan rate of 20 and 5 mV s⁻¹, respectively, in O₂-saturated electrolyte. O₂-saturated electrolytes were prepared by purging 0.1 M KOH with high purity O₂ for at least 25 min at a rate of 0.04 standard litres per minute (SLPM).

Results and discussion

HMCs were prepared using a method developed in our laboratory.²² Silica (SiO₂) nanoparticles (NPs) were synthesized via the Stöber method and used as a sacrificial template. The as-synthesized SiO₂ NPs were coated with dopamine and pluronic F127. The coating was polymerized under basic conditions, after which the polymer shell was carbonized. The SiO₂ core was removed via alcoholic hydrofluoric acid (HF) etching to yield the HMCs on/in which, Mn₃O₄ NPs were deposited.²¹

Fig. 2A and B show electron microscopy images of as-synthesized HMCs. It is clear that HF etching removes the SiO₂ core to yield a hollow structure that retains a spherical morphology with a diameter of 98 ± 20 nm and a shell thickness of 3.8 ± 0.4 nm (Fig. 2B). After addition of Mn(CH₃CO₂)₂ and NaOH followed by 5 h of sonication, the morphology of the HMCs remained intact (Fig. 2C). The SEM image reveals a rough surface on the modified HMCs that presumably arises from the deposition of NPs. Higher magnification inspection using TEM (Fig. 2D) reveals Mn₃O₄ NPs on the surfaces and within the HMCs. The average NP size was determined to be 3.8 ± 0.5 nm (Fig. S6†). The crystallinity of the Mn₃O₄ NPs is clear from the selected area electron diffraction (SAED) pattern (Fig. 3A), which shows rings with d-spacings of 0.312, 0.282, 0.251, 0.204, 0.182, and 0.155 nm, that we confidently assigned to (112), (103), (211), (220), (105), and (224) planes, respectively, of hausmannite Mn₃O₄ (JCPDS card 24-0734).^25,26 These d-spacings were also directly observed via HRTEM (Fig. 3B). PXRD analysis (Fig. 3C) further confirms the reflections are readily indexed to hausmannite Mn₃O₄.²⁶

Fig. 2 Representative SEM secondary electron (SE) (A) and TEM bright field (BF) (B) images of HMC; SEM SE (C) and TEM BF (D) images of Mn₃O₄@HMC.

Fig. 3 Selected area electron diffraction pattern (A), HRTEM image (B), and PXRD pattern (C) for Mn₃O₄@HMC.

EDX mapping of the HMCs (Fig. S1†) shows the N signal overlaps with the bright field image, which is consistent with N being incorporated into the carbon matrix. The oxygen present in the carbon matrix can be reasonably attributed to residual oxygen remaining after the pyrolysis of polydopamine. Fig. 4 shows EDX mapping for the Mn₃O₄@HMC. The C, Mn, and O signals all overlap with the STEM HAADF image. EDX analysis shows that the NPs are uniformly distributed and have a composition consistent with Mn₃O₄.

Fig. 4 EDX mapping of Mn₃O₄@HMC showing uniform distribution of Mn and O.

The Mn₃O₄@HMC hybrid was also characterized using XP and FTIR spectroscopy. As expected, the XP survey spectrum (Fig. S2†) shows emissions associated with Mn, C, N, and O. The deconvoluted N 1s XP spectrum (Fig. 5A) for HMC shows pyridinic and pyrrolic N at 398.2 eV and 400.5 eV, respectively. After incorporation of Mn₃O₄, the pyridinic N emission did not shift, while the pyrrolic N peak shifted from 400.5 eV to 399.9 eV. This observation can reasonably be attributed to pyrrolic N interacting with the Mn₃O₄ NPs and is the subject of ongoing investigation in our laboratory. In the deconvoluted C 1s spectrum (Fig. 5B), the C=O emission shifted to a higher binding energy and merged with the O–C=O feature, consistent with the C=O species also interacting with the Mn₃O₄ surface through the oxygen. The deconvoluted Mn 2p spectrum (Fig. 5C) of the hybrids shows Mn 2p_3/2 at 641.4 eV, confidently assigned to Mn₃O₄.²⁷ A satellite peak corresponding to Mn₃O₄ was also observed at 649.2 eV. The Mn 3s spectrum (Fig. 5D) shows a signature 5.4 eV splitting of the 3s emission that is commonly attributed to the presence of Mn³⁺.^27–29

Fig. 5 High resolution XP spectra for (A) N 1s, (B) C 1s, (C) Mn 2p, and (D) Mn 3s regions.

FTIR spectra for HMC and Mn₃O₄@HMC are shown in Fig. 6. The HMC spectrum shows an absorption at 3434 cm⁻¹ that is attributed to surface O–H and N–H stretching.³⁰ The feature at 2925 cm⁻¹ arises from C–H stretching.³⁰ Features at 1585 and 1435 cm⁻¹ correspond to C–N bending and heterocyclic stretching, respectively, and the feature at 1172 cm⁻¹ is assigned to heterocyclic N–H in-plane deformation breathing.³⁰ In addition to all the HMC spectral features, the Mn₃O₄@HMC spectrum shows an absorption at 524 cm⁻¹ corresponding to Mn–O bonding.³¹ The C–N bending and heterocyclic stretching features in the HMC spectrum are qualitatively sharper and located at lower energy (i.e., 1556 and 1410 cm⁻¹, respectively) for the hybrid, which is consistent with HMC-NP interactions suggested by the N-region of the XP spectra (Table 1).

Fig. 6 FTIR spectra for HMC and Mn₃O₄@HMC.

Table 1 HMC and Mn₃O₄@HMC textural properties

Catalyst	BET surface area (m^2 g^−1)	Pore diameter (nm)	Pore volume (cm^3 g^−1)
Pristine HMC	498	29	3.33
Mn3O4@HMC	201	24	1.28

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The textural properties of the pristine HMC and Mn₃O₄@HMC were evaluated by nitrogen sorption analysis. BET surface area, pore diameter, and pore volume were acquired. Isotherms (Fig. S3 and S4†) show a distinct hysteresis loop was observed at high relative pressure for the pristine HMCs giving an average pore diameter is 29 nm, consistent with a mesoporous material.²² In addition, they exhibit a high BET surface area of 498 cm² g⁻¹ was determined. After incorporation of Mn₃O₄ NPs, the BET surface area and pore volume drop to 201 cm² g⁻¹ and 1.28 cm³ g⁻¹, respectively. This observation can be attributed to Mn₃O₄ NPs occupying the space inside the HMCs.

The ORR activity of Mn₃O₄@HMC was first assessed using cyclic voltammetry (CV) in both Ar- and O₂-saturated 0.1 M KOH aqueous solution using a rotating disk electrode (Fig. 7A). For these measurements, a 0.1 M KOH aqueous electrolyte was used to maximize O₂ solubility and minimize background current.³² The Ar-saturated cyclic voltammogram is featureless and only shows capacitive current; in contrast, the cyclic voltammogram from the O₂-saturated system shows a cathodic current at −0.17 V. This observation indicates that the ORR reaction occurs on the Mn₃O₄@HMC. The ORR performance was subsequently evaluated using standard linear sweep voltammetry (LSV) in 1 M KOH aqueous electrolyte. Electrodes were prepared by impregnating a circular GDL with pristine HMC or Mn₃O₄@HMC. LSV curves of pristine HMC, Mn₃O₄ NPs, Mn₃O₄@HMCs, and Pt–Ru are shown in Fig. 7B. The onset potential (defined as the potential at which the current density reaches 10 mA cm⁻²) for the hybrid material was markedly improved from −0.156 V for the pristine HMC to −0.082 V; this suggests synergistic effects resulting from the combination of HMCs with Mn₃O₄ NPs. This performance is comparable to that of Pt–Ru (−0.077 V). In addition, the maximum current density obtained for Mn₃O₄@HMC is 198.1 mA cm⁻² and exceeds that of pristine HMC (i.e., 167.6 mA cm⁻²); it is marginally better than that of the Pt–Ru (i.e., 196.5 mA cm⁻²). Based upon these observations, Mn₃O₄@HMC exhibits improved catalytic activity relative to pristine HMC and meets or exceeds the performance of our Pt–Ru benchmark.

Fig. 7 (A) Cyclic voltammograms for Mn₃O₄@HMC in Ar- and O₂-saturated 0.1 M KOH at a scan rate of 5 mV s⁻¹; (B) LSV curves obtained in O₂-saturated 1 M KOH aqueous solution at a scan rate of 5 mV s⁻¹.

To investigate the kinetics and catalytic mechanism, LSV was also performed using a Mn₃O₄@HMC-coated rotating disk electrode (RDE) at a scan rate of 5 mV s⁻¹ and predefined rotation rates (i.e., 225, 400, 625, 900, 1225, and 1600 rpm) in O₂-saturated 0.1 M KOH electrolyte (Fig. 8A). In Fig. 8A, kinetically controlled (0 to −0.1 V), kinetic-diffusion controlled (−0.1 to −0.3 V), and diffusion controlled (−0.3 to −0.7 V) regions were observed. The Koutecky–Levich (K–L) eqn (1) was used to determine the number of electrons transferred:

i_k = nFAkC_O2 (2)

B = 0.62nAFC_O2ν^−1/6D₀^2/3 (3)

where the measured current (i) is related to the kinetic current (i_k) and the diffusion-limiting current (i_l). ω is the angular velocity of the RDE, n is the number of electrons transferred per O₂ molecule, F is the Faraday constant (96 485 C mol⁻¹), A is the electrode area (cm²), k is the electron rate transfer constant, C_O2 is the saturated O₂ concentration of the electrolyte (1.2 × 10⁻⁶ mol cm⁻³), ν is the kinematic viscosity of the 0.1 M KOH solution (0.01 cm² s⁻¹), and D₀ is the diffusion coefficient of O₂ in the electrolyte (1.9 × 10⁻⁵ cm² s⁻¹).^33,34 As the rotation rate was increased, higher (more negative) current densities were observed as a result of faster O₂ flux to the electrode surface. K–L plots were constructed by plotting i⁻¹ against ω^−0.5. The number of electrons transferred per O₂ molecule was obtained from the slope of the fitted lines. The average n value in the potential window 0.3–0.7 V is 3.95, suggesting that the Mn₃O₄@HMC catalyst predominantly proceeds via the four-electron pathway (eqn (6)) rather than the competing alternative two-electron pathway (eqn (4) and (5)).³⁵

O₂ + 2H₂O + 2e⁻ → HO⁻₂ + OH⁻ (4)

2HO⁻₂ → 2OH⁻ + O₂ (5)

O₂ + 2H₂O + 4e⁻ → 4OH⁻ (6)

Fig. 8 (A) ORR LSV curves for Mn₃O₄@HMC in O₂-saturated 0.1 M KOH at scan rate of 5 mV s⁻¹ using RDE; (B) K–L plots for Mn₃O₄@HMC extrapolated in the potential range of 0.4–0.6 V.

Mn₃O₄@HMC and pristine HMC were incorporated into separate primary Zn–air batteries as the air electrode catalyst to evaluate their performance. Discharge rate tests were performed at predefined current densities (2, 5, 10, and 20 mA cm⁻²). In all cases, the rate discharge curves (Fig. 9A) show the discharge potential of the Mn₃O₄@HMC was markedly better compared with pristine HMC. Mn₃O₄@HMC also outperformed Pt–Ru at the tested current densities, indicating its superior ORR catalytic activity. At 10 and 20 mA cm⁻², the discharge potentials were 1.26 and 1.22 V, respectively for Mn₃O₄@HMC. Our benchmark Pt–Ru catalyst only exhibited 1.25 and 1.20 V discharge potential at 10 and 20 mA cm⁻², respectively. Furthermore, the performance of the present Mn₃O₄@HMC outperforms many transition metal oxide and carbon nanomaterial hybrids (Table S1†). This is also the first time N-doped hollow carbon nanospheres have been combined with transition metal oxide nanoparticles as a Zn–air battery catalyst. Conventional transition metal nanoparticle synthesis often involves high temperature annealing (>300 °C), which is likely to destroy the delicate feature of the HMC (average thickness = 3.8 nm).^36–38 By implementing the sonication procedure, the potential damage to the HMC active sites was avoided.

Fig. 9 (A) Rate discharge curves; (B) polarization and power density curves for a primary Zn–air battery using Mn₃O₄@HMC, pristine HMC, and Pt–Ru as ORR catalysts.

Polarization and power density curves are shown in Fig. 9B. They highlight that the maximum power density was improved from 158 mW cm⁻² to 183 mW cm⁻² after the incorporation of Mn₃O₄ NPs. The maximum power density is much higher than the value obtained for our Pt–Ru benchmark (158 mW cm⁻²) under the same conditions. The hybrid catalyst also exhibits a much lower charge-transfer resistance, evidenced by the smaller size of the semi-circular region of EIS Nyquist plot (Fig. S8†). The excellent catalytic performance can be attributed to the exterior and interior surfaces of the HMCs being decorated with Mn₃O₄ NPs. The number of active sites was maximized and any synergistic effects were amplified.

The stability of the as-synthesized hybrid material was further investigated by cycling a rechargeable Zn–air battery at 20 mA cm⁻² using Mn₃O₄@HMC on GDL as the ORR electrode. Each cycle was 30 min and 235 cycles (117.5 h) were performed. Ni foam was used as the OER electrode due to its high surface area, good OER catalytic activity, and stability.³⁹ A tri-electrode configuration was utilized so that the ORR performance could be evaluated independently of the OER performance. As shown in the discharge/charge curves (Fig. 10A), the HMC hybrid initially exhibits a discharge potential of 1.21 V. After 117.5 h of cycling (235 cycles), the discharge potential decreased slightly to 1.17 V corresponding to a 3.3% loss. A similar tri-electrode battery, using Pt–Ru on GDL as the ORR electrode and Ni foam as OER electrode, was tested under the same conditions (Fig. 10B). The discharge potential for Pt–Ru decreased from 1.19 to 1.14 V, after 117.5 h of cycling, corresponding to a 4.2% change. This shows the comparable long-term durability of the Mn₃O₄@HMC compared with Pt–Ru. The morphology of the HMC hybrid was not evaluated after cycling due to the difficulty in separating the hybrid from the GDL.

Fig. 10 (A) Discharge/charge cycling curves for a three-electrode Zn–air battery using Mn₃O₄@HMC on GDL and Ni foam as ORR and OER electrodes, respectively. (B) Discharge/charge cycling curves for a three-electrode Zn–air battery using Pt–Ru on GDL and Ni foam as ORR and OER electrodes, respectively.

Conclusions

Mn₃O₄ decorated N-doped hollow mesoporous carbon nanospheres were prepared using a straightforward templated growth/sonication approach. Comparing performance of the hybrid with that of pristine HMCs, we note substantially improved catalytic ORR activity in primary and secondary Zn–air batteries. This synergistic effect, that can reasonably be attributed to Mn–O–C and Mn–N interactions that manifest in the presented XPS analysis, opens the door to the potential of exquisite tuning of catalytic response that could lead to next-generation systems. The primary battery fabricated using Mn₃O₄@HMC generated a maximum power density of 183 mW cm⁻² at a current density of 291 mA cm⁻². The secondary battery with a three-electrode configuration exhibited a small discharge potential drop from 1.21 to 1.17 V after over 117.5 h of cycling at 20 mA cm⁻², displaying excellent durability.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors acknowledge the continued generous funding from the Natural Sciences and Engineering Research Council of Canada (NSERC) Discovery Grant Program, the Alberta/Technical University of Munich International Graduate School for Hybrid Functional Materials (ATUMS) supported by NSERC CREATE, Future Energy Systems (FES T06 P03), and the University of Alberta Department of Chemistry. We thank the staff at Analytical and Instrumentation Laboratory in the Department of Chemistry at the University of Alberta for their help on FTIR. We also thank Dr Shihong Xu and Anqiang He at the UofA Nanofab for their assistance on XPS.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/d0na00428f

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