Manganese Oxide Coated Carbon Materials as Hybrid Catalysts for the Application in Primary Aqueous Metal-Air Batteries

Abstract

One of the major challenges of metal-air batteries is the impeded oxygen reduction reaction (ORR) during discharge occurring at the gas diffusion electrode (GDE) of the battery. Due to the impeded ORR, high overpotentials emerge and result in a loss of energy efficiency. In order to improve the latter, suitable catalysts have to be employed. Transition metal oxides like manganese oxides (e.g., MnO₂, Mn₂O₃, Mn₃O₄, Mn₅O₈, MnOOH) [1,2] are known as good and inexpensive materials for the ORR in alkaline media. A drawback of manganese oxide catalysts is the poor electrical conductivity. Hence, the approach presented in this work aims to enhance the catalytic activity of Mn₃O₄ and γ–MnO₂ by the incorporation of conductive carbon material into the pure manganese oxide. The resulting hybrid catalysts are prepared either by impregnation of Super C 65, Vulcan XC 72, and Kuraray YP 50F via a sol-gel technique employing a MnO₂ precursor sol or by direct precipitation of Mn₃O₄ or γ–MnO₂ particles in the presence of the carbon materials mentioned above. Investigations by rotating disc electrode (RDE) show a noticeably higher catalytic activity of the hybrid catalysts than for the pure materials. For verification of the results measured by RDE, screen printed GDEs are prepared and tested in Zn-air full cells.

1. Introduction

The global demand of energy storage systems for mobile and stationary applications is continuously growing. At present, lithium ion batteries (LIBs) are established in fields of mobile applications such as smart phones and notebooks. Due to higher theoretical specific energy in comparison to LIBs (387 Wh kg⁻¹ [3]) the metal-air technology (Li–O₂: 3503 Wh kg⁻¹ for non-aqueous electrolyte and 3582 Wh kg⁻¹ for aqueous electrolyte; Zn–O₂: 1086 Wh kg⁻¹ [3]; Na–O₂: 1605 Wh kg⁻¹ [4]) is a promising candidate for energy storage systems beyond LIBs [5,6,7,8].

In general, a metal-air battery consists of a metal anode and a gas diffusion electrode (GDE) as cathode separated by electrolyte and separator. The type of electrolyte depends on the anode material. In this article, we focus on GDEs for Zn-air batteries in which an aqueous alkaline electrolyte (such as KOH (aq.)) can be employed [5,9]. Primary Zn-air batteries are commercially used as button cells for hearing heads with a theoretical specific capacity of 820 mAh g⁻¹ Zn and a theoretical cell voltage of 1.65 V [8].

In metal-air batteries, oxygen diffuses from ambient air through the GDE into the battery cell and is reduced at the catalytic centers of the GDE while the metal, in this case zinc, is oxidized. The ORR mechanism follows two different pathways in alkaline solution which are described in the following equations. Equation (1) describes the direct four electron mechanism (n = 4). In this case O₂ is directly reduced to OH⁻ ions.

$$O_2+2H_{2}O+4e^{-}\to 4O{H}^{-}(0.401\mathrm{V}\mathit{vs}.\text{ NHE})$$

(1)

Equations 2 and 3 describe the indirect ORR mechanism with two reactions. Each step includes two electron transfers (n = 2). In the first reaction (Equation (2)), O₂ is reduced forming a peroxide ion (HO₂⁻). In the second reaction (Equation (3)) HO₂⁻ is reduced to OH⁻ [6,10].

$$O_2+H_{2}O+2e^{-}\to H{O}_2^{-}+O{H}^{-}(-0.065\mathrm{V}\mathit{vs}.\text{ NHE})$$

(2)

$$H{O}_2^{-}+H_{2}O+2e^{-}\to 3O{H}^{-}(0.867\mathrm{V}\mathit{vs}.\text{ NHE})$$

(3)

The ORR takes place at the three-phase boundary of the GDE where gaseous oxygen is adsorbed at the catalytically active centers and reduced to hydroxide ions, which are released into the aqueous electrolyte. The consumed electrons are transported through the solid carbon matrix [10].

The ORR exhibit high overpotentials which result in a change of the cell voltage during discharging from theoretical 1.65 V for Zn-air cells down to 1.2 V and below [9]. To reduce these overpotentials, suitable catalysts based on transition metal oxides like manganese oxides, perovskites, spinels or noble metals are required [5,7,8,9,11]. A detailed overview of investigated catalyst for Zn-air batteries was reported by Neburchilov et al. [12]. Especially manganese oxides exhibit a good catalytic activity for the ORR. Moreover, manganese oxides feature a good material availability, low costs and environmental compatibility. [13] Unfortunately, manganese oxides also feature a low electronic conductivity. The drawback can be overcome by the addition of conductive carbon which enhances the catalytic activity [13,14,15,16,17].

In this article, two approaches for a direct functionalization of different carbon materials with Mn₃O₄/γ–MnO₂ are discussed (Figure 1). In the first approach (Figure 1a) carbon is coated with a thin layer of Mn₃O₄ via sol-gel impregnation. Thereby, a hybrid catalyst is prepared which exhibits a thin catalytically active shell of Mn₃O₄ on an electrically conductive carbon core such as Super C 65 or Vulcan XC 72. In the second approach (Figure 1b) different forms of manganese oxides are precipitated in the direct presence of carbon. Depending on the carbon material, γ–MnO₂ or Mn₃O₄ particles grow on the carbon materials with strong adhesion. Both γ–MnO₂ and Mn₃O₄ have been further investigated as catalysts for ORR in GDEs [1,2]. In this paper, all hybrid catalysts are referred to as Mn_xO_y@C.

Figure 1. Different synthesis strategies for Mn_xO_y@C; (a) sol-gel impregnation of carbon and (b) co-precipitation of γ–MnO₂ or Mn₃O₄ in presence of carbon.

Figure 1. Different synthesis strategies for Mn_xO_y@C; (a) sol-gel impregnation of carbon and (b) co-precipitation of γ–MnO₂ or Mn₃O₄ in presence of carbon.

Finally, the synthesized hybrid catalysts are introduced into the reactive layer of a GDE. In Figure 2, the preparation process of the GDEs is depicted. A nickel mesh, the current collector, is hot pressed on the microporous carbon layer of a commercial gas diffusion layer (GDL). Afterwards, the reactive layer consisting of catalyst, carbon, and binder is screen printed on the GDL/nickel mesh substrate.

Figure 2. Schematic preparation of a gas diffusion electrode (GDE).

Figure 2. Schematic preparation of a gas diffusion electrode (GDE).

2. Results and Discussion

2.1. Characterization of Pure and Hybrid Catalysts

The structural properties of all investigated catalysts are analyzed by scanning electron microscopy (SEM), X-ray diffraction (XR), thermogravimetric differential thermal analysis (TG/DTA) and Brunauer-Emmett-Teller (BET) method to evaluate the differences between pure γ–MnO₂, Mn₃O₄, carbon materials and hybrid catalysts.

γ-MnO₂ is synthesized via co-precipitation of MnSO₄·H₂O and (NH₄)₂S₂O₈ according to a procedure presented by Xi et al. [18] and Li et al. [19]. Figure 3a depicts a representative SEM image of spherical catalyst particles of about 4 µm diameter formed by needle-shaped nanochrystals. The associated XRD pattern (Figure 3b) indicates the crystal structure of γ–MnO₂ which is a crystallographic deformation of α–MnO₂ (ramsdellite) and β–MnO₂ (pyrolusite). According to Gyenge and Drillet [20] the peaks at 2θ: 37.5° (101), 43.0° (111), 57.0° (211), and 67.5° (310) can be identified as pyrolusite (JCPDS: 98-002-0229) and the small peak at 2θ = 22.0 (011) matches with ramsdellite (JCPDS: 98-002-0228). The broad peaks are attributed to nanoscale γ–MnO₂ crystallites. The second investigated pure catalyst is a commercial Mn₃O₄ (Sigma Aldrich, purity: 97%). The SEM image in Figure 3c depicts agglomerated Mn₃O₄ particles with a diameter of approximately 1 µm for the primary particles. The corresponding XRD pattern (Figure 3d) displays the expected Mn₃O₄ spinel crystal structure with the following characteristic reflections at 18.0° (101), 28.5° (110), 37.2° (101), 40.9° (200), 42.6° (111), 56.5° (211), 59.2° (220), 64.7° (002), 67.1° (310), 72.1° (301), 72.3° (112) (JCPDS: 024-0734). In addition to the crystal structure, γ-MnO₂ and Mn₃O₄ differ in their specific surface areas. Due to its urchin like structure, γ–MnO₂ exhibits a much higher specific surface area (S_BET = 104 m² g⁻¹) than the smooth Mn₃O₄ particles (S_BET = 1 m² g⁻¹).

To investigate the influence of carbon materials on the hybrid catalysts, three carbon materials with different properties such as specific surface area or carbon modification were selected (Table 1). Super C 65 is a carbon black material which is used as a conductive agent in LIB electrodes [21] and Vulcan XC 72 is as catalyst support in fuel cells [22] which is employed due to its high electric conductivity. The intention in the application of Super C 65 and Vulcan XC 72 is to support the electric conductivity of the hybrid catalyst through a direct link between manganese oxide and carbon black. For a further study on the influence of an increased surface area, Kuraray YP 50F, is chosen. Kuraray YP 50F is an activated carbon material with a surface area of S_BET = 1485 m² g⁻¹ which is known for its application in electric double layer capacitors (ELDC) [23].

Figure 3. (a) Scanning electron microscope (SEM) image and (b) X-ray diffraction pattern of as prepared γ-MnO₂ particles. (c) SEM image and (d) X-ray diffraction pattern of commercial Mn₃O₄ particles.

Figure 3. (a) Scanning electron microscope (SEM) image and (b) X-ray diffraction pattern of as prepared γ-MnO₂ particles. (c) SEM image and (d) X-ray diffraction pattern of commercial Mn₃O₄ particles.

Table 1. Used carbon materials for hybrid catalysts.

Carbon Material	Manufacturer	Specific Surface Area SBET (m² g−1)	Carbon Modification
Super C 65	Imerys (CHE)	63	carbon black
Vulcan XC 72	Cabot (USA)	230	carbon black
Kuraray YP 50F	Kuraray Chemical Co., LTD (JPN)	1485	activated carbon

Table 1. Used carbon materials for hybrid catalysts.

Carbon Material	Manufacturer	Specific Surface Area SBET (m² g−1)	Carbon Modification
Super C 65	Imerys (CHE)	63	carbon black
Vulcan XC 72	Cabot (USA)	230	carbon black
Kuraray YP 50F	Kuraray Chemical Co., LTD (JPN)	1485	activated carbon

The middle row of the SEM images in Figure 4a depicts the pure carbon materials before modification. The fine particulate morphology of both carbon black materials (Super C 65 and Vulcan XC 72) is clearly distinguishable from the coarse irregular shape of the activated carbon particles (Kuraray YP 50F).The first row of the SEM images in Figure 4b pictures the carbon materials coated by sol-gel impregnation. In the SEM images, not much difference in the surface morphology between the pure carbon materials and the coated carbon materials is found. The X-ray diffraction patterns of the sol-gel hybrid catalysts (Figure 5a) show the spinel structure of Mn₃O₄ after crystallization for 4 h at 300 °C.

Figure 4. SEM images of (a) pure carbon materials, (b) hybrid catalysts synthesized via sol-gel impregnation and (c) hybrid catalysts synthesized via co-precipitation.

Figure 4. SEM images of (a) pure carbon materials, (b) hybrid catalysts synthesized via sol-gel impregnation and (c) hybrid catalysts synthesized via co-precipitation.

Figure 5. X-ray diffraction pattern of hybrid catalysts synthesized via (a) sol-gel impregnation and (b) co-precipitation.

Figure 5. X-ray diffraction pattern of hybrid catalysts synthesized via (a) sol-gel impregnation and (b) co-precipitation.

Figure 4c shows the SEM images of the hybrid catalysts which are synthesized via co-precipitation. As a result of this synthesis method, spherical manganese oxide particles with a diameter of a few microns are formed and embedded into the carbon matrix. The X-ray diffraction patterns of the hybrid manganese oxide catalyst in the presence of Super C 65 and Kuraray YP 50F (Figure 5b) offer the crystal structure of Mn₃O₄. In the presence of Vulcan XC 72 a different X-ray diffraction pattern results with the product γ–MnO₂@Vulcan XC 72. The first peak (marked as 1) at 2θ = 24.5° can be correlated with pure Vulcan XC 72. According to the X-ray diffraction of γ–MnO₂ (Figure 3b) the peaks (marked as 2) at 2θ: 37.5° (101), 43.0° (111), and 57.0° (211) can be identified as pyrolusite (JCPDS: 98-002-0229). The peak marked as 3 at 2θ = 31.8° (014) matches with MnCO₃ (JCPDS: 98-003-7242) and the peaks marked as 4 at 2θ: 65.0° (200) and 69.9° (190) matches with γ–MnO₂ (JCPDS: 98-015-0462).

In Figure 6a, TG/DTA measurements of pure γ–MnO₂ and γ–MnO₂@Vulcan XC 72 synthesized via co-precipitation are plotted. Therein, the first exothermic peak for γ–MnO₂ at 206 °C and at 240 °C for γ–MnO₂@Vulcan XC 72 is related to the formation of γ–MnO₂. Furthermore, the endothermic peaks for γ–MnO₂ at 547 °C and at about 950 °C result from the reduction of MnO₂ to Mn₂O₃ to Mn₃O₄ [24]. The big mass loss of about 35 wt % between 400 and 600 °C in the TG curve of γ–MnO₂@Vulcan XC 72 is mainly related to the decomposition of Vulcan XC 72.

The TG/DTA curves of Mn₃O₄@Kuraray YP 50F (Precipitation) in Figure 6b represent a different curve progression. At the first mass loss of about 18 wt % between RT and 100 °C mainly H₂O is detected. The second big mass loss (50 wt %) between 200 and 500 °C as well as the exothermic DTA peak at 368 °C is related to the decomposition of Kuraray YP 50F. The reason for this different TG/DTA behavior between the precipitated hybrid catalysts (based on Vulcan XC 72 and Kuraray YP 50F) can be seen in the pH value of the carbon materials. Kuraray YP 50F and Super C 65 exhibit an alkaline pH value of 9. However, Vulcan XC 72 shows a neutral pH value of about 7. The co-precipitation reaction of γ–MnO₂ without carbon material is proton catalyzed, which means that by addition of Vulcan XC 72 with a neutral pH value the same reaction takes place and the product γ–MnO₂ is formed. The addition of an alkaline material such as Kuraray YP 50F or Super C 65 results in the formation of Mn₃O₄ as reaction product. Dhaouadi et al. [25] reported the formation of Mn₃O₄ in alkaline media. In this reaction, first Mn(OH)₂ is formed, which decomposes into MnO and H₂O (mass loss in Figure 6b between RT and 100 °C in the TG curve). During heat treatment MnO is oxidized to Mn₃O₄.

Figure 6. Thermogravimetric differential thermal analysis (TG/DTA) measurements (before tempering at 300 °C) of (a) pure γ–MnO₂ and γ–MnO₂@Vulcan XC 72 synthesized via co-precipitation; (b) Mn₃O₄@Kuraray YP 50F synthesized via co-precipitation.

Figure 6. Thermogravimetric differential thermal analysis (TG/DTA) measurements (before tempering at 300 °C) of (a) pure γ–MnO₂ and γ–MnO₂@Vulcan XC 72 synthesized via co-precipitation; (b) Mn₃O₄@Kuraray YP 50F synthesized via co-precipitation.

In Table 2, the specific surface areas of all hybrid catalyst materials are depicted. By comparing these values it can be seen that, the co-precipitated hybrid catalysts show a higher specific surface area than the sol-gel impregnated ones, e.g., for Kuraray YP 50F S_BET (Sol) = 239 m² g⁻¹ vs. S_BET (Precipitation) = 869 m² g⁻¹. However, the co-precipitated hybrid catalysts have a higher carbon content (extracted from TG/DTA data). Especially in the case of Vulcan XC 72 (S_BET = 230 m² g⁻¹) and Kuraray YP 50F (S_BET = 1485 m² g⁻¹) the pure carbon materials have a much higher specific surface area than pure γ–MnO₂ (S_BET = 104 m² g⁻¹) or Mn₃O₄ (S_BET = 1 m² g⁻¹). Additionally, the SEM images of Figure 4 demonstrate that in the case of so-gel impregnated hybrid catalysts the carbon materials are covered with a Mn₃O₄ shell. This might causes a blocking of the carbon pores and reduces the specific surface area of these hybrid catalysts in comparison to the corresponding carbon material. For the co-precipitated catalysts γ–MnO₂ or Mn₃O₄ particles are present adjacent to the carbon materials. Therefore, the specific surface area results in an arithmetic mean of carbon and manganese oxide which is much higher compared to the sol-gel impregnated hybrid catalyst.

Table 2. Structural properties of all investigated materials.

Catalyst	Synthesis Pathway	Carbon Content of Hybrid Catalyst (wt %)	Specific Surface Area SBET (m² g−1)
γ-MnO2	Co-precipitation	0	104
Mn3O4	Sigma Aldrich	0	1
Mn3O4@Super C 65	Sol-gel impregnation	32	52
Mn3O4@Super C 65	Co-precipitation	35	73
Super C 65		100	63
Mn3O4@Vulcan XC 72	Sol-gel impregnation	24	50
γ-MnO2@Vulcan XC 72	Co-precipitation	39	116
Vulcan XC 72		100	230
Mn3O4@Kuraray YP 50F	Sol-gel impregnation	18	239
Mn3O4@Kuraray YP 50F	Co-precipitation	28	869
Kuraray YP 50F		100	1485

Table 2. Structural properties of all investigated materials.

Catalyst	Synthesis Pathway	Carbon Content of Hybrid Catalyst (wt %)	Specific Surface Area SBET (m² g−1)
γ-MnO2	Co-precipitation	0	104
Mn3O4	Sigma Aldrich	0	1
Mn3O4@Super C 65	Sol-gel impregnation	32	52
Mn3O4@Super C 65	Co-precipitation	35	73
Super C 65		100	63
Mn3O4@Vulcan XC 72	Sol-gel impregnation	24	50
γ-MnO2@Vulcan XC 72	Co-precipitation	39	116
Vulcan XC 72		100	230
Mn3O4@Kuraray YP 50F	Sol-gel impregnation	18	239
Mn3O4@Kuraray YP 50F	Co-precipitation	28	869
Kuraray YP 50F		100	1485

2.2. Electrocatalytic Activity of Pure and Hybrid Catalysts

The electrocatalytic activity for the oxygen reduction reaction (ORR) is measured by rotating disc electrode (RDE). The polarization curves of all investigated catalysts are depicted in Figure 7a–c. All measurements are carried out in O₂-saturated 1 M KOH (aq.) solution at a rotation rate of 900 rpm. For comparison of the different catalysts the onset potential at −1 mA cm⁻² and the corresponding limiting current density at −0.5 V vs. Hg/HgO are employed (Table 3). The higher the onset potential of the reaction is, the lower the overpotentials for the ORR.

In Figure 7a the polarization curves of Mn₃O₄@Super C 65 (Sol) and Mn₃O₄@Super C 65 (Precipitation) are compared to pure catalysts Mn₃O₄ and γ–MnO₂ as well as to pure Super C 65. The catalytic activity of the latter materials can be ranked in the following ascending order: Mn₃O₄ < Super C 65 < γ–MnO₂. Furthermore, it can be seen that the sol-gel impregnation of Super C 65 with Mn₃O₄ reduces the onset potential compared to Super C 65 by 111 mV vs. Hg/HgO. At the same time, the limiting current density of Mn₃O₄@Super C 65 (Sol) is increased by 0.8 mA cm⁻² compared to Mn₃O₄. The highest ORR activity in this plot is detected for Mn₃O₄@Super C 65 synthesized via co-precipitation. This hybrid catalyst exhibits an onset potential for ORR of −203 mV vs Hg/HgO and a limiting current density of −2.38 mA cm⁻². These results show that the combination of carbon and catalyst strongly enhances the catalytic activity for the ORR in comparison to pure materials Super C 65 and Mn₃O₄ [13,15,16].

The same ranking of ORR activity is also found for the hybrid catalysts containing Vulcan XC 72 and Kuraray YP 50F as carbon material (Figure 7b–c). Independent of the used carbon material, again the catalysts synthesized via co-precipitation show higher ORR activity than the sol-gel impregnated ones (e.g., the difference in onset potentials of the hybrid catalysts based on Super C 65: 19 mV; Vulcan XC 72: 86 mV; Kuraray YP 50F: 35 mV and the difference in limiting current density for Super C 65: 0.70 mA cm⁻², Vulcan XC 72: 0.77 mA cm⁻² and Kuraray YP 50F: 0.62 mA cm⁻²). One reason for this could be that the pores of the carbon as well as of the manganese oxide are more accessible for oxygen, which results in a large three-phase boundary layer for the reduction of oxygen to OH⁻. Another reason for the higher catalytic activity of the co-precipitated catalysts compared to the sol-gel impregnated ones could be the presence of a small amount of Ag in the co-precipitated hybrid catalysts. AgNO₃ is used as catalyst for the co-precipitation synthesis and after heat treatment a little amount of Ag is contained in the co-precipitated catalysts and perhaps gives an additional catalytic effect for the ORR.

For pure carbon materials, Kuraray YP 50F shows the highest onset potential of −194 mV vs. Hg/HgO and almost the same limiting current density of −1.38 mA cm⁻² as Vulcan XC 72. Super C 65 exhibits the lowest limiting current density of −1.09 mA cm⁻² and the poorest onset potential among the investigated carbon materials of −333 mV vs. Hg/HgO. This behavior can be correlated to the specific surface area of the carbon materials—the higher the specific surface area the more active the carbon material for the ORR. In contrast to this, hybrid catalysts show a different behavior. In this case, it appears that the surface area has not such a big influence on the ORR activity.

Figure 7. Oxygen reduction reaction (ORR) polarization curves of pure γ–MnO₂, Mn₃O₄, pure carbon materials and hybrid catalysts based on (a) Super C 65, (b) Vulcan XC 72, (c) Kuraray YP 50F in O₂-saturated 1 M KOH (aq.) solution at a rotations rate of 900 rpm and a scan rate of 5 mV s⁻¹. Corresponding Koutecky-Levich plots of pure γ–MnO₂, Mn₃O₄, pure carbon materials and hybrid catalysts based on (d) Super C 65, (e) Vulcan XC 72, (f) Kuraray YP 50F at −0.5 V vs. Hg/HgO.

Figure 7. Oxygen reduction reaction (ORR) polarization curves of pure γ–MnO₂, Mn₃O₄, pure carbon materials and hybrid catalysts based on (a) Super C 65, (b) Vulcan XC 72, (c) Kuraray YP 50F in O₂-saturated 1 M KOH (aq.) solution at a rotations rate of 900 rpm and a scan rate of 5 mV s⁻¹. Corresponding Koutecky-Levich plots of pure γ–MnO₂, Mn₃O₄, pure carbon materials and hybrid catalysts based on (d) Super C 65, (e) Vulcan XC 72, (f) Kuraray YP 50F at −0.5 V vs. Hg/HgO.

The number of electrons n transferred during ORR and thus the underlying mechanism of the ORR (according to Equations (1–3)) is evaluated by the Koutecky-Levich equation [10,26,27]:

$$\frac{1}{j}=\frac{1}{j_k}+\frac{1}{B{\omega }^{1/2}}$$

(4)

$$B=0.62nFA{\nu }^{-1/6}{c}_{O_2}{D}_{O_2}^{2/3}$$

(5)

(j: measured current density (at −0.5 V vs. Hg/HgO); j_k: kinetic current density; ω: angular velocity; B: slope of the regression line; n: number of electrons transferred; F: faraday constant; ν: kinetic viscosity of the electrolyte, $c_{O_2}$: O₂ concentration in the electrolyte; $D_{O_2}$: diffusion coefficient of O₂ in the electrolyte).

The corresponding Koutecky-Levich plots at –0.5 V vs. Hg/HgO of all investigated catalysts are presented in Figure 7. From the slope of the regression line, the number of transferred electrons n can be calculated according to Equation 5 (Table 3). All investigated pure carbon materials catalyze the ORR via the indirect two electron mechanism (n = 2) [10,17,28,29]. Pure γ–MnO₂ and the hybrid catalysts which are synthesized via co-precipitation follow the direct four electron mechanism (n = 4) [7,17]. The hybrid catalysts synthesized via sol-gel impregnation exhibit electron transfer numbers in the area of about n = 3. In this case both the direct four electron mechanism as well as the indirect two electron mechanism takes place.

Table 3. Electrochemical properties of all investigated catalyst materials.

Catalyst	Synthesis Pathway	Onset Potential at −1 mA cm−² (mV vs. Hg/HgO)	Limiting Current Density at −0.5 V vs. Hg/HgO (mA cm−²)	Electron Transfer Number n
γ-MnO2	Co-precipitation	−238	−1.97	4
Mn3O4	Sigma Aldrich	-	−0.88	2.6
Mn3O4@Super C 65	Sol-gel impregnation	−222	−1.68	3.3
Mn3O4@Super C 65	Co-precipitation	−203	−2.38	4
Super C 65		−333	−1.09	2
Mn3O4@Vulcan XC 72	Sol-gel impregnation	−256	−1.46	3.0
γ-MnO2@Vulcan XC 72	Co-precipitation	−170	−2.23	4
Vulcan XC 72		−245	−1.39	2
Mn3O4@Kuraray YP 50F	Sol-gel impregnation	−238	−1.51	3.3
Mn3O4@Kuraray YP 50F	Co-precipitation	−203	−2.13	4
Kuraray YP 50F		−194	−1.38	2

Table 3. Electrochemical properties of all investigated catalyst materials.

Catalyst	Synthesis Pathway	Onset Potential at −1 mA cm−² (mV vs. Hg/HgO)	Limiting Current Density at −0.5 V vs. Hg/HgO (mA cm−²)	Electron Transfer Number n
γ-MnO2	Co-precipitation	−238	−1.97	4
Mn3O4	Sigma Aldrich	-	−0.88	2.6
Mn3O4@Super C 65	Sol-gel impregnation	−222	−1.68	3.3
Mn3O4@Super C 65	Co-precipitation	−203	−2.38	4
Super C 65		−333	−1.09	2
Mn3O4@Vulcan XC 72	Sol-gel impregnation	−256	−1.46	3.0
γ-MnO2@Vulcan XC 72	Co-precipitation	−170	−2.23	4
Vulcan XC 72		−245	−1.39	2
Mn3O4@Kuraray YP 50F	Sol-gel impregnation	−238	−1.51	3.3
Mn3O4@Kuraray YP 50F	Co-precipitation	−203	−2.13	4
Kuraray YP 50F		−194	−1.38	2

2.3. Discharge Behavior in Zn-Air Full Cells

Subsequent to the RDE characterization, all investigated catalysts and pure carbon materials were introduced into the reactive layer of a GDE (Figure 3). For comparison, also pure carbon materials were processed as well. In Figure 8, the surface morphology of the reactive layer containing 20 wt % γ–MnO₂@Vulcan XC 72 (Precipitation) hybrid catalysts is exemplarily represented. The homogeneous distribution of the hybrid catalyst (red) is visualized by EDS mapping.

Figure 8. SEM image of the reactive layer of an as prepared GDE containing γ-MnO₂@Vulcan XC 72 (precipitation) including energy dispersive X-ray spectroscopy (EDS) mapping (red color corresponds to manganese).

Figure 8. SEM image of the reactive layer of an as prepared GDE containing γ-MnO₂@Vulcan XC 72 (precipitation) including energy dispersive X-ray spectroscopy (EDS) mapping (red color corresponds to manganese).

The prepared GDEs are tested in Zn-air full cells employing 6 M KOH (aq.) as electrolyte, zinc granules as anode material, and a constant flow of ambient air in a commercial El-Cell^® ECC-Air test cell. In Figure 9, the resulting polarization curves of the GDEs are plotted. In graphs a, b, and c the polarization curves of the hybrid catalysts are compared to pure γ–MnO₂ and Mn₃O₄ catalysts as well as the corresponding carbon material to investigate the impact of manganese oxide coating on full cell performance. Within the graphs it can be seen that all cells containing hybrid catalysts via co-precipitation (red lines) show a higher cell voltage than cells prepared with sol-gel impregnated catalysts (blue lines). Thus, it appears that the polarization behavior of co-precipitated catalysts is much better than the polarization behavior of sol-gel impregnated catalysts. This behavior can be correlated with the RDE polarization curves.

Figure 9. Polarization curves of pure γ–MnO₂, Mn₃O₄, pure carbon materials and hybrid catalysts based on (a) Super C 65, (b) Vulcan XC 72, (c) Kuraray YP 50 F, in Zn-air full cells.

Figure 9. Polarization curves of pure γ–MnO₂, Mn₃O₄, pure carbon materials and hybrid catalysts based on (a) Super C 65, (b) Vulcan XC 72, (c) Kuraray YP 50 F, in Zn-air full cells.

The discharge curves of all prepared GDEs are presented in Figure 10. Just like in the polarization behavior measured by RDE and in full cells, the GDEs containing the co-precipitated hybrid catalysts exhibit higher specific capacities compared to the sol-gel impregnated ones (for Super C 65: 75 mAh g⁻¹ higher, for Vulcan XC 72: 132 mAh g⁻¹ higher and for Kuraray YP 50F: 146 mAh g⁻¹ higher) as well as the pure materials. The support of carbon with the co-precipitated synthesis route improves the discharge and polarization behavior. Additionally the amount of manganese oxide is strongly reduced in the hybrid catalysts. The highest specific capacity of the hybrid catalyst is reached with Vulcan XC 72 with 449 mAh g⁻¹ (discharge time: 10.75 h) and a discharge plateau of 1.11 V at 200 mAh g⁻¹. In this case the specific capacity can be increased by 25% (with 39% less amount of γ–MnO₂) compared to pure γ–MnO₂. This improved behavior correlates to the RDE polarization curves of γ–MnO₂@Vulcan XC 72 (precipitation). In the publication of Li et al. [13] a Zn-air discharge curve of a GDE with a Vulcan XC 72 / α-MnO₂ (1:1) mixed catalyst is displayed which is discharged for 11.5 h (at 10 mA/cm²) and exhibits a discharge plateau of 1.2 V. These results are comparable with the here presented hybrid catalyst γ–MnO₂@Vulcan XC 72 synthesized by co-precipitation. The electrochemical properties of all investigated GDEs are shown in Table 4.

Figure 10. Discharge curves of pure γ–MnO₂, Mn₃O₄, pure carbon materials and hybrid catalysts based on (a) Super C 65, (b) Vulcan XC 72, (c) Kuraray YP 50F in Zn-air full cells at a constant current density of 10 mA cm⁻².

Figure 10. Discharge curves of pure γ–MnO₂, Mn₃O₄, pure carbon materials and hybrid catalysts based on (a) Super C 65, (b) Vulcan XC 72, (c) Kuraray YP 50F in Zn-air full cells at a constant current density of 10 mA cm⁻².

Table 4. Electrochemical properties of all investigated gas diffusion electrodes (GDEs).

Catalyst	Synthesis Pathway	Cell Potential at 30 mA cm−² (V)	Cell Potential at 200 mAh g−1 (V)	Specific Capacity (mAh g−1)
γ-MnO2	Co-precipitation	0.89	1.08	344
Mn3O4	Sigma Aldrich	0.84	1.10	314
Mn3O4@Super C 65	Sol-gel impregnation	0.84	1.09	295
Mn3O4@Super C 65	Co-precipitation	0.88	1.10	362
Super C 65		0.86	1.08	330
Mn3O4@Vulcan XC 72	Sol-gel impregnation	0.86	1.08	317
γ-MnO2@Vulcan XC 72	Co-precipitation	0.91	1.11	449
Vulcan XC 72		0.87	1.08	314
Mn3O4@Kuraray YP 50F	Sol-gel impregnation	0.75	1.08	275
Mn3O4@Kuraray YP 50F	Co-precipitation	0.85	1.10	421
Kuraray YP 50F		0.81	1.08	343

Table 4. Electrochemical properties of all investigated gas diffusion electrodes (GDEs).

Catalyst	Synthesis Pathway	Cell Potential at 30 mA cm−² (V)	Cell Potential at 200 mAh g−1 (V)	Specific Capacity (mAh g−1)
γ-MnO2	Co-precipitation	0.89	1.08	344
Mn3O4	Sigma Aldrich	0.84	1.10	314
Mn3O4@Super C 65	Sol-gel impregnation	0.84	1.09	295
Mn3O4@Super C 65	Co-precipitation	0.88	1.10	362
Super C 65		0.86	1.08	330
Mn3O4@Vulcan XC 72	Sol-gel impregnation	0.86	1.08	317
γ-MnO2@Vulcan XC 72	Co-precipitation	0.91	1.11	449
Vulcan XC 72		0.87	1.08	314
Mn3O4@Kuraray YP 50F	Sol-gel impregnation	0.75	1.08	275
Mn3O4@Kuraray YP 50F	Co-precipitation	0.85	1.10	421
Kuraray YP 50F		0.81	1.08	343

3. Experimental Section

3.1. Synthesis of Catalysts

3.1.1. Co-Precipitation

The synthesis via co-precipitation was based on [18,19,30]. AgNO₃ (Alfa Aesar) solution as catalyst was added to an equimolar aqueous solution of MnSO₄·H₂O (Sigma Aldrich) and (NH₄)₂S₂O₈ (Sigma Aldrich). After approximately 60 h and by the oxidative effect of AgNO₃, manganese oxide particles of low solubility were formed. To receive the intended manganese oxide structure, the particles were tempered for 4 h at 300 °C in ambient air.

The same synthesis was used to prepare hybrid catalysts. In this case, carbon materials were added to the aqueous solution of MnSO₄ H₂O, (NH₄)₂S₂O₈ and AgNO₃.

3.1.2. Sol-Gel Impregnation

In order to prepare a stable manganese oxide sol, Mn(NO₃)₂ (Sigma Aldrich) was dissolved in 3 wt % acetylacetone (Fluka Chemie) and 97 wt % propylene glycol monomethyl ether (Merck). After two days, the sol was diluted and stabilized with Superonic F108 (Fluka Chemie) resulting in a manganese oxide content of 1.5 wt %. In the subsequent step, different carbon materials were added to the sol and autoclaved for 5 h at 200 °C. To receive the intended manganese oxide structure on carbon, the hybrid catalysts were tempered for 4 h at 300 °C in ambient air.

3.2. Materials Characterization

All catalysts were analyzed by powder X-ray diffraction (XRD, Cu-K_α radiation, Philips PW 1730/10) at an angular speed of 0.02 s⁻¹ in the range from 2θ = 10 to 2θ = 80.

The morphology of these catalysts and as well as the structure of GDEs were investigated by Scanning Electron Microscopy (SEM, Zeiss Supra 25). The presence of manganese oxide in the GDEs was detected by Energy Dispersive X-ray Spectroscopy (EDS, EDAX). The specific surface area of all catalysts was calculated by the Brunauer-Emmett-Teller (BET) method from the data of N₂ adsorption/desorption isotherms (Quantachrom Autosorb-3B). The carbon content of each hybrid catalyst was evaluated by thermogravimetric analysis and differential thermal analysis (TG/DTA, Netzsch STA 449 C) in synthetic air with a heating rate of 10 K min⁻¹ from room temperature to 1000 °C.

3.3. Preparation of Gas Diffusion Electrodes (GDE)

The gas diffusion electrodes GDE consisted of a commercially gas diffusion layer GDL (Freudenberg FCCT SE & Co. KG, H2315 C2), a nickel mesh as current collector and a reactive layer in which the catalysts were embedded (Figure 2). The GDL was built up of a microporous carbon layer at the electrolyte facing side and of carbon fibers at the air facing side. In a first step, the nickel mesh was hot pressed onto the microporous layer of the GDL. In a second step, a slurry, composed of 20 wt % catalyst, 45 wt % conductive carbon black (Super C 65, Imerys), 20 wt % activated carbon (Kuraray YP 50F , Kuraray), 15 wt % PTFE (Sigma Aldrich, ≤12 µm) and terpineol as solvent (Fluka Chemie, 65% α-terpineol, 10% β-terpineol, 20% γ-terpineol) was mixed in a speedmixer (Hauschild DAC 150 FVZ) and coated on this substrate via screen printing (Ekra X1-SL). After drying at 100 °C for 1 h, a homogeneous reactive layer with a thickness of about 30 µm was obtained.

3.4. Electrochemical Measurements

3.4.1. Rotating Disc Electrode (RDE)

The electrochemical activity of all catalysts was investigated by rotating disc electrode (RDE, Pine Instruments, potentiostat: Autolab M101). Linear sweep voltammetry (LSV) was performed in O₂-saturated 1 M KOH(aq.). For the preparation of the working electrode, 20 mg catalyst was dispersed in 7.96 mL deionized water, 2.0 mL n-propanol (Sigma Aldrich) and 0.04 mL of a Nafion solution (5 wt %, Sigma Aldrich) under ultrasonic mixing (Sonics & Materials VCX130) for 15 min. Then 10 µL of this homogeneous suspension were dropped onto the polished surface of a glassy carbon electrode (5 mm diameter, Pine Instruments) and dried at 35 °C for 15 min. To complete the three electrode assembly, a glassy carbon rod and a Hg/HgO/1 M KOH (aq.) were used as counter electrode and reference electrode. As electrolyte O₂-saturated 1 M KOH(aq.) was used, the LSV was carried out under a constant flow of pure O₂ at various rotations rates of 100, 150, 240, 400, 900, 1600, 2500 rpm with a scan rate of 5 mV s⁻¹ and in a potential range between 0.1 V and −0.6 V vs. Hg/HgO. All experiments were performed three times. The ORR onset potential was determined at −1 mA cm⁻² [31]. The limiting current density was taken at −0.5 V vs. Hg/HgO at a rotation rate of 900 rpm. The number of electrons n, transferred during ORR, was calculated by the Koutecky-Levich equation (Equation 5).

3.4.2. Full Cell Assembly

The Zn-air full cell experiments were performed in El-Cell^® ECC-Air test cells (EL-Cell^®). Herein, the prepared GDEs with a diameter of 18 mm were used as cathode, 0.609 g zinc granules (Grillo GHN20-10) as anode, 6 M KOH (aq.) as electrolyte and a glass fiber separator (EL-Cell^®, ECC-01-0012-C/L). During all measurements, the GDE was provided with a constant flow of ambient air at room temperature.

The discharge curves were recorded at a battery tester (Maccor Series 4000) with a constant current density of 10 mA cm⁻². The polarization curves were taken at different constant current densities between 0 and 30 mA cm⁻².

4. Conclusions

In this work, two different synthesis pathways were shown to combine carbon materials with manganese oxide catalysts. The aim was to increase the catalytic activity of manganese oxides by the support of carbon with a high electric conductivity. The sol-gel impregnation of carbon with manganese oxide results in a core-shell catalyst—whereas the co-precipitation of manganese oxide in presence of carbon results in a hybrid catalyst with spherical manganese oxide particles on carbon. The co-precipitated hybrid catalysts, especially γ–MnO₂@Vulcan XC 72, show the highest catalytic activity for the ORR among the analyzed catalysts measured by RDE. γ–MnO₂@Vulcan XC 72 also exhibits the best discharge and polarization behavior in GDEs measured in Zn-air full cells. In summary, the combination of carbon material with manganese oxide (Mn₃O₄ or γ–MnO₂), especially via the co-precipitation method, provides an electrochemically more active hybrid catalyst than the pure materials.