Preparation of Ag₄Bi₂O₅/MnO₂ Corn/Cob Like Nano Material as a Superior Catalyst for Oxygen Reduction Reaction in Alkaline Solution

Abstract

Ag₄Bi₂O₅/MnO₂ nano-sized material was synthesized by a co-precipitation method in concentrated KOH solution. The morphology characterization indicates that MnO₂ nanoparticles with a size of 20 nm are precipitated on the surface of nano Ag₄Bi₂O₅, forming a structure like corn on the cob. The obtained material with 60% Mn offers slightly higher initial potential (0.098 V vs. Hg/HgO) and limiting current density (−5.67 mA cm⁻²) at a rotating speed of 1600 rpm compared to commercial Pt/C (−0.047 V and −5.35 mA cm⁻², respectively). Furthermore, the obtained material exhibits superior long-term durability and stronger methanol tolerance than commercial Pt/C. The remarkable features suggest that the Ag₄Bi₂O₅/MnO₂ nano-material is a very promising oxygen reduction reaction catalyst.

1. Introduction

With the rapid consumption of fossil energy, the aggravated emission of carbon dioxide (CO₂) leads to severe environmental issues. In order to reduce the consumption of gasoline, fuel cells, especially Zn-O₂, Li-O_2, and the other metal-air batteries, have been considered as clean power sources, which can directly convert chemical energy into electrical energy with higher energy conversion efficiencies compared with traditional internal combustion engines [1]. However, the scarcity and high cost of platinum or platinum-based catalysts [2,3,4] commonly used in the cathodic oxygen reduction reaction (ORR) limit the commercialization of fuel cells. Additionally, the platinum (Pt) or platinum-based materials are sensitive to poisoning by methanol [5]. Recently, intensive research has been conducted to develop platinum free catalysts, such as silver (Ag) [6,7,8,9], manganese oxide [10,11,12,13], and silver/manganese oxide composite [14,15,16], which are promising catalysts with excellent performance at room temperature for alkaline fuel cells. Manganese oxides are promising catalysts for the oxygen reduction reaction owing to their outstanding activity toward oxygen reduction with the advantages of low cost and abundance [17]. Manganese dioxide (MnO₂) has good catalytic performance towards the oxygen reduction reaction through the integral four electrons mechanism, but its application is limited by the instability of the structure during the discharge process [18]. Silver and its complexes have a good ability to catalyze the oxygen reduction reaction. They were also reported to be more stable than Pt and Pt-based catalysts during long-term operation in alkaline media [7]. Ag and Ag-based materials are insensitive to methanol and can be applied in direct methanol fuel cells (DMFCs) [19]. In addition, Ag and its complexes have a lower price than Pt catalysts. The advantages of reasonably high electrochemical activity, long-term stability, methanol tolerance, and lower price compared with Pt make Ag and Ag-based composites attractive as catalysts for the oxygen reduction reaction in alkaline media, especially for DMFCs due to the high methanol tolerance of silver. However, the cost of silver and its complexes is still high compared with some other transition metals. It is urgent to develop a facile and green method to synthetize a new catalyst with low cost, relatively excellent electrocatalytic ability for the oxygen reduction reaction, and good stability and methanol tolerance in an alkaline electrolyte.

In this study, a facile method was proposed to prepare nano Ag₄Bi₂O₅/MnO₂ material. MnO₂ nano-particles were evenly precipitated on the surface of Ag₄Bi₂O₅ nano-rods to form a structure like corn on the cob. The synthesis method has the advantages of low temperature and short synthesis period, and more importantly, MnO₂ can be effectively anchored on the surface of Ag₄Bi₂O₅ nano-rods. This material has a comparable catalytic activity to the commercial Pt/C in terms of the oxygen reduction reaction. The catalyst displays superior stability and methanol tolerance compared with the commercial Pt/C. Therefore, nano Ag₄Bi₂O₅/MnO₂ material can be an effective catalyst for the oxygen reduction reaction in alkaline solution.

2. Results and Discussion

2.1. Illustration of the Synthesis Process

Figure 1 shows the schematic of the synthesis process of Ag₄Bi₂O₅/MnO₂ nano-material. This material was based on Ag₄Bi₂O₅ nano-rods with manganese oxide nanoparticles deposited on the surface to form a structure like corn on the cob. The synthesis method was so facile that manipulation at room temperature was enough to effectively precipitate MnO₂ on the surface of the Ag₄Bi₂O₅ rods. This structure is conducive to the synergistic effect of the three metals (Ag, Bi, and Mn) and the adsorption of O₂ on the surface of the catalyst as well as the disconnection of the O-O bonds. The nanostructure can effectively reduce the size of the catalyst and increase the specific surface area so that more catalytic activity sites are exposed and the oxygen reduction process can be catalyzed in a stable and efficient way [20]. Therefore, Ag₄Bi₂O₅/MnO₂ can be considered as a promising catalyst for the oxygen reduction reaction.

2.2. Structural and Morphological Characterizations

As is clearly illustrated in Figure 2, when Ag₄Bi₂O₅ is added to manganese dioxide, the X-ray powder diffraction (XRD) patterns of the samples (0–60% Mn) still have the typical characteristic peaks of Ag₄Bi₂O₅. From the XRD patterns, it was found that the samples were typical at 2θ = 26.37°, 31.25°, 31.85°, 37.76° and 56.19° corresponding to (112), (411), (312), (600), and (332) of the Ag₄Bi₂O₅, respectively, according to the standard JCPDS 87-0866 of Ag₄Bi₂O₅. The peaks of Ag₄Bi₂O₅/MnO₂ are broad, indicating that the incorporation of Mn affects the crystalline structure of Ag₄Bi₂O₅. There are no typical peaks of MnO₂ because the MnO₂ is amorphous.

Scanning electron microscopy (SEM) images in Figure 3 of the Ag₄Bi₂O₅/MnO₂ samples show the morphological and structural information. It can be seen in Figure 3a that nano Ag₄Bi₂O₅ are smooth rods with length and width of 200 nm and 30 nm, respectively. When the amount of manganese dioxide is 10–60%, there are some nano particles on the rod-like Ag₄Bi₂O₅, and the length and width is still about 200 nm and 30 nm, respectively. With the increase of the ratio of manganese dioxide, there are more nano particles on the nano rods. The structure is beneficial to the synergistic effect of Ag, Bi, and Mn in catalyzing the oxygen reduction reaction. The nanostructure can reduce the size of the catalyst and more catalytic activity sites are exposed so that the oxygen reduction process can be catalyzed in a stable and efficient manner. When the amount of manganese oxide is 70%, the material particles become larger, and the length and width are about 400 nm and 100 nm, respectively.

The results of transmission electron microscope (TEM) and high-resolution transmission electron microscopy (HRTEM) are shown in Figure 4. It can be seen from Figure 4a that MnO₂ is distributed on the surface of rod-like Ag₄Bi₂O₅. The HRTEM image of Ag₄Bi₂O₅ with 60% Mn was used to further demonstrate the detailed structural features of the material. The distances between the lattice planes are 0.333 nm and 0.417 nm corresponding to the planes of Ag₄Bi₂O₅ (112) and Ag₄Bi₂O₅ (301), respectively (Figure 4b). There are no planes of MnO₂ because it is amorphous. This result is consistent with the result of XRD.

In order to explore the elemental distribution of Ag₄Bi₂O₅ with 60% Mn, mapping analysis of Ag₄Bi₂O₅ with 60% Mn was performed as in Figure 5. It can be seen that the four elements of O, Ag, Bi, and Mn are evenly distributed. Energy dispersive spectrometer (EDS) analysis was also performed to obtain the chemical composition shown in Figure 6. It is proved that the presence of O, Ag, Bi, and Mn elements and the successful synthesis of Ag₄Bi₂O₅ with 60% Mn. The atomic ratios of Ag, Bi, and Mn are 15.56%, 7.57%, and 6.15%, respectively. The ratio of Bi and Mn is 1.23, which is close to the theoretical ratio of 1.33. The atomic ratio of Ag:Bi conforms to the atomic ratio 2:1 in the Ag₄Bi₂O₅.

To investigate the elemental compositions and the valences of the elements in the as prepared Ag₄Bi₂O₅ with 60% Mn composite, X-ray photoelectron spectroscopy (XPS) was employed. The results are shown in Figure 7. Figure 7a shows the XPS survey spectra. Figure 7b–d shows the high resolution spectra of Ag 3d, Bi 4f, and Mn 2p of Ag₄Bi₂O₅/MnO₂, respectively. In Figure 7b, it can be seen that the peaks at 368.2 eV and 374.2 eV are Ag 3d_5/2 and Ag 3d_3/2, respectively. These two peaks show that the valence state of Ag is +1 [21]. The two peaks at 158.8 eV and 163.9 eV in Figure 7c correspond to Bi 3f_7/2 and Bi 3f_5/2, respectively, proving the presence of Bi of +3 [21]. Figure 7d shows the XPS spectra of the 2p orbital of the Mn element in Ag₄Bi₂O₅/MnO₂. The two peaks at 642 eV and 653.7 eV correspond to Mn 2p_3/2 and Mn 2p_1/2 of Mn⁴⁺, respectively, which indicates that the valence state of the whole material is dominated by MnO₂ which is on the surface [22].

2.3. Electrocatalytic Performance

To provide an insight into the activity of ORR on the Ag₄Bi₂O₅/MnO₂ material, linear sweep voltammetry (LSV) plots were tested by a rotating disk electrode (RDE) at a speed of 1600 rpm in O₂-saturated 0.1 mol L⁻¹ KOH solution at a scanning rate of 5 mV s⁻¹. MnO₂, Ag₄Bi₂O₅, and commercial Pt/C was also investigated under the same conditions. The starting potential is the potential at the limiting current density of 0.1 mA cm⁻² [23]. It is shown in Figure 8a and

Table 1. Initial potential (v_o), half-wave potential (v_1/2) and limiting current density (j) of commercial Pt/C and Ag₄Bi₂O₅ with different contents of MnO₂.

Parameter	vo (V vs. Hg/HgO)	v1/2 (V vs. Hg/HgO)	J (mA cm−2)
0% Mn	−0.09	−0.227	−2.06
10% Mn	0.042	−0.123	−3.00
20% Mn	0.046	−0.099	−3.086
30% Mn	0.052	−0.090	−3.23
40% Mn	0.052	−0.078	−3.52
50% Mn	0.111	−0.084	−4.70
60% Mn	0.098	−0.047	−5.67
70% Mn	0.107	−0.219	−2.41
100% Mn	−0.014	−0.203	−2.19
Pt/C	0.179	−0.014	−5.09

that the initial potential (v_o), half-wave potential (v_1/2), and limiting current density (j) of Ag₄Bi₂O₅ is −0.09 V, −0.227 V, and −2.06 mA cm⁻², respectively. With the increase of content of MnO₂, the materials have a better ability to catalyze ORR due to the increase of the synergistic effects of Ag, Bi, and Mn. The initial potential, half-wave potential and limiting current density of Ag₄Bi₂O₅ with 60% Mn are 0.098 V, −0.047 V, and −5.67 mA cm⁻², respectively, which shows that this has the best ability to catalyze the oxygen reduction reaction compared to any other ratio of Ag₄Bi₂O₅/MnO₂. In addition, Ag₄Bi₂O₅ with 70% Mn is poor for catalyzing ORR although MnO₂ is increased. This result is due to the particles of the material becoming larger, leading to fewer catalytic activity sites thus reducing the activity for catalyzing the oxygen reduction reaction compared with 10–60% Mn. Figure 8b shows the RDE curves of Ag₄Bi₂O₅ with 60% Mn at different rotation speeds (400–2500 rpm). Figure 8c shows the Koutecky-Levich (K-L) plots which describe the relation between the inverse real current density (j⁻¹) and the inverse of the square root of the rotating rate (ω^−1/2) [22]. The good linearity of the K-L plots reveals that the kinetics of the ORR is first-order with respect to the concentration of dissolved oxygen and similar electron transfer numbers at different potentials [24].

$$\frac{1}{\mathrm{i}}=\frac{1}{{\mathrm{i}}_k}+\frac{1}{{\mathrm{i}}_d}$$

$${\mathrm{i}}_{\mathrm{k}}=nfk{C}_0$$

$${\mathrm{i}}_d=0.62nF{D}_0^{2/3}{\mathrm{v}}^{-1/6}{C}_0{\omega }^{1/2}$$

Here, n is number of electrons involved in the reaction, F is the Faraday constant (96,500 C mol⁻¹), D₀ is the diffusion coefficient of oxygen in the electrolyte (0.1 mol L⁻¹ KOH solution) (1.93 × 10⁻⁵ cm² s⁻¹), v is the viscosity coefficient of the solution (0.1 cm² s⁻¹), C₀ is the concentration of O₂ in 0.1 mol L⁻¹ KOH solution (1.26 × 10⁻³ mol L⁻¹), ω is the speed of the disc (rad s⁻¹) [25,26]. As calculated from Figure 8d in the potential range from −0.25 V to −0.35 V, the average number of transferring electrons of ORR on the Ag₄Bi₂O₅ with 60% Mn is about 3.9, indicating that the ORR catalyzed by the Ag₄Bi₂O₅ with 60% Mn follows the most efficient four-electron mechanism.

To further elucidate the electrochemical performance of the Ag₄Bi₂O₅/MnO₂ materials, the electrochemically active surface area (ECSA) can be evaluated through a simple cyclic voltammetry (CV) measurement as in Figure S1 [27]. The calculated electrochemically effective double-layer capacitances (C_dl) shown in Figure S2 are 0.92, 1.39, 1.51, 2.07, 2.47, 3.78, 5.02, and 1.34 mF cm⁻² for 0%, 10%, 20%, 30%, 40%, 50%, 60%, and 70% Mn, respectively. This result indicates that Ag₄Bi₂O₅ with 60% Mn can expose most electrochemically active sites during the ORR process.

It is inferred that the obtained Ag₄Bi₂O₅ with 60% Mn catalyst offers a larger enhanced electrocatalytic activity than Ag₄Bi₂O₅ and MnO₂ in Figure 9a,b. The initial potential (v_o), half-wave potential (v_1/2) and limiting current density (j) of Ag₄Bi₂O₅ with 60% Mn are 0.098 V, −0.047 V, and −5.67 mA cm⁻², respectively. Compared with commercial Pt/C, the initial potential and half-wave potential of Ag₄Bi₂O₅ with 60% Mn are 0.005 V more positive (Pt/C, 0.093 V) and 0.017 V more negative (Pt/C, −0.03 V), respectively, and the limiting current density is slightly higher than Pt/C, showing the performance of Ag₄Bi₂O₅ with 60% Mn toward oxygen reduction reaction is close to Pt/C in potential and slightly superior in current density. Previous papers showed that the doping of Bi₂O₃ and NaBiO₃ into MnO₂ would enhance the durability during the discharge process [21,28,29]. The new material of Ag₄Bi₂O₅/MnO₂ displays two beneficial functions for both couples of Mn-Bi and Ag-Mn. The former plays an important role of strong support of the structure of MnO₂, which will greatly enhance the durability of the catalyst, and the latter offers remarkable catalytic performance compared to the single Ag or Mn compound. The electrocatalytic activity of Ag₄Bi₂O₅ with 60% Mn towards ORR was examined by CV in 0.1 mol L⁻¹ KOH solution saturated with O₂ and Ar, respectively, at a sweep rate of 50 mV s⁻¹. The oxygen reduction peak of Ag₄Bi₂O₅ with 60% Mn is at −0.09 V and the peak current density is 1.53 mA cm⁻² in electrolyte solution saturated with O₂, as shown in Figure 9c,d shows the Tafel of Ag₄Bi₂O₅ with 60% Mn and commercial Pt/C. The Tafel slope of the Ag₄Bi₂O₅ with 60% Mn and commercial Pt/C are 73 and 68 mV dec⁻¹, respectively. The low ORR Tafel slope of Ag₄Bi₂O₅ with 60% Mn implies a transition from Langmuirian adsorption to Temkin adsorption of adsorbed O/OH groups [30]. Therefore, the electrochemical catalyst activation of Ag₄Bi₂O₅ with 60% Mn is close to that of Pt/C [15]. The long-term stability was assessed in O₂-saturated 0.1 mol L⁻¹ KOH solution by chrono-amperometry. Ag₄Bi₂O₅/MnO₂ has a small decay of −13% in ORR activity after 10,800 s, while Pt/C has an attenuation of 18% shown in Figure 9e. In addition, the current density of the Pt/C shows a much sharper decrease when methanol is introduced in the electrolyte (Figure 9f), while the current density of Ag₄Bi₂O₅/MnO₂ only exhibits a small decrease under the same conditions. This means that Ag₄Bi₂O₅/MnO₂ displays superior stability and methanol tolerance compared to the commercial Pt/C.

Recently, several papers all from prestigious journals have reported several catalysts with good ORR performance. The catalytic properties of these materials and the as-prepared Ag₄Bi₂O₅ with 60% Mn in this study are compared in

Table 2. Comparison of oxygen reduction reaction (ORR) catalytic activities between Ag₄Bi₂O₅ with 60% Mn and the other relevant catalysts.

Catalyst	E0 (V vs. Hg/HgO)	E1/2(V vs. Hg/HgO)	Currents (mA cm−2)	Stability	Reference
Ag4Bi2O5 with 60% Mn	0.098	−0.047	5.67	88% (10,800 s)	This work
Mn3O4/NrGO	−0.1	−0.2	4.4	63% (21,600 s)	[10]
Ag@MnFe2O4/C	−0.08	−0.171	4.8	88.7% (15,000 s)	[31]
50% Ag-MnO2/C	−0.036	−0.216	5.5	91% (50,000 s)	[32]
SC-PMO	−0.037	−0.237	5.0	82% (16,000 s)	[33]
CPANI/Mn2O3	0.108	−0.082	5.61	91.1% (80,000 s)	[34]
rGO/MnO2/Ag	0.034	−0.126	3.4	94% (10,000 s)	[35]
Ag-PBMO5	0.054	−0.056	5.0	91% (50,000 s)	[36]
Ag/Cu37Pd63	0.05	−0.04	2.96	77.6% (48,000 s)	[37]

. Among all the listed catalysts, the Ag₄Bi₂O₅ with 60% Mn shows the most efficient ORR performance with positive initial potential and half-wave potential, high limiting current density, and excellent long-term stability [33,34,35,36,37]. The comparison shows that Ag₄Bi₂O₅ with 60% Mn is a highly efficient catalyst for ORR.

3. Experimental

3.1. Reagents

All the chemicals were analytical grade and used as received without further purification. Ag₂O, Bi₂O₃, Mn(NO₃)₂, HNO₃, and isopropanol were commercially available from Tianjin Fuchen Industry Co. Ltd. (Tianjin, China). KOH was purchased from Chengdu Huarong Chemical Reagent Co. Ltd. (Chengdu, China). Commercial Pt/C (20 wt%, Hispec 3000) was obtained from Johnson Matthey Company (London, UK). Conductive graphite (Timcal-ks6) was available Timical Company (Changzhou, China). Nafion solution (5 wt%, D520) was from DuPont (Wilmington, NC, USA).

3.2. Synthesis and Physical Characterizations

An amount of 100 mL 6.5 mol L⁻¹ KOH was employed as basic solution, marked as solution A. Then 1.16 g Ag₂O and 1.17 g Bi₂O₃ were dissolved in 50 mL 1 mol L⁻¹ HNO₃, and the mixture solution was marked as solution B. The solution B was added into the solution A at a rate of 3 mL min⁻¹. After complete reaction of the solutions A and B, the resulting solution was aged for 1 h and marked as solution C.

Amounts of 0 g, 0.1 g, 0.22 g, 0.38 g, 0.60 g, 0.89 g, 1.34 g, and 2.09 g of 50% Mn(NO₃)₂ solutions were dissolved in deionized water to prepare portions of 20 mL solutions, respectively. These eight Mn(NO₃)₂ solutions were dripped into the above solution C at a rate of 3 mL min⁻¹, respectively. After crystallization for 2 h, the samples were washed with deionized water until pH = 7, and then dried in vacuum at 303.15 K for 8 h. The materials were named 0% Mn, 10% Mn, 20% Mn, 30% Mn, 40% Mn, 50% Mn, 60% Mn, and 70% Mn by ratio of moles.

All the preparation was conducted under conditions of room temperature and a strong agitation at 2000 rpm.

3.3. Physicochemical Characterization

The phase structures of samples were analyzed by a Rigaku D/max2500VB2+/PC X-ray diffractometer (XRD) with a Cu Kα anticathode (40 kV, 200 mA) (Rigaku Corporation, Tokyo, Japan). The morphology and surface structure of Ag₄Bi₂O₅/MnO₂ were investigated by using scanning electron microscopy (SEM, ZEISS, SUPRA 55, Carl Zeiss AG, Oberkochen, Germany), transmission electron microscope (TEM, Hitachi, H-7700, Hitachi Company, Tokyo, Japan) and high resolution transmission electron microscope (HR-TEM, JEOL JEM-2100F, JEOL Company, Tokyo, Japan). The elemental mapping and energy dispersive spectrometer (EDS) was carried out on an X-ray energy instrument (LINK-ISIS300, Oxford, UK). The valences of elements were analyzed by using an X-ray photoelectron spectrometer (XPS, Thermo VG Scientic ESCALAB 250, Thermo Fisher Scientific, Waltham, MA, USA).

3.4. Electrochemical Measurements

The electrochemical performance of the Ag₄Bi₂O₅/MnO₂ samples was tested on a rotating disk electrode with a diameter of 5 mm (RDE, AFMRSCE, Pine Instrument, Grove, PA, USA) at rotation speeds of 400–2500 rpm and a potential sweeping rate of 5 mV s⁻¹ by linear sweep voltammetry (LSV). A Hg/HgO electrode was used as the reference electrode, and a platinum wire as the auxiliary electrode. The working electrode was prepared as follows: 8 mg catalyst and 4 mg conductive graphite were dispersed ultrasonically for 30 min in a mixture of 55 μL Nafion solution, 430 μL isopropanol and 650 μL deionized water to form catalyst ink. Then 5.6 μL of the ink was spread on the glassy carbon electrode (GCE) and dried at room temperature. The working electrode was loaded with a catalyst amount of 0.2 mg cm⁻². Cyclic voltammetry (CV) of Ag₄Bi₂O₅/MnO₂ was conducted using an electrochemical workstation (CHI 760d, Chenhua Instruments, Shanghai, China). The potential range of the CV was between −0.3 V and 0.3 V (V vs. Hg/HgO). The electrochemical tests of LSV, CV, and Tafel were performed in 0.1 mol L⁻¹ KOH solution saturated with oxygen (O₂) or argon (Ar) gas. The i-t chronoamperometric was tested at −0.26 V with O₂ saturation for 10,800 s. The methanol tolerance test was performed in 0.1 mol L⁻¹ KOH solution saturated with O₂ for 1200 s and 3.0 mol L⁻¹ CH₃OH was added at about the 420th second. The results were shown by chronoamperometry curves. For comparison, Ag₄Bi₂O₅, and MnO₂ were also tested by the same procedure.

4. Conclusions

Ag₄Bi₂O₅/MnO₂ nano-material was successfully synthesized by a co-precipitation method. The MnO₂ nano particles were distributed on the surface of rod-like Ag₄Bi₂O₅ crystals to form a structure like corn on the cob. Compared with a commercial Pt/C catalyst, the Ag₄Bi₂O₅ with 60% Mn nano-material has an equivalent ORR activity in alkaline media, but a better stability and methanol tolerance. The excellent electrochemical activity of Ag₄Bi₂O₅/MnO₂ might benefit from the synergistic effects of Ag, Bi, and Mn as well as more active sites. The synergistic effects of the Ag-Mn elements remarkably enhanced the catalytic activity. The Bi-Mn elements and the corn-cob like structure of Ag₄Bi₂O₅/MnO₂ are propitious to strengthen the stability of the catalyst. More active sites contribute to the adsorption of O₂ on the catalyst surface and the breakage of O-O bonds. In alkaline conditions, the catalyst can catalyze the oxygen reduction reaction through the four-electron mechanism. Therefore, nano Ag₄Bi₂O₅/MnO₂ has proved to be a very promising oxygen reduction catalyst under alkaline conditions.