Introduction

Recently, due to the overuse of fossil energy, energy dilemma and environmental issue have become serious increasingly. Hence, the seeking for new projects to deal with the increasingly strained energy dilemma has been one of the most pressing challenges [1,2,3]. The fuel cell has attracted broad attention as a new type of high-efficient power source because it not only converts chemical energy into electric energy directly but also breaks through the limitation of Carnot cycle thermal efficiency. Compared to the fast anode process on the hydrogen electrode, the slow cathode process of oxygen relates to the breaking of O=O and the four-electron reduction process [4,5,6,7]. It is urgent to develop efficient catalysts to enhance the speed of oxygen reduction reaction which has a slow kinetic process with the higher activation energy and to promote the efficiency and specific power of the fuel cell [8,9,10,11,12].

Although platinum catalyst exhibits unique electrochemical performance, the scarcity and high cost limit its large-scale commercialization so that it becomes research focus to develop and improve new platinum free electrocatalysts which can substitute for platinum-based catalysts [13,14,15,16,17,18,19,20]. Moreover, it is more significant to exploit new catalyst used in alkaline electrolyte rather than in acid electrolyte because alkaline electrolyte offers more gentle corrosion to most of the metal oxide contained in catalysts [21]. At present, some hot materials have been proposed and researched, such as metal carbonitride catalysts [22,23,24], metal nitride catalysts [25,26,27], and transition metal oxides catalysts (e.g., MnOx) [28,29,30,31,32,33]. Manganese dioxide has aroused much interest as an alternative catalyst because of its low price, abundant resources, various valence states, and environmental friendliness [28]. Liu Jing et al. have synthesized different morphologies of Mn3O4 and the comparison of them shows that the Mn3O4 nanoflakes possess better overall performance (v1/2 of 0.82 V vs. RHE and j of 0.45 mA) than the morphology of nanoparticles and nanorods [30]. Cheng Fangyi et al. found that the catalyst activity of ORR of MnO2 followed the order of α- > β- > γ-MnO2 and the MnO2 nanowires @Ni nanoparticles compound exhibited great properties with v0 of 0.08 V (vs. Ag/AgCl) and j of 33.5 mA mg−1 [31]. Hazarika et al. have prepared a material in which mesoporous cubical Mn2O3 loaded on an active carbon with good ORR properties, showing a limited current of 4.4 mA cm−2 with high initial potential at 1600 rpm [32]. Manganese dioxide-based catalysts have shown good electrocatalytic properties for reduction of oxygen via 4 electron reaction pathways, while manganese-based catalysts have the disadvantage of being easy to decay in characteristics and the pure structure has unsatisfactory property [34].

Hence, some new research has been investigated on the MnO2 for improving its catalytic performance, and metal cations as dopants have been introduced to alleviate the instability of structure during charging and discharging processes [35,36,37]. Silver and silver-based catalysts have not only high electrochemical activity and good methanol resistance but also strong stability which can support MnO2 to a certain extent [21, 38,39,40,41]. However, the inferior conductivity and dispersibility of nanometal oxide particles have become the bottleneck limiting their further application [42]. The reduced graphene oxide (rGO), especially the wrinkled rGO sheets with high electroconductivity and large surface area, provide numerous nucleation sites to restrain the growth of nanoparticles. The wrinkled rGO as the new carrier can combine with metal oxide to provide more active sites, which can significantly improve their electrical conductivity, accelerate charge transport, and further reduce the polarization of ORR process [43,44,45,46,47,48,49,50,51]. Here, we propose a new ternary catalyst, Ag4Bi2O5/MnOx (1 ≤ x ≤ 2), grown on wrinkled rGO. It has remarkably enhanced performance and overcome the abovementioned drawbacks of simplex catalyst by means of its better natural dispersibility, improved electrical conductivity, and new synergistic effect.

Herein, rGO-supported nano-Ag4Bi2O5/MnOx (rGO-AM) composite was prepared via a facile coprecipitation method in high concentration of KOH solution with rGO dispersed. The obtained material demonstrates the equivalent catalytic activity to that of commercial Pt/C, superior service life, and excellent anti-methanol performance, so that rGO-AM will be a promising catalyst in the application of fuel cells.

Experimental

Synthesis of Ag4Bi2O5/MnOx/rGO

Firstly, 0 mg, 0.69 mg, 1.5 mg, 2.5 mg, 4.2 mg, 6.3 mg, 9.4 mg, and 14.3 mg rGO were added into 8 vessels each containing 100 mL 6.5 M KOH, respectively. Subsequently, they were dispersed into 8 uniform solutions by ultrasonic for 2 h and labeled as a series of solution A. In regard to each proton of solution A, 1.15 g Ag2O and 1.17 g Bi2O3 were reacted with 50 mL 2 M HNO3 to form the acidic Ag+–Bi3+ solution, labeled as solution B. Then the two solutions of A and B were dropped into a 250-mL flask, separately by two peristaltic pumps at a rate of 150 mL h−1 under 30 °C, meanwhile the mixture in the flask was stirred at 2000 rpm, and finally the mixed mother liquor was aged for 1 h. Then 1.0 mL 50% Mn(NO3)2 solution was diluted in deionized water to prepare 20 mL solution, marked as solution C. The solution C was pumped into the above mother liquor at a speed of 3 mL min−1 and the mixture was crystallized gradually for 2 h. The 8 portions of solution A were processed the same way as the above procedure. So, 8 crystallized samples were obtained. All the reagents and materials used were listed in Table S1. The schematic diagram of the synthetic process of Ag4Bi2O5/MnOx/rGO is displayed in Fig. 1.

Fig. 1
figure 1

Schematic diagram of the synthetic process of Ag4Bi2O5/MnOx/rGO

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The crystallized samples were pulpified with 100 mL DI water separately and filtrated until the pH of filt liquor < 8. The obtained solids were placed in a vacuum oven at 30 °C overnight. These products were denoted as 0% rGO-AM, 10% rGO-AM, 20% rGO-AM, 30% rGO-AM, 40% rGO-AM, 50% rGO-AM, 60% rGO-AM, and 70% rGO-AM, separately.

Physicochemical and electrochemical measurements

All the investigated materials were characterized by SEM, XRD, EDS, and XPS, and the related information was also displayed in Table S2.

A three-electrode system containing 0.1 M KOH as electrolyte saturated with O2 or Ar was adopted as follows:

  1. 1.

    Working electrode: a glassy carbon electrode (GCE) coated with catalyst

  2. 2.

    Auxiliary electrode: a platinum wire

  3. 3.

    Reference electrode: a Hg/HgO electrode

In this research, the Hg/HgO electrode was employed in the same electrolyte as the working electrode for the electrochemical assessment, so the potential difference (Hg/HgO vs. RHE) is pH independent, and the relation is as follows:

E(RHE) = E(Hg/HgO) + 0.93 V

For preparation of a working electrode, the catalyst ink was firstly made up with the following components: 8 mg of the obtained materials as catalyst, 4 mg conductive graphite as conductive agent, 450 μL (CH3)2CHOH and 630 μL DI water as dispersing agent, and 55 μL 5% Nafion solution as the raw material of proton exchange membrane which was formed after volatilization of solvent. The mixed catalyst ink was decentralized by ultrasonic for half hour to form a homogenous ink. Whereafter, 5.6 μL ink was applied on the glassy carbon electrode of 5 mm in diameter and solidified at 30 °C. The load amount of catalyst on the working electrodes was approximately 0.2 mg cm−2. Both Pt/C and the obtained materials were prepared following the process and with the same load amount.

Before electrochemical tests, the electrolytes were aerated by O2 or Ar for 30 min to ensure being saturated with the filled gas. Cyclic voltammetry (CV) of the catalyst electrode was carried out in the range from − 0.3 to 0.3 V (vs. Hg/HgO) with a scan rate of 50 mV s−1. The linear sweep voltammetry (LSV) of the obtained rGO-AM catalysts was carried out on the rotating disk electrode at various rotation speeds of 400, 625, 900, 1225, 1600, 2025, 2500 rpm. The limiting current density (j) was read at − 0.30 V where the reaction was at equilibrium, the initial potential (v0) was read as the current density was − 0.02 mA cm−2, and every half-wave potential (v1/2) value was read while the current density was half of the corresponding j. The i–t chronoamperometric test was performed in O2 saturated 0.1 mol L−1 KOH solution at − 0.03 V for 10,800 s. The anti-methanol resistance was checked by chronoamperometric curves test at − 0.03 V for 2000 s in O2 saturated 0.1 mol L−1 KOH solution, and 3.0 mol L−1 methanol provided at about 400 s during the test.

Results and discussion

Morphological and structural characterization

Figure 1 presents the schematic diagram of the synthesis process of AM-rGO composite, which undergoes two steps of co-precipitation in alkaline solution with rGO dispersion. As we expected, the rod-like Ag4Bi2O5 grows on the surface of wrinkled rGO first; afterwards, the nanoparticles are formed on the rods so as to take the shape of corncob. The catalytic activity can be greatly improved by the nanoscale composition, large specific surface area, and numerous exposed active sites.

Figure 2 displays the SEM images of the rGO and AM-rGO composite with different contents of rGO from 10 to 70%. Figure 2a indicates that rGO has a wrinkled surface which can be beneficial to enlarge its specific surface area so as to have more exposed active site to enhance the catalytic performance. It is clear that the morphology of Ag4Bi2O5/MnOx is rod-like with a length of 200 nm and width of 50 nm. As shown in Fig. 2b–h, the rGO is coated by Ag4Bi2O5/MnOx, which is conductive to exerting the synergistic effect of Ag4Bi2O5/MnOx and rGO. In addition, the special structure not only increases the specific surface area of the composite and exposes more uniform active sites but also shortens the transfer pathway for material and electron to benefit improving the catalytic performance of rGO-AM composites.

Fig. 2
figure 2

SEM images of Ag4Bi2O5/MnOx/rGO with different contents of rGO: a pure rGO, b 10% rGO, c 20% rGO, d 30% rGO, e 40% rGO, f 50% rGO, g 60% rGO, h 70% rGO

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In order to realize the element distributions of rod-like Ag4Bi2O5/MnOx, rGO-AM with 60% rGO was examined by mapping method. As clearly displayed in Fig. 3, besides the inherent element Ag, Bi, Mn, and O, the element C is mixed in the prepared material successfully. Moreover, the Ag, Bi, Mn, O, and C elements are uniformly distributed.

Fig. 3
figure 3

a SEM images of 60% rGO-AM; elemental mapping images of 60% rGO-AM b Ag, c Bi, d Mn, e O, and f C

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EDS analysis was employed to check the chemical composition, and the results are shown in Fig. 4a. It is proved that the prepared composite contains Ag, Bi, Mn, O, and C elements, and the element C is uniformly dispersed, inferring that the rGO-AM composite has been synthesized successfully. Meanwhile, it is shown that the atomic ratio of Ag/Bi (18.56:9.26) is approximately equal to the theoretical ratio of 2:1 in the Ag4Bi2O5, while the atomic ratio of Bi/Mn is 1.25, which is closed to the theoretical value of 1.33. Besides, atom C accounts for 1.45% [52].

Fig. 4
figure 4

a EDS spectra of 60% rGO-AM. b XRD of Ag4Bi2O5/MnOx/rGO with different content of rGO (0–70%)

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Figure 4b shows the XRD patterns of the samples with different contents of 10–70% rGO. It can be seen that all samples exhibit sharply characteristic peaks at 2θ = 26.37°, 31.25°, 31.85°, 37.76°, 48.35°, and 56.19°, corresponding to planes of (112), (411), (312), (600), (314), and (332) of Ag4Bi2O5, respectively. That well matches the standard card JCPDS 87-0866 of Ag4Bi2O5. The composite containing rGO still represents the characteristic peaks of Ag4Bi2O5, demonstrating its loading process on rGO has not affected the crystal form and the structure of nano-Ag4Bi2O5. Meanwhile, the pattern indicates that there is no obvious peak of manganese dioxide, indicating it is a kind of amorphous MnOx with the x value of 1–2.

Figure 5 shows the XPS results of the valence state of the elements in the as-prepared composite. Figure 5a shows the XPS spectra of elemental carbon, which present two strong peaks at 288.4 eV and 284.9 eV, indicating the existence of sp2 carbon [52]. The two peaks at 368.3 eV and 374.4 eV correspond to the existence of Ag3d5/2 and Ag3d3/2, respectively, demonstrating element Ag exists in the valence state of + 1 (Fig. 5b) [53]. As seen from Fig. 5c, two peaks (164.0 eV and 158.7 eV) correspond to Bi4f5/2 and Bi4f7/2, respectively. These results prove Bi exists in the valence state of + 3 [53]. Figure 5d shows the 2p orbit of Mn in GO-AM, and the appearance of two peaks at 642.6 eV and 654.1 eV corresponding to Mn2p3/2 and Mn2p1/2 respectively proves the existence of Mn4+ [54]. In general, all the four spectra of Fig. 5 prove that the as-prepared Ag4Bi2O5/MnOx/rGO accords with theory and our expectation [54].

Fig. 5
figure 5

X-ray photoelectron spectroscopy (XPS) spectra of 60% rGO-AM composite a C1s, b Ag3d, c Bi4f, and d Mn2p

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Electrocatalytic performance

Figure 6a illustrates that there is an obvious peak of oxygen reduction reaction at − 0.06 V of rGO-AM in the 0.1 M KOH saturated with O2. linear sweep voltammetry test was conducted via rotating disk electrode (RDE) to investigate the electrocatalytic ability for ORR of the prepared composites with different contents of rGO. Figure 6b shows the LSV plots of nano-rGO-AM composites with different contents of rGO and tested at a rotating speed of 1600 rpm and a scanning rate of 5 mV s−1. Based on the former research [55], the Ag4Bi2O5/MnOx composite without rGO possesses the performance that the initial potential (vo), half-wave potential (v1/2), and limiting current density (j) are 0.089 V, − 0.046 V, and 5.51 mA cm−2, respectively. With addition of 10% rGO, the initial potential and half-wave potential of the new composite become more positive value of 0.093 V and − 0.035 V respectively, together with a great limiting current density of 5.204 mA cm−2. As the amount of rGO increases, v1/2 of rGO-AM composite gradually becomes more positive, and j becomes even higher. When the content of rGO reaches 60%, the half-wave potential attains the most positive value (− 0.0419 V), the initial potential reaches 0.0799 V, and j reaches 6.164 mA cm−2. Nevertheless, when the ratio of rGO is up to 70%, the half-wave potential of rGO-AM starts to become negative, meaning the synergistic effect of Ag, Bi, and Mn is weakened and the catalytic property is decreased due to an excessive amount of rGO. Table S3 exhibits the comparison of vo, v1/2, and j among the rGO-AMs with different contents of rGO.

Fig. 6
figure 6

(a) CV curves of 60% rGO-AM in O2 and Ar-saturated 0.1 mol L−1 KOH, (b) LSV curves of rGO-AM with different ratios of rGO (10% to 70% rGO) in O2-saturated 0.1 mol L−1 KOH, (c) LSV curves of 60% rGO-AM at different rotational speeds, (d) Tafel curves of 60% rGO-AM and commercial Pt/C (e) Koutecky-Levich curves of 60% rGO-AM at different potentials, (f) electron transfer number of 60% rGO-AM at the corresponding potentials, (g) LSV curves of rGO, ABO, ABO-M, 60% rGO-AM, and commercial Pt/C, (h) comparison of kinetic current density jK at −0.03V (A g−1) among rGO, ABO, ABO-M and 60% rGO-AM. (LSV conditions: scan rate of 5 mV s−1, rotational speed of 1600 rpm)

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Figure 6c shows the LSV plots of 60% rGO-AM composite at different rotating speeds. In consideration of the probable side reaction, all of the LSV curves have been treated by taking out the background current of the bare electrode and the curves concerning to Ag4Bi2O5 have been taken out the part of itself (at the same speed but with Ar saturated). The related curves are shown in Figure S1. According to the analysis of separate current densities at − 0.35 V, − 0.30 V, − 0.25 V and − 0.20 V, the Koutecky–Levich curves of 60% rGO-AM are shown in Fig. 6e. The calculations showed the number of transferring electrons for ORR is 3.84, 3.76, 3.72, and 3.66 during the catalytic process, respectively, which are close to four-electron reaction, and the results are shown in Fig. 6f. The data processing was according to the following formula:

$$ {\displaystyle \begin{array}{l}\frac{1}{j}=\frac{1}{j_{\mathrm{L}}}+\frac{1}{j_{\mathrm{K}}}=\frac{1}{{{}_{B\omega}}^{\frac{1}{2}}}+\frac{1}{j_{\mathrm{K}}}\\ {}B=0.62 nF{C}_0D\begin{array}{c}{}_{\frac{2}{3}}\\ {}{}^0\end{array}{V}^{-\frac{1}{6}}\end{array}} $$

Here, j is the measured current density, jK and jL are the kinetic and limiting current densities, ω is the angular velocity of the disk, n is electron transfer number, F is the Faraday constant (96,500 C mol−1), C0 is the concentration of O2 in 0.1 mol L−1 KOH solution (1.26 × 10−3 mol L−1), D0 is the diffusion coefficient of oxygen in 0.1 mol L−1 KOH solution (1.93 × 10−5 cm2 s−1), and V is the viscosity coefficient of the solution (0.1 cm2 s−1) [56].

Figure 6d shows that the Tafel slope of 60% rGO-AM is 73 mV dec−1, close to that of Pt/C (68 mV dec−1), demonstrating the catalytic activity of 60% rGO-AM approximates to that of commercial Pt/C [57].

As shown in Fig. 6g, the initial potential of commercial Pt/C is 0.017 V, half-wave potential is − 0.03 V, and limiting current density (at − 0.30 V) is − 5.09 mA cm−2. By comparison, the half-wave potential of 60% rGO-AM is 11 mV lower than that of Pt/C, while the initial potential of 60% rGO-AM is 62 mV more positive than that of Pt/C and j is higher than that of commercial Pt/C. Figure 6h shows that the ternary catalyst possesses much huger kinetic current density (jK) than the single and binary materials. The kinetic current density of 60% rGO-AM is 6.06 A g−1, while that of the other three materials is about 1–2.2 A g−1. There are three advantages of adding rGO to the Ag4Bi2O5/MnOx composites: the first is to enhance the electrical conductivity of the composite; the second is to realize a synergistic effect of metallic elements Ag, Bi, and Mn to catalyze ORR process; and the last is to provide large specific surface area to improve the catalytic activity. Combined with the above observations, the Ag4Bi2O5/MnOx/rGO composite achieves such an outstanding ORR catalytic performance.

Figure 7a exhibits the i–t chronoamperometric curves of 60% rGO-AM and commercial Pt/C to explore their long-term stability. After the continuous test of 10,800 s, the 60% rGO-AM sample reserves about 86% of its initial performance while the commercial Pt/C undergoes a larger decay of 42% in ORR catalytic activity, which explains that the prepared material possesses greater durability as compared with Pt/C. In addition, Fig. 7b shows the methanol tolerance of Pt/C and 60% rGO-AM by adding 3.0 mol L−1 CH3OH to 0.1 M KOH saturated with O2. It can be seen that the catalytic current of Pt/C drops rapidly while the current of the as-prepared 60% rGO composite almost has no change after the adding process, which reveals 60% rGO-AM has superior methanol tolerance than that of Pt/C catalyst. These experiments prove that 60% rGO-AM is an excellent catalyst for ORR with long durability and great methanol tolerance.

Fig. 7
figure 7

a i–t chronoamperometric curves of 60% rGO-AM and commercial Pt/C. b Methanol tolerance of 60% rGO-AM and commercial Pt/C by the chronoamperometric response with adding 3.0 mol L−1 CH3OH at about 400 s. (Test conditions: in O2-saturated KOH solution, at − 0.03 V and 1600 rpm)

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Furthermore, the composite was contrasted to other related catalysts, and their relevant performance parameters of ORR are listed in Table S4. It can be seen that the prepared catalyst possesses higher ORR catalytic performance than the others.

Conclusions

The rGO-AM composite was synthesized by coprecipitation on the base of rod-like nano-Ag4Bi2O5/MnOx. By the investigation on the content of rGO, it is found that the composite of 60% rGO exhibits the highest catalytic performance for ORR. Besides, the composite with 60% rGO displays more excellent long-term stability and methanol tolerance than that of commercial Pt/C. Its excellent ORR catalytic performance is attributed to the synergistic effect of Ag, Bi, Mn, and C. That behaves as follows: the combination of Ag and Mn increases the catalytic activity for ORR, and the synergy effect of Bi and Mn and the corncob-like structure of Ag4Bi2O5/MnOx composite enhances the stability of the prepared material. In addition, the wrinkle morphology of rGO is beneficial to increase the specific surface area and conductivity of the composite so that more active sites are exposed and short transfer pathway for electron is created. Consequently, the catalytic property is improved. In conclusion, rGO-AM will be a promising high-efficient catalyst for the oxygen reduction reaction.