1 Introduction

Platinum-based catalysts are the most effective catalysts for oxygen reduction reactions (ORRs) in acid media. However, these catalysts are costly and limited in reserve that researchers worldwide have put significant efforts into developing efficient and inexpensive catalysts to overcome such issues in recent years [1–4]. Core–shell catalysts of Pt with other transition metals (Pd, Ni, Cu, Fe, and Ag) can improve the utilization of Pt nanoparticles (NPs) while stimulating catalytic activity [5, 6]. Unfortunately, there are still some drawbacks of the core–shell catalysts, such as poor stability and their potential to poison. Therefore, the activities and stabilities of these Pt-based core–shell catalysts need to be improved. Recently, several studies have been focused on promoting the electrochemical durability of the catalysts [7–11]. For instance, Ying et al. [12] prepared AgPd@Pt NPs by depositing Pt layer on the surface of AgPd alloy NPs, and the specific activity of AgPd@Pt was two times higher than that for Pt/C. Zhao et al. [13] discovered that the nanodiamond@TiN supported Pt NPs by microwave heating polyol method can help to remove the CO or CO-like intermediates and enlarge the Pt/Pt-oxide surface redox couple due to the presence of TiN. Addition of metal oxides (CoO x , MnO x , and CeO x ) can also promote the durability of catalysts owing to their corrosion resistance and durability in catalytic reactions [14–16]. For example, the stability of the Pt/WO3/C catalyst increased 20 % compared with the Pt/C catalyst [17].

Heteropolyacid (HPA), a unique class of inorganic metal−oxygen clusters, is considered to be a promising catalyst in various fields due to its unique physicochemical properties, including non-toxicity, structural variability, ultra-strong Brønsted acidity, low cost, high proton conductivity, and reversible rapid multi-electron transformation under mild conditions [18–20]. Hassan et al. found that HPA as a multi-electron donor accepting conduction band electron can effectively prevent the combination between the hydrogen ions and photoelectrons and improve the photocatalytic activity. Li et al. [21] synthesized photoanodes containing HPA of dye-sensitized solar cells using a solvothermal method, which resulted in an improvement of 49.2 % of the solar electric energy conversion efficiency compared to the performance of photoanodes without HPA. As a novel HPA-based ionic liquid catalyst for n-caprylic acid esterification, Han et al. [22] found that there was only 3 % loss in catalytic activity over six consecutive runs, implying excellent durability and recyclability of the catalyst.

Phosphotungstic acid (H3PW12O40, HPW), a member of the Keggin-structured HPA, is outstanding for use in electrocatalytic fields on account of its strong acidity and high proton conductivity [23–26]. Furthermore, Keggin-structured HPW is a large molecule of stable cage structure, in which tetrahedral P atoms and octahedral W atoms are connected via a strong oxygen bridge. The cage structure has some pores that allow oxygen-containing materials to move freely and improve the contact area among reactants [27]. Thus, Keggin-structured HPW has attracted many researchers’ attention due to its unique physicochemical characteristics [28–32]. For instance, Xiang et al. prepared a composite membrane combination HPW with Nafion, and the maximum power density of the composite membrane increased by 26 % over the cell performance of pristine Nafion under the same conditions. In addition, due to the good catalytic properties of HPW, the mass activity of the hybrid catalyst combination of HPW with Pt/C was about 3 times higher than that of Pt/C catalyst [33]. The HPW-modified PtRu nanocatalyst can reduce the CO poisoning effect, which may be ascribed to the protective effect of the self-assembled HPW layer on the catalyst surface [34]. Dsoke et al. [35] revealed that the Pt-HPW catalyst proceeded through 4e− reduction of O2–H2O, which may be accountable to HPW as a co-catalyst providing a proton-rich environment in the vicinity of the Pt NPs. Some studies have showed that the combination of chitosan (CS) and HPW can greatly enhance the mechanical strength of HPW in solution [36]. Besides, the HPW-CS on carbon can increase the proton conductivity of the carbon support while improving the stability and toxicity tolerance of carbon NPs [37]. Although HPW, as a co-catalyst for ORR, has been the focus of numerous researches, there still exists a certain degree of gaps in meeting the requirements for use in commercialization development.

In this work, inspired by the remarkable works reported and on the basis of our previous studies [38], we immobilized negatively charged HPW on Ag@Pt/MWCNTs NPs attached to positively charged functional groups of CS by electrostatic interaction to obtain HPW-modified Ag@Pt/MWCNTs electrocatalysts. The morphology and catalytic performance of the prepared catalysts were investigated, leading to the understanding of catalytic mechanisms in acidic media and the importance of HPW in hybrid catalysts.

2 Experimental

Multi-walled carbon nanotubes (MWCNTs) were purchased from Shenzhen Nanotech Port Co., Ltd. (Shenzhen, China). Dihydrogen hexachloroplatinate hexahydrate (H2PtCl6·6H2O,) was purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Keggin-type heteropoly phosphotungstic acid (HPW) was obtained from Sigma-Aldrich (Shanghai, China). CS powder was provided by Haidebei, Ltd., China. Commercial 20 % Pt/C [Johnson–Matthey (JM)] catalyst was purchased from Shanghai Hesen electric Co., Ltd. (Shenzhen, China). Nafion (5 % solution in alcohol) was obtained from DuPont. All other chemicals were purchased from the Beijing Chemical Reagent Store (China) and used without further purification. Double-distilled deionized water was used throughout this study.

2.1 Synthesis of Ag@Pt/MWCNTs electrocatalyst

The core–shell Ag@Pt/MWCNTs (Ag:Pt:C = 10:10:80) electrocatalysts used in this work were prepared using NaBH4 and ethylene glycol as reducing agents and performed as described in our previous work [39].

2.2 Synthesis of Ag@Pt/MWCNTs-HPW electrocatalyst

The as-prepared Ag@Pt/MWCNTs NPs were suspended in a CS-acetic acid solution (2 wt%) using ultrasonication for 30 min with CS acting as the functionalization polyelectrolyte. After stirring for 12 h at room temperature, the solution was filtered and washed with deionized water several times. The CS-functionalized Ag@Pt/MWCNTs NPs were dried at 50°C for 24 h. Then, CS-functionalized Ag@Pt/MWCNTs were sonicated in HPW solution (0.38 mol L−1). After stirring for 3 h, the solution was filtered and washed extensively with deionized water and then dried at 50°C for 24 h. The as-synthesized nanoparticle electrocatalysts are denoted by Ag@Pt/MWCNTs-HPW.

Scheme 1 depiction of the synthesis procedure for the Ag@Pt/MWCNTs-HPW. The Ag@Pt/MWCNTs were prepared as described in a previous study [38]. Then, Ag@Pt/MWCNTs were doped with HPW to obtain Ag@Pt/MWCNTs-HPW electrocatalysts.

Scheme 1
scheme 1

Schematic diagram for formation of Ag@Pt/MWCNTs-HPW nanostructure

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2.3 Physical characterizations

The metal phases of the as-prepared electrocatalysts were analyzed with X-ray powder diffraction (XRD, Cu Kα = 1.5406 Å). The high-resolution transmission electron microscopy (HRTEM) was used to observe the particle size and morphology on a JEOL S-520 30 microscope. Scanning electron microscopy (SEM) observations were conducted with a Hitachi S4700 microscope equipped with an electron energy-dispersive X-ray spectrometer (EDX). X-ray photoelectron spectroscopy (XPS) analysis was carried out to obtain information on the catalyst surface using a Thermo VGESCALAB250 spectrometer.

2.4 Electrochemical investigation

The electrocatalytic activity of the synthesized catalysts was evaluated by a Zahner Ennium electrochemical workstation equipped with a three-electrode cell installed with platinum wire and Ag/AgCl as the counter electrode and reference electrode. The working electrode was prepared as follows. First, 19.9 mL distilled water and 5.0 mL isopropanol were ultrasonically mixed with 100 μL Nafion solution (5 wt%, Aldrich) for at least 2 h to form a homogeneous solution to be used as dispersant. Then, a catalyst ink was made by mixing 5.0 mg catalyst and 1.0 mL of as-prepared dispersant ultrasonically for at least 30 min. The catalyst ink (10 μL) was perfectly dropped on the center surface of a glassy carbon (GC) electrode (0.196 cm2) as the working electrode and dried at room temperature. Linear scanning voltammetry (LSV) tests were used to assess the catalytic activity for ORR in O2-saturated 0.1 mol L−1 HClO4 solution. Cyclic voltammetry (CV) and chronoamperometry measurements were conducted in the same solution to study the activity and stability of the electrocatalysts. Electrochemical impedance spectroscopy (EIS) tests were conducted by superimposing a 5 mV ac signal under potentiostatic mode over the frequency range from 0.01 Hz to 100 kHz. All potentials in this study refer to the reversible hydrogen electrode (RHE). All electrochemical measurements were performed at 25 °C.

3 Results and discussion

3.1 Physical characterizations

Figure 1 shows the XRD patterns for the different electrocatalysts recorded in the 2θ range, from 10° to 90°. In Fig. 1, the peak at 2θ = 26.05° is assigned to the characteristic diffraction peak of the carbon support (MWCNTs). For 20 % Pt/MWCNTs, the peaks located at 39.84°, 46.18°, 67.82°, and 81.38° are assigned to the characteristic diffraction peaks of the Pt crystal faces (1 1 1), (2 0 0), (2 2 0), and (3 1 1), respectively. The diffraction peaks of Ag are not obvious in the pattern for Ag@Pt/MWCNTs, indicating that metallic silver formed in the interior of the platinum shell.

Fig. 1
figure 1

XRD patterns for different electrocatalysts. Scanning range: 10°–90°, scanning speed: 5° min−1

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Compared to 20 % Pt/MWCNTs, the characteristic diffraction peak of the Ag@Pt/MWCNTs electrocatalysts shows a negative shift, where larger Ag atoms replaced Pt atoms in the Pt unit, increasing the size of the platinum unit and the lattice constant. In other words, the inflated Pt lattice sheltered the Ag atoms to form an Ag@Pt system. The size of the Ag@Pt NPs was calculated using the Debye–Scherrer equation (Eq. 1):

$$d = 0. 9\lambda /(B\,{ \cos }\theta ),$$
(1)

where d is the average size of the NPs (nm), λ is the wavelength of Cu Kα radiation (=1.54056 Å), B is the width of the half peak of the crystal plane, and θ is the measured crystal face diffraction Bragg’s angle. The characteristic diffraction peak of the Pt crystal faces in HPW-doped electrocatalysts shows a slight negative shift compared to that in Ag@Pt/MWCNTs. There are no obvious characteristic diffraction peaks for HPW in the XRD pattern of Ag@Pt/MWCNTs-HPW composite, which indicates that HPW exists in the form of a single molecule on the Ag@Pt/MWCNTs catalyst.

Figure 2 shows the TEM and HRTEM images of the different electrocatalysts. From Fig. 2a, the catalyst NPs with an average diameter of about 3.26 nm are uniformly dispersed on the MWCNTs support. The Ag@Pt core–shell structure defined from the lattice fringes of the particles is observed in Fig. 2b [38]. After Ag atoms replaced Pt atoms, the platinum lattice expanded, or the platinum lattice atom spacing increased in accordance with the XRD results. The TEM image of Ag@Pt/MWCNT-HPW is displayed in Fig. 2c. After HPW was inserted into Ag@Pt/MWCNT to form a uniform structure, the size of the catalyst NPs increased to 4.0 nm. The surface-specific activity and the electrochemical stability of catalyst improved with the increase in particle size, suggesting that 4.0 nm is the optimum particle size for the maximum ORR mass-specific activity [40]. EDX analysis (Fig. 2d) shows the W and P peaks except Pt and Ag peaks, confirming the existence of HPW on Ag@Pt/MWCNT.

Fig. 2
figure 2

a HRTEM image of Ag@Pt/MWCNTs, b HRTEM image of Ag@Pt, c TEM image of Ag@Pt/MWCNTs-HPW, d EDX analysis of c

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3.2 Electrochemical investigation

The cyclic voltammograms of the different amounts of HPW-modified Ag@Pt/MWCNT catalyst are shown in Fig. 3. The integrated charge of the hydrogen absorption–desorption area of the CV prompted the determination of the corresponding electrochemical active surface areas (EASAs) of the various catalysts [41]:

$${\text{EASA}}\,\left( {{\text{m}}^{ 2} \,{\text{g}}^{ - 1} } \right) = Q_{\text{H}} /( 2. 1\,[{\text{Pt}}]),$$
(2)

where Q H is the charge for hydrogen desorption (C m−2), [Pt] is the Pt loading (g m−2), and 2.1 is the charge (C m−2) required to oxidize a monolayer of H2 on the catalyst. The EASAs of different catalysts were calculated by means of equation (Eq. 2) and listed in Table 1.

Fig. 3
figure 3

Cyclic voltammograms of different electrocatalysts. Electrolyte: 0.1 mol L−1 HClO4, sweep rate: 50 mV s−1

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Table 1 Electrochemical results of various catalysts
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The EASA of the electrocatalyst with HPW content of 25 % (83.62 m2 g−1) is the largest of all the electrocatalysts. The \({\text{PW}}_{12} {\text{O}}_{40}^{3 - }\) group establishes a negative electric field over the catalyst surface that protects complex metal anions from being subjected to electrostatic repulsion so as to conserve the active metal species on the catalyst surface [42]. Thus, the incorporation of HPW can improve the electrical conductivity of Ag@Pt/MWCNTs. However, too much HPW can obstruct the active site of Pt, increasing the resistance of O2 from accessing the surface of Pt NPs.

The ORR polarization curves were studied with the catalyst-coated GC electrode, and the current–potential curves are shown in Fig. 4. It was determined that HPW can ameliorate electrocatalytic activity of the catalyst, and the catalyst with HPW content of 25 % displayed the best electrocatalytic activity from Fig. 4a. The half-wave potential (E 1/2) for the ORR is 0.851 V, which positively shifted 35.0 mV from that of the commercial 20 % Pt/C, revealing that ORR occurs easiest on the surface of the 25 % HPW-modified Ag@Pt/MWCNT catalyst. The mass activity (I m) and specific area activity (I s) were evaluated by normalizing the kinetic current (I k) which was calculated with the Koutecky–Levich equation in consideration of the loading of Pt and ECSAs of the catalysts [43, 44]. The calculated I m and I s of Ag@Pt/MWCNT-25 %HPW are about 3.0 and 2.5 times than those of 20 % Pt/C (JM), respectively, as shown in Table 1.

Fig. 4
figure 4

Polarization curves of different electrocatalysts: a Ag@Pt/MWCNTs-HPW with different contents of H3PW12O40; b different electrocatalysts. Electrolyte: O2-saturated 0.1 mol L−1 HClO4; sweep rate: 5 mV s−1; rotation speed: 1600 rpm

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The polarization curves of Ag@Pt/MWCNT-25 % HPW at different electrode rotational speeds shown in Fig. 5a were used to study the ORR kinetic performance. The Koutecky–Levich equation (Eq. 3) was used to calculate the number of electrons transferred per oxygen molecule (n) involved.

$$j^{ - 1} = j_{k}^{ - 1} + \left( {0. 6 2nF\left( {D_{{{\text{O}}_{ 2} }} } \right)^{ 2/ 3} \upsilon^{ - 1/ 6} C_{{{\text{O}}_{ 2} }} \omega^{ 1/ 2} } \right)^{ - 1} ,$$
(3)

where j and j k are the measured current density (mA cm−2) and the kinetic current density (mA cm−2) of the ORR, respectively; n is the overall number of electrons transferred during the ORR; F(\(D_{{{\text{O}}_{2} }}\))2/3 υ −1/6 \(C_{{{\text{O}}_{2} }}\) is the known constant; and ω is the angular velocity of the disk electrode [45]. The curves of j −1 and ω −1/2 (K–L curves) at the potentials of 0.794, 0.761, 0.696, 0.618, and 0.552 V are displayed in Fig. 5b.

Fig. 5
figure 5

a ORR polarization curves for Ag@Pt/MWCNTs-25 % HPW catalyst in O2-saturated 0.1 mol L−1 HClO4 solution at 25 °C, scan rate of 5 mV s−1, and different rotational speeds: 200, 400, 900, 1600, and 2500 rpm. b Corresponding Koutecky–Levich plots at different potentials

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The n obtained by the Koutecky–Levich equation at each potential is given in Table 2. The results show that the ORR follows a 4e− pathway from O2 to H2O on Ag@Pt/MWCNT-25 % HPW, which indicates that Ag@Pt/MWCNT-25 % HPW has the highest catalytic efficiency [46–48].

Table 2 Number of transferred electrons for various catalysts at various potentials
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Chronoamperometry tests were carried out to study the electrochemical stability of the catalysts at 0.75 V versus NHE in 0.1 mol L−1 HClO4 solution [49–51]. As shown in Fig. 6, there is a sharp decrease in the oxidative reaction currents for all electrocatalysts in the beginning. This is because the active sites are covered gradually by the oxygen species OHads produced during the oxidation reaction process. However, the oxygen-containing species OHads are continuously generated and removed in the reaction process. When they reach equilibrium, the oxidation currents of all electrocatalysts reach a plateau. Since the large molecular HPW can effectively inhibit the formation of the surface oxygen-containing species OHads, the Ag@Pt/MWCNTs-25 % HPW catalyst shows the highest current response for continuous catalysis occurring more than 900 s.

Fig. 6
figure 6

Chronoamperometry curves for different electrocatalysts: Ag@Pt/MWCNTs, Ag@Pt/MWCNTs-HPW and 20 % Pt/C (JM). Electrolyte: 0.1 mol L−1 HClO4; potential: 0.75 V versus NHE; time: 900 s; sweep rate: 50 mV s−1

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EIS measurements were conducted to study the essential actions of the cathodic process [34, 49]. Figure 7 shows the corresponding EIS Nyquist plots of various catalysts at 0.75 V versus NHE in 0.1 mol L−1 HClO4 solution. The semicircle diameter at the high frequency used as a measure of catalytic activity is related to the charge transfer resistance of the catalyst for ORR [52, 53]. The diameter of the circular arc for the 25 % HPW-modified Ag@Pt/MWCNTs catalyst is the smallest of all the catalysts in the same frequency range, implying that charge transfer resistance of 25 % HPW-modified Ag@Pt/MWCNTs catalyst for ORR is the lowest. This low resistance helps to accelerate the oxidative removal of oxygen-containing species OHads adsorbed on the surface of Ag@Pt NPs with HPW and enhances the activity of the 25 % HPW-modified Ag@Pt/MWCNTs catalyst. The negatively charged HPW is combined with positively charged CS onto the Ag@Pt/MWCNTs electrocatalyst via electrostatic attraction, which promotes electron transfer and oxygen reaction. The cage structure of HPW provides a tunnel that allows O2 to adsorb on the surface of the catalyst in a rapid and orderly way, thereby reducing the resistance to the diffusion of oxygen to the surface of the catalyst [54].

Fig. 7
figure 7

Nyquist plots of different electrocatalysts: Ag@Pt/MWCNTs, Ag@Pt/MWCNTs-25 % HPW and 20 % Pt/C (JM). Electrolyte: 0.1 mol L−1 HClO4; frequency scan: 0.01 Hz–100 kHz; potential: 0.75 V versus NHE

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XPS measurements were used to distinguish the electronic properties apart from the chemical states and the surface compositions of the catalysts. The XPS spectra obtained for the different catalysts are plotted in Fig. 8. It is obvious that after the incorporation of HPW, W4f , P2p , and N1s peaks appear in the XPS spectrum of Ag@Pt/MWCNTs-25 % HPW (Fig. 8a, b, e, f), which are attributed to HPW and CS. The XPS results reconfirm that HPW molecules have been successfully fixed on the surface of Ag@Pt/MWCNTs. The W4f spectra of Ag@Pt/MWCNTs-25 % HPW are presented in Fig. 8b. The W4f peaks can be divided into two types of doublets with W4f7/2 centered at binding energies of 36.5 [W(IV)] and 38.3 eV [W(VI)], and the curve-fitting data are listed in Table 3. In Ag@Pt/MWCNTs-25 % HPW, tungsten exhibits mixed chemical states with surface contents of W(IV) and W(VI).

Fig. 8
figure 8

a XPS survey spectrum of Ag@Pt/MWCNTs and Ag@Pt/MWCNTs-HPW catalysts; b W4f of Ag@Pt/MWCNTs-HPW; c Pt4f of Ag@Pt/MWCNTs-HPW; d Pt4f of Ag@Pt/MWCNTs; e P2p of Ag@Pt/MWCNTs-HPW; f N 1s of Ag@Pt/MWCNTs-HPW

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Table 3 XPS spectra data of the catalysts
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The Pt4f spectra of Ag@Pt/MWCNTs-25 % HPW and Ag@Pt/MWCNTs are presented in Fig. 8c, d, respectively. As compared with Ag@Pt/MWCNTs, the binding energy of Pt4f shifts negatively after incorporation of HPW. The existence of HPW in Ag@Pt/MWCNTs leads to a downward shift in the d-band center of Pt and, therefore, HPW can weaken the adsorption of the oxygen-containing species OHads on the surface of the Pt NPs and increase the catalytic activity of Ag@Pt/MWCNTs-HPW electrocatalyst [37].

According to the above results, the mechanism of ORR on Ag@Pt/MWCNTs-HPW is summarized in Scheme 2. As described in our previous study [39], O2 easily accepts e− and H+ to form OHads and undergoes double adsorption on the surface of Pt (Pt-OH, Scheme 2a). The double adsorption is the rate-determining step of ORR, but the surface oxide formation (Pt + H2O → Pt-OH + H+ + e−) hinders further adsorption of O2. The Ag@Pt structure can weaken the adsorption of the OHads on the surface of the catalyst. However, the catalyst doped with HPW is more able to reduce the adsorption energy of the Pt-OH (Scheme 2b). On one hand, W–O–Pt bonds can form between W and the oxygenated Pt species because the d orbitals of W (5d 46s 2) have a stronger ability to accept electrons than that of Pt (5d 96s 1), and electrons are transferred to HPW from the oxygenated Pt species (Scheme 2c). Pt, after losing OHads, can adsorb O2 continuously, resulting in acceleration of the ORR and improvement of the activity of the catalyst. Contrarily, HPW accepts protons and electrons to form H4HPW (HPW + 4H+ + 4e− → H4HPW) (Scheme 2c) [55], and then O2 chemisorbed onto the H4HPW surface is reduced to H2O (H4HPW + O2 → HPW + H2O) (Scheme 2d) [56]. The removal of the oxygen-containing OHads from the surface of the Pt accelerates the double adsorption of O2 and increases the electron transfer coefficient of the ORR rate-determining step, leading to the enhancement of the activity and stability of the catalyst for ORR [57]. Therefore, the synergic effect of the HPW and the Ag@Pt augments the rate of electron transfer and increases the catalytic efficiency for ORR.

Scheme 2
scheme 2

Schematic diagram for mechanism of ORR on Ag@Pt/MWCNTs-HPW nanostructure

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4 Conclusion

In this study, an HPW-modified Ag@Pt/MWCNTs electrocatalyst was successfully prepared using the chemical impregnation method. Physical characterization indicated that the HPW molecules inserted into Ag@Pt/MWCNT formed a uniform structure, and the diameter of the catalyst was about 4.0 nm. Electrochemical investigation results indicated that Ag@Pt/MWCNTs catalyst incorporation with 25 % HPW displayed excellent electrocatalytic activities, which is attributed to the high utilization of Pt and the protective effect of the HPW layer on the catalyst surface. The synergic effect of the HPW and Ag@Pt enhanced the rate of electron transfer and increased the catalytic efficiency of oxygen reduction reaction, influencing 4e− reduction reactions on Ag@Pt/MWCNTs-HPW catalysts. Thus, Ag@Pt/MWCNTs-HPW can be used as a promising cathode catalyst for ORR.