1 Introduction

Proton exchange membrane fuel cells (PEMFCs) are considered as highly potential energy convention devices without emission of pollution [1,2,3]. The challenge for PEMFCs is the poor kinetics of the cathodic reaction that is several orders of magnitude slower than that of the anodic reaction [4,5,6,7]. Platinum is an ideal catalyst to promote the kinetics of cathodic reaction, but its scarcity and high cost limit its wide applications [8,9,10]. Hence, it is important to develop Pt-based alloy electrocatalysts with less platinum and high performance. Current strategies usually focus on modifying the microstructures of the electrocatalysts to enhance activity and durability. For example, fabricating the microstructures, such as nanotubes [11], nanocages [12,13,14], nanowires [15,16,17], nanoframes [18,19,20] and nanodendrites [21, 22], can lead to more active sites and higher specific electrochemical surface area. Reducing the particle size of the catalysts can promote the utilization of Pt, which can enhance the mass activity (MA) and specific activity (SA). However, reducing the size will lead to the agglomeration of particles and further growth due to the Ostwald ripening [23,24,25]. Indeed, platinum has the superior ability to weaken the binding energy of the oxygen reduction reaction intermediates, which limits the kinetics of the cathodic reaction [26, 27]. To decrease the amount of Pt and improve the oxygen reduction reaction (ORR) activity, combining Pt with some non-precious metals is an effective strategy to tune the ORR activity.

Previous reported results have demonstrated that alloying Pt with 3d transition metals such as Fe, Co and Ni will downshift the d-band state, which could change the electronic structure of Pt-based catalysts, promoting high ORR activity and durability [28,29,30,31,32,33]. Among various Pt-based alloys, Pt–Co catalysts are considered as promising ORR electrocatalysts due to high activity and reasonable durability [34, 35]. However, dispersing agents such as surface-active substances have been generally employed during the synthetic process, and it is hard to remove them in the post-treatment process [36, 37], which might have a negative effect on ORR activity. In addition, the prepared catalysts need to be loaded on carbon or other supports before electrochemical tests [38, 39]. Herein, we deployed a facile strategy via one-pot synthesis without adding surfactant to prepare a series of carbon-supported PtxCo (x = 1, 2, 3) catalysts. The particle size of as-prepared PtxCo/C catalysts was around 4 nm, and the shape of particles was uniform. All the as-prepared PtxCo/C (x = 1, 2, 3) catalysts showed improved ORR activity and durability compared to the commercial Pt/C catalyst. Notably, the Pt3Co/C catalyst showed remarkable durability compared to the commercial Pt/C catalyst and its analogs (PtCo/C and Pt2Co/C catalysts).

2 Experimental

2.1 Materials

Chloroplatinic acid hexahydrate (H2PtCl6·6H2O, Aladdin), cobalt chloride hexahydrate (CoCl2·6H2O, Aladdin), ethylene glycol (EG, 99%, Aladdin), carbon black (EC300, Cabot Corporation), acetaldehyde (99%, Aladdin), Nafion (5%, Dupont), and ultrapure water (18.2 MΩ·cm, 25 °C) were used in the experiment.

2.2 Preparation of PtxCo/C catalysts

50 mg EC300 was scattered in 100 ml ultrapure water by ultrasonicating for 30 min till thoroughly dispersed. After that, 50 ml EG solution containing 120.5 mg H2PtCl6·6H2O, 18.5 mg CoCl2·6H2O and 0.2 mmol·L−1 acetaldehyde was added into the carbon solution suspension to form a mixed solution, which was then stirred for 1 h at 80 °C. The mixed solution was filtered, washed with ultrapure water and dried in oven, and finally, Pt3Co/C catalyst was attained. At the same time, PtCo/C and Pt2Co/C catalysts were attained via the same method only by varying the Pt and Co ratios.

2.3 Physical characterization

Transmission electron microscope (TEM) images were investigated by FEI TECNAI F20 operated at 200 kV. The crystal structure of the catalysts was checked by X-ray diffraction (XRD) on a Bruker D8 Advance. The electronic structure information was recorded by X-ray photoelectron spectroscopy (XPS). The metal content of PtxCo/C was measured by inductively coupled plasma (ICP).

2.4 Electrochemical measurements

The electrochemical tests were performed on a CS300 electrochemical station with a standard three-electrode cell at room temperature. The working electrode was a rotating disk electrode (RDE). The active area of the electrode was 0.196 cm2. The counter electrode was a Pt wire, and the reference electrode was a reversible hydrogen electrode (RHE). The electrolyte was 0.1 mol·L−1 HClO4 aqueous solution. The cathode was prepared as follows: 30 mg sample, 5 ml ethanol, 5 ml ultrapure water and 120 μl 5 wt% Nafion were mixed thoroughly by ultrasonic method. 2 μl catalysts ink was pipetted on the clear glassy carbon disk. Cyclic voltammetry (CV) tests were recorded in N2-saturated HClO4 (0.1 mol·L−1) from 0.05 to 1.05 V at a scan rate of 50 mV·s−1. Liner scan voltammetry (LSV) tests were conducted in O2-saturated HClO4 (0.1 mol·L−1) at a scan rate of 5 mV·s−1 under 1600 r·min−1 between 0.30 and 1.05 V. The accelerated degradation tests (ADTs) were conducted at a sweep rate of 100 mV·s−1 from 0.6 to 1.2 V in N2-saturated environment.

2.5 PEMFC single-cell tests

The single-cell tests were carried out in a fuel cell system. The Pt3Co/C catalyst, isopropanol, ultrapure water and 5 wt% Nafion were mixed by ultrasonication to obtain the catalyst ink. Then, we sprayed the catalyst ink on one side of a Nafion 211 membrane as cathode. The commercial Pt/C was loaded to the anode. The Pt loadings were 200 and 400 μg·cm−2, respectively, at the cathode and anode. Membrane electrode assembly with an area of 5 cm2 was fabricated by assembling a gas diffusion layer, anode and cathode. The temperature of the cell was controlled at 75 °C. The back pressures of the cathode and anode were 70 and 80 kPa, respectively. The anode was fed with humidified hydrogen and cathode with air.

3 Results and discussion

3.1 Composition, structure and morphology characterization

The real metal content of PtxCo/C catalysts was recorded by ICP, as shown in Table 1. The measured Pt/Co ratios for PtCo/C, Pt2Co/C and Pt3Co/C catalysts are 54:46, 69:31 and 76:24, respectively, which are approximately equal to the ideal values. The results demonstrate that the one-pot method is effective for preparation of PtxCo/C catalysts.

Table 1 Compositions of PtxCo/C catalysts
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Figure 1 shows XRD patterns of PtxCo/C catalysts and commercial Pt/C catalyst. For PtxCo/C, obvious face-centered cubic (fcc) structure is indexed. The slight peak at 2θ = 25° belongs to C(002) crystal plane, and the other four diffraction peaks at around 39.8°, 46.2°, 67.6° and 81.4° correspond to Pt(111), (200), (220) and (311) planes, respectively [40, 41]. Incorporating Co into Pt lattice would shorten Pt–Pt distance due to compressive strain. XRD patterns show that the platinum characteristic diffraction peaks of the PtxCo/C catalysts shift positively compared to those of commercial Pt/C, indicating the lattice shrinkage. Besides, the shift of the Pt peaks increases with Pt contents decreasing, demonstrating that a number of Co atoms are incorporated into the Pt lattice to form an alloy phase [42,43,44], which can be further confirmed by that no any characteristic peaks of pure Pt or Co are observed.

Fig. 1
figure 1

XRD patterns of carbon-supported and PtxCo/C catalysts

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Figure 2 shows TEM morphologies of the PtxCo/C catalysts. It can be seen that PtxCo/C catalysts are evenly distributed on the surface of carbon support. The average particle diameters of PtCo/C, Pt2Co/C and Pt3Co/C are (3.45 ± 0.42), (3.92 ± 0.52) and (4.25 ± 0.88) nm, respectively. This indicates that the current synthesis method is available to obtain the PtxCo/C catalysts without adding any other surfactants.

Fig. 2
figure 2

TEM images and corresponding size distribution diagrams: a1, a2 PtCo/C; b1, b2 Pt2Co/C; c1, c2 Pt3Co/C

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3.2 XPS analysis

The electronic structure information of the Pt alloys with Co was investigated by XPS in Fig. 3. It can be seen that the Pt 4f peaks of PtxCo/C positively shift compared with Pt/C, which can be ascribed to the decrease in Pt–Pt distance caused by the electronic structure modification of Pt [45,46,47]. As is well known, the catalysts activity and durability are influenced by the chemical adsorption/desorption energy between oxygen species and Pt. The positive shift of Pt 4f spectra indicates that alloying Pt with Co would weaken the interaction between Pt and oxygen species. The influence benefits to expose more catalytic sites on the catalyst surface, leading to promoted catalysts electrocatalytic activity. The Pt 4f peaks of Pt-based catalysts can be splitted into two peaks, which are regarded as Pt 4f5/2 and Pt 4f7/2 [48, 49]. The binding energy of Pt 4f spectra for PtxCo/C and Pt/C is shown in Table 2. In terms of Pt3Co/C, the peaks located at 71.62 (Pt 4f7/2) and 75.00 eV (Pt 4f5/2) are assigned to pure Pt0, while the peaks located at 72.50 (Pt 4f7/2) and 76.10 eV (Pt 4f5/2) are attributed to Pt2+ species. The proportions of Pt0 and Pt2+ are determined by XPS, showing that the metallic Pt0 proportion (71.34%) of Pt3Co/C on the surface is much higher than that of Pt/C (57.12%). Same phenomenon is observed for PtCo/C and Pt2Co/C. It has been demonstrated that Pt0 is the most active valence state of Pt for oxygen reduction reaction. The higher the proportion of Pt0 is on the catalyst surface, the more the Pt catalytic sites are for oxygen reduction, which is the reason that as-prepared PtxCo/C catalysts exhibit remarkably improved kinetics for the cathodic reaction as compared with the commercial Pt/C catalyst. As shown in Fig. S1, the Co 2p3/2 peak of PtxCo/C is located at around 780.0 eV, corresponding to Co0. The Co0 2p peak shows a positive shift in comparison with bulk Co (778.3 eV), confirming the electron donation from Co to Pt.

Fig. 3
figure 3

XPS analysis for Pt 4f spectra: a Pt/C; b PtCo/C; c Pt2Co/C; d Pt3Co/C

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Table 2 Binding energy (BE) of Pt 4f spectra and percentages of Pt0 and Pt2+ in PtxCo/C and commercial Pt/C catalysts
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3.3 Electrochemical performance

Electrocatalytic activities of PtxCo/C catalysts were assessed by CV and LSV tests by comparing with commercial Pt/C. In order to remove the impurities on the catalysts surface and activate the active sites of the catalysts, we scanned several cycles from 0.05 to 1.05 V before recording CV curves. The CV curves are shown in Fig. 4. The characteristic peaks in the potential between 0.05 and 0.50 V are assigned to the hydrogen adsorption/desorption on Pt (Fig. 4a), which we use to calculate the specific electrochemical surface area (ECSA). The specific electrochemical surface area (ECSA) was calculated from the following equation:

$${\text{ECSA}} = \frac{Q}{{0.21*m_{\text{Pt}} }}$$
(1)

where Q is the Coulombic charge associated with hydrogen desorption (mC), \(m_{{{\text{Pt}}}}\) is Pt loading (mg). The ECSA values are 74.48, 68.09 and 55.09 m2·g−1 for PtCo/C, Pt2Co/C and Pt3Co/C, respectively, which are lower than that of commercial Pt/C (80.46 m2·g−1), indicating that the size of nanoparticles has some influences on ECSA. The characteristic potential peak between 0.6 and 1.0 V is assigned to oxidation/reduction of Pt. For PtxCo/C, the peak at around 0.9 V is more positive than that of Pt/C. This phenomenon indicates that the bond between oxygen intermediate such as OHad or OOHad and Pt in the PtxCo/C catalysts is weaker, which is helpful to improve ORR catalytic activity. Figure 4b shows ORR polarization curves of PtxCo/C and commercial Pt/C catalysts. The half-wave potential (E1/2) of PtCo/C, Pt2Co/C and Pt3Co/C are 907, 902 and 900 mV, higher than that of Pt/C (866 mV). This proves that alloying Pt with Co leads to improved ORR activity. To further evaluate electrocatalytic activities of the PtxCo/C and Pt/C catalysts, mass activities and specific activities at 0.9 V are calculated, as displayed in Table 3. The mass activities of PtCo/C, Pt2Co/C and Pt3Co/C are 0.526, 0.462 and 0.441 A·mgPt−1, which are, respectively, 2.60, 2.31 and 2.22 times higher than that of Pt/C (0.200 A·mgPt−1). The specific activities of PtCo/C, Pt2Co/C and Pt3Co/C are 706.59, 679.41 and 801.83 μA·cm−2, which are accordingly 2.89, 2.76 and 3.28 times higher than that of Pt/C (244.75 μA·cm−2). In addition, LSV curves of PtxCo/C at various rotation speeds and Koutechy-Levich (K-L) plots are shown in Fig. 4d. The Koutechý-Levich equation is as follows:

$$\frac{1}{j} = \frac{1}{{j_{{\text{k}}} }} + \frac{1}{{j_{{\text{d}}} }} = \frac{1}{{j_{{\text{k}}} }} + \frac{1}{{0.62nFcD^{2/3} v^{ - 1/6} \omega^{1/2} }}$$
(2)

where \(j_{{\text{k}}}\) is kinetic current density, \(j_{{\text{d}}}\) is diffusion-limiting current density, \(n\) is the number of electrons transferred per oxygen molecule, F is Faraday constant (96,485 C·mol−1), \(c\) is O2 concentration (1.26 \(\times\) 10–6 mol·cm−3), D is oxygen diffusion coefficient in the electrolyte (1.9 \(\times\) 10–5 cm2·s−1), \(v\) is kinematic viscosity of the electrolyte solution (1.009 \(\times\) 10–2 cm2·s−1), and ω is the electrode rotation speed (rad·s−1). The K-L plots were calculated at different potentials from 0.8 to 0.9 V. The transfer numbers of PtCo/C, Pt2Co/C and Pt3Co/C are 3.99, 3.92 and 3.93, respectively, indicating that their oxygen reduction is 4e− transfer reaction. The Tafel slopes that reflect the kinetics of electrochemical reactions were extracted from curves of Pt/C and PtxCo/C. As shown in Fig. S2, PtCo/C (87 mV·dec−1), Pt2Co/C (91 mV·dec−1) and Pt3Co/C (93 mV·dec−1) show lower Tafel slopes than Pt/C (96 mV·dec−1) in the low overpotential region (> 0.8 V), indicating that PtxCo/C catalysts show more beneficial kinetics.

Fig. 4
figure 4

a CV curves in N2-saturated 0.1 mol·L−1 HClO4 scanning at 50 mV·s−1; b LSV curves in O2-saturated 0.1 mol·L−1 HClO4 scanning at 5 mV·s−1; c mass and specific activities; d K-L plots of PtxCo/C and commercial Pt/C catalyst

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Table 3 ORR activities of PtxCo/C and commercial Pt/C catalysts
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The durability of PtxCo/C catalysts was evaluated by CV testing between 0.6 and 1.2 V (vs. RHE) in N2-saturated environment. LSV curves were recorded after 5000 cycles, as shown in Fig. 5. After 5000 cycles, the half-wave potentials of PtCo/C, Pt2Co/C, Pt3Co/C and Pt/C negatively shift by 18, 30, 18, 6 mV, respectively; their mass activities decay 50.4%, 30.0%, 12.1% and 21.5% compared with the initial values; and their specific activities decay 45.6%, 28.0%, 18.1% and 30.2%. Among the PtxCo/C catalysts, Pt3Co/C catalyst shows the highest activity and durability.

Fig. 5
figure 5

LSV curves of a commercial Pt/C, b PtCo/C, c Pt2Co/C, and d Pt3Co/C; comparison of e mass activities and f specific activities between PtxCo/C and commercial Pt/C before and after 5000 cycles with scanning rate of 100 mV·s−1 in O2-saturated 0.1 mol·L−1 HClO4

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3.4 PEMFC single-cell tests

We focused on Pt3Co/C catalyst and further evaluated its electrocatalytic activity in a single cell by comparing with Pt/C catalyst. Figure 6a shows I-V curves of electrodes consisting Pt3Co/C and Pt/C cathodes. Significantly, the maximum power density of the Pt3Co/C electrode is 0.931 W·cm−2, much higher than that of the Pt/C (0.834 W·cm−2). The Pt loadings of Pt3Co/C and Pt/C in cathode are 0.2 and 0.4 mg·cm−2, respectively (Table 4). The I-V curves of the electrodes are normalized to the Pt loading for further comparation. The Pt3Co/C shows superior maximum power density of 4.655 W·mgPt−1, which is 2.23 times higher than that of the Pt/C (2.085 W·mgPt−1).

Fig. 6
figure 6

a I-V curves of electrodes consisting Pt3Co/C and commercial Pt/C based cathodes; b I-V curves with current density normalized to Pt loading

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Table 4 Comparison between Pt3Co/C and Pt/C based cathodes in single cell tests
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4 Conclusion

In summary, we have successfully prepared a series of PtxCo/C catalysts by using a facile one-pot method without adding any surfactants. The as-obtained PtxCo/C nanoparticles are evenly distributed on the carbon support. All the PtxCo/C catalysts exhibit significantly improved mass activity and specific activity than the commercial Pt/C catalyst. Among them, Pt3Co/C shows the best electrochemical performance and durability. Its maximum power density in air-H2 fuel cell is 4.655 W·mgPt−1, which is 2.23 times higher than that of Pt/C. The remarkable improvement on the enhanced ORR activity and durability is attributed to the change of the electronic structure on the surface Pt by alloying Pt with Co. We believe that the present work can provide an effective strategy to improve the activity and durability of Pt-based catalysts for the commercialization of PEMFCs.