English

• 0.   Introduction

  With the gradual depletion of fossil energy, the research and development of new energy sources have become global population^[1-6]. Proton exchange membrane fuel cell (PEMFC) can efficiently use hydrogen energy, and has a higher energy conversion efficiency than internal combustion engine^[7-10]. At the same time, the product is water, producing no greenhouse gases and environmental harmful gases^[11-17]. However, PEMFC also faces a few problems, such as high cost of graphite bipolar plates, service life durability of metal bipolar plates not reaching 10 000 h, and difficulties in hydrogen storage and transportation^[18]. Among them, how to reduce the loading of platinum in fuel cell cathode and maintain high performance is one of the problems that needs to be solved urgently.

  The oxygen reduction reaction (ORR) at the cathode of PEMFC is the decisive step in the overall reaction and consumes more platinum than anode. Although there has been a lot of research and development of many non-Pt catalysts^{[14-15, 17, 19-23]}, having good performance to replace Pt in cathode, because of the acidic chemical environment in cathode, it is difficult for non-Pt catalysts to maintain their own stability^[24]. Therefore, for a period of time, cathode catalysts still need to use Pt/C catalysts. For the synthesis of Ptcontaining catalysts, there have also been many synthetic methods, some of which reduce the Pt loading by synthesizing PtM alloy catalysts^{[4, 25-38]} and show great performance. However, many of these methods use more expensive Pt precursors or oleylamine, increasing the cost and difficulty of subsequent processing, there^- by increasing the difficulty of industrialization.

  In this work, we synthesized a series of PtCo/C catalysts through a step-by-step synthesis method at a relatively mild condition. We first used liquid phase synthesis, using ethylene glycol as the reducing agent, to synthesize Pt/C catalyst precursor. Subsequently, in order to improve the performance of Pt/C catalyst, we used sodium borohydride to reduce Co on Pt/C sample, and then used 400 ℃ to form alloy between Pt and Co. Finally, nitric acid was used to etch the excess Co particles to obtain the final PtCo/C catalysts.

  1.   Experimental

  1.1   Synthesis

  First, we dispersed 200 mg black pearls 2000 (BP2000) conductive activated carbon in 30 mL of ethylene glycol, and sonicated the mixture for 20 minutes. Then, the mixture was added 3 mL benzaldehyde and sonicated for another 5 min. After that, 133.3 µL 100 mg·mL^-1 chloroplatinic acid solution was pipetted into the dispersed solution and sonicated for 20 min until the solution was uniform. The mixed solution was put into a microwave reactor and reacted with 800 W power for 2 min under stirring conditions. After the solution was naturally cooled, it was filtered by sand core funnel and rinsed with ethanol, 100 mL three times. Then, Pt/C catalyst was obtained.

  After drying 125 mg of the obtained Pt/C catalyst and 30 mg cobalt nitrate hexahydrate were dispersed in 20 mL of water and sonicated for 1 h. We took 4 mL 20 mg·mL^-1 NaBH₄ aqueous solution, and added it dropwise to the dispersed Pt/C precursor. The mixture was stirred for 12 h. The solid was filtered and dried. The dried sample was placed in a tube furnace and kept at 400 ℃ for 2 h under argon conditions, to get PtCo/C precursor. We used 1, 2, 3, 4 and 5 mL concentrated HNO₃ (mass ratio of 65%) to prepare 10 mL aqueous solution, which respectively processed 100 mg of the sample. And the samples used 1, 2, 3, 4 and 5 mL concentrated HNO₃ were stirred for 12 h, filtered and washed to obtain the final 1mLHNO₃-PtCo/C, 2mL HNO₃-PtCo/C, 3mLHNO₃-PtCo/C, 4mLHNO₃-PtCo/C and 5mLHNO₃-PtCo/C catalysts, respectivity.

  1.2   Material characterizations

  Transmission electron microscope (TEM) images of PtCo/C catalyst treated with different concentrations of nitric acid were obtained by Hitachi HD7700 TEM at 100 kV and high resolution TEM (HR-TEM) by a Titan ETEM microscope (FEI) working at 200 kV. Powder X-ray diffraction (XRD) patterns of PtCo/C catalysts were measured on a Rigaku Miniflex-600 operating at 40 kV voltage and 15 mA current with Cu Kα radiation (λ =0.154 06 nm) and 2θ =15° ~75°. Inductively coupled plasma mass spectrometry (ICP-MS) was used to measure the content of Pt and Co in PtCo/C catalysts.

  1.3   Membrane electrode preparation and fuel cell measurements

  100 mg each of the above five PtCo/C catalysts was added to a mixed solution of 3.00 mL deionized water, 4.00 mL isopropanol, and 1.26 mL Nafion solution (mass ratio of 5%). The mixture was sonicated for 2 h to make the catalysts evenly dispersed in the ink. The dispersed PtCo/C catalysts slurry was sprayed evenly on 3.5 cm×3.7 cm carbon papers. The weight gain of the carbon paper was about 15 mg each time so that PtCo/C catalysts loading was 15×65%/(3.5×3.7) = 0.75 mg·cm^-2, and Pt loading was 0.75w (w was the mass ratio of Pt). The anode adopted Johnson Matthey 20% Pt/C catalyst, and Pt loading was 0.15 mg·cm^-2. The sprayed catalyst-loaded carbon papers were tested by Qunyi 850e to obtain PEMFC performance data of PtCo/C catalysts.

  2.   Results and discussion

  2.1   Structure and morphology of PtCo/C catalysts

  TEM images (Fig. 1) show that excess Co particles in the samples that have not been treated with nitric acid can be seen clearly, while PtCo particles on the catalysts treated by nitric acid were relatively uniformly distributed on the BP2000 conductive activated carbon. The size of PtCo particles in 1mLHNO₃-PtCo/ C, 2mLHNO₃-PtCo/C, 3mLHNO₃-PtCo/C, 4mLHNO₃-PtCo/C, 5mLHNO₃-PtCo/C catalysts were 4.76, 4.58, 4.85, 3.87, 3.69 nm, respectively (Fig. S1, Supporting information).

  Figure 1

  

  Figure 1.  TEM images of (a) synthesized Pt/C precursor, (b) Pt/C precursor after reacting with Co(NO₃)₂·6H₂O, (c) PtCo/C precursor, (d) 1mLHNO₃-PtCo/C, (e) 2mLHNO₃-PtCo/C, (f) 3mLHNO₃-PtCo/C, (g)4mLHNO₃-PtCo/C and (h) 5mLHNO₃-PtCo/C catalysts

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  Through the XRD patterns (Fig. 2), it is found that before being treated by nitric acid, the sample′s XRD peak shifted to the right compared with the overall PtCo/C catalysts. This result is due to the presence of a large amount of Co. Then, mass fraction of Pt and Co were measured by ICP-MS and the molar ratio of Pt to Co was calculated (Table 1). So, Pt loadings of 1mLHNO₃-PtCo/C, 2mLHNO₃-PtCo/C, 3mLHNO₃-PtCo/C, 4mLHNO₃-PtCo/C and 5mLHNO₃-PtCo/C catalysts were 0.146, 0.141, 0.125, 0.134 and 0.145 mg·cm^-2, respectively. From both XRD and ICP-MS data, Pt existed in the form of Pt₃Co and a few Pt nanoparticles. 2mLHNO₃-PtCo/C catalyst had the highest peak at about 40.4°, illustrating that it exposes the most (111)crystal surfaces among Pt/C precursor and all of PtCo/C catalysts.

  Figure 2

  

  Figure 2.  XRD patterns of Pt/C precursor, Pt/C precursor after reacting with Co(NO₃)₂·6H₂O, 1mLHNO₃-PtCo/C, 2mLHNO₃-PtCo/C, 3mLHNO₃-PtCo/C, 4mLHNO₃-PtCo/C and 5mLHNO₃-PtCo/C catalysts

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  Table 1

  

  Table 1.  Mass fraction of Pt and Co, and molar ratio of Pt to Co calculated by ICP data

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  Sample	wPt/%	wCo/%	nPt∶nCo
  1mLHNO3-PtCo/C	18.50	1.04	5.38
  2mLHNO3-PtCo/C	18.74	0.93	6.08
  3mLHNO3-PtCo/C	18.04	0.77	7.06
  4mLHNO3-PtCo/C	17.01	0.59	8.67
  5mLHNO3-PtCo/C	17.80	0.56	9.60

  The energy-dispersive X-ray spectroscopy (EDX) (Fig. 3) shows the distributions of C, Pt and Co atoms. Co single atoms can be seen on the whole support and the existence of Pt-Co alloys can also be proved as well. The HR-TEM revealed the cubic crystal structures. The lattice fringes of Pt-Co alloys displayed the interplanar spacings of 0.228 and 0.229 nm in average (Fig. 4 and Fig. S3) ^[39-40], which proves the particles are Pt₃Co nanoparticles.

  Figure 3

  

  Figure 3.  EDS-mappings of 2mLHNO₃-PtCo/C

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  Figure 4

  

  Figure 4.  (a) HR-TEM image of 2mLHNO₃-PtCo/C catalysis; (b) Enlarged image of selected-area in (a)

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  2.2   Effect of amount of nitric acid on catalysts performance

  The performance of the catalysts was tested by Qunyi 850e fuel cell test device (Fig. 5). Among the obtained PtCo/C catalysts, 2mLHNO₃-PtCo/C catalyst had the best performance (Fig. 5f). Under H₂-O₂ condition, the maximum power can reach 740 mW·cm^-2 under normal pressure, 1 055 mW·cm^-2 under 50 kPa back pressure, 1 241 mW·cm^-2 under 100 kPa back pressure, and 1 340 mW·cm^-2 under 200 kPa back pressure. At the same time, high current was achieved under high voltage. Under 50 kPa back pressure, the current density at 0.9 V reached 44 mA·cm^-2, and the current density at 0.8 V reached 405 mA·cm^-2 (Fig. 5b).

  Figure 5

  

  Figure 5.  Single cell performance curves of (a) 1mLHNO₃-PtCo/C, (b) 2mLHNO₃-PtCo/C, (c) 3mLHNO₃-PtCo/C, (d)4mLHNO₃-PtCo/C and (e) 5mLHNO₃-PtCo/C catalysts at no back pressure, 50 kPa back pressure, 100 kPa back pressure, and 200 kPa back pressure, and (f) five kinds of PtCo/C catalysts at 50 kPa back pressure

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  Compared with JM Pt/C catalyst (Pt loading was about 0.2 mg·cm^-2), PtCo/C catalysts have a lower usage. What′s more, although PtCo/C catalysts cannot reach a higher power density than JM Pt/C, when the voltage was higher than 0.5 V, 2mLHNO₃-PtCo/C and 3mLHNO₃-PtCo/C catalysts had higher power density and current density. And the voltage region, higher than 0.5 V is the one which is used in PEMFC cars. So, in this case, a higher current can be reached under high voltage. The current magnitude under high voltage affects the start of PEMFC vehicles, and higher current will be more beneficial to the application of fuel cell cathode catalysts. From Fig. 6, PtCo/C catalysts also had great mass activity. At high voltage range, PtCo/C catalysts mass activities were higher than JM 20% Pt/C, so that′s one way to decrease the loading of Pt and keep and improve the activity of cathode catalyst.

  Figure 6

  

  Figure 6.  (a) Single cell performance curves of 2mLHNO₃-PtCo/C and 3mLHNO₃-PtCo/C, and (b) five kinds of PtCo/C catalysts′ mass activity compared with JM 20% Pt/C at 50 kPa back pressure

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  The catalytic performances of PtCo/C catalysts treated with different amounts of nitric acid were also different, and the catalytic performances showed a trend of increasing first and then decreasing. The activity of the catalyst treated with only 1 mL of nitric acid was not ideal, which may be caused by the high Co content on the surface of Pt nanoparticles. After the use of nitric acid exceeded 2 mL, the performance gradually decreased. 2mLHNO₃-PtCo/C catalyst exposed the most (111) crystal surfaces, leading to the highest activity. What′s more, in 2018, Liu pointed out in an article that PtCo particles in ORR had a synergistic effect with the Co single atom on the carrier and they jointly catalyze the reaction^[16]. Therefore, it is speculated that due to the increase of nitric acid used, the monoatomic Co on the carrier will decrease, resulting in a slight decrease in performance.

  2.3   Activation and stability of catalysts and reaction mechanism

  In fuel cell test of the catalysts, a certain period of activation was required at the beginning to achieve the highest performance. In industry, there are also certain requirements for the activation of fuel cell stacks. For example, it is necessary to control the activation time to control the amount of hydrogen used, thereby controlling the cost of fuel cell stack, because a large amount of hydrogen is consumed during activation. In this experiment, through the device, we made a cycle of V_oc-0.2 V-V_oc (V_oc =open circuit voltage) reciprocating test to activate the catalysts, under 50 kPa back pressure. After 6 cycles, 2mLHNO₃-PtCo/C catalysts can basically reach 90% of the maximum power shown in Fig. 7. It shows that 2mLHNO₃-PtCo/C catalyst can quickly show its own activity during the activation process. After 2mLHNO₃-PtCo/C catalyst performance reached the maximum value, in subsequent tests, the maximum power can still maintain under different back pressures, which shows great stability.

  Figure 7

  

  Figure 7.  Single cell performance curves of 6th and 50th compared with max power density cycle

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  As the linear sweep voltammetry (LSV) curves of 2mLHNO₃-PtCo/C catalyst (Fig.S5) and K-L plots (Fig. S5) display the value of electron transfer number (n) during ORR over PtCo/C catalyst is close to be 4, the mechanism is fourelectron reaction mechanism, which reduces oxygen to H₂O through four electron transfers^{[16, 21, 41]}. The four-electron reaction mechanism is divided into the reaction mechanism of association and dissociation in PEMFC. According to relevant literature reports^{[16, 41]}, Pt₃ Co adopts an association reaction mechanism, and the reaction takes place as follows:

  O₂+site → O₂-site

  O₂-site+H⁺+e^- → OOH-site

  OOH-site+H⁺+e^- → O-site+H₂O

  O-site+H⁺+e^- → OH-site

  OH-site+H⁺+e^- → H₂O+site

  3.   Conclusions

  In order to further reduce the Pt loading of PEMFC cathode catalyst, we synthesized a series of PtCo/C catalysts. Through further characterization, we found that Pt in PtCo/C catalyst exists in the form of Pt₃Co with a small amount of Pt nanoparticles. In the test, PtCo/C catalysts showed great performance. Among them, 2mLHNO₃-PtCo/C catalyst had the best performance. Under the condition of H₂-O₂, it reached the performance index of DOE2020 targets at high voltage. The current density at 0.9 V reached 44 mA·cm^-2, the current density at 0.8 V exceeded 300 mA·cm^-2, and the maximum power density exceeded 1 300 mW·cm^-2. We speculate that the reason for the highest performance of 2mLHNO₃-PtCo/C catalyst is that proper nitric acid treatment removes the excess Co particles in the catalyst, while retaining most of the Co single atoms, making the single atoms and Pt₃Co in the catalytic process, so that synergistic effect takes place. However, although the cathode Pt loading in the catalyst is relatively low, it has not yet reached the DOE2020 target of 0.125 mg·cm^-2. Therefore, in future research, how to reduce the Pt loading and improve the performance of the catalyst is still a worldwide problem, and more research is needed to solve it.

  
  Supporting information is available at http://www.wjhxxb.cn
  
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• 

  Figure 1  TEM images of (a) synthesized Pt/C precursor, (b) Pt/C precursor after reacting with Co(NO₃)₂·6H₂O, (c) PtCo/C precursor, (d) 1mLHNO₃-PtCo/C, (e) 2mLHNO₃-PtCo/C, (f) 3mLHNO₃-PtCo/C, (g)4mLHNO₃-PtCo/C and (h) 5mLHNO₃-PtCo/C catalysts

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  Figure 2  XRD patterns of Pt/C precursor, Pt/C precursor after reacting with Co(NO₃)₂·6H₂O, 1mLHNO₃-PtCo/C, 2mLHNO₃-PtCo/C, 3mLHNO₃-PtCo/C, 4mLHNO₃-PtCo/C and 5mLHNO₃-PtCo/C catalysts

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  Figure 3  EDS-mappings of 2mLHNO₃-PtCo/C

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  Figure 4  (a) HR-TEM image of 2mLHNO₃-PtCo/C catalysis; (b) Enlarged image of selected-area in (a)

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  Figure 5  Single cell performance curves of (a) 1mLHNO₃-PtCo/C, (b) 2mLHNO₃-PtCo/C, (c) 3mLHNO₃-PtCo/C, (d)4mLHNO₃-PtCo/C and (e) 5mLHNO₃-PtCo/C catalysts at no back pressure, 50 kPa back pressure, 100 kPa back pressure, and 200 kPa back pressure, and (f) five kinds of PtCo/C catalysts at 50 kPa back pressure

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  Figure 6  (a) Single cell performance curves of 2mLHNO₃-PtCo/C and 3mLHNO₃-PtCo/C, and (b) five kinds of PtCo/C catalysts′ mass activity compared with JM 20% Pt/C at 50 kPa back pressure

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  Figure 7  Single cell performance curves of 6th and 50th compared with max power density cycle

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  Table 1.  Mass fraction of Pt and Co, and molar ratio of Pt to Co calculated by ICP data

  Sample	wPt/%	wCo/%	nPt∶nCo
  1mLHNO3-PtCo/C	18.50	1.04	5.38
  2mLHNO3-PtCo/C	18.74	0.93	6.08
  3mLHNO3-PtCo/C	18.04	0.77	7.06
  4mLHNO3-PtCo/C	17.01	0.59	8.67
  5mLHNO3-PtCo/C	17.80	0.56	9.60

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•