Oxygen reduction reaction (ORR) is the core reaction of many promising energy conversion technologies, such as fuel cells, metal-air batteries etc. [1–4]. Pt/C has been widely recognized as the commercial and commonly adopted electrocatalysts for efficiently catalyzing ORR due to its moderate desorption-adsorption energy of Pt for ORR intermediate products, thus leading to a desirable electrocatalytic performance [5–7]. So the detailed investigation of Pt/C preparation technology has been attracted more attention for its future mass production [8–9, 44, 45]. As of note, the preparation of carbon-supported Pt-based catalysts via polyol method is a common method [10]. In the preparation process, polyol is not only the reducing agent, but also acts as the protective agent and dispersant [11, 12], thus making the as-prepared catalysts homogeneous with high dispersion. Here, taking the reduction of H2PtCl6 by ethylene glycol (EG) as an example, the reaction mechanism can be described as follows:
EG can be firstly decomposed into H2O and CH3CHO under the activation of external energy [13]. Then CH3CHO is used as the reducing agent to reduce Pt4+ ions into Pt nanoparticles. The activation methods of external energy mainly include pulse microwave method [14–16], reflux method [17, 18], and solvothermal method [19–21]. Among them, microwave method calls for high requirements for equipment, especially for mass production, though it possesses the advantage of short reaction time [22]. In addition, reflux method suffers from a long heating time [23, 24]. Fortunately, solvothermal method, adopting high-temperature and high-pressure as reaction conditions, possesses strong anti-interference ability, simple operation, fast reaction rate and easy to realize industrial production, thus leading to a growing attention [25–27]. It is worth emphasizing that the alkaline content in the precursor mixture has a significant effect on the electrocatalytic ORR activity of the obtained Pt/C catalysts. Too little alkaline content leads to the insufficient negative ions in precursor mixture solution, resulting in a bad dispersion of Pt species. On the contrary, excessive alkaline content will be not propitious to the deposition of Pt nanoparticles onto carbon support. Therefore, it is imperative to track the change path of Pt-containing species and reveal the coordination mechanism during the preparation process of precursor mixture, further quantify the added alkaline content, which is of great significance for large-scale preparation of highly efficient Pt/C catalyst.
Regalbuto et al. [28, 29] demonstrated that the Pt-containing species in H2PtCl6 aqueous solution varied according to the content of alkaline in the solution, which was determined by the extended X-ray absorption fine structure characterization technology. Siani et al. [30] found that the coordination between the water and H2PtCl6 might occur in H2PtCl6 aqueous solution. Also, Pt4+ maintained octahedral coordination in the hydrolysis process of H2PtCl6 and pH had a great influence on this process [30]. It can be seen that most of the already reported studies on the above-mentioned challenge were carried out in a mixed system containing a large amount of H2O, while science researches have done a lot and made remarkable progress in this issue. On the other hand, excessive water introduced in the preparation system will lead to a bad dispervisity of the Pt species, and thus increase the particle size of Pt, leading to the poor electrocatalytic activity. Furthermore, the heating temperature and time during the Pt/C preparation process via a solvotherml assisted EG reduction method will have an immense influence on the physical and electrocatalytic properties of the final catalysts as demonstrated in the succeeding section as appropriate.
Inspired by this viewpoint, in the pure EG system, the variation of Pt species in the H2PtCl6 precursor solution with the increment of the added NaOH content were investigated by innovatively using Ultraviolet-visible spectroscopy (UV-vis) technology and H+ concentration detector. As investigated, the neutralization reaction takes place firstly once a little NaOH is added into the H2PtCl6 precursor solution. With the NaOH content increasing, the Pt species gradually changes from [PtCl6]2– to [PtCl6–x(OH)x]2– until all Cl– ions are replaced by OH– ions, and simultaneously the coordination of Pt-EG changes into the coordinations of Pt-EG and Pt-H2O, at this point, the coordination of Pt species is considered to be completed and the mass ratio between NaOH and Pt (m(NaOH):m(Pt)) gets to approximately 2. Next, the heating temperature and time during the Pt/C preparation process via solvotherml assisted EG reduction method were optimized. From the characterizations result, it can be found that the 20-(1/22)-140-2 Pt/C (Pt loading: 20 wt%; m (Pt): m (NaOH) = 1/22; heating temperature: 140 ℃, reaction time: 2 h) exhibits better electrocatalytic ORR activity with respect to the commercial 20 wt% Pt/C (E-TEK). This work reveals the coordination mechanism of Pt species during the Pt/C preparation process via solvotherml assisted EG reduction method, qualifies the alkaline content, providing abundant theoretical reserve and technical accumulation for mass production of highly efficient Pt/C catalysts for ORR.
Preparation of precursor mixture: in a 100.0 mL beaker, an appropriate amount of H2PtCl6-EG solution was added with stirring. After 3 h, 1.0 M NaOH-EG solution was added drop by drop to the above mixture with a total volume of 60.0 mL reaction soltuion. It is noted that the mass ratio of the added NaOH content and Pt denotes as m (NaOH):m(Pt).
The Pt/C catalysts were prepared by a solvotherml assisted EG reduction method with H2PtCl6 as Pt precursor. It was carefully prepared by controlling the main synthetic parameters, such as NaOH content in precursor mixture, heating temperature and time. A typical preparation consisted of the following step. In a 100.0 mL reactor, 80.0 mg of Vulcan XC-72 carbon with 25.0 mL of EG was added and then sonicated. Next, 5.10 mL of 3.92 mgPt/mL H2PtCl6·6H2O in EG was added into the mixture and stirred for 3 h. After that 11.0 mL of 1.0 M NaOH/EG was added dropwise with a mass ratio of m(Pt):m (NaOH) = 1:22 and the final total volume was 60.0 mL. The reaction system temperature was raised to 140 ℃ and kept at this temperature for 2 h. After that, the system was cooled down to room temperature, and the pH value of the suspension was adjusted to 2.0 by adding 0.5 M H2SO4, then stirring overnight. The suspension was filtered and washed by hot deionized water till no Cl– was detected in the filtrate, and then the cakes were dried at 80 ℃ for 12 h in vacuum. The finally obtained 20.0 wt% Pt/C catalysts were denoted as 20-(1/22)-140-2.
Transmission electron microscopy (TEM) was obtained on Tecnai G2 Spirit electron microscopy. Specimens were prepared for TEM analysis by ultrasonically suspending the catalyst powder in ethanol. A drop of the suspension was then applied onto clean holy copper grids and dried in air. X-ray diffraction (XRD) was obtained on the Rigaku (D/max-IIIA) with Cu-Kα radiation (l ¼ 1.5406 Å). UV-vis spectrophotometry was applied to trace the coordination process on a PUXI Tu-1901 recording spectrophotometer.
Electrochemical experiments were carried out on AUTOLAB electrochemical work station (Auto84480) with a standard three electrode cell in 0.5 M H2SO4 aqueous solution at room temperature. The Pt/C catalyst slurry was prepared by sonicating for 1 h in a mixture of 5.0 mg of Pt/C catalyst, 0.40 mL of deionized water, 0.55 mL of isopropanol, and 0.05 mL of Nafion® solution (Aldrich: 5 wt% Nafion). Then Pt/C catalyst slurry (6.0 μL) were dropped onto the surface of glassy carbon (GC) electrode (d = 0.5 cm) to form the thin film electrode with a catalyst loading of 30.6 µg Pt/cm2 as the working electrode. A Pt foil and a saturated calomel electrode (SCE) were used as the counter and reference electrode, respectively. Without specification, all potentials were referenced to reversible hydrogen electrode (RHE), which was obtained from Eq. (1) [31].
Where 0.242 V is the standard potential of SCE reference electrode in saturated KCl aqueous solution at 25 ℃).
For the preparation of precursor mixture, NaOH-EG solution was added into the H2PtCl6-EG solution dropwise. From the change of pH value in precursor mixture (Fig. 1.), it can be obtained that the VNaOH-pH is different from the general neutralization curve of acid-base titration that always presents s sudden curve shape, which is approximately linearly correlated, demonstrating that not only the acid-base neutralization reaction of NaOH and H2PtCl6 in pure EG system occurs. Simultaneously, the corresponding CH+ decreases sharply with the increment of VNaOH. When the added VNaOH gets to 0.50 mL, at this point, the corresponding m (NaOH): m (Pt) (originates from H2PtCl6)) is 1:1. Accordingly, the CH+ in the precursor mixture has been reduced by nearly 95%, identifying that the acid-base neutralization reaction takes place firstly in the system.
In order to illustrate the coordination mechanism of Pt species during the preparation of precursor mixture, UV-vis technology was employed and the results are shown in Fig. 2. The UV adsorption spectra of H2PtCl6-EG solution without adding NaOH-EG solution in Fig. 2(a) exhibits two obvious peaks at 218 and 250 nm, corresponding to Pt-EG coordination and [PtCl6]2– species [32], respectively. From Fig. 2(b), it can be found that the absorption peak intensity of [PtCl6]2– species decreases with the increment of the added NaOH content, and simultaneously new absorption peaks corresponding to [PtCl6–x(OH)x]2– species appear between 268 and 277 nm, indicating that Cl– in [PtCl6]2– can be replaced by OH– until the absorption peak of [PtCl6]2– completely disappears [33]. Fig. 2(c) manifests an obvious absorption peak at 218 nm for H2PtCl6-EG solution corresponding to Pt-EG coordination. As of note, the peak intensity of Pt-EG coordination apparently decreases with the m(NaOH):m(Pt) increasing, meanwhile, a new absorption peak at 230 nm appears, corresponding to Pt-H2O coordination [33]. The variation of the coordination of the Pt species is reasonably attributed to the fact that the little H2O produced from the acid-base neutralization reaction between H+ in H2PtCl6 and OH– in NaOH partially replaces EG to coordinate with Pt. After the m(NaOH):m(Pt) gets to 2:1, further increasing the NaOH content, the coordination of Pt-EG-H2O is almost unchanged, at this point, it can be concluded that the coordination of Pt species in the precursor mixture completes.
Obviously, alkaline content in the precursor mixture during the preparation process of Pt/C has significant effect on the coordination of Pt species and consequently it affects the physiochemical and electrocatalytic ORR performance of the as-prepared catalysts, which inspires us to optimize and then qualify the added alkaline content under the guarantee of the fixed heating temperature (140 ℃) and time (2 h). As shown in Fig. 3 and Table 1, whatever the mass percentage of Pt in the catalysts, the electrocatalytic ORR performance of the Pt/C firstly increases and then decreases with the added NaOH content increasing, demonstrating the importance of the suitable NaOH content. The optimized m(Pt):m(NaOH) for 20, 40 and 60 wt% Pt/C catalysts is 1:22, 2:24 and 3:26, respectively. Considering that the total volume of the reaction system remains unchanged, when the Pt loading increases, it can be seen that as long as the content of NaOH, which is used for the coordination of the increased H2PtCl6, increases by 1:2, the redundant NaOH without coordination remains unchanged, finally realizing the quantification of the added NaOH. This not only verifies the optimized m (NaOH): m (Pt) for the coordination of Pt species as mentioned in Fig. 2, but also provides great significance for the industrial mass production of high-efficiency Pt/C catalyst with the pre-calculated amount of the raw materials.
The heat-treatment temperature plays a crucial role in the proprties of Pt/C catalysts prepared by solovthermal assisted EG reduction method. Here, we investigated the properties of the 20-(1/22)-X-2 (X represents the heating temperature) as a function of pyrolysis temperature in the range of 130‒150 ℃ to optimize this parameter. Fig. 4(a) shows the 20-(1/22)-X-2 Pt/C electrocatalysts are successfully prepared at different heating temperatures. Also, with the increment of the heating temperature, the half-peak width of Pt (220) decreases, illustrating that the particle size of Pt increases (Fig. 4(b)). The change of the particle size of Pt with the heating temperature increasing are summarized as in Table 2. When the heating temperature is 140 ℃, the particle size of Pt is 3.2 nm, which is among the best reported particle size of ~3.0 nm [34–36].
As depicted in Fig. 5 and Table 3, the 20-(1/22)-140-2 Pt/C manifests higher onset potential (Eonset = 0.95 V), half-wave potential (E1/2 = 0.82 V), limiting current density (JL = –4.74 mA/cm2) and ECSA (ECSA = 58.03 m2/g), much better than those of 20-(1/22)-130-2 and 20-(1/22)-150-2. Consequently, the optimized temperature is 140 ℃.
Similarly, the effect of heating time on the properties of the obtained 20-(1/22)-140-X (X represents heating time) was also investigated. As exemplified in Fig. 6 and Table 4, it can been found 20-(1/22)-140-X are successfully synthesized, and the increment of heating time results in higher particle size of Pt. When the heating temperature is 2 h, the particle size is calculated to be 3.2 nm according to the Sherrer formula (see Table 4), which is within the optimized size range reported [37–39].
From Fig. 7 and Table 5, it can obtained that the onset ORR potential of the 20-(1/22)-140-X firstly increased, and then gradually decreases, reaching its peak value at 2 h. At this point, the 20-(1/22)-140-2 Pt/C exhibits higher ORR onset potential (Eonset = 0.95 V) and half-wave potential (E1/2 = 0.82 V), indicating the optimized heating time is 2 h. In summary, the optimized heating temperature and time is 140 ℃ and 2 h, respectively. This condition is just we used to explore the effect of added alkaline content on the performance of the Pt/C catalysts.
According to the above characterizations and discussions, it can be concluded that when the heating temperature is too low or the heating time is too short, the Pt species in the precursor mixture cannot be completely reduced to Pt crystalline grains. Also, in this condition, the Pt crystalline grains originating from already reduced Pt species cannot be fully aggregated and grown, thus leading the Pt nanoparticles with too small particle size, further decreasing the specific surface activity of the as-prepared catalysts [42]. By contrast, when the heating temperature is too high or the heating time is too long, the reduction rate of the Pt species in the precursor mixture is too fast, the Pt crystalline grains is easy to be aggregated and grown excessively, resulting in the Pt nanoparticles with too large particle size, decreasing the specific surface area of the obtained catalysts, ultimately leading to a poor electrocatalytic ORR performance [43].
From Fig. 8, it can be seen that for both the commercial 20 wt% Pt/C (E-TEK) and the 20-(1/22)-140-2 Pt/C prepared via solvothermal assisted EG reduction method Pt nanoparticles are uniformly dispersed on the surface of XC-72 carbon support. It is worth pointing out that the average particle size and distribution range of the 20-(1/22)-140-2 Pt/C is slightly smaller and narrower with respect to that of the commercial 20 wt% Pt/C (Fig. 9), identifying that the 20-(1/22)-140-2 Pt/C possesses better dispersivity, and thus leading to a better electrocatalytic ORR activity.
In order to further determine the accurate Pt loading in the catalyst, thermogravimetric (TG) tests were carried out on the commercial 20 wt% Pt/C and the 20-(1/22)-140-2 Pt/C. As shown in Fig. 10, it can be observed that the TG curves of these two catalysts is almost the same, carbon and light H2O in the catalyst is almost completely removed when the heating temperature in air gets to 500 ℃, and the proportion of the remaining Pt is very close to the theoretical loading of 20 wt% [40, 41].
Comparing the electrocatalytic ORR activity of the 20-(1/22)-140-2 Pt/C with the commercial 20 wt.% Pt/C catalyst, it can be obviously found that the ORR onset potential (Eonset = 0.95 V) of the 20-(1/22)-140-2 Pt/C increases by 50 mV over the commercial Pt/C (Eonset = 0.90 V) (Fig. 11(a)). In addition, its corresponding kinetic current density of the 20-(1/22)-140-2 at 0.9 V is 3.69 times higher than that of commercial Pt/C (Fig. 11(b)). Therefore, the 20-(1/22)-140-2 Pt/C possesses better electrocatalytic activity.
In this work, the variation of Pt species in the preparation of Pt/C catalyst via a solvothermal assisted EG reduction method was studied by adopting UV-vis and H+ concentration detector. As investigated, the Pt species in the precursor mixture change from the Pt-EG and [PtCl6]2– specie to the Pt-EG, Pt-H2O and [PtCl6–x(OH)x]2– species with increasing the NaOH content, and when the m (NaOH): m (Pt) gets to 2:1, the coordination of Pt species in the precursor mixture completes, further achieving the qualification of the added NaOH content. Next, the heating temperature and time during the preparation process of Pt/C were optimized. Consequently, the 20-(1/22)-140-2 Pt/C exhibits higher electrocatalytic ORR activity than commercial Pt/C in acidic media. This work is of great significance for industrialized mass production of efficient Pt/C catalyst.
2020,
Vol. 41 Issue (5): 820-829 DOI: S1872-2067(19)63456-X 
PDF
2020,
Vol. 41 Issue (5): 820-829 DOI: S1872-2067(19)63456-X PDF