Аннотация:Hydrogen-air proton exchange membrane fuel cells are actively developing energy sources owing to their high specific power and environmental friendliness. However, one of the key limitations to their widespread implementation remains the high cost of the OOR catalysts that are traditionally platinum-based. 1,2 To reduce the cost and increase the activity of ORR catalysts, attempts has been made to use platinum alloys with transition metals, e.g., Ni, Co and Cu. 3-5 However, despite the relatively high catalytic characteristics, such materials have a significant drawback, viz., the dissolution of the transition metal during the operation of the catalyst, which is destructive for the proton exchange membrane because of its degradation caused by secondary processes involving cations of the dissolved metal. 6,7 Nevertheless, at present, research in this area are relevant and popular. 3,8-12 The main method of the synthesis of bimetallic nanoparticles containing platinum is a multi-step process of chemical deposition from salts of the corresponding metals. At the same time, electrochemical dispersion of metals is considered as a promising method for obtaining nanoparticles of both individual metals and alloys. 13-16 Here, a pulse electrochemical synthesis technique has been proposed to produce highly active catalysts based on Pt 70 Cu 30 alloy, which allows one to reduce the platinum content without the loss of efficiency. Few-layer graphene structures (FLGS) were obtained via the plasma-electrochemical exfoliation of graphite in 1 m Na 2 SO 4 solution as described earlier. 17,18 Then, electrochemical dispersion of Pt or Pt 70 Cu 30 alloy was carried out in 100 ml of 1 m NaOH solution containing 10 mg of the obtained FLGS. The synthesis of the composite based on Pt nanoparticles was carried out in the anodic-cathodic plasma mode (300 V/-150 V) that ensured the highest productivity and allowed one to get the most active catalysts based on platinum (Pt/FLGS). 19,20 Unlike Pt, this mode has not resulted in obtaining nanoparticles with a composition close to the original alloy by the sputtering of platinum-copper alloys. The probable cause of this is the selective dissolution of copper from the alloy caused by exposure to the anodic plasma, 7 that results in the depletion of the alloy in copper and its sputtering with the predominant formation of platinum nanoparticles and copper oxide/hydroxide. In contrast with pure Pt, for the alloy, using only cathode plasma has not resulted in the quantitative dispersion of the alloy. Nevertheless, the electrolytic (plasma-free) sputtering mode (100 V, 10 ms/-10 V, 2 ms) was found, where along with partial copper dissolution, alloy nanoparticles were formed and thus the PtCu/ FLGS catalyst was obtained. In all cases, such an amount of metal was sputtered to obtain a composite containing ca. 50 wt% of metal nanoparticles. It it worth mentioning that in the process of the alloy sputtering in the electrolytic mode, partial dissolution of copper during a long positive pulse should result in the 'loosening' of the alloy electrode surface, which cause the destruction of this surface layer during both positive and negative pulses. Further optimization of the pulse parameters was not performed; this would be the subject of further investigations. The SEM image of the PtCu/FLGS [Figure 1(a)] shows that partially agglomerated alloy nanoparticles (white colour) are distributed over the surface of thin-layer graphene structures (gray colour) with characteristic lateral dimensions of 0.1-0.7 μm. The TEM image of these particles [Figure 1(b)] indicates their heterogeneous composition with a characteristic size of ca. 5 nm. Moreover, for some particles, one can see a dark core covered by a light shell, which indicates the formation of core-shell type nanoparticles. According to the EDX analysis, the bulk content of Pt and Cu in the PtCu/FLGS composite is 18.0 and 8.0 at%, respectively, which is close to the ratio of metals in the initial Pt 70 Cu 30 alloy. 200 400 600 800 1000 E/mV (vs. RHE)-7.0-6.0-5.0-4.0-3.0-2.0-1.0 0.0 j /mA cm-2 O2 + 4e-+ 4H + ® 2H2O +100 V/-10 V PtCu Electrochemical dispersion of Pt 70 Cu 30 alloy implemented by applying repeated 100 V/-10 V voltage pulses is proposed as a one-step method to produce a composite electrocatalyst of the oxygen reduction reaction (ORR). A comparative study of the catalytic activity towards ORR of the obtained composites based on few-layer graphene structures decorated with PtCu nanoalloy and pure Pt nanoparticles was carried out. The prospects of using the electrochemical sputtering of PtCu alloy for the one-step production of effective electrocatalysts were substantiated.
