Various T off for pulse electroplating

Previously, several reports have identified the nitroso compounds of Ru to be promising complexes as the Ru sources for PtRu electrodepositions.[31, 35, 36] When dissolved in an acidic electrolyte, a spontaneous Ru deposition was not observed in nitroso precursors as compared to the typical RuCl₃.[87] This contributed to a longer lifetime and minimal bath management. In addition, Gavrilov et al. indicated that the presence of nitroso ligands shifts Ru/Ru(III) redox potential to more positive values.[88] Therefore, with the selection of nitroso precursor, similar Pt:Ru ratios are expected to be obtained in both deposit and solution states. This is especially critical because previous studies using the RuCl₃ and RuCl₅ reported substantial Pt enrichments with respect to their concentration ratios in the electrolyte. Consequently, their fabrications of desirable PtRu compositions hinged on empirical determination entirely.

Our preparation steps of nitroso Ru precursor followed earlier documentation in which the dissolution of RuCl₃ in excess NaNO₂ at elevated temperatures was carried out.[89] The resulting complex was confirmed by Blake et al. to be Ru(NO)(NO2)4(OH)²⁻.[90] However, in our electrolyte we also added H₂PtCl₆ after the formation of Ru nitroso complex. Because the molar ratio for Pt:Ru:NaNO2 was 1:1:10, we surmise that the simultaneous presence of PtCl62−

and Pt(NO2)xCl6−x2−

is likely. The color for the plating bath is in light yellow. This is in sharp contrast with the

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electrolytes of RuCl₃ and RuCl₅, which appear in dark brown.

For the pc depositions, intricate interactions between Ton, Toff, and Ja exert significant influences over the PtRu catalyst loadings, compositions, distributions, as well as particle sizes.

Therefore, careful scrutiny in the plating variables would be necessary. In our first set of experiments, pc depositions were conducted at various T_off, while the T_on and J_a remained unchanged at 50 ms and 50 mA/cm², respectively. Because we maintained the total charge constant at 8.0 C/cm², the entire deposition process lasted approximately 4–35 min. Table 3.1 lists the experimental parameters as well as results from TEM and ICP-MS for the PtRu nanoparticles. As presented, the PtRu loadings were in the range of 67.6–128.5 µg/cm². Following faradaic law, the coulombic efficiencies were estimated at 1.8–3.6%. These reduced values are attributed to the parasitic hydrogen evolution occurring on the Pt surfaces. Currents from the capacitive charge and discharge were not expected to be substantial, as the effective working range for the capacitance component were less than 1.0 ms for T_on and T_off.[75] We observed a notable trend for the PtRu composition on different pulses. The ratio for the Pt in the PtRu nanoparticles increased considerably with a longer T_off. At the shortest T_off of 100 ms, we determined the composition to be Pt52.7Ru47.3. In contrast, at the largest Toff of 600 ms, the makeup was confirmed as Pt83.4Ru16.6.

Table 3.1. Results from materials characterizations on the PtRu nanoparticles with fixed values of T_on (50 ms), J_a (50 mA/cm²), and total coulombic charge (8.0 C/cm²).

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TEM observations on the particles sizes indicated a slightly wider distribution as compared to those from conventional chemical reduction methods. For example, at a Toff of 200 ms, the PtRu nanoparticles were an average of 12.9 nm with a standard deviation of 8.7 nm. Moderate size distributions are typical, because the nucleation and growth took place during each individual pulse.

A recent report by Bennett et al. also observed similar behaviors when they prepared Pt nanoparticles on diamond thin films.[91] Figure 3.1 provides the representative TEM images for the PtRu nanoparticles from T_off of 400 and 600 ms, respectively. The average size of the PtRu nanoparticles from the Toff of 400 ms was 4.1 nm, while the Toff of 600 ms revealed a somewhat larger size of 11.3 nm. These TEM images also indicated that the PtRu nanoparticles were dispersed uniformly on the carbon substrates with negligible aggregation.

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Figure 3.1. Representative TEM images for the PtRu nanoparticles with fixed values of Ton (50 ms), Ja (50 mA/cm²), and coulombic charge (8.0 C/cm²), as well as Toff of (A) 400 and (B) 600 ms.

Fig. 3.2 exhibits the CV profiles in mass activity for the PtRu catalyzed electrodes at various T_off. Critical information from the CV responses, including the onset potentials, peak current (i_a) and potential (Va) at anodic scans, peak current (ic) and potential (Vc) at cathodic scans, as well as values for ECSA, are listed in Table 3.2. For the samples with T_off in 100 and 200 ms, their CV curves revealed moderate current outputs. This is unexpected, because compositions for these two samples

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were Pt_52.8Ru_47.2 and Pt_54.6Ru_45.4, respectively. In contrast, the CV profiles of the remaining samples demonstrated obvious anodic signals with relatively reduced cathodic signals, suggesting considerable abilities for the methanol electro-oxidation. In addition, the onset potentials became smaller when the amount of Ru was increased, a fact that is consistent with what was reported in literature that alloying with Pt promotes methanol electro-oxidation. The ratio for the i_a/i_c indicates the capabilities to remove CO after methanol dehydrogenation. Among these samples, the one with T_off of 400 ms demonstrated the highest value. As expected, for the mass activities, the sample with Toff in 400 ms exhibited the highest value of 213 mA/mgPt. Moreover, the general trend for the ECSA was consistent with that of i_a, in which a larger ECSA is associated with a higher i_a.

Table 3.2. Electrochemical parameters from the CV scans in mass activity of the PtRu-Catalyzed carbon cloths with fixed values of Ton (50 ms), Ja (50 mA/cm²), and total coulombic charge (8.0

a) potential at peak current density in anodic scan b) peak mass activity in anodic scan

c) potential at peak current density in cathodic scan d) peak mass activity in cathodic scan

e) ECSA from hydrogen desorption data

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Figure 3.2. CV profiles in mass activity for the PtRu-catalyzed carbon cloths with fixed values of Ton (50 ms), Ja (50 mA/cm²), and coulombic charge (8.0 C/cm²), as well as Toff of (a) 100, (b) 300, (c) 500, (d) 200, (e) 400, and (f) 600 ms.

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