4A.3 Electrocatalytic hydrogen evolution catalysis
The three CNT-supported Ru-based nanomaterials previously described (Ru@CNT, RuO2250@CNT and RuO2300@CNT) were suspended in THF (2 mg·mL-1), drop-casted onto the surface of a glassy carbon rotating disk electrode (RDE/GC), and led dry under air for HER studies. The so-obtained electrodes were called as Ru@CNT@GC, RuO2250@CNT@GC and RuO2300@CNT@GC. They were tested as working electrodes (WE) in 1 M H2SO4 degassed solution, in a three-electrode configuration with a Pt-mesh as counter (CE) and a saturated calomel (SCE) as reference (RE) electrodes.
First, polarization curves under reductive potentials were recorded for the three systems (Figure 7-Figure 8). NPs’ mean size determined by TEM analysis together with the composition of the NPs, namely Ru, RuO2 or a mixture of both, are summarized in Table 1, where electrochemical benchmarking parameters as the onset overpotential (η0), overpotential at |j| = 10 mA·cm-2 (η10) and Tafel slope (b) are also given. From the literature, it is generally accepted for a good HE catalyst to display both, η0 and η10
<100 mV, and the smallest possible b. Thus, Pt/C e.g., has a η0 ≈ 0 mV and η10 < 50 mV, depending on the loading, with b = 30 mv·dec-1. From Table 1 it can be seen that the three modified WE show high HER overpotentials (both η0 and η10, entries 1, 3 and 5) with η0 already >100 mV. However, a shift on the polarization curves was observed for the three electrodes after performing a bulk electrolysis experiment at fixed j = -10 mA·cm-2 (Entries 2, 4 and 6 in Table 1).
Figure 7. Left, polarization curves of Ru@CNT@GC (dashed red) and Ru@CNT@GC-r (dark red) before and after reductive process at j = -10 mA·cm-2 in 1M H2SO4, respectively; right, Ru@CNT@GC-r (dark red), RuO2250@CNT@GC-r (blue) and RuO2300@CNT@GC-r (green) after reductive process at j = -10 mA·cm-2 in 1M H2SO4.
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Table 1. Main physico-chemical and electrochemical data of the CNT-supported RuNPs for HER.
Entry System NPs’ mean
i These samples were treated under reductive conditions (j = -10 mA·cm-2) for 20 minutes. ii The oxidation state being not yet analyzed, it is an assumption given literature data as well as previous observations in the lab.
Indeed a decrease of 50 mV on both the η0 and the η10 is observed after the reductive bulk electrolysis experiment, leading to 150 and 220 mV, respectively, for Ru@CNT@GC-r (entry 2 in Table 1). It is worth mentioning that an analogous trend has been already observed using Ru1 nanomaterial (see Chapter 3C). In that case, XPS analysis evidenced the presence of only metallic Ru after bulk electrolysis, thus indicating the reduction of the passivating RuO2 layer in the applied electrocatalytic conditions. Given that, we can assume that a similar reduction happened here also and led to a Ru0-based nanomaterial. Such a phenomenon can explain the enhancement observed on the HER activity.
Concerning RuO2250@CNT@GC and RuO2300@CNT@GC, higher shifts are visible after the reductive electrolysis (see entries 3/4 and 5/6 in Table 1, respectively), this being especially pronounced for RuO2300@CNT@GC. With those nanomaterials that are initially mainly composed of RuO2, the activity enhancement observed after reductive bulk electrolysis may derive from the formation of a metallic Ru layer on the surface of the RuO2-NPs during the reductive process. If we compare with the data achieved with the full Ru0 nanomaterial (η0 ≈ 150 mV and η10 = 220 mV, entry 2), the values obtained with the reduced RuO2250@CNT@GC-r electrode (η0 ≈ 80 mV and η10 = 140 mV, entry 4) indicate a higher activity. Such a difference can be attributed to 1st) the formation of Ru0 species at the surface of the RuO2-based nanomaterial under reductive conditions, giving rise to a core/shell-like RuO2/Ru0 structure; and 2nd) the increase on the exposed surface derived from the formation of Ru0-species, which introduces irregularities on the crystalline structure. A similar behavior has been already reported by H. You et al.
in 2003,21 who studied the change on the catalytic activity induced by the formation of Ru0 sites on RuO2 (1 1 0) and (1 0 0) single crystal surfaces, after cathodic polarization.
Indeed, they observed an increase on the current density while cycling their
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4A
electrochemical set-up under reductive potentials that they correlated to a change of the crystalline structure evidenced by synchrotron X-Ray surface scattering, and thus suggested the reduction of the oxide surface as the driving force of the enhanced HER activity.
Figure 8. Polarization curves of RuO2@CNT systems before and after reductive process at |j| = 10 mA·cm-2 in 1 M H2SO4. Left, RuO2250@CNT@GC (dashed blue) and RuO2250@CNT@GC-r (dark blue);
right, RuO2300@CNT@GC (dashed green) and RuO2300@CNT@GC-r (dark green).
A similar catalytic behavior has been noticed with the RuO2300@CNT@GC system. In that case η0 and η10 of 130 and 320 mV are first observed, respectively (entry 5). After reduction treatment (entry 6), lower η0 ≈ 50 mV and η10 = 115 mV are reached, indicating a higher activity, as the result of the formation of Ru metal active sites and thus structure modification at the material surface. The values achieved are better than those previously obtained with RuO2250@CNT-GC-r electrode. In addition to the formation of Ru0-species on the surface of RuO2-crystals, the improving on the catalytic activity of RuO2-based materials compared to the Ru0-NPs precursor may rely on another parameter. The modification of both, the particles but also CNTs, may improve the electron transfer from the electrode to the active sites, thus increasing the electrocatalytic performance of the cathodic system.
Figure 9 (left) presents the Tafel plots obtained for the three electrodes after reductive treatment. The Tafel slope (b) allows defining the rate determining step (rds) of the catalytic reaction as described in Ch. 1. Ru@CNT@GC-r (dark red) shows a Tafel slope of 115 mv·dec-1, typical for catalysts having the Volmer step as rds (hydride adsorption on the surface of the NP, typically b ≈ 120 mv·dec-1). RuO2250@CNT@GC-r (dark blue) and RuO2300@CNT@GC-r (dark green) present Tafel slopes of 109 and 77 mv·dec-1, respectively, that reveal for both electrodes a situation in between the Volmer and Heyrovsky steps as rds (Heyrovsky step: H2 electrodesorption with a proton from the solution presents values of b ≈ 40 mv·dec-1).
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Figure 9. Tafel plot of left, Ru@CNT@GC-r (dark red), RuO2250@CNT@GC-r (dark blue) and
RuO2300@CNT@GC-r (dark green), and right, Ru@CNT@GC (red), RuO2250@CNT@GC (light blue) and RuO2300@CNT@GC (light green), in 1 M H2SO4.
Tafel plots of non-reduced systems Ru@CNT@GC (red), RuO2250@CNT@GC (light blue) and RuO2300@CNT@GC (light green), are shown in the right part of Figure 9 for comparison with those of the reduced systems. For both RuO2250@CNT and RuO2300@CNT, an important decrease on the Tafel slope after the reduction process can be thus observed, as expected for Ru0-species which are more active towards the HER than RuO2. In contrast, b ≈ 115 mv·dec-1,almost does not change in the case of Ru@CNT@GC. This can be explained by the fact that for Ru@CNT@GC before the reduction process, the activity is already attributed to Ru0 species that were not passivated when exposed to air, thus maintaining the kinetics after the modification of the superficial RuO2 to Ru0. In contraposition, RuO2250@CNT and RuO2300@CNT improve their kinetics due to the Ru0-species formation, and presumably due to the enhancement on the electron transfer.
In comparison to other electrocatalytic studies with Ru0/RuO2-based materials,5,6,7 the data reached with RuO2300@CNT@GC-r electrode show this material is the best system in this work and that its results are among the best ones reported so far (η0 ≈ 50 mV, η10 = 115 mV and b = 80 mv·dec-1), being even superior than those of Rub material presented in Chapter 3C (see Table A1 in the Annex part). In terms of η0, 50 mV is a small value, but the slow kinetics reflected by the high Tafel slope (further evaluated in the following paragraph) leads to a relatively high η10 value. If we compare these results with Ru-GC (from Chapter 3A; Ru-MeOH/THF system) and Ru1 (from Chapter 3C; Ru-0.2PP system), we can see that a similar η0 is observed as for Ru-GC (40 mV in that case), but the higher Tafel slope (46 mV·dec-1 for Ru-GC) leads also to a higher η10 (83 mV vs. 115 mV). This demonstrates that RuO2300@CNT@GC-r is not far from the MeOH/THF stabilized RuNPs. Nevertheless Ru1 outstands these values,
y = 289.49x + 29.784