CV and ORR of [Pt n(100)x(111)]

Results

Figure 4. Voltammetry of [Pt n{100}x{111}] surfaces in contact with 0.1 M perchloric acid electrolyte, sparged with N2 and an applied scan rate of 50 mVs-1_.

Figure 4 shows the voltammetric profiles of [Pt n{100}x{111}] stepped surfaces in 0.1 mol dm-3 perchloric acid. The most stepped surface, {311}, lies on the turning point in the zone and therefore could equally be considered as belonging to both the [Pt n{100}x{111}] and [Pt n{111}x{100}] series of surfaces (where n = 2). Here though, it shall be considered as a Pt[2{100}x{111}] as its ORR activity will be shown to fit in with the [Pt n{100}x{111}] trend. The voltammetry of this series matches previous reports [30, 31].

The features of the voltammetry of the basal plane Pt{100} surface, as well as the other voltammetry in the series are summarised in reference [31]. With regards to the Pt{100} surface shown in black in figure 4, charge displacement studies indicate that peaks below 0.4 V are associated with Hads formation and the ‘bump’ between 0.4 - 0.6 V being ascribed to anion

adsorption, most likely OHads in perchloric acid [31, 32]. In contrast to Pt{111}, where there is

separation of these two process, Pt{100} exhibits significant overlap of Hads and OHads [33].

Much less work theoretical has been carried out concerning the structure of the Pt{100}- OHads/H2O interface in comparison to Pt{111}. Nevertheless, DFT predicts a complex OHads-

H2O interfacial layer exhibiting long-range interactions, despite the mismatch between

Pt{100}’s square atomic arrangement and water’s preferred hexagonal arrangement [34]. This interfacial layer was found to be important in modelling Pt{100}’s ORR activity [34].

Increasing the step density in this series causes many changes in voltammetry. Firstly, Hads

occurring at Pt{100} terraces diminishes at 0.3 V and a new peak (relating to the step) grows at 0.22 V, the same potential observed in the [Pt n{111}x{100}] series. As the symmetry of the Pt{100}x{111} step is also the same, the explanation used in the previous section will be used for the nature of this peak. Namely, the replacement of step Hads with OH/Oads. Increasing

step density also diminishes the charge associated with OHads at {100} terraces and shifts it to

more positive potentials. This results in separation of previously overlapping Hads and OHads

features, as also noticed in reference [31]. This separation becomes obvious at Pt{911}, where n = 5, and complete separation of the two features is observed in the voltammetry from the Pt{511} surface, where n = 3. These two surfaces exhibit a current minimum at ~0.4 V, between the Hads and OHads terrace features. This positive shift in OHads could be interpreted

as disruption of an ordered {100}-OHads/H2O terrace interfacial layer by the presence of steps,

interfacial layer is not present, the positive potential shift still implies weaker OHads binding

upon increasing step density. The onset potential of OHads formation has been shown in many

studies to be a determining factor in ORR activity [35, 36].

The large peak at ~0.95 V is associated with the electrooxidation of OHads at {100} terrace sites

and therefore, diminishes with step density. Oads is the likely product of this oxidation. This

feature will not be considered in great detail as it occurs at a potential where the oxygen reduction activity has already diminished to zero. The peak centred at 0.85 V (again the same oxidation potential observed in the [Pt n{111}x{100}] series) grows with step density and is due to further oxidation of the step site. As this step site is considered to be already covered by OH/Oads, further oxidation should increase the Oads coverage.

Results

Figure 5. Rotating disc electrode oxygen reduction reaction voltammetry in 0.1 M perchloric acid of [Pt n{100}x{111}] surfaces shown in figure 4. A 1600 rpm rotation rate, 30 mVs-1 _{scan rate and 1 atm oxygen} pressure were implemented. (a), top, shows the whole potential range used, whereas (b), bottom, shows a close

up of the kinetic/mixed kinetic diffusion regimes.

[Pt n{100}x{111}] surfaces give rise to lower ORR activity in perchloric acid than the [Pt n{111}x{100}] and [Pt n{111}x{111}] series, as seen from figure 3 and in other studies [6, 7, 22]. The activity of the [Pt n{100}x{111}] series (see figure 5, above) also diminishes over a larger potential range than [Pt n{111}x{100}], ~ 0.6 - 0.9 V in comparison with ~ 0.7 - 0.9 V. Figure 4 shows that 0.6 V corresponds with completion of the OHads layer on Pt{100}.

Therefore, OHads electrosorption at terraces is concurrent with the initial decrease in activity,

as was observed with Pt{111}. Pt{13,1,1} shows negligible change in ORR activity and activity begins to increase at Pt{911}, where n = 5 is the average terrace width. At potentials less than 0.8 V, a general increase in activity is observed upon increasing step density which correlates well with the positive shift in OHads formation discussed above. Therefore, a bond

strength weakening of site blocking {100}-OHads and/or the disruption of a stable {100} terrace

OHads/H2O interfacial layer are consistent explanations for the increase in activity observed at

these potentials.

Between 0.8 - 0.9 V, the electrooxidation at step sites exerts a substantial effect on ORR activity and the general increase in activity with step density is no longer seen. In this potential range the Pt{311} surface displays the lowest activity, lower even than Pt{100}, and also the largest oxidation peak at ~0.85 V. Again it is shown that oxidation at steps causes this great decrease in activity at low overpotentials.

An alternative explanation for the trends observed in ORR activity over both the [Pt n{100}x{111}] and [Pt n{111}x{100}] electrode surfaces is that the step sites are more active than terraces. Increasing step density increases would then increase the amount of high activity step sites, resulting in a positive shift of the ORR curve. These high activity steps sites are then blocked as they are oxidised at larger potentials. It is currently not known whether either terrace or step sites dominate the ORR activity, or whether they exhibit comparable activity.