2.1. Physicochemical Properties of Carbon-Supported Pt–BeO
The morphology of carbon-supported Pt–BeO (Pt:Be mole ratio of 1:2.3) was investigated by TEM, as shown in Figure 1
. Pt particles with an average diameter of 4 nm are distributed on a carbon support. Although the existence and amount of Be can be confirmed by either ICP or XPS, locating the position of BeO in the TEM image is not straightforward because the heat treatment temperature was not so high as to render BeO to a crystalline form, and it is assumed that amorphous BeO was formed on the carbon support. Indeed, although their system is not exactly the same as ours, Lim et al.
] reported that amorphous Be-rich oxide layer becomes crystalline BeO in between 500 and 600 °C. In contrast, the morphology of Pt–BeO (1:11.5) was fairly different from that of Pt–BeO (1:2.3) in that Pt nanoparticles were hardly seen with merely the detection of amorphous layers (the inset in Figure 1
is a TEM image of Pt–BeO (1:11.5) on carbon support). For the purposes of comparison, we synthesized carbon-supported BeO without Pt to examine the morphology of BeO more easily. The morphology of Pt–BeO (1:11.5) was very similar to BeO on carbon support, and we understand that the amorphous nature of BeO can make it difficult to distinguish BeO particles from the underlying poorly-crystallized Ketjenblack.
The XPS analysis in Figure 2
shows that a peak value of the binding energy of Be matches that of beryllium oxide, and oxygen can be bound with Be or carbon. This could indicate that Be exists as an oxide rather than forming an alloy with Pt.
2.2. Electrochemical Properties of Carbon-Supported Pt–BeO
is a linear sweep voltammetry result comparing the HOR activity of carbon-supported Pt and Pt–BeO with different BeO content (Pt-2.3BeO and Pt-11.5BeO). The HOR current of the Pt-2.3BeO catalyst is slightly higher than that of the Pt catalyst, indicating that the Pt–BeO catalyst can be used as an anode catalyst of PEMFCs. However, if an excessive amount of BeO is around Pt as in the case of Pt-11.5BeO, the HOR current is significantly reduced to an impractical level of HOR activity. This reduction in HOR activity can be attributed to a loss in electrochemical surface area (ESA) of Pt blocked by the excessive amount of BeO, as can be confirmed in Figure 4
, where the ESA of Pt-11.5BeO is indeed negligible compared to that of Pt-2.3BeO. The BET specific surface area (SSA) and ESA of the catalysts are summarized in Table 1
. Regarding the stability of an anode catalyst in a PEMFC environment, the catalyst stability at potentials close to 0 V vs.
RHE is usually important considering anode operating potential ranges. However, the catalyst stability at potentials above 1 V would need to be confirmed in case of a potential reversal incident. The current level of both Pt-2.3BeO and Pt-11.5BeO at potentials between 0.05 V and 1.05 V in Figure 4
was maintained for the first 50 cycles, which could indicate the stable nature of BeO in the PEMFC environment.
is a CO stripping voltammetry result, which compares the CO tolerance of carbon-supported Pt, Pt-2.3BeO, Pt-11.5BeO, and PtRu. Initially adsorbed CO on the catalysts at 0.05 V is oxidized by increasing potential, and their oxidation peak or onset potentials are compared. First, Pt shows a peak oxidation potential around 0.85 V, which is a similar value to Schmidt et al.
], and that of Pt-2.3BeO is ~0.75 V, which is lower than the peak oxidation potential of Pt by 100 mV. However, Pt-11.5BeO with excessive BeO has no oxidation peak, which would indicate that there are no available Pt sites for CO adsorption, resulting in absent CO oxidation peaks. On the other hand, the oxidation peak potential of a commercial PtRu is measured to be 0.55 V, which is the most CO-tolerant among the samples. Although Pt–BeO is not so CO-tolerant as PtRu, it is evident that an addition of an optimum amount of BeO to Pt makes Pt–BeO CO-tolerant compared to Pt.
Meanwhile, while a significant amount of hydrogen can adsorb on Pt–BeO (as seen in the hydrogen oxidation current in Figure 3
), the lower coverage of CO is simultaneously observed as deduced by the lower CO oxidation peak. This can be more significant than the shift of the oxidation potential as compared to Pt. The latter is not always related to the strength of the bond of CO with the Pt surface but also to the potential at which the oxidic species are formed on the surface. In fact, the lower CO coverage accompanied by the increase in hydrogen coverage can be a strong indication of the lower binding strength of CO on the Pt surface, which is affected by the BeO deposits. Kolla et al.
] also pointed out that the direct removal of CO from the Pt surface dominates the sequential CO oxidation process, where hydroxyl species surface diffusion is involved when metal oxides are introduced to PtRu/C.
The origin of BeO-induced CO tolerance is not clear yet, but the following possibilities could be considered. First, the bifunctional mechanism could operate as in other CO-tolerant metal oxides such that the adsorption of hydroxyl species is enhanced on the BeO side, and CO on the Pt side is combined to CO2
through surface diffusion. Another CO conversion scheme to formaldehyde in the presence of BeO has also been suggested [19
]. Stoffelsma et al.
] reported that CO oxidation at Pt is promoted in the presence of Be ions in alkaline solutions, which would be more or less related to the origin of BeO-induced CO tolerance in this study.