Degradation of Pt Catalysts based on Nanocomposites

5.3 Electrochemical Characterization of Pt Catalysts

Figure 57 Selected potential cycles during AST with inset of ECSA change for Pt/ITO–rGO (a) and ITO–rGO (b) and for Pt/FTO–rGO (c) and FTO–rGO (d).

Figure 58 contrasts cyclic and CO stripping voltammetry of Pt/ITO–rGO and Pt/FTO–rGO after potential cycling. CV curves show decreased Pt-related signals in both materials after stress testing. CO stripping experiments show decreased and altered signals of the carbon monoxide oxidation. While CO oxidation does not exhibit a notable change in shape and potential for Pt/ITO–rGO in Figure 58a, the CO oxidation on Pt/FTO–rGO in Figure 58b changed dramatically. The peak at 0.86 VRHE almost disappeared due to stress testing. The other peak at 0.76 VRHE is still present after testing, however, in narrower shape and with less intensity. In-stead, CO oxidation now partially appears in a lower potential range. This means that a distinct asymmetry of the signal at 0.76 VRHE on the lower potential side has arisen and a further very broad low intensity peak starts already at 0.4 VRHE.

This illustrates that degradation of platinum on FTO–rGO strongly weakens the CO adsorption. Especially the signal at 0.86 VRHE was assigned to CO interaction with platinum on rGO and is not present anymore after stress testing. An explana-tion could be that CO adsorpexplana-tion/desorpexplana-tion is stronger influenced by the FTO

particles after AST. To remember, an enhanced CO tolerance was assigned to SnO2 containing catalysts.^[141] It is postulated that known Pt degradation like disso-lution and re-precipitation events^{[15, 71]} occur during the potential cycling and that FTO competes with rGO during these Pt degradation events. This might influence the Pt degradation and re-precipitation. Investigation by IL-TEM in Sec-tion 5.3.3.3 later gives microscopic insights into the catalyst degradaSec-tion. But still we have to consider for interpretation of CO stripping experiments, that various impact factors play a role. Next to impact of the support, Pt surface sites are im-portant as well for CO adsorption/desorption.[250, 274] Pt/ITO–rGO in Figure 58a was also subjected to degradation, because the intensity of CO oxidation signal decreased after exposing to 1,000 potential cycles. However, degradation obvious-ly took place with less change of interactions between Pt and ITO–rGO due to comparable peak shape of CO oxidation in Figure 58a.

ECSA losses are listed in Table 13. Pt/ITO–rGO has a 20 % lowered ECSA using HUPD and a 34 % lowered ECSA using CO sorption. Pt/FTO–rGO has a 24 % reduced ECSA using HUPD and a 30 % reduced ECSA using CO stripping. Thus, an average loss of exactly 27 % is given for both composites and is highly compara-ble with Pt/rGO, which lost 26 % of ECSA. Although electrochemical surface are-as of Pt/ITO–rGO and Pt/FTO–rGO during their initial characterization were approximately one third lower than the area of Pt/rGO, differences towards the ECSA stability between Pt/rGO, Pt/ITO–rGO and Pt/FTO–rGO are not distin-guishable.

Figure 58 Comparison of cyclic and CO stripping voltammetry curves with insets of HQ/Q redox activities before and after AST. Pt/ITO–rGO (a) and Pt/FTO–

rGO (b).

5.3 Electrochemical Characterization of Pt Catalysts Figure 58 further shows insets with a zoomed potential range in CV curves, where HQ/Q redox activity is visible. For both composites HQ/Q signals are negligible before exposure to electrochemical stress (dashed line). For Pt/ITO–rGO in Figure 58a HQ/Q redox activity is arisen after AST (solid line). This indicates partial oxi-dation of the carbon surface in Pt/ITO–rGO. On the contrary, Pt/FTO–rGO in Figure 58b still shows the absence of such a signal after AST (solid line), so that no redox active oxygen species is formed during potential cycling.

Basically, HQ/Q redox activity is known for rGO^[272] and was shown for this study in Figure 46. And metal oxides were shown to suppress HQ/Q redox activity of rGO in Figure 54, where fresh Pt/ITO–rGO and fresh Pt/FTO–rGO were dis-cussed. This behavior stays unchanged for Pt/FTO–rGO but changed for Pt/ITO–

rGO during potential cycling caused by two possible reasons. First, EDS demon-strated that platinum does not prefer the deposition on ITO particles. In conse-quence, Pt must be deposited in higher extent on rGO and is known to catalyze carbon corrosion.^{[19, 127]} Second, Liu et al.^[205] reported disappeared parts of ITO after very similar potential cycling. Schmies et al.^[209] furthermore localized ITO dissolution and re-precipitation at potentials below 1.0 VRHE. Thus, oxygen sur-face groups of rGO may anchor less ITO particles so that HQ/Q redox activity becomes visible again. IL-TEM gives insights into the catalyst aging later.

Furthermore, Geiger et al.^[211] directly compared ITO and FTO dissolution in the fuel cell relevant potential window and reported higher stability of FTO. Indeed FTO reliably protects rGO from corrosion here, because HQ/Q redox reaction is still absent in Figure 58 even after harsh potential cycling. As a reminder, Pt/rGO without incorporated metal oxides had highly visible HQ/Q redox activity and an additional increase of QHQ due to stress testing.

Table 13 Change of electrochemical parameters due to stress testing on Pt catalysts based on nanocomposites.^{[230, 231]}

Method Parameter Pt/ITO-rGO

Pt/FTO-rGO

CV ECSAHUPD / % -20 -24

QHQ / % +568 0

CO Stripping

ECSACO / % -34 -30

CDL / % -10 -8

ORR

Eonset / % -1 -2

MA / % -25 -57

SA / % -7 -43

Figure 59 in combination with Table 13 shows the change in DL capacitances.

They are slightly reduced by -10 % and -8 % for Pt/ITO–rGO and Pt/FTO–rGO, whereas the capacitance of Pt/rGO in Table 11 was increased by +12 %. These op-posite trends must be traced back to the presence of ITO and FTO, respectively.

Furthermore, if the higher capacitance of untested Pt/rGO with 8.9 mF cm^-2 is compared to untested composites with 3.7 mF cm^-2 and 5.1 mF cm^-2, the absolute CDL change of Pt/rGO is more pronounced than changes of Pt/ITO–rGO and Pt/FTO–rGO. So, incorporation of metal oxides leads to a more constant double layer capacitance. For Pt/rGO, an increasing CDL can be caused by carbon corro-sion known for enhancing porosity and oxygen surface groups. For Pt/ITO–rGO and Pt/FTO–rGO, the very slight decreasing CDL might be caused by platinum particle or metal oxide particle aging. Larger particles due to degradation have of course lower surface areas.

5.3 Electrochemical Characterization of Pt Catalysts

Figure 59 Comparison of CV curves after CO sorption before and after AST.

Pt/ITO–rGO (a) and Pt/FTO–rGO (b).

Last, Figure 60 compares the ORR data. Both materials contain increased overpo-tentials for O2 reduction due to the stress test, which is visible through the negative curve shift to lower potentials. The insets show Tafel plots containing this potential shift. Tafel slopes are unchanged so that ORR mechanisms for both catalysts are not affected by the stress test. Changes in onset potential, mass activity and specific activity are listed in Table 13. On the one hand, the fresh FTO-containing catalyst showed a more than 30 % higher specific activity for ORR than fresh ITO-containing catalyst in Table 12. On the other hand, activity losses in Table 13 caused by degradation are more pronounced for Pt/FTO–rGO despite comparable ECSA losses. Pt/ITO–rGO lost 25 % of mass activity and 7 % of specific activity, while Pt/FTO–rGO lost 57 % of mass activity and 43 % of specific activity.

The very low change of specific activity of 7 % for Pt/ITO–rGO demonstrates the decreased ECSA as main cause for the activity loss. However, this is not the case for activity loss of Pt/FTO–rGO, whose specific activity changed much stronger by 43 %. Next to a change in ECSA, additional degradation effects must be relevant.

For instance, CO stripping experiments in Figure 58 showed remarkably changed CO sorption, which indicates changed electronic states in Pt–FTO–rGO interac-tion. Basically, d-electronical interaction appears between SnO2 and Pt nanoparti-cles,^{[25, 47]} which can change the electronic band structure of platinum and influ-ence the chemisorption of O2 or CO molecules. For ORR, downshifting the d -band state of platinum is reported to enhance the catalytic activity.^[67-69] Here, deg-radation of platinum takes place during the stress test. The paths illustrated in Fig-ure 6 could appear due to potential cycling and include Pt dissolution und

re-precipitation or the migration of Pt particles. Thereby, Pt anchoring to other sites of the support is assumed, so that especially d-electronical interaction between FTO substrate and Pt catalyst can be changed. This might affect the activity for ORR. Moreover, surface sites of FTO and Pt particles can alter provoked by po-tential cycling, which might cause a different catalyst–substrate interaction. Change of Pt and FTO particles regarding their location on rGO after the test is investigat-ed by IL-TEM in the next section.

Figure 60 Comparison of cathodic ORR scans at 1,600 rpm with insets of Tafel plots before and after AST. Pt/ITO–rGO (a) and Pt/FTO–rGO (b).

5.3.3.3 Identical Location TEM of Pt Catalysts based on