Electrochemical degradation of Nafion ionomer

Fig. 5.2 provides the CV profiles at various cycles for the electrodes containing carbon cloth, Nafion ionomer, and XC-72R with the supply of ambient oxygen. As shown, the CV profiles exhibited a characteristic behavior for capacitors with symmetric responses in which considerable anodic and cathodic currents were observed above 0.9 V and below −0.1 V, respectively. Notably, the current from the anodic scan for the first cycle was negligible until 0.9 V when a sharp rise occurred. After that, there appeared obvious currents for the cathodic scan, suggesting surface activation at an oxidative potential above 0.9 V in the first cycle. Interestingly, both the anodic and

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cathodic currents demonstrated steady increments with increasing CV cycles. We understood that the recorded currents were mostly from the XC-72R as the carbon cloth contributed an insignificant amount with its relatively reduced surface area. However, in our observation, samples of XC-72R deposited on the carbon cloth revealed CV curves that were insensitive to increasing cycles, a generic behavior for electrochemical double-layer capacitors. Therefore, we realized that there was chemical degradation of Nafion ionomer that led to the increasing currents.

-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2

Figure 5.2. Profiles from multiple CV scans with ambient oxygen for electrodes containing carbon cloth, XC-72R, and Nafion ionomer.

In order to observe the effect of Nafion ionomer degradation more clearly, we need to remove the capacitive currents from the XC-72R. Therefore, we carried out additional experiments with the electrodes containing carbon cloth and Nafion ionomer only. Fig. 5.3(a) exhibits the CV profiles for the samples with the supply of ambient oxygen. As shown, there appeared obvious oxidation and reduction peaks centering around 0.55 and 0.34 V, respectively. In addition, these signals increased steadily with increasing cycles. According to literature, these peaks were attributed to hydroquinone-quinone redox couple on the carbon substrates, suggesting the formation of oxygenated functional groups on the surface.[150-152] Also shown is the carbon cloth without the

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addition of Nafion ionomer but with the oxygen supplied from ambient. Obviously, there was negligible current in the CV scans, indicating that without Nafion ionomer, oxygenated functional groups on the carbon surface were not formed at noticeable amount.

Earlier studies on the Nafion membrane degradation have identified the hydroxyl (‧OH) and peroxy (‧OOH) radicals to be the active species to attack the chemical structure of Nafion. It was suggested that the dissolved oxygen diffuses to the anode side reacting with the hydrogen to form hydrogen peroxide.[20, 153] In our case, with sufficient supply of ambient oxygen, the CV scans in an acidic environment on the carbon electrodes were likely to initiate the oxygen reduction reaction by a two-electron route which led to the formation of hydrogen peroxide.[60, 162] This hydrogen peroxide subsequently engendered the decomposition of Nafion ionomer that further accelerated the oxidation of carbon. To verify the significance of oxygen in this process, we repeated the experiments with the electrodes containing carbon cloth and Nafion ionomer but without the supply of ambient oxygen. The elimination of oxygen was achieved by immersing the working electrode to the electrolyte completely in a sealed three-electrode cell in conjunction with sufficient argon purging to remove any dissolved oxygen. The resulting CV profiles are displayed in Fig. 5.3(b).

Interestingly, there was negligible difference for the CV responses between the first and 20^th cycle, and the absence of the hydroquinone-quinone redox couple was obvious. This behavior indicated that without the simultaneous presence of oxygen and Nafion ionomer, the formation of oxygenated functional groups on the carbon surface was rather unlikely. An alternative approach to produce the hydroxyl radical (‧OH) is the direct oxidation of water.[20] This is a scenario that is possible in the CV scans without the supply of ambient oxygen. However, from Fig. 5.3(b) we concluded that the direct oxidation of water was unable to produce sufficient hydroxyl radicals (‧OH) for Nafion iomomer degradation. Therefore, the principal cause for the formation of oxygenated functionalized groups on the carbon surface was the oxygen reduction route that engendered the decomposition of Nafion ionomer.

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To further validate the contributory role of Nafion ionomer and oxygen for carbon functionalizations, additional experiments on the carbon cloth and Nafion ionomer were carried out.

We performed the CV scans with and without the supply of ambient oxygen, and recorded their anodic currents at the 20^th cycle. Fig. 5.4 exhibits the comparison for the anodic current at 0.5 V for both samples, as well as data from Fig. 5.2 and Fig. 5.3(a), respectively. Apparently, without the supply of ambient oxygen, the anodic current became relatively subdued for every sample. In general, the presence of oxygen promoted the oxidation of carbon and hence resulted in a larger oxidation current. However, with the addition of Nafion ionomer, the effect of oxygen became more pronounced. It is therefore concluded that the degradation of Nafion ionomer, promoted by the presence of oxygen, led to accelerated carbon functionalization.

1E-5

Figure 5.4 Comparison in the current value obtained at 0.5 V from the 20th CV cycle for electrodes containing carbon cloth (CC), Nafion ionomer, CC/Nafion ionomer,and CC/XC-72R/Nafion ionomer. These CV experiments are performed with ambient oxygen and without ambient oxygen, respectively.

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