Cyclic voltammograms of polycrystalline and the carbon supported Pt electrode (loading=0.02mg/cm2) were obtained for different concentrations of -caprolactam solutions in 0.1M HClO4 electrolyte to investigate the Pt contamination effects. Figure 6.6 shows the effects of -caprolactam on hydrogen desorption/adsorption at room temperature. The results show no sifnificant loss of ECSA (e.g. ECSA reduced about 10% in 0.4mM of -caprolactam in 125mL). However, the more loss of ECSA for polycrystalline Pt electrode would lead us to assume that the three phase carbon
supported nano particle size of Pt catalyst would robust contamination effect than plain Pt catalyst. We speculated that the carbon support acted as a filter due to its higher PZC compared to pH of electrolyte. For the Vulcan XC-72R carbon support, the point of zero charge (PZC) is 8.6 and RDE experiment conducted at pH 1 (i.e. 0.1M HClO4) so that the anion, carboxylic acid (-COOH) can be adsorbed onto the surface of carbon. The anion (- COO-) of -amino caproic acid could adsorb onto the surface of Pt as well as carbon
Figure 6.5 Comparison of ion-exchange isotherm of Nafion® membrane for (a) ammonium (b) sodium (c) aniline (d) -caprolactam
support so that polycrystalline Pt electrode showed more loss of ECSA than Pt/C
electrode system. The Nafion® ionomer used for Pt/C electrode RDE experiment may be other reason to show lower loss of ECSA since contaminants should penetrate through Nafion® ionomer to adsorb on the surface of Pt active sites.
The Pt oxidation and PtOx reduction peaks are decreased as -caprolactam is increased and the oxidation shoulder peak was observed around 0.25V (vs. RHE). The reduction of Pt oxidation and PtOx reduction peak may be explained due to the adsorption of carboxylic acid (-COOH) of -amino caproic acid onto PtOx surface. Whether the oxidation shoulder peak shown around 0.25V (vs. RHE) comes from the adsorption of -amino caproic acid on the Pt surface or another electro-organic reaction from the increase of current density from1.0V to 1.2V (vs. RHE), the oxidation shoulder peak at 0.25V (vs. RHE) was more clearly observed with CV measured at higher temperature T=73℃ as shown in Figure 6.6b. Therefore, we can assume that the oxidation peak around 0.25V vs. RHE is related with ε-amino caproic acid.
As noted above, we also expected sigifnicant Pt loss by the ion-exchange of protonated ε-amino caproic acid with the acid sites in the Pt/C electrode at 73℃. However, our experimental data did not show much loss of ECSA in ex-situ RDE experiment. One possible explanation of this CV result in Figure 6.6 is that the pKa (2~3) of carboxylic acid group.
We conducted RDE experiment pH=1 condition, in other words, the carboxylic acid of -amino caproic acid did not dissociate at pH=1 (pH<pKa of -COOH). Thus, there would be relatively difficulty of carboxylic acid (-COOH) to adsorb onto the surface of Pt electrode than carboxlyate (-COO-) which is dissociate anion.
(a) -8 10-5 -6 10-5 -4 10-5 -2 10-5 0 2 10-5 4 10-5 0 0.1 0.2 0.3 0.4 0.5 0.6
Cu
rr
en
t d
ens
ity
, A
/c
m
2 Potential, V(RHE) Blue: Baseline Red: w/ Caprolactam N 2, 20 mV/s(b)
Figure 6.6 (a) Cyclic voltammograms of 45.5% Pt/C and polycrystalline Pt catalyst as concentration of caprolactam increasing a) 0M b) 8M (100ML) c) 40M (500ML) d) 80M (1000ML) e) 0.4mM (5000ML) f) 0.8mM (10000ML)
(b) Caprolactam effect on cyclic voltammograms of Pt/C catalyst electrode at 73℃ (a) 0 (b) 8M (c) 16M (d) 40M (e) 80M (f) 0.4mM
The other RDE experiment conducted at pH=2 showed more loss of ECSA (30%) compared to that loss (17%) conducted at pH=1 as shown in Figure 6.7. Note that CV at pH=2 did not show oxidation shoulder peak. In other words, carboxlyate anion (-COO-) adsorbed onto the surface of Pt rather than carboxylic acid (-COOH). The other possible explanation is that the protonated ε-amino caproic acid can be regarded as a proton conductor (-COO- H+) even though proton conductivity is less than sulfonic acid group (- SO3- H+). As a consequence, ε-amino caproic acid might be acting as another proton conductor at the ionomer/PFSA membrane thus, -caprolactam could not affect on Pt active site by adsorption but ion exchange with ionomer.
The oxidation shoulder peak at 0.25V vs. RHE can expalin with the oxidation of carboxylic acid (-COOH) onto Pt/C surface. Once the carboxylic acid (–COOH) adsorbed onto the Pt/C, the Pt catalyst may aid to oxidize carboxylic acid at relatively low electro- potential so as to produce proton from the oxidation of carboxylic acid: -COOH + Pt → - COO-Pt + H+ + e-. The oxidation shoulder peak increased more at a higher temperature (73℃ as shown in Figure 6.6) compared to that results at room temperature (23℃ as shown in Figure 6.6). The above reaction scheme, carboxylic acid adsorption and electro- organic oxidation, may be possible to support an oxidation peak. The increase of current density about 1.0V to 1.2V also can be explained by the oxidation of carboxylic acid to produce carbon dioxide: -COO-Pt → CO2 + e-
.
The kinetic current of oxygen reduction reaction (ORR) of Pt/C electrode was also measured as increasing of -caprolactam concentration into solution as shown in Figure 6. 8a. The limiting current and Tafel slopes of 45.5% Pt/C catalyst electrode showed that there were no significant change by adding -caprolactam. The Tafel slopes were
Figure 6.7 ORR kinetic current density and Tafel slopes of 45.5% Pt/C catalyst as concentration of -caprolactam is increasing; ORR at pH=1; a) 0 b) 8M c) 80M d) 0.4mM
a)
b)
Figure 6.8 pH effect on ECSA loss of Pt/C electrode in the presence of -caprolactam at T=23℃ (a) pH=1 (b) pH=2
changed less than 5% by adding 5000ML (0.4mM) of -caprolactam into electrolyte. Again, the contamination effect on ORR by -caprolactam may be significant at higher pH likewise CV results. In sum, the open ring structure of -caprolactam from the acid- catalyzed hydrolysis can lead the loss of Pt activity due to the adsorption of carboxlyate anion (-COO-). Their impact also depends on the pH. Likewise the conductivity loss by the open ring reaction of -caprolactam, the temperature impacts on Pt contamination are also shown.