5.3. R ESULTS AND D ISCUSSION
5.3.5. ORR Activity and O 2 Temperature Programmed Desorption
The ORR electrocatalytic performances of the Fe-Phen_1.0 and Fe-Phen_2.0 were examined using the rotating ring-disk electrode (RRDE) technique in both 0.1 M KOH and 0.1 M HClO4. The linear sweep voltammetry (LSV) curves of the catalysts in 0.1 M KOH electrolyte demonstrated that the Fe-Phen_2.0 exhibits a much higher ORR activity than that of the Fe-Phen_1.0, indicated by a positive shift of the half-wave potential by 70 mV in the Fe-Phen_2.0 (Figure 5.9a). The kinetic current of the Fe-Phen_2.0 at 0.90 V is 24 times higher than that of the Fe-Phen_1.0 (Table 5.3). To compare the intrinsic activities of the Fe-Phen_1.0 and Fe-Phen_2.0 catalysts, we calculated the turnover frequency (TOF) for the O2-to-H2O conversion of the catalysts, assuming that the available Fe–Nx sites were determined by CO cyro adsorption amount. The TOF of the Fe-Phen_2.0 at 0.9 V (vs.
RHE) was 7.77 electron site−1 s−1, which was approximately 31 times higher than that of the Fe- Phne_1.0 (0.25 electron site−1 s−1 at 0.90 V) (Figure 5.9c and Table 5.3). The Fe-Phen_2.0 also shows high ORR activity in acidic media (Figure 5.9b and Table 5.4). The LSV curve of Fe-Phen_2.0 for the ORR in 0.1 M HClO4 reveals a half-wave potential at 0.80 V, which is much better than the ORR activity of Fe-Phen_1.0 catalyst with half-wave potential of 0.74 V.
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Figure 5.9. ORR polarization curves of the Fe-Phen_1.0 and Fe-Phen_2.0 catalysts in (a) 0.1 M KOH and (b) 0.1 M HClO4. (c) TOF values at 0.9 V (alkaline) and at 0.8 V (acid). ORR performances of the catalysts from the RRDE test at a rotating speed of 1600 rpm and a scan rate of 5 mV s−1. The catalyst loading on the glassy carbon electrode was 0.6 mg cm−2.
Table 5.3. TOF values of the Fe-Phen_X catalysts in 0.1 M KOH.
Catalyst Name CO Chemisorption (nmol mg−1) a
jk @ 0.9 V (mA cm−2) b
MA0.9 V
(mA mg−1) c
TOF (electron site−1 s−1) d
Fe-Phen_1.0 77.3 1.1 1.83 0.25
Fe-Phen_1.25 58.6 3.8 6.33 1.12
Fe-Phen_1.5 74.3 14.8 24.7 3.44
Fe-Phen_1.75 61.3 15.0 25.0 4.23
Fe-Phen_2.0 57.6 25.9 43.2 7.77
a values are obtained by CO cryo adsorption. SD (site g−1) = nCO (nmol mg−1) × 10−6 × 6.023 × 1023 (site mol)
b calculated from the ORR polarization curve in 0.1 M KOH.
c values are the values of [b] divided by the catalyst loading.
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d calculated as follows: TOF (electron site−1 s−1) × SD (site g−1) × e (C electron−1) = MA0.9 V (A g−1) Table 5.4. TOF values of the Fe-Phen_X catalysts in 0.1 M HClO4.
Catalyst Name CO Chemisorption (nmol mg−1) a
jk @ 0.8 V (mA cm−2) b
MA0.8 V
(mA mg−1) c
TOF (electron site−1 s−1) d
Fe-Phen_1.0 77.3 0.8 1.33 0.18
Fe-Phen_1.25 58.6 1.9 3.17 0.56
Fe-Phen_1.5 74.3 5.2 8.67 1.21
Fe-Phen_1.75 61.3 4.7 7.83 1.32
Fe-Phen_2.0 57.6 6.3 10.5 1.89
a values are obtained by CO cryo adsorption. SD (site g−1) = nCO (nmol mg−1) × 10−6 × 6.023 × 1023 (site mol)
b calculated from the ORR polarization curve in 0.1 M HClO4.
c values are the values of [b] divided by the catalyst loading.
d calculated as follows: TOF (electron site−1 s−1) × SD (site g−1) × e (C electron−1) = MA0.8 V (A g−1) We conducted a low loading test and H2O2 reduction experiment to determine whether the difference in ORR performance is due to the difference in ORR mechanism.32,33 The low loading test was measured by sequentially decreasing the catalyst loading of 600, 200, and 100 μg cm−2 (Figures 5.10 and 5.11). Regardless of the pH of the electrolyte, the H2O2 yield of both catalysts increased as the loading amount decreased, but the H2O2 yield was less than 25% even at the lowest catalyst loading. In other words, the ORR in both catalysts mostly proceed through a 4 electron pathway where O2 becomes H2O, and some ORRs proceed through a 2 x 2 electron pathway where O2
becomes H2O2 and then further reduced to H2O.32
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Figure 5.10. Catalyst loading test in 0.1 M HClO4. ORR polarization curves of the Fe-Phen_1.0 and Fe-Phen_2.0 catalysts with (a) 600, (c) 200, and (e) 100 μg cm−2. H2O2 selectivity of the Fe-Phen_1.0 and Fe-Phen_2.0 catalysts with (b) 600, (d) 200, and (f) 100 μg cm−2.
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Figure 5.11. Catalyst loading test in 0.1 M KOH. ORR polarization curves of the Fe-Phen_1.0 and Fe-Phen_2.0 catalysts with (a) 600, (c) 200, and (e) 100 μg cm−2. H2O2 selectivity of the Fe-Phen_1.0 and Fe-Phen_2.0 catalysts with (b) 600, (d) 200, and (f) 100 μg cm−2.
In H2O2 reduction reaction in N2-saturated electrolyte, Fe-Phen_2.0 and Fe-Phen_1.0 showed similar reduction currents (Figure 5.12), meaning that the H2O2 reduction capability of the two catalysts is similar.33 The results of low loading test and H2O2 reduction experiment suggest that the
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ORR in Fe-Phen_1.0 and Fe-Phen_2.0 proceed with the same reaction mechanism. Therefore, the difference in ORR performance of the two catalysts can be attributed to the changes in O2 binding energy caused by the structural difference.
Figure 5.12. H2O2 reduction current density of the Fe-Phen_1.0 and Fe-Phen_2.0 catalysts in (a) 0.1 M HClO4 and (b) 0.1 M KOH electrolytes with various H2O2 concentrations.
We confirmed that Fe-Phen_1.0 with tube structure has 3–4 carbon layers and Fe-Phen_2.0 with rod structure has stacked 20 carbon layers through TEM images and N2 adsorption-desorption analysis. We also performed O2 TPD experiments (Figure 5.13) to obtain the binding strengths of chemisorbed O2 on the Fe-Phen_1.0 and Fe-Phen_2.0 catalysts.31 The rod-structured Fe-Phen 2.0 had a lower peak temperature for O2 desorption from the catalyst than the tube-structured Fe-Phen 1.0 (Figure 5.13b), indicating that Fe-Phen_2.0 had a weaker binding strength with oxygen than Fe- Phen_1.0. In addition, Rossmeisl et al. suggested that the diporphyrin structures would act as promising ORR active sites by creating a dissociative mechanism that is accessible even at weak binding sides due to its structural advantages.34
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Figure 5.13. (a) O2 TPD profiles of Fe-Phen_1.0 and Fe-Phen_2.0 catalysts. (b) The second O2
desorption peak temperature of Fe-Phen_1.0 and Fe-Phen_2.0 catalysts.