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Fig. 6. (A) Development of oxygen reduction activity during step by step synthe- sis of ORR catalysts onto GC. MWCNTs were drop-coated onto GC (dashed line), modified with FeTMPP-Cl by electrodeposition (dotted line) and heat treated (full line and dash-dot line; the two heat treated samples were prepared independently). O2-saturated 0.5 M H2SOx, RE: relative hydrogen electrode, CE: platinum wire. Mea- surements performed at room temperature, electrode rotated at 1600rpm, scan rate 5mVs~!. (B) Tafel plots displaying the electrocatalytic activity towards oxy- gen reduction for FETMPP-Cl/MWCNT composite heat-treated at 650°C and 850°C, respectively, as derived from RDE measurements as in (A). to the Koutecky-Levich equation possible. The stability of the heat-treated samples was sufficiently high that no decrease in activity was observed during the RDE measurements. The resulting Tafel plots for the heat-treated FeTMPP-Cl/MWCNT composites are displayed in Fig. 6b. The kinetic currents achieved at both heat treat- ment temperatures are nearly identical, as already expected from Fig. 6a. Two regions with different Tafel slopes can be distinguished. The first with a slope of 76.5 mV dec~! (650°C) and 72.7 mV dec"! (850°C) in the region between around 0.7 V and 0.85 V. At poten- tials below 0.6V, the slope is likely affected by other factors than kinetic effects, e.g. mass transfer within the porous film or, at less positive potentials, where the reaction zone will be restricted to the outer layer of the catalyst film, electron transfer through the film. The calculation of the kinetic currents has also been extended to potentials more positive than the onset potential for oxygen reduc- tion (i.e. more positive than ca. 0.9 V). Although it may appear that the current densities calculated at potentials positive from the min- imum are not correct (since the Levich constant for the reduction process is used), this procedure allows to determine correctly the potential of minimum in currents, which is considered to be the real onset potential for oxygen reduction and a much more reli- able value compared to the values given by visual inspection of cyclic voltammograms, which are very frequently found in liter- ature. From the slope of the Koutecky-Levich-plot (not shown), average numbers of electrons transferred per oxygen molecule

Figure 6 (A) Development of oxygen reduction activity during step by step synthe- sis of ORR catalysts onto GC. MWCNTs were drop-coated onto GC (dashed line), modified with FeTMPP-Cl by electrodeposition (dotted line) and heat treated (full line and dash-dot line; the two heat treated samples were prepared independently). O2-saturated 0.5 M H2SOx, RE: relative hydrogen electrode, CE: platinum wire. Mea- surements performed at room temperature, electrode rotated at 1600rpm, scan rate 5mVs~!. (B) Tafel plots displaying the electrocatalytic activity towards oxy- gen reduction for FETMPP-Cl/MWCNT composite heat-treated at 650°C and 850°C, respectively, as derived from RDE measurements as in (A). to the Koutecky-Levich equation possible. The stability of the heat-treated samples was sufficiently high that no decrease in activity was observed during the RDE measurements. The resulting Tafel plots for the heat-treated FeTMPP-Cl/MWCNT composites are displayed in Fig. 6b. The kinetic currents achieved at both heat treat- ment temperatures are nearly identical, as already expected from Fig. 6a. Two regions with different Tafel slopes can be distinguished. The first with a slope of 76.5 mV dec~! (650°C) and 72.7 mV dec"! (850°C) in the region between around 0.7 V and 0.85 V. At poten- tials below 0.6V, the slope is likely affected by other factors than kinetic effects, e.g. mass transfer within the porous film or, at less positive potentials, where the reaction zone will be restricted to the outer layer of the catalyst film, electron transfer through the film. The calculation of the kinetic currents has also been extended to potentials more positive than the onset potential for oxygen reduc- tion (i.e. more positive than ca. 0.9 V). Although it may appear that the current densities calculated at potentials positive from the min- imum are not correct (since the Levich constant for the reduction process is used), this procedure allows to determine correctly the potential of minimum in currents, which is considered to be the real onset potential for oxygen reduction and a much more reli- able value compared to the values given by visual inspection of cyclic voltammograms, which are very frequently found in liter- ature. From the slope of the Koutecky-Levich-plot (not shown), average numbers of electrons transferred per oxygen molecule

Related Figures (7)

Fig. 1. (A) CV of FeTMPP-Cl (3 mM) dissolved in DMF/0.1 M TBABF,, (B) CV of 3m FeSO4:7H20/1,10-phenathroline dissolved in pure H20. WE: glassy carbon, CE: Pt wire, RE: Ag wire, scan rate 100 mVs"!.
Fig. 1. (A) CV of FeTMPP-Cl (3 mM) dissolved in DMF/0.1 M TBABF,, (B) CV of 3m FeSO4:7H20/1,10-phenathroline dissolved in pure H20. WE: glassy carbon, CE: Pt wire, RE: Ag wire, scan rate 100 mVs"!.
Fig. 3. RDE polarisation curves (1600rpm) of MWCNTs modified with FeTMPP- Cl films applying different numbers of deposition pulses as indicated. Solid line: unmodified MWCNTs. O2-saturated 0.5 M H2SOxz, RE: relative hydrogen electrode, CE: platinum wire. Measurements performed at room temperature with a scan rate of 5mVs-!.
Fig. 3. RDE polarisation curves (1600rpm) of MWCNTs modified with FeTMPP- Cl films applying different numbers of deposition pulses as indicated. Solid line: unmodified MWCNTs. O2-saturated 0.5 M H2SOxz, RE: relative hydrogen electrode, CE: platinum wire. Measurements performed at room temperature with a scan rate of 5mVs-!.
Fig. 4. CVs of Fe(phen)3/MWCNT composites prepared by pulsed electrodeposi- tion. Number of pulses as indicated, solid line: unmodified MWCNTs. O2-saturated 0.5M H2SOuz, RE: relative hydrogen electrode, CE: platinum wire. Measurements performed at room temperature with a scan rate of 100 mVs~!.
Fig. 4. CVs of Fe(phen)3/MWCNT composites prepared by pulsed electrodeposi- tion. Number of pulses as indicated, solid line: unmodified MWCNTs. O2-saturated 0.5M H2SOuz, RE: relative hydrogen electrode, CE: platinum wire. Measurements performed at room temperature with a scan rate of 100 mVs~!.
Fig.5. CVs of FETMMP-Cl/MWCNT composites prepared by electrodeposition before and after heat treatment. Dashed line: before HT, dotted line: heat treated at 650°C, solid line: heat treated at 850°C. Deaerated 0.5 M H2SOq, RE: relative hydrogen elec- trode, CE: platinum wire. Measurements performed at room temperature with a scan rate of 100 mVs~!.
Fig.5. CVs of FETMMP-Cl/MWCNT composites prepared by electrodeposition before and after heat treatment. Dashed line: before HT, dotted line: heat treated at 650°C, solid line: heat treated at 850°C. Deaerated 0.5 M H2SOq, RE: relative hydrogen elec- trode, CE: platinum wire. Measurements performed at room temperature with a scan rate of 100 mVs~!.
Kinetic current densities and onset potentials derived from Tafel plots of catalysts obtained by heat treatment of electrodeposited FeTMPP-Cl/MWCNT composites compared with similar catalysts prepared by wet-impregnation [41]. of 2.4 (650°C, 0.2V), 2.7 (650°C, OV), 2.7 (850°C, 0.2V) and 3.1 (850°C, OV) have been calculated, indicating either that both 2- electron reduction to hydrogen peroxide and 4-electron reduction to water are running in parallel or that parts of the hydrogen per- oxide formed in the first step are further reduced to water in a second two-electron reduction. However, compared to the non- heat treated films, the heat treated catalysts showed a much higher stability at least on a short timescale, indicating that the decom- posed polymeric porphyrin is much less prone to attack by H202, in agreement with earlier results on heat treated porphyrins [21].
Kinetic current densities and onset potentials derived from Tafel plots of catalysts obtained by heat treatment of electrodeposited FeTMPP-Cl/MWCNT composites compared with similar catalysts prepared by wet-impregnation [41]. of 2.4 (650°C, 0.2V), 2.7 (650°C, OV), 2.7 (850°C, 0.2V) and 3.1 (850°C, OV) have been calculated, indicating either that both 2- electron reduction to hydrogen peroxide and 4-electron reduction to water are running in parallel or that parts of the hydrogen per- oxide formed in the first step are further reduced to water in a second two-electron reduction. However, compared to the non- heat treated films, the heat treated catalysts showed a much higher stability at least on a short timescale, indicating that the decom- posed polymeric porphyrin is much less prone to attack by H202, in agreement with earlier results on heat treated porphyrins [21].
Fig. 7. Development of oxygen reduction activity during step by step synthesis of ORR catalysts onto GC. MWCNTs were drop-coated onto GC (dashed line), modified with Fe(phen)3 by electrodeposition (solid line) and heat treated (dash-dash-dot and dash-dot line; the two heat treated samples were prepared independently). O2-saturated 0.5 M H2SOx, RE: relative hydrogen electrode, CE: platinum wire. Mea- surements performed at room temperature, electrode rotated at 1600 rpm, scan rate 5mvVs-!.
Fig. 7. Development of oxygen reduction activity during step by step synthesis of ORR catalysts onto GC. MWCNTs were drop-coated onto GC (dashed line), modified with Fe(phen)3 by electrodeposition (solid line) and heat treated (dash-dash-dot and dash-dot line; the two heat treated samples were prepared independently). O2-saturated 0.5 M H2SOx, RE: relative hydrogen electrode, CE: platinum wire. Mea- surements performed at room temperature, electrode rotated at 1600 rpm, scan rate 5mvVs-!.
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EngineeringOxygen Reduction ReactionRotating Disc Electrode
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