Oxygen reduction on AQ-modified

bare and AQ-modified GC, HOPG, Ni-Gra and Cu-Gra electrodes in alkaline solution. Several O2 reduction studies have been performed on AQ-modified GC electrodes [53–55, 121, 130, 131, 134, 195, 218], only very recently the ORR was investigated on GC substrates modified with thick AQ films (up to ΓAQ = 6×10^–9 mol cm^–2) [219]. In the latter study, the results of kinetic para-meters revealed that in spite of the thick AQ film on GC electrode, these electrodes are not advantageous for the electrocatalysis of O2 reduction and a good electrocatalytic effect can be observed already for the ΓAQ values of 10^–9 mol cm^–2 [219]. As can be seen from Figure 37a, these results are in good agreement with those obtained in our previous study [219]. Briefly, the O2

reduction wave on AQ grafted GC electrode started at more negative potential (ca –0.3 V) compared to bare GC (ca –0.2 V). Furthermore, the O2 reduction wave which is usually caused by the native quinone-type groups present on the bare GC surface [53, 54] shifted to more negative direction on GC/AQ electrodes (Figure 37a) indicating that the electrochemically grafted AQ groups inhibit O2 reduction on GC electrodes at these potentials. However, at lower potentials (E < –0.6 V) the reduction current densities were larger on AQ-modified GC electrode compared to bare GC and this is due to the electro-catalytic effect of AQ groups attached to GC electrode surface during the electrografting process (Figure 37a) [131, 219].

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The LSV curves for O₂ reduction on bare HOPG, Ni-Gra and Cu-Gra (Figures 37b-d) all show a major reduction wave at ca 0.75 V and a minor wave at around 1.1 V. This shows that the ORR activity is significantly lower compared to that of bare GC which is consistent with earlier studies [135, 208].

The small pre-peak seen in Figure 37b at 0.55 V on bare HOPG is suggested to occur from O2 reduction at quinone-type groups present on HOPG defect sites.

Moreover, its relatively low intensity implies that mainly the basal plane is exposed [208]. The pre-peak is absent in case of CVD-grown graphene on Ni or Cu substrate (Figures 37c,d) implying that graphene sheets cover the corresponding underlying surface without edge sites. These findings also indicate that the substrates (basal plane HOPG and CVD-grown graphene electrodes) are suitable to study the properties of electrocatalytically active functional groups as reported earlier [53, 54, 134, 135, 195, 218] and therefore anthraquinone is a good candidate for that.

As can be seen in Figure 37b, the O₂ reduction wave started at a more positive potential (at ca –0.3 V) for AQ-modified HOPG compared to bare HOPG. Sarapuu et al. [135] studied the reduction of O₂ in 0.1 M KOH at AQ-modified basal plane HOPG with very low surface concentration of AQ groups (8×10^11 mol cm^2). The authors revealed that in case of HOPG/AQ electrodes, the first O₂ reduction peak was around –0.8 V and larger current values were

observed compared to bare HOPG, suggesting the catalytic effect due to the presence of surface-confined AQ [135].In contrast to the latter study, the first oxygen reduction peak of the AQ-modified HOPG electrodes was already observed at ca –0.55 V (see Figure 37b). At this point we do not know the exact reason for this observation. However there might be several reasons for that.

First the presence of this cathodic peak might be associated with edge plane sites which might have been formed during the grafting of HOPG surface with AQ moieties as suggested above. Therefore the native quinone-type functionalities which are present only on defect sites of HOPG enhance the electrocatalytic activity for O₂ reduction at these potentials. Another explanation might also be attributed to the thicker AQ layer on HOPG (Figure 37b). Then in this case, the O₂ reduction peak observed at bare HOPG (ca –0.8 V) has been shifted to more positive potentials (–0.55 V) because of the AQ-modified HOPG electrodes. For example, Kocak et al. [136] performed the O₂ reduction studies in 0.1 M acetate buffer (pH=5) at basal plane and edge plane HOPG modified with AQ through –NH₂CH₂C₆H₄– linker. Amongst other things the authors investigated the influence of edge plane HOPG electrodes of different AQ coverage on the ORR and the results showed that as the surface concentration of AQ groups increased the reduction peak shifted to a more positive direction and the peak current increased as well [136]. A similar tendency was observed in this study (Figure 37b). Therefore it may be concluded that the current density values depend on the surface concentration of the AQ groups as observed earlier [136].

Compared with bare CVD-grown graphene substrates, the AQ-modified graphene-based electrodes greatly increase the ORR current density values (see Figures 37c,d). In both cases, the oxygen reduction peaks shifted to more positive potentials (at around –0.55 V) compared to the bare substrates where the first peak potential was at ca –0.8 V (Figures 37c,d). Similarly to GC/AQ electrodes, a reduction peak close to –0.9 V was observed for all the AQ-modified carbon-based electrodes (see Figures 37b-d). This peak is most probably due to the electrocatalysis by the surface-bound AQ groups.

However, there is an additional peak at ca –0.65 V in case of GC, HOPG and Ni-Gra grafted with AQ groups. Unfortunately the reason for that remains unclear, but according to the study by Kullapere et al. [121] this peak might indicate the involvement of the AQ sites in the reduction of oxygen.

On the basis of these results we may conclude that all HOPG and graphene-based electrodes modified with thick AQ films display enhanced ORR performance compared to bare substrates. This is caused by the intrinsic electro-catalytic property of AQ groups for O₂ reduction in alkaline media.