Oxygen Reduction at the Liquid–Liquid Interface

DOI: 10.1002/celc.201500343

Articles

Oxygen Reduction at the Liquid–Liquid Interface: Bipolar Electrochemistry through Adsorbed Graphene Layers Andrew N. J. Rodgers and Robert A. W. Dryfe*[a] The reduction of oxygen and protons at the interface between two immiscible electrolyte solutions (ITIES) has received a great deal of interest over the last decade, with various materials being used to catalyse these reactions. Probing the mechanisms through which these reactions proceed when using interfacial catalysts is important from both from the perspective of fundamental understanding and for catalyst optimisation. Herein, we have used interfacial-assembled graphene

to probe the importance of simple electron conductivity towards the catalysis of the oxygen reduction reaction (ORR) at the ITIES, and a bipolar setup to probe the homogeneous/heterogeneous nature of the ORR proceeding through interfacial graphene. We found that interfacial graphene provides a catalytic effect towards the reduction of oxygen at the ITIES, proceeding via the heterogeneous mechanism when using a strong reducing agent.

1. Introduction Study of the interface between two immiscible electrolyte solutions (ITIES) has a long history, dating back to the first experiment by Nernst and Riesenfeld in 1902.[1] Interest in this area has grown rapidly since the 1970s, driven by potential applications such as sensing, nuclear waste reclamation, nanoparticle assembly, mimicking biological membranes and predicting drug transport in vivo.[2–4] Over the last decade, the use of ITIES systems for reduction reactions has received a great deal of attention, with both the oxygen reduction[5–27] and hydrogen evolution[28–38] reactions (ORR and HER, respectively) widely studied.[39–41] Equations (1)–(3) define the two- and four-electron ORRs as well as the HER, respectively. O2ðaq=orgÞ þ 2 Hþ ðaqÞ þ 2 RedðorgÞ ! H2 O2ðaqÞ þ 2 Redþ ðorgÞ

ð1Þ

O2ðaq=orgÞ þ 4 Hþ ðaqÞ þ 4 RedðorgÞ ! 2 H2 OðaqÞ þ 4 Redþ ðorgÞ

ð2Þ

Hþ ðaqÞ þ RedðorgÞ ! 1=2 H2ðgÞ

ð3Þ

ethane (DCE) and 1,2-dichlorobenzene (DCB) are commonly used as the organic phase in ITIES studies. In the case of the ORR, oxygen is also required and can partition between both liquid phases under ambient conditions, with the source dependent upon the reaction mechanism. Part of the interest in using ITIES systems for these reactions stems from the inherent advantages compared to single-liquid-phase systems, including the ability to isolate reactants until a sufficient Galvani potential (D@) is applied, availability of lipophilic reducing agents and partitioning of reaction products out of the organic phase and away from the reducing agent:[7, 41] A range of catalysts have been used to increase the rate of both reactions at the ITIES, including Pt, Pd[29] and Cu[38] nanoparticles (NPs) as well as various transition-metal carbides, borides and dichaldogenides[30, 33] for the HER and porphyrins (with Co[6, 8, 9, 13, 14, 16, 21] or metal free[10, 11, 22, 24]), aniline derivatives,[12, 17] phthalocyanines[20, 25] as well as Pt[5] and Au[23] NPs for the ORR. These catalysts are generally either present in the organic phase or adsorbed at the ITIES. Owing to the biphasic nature of ITIES systems, there are two principal mechanisms through which the HER and ORR can proceed: heterogeneous and homogeneous. In the former scenario, all reactants are confined to their respective liquid phases and heterogeneous transfer of an electron occurs, from the organic reducing agent to the aqueous species, whereas in the latter case, aqueous protons are forced into the organic phase followed by homogeneous reduction within that phase. Both mechanisms are favoured by the application of a positive D@ and differentiation between them is not possible from analysis of the voltammetric response. With regards to thermodynamics, both the HER and ORR require a lower driving force in the organic phase than in the aqueous phase. Indeed, both reactions are spontaneous in DCE and DCB when using either of the two commonly used lipophilic reducing agents, dimethylferrocene (DiMFc) and dec-

The experimental setup for these reactions comprises an acidic aqueous phase, as the source of the protons required for both reactions, and a reducing agent located in the organic phase, typically ferrocene or a derivate thereof. 1,2-Dichloro[a] A. N. J. Rodgers, Prof. R. A. W. Dryfe School of Chemistry, University of Manchester Oxford Road, Manchester, M13 9PL (UK) E-mail: [email protected] Supporting Information for this article is available on the WWW under http://dx.doi.org/10.1002/celc.201500343. T 2016 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited. An invited contribution to a Special Issue on Bipolar Electrochemistry

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Articles amethylferrocene (DcMFc).[16] In the absence of any catalyst, the homogeneous mechanism is believed to dominate for both the HER and ORR, with the transfer of aqueous protons into the organic phase assisted by complexation with the organic reducing agent.[15, 30, 31, 35] In the presence of a catalyst residing in the organic phase, such as the aniline derivatives,[17] Co porphyrins[21] and freebase porphyrins[11] for the ORR, the homogeneous mechanism still appears to dominate. This has been supported by the detection of protonated free-base[11] and Co porphyrins[21] during oxygen reduction at the ITIES and the greater catalytic effect displayed by more basic (H + -transfer favouring) aniline derivatives.[17] The mechanism is more difficult to determine when the catalyst is adsorbed at the ITIES. It has been suggested that such interfacial particles act as bipolar electrodes,[40, 42, 43] which would favour the heterogeneous mechanism. However, there have been relatively few studies probing the reduction mechanism when using an interfacial catalyst. This is surprising given the number of studies on reduction reactions at the ITIES, though it is, in part, likely to be associated with the difficulty in differentiating between a genuine heterogeneous reaction and a homogeneous reaction that takes place on one side of the ITIES.[44] Previously, in our lab,[23] a bipolar setup employing a Au wire bridging two discrete liquid phases was used to study the ORR catalysed by Au. In the bipolar setup, only the heterogeneous mechanism is possible and it was found that the bipolar Au wire displayed a similar catalytic response to Au NPs adsorbed at the interface in a traditional ITIES setup, which supported the aforementioned hypothesis that interfacial particles act as bipolar electrodes. The study by Jedraszko et al.,[26] in which the two-electron ORR to H2O2 was observed at a D@ value less positive than the standard ion-transfer potential of H + (D@0Hþ ), also strongly indicated that the heterogeneous mechanism is possible in the absence of any catalyst. Recently, reduced graphene oxide (rGO)[32] and multiwalled carbon nanotube (MWCNT)[34] graphitic supports were used by the Girault group to improve the catalytic effect of MoS2 and Mo2C, respectively, toward the HER at the ITIES. The catalytic improvements were attributed both to improved electron transfer between the lipophilic reducing agent and catalyst and an increased cross-sectional area over which the reducing agent could transfer electrons to the nano-sized catalysts. These properties are thought to be aided by the ability of graphene to “pool” electrons taken from the reducing agent. Recent studies in our laboratory[45–47] have also shown that graphene (both exfoliated and CVD grown) and CNTs assembled at the water j DCE interface act as conduits for heterogeneous electron transfer across the ITIES. We expect that this combination of properties will result in a catalytic effect displayed by using interfacial-assembled graphene (essentially a simple conductor in this scenario) towards heterogeneous reduction reactions at the ITIES by virtue of an increase in the reaction cross section. Interfacial graphene oxide (GO) has previously been shown to catalyse the two-electron ORR at the ITIES, although the catalytic properties were attributed to GO, owing to the surface-bound quinone groups.[27] ChemElectroChem 2016, 3, 472 – 479

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Here, we report the inherent catalytic properties of solutionphase exfoliated graphene, assembled in situ at the water j DCE interface, towards the ORR mediated by DiMFc and DcMFc reducing agents located in the DCE phase. A bipolar configuration, consisting of discrete water and DCE phases electrically connected through electrodes prepared from solution-phase exfoliated graphene, was subsequently used to probe the homogeneous/heterogeneous nature of the reaction at the ITIES.

2. Results and Discussion The basic cell used for the ITIES work consisted of water and DCE phases containing the supporting electrolytes HCl and bis(triphenylphosphoranylidene)ammonium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (BTPPATFPB), respectively. The HCl in the aqueous phase also acted as a source of the protons that are necessary for the ORR. Scheme 1 depicts the full ITIES cell setup, with the reference phase and reference electrodes. The aqueous-phase pH (x) was varied between 1, 2 and 3, and the concentration of the organic reducing agent (y) was either 0 or 5 mm. For convenience, abridged schematics, depicting only the two bulk-liquid phases, have been included on each cyclic voltammogram (CV) throughout the paper.

Scheme 1. Schematic of the ITIES cell composition used to study the ORR: x was varied between 1, 2 and 3, whereas y was either 0 or 5. All experiments were conducted under ambient conditions.

2.1. Electrochemical Response with Graphene at the ITIES Graphene dispersions in DCE were prepared using the method reported by Toth et al.[47] Natural graphite flakes (NGF) were exfoliated in DCE through bath sonication and subsequently letting the dispersion stand overnight before applying a centrifugation step; the supernatant was taken as the final dispersion. The weight concentration of the NGF dispersion was determined as 19 mg L@1 by using UV/Vis spectroscopy and the absorption coefficient of 2305 mL mg@1 m@1 determined previously in our laboratory.[47] After addition of the aqueous phase on top of the DCE dispersion, NGF flakes were assembled at the water j DCE interface by applying a brief (< 1 min) sonication to the full ITIES cell. This sonication step resulted in some emulsification of the liquid j liquid interface. The ITIES cells were then left to stand overnight to allow emulsion coalescence prior to electrochemical measurements. The inset in Figure 1 shows a photograph of an ITIES cell with graphene assembled at the water j DCE interface. Assuming that all of the NGF, previously dispersed in the DCE phase, assembled at the ITIES, the density of graphene at the interface was 32.5 mg cm@2 (see the Supporting Information for the calculation). Figure 1 shows the CVs with graphene assembled at the ITIES and the supporting electrolytes present in the two liquid phases. The CVs resemble those in the absence of graphene 473

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Figure 2. CVs of the cell depicted in Scheme 1 with 5 mm DiMFc in the DCE phase and varying aqueous-phase pH. No graphene was present at the ITIES.

Figure 1. CVs of the cell depicted in Scheme 1 with varying aqueous-phase pH, in the presence of 32.5 mg cm@2 graphene at the ITIES and in the absence of a reducing agent. The solid-blue curve shows the CV upon addition of TMACl to the aqueous phase, with the peaks corresponding to TMA + transfer across the ITIES. The inset photograph shows the ITIES cell with graphene assembled at the liquid j liquid interface.

depend on the pH of the aqueous phase. These peaks were present in the first CV cycle and are attributed to the transfer of the oxidised form of the reducing agent, dimethylferricenium (DiMFc + ), across the water j DCE interface .[49] Previously in the literature, the presence of DiMFc + has been attributed to slow formation in air-saturated stock solutions of DiMFc.[9, 20] However, no DiMFc + was detected here in the stock solution immediately prior to the voltammetry measurements (see Figure S2 in the Supporting Information). The apparent dependence of the DiMFc + transfer peak-current magnitude on the aqueous-phase pH has not previously been noted in the literature. This dependence indicates that DiMFc + formation is associated with a reaction in which aqueous protons are involved and the appearance of the peaks on the first voltammetric cycle suggests that the reaction occurs at low Galvani potentials. Further investigation of this relationship was not carried out here, but warrants future study. CVs of the cell outlined in Scheme 1 with 5 mm DiMFc in the DCE phase, graphene at the water j DCE interface and an aqueous-phase pH of 1 are shown in Figure 3. Three different runs are shown, along with the same cell in the absence of graphene for comparison. There is no immediate evidence from the CVs that interfacial graphene has any significant catalytic effect on the ORR mediated by DiMFc. Typically, a catalytic effect is manifested in CVs as an irreversible positive current rising at a D@ value that is more negative than the positive limit of the potential window. Despite the apparent lack of oxygen reduction, a comparison of the DiMFc + transfer peakcurrent magnitude indicates a greater concentration of oxidised DiMFc + in the DCE phase when graphene is present. This may be attributed to n doping of the graphene by electrons from DiMFc, as found when using DcMFc as the reducing agent (see Table 1 in Section 2.3), although the extent of doping imparted by DiMFc is expected to be lower, given that it is a weaker reducing agent than DcMFc.[16] It is also possible that some oxidation of DiMFc occurs during the sonication step used to bring about graphene assembly at the water j DCE interface.

(see Figure S1 in the Supporting Information), with no additional faradaic current resulting from the presence of graphene at the interface. The potential window is limited by the transfer of the aqueous supporting electrolyte ions, H + and Cl@ , as shown by the approximate 59 mV increase in the potential window limits upon each order of magnitude decrease in the aqueous electrolyte concentration (increase in aqueous-phase pH). This is a result of the lower magnitude of the Gibbs transfer energies from water to DCE (DG0;w!DCE ) of the aqueous tr 0;w!DCE electrolyte ions (DG0;w!DCE = 56.0 kJ mol@1, DGtr;Cl = @ tr;Hþ @1 51.1 kJ mol ), compared to the organic electrolyte ions @1 (DG0;w!DCE tr;BTPPAþ = @67.5 kJ mol ). There are no data on the Gibbs @ transfer energy of TFPB , though the similarly structured tetra0;w!DCE kis(pentafluorophenyl)borate (TPFB@) has DGtr;TPFB @ = @1 [48] @68.5 kJ mol . The ion-transfer-limited potential window in the presence of interfacial graphene shows that the graphene assemblies are permeable to ions. This is further illustrated by the transfer of tetramethylammonium (TMA + ) across the ITIES upon addition of TMACl to the aqueous phase, as shown in Figure 1. An important consequence of this ion permeability, when considering any potential catalytic effects of interfacial graphene on the ORR, is that the homogeneous mechanism cannot be ruled out in this ITIES configuration. 2.2. Dimethylferrocene as the Reducing Agent The effect of interfacial graphene on the ORR at the water j DCE interface mediated by DiMFc in the DCE phase was investigated. Prior to this, the voltammetric response in the absence of graphene was probed. Figure 2 shows CVs of the ITIES cell outlined in Scheme 1 with 5 mm DiMFc(DCE) present. Again, the potential window is limited by the transfer of the aqueous supporting electrolyte, but there are now additional faradaic peaks at @0.2 and @0.1 V, the magnitudes of which ChemElectroChem 2016, 3, 472 – 479

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Figure 3. CVs of the cell depicted in Scheme 1 with 5 mm DiMFc in the DCE phase, 32.5 mg cm@2 of graphene at the ITIES and an aqueous phase of pH 1. Also shown is the same system in the absence of interfacial graphene (solid black line) for comparison.

Figure 4. CVs of the ITIES cell outlined in Scheme 1, with a pH 1 aqueous phase and with 5 mm DcMFc(DCE) and 32.5 mg cm@2 of interfacial graphene added separately and together. Three runs are shown with both graphene and DcMFc present. The vertical lines represent the Galvani potentials at which the 5 mm DcMFc cell (orange, dash–dot line) and 5 mm DcMFc + 32.5 mg cm@2 graphene (dashed green line) were held for 30 min prior to UV/ Vis characterisation (Figure 5).

Table 1. UV/Vis absorbance values at 779 nm and corresponding DcMFc + concentrations in the DCE phase before and after the application of a positive D@.

ITIES cell[a]

Applied D@ [b] [V]

A779 nm[c]

[DcMFc + ] [mM]

5 mM DcMFc

0 0.2

0.004 0.006

0.03 0.05

5 mM DcMFc + graphene

0 0.17

0.021 0.319

0.17 2.52

Li + -containing aqueous supporting electrolyte in the literature studies. Although the standard ion-transfer potential from water to DCE of Li + (DwDCE @0Liþ = 0.65 V)[48] is more positive than that of H + (DwDCE @0Hþ = 0.58 V),[48] some low concentrations of Li + will partition to the DCE phase and are also likely to transfer concurrently with H + at the positive limit of the potential window. Deng et al.[50] recently reported that hydrated Li + cations catalyse the homogeneous ORR mediated by DcMFc. Therefore, the reversible nature of the positive limit of the potential window, seen here, is likely a result of slower proton consumption by DcMFc in the DCE phase in the absence of Li + . Upon addition of both graphene to the water j DCE interface and 5 mm DcMFc to the DCE phase, a large positive current was observed, rising at Galvani potentials well before the positive limit of the potential window. The irreversible nature of the current shows that a reaction is occurring in tandem with this charge-transfer process, in which both DcMFc and graphene are involved. The origin of this current was further probed by using a combination of chronoamperometry and UV/Vis spectroscopy. A cell containing both graphene and DcMFc was held at D@ = 0.17 V (dashed green line in Figure 4) for 30 min, which resulted in a colour change of the organic phase from yellow to green (Figures 5 a and c), corresponding to oxidation of DcMFc to DcMFc + . The chronoamperometric response is shown in Figure S3 in the Supporting Information. The formation of DcMFc + is confirmed by the evolution of an intense peak at 779 nm in the UV/Vis spectrum of the DCE phase recorded after chronoamperometry (Figure 5 d). In the absence of interfacial graphene, there was no visible colour change of the DCE phase after the application D@ = 0.20 V for 30 min (Figure 5 b). By using the DcMFc + extinction coefficient at 779 nm of 632 m@1 cm@1,[30] the DcMFc + concentration in the organic

[a] ITIES cell as outlined in Scheme 1, with the addition DcMFc/graphene as stated. [b] Galvani potential applied for 30 min, where D@ = 0, the DCE phase stock solution was measured. [c] Absorbance values were recorded in a 2 mm path-length cell.

2.3. Decamethylferrocene as the Reducing Agent DcMFc was used as a stronger reducing agent than DiMFc (standard reduction potentials, E0, in DCE are 0.03 and 0.48 V, respectively).[16] The voltammetric responses of the ITIES cell outlined in Scheme 1 with 5 mm DcMFc present in the DCE phase, graphene assembled at the water j DCE interface and an aqueous phase of pH 1 are shown in Figure 4 for three different runs. Also shown are the CVs in the absence of DcMFc, the absence of graphene and the absence of both DcMFc and graphene, for comparison. In the absence of graphene, the addition of DcMFc resulted in a slight negative shift of the positive limit of the potential window, at which proton transfer occurs from the aqueous phase to the DCE phase. Previously, this has been observed in combination with an irreversible appearance (lack of a backtransfer peak) to the positive end of the potential window, attributed to the consumption of the transferring protons in the DCE phase by DcMFc in the homogeneous ORR.[7, 9, 12, 17, 20] Here, there is a clear back peak, suggesting that proton consumption by DcMFc in the DCE phase is slower in our system. This discrepancy is likely caused by the presence of an additional ChemElectroChem 2016, 3, 472 – 479

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Articles Section 2.1, the interfacial graphene assemblies are permeable to ions transferring across the ITIES. Therefore, there are, theoretically, two mechanisms by which oxygen reduction could occur in the presence of interfacial graphene. Either, transfer of aqueous protons to the organic phase followed by homogeneous reduction in the DCE phase, or heterogeneous transfer of an electron from DcMFc across the ITIES to reduce oxygen located in the aqueous phase. It is difficult to conceive how interfacial-assembled graphene could catalyse the transfer of protons across the ITIES and, therefore, increase the rate of the homogeneous mechanism. Additionally, the observed irreversible positive current (Figure 4) rises at Galvani potentials significantly lower than DwDCE @0Hþ (0.58 V).[48] We expect that the interfacial graphene acts as a bipolar electrode, as suggested previously,[40, 42, 43] and is therefore doped by electrons from DcMFc in the DCE phase, which then reduce aqueous oxygen. This n doping is evident from the greater oxidation of DcMFc + observed in the DCEphase stock solution when graphene was present, as compared to the stock solution containing just DcMFc and the organic supporting electrolyte (Table 1). In the absence of interfacial graphene, all of the reactants would have to meet at the same time and place for heterogeneous reduction to occur. With interfacial graphene present, the reactants no longer have to meet at the same time (owing to the charging of graphene) and the area over which the DcMFc electrons can reduce the aqueous oxygen is increased, as illustrated in Figure 6.

Figure 5. Photographs of the ITIES cell outlined in Scheme 1 with the addition of 5 mm DcMFc(DCE) and 32.5 mg cm@2 of interfacial graphene before (a) and after (c) application of a 0.17 V Galvani potential for 30 min, showing a colour change from yellow to green. b) The same cell in the absence of interfacial graphene, after application of 0.20 V for 30 min, showing the lack of colour change. d) UV/Vis spectra of the DCE phase of both ITIES cells before and after application of the positive Galvani potentials, showing the evolution of a peak at 779 nm that was attributed to DcMFc + after the 0.17 V Galvani potential application when interfacial graphene was present. UV/Vis spectra were recorded in a 2 mm path-length cell.

phase, before and after the 30 min chronoamperometry, was calculated for the two cells, with and without interfacial graphene. These concentrations are presented in Table 1. The onset of the irreversible positive current was also seen to shift to more positive Galvani potentials with increasing aqueous phase pH values (decreasing [H + (aq)]), as shown in Figure S4 in the Supporting Information. This illustrates the involvement of aqueous protons in the process, giving rise to the irreversible positive current, which is consistent with the reduction of O2 to either H2O2 or H2O. If the selectivity between the two- and four-electron ORR mechanisms was the same at each different aqueous-phase pH, then the Nernst equation would dictate a shift of 59 mV per pH unit in the irreversible positive current.[8] However, this shift was @ 59 mV when switching from an aqueous-phase pH of 1 to a pH of 2 (Figure S4). It is possible that this behaviour is caused by a change in selectivity between the two- and four-electron mechanisms upon increasing the aqueous-phase pH from 1 to 2. It is evident that the irreversible positive current is associated with a reaction involving aqueous protons, in which DcMFc acts as a reducing agent and that, in the absence of interfacial graphene, this reaction occurs at a significantly slower rate. We attribute this reaction to the ORR, given that the experiments were conducted under an ambient atmosphere. As stated in ChemElectroChem 2016, 3, 472 – 479

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Figure 6. Illustration of the heterogeneous ORR with DcMFc at a bare ITIES interface (a) and at an ITIES with interfacial graphene (b), showing the greater area of the interface over which heterogeneous electron transfer can occur in the presence of interfacial graphene.

2.4. Decamethylferrocene Bipolar Electrochemistry To test the hypothesis that graphene acts as a bipolar electrode and promotes the heterogeneous ORR, a bipolar electrochemical setup was employed. This consisted of discrete water and DCE phases that were electrically connected by two graphene electrodes, which were themselves connected by a silver wire. The electrodes were inserted into the liquid phases, such that the silver wire was not in contact with the liquids. This setup is illustrated in Figure S4 of the Supporting Information. The two electrodes were prepared from dispersions of the same NGF used to assemble graphene at the water j DCE interface. Scheme 2 outlines the full composition of the ITIES phases in the bipolar setup. The bipolar ITIES configuration prevents the transfer of ions between the two liquid phases by physically separating them, but still allows the passage of electrons between the liquid 476

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Articles the current attributed to the ORR is similar for both setups (ca. 0 V), although the current rises more steeply in the traditional ITIES cell. This may be a result of the differing morphologies of graphene in the ITIES assemblies and in the bipolar electrodes.

Scheme 2. Schematic of the bipolar ITIES cell composition used to study the ORR through graphene: x was varied between 0 and 5. All experiments were conducted under ambient conditions.

3. Conclusions

phases through the graphene electrodes. CVs of the bipolar ITIES configuration are shown in Figure 7, in the absence (grey curves) and presence (orange curves) of 5 mm DcMFc in the DCE phase. In the absence of DcMFc, it is clear that the potential window is no longer limited by the transfer of the supporting electrolyte and was, consequently, larger than in the traditional ITIES configuration (Figure 1). The current within the potential window appears to be largely capacitive, although there are two broad peaks in the region of 0–0.3 V. These peaks may be the result of adsorption or intercalation of supporting electrolyte ions on/into the graphene electrodes.

We have shown that interfacial-assembled graphene, acting as a simple electron conductor, displays a catalytic effect towards the ORR at the water j DCE interface, mediated by a DcMFc reducing agent in the DCE phase. The catalytic effect was attributed to both the storage of electrons from DcMFc by graphene (i.e. doping) and an increase in the cross section of the heterogeneous ORR mechanism, thus allowing electron transfer over a larger area of the water j DCE interface than possible in the absence of interfacial graphene. Use of a bipolar setup with discrete liquid phases confirmed the viability of the heterogeneous mechanism proceeding through interfacial graphene. A similar catalytic effect on the ORR has been reported for interfacial GO; although, in the GO case, it was attributed to surface-bound quinone groups present as a result of the high oxygen content in GO.[27] When using a weaker reducing agent, DiMFc, a similar catalytic effect was not observed. This shows that graphene is not an inherently good catalyst towards the ORR, with the increase in reaction cross section provided by graphene only being significant when the driving force for reduction is already high. By varying the aqueous-phase pH from 1 to 2, a switch in selectivity between the two- and four-electron ORRs appears to occur and further probing of this by using the common ion shake-flask methodology and detection of the H2O2 concentration produced would be of interest.[7] The bipolar configuration employed here opens up the potential to gain significant insights into the mechanism of reduction reactions at the ITIES, which have been catalysed by various materials supported on graphitic structures.[32, 34] By individually functionalising the two graphene electrodes, the importance of the location of the catalytic materials with respect to the distinct liquid phases could be probed.

Figure 7. CVs of the bipolar ITIES cell outlined in Scheme 2, in the presence (solid-orange line) and absence (grey dotted line) of 5 mm DcMFc in the DCE phase. The response after purging the aqueous phase with N2 gas for 30 s prior to the measurement (with 5 mm DcMFc in the DCE phase) is shown by the dashed blue curve.

When 5 mm DcMFc was added to the DCE phase, a positive current was observed that appeared to be irreversible (orange curves, Figure 7). This response is very similar to that seen in the traditional ITIES cell when both DcMFc(DCE) and interfacial graphene are present (Figure 4). Upon purging the aqueous phase with N2 gas for 30 s, the magnitude of the current was dampened, confirming the involvement of aqueous oxygen in the process giving rise to the current. We, therefore, attribute this current response to the heterogeneous reduction of aqueous oxygen by DcMFc located in the DCE phase. Unfortunately, the small volumes (2–3 mL) of the liquid phases used in this experiment prevented purging of the aqueous phase for long time periods without significant evaporation occurring. These bipolar studies have shown that graphene acts as an electron conduit in the heterogeneous ORR. When comparing the CVs in the bipolar case (Figure 7) to those in the traditional ITIES setup (Figure 4), it is apparent that the onset potential for ChemElectroChem 2016, 3, 472 – 479

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Experimental Section Chemicals and Materials All chemicals were used without further purification. Acetone (+ 99 %), bis(triphenylphosphoranylidene)ammonium chloride (BTPPACl, 97 %), decamethylferrocene (DcMFc, 97 %), 1,2-dichloroethane (DCE, Chromasolv, + 99.8 %), 1,1-dimethylferrocene (DiMFc, 95 %), ethanol (+ 99.8 %), tetramethylammonium chloride (TMACl, + 99 %) and tetrapropylammonium chloride (TPrACl, + 99.8 %) were purchased from Sigma–Aldrich, UK. Sodium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (NaTFPB, 97 %) was purchased from Alfa Aesar, USA. Natural graphite flakes (NGF, 2369) were supplied by Graphexel Ltd., UK. The platinum wire (0.5 mm diameter, 99.99 %), platinum mesh and silver wire (1 mm diameter, 99.9 %) were purchased from Advent Research Materials Ltd., UK. Silver conductive paint was purchased from HK Wentworth Ltd., UK. Hydrophobic polyvinylidene fluoride (PVDF) membranes (Durapore, 0.45 mm pore size, 13 mm diameter) were purchased from Millipore

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Articles Corporation, USA. Parafilm was purchased from Pechiney Plastic Packaging, USA. Bis(triphenylphosphoranylidene)ammonium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (BTPPATFPB) was synthesised through metathesis of BTPPACl and NaTFPB, as described previously.[51] A deviation in this procedure was used here, as a 2:1:1 acetone/ethanol/water mixture was used for the initial metathesis reaction and a 1:1 acetone/ethanol mixture was used for the recrystallisation step. All water was deionised (18.2 MW cm) and obtained from a Milli-Q Direct 8 water purification system (Millipore Corporation, USA).

tivity. The DCE electrode was transferred to a polyethylene terephthalate (PET) substrate by compression at approximately 2–4 MPa. This was to avoid wetting of the substrate on which the DCE electrode was supported. The two graphene electrodes were then connected by a silver wire, which was contacted to each electrode with silver paint. The electrodes were inserted into each liquid phase such that only the graphene, and no silver, was in contact with the liquids. It was not possible to correct the applied potential to a Galvani potential difference by using ion transfer in the bipolar setup. The thermodynamics of a bipolar ITIES should be equivalent to those of the equivalent setup with contacted liquid phases.[55] Therefore, the average correction determined in the equivalent standard ITIES cells was used to correct the applied potentials in the bipolar cell to Galvani potential differences.

Graphene Dispersion and Assembly NGF flakes were dispersed in DCE at an initial concentration of 1 mg mL@1 by using a sonication bath (Elmasonic P 70 H sonic, 70 % power, 37 kHz) for 2 h. To remove large aggregates and particles, the dispersions were left to stand for 24 h before the supernatant was removed and centrifuged for 45 min (587 rpm/30 G, Sigma 2–16 Benchtop Centrifuge). The subsequent supernatant was removed as the final dispersion.[52] The dispersions of NGF prepared by using this method have been characterised previously and shown to be composed, primarily, of few-layer graphene, ranging between 2 and 20 layers.[47] Any supporting electrolytes required for electrochemical studies were dissolved in the dispersion at this stage. The assembly of dispersed NGF at the water j DCE interface was performed within the cell used for the electrochemical measurements of the ITIES. The aqueous phase was added on top of the NGF dispersion in DCE and the entire cell was sonicated (Elmasonic P 70 H sonic, 30 % power, 37 kHz) for < 1 min. Some emulsification of the liquid phases occurred during sonication, so the cells were left to stand overnight to allow emulsion coalescence prior to electrochemical measurements. After this procedure, all graphene appeared to be adsorbed at the water j DCE interface. Parafilm was used to cover the open top of the ITIES cells during assembly to minimise evaporation of the liquid phases.

Acknowledgements The authors would like to thank the EPSRC for funding (grant ref. EP/K007033/1 and a DTA studentship for A.N.J.R.). Keywords: bipolar electrochemistry · catalysis · graphene · interfaces · reaction mechanisms [1] W. Nernst, E. H. Riesenfeld, Ann. Phys. 1902, 313, 600 – 608. [2] F. Reymond, D. Fermin, H. J. Lee, H. H. Girault, Electrochim. Acta 2000, 45, 2647 – 2662. [3] P. Vany´sek, L. B. Ramirez, J. Chil. Chem. Soc. 2008, 53, 1455 – 1463. [4] M. Velicky´, A. N. J. Rodgers, R. A. W. Dryfe, K. Y. Tam, Admet Dmpk 2014, 2, 143 – 156. [5] A. Troj#nek, J. Langmaier, Z. Samec, Electrochem. Commun. 2006, 8, 475 – 481. [6] A. Troj#nek, V. Marecˇek, H. J-nchenov#, Z. Samec, Electrochem. Commun. 2007, 9, 2185 – 2190. [7] B. Su, R. P. Nia, F. Li, M. Hojeij, M. Prudent, C. Corminboeuf, Z. Samec, H. H. Girault, Angew. Chem. Int. Ed. 2008, 47, 4675 – 4678; Angew. Chem. 2008, 120, 4753 – 4756. [8] I. Hatay, B. Su, F. Li, M. A. M8ndez, T. Khoury, C. P. Gros, J.-M. Barbe, M. Ersoz, Z. Samec, H. H. Girault, J. Am. Chem. Soc. 2009, 131, 13453 – 13459. [9] R. Partovi-Nia, B. Su, F. Li, C. P. Gros, J.-M. Barbe, Z. Samec, H. H. Girault, Chem. Eur. J. 2009, 15, 2335 – 2340. [10] A. Troj#nek, J. Langmaier, B. Su, H. H. Girault, Z. Samec, Electrochem. Commun. 2009, 11, 1940 – 1943. [11] I. Hatay, B. Su, M. A. M8ndez, C. Corminboeuf, T. Khoury, C. P. Gros, M. Bourdillon, M. Meyer, J.-M. Barbe, M. Ersoz, S. Z#lisˇ, Z. Samec, H. H. Girault, J. Am. Chem. Soc. 2010, 132, 13733 – 13741. [12] B. Su, I. Hatay, F. Li, R. Partovi-Nia, M. A. M8ndez, Z. Samec, M. Ersoz, H. H. Girault, J. Electroanal. Chem. 2010, 639, 102 – 108. [13] B. Su, I. Hatay, A. Troj#nek, Z. Samec, T. Khoury, C. P. Gros, J.-M. Barbe, A. Daina, P.-A. Carrupt, H. H. Girault, J. Am. Chem. Soc. 2010, 132, 2655 – 2662. [14] R. Partovi-Nia, B. Su, M. A. M8ndez, B. Habermeyer, C. P. Gros, J.-M. Barbe, Z. Samec, H. H. Girault, ChemPhysChem 2010, 11, 2979 – 2984. [15] A. J. Olaya, P. Ge, J. F. Gonthier, P. Pechy, C. Corminboeuf, H. H. Girault, J. Am. Chem. Soc. 2011, 133, 12115 – 12123. [16] P. Peljo, T. Rauhala, L. Murtom-ki, T. Kallio, K. Kontturi, Int. J. Hydrogen Energy 2011, 36, 10033 – 10043. [17] I. Hatay Patir, J. Electroanal. Chem. 2012, 685, 28 – 32. [18] H. Deng, P. Peljo, F. Cort8s-Salazar, P. Ge, K. Kontturi, H. H. Girault, J. Electroanal. Chem. 2012, 681, 16 – 23. [19] A. J. Olaya, D. Schaming, P.-F. Brevet, H. Nagatani, T. Zimmermann, J. Vanicek, H.-J. Xu, C. P. Gros, J.-M. Barbe, H. H. Girault, J. Am. Chem. Soc. 2012, 134, 498 – 506. [20] Y. Li, S. Wu, B. Su, Chem. Eur. J. 2012, 18, 7372 – 7376.

Electrochemical Measurements Electrochemical measurements were performed by using a fourelectrode setup, with a Pt-mesh counter electrode (CE) and a Ag/ AgCl reference electrode (RE) in each liquid phase. A PGSTAT100 potentiostat (Metrohm Autolab B.V., Netherlands) was used to control the Galvani potential and monitor the passage of current. The standard ITIES setup used a three-arm cell, with two capillary arms for the REs and a third arm allowing direct access to the DCE for a CE without disruption of the liquid j liquid interface. The aqueous RE was inserted through the open top of the cell. Applied potentials were corrected to the Galvani potential difference by using the standard ion transfer of either TMA + (DwDCE @0TMAþ = 0.16 V)[53] or TPrA + (DwDCE @0TPrAþ = @0.091 V).[54]

Bipolar Setup The bipolar ITIES configuration consisted of two separate liquid phases, using the same four-electrode setup as the standard ITIES configuration, as illustrated in Figure S4 of the Supporting Information. The discrete liquid phases were electrically connected by two graphene electrodes, one immersed in each phase. The electrodes were prepared by vacuum filtering approximately 0.5 mg of NGF dispersed in DCE through a PVDF membrane. The aqueous electrode remained on the hydrophobic PVDF and was compressed at 2–4 MPa to improve flake-to-flake contacts and electrode conducChemElectroChem 2016, 3, 472 – 479

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Manuscript received: August 9, 2015 Accepted Article published: September 21, 2015 Final Article published: October 22, 2015

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