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Altered Electrochemistry at Graphene or Alumina Modified Electrodes: Catalysis Vs. Electrocatalysis in Multistep Electrode Processes Jeffrey Chun Yin Poon, Qianqi Lin, Christopher BatchelorMcAuley, Chris Salter, Colin Johnston, and Richard G Compton J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b04335 • Publication Date (Web): 22 May 2015 Downloaded from http://pubs.acs.org on June 4, 2015

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The Journal of Physical Chemistry

Altered Electrochemistry at Graphene or Alumina Modified Electrodes: Catalysis vs. Electrocatalysis in Multistep Electrode Processes Jeffrey Poon,†‡ Qianqi Lin,†‡ Christopher Batchelor-McAuley,*‡ Chris Salter,§ Colin Johnston,§ Richard G. Compton*‡ † *

‡

Both authors contributed equally to this paper To whom correspondence should be addressed

Department of Chemistry, Physical and Theoretical Chemistry Laboratory, Oxford University, South Parks Road, Oxford OX1 3QZ, U.K. §

Department of Materials, Oxford University, Begbroke Science Park, Begbroke Hill, Oxford OX5 1PF, U.K.

* Email: [email protected] Phone: +44 (0)1865 275448 *Email: [email protected] Phone: +44 (0) 1865 275957

Fax: +44 (0) 1865 275410

KEYWORDS Graphene, alumina, modified electrodes, porous layer, thin layer voltammetry, electrocatalysis, multistep electrode processes

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ABSTRACT The voltammetric behaviour of glassy carbon electrodes modified with either sub-monolayer quantities of alumina or graphene is examined in respect of the two electron, two proton reaction of catechol (1,2-dihydroxybenzene) in aqueous solution. Whilst the voltammetric behaviour superficially hints at a change of electron transfer rate, an alternative explanation in terms of the changed thermodynamics of the intermediate species is advanced and rationalised. 1.

INTRODUCTION

A vast effort is currently being undertaken in respect of seeking modified electrodes, so as to ‘improve’ the electrochemical response of less than optimal surfaces. In many cases this involves the modification of relatively low cost electrodes, such as those made of carbon. Small quantities of a diverse range of materials, often nanomaterials, are used to bring about ‘electrocatalysis’. Both carbon nanotubes (single-walled,1 multiwalled,2 doped,3 etc.) and graphene4-5 are highly popular nanomaterials, and have been championed as especially effective for this purpose.6 Although the ultimate use of the modified electrodes would often be in energy transformation technologies, such as batteries and fuel cells, the method adopted for the evaluation of potential electrocatalysts is almost exclusively voltammetry. In this case, the electrode process of interest:

(1) is examined in an electrochemical cell through the variation of current with applied potential, the latter usually applied in the form of ‘cyclic voltammetry’ with a triangular potential waveform. The voltammetry generates a voltammetric peak in the forward scan corresponding to the conversion of A to B local to the electrode and then a reverse scan used to bring about the reverse transformation, namely of B to A. Typically the separation of the two peak in terms of voltage, ∆Epp, is used as a measure of the electron transfer rate. Assuming A and B are both solution phase species free to diffuse, then the limit of ∆Epp corresponding to fast electron transfer kinetics (relative to prevailing mass transport rates) gives the so-called ‘reversible’ limit and a peak-to-peak separation of:7-8 2 ACS Paragon Plus Environment

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∆ = 2.218

(2)

∆ = 57 (298K) For slower electron transfer kinetic, in the ‘irreversible’ limit:

ln +

∆ =

=

.

(3)

!"#$ + (in mV, 298K)

where α (or β, β=1-α) is the transfer coefficient for the process occurring on the forward going reaction, R is the ideal gas constant (8.314 J K-1 mol-1), T is the absolute temperature, F is the Faraday constant (96485 C mol-1), and v is the voltammetric scan rate; α corresponds to a reduction, β to an oxidation in the Butler-Volmer parameterisation.9 It is important to note that it is the peak-to-peak separation which indicates fast or slow electron transfer rather than the size of the current peak; the change in the latter for a simple one electron transfer is merely ca. 30% between the fully reversible and irreversible limits.10 Experimental analysis is however not usually as straightforward in practice as the above implies. Underpinning this is the key notion that ‘fast’ and ‘slow’ electron transfer refers to that relative to the local, prevailing rates of mass transport at the relevant electrode surface as quantified by the mass transport coefficient, mT (cm s-1).7 It follows that ‘reversibility’ is not an intrinsic property of the A/B redox couple of interest, but also of the electrode used to probe the voltammetry, and indeed the selected voltage scan rate; put simply the rates of mass transport increase from macro- to micro-electrodes (and to nano-electrodes) and also as the voltage scan rate increase. Hence the degree of reversibility decreases and the electron transfer kinetics are ‘apparently’ slower. Furthermore, the change between the reversible and the irreversible limits also reflects the electrode surface morphology. Equations (2) and (3) assume semi-infinite diffusion to a planar electrode. Any change in these mass transport conditions can indirectly give the illusion of altered electrode kinetics.



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It has been pointed out that there can be extreme situations of altered mass transport profoundly affecting the apparent electrode transfer kinetics when electrodes are modified with relatively thick porous layers of conductive material.11-14 Examples might include graphene or carbon nanotubes, and by ‘relatively’ thick the key distance scale is that of the diffusion layer to the unmodified electrode.13,

15

Under these conditions the voltammetric responses reflect first a ‘thin

layer’ contribution for the electrolysis of material occluded within the porous, conductive layer, and second a contribution from diffusion from bulk solution to the surface of the layer. The facilitated mass transport occurring via the first mode is such that the voltage required for electrolysis (and the generation of a voltammetric peak) is often less than that required under conditions of semi-infinite diffusion to a planar electrode; voltammetry under thin layer conditions is more ‘reversible’. Indeed under extreme conditions it is even possible to see the two peaks for an A/B couple in voltammograms, one from each mode of transport.16 Considerations of electrode modification with graphene17-18 and with various types of carbon nanotubes11-12, 14, 19-20, has suggested that the altered mass transport resulting from porous conducting layers used to modify the electrodes has played a significant role in possibly overestimating the electrocatalytic effect of the modifying layers. Indeed Punckt credibly asserts that this is the dominant effect seen in graphene modified electrode and the major role of the modification lies in altered mass transport rather than in enhanced electron transfer kinetics.17 Three related points are important. First the modification can be beneficial even if the mass transport is the dominant effect; specifically since in the thin layer reversible limit the voltammetric peaks shift to occurring at potentials reflecting the formal potential of the various A/B couples, the modification can then usefully improve analytical selectivity16-17 and different voltammetric peaks which are superimposed under purely diffusional conditions can become resolved. Second even if the ‘modifying’ layer is not thick in size but merely ‘roughens’ the electrode surface, then measureable effects can be seen.21 Equations have been devised for the effective electrode rate constants both for porous layers and rough surfaces.21-22 Third the use of hydrodynamic electrodes does not in itself overcome the porous/non-porous issue but may give the impression of minimising the effects23-24; this


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is particularly noteworthy in the case of the search for oxygen reduction catalysts,25 where rotating disc electrodes are often employed, usually in a double-electrode (ring-disc) configuration to probe the intermediacy of hydrogen peroxide or not and have established 2 or 4 electron processes.13, 25 Most studies of electron transfer processes at nanocarbon modified electrodes has focused on simple one electron processes such as the [Ru(NH3)6]2+/3+, [Fe(CN)6]2+/3+ redox couples. Notwithstanding the potential complexity of even the relatively simple systems as discussed above, it is of considerable interest to extend the discussion to a consideration of multi-step electron process, not least since systems such as O2/MeOH, O2/H2O, N2H4/N2 etc. are often the ultimate targets for the design of improved electron transfer catalysts. Moreover a multistep process offers additional scope for mechanistic complexity and the probability of say graphene or carbon nanotube modifications acting in hitherto unexpected roles. We have selected the two electron, two proton oxidation of catechol (1,2-hydroxybenzene, H2C) as a model multistep process. The process focuses on 1,2-benzoquinone (Q) as a product:

Scheme 1: Two proton, two electron oxidation of catechol The main reason for this choice is because the mechanism of this process at unmodified glassy carbon electrodes has recently been studied in depth via a (partial) scheme of squares type analysis.26 Specifically the process proceeds via a semiquinone radical/radical ion intermediate (SH) and the voltmmetry suggests a behaviour where the H2C/SH potential occurs at more positive values than the SH/Q potential. This phenomenon is often referred to as a ‘potential inversion’ and has been seen in some other quinone systems but not in all.27 In the case of catechol both the oxidation and reduction peaks behave voltammetrically in a seemingly electrochemically reversible manner but with a significantly wider peak-to-peak separation than expected for a single Nernstian two electron transfer.



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In particular for pH 5.0, the following oxidation process was suggested, supported by DigiSim (Bioanalytical System Inc., IN, USA) modelling:

(4)

(5) For the reverse, reductive process:

(6)

(7) We previously reported the apparent modification of the above mechanism by alumina particles used to modify the glassy carbon electrode surface. In the present paper we build on these preliminary results and also make comparison with graphene modified electrodes. Qualitatively, but not quantitatively, similar behaviour is seen with the two systems which is interesting since the alumina is, unlike graphene, not electronically conducting and moreover extremely small coverages of alumina are found to be effective. The systems individually and comparatively provide a new insight into a possible route for electrode modifier effects in changing the apparent electrode kinetics of a multistep redox system. 2.

EXPERIMENTAL

2.1 Chemicals All chemicals were provided by Sigma-Aldrich at reagent grade unless specified otherwise. The graphene nanoplatets (GNPs) were procured from Strem Chemicals, MA, USA. All reagents were used without further purification. All solutions prepared were prepared with deionised water of resistivity no less than 18.2 MΩ cm-1 at 298K (Millipore, Billerica, MA, USA). Buffer solutions for a range of pH values, from 1 to


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13, were prepared using various recipes: 0.10 M hydrochloric acid for pH 1.0, 0.10 M sodium citrate (BDH Merck, Dorset, UK)/citric acid for pH 3.0 and pH 5.0, 0.10 M potassium phosphate monobasic/potassium phosphate dibasic for pH 7.0, 0.10 M sodium carbonate (BDH Merck)/sodium bicarbonate for pH 9.0, and sodium hydroxide for pH 11.0 and 13.0. 0.10 M potassium chloride was used as the supporting electrolyte for all experiments. Solution pH values were measured using a Hannah pH213 pH meter (Hannah Instruments, Bedfordshire, UK). 0.50 mM catechol solutions of various pH values were prepared freshly prior to any experiments using the buffer solutions mentioned above. Buffer solutions were saturated with pure nitrogen gas prior to solution preparation, by bubbling the solutions for 30 minutes to prevent solution degradation by atmospheric oxygen. 2.2 Equipment Voltammetric measurements were recorded using an Autolab PGSTAT101 (Autolab, The Netherlands) with a standard three-electrode configuration. Representative voltammograms are all baseline corrected for ease of comparison. Two glassy carbon electrodes (GCEs, diameter 3.0 mm, ALS distributed by BASi, Tokyo, Japan) were used one at a time as working electrodes. One electrode (electrode 1) was used exclusively for alumina electrode modification and the other (electrode 2) was used exclusively for GNP electrode modification. This was to avoid cross contamination of the modified electrode by another modification agent. A saturated calomel electrode (SCE, ALS distributed by BASi, Tokyo, Japan) was used as the reference electrode, and a platinum gauze as the counter electrode. All electrochemical experiments were performed in a three necked flask placed inside a Faraday cage in room temperature. For an unmodified surface, the GCE working electrode was polished by using diamond sprays (Kemet, Kent UK) of decreasing particles sizes: 3.0 - 0.1 µm. To obtain a modified surface for electrode 1, the electrode was polished using alumina slurries of decreasing particle sizes: 1.0 – 0.05 µm (Buehler, IL, USA). Electrode 1 was then modified by polishing, 200 figure of eight cycles with each figure of eight polish constituting one cycle, the electrode surface by using 1.0 µm alumina and 7 ACS Paragon Plus Environment


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rinsed with water. To modify the surface for electrode 2, the electrode was polished using diamond sprays of decreasing particles sizes to obtain an unmodified surface. The electrode was then modified by polishing the electrode surface using slurry of GNP for 200 figure of eight cycles, and subsequently rinsed with water. Both electrodes 1 and 2 were dried by pure nitrogen before measurements.

2.3 Simulation Cyclic voltammetric responses were simulated using the commercial software package DIGISIM (version 3.03b, BASi Technicol, West Lafayette, IN). DIGISIM is based on Butler-Volmer kinetics, and use the fully implicit finite difference (IFD) method proposed by Rudolph.

28-29

All simulations

were set as two consecutive one electron transfers. Simulations were done in a potential range from 0.4 to +1.0 V with a scan rate of 100 mV s-1. Pre-equilibration was enabled. Concentration of catechol (H2C) was set as 0.50 mM, while the initial concentrations of the semiquinone intermediate (SH) and 1,2-benzoquinone product (Q) were set as 0 mM. Diffusion coefficients of all species involved were set to be 1.1 × 10-5 cm2 s-1 as determined by Lin et al.26 The diffusion mode was set as semi-infinite for a geometrically planar electrode with an area of 0.0707 cm2.

2.4 Scanning Electron Microscopy The imaging of the unmodified, alumina modified and graphene nanoplatelets (GNP) modified glassy carbon electrode (GCE) surfaces was done by Scanning Electron Microscopy (SEM). For the unmodified and alumina modified GCE surfaces, a JEOL JSM-6500F Scanning Electron Microscope (JEOL GmbH, Eching b. München, Germany) was used for the SEM imaging using an acceleration voltage of 10.0 kV and a detector in in-lens geometry. The GNP modified GCE surface was imaged with a Zeiss Merlin-Analytical Scanning Electron Microscope (Zeiss, Oberkochen, Germany) using an acceleration voltage of 1.0 kV and a detector in in-lens geometry. The GCE surface modification procedures are described in detail in the Equipment section. 3. RESULTS AND DISCUSSION


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We first report the voltammetry of catechol with unmodified and with alumina modified glassy carbon electrodes (GCE). Second we discuss the corresponding voltammetry at a graphene modified GCE and similar effects are observed as for alumina. Third we analyse the graphene voltammetry and show that the causes other than changed electrode kinetic, mass transport may operate in this multistep process. 3.1 Catechol Catalysis at Alumina Modified Electrodes 25 20 15 10

Current / µA

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5 0 -5 -10 -15 -20 -0.4

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

Potential / V vs. SCE Figure 1 Representative voltammogram taken in 0.50 mM catechol solution in 0.10 M citric acid/sodium citrate buffer (pH 5.0) in 0.10 M KCl. Red: alumina modified GCE. Black: Unmodified GCE. Scan rate = 100 mV s-1. Figure 1 shows the voltammogram in a 0.50 mM solution of catechol in pH 5.0 buffer measured at a scan rate of 100 mV s-1 using an unmodified GCE (black line). Our previous work26 has shown that the scan rates, concentration and pH behaviour of this system is consistent with a potential inversion in which EH2C/SH – ESH/Q = 965 mV. The following other parameters were found to model the system when simulated using DigiSim: k0 = 10 cm s-1, α = 0.50, diffusion coefficients of H2C as (1.1 ± 0.1) × 105 cm2 s-1, and disproportionation rate constant (kf) of SH as at the diffusion controlled limit (~ 1010 dm3 mol-1 s-1). Figure 1 (red line) shows the corresponding voltammogram measured with a GCE surface modified with a sparse coverage of alumina. The comparison of voltammograms across the full pH range was also obtained (from 1.0 to 13.0, see Supporting Information Figure SI54 to SI16).



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The GCE surface was modified following the method described by Lin et al.26 The GCE (electrode 1) was polished by using alumina slurry of decreasing particle size: 1.0, 0.3, and 0.05 µm. Afterwards the electrode was clean by sonication. The electrode was then polished by using 1.0 µm alumina. The surface was then rinsed with water and blow-dried with a jet of pure nitrogen gas.

Figure 2 Scanning electron microscope image of alumina modified glassy carbon electrode surface.

It can be seen that the voltammogram seen from the unmodified electrode with its widely separated peaks (~ 385 ± 7 mV) is replaced by one in which the peak-to-peak separation is much reduced (~ 80 ± 4 mV) at the alumina modified electrode surface. The peak-to-peak separation can also be seen reduced at the pH range from 1.0 to pH 9.0 (see Supporting Information Figure SI4). Figure 2 shows the SEM image of the alumina modified surface; it can be seen that ca. 2 % of the surface is covered with particles of ca. 1 µm diameter which, are on average 11 ± 4 µm apart. It is evident that the tiny amount of alumina has a major impact on the observed voltammogram whilst the extent of coverage and the surface roughness resulting from the modification can have only minimal impact on the mass transport prevailing to the electrode surface. Moreover the alumina is nonconducting so cannot directly impact the voltammetry. It is likely26 that the role of the alumina is to provide adsorption sites which modify the reaction pathway. Note however the midpoint potential of the voltammograms in Figures 1 (black and red) are the same (Emid = 0.278 ± 0.006 V) to within the experimental error suggesting that the process:


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(8) remains overall essentially unchanged thermodynamically. It is likely therefore that the adsorption involves the intermediate semi-quinone species. Note that the separation distance between the alumina particles is less than √2&, where t is the voltammetric timescale of the experiments so that Case 4 of a partially modified electrode operates7,

30-31

and hence peak shaped voltammograms are seen

reflecting catechol diffusion to the full geometric area of the electrode. 3.2 Catechol Oxidation at Graphene Modified Electrodes

Graphene nanoplatelets (GNPs) were first characterised using SEM imaging. The nanoplatelets were determined to have an average width of 16.5 ± 5 µm and an average thickness of 7.1 ± 2 nm.32 This is in concordance to the specification sheet provided by the supplier (Strem). The GCE (electrode 2) was polished using diamond sprays of decreasing particle sizes mentioned in the Experimental section. The electrode was then modified with GNPs by polishing the electrode in a GNP slurry. 20 15 10

Current / µA

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5 0 -5 -10 -15 -20 -0.4

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

Potential / V vs. SCE Figure 3: Representative voltammogram taken in 0.50 mM catechol solution in 0.10 M citric

acid/sodium citrate buffer (pH 5.0) in 0.10 M KCl. Blue: GNP modified GCE. Black: Unmodified GCE. Scan rate = 100 mV s-1. Next voltammograms were recorded in 0.50 mM catechol solution in 0.10 M citric acid/sodium citrate (pH 5.0) buffer with 0.10 M potassium chloride as the supporting electrolyte, for



both unmodified and GNP modified GCEs. Typical voltammograms are shown in Figure 3. At a scan rate of 100 mV s-1, the peak potentials are altered and the wide peak-to-peak separation of the unmodified GCE (~385 ± 7 mV) is replaced by a narrower separation of the GNP modified GCE (~123 ± 3 mV). 25 20 15

Current / µA

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10 5 0 -5 -10 -15

increasing pH

-20 -0.4 -0.3 -0.2 -0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

a)

Potential / V vs. SCE

b)

Figure 4: GNP modified GCE voltammograms taken in 0.50 mM catechol in 0.10 M KCl in solutions

of various pH values. a) Representative voltammograms in various pH values. Black: pH 1.0. Red: pH 3.0. Blue: pH 5.0. Pink: pH 7.0. Green: pH 9.0. Dark Blue: pH 11.0. Purple: pH 13.0. b) Average forward (upwards triangle), backward (downwards triangle) peak positions, and midpoint potential (square) with standard deviation error bars fitted. Black: unmodified GCE. Red: GNP modified GCE. Blue line: gradient of the midpoint potential squares. Scan Rate = 100 mV s-1. Figure 4 presents data obtained across the full pH range (from 1.0 to 13.0). It is evident that modification using graphene has an influence on the voltammetry, albeit not to the same extent as seen for the case of alumina modifying the GCE surface. For example, at pH 5.0 and a scan rate of 100 mV s-1, the alumina modification causes a change of peak-to-peak separation from 385 ± 7 mV for unmodified electrode to 80 ± 4 mV for the alumina modified GCE electrode 1, a 305 ± 8 mV decrease. In contrast the graphene nanoplatelets modification causes a change from 385 ± 7 mV for the unmodified electrode to 123 ± 3 mV for the GNP modified electrode 2, a 262 ± 8 mV decrease.


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It is worth noting the peak potential varies significantly with respect to changes in pH. For a fully electrochemically reversible electrode process, the midpoint potential (Emid) obeys the following equation: '() ~ − 2.303

'

/0 .

(9)

where m and n are respectively the number of protons and electrons involved in the electrochemical reaction, R is the ideal gas constant (8.314 J K-1 mol-1), T is the absolute temperature, and F is the Faraday constant (96485 C mol-1). From Equation (9), it can be seen that Emid varies by 59(m/n) mV per pH unit. At pH values ranging from pH 1.0 to 9.0, the experimental measurements shown in Figure 4b demonstrates a 59 mV/pH shift from the slope of observed midpoint potential. This is consistent with a two electron, two proton transfer in the electrochemical oxidation of catechol. We next used DigiSim to model the observed voltammetry. Due to the multiparameter nature of the redox process the measured diffusion coefficients of H2C was applied to the species SH and Q. The rate constant, k0, and transfer coefficient, α = 0.50, were approximated to be the same for both redox couples H2C/SH and SH/Q. The two key parameters in controlling the observed peak-to-peak separation are the electron transfer rate constants (k0) and the two formal potentials of the H2C/SH and SH/Q redox couples. The difference of the formal potentials, ∆Ef is defined as: ∆1 = 1$ H3 C/SH − 1$ SH/Q

(10)

where Ef 0(H2C/SH) is the formal potential of the H2C/SH redox couple and Ef0(SH/Q) is the formal potential of the SH/Q redox couple.



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Figure 5: Simulated voltammograms with ∆Ef = 1.0 V with varying k0. Black: k0 = 1 cm s-1. Red: k0 =

10 cm s-1. Blue: k0 = 100 cm s-1. Inlay: simulation demonstration of the effect at the parameters k0 and ∆Ef’s on the voltammetric peak currents. Black squares: k0 = 1 cm s-1. Red Squares: k0 = 10 cm s-1. Blue squares: k0 = 100 cm s-1. Green line: Reversible current limit. Purple line: Irreversible current limit. Figure 5 shows how the two parameters influence the peak current which, within the equal diffusion coefficient approximation, can give a partial resolution of the true parameters in terms of their control over the overall process. Specifically for very fast electrode kinetics the voltammetric peak behaves as a fully reversible, Nernstian, peak such that the peak current, Ip, is proportional to n3/2 where n (=2) is the number of electrons transferred. For less fast rate constants the peak current scales with n which reflects twice the current seen from a single one electron process. This changeover is evident in Figure 5. At a constant k0 value, the increase of ∆Ef value increases the peak-to-peak separation (∆Epp) after surpassing a particular ∆Ef limit. For example in Figure 5, with k0 = 1 cm s-1, the ∆Epp starts to increase from ∆Ef ≈ 0.2 V onwards. In contrast, with k0 = 10 cm s-1, the ∆Epp starts to increase from ∆Ef ≈ 0.4 V onwards.


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a)

b)

Figure 6: Contour plots of combined percentage errors of simulation fitting for a) unmodified GCE

and b) GNP modified GCE. Any error above 400% is coloured grey. Experimental scan rate = 100 mV s-1. Black dots: data points of simulation. Figure 6 shows the approximate modelling of the peak-to-peak separation and peak current using the difference in formal potential, ∆Ef, and the average rate constant, k0, as simulation fitting parameters. In the fitting, parameters k0 and ∆Ef are varied. The simulated peak-to-peak separation and forward peak current value for each k0 and ∆Ef setting generates its own percentage error with respect to the experimental values. The combined error (see Supporting Information for details of the calculation method) allows the contour plots of Figure 6 to reflect the goodness of fit for both properties concerned, the peak-to-peak separation (∆Epp) and forward peak current (Ip). In the contour plot, the colour scheme follows a rainbow palette where the violet end denotes the lowest percentage error and the red end of the spectrum denotes the highest percentage error. A clear ‘trench’ of low error fitting can be seen in both Figure 6a and 6b. Any ‘pits’ of lower error in the ‘trench’ is due to the distribution of the simulation data points. For each plot the ‘trench’ of good fit is of identical gradient, where d∆Ef/dlog10(k0) = 0.24 V. Therefore a good fit is attained for a variety of (log(k0), ∆Ef) points. Drop cast experiments were also done for comparison. The drop cast ∆Epp value reductions are much lower than those of the polishing modification method (see Supporting Information Figure SI1 and SI2). This could be indicative of a passivation of the adsorptive ability of the GNPs due to the use of chloroform as the drop cast suspension medium. It may thus be concluded



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that the shift in relative potential of the oxidative and reductive catechol peaks reflects either changes in k0, ∆E, or a combination of both.

Figure 7: Representative scanning electron microscope image of GNP modified glassy carbon

electrode surface. Within the red circles are the observed nanoplatelets. Figure 7 shows the SEM image of the GNP modified GCE surface; it can be seen that the glassy carbon surface is sparsely populated with GNPs, where the distribution is on average 48 ± 32 µm apart. This is in stark contrast to the pristine surface seen on the unmodified GCE surface (see Supporting Information SI3). It is evident that the tiny amount of GNPs has a significant impact on the observed voltammogram. The sparse coverage implies minimal impact on the surface roughness and mass transport to the electrode surface. However, the effect of GNPs on the catechol oxidation voltammetric response is comparable to the alumina modified GCE images seen in Figure 2. This in turn supports the hypothesis that the changed voltammetry seen in alumina and GNP modified GCE surfaces are due to the provision of adsorption sites by the alumina and GNPs to allow an alternative catechol oxidation pathway.


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3.3 Discussion

The results reported in the two previous sub-sections show a profound influence by either adsorbed alumina or adsorped graphene on the surface of GCEs in respect of the two electron, two proton oxidations of catechol. In the case of the alumina modification the modifier is not conductive and it seems likely that the role of the alumina is to adsorb the semi-quinone intermediate:

(11)

(12)

(13) where the weak adsorption of H2C and Q is suggested by the voltammetry of the catechol immobilised on the alumina and graphene nanoplatelets. The coverage of alumina in these experiments is tiny but SEM shows the interparticle separation to be consistent with Case 4 diffusion.7,

30-31

and here the

observed peak shaped voltammetry reflects catechol transport to the full geometric area of the GCE. In the case of graphene the observed behaviour, although not as catalytically apparent as with alumina, the behaviour can be rationalised in terms of relatively altered formal potentials of the H2C/SH and SH/Q redox couples and/or altered electrode kinetics. A ‘unified’ view of the observed catalysis can be proposed. In particular one mechanism in which a multistep electron transfer process can be catalysed can be understood in terms of the catechol to quinone reaction, via semi-quinone reaction as discussed above. Specifically if the semiquinone radical is stabilised by the presence of an electrode modifier, for example via surface adsorption onto the modifier, then one would expect both of the types of catalysis seen for the systems reported. Thus we have seen that adsorption of semi-quinone is almost essential to understand the alumina catalysis whereas for the graphene enhanced process such as adsorption could explain the relative shifts of the Ef0(H2C/SH) and Ef0(SH/Q) values inferred as one probable cause of the observed



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changed voltammetry. In both cases the energetics of the overall transformation could be altered as proposed in Figure 8, which shows the energetics for an applied electrode potential corresponding to the formal potential of the H2C/Q couple.

Figure 8: Scheme of the energetics of different reaction species where the electrode potential is held

at a value corresponding to Ef0 for the Q/H2C couple. 4.

CONCLUSIONS

The two electron, two proton oxidation of catechol at glassy carbon electrodes has been shown to be catalysed by the presence of either alumina or graphene on the electrode surface. Whilst the precise mechanistic detail of the reaction is necessarily speculative at least in part, the observations are consistent with a stabilisation, probably via adsorption, of the semiquinone radicals on the surface of the modifying materials. Such a phenomena could represent a catalysis of the overall transformation but not an ‘electrocatalytic’ process since the rate of the electron transfer step would only be influenced indirectly. ACKNOWLEDGEMENTS

The research leading to these results has received partial funding from the European Research Council under the European Union’s Seventh Framework Programme (FP/2007-2013)/ERC Grant Agreement no. [320403].


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ASSOCIATED CONTENT Supporting Information

Further information relating to catechol redox on unmodified, alumina modified and graphene modified glassy carbon electrode can be found in the Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org.



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18. Pumera, M., Electrochemistry of Graphene: New Horizons for Sensing and Energy Storage. Chem. Rec. 2009, 9, 211-223. 19. Streeter, I.; Xiao, L.; Wildgoose, G. G.; Compton, R. G., Gold Nanoparticle-Modified Carbon Nanotubes-Modified Electrodes. Using Voltammetry to Measure the Total Length of the Nanotubes. J. Phys. Chem. C 2008, 112, 1933-1937. 20. Keeley, G. P.; Lyons, M. E. G., The Effects of Thin Layer Diffusion at Glassy Carbon Electrodes Modified with Porous Films of Single-Walled Carbon Nanotubes. Int. J. Electrochem. Sci. 2009, 4, 794-809. 21. Ward, K. R.; Gara, M.; Lawrence, N. S.; Hartshorne, R. S.; Compton, R. G., Nanoparticle Modified Electrodes Can Show an Apparent Increase in Electrode Kinetics Due Solely to Altered Surface Geometry: The Effective Electrochemical Rate Constant for Non-Flat and Non-Uniform Electrode Surfaces. J. Electroanal. Chem. 2013, 695, 1-9. 22. Ward, K. R.; Compton, R. G., Quantifying the Apparent ‘Catalytic’ Effect of Porous Electrode Surfaces. J. Electroanal. Chem. 2014, 724, 43-47. 23. Masa, J.; Batchelor-McAuley, C.; Schuhmann, W.; Compton, R., Koutecky-Levich Analysis Applied to Nanoparticle Modified Rotating Disk Electrodes: Electrocatalysis or Misinterpretation. Nano. Res. 2014, 7, 71-78. 24. Guo, S.-X.; Zhao, S.-F.; Bond, A. M.; Zhang, J., Simplifying the Evaluation of Graphene Modified Electrode Performance Using Rotating Disk Electrode Voltammetry. Langmuir 2012, 28, 5275-5285. 25. Gara, M.; Ward, K. R.; Compton, R. G., Nanomaterial Modified Electrodes: Evaluating Oxygen Reduction Catalysts. Nanoscale 2013, 5, 7304-7311. 26. Lin, Q.; Li, Q.; Batchelor-McAuley, C.; Compton, R. G., Two-Electron, Two-Proton Oxidation of Catechol: Kinetics and Apparent Catalysis. J. Phys. Chem. C 2015, 119, 1489-1495. 27. Batchelor-McAuley, C.; Li, Q.; Dapin, S. M.; Compton, R. G., Voltammetric Characterization of DNA Intercalators across the Full Ph Range: Anthraquinone-2,6-Disulfonate and Anthraquinone-2Sulfonate. J. Phys. Chem. B 2010, 114, 4094-4100. 28. Rudolph, M., A Fast Implicit Finite Difference Algorithm for the Digital Simulation of Electrochemical Processes. J. Electroanal. Chem. Interfacial. Electrochem. 1991, 314, 13-22. 29. Rudolph, M., Digital Simulations with the Fast Implicit Finite Difference (Fifd) Algorithm: Part Ii. An Improved Treatment of Electrochemical Mechanisms with Second-Order Reactions. J. Electroanal. Chem. 1992, 338, 85-98. 30. Davies, T. J.; Banks, C. E.; Compton, R. G., Voltammetry at Spatially Heterogeneous Electrodes. J. Solid State Electrochem. 2005, 9, 797-808. 31. Davies, T. J.; Ward-Jones, S.; Banks, C. E.; del Campo, J.; Mas, R.; Muñoz, F. X.; Compton, R. G., The Cyclic and Linear Sweep Voltammetry of Regular Arrays of Microdisc Electrodes: Fitting of Experimental Data. J. Electroanal. Chem. 2005, 585, 51-62. 32. Poon, J.; Batchelor-McAuley, C.; Tschulik, K.; Compton, R. G., Single Graphene Nanoplatelets: Capacitance, Potential of Zero Charge and Diffusion Coefficient. Chem. Sci. 2015, 6, 2869-2876.



TOC Graphic

25 20

Alumina Modified GCE Graphene Modified GCE Unmodified GCE

15

Current / µA

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10 5

–2H+, –2e–

0 -5 -10 -15 +2H+, +2e–

-20 -0.6 -0.4 -0.2 0.0 0.2 0.4

0.6

0.8

1.0 1.2

Potential / V vs. SCE


Altered Electrochemistry at Graphene- or Alumina ... - ACS Publications

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