Assessments of the Effect of Increasingly Severe Cathodic

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Assessments of the effect of increasingly severe cathodic pretreatments on the electrochemical activity of polycrystalline boron-doped diamond electrodes Ricardo F. Brocenschi, Peter Hammer, Claude Deslouis, and Romeu C. Rocha-Filho Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.6b00676 • Publication Date (Web): 21 Apr 2016 Downloaded from http://pubs.acs.org on April 27, 2016

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Analytical Chemistry

Assessments of the effect of increasingly severe cathodic pretreatments on the electrochemical activity of polycrystalline boron-doped diamond electrodes

Ricardo F. Brocenschi,a Peter Hammer,b Claude Deslouis,c Romeu C. Rocha-Filho,a*

a

Departamento de Química, Universidade Federal de São Carlos, C.P. 676, 13560-970 São Carlos – SP, Brazil

b

c

Instituto de Química, Universidade Estadual de São Paulo, 14800-060 Araraquara – SP, Brazil

Laboratoire Interfaces et Systèmes Electrochimiques (LISE), UMR8235, CNRS, Université Pierre et Marie Curie, 4 Place Jussieu, 75005 Paris, France

* Corresponding author:

[email protected] phone: +55 (16) 3351-8078; fax: +55 (16) 3351-8350

ACS Paragon Plus Environment

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ABSTRACT

The electrochemical response of many redox species on boron-doped diamond (BDD) electrodes can be strongly dependent on the type of chemical termination on their surface, hydrogen (HT-BDD) or oxygen (OT-BDD). For instance, on an HT-BDD electrode the [Fe(CN)6]3−/4− redox system presents a reversible voltammetric behavior, whereas the oxidation overpotential of ascorbic acid (AA) is significantly decreased. Moreover, the electrochemical activity of BDD electrodes can be significantly affected by electrochemical pretreatments, with cathodic pretreatments (CPTs) leading to redox behaviors associated to HT–BDD. Here we report on the effect of increasingly severe CPTs on the electrochemical activity of a highly doped BDD electrode, assessed with the [Fe(CN)6]3−/4− and AA redox probes, and on the atomic bonding structure on the BDD surface, assessed by XPS. The hydrogenation level of the BDD surface was increased by CPTs, leading to decreases of the total relative level of oxidation of the BDD surface of up to 36%. Contrary to what is commonly assumed, we show that BDD surfaces do not need to be highly hydrogenated to ensure that a reversible voltammetric behavior is obtained for Fe(CN)6]3−/4−; after a CPT, this was attained even when the total relative level of oxidation on the BDD surface was about 15%. At the same time, the overpotential for AA oxidation was confirmed as being very sensitive to the level of oxidation of the BDD surface, a behavior that might allow the use of AA as a secondary indicator of the relative atomic bonding structure on the BDD surface.


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Boron-doped diamond (BDD) has been increasingly used in diverse electrochemical applications1–17 because of interesting properties such as its very wide potential window, low background currents, reduced fouling, and high oxidation power.4,6,8,10,18 The first three of these properties make BDD a very attractive electrode material for analytical applications5,7–9, in many cases having its versatility extended by nanoparticle functionalization.19–21 Nevertheless, as recently highlighted by Macpherson16, the electrochemical properties of BDD are intimately related to its material properties: boron content, presence of non-diamond impurities, surface morphology, surface termination, and surface finish. Ever since the first investigations on the electrochemical response of redox species on BDD electrodes, it became clear that in many cases such response was strongly dependent on the type of surface termination, i.e. whether predominantly hydrogen (HT-BDD) or different oxygen groups (OT-BDD).22,23 For instance, the voltammetric response of the [Fe(CN)6]3−/4− redox probe is very sensitive to the type of surface termination on the BDD electrode; commonly, a reversible behavior is assumed to be obtainable only on HT-BDD.23,24 On the other hand, as reported by Fujishima and coworkers,25,26 the oxidation peak potential of ascorbic acid (AA) on BDD is highly dependent on its surface terminations and can be modulated by electrochemical pretreatments of the BDD electrode; thus, the AA oxidation overpotential was significantly increased by an anodic pretreatment (APT), but could be largely reversed by cathodic pretreatments (CPTs). Later on, Suffredini et al.27 called attention to the dramatic effect of a CPT on the electrochemical response of BDD for some chlorophenols. They also showed, by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS), that the heterogeneous electron transfer (HET) kinetics of [Fe(CN)6]3−/4− on BDD can be modulated back and forth between slow (irreversible) and fast (reversible) by APTs and CPTs, respectively. Subsequently, CPT–BDD electrodes have been successfully used for the determination of many analytes,28 e.g. synthetic phenolic antioxidants in mayonnaise,29 vanillin in pudding powder,30 bezafibrate31 or nimesulide and paracetamol32 in pharmaceutical formulations, and diclofenac33 or furosemide34 in both pharmaceutical formulations


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and synthetic urine. For some analytes, APT–BDD electrodes led to better electrochemical response, e.g. in the simultaneous determination of sulfamethoxazole and trimethoprim in bovine milk.35 Recently, Trouillon et al.36 reported that CPT improves the resistance of BDD electrodes to dopamine fouling, whereas Duran et al.37 reported that CPT effectively activates a fouled BDD microelectrode in situ. Those reports on electrochemical pretreatments25–27 suggested that hydrogenation or oxidation of the BDD surface could be attainable by proper CPTs or APTs, respectively. SalazarBanda et al.38 reported that BDD surfaces hydrogenated by CPT are oxidized when exposed to air, at a rate that is inversely dependent on the boron-doping content, with significant changes in the HET kinetics of [Fe(CN)6]3−/4−. On the other hand, they also reported XPS results showing that the surface of a BDD electrode (boron-doping content of 800 ppm) that underwent a CPT was still significantly oxidized; similar results were reported by Girard et al.39 Nevertheless, in many instances CPT-BDDs have been carelessly assumed to be HT–BDDs, despite the lack of XPS characterization. The effect of different electrochemical pretreatments on the electrochemical properties of BDD have been further investigated by other laboratories,39–43 but only Hoffmann et al.42 have reported on the use of very severe CPTs to attain a high level of hydrogenation of the BDD surface. By applying a CPT of –35 V in 2 mol L–1 HCl, they obtained a HT–BDD surface even more thoroughly hydrogenated than that of another BDD sample submitted to a hydrogen plasma treatment.42 Nonetheless, recently Macpherson16 noted that “it is still widely believed the only reliable way to reconvert the surface back to its fully H-terminated state is exposure to hydrogen plasma or to hydrogen dose”. In this article we report on the effect of increasingly severe CPTs on the electrochemical properties of highly doped BDD films previously submitted to an APT. The atomic bonding structure on the surface of the differently pretreated BDD electrodes was determined by high-resolution XPS. Their electrochemical properties were assessed using two redox probes, [Fe(CN)6]3−/4− (by CV and


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EIS) and AA (by square-wave voltammetry – SWV), concomitantly investigating whether any of them could be used as a secondary indicator of the hydrogenation level of the BDD surface.

EXPERIMENTAL ASPECTS

The BDD working electrodes (1.2 cm × 1.2 cm; exposed geometric area of 0.32 cm2), acquired from Adamant Technologies, Switzerland (lot WD1344–A), consisted of highly doped BDD thin films (estimated boron-doping level of 10000 ppm; thickness of ~2 µm) deposited by CVD on a polished 1 mm thick p-Si substrate (resistivity of 2 mΩ cm). Before any electrochemical pretreatment, the as-received BDD electrodes were acid washed, as described by Granger and Swain.24 The enrichment of O- or H- terminations on the BDD surface was done electrochemically (APT or CPT, respectively) using a two-compartment glass cell. One compartment contained the BDD working electrode and a reversible hydrogen reference electrode – RHE (in 0.5 mol L–1 H2SO4), whereas the other compartment contained a 1 cm × 1 cm Pt foil counter electrode; the compartments were separated by a porous glass membrane to avoid eventual contamination of the working compartment by platinum ionic species. APT was carried out applying a current density of 200 mA cm–2. On the other hand, CPTs (always preceded by the APT) were carried out at different voltages: −5, −10, −20, and −30 V; mean current densities of –0.14, –0.56, –1.40, and –2.44 A cm–2, respectively. The duration of each electrochemical pretreatment was 300 s (no changes were detected in the BDD surface morphology – SEM data not shown). Highly doped BDD films were used because their CPT hydrogenated surfaces should not be easily oxidized when exposed to air,38 as noted above. High-resolution XPS spectra were used to assess the effect of the different electrochemical pretreatments on the atomic bonding structure of the BDD surfaces. The XPS analyses were carried out at a pressure of less than 0.1 µPa using a commercial spectrometer (UNI-SPECS UHV System).


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The Mg Kα line was used (hν = 1253.6 eV) and the analyzer pass energy was set to 10 eV. The inelastic background of the C 1s and O 1s electron core-level spectra was subtracted using Shirley’s method. The composition (at. %) of the near surface region was determined with an accuracy of ±5% from the ratio of the relative peak areas corrected by the Scofield’s sensitivity factors of the corresponding elements. The spectra were fitted without placing constraints using multiple Voigt profiles. The width at half maximum (FWHM) varied between 0.9 and 1.8 eV and the accuracy of the peak positions was ±0.1 eV. The effect of the different electrochemical pretreatments on the electrochemical activity of the BDD electrode was assessed using two redox probes: [Fe(CN)6]3−/4− (Sigma-Aldrich) in 0.5 mol L–1 H2SO4 (Mallinckrodt, 97%) and ascorbic acid (Sigma-Aldrich) in 0.1 mol L–1 HClO4 (Dinâmica, 72%). For [Fe(CN)6]3−/4− (0.1 mmol L–1), this assessment was done by CV (at 50 mV s–1) and by EIS, recording the spectra at the formal potential (η = 0) of the [Fe(CN)6]3−/4− redox probe, varying the frequency of a 10 mV (rms) ac signal from 10 mHz to 100 kHz. For AA (2.5 mmol L–1), that assessment was done by SWV, using the following values for its instrumental parameters:40 frequency (f), 30 Hz; amplitude (a), 40 mV; scan increment (∆Es), 2 mV. The electrochemical measurements were performed using an Autolab PGSTAT-30 (Eco Chemie) potentiostat/galvanostat, with a frequency response analyzer module, controlled by either the GPES (for CV) or FRA software (for EIS).

RESULTS AND DISCUSSION

Fig. 1a shows the CV responses obtained for 0.1 mmol L–1 [Fe(CN)6]3−/4− (in 0.5 mol L–1 H2SO4) after different electrochemical pretreatments of the BDD electrode. As expected, HET is much slower for the APT-BDD electrode (∆Ep of ~530 mV and significantly attenuated peak currents); this notably non-reversible behavior is similar to others previously reported for APT-BDD


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electrodes (see, e.g. Granger and Swain,24 Suffredini et al.,27 Salazar-Banda et al.,38 Oliveira and Oliveira-Brett41 or Hutton et al.44). On the other hand, similar reversible behaviors are obtained after the different CPTs of the BDD electrode, with ∆Ep = 60 mV and peak currents about 55% higher than those attained with the APT-BDD electrode, indicating that the CPTs greatly enhance the electrochemical activity of the BDD electrode and that the least severe CPT is sufficient to obtain a reversible behavior for the [Fe(CN)6]3−/4− redox system. This reversible behavior is similar to others previously reported for CPT-BDD electrodes (see, e.g. Suffredini et al.,27 Salazar-Banda et al.,38 or Oliveira and Oliveira-Brett41). However, by using the voltammetric response of the [Fe(CN)6]3−/4− redox probe, clearly it is not possible to differentiate the effect of increasingly severe CPTs on the state of the terminations on the BDD surface because that response is invariable with the applied CPTs. From these results, we might infer that the least severe CPT here used (voltage of –5 V) yields a degree of hydrogenation of the BDD surface that leads to a reversible behavior of the [Fe(CN)6]3−/4− redox probe. Thus, any further hydrogenation of the BDD surface ensued from more severe CPTs does not affect the voltammetric response of the [Fe(CN)6]3−/4− redox probe. Therefore, this redox probe cannot be used to differentiate additional effects on the atomic terminations of the BDD surface brought on by increasingly severe CPTs. Consequently, the question was whether these effects could be better differentiated through EIS using the same redox probe? Then, EIS data were also recorded after the different electrochemical pretreatments of the BDD electrode. As can be seen in Fig. 1b, the impedance diagram (Nyquist plot) shows after APT the classical format for a redox system, with a clearly defined, large semicircle related to the charge transfer process (charge transfer resistance, Rct, in parallel with the double-layer capacitance). In contrast, after the different CPTs the Nyquist plots show clearly defined straight lines at 45° with respect to the axes, characteristic of semi-infinite linear diffusion described by a Warburg impedance. Similarly to what has been reported by Deslouis et al.45 on EIS data for a (111) polycrystalline BDD, a Randles equivalent circuit (see Fig. 1c) was fitted to these experimental


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impedance diagrams using a simplex method (imposing a constant relative uncertainty of 1% on the impedance modulus data). Then, the values of the apparent HET rate constants, kapp (cm s–1), were calculated as:

݇ୟ୮୮ =

ܴܶ ‫ ܨ‬ଶ ‫ܴܿܣ‬ୡ୲

where R is the molar gas constant (8.314 J K–1 mol–1), T the thermodynamic temperature (298 K), F the Faraday constant (96485 C mol–1), A the geometrical area (0.32 cm2), c the redox species concentration (1.0 × 10–7 mol cm–3), and Rct the charge transfer resistance (Ω). The thus obtained values of kapp (as well as the values of the parameters of the Randles equivalent circuit) are listed in Table 1. In agreement with the CV results, the value of kapp for the APT-BDD electrode is very low (1.2 × 10–4 cm s–1), reflecting a quite sluggish charge transfer process. In contrast, as expect based on the voltammetric results, the cathodic pretreatments lead to significantly active electrode surfaces, with kapp values in the range of 10–1 cm s–1. Previously, Mahé et al.46 reported a similar value of kapp (1.35 × 10–1 cm s–1) also for a CPT–BDD electrode. According to Hupert et al.4, such high kapp values are characteristic of clean, hydrogen terminated BDD films. From a comparison of the values of kapp obtained for the different CPTs, it is clear that the charge transfer rate increases with the applied CPT voltage. Hence, we can conclude that EIS is more sensitive than CV to differentiate the effect of increasingly severe cathodic pretreatments on the electrochemical activity of the BDD film assessed with the [Fe(CN)6]3−/4− redox system, i.e. the EIS response has a higher sensitivity to the changes on the atomic terminations of the BDD surface produced by these pretreatments. The overpotential for AA oxidation is highly dependent on type of surface terminations on the BDD electrode, as first highlighted by Popa et al.25 and Notsu et al.26 Thus, further investigations were carried out detecting AA (2.5 mmol L–1 in 0.1 mol L–1 LiClO4) by SWV after the different electrochemical pretreatments of the BDD electrode. As can be inferred from the data shown in Fig.


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2, the AA oxidation peak is very sensitive to the BBD surface termination condition. Indeed, compared to the APT-BDD the AA oxidation overpotential decreases with the applied CPT voltage by up to about 200 mV, whereas the peak current increases by about 55%. Thus, it seems that AA might be a good candidate for secondary indicator of the hydrogenation level of the BDD surface. Previously, Notsu et al.26 reported a decrease of the AA oxidation overpotential (determined by CV) of about 650 mV between an APT (2.4 V for 60 min, in 0.1 mol L–1 KOH) and a CPT (–5 V vs. Ag/AgCl for 30 min, in 0.1 mol L–1 HClO4) BBD electrode (boron-doping level of ~500 ppm); this indicates that the extent of activation of the BDD electrode might be dependent on the boron-doping level. From preliminary results obtained using an 800 ppm BDD electrode, we infer that this could be the case; however, this needs to be more thoroughly studied. To obtain further insight on the surface properties, XPS measurements were performed on BDD electrodes that were submitted to the different electrochemical pretreatments. Fig. 3 shows the deconvoluted C1s XPS spectra for (a) APT and (b) CPT (−30 V) BDD electrodes. As the XPS sampling depth for carbon atoms at the used conditions is about 4 nm, the most intense component at ~284.5 eV was attributed to sp3 C−C at the diamond bulk, whereas the second most intense component at ~285.3 eV was assigned to hydrogen terminated sp3 C−C surface groups.42 Components located at higher binding energies were associated with different forms of oxidation, i.e. alcohol or ether, C–O (~286.3 eV), carbonyl, C=O (~287.8 eV), and carboxyl, O–C=O (~289.2 eV).39,40,44 The low energy component at ~284.1 eV was assigned to sp2 C−C, expected at grain boundaries of the polycrystalline material.42 Finally, the position and intensity of the sub-peak at ~282.8 eV are compatible with a contribution from C–B bonds, also detected in the fitted B 1s peak at 186.0 eV (not shown). The quantitative XPS analysis confirmed that the concentration of the dopant (0.7 at.%) is close to the nominal value of 1 at.%. The relative fractions of the chemically shifted components, determined from the fitted XPS spectra, for all electrochemically pretreated BDD films are listed in Table 2. As can be inferred from these data, relative to APT-BDD, the


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atomic bonding structure on the BDD surface gradually changes as increasingly severe CPTs are applied, with a marked decrease in the total contribution of oxygenated groups (C–O, C=O, and O– C=O), i.e., from 17.2% for the APT-BDD to less than 11% for the CPT-BDD (–30 V), which is accompanied by a 5% increase of the C–H component. Hence, these results imply that a significant fraction of surface oxides is substituted by hydrogen-terminated groups. The above-presented XPS data might help to understand the interrelationships between the atomic bonding structure of the BDD surface and the previously presented electrochemical data. First of all, we can infer that, contrary to what is commonly believed,4 the hydrogenation level of the BDD surface that leads to a reversible behavior of the [Fe(CN)6]3−/4− redox probe does not need to be very high, since the total relative fraction of the three forms of oxidation on the surface of the –5 V CPT– BDD electrode is still about 15%. Clearly, there seems to be a threshold for the total relative fraction of these forms of oxidation above which HET for that redox probe is markedly hindered, as previously suggested by Hutton et al.44 Actually, the C–O (ether and alcohol groups) and O–C=O components seem to be the culprit, since their relative fractions decrease to a larger extent (16% and 21%, respectively) between the APT–BDD and –5 V CPT–BDD electrodes. Furthermore, these components decrease continuously as more drastic CPTs are applied, which correlates quite well with the increase of the HET rate constants obtained by EIS for the [Fe(CN)6]3−/4− redox system. A similar, yet stronger, correlation exists for AA oxidation, whose overpotential decreases as the relative fraction of the C–O component decreases, confirming that this redox probe might be a good secondary indicator of the hydrogenation level of the BDD surface. Finally, it is also clear that even more severe CPTs are needed to attain hydrogenation levels similar or higher than those observed for H-plasma treated BDD. However, as shown by Deslouis et al.45 using the CS-AFM technique, such an H-plasma treatment at the end of the deposition can turn insulating facets of BDD nanocrystals by the possible formation of B-H pairing that lowers the doping level by decreasing in a thin superficial layer the amount of free boron atoms. These issues are currently under detailed investigation, while


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we are optimizing our experimental system to perform very severe CPTs with the goal of attaining high hydrogenation levels of the BDD surface.

CONCLUSIONS

In this work, the effect of different electrochemical pretreatments on the electrochemical activity of a highly doped BDD electrode was extensively investigated using two redox probes, [Fe(CN)6]3−/4− and ascorbic acid. In addition, the ensuing changes in the atomic bonding structure on the BDD surface were assessed by high-resolution XPS. From the set of electrochemical and spectroscopic data several interesting results were obtained, as summarized hereinafter. First of all, it is clear that the hydrogenation level of the BDD surface can be increased by adequate cathodic pretreatments (CPTs). Previously, Hoffmann et al.42 reported on the attainment of a high level of hydrogenation (higher than those obtained by hydrogen plasma treatment) of a BDD surface using a very severe CPT. In this work, the total relative level of oxidation of the BDD surface was decreased by about 37%, whereas Hoffmann et al.42 attained 80%; however, due to differences in ohmic resistance of the electrochemical cell used, their CPT was significantly more severe than the most severe one performed in this work. Secondly, contrary to what is commonly assumed in the literature, BDD surfaces do not need to be highly hydrogenated to attain high HET rates with the [Fe(CN)6]3−/4− redox system. In fact, reversible behavior was attained with a CPT–BDD even when the total relative fraction of the three forms of oxidation (C–O, C=O, and O–C=O) on the BDD surface was ~15%. Indeed, as previously suggested by Hutton et al.,44 there seems to be a threshold for the total relative fraction of these forms of oxidation (with a significant contribution from C–O) above which HET of that redox probe is markedly hindered At the moment, we are fine-tuning our CPTs to possibly determine the percent range of surface oxidation that corresponds to this threshold.


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Finally, we confirmed that the overpotential for ascorbic acid oxidation is very sensitive to the hydrogenation level of the BDD surface or, complementarily, to its total relative level of oxidation. Thus, this redox species might be used as a secondary indicator of the relative atomic bonding structure on the BDD surface.

AUTHOR INFORMATION

Corresponding author *Phone: +55 (16) 3351-8078. E-mail: [email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS We gratefully acknowledge the financial support from the Brazilian funding agencies FAPESP – São Paulo Research Foundation (Process no. 2014/11769-6), CNPq, and CAPES.

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 1. Values of the parameters of the Randles equivalent circuit fitted to the impedance data and of the apparent HET constants (kapp) for the [Fe(CN)6]3−/4− redox system. EIS data obtained at the formal potential (see Fig. 1) using an anodically (APT) or cathodically (CPT) pretreated BDD electrode, as indicated (duration of each pretreatment: 300 s).

BDD pretreatment

Rs / Ω

Rct / Ω

Q / (µF sn–1)

nCPE

200 mA cm–2 APT

6.3

6.94 × 104

7.74

0.94

12.8

1.2 × 10–4

–5 V CPT

6.6

84.3

6.71

0.96

14.1

9.9 × 10–2

−10 V CPT

6.6

82.6

9.95

0.93

14.3

1.0 × 10–1

−20 V CPT

6.6

54.2

9.62

0.93

14.3

1.5 × 10–1

−30 V CPT

6.6

37.4

8.12

0.94

14.2

2.2 × 10–1


Ceq / (µF cm–2) kapp / (cm s–1)

17


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Page 18 of 23

Table 2. Relative abundances (peak percentages and associated errors*) of components corresponding to different chemical environments as determined by deconvolution of the C 1s XPS spectra of the APT– and CPT–BDD electrodes (duration of each pretreatment: 300 s).

CPT (%)

200 mA cm–2 APT (%)

−5 V

−10 V

−20 V

−30 V

3.0 (±0.5)

3.5 (±0.5)

3.9 (±0.6)

3.7 (±0.6)

5.4 (±0.8)

sp C−C

9.0 (±1.4)

8.7 (±1.3)

9.1 (±1.4)

6.9 (±1.0)

8.0 (±1.2)

sp3 C−C

54.9 (±8.2)

53.7 (±8.1)

55.5 (±8.3)

57.4 (±8.6)

54.9 (±8.2)

C−H

15.8 (±2.4)

19.5 (±2.9)

17.4 (±2.6)

21.6 (±3.2)

20.9 (±3.1)

C−O

13.8 (±2.1)

11.6 (±1.7)

10.5 (±1.6)

7.2 (±1.1)

7.8 (±1.2)

C=O

2.0 (±0.3)

2.0 (±0.3)

2.8 (±0.4)

2.8 (±0.4)

2.5 (±0.4)

O–C=O

1.4 (±0.2)

1.1 (±0.2)

0.9 (±0.1)

0.4 (±0.1)

0.5 (±0.1)

Component C−B 2

* Error values obtained from three deconvolution procedures maintaining the binding energies of the structural components, according to average literature values, in the interval of ±0.2 eV.


18

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Figure captions

Figure 1. a) Cyclic voltammograms (v = 50 mV s–1) and b) impedance diagrams (at the formal potential) for the [Fe(CN)6]3−/4− redox couple (0.1 mmol L–1) in 0.5 mol L–1 H2SO4. Measurements performed using an APT (200 mA cm–2) or CPT–BDD electrode, as indicated in the figure (duration of each pretreatment: 300 s). c) Randles equivalent circuit used to fit the experimental impedance diagrams.

Figure 2. Square-wave voltammograms (with baseline correction) obtained for ascorbic acid (2.5 mmol L–1) in 0.1 mol L–1 HClO4. Measurements performed using an APT (200 mA cm–2) or CPT– BDD electrode, as indicated in the figure (duration of each pretreatment: 300 s). SWV parameters: f = 30 Hz, a = 40 mV, and ∆Es = 2 ms.

Figure 3. Deconvoluted C 1s spectra for (a) an APT (200 mA cm–2) or (b) a CPT–BDD (–30 V) electrode (duration of each pretreatment: 300 s). Open circles refer to the experimental data, colored lines are Voigt fit functions, and the black line is the fitting envelope.


19


Fig. 1

a)

I / µA

12 9 6 3 0 -3 -6 -9 -12 0.2

0.4 0.6 0.8 1.0 1.2 −1 E vs. RHE (0.5 mol L H2SO4) / V −2

0.8 −Z" / k Ω

80

−2

200 mA cm APT −5 V CPT −10 V CPT −20 V CPT −30 V CPT

1.0

100

−Z" / kΩ

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 23

60

67 Hz

0.6 0.4

8.6 Hz


0.2

b)

0.0 0.0 0.2 0.4 0.6 0.8 1.0

40

0.02 Hz

Z' / kΩ

1.5 Hz

20

0.01 Hz

0 0

20

40

60

80

100

Z' / kΩ Rs

CPE Rct

W

c)


20

Page 21 of 23

Fig. 2

200 150 I / µA

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


−2


100 50 0 0.6 0.8 1.0 1.2 1.4 −1 E vs. RHE (0.5 mol L H2SO4) / V


1.6

21


Fig. 3

−2

200 mA cm APT 3

Intensity / a.u.

sp C−C

C−Hx 2

sp C−C

C−O O−C=O

290

C=O

C−B

288

286

284

282

280

Binding energy / eV

−30 V CPT

3

sp C−C

Intensity / a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 23

C−Hx

O−C=O

290

2

C−O C=O

288

sp C−C C−B

286

284

282

280

Binding energy / eV


22

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TOC graphic (for TOC only)


23

Assessments of the Effect of Increasingly Severe Cathodic

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