Revisiting the oxidation peak in the cathodic scan of the cyclic voltammogram of alcohol oxidation on noble metal electrodes

Abstract

This work reports straightforward, intuitive, and convincing evidence to elucidate the origin of the oxidation peak in the cathodic scan of the cyclic voltammogram of alcohol oxidation on noble metal electrodes. Consequently, three new indicators are also proposed for assessing the electrocatalytic performance of electrodes.

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1. Introduction

Direct alcohol fuel cells (DAFCs), which “burn” alcohols (such as methanol CH₃OH) at ambient temperatures to generate electricity, are important energy conversion and storage devices for clean and sustainable technologies.^1–5 The electrooxidation of alcohols relies on the catalytic effect of anodes, which are predominantly noble metals and their alloys.³ In acidic solutions, platinum (Pt) is the most efficient electrocatalyst among all monometallic electrodes.³ In alkaline solutions, palladium (Pd) turns out to be the most efficient one.³ Being consistent with the fact that the alcoholic electrooxidation is a multi-electron reaction, a variety of carbonaceous chemicals have been identified as intermediates, among which carbon monoxide (CO) is widely believed to strongly adsorb on the electrode surface and impair the catalytic performance due to the poisoning effect.^6,7 The seriousness of the CO poisoning has long been indexed by an oxidation peak in the cathodic scan of the cyclic voltammogram (CV).^6–8 More specifically, the intensity ratio (J_f/J_b) of the peak current in the anodic (forward) scan (J_f) versus that in the cathodic (backward) scan (J_b) is used to describe the “CO-tolerance”;⁶ the higher the value, the better the tolerance.^9–17 This criterion is generally attributed to a paper published in 1992.⁶ The conjecture was based on (1) an assumption that the anodic current beyond the methanol oxidation peak J_f results from oxidation of surface-adsorbed CO to CO₂;⁶ and (2) a fact that J_b weakened when the anodic switching potential in the CV was increased.^6,9 Later on, the CO adsorption on electrode surfaces was confirmed by spectroscopic measurements;^7–9 since then, the “J_f/J_b” criterion for indexing the “CO-tolerance” has been widely used in the literature for alcohol fuel cells and referred to when searching for high-performance electrocatalysts.^9–21

In 2012, Tong et al. published a pioneering article to question the validity of this criterion for methanol oxidation on Pt/C and PtRu/C electrodes in acidic solutions.²² By using in situ surface enhanced infrared spectroscopy, they observed that both J_f and J_b presented opposite correlations with the amount of methanol (but not CO) adsorbed on electrode surfaces.²² The observed correlations underlay the conclusion that both oxidation peaks originated from the oxidation of surface-adsorbed methanol and the peak intensity ratio was an inadequate parameter to gauge CO-tolerance.²² However, to date, this opinion has not been well adopted in the community;^23–34 the vast majority of subsequent publications still embrace the old opinion.^{10–14,18,35–84} Some papers quoted both opinions without preference.^85–88 Thus, further clarification on this question is needed.

Herein we provide simple, intuitive, and convincing evidence to clarify the origin of J_b, by revealing the cause-and-effect relationship straightforwardly on the basis of adding alcohol only during the cathodic scan (Fig. 1). That is, the apparent J_b is the net current of fresh alcohols' oxidation (J′_b) triggered and counteracted by the reduction of catalyst oxides (J_MOx→M), because (1) the peak potential E_b emerges right after E_MOx→M and shifts correspondingly when E_MOx→M changes; (2) J_b is strongly dependent on J_MOx→M, essentially with J′_b = J_b + J_MOx→M; and (3) the occurrence of J_b needs the alcohol to be present only before E_MOx→M during the cathodic scan, where it is impossible to generate CO. Thus, opposite to the conventional criterion, a higher ratio of J_b/J_f is believed to index higher reactivation efficiency, which could actually be more desirable for an electrocatalyst. Additionally, we also propose two other performance indicators to index the activity of a given catalyst, that is, the intensive activity and the extensive activity.

Fig. 1 Typical cyclic voltammograms to study the origin of J_b by adding alcohol during the potential window indicated.

2. Experimental section

Chemicals

Pd wire (4 N, 0.5 mm in diameter), Au wire (4 N, 0.5 mm in diameter), and Pt wire (4 N, 0.5 mm in diameter) were purchased from ESPI Metals. Sodium hydroxide (NaOH, 99%, Mallinckrodt), methanol (CH₃OH, Pharmco-AAPER, ACS reagent), ethanol (CH₃CH₂OH, Pharmco-AAPER, ACS reagent), were purchased from Fisher. Perchloric acid (HClO₄, 70%) was purchased from Sigma-Aldrich. All chemicals were used as received. The nano-pure water (18.2 MΩ cm⁻¹) was from a Barnstead water purification system.

Data collection

All electrochemical data were collected by using a conventional three-electrode cell controlled by a Reference 600 electrochemical workstation (Gamry Instruments, Inc., USA). The working electrode was a Pd (Au, or Pt) wire with only 5 mm in length exposed in the electrolyte solution; the counter electrode was a coiled Pt wire; and the reference electrode was a saturated calomel electrode (SCE), relying on which the potential versus standard hydrogen electrode (SHE) was calculated. Before being presented in figures, all potentials in the Pd system were further corrected by the IR drop compensation, where R (the solution resistance) was determined via electrochemical impedance spectroscopy (EIS). The EIS was conducted at −0.34 V vs. SHE, by applying an alternating voltage of 5 mV in the frequency range of 100 kHz to 10 mHz. As indicated respectively in the manuscript, the electrolyte solution was 0.5 M NaOH, 0.5 M NaOH + 1.0 M CH₃OH, 0.5 M NaOH + 1.0 M CH₃CH₂OH, or 0.1 M HClO₄ + 1.0 M CH₃OH. Before the measurement, the electrolyte solution was deaerated by argon for 15–20 min and maintained with a slight overpressure afterwards. All cyclic voltammograms (CVs) were collected at the potential scan rate of 20 mV s⁻¹.

3. Results and discussion

The origin of the referred oxidation peak

We initiated our study from observing methanol oxidation on a polycrystalline Pd electrode in alkaline solutions. Fig. 2A shows a typical CV of methanol oxidation on Pd in 0.5 M NaOH + 1.0 M CH₃OH, featured with a large J_f and a small J_b. Compared with the CV taken from 0.5 M NaOH without CH₃OH (Fig. 2B), two interesting features can be noticed: (1) the anodic current at the potential beyond J_f decreases to virtually zero, which can be assigned to the loss of activity induced by the oxidation of Pd;³³ and (2) the onset (peak) potential of J_b in Fig. 2A nicely matches the onset (peak) potential of J_PdOx→Pd in Fig. 2B. From this observation, we hypothesize that the oxidation peak J_b originates from but not merely the oxidation of fresh methanol, and also that the trigger is the reactivation of the previously deactivated electrode surface via reduction of PdO_x.

Fig. 2 Study on the origin of the oxidation peak (J_b) in the cathodic scan of CV of methanol oxidation on Pd, by preparing the electrolyte solutions in different ways. (A) The premade solution of 0.5 M NaOH + 1.0 M CH₃OH; (B) the premade solution of 0.5 M NaOH; (C) the online made solution of 0.5 M NaOH + 1.0 M CH₃OH by adding 10.0 mL of 2.0 M CH₃OH/0.5 M NaOH into 10.0 mL of 0.5 M NaOH solution during the indicated potential window; and (D) the online made solution of 0.5 M NaOH + 1.0 M CH₃OH by adding 10.0 mL of 2.0 M CH₃OH/0.5 M NaOH into 10.0 mL of 0.5 M NaOH solution at the indicated potential.

To prove the hypothesis, we conducted two experiments to monitor their CVs before and after the addition of methanol into the electrolyte solution. In the first case (Fig. 2C), one cycle of CV was first collected in 0.5 M NaOH to confirm the normal behavior of Pd as in Fig. 2B. Then, after the second anodic scan (line 1), the solution was quickly converted to 0.5 M NaOH + 1.0 M CH₃OH by adding an equal volume of 0.5 M NaOH + 2.0 M CH₃OH into the electrochemical cell. During the potential window of adding methanol (line 2, as indicated), there is no generation of CO from the oxidation of methanol within the instrument sensitivity, because the corresponding current is actually zero. Once the potential reached the onset of J_PdOx→Pd (−0.03 V), an oxidation wave burst and peaked at −0.10 V (line 2), exactly as in Fig. 2A. In the subsequent cycles (lines 3 and 4) the J ∼ E profiles are virtually identical and both are well consistent with that in Fig. 2A. The complete scenario of the CVs before and after the methanol addition is shown in ESI-Fig. 1.† In the second case (Fig. 2D), methanol was not added until the J_PdOx→Pd peak emerged half way; the cathodic current was immediately reversed into an anodic current.

These results clearly suggest that: (1) J_PdOx→Pd triggers the occurrence of J_b; (2) the oxidation of CH₃OH but not of CO is responsible for J_b; (3) the pure oxidation current of fresh CH₃OH (J′_b, shown in ESI-Fig. 2A†) is larger than the apparent current J_b, essentially J′_b = J_b + J_PdOx→Pd; (4) the J_PdOx→Pd process is not actually suppressed by the J′_b process, but just concealed in the CV; and (5) the oxidized surface is not capable of oxidizing CH₃OH, as supported by the zero anodic current beyond 0.36 V in the anodic scan and between 0.66 V and 0.0 V in the cathodic scan.³³ The last point is also further supported by the fact that there is no oxidation current in the anodic scan even if switching the cathodic potential before the occurrence of J_PdOx→Pd (ESI-Fig. 3†). Further evidence to support the first four points would be to study the direct electrooxidation of CO. Coincidently, Mota-Lima et al. have reported recently that the J_PdOx→Pd profile during the repeated CV scans showed no change when the solution was saturated with CO,⁸⁹ which suggested no oxidation of CO in the cathodic scan and its irrelevance to J_b in the presence of CH₃OH. This supports our conclusions above. While some correlation between J_MOx→M and J_b was proposed by several researchers before,^23,26,33,34 explicit evidence and quantitative analysis of the trigger-consequence relationship were not provided. When studying the oxidation of formic acid in acidic solutions, Conway et al. proposed that the observed J_b resulted from an autocatalytic reduction of Pd oxide by formic acid or CO.⁹⁰ Thus, the evidence we have presented is convincing and original to prove our hypothesis and elucidate the origin of J_b. Nevertheless, please note that our conclusions here do not conflict with the generation and tolerance of CO during the forward scan, as demonstrated many times in the literature.^9–21 Also, we do not mean that there is absolutely no component of CO oxidation in J_b during the repeated CV scans in the presence of CH₃OH, but rather we believe that the CO contribution (if any at all) would be below the instrumental detection limit. The CO generated during the forward scan must have been essentially desorbed from the oxidized electrode surface before the potential reaches E_PdOx→Pd. Evidently, a large J_b may not be a bad indication. It would be desirable to establish new criteria associated with J_b for searching high-performance electrocatalysts.

Performance indicators associated with the referred peak

Next, we would like to understand what more information this peak can disclose for understanding a catalyst's performance. Three interesting questions could be asked: (1) how does this peak (in terms of peak potential and intensity) respond to the oxidation extent of Pd? (2) Is there any correlation between peak J_b (or J′_b) and peak J_f to indicate the catalytic performance of Pd? (3) Can the trigger-consequence relationship be generalized to other alcohols, other catalysts, and acidic solutions?

To answer the first two questions, two series of CV experiments in both the CH₃OH-containing solution and the NaOH-only solution were conducted, by changing the switching potentials in anodic (Fig. 3) and cathodic (ESI-Fig. 4†) scans, respectively. The first glance on Fig. 3A (the CH₃OH-containing solution) and Fig. 3B (the NaOH-only solution), which study the anodic switching potential (E₊), could come to the following summary: with increasing E₊ (that is, the oxidation extent), E_b and E_PdOx→Pd both shifted towards more negative, J_b became smaller, J_PdOx→Pd grew larger; E_f stayed no shift at 0.03 V and J_f grew larger; and the hydrogen adsorption/desorption current due to water-decomposition (below −0.3 V) shrank in the presence of CH₃OH and grew larger in the absence of CH₃OH. As shown in ESI-Fig. 4† with changing the cathodic switching potential (E₋), none of the three peaks showed appreciable changes; the only change was the expected diminishing of the hydrogen adsorption/desorption current. We note that the dependence of J_b and J_PdOx→Pd on E₊ is not new in the literature;^6,9,90 however, all explanations are in the context of CO-tolerance but not of J′_b. A further quantitative analysis of the effect of E₊ herein is desired.

Fig. 3 Study on the effect of oxidation extent of the Pd electrode on J_b and J_f, by tuning the switching potential in anodic scans (E₊ = 0.66 V, 0.56 V, 0.46 V, 0.36 V, and 0.26 V). (A) CVs in 0.5 M NaOH + 1.0 M CH₃OH and (B) CVs in 0.5 M NaOH.

Fig. 4 displays the dependence of various factors on E₊. As shown in Fig. 4A, less (or more) positive E₊ induces less (or more) negative E_PdOx→Pd and E_b as well as larger (or smaller) potential gaps between them. Referring to the literature,⁹¹ the hysteresis for E_PdOx→Pd can be assigned to the activation energy involved in the place exchange between Pd, OH, and O during both the oxidation and reduction processes. Fig. 4B shows that the error of using the apparent peak current J_b instead of the actual methanol oxidation current J′_b could be significant for large E₊. Likewise, the difference between J_b/J_f and J′_b/J_f is also obvious (Fig. 4C). The results in Fig. 4A–C show that J′_b would be more appropriate than J_b to index a catalyst's performance and that fair comparisons between different catalysts (particularly across different research groups) require the same experimental conditions, such as the potential window. Since J′_b is induced by and synchronizes with the E₊ dependent J_PdOx→Pd, we term J′_b/J_f as the reactivation efficiency to index how efficient the PdO_x-derived Pd surface is, when compared with the pristine Pd surface. As reported in the literature, J′_b/J_f could be bigger than one.^22,92–94 While the underlying reason is unclear at this moment and also beyond the scope of this work, it might be associated with the formation of advantageous grain boundaries; which have been observed to account for the much superior activity of the oxide-derived copper catalysts to their pristine counterparts in CO reduction.^95,96 Thus, among different catalysts whose other properties are comparable, the one with higher J′_b/J_f might actually be more desirable.

Fig. 4 The dependence of various factors on E₊. (A) E_b and E_PdOx→Pd; (B) J_b and J′_b; (C) J_b/J_f and J′_b/J_f; and (D) J_S = J′_b + J_f (red) and Q_S = Q′_b + Q_f (blue).

With the elucidation of J′_b to also result from MOR, J′_b needs to be included for indexing the catalytic activity. Different from the conventional style of using only J_f, we propose using J_S = J′_b + J_f and name it as the intensive activity, since current density is an intensive variable. While current density is used for calculating the power density of a fuel cell, charge density (Q) is required for calculating the energy density. Because J_S could not differentiate catalysts that have comparable peak intensities but different peak widths, we propose a new term – extensive activity (Q_S = Q′_b + Q_f), to index a catalyst's activity from a different angle than J_S. As shown in Fig. 4D, both J_S and Q_S are strongly dependent on E₊, but in opposite trends. According to J_S, the best E₊ is 0.56 V; according to Q_S, the best E₊ is 0.36 V. This means that both J_S and Q_S are needed to index the activity of a catalyst. The desirable E₊ could then be assigned to a balanced range of [0.36 V, 0.56 V], as indicated by the shadow. In addition, it is noteworthy that the peak potentials in CVs do not describe steady states but dynamic states. Thus, further studies on other properties of the J_f and J′_b peaks are worthwhile.

Then, we calculated the Tafel slopes (S_T) for J_f and J′_b and employed the technique of differential pulse voltammetry (DPV, ESI-Scheme 1†).^97,98 ESI-Fig. 2† shows the J′_b profiles and the corresponding Tafel plots by fitting the linear ranges with E − E^o′ = S_T log(J/J_o), where E^o′ is the formal potential, and J_o is the exchange current density.⁹⁷ ESI-Fig. 5† shows the J_f profiles and the corresponding Tafel plots. The calculated S_T values are graphed against E₊ in Fig. 5A. In the range of 0.26 V to 0.36 V, S_T is 115 mV dec⁻¹ for both J_f and J′_b. With increasing E₊, S_T for J_f increases gradually to 130 mV dec⁻¹, consistent with the literature value;^99,100 in contrast, S_T for J′_b decreases to 89 mV dec⁻¹. The change of S_T with E₊ reflects the E₊ dependent surface properties.^91,97 E^o′ for J_f and J′_b are −0.22 V and −0.34 V (ESI-Fig. 6A†), respectively. The corresponding J_o for J_f increases slightly in the range of 10^−1.54 A m⁻² to 10^−1.20 A m⁻² with increasing E₊; and the J_o for J′_b is around 10^−1.72 A m⁻² (ESI-Fig. 6B†). Different values of E^o′ and J_o for J′_b and J_f are due to the E₊ dependent oxidation/reduction hysteresis.⁹¹ Now we understand that a larger J′_b associated with a smaller E₊ is essentially due to a larger overpotential η = E_b − E^o′ in addition to a smaller J_PdOx→Pd, despite a larger S_T.⁹⁷ Fig. 5B shows the plot of current density versus potential measured by DPV, a technique to measure steady states and minimize the capacitive background currents.^97,98 E_f and E_b are observed at 0.03 V and −0.11 V, respectively, which are consistent with those (0.03 V and −0.10 V) in typical CVs. In contrast, the corresponding S_T for J_f has different values in two potential regions (ESI-Fig. 7A†), that is, 170 mV dec⁻¹ for [−0.15 V, −0.04 V] and 73 mV dec⁻¹ for [−0.04 V, −0.01 V]. The corresponding S_T for J′_b is 67 mV dec⁻¹ (ESI-Fig. 7B†). The different S_T values indicate different rate determining steps involved in the MOR.^101,102 Overall, this implies that CV seems less sensitive than DPV to determine S_T for multi-electron reactions. Smaller S_T and more negative E^o′ for J′_b than for J_f indicate that the oxide-derived surface generated during the cathodic scan is more active than the pristine surface in the anodic scan. On the other hand, however, smaller J′_b than J_f could mean that the oxide-derived surface is less stable, as their overpotentials of 0.24 V (η = −0.10 V − (−0.34) V) for J′_b and 0.25 V (η = −0.03 V − (−0.22) V) for J_f are comparable.

Fig. 5 (A) Tafel slopes for the J_f and J′_b peaks in Fig. 2A; and (B) differential pulse voltammogram to determine the relationship of current density versus potential under steady states for Pd in 0.5 M NaOH + 1.0 M CH₃OH.

Generalization of our understanding to other systems

Furthermore, we would like to test the generality of the trigger-consequence relationship between J_MOx→M and J_b for other systems. In the case of electrooxidation of ethanol (CH₃CH₂OH) on Pd in an alkaline solution (Fig. 6A), we observed the same behavior as for methanol oxidation, except that J_b occurred at a more negative potential (−0.18 V) and J_b/J_f was much larger (0.80), illustrating the high activity of Pd for ethanol oxidation as reported in the literature.¹³ Similar phenomenon was also observed for the electrooxidation of CH₃CH₂OH on Au (ESI-Fig. 8†). In the case of electrooxidation of methanol on Pt in an acidic solution (Fig. 6B and ESI-Fig. 9†),²² the trigger-consequence relationship between J_PtOx→Pt and J_b was clearly observed as well. Moreover, as expected, J_b was observed at a more negative potential (0.65 V) than that of J_PtOx→Pt (0.78 V). The sharp rise of the oxidation current at the methanol addition moment was due to the methanol oxidation on PtO_x surface.²²

Fig. 6 CVs for electrooxidation of ethanol on Pd electrode in 0.5 M NaOH solution (A) and electrooxidation of methanol on Pt electrode in 0.1 M HClO₄ (B). (A) During the first cycle (lines 1 and 2) the solution is 10.0 mL of 0.5 M NaOH. In the second cycle (lines 3 and 4) 10.0 mL of 2.0 M CH₃CH₂OH/0.5 M NaOH is added during the cathodic scan as indicated. Afterwards, another anodic scan (line 5) is conducted. (B) During the first cycle (lines 1 and 2) the solution is 10.0 mL of 0.1 M HClO₄. In the second cycle (lines 3 and 4) 10.0 mL of 1.0 M CH₃OH/0.1 M HClO₄ is added during the cathodic scan as indicated. Afterwards, another cycle (lines 5 and 6) is conducted.

Last, the scanning electron microscope (SEM) images in Fig. 7 show the morphologies of the polycrystalline Pd and Pt electrodes used in this work. It is noteworthy that, when using monometallic electrodes, whether the J_b peak occurs or not does not depend on the electrode morphology and feature scale, but its intensity does.^{1–4,14–19,22,68,92}

Fig. 7 SEM images of polycrystalline Pd and Pt electrodes used in this study.

4. Conclusions

In summary, we have reported clear-cut and convincing evidence to elucidate the origin of the oxidation peak (J_b) observed in the cathodic scan of cyclic voltammogram of alcohol oxidation on noble metal electrodes. This work corrects a long-held misapprehension of J_b as the oxidation of carbon monoxide and J_f/J_b (J_f is the peak current in the anodic scan) as the indicator of carbon monoxide tolerance, critically amending the previous work by Tong et al.²² In fact, the peak originates from the oxidation of fresh alcohols (J′_b), being triggered and counteracted by the reduction of electrode oxides (J_MOx→M) formed in the proceeding anodic scan. J′_b essentially synchronizes with J_MOx→M. J_b does not involve the oxidation of the anodically produced CO. During our preparation of this manuscript, Tong et al. further demonstrated that the CO-tolerance is totally irrelevant to the CH₃OH oxidation even in the forward CV scan for a PtRu electrocatalyst.¹⁰³ The peak intensity ratio J′_b/J_f, which is strongly dependent on the anodic switching potential (E₊), can indicate the reactivation efficiency of a given electrocatalyst. Two other E₊ dependent performance indicators are also proposed, that is, the intensive activity J_S = J′_b + J_f and the extensive activity Q_S = Q′_b + Q_f. Thus, the same experimental conditions, particularly the potential window, are imperative for comparing different catalysts. Smaller Tafel slope (S_T), more negative formal potential (E^o′) and lower current intensity for J′_b than for J_f indicate that the oxide-derived surface has higher activity but lower stability than the pristine surface. This work suggests new criteria and directions for searching high-performance electrocatalysts used in direct alcohol fuel cells. Intriguing questions for future studies include how the electrode morphology and interface species evolve during the CV scan and how to improve the stability of oxide-derived surfaces.

Acknowledgements

This material is based upon work supported by the Start-up Fund for YY from the Colorado School of Mines and National Science Foundation through the REU program from the Renewable Energy Materials Research Science and Engineering Center under Grant no. DMR-0820518.

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Footnote

† Electronic supplementary information (ESI) available: More experimental details, cyclic voltammograms, Tafel plots, graphs of formal potentials and exchange currents against the switching potential in anodic scans, and cyclic voltammograms for an Au electrode. See DOI: 10.1039/c5ra24249e

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