Application of Derivative Voltammetry in the Analysis of Methanol

Article pubs.acs.org/JPCC

Application of Derivative Voltammetry in the Analysis of Methanol Oxidation Reaction Arun Murthy and Arumugam Manthiram* Electrochemical Energy Laboratory & Materials Science and Engineering Program, University of Texas at Austin, Austin, Texas 78712, United States S Supporting Information *

ABSTRACT: Application of derivative voltammetry to methanol oxidation reaction (MOR) has been studied, and its advantage in the analysis of various aspects of MOR with regard to direct methanol fuel cell has been delineated. The derivative technique of voltammetric analysis was employed in evaluating and comparing MOR activities of carbon-supported mono-, bi-, and tri-metallic electrocatalysts. Significant enhancement in the accuracy of estimating voltammetric peak potential and onset potential of methanol oxidation current can be achieved in derivative voltammetry. Furthermore, the better signal-to-noise ratio of derivative voltammetry practically eliminates charging current. From a mechanistic point of view, derivative voltammetry is highly sensitive in resolving a peak due to parallel path mechanism. Electrochemical stability of anode catalysts can be better evaluated and monitored employing derivative voltammetry. Nevertheless, the derivative technique is simple and does not require further instrumentation or cell assembly.

1. INTRODUCTION Direct methanol fuel cell (DMFC) is considered to be a potential energy source for near-future energy demands.1,2 Among the various low-temperature fuel cell systems, DMFC stands out owing to several attractive features such as high energy density and efficiency, low weight and compact cell assembly, easy handling and distribution of liquid fuel, and no preprocessing of fuel.3−6 However, one of the short comings of DMFC is the low methanol oxidation reaction (MOR) kinetics at the anode.7,8 Platinum and platinum-based alloys with oxophilic metals have been widely investigated as promising electrocatalysts for MOR. Cyclic voltammetry is almost the invariably and routinely used electrochemical technique to test and evaluate new catalysts for MOR activity and stability. Cyclic voltammetry is the preferred half-cell technique for evaluating catalysts over the single-cell technique, which is timeconsuming and expensive.9 Simplicity, precision, and sensitivity are some of the attributes that make voltammetry a widely used technique compared with other related electrochemical techniques. In this regard, voltammetric derivative technique offers several advantages over the conventional voltammetric method. Derivative voltammogram represents the rate of change of voltammetric current i with respect to electrode potential E (di/dE) and has more voltammetric features than conventional voltammogram. Characteristic features of derivative voltammetry that are advantageous over conventional voltammetric method are given below. (1) Derivative technique does not involve further instrumentation than that required for conventional voltammetry, and most of the potentiostats operated through software © 2012 American Chemical Society

readily compute and graphically display di/dE. (2) Derivative voltammetry is more sensitive than conventional voltammetry. (3) Being a derivative, di/dE eliminates charging current in most cases or minimizes background interference where double-layer capacity changes slowly compared with faradaic current with respect to potential. (4) Peak potential (Ep) is one of the key thermodynamic features of voltammetric curve of MOR, but precise location is somewhat difficult in broad voltammetric wave, wherein the voltammetric peak spreads over a potential range, whereas di/dE passes through potential axis exactly at Ep and hence Ep can be located more precisely in derivative voltammogram. (5) Derivative peak occurs closer to the foot of the conventional voltammetric wave, and hence the onset potential of MOR can be estimated with higher accuracy in derivative voltammogram. (6) Closely placed voltammetric peaks or shoulders can be resolved in derivative voltammograms, which is highly valuable in distinguishing methanol oxidation peak due to parallel path mechanism from the main peak in the forward scan. Accordingly, we apply in this Article the derivative technique in the analysis of MOR and delineate its benefits over the widely employed conventional voltammetry. Evaluation and comparison of MOR activities of different electrocatalysts are illustrated with carbon-supported mono-, bi-, and tri-metallic electrocatalysts prepared by a polyol reduction method. The advantage of derivative voltammetry in resolving the methanol Received: September 26, 2011 Revised: December 26, 2011 Published: January 13, 2012 3827

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Impedance analysis was conducted with a Solartron 1260 frequency response analyzer and Solartron 1286 potentiostat/ galvanostat controlled by ZPLOT2 software obtained from Scribner. The frequency range employed was 100 mHz to 5 kHz with an amplitude of 5 mV. X-ray photoelectron spectroscopic (XPS) data of the catalysts were obtained with Kratos Analytical XPS instrument using a monochromatic Al Kα X-ray source. The spectra were fitted with a Gaussian− Lorentzian function (G/L 70:30) and a Shirley background function in CASAXPS software. Transmission electron microscopic (TEM) images of the carbon-supported nanoparticles were obtained with a JEOL 2010F TEM.

oxidation peak at carbon-supported platinum nanoparticles is described. Mechanistic analysis of MOR in derivative voltammetry is compared with that in electrochemical impedance spectroscopy (EIS). Because CO poisoning of the electrocatalyst is important in MOR, derivative voltammograms of COad stripping voltammetry have also been presented. Electrochemical stability of the well-known carbon supported PtRu is evaluated through accelerated degradation cycles (ADCs).

2. EXPERIMENTAL SECTION Carbon-supported Pt, PtSn (3:1), PtRu (1:1), and PtSnRu (2:1:1) catalysts were prepared on carbon black (Vulcan XC72R, 40 wt % metal loading) by a polyol reduction method, employing H2PtCl6·6H2O (Strem Chemicals), SnCl2·2H2O (Acros Organics), and RuCl3·3H2O (Strem Chemicals) as metal sources and ethylene glycol (Fisher Scientific) as the solvent and reducing agent. Required amounts of metal precursors were first dissolved in 120 mL of ethylene glycol having 0.1 M NaOH (Fisher Chemical) to obtain 60 mg of total metal weight on carbon. The solution was then refluxed at 160 °C for 3 h under open atmosphere, during which the color of the solution changed to dark brown, indicating the formation of colloid of metal nanoparticles. Carbon black (90 mg) was dispersed separately in 200 mL of ethylene glycol/water (1:1 ratio by volume) by ultrasonication, followed by vigorous stirring. The metal colloid was slowly added to the carbon dispersion with constant stirring. A few drops of aqueous 2 M H2SO4 was added until the pH of the mixture reached 2, and the resulting dispersion was stirred vigorously for 8 h to facilitate the adsorption of colloidal nanoparticles onto the carbon surface. The mixture was then filtered and washed with a copious amount of water; finally, the residue was dried overnight in a vacuum oven at 90 °C. Voltammetric experiments were carried out with a CH Instruments potentiostat in a conventional single compartment three-electrode cell. A platinum wire and a Hg/HgSO4 electrode (in saturated K2SO4 solution) were used, respectively, as counter and reference electrodes. The potentials were, however, referenced with respect to the reversible hydrogen electrode (RHE). A glassy carbon substrate (3 mm diameter, CH Instruments) was polished to a mirror-like finish using 0.05 μm alumina media (Buehler) and coated with a thin layer of the catalyst so as to serve as a working electrode. The catalyst ink was prepared by dispersing 2 mg of 40 wt % catalyst in a mixture of deionized water, ethanol, and Nafion (Electrochem, obtained as 5 wt % solution) by ultrasonic vibration. Two μL of the ink was drop cast onto the glassy carbon substrate and subsequently dried in air. The electrode potential was initially scanned between 0.02 and 1.0 V for 10 cycles at a rate of 50 mV/s so as to clean the catalyst surface in 0.5 M H2SO4. MOR activities of various catalysts were evaluated in a nitrogensaturated 0.5 M H2SO4 + 1 M methanol solution at a scan rate of 50 mV/s. CO stripping voltammetry was performed by purging high purity CO gas for 30 min at a holding potential of 0.1 V. The purging gas was then switched to nitrogen to remove dissolved CO in the solution. Adsorbed CO was then stripped by scanning the potential between 0.05 and 1.2 V at a scan rate of 20 mV/s. Voltammetric data were subsequently transferred to Matlab workspace, and the derivatives were computed in Matlab. Electrochemical stability of PtRu/C was evaluated by scanning the electrode potential for 100 cycles between 0.02 and 1.2 V in 0.5 M H2SO4 + 1 M methanol.

3. RESULTS AND DISCUSSION The first derivative of the voltammetric wave as a function of time when the electrode potential is varied linearly with time at a plane stationary electrode was predicted by Shekun.10 Nicholson and Shain11 current−voltage expression for stationary electrode polarography was differentiated and experimentally verified by Perone et al.12,13 in the cases of diffusioncontrolled reversible and irreversible electrode reactions. Moreover, the application of derivative technique was also extended to anodic stripping voltammetry and kinetic systems.14,15 Carbon supported Pt, PtSn, PtRu, and PtSnRu electrocatalysts were prepared by a polyol reduction method to evaluate and compare the MOR activities of the various electrocatalysts by derivative voltammetry. The surface compositions of the bimetallic PtSn/C (3:1) and PtRu/C (1:1) and trimetallic PtSnRu/C (2:1:1) electrocatalysts were obtained from XPS quantification analysis. XPS and TEM characterization of Pt/C, PtSn/C, PtRu/C, and PtSnRu/C are shown in the Supporting Information (Figures S1 and S2 and Table S1). Figure 1 shows the forward scans of the

Figure 1. Anodic scans of (a) cyclic voltammograms and (b) derivative voltammograms of MOR at Pt/C, PtSn/C, and PtSnRu/C in 0.5 M H2SO4 + 1 M methanol. Scan rate = 50 mV/s.

voltammetric waves of MOR at various carbon-supported electrocatalysts and their respective derivatives. Derivative 3828

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shoulder around 0.6 V in the conventional voltammogram for the methanol oxidation at Pt/C can be resolved into a welldefined peak using derivative method even at a scan rate as low as 1 mV/s. Figure 2 shows the conventional and derivative

voltammograms are characterized by positive and negative peaks representing, respectively, the positive and negative slopes of the voltammetric curves. Both conventional and derivative voltammograms of PtSnRu/C occur at lower potentials, whereas those of Pt/C and PtSn/C occur at comparatively higher potentials. Derivative voltammograms of Pt/C and PtSn/C present multiple positive peaks, whereas the corresponding conventional voltammograms show only weak shoulders at the positive slopes. The derivative voltammetric current crosses the potential axis at Ez, where di/dE becomes zero, which is equal to the peak potential Ep of the voltammetric wave, viz., Ez = Ep. Ez can be accurately measured, whereas it is somewhat difficult in the case of Ep, especially when the voltammetric wave is irreversible and the peak is spread over a considerable potential range. For example, the voltammetric peaks of Pt/C (Ep = 0.874 V) and PtSn/C (Ep = 0.840 V) are sharper, whereas that of PtSnRu/C (Ep = 0.720 V) is broader. The onset of methanol oxidation current and the corresponding potential Eonset is one of the vital criteria for evaluating the MOR activity of a catalyst. The lower the value of Eonset, the better the activity toward MOR. Because the derivative positive peak occurs closer to the foot of the conventional voltammogram, it is easier to locate Eonset on a derivative voltammogram than on a conventional voltammogram. Another important feature is that derivative voltammograms are devoid of charging/background current, whereas methanol oxidation currents are interlaced with charging currents in conventional voltammograms. Even though PtSnRu/C has lower Eonset, the peak height is lower than those of Pt/C and PtSn/C, both in conventional and derivative voltammograms due to the presence of Ru.16 Oxidation of one carbon molecules such as formic acid, formaldehyde, and methanol on polycrystalline platinum leads to serial pathway mechanism through the formation of adsorbed CO, viz., in the case of methanol

CH3OH → COad + 4H+ + 4e‐

Figure 2. Anodic scans of cyclic and derivative voltammograms of MOR at Pt/C in 0.5 M H2SO4 + 1 M methanol at: (a) 50 and (b) 1 mV/s showing the various potential regions and mechanistic aspect of MOR.

voltammograms of oxidation of methanol (1 M) on Pt/C at 1 and 50 mV/s. At 1 mV/s, the shoulder around 0.6 V is barely visible in the conventional voltammogram, whereas it is wellresolved into a peak in the derivative voltammogram. The shoulder in the conventional voltammogram slowly develops as the scan rate is increased (Figure 2a),whereas the respective derivative counterpart shows prominent peaks. The mechanistic insight that can be gained through the derivative voltammogram is illustrated in the case of MOR at Pt/C,as seen in Figure 2a. At lower potentials (point a), dissociative adsorption and subsequent removal of hydrogen atoms from methanol occur, leading to coverage of active surface sites of Pt with adsorbed carbon monoxide. Voltammetric current as well as di/dE are almost zero up to ∼.45 V (point b), and methanol dehydrogenation can be considered to be the rate-determining step at lower potentials (reactions 3 and 4)

(1)

A parallel pathway also operates through the formation of CO2 or other soluble oxidation products without COad formation17

CH3OH + H2O → CO2 + 6H+ + 6e‐

(2)

Because of the differences in the rates of COad formation through serial path among the aforementioned species, the oxidation peak due to parallel path fully appears and partially appears, respectively, in formic acid and formaldehyde at ∼0.6 V.18 The voltammograms of oxidation of formic acid, formaldehyde, and methanol appear different because of the above feature.18 In particular, the oxidation of methanol on polycrystalline platinum shows only a weak shoulder around 0.6 V due to parallel path in the forward scan even though the elementary reaction steps for all three species are essentially the same. Okamoto et al.18 studied the voltammetric conditions such as scan rate, sweep potential range, and concentration of methanol and observed a current peak above 0.6 V, representing a parallel path mechanism. Seland et al.19 used combined pulse-sweep method to show that methanol oxidation at short times can occur on platinum surface free of COad and oxides through parallel pathway. Kinetic parameters of parallel path oxidation at short times leading to soluble products at polycrystalline platinum were obtained by Xu et al.17 through normal pulse voltammetry. The weak

Pt + CH3OHsol → Pt − CH2OH + H+ + e‐

(3)

Pt − CH2OH → Pt − CO + 3H+ + 3e‐

(4)

Evidently, further increase in potential results in a rapid decrease in COad coverage due to its oxidation to CO2, freeing more sites to reactions 3 and 4. The di/dE value increases from point b to d, and the oxidation of COad to CO2 becomes the rate-determining step. Between points b and d, there exists a peak c due to oxidation of methanol through parallel path, as previously discussed (reaction 2). Without a direct path mechanism, di/dE would have followed the trace indicated by c′. This direct path is indicated by a weak shoulder in the conventional voltammogram (Figure 2a), and because of high 3829

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potentials, a large arc appears, and its diameter slowly decreases with potential. The initial slow kinetics is caused by COad from methanol dehydrogenation that blocks further adsorption and dehydrogenation of methanol. Impedance plots in this potential region show only capacitive behavior that can be identified as point a to b in the derivative voltammogram (Figure 2a). The arc enters into the fourth quadrant at the low-frequency region with further increase in potential (Figure 3a). This behavior is called pseudoinductive, indicating that COad starts decreasing and thereby more methanol oxidation occurs at free active sites.20 Analogously, in the derivative voltammogram di/dE starts increasing rapidly after Eonset, showing a rapid decrease in COad with electrode potential. The di/dE plot shows a peak c in this region that has no clear mechanistic counterpart in EIS.21 The arc then abruptly flips into the second and third quadrants with potential, as seen in Figure 3b. Similarly, in the derivative voltammogram di/dE decreases from point d and reaches zero at point e. This is because the surface sites are now rapidly covered with OHad that slows down the methanol oxidation rate. After point e (Figure 2a), di/dE becomes negative until it reaches its lowest point f, where the catalyst surface is almost covered with OHad effectively blocking methanol oxidation. At this potential region, EIS plots flip back to the first quadrant again (Figure 3b). Durability of DMFC is another important issue that affects realization of its commercialization.23,24 PtRu/C is customarily employed as the highly active anode electrocatalyst,25−27 but it undergoes dissolution and particle aggregation under the fuel cell conditions.28,29 High anode potential is one of the important factors that accelerates oxidation of Ru in PtRu and subsequent deterioration of the membrane-electrode assemblies in DMFC.30,31 Accordingly, the stability of the anode electrocatalyst is evaluated in voltammetry through ADC,32,33 wherein the electrode potential is scanned for an extended number of cycles between a selected potential range. Figure 4 shows the derivative voltammograms of ADC between 0.02 and 1.2 V for 100 cycles. Initially, the positive peak at 0.6 V

sensitivity of the derivative technique, a well-formed peak could be observed. However, such a peak or a shoulder is not observed in methanol oxidation on PtRu/C in either the derivative or conventional voltammograms. This is probably because the species that oxidizes COad (for example, OH) is generated by the secondary metals M added to Pt (reactions 5 and 6). It can be noted that at these potentials, Pt is unable to produce OH species, so a higher onset potential is observed in Pt compared with that in Pt-M catalysts. The rapid oxidation of COad at Pt-M catalysts at lower potentials results in the methanol oxidation current predominantly through a serial path mechanism.

M + H2O → M − OH + H+ + e‐

(5)

M − OH + Pt − CO → CO2 + Pt + M + H+ + e‐

(6)

Further increase in the potential results in the generation of more OH on Pt sites, which tends to block methanol adsorption and its subsequent oxidation. The above process is indicated by decreasing di/dE from point d. Even though the voltammetric current continues to increase at this potential, its rate of increase (di/dE) falls, as indicated in the derivative voltammogram (point d in Figure 2a). The oxidative removal of OHad may be thought of as the rate-determining step in this stage. At point e, the di/dE value reaches zero, at which point the voltammetric current reaches a maximum and then starts decreasing. The di/dE value becomes negative after point e because the beneficial effect of OHad is now inimical as OH species are strongly adsorbed to Pt sites, blocking methanol adsorption from the bulk solution. The di/dE value reaches its lowest at point f, where the active sites of the catalyst are almost covered by OHad. The mechanistic aspects of MOR discussed above can be compared with those obtained from EIS.20−22 The impedance behavior of MOR at Pt/C is illustrated in Figure 3 in complex plane plots as a function of electrode potential. At initial lower

Figure 4. Anodic scans of derivative voltammograms of MOR after specified number of potential cycles at PtRu/C in 0.5 M H2SO4 + 1 M methanol. Scan rate = 50 mV/s.

grows in height and shifts anodically until 20 cycles. After 20 cycles, the peak height decreases but still continues to move anodically. Meanwhile, another peak at ∼0.8 V grows with potential cycles until it becomes prominent after 100 cycles. After 100 cycles, the derivative voltammogram resembles that of Pt/C shown in Figure 1. The transformation of PtRu/C catalyst surface into Pt-rich surface because of the dissolution of Ru component has been noticed in our previous study

Figure 3. Complex plane impedance plots of MOR at Pt/C as a function of electrode potential in 0.5 M H2SO4 + 1 M methanol. 3830

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better evaluated and monitored in derivative voltammetry, revealing subtle changes in the voltammogram during the ADC. Application of derivative analysis in COad stripping voltammetry has also been indicated. Despite being advantageous, the derivative technique does not require further instrumentation, and most of the commercial potentiostats operated with the available software are capable of computing and graphically exhibiting di/dE as a function of electrode potential.

employing ADC, COad stripping voltammetry, inductively coupled plasma−optical emission spectroscopy (ICP-OES), and normal pulse voltammetry.34 The stability test of PtRu/C using derivative voltammetry reveals changes taking place at the catalyst surface as well as formation of low potential peaks due to parallel path oxidation. Figure 5 shows COad stripping voltammograms of Pt/C and PtRu/C along with their derivative counterparts. CO ad stripping derivative voltammograms show positive and negative

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ASSOCIATED CONTENT

S Supporting Information *

XPS and TEM characterization data of Pt/C, PtSn/C, PtRu/C, and PtSnRu/C. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +1 512 471 1791. Fax: +1 512 471 7681.

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ACKNOWLEDGMENTS Financial support by the Office of Naval Research MURI grant no. N00014-07-1-0758 is gratefully acknowledged.

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REFERENCES

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Figure 5. First anodic scans of (a) COad stripping voltammograms and (b) COad stripping derivative voltammograms of Pt/C and PtRu/C in 0.5 M H2SO4 at a scan rate of 20 mV/s.

peaks representing, respectively, positive and negative slopes of the conventional voltammograms. The voltammetric peak of PtRu/C occurs about 200 mV lower than that of Pt/C, indicating facile oxidation of COad and PtRu/C as a better catalyst for MOR. COad stripping derivative voltammograms are devoid of charging current. The voltammetric peak Ep can be located in the derivative voltammograms with more accuracy, especially in the case of PtRu/C, where the voltammetric peak is broader (Figure 5a). The lower COad oxidation potential of PtRu/C can be attributed to the ability of Ru component in PtRu to produce oxygenated species at a lower potential. In contrast, Pt is unable to generate oxygenated species at lower potentials, so COad oxidation occurs at a higher potential in Pt/C (reactions 5 and 6).

4. SUMMARY Advantages of the derivative voltammetry technique over the widely used conventional voltammetry in evaluating and comparing MOR activities of electrocatalysts have been demonstrated using Pt/C, PtSn/C, PtRu/C, and PtSnRu/C. In particular, the derivative technique has better signal-to-noise ratio by virtue of minimizing or eliminating the charging current in methanol oxidation. Accuracy of locating the voltammetric peak potential and onset potential of methanol oxidation current is enhanced owing to the higher resolution and sensitivity of the derivative technique. The weakly manifested important mechanistic feature of MOR in conventional voltammogram is well-exhibited in derivative voltammograms. Furthermore, the durability of a catalyst for MOR can be 3831

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