Article pubs.acs.org/ac
Identification and Analysis of Electrochemical Instrumentation Limitations through the Study of Platinum Surface Oxide Formation and Reduction Ashley A. McMath, Julia van Drunen, Jutae Kim, and Gregory Jerkiewicz* Department of Chemistry, Queen’s University, 90 Bader Lane, Kingston, Ontario K7L 3N6, Canada S Supporting Information *
ABSTRACT: Anodic polarization of Pt electrodes in aqueous H2SO4 leads to the formation of a surface oxide (PtO). Herein, the surface oxide growth is accomplished using three different approaches: (i) chronoamperometry (CA); (ii) chronocoulometry (CC); and (iii) a combination of cyclic voltammetry (CV) and CA. The PtO reduction is accomplished potentiodynamically using voltammetry. The oxide growth takes place at defined polarization potentials (Ep), polarization times (tp), and temperatures (T). The oxide charge density (qox) is determined for both the formation (qox,form) and reduction (qox,red) processes. The oxide reduction CV profiles are integrated to determine the charge density values for oxide reduction (qox,red,CV) which are compared with the qox,form,CA and qox,form,CC values. The values of qox,form,CC are greater than those of qox,form,CA, but both potentiotatic methods (CA and CC) produce qox,form values that are consistently lower than those of qox,red,CV. In the case of oxide formation with combined CV and CA, the values of qox,form,CV+CA are found to be lower than the values of qox,red,CV, although the difference is small. Electrochemical quartz crystal nanobalance (EQCN) is used to monitor the mass variation at the electrode surface during the oxide formation and reduction process at Ep = 1.20 V with various tp values. Equal mass changes during oxide formation and reduction are detected by the EQCN. The nature of the differences in qox,form and qox,red encountered with the different experimental methods are discussed in terms of instrumental limitations.
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techniques and to evaluate their applicability to research on electrochemical surface oxide formation. Our recent cyclic voltammetry (CV), electrochemical quartzcrystal nanobalance (EQCN), and ex situ Auger electron spectroscopy (AES) results demonstrate that the surface oxidation of Pt in the 0.85 ≤ E ≤ 1.55 V range leads to the formation of PtO.13 The proposed mechanism for PtO development in aqueous H2SO4 involves (i) the interaction of water molecules (physisorption) with the metal surface (E = 0.27−0.85 V); (ii) the development of ca. the first half monolayer (ML) of chemisorbed oxygen (Ochem) on the Pt surface through electro-adsorption (E = 0.85−1.15 V); (iii) the electro-adsorption of ca. the second half ML of O that is accompanied by an interfacial structural transformation (place exchange) between Ochem and Pt surface atoms and completion of the charge transfer between O and Pt with the development of a quasi-three-dimensional lattice comprised of Pt2+ and O2− (E = 1.15−1.40 V). In the case of polycrystalline Pt (Pt(poly)), the interfacial place exchange process occurs in the 1.10−1.20 V range.13 Due to the structure that the surface PtO adopts, 1 ML of Ochem is equivalent to ca. 2 ML of PtO due to the chessboard-like arrangement of O2− and Pt2+ surface species. The
nterest in the electrochemical properties of Pt continues to increase as the demand for new energy conversion and storage options drives research in the field of polymer electrolyte membrane fuel cells (PEMFCs). Platinum in the form of nanoparticles is the principal electrocatalyst used in both the anode and cathode of PEMFCs.1 The standard potential of a PEMFC is E° = 1.229 V, but due to the overpotentials of the reactions taking place at the anode and cathode as well as the ohmic resistance of the system, the operating potential (voltage) of individual cells in a PEMFC stack falls within the 0.6−1.0 V range.2 Platinum nanoparticles used in PEMFCs and bulk Pt that is used in other electrochemical systems undergo slow chemical and electrochemical dissolution.3−8 The process was demonstrated to occur in the potential range corresponding to the platinum surface oxide (PtO) formation and reduction.9−11 The onset of PtO formation in aqueous H2SO4 is at E = 0.85 V, which is well within the operating potential range of PEMFCs.12 At present, the mechanism of Pt chemical and electrochemical dissolution is poorly understood, although there exists indisputable evidence that the process occurs. Because the Pt dissolution accompanies the PtO formation and reduction, better understanding of these processes is required. Since a variety of experimental methods can be employed to study the process, it is important to compare results obtained using different © XXXX American Chemical Society
Received: November 8, 2015 Accepted: February 13, 2016
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DOI: 10.1021/acs.analchem.5b04239 Anal. Chem. XXXX, XXX, XXX−XXX
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level above that of the electrolyte; the temperature was maintained constant at T = 293, 313, or 333 ± 1.0 K. The attainment of reproducible CV profiles characteristic for a clean Pt(poly) electrode demonstrated that the environment was inert and the experimental setup was free of contaminants. Electrode Preparation. The wire-shaped Pt(poly) (99.999% in purity, Alfa Aesar) working electrode (WE) was prepared according to the steps outlined elsewhere.23 The electrochemically active surface area (Aecsa) of the Pt(poly) electrode was determined by measuring the charge associated with the adsorption of under-potential deposited H (HUPD) during potential cycling and dividing it by the charge density required to form a monolayer of HUPD on Pt(poly) electrode (qML,HUPD = 210 μC cm−2).28,30,31 The Aecsa of the Pt WE is 1.19 ± 0.01 cm2. The counter electrode (CE) consisted of high-purity Pt gauze (99.98% in purity, Alfa Aesar) spot-welded to a Pt wire (99.98% in purity, Alfa Aesar). The surface area of the CE was at least ten times larger than that of WE, and the distance between WE and CE was ca. 3 cm. The reference electrode (RE) was a reversible hydrogen electrode (RHE), which consisted of Pt foil covered with a thin layer of electrodeposited Pt black. Ultrahigh purity H2(g) (Praxair 5.0 grade) was bubbled through the RE compartment at a pressure of ca. 1 bar. The temperature of the RE and WE compartments of the cell was always the same and agreed to ±1 K. Instrumentation and Software. The oxide formation and reduction measurements were performed using an Autolab PGSTAT302 potentiostat (Metrohm). The instrument was equipped with the SCAN250 analog module for CV linear sweep measurements and the F120 Filter/Integrator for realtime, analog charge integration. Nova Advanced Electrochemical Software (Metrohm) was used to control experimental parameters and to acquire data. Data analysis and graph preparation were accomplished using SigmaPlot 12.3 software. In order to demonstrate that the results presented herein are not uniquely related to one instrument, model, or type of programming/software, experiments were duplicated using three other independent systems: (i) a Princeton Applied Research (PAR) VersaSTAT 4 model potentiostat equipped with VersaStudio software, (ii) a Bio-Logic model SP-150 potentiostat equipped with EC-Lab software, and (iii) a Solartron model 1285A potentiostat equipped with CorrWare software. The details and results of these replicate experiments can be found in the Supporting Information. All electrochemical measurements were performed in triplicate. Electrochemical Quartz Crystal Nanobalance. Electrochemical quartz crystal nanobalance (EQCN) measurements were carried out in order to monitor the mass variation at the surface of the Pt electrode during the electrochemical oxide formation and reduction processes. The EQCN setup was comprised of a Bio-Logic Potentiostat/Galvanostat (model SP150) and a Seiko-EG&G quartz-crystal analyzer (model QCA 922). Electrochemical experiments of the type described below in Method 1 were performed using a custom-made Pyrex twocompartment electrochemical cell. A Pt-coated quartz-crystal resonator having ca. 9 MHz resonant frequency was placed in a Teflon holder that was attached to the bottom of the WE compartment with a horizontal configuration. One of the Ptcoated sides of the quartz crystal was in contact with the electrolyte and served as the working electrode; its geometric surface area was 0.196 cm2. A Pt gauze (Alfa Aesar, 99.9%) was
assumption that PtO adopts a chess-board-like lattice for Pt(poly) is an approximation based on the behavior of an oxide layer on Pt(100), provided that the process takes place preserving the substrate’s structure. It is conceivable that, in the case of low-Miller index Pt surfaces, the interfacial place exchange sets in at O coverage values (θO) other than 0.5 depending on the actual strength of the lateral interactions between the Ptδ+−Oδ− surface dipoles. The important observation is that, depending on the magnitude of the Ptδ+− Oδ− surface dipole moment and the strength of lateral repulsions between such dipoles, the interfacial place exchange would set in at some optimal θO value that is unique to each Pt(hkl) surface. The mechanism of PtO formation is still an intriguing topic of both experimental and theoretical research that stimulates debate in the electrochemical community.14−27 At present, it is accepted on the basis of experimental data that the electrochemical surface oxidation of Pt(poly) in acidic media results in the transfer of two electrons and the formation of PtO; the charge density associated with the formation of 1 ML of PtO is q1 ML,PtO = 420 μC cm−2.28 Kinetic analysis of PtO formation and reduction is the topic of many studies; the kinetic models continue to evolve as the mechanism gradually becomes better understood.19,21−23 The initial step in PtO formation (chemisorption of oxygen at E = 0.85−1.15 V) is a fast process. The interfacial phase transformation (the interfacial place exchange) that leads to the formation of the PtO lattice is slow and only occurs when E > 1.20 V is applied. The electrochemical reduction of PtO does not progress through two well-defined steps, as is the case with PtO formation. Regardless of the O coverage (greater or less than 1 ML of O), the electrochemical PtO reduction is a fast process. In this paper, we report a comprehensive analysis of the formation and reduction of PtO under well-defined experimental conditions (0.90 ≤ Ep ≤ 1.50 V, 1.00 × 100 ≤ tp ≤ 1.00 × 104 s, and 293 ≤ T ≤ 333 K, where Ep is the polarization potential; tp is the polarization time; T refers to temperature) using CV, chronomperometry (CA), and chronocoulometry (CC). This electroanalytical data is presented alongside of an EQCN study in order to evaluate these experimental approaches. The oxide charge density values determined using different experimental approaches do not agree, and this discrepancy is discussed in terms of limitations of electrochemical instrumentation.
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EXPERIMENTAL SECTION Electrolyte and Electrochemical Cell. Electrochemical measurements were carried out in an all Pyrex two-compartment electrochemical cell with three electrodes. The glassware was cleaned according to well-established procedures.29 A highpurity 0.5 M aqueous H2SO4 electrolyte solution was prepared using concentrated H2SO4 (Fluka Analytical, TraceSELECT ≥95%) and ultrahigh purity (UHP) water (Millipore, 18.2 MΩ cm). Ultrahigh purity Ar(g) (5.0 grade, Praxair) was purged through the electrolyte in both the working and reference compartments of the cell for 60 min prior to electrochemical measurements in order to expel any oxygen or other reactive gases from the system. Throughout the duration of the experiments, UHP Ar(g) was passed through the electrolyte in the working electrode compartment and UHP H2(g) was passed through the reference electrode compartment. To ensure temperature control, the electrochemical cell was submerged into a thermostatic water bath with the water B
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studies.23,32−34 In all of these, the Pt oxide was formed anodically but its amount, expressed as qox, was determined through its reduction. The Supporting Information provides an in-depth discussion of this approach and examples of the data obtained for PtO formation and reduction in 0.5 M H2SO4 in various Ep, tp, and T conditions. Figure S2 also provides an outline of the method used to obtain the qox,red,CV values from the CV reduction profiles. During the CA growth of Pt oxide layers at predetermined Ep, tp, and T values, the j is monitored as a function of tp and its integration yields qox,form,CA. Evaluation of the CA formation and CV reduction charge density values generates a set of qox,form,CA and qox,red,CV data that can be compared. Presently, we are aware of only one study (a recent communication27) that applies this approach to PtO formation, but in a different environment (alkaline electrolyte), and without consideration of the CC approach. Thus, we believe it is valuable to carry out this type of analysis using a broader range of experimental methods. Figure 1 presents two graphs, each showing a set of qox,form,CA and qox,red,CV values. Figure 1A presents results for a fixed tp,
used as the CE, and an RHE was used as the reference electrode. The experiments were performed in a home-built Faraday cage at room temperature (298 ± 1 K). Electrochemical Oxide-Growth Procedures. Method 1: Oxide Formation via CA, Oxide Reduction via CV. See Figure S1A. Prior to the commencement of all oxide growth experiments, the Pt electrode was conditioned by potential cycling 80 times using a scan rate of s = 100 mV s−1 in the 0.05 ≤ E ≤ 1.40 V range with the last CV scan ending at E = 0.50 V; this procedure shall be referred to as CV preconditioning. After CV preconditioning, the potential was stepped to a Ep value at which an oxide layer was formed. The amount of oxide grown at a given Ep and for a given tp was determined using CA that measured the current density (j) as a function of time (t); integration of a j versus t profile yielded the oxide formation charge density (q). In each separate experiment, a constant Ep was applied for a given tp at a T value. Then, the surface oxide was reduced in a single cathodic CV transient at a scan rate of s = 50 mV s−1 starting at Ep and ending at 0.05 V. The oxide reduction CV profile was integrated, and the oxide charge density was determined. For the analysis that follows, we introduce a methodology of referring to oxide charge density values determined during its formation (abbreviated as “form” in a subscript) or reduction (abbreviated as “red” in a subscript) using a given technique. Thus, the symbol qox,form,CA refers to the oxide charge density measured during its formation using CA, and qox,red,CV refers to the oxide charge density measured during its reduction using CV. Method 2: Oxide Formation via CC, Oxide Reduction via CV. See Figure S1B. After CV preconditioning, the potential was stepped to Ep at which an oxide layer was formed for tp. The values of qox were determined using CC using an analog charge integrator. Subsequently, the surface oxide was reduced in a single cathodic CV transient as described in Method 1. The symbol qox,form,CC refers to the oxide charge density measured during its formation using CC. Method 3: Oxide Formation by E Sweep to Ep, Oxide Reduction via CV. See Figure S1C. After CV preconditioning, the potential was linearly increased at s = 250 mV s−1 from 0.50 V to Ep at which an oxide layer was formed for tp. Because oxide formation occurs during both the fast CV scan and the potentiostatic polarization, the resulting qox,form has two contributions and is abbreviated as qox,form,CV+CA. Subsequently the surface oxide was reduced in a single cathodic CV transient as described in Method 1. Method 4: Oxide Formation by CC, Oxide Reduction via CV with an Adjusted Scan Time. See Figure S1D. After CV preconditioning, the potential was stepped to Ep at which an oxide layer was formed for tp; the values of qox,form were determined using CC. Subsequently, the surface oxide was reduced in a single cathodic CV transient starting at Ep and ending at 0.05 V. Importantly, the value of s was selected so that the duration of the oxide reduction scan is equal to tp. The symbol CV(s) is used to refer to CV at a variable scan rate, and the symbol qox,red,CV(s) refers to the determined oxide charge density.
Figure 1. (A) qox,form,CA values calculated from CA transients (circles) and the qox,red,CV values calculated from CV profiles (squares) presented as a function of Ep for tp = 1.00 × 102 s. (B) The qox,form,CA values calculated from CA transients and the qox,red,CV values calculated from CV profiles as a function of tp for Ep = 1.20 V. In both data sets, the colors of the points correspond to the following T values: T = 293 K (blue), 313 K (green), and 333 K (red).
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with variable Ep values, at three temperatures. Figure 1B presents results for a fixed Ep, with variable tp values, also at three temperatures. The data clearly demonstrate that the values of qox,red,CV are significantly higher than those of qox,form,CA. In the case of tp = 1.00 × 102 s (Figure 1A), the difference between qox,red,CV and qox,form,CA increases with increasing Ep, apart from one point: Ep = 1.50 V and T =
RESULTS AND DISCUSSION Pt Oxide Formation via Chronoamperometry and Pt Oxide Reduction via Cyclic Voltammetry. The effect of polarization parameters (Ep, tp, and T) and environments (neutral gas versus molecular oxygen) on the extent of oxide formation on Pt(poly) electrode was a subject of several C
DOI: 10.1021/acs.analchem.5b04239 Anal. Chem. XXXX, XXX, XXX−XXX
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Analytical Chemistry 333 K. In the case of Ep = 1.20 V (Figure 1B), the difference between qox,red,CV and qox,form,CA appears to be relatively constant and independent of tp. In addition, the values of qox,form,CA only rarely exceed 210 μC cm−2 suggesting that the surface coverage of O is very low. This observation contradicts several existing studies that demonstrate that the electro-oxidation of Pt(poly) under these Ep, tp, and T conditions results in oxide layers with θO gradually increasing up to θO = 2.23,33 These unexpectedly low qox,form,CA values are the motivation for the extensive examination reported herein. Pt Oxide Formation via Chronocoulometry and Pt Oxide Reduction via Cyclic Voltammetry. In the case of CA measurements commencing after a potential holding in the double layer region (E = 0.50 V), the instrument applies Ep but starts measuring j with a time delay related to the hardware switching from one operational mode to another. The implications of this time delay are discussed in detail in the Supporting Information. To gain further insight into the potentiostatic PtO formation process, the CA measurement was replaced with a CC measurement (described in Method 2 and illustrated in Figure S1B). The principle of the measurement remains the same: PtO is formed at a constant Ep value by stepping the potential from 0.50 V to Ep and holding for tp at a given T. Instead of measuring the j as a function of time, the charge density is measured directly using an analog integrator built into the instrument. The result is displayed in a graph of q versus t, where q corresponds to the cumulative charge density measured during the corresponding oxidation time. Not all commercial potentiostats with a CC program written into the software are actually capable of measuring the charge directly. The Supporting Information elaborates on the use of CA and CC methods by presenting data collected using an instrument that does not possess an analog integrator to measure charge. Figure 2A shows a plot of q versus t for PtO formation at Ep = 1.20 V, tp = 1.00 × 101 s, and at T = 293 K. The label qA corresponds to the cumulative charge density collected as the electrode is held at E = 0.50 V for 2 s prior to applying the potential step. The label qB corresponds to the cumulative charge density at the end of the CC experiment. The charge density value corresponding to PtO formation is obtained by subtracting qA from qB (qox,form,CC = qB − qA). Figure 2A reveals a rapid increase in the value of charge density within the initial 1 s right after the potential step to Ep. Although the result of chronocoulometry measurement is displayed as q versus t with a certain sampling interval, the charge measured is cumulative and thus no charge density is expected to be lost due to a time delay when the instrument switches between experimental techniques. Figure 2B presents three sets of qox values for PtO formation at 0.90 ≤ Ep ≤ 1.50 V, for tp = 1.00 × 102 s, and at T = 293 K determined from (i) CV oxide reduction profiles (qox,red,CV), (ii) CA oxide-formation transients (qox,form,CA), and (iii) CC oxide formation measurements (qox,form,CC). Figure 2C presents analogous results for PtO formation at Ep = 1.20 V, for 1.00 × 100 ≤ tp ≤ 1.00 × 103 s, and at T = 293 K. The results demonstrate that in all instances the values of qox,red,CV are significantly higher than those of qox,form,CA and qox,form,CC and that the values of qox,form,CC are higher than the respective values of qox,form,CA. In addition, the values of qox,form,CC do not exceed 420 μC cm−2 suggesting that the surface coverage of O is less than 1 equiv ML. Again, such an observation contradicts studies that clearly demonstrate that the electro-oxidation of Pt(poly)
Figure 2. (A) Charge density (q) versus time (t), chronocoulometry, profile obtained for the formation of PtO at Ep = 1.20 V for tp = 1.00 × 101 s and at T = 293 K. (B) Three sets of qox values for PtO formation at 0.90 ≤ Ep ≤ 1.50 V for tp = 1.00 × 102 s and at T = 293 K determined from (i) CV oxide-reduction profiles (qox,red,CV, blue squares), (ii) CA oxide formation transients (qox,form,CA, blue circles), and (iii) CC oxide-formation measurements (qox,form,CC, orange circles). (C) Three sets of qox values for PtO formation at Ep = 1.20 V for 1.00 × 100 ≤ tp ≤ 1.00 × 103 s and at T = 293 K determined from (i) CV oxide-reduction profiles (qox,red,CV, blue squares), (ii) CA oxide-formation transients (qox,form,CA, blue circles), and (iii) CC oxideformation measurements (qox,form,CC, orange circles).
under the Ep, tp, and T conditions reported above results in oxide layers with θO gradually increasing up to almost θO = 2. It is important to add that, in both cases of oxide formation (CA or CC), we employed CV to determine the values of oxide charge density by recording and integrating CV oxide reduction profiles. In both sets of experiments, the respective values of qox,red,CV (for the same Ep, tp, and T values) agreed to within 2− 3%. The oxide formation charge density values produced using CC are higher than those obtained using CA; this is attributed to the charge “lost” (unrecorded) during the initial time of a CA potential step which is described in the Supporting Information. However, both the CA and CC oxide formation D
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Analytical Chemistry charge density values are still significantly lower than the values achieved for the corresponding oxide reduction using CV. Thus, we decided to continue to explore the differences between oxide formation using a potentiostatic method (CA or CC) and oxide reduction using a potentiodynamic method (CV). Comparison of Pt Oxide Formation under Potentiostatic and Potentiodynamic Conditions. In Methods 1 and 2, the potential step that takes place from E = 0.50 V to Ep is assumed to be instantaneous; however, it is important to understand that a potential step can not be instantaneous, and it is more realistic to represent it as a very fast potential scan at a rate of millions of V s−1.35 The time required to perform a potential step is instrument specific and depends on the magnitude of the step. For example, the PAR VersaSTAT performs a 10 mV step in 2 μs; it corresponds to a scan rate of 5 × 103 V s−1. In order to unravel the origin of the difference between the qox values determined on the basis of CV, CA, and CC experiments, we employed Method 3 which is presented in Figure S1C. This approach entails the formation of PtO by combining two techniques: (i) potentiodynamic oxide formation using linear sweep voltammetry from E = 0.50 V to a given Ep at a scan rate of s = 250 mV s−1 and (ii) potentiostatic oxide formation at Ep for a given tp. Figure 3A shows the portion of the CV profile for the potentiodynamic oxide formation, and Figure 3B presents the CA transient for the potentiostatic oxide formation. Integration of these transients yields the total oxide formation charge density that is abbreviated as qox,form,CV+CA. Following the PtO formation using CV+CA, we recorded the subsequent CV oxide reduction profile and determined the corresponding qox,red,CV values. Figure 3C presents qox,form,CV+CA and qox,red,CV values for 0.90 ≤ Ep ≤ 1.50 V, for tp = 1.00 × 103 s, and at T = 293 K. The results demonstrate that in most cases the values of qox,red,CV are higher than the values of qox,form,CV+CA, although the discrepancy is small. We also observe that for these tp and T values the two data sets intersect between Ep = 1.40 V and Ep = 1.50 V, where the O2(g) evolution begins to play a role. The intersection of these two data sets also indicates that the discrepancy between qox,form,CV+CA and qox,red,CV becomes smaller at higher Ep values. An alternative approach to oxide formation combining CV + CA would be to perform CV + CC. Unfortunately, limitations of commercial instruments prevent such measurements. During the switch from CV to CC, there is a wait time associated with the transition from the digital signal sampler used in CV measurement to the analog integrator used in CC measurement. Pt Oxide Formation via Chronocoulometry and Pt Oxide Reduction via Cyclic Voltammetry at Different Potential Scan Rates. In a separate series of experiments, we examined the influence of scan rate on the oxide reduction behavior and on the values of qox. The oxide was formed at 0.90 ≤ Ep ≤ 1.50 V, for tp = 1.00 × 101 s, and at T = 293 K, and its charge density was determined using CC. The oxide reduction was accomplished in two different ways: (i) as described in Method 1 (Figure S1A), thus at a scan rate of 50 mV s−1, or (ii) as described in Method 4 (Figure S1D), thus at a selected value of scan rate so that the duration of the oxide reduction scan was equal to tp. The values of the oxide reduction charge densities are abbreviated as qox,red,CV and qox,red,CV(s) and are presented along with the qox,form,CC values in Figure 4. In almost all instances (apart from Ep = 0.90 V where the amount of oxide is very small), the values of qox,form,CC are significantly lower than
Figure 3. (A) A section of the CV profile for oxide formation using cyclic voltammetry (at s = 250 mV s−1) up to Ep = 1.20 V and at T = 293 K. (B) CA transient for the potentiostatic oxide formation at Ep = 1.20 V, for tp = 1.00 × 103 s, and at T = 293 K recorded directly after the CV transient shown in (A); there is no change in the applied potential as the instrument switches from CV to CA. (C) qox,form,CV+CA (circles) and qox,red,CV (squares) values for 0.90 ≤ Ep ≤ 1.50 V, tp = 1.00 × 103 s, and at T = 293 K.
those of qox,red,CV and qox,red,CV(s). The values of qox,red,CV and qox,red,CV(s) obtained from two different sets of experiments are in good agreement. The slight difference between qox,red,CV and qox,red,CV(s) values is due to the double layer charging contribution. Modification of the scan rate gives rise to a slightly different double layer current density in each case. EQCN Analysis of the Mass Difference during Platinum Oxide Formation and Oxide Reduction. Electrochemical quartz crystal nanobalance was used to monitor the mass changes (Δm) during the surface oxide formation and reduction. Figure 5A demonstrates the Δm response overlaid with the CV profile of a Pt electrode in 0.5 M aqueous H2SO4 electrolyte in the 0.05 ≤ E ≤ 1.40 V potential region. During potentiodynamic (CV) oxide formation (0.80 ≤ E ≤ 1.40 V), the Δm increases from ca. 60 ng cm−2 to ca. 115 E
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potential program. During the initial potential step, the Δm signal increases drastically by more than 50 ng cm−2. During the subsequent potential holding at Ep, the Δm signal increases only slightly. When the negative-going potential sweep is applied, the Δm signal decreases back to the original value. Thus, the EQCN response demonstrates that the majority of oxide formation occurs during the potential step; throughout the potential holding, the oxide growth continues but at a very slow rate. Table 1 presents the Δm values obtained from the EQCN study for various tp values with Ep = 1.20 V and T = 298 K. The Table 1. Mass Difference Values Measured at the Electrode Surface Using EQCN during Potentiostatic PtO Formation and the Subsequent Potentiodynamic PtO Reductiona
Figure 4. Three sets of qox values for PtO formation at 0.90 ≤ Ep ≤ 1.50 V, for tp = 1.00 × 101 s, and at T = 293 K determined from (i) CV oxide-reduction profiles (qox,red,CV, blue squares), (ii) CV oxidereduction profiles at 40 ≤ s ≤ 100 mV s−1 (qox,red,CV(s), green squares), and (iii) CC oxide formation measurements (qox,form,CC, circles).
tp (s)
Δmstep (ng cm−2)
Δmhold (ng cm−2)
Δmtotal,form (ng cm−2)
Δmtotal,red (ng cm−2)
10 20 100 500
46 48 45 49
6.5 9.0 12.5 20.5
53 58 58 69
−55 −58 −62 −64
a
Ep = 1.20 V and T = 298 K.
Δm values for potentiostatic oxide formation have two contributions: (i) the mass change values during the potential stepping from 0.50 to 1.20 V (Δmstep) and (ii) the mass change values during the potential holding (Δmhold). The total mass change values for potentiostatic oxide formation correspond to the sum of these two values (Δmtotal,form). The total mass change for the potentiodynamic oxide reduction (Δmtotal,red) has only one contribution that is given in the last column. A comparison of the Δmstep, Δmhold, and Δmtotal,form values reveals that depending on tp between 70% and 88% of the overall oxide develops during the very initial potential step. Although one would expect the duration of the step to be the same in all instances, the scan rate to which the step corresponds is unknown and can not be controlled. The values of Δmstep also reveal a certain scatter that is expected because the EQCN is not as accurate as, for instance, CV due to the inherent experimental fluctuations associated with mechanical and electromagnetic interference. An average of the four Δmstep values in Table 1 is 47 ng cm−2. As expected, the Δmhold values increase with increasing tp because the oxide layer becomes progressively thicker. An analysis of the Δmtotal,form and Δmtotal,red values also reveals small differences that are clearly due to the detection limit of this technique. In the section dealing with PtO formation using CA (Method 1), we explain that significant charge density associated with the process is “lost” (unrecorded) due to instrumental limitations. The EQCN measurements, which employ exactly the same potential versus time program as CA experiments but measure mass changes instead of current density, confirm that the majority of surface oxide is formed during the potential step. This observation supports our proposal that CA and CC measurements used to monitor PtO oxide growth (and possibly other electrode processes) using commercially available instrumentation substantially underestimate the overall charge density associated with the process. Discussion of the Origin of Discrepancy between the Values of qox,form and qox,red Using Different Experimental Approaches. The results presented herein reveal discrepancies between the values of qox,form,CA, qox,form,CC, and
Figure 5. (A) EQCN mass variation transient (red) overlaid with the CV profile of a Pt electrode (black) in 0.5 M aqueous H2SO4 recorded at s = 50 mV s−1 and T = 298 K. (B) The mass variation profile overlaid with the E vs t profile during potentiostatic PtO formation and potentiodynamic PtO reduction using Method 1.
ng cm−2. When the direction of potential sweep is reversed, the Δm value remains constant in the 1.10 ≤ E ≤ 1.40 V potential region, prior to the onset of PtO reduction. As the E is further decreased, PtO reduction begins and the Δm value decreases from ca. 115 ng cm−2 back to ca. 60 ng cm−2. The mass decrease due to PtO reduction falls in the same potential window as the PtO reduction peak that is observed in the CV profile. In this study, the oxide growth Method 1 was applied and the EQCN was used to monitor the mass difference. An Ep of 1.20 V was applied for various tp values (tp = 10, 20, 100, and 500 s). Figure 5B shows the Δm response overlaid with the applied F
DOI: 10.1021/acs.analchem.5b04239 Anal. Chem. XXXX, XXX, XXX−XXX
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Analytical Chemistry qox,red,CV and demonstrate that the values of qox,form,CA and qox,form,CC are lower than the respective values of qox,red,CV. In the case of CA measurements, an uncontrollable delay time following the potential step is responsible for the discrepancy between oxide formation and reduction charge densities. Importantly, this delay cannot be eliminated in the case of commercial potentiostats that switch from one operational mode to another. However, the duration of the delay may be minimized through the use of specialized fast sampling accessories that allow the current signal to be sampled on the nanosecond scale (an extended discussion can be found in the Supporting Information). Figure 2A illustrates the method used to determine the qox,form,CC values from CC experimental data. Over the course of these measurements, the charge is sampled approximately every millisecond. The potential step occurs after a 2 s wait time during which the E value is held at 0.50 V. We use the charge density value after two seconds (qA) as the starting point and calculate qox,form,CC relative to the well-defined qA value in order to avoid any offsets of the analog integrator. Since qB is a cumulative charge density value over tp, the values of qox,form,CC are not susceptible to the same experimental uncertainty encountered in the CA measurements because the charge sampling commences prior to the potential step. However, the discrepancy between the qox,form,CC and qox,form,CA values is clearly visible in Figure 2B,C. This difference between the values of qox,form,CA and qox,form,CC increases as a function of polarization potential; in other words, it increases with the magnitude of the potential step. On the other hand, Figure 2C shows that the difference between the values of qox,form,CC and qox,form,CA does not depend on tp and remains ca. 71 ± 7.7 μC cm−2 for all tp values. This difference is assigned to the delay in the current density measurements within the initial microseconds of the CA experiment. As a result of these observations, we believe that the qox,form,CC values are closer to the true qox,form than the qox,form,CA values. We rationalize the difference between the qox,form,CC and qox,red,CV values as follows. Elsewhere,36 it is explained that chronocoulometry measurements using an analog charge integrator are prone to offsets that result in under-evaluation of charge density values. Accurate chronocoulometry measurements require one to record chronoamperometry curves with a sampling time of ca. 50 μs or shorter and for various potential steps. Their integration and verification for uncompensated resistance can yield accurate oxide charge density values. Since the CC analog charge integrator does not employ this elaborate procedure, the difference between the qox,form,CC and qox,red,CV values is assigned to the baseline offset and/or uncompensated resistance. In this contribution, we demonstrate explicitly the limitations of calculating charge density values via CA and CC measurements following a potential step. In many cases, the charge density values determined using commercial instrumentation run by proprietary software were underestimated by as much as 50%. It is also worth noting that the charge densities associated with anion adsorption/desorption contribute to the qox,form and qox,red values calculated in this study. These processes are discussed in the Supporting Information; they do not account for the overall behavior described herein. Significance of the Reported Results and Instrumentation Limitations. The selection of experimental techniques applied to the study of electrochemical oxidation and dissolution of noble metals is important because the j and q values can be correlated to actual material loss. This requires
that the applied technique be capable of accurately measuring the entire current density or charge density associated with the process under investigation. The results presented in this contribution point to some important limitations of commercial instruments. In this study, the limitations result in significant under-estimation of the charge density values in potential step measurements or in experiments that involve two subsequent techniques. We wish to stress that these results were reproduced using four different commercial potentiostats from different manufacturers (data provided in the Supporting Information). Thus, this work is not a criticism of any particular system or supplier but a cautionary example of the importance of considering experimental limitations and verifying results with multiple independent techniques. This study demonstrates that charge density values based on the cathodic reduction of surface oxide layers using voltammetry provide the most reliable data. The results presented herein call for a very careful and thoughtful selection of electrochemical experimental techniques when studying electrochemical oxidation and dissolution of noble metals.
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CONCLUSIONS Anodic polarization of platinum electrodes in 0.5 M aqueous H2SO4 at 0.90 ≤ Ep ≤ 1.50 V, for 1.00 × 100 ≤ tp ≤ 1.00 × 104 s, and at 293 ≤ T ≤ 333 K leads to the formation of the PtO surface oxide. The surface coverage of O increases with increasing Ep, tp, and/or T. Oxide growth was accomplished using CA, CC, and a two-step procedure that combines voltammetry and CA. In all cases, the surface oxide was reduced using CV. The values of qox,form,CA and qox,form,CC are significantly lower than those of qox,red,CV, although the values of qox,form,CC are greater than those of qox,form,CA. In most cases, the values of qox,form,CV+CA are lower than the values of qox,red,CV, although the difference is small. An electrochemical quartz crystal nanobalance is used to determine the mass variation associated with surface oxide formation during potential stepping and holding and oxide reduction upon potential scanning. The Δm values for the PtO formation and reduction processes are in agreement with each other and demonstrate that the nature of the discrepancy between the qox,form,CA and the qox,red,CV is the result of instrumental limitations of commercial potentiostats controlled using proprietary software. Equipment from four different manufacturers demonstrate the same limitations in CA and CC measurements. A careful analysis of the results leads to the conclusion that an accurate determination of the amount of Pt surface oxide using an electrochemical technique and expressed as an oxide charge density should be based on the oxide reduction CV profiles.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.analchem.5b04239. Illustrative schemes for Methods 1−4; background information on PtO formation at various Ep, tp, and T conditions; discussion of potential step measurements; reproducibility of the experiments using various commercial potentiostats and software packages; discussion of the impact of anion adsorption and desorption. (PDF) G
DOI: 10.1021/acs.analchem.5b04239 Anal. Chem. XXXX, XXX, XXX−XXX
Article
Analytical Chemistry
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(26) Furuya, Y.; Mashio, T.; Ohma, A.; Tian, M.; Kaveh, F.; Beauchemin, D.; Jerkiewicz, G. ACS Catal. 2015, 5 (4), 2605−2614. (27) Grdeń, M. Electrochem. Commun. 2015, 61, 14−17. (28) Trasatti, S.; Petrii, O. A. J. Electroanal. Chem. 1992, 327 (1−2), 353−376. (29) Angerstein-Kozlowska, H.; Conway, B. E.; Sharp, W. B. A. J. Electroanal. Chem. Interfacial Electrochem. 1973, 43 (1), 9−36. (30) Zhan, D.; Velmurugan, J.; Mirkin, M. V. J. Am. Chem. Soc. 2009, 131 (41), 14756−14760. (31) Chen, D.; Tao, Q.; Liao, L. W.; Liu, S. X.; Chen, Y. X.; Ye, S. Electrocatalysis 2011, 2, 207−219. (32) Conway, B. E.; Tremiliosi-Filho, G.; Jerkiewicz, G. J. Electroanal. Chem. Interfacial Electrochem. 1991, 297 (2), 435−443. (33) Tremiliosi-Filho, G.; Jerkiewicz, G.; Conway, B. E. Langmuir 1992, 8 (2), 658−667. (34) Kongkanand, A.; Ziegelbauer, J. M. J. Phys. Chem. C 2012, 116 (5), 3684−3693. (35) Nagy, Z. J. Electrochem. Soc. 1982, 129 (9), 1943−1950. (36) Richer, J.; Lipkowski, J. J. Electrochem. Soc. 1986, 133 (1), 121− 128.
AUTHOR INFORMATION
Corresponding Author
*Phone: +1 613 533 6413. E-mail: gregory.jerkiewicz@ queensu.ca. Author Contributions
The manuscript was written through contributions of all authors and all authors have given approval to the final version of the manuscript. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Grateful acknowledgements are made to the Natural Sciences and Engineering Research Council of Canada, Automotive Partnership Canada, and Queen’s University for their financial support. A.A.M. gratefully acknowledges the NSERC undergraduate student research award program.
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DOI: 10.1021/acs.analchem.5b04239 Anal. Chem. XXXX, XXX, XXX−XXX
