Article pubs.acs.org/ac
Two Electrode Collector−Generator Method for the Detection of Electrochemically or Photoelectrochemically Produced O2 Benjamin D. Sherman, Matthew V. Sheridan, Christopher J. Dares, and Thomas J. Meyer* Department of Chemistry, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599, United States ABSTRACT: A dual working electrode technique for the in situ production and quantification of electrochemically or photoelectrochemically produced O2 is described. This technique, termed a collector−generator cell, utilizes a transparent fluorine doped tin oxide electrode to sense O2. This setup is specifically designed for detecting O2 in dye sensitized photoelectrosynthesis cells.
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introducing substantial noise to the oxygen concentration measurements. Electrochemical detection of oxygen traces back to the development of a Clark electrode. Consisting of two platinum electrodes separated from the sample solution by an oxygen permeable membrane, it allows for unambiguous detection and quantification of O2. Complications arise from the large size of the electrodes, which require a large volume cell, thereby diluting the O2 product making accurate measurements laborious. Some manufacturers now offer electrochemical O2 detection systems housed within small enclosures with needle endings (Unisense, Aarhus, Denmark). Measuring meaningful changes in O2 as generated by dye sensitized photoanodes still remains a challenge even with the better size resolution of these systems. We describe here a two electrode technique with O2 analysis carried out by a “collector” which senses O2 produced electrochemically or photoelectrochemically by the “generator” electrode. In a “collector−generator” (C−G) assembly, the two electrodes are held in close proximity with a 1 mm glass spacer between their electroactive surfaces. The magnitude of current resulting from the electrochemical reduction of dissolved oxygen at the collector, a fluorine doped tin oxide electrode (FTO), is used to detect and quantify O2. Other dual working electrode techniques, with the electrodes placed in close proximity, exist such as rotating ring disk electrodes (RRDE),4 twin electrode thin layer electrochemistry,5−8 dual-plate trench electrodes,9,10 and interdigitated array electrodes.11,12 By producing steady-state concentration gradients of electroactive species, these methods enable the
hotoelectrochemical water splitting into H2 and O2 or reduction of CO2 to carbon-based fuels by artificial photosynthesis represents a promising alternative to carbon fossil fuels.1,2 Research developments in this area hinge on the reliable, rapid detection of target products including H2 and O2 in order to demonstrate the viability of emerging approaches. A variety of techniques enable the detection of O2 produced by electro- or photoelectrochemical catalysis including gas chromatography, electrochemistry, and spectroscopy. In the context of measuring photochemically formed O2, especially from dye sensitized semiconductor based systems as emphasized here, each of these methods offer benefits and drawbacks. Widely utilized, gas chromatography (GC) provides unambiguous and quantitative detection of O2. In the absence of a photochemical or photoelectrochemical reactor plumbed in line with a gas chromatograph and isolated from the atmosphere, avoiding erroneous detection of atmospheric O2 presents a significant challenge. With syringe-based head space sampling in a septa sealed photochemical reactor, air permeability through the septa can cause misleading O2 readings. Side stepping this challenge requires the use of O18 labeled water combined with GC−mass spectrometry measurements to establish the solvent water as the origin of the evolved O2. Spectroscopic means of detecting oxygen typically rely on phosphorescence quenching measurements,3 with the lifetime of the probe excited state varying with the concentration of O2 in contact with the probe. Commercial electrodes (Ocean Optics, Dunedin, FL, USA) can be purchased with small probe housings with needle endings. This method depends on the change in total concentration within a sealed cell which can be small for modest photocurrents, low faradaic efficiencies, or large solution volumes. Complications can arise from the light source of the photochemical cell interfering with the probe, © XXXX American Chemical Society
Received: February 24, 2016 Accepted: June 24, 2016
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DOI: 10.1021/acs.analchem.6b00738 Anal. Chem. XXXX, XXX, XXX−XXX
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determination of heterogeneous electron transfer kinetics,5 chemical reaction kinetics in solution following electron transfer,13 the determination of diffusion coefficients,5 pH changes,14 and, most pertinent here, detection of target analytes.10,14 The collector−generator cell described herein is specifically targeted at the detection and quantification of O2 produced either electrochemically or photoelectrochemically at the generator electrode. We have used this technique in a series of recent reports to monitor catalytic O2 generation both electrochemically15,16 and photoelectrochemically.17−20 The Mallouk group first reported the use of a collector electrode for the amperometric detection of photochemically produced O2 from dye sensitized photoanodes,21 with later publications reflecting the development and utility of the approach.22−24 In addition to detailed descriptions of the fabrication and utilization of the collector−generator dual working electrode method, we also present important experimental considerations for avoiding erroneous current readings and to determine the faradaic efficiency for the formation of O2 reliably and reproducibly.
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RESULTS AND DISCUSSION We focus here on a description of the important experimental details and technical considerations for the unambiguous determination of the faradaic efficiency for O2 production using a collector−generator dual working electrode setup. Specifically, the use of unmodified FTO electrodes for oxygen reduction will be described with a voltage window and conditions for the sensitive detection of O2 defined. The importance of using high ionic strength solutions to avoid capacitive interference between the working electrodes as well as the determination of the typical collection efficiency of the C−G cell under the conditions used here will be described. Collector−Generator Design and Method for Determining O2 Faradaic Efficiencies. As shown in Figure 1, the
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EXPERIMENTAL SECTION Fluorine doped tin oxide (FTO) was purchased from Hartford Glass (Hartford, IN, USA). All chemicals were purchased from Sigma-Aldrich or Alpha Aesar and used as received unless otherwise noted. A CH Instruments 760E bipotentiostat was used for all electrochemical experiments. The electrochemical experiments utilized a collector−generator dual working electrode assembly with a saturated calomel reference electrode and platinum wire counter electrode. Electrochemical experiments were performed in a two compartment cell with the reference placed near the working electrode(s) and the platinum counter electrode separated from the working solution by a glass frit. The reference electrode was placed within 5 mm of the working electrode(s), and the resistance between the reference and working electrode(s) was measured to ensure this did not introduce any significant error in the measurements. Collector−Generator Fabrication. The collector−generator electrode setup consisted of two FTO-based working electrodes. In some cases the FTOgenerator was modified with a mesoporous tin doped indium oxide (ITO) layer. The second electrode, FTOcollector, did not undergo any surface modification. Fabrication of a collector−generator assembly began with cutting 10 mm by 40 mm pieces of FTO. The upper right corner of each cell was removed with a diagonal cut from upper left to approximately 8−10 mm down from the top along the right lateral edge. Wire leads were bonded to the upper left corner of the electrodes using conductive epoxy (Chemtronics CW2400). Thin (2−3 mm wide) pieces of 1 mm thick microscope slides were epoxied (Hysol E-00CL) along the lower lateral edges of the conductive face of the FTO collector electrode. Using the same inert epoxy (Hysol E-00CL), the generator electrode was then bonded to the collector with the conductive sides facing, with care taken to ensure that the conductive contacts did not short. This resulted in a continuous seal along the edges of the assembly with openings at the top and bottom allowing the internal volume to fill by capillary action when placed in solution.
Figure 1. (A) Collector−generator schematic and photographs of the front (B) and bottom (C) of an example C−G assembly consisting of a FTO|nanoTiO2 photoanode derivatized with a chromophore− catalyst assembly and a FTO collector electrode.
collector−generator cell consists of two FTO-based electrodes, with the conductive surfaces facing and separated by a 1 mm gap. The lateral edges of the two electrode assembly were completely sealed with epoxy leaving openings at only the top and bottom edges. During electrochemical measurements, the cell is partially submerged in an electrolyte solution and the two electrodes held under independent potential control by a bipotentiostat. The architecture of the two electrode assembly plays an important role in its function for measuring the oxygen concentration in the volume between the electrodes. With O2 generation at the generator electrode, the local concentration of O2 in the volume between the electrodes (generally 90−120 μL) rapidly increases compared to the bulk electrolyte. Because the current density at the generator dictates the quantity of O2 produced, the small confined volume of solution between the collector and generator serves to amplify the O2 signal as compared a more conventional O2 probe which monitors the O2 concentration of the bulk solution (a volume of 5−8 mL for the glass cells used here). In addition, the small aspect ratio of the top and bottom openings (0.05−0.07 cm2) of the C−G assembly compared to the working areas of either electrode (0.75−1 cm2) ensures limited loss of O2 to the head space or bulk solution by diffusion. The spacing between the collector and generator dictates the time constant for establishing a steady-state concentration gradient, and therefore steady-state currents, between the B
DOI: 10.1021/acs.analchem.6b00738 Anal. Chem. XXXX, XXX, XXX−XXX
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Analytical Chemistry electrodes. Starting from an initially O2 depleted solution between the electrodes, simultaneously poising the generator at a potential capable of forming O2 and the collector at a potential for selectively reducing O 2 will establish a concentration gradient between the electrodes. This gradient will reach a steady-state flux in a time (t) dependent on the diffusion coefficient of O2 (DO2) and the distance between the electrodes (l) according to5,14,25 t = l 2/2DO2
(1)
Using a 1 mm distance between the electrodes and taking the diffusivity of O2 in water at 25 °C with 1 M electrolyte as 1.65 × 10−5 cm2 s−1 give a characteristic time of ∼300 s for reaching a steady-state regime.26 This has critical implications for the interpretation of results since it dictates the waiting time required before evaluating the faradaic efficiency of O2 production (ηO2) directly from the current at each electrode. Using the current magnitudes at any time point prior to establishing a steady state will not reflect a true measure of the faradaic efficiency. ηO = (Q col /Q gen)(1/ηcol eff ) 2
(2)
As an alternative to comparing steady-state currents, a chronocolumbic method can be used to determine the faradaic efficiency according to eq 2, where Qcol and Qgen are the total charge passed at the collector and generator electrodes and ηcol eff is the collection efficiency of the cell. The use of this method requires a chronopotentiometric experiment where the generator electrode is held for a defined period at a potential sufficient to produce O2 and then stepped to a more negative potential where O2 is not generated; during the time of the two potential periods at the generator, the collector electrode is held at a potential sufficient to selectively reduce O2. The charge from the production O2, i.e., the total charge passed during the initial, high potential step at the generator (Qgen) is compared with the charge passed at the collector during the entire time of the experiment (Qcol). Stepping the generator to a more negative potential, and thereby terminating O2 production, allows for the depletion of O2 in the solution between the electrodes by reduction at the collector. This ensures a full accounting of the O2 produced during the experiment. Correcting for the collection efficiency accounts for any O2 lost by diffusion to the head space or to the bulk solution. Generally, a 300−600 s period for the high potential phase and a 600−900 s period for the low potential phase allow for an effective measure of ηO2. For photoelectrochemical O2 generation, dark and light phases dictate the periods of quiet or O2 formation at the generator, respectively. Figure 2 shows a representative chronoamperometic collector−generator experiment using an FTO collector and FTO generator. During the initial period, the generator was poised at a potential of 1.6 V vs SCE, a sufficiently high potential to initiate water oxidation to O2. After 900 s, the applied potential at FTOgen switches to 0.4 V vs SCE, ceasing any production of O2. During the entire trace, the FTOcol is held at −0.85 V vs SCE. Integration of the current traces in Figure 2a, as shown in Figure 2b, gives the charge passed as a function of time allowing for a straightforward visual comparison of the faradaic efficiency. To correct for background current at the collector resulting from the diffusion of O2 from the bulk, an experiment with the generator at a
Figure 2. (A) Current vs time plot with (top) FTOgen poised at 1.6 V vs SCE from 0 to 900 s followed by 0.4 V vs SCE from 900 to 1800 s and (bottom) FTOcol poised at −0.85 V vs SCE for the entire experiment. (B) Charge vs time plot of the data shown in panel A. The solid line shows the charge passed at FTOgen and the dashed trace the charge passed at FTOcol. (C) Cyclic voltammogram taken with the FTOgen with the dashed line indicating the potential used to produce O2 at the FTO electrode (1.6 V vs SCE).
potential negative of the onset of oxygen production with the collector held at a potential for O2 reduction gives the total charge resulting from residual O2 present in the cell. Careful application of this approach enables the measurement of ηO2 even without an entirely airtight seal from the atmosphere. In these experiments it is expedient to first subject the C−G cell to an electrochemical purge by poising the collector at a potential sufficient to reduce O2 without a potential applied to the generator. With this procedure, any residual O2 in the solution volume between the electrodes is removed before initiating an experiment to determine ηO2. Similar to a rotating ring-disc electrode, the collector− generator cell allows the determination of an electrode dependent collection efficiency as dictated by its geometry. The collection efficiency is defined as the percentage of charge C
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compromises the assignment of oxygen production solely from the original state of the generator. The use of an FTO collector electrode avoids any complication from employing an FTO-Pt collector. Although the Pt surface carries out oxygen reduction with a smaller overpotential than FTO, in the proper potential window, FTO shows a good sensitivity to the presence of dissolved O2 as seen in Figure 4. In addition, the onset of H2 formation occurs at a
from reduction of O2 at the collector relative to that resulting from the oxidation of water to form O2 at the generator. To empirically determine the collection efficiency, five separate FTOgen−FTOcol cells were prepared and examined with the FTOgen held at 1.6 V vs SCE to form O2 and the FTOcol at −0.85 V vs SCE to reduce O2. From this sampling of cells, an average collection efficiency of 68.9 ± 4.0% was observed which matched the collection efficiency of 70% previously reported.15,17,18 The observed collection efficiency below 100% results from diffusive loss of O2 through the bottom and top gaps in the assembly as well as an electron count of less than 4 per molecule O2 at the collector as described below. As with the use of a rotating ring-disc electrode, proper use of the C−G method as described here should include independent verification of the collection efficiency under the specific conditions of the application. Oxygen Reduction Using FTO. Earlier iterations of the dual electrode strategy for detecting O2 described here used a platinized glass surface for carrying out the reduction of O2.21,22 While Pt offers a good surface for promoting O2 reduction, collector−generator cells constructed with a platinized FTO surface gave some discouraging results indicating migration of the Pt from one electrode surface to the other. As seen in Figure 3, with a dual electrode cell consisting of one FTO
Figure 4. (Top) Cyclic voltammograms taken at a planar FTO electrode in 0.1 M phosphate buffer at pH 7 with 0.1 M NaClO4 supporting electrolyte saturated with air (black), oxygen (dark gray), or nitrogen (light gray). (Bottom) Cyclic voltammograms taken at a planar FTO electrode in nitrogen saturated solution showing the onset of hydrogen formation at potentials more negative than −1 V vs SCE. Figure 3. Cyclic voltammograms of an FTOgen electrode before (black) and after (gray) operation of the collector−generator cell and of an FTO-Ptcol electrode (cyan) after 1 h.
more negative potential as compared with Pt, leaving a wide potential range for sensing O2. Finally, given that the collector− generator cell is designed for detecting photoelectrochemically formed O2, the use of a platinized FTO electrode can cause reflection of light back onto the photoanode or block light from the photoanode depending on the orientation of the cell relative to the light source, which complicates interpretation of the results. The use of transparent FTO as the collector surface avoids any interference with visible light in the cell. Previous studies have remarked on the reduction of oxygen at FTO electrodes27 and used this as a diagnostic for the presence of O2.28 Figure 4 shows a clear increase in cathodic current as a function of the amount of O2 present in solution. Current magnitudes at −0.85 V vs SCE increase from a N2 saturated solution as the lowest, to an air saturated solution, to O2 saturated solution. With the electrolyte conditions identical, and only [O2] variable, the magnitude of cathodic current at −0.85 V vs SCE directly corresponds to the oxygen concentration in solution. In the absence of O2, the voltammogram shows the same behavior as documented previously for FTO electrodes with catalytic currents observed
electrode and one FTO-Pt electrode, cyclic voltammetric (CV) scans at the FTO electrode initially showed a waveform consistent with a clean FTO surface. Following a series of experiments with the FTO poised to form O2 (1.5 V vs SCE) and the FTO-Pt poised to reduce O2 (−0.4 V vs SCE), a CV scan of the initially clean FTO electrode (black line) showed clear indication of surface adsorbed Pt (gray line). The anodic peak at 0.7 V vs SCE and cathodic peak at −0.2 V vs SCE in the voltammogram following 1 h of cell operation are consistent with the formation and striping of a platinum oxide layer and match the same features seen at the FTO-Pt electrode. Additionally, increased catalytic currents at high potential (O2 formation) and low potential (H2 formation) regimes over time as compared to the initial FTO surface indicate the adsorption of Pt on the FTO. The growth of Pt features at the FTO must arise from migration of Pt from the FTO-Pt surface and, if using an FTO-Pt collector electrode, the possibility of seeding the generator surface with Pt, a robust water oxidation catalyst, D
DOI: 10.1021/acs.analchem.6b00738 Anal. Chem. XXXX, XXX, XXX−XXX
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Analytical Chemistry at low potentials (≤−1 V vs SCE) resulting from the generation of H2.29 Importantly, comparison of the CVs with and without the presence of O2 shows an onset for cathodic current relating to the presence of O2 around −0.2 V vs SCE. Given the large contrast in the currents observed at −0.85 V vs SCE, without appreciable contribution from H2 generation, this potential was chosen for sensing O2 at pH 7. The choice of potential for the collector electrode has important consequences in ensuring that the signal arises exclusively from O2 and not from another electrochemical process. A suitable potential should provide sufficient overpotential for O2 reduction including its pH dependence. Monitoring at more negative potentials (≤−1 V vs SCE) causes irreversible damage to the FTO surface and should be avoided.29,30 Determining the mechanism of oxygen reduction occurring at the FTO surface at the potential applied to the collector electrode has major importance for quantifying the amount of O2 sensed at the collector electrode. A previous study suggested that the four electron reduction of O2 predominates at FTO surfaces.27 To investigate this point, chronoamperometic measurements were carried out in air saturated solutions. At a constant potential of −0.85 V vs SCE in pH 7 solution, analysis of the charge passed vs time (Anson plot) using the integrated form of the Cottrell equation (eq 3) can determine the number of electrons transferred.4 In this equation, n is the number of electrons transferred, F Faraday’s constant, A the electrode area, C the concentration of O2, and D the diffusion coefficient of O2. Q = 2nFACπ −1/2D1/2t 1/2
Figure 5. Collector−generator experiment to monitor for the possible formation of H2O2 from O2 reduction and the subsequent oxidation of H2O2 at the collector electrode. The FTOgen electrode was poised at −0.85 V vs SCE from 0 to 900 s and then 0 V vs SCE from 900 to 1800 s. The air saturated electrolyte containing 1 M NaClO4 was buffered at pH 7 using 0.1 M phosphate buffer. The FTOcol was poised at 0.8 V vs SCE during the entire experiment to monitor for H2O2. The inset shows CV scans performed with an FTO working electrode in air saturated solution (black), N2 saturated solution (blue), and N2 saturated solution containing 0.05% H2O2 (red).
dictate the time delay for observing product from the generator electrode at the collector electrode, the 1 mm distance separating the two creates a delay between producing and sensing O2. Despite this, the results of early experiments gave an apparent instantaneous increase in current at the collector electrode with O2 production at the generator. The initial currents observed at the collector electrode showed a magnitude comparable to the generator current and decayed with a similar rate. This response appeared to defy the physical consequence of the gap width. Attributing this instantaneous collector current as representative of O2 reduction would imply unrealistically rapid transport of O2 between the electrodes. Alternatively, the response could be attributed to a capacitance effect arising between the two working electrodes. To investigate a capacitive interference between the collector and generator electrodes, experiments were carried out as a function of supporting electrolyte concentration from 0.05 to 1 M NaClO4. Two different C−G configurations were investigated. One consisted of planar FTO generator and collector electrodes and the other a high surface area mesoporous FTOnanoITO32 generator and planar FTO collector. In these experiments, the electrolyte concentration varied in the order 50 mM, 100 mM, 200 mM, 500 mM, and 1 M NaClO4 with the solution pH held at 7 with 20 mM phosphate buffer at each concentration. For both the FTOgen−FTOcol and FTO-nanoITOgen−FTOcol cells a current mirroring between the collector and generator electrodes was observed, with the effect diminishing as the supporting electrolyte increased (Figure 6). The interference observed with the FTOgen−FTOcol assembly was not as pronounced as for the FTO-nanoITOgen−FTOcol cell. The apparent faradaic efficiency for O2 production was also investigated as a function of electrolyte concentration. Again, the concentration of electrolyte affected the faradaic efficiency for O2 production, with a lower efficiency observed at low concentrations and the efficiency plateauing at higher concentrations. For either configuration, a supporting electrolyte concentration of 0.5 M was sufficient to minimize the
(3)
According to eq 3, when the current depends only on diffusion, the slope of the charge vs t1/2 plot gives n with the other variables known. Using FTO electrodes with the application of insulating epoxy to define the active geometric area, charge vs time plots were recorded at −0.85 V vs SCE in air saturated solutions with 1 M NaClO4 electrolyte at pH 7 (20 mM phosphate buffer). Analysis of the data with [O2] = 215 μM and assuming the diffusion coefficient for O2 in water at 25 °C as 1.65 × 10−5 cm2 s−1 gave an n value of 2.9 ± 0.2 e− as the average of three FTO electrodes with five independent measurements taken for each electrode.26,31 Assuming the four electron oxidation of water to O2 at the generator, an n value of ∼3 for O2 reduction coincides with the observed collection efficiency of ∼70% discussed above. The observation of an overall electron count of 3 per molecule of O2 reduced at the collector raises the possibility of H2O2 contributing to the anodic current observed at the generator electrode over the course of a water oxidation experiment. To investigate this possibility, a C−G experiment was carried out in air saturated solution as shown in Figure 5. In this case, the FTOgen is poised at −0.85 V vs SCE to carry out the reduction of O2 from 0 to 900 s and 0 V vs SCE from 900 to 1800 s. The FTOcol is poised at 0.8 V vs SCE to sense for H2O2 during the entire experiment. As shown in the inset to Figure 5, CVs taken with an FTO working electrode showed increased anodic current at 0.8 V vs SCE in the presence of H2O2 as compared with air or N2 saturated solution absent of H2O2. Importantly, no increase in anodic current is observed at the FTOcol during the experiment demonstrating that if formed during O2 reduction, H2O2 does not contribute to the current observed at the opposing electrode. Influence of Ionic Strength. Since the distance between the two electrodes and the diffusion coefficient of the analyte E
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degradation of the chromophore or chromophore−catalyst assembly.
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CONCLUSION We describe here a functional, accurate, and reliable technique for evaluating evolved O2 as a product of electrochemical or photoelectrochemical water oxidation. It has the advantage of rapid monitoring in small solution volumes. The technique uses a transparent FTO electrode to sense O2, with a small separation between it and the O2 producing electrode. The technique enables detection of small amounts of dissolved O2 and avoids interference of incident illumination with the monitoring of O2. The reliance on a low cost material for O2 detection (FTO), with the only other requirement being access to a bipotentiostat, should make this an attractive method for detection and quantification of oxygen produced either electrochemically or photoelectrochemically at metal oxide based electrodes.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research was supported solely by the UNC EFRC Center for Solar Fuels, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Award No. DE-SC0001011.
Figure 6. (Top) Zoomed view of current vs time plots of (upper frame) FTO-nanoITOgen and (bottom frame) FTOcol at pH 7 (0.02 M phosphate buffer) with the indicated supporting electrolyte concentration (NaClO4). The potential at the FTO-nanoITOgen was switched from 0.4 to 1.6 V vs SCE at 2 s during the experiment while the FTOcol was held at −0.85 V vs SCE. (Bottom) Plot showing the apparent faradaic efficiency vs electrolyte concentration for the indicated collector−generator cells.
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REFERENCES
(1) Ashford, D. L.; Gish, M. K.; Vannucci, A. K.; Brennaman, M. K.; Templeton, J. L.; Papanikolas, J. M.; Meyer, T. J. Chem. Rev. 2015, 115, 13006−13049. (2) Sherman, B.; Vaughn, M.; Bergkamp, J.; Gust, D.; Moore, A.; Moore, T. Photosynth. Res. 2014, 120, 59−70. (3) Xie, Z.; Ma, L.; deKrafft, K. E.; Jin, A.; Lin, W. J. Am. Chem. Soc. 2010, 132, 922−923. (4) Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals and Applications, 2nd ed.; Wiley: New York, 2001. (5) Anderson, L. B.; Reilley, C. N. J. Electroanal. Chem. 1965, 10, 295−305. (6) Christensen, C. R.; Anson, F. C. Anal. Chem. 1963, 35, 205−209. (7) Huang, J.; Zhang, C.; Zhang, W.; Zhou, X. J. Electroanal. Chem. 1997, 433, 33−39. (8) McDuffie, B.; Anderson, L. B.; Reilley, C. N. Anal. Chem. 1966, 38, 883−890. (9) Hasnat, M. A.; Gross, A. J.; Dale, S. E. C.; Barnes, E. O.; Compton, R. G.; Marken, F. Analyst 2014, 139, 569−575. (10) Gross, A. J.; Marken, F. Electrochem. Commun. 2014, 46, 120− 123. (11) Aoki, K.; Morita, M.; Niwa, O.; Tabei, H. J. Electroanal. Chem. Interfacial Electrochem. 1988, 256, 269−282. (12) Chidsey, C. E.; Feldman, B. J.; Lundgren, C.; Murray, R. W. Anal. Chem. 1986, 58, 601−607. (13) Anderson, L. B.; Reilley, C. N. J. Electroanal. Chem. 1965, 10, 538−552. (14) Dengler, A. K.; Wightman, R. M.; McCarty, G. S. Anal. Chem. 2015, 87, 10556−10564. (15) Sherman, B. D.; Sheridan, M. V.; Wee, K.-R.; Song, N.; Dares, C. J.; Fang, Z.; Tamaki, Y.; Nayak, A.; Meyer, T. J. Inorg. Chem. 2016, 55, 512−517. (16) Sheridan, M. V.; Sherman, B. D.; Marquard, S. L.; Fang, Z.; Ashford, D. L.; Wee, K. R.; Gold, A. S.; Alibabaei, L.; Rudd, J. A.;
mirrored charging currents at short times and the influence on the observed faradaic efficiency over long electrolysis periods. Photochemically Generated O2. The development of the C−G method came as a result of the specific challenges presented by measuring photochemically produced oxygen in dye sensitized photoelectrosynthesis cell (DSPEC) applications. These challenges include the following: small overall changes in [O2] resulting from low current densities and faradaic efficiencies, the desire for rapid determination of O2 generation (avoiding long photoelectrolysis experiment times), and the need for a detector that does not physically block illumination of the photoelectrode or require the fabrication of complicated, custom glassware with large volumes. Recently our group has applied this method to several DSPEC systems.17−20 As a specific example, we recently used this method for analyzing the production of O2 photoelectrochemically by electroassembled films containing a ruthenium(II) polypyridyl based chromophore and Ru(2,2′-bipyridine-6,6′-dicarboxylic acid)(pyridine)2 water oxidation catalyst. In these experiments a faradaic efficiency of 22% was measured over a 5 min photolysis period.18 Application of the collector−generator method revealed a decrease in the faradaic efficiency for O2 production over time likely due to competitive decomposition of the oxidized form of the chromophore. This result points to the importance of quantitative O2 measurements in evaluating photochemical and photoelectrochemical efficiencies in water splitting experiments in order to rule out competitive oxidative F
DOI: 10.1021/acs.analchem.6b00738 Anal. Chem. XXXX, XXX, XXX−XXX
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Analytical Chemistry Coggins, M. K.; Meyer, T. J. J. Phys. Chem. C 2015, 119, 25420− 25428. (17) Ashford, D. L.; Sherman, B. D.; Binstead, R. A.; Templeton, J. L.; Meyer, T. J. Angew. Chem. 2015, 127, 4860−4863. (18) Sherman, B. D.; Ashford, D. L.; Lapides, A. M.; Sheridan, M. V.; Wee, K. R.; Meyer, T. J. J. Phys. Chem. Lett. 2015, 6, 3213−3217. (19) Lapides, A. M.; Sherman, B. D.; Brennaman, K.; Dares, C. J.; Skinner, K. R.; Templeton, J.; Meyer, T. J. Chem. Sci. 2015, 6, 6398− 6406. (20) Wee, K.-R.; Sherman, B. D.; Brennaman, M. K.; Sheridan, M. V.; Nayak, A.; Alibabaei, L.; Meyer, T. J. J. Mater. Chem. A 2016, 4, 2969− 2975. (21) Youngblood, W. J.; Lee, S. H. A.; Kobayashi, Y.; HernandezPagan, E. A.; Hoertz, P. G.; Moore, T. A.; Moore, A. L.; Gust, D.; Mallouk, T. E. J. Am. Chem. Soc. 2009, 131, 926−927. (22) Lee, S. H. A.; Zhao, Y.; Hernandez-Pagan, E. A.; Blasdel, L.; Youngblood, W. J.; Mallouk, T. E. Faraday Discuss. 2012, 155, 165− 176. (23) Zhao, Y.; Swierk, J. R.; Megiatto, J. D.; Sherman, B.; Youngblood, W. J.; Qin, D.; Lentz, D. M.; Moore, A. L.; Moore, T. A.; Gust, D.; Mallouk, T. E. Proc. Natl. Acad. Sci. U. S. A. 2012, 109, 15612−15616. (24) Swierk, J. R.; Mendez-Hernandez, D. D.; McCool, N. S.; Liddell, P.; Terazono, Y.; Pahk, I.; Tomlin, J. J.; Oster, N. V.; Moore, T. A.; Moore, A. L.; Gust, D.; Mallouk, T. E. Proc. Natl. Acad. Sci. U. S. A. 2015, 112, 1681−1686. (25) Zevenbergen, M. A. G.; Wolfrum, B. L.; Goluch, E. D.; Singh, P. S.; Lemay, S. G. J. Am. Chem. Soc. 2009, 131, 11471−11477. (26) Gubbins, K. E.; Walker, R. D. J. Electrochem. Soc. 1965, 112, 469−471. (27) Ichinose, K.; Kimikado, Y.; Yoshida, T. Electrochemistry (Tokyo, Jpn.) 2011, 79, 146−155. (28) Kent, C. A.; Concepcion, J. J.; Dares, C. J.; Torelli, D. A.; Rieth, A. J.; Miller, A. S.; Hoertz, P. G.; Meyer, T. J. J. Am. Chem. Soc. 2013, 135, 8432−8435. (29) Armstrong, N. R.; Lin, A. W. C.; Fujihira, M.; Kuwana, T. Anal. Chem. 1976, 48, 741−750. (30) Longo, C.; Abadal, G.; Sanz, F.; Sumodjo, P. T. A. J. Electrochem. Soc. 1999, 146, 2169−2174. (31) MacArthur, C. G. J. Phys. Chem. 1915, 20, 495−502. (32) Hoertz, P. G.; Chen, Z.; Kent, C. A.; Meyer, T. J. Inorg. Chem. 2010, 49, 8179−8181.
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DOI: 10.1021/acs.analchem.6b00738 Anal. Chem. XXXX, XXX, XXX−XXX
