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Ind. Eng. Chem. Res. 1996, 35, 1316-1321
In Situ Electrode Formation in a Swiss Roll Reactor John F. Patzer, II* Departments of Chemical Engineering and Surgery, University of Pittsburgh, Pittsburgh, Pennsylvania 15261
Shang J. Yao Department of Surgery, University of Pittsburgh, Pittsburgh, Pennsylvania 15219
S. K. Wolfson, Jr. Departments of Surgery and Neurological Surgery, University of Pittsburgh, Pittsburgh, Pennsylvania 15219
The Swiss roll electrochemical reactor design, which is comprised of concentric spiral electrode coils, has a superior electrode packing density and reaction capacity in comparison with more conventional designs. Because platinized-titanium surfaces are mechanically unstable at low platinum loadings, use of the Swiss roll design with such electrodes requires in situ electrodeposition of platinum onto the titanium surfaces after fabrication. Methodology is described to examine the resultant Pt distribution across the width and along the length of expanded Ti mesh after in situ electrodeposition. Four major control variables, i.e., total electroplating charge, plating current density, number of pulses used to deliver the charge, and first coil to receive a current pulse, explain 86% of the experimental variance in a linear effects model. Resulting Pt distribution, when normalized for the extent of platinization, is surprisingly uniform on the entire extent of the electrode coils. The major discrepancies from uniform deposition occurred on the Ti electrode coil which acted as the counter electrode during the first electrodeposition, indicating a surface modification which affects the electrodeposition process. Electrochemical reaction engineering has led to the development of several different reactor configurations which can be chosen dependent upon the process being considered. The Swiss roll electrochemical reactor, which is comprised of concentric spiral working and counter electrode coils separated by a nonconducting membrane or screen, has been shown to have superior electrode packing density and electrochemical reaction capacity in comparison with more conventional stacked plate designs (Robertson et al., 1975, 1979; Robertson, 1977). A major problem with the Swiss roll reactor, however, is fabrication of electrodes with reactive surface areas greater than the geometric surface area. This is especially true with platinized-titanium electrode surfaces since Pt adhesion on Ti is primarily mechanical in nature (Hayfield, 1983; Cotton, 1958; Preiser, 1959; Baumgartner and Raub, 1988). As such, Pt deposits are stress sensitive and easily abraded off the Ti surface during fabrication of the concentric electrode coils. Our laboratory has been actively pursuing the development of an electrochemical dialysate regeneration system for portable artificial kidney applications as a treatment modality for end-stage renal disease (ESRD), which is a major medical endeavor in the United States (U.S. Renal Data System, 1994). Urea is the main metabolic waste that needs to be removed by the process. Key to our electrochemical approach to urea removal is low potential, less than about 1.0 V versus Ag/AgCl, oxidation on Pt surfaces. Our current, best estimate of the amount of Pt surface area which is required for a reactor which can oxidize 8 g of urea/h is 27 m2 (Patzer et al., 1989). This much Pt area would be prohibitively expensive and bulky if solid Pt electrodes were used. When Pt is electrodeposited onto Ti surfaces, the requisite surface area can be obtained with as little as 10 g of Pt on a greatly reduced geometric area (Patzer et al., 1991a,b). * Author to whom correspondence is addressed.
0888-5885/96/2635-1316$12.00/0
Reaction engineering considerations of electrochemical urea oxidation have led to the choice of the Swiss roll electrochemical reactor design (Patzer et al., 1991b). To counter the problem of Pt deposits abrading from a Ti surface during formation of the reactor electrode coils, we have been developing an in situ electrodeposition process to electroplate the concentric Ti coils with Pt after fabrication and placement into the reactor (Patzer et al., 1991a,b). The approach is an adaptation of that reported by Marrese (1986) for preparation of strongly adherent Pt black coatings from chloride-based baths onto Pt surfaces. Our prior results found that the amount of Pt deposition was expressed by the relation
RPt/Ti ) (3.35 × 10-4)Qep - 0.830
(1)
with a correlation coefficient r2 ) 0.36. Scanning electron microscopy revealed that the Pt deposits were self-similar in appearance over a wide range of Pt island sizes. The self-similar nature of the surface was also seen in urea oxidation experiments as a function of the extent of Pt deposition. Our prior work did not address the distribution of Pt deposits across the width of the electrode. The large nonzero intercept of eq 1 implies that Pt electrodeposition on the Ti current distribution plates in addition to the electrode coils was a significant part of the process. Indeed, significant Pt deposition on the current distribution plate was observed. The reciprocal of the slope of eq 1, 2985 C/g of Pt, represents the actual current used to electrodeposit Pt on Ti. The value is about 50% greater than the stoichiometrically expected slope of 1978 C/g of Pt. The excess current requirement can be interpreted to confirm the observation that deposition on the current distribution plates is acting as a current sink. Or, an alternative interpretation is that other competitive electrochemical reactions, such as Ti surface modifications, are acting as a current sink. The relatively low © 1996 American Chemical Society
Ind. Eng. Chem. Res., Vol. 35, No. 4, 1996 1317
correlation coefficient makes choosing between these two interpretations difficult: we can only infer that factors other than the simple consideration of mass of Pt deposited being proportional to the amount of charge passed are important in describing the electrodeposition process. The present work was undertaken to address the large nonzero intercept in eq 1, to identify other salient factors in the in situ electrodeposition of Pt on Ti, and to develop information about the distribution of Pt deposits across the electrode width. A good phenomenological picture of what is happening during the electrodeposition process is the first step in developing a suitable physicochemical/mathematical model of the process for scaling to larger systems. Experimental Section Industrial-grade, annealed, and expanded Ti metal mesh (4Ti6-050; Delkar Corp., Branford, CT) was used for the concentric spiral electrode coils. Coils were formed using an apparatus designed to feed four alternating layers of expanded Ti mesh and a nonconducting fiberglass screen onto a central winding spool. The expanded Ti mesh strips were offset from the center to opposite sides of the coil by about 2 mm in order to provide separate, individual contact with Ti current distribution plates, one on each side of the coil. The fiberglass strips were centered, allowing the edges of each Ti electrode coil to contact its distribution plate without contacting the other Ti electrode coil. After winding, the coil was placed in a cylindrical plexiglass tube which formed the reactor walls and maintained the compressed concentric spiral geometry. Thus, the reactor consists of two concentric spiral coils of expanded Ti mesh separated by fiberglass screens. Each coil is separately contacted with a Ti current distribution plate. In departure from previous experimental protocol, however, the current distribution plates were coated with Silastic Brand Medical Adhesive, Silicone Type A (Dow Corning Corp., Midland, MI), prior to use by simply spreading the silicone polymer over the distribution plate surface with a razor blade. The sharp coil-edge points of the expanded Ti mesh easily penetrated the silicone polymer and made good electrical contact with the current distribution plate. The silicone polymer coating served to remove the Ti current distribution plate surfaces as an electrodeposition site from the system. Electroplating was performed using a composite chloroplatinic acid solution that was assayed at 13.1 g of Pt/L (3.5% chloroplatinic acid). The reactor was placed in a vertical position with one current distribution plate above the other. A potentiostat-galvanostat (Amel Model 555, Apparecchiature di Misura Elettroniche, Milan, Italy) was attached to the current distribution plates. Starting with two initial 15-s conditioning pulses, one to each coil, pulses of current as specified in Table 1 were applied alternatively to the two Ti coils in the reactor. The time duration of an electroplating pulse necessary to deliver the desired total charge for a specified set of experimental conditions can be determined as Qep/(jepNpulses). The first column in Table 1 contains two experimental conditions: electrodes with a U designation received the first electrodeposition current pulse, while those with the L designation served as the counter electrode. Thus, electrodes with an L designation served as a counter electrode prior to receiving a plating pulse.
Table 1. Experimental Plan and Results experimental settings coil no.
Qep
jep
Npulses
RPt/Ti
EP092791D-U EP092791D-L EP093091A-U EP093091A-L EP101091A-U EP101091A-L EP101091D-U EP101091D-L EP101691A-U EP101691A-L EP101791C-U EP101791C-L EP101891F-U EP101891F-L EP102191C-U EP102191C-L EP102191E-U EP102191E-L
12 500 12 500 12 500 12 500 12 500 12 500 10 000 10 000 7 500 7 500 7 500 7 500 12 500 12 500 7 500 7 500 7 500 7 500
40 40 20 20 20 20 30 30 40 40 20 20 40 40 40 40 20 20
4 4 4 4 2 2 3 3 2 2 2 2 2 2 4 4 4 4
3.37 3.23 2.54 1.81 2.33 2.21 2.62 2.18 1.29 1.49 1.20 1.11 2.50 2.60 1.96 1.85 1.32 0.97
Figure 1. Sectioning of an electrode for Pt distribution profile determination. Section A was always toward the outer circumference of the electrode coil, near the reactor wall. Section E was always toward the inner circumference of the electrode coil, near the reactor center. Section C1 was always adjacent to the Ti current distribution plate. Section C5 was always most distant from the current distribution plate.
After an electrodeposition experiment, the concentric coils were disassembled and the individual components were washed with deionized water, oven-dried overnight at 150 °C, and weighed. The Pt/Ti ratio, RPt/Ti, was determined from a material balance in which the mass of an electrode after Pt deposition less the original mass was equal to the mass of Pt deposition. Each electrodeposition experiment produces two platinizedtitanium electrodes: one from the electrode in contact with the upper current distribution plate and one from the electrode in contact with the lower current distribution plate. Equal-sized sections, as shown in Figure 1, were taken at approximately evenly spaced intervals along each electrode coil by simply cutting the platinizedtitanium mesh with stainless steel scissors. Section A was always taken from the outer circumference of the coil, that is, the end closest to the reactor wall. Section E was always taken from the inner circumference of the coil, that is, the end closest to the winding spool. Section C1 was always the section of the coil adjacent to the Ti current distribution plate. Section C5 was always the section most distant from the current distribution plate. Prior work has shown that urea decomposition products, primarily ammonium ion, act to reversibly poison
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the Pt surface during prolonged oxidation at potentials above 0.5 V relative to Ag/AgCl (Yao et al., 1983, 1984). The effect is easily reversed by lowering the electrode potential below -0.4 V relative to Ag/AgCl. This has been the subject of appropriate strategies to maximize current utilization for urea decomposition (Patzer et al., 1990, 1991c). Since the cyclic voltammetry protocol for evaluating Pt electrodeposition cycles between -0.9 and +1.1 V relative to Ag/AgCl, the electrode surface is effectively rejuvenated and poisoning effects in the interpretation of the data are minimized. The Pt deposition profile was determined by measuring the specific urea oxidation charge via cyclic voltammetry and calculating the resultant specific urea conversion. Our previous work (Patzer et al., 1991a,b) has shown that specific urea conversion is directly proportional to, and thus a measure of, Pt surface area as obtained by the more traditional determinations made using sulfuric acid (Biegler et al., 1971). The CV scans were conducted at 50 mV s-1 in the range -0.9 to +1.1 V relative to Ag/AgCl in a physiologic buffer (26.1 mg dL-1 of KCl, 608.7 mg dL-1 of NaCl, 15.2 mg dL-1 of MgCl2, 12.3 mg dL-1 of CaCl2, 514.5 mg dL-1 of Na acetate, 200 mg dL-1 of dextrose) with and without 20 mg dL-1 of urea. The electrode urea oxidation charge is a measure of how much electrode mass is required for a given process. It is calculated from the current-potential, or, equivalently, current-time, profiles in the potential range 0.5-1.1 V relative to Ag/AgCl as
Uox ) (Aw/urea - Aw/o urea)/(vmel)
(2)
Specific urea conversion, which is directly proportional to the actual Pt surface area, is a measure of how efficiently Pt has been plated on the electrode (Patzer et al., 1991a,b). It can be calculated as
σurea ) Uox(100 + RPt/Ti)/(nureaFRPt/Ti)
(3)
where nurea ) 2 (Patzer et al., 1989). Thus, the electrode urea oxidation charge is directly related to the Pt electrodeposition and resultant Pt surface area. Results The silicone polymer coating on the Ti current distribution plates effectively removed the current distribution plates as Pt electrodeposition sinks. The coating was intact after electrodeposition. Furthermore, the current distribution plates did not show evidence of any Pt deposits such as were previously observed (Patzer et al., 1991b). The experimental series in Table 1 is a randomized 23 factorial design to garner information on the effects of total charge, number of pulses, and pulse duration on the resultant extent of Pt deposition, RPt/Ti. Application of Yate’s algorithm (Box et al., 1978) to estimate the main effects revealed that (1) the average RPt/Ti was 2.03, (2) increasing the total charge from 7500 to 12 500 C/100 g of Ti increases RPt/Ti by 1.18, (3) increasing the current density from 20 to 40 A/100 g of Ti increases RPt/Ti by 0.60, (3) increasing the number of pulses from 2 to 4 increases RPt/Ti by 0.29, and (4) the electrode receiving the first pulse (the upper electrode) had an average increase in RPt/Ti of 0.19. A simple linear regression fit of RPt/Ti as a function of Qep alone was performed to determine whether silicone coating of the current distribution plates improves the
Figure 2. Parity plot of measured RPt/Ti values versus regression equation (5) fit to the data. Stars are upper electrode data; open squares are lower electrode data.
prior correlation reported in eq 1. The result
RPt/Ti ) (2.35 × 10-4)Qep - 0.318
(4)
with a correlation coefficient r2 ) 0.64 is improvement both with respect to reducing the value of the intercept and in improving the correlation coefficient for the extent of platinization with respect to the primary control variable. The reciprocal of eq 4, 4255 C/g of Pt, however, is now approximately twice the stoichiometric requirement of 1978 C/g of Pt. Since Yate’s analysis revealed that each of the control variables (Qep, jep, Npulses, X) affected the average RPt/Ti by 9% or more, a simple linear regression with each of the control variables was performed. The dummy first current pulse variable, X, was assigned a value of +1 if the electrode received the first pulse and a value of -1 otherwise. The result
RPt/Ti ) (2.35 × 10-4)Qep + 0.030jep + 0.145Npulses + 0.093X - 1.65 (5) has a correlation coefficient of r2 ) 0.86. A parity plot for this relation is shown in Figure 2. Data scatters fairly uniformly on either side of the trend line. Thus, a simple linear effects model is capable of explaining 86% of the experimental results. Pt distribution across the width and along the length of a coil was evaluated by specific urea oxidation charge measurements on each section shown in Figure 1. The resulting data are reported in Table 2. Since we are interested in knowing the relative distribution of Pt across and along a coil irrespective of the extent of Pt deposition, the data in Table 2 were normalized for graphical presentation. Weighted-average specific urea oxidation charge for section C was determined from the individual specific urea oxidation charges for section C by the relation
UC,ave )
∑(mel,iUox,Ci)/∑(mel,i)
(6)
The specific urea oxidation charge along the length of the coil was normalized with respect to the specific urea oxidation charge of section A. The normalized relative distributions of Pt across the width of the coil and along the length of the coil are shown in Figures 3 and 4, respectively. Overall, the Pt deposition appears to be fairly uniform both across the
Ind. Eng. Chem. Res., Vol. 35, No. 4, 1996 1319 Table 2. Pt Distribution across the Electrode Coil As Determined by Specific Urea Oxidation Activity specific urea oxidation activity in each section, mC/g coil no.
C1
C2
C3
C4
C5
Cave
A
B
D
E
EP092791D-U EP092791D-L EP093091A-U EP093091A-L EP101091A-U EP101091A-L EP101091D-U EP101091D-L EP101691A-U EP101691A-L EP101791C-U EP101791C-L EP101891F-U EP101891F-L EP102191C-U EP102191C-L EP102191E-U EP102191E-L
458 425 328 536 326 322 365 449 208 353 232 149 410 233 378 438 248 142
484 496 384 484 349 438 376 490 193 466 319 317 299 426 347 433 270 254
518 534 391 438 314 287 383 632 181 427 434 353 305 412 327 464 312 305
495 566 371 284 329 336 334 501 165 435 271 359 274 325 320 419 249 339
478 605 404 235 337 428 367 537 126 334 329 374 295 404 309 442 268 359
489 524 375 395 330 342 364 521 177 404 315 305 315 355 335 441 268 285
491 537 369 405 351 368 385 481 329 317 336 362 168 479 363 477 295 268
479 539 361 399 339 300 347 451 182 418 349 298 279 307 319 494 286 282
480 492 388 335 322 316 362 483 266 387 267 309 348 586 335 397 259 286
463 509 350 261 310 217 333 402 240 288 255 270 321 470 335 373 268 285
a
a
b
b
Figure 3. (a) Normalized Pt distribution across the width of the upper electrode (which received the first current pulse) at section C. (b) Normalized Pt distribution across the width of the lower electrode (which received the second current pulse) at section C. Error bars are (1 standard deviation.
Figure 4. (a) Normalized Pt distribution at specified locations along the length of the upper electrode (which received the first current pulse). (b) Normalized Pt distribution at specified locations along the length of the lower electrode (which received the second current pulse). Error bars are (1 standard deviation.
width of a coil and along the length of a coil. There is some variation. For example, Pt deposition across the width of the upper coil appears to have a slight maximum in the center and dips toward the edge away from the current distribution plate. In contrast, Pt deposition across the width of the lower coil appears to have a minimum at the current distribution plate edge. The Pt distribution along the length of the lower coil appears to be lower at the inner core end than elsewhere along the coil. The large error bars shown in Figure 4a
for Pt distribution along the length of the upper coil result from the outlier electrode EP101891F-U, which, for some reason, had an inexplicably low Pt deposition in section A. Since section A was the basis for normalization, the result skews the normalized distribution for this electrode and creates the high standard deviations for the entire upper electrode series. Deletion of this electrode from the data set would result in error bars comparable to those in the other figures. We have no experimentally valid reason to delete this electrode from the data set.
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Discussion In the absence of competing electrochemical reactions, deposition at points not on the Ti electrodes, and loss from physical handling, the amount of electrodeposited Pt should be directly proportional to the total charge applied
RPt/Ti ) QepMPt/FnPt ) (5.06 × 10-4)Qep
(7)
This relation assumes that the applied current density is below the limiting current density for supply of Pt ions to the surface by diffusion and convection,
jL ) nPtFDPtCPt/δ
(8)
For the present work, with nPt ) 4 equiv/mol, DPt about 1.5 × 10-5 cm2/s, CPt ) 6.7 × 10-5 mol/cm3, and a maximum δ of about 0.05 cm based upon the gap width between electrodes, jL is estimated to be in excess of 7 mA/cm2. Using the estimated surface area for the expanded Ti mesh (Patzer et al., 1991a), 70 cm2/g, the applied current densities ranged from 2.9 to 5.7 mA/ cm2, which is less than the limiting current density. The decrease in the coefficient on Qep from 3.35 × 10-4 in eq 1 to 2.35 × 10-4 in eq 4 reflects an increase in charge requirements for electrodeposition to nearly twice the theoretical stoichiometric requirement of 1978 C/g of Pt where we would have expected a decrease in current requirements with the more tightly controlled electrodeposition process reported here. Although we did not notice any gas evolution during electrodeposition, the probable explanation is that an unidentified competing electrochemical reaction is taking place, possibly one which is producing gas. This hypothesis also supports the nonzero intercept in that nucleating and growing gas bubbles could be quite effective in dislodging mechanically retained Pt deposits. Our prior work (Patzer et al., 1991b) determined the relation provided in eq 1, which has a coefficient comparable to that in eq 7 but has an intercept quite different from zero. Physically, the intercept represents electrodeposited Pt which is not found on the electrode coil after opening the reactor, separating, rinsing and oven-drying the separate concentric spiral coils, and, finally, weighing the coils to determine Pt gain. Two sources exist for Pt loss: Pt electrodeposited on the Ti distribution plates and Pt lost through the mechanical handling of the coils. A major difference between the experimental procedures of this work and that of our prior work (Patzer et al., 1991b) was the coating of the Ti current distribution plates with a silicone polymer coating. The result of this simple change was no Pt electrodeposition on the current distribution plates. This effect was sufficient to significantly reduce the intercept of the charge/deposition equation from 0.830 in eq 1 to 0.318 in eq 4 and to improve the correlation coefficient, r2, from 0.36 to 0.64. The significant reduction in the intercept leads us to conclude that Pt electrodeposition on the Ti current distribution plates was a primary contributor to the value of the intercept in previous work. The remaining nonzero portion of the intercept is attributed to a combination of Pt loss from the electrode coil due to surface abrasion during handling and to nondifferentiated contributions from the other control variables, jep, Npulses, and X. From Yate’s analysis of the linear effects we know that each of the experimental control variables contributes significantly to the overall electrodeposition pat-
tern. Thus, the linear effects correlation of eq 5, which includes the first current pulse effect, was developed. The positive contribution of current density, jep, to the overall deposition has been reported for other systems (Turner, 1981; Landau, 1983) and is believed due to more nucleation sites for electrodeposition growth. The local rate of nucleation of Pt deposits is governed by the local electrode overpotential, with greater overpotentials resulting in more nucleation sites being formed. In electrodeposition using constant current density, the overpotential will be greater for higher current density at the initial stages of nucleation because too few electrodeposition sites exist to handle the current flux. As more nucleation sites are formed, the overpotential will decrease. More nucleation sites, in turn, result in greater Pt deposition because less diffusional limitations for a Pt ion approach to an active deposition site exist. The increased deposition effect with increasing Npulses is probably due to relaxation in the diffusing Pt concentration profile between pulses. Such relaxation should theoretically allow the development of more compact deposition islands because more Pt is near the Ti surface at the start of a pulse. More compact depositions will be more resistant to abrasion and loss during the physical handling in the analysis procedures. Whether pulsed deposition actually produces more compact profiles in industrial practice is somewhat controversial (Landau, 1983). The electrode which received the first current pulse, control variable X, had a higher average Pt deposition than the one receiving the second pulse. Since the electrode which received the second pulse was acting as the counter electrode to the plating electrode during the first pulse, this effect is most probably due to a surface modification resulting from the electrochemical processes taking place at the counter electrode. Further work is necessary to determine the nature of the surface modification and whether the surface modification can be mitigated so that the electrodeposition on both electrodes is comparable. While the enhanced linear effects model described by eq 5, which uses all of the control variables, satisfactorily accounts for 86% of the experimental variance, it does not account for the large excess of current required for Pt electrodeposition. Given the tighter control on the variance effects in the model, the large excess is likely due to competing electrochemical reactions which need to be identified. Also, additional work will be required to identify whether other suitable control variables exist which can account for the remaining 14% of the variance or whether the remaining variance is due to experimental and analysis procedures. A major goal of this work was to begin to develop information about how Pt is distributed across and along the electrode coils. The normalized, average distribution appears to be fairly uniform across and along the coils as reflected by the fairly constant normalized relative activity value near 1 in Figures 3 and 4. Such uniform distribution is expected because the primary and secondary current distributions are uniform in the concentric coil geometry, force convection is not used, and natural convection is unlikely with the narrow gap between electrodes. The relatively large error bars reflect greater experimental variance than is desirable for reproducible production of platinized-titanium electrodes. The major differences in the normalized Pt distribution appear to be associated with which coil received the first current pulse. From Yate’s analysis,
Ind. Eng. Chem. Res., Vol. 35, No. 4, 1996 1321
receiving the first current pulse affected the amount of Pt deposition by increasing deposition by about 10%. The upper coil, which received the first current pulse, has reasonably uniform Pt distributions across and along the coil, as displayed in Figures 3a and 4a, respectively. The lower coil, which received the second current pulse, shows marked deviation in deposition. Less Pt is deposited along the current distribution plate and less Pt is deposited near the center of the coil. The probable cause of this behavior is, again, a Ti surface modification incurred during the first current pulse to the upper coil. The nature of the surface modification needs to be determined. Conclusions Understanding of in situ Pt electrodeposition on expanded Ti mesh in a concentric spiral coil, Swiss roll electrochemical reactor geometry has been improved to where a simple linear effects model accounts for 86% of the experimental variance. Experimental methodology to measure the extent of Pt deposition across and along a spiral coil was developed. The results show that Pt deposition is relatively uniform across and along the spiral coil regardless of the absolute level of Pt loading. The experimental results also indicate that the initial current pulse can modify the Ti surface with respect to the extent of Pt electrodeposition and resulting Pt distribution. Additional experimental work to elucidate the nature of the Ti surface modifications is necessary. A reaction engineering model describing the electrodeposition process also needs to be developed for more quantitative descriptions of the electrodeposition process. Acknowledgment This work was partially supported through NIH Grant DK44474. The experimental work was performed by Craig A. Refosco. Nomenclature APt ) specific Pt surface area, m2/g Aw/urea ) cyclic voltammetry area with urea present, VA Aw/o urea ) cyclic voltammetry area without urea present, VA CPt ) platinum ion concentration, mol/cm3 DPt ) platinum ion diffusivity, cm2/s F ) Faraday constant, 96 487 C/equiv jep ) electroplating current density, A/100 g of Ti jL ) limiting current density, mA/cm2 mel ) cyclic voltammetry electrode mass, g msection ) electrode section mass, g mPt ) Pt mass, g mTi ) Ti mass, g MPt ) Pt molecular weight, 195.1 g/mol nPt ) amount of platinum electrodeposited, equiv/mol nurea ) urea conversion stoichiometry, equiv/mol Npulses ) number of electrodeposition current pulses Qep ) normalized total electrodeposition charge, C/100 g of Ti RPt/Ti ) Pt/Ti ratio, g of Pt/100 g of Ti UC,ave ) weighted specific urea oxidation charge for section C, C/g of Pt Uox ) specific urea oxidation charge, C/g of Pt v ) cyclic voltammetry scan rate, V/s X ) dummy variable for first pulse: +1 for first pulse, -1 otherwise
Greek Symbols δ ) electrodeposition diffusion layer thickness, cm σurea ) specific urea conversion, mol/g Pt
Literature Cited Baumgartner, M.; Raub, C. J. The corrosion behavior of objects electroplated with platinum: important influence of intermediate layers. Platinum Met. Rev. 1988, 32, 188. Biegler, T.; Rand, D. A. J.; Woods, R. Limiting oxygen coverage on platinized platinum: relevance to determination of real platinum area by hydrogen adsorption. J. Electroanal. Chem. 1971, 29, 269. Box, G. E. P.; Hunter, W. G.; Hunter, J. S. Statistics for Experimenters; Wiley: New York, 1978. Cotton, J. Platinum-faced titanium for electrochemical anodes: a new electrode material for impressed current cathodic protection. Platinum Met. Rev. 1958, 2, 45. Hayfield, P. C. S. Platinized titanium electrodes for cathodic protection: extensive use has provided new insights into the electrochemical properties of platinum. Platinum Met. Rev. 1983, 27, 2. Landau, U. Morphology and distribution of electrodeposits. Tutorial Lectures in Electrochemical Engineering and TechnologysII; AIChE Symposium Series 229; AIChE: New York, 1983; Vol. 79, p 218. Marrese, C. A. Preparation of strongly adherent platinum black coatings. Anal. Chem. 1986, 59, 217. Patzer, J. F., II; Yao, S. J.; Wolfson, S. K., Jr.; Ruppel-Kerr, R. Urea oxidation kinetics via cyclic voltammetry: application to regenerative hemodialysis. Bioelectrochem. Bioenerg. 1989, 22, 341. Patzer, J. F., II; Yao, S. J.; Wolfson, S. K., Jr. Reactor control and reaction kinetics for electrochemical urea oxidation. Chem. Eng. Sci. 1990, 45, 2777. Patzer, J. F., II; Yao, S. J.; Wolfson, S. K., Jr. Platinized-titanium electrodes for urea oxidation. I. Demonstration of efficacy. J. Mol. Catal. 1991a, 70, 217. Patzer, J. F., II; Wolfson, S. K., Jr.; Yao, S. J. Platinized-titanium electrodes for urea oxidation. II. Concentric coil geometry. J. Mol. Catal. 1991b, 70, 231. Patzer, J. F., II; Yao, S. J.; Wolfson, S. K., Jr. Voltage polarity relay: optimal control of electrochemical urea oxidation. IEEE Trans. Biomed. Eng. 1991c, 38, 1157. Preiser, H. Cathodic protection application using platinum anodes. Platinum Met. Rev. 1959, 3, 38. Robertson, P. M. The variation of current density and electrode potential with electrode resistance and its role in cell design. Electrochim. Acta 1977, 22, 411. Robertson, P. M.; Schwager, F.; Ibl, N. A new cell for electrochemical processes. J. Electroanal. Chem. 1975, 65, 883. Robertson, P.; Cettou, P.; Matic, D.; Schwager, F.; Storck, A.; Ibl, N. Electrosynthesis with the Swiss Roll cell: properties of the cell components and their selection for electrosynthesis. Electroorganic Synthesis Technology; AIChE Symposium Series 185; AIChE: New York, 1979; Vol. 75, p 115. Turner, D. R. Electroplating as an engineering science. Tutorial Lectures in Electrochemical Engineering and Technology; AIChE Symposium Series 204; AIChE: New York, 1981; Vol. 77, p 178. U.S. Renal Data System. USRDS 1994 Annual Data Report; The National Institutes of Health, National Institute of Diabetes and Digestive and Kidney Diseases: Bethesda, MD, Aug 1989. Yao, S. J.; Wolfson, S. K., Jr.; Krupper, M. A.; Wu, K. J. Controlled electrolysis of urea in biological fluids. In Charge and Field Effects in Biosystems; Allen, M. J., Usherwood, P. N. R., Eds.; Abacus Press: Brookline Village, MA, 1983; pp 409-411. Yao, S. J.; Wolfson, S. K., Jr.; Krupper, M. A.; Wu, K. J. Controlledpotential controlled-current electrolysis: In vitro and in vivo electrolysis of urea. Bioelectrochem. Bioenerg. 1984, 13, 15.
Received for review March 23, 1995 Revised manuscript received October 12, 1995 Accepted December 12, 1995X IE950196P
Abstract published in Advance ACS Abstracts, February 1, 1996. X
