Factors that Determine the Length Scale forUniform Tinting in Dynamic Windows Basedon Reversible Metal ElectrodepositionMichael T. Strand,† Christopher J. Barile,‡ Tyler S. Hernandez,§ Teresa E. Dayrit,† Luca Bertoluzzi,†

Daniel J. Slotcavage,† and Michael D. McGehee*,†,∥

†Department of Materials Science and Engineering, Stanford University, Stanford, California 94305, United States‡Department of Chemistry, University of NevadaReno, Reno, Nevada 89557, United States§Department of Chemistry, Stanford University, Stanford, California 94305, United States∥Department of Chemical Engineering, University of ColoradoBoulder, Boulder Colorado 80303, United States

*S Supporting Information

ABSTRACT: Dynamic windows based on reversible metalelectrodeposition are attractive compared to conventionalelectrochromics because they can have neutral color, highcontrast, and potentially lower cost, yet they are not nearlyas developed, and the design rules for making themfunction at large scale are not presented in the literature.We model the voltage drops that occur in the transparentelectrodes to get insight on how to obtain uniformelectrodeposition of metals over large area. By optimizingthe surface and density of the Pt nanoparticles used tonucleate metal growth, we lower the nucleation barrier forelectrodeposition by 70 mV. We show that the growth rateof the metal films is determined by diffusion rather than reaction kinetics, which makes it possible to achieve uniform filmdeposition over a range of potentials from −300 to −700 mV. We demonstrate 100 cm2 dynamic windows that are color-neutral and tint uniformly from a clear state (>60%) to a dark state (<5%) in less than 1 min.

Dynamic windows that allow electronic control ofvisible light and solar heat gain are desirable forimproving energy efficiency in buildings and

automobiles and reducing glare without obstructing views.1−3

Despite decades of research, however, traditional approachesto dynamic windows have not widely penetrated the marketdue to problems with color, cost, speed, and durability.4

Dynamic windows based on reversible metal electrodeposition(RME) represent an undeveloped class of electrochromicdevices posed to overcome the challenges inherent to existingtechnologies. We recently demonstrated dynamic windowsbased on the reversible electrodeposition of Bi, Cu, and Ag thatswitch uniformly between transparent and opaque states overthousands of cycles.5,6 We developed transparent electrodesdecorated with Pt nanoparticles that enable uniform electro-deposition of metal films on the 25 cm2 scale.5 We verified thatdynamic windows using metals can be color-neutral andrequire no additional power to be held in the desired opticalstate.6 Finally, we proposed that our design allows dynamicwindows to be manufactured at a fraction of the cost ofconventional technologies. In this Letter, we evaluate what is

needed for dynamic windows based on RME to be realized at apractical scale.A challenge hindering large-scale RME windows is

associated with the limitations of transparent conductors.The majority of electrochromic devices use glass coated withtransparent conducting oxides as the conductive electrodes.7

Indium tin oxide (ITO) is the most commonly usedtransparent conductor and possesses a resistivity of ∼10−4 Ω·cm and a visible transmittance greater than 80%.8 The sheetresistance is typically 10 Ω/sq. The resistivity is 2 orders ofmagnitude higher than that of opaque conductors like metals,and maintaining a uniform current density leaving the ITOelectrodes during electroplating or stripping necessitates avoltage drop that increases with the electrode area.9 Thevoltage distribution can be calculated by integrating Ohm’sLaw over a two-dimensional surface. This calculation yields thefollowing equation

Received: September 20, 2018Accepted: October 23, 2018Published: October 23, 2018

Letterhttp://pubs.acs.org/journal/aelccpCite This: ACS Energy Lett. 2018, 3, 2823−2828

© XXXX American Chemical Society 2823 DOI: 10.1021/acsenergylett.8b01781ACS Energy Lett. 2018, 3, 2823−2828

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where J is the current density, ρ is the resistivity, t is the filmthickness, L is the electrode length, and x and y are positionson the electrode surface defined by a Cartesian coordinatesystem with the origin at the corner of the electrode (a detailedderivation is presented in the Supporting Information). Figure1 shows a schematic view of a 100 cm2 dynamic window andthe simulated voltage distribution across an ITO electrode witha sheet resistance of 10 Ω/sq using the maximum currentdensity observed experimentally in a device that switches fromT = 60% to 10% in 30 s. As shown, these devices must toleratea 0.3 V difference from edge to center. Large-scale RMEwindows, then, must be engineered with a tolerance to thevoltage drop across the transparent electrodes to maintainoptical uniformity during device operation.The devices with the best performance to date use aqueous

electrolytes because metal salts have high solubility anddissociation constants in water, and thus, water-based electro-lytes benefit from relatively low deposition overpotentials.10

However, undesired side reactions like hydrogen evolution setstrict limits on the potentials that may be applied in devices.This limit poses a challenge at scale where higher potentials

may be required for sufficient current to travel across sizableelectrodes. The ideal electrodes will enable uniform nucleationand growth of metal films over the potential range set by sidereactions inherent to the electrolyte and the voltage dropinherent to transparent conductors.In this Letter, we investigate the nucleation and growth

kinetics in an aqueous electrolyte based on the reversibledeposition of Bi and Cu and identify design criteria forachieving uniform deposition at scale. We improve upon our Ptnanoparticle seed layer design and demonstrate electrodes thatexhibit a 2.5× improvement in contrast ratio at a fixed voltagesweep and a 70 mV reduction in deposition overpotentialcompared to our previous work.5,6 We show that film growthin our system is diffusion-limited and that enhanced electrodesenable uniform switching curves over a 400 mV potentialrange. Thus, we verify that lowering the nucleation barrier (i.e.,deposition overpotential) and achieving a high nucleationdensity allow us to deposit uniform metallic films over 100 cm2

electrodes, despite the significant voltage drop. Finally, wedemonstrate 100 cm2 dynamic windows that switch uniformlyfrom a clear state (>60% transmission) to an opaque state(<5% transmission) in less than 1 min.Electrode Morphology and Nucleation. In RME-based dynamic

windows, the working electrode serves two primary functions:

Figure 1. Schematic of a 100 cm2 dynamic window. The device consists of a 100 cm2 Pt-modified ITO working electrode, a Bi−Cu gelelectrolyte, and a semitransparent Cu grid counter electrode mounted to a 100 cm2 glass backing. The device is sealed using 2 mm thickbutyl rubber edge sealant that doubles as an electrode spacer.

Figure 2. (a) Cyclic voltammetry in a Bi−Cu electrolyte comparing bare ITO, an evaporated Pt film [3 nm Pt/3 nm Ti/ITO], and a Ptnanoparticle array on ITO and (b) corresponding transmission (at 550 nm) versus voltage curves. Measurements were taken versus a Ag/AgCl reference at a scan rate of 20 mV/s.

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(i) as a source of electrons to reduce solvated metal ions totheir elemental form and (ii) as a scaffold for metal nucleationand growth. The working electrode must be transparent,conductive, and inert and must promote uniform andreversible electrocrystallization of metal films. Transparentconducting oxides like ITO and fluorine-doped tin oxide(FTO) satisfy the first three requirements; however, directelectroplating onto transparent conductive oxides is difficult.11

The poor wettability and heterogeneous surface chemistry ofITO, for example, leads to a large excess energy for metalnucleation and nonuniform deposition.12 Pt, in contrast, is anexcellent template for RME. Therefore, modifying ITOelectrodes with Pt offers a pathway for fabricating high-performing electrodes.Figure 2a shows cyclic voltammograms (CVs) in a Bi−Cu

aqueous electrolyte on a bare ITO surface, an ITO electrodecoated with a 3 nm thick Pt film that was thermally evaporated,and an ITO electrode coated with an array of Pt nanoparticlesanchored by a self-assembled monolayer of 3-mercaptopro-pionic acid (a thiol-terminated surface modifier). The bareITO and the evaporated Pt film represent two surfaceextremes, while the Pt nanoparticle array combines thebeneficial properties of both materials (e.g., high transparencyand high catalytic activity). Both surfaces with Pt show a + 70mV shift for the onset potential for metal electrodeposition.This result confirms that Pt surfaces offer a lower overpotentialfor nucleation compared to ITO. Figure 2b shows correspond-

ing transmission versus voltage curves measured in aspectroelectrochemical half-cell during the CV experiments.The Pt nanoparticle array yields a 75% contrast ratio over oneCV scan compared to 40% for the bare ITO and theevaporated Pt film. This improved contrast ratio with Pt occursbecause the Pt nanoparticles act as preferred nucleation sitesand enable uniform nucleation and growth of metal electro-deposits without sacrificing any transmission in the transparentstate. While the evaporated Pt film also exhibits a lowerednucleation barrier, the maximum transmission is limited to45% (at 550 nm) because the continuous Pt film along withthe nontransparent Ti adhesion layer that is necessary toprevent the Pt film from delaminating from the ITO surfacereflects and absorbs more light than an array of Ptnanoparticles.13 SEM images demonstrate that the evaporatedPt film displays a similar morphology to the bare ITO surface(Figure 3). The “bright” particles on the micrograph of the Ptnanoparticle array are the 3 nm Pt nanoparticles used, and theparticle surface density is about 8000 particles/μm.2

Overcoming Electrode Voltage Drop. While lab-size prototypeswith a 25 cm2 active area demonstrate impressive performance,significant challenges arise when increasing the size ofelectrochromic devices, particularly due to nonuniform voltagedistribution across the electrode surfaces. The current densityrequired for device operation and the sheet resistance of thetransparent electrodes set the voltage drop inherent to dynamicwindows. As shown in Figure 1 and calculated in eq 1, there is

Figure 3. SEM images of bare ITO (left), the evaporated Pt film [3 nm Pt/3 nm Ti/ITO] (center), and the Pt nanoparticle array on ITO(right). The “white” particles in the image on the right are the 3 nm Pt nanoparticles.

Figure 4. Transmission versus time curves for switching at a series of deposition voltages. Each set of curves represents electrodeposition atdifferent voltages from −200 to −700 mV. Deposition was performed for 30 s followed by a 90 s stripping sequence at +800 mV. The Pt-modified ITO allows facile nucleation of metal electrodeposits and thus uniform transmission change over a wide potential window.

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a 0.3 V drop from edge to center for a 100 cm2 dynamicwindow using the experimental conditions in our system. Thisvoltage drop, of course, presents a significant challenge whenuniform deposition of metallic films is critical to deviceoperation. However, dynamic windows based on reversibleelectrodeposition have unique current transient properties.A typical current transient in a RME device, analogous to a

standard electroplating experiment under Cottrell conditions,is an exponential decay (Figure S2). The large, initial currentdensity is followed by a rapid decay because nanoscale metallicnuclei form on the electrode surface at early times, followed bya decay in current as the metallic clusters coalesce and theconcentration of active ions near the surface is depleted (i.e.,the electrodeposition becomes diffusion-limited).14 Withsufficient potential for nucleation to occur everywhere, uniformgrowth of the metal film is possible once diffusion takes over.Of course, promoting nucleation over the wide range ofvoltages present during the first few seconds is essential toachieving uniformity at a large scale.The key to solving the voltage drop challenge is controlling

the nucleation process by using an inert nanoparticle seedlayer. By increasing the nanoparticle surface density, wedemonstrate increased current density at a fixed voltage sweepand a corresponding improvement in contrast ratio (Figures S3and S4). We show that a 250 °C anneal effectively removessurfactant molecules from the nanoparticle surfaces and yieldsthe catalytic properties of a continuous Pt film (Figure S5). Weattribute the +70 mV shift in onset potential compared to ourprevious work (Figure 2a) to the removal of surfactantmolecules. The Pt nanoparticle nucleation layer developed inthis study exhibits significant performance improvementcompared to surface modifications presented in previouswork.5,6 Most importantly, the Pt nanoparticle array enablesuniform transmission modulation over a wide potential

window. This result demonstrates a route to overcoming theproblems with nonuniform voltage distribution across theelectrode surface. Despite the variance in electrostatic potentialacross the surface, the diffusion-limited nature of thedeposition process enables uniform transmission−time rela-tionships because the concentration gradient becomes uniformacross the electrode area.Figure 4 shows transmission versus time curves for a bare

ITO electrode and an ITO electrode modified with a Ptnanoparticle array. We performed measurements in aspectroelectrochemical half-cell during potentiostatic deposi-tion and stripping sequences at a series of applied voltagesranging from −200 to −700 mV. The difference between thetwo electrodes is apparent in comparing the two curves atlower applied voltages. At an applied voltage of −200 mV, forexample, negligible transmission change (and hence minimalmetal deposition) is observed on the bare ITO electrode, whiletransmission through the Pt-modified electrode decreasesbelow 40% after 30 s. This result is expected because the Ptnanoparticles act as catalysts for nucleation of metal depositsand lower the onset potential for electrodeposition, as shownin Figure 2. As the overpotential is increased, the transmissionversus time relationship for the two electrodes saturatesbecause there is sufficient energy available for metal to nucleateon either surface (Pt nanoparticle or ITO). The nucleation inthis system is governed by a three-dimensional instantaneousnucleation model, followed by diffusion-limited growth asdescribed by the Cottrell equation (Figure S7). Theimplication of the growth being diffusion-limited is that it ispossible to achieve even transmission change in a windowdespite a nonuniform electrostatic potential across theelectrode surface.100 cm2 Devices with Uniform Tinting. With indication that

uniform switching is possible on a larger scale (i.e., tolerance of

Figure 5. Transmission measurements at the device edge and center and photographs of 100 cm2 dynamic windows made with a Pt-modifiedITO electrode (top) and a bare ITO electrode (bottom). The devices were switched under an applied bias of −600 mV for 60 s.Transmission was measured at 550 nm. A coin with 17.91 mm diameter is shown for scale.

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the 0.3 V drop calculated above), we fabricated dynamicwindows with an active area of 100 cm2. The devices consist ofa Pt-modified ITO working electrode, a Bi−Cu aqueous gelelectrolyte, and a semitransparent copper mesh counterelectrode to provide a uniform source of metal ions (seeFigure 1). We sealed the devices using 2 mm thick butyl rubberedge sealant that doubles as an electrode spacer. Figure 5shows transmission versus time curves and photographs of twodevices: one constructed with the Pt-modified electrodedeveloped in this work and one with a bare ITO electrode.Transmission (at 550 nm) was measured at the device edgeand the center during a 60 s tinting sequence under an appliedpotential of −600 mV. The potential of −600 mV was chosenbased on the onset potential for hydrogen adsorption on thePt-modified surface (Figure S8). Complete optical propertiesmeasured using integrating sphere measurements are shown inFigure S9. The measured transmission through the device witha Pt nanoparticle array is relatively uniform from edge to center(±5%), showing little evidence of “irising” or uneven switchingdue to the voltage drop. The deposited metal is color-neutral,and the haze through the device is less than 6%. The bare ITOdevice, in contrast, is unacceptably nonuniform in the darkstate. Therefore, the electrodes developed in this work yield arobust platform for uniform switching under nonuniformpotential conditions.In this Letter, we demonstrated large-area dynamic windows

based on RME. We derived the voltage distribution across theelectrode in eq 1 and modeled the voltage drop that must betolerated in RME devices. We tested electrodes modified withPt seed layers to determine the optimal coating for facilenucleation of metal deposits. We showed that the rapid metalnucleation followed by diffusion-limited metal growth in oursystem enabled uniform transmission change over a 400 mVpotential range. Finally, we successfully fabricated 100 cm2

dynamic windows based on the reversible electrodeposition ofBi and Cu that tint uniformly from a clear state (>60%transmission) to a dark state (<5% transmission) in less than 1min.Several challenges remain before RME windows achieve

commercial success. While this work is a critical step forsolving challenges with nonuniformity on the workingelectrode surface, the counter electrode limits the durabilityof the devices. We manage 300 cycles with no signs ofdegradation, but after some time, nonuniform growth occurson the Cu mesh counter electrode, which leads to a decay inmaximum transmission (Figure S10). This growth occursbecause we are repeatedly reducing and oxidizing the coppercounter electrode over many cycles. Further, ions from thecounter electrode may leach into the system over time andincrease the overall concentration in the electrolyte. Theseeffects are limiting factors in surpassing the 1000 stable cyclesthat we demonstrated for this electrolytic system in previouswork.6 Thus, future work must target a transparent and inertcounter electrode material to achieve the durability needed forthe 25 year lifetime expected for commercial building windows.Further scalability may be achieved by relaxing the voltage

drop in RME devices. Equation 1 predicts that a decrease incurrent density and electrode resistivity would lower thepotential difference from edge to center. Lower current densitycould be achieved by improving the coloration efficiency(defined as the change in optical density per unit charge) ofmetal-based systems. Leveling agents used in electroplatingbaths could yield a denser metal film, which would allow

greater contrast for the same amount of charge passed (ormetal ions reduced) but could make the film smoother andmore reflective, which might not be appealing in allapplications.10 Transparent conductive electrodes based onmeshes of metal lines with width less than 10 μms can havetransmission greater than 95% and sheet resistance < 1 Ω/sq.15They could be used to further mitigate the nonuniformpotential distribution in large-scale dynamic windows. Effortsinvolving the proposed electrolytic and electrode improve-ments could bring RME-based window technology anotherstep closer to commercial viability.

■ EXPERIMENTAL SECTIONPt-Modif ied Electrodes. Pt nanoparticles with average diametersof 3 nm dispersed in water were purchased from Sigma-Aldrich. For working electrodes modified with a SAM of Ptnanoparticles, ITO-coated glass substrates (10 Ω/, Xin Yan)were immersed in an ethanolic solution of 10 mM 3-mercaptopropionic acid for 24 h. Next, the substrates werethoroughly rinsed with ethanol and deionized H2O beforeimmersing them in a Pt nanoparticle suspension for 24 h. AfterPt immersion, the substrates were rinsed with water and driedwith a N2 gun. The substrates were then annealed at 250 °C ona hot plate for 30 min. For working electrodes modified with aPt film, 3 nm of Ti was evaporated on ITO as an adhesionlayer, followed by 3 nm of evaporated Pt. For three-electrodeexperiments, electrochemical potentials were measured andreported using a Biologic SP-150 potentiostat with respect to a“no-leak” Ag/AgCl (3 M KCl) reference electrode.Window Assembly. Dynamic windows were constructed in

two-electrode configurations. Pt-modified or bare ITO (100cm2) on glass served as working electrodes. Transparentcopper mesh (TWP, Inc., wire diameter: 0.0012 in.) served asboth the counter and reference electrodes. The Bi−Cu gelelectrolyte was prepared according to our previous work.6

Butyl rubber edge sealant (Quanex, 2 mm) served as anelectrode spacer and as containment for the electrolyte.Characterization. Transmission spectra recorded were

measured with an Ocean Optics USB2000 spectrometercoupled with an Ocean Optics halogen light source (HL-2000-FHSA). SEM images were obtained using an FEIMagellan 400 XHR scanning electron microscope operatedat an accelerating voltage of 10 kV.A detailed experimental section can be found in the

Supporting Information.

■ ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acsenergy-lett.8b01781.

Derivation of the voltage drop in a square resistor,Figures S1−S12, showing a transmission−time plot,chronoamperometry transients, cyclic voltammograms,SEM images, XPS spectra, linear scan voltammograms,and transmission, absorbance, and reflectance plots, andan experimental section with expanded detail (PDF)

■ AUTHOR INFORMATIONCorresponding Author*E-mail: michael.mcgehee@colorado.edu.

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ORCIDMichael T. Strand: 0000-0002-2144-2735Christopher J. Barile: 0000-0002-4893-9506Tyler S. Hernandez: 0000-0003-1885-4656Michael D. McGehee: 0000-0001-9609-9030NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTS

This research was funded by the U.S. Department of Energy(DOE) under DE-EE0008226. Part of this work wasperformed at the Stanford Nano Shared Facilities (SNSF),supported by the National Science Foundation under AwardECCS-1542152. M.T.S. and T.S.H. acknowledge financialsupport of National Science Foundation Graduate ResearchFellowships (No. NSF DGE-1656518). M.T.S. also acknowl-edges financial support of a Stanford Graduate Fellowship.C.J.B. acknowledges Research & Innovation at the Universityof Nevada, Reno.

■ REFERENCES(1) View Dynamic Glass. Energy Benefits of View Dynamic Glass inWorkplaces. https://viewglass.com/assets/pdfs/workplace-white-paper.pdf#zoom=100 (2017).(2) Hedge, A.; Nou, D. Worker Reactions to Electrochromic andLow e Glass Office Windows. EOIJ 2018, DOI: 10.23880/EOIJ-16000166.(3) Barile, C. J.; Slotcavage, D. J.; McGehee, M. D. Polymer−Nanoparticle Electrochromic Materials That Selectively ModulateVisible and Near-Infrared Light. Chem. Mater. 2016, 28 (5), 1439−1445.(4) Runnerstrom, E. L.; Llordes, A.; Lounis, S. D.; Milliron, D. J.Nanostructured Electrochromic Smart Windows: Traditional Materi-als and NIR-Selective Plasmonic Nanocrystals. Chem. Commun. 2014,50 (73), 10555−10572.(5) Barile, C. J.; Slotcavage, D. J.; Hou, J.; Strand, M. T.; Hernandez,T. S.; McGehee, M. D. Dynamic Windows with Neutral Color, HighContrast, and Excellent Durability Using Reversible Metal Electro-deposition. Joule 2017, 1 (1), 133−145.(6) Hernandez, T. S.; Barile, C. J.; Strand, M. T.; Dayrit, T. E.;Slotcavage, D. J.; McGehee, M. D. Bistable Black ElectrochromicWindows Based on the Reversible Metal Electrodeposition of Bi andCu. ACS Energy Lett. 2018, 3 (1), 104−111.(7) Niklasson, G. A.; Granqvist, C. G. Electrochromics for SmartWindows: Thin Films of Tungsten Oxide and Nickel Oxide, andDevices Based on These. J. Mater. Chem. 2007, 17 (2), 127−156.(8) Chen, Z.; Li, W.; Li, R.; Zhang, Y.; Xu, G.; Cheng, H.Fabrication of Highly Transparent and Conductive Indium−TinOxide Thin Films with a High Figure of Merit via SolutionProcessing. Langmuir 2013, 29 (45), 13836−13842.(9) Gordon, R. G. Criteria for Choosing Transparent Conductors.MRS Bull. 2000, 25 (08), 52−57.(10) Schlesinger, M.; Paunovic, M. Modern Electroplating, 5th ed.;John Wiley: Hoboken, NJ, 2010.(11) Su, H.; Zhang, M.; Chang, Y.-H.; Zhai, P.; Hau, N. Y.; Huang,Y.-T.; Liu, C.; Soh, A. K.; Feng, S.-P. Highly Conductive and LowCost Ni-PET Flexible Substrate for Efficient Dye-Sensitized SolarCells. ACS Appl. Mater. Interfaces 2014, 6 (8), 5577−5584.(12) Feng, H.-P.; Paudel, T.; Yu, B.; Chen, S.; Ren, Z. F.; Chen, G.Nanoparticle-Enabled Selective Electrodeposition. Adv. Mater. 2011,23 (21), 2454−2459.(13) Habteyes, T. G.; Dhuey, S.; Wood, E.; Gargas, D.; Cabrini, S.;Schuck, P. J.; Alivisatos, A. P.; Leone, S. R. Metallic Adhesion LayerInduced Plasmon Damping and Molecular Linker as a NondampingAlternative. ACS Nano 2012, 6 (6), 5702−5709.

(14) Radisic, A.; Ross, F. M.; Searson, P. C. In Situ Study of theGrowth Kinetics of Individual Island Electrodeposition of Copper. J.Phys. Chem. B 2006, 110 (15), 7862−7868.(15) Ellmer, K. Past Achievements and Future Challenges in theDevelopment of Optically Transparent Electrodes. Nat. Photonics2012, 6 (12), 809−817.

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