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Factors that Determine the Length Scale for Uniform Tinting in Dynamic Windows based on Reversible Metal Electrodeposition Michael Strand, Christopher J Barile, Tyler S. Hernandez, Teresa Dayrit, Luca Bertoluzzi, Daniel Slotcavage, and Michael D. McGehee ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.8b01781 • Publication Date (Web): 23 Oct 2018 Downloaded from http://pubs.acs.org on October 24, 2018
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ACS Energy Letters
Factors that Determine the Length Scale for Uniform Tinting in Dynamic Windows based on Reversible Metal Electrodeposition
Michael T. Strand1, Christopher J. Barile2, Tyler S. Hernandez3, Teresa E. Dayrit1, Luca Bertoluzzi1, Daniel J. Slotcavage1, Michael D. McGehee1,4
1
Department of Materials Science and Engineering, Stanford University, Stanford, CA 94305
2
Department of Chemistry, University of Nevada – Reno, Reno, NV 89557
3
Department of Chemistry, Stanford University, Stanford, CA 94305
4
Department of Chemical Engineering, University of Colorado – Boulder, Boulder CO 80303
*E-mail:
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Abstract Dynamic windows based on reversible metal electrodeposition are attractive compared to conventional electrochromics because they can have neutral color, high contrast, and potentially lower cost, yet they are not nearly as developed and the design rules for making them function at large scale are not presented in the literature. We model the voltage drops that occur in the transparent electrodes to get insight on how to obtain uniform electrodeposition of metals over large-area. By optimizing the surface and density of the Pt nanoparticles used to nucleate metal growth, we lower the nucleation barrier for electrodeposition by 70 mV. We show that the growth rate of the metal films is determined by diffusion rather than reaction kinetics, which makes it possible to achieve uniform film deposition over a range of potentials from -300 mV 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 one minute. For Table of Contents Only
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ACS Energy Letters
Dynamic windows that allow electronic control of visible light and solar heat gain are desirable for improving energy efficiency in buildings and automobiles and reducing glare without obstructing views.1–3 Despite decades of research, however, traditional approaches to dynamic windows have not widely penetrated the market due to problems with color, cost, speed, and durability.4 Dynamic windows based on reversible metal electrodeposition (RME) represent an undeveloped class of electrochromic devices posed to overcome the challenges inherent to existing technologies. We recently demonstrated dynamic windows based on the reversible electrodeposition of Bi, Cu, and Ag that switch uniformly between transparent and opaque states over thousands of cycles.5,6 We developed transparent electrodes decorated with Pt nanoparticles that enable uniform electrodeposition of metal films on the 25 cm2 scale.5 We verified that dynamic windows using metals can be color neutral and require no additional power to be held in the desired optical state.6 Finally, we proposed that our design allows dynamic windows to be manufactured at a fraction of the cost of conventional technologies. In this manuscript, we evaluate what is needed for dynamic windows based on reversible metal electrodeposition to be realized at a practical scale. A challenge hindering large-scale RME windows is associated with the limitations of transparent conductors. The majority of electrochromic devices use glass coated with transparent conducting oxides as the conductive electrodes.7 Indium tin oxide (ITO) is the most commonly used transparent conductor and possesses a resistivity of ~10−4 Ω·cm and a visible transmittance greater than 80%.8 The sheet resistance is typically 10 Ω/sq. The resistivity is two orders of magnitude higher than opaque conductors like metals, and maintaining a uniform current density leaving the ITO electrodes during electroplating or stripping necessitates a voltage drop that
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increases with the electrode area.9 The voltage distribution can be calculated by integrating Ohm’s Law over a two-dimensional surface. This calculation yields the following equation: ∆𝑉𝑉 =
𝐽𝐽𝐽𝐽 2𝑡𝑡
(1)
�(𝐿𝐿2 − 𝑥𝑥 2 )(𝐿𝐿2 − 𝑦𝑦 2 )
where J is the current density, 𝜌𝜌 is the resistivity, t is the film thickness, L is the electrode length, and x and y are positions on the electrode surface defined by a Cartesian coordinate system with
the origin at the corner of the electrode (a detailed derivation is presented in the Supporting Information). Figure 1 shows a schematic view of a 100 cm2 dynamic window and the simulated voltage distribution across an ITO electrode with a sheet resistance of 10 Ω/sq using the maximum current density observed experimentally in a device that switches from T = 60% to T = 10% in 30 seconds. As shown, these devices must tolerate a 0.3 V difference from edge to center.
Figure 1: Schematic of a 100 cm2 dynamic window. Device consists of a 100 cm2 Pt-modified ITO working electrode, a Bi-Cu gel electrolyte, and a semi-transparent Cu grid counter electrode mounted to a 100 cm2 glass backing. The device is sealed using 2-mm-thick butyl rubber edge sealant that doubles as an electrode spacer.
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ACS Energy Letters
Large-scale RME windows, then, must be engineered with a tolerance to the voltage drop across the transparent electrodes to maintain optical uniformity during device operation. The devices with the best performance to date use aqueous electrolytes because metal salts have high solubility and dissociation constants in water, and thus water-based electrolytes benefit from relatively low deposition overpotentials.10 However, undesired side reactions like hydrogen evolution set strict 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 sizable electrodes. The ideal electrodes will enable uniform nucleation and growth of metal films over the potential range set by side reactions inherent to the electrolyte and the voltage drop inherent to transparent conductors. In this manuscript, we investigate the nucleation and growth kinetics in an aqueous electrolyte based on the reversible deposition of Bi and Cu and identify design criteria for achieving uniform deposition at scale. We improve upon our Pt nanoparticle seed layer design and demonstrate electrodes that exhibit a 2.5x improvement in contrast ratio at a fixed voltage sweep and a 70 mV reduction in deposition overpotential compared to our previous work.5,6 We show that film growth in our system is diffusion-limited and that enhanced electrodes enable uniform switching curves over a 400 mV potential range. Thus, we verify that lowering the nucleation barrier (i.e. deposition overpotential) and achieving a high nucleation density allows us to deposit uniform metallic films over 100 cm2 electrodes, despite the significant voltage drop. Finally, we demonstrate 100 cm2 dynamic windows that switch uniformly from a clear state (>60% transmission) to an opaque state (60% transmission) to a dark state (
