Electrochromic performance of nanocomposite nickel oxide counter electrodes containing lithium and zirconium

https://doi.org/10.1016/j.solmat.2013.11.023Get rights and content

Highlights

  • Nanocomposite nickel oxide electrochromic counter electrodes were fabricated via RF magnetron co-sputtering.

  • The structure (i.e., crystal structure, electronic structure) of the resulting electrodes was characterized.

  • Electrochromic performance is highly dependent on the Li stoichiometry and hole doping.

Abstract

Nickel oxide materials are suitable for counter electrodes in complementary electrochromic devices. The state-of-the-art nickel oxide counter electrode materials are typically prepared with multiple additives to enhance peformance. Herein, nanocomposite nickel oxide counter electrodes were fabricated via RF magnetron co-sputtering from Ni–Zr alloy and Li2O ceramic targets. The as-deposited nanocomposite counter electrodes were characterized with inductively coupled plasma mass spectrometry (ICP-MS), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD), and X-ray absorption spectroscopy (XAS). It was found that the stoichiometry, crystal structure and electronic structure of the nickel oxide-based materials could be readily tuned by varying the Li2O sputter deposition power level. Comprehensive electrochromic evaluation demonstrated that the performance of the nickel oxide-based materials was dependent on the overall Li stoichiometry. Overall, the nanocomposite nickel oxide counter electrode containing lithium and zirconium synthesized with a Li2O deposition power of 45 W exhibited the optimal performance with an optical modulation of 71% and coloration efficiency of 30 cm2/C at 670 nm in Li-ion electrolyte.

Introduction

Electrochromic effects in transition metal oxide materials (e.g., nickel oxide, tungsten oxide, titanium oxide) have received extensive attention for smart windows, rear-view mirrors and non-emissive displays. A cathode/anode complementary electrode configuration in electrochromic devices is the most prevalent configuration due to the advantages this configuration offers, i.e., color neutrality and coloration efficiency [1], [2]. Tungsten oxide and nickel oxide are the most investigated electrochromic cathodic and anodic materials, respectively [2], [3]. The successful operation of a complementary electrochromic device requires compatibility between the cathodic and anodic electrodes. The critical electrode compatibility requirements include a balanced charge capacity [4], [5], [6] and equivalent switching kinetics [7]. Since the 1970s, extensive efforts have been devoted to improve the performance and mechanistic understanding of the electrochromic process in cathodic tungsten oxide materials [8], [9], [10], [11]. Further research has been directed towards developing cost effective syntheses and manufacturing processes for these cathodic materials [2], [12]. Conversely, many challenges remain unresolved for anodic nickel oxide counter electrodes, including slow switching kinetics, inferior optical modulation, and poor bleached-state transparency [13], [14], [15], [16].

In recent years, significant efforts have been dedicated to optimize the performance of nickel oxide counter electrode materials, with the emphasis on controlling morphology on the nanoscale (nanocomposites) [1], [7], [17], crystal structure (lattice order, crystallinity) [17], [18] and electronic structure [19]. The most promising nickel oxide counter electrodes are composed of complex mixtures of transition metal oxides including secondary additives such as lithium oxide and lithium peroxide [20]. Recently, multicomponent nickel oxide counter electrodes (i.e., nickel oxide materials that contain at least two additives) have shown superior performance relative to the traditional nanocomposite nickel oxide electrodes that contain only one additive [1], [20]. The addition of co-additives (i.e., a transition metal and lithium) to the nickel oxide electrode material enables simultaneous improvement of bleached-state transparency, switching kinetics, and optical modulation [1], [19], [20]. In addition, the nickel oxidation state and hole concentration in nickel oxide-based electrochromic materials are critical for favorable electrochromic performance and relevant specifically to the coloration/bleaching mechanism [19], [20]. In our previous study, we demonstrated that lithium is an effective additive for tuning the nickel oxidation state and hole concentration in nickel oxide materials containing aluminum [19]. However, the electrochromic performance (e.g., optical modulation and bleached-state transparency) of these Al-containing nickel oxide nanocomposites was modest compared to the performance reported for nickel oxide materials containing lithium and tungsten [1] or lithium and zirconium [20] additives. The nickel oxide nanocomposite materials containing lithium and zirconium exhibited extremely high electrochromic performance relative to previous electrochromic transition metal oxide materials [20]. However, it is unclear whether or not the hole concentration in these high performing nickel oxide materials containing lithium and zirconium can be optimized further to improve electrochromic performance.

Herein, we extend our previous studies by systematically tuning the Li stoichiometry in radio frequency (RF) magnetron co-sputtering deposition of nanocomposite nickel oxide materials. It is found that the sputtering conditions (i.e., deposition power levels) provide an efficient route for tuning the Li stoichiometry, crystal structure, and electronic structure in nanocomposite nickel oxide materials. The chemical, structural, and electronic properties of these nickel oxide-based materials are characterized in detail and an optimal lithium concentration is determined.

Section snippets

Electrode preparation

RF magnetron co-sputtering was preformed on an Angstrom EvoVac deposition system housed in a glove box under an argon atmosphere following a previously described method (see Scheme 1) [19]. Three-inch diameter metal alloy target, Ni–Zr (80–20 at%), was purchased from ACI Alloys, while a three-inch diameter Li2O ceramic target (99.9%) supported on a molybdenum backing plate was purchased from Plasmaterials, Inc. The deposition power level for the metal alloy target was fixed at 60 W, while the

Results and discussion

The stoichiometric information of the as-deposited counter electrode materials was quantified using ICP-MS and shown in Table 1 and Fig. 1. The atomic ratio between Li and Ni (Li/Ni) generally increases with the increase of Li2O deposition power, while no obvious difference in the Li/Ni ratio is observed between Li2O/45 W and Li2O/60 W. The atomic ratio between Zr and Ni is fairly stable when the Li2O deposition power was varied. We note that the stoichiometry of Li2O/45 W electrode is slightly

Conclusions

Nanocomposite nickel oxide counter electrodes were fabricated via RF magnetron co-sputtering using Ni–Zr alloy and Li2O ceramic targets. The Li2O sputter power level was tuned to control the Li stoichiometry in the resulting nanocomposite nickel oxide counter electrodes containing lithium and zirconium. Controlling the Li stoichiometry (i.e., nickel oxidation state, hole concentration) was critical for obtaining superior nanocomposite nickel oxide electrodes. The optimal electrochromic

Acknowledgments

This paper is dedicated to the loving memory of Anne C. Dillon. This research was supported by the U.S. Department of Energy under Contract number DE-AC36-08-GO28308 with the National Renewable Energy Laboratory as part of the DOE Office of Energy Efficiency and Renewable Energy Office of Building Technologies Program. Portions of this research were carried out at the Stanford Synchrotron Radiation Laboratory, a national user facility operated by Stanford University on behalf of the U.S.

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