Anodic Electrochromism for Energy‐Efficient Windows: Cation/Anion‐Based Surface Processes and Effects of Crystal Facets in Nickel Oxide Thin Films

Abstract

Anodic electrochromic (EC) oxides are of major interest as counter electrodes for smart window applications owing to their unique optical properties upon charge insertion and extraction. However, performance optimization of such oxides has been hampered by limited understanding of their EC mechanism, particularly in Li + -conducting electrolytes. This paper reports on NiO x films with 1.16 ≤ x ≤ 1.32, prepared by sputter deposition. These films were immersed in an electrolyte of lithium perchlorate in propylene carbonate, and EC properties were studied by cyclic voltammetry and in situ optical transmittance measurements. The electrochromism was significantly enhanced at large values of x. We find that charge exchange in Ni oxide is mainly due to surface processes and involves both cations and anions from the electrolyte, which is different from the case of cathodic EC materials such as WO 3 .Whereas previous studies of Ni oxide have focused on cation intercalation, the cation/anionbased mechanism presented here offers a new paradigm for designing and developing EC devices such as smart windows for energy efficient buildings.2

Electrochromic Iridium-Containing Nickel Oxide Films with Excellent Electrochemical Cycling Performance

“…The magnitude of the charge density indicates that the electrochromism is probably due to surface processes in these films, as it is for Ni oxide. 21 We were unable document any influence of the Ir-dependent preferred crystallite orientation (cf. Fig.…”

“…7,21,55 For γ = 18.3% one may associate the lowered transmittance with an increased content of Ir 4+ (cf. Fig.…”

“…[15][16][17][18] Ni oxide was put forward as an alternative 19,20 and has been of sustained interest for almost 30 years. We recently demonstrated that the coloration is a surface process 21 and that the degradation of charge density in Ni oxide obeys a power law 22 upon cycling in a Li-conducting electrolyte, which implies that a monotonic decline of the optical properties takes place and eventually leads to device failure. Many studies during the last years have focused on ways to improve the performance of Ni oxide films with regard to optical modulation speed and span, bleachedstate transmittance and electrochemical cycling durability by use of additives such as Li, [23][24][25] C, 26, 27 N, 28 F, 29 Al, 30 Ti, 31, 32 V, 33,34 Mn, 35 Co, 36 Cu, 37 W, 38,39 (Li,W), 40 (Li,Al), 41 (Li,Zr), 42 and LiPON.…”

Effect of Sensitization on the Electrochemical Properties of Nanostructured NiO

“…p-type nickel oxide (NiO) [1,2]; is widely employed as electrodic material in photoelectrochemical cells [3][4][5][6][7][8][9], electrochromic windows [10][11][12][13][14], and charge storage systems [15][16][17], among others [18][19][20]. The wide range of NiO applicability derives from the fact that NiO can constitute a functional material in both bulk [21] and nanostructured [22,23] versions.…”

“…Moreover, nanostructured films of NiO present solid-state electroactivity [25][26][27] due to the verification of a series of reversible electrochemical processes that switch the properties of electrical transport [28] and optical absorption [29] of the oxide itself. The optical [10][11][12][13][14]30,31], magnetic [32,33], electrochemical [34], and photoelectrochemical [35,36] properties of NiO (either in the bulk state or in the nanostructured version) are considerably altered when the surface of the oxide is dye-sensitized [37]. Such a type of electrode modification consists mainly in the impartation of additional optical absorption [38][39][40][41][42][43][44][45][46][47][48][49] and photoelectroactivity to NiO in the NIR-Vis range, i.e., in a spectral range of lower energies with 2 of 18 respect to the intrinsic optical absorption of pristine NiO (typically in the near ultraviolet (UV)) [50].…”