Article pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Mechanistic Insights into the Coloration, Evolution, and Degradation of NiOx Electrochromic Anodes
Junji Guo,† Mei Wang,† Guobo Dong,† Zhibin Zhang,‡ Qianqian Zhang,† Hang Yu,† Yu Xiao,† Qirong Liu,† Jiang Liu,§ and Xungang Diao*,† †
Electrochromic Center, School of Physics and Nuclear Energy Engineering, Beihang University, Beijing 100191, P. R. China Solid State Electronics, Ågströmlaboratoriet, Uppsala University, Uppsala 75121, Sweden § Jiangsu Fanhua Glass Co., Ltd. Hai’an, Nantong, Jiangsu Provience, P. R. China Downloaded via STEPHEN F AUSTIN STATE UNIV on July 26, 2018 at 16:21:01 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
‡
S Supporting Information *
ABSTRACT: NiOx is recognized as the leading candidate for smart window anodes that can dynamically modulate optical absorption, thereby achieving energy efficiency in construction buildings. However, the electrochromic mechanism in NiOx is not yet clear, and the ionic species involved are sometimes ambiguous, particularly in aprotic electrolytes. We demonstrate herein that the “net coloration effect” originates from newly generated high-valence Ni3+/Ni4+ ions during anion-dependent anodization, and the Li+ intercalation/deintercalation only plays a role in modulating the oxidation state of Ni. Unambiguous evidences proving the occurrence of anodization reaction were obtained by both chronoamperometry and cyclic voltammetry. Benefiting from the irreversible polarization of Ni2+ to Ni3+/Ni4+, the quantity of voltammetric charge increases by ∼38% under the same test conditions, enhancing the corresponding electrochromic modulation by ∼8%. Strong linkages between the coloration, evolution, and degradation observed in this work provide in-depth insights into the electrocatalytic and electrochromic mechanisms. coloring under cation deintercalation.1,9−12 Among these promising candidates, nickel oxide certainly stands out as the most extensively studied one because of the advantages of lowcost, structural flexibility, and, of most importance, excellent electrochromic properties.12,13 Several models have been proposed to rationalize the underlying chromic mechanisms including the small polaron motion, bandstructure effects, Maxwell Garnett effective medium theory, etc.1,9−11 Nevertheless, despite decades of exhaustive scientific inquiry, a comprehensive description of electrochromism has remained elusive due to the complex evolving nature during the electrochemical reactions. The electrochemical reaction scheme for sputter-deposited Ni oxide films highly depends on the type of ionic species in electrolyte. During the initial electrochemical cycling in aqueous KOH electrolyte, the bulk (B) NiOx phases undergo a “hydrogenation” process: NiOx |B → NiOxHy|S and eventually form stable electrochromic NiOOH/Ni(OH)2 two-phase system on the surface (S), the operation of which is associated with the intercalation/ deintercalation of protons, as described by the Bode scheme:1,14,15
I. INTRODUCTION Electrochromes can be transformed from one crystalline phase to another by using an electric field to control charge transfer, with associated change of electronic absorption bands as well as optical properties.1−3 Incorporating electrochromes into future commercial photoelectrochemical applications, such as artificial muscles, active camouflages, switchable goggles, and solar control windows, will require a clear picture of their working mechanism and proper means for controlling charge− exchange.4−7 Solid-state electrochromism based on cathodic coloration was reported for the highest oxidation state of group V and VI metals Nb, Mo, Ta, and W, with tungsten oxide being by far the most extensively studied electrochromic material,1,2,8 the coloration/bleaching of which is associated with the simultaneous intercalation/deintercalation of cations M+ into/from the host and charge balancing electrons via WO3 − y + x M+ + x e− F MxWO3 − y
(1)
with the intercalation level x.8 Chromism is fundamentally an electronic effect, and the optical absorption originates from the Drude-like absorption from delocalized electrons (for crystalline case) or small polaron transition between two adjacent nonequivalent W sites (for amorphous case). On the other hand, some oxides of metal elements Cr, Mn, Fe, Ni, Rh, and Ir have been identified as the anodic electrochromes that become © XXXX American Chemical Society
NiOOH + H+ + e− F Ni(OH)2
(2)
Received: March 25, 2018
A
DOI: 10.1021/acs.inorgchem.8b00793 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry
Figure 1. (a) XRD pattern of as-deposited NiOx film; the contributions from the underlying ITO/glass substrate were eliminated. (b) Threedimensional tunnel structure of hexagonal NiOOH/Ni(OH)2. (c) Ni 2p XPS.
should be highly stable, but in fact it is not). This Article addresses these issues by combining the Li+ intercalation/ deintercalation and anion-dependent anodization; it not only highlights the role of electrocatalytic activity in determining the net coloration effect but also elucidates some details of the evolution and degradation mechanisms.
It has long been assumed that nickel oxide colors anodically only in its hydrated form in aqueous media, but the net and reversible electrochromic effect was also reported under rigorously dry conditions by Passerini.16 In the case of the nonaqueous PC−LiClO4 electrolyte, basing on the notion that hydrogen in NiOOH migrates from the surface to the bulk, Campet et al. put forward an irreversible transformation mechanism between the bulk and surface phases during the lithiation process:13,17 S
B
+
2x NiOOH| + (1 − 2x)NiO| + 2x Li + 2x e
II. EXPERIMENTAL METHODS Thin films of NiOx were prepared by means of reactive DC magnetron sputtering in Ar (99.99%) and O2 (99.99%) ambient. Base pressure of the sputter main chamber was under 2.0 × 10−3 Pa. Unheated transparent glass, precoated 120 nm thick In2O3:Sn (socalled ITO) having a sheet resistance of 17 Ω/sq, was utilized as substrate . The sputter target was a 2 mm thick metallic Ni plate (99.9%) with 100 mm diameter, and the distance from substrate to target was kept at 7 cm. Prior to the deposition, the target was presputtered with the chosen working gases mixture to reach a steady state. During deposition, the power to the target, gas-flow ratio O2/Ar, and working pressure were maintained at 220 W, 13%, and 1.4 Pa, respectively. Thickness was measured by high-precision step profiler (Dektak II, Bruker Corp.). Film structure was determined by X-ray diffraction (XRD) using a Rigaku D/Max 2200 diffractometer with Cu Kα (λ = 1.5406 Å) radiation source and Bragg−Brentano θ−2θ geometry (40 kV and 40 mA). Elemental composition and chemical bonding state were determined by X-ray photoelectron spectroscopy (XPS, Physical Electronics PHI-5600 system) using an Al Kα X-ray source operated at 200 W, 23.5 eV. Analysis of XPS data was performed using the Shirley background subtraction method. High-resolution FEI-Phillips XL30 S-FEG scanning electron microscope (SEM) was employed to measure the surface morphology of the film. Prior to imaging by SEM, the film was sputter-coated with 10 nm gold for improving the conductivity. All electrochemical measurements [chronoamperometry (CA), cyclic voltammetry (CV), and linear sweep voltammetry (LSV)] were made using a CHI660C electrochemical working station (Chenhua) equipped with the three-electrode electrochemical cell. The NiOx film on ITO-coated glass substrate served as the working electrode. The counter and reference electrode was Pt rod and Ag/ AgCl, respectively. The nonaqueous electrolyte was prepared by dissolving 1 M LiClO4 in propylene carbonate. The in situ optical
−
→ x Ni(OH)2 |B + Li 2xNi1 − xO|S
(3)
Afterward, the absorption strength of lithiated species Li2xNi1−xO on the surface can be further modulated by reversible Li+ intercalation/deintercalation via Li +2x − yNi12−+x − yNi3y+O + y Li+ + ye− F Li +2xNi12−+xO
(4)
In this way, this modulation process is the same as that related to the mechanism of electrochromic reaction of tungsten oxide in format. Accordingly, during both the initial lithiation and subsequent modulation, the charge exchange involves solely Li+ ions from the electrolyte, and the cationic redox itself does not generate new high-valence Ni3+/Ni4+ species in the film. These widely accepted explanations have satisfactorily correlated the optical absorption strength with intervalence charge transfer between the adjacent Ni2+ and Ni3+ sites. However, particularly in nonaqueous media, several basic but critical issues are still controversial, that is, what is the mechanism of “net coloration effect” [reactions (3) and (4) produce no new high-valence Ni3+/Ni4+ ions, and hence the film will not be darker than its initial state during Li+ intercalation/deintercalation], how do Ni4+ species participate in the electrochromism (their existence has been evidenced,18,19 which is not included in above reactions), and what is the reason for the degradation of electrochromism (if charge exchange involves only Li+ ions, the cyclic reversibility B
DOI: 10.1021/acs.inorgchem.8b00793 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
Figure 2. (a) Comparison of in situ optical response at 550 nm of the same NiOx film under different applied potentials. (b) Corresponding CA curves in PC−LiClO4; 0 V was applied for ∼60 s before performing various values of E. (c) The quantities of inserted and extracted charge vs E derived from CA data; the symbolic curve represents the difference between inserted and extracted charge in quantity.
suggest that the Ni3+ species in the film derives from Ni2+ vacancies on surfaces of nickel crystals rather than from the native bulk phases. B. Kinetic Analysis of the Electrochromic Behaviors. Our first experiments employed a commonly used multicycle single potential chronoamperometry together with in situ optical transmittance measurement on NiOx film under various bias potential E. During the reduction processes, the injected Li+ ions (with ionic radius of 0.059 nm) rapidly overcome the barrier at NiOx/electrolyte interface even under lower potential (0.1 V) operation, then they partly may be incorporated in fixed sites, where ions become immobile once trapped inducing a permanent modification of the lattice.27 Correspondingly, the optical transmittance is increased by ∼13% during the first lithiation but hard to regain its initial value of ∼62% if cycled under the same conditions (Figure 2a), showing a characteristic of fast reduction and slow oxidation rate (Figure 2b). Because of both the large relative molecular weight (49) and ionic radius (0.240 nm), the ClO−4 ions cannot directly diffuse into the film, and they are instead absorbed on surfaces of nickel crystals. Significant enhancement of optical modulation can be achieved with increasing E, and the transmittance for colored states is less than its initial value as E exceeds 0.6 V, indicating the generation of new highvalence Ni3+/Ni4+ ions in the film. The generation of new highvalence nickel ions can also be confirmed by comparing the color changes of the film (Supporting Information S2). Noteworthy, the Li+ intercalation/deintercalation does not have the ability to generate new Ni3+/Ni4+ ions, and hence no contribution to the net coloration effect. The insertion/ extraction charge involved in the reduction/oxidation is calculated by ∫ IR/Odt, where IR/O is the cathodic/anodic current in CA curves. It is found that the quantity of insertion charge is less than that of the subsequently extracted one, and the higher the applied potential is, the bigger the difference occurs. This
responses were measured at a wavelength of 550 nm during electrochemical tests by UV−visible−near-infrared (NIR) spectrometer (Evolution 100, Thermo Electron Corporation). The threeelectrode electrochemical cell was positioned between the light source and detector in the chamber of spectrometer. The cell filled with electrolyte was used as the basic reference.
III. RESULTS AND DISCUSSION A. Sample Characterizations. The crystalline structure of the as-deposited NiOx film was detected by XRD and shown in Figure 1a. According to JCPDS card (No. 47-1049), the (111), (200), (220), and (311) peaks typical to NiO are indexed. For sputter-deposited NiOx films the extra oxygen cannot be placed inside the NaCl structure; instead, vacancies of Ni2+ are created, giving irregular crystal facets.13,20 When the film is exposed to ambient atmosphere, these crystal facets would be partly or fully covered with hydroxyl groups by adsorbing H+/ OH− ions on the terminal O/Ni sites, resulting in a two-phase mixture of hexagonal Ni(OH)2 and NiOOH on surfaces of nickel crystals.13,21,22 These hydroxyl groups are composed of distored octahedral NiO6 units with predominant edge-sharing, which produce a three-dimensional network of “tunnels” that are sufficiently large for guest intercalation/deintercalation (Figure 1b).1,15 The high-resolution XPS spectrum was obtained for Ni 2p core states, to confirm the existence of hydroxyl groups on the film surface (Figure 1c). According to previous researches, the Ni 2p 3/2 spectrum can be deconvoluted into five core-level peaks [852.7 eV Ni, 853.8 eV NiO, 855.4 eV Ni(OH)2, 856.8 eV Ni2O3, 857.3 eV NiOOH] and two satellite peaks.19,23−26 The binding energy was calibrated to C 1s signal at 284.8 eV (Supporting Information S1). The small amount of metal Ni is mainly from the inadequate reaction between target atoms and reaction gas. From the fitting peak areas, the relative content of Ni(OH)2 and NiOOH was estimated to be 56.7% and 5.5%, respectively. The results of complementary XRD and XPS characterizations C
DOI: 10.1021/acs.inorgchem.8b00793 Inorg. Chem. XXXX, XXX, XXX−XXX
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Figure 3. (a) CV curve of NiOx film in PC−LiClO4 in the range from −0.5 to 1.5 V at 100 mV/s; the arrow indicates sweep direction. (b) Corresponding in situ transmittance at 550 nm. (c) Anodic branch of the CV curve during the positive sweep; the blue reference curve is obtained by performing LSV measurement in the positive-going potential scan range from −0.3 to 1.5 V at 100 mV/s on as-deposited film. (d) First-order potential derivative of instantaneous currents.
Figure 4. (a) Comparison of in situ optical response at 550 nm of the same NiOx film in different polarized degrees under the same CV test conditions: in the range from −0.7 to 1.5 V at υ = 300 mV/s in PC−LiClO4. (b) CV data for different polarized degrees after stable cycle; the arrow indicates sweep direction. (c) Redox charge variation with respect to the polarization degree. Anodic/cathodic polarizations were implemented through LSVs in the range from 0 to 1.5/−1 V at 10 mV/s.
by many researchers.28−33 Two controversial reaction mechanisms for the electrocatalytic reduction of ClO−4 ions on electrodes can be found in the existing literatures.28,29 From the classical point of view, participation of adsorbed H atoms is essential for the electrocatalytic process. In contrast, some researchers pointed out that the adsorbed H atoms are unlikely to play any role in the electrocatalytic process, and the ClO−4
clearly indicates the existence of additional processes besides Li+ deintercalation during the oxidation processes. For a quite long time, the perchlorate was considered to be inert in various electrochemical studies, due to the stable structure of ClO−4 . However, unambiguous evidences for ClO−4 reduction (which proceeds through the release of oxygen) on a great variety of electrocatalytic electrodes have been obtained D
DOI: 10.1021/acs.inorgchem.8b00793 Inorg. Chem. XXXX, XXX, XXX−XXX
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Figure 5. Evolution of the CV behaviors during the activation (a) and degradation (b) periods in the range from −0.7 to 1.5 V at 100 mV/s in PC−LiClO4; the arrow indicates sweep direction. (c) Surface morphologies of NiOx film in as-deposited state and after 1000 CV cycles.
intensity of potential increasing, and the ClO−4 ions start to release oxygen by electrocatalytic reduction, resulting in the anodization reactions of Ni2+ to Ni3+; (vi) when E reaches the second oxidation potential, the Ni3+ species would be further oxidized to Ni4+. Next, we discuss how the above reaction schemes can give rise to the enhancement of electrochromic modulation during the activation processes and highlight the role of scan rate ν in determining the ion diffusion depth. CV was first performed on NiOx film in the potential window from −0.7 to 1.5 V at ν = 300 mV/s, followed by LSV in the positive-going potential scan range from 0 to 1.5 V at ν = 10 mV/s. Near-steady state of the CV scans can be reached after ∼15 cycles. Because of the deeper ion diffusion depth for oxygen at a relatively low scan rate, more Ni2+ in the film would be oxidized into Ni3+ /Ni 4+ by anodic polarization during LSV. These newly generated high-valence oxide species are then lithiated in subsequent CV cycles via reaction (4), consequently bringing about the enlargement of CV area (Figure 4b). The charge involved in oxidation/reduction was determined from corresponding CV data by ∫ IO/RdV/υ, where ∫ IO/RdV is the integrated area of the anodic/cathodic branch of the CV curves. It is found that the quantity of redox charge involved increases by ∼38% after the fifth anodic polarization (Figure 4c).34 This evolution also can be observed clearly in the corresponding optical responses; specifically, the transmittance for colored states decreases significantly with the increase of anodic polarization degree, enhancing electrochromic modulation by ∼8%. In contrast, the calculated redox charge from CV data remain almost unchanged after a series of cathodic polarization in the negative-going potential scan range from 0 to −1.0 V at ν = 10 mV/s, indicating the irreversibility of the activation mechanism. Accordingly, there is no obvious variation in electrochromic modulation. The slight increase in transmittance for bleaching states, ∼1%, should be assigned to the cation trapping-induced band gap opening.11 C. Kinetic Analysis of Degradation Behaviors. Theoretical and experimental studies indicated the degradation of electrochromic performance in metal oxides is mainly
ions could also be decomposed by the participation of free metal sites in their neighborhoods. In terms of our sputterdeposited NiOx film, all the metal Ni, oxy-, and hydroxy-Ni2+/ Ni3+ species on the surface are available to stabilize oxygen (by providing free metal sites and/or protons), thereby enhancing the electrocatalytic activity for perchlorate reduction.28,30−32 To determine the reaction pathways and verify the occurrence of ClO−4 reduction process, cyclic voltammetry together with in situ optical transmittance measurement were performed on NiOx film. For analytical purposes, the CV curve can be divided into two parts: the cathodic and anodic branches (associated with the increase in transmittance in reduction and decrease in transmittance in oxidation, respectively). In cathodic branch, there are two broad current waves, which are generally attributed to the Li+-intercalationinduced reduction of Ni4+ (peak I) and Ni3+ (peak II), while the anodic branch presents one weak (peak III) and two strong (peaks VI and V) current waves, due to the combined effect of the Li+ deintercalation and anion-dependent anodization. To discriminate the contribution from Li+ ions, parallel linear sweep voltammetry measurement was also performed on the as-deposited film in the positive-going potential scan range from −0.3 to 1.5 V, as shown by the blue curve in Figure 3c. The initial negative values in LSV curve might arise from the capacitive charges caused by the inevitable, tiny transient current. The only characteristic distinction between the LSV and anodic branch of CV is the absence of peak VI signal in LSV curve. This indicates that the oxidation peak VI in CV curve is mainly caused by the Li+ deintercalation. Clearly, multiple peaks can be observed in both the derivative curves of LSV and anodic branch of CV (Figure 3d), indicating the multistep mechanism of chemical reaction for charge transfer. According to the involved charges, four consecutive steps may be roughly distinguished in anodic branch: (i) the capacitive charges are rapidly desorbed, showing a fast response rate (see the CV derivative curve) but weak chromic activity; (ii) the applied potential breaks the Li−O bonds (the bond strength is 341 kJ/mol) leading to the Li+ deintercalation and observed chromic change; (iii) the role of Li+ gradually weakens with the E
DOI: 10.1021/acs.inorgchem.8b00793 Inorg. Chem. XXXX, XXX, XXX−XXX
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attributable to the existence of “deep cations-trapping sites” (which immobilize the diffusing cations).27,35−38 For cathodic electrochromic materials WO3, TiO2, and MoO3, previous studies have demonstrated that the degraded films can be rejuvenated (regain their initial electrochromic modulation and reversibility) by constant-current-driven detrapping.35−37 This technique was also applied to the degraded NiOx film, but it failed to regain the initial cyclic stability of the film, though it obtained higher electrochromic modulation.38 To study the effect of anodic polarizations on degradation behavior, stability test was performed on NiOx film by cyclic voltammetry. During the initial activation, more electroactive species can be achieved at the expense of inner NiOx by the anodic polarization, thereby causing an ongoing increase of CV area (Figure 5a). Though this is a minor effect in each CV scan, the formation of Ni vacancies (Ni3+/Ni4+ species) never ends with cycling. Degradation mechanisms result from such an irreversible process mainly through two pathways as follows: (i) the generated electroactive species provide new deep cations-trapping sites and hence obliterate the electrochemical activity (Figure 5b). (ii) the large numbers of Ni vacancies result in cracking and crumbling (Figure 5c), due to the stress accumulation in the structure.39,40 For the former, the deeper trapped cations can be detrapped by waiting for a suffciently long time at high operating potential. But for the latter, the structural damage will lead to the irreversible change in electrochromic performance. As a consequence, capacity is ultimately always degraded on prolonged cycling, thus hindering its application in electrochromic devices, whatever the preparation and characteristics of film. Aiming to alleviate the performance degradation, researchers have done a series of studies and found that Ir, V, and Ti ion dopants to Ni oxide gave strongly improved properties, due to the suppressed evolution rates of phase structure.25,41
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Zhibin Zhang: 0000-0003-0244-8565 Xungang Diao: 0000-0001-9997-6895 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was financially supported by the National Program on Key Research Project (2016YFB0303901), the Beijing Natural Science Foundation (2161001), and the Fundamental Research Funds for the Central Universities (Grant No. YWF16-JCTD-B-03).
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REFERENCES
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IV. CONCLUSION In conclusion, we have answered the issues posed in the introduction and revealed the evolving chemical nature of NiOx during electrochromic process. Unambiguous evidences proving the occurrence of anodization reactions have been obtained in both the voltammetric and chronoamperometric curves. Arising from the irreversible polarizations of Ni2+ to Ni3+/Ni4+, the quantity of redox charge involved in same cyclic voltammetry tests increases by ∼38%, enhancing the corresponding electrochromic modulation by ∼8%. The results presented indicate that the anion-dependent anodization is responsible for both the net coloration effect and degradation behaviors, which is different from the commonly accepted reaction paths. Our work may provide insight into the primary factors that determine charge exchange NiOx electrochromic anodes and guide future designs for highly efficient electrochromic devices.
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Article
ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b00793. XPS spectra of as-deposited and postanodic polarization NiOx film surfaces, photographs of ITO/glass substrate and NiOx film in different states (PDF) F
DOI: 10.1021/acs.inorgchem.8b00793 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
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DOI: 10.1021/acs.inorgchem.8b00793 Inorg. Chem. XXXX, XXX, XXX−XXX
