Surface Deposition of Ni(OH)2 and Lattice Distortion Induce the Electrochromic Performance Decay of NiO Films in Alkaline Electrolyte

NiO, an anodic electrochromic material, has applications in energy‐saving windows, intelligent displays, and military camouflage. However, its electrochromic mechanism and reasons for its performance degradation in alkaline aqueous electrolytes are complex and poorly understood, making it challenging to improve NiO thin films. We studied the phases and electrochemical characteristics of NiO films in different states (initial, colored, bleached and after 8000 cycles) and identified three main reasons for performance degradation. First, Ni(OH)2 is generated during electrochromic cycling and deposited on the NiO film surface, gradually yielding a NiO@Ni(OH)2 core–shell structure, isolating the internal NiO film from the electrolyte, and preventing ion transfer. Second, the core–shell structure causes the mode of electrical conduction to change from first‐ to second‐order conduction, reducing the efficiency of ion transfer to the surface Ni(OH)2 layer. Third, Ni(OH)2 and NiOOH, which have similar crystal structures but different b‐axis lattice parameters, are formed during electrochromic cycling, and large volume changes in the unit cell reduce the structural stability of the thin film. Finally, we clarified the mechanism of electrochromic performance degradation of NiO films in alkaline aqueous electrolytes and provide a route to activation of NiO films, which will promote the development of electrochromic technology.


Introduction
As intelligent materials, electrochromic materials exhibit stable and reversible changes in their optical state upon the application of a low external direct current (DC) voltage.8] For example, the electrochromic in the cathodic electrochromic material WO 3 [9,10] and intermediate electrochromic oxide V 2 O 5 [11] are caused by changes in the oxidation state of the central metal ions upon the implantation of ions and electrons and application of a voltage. [12,13]As a typical anode electrochromic film, NiO is often combined with WO 3 and other cathodic electrochromic films to form electrochromic devices, which is of great significance to promote the engineering application of electrochromic materials.In contrast to those of WO 3 and V 2 O 5 , the electrochromic mechanism of NiO is relatively complex and remains under debate.
Depending on the electrolyte, cation intercalation and the formation of surface hydroxide are widely considered to be two different discoloration mechanisms. [13,14][17] However, NiO crystals have a facecentered cubic structure, and the close-packed crystal structure has only a narrow channel for Li-ion diffusion, resulting in low ion-transport efficiency and slow color change.Wen et al. [18] compared the electrochromic performance of NiO in KOH and LiClO 4 /propylene carbonate (PC) electrolyte and found that the electrochromic performance of NiO in the alkaline electrolyte is significantly better than that in LiClO 4 /PC.Therefore, the study of the electrochromic mechanism of NiO films in alkaline electrolytes is required.
The formation of hydroxides on the surface of NiO films is the key driver of the electrochromic mechanism in alkaline electrolytes. [19]owever, in alkaline electrolytes such as KOH and LiOH, the electrochromic process may also involve the formation of bivalent NiO, Ni (OH) 2 , trivalent Ni 2 O 3 and NiOOH, and tetravalent NiO 2 .Currently, there are two different theories for this behavior: one theory is that OH À ions in the solution participate in the reaction, another explanation is that the electrochromism of NiO under alkaline conditions is a proton reaction mechanism. [20]The hydroxide generated on the surface of NiO is very thin, has a complex structure and composition, and has poor stability, which makes the experimental research challenging.[23] For example, Rougier et al. studied the mechanism of the electrochromic degradation of NiO and reported that NiO was spontaneously transformed into Ni(OH) 2 upon immersion in an alkaline solution. [19]Subsequently, the Ni(OH) 2 formed on the surface undergoes reversible oxidation upon the application of an electric field, thus generating black NiOOH.But, the formed NiOOH is not stable, and self-discharge occurs easily, resulting in the peeling of the film upon conversion to Ni(OH) 2 .Consequently, on repeated redox reaction, the NiO film degrades completely.However, there remains speculation as to whether this is the correct mechanism. [24]n this study, we combined with the density functional theory (DFT) calculations and experimental results, discussed, and proposed the following electrochromic process and performance degradation mechanism of NiO in alkaline electrolyte: First, the NiO film undergoes oxidation as a result of the applied electric field, generating NiOOH.Subsequently, on voltage reversal, NiOOH is converted to NiO.It is noteworthy that the opaque NiOOH is easily reduced to transparent Ni (OH) 2 due to the embedding of protons.On electrochromic cycling, the generated Ni(OH) 2 is deposited on the surface of the NiO film.The amount of Ni(OH) 2 increases gradually until it coats the NiO film, forming a NiO@Ni(OH) 2 core-shell structure.Thus, the current must pass from the conductive substrate through the internal NiO film to reach the surface Ni(OH) 2 , and the conduction mechanism changes from firstto second-order conduction.This results in a longer electron conduction pathway to the surface Ni(OH) 2 and slower ion transport.At the same time, the surface Ni(OH) 2 prevents contact between the internal NiO and the electrolyte, thus making the coloring and bleaching processes more challenging.Considering the structural similarity of Ni(OH) 2 and NiOOH, the key to the color change mechanism is the removal and insertion of H + ions.However, the interconversion of Ni(OH) 2 and NiOOH causes a change in the b-axis lattice constant of 10%, thereby resulting in structural instability, loss of active materials during electrochromic cycling, and performance degradation.These three processes together deteriorate the electrochromic properties of NiO films.Our findings are expected to aid in the production of new color change materials, enhancement of the electrochromic properties of NiO thin films, and development of electrochromic technology.The most stable state of NiO mp-19009 (Figure 1a) is a halite/rock salt structure in the cubic space group Fm3m.In this structure, Ni 2+ is bonded to six equivalent O 2À atoms to form NiO 6 octahedra, and the neighboring octahedra share corners and edges.The next most stable configuration is that of Ni(OH) 2 (mp-27912, Figure 1b), which is in the P3m1 space group, and has a two-dimensional structure.Specifically, edge-sharing NiO 6 octahedra form Ni-O layers, in which H atoms bond with O and lie in the gaps between the neighboring Ni-O layers.The other configurations of Ni(OH) 2 (mp-1275121 (Figure 1c), mp-1180084 (Figure 1d), mp-625074 (Figure 1e), mp-1274857 (Figure 1f), mp-626794 (Figure 1g), and mp-626843 (Figure 1h)) share the main structural characteristics of Ni(OH) 2 (mp-27912), and the main differences are the degree of slippage and the distance between neighboring Ni-O layers.For NiOOH mp-1067482 (Figure 1i) and mp-999337 (Figure 1j), the structures are still twodimensional, but there is only one H per oxygen in the interlayer space and the degrees of slippage between layers differ.All configuration information is listed in Table 1.

Results and Discussion
Based on the energetic information in the MP database, we assessed the possible phases that could be formed at room temperature during electrochromic cycling.To achieve this, we used the energy above hull (the energy surface of the equilibrium phases).Note that a theoretical phase that is metastable at 0 K could be prepared at room temperature if the energy above the hull is <0.03 eV atom À1 .For Ni(OH)  2b).However, the formation energy for NiOOH mp-999337 is large (0.088 eV atom À1 ), as shown in Figure 2c.Because this is larger than the 0.03 eV atom À1 criterion, the mp-999337 phase is unlikely to be stable at room temperature.In contrast, for NiOOH mp-1067482, the formation energy is 0.03 eV atom À1 , suggesting that it could be stable at room temperature.In summary, the possible phases of Ni(OH) 2 cannot be distinguished based on the formation energies, but the only stable phase of NiOOH is mp-1067482.The associated formation energies and reference states are listed in Table 2.
Figure 3 shows the band structures of the selected compounds.As shown, the band gap of NiO is 3.3 eV (Figure 3a), which is consistent with the experimental value of 3.11 eV. [25]The wide band gap is consistent with the observed transparency of NiO films.The calculated band gaps can be used to make the following conclusions: there are two ranges of band gaps for Ni(OH) 2 : from 3.25 to 3.33 and 2.01 to 2.06 eV, and this hydrolysis product of NiO is, thus, transparent.The theoretical calculations, however, reveal narrow band gaps (from 2.01 to 2.06 eV) for mp-1274857, mp-626794, and mp-626843, which suggest colored films.Therefore, these phases are not consistent with the observed transparent phase.As a result, there are four possible phases of Ni(OH) 2 that are consistent with the experimental and theoretical results and have similar band gaps to NiO: mp-27912, mp-1275121, mp-1180084, and mp-625074.Considering their similar energies and structural characteristics, these phases could all be stable at room temperature.Furthermore, a change in the relative slip between the layers in each layer may result in interphase transformations.For Energy Environ.Mater.2024, 7, e12652 NiOOH mp-1067482, the calculated band gap is only 0.52 eV, suggesting enhanced light absorption.
Based on the theoretical findings, a possible discolouration mechanism of NiO is proposed (Figure 4 Next, the electrochromic mechanism and reason for the performance decay of NiO thin films on electrochromic cycling were studied based on electrochemical experiments of the film in different color states, as well as the theoretical results.As discussed in Preparation of the FTO electrode coated with NiO nanosheets, a thin film of NiO nanosheets was grown on a FTO-coated glass substrate (Figure 5a 1 ) using a hydrothermal method.In its initial state, the film shows good light transmittance (Figure 5a 2 ).Furthermore, when the film was immersed in KOH electrolyte, the transmittance of the film did not change significantly (Figure 5a 3 ).However, after applying a direct current at +1 V, the film quickly became black (Figure 5a 4 ).In contrast, when applying a potential of À1 V, the film rapidly became transparent (Figure 5a 5 ).
SEM micrographs of the NiO films in different states are shown in Figure S1, Supporting Information.Figure S1a, Supporting Information shows the micromorphology of the sample in the initial state.As shown, the film is composed on thin nanosheets, and there are large gaps between the nanosheets, which is conducive for contact between the film and electrolyte.The micromorphologies of the colored and bleached states are shown in Figure S1b,c, Supporting Information, and they slightly differ from that of the initial state.After 8000 electrochromic cycles, the nanosheet structure of the initial state is maintained, although there are obvious signs of other materials attached to the surface of the nanosheets (Figure S1d, Supporting Information).This is consistent with the formation and deposition of Ni(OH) 2 during the color change reaction.As discussed, the large changes in the lattice parameters reduce structural stability, and, after thousands of electrochromic cycles, the NiO film has peeled away from the electrode surface (see Figure S1e, Supporting Information for digital photographs of the film after 8000 cycles).
Next, XRD measurements were used to assess the crystal structures of the films in different states, as shown in Figure 5b.In the XRD patterns of the initial, colored, and bleached states, as well as after 8000 cycles, reflections at 37.3°, 43.3°, 62.9°, 75.2°, and 79.5°were observed, and these correspond to the (111), ( 200), ( 220), (311), and (222), respectively, crystal planes of cubic NiO.The intensities of the reflections also suggest preferential growth along the (200) crystal plane (JCPDS No. 47-1049). [25,26]After 8000 cycles, the intensities of the peaks corresponding to NiO decrease significantly, and the FWHM increases.On the basis of our theoretical calculations, NiO is converted to NiOOH in the colored state and then to a mixture of NiO and Ni (OH) 2 in the bleached state.However, reflections corresponding to NiOOH and Ni(OH) 2 were not detected in the XRD patterns, possibly because these phases are amorphous or the films are thin and the content of these phases is low, rendering them invisible in the XRD patterns.
Next, XPS measurements were used to assess the states of the elements in the films.All spectra were calibrated against the C 1s peak at a binding energy of 284.8 eV.Because we are interested in the surface states only, no etching was carried out.In the obtained spectra, the Ni 2p 1/2 and Ni 2p 3/2 peaks were symmetrical, and only the Ni 2p 3/2 peak was analyzed. [27]The high-resolution Ni 2p 3/2 and O 1s spectra of the film in different states are shown in Figure 5c,d.The peak deconvolution of the Ni 2p 3/2 peaks yielded four peaks, and those at 854.1 and 855.9 eV correspond to Ni-O and Ni-OH, respectively, and the remaining two peaks are satellites.The O 1s peak was also fit into four components, and those at 529.34 and 531.07 eV correspond to lattice oxygen (Ni-O and Ni-OH, respectively), whereas the remaining two peaks correspond to adsorbed oxygen species. [28,29]he Ni 2p 3/2 and O 1s peaks of the film in the initial state are consistent with the reported data for NiO.In contrast, in the spectrum of the colored state, there are significant changes in the Ni 2p 3/2 and O 1s, and the Ni-OH peak becomes dominant, whereas the intensity of the Ni-O peak is significantly reduced, indicating a reduction in the amount of NiO in the film.However, it was not possible to determine if the observed peaks correspond to NiOOH or Ni(OH) 2 because of the presence of Ni-OH bonds in both species.Nevertheless, the results of our DFT calculations and the observed film color suggest that the film is NiOOH.In the spectrum of the bleached state, the intensity of the peak corresponding to Ni-O peak increased, but that of the Ni-OH peak increased significantly.Again, based on the observed colors and results of our calculations, the film is likely composed of Ni(OH) 2 and NiO.In the spectrum of the film after 8000 electrochromic cycles in the bleached state, the intensity of the peak corresponding to Ni-O peak is significantly reduced, suggesting that the NiO content is reduced and the film contains a significant amount of nickel   In addition, Raman measurements of the film in different states were carried out (Figure 5e).The Raman spectra of the film in the initial, colored, and bleached states contain peaks at 501, 480 and 560, and 391 and 454 cm À1 , which correspond to NiO, NiOOH, and Ni(OH) 2 , respectively.Furthermore, in the spectrum of the bleached state, the NiO peak at 501 cm À1 is dominant, again suggesting the presence of a mixture of NiO and Ni(OH) 2 .32] Based on the Raman and XPS results, it is proved that the electrochromic mechanism of NiO in alkaline electrolyte proposed by our  theoretical analysis above is reasonable.The electrochromic process of NiO film in alkaline electrolyte can be defined as follows: First, the initial NiO film undergoes oxidation as a result of the applied electric field, generating NiOOH.Subsequently, on voltage reversal, NiOOH is converted to NiO.Notably, NiOOH is easily reduced as a result of the embedding of protons, resulting in a change from the opaque NiOOH to transparent Ni(OH) 2 .On electrochromic cycling, the generated Ni (OH) 2 is deposited on the surface of the NiO film; then, the coloring and bleaching process becomes dependent on the transformation between NiO, Ni(OH) 2 and NiOOH as shown in the following equations: After thousands of electrochromic cycles, the continuously generated Ni(OH) 2 is deposited on the surface to yield a core-shell structure, denoted NiO@Ni(OH) 2 .A schematic of this mechanism, which differs from those reported previously, is shown in Figure 6.
The electrochromic and performance decay mechanisms of the NiO thin films were further investigated using CV measurements.The first 20 CV cycles are shown in Figure 7a.During the potential scan from À1.2 to +1.2 V, the NiO film is oxidized, yielding two clear oxidation peaks, labeled A-I and A-II, at 0 and +0.8 V, respectively.In contrast, during the reverse scan, the NiO film is reduced, yielding a clear reduction peak near 0 V, and this peak shifted from 0 to À0.3 V with increase in cycle number.The difference in the potential between the oxidation and reduction peak is small, about 1.1 V, and the current density corresponding to these peaks is about 6 mA cm À2 , indicating the improved electrochemical stability of the NiO film.Furthermore, with increase in the number of cycles, the electrochemical activity of the film increased, and the area of the CV curve gradually increased.In particular, during the 1st CV cycle, there was only one oxidation peak (A-I).However, from the 5th to the 20th CV cycle, two oxidation peaks (A-I and A-II) were observed.This is because during the first CV cycle, the film only contained NiO, which gives rise to the A-I peak.However, after several cycles, NiOOH was reduced and Ni(OH) 2 was generated, resulting in the appearance of the second oxidation peak (A-II).In summary, we believe that A-I can be assigned to the oxidation of NiO, whereas A-II can be attributed to the oxidation of Ni(OH) 2 .In contrast, there is only one reduction peak, which corresponds to the reduction of NiOOH. Figure 7b shows the CV curve of the NiO film during cycling, showing the performance decay.With increase in the number of cycles, the CV curve area first increases and then decreases, indicating that ion transport in the film first increases and then decreases.After 2000 cycles, the reduction peak potential shifted to around À0.5 V, and the potential difference between the oxidation and reduction peaks increased, indicating that the stability of the film had deteriorated.In addition, the intensity of the A-I peak decreased.After 4000 cycles, the A-I peak disappeared, leaving only the A-II peak, indicating that, after thousands of cycles, the film coloring process changed from a two-step to a one-step process.Next, the ion-transport properties of the NiO film were characterized using EIS measurements.
The EIS curve of the NiO film from the initial state to performance decay is shown in Figure 7c.Obviously, with the increase in the Energy Environ.Mater.2024, 7, e12652 number of electrochromic cycles, the semicircle diameter in the lowfrequency region gradually increases, indicating that the ion-transport performance of the film gradually decreases, and the charge transfer resistance gradually increases. [33]In addition, after the number of cycles exceeds 6000, two semicircles appear in the low-frequency region, indicating that the charge transfer of the film has changed from firstorder to second-order conduction and that the film has a new interface layer.This result is consistent with the results of XPS and Raman Energy Environ.Mater.2024, 7, e12652 spectroscopy measurements that indicate the deposition of Ni(OH) 2 on the film surface and the formation of a core-shell NiO@Ni(OH) 2 structure.As a result, after 6000 cycles, the charge is transferred from the FTO conductive substrate to the Ni(OH) 2 layer on the surface through the internal NiO layer.This two-step conduction results in a reduction in the electrochemical properties of the film.
Next, we combined electrochemical and spectrophotometric measurements to measure the visible light transmittance of the films in situ during voltage cycling.For these measurements, the transmittance was measured at  650 nm during 20 electrochromic cycles (Figure 7d).The light transmittance of the film after bleaching and coloring are 80% and 5%, respectively; thus, the light modulation is 75%.In the 1st, 10th, and 20th cycles, the coloring and bleaching times were 4 and 2, 5 and 3, and 7 and 4 s, respectively.Therefore, with increase in the number of cycles, the times required for both processes increased.Further analysis of the coloring/bleaching process revealed that after the 10th cycle, the process involved two stages in both coloring (t c ) and bleaching (t b ), each stage is labeled as t c -I, t c -II and t b -I, t b -II, respectively.As the cycle number increased, t c -I showed little change, whereas t c -II increased.Therefore, t c -II is the longest process during coloring.In contrast, t b -I and t b -II account for half of the bleaching time.Therefore, to improve performance, the speed of the t c -II process should be increased.The change in the transmittance from the initial state to performance decay is shown in Figure 7e.As shown, the coloring time, which corresponds to t c -II, first increased and then decreased, and the bleaching process, which corresponds to t b -II, continuously increased.In Figure 7f, the light modulation range is highlighted.As shown, after 6000 cycles, the electrochromic performance of the film decreases rapidly, consistent with the EIS data and electrochromic response curves.After 8000 cycles, the maximum transmittance is only 50%.
In this study, we discuss the electrochemical performance decay of the film over several thousands of electrochromic cycles.In addition, to investigate the performance decay further, the effect of a stepped voltage on the light transmittance was investigated.In these experiments, a potential of +1 V was applied for 10 s and, then, bleaching for 20 s at potentials of À1.2, À1.4,À1.6, À1.8, and À2.0 V was applied (Figure 7g).Notably, the bleaching speed at À2 V was significantly faster than that at À1 V: 5.5 and 20 s, respectively.As shown, the coloring and bleaching processes still involve two stages.The coloring time is stable at 5.5 s, but the bleaching rate of the film changes significantly.Thus, after thousands of cycles, the film does not lose its electrochromic function, but the bleaching process requires a higher voltage and greater kinetic energy for ion transport.This result shows that the fundamental cause of film performance degradation is the increase in the iontransport resistance.This result is also consistent with the formation of a core-shell structure because charge must be transferred from the conductive FTO substrate to the Ni(OH) 2 layer on the surface through the internal NiO layer.Thus, the application of higher voltage enables the ions to be transferred to the Ni(OH) 2 layer on the surface rapidly.

Conclusions
In this study, the phase structure and electrochemical characteristics of NiO thin films in different states were studied by combining theoretical calculations and experimental, and the most likely mechanism for the electrochromic performance decay of NiO thin films was identified.Our results indicated that, the NiO film undergoes oxidation reaction as a result of the applied electric field, generating NiOOH and coloring.Subsequently, on voltage reversal, NiOOH is converted to NiO.It is noteworthy that the opaque NiOOH is easily reduced to transparent Ni(OH) 2 due to the embedding of protons.Both NiO and Ni(OH) 2 are transparent.With the number of electrochromic cycles increases, Ni(OH) 2 is deposited on the surface of the NiO film, forming a core-shell NiO@Ni(OH) 2 structure, resulting in the ion-transport change from firstto second-order conduction and an increase in the ion-transport distance.In addition, the surface Ni(OH) 2 layer prevents contact between the internal NiO and the electrolyte, making the coloring and bleaching processes more difficult.The inter conversion of the Ni(OH) 2 and NiOOH phases results in large changes in the baxis lattice parameter, thus resulting in structural instability and the structural slip or separation of active substances from the electrode during operation.Overall, we clarified the electrochromic and performance decay mechanisms of NiO thin films in alkaline aqueous electrolytes and provided a theoretical basis for the development of new color change materials and enhancement of the electrochromic performance of NiO thin films.Furthermore, we expect our findings to promote the development of electrochromic technology.
Preparation of the FTO electrode coated with NiO nanosheets: First, the FTO glass (2 cm × 5 cm) was soaked in a 5% NaOH-hydrogen peroxide (20:1) mixture for 5 h.Thereafter, the glass was cleaned ultrasonically in acetone and echanol alcohol successively for 30 min.Subsequently, 0.125 M nickel nitrate hexahydrate and HMT were dissolved in 50 mL of deionized water and magnetically stirred for 30 min to ensure complete dissolution.
Then, the cleaned FTO glass was placed at an oblique angle in a hightemperature reactor with the conductive layer faces downward, and the above solution was poured into a polytetrafluoroethylene (PTFE) lining.The reactor was then sealed and placed in an oil bath for reaction at 120 °C for 4 h.After reaction, the FTO glass was removed and heat treatment at 350 °C for 3 h in an Ar atmosphere was carried out to obtain the NiO films.
Characterization: The microstructures of the samples were characterized by field-emission scanning electron microscopy (SEM, ZEISS Sigma 300) and highresolution transmission electron microscopy (HR-TEM, JEOL JEM-2010F).The crystal phases in the samples were analyzed using X-ray diffractometry (XRD, Bruker D8) with Cu-K α irradiation generated at 40 kV and 40 mA between 20°and 90°i n 2θ in a step size of 2°min À1 .The structure and phase composition of the films were determined by comparison with standard PDF card data.
X-ray photoelectron spectroscopy (XPS) (ThermoFisher; ESCALAB 250Xi) was used to characterize the elemental content and valence states of films.During the XPS measurements, the analysis chamber was maintained under a vacuum of 4 × 10 À9 mbar, the excitation source was Al-K α X-rays (hν = 1486.6eV), the working voltage was 14.6 kV, the filament current was 13.5 mA, and signal accumulation was carried out for 20 cycles.The pass energy was 20 eV, and the step size was 0.1 eV.All peaks were normalized to the C 1s peak at a binding energy of 284.8 eV.
To analyze the chemical state of the samples, the XPS data were fitted with a Gauss-Lorentz function in CasaXPS.Peak fitting was carried out after the spectra had been processed using Shirley background removal.In addition, the binding energy separation and the full width at half maximum (FWHM) values (Ni 2p 3/2 : 1.36 eV) were fixed.
The surface composition of the film was analyzed by Raman spectroscopy (HORIBA Scientific LabRAM HR Evolution).The laser wavelength is 532 nm.
Electrochemical measurements: The electrochemical properties of the samples were tested at room temperature using an electrochemical workstation (CS 310).The electrolyte was 1 M aqueous KOH, and the working, counter, and reference electrodes were the NiO film sample, graphite, and Ag/AgCl, respectively.
The cyclic voltammetry (CV) data were obtained at AE1.2 V at a rate of 100 mV s À1 .The electrochemical impedance spectroscopy (EIS) measurements of the samples were carried out within the frequency range of 0.01-100 kHz at an applied voltage of +0.5 V.
Energy Environ.Mater.2024, 7, e12652 The transmittance between 350 and 850 nm, number of electrochromic cycles, and color change response time (at 650 nm) of the thin film samples were measured in situ using the electrochemical workstation combined with an ultraviolet-visible spectrophotometer (Hitachi UH 4150).The transmittance of the film was measured after coloring and fading at +1 or À1 V, and the response time was tested with a pace voltage of AE1 V sustained for 10 s.In contrast, the interval was 5 s when testing the number of electrochromic cycles.
[39] Structure-energy calculations were carried out using the Perdew-Burke-Ernzerhof (PBE) functional for exchange correlation (XC). [40,41]For geometric relaxation of the structures, summation over the Brillouin zone (BZ) was performed with Monkhorst-Pack kpoint intervals limited below 0.04-1 Å. [42][43][44] A plane-wave energy cut-off of 600 eV was used in all calculations. [45,46]For the electronic structure calculations, we use the GGA + U approach of Dudarev et al. [47] to treat the 3d electrons of Ni with the effective Hubbard on-site Coulomb interaction parameter (U 0 = U À J).We choose U 0 = 6.2 eV based on a recent proposal. [35]For the energy band structure calculations, we used the HSE06 functional, which considers non-local contributions to the XC. [48]We employed a convergence criterion of 10 À6 eV in the self-consistent field cycles, [49] and spin polarization was included.The known structures for NiO, Ni(OH) 2 , and NiOOH (e.g., NiO mp-19009, Ni(OH) 2 mp-27912, and NiOOH mp-1067482, where the number following mp-is the id number of the structure in the MP database) were obtained from the Materials Project (MP) database.

First, we assessed
the potential phases involved in the electrochromic process based on DFT calculations of the electronic structures of NiO, Ni(OH) 2 , and NiOOH.Structures of selected phases are shown in Figure 1.
): First, NiO (transparent, band gap 3.3 eV) is transformed into NiOOH (opaque, colored) after absorbing OH À on the application of a voltage.On the application of the inverse voltage, in addition to the partial reduction of NiOOH to NiO, NiOOH can be easily reduced by the embedding of H + ions, resulting in a change from NiOOH (opaque, colored state) to Ni(OH) 2 (transparent state).As the number of electrochromic cycles increases, the generated Ni(OH) 2 is deposited on the surface of the film, and the electrochromic process becomes a coloring and bleaching process involving NiO and Ni(OH) 2 .Considering the structural similarity of Ni(OH) 2 and NiOOH, the key to the color change is the removal and insertion of H + .However, as mentioned previously, the b-axis lattice constant changes by 10% during the transformation of Ni(OH) 2 and NiOOH, which results in structural instability, resulting in film separation and performance degradation (the POSCAR files of NiOOH with ID of mp1067482 and Ni(OH) 2 with ID of mp27912 have been provided in the Supporting Information).

Figure 2 .
Figure 2. a) Energy above hull of Ni(OH) 2 showing the equilibrium phases (NiO and H 2 O) as references.b) Phase diagram of NiOOH with equilibrium phases (Ni 3 O 4 , O 2 and H 2 O) as references.c) Formation energy of NiOOH with respect to equilibrium phases.
hydroxides.In order to study the change of valence information on the surface and inside of the film after 8000 electrochromic cycles, the surface material was removed by argon ion etching, and then XPS test was performed again, the results are shown in FigureS2, Supporting Information.The peaks of Ni 2p 3/2 and O 1s are obviously different from those before etching (Figure 5c,d, bleached state 8000 cycles), but they show peaks consistent with the initial bleached state, which are mainly characterized by Ni-OH characteristic peaks before etching, and the content of Ni-O characteristic peaks increases significantly after etching.This result shows that a thin layer of hydroxide was deposited on the surface of the film, but a large amount of NiO remained in the film.After etching, Ni 2p 3/2 has a characteristic peak of pure nickel (at 852.2 eV), which is due to the destruction of Ni-O bond during argon ion etching and the reduction of part of NiO to pure nickel.

Figure 5 .
Figure 5. Digital photos (a) (a 1 : FTO conductive substrate, a 2 : initial state of NiO film, a 3 : NiO film in KOH electrolyte, a 4 : colored state, and a 5 : bleached state), X-ray diffraction patterns (b), Ni 2p 3/2 (c) and O 1s (d) X-ray photoelectron spectra and Raman spectra (e) of the film in different states.

Figure 6 .
Figure 6.Schematic of the mechanism of electrochromic performance degradation of NiO thin films in alkaline electrolyte.

Figure 7 .
Figure 7. a) First 20 CV curves of the NiO film.b) CV and c) EIS curves of the NiO film from the initial state to performance decay.d) In situ transmittance changes of the film in the 1st, 10th, and 20th cycles.e) In situ transmittance changes and f) electrochromic cycle stability from initial state showing performance decay.g) Change in transmittance during voltage cycling.

Table 1 .
Crystal structure information of NiO, NiOOH, and Ni(OH) 2 obtained from the MP structural library.

Table 2 .
Equilibrium phases and formation energies of NiO, NiOOH, and Ni (OH) 2 from the MP structural library.