Optimized V‐Doped Defective TiO2/α‐(Fe2O3)1‐x(Cr2O3)x Heterojunctions for Photo‐Assisted Supercapacitor Devices: Insights on the Materials Integrity and Dual Conversion‐Storage Mechanism

Energy conversion and storage integrated power units suffer from multiple engineering issues. Replacing two devices (solar cell and supercapacitor assembly) with one device (photo‐enhanced supercapacitor‐ PSC) requires materials with emerged dual solar‐electrochemical storage attributes. Herein, a propitious approach is developed to fabricate all visible light‐enhanced semisolid flexible PCS. Nanoflakes‐based p‐n junction α‐(Fe2O3)1‐x(Cr2O3)x photocathode is synthesized directly on industrial waste stainless steel mesh (316L‐SS). Alongside, three TiO2‐based electrodes are utilized as positive photoactive electrodes. Tuning the optical properties of TiO2 is displayed via doping with mixed valence vanadium (V4+/V5+) together with thermal hydrogen annealing. This is revealed via the reduction of the bandgap energy from 2.89 to 2.15 eV, which can be ascribed to the induced oxygen vacancies. The device can sustain up to 1.6 V potential window with 91% stability after 350 successive charge/discharge cycles with the possibility of performance regeneration to its 100% retention. An illustration of the photo‐storage mechanism is proposed based on the X–ray photoelectron spectroscopy, X–ray crystallography, and band position/alignment results. Quasi‐reversible water splitting/formation is concluded as the main storage mechanism in the semisolid state electrolyte under illumination conditions.


Introduction
3] Extrapolation of this notion emerges in myriad state-of-the-art storage devices to eliminate the constraints of fossil fuel utilization. [3,4][6] Although photo-rechargeable supercapacitors (solar cells coupled with supercapacitors) can retain a decent energy density with relatively good conversion efficiency, these units suffer from multiple engineering issues that hinder their practical scaling up. [7,8]Functional photo-assisted supercapacitors (PSCs) are simplydesigned state-of-the-art alternative devices with few recent studies reported, and various milestones are still ahead.Optically-active material with appropriate storage features can be deployed, thereby storage capacity is boosted intrinsically upon illumination. [9,10]Fabricating the appropriate material with emerged dual solar-electrochemical storage attributes is quite challenging. [4,11,12]Enhancing the conductivity of a semiconductor material without scarfing its favorable optical properties requires smart material engineering.Additionally, understanding the light impact on the structure-activity relationship is still overlooked and yet not well documented. [12]ltaf et al. and Buldu-Akturk et al. have attempted to develop UV-assisted photo-SCs based on ZnO and its graphene composites.[15] Nevertheless, it seems like an uphill struggle to fabricate cheap photo-capacitive material that absorbs in the visible region of the light spectrum, which represents ≈43.9% of the solar spectrum energy. [12][18] Those low bandgap materials would exhibit the good conductivity that is required to ensure good supercapacitive performance. [3,19]On the other hand, wide bandgap semiconductor materials, such as TiO 2 , with salient performance in photo-based applications suffer from low conductivity with weak energy storage capabilities. [20,21]To mitigate this issue, alloying/doping strategies are shown to boost the material's conductivity. [3,19,22]Similarly, defect engineering is a promising protocol to tune the material's intrinsic electronic conductivity and tailor its electronic band structure. [19,23,24]erein, stainless steel and black titanium meshes are used as prevalent substrates that are conductive and would effectively work in alkaline media with high corrosion resistance. [25]According to recent reports, creating an intrinsic nanoporous texture for a pure or even commercial SS electrodes could be realized by an electrochemical anodization protocol, followed by thermal annealing. [17,25,26]To the best of our knowledge, the possibility of SS nano-structuring at ambient temperature is yet to be reported.In the current work, a dry etching approach followed by simple electrochemical activation (ECA) under illumination is demonstrated for nano-structuring stainless steel at ambient temperature.The resulting material (LA SS) is used as the photocathode in the assembled PSC device.As for the photoanode, three electrodes were tested in assembled devices; namely P 25 (mixed anatase and rutile phases), HP 25 thermal hydrogen annealed P 25, and VHP 25 (defect-engineered Vdoped TiO 2 -based composite).Multiple synergistic effects were observed upon the combined thermal hydrogen annealing and alloying with VO x .The VHP 25 composite was able to harvest light in the visible/UV regions with superb super-capacitive performance.The photo-enhanced performance of the fabricated electrodes is demonstrated by conducting cyclic voltammetry (CV) and galvanic charge-discharge (GCD) measurements on the assembled devices in the dark and under illumination.The highest performance device (LA SS // VHP 25) revealed an areal energy of ≈2.3 μWh cm −2 at an areal power of 1.28 mW cm −2 .Thanks to the flexible SS and Ti mesh substrates, the device sustained bending tests while retaining 100% of its capacitance.Besides, the device maintains 91% of its initial capacitance over 350 successive galvanic charge/discharge (GCDs) cycles, with the possibility of 100% regeneration performance upon rewetting the device.

Morphological, Compositional, and Surface Analyses
Figure 1a demonstrates the smooth texture of pristine stainless steel (SS) mesh.Instead of the usually employed electrochemical etching of the SS mesh in highly concentrated ammonium florid electrolytes, an electroless dry-etching process is demonstrated utilizing sulfuric acid-based electrolyte gel. [17,25]he hydrogel promotes the dissolution of Fe, Cr, and Ni from the mesh's surface and preserves a highly nanoporous structure, as shown in Figure 1b,c.The etched stainless steel (E-SS) shows uniformly engraved surfaces at high and low magnifications.To unveil the impact of light /dark electrochemical activation on the E-SS morphology, SEM images of the dark-activated (DA SS) and light-activated (LA SS) functional electrodes are shown in Figure 1d-g.A nano-flakes heterostructure of probable Fe-Cr oxides/hydroxides is generated upon the electrochemical activation (ECA) process.A layer thickness of ≈1 μm is revealed from the side-view SEM images in Figure S5a,b (Supporting Information).Although LA SS and DA SS exhibit similar morphology, thinner flakes were observed under illuminated ECA conditions.It is suggested that the light generated electrons and holes affected the kinetics of the redox electrochemical reactions that occur during the ECA. [27]It is supposed that this has positively impacted the resulting morphology of the nano-flakes heterostructure.Thin flakes of ≈4-7 nm would ideally mitigate the issue of the short-diffusion length of the iron-based photocatalyst. [28]In other words, reducing the nano-flakes' thickness would delay the rate of charge carriers recombination and would indeed improve the photon-to-current efficiency. [20]Thus, the LA SS is used as a photocathode in an all-visible light-assisted supercapacitor.
As for the photoanode, Figure 1h implies the nano-spherical morphology of the P 25.Notably, the hydrogen annealing step depicts no rupture on the intact P 25 morphology, see Figure S6a (Supporting Information).Figure 1i-k reveals the multiwalled carbon-vanadium oxide framework.Further information on VO x nanotubular structure was obtained from the TEM imaging, Figure S6b,c (Supporting Information).It is confirmed that the average inner and outer diameters are ≈20 and 60 nm, respectively.Note that up to ten walls of ≈40 nm each are observed, varying in length from 0.5 to 2.5 μm.This unique morphology would provide myriad benefits, such as superlative specific surface area. [29]Close inspection at the BET isotherm, Figure S7a (Supporting Information), demonstrates a BET surface area of 247 m 2 g, which would be rarely obtained by metal oxides.31] It is speculated from previous reports that the nanotubular carbon framework with titania can ideally hinder electron/hole recombination. [32]Figure 1l shows the VHP 25 composite and confirms its formation.
As the surface composition has a pivotal role in the photostorage performance, XPS analysis was conducted for the LA SS as well as the VHP 25 samples.The XPS survey of the LA SS sample, Figure 2a, discloses the presence of Fe, Cr, and O assigned at 713.0, 578.3, and 532.7 eV, respectively. [33,34]No peaks were observed for Ni, which illustrates the impact of both chemical etching and ECA processes on the complete dealloying of Ni from the surface of the LA SS.That is consistent with ICP measurements, where a considerable concentration of Ni was detected, as indicated for the E-SS and LA SS samples in Table S3 (Supporting Information).According to the XPS compositional atomic quantification, the Fe:Cr ratio is about 1.08.Figure 2b displays the Fe 2p spectrum, revealing two peaks, namely Fe,2p 3/2 and Fe2p 1/2 , centered at 711.5 and 725.4 eV, respectively. [33,34]The spin orbit splitting of ≈13.8 eV, which deviated from other experimental data probed for pure iron oxide, suggests clear changes in the electronic structure. [34]As for Cr 2p spectrum, Figure 2c, four peaks at 577.6, 580.8, 587.0, and 587.9 are demonstrated for 2p 3/2 and 2p 1/2 of Cr 3+ and Cr 6+ , respectively.Similarly, a chemical shift of ≈ 1 eV was observed compared to previously reported data for Cr 2 O 3 and adsorbed CrO 4 −2 . [35]This emphasizes that Fe and Cr are likely to exist as (Fe 2 O 3 ) 1-x (Cr 2 O 3 ) x .The O 1s spectrum was fitted into three peaks, particularly located at 531.1, 532.0, and 533.4,38] On the other side, the survey spectra of VO x and VHP 25 are demonstrated in Figure 2a.The titanium 2p, vanadium 2p, oxygen 1s, carbon 1s, and nitrogen 2s spectra are observed at 459.2, 517.1, 530.5, 285.4,and 400.6 eV, respectively. [29]In Figure 2e, titanium is identified in two oxidation states, namely Ti 4+ and Ti 3+ .The stoichiometric TiO 2 peak (p 3/2 ) is shown at 458.3 eV, while that of Ti 3+ is observed at 457.6 eV. [39,40]Notably, the calculated splitting between the 2p 1/2 and 2p 3/2 spin−orbit coupled state is about 5.7 eV. [40]As for Figure 2f, the O 1s spectrum of the VHP 25 sample shows a metal-oxygen bond within the oxide lattice structure at ≈529.5 eV. [41]Another deconvoluted peak is shown at 531.5 eV, confirming the existence of oxygen-deficient oxide (TiO 2-x ). [39]Pointedly, H 2 thermal treatment induces oxygen vacancies within the oxide lattice. [19,39]In the chemical sense, the presence of oxygen vacancies and reduced species Ti 3+ will positively impact the conductivity of the semiconductor functional material via the formation of donor states. [19,39]Besides, such material engineering provides excess active sites to execute supplement electrochemical redox reactions. [19,39]The deconvolution of the vanadium 2p in the composite, Figure 2g, demonstrates a mixture of V 4+ and V 5+ . [29]Further information on C1s and N1s deconvolution is depicted in Figure S8a,b (Supporting Information).Moreover, V2p and O1s spectra of VO x are shown in Figure S8c (Supporting Information).

Optical Properties
To unveil the interplay between the impact of ECA, H 2 thermal annealing, composition, and the photo-response of the material, the diffuse reflectance spectra (DRS) of the electrodes were probed and analyzed.Figure 3c,e depicts the UV-vis absorption spectra in the range of 250-800 nm for all samples.In particular, the LA SS exhibits two extended-absorption edges with onsets at ≈460 and 770 nm.Compared to the DA SS and SS electrodes, a blue shift in their onset wavelengths was observed (starts at ≈655 and 544 nm, respectively), as shown in Figure 3c.Such prominent optical activity of LA SS in the visible wavelengths can be ascribed to the formed heterostructure upon ECA under illumination. [43,49]To well-relate the impact of the synthesis method on the backbone SS, Tauc plots were drawn to estimate the bandgap energy using the Kubelka-Munk function according to Equation 1, see Figure 3d,f: [27] (hv where  is the absorption coefficient obtained from the UV-vis spectra, hv is the photon's energy, A is a constant, E g is the optical bandgap energy, and n depends on the nature of the transition.As shown in Figure S10a-c (Supporting Information), straight lines are obtained upon plotting (hv) 2 versus photon energy (hv), indicating direct transition (n = 2).The estimated bandgap energies of LA SS were found to be 1.61 and 2.03 eV that are assigned to Cr 2 O 3 and Fe 2 O 3 heterostructure. [43,49]These values are much lower than those of the untreated bare SS (3.14 eV).It is worth mentioning that this band gap narrowing could upsurge the conductivity and modify the capacitive characteristics of the LA SS electrode upon use in electrochemical devices. [3,24]s for the photoanodes, it is realized that hydrogen thermal annealing can plummet the optical bandgap of a semiconductor by creating shallow impurity states. [19,50]Figure 3f reveals that HP 25 exhibits a lower bandgap (2.83 eV) than the P 25 counterpart (2.93 eV).Moreover, the VHP 25 displays a bandgap of 2.89 eV together with an absorption tail at 2.15 eV.Apparently, the defective vanadium-doped titania shifts the adsorption of titania into the visible region, unlike P 25 and HP 25, which can only adsorb in the UV region of the light spectrum. [19,50]This trend was con-sistent with XPS and XRD findings that reveal the presence of shallow donor states (Ti 3+ and V 4+ ) that could assist charge carriers transfer to the CBM without an extra supply of energy. [3,19]igure S10 and Table S6 (Supporting Information) show the estimated bandgaps of all prepared photocathodes and photoanodes.

Electrochemical Three-Electrodes Measurements
Nano-pattering of stainless steel 316L encountered some sort of morphology reconstruction upon electrochemical activation (ECA), whether it is done under AM 1.5 illumination or under dark conditions.A suggested in situ reconstruction to a more stable and active iron-based functional material was confirmed by the change in the redox peaks' positions and their corresponding intensities. [41]For the first activation cycle, Figure 4a depicts three anodic peaks; two overlapped reversible ones assigned as PA1,2 at −0.79 and −0.62 V versus Hg/HgO, and a third irreversible peak observed at 0.37 V versus Hg/HgO. [51]In contrast, two cathodic peaks assigned as PC1,2 are shown at −0.70 and −0.87 V versus Hg/HgO.According to the literature, PA 1,2 are related to two successive transitions of Fe to Fe(OH) 2 and Fe 3 O 4 , which would be further converted to FeOOH/Fe 2 O 3 at a potential > −0.2 V. [52,53] The following equations (Equations 2 and 3) illustrate the possible redox transitions for PA 1,2, and Equations 4-6 explain the passive film formation at potential > -0.2 V versus Hg/HgO. [52,53]e As for the PC1,2 peaks, these are representing the reduction of the aforementioned equations. [52,53]There is an irreversible peak (PA3) at 0.37 V that is related to the conversion of Cr 3+ to soluble chromate species CrO 4 2− .The following suggested equation examples (Equations 7 and 8) can stand for the PA3 peak transition: [54] Cr It is worth mentioning that the soluble CrO 4 2− can be electroadsorbed back on the electrode surface and undergoes further reduction to Cr 2 O 3 in the same region of PC1,2. [55,56]The ICP measurements confirmed the dissolution of the redox active chromium-based species during the ECA, see Table S3 (Supporting Information).Ultimately, Cr 2 O 3 is a passive p-type oxide that can retain its stability along the used potential window consistent with Hukovi's previous study. [52,55]Figure 4b shows the 100 th cycle of ECA that displays gradual changes on the redox peaks; one anodic peak with higher intensity of PA2 and two peaks of reduction.This would explain the less reversible oxidation of some surface species, i.e., Cr 2 O 3 . [55]For example, Knight et al. claimed that FeOOH and Fe 2 O 3 are of an irreducible nature, and they can exist even at very negative potential. [52]To this end, the interpretation of XPS, XRD, and UV-vis agrees with the analysis of the CV profiles of the ECA.For instance, the existence of the predicted oxidized species from CVs such as Cr 2 O 3 and Fe 2 O 3, as well as the electro-adsorbed CrO 4 2− are confirmed.Figure S11a (Supporting Information) depicts the capacitance, anodic, and cathodic charges calculated from the integrated area under the anodic and cathodic peaks of the LA and DA SS electrodes.For instance, the capacitance values of LA and DA SS electrodes are 74.3 and 65.3 mF cm −2 , respectively.Interestingly, both electrodes exhibit prominent redox peaks, representing battery-like storage behavior.However, the LA electrode demonstrates higher capacitance than the DA SS counterpart. [57]Probably, the illumination conditions can sustain a supplement kinetic effect to the redox electrochemical reactions via the produced holes and electrons, as indicated by the CV curves. [19]Thereby, the LA SS is chosen to conduct further electrochemical analysis and to be used as the photocathode for all fabricated photo-assisted SC devices.
In a bid to examine the ultimate influence of light on the functional material behavior, J-E measurements were probed in 1.0 m KOH solution using a 3-electrode electrochemical cell under dark and AM 1.5 illumination.Figure 4c depicts the photocurrent density of LA SS in the potential window of −0.9 to 0.0 V versusHg/HgO.The current collected from LA SS at −0.9 V is ≈−7.7 and −7.4 mA for light and dark conditions, respectively, revealing the enhanced conductivity together with electron transfer kinetics under illumination.Substantially, the Mott-Schottky plots (MS) of LA SS electrode imply a significant enrichment in the overall carrier concentration under light compared to dark conditions, Figure 4d.Table S7 (Supporting Information) summarizes the carrier concentration (N A ) and flat band potentials extracted from the MS profiles. [16,19]As can be noticed, the negative MS slope confirms the typical p-type semiconductor merit of the LA SS electrode in the cathodic potential region.Besides, the cathodic LSV photocurrent refers that p-type Cr 2 O 3 having the predominated impact at this potential domain.Conversely, Fe 2 O 3 can show its n-type character in the positive potential regime, see Figure S11b (Supporting Information).This reflects the development of p-n junction between Cr 2 O 3 and Fe 2 O 3 at the cathode's surface. [58]The detailed conditions and calculations of the MS measurements are listed in the supporting information.
Regarding the photoanode, three electrodes were evaluated from storage as well as photo-response perspectives; namely P 25, HP 25, and VHP 25.The galvanostatic charge-discharge (GCDs) curves were measured at 2 mA cm −2 , as shown in Figure 4e.The results reveal the superlative performance of VHP 25 compared to its counterparts.For instance, Figure 4f demonstrates the areal capacitance exhibited by VHP 25, HP 25, and P 25, where VHP 25 displays the most desirable storage performance.The photoresponse of VHP 25 is estimated from the J-E measurements similar to the LA SS electrode.Figure 4g compares the photocurrent density of VHP 25 in the potential window of 0.0 to 0.7 V versus Hg/HgO.The current revealed at 0.6 V is ≈0.71 mA cm −2 in the dark and 0.92 mA cm −2 under illumination.In a similar manner to LA SS, the MS profiles shown in Figure 4h reveal quite high carriers' concentration under light conditions, see Table S7 (Supporting Information).

Device (Two-Electrode) Measurements
To manifest the practical functionality of the prepared photoanodes and LA SS electrodes, three hybrid semi-solid-state devices were assembled; namely LA SS//P 25, LA SS//HP 25, and LA SS//VHP 25.While both photoanode and photocathode possess optical characteristics, the light was directed to the photocathode side while keeping the photoanode side in the dark.Then, the device was flipped to evaluate the photoanode under illumination while keeping the photocathode under dark.In addition, both sides in dark measurement was collected for each device.The dual photo-capacitive storage performance is evaluated via both CV and GCD profiles for each device.Figure 5a-f shows the different abilities of each device to sustain various potential windows.For instance, the LA SS//P 25 device can sustain 1.3 V, while the LA SS//HP 25 and LA SS//VHP 25 devices can retain up to 1.4 and 1.6 V, respectively, Figure 5g.Overall, the photostorage behavior of the photoanodes is more efficient than that of the photocathode.This can be related to the highly generated carrier concertation as revealed from the MS plot and Table S7 (Supporting Information).For instance, the N D of VHP 25 is four orders of magnitude higher than that of the LA SS.Meanwhile, the substantial upsurge in shallow donor states upon hydrogen annealing and VO x doping help facilitate the charge separation and subsequently the solar to charge efficiency. [16,19,59]The apparent minority carrier concentration can be calculated from the difference in N D and N A values revealed from the MS plots in dark and under illumination.For VHP 25, it is 7.8 × 10 23 cm −3 , while it is 2.4 × 10 20 cm −3 for the LA SS electrode.The viable role of the minority carrier will be revealed later in the next section.Testing the LA SS//VHP 25 device at various potential regimes (1.3, 1.4, and 1.7 V) was performed, as shown in Figure S12 (Supporting Information).The 1.6 V is the optimum potential window for having dual photo-capacitive performance beyond that photo-response is faded, see Figure 5 h.The dark capacitance at 1.6 V is ≈1.48 mF cm −2 that increases by 33.7% (1.98 mF cm −2 ) and 89.1% (2.80 mF cm −2 ) upon illumination from the photocathode and photoanode sides, respectively.This record enhancement percentage of VHP 25 electrode stands the highest among other recent reports as in Table S8 (Supporting Information).Figure 5i unveils the decent storage merits of the LA SS//VHP 25 device compared to its counterparts.The capability retention of all hydrogel LA SS//VHP 25 device is ≈35%, demonstrating the slight downward diffusion affinity of ions at high current densities.
To practically test the functionality of these devices, the areal energy and power densities were estimated as presented in Figure 5j.The Ragone plot manifests a maximum areal energy of the hybrid LA SS//VHP 25 device of 2.3 μWh cm −2 at 1.28 mW cm −2 .In addition, the device could preserve a high areal energy of 0.13 μWh cm −2 at an areal power of 2.6 mW cm −2 .Comparison of the areal capacitance, energy, and power of our device with those previously reported in the literature is listed in Table S9 (Supporting Information), revealing the superiority of our fabricated device.Interestingly, the device can undergo a bending test, thanks to the flexible characteristics of the backbone substrates.Figure 5k discloses the eminent capacitive retention of 100% under the bending conditions.Although the CV profile under bending is a slightly different from that of its original state, the device could sustain its typical areal capacitance.It can be explained that the changes in the light distribution upon bending could be the reason behind the CV altered features.Finally, the stability test (Figure 5l) showed the ability of the device to keep 91% of its first cycle capacitance after 350 successive chargedischarge cycles with decent efficiency.Note that the device efficiency is affected by the suggested quasi-reversible water splitting/formation in the semisolid state electrolyte under illumination conditions as concluded in the next section discussing the storage mechanism.Figure S13 (Supporting Information) discloses the practical performance of up to 1250 cycles.As can be understood, the device capacitance drastically declined after the first 600 cycles with visual observation of electrolyte drying.Performance regeneration can be obtained via drop-casting of fresh gel electrolyte atop the device's electrodes.Basically, packing the device in a coin cell was intentionally avoided as light would not reach the photo-electrodes, which adversely affected the device's long-term stability (fast electrolyte drying).These results further highlight that the activity loss is not related to the electrode material.Thus, demonstrating high water retention electrolytes rather than the traditional KOH-PVA electrolyte would be practically favored as the water plays a significant role in the enhanced capacitance upon illumination, as will be revealed in the next section.

Photo-Assisted Supercapacitor Working Mechanism
To scrutinize the working storage mechanism, band structure and band alignment were investigated.The valence band plots (XPS-VB) of the LA SS and VHP 25 electrodes are shown in Figure 6a,b.Notably, the valance band position with respect to the Fermi level could be approximately determined by the linear extrapolation of the most linear part to the baseline.Indeed, the presence of two valence bands and/or band tail can be easily evaluated from the VB-XPS. [60,61]For instance, two valence band positions are assigned for the LA SS, consistent with the MS findings.While the zero eV in such plot is considered the Femi level position, the band nearer to it (0.45 eV) is suggested to be of the p-type Cr 2 O 3 .In contrast, the one away from the Fermi (1.92 eV) can be assigned to the VB of the n-type Fe 2 O 3 . [58]As for VHP 25, the valance band (2.01 eV) together with the band tail are observed, Figure 6b.That tail can be attributed to the formation of donor states and non-stoichiometric oxides in agreement with XPS and XRD results. [39]he band structure of the photoanode and photocathode are presented in Figure 6c,d.Moreover, the band alignment is estimated from the MS data and plotted in Figure 6e, indexed to 0.0 eV versus normal hydrogen electrode (NHE), and referenced to 4.5 eV versus vacuum. [19]As can be shown, the LA SS could appropriately straddle the H 2 redox potential, while VHP 25 can straddle the two redox potentials of water.
Despite the fact that water electrolysis is considered an irreversible parasitic reaction in traditional aqueous-based supercapacitor devices due to the severe electrolyte degradation. [62]It is suggested that photoelectrochemical water splitting here can be of quasi-reversible nature and the produced H 2 and O 2 gases products can be trapped in the semisolid electrolyte and further reduced when the scan is reversed to more negative potentials.As shown in Figure 6f-i, the suggested mechanism of water splitting is illustrated.For instance, when the light is illuminated from the LA SS side while scanning the potential along the cathodic potentials, carriers are created.Then, a superlative separation inside the p-n junction is achieved. [58,63]A possible explanation for the observed enhanced photocurrent density upon illumination from the photoanode side over the photocathode side, coincidently with the earlier onset potential (Figure 6g,i), is that VHP 25 can straddle both H 2 and O 2 redox potentials with favorable reaction kinetics.Consequently, outstanding photoelectrochemical activities are achieved at the electrolyte/electrode photoanode's interface.To clarify, it can be depicted that the hydrogel matrix acts as a gas-trapping framework, which hinders the bubbles' detachment and consequently facilitates its quasi-reversible reduction.Combining these findings would also illustrate the aggressive decline in the stability upon electrolyte drying, where the gas bubbles could easily be diffused from the solid-state network, which deteriorates the device's performance.

Conclusion
This study outlines the reliable performance of all visible lightenhanced supercapacitor device that is based on commercial stainless steel and Ti meshes.Smart optimization of the photocathode, photoanode, and semisolid electrolyte is demonstrated.The successful fabrication of -(Fe 2 O 3 ) 1-x (Cr 2 O 3 ) x nano-flakes pn junction photocathode is realized via dry etching followed by electrochemical activation.Additionally, defect engineering of TiO 2 upon hydrogen annealing and doping with mixed (V 4+ /V 5+ ) helped to introduce donor states that proved its effectiveness in upsurging the anode storage performance.The assembled hydrogel device manifests a high energy of 2.30 μWh cm −2 at 1.28 mW cm −2 with quite good capability.This study paves the doors to affordable and reliable PSCs, and encourages further manipulation of functional electrolytes that would have decent water retention even at high temperatures to hamper the noticed plummeted performance upon prolonged cycling.

Experimental Section
Nano-Pattering of Stainless Steel 316L Commercial The asreceived mesh was washed and cleaned by ultrasonication in ethanol, and deionized water for ≈10 min, followed by air drying.The average elemental composition was determined by EDX (three spots), see Figure S1 and Table S1 (Supporting Information).On average, the Fe: Cr: Ni: O ratio was ≈33.1:7.86:3.41:1.To dry etch the pristine smooth surface, an acidic gel electrolyte was prepared and used as follows: 1 g of PVA (high molecular weight) was dissolved in 10 mL distilled water that contained 2 mL H 2 SO 4 (stock).Heating the acidic polymer solution gradually to 90 °C is quintessential to dissolve the PVA completely and form a transparent hydrogel.The cleaned stainless steel 316L mesh electrodes (SS bare) were set in a petri-dish and a drop of electrolyte gel was placed on its surface for a certain time.Afterward, the polymeric residuals were removed, and the etched mesh (E-SS) was further cleaned by ultrasonication at ≈20% maximum amplitude in ethanol and deionized water for ≈10 min, followed by air drying.The dry-etching process is depicted in Figure S2 (Supporting Information).The EDX mapping of the gel-etched electrodes shows a lower percent of Fe, Ni, and Cr and a higher oxygen amount, demonstrating the rusting process, see Figure S3 and Table S2 (Supporting Information).Note that ICP measurements were performed on the removed polymeric residuals used in the etching process by dissolving them in concentrated acid.As can be indicated in Table S3 (Supporting Information), all elements were detected with various extents.
Light and Dark Activated Stainless-Steel Electrodes (LA SS and DA SS): The E-SS was then activated electrochemically under light and dark in a three-electrode cell setup.The activation process included cycling the electrodes from 0.5 to −1.2 V versus Hg/HgO in 1 m KOH at a high potential scan rate for 100 cycles.The light and dark activated samples were labeled LA SS and DA SS, respectively.
Synthesis of VOx -Carbon Framework Powder: A suspension of V 2 O 5 and a primary amine (C 16 H 33 NH 2 ) was prepared by adding equimolar (15 mmol) in 5 mL of ethanol and stirring the suspension for 2.5 h.Then, 15 mL of water was added to the suspension followed by further stirring for another 48 h.The mixture was then transferred to an autoclave for hydrothermal treatment that takes seven days at 180 °C.The generated black product was washed with hexane and ethanol and then dried overnight at 80 °C.
Synthesis and Fabrication of the Photoanode: Three photoanode electrodes, namely P 25, HP 25, and VHP 25 were fabricated and tested in both three-and two-electrode cell configurations.Commercial black Ti mesh was used as a flexible substrate after washing and cleaning in ethanol and distilled water.As indicated from three different samples, the Ti mesh elemental composition includes Ti, O, and C with an average ratio of 11.35: 7.23: 1, see Figure S4 and Table S4 (Supporting Information).Detailed fabrication of P 25, HP 25, and VHP 25 is included in the Supporting Information file.
Semisolid State Device Fabrication: Poly(vinyl alcohol) purchased from Sigma-Aldrich, 87-90% hydrolyzed, and with an average molecular weight of 30 000-70 000 was used to prepare the hydrogel.1 g of PVA was dissolved in 8 mL of hot distilled water at 90 °C, and then the gel solution was left to cool.Afterward, 2 mL distilled water that contains 0.56 g of dissolved KOH was added to the gel solution under stirring.The hybrid device was assembled by sandwiching photoanode and photocathode, and a filter paper wetted with KOH-PVA electrolyte gel was placed in between.Materials and chemicals, photoelectrochemical/electrochemical methods, and calculations are shown in the Supporting Information.
Materials Characterization: The structure and morphology were elucidated using (Zeiss SEM Ultra 60 FESEM machine) with an accelerat-ing voltage of 4 KV and high-resolution transmission electron microscope (JEM-2100).The crystal structure of the various samples was explored using PANalytical X-pert/ Pro PW3040 MPD X-ray diffractometer (Cu-K,  = 0.15406 nm, 50 mA, 40 KV) in the range of 5.0-89.9o with a glancing angle of 3 and 4 o , step size = 0.013 and scan step time 37. Raman spectroscopy of the photoanode samples was conducted to evaluate the phase composition using an excitation laser beam of wavelength 532 nm.In addition, X-ray photoelectron spectroscopy (XPS) measurements were demonstrated in a UHV chamber equipped with (SPHERA U7) energy analyzer, Al K monochromator X-ray source (E = 1486.6eV).Moreover, the spectrum at the lowest binding energy (valence band VB-XPS) was analyzed to prob the valence band maximum edge.CASA-XPS software was viable in fitting and deconvolution of the obtained high-resolution XPS spectra.The optical features of the photoelectrodes were determined along the wavelength range of 200-800 nm using a Shimadzu UV-vis diffuse reflectance spectrometer.The Brunauer-Emmett-Teller (BET) isotherm is used to determine the specific surface area of the VO x powder.The vacuum degassing was pre-performed for 2 h at 100 °C, heating rate of 5 °C min −1 .The BET analysis was carried out in N 2 gas adsorption/desorption at 77.35 K bath temperature.Energy Dispersive X-ray Analysis (EDX) model JCM-6000PLUS with (E = 15.0 kV and prob I = 7.47500 nA) was used for mapping composition analysis of thin film LA SS and VHP 25 electrodes.Microwave Plasma Atomic Emission Spectroscopy (MP-AES) model 4210 MP-AES.

Figure 1 .
Figure 1.Field emission scanning electron microscopy (FESEM) images of a) pristine SS mesh with high magnification inset, b,c) dry etched electrodes and nano-pattering of the surface, d,e) DA SS film, f,g) LA SS mesh, h) P 25 over Ti mesh electrode, i) tubular VO x , j,k) TEM images of VO x , and l) SEM of VHP 25 composite film.

Figure 2 .
Figure 2. XPS spectra of the prepared samples: a) survey spectra of LA SS, VHP 25, and VO x .High-resolution spectra of b) Fe 2p, c) Cr 2p, and d) O 1s of LA SS.High-resolution spectra of e) Ti 2p, f) O 1s, and g) V 2p of VHP 25.

Figure 4 .
Figure 4. Electrochemical measurements in 1.0 m KOH electrolyte solution: a) CV plots of DA SS and LA SS at 100 mV s −1 potential scan rate for the 1 st cycle, b) CV plots of DA SS and LA SS for the 100 th cycle, c) J-V of LA SS under dark and illumination conditions at 20 mV s −1 potential scan rate, d) Mott-Schottky plots of LA SS under dark and illumination conditions, e) GCD plots at 2 mA cm −2 , f) Areal capacitance comparison of P 25, HP 25, and VHP 25, g) J--V of VHP 25 at 20 mV s −1 potential scan rate under dark and illumination conditions, and h) Mott-Schottky plots of VHP 25 dark and illumination conditions.

Figure 5 .
Figure 5. a,c,e) CV and b,d,f) GCD profiles of the LA SS//P 25, LA SS//HP 25, and LA SS//VHP 25 devices, g) areal capacitance comparison of the three devices calculated at 0.33 mA cm −2 under light, h) areal capacitance of LA SS// VHP 25 measured at various potential windows at 100 mV s −1 potential scan rate under different dark and light conditions, i) the values of capacitance under different current densities, j) areal energy and power densities of the three tested devices.k) Bending test of the LA SS//VHP 25 device and the probed CVs at original and bending conditions.l) Long-term stability and coulombic efficiency of the LA SS//VHP 25 device measured at 1 mA cm −2 .

Figure 6 .
Figure 6.Valence band (XPS-VB) of a) LA SS and b)VHP 25.Band structure of c) photocathode and d) photoanode, e) band energy diagram with respect to water redox potentials of LA SS and VHP 25 photoelectrodes.Schematic illustration of the enhanced faradaic storage behavior f,g) at LASS and h,i) at VHP 25 illumination.