Hindered Aluminum Plating and Stripping in Urea/NMA/Al(OTF) 3 as a Cl-Free Electrolyte for Aluminum Batteries

Conventional electrolytes for aluminum metal batteries are highly corrosive because they must remove the Al 2 O 3 layer to enable plating and stripping. However, such corrosiveness impacts the stability of all cell parts, thus hampering the real application of aluminum-metal batteries. The urea/NMA/Al(OTF) 3 electrolyte is a non-corrosive alternative to the conventional [EMImCl]: AlCl 3 ionic liquid electrolyte (ILE). Unfortunately, this electrolyte demonstrates poor Al plating/stripping, probably because (being not corrosive) it cannot remove the Al 2 O 3 passivation layer. This work proves that no plating/stripping occurs on the Al electrode despite modifying the Al surface. We highlight how urea/NMA/Al(OTF) 3 electrolyte and the state of the Al electrode surface impact the interphase layer formation and, consequently, the likelihood and reversibility of Al plating/stripping. We point up the requirement for carefully drying electrolyte mixture and components, as water results in hydrogen evolution reaction and creation of an insulating interphase layer containing Al(OH) 3 , AlF 3 , and re-passivated Al oxide, which ﬁ nally blocks the path for the possible Al plating/stripping. ©

Aluminum batteries (AlBs) are considered a desirable alternative to lithium-ion batteries (LiBs) because Al, with a sustainable raw material supply owing to a mature Al foil industry 1 is recyclable, and abundant.Al metal has the highest theoretical volumetric specific capacity (8046 mAh cm −3 ) 2-4 compared to other metals.4][5] AlBs can be classified into two main groups based on whether they utilize an aqueous or a non-aqueous electrolyte.The unsuccessful Al plating/stripping in aqueous electrolytes, due to the competitive H 2 evolution reaction, motivates scientists to search for alternative electrolytes that allow reversible Al plating/stripping.As a non-aqueous alternative, room-temperature ionic liquid electrolytes (RTILEs) with a wide electrochemical potential window and low vapor pressures are widely used in AlBs. 6,7The Al 3+  ) anions exist, the latter being the only active species, which allow reversible Al plating/stripping 6 Several RTILEs have been reported, [8][9][10][11] but the most utilized non-aqueous ionic liquid electrolyte (ILE) compositions, which enable reversible Al plating/ stripping, are melts based on imidazolium Chloride and AlCl 3 . 4,12he commonly used Imidazole-based IL, a combination of 1-Ethyl-3-methylimidazolium chloride and Aluminum Chloride salts ([EMImCl]: AlCl 3 (1:1.5)),has good ionic conductivity and outstanding plating and stripping behavior. 13Its ability to successfully plate and strip Al is due to the presence of Al 2 Cl 7 -, which forms only when the molar ratio of AlCl 3 to [EMIm]Cl is higher than one. 8,11,14lthough [EMImCl]: AlCl 3 is the most advanced chloroaluminate system for AlBs, attempts have been made to discover substitutes, as it is expensive, highly corrosive, hygroscopic, moisture-sensitive, and susceptible to hydrolysis. 15,16The most intuitive way to cope with chloroaluminate's reactivity is to use an alternative Cl-free electrolyte. 16Recently, many AlCl 3 -free electrolytes (presented in Table I) have been explored, such as mixtures containing aluminum trifluoromethanesulfonate (Al(OTF) 3 ) and aluminum bis (trifluoromethanesulfonyl)imide (Al(TFSI) 3 ) as non-corrosive alternative salts.
The urea/NMA/Al(OTF) 3 electrolyte shows an electrochemical stability window broader than [EMImCl]: AlCl 3 , 26 but, unfortunately, it displays inadequate electrochemical Al plating/stripping on Al substrate, as demonstrated by our group. 27Many other AlCl 3 -free electrolytes (resumed in Table I) show poor performance in terms of plating and stripping.The Al(OTF) 3 salt shows poor plating and stripping not only in a mixture with urea and NMA, but also with other solvents like diglyme. 20,21,26,27Aluminum electrodeposition on molybdenum substrate from 1-butylimidazole bis(trifluoromethanesulfonyl) imide demonstrates quasi-reversible plating/stripping, 17 but this is reversible only for a limited number of cycles or with significant side reactions.Al(TFSI) 3 in acetonitrile on molybdenum substrate and Al(PF 6 ) 3 in dimethyl sulfoxide on copper substrate show similar quasi-reversible behavior with significant plating/stripping overpotentials (> 1.5 V). 17,18 To achieve successful Al plating/ stripping in such non-corrosive environment, a proposed method is the modification of the aluminum anode (anode amorphization, 28 anode alloying, 29,30 and surface modification [31][32][33][34] ) because the electrode-electrolyte interphase has a vital role for the plating/ stripping process.The other method is the modification of the electrolyte by developing concentrated electrolytes, 35 adding water scavengers, 36,37 and additives. 25,38Moreover, the amount of active Al species in the electrolyte influences the appearance of Al deposit. 3A chlorine-free electrolyte cannot ensure reversible Al plating/stripping without an appropriate and ionically conductive interphase, which can permit the Al plating/stripping process. 15,39][42] The study of Loaiza, et al. 32 revealed that the initial passivation layer formed upon contact with ILE is porous and intricate, comprised of an outer inorganic/organic layer and an inner oxide-rich layer.The cyclic stability of the cell can be improved by preventing additional solvent reduction and anode component disintegration by creating a stable and robust SEI layer (as it is well-known from the lithium battery technologies). 9,34Without this protection, byproducts of ILE z E-mail: fatemehsadat.rahide@kit.edu;sonia.dsoke@kit.eduJournal of The Electrochemical Society, 2023 170 120534 breakdown can be deposited again onto newly generated surfaces, and the interfaces may become unstable due to the ongoing process of dissolution and deposition. 43However, regardless of the type of electrolyte, Al metal electrodes should serve as the state-of-the-art anode material in AlBs. 4,13,32,444][45][46] The ioninsulator Al 2 O 3 oxide film covering the metallic Al anode blocks the anode's activation and makes it more difficult to attain a reversible reaction of Al plating and stripping, resulting in a substantial overpotential. 3,32,43However, at the same time, this passivation layer provides protection against ILE-induced corrosion, so a good balance between exposed and covered Al sites is desirable. 32,47Al 2 O 3 oxide film can be dissolved locally, enabling adequate Al plating/stripping reaction when it is in contact with ILE 44,47,48 ; this can be considered as an Al surface modification that can be done before cell assembly.However, this modification as an extra and time-consuming step has hampered the development and real application of alternative and non-corrosive non-aqueous electrolytes. 15The Al 2 O 3 passivation layer removal, as a surface modification, determines the reversibility of the Al plating/stripping at the electrode/electrolyte interface. 14,15,47o et al. 34 claimed that the etched and electropolished Al foil has the greatest effect on AIBs' performance. 34A surface pretreatment (as a type of surface modification) can typically be achieved by immersing the Al foil in ILE to partially remove or modify the Al 2 O 3 passivation layer and build an Al, Cl, and N-rich layer at the surface, 47 thus creating an "artificial interphase". 39Long et al. 49 demonstrated that, with this method, the dissolved Al 2 O 3 oxide film in the ILE is replaced by an SEI layer rich in Cl and O species.On the other side, a complete removal of Al 2 O 3 may be detrimental as this oxide prevents Al metal electrode disintegration. 4,50The morphological changes of Al metal as a function of immersion time in the ILE were also examined by Lee et al., 43 who found that a new oxide layer with a particular lattice plane was grown on the Al surface.These findings confirmed that removing the thick oxide film layer during pretreatment is crucial for enhancing battery cell performance. 15However, the function of the native and electrolyte-derived passivation layer is still poorly understood despite its significant influence on the electrochemical performance of the Al anode, in both the aqueous 51 and non-aqueous 15,39,43,47 systems.In addition, the practical application of the active metallic Al anode material in urea/NMA/Al(OTF) 3 as a Cl-free non-corrosive electrolyte has not been evaluated, as no study on interphase layer formed on the Al anode has been reported.Therefore, our work aims to define the crucial issues hindering Al plating (on metallic aluminum) and the performance of surface (non-) modified Al anodes in a urea/NMA/Al(OTF) 3 electrolyte.With that, we highlight the bottlenecks of the urea/NMA/Al(OTF) 3 electrolyte in terms of Al plating/stripping on the Al substrate.The main issue may arise from hydrogen evolution reaction (HER) and the creation of interphase layer containing Al(OH) 3 , Al-F, and re-passivated Al oxide, which consequently blocks the path for Al ions through the electrode-electrolyte interphase.
Electrochemical setup.-Allelectrochemical cells were assembled/opened inside an argon-filled glovebox (MBraun, <0.5 ppm O 2 , <0.5 ppm H 2 O).The electrochemical techniques, including cyclic voltammetry (CV) and chronoamperometry (CA) have been applied in sealed, closed, and airtight TSC surface cells.The TSC surface cell was supplied by rhd Instruments GmbH & Co. KG (Germany).In this cell, an Al and a Pt foil were taken as working electrodes (WE, with a geometric area of 0.28 cm 2 ), depending on the particular experiment.A glassy carbon (GC) disc was used as a counter electrode (CE, with 6 mm diameter), and an aluminum (Al) and silver (Ag) wire as quasireference electrodes.To avoid moisture in the cell, all cell auxiliaries were transported to the argon-filled glovebox after being dried in an oven at 80 °C for 24 h before each cell assembly.Before and after each electrochemical test, the GC electrode was first polished with 250 nm diamond polishing paste, then rinsed in distilled water to remove any dirt or contamination from the electrode surface. 27A mixture of H 2 SO 4 /H 3 PO 4 /HNO 3 (25/70/5 by volume) was used to polish and clean the Al quasi-reference electrode from any dirt and residual oxide. 26,27The Ag quasi-reference electrode was polished with a 1 μm diamond suspension and then rinsed in distilled water. 27 Considering the stability window of the urea/NMA/Al(OTF) 3 electrolyte and the stability of the Ag quasi-reference electrode for Al plating and stripping, this potential range (−1.7 V to 0.5 V), was derived from prior study 27 where it had proven to be a reliable and effective window for Al electrodeposition and dissolution.It's notable to mention that according to the evidence, the calibration of the quasi-reference electrode showed notable shifts in the redox peaks of Ferrocene (utilized as an internal reference) against the Al wire.Conversely, using the Ag wire as the quasi-reference maintained a remarkably stable potential throughout the 24 h period. 27  and energy-dispersive X-ray spectroscopy (EDX) of the Al and Pt electrodes were collected using a JEOL JSM 7500 F machine with acceleration voltages of 5 kV and 10 kV, respectively.The X-ray photoelectron spectroscopy (XPS) measurements of the Al and Pt electrodes were done with a Specs EnviroESCA NAP-XPS 53 (without making use of the near-ambient pressure (NAP) features, so at roughly 10 −6 mbar) via a Nitrogen-filled glovebox (GS, <5 ppm O 2 , <0.5 ppm H 2 O).More specifically, not using NAP features means that we are operating at the minimum of roughly 10 −6 mbar, instead of the up to 10 mbar that the machine can reach.Survey spectra were taken with a pass energy of 100 eV and an energy resolution of 1 eV, while fine spectra were taken with a pass energy of 30 eV and a resolution of.1 eV.

Results and Discussion
Electrochemical characterization.-Figure 1 depicts the CVs recorded on the pristine Al foil in the Al(OTF) 3 -based electrolyte.(Fig. 1b) shows that only capacitive current, i.e., no Al plating/ stripping reaction, can be observed on pristine Al foil in the Al(OTF) 3 -based electrolyte.On the other hand, the Al redox reaction is visible on pristine Al foil in the AlCl 3 -based electrolyte (Fig. 1a).Concerning the AlCl 3 -based electrolyte, the peak current density related to Al stripping increases from 0.4 to 4.027 mAcm −2 from the 1st cycle to the 100th cycle, highlighting the activation process (which should imply progressive Al 2 O 3 dissolution) as the cycle number increases.During the initial cycles, the native Al 2 O 3 oxide film should have a few defect sites that allow electrolytes to pass through and react with internal Al below the cracks and defect sites. 4During the following cycles, the native Al 2 O 3 oxide film gradually dissolves into the ILE since it cannot tolerate the acidic environment. 32At this point, the newly exposed portion of the Al foil begins to participate in the Al plating/stripping. 4 The full progression of 100 CV cycles for both electrolytes is presented in Fig. S1.In agreement with the CV results, SEM images and EDX of the pristine and cycled Al electrode in Al(OTF) 3 -based electrolyte reveal no change as no electrochemical reaction occurs on the Al electrode (Fig. 2).Complementary to SEM images, the observed elements of the pristine and cycled electrodes  S1.It is assumed that Al 2 O 3 is not removed due to the non-acidic nature of the electrolyte therefore activation of Al plating/stripping has been impeded. 1,17Al 2 O 3 oxide film dissolves in Lewis acidic [EMImCl]/AlCl 3 (1:1.5)electrolytes during cycling due to the existence of the chloroaluminate complexes, which explains the increase of the Al plating/stripping capacity with the cycle number.However, the Al 2 O 3 oxide film deactivates the Al surface for any electrochemical reactions in the AlCl 3 -free electrolyte. 15This suggests that in the case of using Al(OTF) 3 -based electrolyte, the Al surface must be modified by pretreatment 15 to activate electrode-electrolyte interfaces for desired electrochemical reactions.Two crucial factors affect the possibility and reversibility of the Al plating/stripping process: 1) the appropriate electrode-electrolyte interphase driven by the state of Al 2 O 3 oxide film covering the Al surface, 2) the water content of utilized electrolyte with the right ionic Al species.Therefore, firstly, we investigated if the state of the Al surface after 18 h of immersion in 900 μl of AlCl 3 -based electrolyte enables Al plating/stripping.Figure 3a shows the recorded CV on surfacemodified Al (MAl) foil in Al(OTF) 3 -based electrolyte.Figure 3b indicates the creation of an SEI rich in Al, N, Cl species during immersion pretreatment, and it agrees with previous studies. 4,33,34owever, contrary to our expectation, no electrochemical activity can be observed for the MAl electrode.Although an oxidation peak with high current density is observed, no corresponding reduction peak is present, indicating that this is an irreversible reaction, as shown in Fig. 3b.This result is different from what was presented in Al(OTF) 3 /[BMIM]OTF ionic liquid electrolyte by Wang et al. 15 The reactivity of the Al surface and the Al(OTF) 3 -based electrolyte with Cl -ions form an insulating interphase layer resulting in a high anodic current density.In order to get more insight into the possible Al deposition in Al(OTF) 3 -based electrolyte, we investigated the possibility of Al electrodeposition on a Pt working electrode 27 and then compared it with the electroplated Al on Pt from an AlCl 3 -based electrolyte.Furthermore, the XPS spectra of Cl2p support this notion, indicating the continued bonding of chlorine to both aluminum and [EMIm] + (Fig. 5).Moreover, the recorded slow current reduction presented in Fig. 4a is probably due to the reduced active surface area of the Pt substrate and Al deposition. 23The other reason could be the facilitation of further Al deposition from the electrolyte because of the freshly deposited Al 3 .XPS analysis of the Al and Pt electrodes was performed to see if the reductive current shown in Chronoamperograms correlates with the metallic Al deposition from active Al species of the electrolytes.
XPS analysis was performed on the three electrodes: a Pt foil, which had undergone CA to electroplate Al from an AlCl 3 -based electrolyte, and on Al and Pt electrodes after CA in an Al(OTF) 3 -based electrolyte.All spectra were analyzed in CasaXPS.Fits of the spectrum for each element and each electrode are shown in Figs.5-7.The lists of peaks for each spectrum are shown in Tables S2-S4.The surface of the Pt foil after Al electroplating with AlCl 3 -based electrolyte was thickly covered with Al deposition products.Thus, as expected, Pt is not visible in the XPS spectrum.Fine spectra were performed on the C1s, N1s, O1s, Al2p, and Cl2p regions and presented in Fig. 5.The Al peaks are clearly bimodal, with the two Al2 p3 2 peaks occurring at 74.6 and 71.6 eV, due to Al 3+ and Al metal respectively.The Cl2p spectrum contains two overlapping doublets, demonstrating that chlorine remains bonded to both aluminum and [EMIm].The AlCl 3 Cl2p 3/2 occurs at 198.7 eV and that of [EMIm]Cl at 197.6 eV, consistent with the results of Calisi et al. 54 The C1s spectrum was fit with three peaks: a C-C/C-H peak, which was calibrated to 285 eV, a C-O/ C-N peak at 286.1 eV, and an O-C=O peak at 289.1 eV.The N1s spectrum displays a larger peak at 401.7 eV coming from cationic nitrogen in imidazolium, 55 and a smaller neutral C-N peak at 399.9 eV, showing either a decomposition product of the imidazolium or residual acetonitrile from washing.Finally, the O1s spectrum is fit with two overlapping peaks, a smaller peak at 530.8 eV, and a much larger one at 532.2 eV.In summary, we reconfirm [EMImCl]/AlCl 3 as an effective electrolyte for Al plating, 56 and the O1s peak at 532.2 eV suggests that the primary surface Al 3+ compound in the sample is Al(OH) 3 . 57This significant hydroxide peak observed on the surface might stem from the glovebox atmosphere.The intricate composition of the oxide passivation layer on the Al metal surface is susceptible to alterations caused by storage conditions. 58,59Factors such as temperature and humidity impact the absorption of elements like water, hydroxides, and carbon dioxide, thereby influencing the layer's overall composition. 58It's notable that the observed fluctuation in CV (Fig. 1a) can be attributed to the uneven current distribution across the electrode surface, highlighting the possibility of gas evolution from the Al metal's surface during cycling.This connection aligns with XPS observations, reinforcing the correlation between current distribution and gas evolution due to the presence of Al(OH) 3 .Moreover, this fluctuation is directly proportional to the scan rate.
Analysis of the Al electrode after CA measurement in Al(OTF) 3 -based electrolyte was complicated by the impossibility of distinguishing deposited Al metal from that already present on the sample.Visually, the measured Al foil lacks the thick deposition layer visible on the Pt foil.Fine spectra were taken for F1s, O1s, C1s, and Al2p.The Al2p spectrum shows again two doublets, ascribed to Al metal and Al 3+ .The O1s splits into two peaks, at 532.3 eV and at 531.1 eV, which are again attributed to Al(OH) 3 /C-O and Al 2 O /O-C=O. 57The C1s spectrum contains a number of well-defined peaks at 285, 285.8, 289.5, 293.1, and 296.3 eV.The first four of these are assigned to C-C, C-O, O-C=O/ C-F, and -CF 3 , respectively, with the C-F compounds clearly derived from the reduction of OTF -.The peak at 296.3 eV is tentatively assigned to CF 4 , 60 but may just be a satellite structure.The F1s spectrum has two peaks: a larger one at 688.7 eV, which is also characteristic of C-F bonding, and a smaller one at 685.6 eV, which is ascribed to a minuscule amount of AlF 3 , 61 though any corresponding peak in the Al2p spectrum is too small to be resolved.The observed AlF 3 results from the reduction of OTF − anion containing fluorine. 39,62,63Moreover, AlF 3 participates in the "repassivation" process and may cause overpotential of the HER over time. 39We verified that, although the components have been dried as reported in previous, 27 high water content (about 28466 ppm) was still present in the Al(OTF) 3 -based electrolyte.Another H 2 source could also arise from the decomposition of urea. 27hermodynamically and kinetically, an excess of H + /H 3 O + assists even in the earlier onset of hydrogen evolution, 64 which may explain the trace hydroxide present in the sample.We assume that the water content is produced during electrolyte preparation, as all electrolyte components have been vacuumed and dried before the electrolyte preparation, as previously reported in the literature. 27However, the presence of water and its effect on Al(OTF) 3 -based electrolytes have not been explored before 24,26 .This high water content would interfere with Al plating 41 because water in the electrolyte solution would result in HER in the presence of Al: As a primary cathodic   reaction, the HER (2Al + 6H 2 O → 2Al(OH) 3 + 3H 2 ) prevents the possibility of reversible Al plating and stripping. 41Water molecules serve as a source of oxygen for the creation of oxide films. 65he oxide film is also re-passivated when the organic component degrades or dissolves in the electrolyte, although the first oxide layer should be much less uniform and probably thinner than those on pristine aluminum.Passive oxide film production also occurs in the pH-neutral range (4 to 8). 16As shown in Fig. S2, the measured pH of the electrolyte is attributed to the polarized O-H bonds of water molecules coordinating Al 3+ . 39,66The Al 3+ transport would be hindered by the formed SEI containing AlF 3 , Al(OH) 3 , and repassivated Al oxide.We should also note that the Al-metal peak is not notably enhanced compared to untreated samples, and the O1s structure shows a mixture of oxide and hydroxide similar to what we have seen in pristine foil samples.It is thus also possible that the oxide-hydroxide layer of the pristine foil was not attacked at all, or that the oxide layer could have formed from trace oxygen and water between cycling and measurement.Regardless of the cause, given the highly regular character of the oxide layer, and low concentration of electrolyte deposition products, we infer that significant Aldeposition did not take place.A summary and approximate breakdown of the relative signal between spectra can be found in Table S3.
Finally, Pt foil, which had undergone CA to electroplate Al from Al(OTF) 3 -based electrolyte, shows nothing indicating any interaction with the electrolyte.Only Pt metal is observed, with trace impurities and adventitious carbon on the surface.This result apparently contradicts the reduction current seen in a recorded CV at a Pt electrode. 27However, this current could derive from hydrogen evolution via the electrochemical reduction of urea since the overpotential for hydrogen evolution is low at Pt electrode substrates and it is currently under investigation in our group.A summary and approximate breakdown of the relative signal between spectra can be found in Table S4.The evident trace of the electrolyte is a small proportion of C-F compounds, related to the reduction of OTF -.In addition, the created interphase layer on Al electrode containing AlF 3 , Al(OH) 3 , and re-passivated Al oxide correlates with occurred HER owing to the high amount of water content in the electrolyte, despite the electrolyte components have been dried as reported in the literature. 27It is confirmed that the hindered Al plating is due to the formed insulating interphase layer containing AlF 3 , Al(OH) 3 , and re-passivated Al oxide.XPS analysis of the Pt electrode after CA in Al(OTF) 3 -based electrolyte shows no measurable deposition of any electrolyte material.As an outlook of the presented study, the contribution of the electrolyte should be explored more in terms of the other possible side reactions aside from the HER and Al(OTF) 3 degradation.A better approach to reduce the interaction between Al and H 2 O is the addition of water scavengers or additives, waterbinding polymers, and additive-driven interfacial engineering.
CVs and CAs were recorded with a biologic potentiostat (VMP12) at 25 °C.CVs have been recorded with a scan rate of 20 mV s −1 in a potential range of −0.5 to 1.0 V vs. Al in AlCl 3 -based electrolyte, and in a potential range of −1.7 to 0.5 V vs. Ag in Al(OTF) 3 -based electrolyte.
CVs were carried out to study the possible Al reduction and oxidation reactions.CA with a constant voltage of −1 V (vs.Ag in Al(OTF) 3 -based electrolyte, and vs. Al in AlCl 3 -based electrolyte) for 5 h was performed to study the possible electroplated Al on the Al and Pt electrodes.Ex-situ electrodes characterization.-Allhandling and preparation of the ex situ Al and Pt electrodes took place inside a glovebox filled with argon (MBraun, <0.5 ppm O 2 , <0.5 ppm H 2 O).To remove the residual electrolyte, surface-modified and cycled electrodes were rinsed in anhydrous acetonitrile or methanol, then dried for 12 h under a vacuum at room temperature in a glass oven (BÜCHI Glass Oven B-585).The pretreatment of the Al foil (1 cm × 1 cm with 0.025 mm thickness) was accomplished by immersion for 18 h in 900 μl of AlCl 3 -based electrolyte.The Pt foil after CA technique in AlCl 3 -based electrolyte, as well as the immersed Al electrode before being used as WE in Al(OTF) 3 -based electrolyte, have been washed three times with anhydrous acetonitrile to make sure the residual electrolyte is removed and then vacuum-dried in a glass oven (BÜCHI Glass Oven B-585) at room temperature.The Al and Pt electrodes, after each applied electrochemical technique in Al(OTF) 3 -based electrolyte, have been rinsed three times in fresh anhydrous methanol to remove the residual electrolyte and then dried under vacuum in a glass oven (BÜCHI Glass Oven B-585) at room temperature.Scanning electron microscope (SEM) imaging

Figure 1 .
Figure 1.CVs recorded with a scan rate of 20 mV s −1 on pristine Al foil in (a) AlCl 3 -based electrolyte and (b) Al(OTF) 3 -based (c) CVs comparison.Al and Ag wires are used as quasi-reference electrodes with AlCl 3 -based and Al(OTF) 3 -based electrolytes, respecitvely.

Figure 4 shows
CAs in the AlCl 3 -based and Al(OTF) 3 -based electrolytes.Regardless of the electrolyte, a cathodic current appears during the CAs experiments.Oscillations in reductive currents during CAs could be attributed to the reductive decomposition of the anions in the AlCl 3 -based electrolyte, as also observed by Slim et al. for other electrolyte compositions.

Figure 3 .
Figure 3. (a) CV recorded with a scan rate of 20 mV s −1 on MAl foil in Al(OTF) 3 -based electrolyte.Ag wire is used as a quasi-reference electrode.(b) EDX results of cycled MAl foil.

Figure 4 .
Figure 4. Chronoamperograms recorded at −1 V, corresponding to Al deposition on Pt electrode from (a) AlCl 3 -based electrolyte and on (b) Pt (c) Al electrode from Al(OTF) 3 -based electrolyte.Al and Ag wires are used as quasi-reference electrodes.

Figure 5 .
Figure 5. XPS fine spectra with peak assignments for Pt foil after Al electrodeposition from AlCl 3 -based electrolyte.Intensities are normalized for each spectrum individually.

Figure 6 .
Figure 6.XPS fine spectra with peak assignments for Al electrode after applied chronoamperometry technique in Al(OTF) 3 -based electrolyte.Intensities are normalized for each spectrum individually.
Conclusion and outlooks.-Therecorded CVs on pristine and surface-modified Al foil reveal a lack of any successful Al plating/ stripping in Al(OTF) 3 -based electrolyte.XPS analysis of the electrodeposited Al on Pt electrode from AlCl 3 -based electrolyte indicates significant deposition of Al metal.Al metal may undergo oxidation after deposition.Concerning the Al(OTF) 3 -based electrolyte, XPS analysis of the Al and Pt electrodes reveals no electroplated Al.

Figure 7 .
Figure 7. XPS fine spectra with peak assignments for Pt electrode after applied chronoamperometry technique in Al(OTF) 3 -based electrolyte.Intensities are normalized for each spectrum individually.Note that energy window of the Pt4f spectrum does not extend far enough to capture the full asymmetric tail.

Table I .
List of electrolyte compositions reported in the literature.