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MnO films: formation of an electrochemical capacitor
Acta Materialia 53 (2005) 957–965 www.actamat-journals.com
Electrochemical oxidation of Mn/MnO films: formation of an electrochemical capacitor B. Djurfors a b
a,*
, J.N. Broughton b, M.J. Brett b, D.G. Ivey
a
Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta, Canada T6G 2G6 Department of Electrical and Computer Engineering, University of Alberta, Edmonton, Alberta, Canada T6G 2G6 Received 8 December 2003; received in revised form 26 October 2004; accepted 27 October 2004 Available online 23 November 2004
Abstract An in-depth study of the oxidation step required to produce electrochemical capacitors from porous manganese-oxide materials was carried out. The oxidation process takes place under the application of a small anodic current in a solution of 1 M Na2SO4, resulting in the formation of a three-layered structure. During the oxidation process, a base layer of undisturbed zigzag material oxidizes from Mn/MnO to Mn3O4. At the same time, a second layer forms directly on the surface of the zigzag material. The layer is partly crystalline Mn3O4 and partly amorphous. The final and most important layer is the amorphous, hydrated MnO2 surface film. It is believed that this layer is solely responsible for the capacitive behavior of these films. The porosity of the electrode prior to oxidation is shown to be immaterial as oxidation of a fully dense film results in similarly high capacitive values attributed to the formation of a porous surface layer. 2004 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Transmission electron microscopy; Manganese oxide; Electrochemical supercapacitor; Physical vapor deposition; XPS
1. Introduction Electrochemical capacitors make use of electrochemical phenomena in order to store charge, and they are primarily used for applications that require a high power output, and a high cycle capacity [1,2]. The redox capacitor, in particular, makes use of both reversible redox reactions and double layer charging in order to store charge. This behavior is typically termed ‘‘pseudocapacitance’’ and tends to resemble a re-chargeable battery more than a traditional capacitor [1]. Traditionally, ruthenium oxide has been the material focused on for pseudocapacitors as it exhibits specific capacitance values of up to 750 F/g. However, raw material *
Corresponding author. Tel.: +1 780 492 6192; fax: +1 780 492 2881. E-mail address: [email protected] (B. Djurfors).
costs are prohibitive to large-scale commercial production [2]. Manganese and its oxides have recently been proposed as replacement materials for the more costly ruthenium oxides in capacitor applications because of their relatively low cost and toxicity [3]. Typically, MnO2 films have been created using chemical reactions or electrochemical depositions and often result in an amorphous hydrated MnO2 product or a weakly crystalline hydrated MnO2 product [3–8]. Broughton and Brett [9] developed a new procedure making use of physical vapor deposition and an oblique vapor incidence angle in order to produce a chevron-type porous metallic structure that is then electrochemically oxidized and subsequently used as a capacitor. Prior to this work, the specific capacitance realized by these films, 225 ± 25 F/g, had not yet achieved the thin-film levels of Chin et al. [10] (720 F/g).
1359-6454/$30.00 2004 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.actamat.2004.10.041
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Preliminary studies indicated that the chevron structure was deposited as a two-phase mixture of Mn and MnO. The film that had been fully electrochemically oxidized and cycled over 100 times had converted to Mn3O4 while maintaining the unique structure and exhibiting good capacitance [10]. However, it was discovered that thermally producing the Mn3O4 phase does not result in an electrode exhibiting any pseudocapacitance. Therefore, there is some critical component of the electrochemical oxidation step that produces a material that is pseudocapacitive. The purpose of this work is to study the electrochemical oxidation process in this system in order to identify a process that may account for the capacitive behavior.
2. Experimental procedure The manganese films were fabricated in a vacuum evaporator, that was fitted with a glancing angle deposition (GLAD) system as described previously, using a pure manganese source [9]. The manganese films were deposited on Si substrates coated with a Ti/Pt metallization layer for adhesion and to provide an inert underlayer. The Ti layer thickness is approximately 25 nm and the Pt layer is 300 nm. The films were made with a chevron texture utilizing an incidence flux angle of 80, with a base pressure of approximately 7 · 10 5 Pa. The resulting deposited chevron layer was approximately 460 nm thick. Dense films were evaporated using the same substrate, but without substrate rotation, resulting in a 38 nm thick, dense Mn layer. The electrochemically oxidized samples were produced by anodizing in a 1 M Na2SO4 electrolyte using a CHA660a potentiostat, with an Ag/AgCl reference electrode and a Pt wire as the counter electrode. A sample was electrochemically oxidized by applying a constant anodic current over a specified time period. The oxidation process is considered complete and the film becomes capacitive once a red surface finish has been obtained and the potential has reached 0.9 V. A current density of 10 4 A/cm2 was used. The first sample was partially oxidized for 1150 s which corresponded to a green color, the second sample was partially oxidized for 1700 s and was still green in color, although the surface finish had darkened, and the final sample was oxidized for 4750 s to completion and a shiny red color. The samples were imaged first in a JEOL field emission scanning electron microscope (FE-SEM) at 5 kV without any conductive coating. Higher resolution imaging was done primarily in a JEOL 2010 transmission electron microscope (TEM) with a Noran ultra-thin window (UTW) X-ray detector for compositional analysis using the energy dispersive X-ray (EDX) technique. Both plan view and cross-section samples were imaged
at 200 kV. Crystal structure analysis was done using selected area diffraction (SAD) patterns. Cross-section samples were made using a standard grinding and dimpling process. The sample was then finished in an ion mill by sputtering from both sides, at an energy of 5 kV with a current of 0.5 mA per gun and an incidence angle of 85 for 15 min. The incident angle was then increased to 87 and the current and gun energy lowered to 0.3 mA per gun and 4 kV, respectively, for a further 45 min. All samples were cooled with liquid nitrogen prior to and during sputtering, to reduce preferential sputtering effects. Plan view samples were made by dimpling from the backside of the material to approximately 10 lm in thickness. Then, samples were sputtered from the substrate side only using the same conditions as above. Surface analysis of various films was carried out by X-ray photoelectron spectroscopy (XPS) using a Kratos AXIS 165 X-ray Photoelectron Spectrometer. A monochromatic Al source was used operating at 210 W giving a pass energy of 20 eV and a step of 0.1 eV.
3. Results During electrochemical oxidation processing of these films in 1 M Na2SO4, it is possible to identify at least one intermediate step based on a visual change in the color of the film surface. In the as-deposited state, the film surface is a grey/black color. During the oxidation process, the color changes first to green and then to red. Once the film has achieved a red color, it will display a typical capacitive response under cyclic voltammetry (CV) cycling. After several hundred cycles, the film surface remains red in color with no other visible changes. 3.1. Chevron films 3.1.1. Partial oxidation-green film (1150 and 1700 s) The first visual change during the electrochemical oxidation process is the formation of a green surface film indicating a significant chemical change. FE-SEM images of this initial step (Fig. 1) reveal that the bulk of the zigzag layer is still intact. Samples oxidized for a shorter time show minimal surface disruption (Fig. 1(b)), however, samples oxidized for a longer period (Fig. 1(c)) reveal that two additional surface layers have formed (Fig. 1(c)). Directly on the surface of the zigzag is a dense layer of disturbed material. On top of this layer is a fairly thick, extremely porous layer protruding from the surface. These layers are hereafter referred to as the dense intermediate layer and the porous surface layer, respectively. Cross-sectional TEM images (Fig. 2(a) and (b)) clearly show the porous surface layer on the surface of the denser Mn zigzag layer. At 1150 s, the porous sur-
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Fig. 1. (a) FE-SEM image of a film in the as-deposited state, (b) partially oxidized for 1150 s and (c) partially oxidized for 1700 s.
Fig. 2. (a) TEM BF cross-sectional image of the sample oxidized for 1150 s. (b) TEM BF cross-sectional image of the sample oxidized for 1750 s with inset diffraction pattern showing some crystallinity, indexed as single phase Mn3O4.
face layer is 250 nm thick and just over 500 nm thick at 1750 s. EDX analysis from the porous layer reveals that it is predominantly manganese and oxygen based suggesting a portion of the original zigzag layer has been re-structured to produce this new layer.
A diffraction pattern from porous surface layer of the sample oxidized for 1700 s (inset Fig. 2(b)) indicates partial crystallinity and was indexed as the Mn3O4 phase. Despite the weak intensities, there were no MnO rings identified, suggesting the porous surface layer is single
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phase Mn3O4 with mixed crystalline and amorphous content. Unfortunately, there is no certainty that the intermediate layer was not sampled as well in the diffraction pattern. No pattern was obtained for the other sample due to the small volume of surface film available. Because of the difficulty in trying to identify the surface layer in cross-section (due to the small volume of material), plan view sections were made in order to attempt to identify the structure of the porous surface layer that is forming. A representative plan view section (Fig. 3) reveals a layer that is partially crystalline in nature but with some amorphous content. However, the morphology presented in the plan section suggests that the image in Fig. 3 do not represent the porous surface layer, but rather, the denser intermediate layer just beneath. For the sample that was oxidized for 1150 s (Fig. 3), the diffraction pattern reveals that both MnO and Mn3O4 are present. Thus, the transition to Mn3O4 is not complete at this point, and likely the residual green color of the film is due to the presence of MnO which is also green in color. The sample oxidized for 1700 s (not shown) also indicates a mixture of MnO and Mn3O4. This contradicts the results obtained from the cross-sections further suggesting that the plan section is actually sampling the dense intermediate layer, rather than the porous surface layer. The TEM is unable to completely characterize the porous surface layer, and thus XPS (Section 3.1.4) is used to identify the structure of the surface layer. In both samples, despite the growth of a porous surface layer, the bulk of the original zigzag structure remains intact (Fig. 1). Unfortunately, in cross-section, it is not possible to image all layers at one time because of preferential sputtering effects during sample preparation. Fig. 4 shows a higher magnification image of the
Fig. 3. TEM DF plan view image of porous surface layer taken from boxed area of inset diffraction pattern showing partial crystallinity.
Fig. 4. (a) TEM BF cross-sectional image of zigzag layer from sample electrochemically oxidized for 1150 s. (b) SAD pattern from zigzag layer indexed as MnO and Mn3O4. (c) SAD pattern from zigzag layer indexed as MnO. (d) TEM BF cross-sectional image of zigzag layer from sample electrochemically oxidized for 1750 s. (e) SAD pattern from the dense intermediate layer indexed as a mixture of MnO and Mn3O4. (f) SAD pattern from zigzag layer indexed as single phase MnO.
bottom layers of the electrochemically oxidized structure for both samples with accompanying diffraction patterns. The sample that was oxidized for 1150 s (Fig. 4(a)– (c)) results in the remaining zigzag layer being converted to a two-phase mixture of MnO and Mn3O4. In some areas, the reflections for MnO are very weak (Fig. 4(b)) suggesting the bulk of the zigzag material is Mn3O4. In other areas, it seems only MnO is present in the zigzag layer (Fig. 4(c)), suggesting a non-uniform oxidation. For the sample oxidized for 1700 s, the undisturbed zigzag layer and the dense intermediate layer that were visible in Fig. 1(b) are also visible in Fig. 4(d). The dense intermediate layer is 80 nm thick and identified through diffraction as a mixture of MnO and Mn3O4 (Fig. 4(e)). Again, the reflections for MnO are extremely weak indicating the bulk is Mn3O4. In addition, the remaining portion of the zigzag layer is 200 nm thick and identified as single phase MnO (Fig. 4(f)). While it appears that the longer oxidation time has resulted in less oxidation of the zigzag layer because no Mn3O4 is found near the substrate as with the shorter oxidation time, this may be a sampling artifact. The thin area sam-
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pled did not contain any Mn3O4, but it is likely still present elsewhere in the sample. Thus the Mn in the zigzag layer is first oxidized from Mn to MnO, which is then oxidized to Mn3O4. 3.1.2. Full oxidation-red film Once the oxidation cycle is complete (4750 s), the final color of the film is red and the film exhibits the capacitive tendencies previously reported [9]. Fig. 5 is a FE-SEM image of the film showing both a porous surface layer that was visible in the previous samples and the zigzag layer, which is still intact. Thus, even at the completion of the oxidation process, the zigzag film is not destroyed. Fig. 6(a) and (b) show two TEM crosssection views of the film. Fig. 6(a) shows a region where two samples have been glued face-to-face, so that the porous surface layer on each is visible and measured to be 120 nm thick. Fig. 6(b) shows the intact zigzag layer, 420 nm thick, with an inset diffraction pattern from the area indexed as single phase Mn3O4. The continued oxidation of the structure does not destroy the porous, surface layer that is growing, it simply further oxidizes the material underneath from MnO
Fig. 5. FE-SEM cross-section image of the sample electrochemically oxidized for 4750 s.
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(Fig. 4) to Mn3O4. Indeed, the plan view (Fig. 7), which is truly representative of the outermost, surface layer, reveals that the continued oxidation results in the formation of an entirely amorphous layer, where before it was still partially crystalline (Fig. 2(b)). Unfortunately, because the layer is amorphous, its structure cannot be identified through electron diffraction, however, it is likely either Mn3O4 or a phase with a higher Mn valence (i.e. Mn4+). After complete oxidation of the layer, 400 nm of the zigzag layer remains from the original 450 nm. Thus, 50 nm of zigzag material have been consumed to produce over 100 nm of the porous surface layer. 3.1.3. Fully cycled film Once the sample has been completely oxidized, it exhibits supercapacitive properties. Even after the films are cycled as a supercapacitors several hundred times, the three-stage structure (underlayer, intermediate layer and porous surface layer) is not destroyed. However, the thickness of the porous surface layer becomes slightly reduced (by approximately 20%). This further suggests that the outermost porous surface layer is directly involved in the capacitance mechanism. As reported previously [10], the zigzag structure is single-phase Mn3O4. Thus, there is no further oxidation of the structure occurring while it is being cycled. Presumably, the only changes occurring during cycling should be reversible and this appears to be the case. In addition, the previously reported surface-active layer [10] has now been identified as the dense intermediate layer that is just beneath the porous surface layer. This layer is not evidence of irreversibility in the process as previously thought, but more likely is linked to the capacitive mechanism. The porous surface layer is likely the only one involved in charging and discharging since it is the only portion accessible to the electrolyte. 3.1.4. XPS SAD in the TEM can only provide limited structural information about the porous surface layer because the
Fig. 6. (a) TEM BF cross-section image showing growth of the porous surface layer. (b) TEM BF cross-section image of zigzag layer with inset diffraction pattern indexed as single phase Mn3O4.
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Fig. 7. (a) TEM BF plan view image of porous surface layer. (b) Amorphous SAD pattern from (a).
layer is amorphous. As a result, XPS was employed to determine the structure of the surface layer. Unfortunately, the C1s peaks exhibit a low intensity shoulder at a binding energy greater than 285 eV suggesting there is some contamination. This effect has been linked to differential charging in the past and has made it difficult to determine the manganese valence from the Mn 2p peaks [11]. Instead, the Mn 3s peaks will be used to approximate the valence using 3s peak splitting widths from Chigane and Ishikawa [11]. The data from the Mn 3s peak splitting widths is tabulated in Table 1. The valence and phase of the sample are identified through direct comparison to standard values from literature. For instance, a splitting width of 5.54 eV falls between the splitting width values for MnO (5.79 eV) and Mn3O4 (5.50 eV) and therefore must contain some amount of both oxides. Because the value, 5.54 eV is closer to the value for the standard Mn3O4 (5.50 eV) it can be assumed that there is more Mn3O4 present than MnO. This procedure was used to determine the valence and phase of each sample listed in Table 1.
Table 1 XPS results from the Mn 3s peak showing approximate valence of each sample (standard manganese oxides from literature are also includeda) Sample
Mn 3s splitting width, DE (eV)
Valence of Mn
Phase
1150 s 1700 s 4750 s Fully cycled MnOa Mn3O4a Mn2O3a MnO2a
5.54 5.48 4.79 5.79 5.79 5.50 5.41 4.78
2, 3 2, 3 4 2 2 2, 3 3 4
MnO and Mn3O4 Mn3O4 and Mn2O3 MnO2 MnO MnO Mn3O4 Mn2O3 MnO2
a
Data taken from [11].
Samples that are partially oxidized and still green in color (1150 and 1700 s) are both shown to be a mixture of Mn2+ and Mn3+. At 1150 s, MnO and Mn3O4 are present, however by 1700 s, Mn3O4 and Mn2O3 are present indicating the continued oxidation of the film. Once the film has been completely oxidized with a color change from green to red (4750 s), the valence of the surface film is entirely Mn4+, or MnO2. Thus, there is a conversion from the as-deposited 0/2+ state (Mn/MnO) to an intermediate 2+/3+ (MnO/Mn3O4 for 1150 s and Mn3O4/ Mn2O3 for 1700 s) state and finally a 4+ state (MnO2) when fully oxidized. This is the expected oxidation path for a Mn sample in an alkaline solution [12] ultimately ending with the production of MnO2 (Mn4+). These results are in keeping with the oxidation of the other layers of the structure (dense intermediate and zigzag) as outlined by the TEM analysis. Deconvolution of the O1s peak can give useful information regarding the state of oxygen bonding and thus the hydration of the sample. A typical O1s spectrum that is deconvoluted into its three components is shown in Fig. 8 with the results tabulated in Table 2. Both samples that have a green surface film (Section 3.1.1) have approximately the same amount of hydrated material in the structure (45%) with the balance being oxide and free water. By the time the oxidation is complete (Section 3.1.2), the hydrated content in the film has been reduced to only 32%. However, relatively speaking, this is a fairly significant amount of hydration. Other researchers have reported values from 14% to 52% hydroxide content depending on the deposition conditions [11,13,14]. The formation of the hydrated film is thought to be critical to the capacitance mechanism. The fraction of reaction sites that are accessible to the Faradic process is larger when the film is in a hydrous form. It has been suggested that the hydrated structure has improved protonic and electronic conductivity,
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Fig. 8. XPS O1s spectrum from the sample oxidized for 4750 s. The contribution of each different oxygen bond to the total peak (oxide, hydroxide, and water) is indicated. Table 2 XPS data from the de-convolution of the O1s peak Sample
Eb (eV)
at.%
Bond Type
1150 s
530.1 531.6 532.3
51 45 4
Mn–O–Mn Mn–O–H H–O–H
1700 s
530.2 531.4 532.2
36 45 19
Mn–O–Mn Mn–O–H H–O–H
4750 s
530.2 531.7 533.3
63 32 5
Mn–O–Mn Mn–O–H H–O–H
Fully cycled
529.7 531.1 532.0
27 33 40
Mn–O–Mn Mn–O–H H–O–H
32 h anneal
529.9 – 531.9
50 – 50
Mn–O–Mn Mn–O–H H–O–H
resulting in superior capacitance for hydrated structures vs. unhydrated structures [1]. Once the sample has been cycled several hundred times (Section 3.1.3), there does not appear to be any visible change in the three layers; however, the XPS results show that some changes are occurring. The Mn 3s data indicates a valence of 2+, corresponding to the MnO phase. This result reveals that changes have occurred during the cycling process and the porous surface film has been reduced from the 4+ valence to the 2+ valence. This valence change is indicative of irreversibility in the process where non-reversible reduction reactions are occurring in addition to the reversible pseudo-capac-
itive reaction, ultimately resulting in a permanent valence change to the film. The hydrated content of the film (33%) has not changed noticeably from the precycled condition, suggesting that the hydrated structure is not being destroyed through the reduction process. It was reported in previous work that thermal oxidation of Mn chevron films in an air at 300 C was detrimental to capacitance [9,10]. The reason for this decrease in capacitance was previously thought to be due to the formation of a capping layer on the surface of the zigzag film that prevented solution penetration [10] and reduced the usable surface area. The thermal oxidation process ultimately forms dense Mn3O4 as the surface layer [10]; however, the recent XPS analysis (Table 2) confirms that the thermally annealed sample contains no hydrated manganese (Mn–O–H) only anhydrous compounds (Mn–O–Mn) and free water (H–O– H). There is a significant quantity of free water (50%) in the original scan, but heating the sample to 100 C for a short time in the XPS chamber resulted in a significant reduction in the amount of water present (30% reduction). The removal of water upon low temperature heating confirms that it is surface adsorbed water and not structural water and thus does not provide any useful chemical hydration (i.e., Mn–O–H type bonds) to develop capacitance. Furthermore, when a thermally annealed sample is put into the Na2SO4 solution and the oxidizing current is applied, the distinct porous surface layer that has been shown to provide significant capacitance does not form, and the film does not follow a similar oxidation trend (potential vs. time) as the Mn/MnO materials. It is believed that thermal oxidation of the film prior to electrochemical treatment
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has altered the surface phases and chemistry such that it is no longer possible to produce the porous, hydrated, amorphous layer upon further electrochemical oxidation. The reason that the porous surface layers will no longer form after thermal oxidation is likely because the layer is already in an advanced state of oxidation (Mn3O4), and there is insufficient opportunity during the oxidation to MnO2 to form a porous layer with the necessary hydrated and amorphous structure. The lack of hydration and an amorphous structure prevents ion insertion and thus no capacitance is measured. The formation of the oxidized capping layer during thermal annealing essentially prevents the formation of the amorphous, hydrated porous surface film that forms during electrochemical oxidation, which is now seen to be critical to the capacitance mechanism. 3.1.5. Dense films The suggestion that the hydrated surface layer produced during electrochemical oxidation is the only surface involved in the capacitance mechanism has potentially eliminated the need for a porous starting surface, such as the chevron films. As a result, dense films, 38 nm in thickness, were deposited and their behavior during electrochemical oxidation was studied. The dense films have a typical columnar thin-film morphology and are a two-phase mixture of Mn and MnO (the same composition as the chevron films [10]). With the same initial starting point, the dense films were subjected to the same oxidation process at the same current density as the chevron films. This particular film was oxidized for 900 s, after which a red surface film formed as with the chevron structures. Fig. 9(a) shows the final oxidized structure. Note that the porous surface film that forms is the same film that had formed on the chevron films with a thickness of approximately 100 nm. The accompanying diffraction pattern (Fig. 9(b)) confirms that this film is amorphous in nature. Thus, despite the significant
reduction in initial surface area and porosity, the electrochemical oxidation process results in a porous surface film that is similar to the one seen with the chevron structure. The electrochemical behavior of these dense films will be discussed in a separate paper. The important conclusion at this point is that the morphology of the starting material does not matter, as both porous and dense starting layers will form the porous surface layer upon oxidation that is responsible for the pseudocapacitive tendencies.
4. Discussion The results indicate that any starting material, in any form, could be used to produce pseudocapacitance provided it is pure Mn or a mixture of Mn/MnO. If the valence of the starting material is too high (i.e. 3+), such that the electrochemical oxidation will not produce any further oxidation, then the porous surface film cannot be formed, and no capacitance can be measured. This phenomenon was found during previous studies [10], where it was discovered that dense films of mixed MnO/Mn3O4 did not exhibit pseudocapacitance because the electrochemical oxidation process did not produce a porous surface film, but rather had a destructive impact on the film. It is suspected that the starting valence of the film was too high, and as Mn3O4 is extremely stable in alkaline solutions [13], the application of the anodic current to the film resulted in destruction rather than controlled oxidation. The electrochemical oxidation process in Na2SO4 results in the formation of an extremely porous surface layer. Although the underlying zigzag layer is oxidized from Mn/MnO to single phase Mn3O4, the more important result is the formation of the porous surface layer. Electron microscopy has confirmed the porous nature of this film, and selected area diffraction has confirmed
Fig. 9. (a) TEM BF image of the dense film electrochemically oxidized for 900 s. (b) SAD pattern from the thinnest portion of the plan view revealing its amorphous nature.
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its amorphous nature. XPS results indicate that the surface of the film is made up of hydrated manganese oxide in the 4+ valence state. In addition, it has been shown that the morphology of the underlying substrate is immaterial as a dense film of manganese oxide also forms the extremely porous, amorphous, hydrated surface film. Since this film has superior specific capacitance to the thicker zigzag films, the importance of the surface film in the capacitance mechanism is clearer. It is likely that the bulk film does not take part in the charge/discharge process, only the surface film does. The relatively smaller amount of ‘‘carrier’’ material in the denser film results in a larger capacitance per mass. However, it is likely that both bulk and thin films would exhibit similar ‘‘capacitance per area’’ values, since the area of porous surface film produced is likely very similar for both bulk and thin films. In addition, capacitance per area would be a more appropriate measure of capacitance, since it is suspected that only that portion of the film is involved in the charge/ discharge; the rest of the film provides substrate support. The dense intermediate layer directly beneath the porous layer may simply be a physical marker of the separation of the active portion of the electrode from the carrier/substrate portion of the electrode. These results should greatly simplify the electrode fabrication process, since very little attention need be paid to the porosity of the starting material, as the electrochemical oxidation process produces its own suitably porous surface. This finding eliminates the need to produce the porous amorphous films using more complicated, multi-step processes. Instead, supercapacitive processing requires only a single solution of Na2SO4 and the electrochemical oxidation process can be done in-situ after the capacitor is packaged. In addition, the starting electrode material need not be pure Mn. This process works extremely well using a two-phase mixture of Mn/MnO, which is easily produced in any PVD system since there is no requirement for low oxide content.
5. Conclusions In summary, the electrochemical oxidation process of Mn/MnO starting films, of any morphology, results in
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the formation of a three-layer structure consisting of undisturbed base layer, a dense intermediate layer directly on top of the base layer and a porous surface layer. It is likely that the porous surface layer is responsible for the capacitive tendencies of the film. During the electrochemical oxidation process, the undisturbed base layer and the dense disturbed layer oxidize to Mn3O4. The porous surface layer, however, oxidizes to an amorphous form of MnO2. XPS results indicate that this outermost porous layer is a hydrated compound accounting for the excellent capacitance values that can be measured with these films.
Acknowledgements The authors acknowledge funding contributions from the Natural Sciences and Engineering Research Council of Canada (NSERC) in addition to the Alberta Informatics Circle of Research Excellence (iCORE) and Micralyne.
References [1] Conway BE. Electrochemical supercapacitors. New York: Kluwer/Plenum Publishers; 1999. [2] Kotz R, Carlen M. Electrochim Acta 2000;45:2438. [3] Toupin M, Brousse T, Belanger D. Chem Mater 2002;14:3946. [4] Jeong YU, Manthiram A. J Electrochem Soc 2002;149(11):A1419. [5] Pang S, Anderson MA, Chapman TW. J Electrochem Soc 2000;147(2):444. [6] Pang S, Anderson MA. J Mater Res 2000;15(10):2096. [7] Hu C, Tsou T. Electrochim Acta 2002;47:3523. [8] Lee HY, Kim SW, Lee HY. Electrochem Solid-State Lett 2001;4(3):A19. [9] Broughton JN, Brett MJ. Electrochem Solid State Lett 2002;5(12):A279. [10] Djurfors B, Broughton JN, Brett MJ, Ivey DG. J Mater Sci 2003;38(24):4817. [11] Chigane M, Ishikawa M. J Electrochem Soc 2000;147(6):2246. [12] Messaoudi M, Joiret S, Keddam M, Takenouti H. Electrochim Acta 2001;45:2487. [13] Chigane M, Ishikawa M, Izaki M. J Electrochem Soc 2001;148(7):D96. [14] Hu C, Wang C. J Electrochem Soc 2003;150(8):A1079.
Electrochemical oxidation of Mn/MnO films: formation of an electrochemical capacitor B. Djurfors a b
a,*
, J.N. Broughton b, M.J. Brett b, D.G. Ivey
a
Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta, Canada T6G 2G6 Department of Electrical and Computer Engineering, University of Alberta, Edmonton, Alberta, Canada T6G 2G6 Received 8 December 2003; received in revised form 26 October 2004; accepted 27 October 2004 Available online 23 November 2004
Abstract An in-depth study of the oxidation step required to produce electrochemical capacitors from porous manganese-oxide materials was carried out. The oxidation process takes place under the application of a small anodic current in a solution of 1 M Na2SO4, resulting in the formation of a three-layered structure. During the oxidation process, a base layer of undisturbed zigzag material oxidizes from Mn/MnO to Mn3O4. At the same time, a second layer forms directly on the surface of the zigzag material. The layer is partly crystalline Mn3O4 and partly amorphous. The final and most important layer is the amorphous, hydrated MnO2 surface film. It is believed that this layer is solely responsible for the capacitive behavior of these films. The porosity of the electrode prior to oxidation is shown to be immaterial as oxidation of a fully dense film results in similarly high capacitive values attributed to the formation of a porous surface layer. 2004 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Transmission electron microscopy; Manganese oxide; Electrochemical supercapacitor; Physical vapor deposition; XPS
1. Introduction Electrochemical capacitors make use of electrochemical phenomena in order to store charge, and they are primarily used for applications that require a high power output, and a high cycle capacity [1,2]. The redox capacitor, in particular, makes use of both reversible redox reactions and double layer charging in order to store charge. This behavior is typically termed ‘‘pseudocapacitance’’ and tends to resemble a re-chargeable battery more than a traditional capacitor [1]. Traditionally, ruthenium oxide has been the material focused on for pseudocapacitors as it exhibits specific capacitance values of up to 750 F/g. However, raw material *
Corresponding author. Tel.: +1 780 492 6192; fax: +1 780 492 2881. E-mail address: [email protected] (B. Djurfors).
costs are prohibitive to large-scale commercial production [2]. Manganese and its oxides have recently been proposed as replacement materials for the more costly ruthenium oxides in capacitor applications because of their relatively low cost and toxicity [3]. Typically, MnO2 films have been created using chemical reactions or electrochemical depositions and often result in an amorphous hydrated MnO2 product or a weakly crystalline hydrated MnO2 product [3–8]. Broughton and Brett [9] developed a new procedure making use of physical vapor deposition and an oblique vapor incidence angle in order to produce a chevron-type porous metallic structure that is then electrochemically oxidized and subsequently used as a capacitor. Prior to this work, the specific capacitance realized by these films, 225 ± 25 F/g, had not yet achieved the thin-film levels of Chin et al. [10] (720 F/g).
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Preliminary studies indicated that the chevron structure was deposited as a two-phase mixture of Mn and MnO. The film that had been fully electrochemically oxidized and cycled over 100 times had converted to Mn3O4 while maintaining the unique structure and exhibiting good capacitance [10]. However, it was discovered that thermally producing the Mn3O4 phase does not result in an electrode exhibiting any pseudocapacitance. Therefore, there is some critical component of the electrochemical oxidation step that produces a material that is pseudocapacitive. The purpose of this work is to study the electrochemical oxidation process in this system in order to identify a process that may account for the capacitive behavior.
2. Experimental procedure The manganese films were fabricated in a vacuum evaporator, that was fitted with a glancing angle deposition (GLAD) system as described previously, using a pure manganese source [9]. The manganese films were deposited on Si substrates coated with a Ti/Pt metallization layer for adhesion and to provide an inert underlayer. The Ti layer thickness is approximately 25 nm and the Pt layer is 300 nm. The films were made with a chevron texture utilizing an incidence flux angle of 80, with a base pressure of approximately 7 · 10 5 Pa. The resulting deposited chevron layer was approximately 460 nm thick. Dense films were evaporated using the same substrate, but without substrate rotation, resulting in a 38 nm thick, dense Mn layer. The electrochemically oxidized samples were produced by anodizing in a 1 M Na2SO4 electrolyte using a CHA660a potentiostat, with an Ag/AgCl reference electrode and a Pt wire as the counter electrode. A sample was electrochemically oxidized by applying a constant anodic current over a specified time period. The oxidation process is considered complete and the film becomes capacitive once a red surface finish has been obtained and the potential has reached 0.9 V. A current density of 10 4 A/cm2 was used. The first sample was partially oxidized for 1150 s which corresponded to a green color, the second sample was partially oxidized for 1700 s and was still green in color, although the surface finish had darkened, and the final sample was oxidized for 4750 s to completion and a shiny red color. The samples were imaged first in a JEOL field emission scanning electron microscope (FE-SEM) at 5 kV without any conductive coating. Higher resolution imaging was done primarily in a JEOL 2010 transmission electron microscope (TEM) with a Noran ultra-thin window (UTW) X-ray detector for compositional analysis using the energy dispersive X-ray (EDX) technique. Both plan view and cross-section samples were imaged
at 200 kV. Crystal structure analysis was done using selected area diffraction (SAD) patterns. Cross-section samples were made using a standard grinding and dimpling process. The sample was then finished in an ion mill by sputtering from both sides, at an energy of 5 kV with a current of 0.5 mA per gun and an incidence angle of 85 for 15 min. The incident angle was then increased to 87 and the current and gun energy lowered to 0.3 mA per gun and 4 kV, respectively, for a further 45 min. All samples were cooled with liquid nitrogen prior to and during sputtering, to reduce preferential sputtering effects. Plan view samples were made by dimpling from the backside of the material to approximately 10 lm in thickness. Then, samples were sputtered from the substrate side only using the same conditions as above. Surface analysis of various films was carried out by X-ray photoelectron spectroscopy (XPS) using a Kratos AXIS 165 X-ray Photoelectron Spectrometer. A monochromatic Al source was used operating at 210 W giving a pass energy of 20 eV and a step of 0.1 eV.
3. Results During electrochemical oxidation processing of these films in 1 M Na2SO4, it is possible to identify at least one intermediate step based on a visual change in the color of the film surface. In the as-deposited state, the film surface is a grey/black color. During the oxidation process, the color changes first to green and then to red. Once the film has achieved a red color, it will display a typical capacitive response under cyclic voltammetry (CV) cycling. After several hundred cycles, the film surface remains red in color with no other visible changes. 3.1. Chevron films 3.1.1. Partial oxidation-green film (1150 and 1700 s) The first visual change during the electrochemical oxidation process is the formation of a green surface film indicating a significant chemical change. FE-SEM images of this initial step (Fig. 1) reveal that the bulk of the zigzag layer is still intact. Samples oxidized for a shorter time show minimal surface disruption (Fig. 1(b)), however, samples oxidized for a longer period (Fig. 1(c)) reveal that two additional surface layers have formed (Fig. 1(c)). Directly on the surface of the zigzag is a dense layer of disturbed material. On top of this layer is a fairly thick, extremely porous layer protruding from the surface. These layers are hereafter referred to as the dense intermediate layer and the porous surface layer, respectively. Cross-sectional TEM images (Fig. 2(a) and (b)) clearly show the porous surface layer on the surface of the denser Mn zigzag layer. At 1150 s, the porous sur-
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Fig. 1. (a) FE-SEM image of a film in the as-deposited state, (b) partially oxidized for 1150 s and (c) partially oxidized for 1700 s.
Fig. 2. (a) TEM BF cross-sectional image of the sample oxidized for 1150 s. (b) TEM BF cross-sectional image of the sample oxidized for 1750 s with inset diffraction pattern showing some crystallinity, indexed as single phase Mn3O4.
face layer is 250 nm thick and just over 500 nm thick at 1750 s. EDX analysis from the porous layer reveals that it is predominantly manganese and oxygen based suggesting a portion of the original zigzag layer has been re-structured to produce this new layer.
A diffraction pattern from porous surface layer of the sample oxidized for 1700 s (inset Fig. 2(b)) indicates partial crystallinity and was indexed as the Mn3O4 phase. Despite the weak intensities, there were no MnO rings identified, suggesting the porous surface layer is single
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phase Mn3O4 with mixed crystalline and amorphous content. Unfortunately, there is no certainty that the intermediate layer was not sampled as well in the diffraction pattern. No pattern was obtained for the other sample due to the small volume of surface film available. Because of the difficulty in trying to identify the surface layer in cross-section (due to the small volume of material), plan view sections were made in order to attempt to identify the structure of the porous surface layer that is forming. A representative plan view section (Fig. 3) reveals a layer that is partially crystalline in nature but with some amorphous content. However, the morphology presented in the plan section suggests that the image in Fig. 3 do not represent the porous surface layer, but rather, the denser intermediate layer just beneath. For the sample that was oxidized for 1150 s (Fig. 3), the diffraction pattern reveals that both MnO and Mn3O4 are present. Thus, the transition to Mn3O4 is not complete at this point, and likely the residual green color of the film is due to the presence of MnO which is also green in color. The sample oxidized for 1700 s (not shown) also indicates a mixture of MnO and Mn3O4. This contradicts the results obtained from the cross-sections further suggesting that the plan section is actually sampling the dense intermediate layer, rather than the porous surface layer. The TEM is unable to completely characterize the porous surface layer, and thus XPS (Section 3.1.4) is used to identify the structure of the surface layer. In both samples, despite the growth of a porous surface layer, the bulk of the original zigzag structure remains intact (Fig. 1). Unfortunately, in cross-section, it is not possible to image all layers at one time because of preferential sputtering effects during sample preparation. Fig. 4 shows a higher magnification image of the
Fig. 3. TEM DF plan view image of porous surface layer taken from boxed area of inset diffraction pattern showing partial crystallinity.
Fig. 4. (a) TEM BF cross-sectional image of zigzag layer from sample electrochemically oxidized for 1150 s. (b) SAD pattern from zigzag layer indexed as MnO and Mn3O4. (c) SAD pattern from zigzag layer indexed as MnO. (d) TEM BF cross-sectional image of zigzag layer from sample electrochemically oxidized for 1750 s. (e) SAD pattern from the dense intermediate layer indexed as a mixture of MnO and Mn3O4. (f) SAD pattern from zigzag layer indexed as single phase MnO.
bottom layers of the electrochemically oxidized structure for both samples with accompanying diffraction patterns. The sample that was oxidized for 1150 s (Fig. 4(a)– (c)) results in the remaining zigzag layer being converted to a two-phase mixture of MnO and Mn3O4. In some areas, the reflections for MnO are very weak (Fig. 4(b)) suggesting the bulk of the zigzag material is Mn3O4. In other areas, it seems only MnO is present in the zigzag layer (Fig. 4(c)), suggesting a non-uniform oxidation. For the sample oxidized for 1700 s, the undisturbed zigzag layer and the dense intermediate layer that were visible in Fig. 1(b) are also visible in Fig. 4(d). The dense intermediate layer is 80 nm thick and identified through diffraction as a mixture of MnO and Mn3O4 (Fig. 4(e)). Again, the reflections for MnO are extremely weak indicating the bulk is Mn3O4. In addition, the remaining portion of the zigzag layer is 200 nm thick and identified as single phase MnO (Fig. 4(f)). While it appears that the longer oxidation time has resulted in less oxidation of the zigzag layer because no Mn3O4 is found near the substrate as with the shorter oxidation time, this may be a sampling artifact. The thin area sam-
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pled did not contain any Mn3O4, but it is likely still present elsewhere in the sample. Thus the Mn in the zigzag layer is first oxidized from Mn to MnO, which is then oxidized to Mn3O4. 3.1.2. Full oxidation-red film Once the oxidation cycle is complete (4750 s), the final color of the film is red and the film exhibits the capacitive tendencies previously reported [9]. Fig. 5 is a FE-SEM image of the film showing both a porous surface layer that was visible in the previous samples and the zigzag layer, which is still intact. Thus, even at the completion of the oxidation process, the zigzag film is not destroyed. Fig. 6(a) and (b) show two TEM crosssection views of the film. Fig. 6(a) shows a region where two samples have been glued face-to-face, so that the porous surface layer on each is visible and measured to be 120 nm thick. Fig. 6(b) shows the intact zigzag layer, 420 nm thick, with an inset diffraction pattern from the area indexed as single phase Mn3O4. The continued oxidation of the structure does not destroy the porous, surface layer that is growing, it simply further oxidizes the material underneath from MnO
Fig. 5. FE-SEM cross-section image of the sample electrochemically oxidized for 4750 s.
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(Fig. 4) to Mn3O4. Indeed, the plan view (Fig. 7), which is truly representative of the outermost, surface layer, reveals that the continued oxidation results in the formation of an entirely amorphous layer, where before it was still partially crystalline (Fig. 2(b)). Unfortunately, because the layer is amorphous, its structure cannot be identified through electron diffraction, however, it is likely either Mn3O4 or a phase with a higher Mn valence (i.e. Mn4+). After complete oxidation of the layer, 400 nm of the zigzag layer remains from the original 450 nm. Thus, 50 nm of zigzag material have been consumed to produce over 100 nm of the porous surface layer. 3.1.3. Fully cycled film Once the sample has been completely oxidized, it exhibits supercapacitive properties. Even after the films are cycled as a supercapacitors several hundred times, the three-stage structure (underlayer, intermediate layer and porous surface layer) is not destroyed. However, the thickness of the porous surface layer becomes slightly reduced (by approximately 20%). This further suggests that the outermost porous surface layer is directly involved in the capacitance mechanism. As reported previously [10], the zigzag structure is single-phase Mn3O4. Thus, there is no further oxidation of the structure occurring while it is being cycled. Presumably, the only changes occurring during cycling should be reversible and this appears to be the case. In addition, the previously reported surface-active layer [10] has now been identified as the dense intermediate layer that is just beneath the porous surface layer. This layer is not evidence of irreversibility in the process as previously thought, but more likely is linked to the capacitive mechanism. The porous surface layer is likely the only one involved in charging and discharging since it is the only portion accessible to the electrolyte. 3.1.4. XPS SAD in the TEM can only provide limited structural information about the porous surface layer because the
Fig. 6. (a) TEM BF cross-section image showing growth of the porous surface layer. (b) TEM BF cross-section image of zigzag layer with inset diffraction pattern indexed as single phase Mn3O4.
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Fig. 7. (a) TEM BF plan view image of porous surface layer. (b) Amorphous SAD pattern from (a).
layer is amorphous. As a result, XPS was employed to determine the structure of the surface layer. Unfortunately, the C1s peaks exhibit a low intensity shoulder at a binding energy greater than 285 eV suggesting there is some contamination. This effect has been linked to differential charging in the past and has made it difficult to determine the manganese valence from the Mn 2p peaks [11]. Instead, the Mn 3s peaks will be used to approximate the valence using 3s peak splitting widths from Chigane and Ishikawa [11]. The data from the Mn 3s peak splitting widths is tabulated in Table 1. The valence and phase of the sample are identified through direct comparison to standard values from literature. For instance, a splitting width of 5.54 eV falls between the splitting width values for MnO (5.79 eV) and Mn3O4 (5.50 eV) and therefore must contain some amount of both oxides. Because the value, 5.54 eV is closer to the value for the standard Mn3O4 (5.50 eV) it can be assumed that there is more Mn3O4 present than MnO. This procedure was used to determine the valence and phase of each sample listed in Table 1.
Table 1 XPS results from the Mn 3s peak showing approximate valence of each sample (standard manganese oxides from literature are also includeda) Sample
Mn 3s splitting width, DE (eV)
Valence of Mn
Phase
1150 s 1700 s 4750 s Fully cycled MnOa Mn3O4a Mn2O3a MnO2a
5.54 5.48 4.79 5.79 5.79 5.50 5.41 4.78
2, 3 2, 3 4 2 2 2, 3 3 4
MnO and Mn3O4 Mn3O4 and Mn2O3 MnO2 MnO MnO Mn3O4 Mn2O3 MnO2
a
Data taken from [11].
Samples that are partially oxidized and still green in color (1150 and 1700 s) are both shown to be a mixture of Mn2+ and Mn3+. At 1150 s, MnO and Mn3O4 are present, however by 1700 s, Mn3O4 and Mn2O3 are present indicating the continued oxidation of the film. Once the film has been completely oxidized with a color change from green to red (4750 s), the valence of the surface film is entirely Mn4+, or MnO2. Thus, there is a conversion from the as-deposited 0/2+ state (Mn/MnO) to an intermediate 2+/3+ (MnO/Mn3O4 for 1150 s and Mn3O4/ Mn2O3 for 1700 s) state and finally a 4+ state (MnO2) when fully oxidized. This is the expected oxidation path for a Mn sample in an alkaline solution [12] ultimately ending with the production of MnO2 (Mn4+). These results are in keeping with the oxidation of the other layers of the structure (dense intermediate and zigzag) as outlined by the TEM analysis. Deconvolution of the O1s peak can give useful information regarding the state of oxygen bonding and thus the hydration of the sample. A typical O1s spectrum that is deconvoluted into its three components is shown in Fig. 8 with the results tabulated in Table 2. Both samples that have a green surface film (Section 3.1.1) have approximately the same amount of hydrated material in the structure (45%) with the balance being oxide and free water. By the time the oxidation is complete (Section 3.1.2), the hydrated content in the film has been reduced to only 32%. However, relatively speaking, this is a fairly significant amount of hydration. Other researchers have reported values from 14% to 52% hydroxide content depending on the deposition conditions [11,13,14]. The formation of the hydrated film is thought to be critical to the capacitance mechanism. The fraction of reaction sites that are accessible to the Faradic process is larger when the film is in a hydrous form. It has been suggested that the hydrated structure has improved protonic and electronic conductivity,
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Fig. 8. XPS O1s spectrum from the sample oxidized for 4750 s. The contribution of each different oxygen bond to the total peak (oxide, hydroxide, and water) is indicated. Table 2 XPS data from the de-convolution of the O1s peak Sample
Eb (eV)
at.%
Bond Type
1150 s
530.1 531.6 532.3
51 45 4
Mn–O–Mn Mn–O–H H–O–H
1700 s
530.2 531.4 532.2
36 45 19
Mn–O–Mn Mn–O–H H–O–H
4750 s
530.2 531.7 533.3
63 32 5
Mn–O–Mn Mn–O–H H–O–H
Fully cycled
529.7 531.1 532.0
27 33 40
Mn–O–Mn Mn–O–H H–O–H
32 h anneal
529.9 – 531.9
50 – 50
Mn–O–Mn Mn–O–H H–O–H
resulting in superior capacitance for hydrated structures vs. unhydrated structures [1]. Once the sample has been cycled several hundred times (Section 3.1.3), there does not appear to be any visible change in the three layers; however, the XPS results show that some changes are occurring. The Mn 3s data indicates a valence of 2+, corresponding to the MnO phase. This result reveals that changes have occurred during the cycling process and the porous surface film has been reduced from the 4+ valence to the 2+ valence. This valence change is indicative of irreversibility in the process where non-reversible reduction reactions are occurring in addition to the reversible pseudo-capac-
itive reaction, ultimately resulting in a permanent valence change to the film. The hydrated content of the film (33%) has not changed noticeably from the precycled condition, suggesting that the hydrated structure is not being destroyed through the reduction process. It was reported in previous work that thermal oxidation of Mn chevron films in an air at 300 C was detrimental to capacitance [9,10]. The reason for this decrease in capacitance was previously thought to be due to the formation of a capping layer on the surface of the zigzag film that prevented solution penetration [10] and reduced the usable surface area. The thermal oxidation process ultimately forms dense Mn3O4 as the surface layer [10]; however, the recent XPS analysis (Table 2) confirms that the thermally annealed sample contains no hydrated manganese (Mn–O–H) only anhydrous compounds (Mn–O–Mn) and free water (H–O– H). There is a significant quantity of free water (50%) in the original scan, but heating the sample to 100 C for a short time in the XPS chamber resulted in a significant reduction in the amount of water present (30% reduction). The removal of water upon low temperature heating confirms that it is surface adsorbed water and not structural water and thus does not provide any useful chemical hydration (i.e., Mn–O–H type bonds) to develop capacitance. Furthermore, when a thermally annealed sample is put into the Na2SO4 solution and the oxidizing current is applied, the distinct porous surface layer that has been shown to provide significant capacitance does not form, and the film does not follow a similar oxidation trend (potential vs. time) as the Mn/MnO materials. It is believed that thermal oxidation of the film prior to electrochemical treatment
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has altered the surface phases and chemistry such that it is no longer possible to produce the porous, hydrated, amorphous layer upon further electrochemical oxidation. The reason that the porous surface layers will no longer form after thermal oxidation is likely because the layer is already in an advanced state of oxidation (Mn3O4), and there is insufficient opportunity during the oxidation to MnO2 to form a porous layer with the necessary hydrated and amorphous structure. The lack of hydration and an amorphous structure prevents ion insertion and thus no capacitance is measured. The formation of the oxidized capping layer during thermal annealing essentially prevents the formation of the amorphous, hydrated porous surface film that forms during electrochemical oxidation, which is now seen to be critical to the capacitance mechanism. 3.1.5. Dense films The suggestion that the hydrated surface layer produced during electrochemical oxidation is the only surface involved in the capacitance mechanism has potentially eliminated the need for a porous starting surface, such as the chevron films. As a result, dense films, 38 nm in thickness, were deposited and their behavior during electrochemical oxidation was studied. The dense films have a typical columnar thin-film morphology and are a two-phase mixture of Mn and MnO (the same composition as the chevron films [10]). With the same initial starting point, the dense films were subjected to the same oxidation process at the same current density as the chevron films. This particular film was oxidized for 900 s, after which a red surface film formed as with the chevron structures. Fig. 9(a) shows the final oxidized structure. Note that the porous surface film that forms is the same film that had formed on the chevron films with a thickness of approximately 100 nm. The accompanying diffraction pattern (Fig. 9(b)) confirms that this film is amorphous in nature. Thus, despite the significant
reduction in initial surface area and porosity, the electrochemical oxidation process results in a porous surface film that is similar to the one seen with the chevron structure. The electrochemical behavior of these dense films will be discussed in a separate paper. The important conclusion at this point is that the morphology of the starting material does not matter, as both porous and dense starting layers will form the porous surface layer upon oxidation that is responsible for the pseudocapacitive tendencies.
4. Discussion The results indicate that any starting material, in any form, could be used to produce pseudocapacitance provided it is pure Mn or a mixture of Mn/MnO. If the valence of the starting material is too high (i.e. 3+), such that the electrochemical oxidation will not produce any further oxidation, then the porous surface film cannot be formed, and no capacitance can be measured. This phenomenon was found during previous studies [10], where it was discovered that dense films of mixed MnO/Mn3O4 did not exhibit pseudocapacitance because the electrochemical oxidation process did not produce a porous surface film, but rather had a destructive impact on the film. It is suspected that the starting valence of the film was too high, and as Mn3O4 is extremely stable in alkaline solutions [13], the application of the anodic current to the film resulted in destruction rather than controlled oxidation. The electrochemical oxidation process in Na2SO4 results in the formation of an extremely porous surface layer. Although the underlying zigzag layer is oxidized from Mn/MnO to single phase Mn3O4, the more important result is the formation of the porous surface layer. Electron microscopy has confirmed the porous nature of this film, and selected area diffraction has confirmed
Fig. 9. (a) TEM BF image of the dense film electrochemically oxidized for 900 s. (b) SAD pattern from the thinnest portion of the plan view revealing its amorphous nature.
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its amorphous nature. XPS results indicate that the surface of the film is made up of hydrated manganese oxide in the 4+ valence state. In addition, it has been shown that the morphology of the underlying substrate is immaterial as a dense film of manganese oxide also forms the extremely porous, amorphous, hydrated surface film. Since this film has superior specific capacitance to the thicker zigzag films, the importance of the surface film in the capacitance mechanism is clearer. It is likely that the bulk film does not take part in the charge/discharge process, only the surface film does. The relatively smaller amount of ‘‘carrier’’ material in the denser film results in a larger capacitance per mass. However, it is likely that both bulk and thin films would exhibit similar ‘‘capacitance per area’’ values, since the area of porous surface film produced is likely very similar for both bulk and thin films. In addition, capacitance per area would be a more appropriate measure of capacitance, since it is suspected that only that portion of the film is involved in the charge/ discharge; the rest of the film provides substrate support. The dense intermediate layer directly beneath the porous layer may simply be a physical marker of the separation of the active portion of the electrode from the carrier/substrate portion of the electrode. These results should greatly simplify the electrode fabrication process, since very little attention need be paid to the porosity of the starting material, as the electrochemical oxidation process produces its own suitably porous surface. This finding eliminates the need to produce the porous amorphous films using more complicated, multi-step processes. Instead, supercapacitive processing requires only a single solution of Na2SO4 and the electrochemical oxidation process can be done in-situ after the capacitor is packaged. In addition, the starting electrode material need not be pure Mn. This process works extremely well using a two-phase mixture of Mn/MnO, which is easily produced in any PVD system since there is no requirement for low oxide content.
5. Conclusions In summary, the electrochemical oxidation process of Mn/MnO starting films, of any morphology, results in
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the formation of a three-layer structure consisting of undisturbed base layer, a dense intermediate layer directly on top of the base layer and a porous surface layer. It is likely that the porous surface layer is responsible for the capacitive tendencies of the film. During the electrochemical oxidation process, the undisturbed base layer and the dense disturbed layer oxidize to Mn3O4. The porous surface layer, however, oxidizes to an amorphous form of MnO2. XPS results indicate that this outermost porous layer is a hydrated compound accounting for the excellent capacitance values that can be measured with these films.
Acknowledgements The authors acknowledge funding contributions from the Natural Sciences and Engineering Research Council of Canada (NSERC) in addition to the Alberta Informatics Circle of Research Excellence (iCORE) and Micralyne.
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