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Hierarchically mesostructured aluminum current collector for enhancing the performance of supercapacitors Yilun Huang, Yuyao Li, Qianming Gong, Guanlei Zhao, Pengjie Zheng, Junfei Bai, Jianning Gan, Ming Zhao, Yang Shao, Dazhi Wang, Lei Liu, Guisheng Zou, Daming Zhuang, Ji Liang, Hongwei Zhu, and Ce-Wen Nan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b03647 • Publication Date (Web): 27 Apr 2018 Downloaded from http://pubs.acs.org on April 27, 2018

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ACS Applied Materials & Interfaces

Hierarchically mesostructured aluminum current collector for enhancing the performance of supercapacitors Yilun Huang†,‡,§, Yuyao Li†,‡,§, Qianming Gong†,‡,§*, Guanlei Zhao∥,⊥, Pengjie Zheng∥, Junfei Bai†,‡,§, Jianning Gan†,‡,§, Ming Zhao†,‡,§, Yang Shao†,§, Dazhi Wang▽, Lei Liu∥,⊥*, Guisheng Zou∥,⊥, Daming Zhuang†,‡,§, Ji Liang∥, Hongwei Zhu†,‡,§, Cewen Nan†,‡,§ † School of Materials Science and Engineering, Tsinghua University, Beijing 100084, PR China. ‡ State Key Laboratory of New Ceramics and Fine Processing, Tsinghua University, Beijing 100084, PR China. § Key Laboratory for Advanced Materials Processing Technology, Ministry of Education, Beijing 100084, PR China. ∥ Department of Mechanical Engineering, Tsinghua University, Beijing 100084, PR China. ⊥ State Key Laboratory of Tribology, Tsinghua University, Beijing 100084, PR China. ▽ Beijing HCC Energy Technology Co., Ltd, Beijing 100085, PR China.

KEYWORDS

Picosecond laser, Micro-nano structures, Aluminum foil, Current collector, Supercapacitor

ABSTRACT

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Aluminum (Al) current collector is one of the most important components of supercapacitors, and its performance has vital effects on the electrochemical performance and cyclic stability of supercapacitors. In the present work, a scalable and low-cost, yet highly efficient picosecond laser processing method of Al current collectors was developed to improve the overall performance of supercapacitors. The laser treatment resulted in hierarchical micro-nano structures on the surface of the commercial Al foil, and reduced the surface oxygen content of the foil. The electrochemical performance of the Al foil with the micro-nano surface structures was examined in the symmetrical activated carbon (AC)-based coin supercapacitors with an organic electrolyte. The results suggest that the laser-treated Al foil (Laser-Al) increased the capacitance density of supercapacitors up to 110.1 F g−1 and promoted the rate capability due to its low contact resistance with the carbonaceous electrode and high electrical conductivity derived from its larger specific surface areas and deoxidized surface. In addition, the capacitor with Laser-Al current collector exhibited high cyclic stability with 91.5% capacitance retention after 10,000 cycles, 21.3% higher than that with pristine Al current collector due to its stronger bonding with the carbonaceous electrode that prevented any delamination during aging. Our work has provided a new strategy for improving the electrochemical performance of supercapacitors.

Figure TOC (8cm*4cm) 2


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INTRODUCTION

With the development of modern electric industry, supercapacitor has become a hot research topic in the field of energy storage, especially for the automotive industry, public transportation, and wind power generation, wherein energy-storage devices with rapid charge/discharge properties and long life spans under poor working conditions are needed.1, 2 Based on their energy storage principles, supercapacitors can be divided into two types including electric double layer capacitors (EDLCs) and electrochemical pseudocapacitors.3-5 At present, EDLCs have been extensively commercialized due to their high capacitance densities, long life spans, high power densities, environmental friendliness, and low cost. A typical supercapacitor of ELDCs consists of activated carbon electrodes, aluminum current collectors, a separator, electrolyte, and a sealing shell. More efforts have been devoted to developing novel electrode materials, especially carbonaceous electrode materials including hierarchical porous graphene and carbon nanotubes with high capacitances, power densities, and energy densities,6-13 rather than improving other components of supercapacitor. The electric current collector of supercapacitor is usually made of aluminum foil. It plays two significant roles: collecting and conducting electric current from the carbon electrode to power sources or electrical appliances, and supporting the activated carbon electrode. Therefore, an ideal current collector should possess high electrical conductivity, low contact resistance with the carbon electrode, and a strong and stable bonding with the carbon electrode. Portet et al.14 reported an Al current collector coated with carbon nanofibers that exhibited a low internal resistance of 0.4 Ω cm−2 and good cyclic stability. Sumboja et al.15 prepared a nano-architectured current collector for supercapacitor electrodes by fixing electrode particles and conferring an efficient conductivity 3



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path with directly grown indium tin oxide nanowire array. Recently, Wang et al.16 and Kim et al.17 demonstrated the advantages of graphene-modified Al and Cu current collectors, respectively. They found that the graphene covered on the current collectors improved the electrochemical performance and cyclic stability of supercapacitors by reducing the charge transfer resistance and protecting the Al and Cu current collectors from corrosion. Their work have inspired more studies on current collectors to improve the electrochemical performances of supercapacitors via (1) increasing the electrical conductivity and current collecting ability or (2) improving the bonding strength between current collector and carbonaceous electrode to prevent delamination, especially under poor working conditions, such as alternating high and low temperatures. Huang et al.18 and Zhu et al.19 found that the carbon layer was peeled from the Al current collector in activated carbon-based supercapacitors with organic electrolytes after long-time aging and processing, resulting in declined electrochemical performances. Therefore, a novel and scalable approach to modifying Al current collectors is significant for improving the performance of supercapacitors. In recent years, lasers have been applied as a convenient technique for constructing surface patterns on Al or other metal materials to induce super-hydrophobic or super-hydrophilic effects.20, 21 The wettability and adhesive bonding strength of the laser-treated metal can be tuned by varying laser power, scan speed, and laser types.22-25 However, to the best of our knowledge, the laser treatment of Al foil for the fabrication of supercapacitors has not been reported. Picosecond lasers possess ultra-short pulse width, adjustable frequency, and high pulse energy, which can be used to construct abundant micro-nano structures on Al surfaces. In addition, the industrial picosecond laser devices are usually equipped with array lens and a 4


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numerically controlled rotary table, which can process samples quicker and more efficiently than femtosecond laser devices. In the present work, we reported a low-cost, environment-friendly, and easy-to-scale-up method for preparing high performance Al current collectors of supercapacitors. A commercial aluminum foil collector was treated with an industrial picosecond laser device to form a rough patterned surface with abundant hierarchical micro-nano structures, which enlarged the contact area with the carbonaceous electrode composed of micro- and nano-sized particles. Compared with that with the untreated pristine Al current collector, the capacitor with the laser treated Al current collector exhibited lower internal resistances, higher capacitance densities, and better cyclic stability.

RESULTS AND DISCUSSIONS

Surface morphology and microstructure The physical and chemical properties of the picosecond laser treated Al current collectors were characterized. The micro-nano structures on Al foil formed by picosecond laser treatment were imaged by SEM (Figure 1). The parameters of laser for the treatment were optimized to avoid too severe (Figure S1a) or too mild ablations (Figure S1b). Regular latticed patterns with the spacing of ~50 μm were observed (Figure 1a), which conformed to the preset space between two laser scans. In addition, the space could be adjusted to obtain multiple surface patterns (Figure S2). The uniformly distributed dark quadrate “islands”, the area not ablated by the laser, are surrounded by grey grids. The initial dark zones with some remnant rolling stripes (Figure 1c) became a rough net-like surface with randomly distributed pits and protuberances 5



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as shown in the grey orthogonal “ribbons” in Figure 1b. The sizes of the laser-induced macropores were measured to be 50-500 nm. The untreated areas exhibited the similar morphology to that of Pristine-Al (Figure 1c vs S3), where only mechanical rolling traces were observed. The ratio between the untreated and treated areas can be varied by adjusting the scanning space, although which is beyond the scope of current study. The hierarchical micronano structures formed on the Al foil are similar to those on the laser-modified magnesium.2628

The formation of these net-like structures and “pits” may be due to the unstable capillary

wave of the smelting and evaporation on the Al surface induced by the ultrahigh energy of pulsed laser. 28 The small “pits” may cause the capillarity phenomenon or “trap” AC and carbon black nanoparticles tightly as “pitfalls.” Such structure with a larger specific surface area (SSA) can enhance the bonding strength with carbonaceous electrodes.29, 30 The SSA of Pristine-Al and Laser-Al were measured by N2 adsorption/desorption method and Brunauer-Emmett-Teller (BET) model to be 2.622 m2 g-1 and 6.469 m2 g-1, respectively (Figure S4a). The distinct ~146.7% enhancement achieved by laser surface modification is consistent with SEM observations and those results reported in literatures.31, 32 It can be seen that Laser-Al has higher pore volume than Pristine-Al. In addition, some mesopores with the average width of about 12.5 nm were produced (Figure S4b). The section SEM image revealed a “mountainous” morphology of the Laser-Al (Figure 1d). The 3D profilometric images directly demonstrated the grooved surface of the Laser-Al (Figure 2a). The regularly aligned square convex plates or “plateau” ablated by the laser were measure to be 40–50 μm long and the orthogonal grooves were ~20 μm wide and ~0.35 μm deep (Figure 2b). These convex plates and grooves increased the contact area between the Al current collector and carbonaceous electrode, and thus lowered 6


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the contact resistance of the whole electrode. The mutual imbedding among the carbon particles, grooves, and quadrate convex plates also strengthened the mechanical interlocking between the carbonaceous electrode and Al current collector.

Figure 1. SEM images of the Laser-Al surface. (a) The latticed pattern on the Al surface at low magnification. The selected zones framed by yellow dotted lines are magnified (b) laser-treated area and (c) untreated area, respectively. (d) A section-view image shows the laser-treated surface turning into rough “mountainous” structures.

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Figure 2. (a) Three-dimensional image of profilometry analyses showing surface morphology of laser-treated aluminum. (b) The partial depth-position (X) curve showing the surface roughness of Laser-Al. The average depth of laser-induced grooves is about 3.0–3.5 μm.

Surface chemical composition In addition to the surface morphology evolution caused by laser treatment, the chemical composition of Al foil was also alternated during the treatment. XPS tests were taken for both Pristine-Al and Laser-Al. As shown in Figure 3a, the contents in the surface of the two samples are mainly composed of aluminum (Al) and oxygen (O) while the ratios are slightly different (Table 1). Specifically, the laser treatment led to the O content decreased from 56.35 at. % to 55.39 at. %, and accordingly brought about an increase of Al content by 0.96 at. % (Figure 3b and c). The variations of the element contents were further confirmed by EDX analyses (Figure S5). It can be explained that the extremely high energy density or power density of the picosecond laser dissociated the O from the thin Al2O3 oxide layer on the Al foil, leaving vacancies 33, 34 and causing surface annealing. 32, 35 We also found that the O content decreasing effect would weaken once the laser fluence exceeded certain range (Table S1 and Figure S6). Al2O3 is not electrically conductive. Higher ratios of O in Al film suggest lower electrical conductivities. In the present work, the decreased oxygen content reduced the average electrical resistance of the Al foils from 0.68 Ω m−1 to 0.33 Ω m−1 (Table S2). The electrical conductivity of the Al foil was increased accordingly. Although no difference was found between the XRD patterns of Laser-Al and Pristine-Al (Figure S7), it can be anticipated that the increased electrical conductivity of Laser-Al would conduce to improved performances of 8


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supercapacitors.

Figure 3. (a) XPS spectra of Pristine-Al and Laser-Al. (b) and (c) are O1s spectra (left) and Al 2p spectra (right) of the samples.

Table 1 Concentration of elements in the samples determined by XPS analyses Sample Pristine-Al Laser-Al

Concentration of elements (at. %) O Al 56.35 43.64 55.39 44.60

Wettability Supercapacitor electrodes are usually prepared by coating a AC/CB mixed slurry on current collectors. The slurry is usually prepared in deionized water or N-methyl pyrrolidone (NMP). The operability and uniformity of the coating are significantly affected by the wettability of the 9



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current collector, which has been ignored in most studies. The static contact angles of deionized water, NMP, and organic electrolyte (1 M ET4N-BF4/ACN) on the surfaces of Laser-Al and Pristine-Al were measure by the sessile drop technique. Neither of Pristine-Al nor laser-treated Al foil repelled the solvents or electrolyte (Figure 4), yet the latter exhibited much better wettability. Concretely, the laser treatment decreased the contact angle (CA) of deionized water decreased from 93.9°to 29.6°, and that of NMP from 38.8°to 7.0°. The CA of the electrolyte could not be measured because of complete wetting (Supplementary materials). After laser treatment, there were orthogonal grooves on the Al surface with hierarchical porous structures. With the surface area increasing, the contact angles of water, NMP and organic electrolyte on Laser-Al decreased remarkably and this is in accordance with Wenzel regime.36 The excellent wettability of Laser-Al is favorable to the formation of a uniform and stable carbonaceous layer with AC/CB slurries in deionized water and NMP. In addition, the highly wettable Laser-Al current collector surface are accessible for electrolyte ions. In all, the laser treatment can not only facilitate the fabrication of high-quality electrodes, but also promote the organic electrolyte wettability of Al current collectors.

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Figure 4. Contact angles test of Pristine-Al (a, c, e) and Laser-Al (b, d, f) samples. The tested solutions were deionized water (a, b), NMP (c, d), and organic electrolyte (e, f).

Electrochemical properties Considerable efforts have been devoted to developing novel electrode materials and new electrolytes to promote the electrochemical properties of supercapacitors, and the effects of current collector are underestimated. However, the electrical conductivity of current collector, the bonding between current collector and carbonaceous layer, and more importantly, the wettability of current collector can significantly affect the electrochemical behaviors, and even the life span of supercapacitors. We fabricated symmetrical coin supercapacitors with Laser-Al 11



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and Pristine-Al current collectors, respectively, and examined their electrochemical performances using ET4N-BF4 as the electrolyte. As shown in Figure 5a and Figure S8a, the coin supercapacitor with Laser-Al and Pristine-Al current collector exhibited similar rectangular-shaped voltage-current curves in the range of 0–2.7 V at the scan rates ranging from 5 mV s−1 to 100 mV s−1, which are consistent with those of the activated carbon electrodes based EDLCs. The electrochemical performance of Laser-Al current collector was further examined by electrochemical impedance spectroscopy (EIS) analysis. Both Laser-Al and Pristine -Al were coated with a carbonaceous layer, and thus any difference between the EIS patterns of electrodes was attributed to the difference between the Al current collectors. The electrodes on Laser-Al and Pristine-Al exhibited the Nyquist plots with similar shapes consisting of a small semicircle and a straight line (Figure 5b). The intercept at Z′ axis is considered as the equivalent series resistance (ESR), which is the combinational resistance of ionic resistance of electrolyte, intrinsic resistance of activated carbon material and Al current collector, and contact resistance at the active carbon/current collector interface. 37,38 The ESR values of LaserAl and Pristine-Al were measured to be 3.3 Ω and 13.6 Ω, respectively (Figure 5b). The EIS results demonstrate that laser treatment can sharply reduce the ESR of supercapacitors. This could be attributed to the increased electrical conductivity of Laser-Al and contact area between Laser-Al and the AC layer.Besides, the semicircle in the high-frequency range corresponds to the charge-transfer resistance (Rct) caused by the Faradaic reactions and the double-layer capacitance on the activated carbon surface. 39 It would be affected by some factors such as the thickness of activated carbon layer, the added amount of electrolyte and the pore size 12


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distribution of the electrode material.

37,40

These factors would be a little fluctuant during the

manual supercapacitor assembling process in this work, so the difference about the Rct values (i.e., 7.3 Ω and 6.5 Ω for Laser-Al and Pristine-Al, respectively) in Figure 5b should not be necessarily associated with laser treatment. Galvanostatic charge–discharge (GCD) was used to characterize the capacitive behaviors and measure the specific capacitances of the supercapacitors with Laser-Al and Pristine-Al current collectors. The specific capacitances of both supercapacitors are the functions of charge– discharge current density and decreased with the increase of current density in the range of 0.1100 A g−1 due to the limited ion diffusion in the electrode and increased internal impedance at high current densities (Figure 5c and d, Figure S8 b and c). The coin supercapacitor with laser scanned current collector exhibited higher specific capacitances up to 110.1 F g−1 at 0.1 A g−1, even at the current density of 100 A g−1, than those of the supercapacitor with Pristine-Al current collector (79.7 F g−1) due to the high electrical conductivity of Laser-Al and low contact resistance between the current collector and carbonaceous electrode derived from the larger contact area. In addition, the large specific surface area of Laser-Al current collector may also directly contribute to the high capacitance to some extent.

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Figure 5. (a) CV curves for the coin supercapacitors with Laser-Al as current collectors at different scan rates from 5 to 100 mV s−1. (b) Nyquist plots in the frequency range from 10 kHz to 10 mHz. The inset shows the enlarged plot at high frequency. (c, d) GCD curves for LaserAl samples obtained at different current densities from 0.1 to 100 A g−1. (e) Specific capacitances at different current densities from 0.1 to 100 A g−1. (f) Cycling performances at the current density of 20 A g−1. The inset shows the GCD curves of the first 10 cycles and the last 10 cycles. 14


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Life span Cyclic stability is a crucial parameter of supercapacitors for their practical application. The long-term cyclic stability of the coin supercapacitors with Laser-Al and Pristine-Al current collectors was evaluated by repeating the GCD test between 0 and 2.7 V at the current density of 20 A g−1 for 10,000 cycles. Figure 5f shows the relationship between the capacitance retention and cycle number. The capacitance of the supercapacitor with Laser-Al current collector only decreased 8.5% after 10,000 cycles, while that of the supercapacitor with Pristine-Al current collector declined 29.8%, suggesting the much better electrochemical stability of the electrode on Laser-Al current collector. To further understand how Laser-Al current collector improved the cyclic stability of supercapacitors, the supercapacitors after the GCD cycling ageing test were disassembled and examined. As shown in Figure 6a, the carbonaceous electrode layer on the Pristine-Al current collector partially fell off and only some carbonaceous fragments were attached to the separator. The fallen carbonaceous layer and minor cracks on the electrode could not provide capacitance during the charge–discharge process, and thus discounted the utilization percentage of the electrode. In contrast, less carbonaceous layer fell from the Laser-Al current collector (Figure 6b), which contributed to the higher capacitance of the corresponding supercapacitor. The SEM imaging revealed a giant gap between the Pristine-Al current collector and carbonaceous electrode (Figure 6c) because of the small contact area between the smooth Pristine-Al surface and carbon particles. The rough surface with randomly dispersed pits caused by the laser treatment could interlock the activated carbon or carbon black particles with comparable sizes, 15



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resulting in a firmly attached carbonaceous layer on the Laser-Al current collector (Figure 6d). Evidently, the more sufficient contact offered more electron transport channels, and thus reduced the contact resistance (Figure 5b). The reinforced interfacial bonding also improve the anti-ageing performance of the supercapacitor.

Figure 6. (a, b) Digital images of the top view of electrodes and separators of disassembled Pristine-Al (a) and Laser-Al (b) coin supercapacitors after ageing. More cracks and fallen parts can be seen in the Pristine-Al sample than in the Laser-Al sample. (c, d) SEM images of the section view of the electrodes showing the bonding between the Pristine-Al (c) or Laser-Al (d) with the carbon layer after ageing. As shown, a gap about ~3 μm between the Pristine-Al and carbon layer was produced during the ageing process. On the contrary, the carbon layer attaches to the Laser-Al firmly.

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The stronger bonding between Laser-Al current collector and carbonaceous electrode was further confirmed by the tensile strength of a specially designed “Al foil layer–carbonaceous layer–Al foil layer” sandwich structure (Figure 7a). The average tensile strength of the Pristine-Al–carbonaceous layer–Pristine-Al sandwich structure was measured to be 14.9 N, while that of the Laser-Al–carbonaceous layer–Laser-Al structure was 16.7 N, 12.1% higher (Table S3 and Figure S9). Due to the lack of any standard, the testing results are only convincing to highlight the difference between the bonding of different Al foils with the carbonaceous layer. In addition, replacing one of the Pristine-Al layers in the Pristine-Al– carbonaceous layer–Pristine-Al with Laser-Al resulted in fractures or delamination only at the interface between Pristine-Al and the middle carbonaceous layer (Figure 7d). Supercapacitors are usually subject to significant temperature variations from subzero degree Celsius to extremely hot environment. Therefore, the sandwich structures of Al foil– carbonaceous layer–Al foil were examined by a special 100 h thermal shock ageing experiment in the temperature range from −20 °C to 80 °C (Figure 7b) with the holding time of 1 h. The average tensile strength of the Laser-Al sandwich structure only decreased 5.8%, while that of the Pristine-Al sandwich structure was reduced by 38.9% (Figure 7c) after the ageing, indicating that the laser treatment markedly enhanced the stability of the bonding between Al foil and carbonaceous electrode. Although the laser treatment slightly increased the tensile strength between Al foil and the carbonaceous layer in this design, the thermal shock ageing experiment demonstrated its intensive significance to the performance of Al current collectors. The high anti-thermal shock resistance of Laser-Al current collector would facilitate prolonging the life span of supercapacitors, especially under harsh environments with great 17



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temperature variations. The related work is ongoing in our lab.

Figure 7. (a) Schematic illustration of specially designed sandwich-like specimens. (b) Schematic of thermal shock ageing experiment. The sandwich-like specimens were repeatedly heated up to 80 °C for 1 h and cooled down to −20 °C for 1 h, and this process was repeated 50 times lasting for 100 h. (c) Average tensile strength of the fresh and aged samples after the thermal shock aging experiment. (d) Digital image of fractured Al foil/AC layer joint showing the AC layer is mostly attached on the laser-treated zone of Laser-Al instead of the Pristine-Al. This is an intuitive phenomenon elucidating that the bonding strength of Laser-Al/AC is higher than that of Pristine-Al/AC.

Conclusion In summary, we proposed an efficient approach to constructing hierarchical micro-nano 18


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structures on the surface of Al current collector with picosecond laser scanning at suitable laser powers. The laser treated Al foil (Laser-Al) exhibited a higher specific surface area of 6.469 m2 g−1 and a much lower electrical resistance of 0.33 Ω m−1 due to the reduced oxygen content and enhanced bonding with carbonaceous electrodes, even after 100 h extreme rough thermal shock ageing. In addition, the laser treatment significantly improved the wettability between Al foil and deionized water, NMP, and organic electrolyte, which contributed to the high electrochemical performance of the corresponding supercapacitor. The capacitance of supercapacitor with Laser-Al current collector was as high as 110.1 F g−1 at 0.1 A g−1, higher than that of supercapacitor with Pristine-Al current collector (79.7 F g−1). In addition, the LaserAl current collector also increased the capacitance retention of 21.3% at 20 A g−1 after 10,000 cycles owing to the reduced internal electrical resistance and enhanced interfacial bonding with carbonaceous electrodes. Our work has provided a low-cost, environment-friendly, and easyto-scale-up picosecond laser processing of Al current collectors for improving the performances of supercapacitors.

METHODS

Commercial aluminum foils used for current collectors of supercapacitors (thickness ~16±2 μm, Shenzhen MTI CO., LTD.) were selected for surface patterning by picosecond laser. The purchased Al foil was named as pristine aluminum (Pristine-Al). The Pristine-Al samples were cleaned by ethanol and flatted by mechanical compaction before laser treatment. A coherent picosecond laser source was adopted to construct micro-nano structures in the surface of aluminum foils (Laser-Al). The laser beam was focused by spherical lens (focal length of 170 19



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mm) in order to obtain a spot shape and a computer controlled two axes (x and y) array lens system was applied to control the patterning process. The patterns processed on the surface of aluminum foils by laser were consisted of evenly spaced lines and rows (orthogonal lines). The laser process parameters are summarized in Table 2.

Table 2. Summarized laser process parameters. Process parameter Laser radiation wavelength Pulse repetition rate Pulse duration Pulse energy Fluence Average power Lasing scan speed Programmed line and row spacing Spot overlap rate Scan times

Value 532 nm 5000 kHz 24 ps 2.98 μJ 0.15 J cm-2 14.9 W 4 m s-1 50 μm 98.40% 1

Characterizations Scanning electron microscope (SEM, SU-8010, Hitachi Co., LTD.) with attached energy dispersive X-ray detector (EDX) was used to obtain the morphology and microstructures of Pristine-Al and Laser-Al samples. X-ray photoelectron spectra (XPS, 250Xl, Thermo Fisher, Co.) was used to analyze the chemical elements variation, such as oxygen and aluminum of the samples. Three-dimensional optical surface profiler (Nexview, ZYGO, CO.) was used to measure the surface micro-nano structures of Laser-Al samples, moreover it could measure the flatness, roughness and feature heights of the surface. The specific surface areas of the aluminum foil were calculated in the range from P/P0=0.01 to 0.1 by Brunauer-Emmett-Teller (BET). A video-based optical contact angle measuring device was used to characterize the 20


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wettability of the laser modified aluminum foil surface. Deionized water, N-methyl pyrrolidone (NMP) and organic electrolyte (1 M ET4N-BF4/ACN) were applied in the contact angle tests, because these two solvents were the dispersant of activated carbon powder of the electrode. Four probe resistance tester was employed to measure the sheet resistance and specific resistance of the samples. Specially designed sandwich structure of Al foil–carbonaceous layer–Al foil specimens were adopting to characterize the bonding strength of aluminum current collector and the carbonaceous electrode layer. The schematic depiction of the specimens (Figure 7a) shows the geometry of the junction (10 mm×10 mm×0.01 mm) and the application of tensile forces during mechanical testing. A universal testing machine (Instron 5843) was applied for this mechanical test. The load was imposed under displacement control with the speed of 0.02 mm s -1. In addition, for investigating the bonding strength of Laser-Al and carbon layer under malconditions, an imitation of real-life thermal shock ageing experiment was designed (Figure 7b). The thermal shock ranged from –20 °C to 80 °C and lasted for 100 h. Electrochemical tests CR2032 coin cells were fabricated for electrochemical performance evaluation of symmetric supercapacitors. The electrodes were consisted of 80 wt.% of commercial activated carbon powder (YP-50F, KURARAY, CO.), 10 wt.% of polyvinylidene fluoride (PVDF) bender powder, and 10 wt.% of conductive black carbon powder (BLACK PEARLS 2000, CABOT, CO.). These components were dispersed in N-methyl pyrrolidone (NMP) solution and were then coated on pristine and laser modified aluminum foil current collectors, respectively. The thickness of activated carbon layer was approximately 15–20 μm. The electrolyte of the 21



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supercapacitor was 1 M (mol L-1) tetraethylammonium tetrafluoroborate (ET4N-BF4) in acetonitrile (CH3CN, ACN). The electrochemical properties of the supercapacitors were studied by cyclic voltammetry (CV), galvanostatic charge-discharge (GCD) and electrochemical impedance spectroscopy (EIS), using a supercapacitor testing system (BT 2000,

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workstation

(VSP-300

electrochemical interface), respectively. CV tests of the coin supercapacitors were investigated between 0–2.7 V at different scan rates from 5–100 mV s-1. The GCD curves and specific capacitance were characterized under different current density of 0.1–100 A g-1 with the voltage of 0–2.7 V. The EIS was recorded in the frequency range from f=10 kHz to 10 MHz at the voltage of 2.7 V. The cyclic ageing tests were carried out at 20 A g-1 for 10,000 cycles. ASSOCIATED CONTENT Supporting Information The Supporting Information provides experimental details for material preparation and characterization. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author * E-mail: (Qianming Gong) [email protected] * E-mail: (Lei Liu) [email protected] Notes The authors declare no competing financial interest. 22


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