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Combining Al-air battery with paper-making industry, a novel type of flexible primary battery technology
Electrochimica Acta 319 (2019) 947e957
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Combining Al-air battery with paper-making industry, a novel type of flexible primary battery technology Yifei Wang a, *, Holly Y.H. Kwok a, Wending Pan a, Yingguang Zhang a, Huimin Zhang b, Xu Lu c, Dennis Y.C. Leung a, ** a b c
Department of Mechanical Engineering, The University of Hong Kong, Hong Kong, China School of Civil Engineering and Architecture, East China Jiao Tong University, Nanchang, China Department of Chemistry, Yale University, New Haven, CT, 06520, United States
a r t i c l e i n f o
a b s t r a c t
Article history: Received 6 May 2019 Received in revised form 26 June 2019 Accepted 11 July 2019 Available online 12 July 2019
Al-air batteries are generally fabricated by rigid and heavyweight materials such as metal or plastic, meanwhile a bulky electrolyte solution either in static or circulation is requisite to ensure their steady operation. Therefore, they are less suitable for powering flexible and portable devices with lower power demand, such as wearable electronics, point-of-care test assays, RFID tags, etc. This work develops a flexible and lightweight Al-air battery with much less electrolyte storage, which is totally fabricated on a cellulose paper. The Al foil anode is embedded inside the paper substrate during paper-making process, while the air-breathing cathode is deposited onto the paper surface using an oxygen reduction ink. Despite its simple structure and low cost, this flexible Al-air battery can deliver a satisfactory power density of 19 mW cm2 with alkaline electrolyte, and its operation lifespan is as long as 58 h with only 25 mg Al when saline electrolyte is employed. The corresponding Al specific capacity is as high as 2338 mA h g1. In addition, this battery exhibits an excellent flexibility when facing different bending angles from 60 to 180 and multiple bending times greater than 1000. Furthermore, a flexible current or voltage output can be easily obtained by scaling up or stacking the present flexible Al-air battery, respectively, and the corresponding stacking efficiency is as high as 94%. In the future, non-metal oxygen catalyst will be employed to improve its environmental friendliness, and hot-pressing will be adopted to further increase its robustness to external deformation. © 2019 Elsevier Ltd. All rights reserved.
Keywords: Paper-based Al-air battery Flexible battery Embedded Al Battery bending
1. Introduction Since its first appearance, metal-air battery (MAB) has become a fascinating power technology. Compared with conventional allsealed batteries, one of the greatest advantages of MAB is its open-ended and lightweight cathode, which can directly breathe oxygen from ambient air. In this manner, MAB can achieve much higher energy density, simpler battery system and reduced fabrication cost benefitted from the elimination of oxidant storage [1]. Till now, various MABs have been developed according to their different metal anodes, such as Li-air battery [2,3], Mg-air battery [4,5], Zn-air battery [6,7] and Al-air battery [8,9]. Each type of these
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (D.Y.C. Leung). https://doi.org/10.1016/j.electacta.2019.07.049 0013-4686/© 2019 Elsevier Ltd. All rights reserved.
(Y.
Wang),
[email protected]
MABs has its specific advantages and disadvantages for different applications. Among them, the primary Al-air battery has received numerous R&D attentions since its first appearance in the 1960s [10]. This is because Al has a comparatively high specific capacity of 2.98 A h g1 (3.86 for Li, 2.20 for Mg, 0.82 for Zn) and a very low market price of 1.75 $ kg1 (68 for Li, 2.75 for Mg, 1.85 for Zn) at the same time, not to mention its huge reserve which is the most abundant metal in the crust of the earth [6,11,12]. During the Al-air battery discharge, the following reactions will occur at the electrode-electrolyte interface, generating useful voltage and current output until the Al anode is exhausted. In addition, the Al corrosion side-reaction also exists, which is negative to the battery performance and practical application. As for the cathode, oxygen from the ambient air will diffuse continuously into the porous catalyst layer due to its concentration gradient, which will be reduced at the triple-phase boundary of the solid catalyst, liquid electrolyte and air.
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Anode : Al þ 3OH /AlðOHÞ3 þ 3e
(1)
Cathode:O2 þ 2H2 O þ 4e /4OH
(2)
Overall:4Al þ 3O2 þ 6H2 O/4AlðOHÞ3
(3)
3 Al corrosion:Al þ OH þ 3H2 O/AlðOHÞ 4 þ H2 2
(4)
Till now, great efforts have been made on Al-air batteries to improve their power output, energy efficiency and system practicability [13]. On the anode side, various Al alloys doped with different species and ratios of trace elements (Ga, Ti, In, Sn, Zn, Bi, Mn and Mg) have been developed to both decrease the anodic overpotential loss and suppress the Al self-corrosion [14e16], while novel anode structures such as 3D printed Al nanoparticles were also reported [17]. On the cathode side, developing low-cost yet highly-efficient oxygen reduction reaction (ORR) catalyst other than Pt is of great importance to its large-scale application, which is a common research target for all kinds of primary metal-air batteries and fuel cells. These non-noble ORR catalysts include but not restrict to manganese dioxide [18e20], cobalt oxides [21,22], perovskite oxides [23,24], bismuth oxyhalide [25], nitrogen-doped carbons [26] and seed-mediated silver manganate nanoplate [27], among which MnO2 is currently the most popular choice. As for the electrolyte, both alkaline and saline solutions are commonly adopted, with various Al corrosion inhibitors developed for the alkaline electrolyte [28e30]. In addition, the recently-emerged gel electrolyte is also a promising research direction especially for portable applications [31e33]. By gelling of the aqueous electrolyte, the water management issue is eliminated together with the potential leakage hazard. Other research works focus on either the system-level development of Al-air battery [34,35] or the numerical investigation of its working mechanisms [36,37]. Most of the existing Al-air batteries are in rigid form with heavy and bulky battery components, which are utilized for better assembling and sealing purpose. In recent years, the flexible battery is getting more and more attention and its application prospect is growing rapidly [38]. Unlike the conventional rigid battery, a flexible battery can be employed in more sophisticated environment which has fewer requirements on the specific battery shape. In addition, a flexible battery can tolerate repetitive deformation without significantly influencing its electricity output, which can be adopted as power unit in dynamic situations, such as wearable electronics. Moreover, unlike the modular fabrication of conventional rigid batteries, the size of flexible batteries can be easily tailored according to the specific space or power requirement, which is another important advantage for real applications. Considering the great advantage of Al in energy density, cost, abundance and eco-friendliness, a flexible version of Al-air battery is highly promising and competitive for powering various singleuse and disposable devices. Fotouhi et al. [39] fabricated a cableshaped flexible Al-air battery by wrapping a paper layer and a CNT-paper matrix on an Al wire. When the bending angle increased, the voltage and current output slightly increased by 9.5 and 15.7%, respectively. Xu et al. [40] also developed a fiber-shaped flexible Al-air battery by covering an Al spring anode with hydrogel electrolyte and Ag-coated CNT sheet cathode. The battery displayed a high Al specific capacity of 935 mA h g1 and the flexibility is also quite good when facing different bending angles and bending times. Nevertheless, both of the two flexible Al-air batteries achieved relatively low power output (1.5 mW cm1 and 1.33 mW cm2, respectively) probably due to the tubular cell configuration, and the employment of high-purity Al anode
(99.999%) will greatly increase the battery cost. Therefore, it is requisite to develop flexible Al-air batteries with improved power output and low-purity Al anode. In this work, we have proposed a novel type of flexible Al-air battery, which was totally fabricated on a single piece of cellulose paper. The Al foil anode was embedded inside the paper substrate during the paper-making process, while the air-breathing cathode was directly deposited onto the paper surface by using an ORR ink. In this manner, a flexible, ultrathin and lightweight battery was successfully obtained. Its electrochemical performance was studied first by using both alkaline and saline electrolytes. In addition, different types of ORR catalysts and different loadings of embedded Al were investigated in order to improve its power output and operation lifetime, respectively. To inspect its flexibility, the battery performances under different bending angles and after different bending times were also tested. Moreover, an electrolyte predeposition technique was studied for more practical battery application, so that only water was needed for battery activation. Finally, the flexible Al-air battery was both scaled up to improve its current output and stacked to increase its voltage output, demonstrating its high potential for practical applications with diverse current/voltage requirements. 2. Experimental 2.1. Preparation of the Al-paper complex In order to integrate the Al foil with the paper substrate without any external force, we innovatively embed the Al anode inside the paper during the paper-making process. By adding an extra step during the conventional paper-making procedure, the Al-paper complex can be easily fabricated. As shown in Fig. 1, a thin layer of paper pulp was first spread uniformly on a stainless steel mesh (18 meshes). Next, a piece of Al foil (98.2% purity, 0.01 mm thickness, from kitchen foil) with a reaction area of 1 cm2 (equivalent to 3.5 mg) was placed on the top of the paper pulp, whose size was slightly smaller than the paper pulp except for a tentacle which reached out of the paper pulp for connection purpose. Afterwards, another layer of paper pulp was slowly poured onto the existing two layers, which not only covered the Al foil surface but also combined with the previous paper pulp at the periphery. Next, the three layers were taken out from the mesh, which were pressed to squeeze the water inside and to achieve better combination of them. Finally, the as-prepared Al-paper complex was dried in an oven at 60 C for half an hour. Its thickness was around 0.57 ± 0.02 mm, which was well controlled by the depth of the pulp-containing groove. Theoretically, a smaller thickness of the paper will benefit the battery performance because of the reduced ionic resistance, which is a common phenomenon in all kinds of electrochemical power sources. Nevertheless, in the present study, a smaller paper thickness also means less electrolyte storage at the vicinity of the electrodes, which is negative to the battery longterm stability. Therefore, thinner papers were not included in this work. 2.2. ORR ink preparation and battery fabrication To obtain a flexible battery, the conventional carbon paperbased cathode is no longer appropriate. Instead, an ORR ink was developed, which could be deposited directly onto the paper substrate as air-breathing cathode. The ORR ink was mainly composed of a non-noble ORR catalyst, a catalyst support and a polymer binder, which were all dispersed in an ethanol-water solvent (1:1 vol ratio). In this work, MnO2 was selected as the ORR catalyst considering its excellent balance between catalytic activity and
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Fig. 1. Flow chart of the battery fabrication process and exploded view of the battery structure.
cost. To prevent the penetration of MnO2 nanoparticles deep into the paper substrate, multi-walled carbon nanotube (MWCNT, aladdin®) with a length of 50 mm was employed as catalyst support, and the metal loading of MnO2/CNT was controlled to be 60 wt%. To prepare this ink, 50 mg of the MnO2/CNT and 1.7 mg of the Nafion binder (DuPont®) were dispersed in 1 mL ethanol-water solvent, which was next sonicated for 30 min to achieve a uniform distribution. Before ink deposition, a silver grid (5 5, 1 cm2 total area) was deposited on the paper surface, which fully corresponded to the Al foil anode inside. This silver grid was added to not only collect the generated current but also confine the cathode area. Afterwards, the as-prepared ORR ink was dip-coated within the Ag grid by a pipette, which was dried next at 60 C for 30 min. The catalyst loading was controlled by selecting the volume of deposited ORR ink.
was connected between the battery cathode and the reference electrode to obtain the cathodic potential during the polarization test. Furthermore, to obtain the long-term discharge stability and Al utilization efficiency, the battery was also discharged at various current densities from 1 to 20 mA cm2, until the embedded Al anode was completely consumed. All the above tests were conducted by an electrochemical workstation (CHI760E) at room temperature, and fresh batteries were utilized for each time of the test.
2.3. Battery characterization
Before testing, the flexible Al-air battery was cut off by the middle of its electrode in order to obtain its cross-sectional view. As shown in Fig. 2(a), the Al foil was well sandwiched by the two layers of cellulose paper. On the top layer, a thin layer of deposited ORR catalyst could be observed, whose edge was slightly cracked due to the cutting process. Here, a 0.1 mm-thick Al foil was adopted instead of the 0.01 mm-thick one in order to better exhibiting the different layers. From this cross-sectional view, it can be concluded that the ORR ink did not penetrate into the paper layer and contact the Al anode, therefore eliminating the issue of battery shortcircuiting. In addition, a thin gap was observed between the Al foil and its paper envelope, indicating that they were not contacting seamlessly with each other. However, when the paper absorbed electrolyte, its cellulose fibers would expand slightly and sandwich the Al foil tightly, ensuring a normal battery operation as will be demonstrated in the following sections. Fig. 2(b) and (c) exhibit the microstructure of the deposited MnO2/CNT catalyst and the homemade cellulose paper, respectively. Since the length of the CNT (~50 mm) was equivalent to the pore size of the cellulose network (~50 mm), it is believed that the long and intertwined CNTs prevented the MnO2 nanoparticles (0.1e0.2 mm) from penetrating into the cellulose paper (Element mapping of the battery cross-section available in Fig. S1). Otherwise, the deposited cathode ink might contact the Al foil inside and cause battery short-circuit. Besides this, the CNT support also helped to improve the electrical conductivity of cathode significantly.
Due to the employment of ink-based cathode, one of the most important issues during battery fabrication is to prevent the ORR ink from penetrating into the paper substrate. Otherwise, it may contact the Al anode inside and lead to battery short-circuit. To inspect this, SEM observation (Hitachi S4800) of the battery crosssectional view was conducted before testing, together with the surface morphology of the home-made paper and ink-based cathode. To activate the battery, 0.5 mL of either 3 M NaOH solution or 4 M NaCl solution was added to the battery end as electrolyte. The OCV was recorded as soon as the electrolyte solution reached the battery electrodes. After the electrolyte flew through all the electrode area, polarization curve of the battery was obtained either by linear weep voltammetry (LSV, for NaOH electrolyte) or by galvanostatic discharge at different current steps (for NaCl electrolyte). This difference in testing method for different types of electrolyte was mainly due to the Al2O3 protective layer on the anode surface. In alkaline electrolyte, this Al2O3 layer can be quickly removed by the abundant hydroxyl ions. However, in neutral saline electrolyte, it will take a certain period of time to remove the Al2O3 layer by a pitting process [41], so that the transient LSV method is no longer appropriate which will greatly underestimate the battery performance. To obtain the single electrode polarization, the battery end was ionically connected to the end of a reference electrode (Ag/AgCl in saturated KCl) via the electrolyte, and a multimeter (Fluke 15B)
3. Results & discussion 3.1. Physical characterization
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Fig. 2. SEM images of the flexible Al-air battery: (a) Cross-sectional view; (b) Surface morphology of the MnO2/CNT catalyst; (c) Surface morphology of the home-made paper.
3.2. Parametric study on battery performance 3.2.1. Effect of electrolyte species In the literature, both strong alkaline and neutral saline electrolytes were commonly adopted for Al-air battery study. Therefore, performance of the present flexible Al-air battery was tested with both NaOH and NaCl solutions. According to our previous study, 4 M NaOH and 4 M NaCl lead to the highest battery power output compared with other electrolyte concentrations [42]. However, since 4M NaOH can significantly distort the cellulose paper, which consequently undermines the cathode integrity on it, therefore, a lower concentration of 3 M NaOH was adopted instead to protect the battery structure. Fig. 3(a) compares the battery performance with different electrolyte species. With 3 M NaOH as electrolyte, the battery achieved a peak power density of 19.0 ± 2.1 mW cm2 and a maximum current density of 47.3 ± 4.0 mA cm2. However, with 4 M NaCl as electrolyte, the battery obtained a much lower peak power density of 8.0 ± 0.3 mW cm2, and the maximum current density was between 40 and 50 mA cm2. From Fig. 3(b), it was found that the anode side encountered much higher overpotential loss in NaCl than in NaOH, which was mainly because of the lower reaction kinetics of Al oxidation in saline environment. In addition, the Al2O3 protective layer on the anode surface was also difficult to be removed under NaCl, which restricted the effective anodic reaction sites. This inferior anode performance is the major reason behind the poorer battery output when using NaCl. As for the cathode, it exhibited a higher open circuit potential (OCP) of 0.5 V in saline environment compared with 0 V in alkaline solution. This is mainly because of the different pH values of the electrolyte. According to Nernst Equation, the standard electrode potential of ORR in 4 M NaCl solution (pH z 7) is calculated to be 0.82 V, while the value for 3 M NaOH is 0.37 V. Considering the activation overpotential loss, the practical values of 0.5 V and 0 V are quite reasonable. As for the whole cathodic polarization curve, the one in NaCl electrolyte is generally above that in NaOH electrolyte, which is also due to this pH difference. Even though a much higher voltage and power output can be delivered by NaOH electrolyte, the discharge lifetime and Al utilization efficiency will be reduced due to the inevitable Al corrosion. Fig. 3(c) and (d) compare the galvanostatic discharge of the present battery using 3.5 mg Al as anode, with NaCl and NaOH as electrolyte, respectively. In general, the battery could discharge stably at various current densities from 1 to 20 mA cm2 until a sudden drop
of the voltage due to Al exhaustion. With NaOH electrolyte, the battery voltage was almost two times of that with NaCl electrolyte, due to its much lower anodic overpotential. However, the battery lifetime was much shorter. At 1 mA cm2, the battery with NaCl could continuously work for 6.5 h, while it could only last for less than 50 min with NaOH. This shorter discharge time led to a limited Al specific capacity of 230 mA h g1 with NaOH, which is only 7.7% of the theoretical value. On the contrary, the Al specific capacity with NaCl electrolyte is as high as 1852 mA h g1 (62.1% of the theoretical value). The major reason behind the low Al utilization efficiency with NaOH electrolyte is the notorious Al self-corrosion, which consumes most of the Al anode without electricity generation at low discharge rate. However, with the increase of discharge current density, the Al oxidation reaction could utilize a larger portion of the Al anode, so that the Al specific capacity was gradually increased to 1732 mA h g1 (58.1% of the theoretical value) at 20 mA cm2. As for the NaCl electrolyte, since the Al self-corrosion was extremely mild, the battery achieved similar Al specific capacities under different current densities, which were all around 55.5e62.1% of the theoretical value. As for the lost efficiency, it was actually due to a mechanical loss of the anode rather than the chemical corrosion loss. As shown by the inset, due to the pitting consumption pattern of the Al anode in NaCl, the thin foil anode tended to be split into pieces at the later discharge stage, so that part of it would be separated from the current collector and be wasted without discharge. In conclusion, the alkaline electrolyte is more suitable for short-term operation with high voltage requirement, while the saline electrolyte is more appropriate for long-term missions with less voltage requirement. Here, it is also worth mentioning that when using NaOH as electrolyte, the battery temperature would slightly increase due to the exothermic reaction of Al corrosion. However, the extent was very limited (within 1 C) because of the abundant aqueous electrolyte inside the paper. Meanwhile, H2 gas was also generated, which would be released to the ambient. On the contrary, when using NaCl as electrolyte, the Al corrosion reaction was negligible, so that the temperature increase and H2 release were also negligible.
3.2.2. Effect of Al loading The operation lifespan of the present flexible Al-air battery can be conveniently tailored by adjusting the loading of the embedded Al foil inside the paper substrate. To demonstrate this, a 0.1 mm thick Al foil (3N purity, aladdin®) with a weight loading of 25 mg cm2 was adopted as battery anode. Since this Al foil has a
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Fig. 3. Effect of electrolyte species on the battery performance: (a) Battery polarization curve; (b) Single electrode polarization; (c) Galvanostatic discharge with 3.5 mg Al as anode and 3 M NaOH as electrolyte; (d) Galvanostatic discharge with 3.5 mg Al as anode and 4 M NaCl as electrolyte (inset: pitting consumption of the Al foil anode).
different metal composition with the previous kitchen foil, their battery performance should be compared first before studying their discharge lifetime. As shown in Fig. 4(a), the battery with either the kitchen foil or the 3N Al foil exhibited a similar performance with each other, with only a slight difference at the high current density
region. From the inset, it can be seen that the anodic performance was identical while the tiny performance difference was resulted from the cathode side. This result indicates that performance of the present battery has a negligible dependence on the Al anode purity, so that the subsequent comparison of battery lifetime can be
Fig. 4. Effect of Al loading on the battery performance: (a) Battery polarization curve (inset: single electrode polarization); (b) Galvanostatic discharge of the battery with 25 mg Al as anode and 4 M NaCl as electrolyte.
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regarded as reasonable. In addition, this result also favors the battery cost-efficiency since low-purity industrial Al can be directly adopted. Fig. 4(b) exhibits the battery discharge result at different current densities with the 3N Al anode (25 mg in total). Compared with Fig. 3(d), the battery operation time was greatly extended, while the corresponding Al specific capacity was also slightly improved. This is probably because that a thicker Al anode leads to a more robust anode current collection process, which is less affected by the pitting-derived dismemberment. In conclusion, operation lifetime of the present battery can be easily tailored by controlling the loading of embedded Al, which is especially suitable for powering various single-use devices requiring low power but long-term operation. 3.2.3. Effect of ORR catalyst loading In this study, MnO2 was selected as the ORR catalyst because of its low cost together with satisfactory catalytic activity, which was commonly employed for Al-air batteries in the literature [18,19,29,43]. Nevertheless, since Mn is a heavy metal and may cause pollution to the environment (soil & water), the loading of this ORR catalyst should be controlled as small as possible. To obtain the optimum catalyst loading, different volumes of the ORR ink from 0 to 40 mL was deposited within the Ag grid, corresponding to the catalyst loading from 0 to 2 mg cm2. Fig. 5(a) compares the battery performance with different MnO2/CNT loadings. With no MnO2/CNT deposited, the battery could still generate a peak power density of 0.36 mW cm2 and a maximum current density of 3 mA cm2, which was attributed to the Ag catalyst contained in the Ag grid current collector [44]. However, due to the limited Ag loading and reaction area of the Ag grid, this performance was negligible when compared with batteries with ink deposition. Therefore, its influence on the present comparative study can be ignored. When 0.5 mg cm2 MnO2/CNT was added to the cathode, the peak power density was significantly increased to 3.2 mW cm2, and a much higher current density of 20 mA cm2 can be obtained. By doubling the loading to 1 mg cm2, the peak power density was further improved to 8.0 mW cm2, and the current density output was extended to 40 mA cm2. However, further increment of the loading to 2 mg cm2 had only a slight benefit to the battery power output (8.9 mW cm2). From Fig. 5(b), it is evident that by increasing the MnO2/CNT loading from 0 to 1 mg cm2, the cathodic performance was significantly improved, while the anodic performance was almost identical. In addition, the
inset of Fig. 5(b) shows the surface morphology of the ink-based cathode with different catalyst loadings, in which only the loading of 1 mg cm2 exhibited an intact catalyst layer. With a lower loading of 0.5 mg cm2, the cathode would be separated by the Ag grid, leading to poorer electrical conductivity; while with a higher loading of 2 mg cm2, cracks would occur inside the catalyst layer, leading to poorer mechanical integrity (SEM image available in Fig. S2, supplementary information). To sum up, a moderate catalyst loading of 1 mg cm2 is competent for the present Al-air battery, which achieves a fine balance between battery performance, environmental-friendliness and electrode integrity. 3.2.4. Effect of ORR catalyst species To replace the MnO2 catalyst, other non-metal ORR catalysts such as CNT and N-doped CNT were also investigated in order to obtain a completely green battery technology. In addition, the conventional Pt/C catalyst was also tested as a benchmark case. As shown in Fig. 6(a), among the four different ORR catalysts, Pt/C achieved the highest peak power density of 10.7 mW cm2, which was followed by 8.0 mW cm2 of the MnO2/CNT. This result is reasonable since Pt/C is currently the most powerful catalyst for ORR. However, when it comes to the cost, the 4 orders of magnitude more expensive Pt is apparently not a suitable choice and much less competitive than MnO2. As for CNT, it could still provide a peak power density of 5.5 mW cm2 without MnO2, but the battery voltage dropped quickly when the current density exceeded 20 mA cm2. To improve the performance of carbonaceous ORR catalysts, heteroatom-doping is proved to be an effective strategy in the literature [26]. Therefore, N-doped CNT (aladdin®) was utilized instead of pure CNT, and this time the peak power density was improved by 22% to 6.7 mW cm2, which was 84% of the MnO2/CNT case. In addition, the battery performance at high current density was also improved. From Fig. 6(b), it can be observed that the major difference of battery performance was located on the cathode side, which had different extents of overpotential loss with different species of ORR catalyst. Among them, the Pt/C possessed the lowest overpotential, while the MnO2/CNT and N-doped CNT had very close cathodic curves with each other. In conclusion, the N-doped CNT has shown its high potential to replace the commonly-adopted MnO2 for the present battery, enabling its free disposal after usage without any environmental concerns. Nevertheless, for the consistency of this work, MnO2/CNT was still utilized in the following sections, unless otherwise specified.
Fig. 5. Effect of ORR catalyst loading on the battery performance: (a) Battery polarization curve; (b) Single electrode polarization (inset: surface morphology of the deposited cathode with different catalyst loadings).
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Fig. 6. Effect of ORR catalyst species on the battery performance: (a) Battery polarization curve; (b) Single electrode polarization.
3.3. Parametric study on battery flexibility 3.3.1. Effect of bending angle Bending is a common type of deformation during the practical application of flexible batteries, which has been frequently investigated in the literature [39,40]. By bending the battery to a specific angle, the contact between different battery components may be impaired, and the structural integrity of the electrode/electrolyte layer may also be undermined. As a consequence, the battery performance will be reduced significantly or even result in a total failure. As shown in Fig. 7(a), the present flexible Al-air battery was bended by the middle of its electrode at different angles from 60 to 180 , and the corresponding battery performance was compared in
Fig. 7(b). Apparently, the bending angle had only a slight influence on the battery polarization, with peak power density decreased by 2.1e9.4%. As indicated by the dash line, this influence mainly occurred at the high current density region between 20 and 40 mA cm2, while it was negligible at its intended operation range between 0 and 20 mA cm2. From Fig. 7(c), it can be observed that the major influence of the bending angle was on the cathode side, which caused a slightly higher overpotential loss for bended batteries when the discharge current density exceeded 20 mA cm2. This is probably because that the bending process has slightly impaired the electrical connection of the cathode catalyst layer especially at the crease site, so that the ohmic loss increased. In addition, the bending process might also impede the oxygen
Fig. 7. Effect of bending angle on the battery performance: (a) Images of the bended flexible Al-air batteries at different angles; (b) Battery polarization curve; (c) Single electrode polarization.
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diffusion into the air cathode due to the reduced space above it, so that the mass transport loss also increased. These two factors mainly affected the middle and high current density regions of the polarization curve, respectively, while the low current density region was mainly controlled by the electrode activation loss. As for the anode side, the performance was nearly unaffected by bending due to the high ductility of Al foil. 3.3.2. Effect of bending times In practical situations, a flexible battery may encounter repetitive deformation due to the continuous movement of its host device, which imposes strict requirement to the battery durability. To investigate the durability of the present flexible Al-air battery, the cell was bended repeatedly by the middle of its electrode with an angle of 90 for multiple times, ranging from 100 to 1000. This high bending time is believed to be sufficient for the primary battery case. As shown in Fig. 8(a), the repetitive bending had only a mild influence on the battery polarization curve, especially at lower current densities below 20 mA cm2. The peak power density was slightly decreased by 8.7%e16.1%. From Fig. 8(b), it was found that the repetitive bending mainly affected the cathode side, while the Al anode was nearly unaffected. Furthermore, Fig. 8(c) shows the surface morphology of the deposited cathode after different times of bending. By increasing the bending time, integrity of the cathode surface was gradually undermined due to the peeling off of the catalyst layer. However, this process generally occurred at the site
of the Ag grid area rather than the paper substrate area. This is probably because that the combination between the catalyst ink with the paper was much stronger than that with the Ag grid. In addition, since the Ag grid was hydrophobic and could not absorb electrolyte during battery operation, the ORR catalyst deposited on it would contribute negligibly to electricity generation, so that the influence of its peeling off on battery overall performance was negligible. However, this would indeed weaken the cathode electric conductivity, which consequently increased the cathode overpotential loss as shown in Fig. 8(b). In addition to the macro-scale surface morphology, micro-scale SEM observation of the MnO2/ CNT catalyst at the bending site was also presented in Fig. 8(c). Apparently, the bending process imposed negligible influence on the distribution of the MnO2 particles on the CNT network. In conclusion, the present flexible Al-air battery possesses satisfactory durability to repetitive bending deformation, which can be applied in highly dynamic circumstances. In the future, its durability may be further improved by hot-pressing treatment after the ink deposition. 3.4. Water-activated battery In practical applications, a water-activated battery design is more convenient than that activated by electrolyte solution, as long as the solid-form electrolyte can be pre-deposited inside the paper substrate and stored stably. For this purpose, different volumes of
Fig. 8. Effect of bending times on the battery performance: (a) Battery polarization curve; (b) Single electrode polarization (inset: schematic diagram of the bending process); (c) Surface morphology of the deposited cathode after different times of bending, including both macro-scale and micro-scale images.
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4 M NaCl solution (25, 125, 250 and 375 mL) were deposited into the as-fabricated flexible Al-air batteries, followed by a complete drying process at 60 C. The amounts of deposited NaCl were determined to be 0.1, 0.5, 1.0 and 1.5 mmol, respectively. Fig. 9(a) shows the corresponding performance of these NaCl pre-deposited batteries, which were all activated by 0.5 mL DI-water. Apparently, the battery with 0.1 mmol NaCl encountered a significant loss of performance due to the insufficient storage of NaCl, which resulted in a very low NaCl concentration inside the paper substrate after water uptake. When the NaCl loading increased to 0.5 mmol, the battery performance was much improved and the peak power density reached 77.5% of the benchmark case with 4 M NaCl solution as electrolyte. However, further increment of the NaCl loading had no distinct benefit to the battery power output any more, which reached a plateau at around 6e6.5 mW cm2. This phenomenon can be explained by the limited solubility of NaCl in water (360 g NaCl in 1 L water at 25 C, which equivalents to about 5.4 mol L1) and the limited water uptake ability of the present flexible Al-air battery (about 0.1 mL for fully wetted). When 0.5 mmol NaCl was pre-deposited, the local NaCl concentration inside the paper after water uptake was already near 5 M, which was very close to the saturation point. Therefore, when 1.0 and 1.5 mmol NaCl was predeposited, the local NaCl concentration would hardly further increase due to the limited water uptake. Nevertheless, increasing the loading of pre-deposited NaCl do have a positive effect on the battery long-term discharge. As shown in Fig. 9(b), when 0.5 mmol NaCl was pre-deposited, the discharge voltage kept decreasing from 0.6 V to 0.4 V, and finally dropped to 0 V due to Al depletion. By increasing the NaCl loading to 1.0 and 1.5 mmol, the discharge curve became more stable, and the discharge voltage was also improved. Therefore, it is concluded that the present flexible A-air battery can be water-activated by pre-depositing the NaCl electrolyte inside, while increasing the NaCl loading helps to stabilize its discharge performance. In practical applications, the NaCl predeposited battery should be stored in a well-sealed package in case of NaCl dissolution due to air humidity.
3.5. Battery scaling up and stacking To further increase its power output, both battery scaling-up and battery stacking have been investigated in this section, in order to provide higher current output and voltage output, respectively. As shown in Fig. 10(a), a scaled-up battery prototype with an
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electrode area of 1 cm 5 cm has been fabricated, whose performance was tested with 4 M NaCl and compared with the single cell case (1 cm 1 cm). Apparently, by scaling-up the electrode area by 5 times, the current output could be extended from 40 mA to 200 mA, while the peak power was also improved from 8.0 mW to 37.4 mW. Theoretically, peak power of the 5 cm2 battery should be 40 mW, so that the calculated scaling-up efficiency was as high as 93.5%. The lost efficiency was probably due to the higher electrical resistance of the scaled-up battery, especially for the part of electrode away from the Cu foil end. Moreover, Fig. 10(b) demonstrates an even longer battery prototype with an electrode area of 1 cm 10 cm. This flexible battery was rolled in a spiral shape for powering a small fan (Video S1 available in the supplementary information), which further demonstrated the excellent battery flexibility. Supplementary data related to this article can be found at https://doi.org/10.1016/j.electacta.2019.07.049. In addition to battery scaling up, Fig. 10(c) exhibits a 5-cell battery pack connected in series and compares its performance with the single cell case. The battery pack could achieve an OCV of 6.6 V, which was almost 5 times of the single cell case (1.33 V). In addition, the peak power was improved from 8.0 mW to 37.5 mW, which was 94% of the theoretical value. The negligible loss of power should be arrived from the interconnection of the individual cells, which brought extra contact resistance to the whole system. Finally, a concept map of a tape-shaped flexible Al-air battery is demonstrated in Fig. 10(d). By incorporating several existing techniques such as paper-making, roll-pressing, screen printing, hot-pressing etc., the present flexible Al-air battery can be easily fabricated and stored in a tape form. During practical application, a desired voltage or current output can be delivered simply by selecting the length of the battery or the number of single cells in the battery pack, respectively.
4. Conclusion In this work, an innovative flexible Al-air battery has been successfully developed by embedding its Al foil anode inside the paper substrate and depositing its ORR ink-based cathode on the surface of the paper, which is lightweight, structurally simple and cost-efficient. This primary battery is mainly targeted for single-use and disposable applications with less power requirement. Both alkaline and saline electrolyte can be adopted, of which the former
Fig. 9. Effect of NaCl loading on the water-activated flexible Al-air battery performance: (a) Polarization curve; (b) Galvanostatic discharge at 1 mA cm2 with 3.5 mg Al as anode.
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Fig. 10. Scaling up and stacking of the flexible Al-air battery for higher current, voltage and power output: (a) Performance of a 1 cm 5 cm scaled-up battery; (b) Demonstration of a 1 cm 10 cm scaled-up battery for powering a fan; (c) Performance of a 5-cell battery pack; (d) Concept map of a flexible Al-air battery tape for practical application.
case can achieve a much higher power output (19 mW cm2) but less operation lifetime (48 min at 1 mA cm2 with 3.5 mg Al), while the latter case can obtain a much longer operation lifetime (6.5 h at 1 mA cm2 with 3.5 mg Al) but less power output (8 mW cm2). In addition, by increasing the embedded Al loading to 25 mg, the battery lifespan with saline electrolyte can be further extended to 58.4 h at 1 mA cm2. The battery flexibility has also been investigated by bending it at different angles and with different times, whose performance is negligibly affected, indicating an excellent flexibility and robustness against dynamic deformation. Moreover, the present battery can simply be activated by water, as long as the saline electrolyte is pre-deposited inside the paper substrate. Finally, scaling-up and stacking of the flexible Al-air battery have been successful demonstrated, of which both the scaled-up battery and the 5-cell battery pack can maintain 93.5e94% of the theoretical power output, demonstrating a very high scaling-up and stacking efficiency. Considering its application potential, a PCT patent (PCT/CN2018/118428) has been filed for the present flexible Al-air battery technology. Acknowledgement The authors would like to acknowledge the CRCG grant of the University of Hong Kong (201711160009) and the SZSTI of Shenzhen Municipal Government (JCYJ20170818141758464) to provide funding support to this project.
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.electacta.2019.07.049. References [1] M.A. Rahman, X. Wang, C. Wen, High energy density metal-air batteries: A review, J. Electrochem. Soc. 160 (2013) A1759eA1771. [2] P. Tan, H. Jiang, X. Zhu, L. An, C. Jung, M. Wu, L. Shi, W. Shyy, T. Zhao, Advances and challenges in lithium-air batteries, Appl. Energy 204 (2017) 780e806. [3] S. Huang, Z. Cui, N. Zhao, J. Sun, X. Guo, Influence of ambient air on cell reactions of Li-air batteries, Electrochim. Acta 191 (2016) 473e478. [4] T. Zhang, Z. Tao, J. Chen, Magnesiumeair batteries: From principle to application, Mater. Horiz. 1 (2014) 196e206. [5] N. Wang, R. Wang, C. Peng, B. Peng, Y. Feng, C. Hu, Discharge behaviour of MgAl-Pb and Mg-Al-Pb-In alloys as anodes for Mg-air battery, Electrochim. Acta 149 (2014) 193e205. [6] Y. Li, H. Dai, Recent advances in zinceair batteries, Chem. Soc. Rev. 43 (2014) 5257e5275. [7] J. Hu, Z. Shi, X. Wang, H. Qiao, H. Huang, Silver-modified porous 3D nitrogendoped graphene aerogel: Highly efficient oxygen reduction electrocatalyst for Zn Air battery, Electrochim. Acta 302 (2019) 216e224. [8] B. Xu, H. Lu, W. Cai, Y. Cao, Y. Deng, W. Yang, Synergistically enhanced oxygen reduction reaction composites of specific surface area and manganese valence controlled a-MnO2 nanotube decorated by silver nanoparticles in Al-air batteries, Electrochim. Acta 305 (2019) 360e369. [9] Y. Wang, H. Kwok, W. Pan, H. Zhang, D.Y.C. Leung, Innovative paper-based Alair batteries as a low-cost and green energy technology for the miniwatt market, J. Power Sources 414 (2019) 278e282. [10] S. Zaromb, The use and behavior of aluminum anodes in alkaline primary batteries, J. Electrochem. Soc. 109 (1962) 1125e1130.
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Combining Al-air battery with paper-making industry, a novel type of flexible primary battery technology Yifei Wang a, *, Holly Y.H. Kwok a, Wending Pan a, Yingguang Zhang a, Huimin Zhang b, Xu Lu c, Dennis Y.C. Leung a, ** a b c
Department of Mechanical Engineering, The University of Hong Kong, Hong Kong, China School of Civil Engineering and Architecture, East China Jiao Tong University, Nanchang, China Department of Chemistry, Yale University, New Haven, CT, 06520, United States
a r t i c l e i n f o
a b s t r a c t
Article history: Received 6 May 2019 Received in revised form 26 June 2019 Accepted 11 July 2019 Available online 12 July 2019
Al-air batteries are generally fabricated by rigid and heavyweight materials such as metal or plastic, meanwhile a bulky electrolyte solution either in static or circulation is requisite to ensure their steady operation. Therefore, they are less suitable for powering flexible and portable devices with lower power demand, such as wearable electronics, point-of-care test assays, RFID tags, etc. This work develops a flexible and lightweight Al-air battery with much less electrolyte storage, which is totally fabricated on a cellulose paper. The Al foil anode is embedded inside the paper substrate during paper-making process, while the air-breathing cathode is deposited onto the paper surface using an oxygen reduction ink. Despite its simple structure and low cost, this flexible Al-air battery can deliver a satisfactory power density of 19 mW cm2 with alkaline electrolyte, and its operation lifespan is as long as 58 h with only 25 mg Al when saline electrolyte is employed. The corresponding Al specific capacity is as high as 2338 mA h g1. In addition, this battery exhibits an excellent flexibility when facing different bending angles from 60 to 180 and multiple bending times greater than 1000. Furthermore, a flexible current or voltage output can be easily obtained by scaling up or stacking the present flexible Al-air battery, respectively, and the corresponding stacking efficiency is as high as 94%. In the future, non-metal oxygen catalyst will be employed to improve its environmental friendliness, and hot-pressing will be adopted to further increase its robustness to external deformation. © 2019 Elsevier Ltd. All rights reserved.
Keywords: Paper-based Al-air battery Flexible battery Embedded Al Battery bending
1. Introduction Since its first appearance, metal-air battery (MAB) has become a fascinating power technology. Compared with conventional allsealed batteries, one of the greatest advantages of MAB is its open-ended and lightweight cathode, which can directly breathe oxygen from ambient air. In this manner, MAB can achieve much higher energy density, simpler battery system and reduced fabrication cost benefitted from the elimination of oxidant storage [1]. Till now, various MABs have been developed according to their different metal anodes, such as Li-air battery [2,3], Mg-air battery [4,5], Zn-air battery [6,7] and Al-air battery [8,9]. Each type of these
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (D.Y.C. Leung). https://doi.org/10.1016/j.electacta.2019.07.049 0013-4686/© 2019 Elsevier Ltd. All rights reserved.
(Y.
Wang),
[email protected]
MABs has its specific advantages and disadvantages for different applications. Among them, the primary Al-air battery has received numerous R&D attentions since its first appearance in the 1960s [10]. This is because Al has a comparatively high specific capacity of 2.98 A h g1 (3.86 for Li, 2.20 for Mg, 0.82 for Zn) and a very low market price of 1.75 $ kg1 (68 for Li, 2.75 for Mg, 1.85 for Zn) at the same time, not to mention its huge reserve which is the most abundant metal in the crust of the earth [6,11,12]. During the Al-air battery discharge, the following reactions will occur at the electrode-electrolyte interface, generating useful voltage and current output until the Al anode is exhausted. In addition, the Al corrosion side-reaction also exists, which is negative to the battery performance and practical application. As for the cathode, oxygen from the ambient air will diffuse continuously into the porous catalyst layer due to its concentration gradient, which will be reduced at the triple-phase boundary of the solid catalyst, liquid electrolyte and air.
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Anode : Al þ 3OH /AlðOHÞ3 þ 3e
(1)
Cathode:O2 þ 2H2 O þ 4e /4OH
(2)
Overall:4Al þ 3O2 þ 6H2 O/4AlðOHÞ3
(3)
3 Al corrosion:Al þ OH þ 3H2 O/AlðOHÞ 4 þ H2 2
(4)
Till now, great efforts have been made on Al-air batteries to improve their power output, energy efficiency and system practicability [13]. On the anode side, various Al alloys doped with different species and ratios of trace elements (Ga, Ti, In, Sn, Zn, Bi, Mn and Mg) have been developed to both decrease the anodic overpotential loss and suppress the Al self-corrosion [14e16], while novel anode structures such as 3D printed Al nanoparticles were also reported [17]. On the cathode side, developing low-cost yet highly-efficient oxygen reduction reaction (ORR) catalyst other than Pt is of great importance to its large-scale application, which is a common research target for all kinds of primary metal-air batteries and fuel cells. These non-noble ORR catalysts include but not restrict to manganese dioxide [18e20], cobalt oxides [21,22], perovskite oxides [23,24], bismuth oxyhalide [25], nitrogen-doped carbons [26] and seed-mediated silver manganate nanoplate [27], among which MnO2 is currently the most popular choice. As for the electrolyte, both alkaline and saline solutions are commonly adopted, with various Al corrosion inhibitors developed for the alkaline electrolyte [28e30]. In addition, the recently-emerged gel electrolyte is also a promising research direction especially for portable applications [31e33]. By gelling of the aqueous electrolyte, the water management issue is eliminated together with the potential leakage hazard. Other research works focus on either the system-level development of Al-air battery [34,35] or the numerical investigation of its working mechanisms [36,37]. Most of the existing Al-air batteries are in rigid form with heavy and bulky battery components, which are utilized for better assembling and sealing purpose. In recent years, the flexible battery is getting more and more attention and its application prospect is growing rapidly [38]. Unlike the conventional rigid battery, a flexible battery can be employed in more sophisticated environment which has fewer requirements on the specific battery shape. In addition, a flexible battery can tolerate repetitive deformation without significantly influencing its electricity output, which can be adopted as power unit in dynamic situations, such as wearable electronics. Moreover, unlike the modular fabrication of conventional rigid batteries, the size of flexible batteries can be easily tailored according to the specific space or power requirement, which is another important advantage for real applications. Considering the great advantage of Al in energy density, cost, abundance and eco-friendliness, a flexible version of Al-air battery is highly promising and competitive for powering various singleuse and disposable devices. Fotouhi et al. [39] fabricated a cableshaped flexible Al-air battery by wrapping a paper layer and a CNT-paper matrix on an Al wire. When the bending angle increased, the voltage and current output slightly increased by 9.5 and 15.7%, respectively. Xu et al. [40] also developed a fiber-shaped flexible Al-air battery by covering an Al spring anode with hydrogel electrolyte and Ag-coated CNT sheet cathode. The battery displayed a high Al specific capacity of 935 mA h g1 and the flexibility is also quite good when facing different bending angles and bending times. Nevertheless, both of the two flexible Al-air batteries achieved relatively low power output (1.5 mW cm1 and 1.33 mW cm2, respectively) probably due to the tubular cell configuration, and the employment of high-purity Al anode
(99.999%) will greatly increase the battery cost. Therefore, it is requisite to develop flexible Al-air batteries with improved power output and low-purity Al anode. In this work, we have proposed a novel type of flexible Al-air battery, which was totally fabricated on a single piece of cellulose paper. The Al foil anode was embedded inside the paper substrate during the paper-making process, while the air-breathing cathode was directly deposited onto the paper surface by using an ORR ink. In this manner, a flexible, ultrathin and lightweight battery was successfully obtained. Its electrochemical performance was studied first by using both alkaline and saline electrolytes. In addition, different types of ORR catalysts and different loadings of embedded Al were investigated in order to improve its power output and operation lifetime, respectively. To inspect its flexibility, the battery performances under different bending angles and after different bending times were also tested. Moreover, an electrolyte predeposition technique was studied for more practical battery application, so that only water was needed for battery activation. Finally, the flexible Al-air battery was both scaled up to improve its current output and stacked to increase its voltage output, demonstrating its high potential for practical applications with diverse current/voltage requirements. 2. Experimental 2.1. Preparation of the Al-paper complex In order to integrate the Al foil with the paper substrate without any external force, we innovatively embed the Al anode inside the paper during the paper-making process. By adding an extra step during the conventional paper-making procedure, the Al-paper complex can be easily fabricated. As shown in Fig. 1, a thin layer of paper pulp was first spread uniformly on a stainless steel mesh (18 meshes). Next, a piece of Al foil (98.2% purity, 0.01 mm thickness, from kitchen foil) with a reaction area of 1 cm2 (equivalent to 3.5 mg) was placed on the top of the paper pulp, whose size was slightly smaller than the paper pulp except for a tentacle which reached out of the paper pulp for connection purpose. Afterwards, another layer of paper pulp was slowly poured onto the existing two layers, which not only covered the Al foil surface but also combined with the previous paper pulp at the periphery. Next, the three layers were taken out from the mesh, which were pressed to squeeze the water inside and to achieve better combination of them. Finally, the as-prepared Al-paper complex was dried in an oven at 60 C for half an hour. Its thickness was around 0.57 ± 0.02 mm, which was well controlled by the depth of the pulp-containing groove. Theoretically, a smaller thickness of the paper will benefit the battery performance because of the reduced ionic resistance, which is a common phenomenon in all kinds of electrochemical power sources. Nevertheless, in the present study, a smaller paper thickness also means less electrolyte storage at the vicinity of the electrodes, which is negative to the battery longterm stability. Therefore, thinner papers were not included in this work. 2.2. ORR ink preparation and battery fabrication To obtain a flexible battery, the conventional carbon paperbased cathode is no longer appropriate. Instead, an ORR ink was developed, which could be deposited directly onto the paper substrate as air-breathing cathode. The ORR ink was mainly composed of a non-noble ORR catalyst, a catalyst support and a polymer binder, which were all dispersed in an ethanol-water solvent (1:1 vol ratio). In this work, MnO2 was selected as the ORR catalyst considering its excellent balance between catalytic activity and
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Fig. 1. Flow chart of the battery fabrication process and exploded view of the battery structure.
cost. To prevent the penetration of MnO2 nanoparticles deep into the paper substrate, multi-walled carbon nanotube (MWCNT, aladdin®) with a length of 50 mm was employed as catalyst support, and the metal loading of MnO2/CNT was controlled to be 60 wt%. To prepare this ink, 50 mg of the MnO2/CNT and 1.7 mg of the Nafion binder (DuPont®) were dispersed in 1 mL ethanol-water solvent, which was next sonicated for 30 min to achieve a uniform distribution. Before ink deposition, a silver grid (5 5, 1 cm2 total area) was deposited on the paper surface, which fully corresponded to the Al foil anode inside. This silver grid was added to not only collect the generated current but also confine the cathode area. Afterwards, the as-prepared ORR ink was dip-coated within the Ag grid by a pipette, which was dried next at 60 C for 30 min. The catalyst loading was controlled by selecting the volume of deposited ORR ink.
was connected between the battery cathode and the reference electrode to obtain the cathodic potential during the polarization test. Furthermore, to obtain the long-term discharge stability and Al utilization efficiency, the battery was also discharged at various current densities from 1 to 20 mA cm2, until the embedded Al anode was completely consumed. All the above tests were conducted by an electrochemical workstation (CHI760E) at room temperature, and fresh batteries were utilized for each time of the test.
2.3. Battery characterization
Before testing, the flexible Al-air battery was cut off by the middle of its electrode in order to obtain its cross-sectional view. As shown in Fig. 2(a), the Al foil was well sandwiched by the two layers of cellulose paper. On the top layer, a thin layer of deposited ORR catalyst could be observed, whose edge was slightly cracked due to the cutting process. Here, a 0.1 mm-thick Al foil was adopted instead of the 0.01 mm-thick one in order to better exhibiting the different layers. From this cross-sectional view, it can be concluded that the ORR ink did not penetrate into the paper layer and contact the Al anode, therefore eliminating the issue of battery shortcircuiting. In addition, a thin gap was observed between the Al foil and its paper envelope, indicating that they were not contacting seamlessly with each other. However, when the paper absorbed electrolyte, its cellulose fibers would expand slightly and sandwich the Al foil tightly, ensuring a normal battery operation as will be demonstrated in the following sections. Fig. 2(b) and (c) exhibit the microstructure of the deposited MnO2/CNT catalyst and the homemade cellulose paper, respectively. Since the length of the CNT (~50 mm) was equivalent to the pore size of the cellulose network (~50 mm), it is believed that the long and intertwined CNTs prevented the MnO2 nanoparticles (0.1e0.2 mm) from penetrating into the cellulose paper (Element mapping of the battery cross-section available in Fig. S1). Otherwise, the deposited cathode ink might contact the Al foil inside and cause battery short-circuit. Besides this, the CNT support also helped to improve the electrical conductivity of cathode significantly.
Due to the employment of ink-based cathode, one of the most important issues during battery fabrication is to prevent the ORR ink from penetrating into the paper substrate. Otherwise, it may contact the Al anode inside and lead to battery short-circuit. To inspect this, SEM observation (Hitachi S4800) of the battery crosssectional view was conducted before testing, together with the surface morphology of the home-made paper and ink-based cathode. To activate the battery, 0.5 mL of either 3 M NaOH solution or 4 M NaCl solution was added to the battery end as electrolyte. The OCV was recorded as soon as the electrolyte solution reached the battery electrodes. After the electrolyte flew through all the electrode area, polarization curve of the battery was obtained either by linear weep voltammetry (LSV, for NaOH electrolyte) or by galvanostatic discharge at different current steps (for NaCl electrolyte). This difference in testing method for different types of electrolyte was mainly due to the Al2O3 protective layer on the anode surface. In alkaline electrolyte, this Al2O3 layer can be quickly removed by the abundant hydroxyl ions. However, in neutral saline electrolyte, it will take a certain period of time to remove the Al2O3 layer by a pitting process [41], so that the transient LSV method is no longer appropriate which will greatly underestimate the battery performance. To obtain the single electrode polarization, the battery end was ionically connected to the end of a reference electrode (Ag/AgCl in saturated KCl) via the electrolyte, and a multimeter (Fluke 15B)
3. Results & discussion 3.1. Physical characterization
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Fig. 2. SEM images of the flexible Al-air battery: (a) Cross-sectional view; (b) Surface morphology of the MnO2/CNT catalyst; (c) Surface morphology of the home-made paper.
3.2. Parametric study on battery performance 3.2.1. Effect of electrolyte species In the literature, both strong alkaline and neutral saline electrolytes were commonly adopted for Al-air battery study. Therefore, performance of the present flexible Al-air battery was tested with both NaOH and NaCl solutions. According to our previous study, 4 M NaOH and 4 M NaCl lead to the highest battery power output compared with other electrolyte concentrations [42]. However, since 4M NaOH can significantly distort the cellulose paper, which consequently undermines the cathode integrity on it, therefore, a lower concentration of 3 M NaOH was adopted instead to protect the battery structure. Fig. 3(a) compares the battery performance with different electrolyte species. With 3 M NaOH as electrolyte, the battery achieved a peak power density of 19.0 ± 2.1 mW cm2 and a maximum current density of 47.3 ± 4.0 mA cm2. However, with 4 M NaCl as electrolyte, the battery obtained a much lower peak power density of 8.0 ± 0.3 mW cm2, and the maximum current density was between 40 and 50 mA cm2. From Fig. 3(b), it was found that the anode side encountered much higher overpotential loss in NaCl than in NaOH, which was mainly because of the lower reaction kinetics of Al oxidation in saline environment. In addition, the Al2O3 protective layer on the anode surface was also difficult to be removed under NaCl, which restricted the effective anodic reaction sites. This inferior anode performance is the major reason behind the poorer battery output when using NaCl. As for the cathode, it exhibited a higher open circuit potential (OCP) of 0.5 V in saline environment compared with 0 V in alkaline solution. This is mainly because of the different pH values of the electrolyte. According to Nernst Equation, the standard electrode potential of ORR in 4 M NaCl solution (pH z 7) is calculated to be 0.82 V, while the value for 3 M NaOH is 0.37 V. Considering the activation overpotential loss, the practical values of 0.5 V and 0 V are quite reasonable. As for the whole cathodic polarization curve, the one in NaCl electrolyte is generally above that in NaOH electrolyte, which is also due to this pH difference. Even though a much higher voltage and power output can be delivered by NaOH electrolyte, the discharge lifetime and Al utilization efficiency will be reduced due to the inevitable Al corrosion. Fig. 3(c) and (d) compare the galvanostatic discharge of the present battery using 3.5 mg Al as anode, with NaCl and NaOH as electrolyte, respectively. In general, the battery could discharge stably at various current densities from 1 to 20 mA cm2 until a sudden drop
of the voltage due to Al exhaustion. With NaOH electrolyte, the battery voltage was almost two times of that with NaCl electrolyte, due to its much lower anodic overpotential. However, the battery lifetime was much shorter. At 1 mA cm2, the battery with NaCl could continuously work for 6.5 h, while it could only last for less than 50 min with NaOH. This shorter discharge time led to a limited Al specific capacity of 230 mA h g1 with NaOH, which is only 7.7% of the theoretical value. On the contrary, the Al specific capacity with NaCl electrolyte is as high as 1852 mA h g1 (62.1% of the theoretical value). The major reason behind the low Al utilization efficiency with NaOH electrolyte is the notorious Al self-corrosion, which consumes most of the Al anode without electricity generation at low discharge rate. However, with the increase of discharge current density, the Al oxidation reaction could utilize a larger portion of the Al anode, so that the Al specific capacity was gradually increased to 1732 mA h g1 (58.1% of the theoretical value) at 20 mA cm2. As for the NaCl electrolyte, since the Al self-corrosion was extremely mild, the battery achieved similar Al specific capacities under different current densities, which were all around 55.5e62.1% of the theoretical value. As for the lost efficiency, it was actually due to a mechanical loss of the anode rather than the chemical corrosion loss. As shown by the inset, due to the pitting consumption pattern of the Al anode in NaCl, the thin foil anode tended to be split into pieces at the later discharge stage, so that part of it would be separated from the current collector and be wasted without discharge. In conclusion, the alkaline electrolyte is more suitable for short-term operation with high voltage requirement, while the saline electrolyte is more appropriate for long-term missions with less voltage requirement. Here, it is also worth mentioning that when using NaOH as electrolyte, the battery temperature would slightly increase due to the exothermic reaction of Al corrosion. However, the extent was very limited (within 1 C) because of the abundant aqueous electrolyte inside the paper. Meanwhile, H2 gas was also generated, which would be released to the ambient. On the contrary, when using NaCl as electrolyte, the Al corrosion reaction was negligible, so that the temperature increase and H2 release were also negligible.
3.2.2. Effect of Al loading The operation lifespan of the present flexible Al-air battery can be conveniently tailored by adjusting the loading of the embedded Al foil inside the paper substrate. To demonstrate this, a 0.1 mm thick Al foil (3N purity, aladdin®) with a weight loading of 25 mg cm2 was adopted as battery anode. Since this Al foil has a
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Fig. 3. Effect of electrolyte species on the battery performance: (a) Battery polarization curve; (b) Single electrode polarization; (c) Galvanostatic discharge with 3.5 mg Al as anode and 3 M NaOH as electrolyte; (d) Galvanostatic discharge with 3.5 mg Al as anode and 4 M NaCl as electrolyte (inset: pitting consumption of the Al foil anode).
different metal composition with the previous kitchen foil, their battery performance should be compared first before studying their discharge lifetime. As shown in Fig. 4(a), the battery with either the kitchen foil or the 3N Al foil exhibited a similar performance with each other, with only a slight difference at the high current density
region. From the inset, it can be seen that the anodic performance was identical while the tiny performance difference was resulted from the cathode side. This result indicates that performance of the present battery has a negligible dependence on the Al anode purity, so that the subsequent comparison of battery lifetime can be
Fig. 4. Effect of Al loading on the battery performance: (a) Battery polarization curve (inset: single electrode polarization); (b) Galvanostatic discharge of the battery with 25 mg Al as anode and 4 M NaCl as electrolyte.
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regarded as reasonable. In addition, this result also favors the battery cost-efficiency since low-purity industrial Al can be directly adopted. Fig. 4(b) exhibits the battery discharge result at different current densities with the 3N Al anode (25 mg in total). Compared with Fig. 3(d), the battery operation time was greatly extended, while the corresponding Al specific capacity was also slightly improved. This is probably because that a thicker Al anode leads to a more robust anode current collection process, which is less affected by the pitting-derived dismemberment. In conclusion, operation lifetime of the present battery can be easily tailored by controlling the loading of embedded Al, which is especially suitable for powering various single-use devices requiring low power but long-term operation. 3.2.3. Effect of ORR catalyst loading In this study, MnO2 was selected as the ORR catalyst because of its low cost together with satisfactory catalytic activity, which was commonly employed for Al-air batteries in the literature [18,19,29,43]. Nevertheless, since Mn is a heavy metal and may cause pollution to the environment (soil & water), the loading of this ORR catalyst should be controlled as small as possible. To obtain the optimum catalyst loading, different volumes of the ORR ink from 0 to 40 mL was deposited within the Ag grid, corresponding to the catalyst loading from 0 to 2 mg cm2. Fig. 5(a) compares the battery performance with different MnO2/CNT loadings. With no MnO2/CNT deposited, the battery could still generate a peak power density of 0.36 mW cm2 and a maximum current density of 3 mA cm2, which was attributed to the Ag catalyst contained in the Ag grid current collector [44]. However, due to the limited Ag loading and reaction area of the Ag grid, this performance was negligible when compared with batteries with ink deposition. Therefore, its influence on the present comparative study can be ignored. When 0.5 mg cm2 MnO2/CNT was added to the cathode, the peak power density was significantly increased to 3.2 mW cm2, and a much higher current density of 20 mA cm2 can be obtained. By doubling the loading to 1 mg cm2, the peak power density was further improved to 8.0 mW cm2, and the current density output was extended to 40 mA cm2. However, further increment of the loading to 2 mg cm2 had only a slight benefit to the battery power output (8.9 mW cm2). From Fig. 5(b), it is evident that by increasing the MnO2/CNT loading from 0 to 1 mg cm2, the cathodic performance was significantly improved, while the anodic performance was almost identical. In addition, the
inset of Fig. 5(b) shows the surface morphology of the ink-based cathode with different catalyst loadings, in which only the loading of 1 mg cm2 exhibited an intact catalyst layer. With a lower loading of 0.5 mg cm2, the cathode would be separated by the Ag grid, leading to poorer electrical conductivity; while with a higher loading of 2 mg cm2, cracks would occur inside the catalyst layer, leading to poorer mechanical integrity (SEM image available in Fig. S2, supplementary information). To sum up, a moderate catalyst loading of 1 mg cm2 is competent for the present Al-air battery, which achieves a fine balance between battery performance, environmental-friendliness and electrode integrity. 3.2.4. Effect of ORR catalyst species To replace the MnO2 catalyst, other non-metal ORR catalysts such as CNT and N-doped CNT were also investigated in order to obtain a completely green battery technology. In addition, the conventional Pt/C catalyst was also tested as a benchmark case. As shown in Fig. 6(a), among the four different ORR catalysts, Pt/C achieved the highest peak power density of 10.7 mW cm2, which was followed by 8.0 mW cm2 of the MnO2/CNT. This result is reasonable since Pt/C is currently the most powerful catalyst for ORR. However, when it comes to the cost, the 4 orders of magnitude more expensive Pt is apparently not a suitable choice and much less competitive than MnO2. As for CNT, it could still provide a peak power density of 5.5 mW cm2 without MnO2, but the battery voltage dropped quickly when the current density exceeded 20 mA cm2. To improve the performance of carbonaceous ORR catalysts, heteroatom-doping is proved to be an effective strategy in the literature [26]. Therefore, N-doped CNT (aladdin®) was utilized instead of pure CNT, and this time the peak power density was improved by 22% to 6.7 mW cm2, which was 84% of the MnO2/CNT case. In addition, the battery performance at high current density was also improved. From Fig. 6(b), it can be observed that the major difference of battery performance was located on the cathode side, which had different extents of overpotential loss with different species of ORR catalyst. Among them, the Pt/C possessed the lowest overpotential, while the MnO2/CNT and N-doped CNT had very close cathodic curves with each other. In conclusion, the N-doped CNT has shown its high potential to replace the commonly-adopted MnO2 for the present battery, enabling its free disposal after usage without any environmental concerns. Nevertheless, for the consistency of this work, MnO2/CNT was still utilized in the following sections, unless otherwise specified.
Fig. 5. Effect of ORR catalyst loading on the battery performance: (a) Battery polarization curve; (b) Single electrode polarization (inset: surface morphology of the deposited cathode with different catalyst loadings).
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Fig. 6. Effect of ORR catalyst species on the battery performance: (a) Battery polarization curve; (b) Single electrode polarization.
3.3. Parametric study on battery flexibility 3.3.1. Effect of bending angle Bending is a common type of deformation during the practical application of flexible batteries, which has been frequently investigated in the literature [39,40]. By bending the battery to a specific angle, the contact between different battery components may be impaired, and the structural integrity of the electrode/electrolyte layer may also be undermined. As a consequence, the battery performance will be reduced significantly or even result in a total failure. As shown in Fig. 7(a), the present flexible Al-air battery was bended by the middle of its electrode at different angles from 60 to 180 , and the corresponding battery performance was compared in
Fig. 7(b). Apparently, the bending angle had only a slight influence on the battery polarization, with peak power density decreased by 2.1e9.4%. As indicated by the dash line, this influence mainly occurred at the high current density region between 20 and 40 mA cm2, while it was negligible at its intended operation range between 0 and 20 mA cm2. From Fig. 7(c), it can be observed that the major influence of the bending angle was on the cathode side, which caused a slightly higher overpotential loss for bended batteries when the discharge current density exceeded 20 mA cm2. This is probably because that the bending process has slightly impaired the electrical connection of the cathode catalyst layer especially at the crease site, so that the ohmic loss increased. In addition, the bending process might also impede the oxygen
Fig. 7. Effect of bending angle on the battery performance: (a) Images of the bended flexible Al-air batteries at different angles; (b) Battery polarization curve; (c) Single electrode polarization.
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diffusion into the air cathode due to the reduced space above it, so that the mass transport loss also increased. These two factors mainly affected the middle and high current density regions of the polarization curve, respectively, while the low current density region was mainly controlled by the electrode activation loss. As for the anode side, the performance was nearly unaffected by bending due to the high ductility of Al foil. 3.3.2. Effect of bending times In practical situations, a flexible battery may encounter repetitive deformation due to the continuous movement of its host device, which imposes strict requirement to the battery durability. To investigate the durability of the present flexible Al-air battery, the cell was bended repeatedly by the middle of its electrode with an angle of 90 for multiple times, ranging from 100 to 1000. This high bending time is believed to be sufficient for the primary battery case. As shown in Fig. 8(a), the repetitive bending had only a mild influence on the battery polarization curve, especially at lower current densities below 20 mA cm2. The peak power density was slightly decreased by 8.7%e16.1%. From Fig. 8(b), it was found that the repetitive bending mainly affected the cathode side, while the Al anode was nearly unaffected. Furthermore, Fig. 8(c) shows the surface morphology of the deposited cathode after different times of bending. By increasing the bending time, integrity of the cathode surface was gradually undermined due to the peeling off of the catalyst layer. However, this process generally occurred at the site
of the Ag grid area rather than the paper substrate area. This is probably because that the combination between the catalyst ink with the paper was much stronger than that with the Ag grid. In addition, since the Ag grid was hydrophobic and could not absorb electrolyte during battery operation, the ORR catalyst deposited on it would contribute negligibly to electricity generation, so that the influence of its peeling off on battery overall performance was negligible. However, this would indeed weaken the cathode electric conductivity, which consequently increased the cathode overpotential loss as shown in Fig. 8(b). In addition to the macro-scale surface morphology, micro-scale SEM observation of the MnO2/ CNT catalyst at the bending site was also presented in Fig. 8(c). Apparently, the bending process imposed negligible influence on the distribution of the MnO2 particles on the CNT network. In conclusion, the present flexible Al-air battery possesses satisfactory durability to repetitive bending deformation, which can be applied in highly dynamic circumstances. In the future, its durability may be further improved by hot-pressing treatment after the ink deposition. 3.4. Water-activated battery In practical applications, a water-activated battery design is more convenient than that activated by electrolyte solution, as long as the solid-form electrolyte can be pre-deposited inside the paper substrate and stored stably. For this purpose, different volumes of
Fig. 8. Effect of bending times on the battery performance: (a) Battery polarization curve; (b) Single electrode polarization (inset: schematic diagram of the bending process); (c) Surface morphology of the deposited cathode after different times of bending, including both macro-scale and micro-scale images.
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4 M NaCl solution (25, 125, 250 and 375 mL) were deposited into the as-fabricated flexible Al-air batteries, followed by a complete drying process at 60 C. The amounts of deposited NaCl were determined to be 0.1, 0.5, 1.0 and 1.5 mmol, respectively. Fig. 9(a) shows the corresponding performance of these NaCl pre-deposited batteries, which were all activated by 0.5 mL DI-water. Apparently, the battery with 0.1 mmol NaCl encountered a significant loss of performance due to the insufficient storage of NaCl, which resulted in a very low NaCl concentration inside the paper substrate after water uptake. When the NaCl loading increased to 0.5 mmol, the battery performance was much improved and the peak power density reached 77.5% of the benchmark case with 4 M NaCl solution as electrolyte. However, further increment of the NaCl loading had no distinct benefit to the battery power output any more, which reached a plateau at around 6e6.5 mW cm2. This phenomenon can be explained by the limited solubility of NaCl in water (360 g NaCl in 1 L water at 25 C, which equivalents to about 5.4 mol L1) and the limited water uptake ability of the present flexible Al-air battery (about 0.1 mL for fully wetted). When 0.5 mmol NaCl was pre-deposited, the local NaCl concentration inside the paper after water uptake was already near 5 M, which was very close to the saturation point. Therefore, when 1.0 and 1.5 mmol NaCl was predeposited, the local NaCl concentration would hardly further increase due to the limited water uptake. Nevertheless, increasing the loading of pre-deposited NaCl do have a positive effect on the battery long-term discharge. As shown in Fig. 9(b), when 0.5 mmol NaCl was pre-deposited, the discharge voltage kept decreasing from 0.6 V to 0.4 V, and finally dropped to 0 V due to Al depletion. By increasing the NaCl loading to 1.0 and 1.5 mmol, the discharge curve became more stable, and the discharge voltage was also improved. Therefore, it is concluded that the present flexible A-air battery can be water-activated by pre-depositing the NaCl electrolyte inside, while increasing the NaCl loading helps to stabilize its discharge performance. In practical applications, the NaCl predeposited battery should be stored in a well-sealed package in case of NaCl dissolution due to air humidity.
3.5. Battery scaling up and stacking To further increase its power output, both battery scaling-up and battery stacking have been investigated in this section, in order to provide higher current output and voltage output, respectively. As shown in Fig. 10(a), a scaled-up battery prototype with an
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electrode area of 1 cm 5 cm has been fabricated, whose performance was tested with 4 M NaCl and compared with the single cell case (1 cm 1 cm). Apparently, by scaling-up the electrode area by 5 times, the current output could be extended from 40 mA to 200 mA, while the peak power was also improved from 8.0 mW to 37.4 mW. Theoretically, peak power of the 5 cm2 battery should be 40 mW, so that the calculated scaling-up efficiency was as high as 93.5%. The lost efficiency was probably due to the higher electrical resistance of the scaled-up battery, especially for the part of electrode away from the Cu foil end. Moreover, Fig. 10(b) demonstrates an even longer battery prototype with an electrode area of 1 cm 10 cm. This flexible battery was rolled in a spiral shape for powering a small fan (Video S1 available in the supplementary information), which further demonstrated the excellent battery flexibility. Supplementary data related to this article can be found at https://doi.org/10.1016/j.electacta.2019.07.049. In addition to battery scaling up, Fig. 10(c) exhibits a 5-cell battery pack connected in series and compares its performance with the single cell case. The battery pack could achieve an OCV of 6.6 V, which was almost 5 times of the single cell case (1.33 V). In addition, the peak power was improved from 8.0 mW to 37.5 mW, which was 94% of the theoretical value. The negligible loss of power should be arrived from the interconnection of the individual cells, which brought extra contact resistance to the whole system. Finally, a concept map of a tape-shaped flexible Al-air battery is demonstrated in Fig. 10(d). By incorporating several existing techniques such as paper-making, roll-pressing, screen printing, hot-pressing etc., the present flexible Al-air battery can be easily fabricated and stored in a tape form. During practical application, a desired voltage or current output can be delivered simply by selecting the length of the battery or the number of single cells in the battery pack, respectively.
4. Conclusion In this work, an innovative flexible Al-air battery has been successfully developed by embedding its Al foil anode inside the paper substrate and depositing its ORR ink-based cathode on the surface of the paper, which is lightweight, structurally simple and cost-efficient. This primary battery is mainly targeted for single-use and disposable applications with less power requirement. Both alkaline and saline electrolyte can be adopted, of which the former
Fig. 9. Effect of NaCl loading on the water-activated flexible Al-air battery performance: (a) Polarization curve; (b) Galvanostatic discharge at 1 mA cm2 with 3.5 mg Al as anode.
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Fig. 10. Scaling up and stacking of the flexible Al-air battery for higher current, voltage and power output: (a) Performance of a 1 cm 5 cm scaled-up battery; (b) Demonstration of a 1 cm 10 cm scaled-up battery for powering a fan; (c) Performance of a 5-cell battery pack; (d) Concept map of a flexible Al-air battery tape for practical application.
case can achieve a much higher power output (19 mW cm2) but less operation lifetime (48 min at 1 mA cm2 with 3.5 mg Al), while the latter case can obtain a much longer operation lifetime (6.5 h at 1 mA cm2 with 3.5 mg Al) but less power output (8 mW cm2). In addition, by increasing the embedded Al loading to 25 mg, the battery lifespan with saline electrolyte can be further extended to 58.4 h at 1 mA cm2. The battery flexibility has also been investigated by bending it at different angles and with different times, whose performance is negligibly affected, indicating an excellent flexibility and robustness against dynamic deformation. Moreover, the present battery can simply be activated by water, as long as the saline electrolyte is pre-deposited inside the paper substrate. Finally, scaling-up and stacking of the flexible Al-air battery have been successful demonstrated, of which both the scaled-up battery and the 5-cell battery pack can maintain 93.5e94% of the theoretical power output, demonstrating a very high scaling-up and stacking efficiency. Considering its application potential, a PCT patent (PCT/CN2018/118428) has been filed for the present flexible Al-air battery technology. Acknowledgement The authors would like to acknowledge the CRCG grant of the University of Hong Kong (201711160009) and the SZSTI of Shenzhen Municipal Government (JCYJ20170818141758464) to provide funding support to this project.
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