Xing Zhong. [ a ] Hua Zhang. [ B ] Yuan Liu. [ B ] Jingwei Bai. [ B ] Lei Liao. [ a ] Yu Huang. [ B ] and Xiangfeng Duan* [ a ] The ever-increasing demand for portable power beginnings has motivated considerable research attempts towards a assortment of power and energy systems. [ 1–9 ] The metal–air battery. utilizing the decrease of O from the ambiance as cathode reaction. is known for its high energy denseness. [ 10 ] The zinc–air battery. being the first commercialised metal–air battery. has received important attending since the sixtiess. [ 11–15 ] More late. there has been renewed involvement in this battery for application in electric vehicles. [ 16 ] However. the zinc–air battery can supply a practical energy denseness of merely 470 Wh kgA1. from a theoretical value of 1370 Wh kgA1. [ 17 ] The aluminum–air battery has a high theoretical energy denseness ( 8100 Wh kgA1 ) . [ 18. 19 ] but is limited to military applications owing to its high self-discharge rate. [ 20 ]
Debasing the aluminium with Sn or with other elements ( in proprietary preparations ) has made the battery’s electrodes less caustic in alkalic solutions. [ 21. 22 ] As an option to aluminum– and zinc–air batteries. the lithium–air battery possesses a higher theoretical energy denseness of 13 000 Wh kgA1 and an expected practical value of 1700 Wh kgA1. [ 23–25 ] but it suffers from possible safety and cost issues. [ 26–28 ] The silicon–air battery is another interesting system. with a theoretical energy denseness of 8470 Wh kgA1. This is less than lithium–air systems but compares favourably to the zinc– and aluminum–air systems. In add-on Si. unlike Li. is one of the most abundant elements on Earth and hence may offer a cost-efficient option. Recently. a silicon–air battery was reported utilizing EMI· ( HF ) 2. 3F ionic liquid-based electrolyte. [ 29. 30 ] The battery system showed a practically limitless shelf-life with a working potency in the scope of 1. 0–1. 2 V.
The practical application of this battery system. nevertheless. might be complicated by serious chemical safety issues. associated with the usage of a fluoride-based electrolyte. Herein. we report a high capacity silicon–air battery utilizing nanostructured Si and alkalic solution based electrolyte that lone involves environmentally friendly elements such as Si. K. O. and H.
The Si surface is foremost modified by the metal-assisted electroless chemical etching method. [ 31–35 ] The assembled battery displays a level and stable discharge curve with a electromotive force runing from 0. 9 to 1. 2 V ( under different discharge current densenesss ) over yearss. In con [ a ] X. Zhong. + Dr. L. Liao. Prof. X. Duan Department of Chemistry and Biochemistry California Nanosystems Institute University of California. Los Angeles 607 Charles E. Young Drive East. Los Angeles. CA 90095 ( USA ) Electronic mail: [ electronic mail protected ][ B ] H. Zhang. + Y. Liu. Dr. J. Bai. Prof. Y. Huang Department of Materials Science and Engineering California Nanosystems Institute University of California. Los Angeles 410 Westwood Plaza. Los Angeles. CA 90095 ( USA ) [ + ] These writers contributed every bit to this work.
trast. the unmodified Si wafer becomes passivated rapidly in the alkalic solution and hence the possible beads quickly after dispatching for a short period of clip ( proceedingss ) . We propose that the formation of the porous surface construction increases the overall Si ( OH ) 4 fade outing rate in the KOH electrolyte. which efficaciously removes the oxide and reactivates the Si surface. The corrosion of the Si in the KOH electrolyte is besides carefully investigated to minimise self-discharge. Corrosion of the Si is efficaciously minimized by utilizing a lower KOH concentration ( 0. 6 m ) . enabling a specific capacity every bit high as 1206. 0 ma H gA1. which is about 2 times the practical value of a commercial zinc–air battery ( ca. 650 ma H gA1. Energizer ) [ 36 ] and 3 times that of a commercial aluminum–air battery ( ca. 320 ma H gA1. Altek Fuel Group Inc. ) . [ 37 ] Figure 1 shows a conventional illustration of a silicon–air battery every bit good as a exposure of a existent one.
We employ a simple device architecture. consisting of a surface modified silicon wafer as the anode. an air diffusion electrode as the cathode. a polydimethylsiloxane ( PDMS ) cast with an openthrough hole sandwiched between the Si wafer and air diffusion electrode as the cell. and variable concentrations of K hydroxide solution as the electrolyte. Top-view and cross-sectional scanning negatron microscopy ( SEM ) images of a Si wafer after the metal-assisted electroless etching procedure are shown in Figure 2 a and b. severally. This procedure creates a microporous bed of Si nanowire packages of ca. 1. 5 mm thickness on top of the Si surface. thereby significantly increasing the raggedness of the substrate surface. The roughed Si substrate is so used as the anode in the air battery system.
Typical galvanostatic discharge features of the device show that it can be continuously discharged before the Si beginning is used up ( merely data for 30 Hs shown here ) with an runing potency of 1. 2 V ( with discharge current denseness of 0. 05 mA cmA2 ; Figure 2 degree Celsius ) . In contrast. in a control experiment utilizing an unmodified Si wafer. the device can merely be discharged for less than 10 min at a lower potency of 1. 1 V before the possible rapidly drops to zero ( Figure 2 vitamin D ) . These surveies clearly demonstrate that the unsmooth surface is a critical factor. responsible for the sustained discharging.
Figure 1. a ) Schematic of an alkaline-based silicon–air battery. B ) Photograph of a silicon–air battery. As a consequence. the oxide at the Si surface can be continuously etched off and the surface is continuously refreshed for sustained discharge. Figure 2 vitamin E and degree Fahrenheit are SEM images of the surface morphologies of modified and unmodified Si wafers. severally. taken instantly after discharge. The modified Si wafer presents a extremely porous surface construction while the unmodified wafer retains its smooth surface.
To further examine the discharge procedure. a 5 H stepped discharge measuring is performed under assorted discharge current densenesss ( Figure 3 a. black curve ) . The current densenesss are increased stepwise from 0. 01 mA cmA2 to 0. 1 mA cmA2 and so stepped back to 0. 01 mA cmA2. With increasing discharge current denseness. the operating potency decreases. This possible bead might be attributed to the internal opposition nowadays between silicon/electrolyte interfaces. We besides investigated the impact of the dopant concentration of the Si wafer on the cell public presentation ( Figure 3 a ) . In general. a Si wafer with higher dopant concentration displays a higher operating electromotive force. which we believe can be attributed to the lower internal electric resistance of the extremely doped Si wafer.
The self-discharge is normally a serious issue in the alkalic solution-based metal–air system. peculiarly for the aluminum– Figure 2. a ) Top-view SEM image of the Si wafer after surface alteration. B ) Cross-sectional position SEM image of the Si wafer after surface alteration. degree Celsius ) Galvanostatic discharge curve of a modified silicon–air battery. The discharge current denseness is 0. 05 mA cmA2. vitamin D ) Galvanostatic discharge curve of an unmodified silicon–air battery. The discharge current denseness is 0. 05 mA cmA2. vitamin E ) Top-view SEM image of modified Si after discharge. degree Fahrenheit ) Top-view SEM image of unmodified Si after discharge. The chief graduated table bars are 5 millimeter. and the graduated table bars in the insets are 1 millimeters.
The discharge procedure can be described as electrochemical reactions of the anode and cathode: [ 38. 39 ] anode: Si ? 4 OHA ! Si?OH?4 ? 4 eA ?E 0 ? 1:69 V? cathode: O2 ? 2 H2 O ? 4 eA ! 4 OHA ?E 0 ? 0:40 V? ?1? ?2?
The anode oxidization merchandise Si ( OH ) 4 demands to be quickly removed from the electrode surface to guarantee uninterrupted discharge. The presence of alkaline ensures that the Si ( OH ) 4 is dissolved and keeps the silicon surface free of oxide. However. when the rate of disintegration of Si ( OH ) 4 in KOH is slower than its production rate. Si ( OH ) 4 can construct up on the Si surface. taking to the formation of the SiO2 that passivates the surface and prevents the battery from uninterrupted discharging.
In our experiment. the oxidization rate was calculated to be 50– 100 nm hA1 ( for a smooth planar wafer ) under a discharge current denseness of 0. 05 mA cmA2 while the SiO2 fade outing rate was merely about 1 ~ 2 nm hA1 at room temperature. [ 38. 39 ] Therefore. for the unmodified Si with level surface. the silicon oxide formation rate far exceeds its dissolution rate. and the surface is covered by silicon oxide and passivated really rapidly. ensuing in a short battery life-time. On the other manus. the surface country of the Si substrate can be well increased by the surface alteration. up to orders of magnitude. [ 31 ] and the electrolyte can easy spread into the pores. With this increased surface country. the overall oxide fade outing rate can be increased
Figure 3. a ) Galvanostatic discharge curve of surface-modified silicon–air battery with assorted dopant concentrations and dispatch current densenesss ( mA cmA2 ) . 1 ( black line ) : 0. 001–0. 002 W centimeter ; 2 ( light Greies line ) : 0. 008– 0. 01 W centimeter ; 3 ( dark Grey line ) : 0. 3–0. 8 W centimeter. B ) Linear sweep voltammograms of modified Si wafer as electrode in KOH solutions with assorted concentrations as the electrolyte. degree Celsius ) Open-circuit electromotive force secret plans measured for 24 H with assorted KOH concentrations. vitamin D ) Galvanostatic discharge curve of modified silicon–air battery in KOH solutions with assorted concentrations and dispatch current densenesss. Trace 1: 6 m KOH with 0. 05 mA cmA2 ; trace 2: 2 m KOH with 0. 05 mA cmA2 ; trace 3: 0. 6 thousand KOH with 0. 05 mA cmA2 ; trace 4: 0. 6 thousand KOH with 0. 1 mA cmA2. vitamin E ) Step highs between the reacting and non-reacting country of the modified Si in KOH solutions with assorted concentrations and dispatch current densenesss. degree Fahrenheit ) Lighting an LED with silicon–air batteries.
Similarly. it is besides a critical challenge in silicon–air battery systems. Since the self-discharge rate is extremely dependent on the electrolyte composing. we have investigated the consequence of KOH concentration on the silicon–air battery. Figure 3 B is the polarisation curve of surface-modified Si in assorted KOH concentrations. As expected. with higher KOH concentration. the anodal disintegration potency is higher ( more negative ) . Generally. a more negative anodal disintegration potency is favourable for higher open-circuit electromotive force ( OCV ) or runing electromotive force. The OCV measurings with assorted KOH concentrations in Figure 3 degree Celsius are consistent with polarisation curves. The values at 6 m. 2 m. and 0. 6 m KOH are 1. 32 ? 0. 01 V. 1. 23 ? 0. 01 V. and 1. 10 ? 0. 01 V for 24 hmeasurements.
It is found that the OCV of the cell at high KOH concentration ( 6 m ) is still much lower than the theoretical value of 2. 09 V. This phenomenon has besides been observed in an aluminum–air battery system. which can be partly attributed to activation polarisation ( over-potential ) and the partial formation of an anodal passivation bed. [ 17 ] The discharge secret plans with assorted KOH concentrations in Figure 3 vitamin D are besides consistent with polarisation curves. With increasing KOH electrolyte concentration ( 0. 6 m. 2 m. 6 m ) . the operating potencies are 1. 01 V. 1. 06 V. and 1. 18 V at a discharge current denseness of 0. 05 mA cmA2. severally. However. the tradeoff for the high operating potency brought by the high KOH concentration is the high Si corrosion ( self-discharge ) rate. The overall chemical corrosion reaction by alkaline is given by [ 38. 39 ] Si ? 2 OHA ? 2 H2 O! SiO2 ?OH?2A ? 2 H2 2 ?3?
Here. silicon reacts with hydroxide ions and produces SiO2 ( OH ) 2 ions and H gas. To quantify the corrosion consequence of KOH in our device. batteries were filled with KOH with assorted concentrations and discharged for 7 H at a current denseness of 0. 05 or 0. 1 ma cmA2 ( Figure 3 vitamin D ) . After the reaction. we measured the measure difference between the reacting and non-reacting country of the Si ( Figure 3 vitamin E ) . The measure height differences between the reacting and non-reacting part ( including the 1. 5 mm microporous bed ) were measured to be 11. 0 millimeter. 8. 2 millimeter. and 3. 3 millimeter for 6 m. 2 m. and 0. 6 thousand KOH electrolyte. severally. Sing the volume per centum of the microporous bed is about 20 % . the entire sums of Si consumed were approximately 9. 8 millimeter. 7. 0 millimeter and 2. 1 millimeter for 6 m. 2 m. and 0. 6 thousand KOH electrolyte.
Additionally. we can cipher the Si consumed by the oxidative discharge to be 0. 4 millimeters based on the discharge current denseness and clip. Therefore. the sums of Si consumed by self-corrosion are about 9. 4 millimeter. 6. 6 millimeter. and 1. 7 millimeter in the 6 m. 2 m. and 0. 6 thousand KOH solutions. matching to mean corrosion rates of 1. 34 mm hA1. 0. 95 mm hA1. and 0. 24 mm hA1. severally. [ 38 ] The appraisal suggests that one can in rule expect a life-time of ca. 2000 H for a 500 millimeter thick Si wafer when utilizing a low KOH concentration. The specific capacities of the silicon–air battery with assorted KOH concentrations and dispatch current densenesss were calculated and are summarized in Table 1. To do a just comparing with other anode stuffs. the weight of the anode siliChemSusChem 2012. 5. 177 – 180 con consumed is used for the electrical capacity computation.
The loss of the specific capacity is exaggerated in the concentrated KOH solution ( 6 m ) as most of the Si is wasted in the selfdischarge procedure. However. as self-corrosion is well reduced by take downing the KOH concentration. the specific capacity increases significantly. With a diluted KOH concentration ( 0. 6 m ) and a discharge current denseness of 0. 1 mA cmA2. the silicon–air battery can make a specific capacity every bit high as 1206. 0 ma H gA1. which is much higher than the practical values reached by commercial zinc–air battery ( 650 ma H gA1. electrical capacity 620 ma h. cell weight. 1. 9 g. Zn anode weight per centum ca. 50 % . Energizer ) and aluminum–air battery ( ca. 320 ma H gA1. electrical capacity 120 A h. aluminium anode weight 0. 37 kilogram. Altek Fuel Group Inc. ) . [ 36. 37 ] Nonetheless. sing that the theoretical specific energy denseness of the silicon–air system is 8470 Wh kgA1. our device can be farther optimized in footings of device constellation and other experimental parametric quantities to make even higher energy denseness.
We besides note the solubility of Si ( OH ) 4 in the electrolyte solution can impact the eventual capacity of the practical silicon–air battery. Extra work is clearly needed to to the full understand oxidative discharge and self-corrosion procedure every bit good as their dependance on surface constructions. anode and electrolyte composings. and hence to develop a practical system with optimized discharge current denseness and minimal self-corrosion. The formation of the silicon–air battery can be readily used to drive practical devices. To show this point. two silicon–air batteries were assembled in consecutive connexion for battery testing. The serially connected silicon–air batteries can be used to illume up a semiconducting material light-emitting rectifying tube ( LED ; 2. 1 V required ; Figure 3 degree Fahrenheit ) .
In decision. we have successfully fabricated a new alkalic solution-based silicon–air battery and show its possible as a high-capacity power beginning. The sustainable discharge profile in the alkalic solution can be attributed to the surface alteration of the get downing Si wafer. which well enlarges the surface contact country between the Si and electrolyte and later increases the Si ( OH ) 4 fade outing rate. The assembled battery can supply an operating potency runing from 0. 9 to 1. 2 V. with assorted current densenesss of 0. 01 to 0. 1 mA cmA2. The self-corrosion of the Si by the alkalic solution can be efficaciously reduced by take downing the concentration of the electrolyte to some extent. nevertheless. with a partial forfeit of the end product potency. Specific capacity every bit high as 1206. 0 ma H gA1 is achieved. which is well larger than the practical value of a commercialised zinc–air battery ( ca. 650 ma H gA1 ) [ 36 ] and that of a commercial aluminum–air battery ( ca. 320 ma H gA1 ) . [ 37 ]
Further betterments in the constellation of the cell. material surface raggedness. electrolyte concentration. wafer dopant type and concentration. and air diffusion electrode are expected to let the design of eco-friendly silicon–air batteries with higher capacity and energy denseness. Importantly. unlike many other battery systems. the alkalic solution based silicon–air battery system described here merely involves common elements such as Si. K. O. and H. and hence may offer an environmentally friendly solution for future nomadic power demands. In combination with the established Fieldss of Si industry. this alkaline-based silicon–air battery may take to a new category of embedded power systems. opening up a new coevals of self-powered Si based device applications such as microelectro-mechanical systems ( MEMS ) . integrated circuits ( ICs ) . and electrical vehicles ( EVs ) .
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