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High Capacity Silicon Air Battery sample essay

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 sources has motivated considerable research efforts towards a variety of power and energy systems.[1–9] The metal–air battery, using the reduction of oxygen from the atmosphere as cathode reaction, is known for its high energy density.[10] The zinc–air battery, being the first commercialized metal–air battery, has received significant attention since the 1960s.[11–15] More recently, there has been renewed interest in this battery for application in electric vehicles.[16] However, the zinc–air battery can provide a practical energy density of only 470 Wh kgÀ1, from a theoretical value of 1370 Wh kgÀ1.[17] The aluminum–air battery has a high theoretical energy density (8100 Wh kgÀ1),[18, 19] but is limited to military applications owing to its high self-discharge rate.[20]

Alloying the aluminum with tin or with other elements (in proprietary formulations) has made the battery’s electrodes less corrosive in alkaline solutions.[21, 22] As an alternative to aluminum– and zinc–air batteries, the lithium–air battery possesses a higher theoretical energy density of 13 000 Wh kgÀ1 and an expected practical value of 1700 Wh kgÀ1,[23–25] but it suffers from potential safety and cost issues.[26–28] The silicon–air battery is another interesting system, with a theoretical energy density of 8470 Wh kgÀ1. This is less than lithium–air systems but compares favorably to the zinc– and aluminum–air systems. In addition silicon, unlike lithium, is one of the most abundant elements on Earth and therefore may offer a cost-effective alternative. Recently, a silicon–air battery was reported using EMI·(HF)2.3F ionic liquid-based electrolyte.[29, 30] The battery system showed a practically unlimited shelf-life with a working potential in the range of 1.0–1.2 V.

The practical application of this battery system, however, might be complicated by serious chemical safety issues, associated with the use of a fluoride-based electrolyte. Herein, we report a high capacity silicon–air battery using nanostructured silicon and alkaline solution based electrolyte that only involves environmentally friendly elements such as silicon, potassium, oxygen, and hydrogen.

The silicon surface is first modified by the metal-assisted electroless chemical etching method.[31–35] The assembled battery displays a flat and stable discharge curve with a voltage ranging from 0.9 to 1.2 V (under different discharge current densities) over days. 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) E-mail: [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 authors contributed equally to this work.

trast, the unmodified silicon wafer becomes passivated quickly in the alkaline solution and therefore the potential drops rapidly after discharging for a short period of time (minutes). We propose that the formation of the porous surface structure increases the overall Si(OH)4 dissolving rate in the KOH electrolyte, which effectively removes the oxide and reactivates the silicon surface. The corrosion of the silicon in the KOH electrolyte is also carefully investigated to minimize self-discharge. Corrosion of the silicon is effectively minimized by using a lower KOH concentration (0.6 m), enabling a specific capacity as high as 1206.0 mA h gÀ1, which is about 2 times the practical value of a commercial zinc–air battery (ca. 650 mA h gÀ1, Energizer)[36] and 3 times that of a commercial aluminum–air battery (ca. 320 mA h gÀ1, Altek Fuel Group Inc.).[37] Figure 1 shows a schematic illustration of a silicon–air battery as well as a photograph of a real 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) stamp with an openthrough hole sandwiched between the silicon wafer and air diffusion electrode as the cell, and variable concentrations of potassium hydroxide solution as the electrolyte. Top-view and cross-sectional scanning electron microscopy (SEM) images of a silicon wafer after the metal-assisted electroless etching process are shown in Figure 2 a and b, respectively. This process creates a microporous layer of silicon nanowire bundles of ca. 1.5 mm thickness on top of the silicon surface, thereby significantly increasing the roughness of the substrate surface. The roughed silicon substrate is then used as the anode in the air battery system.

Typical galvanostatic discharge characteristics of the device show that it can be continuously discharged before the silicon source is used up (only data for 30 h shown here) with an operating potential of 1.2 V (with discharge current density of 0.05 mA cmÀ2 ; Figure 2 c). In contrast, in a control experiment using an unmodified silicon wafer, the device can only be discharged for less than 10 min at a lower potential of 1.1 V before the potential quickly drops to zero (Figure 2 d). These studies clearly demonstrate that the rough 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 result, the oxide at the silicon surface can be continuously etched away and the surface is continuously refreshed for sustained discharge. Figure 2 e and f are SEM images of the surface morphologies of modified and unmodified silicon wafers, respectively, taken immediately after discharge. The modified silicon wafer presents a highly porous surface structure while the unmodified wafer retains its smooth surface.

To further probe the discharge process, a 5 h stepped discharge measurement is performed under various discharge current densities (Figure 3 a, black curve). The current densities are increased stepwise from 0.01 mA cmÀ2 to 0.1 mA cmÀ2 and then stepped back to 0.01 mA cmÀ2. With increasing discharge current density, the operating potential decreases. This potential drop might be attributed to the internal resistance present between silicon/electrolyte interfaces. We also investigated the impact of the dopant concentration of the silicon wafer on the cell performance (Figure 3 a). In general, a silicon wafer with higher dopant concentration displays a higher operating voltage, which we believe can be attributed to the lower internal resistivity of the highly doped silicon wafer.

The self-discharge is usually a serious issue in the alkaline solution-based metal–air system, particularly for the aluminum– Figure 2. a) Top-view SEM image of the silicon wafer after surface modification. b) Cross-sectional view SEM image of the silicon wafer after surface modification. c) Galvanostatic discharge curve of a modified silicon–air battery. The discharge current density is 0.05 mA cmÀ2. d) Galvanostatic discharge curve of an unmodified silicon–air battery. The discharge current density is 0.05 mA cmÀ2. e) Top-view SEM image of modified silicon after discharge. f) Top-view SEM image of unmodified silicon after discharge. The main scale bars are 5 mm, and the scale bars in the insets are 1 mm.

The discharge process can be described as electrochemical reactions of the anode and cathode:[38, 39] anode : Si þ 4 OHÀ ! SiðOHÞ4 þ 4 eÀ ðE 0 ¼ 1:69 VÞ cathode : O2 þ 2 H2 O þ 4 eÀ ! 4 OHÀ ðE 0 ¼ 0:40 VÞ ð1Þ ð2Þ

The anode oxidation product Si(OH)4 needs to be promptly removed from the electrode surface to ensure continuous 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 dissolution of Si(OH)4 in KOH is slower than its production rate, Si(OH)4 can build up on the silicon surface, leading to the formation of the SiO2 that passivates the surface and prevents the battery from continuous discharging.

In our experiment, the oxidation rate was calculated to be 50– 100 nm hÀ1 (for a smooth planar wafer) under a discharge current density of 0.05 mA cmÀ2 while the SiO2 dissolving rate was only around 1 ~ 2 nm hÀ1 at room temperature.[38, 39] Therefore, for the unmodified silicon with flat surface, the silicon oxide formation rate far exceeds its dissolving rate, and the surface is covered by silicon oxide and passivated very quickly, resulting in a short battery lifetime. On the other hand, the surface area of the silicon substrate can be considerably increased by the surface modification, up to orders of magnitude,[31] and the electrolyte can easily diffuse into the pores. With this increased surface area, the overall oxide dissolving rate can be increased

Figure 3. a) Galvanostatic discharge curve of surface-modified silicon–air battery with various dopant concentrations and discharge current densities (mA cmÀ2). 1 (black line): 0.001–0.002 W cm; 2 (light grey line): 0.008– 0.01 W cm; 3 (dark grey line): 0.3–0.8 W cm. b) Linear sweep voltammograms of modified silicon wafer as electrode in KOH solutions with various concentrations as the electrolyte. c) Open-circuit voltage plots measured for 24 h with various KOH concentrations. d) Galvanostatic discharge curve of modified silicon–air battery in KOH solutions with various concentrations and discharge current densities. Trace 1: 6 m KOH with 0.05 mA cmÀ2 ; trace 2: 2 m KOH with 0.05 mA cmÀ2 ; trace 3: 0.6 m KOH with 0.05 mA cmÀ2 ; trace 4: 0.6 m KOH with 0.1 mA cmÀ2. e) Step heights between the reacting and non-reacting area of the modified silicon in KOH solutions with various concentrations and discharge current densities. f) Lighting an LED with silicon–air batteries.

Similarly, it is also a critical challenge in silicon–air battery systems. Since the self-discharge rate is highly dependent on the electrolyte composition, we have investigated the effect of KOH concentration on the silicon–air battery. Figure 3 b is the polarization curve of surface-modified silicon in various KOH concentrations. As expected, with higher KOH concentration, the anodic dissolution potential is higher (more negative). Generally, a more negative anodic dissolution potential is favorable for higher open-circuit voltage (OCV) or operating voltage. The OCV measurements with various KOH concentrations in Figure 3 c are consistent with polarization curves. The values at 6 m, 2 m, and 0.6 m KOH are 1.32 Æ 0.01 V, 1.23 Æ 0.01 V, and 1.10 Æ 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 also been observed in an aluminum–air battery system, which can be partially attributed to activation polarization (over-potential) and the partial formation of an anodic passivation layer.[17] The discharge plots with various KOH concentrations in Figure 3 d are also consistent with polarization curves. With increasing KOH electrolyte concentration (0.6 m, 2 m, 6 m), the operating potentials are 1.01 V, 1.06 V, and 1.18 V at a discharge current density of 0.05 mA cmÀ2, respectively. However, the trade-off for the high operating potential brought by the high KOH concentration is the high silicon corrosion (self-discharge) rate. The overall chemical corrosion reaction by alkaline is given by[38, 39] Si þ 2 OHÀ þ 2 H2 O ! SiO2 ðOHÞ2À þ 2 H2 2 ð3Þ

Here, silicon reacts with hydroxide ions and produces SiO2(OH)2 ions and hydrogen gas. To quantify the corrosion effect of KOH in our device, batteries were filled with KOH with various concentrations and discharged for 7 h at a current density of 0.05 or 0.1 mA cmÀ2 (Figure 3 d). After the reaction, we measured the step difference between the reacting and non-reacting area of the silicon (Figure 3 e). The step height differences between the reacting and non-reacting region (including the 1.5 mm microporous layer) were measured to be 11.0 mm, 8.2 mm, and 3.3 mm for 6 m, 2 m, and 0.6 m KOH electrolyte, respectively. Considering the volume percentage of the microporous layer is about 20 %, the total amounts of silicon consumed were about 9.8 mm, 7.0 mm and 2.1 mm for 6 m, 2 m, and 0.6 m KOH electrolyte.

Additionally, we can calculate the silicon consumed by the oxidative discharge to be 0.4 mm based on the discharge current density and time. Therefore, the amounts of silicon consumed by self-corrosion are about 9.4 mm, 6.6 mm, and 1.7 mm in the 6 m, 2 m, and 0.6 m KOH solutions, corresponding to average corrosion rates of 1.34 mm hÀ1, 0.95 mm hÀ1, and 0.24 mm hÀ1, respectively.[38] The estimation suggests that one can in principle expect a lifetime of ca. 2000 h for a 500 mm thick silicon wafer when using a low KOH concentration. The specific capacities of the silicon–air battery with various KOH concentrations and discharge current densities were calculated and are summarized in Table 1. To make a fair comparison with other anode materials, the weight of the anode siliChemSusChem 2012, 5, 177 – 180 con consumed is used for the capacitance calculation.

The loss of the specific capacity is exaggerated in the concentrated KOH solution (6 m) as most of the silicon is wasted in the selfdischarge process. However, as self-corrosion is substantially reduced by lowering the KOH concentration, the specific capacity increases significantly. With a diluted KOH concentration (0.6 m) and a discharge current density of 0.1 mA cmÀ2, the silicon–air battery can reach a specific capacity as high as 1206.0 mA h gÀ1, which is much higher than the practical values reached by commercial zinc–air battery (650 mA h gÀ1, electrical capacity 620 mA h, cell weight, 1.9 g, zinc anode weight percentage ca. 50 %, Energizer) and aluminum–air battery (ca. 320 mA h gÀ1, electrical capacity 120 A h, aluminum anode weight 0.37 kg, Altek Fuel Group Inc.).[36, 37] Nonetheless, considering that the theoretical specific energy density of the silicon–air system is 8470 Wh kgÀ1, our device can be further optimized in terms of device configuration and other experimental parameters to reach even higher energy density.

We also note the solubility of Si(OH)4 in the electrolyte solution can impact the eventual capacity of the practical silicon–air battery. Additional work is clearly needed to fully understand oxidative discharge and self-corrosion process as well as their dependence on surface structures, anode and electrolyte compositions, and therefore to develop a practical system with optimized discharge current density and minimum self-corrosion. The formation of the silicon–air battery can be readily used to drive practical devices. To demonstrate this point, two silicon–air batteries were assembled in serial connection for battery testing. The serially connected silicon–air batteries can be used to light up a semiconductor light-emitting diode (LED; 2.1 V required; Figure 3 f).

In conclusion, we have successfully fabricated a new alkaline solution-based silicon–air battery and demonstrate its potential as a high-capacity power source. The sustainable discharge profile in the alkaline solution can be attributed to the surface modification of the starting silicon wafer, which substantially enlarges the surface contact area between the silicon and electrolyte and subsequently increases the Si(OH)4 dissolving rate. The assembled battery can provide an operating potential ranging from 0.9 to 1.2 V, with various current densities of 0.01 to 0.1 mA cmÀ2. The self-corrosion of the silicon by the alkaline solution can be effectively reduced by lowering the concentration of the electrolyte to some extent, however, with a partial sacrifice of the output potential. Specific capacity as high as 1206.0 mA h gÀ1 is achieved, which is substantially larger than the practical value of a commercialized zinc–air battery (ca. 650 mA h gÀ1)[36] and that of a commercial aluminum–air battery (ca. 320 mA h gÀ1).[37]

Further improvements in the configuration of the cell, material surface roughness, electrolyte concentration, wafer dopant type and concentration, and air diffusion electrode are expected to allow the design of eco-friendly silicon–air batteries with higher capacity and energy density. Importantly, unlike many other battery systems, the alkaline solution based silicon–air battery system described here only involves common elements such as silicon, potassium, oxygen, and hydrogen, and therefore may offer an environmentally friendly solution for future mobile power requirements. In combination with the established fields of silicon industry, this alkaline-based silicon–air battery may lead to a new class of embedded power systems, opening up a new generation of self-powered silicon based device applications such as microelectro-mechanical systems (MEMS), integrated circuits (ICs), and electrical vehicles (EVs).

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