Cost-Effective Microfabricated Silver-Based Paper Cathodes for High-Discharge-Rate Aluminum-Air Batteries
Authors: Huang Yanghang , Lyu Haoxuan , Vyletel Jackson , Fluker Edward Coleman , Allen Mark George , Bidstrup Allen Sue Ann 
Published: August 2024

We report a cost-effective microfabricated silver-based air-cathode for high-discharge-rate aluminum-air batteries which are suitable for micro-drone applications. A challenge for batteries for aerial drones is that both high gravimetric energy density and power density are required for extended operation. While air batteries can address the energy density issue, achieving high power density necessitates high loadings of expensive catalytic materials such as platinum (Pt), which can constitute over 99% of the cost in commercial air batteries. [1] Building upon prior research that studied the power performance of an aluminum-air battery (AAB) system with a silver-based cathode, this study further explores the influence of silver (Ag) sputtering conditions and Ag loadings on AAB power performance. [2]
The power performance of AAB is significantly influenced by both air flux through the air cathode and electrochemically active catalyst surface area ratio (ECSA). The ECSA is the ratio of the electrochemically active catalyst surface area to its cathode area. Sputtered Ag cathodes with catalyst loadings ranging from 0.03 mg-Ag/cm2 to 0.3 mg-Ag/cm2 consistently show high porosity, indicating good air flux for the microfabricated cathode. Increasing the loading of the sputtered Ag catalyst further can lead to particle agglomeration. To improve power performance and ECSA, a silver-copper (AgCu) co-sputtering technique has been developed, significantly increasing the surface area and power performance.
Figure 1 shows cathodes with Ag loadings ranging from 0.038 mg-Ag/cm2 to 0.267 mg-Ag/cm2 under two silver sputtering conditions: 400W+5mTorr and 100W+10mTorr. Within the examined range, the latter sputtering condition shows superior power performance compared to the former. For Ag loadings exceeding 0.1 mg-Ag/cm2, both conditions show decreasing power performance as Ag loading increases, and the 100W+10mTorr curve shows a maximum power at 0.088 mg-Ag/cm2. Figure 2 shows SEM images of 0.038 mg-Ag/cm2, 0.088 mg-Ag/cm2, and 0.19 mg-Ag/cm2 films, using the 100W+10mTorr sputtering condition. Ag particle size increases with higher Ag loading, which can result from particle agglomeration. Figure 3 shows the catalyst layer porosity and the ECSA calculated from the SEM images. All samples show porosity over 90%, indicating good air flux for the microfabricated Ag cathode. The trend of ECSA for different Ag loading correlates well with the power performance, suggesting that the power performance of the microfabricated Ag cathode primarily depends on the ECSA.
Further enhancement of the ECSA through AgCu co-deposition is investigated, involving two parent alloy compositions (at%): Ag28Cu72 and Ag16Cu84. Following the co-sputtering, the parent alloy undergoes selective etching of Cu using hydrochloric acid (HCl). Figure 4 compares power performance between Ag28Cu72 and Ag16Cu84. Both curves exhibit an upward trend as the silver loading increases from 0.09 mg-Ag/cm2 to 0.4 mg-Ag/cm2, with Ag28Cu72 samples showing superior power performance. Figure 5 shows an SEM image of Ag28Cu72 parent alloy with 0.32 mg-Ag/cm2 loading after Cu etching, demonstrating that the ECSA increased to 47.52 while maintaining a high porosity of 90.2%. The peak power of Ag28Cu72 0.32 mg-Ag/cm2 sample is increased to 200 mW/cm2. By comparison, a 4mg-Pt/cm2 commercial cathode shows a peak power of 280 mW/cm2.
Figure 6 compares the discharge performance of the 0.088 mg-Ag/cm2 cathode with a commercial 4 mg-Pt/cm2 cathode. Both cells can discharge under 1.1 Amp with a power density exceeding 500 W/kgbattery, a threshold sufficient to lift a quadrotor drone. [3] Battery weight encompasses anode, cathode, electrolyte and battery packaging. The 0.088 mg-Ag/cm2 cathode shows a discharge power plateau above 600 W/kgbattery (86% of the 4 mg-Pt/cm2) and an energy density of 228 Wh/kgbattery when discharge power density exceeding 500 W/kgbattery (95% of the 4 mg-Pt/cm2). Remarkably, the material cost of the 0.088 mg-Ag/cm2 cathode is 5000 times less than the 4 mg-Pt/cm2 cathode.
References:

Huang, Lei, et al. "An integrated platinum-nanocarbon electrocatalyst for efficient oxygen reduction." nature communications 13.1 (2022): 6703.

Huang, Yanghang, et al. "Microfabrication-Enhanced Carbon Fiber Cathodes for High Discharge Rate Aluminum-Air Batteries." 2022 21st International Conference on Micro and Nanotechnology for Power Generation and Energy Conversion Applications (PowerMEMS). IEEE, 2022.

Mulgaonkar, Yash, et al. "Power and weight considerations in small, agile quadrotors." Micro-and Nanotechnology Sensors, Systems, and Applications VI. Vol. 9083. SPIE, 2014.

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