Direct Ethanol Fuel Cell for Portable Power

Abstract

Introduction Portable Non-Destructive Inspection (NDI) equipment, such as Eddy Current (EC) and Ultrasonic Testing (UT), provide important tools for the Air Force to characterize flaws, cracks, and/or defects in aircraft, thus monitoring potential safety issues. Most NDI equipment is powered by a battery (alkaline, NiCad, NiMH, or Li-ion) that typically comprises a large percentage of the size and weight of the equipment. The Air Force is interested in finding alternative forms of energy for NDI equipment, due to issues with existing battery technologies that include logistical challenges, battery reliability/degradation, limited portability, and safety. Direct Ethanol Fuel Cell (DEFC) systems could potentially provide a significant advantage over conventional battery technologies, including nearly instantaneous recharging, lighter weight, longer operational times, higher power, and improved cycle life. Further, the 25-Watt power requirements of the NDI equipment are similar to a number of other portable electronics that could benefit from the technology. Materials and Methods Single cell direct ethanol fuel cells were prepared using both commercially available anion exchange membranes and experimental reduced crossover membranes. The cell anode was prepared by depositing a hydrothermally synthesized Pd-based catalyst onto a conductive carbon backing. The cell cathode consisted of pH Matter’s non-metal based oxygen reduction reaction catalyst (COR). The COR catalyst was treated with a PTFE additive to reduce ethanol crossover. The catalyst was deposited onto a commercial carbon paper (Toray) and annealed in air. Testing was completed in a nickel metal set-up with PTFE seals and consisted of mechanical compression of each component to form the cell assembly. The anode was supplied with a fuel composition consisting of 5M KOH and 5M ethanol in distilled water. Air was supplied to the cathode at approximately 25 sccm/cm2 and at ambient pressure. Cell temperature was controlled between ambient and 60°C as to prevent degradation of the cell membrane. Cyclic voltammetry curves were obtained using a PAR Versastat 3 potentiostat between OCV and 100 mV. Simulated load testing was carried out by operating the cell at a steady state current of 65 mA/cm2. Significance Fuel cells provide electricity through an efficient, pollution free, and quiet electrochemical process. However, the adoption of commercial fuel cell systems has been hampered by the logistics of providing a reliable, high purity, and low cost hydrogen [1]. Some fuel cell systems, such as SOFC and Direct Methanol, offer an opportunity to use more readily available fuels, such as liquid methanol or methane. However, these systems have been unable to achieve wide-scale commercialization due to technological challenges, such as stability and lifecycle performance.[2-3] Furthermore, methanol is considered toxic and poses environmental disposal challenges [4]. In contrast, ethanol is readily available in most markets, is liquid at room temperature, has very low toxicity, and can be readily disposed upon dilution with water. Furthermore, logistics of alcohol transport are minimal, a critical requirement for adoption in the Air Force application. In this effort, direct ethanol fuel cells were demonstrated to achieve transient power densities of 80 mW/cm2 at an operational temperature of 60°C and steady state power in excess of 20 mW/cm2, the target power required for use in NDI equipment, for greater than 20 hours. Furthermore, the replacement of fuel allowed for performance recovery of the fuel cell and continued operation in excess of 100 hours. Finally, the cells were incorporated into a full-scale stack and demonstrated under NDI operating conditions. In future work, the stack will be demonstrated with a 25-Watt prototype power system. References C. Huang, et al, Journal of Power Sources 303 (2016) 267-277T. Skalar, et al, Journal of Thermal Analysis and Calorimetry (2017) 127:265–271C. Xu, et al, Journal of Power Sources 168 (2007) 143–153Y.S. Li, et al, Journal of Power Sources 187 (2009) 387–392 Figure 1