Effects of Impurities and Operating Mode on The Dynamics of Direct Butane Proton-Exchange Membrane Fuel Cells

Abstract

Maximizing the energy density of a portable power system is an important design factor since those systems have stringent maximum mass constraints. While Lithium-ion batteries are currently the standard choice for power systems in the scale of 10-100 watts, direct hydrocarbon Proton-exchange membrane fuel cells (PEMFC) have the potential to be more energy-dense than batteries at those scales even with efficiencies as low as %10. PEMFC are simple and operate at low temperatures which make them a suitable choice for portable power generation. Because of their low operating temperature, they are usually fueled by hydrogen. However, hydrogen is hard to store and handle, which makes it an unsuitable fuel for portable power applications. Alternatively, Hydrocarbon fuels, such as butane, are easier to store and safer to handle. Additionally, hydrocarbon fuels are ~50 times more energy dense than state of the art Lithium-ion batteries. For those reasons, it is desirable to develop a direct butane PEMFC for portable power applications. Limited work has been done in the past to demonstrate the viability of direct butane fuel cells, mainly because of the low reactivity of hydrocarbon in low temperature PEMFC. EH Kong et. al. demonstrated a working direct butane PEMFC by adding trace amounts of unsaturated hydrocarbons to the butane and operating the cell in “load-interrupt” mode. The authors proposed that butane might be converting active catalyst sites to inactive sites, causing the fuel cell to ‘extinguish’. Thus, the addition of unsaturated hydrocarbons and the load-interrupt operating mode can circumcise this saturation process. In this work, we demonstrate the repeatability of the direct butane PEMFC results produced by EH Kong et. al. on a custom-built fuel cell testing apparatus and explore methods to explain the current dynamics of the cell.For the experimental setup, we used a 25cm2 membrane electrode assembly (MEA). It consisted of treated Nafion N-177 layer directly painted with a catalyst layer on the anode and cathode sides. The catalyst layer is obtained from an ink that is prepared by mixing platinumblack, water, and 5% Nafionsolution. The Nafion N-177 layer is prepared using the manufacturer standard pretreatment procedure. Once assembled, the MEA layer is then pressed at 120 °C and 500 lbs. for 25 min. We then load the MEA layer into the fuel cell hardware, which includes the fuel cell, humidifiers, and heaters, and test the cell electrical performance at temperatures below (100C).Using the fuel cell testing apparatus, we confirm that for high purity butane (>%99.99) the PEMFC does not start and no open circuit voltage is detected. However, the PEMFC is started when unsaturated hydrocarbons are added to the butane fuel stream as shown in previous work. Specifically, the addition of ethylene at concentrations of 2000 ppm allows the PEMFC to operate continuously at low temperatures (<100C). Previously observed current dynamics were also reproduced. The current density of the fuel cell peaks in the beginning of the cell operation and decreases over time until it reaches steady state. It was observed that operating the cell in a load-interrupt mode, where the current is set to zero for 5 seconds every 25 second of operation, increases the average power density of the cell, when compared to the galvanostatic operating mode. Further we investigate the effect of different additives to butane. Specifically, we explore the effect of adding different concentration of hydrogen to the butane stream to analyze whether the interaction between the two fuels is independent or synergetic. Furthermore, we produce Electrochemical Impedance spectrums of the direct butane PEMFC to gain insights about cell resistances including mass transport limitations.For future work, we are interested in in-situ measurement techniques that allow us to observe the intermediates on the catalyst surface. Such data might allow us to verify and inform the empirical dynamical model proposed in previous work to explain the direct butane PEMFC current dynamics.