Overpotential Analysis of Low Loading Pt/C Electrodes at Low Humidity Conditions

Abstract

In 2014, the transportation sector accounted for 35.3% of all greenhouse gas emissions in the State of Connecticut [1]. A significant amount of greenhouse gases emitted can be primarily attributed to the use of fossil fuels in passenger vehicles and other light-duty trucks. In an effort to reduce the amount of transportation-related emissions, there is a need to produce low- and zero-emission vehicles, such as fuel cell vehicles. Compared to vehicles with traditional internal combustion engines which emit greenhouse gases, fuel cell vehicles only emit water vapor. Through research and development of fuel cell and electrolyzer technologies, fuel cell vehicles have the potential to significantly reduce the greenhouse gas emissions related to the transportation sector. In order to improve the viability of fuel cell vehicles, fuel cell performance must be optimized at low relative humidity conditions while keeping production costs low. One such method for improving the fuel cell performance at low relative humidity is examined by changing the ionomer used in the fuel cell electrodes. Research has been shown that reducing the ionomer side chain length as well as reducing the ionomer equivalent weight may be beneficial to low humidity operation [2]. While low humidity operation is important, fuel cell production costs must also be kept to a minimum in order to offer these zero-emission vehicles to a broader market. The primary method that will be used in this work to reduce the cost of fuel cell production is Reactive Spray Deposition Technology (RSDT). RSDT is a one-step flame-based process which can directly spray a Pt/C electrode onto a membrane using without additional drying steps. Previous work by Yu et al examined the use of RSDT to create low Pt loading electrodes for fuel cell applications [3]. With the cost-effective RSDT method chosen for the development of the fuel cells, the RSDT-produced catalyst coated membranes (CCMs) will be tested in 100% relative humidity conditions as a baseline as well as reduced humidity conditions used in fuel cell vehicle applications. While it is important to improve low humidity fuel cell performance, it is important to have a better understanding of how low humidity directly impacts the electrochemical mechanism. This work will analyze collected polarization and impedance data using a six-step overpotential analysis technique used by Yu et al [3, 4]. This overpotential analysis looks to separate the contributions of different polarization sources to have a better understanding of what is impacting the cell performance. This technique used at low humidity should allow us to have a better understanding about low humidity operation and how to improve fuel cell production by minimizing losses. The work presented will incorporate the RSDT process to make the change in ionomer with the target of improving and better understanding the performance of the fuel cells at low humidity conditions. Along with utilizing the six levels of overpotential analysis technique, we can determine how the changes to the ionomer and membrane as well as the carbon support all affect the polarization sources which will allow to better optimize the fuel cells moving and improve the future viability of fuel cell vehicles.