Application of of polymer electrolyte fuel cells (PEFCs) in automotive sector shows great promise in arresting environmental degradation. They use hydrogen gas as a fuel that electrochemically reacts with air to produce electrical energy and water as a by-product. In a fuel cell electric vehicle (FCEV), these zero tail pipe emission systems offer high efficiency and power density for medium-heavy duty and long range transportation. However, PEFC technology is currently challenged by its high cost due to expensive platinum and limited durability when subjected to harsh and adverse operating conditions that can arise during the normal course of vehicle operation. A severe cause of PEFC degradation is “cell reversal”, resulting from partial and complete fuel starvation. Hydrogen starvation at the anode can arise from blokage in the hydrogen supply system by foreign impurities, water flooding or ice formation during winter [1,2]. Hydrogen starvation could be aggravated when FCEVs are operated under transient conditions such as start-up and rapid load change especially at the cells downstream in a cell stack. When the anode of a particular cell in the stack is starved of hydrogen, the anode requires an additional source of electrons and protons to complete the load circuit. At such a condition, the anode starts generating electrons and protons through water electrolysis reaction which is quickly followed by carbon corrosion reaction (in the presence of water). It causes the anode to consume itself to sustain the load demand, leading to severe degradation [3]. This process increases the anode potential while the cathode potential remains unchanged, leading to cell potential being reversed [1,3]. The existing concept of material-based solution involves adding a water electrolysis catalyst, i.e. an oxygen evolution reaction (OER) catalyst into the PEFC anode, which helps prolong the water electrolysis during hydrogen starvation and in the process preventing the anode potential from increasing further (i.e. cell potential plummeting down). These anodes are termed as Reversal Tolerant Anodes (RTA) [4]. The OER catalyst thus keeps the driving potential for carbon corrosion minimal and protects the cell by preventing self-consumption of the anode while fuel-starved under load demand. However, studies [2,4] indicate that the current strategy of adding expensive OER catalyst to enhance the durability of automotive fuel cells is only a temporary solution. The cell potential still plummets down to -2 V and lower, unless the load is terminated intentionally. Although the durability is increased to certain extent by slowing down the cell performance degradation caused by fuel starvation, the protection is not guaranteed for an indefinite time. The reversal tolerance increases with increasing loading of precious metal-based OER catalysts, however it further increases the PEFC cost. The question that we want to address is the cause of the ultimate failure or deactivation of the OER catalyst in the anode. The complexity of the electro-thermo-chemical phenomena occurring in a PEFC makes it difficult to pin-point the exact cause of degradation. In order to delineate the electrochemical phenomena under cell reversal condition, we need to subject each material component of the RTA to reversal condition and study its behavior. We propose to design well controlled experiment to explore the limitations of the materials used in PEFC electrodes and their vulnerability through insitu electrochemical diagnostics (Cyclic voltammetery, Chronopotentiometry, Electrochemical Impedance Spectroscopy, etc.) and also correlate the observations to the changes the material chemical-electronic and physical properties obtained through exsitu material characterization of the anode electrocatalyst layer. Custom anodes will be fabricated using (1) OER catalyst IrO2 with Nafion® ionomer binder, to study any intrinsic changes in IrO2 phisico-chemical properties and activity towards water electrolysis reaction with time. (2) IrO2 and different loadings of Carbon black (Vulcan, XC 72R) with Nafion ®ionomer binder, to find out if carbon corrosion alongside OER causes deactivation of the catalyst or if the deactivation is caused by electronic isolation of the OER aggregates in the anode. By gaining insight of how each of these materials influence the overall degradation mechanism when put together in an RTA we can eventually design better RTA by engineering new microstructure and material in an economical way. [1] A. Taniguchi, T. Akita, K. Yasuda, and Y. Miyazaki, J. Pow. Sources 130 (2004), 42-49. [2] T.P. Ralph, M. P. Hogarth, Platinum Met. Rev. 46 (2002) 117-135. [3] P. Mandal, B-K Hong, J-G Oh, S. Litster, ECS Trans. 69 (2015), 443-457. [4] T. R. Ralph, S. Hudson, D. P. Wilkinson, ECS Trans. 1 (2006), 67-84.
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Apr 29, 2016
Mir Ghasem Hosseini + 1
Open Access