Experimental and Modeling Studies of High Temperature Polymer Electrolyte Membrane Fuel Cell Performance Under Poisoning Conditions

Unnikrishnan, Anusree and Janardhanan, V (2019) Experimental and Modeling Studies of High Temperature Polymer Electrolyte Membrane Fuel Cell Performance Under Poisoning Conditions. PhD thesis, Indian institute of technology Hyderabad.

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Abstract

An accelerated approach towards the development and commercialization of hydrogen fuel cells in the recent years has been necessiated owing to the adverse effects that follows the use of fossil fuels for all our energy requirements. Amongst the hydrogen driven fuel cells, Polymer electrolyte membrane fuel cells (PEMFCs) are popular for their application in both stationary and transportation sectors. PEMFCs, however are restricted by a number of factors, one of them being their low tolerance to impurities present in the feed streams. One method to enhance their tolerance towards impurities is to operate at higher temperatures (125-200 oC). The transition in the operating temperature is facilitated either by altering the hydrated nafion membranes employed in PEMFCs or by using phosphoric acid doped polybenzimidazole (PBI) membrane. The membrane functions both as a proton conductor as well as a separator between the anode and cathode. The use of PBI allows higher operating temperatures for PEMFCs and are classified as High Temperature PEM fuel cells (HT-PEM). The ones which use Nafion and operates below 100 oC are classified as low temperature PEM fuel cells (LT-PEM) The way forward for the advancement of HT-PEM fuel cell technology is experimentation assisted by modeling studies. Numerical modeling helps to understand the intricate physical and chemical processes within the cell. Models that range from simple analytical models to three dimensional and system level models are available in the literature. However, the ones that study the fundamental aspects of oxygen reduction reaction (ORR) and hydrogen oxidation reaction (HOR) under HT-PEM fuel cell operating conditions are rather scarce in the literature. The fundamental aspects of HOR and ORR are experimentally studied using rotating disc electrodes (RDE) typically at room temperature. The exchange current densities are then calculated from RDE measurements by extrapolating the data from room temperature to fuel cell operating temperatures. There are reports which points to the draw back of RDE to accurately measure HOR rates in acid electrolytes due to the very fast kinetics of HOR in acid media compared to alkaline media. It has been found that the exchange current densities calculated using Butler-Volmer equations are much higher than the ones measured using RDE. When it comes to ORR it is well known that the performance of PEM fuel cells is limited by the sluggish ORR, which leads to significant activation losses. This thesis presents the experimental and modeling studies of HT-PEM fuel cell performance. The durability of electrocatalysts is limited in presence of impurities in the electrochemical systems. The reformate hydrogen fuel used in fuel cells contain ppm level of CO which is deleterious for the fuel cell performance. HT-PEM fuel cells are advantageous because of high operating temperatures and can tolerate CO upto percentage levels. In our study, we explore the effect of trace amount of chloride when present in the anode catalyst layer at HT-PEM operating conditions. The chloride poisoning studies are performed using binderless electrodes and its effect on the performance on commercial Pt/C and PtRu/C anode catalysts is determined using half-cell and single cell studies. The influence on CO poisoning when trace amount of chloride is present in the anode catalyst layer is also studied using electrochemical impedance analysis for Pt/C catalysts and using potentiostatic studies for PtRu/C catalysts. The observed effects are discussed in detail in this thesis. Further, the performance on pure H2 and CO contaminated H2 at various operating temperatures is evaluated using modeling and simulation studies, and is validated experimentally using a 2.25 cm2 membrane electrode assembly (MEA). Starting from the elementary step reactions for HOR and ORR, six different models for the kinetics of HOR and three different models for ORR are derived. The derived models have the advantage that the exchange current density formulation is free from the arbitrary order dependency with respect to the partial pressures of reactants and products and depends on the symmetry factor. Using the derived expressions, two different charge transfer models are implemented as part of this thesis. One model assumes that the charge transfer reactions occur only at the interface between the electrolyte membrane and catalyst layer (interface charge transfer model), while the other accounts for the charge transport within the catalyst layer of the electrodes (distributed charge transfer model). These different charge transfer models with different HOR and ORR kinetics are incorporated into a multi-physics model of the cell, which accounts for the convective and diffusive transport within the flow channels and the electrodes. The model predictions are compared with experimental measurements performed as a part of this thesis and data reported in literature.

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