Novel Anhydrous Superprotonic Ionic Liquids and Membranes for Application In
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Novel Anhydrous Superprotonic Ionic Liquids and Membranes for Application in Mid-temperature Fuel Cells by Younes Ansari A Dissertation Presented in Partial Fulfillment of the Requirements for the Degree Doctor of Philosophy Approved May 2013 by the Graduate Supervisory Committee: C. Austen Angell, Chair Andrew Chizmeshya Ranko Richert George Wolf ARIZONA STATE UNIVERSITY August 2013 ABSTRACT This thesis studies three different types of anhydrous proton conducting electrolytes for use in fuel cells. The proton energy level scheme is used to make the first electrolyte which is a rubbery polymer in which the conductivity reaches values typical of activated Nafion, even though it is completely anhydrous. The protons are introduced into a cross-linked polyphospazene rubber by the superacid HOTf, which is absorbed by partial protonation of the backbone nitrogens. The decoupling of conductivity from segmental relaxation times assessed by comparison with conductivity relaxation times amounts to some 10 orders of magnitude, but it cannot be concluded whether it is purely protonic or due − − equally to a mobile OTf or H(OTf) 2 component. The second electrolyte is built on the success of phosphoric acid as a fuel cell electrolyte, by designing a variant of the molecular acid that has increased temperature range without sacrifice of high temperature conductivity or open circuit voltage. The success is achieved by introduction of a hybrid component, based on silicon coordination of phosphate groups, which prevents decomposition or water loss to 250ºC, while o enhancing free proton motion. Conductivity studies are reported to 285 C and full H 2/O 2 cell polarization curves to 226 oC. The current efficiency reported here (current density per unit of fuel supplied per sec) is the highest on record. A power density of 184 (mW.cm -2) is achieved at 226 oC with hydrogen flow rate of 4.1 ml/minute. The third electrolyte is a novel type of ionic liquids which is made by addition of a super strong Brønsted acid to a super weak Brønsted base. Here it is shown that by allowing the proton of transient HAlCl 4, to relocate on a very weak base that is also i stable to superacids, we can create an anhydrous ionic liquid, itself a superacid, in which the proton is so loosely bound that at least 50% of the electrical conductivity is due to the motion of free protons. The protic ionic liquids (PILs) described, pentafluoropyridinium tetrachloroaluminate and 5-chloro-2,4,6-trifluoropyrimidinium tetrachloroaluminate, might be the forerunner of a class of materials in which the proton plasma state can be approached. ii This thesis is dedicated to my late father, my mother, my sister and my brothers Reza and Yousef iii ACKNOWLEDGMENTS Firstly, I would like to express my deepest gratitude to my advisor regent Professor C. Austen Angell for having accepted me in his research group and for teaching me all the experimental skills and guiding me through the concepts of my research. I am very thankful for his support. I would like to thank J.P. Belieres. He laid much of the groundwork for my work. I also would like to thank my dissertation committee members, Professor Andrew Chizmeshya, Professor George Wolf and Professor Ranko Richert for their support and for giving me their time and expertise to improve my thesis. My special thanks go to Professor Timothy Steimle. I was honored to be his TA for one semester. I also thank Professor Steimle’s postdoc, Dr. Fang Wang, for her help and support. Many thanks to my friend, Dr. Kazuhide Ueno. I have learned a lot from him. I would like to thank Dinesh Medpelli for taking the TEM images of our E-TEK electrodes. I am grateful to Dr. Wei Huang for preliminary NMR studies that will become the subject of a future paper. I thank John Blanchard and Stephen Davidowsky for the proton NMR spectrum of our molecular solutions made at an early stage of my research (2010). I must acknowledge Dr. Jennifer Green and my lab mates Iolanda Klein, Dr. Lee, Dr. Zhao, and Greg Tucker for all their help and support and for their productive collaborations. I thank all of you for making the work in the lab an enjoyable experience. I must also acknowledge Waunita Parrill, our executive assistant, for placing orders, arranging paperwork, and submitting papers. iv Here I would like to thank my best friend Behnam Kia for his friendship. He was the one that I could count on. Last, but not least, I would like to thank my family especially my mother and my brothers, Reza and Yousef for their countless support, without their encouragements this thesis would not have existed. This thesis was supported by Grants W911NF0710423 and W911NF-11-1-0263 from the Army Research Office (ARO). v Table of Contents Page List of Tables …………………………………………….…….………………................x List of Figures……….…………………..……………………………………..................xi CHAPTER 1 Introduction to Ionic Liquids ...................................................................................1 1.1 Ionic Liquids .......................................................................................................1 1.1.1 Inorganic Ionic Liquids (IILs) ............................................................................2 1.1.2 Solvate (chelate) ionic liquids .............................................................................3 1.1.3 Aprotic Ionic Liquids (APILs) ............................................................................4 1.1.4 Protic Ionic Liquids (PILs) .................................................................................6 1.2 Protic Ionic Liquids and Ionicity .........................................................................8 1.3 Ionic conductivity ..............................................................................................12 1.4 Proton Free Energy Level Diagram ..................................................................15 2 Introduction to Fuel cells .......................................................................................18 2.1 Hydrogen Fuel Cell ...........................................................................................19 2.1 What limits the current in fuel cells? ................................................................23 2.3 Different types of Fuel Cells .............................................................................27 2.3.1 Polymer Electrolyte Membrane Fuel Cell or Proton Exchange Membrane Fuel Cell (PEMFC) ..................................................................................................27 2.3.2 Direct Methanol Fuel Cells (DMFCs) ..............................................................29 vi CHAPTER Page 2.3.3 Alkaline Fuel Cells (AFCs) ..............................................................................30 2.3.4 Phosphoric acid fuel cells (PAFCs) ..................................................................31 2.3.5 Molten Carbonate fuel cells (MCFCs) ..............................................................32 2.3.6 Solid oxide fuel cells (SOFCs) .........................................................................33 2.3.7 Biological fuel cells (BFCs)..............................................................................34 2.3.8 Regenerative fuel cells (RFCs) .........................................................................35 2.4 Efficiency ( ε ) in Fuel Cells ..............................................................................37 2.5 Polarization curves in fuel cells ........................................................................42 2.5.1 Activation polarization......................................................................................44 2.5.2 Ohmic polarization............................................................................................48 2.5.3 Concentration polarization ................................................................................51 2.5.4 Other types of polarization losses .....................................................................51 2.6 Cost and production in fuel cells .......................................................................52 2.6.1 Factors affecting fuel cell’s cost .......................................................................52 3 Experimental ..........................................................................................................54 3.1 Electrochemical reaction ...................................................................................54 3.2 Impedance spectroscopy ...................................................................................56 3.3 Viscosity measurements ....................................................................................60 3.4 Fuel cell polarization .........................................................................................63 vii CHAPTER Page 3.5 Infrared (IR) Spectroscopy ................................................................................65 3.6 Nuclear Magnetic Resonance (NMR) ...............................................................67 3.7 Differential Thermal Analysis (DTA) ...............................................................67 3.8 Differential Scanning Calorimetry (DSC) .........................................................69 3.9 Inductively Coupled Plasma (ICP) ....................................................................71 4 Anhydrous Superprotonic Polymer by Superacid Protonation of Cross-linked (PNCl 2)n .................................................................................................................72