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Novel anhydrous superprotonic ionic liquids and membranes for application in mid-temperature fuel cells

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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

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 enhancing free proton motion. Conductivity studies are reported to 285ºC and full H2/O2 cell polarization curves to 226ºC. 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 226ºC 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 HAlCl4, to relocate on a very weak base that is also 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.

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  • 2013

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A new class of solid state, single-ion conductors (H+ and Li+): silicon-based plastic crystals

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Plastic crystals as a class are of much interest in applications as solid state electrolytes for electrochemical energy conversion devices. A subclass exhibit very high protonic conductivity and its members

Plastic crystals as a class are of much interest in applications as solid state electrolytes for electrochemical energy conversion devices. A subclass exhibit very high protonic conductivity and its members have been investigated as possible fuel cell electrolytes, as first demonstrated by Haile’s group in 2001 with CsHSO4. To date these have been inorganic compounds with tetrahedral oxyanions carrying one or more protons, charge-balanced by large alkali cations. Above the rotator phase transition, the HXO4- anions re-orient at a rate dependent on temperature while the centers of mass remain ordered. The transition is accompanied by a conductivity "jump" (as much as four orders of magnitude, to ~ 10 mScm-1 in the now-classic case of CsHSO4) due to mobile protons. These superprotonic plastic crystals bring a “true solid state” alternative to polymer electrolytes, operating at mild temperatures (150-200ºC) without the requirement of humidification. This work describes a new class of solid acids based on silicon, which are of general interest. Its members have extraordinary conductivities, as high as 21.5 mS/cm at room temperature, orders of magnitude above any previous reported case. Three fuel cells are demonstrated, delivering current densities as high as 225 mA/cm2 at short-circuit at 130ºC in one example and 640 mA/cm2 at 87ºC in another. The new compounds are insoluble in water, and their remarkably high conductivities over a wide temperature range allow for lower temperature operations, thus reducing the risk of hydrogen sulfide formation and dehydration reactions. Additionally, plastic crystals have highly advantageous properties that permit their application as solid state electrolytes in lithium batteries. So far only doped materials have been presented. This work presents for the first time non-doped plastic crystals in which the lithium ions are integral part of the structure, as a solid state electrolyte. The new electrolytes have conductivities of 3 to 10 mS/cm at room temperature, and in one example maintain a highly conductive state at temperatures as low as -30oC. The malleability of the materials and single ion conducting properties make these materials highly interesting candidates as a novel class of solid state lithium conductors.

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  • 2016