Chemical and Biomolecular Engineering Faculty Research

Title

Lithium Fuel Cells:I. Lithium/poly (Organophosphazene) Membrane Anodes in Koh and Seawater

Document Type

Article

Publication Date

Summer 2002

Abstract

The main goal of our research project is to design safe, high energy and power density lithium/water systems. We explored the feasibility of substituting the natural bilayer (formed on the lithium surface when lithium is in contact with water), for a thin polymeric film. By substituting the natural bilayer film we hope to reduce the parasitic reactions occurring at the lithium/water interface, thus yielding an increase in the anodic efficiency. We investigated the effect of placing or casting a thin, (lithium/ion-conducting) polymer layer on the lithium metal surface. This paper is part one in a series of two papers. Paper I presents the results obtained with a lithium/polymer system, where the polymer was a monolayer of a polyphosphazene with 90% trifluoromethylphenoxy and 10% lithium carboxyphenoxy side groups (Polymer 4), or a multilayer film formed of one layer of poly[bis(methoxyethoxyethoxy)phosphazene] (MEEP) and one to three layers of Polymer 4 containing from 0 to 75 wt.% of lithium triflate salts. Paper II presents results obtained when the polymer layers were prepared using a polymer with equal amounts of methoxyethoxyethoxy and phenoxy side groups containing from 0 to 75 wt.% of lithium triflate salts. Phosphazene membranes have been designed and tailored to allow lithium ion conduction and prevent water migration to the surface of lithium metal. The phosphazene membranes enhance the safety of an aqueous lithium cell by inhibiting (or reducing) the reaction of lithium with water that evolves hydrogen at the anode. Original tests of lithium/phosphazene systems led to unpredictable open circuit voltages (OCVs). When the adhesion of the membrane to the lithium metal was improved, the OCV stabilized. The OCVs for the half-cell of lithium polymer aqueous electrolytes varies between −3.1 and −2.8 VSCE, depending on the membrane. The current densities for this polymer system are in the range of 10−6–10−3 A/cm2. The Columbic anodic efficiency is assumed to be near 100%—as hydrogen evolution is not measurable. Some of the polymeric membranes developed pinholes with use. Layered systems have also been designed to avoid the development of pinholes over time. In this paper, we present the results obtained by using polyphosphazenes with a 9:1 ratio of trifluoromethylphenoxy and p-carboxyphenoxy side groups and the lithium salt of the carboxylate function. Poly(organophosphazene) membranes with a single layer and a multilayer structure were tested in 8 M KOH or synthetic seawater for up to 5 days.

Volume

47

Issue

15

First Page

2495

Last Page

2503