Non-Fluorinated Polymer Materials for Proton Exchange Membrane Fuel Cells

TLDR

Non‑fluorinated polymer membranes have emerged over the past decade as promising alternatives to perfluorosulfonic acid membranes, aiming to match their performance while extending operating temperatures toward those of phosphoric acid fuel cells and requiring adequate mechanical strength, hydration, proton conductivity, and chemical stability. This review surveys advances in proton‑conducting hydrocarbon and heterocyclic polymers for proton exchange and direct methanol fuel cells, outlining synthesis strategies and key material properties. The authors compare microstructures of non‑fluorinated polymers to perfluorosulfonic acid membranes using small‑angle X‑ray and neutron diffraction, and discuss degradation pathways and aging processes affecting chemical and morphological stability in MEAs for direct methanol and hydrogen‑air fuel cells. Several non‑fluorinated polymer membranes have lifetimes of 500–4000 h, but only a few systems show promise for long‑term operation above 100 °C. 1.

Abstract

▪ Abstract The past 10 years have witnessed a tremendous acceleration in research devoted to non-fluorinated polymer membranes, both as competitive alternatives to commercial perfluorosulfonic acid membranes operating in the same temperature range and with the objective of extending the range of operation of polymer fuel cells toward those more generally occupied by phosphoric acid fuel cells. Important requirements are adequate membrane mechanical strength at levels of functionalization (generally sulfonation) and hydration allowing high proton conductivity, and stability in the aggressive environment of a working fuel cell, in particular thermohydrolytic and chemical stability. This review provides an overview of progress made in the development of proton-conducting hydrocarbon and heterocyclic-based polymers for proton exchange and direct methanol fuel cells and describes the various approaches made to polymer modification/synthesis and salient properties of the materials formed, including those relating to proton transport and proton conductivity, e.g., water diffusion and electro-osmotic drag. The microstructure, deduced from small angle X-ray and neutron diffraction measurements of representative non-fluorinated polymers is compared with that of perfluorosulfonic acid membranes. Different degradation mechanisms and aging processes that can result in chemical and morphological alteration are considered, and recent characterization of membrane-electrode assemblies (MEAs) in direct methanol and hydrogen-air (oxygen) fuel cells completes this review of the state of the art. While several types of non-fluorinated polymer membrane have demonstrated lifetimes of 500–4000 h, only a limited number of systems exist that hold promise for long-term operation above 100°C. 1