(Invited) Ten Years on: Evolution, Status and Future of Alkaline Exchange Membrane Fuel Cell Systems

Abstract

In 2009, CellEra Technologies began work on an Alkaline Exchange Membrane Fuel Cell (AEMFC) stack. Shortly thereafter, at a time when there was relatively little interest in the technology, a stack with 100 cm2 active area, yielding slightly under 100 mW/cm2, running on H2 and O2 was achieved. This result was the first demonstration of an AEMFC stack in operation and generated significant market interest. The membrane-electrode assemblies (MEA’s) used therein yielded up to 200 mW/cm2 in single-cell tests, a comfortably world-leading AEMFC power density at the time. Nevertheless, critical and potentially insurmountable obstacles were also in sharp focus: Low performance vs PEM fuel cells, low durability arising from chemical instability of ionomer in alkaline conditions, sensitivity to atmospheric CO2 and more. There exists in addition a special challenge of water management in the AEMFC. Though not traditionally assigned the same weight as the above obstacles in general AEMFC mythology, it was flagged as critical early on in our development program, and has since remained thematic, as it was determined to be a root cause in a large proportion, if not the majority, of identified device technology limitations. Active water consumption in the cathode electrode, ½O2 + H2O + 2e- → 2OH- (1) balanced by double the overall water generation rate at the anode, H2 + 2OH- → 2H2O + 2e- (2) leads to a wide variety of mutually reinforcing detrimental phenomena that must be mitigated holistically to achieve sustained high performance and durability. The existence of the additional reactant – water – in the cathode (Eq 1) is especially significant, with strong multi-scaled implications, from requirements for the chemistry of the component materials, to the design of catalyst layers, structural aspects of the MEA, optimization of operating conditions, design of flow fields and stack engineering. Even today, ten years on, this remains the core engineering and materials science challenge of the technology in spite of impressive progress made in the research field as a whole. In this talk we review the developments in, and understanding of, the PO-CellTech (formerly CellEra) MEA, stack and fuel cell system that have by now resolved all the fundamental questions of AEMFC viability as a technology, and allow us to target the nascent consumer automotive “FCEV” market: Ultra-low Platinum Group Metal (PGM) loadings, next-generation membrane technology, high areal (MEA) and volumetric (stack) power density, CO2 tolerance, and significant room for continued improvement. We will analyze the status and limitations of AEMFC power density, as well as the major outstanding challenge of device durability, with special reference to the ever-present water management imperative as well as increasingly stringent demands that need to be met, in order successfully to address a rapidly maturing exchange membrane fuel cell market.