(500e) An Environmentally-Friendly Process for Fuel Cell Electrode Reclamation

One of the next-generation power sources is the proton exchange membrane (PEM) fuel cell, which runs on pure hydrogen or hydrogen-rich reformate. At the heart of the PEM fuel cell is a membrane electrode assembly (MEA). The MEA is a laminate composed of electrode layers sandwiched between outer layers, fabricated from either carbon fiber or fabric and which control the diffusion of reactant gases, and the inner polymer membrane. Hydrogen is oxidized at the anode to form protons, which migrate through the membrane and react with oxygen at the cathode to form water. In this type of fuel cell, platinum catalyzes the reactions at both electrodes.

Realization of a future that includes ubiquitous use of hydrogen fuel cell-powered vehicles will be partially contingent on a process for recycling components of fuel cell membrane electrode assemblies. In aggregate, the platinum used for the fuel cell electrodes will represent a large pool of this precious metal, and the efficient recycling of Pt from MEAs will be a cost-enabling factor for success of this technology. Care must be taken in the reclamation process because of the presence of fluoropolymers in the MEA. While Pt is normally recovered with high yield, the combustion process commonly applied to remove an organic matrix will also liberate a large volume of HF, a gas which is both toxic and corrosive. Carbonyl fluoride, which has a recommended exposure limit of 2 ppmv, is another undesirable product of fluoropolymer combustion.

In 2003, The U.S. Department of Energy awarded Engelhard Corporation an 80% cost share grant for a five-year project budgeted at $5.9M. The principal objective is reclaiming platinum from fuel cell MEAs without producing fluorine-containing emissions. Over the last three years, Engelhard has approached the problem from several directions in balancing the two goals: a commercially viable recycling process and an environmentally favorable one. Working with both fresh and aged fuel cells, it has been shown that precious metals can be liberated at high yield using microwave-assisted acid digestion, but exposure of the gas diffusion layer-shielded electrode surfaces is required. A low-cost solvent-stripping process has been identified for two geometries of fuel cell MEAs: GDL and GDE.

This paper will detail progress made in realizing a practical, “green”, process for PEM fuel cell MEAs. The process will be based on the flow diagram shown below: