High oxygen permeation rates through poly(dimethyl siloxane) for fuel cells and oxygen reduction reaction mechanistic studies

The electrochemically catalyzed oxygen reduction reaction (ORR) is essential to operation of most fuel cells and many types of batteries; oxygen, being prevalent in air, provides a costless, ambient, high-energy (1.23 V vs. normal hydrogen electrode) hole source to complete the electrochemical redox circuit. Oxygen mass transport and electrochemical overpotential losses however represent the largest efficiency losses in conventional polymer electrolyte membrane (PEM) fuel cells. Mass transport is limited by the inherent low solubility of oxygen in water. This low solubility and diffusivity can be overcome through reduction of electrolyte thickness, and by utilizing a thin cell wall of highly a permeable material, such as poly(dimethylsiloxane) (PDMS). The well-characterized utility of PDMS in microfluidics as well as its stability in a wide array of aqueous electrolytes, proves PDMS an excellent material for electrochemical devices. The results presented herein utilize microfluidics to reduce the electrolyte thickness (and therefore the thickness of the diffuse layer), and control the thickness of the PDMS layer. This in turn allows large, highly controllable and stable ORR currents of a direct formic acid fuel cell (DFAFC) in a fully passive, planar form-factor. Lessons learnt from this DFAFC study have enabled us to design an in-situ electrochemical X-ray Absorption Spectroscopy (XAS) cell for the study of mechanistic details of ORR electrocatalysis on platinum, the most commonly utilized ORR cathode material. The large ORR overpotential on Pt (~0.33 V loss at onset potential) accounts for the majority of polymer electrolyte membrane (PEM) fuel cell efficiency losses, as well as a huge cost that prevents fuel cell economic viability. Maximum ORR activity is correlated to related properties such as d-band center, and metal-oxide binding energy and bond distances. These properties are observable through XAS. XAS at lower energies, X-ray absorption near edge spectroscopy (XANES) can define d-band occupancy changes, related to oxidation states of the metal. XAS at higher energies, extended X- ray absorption fine structure (EXAFS) can derive bond distances through X-ray interaction with backscattered electrons. From these bond distances, bond types may be inferred, as well as coordination numbers and degree of bond disorder. The results described herein exploit the high oxygen permeability through PDMS in the design of an in-situ electrochemical XAS cell to study the evolution of the Pt electrocatalyst during ORR electrocatalysis. Oxygen is found to adsorb to platinum over a wide potential range, causing an ORR current-dependent Pt-Pt bond expansion