Investigation of Hydrogen Peroxide Production and Transport in a Proton Exchange Membrane Fuel and the Atom Resolved Micro-characterization of its Catalyst

by Pelsozy, Michael C.

The production and prevalence of H2O2 in a proton exchange membrane fuel cell (PEMFC) is concern due to its purported role in the long term durability of the fuel membrane. Several approaches to the detection of H2O2 using microelectrodes inserted into working PEMFCs were explored. Au and Pt microelectrodes were used to carry out various electrochemical methods to probe reactions in situ at steady state. Substantial difficulties were encountered in deploying these methods. A detailed study of various aspects of both Au and Pt microelectrodes in operating cells revealed a number of interesting phenomena as well as difficulties in peroxide detection in the presence of hydrogen cross-over. Interactions between hydrogen and H2O2 in the presence of the electrode surface leads to enhanced oxidation currents at surprisingly low potentials on Au. Steady state and transient voltammetry exhibit different behavior on both Au and Pt in the -OH adsorption region, possibly due to slow catalytic removal of the Au-OH and Pt-OH films by hydrogen. Careful calibration of the response to peroxide in the presence of hydrogen in a fuel cell configuration revealed that H2O2 can be detected at Pt under favorable conditions. However, in an operating fuel cell, H2O2 making it to the probe electrode is insufficient to yield a signal under the conditions of testing. Also, the transport of H2O2 in a PEMFC membrane material was studied. This was done by measuring the diffusion coefficient of H2O2 in recast a Nafion® film on a Pt rotating disk electrode (RDE). Finally, the PEMFC catalyst most widely deployed, nanometer sized Pt particles, was investigated using methods of high-resolution transmission electron microcopy (HRTEM). Attempts were made to obtain images of these particles eliminating or substantially reducing the effects of artifacts arising from the microscope optics. The experimental difficulties in obtaining such images is demonstrated and discussed. An image of a Pt particle substantially reduced in artifact due to the use of aberration corrected HRTEM is used order to show that the displacement of atom columns comprising the particle exists. In the particular case studied the displacement causes an expansion of the particle on the order of 3% - 5% and is attributed to interaction with a neighboring particle via van der Waals forces.