Making Polymer Membrane Fuel Cells Work

with Practical Fuels

Jay Benziger

Department of Chemical Engineering

Princeton University

The news has been filled with the eminent arrival of fuel cells as the solution to the zero emission vehicles and improved fuel economy. If we go back 100 years one can find the same claims about fuel cells. In this talk we will review the major types of fuel cells and what has limited their adoption. We will then focus on Polymer Electrolyte Membrane Fuel Cells which are an attractive power source for “zero” emission vehicles. PEM fuel cells are very sensitive to carbon monoxide in the hydrogen fuel source; at typical operating temperatures of 80°C they are shut down by the presence of 10-100 ppm of CO in the hydrogen feed. We show that CO tolerance depends on the differential heats of adsorption of CO and H₂ on the electrocatalysts. We have demonstrated CO tolerant operation of PEM fuel cells at 400 K. The operation of Polymer Electrolyte Membrane Fuel Cells to be tolerant of reformed fuels requires proper temperature and humidification control. At temperatures above 100 C we demonstrate the fuel cell must be operated at elevated pressures to keep the polymer membrane properly humidified and the fuel cell functioning.

Elevated temperature and pressure operation of PEM fuel cells requires a systems approach to achieve proper control. Humidification control over the fuel cell is critical. We show that multiple steady states occur in PEM fuel cells due to the dependence of proton conduction in the membrane with relative humidity. Fuel cell operation can be shut down due to small changes in operating temperature of the cell, or changes in the humidification. We will show how these multiple steady states can be predicted based on simple reaction engineering models.

The steady state multiplicity has important implications in the design of the flow channels for supply of the reactants. It is critical to match convective and diffusive transport rates in the reactor to maintain the humidification of the membrane. Parallel co-current flow requires humidification of both of the reactant streams for sustained operation. Parallel countercurrent flow only requires humidification of the hydrogen stream. Re-entrant flow schemes can minimize the need for humidification of the reactant streams.

Jay Benziger is Professor of Chemical Engineering at Princeton University. He was educated at Carleton College (BA Mathematics), Columbia University (MS Chemical Enigneering) and Stanford University (PhD Chemical Eningeering). He has been on the faculty at Princeton for 22 years, where his research has been principally in the area of heterogeneous catalysis and surface chemistry. His group pioneered novel methods of infrared spectroscopy for surfaces including infrared photoacoustic spectroscopy. He has extensively studied partial oxidation with vanadium phosphate catalysts, developing new catalysts with improved performance. Over the past two years Professor Benziger has teamed with Professor Andrew Bocarsly to develop new membranes for CO tolerant PEM fuel cells. Professor Benziger has co-authored over 100 papers in refereed journals.. He has received the Exxon Award in Solid State Chemistry from the American Chemical Society, and the Purdy Award from the American Ceramic Society.