Fuel cells have the potential to reduce the country’s energy consumption through increased energy conversion efficiency and dependence on imported petroleum through the use of hydrogen from renewable resources. The US DOE Fuel Cell sub-program emphasizes polymer electrolyte membrane (PEM) fuel cells as a replacement for internal combustion engines in light commercial vehicles to support the goal of reducing oil consumption in the transportation sector. PEM fuel cells are the focus for light commercial vehicles because they can start quickly, have high operational efficiency, and can operate at low temperatures.
The program also supports fuel cells for stationary power, portable power and auxiliary power applications where earlier market entry would support the development of a fuel cell manufacturing and supply base. The technical focus is on the development of materials and components that enable fuel cells to achieve the objectives of the fuel cell sub-program, mainly related to system cost and durability.
DESCRIPTION OF THE FUEL CELL
Like batteries, a fuel cell generates electricity electrocatalytically. In a fuel cell, however, the electrodes are not consumed. Rather, a fuel cell consumes fuel (hydrogen for PEM fuel cells) at the anode and oxygen from the air at the cathode.
A catalyst is used at the anode to promote the separation of the protons and electrons of the hydrogen. The protons travel through a membrane material to the cathode, while the electrons travel to the cathode via an external circuit, where they combine with the protons and oxygen on the cathode catalyst to form water and complete the cycle. The combination of the anode/membrane/cathode layers is called the membrane electrode assembly (MEA).
To complete a cell, the MEA is typically placed between gas diffusion layers and gas flow fields that distribute reactants to the electrodes and collect the current from the reaction. Each cell produces less than one volt, so many cells are “stacked” in series to produce voltage at useful levels.
Most of the current research on catalysts for PEM fuel cells focuses on the cathode. The general objectives are: to reduce Pt content (and hence cost); to obtain higher catalytic activity than the standard carbon-supported platinum catalysts; and to increase the durability of the catalyst/carrier system, particularly during transients and shutdown/start-up cycles.3
The state-of-the-art membrane material is based on perfluoro sulfonic acid, which relies on the presence of water in the membrane to conduct the protons. The primary deficiencies of this material are: loss of conductivity at temperatures above 100 °C and low humidity; insufficient conductivity at low temperature (-20 °C); insufficient mechanical integrity; during humidity cycles causing swelling and shrinking; and chemical stability. Most DOE membrane research focuses on durability and operation at temperatures above 100°C. Both mechanical and chemical resistance are addressed through physical reinforcement and through changes in ionomer chemistry and structure or its end groups.
FCEVs use a propulsion system similar to that of electric vehicles, in which energy stored as hydrogen is converted into electricity by the fuel cell. Unlike traditional internal combustion engine vehicles, these vehicles do not produce harmful tailpipe emissions. Other benefits include increasing U.S. energy resilience through diversity and strengthening the economy.