Proton Exchange Membrane (PEM) Fuel Cells
Introduction
Fuel cells are an alternative energy technology that generate electric energy through the reaction between hydrogen (or a hydrogen-rich fuel source) and oxygen. These devices are particularly interesting due to high efficiencies relative to traditional combustion engines and low emissions, producing only heat and water as waste products. The development of new component materials with increased performance and cost-effectiveness is a critical part of emerging fuel cell research.
This spotlight focuses on materials for Proton Exchange Membrane (PEM) fuel cells, also referred to as Polymeric Electrolyte Membrane fuel cells, which operate at relatively low temperatures (~ 80 °C). For more information about high temperature fuel cells, please visit our technology spotlight on Solid Oxide Fuel Cells (SOFC).
Fuel Cell Components
Fuel cell devices are often composed of multiple fuel cells connected in series to form a stack (Figure 1), which increases the total amount of generated electricity. Each individual fuel cell contains three primary components: two electrodes (anode and cathode) and a conductive electrolyte. In the case of PEM fuel cells, each electrode is comprised of a porous, high-surface area material impregnated with an electrocatalyst, typically platinum or a platinum alloy. The electrolyte material is a polymeric membrane and serves as an ionic conductor.1
Electrical generation in a fuel cell is driven by two primary chemical reactions, as illustrated in (Figure 2). For fuel cells operating on pure H2, hydrogen gas is split into protons and electrons at the anode. The protons are conducted through the electrolyte membrane, and the electrons flow around the membrane, generating an electrical current. The charged ions (H+ and e-) combine with oxygen at the cathode, producing water and heat.2
Figure 1.Schematic illustrating multiple fuel cells combined in a stack.
Figure 2.Schematic diagram of the major components and electrochemical reactions in a PEM fuel cell.
Fuel Cell Catalysts
Platinum exhibits high activity for hydrogen oxidation and continues to be a frequently used electrocatalyst material. One major area of fuel cell research has been the reduction in platinum content without a concurrent decrease in cell performance, giving rise to an overall increase in cost effectiveness for the device.3 This is achievable through the use of engineered catalysts fabricated from platinum nanoparticles supported on conductive carbon (Prod. Nos. 738581, 738549, and 738557). These materials have the advantage of highly dispersed nanoparticles (Figure 3), high electrocatalytic surface area (ESA), and minimal particle growth at elevated temperatures, even at higher levels of Pt loading.
Pt-containing alloys are useful for devices operating on specialized fuel sources, such as methanol or reformate (H2, CO2, CO, and N2). Pt/Ru alloys, for example, have shown increased performance relative to pure Pt electrocatalysts with respect to methanol oxidation and carbon monoxide poisoning.4 Pt3Co is another catalyst of interest (particularly for PEMFC cathodes) and has demonstrated enhanced performance for the oxygen reduction reaction as well as high stability.5
Figure 3.Representative TEM images of Pt/C catalysts (left) and Pt3Co/C catalysts (right) demonstrating highly dispersed nanoparticles on high-surface area carbon supports.
Fuel Cell Membranes
Several key requirements are considered when selecting a fuel cell electrolyte. Desirable properties include high proton conductivity, high chemical and thermal stability, and low gas permeability.4,6 Highly favored materials are typically fluorinated polymers functionalized with sulphonic acid moieties, such as Nafion™ membranes.
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