Nanowerk Spotlight - Fuel cells have gained a lot of attention because they provide a potential solution to our addiction to fossil fuels. Energy production from oil, coal and gas is an extremely polluting, not to mention wasteful, process that consists of heat extraction from fuel by burning it, conversion of that heat to mechanical energy, and transformation of that mechanical energy into electrical energy. In contrast, fuel cells are electrochemical devices that convert a fuel's chemical energy directly to electrical energy with high efficiency and without combustion (although fuel cells operate similar to batteries, an important difference is that batteries store energy, while fuel cells can produce electricity continuously as long as fuel and air are supplied).
Despite their modern day high-tech aura, fuel cells actually have been around since the early 1800s. In 1839, William Robert Grove developed the Grove cell, which used zinc and platinum electrodes exposed to two acids and separated by a porous ceramic pot to generate about 12 amps of current at about 1.8 volts.
Modern fuel cells have the potential to revolutionize transportation. Like battery-electric vehicles, fuel cell vehicles are propelled by electric motors. But while battery electric vehicles use electricity from an external source and store it in a battery, fuel cells onboard a vehicle create electricity through a chemical process using hydrogen fuel and oxygen from the air (although the operation of a fuel cell vehicle is pollution free, the question is how the hydrogen for the fuel cell is produced – see Nanotechnology could clean up the hydrogen car's dirty little secret).
One of the leading fuel cell technologies developed in particular for transportation applications is the proton exchange membrane fuel cell, also known as polymer electrolyte membrane fuel cells – both resulting in the same acronym PEMFC. The proton conductivity of the polymer electrolyte membrane (PEM) is one of the key factors limiting the performance of PEMFCs, which depends on the relative humidity, and controls the cost and durability of these fuel cells. Furthermore, one of the major barriers preventing commercialization of PEMFCs is the lack of suitable materials to make them affordable. Nanotechnology promises cheap bipolar materials using nanocomposites, more efficient non-platinum electrocatalysts, and thermally stable and more durable membranes to become available in the near future. Researchers in India now have used nanotechnology to develop a carbon nanotube composite material which has the potential to make better fuel cell modules due to enhanced electrolyte properties.
"An improvement in the proton conductivity of the electrolyte membrane even by only one order of magnitude could change the performance of fuel cells dramatically" Dr. Vijayamohanan Pillai explains to Nanowerk. "Currently, Nafion (a sulfuric acid in a solid polymer form)-based membranes are widely used as the PEM in fuel cells that operate from 60 to 80°C. Although these state-of-the-art membranes show good proton conductivities from 0.1 to 0.01 Siemens per cm in a humid environment, they have many limitations, such as: 1) dependence on water for conductivity; 2) high methanol permeability; 3) a tendency to disintegrate in the presence of hydroxyl radicals, an intermediate in the cathode reaction; and 4) moderate mechanical and chemical stability."
Pillai, a researcher at the National Chemical Laboratory in Pune, India, and head of its Materials Electrochemistry Group, together with his team has developed a chemical strategy to increase the sulfonic acid content of Nafion membranes by incorporating sulfonic acid functionalized single-walled carbon nanotubes (S-SWCNTs), and demonstrated the remarkable utility of this composite membrane as electrolyte in PEMFC applications.
The researchers reported their findings in a recent issue of Angewandte Chemie International Edition ("Polymer Electrolyte Fuel Cells Using Nafion-Based Composite Membranes with Functionalized Carbon Nanotubes").
Pillai notes that significant efforts by fuel cell researchers go into developing composite materials that aim to increase the water retention capabilities of Nafion so that it doesn't lose its proton conduction at high temperatures. "In our work, we introduced sulfonic acid functionalized single wall carbon nanotubes into the Nafion matrix, thereby increasing the number of sulfonic acid groups – which is the key for its conduction – in the membrane" he says.
While the incorporation of SWCNTs would also help increase the mechanical stability of the composite membranes over that of Nafion membranes, a major benefit would be a much needed cost reduction in PEMFC technology.
Pillai explains that the high proton conductivity of Nafion is attributed to a mechanism in which the reorganization of hydrogen bonds plays a key role. "As the extra sulfonic acid moieties are anchored on the surface of our SWCNTs, these could provide more facile hopping of protons, which in turn would help to increase the proton mobility, thus accounting for the observed enhancement in conductivity (the Nafion/S-SWCNT composite membranes showed an almost one order of magnitude higher conductivity). In contrast, we found that an unmodified SWCNT/Nafion composite membrane does not show any improvement in proton conductivity."
This work has opened a new method of designing the polymer electrolytes for electrochemical power sources like fuel cells. It also demonstrates that, apart from a remarkable improvement in proton conductivity, properly designed membranes with appropriate incorporation of functionalized carbon nanotubes could help decrease the methanol cross-over without sacrificing the proton conductivity of the membrane.
Pillai cautions that several challenges have to be overcome before these advantages can be commercially exploited, including a rigorous evaluation of the chemical stability and durability of the membranes, lifetime studies of membrane-electrode assemblies, and possible corrosion problems of the electrode materials because of the higher sulfonic acid content.
By Michael Berger.
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