NIST researchers devise method for high-speed, in-line process control of fuel cell membrane catalyst layers
Researchers at the National Institute of Standards and Technology (NIST) have devised a method for high-speed, in-line process control of platinum-based catalyst layers in the membrane electrode assembly of the fuel cell. Their system may have overcome a significant hurdle to manufacturing hydrogen fuel cells by creating a way to check whether the expensive catalysts the cells need have been incorporated quickly and effectively.
Reported in a paper in the Journal of Power Sources, they demonstrated the use of multiple reflectivity-based optical methods, such as optical scatterometry and large aperture projection scatterometry (LAPS)—a new high-throughput approach developed at the National Institute of Standards and Technology specifically for fuel cell manufacturing metrology—to take in-line catalyst loading measurements of carbon-supported Pt nanoparticle and Pt-alloy nano-structured thin film catalyst coated membranes.
The proton exchange membrane (PEM) fuel cell remains one of the most researched types of fuel cells for automotive and portable power applications. Commercial viability continues to be limited by high-cost and performance levels that fall short of other power generation technologies currently in use (e.g., internal combustion engine, battery).… Advances in fuel cell manufacturing metrology are needed to facilitate improvements in the ability to measure certain manufacturing parameters such as microstructure defects, surface roughness, and coating quality in the electrode catalyst layer of the Membrane Electrode Assembly (MEA).
One of the key materials in PEM fuel cells is the electrocatalyst, responsible for promoting the electrochemical reactions that occur in the MEA electrodes that generate the electricity. … DOE identifies total PGM catalyst loading targets for 2020 that are at 0.125 mg/cm2 of Pt. To reach this target more economically, essential improvements are needed in fuel cell manufacturing quality control and catalyst loading process control.—Stocker et al.
The researchers found an answer stemming from their experience measuring small objects for a completely different industry: computer chip manufacturing. But their usual approach, based on reflecting a laser’s light from a chip surface, demanded a rethink. Although expertise in optical methods for measuring features smaller than 10 nanometers on chips exists, the chips don’t speed by on a production line at 30 meters per minute, said NIST physical scientist Michael Stocker. In addition, the membrane is black, limiting the reflected light for measurement.
NIST’s large aperture projection scatterometer (LAPS) has a spot size two to three orders-of-magnitude larger in area compared to the focused beam of a commercial ellipsometer. The LAPS system is designed with variable illumination and collection numerical apertures which enable a range of large measurement spot sizes to be selected.
A large spot size permits a stable measurement of the Pt loading in addition to meeting the requirement of a large coverage area. The LAPS, in its current configuration, is an angle-resolved instrument allowing the acquisition of data as a function of the illumination and collection angles.
The team’s demonstration instrument can detect the low levels of light reflected off the tiny platinum particles as the sheet moves past at a meter or two per minute. Stocker said there are no fundamental barriers to scaling up the method or increasing the speed to meet the industry’s future needs.
For example, a manufacturer could array a row of these instruments to scan a meter-wide sheet, with each one identifying trouble spots in a particular section. Though the method would likely need to be combined with other techniques such as X-ray fluorescence to form a complete solution, Stocker said that it leaves fuel cell manufacturers in a good place.
It’s all just optical engineering from this point onward. Industry can take it from here.—Michael Stocker
>M.T. Stocker, B.M. Barnes, M. Sohn, E. Stanfield and R.M. Silver (2017) “Development of large aperture projection scatterometry for catalyst loading evaluation in proton exchange membrane fuel cells,” Journal of Power Sources doi: 10.1016/j.jpowsour.2017.07.092