KEY CONCEPTS
• The high cost and low availability of platinum is leading researchers to find alternative catalyst systems for fuel cells.
• A new catalyst has been developed where a very low amount of platinum is spread uniformly across a platinum-group metal-free catalyst.
• The new catalyst, known as a low platinum over platinum-group metal-free catalyst, exhibited higher catalytic activity and durability than a commercial membrane with 10 times more platinum per square centimeter. 

Fuel cells are emerging as one of the alternative propulsion technologies to be used to replace the internal combustion engine. But identifying inexpensive catalysts to promote the electrochemical reactions of hydrogen and oxygen and lower the overall fuel cell system cost has proved challenging. 

Dr. Di-Jia Liu, senior scientist, chemical sciences and engineering division, Argonne National Laboratory in Argonne, Ill., says, “Oxidation of hydrogen at the anode is not as demanding a process as reduction of oxygen at the cathode. The latter reaction is more sluggish due to the much higher kinetic barrier leading to high ‘overpotential’ (i.e., the difference between the theoretical and actual operating voltages). Platinum, the desired fuel cell catalyst, is required at a three to five times greater concentration for the cathode reaction compared to that for the anode.”

The high cost and low availability of platinum is leading researchers to find alternative catalysis systems. In a previous TLT article, one such approach known as a solid oxide fuel cell is discussed (1). The cathode used is a LSCF (based on lanthanide, strontium, cobalt and iron). One of the problems with the LSCF is caused by strontium migration. Researchers minimized this issue through application of a nanoparticle coating. 

With platinum being the preferred but costly catalyst material, researchers have attempted to minimize its use in fuel cell catalysts. Liu indicates that some excellent works have been reported in further improving platinum catalyst activity through the development of unique 3D structures. He says, “Many of these approaches show excellent performance in liquid electrolyte when tested using the rotating disk electrode method but yet to demonstrate their benefits in the operating fuel cells where they are facing different mass and charge transport requirements.”

Part of the challenge in reducing the platinum content in fuel cell electrodes is how to ensure the sufficient interaction with the reactant by a limited number of platinum catalytic sites. Liu says, “As the oxygen molecules enter the fuel cell, they may not encounter platinum if the number of catalyst sites is spread thin. Therefore, oxygen molecules may not get reduced before they exit the electrode.”

An alternative option for the cathode catalyst is not to use platinum at all. The platinum-group metal (PGM)-free catalysts, by name, do not use precious metals. Instead, they use cheaper more readily available transition metals (iron and cobalt). PGM-free catalysts are inspired by oxygen reduction reactions occurring in nature such as biological systems. He says, “Hemoglobin in the human body contains a porphyrin molecule with an iron at its center coordinated by four nitrogen atoms. This molecule is effective at reducing oxygen to water as we breathe.”

In the search for finding the right catalyst to more effectively reduce oxygen in the fuel cell, Liu and his colleagues developed a catalyst using the combination of platinum and PGM-free materials derived from metal-organic frameworks (MOFs). These three-dimensional crystalline structures contain metal species joined to organic linkers. Their original use was to adsorb gases such as carbon dioxide as was discussed in a previous TLT interview (2) with the developer of MOFs, Dr. Omar Yaghi.

LP@PF
Liu’s team has been working on a PGM-free catalyst derived from a zeolitic imidazolate framework (ZIF) that has been a subgroup of MOFs for quite some time. They prepared this catalyst by heating cobalt ZIF under high temperature. Liu says, “We found that the cobalt ZIF exhibits promising catalytic activity toward oxygen reduction.” 

They also noted that this PGM-free catalyst is not as effective nor as durable as platinum. However, when they applied a very low amount of platinum on this catalyst, a platinum-cobalt alloy was produced over the cobalt-nitrogen embedded catalytic sites. The oxygen reduction reaction was enhanced through a synergistic interaction between the two. 

Liu’s team named the new catalysts LP@PF (low platinum over platinum-group metal-free catalyst). Figure 2 shows a sample of the catalyst being held by one of the researchers. 


Figure 2. A new catalyst, known as LP@PF, with a reduced level of platinum has been found to have potential for use in fuel cells. (Figure courtesy of Argonne National Laboratory.)

Liu says, “Our approach enables platinum to be spread uniformly across the ZIF-derived PGM-free catalyst with uniformly distributed active sites throughout the catalyst, leading to an efficient catalytic interaction with oxygen in the fuel cell.”

Two LP@PF catalysts were synthesized and deposited on the cathode of a membrane electrode assembly and their performance evaluated in a proton-exchange membrane fuel cell. The platinum loading was as low as 0.033 milligrams per square centimeter. In a test comparing a commercial membrane electrode assembly with 10 times more platinum per square centimeter, the LF@PFs displayed higher catalytic activity and durability.

The researchers also performed computational simulation and found that the underlining mechanism of excellent performance by LP@PF catalysts can be attributed to a synergistic catalytic reaction involving the reduction of hydrogen peroxide, which is present as an intermediate, between platinum and PGM-free active sites. “Although the study showed an excellent initial result,” Liu says, “this work only opens up a new direction in research and many challenges remain to be resolved. We will need to further improve catalytic properties so it can perform better under different fuel cell humidity and loading swing. Of course, continuously lowering the platinum usage in the catalyst will always help to reduce fuel cell cost for widespread commercialization. 

Further information can be found in a recent article (3) or by contacting Liu at djliu@anl.gov. 

REFERENCES
1. Canter, N. (2018), “Enhancing the rate of oxygen reduction in fuel cells,” TLT, 74 (6), pp. 22-23.
2. Canter, N. (2006), “MOFs: More effective gas adsorbers,” TLT, 62 (4), pp. 12-15.
3. Chong, L, Wen, J., Kubal, J., Sen, F., Zou, J., Greeley, J., Chan, M., Barkholtz, H., Ding, W. and Liu, D.J. (2018), "Ultralow-loading platinum-cobalt fuel cell catalysts derived from imidazolate frameworks," Science, 362 (6420), pp. 1276-1281.