Researchers in the US and China have developed a catalyst that solves three key problems long associated with direct ethanol fuel cells (DEFCs): low efficiency, the cost of catalytic materials and the toxicity of chemical reactions inside the cells.

The team from the University of Central Florida, Oregon State University, the University of Pittsburgh, and Southern University of Science and Technology, Shenzhen, China, found that putting fluorine atoms into palladium-nitrogen-carbon catalysts had a number of positive effects—including keeping the power-dense cells stable for nearly 6,000 hours. The findings from the study are published in Nature Energy.

The local coordination environment (LCE) has recently been shown to play a vital role in boosting the reaction kinetics in many emerging electrocatalytic systems that are traditionally considered to be ‘sluggish’. For instance, the widespread implementations of high-energy direct ethanol fuel cells (DEFCs) are always impeded by the ‘sluggish’ 12-electron ethanol oxidation reaction (EOR) and 4-electron oxygen reduction reaction (ORR).

A classic design is to improve the ORR kinetics by embedding metal–nitrogen (M–N) active moieties within the conducting carbon supports, forming M–N–C coordination. Other heteroatoms (X = P, S, B and so on) have also been embedded into carbon supports, forming M–X–C coordinations to improve the ORR kinetics, most likely via interatomic synergism. The LCE has been suggested as a prominent regulator of low-dimension catalysts such as single atomic site catalysts owing to their high sensitivities to LCE. However, the fundamental question regarding how to implement the concept of LCE in large-dimension catalysts (that is, nanomaterials and commercially available materials) is still unset-tled. The prerequisite for EOR (that is, at least three continuous atomic sites) makes it daunting to controllably regulate the LCE.

… Herein, we designed an efficient strategy to regulate the LCE and create catalytic M–X moieties in the M/X–C catalysts, in which F coordination was introduced to weaken the C–X bonds and drive the X atoms to metal sites. Particularly, we used classic Pd/N–C as a model catalyst to validate the concept and explore the underly-ing materials chemistry, due to its bifunctionality towards EOR and ORR. … The strategy also proved efficient and versatile for regulating the LCE of other Pd/X–C (X = P, S, B) and commercially available catalysts (Pd/C and Pt/C).

—Chang et al.

The researchers are in the process of soliciting funding to develop prototypes of DEFC units for portable devices and vehicles.

If this is successful, we can deliver a device for commercialization in five years. With more industrial collaborators, the DEFC vehicle can be implemented in 10 years, hopefully.

—Zhenxing Feng of the OSU College of Engineering

The first vehicle powered by an ethanol-based fuel cell was developed in 2007, Feng said.

However, the further development of DEFC vehicles has significantly lagged due to the low efficiency of DEFC, the costs related to catalysts and the risk of catalyst poisoning from carbon monoxide produced in reactions inside the fuel cell.

—Zhenxing Feng

To tackle those problems the research team developed high-performance palladium alloy catalysts that use less of the precious metal than current palladium-based catalysts.

Our team showed that introducing fluorine atoms into palladium-nitrogen-carbon catalysts modifies the environment around the palladium, and that improves both activity and durability for two important reactions in the cell: the ethanol oxidation reaction and the oxygen reduction reaction.

Advanced synchrotron X-ray spectroscopy characterizations made at Argonne suggest that fluorine atom introduction creates a more nitrogen-rich palladium surface, which is favorable for catalysis. Durability is enhanced by inhibiting palladium migration and decreasing carbon corrosion.

—Zhenxing Feng

Supporting this research were the National Science Foundation and the US Department of Energy.

Resources

• Chang, J., Wang, G., Wang, M. et al. (2021) “Improving Pd–N–C fuel cell electrocatalysts through fluorination-driven rearrangements of local coordination environment.” Nat Energy doi: 10.1038/s41560-021-00940-4