In an article recently published in ACS Applied Materials & Interfaces, investigations were made to uncover where the active sites are located in Oxygen Reduction Reactions (ORR) as well as how they operate in Platinum-Gadolinium and Platinum-Praseodymium catalyst reactions for hydrogen fuel cells.
Study: Revealing the Nature of Active Sites on Pt–Gd and Pt–Pr Alloys during the Oxygen Reduction Reaction. Image Credit: Corona Borealis Studio/Shutterstock.com
Significance of Hydrogen Fuel Cells
Hydrogen acts as a renewable fuel that produces water when employed in a fuel cell. Hydrogen may be obtained from a variety of naturally occurring sources, including hydrocarbons, biomass, as well as green energy sources such as wind or solar power. These features make it a desirable component for locomotion and energy generation.
A fuel cell harvests electrical energy from the chemical energy stored in hydrogen as well as other constituents in an effective and clean manner. Only heat, water, and electricity are formed when the employed fuel source is hydrogen, thus making it highly desirable for green uses. Fuel cells are not uncommon as they use a broad assortment of fuels and are power sources for massive components such as power plants or as tiny as a smartwatch.
Working of Fuel Cells
A fuel cell is constructed using a set of anode and cathode (negatively and positively charged, respectively) surrounding an electrolytic cell. The fuel source accumulates, such as hydrogen at the anode, while air may be used at the cathode. Catalysis occurs inside the electrolytic cell, splitting hydrogen molecules into their primary constituents; electrons and protons travel to the negatively charged anode and positively charged cathode, respectively.
Electricity is observed inside the cell, which may be correctly attributed to the flow of electrons. Meanwhile, protons travel inside the cell to reach the cathode, after which they react with oxygen and the already accumulated electrons to evolve heat and form water.
Presently, hydrogen fuel may be manufactured in numerous ways. A thermal process, namely, natural gas reformation and electrolysis, are among the most effective methods. To generate this hydrogen, processes such as steam reforming are used. It is an elevated temperature process where steam reacts with hydrocarbons to create hydrogen, making it a widespread method for the generation of hydrogen.
Problems With Large Scale Production
Although hydrogen fuel cells have many industrial applications, commercial production poses another challenge. It is worth mentioning that oxygen reduction reactions, which involve slow reaction kinetics, are the limiting factors for the large-scale application of hydrogen fuel cells.
As of now, platinum-based catalysts are the only catalysts that have demonstrated sufficient stability and performance. Alloying Pt with other metals is a well-known method for increasing efficiency while lowering Pt loading. The working of the active sites, however, is critical for catalyst tuning.
Producing nanoparticles of specific sizes and precise shapes with an increased density of active centers accessible to reactants as well as reaction intermediates is one way to boost the efficiency of Pt-based catalysts. The evaluation of active sites, on the other hand, is critical for developing effective design solutions.
An Investigation into the Mechanism of Active Sites
The authors of this paper probed the nature of ORR active zones in acidic fluids by using electrochemical scanning tunneling microscopy (EC-STM). Localized oscillations inside the EC-STM signal are detected by this approach, indicating changes in local activity. It was found that the Pt-based lanthanide terraces are the heaviest contributors to the whole reaction, according to in-situ studies confirmed by density-functional calculation-based coordination charts.
Advanced-coordination sites, such as those located at the lowest step concavities and edges, are generally passive. Lower coordination sites, such as around the top of step edges, display more activity due to steric and strain hindrance effects.
As a result, traditional "round" nanoparticles should outperform their convex and faulty alternatives.
The Future – What To Look Forward To?
The information obtained here should help to improve the electro-catalytic activity of alloyed nanocomposite Pt-lanthanides. It is also evident that an understanding of the characteristics of active sites for every unique catalysis material is critical for effective catalyst design. Surface-sensitive approaches that provide access to active areas and simulation methods based on structure-sensitive descriptors are also needed.
Kluge, R. M., Psaltis, E. et al. (2022, April 21). Revealing the Nature of Active Sites on Pt−Gd and Pt−Pr Alloys during the Oxygen Reduction Reaction. ACS Applied Materials & Interfaces. Available at: https://pubs.acs.org/doi/10.1021/acsami.2c03604