Effect of Transition Metals on the Oxygen Reduction Reaction Activity at Metal-N3/C Active Sites
Graphical Abstract
“Understanding the role the metal plays in M-N3/C active site. Using a terpyridine-based molecular receptor, the surface of the carbon support can be modified with highly active pyridinic nitrogen. The N3/C site is used to assess the effect of altering the metal center, using transition and post-transition metals, in terms of the activity for the oxygen reduction reaction…“ Learn more about the story behind the research featured on the front cover in this issue's Cover Profile. Read the corresponding Article at 10.1002/celc.202000954.
Abstract
Invited for this month's cover picture are the groups of Brad Easton and Olena Zenkina at Ontario Tech University (Canada). The cover picture shows an artistic dipcition of a treasure map quest for unique M-N3 non-platinum group metal fuel cell catalysts. Read the full text of the Article at 10.1002/celc.202000954.
What was the inspiration for this cover design?
The search for better fuel cell catalysts is in many ways a quest for treasure, though we are now seeking to search for something other than noble or tradition platinum group metal (PGM). The cover image represents our exploration of Fe-C/N catalysts system, specifically terpyridine (terpy) units covalently anchored to Vulcan carbon support (V-tpy). Our ship of exploration is named after the lead author in this study (and yes she does have a terpy tattoo). Coming ashore on Vulcan Carbon Island we have explored how the terpyridine motif interacts with non-precious metals, which are visualized as gems on top of the pyramids. Interestingly, V-tpy system cannot hold Sn, instead, tin oxide species were found on the surface that is visualized as magenta spheres at the foot of the pyramid. Like any good treasure map, there are places to visit, pitfalls to avoid, and uncharted place that still needs to be explored. The cover image was designed Dr. Ebralidze in collaboration with the other authors.
Why is the proposed N3-M catalytic cite is interesting and how it was designed and incorporated into the support material?
Many groups that work in the field use drastically different strategies and methodologies to prepare nitrogen-enriched catalysts that involve extensive heating. That causes a wide distribution of various catalytic sites on the supports with the major active site suggested to be N2+2/M motif. Some groups have proposed the formation of an N3 site being essentially a “defect site”, where an N2+2 is missing a N. In our work, the N3 site is not a defect, rather it is done by design. Our system is rationally designed in the way that we introduce molecularly defined N3 motifs into the support under mild reaction conditions and use coordination chemistry to introduce a metal center in the N3 motif pre-immobilized on the surface. The strong coordinative affinity between chelating terpyridine unit and transition metals (Fe, Ni, Mn or Co) defines the geometry of this catalytic active site.
Is your current research mainly curiosity driven (fundamental) or rather applied?
I would say that we are most interested in research that combines both fundamental and applied research. Many so-called applied research areas are often faced with challenges that are truly fundamental in nature. In the case of fuel cells, reducing electrocatalyst costs can often be seen as rather applied challenge, especially when researchers are striving for performance targets for automotive applications. However, we have taken a more fundamental approach, using a surface modification methodology to create model catalytic systems so that we can better understand how certain catalytic sites works. An in-depth mechanistic understanding of the catalytic performance of well-defined active sites may open the doors to successful rational design, fine-tuning, and optimization of the next generation of catalysts to grant wide practical applications. Our goal is that the fundamental methods and knowledge can help advance the applied research into higher performing non-PGM catalysts.
How did the collaboration on this project start?
The Easton and Zenkina labs began collaborating in 2016. At the time Zenkina's group had been doing work on modifying transparent surfaces with terpy-derivatives for metal ion sensing. The modified surfaces would undergo a distinct colour change when metal ions were coordinated to molecularly defined terpy units, allowing for optical sensing of different metal ions. Easton's group had been working in electrocatalysts for fuel cells and sensors for a while and our conversions led us to believe these terpy-modified surfaces would show very interesting electrochemical properties. Soon after we started working together on applying this interdisciplinary approach into the creation of stable and atom efficient metalorganic/inorganic electrochromic devices, where an applied potential can selectively and reversibly turn on and off the colour. From there, we began co-supervising a PhD student (Holly Fruehwald) who expanded this into the modification of fuel cell catalyst supports.
