Beating platinum is akin to finding the Holy Grail for material scientists working on fuel cells. We know very little about the microscopic workings of catalysis, but platinum – beautiful, shiny, and and far too expensive to base an economy on its production – has always topped the charts in terms of catalyzing chemical reactions at the electrodes in fuel cells. That’s too bad, because it would be great to find a cheap, earth-abundant material that works well catalyzing (speeding up) the reactions we need for effective fuel cells, photocatalytics, etc.
Enter an exciting new study published in ACS Nano! Rice University researchers have developed an electrode to use in fuel cells that replaces platinum with graphene quantum dots (GQDs) adhered to flakes of graphene and doped with boron and nitrogen. The performance, in terms of onset potential and current densities, are as good as platinum. The beauty of this study is that the carbon-based electrodes can be made from coal and graphite, two extremely abundant materials that are orders of magnitude cheaper than platinum. This is a huge step in the right direction for getting fuel cells market-ready!
Making these electrodes seems like a beautifully simple process. Here’s the basic flowchart:Start with some coal. Oxidize it by placing it in a mix of sulfuric and nitric acid to create 15-20 nm GQDs (this process is actually a bit novel and developed by the Rice group. I believe placing the coal in acid allows the acid to attack some edges/defects/broken bonds in the coal to create the GQDs). The authors then mixed the GQDs with graphene oxide (GO) in solution and heated for 14 hours (the hydrothermal step above). Graphene oxide is graphene – a one-atom thick layer of carbon atoms in a honeycomb – with functional groups like COOH attached to it. These strongly interact with the GQDs and lead to the dots peppering the GO surface. (bottom left on figure above). The bottom right of the figure shows a zoomed-in graphene quantum dot, with defects in the graphene lattice, and shows an example of the oxygen reduction reaction happening red that’s crucial to speed up for high-performing fuel cells.
Finally, this structure was heated up for varying amounts of time with boron and nitrogen to create the B- and N-doped GQDs. This can help with the catalytic activity when boron nor nitrogen replaces carbon in the GQDs, creating more favorable sites for reducing oxygen and improving performance. Here’s a transmission electronic microscopy image of the final product:See all those sharp lines and edges? That’s the beauty of the graphene quantum dots. More edges mean more places for oxygen to reduce, so all these GQDs provide much more surface area and edges to chemically favor this reaction. That’s the basic idea behind this type of electrode construction! The authors found they needed just the right ratio of GQDs to GO – too much GQDs would lead to the dots aggregating together and reducing the amount of edges showing. So the graphene oxide was crucial as a substrate to then allow these GQDs to show all their sides, if you will.
And, finally, how did the electrodes perform at reducing oxygen? The key parameter here is the kinetic current density, which the authors use to represent catalytic activity. Higher kinetic current density means more oxygen reduction reactions happening per unit time, release more electrons per unit time, meaning more current. Here’s a graph comparing several types of electrodes listed along the x-axis and their respective kinetic current densities along the y-axis:
The electrodes are labeled based on their composition: DF means dopant-free (no B or N), N or B stand for nitrogen and/or boron doping, the numbers stand for the minutes of heating the GQD/GO with the boron/nitrogen. The far left shows the platinum/carbon (Pt?C) state-of-the-art electrode for comparison. You can see that the GQD/GO electrode with BN doping and 30 minutes annealing performs better than platinum! Annealing time and the use of boron and nitrogen are definitely crucial for high performance, base on this data, as the dopant-free (DF) electrode shows much lower activity. Annealing too short (10 minutes) or too long (60 minutes) both decrease performance as well. Reasons for this are not yet understood and will require some careful experimental and computational work to understand what’s happening at the atomic/molecular level with this doping and heating.
Also notice that removing the quantum dots also severely hinders performance (second bar from the right). This emphasizes the importance of the edge states that the GQDs provide as additional catalytic sites to quicken the reduction reaction.
Remember, performing as well as platinum is a huge achievement. The main barrier to using platinum for large-scale production is its price and scarcity, so finding something cheap that performs at the same level is excellent!
So, all in all, great news on the material science front. I think fuel cells are a great future option when thinking about alternative energy for transportation. We’re doing a good job finding other energies, such as natural gas and renewables, for electricity generation, but the bulk of our oil consumption is still in the transportation sector, and our consumption does not seem to be going down much. Cheaper fuel cell technology is a great path to pursue to continue to remove this dependence on fossil fuels.
Fei, H., Ye, R., Ye, G., Gong, Y., Peng, Z., Fan, X., Samuel, E., Ajayan, P., & Tour, J. (2014). Boron- and Nitrogen-Doped Graphene Quantum Dots/Graphene Hybrid Nanoplatelets as Efficient Electrocatalysts for Oxygen Reduction ACS Nano DOI: 10.1021/nn504637y