The global energy and environmental crises call for humanity to act quickly and wisely. Harvesting and using energy more efficiently are among the top priorities. Clean energies, especially hydrogen fuel generated from renewable energy resources, are becoming increasingly important.
Electrocatalysis' role here is to use the electrons generated from solar or wind energy to run the chemical reactions needed to produce sustainable fuels. Decades of great efforts from the electrocatalysis community have been put into seeking more efficient catalysts. Most efforts focus on regulating the interactions between the catalysts and the reaction intermediates. Common strategies include nanostructuring and alloying. As an electrochemist with a physics degree, I had also been looking at the atomic structures of the active sites and considering mostly strain and ligand effects.
But what about playing with the intrinsic properties of the electron itself?
Ever since I joined the Lingenfelder Lab at EPFL, the broad experience of Magalí (Maggie), especially in solid/liquid interfaces, self-assembly and chirality, has led my attention to unconventional strategies to boost electrocatalytic reactions using hybrid materials. The pioneering works from the group of Prof. Naaman showed the potential of controlling electron spin polarization via chiral-induced spin selectivity (CISS) effect in electrocatalytic oxygen evolution.1,2 At the beginning, I was both impressed and skeptical because there are so many factors contributing to electrocatalytic activity that it is difficult to ascribe them to a single mechanism! Therefore, we decided to set our own investigation into electron spin polarization effects. We designed experiments to decouple all the other possible effects and look for a guideline for optimal activity enhancement by introducing chiral layers on already state-of-the-art catalysts.
The samples consisted of three key parts: the substrate, the catalyst, and the spin polarizer. Our partner synthesized state-of-the-art 2D Ni- and NiFe-based materials as the catalysts.3,4 The "homemade" chiral carbohelicenes with high racemization barriers and rigid structure served as the spin polarizers. To ensure the stability of the samples, we used Au(111) thin films as the substrates. With all materials selected, we started the first set of experiments. We quickly realized that a stable attachment of the helicene molecules on the samples is essential for a valid assessment of their effects. Therefore, we added thiadiazole groups onto the helicene molecules to strengthen the stability. As expected, the functionalized helicene molecules can make stable characteristic self-assembly layers on the Au substrate (Figure 1a and b). Afterwards, we obtained the first good news: adding chiral helicene molecules significantly improved the oxygen evolution reaction (Figure 1c). However, such improvement came with careful positioning of the molecules. Directly depositing helicene molecules on a catalyst surface blocked the active sites and hindered the reaction (Figure 1d).
To firmly ascribe the activity enhancement to the chirality of the helicene molecules, we tested their achiral imposter: a molecule that contains the "footprint" of the chiral helicene but doesn't have the helical structure. It has the same bonding with the substrate and the catalyst. However, it provided no enhancement to the reaction. The next concern was the influence of the chemical composition of the catalyst and the electrolyte. After testing different catalysts in electrolytes with and without Fe impurities, we confirmed that the effect was fully related to the chirality of the molecules.
We also have good news for researchers struggling with reactions having too complicated reaction pathways and product selectivity issues. For spin-selective reactions, chiral molecular functionalization can control "spin-forbidden" pathways and significantly improve product selectivity.
As Richard Feynman visionarily talked about the nanoscales: "There's Plenty of Room at the Bottom: An Invitation to Enter a New Field of Physics"4, we started this study with many doubts, and we are having many exciting findings! The electron spin-related mechanisms should not be neglected in electrocatalysis. Moreover, the use of chiral molecules to control electron spin polarization seems more direct and efficient than the use of an external magnetic field. We expect this work will be an inspiration for many new attempts. There is plenty of room for chiral molecules to boost achiral reactions via spin selective chemistry! We are currently working on CO2 electroreduction and ammonia synthesis, and we would like to encourage other researchers to join our journey.
For more details, please read our recent publication in Nature Communications:
1 Mtangi, W. et al. Control of Electrons' Spin Eliminates Hydrogen Peroxide Formation During Water Splitting. J. Am. Chem. Soc. 139, 2794–2798 (2017).
2 Zhang, W., Banerjee-Ghosh, K., Tassinari, F. & Naaman, R. Enhanced electrochemical water splitting with chiral molecule-coated Fe3O4 nanoparticles. ACS Energy Lett. 3, 2308–2313 (2018).
3 Song, F. & Hu, X. Exfoliation of layered double hydroxides for enhanced oxygen evolution catalysis. Nat. Commun. 5, 4477 (2014).
4 Feynman, R.P. There's plenty of room at the bottom: An invitation to enter a new field of physics. Miniaturization, Reinhold. (1961).