Closing the solar fuel cycle: Investigating organic amines and water as reversible electron donors

Carnegie Mellon University, Pittsburgh PA

Investigators

Abstract

This award in the Catalysis program supports the work of Professor Stefan Bernhard at Carnegie Mellon University. This project focuses on the development of light rechargeable batteries as a cheap, practical way to capture and store the sun's energy for powering common electronic devices. The batteries are inspired by photosynthesis in plants which drives a fuel producing reaction called reduction via oxygen generation. For functional batteries to be produced, both of these types of reactions must be optimized and then combined with forward reactions representing charging, while the back reaction of the products serves as the discharge cycle. Possible reduction reactions targeted here produce hydrogen and metal products or compounds produced from carbon dioxide. To optimize the reduction, new catalysts and light absorbers are being designed to push the limits of performance and stability. On the oxidation side, the same kind of oxygen evolution in plants has already been widely studied, but here, work looks at a different chemistry, amine oxidation, as a potentially more practical alternative for use in a battery. This unexplored process can be established as a new tool for solar energy storage. Because this research project is aimed at a critical societal challenge, it inspires students to gain chemical knowledge and to become involved in chemical research. Moreover, the Bernhard laboratory continues to reach out to and involve researchers from underrepresented groups in the fields of science, technology and engineering. Educational activities at the undergraduate and graduate student level focus on the theme of renewable energy while outreach activities for non-scientists both educate the public on energy issues and instill excitement for science in general. Light to chemical energy conversion systems commonly comprise of a light-absorbing chromophore, a catalytic cycle producing reduced fuel, and a complementary oxidative cycle. Work proposed here aims to optimize these elements for unification in a solar rechargeable battery which can serve as a cheap, practical means of simultaneously capturing and storing the sun's energy. Such batteries would charge through solar driven forward reduction and oxidation while back reaction of the products would enable discharge. Work towards new batteries builds upon previous efforts in the Bernhard lab which have pioneered the use of Ir(III)-based chromophores as highly efficient drivers for photo-induced water and metal ion reduction. In the established schemes, these chromophores pass excited electrons to a reduction catalyst or substrate while receiving electrons from a sacrificial amine. This amine then undergoes irreversible C-N bond dissociation prohibiting any back electron transfer. However, translating this system to a rechargeable battery means that the amine donor must be designed to not suffer oxidation-induced degradation while still preventing back reaction. To this end, amines are being pursued with reversible electrochemistry as well as an oxidized form which is heavily stabilized via deprotonation and aromatization for prevention of back electron transfer. Initial electrochemical studies on amines pinpoint promising structures with suitable oxidation potentials. Subsequent studies in a galvanic cell then insure that the stable oxidized form of the amines can be reduced back to their original structures. Champion donors are ultimately utilized in systems where their oxidation is unified with a photo-induced reduction reaction producing metal or hydrogen fuel. Nuclear magnetic resonance and Raman spectroscopy are used to carefully monitor the fate of the amine upon illumination. While ongoing efforts are largely focused on amine oxidation, water oxidation is still a desirable counterpart for reductions in solar fuel schemes. Thus, some work is aimed at developing families of Ir(III) catalysts for water oxidation. The oxygen product of this reaction is a strong oxidant, so catalysts are being designed specifically to resist ligand oxidation for a longer lifetime. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.