(Invited) On the Effective Design of Electrochemical Interfaces for Solar Production of Fuels

Extensive research has been conducted on man-made systems for the solar production of fuels in which many different chemical and physical constructs and prototypes have been assembled to demonstrate the possibility of creating fuels from sunlight. These efforts comprise a broad array of chemical and electrochemical work that originates in the Fujishima and Honda work on photoelectrochemical water splitting and has moved on to molecular-based catalysts for water splitting as well as designed nanostructures of semiconductors for light absorption and charge separation. This presentation examines how the mechanistic design of these systems for solar fuels production constrain and define the chemistry and electrochemistry of the solid and molecular components required for effective operation. It will also identify alternative approaches that show promise. The discussion will begin with design analyses of the photosynthetic system for carbon fixation and man-made photosystems for the oxidation of water as a sacrificial electron donor for fuels production. It will be shown how the kinetics inherent in their operational design imposes demands upon the photosystem components, in particular the catalyst TOF and TON figures. With this collection of designs, full structures for water or carbon dioxide reduction can be composed and consequent demands upon catalysts can be quantified. While many research systems for solar photoconversion are given as examples of “artificial photosynthesis,” few actually take full advantage of the important design principles that underlie the effectiveness of photosynthesis as a means of transducing solar energy into the biochemistry of a life form. A tracing of the flow of energy and charge through the photosynthetic reaction centers and then to RuBisCO for CO2 fixation reveals the significance of impedance matching at various coupling points in the system. A direct coupling of Photosystem II with the oxygen evolving catalyst is evident as is indirect coupling at other points where protons and redox agents are produced and their diffusion creates a Nernstian concentration potential at distant locations to initiate further reaction. These types of coupling are of consequence in man-made systems. The oxidation of water provides the electrons and protons for solar driven reduction reactions, whether it be natural or artificial. Nature has its own oxygen evolving complex in photosynthesis to effect this reaction and there are many man-made structures that have been assembled and studied to oxidize water as well. A kinetic design of the natural system will be drawn in this presentation that will be compared with the kinetic designs of representative artificial systems and a critique will be made. These include semiconductor photoelectrolysis of water, dye sensitized electrochemical water oxidation, molecular water oxidation catalysts, and particulate and nanostructured systems for water splitting. The relationship between the kinetic design of the system and the component solids, catalysts and associated species will be shown. The catalytic reduction of CO2 or water to a fuel are wide spread and dynamic areas of research. Whereas the electrochemistry of water reduction to hydrogen has long been an object of study, the catalysis of CO2 reduction is much less developed. Molecular catalysts are being sought for this role as are interfacial metal deposits with both being accompanied by theoretical backing. Given the recognition and analysis of the various designs identified in this discussion, a more effective effort can be made in the development of photosystems for production of these fuels. They can help define not only the form of the catalyst, be it solid or molecular, but also its required reaction velocity and durability.