Goal 2
Discover photon, electron, and molecular processes, from light excitation to the catalytic center: realizing ensembles of molecules and materials that achieve efficiency and selectivity beyond those available with conventional electrochemical processes.
The State of the Art in 2020: There has been extensive study of key individual electrochemical reaction pathways involving CO₂. These studies have provided important insights, but in general the electrochemical mechanisms are not well-understood and therefore not well-controlled. There have also been investigations of charge carrier generation and factors that influence mobility within semiconductors, which are usually the main source of electrons and holes for solar fuels generation. These charge carriers drive reactions that are primarily electrochemical at the catalytic center, and the full spectrum of sunlight is not fully utilized.
LiSA’s Research: A fundamental challenge in solar-to-chemical energy conversion involves the control of mul-tiple-electron/multiple-proton chemical transformations at catalytic sites in order to control selectivity. These transformations involve transfer of energy from short-lived excited states created within light absorbers through multiple heterogeneous interfaces to the catalytic interface. Carrier generation, relaxation, transport, and storage must be understood and con-trolled to accumulate the requisite charge at a catalytic site to drive chemical reactions with high selectivity compared to competing degradation reactions such as corrosion. Light is a powerful and unique tool that can be utilized beyond its primary role of creating separated charges within semiconducting photoabsorbers. For example, it can be used to preferably shift the energy landscape or conditions of different microenvironments in both time and space to achieve deterministic control of the synthesis of liquid products. Specific wavelengths of light can selectively initiate or accelerate events that would be highly unlikely to occur otherwise. Techniques for timed addition or removal of electrons, holes, protons, or hydroxides into or near catalytic centers using light can trigger favorable chemistry. Realizing these strategies in the integrated systems described in Goal 1 requires establishment of a scientific foundation that LiSA is building through development and application of experiment and theory to bridge time and length scales from photo-generation of carriers and photo-modulation of molecular function to catalysis. Management of systems of photo-mediated microenvironments, whose co-design and cooperative functions will enable control of the dynamic energy landscape of chemical reactions, facilitates photocatalyst discovery and durability efforts under Goal 3.
Team Contributions: LiSA’s Photodynamics team leads work exploring chemical dynamics in space and time, and the use of light to control reactivity in microenvironments. The photoactive Materials team designs and evaluates the photoactivity of anode and cathode photocatalysts. The Chemical Microenvironments team establishes mechanisms for light-driven transport and works with the Durability team on establishing the dynamics underlying degradation processes within microenvironments. Using the mechanistic understanding, the Systems and Integration team develops ways to control photoelectrode potential and current to control catalytic selectivity.
Artwork: Darius Siwek