We are interested in learning how complex reacting systems work, and through this work see that there are interesting patterns across nature. Understanding these patterns can help us build models for new systems by re-using model framework elements and results for one system to gain new insights to others that have similar mechanistic elements.
Some of the elements we are finding to be important include uptake and release of molecules across gas-liquid, liquid-solid and gas-solid phase boundaries, diffusion through nanoscale channels, complex oxidation-reduction reaction networks, and influences of continuously changing environments on reactivity.
We are currently funded by Department of Energy Basic Energy Sciences and the National Institutes of Health.
Durability science for artificial photosynthesis systems
Lab members: Sirui Li, Chenqi Fan, and Ethan Yu
The DOE Fuels from Sunlight Hub, the Liquid Sunlight Alliance, is focused on creating fully integrated systems that can generate liquid fuels directly from sunlight, CO2 and water. Durability of these systems is currently limited due to interfacial corrosion and catalyst remodeling, and we are working on how interfacial reactions and rare, sporadic events in the surrounding electrolyte volumes can influence photo-driven and dark reactions over time. We are particularly interested in how to think about water ion and solute chemistry in volumes too small to have a well-defined pH.
Photoelectrochemical energy conversion systems
Lab members: Thomas Cheshire, Ramzi Massad, Chenqi Fan and Jeb Boodry
Experimental collaborators: Thomas Meyer, Gerald Meyer, Andrew Moran, John Papanikolas, Kyle Brennaman (UNC). Theory collaborators: Martin Head-Gordon and Justin Talbot
Systems like dye-sensitized solar cells (DSSC) and dye-sensitized photoelectrochemical cells (DSPEC) have the potential for inexpensive, efficient conversion of sunlight into electricity and chemicals, but have performance limitations that have not been solved. By performing multiscale simulations of complete DSSC systems, we are connecting molecular-level events in the dye molecules and the electrolyte to external observables such as current density, and learning about some of the factors that affect the current flow. We have made an in-depth investigation of factors that control the kinetics and performance of porous electrodes. Through this work we learned that the internal structure of the pores play a very important role in governing the kinetics, but do not induce transport limitations under typical DSSC conditions. We have also studied the influence of electron trapping on device efficiency, and have discovered that the electrolyte plays a key role in making the system resilient to losses.
We have also analyzed transient absorption data for photoexcited dyes to extract rate constants for excitation and relaxation processes that are currently analyzed in terms of lifetimes. The work showed that ultrafast non-radiative processes appear to reduce the singlet-triplet conversion efficiency to less than unity, and that multiple states in the singlet manifold need to be considered for a complete description of the kinetics. We have also been able to extract estimates of excited state transition dipoles using the simulation results and are currently working on estimates of the charge injection timing relative to excitation events. We have used the scheme to calculate excitations under natural sunlight without charge injection, and have found significant excited state absorptions even under low intensity illumination.
Current work couples the ultrafast dye kinetics to molecular water oxidation catalysis, constructing a more detailed photoanode model for DSPEC systems. These studies have shown that progress through the catalytic cycle is inherently stochastic, and that much of the light absorbed by the dyes cannot be used to advance the chemistry because most of the catalyst states are in non-photoactive intermediates.
Permeation of small molecules through polymeric membranes
Experimental and computational collaborators: Marielle Soniat Pointer, Vy Nguyen, Adam Weber, Lien-Chun Weng and Meron Tesfaye, Dan Miller and Sarah Dischinger (LBNL).
Theory collaborator: Bill Goddard (Caltech)
At the Joint Center for Artificial Photosynthesis, a recently completed DOE Energy Innovation Hub, we focused on discovering materials for solar conversion of CO2 and water into transportation fuels via photoelectrochemical processes. These materials are integrated into test beds to assess how well they function. Test beds incorporate membranes to separate the anode and cathode chambers and block molecular crossover, thus inhibiting back reactions of CO2 reduction products, while permitting ion conduction across them. Because the flux of sunlight varies continuously during a 24 hour period, it is important to develop an understanding of how membranes (usually studied and used at steady state) respond to varying conditions. This is especially important for CO2 reduction systems, where product blocking is difficult to achieve. Knowledge of how the permeant and the membrane interact in real time will provide insight to improved membrane polymer designs.
We have developed a kinetic reaction-diffusion description for uptake and permeation of small gaseous molecules through rubbery and glassy membranes that allows non-steady-state membrane function to be simulated. The simulations are validated by new time-dependent permeation measurements, and use sticking probabilities calculated from theory to describe the gas-polymer interfacial interactions. Through this work we have learned that even nominally inert permeants interact with the polymeric matrix, influencing their solubility as a function of time. We have also investigated sorption of methanol, a typical CO2 reduction product, into Nafion. This is a strongly interacting system that undergoes significant swelling during permeation. We have found that methanol causes the polymer chains to contract as the membrane swells, increasing the space available for methanol in the polymer, which would promote undesired crossover. We have also developed rigorous experimental-computational methods for determining predictive permeabilities, using methanol permeation through Nafion as a model system.
As a final project, we examined the dehydration of Nafion over a range of relative humidities, showing that the dehydration process is a highly dynamic balance of water evaporation and desorption.
Artificial photosynthesis systems
We are very interested in how full artificial systems work, especially identifying factors that control their efficiency and lifetimes, and thinking about how science can support development of artificial photosynthesis technologies in the future. Working with teams of experimentalists and modelers, we have participated in lifecycle assessments, critical evaluations of solar fuels technology, and device studies.
Evaporation of aqueous droplets
Lab member: Connor Pollack
Experimental collaborators: Jonathan Reid and Rachael Miles (U. Bristol)
Chemical reactions in aerosols compete with evaporation when they contain volatile components either as reaction products or as solvents. In modeling such processes, it is crucial that aerosol volume changes that result from evaporation during a reaction be closely tracked since they cause concentrations and hence reaction rates in the aerosol to change continually. While the physics of evaporation has been studied for many decades, the resulting description is expressed in terms of thermodynamics or statistical mechanics, which cannot be combined directly into a typical reaction-diffusion kinetics scheme describing transformation of reactants into products. We are currently working on development of a kinetic description for evaporation of water from pure water droplets and aqueous mixtures that incorporates known physics. This description will allow us to understand the elementary processes involved, and use them to investigate reactivity in nanoscale volumes.