Farzaneh Jahanbakhshi – Postdoctoral Fellow
How to properly predict CO2 reduction reaction kinetics out of reach of experiment?
The electrochemical CO2 reduction reaction (CO2RR) to energy-rich hydrocarbons and oxygenates could contribute to sustainable fuel generation, yet efficient electrocatalysts for this reaction still do not exist. Understanding the reaction mechanism on the best metal catalyst identified so far (copper) could aid in the design of better electrocatalysts by pinpointing bottlenecks for activity and selectivity. Because in situ electrochemical mechanism determination by experiment is not currently possible, such mechanistic analysis typically is conducted using quantum mechanical simulations, specifically density functional theory (DFT). However, the exchange-correlation density functional approximations most often used to model such reactions exhibit fundamental errors, leading to the wrong adsorption site for CO on metal surfaces, thereby casting doubt on previous DFT-predicted CO2RR kinetics. The Carter group has been rigorously re-examining CO2RR mechanisms on copper, by means of state-of-the-art embedded correlated wavefunction (ECW) theory, which regionally corrects for errors inherent in DFT approximations, enabling it to describe charge transfer much more accurately than standard DFT. With rigorous ECW theory, one can properly describe electrochemistry and unearth chemical insights out of reach of experiments while predicting observables (reaction energies and barriers) fully consistent with experiments, unlike with standard DFT. My research focuses on the application of ECW theory to predict CO2RR kinetics by evaluating potential-independent surface hydrogen transfer, potential-dependent proton-coupled electron transfer, as well as C-C bond formation toward C2 products, on metal-doped copper surfaces, with the goal to uncover superior CO2RR electrocatalysts based on state-of-the-art quantum simulation techniques.
Lesheng Li – Postdoctoral Fellow
Is 2-PyH-* formation favored on CdTe(111) and CuInS2(112) surfaces?
Adsorbed 2-pyridinide (2-PyH-*) was found to be the key intermediate for the CO2 reduction reaction on diverse semiconductor surfaces. Previous research conducted in Prof. Carter’s group has confirmed that 2-PyH-*formation is thermodynamically and kinetically favored on GaP(110) and GaP(111) surfaces. The results concluded that 2-PyH-* is a valid candidate as an active co-catalyst for CO2 reduction on p-GaP photoelectrodes, acting as a viable hydride shuttle to CO2. One of my current projects aims to explore whether or not such an adsorption of 2-PyH-*also is favored on the other two technologically important semiconductor surfaces, i.e., CdTe(111) and CuInS2(112). Density functional theory is employed here to calculate the reaction free energy and activation free energy for the formation of adsorbed 2-PyH-* on the CdTe(111) and CuInS2(112) surfaces. By comparing the results from different semiconductor surfaces, it may provide us with a deeper insight of the distinct performance of these electrodes.
Mark Martirez – Assistant Project Scientist
Can we use light to speed up N2 dissociation for NH3 synthesis?
Ammonia (NH3) is one of the most essential agricultural compounds being used as the primary source of nitrogen in fertilizers. The Haber-Bosch process is a time-tested synthetic method for NH3 in large quantities from N2 and H2. While the reaction of N2 and H2 gas is thermodynamically allowed at room temperature and pressure, the initial dissociation of both reactants required to facilitate their combination is highly unfavorable, both thermodynamically and kinetically. Although a transition metal catalyst, such as Fe, may lower the barrier for the dissociation of N2, high temperature is still required to hasten this process, which consequently demands high pressure to remain nearly spontaneous. Through first-principles methods, I investigate alternative metal catalysts that could not only lower dissociation barrier for N2 but also harness the energy of light, via plasmon resonance, to facilitate this process. With light instead of heat as an additional driving force, a high-dissociation rate at lower temperatures may be achieved, which would then relax the need for higher pressures and thus improve the energy efficiency of the process overall.
Xuelan Wen – Postdoctoral Fellow
What is the mechanism of plasmon-enhanced photocatalysis on antenna-reactor complexes?
Traditional plasmonic metals (Au/Ag/Cu/Al) have limited surface chemistry, while conventional catalytic metals (Pd/Pt/Ru/Rh) are poor optical absorbers. In antenna-reactor complexes, a catalytic reactor is placed adjacent to a plasmonic antenna. This combination dramatically increases photocatalytic efficiency, and sometimes selectivity too. Using various first-principles quantum mechanics techniques, such as density functional theory (DFT) and embedded correlated wavefunction (ECW) theory, we want to understand the reaction mechanisms involving ground state and multiple excited states. These insights further help us design better photocatalysts.
Robert Wexler – Postdoctoral Fellow
How can we improve the performance of solar cells?
Every hour, the sun delivers more energy to the Earth than we consume in a year. As such, the potential impact of solar energy is unmistakable as it far-exceeds global energy needs while simultaneously combating global warming due to its sustainability and carbon-neutrality. To capitalize on this opportunity, the scientific community has spent several decades searching for materials that efficiently absorb sunlight. A number of solar technologies have been commercialized, most notably those based on Si and Ru dyes. These and other promising materials, however, are unable to completely supplant fossil fuels because they are too expensive and/or toxic. Consequently, sunlight absorption, cost, and toxicity all must be considered concurrently to design an efficient, scalable, and environmentally friendly solar infrastructure. Recently, there has been growing interest in the solar material Cu2ZnSnS4 (CZTS), which contains inexpensive and nontoxic elements and possesses ideal sunlight absorption characteristics (namely, a band gap of ~1.4-1.6 eV). Unfortunately, under processing conditions, atoms can exchange sites (e.g., Zn and Sn), which leads to band gap reduction and the formation of localized electronic states in the band gap that can trap charge carriers. A fundamental theoretical understanding of these so-called antisite defects is still lacking, thus hindering the design of improved CZTS-based materials. In my research, I apply density functional theory calculations and thermodynamics both to enrich understanding of defect stability and structure and to identify promising doping schemes to limit the formation of defects detrimental to solar cell performance.
Qing Zhao – Postdoctoral Fellow
What is the mechanism of CO2 reduction reaction in heterogeneous catalysis using embedded correlated wavefucntion theory?
The electrocatalytic CO2 reduction reaction to useful C2 hydrocarbons products remains a challenging problem in heterogeneous catalysis. Density functional theory (DFT) provides unique insights into identifying reaction mechanisms in catalysis modeling. However, the approximations made in semi-local DFT produce delocalization errors that limit its predictive accuracies for determining reaction energetics and kinetics. In contrast, correlated wavefunction (CW) theory is more accurate, but the high computational cost prohibits its applications to large systems. Instead, embedded CW theory, in which the system is split into two subsystems and the catalytic center is treated with CW theory and the environment is studied with efficient DFT, is used to understand the mechanisms of CO2 reduction reaction and to design new catalysts.