Session M21: Energy Related Catalysis and Photo/Thermo effects

High throughput theoretical and experimental screening of photocatalysts for water splitting

We apply a high-throughput computational screening process with feedback from experimental results to identify promising photocatalysts for water splitting. Using an automated linear response framework for predicting Hubbard parameters, we overcome the limitations of local and semilocal density-functional theory calculations in predicting band gaps. From our computational screening, we identify 28 promising photocatalysts, many of which have not been previously reported. We then synthesized some along with chemical derivatives from our selection, for a total of 18 synthesized materials using solid-state synthesis techniques. We report hydrogen production in a number of the materials tested for photocatalysis using gas chromatography; many of which have been previously unreported for water splitting. We further measure cyclic voltammograms, demonstrating high levels of photoactivity in several samples.   

High-Throughput DFT of Solar Thermochemical Perovskite Oxides: Comparison of Computed and Experimental Thermodynamics

Thermochemical water-splitting (TWS) offers a renewable alternative to fossil fuels by utilizing solar energy for the production of hydrogen via a two-step redox reaction sequence involving a metal oxide. Current and past efforts have been aimed at identifying the best compounds for such reactions based on thermodynamic and kinetic properties. In this context, high throughput density functional theory (HT-DFT) represents an attractive tool for a quick and efficient refinement of the pool of potential candidates. This work concentrates on ABO3 perovskite compounds, conducting a high throughput study of their reduction enthalpy, and selecting a group of 12 compounds for a critical, quantitative comparison with experimental data generated by our collaborators. Building on, and significantly extending, a previous HT-DFT study, we have confirmed the use of high throughput DFT as a reliable method for screening for potential candidates for water splitting reactions, while also demonstrating the importance of choosing the appropriate structure for the compound under investigation. Furthermore, we provided predictions for several promising compounds, highlighting the crucial role played by the B site cation and its oxidation state.   

Increased Hydrogen Production of Perovskite Solar Thermochemical Water Splitters by Joint Reduction on A and B Sites

Oxide based solar-thermochemical hydrogen (STCH) production operates by cyclic creation of oxygen vacancies that then react with steam to produce hydrogen. STCH performance is strongly dependent on both the change in vacancy stoichiometry (��) during cycling (����) and the output gas ratio of H₂/H₂O. Perovskites (ABO₃) have been investigated by controlled synthesis and X-ray adsorption spectroscopy (XAS) as STCH oxides in-part due to their tunable cation-dependent oxygen vacancy enthalpies. Classically, only one site, either A or B, has been responsible for charge compensation of oxygen vacancies. However, in this work we investigate water splitting by computationally designed perovskites expected to possess dual reduction on both A and B sites, which is predicted to increase vacancy formation, and hence performance, through increased formation entropy of vacancies. In-operando X-ray absorption spectroscopy (XAS) results at Ce L-edge and Mn and V K- and L-edges will demonstrate changing oxidation states of A and B site elements. Additional in-operando diffraction and mass loss experiments will correlate ���� and structural changes with cation reduction. This work will demonstrate the effectiveness of dual site reduction as a design criterion for STCH material design.   

Photocatalytic Reaction driven by hot electrons on Plasmon-resonant Grating Nanostructures

We demonstrate hot electron injection in a Ag-based plasmon resonant grating nanostructure. By varying the incident angle of irradiation, sharp dips are observed in the photoreflectance with p-polarized light when there is wavevector matching between the incident light and the plasmon resonant modes of the grating. With 633nm light, we observe a 12-fold enhancement in the photocurrent between resonant and non-resonant polarizations at incident angles of ±7.6^o from normal. At 785nm irradiation, we observe similar resonant profiles to those obtained with 633nm wavelength light but with a 44-fold enhancement factor. Using 532nm light, we observe two resonant peaks (10X enhancement) in the photocurrent at 19.4^o and 28.0^o incident angles. Finite difference time domain (FDTD) simulations of these grating structures confirm the resonant profiles observed in the angle-dependent spectra of these gratings and provide a detailed picture of the electric field profiles on and off resonance.   

Using density functional theory to evaluate Ca-Ce-M-O (M = 3d transition metal) oxide perovskites for solar thermochemical applications

Solar thermochemical (STC) processes that use redox-active, off-stoichiometric, transition-metal oxide substrates to split water and/or CO₂ are an efficient way to generate reusable fuels or fuel precursors using concentrated solar flux. However, STC processes require oxides that are thermally stable, tolerate high oxygen off-stoichiometry, and be resistant to adverse phase transformations. In this work, we explore the chemical space of Ca-Ce-M-O (M=3d transition metal) oxide perovskites as potential STC candidates using density functional theory based calculations. Specifically, we use the strongly constrained and appropriately normed (SCAN) functional, with an appropriately determined Hubbard U correction to reduce the self-interaction errors of the highly-correlated 3d and 4f electrons within M and Ce, respectively. While, we consider Ca and Ce on the A site (in an ABO₃ perovskite framework) because of their similar ionic radii and the potential redox-activity of Ce, we consider all 3d transition metals except Zn on the B-site. Subsequently, we evaluate the oxygen vacancy formation energy, electronic properties, and thermodynamic stability of ternary Ca-M-O, Ce-M-O, and quaternary Ca-Ce-M-O perovskites and identify promising candidates that can improve STC efficiencies.   

Nanoparticle Enhancement of Plasma-driven CO2 reduction to Higher-order Hydrocarbons

By discharging ns high voltage pulses across an insulating substrate containing metal nanoparticles, we observe a significant enhancement in the generation of plasma through local field enhancement on the surface of the nanoparticles. Electromagnetic simulation shows local field enhancement on the order of 10X. Since the plasma is initiated by field emission of electrons, which depends exponentially on the electric field, we believe that this 10-fold increase can result in several orders of magnitude increases in the generation of plasma. By discharging ns high voltage pulses in CO₂-saturated water, we observe CO₂ reduction to higher-order hydrocarbons. Here, the plasma emission spectra exhibit Swan bands, which correspond to C₂ species, indicating that, in addition to reducing CO₂, C₂-species are formed, presenting the exciting possibility of converting a notorious greenhouse gas into an energy dense hydrocarbon fuel. We have also performed cryogenic NMR spectroscopy of various products in water and liquid ion chromatography(IC) ex-situ. Here, we observe clear peaks corresponding to formic acid, and acetic acid, which corresponds to a C₂-hydrocarbon species. We have also observed the presence of oxalates (i.e., C₂O₄²⁻), which is C₂ species, at approximately 150g/L using IC.   

Excitonic Effects in Absorption Spectra of Carbon Dioxide Reduction Photocatalysts

We study the quasiparticle bandstructure and excitonic properties of 52 selected materials. These materials were recently shortlisted for their potential as a photocatalyst in CO₂ reduction, through rigorous first-principles computation-based screening strategy. Many body perturbation theory within GW approximation has been used to explore the electronic structure of these materials. We use state-of-the-art Bethe-Salpeter formalism to inspect the excitonic effects. A high-throughput computational workflow using the “atomate” package has been used to perform the GW-BSE calculations and analyze the results. We validate our results with 10 previously studied materials found in literature and report the results for 42 promising unexplored materials. Furthermore, our study investigates the suitability of these materials in applications such as CO₂ photo-reduction, efficient solar cells, etc. by examining their absorption spectra and excitonic properties.   

Understanding the C1 Selectivity Descriptors in Electrochemical CO2 Reduction for Production of Solar Fuels

Generating solar fuels from carbon dioxide (CO2) and water offers an intriguing opportunity for a carbon-neutral, sustainable, and scalable source of energy. Electrochemical reduction of CO2 is a key reaction for the production of liquid fuels, but it follows a complex reaction network. Even for products with a single carbon atom (C1 products), two bifurcated pathways exist. In this study, we combine evidence from the experiments with a theoretical analysis of energetics to rationalize that not all steps in the reduction of CO2 are electrochemical. This insight enables us to create a selectivity map for two-electron products (carbon monoxide (CO) and formate) on elemental metal surfaces using only two energy descriptors. In the further reduction of CO*, we also find bifurcated pathway one for CHO* and one for COH*. We find Cu to be the only elemental metal capable of reducing CO2 to products beyond 2e− via the proposed COH pathway. Our analysis also rationalizes experimentally observed differences in products between the thermal and electrochemical reduction of CO2 on Cu.   

CO2 conversion on defect-induced single-layer h-BN

Finding effective heterogeneous catalysts, consisting of abundant elements, for hydrogenation of waste gas carbon dioxide into value added molecules is a challenging task for global energy and sustainability solutions. In this talk, we will present results of a closely coupled computational and experimental effort that shows that reconfiguration of the frontier orbital in defect-laden hexagonal boron nitride (dh-BN) can effectively activate the CO₂ molecule for hydrogenation. Our density functional theory (DFT) based calculations of reaction pathways and activation energy barriers demonstrate that activation occurs through back-donation to the π* orbitals of CO₂ from frontier orbitals (defect state) of the h-BN sheet localized near a nitrogen vacancy (V_N). Subsequently, CO₂ is hydrogenated to formic acid (HCOOH) and methanol (CH₃OH). These results were experimentally confirmed in a reactor designed to continuously produce defects in h-BN by the application of mechanical force. We find temperature-dependent switchable catalysis with formic acid formation observed at reaction temperatures above 160 ^οC and methanol formation at lower temperatures (as low as 20 ^οC).   

Optimizing Power Output of Graphene Energy Harvesting

Free standing graphene has been found to invert its curvature over time by researchers at the University of Arkansas. Recently, they have been working on building a chip for converting kinetic energy from the system to electrical energy by using a variable capacitor principle. In this study we investigate the macroscopic models of graphene energy harvesting (GEH) with the goal of finding the output power optimal conditions. We present a GEH circuit model, which contains a manually-driven variable capacitor in series with a DC voltage source, diodes components for rectification, and a storage capacitor for storing the harvested charge. The DC experiment, however, presents difficulties with measuring efficiency, controlling the frequency, and reaching high voltage limits. Therefore, we design an equivalent AC experiment, in which we replace the DC voltage with an AC source and the variable capacitor with a fixed one. Our power studies reveal that the maximum efficiency of is reached at the maximum power output point, which happens at the time of times the time constant of the circuit.   

Defect chemistry in La/Sr-based oxyhydrides

Oxyhydrides in the series La_2-ySr_yLiH_1+yO_3-y have attracted interest for solid-state hydrogen electrolytes on account of their high H^- ionic conductivity. Here, we use first-principles calculations to examine the prevalence of native point defects in La₂LiHO₃ and Sr₂LiH₃O and connect our results to ionic transport properties. We find that both oxyhydrides have high concentrations of point defects. Sr₂LiH₃O, in particular, experiences a stabilizing effect from disorder caused by the presence of pairs of compensating species, such as O_H^- and V_H⁺, or O_H^- and H_i⁺. V_H⁺ serves as the most important point defect for ionic conduction; its presence explains the high conductivity in Sr₂LiH₃O. We identify O-rich and moderately H-rich synthesis conditions as optimal for ensuring stability and maximizing ionic conductivity.
  

First-Principles Investigation of photo-induced structural change of WO3

The visible light region accounts for approximately 40% of the energy of solar radiation. Therefore, tungsten oxide, which is the visible light-driven photocatalysts, has been receiving much attention. It is known that photoexcited tungsten oxide has long-lived photocarriers, which have an important role in photocatalytic reactions. In a recent study, the dynamics of photoelectrons and structural changes of tungsten oxide has been observed by femtosecond transient XAFS. They have found that the electronic state was the first to change followed by the local structure, which was affected within 200 ps of photoexcitation. However, little is known about how the structure changes and how it affects the photocarrier. Therefore, in this study, tungsten oxide was investigated by performing first-principles calculations based on density functional theory and many-body perturbation theory. We discuss the light absorption related by excitation, photoexcited carriers, and structural changes from electronic, optical, phonon, electron-phonon properties, et cetra.   

Ab-initio investigation of a novel photocathode: bulk and surface properties of CuFeO2

Photoelectrochemical (PEC) devices offer the possibility to convert solar radiation into chemical fuels, mimicking the natural process of photosynthesis. Recent experiments have highlighted CuFeO₂ (CFO) as a promising candidate in the role of photocathode [1, 2], yet in spite of these encouraging results, improvement in the catalytic activity and charge separation is required.
In this work, we present a theoretical characterization of CFO based on the DFT+U approach and refined with hybrid calculations. We characterized the stability of bulk CFO with respect to other iron and copper oxides in air and in an aqueous environment, coupling DFT with ab-initio thermodynamics. On this basis, we studied the formation energy of native defects of CFO. The low formation energy of a copper antisite defect could be detrimental for the PEC operations of CFO since states inside the energy gap could favour the electron-hole recombination.
Based on our thermodynamic screening we focused our investigation on two energetically relevant surfaces. We interfaced these surfaces with water and we performed ab-initio molecular dynamics simulations to determine the alignment between the band edges and the redox potentials of water.
[1] Adv. Mater. 28, 9308 (2016)
[2] Nature Rev. Mat. 1, 15010 (2016)   

Linking Nanoscale Grain Boundary Composition and Energetic Properties in Ceramic Oxides

Ceramic oxides are used for a wide variety of technologically relevant applications from electrochemical devices, novel resistive switching devices and oxygen sensors. Applications such as these typically rely upon the ability of oxides to conduct ions efficiently through the lattice. Recent nanoscale compositional characterization of the GB composition has shown different nominal concentrations of solutes could result in orders of magnitude increase in GB ionic conductivity relative to the undoped samples. This study aims to predict the optimal dopants that, when present in high concentrations, increase the ionic conductivity across the GB. Computational modeling is employed using density functional theory to optimize the GB interfacial structure for various GB misorientations in CeO2. This study further develops our understanding of high solute GB composition enabling the development of methods such as selective doping to improve macroscopic ionic conductivity for both the grain and GB.