Session K38: Photocatalysis, Water Splitting and CO2 Reduction

Optimizing photoabsorbers for water splitting: ab initio calculation of defective WO$_{3}$

Tungsten trioxide (WO$_{3}$) is a promising photoabsorber for water splitting [1], widely studied in the last decade. By means of ab initio simulations with dielectric-dependent hybrid functionals [2], we investigated a realistic model of the most stable surface of WO$_{3}$, which presents a high concentration of oxygen vacancies. We found multiple, stable local minima of the WO$_{3}$ surface, that may be attained depending on the type of lattice distortions occurring close to defects at finite temperature. Our results showed that the potential energy surface of the defective WO$_{3}$ surface is highly corrugated, with singlet and triplet states close in energy, and associated frontier orbitals with different localization properties. We gained insight on the effect on transport properties and of charge localization at the surface by using first principles molecular dynamics. [1] Q.X. Mi, Y. Ping, Y. Li, B.F. Cao, B.S. Brunschwig, P.G. Khalifah, G. Galli, H.B. Gray, and N.S. Lewis, J. Am. Chem. Soc. 134, 18318 (2012) [2] J.H. Skone, M. Govoni, and G. Galli, Phys. Rev. B 89, 195112 (2014)   

Molecular Packing Details of Photocatalytic Perylene Amphiphile Nanosheets via X-ray Scattering

Molecular packing in light harvesting 2D assemblies of photocatalytic materials is a critical factor for solar-to-fuel conversion efficiency. However, the structure-function correlations have not yet been fully established. This is partly because of the difficulties in extracting the molecular arrangements from the complex 3D powder averaged diffraction patterns of 2D lattices, obtained via in-situ wide-angle X-ray scattering. Here, we develop a scattering theory formalism and couple it with simple geometrical model for the molecular shape of chromophore 9-methoxy-PMI (MeO-PMI) used in our study. This generally applicable method fully reproduces the measured diffraction pattern including the asymmetric line-shapes for the Bragg reflections and yields the molecular packing arrangement within a 2D crystal with a remarkable degree of detail. We find an approximate edge-centered herringbone structure for the PMI fused aromatic rings and ordering of the chains. This packing arrangement differs from the more symmetric face-to-face orientation of the unsubstituted PMI rings, which is correlated to our measurement of the reduced catalytic performance of MeO-PMI nanosheets as compared to the mesoscopically similar unsubstituted PMI assemblies.   

Optoelectronic and transport properties of epitaxially strained BiVO$_4$ from first principles

Bismuth vanadate (BiVO$_4$) is a promising photo-catalyst for water-splitting. However, the photo-electrochemical performance of BiVO$_4$ is limited by a relatively large band gap ($\sim$2.5eV) and low electron mobilities. Previous theoretical work has focused on the role of extrinsic and intrinsic defects to control and tune the optical and transport properties of BiVO$_4$; however, the effect of anisotropic strain remains largely unexplored. Recently, thin films BiVO$_4$ have been grown using molecular beam epitaxy, opening new possibilities to design BiVO$_4$-based renewable solar-energy devices. In this work, we use density functional theory and GW/BSE many-body perturbation theory calculations to investigate the effect of epitaxial strain in the structural, optoelectronic and transport properties of BiVO$_4$. We find that compressive epitaxial strain leads to a moderate decrease of the band gap and an enhancement of hole effective mass and majority carrier small polaron formation energy. In addition, we determine the effect of epitaxial strain on the transport properties of electron and hole polarons and their interaction with oxygen vacancies. This work is supported by DOE, computational resources are provided by NERSC.   

A joint theoretical and experimental study of the (Ni,Fe)-oxyhydroxide oxygen evolution catalyst for water splitting

Recent experiments showed that iron doped nickel oxyhydroxides are excellent catalysts for driving the oxygen evolution reaction (OER) of water splitting; however, the role played by iron is still controversial.[1, 2, 3] In a joint theoretical and experimental study, we investigated the optoelectronic properties and oxidation potentials of the (Ni,Fe)-oxyhydroxide layered materials, and we determined oxidation states, band gaps and analyzed the role of iron doping. We found that Fe(IV) is present in catalytically active, doped oxyhydroxides, consistent with the suggestions of recent in operando Mossbauer experiments,[3] and that oxygen atoms bound to the Fe dopants might be the most active sites for OER in oxyhydroxide films. We also showed that hybrid functionals give a more accurate account of the optical properties of these materials than the widely adopted DFT+U level of theory. [1] L. Trotochaud et al., JACS 136, 6744 (2014). [2] D. Friebel et al., JACS 137, 1305 (2015). [3] J. Y. C. Chen et al., JACS 137, 15090 (2015).   

Visible Light Photoelectrochemical Performance of Zinc Oxide Nanotube-Nanorod Hybrid Structure.

Zinc oxide (ZnO) is a scientifically and technologically important functional material that finds applications in a wide range of fields including electronics, optoelectronics and photovoltaics. It is a low cost, abundant and non-toxic material and hence, it is one of the most appropriate materials for sustainable processes such as solar energy conversion. Nevertheless, the bandgap of ZnO is about 3.37 eV and as a result it can absorb radiations in the ultraviolet range only. We have recently developed a ZnO nanotube-nanowire architecture through a sustainable process that consists of anodization of zinc in a non-toxic electrolyte. We have successfully engineered the band gap of this nanomaterial, which resulted in a dramatic improvement in the visible light photoactivity of the material. We will discuss the details of this study and the performance of the material as a photoanode for photoelectrochemical water splitting.   

Density functional theory studies of TiO$_{\mathrm{2}}$ for photocatalysis and Li storage applications

We present two theory-experiment collaboration studies of anatase TiO$_{\mathrm{2}}$ for energy applications [1,2]. First, we discuss a hydrogen-nitrogen co-doped TiO$_{\mathrm{2}}$ (HN-TiO$_{\mathrm{2}})$ as a photocatalyst, and show that the interstitially introduced HN contributes to the increase of solar-to-fuel conversion efficiency. We find that the variation of valence band maximum (VBM) of NH-TiO$_{\mathrm{2}}$ extends the photoactive spectrum to the visible light, and argue that created mid-gap states produce efficient electron and hole conduction channels. Next, we consider experimentally fabricated hierarchical TiO$_{\mathrm{2}}$ nanocrystals integrated with binder-free porous graphene (PG) network foam [2] for a Li storage application. It was found that the TiO$_{\mathrm{2}}$-PG facilitated rapid ionic transfer during the Li-ion insertion/extraction process. We clarify the mechanisms by showing that Li ion migration into the TiO$_{\mathrm{2}}$-PG interface stabilize the binder-free oxide-graphene interface. Atomistic mechanism of Li ion insertion and migration is discussed by comparing cases between an isolated Li ion, when the crowding effect is included, and when the surface Li ions are present. We found that the supply of additional surface Li ions significantly reduce the Li insertion barrier, driving a spontaneous domino-like concerted Li insertion at the oxide surface region. [1] Adv. Energy Mater. \textbf{6}, 1600583 (2016). [2] Adv. Funct. Mater. \textbf{26}, 5139 (2016).   

Theoretical explanation for the enhanced water-splitting catalytic activity in delithiated LiCoO$_2$

Layered LiCoO$_2$ (LCO) is important battery material not well know for its electrochemical catalytic activity. On the other hand, layered $3d$-metal-oxyhydroxides, including CoOOH, are very active water-splitting (oxygen-evolution reaction, OER) catalysts in alkaline media with structure very similar to LCO. In this work, we use DFT+U calculations to explain the enhancement effect in OER catalytic activity in the electrochemically delithiated LCO (De-LCO). We analyze the surface stability and OER activity of the 5 most stable surface facets of LCO and the De-LCO, with 50\% Li removed. Using standard thermodynamic approach of the theoretical OER overpotential, we show that all surface facets have more active sites under Li removal. Particularly, the non-polar facets are identified to be highly active and are further activated by the delithiation process, while the dominant (0001) surface has negligible activity. These findings are in very good agreement with our experimental investigation of LCO and De-LCO nanosheets and nanoparticles with well defined surface morphology.   

First-Principles Study of Single Atom Catalyzed Photoelectrochemical Reduction of CO$_{\mathrm{2}}$

Production of synthetic chemical fuels from solar energy is critical for us to meet the globally growing need of energy as the fossil fuels are depleting fast. The greenhouse gas CO$_{\mathrm{2}}$, as the major product of consumption of both fossil fuels and solar fuels, can be used as the feedstock for solar fuels, thereby providing a sustainable way of closing the carbon cycle. The conversion rate of CO$_{\mathrm{2}}$ to fuels is, however, still too low to be practical besides the poor selectivity of products. In this study, we have investigated the use of single metal atoms supported on graphene sheets as catalysts for the photoelectrochemical reduction of CO$_{\mathrm{2}}$ using the first-principles approach. Reaction pathways to produce a variety of products such as CO, HCOOH, HCHO, CH$_{\mathrm{3}}$OH and CH$_{\mathrm{4}}$ will be presented to demonstrate the differences in metals with a focus on their efficiency and selectivity. Potential candidates of better catalytic performance for production of fuels are identified through computational screening.   

Fundamentals of CO2 adsorption on model copper oxide surfaces

Copper oxides can catalyze the conversion of CO$_{2}$ selectively into methanol. We are studying fundamental aspects of the adsorption of CO$_{2}$ on model copper oxide surfaces using low temperature (5 K) scanning tunneling microscopy (STM) and spectroscopy (STS). We prepared an oxygen adlayer by exposing Cu(100) to O$_{2}$ at 575K, which results in a (2$\sqrt{2}$x$\sqrt{2}$)R45$^{\circ}$ missing row reconstruction. STM images of the pristine surface show a ladder-like contrast with distinct domains. STS reveals a series of image potential states consistent with an increase in work function and two additional unoccupied states. We introduce CO$_{2}$ to the surface in situ, which allows us to unambiguously identify individual CO$_{2}$ molecules and their adsorption sites. We find that the CO$_{2}$ molecules sit in between the O atoms in the missing row reconstruction. The CO$_{2}$ molecules are easily perturbed by the STM tip under typical imaging conditions, suggesting that the molecules are weakly bound to the surface. STS is used to probe molecular orbital states of CO$_{2}$ molecules. Density functional theory calculations of adsorption sites, vibrational modes, and diffusion barriers are in qualitative agreement with the experiment.   

Morphological Control of Au Dendrite Electrocatalysts for CO$_2$ Reduction

Au has demonstrated the highest catalytic selectivity, activity, and stability for CO$_2$ reduction to CO of any metal, but the mechanism for this performance remains unclear. Studies of nanoparticle films have shown that higher index facets have improved performance, but the preeminent nanoparticle films, from oxide-derived Au, lack well-defined facets and morphological stability to illuminate their enabling mechanism. More recent work has shown Au needles with a sub 5 nm radius of curvature have excellent performance and stability, independent of crystal facet. The same studies, however, still show calculations expecting a facet dependance. Here we demonstrate a facile and novel dendrite fabrication process with tunable morphology. The dendrites show high catalytic selectivity, activity, and stability for CO$_2$ reduction to CO, along with morphological stability after 18 hours of operation, allowing correlation between morphology and performance. The influence of exposed facets will be discussed.   

Electrochemical CO$_2$ Reduction via Gas-Phase Catholyte

Reducing CO$_2$ to CO through electrolysis, for the eventual conversion to hydrocarbons, provides a path towards utility-scale seasonal storage of renewable energy. Electrochemical reduction of CO$_2$ has previously been achieved using a two chamber system. The chambers are typically separated by a semipermeable Nafion membrane, with an oxygen evolution catalyst anode on one side, a gold cathode on the other, and a solution containing CO$_2$ on both sides. If instead, CO$_2$ gas was in the second chamber, the reaction should yield more CO formed from CO$_2$ at a given overpotential; this would result from the increased concentration of CO$_2$ at the cathode surface and more facile mass transport of the CO and CO$_2$. With liquid in one chamber and gas in the other, electrolysis is performed by integrating the cathode onto the semipermeable Nafion membrane. This membrane electrode assembly is fabricated via nanoimprint lithography (NIL), simultaneously achieving high active surface area and permeability. Challenges to the Nafion NIL process, and the performance of the system in CO$_2$ reduction, will be presented.   

Superalkalis as building blocks of noble-metal-free CO$_{\mathrm{2}}$ activation

One of the great challenges to treat greenhouse effect is to convert CO$_{\mathrm{2}}$~into fuels. The key step for this process requires activation of the CO$_{\mathrm{2}}$~molecule. Recent experiments have shown that this can be accomplished by reacting CO$_{\mathrm{2}}$~with noble metal Au. Realizing that the addition of an electron transforms the neutral CO$_{\mathrm{2}}$~from linear to a bent structure, it was argued that the key parameter that promotes electron transfer from a metal atom to CO$_{\mathrm{2}}$~depends upon its ionization potential. We note that hetero-atomic clusters known as super-alkalies can be designed such that their ionization potential is smaller than those of alkali atoms. Using first-principles theory we have designed a variety of super-alkali species using different electron counting rules and studied their thermodynamic stability using molecular dynamics simulation. Reaction of these super-alkalies with CO$_{\mathrm{2}}$~shows that they can be ideal noble-metal free particles for CO$_{\mathrm{2}}$~activation. ~   

Solvent effect in biomass conversions: Catalytic reactions of cresol at the liquid-solid interface

Liquid water plays a very important role in biomass conversions. The specific mechanisms for the solvent effect in various catalytic reactions remain elusive, however. We employed first-principles density functional theory and \textit{ab initio} molecular dynamic simulations to explore the mechanism for the catalytic hydrogenation of $o$-cresol at the water/Pt(111) interface. We found that the hydrogen atom of the hydroxyl group of $o$-cresol dissociates into water with a barrier of essentially 0 kJ/mol. We also found that the reaction barriers for the stepwise hydrogenation of $o$-cresol at the water/Pt(111) interface are lower by 10-30 kJ/mol than those on the Pt(111) surface in the absence of water, suggesting that water promotes hydrogenation of $o$-cresol. In addition, we determined the detailed configurations for the transition states. Furthermore, we found that, while 2-methyl-cyclohexanone is an intermediate product when water is absent, the barrier for further hydrogenation of 2-methyl-cyclohexanone is significantly reduced when water is present, indicating that water facilitates the formation of the final product, 2-methyl-cyclohexanonal.