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Aerogels are a very interesting group of materials owing to their unique physical and chemical properties. In the context of electrocatalysis, the focus has been on their physical properties, and they have been used primarily as catalyst supports so far. In this work, we synthesized porphyrin aerogels containing Ni and NiFe mixed metal materials and studied them as catalysts for the oxygen evolution reaction (OER). Different Ni : Fe ratios were synthesized and studied in electrochemical cells, and DFT calculations were conducted in order to gain insight into their behavior. The activity trends were dependent on the metal ratios and differ from known NiFeOOH materials due to the change in the oxidation states of the metals to higher numbers. Herein, we show that Ni and Fe have a synergistic effect on the OER, despite being structurally separated. They are connected electronically, though, through a large organic aromatic system that facilitates electron sharing between them. Among the mixed metal porphyrin aerogels, the best ratio was found to be Ni : Fe = 35 : 65, in contrast to oxide/oxyhydroxide materials in which a ratio of 80 : 20 was found to be ideal.

Electrolysis of water to produce green and renewable hydrogen fuel is of great interest in the clean energy field. Water molecules can be decomposed to hydrogen and oxygen through catalysis. Catalytic materials under electrochemical operation are subject to harsh chemical environments, and as a result mechanical changes may appear in the material. In two dimensional materials, the weak van der Waals (vdW) forces holding the layers together may cause a change in the stacking order of the material. The big challenge is to understand the effect of the interlayer arrangements of two dimensional materials on their catalytic performance. In this research we use Density Functional Theory in order to explore the catalytic performance of β-NiOOH, a two dimensional material that is one of the best known catalysts for the oxygen evolution reaction (OER), under different displacements. Our results indicate that changes in the structural stacking of NiOOH could affect the catalytic properties of the system. Particularly, we find that small shifts between the layers enhance the OER activity by reducing the overpotential down to 240 [mV] due to the formation of an unstable state and the formation of new vdW bonds between the layers. The potential ability to lower the overpotential of NiOOH could give exceptional results in increasing the efficiency of the OER.

A solid oxide fuel cell (SOFC) plays a significant role in converting chemically stored energy to electrical energy by using clean and renewable fuels, such as H2 and CO. The LSFCr (La0.3Sr0.7Fe0.7...

Hematite has been widely studied for catalytic water splitting, but the role of the interactions between catalytic sites is unknown. In this paper, we calculate the oxygen evolution reaction free energies and the surface adsorption distribution using a combination of density functional theory and Monte Carlo simulations to "cover the waterfront," or cover a wide range of properties with a simulation of the hematite surface under working conditions. First, we show that modeling noninteracting catalytic sites provides a poor explanation of hematite's slow reaction kinetics. The interactions between the catalytic site may hinder catalysis through the strong interactions of *OH

Hematite is a possible photoanode for photoelectrochemical cells (PECs) that is widely studied using density functional theory (DFT). In this paper we perform more accurate calculations of the absorption spectrum of hematite using the one-shot Green's function (G0W0) and Bethe-Salpeter equation (BSE) methods, which take excited states into account and compare the spectrum to experimental data. We found a match between our calculations and the observed absorption spectra in peak locations. Furthermore, there is anisotropy of the absorption spectra that is concurrent with the crystal structure. We also calculated the absorption spectrum of hematite intermediates during catalysis of the oxygen evolution reaction to better understand which intermediate is dominant during the reaction and the contribution of excited states to catalysis. The *O intermediate was found to be the most optically and chemically dominant species during catalysis.

The world’s shift to the production of energy from sustainable sources requires the development of large energy storage. One of the best methods to store surplus energy produced from environmentall...

Understanding the water splitting mechanism in photocatalysis is a rewarding goal as it will allow producing clean fuel for a sustainable life in the future. However, identifying the photocatalytic mechanisms by modeling photoactive nanoparticles requires sophisticated computational techniques based on multiscale modeling. In this review, we will survey the strengths and drawbacks of currently available theoretical methods at different length and accuracy scales. Understanding the surface-active site through Density Functional Theory (DFT) using new, more accurate exchange-correlation functionals plays a key role for surface engineering. Larger scale dynamics of the catalyst/electrolyte interface can be treated with Molecular Dynamics albeit there is a need for more generalizations of force fields. Monte Carlo and Continuum Modeling techniques are so far not the prominent path for modeling water splitting but interest is growing due to the lower computational cost and the feasibility to compare the modeling outcome directly to experimental data. The future challenges in modeling complex nano- photocatalysts involve combining different methods in a hierarchical way so that resources are spent wisely at each length scale, as well as accounting for excited states chemistry that is important for photocatalysis, a path that will bring devices closer to the theoretical limit of photocatalytic efficiency.

Efforts to improve energy storage depend greatly on the development of efficient electrode materials. Recently, strain has been employed as an alternate approach to improve ion mobility. While lattice strain has been well-researched in catalytic applications, its effects on electrochemical energy storage are largely limited to computational studies due to complexities associated with strain control in nanomaterials as well as loss of strain due to the phase change of the active material during charging–discharging. In this work, we overcome these challenges and investigate the effects of strain on supercapacitor performance in Li-ion-based energy devices. We synthesize epitaxial Fe3O4@MnFe2O4 (core@shell) nanoparticles with varying shell thickness to control the lattice strain. A narrow voltage window for electrochemical testing is used to limit the storage mechanism to lithiation–delithiation, preventing a phase change and maintaining structural strain. Cyclic voltammetry reveals a pseudocapacitive behavior and similar levels of surface charge storage in both strained- and unstrained-MnFe2O4 samples; however, diffusive charge storage in the strained sample is twice as high as the unstrained sample. The strained-MnFe2O4 electrode exceeds the performance of the unstrained-MnFe2O4 electrode in energy density by ∼33%, power density by ∼28%, and specific capacitance by ∼48%. Density functional theory shows lower formation energies for Li-intercalation and lower activation barrier for Li-diffusion in strained-MnFe2O4, corresponding to a threefold increase in the diffusion coefficient. The enhanced Li-ion diffusion rate in the strained-electrodes is further confirmed using the galvanostatic intermittent titration technique. This work provides a starting point to using strain engineering as a novel approach for designing high performance energy storage devices.

Flicker noise causes decoherence in Josephson junction-based superconducting qubits, thus limiting their practical potential as building blocks for quantum computers. This is due to limited length and complexity of executable algorithms, and increased dependency on error-correcting measures. Therefore, identifying and subsiding the atomic sources of flicker noise are of great importance to the development of this technology. We developed a method that combines ab initio DFT calculations and quantum dynamics to model charge transport across a Josephson junction, by which it is possible to more accurately assess different defects as sources of flicker noise. We demonstrate the use of our method in an investigation of various atomic defects, including vacancies, trapping, and substitutions, in an Al|Al2O3|Al Josephson junction. This demonstration both reveals weaknesses in previous attempts to pinpoint the atomic sources of flicker noise and highlights new candidates.

Non-polluting fuels, such as hydrogen, are widely considered to be a promising way to decrease the environmental problems caused by the burning of fossil fuels. Finding materials that can help us to generate these “clean” fuels is a difficult and time-consuming process. Computer simulation technology is the most important tool in this area, because it helps scientists to understand existing materials and can also help them to design new types of materials. In this article, we explain how certain materials can help us to produce hydrogen, a clean fuel, from water, and we also describe how computer simulation technology can offer a route for the discovery of new efficient materials.

The size of the Field Effect Transistor (FET) has been decreasing exponentially each year to keep up with the demands of the microelectronic industry. However, this scaling is now reaching its limits due to the performance of the materials currently used in the FET. Specifically, the leakage through silicon dioxide, the current gate dielectric, is beginning to inhibit the FET's performance. Recently, attention has been turning towards two-dimensional materials for electronic devices. However, insulating is especially challenging for two-dimensional materials due to the short diffusion length for charge carriers. We consider several materials that have been shown to have a stable two-dimensional honeycomb structure. In order to characterize these materials, we use Density Functional Theory (DFT) to calculate key properties that determine performance, including the dielectric function, band gap, and effective mass. We check the compatibility of these materials with Molybdenum Disulfide (MoS2), a two-dimensional material that has emerged as a promising candidate for the semiconducting channel of a FET. We have identified several promising candidates, including hBN, which is the common choice as a dielectric material, and LiF and BeO, which are materials that have not been used before as gate dielectrics. We anticipate that this study will contribute to advancing the development of transistors that are made completely of two-dimensional materials.

Understanding the mechanisms of charge transport in ternary and higher order spinels would enable design principles for enhancing their electronic and electrochemical properties for energy applications. Here we investigate the ternary CoxMn3–xO4 spinel system and determine the relationship between electronic properties and cation site occupation, based on the octahedral site, donor–acceptor formalization introduced in the polaron hopping model. We employ synchrotron X-ray emission spectroscopy to characterize the donor–acceptor cation pairs in the lattice. We find that the stochiometric volcano trend in electronic conductivity correlates well with the concentration of Mn4+/Mn3+ hopping pairs, while the concentration of Mn3+/Mn2+ pairs is unchanged with stoichiometry. We also find that Co does not directly contribute to conductivity; however, the Co does create configurational disorder that leads to the generation of hopping pairs for Mn at octahedral sites. From these results we conclude that Mn4+/Mn3+ pa...

Nickel-, cobalt-, and iron-based (oxy)hydroxides comprise the most-commonly studied electrocatalysts for the oxygen-evolution reaction (OER) in alkaline solution. A fundamental understanding of composition-structure-activity relationships for mixed-metal Ni-Co and Ni-Co-Fe (oxy)hydroxides is important to guide the design of advanced OER catalysts. Here we use cyclic voltammetry, chronopotentiometry, inductively-coupled plasma-optical emission spectroscopy, and in situ electrical conductivity measurements to characterize the properties and activity of various compositions of Ni-Co-Fe (oxy)hydroxides prepared by cathodic co-electrodeposition. Consistent with previous studies, we find Fe is essential for the mixed-metal (oxy)hydroxides to achieve high OER activity. In the rigorous absence of Fe (achieved by using specially cleaned electrolytes), the most-active Ni-Co (oxy)hydroxide composition has an OER turn-over frequency only twice that of pure Co (oxy)hydroxide, suggesting minimal synergism between the two metals. The addition of Co to Ni-Fe (oxy)hydroxides shifts the onset of electrical conductivity to lower potentials, but has little effect on the intrinsic OER activity, with the most-active Ni-Co-Fe (oxy)hydroxide having an OER turn-over frequency only ~ 1.5 times that of the Ni-Fe (oxy)hydroxides. The magnitudes of the electrical conductivities are similar for all the compositions measured. Density-functional-theory-calculated projected density of states show a significant contribution of all chemical elements at the valence band edge of the mixed-metal oxyhydroxide electronic structure, demonstrating significant electronic hybridization between the elements. The calculations suggest the involvement of all the elements in modulating the electronic structure at putative Fe-based active sites that are probably located at edges or defects in the two-dimensional oxyhydroxide sheets.

Hematite ( α -Fe2O3) is one of the most widely studied materials for the application of water oxidation in the field of renewable energy. In order to push hematite toward maximal efficiency, a better understanding of the intrinsic limitations is essential. In this review, we cover our latest results on identifying and finding ways to overcome bottlenecks by modelling the electrochemical processes in hematite at three regimes: 1. at the bulk, the calculated hole effective mass is unfavorably high and conductivity can be improved through doping and alloying, 2. at the surface, catalysis is slow and involves long-lived surface states, but the catalytic overpotential can be reduced by adding a doping gradient or overlayers without strain, and 3. at the back contact the losses can be improved by selecting a proper metal contact. We provide unique analysis approaches that reveal fundamental understanding of electrochemistry with hematite. These models have made an impact on understanding experimental observations and have influenced subsequent theoretical investigations.

Electrocatalytic water-splitting is considered as a highly promising technology for the sustainable production of hydrogen. Currently, the commercial distribution of such devices is limited by the high price of noble metal (e.g. Pt) electrocatalysts, which still show the best activity toward the hydrogen evolution reaction (HER). Here it is shown that nickel phosphide (Ni-P) decorated with trace amounts of Pt fulfils both performance and cost requirements for HER electrocatalysis. The catalyst was prepared via a facile and safe two-step wet chemical process. Its excellent performance relies on the synergistic effect between Ni-P and Pt, which leads to a stronger reductive power of Pt and better hydrogen adsorption and reaction kinetics. These findings demonstrate the possibilities to advance the field of water electrolysis and other fields of catalysis by utilizing low-cost catalysts with noble metal-like activity.

In order to solely rely on renewable and efficient energy sources, reliable energy storage and production systems are required. Hydrogen is considered an ideal solution as it can be produced electrochemically by water electrolysis and renewably while no pollutants are released when consumed. The most common catalysts in electrolyzers are composed of rare and expensive precious group metals. Replacing these materials with Earth-abundant materials is important to make these devices economically viable. Metal organic frameworks are one possible solution. Herein we demonstrate the synthesis and characterization studies of metal benzene-tri-carboxylic acid-based metal-organic frameworks embedded in activated carbon. The conductive composite material was found to be electrocatalytically active for both the oxygen evolution reaction and the hydrogen evolution reaction. Furthermore, several metal organic frameworks sharing the same ligand but with different first-row transition metals (M = Co, Cu, Fe, Mn) were compared, and the trend of their activity is discussed. Cobalt was found to have the highest activity among the studied metal centers, and therefore has the best potential to serve as a bifunctional catalyst for alkaline electrolyzers.

Graphene is a promising material in the field of interface science, especially for noncovalent functionalization, sensing, and for applications in catalysis and nanoelectronics. The noncovalent self-assembly of aromatic molecules on graphene promotes electronic coupling through π-π interactions that allows for quenching of the fluorescence of adsorbent molecules and the enhancement of their Raman spectra via graphene-enhanced Raman spectroscopy (GERS). Although recent work has explored the Raman enhancement on mono- and bilayer graphene, the layer dependence of both electronic phenomena (i.e., fluorescence quenching and Raman enhancement) has largely remained underexplored. Similarly, the effect of near-surface molecular dipoles on GERS has sparsely been examined. In this work, we employ self-assembled monolayers of azobenzene-decorated triazatriangulene molecules (AzoTATA) on graphene terraces to examine the effect of switchable molecular dipoles on the GERS effect, which occurs as a function of azobenzene photoisomerization. Furthermore, using empirical and computational methods, we present a systematic study for deriving the mechanism of GERS enhancement and fluorescence quenching on graphene terraces.

Trimetallic double hydroxide NiFeCo-OH is prepared by coprecipitation, from which three different catalysts are fabricated by different heat treatments, all at 350 °C maximum temperature. Among the prepared catalysts, the one prepared at a heating and cooling rate of 2 °C min-1 in N2 atmosphere (designated NiFeCo-N2 -2 °C) displays the best catalytic properties after stability testing, exhibiting a high current density (9.06 mA cm-2 at 320 mV), low Tafel slope (72.9 mV dec-1 ), good stability (over 20 h), high turnover frequency (0.304 s-1 ), and high mass activity (46.52 A g-1 at 320 mV). Stability tests reveal that the hydroxide phase is less suitable for long-term use than catalysts with an oxide phase. Two causes are identified for the loss of stability in the hydroxide phase: a) Modeling of the distribution function of relaxation times (DFRT) reveals the increase in resistance contributed by various relaxation processes; b) density functional theory (DFT) surface energy calculations reveal that the higher surface energy of the hydroxide-phase catalyst impairs the stability. These findings represent a new strategy to optimize catalysts for water splitting.

A catalyst surface may be exposed to strain due to application of load or interfacing with a substrate with large lattice mismatch. In order to test the effect of strain on catalytic efficiency, we use density functional theory + U (DFT + U) to model water oxidation on expanded and contracted surfaces of the Ni1−xFexOOH alloy, one of the best known catalysts for water splitting. We find that a low amount of iron content has a similar effect as of applying compressive strain. Due to high oxidation state of Fe4+ at the active site, the Fe–O bond is shorter than in pure FeOOH, which is beneficial for extracting electrons from states delocalized on Fe and Ni atoms. At 33% of Fe content, the efficiency is even better since Fe3+ is at the active site and can easily change oxidation state during the reaction. However, the efficiency drops at higher iron percentages since the surface is unstable and may form FeOOH aggregates. We find that the best performance is obtained at 33% iron content and 5% expansion. Therefore, in addition to changing iron content, strain can also be used as a control handle for improving water splitting catalysis.

The charge transport properties in solids play an important role in the selection of materials for electrochemical devices. Spinels are a special class of solids that are very versatile and possess different properties based on changes in stoichiometry and cation distribution. In that way, their properties can be tailored to fit certain uses. Here we report a density functional theory study of the electronic structures of nine normal and inverse ternary AB2O4 (A, B = Fe, Co, Ni, Mn) and A3O4 spinels. We found that changing the cation distribution of CoMn2O4 into (Mn)[CoMn]O4 lowers the bandgap by about three times. Additionally, charge transport occurs mostly through octahedral sites while in (Co)[CoNi]O4 it occurs through tetrahedral sites. Bulk-based band alignment results are also reported for the spinels in this work in order to design materials with preferred charge transport pathways.

Modeling charge transport across material interfaces is important for understanding the limitations of electronic devices such as transistors, electrochemical cells, sensors, and batteries. However, modeling the entire structure and full dimensionality of an interface can be computationally demanding. In this study, we investigate the validity of an efficient reduced one-dimensional Hamiltonian for calculating charge transport along interfaces by comparing to a two-dimensional model that accounts for additional charge transport pathways. We find that the one-dimensional model successfully predicts the qualitative trend of charge transmission probability among Pt/Fe2O3 and Ag/Fe2O3 interfaces. However, the two-dimensional model provides additional information on possible pathways that are not perpendicular to the interface direction. These charge transport pathways are directed along the lowest potential energy profile of the interface that correlates with the crystal structure of the constituting materials. However, the two-dimensional paths are longer and take more scattering time. Therefore, the one-dimensional model may hold sufficient information for qualitative estimation of charge transport through some material interfaces.

The small-polaron hopping model has been used for six decades to rationalize electronic charge transport in oxides. The model was developed for binary oxides, and, despite its significance, its accuracy has not been rigorously tested for higher-order oxides. Here, the small-polaron transport model is tested by using a spinel system with mixed cation oxidation states (Mnx Fe3-x O4 ). Using molecular-beam epitaxy (MBE), a series of single crystal Mnx Fe3-x O4 thin films with controlled stoichiometry, 0 ≤ x ≤ 2.3, and lattice strain are grown, and the cation site-occupation is determined through X-ray emission spectroscopy (XES). Density functional theory + U analysis shows that charge transport occurs only between like-cations (Fe/Fe or Mn/Mn). The site-occupation data and percolation models show that there are limited stoichiometric ranges for transport along Fe and Mn pathways. Furthermore, due to asymmetric hopping barriers and formation energies, the MnOh2+ polaron is energetically preferred to the FeOh2+ polaron, resulting in an asymmetric contribution of Mn/Mn pathways. All of these findings are not contained in the conventional small-polaron hopping model, highlighting its inadequacy. To correct the model, new parameters in the nearest-neighbor hopping equation are introduced to account for percolation, cross-hopping, and polaron-distribution, and it is found that a near-perfect correlation can be made between experiment and theory for the electronic conductivity.

Hematite is a possible photoanode for photoelectrochemical cells (PECs) that is widely studied using density functional theory (DFT). In this paper we perform more accurate calculations of the absorption spectrum of hematite using the one-shot Green's function (

Physical and chemical properties of the materials are examined by creating point defects and imposing impurities that have a strong impact in the performance of device applications. Experimental defect identification is typically difficult and indirect, usually requiring an ingenious combination of different techniques. First-principles calculations have emerged as a powerful approach in the defect chemistry to complement experimental measurements and have become a reliable predictive tool to identify and characterize defects. In this chapter, we have reviewed theoretical models of point defects in crystalline materials that involve electronic-structure calculations based on density functional theory (DFT) methods and include approximations to the exchange-correlation functional (DFT+U and hybrid functionals). Our main goals are (1) to discuss the effect and importance of defects in two-dimensional and three-dimensional materials; (2) to present key quantities that are necessary in calculating the vacancy formation energies of acceptor and donor defects; and (3) to understand the conducting properties of semiconductor oxides through defect chemistry.

Iron cations are essential for the high activity of nickel and cobalt-based (oxy)hydroxides for the oxygen evolution reaction, but the role of iron in the catalytic mechanism remains under active investigation. Operando X-ray absorption spectroscopy and density functional theory calculations are used to demonstrate partial Fe oxidation and a shortening of the Fe-O bond length during oxygen evolution on Co(Fe)Ox Hy . Cobalt oxidation during oxygen evolution is only observed in the absence of iron. These results demonstrate a different mechanism for water oxidation in the presence and absence of iron and support the hypothesis that oxidized iron species are involved in water-oxidation catalysis on Co(Fe)Ox Hy .

Understanding catalytic mechanisms is important for discovering better catalysts, particularly for water splitting reactions that are of great interest to the renewable energy field. One of the best performing catalysts for water oxidation is nickel oxyhydroxide (NiOOH). However, only one mechanism has been adopted so far for modeling catalysis of the active plane: β-NiOOH(01̅5). In order to understand how a second reaction mechanism affects catalysis, we perform Density Functional Theory + U (DFT+U) calculations of a second mechanism for water oxidation reaction of NiOOH. Then, we use a Metropolis Monte Carlo algorithm to calculate how many catalytic cycles are completed when two reaction mechanisms are competing. We find that within the Metropolis algorithm, the second mechanism has a higher overpotential and is therefore not active even for large applied biases.

Transition metal layered double hydroxides can be fabricated in two-dimensions. Specifically, two-dimensional monolayers, such as Fe(OH)2, Co(OH)2 and Ni(OH)2, and their corresponding heterostructure bilayers can be potentially useful for solar energy conversion. In order to determine the electronic structure properties of these unique materials, we perform Density Functional Theory calculations. We find that the adhesion energy between the monolayers is small and should allow separating the layers. Furthermore, the calculated band gaps are in the range of 1.4–2.3 eV, which is suitable for good optical absorption of solar energy. All of the studied hydroxides have different chemical character at the band edges, i.e. transition metal states at the valence band and oxygen states at the conduction band, which may allow good charge separation after optical excitation.