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In this work, we use a Bayesian optimization (BO) algorithm to sample the space of covalent organic framework (COF) components aimed at the design of COFs with a high hole conductivity. COFs are crystalline, often porous coordination polymers, where organic molecular units—called building blocks (BBs)—are connected by covalent bonds. Even though we limit ourselves here to a space of three-fold symmetric BBs forming two-dimensional COF sheets, their design space is still much too large to be sampled by traditional means through evaluating the properties of each element in this space from first principles. In order to ensure valid BBs, we use a molecular generation algorithm that, by construction, leads to rigid three-fold symmetric molecules. The BO approach then trains two distinct surrogate models for two conductivity properties, level alignment vs a reference electrode and reorganization free energy, which are combined in a fitness function as the objective that evaluates BBs' conductivities. These continuously improving surrogates allow the prediction of a material's properties at a low computational cost. It thus allows us to select promising candidates which, together with candidates that are very different from the molecules already sampled, form the updated training sets of the surrogate models. In the course of 20 such training steps, we find a number of promising candidates, some being only variations on already known motifs and others being completely novel. Finally, we subject the six best such candidates to a computational reverse synthesis analysis to gauge their real-world synthesizability. [ABSTRACT FROM AUTHOR]

We report the free energy barriers for the elementary reactions in the 2e− and 4e− oxygen reduction reaction (ORR) steps on Au(100) in an alkaline solution. Due to the weak adsorption energy of O2 on Au(100), the barrier for the association channel is very low, and the 2e− pathway is clearly favored, while the barrier for the O–O dissociation channel is significantly higher at 0.5 eV. Above 0.7 V reversible hydrogen electrode (RHE), the association channel becomes thermodynamically unfavorable, which opens up the O–O dissociation channel, leading to the 4e− pathway. The low adsorption energy of oxygenated species on Au is now an advantage, and residue ORR current can be observed up to the 1.0–1.2 V region (RHE). In contrast, the O–O dissociation barrier on Au(111) is significantly higher, at close to 0.9 eV, due to coupling with surface reorganization, which explains the lower ORR activity on Au(111) than that on Au(100). In combination with the previously suggested outer sphere electron transfer to O2 for its initial adsorption, these results provide a consistent explanation for the features in the experimentally measured polarization curve for the alkaline ORR on Au(100) and demonstrate an ORR mechanism distinct from that on Pt(111). It also highlights the importance to consider the spin state of O2 in ORR and to understand the activation barriers, in addition to the adsorption energies, to account for the features observed in electrochemical measurements. [ABSTRACT FROM AUTHOR]

The development of water electrolysis catalysts that accelerate the oxygen evolution reaction (OER) is a crucial challenge. Ni-based oxides are promising OER catalysts; however, quantitative studies of Ni-based oxides remain unexplored. In this study, we quantitatively evaluated the OER activity of LiNi0.5Mn1.5O4 as a thin-film electrode catalyst. The LiNi0.5Mn1.5O4 thin film fabricated using a sputtering method exhibited a current density of 6.6 and ∼2.6 mAcm−2 for geometric and estimated areas, respectively, at 1.78 V vs. a reversible hydrogen electrode. X-ray photoelectron spectroscopy indicated the presence of Ni3+ in the as-grown and post-OER LiNi0.5Mn1.5O4 thin films. These results suggest that Ni3+ plays a key role in the OER of LiNi0.5Mn1.5O4. [ABSTRACT FROM AUTHOR]

We have developed and implemented an implicit electrolyte model in the Vienna Ab initio Simulation Package (VASP) that includes nonlinear dielectric and ionic responses as well as a nonlocal definition of the cavities defining the spatial regions where these responses can occur. The implementation into the existing VASPsol code is numerically efficient and exhibits robust convergence, requiring computational effort only slightly higher than the original linear polarizable continuum model. The nonlinear + nonlocal model is able to reproduce the characteristic "double hump" shape observed experimentally for the differential capacitance of an electrified metal interface while preventing "leakage" of the electrolyte into regions of space too small to contain a single water molecule or solvated ion. The model also gives a reasonable prediction of molecular solvation free energies as well as the self-ionization free energy of water and the absolute electron chemical potential of the standard hydrogen electrode. All of this, combined with the additional ability to run constant potential density functional theory calculations, should enable the routine computation of activation barriers for electrocatalytic processes. [ABSTRACT FROM AUTHOR]

Electrochemical reduction of CO2 and H2O to synthesize gas (H2 and CO mixture) is of significant interest due to established industrial pathways to tune the H2 to CO composition to generate an array of valuable products including methanol, synthetic fuel, synthetic natural gas, and hydrogen. However, controlled H2:CO ratios are challenging on CO-active electrocatalysts like silver. We demonstrate that applying laser engineering to adjust the surface wetting state of a silver electrocatalyst with water contact angles θw ranging from 47° and 135°, H2:CO ratios can be tuned from 1 to 4 at modest potentials (−0.7 V versus RHE, RHE—reversible hydrogen electrode) with almost total unity Faradaic efficiency. Both hydrophilic (θw = 47°) and more hydrophobic (θw = 135°) samples showed an increasing H2:CO trend with rising potentials (0.7–1.2 V versus RHE) due to mass transport. Conversely, silver electrocatalyst with θw = 110° exhibited a constant H2:CO ratio of 4. This indicates catalyst wettability potentially affects *H and *HOCO intermediates' adsorption, impacting H2:CO ratios. Our results show the feasibility of syngas composition control on silver catalysts via surface wettability, providing a simpler alternative to complex multicomponent electrocatalytic systems. [ABSTRACT FROM AUTHOR]

The computational modeling of electrochemical interfaces and their applications in electrocatalysis has attracted great attention in recent years. While tremendous progress has been made in this area, however, the accurate atomistic descriptions at the electrode/electrolyte interfaces remain a great challenge. The Computational Hydrogen Electrode (CHE) method and continuum modeling of the solvent and electrolyte interactions form the basis for most of these methodological developments. Several posterior corrections have been added to the CHE method to improve its accuracy and widen its applications. The most recently developed grand canonical potential approaches with the embedded diffuse layer models have shown considerable improvement in defining interfacial interactions at electrode/electrolyte interfaces over the state-of-the-art computational models for electrocatalysis. In this Review, we present an overview of these different computational models developed over the years to quantitatively probe the thermodynamics and kinetics of electrochemical reactions in the presence of an electrified catalyst surface under various electrochemical environments. We begin our discussion by giving a brief picture of the different continuum solvation approaches, implemented within the ab initio method to effectively model the solvent and electrolyte interactions. Next, we present the thermodynamic and kinetic modeling approaches to determine the activity and stability of the electrocatalysts. A few applications to these approaches are also discussed. We conclude by giving an outlook on the different machine learning models that have been integrated with the thermodynamic approaches to improve their efficiency and widen their applicability. [ABSTRACT FROM AUTHOR]

Electrochemical nitrogen reduction reaction (NRR) is imperatively countered with the oxygen evolution reaction (OER) on a conventional Pt counter electrode. Upon focusing on the development of suitable cathode catalysts, it is usually overseen that OER on Pt seeks a significant energy input to overcome the slow reaction kinetics, regardless of the efficiency of the NRR catalyst. Here, we unveil an out-of-the-box concept with state-of-the-art catalysts that, on pursuing OER with RuO2 in KOH, the NRR process reinforces thermodynamically. In this work, it has been shown how both the electrode and electrolyte simultaneously help to elevate a reaction mechanism in terms of Gibbs' energy and equilibrium constant. As a proof of concept, we assembled RuO2 with an NRR catalyst, iron phthalocyanine (FePc), in an electrolyzer, preferably in a two-electrode setup, where the catholyte consisted of 0.5M NaBF4. This system achieved selective cathodic conversion of N2 to NH3 with 67.6% Faradaic efficiency at 0.0 V (vs reversible hydrogen electrode) and simultaneous anodic water oxidation to O2 with a high electricity-to-chemical energy conversion efficiency of 46.7%. The electrolyzer forecasted a full cell voltage of 2.04 V, which demands only 603 mV overpotential to attain 0.5 mA current to drive forward the chemical equilibrium of the overall cell reaction. This study not only emphasized the importance of electrode–electrolyte improvisation but also provided a wider outlook in terms of different thermodynamic parameters to be considered to determine the efficiency of the overall NRR coupled OER process. [ABSTRACT FROM AUTHOR]

The bismuth vanadate (BiVO4) photoanode receives extensive attention in photoelectrochemical (PEC) water splitting. However, the high charge recombination rate, low electronic conductivity, and sluggish electrode kinetics have inhibited the PEC performance. Increasing the reaction temperature for water oxidation is an effective way to enhance the carrier kinetics of BiVO4. Herein, a polypyrrole (PPy) layer was coated on the BiVO4 film. The PPy layer could harvest the near-infrared light to elevate the temperature of the BiVO4 photoelectrode and further improve charge separation and injection efficiencies. In addition, the conductive polymer PPy layer acted as an effective charge transfer channel to facilitate photogenerated holes moving from BiVO4 to the electrode/electrolyte interface. Therefore, PPy modification led to a significantly improved water oxidation property. After loading the cobalt–phosphate co-catalyst, the photocurrent density reached 3.64 mA cm−2 at 1.23 V vs the reversible hydrogen electrode, corresponding to an incident photon-to-current conversion efficiency of 63% at 430 nm. This work provided an effective strategy for designing a photothermal material assisted photoelectrode for efficient water splitting. [ABSTRACT FROM AUTHOR]

Surface platinum hydride structures may exist and play a potentially important role during electrocatalysis and cathodic corrosion of Pt(111). Earlier work on platinum hydrides suggests that Pt may form clusters with multiple equivalents of hydrogen. Here, using thermodynamic methods and density functional theory, we compared several surface hydride structures on Pt(111). The structures contain multiple monolayers of hydrogen in or near the surface Pt layer. The hydrogen in these structures may bind the subsurface or reconstruct the surface both in the set of initial configurations and in the resulting (meta)stable structures. Multilayer stable configurations share one monolayer of subsurface H stacking between the top two Pt layers. The structure containing two monolayers (MLs) of H is formed at −0.29 V vs normal hydrogen electrode, is locally stable with respect to configurations with similar H densities, and binds H neutrally. Structures with 3 and 4 ML H form at −0.36 and −0.44 V, respectively, which correspond reasonably well to the experimental onset potential of cathodic corrosion on Pt(111). For the 3 ML configuration, the top Pt layer is reconstructed by interstitial H atoms to form a well-ordered structure with Pt atoms surrounded by four, five, or six H atoms in roughly square-planar and octahedral coordination patterns. Our work provides insight into the operando surface state during low-potential reduction reactions on Pt(111) and shows a plausible precursor for cathodic corrosion. [ABSTRACT FROM AUTHOR]

Structural modulation of the active site with atomic-level precision is of great importance to meet the activity and selectivity challenges that electrocatalysts are commonly facing. In this work, we have designed a metal (M)–nonmetal diatomic site embedded in graphene-like C2N (denoted as Mo–B@C2N), where the electrocatalytic N2 reduction reaction (eNRR) was thoroughly explored using density functional theory combined with the computational hydrogen electrode method. Compared to M–M diatomic sites, the Mo–B site can generate a pronounced synergistic effect that led to eNRR proceeding via a novel quasi-dissociative reaction mechanism that has not been reported relative to the conventional enzymatic, consecutive, distal, and alternating associative mechanism. This newly uncovered mechanism in which N–N bond scission takes place immediately after the first proton-coupled electron transfer (PCET) step (i.e., *NH–*N + H+ + e− → *NH2*N) has demonstrated much advantage in the PCET process over the four conventional mechanism in terms of thermodynamic barrier, except that the adsorption of side-on *N2 seemed thermodynamically unfavorable (ΔGads = 0.61 eV). Our results have revealed that the activation of the inert N≡N triple bond is dominated by the π*-backdonation mechanism as a consequence of charge transfers from both the B and Mo sites and, unexpectedly, from the substrate C2N itself as well. Moreover, the hybrid Mo–B diatomic site demonstrated superior performance over either the Mo–Mo or B–B site for driving eNRR. Our study could provide insight into the delicate relationships among atomic site, substrate, and electrocatalytic performance. [ABSTRACT FROM AUTHOR]

Revealing the effect of external applied potential on the reaction mechanism and product selectivity is of great significance in electrochemical studies. In this work, the grand canonical density functional theory method was applied to simulate the explicit electrocatalytic process of oxygen evolution reaction and electrochemical ozone production due to the O3 product sensitivity toward the applied potential. Over the Pt/Pd single atom embedded on B/N co-doped graphene (Pt/Pd-BNC) surface, crossover points of O2/O3 selectivity inversion were predicted to be 1.33 and 0.89 V vs standard hydrogen electrode, which were also consistent with the previous experimental results. An in-depth analysis of the energetic terms in the reaction free energies also found the considerable impact of the applied potential on the Helmholtz free energy term, with optimal potential predicted for the key elementary steps, and linear correlations between electrode potential (U) and reaction free energy were found for each elementary step. This study offers extensive knowledge on the potential effect on the O2/O3 selective formation on two-dimensional anode surfaces and provides new insights for investigating the reactivity/selectivity on electrode surfaces in real reaction conditions. [ABSTRACT FROM AUTHOR]

Magnesium has attracted growing interest for its use in various applications, primarily due to its abundance, lightweight properties, and relatively low cost. However, one major drawback to its widespread use remains to be its reactivity in aqueous environments, which is poorly understood at the atomistic level. Ab initio density functional theory methods are particularly well suited to bridge this knowledge gap, but the explicit simulation of electrified water/metal interfaces is often too costly from a computational viewpoint. Here, we investigate water/Mg interfaces using the computationally efficient implicit solvent model VASPsol. We show that the Mg (0001), (10 1 ̄ 0), and (10 1 ̄ 1) surfaces each form different electrochemical double layers due to the anisotropic smoothing of the electron density at their surfaces, following Smoluchowski rules. We highlight the dependence that the position of the diffuse cavity surrounding the interface has on the potential of zero charge and the electron double layer capacitance, and how these parameters are also affected by the addition of explicit water and adsorbed OH molecules. Finally, we calculate the equilibrium potential of Mg2+/Mg0 in an aqueous environment to be −2.46 V vs a standard hydrogen electrode, in excellent agreement with the experiment. [ABSTRACT FROM AUTHOR]

Phenol is an important model compound to understand the thermocatalytic (TCH) and electrocatalytic hydrogenation (ECH) of biomass to biofuels. Although Pt and Rh are among the most studied catalysts for aqueous-phase phenol hydrogenation, the reason why certain facets are active for ECH and TCH is not fully understood. Herein, we identify the active facet of Pt and Rh catalysts for aqueous-phase hydrogenation of phenol and explain the origin of the size-dependent activity trends of Pt and Rh nanoparticles. Phenol adsorption energies extracted on the active sites of Pt and Rh nanoparticles on carbon by fitting kinetic data show that the active sites adsorb phenol weakly. We predict that the turnover frequencies (TOFs) for the hydrogenation of phenol to cyclohexanone on Pt(111) and Rh(111) terraces are higher than those on (221) stepped facets based on density functional theory modeling and mean-field microkinetic simulations. The higher activities of the (111) terraces are due to lower activation energies and weaker phenol adsorption, preventing high coverages of phenol from inhibiting hydrogen adsorption. We measure that the TOF for ECH of phenol increases as the Rh nanoparticle diameter increases from 2 to 10 nm at 298 K and −0.1 V vs the reversible hydrogen electrode, qualitatively matching prior reports for Pt nanoparticles. The increase in experimental TOFs as Pt and Rh nanoparticle diameters increase is due to a larger fraction of terraces on larger particles. These findings clarify the structure sensitivity and active site of Pt and Rh for the hydrogenation of phenol and will inform the catalyst design for the hydrogenation of bio-oils. [ABSTRACT FROM AUTHOR]

Hydrogen defects sometimes form shallow impurity levels in semiconductors, and it is an important topic for semiconductor research to investigate their details. One of the experimental methods to determine the state of hydrogen is the muon spin rotation (μ SR) experiment. By observing formation of a pseudo-hydrogen atom, called muonium, it is possible to investigate the hydrogen defect levels. In a previous theoretical study, the pinning levels were calculated for various materials as a reference for hydrogen defect levels, and these levels were universally distributed near the hydrogen electrode potential. Based on the prediction, μ SR experiments were performed for germanium sulfide (GeS) and germanium telluride (GeTe), where the hydrogen electrode potential is located in the bandgap for GeS, but not for GeTe. As a result, the μ SR spectra showed that the muonium forms in GeS, while it does not in GeTe. In GeS, 58% of the muons formed muoniums. The activation energy was obtained as Δ E = 26.2 ± 6.9 meV. The hyperfine coupling frequency was ω c (2 π) − 1 = 1.95 ± 0.17 GHz, and the Bohr radius of muonium was 1.3 times larger than that in vacuum. These properties indicated that the identified muonium does not form a typical impurity level that affects the electrical properties. [ABSTRACT FROM AUTHOR]

The activity of Pt(111) electrodes for the hydrogen evolution reaction (HER) in 0.5M H2SO4 solution is found to increase with continuous potential cycling in the HER potential region. In addition, the basic cyclic voltammograms obtained in 0.5M H2SO4 saturated with N2 after HER show several characteristic changes: the current waves for hydrogen adsorption in the region of0.2 < E < 0.35 V and for sulfate adsorption at 0.35 < E < 0.5 V decrease and the current spike at 0.44 V for the phase transition of the sulfate adlayer gradually disappears. We suggest that these changes are caused by the absorption of a small amount of hydrogen in the subsurface layer and propose a mechanism by which this enhances hydrogen evolution. [ABSTRACT FROM AUTHOR]

The electrochemical reduction of CO2 into valuable chemicals under mild conditions has become a promising technology for energy storage and conversion in the past few years, receiving much attention from theoretical researchers investigating the reaction mechanisms. However, most of the previous simulations are related to the key intermediates of *COOH and *CO using the computational hydrogen electrode approach under vacuum conditions, and the details of the CO2 activation are usually ignored due to the model simplicity. Here, we study the CO2 activation at the Au–water interfaces by considering the dynamics of an explicit water solvent, where both regular ab initio molecular dynamics and constrained ab initio molecular dynamics simulations are carried out to explore the CO2 adsorption/desorption reactions from the atomic level. By introducing K+ cations into Au(110)–water interfacial models, an electrochemical environment under reducing potentials is constructed, where the reaction free energy (0.26 eV) and activation energy (0.61 eV) are obtained for CO2 adsorption based on the thermodynamic integration. Moreover, the Bader charge analysis demonstrates that CO2 adsorption is activated by the first-electron transfer, forming the adsorbed CO2− anion initiating the overall catalytic reaction. [ABSTRACT FROM AUTHOR]

Undercoordinated metal nanoclusters have shown great promise for various catalytic applications. However, their activity is often limited by the covalently bonded ligands, which could block the active surface sites. Here, we investigate the ligand removal process for Au25 nanoclusters using both thermal and electrochemical treatments, as well as its impact on the electroreduction of CO2 to CO. The Au25 nanoclusters are synthesized with 2-phenylethanethiol as the capping agent and anchored on sulfur-doped graphene. The thiolate ligands can be readily removed under either thermal annealing at ≥180°C or electrochemical biasing at ≤−0.5 V vs reversible hydrogen electrode, as evidenced by the Cu underpotential deposition surface area measurement, x-ray photoelectron spectroscopy, and extended x-ray absorption fine structure spectroscopy. However, these ligand-removing treatments also trigger the structural evolution of Au25 nanoclusters concomitantly. The thermally and electrochemically treated Au25 nanoclusters show enhanced activity and selectivity for the electrochemical CO2-to-CO conversion than their pristine counterpart, which is attributed to the exposure of undercoordinated Au sites on the surface after ligand removal. This work provides facile strategies to strip away the staple ligands from metal nanoclusters and highlights its importance in promoting the catalytic performances. [ABSTRACT FROM AUTHOR]

Progress in electrochemical technologies, such as automotive batteries, supercapacitors, and fuel cells, depends greatly on developing improved charged interfaces between electrodes and electrolytes. The rational development of such interfaces can benefit from the atomistic understanding of the materials involved by first-principles quantum mechanical simulations with Density Functional Theory (DFT). However, such simulations are typically performed on the electrode surface in the absence of its electrolyte environment and at constant charge. We have developed a new hybrid computational method combining DFT and the Poisson–Boltzmann equation (P–BE) capable of simulating experimental electrochemistry under potential control in the presence of a solvent and an electrolyte. The charged electrode is represented quantum-mechanically via linear-scaling DFT, which can model nanoscale systems with thousands of atoms and is neutralized by a counter electrolyte charge via the solution of a modified P–BE. Our approach works with the total free energy of the combined multiscale system in a grand canonical ensemble of electrons subject to a constant electrochemical potential. It is calibrated with respect to the reduction potential of common reference electrodes, such as the standard hydrogen electrode and the Li metal electrode, which is used as a reference electrode in Li-ion batteries. Our new method can be used to predict electrochemical properties under constant potential, and we demonstrate this in exemplar simulations of the differential capacitance of few-layer graphene electrodes and the charging of a graphene electrode coupled to a Li metal electrode at different voltages. [ABSTRACT FROM AUTHOR]

Graphene flow sensors not only hold great prospects for applications but also encounter many difficulties, such as unwanted electrochemical phenomena, low measurable signal, and limited dependence on the flow direction. This study proposes an approach allowing for the detection of a flow direction-dependent electric signal in aqueous solutions of salts, acids, and bases. The key element in the proposed solution is the use of a reference electrode which allows external gating of the graphene structure. Using external gating enables to enhance substantially the amplitude of the flow-generated signal. Simultaneous measurement of the reference electrode current allows us to recover a flow-direction-sensitive component of the flow-induced voltage in graphene. The obtained results are discussed in terms of the Coulomb interaction and other phenomena, which can be present at the interface of graphene with the aqueous solution. [ABSTRACT FROM AUTHOR]

Redox-flow batteries (RFBs) are promising electrochemical energy storage devices to load-level intermittent power from renewable energy. In particular, aqueous RFBs using aqueous electrolytes possess several advantages over nonaqueous ones, such as low fabrication cost, nontoxicity, safety, and environmental benignity. Therefore, developing high-performance, abundant, less-expensive iron-based catholytes for aqueous RFBs is essential toward their wide deployment in a power grid. In this Perspective, we summarize the recent progress of iron-based catholytes for aqueous RFBs. We emphasize that iron-based catholytes possess widely ranged redox potentials (-1.0 to 1.5 V vs standard hydrogen electrodes) and solubility in water (0.2-4.0 mol L-1), thereby providing a wide range of cell performance. The molecular design, such as ligand functionalization, counter ion mixing, and asymmetrization, allows for rationally improving solubility, redox potential, and energy density. Furthermore, we demonstrate a simple evaluation method of the redox potential of iron-based catholytes using the calculated energy levels of the lowest unoccupied molecular orbital of ligand molecules. Finally, we rationalize the design strategy of iron-based catholytes for advanced aqueous RFBs. [ABSTRACT FROM AUTHOR]

The outstanding performance of NiOOH/FeOOH-based oxygen evolution reaction (OER) catalysts is rationalized in terms of a bifunctional mechanism involving two distinct active sites. In this mechanism, the OOHads reaction intermediate, which unfavorably affects the overall OER activity due to the linear scaling relationship, is replaced by O2 adsorbed at the active site on FeOOH and Hads adsorbed at the NiOOH substrate. Here, we use the computational hydrogen electrode method to assess promising models of both the FeOOH catalyst and the NiOOH hydrogen acceptor. These two materials are interfaced in various ways to evaluate their performance as bifunctional OER catalysts. In some cases, overpotentials as low as 0.16 V are found, supporting the bifunctional mechanism as a means to overcome the limitations imposed by linear scaling relationships. [ABSTRACT FROM AUTHOR]

The kinetics of the inner-sphere electron transfer reaction between a gold electrode and CO2 was measured as a function of the applied potential in an aqueous environment. Extraction of the electron transfer rate constant requires deconvolution of the current associated with CO2 reduction from the competing hydrogen evolution reaction and mass transport. Analysis of the inner-sphere electron transfer reaction reveals a driving force dependence of the rate constant that has similar characteristics to that of a Marcus–Hush–Levich outer-sphere electron transfer model. Consideration of simple assumptions for CO2 adsorption on the electrode surface allows for the evaluation of a CO2,ads/CO2•−ads standard potential of ∼−0.75 ± 0.05 V vs Standard Hydrogen Electrode (SHE) and a reorganization energy on the order of 0.75 ± 0.10 eV. This standard potential is considerably lower than that observed for CO2 reduction on planar metal electrodes (∼>–1.4 V vs SHE for >10 mA/cm2), thus indicating that CO2 reduction occurs at a significant overpotential and thus provides an imperative for the design of better CO2 reduction electrocatalysts. [ABSTRACT FROM AUTHOR]

Thin-film material libraries in the ternary and quaternary metal oxide systems Fe–V–O, Cu–V–O, and Cu–Fe–V–O were synthesized using combinatorial reactive co-sputtering with subsequent annealing in air. Their compositional, structural, and functional properties were assessed using high-throughput characterization methods. Prior to the investigation of the quaternary system Cu–Fe–V–O, the compositions (Fe61V39)Ox and (Cu52V48)Ox with promising photoactivity were identified from their ternary subsystems Fe–V–O and Cu–V–O, respectively. Two Cu–Fe–V–O material libraries with (Cu29-72Fe4-27V22-57)Ox and (Cu11-55Fe27-73V12-34)Ox composition spread were investigated. Seven mixed ternary and quaternary phase regions were identified: I (α-Cu3FeV6O26/FeVO4), II (Cu5V2O10/FeVO4/α-Cu3Fe4V6O26), III (Cu5V2O10), IV (Cu5V2O10/FeVO4, V (FeVO4/γ-Cu2V2O7/α-Cu3Fe4V6O26), VI (β-Cu2V2O7/α-Cu3Fe4V6O26/FeVO4), and VII (β-Cu3Fe4V6O26/FeVO4). In the investigated composition range, two photoactive regions, (Cu53Fe7V40)Ox and (Cu45Fe21V34)Ox, were identified, exhibiting 103 μA/cm2 and 108 μA/cm2 photocurrent density for the oxygen evolution reaction at 1.63 V vs reversible hydrogen electrode, respectively. The highest photoactive region (Cu45Fe21V34)Ox comprises the dominant α-Cu3Fe4V6O24 phase and minor FeVO4 phase. This photoactive region corresponds to having an indirect bandgap of 1.87 eV and a direct bandgap of 2.58 eV with an incident photon-to-current efficiency of 30% at a wavelength of 310 nm. [ABSTRACT FROM AUTHOR]

Engineering the interface and surface structures of semiconductor-based photoelectrodes for improved charge transfer dynamics and promoted water redox reaction kinetics is essential to achieve efficient photoelectrochemical (PEC) water splitting. In this work, α-Fe2O3 nanorods, successively coated with TiO2 and CoOx thin layers, were reported as the photoanode for solar-driven water oxidation. The obtained α-Fe2O3/TiO2/CoOx photoanode exhibits superior PEC performance as compared to bare α-Fe2O3, with a 3.3-time improvement in photocurrent density at 1.23 V vs reversible hydrogen electrode. This significant enhancement results from the formed heterojunction between α-Fe2O3 and TiO2 for the accelerated photogenerated charge separation and transfer as well as the passivated surface defects by the TiO2 overlayer for reduced charge recombination. Additionally, the existence of CoOx as the oxygen evolution catalyst significantly facilitates the surface reaction kinetics and thus reduces the overpotential for water oxidation. This study demonstrates a collaborative strategy of interface and surface engineering to design novel structures of α-Fe2O3 based photoanodes for highly efficient solar water oxidation. [ABSTRACT FROM AUTHOR]

We demonstrate the construction of heterojunction arrays for boosting solar water splitting by combining in situ guided growth of heterointerfaces and energy band tuning. By directly growing an ultrathin Fe2WO6 layer on a preformed WO3 nanosheet array in full coverage, a uniform and dense array of intimately contacted WO3/Fe2WO6 heterojunction was created. The heterojunction array shows not only a largely broadened visible light absorption range but also a built-in interface polarization to accelerate hole transfer from WO3 to Fe2WO6. Meanwhile, fine-tuning to match energy levels at the heterojunction was achieved by doping WO3 with Fe (Fe-WO3), leading to improved electrical conductivity and reduced charge recombination. Photoanodes based on such heterojunction arrays have shown significantly enhanced photoelectrochemical (PEC) water splitting performance, clearly arising from the above-mentioned efforts. Furthermore, by decorating FeOOH/NiOOH cocatalysts on the heterojunction arrays in tandem, the surface water oxidation kinetics was considerably accelerated, and the resulting Fe-WO3/Fe2WO6/FeOOH/NiOOH photoanode achieved a photocurrent density of 2.78 mA/cm2 at 1.23 V vs reversible hydrogen electrode. This work highlights the benefits of in situ construction of heterojunction arrays for enhancing the PEC performance. [ABSTRACT FROM AUTHOR]

The production of solar hydrogen with a silicon based water splitting device is a promising future technology, and silicon-based metal–insulator–semiconductor (MIS) electrodes have been proposed as suitable architectures for efficient photocathodes based on the electronic properties of the MIS structures and the catalytic properties of the metals. In this paper, we demonstrate that the interfaces between the metal and oxide of laterally patterned MIS electrodes may strongly enhance the catalytic activity of the electrode compared to bulk metal surfaces. The employed electrodes consist of well-defined, large-area arrays of gold structures of various mesoscopic sizes embedded in a silicon oxide support on silicon. We demonstrate that the activity of these electrodes for hydrogen evolution reaction (HER) increases with an increase in gold/silicon oxide boundary length in both acidic and alkaline media, although the enhancement of the HER rate in alkaline electrolytes is considerably larger than in acidic electrolytes. Electrodes with the largest interfacial length of gold/silicon oxide exhibited a 10-times larger HER rate in alkaline electrolytes than those with the smallest interfacial length. The data suggest that at the metal/silicon oxide boundaries, alkaline HER is enhanced through a bifunctional mechanism, which we tentatively relate to the laterally structured electrode geometry and to positive charges present in silicon oxide: Both properties change locally the interfacial electric field at the gold/silicon oxide boundary, which, in turn, facilitates a faster transport of hydroxide ions away from the electrode/electrolyte interface in alkaline solution. This mechanism boosts the alkaline HER activity of p-type silicon based photoelectrodes close to their HER activity in acidic electrolytes. [ABSTRACT FROM AUTHOR]

The bifunctional mechanism for the oxygen evolution reaction (OER) involving two distinct reaction sites is studied through the computational hydrogen electrode method for a set of catalyst materials including rutile TiO2(110), anatase TiO2(101), SnO2(110), RuO2(110), IrO2(110), Ni2P(0001), and BiVO4(001). The calculations are performed both at the semilocal level and at the hybrid functional level. Moreover, anodic conditions are modeled and their effect on the OER free energy steps is evaluated. The free energies of the reaction steps indicate that for specific combinations of catalysts, the limitations due to the linear scaling relationship can be overcome, leading to smaller overpotentials for the overall OER. At the same time, a detailed analysis of the results reveals a strong dependence on the adopted functional. For both functionals, it is shown that the energy level of the highest occupied electronic state can serve as a descriptor to guide the search for the optimal catalyst acting as a hydrogen acceptor. These results support the bifunctional mechanism as a means to break the linear scaling relationship and to further reduce the overpotential of the OER. [ABSTRACT FROM AUTHOR]

Metal oxides are promising photoanode materials for photoelectrochemical reactions due to their chemical stability and low-cost synthesis. However, their high photocarrier recombination rate limits their commercial applications. One strategy to promote photocarrier separation efficiency in metal oxides is building an inner electric field through band engineering techniques. In this study, a gradient band structure was prepared in (Ga, Fe)2O3 thin films on F:SnO2 (FTO) coated glass substrate by varying the Ga/Fe ratio. The performance of the gradient photoanode was evaluated by measuring its photocatalytic activity with photocurrent. The forward gradient photoanode, with increasing Ga/Fe ratio from bottom substrate to photoanode surface, exhibited a higher photocurrent density (0.44 mA / cm 2 at 1.23 V vs reversible hydrogen electrode (RHE) under AM 1.5G illumination) compared to the reverse gradient photoanode with decreasing the Ga/Fe ratio in the same situation. The forward gradient band structure, which facilitates photocarrier separation and transportation, was believed to play a dominant role in improving the photocatalytic performance. This work demonstrates an effective strategy to improve photocarrier separation and photocatalytic performance of metal oxide photocatalysts through band engineering. [ABSTRACT FROM AUTHOR]

In this work, we examine metal electrode-ionomer electrolyte systems at high voltage (negative surface charge) and at high pH to assess factors that influence hydrogen production efficiency. We simulate the hydrogen evolution electrode interface investigated experimentally in the work of Bates et al. [J. Phys. Chem. C 119, 5467 (2015)] using a combination of first principles calculations and classical molecular dynamics. With this detailed molecular information, we explore the hypotheses posed in the work of Bates et al. In particular, we examine the response of the system to increased bias voltage and oxide coverage in terms of the potential profile, changes in solvation and species concentrations away from the electrode, surface concentrations, and orientation of water at reactive surface sites. We discuss this response in the context of hydrogen production. [ABSTRACT FROM AUTHOR]

Due to the theoretical high specific capacity (3860 mAh/g) and the low standard electrode potential (−3.040 V vs. standard hydrogen electrode), rechargeable lithium metal batteries are considered as excellent energy storage systems. Unfortunately, security concerns related to dendrite formation during charge/discharge cycles still hinder the commercial use of Li metal-based batteries. Using density functional theory, we have studied the bulk and surface properties of metallic lithium at an atomistic level. In this process, bcc Li(100) proved to be the most stable metallic lithium surface. Subsequently, possible self-diffusion mechanisms on perfect and imperfect Li(100) surfaces were examined. For this purpose, nudged elastic band calculations were performed to characterize the respective diffusion processes and to determine the relevant pre-exponential factors and activation barriers. On the basis of the acquired data, it became possible to derive activation temperatures and reaction rates for the respective processes, which are useful for experimental verification as well as for the implementation in long-scale kinetic Monte Carlo simulations. [ABSTRACT FROM AUTHOR]

The absolute intrinsic hydration free energy G H + , w a t ◦ of the proton, the surface electric potential jump χ w a t ◦ upon entering bulk water, and the absolute redox potential V H + , w a t ◦ of the reference hydrogen electrode are cornerstone quantities for formulating single-ion thermodynamics on absolute scales. They can be easily calculated from each other but remain fundamentally elusive, i.e., they cannot be determined experimentally without invoking some extra-thermodynamic assumption (ETA). The Born model provides a natural framework to formulate such an assumption (Born ETA), as it automatically factors out the contribution of crossing the water surface from the hydration free energy. However, this model describes the short-range solvation inaccurately and relies on the choice of arbitrary ion-size parameters. In the present study, both shortcomings are alleviated by performing first-principle calculations of the hydration free energies of the sodium (Na+) and potassium (K+) ions. The calculations rely on thermodynamic integration based on quantum-mechanical molecular-mechanical (QM/MM) molecular dynamics (MD) simulations involving the ion and 2000 water molecules. The ion and its first hydration shell are described using a correlated ab initio method, namely resolution-of-identity second-order Møller-Plesset perturbation (RIMP2). The next hydration shells are described using the extended simple point charge water model (SPC/E). The hydration free energy is first calculated at the MM level and subsequently increased by a quantization term accounting for the transformation to a QM/MM description. It is also corrected for finite-size, approximate-electrostatics, and potential-summation errors, as well as standard-state definition. These computationally intensive simulations provide accurate first-principle estimates for G H + , w a t ◦ , χ w a t ◦ , and V H + , w a t ◦ , reported with statistical errors based on a confidence interval of 99%. The values obtained from the independent Na+ and K+ simulations are in excellent agreement. In particular, the difference between the two hydration free energies, which is not an elusive quantity, is 73.9 ± 5.4 kJ mol−1 (K+ minus Na+), to be compared with the experimental value of 71.7 ± 2.8 kJ mol−1. The calculated values of G H + , w a t ◦ , χ w a t ◦ , and V H + , w a t ◦ (−1096.7 ± 6.1 kJ mol−1, 0.10 ± 0.10 V, and 4.32 ± 0.06 V, respectively, averaging over the two ions) are also in remarkable agreement with the values recommended by Reif and Hünenberger based on a thorough analysis of the experimental literature (−1100 ± 5 kJ mol−1, 0.13 ± 0.10 V, and 4.28 ± 0.13 V, respectively). The QM/MM MD simulations are also shown to provide an accurate description of the hydration structure, dynamics, and energetics. [ABSTRACT FROM AUTHOR]

Composites of MnO2/multi-wall carbon nanotubes (MWCNTs) were prepared using different weight ratios of MWCNTs: KMnO4 (1:2, 1:5, 1:10, 1:15, 1:20, and 1:25) using a one-pot hydrothermal method. The synthesized materials were physically characterized by x-ray diffraction, transmission electron microscopy (TEM), field emission-scanning electron microscopy (FE-SEM), (Brunauer–Emmett–Teller) BET, and thermogravimetric analysis. TEM and SEM studies indicate that MnO2 is homogeneously entangled with MWCNTs. The electrochemical performance evaluation was performed in a 3-electrode system using MnO2/MWCNT electrodes coated onto a Ni mesh as the working electrode, a Pt foil as the counter electrode, and Ag/AgCl as the reference electrode. The specific capacitance was obtained from charge–discharge studies at varying current densities between 0.5 and 5 A/g. The specific capacitance of MWCNT-KMnO4 (1:10, 1:15, and 1:25) samples was obtained as 114, 164, and 100 F/g, respectively, at a current density of 1 A/g. [ABSTRACT FROM AUTHOR]

Composites of MnO2/multi-wall carbon nanotubes (MWCNTs) were prepared using different weight ratios of MWCNTs: KMnO4 (1:2, 1:5, 1:10, 1:15, 1:20, and 1:25) using a one-pot hydrothermal method. The synthesized materials were physically characterized by x-ray diffraction, transmission electron microscopy (TEM), field emission-scanning electron microscopy (FE-SEM), (Brunauer–Emmett–Teller) BET, and thermogravimetric analysis. TEM and SEM studies indicate that MnO2 is homogeneously entangled with MWCNTs. The electrochemical performance evaluation was performed in a 3-electrode system using MnO2/MWCNT electrodes coated onto a Ni mesh as the working electrode, a Pt foil as the counter electrode, and Ag/AgCl as the reference electrode. The specific capacitance was obtained from charge–discharge studies at varying current densities between 0.5 and 5 A/g. The specific capacitance of MWCNT-KMnO4 (1:10, 1:15, and 1:25) samples was obtained as 114, 164, and 100 F/g, respectively, at a current density of 1 A/g. [ABSTRACT FROM AUTHOR]

This research investigated responses of the Belousov–Zhabotinsky (BZ) reaction to the presence of a chemically inert Pt wire in solution. Experiments showed that connecting the Pt wire to a neutral ground caused a spontaneous drastic shift in the redox potential and might even induce complex behavior. Characterizations using an unstirred ferriin solution demonstrated the formation of a red colored propagating front at the grounded Pt wire, suggesting the reduction of ferriin to ferroin. Measurements with different combinations of electrodes in both stirred and reaction-diffusion media further confirmed the reduction of BZ metal catalysts at the Pt wire and the accompanying oxidation reaction at the reference electrode. The observed drastic change in redox potential and oscillation waveform can be understood based on the passive reduction reaction at the indicator electrode that is connected to the reference electrode through a potential meter. The obtained influence can be further manipulated by adding a resistor between the Pt wire and the neutral ground, making this convenient perturbation method attractive for the study of redox chemical reaction dynamics. [ABSTRACT FROM AUTHOR]

Traditional thermodynamic considerations show that the standard electrode potentials, E0, of metal/metal ion electrodes are linearly related to the Fermi energies of the metal electrons, μ. Expressing E0 in volts, μ in volts/ion, and accounting for the dielectric screening of the solution, the expected slope of the straight line equals 1. Having evaluated μ from the electronic contributions of the specific heats of 11 sp metals in terms of Sommerfeld's theory, we found a linear relationship between μ and E0. The slope of that line depends on the relative permittivity of water at the metal surface, ɛin. Taking ɛin = 6, in accordance with the general practice in electrochemistry, the expected slope was obtained without any parameter fitting. The intercept of the straight line can be interpreted as the absolute standard potential of the hydrogen electrode finding E H + / H 2 0 a b s = 3.86 ± 0.13 V. The calculations being based on equilibrium thermodynamics are independent of any actual mechanism. [ABSTRACT FROM AUTHOR]

Negative hydrogen (H−) ion sources are operated with Cs injection to reduce coextracted electron current. Injected Cs atoms adhere on the surface of the electrode with extraction holes and increase the local density of H− ions. The surface work function of the extraction electrode is the key parameter determining electron transfer from the electrode to hydrogen atom leaving the surface. Thus, the work functions of the target surfaces for obtaining fundamental process data were measured with the work function diode, Kelvin probe, and photoelectric method. The measurement of the work function of the extraction electrode accompanies difficulties as probe insertion blocks the plasma flow and photoelectric signals are small. Attempts made to monitor the surface conditions for realizing better ion source operations are summarized. [ABSTRACT FROM AUTHOR]

Solid solutions between ZnSe and Cu(In, Ga)Se2 (ZnSe:CIGS) have promising properties as photocathodes for solar hydrogen production from water, such as a long absorption edge of about 900 nm and a large driving force for reaction, >0.9 V, originated by a deep valence band maximum of 1.0–1.1 V vs normal hydrogen electrode (NHE). However, their performance is limited with an incident photon-to-current conversion efficiency (IPCE) of 65% at 400 nm and a half-cell solar-to-hydrogen energy conversion efficiency (HC-STH) of 3.6% so far. Te addition during ZnSe:CIGS thin film deposition by vacuum co-evaporation clearly decreased the optimal deposition temperature from 450 to 380 °C and resulted in columnar shaped grains of submicrometer size in diameter and with almost no grain boundary between the film surface and the backside electrode of Mo, which is significantly larger than the case of without Te addition. Interestingly, a Ga/In compositional ratio gradient was further introduced to the depth profile, which can facilitate charge separation. Structural characterizations using XRD and cross-sectional transmission electron microscopy revealed that the composition gradient was mainly formed by the diffusion of In through grain boundaries in the Ga-rich layer, and thus, the mixing between the Ga-rich and In-rich layers was more significant at a lower deposition temperature owing to the smaller grain size. The photocathode fabricated from the ZnSe:CIGS thin film with the composition gradient showed a very high IPCE of 89% at 540 nm and 0 V vs the reversible hydrogen electrode, and HC-STH of 3.7%, which is higher than values reported thus far. In conclusion, it is clarified that the potential gradient, which can be introduced by composition gradient, is beneficial for photoelectrodes and photocatalysts to achieve higher performance. [ABSTRACT FROM AUTHOR]

Group III-nitride alloys are believed to be promising photoelectrodes for photoelectrochemical water splitting to get hydrogen fuel. Here, we grew the InGaN nanowires (NWs) on silicon (111) as a photoanode using a low-cost chemical vapor deposition method. The photocurrent of an InGaN NWs' photoanode is five times greater than that of a GaN NWs' photoanode. The maximum photocurrent density of 8 mA/cm2 at 0.5 V vs the reverse hydrogen electrode with an applied bias photon-to-current efficiency of 5.8% was observed in the In30Ga70N NWs/Si (111) heterostructure. The incident photon-to-current conversion efficiency of 19.1% at 520 nm was observed for In30Ga70N NWs' photoanodes, which is much higher than GaN NWs. The hydrogen generation rates are 42.3 μmol/cm2 h at 0.15 V under 100 mW/cm2 white light irradiation. This InGaN/Si heterostructure photoanode improves photoelectrochemical performance for hydrogen fuel generation, paving the path for future development at a lower cost and on a larger scale. [ABSTRACT FROM AUTHOR]

Photoelectrochemical (PEC) water splitting directly converts solar energy into clean and sustainable hydrogen energy, representing a promising and effective technique for solving the world's energy crisis. A hole transfer layer (HTL) plays a vital role for extracting holes from a semiconductor layer to a back electrode in PEC cells for water splitting. In this work, a TiOx/Sb2Se3/NiOx photocathode was fabricated by using p-NiOx as HTL. We show that the electrical properties of NiOx films can be well adjusted by the pulsed laser deposition technique. By systematically adjusting the electrical properties of the NiOx HTL and passivating the surface defects through coating with the TiOx layer, we achieved a high photocurrent density of up to −18.08 mA cm−2 at 0 V vs a reversible hydrogen electrode (RHE), an onset potential of 0.43 V vs RHE, and a half-cell solar to hydrogen conversion efficiency of 1.01% for the TiOx/Sb2Se3/NiOx photocathode. This work demonstrated NiOx as a potential hole transport material for a cost-effective Sb2Se3 PEC cell. [ABSTRACT FROM AUTHOR]

Photoelectrochemical activity for water reduction (H2 liberation) over a co-evaporated CuGaSe2 compact thin film modified with a CdS layer and Pt deposits under simulated sunlight (AM 1.5G) radiation was evaluated, specifically focusing on the impact of a Cu-deficient layer (CDL) loaded on the top part of the CuGaSe2 film. It was found that the intentional loading of the CDL with an appropriate thickness was effective for achieving a large current flow and relatively positive photocurrent onset. The half-cell solar-to-hydrogen efficiency reached 6.6% over the best photocathode used. Moreover, the highest photocurrent onset potential of more than 0.9 V vs reversible hydrogen electrode was achieved over the photocathode based on the CuGaSe2 film having an extremely thick CDL (200 nm) with a relatively thick CdS layer (90 nm) due to efficient spatial separation of photogenerated carriers. [ABSTRACT FROM AUTHOR]

Although gallium nitride (GaN) nanostructures are auspicious for photocatalytic activity, geometrical optimization has paid much attention for a significant light trapping in photoelectrochemical applications. To minimize the optical losses, we designed a prototype V-groove textured Si (100) with (111) facets, and GaN nanorods (NRs) were grown over a prototype substrate using plasma-assisted molecular beam epitaxy. The photocurrent density of V-groove textured GaN NRs in the NaOH electrolyte is found to be 801.62 μA/cm2 at 1.14 V vs reversible hydrogen electrode, which was 2.1-fold larger than that of GaN NRs on plain Si (111). Using this prototype V-groove textured Si(100) with (111) facets, a significant light can be trapped and modulated into GaN NRs. Furthermore, the heterostructure between GaN NRs and V-groove textured Si stimulates effective charge separation and transportation. These results represent an important forward step in solar photoelectrolysis. [ABSTRACT FROM AUTHOR]