Electrochemical Doping of Two-Dimensional Superatomic Materials.

We report an electrochemical method for doping two-dimensional (2D) superatomic semiconductor Re6Se8Cl2 that significantly improves the material's electrical transport while retaining the in-plane and stacking structures. The electrochemical reduction induces the complete dissociation of chloride anions from the surface of each superatomic nanosheet. After the material is dehalogenated, we observe the electrical conductivity (σ) increases by two orders of magnitude while the 3D electron carrier density (n3D) increases by three orders of magnitude. In addition, the thermal activation energy (Ea) and electron mobility (µe) decrease. We conclude that we have achieved effective electron-doping in 2D superatomic Re6Se8Cl2, which significantly improves the electrical transport properties. Our work sets the foundation for electrochemically doping and tuning the transport properties of other 2D superatomic materials.

Confinement Effects on Proton Transfer in TiO2 Nanopores from Machine Learning Potential Molecular Dynamics Simulations.

Improved understanding of proton transfer in nanopores is critical for a wide range of emerging applications, yet experimentally probing mechanisms and energetics of this process remains a significant challenge. To help reveal details of this process, we developed and applied a machine learning potential derived from first-principles calculations to examine water reactivity and proton transfer in TiO2 slit-pores. We find that confinement of water within pores smaller than 0.5 nm imposes strong and complex effects on water reactivity and proton transfer. Although the proton transfer mechanism is similar to that at a TiO2 interface with bulk water, confinement reduces the activation energy of this process, leading to more frequent proton transfer events. This enhanced proton transfer stems from the contraction of oxygen-oxygen distances dictated by the interplay between confinement and hydrophilic interactions. Our simulations also highlight the importance of the surface topology, where faster proton transport is found in the direction where a unique arrangement of surface oxygens enables the formation of an ordered water chain. In a broader context, our study demonstrates that proton transfer in hydrophilic nanopores can be enhanced by controlling pore size, surface chemistry, and topology.

Oxide-Encapsulated Silver Electrocatalysts for Selective and Stable Syngas Production from Reactive Carbon Capture Solutions.

Electrolysis of bicarbonate-containing CO2 capture solutions is a promising approach towards achieving low-cost carbon-neutral chemicals production. However, the parasitic bicarbonate-mediated hydrogen evolution reaction (HER) and electrode instability in the presence of trace impurities remain major obstacles to overcome. This work demonstrates that the combined use of titanium dioxide (TiO2) overlayers with the chelating agent ethylene diamine tetra-acetic acid (EDTA) significantly enhances the selectivity and stability of Ag-based electrocatalysts for bicarbonate electrolysis. The amorphous TiO2 overlayers suppress the HER by over 50% at potentials more negative than -0.7 V vs. RHE, increasing the CO faradaic efficiency (FE) by 33% (relative). In situ surface-enhanced Raman spectroscopy (SERS) measurements reveal the absence of near-surface bicarbonate species and an abundance of CO2 reduction intermediates at the Ag|TiO2 buried interface, suggesting that the overlayers suppress HER by blocking bicarbonate ions from reaching the buried active sites. In accelerated degradation tests with 5 ppm of Fe(III) impurity, the addition of EDTA allows stable CO production with >47% FE, while the electrodes rapidly deactivate in the absence of EDTA. This work highlights the use of TiO2 overlayers for enhancing the CO:H2 ratio while simultaneously protecting electrocatalysts from impurities likely to be present in "open" carbon capture systems.

Condensed Layer Deposition of Nanoscopic TiO2 Overlayers on High-Surface-Area Electrocatalysts.

Encapsulating an electrocatalytic material with a semipermeable, nanoscopic oxide overlayer offers a promising approach to enhancing its stability, activity, and/or selectivity compared to an unencapsulated electrocatalyst. However, applying nanoscopic oxide encapsulation layers to high-surface-area electrodes such as nanoparticle-supported porous electrodes is a challenging task. This study demonstrates that the recently developed condensed layer deposition (CLD) method can be used for depositing nanoscopic (sub-10 nm thick) titanium dioxide (TiO2) overlayers onto high-surface-area platinized carbon foam electrodes. Characterization of the overlayers by transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS) showed that the films are amorphous, while X-ray photoelectron spectroscopy confirmed that they exhibit TiO2 stoichiometry. Electrodes were also characterized by hydrogen underpotential deposition (Hupd) and carbon monoxide (CO) stripping, demonstrating that the Pt electrocatalysts remain electrochemically active after encapsulation. Additionally, copper underpotential deposition (Cuupd) measurements revealed that TiO2 overlayers are effective at blocking Cu2+ from reaching the TiO2/Pt buried interface and were used to estimate that between 43 and 98% of Pt surface sites were encapsulated. Overall, this study shows that CLD is a promising approach for depositing nanoscopic protective overlayers on high-surface-area electrodes.

Single-Particle Measurements Reveal the Origin of Low Solar-to-Hydrogen Efficiency of Rh-Doped SrTiO3 Photocatalysts.

Solar-powered photochemical water splitting using suspensions of photocatalyst nanoparticles is an attractive route for economical production of green hydrogen. SrTiO3-based photocatalysts have been intensely investigated due to their stability and recently demonstrated near-100% external quantum yield (EQY) for water splitting using wavelengths below 360 nm. To extend the optical absorption into the visible, SrTiO3 nanoparticles have been doped with various transition metals. Here we demonstrate that doping SrTiO3 nanoparticles with 1% Rh introduces midgap acceptor states which reduce the free electron concentration by 5 orders of magnitude, dramatically reducing built-in potentials which could otherwise separate electron-hole (e-h) pairs. Rhodium states also function as recombination centers, reducing the photocarrier lifetime by nearly 2 orders of magnitude and the maximum achievable EQY to 10%. Furthermore, the absence of built-in electric fields within Rh-doped SrTiO3 nanoparticles suggests that modest e-h separation can be achieved by exploiting a difference in mobility between electrons and holes.

Functional Monolayers on a Superatomic Pegboard.

We advance the chemistry of apical chlorine substitution in the 2D superatomic semiconductor Re6Se8Cl2 to build functional and atomically precise monolayers on the surface of the 2D superatomic Re6Se8 substrate. We create a functional monolayer by installing surface (2,2'-bipyridine)-4-sulfide (Sbpy) groups that chelate to catalytically active metal complexes. Through this reaction chemistry, we can create monolayers where we can control the distribution of catalytic sites. As a demonstration, we create highly active electrocatalysts for the oxygen evolution reaction using monolayers of cobalt(acetylacetonate)2bipyridine. We can further produce a series of catalysts by incorporating organic spacers in the functional monolayers. The structure and flexibility of the surface linkers can affect the catalytic performance, possibly by tuning the coupling between the functional monolayer and the superatomic substrate. These studies establish that the Re6Se8 sheet behaves as a chemical pegboard: a surface amenable to geometrically and chemically well-defined modification to yield functional monolayers, in this case catalytically active, that are atomically precise. This is an effective method to generate diverse families of functional nanomaterials.

Mechanistic Insights on Permeation of Water over Iron Cations in Nanoporous Silicon Oxide Films for Selective H2 and O2 Evolution.

Electrocatalysts encapsulated by an ultrathin and semipermeable oxide layer offer a promising avenue for efficient, selective, and cost-effective production of hydrogen through photoelectrochemical water splitting. This architecture is especially attractive for Z-scheme water splitting, for which a nanoporous oxide film can be leveraged to mitigate undesired, yet kinetically facile, reactions involving redox shuttles, such as aqueous iron cations, by limiting transport of these species to catalytically active sites. In this work, molecular dynamics simulations were combined with electrochemical measurements to provide a mechanistic understanding of permeation of water and Fe(III)/Fe(II) redox shuttles through nanoporous SiO2 films. It is shown that even for SiO2 pores with a width as small as 0.8 nm, water does not experience any energy barrier for permeating into the pores due to a favorable interaction with hydrophilic silanol groups on the oxide surface. In contrast, permeation of Fe(III) and Fe(II) into microporous SiO2 pores is limited due to high energy barriers, which stem from a combination of distortion and dehydration of the second and third ion solvation shells. Our simulations and experimental results show that SiO2 coatings can effectively mitigate undesired Fe(III)/Fe(II) redox reactions at underlying electrodes by attenuating permeation of iron cations, while allowing water to permeate and thus participate in water splitting reactions. In a broader context, our study demonstrates that selectivity of solvated cations can be manipulated by controlling the pore size and surface chemistry of oxide films.

Probing the Effects of Electrode Composition and Morphology on the Effectiveness of Silicon Oxide Overlayers to Enhance Selective Oxygen Evolution in the Presence of Chloride Ions.

Seawater electrolysis offers significant logistical advantages over freshwater electrolysis but suffers from a fundamental selectivity problem at the anode. To prevent the evolution of toxic chlorine alongside the evolution of oxygen, a promising approach is the use of electrochemically inert overlayers. Such thin films can exert a perm-selective effect, allowing the transport of water and oxygen between the bulk electrolyte and the electrocatalytic buried interface while suppressing the transport of chloride ions. In this work, we investigate thin (5-20 nm) overlayer films composed of amorphous silicon oxide (SiO x ) and their application to suppressing the chlorine evolution reaction (CER) in favor of the oxygen evolution reaction (OER) during acidic saltwater electrolysis on three different types of electrodes. While SiO x overlayers are seen to be an effective barrier against the CER on well-defined, smooth Pt thin films, decreasing the CER activity roughly 20-fold, this ability has not been previously explored on Ir-based catalysts with a higher surface area relevant to industrial applications. On amorphous iridium oxide electrodes, the selectivity toward the CER versus the OER was marginally reduced from ∼98 to ∼94%, which was attributed to the higher abundance of defects in overlayers deposited on the rougher electrode. On the other hand, Ir-based anodes consisting of thick mixed metal oxide films supported on Ti showed a significant decrease in CER selectivity, from ∼100 to ∼50%, although this came at the cost of reduced activity toward the OER. These results show that the morphology and composition of the underlying electrode play important roles in the effectiveness of the selective overlayers and provide guidance for further development of high-surface-area OER-selective anodes.

Quantifying the Influence of Defects on Selectivity of Electrodes Encapsulated by Nanoscopic Silicon Oxide Overlayers.

Encapsulation of electrocatalysts and photocatalysts with semipermeable nanoscopic oxide overlayers that exhibit selective transport properties is an attractive approach to achieve high redox selectivity. However, defects within the overlayers─such as pinholes, cracks, or particle inclusions─may facilitate local high rates of parasitic reactions by creating pathways for facile transport of undesired reactants to exposed active sites. Scanning electrochemical microscopy (SECM) is an attractive method to determine the influence of defects on macroscopic performance metrics thanks to its ability to measure the relative rates of competing electrochemical reactions with high spatial resolution over the electrode. Here, we report the use of SECM to determine the influence of overlayer defects on the selectivity of silicon oxide (SiOx) encapsulated platinum thin-film electrocatalysts operated under conditions where two competing reactions─the hydrogen evolution and Fe(III) reduction reactions─can occur. After an SECM methodology is described to determine spatially resolved selectivity, representative selectivity maps are correlated with the location of defects that are characterized by optical, electron, and atomic force microscopies. This analysis reveals that certain types of defects in the oxide overlayer are responsible for ∼60-90% of the partial current density toward the undesired Fe(III) reduction reaction. By correcting for defect contributions to Fe(III) reduction rates, true Fe(III) permeability values for the SiOx overlayers were determined to be over an order of magnitude lower than permeabilities determined from analyses that ignore the presence of defects. Finally, different types of defects were studied revealing that defect morphology can have varying influence on both redox selectivity and calculated permeability. This work highlights the need for spatially resolved measurements to evaluate the performance of oxide-encapsulated catalysts and understand their performance limits.

Carbon-negative cement manufacturing from seawater-derived magnesium feedstocks.

This study describes and demonstrates key steps in a carbon-negative process for manufacturing cement from widely abundant seawater-derived magnesium (Mg) feedstocks. In contrast to conventional Portland cement, which starts with carbon-containing limestone as the source material, the proposed process uses membrane-free electrolyzers to facilitate the conversion of carbon-free magnesium ions (Mg2+) in seawater into magnesium hydroxide [Mg(OH)2] precursors for the production of Mg-based cement. After a low-temperature carbonation curing step converts Mg(OH)2 into magnesium carbonates through reaction with carbon dioxide (CO2), the resulting Mg-based binders can exhibit compressive strength comparable to that achieved by Portland cement after curing for only 2 days. Although the proposed "cement-from-seawater" process requires similar energy use per ton of cement as existing processes and is not currently suitable for use in conventional reinforced concrete, its potential to achieve a carbon-negative footprint makes it highly attractive to help decarbonize one of the most carbon-intensive industries.

Compressed Sensing Image Reconstruction of Scanning Electrochemical Microscopy Measurements Carried Out at Ultrahigh Scan Speeds Using Continuous Line Probes.

Previous studies on scanning electrochemical microscopy (SECM) imaging with nonlocal continuous line probes (CLPs) have demonstrated the ability to increase areal imaging rates by an order of magnitude compared to SECM based on conventional ultramicroelectrode (UME) disk electrodes. Increasing the linear scan speed of the CLP during imaging presents an opportunity to increase imaging rates even further but results in a significant deterioration in image quality due to transport processes in the liquid electrolyte. Here, we show that compressed sensing (CS) postprocessing can be successfully applied to CLP-based SECM measurements to reconstruct images with minimal distortion at probe scan rates greatly exceeding the conventional SECM ″speed limit″. By systematically evaluating the image quality of images generated by adaptable postprocessing CS methods for CLP-SECM data collected at varying scan rates, this work establishes a new upper bound for CLP scan rates. While conventional SECM imaging typically uses probe scan speeds characterized by Péclet numbers (Pe) < 1, this study shows that CS postprocessing methods can allow for an accurate image reconstruction for Pe approaching 5, corresponding to an order of magnitude increase in the maximum probe scan speed. This upper limit corresponds to the onset of chaotic convective flows within the electrolyte for the probes investigated in this work, highlighting the importance of considering hydrodynamics in the design of fast-scanning probes.

Self-terminating electrodeposition of Pt on WC electrocatalysts.

Self-terminated electrochemical deposition is used to grow Pt nanoparticles on tungsten monocarbide (WC) from a pH 4 electrolyte of 3 mmol/L K2PtCl4-0.5 mol/L NaCl. An unconventional potentiodynamic deposition program is used where nucleation is promoted at large overpotentials followed by growth termination at still larger overpotentials to yield a high coverage of Pt nanoparticles. Following three deposition cycles between -0.8 VSCE and -0.45 VSCE, the surface is covered by a monolayer equivalent charge of Pt in the form of ≈3 × 1011 particles/cm2 that are ≈6.7 ± 1.1 nm in diameter. The number and size of nanoparticles increase monotonically for five deposition cycles. Area-normalized kinetics for hydrogen evolution (HER) and oxidation (HOR) on Pt-WC were determined in 0.5 mol/L H2SO4. For the lowest surface coverage of Pt nanoparticles on WC, ≈ 0.01, an exchange current density of ≈ 100 mA/cm2 is achieved, comparable to the highest reported values for Pt nanoparticles and ultramicroelectrodes. The area normalized apparent exchange current density decreases with increasing Pt coverage as the relative contribution of point versus planar diffusion decreases. Self-terminated electrodeposition of Pt provides an attractive approach to achieving ultra-low loadings of well-dispersed Pt nanoparticles on a non-precious metal support like WC.

The Critical Role of Electrolyte Gating on the Hydrogen Evolution Performance of Monolayer MoS2.

According to density functional theory, monolayer (ML) MoS2 is predicted to possess electrocatalytic activity for the hydrogen evolution reaction (HER) that approaches that of platinum. However, its observed HER activity is much lower, which is widely believed to result from a large Schottky barrier between ML MoS2 and its electrical contact. In order to better understand the role of contact resistance in limiting the performance of ML MoS2 HER electrocatalysts, this study has employed well-defined test platforms that allow for the simultaneous measurement of contact resistance and electrocatalytic activity toward the HER during electrochemical testing. At open circuit potential, these measurements reveal that a 0.5 M H2SO4 electrolyte can act as a strong p-dopant that depletes free electrons in MoS2 and leads to extremely high contact resistance, even if the contact resistance of the as-made device in air is originally very low. However, under applied negative potentials this doping is mitigated by a strong electrolyte-mediated gating effect which can reduce the contact and sheet resistances of properly configured ML MoS2 electrocatalysts by more than 5 orders of magnitude. At potentials relevant to HER, the contact resistance becomes negligible and the performance of MoS2 electrodes is limited by HER kinetics. These findings have important implications for the design of low-dimensional semiconducting electrocatalysts and photocatalysts.

Design and operation of a scanning electrochemical microscope for imaging with continuous line probes.

This article describes a home-built scanning electrochemical microscope capable of achieving high areal imaging rates through the use of continuous line probes (CLPs) and compressed sensing (CS) image reconstruction. The CLP is a nonlocal probe consisting of a band electrode, where the achievable spatial resolution is set by the thickness of the band and the achievable imaging rate is largely determined by its width. A combination of linear and rotational motors allows for CLP scanning at different angles over areas up to 25 cm2 to generate the raw signal necessary to reconstruct the desired electrochemical image using CS signal analysis algorithms. Herein, we provide detailed descriptions of CLP fabrication, microscope design, and the procedures used to carry out scanning electrochemical microscopy imaging with CLPs. In order to illustrate the basic operating procedures for the microscope, line scans and images measured in the substrate generation-probe-collection mode for flat samples containing platinum disk electrodes are presented. These exemplary measurements illustrate methods for calibrating the positioning system, positioning and cleaning the CLP, and verifying proper positioning/probe sensitivity along its length.

Scanning Line Probe Microscopy: Beyond the Point Probe.

Scanning probe microscopy (SPM) techniques have become indispensable tools for studying nano- and microscale materials and processes but suffer from a trade-off between resolution and areal scan rate that limits their utility for a number of applications and sample types. Here, we present a novel approach to SPM imaging based on combining nonlocal scanning line probes with compressed sensing (CS) signal analysis methods. Using scanning electrochemical microscopy (SECM) as an exemplar SPM technique, we demonstrate this approach using continuous microband electrodes, or line probes, which are used to perform chemical imaging of electrocatalytic Pt discs deposited on an inert substrate. These results demonstrate the potential to achieve high areal SPM imaging rates using nonlocal scanning probes and CS image reconstruction.

Membraneless electrolyzers for the simultaneous production of acid and base.

This study investigates the use of membraneless electrolyzers based on angled mesh flow-through electrodes for the simultaneous production of acid and base (lye) from aqueous brine solutions. These electrolyte-agnostic flow cells are capable of producing a wide variety of acids and bases with precisely controlled pH using a simple cell design.

Enhanced Performance of Si MIS Photocathodes Containing Oxide-Coated Nanoparticle Electrocatalysts.

Electrodepositing low loadings of metallic nanoparticle catalysts onto the surface of semiconducting photoelectrodes is a highly attractive approach for decreasing catalyst costs and minimizing optical losses. However, securely anchoring nanoparticles to the photoelectrode surface can be challenging-especially if the surface is covered by a thin insulating overlayer. Herein, we report on Si-based photocathodes for the hydrogen evolution reaction that overcome this problem through the use of a 2-10 nm thick layer of silicon oxide (SiOx) that is deposited on top of Pt nanoparticle catalysts that were first electrodeposited on a 1.5 nm SiO2|p-Si(100) absorber layer. Such insulator-metal-insulator-semiconductor (IMIS) photoelectrodes exhibit superior durability and charge transfer properties compared to metal-insulator-semiconductor (MIS) control samples that lacked the secondary SiOx overlayer. Systematic investigation of the influence of particle loading, SiOx layer thickness, and illumination intensity suggests that the SiOx layer possesses moderate conductivity, thereby reducing charge transfer resistance associated with high local tunneling current densities between the p-Si and Pt nanoparticles. Importantly, the IMIS architecture is proven to be a highly effective approach for stabilizing electrocatalytic nanoparticles deposited on insulating overlayers without adversely affecting mass transport of reactant and product species associated with the hydrogen evolution reaction.

H2 evolution at Si-based metal-insulator-semiconductor photoelectrodes enhanced by inversion channel charge collection and H spillover.

Photoelectrochemical (PEC) water splitting represents a promising route for renewable production of hydrogen, but trade-offs between photoelectrode stability and efficiency have greatly limited the performance of PEC devices. In this work, we employ a metal-insulator-semiconductor (MIS) photoelectrode architecture that allows for stable and efficient water splitting using narrow bandgap semiconductors. Substantial improvement in the performance of Si-based MIS photocathodes is demonstrated through a combination of a high-quality thermal SiO2 layer and the use of bilayer metal catalysts. Scanning probe techniques were used to simultaneously map the photovoltaic and catalytic properties of the MIS surface and reveal the spillover-assisted evolution of hydrogen off the SiO2 surface and lateral photovoltage driven minority carrier transport over distances that can exceed 2 cm. The latter finding is explained by the photo- and electrolyte-induced formation of an inversion channel immediately beneath the SiO2/Si interface. These findings have important implications for further development of MIS photoelectrodes and offer the possibility of highly efficient PEC water splitting.

A new class of electrocatalysts for hydrogen production from water electrolysis: metal monolayers supported on low-cost transition metal carbides.

This work explores the opportunity to substantially reduce the cost of hydrogen evolution reaction (HER) catalysts by supporting monolayer (ML) amounts of precious metals on transition metal carbide substrates. The metal component includes platinum (Pt), palladium (Pd), and gold (Au); the low-cost carbide substrate includes tungsten carbides (WC and W(2)C) and molybdenum carbide (Mo(2)C). As a platform for these studies, single-phase carbide thin films with well-characterized surfaces have been synthesized, allowing for a direct comparison of the intrinsic HER activity of bare and Pt-modified carbide surfaces. It is found that WC and W(2)C are both excellent cathode support materials for ML Pt, exhibiting HER activities that are comparable to bulk Pt while displaying stable HER activity during chronopotentiometric HER measurements. The findings of excellent stability and HER activity of the ML Pt-WC and Pt-W(2)C surfaces may be explained by the similar bulk electronic properties of tungsten carbides to Pt, as is supported by density functional theory calculations. These results are further extended to other metal overlayers (Pd and Au) and supports (Mo(2)C), which demonstrate that the metal ML-supported transition metal carbide surfaces exhibit HER activity that is consistent with the well-known volcano relationship between activity and hydrogen binding energy. This work highlights the potential of using carbide materials to reduce the costs of hydrogen production from water electrolysis by serving as stable, low-cost supports for ML amounts of precious metals.