Search

Interactions between a transition metal (oxide) catalyst and a support can tailor the number and nature of active sites, for instance in the methanol oxidation reaction. We here use ambient pressure X-ray photoelectron spectroscopy (AP-XPS) to identify and compare the surface adsorbates that form on amorphous metal oxide films that maximize such interactions. Considering Al

Water electrolysis powered by renewable electricity produces green hydrogen and oxygen gas, which can be used for energy, fertilizer, and industrial applications and thus displace fossil fuels. Pure-water anion-exchange-membrane (AEM) electrolyzers in principle offer the advantages of commercialized proton-exchange-membrane systems (high current density, low cross over, output gas compression, etc.) while enabling the use of less-expensive steel components and nonprecious metal catalysts. AEM electrolyzer research and development, however, has been limited by the lack of broadly accessible materials that provide consistent cell performance, making it difficult to compare results across studies. Further, even when the same materials are used, different pretreatments and electrochemical analysis techniques can produce different results. Here, we report an AEM electrolyzer comprising commercially available catalysts, membrane, ionomer, and gas-diffusion layers operating near 1.9 V at 1 A cm-2 in pure water. After the initial break in, the performance degraded by 0.67 mV h-1 at 0.5 A cm-2 at 55 °C. We detail the key preparation, assembly, and operation techniques employed and show further performance improvements using advanced materials as a proof-of-concept for future AEM-electrolyzer development. The data thus provide an easily reproducible and comparatively high-performance baseline that can be used by other laboratories to calibrate the performance of improved cell components, nonprecious metal oxygen evolution, and hydrogen evolution catalysts and learn how to mitigate degradation pathways.

ConspectusCatalyzing the oxygen evolution reaction (OER) is important for key energy-storage technologies, particularly water electrolysis and photoelectrolysis for hydrogen fuel production. Under ...

The addition of redox-active molecules into electrochemical-capacitor electrolytes provides increased specific energy density. Here we illustrate the underlying operational mechanisms and design pr...

Alkaline-membrane electrolyzers operating in pure water might provide scalable low-cost H2 production but currently lag in performance and durability compared to commercial technologies. Typically,...

Summary Membrane electrolyzers enable the production of high-purity H2 from pure water. There is growing interest in developing this technology to operate in impure water. While proton-exchange-membrane (PEM) electrolyzers are the current industry leaders, anion-exchange-membrane (AEM) electrolyzers might be better suited for impure-water operation with further development. Here, the technical barriers to developing AEM electrolyzers for impure-water splitting are discussed. We highlight progress in the field and present possible research approaches to advance the field.

With suitable water dissociation (WD) catalysts, bipolar membranes (BPMs) can efficiently dissociate water into H+ and OH– at the junction between anion- and cation-exchange layers (AEL and CEL, re...

Plasmonic band-stop filters with tunable optical absorbance in the near- and mid-IR are important for wireless communications, bioimaging, and filtering applications. However, their design is const...

Easing water apartTraditional setups for splitting water into hydrogen and oxygen operate either in acid or in base. A bipolar membrane can potentially enhance efficiency by bridging acidic hydrogen evolution to basic oxygen evolution. Oeneret al.undertook a systematic study of how catalysts paired with such a membrane might accelerate the preliminary step of water dissociation into protons and hydroxide ions. Using insights from this study for optimal catalyst integration, they were able to substantially lower the overpotential of a bipolar membrane electrolyzer.Science, this issue p.1099

Commercialized membrane electrolyzers use acidic proton exchange membranes (PEMs). These systems offer high performance but require the use of expensive precious-metal catalysts such as IrO2 and Pt that are nominally stable under the locally acidic conditions of the ionomer. Alkaline-exchange-membrane (AEM) electrolyzers in principle offer the performance of commercialized PEM electrolyzers with the ability to use earth-abundant catalysts and inexpensive bipolar plate materials. I will present our fundamental work in understanding earth-abundant water-oxidation catalysts, including the use of integrated reference-electrode device architectures and cross-sectional materials analysis, as well as progress in building high-performance AEM electrolyzers. Baseline systems operate at 1 A·cm-2 in pure water feed at < 1.9 V at a moderate temperature of ~70 °C using either IrO2 or Co3O4 anode catalyst layers, PiperION alkaline ionomers, and stainless steel porous transport layers. These devices, however, degrade rapidly compared to PEM electrolyzers. The voltage profile corresponding to degradation has intial fast (~10 mV/h) and steady-state slow components (~1 mV/h), which we link to chemical and structural changes in the ionomer catalyst reactive zone. I will discuss these processes in electrolyzer devices and in model systems that are correlated with this performance loss, as well as present promising new strategies we have developed to mitigate degradation using novel ionomers and electrode catalyst compositions and architectures. Figure 1

Charge-carrier-selective interfaces between electrocatalyst particles and semiconductor light absorbers are critical for solar photochemistry but controlling their properties is challenging. In this talk I will show that the nanoelectrode tip of an atomic-force-microscope cantilever can sense the surface electrochemical potential of thin-film and nanoscale electrocatalysts coating semiconductor photoelectrodes during operation. This technique allowed us to unambiguously show that metal (oxy)hydroxide layers act as both hole collectors and oxygen-evolution catalysts on metal-oxide photoanodes such as Fe2O3 and BiVO4. We also discovered the critical role that heterogeneous interfacial barrier heights, and a related nanoscale pinch-off effect, play in building carrier-selective interfaces in semiconductor photoelectrodes for generating fuel from sunlight. As specific example, thin films and nanoparticle of Pt hydrogen-evolution catalysts on p-InP, a high-performance photocathode material, along with macroscopic and nanoscopic electrical and chemical analysis, are used to show how hydrogen alloying, the pinch-off effect for nanoscale contacts, and the formation of a native surface oxides all play different roles in creating charge-carrier-selective junctions. These measurements are compared and contrasted to a new approach to “wirelessly” measure interfacial for different contact materials by analyzing shifts in element-specific shifts in x-ray photoelectron emission energies. The sum of these new insights can be broadly applied to photocathodes, photoanodes, and overall water-splitting systems to control charge-carrier selectivity and improve performance.

Summary Electrochemistry is an established discipline with modern frontiers spanning energy conversion and storage, neuroscience, and organic synthesis. In spite of the expanding opportunities for academic and industrial electrochemists, particularly in the growing energy-storage sector, rigorous training of electrochemists is generally lacking at academic institutions in the United States. In this perspective, we highlight the core concepts of electrochemistry and discuss ways in which it has been historically taught. We identify challenges faced when teaching inherently interdisciplinary electrochemical concepts and discuss how technology provides new tools for teaching, such as inexpensive electronics and open-source software, to help address these challenges. Finally, we outline example programs and discuss how new tools and approaches can be brought together to prepare scientists and engineers for careers in electrochemical technology where they can accelerate the research, development, and deployment of the clean energy technology essential to combat climate change in the coming decades., Graphical abstract, Applied sciences; Education; Electrochemical materials science; Electrochemistry; Electrochemistry experimental methods; Engineering

The electronic structure of multimetal, amorphous oxides can be varied across a wide range of elemental compositions. Bulk properties such as conductivity, work function, and absorption can thus be tailored to suit a range of applications spanning from carrier-selective contacts to catalysis. Missing, however, is an understanding of how the surface reactivity is impacted in mixed metal-oxide amorphous films. Here we investigate the propensity of Al(1–x)M(x)Oy (M = Fe, Mn) amorphous oxide films to dissociate water into hydroxyl groups in a humid environment and find comparable hydroxylation at the low relative humidity (∼0.3% RH) probed by ambient pressure X-ray photoelectron spectroscopy. In contrast, films with both Al and Fe show an increased formation of methoxy groups upon methanol exposure compared to pure Al- and Fe-oxide end members, indicating that the coordination environment of the amorphous oxide network impacts the acidity and redox character of surface metal and oxygen sites. These results pr...

Semiconductor structures (for example, films, wires, particles) used in photoelectrochemical devices are often decorated with nanoparticles that catalyse fuel-forming reactions, including water oxidation, hydrogen evolution or carbon-dioxide reduction. For high performance, the catalyst nanoparticles must form charge-carrier-selective contacts with the underlying light-absorbing semiconductor, facilitating either hole or electron transfer while inhibiting collection of the opposite carrier. Despite the key role played by such selective contacts in photoelectrochemical energy conversion and storage, the underlying nanoscale interfaces are poorly understood because direct measurement of their properties is challenging, especially under operating conditions. Using an n-Si/Ni photoanode model system and potential-sensing atomic force microscopy, we measure interfacial electron-transfer processes and map the photovoltage generated during photoelectrochemical oxygen evolution at nanoscopic semiconductor/catalyst interfaces. We discover interfaces where the selectivity of low-Schottky-barrier n-Si/Ni contacts for holes is enhanced via a nanoscale size-dependent pinch-off effect produced when surrounding high-barrier regions develop during device operation. These results thus demonstrate (1) the ability to make nanoscale operando measurements of contact properties under practical photoelectrochemical conditions and (2) a design principle to control the flow of electrons and holes across semiconductor/catalyst junctions that is broadly relevant to different photoelectrochemical devices. Although nanocatalysts forming selective contacts are crucial in photoelectrochemistry, the underlying nanoscale interfaces are poorly understood. Using a n-Si/Ni photoanode and potential-sensing AFM, interfacial electron-transfer processes and photovoltage are measured.

FeOx Hy and Fe-containing Ni/Co oxyhydroxides are the most-active catalysts for the oxygen evolution reaction (OER) in alkaline media. However, the activity of Fe sites appears strongly dependent on the electrode-substrate material and/or the elemental composition of the matrix in which it is embedded. A fundamental understanding of these interactions that modulate the OER activity of FeOx Hy is lacking. We report the use of cyclic voltammetry and chronopotentiometry to assess the substrate-dependent activity of FeOx Hy on a number of commonly used electrode substrates, including Au, Pt, Pd, Cu, and C. We also evaluate the OER activity and Tafel behavior of these metallic substrates in 1 M KOH aqueous solution with Fe3+ and other electrolyte impurities. We find that the OER activity of FeOx Hy varies by substrate in the order Au>Pd≈Pt≈Cu>C. The trend may be caused by differences in the adsorption strength of the Fe oxo ion on the substrate, where a stronger adhesion results in more adsorbed Fe at the interface during steady-state OER and possibly a decreased charge-transfer resistance at the FeOx Hy -substrate interface. These results suggest that the local atomic and electronic structure of [FeO6 ] units play an important role in catalysis of the OER as the activity can be tuned substantially by substrate interactions.

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.

Metal-oxide thin films are used extensively in electronic and energy applications. Solution processing offers a potentially scalable and inexpensive deposition method to expand the applications of metal-oxide films and to complement vacuum-deposition techniques. Among precursors for solution deposition, metal nitrates stand out for their ability to form high-quality metal-oxide thin films. This review focuses on unique aspects of metal-nitrate chemistry that have been exploited to advance the development of solution-processed thin films. We discuss the solid-state bulk, solution, and thin-film chemical reactions involving metal nitrates and illustrate how the resulting metal-oxide thin-film properties depend on the entire reaction pathway. To conclude, we offer perspective as to how understanding the chemistry of film formation from metal-nitrate precursors is useful for addressing the primary drivers for industrial manufacturing of solution-processed metal-oxide thin films.

Low-cost methods of III-V deposition are an important component of making high-efficiency III-V solar cells cost-competitive for terrestrial applications. Here, we report the design and testing of a close-spaced vapor transport system for the growth of epitaxial GaAs films using solid precursors and HCl as the transport agent. Previous work on a related system demonstrated growth of high-quality GaAs using H2O as the transport agent, but the use of H2O generates oxide-related defects and limits material compatibility. The new design also enables independent handling of source and substrate material. The effect of process conditions on growth rate, surface morphology, and substrate-orientation-dependent growth is discussed. We demonstrate successful doping of Si and Zn with average transport efficiencies of approximately 30% and 20%, respectively, based on secondary-ion-mass-spectrometry data. Room-temperature hall mobilities approached those achieved for GaAs grown by metal-organic vapor phase epitaxy and water-based close-spaced vapor transport, 2210–3400 cm2 V−1 s−1 for n-GaAs and 70–110 cm2 V−1 s−1 for p-GaAs depending on dopant concentration. Initial results on doping and cross-contamination suggest this system should be capable of homoepitaxial growth of p-n junctions.

Anion-exchange-membrane water electrolyzers (AEMWEs) in principle operate without soluble electrolyte using earth-abundant catalysts and cell materials and thus lower the cost of green H

The desolvation of ions at electrochemical interfaces is widely accepted as being the rate limiting step to charge transfer in devices such as ion-intercalation batteries and electrolyzers. While activation energies can be obtained for processes like ion intercalation, the kinetics and mechanism of desolvation remain elusive because they are often convoluted with resistances arising from complex interfacial chemistries. Here I present the use of a biphasic microfluidic electrochemical cell to simultaneously study the kinetics and stoichiometry of tetrabutylammonium (TBA) ion transfer at the interface between two immiscible electrolyte solutions (ITIES). Ion transfer at the ITIES allows one to mostly eliminate the variable of complex interfacial chemistry in the process of desolvation, and with our flow cell geometry we can separate the two phases after ion transfer to measure reaction products via quantitative nuclear magnetic resonance spectroscopy (qNMR). Thus, our novel microfluidic platform for studying electrochemical ion transfer allows us to correlate desolvation kinetics with ion-solvent shell identity, and eventually inform the design of ion transfer mediators/catalysts for the improvement of energy storage technology. Figure 1

The presence of a carrier selective contact is fundamental for photoelectrochemical (PEC) optoelectronic devices which convert solar energy to chemical/electrical energy. For example, during the overall water splitting (OWS) reaction in an acidic electrolyte, the generated electrons and holes must selectively transport to their respective contacts where they reduce protons and oxidize water, forming H2(g) and O2(g) respectively. Often, bare semiconductor (SEM) absorbers are poor catalysts and metals (or metal oxides) (CAT) are alternatively used, although forming a new junction with the underlying SEM. In some cases, the CAT/SEM junction does not theoretically form the required carrier selective junction. A real photocathode consisting of a p-InP and Pt junction should theoretically form an ohmic or non-selective junction, however, this photocathode efficiently reduces protons in operation. In extension, many single OWS particles decorated with cocatalysts theoretically would not form selective contacts producing sufficient open-circuit voltages required to split water (by simple thermionic-emission theory alone). This is further debated in the mechanism for carrier separation during photoelectrodeposition of catalyst in such OWS particles. Here, we exploited conductive probe microscopies and surface-sensitive spectroscopies to reveal the underlying mechanism driving carrier separation towards selective contacts such as alloying transformation and heterojunction formation in the Pt/p-InP system. Additionally, I will demonstrate the unique behavior of nanoscopic electrical contacts by individual characterization (operando and in-situ), demonstrating selectivity by a manifestation of the pinch-off effect. On the Pt/p-InP model system, individual contact selectivity, by the pinch-off effect, on a chemically controlled surface seen in Figure 1.a later evolves due to the formation of an InOx/p-InP heterojunction after operation (after CV). Furthermore, we investigate SrTiO3 as a planar model system proxy to OWS particles and show only small variation in the oxygen evolution reaction (OER) current in macroscopic cyclic voltammograms of and terminated surfaces (Figure 1.b). By revealing the underlying mechanisms and the degree that they play, PEC devices can be engineered utilizing an optimal absorber and catalyst whilst their pristine junction would theoretically be non-selective or producing an insufficient photovoltage. Figure 1

Charge-carrier-selective interfaces between electrocatalyst particles and semiconductor light absorbers are critical for solar photochemistry but controlling their properties is challenging. In this talk I will show that the nanoelectrode tip of an atomic-force-microscope cantilever can sense the surface electrochemical potential of thin-film and nanoscale electrocatalysts coating semiconductor photoelectrodes during operation. This technique allowed us to unambiguously show that metal (oxy)hydroxide layers act as both hole collectors and oxygen-evolution catalysts on metal-oxide photoanodes such as Fe2O3 and BiVO4. We also discovered the critical role that heterogeneous interfacial barrier heights, and a related nanoscale pinch-off effect, play in building carrier-selective interfaces in semiconductor photoelectrodes for generating fuel from sunlight. As specific example, thin films and nanoparticle of Pt hydrogen-evolution catalysts on p-InP, a high-performance photocathode material, along with macroscopic and nanoscopic electrical and chemical analysis, are used to show how hydrogen alloying, the pinch-off effect for nanoscale contacts, and the formation of a native surface oxides all play different roles in creating charge-carrier-selective junctions. The sum of these new insights can be broadly applied to photocathodes, photoanodes, and overall water-splitting systems to control charge-carrier selectivity and improve performance.

Heterogeneous electrocatalysts for the oxygen evolution reaction (OER) are complicated materials with dynamic structures. They exhibit potential-induced phase transitions, potential-dependent electronic properties, variable oxidation and protonation states, and disordered local/surface phases. These properties make understanding the OER, and ultimately designing higher-efficiency catalysts, challenging. Measurements of intrinsic activity show that, by far, the most-active phases for OER under alkaline conditions are Fe-containing mixed-metal oxyhydroxides, but exactly how they function remains controversial. I will discuss our work to understand the key properties of these catalysts, including morphology, composition, and molecular/electronic structure, and how they evolve and are dynamic under active catalytic conditions. Specifically, I will highlight new work where we have been able to systematically control the concentration of surface Fe species on NiOOH and CoOOH host structures. We discover that the per-Fe turn-over frequency (TOFFe) for the OER increases nearly linearly with the Fe concentration, providing clear evidence for cooperative effects between Fe atoms and suggestive of a mechanism involving multiple Fe sites that is supported by density functional theory calculations. In sum, these concepts inform design strategies for higher-performance catalyst architectures and for their incorporation into practical electrolyzer devices to make clean hydrogen fuel from inexpensive renewable electricity for green long-distance transportation and long-duration energy storage.

Non-potable water represents a vast resource that can enable hydrogen generation to support energy storage and chemical manufacturing, as well as oxygen generation for undersea applications. Ionic impurities present in brackish water, wastewater, and seawater are highly detrimental to the operational stability of state-of-the-art proton-exchange membrane water electrolyzers (PEMWE). Chloride (Cl-) impurities are particularly pernicious, as Cl- can be oxidized to form reactive Cl2/HOCl/OCl- species at the anode which accelerates the degradation of electrolyzer components. In contrast, bipolar membrane water electrolyzer (BPMWE) architectures can be used to control the transport and reactivity of feed impurities by leveraging ion-selective membranes that allow for independent control of electrolyte pH at each electrode, thereby opening a path toward impurity-tolerant electrolyzers. In this work, we compare the performance of BPMWE and PEMWE architectures fed by simulated seawater (0.5M NaClaq) at current densities up to 500mA cm-2 for up to six hours. We present quantification of the transport of ions across the BPM, as well as oxidized chlorine species generation. In the PEMWE construct, we observe up to 14 μM of oxidized species after only 6 hours of operation; in the BPMWE, chloride oxidation is successfully suppressed (-2. Furthermore, we observed that Cl- ion crossover was constant even as current density increased, highlighting that cross-membrane H+/OH- transport is favored at high current densities.

Copper is used in a variety of capacities in the development of renewable energy technologies, including as a catalyst on gas diffusion electrodes in electrochemical devices such as CO2 reduction devices. By gaining a better understanding of Cu corrosion, we can learn more about the durability of devices that utilize copper. However, there are a myriad of factors that influence the kinetics of metal corrosion and deposition, which increases the difficulty to precisely analyze the fundamentals. Factors of concern include adsorbing/desorbing multiple layers of metal, uneven surfaces, defects on the electrode surface, and changes in electrode crystal facets. To mitigate these factors, obtain a more representative picture of Cu corrosion, and investigate the effect of the anions on copper, we created three model systems by employing copper underpotential deposition (UPD) on a single-crystalline Au[111] surface: 1) a phenomenal model system in sulfuric acid; 2) a bridging environment (methane sulfonic acid); and 3) a model microenvironment for gas diffusion (Nafion electrolyte). As a result, a loss in peak sharpness, as well as an increase in the number of peaks that are less resolved were observed in phenomenal and bridging environment, respectively, and a loss in ordered monolayers was seen in model microenvironment. Surprisingly, sulfuric acid and methane sulfonic acid have similar charges passed indicating the same amount of copper is deposited and stripped, whereas much less charge is passed in Nafion, demonstrating that small changes to the supporting anion can have a large effect on Cu UPD structure. As the real world does not exist in a model system, our model microenvironment will provide a better image of Cu corrosion fundamentals in a gas diffusion electrode, which is critical for electrochemical devices application. Figure 1

Water electrolysis powered by renewable sources such as wind or solar produces clean hydrogen gas, which is used for many industrial processes and will be essential in a future clean energy economy. Alkaline water electrolysis (AWE) and proton exchange membrane (PEM) electrolysis are mature technologies at megawatt to gigawatt scale. AWE uses earth abundant electrode and catalyst materials submerged in caustic liquid electrolyte (usually KOH or NaOH) separated by a porous diaphragm. This configuration can suffer from gas crossover and shunt current limitations. PEM electrolyzers circumvent these barriers by using an ionically-conductive solid electrolyte membrane that reduces gas crossover and allows for pure water operation that eliminates shunt currents. However, these membranes create a locally acidic environment that necessitates the use of expensive platinum-group-metal (PGM) catalyst materials and hardware coatings. Anion exchange membrane (AEM) electrolysis is an emerging technology that has the potential to combine the benefits of liquid alkaline and PEM. AEM electrolyzers use an anion-selective membrane, maintaining the pure water operation of PEM but creating a locally-alkaline environment allowing for the use of non-PGM materials. However, AEM electrolysis is still immature. In particular, the hydroxide-conducting membrane and ionomer have not yet achieved sufficient stability in pure water to compete with PEM systems. The specific dominant degradation mechanism and location has also not been conclusively identified. Further, while non-PGM catalysts such as Ni- and Fe-based oxyhydroxide catalysts out-compete PGM catalysts in three-electrode KOH studies, this performance does not carry to the pure water electrolyzer configuration. In my talk I report AEM electrolyzer performance below 2 V at 1 A·cm-2 in pure water for both PGM and non-PGM anode catalysts. I will also discuss the specific degradation mechanism of the ionomer and membrane during electrolyzer operation with both catalysts. Understanding the differences in performance and durability with precious metal and earth abundant materials is key for AEMs to compete with current electrolyzer technologies. I compare the performance and stability of five Ni-, Co-, and Fe-based catalysts and compare to IrOx. XPS and cross-sectional SEM data show oxidation of the ionomer binder at the anode for all catalysts, which results in significant degradation and performance loss. I also show additional degradation is induced by introducing soluble Fe species, indicating separate mechanisms beyond ionomer degradation are present in the non-PGM systems. In addition to the fundamental insights into the dynamic behavior of non-PGM catalysts and oxidative durability of ionomeric polymer materials, these findings contribute to bringing low-cost AEM electrolyzer technology to scale. Figure 1

Solar water splitting provides a mechanism to convert and store solar energy in the form of stable chemical bonds. Water-splitting systems often include semiconductor photoanodes, such as n-Fe2O3 and n-BiVO4, which use photogenerated holes to oxidize water. These photoanodes often exhibit improved performance when coated with metal-oxide/(oxy)hydroxide overlayers that are catalytic for the water-oxidation reaction. The mechanism for this improvement, however, remains a controversial topic. This is, in part, due to a lack of experimental techniques that are able to directly track the flow of photogenerated holes in such multicomponent systems. In this Perspective, we illustrate how this issue can be addressed by using a second working electrode to make direct current/voltage measurements on the catalytic overlayer during operation in a photoelectrochemical cell. We discuss examples where the second working electrode is a thin metallic film deposited on the catalyst layer, as well as where it is the tip of ...

Anion exchange membrane (AEM) electrolysis is a promising technology to produce hydrogen through the splitting of pure water. In contrast to proton-exchange-membrane (PEM) technology, which require...

Developing low-cost and efficient electrocatalysts to accelerate oxygen evolution reaction (OER) kinetics is vital for water and carbon-dioxide electrolyzers. The fastest-known water oxidation catalyst, Ni(Fe)OxHy, usually produced through an electrochemical reconstruction of precatalysts under alkaline condition, has received substantial attention. However, the reconstruction in the reported catalysts usually leads to a limited active layer and poorly controlled Fe-activated sites. Here, we demonstrate a new electrochemistry-driven F-enabled surface-reconstruction strategy for converting the ultrathin NiFeOxFy nanosheets into an Fe-enriched Ni(Fe)OxHy phase. The activated electrocatalyst shows a low OER overpotential of 218 ± 5 mV at 10 mA cm-2 and a low Tafel slope of 31 ± 4 mV dec-1, which is among the best for NiFe-based OER electrocatalysts. Such superior performance is caused by the effective formation of the Fe-enriched Ni(Fe)OxHy active-phase that is identified by operando Raman spectroscopy and the substantially improved surface wettability and gas-bubble-releasing behavior.

Here, we employ a combination of 27Al solid-state nuclear magnetic resonance (SSNMR) and conventional spectroscopic and microscopic techniques to investigate the structural evolution of aqueous aluminum precursors to a uniform and smooth aluminum oxide film. The route involves no organic ligands and relies on dehydration, dehydroxylation, and nitrate loss for condensation and formation of the three-dimensional aluminum oxide structure. Local chemical environments are tracked as films evolve over the temperature range 200–1100 °C. 27Al SSNMR reveals that Al centers are predominantly four- and five-coordinate in amorphous films annealed between 200 and 800 °C and four- and six-coordinate in crystalline phases that form above 800 °C. The Al coordination of the aqueous-deposited aluminum oxide films are compared to data from SSNMR studies on vapor-phase-deposited aluminum oxide thin films. Additionally, dielectric constants of aluminum oxide-based capacitors are measured and correlated with the SSNMR results....

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 .

Oxide/(oxy)hydroxide overlayers such as cobalt (oxy)hydroxide phosphate (CoPi) enhance the performance of BiVO4 water-spitting photoanodes, but the mechanism of this enhancement remains unclear. We show that if the BiVO4 layer is thin and incompletely covers an underlying conductive glass, the performance dramatically decreases as CoPi loading is increased. This is consistent with direct contact between the CoPi and conducting glass that leads to “shunt” recombination of photogenerated holes accumulated in the CoPi. For thicker BiVO4 layers that completely cover the conducting glass, these shunt pathways are blocked. We then use a nanoelectrode atomic force microscopy probe to measure, in operando, the electrochemical potential of CoPi on thick BiVO4 films under illumination. We find that CoPi is charged to a potential necessary to drive water oxidation at a rate consistent with the measured photocurrent. CoPi acts as a hole collector and is the principal driver of water oxidation on BiVO4.

Metal (oxy)hydroxides (MOxHy, M = Fe, Co, Ni, and mixtures thereof) are important materials in electrochemistry. In particular, MOxHy are the fastest known catalysts for the oxygen evolution reacti...