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BACKGROUND: Sodium chlorate (NaClO3) is extensively used in the paper industry, but its production uses strictly regulated highly toxic Na2Cr2O7 to reach high hydrogen evolution reaction (HER) Faradaic efficiencies. It is therefore important to find alternatives either to replace Na2Cr2O7 or reduce its concentration. RESULTS: The Na2Cr2O7 concentration can be significantly reduced by using Na2MoO4 as an electrolyte co-additive. Na2MoO4 in the millimolar range shifts the platinum cathode potential to less negative values due to an activating effect of cathodically deposited Mo species. It also acts as a stabilizer of the electrodeposited chromium hydroxide but has a minor effect on the HER Faradaic efficiency. X-ray photoelectron spectroscopy (XPS) results show cathodic deposition of molybdenum of different oxidation states, depending on deposition conditions. Once Na2Cr2O7 was present, molybdenum was not detected by XPS, as it is likely that only trace levels were deposited. Using electrochemical measurements and mass spectrometry we quantitatively monitored H-2 and O-2 production rates. The results indicate that 3 mu mol L-1 Na2Cr2O7 (contrary to current industrial 10-30 mmol L-1) is sufficient to enhance the HER Faradaic efficiency on platinum by 15%, and by co-adding 10 mmol L-1 Na2MoO4 the cathode is activated while avoiding detrimental O-2 generation from chemical and electrochemical reactions. Higher concentrations of Na2MoO4 led to increased oxygen production. CONCLUSION: Careful tuning of the molybdate concentration can enhance performance of the chlorate process using chromate in the micromolar range. These insights could be also exploited in the efficient hydrogen generation by photocatalytic water splitting and in the remediation of industrial wastewater.

Electrochemical decarboxylation of acetic acid on boron-doped diamond electrodes (BDD) was studied as a possible means to decrease the acidity of pyrolysis oil. It is shown that decarboxylation occurs without competitive OER on BBD electrodes to form methanol and methyl acetate by consecutive reaction of hydroxyl radicals with acetic acid. The performance is little affected by the applied current density (and associated potential), concentration, and the pH of the solution. At current densities above 50 mA/cm2, FEs of 90% towards decarboxylation products are obtained, confirmed by situ electrochemical mass spectrometry (ECMS) investigation showing only small amounts of oxygen formed by water oxidation. Using platinum-modified BDD electrodes, it is shown that ethane-selectivity, the Kolbe product, strongly depends on the shape and geometry of the platinum particles. Using nano-thorn alike Pt particles, a faradaic efficiency of approx. 40 % towards ethane can be obtained, whereas 3D porous platinum nanoparticles showed high selectivity towards OER. Using thin platinum layers, a high FE towards ethane was obtained >70%, which is thickness-independent at layer thicknesses above 20 nm. Comparison with other substrates revealed that BDD is an ideal support for Pt functionalisation giving advantage of stability, high valuable products formation (ethane and methanol). In short, this work provides guidelines for electrode fabrication in the context of the electrochemical upgrading of biomass feedstocks by acid decarboxylation.

This review discusses the developments in CO separation techniques in the last few decades and the gaps in knowledge it leaves, as well as sources of CO, both traditional and emerging, which would benefit from improved CO separation.

Chromium-oxyhydroxide (CrxOyHz)-based thin films have previously been shown in photocatalysis and industrial chlorate production to prevent unwanted reduction reactions to occur, thereby enhancing the selectivity for hydrogen evolution and thus the overall process efficiency. Here, a highly reproducible synthesis protocol was developed to allow for the electrodeposition of CrxOyHz-based thin films with controlled thickness in the range of the sub-monolayer up to (>4) multilayer coverage. Electrodeposited CrxOyHz coatings were electrochemically characterized using voltammetry and stripping experiments, allowing thickness-dependent film selectivity to be deduced in detail. The results are discussed in terms of mass transport properties and structure of the electrodeposited chromium oxyhydroxide films. It is shown that the permeation of diatomic probe molecules, such as O2 and CO, was significantly reduced by films as thin as four monolayers. Importantly, it is shown that the prepared thin film coatings enabled prolonged hydrogen oxidation in the presence of CO (up to 5 vol.%), demonstrating the benefits of thin-film-protected electrocatalysts. In general, this study provides insight into the synthesis and use of thin-film-protected electrodes leading to improvements in (electro)catalyst selectivity and durability.

By photodeposition of CrOx on SrTiO3-based semiconductors doped with aliovalent Mg(II) and functionalized with Ni/NiOx catalytic nanoparticles (economically significantly more viable than commonly used Rh catalysts), an increase in apparent quantum yield (AQYs) from ∼10 to 26% in overall water splitting was obtained. More importantly, deposition of CrOx also significantly enhances the stability of Ni/NiO nanoparticles in the production of hydrogen, allowing sustained operation, even in intermittent cycles of illumination. In situ elemental analysis of the water constituents during or after photocatalysis by inductively coupled plasma mass spectrometry/optical emission spectrometry shows that after CrOx deposition, dissolution of Ni ions from Ni/NiOx-Mg:SrTiO3 is significantly suppressed, in agreement with the stabilizing effect observed, when both Mg dopant and CrOx are present. State-of-the-art electron microscopy and energy-dispersive X-ray spectroscopy (EDX) and electron energy-loss spectroscopy (EELS) analyses demonstrate that upon preparation, CrOx is photodeposited in the vicinity of several, but not all, Ni/NiOx particles. This implies the formation of a Ni-Cr mixed metal oxide, which is highly effective in water reduction. Inhomogeneities in the interfacial contact, evident from differences in contact angles between Ni/NiOx particles and the Mg:SrTiO3 semiconductor, likely affect the probability of reduction of Cr(VI) species during synthesis by photodeposition, explaining the observed inhomogeneity in the spatial CrOx distribution. Furthermore, by comparison with undoped SrTiO3, Mg-doping appears essential to provide such favorable interfacial contact and to establish the beneficial effect of CrOx. This study suggests that the performance of semiconductors can be significantly improved if inhomogeneities in interfacial contact between semiconductors and highly effective catalytic nanoparticles can be resolved by (surface) doping and improved synthesis protocols.

The economic viability of the electrochemical chlorate process depends on toxic chromate to induce cathodic selectivity to hydrogen and mitigate reduction of hypochlorite or chlorate. In this study, it is shown that performance of a pilot plant for chlorate production can be sustained when a 1000-fold reduction in chromate concentration is compensated by addition of molybdate. Laboratory measurements employing a Quartz Crystal Microbalance suggest growth of a nanometre-thick hybrid Mo–Cr-oxide film to induce cathodic selectivity. An optimized energy efficiency for pilot plant operation was obtained using 0.8 mM molybdate and 27 μM chromate, balancing formation of an effective oxide layer and undesired Mo-induced decomposition of hypochlorite to oxygen in solution. Refinement at the pilot scale level is expected to further optimize the energy consumption, thereby increasing safety aspects and the economic viability of chlorate production. Graphical Abstract

We report the design and one-pot synthesis of Ag-doped BiVO4 embedded in reduced graphene oxide (BiVO4:Ag/rGO) nanocomposites via a hydrothermal processing route. The binary heterojunction photocatalysts exhibited high efficiency for visible light degradation of model dyes and were correspondingly used for the preparation of photocatalytic membranes using polyvinylidene fluoride (PVDF) or polyethylene glycol (PEG)-modified polyimide (PI), respectively. The surface and cross-section images combined with elemental mapping illustrated the effective distribution of the nanocomposites within the polymeric membranes. Photocatalytic degradation efficiencies of 61% and 70% were achieved after 5 h of visible light irradiation using BiVO4:Ag/rGO@PVDF and BiVO4:Ag/rGO@PI (PEG-modified) systems, respectively. The beneficial photocatalytic performance of the BiVO4:Ag/rGO@PI (PEG-modified) membrane is explained by the higher hydrophilicity due to the PEG modification of the PI membrane. This work may provide a rational and effective strategy to fabricate highly efficient photocatalytic nanocomposite membranes with well-contacted interfaces for environmental purification.

Kolbe already discovered in 1849 that electrochemical oxidative decarboxylation of carboxylic acids is feasible and leads to formation of alkanes and CO2, via alkyl radical intermediates. We now show for Pt electrodes that Kolbe electrolysis of acetic acid is favored in electrolytes with a pH similar to, or larger than the pKa of acetic acid, suppressing the formation of O2. However extended duration of electrolysis of acetate at basic pH results in loss of Faradaic efficiency to ethane, compensated by the formation of methanol. This change in selectivity is likely caused by the dissolution of CO2 near the electrode-electrolyte interface, resulting in enlarged concentration of bicarbonate/carbonate. On the positively polarized, and oxidized Pt surface, these anions seem to inhibit homocoupling of methyl radicals to ethane. These results demonstrate that reaction selectivity in acetic acid (acetate) oxidation using oxidized Pt electrodes is determined by the pH and the anionic composition of the electrolyte.

Functionalization of semiconductors by metallic nanoparticle is considered to be one of the most effective procedure to improve photocatalytic hydrogen production. Photodeposition is frequently used for functionalization but particle sizes and dispersions are still difficult to control. Here, Pt functionalization is achieved in a one-pot synthesis. The as-prepared samples are compared to reference materials prepared by conventional photodeposition and our results confirm that small and well-dispersed nanoparticles with superior stability are obtained by one-pot synthesis. The enhanced stability is attributed to a limited leaching of Pt nanoparticles during illumination likely caused by the preferable interaction of small, well dispersed Pt nanoparticles with the TiO2 support material. In addition, our results demonstrate that Na-residues are detrimental for the photocatalytic performance and washing in acidic solution is mandatory to effectively reduce the sodium contamination.

Confocal fluorescence microscopy is a proven technique, which can image near-electrode pH changes. For a complete understanding of electrode processes, time-resolved measurements are required, which have not yet been provided. Here we present the first measurements of time-resolved pH profiles with confocal fluorescence microscopy. The experimental results compare favorably with a one-dimensional reaction-diffusion model; this holds up to the point where the measurements reveal three-dimensionality in the pH distribution. Specific factors affecting the pH measurement such as attenuation of light and the role of dye migration are also discussed in detail. The method is further applied to reveal the buffer effects observed in sulfate-containing electrolytes. The work presented here is paving the way toward the use of confocal fluorescence microscopy in the measurement of 3D time-resolved pH changes in numerous electrochemical settings, for example in the vicinity of bubbles., Comment: 10 pages, 7 figures, Supplementary information

Enhancing the efficiency of water electrolysis, which can be severely impacted by the nucleation and growth of bubbles, is key in the energy transition. In this combined experimental and numerical study, in-situ bubble evolution and dissolution processes are imaged and compared to numerical simulations employing the immersed boundary method. We find that it is crucial to include solutal driven natural convection in order to represent the experimentally observed bubble behaviour even though such effects have commonly been neglected in modelling efforts so far. We reveal how the convective patterns depend on current densities and bubble spacings, leading to distinctively different bubble growth and shrinkage dynamics. Bubbles are seen to promote the convective instability if their spacing is large ($\geq4$mm for the present conditions), whereas the onset of convection is delayed if the inter-bubble distance is smaller. Our approach and our results can help devise efficient mass transfer solutions for gas evolving electrodes., 30 pages, 13 figures

Reduction of nitric oxide was investigated using Cu electrodes in acid and neutral pH conditions. Analysis of Cu discs in stagnant electrolyte by Electrochemical Mass Spectrometry (EC-MS), revealed the favorable formation of ammonia (and hydrogen) in acidic electrolyte, while N2O and N2 are formed in significant quantities at neutral conditions. Additional performance evaluation of Cu electrodes in hollow fiber geometry, was performed using 10 vol % NO in Ar supplied through the porous electrode structure and off-line determination of ammonia by 1H NMR spectroscopy. The pH dependent performance of the Cu hollow fiber is in agreement with EC-MS data at low gas flow rates, showing the highest ammonia selectivity in acidic conditions. However, at relatively high gas flow rates, almost 90 % faradaic efficiency and a NH3 production rate of 400 μmol h−2 cm−2 were obtained in neutral electrolyte at −0.6 V vs RHE, likely due to enhanced availability of NO at the electrode surface, suppressing the hydrogen evolution reaction. This approach shows conversion of waste NO gas to valuable green fertilizer components is possible.

Understanding structure and function of solid-liquid interfaces is essential for the development of nanomaterials for various applications including heterogeneous catalysis in liquid phase processes and water splitting for storage of renewable electricity. The characteristic anisotropy of crystalline nanoparticles is believed to be essential for their performance but remains poorly understood and difficult to characterize. We use dual scale Atomic Force Microscopy to measure electrostatic and hydration forces of faceted semiconducting SrTiO3 nanoparticles in aqueous electrolyte at variable pH. We demonstrate (i) the ability to quantify strongly facet-dependent surface charges yielding isoelectric points of the dominant {100} and {110} facets that differ by as much as 2 pH units, ii) facet-dependent accumulation of oppositely charged (SiO2 ) particles, and iii) that atomic scale defects can be resolved but are in fact rare for the samples investigated. Atomically resolved images and facet-dependent oscillatory hydration forces suggest a microscopic charge generation mechanism that explains colloidal scale electrostatic forces. This article is protected by copyright. All rights reserved.

One of the major challenges in reusing cooling tower blowdown water (CTBD) utilizing membrane processes is its remaining organic compounds, e.g., humic substances leading to biofouling. Besides, the possible abundance of chloride in CTBD imposes the concern of the formation of chlorinated by-products. To choose a pre-treatment process for the studied CTBD composition, various advanced oxidation processes (AOPs), including electrooxidation (EO), photocatalytic degradation (PCD), heat-activated persulfate oxidation (PS), UVC/vacuum UV (UVC/VUV), and UVC processes, were evaluated and compared based on two main targets: i) highest removal and mineralization of the organics, especially humic substances; and ii) lowest formation of chlorinated by-products including adsorbable organic halides and oxychlorides. All the processes were conducted in the natural condition of the real CTBD, while solution pH was monitored. Based on results of chemical oxygen demand, total organic carbon, dissolved organic carbon, UV254 absorbance, liquid-chromatography–organic carbon detection (LC-OCD), and fluorescence excitation-emission matrices (FEEM), it is concluded that PS leads to complete removal of organic compounds along with the lowest formation of low molecular weight organic acids and organic neutrals. FEEM and LC-OCD data also indicated that EO, PCD, and UVC/VUV processes brought about substantial removal of organic compounds and broke down the humic substances into low molecular weight building blocks and organics. Besides, EO exhibited the highest AOX and oxychlorides formation, while these were limited when using the other AOPs. Summarizing, PS, PCD, and UVC/VUV were efficient processes for the degradation and mineralization of organics without generating significant amounts of chlorinated by-products.

In electrochemical systems, an understanding of the underlying transport processes is required to aid in their better design. This includes knowledge of possible near-electrode convective mixing that can enhance measured currents. Here, for a binary acidic electrolyte in contact with a platinum electrode, we provide evidence of electroconvective instability during electrocatalytic proton reduction. The current-voltage characteristics indicate that electroconvection, visualized with a fluorescent dye, drives current densities larger than the diffusion transport limit. The onset and transition times of the instability do not follow the expected inverse-square dependence on the current density, but, above a bulk-reaction-limited current density, are delayed by the water dissociation reaction, that is, the formation of ${\mathrm{H}}^{+}$ and ${\mathrm{OH}}^{\ensuremath{-}}$ ions. The dominant size of the electroconvective patterns is also measured and found to vary with the diffusion length scale, confirming previous predictions on the size of electroconvective vortices.

By photodeposition of CrOx on SrTiO3-based semiconductors doped with aliovalent Mg(II) and functionalized with Ni/NiOx catalytic nanoparticles (economically significantly more viable than commonly used Rh catalysts), an increase in apparent quantum yield (AQYs) from ∼10 to 26% in overall water splitting was obtained. More importantly, deposition of CrOx also significantly enhances the stability of Ni/NiO nanoparticles in the production of hydrogen, allowing sustained operation, even in intermittent cycles of illumination. In situ elemental analysis of the water constituents during or after photocatalysis by inductively coupled plasma mass spectrometry/optical emission spectrometry shows that after CrOx deposition, dissolution of Ni ions from Ni/NiOx-Mg:SrTiO3 is significantly suppressed, in agreement with the stabilizing effect observed, when both Mg dopant and CrOx are present. State-of-the-art electron microscopy and energy-dispersive X-ray spectroscopy (EDX) and electron energy-loss spectroscopy (EELS) analyses demonstrate that upon preparation, CrOx is photodeposited in the vicinity of several, but not all, Ni/NiOx particles. This implies the formation of a Ni–Cr mixed metal oxide, which is highly effective in water reduction. Inhomogeneities in the interfacial contact, evident from differences in contact angles between Ni/NiOx particles and the Mg:SrTiO3 semiconductor, likely affect the probability of reduction of Cr(VI) species during synthesis by photodeposition, explaining the observed inhomogeneity in the spatial CrOx distribution. Furthermore, by comparison with undoped SrTiO3, Mg-doping appears essential to provide such favorable interfacial contact and to establish the beneficial effect of CrOx. This study suggests that the performance of semiconductors can be significantly improved if inhomogeneities in interfacial contact between semiconductors and highly effective catalytic nanoparticles can be resolved by (surface) doping and improved synthesis protocols.

TiO2 is one of the most attractive photocatalysts, with applications in water splitting, wastewater treatment, and air purification. Understanding the fundamentals of the functioning of TiO2 requires knowledge of the nature and dynamics of photo-induced excitons and charge carriers. Although photoluminescence (PL) spectroscopy can provide important fundamental insights, photophysical mechanisms are still under debate. To address this problem, the aim of the present work is to investigate the evolution of the PL spectrum in time of nanostructured anatase TiO2 thin films and the nature of associated processes, at room temperature and in aqueous media closely resembling photocatalytic conditions. We show that the PL spectrum of commonly used nanostructured anatase TiO2 thin films in aqueous media is time-dependent, with pH-dependent broadening at the low energy side of the spectrum in time. By global analysis of the spectrotemporal PL behavior and the effect of addition of NaCl at neutral and mildly acidic conditions, we show that this spectral development is due to an increasing contribution of processes sensitive to surface termination relative to bulk processes to the PL in time. The time-dependent PL spectrum and dynamics can be assigned to the recombination of mobile electrons populating the conduction band or shallow traps with immobile hole polarons in deep traps and motion of electrons from the nanoparticle bulk toward the depletion layer/surface in ca. 1 ns. This directionality likely plays an important role in the photocatalytic performance of nanostructured anatase TiO2 and effects of ions such as chloride in aqueous media. Control of the directional motion of electrons and suppression of surface charge recombination via surface engineering show promise to further increase the photocatalytic activity.

The formation, characterization, and dye sensitized solar cell application of nickel(II), zinc(II) and cadmium(II) ferrocenyl dithiophosphonate complexes were investigated. The multidentate monoanionic ligand [S2PFc(OH)]− (L1) was synthesized. The reaction between metal salt precursors and L1 produced Ni(II) complexes of the type [Ni{S2P(Fc)(OH)}2] (1) (molar ratio 1:2), and a tetranickel(II) complex of the type [Ni2{S2OP(Fc)}2]2 (2) (molar ratio (1:1). It also produced a Zn(II) complex [Zn{S2P(Fc)(OH)}2]2 (3), and a Cd(II) complex [Cd{S2P(Fc)(OH)}2]2 (4). Complexes 1–4 were characterized by 1H and 31P NMR, FTIR and elemental analysis, and complexes 1 and 2 were additionally analyzed by X-ray crystallography. The first examples of dye-sensitized solar cells (DSSCs) co-sensitized with ferrocenyl dithiophosphonate complexes 1–4 are reported. Co-sensitization with the ruthenium dye N719, produced the dye materials (3)-N719 (η=8.30%) and (4)-N719 (η=7.78%), and they were found to have a better overall conversion efficiency than the pure Ru N719 dye standard (η=7.14%) under the same experimental conditions. The DSSCs were characterized using UV/vis, cyclic voltammetry, electrochemical impedance spectroscopy (EIS), photovoltaic- (I−V curves), and performing incident photon-to-current efficiency (IPCE) measurements.

In electrochemical production of sodium chlorate from brine solutions, an intriguing function of sodium (di)chromate is to inhibit cathodic reduction of oxychlorides, while maintaining effective reduction of water to form hydrogen. Using an electrochemical Quartz Crystal Microbalance (eQCM) and a Rotating Ring Disk Electrode (RRDE; Au disk, Pt ring), we analyzed the deposition of reduced Cr-species formed from reduction of CrVIO4 2− on Au electrodes. Generally, the current induced by reduction of CrVIO4 2− is significantly larger than the accumulated amount of weight deposited on the Au electrode. Deconvolution of the reductive peak reveals two processes that can be differentiated by varying rotation speed. We therefore propose soluble CrVO4 3− is formed by reduction of CrVIO4 2−, followed by consecutive reduction of CrVO4 3− to primarily soluble CrIII(OH)4 -. Simultaneously, reduction of CrVO4 3− also leads to the formation of a monolayer of CrIII(hydr)oxide. This monolayer significantly inhibits the further reduction of CrVIO4 2−, but allows the film to reach a maximum thickness of approximately 1.85 nm by reduction of surface adsorbed CrVO4 3− and/or de-hydroxylation of CrIII(OH)4 -. The observation that limitation of film growth is due to film-induced inhibition of reduction of CrVIO4 2−, and significant solubility of CrIII(OH)3 in the form of CrIII(OH)4 -, will aid in the search of a non-toxic chrome-free alternative for inhibition of cathodic reduction of oxychlorides and selective hydrogen evolution in the chlorate process.

Ag-Functionalized CuWO 4 /WO 3 heterostructures were successfully prepared via a polyvinyl pyrrolidone (PVP)-assisted sol-gel (PSG) route. Thin films prepared via electrophoretic deposition were used as photoanodes for photoelectrochemical (PEC) water splitting. Compared to pristine CuWO 4 and WO 3 films, a significant enhancement of the photocurrent (3-4 times) at the thermodynamic potential for oxygen evolution (0.62 V vs. Ag/AgCl, pH 7) was obtained for the Ag-functionalized CuWO 4 /WO 3 photoanodes. The obtained enhancement is shown to be derived from a synergic contribution of heterostructure formation (CuWO 4 /WO 3 ) and improvements of light utilization by Ag-induced surface plasmon resonance (SPR) effects. Accordingly, a photocurrent of 0.205 mA cm -2 at 0.62 V vs. Ag/AgCl under neutral conditions (without hole scavengers) under front-side simulated AM1.5G illumination was achieved. A detailed analysis of the obtained PEC data alongside performed impedance measurements suggests that charge seperation is significantly improved for the prepared Ag-functionalized CuWO 4 /WO 3 photoanodes. Our work offers beneficial insights to design new plasmonic metal/heterostructured nanocomposites for energy conversion applications.

Selective water oxidation to hydrogen peroxide has emerged as an economically attractive replacement for oxygen in electrochemical hydrogen production by water splitting. Here, boron-doped diamond (BDD) is shown to be a promising anode material for anodic H2O2 formation. Faradaic efficiencies of up to 31.7% at 2.90 V versus the reference hydrogen electrode and a current density of 39.8 mA cm–2 were observed, corresponding to a H2O2 production rate of 3.93 μmol min–1 cm–2. A techno-economic evaluation based on the experimentally obtained values demonstrates that the corresponding levelized cost of hydrogen (LCH) is significant ($62.0 kg–1). Particularly, the current market price of BDD limits its implementation as a selective water oxidation anode for H2O2 generation. The sensitivity analysis however suggests that the LCH can be significantly improved by either decreasing the anode cost or increasing the current density. Both approaches are in fact feasible to allow for cost-effective electrochemical H2 production and even competition with H2 obtained from steam methane reforming. This study will guide ongoing research efforts toward BDD development and implementation of selective water oxidation to hydrogen peroxide.

Online, on-chip measurement of nitric oxide (NO) in organ-on-chip devices is desired to study endothelial (dys)function under dynamic conditions. In this work, ruthenium oxide (RuOx) is explored as an amperometric NO sensor and its suitability for organ-on-chip applications. For testing purposes, diethylamine NONOate was used as chemical NO donor. The NONOate’s NO generation and electrochemical oxidation of generated NO were confirmed by real-time electrochemical/mass-spectrometry. Using RuOx nanorods electrodes, we show that NO oxidation occurred at a lower onset potential (+675 mV vs. Ag/AgCl) than on bare Pt electrode (+800 mV vs. Ag/AgCl). Due to NO adsorption on the RuOx surface, NO oxidation also delivered a higher current density (33.5 nA.μM−1. cm-2) compared to bare Pt (19.6 nA.μM−1. cm-2), making RuOx nanorods a favourable electrode for NO sensing applications. The RuOx electrode’s suitability for organ-on-chip applications was successfully tested by using the electrode to detect a few micromolar concentration of NO generated by endothelial cell culture. Overall, the RuOx nanorods proved to be suitable for organ-on-chip studies due to their high sensitivity and selectivity. Our chip-integrated electrode allows for online NO monitoring in biologically relevant in vitro experiments.

Defect-engineered or substoichiometric TiOx is of interest for use in photo- and electrocatalytic processes both as active material and catalyst support. Electrochemical doping of TiO2 via cathodic polarization is an appealing preparation method and frequently employed. Here, we explored the electrochemical preparation of TiOx in an undivided cell using iridium-based (iridium mixed-metal-oxide) and boron doped diamond (BDD) counter electrodes. Cyclic voltammetry and impedance spectroscopy revealed superior charge transfer properties of crystalline TiOx electrodes prepared with BDD (TiOx-BDD). It is shown that the electrochemical properties correlate well with intensities of the H-signals determined using Time of Flight - Secondary Ion Mass Spectrometry (ToF-SIMS). Thus, it is concluded that electrochemical preparation using BDD causes favourable H+ intercalation and/or H diffusion into the sub-surface layers of TiOx. Our extensive analysis using a combination of electrochemical and surface characterization (LEIS and XPS) techniques, additionally suggests that cathodic deposition of Ir, originating from the Ir-based counter electrode, present in sub-ppm concentrations only results in less-efficient doping. Instead in the presence of sub-ppm level Ir contamination hydrogen evolution is favoured during cathodic polarization. The results presented within this study highlight the necessity to use inherently stable counter electrodes for electrochemical preparation and reveal the pronounced influence of trace contamination in electrochemistry in general and the doping mechanism of TiOx electrodes in particular.

In this work, spectroscopic and kinetic studies were performed on photocatalytic oxidation of cyclohexanol to cyclohexanone. The photocatalytic experiments were performed according to a three-level full factorial design and the rate of cyclohexanone production was determined by HPLC analysis. In situ ATR-FTIR analysis of the photocatalytic reaction revealed that cyclohexanol can be selectively converted to cyclohexanone, without the formation of significant amounts of carbonates and carboxylates. A reaction mechanism based on different steps from charge separation to cyclohexanone molecule formation is proposed. The results were utilized to determine the kinetic parameters (with the help of genetic algorithm) and validate the model. The developed kinetic model illustrates that the rate of cyclohexanone production increases as a power function with respect to the light intensity and decreases as an exponential function with respect to time. An excellent selectivity of cyclohexanone was confirmed by spectroscopic and chromatographic studies. This study demonstrates that photocatalysis can be a promising technology for formation of cyclohexanone from cyclohexanol.

Photocatalytic oxidation of propane using hydrothermally synthesized TiO2 samples with similar primary crystal size containing different ratios of anatase, brookite and rutile phases has been studied by measuring light-induced propane conversion and in situ DRIFTS (diffuse reflectance Fourier transform infrared spectroscopy). Propane was found to adsorb on the photocatalysts, both in the absence and presence of light. The extent of adsorption depends on the phase composition of synthesized titania powders and, in general, it decreases with increasing rutile and brookite content. Still, the intrinsic activity for photocatalytic decomposition of propane is higher for photocatalysts with lower ability for propane adsorption, suggesting this is not the rate-limiting step. In situ DRIFTS analysis shows that bands related to adsorbed acetone, formate and bicarbonate species appear on the surface of the photocatalysts during illumination. Correlation of propane conversion and infrared (IR) data shows that the presence of formate and bicarbonate species, in excess with respect to acetone, is composition dependent, and results in relatively low activity of the respective TiO2. This study highlights the need for precise control of the phase composition to optimize rates in the photocatalytic oxidation of propane and a high rutile content seems to be favorable. This research was by funded by GV/FEDER (PROMETEO/2018/076), MICINN and FEDER (RTI2018-095291-B-I00), University of Alicante (VIGROB-136 and Mobility Grants from the EDUA to facilitate obtaining the title of International Doctor).

Materials used for photocatalytic overall water splitting (POWS) are typically composed of light-absorbing semiconductor crystals, functionalized with so-called cocatalytic nanoparticles to improve the kinetics of the hydrogen and/or oxygen evolution reactions. While function, quantity, and protection of such metal(oxide) nanoparticles have been addressed in the literature of photocatalysis, the stability and transients in the active oxidation-state upon illumination have received relatively little attention. In this Perspective, the latest insights in the active state of frequently applied cocatalysts systems, including Pt, Rh/Cr2O3, or Ni/NiO x , will be presented. While the initial morphology and oxidation state of such nanoparticles is a strong function of the applied preparation procedure, significant changes in these properties can occur during water splitting. We discuss these changes in relation to the nature of the cocatalyst/semiconductor interface. We also show how know-how of other disciplines such as heterogeneous catalysis or electro-catalysis and recent advances in analytical methodology can help to determine the active state of cocatalytic nanoparticles in photocatalytic applications.

A nanocomposite of ZnV2O6 with hierarchical flower-like structure hybridized with reduced graphene oxide (rGO) was fabricated using a facile hydrothermal approach. The structure, morphology, optical and electronic properties were explored using comprehensive analytical techniques. The results revealed that the rGO sheets were decorated with the in situ-formed ZnV2O6 nanoparticles yielding a well-combined composite structure. The photocatalytic activity of as-prepared ZnV2O6/rGO hybrids is 2.48 times larger than that of pristine ZnV2O6 for the degradation of Rhodamine B (RhB). In parallel to the experimental results, the basic mechanisms of interfacial interaction, charge transfer/separation and subsequently their influence on the photocatalytic activity were theoretically studied by first-principles calculations. The photocatalytic enhancement is attributed to efficient interfacial electron transfer from ZnV2O6 to rGO, leading to a prolonged lifetime of photoinduced charge carriers. We anticipate that these results will lead to new insights in the judicious design of graphene-based semiconductor photocatalysts.

Sodium dichromate is an essential solution additive for the electrocatalytic production of sodium chlorate, assuring selective hydrogen evolution. Unfortunately, the serious environmental and health concerns related to hexavalent chromium mean there is an urgent need to find an alternative solution to achieve the required selectivity. In this study sodium permanganate is evaluated as a possible alternative to chromate, with positive results. The permanganate additive is stable in hypochlorite-containing solutions, and during electrolysis a thin film is reductively deposited on the cathode. The deposit is identified as amorphous manganese oxide by Raman spectroscopic and X-ray diffraction studies. Using different electrochemical techniques (potentiodynamic measurements, galvanostatic polarization curves) we demonstrate that the reduction of hypochlorite is suppressed, while the hydrogen evolution reaction can still proceed. In addition, the formed manganese oxide film acts as a barrier for the reduction of dissolved oxygen. The extent of hydrogen evolution selectivity in hypochlorite solutions was quantified in an undivided electrochemical cell using mass spectrometry. The cathodic current efficiency is significantly enhanced after the addition of permanganate, while the effect on the anodic selectivity and the decomposition of hypochlorite in solution is negligible. Importantly, similar results were obtained using electrodes with manganese oxide films formed ex situ. In conclusion, manganese oxides show great promise in inducing selective hydrogen evolution, and may open new research avenues to the rational design of selective cathodes, both for the chlorate process and for related processes such as photocatalytic water splitting.

Tradition is tradition: Already for the 52nd time the Annual Meeting of the German Catalysis Society (GeCatS) took place from March 13th to 15th, 2019. Following its long‐standing tradition, the meeting was organized in the Neue Weimarhalle in the beautiful city of Weimar, Thuringia.

Facet-engineering and the deposition of co-catalysts lead to significant improvement in efficiency of semiconductors in photocatalytic applications. Here, we demonstrate, using the specific example of bismuth-oxy-bromide (BiOBr) particles, that facet-selective, photo-induced reductive or oxidative deposition of co-catalysts onto plate-like semiconductor particles is strongly pH-dependent. High resolution atomic force microscopy and spectroscopy measurements demonstrate that the effect of pH is caused by a reversal of the surface charge of the [001] facets upon increasing pH from 3 to 9 (isoelectric point ≈5), while the side facets become increasingly negatively-charged. We discuss the effect of facet-surface-charge on particle distributions by band-bending, favoring either electron transfer and metal deposition, or hole transfer and metal-oxide deposition. This finding opens up new ways to design highly effective, photocatalytic composite architectures, containing spatially separated catalytic particles of multiple compositions.

SrTiO3 is a well-known photocatalyst inducing overall water splitting when exposed to UV irradiation of wavelengths < 370 nm. However, the apparent quantum efficiency of SrTiO3 is typically low, even when functionalized with nanoparticles of Pt or Ni@NiO. Here, we introduce a simple solid state preparation method to control the incorporation of magnesium into the perovskite structure of SrTiO3. After deposition of Pt or Ni@NiO, the photocatalytic water splitting efficiency of the Mg:SrTiOx composites is up to 20 times higher as compared to SrTiO3 containing similar catalytic nanoparticles, and an apparent quantum yield (AQY) of 10 % can be obtained in the wavelength range of 300 - 400 nm. Detailed characterization of the Mg:SrTiOx composites revealed that Mg is likely substituting the tetravalent Ti-ion, leading to a favorable surface-space-charge layer. This originates from tuning of the donor density in the cubic SrTiO3 structure by Mg-incorporation, and enables high oxygen evolution rates. Nevertheless, interfacing with an appropriate hydrogen evolution catalyst is mandatory and non-trivial, to obtain high performance in water splitting.

Employing an oxidative photodeposition of CrOx the well-known Rh/CrOx co-catalyst system was prepared on different semiconductors. These photocatalysts showed up to 25% higher overall water splitting activities compared with conventionally prepared materials. The enhancement is attributed to a favorable selective deposition of CrOx caused by charge-directed deposition.

Engineering stable electrodes using highly active catalyst nanopowders for electrochemical water splitting remains a notorious challenge. We report an innovative and general approach for attaining highly stable catalyst films with self-healing capability based on in-situ self-assembly of catalyst particles during electrolysis. The catalyst particles are added to the electrolyte forming a suspension that is pumped through the electrolyzer. Particles with negatively charged surfaces stick onto the anode, while particles with positively charged surfaces stick to the cathode. The self-assembled catalyst films possess self-healing properties as long as a sufficient amount of catalyst particles are present in the electrolyte. The proof-of-concept was demonstrated in a non-zero gap alkaline electrolyzer using NiFe LDH and NixB catalyst nanopowders for anode and cathode, respectively. Steady cell voltages were maintained for at least three weeks during continuous electrolysis at 50-100 mA cm-2

The high (de)lithiation potential of TiO2 (ca. 1.7 V vs Li/ Li+ in 1 M Li+) decreases the voltage and, thus, the energy density of a corresponding Li-ion battery. On the other hand, it offers several advantages such as the (de)lithiation potential far from lithium deposition or absence of a solid electrolyte interphase (SEI). The latter is currently under controversial debate as several studies reported the presence of a SEI when operating TiO2 electrodes at potentials above 1.0 V vs Li/Li+. We investigate the formation of a SEI at anatase TiO2 electrodes by means of X-ray photoemission spectroscopy (XPS) and scanning electrochemical microscopy (SECM). The investigations were performed in different potential ranges, namely, during storage (without external polarization), between 3.0-2.0 V and 3.0-1.0 V vs Li/Li+, respectively. No SEI is formed when a completely dried and residues-free TiO2 electrode is cycled between 3.0 and 2.0 V vs Li/Li+. A SEI is detected by XPS in the case of samples stored for 6 weeks or cycled between 3.0 and 1.0 V vs Li/Li+. With use of SECM, it is verified that this SEI does not possess the electrically insulating character as expected for a "classic" SEI. Therefore, we propose the term apparent SEI for TiO2 electrodes to differentiate it from the protecting and ef fective SEI formed at graphite electrodes.

We have evaluated the applicability of Ni anodes in electrochemical conversion of H2S to form sulfur (polysulfides) and H2. Two different electrolytes containing sulfide were evaluated: a buffered solution of Na2HPO4 at pH 9.2, and a NaOH solution at pH 13. At pH 9.2, deposition of sulfur on the Ni anode was observed, resulting in a significant decrease in electrochemical performance. The composition, morphology, and thickness of the sulfur deposit, as determined by Raman spectroscopy and SEM, was found to strongly depend on the applied potential, and ranged from dense S8 films to highly porous spherical sulfur structures. Oxidation of the anode was also observed by conversion of Ni to NiS2. The formation of the sulfur film was prevented by performing the reaction at pH 13 in NaOH in the range of − 1.0 V to + 0.6 V versus Hg/HgO. It is proposed that at these highly basic pH values, sulfur is dissolved in the electrolyte in the form of polysulfides, such as S2 2− or S8 2−. When using Ni anodes some oxygen evolution was observed at the anode, in particular at pH 13, resulting in a Faradaic efficiency for sulfur removal of ~ 90%. Graphic Abstract: [Figure not available: see fulltext.].

We evaluated electrochemical degradation (ECD) and photocatalytic degradation (PCD) technologies for saline water purification, with a focus on rate comparison and formation and degradation of chlorinated aromatic intermediates using the same non-chlorinated parent compound, 4-ethylphenol (4EP). At 15 mA·cm-2, and in the absence of chloride (0.6 mol·L-1 NaNO3 was used as supporting electrolyte), ECD resulted in an apparent zero-order rate of 30 μmol L-1·h-1, whereas rates of ?300 μmol L-1·h-1 and ?3750 μmol L-1·h-1 were computed for low (0.03 mol·L-1) and high (0.6 mol·L-1) NaCl concentration, respectively. For PCD, initial rates of ?330 μmol L-1·h-1 and 205 μmol L-1·h-1 were found for low and high NaCl concentrations, at a photocatalyst (TiO2) concentration of 0.5 g·L-1, and illumination at λmax ≈ 375 nm, with an intensity ?0.32 mW·cm-2. In the chlorine mediated ECD approach, significant quantities of free chlorine (hypochlorite, Cl2) and chlorinated hydrocarbons were formed in solution, while photocatalytic degradation did not show the formation of free chlorine, nor chlorine-containing intermediates, and resulted in better removal of non-purgeable hydrocarbons than ECD. The origin of the minimal formation of free chlorine and chlorinated compounds in photocatalytic degradation is discussed based on photoelectrochemical results and existing literature, and explained by a chloride-mediated surface-charge recombination mechanism.

H-BiVO4–x:Mo was successfully deposited on microwire-structured silicon substrates, using indium tin oxide (ITO) as an interlayer and BiOI prepared by electrodeposition as precursor. Electrodeposition of BiOI, induced by the electrochemical reduction of p-benzoquinone, appeared to proceed through three stages, being nucleation of particles at the base and bottom of the microwire arrays, followed by rapid (homogeneous) growth, and termination by increasing interfacial resistances. Variations in charge density and morphology as a function of spacing of the microwires are explained by (a) variations in mass transfer limitations, most likely associated with the electrochemical reduction of p-benzoquinone, and (b) inhomogeneity in ITO deposition. Unexpectedly, H-BiVO4–x:Mo on microwire substrates (4 μm radius, 4 to 20 μm spacing, and 5 to 16 μm length) underperformed compared to H-BiVO4–x:Mo on flat surfaces in photocatalytic tests employing sulfite (SO32–) oxidation in a KPi buffer solution at pH 7.0. While we cannot exclude optical effects, or differences in material properties on the nanoscale, we predominantly attribute this to detrimental diffusion limitations of the redox species within the internal volume of the microwire arrays, in agreement with existing literature and the observations regarding the electrodeposition of BiOI. Our results may assist in developing high-efficiency PEC devices., We discuss the effect of the Si architecture on the performance of H-BiVO4:Mo in the oxidation of sulfite to sulfate, providing indications of applicability in solar-to-fuel devices.

In the field of photocatalytic water splitting to produce sustainable hydrogen, significant breakthrough in Apparent Quantum Efficiency (AQE) has been obtained by modification of SrTiO3-based semiconductors. Combining doping with aliovalent Al(III) and functionalization with CrOx-promoted Rh nanoparticles, AQEs of close to 100% have been reported upon UV excitation. In this study it is demonstrated that economically more viable Ni-based nanoparticles allow Mg-doped SrTiO3-based semiconductors to provide for AQEs >25%. A key and novel finding is that photodeposited CrOx not only promotes the performance of the Ni particles (AQY increases from ~10% to 26%), but also enhances the stability, allowing sustained cyclic operation (day-night cycles). In situ elemental analysis shows that CrOx significantly reduces dissolution of Ni from Ni/NiO-Mg:SrTiO3 by formation of a Ni-Cr mixed metal oxide. This is confirmed by state-of-the-art electron microscopy, which furthermore demonstrates that upon preparation, CrOx is photodeposited in the vicinity of several, but not all, Ni/NiO particles. Inhomogeneities in the interfacial contact between the Ni/NiO particles and the Mg:SrTiO3 substrate likely affect the probability of reduction of Cr(VI)-species during photodeposition, explaining the inhomogeneity. In addition it is revealed that the dopant Mg is essential in providing favorable interfacial contact with several NiO/Ni particles and effective promotion of these particles by CrOx. In general this presentation provides synthesis protocols that allow for intensification of interfacial contacts of Ni/NiO and (doped) SrTiO3, and stability enhancement by photodeposition of CrOx.

Si is an excellent photo-absorber for use in dual-band-gap photoelectrochemical water splitting. We investigate photoanodes with n+pp+-Si configuration under back-side illumination, which is suited to work in a tandem device stack. A co-sputtered NiCoOx film coupled to the Si was used as a protective catalyst for the water oxidation reaction in 1 m KOH. The sample showed a high photocurrent (21 mA cm−2) under red-light illumination (38.6 mW cm−2). Long-term stability testing showed a gradual decrease of activity in the beginning, and then the activity increased, yielding a cathodic shift of the onset voltage (>50 mV), likely owing to the divergent response of Ni and Co to the Fe present in KOH. Once the activity of the sample stabilized, no further degradation was observed for the following 6 days, indicating that the demonstrated back-illuminated photoanode configuration can be considered a promising architecture for use as a bottom cell of the tandem water-splitting device under alkaline conditions.

Efficient and financially attractive (photo)electrochemical water splitting for the production of hydrogen (H2) is essential to drive a green hydrogen economy. However, techno-economic analysis predicts that the price of H2 obtained through (P)EC water splitting is over $10 000 ton-1 at solar-to-hydrogen (STH) efficiencies of ca. 10 %, whereas the current market value of H2 produced by conventional steam methane reforming is only $1400 ton-1.[1, 2] Co-production of high-value chemicals at the anode is an appealing strategy to increase the financial attractiveness of the production of 'green hydrogen. In ‘classic’ (P)EC water splitting, oxygen is the main product at the anode, formed at a potential of E0 (O2/H2O) = +1.23 V vs RHE. Though thermodynamically less favorable (E0 (H2O2/H2O) = +1.78 V vs RHE), the selective partial oxidation of water to hydrogen peroxide (H2O2) is of industrial interest.[2] With a market price of $500-1200 ton-1 for H2O2 and given the low market value of O2 (only $35 ton-1), the financial incentive is clearly demonstrated. Considering that the demand for H2O2 is constantly increasing, hydrogen production through a process with concomitant H2O2 has a high potential of becoming competitive with steam methane reforming. Here, we discuss the techno-economics of a PEC water splitting system producing H2O2 and H2. We will clearly show that an electrochemical process holds great financial promise for industrial development. Not only do we demonstrate at which prices H2 and H2O2 can be sold to make a profit, we also discuss which parameters need to be optimized to decrease the hydrogen price. Interestingly, when the H2O2 price is $0.85 kg-1, an STH of only 9.39% is required to compete with hydrogen production by steam methane reforming. As the anodic formation of H2O2 is not yet well explored, it will also be shown that continuous electrochemically production of H2O2 at a rate of 0.092 µmol min-1 cm-2 @ 7.08 mA/cm2 through partial water oxidation can easily be achieved in an non-optimized electrochemical cell. Finally, the importance of electrolyte, electrode and reactor engineering to increase the Faradaic efficiency and the rate of H2O2 production will be revealed. References 1. Shaner, M. R.; Atwater, H. A.; Lewis, N. S.; McFarland, E. W. A Comparative Technoeconomic Analysis of Renewable Hydrogen Production using Solar Energy. Energy Environ. Sci., 2016, 9, 2354-2371. 2. Mei, B.; Mul, G.; Seger, B. Beyond Water Splitting: Efficiencies of Photo-Electrochemical Devices Producing Hydrogen and Valuable Oxidation Products. Adv. Sustainable Syst. 2017, 1, 1600035.

In electrochemical reactions, mass-transfer effects due to ion consumption at the electrode are of significance. The concentration gradient thus formed between the electrode surface and the bulk of the solution also contains valuable information about the surface activity and reactivity. In the context of the hydrogen evolution reaction, H+ ion consumption at the electrode surface increases near-electrode pH, and forms similar gradients between the electrode surface and the bulk. Confocal microscopy has been used in recent years as a tool to measure pH profiles (with a suitable pH probe) near electrodes1–3. The experimental diffusion profiles have generally been compared with a steady state model, and time resolved measurements of the growth of the boundary layer are lacking. In this contribution, the consumption of H+ ions on a transparent platinum electrode is used to study the one-dimensional growth of the ion depleted boundary layer at different current densities. Using fluorescein as the pH probe, it will be shown that ion concentration can be estimated and hence time-dependent diffusion profiles measured. Furthermore we will highlight the importance of using a pH independent dye to monitor electrostatic effects, which are seen to be significant at low supporting electrolyte concentrations. The experimental results are compared with those of a suitable time dependent reaction-diffusion model. Finally, building on these results, an attempt is made to measure pH profiles around growing bubbles during electrochemical hydrogen evolution with an aim to get further insights into the associated reaction resistances4. (1) Rudd, N. C.; Cannan, S.; Bitziou, E.; Ciani, I.; Whitworth, A. L.; Unwin, P. R. Fluorescence Confocal Laser Scanning Microscopy as a Probe of PH Gradients in Electrode Reactions and Surface Activity. (2) Leenheer, A. J.; Atwater, H. A. Imaging Water-Splitting Electrocatalysts with PH-Sensing Confocal Fluorescence Microscopy. J. Electrochem. Soc. 2012, 159 (9), H752–H757. (3) Bouffier, L.; Doneux, T. Coupling Electrochemistry with in Situ Fluorescence (Confocal) Microscopy. Curr. Opin. Electrochem. 2017, 6, 31–37. (4) Peñas, P.; van der Linde, P.; Vijselaar, W.; van der Meer, D.; Lohse, D.; Huskens, J.; Gardeniers, H.; Modestino, M. A.; Rivas, D. F. Decoupling Gas Evolution from Water-Splitting Electrodes. J. Electrochem. Soc. 2019, 166 (15), H769–H776.