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Lead cathodes show decreasing Faradaic Efficiency (FE) in electrochemical conversion of CO2 to formate, favoring hydrogen, within minutes of operation in KHCO3 electrolyte at −1 to −1.3 V vs RHE. Periodic anodic polarization (pulsed electrochemistry) is demonstrated to result in a high time-averaged FE towards formate. Specifically, an anodic polarization time of 0.1–1 s is sufficient to obtain a Pb-surface providing an averaged formate FE of 30–50%, when the cathodic polarization time is limited to a few seconds. A Pourbaix diagram and Raman spectra are provided, which show that PbCO3 is formed on the surface of the cathode in KHCO3 electrolyte at anodic potential (0.05 V vs RHE), which compound is likely inducing the high FE towards formate. Our method thus provides a means to operate Pb electrodes in electrochemical CO2 reduction with high stability, at a low energy penalty.

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.

The overall mass balance of an electrochemical reactor is a key factor in determining the economic potential of a process. We provide a method to estimate the overall mass balance and electrolyte composition of electrochemical conversion systems and apply it to several CO2 conversion systems. Next to the reactions at the electrodes, the influence of transport across the membrane and homogeneous reactions are considered. It is thereby shown that potassium bicarbonate – an often proposed electrolyte for aqueous CO2 reduction - does not provide the conditions required for CO2 conversion to formic acid. The requirements for an electrolyte that does result in that conversion are described. The simplicity and versatility of the simplified method and the necessity of establishing a complete mass balance make the method a valuable addition to the electrochemist’s toolbox.

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.

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.

The origin of instability of p-Cu2O films deposited on a platinized Si substrate when used as photocathode in photoelectrochemical water splitting, was studied in the absence or presence of a protective layer of RuO2. When applied at +0.3 V vs. RHE and at pH 7, p-Cu2O films were found to show a slightly more stable performance as compared to photoelectrochemical measurements reported in the literature at 0 V vs. RHE and under acidic conditions. In addition, the stability and the photocurrent induced by the Cu2O films were significantly improved when H2O2 was added to the electrolyte, which is explained by efficient scavenging of electrons, yielding oxygen and water as confirmed by gas chromatography (GC). Also, other electron acceptors improved the photocatalytic performance of the p-Cu2O films, demonstrating that the transfer of photo-excited electrons to protons adsorbed on the surface is the rate determining step in p-Cu2O based photo-electrochemical water splitting. We confirmed that deposition of RuO2 improves the stability of the films, but to the expense of a decrease in photocurrent density. The results provided in this study rationalize the attachment of an effective H2 evolution catalyst as a means to significantly improve the stability of p-Cu2O electrodes.

Recent advances in technologies for the decentralized, islanded ammonia economy are reviewed, with an emphasis on feasibility for long-term practical implementation. The emphasis in this review is on storage systems in the size range of 1–10 MW. Alternatives for hydrogen production, nitrogen production, ammonia synthesis, ammonia separation, ammonia storage, and ammonia combustion are compared and evaluated. A conceptual process design, based on the optimization of temperature and pressure levels of existing and recently proposed technologies, is presented for an islanded ammonia energy system. This process design consists of wind turbines and solar panels for electricity generation, a battery for short-term energy storage, an electrolyzer for hydrogen production, a pressure swing adsorption unit for nitrogen production, a novel ruthenium-based catalyst for ammonia synthesis, a supported metal halide for ammonia separation and storage, and an ammonia fueled, proton-conducting solid oxide fuel cell for electricity generation. In a generic location in northern Europe, it is possible to operate the islanded energy system at a round-trip efficiency of 61% and at a cost of about 0.30–0.35 € kWh−1.

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.

Platinized TiO2 prepared by photodeposition was evaluated for activity in the simultaneous conversion of methyl orange (MO), and methanol assisted formation of hydrogen. Low concentrations of MO were found ineffective for generation of hydrogen in measurable quantities upon illumination of Pt/TiO2 in water. On the other hand, methanol induced hydrogen formation was significant. Surprisingly, when methyl orange was added to the methanol/water solution, hydrogen formation was significantly suppressed. The origin of this detrimental effect of methyl orange lies in the strong and preferred adsorption of the dye on the Pt sites of the catalyst, resulting in hydrogenation of the azo bond and suppression of the catalyzed formation of hydrogen. The hydrogenation of the azo bond is corroborated by dis-colorization of the solution and the observation of a mass fragment in LC–MS analysis corresponding to a hydrogenated product of MO (m/z = 172). Similar to hydrogen formation, dye dis-colorization is stimulated by the presence of methanol, without the formation of new chemical compounds, confirming the role of methanol as a hole scavenger in the photocatalytic processes. Finally the presence of oxygen (in lean conditions) delays dye hydrogenation and hydrogen formation, which we discuss is due to oxygen adsorption and formation of superoxide anions over the Pt sites (leading to oxidation of methanol), which is preferred over Ndouble bond; length as m-dashN bond hydrogenation, and proton reduction.

We report on the oxidation state of Pt nanoparticles when deposited on SrTiO3 or Rh-doped SrTiO3 under realistic solar water-splitting conditions. The oxidation state was investigated using state-of-the-art analysis of the reaction in a continuously stirred tank reactor (CSTR) connected to a micro gas chromatograph equipped with a pulsed discharge detector (PDD) detector and ex-situ XPS. We demonstrate an initially large, but transient, peak in the oxygen production rate during the conversion of acidified water using SrTiO3 loaded with Pt nanoparticles prepared by photo-deposition (in the absence of methanol). This was confirmed by XPS analysis to be the result of a change in oxidation state from PtOx to metallic Pt. Remarkably, when PtOx was present on SrTiO3 doped with Rhδ+, the transient peak in the O2 production was of much lower magnitude than when in the absence of Rhδ+. The XPS analysis demonstrates that this can be explained by the incomplete reduction of the Pt nanoparticles in the presence of Rhδ+. The average oxidation state of Pt deposited on SrTiO3 during the water-splitting reaction thus appears very much dependent on the absence or presence of dopants. This effect is often neglected, but is highly relevant for the discussion of composition-activity correlations of (doped) semiconductors functionalized with metal(oxide) nanoparticles in overall water splitting.

Hollow fiber electrodes, also known as porous tube electrodes, creatively address common gas related challenges in electrochemistry. By purging gas through the electrode wall an intense three phase contact can be established along the entire electrode-electrolyte interface. The electrochemical behavior of the hollow fiber electrode has been hypothesized based on the CO2 reduction reaction on a copper hollow fiber electrode 1. It was however not studied to any significant extent. The electrochemical behavior of the hollow fiber electrode and it’s relevant parameters have now been clarified and studied in more detail. In this study highly conductive titanium hollow fiber electrodes are used, since it is an attractive base electrode material for a great variety of electrochemical applications 2. These titanium based hollow fibers have an electrical resistivity (4.1-9.6 μΩ·m) 3 that is orders of magnitude lower than those previously reported in literature 4. This results from utilization of the two-step thermal decomposition of the polymer, which is required for the construction of an inorganic hollow fiber in a dry-wet spinning process 3. The investigation of platinum electrodeposition onto titanium hollow fiber electrodes with different flow rates has proven exceptionally insightful with regard to understanding the electrochemical behavior of the hollow fiber electrodes. The effect of volumetric gas flow rate on the resulting platinum deposit is visualized through the platinum electrochemical surface area and SEM images (Figure 1). Looking beyond the volumetric gas flow rate it is found that the exerted pressure and the pore size distribution relate to the area depended flow rate and bubble frequency at a pore, which affect the platinum distribution. In order to deconvolute the mass transfer during the platinum electrodeposition process, the hydrogen evolution reaction and an Fe2+/3+-redox couple have been investigated. These experiments isolate mass transport as an electrical current response to flow rate in the cases of gas evolution and electron transfer in solution respectively. By drawing on analogues with the proven methods of porometry and rotating disk electrodes it was found that many aspects of the electrochemical behavior of hollow fiber electrodes can be explained. The Young-Laplace equation helps in matching pressure fluctuation to the chemical state of the electrode surface and following the principles of the Koutecký-Levich equation it is possible to describe the current changes resulting from gas flow rate. Kas, et al. (2016). Nat. Commun., 7. DOI: 10.1038/ncomms10748 Devilliers & E. Mahé. (2007). Trends in Electrochemistry Research., 1. ISBN: 1-59454-457-3 P. H. Jong, et al. (2020). To be submitted. David, et al. (2014). J. Membr. Sci., 139. DOI: 10.1016/j.memsci.2014.03.010 Figure 1

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.

Ni/NiO-Mg:SrTiO3 is a promising formulation for the overall splitting of water to H2 and O2 when illuminated with light of 365 nm. The preparation prescribes various steps, including solid state synthesis of MgO/TiO2, followed by mixing with SrCO3 and calcination at 1100 C for 10 h. Ni@NiO is deposited by impregnation, calcination, reduction in H2, and passivation with O2. When CrOx is finally photodeposited on such composite, a significant enhancement of the performance is observed, both in activity and stability. A steady state apparent quantum yield at 365 nm of no less than 38% is obtained. The composition of the photo-active material was investigated using state-of-the-art electron microscopy and elemental analysis, which demonstrates that CrOx is photodeposited in the vicinity of several, but not all Ni/NiO particles. This is explained by the inhomogeneity in interfacial contact between the Ni/NiO particles and the Mg:SrTiO3 substrate, affecting the probability of reduction of Cr(VI)-species during the photodeposition process. In situ ICP shows various elements of the composite catalyst leach into solution, including Sr, Mg and Ni. CrOx reduces leaching of Ni/NiO from Ni/NiO-Mg:SrTiO3, which might explain the enhanced stability of CrOx containing composites. The mechanism of activation of the catalyst by photodeposition of CrOx and suggestions for further improvement in the preparation cycle of the composite catalyst, are discussed.

The generation of hydrogen from renewable sources is of great interest in sustainable fuel production and storage. Emerging technologies such as photocatalytic water splitting are being considered as cost effective solutions for the production of hydrogen. In these applications Pt is commonly employed as it is highly effective for the hydrogen evolution reaction (HER), however loss of efficiency can be a problem due to back-reactions (for example the oxygen reduction reaction (ORR)) and to a lesser extent poisoning from contaminants (e.g. CO). CrOx-based protective layers have been shown to improve selectivity towards hydrogen for Pt catalysts, for which lower concentrations of the chromate precursor are preferred due to toxicity issues. Still, there is a lack of fundamental understanding of coating procedures and the levels required. In this study cyclic voltammetry and eQCM have been used to systematically investigate the dependence of concentration and deposition time on the extent of film growth and activity in the relevant reactions for photocatalytic water splitting. Specifically we highlight trends in the HER and ORR activity on the buried Pt surface and evaluate stability as a function of CrOx coverage. It has been shown by the experimental results that an optimum can be found between minimising the overpotential required for HER whilst fully protecting against the oxygen reduction back-reaction. Thus, these results reveal the lowest Cr precursor concentrations that can be effectively applied in photocatalysis.

Understanding of reaction pathways and mechanism plays an important role in determining the kinetics of electrochemical reactions, and thus in the design of electrochemical processes. Attenuated Total Reflection - Surface Enhanced Infrared Absorption Spectroscopy (ATR-SEIRAS) provides a very sensitive tool for analysis of surface-electrochemical transformations. One of the applications of this technique is operando study of the electrochemical reduction of CO2. However, there is still much uncertainty concerning the nature of the intermediates towards CO in the conversion of CO2, and the interpretation of the measured peak positions in the IR spectra. In this study, ATR-SEIRAS allowed identification of notable intermediates of electrochemical CO2 reduction at moderately negative potential ranges using a surface enhanced ~12 nm thick sputtered Cu-film and isotopically labeled water (D2O) and CO2 (13CO2). By increasing the potential from -1.3 V (vs RHE) to 0 V, we could identify the transition in selectivity of surface-adsorbed species. At highly negative potential the formation of OH- and H2 is dominant, resulting in a relatively high concentration of carbonate in solution. When the potential is lowered, the rate of H2 formation decreases, which induces lowering of the concentration of carbonate and increased quantities of bicarbonates, a transition which is spectroscopically well resolved. To explain the formation of CO (clearly visible within the investigated potential range), alternatively or additionally to H+-assisted reduction of CO2, adsorbed CO2 - is shown to react with physisorbed CO2, forming the CO2 dimer radical anion. We could identify this CO2 dimer radical anion (C2O4 −) on polycrystalline copper electrodes, at moderately negative potentials. Decomposition of the dimer leads to CO and (surface) carbonate. Finally, at the lowest potentials investigated for reduction of CO2, the formation of (surface) formate is evident, which cannot be further reduced.

In order to understand the effect of nanoparticle size on photocatalytic activity in gas phase propane oxidation, TiO2 nanoparticles embedded in TUD-1 mesopores were prepared with a high degree of size control in the range 3–8 nm. One sample of TiO2-TUD-1 was prepared with Ti/Si ratio of 2.5, and during the synthesis, it was divided into four parts and hydrothermally treated for different times 8–24 h. The produced samples were characterized by means of XRD, HR-TEM, N2 sorption, and DR-UV-Vis spectroscopy. Characterization data showed that the size of the TiO2 nanocrystals is increasing with the hydrothermal treatment time. The photocatalytic activity of the four investigated samples, with identical TiO2 loading, was performed under the illumination of ultraviolet radiation λ = 335 nm, selectively activating the nanoparticles. Results showed that the initial reaction rate in photocatalytic oxidation of propane was found to be smaller for the TiO2 nanoparticles with an average size of 3.5 nm as compared to the TiO2 nanoparticles with an average size of 7.5 nm. However, the larger total available surface area lead to a higher propane consumption with higher photochemical selectivity toward acetone (desired product), as compared to the 7.5 nm particles.

Made available in DSpace on 2018-12-11T17:37:38Z (GMT). No. of bitstreams: 0 Previous issue date: 2018-11-01 Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) This work addresses the development of a magnetically extractable magnetite-silica-titania photocatalyst to be applied in the degradation of organic compounds in water. MCM-41 silica was successfully deposited on magnetite, providing large surface area for anchoring of TiOx species, and preventing thermally induced conversion of magnetite to hematite. A good correlation between the calculated values and amount of titania deposited on the silica-covered magnetite particles was obtained for a synthesis route involving the treatment of magnetite-silica in boiling ethanolic Ti-precursor solution. Photocatalytic activity in conversion of 4-chlorophenol could only be observed for compositions containing larger than ∼50 wt% titania, whereas increasing the titania content did not significantly improve performance. Experiments carried out at pH ∼ 3.0 and ∼7.2 demonstrated the performance is relatively pH-independent. The structure activity-correlation of the materials is briefly discussed. Universidade Estadual Paulista/UNESP Departamento de Química e Bioquímica – Instituto de Biociências (IB), Caixa Postal 510 Photocatalytic Synthesis Group MESA+ Institute for Nanotechnology Faculty of Science and Technology University of Twente, Meander 229, P.O. Box 217 Instituto de Física de São Carlos (IFSC) Universidade de São Paulo (USP) Catalytic Processes and Materials MESA+ Faculty of Science and Technology University of Twente, P.O. Box 217 Universidade Estadual Paulista/UNESP Laboratório Central – Faculdade de Ciências Agronômicas (FCA), Caixa Postal 1780 Universidade Estadual Paulista/UNESP Departamento de Química e Bioquímica – Instituto de Biociências (IB), Caixa Postal 510 Universidade Estadual Paulista/UNESP Laboratório Central – Faculdade de Ciências Agronômicas (FCA), Caixa Postal 1780 CNPq: 120112/2013-2 FAPESP: 2013/22955-2 FAPESP: 2015/05224-0

Electrochemical conversion of CO2 to fuels is an attractive pathway to store electrical energy in chemical form. Isolated, low-valent Ni species in nitrogen-doped graphene are now demonstrated to selectively convert CO2 to CO electrochemically with high intrinsic activity and stability.

Photocathodes for hydrogen evolution from water were made by electrodeposition of Ni-Mo layers on NiFe

Nanomaterials 8(9), 693 - (2018). doi:10.3390/nano8090693, Published by MDPI, Basel

Formation of platinum nanoparticles on tungsten oxide by photodeposition from acidic [PtCl6]2- solutions in the absence or presence of methanol is studied in detail. Without methanol, [PtCl6]2--ions adsorbed on the WO3 surface are converted to highly dispersed PtO/Pt(OH)2 particles upon illumination, the maximum achievable amount being limited to 10-30% of [PtCl6]2- present in solution. Inclusion of methanol not only promotes adsorption of [PtCl6]2- ions, but upon illumination results in a deposited Pt quantity close to 100% of [PtCl6]2-. The obtained Pt particles are in the metallic state, large, and often clustered. The limited loading and presence of PtO/Pt(OH)2 in the absence of methanol is explained by reduction of WO3-surface sites, accompanied by water oxidation catalyzed by PtO/Pt(OH)2 deposits. The extensive reduction of Pt in methanol solutions is likely induced by methoxy-radicals, oxidatively formed by reaction of methanol with photogenerated holes. This study provides guidelines for optimization of synthesis procedures of Pt/WO3 (photo)catalysts.

Chromium-incorporating mesoporous material denoted as Cr-TUD-1 was synthesized by using a surfactant-free one-pot synthesis procedure. Samples with different chromium loading were prepared and extensively characterized by means of X-ray diffraction (XRD), UV–vis and Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), 29Si MAS NMR, high-resolution transition electron microscopy (HR-TEM), nitrogen physisorption and elemental analysis. Characterization results show that as a function of Cr loading, Cr exists in different morphological structures such as isolated tetrahedrally coordinated Cr6+, nano-particulates, and finally bulky crystalline Cr2O3. The catalytic performance of the Cr-TUD-1 samples was investigated in the gas phase photo-catalytic oxidation of a mixture of light hydrocarbons i.e. methane, ethylene, ethane, propylene, and propane by molecular oxygen. Isolated Cr6+ species were found to be more active than Cr3+. The sample with 5 wt.% Cr loading which contains a mixture of Cr6+ and Cr3+ moieties was found to be the most active sample. A mechanism is proposed to show the synergy between Cr6+ and Cr3+. Cr-TUD-1 was found to be stable under dry conditions, the photocatalytic oxidation rates of the different hydrocarbons were almost the same up to six runs, however, the presence of water vapour in the reaction mixture causes serious deactivation to the catalyst. The activity of Cr-TUD-1 was superior to other metals-incorporated TUD-1 (e.g. Ti, V, and Mo) or other Cr-containing mesoporous materials (e.g. Cr-MCM-41 and Cr–SiO2) under the same reaction conditions.

In this study, the antibacterial effect of a Fe and Cd co-doped TiO2 (Fe–Cd/TiO2) visible light sensitive photocatalyst was optimized by varying operating parameters and using a response surface methodology to evaluate the experimental data. Twenty sets of disinfection experiments were conducted by adjusting three operating parameters, i.e. initial pH of the solution, reaction temperature and catalyst loading, at five levels. Based on the experimental data, a semi-empirical model was established and subsequently applied to predict the final concentration of Escherichia coli after 45 min exposure to the catalyst and visible light irradiation. Using the accurate model (coefficient of determination R2 = 0.963), optimum values were found to be 40.1 °C, 6.3, and 1.0 g/L for reaction temperature, initial pH of solution and photocatalyst loading, respectively. Under the optimized conditions, the Fe–Cd/TiO2 catalyst achieved a bacterial inactivation efficiency of 99.9% after 45 min of visible light illumination

The photocatalytic decomposition of cortisone 21-acetate (CA), a model compound for the commonly used steroid, cortisone, was studied. CA was photocatalytically decomposed in a slurry reactor with the initial rates between 0.11 and 0.46 mg L(-1)min(-1) at 10 mg L(-1) concentration, using the following heterogeneous photocatalysts in decreasing order of their catalytic activity: ZnOEvonik TiO2 P25Hombikat TiO2WO3. Due to the lack of ZnO stability in aqueous solutions, TiO2 P25 was chosen for further experiments. The decomposition reaction was found to be pseudo-first order and the rate constant decreased as a function of increasing initial CA concentration. Changing the initial pH of the CA solution did not affect the reaction rate significantly. The decomposition reaction in the presence of the oxidizing sacrificial agent sodium persulfate showed an observed decomposition rate constant of 0.004 min(-1), lower than that obtained for TiO2 P25 (0.040 min(-1)). The highest photocatalytic degradation rate constant was obtained combining both TiO2 P25 and S2O8(2-) (0.071 min(-1)) showing a synergistic effect. No reactive intermediates were detected using LC-MS showing fast photocatalytic decomposition kinetics of CA.

The nature of the external crystal facets is critical to control the (photo)catalytic properties. Two-dimensional materials can expose only one type of crystal facet among zero-dimensional (0D), one-dimensional (1D), and two-dimensional (2D) materials. Controllable tuning of the nature of the external crystal facets of 2D materials is highly desirable but very challenging. Here, we show that 2D particles with the desirable crystal facet for high-performance photocatalytic H2 generation can be obtained by using 2D metal oxide materials (i.e., nanosheets of Ca2Nb3O10 and Ti0.87O2) as templates taking consideration of the crystal configuration of 2D oxide and target compounds. We demonstrate that anatase TiO2 crystals grown on different nanosheet templates exhibit variations in photocatalytic hydrogen production rates from aqueous phase methanol solutions (6.7%), which can be attributed to the nature of the main crystal facet exposed. In view of the large number of 2D materials that have already been synthesized, this work offers a key to design and synthesize nanoparticles with the desirable specific external crystal facet for (photo)catalysis application.

SrTiO

Solar-to-hydrogen conversion by water splitting in photoelectrochemical cells (PECs) is a promising approach to alleviate problems associated with intermittency in solar energy supply and demand. Several interfacial resistances in photoelectrodes limit the performance of such cells, while the properties of interfaces are not easy to analyze in situ. We applied photoconductive-AFM to analyze the performance of WO3/p+n Si photoanodes, containing an ultra-thin metal interface of either Au or Pt. The Au interface consisted of Au nanoparticles with well-ordered interspacing, while Pt was present in the form of a continuous film. Photoconductive-AFM data show that upon illumination significantly larger currents are measured for the WO3/p+n Si anode equipped with the Au interface, as compared to the WO3/p+n Si anode with the Pt interface, in agreement with the better performance of the former electrode in a photoelectrochemical cell. The remarkable performance of the Au-containing electrode is proposed to be the result of favorable electron-hole recombination rates induced by the Au nanoparticles in a plasmon resonance excited state.

Transients in the composition of Ni@NiOx core-shell co-catalysts deposited on SrTiO3 are discussed on the basis of state-of-the-art continuous analysis of photocatalytic water splitting, and post-XPS and TEM analyses. The formation of excessive hydrogen (H2:O2 ≫ 2) in the initial stages of illumination demonstrates oxidation of Ni(OH)2 to NiOOH (nickel oxyhydroxide), with the latter catalyzing water oxidation. A disproportionation reaction of Ni and NiOOH, yielding Ni(OH)2 with residual embedded Ni, occurs when illumination is discontinued, which explains repetitive transients in (excess) hydrogen and oxygen formation when illumination is reinitiated. (Chemical Equation Presented).

Core–shell nanoparticles containing plasmonic metals (Ag or Au) have been frequently reported to enhance performance of photo-electrochemical (PEC) devices. However, the stability of these particles in water-splitting conditions is usually not addressed. In this study we demonstrate that Ag@SiO2 core–shell particles are instable in the acidic conditions in which WO3-based PEC cells typically operate, Ag in the core being prone to oxidation, even if the SiO2 shell has a thickness in the order of 10 nm. This is evident from in situ voltammetry studies of several anode composites. Similar to the results of the PEC experiments, the Ag@SiO2 core–shell particles are instable in slurry-based, Pt/ZnO induced photocatalytic water-splitting. This was evidenced by in situ photodeposition of Ag nanoparticles on the Pt-loaded ZnO catalyst, observed in TEM micrographs obtained after reaction. We explain the instability of Ag@SiO2 by OH-radical induced oxidation of Ag, yielding dissolved Ag+. Our results imply that a decrease in shell permeability for OH-radicals is necessary to obtain stable, Ag-based plasmonic entities in photo-electro-chemical and photocatalytic water splitting.

Au nanoparticles prepared by deposition–precipitation were evaluated in promoting photocatalytic activity of TiO2 (P25) in the oxidation of methylcyclohexane. At 375 nm and in particular at 425 nm, Au was found to significantly enhance the rate induced by P25. Illumination of Au-promoted P25 at 525 nm did not result in any measureable activity. To validate whether the enhancement at 425 nm is purely catalytic, or if plasmonic effects are relevant, we compared the rates of Au/TiO2 with Pt-promoted TiO2 at 375 and 425 nm. At 375 nm, Pt nanoparticles induced larger catalytic effects than Au nanoparticles, whereas the rate enhancement induced by Pt was much lower than of Au at 425 nm. We assign the thus demonstrated Au based plasmonic phenomena at 425 nm to the so-called plasmon resonance energy transfer, rather than to hot electron transfer, given the absence of activity at 525 nm.

We present a novel high throughput photocatalyst efficiency assessment method based on 96-well microplates and UV-Vis spectroscopy. We demonstrate the reproducibility of the method using methyl orange (MO) decomposition, and compare kinetic data obtained with those provided in the literature for large conventional photoreactors. To demonstrate the example of the method capabilities, we rapidly screened the effects of salts, potentially present in wastewater, on kinetic rates of MO decomposition, and briefly discuss the obtained data comparing it with the existing literature obtained using conventional photoreactors.

A novel TiO2 composite consisting of Anatase interacting with a Ti3+-containing Rutile phase was synthesized by heating a mixture of TiO2 (Hombikat) and Ti2O3 in air at different temperatures ranging from 300 °C up to 900 °C. The preparation of the samples was analyzed by Thermal Gravimetric Analysis (TGA), and the resulting composites characterized by X-ray powder diffraction (XRD), Raman and UV–Vis spectroscopy, X-ray Photoelectron Spectroscopy (XPS), Electron Paramagnetic Resonance (EPR) spectroscopy, and Scanning Electron Microscopy. Characterization data show a phase transformation from Ti2O3 to Ti3+-containing Rutile at temperatures of around 600 °C. Moreover, Hombikat is gradually converted from amorphous to crystalline Anatase. The Ti3+-content and the degree of Anatase crystallinity are respectively inversely and directly proportional to an increasing preparation temperature. The composite which was synthesized at 600 °C showed the highest photocatalytic rate in the decolorization of Methyl Orange (MO). The rate constant was significantly larger than obtained for Evonik P25 after identical thermal treatment (600 °C). Photodeposition of Pt further not only enhanced the photocatalytic activity of the optimized composite, but surprisingly also the stability. The methyl orange degradation results are discussed on the basis of hole and electron transfer phenomena between Anatase and Rutile phases, the latter containing (surface) oxygen vacancies (Ti3+). The presence of surface oxygen vacancies and/or Pt nanoparticles is proposed to be of benefit to the rate determining oxygen reduction reaction

A new method for the formation of coaxial TiO2–Ag nanowires is presented, in which TiO2 nanotubes were formed by the templated electrochemically induced sol–gel method, followed by thermal annealing. The as-formed TiO2 nanotubes have been successfully filled with a Ag core using a subsequent electrodeposition step. Coaxial nanowires have a very suitable architecture for photocatalysis, solar cells or batteries due to the high contact area between the two different phases, the large outer surface area exposed to the reactant, and short electron diffusion paths. The coaxial nanowires showed a higher efficiency than empty TiO2 nanotubes and TiO2 nanotubes with attached Ag nanoparticles in photocatalytic water splitting. Coaxial TiO2–Ag nanowires formed H2 at a rate of 1.23 × 10−3 ± 0.3 × 10−3 mol g−1 h−1 without deactivation for at least 6 h.

Several remarkable observations regarding structure, (surface) composition and visible light induced photocatalytic activity of Bi-promoted Anatase photocatalysts are reported. First, XPS characterization data show that compositions of mixed Bi-Ti-oxide phases obtained by hydrothermal treatment at 180 °C of aqueous solutions of ethanol, titanium n-butoxide, and bismuth nitrate, are surface enriched with a specific fraction of metallic-like bismuth. Second, the formation of highly dispersed nanoparticles of platinum on these composites by photo-deposition is accompanied by significant morphological changes. Third, the platinum functionalized, bismuth-promoted Anatase composites exhibit extraordinary photocatalytic activity in the photocatalytic degradation of organic compounds (acid orange 7 and salicylic acid, respectively) upon illumination at 447 nm, higher than observed for P25 upon UV illumination in similar reactor configuration. An optimized Pt-Bi-Ti-Ox composite consists of 1 wt% Pt and 5 wt% Bi. The high activity of the composite is discussed on the basis of the crystalline morphology and surface composition

The catalytic activity and hydrocarbon selectivity in electrochemical carbon dioxide (CO2) reduction on cuprous oxide (Cu2O) derived copper nanoparticles is discussed. Cuprous oxide films with [100], [110] and [111] orientation and variable thickness were electrodeposited by reduction of copper(ii) lactate on commercially available copper plates. After initiation of the electrochemical CO2 reduction by these oxide structures, the selectivity of the process was found to largely depend on the parent Cu2O film thickness, rather than on the initial crystal orientation. Starting with thin Cu2O films, besides CO and hydrogen, selective formation of ethylene is observed with very high ethylene-to-methane ratios (∼8 to 12). In addition to these products, thicker Cu2O films yield a remarkably large amount of ethane. Long term Faradaic efficiency analysis of hydrocarbons shows no sign of deactivation of the electrodes after 5 hours of continuous experiment. Online mass spectroscopy studies combined with X-ray diffraction data suggest the reduction of the Cu2O films in the presence of CO2, generating a nanoparticulate Cu morphology, prior to the production of hydrogen, CO, and hydrocarbons. Optimizing coverage, number density and size of the copper nanoparticles, as well as local surface pH, may allow highly selective formation of the industrially important product ethylene.

In this paper we report on the photocatalytic behavior in gas phase propane oxidation of well-defined TiO2 nanoparticle (NP) coatings prepared via Supersonic Cluster Beam Deposition (SCBD) on Si-wafers and quartz substrates. The temperature dependent crystal phase of the coatings was analyzed by Raman spectroscopy, and the morphology by High Resolution-Scanning Electron Microscopy. SCBD deposition in the presence of oxygen enables the in situ synthesis of TiO2 layers of amorphous NP at room temperature. Adapting the deposition temperature to 500 °C or 650 °C leads to Anatase crystals of variable size ranges, and layers showing significant porosity. At 800 °C mainly Rutile is formed. Post annealing by wafer heating of the amorphous NP prepared at room temperature results in comparable temperature dependent phases and morphologies. Photocatalytic activity in propane oxidation was dependent on the morphology of the samples: the activity decreases as a function of increasing particle size. The presence of water vapor in the propane feed generally increased the activity of the wafer-heated samples, suggesting OH groups are not profoundly present on SCBD synthesized layers. In addition, a remarkable effect of the substrate (Si or Quartz) was observed: strong interaction between Si and TiO2 is largely detrimental for photocatalytic activity. The consequences of these findings for the application of SCBD to synthesize samples for fundamental (spectroscopic) study of photocatalysis are discussed

Sub-stoichiometric titanium dioxide (TiOx) is known for its corrosion resistance and electrical conductivity.1 Thus, it is recognized as promising material for (photo-)electrochemical applications and used as protective layer in PEC water splitting or electrode material for water treatment.1,2 In general, its properties strongly depend on the synthesis and the conductivity of TiOx can be tuned by its Ti/O ratio.1,3 TiOx is commonly prepared by hydrogenation;3 however, the requirements for high pressure, high temperature and hydrogen atmosphere are cumbersome imposing a potential safety risk. Over the past years, electrochemical preparation of TiOx via cathodic polarization of TiO2 substrates became an interesting alternative.4 Pt is widely used in electrochemistry as counter electrode5 and frequently applied for TiOx preparation;4 however, the suitability of Pt as (anodic) counter electrode (CE) is questionable since the stability of Pt is often overestimated.5 As Pt contamination can severely influence the electrocatalytic properties this study focuses on the preparation of TiOx using three different CE (Pt, Iridium mixed metal oxide, Boron doped diamond) and the influence of the CE will be discussed in detail. It will be shown using state of the art techniques such as SEM, XPS and highly surface sensitive low energy ion scattering (LEIS) spectroscopy that indeed Pt contamination dominates the properties of TiOx (Figure 1a). In contrast, for Iridium mixed metal oxide and Boron doped diamond CEs no related contaminations were observed allowing to determine the intrinsic properties of TiOx electrodes. Using Fe2 +/3+ redox couple experiments and electrochemical impedance spectroscopy we determined the charge transfer properties of the TiOx electrodes prepared with BDD and IrMMO (Figure 1b). Although both BDD and IrMMO appear to be suited for the preparation of TiOx differences in the doping are observed that will be discussed in detail in this contribution. References: F. C. Walsh and R. G. A. Wills, Electrochim. Acta, 55, 6342–6351 (2010), doi: 10.1039/c6ra14507h B. Xu, H. Y. Sohn, Y. Mohassab, and Y. Lan, RSC Adv., 6, 79706–79722 (2016), doi: 10.1039/c6ra14507h. X. Yan, L. Tian, X. Tan, M Zhou, L. Liu and X. Chen, MRS Commun., 6, 192–203 (2016), doi: 10.1557/mrc.2016.33. J. Swaminathan, R. Subbiah, and V. Singaram, ACS Catal., 6, 2222–2229 (2016), doi: 10.1021/acscatal.5b02614. J. G. Chen, C. W. Jones, S. Linic, and V. R. Stamenkovic, ACS Catal., 7, 6392–6393 (2017), doi: 10.1021/acscatal.7b02839. Figure 1