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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.

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.

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

For existing large-scale industrial processes (chlorate process) as well as emerging technologies (photocatalytic water splitting) electrochemical selectivity towards hydrogen evolution is essential to achieve high (energy) efficiencies. Electrode selectivity is widely reported to be achieved in the presence of solution additives such as sodium dichromate (Na2Cr2O7) [1]. In the chlorate process dichromate is known to form a thin membrane-like structure by reductive electrochemical deposition on the cathode. With the growing interest in the electrification of the chemical industry and the general need for clean energy technologies, the development of membrane-coated “buried” electrodes recently received increasing attention [2]. In this presentation, 1) the formation of amorphous oxide electrode coatings from chromate solutions will be described and 2) potential alternative coatings, particularly coatings derived from permanganate and vanadate electrolyte additives, will be introduced [3,4]. It will be shown by different electroanalytical techniques that these amorphous oxide overlayers are selectively permeable to certain electroactive species (Figure 1) and enable efficient hydrogen evolution without significantly altered kinetics. References [1] V. Smulders, N. Simic, A.S.O. Gomes, B. Mei, G. Mul, Electrochim. Acta, 2018, accepted; [2] D.V. Esposito, ACS Catal., 2018, 8, 457-465; [3] B. Endrődi, V. Smulders, N. Simic, M. Wildlock, G. Mul, B. Mei, A. Cornella, Appl. Catal. B, 2018, DOI: 10.1016/j.apcatb.2018.11.038; [4] B. Endrődi, S. Sandin, V. Smulders, N. Simic, M. Wildlock, G. Mul, B. Mei, A. Cornell, J. Clean. Prod. 2018, 182, 529-537.