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Octahedra (oh) PtNiX/C catalysts have attracted attention as cathode catalysts for proton-exchange membrane fuel cells (PEMFCs) due to their exceptional catalytic activities toward the oxygen reduction reaction. Here, we investigate the degradation dynamics of oh-PtNiIr in fuel cell conditions by operando X-ray diffraction (XRD). Two XRD-coupled square-wave accelerated stress tests (0.6 to 0.95) V and (0.7 to 0.95) V (where V is the cell voltage) confirm that, when fixing the upper limit, the dissolution and overall degradation strongly depend on the lower potential limit. By directly observing the extent of metal oxidation during potential cycling, we link the alloy redox dynamics to the stability. The studied catalysts' stability is proportional to both the extent of metal oxidation and, more interestingly, the degree of reduction. Comparing a benchmark Pt catalyst with oh-PtNiIr allows for associating the differences between oxidation and reduction potentials and the optimal usage window for each class of catalysts. This relatively simple method can be employed to find the operation boundaries of the PEMFC to minimize the degradation of a large class of Pt-based catalysts without time-consuming stress tests.

Layered double hydroxides (LDHs) are among the most active and studied catalysts for the oxygen evolution reaction (OER) in alkaline electrolytes. However, previous studies have generally either focused on a small number of LDHs, applied synthetic routes with limited structural control, or used non‐intrinsic activity metrics, thus hampering the construction of consistent structure–activity‐relations. Herein, by employing new individually developed synthesis strategies with atomic structural control, we obtained a broad series of crystalline α‐MA(II)MB(III) LDH and β‐MA(OH)2 electrocatalysts (MA=Ni, Co, and MB=Co, Fe, Mn). We further derived their intrinsic activity through electrochemical active surface area normalization, yielding the trend NiFe LDH > CoFe LDH > Fe‐free Co‐containing catalysts > Fe‐Co‐free Ni‐based catalysts. Our theoretical reactivity analysis revealed that these intrinsic activity trends originate from the dual‐metal‐site nature of the reaction centers, which lead to composition‐dependent synergies and diverse scaling relationships that may be used to design catalysts with improved performance., Catalytic activities for oxygen evolution on crystalline 3d transition metal layered double hydroxides are derived using electrochemical surface area based normalization. Density functional calculations reveal a dual‐metal‐site feature of the reaction centers that provides opportunities to design new catalysts with improved performance.

Replacing fossil fuels with energy sources and carriers that are sustainable, environmentally benign, and affordable is amongst the most pressing challenges for future socio-economic development. To that goal, hydrogen is presumed to be the most promising energy carrier. Electrocatalytic water splitting, if driven by green electricity, would provide hydrogen with minimal CO2 footprint. The viability of water electrolysis still hinges on the availability of durable earth-abundant electrocatalyst materials and the overall process efficiency. This review spans from the fundamentals of electrocatalytically initiated water splitting to the very latest scientific findings from university and institutional research, also covering specifications and special features of the current industrial processes and those processes currently being tested in large-scale applications. Recently developed strategies are described for the optimisation and discovery of active and durable materials for electrodes that ever-increasingly harness first-principles calculations and machine learning. In addition, a technoeconomic analysis of water electrolysis is included that allows an assessment of the extent to which a large-scale implementation of water splitting can help to combat climate change. This review article is intended to cross-pollinate and strengthen efforts from fundamental understanding to technical implementation and to improve the ‘junctions’ between the field's physical chemists, materials scientists and engineers, as well as stimulate much-needed exchange among these groups on challenges encountered in the different domains.

Direct methanol fuel cells (DMFCs) have the major advantage of the high energy density of the methanol (4.33 kWh/l) they use as a liquid fuel, although their costs remain too high due to the high q...

Hot, coastal, hyper-arid regions with intense solar irradiation and strong on- and off-shore wind patterns are ideal locations for the production of renewable electricity using wind turbines or photovoltaics. Given ample access to seawater and scarce freshwater resources, such regions make the direct and selective electrolytic splitting of seawater into molecular hydrogen and oxygen a potentially attractive technology. The key catalytic challenge consists of the competition between anodic chlorine chemistry and the oxygen evolution reaction (OER). This Perspective addresses some aspects related to direct seawater electrolyzers equipped with selective OER and hydrogen evolution reaction (HER) electrocatalysts. Starting from a historical background to the most recent achievements, it will provide insights into the current state and future perspectives of the topic. This Perspective also addresses prospects of the combination of direct seawater electrolysis with hydrogen fuel cell technology (reversible seaw...

The European GAIA project focussed on the development of novel ionomer, membrane, reinforcement, catalyst, catalyst support, gas diffusion and microporous layers, and layer constructions for high power density, high current density automotive membrane electrode assemblies (MEAs). Reaching a sufficiently low degradation rate (11-14 µV/h in an automotive drive cycle including operation at 105 °C) consistent with the 6,000 hour lifetime target while also succeeding in achieving the 1.8 W/cm2 power density at high current density (3 A/cm2) target was a major challenge, and the outcomes of GAIA represent an important step forward for fuel cell transport MEA technology. The results are all the more important that they were obtained with MEAs using materials developed and up-scaled in GAIA. By reaching this high-power density without increasing platinum loading, the Pt-specific power density was reduced to 0.25 g Pt/kW. Costs analysis demonstrated that recycling (catalyst and ionomer) has the potential to significantly reduce MEA cost, and that, with this, the cost per kW of the high power density GAIA MEAs approaches the 6 €/kW target. This presentation will outline the main materials development steps, summarise testing protocols and the results of automotive size cell short stack tests. Acknowledgement. This project has received funding from the Fuel Cells and Hydrogen 2 Joint Undertaking (now Clean Hydrogen Partnership) under grant agreement n°826097. This Joint Undertaking receives support from the European Union’s Horizon 2020 Research and Innovation program, Hydrogen Europe and Hydrogen Europe Research.

The NiFe layered double hydroxides (LDHs) are among the most active electrocatalysts for the oxygen evolution reaction (OER) in alkaline electrolytes.1 The incorporation of Fe dramatically increases the catalytic activity of Ni hydroxides.2 However, even the most active NiFe LDH catalyst still shows a considerable overpotential for the OER. Understanding the nature of the catalytic active sites and the catalytic mechanism are key challenges to develop better OER electrocatalysts. In this contribution, atomic-scale details of the catalytic active phase will be presented, showing that NiFe LDHs are oxidized under applied anodic potentials from as-prepared α-phases to activated γ-phases.3 The interlayer and in-plane lattice parameters of the OER-active γ-phase were obtained by performing wide angle X-ray scattering (WAXS) measurements on NiFe LDH nanoplatelets during operating electrochemical conditions and were characterized by a contraction of about 8%. Operando WAXS was then performed for other selected catalysts belonging to the transition metal LDH family of materials. Structural similarities of the catalytically active phases will be highlighted. Finally, in order to derive activity-structure relationships, an approach is presented to calculate intrinsic catalytic activities, which are challenging to evaluate for this class of materials. The presented method is based on electrochemical active surface area normalization obtained by impedance spectroscopy measurements under OER.4 References F. Dionigi and P. Strasser, Advanced Energy Materials, 2016, 6 1600621. L. Trotochaud, S. L. Young, J. K. Ranney and S. W. Boettcher, Journal of the American Chemical Society, 2014, 136: 6744-6753. F. Dionigi, Z. Zeng, I. Sinev, T. Merzdorf, S. Deshpande, M. B. Lopez, S. Kunze, I. Zegkinoglou, H. Sarodnik, D. Fan, A. Bergmann, J. Drnec, J. Ferreira de Araujo, M. Gliech, D. Teschner, J. Zhu, W.-X. Li, J. Greeley, B. Roldan Cuenya and P. Strasser, Nat Commun, 2020, 11 2522. F. Dionigi, J. Zhu, Z. Zeng, T. Merzdorf, H. Sarodnik, M. Gliech, L. Pan, W.-X. , Li, J. Greeley, and P. Strasser, Angew Chem Int Edit, 60, 14446 – 14457 (2021).

Proton exchange membrane fuel cells have been recently developed at an increasing pace as clean energy conversion devices for stationary and transport sector applications. High platinum cathode loadings contribute significantly to costs. This is why improved catalyst and support materials as well as catalyst layer design are critically needed. Recent advances in nanotechnologies and material sciences have led to the discoveries of several highly promising families of materials. These include platinum-based alloys with shape-selected nanostructures, platinum-group-metal-free catalysts such as metal-nitrogen-doped carbon materials and modification of the carbon support to control surface properties and ionomer/catalyst interactions. Furthermore, the development of advanced characterization techniques allows a deeper understanding of the catalyst evolution under different conditions. This review focuses on all these recent developments and it closes with a discussion of future research directions in the field., The high platinum loadings at the cathodes of proton exchange membrane fuel cells significantly contribute to the cost of these clean energy conversion devices. Here, the authors critically review and discuss recent developments on low- and non-platinum-based cathode catalysts and catalyst layers.

Water oxidation is a crucial reaction for renewable energy conversion and storage. Among the alkaline oxygen evolution reaction (OER) catalysts, NiFe based oxyhydroxides show the highest catalytic activity. However, the details of their OER mechanism are still unclear, due to the elusive nature of the OER intermediates. Here, using a novel differential electrochemical mass spectrometry (DEMS) cell interface, we performed isotope‐labelling experiments in 18O‐labelled aqueous alkaline electrolyte on Ni(OH)2 and NiFe layered double hydroxide nanocatalysts. Our experiments confirm the occurrence of Mars‐van‐Krevelen lattice oxygen evolution reaction mechanism in both catalysts to various degrees, which involves the coupling of oxygen atoms from the catalyst and the electrolyte. The quantitative charge analysis suggests that the participating lattice oxygen atoms belong exclusively to the catalyst surface, confirming DFT computational hypotheses. Also, DEMS data suggest a fundamental correlation between the magnitude of the lattice oxygen mechanism and the faradaic efficiency of oxygen controlled by pseudocapacitive oxidative metal redox charges., Differential electrochemical mass spectrometry using a hanging droplet cell confirmed the occurrence of Mars‐van‐Krevelen lattice oxygen evolution reaction mechanism in both Ni(OH)2 and NiFe layered double hydroxide nanocatalysts. This mechanism involves the coupling of oxygen atoms from the catalyst surface and the electrolyte.

Electrochemical water splitting using renewable energy is a key route to generate green H2 fuel. In this process, however, O2 generation at the anode through the oxygen evolution reaction (OER) is inherently slower by over four orders of magnitude than H2 generation at the cathode. Thus, improving OER efficiency has been a majority effort in electrolysis. Ni-based and Co-based layered double hydroxides (LDHs) are among the most active and studied catalysts for the oxygen evolution reaction (OER) in alkaline electrolytes. Because it happens under extremely oxidative aqueous conditions, however, in-situ crystal structures of the OER active phase are still largely unknown, significantly hindering the establishment of structure-property relationships. In this talk, we provide the first direct atomic-scale evidence that, under applied anodic potentials, NiFe and CoFe LDHs oxidize from as-prepared α-phases to activated γ-phases. The OER-active γ-phases are characterized by about 8% contraction of the lattice spacing and switching of the intercalated ions from carbonate to potassium. The calculated surface phase diagrams indicate that surface O sites are saturated with H by forming bridge OH, and coordinatively unsaturated metal sites are poisoned by OH adsorption under OER conditions. These structures, and the associated reaction free energies, suggest that the OER proceeds via a Mars van Krevelen mechanism, starting with the oxidation of bridge OH at the reaction centers with dual metal sites, i.e., M1-OH-M2. Our study suggests that the compound-dependent activity originates from the dual-metal site feature of the reaction centers. While this feature does not influence the OH-OOH scaling relationship, it leads to diverse OH-O scaling relationships, including those with near-zero slopes and negative slopes. Breaking OH-OOH scaling relationships were frequently discussed in the literature, as it determines the minimum overpotential. However, our study showed that, to approach the minimum overpotential dictated by a specific OH-OOH scaling relationship, the key is to break the OH-O scaling relationship. A possible route is to form binary metal oxyhydroxides with dual metal sites at the reaction centers or introduce a third element into NiFe LDH or CoFe LDH. References: Dionigi, Z. Zeng, I. Sinev, T. Merzdorf, S. Deshpande, M. B. Lopez, S. Kunze, I. Zegkinoglou, H. Sarodnik, D. Fan, A. Bergmann, J. Drnec, J. F. d. Araujo, M. Gliech, D. Teschner, J. Zhu, W.-X. Li, J. Greeley, B. R. Cuenya, P. Strasser, Nature Communications 2020, 11, 2522. Dionigi, J. Zhu, Z. Zeng, T. Merzdorf, H. Sarodnik, M. Gliech, L. Pan, W.-X. Li, J. Greeley, P. Strasser, Angew. Chem., Int. Ed. 2021, 60, 14446-14457.

Identifying the structural response of nanoparticle-support ensembles to the reaction conditions is essential to determine their structure in the catalytically active state as well as to unravel the possible degradation pathways. In this work, we investigate the (electronic) structure of carbon- and oxide-supported Pt nanoparticles during electrochemical oxidation by in situ X-ray diffraction, absorption spectroscopy as well as the Pt dissolution rate by in situ mass spectrometry. We prepared ellipsoidal Pt nanoparticles by impregnation of the carbon and titanium-based oxide support as well as spherical Pt nanoparticles on an indium-based oxide support by a surfactant-assisted synthesis route. During electrochemical oxidation, we show that the oxide-supported Pt nanoparticles resist (bulk) oxide formation and Pt dissolution. The lattice of smaller Pt nanoparticles exhibits a size-induced lattice contraction in the as-prepared state with respect to bulk Pt but it expands reversibly during electrochemical oxidation. This expansion is suppressed for the Pt nanoparticles with a bulk-like relaxed lattice. We could correlate the formation of d-band vacancies in the metallic Pt with Pt lattice expansion. PtOx formation is strongest for platelet-like nanoparticles and we explain this with a higher fraction of exposed Pt(100) facets. Of all investigated nanoparticle-support ensembles, the structural response of RuO2/TiO2-supported Pt nanoparticles is the most promising with respect to their morphological and structural integrity under electrochemical reaction conditions.

Review Article Published: 17 February 2020 Electrolysis of low-grade and saline surface water Wenming Tong, Mark Forster, Fabio Dionigi, Sören Dresp, Roghayeh Sadeghi Erami, Peter Strasser, Alexander J. Cowan & Pau Farràs Nature Energy (2020)Cite this article 1779 Accesses 1 Citations 60 Altmetric Metricsdetails Abstract Powered by renewable energy sources such as solar, marine, geothermal and wind, generation of storable hydrogen fuel through water electrolysis provides a promising path towards energy sustainability. However, state-of-the-art electrolysis requires support from associated processes such as desalination of water sources, further purification of desalinated water, and transportation of water, which often contribute financial and energy costs. One strategy to avoid these operations is to develop electrolysers that are capable of operating with impure water feeds directly. Here we review recent developments in electrode materials/catalysts for water electrolysis using low-grade and saline water, a significantly more abundant resource worldwide compared to potable water. We address the associated challenges in design of electrolysers, and discuss future potential approaches that may yield highly active and selective materials for water electrolysis in the presence of common impurities such as metal ions, chloride and bio-organisms. W.T., M.F., R.S.E., A.J.C. and P.F. acknowledge financial support from INTERREG Atlantic Area programme (Grant reference EAPA_190_2016). P.F. acknowledges support from Royal Society Alumni programme. F.D., S.D. and P.S. gratefully acknowledge financial support by the German Research Foundation (DFG) through Grant reference number STR 596/8-1 and the federal ministry for economic affairs and energy (Bundesministerium für Wirtschaft und Energie, BMWi) under grant number 03EIV041F. P.S. acknowledges partial funding by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) under Germany´s Excellence Strategy – EXC 2008/1 – 390540038 (zum Teil gefördert durch die Deutsche Forschungsgemeinschaft (DFG) im Rahmen der Exzellenzstrategie des Bundes und der Länder – EXC 2008/1 – 390540038). peer-reviewed 2020-08-17

Nitrogen-enriched porous carbons have been discussed as supports for Pt nanoparticle catalysts deployed at cathode layers of polymer electrolyte membrane fuel cells (PEMFC). Here, we present an analysis of the chemical process of carbon surface modification using ammonolysis of preoxidized carbon blacks, and correlate their chemical structure with their catalytic activity and stability using in situ analytical techniques. Upon ammonolysis, the support materials were characterized with respect to their elemental composition, the physical surface area, and the surface zeta potential. The nature of the introduced N-functionalities was assessed by X-ray photoelectron spectroscopy. At lower ammonolysis temperatures, pyrrolic-N were invariably the most abundant surface species while at elevated treatment temperatures pyridinic-N prevailed. The corrosion stability under electrochemical conditions was assessed by in situ high-temperature differential electrochemical mass spectroscopy in a single gas diffusion layer electrode; this test revealed exceptional improvements in corrosion resistance for a specific type of nitrogen modification. Finally, Pt nanoparticles were deposited on the modified supports. In situ X-ray scattering techniques (X-ray diffraction and small-angle X-ray scattering) revealed the time evolution of the active Pt phase during accelerated electrochemical stress tests in electrode potential ranges where the catalytic oxygen reduction reaction proceeds. Data suggest that abundance of pyrrolic nitrogen moieties lower carbon corrosion and lead to superior catalyst stability compared to state-of-the-art Pt catalysts. Our study suggests with specific materials science strategies how chemically tailored carbon supports improve the performance of electrode layers in PEMFC devices.

Identifying the structural response of nanoparticle–support ensembles to the reaction conditions is essential to determine their structure in the catalytically active state as well as to unravel the possibledegradation pathways. In this work, we investigate the (electronic) structure of carbon- and oxide-supported Pt nanoparticles during electrochemical oxidation byin situX-ray diffraction, absorptionspectroscopy as well as the Pt dissolution rate byin situmass spectrometry. We prepared ellipsoidal Pt nanoparticles by impregnation of the carbon and titanium-based oxide support as well as spherical Pt nanoparticles on an indium-based oxide support by a surfactant-assisted synthesis route. Duringelectrochemical oxidation, we show that the oxide-supported Pt nanoparticles resist (bulk) oxideformation and Pt dissolution. The lattice of smaller Pt nanoparticles exhibits a size-induced latticecontraction in the as-prepared state with respect to bulk Pt but it expands reversibly during electrochemical oxidation. This expansion is suppressed for the Pt nanoparticles with a bulk-like relaxedlattice. We could correlate the formation of d-band vacancies in the metallic Pt with Pt lattice expansion. PtOxformation is strongest for platelet-like nanoparticles and we explain this with a higher fraction of exposed Pt(100) facets. Of all investigated nanoparticle–support ensembles, the structural response of RuO2/TiO2-supported Pt nanoparticles is the most promising with respect to their morpho-logical and structural integrity under electrochemical reaction conditions.

This contribution reports the discovery and analysis of a p-block Sn-based catalyst for the electroreduction of molecular oxygen in acidic conditions at fuel cell cathodes; the catalyst is free of platinum-group metals and contains single-metal-atom actives sites coordinated by nitrogen. The prepared SnNC catalysts meet and exceed state-of-the-art FeNC catalysts in terms of intrinsic catalytic turn-over frequency and hydrogen–air fuel cell power density. The SnNC-NH3 catalysts displayed a 40–50% higher current density than FeNC-NH3 at cell voltages below 0.7 V. Additional benefits include a highly favourable selectivity for the four-electron reduction pathway and a Fenton-inactive character of Sn. A range of analytical techniques combined with density functional theory calculations indicate that stannic Sn(iv)Nx single-metal sites with moderate oxygen chemisorption properties and low pyridinic N coordination numbers act as catalytically active moieties. The superior proton-exchange membrane fuel cell performance of SnNC cathode catalysts under realistic, hydrogen–air fuel cell conditions, particularly after NH3 activation treatment, makes them a promising alternative to today’s state-of-the-art Fe-based catalysts. For oxygen reduction and hydrogen oxidation reactions, proton-exchange membrane fuel cells typically rely on precious-metal-based catalysts. A p-block single-metal-site tin/nitrogen-doped carbon is shown to exhibit promising electrocatalytic and fuel cell performance.

The reduction of the amount of platinum used in proton exchange membrane fuel cell cathodes at constant power density helps lower the cell stack cost of fuel cell electric vehicles. Recent screening studies using the thin film rotating disk electrode technique have identified an ever-growing number of Pt-based nanocatalysts with oxygen reduction reaction Pt-mass activities that allow for a substantial projected decrease in the geometric platinum loading at the cathode layer. However, the step from a rotating disk electrode test to a membrane electrode assembly test has proved a formidable task. The deployment of advanced, often shape-controlled dealloyed Pt alloy nanocatalysts in actual cathode layers of proton exchange membrane fuel cells has remained extremely challenging with respect to their actual catalytic activity under hydrogen/oxygen flow, their hydrogen/air performance at high current densities, and their morphological stability under prolonged fuel cell operations. In this review, we discuss some of these challenges, yet also propose possible solutions to understand the challenges and to eventually unfold the full potential of advanced Pt-based alloy oxygen reduction reaction catalysts in fuel cell electrode layers.

We report and study the translation of exceptionally high catalytic oxygen electroreduction activities of molybdenum-doped octahedrally shaped PtNi(Mo) nanoparticles from conventional thin-film rotating disk electrode screenings (3.43 +/- 0.35 A mg(pt)(-1) at 0.9 V-RHE) to membrane electrode assembly (MEA)-based single fuel cell tests with sustained Pt mass activities of 0.45 A mg(pt)(-1) at 0.9 V-cell, one of the highest ever reported performances for advanced shaped Pt alloys in real devices. Scanning transmission electron microscopy with energy dispersive X-ray analysis (STEM-EDX) reveals that Mo preferentially occupies the Pt-rich edges and vertices of the element-anisotropic octahedral PtNi particles. Furthermore, by combining in situ wide-angle X-ray spectroscopy, X-ray fluorescence, and STEM-EDX elemental mapping with electrochemical measurements, we finally succeeded to realize high Ni retention in activated PtNiMo nanoparticles even after prolonged potential-cycling stability tests. Stability losses at the anodic potential limits were mainly attributed to the loss of the octahedral particle shape. Extending the anodic potential limits of the tests to the Pt oxidation region induced detectable Ni losses and structural changes. Our study shows on an atomic level how Mo adatoms on the surface impact the Ni surface composition, which, in turn, gives rise to the exceptionally high experimental catalytic ORR reactivity and calls for strategies on how to preserve this particular surface composition to arrive at performance stabilities comparable with state-of-the-art spherical dealloyed Pt core-shell catalysts.

The sluggish kinetics of the oxygen reduction reaction (ORR) at the cathode of proton exchange membrane fuel cells (PEMFCs) and the harsh environment pose challenges on the development of cheap, active and stable catalysts. As a consequence, Pt catalysts are the most utilized catalysts in commercial PEMFC stacks for automotive applications. While the progress in the recent years allowed a considerable decrease of Pt loadings (from 0.4-0.8 to ~0.1 mgPt cm-2), other challenges emerged at such and lower Pt loadings at high current densities.1 For example, octahedral PtNi nanoparticles have been reported to achieve extremely high mass activity in rotating disk electrode (RDE) experiments2 and the introduction of a third metal as surface dopant has been shown to have beneficial effects on the RDE performance.3 Despite these promising steps toward shape-stable PtNiX octahedral nanoparticles, the morphological stability and the performance in membrane electrode assembly (MEA)-based fuel cell measurements still need to be improved to match and surpass the state of the art Pt and dealloyed Pt-alloy catalysts.4 In this contribution we will show our recent efforts in improving the performance of PtNi based octahedral nanoparticle catalysts towards integration in low Pt loading cathodes for PEMFC. In particular, two strategies will be presented. First, Rh surface doping is introduced to improve the morphological stability. Second, new carbon modified supports by nitrogen plasma treatment have been investigated. References A. Kongkanand and M. F. Mathias, J Phys Chem Lett, 2016, 7, 1127-1137. P. Strasser, Science, 2015, 349, 379-380. X. Q. Huang, Z. P. Zhao, L. Cao, Y. Chen, E. B. Zhu, Z. Y. Lin, M. F. Li, A. M. Yan, A. Zettl, Y. M. Wang, X. F. Duan, T. Mueller and Y. Huang, Science, 2015, 348, 1230-1234. F. Dionigi, C. C. Weber, M. Primbs, M. Gocyla, A. M. Bonastre, C. Spöri, H. Schmies, E. Hornberger, S. Kühl, J. Drnec, M. Heggen, J. Sharman, R. E. Dunin-Borkowski and P. Strasser, Nano Lett, 2019, 19, 6876-6885. Acknowledgements The GAIA project has received funding from the Fuel Cells and Hydrogen 2 Joint Undertaking under grant agreement No 826097. This Joint Undertaking receives support from the European Union’s Horizon 2020 research and innovation programme, Hydrogen Europe and Hydrogen Europe Research. Figure 1. Mass activity from RDE measurements as a function of the electrochemically active surface area obtained by hydrogen under potential deposition, before (black) and after (red) stability tests. The blue arrow and the percentage indicate the loss after stability tests. Pt/Cv and Oh-PtNiMo/Cv data from ref.4. Figure 1

Water splitting to generate O2 and H2 fuel is an ongoing focus of (photo)electrochemical energy storage and conversion efforts. While water oxidation to generate O2 through the oxygen evolution reaction (OER) accounts for the majority of energy loss in this process, water reduction to generate H2 through the hydrogen evolution reaction (HER) in alkaline is also sluggish, with over two orders of magnitude less activity than in acid. For OER, NiFe layered double hydroxides have attracted significant interest due to their comparable performance with precious metal-based RuO2 and IrO2 catalysts. In spite of extensive study, however, the 3D crystal structure of the active phase under catalytic oxygen evolution reaction conditions remains unclear, and the lack of atomic-scale details concerning the crystal structure makes it challenging to choose appropriate structural models for first principles-based mechanistic studies. Similarly, for alkaline HER, ultrathin (oxy)hydroxide films on precious metal substrates have shown impressive activity improvements compared to pure Pt, but the films’ structure and stability are still largely unknown, and the catalytic mechanism remains unclear. In this presentation, we will begin by showing our recent efforts to elucidate the catalytically active phase and OER mechanism on NiFe layered double hydroxides by combining electrochemical measurements, operando experiments, DFT calculations, and ab initio molecular dynamics simulations. Next, for HER, we will turn to monolayer Ni (oxy)hydroxide films, wherein we will demonstrate that these ultrathin films can be dramatically stabilized with respect to the corresponding bulk analogs and that the bifunctional interface provides ideal sites for water activation. Finally, if time permits, we outline our recent work on oxygen reduction reaction electrochemistry, discussing both tunable intrinsic strain in two-dimensional transition metal electrocatalysts for the ORR, as well as recent developments in the study of stability of PGM-free electrocatalysts.

The incorporation of Fe3+ ions in Ni(OH)2 is responsible for a dramatic oxygen evolution reaction (OER) activity enhancement.1 NiFe based (oxy)hydroxides are among the most active electrocatalysts for the OER in alkaline electrolytes.2 Their crystal structure, known as layered double hydroxide (LDH), is composed of layers of edge sharing metal oxygen octahedra that are intercalated with water molecules and charge balancing anions (Figure 1).3 In contrast, Fe-free Ni(OH)2 can be synthesized in the well defined brucite-like crystal structure, which does not have intercalated species and is indicated as β-M(OH)2. Oxidative deprotonation and possibly the electrocatalytic OER reaction lead to reversible structural changes that are only observable with in operando or in situ methods.1, 4 Several reports investigated the oxidized phases, but key aspects of the catalytically OER active site remain elusive and more investigations are necessary. In this contribution our results obtained using in operando wide angle X-ray scattering (WAXS) on crystalline NiFe LDH nanostructured catalysts and β-Ni(OH)2 catalysts will be presented.5 We identified the structure of the active catalyst state and observed that the lattice spacing under operating conditions changes reaching a similar value for both catalysts (Figure 1). Using a unique differential electrochemical mass spectrometry (DEMS) apparatus and isotope labelled experiments we further unravel mechanistic details on how the OER mechanism proceeds on these catalysts. References D. Friebel, M. W. Louie, M. Bajdich, K. E. Sanwald, Y. Cai, A. M. Wise, M. J. Cheng, D. Sokaras, T. C. Weng, R. Alonso-Mori, R. C. Davis, J. R. Bargar, J. K. Norskov, A. Nilsson and A. T. Bell, J Am Chem Soc, 2015, 137, 1305-1313. M. Gong, Y. G. Li, H. L. Wang, Y. Y. Liang, J. Z. Wu, J. G. Zhou, J. Wang, T. Regier, F. Wei and H. J. Dai, J Am Chem Soc, 2013, 135, 8452-8455. F. Dionigi and P. Strasser, Advanced Energy Materials, 2016, 6. M. Gorlin, J. F. de Araujo, H. Schmies, D. Bernsmeier, S. Dresp, M. Gliech, Z. Jusys, P. Chernev, R. Kraehnert, H. Dau and P. Strasser, J Am Chem Soc, 2017, 139, 2070-2082. F. Dionigi, Z. Zeng, I. Sinev, T. Merzdorf, S. Deshpande, M. B. Lopez, S. Kunze, I. Zegkinoglou, H. Sarodnik, D. X. Fan, A. Bergmann, J. Drnec, J. F. De Araujo, M. Gliech, D. Teschner, J. Greeley, B. Roldan Cuenya and P. Strasser, submitted 2019. Figure 1. Shift of the (003) diffraction peak associated to the interlayer distance for NiFe LDH catalysts between the resting state (black) and OER active state (red).5 3D structural model of the as prepared NiFe LDH. Figure 1

Water splitting to generate O2 and H2 fuel has been a major focus of (photo)electrochemical energy storage and conversion efforts, but many challenges remain. While water oxidation to generate O2 through the oxygen evolution reaction (OER) accounts for the majority of energy loss in this process, water reduction to generate H2 through the hydrogen evolution reaction in alkaline is over two orders of magnitude slower than that in acid. For OER, NiFe layered double hydroxides have attracted significant interest due to their comparable performance with precious metal-based RuO2 and IrO2 catalysts. In spite of extensive study, however, the 3D crystal structure of the active phase under catalytic oxygen evolution reaction conditions remains unclear. The lack of the atomic-scale details of crystal structure makes it challenging to choose appropriate structural models for first principles-based mechanistic studies. Therefore, it is of significant interest to identify these materials’ in-situ crystal structure and, subsequently, determine the intrinsic catalytic mechanism. Similarly, for alkaline HER, ultrathin (oxy)hydroxide films on precious metal substrates possess impressive activity improvement, but the films’ structure and stability are still largely unknown, and the catalytic mechanism remains unclear. In this presentation, we will begin by showing our recent efforts to elucidate the catalytically active phase and OER mechanism on NiFe layered double hydroxides by combining electrochemical measurements, operando experiments, DFT calculations, and ab initio molecular dynamics simulations. Next, for HER, we will introduce the methodologies we have recently developed towards the highly accurate prediction of Pourbaix diagram of transition metal (oxy)hydroxides. Subsequently, using monolayer Ni (oxy)hydroxide films as an example, we will describe a simple scheme to study the structures and the stability of these films on precious metal surfaces. We will show how the ultrathin films can be dramatically stabilized with respect to the corresponding bulk analogs. Then, using the hydrogen evolution reaction as an example, we will demonstrate how these techniques can be applied to understand the steady state, the active phases, and the catalytic mechanism of bi-functional interfaces. We will then demonstrate the extension of the present understanding to real-world catalysts, i.e. precious metal nanoparticles supported on ultrathin transition metal (oxy)hydroxide films. Finally, we will show this understanding can be used to design new bi-functional catalysts with improved performances. If time permits, we will also show our recent work on tunable intrinsic strain in two-dimensional transition metal electrocatalysts for the oxygen reduction reaction. References: 1. Z. Zeng, K.-C. Chang, J. Kubal, N. M. Markovic, J. Greeley, Nature Energy 2, 17070 (2017). 2. L. Wang, Y. Zhu, Z. Zeng, C. Lin, M. Giroux, L. Jiang, Y. Han, J. Greeley, C. Wang, J. Jin, Nano Energy 31, 456-461 (2017). 3. L. Wang, Z. Zeng, W. Gao, T. Maxson, D. Raciti, M. Giroux, X. Pan, C. Wang, J. Greeley, Science 363, 870-874 (2019). 4. F. Dionigi, Z. Zeng, I. Sinev, T. Merzdorf, S. Deshpande, M. Bernal Lopez, S. Kunze, I. Zegkinoglou, H. Sarodnik, D. Fan, A. Bergmann, J. Drnec, J. Ferreira de Araujo, M. Gliech, D. Teschner, J. Greeley, B. Roldan Cuenya, P. Strasser, submitted, 2019.

In the face of the global energy challenge and progressing global climate change, renewable energy systems and components, such as fuel cells and electrolyzers, which close the energetic oxygen and carbon cycles, have become a technology development priority. The electrochemical oxygen reduction reaction (ORR) and the direct electrochemical carbon dioxide reduction reaction (CO

The electrochemical hydrogen evolution reaction (HER) is growing in significance as society begins to rely more on renewable energy sources such as wind and solar power. Thus, research on designing...

Using silicon-based μ-reactors, we have studied the photocatalytic water splitting reaction and the catalytic back reaction on the same catalysts. GaN:ZnO without cocatalyst and loaded with Rh, Pt, Cr 2 O 3 /Rh, Cr 2 O 3 /Pt, and Rh–Cr mixed oxide has been tested for gas-phase photocatalytic water splitting. The results confirm the high activity observed in liquid-phase experiments with Cr 2 O 3 /Rh and Rh–Cr mixed oxide as cocatalysts. To investigate the reason of this enhanced activity, the back reaction was studied by reacting stoichiometric H 2 /O 2 and monitoring the water molecules produced. The comparison of the two experiments shows that the suppression of the back reaction with the core/shell cocatalysts and the Rh–Cr mixed oxide corresponds to an increase in the net photocatalytic water splitting activity. The fact that the back reaction is not completely suppressed with Cr 2 O 3 /Pt compared to Cr 2 O 3 /Rh may be the cause of the higher net activity of the Cr 2 O 3 /Rh.

In the face of the global energy challenge and progressing global climate change, renewable energy systems and components, such as fuel cells and electrolyzers, which close the energetic oxygen and carbon cycles, have become a technology development priority. The electrochemical oxygen reduction reaction (ORR) and the direct electrochemical carbon dioxide reduction reaction (CO2 RR) are important electrocatalytic processes that proceed at gas diffusion electrodes of hydrogen fuel cells and CO2 electrolyzers, respectively. However, their low catalytic activity (voltage efficiency), limited long-term stability, and moderate product selectivity (related to their Faradaic efficiency) have remained challenges. To address these, suitable catalysts are required. This review addresses the current state of research on Pt-based and Cu-based nanoalloy electrocatalysts for ORR and CO2 RR, respectively, and critically compares and contrasts key performance parameters such as activity, selectivity, and durability. In particular, Pt nanoparticles alloyed with transition metals, post-transition metals and lanthanides, are discussed, as well as the material characterization and their performance for the ORR. Then, bimetallic Cu nanoalloy catalysts are reviewed and organized according to their main reaction product generated by the second metal. This review concludes with a perspective on nanoalloy catalysts for the ORR and the CO2 RR, and proposes future research directions.

Proton exchange membrane fuel cells (PEMFCs) convert chemical energy into electricity and uses hydrogen and oxygen as fuel. The sluggish kinetics of the oxygen reduction reaction (ORR) and harsh environment pose challenges on the development of cheap, active and stable catalysts. Shape-controlled bi- or tri-metallic Pt alloy nanoparticles, i.e. octahedral PtNi nanoparticles, have been reported to achieve extremely high mass activity in rotating disk electrode (RDE) experiments and for this reason are considered as a highly promising class of ORR catalysts.1 Despite the high mass activity, the low electrochemical surface area (ECSA), often below 50 m2/g, is considered not optimal for low-loaded PGM cathodes and possible to hinder their performance at higher current density in membrane electrode assemblies (MEA).2 A second challenge is their morphological stability under electrochemical testing. Ni loss and other degradation processes often result in shape changes into more “rounded” or concave structures with loss of the (111) facet and lower activity (Figure 1).3 Introduction of a third metal such as Co, Rh and Mo have been shown to have beneficial effects and is considered a promising step toward shape-stable PtNiX octahedral nanoparticles. In particular, an exceptionally high initial mass activity has been reported for Mo surface doped PtNi catalyst, which also showed good stability in an accelerated test in RDE.4 In this contribution we will show our recent efforts in improving the activity and especially the durability and morphological stability of PtNi based octahedral nanoparticle catalysts. A surface treatment involving the introduction of Mo is applied to octahedral PtNi/C. Activity and stability are evaluated from RDE measurements following a protocol developed inside the INSPIRE project.5 Degradation is monitored by in operando wide angle X-ray scattering (WAXS) performed at European Synchrotron Radiation Facility (ESRF). By this techniques is possible to follow the change in lattice parameter a during different stability tests. The results are correlated to Ni leaching from the crystalline alloy phase. P. Strasser, Science, 2015, 349, 379-380. A. Kongkanand and M. F. Mathias, J Phys Chem Lett, 2016, 7, 1127-1137. L. Gan, C. H. Cui, M. Heggen, F. Dionigi, S. Rudi and P. Strasser, Science, 2014, 346, 1502-1506. X. Q. Huang, Z. P. Zhao, L. Cao, Y. Chen, E. B. Zhu, Z. Y. Lin, M. F. Li, A. M. Yan, A. Zettl, Y. M. Wang, X. F. Duan, T. Mueller and Y. Huang, Science, 2015, 348, 1230-1234. S. Martens, L. Asen, G. Ercolano, F. Dionigi, C. Zalitis, A. Hawkins, A. Martinez Bonastre, L. Seidl, A. C. Knoll, J. Sharman, P. Strasser, D. Jones and O. Schneider, J Power Sources, 2018, submitted. The project leading to this application has received funding from the Fuel Cells and Hydrogen 2 Joint Undertaking under grant agreement No 700127. This Joint Undertaking receives support from the European Union’s Horizon 2020 Research and Innovation Programme and Hydrogen Europe and N.ERGHY. Figure 1. Life cycle of an octahedral PtNi nanoparticle showing a degradation pathway to loss of octahedral shape.3 Figure 1

NiFe based (oxy)hydroxides have been identified among the most active catalysts for the oxygen evolution reaction (OER) in alkaline electrolytes.1 Their crystal structure is known as layered double hydroxide (LDH) and is composed of layers of edge sharing metal oxygen octahedra that are intercalated with water molecules and charge balancing anions (Figure 1, insert).2 The incorporation of Fe3+ ions in Ni(OH)2 layers is considered to be responsible for the formation of this structure and its activity enhancement.3 Similarly, incorporation of Fe3+ into Co(OH)2 leads to CoFe LDH. This catalyst has also superior OER activity respect to Fe-free Co(OH)2.4 Despite their structure is well characterized, there are evidences that the OER active structure of LDH catalysts during OER is different than the one of the as prepared materials.3 In particular, oxidative deprotonation and the electrocatalytic reaction might lead to reversible structural differences that are only observable with in operando or in situ methods. More investigations are necessary, since the identification of the OER active structure of a catalyst is important for designing more active and stable catalysts. Theoretical predictions based on density functional theory (DFT) can guide this process, however so far different atomic models have been used in DFT calculations due the lack of clear structural information concerning the crystalline phase of Fe containing LDH under OER active conditions. In this presentation we will discuss the results of an in operando X-ray absorption spectroscopy and diffraction study on crystalline NiFe and CoFe LDH nanostructured catalysts. We identified the structure of the catalytically silent and active catalyst state and observed that for both catalysts the interlayer distance contracts under operating conditions to a similar value (figure 1). Our results show structural modifications under operating conditions which correlate with the electrochemical activity in alkaline electrolyte. Thus they provide new insights into the structure of the active catalysts state which will help to understand the processes under catalytic conditions. Figure 1. Shift of the (003) diffraction peak associated to the interlayer distance for NiFe LDH catalysts between the resting state (black) and OER active state (red) (unpublished). 3D structural model of the as prepared NiFe LDH with intercalated water and carbonate ions shown in the insert. Figure 1

Seawater is an abundant water resource on our planet and its direct electrolysis has the advantage that it would not compete with activities demanding fresh water. Oxygen selectivity is challenging when performing seawater electrolysis owing to competing chloride oxidation reactions. In this work we propose a design criterion based on thermodynamic and kinetic considerations that identifies alkaline conditions as preferable to obtain high selectivity for the oxygen evolution reaction. The criterion states that catalysts sustaining the desired operating current with an overpotential < 480 mV in alkaline pH possess the best chance to achieve 100 % oxygen/hydrogen selectivity. NiFe layered double hydroxide is shown to satisfy this criterion at pH 13 in seawater-mimicking electrolyte. The catalyst was synthesized by a solvothermal method and the activity, surface redox chemistry, and stability were tested electrochemically in alkaline and near-neutral conditions (borate buffer at pH 9.2) and under both fresh seawater conditions. The Tafel slope at low current densities is not influenced by pH or presence of chloride. On the other hand, the addition of chloride ions has an influence in the temporal evolution of the nickel reduction peak and on both the activity and stability at high current densities at pH 9.2. Faradaic efficiency close to 100 % under the operating conditions predicted by our design criteria was proven using in situ electrochemical mass spectrometry.

Multimetallic shape-controlled nanoparticles offer great opportunities to tune the activity, selectivity, and stability of electrocatalytic surface reactions. However, in many cases, our synthetic control over particle size, composition, and shape is limited requiring trial and error. Deeper atomic-scale insight in the particle formation process would enable more rational syntheses. Here we exemplify this using a family of trimetallic PtNiCo nanooctahedra obtained via a low-temperature, surfactant-free solvothermal synthesis. We analyze the competition between Ni and Co precursors under coreduction “one-step” conditions when the Ni reduction rates prevailed. To tune the Co reduction rate and final content, we develop a “two-step” route and track the evolution of the composition and morphology of the particles at the atomic scale. To achieve this, scanning transmission electron microscopy and energy dispersive X-ray elemental mapping techniques are used. We provide evidence of a heterogeneous element distribution caused by element-specific anisotropic growth and create octahedral nanoparticles with tailored atomic composition like Pt1.5M, PtM, and PtM1.5 (M = Ni + Co). These trimetallic electrocatalysts have been tested toward the oxygen reduction reaction (ORR), showing a greatly enhanced mass activity related to commercial Pt/C and less activity loss than binary PtNi and PtCo after 4000 potential cycles. P.S. acknowledges financial support by the German Research Foundation (DFG) through grant STR 596/5-1 (“Nanoscale Pt Alloy electrocatalysts with well-defined shapes”). Partial funding by the German Ministry of Education and Research (BMBF) grant “LOPLAKAT” is gratefully acknowledged. Also, this work was financially supported by the MICINN (Spain) (project 2013-44083-P). R.M.A.A. thanks the funding received from MICINN (EEBB-I-14-08240) to carry out a predoctoral stay in a foreign R&D center. M.H. thanks the Deutsche Forschungsgemeinschaft (DFG) for financial support within the grant HE 7192/1-1.

A NiFe layered double hydroxide (LDH) co-catalyst coated Ta3N5 nanorod electrode is fabricated by vapor deposition of Al on a Ta foil followed by anodizing the Al layer in oxalic acid, (40 V, 0.5 h), pore widening of the as-prepared porous Al2O3 by immersion in 5% aq.

Morphological shape in chemistry and biology owes its existence to anisotropic growth and is closely coupled to distinct functionality. Although much is known about the principal growth mechanisms of monometallic shaped nanocrystals, the anisotropic growth of shaped alloy nanocrystals is still poorly understood. Using aberration-corrected scanning transmission electron microscopy, we reveal an element-specific anisotropic growth mechanism of platinum (Pt) bimetallic nano-octahedra where compositional anisotropy couples to geometric anisotropy. A Pt-rich phase evolves into precursor nanohexapods, followed by a slower step-induced deposition of an M-rich (M = Ni, Co, etc.) phase at the concave hexapod surface forming the octahedral facets. Our finding explains earlier reports on unusual compositional segregations and chemical degradation pathways of bimetallic polyhedral catalysts and may aid rational synthesis of shaped alloy catalysts with desired compositional patterns and properties.

A new Pyrex-based μ-reactor for photocatalytic and optical characterization experiments is presented. The reactor chamber and gas channels are microfabricated in a thin poly-silicon coated Pyrex chip that is sealed with a Pyrex lid by anodic bonding. The device is transparent to light in the UV-vis-near infrared range of wavelengths (photon energies between ∼0.4 and ∼4.1 eV). The absorbance of a photocatalytic film obtained with a light transmission measurement during a photocatalytic reaction is presented as a proof of concept of a photocatalytic reactivity measurement combined with in situ optical characterization. Diffuse reflectance measurements of highly scattering photocatalytic nanopowders in a sealed Pyrex μ-reactor are also possible using an integrating sphere as shown in this work. These experiments prove that a photocatalyst can be characterized with optical techniques after a photocatalytic reaction without removing the material from the reactor. The catalyst deposited in the cylindrical reactor chamber can be illuminated from both top and bottom sides and an example of application of top and bottom illumination is presented.

Electrochemistry will play a critical role in the development of future sustainable energy storage and conversion technologies, where new functional nanocatalyst materials are the key components for application in fuel cells and electrolyzers. In this context, the generation of hydrogen and oxygen from seawater electrolysis is an attractive process for fuel production since the majority of liquid water available to mankind at the earth's surface is salty water [1]. Unfortunately, chlorine evolution reaction (CER) is a competing reaction to oxygen evolution reaction (OER) when seawater is used as electrolyte. Although OER is thermodynamically favored over CER, the poor kinetics of the OER makes CER the dominant reaction in many catalytic systems. NiFe layered double hydroxide (LDH) has been reported to catalyze the OER reaction in alkaline media at low overpotentials comparable with the performance of noble metals catalysts [2]. In our study, its activity and stability for seawater oxidation have been investigated with a rotating disk electrode in chloride containing electrolytes with basic and slightly basic pH. The electrochemical experiments are combined with products analysis (Figure 1) in order to estimate the selectivity for oxygen evolution. In particular, CER and hypochlorite formation are investigated by an in-line quadrupole mass spectrometer (QMS) and iodometric titration respectively. NiFe LDH shows a high selectivity for OER in alkaline electrolytes (pH 13) containing 0.5 M NaCl, a chloride ions concentration typical of seawater. In 0.3 M borate buffer (pH 9.2) with 0.5 M NaCl the electrochemical experiments show degradation in performance at current densities approaching 10 mA/cm2, despite the high selectivity at lower current densities. This degradation of the anodic current is not observed in pH 13 (0.1 M KOH) in the presence of the same concentration of NaCl. References [1] H.K. Abdel-Aal, K.M. Zohdy, M. Abdel Kareem, Hydrogen Production Using Sea Water Electrolysis, The Open Fuel Cells Journal, 3 (2010) 1-7. [2] M. Gong, Y.G. Li, H.L. Wang, Y.Y. Liang, J.Z. Wu, J.G. Zhou, J. Wang, T. Regier, F. Wei, H.J. Dai, An Advanced Ni-Fe Layered Double Hydroxide Electrocatalyst for Water Oxidation, J Am Chem Soc, 135 (2013) 8452-8455.

NiFe-based (oxy)hydroxides are highly active catalysts for the oxygen evolution reaction in alkaline electrolyte solutions. These catalysts can be synthesized in different ways leading to nanomaterials and thin films with distinct morphologies, stoichiometries and long-range order. Notably, their structure evolves under oxygen evolution operating conditions with respect to the as-synthesized state. Therefore, many researchers have dedicated their efforts on the identification of the catalytic active sites employing in operando experimental methods and theoretical calculations. These investigations are pivotal to rationally design materials with outstanding performances that will constitute the anodes of practical commercial alkaline electrolyzers. The family of NiFe-based oxyhydroxide catalysts reported in recent years is addressed and the actual state of the research with special focus on the understanding of the oxygen-evolution-reaction active sites and phase is described. Finally, an overview on the proposed oxygen-evolution-reaction mechanisms occurring on NiFe-based oxyhydroxide electrocatalysts is provided.

We study the photoluminescence from the negatively charged (X−) trions and neutral (X) excitons, in a diluted 2DEG in a magnetic field (B) up to 55 T. At zero B we analyze the evolution of the X− and X emission intensity, tuning it through the optical depletion effect. At non-zero field we find that the emission intensity of the singlet state of the trion X S − and of X are in inverse proposition to B, revealing the effect and the mechanism of the magnetic confinement of X S − and of the excitonic shrinkage on the emission intensity.

We study the quantum Hall liquid and the metal–insulator transition in a high-mobility two-dimensional electron gas, by means of photoluminescence and magnetotransport measurements. In the integer and fractional regime at ½ > 1/3, by analyzing the emission energy dispersion we probe the magneto-Coulomb screening and the hidden symmetry of the electron liquid. In the fractional regime above ½ = 1/3, the system undergoes metal-toinsulator transition, and in the insulating phase the dispersion becomes linear with evidence of an increased renormalized mass.

In the photoluminescence spectra of a two-dimensional electron gas in the fractional quantum Hall regime we observe for the first time the states at filling factor $\ensuremath{\nu}=4/5$, 5/7, 4/11, and 3/8 as clear minima in the emission peak intensity or area. The first three states are described as interacting composite fermions in fractional quantum Hall regime. The minimum in the intensity at $\ensuremath{\nu}=3/8$, which is not explained within this picture, can be an evidence of a suppression of the screening of the coulomb interaction among the effective quasiparticles involved in this intriguing state.

In the field of photocatalysis the batch-nature of the typical benchmarking experiment makes it very laborious to obtain good kinetic data as a function of parameters such as illumination wavelength, irradiance, catalyst temperature, reactant composition, etc. Microreactors with on-line mass spectrometry, on the other hand, allow fast and automated acquisition of quantitative kinetic data. [1,2] As an example, we show how microreactor experiments on water splitting using Pt- or Rh-loaded GaN:ZnO photocatalysts quickly rank different catalysts according to their activity for gas-phase water splitting - but also how the activity scales with relative humidity and the crucial role of CrOx "capping" of the Pt- or Rh-co catalyst in order to prevent the loss of H2/O2 product via backward reaction on the precious metal. [3,4] The data suggests that protons transfer via the catalyst surface between the oxygen-evolving sites and the hydrogen evolving co-catalyst sites. Recently, the microreactor experimental platform is being developed to support in-situ UV-VIS-IR spectroscopy [5] and even the introduction of liquid aqueous electrolyte and electrodes - all while retaining high sensitivity time resolved mass spectrometric product detection. [6] [1] Vesborg et al. Chemical Engineering Journal, 160, p. 738-741 (2010) [2] Vesborg et al. J. Phys. Chem. C, 114, p. 11162-11168 (2010) [3] Dionigi et al. Energy & Env. Sci., 4, p. 2937-2942 (2011) [4] Dionigi et al. J. Catal., 292, p. 26-31 (2012) [5] Dionigi et al. Rev. Sci. Instr., 84, p. 103910 (2013) [6] Bøndergaard et al. "Fast and sensitive method for detecting volatile species in liquids", submitted

Extended abstract of a paper presented at Microscopy and Microanalysis 2012 in Phoenix, Arizona, USA, July 29 – August 2, 2012.

Rh2−yCryO3/GaN:ZnO has been tested for gas phase overall photocatalytic water splitting by dosing water vapor. The sample has been deposited in a μ-reactor and evolves hydrogen and oxygen under illumination of solar light. This experiment proves the possibility to study solar active materials and the mechanism of the water splitting reaction with gas phase experiments. The high impact of the relative humidity on the activity has been shown by changing the water partial pressure and the reactor temperature.