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Herein we present surface sensitive operando XAS L-edge measurements on IrOx/RuO2 thin films as well as mass-selected RuOx and Ru nanoparticles. We observed shifts of the white line XAS peak toward higher energies with applied electrochemical potential. Apart from the case of the metallic Ru nanoparticles, the observed potential dependencies were purely core-level shifts caused by a change in oxidation state, which indicates no structural changes. These findings can be explained by different binding energies of oxygenated species on the surface of IrOx and RuOx. Simulated XAS spectra show that the average Ir oxidation state change is strongly affected by the coverage of atomic O. The observed shifts in oxidation state suggest that the surface has a high coverage of O at potentials just below the potential where oxygen evolution is exergonic in free energy. This observation is consistent with the notion that the metal-oxygen bond is stronger than ideal.

Water splitting is hindered by the sluggish kinetics of the oxygen evolution reaction (OER). The choice of materials for this reaction in acid is limited to the platinum group metals; high loading required of these scarce and expensive elements severely limit the scalability of such technology. Ruthenium oxide is among the best catalysts for OER, however the reported activity and stability can vary tremendously depending on the preparation conditions and pre-treatment. Herein, we investigate the effect of oxidation treatment on mass-selected Ru nanoparticles in the size range between 2 and 10 nm. The effect of two distinct oxidation pre-treatments on the activity and stability have been investigated: (1) thermal oxidation; and (2) oxidation with an oxygen plasma under vacuum. We report that activity and stability can be tuned by using different oxidation pre-treatments. Thermally oxidized particles exhibited the lowest activity, although over an order of magnitude higher than the state of the art, and the highest stability. Plasma-treated particles showed intermediate performance between as-deposited and thermally oxidized NPs.

The high precious metal loading and high overpotential of the oxygen evolution reaction (OER) prevents the widespread utilization of polymer electrolyte membrane (PEM) water electrolyzers. Herein we explore the OER activity and stability in acidic electrolyte of a combined IrOx/RuO2 system consisting of RuO2 thin films with sub-monolayer (1, 2 and 4 Å) amounts of IrOx deposited on top. Operando extended X-ray absorption fine structure (EXAFS) on the Ir L-3 edge revealed a rutile type IrO2 structure with some Ir sites occupied by Ru, IrOx being at the surface of the RuO2 thin film. We monitor corrosion on IrOx/RuO2 thin films by combining electrochemical quartz crystal microbalance (EQCM) with inductively coupled mass spectrometry (ICP-MS). We elucidate the importance of sub-monolayer surface IrOx in minimizing Ru dissolution. Our work shows that we can tune the surface properties of active OER catalysts such as RuO2, aiming to achieve higher electrocatalytic stability in PEM electrolyzers.

Electrochemical production of hydrogen, facilitated in electrolyzers, holds great promise for energy storage and solar fuel production. A bottleneck in the process is the catalysis of the oxygen evolution reaction, involving the transfer of four electrons. The challenge is that the binding energies of all reaction intermediates cannot be optimized individually. However, experimental investigations have shown that drastic improvements can be realized for manganese and cobalt-based oxides if gold is added to the surface or used as substrate. We propose an explanation for these enhancements based on a hydrogen acceptor concept. This concept comprises a stabilization of an *OOH intermediate, which effectively lowers the potential needed for breaking bonds to the surface. On this basis, we investigate the interactions between the oxides and gold by using DFT calculations. The results suggest that the oxygen evolution reaction overpotential decreases by 100–300 mV for manganese oxides and 100 mV for cobalt oxides.

The high precious metal loading and high overpotential of the oxygen evolution reaction (OER) prevents the widespread utilization of polymer electrolyte membrane (PEM) water electrolyzers. Herein we explore the OER activity and stability in acidic electrolyte of a combined IrO

Because of the rising need for energy storage, potentially facilitated by electrolyzers, improvements to the catalysis of the oxygen evolution reaction (OER) become increasingly relevant. Standardized protocols have been developed for determining critical figures of merit, such as the electrochemical surface area, mass activity and specific activity. Even so, when new and more active catalysts are reported, the catalyst stability tends to play a minor role. In this work, we monitor corrosion on RuO2 and MnOx by combining the electrochemical quartz crystal microbalance (EQCM) with inductively coupled plasma mass spectrometry (ICP–MS). We show that a meaningful estimation of the stability cannot be achieved based on purely electrochemical tests. On the catalysts tested, the anodic dissolution current was four orders of magnitude lower than the total current. We propose that even if long-term testing cannot be replaced, a useful evaluation of the stability can be achieved with short-term tests by using EQCM or ICP–MS.

Oxygen evolution was investigated on model, mass-selected RuO2 nanoparticles in acid, prepared by magnetron sputtering. Our investigations include electrochemical measurements, electron microscopy, scanning tunneling microscopy and X-ray photoelectron spectroscopy. We show that the stability and activity of nanoparticulate RuO2 is highly sensitive to its surface pretreatment. At 0.25 V overpotential, the catalysts show a mass activity of up to 0.6 A mg(-1) and a turnover frequency of 0.65 s-1, one order of magnitude higher than the current state-of-the-art.

Future generations require more efficient and localized processes for energy conversion and chemical synthesis. The continuous on-site production of hydrogen peroxide would provide an attractive alternative to the present state-of-the-art, which is based on the complex anthraquinone process. The electrochemical reduction of oxygen to hydrogen peroxide is a particularly promising means of achieving this aim. However, it would require active, selective and stable materials to catalyse the reaction. Although progress has been made in this respect, further improvements through the development of new electrocatalysts are needed. Using density functional theory calculations, we identify Pt-Hg as a promising candidate. Electrochemical measurements on Pt-Hg nanoparticles show more than an order of magnitude improvement in mass activity, that is, A g(-1) precious metal, for H2O2 production, over the best performing catalysts in the literature.

Catalysts are required for the oxygen evolution reaction, which are abundant, active, and stable in acid. MnO2 is a promising candidate material for this purpose. However, it dissolves at high overpotentials. Using first-principles calculations, a strategy to mitigate this problem by decorating undercoordinated surface sites of MnO2 with a stable oxide is developed here. TiO2 stands out as the most promising of the different oxides in the simulations. This prediction is experimentally verified by testing sputter-deposited thin films of MnO2 and Ti–MnO2. A combination of electrochemical measurements, quartz crystal microbalance, inductively coupled plasma mass spectrometry measurements, and X-ray photoelectron spectroscopy is performed. Small amounts of TiO2 incorporated into MnO2 lead to a moderate improvement in stability, with only a small decrease in activity. This study opens up the possibility of engineering surface properties of catalysts so that active and abundant nonprecious metal oxides can be used in acid electrolytes.

Of the different water splitting technologies, polymer electrolyte membrane (PEM) electrolysers are the most amenable towards small-scale delocalized storage of renewable electricity. In order for these devices make a significant impact to the global energy landscape, they will need to be scaled to the TW level. State-of the art PEM electrolysers employ IrO x , which is both expensive and scarce, to catalyse oxygen evolution.(1) Around a decade’s worth of Ir production would be required to scale up PEM electrolysis to the TW scale: this is clearly untenable.(2) It turns out that RuOx has a higher catalytic activity than IrOx, but is more prone to dissolution.(3-5) All other surfaces are completely inactive or unstable. We recently demonstrated that mass-selected RuOx nanoparticles exhibited an order of magnitude improvements in both mass activity and turnover frequency over the state of the art. Should this activity be stabilised for instance, by utilising a more stable oxide such as TiOx or IrOx,(6, 7) PEM electrolysis could indeed be scalable to the TW level. Alternatively, the precious metal catalysts could be eliminated altogether and replaced by abundant, active and stable catalysts. However, this is not a trivial task, given the highly oxidising and corrosive environment under reaction conditions. Herein, we attempt to address this problem by stabilising MnOx with TiOx. Density functional theory calculations suggest that the undercoordinated surface sites on MnOx, which are inactive and also most prone to dissolution, could be stabilised by TiO x . We test this notion by performing oxygen evolution on Mn-TiOx thin films. We probe the composition using X-ray photoelectron spectroscopy measurements and dissolution with inductively coupled plasma mass spectroscopy. We confirm that TiO x does indeed engender MnOx with modest stability. Further development of this strategy opens up the possibility of developing active, stable and abundant non-precious metal oxides for oxygen evolution in acid. References 1. M. K. Debe, S. M. Hendricks, G. D. Vernstrom, M. Meyers, M. Brostrom, M. Stephens, Q. Chan, J. Willey, M. Hamden, C. K. Mittelsteadt, C. B. Capuano, K. E. Ayers and E. B. Anderson, J. Electrochem. Soc., 159, K165 (2012). 2. E. A. Paoli, F. Masini, R. Frydendal, D. Deiana, C. Schlaup, M. Malizia, T. W. Hansen, S. Horch, I. E. L. Stephens and I. Chorkendorff, Chemical Science, DOI: 10.1039/c4sc02685c (2015). 3. T. Reier, M. Oezaslan and P. Strasser, ACS Catalysis, 2, 1765 (2012). 4. S. Cherevko, A. R. Zeradjanin, A. A. Topalov, N. Kulyk, I. Katsounaros and K. J. J. Mayrhofer, ChemCatChem, 6, 2219 (2014). 5. R. Frydendal, E. A. Paoli, B. P. Knudsen, B. Wickman, P. Malacrida, I. E. L. Stephens and I. Chorkendorff, ChemElectroChem, DOI: 10.1002/celc.201402262 (2014). 6. S. Trasatti, Electrochimica Acta, 45, 2377 (2000). 7. N. Danilovic, R. Subbaraman, K. C. Chang, S. H. Chang, Y. Kang, J. Snyder, A. P. Paulikas, D. Strmcnik, Y. T. Kim, D. Myers, V. R. Stamenkovic and N. M. Markovic, Angewandte Chemie International Edition, (2014).

In recent years, hydrogen production from polymer electrolyte membrane (PEM) electrolysis has attracted increasingly interest in the development of a clean/ CO2 free technology for energy storage [1]. The majority of the efficiency losses are at the anode [2], where oxygen is evolved, according to the reaction: 2H2O --> O2 + 4H+ + 4e- The most active and widely used catalysts for the oxygen evolution reaction (OER) are RuO x based materials. However, RuO x corrodes under reaction conditions. Moreover, Ru is very expensive and scarce [3]. In the current investigation, we focus on the evaluation of (a) oxygen evolution activity and (b) corrosion of well defined, mass selected Ru nanoparticles as a function of size and shape. We adapt a methodology previously used in our laboratory to study to investigate the oxygen reduction reaction [4]. The size selected electrocatalysts are prepared using an ultra-high-vacuum (UHV) compatible technique where nanoparticles are aggregated in a magnetron sputter source. This technique has distinct advantages, as it results in a high degree of control over critical parameters such as particle size, coverage and density [5]. By investigating such well-defined catalysts, we can improve our understanding of the relationship between catalyst structure and reactivity. The catalysts are formed from a ruthenium target, deposited under vacuum directly onto a glassy carbon or Au(111) electrode and then oxidized in furnace at 400 °C under 1 bar oxygen. The structure and composition are tested ex-situ before and after annealing using X-ray Photoelectron Spectroscopy (XPS), high resolution Transmission Electron Microscopy (HR-TEM) and Scanning Electron Microscopy (SEM). Electrochemical activity and stability are tested ex-situ in a Rotating Ring Disk Electrode set-up [6]. Furthermore, Electrochemical Scanning Tunnelling Microscopy (EC-STM) is used to directly observe the catalyst dissolution under reaction conditions. The EC-STM images are acquired in Ar saturated 0,05 M H2SO4, while the potential is scanned at a scan rate of 2 mV/s. The metallic and pre-oxidized nanoparticles show distinct behaviour from each other, under OER conditions. The metallic nanoparticles disappear from view, as the potential is raised (Figure 1a), due to dissolution, while the pre-oxidised particles (Figure 1b) exhibit no evidence of corrosion in the same potential range. These microscopic data correlate closely with the activity data. The observed activity is significantly higher than the state-of-the-art reported in literature [7], [8]. If this activity could be replicated in an electrolyser, the precious metal loading could be lowered by one order of magnitude. [1] H. Dau, C. Limberg, T. Reier, M. Risch, S. Roggan, and P. Strasser, ChemCatChem, vol. 2, no. 7, pp. 724–761, Jun. 2010. [2] A. Marshall, B. Børresen, G. Hagen, M. Tsypkin, and R. Tunold, Energy, vol. 32, no. 4, pp. 431–436, Apr. 2007. [3] P. C. K. Vesborg and T. F. Jaramillo, RSC Adv., vol. 2, no. 21, p. 7933, 2012. [4] F. J. Perez-Alonso, D. N. McCarthy, A. Nierhoff, P. Hernandez-Fernandez, C. Strebel, I. E. L. Stephens, J. H. Nielsen, and I. Chorkendorff, Angew. Chem. Int. Ed. Engl., vol. 51, no. 19, pp. 4641–3, May 2012. [5] S. Murphy, R. M. Nielsen, C. Strebel, M. Johansson, and J. H. Nielsen, Carbon N. Y., vol. 49, no. 2, pp. 376–385, Feb. 2011. [6] M. Vukovic, J. Chem. Soc. Faraday Trans., vol. 86, no. 22, pp. 3743–3746, 1990. [7] R. Forgie, G. Bugosh, K. C. Neyerlin, Z. Liu, and P. Strasser, Electrochem. Solid-State Lett., vol. 13, no. 4, p. B36, 2010. [8] Y. Lee, J. Suntivich, K. J. May, E. E. Perry, and Y. Shao-horn, J. Phys. Chem C Lett., vol. 3, p. 394, 2012.