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H2O2 is a valuable, environmentally friendly oxidizing agent, with a wide range of uses, from the provision of clean water to the synthesis of valuable chemicals. The on-site electrolytic production of H2O2 would bring the chemical to applications beyond its present reach. The successful commercialization of electrochemical H2O2 production requires cathode catalysts with high activity, selectivity and stability. In this Perspective, we highlight our current understanding of the factors that control the cathode performance. We review the influence of catalyst material, electrolyte and the structure of the interface at the mesoscopic scale. We provide original theoretical data on the role of the geometry of the active site and its influence on activity and selectivity. We have also conducted a series of original experiments on (i) the effect of pH on H2O2 production on glassy carbon, pure metals, and metal-mercury alloys, and (ii) the influence of cell geometry and mass transport in liquid half-cells in comparison to membrane electrode assemblies.

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.

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

The oxygen evolution reaction in acidic electrolyzers requires the presence of stable catalysts and current collectors at the anode. IrO2 catalysts and Ti current collectors are among the best in this regard. We show evidence of iridium and titanium corrosion and subsequent membrane crossover in long-term experiments of proton-exchange membrane electrolyzers for H2O2 production. The accumulation of trace iridium at the cathode was linked to degraded performance and increased cathodic current from hydrogen evolution. Detection of trace metal content at the cathode electrodes was enabled by prompt-gamma ray activation analysis and neutron activation analysis. These findings are not just relevant for H2O2 electrolyzers but to any system using iridium-based anode catalysts, including CO2 electroreduction.

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.

Stabilizing efficient photoabsorbers for solar water splitting has recently shown significant progress with the development of various protection layers. Suitable protection layers for tandem devices should be conductive, transparent, and stable in strongly acidic or alkaline solutions. This paper shows that under certain conditions n-type semiconductors, such as TiO2, can be used as protection layers for Si-based photoanodes. It also provides evidence that even in a photoanode assembly TiO2 is conducting only electrons (not holes as in p-type protection layers), and therefore TiO2 can be described as a simple ohmic contact. This renders n-type semiconductors, such as TiO2, to be versatile and simple protection layers, which can be used for photoanodes and as previously shown for photocathodes. The ohmic behavior of n-type TiO2 in a Si/TiO2–photoanode assembly is demonstrated under dark and illuminated conditions by performing the oxygen evolution reaction (OER) and using the Fe(II)/Fe(III) redox couple. ...

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

Herein we present surface sensitive operando XAS L-edge measurements on IrO

The electrochemical splitting of water holds great potential as a method for producing clean fuels by storing electricity from intermittent energy sources. The efficiency of such a process would be greatly facilitated by incorporating more active catalysts based on abundant materials for the oxygen evolution reaction. Manganese oxides are promising as catalysts for this reaction. Recent reports show that their activity can be drastically enhanced when modified with gold. Herein, we investigate highly active mixed Au-MnOx thin films for the oxygen evolution reaction, which exhibit more than five times improvement over pure MnOx. These films are characterized with operando X-ray Absorption Spectroscopy, which reveal that Mn assumes a higher oxidation state under reaction conditions when Au is present. The magnitude of the enhancement is correlated to the size of the Au domains, where larger domains are the more beneficial.

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.

Silicon is a promising contender in the race for low-bandgap absorbers for use in a solar driven monolithic water splitting cell (PEC). However, given its role as the low-bandgap material the silicon must sit behind the corresponding high-bandgap material and as such, it will be exposed to (red) light from the dry back-side – not from the wet front side, where the electrochemistry takes place.[1,2] Depending on the configuration of the selective contacts (junctions) this may lead to compromises between high absorption and low recombination.[2,3] We discuss the tradeoffs and compare modeling results to measurements. Regardless of configuration, the wet surface of the silicon is prone to passivation or corrosion and must therefore be carefully protected in service in order to remain active. We demonstrate the use of TiO2 as an effective protection layer for both photoanodes and photocathodes in acid electrolyte [4] and NiCoOx for photoanodes in alkaline electrolyte. [3] References: [1]: B. Seger et alia, Energ. Environ. Sci., 7 (8), 2397-2413 (2014), DOI:10.1039/c4ee01335b [2]: D. Bae et alia, Energ. Environ. Sci., 8 (2), 650-660 (2015), DOI: 10.1039/c4ee03723e [3]: D. Bae et alia, submitted, (2016) [4]: B. Mei et alia, J. Phys. Chem. C., 119 (27), 15019-15027 (2015), DOI: 10.1021/acs.jpcc.5b04407

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.

To enable the development of low temperature fuel cells, significant improvements are required to the efficiency of the Pt electrocatalysts at the cathode, where oxygen reduction takes place. Herein, we study the effect of subsurface solute metals on the reactivity of Pt, using a Cu/Pt(111) near-surface alloy. Our investigations incorporate electrochemical measurements, ultrahigh vacuum experiments, and density functional theory. Changes to the OH binding energy, ΔE(OH), were monitored in situ and adjusted continuously through the subsurface Cu coverage. The incorporation of submonolayer quantities of Cu into Pt(111) resulted in an 8-fold improvement in oxygen reduction activity. The most optimal catalyst for oxygen reduction has an ΔE(OH) ≈ 0.1 eV weaker than that of pure Pt, validating earlier theoretical predictions.

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.

The 2-photon tandem device for photocatalytic water splitting has been theoretically shown to provide a higher efficiency than a single photon device(1). This increased efficiency can be achieved by having one material optimized to absorb high energy photons (large bandgap) and another material optimized to absorb low energy photons (small bandgap). To a large degree this approach has been hindered by corrosion issues. In this talk I will first discuss how our computational screening of 2,400 materials showed that very few materials can efficiently absorb light without corroding in water splitting conditions.(2) I will follow this up by discussing how protection layers bypass the corrosion issue by creating a buffer layer.(3) Finally I will show how we integrated a photocatalyst/protection layer/(co-catalyst) scheme to produce highly efficient H2 evolution photocathodes and O2 evolution photoanodes.(3, 4) 1. A. B. Laursen, S. Kegnaes, S. Dahl and I. Chorkendorff, Energy & Environmental Science, 5 (2012). 2. B. Seger, I. E. Castelli, P. C. K. Vesborg, K. W. Jacobsen, O. Hansen and I. Chorkendorff, Energy & Environmental Science, 7, 2397 (2014). 3. B. Seger, T. Pedersen, A. B. Laursen, P. C. K. Vesborg, O. Hansen and I. Chorkendorff, Journal of the American Chemical Society, 135, 1057 (2013). 4. B. Mei, A. A. Permyakova, R. Frydendal, D. Bae, T. Pedersen, P. Malacrida, O. Hansen, I. E. L. Stephens, P. C. K. Vesborg, B. Seger and I. Chorkendorff, The Journal of Physical Chemistry Letters, 5, 3456 (2014). Figure 1

Sputter deposition of 50 nm thick NiO films on p(+)-n-Si and subsequent treatment in an Fe-containing electrolyte yielded highly transparent photoanodes capable of water oxidation (OER) in alkaline media (1 M KOH) with high efficiency and stability. The Fe treatment of NiO thin films enabled Si-based photoanode assemblies to obtain a current density of 10 mA/cm(2) (requirement for10% efficient devices) at 1.15 V versus RHE (reversible hydrogen electrode) under red-light (38.6 mW/cm(2)) irradiation. Thus, the photoanodes were harvesting ∼80 mV of free energy (voltage), which places them among the best-performing Si-based photoanodes in alkaline media. The stability was proven by chronoamperometry at 1.3 V versus RHE for 300 h. Furthermore, measurements with electrochemical quartz crystal microbalances coupled with ICP-MS showed minor corrosion under dark operation. Extrapolation of the corrosion rate showed stability for more than 2000 days of continuous operation. Therefore, protection by Fe-treated NiO films is a promising strategy to achieve highly efficient and stable photoanodes.