Dynamic Potential-Ph Diagrams Application to Electrocatalysts for Water Oxidation

The construction and use of "dynamic potential-pH diagrams" (DPPDs), that are intended to extend the usefulness of thermodynamic Pourbaix diagrams to include kinetic considerations is described. As an example, DPPDs are presented for the comparison of electrocatalysts for water oxidation, i.e., the oxygen evolution reaction (OER), an important electrochemical reaction because of its key role in energy conversion devices and biological systems (water electrolyses, photoelectrochemical water splitting, plant photosynthesis). The criteria for obtaining kinetic data are discussed and a 3-D diagram, which shows the heterogeneous electron transfer kinetics of an electrochemical system as a function of pH and applied potential is presented. DPPDs are given for four catalysts: IrO(2), Co(3)O(4), Co(3)O(4) electrodeposited in a phosphate medium (Co-Pi) and Pt, allowing a direct comparison of the activity of different electrode materials over a broad range of experimental conditions (pH, potential, current density). In addition, the experimental setup and the factors affecting the accurate collection and presentation of data (e. g., reference electrode system, correction of ohmic drops, bubble formation) are discussed.Ministry of Education, University and Research PRIN 2008PF9TWZ, 2008N7CYL5Universita degli Studi di MilanoNational Science Foundation CHE-0808927Robert A. Welch Foundation F-0021Center for Electrochemistr

Special issue on the 2nd E3 Mediterranean symposium foreword

As Guest Editors, we are delighted to introduce this Special Issue made to celebrate the '2nd E3 Mediterranean Symposium: Electrochemistry for Environment and Energy', which was held in Palazzo Feltrinelli, Gargnano, Italy, from 14 to 16 September 2016 following the 'Giornate dell'Elettrochimica Italiana'

Metastable Ni(I)-TiO _2-x Photocatalysts: Self-Amplifying H₂ Evolution from Plain Water without Noble Metal Co-Catalyst and Sacrificial Agent

Decoration of semiconductor photocatalysts with cocatalysts is generally done by a step-by-step assembly process. Here, we describe the self-assembling and self-activating nature of a photocatalytic system that forms under illumination of reduced anatase TiO2 nanoparticles in an aqueous Ni2+ solution. UV illumination creates in situ a Ni+/TiO2/Ti3+ photocatalyst that self-activates and, over time, produces H-2 at a higher rate. In situ X-ray absorption spectroscopy and electron paramagnetic resonance spectroscopy show that key to self-assembly and self-activation is the light-induced formation of defects in the semiconductor, which enables the formation of monovalent nickel (Ni+) surface states. Metallic nickel states, i.e., Ni-0, do not form under the dark (resting state) or under illumination (active state). Once the catalyst is assembled, the Ni+ surface states act as electron relay for electron transfer to form H-2 from water, in the absence of sacrificial species or noble metal cocatalysts.Web of Science14548261322612

Rapid Characterization of Oxygen-Evolving Electrocatalyst Spot Arrays by the Substrate Generation/Tip Collection Mode of Scanning Electrochemical Microscopy with Decreased O-2 Diffusion Layer Overlap

A simple approach for the screening of oxygen evolution reaction (OER) electrocatalyst arrays by scanning electrochemical microscopy (SECM) in the substrate generation/tip collection (SG/TC) mode is described. The methodology is based on the application of a series (9-10 replicates) of double-potential steps to a catalytically active substrate electrode, which is switched between potentials where it displays OER activity and inactivity. With an SECM tip coaligned to a given electrocatalyst spot, the dual potential step is applied for a relatively short time in order to restrict the growth of the resulting O2 diffusion layer. The SECM is then able to measure the O2 produced while the potential sequence prevents the overlap of the diffusion layer from neighboring spots. With this approach, each spot of material in an array of Ir:Sn oxide compositions (disk shaped, about 150 μm radius) was examined independently at a constant distance. The method was tested for a series of oxygen evolution catalysts made of SnO2-IrO2 mixtures, with compositions varying between Ir:Sn 100:0 to Ir:Sn 0:100. Optimal conditions for avoiding overlapping of the diffusion profiles generated at each spot of the substrate were evaluated by digital simulation. The results obtained for the activity of SnO2-IrO2 mixtures using this new technique were validated by comparison to reported results using SECM and other technique

Investigating the platinum electrode surface during Kolbe electrolysis of acetic acid

Platinum is commonly applied as the anode material for Kolbe electrolysis of carboxylic acids thanks to its superior performance. Literature claims that the formation of a barrier layer on the Pt anode in carboxylic acid electrolyte suppresses the competing oxygen evolution and promotes anodic decarboxylation. In this work, we show by using a combination of complementary in situ and ex situ surface sensitive techniques, that the presence of acetate ions also prevents the formation of a passive oxide layer on the platinum surface at high anodic potentials even in aqueous electrolyte. Furthermore, Pt dissolves actively under these conditions, challenging the technical implementation of Kolbe electrolysis. Future studies exploring the activity-structure-stability relation of Pt are required to increase the economic viability of Kolbe electrolysis

Evidence of Facilitated Electron Transfer on Hydrogenated Self-Doped TiO2 Nanocrystals

International audienceTiO2 nanocrystals are widely used as semiconductor photocatalysts, despite their limited sunlight absorption. The hydrogenation of TiO2 has recently been proposed as a promising new method for enhancing its solar-driven photocatalytic activity. But, the ability of this hydrogenated TiO2 to transfer electrons across the interface has not been studied in detail. For this reason, the electrochemical properties of hydrogenated self-doped TiO2−x nanocrystals are studied and compared to commercial anatase TiO2 nanocrystals. Here we use cyclic voltammetry (CV) and scanning electrochemical microscopy (SECM) as tools to characterize the semiconductors and, in particular, to evaluate their reactivity towards redox species in solution. CV is used to evaluate the doping level; we propose the co-presence of permanent defects, induced by the hydrogenation reductive treatment, as well as temporary ones, induced electrochemically. By using ferrocenemethanol as the redox-active species, SECM, in a substrate/tip configuration, demonstrates that the electron transfer is facilitated on hydrogenated self-doped TiO2−x by a potential-shift of approximately 0.3–0.4 V

Dewetting of PtCu Nanoalloys on TiO2 Nanocavities Provides a Synergistic Photocatalytic Enhancement for Efficient H2 Evolution

We investigate the co-catalytic activity of PtCu alloy nanoparticles for photocatalytic H2 evolution from methanol-water solutions. To produce the photocatalysts, a few nm-thick Pt-Cu bilayers are deposited on anodic TiO2 nanocavity arrays and converted by solid state dewetting, i.e. a suitable thermal treatment, into bimetallic PtCu nanoparticles. XRD and XPS results prove the formation of PtCu nanoalloys that carry a shell of surface oxides. XANES data support Pt and Cu alloying and indicate the presence of lattice disorder in the PtCu nanoparticles. The PtCu co-catalyst on TiO2 shows a synergistic activity enhancement and a significantly higher activity towards photocatalytic H2 evolution than Pt- or Cu-TiO2. We propose the enhanced activity to be due to Pt-Cu electronic interactions, where Cu increases the electron density on Pt favoring a more efficient electron transfer for H2 evolution. In addition, Cu can further promote the photo-activity by providing additional surface catalytic sites for hydrogen recombination. Remarkably, when increasing the methanol concentration up to 50 vol% in the reaction phase, we observe for PtCu-TiO2 a steeper activity increase compared to Pt-TiO2. A further increase in methanol concentration (up to 80 vol%) causes for Pt-TiO2 a clear activity decay, while PtCu-TiO2 still maintains a high level of activity. This suggests an improved robustness of PtCu nanoalloys against poisoning from methanol oxidation products such as CO

Dewetting-Alloying of Nicu Bilayers on TiO2TiO_{2} Surfaces for Noble Metal-Free Photocatalytic H2H_{2} Evolution

We present an approach to fabricate an efficient noble metal-free photocatalytic platform for H2 evolution and provide evidences that the photocatalyst active state forms via a photo-induced redox process in organic-water mixtures.To fabricate the photocatalytic platform, NiCu bilayers (thickness in the range of a few nm) are deposited by Ar-plasma sputtering on anodic TiO2 nanocavity arrays.[1] A subsequent thermal treatment triggers solid-state dewetting[2,3] of the metal bilayer, i.e. owing to surface diffusion, the Ni and Cu films agglomerate and inter-mix forming NiCu bimetallic nanoparticles at the TiO2 surface.[4] This approach allows for a full control over key features of the NiCu nanoparticles, e.g. size, loading, composition and co-catalytic H2 generation ability. We found that dewetted-alloyed NiCu nanoparticles not only are significantly more reactive than their pure Ni or Cu counterparts, but also lead to H2 generation rates that approach those of noble metal (Pt) modified TiO2 nanocavities.Characterization results (EDS-TEM, XPS and XRD) of the as-formed photocatalyst suggest the co-catalyst nanoparticles to feature a NiCu bimetallic core and an oxide (or hydroxide) shell – the latter presumably forms by surface oxidation under ambient conditions.To identify the chemical state of the NiCu nanoparticles during photocatalysis, we carried out XAS operando experiments[5,6] (beam line P65 at DESY – Petra III, Hamburg, Germany) at the Cu K and Ni K edges, in fluorescence mode, under UV light illumination in degassed ethanol-water solutions.Our results demonstrate that under operando conditions the co-catalyst is subjected to changes of the Ni and Cu chemical state:[7–9] we observe that surface Ni and Cu oxide species are reduced (by TiO2 conduction band electrons) in the early stage of illumination – this converts the co-catalyst nanoparticles into the active metallic NiCu phase.[1] J. E. Yoo, K. Lee, M. Altomare, E. Selli, P. Schmuki, Angew. Chemie Int. Ed. 2013, 52, 7514–7517.[2] C. V. Thompson, Annu. Rev. Mater. Res. 2012, 42, 399–434.[3] M. Altomare, N. T. Nguyen, P. Schmuki, Chem. Sci. 2016, 7, 6865–6886.[4] D. Spanu, S. Recchia, S. Mohajernia, O. Tomanec, Š. Kment, R. Zboril, P. Schmuki, M. Altomare, ACS Catal. 2018, 8, 5298–5305.[5] M. Fracchia, P. Ghigna, A. Vertova, S. Rondinini, A. Minguzzi, Surfaces 2018, 1, 138–150.[6] A. Minguzzi, O. Lugaresi, C. Locatelli, S. Rondinini, F. D'Acapito, E. Achilli, P. Ghigna, Anal. Chem. 2013, 85, 7009–7013.[7] J. S. Schubert, J. Popovic, G. M. Haselmann, S. P. Nandan, J. Wang, A. Giesriegl, A. S. Cherevan, D. Eder, J. Mater. Chem. A 2019, 7, 18568–18579.[8] B. Mei, K. Han, G. Mul, ACS Catal. 2018, 8, 9154–9164.[9] M. J. Muñoz-Batista, D. Motta Meira, G. Colón, A. Kubacka, M. Fernández-García, Angew. Chemie Int. Ed. 2018, 57, 1199–1203