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

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

Influence of Strain on the Band Gap of Cu2O

Cu2O has been considered as a candidate material for transparent conducting oxides and photocatalytic water splitting. Both applications require suitably tuned band gaps. Here we explore the influence of compressive and tensile strain on the band gap by means of density functional theory (DFT) modeling. Our results indicate that the band gap decreases under tensile strain while it increases to a maximum under moderate compressive strain and decreases again under extreme compressive strain. This peculiar behavior is rationalized through a detailed analysis of the electronic structure by means of density of states (DOS), density overlap region indicators (DORI), and crystal overlap Hamilton populations (COHP). Contrary to previous studies we do not find any indications that the band gap is determined by d10-d10 interactions. Instead, our analysis clearly shows that both the conduction and the valence band edges are determined by Cu-O antibonding states. The band gap decrease under extreme compressive strain is associated with the appearance of Cu 4sp states in the conduction band region

Electrochemically assisted deposition on TiO2 scaffold for Tissue Engineering: An apatite bio-inspired crystallization pathway

The interactions among inorganic materials and biological environments occurring at the nanoscale level are crucial to understand the mechanisms behind the bioactivity of biomaterials and for their better design. In the present study, we prepare TiO2 and TiO2/hydroxyapatite (HA) scaffolds for Tissue Engineering (TE) via a sol-gel/polymeric sponge method. The obtained architectures have a biomimetic morphology (trabecular bone) and an average pore dimension of 100 m. Moreover, we introduce a versatile electrochemically assisted deposition method for non-conductive substrates that allows the formation of a stable coating on the pore walls of the scaffolds with the morphology and phase composition determined by SEM and DRIFTS. Both the materials properties are strictly correlated to the scaffold composition: pure TiO2 scaffolds are coated with amorphous calcium carbonate while onto TiO2/HA scaffolds we observe the growth of octacalcium phosphate nanocrystals. The bioactivity of the TiO2-based scaffolds is then predicted examining the apatite formation on their surface in simulated body fluid. The presence of apatite crystals is observed only for pure TiO 2 scaffolds due to the transformation of amorphous calcium carbonate into crystalline biomimetic HA: a novel bio-inspired crystallization pathway toward the bioactivity of scaffold for TE

Copper oxide-based photocatalysts and photocathodes: Fundamentals and recent advances

Funding Information: This research was funded by the Jane and Aatos Erkko Foundation, grant: ?Renewable energy storage to high value chemicals?; the Italian Ministry of University, grant number Prot. 2017YH9MRK; the Universit? degli Studi di Milano, grant number PSR 2020. Publisher Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland.This work aims at reviewing the most impactful results obtained on the development of Cu-based photocathodes. The need of a sustainable exploitation of renewable energy sources and the parallel request of reducing pollutant emissions in airborne streams and in waters call for new technologies based on the use of efficient, abundant, low-toxicity and low-cost materials. Photoelectrochemical devices that adopts abundant element-based photoelectrodes might respond to these requests being an enabling technology for the direct use of sunlight to the production of energy fuels form water electrolysis (H2) and CO2 reduction (to alcohols, light hydrocarbons), as well as for the degradation of pollutants. This review analyses the physical chemical properties of Cu2O (and CuO) and the possible strategies to tune them (doping, lattice strain). Combining Cu with other elements in multinary oxides or in composite photoelectrodes is also discussed in detail. Finally, a short overview on the possible applications of these materials is presented.Peer reviewe

Gas-phase volatile organic chloride electroreduction: A versatile experimental setup for electrolytic dechlorination and voltammetric analysis

Silver is a well-known and largely studied electrocatalytic material for (hydro)dehalogenation reactions of organic halides, a class which includes several priority pollutants. Silver nanoparticles are particularly attractive since exhibit similar or even better performances than bulk silver allowing a significant reduction of precious metal loading. In this study the hydrodehalogenation of CHCl3 is selected as model reaction in order to evaluate the electrocatalytic activity of 2-15 nm silver nanoparticles. For the electrochemical characterization a versatile experimental setup was purportedly in-house designed and assembled that allows both cyclic voltammetry and electrolysis investigations. All the tests are performed in aqueous solution using a gas diffusion electrode as working cathode. The progressive conversion of gaseous trichloromethane to less chlorinated compounds is quantitatively evaluated in terms of concentration Cl- ions produced in the aqueous electrolyte solution. (C) 2014 Elsevier B.V. All rights reserved

Observation of charge transfer cascades in α-Fe2O3/IrOx photoanodes by operando X-ray absorption spectroscopy

Electrochemical devices for energy conversion and storage are central for a sustainable economy. The performance of electrodes is driven by charge transfer across different layer materials and an understanding of the mechanistics is pivotal to gain improved efficiency. Here, we directly observe the transfer of photogenerated charge carriers in a photoanode made of hematite (a-Fe2O3) and a hydrous iridium oxide (IrOx) overlayer, which plays a key role in photoelectrochemical water oxidation. Through the use of operando X-ray absorption spectroscopy (XAS), we probe the change in occupancy of the Ir 5d levels during optical band gap excitation of a-Fe2O3. At potentials where no photocurrent is observed, electrons flow from the a-Fe2O3 photoanode to the IrOx overlayer. In contrast, when the composite electrode produces a sustained photocurrent (i.e., 1.4 V vs. RHE), a significant transfer of holes from the illuminated a-Fe2O3 to the IrOx layer is clearly demonstrated. The analysis of the operando XAS spectra further suggests that oxygen evolution actually occurs both at the a-Fe2O3/electrolyte and a-Fe2O3/IrOx interfaces. These findings represent an important outcome for a better understanding of composite photoelectrodes and their use in photoelectrochemical systems, such as hydrogen generation or CO2 reduction from sunlight