Methanol oxidation at platinized copper particles prepared by galvanic replacement

Bimetallic Pt-Cu particles have been prepared by galvanic replacement of Cu precursor nanoparticles, upon the treatment of the latter with a chloro-platinate acidic solution. The resulting particles, typically a few tens of nm large, were supported on high surface area carbon (Vulcan® XC–72R, Cabot) and tested as electrodes. Surface electrochemistry in deaerated acid solutions was similar to that of pure Pt, indicating the existence of a Pt shell (hence the particles are denoted as Pt(Cu)). Pt(Cu)/C supported catalysts exhibit superior carbon monoxide and methanol oxidation activity with respect to their Pt/C analogues when compared on a per electroactive surface area basis, due to the modification of Pt activity by Cu residing in the particle core. However, as a result of large particle size and agglomeration phenomena, Pt(Cu)/C are still inferior to Pt/C when compared on a mass specific activity basis

Nanosized Chevrel phases for dendrite-free zinc–ion based energy storage: unraveling the phase transformations

The nanoscale form of the Chevrel phase, Mo$_6$S$_8$, is demonstrated to be a highly efficient zinc-free anode in aqueous zinc ion hybrid supercapacitors (ZIHSCs). The unique morphological characteristics of the material when its dimensions approach the nanoscale result in fast zinc intercalation kinetics that surpass the ion transport rate reported for some of the most promising materials, such as TiS$_2$ and TiSe$_2$. In situ Raman spectroscopy, post-mortem X-ray diffraction, Hard X-ray photoelectron spectroscopy, and density functional theory (DFT) calculations were combined to understand the overall mechanism of the zinc ion (de)intercalation process. The previously unknown formation of the sulfur-deficient Zn$_{2,9}$Mo$_{15}$S$_{19}$ (Zn$_{1,6}$Mo$_6$S$_{7,6}$) phase is identified, leading to a re-evaluation of the mechanism of the (de)intercalation process. A full cell comprised of an activated carbon (YEC-8A) positive electrode delivers a cell capacity of 38 mA h g$^{-1}$ and an energy density of 43.8 W h kg$^{-1}$ at a specific current density of 0.2 A g$^{-1}$. The excellent cycling stability of the device is demonstrated for up to 8000 cycles at 3 A g$^{-1}$ with a coulombic efficiency close to 100%. Post-mortem microscopic studies reveal the absence of dendrite formation at the nanosized Mo$_6$S$_8$ anode, in stark contrast to the state-of-the-art zinc electrode

The renaissance of electrowetting

Control of wetting on conducting surfaces using external stimuli such as electricity, underlies the operation of devices in a broad range of technologies including micro-/nano-fluidics, energy conversion/storage and filtration systems. Electrowetting is the change in contact angle of a liquid relative to its equilibrium value upon application of a potential bias. The phenomenon, being identified almost a century ago, is fundamentally an electrochemical process. However, the majority of the most recent research in this area focuses on electrowetting from the "applications" perspective. Device-based electrowetting uses substrates with insulating overlayers to eliminate charge transfer at the solid|liquid interface and hence suppress the electrochemical character of the overall process. In this short review, we focus on electrowetting directly on conductors and discuss the purely electrochemical aspects of the phenomenon along with the open questions related to this rejuvenated topic

Electrochemical, microscopic and spectroscopic characterization of iridium electrocatalysts for hydrogen and oxygen evolution reactions

The aim of the present thesis was the development of iridium based cathodes and anodes with enhanced electrocatalytic activity towards hydrogen (HER) and oxygen (OER) evolution reactions respectively. In particular, iridium or iridium oxide shell – iridium – nickel core particulate films have been prepared by galvanic replacement technique. The resulting catalysts exhibit superior intrinsic catalytic activity for hydrogen and oxygen evolution reactions compared to that of plain iridium and iridium oxide. Additionally, iridium oxide electrodes used as oxygen evolving anodes were studied by means of electrochemical impedance spectroscopy (EIS) to establish criteria for the assessment of their electroactive surface area and intrinsic catalytic activity. It is proved that the total capacitance of the catalysts (as derived from EIS data) can be used to correct direct current vs. electrode potential OER data for electroactive surface area effects, while the product of OER charge transfer resistance with total electrode capacitance can be taken as an appropriate parameter to characterize the catalyst intrinsic catalytic activity. Finally, scanning electrochemical microscopy in the surface interrogation mode (SI-SECM) has been used (for the first time) to probe the strength of metal-adsorbed hydrogen bonds at several noble metal catalysts.Σκοπός της διατριβής ήταν η ανάπτυξη και μελέτη καθόδων και ανόδων βασισμένων στο ιρίδιο, με καταλυτικές ιδιότητες ως προς τις αντιδράσεις έκλυσης υδρογόνου και οξυγόνου αντίστοιχα. Ειδικότερα, παρασκευάστηκαν διμεταλλικοί καταλύτες ιριδίου – νικελίου και οξειδίου του ιριδίου – νικελίου με δομή κελύφους του ευγενούς μετάλλου – πυρήνα μίγματος των δύο μετάλλων, με τη μέθοδο της γαλβανικής αντικατάστασης. Οι καταλύτες που παρασκευάστηκαν εμφανίζουν αυξημένη εγγενή ηλεκτροκαταλυτική ενεργότητα ως προς τις αντιδράσεις έκλυσης υδρογόνου και οξυγόνου συγκριτικά με το καθαρό μέταλλο/οξείδιο. Επιπλέον, αναπτύχθηκε μία μέθοδος προσδιορισμού της εγγενούς καταλυτικής ενεργότητας των καταλυτών οξειδίου του ιριδίου, μέσω του υπολογισμού της συνολικής χωρητικότητας αυτών από μετρήσεις ηλεκτροχημικής φασματοσκοπίας εμπέδησης. Τα δεδομένα που προέκυψαν, εφαρμόστηκαν σε μετρήσεις συνεχούς (κανονικοποίηση καμπυλών πόλωσης) και εναλλασσόμενου (υπολογισμός γινομένου αντίστασης μεταφοράς φορτίου της δράσης έκλυσης οξυγόνου επί τη συνολική χωρητικότητα του καταλύτη) ρεύματος. Τέλος, χρησιμοποιήθηκε για πρώτη φορά, η τεχνική της ηλεκτροχημικής μικροσκοπίας σάρωσης υπό λειτουργία διερεύνησης επιφάνειας για την ανίχνευση των υποτασικά αποτιθέμενων υδρογόνων στην επιφάνεια υποστρωμάτων ευγενών μετάλλων. Τα αποτελέσματα που καταγράφηκαν ερμηνεύονται με βάση τη συσχέτιση της προσροφητικής ικανότητας ως προς το υδρογόνο των υποστρωμάτων/ισχύς δεσμού ευγενούς μετάλλου – υδρογόνου και του χρόνου εκρόφησης των προσροφημένων υδρογόνων

Formation and reduction of anodic film on polycrystalline Bi electrode in pure methanol solutions

The processes of film formation and reduction of bismuth in pure methanol are phenomenologically studied by means of cyclic voltammetry, ac voltammetry and electrochemical impedance spectroscopy methods. Film formation takes place under low electrode potentials within the potential range from -0.1 to about 0.2 V vs. Ag|AgCl resulting in the development of Bi(CH3O)ads layer. The scan rate effect on the anodic current profile is interpreted in terms of a gradual variation of uncompensated resistance, accompanying the processes of film formation and reduction. Phase sensitive ac voltammetry measurements suggest leaky insulating character of a thin anodic film in agreement with the results of electrochemical impedance experiments

Electrocatalysts Prepared by Galvanic Replacement

Galvanic replacement is the spontaneous replacement of surface layers of a metal, M, by a more noble metal, Mnoble, when the former is treated with a solution containing the latter in ionic form, according to the general replacement reaction: nM + mMnoblen+ → nMm+ + mMnoble. The reaction is driven by the difference in the equilibrium potential of the two metal/metal ion redox couples and, to avoid parasitic cathodic processes such as oxygen reduction and (in some cases) hydrogen evolution too, both oxygen levels and the pH must be optimized. The resulting bimetallic material can in principle have a Mnoble-rich shell and M-rich core (denoted as Mnoble(M)) leading to a possible decrease in noble metal loading and the modification of its properties by the underlying metal M. This paper reviews a number of bimetallic or ternary electrocatalytic materials prepared by galvanic replacement for fuel cell, electrolysis and electrosynthesis reactions. These include oxygen reduction, methanol, formic acid and ethanol oxidation, hydrogen evolution and oxidation, oxygen evolution, borohydride oxidation, and halide reduction. Methods for depositing the precursor metal M on the support material (electrodeposition, electroless deposition, photodeposition) as well as the various options for the support are also reviewed

The electrochemical double layer at the graphene/aqueous electrolyte interface: what we can learn from simulations, experiments, and theory

The physical-chemistry of the graphene/aqueous-electrolyte interface underpins the operational conditions of a wide range of devices. Despite its importance, this interface is poorly understood due to the challenges faced in its experimental characterization and the difficulty of developing models that encompass its full physics. In this review we first summarize the classical theory of the electrochemical double layer, with the aim of defining a universal nomenclature to link experiments and simulations within a single unified framework. We then present the most recent experimental, theoretical and computational data and discuss how they compare with standard theory. The review ends with some remarks about how to compare simulations and experimental data and how this technology might evolve in the future

Electrowetting on Glassy Carbon substrates

The wetting properties of carbon surfaces are important for a number of applications, including in electrochemistry. An under-studied area is the electrowetting properties of carbon materials, namely the sensitivity of wetting to an applied potential. In this work we explore the electrowetting behaviour of glassy carbon substrates and compare and contrast the observed response with our previous work using highly oriented pyrolytic graphite. As with the graphite substrate, “water-in-salt” electrolytes are found to suppress Faradaic processes, thereby enlarging the electrochemical potential window. A notable difference response to positive and negative polarity was seen for the graphite and glassy carbon substrates. Moreover, whereas graphite has previously been shown to give a reversible electrowetting response over many cycles, an irreversible wetting was observed for glassy carbon. Similarly, the timescales of the wetting process were much faster on the graphitic substrate. Reasons underlying these marked changes in behaviour on the different carbon surfaces are suggested. <br/