Microfluidic Platform for Seawater Desalination by Coulometric Removal of Chloride Ions through Printed Ag Electrodes

This paper reports on the electrochemical characterization and comparison of printable silver inks for the fabrication of planar electrodes to integrate in a microfluidic platform for in situ desalination of seawater prior to detection of nutrients in marine environment. Screen printing and inkjet printing were investigated to overcome limitations of more conventional deposition techniques. The screen printed ink DuPont 5064H was chosen for the fabrication of the desalination platform as it displayed the best properties in terms of oxidation ability (15.5 mC/mm(2)) and absence of sample contamination. The platform was tested at different oxidation conditions; the optimum potential for desalination was found to be +0.9 V. The device desalination performance was then evaluated through conductivity measurements of the treated sample and a proof-of-concept was achieved: for a potential of +0.9 V, conductivity (hence concentration) could be lowered from an initial value of 42.13 mS (0.6 M) to 20.36 mS (0.206 M), 17.37 mS (0.157 M) and 6.38 mS (0.0516 M) for oxidation times of 120 s, 240 s and 360 s, respectively. The operating conditions allowed the regeneration of the silver electrode and the possibility to deploy it for six to ten cycles before depletion of the silver working electrode. (C) 2017 The Electrochemical Society. All rights reserved.

A DEMS study of methanol and formic acid oxidation on boron-doped diamond electrode

Christos Comninellis, György Foti, Agnieszka Kapalka

The electrochemical oxidation of methanol and formic acid in 1 M HClO4 is studied on boron-doped diamond electrode using differential electrochemical mass spectrometry (DEMS). DEMS is used to identify products and intermediates of the reaction monitored online during the potential sweep. The measurements show that the oxidation of both methanol and formic acid proceeds close to the potential of the oxygen evolution reaction, resulting in the appearance of an oxidation wave. During the oxidation, the oxygen evolution reaction shifts toward a higher potential, i.e., becomes a secondary reaction. Depending on the potential, either the formation of intermediates (partial oxidation) or the complete mineralization of organics, simultaneous with oxygen evolution, is observed. Diffusion limitation of the oxidation is clearly observed in parallel to the increase in O-2 formation. Methylformate is formed from methanol at potentials at which the local concentration of OH radicals is not sufficient for a complete mineralization. Formic acid oxidation proceeds in two waves; before the onset of oxygen evolution, an intermediate (probably oxalic acid) is produced in parallel to CO2 formation. Only in parallel to oxygen evolution is mineralization to CO2 is complete. (C) 2009 The Electrochemical Society. [DOI: 10.1149/1.3207009] All rights reserved.

2009

Mesoscopic thin films of hematite as photoanode for water photoelectrolysis

Alexis Joseph Duret

Due to the limiting amount of fossil fuel available and to the continuous growth of the world energy consumption, it becomes important to find alternative energy sources. Hydrogen produced by the photoelectrolysis of water is a perfect candidate as a clean energy source since it can be produced and used efficiently without generating any pollution. The water oxidation to oxygen is a slow reaction that requires an important overpotential in order to reach a significant current. This problem is crucial in water photoelectrolysis: a high overpotential means more solar cells in series in order to obtain high enough conversion efficiency. In order to lower the number of solar cells in series, a photoactive anode can be used. One potentially good material for a photoanode is hematite: it is cheap, stable and it has a bandgap small enough to absorb a part of the visible light. The objective of the present work is to develop and characterize highly photoactive semi transparent thin films of hematite that could be used efficiently to photo oxidize water. Three different deposition methods have been tested: Spray Pyrolysis (SP), Ultrasonic Spray Pyrolysis (USP) and Electrostatic Spray Deposition (ESD). The films obtained were characterized using spectroscopic (UV-Vis absorption, SEM and Raman spectroscopy) and electrochemical techniques (current potential curves in dark and under light, IPCE spectra, photopotential measurements and electrochemical impedance spectroscopy). The films made by ESD were not photoactive. The films made by SP were weakly photoactive and the films made by USP gave the highest photocurrent. After an optimization step, films with a photocurrent of 0.5 and 1.07mA/cm-2 at 1200mV vs RHE and under 1.5AM were produced by SP and USP respectively. Ti[IV] doping improved the photoresponse of the films deposited by SP. An inverse effect was observed for the films deposited by USP: the introduction of a dopant lowered the photocurrent and increased the dark current. The properties of layers made by USP were compared to those deposited by SP. Although both types of films show similar XRD, UV-visible and Raman spectra they differ greatly in their morphology and in their photoactivity. The mesoscopic α-Fe2O3 layers produced by USP consist mainly of 100 nm- sized platelets with a thickness of 5-10 nanometers. These nanosheets are oriented perpendicularly to the FTO support, their flat surface exposing (001) facets. The films made by SP consist of spherical hematite particles of 50-100nm in diameter densely packed. Open circuit photovoltage measurements indicated a lower surface state density for the films made by USP as compared to the films made by SP. This factor explained the much higher photoactivity of the USP compared to the SP deposited α-Fe2O3 films. Addition of hydrogen peroxide to the alkaline electrolyte further improves the photocurrent-voltage characteristics of films generated by USP and SP indicating hole transfer from the valence band of the semiconductor oxide to the adsorbed water to be the rate limiting kinetic step in the oxygen generation reaction. Electrochemical impedance spectroscopy (EIS) experiments have been performed on the most photoactive USP films. The construction of a frequency independent Mott-Schottky plot showed that the hematite flat band potential lies at 0.26V vs RHE in 1M NaOH and that the donor concentration is 5.6 1017cm-3. EIS allowed the identification of a deep donor level 0.3V above the flat band potential. This level explains the difference between the onset potential and the flat band potential. The electrochemical and photoelectrochemical behaviors of the most photoactive hematite thin films deposited by USP were found to be really close to those of single crystal hematite indicating that the influence on the photoactivity of the grain boundaries can be neglected.

EPFL2005

Self and induced stabilized nanoparticles for electrocatalysis

Mériadec Limat

Since few decades the search of new catalysts formulations for fuel cell applications has motivated numerous projects especially due to the future lack of fossil fuel, environmental and economical reasons. Consequently, Direct Alcohol Fuel Cell (DAFC) or Direct Formic Acid Fuel Cell domains have received great attention. In spite of the promising results reported recently, these types of fuel cell still exhibit some problems. In fact, platinum, bulk or nanoparticles, undergoes surface poisoning involved by intermediates (as acetaldehyde, acetic acid, carbon monoxide) which are strongly bonded to the surface. These poisoning species block the active sites at the surface electrode and drastically decrease the catalytic activity and also the life-time of the cell. Probable solutions are suggested essentially in Pt-based alloys catalysts or nanocatalysts in order to avoid this phenomenon. Therefore, multicomponent catalysts deposited at carbon electrodes (Boron-Doped Diamond and Highly Oriented Pyrolytic Graphite) has been investigated during this thesis. These two substrates have been chosen due to their outstanding properties. In the first part of this study, self stabilized nanoparticles at BDD substrate have been studied. Ex-situ ternary Pt-Ru-Sn microemulsion-synthesized nanoparticles deposited at BDD substrate were investigated towards both methanol and ethanol electrooxidation. Such type of composite electrode seemed to possess a greater efficiency towards methanol oxidation than observed for ethanol, leading essentially for the latter to C2 oxidation products. The inability of this catalyst to activate the C-C bond scission can explain this observation. However, the major enhancement was observed for the methanol electrooxidation. Recent studies have shown that gold nanoparticles exhibited surprising catalytic activity, very different from the bulk, making them a privileged candidate to replace or be added to platinum. Gold nanoparticles prepared with a twofold technique (sputtering followed by a heat treatment in air) have been characterized with both outer and inner-sphere redox reactions. It has been observed that the resulting electrochemical behavior of such type of composite electrode was similar to a gold microelectrodes array, due to the high difference between the two material electrochemical rate constants. In a morphological point of view, the processes of nucleation and growth engaged in the stabilized and well-dispersed gold nanoparticles formation have especially attracted a great attention. At BDD surface, the gold nanoparticles were located at specific sites (grain boundaries, defects, impurities,…). This phenomenon has motivated further investigations at HOPG substrate, known to possess a quite smooth and perfect surface. In the second part of this thesis, induced stabilized nanoparticles at HOPG surface were studied. In a first time, HOPG was morphologically and electrochemically characterized. The surface state has appeared to be a key parameter, essentially with the presence of the two surface species, the edge plane and the basal plane. High modifications in the electrochemical behavior of HOPG electrode were observed after an anodic pretreatment in acidic media, due to surface concentration increase of the edge plane species, which are highly reactive. Based on these results a tailoring of the HOPG has been performed in order to create nanocontainers for a further gold deposition. HOPG surface has been bombarded by gold clusters and then oxidized (etching step) under well-defined experimental conditions, leading in a tailored pattern of nanopits with high reliability and reproducibility. Gold deposition was performed by evaporation. The gold nanoparticles were located in the created nanocontainers and were found to be stable under cyclic voltammetric measurements in acidic media. A new synthesis method for a bimetallic or alloy system formed by Au and Pt nanoparticles stabilized at tailored HOPG surface was developed. Pt electrodeposited at Au/HOPG electrode surface have been observed to preferentially deposit on Au. In fact, gold nanoparticles were used as nucleation sites for the platinum particles. The resulting Pt-Au/HOPG electrodes were then heated at 400 °C in vacuum conditions to improve interactions between both metals. The appearance of an alloy was observed and characterized by a negative shift in the peak potential of Pt oxides reduction. Thanks to these results, Pt-Au/HOPG composite electrodes were applied for electrocatalytic oxidation. At a surface composition of P55-Au45 alloy, Pt-Au/HOPG after the heat treatment has demonstrated a higher electrocatalytic activity than before the heat treatment towards formic acid electrooxidation. It seemed that electronic and geometrical effect was responsible of such enhancement. Finally, bismuth adsorption at both P55-Au45/HOPG and polycrystalline platinum was investigated towards formic acid electrooxidation. The effect of Bi adatoms has been essentially ascribed to geometrical restrictions for CO to adsorb at platinum active sites. Drastic enhancement in current density at a fixed potential and dramatic decrease of the onset potential of formic acid oxidation were observed. Thus, it seemed that avoiding the CO poisoning can be the most judicious development for new electrocatalysts in fuel cell applications.