Alexandros Katsaounis | University of Patras (original) (raw)

Papers by Alexandros Katsaounis

Electrochemistry Communications, 2010

The electrochemical oxidation of ammonia (NH 3 and/or NH 4 + ) in the presence of chloride was in... more The electrochemical oxidation of ammonia (NH 3 and/or NH 4 + ) in the presence of chloride was investigated on a Ti/PtO x -IrO 2 electrode. It was shown that ammonia is effectively removed from solution via electrogenerated active chlorine. Based on mass balances, nitrogen is postulated to be the main product of ammonia electrolysis. In the bulk, the concentration of chloramines was low. This could be explained by the fact that the oxidation of ammonia takes place close to the electrode surface where an excess of chlorine relative to ammonia is ensured during the process. This results in the oxidation of ammonia to N 2 and in a local pH decrease. As a result, chloramines were decomposed in the proximity of the electrode prior to diffusing into the bulk.

Applied Catalysis B: Environmental, 2005

Proton migration through hydrated Nafion membranes in polymer electrolyte membrane (PEM) fuel cel... more Proton migration through hydrated Nafion membranes in polymer electrolyte membrane (PEM) fuel cells occurs both in the aqueous phase of the membrane and on the sulfonate groups on the surface of the membrane pores. Here we show using D2 and H2 fuel and basic quantum mechanical equations that this surface proton migration is largely due to proton tunneling between adjacent

Journal of Catalysis, 2013

A simple kinetic model has been developed to describe the catalytic and electrocatalytic performa... more A simple kinetic model has been developed to describe the catalytic and electrocatalytic performance of Ni/GDC and Au-Ni/GDC anodes of SOFCs operating under internal methane-steam reforming reaction conditions, at low and high steam-to-carbon ratio values. The model accounts for the surface dissociation of CH 4 to form methyl species which then react with H 2 O to form CO and H 2 . Under fuel cell operation conditions, two cases have been distinguished according to the observed electrochemical behavior; the high and the low steam-to-carbon ratio feed conditions. The former is characterized by electrochemical consumption of H 2 and CO, produced by internal CH 4 steam reforming, while the latter by electrochemical partial oxidation of CH 4 , to form H 2 and CO, and oxidation of H 2 . Interestingly, the coverage of methyl-type species of the catalyst surface, as extracted from the model and the catalytic kinetic data, was found to coincide with the methyl species coverage at the three-phase boundaries, as extracted from the electrocatalytic experiments. The model is in good agreement with experiment under both open-circuit and fuel cell operation conditions.

Olive mill wastewater Biodegradability a b s t r a c t

Purpose The disinfection efficiency of water and secondary treated wastewater by means of photoel... more Purpose The disinfection efficiency of water and secondary treated wastewater by means of photoelectrocatalytic oxidation (PEC) using reference strains of Enterococcus faecalis and Escherichia coli as faecal indicators was evaluated. Operating parameters such as applied potential (2-10 V), initial bacterial concentration (10 3 -10 7 CFU/mL), treatment time (up to 90 min) and aqueous matrix (pure water and treated effluent) were assessed concerning their impact on disinfection.

Titania Water a b s t r a c t

Journal of Applied Electrochemistry, 2010

Electrochemical Promotion of Catalysis (EPOC or NEMCA effect) is one of the most exciting discove... more Electrochemical Promotion of Catalysis (EPOC or NEMCA effect) is one of the most exciting discoveries in Electrochemistry with great impact on many catalytic and electrocatalytic processes. According to the words of John O'M. Bockris, EPOC is a triumph, and the latest in a series of advances in electrochemistry which have come about in the last 30 years. It has been shown with more than 80 different catalytic systems that the catalytic activity and selectivity of conductive catalysts deposited on solid electrolytes can be altered in a very pronounced, reversible and, to some extent, predictable manner by applying electrical currents or potentials (typically up to ±2 V) between the catalyst and a second electronic conductor (counter electrode) also deposited on the solid electrolyte. The induced steady-state change in catalytic rate can be up to 135 9 10 3 % higher than the normal (open-circuit) catalytic rate and up to 3 9 10 5 higher than the steady-state rate of ion supply. EPOC studies in the last 7 years mainly focus on the following four areas: Catalytic reactions with environmental impact (such as reduction of NO x and oxidation of light hydrocarbons), mechanistic studies on the origin of EPOC (using mainly oxygen ion conductors), scale-up pf EPOC reactors for potential commercialization via development of novel compact monolithic reactors and application of EPOC in high or low temperature fuel cells via introduction of the concept of triode fuel cell. The most recent EPOC studies in these areas are discussed in the present review and some of the future trends and aims of EPOC research are presented.

Applied Catalysis B-environmental, 2010

The effect of electrochemical promotion of catalysis (NEMCA effect or EPOC) has been studied for ... more The effect of electrochemical promotion of catalysis (NEMCA effect or EPOC) has been studied for the methane oxidation reaction over Rh catalytic films interfaced with YSZ, an oxygen ion conductor, at temperatures from 350 to 550 °C, under reducing, stoichiometric and oxidizing conditions. CO2 is the main reaction product; however, CO is produced in small amounts at high temperatures. The effect of electrochemical promotion of catalysis on the reaction catalytic rate has been found to decrease by increasing partial pressure of oxygen and temperature. Under reducing conditions, at 430 °C, positive current application can cause a 3-fold increase of the catalytic rate, while the apparent Faradaic efficiency is 170. After positive current interruption the catalytic rate reversibly returns to the initial open-circuit state value. Negative current application results in a 57% decrease of the catalytic rate with an apparent Faradaic efficiency equal to 40. After negative current interruption the catalytic rate slowly increases but remains lower than the initial value. This permanent poisoning effect has been interpreted by formation of a surface oxide layer by the strongly adsorbed oxygen from the gas phase upon negative polarization. A poisoning index, β, has been defined to quantify the magnitude of the effect. Moreover, under stoichiometric conditions, a periodical oscillation of the rate is observed both under open-circuit and polarization conditions in a narrow temperature window between 480 and 520 °C, which is attributed to catalyst phase transition phenomena.▶ Methane oxidation can be electrochemically enhanced on Rh catalytic films interfaced with YSZ at temperatures from 350 to 550 °C. ▶ CO2 is the main product; however, CO is produced in small amounts at high temperatures. ▶ Electrochemical promotion favored at reducing conditions and low temperatures. ▶ Rate enhancements up to 3 and Faradaic efficiencies up to 170 can be observed. ▶ Permanent poisoning effect is taking place under oxidizing conditions and negative polarization. This can be interpreted by formation of a surface oxide layer.

Cheminform, 2010

ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was e... more ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Journal of Applied Electrochemistry, 2010

Electrochemical oxidation of an azo dye (Reactive Red 120) was studied in acidic media (1 M HClO ... more Electrochemical oxidation of an azo dye (Reactive Red 120) was studied in acidic media (1 M HClO 4 ) using DSA type (Ti/IrO 2 -RuO 2 ) and boron doped diamond (BDD) anodes. Ti/IrO 2 -RuO 2 exhibited low oxidation power with high selectivity to organic intermediates and low TOC removal (10% at 25°C and 40% at 80°C). On the other hand BDD was found to be suitable for total mineralization of the organic loading to CO 2 . In both cases, the decoloration of the solution was almost 100% achieved very quickly with BDD (2 Ah L -1 ) but only after long treatment with Ti/IrO 2 -RuO 2 (25 Ah L -1 ). The instantaneous current efficiency (ICE) was up to 0.13 in the case of Ti/IrO 2 -RuO 2 and up to 0.45 in the case of BDD.

Journal of Applied Electrochemistry, 2010

The electrochemical oxidation of olive mill wastewater (OMW) over a Ti/RuO 2 anode was studied by... more The electrochemical oxidation of olive mill wastewater (OMW) over a Ti/RuO 2 anode was studied by means of cyclic voltammetry and bulk electrolysis and compared with previous results over a Ti/IrO 2 anode. Experiments were conducted at 300-1,220 mg L -1 initial chemical oxygen demand (COD) concentrations, 0.05-1.35 V versus SHE and 1.39-1.48 V versus SHE potential windows, 15-50 mA cm -2 current densities, 0-20 mM NaCl, Na 2 SO 4 , or FeCl 3 concentrations, 80°C temperature, and acidic conditions. Partial and total oxidation reactions occur with the overall rate being near first-order kinetics with respect to COD. Oxidation at 28 Ah L -1 and 50 mA cm -2 leads to quite high color and phenols removal (86 and 84%, respectively), elimination of ecotoxicity, and a satisfactory COD and total organic carbon reduction (52 and 38%, respectively). Similar performance can be achieved at the same charge (28 Ah L -1 ) using lower current densities (15 mA cm -2 ) but in the presence of various salts. For example, COD removal is less than 7% at 28 Ah L -1 in a salt-free sample, while addition of 20 mM NaCl results in 54% COD reduction. Decolorization of OMW using Ti/RuO 2 anode seems to be independent of the presence of salts in contrast with Ti/IrO 2 where addition of NaCl has a beneficial effect on decolorization.

Electrochimica Acta, 2007

A first principles model is developed to describe and predict the protonic conductivity of fully ... more A first principles model is developed to describe and predict the protonic conductivity of fully hydrated Nafion membranes and its peculiar non-linear dependence on membrane thickness, potential and PH2PH2. The model focuses on the surface migration of protons between adjacent sulfonate groups and utilizes the Poisson–Boltzmann charge distribution around each proton combined with the basic Gamow equation of quantum mechanics for proton tunneling. It is shown that the proton tunneling distance equals the proton wavelength and that each proton surrounded by its Debye–Hückel cloud behaves as a leaking nanobattery.The model, which contains no adjustable parameters, is solved analytically and its predictions are in semiquantitative agreement with experiment, including the magnitude of the conductivity, its linear increase with membrane thickness, its exponential increase with potential and its strong dependence on PH2PH2.

Topics in Catalysis, 2006

The effect of catalyst film thickness on the magnitude of the effect of electrochemical promotion... more The effect of catalyst film thickness on the magnitude of the effect of electrochemical promotion of catalysis (EPOC or NEMCA effect) was investigated for the model catalytic reaction of C 2 H 4 oxidation on porous Pt paste catalyst-electrodes deposited on YSZ. It was found that the catalytic rate enhancement q is up to 400 for thinner (0.2 lm) Pt films (40,000% rate enhancement) and gradually decreases to 50 for thicker (1 lm) films. The results are in good qualitative agreement with model predictions describing the diffusion and reaction of the backspillover O 2) species which causes electrochemical promotion.

Journal of Applied Electrochemistry, 2008

The origin of the effect of non-faradaic electrochemical modification of catalytic activity (NEMC... more The origin of the effect of non-faradaic electrochemical modification of catalytic activity (NEMCA) or Electrochemical Promotion was investigated via temperature-programmed-desorption (TPD) of oxygen, from polycrystalline Pd films deposited on 8 mol%Y 2 O 3 -stabilized-ZrO 2 (YSZ), an O 2conductor, under high-vacuum conditions and temperatures between 50 and 250°C. Oxygen was adsorbed both via the gas phase and electrochemically, as O 2-, via electrical current application between the Pd catalyst film and a Au counter electrode. Gaseous oxygen adsorption gives two adsorbed atomic oxygen species desorbing at about 300°C (state b 1 ) and 340-500°C (state b 2 ). The creation of the low temperature peak is favored at high exposure times (exposure [1 kL) and low adsorption temperatures (T ads \ 200°C). The decrease of the open circuit potential (or catalyst work function) during the adsorption at high exposure times, indicates the formation of subsurface oxygen species which desorbs at higher temperatures (above 450°C). The desorption peak of this subsurface oxygen is not clear due to the wide peaks of the TPD spectra. The TPD spectra after electrochemical O 2pumping to the Pd catalyst film show two peaks (at 350 and 430°C) corresponding to spillover O ads and O dÀ ads according to the reaction:

Applied Catalysis B-environmental, 2005

Proton migration through hydrated Nafion membranes in polymer electrolyte membrane (PEM) fuel cel... more Proton migration through hydrated Nafion membranes in polymer electrolyte membrane (PEM) fuel cells occurs both in the aqueous phase of the membrane and on the sulfonate groups on the surface of the membrane pores. Here we show using D2 and H2 fuel and basic quantum mechanical equations that this surface proton migration is largely due to proton tunneling between adjacent sulfonate groups, leading to an exponential variation of Nafion conductivity with cell potential. This amphibious mode of proton migration, particle-like in the aqueous phase and wave-like in the narrow pores, is shown to be the major cause of cell overpotential, bistability and oscillations of state-of-the-art PEM fuel cells operating on H2, reformate or methanol fuel. We also show that this phenomenon can be exploited via introduction of a third auxiliary electrode to independently control the anode–cathode potential difference and dramatically enhance fuel cell power output even in absence of noble metals at the anode.

Journal of Catalysis, 2004

The oxidation of CO by gaseous 18O2 was investigated on electropromoted Pt films deposited on Y2O... more The oxidation of CO by gaseous 18O2 was investigated on electropromoted Pt films deposited on Y2O3-stabilized ZrO2 (YSZ) and on nanodispersed Pt/YSZ catalysts under high vacuum and under atmospheric pressure conditions. For both catalyst systems and in both cases it was found that the temperature dependence of the catalytic oxidation rate can be correlated directly with the corresponding TPD spectra of 18O2, 16O18O, and 16O2 and that lattice oxygen plays a key role in the oxidation reaction, acting both as a reactant and as a sacrificial promoter. For both systems the results confirm the sacrificial promoter model of electrochemical promotion and metal–support interactions with O2−-conducting supports. This mechanism contains as limiting cases the promoted Langmuir–Hinshelwood and the Mars–van Krevelen mechanisms, which predominate at low and high temperatures, respectively.

Topics in Catalysis, 2006

The effect of catalyst film thickness on the magnitude of the effect of electrochemical promotion... more The effect of catalyst film thickness on the magnitude of the effect of electrochemical promotion was investigated for the model catalytic reaction of C 2 H 4 oxidation on porous Pt paste catalyst-electrodes deposited on YSZ. It was found that the catalytic rate enhancement q is up to 400 for thinner (0.2 lm) Pt films (40,000% rate enhancement) and gradually decreases to 50 for thicker (1 lm) films. The Faradaic efficiency L was found to increase moderately with increasing film thickness and to be described semiquantitatively by the ratio 2Fr o /I 0 , where r o is the unpromoted rate and I 0 is the exchange current of the catalyst-electrolyte interface. The results are in good qualitative agreement with model predictions describing the diffusion and reaction of the backspillover O 2species, which causes electrochemical promotion.

Electrochimica Acta, 2005

The current–potential curves of polymer electrolyte membrane (PEM) fuel cells exhibit under certa... more The current–potential curves of polymer electrolyte membrane (PEM) fuel cells exhibit under certain conditions steady-state multiplicity, i.e. for a fixed current value, there are two corresponding cell potential values and thus the current versus potential curve exhibits a local maximum. This behaviour cannot be described by any of the known classical mathematical expressions for overpotential. We have studied this phenomenon in PEM fuel cells operating on H2 with Pt cathodes and Pt–Ru- or C-based anodes and have found that the steady-state multiplicity results from the significant dependence of the Nafion membrane conductivity on cell potential. A simple mathematical model accounting for this non-linear behaviour of Nafion conductivity describes semiquantitatively the results both of the present work and of previous literature studies, where the current–potential curves exhibit either steady-state multiplicity or a significant downward bending of the current–potential curve. The role of the non-linear Nafion membrane conductivity in the current and potential oscillations observed in presence of CO at the anode is also briefly discussed together with its implications about the proton transfer mechanism in the Nafion membrane.

Electrochimica Acta, 2006

The conductivity of fully hydrated Nafion 112, 1135, 115 and 117 membranes was measured via ac im... more The conductivity of fully hydrated Nafion 112, 1135, 115 and 117 membranes was measured via ac impedance spectroscopy and steady-state current–potential measurements both in symmetric H2, Pt|Nafion|Pt, H2 and D2, Pt|Nafion|Pt, D2 PEM cells and in H2, Pt|Nafion|Pt, air and D2, Pt|Nafion|Pt, air PEM fuel cells. In agreement with recent studies, it was found that the conductivity, σ, increases almost linearly with membrane thickness L and also depends exponentially on potential and almost linearly on PH21/2. These and other observations, including the strong isotope effect obtained upon switching between H2 and D2 at the anode, show that the conductivity of Nafion contains two components, one due to proton migration in the aqueous phase, the other due to proton tunneling between adjacent sulfonate groups in narrow pores. The observed near-linear increase of σ with L is consistent with the proton tunneling mechanism but can also be explained by the existence of skin layers with lower conductivity at the ionomer interfaces with the anode and cathode.

Electrochemistry Communications, 2010

The electrochemical oxidation of ammonia (NH 3 and/or NH 4 + ) in the presence of chloride was in... more The electrochemical oxidation of ammonia (NH 3 and/or NH 4 + ) in the presence of chloride was investigated on a Ti/PtO x -IrO 2 electrode. It was shown that ammonia is effectively removed from solution via electrogenerated active chlorine. Based on mass balances, nitrogen is postulated to be the main product of ammonia electrolysis. In the bulk, the concentration of chloramines was low. This could be explained by the fact that the oxidation of ammonia takes place close to the electrode surface where an excess of chlorine relative to ammonia is ensured during the process. This results in the oxidation of ammonia to N 2 and in a local pH decrease. As a result, chloramines were decomposed in the proximity of the electrode prior to diffusing into the bulk.

Applied Catalysis B: Environmental, 2005

Proton migration through hydrated Nafion membranes in polymer electrolyte membrane (PEM) fuel cel... more Proton migration through hydrated Nafion membranes in polymer electrolyte membrane (PEM) fuel cells occurs both in the aqueous phase of the membrane and on the sulfonate groups on the surface of the membrane pores. Here we show using D2 and H2 fuel and basic quantum mechanical equations that this surface proton migration is largely due to proton tunneling between adjacent

Journal of Catalysis, 2013

A simple kinetic model has been developed to describe the catalytic and electrocatalytic performa... more A simple kinetic model has been developed to describe the catalytic and electrocatalytic performance of Ni/GDC and Au-Ni/GDC anodes of SOFCs operating under internal methane-steam reforming reaction conditions, at low and high steam-to-carbon ratio values. The model accounts for the surface dissociation of CH 4 to form methyl species which then react with H 2 O to form CO and H 2 . Under fuel cell operation conditions, two cases have been distinguished according to the observed electrochemical behavior; the high and the low steam-to-carbon ratio feed conditions. The former is characterized by electrochemical consumption of H 2 and CO, produced by internal CH 4 steam reforming, while the latter by electrochemical partial oxidation of CH 4 , to form H 2 and CO, and oxidation of H 2 . Interestingly, the coverage of methyl-type species of the catalyst surface, as extracted from the model and the catalytic kinetic data, was found to coincide with the methyl species coverage at the three-phase boundaries, as extracted from the electrocatalytic experiments. The model is in good agreement with experiment under both open-circuit and fuel cell operation conditions.

Olive mill wastewater Biodegradability a b s t r a c t

Purpose The disinfection efficiency of water and secondary treated wastewater by means of photoel... more Purpose The disinfection efficiency of water and secondary treated wastewater by means of photoelectrocatalytic oxidation (PEC) using reference strains of Enterococcus faecalis and Escherichia coli as faecal indicators was evaluated. Operating parameters such as applied potential (2-10 V), initial bacterial concentration (10 3 -10 7 CFU/mL), treatment time (up to 90 min) and aqueous matrix (pure water and treated effluent) were assessed concerning their impact on disinfection.

Titania Water a b s t r a c t

Journal of Applied Electrochemistry, 2010

Electrochemical Promotion of Catalysis (EPOC or NEMCA effect) is one of the most exciting discove... more Electrochemical Promotion of Catalysis (EPOC or NEMCA effect) is one of the most exciting discoveries in Electrochemistry with great impact on many catalytic and electrocatalytic processes. According to the words of John O'M. Bockris, EPOC is a triumph, and the latest in a series of advances in electrochemistry which have come about in the last 30 years. It has been shown with more than 80 different catalytic systems that the catalytic activity and selectivity of conductive catalysts deposited on solid electrolytes can be altered in a very pronounced, reversible and, to some extent, predictable manner by applying electrical currents or potentials (typically up to ±2 V) between the catalyst and a second electronic conductor (counter electrode) also deposited on the solid electrolyte. The induced steady-state change in catalytic rate can be up to 135 9 10 3 % higher than the normal (open-circuit) catalytic rate and up to 3 9 10 5 higher than the steady-state rate of ion supply. EPOC studies in the last 7 years mainly focus on the following four areas: Catalytic reactions with environmental impact (such as reduction of NO x and oxidation of light hydrocarbons), mechanistic studies on the origin of EPOC (using mainly oxygen ion conductors), scale-up pf EPOC reactors for potential commercialization via development of novel compact monolithic reactors and application of EPOC in high or low temperature fuel cells via introduction of the concept of triode fuel cell. The most recent EPOC studies in these areas are discussed in the present review and some of the future trends and aims of EPOC research are presented.

Applied Catalysis B-environmental, 2010

The effect of electrochemical promotion of catalysis (NEMCA effect or EPOC) has been studied for ... more The effect of electrochemical promotion of catalysis (NEMCA effect or EPOC) has been studied for the methane oxidation reaction over Rh catalytic films interfaced with YSZ, an oxygen ion conductor, at temperatures from 350 to 550 °C, under reducing, stoichiometric and oxidizing conditions. CO2 is the main reaction product; however, CO is produced in small amounts at high temperatures. The effect of electrochemical promotion of catalysis on the reaction catalytic rate has been found to decrease by increasing partial pressure of oxygen and temperature. Under reducing conditions, at 430 °C, positive current application can cause a 3-fold increase of the catalytic rate, while the apparent Faradaic efficiency is 170. After positive current interruption the catalytic rate reversibly returns to the initial open-circuit state value. Negative current application results in a 57% decrease of the catalytic rate with an apparent Faradaic efficiency equal to 40. After negative current interruption the catalytic rate slowly increases but remains lower than the initial value. This permanent poisoning effect has been interpreted by formation of a surface oxide layer by the strongly adsorbed oxygen from the gas phase upon negative polarization. A poisoning index, β, has been defined to quantify the magnitude of the effect. Moreover, under stoichiometric conditions, a periodical oscillation of the rate is observed both under open-circuit and polarization conditions in a narrow temperature window between 480 and 520 °C, which is attributed to catalyst phase transition phenomena.▶ Methane oxidation can be electrochemically enhanced on Rh catalytic films interfaced with YSZ at temperatures from 350 to 550 °C. ▶ CO2 is the main product; however, CO is produced in small amounts at high temperatures. ▶ Electrochemical promotion favored at reducing conditions and low temperatures. ▶ Rate enhancements up to 3 and Faradaic efficiencies up to 170 can be observed. ▶ Permanent poisoning effect is taking place under oxidizing conditions and negative polarization. This can be interpreted by formation of a surface oxide layer.

Cheminform, 2010

ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was e... more ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Journal of Applied Electrochemistry, 2010

Electrochemical oxidation of an azo dye (Reactive Red 120) was studied in acidic media (1 M HClO ... more Electrochemical oxidation of an azo dye (Reactive Red 120) was studied in acidic media (1 M HClO 4 ) using DSA type (Ti/IrO 2 -RuO 2 ) and boron doped diamond (BDD) anodes. Ti/IrO 2 -RuO 2 exhibited low oxidation power with high selectivity to organic intermediates and low TOC removal (10% at 25°C and 40% at 80°C). On the other hand BDD was found to be suitable for total mineralization of the organic loading to CO 2 . In both cases, the decoloration of the solution was almost 100% achieved very quickly with BDD (2 Ah L -1 ) but only after long treatment with Ti/IrO 2 -RuO 2 (25 Ah L -1 ). The instantaneous current efficiency (ICE) was up to 0.13 in the case of Ti/IrO 2 -RuO 2 and up to 0.45 in the case of BDD.

Journal of Applied Electrochemistry, 2010

The electrochemical oxidation of olive mill wastewater (OMW) over a Ti/RuO 2 anode was studied by... more The electrochemical oxidation of olive mill wastewater (OMW) over a Ti/RuO 2 anode was studied by means of cyclic voltammetry and bulk electrolysis and compared with previous results over a Ti/IrO 2 anode. Experiments were conducted at 300-1,220 mg L -1 initial chemical oxygen demand (COD) concentrations, 0.05-1.35 V versus SHE and 1.39-1.48 V versus SHE potential windows, 15-50 mA cm -2 current densities, 0-20 mM NaCl, Na 2 SO 4 , or FeCl 3 concentrations, 80°C temperature, and acidic conditions. Partial and total oxidation reactions occur with the overall rate being near first-order kinetics with respect to COD. Oxidation at 28 Ah L -1 and 50 mA cm -2 leads to quite high color and phenols removal (86 and 84%, respectively), elimination of ecotoxicity, and a satisfactory COD and total organic carbon reduction (52 and 38%, respectively). Similar performance can be achieved at the same charge (28 Ah L -1 ) using lower current densities (15 mA cm -2 ) but in the presence of various salts. For example, COD removal is less than 7% at 28 Ah L -1 in a salt-free sample, while addition of 20 mM NaCl results in 54% COD reduction. Decolorization of OMW using Ti/RuO 2 anode seems to be independent of the presence of salts in contrast with Ti/IrO 2 where addition of NaCl has a beneficial effect on decolorization.

Electrochimica Acta, 2007

A first principles model is developed to describe and predict the protonic conductivity of fully ... more A first principles model is developed to describe and predict the protonic conductivity of fully hydrated Nafion membranes and its peculiar non-linear dependence on membrane thickness, potential and PH2PH2. The model focuses on the surface migration of protons between adjacent sulfonate groups and utilizes the Poisson–Boltzmann charge distribution around each proton combined with the basic Gamow equation of quantum mechanics for proton tunneling. It is shown that the proton tunneling distance equals the proton wavelength and that each proton surrounded by its Debye–Hückel cloud behaves as a leaking nanobattery.The model, which contains no adjustable parameters, is solved analytically and its predictions are in semiquantitative agreement with experiment, including the magnitude of the conductivity, its linear increase with membrane thickness, its exponential increase with potential and its strong dependence on PH2PH2.

Topics in Catalysis, 2006

The effect of catalyst film thickness on the magnitude of the effect of electrochemical promotion... more The effect of catalyst film thickness on the magnitude of the effect of electrochemical promotion of catalysis (EPOC or NEMCA effect) was investigated for the model catalytic reaction of C 2 H 4 oxidation on porous Pt paste catalyst-electrodes deposited on YSZ. It was found that the catalytic rate enhancement q is up to 400 for thinner (0.2 lm) Pt films (40,000% rate enhancement) and gradually decreases to 50 for thicker (1 lm) films. The results are in good qualitative agreement with model predictions describing the diffusion and reaction of the backspillover O 2) species which causes electrochemical promotion.

Journal of Applied Electrochemistry, 2008

The origin of the effect of non-faradaic electrochemical modification of catalytic activity (NEMC... more The origin of the effect of non-faradaic electrochemical modification of catalytic activity (NEMCA) or Electrochemical Promotion was investigated via temperature-programmed-desorption (TPD) of oxygen, from polycrystalline Pd films deposited on 8 mol%Y 2 O 3 -stabilized-ZrO 2 (YSZ), an O 2conductor, under high-vacuum conditions and temperatures between 50 and 250°C. Oxygen was adsorbed both via the gas phase and electrochemically, as O 2-, via electrical current application between the Pd catalyst film and a Au counter electrode. Gaseous oxygen adsorption gives two adsorbed atomic oxygen species desorbing at about 300°C (state b 1 ) and 340-500°C (state b 2 ). The creation of the low temperature peak is favored at high exposure times (exposure [1 kL) and low adsorption temperatures (T ads \ 200°C). The decrease of the open circuit potential (or catalyst work function) during the adsorption at high exposure times, indicates the formation of subsurface oxygen species which desorbs at higher temperatures (above 450°C). The desorption peak of this subsurface oxygen is not clear due to the wide peaks of the TPD spectra. The TPD spectra after electrochemical O 2pumping to the Pd catalyst film show two peaks (at 350 and 430°C) corresponding to spillover O ads and O dÀ ads according to the reaction:

Applied Catalysis B-environmental, 2005

Proton migration through hydrated Nafion membranes in polymer electrolyte membrane (PEM) fuel cel... more Proton migration through hydrated Nafion membranes in polymer electrolyte membrane (PEM) fuel cells occurs both in the aqueous phase of the membrane and on the sulfonate groups on the surface of the membrane pores. Here we show using D2 and H2 fuel and basic quantum mechanical equations that this surface proton migration is largely due to proton tunneling between adjacent sulfonate groups, leading to an exponential variation of Nafion conductivity with cell potential. This amphibious mode of proton migration, particle-like in the aqueous phase and wave-like in the narrow pores, is shown to be the major cause of cell overpotential, bistability and oscillations of state-of-the-art PEM fuel cells operating on H2, reformate or methanol fuel. We also show that this phenomenon can be exploited via introduction of a third auxiliary electrode to independently control the anode–cathode potential difference and dramatically enhance fuel cell power output even in absence of noble metals at the anode.

Journal of Catalysis, 2004

The oxidation of CO by gaseous 18O2 was investigated on electropromoted Pt films deposited on Y2O... more The oxidation of CO by gaseous 18O2 was investigated on electropromoted Pt films deposited on Y2O3-stabilized ZrO2 (YSZ) and on nanodispersed Pt/YSZ catalysts under high vacuum and under atmospheric pressure conditions. For both catalyst systems and in both cases it was found that the temperature dependence of the catalytic oxidation rate can be correlated directly with the corresponding TPD spectra of 18O2, 16O18O, and 16O2 and that lattice oxygen plays a key role in the oxidation reaction, acting both as a reactant and as a sacrificial promoter. For both systems the results confirm the sacrificial promoter model of electrochemical promotion and metal–support interactions with O2−-conducting supports. This mechanism contains as limiting cases the promoted Langmuir–Hinshelwood and the Mars–van Krevelen mechanisms, which predominate at low and high temperatures, respectively.

Topics in Catalysis, 2006

The effect of catalyst film thickness on the magnitude of the effect of electrochemical promotion... more The effect of catalyst film thickness on the magnitude of the effect of electrochemical promotion was investigated for the model catalytic reaction of C 2 H 4 oxidation on porous Pt paste catalyst-electrodes deposited on YSZ. It was found that the catalytic rate enhancement q is up to 400 for thinner (0.2 lm) Pt films (40,000% rate enhancement) and gradually decreases to 50 for thicker (1 lm) films. The Faradaic efficiency L was found to increase moderately with increasing film thickness and to be described semiquantitatively by the ratio 2Fr o /I 0 , where r o is the unpromoted rate and I 0 is the exchange current of the catalyst-electrolyte interface. The results are in good qualitative agreement with model predictions describing the diffusion and reaction of the backspillover O 2species, which causes electrochemical promotion.

Electrochimica Acta, 2005

The current–potential curves of polymer electrolyte membrane (PEM) fuel cells exhibit under certa... more The current–potential curves of polymer electrolyte membrane (PEM) fuel cells exhibit under certain conditions steady-state multiplicity, i.e. for a fixed current value, there are two corresponding cell potential values and thus the current versus potential curve exhibits a local maximum. This behaviour cannot be described by any of the known classical mathematical expressions for overpotential. We have studied this phenomenon in PEM fuel cells operating on H2 with Pt cathodes and Pt–Ru- or C-based anodes and have found that the steady-state multiplicity results from the significant dependence of the Nafion membrane conductivity on cell potential. A simple mathematical model accounting for this non-linear behaviour of Nafion conductivity describes semiquantitatively the results both of the present work and of previous literature studies, where the current–potential curves exhibit either steady-state multiplicity or a significant downward bending of the current–potential curve. The role of the non-linear Nafion membrane conductivity in the current and potential oscillations observed in presence of CO at the anode is also briefly discussed together with its implications about the proton transfer mechanism in the Nafion membrane.

Electrochimica Acta, 2006

The conductivity of fully hydrated Nafion 112, 1135, 115 and 117 membranes was measured via ac im... more The conductivity of fully hydrated Nafion 112, 1135, 115 and 117 membranes was measured via ac impedance spectroscopy and steady-state current–potential measurements both in symmetric H2, Pt|Nafion|Pt, H2 and D2, Pt|Nafion|Pt, D2 PEM cells and in H2, Pt|Nafion|Pt, air and D2, Pt|Nafion|Pt, air PEM fuel cells. In agreement with recent studies, it was found that the conductivity, σ, increases almost linearly with membrane thickness L and also depends exponentially on potential and almost linearly on PH21/2. These and other observations, including the strong isotope effect obtained upon switching between H2 and D2 at the anode, show that the conductivity of Nafion contains two components, one due to proton migration in the aqueous phase, the other due to proton tunneling between adjacent sulfonate groups in narrow pores. The observed near-linear increase of σ with L is consistent with the proton tunneling mechanism but can also be explained by the existence of skin layers with lower conductivity at the ionomer interfaces with the anode and cathode.