Effect Of Electrochemical Promotion Research Articles

Highly efficient La/Ni co-doped strontium titanate catalyst for co-production of propylene and hydrogen from propane in protonic ceramic electrochemical cells

A highly efficient La/Ni co-doped strontium titanate (LSNT) perovskite catalyst is developed and integrated in a protonic ceramic electrochemical cell for co-production of propylene and high-purity hydrogen from commercial propane feedstock. Propane conversion and hydrogen production rate can be effectively enhanced under an applied current due to the electrochemical promotion effect and/or shifted reaction equilibrium induced by rapid separation of hydrogen product. Water vapor in the feed gas could significantly improve the catalyst stability by suppressing the coke formation. The propane conversion could reach up to 53% at 600 °C under a current density of 90 mA cm−2. The LSNT catalyst also shows excellent tolerance for the sulfur contaminant in commercial propane gas. The excellent performance of the LSNT catalyst is attributed to the highly active and selective Ni species at the interface with the perovskite substrate, which are formed in situ via reduction-induced exsolution under reaction conditions.

BaZr0.1Co0.4Fe0.4Y0.1O3-SDC composite as quasi-symmetrical electrode for proton conducting solid oxide fuel cells

Symmetric solid oxide fuel cells (SSOFCs) with the identical anode and cathode electrocatalysts show promise to reduce material and system cost while increasing the cell lifespan. In this work, BaZr0.1Co0.4Fe0.4Y0.1O3 (BZCFY) oxide perovskite is proposed as a symmetric electrode for SSOFCs based on proton conducting electrolyte, with targets of reducing temperature and high-performance application. Active oxygen ionic conductor and catalyst, SDC, is composited to improve the cell performance and electrode durability. Those materials show good chemical compatibility while BZCFY is decomposed to alloy and mixed oxide composite, which significantly affects electrode activity. SDC-BZCFY composite gives an electrode polarization resistance of 1.35–13.7 Ω cm2 and 0.32–1.59 Ω cm2 for hydrogen oxidation reaction and oxygen reduction reaction on the proton conducting electrolyte, BZCY, at the temperature range of 700–550 °C, respectively. Moreover, it displays an excellent oxygen reduction kinetics with an impressive activation energy of 0.91 eV. The polarization resistances are significantly reduced in the fuel cell condition owning to the electrochemical promotion effect under open-circuit condition. Quasi-SSOFCs with BZCY electrolyte in a thickness of 480 μm and electrode thickness of 25 μm give a peak power density of 114.8 and 74.3 mW cm−2 at 650 and 600 °C, respectively. In addition, SSOFC also displays acceptable durability under constant voltage operational condition for 25 h. This work highlights alternative active electrode material for symmetric solid oxide fuel cells for low temperature operation.

Electrochemical promotion of carbon supported Pt, Rh and Pd catalysts for H2 oxidation in aqueous alkaline media

AbstractBACKGROUNDThe effect of electrochemical promotion of catalysis (EPOC) for the case of H2 oxidation reaction over commercial M (M=Pt, Rh, Pd)/Vulcan XC72 (ETEK) electrodes immersed in alkaline media was explored in this study. Various reactant mixtures (H2 and O2) fed simultaneously through a glass frit to the anodic electrode which was immersed in the electrolyte. Both positive and negative currents were applied between anode and cathode while all the reactants were followed by mass spectrometry.RESULTSIn the case of Pt electrode, it was found that application of small positive currents resulted in a reversible enhancement of both oxygen and hydrogen consumption rate, higher than the expected Faradaic one (I/2F). Pd and Rh electrodes exhibited lower catalytic activity at ambient temperature while positive current applications induced an acceleration of the catalytic reaction. Interestingly, modification of the catalytic activity of Pd and Rh electrodes was found to be irreversible even under negative current applications.CONCLUSIONCommercially available, carbon supported metal (Pt, Rh, Pd) catalysts, can be electrochemically promoted during hydrogen oxidation reaction in alkaline media. The rate enhancement could be explained by modification of the work function of the catalytic surface (thus the strength of the reactant's adsorption bond) during potential or current application as well as modification of the catalyst surface composition. © 2018 Society of Chemical Industry

Multifunctional plasmonic Ag-hematite nano-dendrite electro-catalysts for methanol assisted water splitting: Synergism between silver nanoparticles and hematite dendrites

Hydrogen is the most environment friendly fuel and has the largest energy density but still much away from being a viable technology due to the cost associated with its production on-site on-demand. However, hydrogen production via water splitting could become potential commercial technology by designing new catalyst materials with low cost, desired surface structures and properties that govern hydrogen evolution reaction (HER) activity and stability. Here, we report the methanol assisted electrochemical water splitting using silver nanoparticles decorated hematite (Ag-hematite) dendrite nanostructures. Ag-hematite nano-dendrites prepared via two different methods viz. chemical co-precipitation and hydrothermal treatment are analysed and compared for their potential applications towards methanol assisted water splitting. It is found that Ag-hematite nano-dendrites prepared by chemical precipitation method shows much better activity as compared to both the parent materials (i.e. Ag NPs and hematite nano-dendrites) as well as Ag-hematite nano-dendrites synthesized by hydrothermal treatment. A baseline study showing the influence of methanol concentration, catalyst, catalyst support, and operating mode has been established. The analysis of the system was carried out as a function of onset potentials and kinetic parameters, including the Tafel slopes and exchange current densities. The effect of electrochemical promotion was investigated to see if it can increase the efficiency and performance of H2 production through electrochemical processes. The observed electro-catalytic enhancement could be attributed to the synergistic effect of hematite dendrites, larger surface area of dendrite structure leading to higher loading of Ag NPs on the surface of HDs. Moreover, the endurance study was performed to check the stability of the presented electrocatalyst in acidic medium under both dark and light illumination conditions which shows that the presented composite catalyst is stable for minimum 100 scans even under light illumination with no signs of photo-corrosion.

Mechanistic modeling of NO electrochemical reduction in a micro-tubular cell: Effects of CO2/H2O components and electrochemical promotion

A two dimensional model coupling the mass balance and momentum balance processes was developed for a micro-tubular electrochemical cell for NOx reduction. The effect of electrochemical promotion of catalysis (EPOC) on NO decomposition was considered in the kinetic model. The cell was consisted of La0.6Sr0.4Co0.95Cu0.05O3−δ (LSCC)–Ce0.9Gd0.1O1.95 (GDC) catalytic electrode, YSZ electrolyte, Ni–YSZ anode. The model was validated by comparing the simulation results with experimental data from the literature. Parametric simulations were conducted to investigate the effects of CO2 and H2O on the NO conversion performance. Results revealed that the presence of CO2 and H2O are beneficial to the NO reduction. The improvement by H2O was stronger than that by CO2. The effects of temperature on NO conversion were investigated. The distributions of the gas compositions along the channel and catalytic electrode were discussed to analyze the inner diffusion and reaction processes. Gas flow rate was another influential factor and low gas velocity was advantageous for the NOx reduction reaction kinetics.

Electrochemical promotion of propane oxidation over Pd, Ir, and Ru catalyst-electrodes deposited on YSZ

The effect of electrochemical promotion of catalysis was investigated for the oxidation of propane using Pd, Ir, and Ru catalyst-electrodes sputter-deposited on YSZ disks in the temperature range of 250–450 °C. Electrophobic type behavior was observed, i.e., the catalytic reaction rate was found to increase with catalyst potential. The observed rate changes under polarization were strongly non-Faradaic and exceeded under anodic potential application the electrocatalytic rate of O2− supply to the catalyst surface, I/2F, by up to a factor of 250 for Pd, 125 for Ir, and 15 for Ru catalyst-electrodes.

Hydrogenation of CO2 over Ru/YSZ Electropromoted Catalysts

The effect of electrochemical promotion of catalysis (EPOC or NEMCA effect) was investigated for the hydrogenation of CO2 using Ru catalyst electrodes supported on YSZ solid electrolyte pellets at ...

Electrochemical activation of the catalytic methanol reforming reaction for H2 production

The effect of electrochemical promotion has been investigated for the first time in the methanol reforming reaction for H2 production using a thin Pt film deposited on a K-βAl2O3 beta solid electrolyte. The electrochemical pumping of K+ ions from the solid electrolyte support to the metal thin film allowed enhancing strongly the H2, CO2 and CO production rates at all explored reaction conditions. Specifically, H2 production rate was five-fold increased under the optimal potential conditions. The promotional phenomenon was found to be reversible due to the activation of H2O molecules that led to the removal of carbonaceous species intermediates occurring in secondary reactions of the methanol reforming reaction mechanism.

Electrochemical promotion of CO combustion over Pt/YSZ under high vacuum conditions

Electrochemical promotion of CO combustion over Pt/YSZ was investigated under high vacuum conditions. A galvanostatic step was coupled to mass spectrometric gas analysis using an electrochemical mass spectrometric monitoring device. Non-Faradaic electrochemical promotion of catalysis took place at 300°C while only electrochemical oxidation was observed at 400°C. Oxygen evolution measurements revealed that electrochemical promotion is related to the thermodynamically stable PtOx species over the Pt/gas interface. The polarization time and O2 pressure show strong influence on the relaxation transient upon current interruption. We propose that during anodic polarization, PtOx is first formed at the Pt/YSZ interface. With prolonged polarization time, the formed PtOx either migrates over the Pt/gas interface inducing electrochemical promotion or diffuses into the Pt bulk leading to the oxygen storage. After polarization, the stored O species is released and acts as sacrificial promoter causing the persistent electrochemical promotion effect.

Electrochemical promotion of the water–gas shift reaction on Pt/YSZ

The effect of electrochemical promotion of catalysis was investigated for the water–gas shift reaction over porous Pt catalyst electrodes interfaced with 8%mol Yttria-stabilized Zirconia. A fuel cell type electrochemical reactor was used at temperatures from 300 °C to 400 °C, under P H 2 O / P CO ratio values from 2.85 to 31. A negative order dependence of the catalytic reaction rate on P CO and a positive one on P H 2 O was found under open-circuit and polarization conditions. Positive potential application (+2.5 V), i.e., O 2− supply to the catalyst surface, causes a small decrease in the catalytic reaction rate, while negative potential application (−1.5 V) results in a pronounced rate increase, up to 200%, with apparent faradaic efficiency values up to 110. The rate increase obtained with negative polarization can be attributed to the weakening of the Pt–CO bond strength but also, to the increase in surface concentration of oxygen ion vacancies near the Pt-gas-support three-phase boundaries necessary for water dissociation.

Noble Metal-Free Hydrazine Fuel Cell Catalysts: EPOC Effect in Competing Chemical and Electrochemical Reaction Pathways

We report the discovery of a highly active Ni-Co alloy electrocatalyst for the oxidation of hydrazine (N(2)H(4)) and provide evidence for competing electrochemical (faradaic) and chemical (nonfaradaic) reaction pathways. The electrochemical conversion of hydrazine on catalytic surfaces in fuel cells is of great scientific and technological interest, because it offers multiple redox states, complex reaction pathways, and significantly more favorable energy and power densities compared to hydrogen fuel. Structure-reactivity relations of a Ni(60)Co(40) alloy electrocatalyst are presented with a 6-fold increase in catalytic N(2)H(4) oxidation activity over today's benchmark catalysts. We further study the mechanistic pathways of the catalytic N(2)H(4) conversion as function of the applied electrode potential using differentially pumped electrochemical mass spectrometry (DEMS). At positive overpotentials, N(2)H(4) is electrooxidized into nitrogen consuming hydroxide ions, which is the fuel cell-relevant faradaic reaction pathway. In parallel, N(2)H(4) decomposes chemically into molecular nitrogen and hydrogen over a broad range of electrode potentials. The electroless chemical decomposition rate was controlled by the electrode potential, suggesting a rare example of a liquid-phase electrochemical promotion effect of a chemical catalytic reaction ("EPOC"). The coexisting electrocatalytic (faradaic) and heterogeneous catalytic (electroless, nonfaradaic) reaction pathways have important implications for the efficiency of hydrazine fuel cells.

Electrochemical promotion of the SO2 oxidation over thin Pt films interfaced with YSZ in a monolithic electropromoted reactor

The effect of electrochemical promotion of catalysis on the SO2 oxidation reaction was investigated over thin (∼40nm) Pt catalyst electrodes interfaced with YSZ, in a monolithic electrochemically promoted (MEP) reactor equipped with five or twenty-two electrocatalytic plates. A mildly oxidizing gas mixture was used at temperatures from 330 to 370°C and flowrates between 1 and 30Lmin−1.It was found that positive potential application, i.e. O2− supply to catalyst surface, can cause an increase of up to 200% in the catalytic oxidation rate of SO2 with Faradaic efficiency values up to 30 at flowrates as high as 30Lmin−1 which corresponds to 3×104h−1 space velocity or 0.1s residence time.The results show the strong potential of MEP reactors for practical applications of the electrochemical promotion of catalysis (EPOC) effect, in gas treatment units and chemical synthesis and destruction processes.

Electrochemical promotion of methane oxidation on Rh/YSZ

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.

Origin of non-Faradayicity in electrochemical promotion of catalytic ethylene oxidation

The electrochemical promotion of catalytic C(2)H(4) oxidation has been investigated under low pressure conditions (p approximately 10(-6)-10(-4) mbar) with a Pt film on yttrium stabilized zirconia (YSZ) as catalyst. All measurements were conducted with a UHV system with a differentially pumped quadruple mass spectrometer (QMS) for rate measurements and a photoelectron emission microscope (PEEM) for spatially resolved measurements. A pronounced rate hysteresis upon cyclic variation of p(C(2)H(4)) was observed under open circuit conditions which is attributed to carbonaceous CH(x) adlayer inhibiting O(2) adsorption and hence poisoning the reaction. Application of a positive potential causes a partial removal of the inhibiting CH(x) adlayer by spillover oxygen thus triggering a transition from thus poisoned state of the surface to an active state with reduced carbon coverage. The ignition effect and therefore also the electrochemical promotion effect are linked to the presence of a carbon adlayer on the surface. The non-Faradayicity which is observed in this reaction system is explained as an ignition effect.

Electrochemical promotion of NO reduction by C3H6 on Rh catalyst-electrode films supported on YSZ and on dispersed Rh/YSZ catalysts

The selective catalytic reduction of NO by C3H6 in the presence of O2 was investigated on porous polycrystalline Rh catalyst-electrode films deposited on YSZ (Y2O3-stabilized ZrO2) and dispersed Rh/YSZ (0.5 wt%) catalysts. In the former case, it was found that the application of current or potential between the Rh catalyst-electrode and an Au counterelectrode enhanced the rate of NO reduction and CO2 formation by factors of up to 55 and 200, respectively. These rate increases were strongly non-Faradaic with apparent Faradaic efficiencies, Λ, of up to 7000 for the formation of CO2, demonstrating a significant effect of electrochemical promotion or non-Faradaic electrochemical modification of catalytic activity. A comparison of the activity between the electropromoted Rh paste catalyst and the dispersed Rh/YSZ catalyst shows that the TOFs on the dispersed Rh/YSZ catalyst fall between the corresponding TOFs on the unpromoted and electropromoted Rh catalyst-electrode film.

Electrochemical promotion in O2− cells during propane oxidation

The catalytic oxidation of propane was studied in an oxygen ion conducting solid electrolyte cell at 623–773 K and atmospheric total pressure. Three catalyst-electrodes were tested, Pt, Pd and Ag. Under open circuit, the technique of Solid Electrolyte Potentiometry (SEP) was used to monitor the thermodynamic activity of oxygen adsorbed on the catalyst surface during reaction. Under closed circuit, the effect of electrochemical promotion was investigated. On Pt and Pd, the promoting effect was of the “inverted volcano” type. On Pt, the reaction exhibited a strong NEMCA effect; the closed circuit rate exceeded the open circuit rate by more than three orders of magnitude. On Pd, on the other hand, the effect was hardly Faradaic. On Ag, a moderate NEMCA behavior was observed and the promoting effect was purely electrophobic. Both open and closed circuit results obtained with these three catalyst-electrodes, are evaluated and compared to each other.

In situ electrochemical modification of catalytic activity for propane combustion of Pt/β′′-Al2O3 catalyst-electrodes

The effect of electrochemical promotion (EP) of propane combustion was studied over a platinum film catalyst deposited on sodium β′′-Al2O3, a Na+ conductor, in the temperature range 320–440 °C. It was found that electrochemical pumping of sodium to the platinum surface markedly modifies its catalytic properties. For stoichiometric oxygen to propane ratio the system exhibited electrophobic behavior, i.e., addition of sodium resulted in decrease of the CO2 production rate. Relative changes in the catalytic rate by up to 60 times larger than the corresponding change in sodium coverage were measured. The observed behavior is explained by taking into account the reaction mechanism and the effect of the electrochemically controlled sodium coverage on the bonding of coadsorbed reactant species.

Electrocatalysis and electrochemical promotion of CO oxidation in PEM fuel cells: the role of oxygen crossover

The electrochemical promotion of catalysis (or NEMCA effect) was studied for the CO oxidation and water gas shift reaction on a Pt anode in a polymer electrolyte membrane (PEM) fuel cell. It was found that this phenomenon plays a significant role in a normal fuel cell operation (fuel mixture – air) but not in a hydrogen pumping operation (fuel mixture – H2). This implies that the role of oxygen crossover in the electropromotion (EP) of CO oxidation is vital. During fuel cell operation, the increase in the rate of CO consumption is 2.5 times larger than the electrochemical rate, I/2F of CO oxidation, while for oxygen bleeding conditions (fuel mixture + O2−air) the increase is five times larger than I/2F. This shows that the catalytic properties of the Pt anode are significantly modified by varying the catalyst potential. In order to confirm the role of oxygen crossover, Nafion membranes (117, 1135) with different thickness, were studied. The results show that upon decreasing the membrane thickness the crossover is increased and thus the electrochemical promotion effect becomes more pronounced.

Electrochemical activation of Pt catalyst by potassium for low temperature CO deep oxidation

This study exposes, for the first time, the effect of Electrochemical Promotion (EPOC) by potassium cations on Pt catalyst for low temperature CO oxidation process. The effect of catalyst polarization drastically increased the catalytic rate for more than 11 times by decreasing the light-off temperature by almost 40 °C. The promotional phenomenon was found to be reversible due to the formation and decomposition of potassium compounds on Pt surface that enhanced the adsorption of O 2 at the expense of CO. The achieved results demonstrated that Electrochemical Promotion of Catalysis could be applied to improve the performance of catalytic converters at lower temperatures.

Methanol reformate treatment in a PEM fuel cell-reactor

The treatment of methanol reformate, containing up to 2500 ppm CO, by the anode of a PEM fuel cell, operating as a preferential oxidation (PROX) reactor, was investigated in order to examine the possibility of electrochemically promoting the water–gas-shift (WGS) reaction and thus making the gas mixture suitable for anodic oxidation. It was found that the electrochemical promotion effect plays a significant role in a normal fuel cell operation (air at the cathode) but not in a hydrogen pumping operation (H 2 at the cathode). This implies that the role of oxygen crossover in the electropromotion (EP) of the WGS reaction and in the CO oxidation is vital. During fuel cell operation, the increase in the rate of CO consumption over a Pt/C anode is 2.5 times larger than the electrochemical rate, I/2 F of CO consumption, while for oxygen bleeding conditions (fuel mixture + 1% O 2 at the anode) the increase is up to five times larger than I/2 F, i.e. the Faradaic efficiency is up to 5. This shows that the catalytic properties of the Pt anode are significantly modified by varying catalyst potential and by the extent of O 2 crossover. The effect of temperature, gas composition, membrane thickness and Pt anode alloying with Cu was studied. It was found that the rate of CO consumption is significantly enhanced by increasing T, p H 2 and increasing O 2 crossover rate. Also the Faradaic efficiency reaches even higher values (up to 9) when using PtCu/C anodes. However, the Faradaic efficiency drops in general below 100% at high current densities and CO conversion levels.