Electrochemical Modification Of Catalytic Activity Research Articles

Electrochemical Promotion of Bimetallic Palladium‐Cobalt Nano‐Catalysts for Complete Methane Oxidation

AbstractThe catalytic activity and electrochemical promotion of catalysis (EPOC) of the bimetallic Pd−Co nanoparticles (10 nm) deposited on yttria‐stabilized zirconia were investigated for complete methane oxidation. The reaction was conducted under open‐circuit and under polarization in a temperature range of 320–400 °C in reducing, stoichiometric, and oxidizing conditions. The catalytic activity of the catalyst nanoparticles increased to 40 % upon positive polarization in all gaseous compositions. A comparison of three reaction conditions revealed that the highest reaction rate increase (enhancement ratio, ρ=1.4) occurs under reducing conditions. The reaction rate increased upon anodic polarization, resulting in non‐Faradaic electrochemical modification of catalytic activity (Λ≫1) in reducing and oxidizing conditions and Faradaic or electrochemical enhancement (Λ<1) in stoichiometric reaction condition. This work demonstrates that the formation of different Pd and Co oxide phases can be accurately controlled by electrochemical stimuli and, in reducing conditions, result in pseudo‐capacitor behaviour.

First-Principles Studies for Optimal Model of the Ni/YSZ Triple Phase Boundary in Solid Oxide Cells

Non-Faradaic electrochemical modification of catalytic activity (NEMCA) with electric field applications in solid oxide cells (SOCs) is thought to be induced by spillover effects of lattice oxygen from the bulk, although the detailed mechanism has not still been clear. In SOCs, important phenomena such as fuel decomposition, charge transfer, etc. occur at the triple phase boundary (TPB) as a highly active site that consists of catalyst, electrolyte, and gas phases. NEMCA is expected to be also induced strongly by the surface mechanism on TPB, and understanding surface reactions on TPB is essential for improving catalyst and cell performances. However, the reliable TPB model has not been still uniquely defined to discuss the property of TPB although various studies have been reported. Therefore, in this study, we have focused on the TPB model comprising Ni catalyst cluster; YSZ electrolyte; and gas phase, and aimed to identify a reliable TPB model for theoretical studies by using first-principles calculations as an initial step. In concrete, we identified firstly the stable structures of YSZ surface models by using DFT calculations taking into account oxygen vacancy positions, yttrium atom arrangements, yttria concentration, and other factors. Thereafter, we discussed a reliable Ni/YSZ interface model based on the most stable YSZ model proposed above results by evaluating the Ni structure, interface stability, and so on.In this study, DFT calculations with a plane-wave basis set were implemented using CASTEP, and GGA-PBE exchange-correlation functional was used. The plane-wave cutoff energy was set as 489.8 eV, the OTFG-ultrasoft was used as the pseudopotentials, and the spin-polarization is considered because YSZ is a ferromagnetic substance. In YSZ surface models, yttria concentrations are set to 4.35 mol% and 9.1 mol% which shows the maximum ion conductivity of ZrO2. The three-layer YSZ (111) slabs with 15 Å vacuum layer with 2×2 and 2×4 unit cells were used for repeated slab models. The DFT+U method is used to obtain the correct electric structure of metal oxides with partially filled d or f-orbital shells, and k-points were set to 4×2×1. In Ni/YSZ interface models, we considered Ni cluster and Ni belt type models based on the most stable 2×4 YSZ surface model (9.1 mol%) proposed in this study. We also considered both cases of (111) and (100) facets for the contact interface. In the case of Ni/YSZ interface models, the DFT+U method was not considered to improve the calculation convergence and k-points were set to 4×2×1. A schematic diagram of the Ni/YSZ interface model is shown in the attached Figure.In the case of the 2×2 YSZ model with the yttria concentration of 9.1 mol%, the YSZ model is stabilized when oxygen vacancy is on the second O atom layer and the second neighbor to Y atoms, indicating that improvement in the geometry instability of ZrO2 for 8-coordination is more important than keeping local electron neutrality. We have also found that oxygen vacancy positions are more sensitive to the YSZ surface stability than Y atom arrangements. In the case of the 2×4 YSZ model with the yttria concentration of 4.35 mol%, the YSZ model where there are Y atoms on the first layer is stabilized. The crystal structure achieves a more stable structure by varying bond lengths when Y3+ with the larger ionic radius is replaced with Zr4+. Therefore, the structure is easier to stabilize when the Y atom exists on the surface than in the bulk due to the higher degree of freedom of the Y atom. The most stable structure of the 2×4 YSZ model with the yttria concentration of 9.1 mol% given based on the above results is 0.18 eV more stable than the previously reported structure [1]. This is because the number of Y atoms on the first layer with the second neighbor from oxygen vacancy is larger than the previously reported structure. We evaluated then the structural stability of the Ni/YSZ interface model based on the above YSZ model. As a result, we have found that adhesion energy between Ni and YSZ is independent of the relative position of Ni atoms. In addition, the larger the number of Ni atoms is, the more stable the structure is. This is because the electronic property of Ni atoms approaches the metal as increasing the number of Ni atoms. Other results and a detailed discussion will be reported in our meeting publications (ECS Transactions) and the presentation.[1] M. Shishkin and T. Ziegler, Phys. Chem. Chem. Phys., 16, 1798-1808 (2014). Figure 1

Non-Faradaic Electrochemical Modification of Catalytic Activity: LSCF As Electrocatalyst for Nitrous Oxide Electrochemical Reduction

Nitrous oxide (N2O) mitigation is of high importance as this gas is responsible for several environmental problems, such as photochemical smog, acid rain, ozone layer depletion and global warming. While N2O is not the principal contributor to global warming (~ 6 %), it has 310 and 21 times the Global Warming Potential (GWP) of the most common anthropogenic greenhouse gases, CO2 and CH4, respectively [1]. European Union directives currently aim at a reduction in the emissions of these gases by 42 % from their 2013 levels by 2030 [2].The current work aims to bridge solid-state chemistry and heterogenous catalysis, by studying the electrochemical promotion of catalysis (EPOC), also known as non-Faradaic electrochemical modification of catalytic activity (NEMCA), of the N2O reduction reaction. Here, the catalytic activity of an electrode surface is radically changed upon polarization, typically applying large electrical potentials (> 2 V) that lead to considerable changes in the conversion and the selectivity of the products [3].For this work, the well-known mixed electronic and ionic conductor La0.6Sr0.4Co0.2Fe0.8O3-δ LSCF) was utilised as the electrocatalyst, due to its high availability and excellent performance as a cathode in solid oxide fuel cells (SOFCs). This composition exhibits a very high oxygen surface exchange coefficient (K* = 1.1 x 10-3cm s-1 at 800 °C) as well as high electronic (230 S cm-1 at 900 °C) and ionic conductivities (0.2 S cm-1 at 900 °C), essential requisites for this application [4].A 3-probe cell-based reactor was developed, composed by a solid oxide-ion-conducting electrolyte substrate made of 8 % mol Y2O3 – stabilized ZrO2, a working electrode made of LSCF and counter and reference electrodes made of inert gold (Au), to avoid any influence of the latter parts in the electrocatalytic activity of the main LSCF electrocatalyst. A buffer layer of Ce0.9Gd0.1O2-δ (CGO) was additionally used between the working electrode and the substrate, to prevent the inter-reaction between the YSZ and LSCF.The key aspect of this work was the use of an integrated zirconia oxygen sensor in the studies, to determine variations in the values of oxygen partial pressure (pO2 ) upon polarisation, which were subsequently correlated with the extent of N2O conversion. This methodology proved to be highly effective and avoids the need for additional techniques, such as gas chromatography.From the experiments, it was found that by applying cathodic polarisation (i.e -0.05 and -0.25 A), an increase of the catalytic rate (r O2 mol s-1) was observed, resulting in a significant increase of N2O conversion. An electrophobic behaviour of the electrocatalyst, i.e., a reaction rate that increases upon cathodic polarization, was observed for all temperatures with a maximum rate enhancement ratio (ρ=r/r0) up to 3.47 was achieved at -0.05 A at 600 ºC. The Faradaic efficiency (Λ) was also calculated for the tested experimental conditions, and showed values that exceed unity, evidencing non-Faradaic electrochemical behaviour. The highest Λ value of 8.8 was obtained at the lower temperature and currents tested at 600 °C. The nature of the mechanism was also assessed, showing a completely reversible effect, where the electrochemically promoted reaction rate returns to its open circuit value upon removal of polarisation. This work, therefore, highlights that non-Faradaic activation of heterogenous catalysis can be an effective tool to promote the reductive treatment of this greenhouse gas, with great environmental importance.

A Discussion on the Unique Features of Electrochemical Promotion of Catalysis (EPOC): Are We in the Right Path Towards Commercial Implementation?

The phenomenon of “Non-Faradaic Electrochemical Modification of Catalytic Activity (NEMCA)” or “Electrochemical Promotion of Catalysis (EPOC)” has been extensively studied for the last decades. Its main strength, with respect to conventionally promoted catalytic systems, is its capability to modify in-situ the activity and/or selectivity of a catalyst by controlling the supply and removal of promoters upon electrical polarization. Previous reviews have summarized the main achievements in this field from both the scientific and technological points of view. However, to this date no commercial application of the EPOC phenomenon has been developed, although numerous advances have been made on the application of EPOC on catalyst nanostructures (closer to those employed in conventional catalytic systems), and on the development of scaled-up reactors suitable for EPOC application. The main bottleneck for EPOC commercialization is likely the choice of the right chemical process. Therefore, from our point of view, future efforts should focus on coupling the latest EPOC advances with the chemical processes where the EPOC phenomenon offers a competitive advantage, either from an environmental, a practical or an economic point of view. In this article, we discuss some of the most promising cases published to date and suggest future improvement strategies. The considered processes are: (i) ethylene epoxidation with environmentally friendly promoters, (ii) NOx storage and reduction under constant reaction atmosphere, (iii) CH4 steam reforming with in-situ catalyst regeneration, (iv) H2 production, storage and release under fixed temperature and pressure, and (v) EPOC-enhanced electrolysers.

Electrochemical promotion of propane oxidation at Pt/BiCuVOx-YSZ thin-film cells: Economical use of YSZ

Abstract Electrochemical promotion of catalysis (EPOC) or Non-faradaic electrochemical modification of catalytic activity (NEMCA) is a phenomenon that may be exploited in catalytic converters to reduce the usage of precious metal catalysts. Nevertheless, the conventional structure of NEMCA cells employs a thick piece of relatively expensive solid electrolyte such as yttria-stabilised zirconia (YSZ). Minimising the amount of this costly material may bring EPOC closer to commercial application. NEMCA cells were fabricated with thin films of YSZ or BiCuVOx-YSZ as the electrolytes and sputtered Pt as the working catalyst electrodes. The catalytic activity for propane combustion was studied at 200, 300, 400, and 500 °C for cells with YSZ, but only at 400 °C for cells with BiCuVOx-YSZ. The effects of working electrode potentials and BiCuVOx contents were also studied. Most cells made with only YSZ could not function as electrochemical cells. Nevertheless, only one cell survived and exhibited NEMCA. On the other hand, all BiCuVOx-YSZ cells functioned properly. Electrophobic NEMCA behaviour was observed from both cell types. Addition of BiCuVOx improved sintering and resulted in films of higher conductivity. Nonetheless, diminished surface areas and suppressed faradaic efficiencies were observed. Undesirable electronic conductivity was also introduced by BiCuVOx and hence an optimum BiCuVOx content of 3.5 wt % was observed to offer the highest rate enhancement ratio, ρ, and the highest faradaic efficiency, Λ, of 5.3 and 339, respectively, at 400 °C.

First-Principles Investigation of Promoting Effect in CO2 Methanation with an Electric Field in SOEC

Non-faradaic electrochemical modification of catalytic activity (NEMCA) by impression of electric field is one of the methods to improve catalyst performance. In this study, we have theoretically investigated the mechanism of NEMCA in CO2 methanation in solid oxide electrolysis cell using the density functional theory (DFT). DFT calculations have been performed for hydrocarbon species on Ni (111) facet with direct electric field or co-adsorbed oxygen atoms, and their dependence for the rate determining steps (RDSs) in CO2 methanation from our kinetic simulation has discussed. We have found that direct effects of electric field in surfaces adsorption are different in each intermediate in RDSs and all RDSs accelerate with electric field. While, all intermediates bind on the surface less strongly as increasing the number of co-adsorbed oxygen atoms and some reactions decelerate and the other accelerate with co-adsorbed oxygen atoms.

Direct CO2 conversion to syngas in a BaCe0.2Zr0.7Y0.1O3- δ-based proton-conducting electrolysis cell

Electrolysis of steam and CO2 is considered to be a promising instrument for energy storage via sustainable H2 and hydrocarbon production. A model electrolysis cell was assembled using a thick BaCe0.2Zr0.7Y0.1O3-δ (BCZY27) electrolyte and two distinct electrodes, i.e., a (H2-cathode) porous Pt layer; and (steam-anode) a composite made of 60 vol. % La0.8Sr0.2MnO3-δ (LSM) and 40 vol. % BCZY27. The as-sintered steam electrode was catalytically-activated with Pr6O11-CeO2 nanoparticles. The cell was characterized by means of voltamperometry and impedance spectroscopy. Different operation parameters were analyzed: temperature; water concentration in the anode chamber; and H2 and CO2 concentration in the cathode chamber. Increasing H2O concentration (in the anode) and presence of CO2 (in the cathode) positively affected the electrode performance giving rise to lower cell overpotential and, consequently, substantial improvement in Faradaic efficiency. The high electrolyte thickness and the non-optimized Pt cathode limited the range of current density and the achieved peak power densities. The Faradaic efficiency for water electrolysis reached a value of 39% at 10.4 mA/cm2, as determined by the analysis of the H2 production. During co-electrolysis, the CO2 reaction was fostered by co-feeding a minimum H2 amount. CO formation took place through the reverse water gas shift (RWGS) reaction. When the current density was applied, CO2 conversion increased due mainly to the non-Faradaic electrochemical modification of catalytic activity (NEMCA effect) that allowed for the improvement of CO2 hydrogenation kinetics.

Electrochemical Promotion of Bimetallic Nanoparticles with Low-Noble Metal Content Interfaced with Yttria-Stabilized Zirconia Solid-Electrolyte

Heterogeneous catalysts play a central role in production of a variety of useful chemicals, energy generation and greenhouse gas emissions reduction. A heterogeneous catalyst usually consists of fine metal or metal oxide particles in the nanometer range dispersed on a high surface area supports. The catalytic activity and/or selectivity of the catalyst could be tuned in by using chemical promoters that are added to the catalyst during its fabrication or by choosing a suitable catalyst supports via metal-support interaction (MSI) effect. Another way to efficiently promote catalytic activity and in some instances catalyst selectivity is the use of electrochemistry via Electrochemical Promotion of Catalysis (EPOC), also called Non-faradaic Electrochemical Modification of Catalytic Activity (NEMCA) phenomenon [1, 2]. EPOC or NEMCA is ascribed to significant changes in the catalytic properties of metal or metal oxide catalysts deposited on solid-electrolytes caused by the application of small current (in the range of a few μA) or potential (up to 2V). The application of electric stimuli results in the supply of ionic species from the solid-electrolyte to the active surface, where the catalytic reaction takes place. These ionic species act as promotes and their addition results in significant catalytic rate increase due to changes in the catalyst work function [2-4]. This phenomenon has been applied to over 100 catalytic systems and reactions and continues to make an impact to surface science, catalysis and solid-state electrochemistry. The application of EPOC to nanostructured, highly dispersed catalysts has attracted significant interest in the last decade and several studies have demonstrated dramatic EPOC effect with noble metal (Pt, RuO2, Pd, etc.) nanoparticles (NPs) as low as 1.5 nm in size and metal loading down to 0.1 mg metal/cm2 [5-9]. Despite this tremendous progress, further improvement in metal utilization for practical advancement of EPOC is essential. In the present work, we investigated EPOC of bi-metallic, highly dispersed Ni90Pd10 and Ru45Fe55 (at. %) nanoparticles for two environmentally important reactions: CH4 combustion and CO2 hydrogenation. Both CH4 and CO2 are greenhouse gases that have negative impact on the environment and contribute to climate change, therefore methane conversion to CO2 and CO2 transformed to useful chemicals have several advantages. Ni90Pd10 and Ru45Fe55 NPs were synthesized using polyol reduction method by using ethylene glycol as reducing and stabilizing agent. TEM/SEM and XRD analysis were carried out to determent the particle and crystallite size, as well as alloy formation in bi-metallic NPs. The resulting colloidal solution containing NPs were deposited on yttria-stabilized zirconia (8 mol % Y2O3-ZrO2, (YSZ)) solid-electrolyte disk (D = 19 mm, thickness = 1 mm) with loading of 0.2 - 0.3 mg metal/cm2 and served as a catalyst-working electrode. Inert gold counter and pseudo-reference electrode were deposited on the opposite side of YSZ disk. Gold mesh served a current collector for NPs. The solid-electrolyte cell was placed in the CSTR-type quartz reactor (atm. pressure) [9] for CH4 combustion (T = 400 – 500 oC) and CO2 hydrogenation (T = 275 – 400 oC) reactions. Galvanostatic and potentiostatic EPOC transient experiments were carried out under various gas compositions and temperatures. The EPOC results on bi-metallic nanoparticles will be presented and the role of Ni and Fe on the catalytic activity and EPOC efficiency will be discussed. References Stoukides, C.G. Vayenas, J. Catal., 70 (1981), 137-146G. Vayenas, S. Bebelis, S. Ladas, Nature 343 (1990) 625–627.G. Vayenas, S. Bebelis, C. Pliangos, S. Brosda, D. TsiplakidesElectrochemical Activation of Catalysis: Promotion, Electrochemical Promotion and Metal-support Interactions, Kluwer Academic/Plenum, New York (2001)Vernoux, L. Lizarraga, M.N. Tsampas, F.M. Sapountzi, A. De Lucas-Consuegra, J.L. Valverde, S. Souentie, C.G. Vayenas, D. Tsiplakides, S. Balomenou, E.A. Baranova, Chem. Rev. 113 (2013) 8192–8260.Kambolis, L. Lizarraga, M.N. Tsampas, L. Burel, M. Rieu, J.P. Viricelle, Vernoux Electrochem. Commun., 19 (2012), pp. 5-8.A.E. Dole, E.A. Baranova, Implementation of Nanostructured Catalysts in the Electrochemical Promotion of Catalysis in Handb. Nanoelectrochemistry M. Aliofkhazraei, H.A.S. Makhlouf (Eds.), Springer International Publishing, Cham (2015), 1-27A.E. Dole, A. Costa, M. Couillard, E.A. Baranova, J. Catal. 333 (2016) 40–50.M. Hajar, K.D. Patel, U. Tariq, E.A. Baranova, J. Catal. 352 (2017) 42–51.

Electrochemical Promotion of Catalysis for CO2 Hydrogenation on Ru-Based Catalyst Using Ionically Conducting Ceramics

The release of carbon dioxide (CO2) into the atmosphere has led to effects of climate change resulting in an increase in global temperature, ocean acidification and many other environmental issues. Hydrogenation of CO2 into synthetic hydrocarbons is a promising solution in decreasing anthropogenic dependence on fossil fuels and providing an energy source that is carbon-neutral. The reverse water gas shift (RWGS) reaction is a feasible hydrogenation reaction that requires a 2-electron transfer to yield syngas (CO + H2) to be used in the Fischer-Tropsch reaction to synthesize synthetic hydrocarbons. In previous work (submitted to the Journal of CO2 Utilization), the conversion of CO2 into CO using Ru-nanostructured metal nanoparticles dispersed on ionically conducting ceramic supports like ceria (CeO2), doped-ceria (x-CeO2) and yttria-stabilized zirconia (YSZ) was studied with promising results. The activity of the Ru-based nanoparticles was improved due to the ionically conductive properties of the support, which contain oxygen (Oδ-) ionic species that promote the reaction. This promotional effect is known as the metal-support interaction (MSI) where nanoparticles are dispersed on a powder support, allowing Oδ- species to migrate from support to nanoparticle by an increase in temperature [1,2]. The MSI effect has been observed using the best Ru-based powder catalyst supported on samarium-doped ceria (SDC) - Ru45Fe55/SDC (2 wt.%), which yielded high CO amounts between 300-750°C. Current research aims at improving the overall RWGS reaction at lower temperatures through the utilization of the electrochemical promotion of catalysis (EPOC) or non-faradaic electrochemical modification of catalytic activity (NEMCA) effect [3,4]. EPOC allows to control in-situ the migration of ionic species to and from the metal surface through the application of a potential difference or current between the catalyst-working electrode and an inert counter electrode. This migration of species leads to the formation of a neutral double layer encapsulating Ru nanoparticles, promoting the reaction. The catalyst setup resembles an electrocatalytic cell where metal nanoparticles act as the working electrode deposited on a solid support in the form of a disc. The support (YSZ in this case) represents a fixed layer of electrolytes that conducts Oδ- ions to migrate to and from the active catalyst. As shown in Fig. 1, a promotional effect is observed for Ru on YSZ at 350°C under constant potential of 0.25 V, favoring the formation of CO through an enhancement ratio of ~2 and Faradaic efficiency of ~19, which is attributed to the synergistic effect between the metal and promoted ionic species Oδ­-. Additionally, density functional theory (DFT) calculations are being conducted for the hydrogenation of CO2 on Ru nanoparticles and will be discussed in correlation with the experimental findings to confirm the mechanisms occurring during the reaction. [1] P. Vernoux, M. Guth, X. Li, Ionically Conducting Ceramics as Alternative Catalyst Supports, Electrochem. Solid-State Lett. 12 (2009) E9–E11.[2] S. Ntais, R.J. Isaifan, E.A. Baranova, An X-ray photoelectron spectroscopy study of platinum nanoparticles on yttria-stabilized zirconia ionic support: Insight into metal support interaction, Mater. Chem. Phys. 148 (2014) 673–679.[3] D. Vayenas, C.G., Bebelis, S., Pliangos, C., Brosda, S., Tsiplakides, Electrochemical Activation of Catalysis, Springer US, (2001).[4] P. Vernoux, L. Lizarraga, M.N. Tsampas, F.M. Sapountzi, A. De Lucas-Consuegra, J.L. Valverde, S. Souentie, C.G. Vayenas, D. Tsiplakides, S. Balomenou, E.A. Baranova, Ionically Conducting Ceramics as Active Catalyst Supports, Chem. Rev. 113 (2013) 8192–8260. Figure 1

Electrochemical Promotion of Propene Combustion on Ag Catalytic Coatings

The non-faradaic electrochemical modification of catalytic activity (NEMCA effect) or the electrochemical promotion of catalysis (EPOC) has been investigated thoroughly for more than 120 catalytic reaction systems [1,2]. In electrochemical promotion studies, the conductive catalyst-electrode is in contact with an ionic conductor and the catalyst is electrochemically promoted by applying a current or potential between the catalyst film and a reference electrode. Numerous surface science and electrochemical techniques have shown that EPOC is due to the electrochemically controlled migration of promoting or poisoning ionic species (O2-in case of YSZ) between the ionic conductor and the gas exposed catalytic surface [1]. Propene is one of the major unburnt hydrocarbons containing in diesel cars exhausts. The most effective materials for propene combustion are Platinum-Group Metals. In spite of their high efficiency, these catalysts cannot be considered in the medium-term due to their excessive cost, which makes necessary stages of recovery and recycling [1]. Ag-based catalysts represent a possible alternative. This study reports the electrochemical promotion of the propene combustion on Ag films deposited on 8 mol% Y2O3 stabilized ZrO2 solid electrolyte, an O2-ionic conductor. Nanostructured electrochemical catalysts were prepared by screen-printing and reactive Physical Vapor Deposition (PVD) method. Screen-printing technique is a flexible tool to prepare few µm thick porous films at low cost whereas extremely thin coatings of Ag can be produced by PVD. Thickness and porosity of Ag coatings were modified by changing the deposition parameters (duration and pressure for PVD, nature of the ink and calcination temperature for screen-printing) to optimize the catalytic properties. Catalytic and electrocatalytic tests have been carried out in a quartz reactor [3] which operated under continuous flowing conditions at atmospheric pressure. The catalytic activity was monitored in a temperature range of 100 to 400oC under lean-burn conditions, as encountered in Diesel exhausts. The most active Ag films were also evaluated under closed circuit conditions (± 2V) in order to measure the effect of polarisation between the silver working electrode and an Au reference electrode. Both electrodes were exposed to the same atmosphere in a single chamber configuration. The catalytic activity of samples is depicted in Figure 1. All Ag coatings are effective from around 200°C. The catalytic performances were correlated with the microstructure of the films. Furthermore, the propene combustion was found to be electropromoted on the most active Ag films at low temperature with Faradaic efficiencies larger than 100.

Electrochemical Promotion of Catalysis

Electrochemical Promotion of Catalysis (EPOC), also called Non-Faradaic Electrochemical Modification of Catalytic Activity (NEMCA), is a promising concept for boosting catalytic processes and advancing the frontiers of catalysis [1]. This innovative field aims to modify in-operando both the activity and the selectivity of catalysts, in a reversible and controlled manner. EPOC utilizes solid electrolyte materials (ionically conducting ceramics) as catalytic carriers. Ions contained in these electrolytes are electrochemically supplied to the catalyst surface and act as promoting agents to modify the catalyst electronic properties in order to achieve optimal catalytic performance. It thus provides a unique means of varying promoter levels at the metal surface under reaction conditions by simply changing the potential of the catalyst film. The main advantage of EPOC is that the electrochemical activation magnitude is much higher than that predicted by Faraday’s law. Therefore, EPOC requires low currents or potentials. Moreover, promoting species such as O2- cannot be formed via gaseous adsorption and cannot be easily dosed by chemical ways. The EPOC technology consists in the implementation of catalysis in an electrochemical cell by using electrochemical catalysts which consist of a catalytic layer interfaced on an ionically conducting ceramic support. The latter serves as an electrically controlled source or sink of ionic promoter species that activate and modulate the behavior of the catalytic surface. Figure 1a displays a typical EPOC reactor. The catalytic layer, typically 40 nm to 10 mm thick, is deposited on a dense solid electrolyte membrane (Figure 1a). This catalytic layer must be electronic conductor to allow the polarization. Therefore, the catalyst is also an electrode and is then named catalyst-electrode. A counter-electrode and a reference electrode, both catalytically inert, are deposited on the other side of the membrane (Figure 1a). An electrical current density (1-500 µA/cm2) or potential (±2 V) is applied between the catalyst and the counter electrode. The reactants are co-fed over the porous electrochemical catalytic layer. Figure 1b displays a typical example of an EPOC experiment for the propane deep oxidation. The electrochemical catalyst was composed of a Pt thin film deposited by Physical Vapor Deposition (PVD) on a dense membrane of Yttria-Stabilized Zirconia (YSZ), an O2- ionic conductor. A positive current (from +10 up to +500µA) can strongly increase the propane conversion with a non-Faradaic manner, since the activation is up to 300 times higher than that predicted by the Faraday’s law through the electrochemical oxidation of propane. This latter reaction will be used to highlight recent advances of the EPOC technology both for the understanding (18O2 isotopic exchange) of the process and its applications via the proof of concept of electropromoted nanoparticles. [1] P. Vernoux, L. Lizarraga, M.N. Tsampas, F.M. Sapountzi, A. De Lucas-Consuegra, J.L. Valverde, S. Souentie, C.G. Vayenas, D. Tsiplakides, S. Balomenou, E.A. Baranova, Chem. Rev. 113 (2013) 8192. Figure 1

High Faradaic Yields of Non-Faradaic Electrochemical Modification of Catalytic Activity of Propane Oxidation at Pt-YSZ

Non-faradaic Electrochemical Modification of Catalytic Activity (NEMCA) of propane oxidation at Pt dispersed on yttria-stabilized zirconia (YSZ) discs by strong electrostatic adsorption (SEA) and wet impregnation (WI) in wireless configuration was studied and compared. With a stoichiometric gas mixture of propane and oxygen diluted in helium, an exceptionally large faradaic yield of 5.2 × 104 was achieved from a cell prepared by SEA at a reaction temperature of 200°C and a cell voltage of 6 V, compared to a smaller value of 1.0 × 104 obtained from a cell prepared by WI. A conceptual mechanism explaining the dependence of the faradaic yields and the rate enhancement ratios on Pt particle sizes is proposed.

Recent Progress on the Application of BASE in Energy Storage and Environmental Protection

Beta-alumina solid electrolyte (BASE) is a fast ion conductor material which shows high conductivity of sodium ions and low electronic conductivity. Recent Progress on the Application of BASE in energy storage such as Na Based Batteries (NBBS) and Alkali metal thermal to-electric converter (AMTEC), and Non-Faradaic Electrochemical Modification of Catalytic Activity (NEMCA) in environmental protection were reviewed.

Methane Conversion Assisted by Plasma or Electric Field

Direct conversion of methane to other valuable products such as ethylene, methanol, benzene, carbon, hydrogen, and syngas has been widely investigated. Such conversion requires high temperatures because of the strong C–H bond in CH4, and such high-temperature reactions present various problems: sequential reaction to decrease the selectivity to target products, need for multiple heat exchangers to use or recover high-temperature waste heat, materials that are stable at high temperatures, etc. To solve these problems, several trials have been undertaken to lower reaction temperatures using plasma, Non-Faradaic Electrochemical Modification of Catalytic Activity (NEMCA), and electric fields. This review describes recent trends in methane conversion, and describes our methods to lower the reaction temperature.

Electrochemical promotion of catalytic reactions: Thermodynamic analysis and calculation of the limits in Faradaic Efficiency

In the past three decades, more than 70 catalytic systems have been reported to exhibit the NEMCA phenomenon (Non-Faradaic Electrochemical Modification of Catalytic Activity). NEMCA has been observed in O2−, Na+, K+, Pb2+ and H+ conductors. As opposed to catalytic oxidations, however, in which dramatic increases in catalytic reaction rates have been observed, hydrogenation reactions exhibited weak NEMCA characteristics. The maximum Faradaic Efficiency (Λ) that can be obtained in a reaction with limited equilibrium conversion was calculated using a thermodynamic analysis. The values of Λ predicted by the present analysis are compared to those obtained experimentally during the reaction of NH3 synthesis in a H+ cell.

Bridging electrochemistry and heterogeneous catalysis

The discovery of the effect of the electrochemical promotion of catalysis (EPOC) or non-Faradaic electrochemical modification of catalytic activity (NEMCA effect) is described together with the key steps of its exploration at high pressure and high vacuum and its rationalization via electrochemical and surface science techniques. Recent attempts for its practical utilization are also surveyed.

ChemInform Abstract: Non-Faradaic Electrochemical Modification of Catalytic Activity (NEMCA) . Part 4. The Use of β′′-Al2O3 as the Solid Electrolyte.

ChemInform Abstract: Non-Faradaic Electrochemical Modification of Catalytic Activity (NEMCA) . Part 4. The Use of β′′-Al2O3 as the Solid Electrolyte.

Electrochemical characterization of the Pt/β′′–Al2O3 system under conditions of in situ electrochemical modification of catalytic activity for propane combustion

Linear sweep and cyclic voltammetry were used for electrochemical characterization of the Pt/sodium-β′′-Al2O3 system and investigation of the phases formed upon electrochemical pumping of sodium ions to the Pt catalyst-electrode under conditions of propane combustion or under mixtures of O2 or CO2 with helium, at temperatures between 320 and 480 °C. The number, position and magnitude of the peaks in the obtained voltammograms were found to depend on gas phase composition, temperature, and pre-scan (initial polarization) conditions. The results showed that under conditions of propane combustion more than one sodium phases can be formed as a result of electrochemical pumping of sodium ions to the Pt catalyst. The related electrochemical reactions and the identity of these sodium phases, which correspond mainly to carbonate or bicarbonate phases but also to oxide phases, are discussed on the basis of the obtained results and those of former studies.

Electrochemical catalysis for propane combustion using nanometric sputtered-deposited Pt films

The catalytic activity of sputtered-Pt films onto Y 2O 3-stabilized ZrO 2 (YSZ) pellets was investigated on the propane combustion. Catalytic performances were compared with conventional electrochemical catalysts based on Pt-paste films. Sputtered electrochemical catalysts presented higher Pt dispersion and significantly larger intrinsic catalytic activity. Furthermore, the catalytic activity of both systems was improved when electrochemical promotion of catalysis (EPOC) was applied to them. The magnitude of the electrochemical activation was higher for sputtered electrochemical catalysts than for Pt-paste films. However, no permanent Non-faradaic Electrochemical Modification of Catalytic Activity (NEMCA) effect was observed on sputtered-Pt films. Nevertheless, the kinetic behaviour of the electrocatalytic activity exhibited temperature dependence, which may be related with the formation of PtO x species in the bulk of the Pt film during the anodic polarization.

Preparation and characterization of catalyst thin films

Many devices used in catalysis are based on high surface area materials in which catalytic reactions are carried out inside pores, channels and other confined cavities for which the deposition of catalyst thin films is required. This paper provides an overview of the methods in use for the preparation and characterization of catalyst thin films, and focuses more specifically on thin films involved in the electropromotion of catalysis (EPOC). In fact, EPOC or NEMCA (Non-Faradic Electrochemical Modification of Catalytic Activity) have shown the importance of being able to combine electrical contacts between catalytic metals and ion conducting oxide layers as well as to develop in the same system catalytic materials with large specific surface area. The different aspects of thin film preparation and characterization are described in relation to catalyst thin films deposition. Multimodal and hierarchic porous structures can be obtained from the assembly of catalyst thin films with various carrier materials, anticipating more efficient catalytic systems. Chemical and physical coating techniques are compared with a special attention on those useful for the preparation of thin films with controlled porous structure and morphology. With regard to EPOC systems, electrode and electrolyte materials of interest for electrochemical catalytic devices are listed and typical examples of systems based on electrocatalyst thin films are given.