Anodic Overvoltage Research Articles

The transformations of thiols and their dimers in the redox-mediated thiol-disulfide exchange reaction

A search for new approaches to sulfurous waste utilization is one of the urgent tasks of chemical technology. Thiol-disulfide exchange reaction (TDE) is one of the possible ways to involve technogenic wastes in organic synthesis. Electricity can promote such type of interactions. In this paper, we have studied TDE reactions involving low molecular weight thiols or their dimers under electrochemical conditions. The exchange processes were examined using the model reaction between 1-propanethiol and phenyl disulfide. Electrolysis was performed in the presence of redox mediators such as arylphosphines, substituted amines, o-, p-aminophenols or catechol. These compounds can initiate a TDE process with a formation of unsymmetrical disulfides. 4-Amino-2,6-diphenylphenol was chosen as the most effective redox mediator, which reduces the anodic overvoltage of a thiol oxidation by 1.20 V. The advantage of electrolysis in an undivided cell is the increased yield of target unsymmetrical disulfides due to the possibility of reduction ofhomodimers at the cathode. The involvement of refining waste, such as C3–C4 disulfide oil, in the reaction with substituted thiophenols made it possible to obtain a number of unsymmetrical arylalkyl disulfides with biologically active fragments in a high yield (up to 97%) under indirect electrolysis conditions.

Degradation Analysis of SOFC Performance for Long-Term Operation with High Fuel Utilization (2)

Solid oxide fuel cells (SOFCs) are a power generation technology with its high efficiency and low environmental impact, and the most important issue for SOFC for practical use is to improve reliability as well as high efficiency. A national project on SOFC development in Japan, “Development of advanced evaluation and analysis technologies for the durability of Solid Oxide Fuel Cells stacks (2020-),” has been initiated as a NEDO project in order to clarify their long-term durability and find dominant degradation factor for each SOFC stacks. Central Research Institute of Electric Power Industry (CRIEPI) has operated three kinds of different SOFC cell/stacks (flat-tubular and two kinds of planer) for durability under NEDO’s project, which have been developed by KYOCERA, MORIMURA SOFC Technology, and DENSO.Long-term durability tests of 20,000 hours have been conducted under high fuel utilization of 80% and 85%, which is close to the operating limit of each SOFC cell/stack for high efficiency, to evaluate the effects on the performance and degradation behavior of the cell/stack. In previous projects, we have developed an electrode polarization model that can accurately represent SOFC performance, including cell voltage and each overvoltage, under a variety of conditions. The degradation factors for durability of each SOFC cell/stack have been analyzed by using this model. The analysis revealed that the dominant degradation factors were increased cathode overvoltage and IR loss, similar to the SOFC cell/stacks that were modified from the previous project to improve their long-term durability. On the other hand, the degradation rate of cell voltage was higher at 85% than at 80% fuel utilization in one cell/stack, and the performance degradation factor analysis revealed that this difference was due to an increase in anode overvoltage at 85% than at 80% fuel utilization, where anode overvoltage was not the primary degradation factor.This work has been carried out under the projects (JPNP13001 and JPNP20003) of the New Energy and Industrial Technology Development Organization (NEDO), Japan as contract research. We would like to express our gratitude to NEDO, many SOFC developers, as well as to university partners and National Institute of Advanced Industrial Science and Technology (AIST).

(Invited) Electrowinning of Aluminium and Carbon Footprint

Primary aluminium is produced by the Hall-Heroult process which is based on electrolysis in molten fluoride electrolyte, Na3AlF6-AlF3, at ~960 oC in which the raw material alumina is dissolved and decomposed into pure aluminium and CO2 gas due to the use of carbon anodes. The current efficiency with respect to aluminium can be as high as 96 % in Hall-Heroult cells. The total cell reaction is: Al2O3 (diss) + 3/2 C (s) = 2 Al (l) + 3/2 CO2 (g) The electrical energy consumption is around 12 - 15 kWh/kg Al, which corresponds to an energy efficiency of about 50 - 60 %. The main losses are due to a large ohmic voltage drop in the electrolyte and high anodic overvoltage. There are considerable emissions of CO2 from generation of electricity and the anode reaction. In addition there are emissions of CF4 and C2F6 (PFC gases) during so-called anode effect. Several efforts are under way to reduce the carbon footprint of the electrolysis process. One approach is to develop inert anodes to produce oxygen to eliminate the direct emissions of CO2 and PFC. The most promising inert anode candidates are based on alloys (Fe, Ni, Cu) in an electrolyte rich in KF with a lower liquidus temperature. Another idea is to produce aluminium based alloys directly during electrolysis by adding metal oxides in order to co-deposit alloying elements such as silicon, titanium and manganese. Such a method may give significant savings also in terms of carbon footprint since the current method for producing alloys is to add pure metals to liquid aluminium before casting. These alloying metals are associated with high emissions of CO2 durning their primary production processes.

Electrochemical Performance of a Spatially Distributed ECPrOx Reactor

In this contribution a spatially distributed Electrochemical Preferential Oxidation (ECPrOx) reactor is evaluated and the influence of the temperature, CO inlet concentration, feed flow rate and relative humidity on the polarization curve, the local current distribution and the CO outlet concentration, selectivity and conversion are investigated. For that, six different cases are studied under galvanostatic and potentiostatic operation. For Galvanostatic operation, it was observed that depending on the operation conditions, the bifurcation point as well as the frequency and amplitude of the oscillations can be modified. Additionally, several spatiotemporal profiles of the local current density were found. For potentiostatic operation, the spatiotemporal profiles are constant with time, however they are affected by the anode overvoltage. Finally, after the bifurcation point, the temporal averaged anode overvoltage and the CO outlet concentration is always lower for galvanostatic control.

Online observer of the voltage components of the PEM electrolyzer based on the time-varying linearization of the semi-empirical model

This paper presents a method to estimate the components of the PEM electrolyzer voltage with the dynamic input current. The total voltage can be divided into reversible voltage and irreversible voltage that the former always can be described as a function of temperature and pressure. For the irreversible voltage, a dynamic semi-empirical model is linearized by a dynamic expansion point varying with the input power. A Luenberger observer is established from the linearized model to estimate the anode overvoltage, cathode overvoltage and ohmic overvoltage effectively and rapidly, for which the stability is proved by LMI approach. Considering the deviation caused by local linearization, a switching scheme of the expansion point is also analyzed to provide an appropriate solution for fluctuating current when accessed to renewable energy power. Simulations have been carried out to testify and verify the method with varying input current, different switching expansion points, and uncertain parameters. The results show that the online Luenberger observer can achieve fine accuracy. And for the varying input current, updating the expansion point with a certain scale can further improve the observation accuracy, which enhances the performance of the observer to adapt the hydrogen production with renewable energy to evaluate the behavior of the PEM electrolyzer.

Relationship between Evaluation of Oer Performance and Oxygen Bubble Generation Behavior in Alkaline Water Electrolysis

Introduction Alkaline Water Electrolysis (AWE) is anticipated as a mean of producing hydrogen as an energy carrier for sustainable development goals. One of the problems in alkaline water electrolysis is the rise in anode overvoltage at the high current density range. To further improve the efficiency of hydrogen production, grasping the bubble generation behavior and development of electrode materials based on it are important. In this study, using nickel electrodes with different diameters, the formation behavior of oxygen bubbles on the electrodes was observed with a high-speed video camera, and the relationship between the current density / electrode potential and the bubble formation behavior was investigated. Experimental Working electrode were Ni wire (NiW) (two types with diameters of 100 and 200 μm). A reference and a counter electrode were a reversible hydrogen electrode (RHE) and Ni rods (diameter 3 mm), respectively. All measurements were performed with a three-electrode electrochemical cell [1] at 303±1K with KOH aqueous solution. To reduce the solution resistance as much as possible, the working electrode and the reference electrode were placed as close as possible (about 1 mm). To equalize the current density distribution, Ni rods counter electrode were inserted from both sides of the cell. Observation windows were fitted on the sides of the cell. The working electrode was exposed only the horizontal part about 3 mm where we want to observe the electrolytic reaction, and the other parts were covered by PTFE heat-shrink tube and epoxy resin. After appropriate pretreatments, cyclic voltammetry (CV) measurement was performed at a scan rate of 5 mV s-1 to obtain polarization curves. Electrochemical impedance spectroscopy (EIS) measurement was performed at 1.5-1.8 V vs. RHE with a potential amplitude 10 mV, a frequency range of 1-106 Hz and 20 repeated measurements for each frequency. The solution resistance was obtained from the Nyquist plots, and the potential loss was corrected. The current density i geo / A cm-2 was normalized by the geometric surface area of the cylindrical shaped working electrode. Constant current electrolysis was performed under five conditions of i geo = 0.05-1.0 A cm-2. Under each condition, the formation behavior of oxygen bubbles was photographed with a high-speed video camera. From the obtained photographed images, the diameters of hundreds of oxygen bubbles detached from the electrodes were analyzed to calculate the Sauter mean diameter d 32. Results and discussion Figure 1 shows the polarization curves of the Ni wire electrodes with different diameters. In the range of i geo < 0.3 A cm-2 where the logarithmic value of the current density i geo changed linearly with respect to the potential, there was no significant difference in an OER performance among the two types of electrodes. In the range of i geo > 0.3 A cm-2, the overvoltage increased with i geo regardless of the electrode diameter, and it was confirmed that the smaller the diameter of the electrode was, the smaller the overvoltage at the same current density was, especially in the high current density range.Figure 2 shows the relationship between current density and overvoltage. It was confirmed that the overvoltage increased as the current density increased, and the smaller the surface curvature of the electrode was, the larger the overvoltage was.Figure 3 shows the relationship between the current density and oxygen bubble diameter. The larger the current density was, the larger the oxygen bubble diameter was, and the oxygen bubble diameter was in proportion to the 1/3 power of i geo. From the in-situ observation of the oxygen bubble generation behavior on the electrode with a high-speed video camera [2], the oxygen bubbles generated on the electrode coalesced and detached from the upper end of the Ni wire electrode placed horizontally. From the above, it was considered that the larger the surface curvature of the electrode was, i.e. the smaller the electrode diameter was, the smoother the detachment of oxygen bubbles from the electrode was. It is implied that large curvature of electrode surface contributed to the smaller overvoltage. Acknowledgements A part of this study was based on results obtained from the Development of Fundamental Technology for Advancement of Water Electrolysis Hydrogen Production in Advancement of Hydrogen Technologies and Utilization Project (JPNP14021) commissioned by the New Energy and Industrial Technology Development Organization (NEDO). Reference [1] Y. Kojima, R. Misumi, M. Kaminoyama, S. Mitsushima, 43rd Electrolysis Technology Debate, 14 (2019)[2] H. Ikeda, Y. Kojima, R. Misumi, M. Kaminoyama, S. Mitsushima, 22nd Society of Chemical Engineers Student Presentation, D08 (2020) Figure 1

In Situ X-Ray Diffraction Study of Iridium Crystalline Structure Under Working Conditions of Proton Exchange Membrane Water Electrolysis

Proton exchange membrane water electrolysis (PEMWE) has received much attention as a hydrogen production process [1, 2]. Nowadays, due to its high efficiency, compact design, and easy maintenance, it offers the possibility of integrating different types of renewable energy such as solar and wind energy.However, to promote PEMWE, the performance of PEMWE needs to be higher than the current status to achieve wide application. To achieve this, there are many problems to be solved. For example, catalyst layer degradation (CL) is one of the main problems that need to be solved. CL plays an important role in promoting an efficient chemical reaction inside an operating PEMWE cell, but it is easily degraded due to the high anodic overvoltage and oxygen/water mixture, which reduces the performance of PEMWE. Some previous works have tried to elucidate the degradation mechanism, but most of them focused on static studies and the in-situ conditions in the cell were not carefully considered. Recently, iridium or oxide-iridium is often used as the material of anode electrode because it has high durability at a high electric potential. Therefore, focusing on the degradation mechanism of iridium during the operation of the PEMWE cell will be effective for the further development of electrodes for PEMWE.In this study, the PEMWE cell with a viewing window on the top was originally designed to study the degradation mechanism of the anode. The in-house cell was used to study the degradation process during the operation of the PEMWE cell. X-ray diffraction (XRD) was chosen as a method to follow the Phase change of the catalyst layer on the electrode surface due to the chemical reaction during operation. Reference [1] M. Carmo et al., Int. J. Hydrogen, 4901-4934, 38 (2013)[2] M. Steahler et al., Int. J. Hydrogen, 7053-7058,44 (2019)

Degradation Analysis of SOFC Performance for Long-Term Operation with High Fuel Utilization

A national project in Japan for SOFC development has been initiated from FY2020 to FY2024 in NEDO’s project in order to establish the method for degradation diagnosis of SOFC stacks with durability and reliability for 130,000 hours or the method for evaluation of expanding application. Central Research Institute of Electric Power Industry (CRIEPI) has operated three types of SOFC stacks, which are developed by KYOCERA, MORIMUA SOFC TECHNOLOGY, and DENSO and investigated their long-term durability in this project. We have developed the electrode polarization model for SOFC performance which is able to express the SOFC performance such as Nernst loss, cathode overvoltage, anode overvoltage, and internal resistance (IR) loss under various conditions with high prediction accuracy. Each SOFC stack has been operated with high fuel utilization (Uf = 80% and 85%) as a severe condition for high efficiency towards 20,000-hours operation, and degradation factors for SOFC durability has been analyzed by using our electrode polarization model. Analysis results until now showed that dominant degradation factors for both Uf = 80% and 85% were increase of cathode overvoltage or IR loss as same as that of the SOFC stacks in the previous term [1]. Although anode overvoltage was a minor degradation factor, increase rate of anode overvoltage under Uf = 85% was relatively larger than that under Uf = 80%. In addition, thermal property testing will be conducted for each SOFC stack in order to include temperature term into our electrode polarization model. This new model can predict SOFC performance more accurately under various operating conditions. This work has been carried out under the project of the New Energy and Industrial Technology Development Organization (NEDO), Japan as a contract research. We would like to express our gratitude to NEDO, KYOCERA, MORIMUA SOFC TECHNOLOGY, Denso as well as to university partners and National Institute of Advanced Industrial Science and Technology (AIST).Reference[1] A. Ido et al., ECS Transactions, 91(1), 801-808 (2019).

CATALYTIC PROPERTIES OF Ni-V COATING IN THE PROCESS OF HYDROGEN RELEASE

Solar and hydrogen energy play an important role in providing a variety of industrial facilities with electricity and heat. One of the priorities of modern industry is to increase the production of environmentally friendly energy source – electrochemical synthesis of hydrogen. Modern methods of electrolysis of water do not meet the need for its use, due to the high cost of electrosynthesis of water-alkaline electrolysis, which depends on the material and energy consumption of electrolysis.&#x0D; The useful energy consumption for the production of energy – hydrogen at the cathode and "unnecessary" costs - for the release of oxygen at the anode, depend on the overvoltage of the respective reactions. Therefore, the most important problem of hydrogen energy is the synthesis of electrode materials with low overvoltage of O2 and H2. Electrode materials with low overvoltage will reduce the specific consumption of electricity in obtaining hydrogen by "classical" electrolysis.&#x0D; The prospects of reducing the cathodic and anodic overvoltage, which is a significant part of the voltage at the terminals of the cell, for the development of highly efficient and competitive technologies for hydrogen production by low-temperature electrolysis of an alkaline solution have been theoretically substantiated and experimentally confirmed. To reduce the overvoltage of the cathodic hydrogen evolution, it is proposed to modify the surface of the cathodes. The application of a small amount of electrolytic alloys of metals of the iron family with molybdenum and tungsten on nickel, cobalt, titanium and steel electrodes significantly (by 40–50 %) reduces the overvoltage of cathodic release of hydrogen from alkali solution.&#x0D; The use of steel electrodes, the surface of which is modified with vanadium and ni-ckel, reduces the voltage drop on the cell during the synthesis of H2 and O2 by 0.2–0.3 V, which creates conditions for reducing energy costs and energy savings.

Some future works of research in electrochemistry

From the brief critical overview of such energy conversion processes as fuel cell (FC), electrolysis of water/hydrochloric acid and discharge/charge of lithium ion batteries, and corrosion process with aid of the conception of anodic and cathodic overvoltages, we derived some key unsolved/unanswered questions which are proposed as future key issues of electrochemistry which can be split into 4 thematic issues. Furthermore, 2 issues of energy conversion at “mixing entropy batteries” by reverse electrodialysis/desalination by electrodialysis and solar galvanic cell/reverse photovoltaic cell with a semiconductive organic light emission diode (OLED) are also briefly supplemented.

Functional properties and electrochemical performance of Ca-doped Sr2-xCaxFe1.5Mo0.5O6-δ as anode for solid oxide fuel cells

Performance of Ca-doped strontium ferrite-molybdate Sr2-xCaxFe1.5Mo0.5O6-δ (x = 0, 0.05, 0.15, 0.3, 0.5) has been studied under reducing conditions. Thermogravimetry (TG) analysis in 5% H2/Ar and DSC measurements in high-purity argon demonstrated that the compositions are highly stable and there are no phase transitions. The best electrochemical and electrical performances of about 0.12 Ω cm2 and 33 S/cm at 800 °C in wet hydrogen, respectively, were obtained for the Sr1.85Ca0.15Fe1.5Mo0.5O6-δ anode. The EIS spectra were analyzed by means of distribution relaxation times and non-linear least squares methods, which made it possible to determine the behavior of the rate-limiting steps of hydrogen oxidation. Solid oxide fuel cells (SOFC) of planar design with the 1.5 mm LaGaO3-based supporting electrolyte, with the Sr1.85Ca0.15Fe1.5Mo0.5O6-δ symmetrical electrodes and with the Sr2-xCaxFe1.5Mo0.5O6-δ (x = 0.15 and 0.3) anode and the 70 wt% Pr0.9Y0.1BaCo2O6–δ + 30 wt% SDC cathode have been studied. The obtained results demonstrated a low anode overvoltage, which was about 0.12 V at 1 A/cm2 and 800 °C in wet hydrogen. The maximum power density of SOFC with the Sr1.85Ca0.15Fe1.5Mo0.5O6-δ anode and cobaltite cathode was about 0.18 W/cm2 at 800 °С under air/wet hydrogen conditions. The obtained results elucidate that Sr1.85Ca0.15Fe1.5Mo0.5O6-δ is a promising anode for solid oxide fuel cells.

Simulation and Visualization of Cell Performance Distribution of SOFCs

Introduction It has been reported for SOFC systems that high electrical efficiency beyond 70% could be achieved theoretically in a recent study[1]. It is important to understand the internal characteristics to design highly efficient SOFC. However, it is difficult to visualize internal reactions experimentally. From these reasons, it is desired to develop a simulation technique for visualizing various phenomena within fuel cell stacks. Here, we focus on numerical analysis on electrode kinetics. Exchange current density is an important phenomenological parameter in considering electrochemical reaction kinetics at SOFC electrodes and in simulating cell performance. Here, the exchange current density of fully pre-reformed methane-fueled SOFC anodes is measured and phenomenological equations of anode exchange current density are derived. Furthermore, we combine numerical analysis with the phenomenological relations to simulate the spatial distribution of anode overvoltages. Experimental conditions In this study, electrolyte-supported cells were used to measure the anode exchange current density. The electrolyte material was ScSZ (ScSZ:10mol% Sc2O3–1mol% CeO2–89mol% ZrO2) and the electrode prepared using NiO-ScSZ was applied to the anode. The electrode prepared using LSM（LSM: (La0.8Sr0.2)0.98MnO3）and LSM-ScSZ was applied to the cathode. By changing operation temperature, fuel utilization (Uf), and steam-to-carbon ratio (S/C), anode overvoltage was measured at each current density (0.25, 0.3, 0.35, and 0.4 cm2) for methane-fueled pre-reformed SOFCs by the current interrupt method. The anode exchange current density was then derived from the current density and anode overvoltage values. In simulation, three-dimensional cell model was created using a numerical analysis software. A parallel co-flow model was considered. The inlet gas temperature was 800°C and the anode gas composition was fixed with the S/C of 2.5. Results and discussion The equation of anode exchange current density based on hydrogen-fueled model[2] was revised to consider the equilibrium reforming reaction, the water gas shift reaction, and the conservation law of anode gas partial pressure. It has been reported that the electrochemical reaction, where the adsorbed hydrogen atom (Had) in hydrogen-based fuels is rate-limiting[2], an Arrhenius-type equation shown in Eq. 1 has been proposed based on the assumption that the electrochemical reaction of Had is rate- limiting: i 0,a = a·exp(b/RT)·PH2(A) 0.5·PH2O(A) 0.5 (1) The partial pressure terms were modified considering the equilibrium reforming reaction and the water gas shift reaction. Pre-exponential factor a and activation energy b were fixed using the experimental results. Fig. 1 shows the experimental results of anode exchange current density. The phenomenological values derived from Eq. 1 are shown in Fig. 1. As the fuel utilization varied, the exchange current density increased and reached the maximum at around 40% of Uf. The experimental results could be well fitted by Eq.1. The SOFC cell performance simulation was performed using the three-dimensional SOFC model. Fig. 2 shows the anode overvoltage distribution along the fuel flow direction from the left side to the right side. The overvoltage decreased from the inlet towards the outlet. This is partly because oxygen partial pressure on the anode increased to the outlet and the electromotive force and the exchange current decreased.

Preparation and Characterization of Alternative Ni-Based Anode Materials for SOFCs

Introduction Ni is widely used as the anode material of SOFCs as a cermet containing ionic conductor and electronic conductor. In the SOFC power generation, the oxygen partial pressure in the downstream region of the SOFC systems increases, the metallic Ni can be oxidized to NiO, and thus volume expansion occurs1. Aggregation of adjacent Ni particles can degrade the electrode structure, which makes it difficult to fully utilize the fuel supplied. We have succeeded in fabricating an SOFC anode which is resistant to redox cycling using oxide conductors, La-doped SrTiO3 (LST) and Gd-doped CeO2 (GDC). In this study, we aim to develop (i) a novel electrode in which a metallic Ni catalyst is dissolved from the electrode material and precipitated on the surface of the anode in a reducing atmosphere (Ni-LST-GDC anode), and (ii) a novel electrode using a Ni-based alloy in the electrode framework (Ni-alloy-GDC anode)2,3. Experimental Alternative anode materials were prepared via wet process2,3. The cells were prepared by screen-printing the anode and the cathode followed by each sintering process in air. A platinum mesh was used for the current collector, and a Pt-based reference electrode was deposited on the cathode side for overvoltage separation between the anode and the cathode. The electrochemical performance of the fuel cells was evaluated from the I-V characteristics at the operating temperature of 800°C, using 3%-humidified hydrogen fuel. In order to evaluate the electrode activity, the electrode overvoltage was measured. The current interruption method was applied for evaluating the electrode overvoltages. Results and discussion Figure 1 shows the I-V characteristics of the cells with the newly prepared Ni-LST-GDC anode, the Ni-impregnated anode, and the LST-GDC anode without adding Ni, at 800°C, in 3%-humidified hydrogen fuel. Regarding the anode voltage and the anodic overvoltage, the Ni-LST-GDC anode exhibited better performance than the anode composed only of LST and GDC. In comparison with the Ni-impregnated anode, however, the cell performance was still lower both in anode voltage, overvoltage, and IR loss. Figure 2 shows STEM micrographs indicating that the Ni catalyst particle precipitated could locate on the GDC. Anode materials design principles of alternative Ni-based anodes will be discussed. But most of Ni particles located on the electron-conductive LST. In contrast, the cell with Ni-alloy anode exhibited comparable performance compared to that with the Ni-SSZ cermet.

Effect of TiO2 Addition on Grain Growth, Anodic Bubble Evolution and Anodic Overvoltage of NiFe2O4-Based Composite Inert Anodes

A two-step powder compaction and sintering process was employed to fabricate TiO2-doped NiFe2O4 ceramic-based inert anodes. Grain growth during isothermal sintering was analyzed using Brook grain growth model. The bubble behavior of NiFe2O4 ceramic-based inert anodes was investigated in a two-compartment see-through quartz cell for aluminum electrolysis process. Anodic overvoltage and potential decay curves of the inert anodes were measured by using the steady state and current interruption technique. The results showed that the kinetic index of grain growth decreased with an increase in temperature. The average activation energy of grain growth for 1.0 wt.% TiO2-doped NiFe2O4 ceramic samples with a sintering temperature range from 1373 to 1673 K dropped from 675.30 to 183.47 kJ/mol. The diameter size of bubbles before releasing from the bottom surface of the anodes was reduced with increasing the current density, and the larger average releasing bubble size for carbon anode at the same current density could be obtained, which was compared to the NiFe2O4 inert anodes. Besides, the cell voltage of carbon anodes fluctuated much more violently under the same experimental conditions. After adding small amount of TiO2, a minor reduction in anodic overvoltage of NiFe2O4-based anodes can be observed.

Redox cycling induced Ni exsolution in Gd0.1Ce0.8Ni0.1O2 - (Sr0.9La0.1)0.9Ti0.9Ni0.1O3 composite solid oxide fuel cell anodes

Oxide anodes composed of 60 wt% Gd0.1Ce0.8Ni0.1O2 (GDCN)- 40 wt% (Sr0.9La0.1)0.9Ti0.9Ni0.1O3 (SLTN) composites were prepared and tested on (ZrO2)0.89(Sc2O3)0.1(CeO2)0.01 (SSZ) electrolyte-supported SOFC cells utilizing a (La0.75Sr0.25)0.98MnO3 (LSM)-SSZ cathode, in 3%-humidified hydrogen fuel at 800 °C. Improved electrochemical performance was found compared to the cell using Ni-free 60 wt% Gd0.1Ce0.9O2 (GDC) - 40 wt % Sr0.9La0.1TiO3 (SLT) that was attributed to the exsolution of nano-sized Ni particles from the Ni-doped system. This exsolution process represents a simpler, more attractive method to improve performance than the more conventional but more complicated infiltration method for introducing catalytic nanoparticles. Redox cycling testing was performed to investigate the performance and structural stability of the Ni-doped GDC-SLT anode. The results indicated that the Ni exsolution and aggregation occurred while redox cycling proceeded, resulting in a gradually reduced anodic overvoltage. Symmetric cells with dense thin film Gd0.1Ce0.9-xNixO2 (x = 0, 0.05, 0.1, 0.15) electrodes were also tested, demonstrating lower area-specific resistances with increasing Ni content on the surface under reducing conditions. The steady improvement during redox cycling, despite Ni agglomeration, is related to the continuous increase in the overall Ni content on the anode surface, which may be enabled by kinetic limitations to Ni re-dissolving under oxidizing transients.

Effect of Exchange Current Density on Current Distribution at Planar-Type SOFC Anodes

Introduction It has been reported for SOFC systems that high electrical efficiency of more than 70-80% could be achieved theoretically in a recent study [1]. However, their electrochemical performance would become lower due to an increase in ohmic and nonohmic resistances and possible fuel starvation. From these reasons, it is desired to develop a simulation technique for visualizing various phenomena in fuel cells. Here, we focus on numerical analysis on electrode kinetics. Since exchange current density is an important phenomenological parameter in considering electrochemical reaction kinetics at an SOFC electrode and in simulating the performance, the exchange current density of methane-fueled pre-reformed SOFC anodes was measured and compared with that of hydrogen-based fuel. Furthermore, the spatial distribution of current density was simulated for given average current density in case anode exchange current density i0 is either constant or variable. Experiment In this study, electrolyte-supported cells Ni-ScSZ/ScSZ/LSM-ScSZ (ScSZ:10mol%Sc2O3- 1mol%CeO2-89mol% ZrO2) were used. The electrode area was 8×8mm2 for i0 measurements. Pt mesh was used as a current collector. In this study, by changing operation temperature, fuel utilization (Uf), and steam-to-carbon ratio (S/C), anode overvoltage was measured at each (average) current density (0.25, 0.3, 0.35, and 0.4 cm2) for methane-fueled pre-reformed SOFCs by the current interrupt method. The anode exchange current density was then derived from the current density and anode overvoltage values. Results and discussion Anode exchange current density for hydrogen-fueled SOFCs may be described by Eq. (I) after Hosoi et al. [2]. Anode exchange current density could also be described by Eq. (II) in case vacant surface adsorption sites are predominant [2,3]. Figure 1(a) shows experimental values derived for the pre-reformed fuel in this study. The values derived from Eq. (i) are also shown with solid lines in Fig. 1(a) and (b), while the values derived from Eq. (II) are shown with dashed lines in Fig. 1(b). All these figures indicate gradual decrease in anode exchange current density in the high fuel utilization (Uf) region. i0,a = 3.5・exp(-6.2×104[Jmol-1] / RT)・(PH2 / PH2,ref)0.41・(PH2O/PH2O,ref)0.4 Eq. (I) i0,a = a’・exp(b’ [Jmol-1] /RT)・PH2 0.5 ・PH2O 0.5 Eq. (II) 3D-numerical simulation has been made using the software COMSOL to derive 2D current density of the cell shown in Fig. 1(c). Current density distribution for various average current densities is derived as a function of Uf and shown in Figs. 1(d) and 1(e) for constant i0 and variable i0 , respectively. While Fig. 1(d) suggests continuous decrease in current density from inlet to outlet, Fig. 1(e) suggests almost constant current density near the inlet and its gradual decrease near the outlet. The tendency of the latter is rather consistent with the dependence of anode exchange current density as a function of Uf in Figs. 1(a) and 1(b) for hydrogen-based fuels. Figure 1

Alternative SOFC Anode Materials with Ion– and Electron–Conducting Backbones for Higher Fuel Utilization

Introduction Solid oxide fuel cells (SOFC) have a great potential to generate electricity with a high efficiency. Currently, Ni-YSZ cermet is widely used as the SOFC anode material. As anode Ni can be oxidized under a high fuel utilization, however, SOFC fuel cannot be fully used [1]. In this study, we analyze alternative anode materials composed of ionic conducting GDC (Ce0.9Gd0.1O2) and electronic conducting LST (Sr0.9La0.1TiO3), both of which act as stable ion- and electron-conducting frameworks, respectively, and Ni acts only as an electrocatalyst, as described in Figure 1. In addition, Rh, despite Ni, is also impregnated for higher fuel utilization. Experimental Electrolyte-supported cells with ScSZ (10 mol% Sc2O3 - 1 mol% CeO2 - 89 mol% ZrO2) plate (20 mmφ×0.2 mmt) were used in this study. Mixture of LST and GDC (50:50 in volume) was used for the anode and was sintered at 1300oC for 3h. Mixture of (La0.8Sr0.2)0.98MnO3 (LSM) and ScSZ with a weight ratio of 50:50 was used for the cathode. In order to further improve the anodic performance, Ni and GDC were impregnated into the porous LST-GDC composite anode. Electrode area was 8×8 mm2 and Pt mesh was used as the current collector. Cell performance such as I-V characteristics, anodic overvoltage, and anode-side ohmic loss were measured at 800oC by feeding humidified fuel. This Ni-GDC co-impregnated anode was evaluated through the I-V measurements, FESEM analysis, and Red-Ox cycling tests [2]. Moreover, in order to investigate the stability for the operation under high fuel utilization in the downstream of practical SOFC systems, the long-term durability test (fuel: 95%-humidified H2, constant current density: 0.2 A cm-2) was conducted. In addition, the Rh-GDC co-impregnated anode performance was investigated for higher fuel utilization. Results and discussion The Ni-GDC co-impregnated anode (Ni:GDC = 1:5 in volume) showed comparable performance to the conventional Ni-ScSZ anode, as shown in Figure 2. The Ni loading was 0.167 mg-Ni/cm2 and the I-V performances were investigated using 3%-humidified H2 fuel. This anode was more stable against redox cycling than the conventional anode. The FIB-SEM micrograph of the Ni-GDC co-impregnated anode is shown in Figure 3, showing that catalytic Ni nanoparticles were supported on the LST-GDC composite frameworks in a highly dispersed manner, which could prevent Ni agglomeration. Although the Ni-GDC co-impregnated anode was stable against the redox cycling and its performance was comparable to the conventional one, it deteriorated in the durability test at high fuel utilization. Ni particles were oxidized to lose their catalytic activity because the initial anode voltage of the co-impregnated anodes was lower (< 0.701 V [3]) under the heavily-humidified hydrogen fuel. On the other hand, Rh-GDC co-impregnated (0.232 mg-Rh/cm2, Rh:GDC = 1:5 in volume) anode was stable against this durability test because Rh is a stable catalyst even under high oxygen partial pressure. Consequently, these anodes could be promising alternative anodes for high fuel utilization operation of SOFCs.

Voltammetric Sensor Based on Fe-doped ZnO and TiO2 Nanostructures-modified Carbon-paste Electrode for Determination of Levodopa

In this study, undoped and 1 wt.% Fe-doped with ZnO, and TiO2 nanostructures were synthesized by a simple hydrothermal method without using templates. The influence of the Fe dopant on structural, optical and electrochemical response was studied by x-ray diffraction, scanning electron microscopy, UV–Vis spectra, photoluminescence spectra and electrochemical characterization system. The electrochemical response of the carbon paste electrode modified with synthesized nanostructures (undoped ZnO and TiO2 as well as doped with Fe ions) toward levodopa (L-Dopa) was studied. Cyclic voltammetry using provided modified electrodes showed electro-catalytic properties for electro-oxidation of L-Dopa and a significant reduction was observed in the anodic overvoltage compared to the bare electrode. The results indicated the presence of the sufficient dopants. The best response was obtained in terms of the current enhancement, overvoltage reduction, and reversibility improvement of the L-Dopa oxidation reaction under experimental conditions by the modified electrode with TiO2 nanoparticles doped with Fe ions.

Oscillatory Reaction Rates in a Direct Formic Acid Fuel Cell (DFAFC)

Formic acid has been studied as a promising fuel for its direct oxidation in fuel cells since, in comparison with other organic molecules, it is more easily oxidized to CO2, has a high thermodynamic voltage and fewer problems associated with crossover [1]. It is accepted that the electro-oxidation of formic acid on platinum proceeds through parallel reaction pathways, including an indirect path through carbon monoxide poisoning, the active intermediate (or formate) pathway and the direct pathway [2]. From another point of view, the electro-oxidation of formic acid on Pt and on Pt-group metals is a typical system showing periodic oscillations. Basically, the mechanism that gives rise to this type of oscillatory dynamics is the interplay between the adsorption isotherm of a catalytic poison, for example θCO, and of oxygenated species, θOH, both of which are potential dependent [3]. A key question underlying this dynamics is how it can affect the efficiency of a DFAFC. The purpose of this study was to evaluate the performance of a DFAFC under oscillatory regime. The experiments were performed using the typical setup of a unit cell, with a special approach (hydrogen on the cathode) that allows reading the anodic overvoltage directly. The conditions were: Pt/C (50 wt%) was used as anodic and cathodic catalysts with a metal loading of 1 mgPt cm-2. The anode was fed with 8 mol L-1 of HCOOH and the cathode was fed with H2. The unit cell (≈ 4.6 cm2) was operated at 30oC under atmosphere pressure. Figure 1 exhibits the current-potential curves obtained under potentiostatic and galvanostatic control modes for the electro-oxidation of formic acid and the theoretical curve for oxygen reduction. It is worth noting that the high overpotential required for the oxygen reduction and for the formic acid oxidation on Pt drastically reduces the DFAFC voltage, as it does for all fuel cells, given the sluggish kinetics. However, during the slow galvanodynamic sweep, spontaneous voltage oscillations emerge and this kinetic instability periodically decreases the anodic overpotential. This fact is interesting since the oscillations increase the average cell voltage and could result in the deceleration of long-term surface deactivation [4]. It also testifies to the importance of evaluating its dynamics to improve the fuel cell efficiency. The occurrence of such oscillatory behavior during the operation of a Direct Methanol Fuel Cell (DMFC) has been report, however, due to the small amplitude and waveform of the oscillations that takes place during the methanol electro-oxidation, no net improvement was observed in this system [5]. Although, a decrease in long-term instabilities can still play an important role. Probably, the self-cleaning process is more efficient for formic acid oxidation than that for methanol due to its larger amplitude and also because during a cycle the anodic overpotential remains longer at lower values [4]. Here we report a self-organization process observed in a DFAFC and its influence on the device efficiency. It was observed that at certain moments the anodic potential is lower than that on a steady state and this phenomena together with a self-cleaning process could improve the overall fuel cell performance. Acknowledgements J.A.N. and H.V. acknowledge São Paulo Research Foundation (FAPESP) for the scholarship (grant #2015/09295-9) and financial support (grants #2012/21204-0, and #2013/16930-7). H.V. (#306151/2010-3) also acknowledges Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) for financial support.

Electrochemical performance and stability of Ni1-xCox-based cermet anode for direct methane-fuelled solid oxide fuel cells

Carbon deposition on Ni-based anode is well-known as a major barrier for the practical use and commercialization of hydrocarbon-fuelled solid oxide fuel cells (SOFCs). In this work, Co alloying in Ni-YSZ was studied as an alternative anode material for using CH 4 as a fuel. The Ni-YSZ and Ni-Co alloyed-YSZ were prepared by the traditional impregnation method without further mixing processes. After sintering and reduction in H 2 atmosphere, the introduced Co can completely dissolved into the Ni lattice and changed the morphology with an increase in the Ni-YSZ grain size and showed a better uniform microstructure. The Co alloying also enhanced the electrochemical performance under CH 4 fuel by reducing the resistance and anodic overvoltage. Moreover, the Co addition enhanced the stability of the cell with CH 4 a constant load current of 80 mA for 60 h. This performance related to the carbon deposition on the anode surface. The Co alloying showed a high efficiency to suppress the carbon deposition and improved the electrochemical performance of an SOFC cell operating under CH 4 fuel.