Steady-state Polarization Research Articles

Electrochemical impedance spectroscopy measurements on time-variant systems: the case of the Volmer-Heyrovský corrosion reaction. Part I: theoretical description

One of the theoretical requirements of electrochemical impedance spectroscopy measure­ments is that the studied system should not be varying with time. Unfortunately, this is rarely the case of physical systems. In the literature, quite a few methods exist to check and correct a posteriori the effect of time-variance, allowing to use conventional equivalent circuit models to fit and interpret the data. We suggest a different approach where, for a given electro­chemical mechanism and specific experimental conditions, assuming stationarity during each measurement, a time- and frequency-dependent expression of the Faradaic impe­dance is derived from the kinetic equations. The case of a potential relaxation at zero current following an anodic steady-state polarization is considered for a system where a Volmer-Heyrovský corrosion mechanism is supposed to take place.

Confrontation of Capitalism and Socialism in Wikipedia Networks

We introduce the Ising Network Opinion Formation (INOF) model and apply it to the analysis of networks of six Wikipedia language editions. In the model, Ising spins are placed at network nodes/articles and the steady-state opinion polarization of spins is determined from the Monte Carlo iterations in which a given spin orientation is determined by in-going links from other spins. The main consideration was the opinion confrontation between capitalism, imperialism (blue opinion) and socialism, communism (red opinion). These nodes have fixed spin/opinion orientation while other nodes achieve their steady-state opinions in the process of Monte Carlo iterations. We found that the global network opinion favors socialism, communism for all six editions. The model also determined the opinion preferences for world countries and political leaders, showing good agreement with heuristic expectations. We also present results for opinion competition between Christianity and Islam, and USA Democratic and Republican parties. We argue that the INOF approach can find numerous applications for directed complex networks.

Insights into the Interaction Landscape of the EVH1 Domain of Mena.

The Enabled/VASP homology 1 (EVH1) domain is a small module that interacts with proline-rich stretches in its ligands and is found in various signaling and scaffolding proteins. Mena, the mammalian homologue of Ena, is involved in diverse actin-associated events, such as membrane dynamics, bacterial motility, and tumor intravasation and extravasation. Two-dimensional (2D) 1H-15N HSQC NMR was used to study Mena EVH1 binding properties, defining the amino acids involved in ligand recognition for the physiological ligands ActA and PCARE, and a synthetic polyproline-inspired small molecule (hereafter inhibitor 6c). Chemical shift perturbations indicated that proline-rich segments bind in the conserved EVH1 hydrophobic cleft. The PCARE-derived peptide elicited more perturbations compared to the ActA-derived peptide, consistent with a previous report of a structural alteration in the solvent-exposed β7-β8 loop. Unexpectedly, EVH1 and the proline-rich segment of PTP1B did not exhibit NMR chemical shift perturbations; however, the high-resolution crystal structure implicated the conserved EVH1 hydrophobic cleft in ligand recognition. Intrinsic steady-state fluorescence and fluorescence polarization assays indicate that residues outside the proline-rich segment enhance the ligand affinity for EVH1 (Kd = 3-8 μM). Inhibitor 6c displayed tighter binding (Kd ∼ 0.3 μM) and occupies the same EVH1 cleft as physiological ligands. These studies revealed that the EVH1 domain enhances ligand affinity through recognition of residues flanking the proline-rich segments. Additionally, a synthetic inhibitor binds more tightly to the EVH1 domain than natural ligands, occupying the same hydrophobic cleft.

The nonlinear frequency response method for the diagnosis of PEM water electrolyzer performance

A better grasp of the underlying phenomena occurring in electrochemical technologies is crucial for their further development and, consequently, a much-needed step forward to a greener economy. Diagnostic methods that can reliably determine the state of health and causes of the performance shortcomings are indispensable. The ease of obtaining electrochemical data makes the analysis of current and voltage responses the preferred diagnostic approach. Traditional techniques, like steady-state polarization and electrochemical impedance spectroscopy are limited by their inability to distinguish between different processes due to the constraints of steady-state and linearity of system response, respectively. The nonlinear frequency response (NFR) method is an advanced diagnostic method that has the potential to overcome these issues. In this work, the NFR method was applied both experimentally and theoretically to study polymer electrolyte membrane water electrolysis (PEMWE). The model-based analysis provides insights into the losses in the PEMWE at different current densities. It shows that the contributions of the cathode to the overpotential losses at high current densities cannot be neglected. This has been much discussed in the literature and was often attributed only to mass transport losses. The contribution of mass transport has also been identified at higher current densities but is less pronounced than the kinetic contributions. Furthermore, we show that including the nonlinear dynamics in the analysis was crucial for identifying the appropriate parameter set. Overall, this work showed a considerable potential of the NFR method for the diagnosis of PEMWE due to its prospects of identifying different processes occurring within.

Mechanistic Insights into Cellular and Molecular Targets of Zinc Oxide Quantum Dots (ZnO QDs) in Fungal Pathogen, Candida albicans: One Drug Multi-Targeted Therapeutic Approach.

Rationally designed multitargeted drugs, known as network therapeutics/multimodal drugs, have emerged as versatile therapeutic solutions to combat drug-resistant microbes. Here, we report novel mechanistic insights into cellular and molecular targets of ZnO quantum dots (QDs) against Candida albicans, a representative of fungal pathogens. Stable, monodispersed 4-6 nm ZnO QDs were synthesized using a wet chemical route, which exhibited dose-dependent inhibition on the growth dynamics of Candida. Treatment with 200 μg/mL ZnO QDs revealed an aberrant morphology and a disrupted cellular ultrastructure in electron microscopy and led to a 23% reduction in ergosterol content and a 53% increase in intracellular reactive oxygen species. Significant increase in steady-state fluorescence polarization and fluorescence lifetime decay of membrane probe 1,6-diphenyl-1,3,5-hexatriene (DPH) in treated cells, respectively, implied reduction in membrane fluidity and enhanced microviscosity. The observed reduction in passive diffusion of fluorescent Rhodamine 6G across the membrane validated the intricate relationship between ergosterol, membrane fluidity, and microviscosity. An inverse relationship existing between ergosterol biosynthetic genes, ERG11 and ERG3 in treated cells, related well with displayed higher susceptibilities. Furthermore, treated cells exhibited impaired functionality and downregulation of ABC drug efflux pumps. Multiple cellular targets of ZnO QDs in Candida were validated by in silico molecular docking. Thus, targeting ERG11, ERG3, and ABC drug efflux pumps might emerge as a versatile, nano-ZnO-based strategy in fungal therapeutics to address the challenges of drug resistance.

Investigation of Iron Dissolution Mechanism in Acidic Solutions with and without Dissolved CO2—Part I: Electrochemical Impedance Spectroscopy Measurements

Aqueous CO2 corrosion of mild steel is one of the major problems in the oil and gas industry. While current understanding primarily focuses on cathodic reaction mechanisms, less attention has been given to the impact of aqueous CO2 on the anodic iron dissolution reaction. In contrast, the mechanism of iron dissolution in strongly acidic environments has been thoroughly investigated. Among the reaction mechanisms found in the open literature, a multipath mechanism was identified that could explain the iron dissolution in strong acidic sulfate solution; both in terms of steady-state polarization sweeps and impedance data at various pH values and current densities. However, the role of aqueous CO2 in solutions containing chlorides on the mechanism of iron dissolution remained an open question. The present study used electrochemical impedance spectroscopy (EIS) as the main technique, to study the mechanism of iron dissolution in strong acid chloride solution with and without the presence of CO2. Results showed that the presence of chloride ions (0.5 M) decreased the rate of iron dissolution by competing with hydroxide ions to adsorb on the metal surface, forming chloride-containing intermediate species that participate in the iron dissolution reaction. The resulting decrease in the availability of hydroxide intermediates, which are more effective at enhancing the reaction rate compared to chloride-containing intermediates, leads to an overall decrease in the rate of iron dissolution. While the presence of CO2 increases anodic current density, EIS investigation revealed that neither aqueous CO2 nor other carbonic species directly react on the bare metal surface to form adsorbed intermediates involved in the anodic reaction. EIS investigation suggested that aqueous CO2 may induce changes in the chemical composition of adsorbed species, rate constants, and surface coverage, thereby altering the kinetics of the underlying reactions.

Study of Resistance Extraction Methods for Proton Exchange Membrane Fuel Cells Based on Static Resistance Correction

Accurate extraction of polarization resistance is crucial in the application of proton exchange membrane fuel cells. It is generally assumed that the steady-state resistance obtained from the polarization curve model is equivalent to the AC impedance obtained from the electrochemical impedance spectroscopy (EIS) when the frequency approaches zero. However, due to the low-frequency stability and nonlinearity issues of the EIS method, this dynamic process leads to an additional rise in polarization resistance compared to the steady-state method. In this paper, a semi-empirical model and equivalent circuit models are developed to extract the steady-state and dynamic polarization resistances, respectively, while a static internal resistance correction method is proposed to represent the systematic error between the two. With the correction, the root mean square error of the steady-state resistance relative to the dynamic polarization resistance decreases from 26.12% to 7.42%, indicating that the weighted sum of the static internal resistance and the steady-state resistance can better correspond to the dynamic polarization resistance. The correction method can also simplify the EIS procedure by directly generating an estimate of the dynamic polarization resistance in the full current interval.

Product Distribution of Steady-State and Pulsed Electrochemical Regeneration of 1,4-NADH and Integration with Enzymatic Reaction.

The direct electrochemical reduction of nicotinamide adenine dinucleotide (NAD+) results in various products, complicating the regeneration of the crucial 1,4-NADH cofactor for enzymatic reactions. Previous research primarily focused on steady-state polarization to examine potential impacts on product selectivity. However, this study explores the influence of dynamic conditions on the selectivity of NAD+ reduction products by comparing two dynamic profiles with steady-state conditions. Our findings reveal that the main products, including 1,4-NADH, several dimers, and ADP-ribose, remained consistent across all conditions. A minor by-product, 1,6-NADH, was also identified. The product distribution varied depending on the experimental conditions (steady state vs. dynamic) and the concentration of NAD+, with higher concentrations and overpotentials promoting dimerization. The optimal yield of 1,4-NADH was achieved under steady-state conditions with low overpotential and NAD+ concentrations. While dynamic conditions enhanced the 1,4-NADH yield at shorter reaction times, they also resulted in a significant amount of unidentified products. Furthermore, this study assessed the potential of using pulsed electrochemical regeneration of 1,4-NADH with enoate reductase (XenB) for cyclohexenone reduction.

The Effect of SnO<sub>2</sub> and Rh on Pt Nanowire Catalysts for Ethanol Oxidation

In this study, we synthesized Pt-Rh nanowires (NWs) through chemical reduction of metallic precursors using formic acid at room temperature, excluding the use of surfactants, templates, or stabilizing agents. These NWs were supported on two substrates: carbon (Vulcan XC-72R) and carbon modified with tin oxide (SnO<sub>2</sub>) via the sol-gel method (10 wt.% SnO<sub>2</sub>). We explored the electroactivity of Pt/SnO<sub>2</sub>/C, Pt-Rh/C, Pt-Rh/SnO<sub>2(commercial)</sub>/C (commercial SnO<sub>2</sub>), and Pt-Rh/SnO<sub>2</sub>/C NWs toward electrochemical oxidation of ethanol in acidic media using various techniques, including CO-stripping, cyclic voltammetry, derivative voltammetry, chronoamperometry, and steady-state polarization curves. Physical characterization involved X-ray diffraction and transmission electron microscopy. The synthesized NWs exhibit higher ethanol oxidation activity than the commercial Pt/C (Johnson Matthey™) catalyst. Rh atoms are hypothesized to enhance complete ethanol oxidation, while the NW morphology improves ethanol adsorption at the catalyst surface for subsequent oxidation. Additionally, the choice of support material plays a significant role in influencing the catalytic activity. The superior catalytic activity of Pt-Rh/SnO<sub>2</sub>/C NWs may be attributed to the facile dissociation of the C-C bond, low CO adsorption (electronic effect due to Rh presence), and the bifunctional mechanism facilitated by the oxophilic nature of the SnO<sub>2</sub> support.

Electrocatalytic Reduction of Cobalt Oxide Spinel for Oxygen Reduction Reaction in Alkaline Media

The cathode material were prepared by dispersing Co3O4 on vulcan carbon in situ and manually grinding in pestle and mortar. Co3O4 prepared by hydrazine based combustion method showed nanocrystalline nature. Surface area was found to increase on dispersing Co3O4 on vulcan carbon by manually grinding it in pestle and mortar. The kinetics of the oxygen reduction reaction (ORR) was studied on Co3O4/C electrodes in KOH electrolyte by using cyclic voltammetry and steady state polarization measurements using Hg/HgO as the reference electrode. The morphological features of the electrocatalyst was studied by scanning electron microscopy (SEM), X-ray diffraction (XRD) & BET surface area. The polarization results showed higher electrocatlytic activity for the ORR on Co3O4/C (G) cathode in comparison to Co3O4/C (P) prepared by dispering Co3O4 on a vulcan carbon. Electrocatalytic activity of N(G) & N(P) were compared with Pt/C electrodes.

Measurement of Enthalpy and Entropy of a Model Electrocatalyst for the Oxygen Evolution Reaction

AbstractExperimentally determined thermodynamic parameters are rarely reported for electrocatalytic reactions including the oxygen evolution reaction (OER). Yet, they contain unique and valuable mechanistic insight and present a missing link to theoretical investigations. Herein, a protocol for determining thermodynamic properties of the rate determining state and intermediate (RDSI) of the OER is presented. Cobalt oxide is investigated at pH 7 as a case study. Two different approaches are employed: steady state polarization (SSP) that uses chronopotentiometry at different temperatures and current values, and potentiostatic electrochemical impedance spectroscopy (PEIS) at different DC voltages and temperatures. The data is used to fit a 3D plane from which entropy and enthalpy of the RDSI are obtained. The data analysis requires an appropriate filtering of the data. Hence, we discuss suitable figures of merit for establishing appropriate filtering criteria. The values obtained are 0.72 and −0.39 eV (at 298 K) for enthalpic and entropic contributions, respectively. The obtained values are reproducible for both approaches and consistent with literature.

Overpotential on Oxygen-Evolving Platinum and Ni-Fe-Cu Anode for Low-Temperature Molten Fluoride Electrolytes

To eliminate climate gas emissions from aluminum electrolysis, modifying a cryolite-based electrolyte partly replacing Na with K reduces liquidus, allowing a process temperature of 800°C. This enables the use of various metallic alloys for oxygen-evolving inert anode technology. This alternative process requires a higher energy efficiency to compensate for an increased reaction voltage, which highlights the importance of evaluating the kinetics and overpotential on oxygen-evolving anodes. This study evaluates anodic overpotentials using steady-state polarization on platinum and three Ni-Fe-Cu-based alloy compositions in a KF-NaF-AlF3-Al2O3(sat.) electrolyte at 800°C. The polarization curve on the platinum anode reveals two linear Tafel regions, while Ni-Fe-Cu anodes exhibit a single Tafel region. Notably, Ni-Fe-Cu anodes treated with high-temperature air oxidation to develop a pre-formed oxide layer exhibit better electrocatalytic activity than untreated anodes of corresponding composition. The kinetic equations, based on a theoretical model for the proposed mechanism of the oxygen evolution reaction, are derived and utilized to simulate overpotential and current, taking into account surface coverage. This model accurately predicts the two experimentally observed Tafel regions on the platinum anode, indicating a two-step charge transfer-controlled mechanism. We illustrate that multiple Tafel slopes can be attributed to the potential-dependent surface coverage of an adsorbate and can be correlated with the particular rate-determining step.

Overpotential Behavior of Electrolysis and Fuel Cell Modes of Molten Carbonate Cells

Molten carbonate is an high operating temperature electrolyte for the fuel cell (MCFC) and electrolysis cells (MCEC). In particular, the liquid molten carbonates allow the reversible operation between fuel cell and electrolysis cells. The electrode kinetics and mass transfer characteristics of MCFC have been rather extensively investigated compared with those of MCEC. In this work, the mass transfer behaviors between MCFC and MCEC have been focused by experimental way with 100 cm2 class bench cells.The electrodes and matrix in the cell were supplied by KIST in Korea. The cells were operated at 923K under atmospheric pressure. The hydrogen electrode was supplied by a mixture of H2:CO2=1:1, 2:1, 1:2 ratios at 20% or 40% H2O. The oxygen electrode was fed by Air:CO2=0.7:0.3 atm. The steady state polarization (SSP) from 0 to 150 mA cm-2, inert gas step addition (ISA), and reactant gas addition (RA) methods were employed. The ISA adds inert gas to the hydrogen or oxygen electrodes, then the addition varies reactants flow rate and partial pressures. Consequently, the variations are reflected to the cell voltage at open circuit and polarization states. The voltage shift by the flow and partial pressure change represents overpotential behaviors at the electrodes [1]. The RA method uses a reactant gas instead of inert gas in the ISA. Therefore, the RA provides the overpotential information of the added reactant species [2].From the experimental investigations, the total voltage loss at the FC is larger than that of EC mode. In addition, the hydrogen electrode has significant gas phase mass transfer resistance due to the H2, CO2, and H2O species at the FC and EC modes. A similar overpotential was observed at the hydrogen electrode. On the other hand, the oxygen electrode has much less overpotential at the EC mode than the FC mode. Since the oxygen reduction at the FC mode contains O2 dissolution and mass transfer through the carbonate electrolytes, liquid phase mass transfer resistance is dominant. However, the oxidation of carbonate ions to oxygen does not contain liquid phase mass transfer process at the EC mode. Therefore, much less overpotential was observed at the oxygen electrode at the EC mode than the FC mode. This can be a reason of smaller total voltage loss at the MCEC than the MCFC.[1] Samuel Koomson, Choong-Gon Lee, J. Electroanalytical Chemistry, 925, 116896 (2022).[2] C.-G. Lee, H.-C. Lim, J. Electrochem. Soc., 152, A219-A228 (2005).

Multi-scale model of axonal and dendritic polarization by transcranial direct current stimulation in realistic head geometry

BackgroundTranscranial direct current stimulation (tDCS) is a non-invasive brain stimulation modality that can alter cortical excitability. However, it remains unclear how the subcellular elements of different neuron types are polarized by specific electric field (E-field) distributions. ObjectiveTo quantify neuronal polarization generated by tDCS in a multi-scale computational model. MethodsWe embedded layer-specific, morphologically-realistic cortical neuron models in a finite element model of the E-field in a human head and simulated steady-state polarization generated by conventional primary-motor-cortex–supraorbital (M1–SO) and 4 × 1 high-definition (HD) tDCS. We quantified somatic, axonal, and dendritic polarization of excitatory pyramidal cells in layers 2/3, 5, and 6, as well as inhibitory interneurons in layers 1 and 4 of the hand knob. ResultsAxonal and dendritic terminals were polarized more than the soma in all neurons, with peak axonal and dendritic polarization of 0.92 mV and 0.21 mV, respectively, compared to peak somatic polarization of 0.07 mV for 1.8 mA M1–SO stimulation. Both montages generated regions of depolarization and hyperpolarization beneath the M1 anode; M1–SO produced slightly stronger, more diffuse polarization peaking in the central sulcus, while 4 × 1 HD produced higher peak polarization in the gyral crown. The E-field component normal to the cortical surface correlated strongly with pyramidal neuron somatic polarization (R2>0.9), but exhibited weaker correlations with peak pyramidal axonal and dendritic polarization (R2:0.5–0.9) and peak polarization in all subcellular regions of interneurons (R2:0.3–0.6). Simulating polarization by uniform local E-field extracted at the soma approximated the spatial distribution of tDCS polarization but produced large errors in some regions (median absolute percent error: 7.9 %). ConclusionsPolarization of pre- and postsynaptic compartments of excitatory and inhibitory cortical neurons may play a significant role in tDCS neuromodulation. These effects cannot be predicted from the E-field distribution alone but rather require calculation of the neuronal response.

Metastability exchange optical pumping of 3He at low pressure and high magnetic field

Systematic studies on metastability exchange optical pumping of 3He nuclei have been performed at Jefferson Lab using a 1-torr sealed cell at magnetic fields from 2 to 4T. The effects of the discharge intensity, pump laser power and pumping transition schemes on achievable nuclear polarization and pumping rate have been investigated. A maximum steady-state nuclear polarization of about 75% has been obtained. This work provides a baseline for the development of the novel polarized 3He target for CLAS12 at Jefferson Lab.

Effect of temperature on the electrode overpotential of molten carbonate electrolysis and fuel cells with inert-gas step addition method

This study explores molten carbonate electrolysis and fuel cell behaviour across temperatures from 823 to 973 K using steady-state polarization (SSP), electrochemical impedance spectroscopy (EIS), and inert-gas step addition (ISA) techniques. The findings reveal that total voltage loss decreases with rising temperature in fuel cell (FC) and electrolysis (EC) modes. The ohmic loss similarly declines as temperature increases. The ISA method determines the gas-phase mass transfer-induced overpotential, which increases as temperature increases in both modes at the hydrogen electrode (HE). This indicates that pore diffusion resistance strongly affects the HE in both modes. The liquid-phase mass transfer-induced overpotential at the oxygen electrode (OE) exhibits a much smaller value in EC mode than in FC mode at all temperatures. The increased O2 and CO2 partial pressures and decomposition of sufficient amounts of carbonate electrolytes to generate O2 in EC mode would reduce the liquid-phase resistance and its induced overpotential at the OE in the temperature range.

Anodic Overpotential on Metallic Oxygen Evolving Anodes for Aluminum Electrolysis in Low Temperature Electrolyte of KF-Naf-AlF₃-Al₂O₃

To eliminate climate gas emissions from aluminum electrolysis, significant research and development efforts focus on non-consumable oxygen evolving anodes to replace the carbon anodes used in the Hall Héroult process. Modifying the cryolite-based electrolyte currently used, by partly replacing Na with K in the molten fluoride, opens up a range of metallic alloys as feasible materials for inert anodes. In this work, the performance of Cu-Ni-Fe anodes, with or without surface treatment, is benchmarked against Pt and the pure components of the alloy.Anodic overpotential using an inert anode with low-temperature electrolyte, 750–800 ⁰C, for aluminum reduction should be low to compensate for ~1.10 V higher reversible potential for O₂ evolution than in conventional Hall-Héroult process at 960 ⁰C. In this study, voltammetry methods were applied in a low-temperature electrolyte mixture of KF-NaF-AlF₃-Al₂O₃ at 800 ⁰C. Oxygen evolution overpotential on pure Pt anode was measured using steady-state polarization. Tafel plots were obtained, and kinetic parameters were derived in the form of Tafel coefficients and exchange current density. A theoretical treatment for anodic process on Pt electrode with a two-step electron transfer mechanism is developed.Some anodic polarization curves were obtained for pure metals of Cu, Ni, Fe, and alloy of Cu-Ni-Fe in KF-NaF-AlF₃-Al₂O₃ at 800 ⁰C. Prior to use, some anodes of Cu-Ni-Fe alloy were oxidized in air at 800 ⁰C for 8 h and found to perform relatively better than an untreated alloy of the same composition. Anodes following electrochemical measurements were characterized by scanning electron microscope (SEM) coupled energy dispersive spectroscopy (EDS).

(Invited) Investigations of Electrochemical CO2 Reduction with Differential Electrochemical Mass Spectrometry

Differential electrochemical mass spectrometry (DEMS) is an analytical technique wherein an electrochemical reactor is interfaced with a mass spectrometer using a pervaporation membrane. This configuration enables volatile electrochemical reaction products to be continuously collected, identified, and quantified during steady state and dynamic polarization. The capabilities of this analytical technique are highly dependent on the design of the electrochemical reactor and how it is interfaced to the mass spectrometer. This presentation will introduce a variety of different DEMS cell designs and will compare their capabilities and limitations in terms of product sensitivity, product quantifiability, and time response. These comparisons will be illustrated through a series of vignettes investigating the electrocatalysis of CO2 reduction over Cu, Ag, and Au. This reaction is particularly difficult to investigate with DEMS since many of the reaction products yield identical mass fragments upon electron impact ionization. A general strategy for deconvoluting the extent to which a given product contributes to the observed mass ion currents will be presented. The utilization of DEMS for the direct observation of both the composition of the local reaction environment and the transient formation of intermediate reaction products will be discussed, as well as how these insights can be leveraged to guide rational electrocatalyst design. Finally, a new type of DEMS setup capable of quantifying the steady state surface coverage and surface lifetimes of electrochemical reaction intermediates will be presented. Figure 1

Anodic Behaviour of NixFeyCu5 and NixFeyCu20 in Potassium-Rich Naf-KF-AlF3-Al2O3 Melts

Using inert anode technology to produce aluminium would substantially reduce the aluminium industry's carbon footprint. Replacing the Hall-Héroult process with an alternative process based on vertical electrode cell (VEC) design using inert anodes and wettable cathodes is a promising alternative, but must be developed to be economically viable at scale. Three material types have been explored as inert anodes: metals, cermets and ceramics. Metallic anodes have been widely studied for their attractive properties, such as superior mechanical strength, high electrical conductivity and ease of fabrication. Of metallic anodes, Cu-Al, Ni-Fe and Ni-Fe-Cu-based alloys are the most commonly examined alloy systems for anode applications. It is also necessary to use low-temperature melts during the electrolysis process to ensure that the anodes are not damaged. In NaF-KF-AlF3 melts, increasing the KF content in the mixture would reduce the liquidus temperature and increase the alumina solubility in the melt.In this work, we examine the anodic behaviour of NixFeyCu5 and NixFeyCu20 electrodes, without prior surface treatment, in potassium-rich NaF-KF-AlF3 melts at 770°C. The cryolite ratio (xNaF+xKF/xAlF3) and potassium ratio (xKF/xKF+xNaF) of the melt used for the studies are 1.3 and 0.7, respectively. Electrochemical techniques such as chronopotentiometry, chronoamperometry and linear sweep voltammetry were performed to examine the anodic behaviour of these metallic anodes under an N2 atmosphere.Figure 1 shows the steady-state anodic polarisation curves on NixFeyCu5 and NixFeyCu20 anodes before and after galvanostatic polarisation with an applied current density of 0.5 Acm-2 for 120 minutes. It can be noted that the anodic potential at the same current density is more for the polarised anode compared to the non-polarised in both cases. This is due to the formation of a stable oxide layer during the galvanostatic polarisation of the anode. Oxidation of the anode is expected instead of fluoridation when the melt is saturated with alumina. The stabilisation of anodic current can be seen between 1.9 and 2.4 V. This stabilisation corresponds to the passivation of anodes due to the oxide layer formation. A rapid increase in the current after 2.4 V is due to the oxygen evolution reaction. The anodic current density in the passive region is much lower in anodes with Cu 20%, meaning a more protective layer is formed. After the galvanostatic polarisation of the anode, anodes were examined using scanning electron microscopy to understand the oxide layer composition. Anodes performed stably in potassium-rich melts, and Cu content plays a vital role in the anode's performance. Figure 1. Stationary state polarisation curves of NixFeyCu5 and NixFeyCu20 anodes before and after the polarisation at 0.5 A cm-2 Figure 1

Performances of Ni−SDC Hydrogen Electrodes in Reversible Operation Between SOEC and SOFC-Modes

A reversible solid oxide cell (R-SOC), which can be operated as solid oxide electrolysis cell (SOEC) and solid oxide fuel cell (SOFC), is a highly efficient direct energy converter between hydrogen and electricity (1). We have engaged in the R&D of high-performance, durable electrodes for the R-SOC (2-13). Recently, it was found that the durability of a double-layer H2 (DL) electrode, consisting of a SDC scaffold [samaria-doped ceria (CeO2)0.8(SmO1.5)0.2] with highly dispersed Ni0.9Co0.1 nanoparticles as the catalyst layer and a thin current collecting layer of Ni–YSZ cermet, was improved greatly by a reversible cycling operation between the SOEC and SOFC modes (12). The microstructure was observed to be stabilized by the cycling operation, i.e., lower parts of many Ni‒Co particles were anchored tightly on the SDC support and some portions were coated with SDC film, with certainty due to a strong interaction between Ni‒Co and SDC (13). However, the procedure for dispersing Ni‒Co nanoparticles on SDC (via an impregnation method) is not always suitable for large-scale fabrication. Aiming to develop a durable, practical hydrogen electrode with such a stabilized microstructure, we prepared new Ni–SDC hydrogen electrodes by a convenient protocol and examined the performances in the reversible operation.We prepared a coin-size cell with an air reference electrode (ARE) (6, 9-12):Ni–SDC H2 electrode│YSZ (0.5 mm)│SDC interlayer│LSCF–SDC O2 electrodeFor the H2 electrode, a paste was prepared by mixing Ni nanoparticles with uniform size (prepared by Noritake Co., Limited), SDC powder, and pore former. The paste was painted on YSZ electrolyte, followed by sintering. The LSCF–SDC (40 vol.% SDC) oxygen electrode was prepared on the SDC interlayer in the same manner as that described in our work (7). Test cells were operated at 800ºC by supplying humidified H2 (p[H2O] = 0.4 atm) to the H2 electrode compartment and dry O2 to the O2 electrode, irrespective of operation modes (SOEC or SOFC). The steady-state IR-free polarization curves (I–E curves) of the electrodes with area-specific ohmic resistances were measured by the current-interruption method in a three-electrode configuration. Figure 1 shows a SEM image acquired with a back-scattered electron (BSE) detector and Ni mapping by an energy-dispersive X-ray spectrometer (EDX) for the cross section of Ni–SDC electrode in pristine condition. In the BSE image, SDC is observed as light gray, Ni as dark gray, and pores as black. The distribution of Ni particles are fairly uniform in the electrode. The Ni–SDC electrode thus prepared exhibited a comparable initial performance to that of our DL-H2 electrode with Ni‒Co dispersed SDC prepared by the impregnation method (12). The durability tests of the electrodes are under progress in reversible cycling between −0.50 A cm−2 (SOEC-mode for 11 h) and 0.50 A cm−2 (SOFC-mode for 11 h). This work was supported by funds for the “Collaborative Industry-Academia-Government R&D Project for Solving Common Challenges toward Dramatically Expanded Use of Fuel Cells” from the New Energy and Industrial Technology Development Organization (NEDO) of Japan. References S. D. Ebbesen, S. H. Jensen, A. Hauch, and M. B. Mogensen, Chem. Rev., 114, 10697 (2014). H. Uchida, N. Osada, and M. Watanabe, Electrochem. Solid-State Lett., 7, A500 (2004). N. Osada, H. Uchida, and M. Watanabe, J. Electrochem. Soc., 153, A816 (2006). Y. Tao, H. Nishino, S. Ashidate, H. Kokubo, M. Watanabe, and H. Uchida, Electrochim. Acta, 54, 3309 (2009). R. Nishida, P. Puengjinda, H. Nishino, K. Kakinuma, M. E. Brito, M. Watanabe, and H. Uchida, RSC Adv., 4, 16260 (2014). H. Uchida, P. Puengjinda, K. Miyano, K. Shimura, H. Nishino, K. Kakinuma, M. E. Brito, and M. Watanabe, ECS Trans., 68 (1), 3307 (2015). K. Shimura, H. Nishino, K. Kakinuma, M. E. Brito, and H. Uchida, Electrochim. Acta, 225, 114 (2017). K. Shimura, H. Nishino, K. Kakinuma, M. E. Brito, and H. Uchida, J. Ceram. Soc. Jon., 125, 218 (2017). P. Puengjinda, H. Nishino, K. Kakinuma, M. E. Brito, and H. Uchida, J. Electrochem. Soc., 164, F889 (2017). H. Uchida, P. Puengjinda, K. Shimura, H. Nishino, K. Kakinuma, and M. E. Brito, ECS Trans., 78 (1), 3189 (2017). H. Uchida, H. Nishino, K. Kakinuma, and M. E. Brito, ECS Trans., 91 (1), 2379 (2019). H. Uchida, H. Nishino, P. Puengjinda, and K. Kakinuma, J. Electrochem. Soc., 167, 134516 (2020). H. Uchida, H. Nishino, and E. Da’as, ECS Trans., 103 (1), 6119 (2021). Figure 1