Electrode Polarization Research Articles

Synergistic Electrode and Electrolyte Polarities Lead to Outstanding Organic Potassium‐based Batteries

AbstractOrganic materials are promising as battery electrodes due to their flexible design, low cost, and sustainability. Although high electrolyte concentrations are known to suppress organic cathode dissolution, the organic cathode solubility depends on the interplay between the electrode and electrolyte polarities, which remains unexplored. Here, we elucidate the delicate interplay of electrode and electrolyte polarities to achieve stable cycling of organic cathode. Notably, we demonstrate that the solubility of low‐polar organic cathodes initially increases and subsequently decreases in electrolytes with increasing polarity. In contrast, high‐polar organic cathodes display increasing solubility in electrolytes with increasing polarity. When the polarities of the organic cathodes and the electrolyte are tuned, the batteries show high‐capacity retention and Coulombic efficiency. For example, polyaniline||potassiated graphite (KC8) pouch cells deliver 2500 cycles with an average Coulombic efficiency of 99.85 %. These new insights into low‐ and high‐polar organic cathode behavior in electrolytes lay a theoretical foundation for designing new organic battery electrodes and optimizing their electrolyte formulation.

Enhanced Sodium Storage and Thermal Safety of NaNi1/3Fe1/3Mn1/3O2 Cathode via Incorporation of TiN and WO3.

This study proposes an efficient, cost-effective, and industrially scalable electrode modulation strategy, which involves directly adding a small amount of high thermal and high conductance TiN and well interface compatible WO3 to NaNi1/3Fe1/3Mn1/3O2 (NaNFMO-TW) cathode slurry, to effectively reduce electrode polarization and interface side reactions, reduce the Ohmic heat and polarization heat of the battery, and ultimately to significantly improve the sodium-ion storage and thermal safety performance of the battery. At room temperature (RT) and 1C rate, the modified NaNFMO-TW electrode exhibits a reversible capacity of ∼95 mAh g-1 after 300 cycles, with a capacity retention rate of 82.6%, being higher than the 50.7% for NaNFMO. Furthermore, the assembled pouch battery with NaNFMO-TW retains 58.2% capacity after 300 cycles at RT&0.5C, being conspicuously superior to the 46.1% achieved by the NaNFMO||HC battery. In particular, adiabatic thermal tests and infrared thermal imaging tests reveal a marked improvement in the thermal safety of the modified battery, with a reduction in surface temperature of ∼1.3 and ∼2.2 °C during 3C charging and discharging, respectively. Moreover, the results confirmed the performance enhancement mechanism of NaNFMO by addition of TiN and WO3. Such electrode modulation strategy provides a practical method for improving battery performance.

Catalytic Performance of B-Site Doped LST Modified NiO/YSZ Cathodes for CO2 Reduction in a Solid Oxide Electrolyzer

Abstract The NiO/YSZ modified with B-site doped lanthanum strontium titanate (LST) was investigated as cathode materials for the reduction of CO2 in a solid oxide electrolysis cell (SOEC). The electrocatalytic activity evaluated in CO2/H2 under solid oxide fuel cell and SOEC conditions both demonstrated that the cobalt-doped LST (LSTC) performs better than pristine NiO/YSZ and is the best among the doped LSTs. The high performance of the LSTC-modified cathode was ascribed to its good CO2 reduction ability, as evidenced by infrared and temperature-programmed surface reaction studies. Furthermore, LSTC was believed to play a role not only in serving active sites for the CO2 reaction, but also in relieving stress induced by Ni oxidation, thus stabilizing the cathode structure. Direct electrolysis in the absence of H2 further confirmed that the LSTC modified cell exhibits better performance. On the contrary, the other LST-modified cells and a CaO-modified one suffered from severe electrode polarization. These results present the potential to construct a cell combining B-site doped LST oxides with Ni cermet in a SOEC for energy conversion.

Nonlinearity Manipulation for Highly Efficient Modal Phase‐matched Second Harmonic Generation on Thin‐Film Lithium Niobate

AbstractWhile modal phase matching (MPM) shows great potential for adaptable engineering in quadratic nonlinear optical processes, poor mode overlap in traditional MPM persists as a key challenge. In this paper, a flexible domain engineering method are proposed and demonstrated for second‐harmonic generation (SHG) on thin‐film lithium niobate (TFLN). By meticulously tuning the electric field intensity applied to the poling electrodes with an Al2O3 layer, the depth of domain inversion layer in TFLN can be controlled flexibly and locally, leading to the manipulation of nonlinearity, which effectively increases the mode overlap. SHG in the dual‐layer TFLN ridge waveguides has been experimentally realized with a normalized conversion efficiency of 8300% W−1 cm−2. The flexible and fabrication‐friendly approach provides a versatile method for diverse MPM scenarios, and presents considerable potential for efficient quadratic nonlinear processes.

Thermoelectrochemical formation of a solid electrolyte interphase on a silicon negative electrode to enhance the durability of silicon-enriched lithium-ion batteries by compositional modification.

The SiO electrode interface is passivated with a SiO2 layer, which hinders the deposition of an inorganic solid electrolyte interphase (SEI) due to its high surface work function and low exchange current density of electrolyte decomposition. Consequently, a thermally vulnerable, organic-based SEI formed on the SiO electrode, leading to poor cycling performance at elevated temperatures. To address this issue, the SEI formation process is thermoelectrochemically activated. Increasing the formation temperature lowers the work function by shifting the electron energy levels and increases the exchange current density for SEI formation. Higher temperatures promote the incorporation of inorganic Li2CO3 into the SEI film, resulting from the two-electron reduction of ethylene carbonate, and hence the thermally stable SEI film leads to stable cycleability. However, excessively high temperatures cause the SEI layer to become thick and resistive, significantly increasing the polarization of the SiO electrode, which leads to a deficient improvement of cycle performance. Therefore, moderate temperature exposure is required to convert the organic SEI into less resistive, inorganic components. The implementation of a mechanism-assisted SEI formation process in pouch cells using identical materials significantly improves the cycling performance, with a 20% enhancement by the 300th cycle. Additionally, the thermoelectrochemical activation of SEI formation reduces cathodic side reactions on SiO electrodes, which helps in preventing coupled failure of the NCM electrode by mitigating intergranular cracking and preserving its structure.

Synergistic Electrode and Electrolyte Polarities Lead to Outstanding Organic Potassium-based Batteries.

Organic materials are promising as battery electrodes due to their flexible design, low cost, and sustainability. Although high electrolyte concentrations are known to suppress organic cathode dissolution, the organic cathode solubility depends on the interplay between the electrode and electrolyte polarities, which remains unexplored. Here, we elucidate the delicate interplay of electrode and electrolyte polarities to achieve stable cycling of organic cathode. Notably, we demonstrate that the solubility of low-polar organic cathodes initially increases and subsequently decreases in electrolytes with increasing polarity. In contrast, high-polar organic cathodes display increasing solubility in electrolytes with increasing polarity. When the polarities of the organic cathodes and the electrolyte are tuned, the batteries show high-capacity retention and Coulombic efficiency. For example, polyaniline||potassiated graphite (KC8) pouch cells deliver 2500 cycles with an average Coulombic efficiency of 99.85%. These new insights into low- and high-polar organic cathode behavior in electrolytes lay a theoretical foundation for designing new organic battery electrodes and optimizing their electrolyte formulation.

A Molecularly Imprinted Electrochemical Sensor for Carbendazim Detection Based on Synergy Amplified Effect of Bioelectrocatalysis and Nanocomposites.

The highly selective and sensitive determination of pesticide residues in food is critical for human health protection. Herein, the specific selectivity of molecularly imprinted polymers (MIPs) was proposed to construct an electrochemical sensor for the detection of carbendazim (CBD), one of the famous broad-spectrum fungicides, by combining with the synergistic effect of bioelectrocatalysis and nanocomposites. Gold nanoparticle-reduced graphene oxide (AuNP-rGO) composites were electrodeposited on a polished glassy carbon electrode (GCE). Then the MIP films were electropolymerized on the surface of the nanolayer using CBD as the template molecule and o-phenylenediamine (OPD) as the monomer. The detection sensitivity of CBD on the heterogeneous structure films was greatly amplified by AuNP-rGO composites and the bioelectrochemical oxidation of glucose, which was catalyzed by glucose oxidase (GOD) with the help of mediator in the underlying solution. The developed sensor showed high selectivity, good reproducibility, and excellent stability towards CBD with the linear range from 2.0 × 10-9 to 7.0 × 10-5 M, and the limit of detection (LOD) of 0.68 nM (S/N = 3). The expected system would provide a new idea for the development of simple and sensitive molecularly imprinted electrochemical sensors (MIESs).

Sensitivity of bulk electrical impedance spectroscopy (bio-capacitance) probes to cell and culture properties: Study on CHO cell cultures.

Bulk electrical impedance spectroscopy (bio-capacitance) probes, hold significant promise for real-time cell monitoring in bioprocesses. Focusing on Chinese hamster ovary (CHO) cells, we present a sensitivity analysis framework to assess the impact of cell and culture properties on the complex permittivity spectrum, εmix, and its associated parameters, permittivity increment, Δε, critical frequency, fc, and Cole-Cole parameter, α, measured by bio-capacitance probes. Our sensitivity analysis showed that Δε is highly sensitive to cell size and concentration, making it suitable for estimating biovolume during the exponential growth phase, whereas fc provides information about cumulative changes in cell size, membrane permittivity, and cytoplasm conductivity during the transition to death phase. The analysis indicated that specific information about cell membrane permittivity or internal conductivity cannot be extracted from εmix spectrum. Based on the sensitivity analysis, we proposed two alternative parameters for monitoring cells in bioprocesses: Δε1 MHz and Δε1 MHz/Δε0.3 MHz, using measurements at 300 kHz, 1 MHz, and 10 MHz. Δε1 MHz is suitable for estimating viable cell density during the exponential growth phase due to its lower sensitivity to cell size. Δε1 MHz/Δε0.3 MHz can replace fc due to similar sensitivities to cell size and dielectric properties. These frequencies are within most bio-capacitance probes' optimal operation range, eliminating the need for low-frequency electrode polarization and high-frequency stray capacitances corrections. Experimental measurements on CHO cells confirmed the results of sensitivity analysis.

Efficient method for simulating ionic fluids between polarizable metal electrodes.

We introduce an efficient method for simulating Coulomb systems confined by conducting planar surfaces. The new approach is suitable for both coarse-grained models and all-atom simulations of ionic liquids between polarizable metal electrodes. To demonstrate its efficiency, we use the new method to study the differential capacitance of an ionic liquid. Our technique is at least two orders of magnitude faster than traditional Ewald-based methods for non-polarizable surfaces, when calculating the electrostatic energy between two ions. This advancement has significant potential to enhance understanding in fields such as materials science and electrochemistry, enabling efficient large-scale simulations of Coulomb systems confined by polarizable metal electrodes.

Numerical Analysis of Geometric Influences in Tetrapolar Electrical Impedance Spectroscopy Using Monte Carlo Simulations

Abstract Electrical impedance spectroscopy is promising for intraoperative cancer diagnosis by differentiating tissue based on its electrical properties, yet faces challenges from material nonlinearities and sensor effects. In this study, 3D finite element simulations were performed to analyze geometric dependencies. Two different tetrapolar ring electrode layouts are placed on virtual piece of normal and tumor tissue with each radius varied using Monte Carlo simulations and the resulting impedances and cell constants were evaluated statistically. Additionally, a novel sensitivity quality metric is introduced, enabling the identification of the geometry more suitable for impedance measurements. The results highlight the superior sensitivity quality of the improved geometry. Future research should integrate the geometric Monte Carlo approach with variations of dielectric properties of tissue and optimizing the quality metric taking electrode polarization effects into considerations.

Stability of sputtered iridium oxide neural microelectrodes under kilohertz frequency pulsed stimulation

Objective. Kilohertz (kHz) frequency stimulation has gained attention as a neuromodulation therapy in spinal cord and in peripheral nerve block applications, mainly for treating chronic pain. Yet, few studies have investigated the effects of high-frequency stimulation on the performance of the electrode materials. In this work, we assess the electrochemical characteristics and stability of sputtered iridium oxide film (SIROF) microelectrodes under kHz frequency pulsed electrical stimulation.Approach. SIROF microelectrodes were subjected to 1.5-10 kHz pulsing at charge densities of 250-1000μC cm-2(25-100 nC phase-1), under monopolar and bipolar configurations, in buffered saline solution. The electrochemical behavior as well as the long-term stability of the pulsed electrodes was evaluated by voltage transient, cyclic voltammetry, and electrochemical impedance spectroscopy measurements.Main results. Electrode polarization was more pronounced at higher stimulation frequencies in both monopolar and bipolar configurations. Bipolar stimulation resulted in an overall higher level of polarization than monopolar stimulation with the same parameters. In all tested pulsing conditions, except one, the maximum cathodal and anodal potential excursions stayed within the water window of iridium oxide (-0.6-0.8 V vs Ag|AgCl). Additionally, these SIROF microelectrodes showed little or no changes in the electrochemical performance under continuous current pulsing at frequencies up to 10 kHz for more than 109pulses.Significance. Our results suggest that 10 000μm2SIROF microelectrodes can deliver high-frequency neural stimulation up to 10 kHz in buffered saline at charge densities between 250 and 1000μC cm-2(25-100 nC phase-1).

Electrochemical regeneration of CO2 absorbent in a microreactor

This study investigates an Electrochemically Mediated Amine Regeneration (EMAR) process for CO2 capture using a novel microreactor design. We examined gas–liquid flow patterns and bubble evolution in microchannels, analyzing the effects of flow rate and electrical current on CO2 output, Faraday efficiency, and energy consumption. A periodic electrode polarity switching protocol significantly extended reactor lifetime. While promising for small-scale applications, the EMAR microreactor faces challenges in energy reduction for large-scale implementation. This work provides insights into electrochemical microreactor design for CO2 capture, contributing to advancements in this field.

(Invited) Investigation of Co-Free Cathode Composited with Proton Conductors for Protonic Ceramic Fuel Cells with Yb-Doped BaZrO3 Electrolyte

Protonic ceramic fuel cells (PCFCs) are expected to gain significant advantage from the use of Co-free cathodes. This shift can lead to reduced manufacturing costs and enhance durability performance. In our previous study, the investigation compared the power generation characteristics of PCFCs with BaZr0.8Yb0.2O3-δ (BZYb) electrolytes utilizing cathodes composed of La0.6Sr0.4FeO3-δ (LSF) as a Co-free cathode material and La0.6Sr0.4CoO3-δ (LSC) or La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) as Co-containing cathode materials composited with BZYb. Consequently, the PCFC with an LSF-BZYb cathode exhibited lower performance compared to the PCFCs with LSC-BZYb and LSCF-BZYb cathodes due to the higher electrode polarization resistance.In this study, the challenges associated with achieving high-performance Co-free cathodes that can match the performance of Co-containing cathodes are presented, along with the mechanism to enhance performance. To improve performance, the use of La0.65Ca0.35FeO3-δ (LCaF), as a Co-free cathode, in addition to LSF, which has been reported to demonstrate high performance in SOFCs, was explored. Additionally, LCaF was composited with BaCe0.7Zr0.1Y0.1Yb0.1O3-δ (BCZYYb), known for its higher proton conductivity and easier sinterability than BZYb.When comparing the power generation characteristics of PCFCs with LCaF-BZYb and LSF-BZYb cathodes, the PCFC with LCaF-BZYb cathode exhibited higher ohmic and electrode polarization resistances. These results were attributed to weak adhesion between the electrolyte and cathode, resulting in lower performance than the PCFC with LSF-BZYb cathode.Furthermore, when comparing the power generation characteristic of the PCFC with a composite cathode of LCaF and BCZYYb with the LCaF-BZYb cathode, the ohmic and electrode polarization resistances were significantly lower. This improvement was attributed to the advanced sintering of BCZYYb and the connected percolation of the proton-conducting network. Consequently, the performance of the PCFC with the LCaF-BCZYYb cathode surpassed that of the LCaF-BZYb cathode and was comparable to Co-containing cathodes such as LSC-BCZYYb cathode.In conclusion, for Co-free cathodes in PCFCs, it is crucial to composite them with proton conductors possessing better sinterability, such as BCZYYb, to achieve higher performance.Acknowledgements: This work is based on the results obtained from a project (JPNP20003) commissioned by the New Energy and Industrial Technology Development Organization (NEDO), Japan.

Room Temperature Fluorination/Defluorination of FeFx Positive Electrode for All-Solid-State Fluoride Ion Batteries

IntroductionA fluoride battery is a new concept battery that uses fluoride ions as charge carriers. It is attracting attention as an innovative battery that surpasses lithium-ion batteries due to its high theoretical energy density. Furthermore, by making an all-solid-state battery using a solid electrolyte, it is possible to construct a kinetically advantageous battery. Iron fluoride (FeF3) is a positive electrode material with a high theoretical capacity of 712 mAh g-1. A charge/discharge capacity exceeding 500 mAh g-1 at 160 °C has been reported for a composite electrode using FeF3 powder.1 The main future challenge for FeFx positive electrode is to improve charge/discharge characteristics at low temperatures and to clarify the fluorination/defluorination mechanism. However, the currently available solid electrolytes have low conductivity, causing an increase in ohmic polarization of the composite electrode and making it difficult to evaluate the FeFx positive electrode at low temperatures. In this study, FeFx thin film electrodes were fabricated, in which the effect of ohmic polarization can be suppressed by reducing the current. Using the thin film samples, the charge/discharge characteristics and electrode reaction mechanism of FeFx positive electrodes at room temperature were investigated in detail.ExperimentalOn a LaF3 single crystal substrate (solid electrolyte), a 10 nm thick FeF3 layer and C layer were sequentially deposited as a positive electrode and a current collector, respectively, by sputtering. A PbF2 layer and a Pb foil were laminated on the opposite side of the substrate as a counter electrode. The fabricated FeFx all-solid-state thin film battery was placed in a cell that could be evacuated and heated, and then charged/discharged at the desired temperature and current rate (1C was defined 712 mAh g-1, the theoretical capacity of FeF3). The discharge/charge cutoff-voltage was set to -1.5/1.5 V or -2/3 V (vs. PbF2/Pb). The FeFx electrode reaction was electrochemically analyzed by steady state polarization and AC impedance measurements. The FeFx thin film structure was investigated by SEM and XPS. The electronic state, coordination structure, and crystal structure of the FeFx during charging/discharging were analyzed by operando XAFS/XRD using synchrotron radiation (SPring-8/BL28XU).Result and DiscussionThe experimental results are summarized below.(1) The reversible capacities at 25 °C and 0.1C were 380 and 629 mAh g-1 (53% and 88% of the theoretical capacity) for the voltage ranges of -1.5/1.5 V and -2/3 V, respectively. No capacity degradation was observed up to 30 cycles in the cycle test at 25 °C and 0.1C.(2) The electrode reaction current was proportional to the exponent of the overpotential. This relationship is known as Tafel behavior and indicates that the fluorination/defluorination of the FeFx positive electrode is rate-limited by the charge transfer process.(3) During discharging/charging, reversible changes in Fe valences (Fe3+, Fe2+, Fe0), atomic bonds (Fe–F, Fe–Fe), and crystal structure (FeF3, FeF2, Fe) were observed. The reaction capacity determined from the change in average Fe valence and the reversible capacity measured electrochemically were almost equal.(4) These results indicate that the FeFx positive electrode can be charged/discharged (fluorinated/defluorinated) at room temperature with high capacities close to the theoretical value, at practical current rates, and with excellent cyclability.AcknowledgementThis work was supported by the New Energy and Industrial Technology Development Organization (NEDO) under RISING3 project (JPNP21006), Japan.

(Invited) Effect of Electrode Polarization on Oxide Ion-Conducting and Proton-Conducting Electrolytes

Polarization at the electrode/electrolyte interface has been a central issue in electrochemistry, which is also true for high-temperature ion conducting devices. Charge transfer limitation does not simply apply to such interfaces where electrons make direct exchange. Thus, overpotentials have been discussed in terms of a shift of local oxygen activity from equilibrium with the gas phase [1]. This approach has been successful in explaining phenomena at high temperature electrodes, such as large chemical capacitance of mixed-conducting film electrodes and appearance of Gerischer-type impedance with porous electrodes [2,3]. As well as the effect on the electrode properties, the interface potential shift can also affect the electrolyte. The Hebb-Wagner polarization technique utilizes the polarization at ion-blocking electrode to control the chemical potential within the electrolyte. The enhanced n-type or p-type conduction of ceria-based or proton conducting oxide electrolyte under water electrolysis operations is recognized as a practical problem of reduced energy conversion efficiency. However, the effect of overpotential at practical semi-blocking electrodes (or incompletely reversible electrodes) on the electrolyte is not well understood. The aims of this study is to elucidate whether the overpotential directly defines the chemical potential shift in the electrolyte near the interface.Two kinds of approaches were taken: i.e. direct measurement of the chemical potential in the electrolyte and non-linear equivalent circuit analysis of electrochemical responses. The direct measurements were made by using embedded probes in an oxide ion conductor. Yttria stabilized zirconia powder was pressed and sintered with five platinum wires of 0.1~0.2 mm thick. Porous platinum electrodes were pasted on the both ends of the sample and direct or alternating current was applied. Electric fields between the probes/electrodes were estimated by high frequency response of ac impedance, and subtracted from the electrical potential of the probes under dc. The distribution of chemical potential of electron, and thus, of oxygen was obtained assuming constant chemical potential of oxide ion. When oxygen exchange on the cathode was blocked using a glass paste, the oxygen potential at each probe shifted to the negative direction, and the change propagated slowly from the cathode side to the anode side. The result was in close agreement with the estimation from the mobility and carrier concentration of electrons in YSZ, suggesting that the probes successfully detected the local oxygen potential. Evaluation of chemical potential shifts with half-blocking electrode is in progress.For the equivalent circuit approach, a model circuit was developed to simulate the linear and nonlinear responses of electrode/electrolyte systems. The circuit consisted of charged carrier transport lines connected by capacitors exhibiting the local defect equilibrium, as proposed by Jamnik and Maier[4]. Unlike an equivalent circuit for a regular impedance analysis, the circuit elements were defined using parameters that varied with local chemical potentials. The responses of the circuit were calculated by using a general-purpose circuit analyzer LT-SPICE that enabled analyses of transient response as well as linear impedance. It also enabled higher harmonic analyses which is useful for detecting electrolyte polarization. The developed model was tested with a simple symmetric cell with mixed conducting electrode. Qualitative correspondence was obtained regarding the oxygen potential dependence in the linear impedance and the second harmonic responses. In addition to the oxide ion conductors, proton conductor with hole and oxide ion as minor carriers were modeled. Calculations with different assumptions of the oxygen (and hydrogen) potential shift will be compared with the ongoing experiments to find the effect of polarization of a half-blocking electrode.

Anodic Dissolution Behavior of Si Electrode in HF Solution Containing H2O2

Silicon is expected to be as anode materials for lithium-ion batteries because the battery performance can be improved by increasing surface area1). Metal-assisted chemical etching (MACE) 2) is a low-cost method to form the nano-silicon structures on Si electrode, which is called silicon nanowires (SiNWs). Though this method enables the fabrication of SiNWs to increase the surface area on Si electrode, the mechanisms on MACE have not been clarified yet. This study aimed to investigate the anodic dissolution behavior of Si electrode to understand the effect of electrolyte concentrations on MACE by electrochemical methods in the HF solution containing H2O2.Electrochemical measurements were carried out by a three-electrode system. An electrochemical cell was made of a polypropylene (PP) to use HF as the electrolyte. The counter electrode was a platinum wire, and the reference electrode was a KCl-saturated Ag/AgCl electrode (SSE) fabricated by the polycarbonate that can be used in the HF solution. The Si, Ag, and Si with deposited Ag particles were used as working electrodes. The Si electrode was made from n-type silicon wafers with a (100) surface. The electrode of Si with deposited Ag particles was prepared using an electroless Ag plating. A mixture of HF and H2O2 was used as the electrolyte solution. To investigate MACE behavior, the measurements of the electrode potential, polarization curve, and electrochemical impedance were carried out at each electrode. The electrode potential of Si with deposited Ag particles was measured for arbitrary time in the electrolyte solution. The potentiostatic polarization of Si electrode was performed to investigate anodic dissolution behavior. The electrochemical impedance measurements were carried out with an AC potential amplitude of 10 mV at arbitrary DC potential. The frequency range was 100 kHz to 0.01 Hz at five frequencies per decade.In the electrolyte solution of HF containing H2O2, the electrode potential of the Si with deposited Ag particles shifted to the less noble potential at the initial stage during MACE regardless of the concentrations of electrolyte solution. The cathodic polarization curve of Ag indicated that the cathodic reactions on Ag electrode were affected by the H2O2 concentrations. The anodic polarization curve of Si electrode demonstrated that the dissolution and film formation may occur on Si electrode, which was depended on the HF concentrations. Based on these results, the electrochemical impedance measurement was performed on the Si electrode. The dissolution and oxide film formation as the anodic reactions and the reduction reactions as the cathodic reactions during MACE were discussed to determine the suitable conditions for fabrication of SiNWs on the Si electrode. References Haibin Li, Shinya Kato, Yosuke Ishii, Yasuyoshi Kurokawa and Tetsuo Soga, Nano Ex., 3, 045010 (2023).Ming-Liang Zhang et al, J. Phys. Chem. C, 112, 4444 (2008).

Quantifying Individual Electrode Polarization and Elucidating Interactive Phenomena in PEM Fuel Cells

Fuel cells (FCs) that utilize hydrogen as fuel represent a clean and advanced energy conversion technology[1]. In the past two decades, FCs have achieved significant breakthroughs through the development of key materials and components. Nevertheless, quantifying the polarization and dynamics of individual electrodes and components under operating conditions remains a challenge for FC systems, which involve multiple physical and electrochemical processes. Electrochemical impedance spectroscopy (EIS), a widely used non-invasive technique, provides extensive information about the charge transport and reaction kinetics in the frequency domain[2]. The distribution of relaxation times (DRT) method further increases the resolution of the impedance in the relaxation domain[3]. In general, various physical and electrochemical processes occurring on the two electrodes manifest as peaks in the DRT plot. The areas corresponding to these peaks quantify the resistance contributed by each respective process. However, the DRT peaks are still highly convoluted, and additional efforts are required to assign the peak contributions and further separate the resistances of cathode processes and anode processes. In this work, we designed a Pt-wire (Au-wire) micro-reference electrode/ probe (MRE), which is laminated between two PEMs (Nafion® 212), and positioned centrally within the active area. The artifact-free impedance spectra of the cathode and anode under operating conditions were recorded by utilizing the pseudo-reference electrode. The DRT functions of the cathode and anode can add up to the total DRT function of the cell. Under these conditions, The ion conduction resistance, capacitance of the electrode, and contact resistance at the electrode∣∣MRE interface can be quantified. Multiple operation conditions including relative humidity of both electrodes, back pressures, and gas compositions are studied to verify the reliability of the DRT method in PEMFC research. Hence, this study offers a novel approach to guiding the development of high-performance PEMFC by quantifying the resistance associated with various processes in PEMFC components. Reference: [1] F. Xiao, T. Chen, Y. Peng, R. Zhang, Fault diagnosis method for proton exchange membrane fuel cells based on EIS measurement optimization, Fuel Cells, 22 (2022) 140-152.[2] J. Kwon, P. Choi, S. Jo, H. Oh, K.-Y. Cho, Y.-K. Lee, S. Kim, K. Eom, Identification of electrode degradation by carbon corrosion in polymer electrolyte membrane fuel cells using the distribution of relaxation time analysis, Electrochim. Acta, 414 (2022).[3] Y. Wang, N. Xu, X.-D. Zhou, Quantifying individual electrode polarization and unraveling the interactive phenomenon in solid oxide fuel cells, Next Energy, 1 (2023).

On Three Fundamental Questions Concerning Activity and Stability in Solid Oxide Cells

The aim of this presentation is to address three fundamental questions related to solid oxide cells: Are the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) indeed the rate-limiting steps in the high-performance of solid oxide cells, and how can we accurately quantify the area specific resistance of the oxygen electrode?What causes the phenomenon known as “electrochemically driven phase transformation,” and how can our understanding of this process be applied to create electrodes that are both high-performing and durable?How can we develop reliable accelerated testing protocols for both solid oxide fuel cells and solid oxide electrolysis cells, and which physical processes are being accelerated through these tests? The accurate quantification of electrode polarization in fuel cells and electrolyzers holds critical importance but presents a significant challenge due to the extensive overlap in polarizations from both the anode and cathode, whether in DC or AC measurements. This overlap creates a notable gap in our understanding of electrode behavior and in the development of electrochemical devices that are both high-performing and durable. In this presentation, I will introduce a reliable method for measuring the polarization of individual electrodes within a solid oxide cell. This technique is versatile and can be easily adapted for use in various types of electrochemical cells. Additionally, we will explore the limitations associated with using the distribution of relaxation times analysis for determining the polarization of individual electrodes.We will then address the origin for the phase transformation observed in an oxygen electrode during operation, as opposed to what is seen with thermally annealed materials. By integrating electronic conduction at the interface layer, we can significantly reduce the occurrence of phase transformation. This understanding of “electrochemically driven phase change” is crucial for the development of reliable accelerated testing protocols. Such protocols are essential in solid oxide fuel cell research to speed up the investigation of durability issues, quickly identify potential failure mechanisms, and ultimately estimate the lifespan of electrochemical cells. In our study, we compared solid oxide fuel cells operated at constant current density with those subjected to accelerated testing methods, which involve intermittent current injections. We have formulated a general accelerated testing framework, which will be presented in detail.Finally, we will present a theoretical model that underlies those three questions and can be used to address stability and durability issues in solid oxide cells. Acknowledgements: The presenter would like to thank Prof. Anil Virkar, Dr. Emir Dogdibegovic, and Dr. Yudong Wang, for their invaluable contributions to the work presented. This work was made possible through the support of projects DE-FE0026097, DE-FE0031667, DE-AR0001346, and DE-FE0032110.

Epitaxy Promotes OER Stability of IrO2-Coated SnO2 Nanoparticles in PEM Electrolysis

The transition from fossil fuel-based and CO2-intensive energy production to a future sustainable energy-based economy requires solutions for efficient energy conversion and storage for later re-electrification of green energy from a renewable source such as solar or wind power. One possibility is to convert energy from sustainable sources into chemical energy in the form of hydrogen by electrolysis of water. To meet this challenge, Proton Exchange Membrane (PEM) electrolysis is a powerful, state-of-the-art technology that uses scarce iridium-based catalysts to promote the oxygen evolution reaction (OER). To improve the sustainability and economics of the PEM process, it is necessary to minimize the use of rare and precious iridium to reduce the overall density of iridium-based catalysts for the OER.Here, we present different strategies for the synthesis of low-total iridium OER catalysts. In addition to the choice of support material and support morphology, we also consider the coating strategy to produce highly active and durable catalysts.The coating of corrosion-resistant high surface area metal oxide supports of various morphologies with an ultrathin layer of amorphous iridium hydroxide by a simple wet chemical synthesis at low temperature and ambient pressure or by a hydrothermal approach leads to the formation of interconnected amorphous hydrous iridium oxide particles. In a subsequent step, the amorphous phase is oxidized at elevated temperatures using an Adams fusion salt melt. This process allows a controllable phase transformation and crystallization to form a layer of interconnected partially crystalline IrOx nanoparticles of ≈1.5-2 nm domain size on the surface of the supporting metal oxides, thus providing an electrical percolation path in the catalyst layer.The appropriate choice of support material in terms of crystal lattice parameters can moderate the crystal growth of the iridium oxide in terms of epitaxy and significantly increase the stability due to tight anchoring to the support. The final core-shell nanostructures comprise highly active iridium oxide with total Ir weight fractions as low as 7at% on SnO2 nanoparticles, compared to a state-of-the-art benchmark catalyst with 56at% Ir supported on TiO2 nanoparticles. Transmission electron microscopy (TEM) imaging was used to elucidate the effect of different core-shell designs, particularly on iridium coating and crystallinity. Rotating disk electrode (RDE) measurements were performed to evaluate the electrochemical properties and assess the activity of respective catalyst as well as the cycling stability. Further, membrane electrode (MEA) polarization curves suggest enhanced stability of epitaxial catalyst layers.

(Invited) Alloying Element-Based Functional Interface for Magnesium Metal Negative Electrode

Rechargeable Mg batteries (RMBs), where Mg metal served as an active material of negative electrodes, are a fascinating candidate of next generation batteries owing to the advantageous properties of Mg, such as large natural abundance, cost-effectiveness, high chemical stability, sufficiently low standard electrode potential, and high specific/volumetric capacities. However, despite of the recent dramatic advancement in development of individual battery components in the last decade, there remains serious challenges that impede realization of practical RMBs. One of the significant obstacles for RMB realization is the instability of [Mg | electrolyte] interface. Such interfacial instability would lead to cell failure due to passivation and consequent three-dimensional growth of spherical Mg deposits.In this study, we investigated the surface engineering approach combined with halide-free electrolytes to achieve robust interface. Simple galvanic replacement reactions were adopted to construct an artificial interface. As the physical structure of metallic electrodes would change successively upon repeated deposition/dissolution cycling, the artificial interface was constructed with the elements those capable to form alloys/intermetallic compounds with Mg, such as Bi, In, Ge, Sn, and Zn. The mechanically polished Mg electrodes were soaked in ethereal solutions of halide or triflate salts incorporating desired alloying elements for pre-determined period, washed with anhydrous dimethoxyethane several times, dried under vacuum, then subjected as working/negative electrodes. Diglyme solutions of Mg[Al(HFIP)4]2, Mg[B(HFIP)4]2, or Mg[TFSA]2 were used as electrolytes and glass fiber filer (GF/A) was served as a separator.The element-dependent electrochemical performance was clearly observed for the galvanostatic Mg deposition/dissolution cycling. Through the systematic survey for a series of pretreatment solutions, the interface derived from ZnCl2 was found to impart outstanding performance with the weakly-coordinated anion-based electrolytes. The ZnCl2-modified Mg electrodes support exceptionally long-term galvanostatic Mg deposition/dissolution cycling over 1500h and 750h at 6.5% and 32.5% utilization ratio of 40 µm-thick Mg electrodes, respectively, while unmodified ones showed substantially inferior performance at the same experimental condition. The microscopic observation combined with chemical and structural analyses revealed that the Mg2+ conductive nature of somewhat amorphous Mg-Zn-Cl integrated interface can stabilize the interface and allow Mg deposition/dissolution beneath the interface. The above Mg-Zn-Cl integrated interface enables the construction of RMBs with sulfide-based positive electrodes. To achieve further high-energy-density RMBs by adopting high-voltage oxide-based positive electrodes, halide-free artificial interface was also developed.AcknowledgementThis work was financially supported by the NEXT Center of Innovation Program (COI-NEXT, Grant Number JPMJPF2016) and GteX Program Japan (Grant Number JPMJGX23S1) of the Japan Science and Technology Agency and Grant-in-Aid for Scientific Research (KAKENHI, JP21K05263) of Japan Society for the Promotion of Science.ReferenceMandai, U. Tanaka, M. Watanabe, “Mg–Zn–Cl-integrated functional interface for enhancing the cycle life of Mg electrodes”, Energy Storage Mater., 2024, 67, 103302.