Anode Impedance Research Articles

Analytical and numerical models for impedance of a button SOFC anode

We report a physics–based model for the impedance of a button–type anode–supported SOFC. The model includes ion and electron charge conservation equations and a Fick’s diffusion transport equation for hydrogen in the anode support layer. In the limit of small overpotentials, an analytical solution for the anode impedance is derived. For typical SOFC anode parameters, this solution is valid from open–circuit voltage (OCV) up to the cell current density of about 5 mA cm−2. Fast least–squares fitting of the model impedance to an experimental spectrum measured at OCV is demonstrated and the resulting fitting parameters are compared with literature data. A high–current numerical version of the model is also suitable for fast fitting of experimental impedance spectra.

Impact of cell design and conditioning on polymer electrolyte membrane water electrolyzer operation

Integration of polymer electrolyte membrane water electrolyzers (PEMWEs) for clean hydrogen generation requires a robust understanding of the impact cell designs and conditioning protocols have on operation and stability. Here, catalyst-coated electrode and catalyst-coated membrane cells employing Pt/C cathode catalyst layer, an IrO2 anode catalyst layer, with a platinized titanium mesh or a carbon paper with a microporous layer as the porous transport layer were developed. The impact of cell conditioning above and below 0.25 A cm−2 was investigated using advanced electrochemical impedance spectroscopy analyses and microscopic imaging, with the electrochemical response related to physicochemical processes. Operation below 0.25 A cm−2 prior to operation above 0.25 A cm−2 resulted in anode corrosion and titanium cation contamination, increasing the cell voltage at 1 A cm−2 by 200 mV compared to uncontaminated cells. Conditioning above 0.25 A cm−2 led to non-negligible hydrogen transport resistances due to cathode flooding that resulted in a ca. 50 mV contribution at 1 A cm−2 and convoluted with the anode impedance response. The presence of a microporous layer increased catalyst utilization but increased the cell voltage by 300 mV at 1 A cm−2 due to increased anodic mass transport resistances. These results yield critical insights into the impact of PEMWE cell design and operation on corresponding cell performance and stability while highlighting the need for application dependent standardized operating protocols and operational windows.

Simultaneous Regulation of Organic and Inorganic Components in Interphase by Fiber Separator for High-Stable Sodium Metal Batteries.

Uncontrollable side reactions at the metal interface have been identified as the root cause of the formation of a fragile solid electrolyte interphase, leading to irreversible sodium loss in sodium metal batteries. Here, we proposed an interface engineering strategy that employed a carboxyl functionalized cellulose separator to provide strong dipole moments and induce the cleavage of P-F bond to construct a solid electrolyte interphase (SEI) rich in NaF. In addition, we employed nuclear magnetic resonance technology confirmed that the separator with strong dipole moments prevented the reduction of organic solvents by attracting electrons, thereby inhibiting the formation of organic oligomers. SEI with high NaF content and few oligomers is smooth and robust, obviously decreasing the interface impedance of the Na anode. The symmetric Na||Na cells, equipped with the functionalized separator, efficiently operated for 1400 hours with a stable 72 mV overpotential at 0.25 mA cm-2, exhibiting low energy barrier and fast ion transport kinetics. The Na||Na3V2(PO4)3 cell also showed stable cycling performance, with the capacity remaining at 94.83 % of the initial capacity after 1000 cycles at 1C. The proposed separator could control the formation and composition of SEI, paving the way for the development of long-life sodium metal batteries.

A non-destructive heating method for lithium-ion batteries at low temperatures

Low temperatures seriously affect the performance of lithium-ion batteries. This study proposes a non-destructive low-temperature bidirectional pulse current (BPC) heating method. Different from existing heating approaches, this method not only optimizes heating frequency and amplitude but also considers the optimization of the charge/discharge pulse duration ratio. To optimize the BPC heating strategy, a precise electro-thermal coupled model is established, and a neural network is employed to delineate the relationship among model parameters, temperature, and state of charge (SOC). Additionally, the interplay between the impedance of the graphite anode and that of the full cell is analyzed by constructing a three-electrode battery. Then, a novel full-cell-oriented lithium plating criterion is introduced. Finally, based on the constructed electro-thermal coupled model, lithium plating criterion, and terminal voltage constraint, a novel non-destructive BPC heating method is proposed. The results show a significant improvement in heating efficiency compared to conventional BPC heating. Especially for high SOCs, the heating power is increased at least 8 times. When the battery SOC is below 40 %, the average heating rate from −10 °C to 10 °C is 11.28 °C/min. Even at 90 % SOC, the heating rate remains at 2.88 °C/min. Furthermore, the capacity and impedance of a battery at 50 % SOC exhibit no significant changes after 60 heating cycles using the optimal BPC heating strategy at 100 Hz. These findings show that the optimized method proposed in this study has high heating efficiency and no damage to the battery.

In-situ analysis of cathode and anode impedances to probe the performance degradation of lithium–sulfur batteries

Improving the performance of Li-metal batteries with sulfur cathodes while maintaining high sulfur loading (>4 mg cm−2) and low electrolyte-to-sulfur ratio (<10 mg μL−1) is a significant challenge. These conditions often result in poor battery cycle life and sudden capacity failure during charge-discharge processes. To address this challenge, we investigate Li–S batteries in a three-electrode (3E) configuration, utilizing optimized materials and geometries for the reference electrode. This configuration allows us to independently track the in-situ variations in cathode and anode resistances, along with the electrode potential difference, unveiling the root causes of abrupt battery failure during cycling. Our 3E approach uncovers previously unnoticed distortion phenomena in the electrochemical impedance spectra of both the cathode and anode, providing essential indicators of imminent micro-short circuiting. Supported by simulations using the finite element method and equivalent circuit analysis software, these insights clarify that the primary reason for battery failure is the growth of Li dendrites and their subsequent contact with the cathode, rather than a sudden increase in cell impedance. By substituting traditional Li foil with a specially designed Li anode less susceptible to Li dendrite growth, we achieve reduced anode resistance and improved battery capacity and cycle life stability. This research reveals the potential of the 3E configuration for precise in-situ impedance monitoring, which is valuable for devising strategies to enhance cycle stability.

Electrode-Resolved Impedance and Potential Measurements to Investigate the Rate Capability of High Loading Micron-Silicon Anodes in All-Solid-State Batteries

In order to enable the market introduction of all-solid-state batteries (ASSBs), several anode concepts are under investigation to increase the energy density compared to graphite anodes, while maintaining long cycle-life. At the same time, the production costs must be the same or even reduced in comparison with classical lithium-ion batteries. Besides lithium metal as anode material, which still poses significant technical challenges, more and more interest is being directed toward materials that form lithium alloys. One of the most attractive candidates is silicon, which has a low delithiation potential of around 0.45 V vs. Li+/Li and an attractive theoretical gravimetric capacity of 3579 mAh/gSi, which in the fully lithiated state corresponds to a volumetric capacity of ≈2190 mAh/cm3, even higher than the one reachable by lithium metal.Silicon-containing anodes are already being introduced in classical lithium-ion batteries. For that system, the lithium-ion conduction across the anode electrode is mainly provided by the liquid electrolyte that fills the pores of the electrode, while the electrical conduction is enhanced by the addition of graphite and/or conductive carbon additives, because crystalline silicon is only a semiconductor. As is well known, the major issue with silicon anodes is the volume expansion of 300% upon lithiation of silicon, inducing severe cracking of µm-sized silicon particles as well as continuous solid electrolyte interphase (SEI) growth, both of which lead to a fast capacity fading.The intrinsic solid nature of all-solid-state batteries opens the possibility of a new electrode concept that has the potential to mitigate the above-mentioned drawbacks of silicon. Indeed, the silicon particles themselves could be used for the lithium-ion conduction across the electrode, so that anode electrodes can be prepared that are free of electrolyte, thereby largely restricting SEI formation to the anode/separator interface .In this work, anodes with >90% weight of silicon are prepared by coating a water-based slurry that contains microscale silicon particles (ca. 2-4 µm diameter) as active material, the rest being made up of LiPAA and NaCMC as binders as well as C65 carbon as conductive additive. The combination of the water-based process and the microscale dimension of the silicon particles make this electrode concept easily scalable and cost-effective.To reach a competitive areal silicon anode capacity of 3 mAh/cm2, with the full utilization of the silicon, only a thin layer is needed (pristine thickness of 8 µm when the anode porosity is 50%). The thickness variation and the anode morphology are being investigated by examining cross-sectional scanning electron microscopy (SEM) images at different states-of-charge (SOC). After the first complete lithiation, the anode consists of a fully dense and compact Li3.75Si alloy block of 15 µm thickness. Upon its complete delithiation, a peculiar porous columnar morphology is formed that on account of the void space between the columns and within the columns retains a thickness of 11 µm. This peculiar morphology was recently reported in the literature (Tan et al., Science 373 (2021), 1494–1499); and seems to be reversible during subsequent lithiation and delithiation processes. For example, our half-cells utilizing an InLi counter electrode and operating at 70 MPa have more than 80% capacity retention after 350 cycles at a rate of C/5, with an average coulombic efficiency higher than 99.9%. The reason for this promising stability could be attributed to the minimal loss of lithium into the SEI, which is limited to the geometrical contact area between the silicon and the solid electrolyte separator.To better understand the electrochemical and mechanical characteristics of these silicon anodes, they are investigated in full-cells, utilizing an NCM composite cathode, while the anode potential is monitored by using a gold wire micro reference electrode (µ-RE) which was recently developed in our group (Sedlmeier et al., J. Electrochem. Soc. 170 (2023) 030536). This specific µ-RE, which has a defined and stable potential, allows to precisely deconvolute the potential curves of cathode and anode, and also to measure and separate the impedance contributions of the two electrodes. Therefore, the impedance of the silicon anode is measured at different SOC to understand how the contact resistances, the charge-transfer resistance, and the solid-state diffusion affect its overall impedance. In particular, the low impedance at high SOC enables to reach a good fast charging rate capability up to 1C without lithium metal plating and dendrite formation, as reported in Figure 1. However, in order to explore whether the fast charging goal of 4C (80% of the capacity in 15 minutes) can be reached, the influence of the carbon additive, operating temperature and applied pressure are being investigated. Figure 1

AMIDR: A Complete Pulse Method for Measuring Cathode Solid-State Diffusivity

Lithium transport within cathode material is driven by diffusion. While fast cathode diffusion is desired, solid-state diffusivity is one of the most difficult cell material properties to measure. However, it is an important property to understand as cathode capacity is often limited by poor diffusivity. GITT (Galvanostatic Intermittent Titration Technique) is a common diffusivity measurement method. This technique is accomplished by applying a series of constant current pulses to a cell and analyzing the voltage response. While easy to use, this method only measures near the surface of the active material by assuming that it takes the geometry of a semi-infinite plane. This approximation means that GITT is only accurate when measuring the very initial voltage response. However, the voltage response cannot be analyzed too early as this may amplify error produced by CDL (double layer capacitance) and RI (interface resistance) found at the cathode-electrolyte interface which GITT does not account for. EIS (Electrochemical Impedance Spectroscopy), an AC method, is better suited for distinguishing CDL/RI from diffusivity but also struggles to evaluate diffusivity beyond the surface of active material within a reasonable time frame1. In addition, instruments capable of quality EIS are often cost prohibitive compared to DC cell testers.The voltage responses after the first few moments of a pulse are relevant when evaluating cell behaviour in real-world applications and should be considered to produce comprehensive measurements of diffusivity and its performance impact. AMID (Atlung Method for Intercalant Diffusion) is a multi-rate pulse method produced by this lab which uses a series of pulses, both short/fast and long/slow, and an approximation-free analytical model (developed by Sven Atlung) for diffusion in a spherical particle, not a semi-infinite plane2. However, AMID cannot differentiate diffusion impedance from resistance and therefore requires that resistance be considered negligible. In addition, while AMID mathematics are free of approximations, the typical multi-rate pulse protocol does require that series of pulses be approximated as singular pulses3. Lastly, the multi-rate pulse protocol limits the state-of-charge resolution of diffusivity measurements.AMIDR (Atlung Method for Intercalant Diffusion and Resistance) incorporates a GITT-style single pulse protocol with AMID-style mathematics modified to account for resistance as shown in Figure 1b). AMIDR collects pulse data with a pseudo-logarithmic time distribution. This means that the same number of data points is collected in 0.1-1.0 seconds, 1.0-10 seconds, 10-100 seconds and so on. Unlike GITT which measures diffusivity at an early arbitrary point in a pulse, AMIDR considers the entire length of a pulse comprehensively. Fractional capacity is calculated for each datapoint by dividing the actual capacity with the ideal “impedance-free” capacity expected at that voltage as shown in Fig 1c). These fractional capacity values, normalized from 0-1, are fitted to the modified Atlung model. This modified Atlung model is nearly the same as the original Atlung model except an additional term is added for capacity limited by resistance. Unlike AMID, AMIDR measures both diffusivity and resistance simultaneously by using both as fitting parameters. Lastly, while CDL is usually small enough for RI to behave simply like ohmic resistance on relatively short time scales (>0.1 seconds), AMIDR has the option to account for CDL/RI when relevant such as near the bottom of a complete discharge.AMIDR is a new diffusivity measurement method tailored for cathode material in Li-ion batteries. By combining the advantages of various previous diffusivity methods, AMIDR eliminates many of their concerns and sources of error such as sampling of only the material surface, alternate confounding sources of impedance, and expensive equipment as seen in Figure 1a). When paired with proper cell design (i.e. ultra-low single layer cathode loading to remove pore impedance and reference electrodes to remove anode impedance), AMIDR can produce high resolution diffusivity measurements with improved accuracy and applicability to real-world cell performance.References C. Deng and W. Lu, Journal of Power Sources, 473, 228613 (2020).A. Liu et al., J. Electrochem. Soc., 168, 070503 (2021).M. Doyle, J. Newman, and J. Reimers, Journal of Power Sources, 52, 211–216 (1994). Figure 1. (a) Comparison of various Diffusivity measurements. (b) The modified Atlung model for diffusion in a spherical particle. (c) How pulse data is converted to the unitless Atlung model. Figure 1

Which Exchange Current Densities Can Be Achieved in Composite Cathodes of Bulk-Type All-Solid-State Batteries? A Comparative Case Study.

The performance of bulk-type all-solid-state Li batteries (ASSBs) depends critically on the contacts between cathode active material (CAM) particles and solid electrolyte (SE) particles inside the composite cathodes. These contacts determine the Li+ exchange current density at the CAM | SE interfaces. Nevertheless, there is a lack of experimental studies on Li+ exchange current densities, which may be caused by the poor understanding of the impedance spectra of ASSBs. We have carried out a comparative case study using two different active materials, namely, single-crystalline LiCoO2 particles and single-crystalline LiNi0.83Mn0.06Co0.11O2 particles. Amorphous 0.67 Li3PS4 + 0.33 LiI particles act as a solid electrolyte within the cathode and separator, and lithiated indium acts as the anode. The determination of the cathode exchange current density is based on (i) impedance measurements on In-Li | SE | In-Li symmetric cells in order to determine the anode impedance together with the anode | separator interfacial impedance and (ii) variation in the composite cathode thickness in order to differentiate between the ion transport resistance and the charge transfer resistance of the composite cathode. We show that under the application of stack pressures in the range of 400 MPa, the Li+ exchange current densities can compete with or even exceed those obtained for CAM | liquid electrolyte interfaces.

Spatially Resolved Operando X-Ray Absorption Spectroscopy in NCA/Graphite to Quantify the Potential-Dependent Transition Metal Dissolution and Its Effect on Capacity Fading

Layered transition metal oxides like NCAs (LiNixCoyAlzO2, with x+y+z=1) and NCMs (LiNixCoyMnzO2, with x+y+z=1) are used as cathode active materials (CAMs) for high energy Li-ion batteries due to their high capacity. However, at high upper cut-off potentials, those CAMs suffer from structural instabilities, resulting in severe capacity fading and thus limiting the accessible capacity that can be obtained. Possible causes for the capacity fade at high cut-off potentials and high state-of-charge (SOC) include the (electro)chemical oxidation of the electrolyte oxidation and transition metal (TM) dissolution from the CAM surface.1 Furthermore, layered TM-oxides are known to release lattice oxygen from the near-surface region at high SOC (i.e., at ≈80% SOC when referenced to the total amount of lithium), resulting in reactive oxygen species that induce electrolyte oxidation and HF formation.2 This release of lattice oxygen results in a surface reconstruction from the pristine layered structure to a more resistive spinel- or rocksalt-like structure, thereby inducing an impedance build-up on the cathode. Diffusion of dissolved transition metals to the anode and their subsequent deposition on the anode active material particles can also have a severe effect on cell aging, as the accumulation of metal species on the graphite anode has shown to catalyze the degradation of the protective anode solid/electrolyte interphase (SEI), eventually resulting in the loss of active lithium and in an anode impedance growth. Since the dissolution of manganese is considered to have the most detrimental effect on the anode SEI compared to cobalt and nickel,3 manganese-free NCAs (e.g., LiNi0.8Co0.15Al0.05O2) might have an advantage over manganese-containing NCMs.In this study, we will examine the potential-dependent dissolution of Ni and Co in NCA/graphite cells using operando XAS, and compare it to the potential-dependent dissolution of Ni, Co, and Mn from LiNi0.6Co0.2Mn0.2O2 (NMC622) that we had determined previously by operando XAS.4 Owing to the specially designed geometry of the operando XAS cell,5 we can spectroscopically access and independently investigate the concentration and oxidation state of transition metals, both dissolved in the electrolyte and deposited within the graphite anode. This is illustrated for an NCA/graphite cell in Figure 1. We will also examine the effect of lattice oxygen release from NCA on the NCA/graphite full-cell performance by applying different techniques: We employ a three-electrode Swagelok® type T-cell with a gold wire micro reference electrode (µ-GWRE)6 to quantify the anode and the cathode impedance over the course of 100 cycles as a function of the upper cutoff voltage. In addition, on-line electrochemical mass spectrometry (OEMS)7 is applied to detect the onset SOC for the release of lattice oxygen. From these comparisons, we aim to get a detailed understanding about the influence of transition metal dissolution from NCA on capacity fade and cycle life.

Study on power generation and Congo red decolorization of 3D conductive PPy-CNT hydrogel in bioelectrochemical system

In this paper, a polypyrrole-carbon nanotube hydrogel (PPy-CNT) with 3D macroporous structure was prepared by secondary growth method. This self-supporting material with good conductivity and biocompatibility can be directly used as anode in a microbial fuel cell (MFC). The prepared material had a uniform structure with rich 3D porosity and showed good water retention performance. The effect of the mass ratio of PPy and CNT in the hydrogel were also investigated to evaluate the electrical performance of MFC. The MFC with 10:1 PPy-CNT hydrogel anode could reached the maximum power density of 3660.25 mW/m3 and the minimal electrochemical reaction impedance of anode was 5.06 Ω. The effects of Congo red concentration, external resistance and suspended activated sludge on decolorazation and electricity generation were also investigated in the MFC with the best performance hydrogel. When the Congo red concentration was 50 mg/L and the external resistance was 200 Ω, the dye decolorization rate and chemical oxygen demand (COD) removal rate could reach 94.35% and 42.31% at 48h while the output voltage of MFC was 480 mV. When activated sludge was present, the decolorization rate and COD removal rate could be increased to 99.55% and 48.08% at 48 h. The above results showed that the porous hydrogel anode had broad application prospects in synchronous wastewater treatment and electricity production of MFC.

Diagnosis of proton exchange membrane fuel cell system based on adaptive neural fuzzy inference system and electrochemical impedance spectroscopy

A new diagnostic method based on adaptive neural fuzzy inference system (ANFIS) and electrochemical impedance spectroscopy (EIS) is proposed for the proton exchange membrane fuel cell (PEMFC) system. Firstly, a new parameter identification method that combines genetic algorithm (GA) and Levenberg–Marquardt (LM) algorithm is proposed to identify the fractional-order equivalent circuit model (ECM), in which the anode impedance, cathode impedance, and mass transfer are all considered. This new method allows better exploitation of the EIS diagrams, and the internal relationships between the fault conditions and the ECM parameters are thoroughly analyzed according to it. Then, based on these relationships, a new diagnostic algorithm based on k-means clustering and ANFIS is designed to precisely identify several faults that can occur in the PEMFC, such as membrane flooding, drying, and mass transfer fault. Finally, the effectiveness of this method is demonstrated experimentally through the exploitation of EIS data under different faults and operating conditions of the PEMFC.

Plating energy as a universal descriptor to classify accelerated cell failure under operational extremes

Lithium plating is an anode-centric degradation process occurring in lithium-ion batteries resulting in irreversible capacity loss and cell failure. Temperature plays a critical role in improving the kinetics and transport, reducing lithium plating propensity. This study quantitively probes the evolution of plating with aging under temperature extremes in commercial Li-ion cells. Plating energy is proposed as a unique descriptor to quantify the extent of lithium plating and state of the electrode using operando analytics at any operating condition. Cells operated at temperature extrema (high/low) experience rapid capacity fade accompanied by a significant rise in anode impedance and exhibit plating energies greater than 1 Wh. Unfavorable intercalation kinetics at low temperatures and favorable solid electrolyte interphase (SEI) kinetics at high temperatures exacerbate anode impedance. These kinetically disparate manifestations on anode impedance adversely impact the interfacial overpotential and reversibility of plating, resulting in localized deposits and preferential stripping, ultimately promoting cell failure.

Investigating the Catalytic Requirements of Perovskite Fuel Electrodes Using Ultra-Low Metal Loadings

Solid Oxide Fuel Cells (SOFC) with La0.3Sr0.7TiO3 (LST)–yttria-stabilized ZrO2 (YSZ) anodes were prepared by impregnation of LST into porous YSZ scaffolds and then modified by Atomic Layer Deposition (ALD) of Ni, Pt, Pd, Fe, Co. and CeO2. Weight loadings as low as 0.01% of Pt, Ni, and Pd were sufficient to decrease anode impedances by orders of magnitude for operation in humidified H2 at 973 K. The effects of CeO2, Co, and Fe were less but still significant. Sintering at higher temperatures was important. Possible ways of stabilizing the metal particles and implications for developing ceramic anodes are discussed.

Enhanced performance of a Li‐ion rechargeable battery at low temperatures: Use of 3,3,3‐trifluoropropyl acetate as an electrolyte additive

AbstractThe addition of 3,3,3‐trifluoropropyl acetate (TFPA) to electrolyte in a Li‐ion rechargeable battery (LIB) provides a means for increasing the discharge performance at low temperatures as a result of the formation of a superior solid electrolyte interphase (SEI) on the graphite anode. For instance, the addition of 2 wt% TFPA to the electrolyte significantly increased the cycle stability and the discharge capacities at low temperatures (‐10°C) even at current rates of 3 C. The SEI films formed on the graphite anodes were characterized by electrochemical and spectroscopic techniques and by computational analysis. Although the formation of LiF on the anode has been recognized, present research revealed that the decomposition of TFPA on the anode surface resulted in the formation of an SEI layer consisting predominantly of organic fluorides. This layer suppressed the decomposition of the electrolyte resulting in a decreased anode impedance and an increase in cycle stability and discharge capacity at low temperatures.

(Invited) Extend Calendar Life of Si Based Li-Ion Batteries

Although significant progress has been made in increasing the specific energy and cycle life of silicon (Si) based Li-ion batteries (Si-LIBs), calendar life of these batteries is still less than two years, which is far less than the 10 year life-time required for electrical vehicle applications.1 Graphite anodes undergo only ~10% volume change during cycling, so SEI formed during the initial process does not experience significant mechanical stress during the subsequent cycles. However, the SEI layer formed on Si anodes experiences tremendous mechanical stress during repeated cycles. Most SEI layers formed on the Si surface in conventional electrolytes break down, exposing fresh, lithiated Si or SiO to electrolyte. Thus, significant Si corrosion and electrolyte consumption continuously occur during battery storage, especially in the fully lithiated conditions and elevated temperatures required for the accelerated calendar life test.2 As a result, the impedance of the Si anode will increase much faster than a graphite anode and shorten its calendar life, as observed in nearly all Si-LIBs. In this work, we will report the results of our recent investigation on the fundamental mechanism behind the limited calendar life of Si-LIBs. Several approaches that alleviated degradation of SEI layer and increasing cycle life of Si-LIBs will be discussed, including the nanostructured Si designs that can minimize the external size expansion, minimized particle surface area, and formation of stable SEI layers by localized high concentration electrolyte tailored for Si anode. The combination of these methods can largely extend the calendar life of Si-LIBs and accelerate the application of these batteries for large-scale electrical vehicles and consumer electronics applications.

Modification of carbon based cathode electrode in a batch-type microbial fuel cells

Microbial fuel cells (MFCs) provide an efficient way to utilize energy from biomass while it offer advantages such as pollution avoidance, high energy conversion efficiency, and flexibility for a wide range of applications. MFCs have gained much attention and are considered as current research hotspot in bioenergy. To further improve the reaction area, microbial compatibility, and to increase the power generation efficiency in the design and manufacturing of a small-scale MFC, the use of 3D printing technology and the employment of a high-efficiency mixing area are proposed in this work. Carbon cloth is employed as the electrode substrate of the MFC. Moreover, the carbon nanotubes and graphene dispersions are used to modify the carbon electrode plates. The results show that the graphene modified carbon cloth electrode MFC has a maximum power density of 199.24 mW m−2 at a current density of 1.52 A m−2. While the unmodified carbon cloth electrode MFC has a current density of 0.63 A m−2. This indicates that the graphene modified carbon cloth electrode MFC achieved a maximum power density of about 232% compared with the unmodified carbon cloth electrode MFC. The lowest internal resistance obtained was 0.66 kΩ when the electron transfer medium was accounted. This indicated a low anode impedance which is desired in MFC systems. Result revealed that the graphene modified carbon cloth electrode is the best electrode material based on the enhanced power density and lowered internal resistance. The results of the work aims to provide insight in the improvement of the MFC electrode modification technology.

An Investigation of the Electrochemical Processes in Magnesium-Sulfur Batteries By Operando Spectroscopy Techniques

The high energy density conversion cathode material like sulfur (S) is promising in combination with Mg anode because of the high theoretical capacity (1672 mA h g-1 or 3459 mA h cm-3), as well as their wide abundance and low cost. Magnesium/sulfur (Mg/S) batteries face drawbacks like large over potential between charge/discharge cycles, rapid capacity fading, poor cycling efficiency and low kinetics. Most of these issues are strongly correlated with the electrolytes used in Mg/S cells and the polysulfide shuttling to the Mg anode side. In spite of recent achievements in Mg/S rechargeable batteries, especially in the field of electrolyte and cathode development, a fundamental understanding of the battery chemistry is still missing and needs further exploration.In the present study, the electrochemical performances of a room-temperature Mg/S battery with a newly designed sulfur composite cathode and a non-nucleophilic magnesium electrolyte (Mg[B(hfip)4]2 in DME solvent) have been investigated [1, 2]. Operando Raman spectroscopy is employed to track the formation of polysulfide species at the cathode of Mg/S cells during the charge/discharge process, while density functional theory (DFT) calculations are used to correlate the Raman modes with a series of polysulfide species (Sx n-, x = 1–8). Operando Raman spectroscopy proves the chemical transformation from elemental sulfur (S8) via long and short chain polysulfides to magnesium sulfide upon discharge and conversion back to elemental sulfur during charging. Furthermore, the spectral measurements indicate the formation of a nano-crystalline magnesium sulfide with a cubic zinc blende phase at the end of discharge [2].Impedance studies of the Mg anode and sulfur cathode electrodes of Mg/S cell under static conditions prove that the total cell impedance is mainly dominated by the anode/electrolyte interface due to the formation of a highly resistive passivation layer. Indeed, upon cycling of the Mg/S cell, the amount of polysulfide species migrated to the anode side via dissolution significantly contributes to the increase of the anode overpotential and resistances. Furthermore, under dynamic conditions, the oxidation/reduction potentials applied to the electrodes dramatically influence the anode impedances due to the oxidation/reduction of chemical species present at the Mg anode interphase. The chemical composition, thickness and morphology of the Mg anode/electrolyte interphase layer is investigated by x-ray photoelectron spectroscopy (XPS) with depth profiling and focused ion beam scanning electron microscope (FIB-SEM) techniques. Reference s [1] Z. Zhao-Karger et al, Journal of Materials Chemistry A, 2017, 5, 10815-10820.[2] Vinayan, B. P. et al., Journal of Materials Chemistry A, 2019, 7, 25490-25502. Figure 1

SOFC Analysis through the Distribution of Relaxation Times for Various Preparation and Operation Parameters

Introduction In order to extend the lifetime of SOFC systems, it is important to separate and analyze the factors affecting their performance and durability quantitatively. The reactions at both electrodes of fuel cells in power generation are rather complicated. Electrode resistances consist of several factors. Here in this study, we systematically evaluate their dependence on fuel composition, operating temperature, and constituent materials, by measuring various correlations through impedance measurements and the distribution of relaxation times (DRT) analysis [1-4]. The aim of this study is to systematically analyze electrode reaction processes of SOFCs for various preparation and operation parameters. Experimental First, electrochemical performance was measured while lowering the operating temperature by 50°C but keeping the oxygen concentration for the cathode and the hydrogen concentration for the anode constant for various cells. After that, the performance was measured by varying oxygen and hydrogen concentrations with the operating temperature fixed at 800°C. Impedance measurement was performed, and each resistance component was separated by the DRT analysis. In the previous DRT studies [1-4], it is reported that the resistances caused by the transport of charges such as ions appear in the high frequency region, while the resistances caused by the interfacial electrode reactions, gas adsorption / dissociation, gas diffusion, etc. appear in the middle and low frequency regions.The materials dependency was also evaluated by using five types of cells with different materials denoted as Anode|Electrolyte|Cathode: Ni-ScSZ|ScSZ|LSM, Ni-ScSZ|YSZ|LSM, Ni-YSZ|ScSZ|LSM, Ni-YSZ|YSZ|LSM, and Ni-ScSZ|ScSZ|GDC/LSCF. The electrode thickness dependency was measured by using the cells with 40 µm, 60 µm, and 80 µm thick anode and the cells with 40 µm and 60 µm thick cathode. The electrode sintering temperature dependency was measured by using the cells with the anodes sintered at 1250°C, 1300°C, and 1350°C, and the cells with the cathodes sintered at 1150°C, 1200°C, and 1250°C. The impedance measurements and DRT analyses were performed for analyzing the temperature dependence and fuel and air concentration dependence of these cells. Results and Discussion For the cells studied, DRT peaks for the anodes appeared at around 10-0.5 Hz, 102 Hz, and 103 Hz, and those for the cathodes appeared at around 101 Hz, 103.5 Hz, and 104.5 Hz. Figures 1 and 2 show the DRT spectra obtained from the anode impedance measurements for various preparation and operation parameters. To mention a few, regarding the DRT peak at around 103 Hz, the resistance is related to the presence of ScSZ (see Fig. 1(a)), dependent on anode thickness (Fig. 1(b)), but independent of H2 concentration (Fig. 2(b)).DRT analysis of SOFCs under various operational conditions and with different cell materials was systematically conducted for model cells, and the relationship between the reaction processes and peak frequency was also investigated. Various DRT peaks related to different electrode processes will be systematically considered and classilfied. References C. Graves, S. D. Ebbesen, and M. Mogensen , Solid State Ionics, 192, 398 (2011).T. H. Wana, M. Saccoccioa, C. Chena, and F. Ciuccia, Electrochim. Acta, 184, 483 (2015).A. Leonide, V. Sonn, A. Weber, and E. Ivers-Tiffée, J. Electrochem. Soc., 155 (1), B36 (2007).H. Sumi, T. Yamaguchi, K. Hamamoto, T. Suzuki, Y. Fujishiro, T. Matsui, and K. Eguchi, Electrochim. Acta, 67, 159 (2012). Acknowledgment This study was partially supported by the Center of Innovation (COI) program (JPMJCE1318) of the Japan Science and Technology Agency. Figure 1

Effects of vertical and horizontal configurations of different numbers of brush anodes on performance and electrochemistry of microbial fuel cells

To maximize wastewater treatment and energy production by microbial fuel cells (MFCs), it is important to design the optimal anode arrangement. In this study, four brushes were tested horizontally or vertically to the cathode as the number of the anodes increased from one to four. In the horizontal configuration, adding the anodes greatly reduce electrode resistance and enhanced cell performance, showing four anodes (H4) was the best. In the vertical configuration, two anodes (V2) showed greatest performance and greatest decrease in anode resistance. Compared with one anode, maximum power increased by 59% in H4 and by 18% in V2; anode polarization resistance decreased by 95% in H4 and by 74% in V2; anode impedance decreased by 91% in H2 and by 73% in V2. Cathode resistance was relatively constant, showing adding anodes had negligible effect on it. Because diffusion resistance increases with increasing distance between an anode and a cathode, the vertical anodes should have different diffusion resistance and performances. In this study, adding more anodes vertically decreased cell performance in V3 and V4. However, in a cyclic voltammetry test, current production was substantially increased when the third and the fourth brush anodes were introduced in the both arrangements. Compared with one anode, current production increased by 200% in H4 and by 205% in V4. It shows that the external electrical input relieved diffusion resistance and increased current generation and that installing anodes away from the cathode is a good strategy to increase current production in a system with external power supply such as microbial electrolysis cell. Based on the results, we suggest the following strategy: i) install multiple anodes horizontally along the cathode; ii) install multiple anodes in the second row horizontally along the cathode; iii) install multiple anodes both horizontally and vertically if there is an external power supply.

Operando investigation of the solid electrolyte interphase mechanical and transport properties formed from vinylene carbonate and fluoroethylene carbonate

The electrolyte additives vinylene carbonate (VC) and fluoroethylene carbonate (FEC) are well known for increasing the lifetime of a Li-ion battery cell by supporting the formation of an effective solid electrolyte interphase (SEI) at the anode. In this study combined simultaneous electrochemical impedance spectroscopy (EIS) and operando electrochemical quartz crystal microbalance with dissipation monitoring (EQCM-D) are employed together with in situ gas analysis (OEMS) to study the influence of VC and FEC on the passivation process and the interphase properties at carbon-based anodes. In small quantities both additives reduce the initial interphase mass loading by 30–50%, but only VC also effectively prevents continuous side reactions and improves anode passivation significantly. VC and FEC are both reduced at potentials above 1 V vs. Li+/Li in the first cycle and change the SEI composition which causes an increase of the SEI shear storage modulus by over one order of magnitude in both cases. As a consequence, the ion diffusion coefficient and conductivity in the interphase is also significantly affected. While small quantities of VC in the initial electrolyte increase the SEI conductivity, FEC decomposition products hinder charge transport through the SEI and thus increase overall anode impedance significantly.