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Investigations on Electrochemical Performance As Well As Thermal Stability of Two New Lithium Electrolyte Salts Compared to LiPF6
Authors: Murmann Patrick , Niehoff Philip , Nowak Sascha , Schmitz Raphael Wilhelm , Sartori Peter , Winter Martin , Cekic-Laskovic Isidora
Published: February 2020

The demand for lithium-ion batteries has increased tremendously in the last few decades.[1] Since many applications require a wide working temperature range, new cell materials, especially regarding the conducting electrolyte salt, are necessary. Besides the temperature stability, the alternating salt should yield comparable results regarding the electrochemical cycling, the conductivity, the ability to form a stable layer on the graphite anode and the specification to avoid aluminum dissolution of the current collector on the cathode side.[2] Additionally, a low price, decent availability and a low toxicity are desirable. Another very important aspect, regularly highlighted by the media, is the safety of the battery.

All these points have to be taken into consideration when testing alternatives for the currently used electrolyte components.[3, 4]

In this work we investigated electrolyte solutions, containing two lithium salts lithium-cyclo-difluoromethane-1,1-bis(sulfonyl)imide (LiDMSI) and lithium-cyclo-hexafluoropropane-1,1-bis(sulfonyl)imide (LiHPSI), dissolved in organic carbonate solvents.[5] These electrolytes were electrochemically investigated on graphite (Figure 1) and LiNi1/3Mn1/3Co1/3O2 (NMC) electrodes and compared to the electrolyte salt LiPF6with regard to conductivity, the electrochemical stability window, the anodic dissolution behavior vs. aluminum as well as the thermal stability behavior at 60 °C (Figure 2). Furthermore, XPS studies were carried out to investigate the influence of the salt on the composition and the thickness of the solid electrolyte interphase (SEI). Constant current cycling experiments proved the potential applicability of the investigated salts for lithium ion batteries.

Figure1:

First 23 charge/discharge cycles of graphite in a EC:DEC (1:1) (by weight %) electrolyte containing 1 M of the electrolyte salts LiDMSI, LiHPSI or LiPF6, cycled at 20°C with Li as CE and RE. The rate of the first three cycles was C/5 and for the following 20 cycles 1C. A 1 h constant potential step at 0.025 V vs. Li/Li+ was implemented into the discharge step. The potential range values from 1.5 V to 0.025 V vs. Li/Li+. Both, the de-intercalation capacity (discharge capacity) and the coulombic efficiency are plotted versus the number of cycles. The graphite electrodes had an average mass loading of about 2.5 mg.

[1] J.B. Goodenough, Accounts Chem Res, (2012).

[2] E. Kramer, T. Schedlbauer, B. Hoffmann, L. Terborg, S. Nowak, H.J. Gores, S. Passerini, M. Winter, J Electrochem Soc, 160 (2013) A356-A360.

[3] K. Xu, Chem Rev, 104 (2004) 4303-4417.

[4] R. Wagner, N. Preschitschek, S. Passerini, J. Leker, M. Winter, J Appl Electrochem, 43 (2013) 481-496.

[5] L.H. Pohl, Volker; Sartori, Peter; Juschke, Ralf, PCT Int. Appl. (1997), WO 9731909 A1 19970904.
Improving Lithium-Air Batteries By Accurately Predicting Lithium Diffusion Coefficients
Authors: Kane Marie , Ward Donald , Templeton Jeremy , Jones Reese , Reyes Karla
Published: February 2020

Abstract

Li-ion battery technology has reached a point in which the energy density can only be increased incrementally and will not meet the needs of future electric vehicles, portable power and military applications. Metal/air batteries have the potential to yield much higher energy density due to the elimination of the cathode to make a lighter battery package and the unlimited availability of oxygen from the environment. In particular, lithium-air batteries are especially attractive due to a high theoretical energy density. However, there are numerous challenges that must be solved before these batteries can be used commercially. In particular, the performance of the battery is highly dependent on the transport of lithium through the electrolyte.

Here, we investigate the effect of carbonate-based electrolyte chemistries on the lithium diffusion coefficient by molecular dynamics (MD) simulations and electrochemical cyclic voltammetry (CV) measurements. Typically, CV measurements have been shown to greatly underestimate the diffusion coefficient (see figure) for the Li-air battery by three orders of magnitude.

We will present a new method to utilize CV measurements to accurately predict lithium diffusion coefficients in Li-air batteries and also propose a new porous silica cathode material to maximize the diffusion of lithium and lithium oxide reaction products for maximum battery efficiency. The improved diffusion of these species is supported by modeling results.

Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000
Surface Film Characterization of a Li-Air Cell Using Electrochemical Impedance Spectroscopy
Authors: Stephens Jamal Frederon , Weatherspoon Mark H. , Nelson Ruben , Moss Pedro L.
Published: February 2020

The development of Li-air batteries has presented many problematic shortcomings that effect the cyclability and efficiency of the cell. More specifically, it is known that as the cell cycles the discharge capacity of Li-air gets much smaller than its theoretical capacity. This can come from both the irreversible and reversible products that build up on the cathode surface, inhibiting the flow of reactants to the active surface. [1] This paper provides preliminary results on a technique used to characterize the surface film build-up on the cathode of a Li-air cell. The surface film capacitance and resistance are extracted from high frequency impedance measurements of the cell.

Experimental Method

The Gamry Reference 3000 was used to perform electrochemical impedance spectroscopy (EIS) measurements on a Li-air cell employing a porous carbon cathode with a electroless deposited nickel and molybdenum (NiMo) composite that was also electrolytically oxidized. The Nyquist plot of the pre-discharge data is presented in Figure 1. The inset in Figure 1 shows a small semi-circle in the high frequency region which is indicative of a film on the surface of the cathode. In this region an associated interfacial capacitance (Cint) and interfacial resistance (Rint) was investigated.

Results

The effective capacitance (Ceff) of the film on the cathode was calculated from the high frequency reactance data in Figure 1 based upon equation (1) and is plotted in Figure 2 and 3 [2]. Zj is the imaginary impedance, and ω is the angular frequency. Cint can be approximated in Figure 2 where Ceff is seen to be frequency independent. Equation (2) was then used to determine Rint with respect to Cint and Ceff. From the calculated Cint and Rint values, the characteristic frequency (fRC) and relaxation characteristic frequency (fC ) were determined using equations (3) and (4) where Re is the solution resistance. These two frequencies are identified in Figure 3 versus Ceff and can be used to help verify the correctness of Cint and Rint. For future work, this technique will be applied to EIS data obtained both after discharge and charge to extract the full set of cathode interfacial parameters versus cycling.

Acknowledgement

This work was supported by the FREEDM ERC program of the National Science Foundation under award number EEC-08212121.

References

Girishkumar, G, B. McCloskey, A. C. Luntz, S. Swanson, and W. Wilcke. "Lithium-Air Battery: Promise and Challenges." Physical Chemistry (2010): 2193-2203. Web.

Orazem, Mark E., and Bernard Tribollet. "Methods for representing Impedance."Electrochemical Impedance Spectroscopy (2008)
LiCoPO4 and Li2CoSiO4 Nanoparticles Synthesis by Supercritical Fluid Process for Lithium Ion Battery Application
Authors: Devaraju Murukanahally Kempaiah , Truong Quang Duc , Honma Itaru
Published: February 2020

In recent years, transition metal polyanionic compounds such as phosphates and silicates are considered as promising positive electrode materials for lithium-ion batteries. These members of the lithium metal polyanion family have been a major focus of research for reasons of high safety, low cost and low environmental impact. Among, metal phosphate cathode material, LiCoPO4 electrode materials have received considerable attention for the application in lithium secondary batteries due to its high stability, non toxicity, good charge/discharge capabilities and cheap cost. Recently, polyanion-containing frameworks are of great research interest in the area of lithium ion batteries, because, these materials can reversibly insert/extract two lithium ions per formula unit would help to increase the capacity and energy density. Regarding this, silicates with the general formula Li2MSiO4 (M=Mn, Fe and Co) have been considered as potential lithium insertion/ extraction cathodes. Among silicates, Li2CoSiO4 is an attractive cathode due to its high operating voltage. In this work, we have used supercritical fluid process for the synthesis of LiCoPO4 and Li2CoSiO4 nanoparticles and are investigated for lithium ion battery applications. LiCoPO4 and Li2CoSiO4 nanoparticles were synthesized from LiOH.H2O/Lithum acetyl acetonate, CoCl3.6H2O/C4H6CoO4.4H2O and H3PO4/TEOS in molar ratio of 1:1:1 for LiCoPO4 and 4:1:1 for Li2CoSiO4 nanoparticles via supercritical fluid process. Cheap solvents such as water, ethanol and water-ethanol mixed solvents were used to prepare precursor solutions and reducing agents were also used in the synthesis. Supercritical synthesis was carried out at 400-450oC for about 5-10 min of reaction time. Then, the products were washed thoroughly in water and ethanol followed by vacuum dry for overnight. The as-synthesized LiCoPO4 and Li2CoSiO4 nanoparticles showed 20-150 nm in diameter, which was analysed by transmission electron microscopy (TEM). Further, LiCoPO4 and Li2CoSiO4 nanoparticles were systematically characterized using powder X-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM), scanning transmission electron microscopy (STEM), energy dispersive spectroscopy (EDS) and charge-discharge measurements. The discharge capacities of 80-110 mAhg-1 for LiCoPO4 nanoparticles and 40-110 mAhg-1 for Li2CoSiO4 nanoparticles were observed. The electrochemical property was measured by galvanostatic discharge method using 1M LiPF6 mixed in EC:DEC as electrolyte. The electrode was prepared using 10 wt% PTFE as binder. The weight ratio of active material:AB:PTFE are 80:10:10.
Enhanced Electrolyte Utilizing Fluorocarbon Additives
Authors: Wijaya Olivia , Rinaldi Ali , Yazami Rachid
Published: February 2020

Perfluorocarbons, the main component of artificial blood, have been investigated as additives for lithium oxygen battery due to their ability to absorb significant volume of oxygen. They are expected to increase the amount of oxygen dissolved in the electrolyte and therefore decrease the polarization during discharge. In this work, fluorocarbon compound (PFC) is incorporated in the ether electrolytes and its performance is tested in lithium oxygen battery. Our results demonstrate that addition of PFC improves the discharge voltage significantly up to 200mV for 20% PFC addition in 0.1 M LiClO4:TEGDME electrolyte. We also discover that the effect of PFC additives is more pronounce at thicker electrode, signifying that PFC additives might enhance the concentration of oxygen and or diffusion of oxygen throughout the electrode thickness. To further investigate the effect of altering oxygen solubility, the pressure of the system was varied. The impedance spectroscopy demonstrates that the charge transfer resistance value decreases with addition of PFC, decrease of cathode thickness and increase of pressure. Further result from rotating ring disk in order to quantify the concentration of oxygen soluble in the electrolyte and its difussion coefficient will be presented. This novel approach of utilizing the phenomenal characteristics of perfluorocarbon reveals an unconventional approach towards enhancing the lithium O2 battery performance.
Effective Methods to Screen out Li-Ion Cells with Subtle Defects
Authors: Jeevarajan Judith A , Jung Kwang , Chung Jae Sik , Park James , Martinez James
Published: February 2020

Several methods are currently being used to predict and reject lithium-ion cells for soft or subtle shorts. Some of the methods include self-discharge tests at full SOC, self-discharge tests at 0% SOC, self-discharge tests at very low SOC, etc. An elaborate test program was carried out to compare these tests and determine if these are a true reflection of contaminants or other defects inside a lithium-ion cell. Cycle life testing of cells that failed the criteria established as well as destructive physical analysis with SEM and EDS analysis were also carried out on the samples tested under these different self-discharge tests protocols. In addition, traditional tests carried out at NASA-JSC to stringently screen cells for defects and internal shorts will also be discussed. This paper will address the topic of whether there are good methods to detect internal cell defects in li-ion cells.
Novel Electronic Structures within Electrochemical Cells
Authors: Selim T , Grebel H.
Published: February 2020

Electrochemical reactions have been studied since the early eighteenth century [1,2]. Two-half cells are typically connected via a permeable membrane, which enables the passage of ions, yet, limits the flow of the bulk electrolyte molecules.

We wish to electrically control the ion flow inside an electrochemical cell. One way to do so is to externally vary the potential between the anode and the cathode electrodes. However, such approach affects the surface potential of both electrodes and, thus changes overall reaction rate. This crude approach is not suitable for control since the ion current depends on the external bias nonlinearly as may be observed by Tafel plots.

We would like to operate the cell at a fix external bias, yet to be able to control only the cell’s current. In our approach a third, permeable electrode (a gate electrode) is placed between the anode and the cathode [3,4]. This mid-electrode presents an electrical barrier to the flowing ions. By applying bias to the gate electrode one may increase, or decrease this barrier, hence controlling the ion flow and as a consequence, the cell’s external current. Examples of relatively low oxidizing, permeable, gate electrode are: graphene – monolayer or a few layers of graphite – and various configurations of functionalized carbon nanotube (CNT) films. From electronic point of view, an electrochemical cell (e.g., a battery) is a two-terminal device. We turn it into a three-terminal device by introducing a central electrode – the gate electrode. Unlike other channel devices, the ions pass across the electrode in what amounts to a true large area (2-D) structure. This area may be on the orders of many cm2 or even larger. We are dealing with a multi-component current species containing both positive and negative charges.

Affecting the ionic current in electrochemical cells may be using the configuration shown in the figure. Two graphite rods are immersed in a 1% NaCl solution. A voltage is applied between the rods, Ecell. The permeable gate electrode is made of two layers of functionalized carbon nanotubes: one layer is made p-type, while the other is made n-type. This composite gate electrode constitutes a permeable p-n junction. We bias the junction(s) with a bias potential V. We measure the cell’s current Icell as a function of V at a given Ecell.

In talk we will report on cyclic voltammetry (CV) as well as current-voltage experiments.

1. Galvani, L. (1791) Commentarii Bononiesi, 7, p. 363.

2. Volta, A., (1800) Phil. Mag., 8, pp. 289-311.

3. Amrita Banerjee and Haim Grebel, Electrochem. Commun. (2010), doi:10.1016/ j.elecom.2009.12.013.

4. Joel Grebel, Amrita Banerjee and Haim Grebel, Electrochimica Acta, 95 308–312 (2013).
Growth of Oxide Nanotube Arrays on Ti-Ni Alloy
Authors: Kim Min Su , Tsuchiya Hiroaki , Fujimoto Shinji
Published: February 2020

Ti-Ni alloys exhibit unique shape memory effect and superelasticity, which render them a wide range of applications in biomedical fields, such as dental and orthopedic as well as cardiovascular devices. However, it has been reported in-vivo implantation of Ti-Ni alloys caused corrosion after a few months. The corrosion will not only degrade mechanical properties of Ti-Ni alloys but also lead to the release of harmful Ni ion. In order to avoid this problem, many surface treatments have been proposed. In the present study, the growth of oxide layers on Ti-Ni alloys was examined using anodization.Materials examined were Ti-Ni alloys fabricated bya conventional arc melting technique. Anodization of Ti-Ni alloy was carried out in ethylene glycol containing 0.25wt.% NH4F and 1.5wt.% deionized water. The anodizations were carried out at various voltages at room temperature in a two-electrode configuration with a Ti-Ni alloy as a working electrode and platinum gauze as a counter electrode. After the anodization, the morphology, crystal structure, composition of the substrates and oxide layers formed were characterized.SEM observations revealed that the morphology of resulting oxide layers strongly depended on applied voltage and chemical composition of Ti-Ni alloy. Under optimized conditions it was found that tubular oxide layers could be formed on the substrates.
Electrochemical Palladium Deposition on Titanium Nitride, Ruthenium and Palladium Substrates for Nanoscale Device Fabrication
Authors: Sahin Asli , Cotte John M , Huang Qiang , Baker-O'Neal Brett
Published: February 2020

With the advances in device fabrication, scaling of electrochemical techniques to molecular scale is becoming essential. Electrochemical deposition of metals in devices with nanoscale dimensions can play an integral role in reducing critical dimensions of such devices. This study focuses on electrochemical deposition of palladium on substrates such as titanium nitride, ruthenium and palladium that are commonly used in manufacturing of nanoscale devices such as nanochannels and nanopores.

Palladium is used in many applications ranging from electrochemical biosensors to catalysts in electronic and optoelectronic devices (1, 2). Electrochemical deposition of palladium is preferred in many of these applications as a result of ease of manufacturing, controllability and economics.

Cyclic voltammetry and hydrodynamic linear sweep voltammetry were used to understand the effects of electrolyte composition and operating conditions on palladium electrodeposition. Blanket samples of titanium nitride, ruthenium and palladium were plated to study nucleation of palladium on these substrates. It was observed that high nucleus densities could be achieved on all three substrates under optimal operating conditions and electrolyte compositions. Ruthenium and palladium substrates behaved similarly in the electrolytes used in this study whereas titanium nitrite behavior was different.

To study palladium plating in small openings two test vehicles were designed. The first test vehicle (panel A) had metal lines buried under oxide layers and the second test vehicle (panel B) had metal lines without the oxide capping layer. Transfer of knowledge from beaker-scale experiments on blanket samples and electroanalytical techniques to plating in these test vehicles will be presented.

References

1. M. Schlesinger, Modern electroplating, Wiley (2010).

2. L. Shaidarova, I. Chelnokova, A. Gedmina, G. Budnikov, S. Ziganshina, A. Mozhanova and A. Bukharaev, Journal of Analytical Chemistry, 61, 375 (2006).

Figure 1. Schematic of test vehicles for palladium plating.
Effect of Copper Particle Size on the Product Distribution of the Electrochemical Reduction of CO2
Authors: Ethier Jeffrey , Kalu Egwu Eric
Published: February 2020

Electroless copper electrocatalyst is explored in the electrochemical reduction of carbon dioxide for renewable energy generation. Non-conductive carbon toray paper is made active for electroless plating with Pd-ink and is followed by the elctroless deposition method where copper particle size is varied with deposition time. The copper catalyzed toray paper is utilized as the cathode in the electrolytic reduction of carbon dioxide. The feasibility of using the electrocatalyst as prepared to electrochemically reduce carbon dioxide is first determined from cyclic voltammogram. The electroless copper particle size deposited on the working electrode for CO2 reduction is estimated via SEM. It was found that the electrocatalyst prepared by electroless deposition was active for the CO2electrochemical reduction reaction. Particle size is shown to have an effect on current measured from the reaction. Results will be presented on the product distributions resulting from use of different electroless Cu particle sizes for the reduction of carbon dioxide. Detailed characterization of the electrode material will also be presented.

Electroless copper electrocatalyst is explored in the electrochemical reduction of carbon dioxide for renewable energy generation. Non-conductive carbon toray paper is made active for electroless plating with Pd-ink and is followed by the elctroless deposition method where copper particle size is varied with deposition time. The copper catalyzed toray paper is utilized as the cathode in the electrolytic reduction of carbon dioxide. The feasibility of using the electrocatalyst as prepared to electrochemically reduce carbon dioxide is first determined from cyclic voltammogram. The electroless copper particle size deposited on the working electrode for CO2 reduction is estimated via SEM. It was found that the electrocatalyst prepared by electroless deposition was active for the CO2 electrochemical reduction reaction. Particle size is shown to have an effect on current measured from the reaction. Results will be presented on the product distributions resulting from use of different electroless Cu particle sizes for the reduction of carbon dioxide. Detailed characterization of the electrode material will also be presented.
Characterization of Micro-Porous Layer of Gas Diffusion Layer of a PEM Fuel Cell Via X-Ray Tomographic Microscopy
Authors: Odaya Silvia , Philips Ryan , Phillion Andre , Hoorfar Mina
Published: February 2020

Abstract

The gas diffusion layer (GDL) performs key functions within the fuel cell, providing access for hydrogen and oxygen gas to flow to and from the catalyst layer, conducting electricity and heat, and removing water byproduct. The GDL is commonly treated with a micro-porous layer (MPL) on the catalyst side to minimize contact resistance by reducing the roughness of the contact surface. The MPL also reduces mass transport losses and hence improves water management in the cell operated at high current densities [1].

This paper describes a method to characterize the GDL/MPL porous structure and their interface using X-ray micro-computed tomographic (µ-CT) microscopy. µ-CT is a relatively new tool that is used for acquiring the internal three-dimensional structure of both fully dense and porous materials [2]. In this work, the structure of the MPL/GDL is evaluated via µ-CT for various GDL samples (such as Toray TGP060 and EP40) loaded with different MPL loadings. Figure 1 shows a 3D image of the EP40 T12 GDL without the MPL (CBT 18). While previous studies have also examined the MPL layer using this technique (e.g. [1,3]), this is the first to examine the GDL/MPL interface, as well as the surface characteristics of the MPL. A new method is presented for separating, or segmenting, various phases (GDL, PTFE, and MPL). Using this methodology, the surface roughness of the MPL, and the porosity within the GDL is identified. The results show that there is considerable overlap between the GDL and MPL. Also, while the MPL has a smooth surface in comparison to the GDL, it remains relatively rough. The correlation between PTFE content, thickness of the MPL, and porosity will be discussed.

References

1. Z. Fishman, A. Bazylak (2011). Journal of The Electrochemical Society, Vol. 158, B846-B851.

2. C. Puncreobutr, P.D. Lee, R.W. Hamilton, A.B. Phillion (2012). JOM, Vol. 64, pp:89-95.

3. A. Pfrang, S. Didas, G. Tsotridis (2013). Journal of Power Sources, Vol. 235, pp:81-86.

Figures

Figure 1: EP40 T12 GDL specimen without MPL (CBT18). The PTFE can be seen between the carbon strands.
Effect of Bias-Voltage in H2/N2 plasma Treatment on Porous Low Dielectric Constant Materials
Authors: Cheng Yi-Lung , Lin Bing-Hong , Bo Tian-Cih
Published: February 2020

Porous low-dielectric constant (low-k) materials are needed for advanced technologies to improve signal propagation. Plasma treatments in interconnection fabrication have been considered to be critical steps to impact the low-k films’ properties. In this study, the degradation of the porous low-k films by plasma exposure using several kinds of treatment conditions was investigated in this study. The increase of the dielectric constant and the degradation of the porous low-k film reliability were found to be related to the decreased amount of Si-CH3 bonds and the absorbed amount of H2O or OH groups in the film. Pure H2 gas plasma treatment extracts more CH3 group from the porous low-k film and forms a larger amount of H2O or OH groups. Although N2 gas plasma treatment can suppress H2O or OH group formation, the increase of the dielectric constant is higher due to the formation of Si-N or C-N bonds. The plasma treatment using a mixture gas of H2 and N2 can obtain a balance result in the dielectric constant increase and absorbed moisture uptake. Furthermore, introducing an increasing bias-voltage on the sample electrode during H2/N2 Plasma treatment can effectively suppress the degradation of the porous low-k films.
Preparation of Porous Yttria-Stabilized Zirconia Ceramics By Thermal Removal of Potassium Iodide
Authors: Muccillo R. , Carvalho Sabrina Macedo
Published: February 2020

Near to full skeletal density ZrO2:8 mol% Y2O3 polycrystalline ceramics with different pore content were prepared by mixing different amounts of KI and thermally removing them during sintering at 1400oC. That removal of the alkali halide was evaluated by thermogravimetric and dilatometric measurements from room temperature to 1500oC.

The sintered pellets were analyzed by X-ray diffraction and X-ray fluorescence (XRF) for structural phase identification and KI content, respectively. Images of the polished and thermally etched surfaces were obtained by scanning probe microscopy (SPM) and scanning electron microscopy (SEM) for evaluation of average pore content and distribution. Electrochemical impedance spectroscopy measurements were carried out in the 300-450oC range in the 5 Hz-10 MHz frequency range.

The thermal removal of KI starts at its melting point, ~ 680oC and ends at ~ 900oC. No traces of KI are found by XRF analysis in sintered pellets.

The final linear shrinkage value is higher for compositions without KI and the temperature of maximum shrinkage rate increases for increasing KI content.

The total oxide ion electrical resistivity, extracted from the impedance plots, also increases for increasing KI content.

The SPM images observed at polished and thermally etched surfaces of sintered pellets show that the larger is the amount of KI addition, the larger is the average pore size and the smaller is the average grain size.

The effect of KI addition before sintering leads, upon sintering, dense porous ceramics with high skeletal density. Moreover, grain growth is inhibited.

The impedance spectroscopy results allowed for the evaluation of the bulk and the grain boundary conductivities. The increase in both contributions with the increase in KI addition was correlated to the observation by scanning probe microscopy of the increase of intragranular as well as of intergranular porosity. Other parameters determined after the deconvolution of the impedance plots, namely, the blocking factor, the frequency factor and the descentralization angle showed that the impedance spectroscopy technique is an important tool for the characterization of porous ionic conducting solids.
Electrocatalysis Involving High Temperature Proton-Conducting Oxides: Intriguing Experimental Observations & Theoretical Implications
Authors: Cheng Zhe
Published: February 2020

High temperature proton conducting oxides were discovered in the 1980s, and they exhibit, depending on temperature and atmosphere, conduction of proton as well as oxygen vacancy and electron hole at temperatures of ~400-900 oC. From electrocatalysis point of view, the conduction of proton together with oxygen vacancy and hole present new opportunity to tailor the electrode reaction process for high temperature electrochemical reactions, which could provide, sometimes surprisingly, new features that may benefit efficient energy conversion and chemical production. Using the electrochemical system of solid oxide fuel cells (SOFC) as an example, this presentation will show incorporating proton conducting oxides as electrolyte and/or (part of) the electrodes in SOFC leads to interesting experimental observations such as improved resistance to poisoning by hydrogen sulfide (H2S), better resistance to deactivation by deposition of carbon, and higher open circuit voltage. The theoretical implications of these intriguing observations will be discussed in terms of the possible changes in the underlying electrochemical reaction mechanisms, and the directions for future research will be pointed out.
Dependence of the Electrocatalytic Activity Towards CO2 Reduction on the Crystal Structure of TiO2
Authors: Sharma Pranav P. , Ke fu-Sheng , Zhou Xiao-Dong
Published: February 2020

Titanium dioxide (TiO2) has been investigated as a possible electrocatalyst for CO2 reduction in a polymer exchange membrane fuel cell (PEMFC) based full electrochemical cell. The electrocatalytic activity of TiO2 for CO2 reduction is discovered to strongly depend on the crystal structure. Interestingly, rutile phase shows a much higher activity for CO2 reduction than anatase, in contradiction to widely reported higher photocatalytic activity of anatase than rutile. Carbon monoxide (CO), methane (CH4), formate (HCOO-) are observed as major CO2 reduction products with ethanol (C2H5OH) in traces for rutile TiO2. This observed much higher electrocatalytic activity of rutile than anatase is mainly attributed to the exposed (110) surfaces for the case of rutile TiO2. X-ray photoelectron spectroscopy (XPS) measurements of the TiO2 gas diffusion electrodes (GDEs) before and after electrolysis also suggest the presence of surface defects such as Ti3+ and oxygen vacancies which significantly influence the electrocatalytic activity for CO2 reduction.
Investigation of Borohydride Oxidation on Graphene Supported Gold-Copper Nanocomposites
Authors: Norkus Eugenijus , Baronaite Asta , Stankeviciene Ina , Vaiciuniene Jurate , Kondrotas Rokas , Juskenas Remigijus , Tamasauskaite-Tamasiunaite Loreta
Published: February 2020

Graphene as catalyst support has incurred intense interest in fuel cell applications due to its unique, outstanding physicochemical properties. Pure gold and gold-based catalysts show an enhanced electrocatalytic activity towards the direct oxidation of borohydride. Combination of graphene and gold-based catalysts allows reducing the amount of noble metal and fabricating novel materials with high efficiency towards borohydride electro-oxidation.

In this study the graphene supported AuCu catalysts (denoted as AuCu/GR) with different Au:Cu molar ratio were prepared by microwave heating of Au(III) and Cu(II) salts in ethylene glycol (EG) solutions at 150 oC for 30 min. It has been determined that the composition of AuCu/GR catalysts depends on pH of reaction mixture. When the reaction mixture pH was 7.0, 10.1 and 12.2, the AuCu/GR catalysts having the Au:Cu molar ratio 37:1, 1:13 and 1:32, respectively, were synthesized. The morphology, structure and composition of the prepared catalysts were examined by transmission electron microscopy (TEM), X-ray diffraction (XRD) and inductively coupled optical emission spectroscopy (ICP-OES). The electrocatalytic activity of the synthesized catalysts was investigated towards the oxidation of borohydride by means of cyclic voltammetry and chronoamperometry.

It was found that Au nanoparticles were successively deposited onto the surface of graphene nanosheets. Au nanoparticles were uniform and well dispersed on the surface of graphene. The analysis of composition of the prepared AuCu/GR catalysts by means of ICP-OES confirms the presence of Au and Cu.

The Au loadings were 0.055 mg Au cm–2 in the Au/GR and 0.107, 0.091 and 0.088 mg Au cm–2 in the synthesized AuCu/GR catalysts having the Au:Cu molar ratio 37:7, 1:13 and 1:32, respectively. It has been determined that the graphene supported AuCu catalysts show an enhanced electrocatalytic activity towards the oxidation of BH4
– ions comparing with the graphene supported bare Au catalyst and graphene supported bare Cu catalyst. Furthermore, borohydride oxidation current densities at the graphene supported AuCu/GR catalysts having the Au:Cu molar ratio 1:12 and 1:32 are ~13 times higher than those obtained at the AuCu/GR catalyst with the Au:Cu molar ratio 37:1.

Acknowledgement

This research was funded by a Grant (No. ATE-08/2012) from the Research Council of Lithuania.
Enhanced Catalytic Efficiency and CO Tolerance of Novel Pt Nanostructures Prepared By Pulsed Electrodeposition for Methanol Oxidation
Authors: Chuang Chen Kang , Yeh Tsung Kuang
Published: February 2020

Innovative support-free platinum (Pt) nanostructures were developed in this study for enhancing the anode catalyst activities for proton exchange membrane fuel cell (PEMFC) application. The platinum nanostructures were directly grown on carbon paper by a pulsed electrodeposition technique. The morphology of Pt nanostructures were investigated by SEM, and shown as dendritic structures instead of nanoparticles. The sizes of the dendritic structures are 2-4 μm in length and 400-600 nm in diameter. A cyclic voltammetry analysis was carried out for characterizing the behavior of methanol oxidation on specimen bearing the Pt dendritic nanostructures in mixed 1 M methanol and 0.5 M sulfuric acid solutions. It was found that the peak current density of methanol oxidation obtained from the cyclic voltammogram on the new Pt dendritic nanostructures specimen was 263 mA/cm2.

Due to its unique morphology, the Pt dendritic nanostructures could exhibit a good carbon monoxide tolerance and high efficiency without the joined Ruthenium (Ru) catalyst. The catalytic efficiency of prepared Pt dendritic nanostructures was much better than that of a commercial Pt-black catalyst. It is noteworthy that the if/ib value of the Pt dendritic nanostructures is higher than 1.4, which is very different from pure Pt nanoparticles. The outcome signified that the novel catalyst morphology of the Pt dendritic nanostructures have remarkable CO tolerance even if Ru or other metal catalyst were not joined.
Advanced Electrocatalysts for Fuel Cells
Authors: Snyder Joshua David , Kang Yijin , Li Dongguo , Strmcnik Dusan , Markovic Nenad M , Stamenkovic Vojislav R
Published: February 2020

The primary barrier to broad applications of electrocatalyst for electrolyzers, fuel cells and metal-air batteries is lack of fundamental understanding of processes that are taking place at solid-liquid interfaces. Moreover, the cost, performance and durability of the current state of the art electrocatalysts are still not at level that would warrant commercialization of these technologies. Major catalytic improvements have been accomplished for the oxygen reduction reaction in case of Pt bi/multi-metallic alloys. It has been demonstrated that tuning of surface properties such as surface and subsurface composition, surface structure, and hence, electronic properties are critical aspects toward novel materials with unique functional properties. For instance, an increase in activity of almost two orders of magnitude compared to conventional Pt/C catalyst was reported for single crystalline Pt-Skin surfaces. This finding inspired subsequent nanoscale catalyst design aiming to synthesize more active, but also more durable catalysts. A synergy between well-defined extended surfaces and practical electrocatalysts remains to be a potent approach in development of systems with advanced catalytic properties and it will be thoroughly discussed in this presentation.

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[2] van der Vliet et al. Nature Mat. 11 (2012) 1051.
Characterisation of Anion Exchnage Membrane Fuel Cells
Authors: Smith Graham T. , Kucernak Anthony R. J.
Published: February 2020

Alkaline membrane fuel cells (AMFC) offer an exciting route to reduce the material cost of low temperature fuel cells. In contrast to traditional proton exchange membrane fuel cells (PEMFC) hydroxide ions rather than protons carry charge [1]. The resulting rise in pH increases the stability and possibly reactivity of some oxygen reduction reaction (ORR) catalysts such as Ag and Ni [2]. Critically, unlike alkaline fuel cells with a liquid electrolyte AMFC are resistant to metal carbonate precipitation and can purge themselves of carbonate anions during operation [3].

We will report techniques to characterise the electrochemical surface area of silver fuel cell electrodes and carry out in-situ measurements of the extent of membrane carbonation. We believe these techniques to be widely applicable and essential in understanding AMFC.

As well as having exceptional conductivity, silver has been shown to have an ORR price activity (£/mA) similar to the archetypal PEM catalyst platinum when in contact with alkaline membranes [4]. It is an outstanding material for both transport layers and cathode catalysts and is used widely in AMFC [5,6]. Unlike platinum, it is not possible to accurately measure the specific surface area of a silver catalyst using hydrogen adsorption or oxide formation. We will show specific surface area measurements on both full silver electrodes and silver nanoparticles down to 10nm in size, using lead under potential deposition (UPD) [7]. Though UPD is a well-known technique, its use on nanoparticles has recently been the subject of increased attention [8]. Our work confirms that lead UPD on silver nanoparticles is possible for select electrolytes.

Anion exchange membranes exhibit their highest conductivity when hydroxide ions are the mobile anion. When membranes are exposed to air carbonate and bicarbonate ions form (carbonation) and conductivity is reduced. In an operating cell the reverse process occurs [3]. Previously this phenomenon has only been studied by online mass spectrometry. We will report a new electrode, which can be placed inside an operating cell, to monitor the extent of carbonation locally during both start-up and steady state operation. Using this technique we will show an optimised method for purging the membrane during start-up.

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Optimization of Gas Diffusion Layers with High-Corrosion Resistance for Pemwe
Authors: Steen Stuart M. , Zhang Feng-Yuan
Published: February 2020

An advanced polymer electrolyte membrane water electrolyzer (PEMWE), which is a reverse PEM fuel cell (PEMFC), has been an effective energy storage medium by producing hydrogen/oxygen from water with electricity from renewable energy sources because of its distinguished efficiency, high energy density, compact and large capacity. In a PEMWE, a gas diffusion layer (GDL) is located between the catalyst layer and the current collecting flow field. The purpose of the GDL is to transport electrons, heat and reactants/products to and from the catalyst layer with minimum voltage, current, thermal, interfacial, and fluidic loss. Carbon materials (carbon paper or carbon cloth), which are typically used in PEMFCs, are unsuitable for PEMWEs due to high potential of the oxygen electrode. The carbon corrosion and consumption will result in poor interfacial contacts which will degrade performance and efficiency. This presentation will highlight recent efforts to optimize the GDL properties at the anode side for high-efficiency PEMWEs. A set of GDLs with different thicknesses and a range of porosities from 30% to 80% are designed and examined with a PEMWE that was developed in lab. Both electropotential performance and impedance measurements are conducted on the different GDLs in order to evaluate the effectiveness of altering the aforementioned parameters. The microscale characterizations of all the GDLs are detailed. From the results and metrics, an optimized set of parameters for an anode-side GDL is created.

Acknowledgements

This research is supported by the U.S. Department of Energy under Grant DE-FE0011585.
Effect of Gas Diffusion Layer Structure on the Performance of Hydrogen Oxygen and Hydrogen Air Polymer Electrolyte Membrane Fuel Cell
Authors: Velayutham Guruviah , Sribollepalli Srinivas , Kalingamuthu P
Published: February 2020

Gas diffusion layer (GDL) plays a very important role in the function of fuel cell performance. Therefore, the requirements of an ideal GDL include diffusion and distribution of reactants to the catalyst layers, good in plane and thro plane conductivity. Depending on the reactants and operating current densities, the GDL must provide an optimum hydrophobicity and hydrophilicity for gas distribution, water and thermal management (1-3). In order to improve the stability of fuel cell stack, a complete understanding of GDL properties are essential. In the present work, AvCarb MGL370 carbon paper and AvCarb 1071 carbon cloth is used as a substrate material. The effect of PTFE content on the substrate and micro-porous layer on the fuel cell performance is compared in both hydrogen/oxygen and hydrogen/air. The properties of GDL were characterized by experimental setup developed in house for the measurement of gas diffusion coefficient. The properties were also measured using Capillary Flow Porosity meter.

The percentage of PTFE on the substrate is varied for both carbon paper as well as carbon cloth ranging from 5 to 20%, however the concentration of PTFE in the micro layer is fixed. The experiments are conducted by keeping constant PTFE loading in the substrate while the concentration PTFE in the micro layer is varied from 15 to 30%. The PEMFC cells were tested at 65oC and at ambient pressure with relative humidity of hydrogen and air kept at 90% and 80%, respectively. The results were analyzed from the polarization curves. The long-term stability of the cell is evaluated by operating the cell at constant current density. PTFE content in the substrate as well as micro porous layer is important for the performance of fuel cell. The current density achieved in the hydrogen/oxygen is about 1.25A/cm2 at 0.6V, while the same electrode when operated with hydrogen air is 0.5A/cm2 at the same operating voltage (GDL1). Alteration of both substrate and micro-layer PTFE concentration improve the performance to 1.6A/cm2 and 0.7A/cm2 at 0.6V for hydrogen/oxygen and hydrogen/air respectively (GDL2),Fig.1. The effect of hot pressing condition on both carbon cloth as well as carbon paper is also discussed. The pressing pressure conditions varied from 40 to 120kg/cm2. In this paper the performance of both carbon paper and carbon cloth is discussed with respect to PTFE content and pressing conditions.

Fig.1: The effect of PTFE content in the substrate and micro layer, ambient pressure, temperature of operation 65oC, H2/O2 - 100/80%, flow rate H2/O2is 300/400cc

References:

1. G. Velayutham, J. Kaushik , N.Rajalakshmi , K.S.Dhathathreyan , Fuel Cell 7, 314-318 (2007)

2. G. Velayutham, International Journal of Hydrogen Energy 36(22), 14845-14850, (2011)

3. E.Passalacqua , G.Squadrito, F.Lufrano , A.Patti, L.Giorgi . J Appl Electrochem , 31, 449-54 ( 2001)
First-Principles Computational Studies on Li-Ion Battery Cathode Materials
Authors: Saavedra-Arias Jose J. , Chitturi Venkateswara Rao , Katiyar Ram S. , Ishikawa Yasuyuki
Published: February 2020

First-principles computation methods play an important role in designing and developing new electrode materials for Li-ion batteries (LIBs). These methods are more useful to study phase transformations, electron and ion mobility processes, and transportation kinetics related to various Li-intercalated materials and investigate mechanisms underlying in the electrochemical/lithium intercalation processes [1-3].

We have examined the influence of Mn substitution by Ni on the structural, electronic, and Li-intercalation properties of LiMn2O4spinel and also identified potential layered cathode material for LIBs by performing first-principles computational studies. The calculations were performed in the local density aproximation (LDA) to density-functional theory (DFT) as implemented in the Vienna Ab Initio Simulation Package (VASP). In order to reduce the number of the plane waves required for simulating the interactions between ions and electrons, ultra-soft Vanderbilt pseusopotentials were used.

In the case of spinel cathode materials, Ni substitution was done systematically for up to 25% by replacing Mn in a Mn16O32 supercell configuration and then Li atoms were intercalated. The influence of Ni substitution on the lithium hoping pathways between the two stable Li positions was studied and Li+ diffusion pathway in the Ni-substituted Mn-O supercell structures was identified. The calculations revealed that Ni substitution for Mn in LiMn2O4 indeed improved Li ion mobility [4].

To identify potential layered cathode materials for LIBs, first-principles calculations in conjunction with alloy metal method were used to evaluate average voltage, phase stability, formation energy, and Li intercalation potential for a series of layered LiMO2 (M = Co, Ni, Mn, and W) systems. According to the negative formation energy (-0.0535 eV), average voltage (3.48 V), and similar phase stability upon lithium extraction compared to LiCoO2, we concluded LiNi0.66Co0.17Mn0.17O2as a promising layered cathode material for lithium-ion batteries [5]. Computational details and results on the spinel and layered cathode materials will be presented in detail.

References

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5) J.J. Saavedra-Arias, C.V. Rao, J. Shojan, A. Manivannan, L. Torres, Y. Ishikawa and R.S. Katiyar, J. Power Sources 211 (2012) 12.