Materials for electrochemical energy conversion - from modular to large-scale energy generation and storage

Resume : A novel technique of electrical explosion has been used for nanomaterial synthesis by a single-step process. However, it was known that in principle the electrical explosion of Si is not possible due to the fact that applying a high voltage on Si wire having relatively high electrical resistance leads to a current flow through surface plasma formed along the wire which disables the resistive heating of Si. In this presentation, we will show the successful application of electrical explosion to silicon nanoparticle synthesis and the electrochemical properties of Si/C nanocomposites derived from the nanoparticles. The electrical explosion of Si was successfully applied using a liquid media to suppress the unfavorable surface plasma generation. The physical and chemical properties of the produced materials were analyzed by XRD, FE-SEM, HR-TEM and the performances of the Si/C composites as an anode for Li ion batteries were investigated by electrochemical charge/discharge measurements. The Si/C composites obtained under the explosion in hexane and ethanol consist of a core-shell structure with a compact graphitic carbon on the Si particle surface which cause detrimental effects against Li ion diffusion. However, we found that Si explosion in methanol minimized the formation of the carbon phase on the surface of nano-sized Si. The Si/C nanocomposite after a simple pyrolysis with carbon precursor showed a high capacity with an excellent cycle performance.

Resume : Solid oxide fuel cells (SOFCs) offer several advantages over low-temperature fuel cells, e.g., high thermodynamic efficiency, fast electrode kinetics, and direct use of hydrocarbon fuels. An interconnect provides electrical connection between adjacent cells and separates fuel and oxidant gases. The interconnects can be divided broadly into two categories – ceramic and metallic interconnects. A thin and gastight ceramic layer is deposited onto a porous support, and metallic interconnects are coated with conductive ceramics to improve their surface stability. Here, we report on SOFC interconnect coatings based on perovskite-type oxides. A dual-layer ceramic interconnect is first discussed that consists of an n-type conducting (Sr,La)TiO3 layer on the anode side and a p-type conducting (La,Sr)FeO3 layer on the cathode side. Then, protective perovskite coatings of (La,Sr)MO3 (M = transition metals) on metallic interconnects are presented, which are capable of mitigating oxide scale growth and inhibiting Cr evaporation. The synthesis of perovskite oxides and their electrical/thermal expansion properties are discussed in detail, followed by a description of coating processes. We demonstrate that the dense interconnect or protective layers are fabricated by a simple and cost-effective coating–sintering process and they exhibit low resistance and high stability under SOFC operating conditions.

Resume : With the expanding need for large electrical energy storage systems in connection with renewable energy sources, flow batteries, have been enormously considered due to their high flexibility in upgrade and low cost associated with scale-up. Of all the flow batteries, vanadium redox flow battery (VRFB) use of same element in both half-cell solutions that overcomes the inherent issue of cross contamination by diffusion of different ions across the membrane. Together with the absence of any toxic emissions, the vanadium redox flow battery has demonstrated its uniqueness in terms of safety and long life cycle. Typical charge–discharge reactions of a VRFB involve two vanadium redox couples, V(II)/V(III) and V(IV)/V(V), in the negative and positive half-cells, respectively. In a fashion similar to most batteries, electrons are transferred between the two electrodes through the external circuit during the charge and discharge processes. In a VRFB, the ion exchange membrane is a key component as an ionic conductor and separator: it not only provides an ionic conduction pathway between the two electrolytes but also prevents mixing of the negative and positive electrolytes. The crossover of ions through the membrane, with the diffusion of vanadium ions from one half-cell to the other due to the concentration gradients between the two electrolytes, will result in self-discharge and thus the loss of the chemical energy. In this study, the hybrid membrane of inorganic materials with perfluorinated organic membrane were fabricated and characterized in terms of ionic conductivity and permeability. The ionic conductivity was measured with four point probe method and the permeability was measured with UV spectroscopy. The hybrid membrane exhibited similar ionic conductivity with perfluorinated organic membrane and 30% lower permeability than perfluorinated organic membrane.

Resume : Carbon papers (CPs) were fabricated using wet-laying carbon fibers (CFs) and polyacrylonitrile (PAN) fibers. Scanning electron microscopy revealed that the PAN fibers tightly interconnected the CF junctions with the pores. The tensile strength of the carbon webs (CWs) increased as the fraction of PAN fibers used as the binder increased. The CW fabricated with 0.15 wt.% PAN fibers had a tensile strength six times greater than that of a bare CW. Moreover, by mixing the CFs with PAN fibers in water, the CFs separated from each other in the webs due to the interruption of hydrophobicity between the CFs. After the CWs were pyrolyzed at 1200 C in the presence of a phenolic resin, the PAN fibers maintained their morphology due to their high carbon content. The resistance of the CPs with high PAN fiber content was significantly lower than that of a bare CP due to the interconnection of the CFs by the carbonized PAN fibers.

Resume : In this work we investigated composite electrolytes of 25 and 50 mol% cross linked polyvinyl alcohol by oxalic and citric acids for fuel cell application. We studied the effects of crosslinking on thermal, structural, conductivity and dielectric relaxations. The glass transition temperature is increased indicating the decrease of the local mobility of the polymer chains. X-ray diffraction showed the decrease of the cristallinity degree. The Ac conductivity studied between 10-1Hz and 1MHz showed a power law response in the high frequency range. This behavior characterizes the charge transport in disordered materials. At low frequencies the Ac conductivity is governed by the electrode/sample interface polarization. The dc conductivity is thermally activated and the activation energy is deduced from an Arrhenius fitting. The use of the dielectric permittivity indicates the presence of a relaxation process attributed to electrode/sample interface polarization and another relaxation process attributed to alpha relaxation of the polyvinyl alcohol matrix.

Resume : The electrolyte plays an important role in Lithium ion batteries(LIB's). Organic solvents such as cyclic and linear carbonates are commonly used in LIB's. It is very important to investigate the structure and dynamics of Li+ ion in pure and mixed solvent compositions. Although there are some DFT studies of Li+ ion with single solvent, in several applications mixtures of three solvent are used to improve stability and transport properties. Here we focus on three carbonates such as cyclic ethylene carbonate(EC), propylene carbonate(PC) and linear dimethyl carbonate(DMC). The DFT method provides a detailed account for the electronic structures of carbonates and Li+ ion including clusters Li+(S)n (S=EC,PC,DMC n=1-4) and mixed cluster Li+(EClDMCmPCn) (l+m+n=4) in gas phase as well as in solvent environment and including the effect of PF6- anion. The results show that carbonates mainly interact through the intermolecular Li+ --O=C. We have studied the coordination shell around Li+ for three solvents. We have investigated the Mulliken charges and vibrational modes with solvent molecules varying from 1 to 4 and thermodynamics properties used to clarify the stability of clusters.Study of clusters of EC and PC reveals that PC tends to substitute EC in the solvation shell. These DFT calculations were performed at the level of GGA/PBE using TZP basis. This study will be used to develop Molecular Dynamics studies that will help to understand the transport properties of Li+ ion in LIB's.

Resume : The aim of this study is to design a hybrid electrolyte with a three-layer structure: a proton conducting polymer layer on one side of a resorcinol-formaldehyde (RF) porous structure, while the other is impregnated with an anion conducting polymer. The sandwich structure enables protons generated at the anode to progress toward the center and meet hydroxyl ions created at the cathode to form water, which is directed through the porous RF network. The hybrid electrolyte ensures an efficient water management by preventing water presence at electrode sites and, hence, related problems: corrosion, low fuel efficiency. Specific structural features are identified by means of Fourier transform infrared spectroscopy, while information on the polymer/porous RF network interfaces are revealed by scanning electron microscopy. Electrochemical characteristics are studied through electrochemical impedance spectroscopy. Further on, the hybrid electrolyte is tested in H2/air fuel cell system, with a Pt catalyst load of 1mg/cm^2 at both anode and cathode, reaching a maximum current density of 400-600mA/cm^2 at a voltage of 0.6-0.7V.

Resume : Major drawbacks in fuel cells performance is determined by the oxygen reduction reaction (ORR) which is sluggish than the hydrogen oxidation reaction (HOR). Extensive researches made over the past decades have focused in improving the ORR kinetics by using different catalysts: noble metals (Pt), alloys, carbon materials, quinone, transition metals chalcogenides, transition metals carbides and heteropolyacids. One approach is to utilize heteropolyacids (HPA) as potential cathode mediators, due to their acidic and redox properties, their potential to carry complex counter-cations (e.g. H+, H3O+, H5O2+) and their thermal stability. Three types of heteropolyacids: phosphomolibdic acid, phosphotungstic acid and tungstosilicic acid where studied as redox mediators in the cathodic chamber. The experimental set-up consists of a bicameral fuel cell with Nafion membrane. The anodic chamber acts as the source of hydrated protons at a given potential (equivalent with a reversible hydrogen electrode); the cathode chamber contains the aqueous HPA mediators recirculate in different conditions of flow and oxygen concentration. Electrochemical impedance spectroscopy, cyclic voltammetry and polarization curves were carried out to identify kinetics mechanisms related to ORR improvement.

Resume : Urea Fuel Cells employ a non-toxic, non-flammable and transport-friendly (without pressurization need) fuel that can be easily recovered from waste water. The present study investigates the urea fuel cells performance, using a new membrane-electrode assembly based on an anion exchange resin in a polyvinyl alcohol polymer matrix. Composite membrane intrinsic properties are studied in terms of ion-exchange capacity (IEC), water content, transport number, and thermal stability. Results show an IEC and water content increase with resin content, while polymer glass transition temperature and melting points shift due to hydrogen bonding between polymer and resin. Urea fuel cell testing is performed using Pt/C at both anode and cathode, for different urea concentrations, ranging from 1 to 7 M.

Resume : We report the vibrational properties and the changes in the degree of crystallinity of poly(ether ether ketone) (PEEK) membranes by using Fourier transform infrared (FTIR) spectroscopy. The changes in the vibrational modes of the functional groups present in the polymer due to sulfonation and the incorporation of functionalized silica particles are discussed. From the infrared spectra, the incorporation of sulfonic acid group in the polymer chain as well as in the functionalized silica nanoparticles was confirmed. We have attempted a non-destructive method to calculate the crystallinity of polymers by measuring the variations in the peak area intensity of the characteristic out-of-plane vibrations of the aromatic rings in the PEEK polymer with respect to a crystallinity-insensitive reference peak. These values were correlated to the crystallinity values calculated by Differential Scanning Calorimetry (DSC). High degree of sulfonation (DS) values decreased the crystallinity of the polymer, indicating the crystalline-to-amorphous phase transition in the polymer, which explains the improved ionic conductivity at high DS values. Keywords: SPEEK, FTIR, sulfonation, silica, crystallinity, DSC

Resume : Reversible oxygen electrochemistry is vital to numerous future renewable energy technologies, such as fuel cells, metal-air batteries, and electrolyzers. Active electrocatalysts for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are known; however, they are typically expensive and based on rare materials such as Pt (for ORR) and Ir or IrOx (for OER). These can also suffer from reaction poisoning and poor electrochemical selectivity, further decreasing their utility. Therefore, catalyst materials that are highly active, stable, and cost-effective are needed. We have recently begun developing catalysts for oxygen electrochemistry in aqueous electrolytes. We will present on Cu-α-MnO2 nanowire electrocatalysts and demonstrate how various Cu-ion doping amounts alter the nanowire surface area, crystalline lattice, conductivity, and the manganese oxidation state; we then rationalize how these factors impact the activity for the ORR. We will demonstrate that when blended with a graphene-like carbon, Cu-α-MnO2 compares well to the Pt/C benchmark for the ORR in alkaline electrolyte. We will also present on our new porous Co3O4 and NixCo3-xO4 spinel films. Electrocatalytic data shows performance rivaling that of commercial benchmarks and recent literature catalysts for both the ORR and the OER. These electrocatalysts hold much promise for improving the efficiency of devices reliant on reversible oxygen electrochemistry. This work was supported by Sandia National Laboratories: Sandia is a multi-program laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy’s National Nuclear Security Administration under Contract DE-AC04-94AL85000.

Resume : Germanium has a significantly higher theoretical energy storage capacity (1600 mAhg-1) than the carbon-based anodes (372 mAhg-1) presently used in Li-ion batteries and Ge has relatively high electronic conductivity and room temperature Li-ion diffusivity. Yet large volume changes during operation severely limit Ge anode lifetimes. To understand the dynamics and failure mechanism of these and other electrode materials, it is essential to image batteries under operating conditions. Here we show significant size dependence on the cycling characteristics of micron-sized Ge particles employing hard X-ray microscopy during typical battery operation. We found smaller Ge particles experience volume expansion before their larger counterparts. However, these small particles rapidly lose electrical contact and only the largest particles contribute to the battery capacity in the second cycle. Finally, we found particles with larger surface area to volume ratios have greater volume changes during lithiation. Our results demonstrate the significant value in high resolution imaging of batteries under operating conditions and suggest possible strategies to mitigate capacity fade in Ge and Si anodes.

Resume : TiO2 (B) is metastable monoclinic modification of titanium dioxide. Zukalova et al. found that Li-insertion into TiO2 (B) is characterized by unusually large faradaic pseudocapacitance. This peculiar effect was ascribed to Li-accommodation in open channels of TiO2 (B) structure allowing fast Li-transport in TiO2 (B) lattice along the b axis (perpendicular to (010) face). Analysis of cyclic voltammograms of Li insertion into TiO2 (B) and anatase provides information about capacitive contributions to overall charge of Li-storage. The enhancement of 30% is found in capacitive contributions (normalized to the total stored charges) in TiO2 (B) compared to that in anatase, in spite of ca. three times smaller surface area of the former. Different charging mechanism explains facilitated Li- insertion in TiO2 (B). The difference is caused mainly by pseudocapacitive Li-storage in the bulk TiO2 (B). Deconvolution of cyclic voltammograms also indicates different capacitive contributions of the two voltammetric peaks, S1 and S2 of TiO2 (B). These results provide novel insight into the Li-storage in TiO2 (B) and its difference from that in anatase. This work was supported by the Grant Agency of the Czech Republic (contract No. 108/12/0814).

Resume : In view of power limitations of organic Li-air batteries, we explore aqueous Li-air batteries with highly soluble discharge products enhancing power performance and energy efficiency, but requiring an electrolyte membrane to prevent a direct contact of Li to the aqueous catholyte. Here we design and test a composite membrane synergizing the robustness of ceramic electrolytes with the flexibility of polymer electrolytes and simplifying design compared to existing multilayer membranes. The optimized membrane contains 70wt% of the NASICON-type high stability electrolyte LAGP (conductivity 5×10^-4 Scm-1) in a polymer matrix of 18wt% PVDF, 6wt% PEO and 6wt% LiBF4. After immersion in acidified 1 M LiCl solution for one month the membrane shows no sign of significant swelling or change in conductivity. An aqueous Li-air cell employing a Li anode, the composite membrane, the catholyte on glass fibre separator and a Pt-impregnated CNT air cathode is constructed and the performance in ambient air is studied. With the LiCl catholyte it showed an open circuit voltage of 3.0 V and the curve has a plateau around 2.9 V (vs. Li). At a constant current density of 1 mAcm-2, the first discharge and charge capacity are 350 and 250 mAh per g of carbon, respectively in the voltage range of 1.0-3.8 (vs. Li/Li+) with low polarization (0.2 V). To investigate the origin of the limited cyclability, the by-products formed during discharge are analyzed by in situ synchrotron studies.

Resume : Needs of electrochemical energy storage devices have driven great interest in batteries and electrochemical capacitors. Nowadays, RuO2 is the most important oxide-based electrode for electrochemical capacitors. However, ruthenium oxide is quite expensive and other transition metal oxides are currently been investigated as cheaper alternatives. In this context, MnO2 is the most plausible choice [1]. Exploration of these oxides in capacitor devices has shown that, as for RuO2, there is a limited contribution of the electrochemical double layer to the total capacitance, its high value being mostly associated to fast redox reactions taking place at the electrode/electrolyte interface. Therefore, and unlike for traditional carbonaceous materials used as supercapacitor electrodes where the charge storage relies on the electrochemical double layer, the role of mesoporosity in transition metal oxide has been somewhat omitted. In this contribution, we show the synthesis and characterization of mesoporous manganese oxides. Their preparation involved a simple, slow thermal decomposition of an oxalate containing manganese (II) ions [2]. For comparison purposes we have also studied manganese oxide obtained by different original methods including: - the oxidation of alkenes assisted by CTAMnO4 (CTA= cetyltrimethylammonium) [3] - the oxidation of alcohol, dimethylformamide, ethylene glycol, and glycerol by KMnO4 [4,5]. The latter syntheses would lead to micro or macroporous materials with different particles aggregation than found for the mesoporous oxides. We analyzed the effect of mesoporosity on the electrochemical performances of the different systems synthesized, in terms of capacitance, device resistance and cycling efficiency with aqueous electrolytes. Furthermore, influence of the alkenes hydrophobic chain, the counter-ion of MnO4- group, and, in general, of the reductor agent on the morphology of the final manganese oxide are also determined according to specific and porosity determination by BET analysis and SEM imaging. In addition, several electrochemical techniques, including electrochemical impedance spectroscopy and cyclic voltammetry, have been applied for this research to determine electrochemical performances of these different materials. After the study on three electrode cells, such oxides were tested as positive electrodes in front of negative electrodes based on activated carbon. [1] D. B?langer, T. Brousse, J.W. Long, The Electrochemical Society Interface (2008) 49?52. [2] L. Guo, H. Arafune, N. Teramae, Langmuir 29 (2013) 4404?4412. [3] S. Dash, S. Patel, B. K. Mishra, Tetrahedron 65 (2009) 707?739. [4] J-L. Liu, L-Z. Fan, X. Qu. Electrochim. Acta 66 (2012) 302?305. [5] Y-T. Wang, A-H. Lu, W-C. Li. Microp. Mesop. Mater. 153 (2012) 247?253.

Resume : In this study, we develop tungstate materials as electrocatalysts sensors systems allowing the degradation of pollutant. In this work, tungsten trioxide (WO3) was prepared by electrodeposition and used as anode for electrochemical degradation of Rhodamin B dye in aqueous solution. Oxide film was prepared using a facile one-pot chronoamperometry approach, from an electrolytic solution containing an equimolar mixture of Na2WO4.2H2O and H2O2. Several parameters were optimized to deposit these thin films such as the precursor concentration, current density, substrate, time deposit. X- Ray Diffraction (DRX) analyze was used to identified the oxide structure, scanning electron microscopy (SEM) coupled with EDX for morphological studies. The synthesized films were applied for the electrodegradation of organic pollutants by anodic oxidation. The Rhodamin B is deteriorated by electrochemical root using the three-electrode assembly: the working electrode based on WO3 thin film, Platinum foil as counter electrode and saturated calomel electrode as the reference. Many parameters were varied to find the optimal conditions for the degradation (supporting electrolyte, current density, temperature). Ultraviolet spectroscopy and chemical oxygen demand measurements were conducted to follow degradation rate and study the reaction kinetics of Rhodamin B degradation. Keywords: Electrodeposition, Tungsten oxide, Anodic oxidation, Rhodamin B, Reaction kinetics.

Resume : Unpredictable risk of nuclear power energy and, in particular, the energy turnaround in Germany have renewed interest on green power based energy and material conversion, and solutions for efficient energy storage. Electrochemical energy storage technologies are of particular importance due to high achievable energy densities. Especially all-solid-state batteries, which were previously reserved for low power energy devices, such as microchips, wireless sensors or low-power medical applications, offer greater potential in respect to energy density, safety and autarchic power supply for short and long term operation applications. These aspects and new concepts of electrochemical storage technologies are in the focus of a joint research project – CryPhysConcept – coordinated by the TU Bergakademie Freiberg. This contribution presents a new solid-state battery concept, based on oxide material that aims to use oxygen defect migration in a crystal as electromotive force to store (and release) electric energy. Furthermore, a solid-state air-battery based on thin-film technology containing an oxygen cathode, is addressed in this work. Since the efficiency of metal-air batteries is significantly limited by the activation of oxygen reduction reaction (ORR) as well as of the ion and electron conductivities, an adequate porosity to allow air oxygen to readily diffuse through the cathode material as well as a controlled metal doping is required. The ion implantation of promising transition metal oxides is a key technology to achieve this goal. A co-implanted with Ni2+- and Co2+-ions strontium titanate (SrTiO3) oxide material combined with millisecond flash lamp annealing to create metal clusters, which act as active centres, is used to improve catalytic properties. The depth profile of the samples is characterized by means of Rutherford backscattering spectrometry (RBS) and X-ray diffraction (XRD). The oxygen catalytic properties of the implanted crystalline SrTiO3 will be demonstrated. FEM-simulations are used to improve functionality.