Cathodic Contact Research Articles

Strategies for hollow cathode command in short electrodynamics tether-based deorbit devices

Heaterless hollow cathodes are a promising solution for reaching a good cathodic contact in electrodynamic tether systems. However, these cathodes cannot keep the plasma discharge if the electric current IC is below a threshold Imin. If the tether is short, and to save power and expellant, an appropriate command strategy is needed to ensure that the cathode is triggered in conditions satisfying IC>Imin. This study investigates four command strategies, including two fully autonomous approaches (one based on satellite position and the other on local plasma density) and two strategies involving ground segment access, differing in command generation location (ground segment and on-board computer). The success rates of the four strategies were studied though numerical simulation and for the specific scenario of the E.T.PACK demonstration mission. The results demonstrate that, as compared to continuous operation of the hollow cathode, the proposed strategies reduce propellant wastage by a factor between 5 and 10. The strategy requiring ground segment access and with the onboard computer making the command decision has the highest success rate (94%). Among the autonomous configurations, the strategy based solely on local plasma density achieves the highest success rate (69%). Additionally, the impact of sensor accuracy on success rates was assessed via Monte Carlo analysis. The command strategy relying on plasma density measurement is the most sensitivity strategy, with a 10% decrease in success rate due to sensor uncertainty.

Feasibility Study of the Bare-Photovoltaic-Tether Concept: Prototypes and Experimental Performance Evaluation of the Photovoltaic Tether Segment

Consumable-free electron emitters are presently not feasible for autonomous tether-based deorbit devices such as E.T.PACK due to their power requirement. The bare-photovoltaic-tether (BPT) concept combines the bare tether electron collection with a tether segment, coated with thin film Copper Indium Gallium Selenide solar cells to harvest additional power for the cathodic contact, potentially enabling propellant-less operation. This thesis presents the first prototype of the photovoltaic tether segment, its architecture, its electrical characteristics, major challenges of the system and possible solutions. Photovoltaic tether segments of up to 3 m in length were manufactured, consisting of parallelized submodules of 25 cm in length. Due to space limitations, only the I-V-characteristics of these submodules were measured under a self-built Class BCA LED Solar-Simulator inside a vacuum chamber and at varying temperatures between −100 °C and 100 °C. In addition, the suitability of the concept for a low Earth orbit environment was assessed by performing atomic oxygen exposure tests using a microwave-based low pressure plasma atomic oxygen source. Based on the experimental data, a model is provided for predicting the performance of the photovoltaic segment in orbit, highlighting the main problems of the BPT: temperature, orientation and partial shading.

A bare-photovoltaic tether for consumable-less and autonomous space propulsion and power generation

State-of-the-art electrodynamic tethers reach a steady electric current by using a bare segment to capture electrons passively from the ambient plasma (anodic contact) and an active electron emitter or a tether segment coated with a low-work-function material (cathodic contact) to emit electrons back and close the electrical circuit. This work proposes to take advantage of recent developments on thin-film solar cells and insert a photovoltaic (pv) tether segment in between the anodic and the cathodic contacts. Since thin-film solar cells can be folded and manufactured with any desired length and the same cross-section dimensions as the bare segment, i.e. width and thickness around few centimeters and tens of microns, the resulting device is compact and preserves bare tether simplicity. Detailed analysis of the current and voltage profiles throughout the tether shows that the electrical power introduced by the pv-segment into the tether-plasma circuit improves the performance and makes them less dependent on ambient conditions. The pv-segment decreases considerably the tether-to-plasma bias at the cathodic contact, thus opening the possibility to emit substantial current while using consumable-less electron emitters like thermionic and electron field emitters. The pv-segment also favors the current collection by increasing the tether-to-plasma bias at the bare segment. Propulsion and power generation applications and alternative architectures of bare-pv tethers are briefly discussed.

Diffusion-Related SOFC Stack Degradation

Solid Oxide Fuel Cell systems for stationary applications should exhibit low degradation (<< 1%/1,000h voltage loss) to ensure long-term operation (> 40,000h). The rather high operation temperature of SOFCs (≥ 700°C) is on the one hand beneficial for oxygen conduction through the ceramic-based electrolyte, but on the other hand all diffusion-related, energetically-driven mechanisms are also increased. This counts less for gas diffusion processes but is more pronounced for solid-state diffusion (surface, bulk, grain boundary). For gas diffusion processes other parameters like gas stream, individual partial pressures and partial pressure gradients are dominating. Within two different stack tests with four-plane short stacks and their intensive post-test characterization two varying diffusion-related degradation mechanisms have been investigated. One is a short-term (~1250h) test with two different chromium evaporation protection layers on the air side metallic interconnect and frame and the other one is a long-term endurance test (~ 35,000h). For the first stack two planes were coated with a manganese oxide layer applied by wet powder spraying (WPS) while the other two planes were coated by atmospheric plasma spraying (APS) with a manganese-cobalt-iron-spinel layer. Thus the protection layers do not only differ by coating technology but also by material. The voltage loss in the planes with WPS-coated interconnect was markedly higher than in those coated by APS. Finally it turned out that the layers microstructure plays the key role in Cr evaporation minimization. In this stack, gas-phase diffusion is dominating degradation. In the long-term operated stack, the influence of a diffusing element from the cathodic contact layer towards and through the electrolyte has been characterized. Manganese, presumably originating from the contact layer (but it’s additionally also part of the protection layer), diffuses via solid-state diffusion and reacts while reaching reducing conditions in the fuel compartment with the zirconia-based electrolyte. This interaction forms a secondary sponge-like porous phase at the boundary electrolyte-anode. This secondary phase causes stresses and leads, after growing to big islands of some ten micrometers in diameter, to electrolyte-anode delamination and finally to complete cell failure. For the Mn-related degradation solid state diffusion has been supposed, as the electrolyte is coated by a dense thin-film diffusion barrier layer which suppresses gas diffusion. Both results indicate that volatile elements (which could either evaporate via gas phase or diffuse via solid state diffusion) should be avoided as they can lead to severe cell, stack and system failure even after thousands of hours of high-temperature operation.

Chemical Degradation of the Cathodic Electrical Contact Between Carbon and Cast Iron in Aluminum Production Cells

The cathodic carbon to cast iron electrical contact degradation is one of the factors to consider in the cathode voltage drop (CVD) increase over the lifetime of aluminum production cells. Lab-scale experiments were carried out to study the cast iron to carbon interface chemical degradation and the impact of important cell parameters like temperature and bath chemistry. Laboratory degradation results were compared with industrial samples. A thermoelectric Ansys numerical model was then used to predict the effect of cast iron surface degradation over CVD. Results show that the aluminum formation on the cast iron surface and its subsequent diffusion creates an immiscible mixture of Fe-Al metal alloy and electrolytic bath. Disparities were also observed between industrial samples taken from two different technologies, suggesting that the degradation can be slowed down. Thermoelectric calculations finally revealed that the impact of the contact resistance augmentation is by far greater than the cast iron degradation.

DC and AC Performance of Graphite Films Supercapacitors Prepared by Contact Glow Discharge Electrolysis

In the present work, we fabricate and characterize carbon graphite films by high-voltage cathodic and anodic contact glow discharge electrolysis (CGDE) for supercapacitor applications. Transmission electron microscopy revealed that the carbon films obtained by anodic CGDE, in which a carbon graphite is used as the positive electrode, are in the form of two-dimensional, relatively large lateral size sheets without major fractures or structural distortions. Whereas with the cathodic configuration, tiny and elongated flakes of graphite attached to larger sheets are obtained. We observed negative Raman wavenumber shifts of the G peak and the appearance of the T peak at ca. 1100 cm− 1, which are attributed to thermal-induced strains and structural damages in the graphite crystal lattice, as well as to some C-C sp3 bonds content. In addition to the aforementioned results and optical emission spectroscopy, it is proposed that both formation mechanisms by electrochemical exfoliation and material evaporation occur depending on the used polarization. All-solid-state symmetric supercapacitors using polyvinyl alcohol/H3PO4 as gel electrolyte were fabricated and tested under dc and ac frequencies. We found two different behaviors: at dc and close-to-dc, the material prepared with cathodic CGDE showed close-to-ideal behavior with higher capacitance at different charging/discharging rates, which makes it more suitable for energy storage applications, whereas at higher frequencies ( ⩾ 100 Hz), the material prepared with anodic CGDE performed better for ac filtering applications that has been verified using an inverting integrator circuit.

Cathodic contact glow discharge electrolysis: its origin and non-faradaic chemical effects

Normal electrolysis (NE), at sufficiently high voltages, breaks down and undergoes a transition to a phenomenon called contact glow discharge electrolysis (CGDE) in which a sheath of glow discharge plasma encapsulates one of the electrodes, the anode or the cathode. The chemical effects of CGDE are highly non-faradaic e.g. a mixture of H2 and H2O2 plus O2 each in excess of the Faraday law value is liberated at the glow discharge plasma electrode from an aqueous electrolyte solution. Studies of cathodic CGDE, particularly its origin and chemical effects, in comparison to those of anodic CGDE have received significantly less attention and have not been studied in detail. The present paper is an attempt towards elucidation of the mechanisms of the growth of cathodic CGDE during NE and its non-faradaic chemical effects. The findings of the study have led to the inference that emission of secondary electrons from the metal cathode with sufficient kinetic energies, vaporization of the electrolyte solvent in the vicinity of the cathode surface induced by Joule heating and the onset of hydrodynamic instabilities in local vaporization contribute to the generation of the plasma at the cathode during NE. The findings have further shown that non-faradaic yields of CGDE at the cathode originate from energy transfer processes in two reaction zones: a plasma phase reaction zone around the cathode which accounts for ~75% of the yields, and a liquid phase reaction zone near the plasma-catholyte solution interface accounting for the remaining ~25% of the yields.

Nonlinear time-series analysis of current signal in cathodic contact glow discharge electrolysis

In the standard two-electrode configuration employed in electrolytic process, when the control dc voltage is brought to a critical value, the system undergoes a transition from conventional electrolysis to contact glow discharge electrolysis (CGDE), which has also been referred to as liquid-submerged micro-plasma, glow discharge plasma electrolysis, electrode effect, electrolytic plasma, etc. The light-emitting process is associated with the development of an irregular and erratic current time-series which has been arbitrarily labelled as “random,” and thus dissuaded further research in this direction. Here, we examine the current time-series signals measured in cathodic CGDE configuration in a concentrated KOH solution at different dc bias voltages greater than the critical voltage. We show that the signals are, in fact, not random according to the NIST SP. 800-22 test suite definition. We also demonstrate that post-processing low-pass filtered sequences requires less time than the native as-measured sequences, suggesting a superposition of low frequency chaotic fluctuations and high frequency behaviors (which may be produced by more than one possible source of entropy). Using an array of nonlinear time-series analyses for dynamical systems, i.e., the computation of largest Lyapunov exponents and correlation dimensions, and re-construction of phase portraits, we found that low-pass filtered datasets undergo a transition from quasi-periodic to chaotic to quasi-hyper-chaotic behavior, and back again to chaos when the voltage controlling-parameter is increased. The high frequency part of the signals is discussed in terms of highly nonlinear turbulent motion developed around the working electrode.

One-pot synthesis of composite NiO/graphitic carbon flakes with contact glow discharge electrolysis for electrochemical supercapacitors

Thanks to their high power density and degree of reversibility, supercapacitors are electrochemical devices that narrow the gap between secondary batteries and traditional dielectric capacitors in the traditional Ragone plot. However, their use is still hindered by their capability to achieve higher energy density. In this work, we present a one-pot synthesis procedure of composite graphitic carbon flake-supported NiO for electrochemical energy storage application. We used cathodic contact glow discharge electrolysis by applying 120 Vdc terminal voltage between a thin Pt wire, slightly submerged in an aqueous solution of NiSO4(H2O)6 + Na2SO4, and a large surface area carbon graphite anode. Strong active species generated within the micro-plasma volume locally reduce the nickel precursors to form NiO materials, while at the anodically polarized graphite rod, the forces holding the graphene layers together are weakened by ion/solvent intercalation producing micrometer-sized graphitic carbon flakes. The morphological characterization is carried out by electron microscopy, energy dispersive X-ray spectroscopy, powder X-ray diffraction, and micro-Raman spectroscopy. Cyclic voltammetry, constant-current charge/discharge, and electrochemical impedance spectroscopy in 5 mol l−1 KOH solution are carried out to evaluate the electrochemical energy storage performance of the material. We show that carbon flake-supported NiO exhibits the dual combination of electric double-layer capacitance with faradic behavior, giving 495 F g−1 specific capacitance at 2 A g−1 current density. Copyright © 2015 John Wiley & Sons, Ltd.

Low work-function thermionic emission and orbital-motion-limited ion collection at bare-tether cathodic contact

With a thin coating of low-work-function material, thermionic emission in the cathodic segment of bare tethers might be much greater than orbital-motion-limited (OML) ion collection current. The space charge of the emitted electrons decreases the electric field that accelerates them outwards, and could even reverse it for high enough emission, producing a potential hollow. In this work, at the conditions of high bias and relatively low emission that make the potential monotonic, an asymptotic analysis is carried out, extending the OML ion-collection analysis to investigate the probe response due to electrons emitted by the negatively biased cylindrical probe. At given emission, the space charge effect from emitted electrons increases with decreasing magnitude of negative probe bias. Although emitted electrons present negligible space charge far away from the probe, their effect cannot be neglected in the global analysis for the sheath structure and two thin layers in between sheath and the quasineutral region. The space-charge-limited condition is located. It is found that thermionic emission increases the range of probe radius for OML validity and is greatly more effective than ion collection for cathodic contact of tethers.

Cathodic Contact Glow Discharge Electrolysis for the Degradation of Liquid Ammonia Solutions

The application of 50 VDC cathodic contact glow discharge electrolysis for six hours in 1.0 mol/L KOH + 0.8, 0.4 and 0.08 g/L ammonia results in the removal of 66, 71 and 77% of ammonia, respectively, following pseudo first‐order kinetics at ca. 0.3 mol/kWh. The yields of dissolved NO2N and NO3N lie between 9 and 29% and are coupled with ammonia removal suggesting a nitrification process. Based on the near‐cathode UV–Vis emissions and ionization products of H2O and NH3 molecules, a mechanism is proposed along with the charge/energy transfer reactions between the plasma‐generated free radicals and ionic species. Joule heating and vaporization are also believed to physically remove ammonia.

Size-dependent capacitance of NiO nanoparticles synthesized with cathodic contact glow discharge electrolysis

NiO nanoparticles of 70, 91 and 107 nm average diameter are synthesized by cathodic contact glow discharge electrolysis at 30, 36 and 42 VDC respectively, in 2 M H2SO4 + 0.5 M ethanol + 2.5 mg ml−1 of PVP, and are investigated for electrochemical energy storage. From the cyclic voltammetry and galvanostatic charge–discharge measurements in 1 M KOH, it was found that a maximum specific capacitance of 218 F g−1 is achieved with the 70 nm NiO nanoparticles at 2.7 A g−1. Larger nanoparticles of 91 and 107 nm diameter exhibit specific capacitances of 106 and 63 F g−1, respectively, suggesting a size-dependent capacitive performance enhanced with decreasing particles size.

Synthesis of Ni and Pt nanomaterials by cathodic contact glow discharge electrolysis in acidic and alkaline media

Ni, NiO, β-Ni(OH)2 and Pt nanomaterials are prepared with cathodic contact glow discharge electrolysis (CGDE) in acidic and alkaline media by alternating the Ni and Pt cathode vs. a large Ni anode. The morphological and crystalline characterizations of the products suggest two reaction mechanisms where in the first, metal ions are locally reduced by the high reducing agents generated by the electrical discharges from the cathode, i.e. the solvated electron, the H radical and secondary species if any. This is supported by CGDE experiments conducted in acidic environment with Ni or Pt cathode that gave Ni-based materials only. The second mechanism for the formation of metals aggregates is due to the cathode material electro-erosion due to the high temperature spots at its surface that lead to the local vaporization of its material and the rapid quenching of the generated powder. This conclusion is drawn after eliminating the source of Ni2+ ions, initially due to the Ni anode dissolution in acidic pH, and conducting the same two previous experiments in concentrated KOH solutions instead. In both arrangements, it is the cathode material that was the source of the synthesized nanoparticles.

Bare-tether cathodic contact through thermionic emission by low-work-function materials

A new material, C12A7:e- electride, which might present a work function as low as 0.6 eV and moderately high temperature stability, was recently proposed as coating for floating bare tethers. Arising from heating under space operation, current is emitted by thermionic emission along a thus coated cathodic segment. A preliminary study on the space-charge-limited (SCL) double layer in front of the cathodic segment is presented using Langmuir’s SCL electron current between cylindrical electrodes and orbital-motion-limited ion-collection sheath. A detailed calculation of current and bias profiles along the entire tether length is carried out with ohmic effects and the transition from SCL to full Richardson-Dushman emission included. Analysis shows that in the simplest drag mode, under typical orbital and tether conditions, thermionic emission leads to a short cathodic section and may eliminate the need for an active cathodic device and its corresponding gas feed requirements and power subsystem, which results in a truly “propellant-less” tether system for such basic applications as de-orbiting low earth orbit satellites.

Opening Electrical Contacts: The Transition from the Molten Metal Bridge to the Electric Arc

This paper presents a comprehensive explanation of the formation of the electric arc between opening contacts in a current carrying electric circuit. As the contacts begin to open a molten metal bridge forms between them. The rupture of this bridge and the initial formation of the electric arc are studied in both atmospheric air and vacuum using experiments to determine the direction of metal transfer between the contacts as a function of time after the rupture of the molten metal bridge. High speed streak photography is also used to show the rupture of the molten metal bridge and the initial formation of the electric arc. Analysis of these data show that a very high-pressure, high-temperature metal vapor zone exists between the contacts after the rupture of the molten metal bridge. Under this condition a pseudo-arc forms where current is carried by metal ions and an anomalous, high net transfer of metal to the cathodic contact occurs. The pressure in this region decreases rapidly and there is a transition to the usual electric arc, which still operates in the metal vapor. In this arc the current is now mostly carried by electrons. The data shows that there is still a net transfer of metal to the cathode, but now its volume is a function of the arcing time.

Protective Coatings of Metallic Interconnects for IT-SOFC Application

The development of high-performing planar solid oxide fuel cell (SOFC) stacks operating at intermediate temperature (700–850°C) is based on thin-electrolyte anode supported cells (ASCs) and interconnects made by ferritic stainless steels. These metallic materials match very well the thermal expansion behavior of the ASCs and can be manufactured and formed using cheaper and easier processes than ceramics or chromium alloys. Nevertheless, some problems remain to be solved with these components as the performance degradation due to the oxide scale growth at the cathodic contact surface and the evaporation of volatile Cr-containing species, which poisons the cathodic materials. Both effects strongly limit the stack performance compared to single cells and increase the degradation rate with time. Providing the steel composition is carefully controlled, the above problems can be limited and some special ferritic stainless steels have been developed in the past years for SOFC application. Unfortunately, no commercial alloy is still able to satisfy the limit in degradation rate required for stationary applications (SECA target is &lt;0.25% upon 1000h on a minimum service life of 40,000h). To achieve these goals a further improvement of composition should be required but this cannot be easily obtained in a cost-effective large-scale metallurgical production. An alternative and probably simpler way is to coat the surface of the steel with a protective layer with the twofold aim to limit Cr evaporation and to develop a conductive scale. In the present work, the effect of different oxide coatings on the chromium evaporation rate and on the contact resistance of ferritic stainless steel has been investigated. To obtain a conductive layer, spinel compositions containing Co, Mn, and Cu have been considered. Steels surfaces have been spray-coated using alcoholic suspensions, and the microstructural evolution of the interface between the metallic substrate and oxide layers has been investigated by scanning electron microscopy and energy dispersive X-ray spectroscopy line-scan analysis for exposure at high temperature. The variation with time of the area-specific resistance at 800°C has been recorded up to 1000h. The evaporation rate of Cr-containing species has been also evaluated by a qualitative method.

Cathodic contact glow discharge electrolysis under reduced pressure

The effects of pressure and electrolyte concentrations on cathodic contact glow discharge electrolysis of aqueous LiOH on platinum have been studied. I determining factor for the onset of a glow discharge, although solvent evaporation due to Joule heating might have some influence. The discharges obtai of the chemistry of energetic species generated at the electrode.

Cathodic contact erosion due to short-duration gas breakdown arcs

The erosion of palladium contacts due to gas breakdown arcs in air and in an activating environment (naphthalene vapour) has been studied. The rate of erosion in air was not constant with arc energy but increased almost linearly from about 0·1 to 1·5 cm3 erg−1 over an energy range of about 100 to 500 erg per discharge. Beyond this energy range a slower increase was observed. Under the activated conditions the erosion rate was much lower than in air (eg at 2000 erg the erosion rate in air was 1·65×10−14 cm3 erg−1 and with naphthalene activation the rate was 1·1×10−14 cm3 erg−1); however, a variation in the erosion rate with energy was also observed. The variation with energy is considered to be caused by part of the available erosion energy being used up in surface film decomposition.

A Reversible Solid‐State Battery with RbAg4 I 5 as Electrolyte

A solid‐state reversible cell of the type , TBAI, C, has been developed. The anodic and cathodic contact polarizations have been minimized by amalgamation of the silver electrode and the use of a suitable mixture made of tetrabutylammonium iodide (TBAI), iodine, and graphite, respectively. The cell has an open‐circuit voltage of 0.56V at 25°C, a total volume of 5 cm3, an internal resistance of 30 ohms, short‐circuit current of 1.5 A/dm2, and is rechargeable with high coulombic efficiency.