Cathode Temperature Research Articles

Evaluation criterion of cathode flow field in air-cooling proton exchange membrane fuel cells

In recent years, air-cooling proton exchange membrane fuel cells (PEMFCs) have become a priority choice for applications due to their lightweight and simple structure. In this work, the mechanism of cathode flow field characteristics on PEMFC performance is revealed, and the evaluation criterion of the cathode flow field is proposed. A 2 kW PEMFC stack of 100 cells is used, and the different uniformity and airflow rate of the cathode flow field are achieved by varying the position and number of cathode fans with a constant total parasitic fan power. The PEMFC power, cathode temperature distribution, and single-cell voltage consistency of different configurations are evaluated. The results show that the maximum difference in output power and maximum temperature of different PEMFC cathode flow field configurations reach 6.5% and 22.3%, respectively. Three key parameters are proposed to evaluate the PEMFC cathode flow field: total uniformity rate can evaluate the overall uniformity of the PEMFC cathode flow field, vertical uniformity rate can be used to predict and evaluate the single-cell voltage consistency of the PEMFC stack, and cathode airflow rate determines the continuous operating power of the PEMFC stack. The evaluation criterion can guide the design and optimization of air-cooling PEMFCs.

Effect of the cathode surface temperature on the cathode fall layer parameters: experiment and simulation

Combined experimental and numerical studies reveal a significant effect of the cathode temperature on the basic parameters (such as the electric field profile, thickness of the cathode fall layer, current density, and gas temperature) of the cathode fall of the self-sustained normal direct current atmospheric pressure glow discharge (APGD) in helium. Numerical models are spatially one- and two-dimensional and based on drift-diffusion theory of gas discharges. It was observed that heating of the cathode, resulting from a flow of the discharge current in APGD with a constricted positive column, leads to an increase of the interelectrode voltage if the cathode is not cooled and its temperature increases. With additional heating of the cathode by an external heat source, the interelectrode voltage tends to decrease. Radially inhomogeneous profiles of the reduced electric field on the uncooled cathode surface were measured. Simulation results exhibit reasonably good agreement with experiment for APGDs with cooled and uncooled cathodes.

Data Driven Model for a Fuel Cell stack development in a complex Multi-source Hybrid Renewable Energy System

Fuel cells based on polymer electrolyte membrane are considered as the most hopeful clean power technology. The operating principles of polymer electrolyte membrane fuel cells (PEMFC) system involve electrochemistry, thermodynamics and hydrodynamics theory for which it is difficult to establish a mathematical model. In this paper a nonlinear data driven model of a PEMFC stack is developed using Neural Networks (NNs). The model presented is a black-box model, based on a set of measurable exogenous inputs and is able to predict the output voltage and cathode temperature of a high power module working at the CNR- ITAE. A 5 kW PEM fuel cell stack is employed to experimentally investigate the dynamic behaviour and to reveal the most influential factors.

Electrical and thermal characteristics of LaB6 thermionic hollow cathodes operating in He, D2, Ar, and Xe at 4–200 Pa and 0.25–5 A cm−2

We report characteristics of lanthanum hexaboride (LaB6) cathodes for conditions relevant to a high-voltage, high-current switching device. LaB6 is selected because of its relatively low work function, stability in the presence of many common gas impurities, and a low evaporation rate (long life) at operating temperatures. We have investigated the effect of cathode geometry, gas type, pressure, and current on a LaB6 hollow cathode in a diode configuration with an opposing planar anode. The gas is stagnant at a relatively low pressure of 4–200 Pa (0.03–1.5 Torr) because of device high-voltage standoff requirements. In all cases, the plasma voltage decreases to a plateau value with increasing current where a practical device would operate. The plateau voltage aligns with the fill gas excitation or ionization voltage, depending on conditions. At high current, the cathode temperature exceeds that required for thermionic emission to supply the total current, a behavior that is understood by accounting for back-diffusing bulk electrons.

The effect of space charge neutralization on the parametric design of photon enhanced thermionic solar converters

Photon-enhanced thermionic emission (PETE) is a promising method for increasing current density at relatively low cathode temperatures, but is susceptible to the space charge effect. In this study, we propose a novel PETE device model in which the space-charge barrier is relaxed by photon-excited cesium ions. The decrease in maximum efficiency with interelectrode gap is more gradual compared to previous studies. We investigate the effects of cathode materials properties (band gap and electron affinity) and operating conditions (cathode temperature and interelectrode gap) on PETE performance, and identify optimal performance conditions. Our findings show that the optimal gap range exceeds 5 μm and near field radiation loss can be minimized, providing an important opportunity for designing PETE device with larger interelectrode gaps and higher solar flux concentrations.

Emissive cathode immersed in a plasma: plasma–cathode interactions, operation and stability

Thermionic emission from a polycrystalline tungsten emissive cathode immersed in a magnetized plasma column is investigated experimentally and numerically. Electrical and optical measurements of the cathode temperature show a highly inhomogeneous cathode temperature profile due to plasma–cathode interactions. The spatially and temporally resolved cathode temperature profile provides an in-depth understanding of the thermionic electron current, in excellent agreement with experimental data. The plasma-cathode coupling leads to a sharp and heterogeneous rise in temperature along the cathode, which can eventually lead to unstable cathode operation, with divergent current growth. A detailed thermal modeling accurately reproduces the experimental measurements, and allows to quantify precisely the relative importance of heating and cooling mechanisms in the operation of the cathode immersed in the plasma. Numerical resolution of the resulting integro-differential equation highlights the essential role of heterogeneous ohmic heating and the importance of ion bombardment heating in the emergence of unstable regimes. Detailed thermal modelling enables operating regimes to be predicted in excellent agreement with experimental results.

Estimation of the representative temperatures of a typical electrochemical battery cell when cooling under natural convection conditions

Commercial thermal management systems for electrochemical batteries use information from temperature sensors on the battery surface as the main process characteristic. The main problem of batteries lies in the intense heat energy dissipation in the closed internal volume due to chemical reactions at cyclic charge and discharge. In addition, increasing in the energy intensity of modern batteries, charge and discharge rates, as well as cases of fires and explosions of the battery energy storage systems (ESS) indicate the need to analyze the thermal state of internal electrode battery structures. A multi-layered battery structure and low thermal conductivity of some materials requires ensuring minimum temperature variations between individual battery layers and the battery housing surface, as well as between the battery cells. Currently known instrumental methods for measuring battery cell temperatures do not provide reliable estimates of cell electrolyte, anode and cathode temperatures. The article presents the results of a theoretical analysis of representative temperatures of a typical cell of a fairly common lead-acid battery in typical operating conditions. The results were obtained using the electrothermal model and the solution of the system of non-stationary equations of heat conductivity in the two-dimensional formulation with the finite difference method. Typical operating conditions (ambient temperature of 20 °C, convective heat transfer coefficient 5 of W/(m2K), charge time of 8 h) were considered. The maximum cell temperature was 36 °C, the average temperature was 31 °C, and the case temperature was 26 °C. It has been concluded that it is impossible to predict real thermal condition of the battery without analysis of temperature differences by its cross-sections. Moreover, the temperature of the external surface of the battery housing cannot be used as a key characteristic of battery life analysis.

Study of the process of preparing amorphous Fe–W(La) alloy plating by induced co-deposition

To obtain high-performance Fe–W alloy plating instead of environmentally hazardous chromium plating, the effects of the cathode material, electrolyte pH, temperature, current density, plating time and rotational speed on the cathode current efficiency and alloy plating are investigated in this paper. The results show that the pH and current density of the plating solution greatly influence the morphology and current efficiency of the cathode. The current efficiency of the cathode can reach 63.56%, and the tungsten content can reach 55% at pH = 8, 60 °C, 12 A/dm2, 100 r/min, 75 min, 0.1 mol/l of Fe2+ and 0.2 mol/l of W6+. After XRD analysis, the plating is found to consist of the Fe7W6 amorphous phase. In addition, La is added to the Fe–W alloy under optimal conditions. By analyzing the polarization curve, the potential of the Fe–W(La) alloy is positively shifted by 0.039 V compared with the Fe–W alloy, which has good corrosion resistance.

Influence of Thermionic Emission on He Ionization and Plasma Enhancement in Thermionic Energy Conversion

In this paper, the influence of thermionic emission on He ionization and plasma enhancement in thermionic energy conversion (TEC) are studied by experiment and numerical simulation. A 1D unsteady plasma TEC model, which includes a He ionization model, plasma conservation equations, and a thermionic emission formula for the wall, is developed. A He plasma thermionic energy conversion device composed of a barium–tungsten cathode and a tungsten anode is established. The volt–ampere curves of the He plasma TEC device are measured at 1050 K, 1150 K, 1250 K, 1300 K, and 1350 K temperatures. Both important cathode parameters, work function and emission area, are estimated. Based on the modelling simulation and the experiment, the He ionization mechanism in plasma TEC is discovered. The effects of cathode temperature on several distributions of plasma reaction rates, particle number density, and potential in He plasma TEC are described. Some important parameters, including electron mobility, resistivity, and plasma equilibrium are analyzed. The relationship of thermionic emission on plasma enhancement to the output power of plasma TEC is presented. The output powers of plasma TEC and vacuum TEC are compared at various cathode temperatures. A dimensionless analyzing method concerning thermionic emission intensity and plasma enhancement power is proposed. A brief dimensionless relationship is deduced regarding thermionic emission intensity and the plasma enhancement contribution of TEC. The principles and methods for quantitative calculations concerning the output power of plasma TEC under the action of thermionic emission are established. It is possible to do quantitative research on the effects of thermionic emission on plasma-enhanced TEC.

Contribution of Electrolyte Decomposition Products and the Effect of Temperature on the Dissolution of Transition Metals from Cathode Materials.

A fundamental understanding of aging processes in lithium-ion batteries (LIBs) is imperative in the development of future battery architectures for widespread electrification. Herein, dissolution of transition metals from cathode active materials of LIBs is among the most important degradation processes. Research has demonstrated that elevated operating temperatures accelerate battery degradation. However, the exact mechanism of transition-metal dissolution at elevated temperatures has still to be clarified. Current literature suggests that the reaction rate of dissolution increases with increasing temperature; moreover, the decomposition of electrolytes results in products that also accelerate dissolution processes. Most studies focus on ex situ analyses of thermally treated full cells. This approach is not appropriate to get detailed insights and to distinguish between different contributions. In this work, with the help of real-time dissolution analysis using an electroanalytical flow cell (EFC) coupled to an inductively coupled plasma mass spectrometer (ICP-MS), we present novel details of the temperature effects on in situ dissolution at the cathode electrolyte interface. With fresh electrolytes, we find increased Mn dissolution even at open-circuit conditions as well as with constant voltage polarization when the electrode sample is heated at constant temperatures between 50 and 80 °C. The release of transition metals also responds in a nuanced manner when applying temperature transients. Utilizing electrolytes preheated at 60 and 100 °C, we demonstrate that decomposition products in the bulk electrolyte have no influence on transition-metal (TM) dissolution when constantly flushing the cell with the thermally aged electrolyte samples. Only when keeping the cathode temperature at 60 °C, the dissolution increases by a factor of 2-3. Our findings highlight the interplay between the cathode and electrolyte and provide new insights into the dissolution mechanism of cathode materials.

Thermoradiative anode for enhanced thermionic energy conversion

Despite its great potential, thermionics has a low Carnot efficiency because a considerable percentage of cathode thermionic energy is wasted to heat the anode. This paper introduces a novel concept of anode-thermoradiative enhanced thermionic (A-TET) conversion, where a metal-n-p junction serves as thermionic anode to induce a cascading effect of thermionic-thermoradiative emission. An elaborated model based on first and second law of thermodynamics is established to evaluate energy transfer and irreversibility of A-TET converter. The simulation results show that the A-TET converter achieves a higher energy efficiency of 36.0 %, up by 56.2 % compared to pure thermionics and outperforms the thermionics & thermoradiatives combined system, with an absolute exergy efficiency improvement of 10.47 % at cathode temperature of 1400 K. A-TET conversion exhibits an extremely minimal exergy loss ratio of thermionic emission of 0.66 %, attributing to the inhibiting effect of thermoradiative on the thermionic emission. A temperature-entropy diagram is further established to pinpoint the irreversibility of A-TET conversion, with the entropy change of thermionic emission of 0.0311 W/(K·m2) and thermoradiative emission of 0.7666 W/(K·m2). This concept of A-TET conversion paves a way for the development of efficient thermionic converter for power-to-heat-to-power system.

Plasma Energy Loss by Cathode Heat Conduction in a Vacuum Arc: Cathode Effective Voltage

The importance of understanding the energy loss specifics by the cathode for vacuum arc metallic plasma generation and its applications were emphasized. To this end, the heat conduction losses per unit current were characterized by the cathode effective voltage uef, which is weakly dependent on the current. In this paper, a physical model and a mathematical approach were developed to describe the energy dissipation due to heat conduction in the cathode body, which is heated by energy outflowed from the adjacent plasma. The arc plasma generation was considered by taking into account the kinetics of the heavy particle fluxes in the non-equilibrium layer near the vaporizing surface. The phenomena of electric sheath, heat and mass transfer at the cathode were taken into account. The self-consistent numerical analysis was performed with a system of equations for a copper cathode spot. The transient analysis starts from the spot initiation, modeled by the plasma arising at the initial time determined by the kind of arc triggering, up to spot development. The results of the calculations show that the cathode effective voltage uef is determined by the cathode temperature as a function of spot time. The calculated evolution of the voltage uef shows that the steady state of uef is approximately 7 V, and it is reached when the cathode temperature reaches a steady state at approximately one microsecond. This essential result provides an explanation for the good agreement with the experimental cathode effective voltage (6–8 V) measured for the arc duration from one millisecond up to a few seconds.

Performance analysis of photon-enhanced thermionic emission systems mediated by quantum tunneling

Reducing the gap between the electrodes to the nanoscale and utilizing quantum effects are an effective way to enhance the performance of a thermionic energy device. In this work, we establish the model of a photon-enhanced thermionic emission system with a nanoscale vacuum gap, where the electron transport due to electron tunneling and the near-field radiation resulting from photon tunneling are introduced. Analytical expressions for the thermionic emission current, electron tunneling current, and heat flux due to the near-field radiation are provided. By using the energy and particle balance equations, the electron concentration and the temperature of the cathode are determined. The impacts of the voltage, electron affinity, and gap distance on the performance are further analyzed. Results show that the suggested system can achieve high efficiency at the low-temperature cathode. Up to 34.7% of solar-to-electricity efficiency is possible at a cathode temperature of 472.5 K. The proposed model provides a strategy for designing highly efficient thermionic emission devices operating at low temperatures.

LiCoO2 sintering aid towards cathode-interface-enhanced garnet electrolytes

Garnet-type Li7La3Zr2O12 (LLZO) has high ionic conductivity and good compatibility with lithium metal. High-temperature processing has been proven an effective method to decrease the interface resistance of cathode|LLZO. However, its application is still hindered by the interlayer co-diffusion with the cathode and high sintering temperature (>1200 °C). In this work, a new garnet-type composite solid-state electrolyte (SSE) Li6.54La2.96Ba0.04Zr1.5Nb0.5O12-LiCoO2 (LLBZNO-LCO) is firstly proposed to improve the chemical stability and electrochemical properties of garnet with high-temperature processing. Small doses of LCO (3%) can significantly decrease the LCO|SSE interface resistance from 121.2 to 10.1 Ω cm2, while the sintering temperature of garnet-type LLBZNO decreases from 1230 to 1000 °C. The all-solid-state battery based on the sintered LLBZNO-LCO SSE exhibits excellent cycling stability. Our approach achieves an enhanced LCO|SSE interface and an improved sintering activity of garnet SSE, which provides a new strategy for optimizing the comprehensive performance of garnet SSE.

Caesium balance of the ISIS H- Penning ion source in long pulse operation

We have developed a semi-empirical model predicting the equilibrium caesium coverage (in monolayers) and the resulting work function of the ISIS Penning ion source cathode (in eV) in long pulse operation. We use the caesium balance model to predict the temporal structure of the extracted H- beam current pulses and compare the model predictions to experimental data with 60 mA, 2 ms beam pulses at 50 Hz. The experimentally observed droop of the beam current is reproduced by the model. The results imply that extracting square beam pulses in the long pulse operation mode requires more efficient cathode cooling. We apply the model for 70 mA, 3 ms beam pulses at 14 Hz, relevant for ESSνSB project. We conclude that achieving these parameters requires reducing the cathode temperature by 200 °C, and ramping the discharge current to cancel the discharge voltage droop.

Breakdown mode and parameter space of micro-discharge sustained by thermionic emission

Gas breakdown driven by thermionic emission in a microgap to produce low-temperature plasma is studied using a 1D implicit particle-in-cell/Monte Carlo collision model. The influence of background gas pressure, external driving voltage, cathode temperature and discharge gap on argon glow micro-discharge in the parallel plate was simulated. Different parameters and conditions have different effects on the gas breakdown at small size. The discharge gap of hundreds of µm has little influence on the gas breakdown and only changes the plasma distribution. As the applied voltage increases, the gas changes from a non-breakdown mode to a breakdown mode, and the gas breakdown is more sensitive to the applied voltage than the gas pressure at low voltages. In all breakdown modes, the gas pressure hardly changes the plasma evolution characteristics. At appropriate cathode temperatures, the density of electrons and ions increases rapidly, forming a stable sheath, and the equivalent resistance of the discharge gap becomes smaller as the temperature rises and the plasma is in abnormal glow discharge.

Self-Sustaining Control Strategy for Proton-Exchange Membrane Electrolysis Devices Based on Gradient-Disturbance Observation Method

This paper proposes a self-sustaining control model for proton-exchange membrane (PEM) electrolysis devices, aiming to maintain the temperature of their internal operating environment and, thus, improve the electrolysis efficiency and hydrogen production rate. Based on the analysis of energy–substance balance and electrochemical reaction characteristics, an electrothermal-coupling dynamic model for PEM electrolysis devices was constructed. Considering the influence of the input energy–substance and the output hydrogen and oxygen of PEM electrolysis devices on the whole dynamic equilibrium process, the required electrical energy and water molar flow rate are dynamically adjusted so that the temperature of the cathode and the anode is maintained near 338.15 K. The analytical results show that the hydrogen production rate and electrolysis efficiency are increased by 0.275 mol/min and 3.9%, respectively, by linearly stacking 100 PEM electrolysis devices to form a hydrogen production system with constant cathode and anode operating temperatures around 338.15 K in the self-sustaining controlled mode.

Theoretical research and simulation of GaAs nanowire arrays in reflection-type photon-enhanced thermionic emission solar converters

In this paper, GaAs was used as the cathode of the photon-enhanced thermionic emission (PETE) solar converter. Based on the one-dimensional continuity equation and boundary conditions, a reflection-type PETE theoretical model was established to simulate the effects of solar concentration, electron affinity, cathode temperature and electron recombination velocity on the current density and the conversion efficiency. For optimizing the performance of the solar converter, we used GaAs nanowire (NW) arrays as the cathode and calculated the influences of NW arrays’ period, duty ratio, and height on the absorbance and the conversion efficiency by combining the theoretical model with the three-dimensional finite-difference time-domain (3D FDTD) method. The results were used to design the structure size of NW arrays, which improved the conversion efficiency of the solar converter to 23.07%. It shows that the GaAs NW arrays are promising PETE cathode materials.

Thermal Assessment of Dielectric Microspacer Technology Using an Advanced Three-Dimensional Simulation Model

Dielectric microspacers (DMS) are a novel micro-technology that can be used to achieve a fixed micron/sub-micron gap distance between two separated surfaces, such as the emitter (cathode) and the PV cell (anode) of a near-field thermophotovoltaic converter (TPV). One of the system’s challenges is the flow of undesirable excess thermal energy from the cathode to the anode that might cause the PV cell to overheat. This work investigates the possibility of integrating this technology into a hybrid thermionic-photovoltaic (TIPV) converter operating at ultra-high temperatures (>1000 °C) without any risk of collector’s overheating, which might lead to its mechanical failure. A steady-state 3-D CFD model was developed in Fluent v17.1 solver to assess the system’s thermal behavior when the two electrodes were separated by a distance of 8–10 μm. The heat transfer through conduction across the system components and the net photon/electron flux between the two electrodes were simulated. Different cathode temperatures within the range of 1500–2500 K and various DMS shapes (capillary, cylindrical), patterns (e.g., ring-shaped) and sizes were studied. Results show that thermal performance is not affected by the DMS pattern, even for thermal conductivities of 80 W/(m·K), whereas the possibility of mechanical failure is considerable for Tcathode > 2000 K.

N-p junction to enhance solar thermionic energy conversion: From a thermodynamic point of view

Solar thermionics, owing to its silent operation and potential effectiveness in converting solar energy into electricity, is becoming a promising advanced technology for concentrated solar power. However, conventional solar thermionic emission and associated solar-to-electricity conversion are weakened because a portion of the cathode’s energy is transmitted to the anode in the form of thermal radiation and eventually dissipated into waste heat. In this paper, a n-p junction is employed as the cathode to couple thermoradiative and thermionic emission into a single process, lowering thermal radiation in the interelectrode while generating additional electricity. An intricate thermodynamic model is developed to investigate the effects of various system factors, such as concentration ratio and cathode electron affinity, on solar cathode-thermoradiative enhanced thermionic (C-TET) conversion. The findings reveal that the increasing induced potential barrier of thermoradiatives raises the cathode temperature, compensating for decreased thermionic current at elevated thermionic voltages. When the concentration ratio is 1000, solar C-TET surpass conventional metal thermionics in terms of solar-to-electricity efficiency by 27.5 % at a cathode bandgap of 0.15 eV and electron affinity of 1.8 eV. The n-p junction boosts thermionic exergy efficiency by 13.9 % and contributes extra thermoradiative exergy of 3.45 %, resulting in a 25.8 % increase in overall power generation over metal thermionics. Temperature-entropy diagram is also established to identify the irreversibility and principal performance bottleneck of solar C-TET conversion. Conduction band electrons transit to the valence band with a modest entropy production of 17.29 W/K·m−2 at n-p junction, while thermionic emission produces 101.91 W/K·m−2.