Thermal Neutral Voltage Research Articles

Simultaneous electrochemical reduction of water and oxidation of carbon in a solid oxide cell for separately producing hydrogen and carbon monoxide

Hydrogen can be produced from reduction of water at the cathode of a solid oxide electrolysis cell (SOEC) and electricity and CO gas can be co-generated from the oxidation of carbon at the anode of a direct carbon solid oxide fuel cell (DC-SOFC). In the present work, a solid oxide cell (SOC) combining the cathode reaction of the SOEC and the anode reaction of the DC-SOFC is proposed, prepared, and investigated. It is demonstrated that hydrogen and CO can be separately produced from the cathode and anode through simultaneous reduction of water and oxidation of carbon in such a H2O-C SOC. Resistances from electrode polarizations are analyzed through measuring current-voltage characteristics, overpotentials, and AC impedances of the cell. The advantage of the H2O-C SOC over two separately operated SOEC and DC-SOFC is verified by the resistance analysis. An electrolyte-supported H2O-C SOC, with a cermet of silver and gadolinium-doped ceria (Ag-GDC) as the symmetrical electrode material and a steam concentration of 50 % at the cathode, gives an operating current density of 0.67 A cm-2 at the thermal neutral voltage (0.7 V). It stably operates at 0.2 A cm-2 under an applied voltage of only 0.16 V, producing pure hydrogen from the cathode and CO with a concentration of 88 % from the anode. The performance is significantly improved by using an anode-supported H2O-C SOC which gives an operating current density of 2.27 A cm-2 at the thermal neutral voltage.

Enhancing durability of solid oxide cells for hydrogen production from seawater by designing nano-structured Sm0.5Sr0.5Co3-δ infiltrated air electrodes

In this work, the nano-structured Sm0.5Sr0.5CoO3-δ (SSC) was infiltrated into La0.6Sr0.4Co0.2Fe0.8O3-δ-Gd0.1Ce0.9O2-δ (LSCF-GDC) electrode of a large-area flat-tube solid oxide cell for hydrogen production. The electrolysis current density under 76% H2O-24% H2 increased only approximately 3% at about the thermal neutral voltage (1.29 V) and 750 °C, but the electrolysis durability with 76% H2O-24% H2 and 500 mA cm−2 improved significantly by more than 90%, with the degradation rate being approximately 1/10th of that of the non-infiltrated cell. This work indicates that SSC infiltration of the air electrode could suppress Sr segregation in LSCF, thus greatly enhancing the cell durability. The study provides a reference for large-scale hydrogen production of high durability SOECs by designing nanostructured electrodes via infiltration.

(Invited) Durability of Electrolyte Supported Solid Oxide Cells for Steam Electrolysis: Results from Cell Testing in the 20,000 h to 50,000 Hours Range

There is a current fast development of water/steam electrolysis for the production of hydrogen from renewable energy sources. (Multi) MW power demonstration units using the different (low and high temperature) technologies are, e.g., subject of the R&D project H2Giga funded by the German Federal Ministry of Education and Research /1/. Steam electrolysis with solid oxide cells (SOC) is characterised by a high energy-conversion efficiency, notably if steam is available, such as via a heat source of moderate temperature for steam generation. Approaches for a matching to a fluctuating load, important in the context of operation with renewable energy sources, are also under development with the SOC technology /2/.For the SOC electrolysers, durability analysis of the cells in a time scale appropriate for application remains an important development step. Cell (or short stack) testing allows a rather precise control of the cell temperature which facilitates quantification of degradation for the practically required degradation rates well below 10 mV/1000 h (cf. /3/). In this contribution, our testing results will be updated, with up to about 50,000 h operation using electrolyte supported cells (ESC) with Ni/GDC hydrogen electrodes and different types of electrolyte (10Sc1CeSZ, 6Sc1CeSZ, 3YSZ). As reference for cell durability serves a 23,000 h steam electrolysis test with a cell with 6Sc1CeSZ electrolyte /4/. However, cells with the known robust 3YSZ electrolyte seem to yield a comparably low long-term degradation, together with better initial stability /5/. The use of an electrolyte of high ionic conductivity, 10Sc1CeSZ, allows a relatively low initial temperature (<800°C) for a cell voltage close to the thermal neutral voltage for 0.6 Acm-2 current density. This, in turn, leaves a wide temperature margin for compensation of voltage degradation via a temperature increase (cf. initial period of the test in /3/). The results from the durability tests serve as examples for a large degree of SOFC/SOEC reversibility achievable with the ESC. Moreover, the relatively thick electrolyte layer of the ESC with its large ohmic resistance comes along with a high ohmic contribution in the overall degradation in most tests. The different degradation contributions were separated with in-situ impedance spectroscopy without interrupting the DC current flow.

A study on solid oxide electrolyzer stack and system performance based on alternative mapping models

In this study, a solid oxide electrolyzer cell (SOEC) stack model is developed based on an alternative mapping concept. The SOEC stack performance in a commercial hot box is systematically studied under different operating currents, flow rates, and flow directions. The results revealed that the SOEC stack operated in a hot box has thoroughly different temperature distributions, resulting in additional efficiency losses and an increase in thermal neutral voltage. The SOEC stack model computation results are summarized into stack performance diagrams and used in the system design. A 6-Nm 3 /h SOEC system with preheaters and recycling cathode materials is designed, and its performance is studied. The system efficiency is greatly influenced by the steam generator, and an external steam source can help increase the total efficiency of the system to more than 83%. Even the current increase may deteriorate the stack performance. It can increase the SOEC system efficiency by saving energy in the steam generator and preheaters. An increase in the flow rate around anode and cathode can improve the system capacity and efficiency. The system's maximum capacity is limited by the preheater heat balance and the stack output temperature. The feasible maximum system capacity is 33.4 kW electrolysis electric power input and 9.93 Nm 3 /h hydrogen production rate. At a constant system capacity, decreasing the air flow rate can minimize the heat losses in anode off-gas and achieve more than 87% nonsteam system efficiency. • A new SOEC stack modeling method based on the alternative mapping concept. • The differences of the stack performances in lab scale test and commercial installation circumstances are fully discussed. • A 6 m 3 /h SOEC demo system is designed and analyzed to get the best capacity and efficiency.

Three‐dimensional Modeling and Performance Optimization of Proton Conducting Solid Oxide Electrolysis Cell▴

AbstractIn this study, a three‐dimensional model is developed to study a novel proton conducting solid oxide electrolysis cell (H‐SOEC) with porous current collector to alleviate the H2O‐starvation problem in oxygen electrode. The comparison between conventional straight‐channel structure and the new porous structure is conducted in detail. It is found that the porous current collector can increase the flow uniformity and facilitate the diffusion of H2O, which in turn improves the average performance of the H‐SOEC. The electrolysis efficiency can be increased by 6.4% compared to straight‐channel structure. It is also found that the thermal neutral voltage is decreased when the porous current collector is adopted. Besides, the evaluation of syngas production for CO2‐supplied H‐SOEC is performed. Compared to H2 production, the efficiency of syngas production can be increased by up to 13.5%.

Solid Oxide Electrolyser Cell Testing Up to the Above 30,000 h Time Range

A running steam electrolysis test with a 10Sc1CeSZ electrolyte supported cell with Ni/GDC hydrogen and LSCF oxygen electrodes has passed 34,000 h with -0.6 Acm-2 current density. The high specific ionic electrolyte conductivity allowed a moderate initial cell temperature (~780°C) for operation with a cell voltage close to the thermal neutral voltage (U th ~1.29 V), i.e. with highest energy-conversion efficiency. Part of the wide margin for temperature increase was used for compensation of the voltage degradation, done after 20 and 31 kh, respectively. The resulting compensation rate of 0.9°C/kh extrapolates to a degradation compensated lifetime beyond current system requirements (>50,000 h). Compensation was traced with voltage-current curves and with impedance. Voltage degradation for constant temperature amounted, after several kh initial stabilisation, to (4 - 5) mV/kh (0.3 - 0.4 %/kh). Degradation was mainly ohmic, without clearly detectable non-ohmic electrode contributions. Non-ohmic terms therefore added to the compensation of ohmic degradation via temperature increase.

Solid Oxide Electrolyser Cell Testing up to the Above 30,000 h Time Range

Steam (and co-) electrolysis demonstration projects with solid oxide cells (SOC) have reached a power range of several hundred kW (e.g., /1/). This generates an urgent demand for verification of the durability of the cells as core electrolyser elements. Cell lifetimes of several years are required, which largely exceeds the commonly reported testing periods. Apart from economic considerations, cell (or short stack) testing allows a more precise control of the cell temperature together with less temperature variation across the surface. This, in turn, facilitates quantification of degradation, in particular for the practically required low degradation rates of <10 mV/kh. Appropriate durability was confirmed with a 23 kh steam electrolysis test with an electrolyte supported cell with a Ni/GDC hydrogen electrode (20 kh at -0.9 Acm-2 with 7.3 mV/kh voltage degradation /2/). This result represents an example for SOFC/SOEC reversibility holding even for a current density magnitude exceeding the commonly applied values under SOFC operation (higher current densities are feasible in the electrolysis mode owing to the more favourable thermal conditions, but they will, in general, imply the development of appropriate cells). Comparably low degradation was confirmed in a one year long test coupling steady-state and ON/OFF switching operation (80,000 cycles) for a cell with a thin 3YSZ electrode instead of the 6Sc1CeSZ in the 23 kh test /3/. ON/OFF stack switching is part of an operation strategy for matching a fluctuating load (such as coupling with renewable energy sources or grid stabilisation).In this contribution, emphasis will be put on a cell test of even longer duration (>30 kh so far) with an electrolyte supported cell, maintaining the current density above the typical SOFC values (initial part of the test in ref. /4/). The operation conditions are chosen for a cell voltage close to the thermal neutral voltage (Uth ~1.3 V), in accord with the requirements for stack operation. The use of an electrolyte of high ionic conductivity, 10Sc1CeSZ, allows a moderate initial temperature below 800°C at the not degraded cell. Temperature is later increased for compensation of degradation. For an appropriate setting of the initial testing parameters together with a low cell degradation and a sufficiently wide temperature window, one should then obtain an operation close to the theoretical efficiency limit with zero voltage degradation during the entire envisaged lifetime. As in our previous work, the long-term cell behaviour is traced with in-situ impedance spectroscopy without interrupting the DC current flow. The main focus of the degradation analysis refers to the ohmic degradation, which typically dominates in the cells of concern /2-4/.

Thermal effects in H2O and CO2 assisted direct carbon solid oxide fuel cells

Thermal effects in a H2O and CO2 assisted tubular direct carbon solid oxide fuel cell (DC-SOFC) are numerically investigated. Parametric simulations are further conducted to study the effects of operating potential, the distance between carbon and anode, inlet gas temperature, and anode inlet gas flow rate on the thermal behaviors of the fuel cell. It is found that the fuel cell with H2O as gasification agent performs considerably better than the cell with CO2 as gasification agent in all cases. It is also found that the temperature field of the fuel cell is highly uneven. The breakdown of the heat sources in the fuel cell shows that the H2O assisted DC-SOFC has much higher heat generation and consumption than the CO2 assisted cell. Interestingly, a thermal neutral voltage is observed, at which no heating or cooling of the cell is needed. In addition, the distance between the anode and the carbon layer is required to be as small as possible, which improves the temperature uniformity of the fuel cell. The results of this study demonstrates the importance of thermal effects in DC-SOFCs and form a solid foundation for DC-SOFC thermal management.

80,000 current on/off cycles in a one year long steam electrolysis test with a solid oxide cell

The current ON/OFF switching of a solid oxide electrolysis cell is treated as elementary step for power variation in a steam electrolyser system. If the cell voltage in the ON mode is adjusted to the thermal neutral voltage, heat generation remains zero in both modes, which largely facilitates the thermal management. To verify whether the cells withstand the switching, an electrolysis durability test with an electrolyte supported solid oxide cell was performed during one year at about 850 °C. The cell consisted of a 3YSZ electrolyte, CGO diffusion-barrier/adhesion layers, a lanthanum strontium cobaltite ferrite (LSCF) oxygen electrode, and a nickel/gadolinia-doped ceria (Ni/GDC) steam/hydrogen electrode. The test included two operation blocks with each 40,000 cycles of 2 min duration and a current density of −0.7 Acm−2 in the ON mode (−0.07 Acm−2 in OFF mode), as well as steady-state ON periods with 5800 h duration. Voltage degradation was 5 mV/1000 h (0.4%/1000 h) and the increase in the area specific resistance 7 mΩcm2/1000 h, without notable dependence on current cycling. Impedance spectroscopic results were in agreement with the only small switching transients seen in the cell voltage; moreover, they confirmed a dominating ohmic degradation together with minor contributions from gas conversion and reaction, respectively. No electrode delamination was detectable after scheduled test completion.

Modelling of effect of pressure on co-electrolysis of water and carbon dioxide in solid oxide electrolysis cell

As a promising renewable energy storage device, the solid oxide electrolysis cell (SOEC) attracts wide attention in the world. In this study, the effect of operating pressure on the co-electrolysis of water and carbon dioxide in SOEC is investigated by a three-dimensional model, in which the reversible water gas shift reaction and direct internal reforming reaction are considered. After comparison with experimental data, the influence of operating pressure on the polarization loss, electrolysis reaction rate, chemical reaction rate and thermal-neutral voltage is also studied in detail. The results show that the cell voltage increases below 8 atm and then decreases with the operating pressure. In addition, the effects of thermal insulation boundary condition, gas flow configuration and gas utilization rate under different operating pressures are also discussed. It is found that the reverse direct internal reforming reaction is activated under the operating pressure higher than 3 atm. Moreover, compared to co-flow arrangement, counter-flow arrangement is more helpful to cell performance improvement at high current densities.

Dynamic electro-thermal modeling of co-electrolysis of steam and carbon dioxide in a tubular solid oxide electrolysis cell

SOEC (Solid oxide electrolysis cell) has great potential to store unstable renewable energy in form of steady chemical energy. For this application, transient behavior in variable working condition is crucial. A 2D (two-dimensional) dynamic model was developed to predict the dynamic response of H2O/CO2 co-electrolysis in TSOEC (tubular solid oxide electrolysis cell). The dynamic model comprehensively considered the macroscopic fluid flow, microscopic diffusion, heat transfer and electrochemical/chemical reactions in the TSOEC. In this paper, the TNV (thermal neutral voltage) of TSOEC was specifically defined and calculated. The time constants of charge/mass/heat transport processes were evaluated. Further, the responses of current, gas concentrations, temperature, efficiency, conversions to electricity/gas flow/temperature step inputs were respectively studied. Through the study, appropriate transient operations were designed to improve efficiency and conversions.

Electrochemical performance and long-term durability of a 200 W-class solid oxide regenerative fuel cell stack

The electrochemical performance and durability of a 200 W-class solid oxide regenerative fuel cell (SORFC) stack are investigated for cyclic mode-changing and long-term operation. Three unit cells (10 cm × 10 cm), each based on a Ni – yttria-stabilized zirconia (YSZ) fuel electrode, a scandia-stabilized zirconia (ScSZ) electrolyte and a Sr-doped LaCoO3 (LSC) – gadolinia-doped ceria (GDC) air electrode, are used for the stack development. Delamination of the air electrode is suppressed by using a mixed ionic- and electronic-conducting air electrode with no oxygen excess non-stoichiometry, and gas leakage is minimized by using novel glass-ceramic composite sealants. Excellent electrochemical performance is achieved in a single cell level by minimizing the ohmic and electrode polarizations, and the three-cell stack is successfully configured without major performance loss associated with electrical contacts, gas supply or sealing. Stable operation is confirmed at a thermal neutral voltage for 1000 h in the solid oxide electrolysis cell (SOEC) mode, and the periodic change of the operation mode between the solid oxide fuel cell (SOFC) and SOEC is found to accelerate the performance degradation. The effect of cyclic mode-changing on the stability of the SORFC stacks is discussed in detail based on the observations from the post-mortem microstructural analysis.

고온열을 이용한 고온수증기전기분해장치(HTSE)에 의한 수소생산 특성에 관한 전산유체해석적 연구

The characteristics for hydrogen production and the thermochemical properties of high temperature steam electrolysis(HTSE) device have been numerically analyzed in a two-dimension, steady-state with using the COMSOL . The main parameters for the calculation are applied voltage, ASR(Area-specific Resistance), temperature and pressure of the inlet gas flow. The results showed that thermal-neutral voltage was 1.2454 V and rather than the cell temperature increases or decreases with increasing applied voltage by thermal-neutral voltage starting this voltage the temperature in high voltage tended to rise and temperature in the low voltage tended to fall. And with, increasing the values of ASR, temperature inside the cell and the hydrogen production rate were decreased.

Modeling of a solid oxide electrolysis cell for carbon dioxide electrolysis

In this study, two models are developed to investigate the performance of a solid oxide electrolysis cell (SOEC) for CO 2 electrolysis at different levels. The first model is a one-dimensional model which is basing on a previously developed electrochemical model for steam electrolysis and considered all overpotentials in the SOEC. The second model is a two-dimensional thermal-fluid model consisting of the 1D model and a computational fluid dynamics (CFD) model. It is found that the mean electrolyte temperature initially decreases with increasing operating potential, reaches the minimum at about 1.1 V and increases considerably with a further increase in potential. At the thermal-neutral voltage (1.463 V at 1173 K), the calculated mean electrolyte temperature matches the inlet gas temperature. Increasing the operating potential increases both the local current density and the electrolyte Nernst potential. The electrochemical performance can be improved by increasing the inlet gas velocity from 0.2 m s −1 to 1.0 m s −1 but further increasing the inlet gas velocity will not considerably enhance the SOEC performance. It is also found that a change of electrode permeability in the order of 10 −16 m 2 to 10 −13 m 2 does not noticeably influence the SOEC performance in the present study, due to negligible convection effect in the porous electrodes.

Performance of Planar High-Temperature Electrolysis Stacks for Hydrogen Production from Nuclear Energy

An experimental program is under way to assess the performance of solid-oxide cells operating in the steam electrolysis mode for hydrogen production in a temperature range from 800 to 900°C. This temperature range is consistent with the planned coolant outlet temperature range of advanced nuclear reactors. Results were obtained from two multiple-cell planar electrolysis stacks with an active area of 64 cm2 per cell. The electrolysis cells are electrolyte-supported, with scandia-stabilized zirconia electrolytes (~140 μm thick), nickel-cermet steam/hydrogen electrodes, and manganite oxygen-side electrodes. The metallic interconnect plates are fabricated from ferritic stainless steel. The experiments were performed in a range of steam inlet mole fractions (0.1 to 0.6), gas flow rates (1000 to 4000 standard cubic centimeters per minute), and current densities (0 to 0.38 A/cm2). Steam consumption rates associated with electrolysis were measured directly using inlet and outlet dewpoint instrumentation. Cell operating potentials and cell current were varied using a programmable power supply. Values of area-specific resistance and stack internal temperatures are presented as a function of current density. Initial stack-average area-specific resistance values <1.5 Ω·cm2 were observed. Hydrogen production rates in excess of 200 normal liters per hour (NL/h) were demonstrated. Internal stack temperature measurements revealed a net cooling effect for operating voltages between the open-cell potential and the thermal neutral voltage. These temperature measurements agreed very favorably with computational fluid dynamics predictions. A continuous long-duration test was run for 1000 h with a mean hydrogen production rate of 177 NL/h. Some performance degradation was noted during the long test. Stack performance is shown to be dependent on inlet steam flow rate.

Electrolyzer Power Requirements for Oxidizer Production on Mars

Proton-exchange-membrane electrolyzers and solid-oxide electrolyzers have both been proposed for propellant production on Mars. Past studies assume the latter requires more energy because solid-oxide electrolyzers have significantly higher operating temperature. However, heat-transfer needs of proton exchange membrane electrolyzers are often neglected. On Mars, where the atmosphere has a mean temperature of 215K, heat transfer must be considered. Notably, theoretical analyses show that the total energy requirements of both electrolyzers for a given oxygen production are equivalent. Also in opposition to common perception, calculations show heat-transfer direction is not dependent upon current but rather operating temperature and voltage. If either electrolyzer is operated above its thermal neutral voltage, excess power will be dissipated as heat. If either electrolyzer is operated below it, the electrolyzer will require additional energy in the form of heat to maintain its operating temperature. If the required thermal energy is available from another component within a propellant production plant, then operating the electrolyzer at its theoretical minimum voltage is more efficient. Calculations show that a solidoxide electrolyzer minimum voltage is at least 20% less than a proton-exchange-membrane electrolyzer minimum voltage. Experimental observations and data are presented to corroborate calculations. Nomenclature � =F araday’s constant ¯ h = molar enthalpy I = current k H = Henry’s constant M = molecular weight