Steam Partial Pressure Research Articles

Reaction kinetics and product evolution of hydrolysis in copper-chlorine thermochemical cycle for hydrogen production

By-products of the hydrolysis reaction in the copper-chlorine thermochemical cycle for hydrogen production are key to the final thermal and hydrogen efficiency. The current study focuses on the influences of reaction time, temperature, and steam partial pressure on the hydrolysis reaction and by-product formation mechanism via experimental and multiple analytical methods. The result demonstrates that the hydrolysis reaction of CuCl2 and steam converts to CuCl and Cu2OCl2, with a high conversion of 97%, while the Cu2OCl2 and CuCl2-CuO product mixtures exhibit interconversion capabilities. The increase of temperature increases the CuCl product content, while the decrease of steam partial pressure increases the by-product CuCl2. Particle morphology and reaction thermodynamics are analyzed for main and by-products formation. Finally, the homogeneous reaction model and shrinking core model are used and compared for the activation energy of the formation of Cu2OCl2 for the hydrolysis reaction and the effect of by-products.

Stack-like Contacting in Solid Oxide Cells: Electrochemical Characterization and Modeling

The integrity of metallic interconnectors (MICs) in a solid oxide cell stack is crucial because contact resistances or limitations in gas supply may occur. In this contribution, a Crofer 22 APU® interconnector with a (Mn, Co, Fe)3O4 spinel oxide (MCO) coating and a lanthanum-strontium-manganese-cobalt oxide (LSMC) contact layer at the air side was investigated. The electrochemical behavior was characterized by means of IV-characteristics, impedance spectroscopy and DRT analysis. In particular, the contact losses at the air side were measured with targeted potential probes. With respect to the contact layer mounted in a dried state, the application of a stack-like clamping pressure of 1MPa showed a significant decrease of the contact resistance. In order to extend an existing zero-dimensional performance model for an electrolyte-supported cell with a Ni/GDC fuel electrode and LSCF air electrode, a method was established to parameterize contact losses at the air electrode. The observed activation energy of the contact losses showed to be independent of the clamping pressure. Additionally, the dependency of the cell´s intrinsic ohmic losses towards the steam partial pressure at the Ni/GDC fuel electrode was quantified and included to the model. Simulation studies were validated with experimental data for technical operating conditions.

Experimental investigation of the corrosion behavior of Eurofer97 steel in contact with Lithium ceramic breeder pebbles under specific Helium Cooled Pebble Bed breeding zone atmosphere

For sub-sized fatigue specimens made of EUROFER97 in unconstrained contact to ceramic breeder pebbles, exposed to purge gas conditions for different durations in an oven, a chemical surface attack was observed which led to a significant reduction of fatigue lifetime [Aktaa et al., 2020]. To better reproduce the flow of the purge gas in the breeding zone of a Helium-Cooled Pebble Bed Breeding Blanket, the experiment was now repeated by placing the same kind of samples in the helium loop HELOKA HEMAT, where the gas is permanently circulated in a closed circuit, and its water content is controllable. The composition of the gas was monitored with a mass spectrometer and humidity sensors. The samples were exposed to a mixture of helium and 0.1vol% hydrogen at DEMO relevant operating conditions (550 °C, 1.2 bars abs.) for a duration of 8, 16, 32, and 64 days, respectively. New sample holders were designed which allow direct contact of the samples with the gas mixture. A special procedure for the handling of the test section was implemented to avoid contact of the hygroscopic pebbles with air humidity during the preparation phase and during the extraction of the samples from the test rig.The present contribution discusses the various considerations on which the test rig design was based, followed by a description of the experimental setup and preliminary results of the testing campaign. These are similar to the results achieved in the oven experiment. In the future, the loop will be used for tests with up to 200 Pa partial pressure of steam in helium; a mixture that is considered relevant for the DEMO purge gas composition.

Thermochemical heat storage performance and structural stability of SiO2-coated CaO particles under fluidization in CaO/Ca(OH)2 cycles

The decline in CaO/Ca(OH)2 heat storage performance of CaO-based material with the number of cycles due to its fast expansion and fragmentation is an problem in the fluidized bed reactor. In this paper, a novel SiO2-coated CaO particle was manufactured from limestone and silica sol via wet-mixing method. Exothermic performance (such as exothermic temperature, effective hydration conversion and volumetric energy density) and structural stability of SiO2-coated CaO particles in a fluidized bed reactor were studied. The effects of SiO2 addition and calcination temperature on the exothermic performance and structural stability were analyzed, respectively. The results show that the mass ratio of limestone to silica sol of 1:1 and calcination temperature of 850 °C are recommended for the preparation of SiO2-coated CaO. The SiO2 layer is observed on the surface of the CaO. SiO2-coated CaO particles exhibit superior structural stability than uncoated CaO particles. The SiO2 coating reduces the increase of average particle size during 10 cycles from 33 % to 11 %. After 10 heat storage cycles, SiO2-coated CaO particles achieve a reduction of 45.4 % in expansion rate and 48.5 % in attrition rate compared with uncoated CaO particles, respectively. The volumetric energy density of SiO2-coated CaO particles after 10 cycles is 33.2 % higher than that of uncoated CaO particles. The highest exothermic temperature of SiO2-coated CaO particles after 10 cycles reaches 455.4 °C (steam partial pressure @ 60 kPa). The SiO2-coated CaO seems a promising candidate for the CaO/Ca(OH)2 thermochemical heat storage in the fluidized bed rector.

Degradation Issues in Solid Oxide Electrolysers

One of the crucial aspects to achieving long lifetime and high-efficient SOCs (Solid oxide cells) is to enhance durability of the current devices.Degradation issues are typically associated with problems in both electrodes. For example, nickel-yttria stabilized zirconia (Ni-YSZ) hydrogen electrodes present under extreme conditions of high steam partial pressures, a low durability due to Ni oxidation resulting in lowered electronic conductivity and catalytic activity of the electrode. Ni agglomeration, depleting and micro crack in Ni-containing hydrogen electrode can also occur leading to a lower activity of the Ni-based electrodes as an SOEC fuel electrode. Furthermore, carbon deposition and sulfur poisoning on the Ni surface are also lead to cell performance degradation and poor durability.In addition, the performance of the oxygen electrode is also particularly more important in electrolysis mode than in fuel cell mode, as it is well established that electrochemically induced oxygen pressure increase at the electrolyte-oxygen electrode interface and subsequent membrane failure have been theoretically predicted and experimentally observed in SOEC mode. For example, several examples in the literature reported the presence on voids at the YSZ electrolyte, leading to delamination of the oxygen electrode. In this sense, lanthanide nickelates (Ln = La, Nd, Pr) have received considerable interest as materials for IT-SOFC electrodes and oxygen separation membranes, and they seem to be very attractive for electrolysis applications. The hyperstoichiometry of some oxygen electrode materials such as these Ruddlesden-Popper phases is believed to be favourable for effective oxygen evolution, as performance of these electrodes is enhanced in SOEC mode.Different strategies to develop optimized electrode structures as well as controlled operating conditions will be discussed in order to improve the durability of SOCs.

Significance of Pressure Drop, Changing Molar Flow, and Formation of Steam in the Accurate Modeling of a Multi-Tubular Fischer–Tropsch Reactor with Cobalt as Catalyst

A Fischer–Tropsch (FT) fixed-bed reactor was simulated with reactor models of different complexities to elucidate the impact of a pressure drop, a change in the total molar volume rate (induced by the reaction) along the tubes, and a change in the axial variation of the external radial heat transfer coefficient (external tube wall to cooling medium, here, boiling water) compared to disregarding these aspects. The reaction kinetics of CO conversion for cobalt as a catalyst were utilized, and the influence of inhibition of syngas (CO, H2) conversion reaction rate by steam, inevitably formed during FT synthesis, was also investigated. The analysis of the behavior of the reactor (axial/radial temperature profiles, productivity regarding the hydrocarbons formed, and syngas conversion) clearly shows that, for accurate reactor modeling, the decline in the total molar flow from the reaction and the pressure drop should be considered; both effects change the gas velocity along the tubes and, thus, the residence time and syngas conversion compared to disregarding these aspects. Only in rare cases do both opposing effects cancel each other out. The inhibition of the reaction rate by steam should also be considered for cobalt as a catalyst if the final partial pressure of steam in the tubes exceeds about 5 bar. In contrast, the impact of an axially changing heat transfer coefficient is almost negligible compared to disregarding this effect.

Analysing the effects of process parameters on HDPE pyrolysis and in-line reforming

This study presents a comprehensive parametric investigation focusing on the pyrolysis and in-line reforming of High-Density Polyethylene (HDPE) at zero time on stream. The research explores the impact of varying operating parameters, specifically examining the effects of temperature, space time, and steam/plastic (S/P) ratio on conversion, hydrogen production, and product yields. The temperature was varied within the range of 600–750 °C, demonstrating notable influence on conversion efficiency and hydrogen production. Additionally, the effect of space-time (2.8–20.8 g catalyst min gHDPE−1) was thoroughly evaluated, revealing substantial improvements in both conversion and hydrogen yields. Moreover, the S/P ratio of 1 to 3 was investigated, showcasing significant enhancements in hydrogen and CO2 yields with increasing steam partial pressure. The study provides valuable insights into optimizing the process parameters for HDPE pyrolysis and in-line reforming, which are crucial for achieving efficient conversion and enhancing hydrogen production.

Numerical Analysis of Humidification Condition Effects on Protonic Ceramic Fuel Cell Performance Considering Concentration Overpotential

Protonic ceramic fuel cells (PCFCs) show relatively high performance at intermediate temperature range (400–600 °C) due to high proton conductivity in electrolyte. However, besides proton, other charges such as electron hole and oxygen vacancy can also be conductive in PCFCs. Especially, the hole conductivity will induce a hole current leakage so that current efficiency and power density will decrease. Besides the operating temperature, the conductivities of proton and hole are also strongly affected by the partial pressures of steam and oxygen in the PCFC [1,2]. Higher steam partial pressure leads higher proton conductivity, so that a higher power density can be expected. However, other influences such as concentration overpotential may be non-negligible. Furthermore, the proton and hole current density are affected by defect concentrations differences between two surfaces of the electrolyte membrane. Numerical simulation method is powerful tool for dealing with the complicated conditions in a PCFC mentioned above.In the present study, the influences of steam partial pressure on a BaZr0.8Yb0.2O3− δ (BZYb20) electrolyte are investigated. Specifically, to reveal the currents induced by the electrostatic potential and gas concentrations, Nernst-Planck model is selected. To relatively obviously reveal the steam concentration influences on the PCFC performances, low (3%) and high (69%) humidification conditions of H2 fuel gas are adopted. The results in Fig. 1(a) and 1(b) show the simulation and experimental current-voltage characteristics under 3% and 69% humidification conditions, respectively. The green solid and dotted lines are external and proton current densities considering concentration overpotential influence, respectively. The blue lines express external current density when no concentration overpotential in the numerical model. The red circles are the measured data. At low steam partial pressure, the simulation results well reproduced the experimental data without concentration overpotential consideration. However, an obvious deviation was found between simulation and experimental results when neglecting concentration overpotential in the simulation model. The performances such as open circuit voltage (OCV) and hole current leakage (difference between proton and external current density) are influenced by the steam partial pressure.

Accelerated Stress Testing of Solid Oxide Electrolysis Cells in a Symmetric Steam-Rich Atmosphere

Temperature, current density, steam utilization variability, and local steam starvation or excess may cause diverse degradation processes to occur in solid oxide electrolysis cells. Typical degradation mechanisms for SOEC with nickel-based fuel electrode materials include nickel migration/evaporation and agglomeration, which may lead to reduced active sites at the triple-phase boundary. However, the phenomenological cause of such degradation mechanisms is still actively debated, making the selection of a durable fuel electrode material challenging. It is desirable to understand the process by which these phenomena occur not only to minimize it, but to intentionally induce it to develop accelerated stress test protocols for cell lifetime prognostics. The degradation of solid oxide electrolyzers has been reported to be accelerated by certain stressful operating conditions. Brisse and Mocotuguy conducted a review of the degradation mechanisms of solid oxide button cells; they reported that high steam partial pressures can lead to the formation of nickel hydroxide at the cathode, which then diffuse to the surface of YSZ1. Sun et al. hypothesize that this is the main mechanism for nickel migration, due to the positive oxidation state of nickel and the direction of the electric potential2. Changes in the surface morphology of the fuel electrode due to redox cycle-induced degradation has been reported, showing the dynamics of nickel migration and agglomeration under these conditions3 . Redox cycling was found to be a destructive test protocol; efforts are currently being made by the group of Daria Vladikova to prevent fracturing of the electrodes and electrolyte during such stressful testing4. Königshofer et al. have reported through multiple investigations that protocols inducing operation at high steam conversion rates (>90%) and large current densities (0.8 A/cm2) have been effective at reproducing long-term SOEC voltage degradation5,6.The objective of this research is to establish a protocol that will help determine the causes that trigger Ni migration or evaporation, and that will mimic the electrode degradation mechanism to standardize the procedure for comparing the long-term performance of solid oxide electrolysis cells. Symmetric button cells of various fuel electrode chemistries with a diameter of 20 mm (active area 1.23 cm2) are tested under symmetric atmospheres with steam/H2 blends. The following two accelerated stress tests (AST) protocols are employed: cycling of steam partial pressure and cycling of current density. In the former, the steam molar fraction is varied from 75% to 95% in a 20-minute cycle for at least 400 cycles (i.e., 1,176 h). In the latter protocol, current density is controlled using a galvanodynamic 10-minute long cyclic ramp profile between 0.8 A/cm2 and 1.5 A/cm2 at a constant steam molar fraction between 75-95% for at least 800 cycles (i.e., 1,176 h).For all tests, the cells are operated at a constant furnace temperature between 700°C and 850°C. The performance characteristics of the various cells are expressed in terms of electrochemical impedance spectroscopy (EIS) and polarization curves every 12 h during the AST. X-ray diffraction (XRD) and fluorescence (XRF) are used to analyze the cells at beginning of life and post-mortem to identify phase changes which occur during operation. Scanning Electron Microscopy - Energy Dispersion Spectroscopy (SEM-EDS) and Glow-discharge spectroscopy (GD) are used to identify and map the nickel concentration on the cathode surface and cross-section. Preliminary results suggest that the high frequency resistance may increase by nearly 500% under a constant high-steam condition (> 90%) in a Ni-YSZ|YSZ|Ni-YSZ cell during a 200 h operation. P. Moçoteguy and A. Brisse, Int J Hydrogen Energy, 38, 15887–15902 (2013).X. Sun, T. L. Skafte, and S. H. Jensen, Fuel Cells (2021).S. Yang et al., J Alloys Compd, 921, 166085 (2022).D. Vladikova et al., Energies (Basel), 15 (2022).B. Königshofer et al., J Power Sources, 523 (2022).B. Königshofer et al., J Power Sources, 497 (2021).

Comparison of the Electrochemical and Degradation Behaviour of Ni-YSZ and Ni-GDC Electrodes Under Steam, Co- and CO2 Electrolysis

The mixed ionic and electronic conductive property of Ni-GDC has ensured that the electrochemical reaction zones of the electrode extend beyond the triple phase boundary of Ni/GDC/fuel gas to the double phase boundary of the GDC and the fuel gas [1, 2]. In addition, ceria has shown good carbon suppression properties when operated in carbon-containing fuels[3, 4]. For these reasons, Ni-GDC has emerged as a possible replacement for the conventional Ni-YSZ electrode. However, a direct comparison of the performance and long-term degradation of Ni-GDC with literature values of the Ni-YSZ would be ambiguous. On one hand, different reports utilize different fuel gas compositions, operating temperatures as well as current densities. On the other hand, the fabrication of fuel electrode-supported Ni-GDC is still a challenge due to the well-known inter-diffusion [5, 6] between the YSZ and the GDC oxide phase at a high sintering temperature (1400 °C) of YSZ electrolytes. Hence, most of the electrode fabrication has remained on electrolyte support. Thus, a direct comparison of electrolyte-supported Ni-GDC with the conventional fuel electrode-supported Ni-YSZ is ineffective due to different degradation behaviour.Therefore, this present study aims to investigate and compare the long-term stability of Ni-GDC and Ni-YSZ under three different electrolysis modes; steam-electrolysis, co-electrolysis and CO2-electrolysis. Firstly, electrolyte-supported single cells of Ni-GDC (NiO-GDC//8YSZ//GDC//LSCF) and Ni-YSZ (NiO-GDC//8YSZ//GDC//LSCF) were fabricated and investigated using electrochemical impedance spectroscopy (EIS) from 750-900 °C temperature range. Furthermore, the impedance data were also recorded under polarization (0.7 to 1.4V) as well as at OCV by varying the partial pressure of steam, CO2 and oxygen (, pH20, pCO2 and pO2). Finally, stability tests of the single cells were carried out under steam electrolysis (H2O: H2, 50:50) and co-electrolysis (H2O: CO2:H2, 40:40:20) conditions at 900 °C with 0.5 A/cm2 current density for 500 h [7]. For the CO2-electrolysis (CO2:CO, 80:20), the stability test was performed up to 1000 h. The post-test characterization of the operated cells was carried out using both SEM-EDX as well as FIB-SEM. The results reveal that Ni-GDC exhibits higher current density than Ni-YSZ in all the electrolysis modes. In the post-test analysis, loss of GDC percolation was observed and the Ni particles were observed to be covered by the GDC oxide phase. Furthermore, both electrodes demonstrate that Ni is migrating away from the electrolyte in all the electrolysis modes. The detailed electrochemical and microstructural properties of the operated cells will be presented and discussed.

Experimental study on kinetic oxidation of graphite IG-110 by steam

Graphite is proposed for use in High-temperature Gas-cooled Reactors (HTGRs) as the fuel matrix, neutron moderator/reflector, and core structural material. One important property of nuclear grade graphite is their resistance to oxidation in high-temperature environment. Extensive investigation has been performed in the literature for graphite oxidation by air. However, available experimental data are still limited for graphite oxidation by steam under conditions comparable to a postulated steam ingress accident in HTGRs. In this study, the oxidation rate of graphite IG-110 by steam in helium flow was measured at temperatures from 850 to 1100 °C with the steam partial pressure varying from 0.5 to 20.0 kPa and the hydrogen partial pressure varying from 0 to 2.0 kPa. Further analysis confirms the oxidation process in this present study is dominated by the chemical kinetics, which lends credit to the data for being used to develop numerical models. It was observed that the increase of the kinetic oxidation rate with the steam partial pressure tends to become less apparent if the steam partial pressure keeps increasing. In addition, it was found that the partitioning of hydrogen inhibits the graphite-steam reaction process even with the steam partial pressure up to 20.0 kPa. However, this inhibiting effect starts to become saturated when the hydrogen partial pressure exceeds 1.0 kPa. The oxidation rates were fitted to the conventional Langmuir-Hinshelwood (LH) and Boltzmann-enhanced Langmuir-Hinshelwood (BLH) models by a multivariable optimization algorithm. The BLH model exhibits a better accuracy than the LH model within the specified experimental conditions. The predicted oxidation rate using the BLH model shows a mean relative difference of about 24% with the maximum difference of about 55% when compared with our experimental data.

Reactivity of Pd–Cu/Al2O3 and Cu/Al2O3 during catalytic hydrogen combustion – In-situ mechanistic study and rate law determination

This study compares the performance, including reactivity and stability, of Pd–Cu/Al2O3 and Cu/Al2O3 catalysts during catalytic hydrogen combustion in a temperature range of 20–600 °C. The physicochemical and catalytic properties of catalysts were characterized using various analytical techniques. The reaction rates were measured using a fixed-bed reactor connected to a micro-gas chromatograph, and the rate law equations containing a term for steam partial pressure were determined. The effects of reaction temperature and catalyst composition on the reaction mechanism were investigated using in-situ Fourier-transform infrared spectroscopy analysis, and the generation of OH groups was analyzed to compare the reaction pathways of catalytic hydrogen combustion over Pd–Cu/Al2O3 and Cu/Al2O3 catalysts. The results showed that at temperatures above 500 °C, Cu/Al2O3 achieved a comparable hydrogen conversion (96.5 and 98%) to that of Pd–Cu/Al2O3. The rate-limiting steps of catalytic combustion over Pd–Cu/Al2O3 and Cu/Al2O3 were the formation and breaking of metal-oxygen bonding, respectively. It was also found that the difference between the reactivity of Pd–Cu/Al2O3 and Cu/Al2O3 was less pronounced under wet conditions.

Accelerated Stress Testing of Solid Oxide Electrolysis Cells in a Symmetric Steam-Rich Atmosphere

Commercialization of solid oxide electrolyzers is restricted by rapid degradation mechanisms of the electrodes, most notably nickel migration and agglomeration which leads to reduced activation sites at the triple-phase boundary. It is desirable to understand the process by which this phenomenon occurs not only to minimize it, but to intentionally induce it in a method which can compare the longevity of various chemistries. The objective of this research is to establish a protocol which mimics long-term electrode degradation. It is hoped that this will lead to a standardized procedure for comparing the projected long-term performance of solid oxide electrolysis cells. To promote degradation, operation at high steam partial pressure (>95% molar concentration) is employed.

Numerical Analysis of Humidification Condition Effects on Protonic Ceramic Fuel Cell Performance Considering Concentration Overpotential

Protonic ceramic fuel cells (PCFCs) operating at an intermediate temperature range (400–600 °C) show high performance due to high proton conductivity in the electrolyte. Especially, because the water steam is generated at the oxygen side, so there is less dilution effect on fuel. Therefore, one of the main merits of PCFCs is that it can operate with extremely high fuel utilization. Furthermore, a higher fuel humidification rate leads to higher proton conductivity. However, under relatively high steam partial pressure, the concentration overpotential effect on cell performance may become non-negligible. In the present study, the performance of PCFC under several fuel humidification conditions is investigated by considering the influence of concentration overpotential by numerical analysis.

Improving Electrocatalytic Activity of Cobalt-Free Barium Ferrite-Based Perovskite Oxygen Electrodes for Proton-Conducting Solid Oxide Cells via Introducing A-Site Deficiency

Proton-conducting solid oxide cells are regarded as promising solid-state energy conversion devices to realize the high-efficiency conversion between electrical energy and chemical energy. However, the high cost, easily reducible ion characteristics, and large thermal expansion coefficient related to the valence state of the traditional high-catalytic cobalt-based oxygen electrode are unavoidable problems. Cobalt-free barium ferrite-based oxides with triple-conducting properties are considered the most promising prospective candidates for highly active oxygen electrode materials because of the increased electrochemical reaction active sites, excellent thermal stability, and low thermal expansion coefficient. In this work, by introducing 5% A-site deficiency at the atomic scale, the oxygen vacancy concentration is increased so that the catalytic reaction kinetics and proton-conduction ability of ABO3-type perovskite oxygen electrode material BaFe0.5Sn0.2Bi0.3O3−δ are improved. The effects of non-stoichiometry on the phase composition, microstructure, hydration capacity, thermal expansion characteristics, conductivity, and chemical stability are investigated. In both fuel cell and electrolysis cell operation modes, the proton-conducting solid oxide cells with the A-site-deficient Ba0.95Fe0.5Sn0.2Bi0.3O3−δ oxygen electrode demonstrate excellent electrochemical performance, giving a peak power density of 0.69 W cm–2 (increased by 19.0%) and an electrolysis current density of 1.64 A cm–2 (increased by 30.2%) at 700 °C. Moreover, even at 600 °C with 50% high steam partial pressure, electrolysis for hydrogen production is maintained for about 100 h without significant attenuation, confirming the vital role of A-site defect engineering in the design of advanced oxygen electrode materials.

High Performance and Stability Enabled by Tuning the Component Thermal Expansion Coefficients of a Proton-Conducting Solid Oxide Cell Operating at High Steam Concentration

The solid oxide cell (SOC) based on a proton-conducting electrolyte is promising to play an important role in large-scale energy storage and hydrogen production. However, its development is hindered by performance degradation caused by the mismatch in thermal expansion coefficients (TECs) between the electrolyte and the oxygen electrode. Although there is work on reducing TECs of the oxygen electrode, it is difficult to find proper materials having a low TEC while maintaining a high performance. In this paper, we propose a new path to solve this problem: increasing the TEC of the electrolyte. We demonstrate that doping Fe into BaZr0.3Ce0.5Y0.1Yb0.1O3−δ (BZCYYb) can increase the TEC from 10.3 × 10–6 K–1 of BZCYYb to 13.4 × 10–6 K–1 of BaZr0.3Ce0.48Fe0.02Y0.1Yb0.1O3−δ (BZCYYbF). Fe doping also enhances the sinterability, electrochemical performance, and stability of the electrolyte. Meanwhile, we find that doping Fe into the oxygen electrode material PrNi0.5Co0.5O3−δ (PNC) decreases the TEC from 19.2 × 10–6 K–1 of PNC to 17.4 × 10–6 K–1 of PrNi0.4Co0.4Fe0.2O3−δ (PNCF). The improved stability of the SOC with the BZCYYbF electrolyte and the PNCF oxygen electrode is verified for steam electrolysis under a high steam partial pressure, 50%. Such a cell operates stably at a current density of 1.76 A cm–2 under 1.30 V and 700 °C for 150 h.

Superprotonic conductivity in RbH2-3y(PO4)1-y: a phosphate deficient analog to cubic CsH2PO4 in the (1 - x)RbH2PO4 - xRb2HPO4 system.

In contrast to CsH2PO4 (cesium dihydrogen phosphate, CDP), a material with a well-established superprotonic transition to a high conductivity state at 228 °C, RbH2PO4 (rubidium dihydrogen phosphate, RDP) decomposes upon heating under ambient pressure conditions. Here we find, from study of the (1 - x)RbH2PO4 - xRb2HPO4 system, the remarkable occurrence of cubic, off-stoichiometric RbH2-3y(PO4)1-y, or α-RDP, with a variable Rb : PO4 ratio. Materials were characterized by simultaneous thermal analysis and in situ X-ray powder diffraction performed under high steam partial pressure, from which the phase diagram between RbH2PO4 (x = 0) and Rb5H7(PO4)4 (x = 1/4) was established. The system displays eutectoid behavior, with a eutectoid transition temperature of 242.0 ± 0.5 °C and eutectoid composition of x = 0.190 ± 0.004. Even the end-member Rb5H7(PO4)4 appears to transform to α-RDP, implying y in the chemical formula of 0.2 and a phosphate site vacancy concentration as high as 20%. Charge balance is attained by a decrease in the average number of protons on the remaining phosphate groups. The cubic lattice parameter at x = 0.180, near the eutectoid composition, and at a temperature of 249 °C is 4.7138(2) Å. This value is substantially smaller than the estimated ambient-pressure lattice parameter of stoichiometric RbH2PO4 of 4.837(12) Å, consistent with the proposal of phosphate site vacancies in the former. The superprotonic conductivity of the x = 0.180 material is 6 × 10-3 S cm-1 at 244 °C, a factor of three lower than that of CDP at the same temperature. While the engineering properties of α-RDP do not suggest immediate technological relevance, the discovery of a superprotonic solid acid with a high concentration of phosphate site vacancies opens new avenues for developing proton conducting electrolytes, and in particular, for controlling their transition behavior.

Hydrothermal Stability of Hydrogen-Selective Carbon-Ceramic Membranes Derived from Polybenzoxazine-Modified Silica-Zirconia.

This work investigated the long-term hydrothermal performance of composite carbon-SiO2-ZrO2 membranes. A carbon-SiO2-ZrO2 composite was formed from the inert pyrolysis of SiO2-ZrO2-polybenzoxazine resin. The carbon-SiO2-ZrO2 composites prepared at 550 and 750 °C had different surface and microstructural properties. A carbon-SiO2-ZrO2 membrane fabricated at 750 °C exhibited H2 selectivity over CO2, N2, and CH4 of 27, 139, and 1026, respectively, that were higher than those of a membrane fabricated at 550 °C (5, 12, and 11, respectively). In addition to maintaining high H2 permeance and selectivity, the carbon-SiO2-ZrO2 membrane fabricated at 750 °C also showed better stability under hydrothermal conditions at steam partial pressures of 90 (30 mol%) and 150 kPa (50 mol%) compared with the membrane fabricated at 500 °C. This was attributed to the complete pyrolytic and ceramic transformation of the microstructure after pyrolysis at 750 °C. This work thus demonstrates the promise of carbon-SiO2-ZrO2 membranes for H2 separation under severe hydrothermal conditions.

Comparison of reaction characteristics and kinetics between tar thermal cracking and steam reforming in a micro fluidized bed reaction analyzer

In this study, a micro fluidized bed reaction analyzer (MFBRA) was used to investigate the reaction characteristics of tar thermal cracking and steam reforming at different temperatures (T, 750, 850, and 950 °C) and steam partial pressures (SP, 10%, 20%, and 30%). The reaction kinetics, including gas components generation (H2, CO, CO2, and CH4) and carbon (C) conversion in gas products, were calculated and further compared. For the two processes, the behaviors of gas generation and C conversion can be classified as Stage-1 and Stage-2 based on the different increases. By raising temperature and SP, the yields of H2/CO/CH4 and C conversion increased obviously, while CO2 showed a growing and subsequently declining trend. A similar phenomenon also existed in the relationship between reaction rate Vs. gas yield/C conversion. Compared to thermal cracking, the reaction time in tar steam reforming became much longer; the yields of H2/CO/CH4 and the C conversion increased significantly, while CO2 decreased. In the meantime, there was a clear reduction in all reaction rates of gas generation and C conversion. Furthermore, in tar steam reforming, the activated energy (Ea) of gas components generation (H2, CO, CO2, and CH4) decreased from 114.92 to 90.48, 86.54–74.49, 32.39–28.87, and 55.34–42.06 kJ/mol, respectively; and Ea of total C conversion lowered from 87.74 to 65.75 kJ/mol. Additionally, SP had no impact on Ea during tar steam reforming. To ensure the validity of the test results by MFBRA, the kinetics of tar thermal cracking was compared with that in the literature.

Falling-Film Absorption Model Considering Surface Wave and Vibration Effects Based on Lattice Boltzmann Method

As one of the effective solutions to recover waste heat, absorption refrigeration systems are used in various industrial or refrigeration places. Flat-plate falling-film absorption is one of the newer types, and the lattice Boltzmann method (LBM) has certain advantages compared with the traditional numerical simulation method. In this work, an LBM is used to analyze flat-plate falling-film absorption. Using the additional calculation of the pressure by the pseudo-force model, a lithium bromide–water working fluid–heat and mass transfer model driven by steam partial pressure is realized. The results show that the turbulence generated in the surface wave has a favorable effect on the absorption process; the degree of turbulence gradually decreases with the increase in the Reynolds number, which weakens the increasing effect of the surface wave on the absorption. When the Reynolds number is moderate, the solitary wave flows forward relative to the front thin liquid film, which promotes concentration and temperature diffusion inside the liquid film and inside the solitary wave. The model of falling-film flow under vibration environment is realized by using the characteristic of imposing inertial force in the model by pseudo-force method. The results show that vibration has a favorable effect on liquid film absorption, increasing the amplitude can increase the gas–liquid contact area and obtain a lower average film thickness, while increasing the vibration frequency can promote the internal diffusion of the solution.