Oxygen Surface Exchange Coefficient Research Articles

Doping effects of alkaline earth element on oxygen reduction property of high-entropy perovskite cathode for solid oxide fuel cells

The nearly infinite arrangement and combination of multiple components in high entropy materials provide a huge platform for the modification and performance regulation of new materials. It is an attractive idea to introduce the concept of high entropy into material design and improve its electrochemical performance through component regulation. In this paper, cobalt-based high-entropy perovskite oxides with different alkaline earth metal contents were successfully synthesized. The phase structure, direct current conductivity, oxygen mobility characters and electrochemical properties of these materials are studied. It is found that with the continuous substitution of rare earth elements by alkaline earth elements in high-entropy materials, the content of oxygen vacancies increases and the electrochemical performance is enhanced. The chemical oxygen diffusion coefficient (Dchem) and surface exchange coefficient (Kchem) of the high-entropy oxide La0.2Pr0.2Ba0.2Sr0.2Ca0.2CoO3 are 2.8 × 10−6 cm2 s−1 and 9.1 × 10−3 cm s−1 at 700 °C, respectively. The electrochemical impedance spectroscopy measurement results show that La0.2Pr0.2Ba0.2Sr0.2Ca0.2CoO3 has the minimum polarization resistance, which is 0.03 Ω cm2 at 700 °C in air. The anode-supported fuel cell with the high entropy oxide reaches a peak power density of 792 mW cm−2 at 700 °C and exhibits attractive long-term stability and excellent CO2 resistant properties.

Oxygen exchange and transport properties of the first-order Ruddlesden-Popper phase La2Ni0.9Co0.1O4+δ

The rare earth nickelate La2Ni0.9Co0.1O4+δ (LNCO) was synthesized via the citrate/EDTA method. X-ray powder diffraction confirmed that the material is single-phase and crystallizes in the orthorhombic K2NiF4-type structure. In-situ dc-conductivity and conductivity relaxation measurements on a bar-shaped sample were applied to determine the electronic conductivity as well as the chemical surface exchange coefficient and chemical diffusion coefficient of oxygen from 600 °C to 850 °C and 0.01 ≤ pO2 / bar ≤ 0.1. The results indicate that substitution of nickel with cobalt in La2Ni0.9Co0.1O4+δ leads to significantly higher values of the surface exchange coefficient compared to La2NiO4+δ, while the electronic conductivity is somewhat reduced. The oxygen non-stoichiometry of LNCO was studied by thermogravimetry in the same temperature and oxygen partial pressure range. The measured thermal expansion coefficients of LNCO fit well with those of common solid electrolytes such as GDC and YSZ. Self-diffusion coefficients of oxygen and ionic conductivities were calculated from the experimentally determined diffusivities and the thermodynamic factor of oxygen by using the Nernst-Einstein relation. The results indicate that LNCO offers an attractive option for application as air electrode in solid oxide cells.

Considerable oxygen reduction activity and durability of BaO nanoparticles-decorated Ln0.94BaCo2O5+δ electrocatalysts

Owing to unique catalytic properties and durability, nanoparticles-containing heterostructured catalysts have attracted much interest in energy storage and conversion fields. Here we report a multi-composite electrocatalyst consisting of BaO nanoparticles and Ln0.94BaCo2O5+δ (Ln = La, Pr, Nd, Sm, and Gd) (e-Ln0.94BCO) though in situ exsolution route. As-obtained e-Ln0.94BCO catalysts exhibit desirable oxygen reduction reaction (ORR) activity and durability toward solid oxide fuel cells (SOFCs) cathode application. When varying Ln3+ from Gd3+ to La3+, the phase transformation of orthorhombic-tetragonal-cubic can be identified in Ln0.94BaCo2O5+δ, significantly demonstrating improved electrochemical performance as well as electrical conductivity, oxygen surface exchange and chemical diffusion coefficients. Resultantly, the e-L0.94BCO cathode exhibits the lowest area-specific resistance (ASR) of 0.03 Ω cm2 at 700 °C, reduced by ≈ 43% relative to pristine La0.94BaCo2O5+δ (L0.94BCO) cathode. Under realistic operating conditions, the e-L0.94BCO cathode-based button cell delivers a peak power density (PPD) 0f 1.05 W cm−2 at 700 °C, along with outstanding long-term stability. Furthermore, an exceptional CO2 durability is achieved in e-L0.94BCO with negligible degradation rate. Density-functional theory (DFT) calculations imply that interfacial BaO makes a movement of O 2p-band center to the Fermi level, which is closely related to promoted electrochemical performance.

Cobalt-Free Double Perovskite Oxide as a Promising Cathode for Solid Oxide Fuel Cells.

Double perovskite oxide PrBaFe2O5+δ is a potential cathode material for intermediate-temperature solid oxide fuel cells. To improve its electrochemical performance, the trivalent element Ga is investigated to partially replace Fe, forming PrBaFe2-xGaxO5+δ (PBFGx, x = 0.05, 0.1, and 0.15). The doping effects on physicochemical properties and electrochemical properties are analyzed regarding the phase structures, element valence states, amount of oxygen vacancies, content of oxygen species, oxygen surface exchange coefficients (kchem), electrochemical polarization resistance, and single-cell performance. Specifically, PBFG0.1 exhibits improved kchem, such as a 19% improvement from 4.09 × 10-4 to 4.86 × 10-4 cm s-1 at 750 °C, due to the increased concentration of reactive oxygen species and oxygen vacancies. Consequently, the interfacial polarization resistance is decreased by 28% from 0.057 to 0.041 Ω cm2 at 800 °C. The subreaction steps of the oxygen reduction reaction in the PBFG0.1 cathode are further investigated, which suggests that the oxygen dissociation process is greatly enhanced by doping Ga. Meanwhile, doping Ga increases the peak power density of the anode-supported single cell by 36% from 629 to 856 mW cm-2 at 800 °C. The single cell with the PBFG0.1 cathode also exhibits good stability in 100 h of long-term operation at 750 °C.

Surface oxygen exchange kinetics of mixed conducting oxides: Dilatometric vs electrical conductivity relaxation study

The surface oxygen exchange kinetics of mixed ionic-electronic conducting (MIEC) oxides play a crucial role in a variety of applications, including solid oxide fuel/electrolysis cells (SOFCs/SOECs), permeation membranes and sensors. To date, a variety of methods, including isotope exchange, electrical conductivity, optical absorptivity and thermogravimetry relaxation, and impedance spectroscopy have been used to measure the oxygen exchange coefficient of MIEC oxides. Each of these methods have their advantages and limitations, depending on the physical and chemical properties of the investigated materials and their sample dimensions and densities. Here, we demonstrate the ability to precisely measure the surface oxygen exchange coefficient (kchem) of MIEC model material Pr0.1Ce0.9O2-δ (PCO) by use of dilatometric relaxation measurements, particularly of interest for materials that exhibit larger chemical expansion coefficients. This is achieved by use of porous bulk specimens to ensure that the contribution of oxygen exchange to the overall kinetics is dominant. We demonstrate that kchem values extracted from chemical expansion relaxation measurements on PCO are nearly identical to those derived from electrical conductivity relaxation measurements. This provides the opportunity to precisely investigate the oxygen exchange and chemical expansion kinetics of a wide range of materials used in high-temperature applications, particularly where more conventional methods are difficult or inappropriate to apply.

Tuning Surface Acidity of Mixed Conducting Electrodes: Recovery of Si-Induced Degradation of Oxygen Exchange Rate and Area Specific Resistance.

Metal oxides are an important class of functional materials, and for many applications, ranging from solid oxide fuel/electrolysis cells, oxygen permeation membranes, and oxygen storage materials to gas sensors (semiconducting and electrolytic) and catalysts, the interaction between the surface and oxygen in the gas phase is central. Ubiquitous Si-impurities are known to impede this interaction, commonly attributed to the formation of glassy blocking layers on the surface. Here, the surface oxygen exchange coefficient (kchem ) is examined for Pr0.1 Ce0.9 O2-δ (PCO), a model mixed ionic electronic conductor, via electrical conductivity relaxation measurements, and the area-specific resistance (ASR) by electrochemical impedance spectroscopy. It is demonstrated that even low silica levels, introduced by infiltration, depress kchem by a factor 4000, while the ASR increases 40-fold and weattribute this to its acidity relative to that of PCO. The ability to fully regenerate the poisoned surface by the subsequent addition of basic Ca- or Li-species is further shown. This ability to not only recover Si-poisoned surfaces by tuning the relative surface acidity of an oxide surface, but subsequently outperform the pre-poisoned response, promises to extend the operating life of materials and devices for which the catalytic oxygen/solid interface reaction is central.

Surface Space‐Charge Potential and Oxygen Surface Exchange Coefficient of SrTiO3: The Effect of Surface Orientation

Abstract18O2/16O2 exchange experiments on terminated (100), (110), and (111) oriented samples of single‐crystal SrTiO3 are used to study the effect of surface orientation on the oxygen surface exchange coefficient and the surface space‐charge potential Φ0. Isotope exchanges are carried out at temperatures 922 < T/K < 1073 at an oxygen activity aO2 = 0.2 in dry conditions (aH2O < 10−7.7); isotope profiles are measured subsequently by secondary ion mass spectrometry. Isothermal values are found to vary by less than one order of magnitude, with . All data are consistent with a change in the activation enthalpy of surface exchange from 3.0 eV at high temperatures to 1.8 eV at lower temperatures. The transition is attributed to a change from a charge‐transfer mechanism at high temperatures to a mechanism involving surface OH species. The transition temperature depends on the surface orientation, Ttr(111) ≈ Ttr(110) > Ttr(100). Values of Φ0 obtained varied between 0.50 V and 0.37 V, with isothermal values obeying Φ0(111) ≈ Φ0(110) < Φ0(100). Φ0 data is analysed to obtain the OH− surface coverage for the low‐temperature regime. Similarities between the behavior of Φ0 and that of are highlighted.

(Pr0.9La0.1)1.9(Ni0.7Cu0.3)0.9Mn0.1O4+δ nanofiber cathode with Pr site cation defect prepared by electrospinning and its application in reversible fuel cells

In this contribution, the Pr-site cation-deficient (Pr0.9La0.1)1.9(Ni0.7Cu0.3)0.9Mn0.1O4+δ nanofiber electrode material is successfully prepared by electrospinning and microwave sintering, and the effect of defects on the crystal structure and electrochemical performance are systematically studied. The resulted cations deficiency within material can form a certain amount of oxygen vacancies, which improves its hydration performance and accelerates the surface exchange coefficient of oxygen. Based on the thermogravimetric analysis (TGA) and electrical conductivity relaxation (ECR) technology, it is concluded that the enhanced proton conductivity of the material benefits from the oxygen vacancies caused by internal crystal defects. The doping of Mn element weakens the binding energy of the Ni–O bond, which avails the release of lattice oxygen. In addition, the distribution of relaxation time (DRT) analysis demonstrates that the main electrode reaction is the dissociation reaction process of the adsorbed oxygen. As a result, the polarization resistance (Rp) of the symmetrical cells at 700 °C is only 0.101 Ω cm2. In the single-cell fuel mode, the power density reaches 894 mW cm−2, and the excited current density in the electrolysis mode is 1563 mA cm−2 at 700 °C.

Surface kinetics and bulk transport in La2Ni0.5Cu0.5O4+δ membranes from conductivity relaxation

This work reports conductivity relaxation measurements on both uncoated (1.2 mm thick) and coated (2.0 mm thick) La2Ni0.5Cu0.5O4+δ membranes in the temperature range between 550 and 850 °C and oxygen partial pressures from 0.01 to 1.0 atm. The results show that surface kinetics has a significant effect on the relaxation profiles, especially at low temperatures and should not be neglected when extracting transport parameters. Oxygen chemical diffusion and surface exchange coefficients have been determined by transient conductivity with surface modification. Higher activation energy of surface exchange compared to bulk diffusion is observed for La2Ni0.5Cu0.5O4+δ, similar to that for La2NiO4+δ. Based on the oxygen partial pressure dependence of the surface exchange coefficient, it has been revealed that oxygen dissociative adsorption rate-limits the surface exchange.

The impact of Mn nonstoichiometry on the oxygen mass transport properties of La0.8Sr0.2Mn y O3±δ thin films

Oxygen mass transport in perovskite oxides is relevant for a variety of energy and information technologies. In oxide thin films, cation nonstoichiometry is often found but its impact on the oxygen transport properties is not well understood. Here, we used oxygen isotope exchange depth profile technique coupled with secondary ion mass spectrometry to study oxygen mass transport and the defect compensation mechanism of Mn-deficient La0.8Sr0.2Mn y O3±δ epitaxial thin films. Oxygen diffusivity and surface exchange coefficients were observed to be consistent with literature measurements and to be independent on the degree of Mn deficiency in the layers. Defect chemistry modeling, together with a collection of different experimental techniques, suggests that the Mn-deficiency is mainly compensated by the formation of antisite defects. The results highlight the importance of antisite defects in perovskite thin films for mitigating cationic nonstoichiometry effects on oxygen mass transport properties.

Restrictions of nitric oxide electrocatalytic decomposition over perovskite cathode in presence of oxygen: Oxygen surface exchange and diffusion

Nitric oxide (NO) abatement from engine exhaust is of great significance to alleviate air pollution and haze. Compared with the traditional selective catalytic reduction (SCR) technology, electrocatalytic decomposition of NO simplifies the reductant supply system and therefore avoids secondary pollution. In this study, typical perovskite La0.6Sr0.4CoxFe1-xO3-δ (LSCF) infiltrated by different dosages of nano ceria Ce0.9Gd0.1O1.9 (GDC) was used as composite cathodes, in order to explore the critical factors to restrain NO conversion in excess of O2. The results show that electron as reactive species transfers among NO, ABO3-type cathode and oxygen vacancy. The maximum of NO removal efficiency can reach 96.27 % in absence of O2 and up to 80.55 % in presence of 1% O2 in case of LSCF infiltrated by moderate dosages LSCF-GDC(2), which is superior to those of LSCF, LSCF-GDC(4) and LSM-GDC(nano) composite cathode. Compared to oxygen storage capacity (OSC) caused by the infiltration of nano ceria, higher surface oxygen exchange coefficient (kδ) and chemical diffusion coefficient (Dchem) lead to the significant decrease in polarization resistance (Rp), and consequently to the enhancement of NO removal in presence of O2. No matter what kind of oxygen deriving from oxygen reduction reaction (ORR) and NO reduction reaction (NORR), GDC infiltration into LSCF improves oxygen transport property and however, the property of cathode in ORR is dominant over in NORR in presence of O2. Moderate GDC loading has the highest oxygen transport kinetics, and oxygen surface exchange is faster than chemical diffusion, due to lower activation energy. Over loading of GDC with greater ohmic resistance (Rs) inversely influences the NO removal.

Pt Current Collectors Artificially Boosting Si-Contaminated Praseodymium Doped Ceria Oxygen Surface Exchange Coefficients

The chemical oxygen surface exchange coefficient (kchem ) values used to quantify and rank oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) catalyst performance for high-temperature, oxygen-exchange-enabled devices (such as Solid Oxide Fuel Cells, Solid Oxide Electrolysis Cells, oxygen sensors, etc.) are often determined electrically, with the aid of precious metal current collectors. In addition, due to ease of introducing siliceous contaminants during the manufacturing and operation of these devices, the performance of their oxygen exchange catalysts are often impacted by Si surface contaminants.Here, Curvature Relaxation (KR), Time-of-Flight Secondary Ion Mass Spectroscopy (ToF-SIMS), X-ray diffraction (XRD), Scanning Electron Microscopy (SEM), and X-Ray Photoelectron Spectroscopy (XPS) analyses performed on Pulsed Laser Deposited thin films of the oxygen exchange catalyst Pr0.1Ce0.9O2-x (PCO) show that siliceous surface phases only a few nanometers thick are capable of 1) reducing the PCO chemical oxygen surface exchange coefficient by approximately 3 orders of magnitude and 2) approximately doubling the activation energy for oxygen incorporation and removal. Further, the results show that unpolarized platinum current collectors dramatically improve the kchem of Si-contaminated PCO by reducing the Si concentration at the PCO surface and/or diffusing into the PCO; even for PCO thin films only exposed to mild temperatures of 500 oC. This suggests that precious metal current collectors are likely responsible for some of the large kchem variation reported in the literature for “identical” materials tested under “identical” conditions. Figure 1

A-site deficient Sr0.9Ti0.3Fe0.7O3−δ perovskite：A high stable cobalt-free oxygen electrode material for solid oxide electrochemical cells with excellent electrocatalytic activity and CO2 tolerance

Recently, SrTi0.3Fe0.7O3−δ (STF) has been investigated as a highly stable oxygen electrode material for solid oxide electrochemical cells (SOCs) with a sufficiently low resistance for cell operation at temperatures of > 700 °C. However, in general, the STF electrode performance is limited at temperatures of ≤ 700 °C due to the low oxygen surface exchange coefficient, which is mainly caused by high Sr surface segregation. To improve the electrode performance, Sr0.9(Ti0.3Fe0.7)O3−δ (A-STF) with an A-site-deficient design is developed to reduce the Sr content and thus reduce the Sr surface segregation, thereby providing a unique combination of excellent oxygen electrode performance and long-term stability. The A-site deficiency reduces the electrode polarization resistance by > 3 times at 600 °C and clearly improves the oxygen diffusion and surface exchange coefficients due to the decrease of Sr surface segregation. The A-STF electrode exhibits stable performance in the fuel cell and electrolysis modes at 1 A cm−2 > 1200 h. The stability of STF-based oxygen electrodes in a CO2-enriched atmosphere is investigated, and the results indicate that A-STF exhibits excellent CO2 tolerance.

Triple Perovskite Nd1.5Ba1.5CoFeMnO9-δ-Sm0.2Ce0.8O1.9 Composite as Cathodes for the Intermediate Temperature Solid Oxide Fuel Cells.

Triple perovskite has been recently developed for the intermediate temperature solid oxide fuel cell (IT-SOFC). The performance of Nd1.5Ba1.5CoFeMnO9−δ (NBCFM) cathodes for IT-SOFC is investigated in this work. The interfacial polarization resistance (RP) of NBCFM is 1.1273 Ω cm2~0.1587 Ω cm2 in the range of 700–800 °C, showing good electrochemical performance. The linear thermal expansion coefficient of NBCFM is 17.40 × 10−6 K−1 at 40–800 °C, which is significantly higher than that of the electrolyte. In order to further improve the electrochemical performance and reduce the thermal expansion coefficient (TEC) of NBCFM, Ce0.8Sm0.2O2−δ (SDC) is mixed with NBCFM to prepare an NBCFM-xSDC composite cathode (x = 0, 10, 20, 30, 40 wt.%). The thermal expansion coefficient decreases monotonically from 17.40 × 10−6 K−1 to 15.25 × 10−6 K−1. The surface oxygen exchange coefficient of NBCFM-xSDC at a given temperature increases from 10−4 to 10−3 cm s−1 over the range from 0.01 to 0.09 atm, exhibiting fast surface exchange kinetics. The area specific resistance (ASR) of NBCFM-30%SDC is 0.06575 Ω cm2 at 800 °C, which is only 41% of the ASR value of NBCFM (0.15872 Ω cm2). The outstanding performance indicates the feasibility of NBCFM-30% SDC as an IT-SOFC cathode material. This study provides a promising strategy for designing high-performance composite cathodes for SOFCs based on triple perovskite structures.

Tuning the Phase Transition of SrFeO3-δ by Mn toward Enhanced Catalytic Activity and CO2 Resistance for the Oxygen Reduction Reaction.

Developing high-performance cathodes with sufficient stability against CO2 rooting in ambient atmosphere is crucial to realizing the practical application of solid-oxide fuel cells. Herein, the Mn dopant is investigated to regulate the phase structure and cathode performance of SrFeO3-δ perovskites through partially replacing the B-site Fe. Compared with parent SrFeO3-δ, Mn-doped materials, SrFe1-xMnxO3-δ (x = 0.05 and 0.1), show stabilized cubic perovskites at room temperature. Meanwhile, doping Mn accelerates the oxygen reduction reaction process, showing a reduced polarization resistance of 0.155 Ω·cm2 at 700 °C for SrFe0.95Mn0.05O3-δ, which is less than 30% of SrFeO3-δ. In addition, the Mn dopant improves the chemical oxygen surface exchange and bulk diffusion coefficients. Furthermore, Mn enhances the tolerance toward CO2 corrosion in various CO2 atmospheres. Density functional theory calculations also reveal that Mn can strengthen the structural stability and increase the activity for the oxygen reduction reaction.

Sr Substituted La2−xSrxNi0.8Co0.2O4+δ (0 ≤ x ≤ 0.8): Impact on Oxygen Stoichiometry and Electrochemical Properties

Lanthanide nickelate Ln2NiO4+δ (Ln = La, Pr, or Nd) based mixed ionic and electronic conducting (MIEC) materials have drawn significant attention as an alternative oxygen electrode for solid oxide cells (SOCs). These nickelates show very high oxygen diffusion coefficient (D*) and surface exchange coefficient (k*) values and hence exhibit good electrocatalytic activity. Earlier reported results show that the partial substitution of Co2+ at B-site in La2Ni1−xCoxO4+δ (LNCO) leads to an enhancement in the transport and electrochemical properties of the material. Herein, we perform the substitution at A-site with Sr, i.e., La2−xSrxNi0.8Co0.2O4+δ, in order to further investigate the structural, physicochemical, and electrochemical properties. The structural characterization of the synthesized powders reveals a decrease in the lattice parameters as well as lattice volume with increasing Sr content. Furthermore, a decrease in the oxygen over stoichiometry is also observed with Sr substitution. The electrochemical measurements are performed with the symmetrical half-cells using impedance spectroscopy in the 700–900 °C temperature range. The total polarization resistance of the cell is increased with Sr substitution. The electrode reaction mechanism is also studied by recording the impedance spectra under different oxygen partial pressures. Finally, the kinetic parameters are investigated by analyzing the impedance spectra under polarization. A decrease in exchange current density (i0) is observed with increasing Sr content.

Promising La2Mo2O9–La2Mo3O12 Composite Oxygen-Ionic Electrolytes: Interphase Phenomena

The La2Mo2O9-La2Mo3O12 composite materials represent a novel class of highly conductive materials demonstrating increased oxygen-ion conductivity. Extensive research of (100 - x)La2Mo2O9-xLa2Mo3O12 composites over a wide range of concentrations (x = 5, 10, 15, 20, 30, and 100) was carried out for the first time. An increase in conductivity, oxygen surface exchange coefficient, and oxygen diffusivity is observed for composites compared to individual oxides, which is associated with the segregation of different ions on the surface of the grains and the formation of a La5Mo3O16 new phase at the contact boundary of La2Mo2O9 and La2Mo3O12. 3D-modeling of the composite microstructure was performed on the basis of SEM-image analysis data in order to estimate the conductivity of the interphase layer between the La2Mo2O9 and La2Mo3O12 grains containing La5Mo3O16. The electrical conductivity values of the composite materials calculated from a 3D-simulated microstructure and the experimentally measured conductivity correlate and demonstrate a composite effect.

Electrode properties of CuBi2O4 spinel oxide as a new and potential cathode material for solid oxide fuel cells

Herein, the spinel oxide CuBi 2 O 4 is prepared by solid-state reaction method and characterized for the first time as a cathode material for intermediate temperature solid oxide fuel cells (IT-SOFCs). CuBi 2 O 4 (CBO) crystallizes in tetragonal structure and is chemically compatible with Ce 0.9 Gd 0.1 O 1.9 (GDC) electrolyte at 800 °C. Electrical conductivity relaxation is conducted to assess the oxygen surface exchange coefficient k chem and oxygen bulk diffusion coefficient D chem of CBO material. It is found that CBO exhibits better oxygen transport properties than the traditional La 0.8 Sr 0.2 MnO 3 (LSM) and La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3-δ (LSCF) cathodes. The electrochemical activity of CBO-based cathode is investigated by impedance spectroscopy on the symmetric cell with GDC electrolyte. The CBO cathode forms good contact with the GDC electrolyte and displays low polarization resistance ( R P ) values, e.g., 0.58 Ωcm 2 at 700 °C in air. The introducing GDC to form a composite cathode with CBO further reduces the R P to 0.39 Ωcm 2 . An anode-supported single cell with the CBO-GDC cathode exhibits a peak power density of 507 mWcm −2 at 750 °C, suggesting the viability of CBO oxide as a cathode material for IT-SOFC. • CuBi2O4 exhibits fast surface oxygen exchange and bulk diffusion properties. • CuBi2O4 forms good contact with GDC electrolyte at low sintering temperatures. • CuBi2O4 cathode shows improved electrochemical performance below 700 °C.

A New Class of High Entropy Perovskite Oxides with Increased Reducibility and Stability for Solar Thermochemical Hydrogen Production

The growing demand for energy due to the increasing world population and industrialization of countries has caused pollution and climate change concerns. A global transition from fossil fuel energy to sustainable energy sources, in particular solar energy, calls for breakthroughs in energy storage and H2 fuel production. Two-step solar thermochemical hydrogen (STCH) production has emerged as an attractive route to produce clean H2 fuel from concentrated solar thermal energy. The two-step chemical looping is readily achieved using nonstoichiometric oxides such as ceria and perovskite oxides because those materials can retain their structural integrity throughout the cycling process. A new class of redox oxides, high-entropy perovskite oxides (HEPOs), are proposed to show great promises in the STCH application These novel HEPO materials have an unprecedented vast compositional space, large tunability, and unique thermodynamic and kinetic behaviors. Herein, we present that a HEPO family of La0.8Sr0.2(Mn[1-x]/3Fe[1-x]/3CoxAl[1-x]/3)O3- δ (x = 0.16, 0.2, 0.25, or 0.4) on the basis of an established STCH active material La0.6Sr0.4Mn0.6Al0.4O3- δ, was developed using high-energy ball milling (HEBM) and sintering. We comprehensively characterized the synthesized HEPOs in terms of phase purity, morphology, homogeneous elemental distribution, identification of redox active elements, reducibility extent (Δδ) and cycling structure stability and explored their STCH production performances. The optimal sample (La0.8Sr0.2(Mn0.2Fe0.2Co0.4Al0.2)O3- δ) showed a large Δδ ≥ 0.15 under the conditions of reduction at 1320 ℃ and oxidation at 870 ℃. All HEPO samples demonstrated R3-c phase stability during the cycling. Elemental mapping showed homogeneous distribution. Characterization of the electrical conductivity relaxation of the materials showed intrinsic material kinetic properties for oxygen surface exchange coefficient k ≥ 7.5 x 10-4 cm/s at 700 ℃, indicating the high kinetics for the HEPO reduction. The STCH production testing in our prototype setup showed that several promising samples had a high H2 yield of more than 300 μmol/g within less than 45 minutes upon exposure to 40 vol % steam and good cycling stability. This work opens up an avenue for the rational design of HEPOs for the solar thermochemical hydrogen production. Figure 1

Silicon Contamination of the Praseodymium Doped Ceria Oxygen Surface Exchange Coefficient

Chemical oxygen surface exchange coefficients are used to quantify and rank the performance of oxygen exchange catalysts used in solid oxide fuel cell, solid oxide electrolysis cell, oxygen separation membrane, catalytic converter, and other oxygen-exchange-enabled devices. Unfortunately, during the manufacture and operation of these devices it is easy to introduce siliceous contaminants that degrade their oxygen exchange performance. Surprisingly, despite the well-known negative impact siliceous impurities have on the oxygen transport properties of ceria-based materials, quantitative measurements of how siliceous impurities impact the chemical oxygen surface exchange coefficient of praseodymium doped ceria are largely absent from the literature. In this work, the crystal structure, film thickness, surface composition, bulk composition, and chemical oxygen exchange coefficient of (100)-oriented PCO thin films with, and without, surface silica contaminants were evaluated via X-ray diffraction (XRD), Scanning Electron Microscopy (SEM), X-ray Photoelectron Spectroscopy (XPS), Secondary Ion Mass Spectroscopy (SIMS), and Curvature Relaxation (κR) experiments, respectively. The results show that siliceous surface phases only a few nanometers thick are capable of (1) reducing the PCO chemical oxygen surface exchange coefficient by approximately 3 orders of magnitude and (2) approximately doubling the activation energy for oxygen exchange.