P-type Electronic Conductivity Research Articles

Elucidation of the Transport Properties of Calcium-Doped High Entropy Rare Earth Aluminates for Solid Oxide Fuel Cell Applications.

Solid oxide fuel cells (SOFCs) are paving the way to clean energy conversion, relying on efficient oxygen-ion conductors with high ionic conductivity coupled with a negligible electronic contribution. Doped rare earth aluminates are promising candidates for SOFC electrolytes due to their high ionic conductivity. However, they often suffer from p-type electronic conductivity at operating temperatures above 500°C under oxidizing conditions caused by the incorporation of oxygen into the lattice. High entropy materials are a new class of materials conceptualized to be stable at higher temperatures due to their high configurational entropy. Introducing this concept to rare earth aluminates can be a promising approach to stabilize the lattice by shifting the stoichiometric point of the oxides to higher oxygen activities, and thereby, reducing the p-type electronic conductivity in the relevant oxygen partial pressure range. In this study, the high entropy oxide (Gd,La,Nd,Pr,Sm)AlO3 is synthesized and doped with Ca. The Ca-doped (Gd,La,Nd,Pr,Sm)AlO3 compounds exhibit a higher ionic conductivity than most of the corresponding Ca-doped rare earth aluminates accompanied by a reduction of the p-type electronic conductivity contribution typically observed under oxidizing conditions. In light of these findings, this study introduces high entropy aluminates as a promising candidate for SOFC electrolytes.

Phase evolution, defect chemistry, and electrical properties of Na0.5Bi0.5TiO3-BiNi0.5Ti0.5O3 solid solutions

Solid solutions of Na0.5Bi0.5TiO3 (NBT) and BiNi0.5Ti0.5O3 (BNiT) were prepared by a solid-state reaction route, and their electrical properties investigated by a combination of impedance spectroscopy and electromotive force measurements to explore the possibility of developing mixed ionic-electronic conductors based on NBT. Phase analysis showed that BNiT has a large solid solution limit in NBT (60 mol% based on X-ray diffraction), and the room temperature crystal structure changes from rhombohedral to pseudo-cubic with increasing BNiT content. Neutron diffraction revealed the coexistence of rhombohedral and tetragonal phases when the BNiT content ≥ 40 mol%. Electrically, incorporation of BNiT induces p-type electronic conduction into NBT by hopping of holes between Ni2+ (NiNix) and Ni3+ (NiNi·), and therefore changes the electrical conduction mechanism systematically from predominant oxide-ion conduction to mixed ionic-electronic conduction and then to predominant p-type electronic conduction. The total conductivity of the solid solutions showed a “V-shape” variation with increasing BNiT content. Possible mechanisms for the phase evolution and the conductivity-composition relationships are discussed. Achieving high levels of ionic and electronic conductivity simultaneously in NBT by introducing elements with variable oxidation states remains challenging due to the competition between an enhanced electronic component and a suppressed ionic component. Low levels of BNiT incorporation are, however, beneficial to reducing the dielectric loss of NBT for dielectric applications.

Inverse-Perovskite Ba3 BO (B = Si and Ge) as a High Performance Environmentally Benign Thermoelectric Material with Low Lattice Thermal Conductivity.

High energy-conversion efficiency (ZT) of thermoelectric materials has been achieved in heavy metal chalcogenides, but the use of toxic Pb or Te is an obstacle for wide applications of thermoelectricity. Here, high ZT is demonstrated in toxic-element free Ba3 BO (B = Si and Ge) withinverse-perovskite structure. The negatively charged B ion contributes to hole transport with long carrier life time, and their highly dispersive bands with multiple valley degeneracy realize both high p-type electronic conductivity and high Seebeck coefficient, resulting in high power factor (PF). In addition, extremely low lattice thermal conductivities (κlat ) 1.0-0.4Wm-1 K-1 at T = 300-600 K are observed in Ba3 BO. Highly distorted O-Ba6 octahedral framework with weak ionic bonds between Ba with large mass and O provides low phonon velocities and strong phonon scattering in Ba3 BO. As a consequence of high PF and low κlat , Ba3 SiO (Ba3 GeO) exhibits rather high ZT = 0.16-0.84 (0.35-0.65) at T = 300-623 K (300-523 K). Finally, based on first-principles carrier and phonon transport calculations, maximum ZT is predicted to be 2.14 for Ba3 SiO and 1.21 for Ba3 GeO at T = 600 K by optimizing hole concentration. Present results propose that inverse-perovskites would be a new platform of environmentally-benign high-ZT thermoelectric materials.

Hydrogen production at intermediate temperatures with proton conducting ceramic cells: Electrocatalytic activity, durability and energy efficiency

Proton conducting ceramic cells (PCCs) are an attractive emerging technology operating in the intermediate temperature range of 500 to 700 °C. In this work, we evaluate the production of hydrogen at intermediate temperatures by proton conducting ceramic cell electrolysis (PCCEL). We demonstrate a high-performance steam electrolysis owing to a composite positrode based on BaGd0.8La0.2Co2O6−δ (BGLC1082) and BaZr0.5Ce0.4Y0.1O3−δ (BZCY541). The high reliability of PCCEL is demonstrated for 1680 h at a current density as high as –0.8Acm−2 close to the thermoneutral cell voltage at 600 °C. The electrolysis cell showed a specific energy consumption ranging from 54 to 66 kW h kg−1 that is comparable to state-of-the-art low temperature electrolysis technologies, while showing hydrogen production rates systematically higher than commercial solid oxide ceramic cells (SOCs). Compared to SOCs, the results verified the higher performances of PCCs at the relevant operating temperatures, due to the lower activation energy for proton transfer comparing with oxygen ion conduction. However, because of the p-type electronic conduction in protonic ceramics, the energy conversion rate of PCCs is relatively lower in steam electrolysis. The faradaic efficiency of the PCC in electrolysis mode can be increased at lower operating temperatures and in endothermic conditions, making PCCEL a technology of choice to valorize high temperature waste heat from industrial processes into hydrogen. To increase the faradaic efficiency by optimizing the materials, the cell design, or the operating strategy is a key challenge to address for future developments of PCCEL in order to achieve even more superior techno-economic merits.

Ba2NiMoO6-δ as a potential electrode for protonic ceramic electrochemical cells

A novel layered double perovskite-based electrode of composition Ba2NiMoO6-δ (BNMO) is proposed for application in Protonic Ceramic Electrochemical Cells (PCECs). Structural characterization by X-ray diffraction (XRD) revealed good chemical compatibility with a BaCe0.7Zr0.1Y0.2O3-δ (BCZY712) electrolyte and phase stability in oxidizing atmospheres. In this study, symmetrical cells of both the pure BNMO electrode and a composite electrode with 50 vol% of BCZY712 material were prepared by screen printing and sintering in air at 950 °C. Microstructural characterization by Scanning Electron Microscopy (SEM) showed good adhesion of the electrode films with the substrate, while Energy-dispersive Spectroscopy (EDS) revealed good percolation of both phases in the case of the composite electrode. Electrochemical studies made by Electrochemical Impedance Spectroscopy (EIS) on the pure BNMO electrode demonstrated that the total polarization resistance was lower in wet conditions in the case of N2 atmospheres, corroborating a potential protonic contribution, while in O2 atmospheres this term was similar between wet and dry conditions, due to dominant p-type electronic conductivity. Conversely, performance was aggravated by the addition of the BCZY712 composite phase; a result clarified to be due to an insufficient level of electronic conductivity in the composite electrode. To the best of our knowledge this is the first study of BNMO for proton conducting applications, providing important information on preparation, compatibility and electrochemical performance, as required for its further optimization.

Characterization of Ruddlesden-Popper La2−xBaxNiO4±δ Nickelates as Potential Electrocatalysts for Solid Oxide Cells

Ruddlesden-Popper La2-xBaxNiO4±δ (x = 0-1.1) nickelates were prepared by a glycine-nitrate combustion route combined with high-temperature processing and evaluated for potential application as electrocatalysts for solid oxide cells and electrochemical NOx elimination. The characterization included structural, microstructural and dilatometric studies, determination of oxygen nonstoichiometry, measurements of electrical conductivity and oxygen permeability, and assessment of chemical compatibility with other materials. The formation range of phase-pure solid solutions was found to be limited to x = 0.5. Exceeding this limit leads to the co-existence of the main nickelate phase with low-melting Ba- and Ni-based secondary phases responsible for a strong reactivity with Pt components in experimental cells. Acceptor-type substitution of lanthanum by barium in La2-xBaxNiO4+δ is charge-compensated by decreasing oxygen excess, from δ ≈ 0.1 for x = 0 to nearly oxygen-stoichiometric state for x = 0.5 at 800 °C in air, and generation of electron-holes (formation of Ni3+). This leads to an increase in p-type electronic conductivity (up to ~80 S/cm for highly porous La1.5Ba0.5NiO4+δ ceramics at 450-900 °C) and a decline of oxygen-ionic transport. La2-xBaxNiO4+δ (x = 0-0.5) ceramics exhibit moderate thermal expansion coefficients, 13.8-14.3 ppm/K at 25-1000 °C in air. These ceramic materials react with yttria-stabilized zirconia at 700 °C with the formation of an insulating La2Zr2O7 phase but show good chemical compatibility with BaZr0.85Y0.15O3-δ solid electrolyte.

Nanoscale Iron Redistribution during Thermochemical Decomposition of CaTi1-x Fe x O3-δ Alters the Electrical Transport Pathway: Implications for Oxygen-Transport Membranes, Electrocatalysis, and Photocatalysis.

Potential applications of the earth-abundant, low-cost, and non-critical perovskite CaTi1-x Fe x O3-δ in electrocatalysis, photocatalysis, and oxygen-transport membranes have motivated research to tune its chemical composition and morphology. However, investigations on the decomposition mechanism(s) of CaTi1-x Fe x O3-δ under thermochemically reducing conditions are limited, and direct evidence of the nano- and atomic-level decomposition process is not available in the literature. In this work, the phase evolution of CaTi1-x Fe x O3-δ (x = 0-0.4) was investigated in a H2-containing atmosphere after heat treatments up to 600 °C. The results show that CaTi1-x Fe x O3-δ maintained a stable perovskite phase at low Fe contents while exhibiting a phase decomposition to Fe/Fe oxide nanoparticles as the Fe content increases. In CaTi0.7Fe0.3O3-δ and CaTi0.6Fe0.4O3-δ, the phase evolution to Fe/Fe oxide was greatly influenced by the temperature: Only temperatures of 300 °C and greater facilitated phase evolution. Fully coherent Fe-rich and Fe-depleted perovskite nanodomains were observed directly by atomic-resolution scanning transmission electron microscopy. Prior evidence for such nanodomain formation was not found, and it is thought to result from a near-surface Kirkendall-like phenomenon caused by Fe migration in the absence of Ca and Ti co-migration. Density functional theory simulations of Fe-doped bulk models reveal that Fe in an octahedral interstitial site is energetically more favorable than in a tetrahedral site. In addition to coherent nanodomains, agglomerated Fe/Fe oxide nanoparticles formed on the ceramic surface during decomposition, which altered the electrical transport mechanism. From temperature-dependent electrical conductivity measurements, it was found that heat treatment and phase decomposition change the transport mechanism from thermally activated p-type electronic conductivity through the perovskite to electronic conduction through the iron oxide formed by thermochemical decomposition. This understanding will be useful to those who are developing or employing this and similar earth-abundant functional perovskites for use under reducing conditions, at elevated temperatures, and when designing materials syntheses and processes.

Effect of the oxidation states on the electrical properties of Fe-doped Pr2Zr2O7 pyrochlore

Pyrochlores with formula Pr2Zr2-xFexO7±δ (x = 0.00, 0.05, 0.10 and 0.15) were synthesized by the coprecipitation method. Pellets of these compositions sintered at 1400 °C for 12 h were characterized by XRD, Rietveld Refining Method, SEM/EDX, XPS, TGA, and Impedance Spectroscopy (IS). The solid solution limit was x ≤ 0.10. All the compositions exhibited a p-type electronic conductivity mechanism, and the total conductivity increased with the presence of iron. A mixture of Pr(IV)/Pr(III) and Fe(II)/Fe(III) ions was observed by XPS measurements in all samples sintered in air. Moreover, when samples were submitted to different atmospheres and temperatures, the IS technique combined with XPS and TGA was used to detect changes in the oxidation state of the elements present. A reduction of Pr(IV) to Pr(III), accompanied by a significant change in conductivity, was observed at temperatures around 500 °C for samples measured in N2. For the iron-doped compositions, a delay in the praseodymium reduction of 50 °C was observed associated with the effect of oxygen retention in the samples. These compositions showed a higher proportion of Pr(IV) in both oxygen and nitrogen atmospheres. The presence of Pr(IV) implies a greater retention of oxygen in the sample. The results presented in this study allow the electrical properties of the pyrochlore to be tailored depending on the oxidation states of the ions present in it and provide information for future applications of this material, for example, as a component in solid-oxide fuel cells, the generation of clean energy, a heterogeneous catalyst, or an oxygen storage material.

Ultrafine Co-Doped NiO Nanoparticles Decorated on Carbon Nanotubes Improving the Electrochemical Performance and Cycling Stability of Li–CO2 Batteries

Rechargeable Li–CO2 batteries offer a sustainable strategy for CO2 utilization and energy storage in an environmentally friendly way. However, the current Li–CO2 batteries are restrained by unsatisfactory electrochemical performance and lack of cost-effectiveness in practical application. Thus, developing low-cost and highly effective electrode materials is crucial. Inspired by the concept of high nickel and low cobalt content in the electrode material, ultrafine Co0.1Ni0.9Ox nanoparticles decorated on carbon nanotubes (Co0.1Ni0.9Ox/CNT) are first developed in this work. Through various characterization techniques, the effects of doping Co2+ cations on the properties and electrochemical performance are systematically studied. At a current density of 100 mA g–1, a battery with the Co0.1Ni0.9Ox/CNT electrode delivers a high capacity of 5871.41 mA h g–1 with a Coulombic efficiency of 92.91% and a low overpotential of 3.94 V with an energy efficiency of 68%. At a cutoff capacity of 500 mA h g–1, the battery based on the Co0.1Ni0.9Ox/CNT electrode can be easily operated for 50 cycles without the trace of capacity degradation, which is nearly twice more than those with the NiO/CNT and CNT electrodes. Compared with the NiO/CNT electrode, the Co0.1Ni0.9Ox/CNT electrode exhibits superior electrocatalytic activity in oxidation and evolution reactions of CO2, which results from doping Co2+ cations in the NiO lattice to improve the p-type electronic conductivity. In addition, the polymer-like discharge products affected by doping Co2+ provide better reaction interfaces, curtailing the overpotential and reducing the life span. The results enlighten new ideas of fabricating high-performance and cost-effective electrode materials for prolonging the cycling life of Li–CO2 batteries toward practical applications.

Oxides with Mixed Protonic and Electronic Conductivity

Oxides with mixed protonic and p-type electronic conductivity (and typically containing also mobile oxygen vacancies) are important functional materials, e.g., for oxygen electrodes in protonic ceramic electrochemical cells or for permeation membranes. Owing to the presence of three carriers, their defect chemical behavior is complex. Deviations from ideal behavior (defect interactions) have to be taken into account, which are related to the partially covalent character of the transition metal–oxygen bonds. Compared to acceptor-doped Ba(Zr,Ce)O3− z electrolytes, perovskites with redox-active transition-metal cations typically show smaller degrees of hydration. Trends in the proton uptake of (Ba,Sr,La)(Fe,Co,Y,Zn)O3−δ perovskites are analyzed and correlated to structural features (local lattice distortions) and electronic properties (the position of oxygen states on an absolute energy scale). The proton mobility in such mixed-conducting perovskites is estimated. Specific aspects of the application of protonic and electronic mixed-conducting oxides in protonic ceramic electrochemical cells are discussed, and an overview of recent materials and device developments is given.

Bi-Layer Structured Co-Ionic Composite Electrolytes Solid Oxide Cells for Stable Fuel Cell and Electrolysis Operations

A bilayer comprising BCY (BaCe0.85Y0.15O3-d) - GDC (Gd0.2Ce0.8O2-d) at anode side and BZY (BaZr0.85Y0.15O3-d) - NDC (Nd0.1Ce0.9O2-d) at cathode side, was designed for improving the stability for intermediate solid oxide fuel cells. Two Pt probes were embedded in each electrolyte layer close to anode and cathode sides of electrolyte for the estimation of the local electronic conduction, which is correlated to the cell stability under negative voltage operation caused by cell imbalance. Mixed ionic and n-type electronic conduction was locally developed in the BCY-GDC region near the anode side while mixed ionic and p-type electronic conduction was locally developed in the BZY-NDC near the cathode side. On the other hand, the middle electrolyte region exhibited predominantly ionic conduction which was an effective electrolyte region. Contrary to single layer structured composite electrolytes, the bi-layered composite electrolyte cell was stably operated without electrode/electrolyte delamination under negative voltage conditions, by means of local electronic conduction. Hence, we suggest the composite electrolytes configuration, such as cathode/ BCY-GDC / BZY-NDC / anode, for stable fuel cell and electrolysis operations.

Electrical properties of calcia-stabilised zirconia ceramics: Voltage-induced p-type conductivity and oxygen redox activity

The electrical properties of oxide ion-conducting Ca0.15Zr0.85O1.85, CSZ, ceramics were investigated using impedance spectroscopy as a function of temperature and applied dc bias. The total resistance showed a time-dependent decrease during application of a dc bias and was attributed to the reversible introduction of p-type semiconductivity, initially at the sample surface followed by grain boundaries and the sample bulk. Electron holes may be located on under-bonded oxygen ions which are associated either with Ca dopant ions or the ceramic surfaces and interfaces, resulting in mixed oxide ion and p-type electronic conduction. This p-type behaviour with a dc bias is different from the behaviour of most well-studied, stabilised zirconias which become n-type mixed conducting due to oxygen loss at low pO2.

From structure to conduction mechanism: investigating the effect of sintering atmosphere and the role of Mn in Ca0.6Sr0.4TiO3 ceramics

The effects of sintering atmospheres (air, O2, and N2) and defect chemistry role of Mn in the structure, electrical properties, and conduction mechanism of Ca0.6Sr0.4TiO3 (CST) ceramics were extensively investigated in this study. O2 atmosphere sintering resulted in a decrease in the concentration of oxygen vacancies, as revealed by the expanded unit cell and increased Ti ions' valence state. The reverse was the case for samples sintered in N2. Interestingly, sintering in both of O2 and N2 resulted in deteriorated bulk resistivity, due to the transition of the conduction mechanism (n-type electronic→ionic→p-type electronic conduction) with increasing oxygen partial pressure (PO2). This was verified by subjecting the samples to annealing treatment in different atmospheres and measuring their high-temperature equilibrium conductivities. Additionally, Mn-doped CST ceramics demonstrated higher resistivity than that of the pure CST ceramics, regardless of the sintering atmospheres, which could be attributed to the self-changing of Mn ions’ valence state in different PO2.

Mixed ionic-electronic conductivity, phase stability and electrochemical activity of Gd-substituted La2NiO4+δ as oxygen electrode material for solid oxide fuel/electrolysis cells

Ruddlesden-Popper La2-xGdxNiO4+δ (x = 0–0.4) nickelates were synthesized by glycerol-nitrate combustion technique and explored as potential oxygen electrode materials for solid oxide fuel/electrolysis cells. Similar to the parent La2NiO4+δ, the metastability of RP-type n = 1 structure limits the applicability of La2-xGdxNiO4+δ to temperatures below 900 °C. These solid solutions are mixed conductors with predominantly p-type electronic conductivity that exceeds 50 S/cm at 500–800 °C in air. Substitution by gadolinium does not change the overstoichiometric oxygen content in air but has a negative impact on the mobility of interstitial oxygen, most likely, due to steric effects associated with the lattice shrinkage on doping. The electrochemical activity of bilayer electrodes comprising functional La2-xGdxNiO4+δ and current collecting LaNi0.6Fe0.4O3-δ + 3 wt% CuO layers in contact with Ce0.8Gd0.2O1.9 electrolyte was studied in air at 550–850 °C. Analysis of electrochemical impedance spectroscopy data employing the ALS (Adler-Lane-Steele) model revealed the limiting role of oxygen-ionic conductivity of functional La2-xGdxNiO4+δ materials in overall electrode performance.

Effect of zinc doping on electrical properties of LaAlO3 perovskite

New solid solution with the general formula of LaAl1-xZnxO3-1/2x was prepared by a solid-state reaction route. According to XRD, the crystal structure of LaAlO3 is rhombohedral, while the solid solution possesses cubic symmetry. Homogeneity region of the solid solution LaAl1-xZnxO3-1/2x was narrow and limited to the maximum concentration of 5 mol. %. Computer simulations using crystalochemistry and density functional theory approaches showed that LaAlO3 has high energy barriers for O2–-ion transport (2.79 eV). These results are in good agreement with the low values of electrical conductivity obtained experimentally. The electrical conductivity of LaAl1-xZnxO3-1/2x was measured by impedance spectroscopy in the temperature range of 200–1000 °C. The partial substitution of Al3+ by Zn2+ was found to increase the electrical conductivity by ~2 order of magnitude. The electrical conductivity of doped phase LaAl0.95Zn0.05O2.975 as a function of oxygen partial pressure was measured, and the partial contributions (oxygen-ionic and electronic) were determined. It was found that the sample has mixed ionic and p-type electronic conductivity, while the electronic contribution increases with the rise of the temperature.

Mixed Ionic-Electronic Conductivity, Redox Behavior and Thermochemical Expansion of Mn-Substituted 5YSZ as an Interlayer Material for Reversible Solid Oxide Cells.

Manganese-substituted 5 mol.% yttria-stabilized zirconia (5YSZ) was explored as a prospective material for protective interlayers between electrolyte and oxygen electrodes in reversible solid oxide fuel/electrolysis cells. [(ZrO2)0.95(Y2O3)0.05]1−x[MnOy]x (x = 0.05, 0.10 and 0.15) ceramics with cubic fluorite structure were sintered in air at 1600 °C. The characterization included X-ray diffraction (XRD), scanning electron microscopy (SEM)/energy dispersive spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), thermogravimetry and dilatometry in controlled atmospheres, electrical conductivity measurements, and determination of oxygen-ion transference numbers by the electromotive force (EMF) technique. Mn-substituted 5YSZ solid solutions exhibit variable oxygen nonstoichiometry with manganese cations in a mixed 2+/3+ oxidation state under oxidizing conditions. Substitution by manganese gradually increases the extent of oxygen content variation on thermal/redox cycling, chemical contribution to thermal expansion and dimensional changes on reduction. It also deteriorates oxygen-ionic conductivity and improves p-type electronic conductivity under oxidizing conditions, leading to a gradual transformation from predominantly ionic to prevailing electronic transport with increasing x. Mn2+/3+→Mn2+ transformation under reducing atmospheres is accompanied by the suppression of electronic transport and an increase in ionic conductivity. All Mn-substituted 5YSZ ceramics are solid electrolytes under reducing conditions. Prolonged treatments in reducing atmospheres, however, promote microstructural changes at the surface of bulk ceramics and Mn exsolution. Mn-substituted 5YSZ with 0.05 ≤ x < 0.10 is considered the most suitable for the interlayer application, due to the best combination of relevant factors, including oxygen content variations, levels of ionic/electronic conductivity and thermochemical expansion.

Electrical properties of Mg-doped and Mg, Si co-doped alumina

Impedance measurements on Mg-doped alumina ceramics, over the temperature range 400−910 °C, showed a combination of oxide ion conductivity and p-type electronic conductivity, depending on temperature and oxygen partial pressure. The oxide ion conductivity is attributed to oxygen vacancies, introduced as charge compensation for the Mg dopant. The p-type conductivity is attributed to hole creation on under-bonded oxide ions and is the dominant conductivity for measurements in air at high temperatures. In samples co-doped with Mg and Si, impedance measurements showed a reduction in oxide ion conductivity and therefore, number of oxide ion vacancies because the self-compensation mechanism of co-doping lead to direct replacement of Al3+by Mg2+ and Si4+; the level of p-type conductivity was also reduced.

La4Ni3O10±δ – BaCe0.9Y0.1O3-δ cathodes for proton ceramic fuel cells; short-circuiting analysis using BaCe0.9Y0.1O3-δ symmetric cells

Protonic ceramic fuel cells are a promising technology for energy conversion and chemical synthesis in the intermediate temperature range (400–600 °C). Nevertheless, a major restriction to their wider implementation concerns their relatively high cathode polarisation resistance that can become performance limiting as working temperatures decrease. As potential new cathode materials, high order nickelates can be attractive candidates as they exhibit high electrical conductivities at lower temperatures due to their metallic behaviour. In the current work, we investigate the performance of La4Ni3O10±δ and La4Ni3O10±δ–BaCe0.9Y0.1O3-δ composite cathodes, deposited on a BaCe0.9Y0.1O3-δ (BCY10), proton-conducting electrolyte in the temperature range 350–550 °C. The study is performed in symmetrical cell configuration to permit separation of the electrode performance from the overall cell behaviour. For such experiments, possible internal short-circuiting of the electrolyte in oxidising conditions can arise from the presence of p-type electronic conductivity in the BCY10 electrolyte, causing underestimation of measured polarisation resistances. The current work corrects for such factors by knowledge of the transference numbers of the BCY10 electrolyte and the use of a parallel short-circuiting resistor in the equivalent circuit model. The results underscore that the cathode characteristics are highly dependent on the electronic leakage, especially at higher temperatures, and that suitable correction of data is imperative before discussion. Corrected values for polarisation resistance in wet O2 reveal the composite cathode shows an improvement of total polarisation resistance by a factor of 1.5. The analysis on the oxygen reduction reaction mechanism allows the rate-limiting processes between the electrodes to be differentiated, with the low-frequency term, corresponding to surface processes, being the dominant polarisation resistance and the most improved on formation of the composite. Overall performance analysis indicates that the results from this work are in line with some of the best cathodes currently reported.

Hierarchical Mesoporous/Macroporous Co-Doped NiO Nanosheet Arrays as Free-Standing Electrode Materials for Rechargeable Li-O2 Batteries.

Lithium-oxygen (Li-O2) batteries have been widely recognized as appealing power systems for their extremely high energy density versus common Li-ion batteries. However, there are still lots of issues that need to be addressed toward the practical application. Here, free-standing Co-doped NiO three-dimensional nanosheets were prepared by a hydrothermal synthesis method and directly employed as the air-breathing cathode of the Li-O2 battery. The morphological phenomenon and electrochemical performance of the as-prepared cathode material were characterized by high-resolution scanning electron microscopy, X-ray diffraction, cyclic voltammetry, galvanostatic charge-discharge tests, and electrochemical impedance spectroscopy measurements. The Co-doped NiO electrode delivered a maximum discharge capacity of around 12 857 mA h g-1 with a low overpotential (0.82 V) at 200 mA g-1. Under upper-limit specific capacities of 500 mA h g-1 at 400 mA g-1, the Li-O2 batteries exhibited a long cycle life of 165 cycles. Compared with the undoped NiO electrode, the Li-O2 battery based on the Co-doped NiO cathode showed significantly higher oxygen reduction reaction and oxygen evolution reaction activities. This superior electrochemical performance is because of the partial substitution of Ni2+ in the NiO matrix by Co2+ to improve the p-type electronic conductivity of NiO. In addition, the morphology and specific surface area of NiO are affected by Co doping, which can expand the electrode-electrolyte contact area and lead to sufficient space for Li2O2 deposition. This approach harnesses the great potential of Co-doped NiO nanosheets for practical applications as advanced electrodes for rechargeable Li-O2 batteries.

Simultaneous Optical Transmission Relaxation and Electrical Conductivity Relaxation Measurements of Oxygen Surface Exchange Kinetics during Crystallization

Oxygen exchange at the surface of mixed ionic and electronic conductors (MIECs) plays an integral role in the efficiency and lifetime of many electrochemical devices and is often the efficiency-limiting process at intermediate temperatures (300-600 °C). Despite development of many MIEC perovskite compositions for this purpose, there is still a need to deeply understand processing-structure-property relationships in relation to optimizing the surface exchange coefficient. In recent work, we observed by a new optical transmission relaxation (OTR) approach that perovskite SrTi0.65Fe0.35O3-d (STF35) films deposited by pulsed laser deposition at room temperature in the amorphous state, and subsequently annealed at the crystallization temperature, show immeasurably fast oxygen surface exchange kinetics in the intermediate temperature range [1,2]. In order to better understand this effect, our ongoing work has three aims: 1) to characterize the structural changes occurring in STF35 thin films upon crystallization using X-ray absorption spectroscopy 2) to relate the structural evolution to emergent functional properties, and 3) to evaluate the accuracy of the OTR technique for the measurement of oxygen surface exchange kinetics during crystallization. Towards this last goal, surface exchange coefficients (kchem) were analyzed using simultaneous electrical conductivity relaxation (ECR) and optical transmission relaxation (OTR) techniques on amorphous and crystalline films at various temperature points from 300-600 °C and as a function of oxygen activity in the films by varying the oxygen partial pressure (pO2). Our prior work has shown that the change in optical absorption of the films is directly proportional to the change in their oxygen concentration and change in hole concentration (via Fe valence changes). Therefore, we hypothesized that as the films oxidize or reduce, the amount of light absorbed increases or decreases in direct correspondence to the increase or decrease in p-type electronic conductivity in the film, assuming minimal changes in the electronic mobility with pO2 [3]. Across the different measurement conditions, our results showed excellent agreement in relaxation profiles and fitted kchem values between these two techniques (OTR and ECR) for crystalline films (within a factor of 2), demonstrating the applicability of the optical technique to the STF35 system. Further, both ECR and OTR showed the emergence of rapid oxygen exchange upon in situ crystallization, validating the earlier optically-based observation of this effect. This novel OTR technique provides a continuous, contact-free, and in situ measurement of the native surface kinetics in realistic operating conditions that has the potential to be applied to many other systems. [1] T. Chen, G.F. Harrington, K. Sasaki, and N.H. Perry, “Impact of microstructure and crystallinity on surface exchange kinetics of strontium titanium iron oxide perovskite by in situ optical transmission relaxation approach” Journal of Materials Chemistry A, 5, 23006-23019 (2017). [2] T. Chen, G.F. Harrington, J. Masood, K. Sasaki, and N.H. Perry, “Emergence of Rapid Oxygen Surface Exchange Kinetics During In Situ Crystallization of Mixed Conducting Thin Film Oxides” ACS Applied Materials and Interfaces (2019). [3] N.H. Perry, N. Kim, E. Ertekin, H.L. Tuller, “Origins and control of optical absorption in a non-dilute oxide solid solution: Sr(Ti,Fe)O3-x perovskite case study” Chemistry of Materials (2019).