Cross-Validation of the Remarkably High Surface Oxygen Exchange Kinetics of PrBa0.5Sr0.5Co1.5Fe0.5O5+δ: A Combined Thin-Film Mass Relaxation and Bulk Electrical Conductivity Relaxation Study.

In this work, we measure the oxygen kinetic properties of double perovskite PrBa0.5Sr0.5Co1.5Fe0.5O5+δ (PBSCF), a material widely used as the air electrode in solid oxide electrochemical cells, by mass relaxation (MR) and electrical conductivity relaxation (ECR) experiments. MR studies are carried out using thin films deposited on a gallium phosphate piezocrystal microbalance, and ECR studies are performed using a bulk bar sample with 97% theoretical density. Measurements are performed at 600 °C over the temperature oxygen partial pressure range from 10-4 to 0.21 atm. Despite the differences in experimental formats and surface microstructural features, the ks values extracted from the two methods are found to be in good agreement with one another. The rate constant is found to increase with oxygen partial pressure with a power law dependence, rising from 1.0 × 10-6 cm/s at 3.2 × 10-4 atm to 1.2 × 10-4 cm/s at 0.24 atm, as averaged over the oxidation and reduction directions. The rates in the oxidation direction are observed to be slightly higher than those in the reduction direction for a given pair of pO2 values, suggesting that the final pO2 value controls the overall relaxation behavior. The power law exponent describing the dependence of ks on pO2 is found to be 0.74 ± 0.01. The ECR study of the bulk sample reveals that even with a diffusion length of 1.8 mm, the relaxation process is largely free of diffusion limitations, indicating that PBSCF has the high bulk transport properties required for a double-phase boundary oxidation/reduction pathway.

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

Correction for 'Superprotonic conductivity in RbH2-3y(PO4)1-y: a phosphate deficient analog to cubic CsH2PO4 in the (1 - x)RbH2PO4 - xRb2HPO4 system' by Grace Xiong et al., Mater. Horiz., 2023, 10, 5555-5563, https://doi.org/10.1039/D3MH00852E.

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.

Molybdenum Oxide Precursors that Promote the Low-Temperature Formation of High-Surface-Area Cubic Molybdenum (Oxy)nitride.

Molybdenum nitrides and oxynitrides have been increasingly realized as (electro)catalysts for a variety of reactions. In this context, the cubic "γ-Mo2N", also known to contain oxygen in the bulk, is of particular interest. The γ phase is typically derived from ammonolysis of MoO3, and a high temperature is needed to fully react the stable MoO2 intermediate that often forms along the reaction pathway. In this study, ammonolysis of atypical bronze (HxMoO3) and peroxo (H2MoO5) precursors was undertaken to avoid the formation of this undesired intermediate with the aim of synthesizing "γ-Mo2N" at reduced temperatures and thus with a high surface area. It was found, using in situ powder diffraction, that, when the phase I bronze (x ≈ 0.3) served as the precursor, MoO2 formed as an intermediate and was retained in the reaction product until 700 °C. In contrast, ammonolysis of the phase III bronze (x ≈ 1.7) and of H2MoO5 circumvented the MoO2 intermediate. From these latter two precursors, "γ-Mo2N" was formed at the lowest maximum reaction temperatures reported in the literature, namely, 480 °C in the case of HxMoO3-III and 380 °C for H2MoO5. The resulting products displayed extremely high surface areas of 206 and 152 m2/g, respectively, presumably as a consequence of the low synthesis temperatures. While the HxMoO3-III precursor showed evidence of a topotactic transformation pathway, with morphological similarity between precursor and product phases, H2MoO5 transformed via amorphization. Electrochemical characterization showed moderate activity for the hydrogen evolution reaction (HER), which increased after exposure to reducing potentials and loosely scaled with the catalyst-specific surface area. This work points toward new low-temperature synthesis pathways for accessing molybdenum (oxy)nitrides with high surface areas.

Broad Applicability of Electrochemical Impedance Spectroscopy to the Measurement of Oxygen Nonstoichiometry in Mixed Ion and Electron Conductors.

Oxygen nonstoichiometry is a fundamental feature of mixed ion and electron conductors (MIECs). In this work, a general electrochemical method for determining nonstoichiometry in thin film MIECs, via measurement of the chemical capacitance, is demonstrated using ceria and ceria-zirconia (Ce0.8Zr0.2O2-δ) as representative materials. A.C. impedance data are collected from both materials at high temperature (750-900 °C) under reducing conditions with oxygen partial pressure (pO2) in the range 10-13 to 10-20 atm. Additional measurements of ceria-zirconia films are made under relatively oxidizing conditions with pO2 in the range 0.2 to 10-4 atm and temperatures of 800-900 °C. Under reducing conditions, the impedance spectra are described by a simple circuit in which a resistor is in series with a resistor and capacitor in parallel, and thickness-dependent measurements are used to resolve the capacitance into interfacial and chemical terms. Under more oxidizing conditions, the impedance spectra (of Ce0.8Zr0.2O2-δ) reveal an additional diffusional feature, which enables determination of the ionic resistance of the film in addition to the capacitance, and hence the transport properties. A generalized mathematical formalism is presented for recovering the nonstoichiometry from the chemical capacitance, without recourse to defect chemical models. The ceria nonstoichiometry values are in good agreement with literature values determined by thermogravimetric measurements but display considerably less scatter and are collected on considerably shorter time scales. The thermodynamic analysis of Ce0.8Zr0.2O2-δ corroborates earlier findings that introduction of Zr into ceria enhances its reducibility.

Electrifying membranes to deliver hydrogen.

An electrochemical membrane reactor enables efficient hydrogen generation.

Structure and Properties of Cs7(H4PO4)(H2PO4)8: A New Superprotonic Solid Acid Featuring the Unusual Polycation (H4PO4).

We report the discovery of a new superprotonic compound, Cs7(H4PO4)(H2PO4)8, or CPP, which forms at elevated temperatures from the reaction of CsH2PO4 and CsH5(PO4)2. The structure, solved using high-temperature single-crystal X-ray diffraction and confirmed by high-temperature 31P NMR spectroscopy, crystallizes in space group Pm3̅n and has a lattice constant of 20.1994(9) Å at 130 °C. The unit cell resembles a 4 × 4 × 4 superstructure of superprotonic CsH2PO4, but features an extraordinary chemical moiety, rotationally disordered H4PO4+ cations, which periodically occupy one of every eight cation sites. The influence of this remarkable cation on the structure, thermodynamics, and proton transport properties of the CPP phase is discussed. Notably, CPP forms at a temperature of 90 °C, much lower than the superprotonic transition temperature of 228 °C of CsH2PO4, and the compound does not appear to have an ordered, low-temperature form. Under nominally dry conditions, the material is stable against dehydration to ∼151 °C, and this results in a particularly wide region of stability of a superprotonic material in the absence of active humidification. The conductivity of Cs7(H4PO4)(H2PO4)8 is moderate, 5.8 × 10-4 S cm-1 at 140 °C, but appears nevertheless facilitated by polyanion (H2PO4-) group reorientation.

Variability and origins of grain boundary electric potential detected by electron holography and atom-probe tomography.

A number of grain boundary phenomena in ionic materials, in particular, anomalous (either depressed or enhanced) charge transport, have been attributed to space charge effects. Developing effective strategies to manipulate transport behaviour requires deep knowledge of the origins of the interfacial charge, as well as its variability within a polycrystalline sample with millions of unique grain boundaries. Electron holography is a powerful technique uniquely suited for studying the electric potential profile at individual grain boundaries, whereas atom-probe tomography provides access to the chemical identify of essentially every atom at individual grain boundaries. Using these two techniques, we show here that the space charge potential at grain boundaries in lightly doped, high-purity ceria can vary by almost an order of magnitude. We further find that trace impurities (<25 ppm), rather than inherent thermodynamic factors, may be the ultimate source of grain boundary charge. These insights suggest chemical tunability of grain boundary transport properties.

A review of defect structure and chemistry in ceria and its solid solutions.

Ceria and its solid solutions play a vital role in several industrial processes and devices. These include solar energy-to-fuel conversion, solid oxide fuel and electrolyzer cells, memristors, chemical looping combustion, automotive 3-way catalysts, catalytic surface coatings, supercapacitors and recently, electrostrictive devices. An attractive feature of ceria is the possibility of tuning defect-chemistry to increase the effectiveness of the materials in application areas. Years of study have revealed many features of the long-range, macroscopic characteristics of ceria and its derivatives. In this review we focus on an area of ceria defect chemistry which has received comparatively little attention - defect-induced local distortions and short-range associates. These features are non-periodic in nature and hence not readily detected by conventional X-ray powder diffraction. We compile the relevant literature data obtained by thermodynamic analysis, Raman spectroscopy, and X-ray absorption fine structure (XAFS) spectroscopy. Each of these techniques provides insight into material behavior without reliance on long-range periodic symmetry. From thermodynamic analyses, association of defects is inferred. From XAFS, an element-specific probe, local structure around selected atomic species is obtained, whereas from Raman spectroscopy, local symmetry breaking and vibrational changes in bonding patterns is detected. We note that, for undoped ceria and its solid solutions, the relationship between short range order and cation-oxygen-vacancy coordination remains a subject of active debate. Beyond collating the sometimes contradictory data in the literature, we strengthen this review by reporting new spectroscopy results and analysis. We contribute to this debate by introducing additional data and analysis, with the expectation that increasing our fundamental understanding of this relationship will lead to an ability to predict and tailor the defect-chemistry of ceria-based materials for practical applications.

Suppression of atom motion and metal deposition in mixed ionic electronic conductors.

Many superionic mixed ionic-electronic conductors with a liquid-like sublattice have been identified as high efficiency thermoelectric materials, but their applications are limited due to the possibility of decomposition when subjected to high electronic currents and large temperature gradients. Here, through systematically investigating electromigration in copper sulfide/selenide thermoelectric materials, we reveal the mechanism for atom migration and deposition based on a critical chemical potential difference. Then, a strategy for stable use is proposed: constructing a series of electronically conducting, but ion-blocking barriers to reset the chemical potential of such conductors to keep it below the threshold for decomposition, even if it is used with high electric currents and/or large temperature differences. This strategy not only opens the possibility of using such conductors in thermoelectric applications, but may also provide approaches to engineer perovskite photovoltaic materials and the experimental methods may be applicable to understanding dendrite growth in lithium ion batteries.

Out-of-Plane Ionic Conductivity Measurement Configuration for High-Throughput Experiments.

An approach for measuring conductivity of thin-film electrolytes in an out-of-plane configuration, amenable to high-throughput experimentation, is presented. A comprehensive analysis of the geometric requirements for success is performed. Using samaria-doped ceria (Ce0.8Sm0.2O1.9, SDC) excellent agreement between bulk samples and thin films with continuous and patterned electrodes, 100-500 µm in diameter, is demonstrated. Films were deposited on conductive Nb-doped SrTiO3, and conductivity was measured by AC impedance spectroscopy over the temperature range from ∼200 to ∼500 °C. The patterned electrode geometry, which encompassed an array of microdot metal electrodes for making top contact, enabled measurements at hundreds of positions on the film, implying the potential for measuring hundreds of composition in a single library.

Chemical surface exchange of oxygen on CeO2-δ in an O2/H2O atmosphere.

The chemical surface reaction rate constant controlling the change of oxidation state of undoped ceria, kChem, was measured at 1400 °C in the range of (∼0 ≤ (pH2O/atm) ≤ 0.163(9)) and (10-3.85 ≤ (pO2/atm) ≤ 10-2.86) via the electrical conductivity relaxation method. In humidified atmospheres, kChem is fully described as the sum of kChem,O2 and kChem,H2O, which are, respectively, the rate constants for oxidation by O2 and by H2O alone. Using measurements under appropriately controlled gas conditions, the total rate constant is found to follow the correlation kChem/cm s-1 = 10-(1.35±0.07) × (pO2/atm)0.72±0.02 + 10-(3.85±0.03) × (pH2O/atm)0.36±0.03 where the pO2 and pH2O values of relevance are explicitly those of the final gas condition. The results suggest that at such high temperatures, the concentrations of surface adsorbed species are too low to influence the independent reaction pathways.

Gas-phase vs. material-kinetic limits on the redox response of nonstoichiometric oxides.

Cerium dioxide, CeO2-δ, remains one of the most attractive materials under consideration for solar-driven thermochemical production of chemical fuels. Understanding the rate-limiting factors in fuel production is essential for maximizing the efficacy of the thermochemical process. The rate of response is measured here via electrical conductance relaxation methods using porous ceria structures with architectural features typical of those employed in solar reactors. A transition from behavior controlled by material surface reaction kinetics to that controlled by sweep-gas supply rates is observed on increasing temperature, increasing volume specific surface area, and decreasing normalized gas flow rate. The transition behavior is relevant not only for optimal reactor operation and architectural design of the material, but also for accurate measurement of material properties.

Extreme high temperature redox kinetics in ceria: exploration of the transition from gas-phase to material-kinetic limitations.

The redox kinetics of undoped ceria (CeO2-δ) are investigated by the electrical conductivity relaxation method in the oxygen partial pressure range of -4.3 ≤ log(pO2/atm) ≤ -2.0 at 1400 °C. It is demonstrated that extremely large gas flow rates, relative to the mass of the oxide, are required in order to overcome gas phase limitations and access the material kinetic properties. Using these high flow rate conditions, the surface reaction rate constant kchem is found to obey the correlation log(kchem/cm s(-1)) = (0.84 ± 0.02) × log(pO2/atm) - (0.99 ± 0.05) and increases with oxygen partial pressure. This increase contrasts the known behavior of the dominant defect species, oxygen vacancies and free electrons, which decrease in concentration with increasing oxygen partial pressure. For the sample geometries employed, diffusion was too fast to be detected. At low gas flow rates, the relaxation process becomes limited by the capacity of the sweep gas to supply/remove oxygen to/from the oxide. An analytical expression is derived for the relaxation in the gas-phase limited regime, and the result reveals an exponential decay profile, identical in form to that known for a surface reaction limited process. Thus, measurements under varied gas flow rates are required to differentiate between surface reaction limited and gas flow limited behavior.

Electrochemical behavior of thin-film Sm-doped ceria: insights from the point-contact configuration.

The electrochemical behavior of chemical vapor deposition (CVD) grown porous films of Sm-doped ceria (SDC) for hydrogen oxidation has been evaluated by impedance spectroscopy using a point contact geometry at a temperature of 650 °C. Porous SDC films, 950 nm in thickness, were deposited on both sides of single-crystal YSZ(100). Pt paste was applied over the surface of one SDC layer to create a high-activity counter electrode. Ni wire was contacted to the surface of the other SDC layer to create a limited contact-area working electrode. The active area of contact at the working electrode was determined using the Newman equation and the electrolyte constriction impedance. The radius of this area varied from 5 to 18 µm, depending on gas composition and bias. The area-normalized electrode impedance (where the area was that determined as described above) varied from 0.03 to 0.17 Ω cm(2) and generally decreased with cathodic bias and decreasing oxygen partial pressure. From an analysis of the dimensions of the active area with bias, it was found that the majority of the overpotential occurred at the SDC|gas interface rather than the SDC|YSZ interface. Overall, the anode overpotential is found to be extremely small, competitive with the best oxide anodes reported in the literature. Nevertheless, the impedance falls in line with expected values based on extrapolations of the properties of dense, flat SDC model electrodes grown by pulsed laser deposition (Chueh et al., Nat. Mater., 2012). The results demonstrate that, with suitable fabrication approaches, exceptional activity can be achieved with SDC for hydrogen electrooxidation even in the absence of metal-oxide-gas triple phase boundaries.

Characterization of the dynamics in the protonic conductor CsH2PO4 by ¹7O solid-state NMR spectroscopy and first-principles calculations: correlating phosphate and protonic motion.

(17)O NMR spectroscopy combined with first-principles calculations was employed to understand the local structure and dynamics of the phosphate ions and protons in the paraelectric phase of the proton conductor CsH2PO4. For the room-temperature structure, the results confirm that one proton (H1) is localized in an asymmetric H-bond (between O1 donor and O2 acceptor oxygen atoms), whereas the H2 proton undergoes rapid exchange between two sites in a hydrogen bond with a symmetric double potential well at a rate ≥10(7) Hz. Variable-temperature (17)O NMR spectra recorded from 22 to 214 °C were interpreted by considering different models for the rotation of the phosphate anions. At least two distinct rate constants for rotations about four pseudo C3 axes of the phosphate ion were required in order to achieve good agreement with the experimental data. An activation energy of 0.21 ± 0.06 eV was observed for rotation about the P-O1 axis, with a higher activation energy of 0.50 ± 0.07 eV being obtained for rotation about the P-O2, P-O3(d), and P-O3(a) axes, with the superscripts denoting, respectively, dynamic donor and acceptor oxygen atoms of the H-bond. The higher activation energy of the second process is most likely associated with the cost of breaking an O1-H1 bond. The activation energy of this process is slightly lower than that obtained from the (1)H exchange process (0.70 ± 0.07 eV) (Kim, G.; Blanc, F.; Hu, Y.-Y.; Grey, C. P. J. Phys. Chem. C 2013, 117, 6504-6515) associated with the translational motion of the protons. The relationship between proton jumps and phosphate rotation was analyzed in detail by considering uncorrelated motion, motion of individual PO4 ions and the four connected/H-bonded protons, and concerted motions of adjacent phosphate units, mediated by proton hops. We conclude that, while phosphate rotations aid proton motion, not all phosphate rotations result in proton jumps.

Platinum-decorated carbon nanotubes for hydrogen oxidation and proton reduction in solid acid electrochemical cells.

Pt-decorated carbon nanotubes (Pt-CNTs) were used to enhance proton reduction and hydrogen evolution in solid acid electrochemical cells based on the proton-conducting electrolyte CsH2PO4. The carbon nanotubes served as interconnects to the current collector and as a platform for interaction between the Pt and CsH2PO4, ensuring minimal catalyst isolation and a large number density of active sites. Particle size matching was achieved by using electrospray deposition to form sub-micron to nanometric CsH2PO4. A porous composite electrode was fabricated from electrospray deposition of a solution of Pt-CNTs and CsH2PO4. Using AC impedance spectroscopy and cyclic voltammetry, the total electrode overpotential corresponding to proton reduction and hydrogen oxidation of the most active electrodes containing just 0.014 mg cm-1 of Pt was found to be 0.1 V (or 0.05 V per electrode) at a current density of 42 mA cm-2 for a measurement temperature of 240 °C and a hydrogen-steam atmosphere. The zero bias electrode impedance was 1.2 Ω cm2, corresponding to a Pt utilization of 61 S mg-1, a 3-fold improvement over state-of-the-art electrodes with a 50× decrease in Pt loading.

Dynamic Nuclear Polarization NMR of Low-γ Nuclei: Structural Insights into Hydrated Yttrium-Doped BaZrO3.

We demonstrate that solid-state NMR spectra of challenging nuclei with a low gyromagnetic ratio such as yttrium-89 can be acquired quickly with indirect dynamic nuclear polarization (DNP) methods. Proton to (89)Y cross polarization (CP) magic angle spinning (MAS) spectra of Y(3+) in a frozen aqueous solution were acquired in minutes using the AMUPol biradical as a polarizing agent. Subsequently, the detection of the (89)Y and (1)H NMR signals from technologically important hydrated yttrium-doped zirconate ceramics, in combination with DFT calculations, allows the local yttrium and proton environments present in these protonic conductors to be detected and assigned to different hydrogen-bonded environments.

High-temperature isothermal chemical cycling for solar-driven fuel production.

The possibility of producing chemical fuel (hydrogen) from the solar-thermal energy input using an isothermal cycling strategy is explored. The canonical thermochemical reactive oxide, ceria, is reduced under high temperature and inert sweep gas, and in the second step oxidized by H2O at the same temperature. The process takes advantage of the oxygen chemical potential difference between the inert sweep gas and high-temperature steam, the latter becoming more oxidizing with increasing temperature as a result of thermolysis. The isothermal operation relieves the need to achieve high solid-state heat recovery for high system efficiency, as is required in a conventional two-temperature process. Thermodynamic analysis underscores the importance of gas-phase heat recovery in the isothermal approach and suggests that attractive efficiencies may be practically achievable on the system level. However, with ceria as the reactive oxide, the isothermal approach is not viable at temperatures much below 1400 °C irrespective of heat recovery. Experimental investigations show that an isothermal cycle performed at 1500 °C can yield fuel at a rate of ~9.2 ml g(-1) h(-1), while providing exceptional system simplification relative to two-temperature cycling.

Carbon nanotubes as electronic interconnects in solid acid fuel cell electrodes.

Carbon nanotubes have been explored as interconnects in solid acid fuel cells to improve the link between nanoscale Pt catalyst particles and macroscale current collectors. The nanotubes were grown by chemical vapor deposition on carbon paper substrates, using nickel nanoparticles as the catalyst, and were characterized using scanning electron microscopy and Raman spectroscopy. The composite electrode material, consisting of CsH2PO4, platinum nanoparticles, and platinum on carbon-black nanoparticles, was deposited onto the nanotube-overgrown carbon paper by electrospraying, forming a highly porous, fractal structure. AC impedance spectroscopy in a symmetric cell configuration revealed a significant reduction of the electrode impedance as compared to similarly prepared electrodes without carbon nanotubes.