Imperial College London

Professor Stephen Skinner

Faculty of EngineeringDepartment of Materials

CeresPower/RAEng Research Chair in Electrochemical Devices
 
 
 
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Contact

 

+44 (0)20 7594 6782s.skinner

 
 
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Location

 

206GoldsmithSouth Kensington Campus

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Summary

 

Publications

Publication Type
Year
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249 results found

Han Y, Li S, Skinner S, 2024, Effect of W and Mo co-substitution on conductivity and structure of LaNbO4, Journal of Solid State Chemistry, Vol: 332, ISSN: 0022-4596

LaNb1-xMxO4+δ (M = W, Mo) which adopts a modulated tetragonal scheelite type structure exhibits enormous potential as an electrolyte material for solid oxide fuel cells. Here we report the solid-state synthesis of a series of W/Mo co-substituted samples with LaNb0.8WxMo0.2-xO4.1 (x = 0.00, 0.02, 0.04, 0.10, 0.16, 0.18, 0.20) which exhibited the highest total conductivity at x = 0.02 in the intermediate temperature range (2.37 × 10−3 S cm−1 at 665 °C in air). Powder X-ray diffraction patterns showed that the tetragonal phases were able to be stabilized at room temperature when x > 0.02. The phase diagram obtained from variable-temperature X-ray diffraction, indicated that the modulated structures could be maintained from room temperature to 900 °C when x ≥ 0.10. With the combination of electrochemical impedance spectroscopy measurements and Rietveld refinements from X-ray diffraction patterns, total conductivities of these materials were found to be proportional to the volume of the unit cells. It is also noted that the existence of modulated structures negatively impacts the total conductivities.

Journal article

Xie Z, Baghdadi Y, Skinner S, 2024, Enhanced performance of Pr4Ni3O10±δ-Ce0.75Gd0.1Pr0.15O2−δ composite electrode via particle size grading, ACS Applied Energy Materials, Vol: 7, Pages: 1640-1646, ISSN: 2574-0962

A particle grading strategy was applied in Pr4Ni3O10±δ (PNO)–Ce0.75Gd0.1Pr0.15O2−δ (CGPO) composite electrodes for intermediate-temperature solid oxide fuel cells. By testing of ungraded and graded electrodes and analysis of the obtained impedance spectra with the help of the distribution of relaxation time, it was found that the increase in oxygen species diffusion resistance, due to greater tortuosity of electrode particles, can be suppressed by the grading strategy. At the same time, the step of charge transfer can be enhanced. Overall, graded electrodes have much lower area specific resistance (ASR) than ungraded electrodes. The lowest ASR obtained for graded electrodes is 0.059 Ω cm2 at 625 °C (0.025 Ω cm2 at 698 °C) and pO2 of 0.21 atm, nearly a one-fold decrease compared to 0.11 Ω cm2 for ungraded electrodes at the same condition.

Journal article

Sukumaran S, Skinner SJ, Lima D, Dawson Ret al., 2023, Probing the Dynamic Na Metal/Nasicon Interface in Sodium-Ion Batteries, ECS Meeting Abstracts, Vol: MA2023-02, Pages: 2901-2901

<jats:p> Interest in solid-state sodium-ion batteries (SSIBs) have resurged remarkably as a means of diversification in energy storage devices to address climate issues and further propelled by the movement of ‘Beyond-Li’. State of the art SSIBs not only offer energy security and sustainability but also comparable power and energy densities to their counter-parts. Yet to fully capitalise on the technology, the fundamental processes taking place between the anode and interface or ‘point of contact’ needs to be understood. Previous iterations of SSIBs have employed various solid-electrolyte candidates – most notable being beta-alumina – however with the recent advancements in synthesis and dopants more prominent solid-electrolytes have become interesting, one in particular being NaSICON (Sodium Super Ionic Conductor). NaSICON has several advantages, one of which is the reduction in operating temperatures of SSIBs due to its inherent fast sodium-ion transport<jats:sup>1</jats:sup>. This would broaden the potential applications of SSIBs, making the technology more accessible to a wider range of markets.</jats:p> <jats:p>Current perspectives on NaSICON solid-electrolytes have focused on optimising conductivity through means of additive dopants<jats:sup>2</jats:sup> and synthesis procedures<jats:sup>3</jats:sup>. Yet at present, the understanding of the fundamentals of interfacial inhomogeneities that affect the performance of these materials in SSIBs is limited. With consideration of the direct implications on performance that surface chemistry has, understanding of this is paramount.</jats:p> <jats:p>Evaluation of the chemical evolution of the Na metal/NaSICON interface has been achieved through assessing the changes of the chemical landscape that take place at the surface and near-surface, through surface characterisation techniques that p

Journal article

Mousley P, Nicklin C, Pramana S, van den Bosch C, Ryan M, Skinner Set al., 2023, Temperature effect on surface structure of single crystal SrLaAlO4(001), APL Materials, Vol: 11, ISSN: 2166-532X

Development of next generation electrochemical devices such as solid oxide cells requires control of the charge transfer processes across key interfaces. Structural strain at electrolyte:electrode interfaces could potentially alter the devices charge transport properties, therefore understanding the structural behaviour of electrode surfaces at operating conditions is important. The functional oxide single crystal substrate SrLaAlO4 has been well-characterised with bulk structure studies, however there are very few studies of SrLaAlO4 surface structures. Here we present an investigation of the surface structure of SrLaAlO4(001) substrates using surface X-ray diffraction, under UHV conditions (10−10 torr) with the substrate held at either room temperature or 650 ◦C. Best-fit models using a 1:1 ratio of Sr:La showed significant distortions to the surface AlO6 octahedra.

Journal article

Sha Z, Kerherve G, van Spronsen MA, Wilson GE, Kilner JA, Held G, Skinner SJet al., 2023, Studying Surface Chemistry of Mixed Conducting Perovskite Oxide Electrodes with Synchrotron-Based Soft X-rays, The Journal of Physical Chemistry C, Vol: 127, Pages: 20325-20336, ISSN: 1932-7447

Journal article

Xie Z, Jang I, Ouyang M, Hankin A, Skinner Set al., 2023, High performance composite Pr4Ni3O10±δ-Ce0.75Gd0.1Pr0.15O2−δ solid oxide cell air electrode, JPhys Energy, Vol: 5, Pages: 1-16, ISSN: 2515-7655

A composite electrode composed of Pr4Ni3O10±δ - Ce0.75Gd0.1Pr0.15O2−δ (50 wt. % - 50 wt. %) was thoroughly investigated in terms of the electrochemical performance as a function of microstructure. The electrochemical performance was characterized by electrochemical impedance spectroscopy and the microstructures, characterized by focused ion beam-scanning electron microscopy and 3D reconstructions, were modified by changing the particle size of Pr4Ni3O10±δ and the electrode thickness. The distribution of relaxation time (DRT) method was applied to help resolve electrochemical processes occurring in the electrodes. It was found that an appropriate increase in electrode thickness and an appropriate decrease in particle size enhanced the oxygen reduction reaction kinetics. The&#xD;lowest area specific resistance obtained in this study at 670 °C under pO2 of 0.21 atm was 0.055 Ω cm2. Finally, a comparison to the Adler Lane Steele (ALS) model was made and the main active site for the oxygen reduction reaction was concluded to be triple phase boundaries. A fuel cell made of the composite material as the cathode was fabricated and tested. The peak power density was 1&#xD;Wcm−2 at 800 °C, which demonstrates this composite material is promising for SOFC cathodes.

Journal article

Skinner S, HAN Y, 2023, Investigation of crystal structure and electrochemical performance of Gd doped LaNb0.9Mo0.1O4.05, Materials Advances, Vol: 4, Pages: 3759-3766, ISSN: 2633-5409

Single phase La1-xGdxNb0.9Mo0.1O4.05 (x=0, 0.20, 0.40, 0.50, 0.60, 0.80 and 1.00) has been synthesized by a solid state reaction route. The crystal structures of all samples were investigated by high temperature X-ray diffraction. Both LaNb0.9Mo0.1O4.05 and La0.8Gd0.2Nb0.9Mo0.1O4.05 were found to possess modulated crystal structures at room temperature. The electrochemical performance of all samples was investigated by electrochemical impedance spectroscopy. LaNb0.9Mo0.1O4.05 exhibited the highest total conductivity of 1.52 × 10−2 Scm−1 at 900 °C in air; however, at intermediate temperature range, the conductivity of La0.8Gd0.2Nb0.9Mo0.1O4.05 was higher than LaNb0.9Mo0.1O4.05. The reason for these phenomena could be related to the influence of the negative effects of a modulated structure on electrochemical performance and both Gd and Mo dopants on the stability of the sample, which was investigated using the Bond Valence Sum approach.

Journal article

Yatoo MA, Skinner SJ, 2023, Oxygen Transport in Higher-Order Ruddlesden-Popper Phase Materials, ECS Meeting Abstracts, Vol: MA2023-01, Pages: 371-371

<jats:p> A series of higher-order Ruddlesden-Popper phase materials – La<jats:sub>3</jats:sub>PrNi<jats:sub>3</jats:sub>O<jats:sub>9.76 </jats:sub>(L3P1N3),<jats:sub> </jats:sub>La<jats:sub>2</jats:sub>Pr<jats:sub>2</jats:sub>Ni<jats:sub>3</jats:sub>O<jats:sub>9.65 </jats:sub>(L2P2N3)<jats:sub> </jats:sub>and LaPr<jats:sub>3</jats:sub>Ni<jats:sub>3</jats:sub>O<jats:sub>9.76</jats:sub> (L1P3N3) – were synthesised and investigated by neutron powder diffraction to understand the oxygen defect structure and propose possible pathways for oxygen transport in these materials. Further complimentary DFT calculations of the materials were performed to support the experimental analysis.</jats:p> <jats:p>All of the phases were hypostoichiometric and it was observed that the majority of the oxygen vacancies were confined to the perovskite layers, with a preference for equatorial oxygen sites. A particular preference for vacancies in O(1) and O(5) sites at high temperatures was observed from neutron diffraction measurements which were further complimented by DFT calculations wherein the vacancy formation energy was found to be lowest at the O(1) site. Also, a preference for a curved oxygen transport pathway around the NiO<jats:sub>6</jats:sub> octahedra was observed which agrees with the published literature for Ruddlesden-Popper phase materials.</jats:p> <jats:p>Lattice parameters for all three compositions showed a linear increase with increasing temperature, but the increase was greatest in the <jats:italic>c</jats:italic> parameter while the <jats:italic>b</jats:italic> parameter showed only a slight increase when compared to the <jats:italic>a</jats:italic> parameter. The thermal expansion coefficient was calculated

Journal article

Bagarinao KD, Celikbilek O, Kerherve G, Fearn S, Skinner SJ, Kishimoto Het al., 2023, Nanostructured LSC Thin Film Electrodes with Improved Electrochemical Performance and Long-Term Stability, ECS Meeting Abstracts, Vol: MA2023-01, Pages: 113-113

<jats:p> Nanostructured La<jats:sub>0.6</jats:sub>Sr<jats:sub>0.4</jats:sub>CoO<jats:sub>3-δ</jats:sub> (LSC) thin film electrodes exhibit exceptionally high oxygen surface exchange properties, surpassing those of conventional microscale electrode structures, which are desirable for application in solid oxide cells (SOC) [1-2]. On the other hand, the LSC nanostructures also tend to undergo significant morphological changes at typically high temperatures required for SOC operation, leading to rapid degradation in performance. Here, towards the goal of improving the long-term stability of electrochemical performance of nanostructured LSC thin films, a systematic investigation of the effect of processing temperatures on long-term stability was carried out [3]. By varying the deposition temperature (500 °C to room temperature), the as-grown characteristic nanostructures of LSC thin films prepared using pulsed laser deposition can be tuned from highly dense nanocolumnar grains to nanofibrous structures with high porosity. Variations in the deposition temperature also resulted to differences in the proportion of surface-bound/lattice-bound Sr and Co<jats:sup>2+</jats:sup>/Co<jats:sup>3+</jats:sup> at the surfaces of the as-grown LSC thin films; however, prolonged annealing at 700 °C in air essentially transforms the surfaces to a final state with mostly lattice-bound Sr and Co<jats:sup>3+</jats:sup>. Nevertheless, LSC films with initially nanofibrous structures are found to be less prone to the grain sintering effect occurring at high temperatures and exhibit less degradation of the electrode polarization resistance as compared to well-dense films. Using lower deposition temperatures, cation interdiffusion occurring at LSC/GDC interfaces is also significantly suppressed, thus leading to better interfacial stability as compared to those prepared at higher deposition temperatures.

Journal article

Xu X, Bi L, Skinner SJ, 2023, Cathode Surface Segregating Modification for Boosting Oxygen Reduction Reactions: Coupling Theory and Experiment in Proton Conducting Solid Oxide Fuel Cells (H-SOFCs), ECS Meeting Abstracts, Vol: MA2023-01, Pages: 192-192

<jats:p> The surface electrochemical catalysis reaction is complex due to the unpredictable surface morphology and severe reaction environment, especially for cathodes in proton conducting solid oxide fuel cells (H-SOFCs) where water vapor is generated and evaporated at a high operating temperature. The first-generation cathode materials (La<jats:sub>x</jats:sub>Sr<jats:sub>1-x</jats:sub>MO<jats:sub>3-δ</jats:sub>, M=Fe, Co, Mn) tend to have A-site cation segregation, and this segregation layer reduces the performance. Surface segregation phenomena tends to be ignored by computer simulations because surface termination is described as a highly crystallized layer, misleading the development of material computing science. Therefore, a combination of theory and surface investigation is essential for future work in this community. In this work, we use density functional theory (DFT) for designing a higher-performing cathode material, La<jats:sub>0.35</jats:sub>Bi<jats:sub>0.15</jats:sub>Sr<jats:sub>0.5</jats:sub>FeO<jats:sub>3-δ</jats:sub> (LBSF), for improvement of both bulk and surface electrochemical character in an intrinsic manner, predicting a candidate material with good performance that can be used in solid oxide proton conducting fuel cells (H-SOFCs). By combining a sensitive surface analysis technique, low energy ion scattering (LEIS), surface segregation is finely considered and determined that the particles are fully covered with a Sr segregation layer on the outer most surface. Aberration-Corrected transmission electron microscopy (ACTEM) is also applied and shows an amorphous structure at the particle edge. With the determination of this segregation layer, a reality-closed model is given, and related surface reactions are modelled based on this layer instead of a conventional cleaved, well-crystallized surface. Also, as bismuth diffuses towards the surfa

Journal article

Williams NJ, Leah R, Mukerjee S, Zhuang D, Bazant MZ, Skinner SJet al., 2023, Multiphase Porous Electrode Theory for the Next Generation of SOFC/SOEC Electrodes, ECS Meeting Abstracts, Vol: MA2023-01, Pages: 59-59

<jats:p> The mixed-ionic electronic conduction (MIEC) of gadolinium doped ceria (CGO) under reduced oxygen conditions makes it an excellent fuel electrode material for SOFC/SOEC applications. As part of a composite electrode (Ni/CGO), the nickel phase offers a fast electronic conduction pathway to the current collector and may act as an electrocatalyst at the three-phase boundary. Although the Ni/CGO cermet provides superior electrochemical performance compared with older technologies such as Ni/YSZ, the models used to study MIEC materials do not capture the unique transport and kinetic physics which makes them an excellent choice for the next generation of fuel electrodes. Within the framework of multiphase porous electrode theory, this work provides a novel set of differential-algebraic equations which captures the effects of the activation overpotential on electronic defect concentration, electrostatic surface potential and ionic transport to accurately predict the current-voltage behaviour of the Ni/CGO electrode. Moreover, through concerted electron and proton tunnelling events, we unify the theory of the electrostatic surface potential with proton-coupled electron transfer kinetics. </jats:p>

Journal article

Yatoo MA, Skinner SJ, 2023, Hydrogen Exchange and Diffusion Kinetics at Elevated Temperatures in Proton-Conducting Solid Oxide Materials, ECS Meeting Abstracts, Vol: MA2023-01, Pages: 454-454

<jats:p> Storage of purified hydrogen is one of the central challenges in addressing climate change and reducing our reliance on fossil fuels for energy conversion and storage, and therefore there is a global surge in research and development concerning hydrogen purification and storage. In this regard, we are studying proton conduction in solid oxide materials at elevated temperatures for applications in hydrogen separation and compression membranes. Hydrogen compression is the most recommended method to store hydrogen for automotive applications as it allows an increase in the hydrogen volumetric energy density.</jats:p> <jats:p>Traditionally the protonic conductivity in these materials is measured by indirect methods. For example, conductivity measurements in mixed gas atmospheres, comparing for example dry N<jats:sub>2</jats:sub> with humidified N<jats:sub>2</jats:sub>, thereby allowing the contribution of protons to be evaluated. In this study, we for the first time report the evaluation of protonic conductivity in BaZr<jats:sub>1-x</jats:sub>Ce<jats:sub>x</jats:sub>Y<jats:sub>0.2</jats:sub>O<jats:sub>3−δ</jats:sub>, BaZr<jats:sub>0.1</jats:sub>Ce<jats:sub>0.7</jats:sub>Y<jats:sub>0.2–x</jats:sub>Yb<jats:sub>x</jats:sub>O<jats:sub>3–δ</jats:sub> and Ba<jats:sub>7</jats:sub>Nb<jats:sub>4</jats:sub>MoO<jats:sub>20</jats:sub> by direct measurements afforded by the Isotope Exchange Depth Profiling technique with deuterium labelling. We also report the kinetics of H/D transport through the bulk materials and across metal-ceramic interfaces with a particular interest in the behaviour of the interface between the key Pd/Pd alloy catalyst component and the hydrogen-transporting oxide ceramic material. The transport and interface behaviour in

Journal article

Skinner S, Williams N, Warburton RE, Seymour I, Cohen AE, Bazant Met al., 2023, Proton-coupled electron transfer at SOFC electrodes, Journal of Chemical Physics, Vol: 158, Pages: 1-10, ISSN: 0021-9606

Understanding the charge transfer processes at solid oxide-fuel cell (SOFC) electrodes is critical to designing more efficient and robust materials. Activation losses atSOFC electrodes have been widely attributed to the ambipolar migration of charges atthe mixed ionic–electronic conductor-gas interface. Empirical Butler-Volmer kineticsbased on transition state theory is often used to model the current-voltage relationship, where charged particles transfer classically over an energy barrier. However, thehydrogen oxidation/water electrolysis reaction H2(g) + O2− ⇀↽ H2O(g) + 2e− must bemodelled through concerted electron and proton tunnelling events, where we unify thetheory of the electrostatic surface potential with proton-coupled electron transfer kinetics. We derive a framework for the reaction rate that depends on the electrostaticsurface potential, adsorbate dipole moment, the electronic structure of the electrondonor/acceptor, and vibronic states of the hydrogen species. This theory was used tostudy the current-voltage characteristics of the Ni/gadolinium doped ceria electrode inH2/H2O(g) where we find excellent validation of this novel model. These results yieldthe first reported quantification of the solvent reorganisation energy for an SOFC material, and suggests that the three-phase boundary mechanism is the dominant pathwayfor charge transfer at cermet electrodes.

Journal article

Xu X, Bi L, Skinner SJ, 2023, Cathode Surface Segregating Modification for Boosting Oxygen Reduction Reactions: Coupling Theory and Experiment in Proton Conducting Solid Oxide Fuel Cells (H-SOFCs), ECS Transactions, Vol: 111, Pages: 1249-1257, ISSN: 1938-5862

<jats:p>Computational simulation, based on Density Functional Theory (DFT), was carried out to develop a more powerful cathode material, La<jats:sub>0.35</jats:sub>Bi<jats:sub>0.15</jats:sub>Sr<jats:sub>0.5</jats:sub>FeO<jats:sub>3-δ</jats:sub> (LBSF), based on a first-generation cathode material, La<jats:sub>0.5</jats:sub>Sr<jats:sub>0.5</jats:sub>FeO<jats:sub>3-δ</jats:sub> (LSF) for use in proton-conducting solid oxide fuel cells (H-SOFCs). The calculation results show that the Bi substitution causes an oxygen p band center shift, which accelerates the oxygen reduction reaction (ORR). To prove this prediction, LBSF and LSF was synthesized successfully, and each surface was also characterized with X-ray photoelectron spectroscopy (XPS) and low energy ion scattering (LEIS), observing a surface Sr segregation and change after Bi substitution. The change proves an effective modification of surface segregation component. Subsequently, LBSF, applied into H-SOFCs, shows excellent performance with a peak power density of 1630 mW cm<jats:sup>-2</jats:sup> at 700 ℃ when this material was employed as a cathode in a full H-SOFC.</jats:p>

Journal article

Yatoo MA, Skinner SJ, 2023, Oxygen Transport in Higher-Order Ruddlesden-Popper Phase Materials, ECS Transactions, Vol: 111, Pages: 2405-2412, ISSN: 1938-5862

<jats:p>A series of Ruddlesden-Popper phase materials – La<jats:sub>3</jats:sub>PrNi<jats:sub>3</jats:sub>O<jats:sub>10-</jats:sub> <jats:sub>δ</jats:sub>,<jats:sub> </jats:sub>La<jats:sub>2</jats:sub>Pr<jats:sub>2</jats:sub>Ni<jats:sub>3</jats:sub>O<jats:sub>10-δ </jats:sub>and LaPr<jats:sub>3</jats:sub>Ni<jats:sub>3</jats:sub>O<jats:sub>10-</jats:sub> <jats:sub>δ </jats:sub>– were synthesised and investigated by neutron powder diffraction to understand the oxygen defect structure. The thermal expansion coefficient was calculated for all compositions and was found to be in the range of 13.0 - 13.4 × 10<jats:sup>-6</jats:sup> ˚C<jats:sup>-1</jats:sup>, which is compatible with the commonly used electrolyte materials for solid oxide fuel cells. A weak preference for Pr occupying the M(2) site in the rocksalt layer was also observed. The majority of the oxygen vacancies were confined to the perovskite layers with a particular preference for O(1) and O(5) sites at high temperatures observed by neutron diffraction measurements. Further, a preference for a curved oxygen transport pathway around the NiO<jats:sub>6</jats:sub> octahedra was observed which agrees with the published literature for these materials.</jats:p>

Journal article

Bagarinao KD, Celikbilek O, Kerherve G, Fearn S, Skinner SJ, Kishimoto Het al., 2023, Nanostructured LSC Thin Film Electrodes with Improved Electrochemical Performance and Long-Term Stability, ECS Transactions, Vol: 111, Pages: 731-736, ISSN: 1938-5862

<jats:p>Nanostructured La<jats:sub>0.6</jats:sub>Sr<jats:sub>0.4</jats:sub>CoO<jats:sub>3-</jats:sub> <jats:italic> <jats:sub>δ</jats:sub> </jats:italic> </jats:p> <jats:p>(LSC) thin film electrodes exhibit exceptionally high oxygen surface exchange properties, surpassing those of conventional microscale electrode structures, which are desirable for application in solid oxide cells (SOCs). Here, towards the goal of improving the long-term stability of electrochemical performance of nanostructured LSC thin films, a systematic investigation of the effect of processing temperatures on long-term stability was carried out. By varying the deposition temperature (from 500 °C to room temperature), the as-grown characteristic nanostructures of LSC thin films prepared using pulsed laser deposition can be tuned from highly dense nanocolumnar grains to nanofibrous structures with high porosity. From the comparison of the properties of the as-grown films and those annealed at 700 °C for 300 h, it was revealed that LSC thin films prepared at lower deposition temperatures are less prone to the grain sintering effect and cation interdiffusion and exhibit better interfacial stability and improved long-term performance.</jats:p>

Journal article

Yatoo M, Seymour I, Skinner S, 2023, Neutron diffraction and DFT studies of oxygen defect and transport in higher-order Ruddlesden-Popper phase materials, RSC Advances: an international journal to further the chemical sciences, Vol: 13, Pages: 13786-13797, ISSN: 2046-2069

A series of higher-order Ruddlesden–Popper phase materials – La3PrNi3O10−δ, La2Pr2Ni3O10−δ and LaPr3Ni3O10−δ – were synthesised and investigated by neutron powder diffraction to understand the oxygen defect structure and propose possible pathways for oxygen transport in these materials. Further complimentary DFT calculations of the materials were performed to support the experimental analysis. All of the phases were hypostoichiometric and it was observed that the majority of the oxygen vacancies were confined to the perovskite layers, with a preference for equatorial oxygen sites. A particular preference for vacancies in O(1) and O(5) sites at high temperatures was observed from neutron diffraction measurements which were further complimented by DFT calculations wherein the vacancy formation energy was found to be lowest at the O(1) site. Also, a preference for a curved oxygen transport pathway around the NiO6 octahedra was observed which agrees with the published literature for Ruddlesden–Popper phase materials. Lattice parameters for all three compositions showed a linear increase with increasing temperature, but the increase was greatest in the c parameter while the b parameter showed only a slight increase when compared to the a parameter. The thermal expansion coefficient was calculated for all compositions and was found to be in the range 13.0–13.4 × 10−6 °C−1, which is compatible with the commonly used electrolyte materials for solid oxide fuel cells.

Journal article

Williams NJ, Osborne C, Seymour I, Bazant MZ, Skinner Set al., 2023, Application of finite Gaussian process distribution of relaxation times on Sofc electrodes, Electrochemistry Communications, Vol: 149, Pages: 1-6, ISSN: 1388-2481

Electrochemical impedance spectroscopy (EIS) is a powerful tool in characterisation of processes in electrochemical systems, allowing us to elucidate the resistance and characteristic frequency of physical properties suchas reaction and transport rates. The essence of EIS is the relationship between current and potential at a givenfrequency. However, it is often the case that we do not understand the electrochemical system well enough to fita meaningful physical model to EIS data. The distribution of relaxation times (DRT) calculation assumes aninfinite series of relaxation processes distributed over a characteristic timescale. The DRT calculation mayidentify the number of processes occurring, as well as their respective resistivity and characteristic timescale, andmay resolve processes which have relatively similar timescales. Using a nonparametric tool known as Gaussianprocess (GP) regression, we showcase a method of finding a unique solution to the ill-posed DRT problem byoptimising kernel hyperparameters as opposed to ad-hoc regularisation. In this work, we use finite GP regressionunder inequality constraints (fGP) to analysed EIS data generated by a (Ni/CGO|CGO|YSZ|Reference Cathode)solid-oxide fuel cell in a gas mixture of 0.5 bar H2/0.5 bar H2O and at a temperature of 600 ◦C. By varying thecurrent density, we can characterise the current-voltage relationship of the electrode and shed light on the reaction mechanism governing charge transfer at the solid-gas interface. Our findings also show that even atrelatively high current densities (±600 mA cm− 2) the electrode process is limited by charge transfer.

Journal article

Skinner S, Sha Z, Shen Z, Kilner J, Cali Eet al., 2023, Understanding surface chemical processes in perovskite oxide electrodes, Journal of Materials Chemistry A, Vol: 11, Pages: 5645-5659, ISSN: 2050-7488

The effect of operating conditions on the surface composition and evolution of (La0.8Sr0.2)0.95Cr0.5Fe0.5O3−δ (LSCrF8255) as a model perovskite oxide was investigated. LSCrF8255 pellets were annealed under dry oxygen (pO2 = 200 mbar), wet oxygen (pO2 = 200 mbar, pH2O = 30 mbar), and water vapour (pO2 < 1 mbar, pH2O = 30 mbar) environments to reflect the applications of perovskite materials as electrodes for oxygen reduction/evolution and H2O electrolysis in electrochemical energy conversion devices such as solid oxide fuel/electrolysis cells (SOFCs/SOECs) and oxygen transport membranes (OTMs). A series of comprehensive surface characterization techniques were applied, including low energy ion scattering spectroscopy (LEIS), X-ray photoelectron spectroscopy (XPS), secondary ion mass spectrometry (SIMS), scanning electron microscopy (SEM), scanning transmission electron microscopy (STEM), and energy-dispersive X-ray spectroscopy (EDX). Our comprehensive study showed that after annealing at 900 °C for 27 hours, a severe level of Sr surface segregation occurred on the sample annealed in both dry oxygen and water vapour but in different manners, whereas on the sample annealed in wet oxygen, Sr segregation was likely suppressed. In addition, the Sr segregation behaviour can be correlated to other mass transport phenomena, such as Cr evaporation and redeposition and Si deposition, as well as to crystal orientation and defects such as grain boundaries and dislocations. Apart from the Sr-enriched surface precipitates, phase separation was consistently observed on the samples annealed in all three conditions. The secondary phase was found to be B-site cation enriched (significantly Fe enriched, relatively Cr enriched) and A-site cation (La and Sr) deficient. Moreover, in contrast to the Sr enriched surface, a La enriched surface was observed on samples annealed in dry oxygen at 600 and 700 °C, which was found to be potentially caused by the Sr and Cr su

Journal article

Williams NJ, Quérel E, Seymour ID, Skinner SJ, Aguadero Aet al., 2023, Operando characterization and theoretical modeling of metal|electrolyte interphase growth kinetics in solid-state batteries. Part II: Modeling, Chemistry of Materials, Vol: 35, Pages: 863-869, ISSN: 0897-4756

Understanding the interfacial dynamics of batteries is crucial to control degradation and increase electrochemical performance and cycling life. If the chemical potential of a negative electrode material lies outside of the stability window of an electrolyte (either solid or liquid), a decomposition layer (interphase) will form at the interface. To better understand and control degradation at interfaces in batteries, theoretical models describing the rate of formation of these interphases are required. This study focuses on the growth kinetics of the interphase forming between solid electrolytes and metallic negative electrodes in solid-state batteries. More specifically, we demonstrate that the rate of interphase formation and metal plating during charge can be accurately described by adapting the theory of coupled ion-electron transfer (CIET). The model is validated by fitting experimental data presented in the first part of this study. The data was collected operando as a Na metal layer was plated on top of a NaSICON solid electrolyte (Na3.4Zr2Si2.4P0.6O12 or NZSP) inside an XPS chamber. This study highlights the depth of information which can be extracted from this single operando experiment and is widely applicable to other solid-state electrolyte systems.

Journal article

Sadia Y, Skinner S, 2023, Electrochemical processes in (La1-xSrx)2(Ni0.9Mn0.1)O4+δ based air electrodes for solid oxide cells, Solid State Ionics, Vol: 390, ISSN: 0167-2738

Solid oxide fuel cells have been one of the most promising alternatives to fossil-fuel based energy in the past few years. One of the most important challenges is increasing the performance of mixed ionic-electronic conducting cathodes at lower temperatures. One such cathode which has been studied lately is based on the Ruddlesden-Popper phase La2NiO4 with Sr and Mn replacements on the A and B sites respectively. Using electrochemical impedance spectroscopy and distribution of relaxation time analysis this paper attempts to separate the different processes in such cathodes. Three different processes were identified with a low, medium, and high frequency. The three processes were related to three different processes respectively: oxygen diffusion, oxygen surface exchange and charge transfer, and finally oxygen transfer through the electrode/electrolyte interface.

Journal article

Quérel E, Williams NJ, Seymour ID, Skinner SJ, Aguadero Aet al., 2023, Operando characterization and theoretical modeling of Metal|Electrolyte interphase growth kinetics in solid-state batteries. Part I: experiments, Chemistry of Materials, Vol: 35, Pages: 853-862, ISSN: 0897-4756

To harness all of the benefits of solid-state battery (SSB) architectures in terms of energy density, their negative electrode should be an alkali metal. However, the high chemical potential of alkali metals makes them prone to reduce most solid electrolytes (SE), resulting in a decomposition layer called an interphase at the metal|SE interface. Quantitative information about the interphase chemical composition and rate of formation is challenging to obtain because the reaction occurs at a buried interface. In this study, a thin layer of Na metal (Na0) is plated on the surface of an SE of the NaSICON family (Na3.4Zr2Si2.4P0.6O12 or NZSP) inside a commercial X-ray photoelectron spectroscopy (XPS) system while continuously analyzing the composition of the interphase operando. We identify the existence of a solid electrolyte interphase at the Na0|NZSP interface, and more importantly, we demonstrate for the first time that this protocol can be used to study the kinetics of interphase formation. A second important outcome of this article is that the surface chemistry of NZSP samples can be tuned to improve their stability against Na0. It is demonstrated by XPS and time-resolved electrochemical impedance spectroscopy (EIS) that a native NaxPOy layer present on the surface of as-sintered NZSP samples protects their surface against decomposition.

Journal article

Skinner S, Tsai C-Y, Mukerjee S, Leah R, Hjalmarrson Pet al., 2023, Electrolyte Materials for Solid Oxide Electrolysis Cells, High Temperature Electrolysis: From fundamrntals to applications, Editors: Sitte, Merkle, ISBN: 9780750339490

Book chapter

Shih P, Aguadero A, Skinner S, 2023, A-site acceptor-doping strategy to enhance oxygen transport in sodium bismuth titanate perovskite, Journal of the American Ceramic Society, Vol: 106, Pages: 100-108, ISSN: 0002-7820

Sodium–bismuth–titanate (NBT) has recently been shown to contain high levels of oxide ion conductivity. Here we report the effect of A-site monovalent ions, M+ = K+ and Li+, on the electrical conductivity of NBT. The partial replacement of Bi3+ with monovalent ions improved the ionic conductivity by over one order of magnitude without an apparent change of the conduction mechanism, which is attributed to an increase in the oxygen vacancy concentration based on an acceptor-doping approach. The 18O tracer-diffusion coefficient (D*) determined by the isotope exchange depth profile method in combination with secondary ion mass spectrometry confirmed that oxygen ions are the main charge carriers in the system. Among these acceptor-doped samples, 4% Li doping provides the highest total conductivity, leading to a further discussion of doping strategies for NBT-based materials to enhance the electrical behavior, is discussed. Comparisons with other oxide-ion conductors and an oxygen-vacancy diffusivity limit model in perovskite lattice suggested that the doped NBT-based materials might already have achieved the optimization of the ionic conductivity.

Journal article

Guo J, Cai R, Cali E, Wilson G, Kerherve G, Haigh S, Skinner Set al., 2022, Low-temperature exsolution of Ni-Ru bimetallic nanoparticles from A-site deficient double perovskites, Small, Vol: 18, ISSN: 1613-6810

Exsolution of stable metallic nanoparticles for use as efficient electrocatalysts has been of increasing interest for a range of energy technologies. Typically, exsolved nanoparticles show higher thermal and coarsening stability compared to conventionally deposited catalysts. Here, A-site deficient double perovskite oxides, La2-xNiRuO6-δ (x = 0.1 and 0.15), are designed and subjected to low-temperature reduction leading to exsolution. The reduced double perovskite materials are shown to exsolve nanoparticles of 2–6 nm diameter during the reduction in the low-temperature range of 350–450 °C. The nanoparticle sizes are found to increase after reduction at the higher temperature (450 °C), suggesting diffusion-limited particle growth. Interestingly, both nickel and ruthenium are co-exsolved during the reduction process. The formation of bimetallic nanoparticles at such low temperatures is rare. From the in situ impedance spectroscopy measurements of the double perovskite electrode layers, the onset of the exsolution process is found to be within the first few minutes of the reduction reaction. In addition, the area-specific resistance of the electrode layers is found to decrease by 90% from 291 to 29 Ω cm2, suggesting encouraging prospects for these low-temperature rapidly exsolved Ni/Ru alloy nanoparticles in a range of catalytic applications.

Journal article

Skinner S, Williams N, Seymour I, Fraggedakis Det al., 2022, Electric fields and charge separation for solid oxide fuel cell electrodes, Nano Letters: a journal dedicated to nanoscience and nanotechnology, Vol: 22, Pages: 7515-7521, ISSN: 1530-6984

Activation losses at solid oxide fuel cell (SOFC) electrodes have been widely attributed to charge transfer at the electrode surface. The electrostatic nature of electrode–gas interactions allows us to study these phenomena by simulating an electric field across the electrode–gas interface, where we are able to describe the activation overpotential using density functional theory (DFT). The electrostatic responses to the electric field are used to approximate the behavior of an electrode under electrical bias and have found a correlation with experimental data for three different reduction reactions at mixed ionic–electronic conducting (MIEC) electrode surfaces (H2O and CO2 on CeO2; O2 on LaFeO3). In this work, we demonstrate the importance of decoupled ion–electron transfer and charged adsorbates on the performance of electrodes under nonequilibrium conditions. Finally, our findings on MIEC–gas interactions have potential implications in the fields of energy storage and catalysis.

Journal article

High M, Patzschke C, Zheng L, Zeng D, Gavalda Diaz O, Ding N, Chien KHH, Zhang Z, Wilson G, Berenov A, Skinner S, Campbell K, Xiao R, Fennell PAUL, Song Qet al., 2022, Precursor engineering of hydrotalcite-derived redox sorbents for reversible and stable thermochemical oxygen storage, Nature Communications, Vol: 13, ISSN: 2041-1723

Chemical looping processes based on multiple-step reduction and oxidation of metal oxideshold great promise for a variety of energy applications, such as CO2 capture and conversion, gasseparation, energy storage, and redox catalytic processes. Copper-based mixed oxides are one of themost promising candidate materials with a high oxygen storage capacity. However, the structuraldeterioration and sintering at high temperatures is one key scientific challenge. Herein, we report aprecursor engineering approach to prepare durable copper-based redox sorbents for use inthermochemical looping processes for combustion and gas purification. Calcination of the CuMgAlhydrotalcite precursors formed mixed metal oxides consisting of CuO nanoparticles dispersed in the MgAl oxide support which inhibits the formation of copper aluminates during redox cycling. The copperbased redox sorbents demonstrated enhanced reaction rates, stable O2 storage capacity over 500 redoxcycles at 900 °C, and efficient gas purification over a broad temperature range. We expect that ourmaterials design strategy has broad implications on synthesis and engineering of mixed metal oxides fora range of thermochemical processes and redox catalytic applications.

Journal article

Yatoo MA, Skinner S, 2022, Direct Measurements of Hydrogen Exchange and Diffusion Kinetics at Elevated Temperatures in Proton-Conducting Solid Oxide Materials, ECS Meeting Abstracts, Vol: MA2022-01, Pages: 1743-1743

<jats:p> Storage of purified hydrogen is one of the central challenges in addressing climate change and reducing our reliance on fossil fuels for energy conversion and storage, and therefore there is a global surge in research and development concerning hydrogen purification and storage. In this regard, we are studying proton conduction in solid oxide materials at elevated temperatures for applications in hydrogen separation and compression membranes. Hydrogen compression is the most recommended method to store hydrogen for automotive applications as it allows an increase in the hydrogen volumetric energy density.</jats:p> <jats:p>Traditionally the protonic conductivity in these materials is measured by indirect methods. For example, conductivity measurements in mixed gas atmospheres, comparing for example dry N<jats:sub>2</jats:sub> with humidified N<jats:sub>2</jats:sub>, thereby allowing the contribution of protons to be evaluated. In this study, we for the first time report the evaluation of protonic conductivity in BaZr<jats:sub>1-x</jats:sub>Ce<jats:sub>x</jats:sub>Y<jats:sub>0.2</jats:sub>O<jats:sub>3−δ</jats:sub> (BZCY) and BaZr<jats:sub>0.1</jats:sub>Ce<jats:sub>0.7</jats:sub>Y<jats:sub>0.2–x</jats:sub>Yb<jats:sub>x</jats:sub>O<jats:sub>3–δ</jats:sub> (BZCYYb) by direct measurements afforded by the Isotope Exchange Depth Profiling (IEDP) technique with deuterium labelling. We also report the kinetics of H/D transport through the bulk materials and across metal-ceramic interfaces with particular interest in the behaviour of the interface between the key Pd/Pd alloy catalyst component and the hydrogen transporting oxide ceramic material. The transport and interface behaviour information will be of significance in designing hydrogen separation and compression membrane

Journal article

Wilson GE, Seymour I, Cavallaro A, Skinner S, Aguadero Aet al., 2022, Screening Ruddlesden-Popper (n=1) Oxide Materials for Thermochemical Water Splitting By Density Functional Theory, ECS Meeting Abstracts, Vol: MA2022-01, Pages: 1595-1595

<jats:p> Thermochemical redox reactors store concentrated solar power by thermally inducing an oxygen deficiency within a metal oxide structure. The metal oxide’s affinity for reoxidation allows it to facilitate the splitting of H<jats:sub>2</jats:sub>O or CO<jats:sub>2</jats:sub> to produce H<jats:sub>2</jats:sub> or CO for syngas formation.[1] Typically, high temperatures (&gt;1400 °C) are used to drive the reduction of the benchmark material, CeO<jats:sub>2</jats:sub>, [2] however there is motivation to lower this temperature and investigate new metal oxides capable of larger fuel productions. Perovskite materials have been thoroughly investigated due to their crystallographic stability and ability to accommodate relatively large oxygen deficiencies. Emery et al. [3] conducted a wide computational screening of this family of materials based on a few simple thermodynamic parameters originally proposed by Meredig and Wolverton. [4]</jats:p> <jats:p>Herein, we extend these previous studies and investigate the A<jats:sub>2</jats:sub>BO<jats:sub>4</jats:sub> Ruddlesden-Popper (RP) oxides family [5], layered perovskites, that have previously demonstrated fast redox kinetics and large oxygen storage as solid oxide fuel cell cathodes.[6] A combination of screening parameters based on charge neutrality, Goldschmidt tolerance and computed defect formation energy, identified 38 possible RP candidate materials. One of which - Ca<jats:sub>2</jats:sub>MnO<jats:sub>4-δ</jats:sub> – was taken forward to experimental testing due to its Earth abundant elements. The powder was synthesized via a modified Pechini method and thermal analysis experiments demonstrated thermally driven oxygen evolution from 800 to 1200 °C equating to a non-stoichiometry of δ=0.18. High-temperature X-ray diffraction alluded to the formation o

Journal article

Williams N, Seymour I, Leah R, Banerjee A, Mukerjee S, Skinner SJet al., 2022, Non-equilibrium thermodynamics of mixed Ionic-electronic conductive electrodes and their interfaces: a Ni/CGO Study, Journal of Materials Chemistry A, Vol: 10, Pages: 11121-11130, ISSN: 2050-7488

Non-equilibrium thermodynamics describe the current–voltage characteristics of electrochemical devices. For conventional electrode–electrolyte interfaces, the local activation overpotential is used to describe the electrostatic potential step between the two materials as a current is generated. However, the activation overpotential for the metal/mixed ionic-electronic conducting (MIEC) composite electrodes studied in this work originates at the MIEC–gas interface. Moreover, we have studied the effects of non-equilibrium on the electrostatic surface potential and evaluated its influence over electrode kinetics. By investigating two phase (2PB) and three phase boundary (3PB) reactions at the Ni/Ce1−xGdxO2−δ (Ni/CGO) electrode, we have demonstrated that the driving force for coupled ion-electron transfer is held at the CGO–gas interface for both reaction pathways. We also determined that the rate of coupled ion-electron transfer via the 3PB scales with the availability of free sites on the metallic surface, revealing a Sabatier-like relationship with regards to the selection of metallic phases. Finally, we demonstrated how the theory of the electrostatic surface potential can be applied to other systems outside of the well-studied H2/H2O electrode environment. These findings therefore provide an insight into the design of future electrode structures for a range of electrochemical devices.

Journal article

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