Solid State Ionics Research Articles

Electrochemical Colloquium and Beyond: Opportunities for Online Education in Electrochemistry

Electrochemistry represents an exceptionally multifaceted and hard-to-teach subject. First, it requires knowledge of very distinct fields, including chemical thermodynamics, surface science, quantum mechanics, solid state ionics, catalysis, photovoltaics, cellular biology, and others. As a result, no electrochemistry curriculum can cover all the material needed for comprehensive understanding of the subject, meaning that the course is often shaped by the teacher’s personal preferences and agenda (e.g., fundamental electrochemistry, energy storage, corrosion, bioelectrochemistry, etc.). Second, the very core of electrochemistry – electrical double layer – is heavily debated within the professional community and does not have a satisfactory and consensual description suitable for textbooks and teaching. Unlike the kinetics of molecular reactions in solutions or at gas-solid interfaces, the kinetics of electrochemical reactions with the double layer region, even in simplest cases, is almost impossible to predict from first principles. All this causes confusion among both students and teachers – how should we teach when the key concept cannot be described adequately? Third, the field of electrochemistry is evolving, older concepts are reconsidered, and the new ones are introduced. This makes it difficult for students to relate course material to what is going on in real research. Finally, electrochemistry is vital for industry, which makes it particularly tempting for teachers to introduce a substantial “engineering” component into the curriculum and turn it into “applied electrochemistry course” (covering energy storage and conversion, corrosion, electroplating, etc.). While being well-intended, this leads to oversimplification of electrochemistry and offers a general (and often confusing) overview of the field.In my talk, I will discuss the difficulties of teaching electrochemistry courses and how online platforms can offer possible solutions. Specifically, I will draw on my experience with the Electrochemical Online Colloquium, which aims to make fundamental science and essential knowledge freely accessible to the public. My argument is that a holistic understanding of electrochemistry cannot be achieved if one ignores knowledge gained in neighboring fields. At the same time, discussion of open questions and critical assessment of concepts by experts helps a student understand what is truly well-established in the field and what is not. I will also discuss future opportunities for online education in electrochemistry and how we can improve the quality of research in this field.

Electrochemical Activity and Stability of Pure Gadolinium Doped Ceria as Fuel Electrode in Solid Oxide Electrolysis Cells

The state-of-the-art fuel electrode materials for solid oxide electrolysis cells (SOECs) are Ni-cermets due to their high electro-catalytic activity, high electrical conductivity and the low price. However, one major concern that must be solved for a widespread commercialisation is their poor degradation behaviour during electrolysis operation caused by for example Ni migration, depletion and agglomeration in the fuel electrode. One strategy to improve the durability of SOECs is the usage of alternative fuel electrode materials. For instance, completely Ni-free fuel electrodes may solve the problems of morphological degradation during operation and further could enhance redox cycling stability. Within possible candidates, gadolinium doped ceria (GDC) is an interesting material due to good stability towards carbon deposition and the possibility to host electro catalysis in electrolysis reactions [1, 2]. Furthermore, GDC (Ce0.8Gd0.2O2-δ) shows electronic conducting (700 °C, 0.028 S·cm-1) and ionic conducting (700 °C, 0.47 S·cm-1) properties at low partial pressures of oxygen and high temperatures [3]. Hence, in this work we have prepared electrolyte supported single cells with GDC (Ce0.8Gd0.2O2- δ) as single phase fuel electrode for their use as SOEC. The single cells (GDC//8YSZ//GDC//LSCF) were characterized by electrochemical impedance spectroscopy (EIS) and current-voltage characteristics in steam electrolysis, co-electrolysis and CO2 electrolysis conditions exhibiting 70 % of the performance compared to similar produced Ni-GDC fuel electrode single cells at 900 °C [4]. Furthermore, the results include process analysis by EIS, long term stability experiments for steam electrolysis and CO2 electrolysis conditions of at least 1000 hours and post characterization of the degraded cells by EIS, SEM and EDX analysis.[1] Nakmura et al. Journal of the Electrochemical Society 155 (2008) B563.[2] Chueh et al. Nature Materials 11 (2012) 155-161.[3] Maricle et al. Solid State Ionics 52 (1992) 173-182.[4] Unachukwu et al. Journal of Power Sources 556 (2023) 232436. Figure 1

Electric field control of RKKY coupling through solid-state ionics

Placing a suitable spacer layer between two magnetic layers can lead to an interaction between the magnetic layers known as Ruderman–Kittel–Kasuya–Yosida (RKKY) coupling. Controlling RKKY coupling, particularly the ability to switch between ferromagnetic and antiferromagnetic coupling, would enable novel magnetic data storage devices. By combining solid-state Li ion battery technology with an out-of-plane magnetized Co/Pt-based stack coupled through a Ru interlayer, we investigate the effects of the insertion of Li ions on the magnetic properties of the stack. The RKKY coupling and its voltage dependence are measured as a function of the Ru interlayer thickness, along with the effects of repeated voltage cycling. The Li ions both change the amplitude of the RKKY coupling and its phase, leading to the ability to switch the RKKY coupling between ferromagnetic and antiferromagnetic with applied voltages.

Electrochemical Measurement of CoO Solubility Product and Co Solubility in Molten Lead-Bismuth Eutectic by Solid-State Ionic Devices

The use of liquid Pb-Bi eutectic alloy (LBE) as primary coolant and spallation target in Lead-cooled Fast Reactors (LFRs) and Accelerator Driven Systems (ADSs) presents a corrosion challenge, as structural materials can release impurities, such as Fe, Cr, Ni and Co, into the coolant. These corrosion product impurities can react with dissolved oxygen, resulting in the formation of solid oxide particles that can block the critical flow channels in the core or heat-exchanger, compromising the safety of the reactor. Of particular concern is the additional radiological risk posed by cobalt, which can undergo neutron capture and be transmuted into the γ-emitter Therefore, it is essential to accurate assess the cobalt oxide solubility product and the cobalt solubility limit. In this work, we characterized the Co-O-LBE system through an electrochemical method that involved the coulometric titration of oxygen. By analyzing the oxygen evolution during CoO formation in LBE while titrating oxygen, we derived the following CoO solubility product (K sp ) and Co solubility (SCo) thermal laws: log10Ksp=1.22−7386T[K]673−778Klog10SCowt%=0.64−2233T[K]673−778K

Mechanisms of photoluminescence in cerium oxide pellets with different types of defects

We systematically investigated the photoluminescence (PL) spectra of sintered pellets of cerium oxide, CeO2. The crystal defects were controlled by commonly used techniques in the field of solid-state ionics. We observed the transitions between the O-2p and Ce-4f energy levels in intrinsic CeO2. By doping with trivalent lanthanide ions, which results in oxygen vacancies by charge compensation, a new transition band appears. It involves electronic levels that are similar to those of Ce2O3 clusters, rather than to those of oxygen vacancies. In reduced cerium oxide CeO2-δ, where the number of Ce3+ ions is larger, electrons partially fill the energy levels of defects similar to Ce2O3 clusters, and thus an optical response derived from the Ce-4f–Ce-5d transition appears.

Lithium ion transport in water-containing Li(SCN) over a wide compositional range: From water doping to hydration

This work explores the Li+ cation transport mechanisms in the system Li(SCN) · x H2O, complementary to our previous studies on crystal structures and hydration properties of lithium thiocyanate Li(SCN). Anhydrous Li(SCN) is a Schottky defective material with lithium vacancies VLi' being the dominant mobile defect. Two water incorporation mechanisms are identified: (i) At very low concentrations H2O acts as a dopant and forms H2OSCN• defects, constituting a substitutive incorporation mechanism. Doping with water not only increases the concentration of VLi' in Li(SCN), but also the vacancy mobility, making H2O-doping a special case. A complete quantitative defect chemical model is derived, including enthalpies and entropies of defect formation, association, and migration. (ii) Higher water activities induce chemical and structural changes in Li(SCN) · x H2O, i.e. hydration by reactive incorporation occurs. Single-phases as well as composites are investigated for their Li+ ion transport mechanisms. In case of x ≤ 1, defect associates determine the conductivity resulting in exceptionally high activation enthalpies. Further hydration to 1 < x < 2 creates composites of solid mono- and dihydrates, in which the low-melting diyhdrate can form percolating pathways and increase the conductivity to values as high as 7 × 10−4 S/cm at room temperature. Further increase of x leads to complete liquefaction. The entire range of x in the Li(SCN) – H2O system was studied regarding ion transport properties, identifying the individual mechanisms and temperature dependences. The results of this work deepen the understanding of hydration effects on solid ion conductors in solid state ionics, and help to elucidate transport phenomena in salt – solvent systems.

COMPOSITION MATERIALS IN THE CaF2–BaF2 SYSTEM

Composite nanomaterials (1 – x)CaF2–xBaF2 with 0.4 ≤ х ≤ 0.6 have been prepared by directional melt crystallization in a fluorinating atmosphere. The structural, optical, mechanical, and electrical characteristics of the synthesized nanocomposites are studied. Biphasic (1 – x)CaF2–xBaF2 composites have a lamellar fine microstructure. The lamella thickness decreases with an increase in the BaF2 content and reaches 30–50 nm for the composition with x = 0.5. The composites retain high transparency in the IR range (close to the values for the initial components). The microhardness (НV ≈ 5.0 GPa) and ionic conductivity (σdc = (1–3) × 103 S/cm at 823 K) of these composites exceed significantly the corresponding parameters of CaF2 and BaF2 crystals. The fluoride nanocomposites under investigation are promising materials for practical applications in photonics and solid-state ionics.

Centimeter level high surface roughness copper / silver nanoheterostructures for highly sensitive SERS detection

A novel method for centimeter level high surface roughness copper nanoparticles (CuNPs)/silver nanowires (AgNWs) heterostructures prepared by vacuum thermal evaporation method and solid-state ionics method under 6 ​μA direct current electric field (DCEF) using fast ionic conductor KAg4I5 was developed. The fractal dimension of the CuNPs/AgNWs heterostructures was calculated by grid-coverage method and the surface-enhanced Raman scattering (SERS) performance was detected by melamine aqueous solution as probe molecules. The results indicate that the macroscopic dendritic CuNPs/AgNWs heterostructures arranged neatly near cathode with the longest length of 6.3 ​cm were prepared, the diameters of CuNPs/AgNWs heterostructures ranged from 40 to 100 ​nm and many regularly arranged CuNPs with the diameter from 10 to 30 ​nm lie in the prepared nanoheterostructures, which lead to the CuNPs/AgNWs heterostructures have high surface roughness. The molar ratio of Ag:Cu is 11.87:1 and the fractal dimension of CuNPs/AgNWs heterostructures was 1.64 which means that there were more dentritic nanoheterostructures. The limiting concentration of melamine for the prepared high surface roughness CuNPs/AgNWs heterostructures SERS substrates is 10−16 ​mol/L, which is far below the strictest safety limits required of melamine in US and China. The EF for R6G/CuNPs/AgNWs heterostructures is estimated to be 4.82 ​× ​1012. These results indicate that the prepared CuNPs/AgNWs heterostructures with facile material synthesis, high surface roughness, low detection concentration and simple detection procedure has promising and strong application prospect for SERS detection in food safety.

From electrochromic phenomena to energy harvesting and storage—an overview of solid state ionics research at the Institute of Solid State Physics, University of Latvia

From electrochromic phenomena to energy harvesting and storage—an overview of solid state ionics research at the Institute of Solid State Physics, University of Latvia

Solid state ionics enabled ultra-sensitive detection of thermal trace with 0.001K resolution in deep sea

The deep sea remains the largest uncharted territory on Earth because it’s eternally dark under high pressure and the saltwater is corrosive and conductive. The harsh environment poses great difficulties for the durability of the sensing method and the device. Sea creatures like sharks adopt an elegant way to detect objects by the tiny temperature differences in the seawater medium using their extremely thermo-sensitive thermoelectric sensory organ on the nose. Inspired by shark noses, we designed and developed an elastic, self-healable and extremely sensitive thermal sensor which can identify a temperature difference as low as 0.01 K with a resolution of 0.001 K. The sensor can work reliably in seawater or under a pressure of 110 MPa without any encapsulation. Using the integrated temperature sensor arrays, we have constructed a model of an effective deep water mapping and detection device.

Solid oxide electrolysis cells – Interplay between operating conditions, fuel electrode overpotential and degradation

With the integration of increased share of energy from renewable, but fluctuating, energy sources there will be an increased demand for large-scale flexible conversion of electrical energy into easily storable chemical fuels. Electrolysis will be a key component in the future of energy conversion and storage solutions. Solid oxide cells (SOC) can provide the most efficient electrolysis technology. Furthermore, solid oxide electrolysis cell (SOEC) technology has improved tremendously in terms of initial performance, long-term durability, and up-scaling of cell and stack sizes; and with respect to commissioning and operation of SOEC plants. Nevertheless, it is evident that further improvement of electrochemical performance and especially long-term durability is crucial for the coming up-scaling and integrated SOEC systems as part of our energy grid. This calls for research within 1) thorough mapping of degradation mechanisms, 2) determination of thresholds for the onset of irreversible degradation processes, 3) safe operating conditions and means to mitigate known degradation processes, etc. However, degradation is not a simple singular property and there is often a puzzling interplay between parameters such as: the chosen electro-catalysts, the manufacturing of cell components, obtained microstructures, the resulting initial electrochemical performance, operating conditions, and the nature and extent of different degradation processes. This work will present11This work has been presented as invited keynote speak at symposium E01.07 as part of the 23rd International Conference on Solid State Ionics, MRS Meeting, Boston (US), July 2022. an overview of the puzzling interplay between initial electrochemical performance and long-term durability. It will then provide examples of multiple complementary characterization techniques applied for the analyses of these features and processes (e.g., SEM, TEM, Raman spectroscopy and electrochemical impedance spectroscopy). Furthermore, strategies to mitigate and/or limit specific degradation processes and insight into data required to establish degradation wise safe operating conditions are discussed. The present work will touch upon multiple performance characteristics and degradation processes: 1) triple-phase-boundaries (TPB) and critical micro/nano-structural parameters of the porous nickel/yttria-stabilized-zirconia (Ni/YSZ) electrode; 2) the perception of electrode overpotential and why this parameter is more important than the set current density or operating cell/stack voltage, 3) the effect of impurities, 4) the interplay between impurities and carbon deposition and 5) the degradation caused by Ni migration and actions to mitigate this degradation process.

Diameter-dependent ultrafast lithium-ion transport in carbon nanotubes.

Ion transport in solids is a key topic in solid-state ionics. It is critical but challenging to understand the relationship between material structures and ion transport. Nanochannels in crystals provide ion transport pathways, which are responsible for the fast ion transport in fast lithium (Li)-ion conductors. The controlled synthesis of carbon nanotubes (CNTs) provides a promising approach to artificially regulating nanochannels. Herein, the CNTs with a diameter of 5.5 Å are predicted to exhibit an ultralow Li-ion diffusion barrier of about 10meV, much lower than those in routine solid electrolyte materials. Such a characteristic is attributed to the similar chemical environment of a Li ion during its diffusion based on atomic and electronic structure analyses. The concerted diffusion of Li ions ensures high ionic conductivities of CNTs. These results not only reveal the immense potential of CNTs for fast Li-ion transport but also provide a new understanding for rationally designing solid materials with high ionic conductivities.

Solid State Ionics 2011–2021: Trends and Prospects

Based on the analysis of publication activity, trends in the development of the main sections of solid state ionics have been formulated by using expertly curated abstract citation database of peer-reviewed scientific literature Scopus. Promising areas of research related to in situ and operando experiments, artificial intelligence (machine learning), and the design of new devices using superionic materials are indicated.

Solid State Ionics 2011–2021: Trends and Prospects

Solid State Ionics 2011–2021: Trends and Prospects

Advancement of Electrochemical Thermoelectric Conversion with Molecular Technology.

Thermocells are a thermoelectric conversion technology that utilizes the shift in an electrochemical equilibrium arising from a temperature difference. This technology has a long history; however, its low conversion efficiency impedes its practical usage. Recently, an increasing number of reports have shown drastic improvements in thermoelectric conversion efficiency, and thermocells could arguably represent an alternative to solid thermoelectric devices. In this Minireview, we regard thermocells as molecular systems consisting of successive molecular processes responding to a temperature change to achieve energy generation. Various molecular technologies have been applied to thermocells in recent years, and could stimulate diverse research fields, including supramolecular chemistry, physical chemistry, electrochemistry, and solid-state ionics. These research approaches will also provide novel methods for achieving a sustainable society in the future.

Microstructure and Electrochemical Performance of Rapidly Sintered LiCoO2 Cathodes

Lithium ion battery technology based upon liquid carbonate electrolytes and intercalation electrodes has achieved widespread commercial success. The batteries power mobile devices like cell phones, laptop computers and cordless power tools. They are used in hybrid and all electric vehicles to reduce greenhouse gas emissions and dependency on limited petrochemical resources. They have been demonstrated to stabilize electric grids at local and national levels under periods of high demand. Even with these remarkable achievements, improvements in energy density are sought in batteries for most applications. Lithium metal batteries are being pursued around the world to meet this need.Rapid, continuous, roll-to-roll sintering of alumina ceramic tapes was recently demonstrated at Corning Incorporated [1]. The process is based on the pioneering work of Ketcham et al [2]. The ceramic tape, because it is thin, is flexible and may be heated, sintered, and cooled back to room temperature in less than one hour without cracking. One application of this technology is for manufacture of thin, <30 µm, solid electrolyte separators. Another is to enable novel, cathode-supported battery architectures. To that end, rapid sintering has been extended to lithium cobaltite (LCO). Continuous sintering and winding of LCO is shown in Fig. 1. The LCO cathode because it is sintered and self-supporting may be used to reduce the proportions of inactive components to dramatically increase energy density. Both composite and dense cathodes can be envisioned.One possible way of creating a solid-state battery with LCO ribbon is to capitalize on the LiPON solid electrolyte originally developed at ORNL for thin-film micro-batteries (TFMB’s) [3, 4]. The materials in TFMB’s have proven track records and attractive performance attributes. The LiPON solid electrolyte in contact with a lithium metal anode is resistant to dendrite formation even at current densities >5 mA/cm²; a claim that cannot be made for lithium garnet and lithium phosphosulfide glass [5, 6].TFMB’s also have tremendous cycling longevity and retain more than 90% of capacity after over 104 charge-discharge cycles [7]. The challenge with the technology is the high cost associated with growth of the cathode by sputtering processes; thicknesses are limited to ~2-3 µm. As a consequence, TFMB’s are a good fit for applications where small size power sources are needed like smart cards, medical implants, RFID tags, and wireless sensing. A thicker, rapidly sintered cathode could address the cost-capacity limitation. The cathode for this architecture could be made in a dense, closed pore form or with 15-25% porosity to host and electrolyte. The later would be expected to offer greater rate performance due to shorter diffusion distances and greater surface area for charge transfer. Energy density of more than 1,500 Wh/L is theoretically achievable.The performance of cathode materials like LCO depends upon synthesis conditions, process history, design and microstructure. We explore the electrochemical attributes of free standing LCO cathodes with thickness of 15 to 80 µm and porosity ranging from closed (dense) to ~25%. Cathodes were tested in 2032 coin cells using Whatman glass fiber as the separator and 1 M LiPF6 in 1:1 EC:DMC. Capacity of the cathodes regardless of microstructure is near the theoretical value. Lithium diffusivity of dense polycrystalline cathodes was characterized by galvanostatic intermittent titration and averages 1.13×10-9 cm²/s over the potential window of 3.0 to 4.3 V. Surface XRD shows the grain texture to be nominally random. Approximately 50% capacity was available at C/2 at a 23 µm thickness. Resistance of porous cathodes decreased with perimeter-to-surface area, and effective resistances below 10 Ωcm² were observed. Cycling endurance of porous sintered cathodes with 2.4 mAh/cm² capacity was evaluated with lithium titanate working electrodes. More than 90% capacity was retained after 1000 cycles between 3.0 and 4.3 V vs Li at 3C rate.[1] C. Kim, et al, “5G mm Wave Patch Antenna on Multi-layered Alumina Ribbon Ceramic Substrates,” IEEE International Symposium on Antennas and Propagation and North American Radio Science Meeting, July 2020, pp. 65-66.[2] T.D. Ketcham, et al., US 5,089,455, Feb 18, 1992.[3] J.B. Bates, et al., Solid State Ionics, 53-56 (1992) 647-654.[4] J.B. Bates, et al., U.S. 5,561,004, Oct 10, 1996.[5] F.D. Han, et al., Nat. Energy, (2019) 1-10.[6] J.C. Li, et al., Adv. Energy. Mater., 5 (2015) 1401408.[7] J. Xie, et al., Solid State Ionics, 178 (2008) 362-370. Figure 1

An Investigation of Tin Electroless Deposition

Electroless metal/alloy deposition can be an efficient process in certain areas of microelectronic fabrication. In fact, it is often easier to obtain coatings of uniform thickness and composition using electroless deposition than with electrodeposition, since one does not have the current density uniformity problem of the latter. For example, we were able to develop a Ni(P) process as a replacement for the final aluminum interconnect level, significantly decreasing wafer processing cycle time, by selectively depositing a Ni(P) capping layer on the Cu bitline wiring level. In STTM MRAM, we successfully employed this Ni(P) capping process to enable the evaluation of memory state retention via functional testing in an air atmosphere at elevated temperatures (1).However, there is a need to explore materials + deposition methods for rapidly developing fields, such as Quantum computing, e.g., materials with superconducting properties. Conventional electroless metal deposition, utilizing a separate reducing agent, can deposit materials with unique and useful properties, such as phosphorus-containing alloys in the case of hypophosphite reducing agents. This talk discusses work we have carried out on electroless tin deposition, including aspects of electroless solution preparation and stability, copper substrate surface preparation and catalyzation, and the mechanisms of electroless deposition and solution decomposition.Electroless processes can deposit a limited number of materials, especially pure metals. This is in part due to conventional electroless processes requiring catalytically active surfaces both to initiate the deposition reaction and to sustain it, the heterogeneous oxidation of the reducing agent being a kinetically hindered process, often with multiple reaction pathways (2). Though not possessing good catalytic activity due to its Periodic Table related, p -block element status, pure Sn, an environmentally robust, superconducting metal, can be electrolessly deposited through a disproportionation reaction involving stannous ions (3) in an alkaline aqueous medium.We achieved electroless Sn deposition rates of up to 8 – 9 μm/hr for tartrate-citrate complexed electroless Sn solutions in the temperature range 80 – 85 ⁰C with sodium and potassium hydroxides to adjust alkalinity. We found that either in-house formulated, or commercially available, immersion Sn solutions deposited a uniform Sn catalyst layer (≤ 0.5 μm) to initiate the electroless Sn deposition reaction on copper; however, improperly formulated immersion Sn solutions rapidly developed precipitates due to tin ion hydrolysis.The biggest technical challenge was minimizing unwanted electroless deposition of tin in bulk solution, i.e., deposition not associated with the catalytically active substrate surface. Tin oxide (SnO) is known to be metastable at ambient conditions and to decompose at temperatures above 300 ⁰C with “noticeable rate” into Sn and SnO2 (4). Thus, removal of filterable hydrolysis products of Sn(II) following solution preparation was important, but not always sufficient, for obtaining solutions that were viable for several days of use. The reasons for, and mechanisms of, electroless Sn solution decomposition do not appear to have been adequately addressed in the literature.We will show SIMS analysis of both immersion and electroless Sn layers along with synchrotron X-Ray analysis results of immersion Sn catalyst films on Cu to determine the extent of Sn-Cu intermetallic formation following their formation. We will discuss the current understanding of the mechanism of electroless Sn deposition including that of concomitant H2 gas evolution. We will conclude with contrasting the Ni(P) and Sn electroless processes in terms of ease of operation and reliability for routine processing. † Present address: Solvay, 1937 West Main Street, Stamford 06902, CT. ‡ Quantum intern at the IBM TJ Watson Research Center, Summer 2019, 2020 and 2021.[1]. E. J. O'Sullivan, C. Camagong et al., 2019 Meet. Abstr. MA2019-02 916; https://doi.org/10.1149/MA2019-02/15/916.[2]. E. J. O'Sullivan, Ch 5, Advances in Electrochemical Science and Engineering, Volume 7, https://doi.org/10.1002/3527600264.ch5.[3]. A. Molenaar and J. W. G. de Bakker, 1989, J. Electrochem. Soc. 136, 378 and refs therein; H. Koyano, M. Kato, and M. Uchida, 1991, Plating and Surface Finishing, 78, 68-74 and refs therein.[4]. H. Giefers et al, 2005, Solid State Ionics, 176, 199-207; https://doi.org/10.1016/j.ssi.2004.06.006. Acknowledgements The authors gratefully acknowledge the efforts of the staff of the Microelectronics Research Laboratory (MRL) at the IBM T. J. Watson Research Center, where some of the fabrication work described in this talk was carried out.

(Invited) Material Porosity

We really like pores in our research group. Big pores, small pores, ordered pores, random pores – they all have a function and as is often found, show behaviour that is not always predicted. I started my research journey trying to put extremely thin films onto near-perfect III-V crystals to control (opto)electronic properties and when the first TEM on our campus showed the image in Fig. 1 almost 21 years ago (1,2), the electrochemical modification of the InP made more sense. In this talk, I will summarise the journey from porous InP that led to studies of other porous semiconductors such as silicon (3-10) and GaN (11-15), periodically ordered photonic crystal porous structures (16-26) and some optical, thermal and electrochemical properties photocatalysis, batteries and related that were modified by the porous structure, leading up to the most recent porous materials (27). References C. O’Dwyer, D. N. Buckley, D. Sutton, M. Serantoni and S. B. Newcomb, J. Electrochem. Soc., 154, H78 (2007).C. O’Dwyer, D. N. Buckley, D. Sutton and S. B. Newcomb, J. Electrochem. Soc., 153, G1039 (2006).C. O'Dwyer, W. McSweeney and G. Collins, ECS J. Solid State Sci. Technol., 5, R3059 (2016).W. McSweeney, C. Glynn, H. Geaney, G. Collins, J. D. Holmes and C. O'Dwyer, Semicond. Sci. Technol., 31, 014003 (2016).W. McSweeney, H. Geaney and C. O'Dwyer, Nano Res., 8, 1395 (2015).W. McSweeney, H. Geaney, C. Glynn, D. McNulty and C. O'Dwyer, ECS Trans., 66, 39 (2015).W. McSweeney, O. Lotty, C. Glynn, H. Geaney, J. D. Holmes and C. O'Dwyer, Electrochim. Acta, 135, 356 (2014).W. McSweeney, O. Lotty, N. V. V. Mogili, C. Glynn, H. Geaney, D. Tanner, J. D. Holmes and C. O'Dwyer, J. Appl. Phys., 114, 034309 (2013).E. Quiroga-González, J. Carstensen, C. Glynn, C. O’Dwyer and H. Föll, Phys. Chem. Chem. Phys., 16, 255 (2014).C. Glynn, K.-M. Jones, V. Mogili, W. McSweeney and C. O'Dwyer, ECS J. Solid State Sci. Technol., 6, N3029 (2017).O. V. Bilousov, J. J. Carvajal, A. Vilalta-Clemente, P. Ruterana, F. Díaz, M. Aguiló and C. O’Dwyer, Chem. Mater., 26, 1243−1249 (2014).O. V. Bilousov, J. J. Carvajal, H. Geaney, V. Z. Zubialevich, P. J. Parbrook, O. Martínez, J. Jiménez, F. Díaz, M. Aguiló and C. O’Dwyer, ACS Appl. Mater. Interface, 6, 17954 (2014).O. V. Bilousov, J. J. Carvajal, H. Geaney, F. Díaz, M. Aguiló and C. O’Dwyer, CrystEngComm, 16, 10255 (2014).O. V. Bilousov, H. Geaney, J. J. Carvajal, V. Z. Zubialevich, P. J. Parbrook, A. Giguère, D. Drouin, F. Díaz, M. Aguiló and C. O’Dwyer, Appl. Phys. Lett., 103, 112103 (2013).O. V. Bilousov, J. J. Carvajal, D. Drouin, X. Mateos, F. Díaz, M. Aguiló and C. O'Dwyer, ACS Appl. Mater. Interfaces, 4, 6927 (2012).S. O'Hanon, D. McNulty, R. Tian, J. Coleman and C. O'Dwyer, J. Electrochem. Soc., 167, 140532 (2020).D. McNulty, H. Geaney, Q. Ramasse and C. O'Dwyer, Adv. Funct. Mater., 30, 2005073 (2020).A. Lonergan, C. Hu and C. O'Dwyer, Phys. Rev. Materials, 4, 065201 (2020).G. Collins, A. Lonergan, D. McNulty, C. Glynn, D. Buckley, C. Hu and C. O’Dwyer, Adv. Mater. Interfaces, 7, 1901805 (2020).D. McNulty, A. Lonergan, S. O'Hanlon and C. O'Dwyer, Solid State Ionics, 314, 195 (2018).D. McNulty, H. Geaney, D. Buckley and C. O'Dwyer, Nano Energy, 43, 11 (2018).A. Lonergan, D. McNulty and C. O'Dwyer, J. Appl. Phys., 124, 095106 (2018).S. O'Hanlon, D. McNulty and C. O'Dwyer, J. Electrochem. Soc., 164, D111 (2017).D. McNulty, E. Carroll and C. O'Dwyer, Adv. Energy Mater., 7, 1602291 (2017).E. Armstrong and C. O'Dwyer, J. Mater. Chem. C, 3, 6109 (2015).E. Armstrong, D. McNulty, H. Geaney and C. O’Dwyer, ACS Appl. Mater. Interfaces, 7, 27006 (2015).A. Lonergan, B. Murphy and C. O'Dwyer, ECS J. Solid State Sci. Technol., 10, 085001 (2021). Figure 1. Bright field TEM of a cross section of an InP electrode after a potential sweep from 0.0 to 0.44 V (SCE) in 5 mol dm-3 KOH at 2.5 mV s-1. The plane of the micrograph is (011). Figure 1

On-Board Strontium Getters for Improved Solid Oxide Cell Durability

Nano-composite electrodes fabricated by introducing nano-sized mixed ionic and electronic conducting (MIEC) catalysts into partially-sintered micro-sized ionic conducting backbones have been used by many researchers to improve the electrochemical performance, and/or reduce the operating temperature, of Solid Oxide Cells (SOCs).1 Unfortunately, these infiltrated electrodes have often possessed poor electrochemical performance stability.2 This presentation will review recent results showing how pre-infiltrated ceria nano-particles3 or atomic layer deposited zirconia overcoats4 can be used as on-board Sr getters to increase the long-term stability of La0.6Sr0.4Co0.8Fe0.2O3-x (LSCF)-infiltrated Ce0.9Gd0.1O1.95-x (GDC) SOC electrodes, without compromising initial performance. This talk will also present results showing that when combined, these surface modification techniques result in LSCF-GDC performance degradation rates that are no better than those obtained from each technique used separately. Lastly, this talk will present GDC pre-infiltration results supporting a newly-emerging consensus that Sr does not surface segregate in Sm0.5Sr0.5CoO3-x (SSC) like it does in LSCF. References D. Ding, X. X. Li, S. Y. Lai, K. Gerdes and M. L. Liu, Energy Environ. Sci., 7, 552 (2014).M. Shah, P. W. Voorhees and S. A. Barnett, Solid State Ionics, 187, 64 (2011).Y. Zhang and J. D. Nicholas, Journal of the Electrochemical Society, 168, 024522 (2021).Y. Zhang, Y. Wen, K. Huang and J. D. Nicholas, ACS Applied Energy Materials, 3, 4057 (2020).

Polarization Resistance of Ceria-Containing Fuel Electrodes in Solid Oxide Cells Studied By Impedance and DRT Analysis

Introduction Solid oxide electrolysis cell (SOEC), which enables steam electrolysis by reversely operating solid oxide fuel cell (SOFC), is capable of highly efficient hydrogen production. Whilst SOFC and SOEC are using similar materials and cell structures, electrode reactions in SOEC operation have to be studied in more details. In our previous study [1], Ni-ScSZ cermet, widely used as SOFC fuel electrodes, exhibited a tendency where electrode resistance increases at a low absolute current density in SOEC operation, while Ni-GDC co-impregnated cells fabricated by the impregnation method exhibited sufficient performance in a wide current density range. Here in this study, various model fuel electrode materials were evaluated and compared in a wide current density range in the SOFC and SOEC modes under the same measurement conditions. Electrode reactions are evaluated through measurements of electrochemical impedance spectra (EIS) and subsequent analysis of distribution of relaxation times (DRT). Experimental Three types of cells with different fuel electrodes were prepared: Ni-ScSZ cermet fuel electrode; Ni-GDC cermet fuel electrode; and Ni-GDC co-impregnated fuel electrode. The Ni-GDC cermet fuel electrodes were fabricated using gadolinia-doped ceria (GDC), widely known as a mixed ionic electronic conductor, besides a pure ionic conductor ScSZ. The Ni-GDC co-impregnated cell was fabricated by preparing a composite electrode backbone of GDC and La-doped SrTiO3 (LST), followed by impregnation of the catalyst particles, Ni and GDC. La-Sr-Ti oxides are known to exhibit sufficient stability in both oxidizing and reducing atmospheres [2, 3]. Moreover, co-impregnated fuel electrodes have shown high tolerance against highly-humidified hydrogen fuel and redox durability [4].Electrode resistances were separated into the ohmic resistance and the nonohmic polarization resistance through electrochemical impedance spectroscopy. Impedance measurements were made by using an impedance analyzer (1255WB, Solartron, UK) in a frequency range between 0.1 Hz and 1 MHz. The fuel, 50%-humidified H2, was supplied to the fuel electrode. The operating temperature was 800 °C. To characterize the electrode processes in both SOFC and SOEC modes, impedance measurements were performed by applying stepwise changes in current density up to ±1.2 A cm-2, for every 0.2 A cm-2. The DRT analysis of the EIS data was performed using a numerical calculation program based on Tikhonov regularization (Z-Assist, Toyo Corporation, Japan) to separate electrochemical processes involved in the electrode reactions. Results and discussion The polarization resistances obtained by the EIS measurements are shown in Fig. 1. The Ni-ScSZ cermet fuel electrode (Fig. 1 (a)) exhibited unique polarization resistance in the SOEC mode. With increasing the absolute value of current density in the SOEC mode, polarization resistance increased significantly at first, reaching a maximum value at -0.4 A cm-2, then decreased. In contrast, for the Ni-GDC cermet fuel electrode (Fig. 1 (b)), polarization resistance increased only slightly with increasing current density in the SOEC mode, and overall polarization resistance remained low. This tendency was also seen for the Ni-GDC co-impregnated fuel electrode (Fig. 1 (c)), where such increase in polarization resistance was further suppressed.Such increase and decrease in polarization resistance observed for the Ni-ScSZ cermet have to be explained by different mechanisms (at least two opposite factors). One possible explanation could be that the reduced and recovered catalytic activity due to e.g. oxidation and reduction of the Ni catalyst surface. The presence of GDC in the fuel electrode has a positive effect reducing the polarization resistance in the SOEC mode. The electrode reaction area may be extended beyond the Ni surface and/or the triple phase boundaries. Furthermore, the Ce ion in ceria undergoes a valence change, which may promote flexible surface exchange reactions, and surface reactions on the Ni catalysts. For the SOEC operation, it will be desirable to clarify the effects of GDC in the r-SOC electrode reactions. Acknowledgments A part of this study was supported by “Research and Development Program for Promoting Innovative Clean Energy Technologies Through International Collaboration” of the New Energy and Industrial Technology Development Organization (NEDO) (Contract No.20001460-0). Collaborative support by Prof. H. L. Tuller, Prof. B. Yildiz, and Prof. J. L. M. Rupp at Massachusetts Institute of Technology (MIT) is gratefully acknowledged. References N. Endo, T. Fukumoto, R. Ushijima, K. Natsukoshi, Y. Tachikawa, J. Matsuda, S. Taniguchi, and K. Sasaki, ECS Trans., 103(1), 1981 (2021).Q. Ma and F. Tietz, Solid State Ionics, 225, 108 (2012).G. Chen, H. Kishimoto, K. Yamaji, K. Kuramoto, and T. Horita, J. Electrochem. Soc., 162(3), F223 (2014).S. Futamura, A. Muramoto, Y. Tachikawa, J. Matsuda, S. M. Lyth, Y, Shiratori, S. Taniguchi, and K. Sasaki, Int. J. Hydrogen Energy, 44(16), 8502 (2019). Figure 1