Protonic Ceramic Research Articles

Solid-Solid Interfaces in Protonic Ceramic Devices: A Critical Review.

The literature concerning protonic ceramic devices is critically reviewed focusing the reader’s attention on the structure, composition, and phenomena taking place at solid–solid interfaces. These interfaces play a crucial role in the overall device performance, and the relevance of understanding the phenomena taking place at the interfaces for the further improvement of electrochemical protonic ceramic devices is therefore stressed. The grain boundaries and heterostructures in electrolytic membranes, the electrode–electrolyte contacts, and the interfaces within composite anode and cathode materials are all considered, with specific concern to advanced techniques of characterization and to computational modeling by ab initio approaches. An outlook about future developments and improvements highlights the necessity of a deeper insight into the advanced analysis of what happens at the solid–solid interfaces and of in situ/operando investigations that are presently sporadic in the literature on protonic ceramic devices.

Advanced Manufacturing of Intermediate-Temperature Protonic Ceramic Electrochemical Cells

Proton conducting oxide (i.e., protonic ceramic) has been thought of as an ideal solid electrolyte for energy conversion and storage applications since Iwahara et al. reported the perovskite-type protonic ceramics represented by doped barium/strontium cerates and zirconates in the 1980s. Proton, as the charge carrier in protonic ceramics, possesses a much lower transport activation energy than oxide-ion, which has rendered protonic ceramics for extensive intermediate-temperature (IT, 400-700°C) electrochemical devices such as protonic ceramic fuel cells (PCFCs), reversible protonic ceramic fuel cells, and protonic ceramic membrane reactors.

SrCe0.9In0.1O3-δ-based reversible symmetrical Protonic Ceramic Cell

In-doped SrCe0.9In0.1O3-δ (SCI) perovskite-type oxide is utilized as the solid electrolyte, as well as a component, together with SrFe0.75Mo0.25O3-δ (SFM) compound, in the composite-type electrodes to construct symmetrical Protonic Ceramic Fuel Cells (PCFC). With good mutual stability of SCI and SFM at high temperatures in water vapor-containing reducing and oxidizing conditions, as well as sufficient ionic conductivity with high proton transference number of SrCe0.9In0.1O3-δ, and high electronic conduction of SrFe0.75Mo0.25O3-δ, effectiveness of the electrochemical reactions is ensured. It is documented that 80:20 wt.% SFM-SCI electrode performs better, comparing to the one having 50:50 wt.% ratio of both compounds, with both electrodes showing lower polarization resistance values in the reducing conditions. Symmetrical SCI-SFM|SCI|SCI-SFM cells can be operated in normal (PCFC) and reversed, Protonic Ceramic Electrolyzer Cell (PCEC) mode, showing similar shape of current density-voltage characteristics. The maximum obtained power density is 32 mW cm−2 at 800 °C.

(Invited) Applications of Protonic Ceramics for Electrochemical Energy Conversion and Storage

Recent exciting advancements in proton-conducting ceramic materials and devices suggest that they are approaching a technology readiness level that rivals more well-established polymeric and oxygen-ion conducting counterparts. Because they can enable proton-mediated electrochemistry under both dry and wet environments at moderate temperatures, protonic ceramics provide unique opportunities to enhance or synergize a diverse range of complementary electrochemical and thermochemical processes. Because of this potential, significant efforts have been devoted to advancing numerous energy-related applications using these materials. Here, we will provide an overview of recent research efforts at the Colorado School of Mines focused on developing protonic ceramics for a number of applications, including: Hydrocarbon-tolerant protonic ceramic fuel cells for electricity generation (PCFCs)Protonic ceramic electrolyzers for fuel synthesis (PCECs)Reversible protonic-ceramic electrochemical cells for energy storage (RePCECs). Recent progress has lead to remarkably high-performance H2 and hydrocarbon-fueled PCFCs; exceptionally efficient (>97% LHV efficiency) PCECs for H2 production and for the co-conversion of steam and carbon dioxide to renewable methane; H2/H2O-based RePCECs with >75% round-trip efficiency (cell-level) for seasonal energy storage, and a reversible ammonia fuel cell for ammonia synthesis/power production. In addition, ongoing collaboration with industrial partners demonstrates promising scale-up and durability progress for protonic ceramic cells and stacks that underscores their potential for eventual commercial application.Acknowledgements: This work was supported by the Advanced Research Projects Agency-Energy (ARPA-E) through the REFUEL (Award No. DEAR0000808) and REBELS programs (Award No. DE-AR0000493) and the Office of Fossil Energy (Award No. DE-FE0031716). Additional support was provided by the Army Research Office under Grant No. W911NF-17-1-0051, the Office of Naval Research via Grant No. N00014-16-1-2780, and faculty research funding from Kansas State University.

Energy to Molecules and Materials (E2M2): Intermediate Temperature Electrosynthesis of Intermediates, Fuels and Chemicals Using Protonic Ceramic Electrochemical Cells (PCECs)

Energy and chemical industry showed increasing interest in energy to molecules and materials (E2M2) due to the widely available distributed feedstock and energy input (renewable and/or nuclear energy associated electricity and excess heat). These factors offer new opportunities to those technologies with smaller scale, modularized/distributed, and driven by an integrated system of renewable and other energy sources that can show promise for increased productivity with decreased energy, capital, and operating costs. Intermediate temperature electrosynthesis through solid oxide cells is a promising strategy because of the following advatanges: (1) it can control the product selectivity by changing the operating potentials and temperatures, (2) it can be operated at ambient pressure, (3) it has high production rate and low overpotential, and (4) it can be easily integrated with power and heat from nuclear energy and renewables. Despite of these promising features, this technology still faces many challenges that include but not limited to immature process, relatively low catalyst activity, and unproven operation durability. At Idaho National Laboratory, we have made significant achievements on electrosynthesis of intermediates, fuels and chemicals using natural gas, steam, carbon dioxide and nitrogen through protonic ceramic membrane reactors. The research outcomes have the potential to establish a globally competitive U.S. industry for converting the Nation’s abundant supply of feedstock into conversion-ready intermediates and/or high demand fuels and chemicals.

Electrode Degradation in Proton-Conducting Ceramic Electrochemical Devices

We present our efforts to identify, quantify, and mitigate sources of degradation in proton-conducting ceramic electrochemical devices. As described in another presentation, researchers at the Colorado School of Mines are currently scaling up proton-conducting ceramics into multi-cell stacks for both fuel-cell and electrolyzer applications. This integration of novel protonic-ceramic materials with stack packaging materials brings questions regarding materials compatibility and stack-level degradation. The lower operating temperature (500 – 600 ºC) of protonic-ceramic devices presents promise for reducing the driving forces behind several well-established degradation mechanisms found in more-prevalent oxygen-ion conducting ceramics. Nickel sintering in the anode support should decrease, as should chromia-scale growth and electrode poisoning from metallic interconnects. That said, other aspects of protonic ceramics could enhance degradation. For example, under fuel-cell operation, water vapor is formed at the cathode, increasing oxygen partial pressure and promoting chromia poisoning. Further, these degradation mechanisms are presented over thousands of hours of stack operation, often happening simultaneously, clouding our understanding.In this presentation, we describe our efforts to identify and mitigate the key degradation mechanisms found in proton-conducting ceramic electrochemical devices and stacks. Through collaboration with the University of Bordeaux, we have developed unique device packaging and experimentation to enable careful measurement of electrochemical performance at electrode-electrolyte interfaces over thousands of hours of operation. As shown in the figure, a ~ 2-mm-thick BaCe0.4Zr0.4Y0.1Yb0.1O3-δ (BCZYYb4411) electrolyte is sandwiched between two identical BaCo0.4Fe0.4Zr0.1Y0.1O3-δ (BCFZY) air electrodes to form an electrolyte-supported symmetric cell. Fuel electrodes can also be studied. The cell is packaged, perhaps with metallic interconnects, within carefully designed alumina fixturing that maintains consistent electrical connections throughout the long testing periods. The assembly is sealed within a quartz reactor and placed within a furnace, and enables exploration of a wide range of operating conditions, gas compositions, and temperatures.In this talk, we will present results focusing on degradation at the cathode-electrolyte interface. The effect of H2O concentration, and the role of metallic interconnects in cathode degradation will be presented. Figure 1

Highly Efficient and Durable Materials for Protonic Ceramic Electrochemical Cells Operated at 400~600 ○c

Protonic ceramic electrochemical cells (PCECs) is an emerging and attractive approach to store intermittent renewable energies which can be converted back to electricity in reversible operation, aiming to achieve the energy conversion and storage in an electrochemical manner. Economically competitive PCEC systems have distinct advantages over conventional oxygen-ion conducting counterpart (O-SOC), but further materials innovation is still needed to improve unit-cell performance and durability at reduced operating temperature (400~600○C), enhance system lifetime and reduce cost. To achieve efficient electrochemical hydrogen and power production with stable operation, highly robust and durable oxygen electrode is urgently desired to facilitate water oxidation and oxygen reduction reactions simultaneously, which are the critical technical barriers for both electrolysis and fuel cell operation at reduced temperatures. The recent efforts have been focused on developing new triple conducting electrode with perovskite perovskite structure for high-performance and stable PCECs. The systematic material characterizations are carried out to evaluate the feasibility of activity, stability and interface compatibility, such as Fourier-transform infrared spectroscopy, high-temperature X-ray diffraction, hydrogen permeation and first-principal computation. The catalytic activity towards electrochemical reactions is demonstrated in various electrochemical performance evaluations at 400~600○C. The long-term durability of the PCEC is examined to show the high resistance against transient current density change in reversible operation and high steam concentration.

Development of Protonic Ceramic Fuel Cells Using Yb-Doped BaZrO3 Electrolytes

Acceptor-doped barium zirconate was used as the electrolyte material because it has CO2 stability. Previous studies have shown that yttrium-doped barium zirconate (BZY) has the highest proton conductivity. However, cells using BZY crack in the conventional ceramic tape-casting and firing process used to fabricate laminated ceramic electronic devices. This is because the NiO used for the anode reacts with BZY to form BaY2NiO5.The reactivity between the acceptor-doped barium zirconate and NiO was examined at the 1500 °C firing temperature. Ytterbium-doped barium zirconate (BZYb) did not form the complex oxide with NiO. Using BZYb revealed no problems such as cell cracking. In the cell using BZYb, the maximum power density at 700 °C and 600 °C were 0.70 Wcm-2 and 0.49 Wcm-2, respectively. Electrochemical impedance spectroscopy of the cell showed that the ohmic resistance was 0.28 Wcm2, approximately three times larger than the resistance value calculated using the bulk conductivity of BZYb and electrolyte thickness. It seems to be due to the solid solution of Ni in the electrolyte. To improve performance, techniques to prevent Ni solid solution diffusion are necessary.

Electrochemical Analysis of Anode Supported PCFC with Pseudo Symmetrical Cell

Protonic Ceramic Fuel Cell (PCFC) is expected to be the next-generation fuel cell for its higher energy conversion efficiency and lower operation temperature. BaZrO3 based oxide are potential materials as PCFC electrolyte because they have a high chemical stability under CO2 condition and high proton conduction. Recently, we have developed PCFC using BaZr0.8Yb0.2O3-δ (BZYb) as the electrolyte. The proton conductivity of the BZYb pellet was 1.1×10-2 S/cm at 600℃ and did not form the complex oxide with NiO. The power density of the developed cell (Ni-BaZr0.8Yb0.2O3-δ| BaZr0.8Yb0.2O3-δ| La0.6Sr0.4CoO3-δ) achieved 0.49 W/cm2 at 600℃.To improve the performance of anode supported PCFC, anode supported pseudo symmetrical cell:Ni-BaZr0.8Yb0.2O3-δ| BaZr0.8Yb0.2O3-δ| Niwas manufactured and analyzed by impedance spectroscopy under the various conditions and temperatures.The thickness dependence of the ohmic resistance was measured for three types of electrolyte (thickness: 8 μm, 16 μm and 25 μm) under 48%H2-49%N2-3%H2O gas condition at 300-700℃. The conductivity was calculated from the relation between electrolyte thickness and the ohmic resistance, which was 0.35×10-2 S/cm at 600℃. The degradation of proton conductivity occurs due to Ni dissolution. The performance of anode supported PCFC might be improved further if the Ni dissolution to electrolyte can be inhibited. Figure 1

Infiltration of Metal Catalysts for NH3 Synthesis in Protonic Ceramic Cells

The electrochemical synthesis of green NH3 representes a promising alternative to the conventional Haber-Bosch process, which currently relies on H2 mainly sourced from fossil fuels. Additionally, it compensates for the thermodynamic limitations of the ammonia synthesis reaction via the utilization of high pressures of 150-300 bar, resulting in high energy consumption. The prospect of solid state ammonia synthesis (SSAS) in proton conducting electrolysis cells (PCEC) has been the subject of increased interest recently [1]. It features steam in place of H2 as a reactant and the electrochemical activation of N2 at the hydrogen electrode (cathode) through the use of a catalyst. Possible candidates are metallic Ru or Fe, analogous to the thermocatalytic Haber-Bosch process.The implementation of the catalyst in the electrochemical cell is critical to the performance of the system. In order to augment the kinetics, high amounts of catalyst near the electrode-electrolyte interface are desired. Therefore, the microstructure of the electrode is of primary interest, as the void fraction of the layer must be large enough to enable a well-distributed deposition of catalyst particles. This may be achieved via the use of a pore-former, e.g. graphite powder, in the wet-route elaboration of the hydrogen electrode (see Fig. 1). However, modifications must not decrease the mechanical properties of the layer.The effect of various parameters (sintering conditions, thickness of the electrode layer, effect of additives to enhance wettability) on the final electrode structure were investigated, as well as the influence of the infiltration protocol (evacuation time, number of infiltration steps, drying procedure). The findings along with preliminary electrochemical characterizations will be discussed.[1] V. Kyriakou, I. Garagounis, E. Vasileiou, A. Vourros, M. Stoukides, Progress in the Electrochemical Synthesis of Ammonia, Catalysis Today, 286 (2017) 2-13. Figure 1

Improved Mechanical Reliability for Protonic Ceramic Electrochemical Cell Using Alumina Dispersed Electrode Support

The protonic ceramic electrochemical cell (PCEC) is increasingly attractive in energy conversion technologies like high temperature steam electrolysis and protonic solid oxide fuel cell. The mechanical durability and robustness of single PCEC, which is critical for technology scale-up and commercialization, is intrinsically lower than the conventional YSZ based oxygen ion conducting cells. In this research, we aim to increase the mechanical reliability of BaZr0.1Ce0.7Y0.1Yb0.1O3−δ (BZCYYb) based PCEC by dispersing alumina into the NiO-BZCYYb electrode support. The influences of alumina dispersion on chemical stability, mechanical properties like bending strength (room and high temperature), residual stress, and electrical conductivity of the PCECs with (NiO–BZCYYb)1−x(Al2O3)x (x = 0.1–5 wt%) anode support are measured and analyzed in both oxidized and reduced conditions.

Deconvolution of Water-Splitting on the Triple-Conducting Ruddlesden-Popper-Phase Anode for Protonic Ceramic Electrolysis Cells.

Triple-conducting materials have been proved to improve the performance of popular protonic ceramic electrolysis cells. However, partially because of the complexity of the water-splitting reaction involving three charge carriers, that is, oxygen (O2-), proton (H+), and electron (e-), the triple-conducting reaction mechanism was not clear, and the reaction conducting pathways have seldom been addressed. In this study, the triple-conducting Ruddlesden-Popper phase Pr1.75Ba0.25NiO4+δ as an anode on the BaCe0.7Zr0.1Y0.1Yb0.1O3-δ electrolyte was fabricated and its electroresponses were characterized by electrochemical impedance spectroscopy with various atmospheres and temperatures. The impedance spectra are deconvoluted by means of the distribution of the relaxation time method. The surface exchange rate and chemical diffusivity of H+ and O2- are characterized by electrical conductivity relaxation. The physical locations of electrochemical processes are also identified by atomic layer deposition with a surface inhibitor. A microkinetics model is proposed toward conductivities, triple-conducting pathways, reactant dependency, surface exchange and bulk diffusion capabilities, and other relevant properties. Finally, the rate-limiting steps and suggestions for further improvement of electrode performance are presented.

Computational Thermal Studies for Transport Properties on Protonic Ceramic Fuel Cell

Computational Thermal Studies for Transport Properties on Protonic Ceramic Fuel Cell

The Role of Space Charge at Metal/Oxide Interfaces in Proton Ceramic Electrochemical Cells

The contact potential and the resulting charge accumulation or depletion at the electrode/electrolyte interface are investigated for Ni and Cu group metals in contact with the state-of-the-art prot...

A Durable Ruddlesden-Popper Cathode for Protonic Ceramic Fuel Cells.

Protonic ceramic fuel cells (PCFCs) have been proved as an efficient energy converter at intermediate temperatures. To accelerate the kinetics of the proton-involved oxygen reduction reaction (p-ORR), developing efficient and durable cathodes is of great importance for improving PCFCs. In this work, a new triple-layered Ruddlesden-Popper (R-P) structure oxide, Sr3 EuFe2.5 Co0.5 O10-δ (3-SEFC0.5 ), was developed as a potential single-phase cathode for PCFCs, showing high oxygen non-stoichiometry and desirable structural thermal stability. By employing this highly active and stable single-phase cathode, the PCFC demonstrated unprecedented low polarization resistances and exceptionally great peak power densities, which were approximately 0.030 Ω cm2 and 900 mW cm-2 measured at 700 °C, respectively. These findings not only manifest the effectiveness of optimal doping in improving the structural stability and electrocatalytic activity in the multi-layered perovskite family, but also highlight the great potential of using multi-layered R-P series oxides as highly active and durable catalysts for PCFCs.

Protonic Ceramic Electrochemical Cell for Efficient Separation of Hydrogen.

Advancement of a hydrogen economy requires establishment of a whole supply chain including hydrogen production, purification, storage, utilization, and recovery. Nevertheless, it remains challenging to selectively purify hydrogen out of H2-containing streams, especially at low concentrations. Herein, a novel protonic ceramic electrochemical cell is reported that can sustainably separate pure H2 out of H2-diluted streams over the temperature regime of 350-500 °C by mildly controlling the electric voltage. With the Faraday's efficiency above 96%, the measured H2 separation rate at 0.51 V and 500 °C is 3.3 mL cm-2 min-1 out of 10% H2 - 90% N2, or 2.4 mL cm-2 min-1 out of 10% H2 - 90% CH4 taken as an example of renewable hydrogen blended in the natural gas pipelines. Such high hydrogen separation capability at reduced temperatures is enabled by the nanoporous nickel catalysts and well-bonded electrochemical interfaces as produced from well-controlled in situ slow reduction of nickel oxides. These results demonstrate technical feasibility of onsite purification of hydrogen prior to their practical applications such as fuels for fuel cell electric vehicles.

Rapid Laser Reactive Sintering for Sustainable and Clean Preparation of Protonic Ceramics.

One of the essential challenges for energy conversion and storage devices based on protonic ceramics is that the high temperature (1600–1700 °C) and long-time firing (>10 h) are inevitably required for the fabrication, which makes the sustainable and clean manufacturing of protonic ceramic devices impractical. This study provided a new rapid laser reactive sintering (RLRS) method for the preparation of nine protonic ceramics [i.e., BaZr0.8Y0.2O3−δ (BZY20), BZY20 + 1 wt % NiO, BaCe0.7Zr0.1Y0.1Yb0.1O3−δ (BCZYYb), BCZYYb + 1 wt % NiO, 40 wt % BCZYYb + 60 wt % NiO, BaCe0.85Fe0.15O3−δ–BaCe0.15Fe0.85O3−δ (BCF), BaCo0.4Fe0.4Zr0.1Y0.1O3−δ (BCFZY0.1), BaCe0.6Zr0.3Y0.1O3−δ (BCZY63), and La0.7Sr0.3CrO3−δ (LSC)] with desired crystal structures and microstructures. Following this, the dual-layer half-cells, comprising the porous electrode and dense electrolyte, were prepared by the developed RLRS technique. After applying the BCFZY0.1 cathode, the protonic ceramic fuel cell (PCFC) single cells were prepared and tested initially. The derived conductivity of the RLRS electrolyte films showed comparable proton conductivity with the electrolyte prepared by conventional furnace sintering. The initial cost estimation based on electricity consumption during the sintering process for the fabrication of PCFC single cells showed that RLRS is more competitive than the conventional furnace sintering. This RLRS can be combined with the rapid additive manufacturing of ceramics for the sustainable and clean manufacturing of protonic ceramic energy devices and the processing of other ceramic devices.

A Novel Laser 3D Printing Method for the Advanced Manufacturing of Protonic Ceramics.

Protonic ceramics (PCs) with high proton conductivity at intermediate temperatures (300–600 °C) have attracted many applications in energy conversion and storage devices such as PC fuel/electrolysis cells, PC membrane reactors, hydrogen pump, hydrogen or water-permeable membranes, and gas sensors. One of the essential steps for fulfilling the practical utilization of these intermediate-temperature PC energy devices is the successful development of advanced manufacturing methods for cost-effectively and rapidly fabricating them with high energy density and efficiency in a customized demand. In this work, we developed a new laser 3D printing (L3DP) technique by integrating digital microextrusion-based 3D printing and precise and rapid laser processing (sintering, drying, cutting, and polishing), which showed the capability of manufacturing PCs with desired complex geometries, crystal structures, and microstructures. The L3DP method allowed the fabrication of PC parts such as pellets, cylinders, cones, films, straight/lobed tubes with sealed endings, microchannel membranes, and half cells for assembling PC energy devices. The preliminary measurement of the L3DP electrolyte film showed a high proton conductivity of ≈7 × 10−3 S/cm. This L3DP technique not only demonstrated the potential to bring the PCs into practical use but also made it possible for the rapid direct digital manufacturing of ceramic-based devices.

(Invited) Elevated Temperature Electrosynthesis of Fuels and Chemicals Using Carbon Dioxide and Nitrogen through Protonic Ceramic Electrochemical Cells (PCECs)

Currently, the main feedstock in chemical and energy industry still comes from nonrenewable petroleum that is being depleted rapidly for fuel and chemical synthesis, leading to significant energy consumption and carbon emissions. To address rising environmental concerns and energy challenges, it is highly desirable to develop green chemical synthesis technologies for a sustainable future. Elevated temperature electrosynthesis through solid oxide cells is a promising strategy because of the following advatanges: (1) it can control the product selectivity by changing the operating potentials and temperatures, (2) it can be operated at ambient pressure, (3) it has high production rate and low overpotential, and (4) it can be easily integrated with other renewable power sources. Despite of these promising features, this technology still faces many challenges. One of the major issues is the relatively low catalyst activity as well as insufficient catalyst stability and durability. Therefore, the development of active and stable electrocatalysts is the key to make high temperature electrosynthesis efficient and practical for industrial scale applications. At Idaho National Laboratory, we have made significant achievements on electrosynthesis for ammonia production and CO2 hydrogenation using protonic ceramic membrane reactors. The outcomes of these researches have the potential to contribute to the US DOE’s goals of growing the energy economy, improving energy efficiency, and innovating clean energy sources.

Manufacture and Electrochemical Test of Short Stacks Based on Protonic Ceramic Electrochemical Cell at Idaho National Laboratory

As the technology of protonic ceramic electrochemical cells (PCEC) emerge for efficient hydrogen production as well as chemical electrosynthesis at reduced operating temperatures (400-600°C), the development of PCEC stack becomes critical to evaluate the feasibility of incorporating current PCEC material system into stacks. Idaho National Laboratory (INL) aims to develop stack manufacturing and electrochemical measurement capability for further enhancing system durability and reducing the overall cost. In this work, the large-scale 5cm×5cm and 10cm×10cm square cells are manufactured by high-temperature roll-to-roll method combining optimized additive manufacturing capability developed in INL. The integration of cells, sealants, current collectors, interconnects and gas manifolds involves significant exploration work of demonstrating high electrolysis performances. Both fuel cell and electrolysis operations are carried out to evaluate the effect of stack components on the performance with goal of achieving >70% performance of button cells. The long-term stability is operated at realistic conditions to assess the stack integration quality for further optimization of sealant materials, cell geometry, interconnect design and operating conditions. This early-stage work can benefit the technical and economic appraise on these new emerging technologies for industry attention and market penetration.