The electronic structure of diatomic nickel oxide

The nature of the Ni-O bond is relevant to catalytic and environmental applications. The vibrational frequency and electronic structure of NiO were calculated using CASSCF, icMRCI+Q, CCSD(T), and DFT. CASSCF predicted a quintet state (5Σ-) ground state for the equilibrium bond distance with a state crossing at 1.65 Å, where the triplet (3Σ-) state becomes of lower energy. These states arise from the 3d8(3F)4s2 (3F) and 3d9(2D)4s1 (3D) configurations of Ni. The icMRCI+Q method predicts a triplet (3Σ-) ground state and does not predict a state crossing with the quintet. This state has significant ionic character with the 2pz of O bonding with the 4s/3dz2 of the Ni to form a σ bond. The NiO frequency at the icMRCI+Q level of 835.0 cm-1 is in excellent agreement with experiment; the value of re is 1.5992 Å at this computational level. CCSD(T) predicts ωe = 888.80 cm-1 when extrapolated to the complete basis set limit. Frequencies predicted using CCSD(T) deviate from experiment consistent with the calculations showing large multireference character. A wide array of density functionals were benchmarked. Of the 43 functionals tested, the ones that gave the best prediction of the frequency are ωB97XD, CAM-B3LYP, and τ-HCTH with respective values of 831.8, 838.3, and 837.4 cm-1 respectively. The bond dissociation energy (BDE) of NiO is predicted to be 352.4 kJ mol-1 at the Feller-Peterson-Dixon (FPD) level in good agreement with one of the experimental values. The calculated BDEs at the DFT level are sensitive to the choice of functional and atomic asymptote. Sixteen functionals predicted the BDE within 20 kJ mol-1 of the FPD value.

Ni-Substituted Sr2FeMoO6-δ as an Electrode Material for Symmetrical and Reversible Solid-Oxide Cells

This work develops a novel perovskite Sr2FeNi0.35Mo0.65O6-δ (SFN0.35M) simultaneously using as a fuel electrode and oxygen electrode in a reversible solid oxide cell (RSOC). SFN0.35M shows outstanding electrocatalytic activity for hydrogen oxidation, hydrogen evolution, oxygen reduction, and oxygen evolution. In situ exsolution and dissolution of Fe-Ni alloy nanoparticles in SFN0.35M is revealed. In a reducing atmosphere, SFN0.35M shows in situ exsolution of Fe-Ni alloy nanoparticles, and then the Fe-Ni alloy is reoxidized into SFN0.35M while converting into an oxidizing atmosphere. The polarization resistances of SFN0.35M electrode are 0.043 Ω cm2 in 20% O2-N2 and 0.064 Ω cm2 in H2 at 850 °C. Moreover, symmetric fuel cells using the SFN0.35M electrode achieves a maximum power density of 0.501 W cm-2 at 850 °C in H2 fuel, while the symmetric electrolysis cell has an electrolysis current density of 0.794 A cm-2 at 1.29 V in 90% H2O-10% H2 at 850 °C. It is the first time we demonstrate that the cell voltage of symmetrical cell at 0.5 A cm-2 in the fuel cell mode and -0.5 A cm-2 in the electrolysis cell mode can be fully recovered in 10 electrode alternating cycles and therefore demonstrate the possibility that SFN0.35M can be used in a fully symmetric RSOC stack with electrode alternating functions.

Performance and Degradation of Electrolyte-Supported Single Cell Composed of Mo-Au-Ni/GDC Fuel Electrode and LSCF Oxygen Electrode during High Temperature Steam Electrolysis

Ni-gadolinia-doped ceria (GDC) based electrode materials have drawn significant attention as an alternative fuel electrode for solid oxide cells (SOCs) owing to mixed ionic conductivity of GDC and high electronic and catalytic activity of Ni. Moreover, the catalytic activity and electrochemical performance of the Ni-GDC electrode can be further improved by dispersing small quantities of other metal additives, such as gold or molybdenum. Therefore, herein, we considered gold and molybdenum modified Ni-GDC electrodes and focused on the upscaling; hence, we prepared 5 × 5 cm2 electrolyte-supported single cells. Their electrochemical performance was investigated at different temperatures and fuel gas compositions. The long-term steam electrolysis test, up to 1700 h, was performed at 900 °C with −0.3 A·cm−2 current load. Lastly, post-test analyses of measured cells were carried out to investigate their degradation mechanisms. Sr-segregation and cobalt oxide formation towards the oxygen electrode side, and Ni-particle coarsening and depletion away from the electrolyte towards the fuel electrode side, were observed, and can be considered as a main reason for the degradation. Thus, modification of Ni/GDC with Au and Mo seems to significantly improve the electro-catalytic activity of the electrode; however, it does not significantly mitigate the Ni-migration phenomenon after prolonged operation.

Integrated Co-Electrolysis and Syngas Methanation for the Direct Production of Synthetic Natural Gas from CO2 and H2 O

The concept of an integrated power-to-gas (P2G) process was demonstrated for renewable energy storage by converting renewable electrical energy to synthetic fuels. Such a dynamically integrated process enables direct production of synthetic natural gas (SNG) from CO₂ and H₂ O. The produced SNG can be stored or directly injected into the existing natural gas network. To study process integration, operating parameters of the high-temperature solid oxide electrolysis cell (SOEC) producing syngas (H₂ +CO) mixtures through co-electrolysis and a fixed bed reactor for syngas methanation of such gas mixtures were first optimized individually. Reactor design, operating conditions, and enhanced SNG selectivity were the main targets of the study. SOEC experiments were performed on state-of-the-art button cells. Varying operating conditions (temperature, flow rate, gas mixture and current density) emphasized the capability of the system to produce tailor-made syngas mixtures for downstream methanation. Catalytic syngas methanation was performed using hydrotalcite-derived 20 %Ni-2 %Fe/(Mg,Al)O_x catalyst and commercial methanation catalyst (Ni/Al₂ O₃ ) as reference. Despite water in the feed mixture, SNG with high selectivity (≥90 %) was produced at 300 °C and atmospheric pressure. An adequate rate of syngas conversion was obtained with H₂ O contents up to 30 %, decreasing significantly for 50 % H₂ O in the feed. Compared to the commercial catalyst, 20 %Ni-2 %Fe/(Mg,Al)O_x enabled a higher rate of CO_x conversion.

Robust Anode-Supported Cells with Fast Oxygen Release Channels for Efficient and Stable CO2 Electrolysis at Ultrahigh Current Densities

High-temperature electrolysis using solid oxide electrolysis cells (SOECs) provides a promising way for the storage of renewable energy into chemical fuels. During the past, nickel-based cathode-supported thin-film electrolyte configuration was widely adopted. However, such cells suffer from the serious challenge of anode delamination at high electrolysis currents due to enormous gaseous oxygen formation at the anode-electrolyte interface with insufficient adhesion caused by low sintering temperatures for ensuring high anode porosity and cathode pulverization because of potential nickel redox reaction. Here, the authors propose, fabricate, and test asymmetric thick anode-supported SOECs with firm anode-electrolyte interface and graded anode gas diffusion channel for realizing efficient and stable electrolysis at ultrahigh currents. Such a specially structured anode allows the co-sintering of anode support and electrolyte at high temperatures to form strong interface adhesion while suppressing anode sintering. The mixed oxygen-ion and electron conducting anode with graded channel structure provides a fast oxygen release pathway, large anode surface for oxygen evolution reaction, and excellent support for depositing nanocatalysts, to further improve oxygen evolution activity. As a result, the as-prepared cells demonstrate both high performance, comparable or even higher than state-of-the-art cathode-supported SOECs, and outstanding stability at a record current density of 2.5 A cm^-2 .

Direct Solid Oxide Electrolysis of Carbon Dioxide: Analysis of Performance and Processes

Chemical industries rely heavily on fossil resources for the production of carbon-based chemicals. A possible transformation towards sustainability is the usage of carbon dioxide as a source of carbon. Carbon dioxide is activated for follow-up reactions by its conversion to carbon monoxide. This can be accomplished by electrochemical reduction in solid oxide cells. In this work, we investigate the process performance of the direct high-temperature CO2 electrolysis by current-voltage characteristics (iV) and Electrochemical Impedance Spectroscopy (EIS) experiments. Variations of the operation parameters temperature, load, fuel utilization, feed gas ratio and flow rate show the versatility of the procedure with maintaining high current densities of 0.75 up to 1.5 A·cm−2, therefore resulting in high conversion rates. The potential of the high-temperature carbon dioxide electrolysis as a suitable enabler for the activation of CO2 as a chemical feedstock is therefore appointed and shown.

A 4 × 4 cm2 Nanoengineered Solid Oxide Electrolysis Cell for Efficient and Durable Hydrogen Production

Despite various advantages of high-temperature solid oxide electrolysis cells (SOECs) over their low-temperature competitors, the insufficient long-term durability has prevented the commercialization of SOECs. Here, we address this challenge by employing two nanoengineered electrodes. The O₂ electrode consists of a La_0.6Sr_0.4CoO_3-δ (LSC) and Gd,Pr-co-doped CeO₂ (CGPO) nanocomposite coating deposited on a Gd-doped CeO₂ (CGO) scaffold, and the H₂ electrode comprises a Ni/yttria stabilized zirconia (YSZ) electrode modified with a nanogranular CGO coating. The resulting cell with an active area of 4 × 4 cm² exhibits a current density exceeding 1.2 A cm^-2 at 1.3 V and 750 °C for steam electrolysis while also offering excellent long-term durability at 1 A cm^-2 with a high steam-to-hydrogen conversion of ∼56%. We further unravel the degradation mechanism of the most commonly used Ni/YSZ electrode under these conditions and describe the mitigation of the discussed mechanism on our nanoengineered electrode. Our findings demonstrate the potential of designing robust SOECs by nanoengineering electrodes through infiltration and have significant implications for the practical integration of SOEC technology in the future sustainable energy system.

A review of high temperature co-electrolysis of H2O and CO2 to produce sustainable fuels using solid oxide electrolysis cells (SOECs): advanced materials and technology

High-temperature solid oxide electrolysis cells (SOECs) are advanced electrochemical energy storage and conversion devices with high conversion/energy efficiencies. They offer attractive high-temperature co-electrolysis routes that reduce extra CO₂ emissions, enable large-scale energy storage/conversion and facilitate the integration of renewable energies into the electric grid. Exciting new research has focused on CO₂ electrochemical activation/conversion through a co-electrolysis process based on the assumption that difficult C[double bond, length as m-dash]O double bonds can be activated effectively through this electrochemical method. Based on existing investigations, this paper puts forth a comprehensive overview of recent and past developments in co-electrolysis with SOECs for CO₂ conversion and utilization. Here, we discuss in detail the approaches of CO₂ conversion, the developmental history, the basic principles, the economic feasibility of CO₂/H₂O co-electrolysis, and the diverse range of fuel electrodes as well as oxygen electrode materials. SOEC performance measurements, characterization and simulations are classified and presented in this paper. SOEC cell and stack designs, fabrications and scale-ups are also summarized and described. In particular, insights into CO₂ electrochemical conversions, solid oxide cell material behaviors and degradation mechanisms are highlighted to obtain a better understanding of the high temperature electrolysis process in SOECs. Proposed research directions are also outlined to provide guidelines for future research.

Sustainable Conversion of Carbon Dioxide: An Integrated Review of Catalysis and Life Cycle Assessment

CO₂ conversion covers a wide range of possible application areas from fuels to bulk and commodity chemicals and even to specialty products with biological activity such as pharmaceuticals. In the present review, we discuss selected examples in these areas in a combined analysis of the state-of-the-art of synthetic methodologies and processes with their life cycle assessment. Thereby, we attempted to assess the potential to reduce the environmental footprint in these application fields relative to the current petrochemical value chain. This analysis and discussion differs significantly from a viewpoint on CO₂ utilization as a measure for global CO₂ mitigation. Whereas the latter focuses on reducing the end-of-pipe problem "CO₂ emissions" from todays' industries, the approach taken here tries to identify opportunities by exploiting a novel feedstock that avoids the utilization of fossil resource in transition toward more sustainable future production. Thus, the motivation to develop CO₂-based chemistry does not depend primarily on the absolute amount of CO₂ emissions that can be remediated by a single technology. Rather, CO₂-based chemistry is stimulated by the significance of the relative improvement in carbon balance and other critical factors defining the environmental impact of chemical production in all relevant sectors in accord with the principles of green chemistry.

Electrochemically Scavenging the Silica Impurities at the Ni-YSZ Triple Phase Boundary of Solid Oxide Cells

Silica impurity originated from the sealing or raw materials of the solid oxide cells (SOCs) accumulating at the Ni-YSZ triple phase boundaries (TPBs) is known as one major reason for electrode passivation. Here we report nanosilica precipitates inside Ni grains instead of blocking the TPBs when operating the SOCs at |i| ≥ 1.5 A cm(-2) for electrolysis of H2O/CO2. An electrochemical scavenging mechanism was proposed to explain this unique behavior: the removal of silica proceeded through the reduction of the silica to Si under strong cathodic polarization, followed by bulk diffusion of Si into Ni and reoxidation of Si in the Ni grain.

Tailoring the Microstructure of a Solid Oxide Fuel Cell Anode Support by Calcination and Milling of YSZ

In this study, the effects of calcination and milling of 8YSZ (8 mol% yttria stabilized zirconia) used in the nickel-YSZ anode on the performance of anode supported tubular fuel cells were investigated. For this purpose, two different types of cells were prepared based on a Ni-YSZ/YSZ/Nd₂NiO_4+δ-YSZ configuration. For the anode preparation, a suspension was prepared by mixing NiO and YSZ in a ratio of 65:35 wt% (Ni:YSZ 50:50 vol.%) with 30 vol.% graphite as the pore former. As received Tosoh YSZ or its calcined form (heated at 1500 °C for 3 hours) was used in the anode support as the YSZ source. Electrochemical results showed that optimization of the fuel electrode microstructure is essential for the optimal distribution of gas within the support of the cell, especially under electrolysis operation where the performance for an optimized cell (calcined YSZ) was enhanced by a factor of two. In comparison with a standard cell (containing as received YSZ), at 1.5 V and 800 °C the measured current density was −1380 mA cm⁻² and −690 mA cm⁻² for the cells containing calcined and as received YSZ, respectively. The present study suggests that the anode porosity for improved cell performance under SOEC is more critical than SOFC mode due to more complex gas diffusion under electrolysis mode where large amount of steam needs to be transfered into the cell.