Ammonia-fed reversible protonic ceramic fuel cells with Ru-based catalyst

Design Refinement of Catalytic System for Scale-Up Mild Nitrogen Photo-Fixation

Ammonia and nitric acid, versatile industrial feedstocks, and burgeoning clean energy vectors hold immense promise for sustainable development. However, Haber-Bosch and Ostwald processes, which generates carbon dioxide as massive by-product, contribute to greenhouse effects and pose environmental challenges. Thus, the pursuit of nitrogen fixation through carbon-neutral pathways under benign conditions is a frontier of scientific topics, with the harnessing of solar energy emerging as an enticing and viable option. This review delves into the refinement strategies for scale-up mild photocatalytic nitrogen fixation, fields ripe with potential for innovation. The narrative is centered on enhancing the intrinsic capabilities of catalysts to surmount current efficiency barriers. Key focus areas include the in-depth exploration of fundamental mechanisms underpinning photocatalytic procedures, rational element selection, and functional planning, state-of-the-art experimental protocols for understanding photo-fixation processes, valid photocatalytic activity evaluation, and the rational design of catalysts. Furthermore, the review offers a suite of forward-looking recommendations aimed at propelling the advancement of mild nitrogen photo-fixation. It scrutinizes the existing challenges and prospects within this burgeoning domain, aspiring to equip researchers with insightful perspectives that can catalyze the evolution of cutting-edge nitrogen fixation methodologies and steer the development of next-generation photocatalytic systems.

Status and outlook of solid electrolyte membrane reactors for energy, chemical, and environmental applications

Solid electrolyte membrane reactors (SEMRs) can be operated at high temperatures with distinct reaction kinetics, or at lower temperatures (300-500 °C) for industrially relevant energy applications (such as solid oxide fuel/electrolysis cells, direct carbon fuel cells, and metal-air batteries), chemical (such as alkane dehydrogenation, C-C coupling, and NH3 synthesis), environmental (De-NO x , CO2 utilization, and separation), as well as their combined (one-step coupled CO2/H2O co-electrolysis and methanation reaction, power and chemical cogeneration) applications. SEMRs can efficiently integrate electrical, chemical, and thermal energy sectors, thereby circumventing thermodynamic constraints and production separation issues. They offer a promising way to achieve carbon neutrality and improve chemical manufacturing processes. This review thoroughly examines SEMRs utilizing various ionic conductors, namely O2-, H+, and hybrid types, with operations in different reactor/cell architectures (such as panel, tubular, single chamber, and porous electrolytes). The reactors operate in various modes including pumping, extraction, reversible, or electrical promoting modes, providing multiple functionalities. The discussion extends to examining critical materials for solid-state cells and catalysts essential for specific technologically important reactions, focusing on electrochemical performance, conversion efficiency, and selectivity. The review also serves as a first attempt to address the potential of process-intensified SEMRs through the integration of photo/solar, thermoelectric, and plasma energy and explores the unique phenomenon of electrochemical promotion of catalysis (EPOC) in membrane reactors. The ultimate goal is to offer insight into ongoing critical scientific and technical challenges like durability and operational cost hindering the widespread industrial implementation of SEMRs while exploring the opportunities in this rapidly growing research domain. Although still in an early stage with limited demonstrations and applications, advances in materials, catalysis science, solid-state ionics, and reactor design, as well as process intensification and/or system integration will fill the gaps in current high temperature operation of SEMRs and industrially relevant applications like sustainable clean chemical production, efficient energy conversion/storage, as well as environmental enhancement.

Transport properties of B-site codoped CaHfO3 proton conductors with octahedral distortion

Perovskite-type solid electrolytes exhibit a diverse range of conductive properties due to the competition and coupling of multiple degrees of freedom. In perovskite structures, B-site and X-site ions form topological octahedral sublattices, which are instrumental in regulating transport properties for various charge carriers. However, research focused on the relationship between octahedral distortion and conductive properties in perovskite-type proton conductors remains limited. In this study, dopants such as Ge, Sn, Pr, and Ce were selected to modify the degree of BO6 octahedral distortion in CaHf0.9Sc0.1O3-δ . The relationships between conductivity, transport number, mobility, and the distortion degree were systematically investigated. The data indicate that both proton and oxygen ion mobilities initially increase with the octahedral distortion angle and then decrease, and CaHf0.8Sn0.1Sc0.1O3-δ with an octahedral distortion angle of 15.6°, exhibited the highest ionic mobilities and conductivities. The BO6 octahedral distortion appears to limit oxide ion conduction while enhancing the proton transport number. However, excessive doping generates additional oxygen vacancies, which adversely affect proton conduction. Under the combined influence of these factors, CaHf0.8Ce0.1Sc0.1O3-δ achieved the highest proton transport number of 0.503 at 800 °C. Overall, this work provides insights into the relationship between octahedral distortion and conductive properties, suggesting that co-doping is a feasible approach for further regulating carrier mobility properties.

Mixed Conduction in A-Site Double-Perovskite Na1+xLa1-xZr2O6-δ Proton Conductors

Perovskite-type proton conductors exist in two structural forms, ABO3 and A2B'B″O6. In this study, novel A-site double-perovskite proton conductors (A'A″B2O6) were proposed. Na1+xLa1-xZr2O6-δ (x = 0, 0.1, 0.2) perovskites were prepared by a solid-state reaction at 1200 °C. However, raising the sintering temperature to 1300 °C resulted in the Na to volatilize, converting the Na1.1La0.9Zr2O6-δ into La0.9Zr2O6-δ. The conductivities of these materials in a humid atmosphere were tested using electrochemical impedance spectroscopy, and their carrier transport numbers were measured using the defect equilibria model and concentration cell method. Na1.1La0.9Zr2O6-δ and Na1.2La0.8Zr2O6-δ are predominantly proton conductors, with Na1.1La0.9Zr2O6-δ exhibiting the highest proton transport number of 0.52 at 800 °C. In contrast, NaLaZr2O6 is predominantly an electronic conductor, while La0.9Zr2O6-δ functions as an oxide ion conductor. Due to their high protonic transport numbers, these Na1+xLa1-xZr2O6-δ A-site double-perovskite oxides present a promising avenue for the development of proton conductors.

Challenges and Advancements in the Electrochemical Utilization of Ammonia Using Solid Oxide Fuel Cells

Solid oxide fuel cells utilized with NH3 (NH3-SOFCs) have great potential to be environmentally friendly devices with high efficiency and energy density. The advancement of this technology is hindered by the sluggish kinetics of chemical or electrochemical processes occurring on anodes/catalysts. Extensive efforts have been devoted to developing efficient and durable anode/catalysts in recent decades. Although modifications to the structure, composition, and morphology of anodes or catalysts are effective, the mechanistic understandings of performance improvements or degradations remain incompletely understood. This review informatively commences by summarizing existing reports on the progress of NH3-SOFCs. It subsequently outlines the influence of factors on the performance of NH3-SOFCs. The degradation mechanisms of the cells/systems are also reviewed. Lastly, the persistent challenges in designing highly efficient electrodes/catalysts for low-temperature NH3-SOFCs, and future perspectives derived from SOFCs are discussed. Notably, durability, thermal cycling stability, and power density are identified as crucial indicators for enhancing low-temperature (550 °C or below) NH3-SOFCs. This review aims to offer an updated overview of how catalysts/electrodes affect electrochemical activity and durability, offering critical insights for improving performance and mechanistic understanding, as well as establishing the scientific foundation for the design of electrodes for NH3-SOFCs.

Nanotechnologies in ceramic electrochemical cells

Although they are emerging technologies for achieving high-efficiency and green and eco-friendly energy conversion, ceramic electrochemical cells (CECs), i.e. solid oxide electrolysis cells (SOECs) and fuel cells (SOFCs), are still fundamentally limited by their inferior catalytic activities at low temperature, poor thermo-mechanical stability, high material cost, etc. The materials used in electrolytes and electrodes, which are the most important components in CECs, are highly associated with the cell performances. Therefore, rational design of electrolytes and electrodes with excellent catalytic activities and high stabilities at relatively low cost is a meaningful and valuable approach for the development of CECs. Nanotechnology is a powerful tool for improving the material performances in CECs owing to the favourable effects induced by the nanocrystallization of electrolytes and electrodes. Herein, a relatively comprehensive review on the nanotechnologies implemented in CECs is conducted. The working principles of CECs and the corresponding challenges were first presented, followed by the comprehensive insights into the working mechanisms of nanocrystalline materials in CECs. Then, systematic summarization and analyses of the commonly used nano-engineering strategies in the fabrication of CEC materials, including physical and chemical methods, were provided. In addition, the frontiers in the research of advanced electrolyte and electrode materials were discussed with a special emphasis on the modified electrochemical properties derived from nanotechnologies. Finally, the bottlenecks and the promising breakthroughs in nanotechnologies were highlighted in the direction of providing useful references for rational design of nanomaterials for CECs.

A Conceptual Approach for the Design of New Catalysts for Ammonia Synthesis: A Metal-Support Interactions Review

The growing interest in green ammonia production has spurred the development of new catalysts with the potential to carry out the Haber-Bosch process under mild pressure and temperature conditions. While there is a wide experimental background on new catalysts involving transition metals, supports and additives, the fundamentals behind ammonia synthesis performance on these catalysts remained partially unsolved. Here, we review the most important works developed to date and analyze the traditional catalysts for ammonia synthesis, as well as the influence of the electron transfer properties of the so-called 3rd-generation catalysts. Finally, the importance of metal-support interactions is highlighted as an effective pathway for the design of new materials with potential to carry out ammonia synthesis at low temperatures and pressures.

Self-Construction of Efficient Interfaces Ensures High-Performance Direct Ammonia Protonic Ceramic Fuel Cells

Direct ammonia protonic ceramic fuel cells (PCFCs) are highly efficient energy conversion devices since ammonia as a carbon-neutral hydrogen-rich carrier shows great potential for storage and long-distance transportation when compared with hydrogen fuel. However, traditional Ni-based anodes readily suffer from severe structural destruction and dramatic deactivation after long-time exposure to ammonia. Here a Sr2 Fe1.35 Mo0.45 Cu0.2 O6-δ (SFMC) anode catalytic layer (ACL) painted onto a Ni-BaZr0.1 Ce0.7 Y0.1 Yb0.1 O3- δ (BZCYYb) anode with enhanced catalytic activity and durability toward the direct utilization of ammonia is reported. A tubular Ni-BZCYYb anode-supported cells with the SFMC ACL show excellent peak power densities of 1.77 W cm-2 in wet H2 (3% H2 O) and 1.02 W cm-2 in NH3 at 650 °C. A relatively stable operation of the cells is obtained at 650 °C for 200 h in ammonia fuel. Such achieved improvements in the activity and durability are attributed to the self-constructed interfaces with the phases of NiCu or/and NiFe for efficient NH3 decomposition, resulting in a strong NH3 adsorption strength of the SFMC, as confirmed by NH3 thermal conversion and NH3 -temperature programmed desorption. This research offers a valuable strategy of applying an internal catalytic layer for highly active and durable ammonia PCFCs.

High-Performance Ammonia Protonic Ceramic Fuel Cells Using a Pd Inter-Catalyst

This study reports the performance and durability of a protonic ceramic fuel cells (PCFCs) in an ammonia fuel injection environment. The low ammonia decomposition rate in PCFCs with lower operating temperatures is improved relative to that of solid oxide fuel cells by treatment with a catalyst. By treating the anode of the PCFCs with a palladium (Pd) catalyst at 500 °C under ammonia fuel injection, the performance (peak power density of 340 mW cm^-2 at 500 °C) is approximately two-fold higher than that of the bare sample not treated with Pd. Pd catalysts are deposited through an atomic layer deposition post-treatment process on the anode surface, in which nickel oxide (NiO) and BaZr_0.2 Ce_0.6 Y_0.1 Yb_0.1 O_3-δ (BZCYYb) are mixed, and Pd can penetrate the anode surface and porous interior. Impedance analysis confirmed that Pd increased the current collection and significantly reduced the polarization resistance, particularly in the low-temperature region (≈500 °C), thereby improving the performance. Furthermore, stability tests showed that superior durability is achieved compared with that of the bare sample. Based on these results, the method presented herein is expected to represent a promising solution for securing high-performance and stable PCFCs based on ammonia injection.

Protonic Ceramic Electrochemical Cells for Synthesizing Sustainable Chemicals and Fuels

Protonic ceramic electrochemical cells (PCECs) have been intensively studied as the technology that can be employed for power generation, energy storage, and sustainable chemical synthesis. Recently, there have been substantial advances in electrolyte and electrode materials for improving the performance of protonic ceramic fuel cells and protonic ceramic electrolyzers. However, the electrocatalytic materials development for synthesizing chemicals in PCECs has gained less attention, and there is a lack of systematic and fundamental understanding of the PCEC reactor design, reaction mechanisms, and electrode materials. This review comprehensively summarizes and critically evaluates the most up-to-date progress in employing PCECs to synthesize a wide range of chemicals, including ammonia, carbon monoxide, methane, light olefins, and aromatics. Factors that impact the conversion, selectivity, product yield, and energy efficiencies are discussed to provide new insights into designing electrochemical cells, developing electrode materials, and achieving economically viable chemical synthesis. The primary challenges associated with producing chemicals in PCECs are highlighted. Approaches to tackle these challenges are then offered, with a particular focus on deliberately designing electrode materials, aiming to achieve practically valuable product yield and energy efficiency. Finally, perspectives on the future development of PCECs for synthesizing sustainable chemicals are provided.