Showing papers by "Zongping Shao published in 2021"

Thermal-expansion offset for high-performance fuel cell cathodes

Abstract: One challenge for the commercial development of solid oxide fuel cells as efficient energy-conversion devices is thermo-mechanical instability. Large internal-strain gradients caused by the mismatch in thermal expansion behaviour between different fuel cell components are the main cause of this instability, which can lead to cell degradation, delamination or fracture1-4. Here we demonstrate an approach to realizing full thermo-mechanical compatibility between the cathode and other cell components by introducing a thermal-expansion offset. We use reactive sintering to combine a cobalt-based perovskite with high electrochemical activity and large thermal-expansion coefficient with a negative-thermal-expansion material, thus forming a composite electrode with a thermal-expansion behaviour that is well matched to that of the electrolyte. A new interphase is formed because of the limited reaction between the two materials in the composite during the calcination process, which also creates A-site deficiencies in the perovskite. As a result, the composite shows both high activity and excellent stability. The introduction of reactive negative-thermal-expansion components may provide a general strategy for the development of fully compatible and highly active electrodes for solid oxide fuel cells.

High‐Quality Ruddlesden–Popper Perovskite Film Formation for High‐Performance Perovskite Solar Cells

Abstract: In the last decade, perovskite solar cells (PSCs) have undergone unprecedented rapid development and become a promising candidate for a new-generation solar cell. Among various PSCs, typical 3D halide perovskite-based PSCs deliver the highest efficiency but they suffer from severe instability, which restricts their practical applications. By contrast, the low-dimensional Ruddlesden-Popper (RP) perovskite-based PSCs have recently raised increasing attention due to their superior stability. Yet, the efficiency of RP perovskite-based PSCs is still far from that of the 3D counterparts owing to the difficulty in fabricating high-quality RP perovskite films. In pursuit of high-efficiency RP perovskite-based PSCs, it is critical to manipulate the film formation process to prepare high-quality RP perovskite films. This review aims to provide comprehensive understanding of the high-quality RP-type perovskite film formation by investigating the influential factors. On this basis, several strategies to improve the RP perovskite film quality are proposed via summarizing the recent progress and efforts on the preparation of high-quality RP perovskite film. This review will provide useful guidelines for a better understanding of the crystallization and phase kinetics during RP perovskite film formation process and the design and development of high-performance RP perovskite-based PSCs, promoting the commercialization of PSC technology.

Defect engineering of oxide perovskites for catalysis and energy storage: synthesis of chemistry and materials science.

Abstract: Oxide perovskites have emerged as an important class of materials with important applications in many technological areas, particularly thermocatalysis, electrocatalysis, photocatalysis, and energy storage. However, their implementation faces numerous challenges that are familiar to the chemist and materials scientist. The present work surveys the state-of-the-art by integrating these two viewpoints, focusing on the critical role that defect engineering plays in the design, fabrication, modification, and application of these materials. An extensive review of experimental and simulation studies of the synthesis and performance of oxide perovskites and devices containing these materials is coupled with exposition of the fundamental and applied aspects of defect equilibria. The aim of this approach is to elucidate how these issues can be integrated in order to shed light on the interpretation of the data and what trajectories are suggested by them. This critical examination has revealed a number of areas in which the review can provide a greater understanding. These include considerations of (1) the nature and formation of solid solutions, (2) site filling and stoichiometry, (3) the rationale for the design of defective oxide perovskites, and (4) the complex mechanisms of charge compensation and charge transfer. The review concludes with some proposed strategies to address the challenges in the future development of oxide perovskites and their applications.

Selenic Acid Etching Assisted Vacancy Engineering for Designing Highly Active Electrocatalysts toward the Oxygen Evolution Reaction.

Abstract: Oxygen evolution electrocatalysts are central to overall water splitting, and they should meet the requirements of low cost, high activity, high conductivity, and stable performance. Herein, a general, selenic-acid-assisted etching strategy is designed from a metal-organic framework as a precursor to realize carbon-coated 3d metal selenides Mm Sen (Co0.85 Se1- x , NiSe2- x , FeSe2- x ) with rich Se vacancies as high-performance precious metal-free oxygen evolution reaction (OER) electrocatalysts. Specifically, the as-prepared Co0.85 Se1- x @C nanocages deliver an overpotential of only 231 mV at a current density of 10 mA cm-2 for the OER and the corresponding full water-splitting electrolyzer requires only a cell voltage of 1.49 V at 10 mA cm-2 in alkaline media. Density functional theory calculation reveals the important role of abundant Se vacancies for improving the catalytic activity through improving the conductivity and reducing reaction barriers for the formation of intermediates. Although phase change after long-term operation is observed with the formation of metal hydroxides, catalytic activity is not obviously affected, which strengthens the important role of the carbon network in the operating stability. This study provides a new opportunity to realize high-performance OER electrocatalysts by a general strategy on selenic acid etching assisted vacancy engineering.

Smart Construction of an Intimate Lithium | Garnet Interface for All‐Solid‐State Batteries by Tuning the Tension of Molten Lithium

Abstract: All-solid-state lithium batteries (ASSBs) have the potential to trigger a battery revolution for electric vehicles due to their advantages in safety and energy density. Screening of various possible solid electrolytes for ASSBs has revealed that garnet electrolytes are promising due to their high ionic conductivity and superior (electro)chemical stabilities. However, a major challenge of garnet electrolytes is poor contact with Li-metal anodes, resulting in an extremely large interfacial impedance and severe Li dendrite propagation. Herein, an innovative surface tension modification method is proposed to create an intimate Li | garnet interface by tuning molten Li with a trace amount of Si3N4 (1 wt%). The resultant Li-Si-N melt can not only convert the Li | garnet interface from point-to-point contact to consecutive face-to-face contact but also homogenize the electric-field distribution during the Li stripping/depositing process, thereby significantly decreasing its interfacial impedance (1 Ω cm2 at 25 °C) and improving its cycle stability (1000 h at 0.4 mA cm−2) and critical current density (1.8 mA cm−2). Specifically, the all-solid-state full cell paired with a LiFePO4 cathode delivered a high capacity of 145 mAh g−1 at 2 C and maintained 97% of the initial capacity after 100 cycles at 1 C.

Electrochemistry and energy conversion features of protonic ceramic cells with mixed ionic-electronic electrolytes

Abstract: Protonic ceramic electrochemical cells (including fuel cells (PCFCs) and electrolysis cells (PCECs)) are positioned as an eco-friendly means for realizing energy/chemical conversion at low (below 500 °C) and intermediate (500–800 °C) temperatures; as a result, R&D of PCFCs and PCECs are compatible with hydrogen energy and CO2 utilization programs that play an increasing role in global environmental practice. However, along with ionic transport, the majority of proton-conducting ceramic materials also exhibit electronic transport under oxidizing conditions and elevated temperatures. This feature negatively affects the performance of cells due to the short-circuit effect leading to a reduction in faradaic and energy efficiencies. In response, in order to achieve a compromise between high performance and high efficiency, the present review article aims at revealing the main factors contributing to undesirable electronic transport of materials used in PCFCs and PCECs, as well as possible solutions leading to its suppression for improving their efficiency.

High-Performance Perovskite Composite Electrocatalysts Enabled by Controllable Interface Engineering

Abstract: Single-phase perovskite oxides that contain nonprecious metals have long been pursued as candidates for catalyzing the oxygen evolution reaction, but their catalytic activity cannot meet the requirements for practical electrochemical energy conversion technologies. Here a cation deficiency-promoted phase separation strategy to design perovskite-based composites with significantly enhanced water oxidation kinetics compared to single-phase counterparts is reported. These composites, self-assembled from perovskite precursors, comprise strongly interacting perovskite and related phases, whose structure, composition, and concentration can be accurately controlled by tailoring the stoichiometry of the precursors. The composite catalyst with optimized phase composition and concentration outperforms known perovskite oxide systems and state-of-the-art catalysts by 1-3 orders of magnitude. It is further demonstrated that the strong interfacial interaction of the composite catalysts plays a key role in promoting oxygen ionic transport to boost the lattice-oxygen participated water oxidation. These results suggest a simple and viable approach to developing high-performance, perovskite-based composite catalysts for electrochemical energy conversion.

Fast operando spectroscopy tracking in situ generation of rich defects in silver nanocrystals for highly selective electrochemical CO2 reduction.

Abstract: Electrochemical CO2 reduction (ECR) is highly attractive to curb global warming. The knowledge on the evolution of catalysts and identification of active sites during the reaction is important, but still limited. Here, we report an efficient catalyst (Ag-D) with suitable defect concentration operando formed during ECR within several minutes. Utilizing the powerful fast operando X-ray absorption spectroscopy, the evolving electronic and crystal structures are unraveled under ECR condition. The catalyst exhibits a ~100% faradaic efficiency and negligible performance degradation over a 120-hour test at a moderate overpotential of 0.7 V in an H-cell reactor and a current density of ~180 mA cm−2 at −1.0 V vs. reversible hydrogen electrode in a flow-cell reactor. Density functional theory calculations indicate that the adsorption of intermediate COOH could be enhanced and the free energy of the reaction pathways could be optimized by an appropriate defect concentration, rationalizing the experimental observation. Efficient electrocatalysts are crucial to the electrochemical carbon dioxide reduction. Here, the authors use the fast operando technique to explore a silver-based catalyst with improved catalytic performance benefiting from the suitable defect structures.

Recent Advances in the Understanding of the Surface Reconstruction of Oxygen Evolution Electrocatalysts and Materials Development

Abstract: The electrochemical oxygen evolution reaction (OER) plays an important role in many clean electrochemical energy storage and conversion systems, such as electrochemical water splitting, rechargeable metal–air batteries, and electrochemical CO2 reduction. However, the OER involves a complex four-electron process and suffers from intrinsically sluggish kinetics, which greatly impairs the efficiency of electrochemical systems. In addition, state-of-the-art RuO2-based OER electrocatalysts are too expensive and scarce for practical applications. The development of highly active, cost-effective, and durable electrocatalysts that can improve OER performance (activity and durability) is of significant importance in realizing the widespread application of these advanced technologies. To date, considerable progress has been made in the development of alternative, noble metal-free OER electrocatalysts. Among these alternative catalysts, transition metal compounds have received particular attention and have shown activities comparable to or even higher than those of their precious metal counterparts. In contrast to many other electrocatalysts, such as carbon-based materials, transition metal compounds often exhibit a surface reconstruction phenomenon that is accompanied by the transformation of valence states during electrochemical OER processes. This surface reconstruction results in changes to the true active sites and an improvement or reduction in OER catalytic performance. Therefore, understanding the self-reconstruction process and precisely identifying the true active sites on electrocatalyst surfaces will help us to finely tune the properties and activities of OER catalysts. This review provides a comprehensive summary of recent progress made in understanding the surface reconstruction phenomena of various transition metal-based OER electrocatalysts, focusing on uncovering the correlations among structure, surface reconstruction and intrinsic activity. Recent advances in OER electrocatalysts that exhibit a surface self-reconstruction capability are also discussed. We identify possible challenges and perspectives for the development of OER electrocatalysts based on surface reconstruction. We hope this review will provide readers with some guidance on the rational design of catalysts for various electrochemical reactions.

Emerging two-dimensional nanomaterials for electrochemical nitrogen reduction.

Abstract: Ammonia (NH3) is essential to serve as the biological building blocks for maintaining organism function, and as the indispensable nitrogenous fertilizers for increasing the yield of nutritious crops. The current Haber–Bosch process for industrial NH3 production is highly energy- and capital-intensive. In light of this, the electroreduction of nitrogen (N2) into valuable NH3, as an alternative, offers a sustainable pathway for the Haber–Bosch transition, because it utilizes renewable electricity and operates under ambient conditions. Identifying highly efficient electrocatalysts remains the priority in the electrochemical nitrogen reduction reaction (NRR), marking superior selectivity, activity, and stability. Two-dimensional (2D) nanomaterials with sufficient exposed active sites, high specific surface area, good conductivity, rich surface defects, and easily tunable electronic properties hold great promise for the adsorption and activation of nitrogen towards sustainable NRR. Therefore, this Review focuses on the fundamental principles and the key metrics being pursued in NRR. Based on the fundamental understanding, the recent efforts devoted to engineering protocols for constructing 2D electrocatalysts towards NRR are presented. Then, the state-of-the-art 2D electrocatalysts for N2 reduction to NH3 are summarized, aiming at providing a comprehensive overview of the structure-performance relationships of 2D electrocatalysts towards NRR. Finally, we propose the challenges and future outlook in this prospective area.

Development of nickel based cermet anode materials in solid oxide fuel cells – Now and future

Abstract: High temperature solid oxide fuel cell (SOFC) is the most efficient and clean energy conversion technology to electrochemically convert the chemical energy of fuels such as hydrogen, natural gas and hydrocarbons to electricity, and also the most viable alternative to the traditional thermal power plants. However, the power output of a SOFC critically depends on the characteristics and performance of its key components: anode, electrolyte and cathode. Due to the highly reducing environment and strict requirements in electrical conductivity and catalytic activity, there are limited choices in the anode materials of SOFCs, particularly for operation in the intermediate temperature range of 500–800 ​°C. Among them, Ni-based cermets are the most common and popular anode materials of SOFCs. The objective of this paper is to review the development of Ni-based anode materials in SOFC from the viewpoints of materials microstructure, performance and industrial scalability associated with the fabrication and optimization processes. The latest advancement in nano-structure architecture, contaminant tolerance and interface optimization of Ni-based cermet anodes is presented. And at the end of this paper, we propose and appeal for the collaborative work of scientists from different disciplines that enable the inter-fusion research of fabrication, microanalysis and modelling, aiming at the challenges in the development of Ni-based cermet anodes for commercially viable intermediate temperature SOFC or IT-SOFC technologies.

Recent advances in functional oxides for high energy density sodium-ion batteries

Abstract: The rapid development of power generation from renewable energy, such as geothermal, wind, and tidal energy, arises the need for efficient and economic electrochemical energy storage systems for integrating electric power smoothly into power grids The lithium-ion battery has been successfully utilized for a variety of portable electronic devices but its application in large-scale energy storage has raised concerns about safety, availability of lithium resources, and its increasing price Alternatively, sodium-ion battery (SIB) has been in acquisition predominantly, because of the abundant sodium resources over the earth’s crust at low cost Developing high energy density electrode materials is one of the most intensive research topics Oxides have been extensively investigated as the cathode and anode materials for high-performance and durable SIBs, owing to their various advantages including facile synthesis, versatile compositions, and easy structural tuning In this review, we provide an in-time thorough summary of recent advances in oxide materials for cathodes and anodes for high energy density SIBs The energy storage mechanism, challenges, categories, and optimizations of both electrodes are discussed Existing research gaps and future perspectives are also outlined This review is expected to accelerate the research for developing new pathways for tuning the properties of the oxide electrodes based on different sodium storage mechanisms

Self-Supported Nickel Phosphide Electrode for Efficient Alkaline Water-to-Hydrogen Conversion via Urea Electrolysis

Abstract: Electrochemical water splitting is an attractive technique to produce renewable hydrogen gas. However, considerable challenges remain before the catalytic anodic oxygen evolution reaction (OER) can...

A Highly Ordered Hydrophilic–Hydrophobic Janus Bi-Functional Layer with Ultralow Pt Loading and Fast Gas/Water Transport for Fuel Cells

Abstract: One of the critical challenges that limit broad commercialization of proton exchange membrane fuel cells (PEMFC) is to reduce the usage of Pt while maintaining high power output and sufficient durability. Herein, a novel bi-functional layer consisting of vertically aligned carbon nanotubes (VACNTs) and nanoparticles of Pt-Co catalysts (Pt-Co/VACNTs) is reported for high-performance PEMFCs. Readily prepared by a two-step process, the Pt-Co/VACNTs layer with a hydrophilic catalyst-loaded side and a hydrophobic gas diffusion side enables a PTFE-free electrode structure with fully exposed catalyst active sites and superior gas–water diffusion capability. When tested in a PEMFC, the bi-functional Pt-Co/VACNTs layer with ultralow Pt loading (~65 μg cm) demonstrates a power density of 19.5 kW g at 0.6 V, more than seven times that of a cell with commercial Pt/C catalyst (2.7 kW g at 0.6 V) at a loading of 400 μg cm tested under similar conditions. This remarkable design of VACNTs-based catalyst with dual functionalities enables much lower Pt loading, faster mass transport, and higher electrochemical performance and stability. Further, the preparation procedure can be easily scaled up for low-cost fabrication and commercialization.