Article

Energy material analysis via in-situ/operando scanning transmission x-ray microscopy: A review

Authors:
To read the full-text of this research, you can request a copy directly from the authors.

No full-text available

Request Full-text Paper PDF

To read the full-text of this research,
you can request a copy directly from the authors.

... Scanning transmission X-ray microscopy (STXM), a nanoscale imaging technique based on soft XAS, provides depth-averaged oxidation state information of specific regions, enabling mapping the chemical composition of compounds with a high spatial resolution ($40 nm) and chemical sensitivity. 15 This soft X-ray microscopy can probe the L-edge of early transition metals (e.g., Fe, Co, and Cu), offering strong absorption contrast and accurately quantifying subtle changes in oxidation states. Thus, STXM has an advantage in visualizing oxidation states of nanomaterials. ...
... Electrochemical cells containing electrolytes should be designed in a way to minimize beam attenuation. Despite these technical difficulties, which include potential beam-induced damage 15 and scarcity of facilities, the first application of in situ electrochemical STXM measurement demonstrated the ability to track the electrochemical changes of polyaniline under applied potential control. 16 Since then, operando STXM technologies have been successfully developed for various applications, including thermal catalysis, [17][18][19] batteries, 20,21 and electrocatalysis. ...
... Bragg coherent diffractive imaging [ 105] Visualization of defect, dislocation, and interactions Improvement of resolution, prevention of beam damage to samples Synchrotron-based scanning transmission X-ray microscopy [ 111] Real-time imaging of chemical composition and morphological Improvement of resolution, prevention of beam damage to samples reconstruct the 3D microstructure of the sample through the tomographic reconstruction algorithm, which can in situ visualize the lithiation behavior and phase transition of the active material during the charge and discharge process of the battery (Figure 9b). [106,107] Although 3D X-ray microscopy has been able to reveal important insights into the morphology, strain, dislocation, and quantitative measurements of local lithium insertion rates in single particles have remained difficult to achieve. ...
... Simultaneously, the potential degradation effect of high-intensity Xray radiation on samples is also a problem that cannot be ignored, which may lead to distortion of material characterization results. [111] The advantages and challenges of various in situ physical characterizations at single particle scale are presented in Table 2. In summary, various physical characterization techniques play an increasingly important role in the structure and composition analysis of energy storage materials. ...
Article
Full-text available
As electric vehicles, portable electronic devices, and tools have increasingly high requirements for battery energy density and power density, constantly improving battery performance is a research focus. Accurate measurement of the structure–activity relationship of active materials is key to advancing the research of high‐performance batteries. However, conventional performance tests of active materials are based on the electrochemical measurement of porous composite electrodes containing active materials, polymer binders, and conductive carbon additives, which cannot establish an accurate structure–activity relationship with the physical characterization of microregions. In this review, in order to promote the accurate measurement and understanding of the structure–activity relationship of materials, the electrochemical measurement and physical characterization of energy storage materials at single‐particle scale are reviewed. The potential problems and possible improvement schemes of the single particle electrochemical measurement and physical characterization are proposed. Their potential applications in single particle electrochemical simulation and machine learning are prospected. This review aims to promote the further application of single particle electrochemical measurement and physical characterization in energy storage materials, hoping to achieve 3D unified evaluation of physical characterization, electrochemical measurement, and theoretical simulation at the single particle scale to provide new inspiration for the development of high‐performance batteries.
... It is an established approach for label-free probing of various samples. [1][2][3][4][5][6][7] This includes samples of importance to materials science, biological matter, as well as drug delivery systems. Its strength lies in the use of tunable soft X-rays delivered from synchrotron radiation sources allowing for element-selective excitations and exploiting chemical shifts of resonant transitions, for probing the chemical environment of the absorber with spatial resolution on the nanoscale. ...
Article
Full-text available
Scanning Transmission X‐ray microscopy (STXM) is a sensitive and selective probe for the penetration of rapamycin which is topically applied to human skin ex vivo and is facilitated by skin treatment with microneedles puncturing the skin. Inner‐shell excitation serves as a selective probe for detecting rapamycin by changes in optical density as well as linear combination modeling using reference spectra of the most abundant species. The results indicate that mechanical damage induced by microneedles allows this drug to accumulate in the stratum corneum without reaching the viable skin layers. This is unlike intact skin which shows no drug penetration at all and underscores the mechanical impact of microneedle skin treatment. These results are compared to drug penetration profiles of other drugs highlighting the importance of skin barriers. High spatial resolution studies also indicate that the lipophilic drug rapamycin is observed in corneocytes. Attempts in data evaluation are reported to probe rapamycin also in the lipid layers between the corneocytes, which was not accomplished before. These results are compared to recent results on rapamycin uptake in skin where barrier impairment was induced by pre‐treatment with the enzyme trypsin and drug formulations leading to occlusion.
... STXM has seen extensive application in energy materials science [12][13][14][15][16][17][18][19][20] since it provides chemical and structural information beyond elemental identity, such as oxidation state, chemical bonding, coordination, orientation, and magnetic properties. However, materials science and technology increasingly require analytical probes at the nanoscale, so the spatial resolution of STXM (practical instruments ~30 nm, state-of-art ~8 nm [21]) is a significant limitation. ...
... Hard X-rays are more suitable for larger samples (>10 μm) because it provides more detailed information from the depth of the sample, which is especially convenient in battery research and other energy applications, but less studied for electrocatalysis. [58] Figure 5. a) Schematic representation of electrochemical cell used in liquid-phase TEM measurements. Reproduced with permission. ...
Article
Full-text available
The world is facing grand challenges in energy security, environmental pollution, and sustainable use (and re‐use) of resources. Electrochemical processes, incorporating electrosynthesis, electrochemical catalysis, and electrochemical energy storage devices, provide pathways to address these challenges via green chemistry. However, the applicability of electrochemical processes for these systems is limited by the required energy input, the “electrons” in electrochemistry. Electrocatalysis as a subset of electrochemistry is set to underpin many of the United Nations Sustainable Development Goals, including “Affordable and Clean Energy” through the production of future fuels and abatement of carbon emissions; “Responsible Consumption and Production” through recycling and degradation of waste; and “Climate Action” through CO2 (and other greenhouse gas) remediation. The rise of green photovoltaic power has lowered the carbon cost of these electrons, making electrocatalysis an even more viable, green(er), chemical conversion pathway. This perspective highlights the need for comprehensive understanding of catalyst structure via in situ and operando analysis to complement device design considerations. The challenges faced by the field of electrocatalysis in data reporting, elimination of electrochemical artifacts, catalyst stability, and scaling to industrial relevance, along with opportunities, emerging tools, are discussed with a view to achieve the maximum ‘potential’ of electrocatalysis.
Article
Catalysis, which is central to the energy and chemical industries, constitutes a dynamic process that involves multiple stages, ranging from activation and deactivation to rapid catalytic cycles. Comprehending these dynamic processes is crucial for optimizing catalytic reactions and developing innovative catalytic systems. This review centers on the integration of operando spectroscopies with transient analysis to acquire quantitative dynamic information directly from spectroscopic investigations of catalytic processes. Operando spectroscopies facilitate real-time observations of dynamic changes occurring during catalysis, while transient experiments yield insights that extend beyond steady-state kinetics by examining catalysts’ responses to and recovery from dynamic conditions. This review aims to underscore the significance of integrating multiple spectroscopic techniques and transient methodologies to yield quantitative insights into catalytic processes, elucidating in real time the relationships between active structures, surface intermediates, and catalytic kinetics.
Article
Full-text available
The scarcity of fuels, high pollution levels, climate change, and other major environmental issues are critical challenges that modern societies are facing, mostly originating from fossil fuels-based economies. These challenges can be addressed by developing green, eco-friendly, inexpensive energy sources and energy storage devices. Electrochemical energy storage materials possess high capacitance and superior power density. To engineer highly efficient next-generation electrochemical energy storage devices, the mechanisms of electrochemical reactions and redox behavior must be probed in operational environments. They can be studied by investigating atomic and electronic structures using in situ x-ray absorption spectroscopy (XAS) analysis. Such a technique has attracted substantial research and development interest in the field of energy science for over a decade. The mechanisms of charge/discharge, carrier transport, and ion intercalation/deintercalation can be elucidated. Supercapacitors generally store energy by two specific mechanisms—pseudocapacitance and electrochemical double-layer capacitance. In situ XAS is a powerful tool for probing and understanding these mechanisms. In this Review, both soft and hard x rays are used for the in situ XAS analysis of various representative electrochemical energy storage systems. This Review also showcases some of the highly efficient energy and power density candidates. Furthermore, the importance of synchrotron-based x-ray spectroscopy characterization techniques is enlightened. The impact of the electronic structure, local atomic structure, and electronically active elements/sites of the typical electrochemical energy storage candidates in operational conditions is elucidated. Regarding electrochemical energy storage mechanisms in their respective working environments, the unknown valence states and reversible/irreversible nature of elements, local hybridization, delocalized d-electrons spin states, participation of coordination shells, disorder, and faradaic/non-faradaic behavior are thoroughly discussed. Finally, the future direction of in situ XAS analysis combined with spatial chemical mapping from operando scanning transmission x-ray microscopy and other emerging characterization techniques is presented and discussed.
Article
This review categorizes subnanometer pores/channels (SNPCs) from structural perspective and demonstrates electrochemical couplings in SNPCs for batteries while proposing corresponding challenges and future research directions.
Article
Full-text available
Carbon capture and utilization technology has been studied for its practical ability to reduce CO 2 emissions and enable economical chemical production. The main challenge of this technology is that a large amount of thermal energy must be provided to supply high-purity CO 2 and purify the product. Herein, we propose a new concept called reaction swing absorption, which produces synthesis gas (syngas) with net-zero CO 2 emission through direct electrochemical CO 2 reduction in a newly proposed amine solution, triethylamine. Experimental investigations show high CO 2 absorption rates (>84%) of triethylamine from low CO 2 concentrated flue gas. In addition, the CO Faradaic efficiency in a triethylamine supplied membrane electrode assembly electrolyzer is approximately 30% (@−200 mA cm ⁻² ), twice higher than those in conventional alkanolamine solvents. Based on the experimental results and rigorous process modeling, we reveal that reaction swing absorption produces high pressure syngas at a reasonable cost with negligible CO 2 emissions. This system provides a fundamental solution for the CO 2 crossover and low system stability of electrochemical CO 2 reduction.
Article
Full-text available
X-ray spectroptychography is an emerging method for the chemical microanalysis of advanced nanomaterials such as catalysts and batteries. This method builds upon established synchrotron X-ray microscopy and spectromicroscopy techniques with added spatial resolution from ptychography, an algorithmic imaging technique. This minireview will introduce the technique of X-ray spectroptychography, where ptychography is performed with variable photon energy, and discuss recent results and prospects for this method.
Article
Full-text available
Hydrogen generation through electrocatalytic splitting of water, i.e., hydrogen evolution reaction (HER), is an attractive method of converting the electricity generated from renewable sources into chemical energy stored in hydrogen molecules. A wide variety of materials have been studied in an effort to develop efficient and cost-effective electrocatalysts that can replace the traditional platinum/carbon catalyst. One family of functional materials that holds promise for this application is perovskite oxides. This mini-review discusses some of the progress made in the development of HER electrocatalysts based on perovskite oxides in the past decade. Given the diverse range of possible compositions of perovskite oxides, various studies have focused on compositional modifications to develop single-phase catalysts, whereas others have investigated heterostructures and composites that take advantage of synergistic interactions of different compounds with perovskite oxides. The recent advances indicate that this family of materials have great potential for utilization in HER electrocatalysis.
Article
Full-text available
Ptychography is a coherent diffraction imaging technique that measures diffraction patterns at many overlapping points on a sample and then uses an algorithm to reconstruct amplitude and phase images of the object and probe. Here, we report imaging, spectroscopy and linear dichroism ptychographic measurements at the carbon K-edge. This progress was achieved with a new generation of scientific Complementary Metal Oxide Semiconductor (sCMOS) X-ray cameras with an uncoated image sensor which has fast image transfer and high quantum efficiency at the carbon K-edge. Reconstructed amplitude and phase contrast images, C 1s spectral stacks, and X-ray linear dichroism of carbon nanotubes at the carbon K-edge were measured with ptychography. Ptychography and conventional Scanning Transmission X-ray Microscopy (STXM) are compared using results acquired from the same area. Relative to STXM, ptychography provides both improved spatial resolution and improved image quality. We used defocus ptychography, with an X-ray beam spot size of 1.0 micron, in order to reduce radiation damage and carbon deposition. Comparable spatial resolution was achieved to that of ptychography performed with a focused beam. Ptycho-graphy at the carbon K-edge offers unique opportunities to perform high resolution spec-tromicroscopy on organic materials important in medicine, biology, environmental science and energy materials.
Article
Full-text available
Transmission X-ray microscopy (TXM), which can provide morphological and chemical structural information inside of battery component materials at tens of nanometer scale, has become a powerful tool in battery research. This article presents a short review of the TXM, including its instrumentation, battery research applications, and the practical sample preparation and data analysis in the TXM applications. A brief discussion on the challenges and opportunities in the TXM applications is presented at the end.
Article
Full-text available
As a consequence of the depletion of fossil fuels and an increasing population, the global energy crisis has driven researchers to explore innovative energy storage and conversion (ESC) devices, such as fuel cells, electrolyzers and chemical looping systems. In order to enhance the energy conversion efficiency of these electrochemical devices, high performance and stable electrocatalysts are essential to accelerate the sluggish electrochemical kinetics, e.g. oxygen reduction reaction (ORR), oxygen evolution reaction (OER), hydrogen evolution reaction (HER) and redox reaction. In recent years, as cost-effective and high-efficient catalysts, perovskite oxides have attracted much attention. In addition, the potential of perovskite electrocatalysts may be further boosted due to their flexible composition and tunable electronic structures. This review provides the readers with a comprehensive understanding and updated information of improvements towards the electrocatalytic performances of perovskite oxides. It will focus on research papers regarding low to intermediate temperature electrochemical devices, e.g., water splitting, fuel cells, chemical looping technology and three-way catalysis (TWC) published over the last five years. Various design strategies for optimizing the conductivity and catalytic activity of perovskite are discussed in detail. In the end, this review discusses challenges for the future researches in regard to perovskite based electrocatalysts.
Article
Full-text available
Transition metal (oxy)hydroxides are promising electrocatalysts for the oxygen evolution reaction1–3. The properties of these materials evolve dynamically and heterogeneously⁴ with applied voltage through ion insertion redox reactions, converting materials that are inactive under open circuit conditions into active electrocatalysts during operation⁵. The catalytic state is thus inherently far from equilibrium, which complicates its direct observation. Here, using a suite of correlative operando scanning probe and X-ray microscopy techniques, we establish a link between the oxygen evolution activity and the local operational chemical, physical and electronic nanoscale structure of single-crystalline β-Co(OH)2 platelet particles. At pre-catalytic voltages, the particles swell to form an α-CoO2H1.5·0.5H2O-like structure—produced through hydroxide intercalation—in which the oxidation state of cobalt is +2.5. Upon increasing the voltage to drive oxygen evolution, interlayer water and protons de-intercalate to form contracted β-CoOOH particles that contain Co³⁺ species. Although these transformations manifest heterogeneously through the bulk of the particles, the electrochemical current is primarily restricted to their edge facets. The observed Tafel behaviour is correlated with the local concentration of Co³⁺ at these reactive edge sites, demonstrating the link between bulk ion-insertion and surface catalytic activity.
Article
Full-text available
The active particle at the electrode of Li‐ion batteries is surrounded by other particles and binders. During the lithiation/delithiation process, these surrounding materials mechanically constrain expansion and contraction of the particle. Since electrochemical and mechanical responses mutually influence each other, the constraining condition can finally affect cell performance. In this paper, we investigate the mechanical and electrochemical responses at the particle and cell levels with consideration of the coupling effect of electrochemistry and mechanics. To study the effect of mechanical constraints on cell performance, we carry out numerical simulations with a particle network electrode model, where binders connect the multi‐sized particles that are enclosed in a binder layer. The simulations show that the constraint from neighboring particles generates nonsymmetric lithium concentration and stress distribution. Stress‐potential coupling in the battery model reduces the stress level by a maximum of 8.4% in the small particles and 30% in the large particles. However, due to the constraint of the binder and connected particles, the reduction in stress is not uniform. The maximum stress difference between the interacting point and the free surface is 13.6%. Besides, the results depict that considering the stress‐potential coupling effect results in increased discharge capacity of the cell by 13.4%. Our study suggests that to design robust electrodes, understanding the mechanical‐electro‐chemo coupled behavior is essential. Highlights • We establish a fully coupled chemo‐mechanical multiparticle model. • Stress‐potential coupling alleviates concentration and stress gradients inside active particles. • Particle‐particle and particle‐binder interactions cause stress inhomogeneity. • Increasing binder modulus enhances stress nonuniformity.
Article
Full-text available
Layered oxides widely used as lithium-ion battery electrodes are designed to be cycled under conditions that avoid phase transitions. Although the desired single-phase composition ranges are well established near equilibrium, operando diffraction studies on many-particle porous electrodes have suggested phase separation during delithiation. Notably, the separation is not always observed, and never during lithiation. These anomalies have been attributed to irreversible processes during the first delithiation or reversible concentration-dependent diffusion. However, these explanations are not consistent with all experimental observations such as rate and path dependencies and particle-by-particle lithium concentration changes. Here, we show that the apparent phase separation is a dynamical artefact occurring in a many-particle system driven by autocatalytic electrochemical reactions, that is, an interfacial exchange current that increases with the extent of delithiation. We experimentally validate this population-dynamics model using the single-phase material Lix(Ni1/3Mn1/3Co1/3)O2 (0.5 < x < 1) and demonstrate generality with other transition-metal compositions. Operando diffraction and nanoscale oxidation-state mapping unambiguously prove that this fictitious phase separation is a repeatable non-equilibrium effect. We quantitatively confirm the theory with multiple-datastream-driven model extraction. More generally, our study experimentally demonstrates the control of ensemble stability by electro-autocatalysis, highlighting the importance of population dynamics in battery electrodes (even non-phase-separating ones).
Article
Full-text available
One hindrance to the development of fuel cells and electrolyzers are the oxygen electrodes, which suffer from high overpotentials and slow kinetics. Perovskite oxides have been shown to be promising oxygen electrode catalysts because of their low cost, flexibility, and tailorable properties. In order to improve perovskite catalysts for the oxygen reduction (ORR) and oxygen evolution (OER) reactions, a better understanding of their reaction mechanisms is needed. This Perspective aims to inform researchers of the current proposed reaction mechanisms for ORR and OER on perovskites and perovskite/carbon composites in order to guide future catalyst development. Additionally, important experimental practices will be recommended. A recent development for OER is the lattice oxygen evolution reaction, which is a possible addition to the conventional four consecutive proton-coupled electron transfer mechanism. Carbon additives are consistently added to perovskites to enhance conductivity and ORR/OER activity. However, carbon plays an active role in ORR, and there is evidence of a synergistic relationship between perovskite and carbon in perovskite/carbon composites.
Article
Full-text available
In-depth knowledge of electrode processes is crucial for determining the electrochemical performance of lithium-ion batteries (LIBs). In particular, the conduction mechanisms of charged species in the electrodes, such as lithium ions (Li ) and electrons, are directly correlated with the performance of the battery because the overall reaction is dependent on the charge⁺ transport behavior in the electrodes. Therefore, it is necessary to understand the different electrochemical processes occur-ring in electrodes in order to elucidate the charge conduction phenomenon. Thus, it is essential to conduct fundamental studies on electrochemical processes to resolve the technical challenges and issues arising during the ionic and electronic conduction. Furthermore, it is also necessary to understand the transport of charged species as well as the predominant factors affecting their transport in electrodes. Based on such in-depth studies, potential approaches can be introduced to enhance the mobility of charged entities, thereby achieving superior battery performances. A clear understanding of the conduction mechanism inside electrodes can help overcome challenges associated with the rapid movement of charged species and provide a practical guideline for the development of advanced materials suitable for high-performance LIBs.
Article
Full-text available
The 3D chemical structure (4D spectromicroscopy) of nanoporous Al2O3 aerogels coated with ZnO by atomic layer deposition (ALD) was studied by multienergy scanning transmission X-ray microscopy. These materials are representative of a class of designer catalysts in which the nanoporous support is prepared separately from the active catalyst material, which is subsequently introduced by ALD, thereby allowing independent optimization of the morphology, chemistry, and spatial distribution of the support and catalyst. The samples studied were prepared by Ga ion and Xe plasma focused ion beam (FIB) milling as well as drop casting from water suspension. Zn L and Al K edge spectra of six samples with three different ZnO loadings were measured to investigate how loading and different sample preparation methods affect the 3D distribution of the ZnO and Al2O3. Scanning transmission X-ray microscopy (STXM) and ptychographic imaging at two energies each at the Zn L3 and Al K edge were measured. The ptychography data were analyzed by using the SHARP reconstruction code to generate quantitative 2D chemical maps of the Al2O3 and the ZnO. The STXM and ptychography maps were then measured at a sequence of tilt angles, covering up to 160° of rotation. The 3D structure of the ZnO and Al2O3 was derived from the tilt series data by tomographic reconstruction using a compressed sensing algorithm. A two-dimensional spatial resolution (half-period) of 6 nm, measured by Fourier ring correlation, and a 3D spatial resolution (half-period) of 9 nm, measured by Fourier shell correlation, were achieved when using the COSMIC beamline at the Advanced Light Source (ALS). The results show that for all of the ZnO loadings studied there is nonuniform coverage of the ZnO on the Al2O3 aerogel framework. In addition, we found that both FIB methods create sample artifacts, although the distortion was less with Xe plasma than Ga ion FIB.
Article
Full-text available
The analysis of chemical states and morphology in nanomaterials is central to many areas of science. We address this need with an ultrahigh-resolution scanning transmission soft x-ray microscope. Our instrument provides multiple analysis tools in a compact assembly and can achieve few-nanometer spatial resolution and high chemical sensitivity via x-ray ptychography and conventional scanning microscopy. A novel scanning mechanism, coupled to advanced x-ray detectors, a high-brightness x-ray source, and high-performance computing for analysis provide a revolutionary step forward in terms of imaging speed and resolution. We present x-ray microscopy with 8-nm full-period spatial resolution and use this capability in conjunction with operando sample environments and cryogenic imaging, which are now routinely available. Our multimodal approach will find wide use across many fields of science and facilitate correlative analysis of materials with other types of probes.
Article
Full-text available
Electrolyte-filled subnanometre pores exhibit exciting physics and play an increasingly important role in science and technology. In supercapacitors, for instance, ultranarrow pores provide excellent capacitive characteristics. However, ions experience difficulties in entering and leaving such pores, which slows down charging and discharging processes. In an earlier work we showed for a simple model that a slow voltage sweep charges ultranarrow pores quicker than an abrupt voltage step. A slowly applied voltage avoids ionic clogging and co-ion trapping—a problem known to occur when the applied potential is varied too quickly—causing sluggish dynamics. Herein, we verify this finding experimentally. Guided by theoretical considerations, we also develop a non-linear voltage sweep and demonstrate, with molecular dynamics simulations, that it can charge a nanopore even faster than the corresponding optimized linear sweep. For discharging we find, with simulations and in experiments, that if we reverse the applied potential and then sweep it to zero, the pores lose their charge much quicker than they do for a short-circuited discharge over their internal resistance. Our findings open up opportunities to greatly accelerate charging and discharging of subnanometre pores without compromising the capacitive characteristics, improving their importance for energy storage, capacitive deionization, and electrochemical heat harvesting.
Article
Full-text available
During the last decades, X-ray absorption spectroscopy (XAS) has become an indispensable method for probing the structure and composition of heterogeneous catalysts, revealing the nature of the active sites and establishing links between structural motifs in a catalyst, local electronic structure, and catalytic properties. Here we discuss the fundamental principles of the XAS method and describe the progress in the instrumentation and data analysis approaches undertaken for deciphering X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectra. Recent usages of XAS in the field of heterogeneous catalysis, with emphasis on examples concerning electrocatalysis, will be presented. The latter is a rapidly developing field with immense industrial applications but also unique challenges in terms of the experimental characterization restrictions and advanced modeling approaches required. This review will highlight the new insight that can be gained with XAS on complex real-world electrocatalysts including their working mechanisms and the dynamic processes taking place in the course of a chemical reaction. More specifically, we will discuss applications of in situ and operando XAS to probe the catalyst’s interactions with the environment (support, electrolyte, ligands, adsorbates, reaction products, and intermediates) and its structural, chemical, and electronic transformations as it adapts to the reaction conditions.
Article
Full-text available
Herein, we report the synthesis of a -Al2O3-supported NiCo catalyst for dry methane reforming (DMR) and study the catalyst using in-situ Scanning Transmission X-Ray Microscopy (STXM) during the reduction (activation step) and under reaction conditions. During the reduction process, the NiCo alloy particles undergo elemental segregation with Co migrating toward the center of the catalyst particles and Ni migrating to the outer surfaces. Under DMR conditions, the segregated structure is maintained, thus hinting at the importance of this structure to optimal catalytic function. Finally, the formation of Ni-rich branches on the surface of the particles is observed during DMR, suggesting that the loss of Ni from the outer-shell may play a role in the reduced stability and hence catalyst deactivation. These findings provide insight into the morphological and electronic structural changes that occur in a NiCo based catalyst during DMR. Further, this study emphasizes the need to study catalysts under operating conditions in order to elucidate material dynamics during the reaction.
Article
Full-text available
Platinum nanocatalysts play critical roles in CO oxidation, an important catalytic conversion process. As the catalyst size decreases, the influence of the support material on catalysis increases which can alter the chemical states of Pt atoms in contact with the support. Herein, we demonstrate that under-coordinated Pt atoms at the edges of the first cluster layer are rendered cationic by direct contact with the Al2O3 support, which affects the overall CO oxidation activity. The ratio of neutral to cationic Pt atoms in the Pt nanocluster is strongly correlated with the CO oxidation activity, but no correlation exists with the total surface area of surface-exposed Pt atoms. The low oxygen affinity of cationic Pt atoms explains this counterintuitive result. Using this relationship and our modified bond-additivity method, which only requires the catalyst–support bond energy as input, we successfully predict the CO oxidation activities of various sized Pt clusters on TiO2.
Article
Full-text available
Highly reactive dense Pt single-atoms stabilized on an oxide support can resolve a grand challenge in the economic use of Pt in catalysis. The maximized number density of reaction sites provided by dense Pt single-atoms guarantees the improved catalytic performance of Pt combined with high efficiency. By manipulating the chemical nature of multi-component interfaces, we synthesized CO-tolerant dense Pt single-atoms highly reactive for the CO oxidation reaction, which governs the key steps for chemical energy conversion and emission control. The addition of 1 wt% of Ce to TiO2 support particles creates a CeOx–TiO2 interface that stabilizes Pt single-atoms by strong electronic interactions. Dense Pt single-atoms formed on CeOx/TiO2 oxides exhibit 15.1 times greater specific mass activity toward CO oxidation at 140 °C compared with a bare Pt/TiO2 catalyst. We elaborate how the CeOx–TiO2 interfaces activate the interface-mediated Mars–van Krevelen mechanism of CO oxidation and protect Pt single-atoms from CO-poisoning. Through a comprehensive interpretation of the formation and activation of dense Pt single-atoms using operando X-ray absorption spectroscopy, density functional theory calculations, and experimental catalyst performance tests, we provide a key that enables the catalytic performance of noble metal single-atom catalysts to be optimized by atomic-scale tuning of the metal–support interface.
Article
Full-text available
Noble metal (Pt, Ru, and Ir)-based electrocatalysts are currently considered the most active materials for the hydrogen evolution reaction (HER). Although they have been associated with high cost, easy agglomeration, and poor stability during the HER reaction, recent efforts to intentionally tailor noble-metal-based catalysts have led to promising improvements, with lower cost and superior activity, which are critical to achieving large-scale production of pure hydrogen. In this mini-review, we focus on the recent advances in noble-metal-based HER electrocatalysts. In particular, the synthesis strategies to enhance cost-effectiveness and the catalytic activity for HER are highlighted.
Article
Full-text available
Electrolytes are the one of the vital constituents of electrochemical energy storage devices and their physical and chemical properties play important role in these devices’ performance, including capacity, power density, rate performance, cyclability and safety. This article reviews the current state of understanding of the electrode-electrolyte interaction in supercapacitors and battery-supercapacitor hybrids devices. The article discuss factors that affecting the overall performance of the devices such as the ionic conductivity, mobility, diffusion coefficient, radius of bare and hydrated spheres, ion solvation, viscosity, dielectric constant, electrochemical stability, thermal stability and dispersion interaction. The requirements needed to design better electrolytes and the challenges that still needs to be addressed for building better supercapacitive devices for the competitive energy storage market have also been highlighted.
Article
Full-text available
The use of renewable energy resources such as solar, wind and biomass will not diminish their availability. Sunlight being a constant source of energy is used to meet the ever-increasing energy need. This review discusses the world's energy needs, renewable energy technologies for domestic use, and highlights public opinions on renewable energy. A systematic review of the literature was conducted between the years 2009 to 2018. During this process, more than 300 articles were classified and 42 papers were filtered for critical review. The literature analysis showed that despite serious efforts at all levels to reduce reliance on fossil fuels by promoting renewable energy as its alternative, fossil fuels continue to contribute 73.5% to the worldwide electricity production in 2017. Conversely, renewable sources contributed only 26.5%. Furthermore, this study highlights that lack of public awareness is a major barrier to the acceptance of renewable energy technologies. The results of this study show that worldwide energy crises can be managed by integrating renewable energy sources in the power generation. Moreover, in order to facilitate the development of renewable energy technologies, this systematic review has highlighted the importance of public opinion and performed a real-time analysis of public tweets. This example of tweet analysis is a relatively novel initiative in a review study that will seek to direct the attention of future researchers and policymakers towards public opinion and recommend the implications to both academia and industries.
Article
Full-text available
Heterogeneous single-atom catalysts involve isolated metal atoms anchored to a support, displaying high catalytic performance and stability in many important chemical reactions. We present a general theoretical framework to establish the thermodynamic stability of metal single atoms and metal nano-particles on a support in the presence of adsorbates. As a case study, we establish for Pt-CeO2 the CO partial pressure and temperature range within which Pt single atoms are more stable than Pt nanoparti-cles. Density functional theory and kinetic Monte-Carlo simulations demonstrate that Pt atoms doped into the CeO2 surface exhibit a very high CO oxidation activity and thermodynamic stability in comparison to models involving Pt single atoms on terraces and steps of CeO2. An intermediate CO adsorption strength is important to explain a high activity. Our work provides a systematic strategy to evaluate the stability and reactivity of single-atoms on a support.
Article
Full-text available
Surface-supported isolated atoms in single-atom catalysts (SACs) are usually stabilized by diverse defects. The fabrication of high-metal-loading and thermally stable SACs remains a formidable challenge due to the difficulty of creating high densities of underpinning stable defects. Here we report that isolated Pt atoms can be stabilized through a strong covalent metal-support interaction (CMSI) that is not associated with support defects, yielding a high-loading and thermally stable SAC by trapping either the already deposited Pt atoms or the PtO2 units vaporized from nanoparticles during high-temperature calcination. Experimental and computational modeling studies reveal that iron oxide reducibility is crucial to anchor isolated Pt atoms. The resulting high concentrations of single atoms enable specific activities far exceeding those of conventional nanoparticle catalysts. This non defect-stabilization strategy can be extended to non-reducible supports by simply doping with iron oxide, thus paving a new way for constructing high-loading SACs for diverse industrially important catalytic reactions.
Article
Full-text available
The oxygen evolution and reduction reactions are two extremely important reactions in terms of energy applications. Currently, the Oxygen Evolution Reaction (OER) hinders the efficient running of electrolyzer devices which convert water into molecular H2. This H2 can subsequently be used in a H2/O2 fuel cell for the renewable generation of electricity with only H2O as a by-product. However, this fuel cell process is not economy feasible due to the sluggish kinetics of the Oxygen Reduction Reaction (ORR) at the device cathode, even with expensive state-of-the-art electrocatalytic materials. As of late, the amount of interest in the OER and ORR, from research laboratories from all over the globe, has risen rapidly in order to find cheap and efficient catalysts to replace the expensive platinum based catalysts currently used in the two aforementioned energy conversion/generation technologies. Layered transition metal oxides, based on the cheap transition metal oxides Mn, Co, Ni and Fe have been reported as viable catalysts for the OER and ORR. Layered structures have an added advantage over non-layered materials as the surface area can be increase by means of exfoliation, with potential for tailoring electrocatalytic activity. It has been shown that the fabrication process and post-synthetic treatments, e.g. anion exchange or exfoliation, of these materials can alter the catalytic activity of these materials. Here we summarise various fabrication methods and modifications utilised in literature to tailor the performance of layered transition metal and hydroxide based catalysts for the ORR and OER toward that of the state-of-the-art materials for these technologies.
Article
Full-text available
Efficient catalysts for the anodic oxygen evolution reaction (OER) are critical for electrochemical H2 production. Their design requires structural knowledge of their catalytically active sites and state. Here, we track the atomic-scale structural evolution of well-defined CoOx(OH)y compounds into their catalytically active state during electrocatalytic operation through operando and surface-sensitive X-ray spectroscopy and surface voltammetry, supported by theoretical calculations. We find clear voltammetric evidence that electrochemically reducible near-surface Co³⁺–O sites play an organizing role for high OER activity. These sites invariably emerge independent of initial metal valency and coordination under catalytic OER conditions. Combining experiments and theory reveals the unified chemical structure motif as µ2-OH-bridged Co2+/3+ ion clusters formed on all three-dimensional cross-linked and layered CoOx(OH)y precursors and present in an oxidized form during the OER, as shown by operando X-ray spectroscopy. Together, the spectroscopic and electrochemical fingerprints offer a unified picture of our molecular understanding of the structure of catalytically active metal oxide OER sites. © 2018, The Author(s), under exclusive licence to Springer Nature Limited.
Article
Full-text available
We demonstrate electrically detected X-ray absorption fine structure spectroscopy (XAFS) using a 65 nm-diameter beam in single n+-i-n+ doped nanowire devices. Spatial scans show a peak of the X-ray beam induced current (XBIC) signal in the middle segment of the nanowire. The XBIC and the X-ray fluorescence (XRF) signals were detected simultaneously as a function of the excitation energy near the Ga K absorption edge at 10.37 keV. The spectra show similar oscillations around the edge, which shows that the XBIC is limited by the primary absorption. Our results reveal the feasibility of the XBIC detection mode for the XAFS investigation in nanostructured devices.
Article
Full-text available
Lithium-ion battery (LIB) technology is the most attractive technology for energy storage systems in today’s market. However, further improvements and optimizations are still required to solve challenges such as energy density, cycle life, and safety. Addressing these challenges in LIBs requires a fundamental understanding of the reaction mechanisms in various physical/chemical processes during LIB operation. Advanced in situ/operando synchrotron-based X-ray characterization techniques are powerful tools for providing valuable information about the complicated reaction mechanisms in LIBs. In this review, several state-of-the-art in situ/operando synchrotron-based X-ray techniques and their combination with other characterization tools for battery research are introduced. Various in situ cell configurations and practical operating tips for cell design and experimental set-ups are also discussed.
Article
Full-text available
The reduction of carbon dioxide to renewable fuels and feedstocks offers opportunities for large-scale, long-term energy storage. The synthesis of efficient CO2 reduction electrocatalysts with high C2:C1 selectivity remains a field of intense interest. Here we present electro-redeposition, the dissolution and redeposition of copper from a sol–gel, to enhance copper catalysts in terms of their morphology, oxidation state and consequent performance. We utilized in situ soft X-ray absorption spectroscopy to track the oxidation state of copper under CO2 reduction conditions with time resolution. The sol–gel material slows the electrochemical reduction of copper, enabling control over nanoscale morphology and the stabilization of Cu+ at negative potentials. CO2 reduction experiments, in situ X-ray spectroscopy and density functional theory simulations revealed the beneficial interplay between sharp morphologies and Cu+ oxidation state. The catalyst exhibits a partial ethylene current density of 160 mA cm–2 (−1.0 V versus reversible hydrogen electrode) and an ethylene/methane ratio of 200. Catalysts that can selectively reduce carbon dioxide to C2+ products are attractive for the generation of more complex and useful chemicals. Here, an electro-redeposited copper catalyst is shown to provide excellent selectivity and high current density for ethylene formation. Detailed characterization and theory link the performance to the catalyst morphology.
Article
This investigation, which is motivated by promising pseudocapacitive properties of Mn3O4 for energy storage in cathodes of supercapacitors, addresses the need to understand both the activation and the charge storage mechanisms of Mn3O4 electrodes. Specific activation protocols are shown to result in significant capacitance increase during cycling. For the first time scanning transmission X-ray microscopy (STXM) is used for analysis of Mn3O4 activation. STXM analyses at the Mn 2p and O 1s edges provide chemical mapping of different oxidation states with high spatial resolution. Mn3O4-carbon nanotube composite electrodes with commercially important high active mass loading of 40 mg cm⁻² are prepared using quercetin dispersant. The catecholate type polyaromatic quercetin facilitates co-dispersion of Mn3O4 with carbon nanotubes and allows enhanced electrode performance at high active mass loadings. Cyclic voltammetry, electrochemical impedance spectroscopy and galvanostatic charge-discharge are used for the capacitance monitoring during electrode activation. Two strategies are used for electrode activation in Na2SO4 electrolyte: electrochemical cycling at different scan rates and continuous cycling at a fixed scan rate. The capacitance variations are linked to STXM observations, which show gradual oxidation of Mn3O4 to form MnO2 on the particle surface and in the bulk. The pseudocapacitive charge storage mechanism involves in situ oxidation of Mn3O4 and redox reactions of Mn⁴⁺/Mn³⁺ species on the particle surface and in the bulk.
Article
The increasing demands of electronic devices and electric transportation necessitate lithium-ion batteries with simultaneous high energy and power capabilities. However, rate capabilities are often limited in high-loading electrodes due to the lengthy and tortuous ion transport paths with their electrochemical behaviors governed by complicated electrode architectures still elusive. Here, we report the electrode-level tortuosity engineering design enabling improved charge storage kinetics in high-energy electrodes. Both high areal capacity and high-rate capability can be achieved beyond the practical level of mass loadings in electrodes with vertically oriented architectures. The electrochemical properties in electrodes with various architectures were quantitatively investigated through correlating the characteristic time with tortuosity. The lithium-ion transport kinetics regulated by electrode architectures was further studied via combining the three-dimensional electrode architecture visualization and simulation. The tortuosity-controlled charge storage kinetics revealed in this study can be extended to general electrode systems and provide useful design consideration for next-generation high-energy/power batteries.
Article
As the global energy shortage and environmental pollution worsen, the development of clean and renewable energy becomes more urgent. Electrochemical technology can achieve high-efficiency, low-cost clean energy conversion, mainly relying on highly efficient electrocatalysts, including noble metals such as Au, Pt, Pd. MXenes, an emerging class of two-dimensional materials, have been widely used in electrocatalysis due to their high conductivity, large specific surface area, and good hydrophilicity. In particular, MXenes and their derivatives have been widely used to support various noble metal nanomaterials. They have shown outstanding catalytic performances and good stability in the hydrogen evolution reaction, oxygen evolution reaction, oxygen reduction reaction, nitrogen reduction reaction, and methanol oxidation reaction. Herein, we review the strategies for the synthesis of noble metal MXene-based catalysts, emphasize the application of noble metal MXene-based catalysts in the field of electrocatalysis, and focus on the strategy of enhancing the electrocatalytic performance of noble metal MXene-based catalysts. Based on this, the limitations and future development of noble metal MXene-based catalysts in electrocatalysis are discussed.
Article
Lithium iron phosphate (LixFePO4), a cathode material used in rechargeable Li-ion batteries, phase separates upon de/lithiation under equilibrium. The interfacial structure and chemistry within these cathode materials affects Li-ion transport, and therefore battery performance. Correlative imaging of LixFePO4 was performed using four-dimensional scanning transmission electron microscopy (4D-STEM), scanning transmission X-ray microscopy (STXM), and X-ray ptychography in order to analyze the local structure and chemistry of the same particle set. Over 50,000 diffraction patterns from 10 particles provided measurements of both structure and chemistry at a nanoscale spatial resolution (16.6–49.5 nm) over wide (several micron) fields-of-view with statistical robustness. LixFePO4 particles at varying stages of delithiation were measured to examine the evolution of structure and chemistry as a function of delithiation. In lithiated and delithiated particles, local variations were observed in the degree of lithiation even while local lattice structures remained comparatively constant, and calculation of linear coefficients of chemical expansion suggest pinning of the lattice structures in these populations. Partially delithiated particles displayed broadly core–shell-like structures, however, with highly variable behavior both locally and per individual particle that exhibited distinctive intermediate regions at the interface between phases, and pockets within the lithiated core that correspond to FePO4 in structure and chemistry. The results provide insight into the LixFePO4 system, subtleties in the scope and applicability of Vegard’s law (linear lattice parameter-composition behavior) under local versus global measurements, and demonstrate a powerful new combination of experimental and analytical modalities for bridging the crucial gap between local and statistical characterization.
Article
Electrochemical ion insertion involves coupled ion–electron transfer reactions, transport of guest species and redox of the host. The hosts are typically anisotropic solids with 2D conduction planes but can also be materials with 1D or isotropic transport pathways. These insertion compounds have traditionally been studied in the context of energy storage but also find extensive applications in electrocatalysis, optoelectronics and computing. Recent developments in operando, ultrafast and high-resolution characterization methods, as well as accurate theoretical simulation methods, have led to a renaissance in the understanding of ion-insertion compounds. In this Review, we present a unified framework for understanding insertion compounds across timescales and length scales ranging from atomic to device levels. Using graphite, transition metal dichalcogenides, layered oxides, oxyhydroxides and olivines as examples, we explore commonalities in these materials in terms of point defects, interfacial reactions and phase transformations. We illustrate similarities in the operating principles of various ion-insertion devices, ranging from batteries and electrocatalysts to electrochromics and thermal transistors, with the goal of unifying research across disciplinary boundaries. Electrochemical ion insertion is rapidly emerging as a powerful materials design strategy. This Review discusses how ion insertion enables reversible transformation and switching of physico-chemical properties, the role of defects and interfacial reactions, and opportunities for ultrafast ionic control.
Article
Supercapacitors are presumed to be ideal for electrochemical energy storage high power applications because they are an intermediate between conventional capacitors and batteries. Various bio-derived activated carbon was venerated for enhanced supercapacitor application and the electrochemical performance of carbon-based materials in aqueous electrolytes was further enhanced to a greater extent with the incorporation of redox additives. In this work, the activated carbon obtained from beetroot is explored as the electrode material for supercapacitors with redox additives in electrolytes. The chemical functionalities and elements present in the prepared activated carbon were confirmed using FT-IR and XPS, respectively. The electrochemical energy storage behavior of the prepared activated carbon was analyzed with hydroquinone (HQ) as a redox additive. The specific capacity of the prepared electrode in 0.01 M HQ/H2SO4 at 3 A/g is 2589 C/g. The calculated power and energy density of the fabricated symmetric cell with HQ/H2SO4 at 3 A/g are 2356 W/kg and 36 Wh/ kg, which is superior to the mere 1 M H2SO4 electrolyte with the gravimetric power and energy density of 1800 W/kg and 13.5 Wh/kg, respectively.
Article
Total electron yield (TEY) imaging is an established scanning transmission X-ray microscopy (STXM) technique that gives varying contrast based on a sample’s geometry, elemental composition, and electrical conductivity. However, the TEY-STXM signal is determined solely by the electrons that the beam ejects from the sample. A related technique, X-ray beam-induced current (XBIC) imaging, is sensitive to electrons and holes independently, but requires electric fields in the sample. Here we report that multi-electrode devices can be wired to produce differential electron yield (DEY) contrast, which is also independently sensitive to electrons and holes, but does not require an electric field. Depending on whether the region illuminated by the focused STXM beam is better connected to one electrode or another, the DEY-STXM contrast changes sign. DEY-STXM images thus provide a vivid map of a device’s connectivity landscape, which can be key to understanding device function and failure. To demonstrate an application in the area of failure analysis, we image a 100 nm, lithographically-defined aluminum nanowire that has failed after being stressed with a large current density.
Article
The capability in spatially resolving the interactions between components in lithium (Li)-ion battery cathodes, especially correlating chemistry and electronic structure, is challenging but critical for a better understanding of complex degradation mechanisms for rational developments. X-ray spectro-ptychography and conventional synchrotron-based scanning transmission X-ray microscopy image stacks are the most powerful probes for studying the distribution and chemical state of cations in degraded Li-rich cathodes. Herein, we propose a chemical approach with a spatial resolution of around 5.6 nm to imaging degradation heterogeneities and interplay among components in degraded Li-rich cathodes. Through the chemical imaging reconstruction of the degraded Li-rich cathodes, fluorine (F) ions incorporated into the lattice during charging/discharging processes are proved and strongly correlate with the manganese (Mn) dissolution and oxygen loss within the secondary particles and impact the electronic structure. Otherwise, the electrode-electrolyte interphase component, scattered LiF particles (100-500 nm) along with the MnF2 layer, is also visualized between the primary particles inside the secondary particles of the degraded cathodes. The results provide direct visual evidence for the Li-rich cathode degradation mechanisms and demonstrate that the low-energy ptychography technique offers a superior approach for high-resolution battery material characterization.
Article
Unique ion transport behaviors in two-dimensional (2D) nanochannels have sparked strong interests in exploring 2D nanomaterials for supercapacitor applications, which rely much on the formation of electric double layers (EDLs) at the solid-liquid interface. However, there is still a crucial missing part on understanding how ions with different kinetic properties could affect the formation of EDLs. Here, we examine the real-time ion transport in the formation of EDLs by electrochemical quartz crystal microbalance (EQCM) and observe distinct charge transport behaviors between activated carbon with a tortuous pore structure and graphene films with 2D channel spacing (<2 nm). Using molecular dynamics (MD) simulations, we find that ions with a higher diffusion coefficient predominantly affect the EDLs formation, leading to the anion- or cation-dominated ion exchange process in the sub-2 nm 2D graphene channels. By expanding to different electrolytes, the kinetics-controlled ion transport mechanism is further confirmed by experimental observations and MD simulations. Such findings will bring new insights for improved electrochemical performance of 2D nanomaterials.
Article
The outbreak of the novel COVID-19 has dominated the world stage, and its consequences, both direct and indirect, are expected to prove to be even more pervasive over time. The COVID-19 pandemic has struck the renewable energy manufacturing facilities, supply chains, and companies and slowed down the transition to the sustainable energy world. The global decline in economy-driven demand could damage the positive trend of green and low-carbon energy progress. Although it is too early to judge how profound the negative effects of the pandemic on the global renewable and sustainable energy systems will be, a significant short-run contraction to the development of renewables is inevitable. Therefore, the energy and climate policies may require to be restructured based on the new circumstances. In this context, several beneficial stimuluses should be offered by the governments to persuade the private sectors and society to invest on renewables. Undoubtedly, intelligent policies could convert the menaces of COVID-19 to the great opportunities for renewables and ultimately the world’s sustainable energy scenario could return to its long-term trajectory toward green power generation and utilization over the next few years.
Article
Assessing the reaction pathway of multi-electron-transfer reactions is an essential yet difficult task for the rational design of electrocatalysts. In this work, we develop a heuristic approach that combines thermodynamic adsorption energetics calculated through density functional theory with microkinetic modeling using the steady state approximation to interpret the potential-dependent Tafel behavior of consecutive electrochemical reactions. In doing so, we introduce a kinetic framework for ab initio calculations that ensures self-consistent adsorption energetics based on kinetically limited adsorbate coverages. The approach is applied to experimental results on CoOx(OH)2-x single crystal electrocatalyst particles yielding coverage dependent mechanistic information and identification of the rate-limiting step with standard rate constants for the oxygen evolution reaction on the (11-20) surfaces of the β-Co(OH)2, β-CoOOH, and CoO2 bulk phases. This generalizable method enables catalyst benchmarking based on determining the active species involved and associated intrinsic reaction rate constants in consecutive multi-electron-transfer reactions.
Article
With the growing need for sustainable energy technologies, advanced characterization methods become more and more critical for optimizing energy materials and understanding their operation mechanisms. In this review, we focus on the synchrotron-based X-ray imaging technologies and the associated applications in gaining fundamental insights into the physical/chemical properties and reaction mechanisms of energy materials. We will discuss a few major X-ray imaging technologies, including X-ray projection imaging, transmission X-ray microscopy, scanning transmission X-ray microscopy, tender and soft X-ray imaging, and coherent diffraction imaging. Researchers can choose from various X-ray imaging techniques with different working principles based on research goals and sample specifications. With the X-ray imaging techniques, we can obtain the morphology, phase, lattice and strain information of energy materials in both 2D and 3D in an intuitive way. In addition, with the high-penetration X-rays and the high-brilliance synchrotron sources, operando/in-situ experiments can be designed to track the qualitative and quantitative changes of the samples during operation. We expect this review can broaden readers’ view on X-ray imaging techniques and inspire new ideas and possibilities in energy materials research.
Article
With promising activity and stability for the oxygen reduction reaction (ORR), transition metal nitrides are an interesting class of non-platinum group catalysts for polymer electrolyte membrane fuel cells. Here, we report an active thin-film nickel nitride catalyst synthesized through a reactive sputtering method. In rotating disk electrode testing in a 0.1 M HClO4 electrolyte, the crystalline nickel nitride film achieved high activity and selectivity to four-electron ORR. It also exhibited good stability during 10 and 40 h chronoamperometry measurements in acid and alkaline electrolyte, respectively. A combined experiment-theory approach, with detailed ex situ materials characterization and density functional theory calculations, provides insight into the structure of the catalyst and its surface during catalysis. Design strategies for activity and stability improvement through alloying and nanostructuring are discussed.
Article
As a high-voltage spinel, LiNi0.5Mn1.5O4 (LNMO) is a promising candidate for high energy density cathodes in lithium-ion batteries (LiBs). The material has not yet achieved any commercial success, as there remain problems with capacity fade after extended charge and discharge cycling. In order to enable improvements, it is necessary to understand the fundamental underlying processes in the material. In this experimental study, we present operando Raman measurements to investigate the potential-resolved structural evolution of ordered LNMO as a cathode material during the charging and discharging process. Using the method of Raman spectroscopy, only two phases can be unequivocally distinguished in the case of ordered LNMO, namely, LiNi0.5Mn1.5O4 and Ni0.5Mn1.5O4 (NMO). The half-delithiated phase, Li0.5Ni0.5Mn1.5O4, cannot be discriminated by using this spectroscopic method. The dynamics of the phase changes between LiNi0.5Mn1.5O4 and Ni0.5Mn1.5O4 differ for lithiation and delithiation. Long-term operando Raman measurements of half-cells prove that a decomposition of the solvent takes place and that the conductive salt LiPF6 is consumed, i.e., the concentration of PF6– is strongly decreasing. The solvent component ethylene carbonate (EC) is preferentially decomposed during the cycling process, and byproducts such as esters and alcohols can be detected.
Article
Ru with variable loadings (0.5-2 wt.%) in nanoparticle form, were deposited over ceria, magnesia modified mesoporous silica by single step synthesis followed by urea hydrolysis of the metal precursor. The obtained materials were tested for methane reforming with CO 2 (DRM). The catalysts were substantially exposed to different characterisation techniques both before and after the reaction to get an insight into the structure-activity relationship. Characterisation results and activity study confirmed, homogeneous dispersion of nano-sized ruthenium particles over a high surface area of modified silica possess strong metal support interaction, which restricts metal sintering and enables coke free CO 2 reforming of methane (DRM). Introduction of ceria and magnesia strongly enhanced the surface oxygen storage capacity (OSC) that readily oxidise the deposited coke over the active metal during the activity analysis. An in-detailed study was carried out to evaluate the effect of reaction parameters and long-term activity analysis on the material. The catalyst systems with lower Ru loading performs a better activity in comparison to higher loading. At a higher Ruthenium loading, the metal particle size gets increase and remains mainly over the surface rather than in the pores which enhances the rate of coke deposition and hence decreases the catalyst activity.
Article
In recent years, development to increase the performance of Pt-based catalysts and reduce the cost has received significant attention. Among them, the preparation of Pt-based catalysts with high atom utilization efficiency can induce more active sites between the Pt atoms and participating molecules, resulting in improved mass activity. In addition, the combination of high atom utilization efficiency with well-controlled surface structure and composition could boost the mass activity for Pt-based catalysts. This review describes recent progress on the design and synthesis of Pt-based catalysts with high atom utilization efficiency and their enhanced catalytic performance in electrochemical catalytic reactions. The significance for the fabrication of nanostructures and single atom catalysts with high atom utilization will be presented in the introduction section. We discuss the synthetic strategies according to two routes: (1) the rational design of Pt nanostructures, including porous, nanowire, core-shell and hollow structures; (2) preparation of Pt single atom catalysts and the stabilization of single atoms. Additionally, we discuss the superior electro-catalytic applications of Pt-based catalysts with high atom utilization efficiency. These recent advancements in rational design of Pt-based catalysts offers numerous cases for potential industrialized catalysts with high mass activity and reduced cost in the future.
Article
Single-atom catalysis has arguably become the most active new frontier in heterogeneous catalysis. Aided by recent advances in practical synthetic methodologies, characterization techniques and computational modelling, we now have a large number of single-atom catalysts (SACs) that exhibit distinctive performances for a wide variety of chemical reactions. This Perspective summarizes recent experimental and computational efforts aimed at understanding the bonding in SACs and how this relates to catalytic performance. The examples described here illustrate the utility of SACs in a broad scope of industrially important reactions and highlight the advantages these catalysts have over those presently used. SACs have well-defined active centres, such that unique opportunities exist for the rational design of new catalysts with high activities, selectivities and stabilities. Indeed, given a certain practical application, we can often design a suitable SAC; thus, the field has developed very rapidly and afforded promising catalyst leads. Moreover, the control we have over certain SAC structures paves the way for designing base metal catalysts with the activities of noble metal catalysts. It appears that we are entering a new era of heterogeneous catalysis in which we have control over well-dispersed single-atom active sites whose properties we can readily tune.
Article
Dry reforming of methane was studied over Ni,Y-promoted KIT-6 ordered mesoporous silicas, prepared by incipient impregnation (nickel content 12 wt%, yttrium content of 4 wt%, 8 wt% or 12 wt%). The catalysts were characterized by XRF, FT-IR, TGA/DSC-MS, N2-adsorption, TEM, HRTEM, XRD and TPR-H2. The promotion with 8 wt% Y (Y/Si = 0.05) resulted in the highest activity and H2/CO molar ratio closer to the stoichiometric value at temperatures from 600 to 750 °C. The characterization results of the yttrium promoted materials showed higher reducibility of the bulk NiO, bigger Ni crystallite size after reduction and DRM test, and better dispersion of nickel in the channels of the KIT-6 support. Additionally, larger Ni particles were observed on the external surface of the support, which may be related to catalytic selectivity towards carbon forming reactions. Upon dry methane reforming the segregated phases of Niº, Y2O3, and possibly Y2Si2O7 were registered. No presence of a Ni,Y alloy was observed.
Article
In operando tracking of the complex phase transformation pathway in battery materials and correlating morphology change, chemical composition, and phase structure with electrochemical performance is critical in exploring advanced battery systems with high energy density and safety. Emerging synchrotron X-ray imaging techniques with high spatial, temporal, and chemical resolution provides unique tools to elucidate the underlying mechanisms in battery electrochemical reactions. Here, the recent significant progress in a number of rapidly growing synchrotron X-ray imaging techniques that have been applied in battery research under in operando conditions is summarized. The basic principle and in operando experimental setup of these X-ray imaging methods are briefly introduced. The unique capabilities of each X-ray technique and the critical achieved scientific insights in a variety of battery materials are discussed, with particular emphasis on how these in operando X-ray imaging techniques can advance fundamental understanding of the complex battery electrochemistry. Perspectives on the challenges and future trends in technology development for in operando X-ray imaging study are also briefly proposed.
Article
X-ray ptychographic microscopy combines the advantages of raster scanning X-ray microscopy with the more recently developed techniques of coherent diffraction imaging. It is limited neither by the fabricational challenges associated with X-ray optics nor by the requirements of isolated specimen preparation, and offers in principle wavelength-limited resolution, as well as stable access and solution to the phase problem. In this Review, we discuss the basic principles of X-ray ptychography and summarize the main milestones in the evolution of X-ray ptychographic microscopy and tomography over the past ten years, since its first demonstration with X-rays. We also highlight the potential for applications in the life and materials sciences, and discuss the latest advanced concepts and probable future developments.