Scientists explore complex metal-support interactions under redox conditions using our Climate system

Scientists explore complex metal-support interactions under redox conditions using our Climate system

Via the DENSsolutions Climate system, a team of scientists uncover the dynamic interplay between platinum nanoparticles and titania support under reaction conditions.

Original article by H. Frey, A. Beck, X. Huang, J.A. van Bokhoven and M. G. Willinger

For catalysts consisting of metal nanoparticles and an oxide support, understanding the synergistic reactions between the metal and support is paramount. In the case of reducible supports, the so-called strong metal-support interaction (SMSI) provides a means of tuning the chemisorption and catalytic properties of supported metal particles. SMSI involves the encapsulation of nanoparticles by a thin layer of partially reduced support material. The encapsulation is typically induced during high-temperature reductive “activation”, i.e., treatment in hydrogen. Notably, the direct imaging of this encapsulated state has mostly been achieved ex situ. Little is known about SMSI under catalytic working conditions, where the application of in situ electron microscopy is invaluable. While environmental transmission electron microscopy (TEM) is generally limited to chamber pressures of around 20 mbar, the DENSsolutions Climate Nano-Reactor can handle pressures 100 times higher, unlocking unprecedented research possibilities for the study of catalysts in their native environment.

In recent research performed at ScopeM, ETH Zurich, where our advanced Climate G+ system is installed, Hannes Frey, Arik Beck, Dr. Xing Huang, Prof. Dr. Jeroen Anton van Bokhoven and Prof. Dr. Marc Georg Willinger investigated the dynamic interplay between metal nanoparticles and oxide support under reaction conditions. More specifically, the scientists revealed the working state of a model catalyst, Pt–TiO, by directly observing the synergistic interactions related to SMSI between the platinum nanoparticles and titania support.

Switching to a redox-active H₂–O₂ mixture

Frey and his fellow collaborators first induced the classical SMSI state by heating the titania-supported platinum nanoparticles (NPs) in H₂. The nanoparticles were then transferred into an O₂ atmosphere via inert gas purging. Interestingly, this treatment resulted in the platinum NPs incurring a nonclassical oxidized SMSI state. After preparing the system, the researchers then exposed it to a redox active atmosphere through the addition of H₂ into the O₂ flow. With the Climate G+ system, the researchers were able to mix the gasses on the fly, while continuing with the high-resolution observation. 

In Figure 1 and Movie 1 below, the morphological change of the platinum NPs upon transition into the redox-active regime is presented. Increasing the partial pressure of H₂ in the Climate Nano-Reactor resulted in the gradual change in the encapsulated state of the Pt NPs. Moreover, this resulted in the ultimate disappearance of the overlayer as soon as the gas composition reached a set mixture of 60 mbar H₂ and 700 mbar O₂ after ~180 s. Once the overlayer was fully removed, the particle was observed to experience particle dynamics like restructuring and migration (shown in Figure 1E and 1F). 

Figure 1: Image series depicting the morphological changes observed when a titania-supported platinum nanoparticle in a nonclassical SMSI state is exposed to a redox-active atmosphere. The composition of the gas in the Nano-Reactor was gradually changed from 700 mbar O₂ to a mixture of 60 mbar H₂ plus 700 mbar O₂. t0 is the time at which the H₂ flow was turned on.

Movie 1: TEM Movie depicting the morphological changes observed when a titania-supported platinum nanoparticle in a nonclassical SMSI state is exposed to a redox-active atmosphere.

Particle and interfacial dynamics in the redox-active regime

The researchers then set out to explore the response of a collection of nanoparticles to reaction conditions. They discovered that the degree of structural dynamics and mobility differed greatly among the nanoparticles. Whereas some NPs remained static and stationary, others incurred structural fluctuations and migrated across the substrate surface. The researchers decided to follow three representative cases of nanoparticles with different orientations. In all cases, it is observed that the redox chemistry at the interface is the driving force for particle reconstruction and migration. In Figure 2 below, the respective structural dynamics of the three selected nanoparticles are presented.

Figure 2: (A–C) Pt NP oriented with (111) planes perpendicular to the interface. (G to J) Pt NP oriented with (111) planes parallel to the interface. (K to N) Pt NP that has its (111) planes inclined toward the interface. The blue shapes indicate the respective positions of the Pt NP in the previous frames.

Case 1

The first platinum NP that was considered was oriented with (111) planes perpendicular to the interface. In Movie 2 below, the recorded time series of this platinum NP at 600°C in an atmosphere containing 700 mbar O₂ plus 60 mbar H₂ is shown. Here, the NP developed pronounced structural dynamics that involved twin formation and shearing along (111) planes in an up-down motion, perpendicular to the interface. 

Movie 2: Recorded time series of the Pt NP presented in Figure 2A–C) at 600°C in an atmosphere containing 700 mbar O₂ plus 60 mbar H₂.

Case 2

The second Pt NP that was considered was oriented with (111) planes parallel to the interface. Movie 3 below shows the image series acquired for this nanoparticle, at 600°C in an atmosphere containing 700 mbar O₂ plus 60 mbar H₂. Here, a continuous step flow motion of the (111)-type facet in contact with the interface is observed.

Movie 3: Image series of the Pt NP presented in Figure 2G–J) at 600°C in an atmosphere containing 700 mbar O₂ plus 60 mbar H₂.

Case 3

The third platinum NP that was considered was oriented with (111) planes inclined towards the interface. Movie 4 below shows the image series acquired for this nanoparticle, at 600°C in an atmosphere containing 700 mbar O₂ plus 60 mbar H₂. In this case, the nanoparticle is observed to engage in redox chemistry-driven directional surface migration which is caused by restructuring at the interface.

Movie 4: Image series of the Pt NP presented in Figure 2K–N) at 600°C in an atmosphere containing 700 mbar O₂ plus 60 mbar H₂.

Retraction of H₂ and reformation of the oxidic SMSI overlayer

The next step for Frey and his fellow researchers was to switch the gas composition from a reactive feed back to a purely oxidizing environment by turning off the H₂ flow. This change in the gas composition resulted in the encapsulated state of the platinum NPs to be reestablished. In Figure 3 below, the morphological change of a Pt NP upon switching the gas from a redox-active environment to a purely oxidizing environment is presented. This switching of the gas composition led to the reformation of a classical particle overgrowth. It is seen that the nanoparticles first adopted a spherical morphology (Figure 3A–C). Then, as soon as H₂ was fully removed from the Nano-Reactor, the support material migrated onto the Pt NPs and the overlayer reformed (Figure 3D–F).

Figure 3: Image series depicting the morphological changes of a Pt NP observed when switching from a redox-active to purely oxidizing environment at 600°C. t0 is the time at which the H₂ flow was set to zero.

Conclusion

The aim of this paper was to reveal the working state of a catalyst via direct observation and to study possible synergistic interactions related to SMSI between metal nanoparticles and a reducible oxide support. The use of our Climate system enabled the researchers to capture in exceptional detail the dynamic and complex metal-support interactions of Pt–TiO₂ under reactive catalytic conditions. This is especially thanks to the advanced capacity of the system to handle pressures of up to 2 bar. 

A key finding of this paper is that the stable configurations of static Pt particles exhibiting encapsulation were observed to exist either in pure H₂ (the classical SMSI state) or in pure O₂ (the nonclassical SMSI state), but not in an environment where both gases were simultaneously present. Indeed, the exposure to the redox-active environment led to the removal of the overlayer and the subsequent emergence of pronounced particle dynamics. Moreover, the in situ observations show that the particle restructuring and migration behavior is dependent on the relative orientation of the particle on the support, and therefore the configuration of the interface. Overall, these findings advance our comprehension of SMSI-induced encapsulation of metal nanoparticles, which can in turn help us better tune the chemisorption and catalytic properties of catalysts. 

Hanglong Wu portrait

“The DENSsolutions Climate System allows us to reveal the so-far unseen: Looking at not only how the gas-phase is changed in the presence of a catalyst, but also studying how the interaction between gas-phase and catalyst leads to the emergence of catalytic function. Direct real-space observation is essential for our understanding of working catalysts and the development of new processes that are urgently needed in view of climate change and limited natural resources.”

Prof. Dr. Marc-Georg Willinger
Professor  |  Technical University of Munich

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Hydrogen vs. syngas: The battle of the reducing gases

Hydrogen vs. syngas: The battle of the reducing gases

Using our Climate system, scientists are able to unveil the most effective method to obtain sintering-resistant metallic cobalt nanoparticles at lower temperatures.

Original article by Ofentse A. Makgae, Tumelo N. Phaahlamohlaka, Benzhen Yao, Manfred E. Schuster, Thomas J. A. Slater, Peter P. Edwards, Neil J. Coville, Emanuela Liberti, and Angus I. Kirkland

The Fischer-Tropsch synthesis (FTS) is an established catalytic chemical process that occurs when carbon monoxide and hydrogen are converted into hydrocarbons. These hydrocarbons can be further developed into high-quality fuels, lubricants as well as raw materials for the production of plastics, rubbers and industrial chemicals. In FTS, supported cobalt catalysts are commonly used, which are active in their reduced metallic state. Many factors can affect the reduction process of cobalt oxide (Co₃3O₄) to metallic cobalt, including the reduction temperature, the composition of the reducing gas, the catalyst promoter, catalyst support and particle size. Despite this, little is known about the effect of using H₂ versus syngas (H₂ & CO) as the reducing agent on carbon-supported cobalt catalysts.

In recent research performed at the David Cockayne Centre for Electron Microscopy and the electron Physical Science Imaging Centre (ePSIC), a number of scientists from highly renowned universities and institutes, including University of Oxford, Johnson Matthey, Diamond Light Source and Cardiff University, investigate the atomic structure and valence state of cobalt nanocrystals on carbon supports under syngas versus hydrogen reduction. Specifically, using ex situ and in situ high-resolution aberration-corrected analytical electron microscopy with our Climate system, the researchers were able to explore the effect of H₂ versus syngas (H₂ & CO) on the reducibility of cobalt oxide nanoparticles supported on hollow carbon spheres. By uncovering highly effective methods to obtain sintering-resistant metallic Co nanoparticles at lower temperatures on hollow carbon sphere supports, this work provides valuable insights that can inform important industrial processes.

Synthesis and reduction of cobalt oxide

Dr. Ofenste Makgae and his fellow collaborators first synthesized the Co₃3O₄ nanoparticles before preparing the hollow carbon sphere supports (HCSs). The synthesized Co₃3O₄ nanoparticles were then loaded on the surface of the HCSs to prepare the precursor catalyst.

The researchers then initiated the reduction process of the Co₃3O₄ nanoparticles for both the ex situ and in situ analysis. For the ex situ analysis, the precursor was reduced via H₂ and syngas, respectively, at 350 °C at atmospheric pressure for 20 hours. For the in situ analysis, the catalysts were studied using the DENSsolutions Climate system in a probe-corrected JEOL ARM200F at ePSIC operated at 200 kV. Notably, the DENSsolutions Climate system is capable of real-time dynamic mixing of 3 gases over a temperature range from room temperature to 1000 °C. Via the Climate Gas Supply System, Dr. Makgae and his fellow collaborators were able to provide a 0.2mL/min flow of gas at a pressure of 500mbar for both the H₂ and syngas reduction. The temperature was ramped from room temperature to 150 °C under a N₂ flow to remove contaminants. Next, it was ramped to 350 °C at 10 °C/min under a reducing environment. Finally, the temperature was held at 350 °C for 2 hours for catalyst activation.

Ex situ post-reduction morphology analysis

The researchers then set out to analyze the overall morphology of the carbon support post reduction, to ensure that its integrity was not compromised during the reduction process. In Figure 1 below, (high-resolution) STEM-ADF images of the as-prepared Co₃3O₄ and ex situ syngas- and H₂-reduced HCS-supported nanoparticles are shown. The researchers were able to determine that the morphology of the HCSs remained intact after 20 hours of reduction at 350 °C.

Figure 1: STEM–ADF images of the Co nanoparticles (a) before and after the ex situ reduction in (b) syngas (H₂/CO = 2) and (c) H₂ at 350 °C. (d–f) are the corresponding high-resolution STEM–ADF images of the as-prepared Co₃3O₄, syngas-, and H₂-reduced nanoparticles.

EELS spectra and elemental maps

Aside from morphological changes, the oxidation state of cobalt also changes during the reduction process as a function of the reducing gas composition. Using STEM–EELS, the researchers were able to thoroughly investigate the Co-oxidation state changes. Figure 2 below compares the EELS spectra and elemental maps of single particles of the as-prepared Co₃3O₄, and the syngas-reduced and H₂-reduced nanoparticles. Via the EELS spectra analysis, the researchers determined that there was indeed a significant decrease in the oxidation state of Co.

Figure 2: EELS spectra of the as-prepared Co₃3O₄ (green), syngas- (orange), and H₂- (blue) ex situ reduced nanoparticles at 350 °C. EELS elemental maps of (b–d) as-prepared, (e–g) syngas-reduced, and (h–j) H₂-reduced Co₃3O₄ nanoparticles.

Importantly, the EELS spectra observations also show that the degree of reduction varies amongst the two methods (H₂ and syngas). To explore this further, the researchers collected additional spectra to compare the ex situ syngas-reduced and H₂-reduced nanoparticles at 350 °C, with the as-prepared Co₃3O₄ as a reference. From this further analysis, it is observed that the Co in the case of the syngas-reduced nanoparticles’ spectra is in a near-metallic state as compared to the H₂-reduced nanoparticles. These results are consistent with the elemental maps obtained, showing that a more intense Co signal (green) is observed in the elemental map from a syngas-reduced nanoparticle (Figure 2g), compared to a more diffuse Co signal in the elemental map of a H₂-reduced nanoparticle (Figure 2j). Ultimately, in the absence of air surface reoxidation, the results of the in situ STEM–EELS and ADF analysis show that syngas achieves a higher degree of Co reduction than H₂.

High-resolution STEM-ADF analysis

The next step for Dr. Makgae and his collaborators was to perform high-resolution STEM–ADF imaging in order to investigate the differences in the atomic structure associated with the observed differences in Co-oxidation states in the syngas- and H₂-reduced nanoparticles. They explored the reduction of the syngas- and H₂-reduced nanoparticles at two different temperatures, 350 °C and 600 °C.

1) At 350 °C

In Figure 3a–e below, the high-resolution STEM–ADF images of syngas-reduced nanoparticles (a–c) and H₂-reduced nanoparticles (d,e) are presented. The results were found to be consistent with the EELS data, showing that syngas reduction produces a majority of metallic Co nanoparticles, whereas H₂ produced particles with a majority of the Co in the 2+ oxidation state. 

Figure 3: High-resolution STEM–ADF images of ex situ (a–c) 350 °C syngas-reduced nanoparticles and (d,e) H₂-reduced nanoparticles.

2) At 600 °C

The temperature was then increased to 600 °C to investigate the effect of temperature on reducing HCS-supported Co₃3O₄ nanoparticles under H₂ versus syngas. It is observed that both the H₂- and syngas-reduced nanoparticles exhibit an atomic structure consistent with cobalt (Figure 4a–d). Indeed, the results are consistent with the EELS data from the 600 °C reduced nanoparticles, which shows the presence of metallic cobalt. Conclusively, in order to achieve a complete reduction of Co₃3O₄ under H₂, the HCS-supported nanoparticles must be reduced at high temperatures. Importantly, it is observed that the high temperature of 600 °C resulted in severe sintering of the nanoparticles reduced in H₂ compared to syngas. The sinter-resistance in syngas-reduced nanoparticles is attributed to the encapsulation of nanoparticles with a carbon shell (indicated by the orange arrow in Figure 4b). This encapsulation prevents the coalescence of the nanoparticles at high temperatures. Interestingly, in the case of the H₂-reduced nanoparticles, a carbon shell is not observed (Figure 4d).

Figure 4: High-resolution STEM–ADF images of 600 °C ex situ (a,b) syngas-reduced nanoparticles and (c,d) H₂-reduced Co nanoparticles.

Conclusively, the presence of CO in syngas is observed to play a critical role in the conversion of Co₃3O₄ to metallic Co at 350 °C. In contrast, the full reduction of Co₃3O₄ to active metallic Co in H₂ is not achieved at 350 °C. Increasing the activation temperature to 600 °C does drive the complete reduction, however, significant particle sintering was still observed. Therefore, the presence of CO in a syngas mixture is indeed crucial to achieving complete Co₃3O₄ reduction at 350 °C.

Conclusion

This paper comprehensively illuminates the most effective method to obtain sintering-resistant metallic cobalt nanoparticles at lower temperatures under carbon supports. By conducting a thorough ex situ and in situ electron microscopy analysis using the DENSsolutions Climate system, Dr. Makgae and his fellow collaborators were able to observe under nanometer resolution how the reduction process under H₂ versus syngas affects the physicochemical properties of the reduced nanoparticles. The findings show that syngas achieves a higher reduction at an industrially relevant Fischer-Tropsch reduction temperature of 350 °C compared to H₂ on carbon supports. Importantly, lower reduction temperatures are more efficient as operating costs are reduced, which is a significant consideration for organizations involved in industrial processes. The discovery of such findings could not be possible without the application of in situ electron microscopy techniques, for which our Climate system has proved to be remarkably valuable.

“The DENSsolutions Climate system allowed us to directly evaluate the reducibility of Fischer-Tropsch catalysts under industrially relevant temperatures. Such observations provide a unique insight into the evolution of catalysts under activation conditions.

Dr. Ofentse Makgae
Post-doctoral Research Fellow  |  Lund University

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Introducing the Stream Liquid Supply System: An integrated solution offering unmatched flexibility

Introducing the Stream Liquid Supply System: An integrated solution offering unmatched flexibility

An interview with DENSsolutions Mechanical Engineer Alejandro Rozene about our latest addition to the Stream product line: the Liquid Supply System (LSS).

DENSsolutions introduces its latest solution: the Stream Liquid Supply System (LSS): an integrated solution designed to offer you ease-of-use, flexibility and reproducibility in your in situ liquid experiments. In this article, we interview our Mechanical Engineer Alejandro Rozene to learn all about the LSS, from what inspired its development, its unique capabilities and the many applications that will benefit from its creation.

What led to the development of the Stream Liquid Supply System?

“Our Liquid Phase Electron Microscopy (LPEM) solutions have been used by researchers around the world in a plethora of research fields, such as protein studies, battery research and molecular self-assembly. With our Stream system, users can fully control the microfluidic environment inside the Nano-Cell, our environmental MEMS sample carrier, while biasing or heating their sample. Considering the wide range of capabilities the system offers and the complexity of LPEM, Stream is quite the advanced system. This is precisely what inspired the next step in the innovation process: the development of an integrated solution designed to offer greater flexibility and ease-of-use. The Liquid Supply System (LSS) is a single scientific instrument that allows microscopists to carry out imaging experiments in fewer steps while introducing new features. The LSS is also designed to be the basic unit of a modular system that can be configured for different research applications.” 

What are the main benefits of the LSS for users?

“The introduction of the LSS to the Stream product line brings forth numerous advantages for your in situ liquid experiments, including greater flexibility, reliability and reproducibility. These benefits are detailed below:

1) Ease of use: Thanks to the clever architecture of the LSS, particularly the moveable base, you can easily relocate, store and set it up in various locations. It is also possible to control the microfluidic environment of the Nano-Cell and to apply different stimuli using a single interface in Impulse, our in situ experiment control and automation software. All of the sensor data is collected centrally. The LSS therefore simplifies the LPEM workflow and allows you to shift the focus from the hardware to the imaging experiment. This will potentially open the door to more elaborate experimental workflows.

2) High resolution imaging and meaningful analytical analysis: It is widely known that LPEM suffers from limited resolution caused by the thickness of the liquid layer inside the sample chamber. Even a 500 nm liquid layer can limit imaging resolution. With the LSS, the capability of controllable and inert gas purging is introduced. This means that you can easily displace the liquid in the Nano-Cell and effortlessly cycle between dry and liquid environments. This can be done with air or with an inert gas for air-sensitive samples. Via purging, you can easily get rid of excess liquid in the sample, thereby allowing you to achieve high resolution imaging, image in electron diffraction mode and perform spectroscopy techniques: EDS and EELS.

3) Reliable and reproducible results: The components of the LSS, such as the inlet and outlet pressure-based pumps as well as the liquid flow meter, introduce an unprecedented level of control during your liquid phase microscopy experiments. The LSS offers the unique ability to actively measure the liquid flow, making it possible for you to compare results from different experiments. Moreover, this means that you can easily detect potential clogs in the system and act fast, allowing you to spend your TEM time efficiently and effectively. Ultimately, the combination of our LSS and unique Nano-Cell design, having an on-chip inlet and outlet, enables the liquid delivery to be both reliable and reproducible with a success rate of more than 95%.”

Which applications will benefit most from the LSS?

“The LSS is beneficial for any researcher who wants to use LPEM to observe hydrated samples, matter suspended in liquid or liquid itself in a dynamic environment. Some of the many fields that will benefit from the system include:

  1. Life Science: DNA imaging, biomineralization, cell imaging and protein studies
  2. Electrochemistry: studies of aqueous electrolytes and electrocatalysis
  3. Material Science: studies of nanoparticle formation, self-assembly and growth

These are just a few examples of the several fields that can benefit from the Stream system. Furthermore, the door is open to any researcher who may want to exploit the unique capabilities of the LSS for any liquid-based study outside of the TEM.”

What kind of challenges were tackled during development?

“The LSS is a state-of-the-art scientific instrument. As with any other mechatronic development, integrating the sensing, actuating and microfluidic components into a single, robust machine was in itself a challenge. Also, our market is especially demanding since the users of this instrument already work with some of the most advanced equipment out there, which undoubtedly sets the bar very high. However, it was a very enjoyable experience thanks to the incredible teamwork and collaboration of our MEMS Project Manager, Tijn van Omme, and our Software Developer, Emil Svensson, as well as our manufacturing partners.”

What is the compatibility of the LSS?

“The LSS is an integral part of our Stream product line. It is therefore fully compatible with the modular Stream sample holder, the Stream liquid biasing and liquid heating Nano-Cells, as well as with Impulse. With regards to the microscopes, compatibility is given by the holder type. We currently offer compatibility with JEOL and Thermo Fisher Scientific microscopes.”

Has the LSS already been installed?

“Yes, the Liquid Supply System has been installed at the University of Alberta Nanofabrication and Characterization Facility (nanoFAB) in Canada. The nanoFAB is a national, open-access training, service, and collaboration centre, focused on academic and industrial applications in micro- and nanoscale fabrication and characterization. The installation was carried out by our MEMS Project Manager, Tijn van Omme, with the support of our close distributor Colt Murray from Nanoscience Instruments. It is the first of several installations planned in the upcoming months.”

From left to right: XueHai Tan, Colt Murray, Tijn van Omme and Haoyang Yu

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Unraveling the dynamic behavior of zinc electrodes in aqueous electrolytes using our Stream system

Unraveling the dynamic behavior of zinc electrodes in aqueous electrolytes using our Stream system

Scientists are able to observe for the first time using in situ TEM the morphology and phase evolution of zinc anodes during zinc dissolution and deposition in aqueous electrolytes.

Original article by Yongfeng Huang, Qingqing Gu, Zhanglong Guo, Wenbao Liu, Ziwen Chang, Yuefeng Liu, Feiyu Kang, Liubing Dong, Chengjun Xu

Marc Willinger TOC 1200x628

Metallic zinc is a widely used electrode material for aqueous rechargeable zinc-ion batteries (ZIBs) due to its high theoretical capacity, low redox potential, natural abundance and low cost. Despite these valuable benefits, zinc electrodes suffer from short cycling stability due to many factors such as the growth of zinc dendrites as well as corrosion. Little is known about the underlying reasons for the failure of zinc anodes on a deeper level. Indeed, the use of in situ transmission electron microscopy can serve as a means to bridge this gap in knowledge, unraveling the morphological evolution of zinc anodes during zinc dissolution and deposition in aqueous electrolytes. 

In recent research performed at the DICP-DENS Microscopy Centre by Dr. Yuefeng Liu from the Dalian Institute of Chemical Physics, Yongfeng Huang from Tsinghua University, and many more collaborators, they were able to obtain a deep understanding of the stripping/plating behaviors of zinc in various aqueous electrolytes using the DENSsolutions Stream system. Specifically, the researchers investigated the effects of adding Mn²⁺ and CF₃SO₃⁻ in zinc-salt aqueous electrolytes on the zinc plating/stripping behavior. This work not only sheds light on the mechanism of zinc anodes during zinc dissolution/deposition processes in aqueous electrolytes, but also provides effective strategies to achieve long-term stable rechargeable zinc-ion batteries (ZIBs). 

Three aqueous electrolytes

As the high reversibility of zinc anodes is a prerequisite for large- scale applications of ZIBs and ZHSs, many efforts have been dedicated to achieve long-term stable zinc anodes. One effective way to stabilize and acquire dendrite-free zinc anodes is by introducing additives in aqueous electrolytes. In previous studies, it was found that the addition of Mn²⁺ in the form of MnSO₄ to a ZnSO₄ aqueous electrolyte can actually suppress the dissolution of the MnO₂ cathode. Similarly, in comparison to a ZnSO₄ electrolyte, a Zn(CF₃SO₃)₂ electrolyte is far more effective in achieving better electrochemical performance of many ZIB systems.

For these reasons, aside from a pure ZnSO₄ electrolyte, ZnSO₄/MnSO₄ and Zn(CF₃SO₃)₂ are commonly used aqueous electrolytes for ZIBs. Despite this, zinc plating/stripping behaviors in ZnSO₄/MnSO₄ and Zn(CF₃SO₃)₂ and their effects on electrochemical stability of metallic zinc anodes are unknown. In this study, the researchers combine operando TEM and electrochemical analysis to unravel the zinc stripping/plating behaviors in the three aqueous electrolytes detailed above (ZnSO₄, ZnSO₄/MnSO₄ and Zn(CF₃SO₃)₂). In the following sections, in situ TEM images and videos of the zinc stripping/plating process in the three different electrolytes are shown.

1) Zinc plating/stripping process in a ZnSO₄ electrolyte

The researchers first constructed a micro zinc-ion battery inside the Stream holder, allowing them to monitor the zinc plating/stripping processes using the system’s on-chip flow channel. After assembling the Stream Nano-Cell and leak-testing it, the researchers were then able to flow the electrolytes into the liquid cell, using the pressure-based liquid pump to fully control the rate of the flow. 

Next, the researchers set out to observe the zinc stripping/plating behavior in a pure ZnSO₄ aqueous solution. In Figure 1 below, the in situ TEM images detailing the zinc plating and stripping process at different times in 2 M ZnSO₄ are shown. During the zinc plating process exhibited in Figure 1a–d, the thickness of zinc deposited on the platinum substrate increases. Simultaneously, dendrite-like structures form and develop gradually. This confirms the dendrite issue of metallic zinc electrodes in the 2 M ZnSO₄ electrolyte, which leads to a short-circuit occurrence. After this, the zinc stripping process occurs, which is demonstrated in Figure 1d–f. During this process, it is observed that the deposited zinc is unable to fully strip away from the platinum substrate and dissolve into the electrolyte under the same charge and discharge time conditions. As a result, dead zinc is generated (see Figure 1f).

Figure 2: Marc Willinger

Figure 1: In-situ TEM images of zinc plating/stripping on the surface of Pt working electrode in 2 M ZnSO₄ electrolyte at different times: (a) 0 s, (b) 3 s, (c) 10 s, (d) 28 s, (e) 80 s and (f) 261 s.

Electrolytes of high concentrations are generally not conducive for monitoring zinc stripping/plating behaviors using in situ TEM, as they can block out the liquid inlet hole of the holder. Due to this, along with the valuable insights that further investigating the effect of concentration on dendrite growth can provide, the ZnSO₄ electrolyte was diluted from 2 M to 20 mM. The researchers then observed the zinc stripping/plating process in the 20 mM ZnSO₄ aqueous electrolyte. The in situ TEM images and corresponding movie detailing this process can be seen below in Figure 2 and Video 1, respectively. 

The zinc stripping/plating process was controlled using the cyclic voltammetry (CV) technique at 20 mV/s. Figure 2g shows the corresponding CV curve, where the red dots marked as a–f correspond to the states in Figure 2a–f. At 71s (see Figure 2b), the researchers observed that the visible zinc particles in the electrolyte, as well as the dense dispositioned zinc and the zinc dendrites (marked in green) on the surface of the Pt substrate occur at the same. Between 76s–95s (Figure 2c-d), they noticed that more zinc was plating on the platinum surface, with the potential ranging from − 1.41 V to − 1.1 V. Next, the process of zinc dissolution occurs, where the zinc frontier line is observed to shrink back to area II at 114s (Figure 2e) and to the edge of the Pt substrate at 144s (Figure 2f). It is observed that the dendrite is still not completely removed when the potential returns to 0 V. Conclusively, although reducing the concentration can slow down dendrite formation, it does not fully suppress it.

Figure 2: In-situ TEM images of zinc plating/stripping on the surface of Pt working electrode in 20 mM ZnSO₄ electrolyte at different times: (a) 0 s (b) 71 s, (c) 76 s, (d) 95 s, (e) 114 s and (f) 144 s. (g) CV curve of zinc stripping/plating process on the surface of Pt working electrode in 20 mM ZnSO4 electrolyte at a scan rate of 20 mV/s. (h) XRD pattern after zinc plating/stripping at Pt. (i) XPS result after zinc plating/stripping at Pt.

Video 1: Video of zinc plating/striping at the interface of Pt working electrode in 20 mM ZnSO₄ electrolyte.

2) Zinc plating/stripping process in a ZnSO₄/MnSO₄ electrolyte mixture

The next step for the researchers was to observe the zinc stripping/plating process in a ZnSO₄/MnSO₄ electrolyte mixture, as MnSO₄ additives in ZnSO₄ electrolytes can actually optimize the electro-chemical stability of metallic zinc electrodes. They first added 5 mM MnSO₄ to the 20 mM ZnSO₄ aqueous electrolyte. The in situ TEM images and corresponding movie detailing the resulting zinc plating/stripping process can be seen below in Figure 3 and Video 2, respectively. 

Figure 3a–d depicts the zinc plating process, in which Mn spherical particles appear and gradually increase in size. At 124s (see Figure 3e), the zinc stripping process starts to occur, and these spherical particles in the electrolyte begin to disintegrate and finally disappear at 259s (see Figure 3i). Aside from the spherical particles in the electrolyte, a visible amount of deposited zinc (marked in green in Figure 3e) appears on the Pt working electrode. Notably, the sphere species, both in the electrolyte and also on the surface of the Pt substrate, disintegrate. This results in the further precipitation of zinc and leads to the increased thickness of zinc. Interestingly, zinc dendrites were not observed to form in the ZnSO₄/MnSO₄ electrolyte mixture, unlike in the pure ZnSO₄ electrolyte. This directly suggests that the Mn²⁺ additive is indeed beneficial for suppressing the formation of zinc dendrites.

Figure 3: Zinc stripping/plating behaviors in 20 mM ZnSO₄ + 5 mM MnSO₄ electrolyte at different times: (a) 0 s (initial state), (b) 60 s (c) 69 s, (d) 119 s, (e) 124 s, (f) 177 s, (g) 199 s, (h) 238 s and (i) 259 s. (j) Cyclic voltammogram curve of Zn plating and striping at the interface of Pt working electrode at a scan rate of 20 mV/s, the red dots are assigned to Figure a-i, (k) XRD pattern after zinc plating/stripping at Pt (CaCO₃ is from the XRD bonding adhesives), and (l) E-pH diagram of Mn.

Video 2: Video of zinc plating/striping at the interface of Pt working electrode in the 20 mM ZnSO₄ + 5 mM MnSO₄ electrolyte.

3) Zinc plating/stripping process in a Zn(CF₃SO₃)₂ electrolyte

Finally, the researchers set out to explore the zinc plating/stripping process in a 20 mM Zn(CF₃SO₃)₂ electrolyte. The in situ TEM images and corresponding movie detailing the resulting zinc plating/stripping process in this electrolyte is shown in Figure 4 and Video 3, respectively. The zinc plating process is depicted in Figure 4a–d, where particle-like zinc uniformly deposits on the surface of the Pt substrate, and the thickness of the deposited zinc layer increases. Here, no zinc dendrites form during the plating process. At 175s (see Figure 4e), the zinc stripping process starts, and the deposited zinc particles on the Pt substrate gradually dissolve into electrolyte. Finally, at 240s (see Figure 4f), all zinc particles disappear, demonstrating the high reversibility of zinc deposition/dissolution in the Zn(CF₃SO₃)₂ electrolyte. Importantly, the deposition of zinc on the Pt substrate in the Zn(CF₃SO₃)₂ electrolyte is more uniform and homogeneous than that in the ZnSO₄ electrolyte and ZnSO₄/MnSO₄ electrolyte mixture.

Figure 4: Zinc stripping/plating behaviors in 20 mM Zn(CF₃SO₃)₂ electrolyte at different times: (a) initial, 1.0 V (b) 124 s, -1.48 V, (c) 130 s, -1.40 V, (d) 138 s, -1.24 V, (e) 175 s, -0.40 V, (f) 240 s, 0.80 V, (g) Cyclic voltammogram curve at the scan rate of 20mV/s about Zn plating and striping at the interface of Pt working electrode in 20 mM Zn(CF₃SO₃)₂ electrolyte, the red dots are assigned to Figure a-f (h) XRD pattern after zinc plating/stripping at Pt (CaCO3 is from the XRD bonding adhesives). (i) SEM image after Zn plating on Pt.

Video 3: Video of zinc plating/striping at the interface of Pt working electrode in the 20 mM Zn(CF₃SO₃)₂ electrolyte.

Conclusion

This paper provides a novel understanding of the failure process of metallic zinc electrodes in a ZnSO₄ electrolyte, as well as the effects and acting mechanism of Mn²⁺ in the ZnSO₄/MnSO₄ electrolyte mixture and CF₃SO₃⁻ in the Zn(CF₃SO₃)₂ electrolyte on the zinc plating/stripping behavior. These deep-level insights serve to close the gap in our understanding of how we can create long-term stable ZIBs with metallic zinc anodes and optimal aqueous electrolytes, beneficial for several critical applications, including large-scale energy storage and portable electronic applications. The discovery of such findings could not be possible without the application of in situ characterization techniques, for which our Stream system has proved to be remarkably effective.

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Scientists develop a novel approach to generate extreme thermal gradients using our Wildfire Nano-Chip

Via a simple modification to our Wildfire Nano-Chip, scientists show that extreme thermal gradients across a TEM specimen can be generated.

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Scientists develop a novel approach to generate extreme thermal gradients using our Wildfire Nano-Chip

Scientists develop a novel approach to generate extreme thermal gradients using our Wildfire Nano-Chip

Via a simple modification to our Wildfire Nano-Chip, scientists show that extreme thermal gradients across a TEM specimen can be generated.

Original article by Sriram Vijayan, Rongxuan Wang, Zhenyu Kong and Joerg R. Jinschek

Marc Willinger TOC 1200x628

In traditional manufacturing processes, it is typically observed that the material experiences steady-state conditions. This results in the formation of stable equilibrium phases in the material. However, for those phases in materials that develop under ‘far-from equilibrium’ conditions, the same does not apply. In fact, for certain processing techniques that involve rapid thermal cycling and/or extreme thermal gradients, changes in the local phase equilibria, and therefore the local microstructure, often occur. Currently, observing these dynamic processes in materials under such conditions remains a great challenge. This is particularly because MEMS-based in situ TEM heating devices that can generate thermal gradients, combined with the capability to thermally cycle a specimen between different temperatures, are not currently available.

In recent research performed at The Ohio State University by Dr. Sriram Vijayan and Dr. Joerg Jinschek (now at Technical University of Denmark), as well as Rongxuan Wang and Dr. Zhenyu Kong from Virginia Tech, it is shown that controlled thermal gradients across a TEM specimen can be generated via a simple modification to our Wildfire Nano-Chip. Such a device can dramatically improve our understanding of material performance under extreme thermal conditions. Until now, thermally activated processes that affect microstructural stability of a material exposed to such thermal transients could only be investigated postmortem. Therefore, the need for new approaches to study thermally activated phenomena in materials in real time under such conditions is required. Importantly, this work is a major step forward in paving the way for scientists to be able to perform site-selective studies on materials under transient thermal conditions.

The study

As a first step, Dr. Vijayan and his fellow collaborators made modifications to a standard DENSsolutions Wildfire Nano-Chip. They then measured the local temperature distribution across the modified Nano-Chip and sample, and thereby the thermal gradient. The magnitude of the thermal gradient generated by this modified MEMS-based microheater was determined by measuring the temperature distribution using a combination of ex situ and in situ temperature measurement techniques. Regarding the ex situ temperature calibration, Dr. Vijayan and Dr. Jinschek teamed up with their MURI partners at Virginia Tech. As for the in situ temperature calibration, this was done at CEMAS, the state-of-the-art electron microscopy facility at The Ohio State University, using the previously established ‘Ag nanocube sublimation method’. 

The Nano-Chip modification

Using a Helios Nanolab 600 DualBeam focused ion beam (FIB)/scanning electron microscope (SEM), the researchers were able to modify a standard Nano-Chip. The Ga+ ion-based FIB was operated at an accelerating voltage of 30 kV and 6.4 nA beam current to mill a rectangular window next to the spiral metallic heater. This milled window can be seen in Figure 1b below. Importantly, after the FIB modification, the modified Nano-Chip exhibited no signs of deterioration in performance. 

Figure 2: Marc Willinger

Figure 1: SEM images of the (a) unmodified Nano-Chip and (b) modified Nano-Chip.

Lamella preparation and transfer

Next, the researchers worked on preparing and transferring the FIB lamella from a (001) oriented single crystal silicon wafer surface onto the Nano-Chip. This workflow is detailed in Figure 2a–f below. The FIB-cut Si lamella was placed across the larger window (Figure 2e), with one end of the lamella on the heater and the other end on the SiN membrane acting as the heat sink. The Si lamella was welded to the MEMS heater using a Pt-based Gas Injection System (GIS) needle with the aid of the Ga ion beam. The Pt was welded at the top left edge and top right edge of the lamella as seen from the SE Image in Figure 2f.

Figure 2: Marc Willinger

Figure 2: Secondary electron (SE) and ion (SI) images of the FIB preparation and transfer of the lamella over the modified window in the FIB.

Ex situ temperature calibration

At Dr. Kong’s lab (Virginia Tech), the researchers then performed infrared thermal imaging (IRTI) experiments on the MEMS devices under ambient air conditions. IRTI is a particularly useful ex situ approach to measure the spatial distribution of temperature across the MEMS-device with a reasonable degree of accuracy. Using IRTI, the temperature distribution was measured across both the unmodified and modified Nano-Chip. 

1) Temperature distribution across the unmodified Nano-Chip

First, the researchers measured the temperature distribution across the unmodified Nano-Chip to obtain reference temperature measurements. This distribution indicated that the metallic spiral heating element surrounding the viewing area of the microheater exhibited the highest temperature. Moreover, based on the temperature profile of the MEMS microheater, the spiral metallic heating element is the “hottest” region, while the surrounding SiN membrane is the “coldest” region on the MEMS device. These thermal conditions result in a large thermal gradient between the spiral metallic heater (= the “heat source”) and the SiN membrane (= the “heat sink”). 

2) Temperature distribution across the modified Nano-Chip

In order to generate a thermal gradient across a specimen, a FIB-cut lamella must be placed such that one end of the specimen is on or adjacent to the heater and the other end is on the SiN membrane, away from the heater. 

The researchers then performed IRTI again, but on the modified Nano-Chip with a FIB-cut lamella of Si placed across the modified window. As shown in Figure 3a below, the “hot end” of the Si lamella was adjacent to the outer spiral of the metallic heater, represented by the region enclosed in the black dotted square. On the other hand, the “cold end” was across the window in the SiN membrane, represented by the region enclosed in the yellow dotted square. Next, the thermal gradients across the FIB-cut TEM specimen were measured via an established technique that allows temperature measurement inside the TEM with high spatial resolution. 

Figure 2: Marc Willinger

Figure 3: Overlay of the SEM image of the modified device with Si lamella on the thermal map of the device at 1300°C (unit of heatmap scale is in C), (b) the horizontal temperature line profile along the “hot end” (enclosed in dotted black square) and “cold end” (enclosed in dotted yellow square) of the Si lamella.

In situ temperature calibration

In order to validate the ex situ results and accurately measure the temperature across the FIB-cut specimen, the researchers measured the temperature via the established Ag nanocube (NC) sublimation method. The lamella was again placed such that the “hot end” was over the central spiral metallic heater and the “cold end” across the modified window. Next, monodispersed Ag NCs were dispersed onto the FIB lamella. The modified Nano-Chip was then heated to a set point temperature of 1,000°C and held until the Ag NCs completely sublimate. In Figure 4a below, the Ag NCs that dispersed across the FIB lamella are highlighted with colored boxes and labeled as regions 1–7. After sufficient sublimation events on the lamella were observed, the sublimation experiment was halted. Notably, the Ag NCs in region 6 and 7 did not completely sublimate, as the sublimation kinetics slowed down at lower temperatures. 

“When exposing the FIB-cut TEM lamella to a combination of extreme thermal gradient heating and thermal cycling, the DENSsolutions Nano-Chip was observed to be extremely stable. The impulse software was very useful for these experiments as it allowed us to design complex temperature profiles during the heating experiments.” – Dr. Sriram Vijayan, Post-Doctoral Researcher, Center for Electron Microscopy & Analysis (CEMAS), The Ohio State University

The researchers then estimated the mean temperature of the Ag NCs on the FIB lamella found within regions 1–5. Subsequently, the mean temperatures obtained from regions 1 to 5 were plotted against the relative distance from the heater. In figure 4b below, a linear fit of the temperature data with 95% prediction intervals is presented. The thermal gradient of ΔT/Δx = 6.3 x 10⁶ °C/m was calculated at the temperature set point of 1,000°C. 

The in situ results confirm that the researchers’ proposed modification of our Wildfire Nano-Chip has indeed generated a large thermal gradient across the FIB-cut TEM specimen. In fact, this resulting thermal gradient was found to be in the range of gradients observed in several engineering and manufacturing applications. Ultimately, this innovative approach now opens up the scope of research to address a wide range of material science problems observed under transient thermal conditions.

Figure 2: Marc Willinger

Figure 4: (a) SEM image of different regions (1–7) across the Si lamella where the temperature of the lamella was measured. (b) The linear fit of the temperature from the “hot end” to the “cold end” superimposed over the plot of the measured sublimation temp from region 1 to 5 versus distance from the “hot end”.
.

Conclusion

This paper presents a very simple methodology to study non-equilibrium processes in materials under extreme thermal gradients and rapid thermal cycling conditions. Specifically, this modified Nano-Chip can enable dynamic studies of the microstructural evolution in materials used in several critical applications, including nuclear reactor components, microelectronic packages and additively manufactured builds. 

In the case of metal-based additive manufacturing (AM), extreme spatial and temporal thermal transients significantly impact the material microstructure within the melt pool and regions below the melt pool (‘previously melted’ layers of an AM build). Using this novel in situ TEM heating approach, we can now begin to observe and understand physical processes that contribute to the microstructural evolution in ‘previously melted’ layers during AM, thereby closing gaps in our understanding of processing-structure-property relationships in metal-based AM.

Ultimately, this powerful device can accurately simulate these complex thermal conditions inside the TEM and capture the microstructural evolution of materials during such extreme processing conditions. We are truly proud of the role that our Wildfire system has played in making this research possible and strive to continue enabling groundbreaking research now and in the future.

Hanglong Wu portrait

“Our modified DENSsolutions Wildfire Nano-Chip is well-suited for in-situ TEM heating experiments that require ‘far-from-equilibrium’ thermal conditions. This enables the observation of dynamic processes in a wide range of material systems at the nanoscale, which will further our fundamental understanding on microstructural stability of materials during processing and under ‘in-service’ conditions.” 

Dr. Joerg Jinschek
Professor |  Technical University of Denmark

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New measuring technique proves exceptional temperature accuracy of our Wildfire Nanochip

In collaboration with Utrecht University, we develop a novel technique to measure temperature at the nanoscale, showing the remarkable temperature accuracy and homogeneity of our Wildfire Nanochip.

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DENSsolutions has installed yet another Stream system in Germany at Forschungszentrum Jülich

DENSsolutions has installed yet another Stream system in Germany at Forschungszentrum Jülich

DENSsolutions Installing South Korea's second Stream system at Seoul National University

From left to right: Andreas Körner and Dr. Andreas Hutzler

We are proud to announce that DENSsolutions has installed yet another Stream system in Germany at the esteemed Forschungszentrum Jülich, one of the largest interdisciplinary research centres in Europe. In this article, we interview Dr. Andreas Hutzler, the new head of the TEM lab in the Helmholtz Institute Erlangen-Nürnberg for Renewable Energy (HI ERN) at Forschungszentrum Jülich, to learn more about their advanced microscopy facility, its research direction, as well as how our Stream system is advancing their research.

Can you tell me more about the microscopy facility at HI ERN?

“The Helmholtz Institute Erlangen-Nürnberg for Renewable Energy (HI ERN) is part of the Forschungszentrum Jülich. It specializes in providing crucial research on technologies needed to utilize renewable energies in the decades to come. Our research is centered around fuel cells, electrolyzers and hydrogen storage. The institute was founded in 2013 and has been growing ever since. In 2021, its new research building was inaugurated, hosting the space for a new transmission electron microscope, the Talos F200i from Thermo Fisher Scientific. This tool provides in-house structural analysis on the nanoscale for catalysts, support systems and membranes.”

What type of applications are the users at HI ERN using the Stream system for?

“Our goal is to study electrochemical processes taking place on electrode and catalyst surfaces within electrolyzers and fuel cells down to the atomic scale. We aim to understand which reactions take place, and which conditions enhance the performance of the cells or disintegrate the structures involved.

In order to understand this, we consider beam-induced effects onto the solution chemistry we investigate. For this, we utilize a comprehensive radiolysis model for unraveling the influence of electron irradiation onto the sample and compare the results to non-biased experimental observations. Once this is understood, we continue with analyzing dynamic processes at the nanoscale to gain insights into reaction pathways and degradation mechanisms in P2X and X2P applications.”

What particular features of the DENSsolutions Stream solution attracted you to the system?

“In order to understand observable processes and their correlated chemistry, it is necessary to accurately tune experimental conditions while operating the system. The ability of the Stream system to flexibly adjust pressure, flux, temperature and potential allows to run a manifold of experiments in a wide parameter space. This is needed in order to verify the stability of our reaction kinetic models and for testing electrolysis at borderline conditions. Before, the structures could only be studied after the reaction has taken place. But the ability to directly observe dynamic processes on-site in real time gives valuable insights in the chemistry at hand.”

Can you tell me about the grant that was won to acquire the system?

“One of our key research interests is the development of new methods for characterizing fundamental and applied processes in electrocatalysis relevant to electrochemical energy conversion. After establishing identical-location TEM (IL-TEM) for energy applications and with the start of my team, a new transmission electron microscope as well as equipment needed for in situ liquid-phase TEM was funded by and installed at HI ERN. This particular toolbox will be a great asset for the nanoanalysis of electrochemical processes in my team which will enable unique insights in energy research.”

In your experience so far, how have you found the Stream system?

“The modular architecture of the Stream system enables a very versatile applicability without risking leakage or cross-contaminations. The performance of LP-TEM is considerably enhanced due to the controllability of liquid flow, the ever-present window bulging via the utilization of a novel chip design as well as a differential pumping system as a standard. Moreover, DENSsolutions came forward with providing non-standard solutions in order to provide compatibility with other setups at our institute.”

DENSsolutions Prof. Jungwon Park
Dr. Andreas Hutzler
Head of the Transmission Electron Microscopy lab| HI ERN, Forschungszentrum Jülich

Dr. Andreas Hutzler is the new head of the Transmission Electron Microscopy lab at HI ERN, PI of multiple projects at HI ERN and university and is currently setting up a team for nanoanalysis of electrochemical processes. His research interests mainly focus on methodological aspects of LP-TEM and its application in electrochemical energy conversion.

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