Platinum (Pt) is recognized as the most active material for the hydrogen evolution reaction in acidic media; however, its catalytic activity is often underestimated in proton exchange membrane water electrolysis (PEMWE) due to poor utilization of the cathode catalyst layer. In this study, we present the synthesis, characterization, and application of Pt nanoparticles with atomic precision on a microporous-layer-coated gas diffusion layer for PEMWE. The Pt nanoparticles were synthesized via atomic layer deposition, a technique that enables precise control over loading and particle size at the atomic scale. The resulting gas diffusion electrode with an exceptionally low platinum loading (1.08–5.40 μg cm^-2) demonstrated mass activity at least one order of magnitude higher than that of benchmark Pt. Furthermore, the electrode exhibited exceptional stability at a current density of 1 A cm^-2 over 200 hours. It also showed robust performance under dynamic operation, enduring 25,000 cycles of alternating cell voltages between 1.45 V and 2 V.

The long-term operation of CO₂ electrolyzers using membrane electrode assemblies (MEAs) is limited by challenges related to water management. However, the water balance in CO₂ electrolyzer cells still has not been fully understood, and conflicting observations have been reported in the literature. In this study, a one-dimensional non-isothermal multiphysics model of an exchange MEA CO₂ electrolyzer with a Tokuyama A201 anion exchange membrane is developed to investigate the role of different physical and chemical phenomena on the water balance. The relative contributions of these processes vary with current density and membrane transport properties, which shift the dominant water transport mechanism in the cell. Our results highlight the significant contribution of homogeneous reactions, particularly OH⁻, to the water balance across the membrane. At low currents (i < 130 mA cm⁻²), homogeneous buffer reactions dominate the water balance and result in net water production near the catalyst layer. At higher currents (i > 130 mA cm⁻²), the flux is governed by electro-osmotic drag and a temperature gradient over the cathode gas diffusion electrode (GDE) with their relative contributions depending on membrane properties. Homogeneous buffering can re-emerge as the dominant mechanism at high currents if the hydroxide ion concentration in the membrane increases, for example under CO₂-limited cathode conditions, allowing hydroxide ions to react with depleted bicarbonate near the anode.

Atomic layer deposition (ALD) is widely studied for numerous applications and is commercially employed in the semiconductor industry, where planar substrates are the norm. However, the inherent ALD feature of coating virtually any surface geometry with atomistic thickness control is equally attractive for coating particulate materials (supports). In this review, we provide a comprehensive overview of the developments in this decades-old field of ALD on particulate materials, drawing on a bottom-up and quantitative analysis of 799 articles from this field. The obtained dataset is the basis for abstractions regarding reactor types (specifically for particles), coating materials, reactants, supports and processing conditions. Furthermore, the dataset enables direct access to specific processing conditions (for a given material, surface functionality, application etc.) and increases accessibility of the respective literature. We also review fundamental concepts of ALD on particles and discuss the most common applications, i.e., catalysis (thermo-, electro-, photo-), batteries, luminescent phosphors and healthcare. Finally, we identify historical trends and provide an outlook on prospective developments.

Electrochemical CO2 reduction (CO2R) is a promising technology for carbon recycling and energy storage. While gas-fed CO2R is currently the best practice because it facilitates fast mass transport, CO2R in water offers potential advantages such as avoiding salt formation, facile water control, and easier integration with CO2 capture. In this work, we enhance mass transport in an aqueous CO2 electrolyzer using fast pressure pulses (50 Hz, 1.2 bar) with a vibratory pump typically found in coffee machines. We demonstrate a limiting current density of 87 mA cm−2 toward CO2R products—nearly three times higher than without pulses. The current density can be further increased by leveraging the peak-to-peak pressure amplitude or pump frequency, as shown through particle image velocimetry (PIV) and an order-of-magnitude scaling analysis. Although challenges remain, such as pump energy consumption, contamination, heating, and pressure-wave damping, the pressure-pulsed concept is a promising direction for aqueous CO2R.

The growing need for advanced materials with tunable properties has triggered an increasing interest in innovative surface modification techniques. Fluidized-bed atomic layer deposition (FB-ALD) offers a powerful solution for surface engineering and functionalization of powder-based materials for a variety of applications. By relying on its capability for controlling uniformity and conformality of the coatings precisely at the atomic scale, ALD can effectively modify surface characteristics to improve the functionality and durability of the materials. In this review, we will provide comprehensive fundamentals and strategies to improve the fluidization of nanopowders and reveal the potential of FR-ALD in two emerging applications. The first application is in energy devices, where FB-ALD is employed to develop Pt-based electrocatalysts for fuel cells and other catalytic reactions. We demonstrate that FB-ALD enables precise control of size, composition, and dispersion of Pt nanoparticles over the support surface, resulting in a strong enhancement in catalytic performance. We additionally discuss the application of FB-ALD in boosting the stability and durability of catalysts by surface engineering with ultrathin films and ultrasmall nanoparticles without compromising their activity. These capabilities open new avenues for the development of high-performance and durable catalysts for energy applications. The second application is in pharmaceutical research, where FB-ALD is employed to coat active pharmaceutical ingredients with thin films of biocompatible materials, such as Al₂O₃, ZnO, SiO₂, and TiO₂, to control their release profiles and improve their physical properties, such as wettability, dispersibility, flowability, and solubility, which are essential for enhancing therapeutic efficacy and patient compliance. The versatility and precision of FB-ALD position it as a key technology for the development of next-generation materials, addressing the critical challenges of performance, stability, and functionality of powder-based materials for different fields.

Conventional fluidization of cohesive powders is challenging due to their strong interparticle forces, often requiring assistance methods. In this study, the hydrodynamics of pulsed and vibrated beds of cohesive Geldart C silica powder (Sauter mean diameter d₃₂=7.9μm) in a 19.2cm diameter column were investigated using X-ray imaging. The results show that low-frequency, moderate-amplitude gas pulsation improves fluidization by disrupting long, persistent gas channels. Higher-frequency pulsation is dampened throughout the bed, resulting in negligible improvement over unassisted systems. When coupled with mechanical vibration, gas pulsation slightly mitigates solid compaction at the bottom section, but the overall flow pattern remains largely unchanged compared to vibration alone. The findings highlight the potential of integrating gas pulsation with other assistance methods to enhance fluidization in practical applications.

The fractal structure of aggregates consisting of primary nanoparticles naturally arises during their synthesis. While typically considered to be a fully stochastic process, we suspect long-range interactions, in particular van der Waals forces, to induce an active pull on particles, altering the clustering process. Using an off-grid 3D model, we show that an active pull decreases the density and fractal dimension of formed clusters. These findings could not be reproduced by 2D models, which underestimate screening effects. Additionally, we determined the range within which van der Waals forces dominate the aggregation process.

Evaluation of the hydrodynamics of opaque multi-phase flows remains a challenging task, with implications for various industrial processes such as chemical processing, pharmaceutical, and mineral processing. Understanding how design and operational variables affect the complex behavior of multi-phase flow systems is essential for optimizing processing conditions and improving efficiency. Radioactive particle tracking (RPT) has been a proven measurement technique to evaluate hydrodynamics in multi-phase flow systems. However, a limitation of the classical RPT technique exists in the assumptions made in the simulation of the count rate received by the detectors in correcting for varying flow-induced fluctuations in the volume fraction of the dispersed phase, often encountered in industrial multi-phase flow systems. In this paper, we introduce a fundamentally novel experimental RPT method that directly uses detected incident photon hit locations for the reconstruction of the three-dimensional radioactive tracer particle position. We argue that this approach is inherently more robust as varying attenuation does not affect the reconstruction. The RPT setup consists of three identical γ-radiation slit collimator detectors that are placed equidistantly at 120° intervals. A subsequent calibration-experimentation procedure is established that allows reconstruction of the tracer particle position with spatial accuracy and precision in the order of 1 mm. We demonstrate the applications of this technique in evaluating hydrodynamics in multi-phase systems by characterizing the flow field of industrial-grade polypropylene reactor powder in a laboratory-scale horizontal stirred bed reactor.

Chelator-impregnated resins have been studied earlier for the chemical separation of elements in aqueous solutions, but issues with their chemical stability have limited their use in the separation of (medical) radionuclides from their respective irradiated targets. We developed a polydimethylsiloxane (PDMS)-based chelator-impregnated resin that showed a high chemical stability against leaching. Several different chelators were tested in this study. After impregnation of the PDMS beads with the di-2-ethylhexylphosphoric acid (D2EHPA) chelator, an in-flow separation study with various radionuclides (Y-90, La-140, and Ac-225) was conducted. These three radionuclides have potential use in nuclear medicine and a production route through irradiation of Sr-, Ba-, and Ra-targets respectively, necessitating their chemical separation. The D2EHPA-impregnated beads achieved high adsorption efficiencies of 99.89% ± 0.14%, 99.50% ± 0.10%, and 98.51% ± 0.25%, for Y-90, La-140, and Ac-225, respectively, while co-adsorption of minor amounts (< 3%) of the targets were reported. These results, together with the high chemical stability of the PDMS-based resin, highlight the potential of chelator-impregnated resins in the rapidly growing field of (medical) radionuclide production.

Atomic layer deposition (ALD) of platinum (Pt) has gained significant interest in the recent years due to its capability of depositing various Pt nanostructures for applications in different fields, such as Pt nanoparticles (NPs) for catalytic reactions and energy devices and Pt thin films for microelectronic technology. Among various developed processes, Pt ALD using MeCpPtMe₃as the precursor has been most popularly employed owing to the high reactivity, volatility, and thermal stability of the precursor, which enable controlled deposition of Pt nanostructures in a broad range of temperatures. Typical MeCpPtMe₃-based Pt ALD processes use O₂and H₂as the coreactants. In this study, we explore atomic hydrogen as an alternative and reveal its exceptional reactivity that outperforms H₂and O₂. Specifically, atomic hydrogen enables the deposition of highly dispersed Pt NPs with narrow particle size distributions (i.e., standard deviation <0.3 nm) on various oxide surfaces, including TiO₂, SiO₂, CeO₂and V₂O₅, which is unattainable with H₂under identical experimental conditions. In addition, it facilitates the deposition of Pt NPs with improved size uniformity and accelerates the closure of Pt films compared to ALD processes using O₂as the coreactant. The results demonstrate a significant potential of atomic hydrogen as a highly effective coreactant for ALD of Pt NPs and thin films.

Electrochemical CO2 reduction presents an opportunity to transform waste flue gas with water and renewable electricity into chemicals or fuels. However, the energy-intensive nature of purification of flue gas underscores the appeal of directly utilizing the flue gas streams containing impurities. In this study, we investigate the impact of SO2 impurities on CO2 electroreduction in two electrochemical cell geometries: an H-cell and a membrane electrode assembly (MEA) cell. We observe distinctly different behavior of the Ag on carbon black (Ag/CB) catalyst under SO2 impurities in the H-cell compared to the MEA cell, where SO2 impurities exhibit a more pronounced effect on Ag/CB catalysts in the H-cell than in the MEA cell. This difference is attributed to the higher solubility of SO2 in the electrolyte compared to CO2, resulting in an accumulation effect and causing differences in the SO2 concentration near the electrode between the H-cell and the MEA system. By depositing a very thin SiO2 coating on the outermost surface of the Ag/CB catalyst using atomic layer deposition (ALD), the impact of SO2 on the catalyst's selectivity is diminished. This is attributed to the permeability difference between CO2 and SO2 through the SiO2 coatings and results in a local SO2 concentration difference between samples with and without SiO2 coatings.

An integrated battery-electrolyzer stores renewable electricity as a battery and produces hydrogen when overcharged. This dual application requires electrode concepts that ideally enhance both battery and electrolysis operation without compromising either. One such concept is 3D structured electrodes including channels that improve ionic conductivity and material utilization as well as facilitating bubble removal during electrolysis. In this work, we first develop a 1D model of a porous sintered nickel electrode that takes the void fraction of the 3D geometry into account and allows for the determination of the current and potential distribution for both battery charging and oxygen evolution. An optimized void fraction that maximizes the reactive surface area for oxygen evolution is determined, and we discuss under what circumstances a 3D geometry is beneficial. Finally, we show how the improved ionic conductivity of 3D electrodes also results in more homogeneous battery charging, increasing charging efficiency in nickel electrodes.

Noble metal nanoparticles (NPs), particularly platinum (Pt), are widely used in heterogeneous catalysis due to their exceptional activity. However, controlling their size and preventing sintering during synthesis remains a major challenge, especially when aiming for high dispersion and stability on supports such as graphene. Atomic layer deposition (ALD) has emerged as a promising method to address these issues, yet conventional processes often lead to broad particle size distributions (PSDs). This work introduces a new approach for the deposition of size-controlled and sintering-resistant Pt NPs on graphene by atmospheric-pressure ALD using MeCpPtMe₃and O₂. In this approach, the deposition temperature varies in a cyclic manner in accordance with the Pt precursor and the O₂exposure steps. In every ALD cycle, the MeCpPtMe₃exposure is carried out at either 150 or 200 °C, and the O₂exposure is at room temperature. The room-temperature step hinders the diffusion and coalescence of Pt NPs, resulting in significantly narrower PSDs compared to those achieved by the conventional ALD processes at 150 and 200 °C. Importantly, Pt NPs with narrower PSDs exhibit higher catalytic activity and improved stability, which are demonstrated for the propene oxidation reaction, despite having a significantly lower Pt loading. Our approach may open a new avenue toward the size-selection synthesis of noble metal NPs for catalytic applications.

Horizontal stirred bed reactors (HSBRs) are widely used in the commercial production of polypropylene (PP). Despite their commercial significance, a comprehensive understanding of the flow behavior in HSBRs remains elusive, primarily due to the lack of detailed experimental data. This study investigates the influence of operating parameters on the particle flow behavior of two types of PP reactor powder in a laboratory-scale HSBR using X-ray imaging. Our results indicate that the overall flow behavior and phase holdup in the HSBR are dominated by agitation. Moreover, gas injection through the inlet points at the bottom of the HSBR results in spouting behavior, which can lead to reduced gas–solid contacting and, in extreme cases, complete bypass. Finally, the presence of liquid (in this study, isopropyl alcohol) adversely affects the flow behavior of the PP reactor powder due to liquid bridging at the contact points of particles. Powders that comprise particles with relatively small sizes and dense surface morphology are particularly prone to reduced flow behavior when exposed to liquid.

Vibro-assisted fluidization of cohesive micro-silica has been studied by means of X-ray imaging, pressure drop measurements, and off-line determination of the agglomerate size. Pressure drop and bed height development could be explained by observable phenomena taking place in the bed; slugging, channeling, fluidization or densification. It was observed that channeling is the main cause of poor fluidization of the micro-silica, resulting in poor gas-solid contact and little internal mixing. Improvement in fluidization upon starting the mechanical vibration was achieved by disrupting the channels. Agglomerate sizes were found to not significantly change during experiments.

In this study, the impact of different vibrational modes on the fluidization characteristics of cohesive micro- and nano-silica powder was examined. Fractional pressure drop, bed expansion measurements, and X-ray imaging were utilized to characterize the fluidization quality. The densities of the emulsion phase at the top and bottom of the column were quantified and compared, providing insights into the solid distribution within the fluidized bed. In the absence of vibration, neither powder could be fluidized within the considered range of superficial gas velocities. Vertical vibration was found to initiate fluidization for both powders. In contrast, elliptical vibration failed to overcome the channelling behavior when fluidizing the micro-powder. For nano-powder, combined channelling and powder compaction occurred when the bed was subjected to elliptical vibration. For the micro-powder, it was observed that bed homogeneity was independent of vertical vibration intensity but improved with increasing superficial gas velocity. For nano-powder, intensifying vertical vibration led to segregation, likely due to agglomerate densification. Furthermore, fractional pressure drop measurements proved to be a strong tool in assessing fluidization quality, providing insights that could not be attained by conventional indicators.

To make green hydrogen more economically attractive, the energy losses in alkaline electrolysis need to be minimized while operating at high current densities (1 A cm⁻²). At these current densities the ohmic resistance and gas bubbles effects contribute largely to the energy losses. To mitigate the gas bubbles losses, we demonstrate, for the first time, a pressure swing to remove gas bubbles in a zero-gap alkaline water electrolyzer. The pressure swing leverages the ideal gas law to increase the volume of gas in the system periodically, for a short duration (<2 s). This temporal volume increase effectively removes bubbles from the electrolyzer. We show that pressure swing can be used to measure the effect of bubbles on the ohmic resistance (R_Bubbles). Our results reveal that foam electrodes have a significantly larger R_Bubbles than perforated plate electrodes (1.8 Ω cm² vs 0.3 Ω cm²). The time-averaged cell voltage reduces by 170 mV when applying pressure swings to an electrolyzer operating at 200 mA cm⁻² in 1 M KOH with foam electrodes. The bubble resistance further depends on the electrolyte conductivity (inversely proportional) and is only moderately affected by operating pressure (25 % lower when increasing pressure amplitude from 1–2 to 1–5 bar). By implementing these findings in a model, we estimate that the pressure swing could reduce the cell voltage by ∼0.1 V for an electrolyzer operating at industrial conditions (6 M KOH, 80 °C, 1 A cm⁻²) for foam electrodes. For perforated plate electrodes, however, the reduced cell voltage is lower and does not outweigh the additional compression energy.

In this study, we investigate the correlation between fluidization behavior and flow properties of 10 commercially available cohesive powders through fluidization and rotating drum experiments. The rotating drum was operated at high speeds to aerate the powder, creating flow fields and stress conditions comparable to those in gas–solid fluidized beds. We introduce a fluidization quality index (FQI) computed from X-ray imaging, which addresses the limitations of conventional analyses for assessing fluidization quality of cohesive powders. By analyzing flow patterns, the FQI integrates gas holdup and its temporal variation, providing a reliable measure of fluidization quality. The findings establish a positive correlation between flowability and fluidization quality, demonstrating how flowability measurements can predict the effectiveness of mechanical vibration in enhancing fluidization characteristics. These results suggest that fast, user-friendly flowability assessments in a rotating drum can effectively predict fluidization potential, contributing to process optimization and advancing fluidization studies for cohesive powders.

Horizontal stirred bed reactors are widely used in the commercial manufacturing of polypropylene. However, a comprehensive understanding of the particle dynamics in horizontal stirred bed reactors remains elusive, primarily due to the lack of detailed experimental data. In this work, we studied the influence of operating parameters on the particle flow dynamics in a laboratory-scale horizontal stirred bed reactor using single-photon emission radioactive particle tracking. The results show that the general solids flow behavior is strongly affected by both the agitator rotation speed and reactor fill level. Operation at low rotation speed and low fill level results in solids flow with poor radial and circumferential distribution due to internal bed circulation. On the contrary, at increased rotation speeds and fill levels, solids motion throughout the bed is continuous resulting in excellent solids distribution. The solids circulation was found to increase for both an increase in rotation speed and reactor fill level. The axial dispersion coefficient, on the other hand, shows a linear relation with the rotation speed, but no conclusive relation between the axial dispersion coefficient and the reactor fill level was found.

Extending the lifetime of electrocatalytic materials is a major challenge in electrocatalysis. Here, we employ atomic layer deposition (ALD) to coat the surface of carbon black supported platinum nanoparticles (Pt/CB) with an ultra-thin layer of silicon dioxide (SiO2) to prevent deactivation of the catalyst during H2 evolution. Our results show that after an accelerated durability test (ADT) the current density at −0.2 V vs. reversible hydrogen electrode (RHE) of the unprotected Pt/CB catalyst was reduced by 34%. By contrast, after coating the Pt/CB catalyst with 2 SiO2 ALD cycles, the current density at the same potential was reduced by 7% after the ADT procedure, whereas when the Pt/CB sample was coated with 5 SiO2 ALD cycles, the current density was reduced by only 2% after the ADT. Characterization of the Pt particles after electrochemical testing shows that the average particle size of the uncoated Pt/CB catalyst increases by roughly 16% after the ADT, whereas it only increases by 3% for the Pt/CB catalyst coated with 5 cycles of SiO2 ALD. In addition, the coating also strongly reduces the detachment of Pt nanoparticles, as shown by a strong decrease in the Pt concentration in the electrolyte after the ADT. However, 20 cycles of SiO2 ALD coating results in an over-thick coating that has an inhibitory effect on the catalytic activity. In summary, we demonstrate that only a few cycles of SiO2 ALD can strongly improve the stability of Pt catalyst for the hydrogen evolution reaction.